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Symposium B: Concepts in Molecular and

Organic Electronics

Concepts in Molecular and Organic Electronics

April 13 - 17, 2009

Chairs

Norbert Koch Institut fuer Physik

Humboldt-Universitaet zu Berlin

Newtonstr. 15

Berlin, 12489 Germany

49-30-2093-7819

Egbert Zojer Institut fuer Festkoerperphysik

Technische Universitaet Graz

Petersgasse 16

Graz, 8010 Austria

43-316-873-8475

Saw-Wai Hla Dept. of Physics and Astronomy

Ohio University

Xiaoyang Zhu Dept. of Chemistry

University of Minnesota

Clippinger Lab 251B

Athens, OH 45701

740-593-1727

207 Pleasant St. SE

Minneapolis, MN 55455-0431

612-624-7849

Symposium Support Air Force Research Laboratory

Proceedings to be published in both book form and online

(see MRS Online Proceedings Library at www.mrs.org/opl)

as volume 1154

of the Materials Research Society

Symposium Proceedings Series.

* Invited paper

SESSION B1: Molecular Scale Electronics I

Chair: Norbert Koch

Monday Morning, April 13, 2009

Room 2001 (Moscone West)

8:30 AM *B1.1

SAMs, the ``Liquid-Metal Junction, and the Study of Electron Transport through Organic

Monolayer Films." George Whitesides1, E. Weiss

1, R. Chiechi

1, C. Nihuis

1, W. Reus

1 and M. A

Rampi2;

1Chemistry, Harvard University, Cambridge, Massachusetts;

2Chemistry, University of

Ferrara, Ferarra, Italy.

Self-assembled monolayers (SAMs) are now accepted as the most flexible system available for

use in studying the physical-organic chemistry of surfaces and monolayer films. Superficially,

the most widely studied classes of SAMs—alkanethiolates on metals (Au, Ag, Pd, Pt) seem to be

relatively simple structurally: in essence, defective 2D organic crystals supported epitaxially on

the metal film. The underlying molecular reality is much more complicated, and SAMs have

many types of local irregularities and heterogeneities. In some types of experiments, those

irregularities may be relatively unimportant; in others, very important. This talk will describe

studies of electron transport across SAMs using “liquid-metal drop” junctions—that is, junctions

of the form metal/SAM//SAM/liquid metal (optimally eutectic GaIn)--and the use of SAMs to

study the mechanisms of this process. Although improving the reproducibility, and defining the

mechanism of electron transport, in these (and related) systems is a challenging experimental

problem, the metal/SAM//SAM/liquid-metal junction (especially using so-called “ultrasmooth”

metals as supports for the SAM) is probably the most useful one now available for surveying the

relationship between organic structure, current density, and the parameter characterizing

attenuation of current distance with thickness (β). For most SAMs, tunneling is probably the

most important contribution to electron transport. The talk will describe studies of the influence

of a number of other characteristics of the system—the structure of the metal-organic interface,

the method of preparation of the SAM, and the structure of the organic molecules making it up--

on current density.

9:00 AM *B1.2

Electronic Processes in Organic Electronic and Photonic Devices: A Theoretical

DescriptionJean-Luc Bredas, School of Chemistry and Biochemistry, Georgia Tech, Atlanta,

Georgia.

Conjugated organic oligomer and polymer materials are now increasingly used as the active

semiconductor elements in devices such as photo-voltaic cells, light-emitting diodes, field-effects

transistors, electro-optic modulators, or all-optical signal processors. In this presentation, we will

discuss the progress we have recently achieved in the theoretical understanding of: (i) the

mechanisms for charge dissociation and charge separation at the donor/acceptor interface in

organic solar cells; and (ii) the linear and nonlinear optical properties of a new class of cyanine

molecules that present a strong bond-alternated geometric structure.

9:30 AM B1.3 Is a Molecular Electronics Technology Useful? - A Device Theorist’s Perspective.Avik

Ghosh, Electrical Engineering, University of Virginia, Charlottesville, Virginia.

Molecular electronics is often touted as a potential successor to silicon-based CMOS, due to the

advantages related to size, cheapness of fabrication, chemical tunability and „bottom-up‟ self-

assembly. However, progress has been stymied by difficulties of creating stable junctions and

reproducible measurements of molecular currents. The situation has improved considerably over

the last few years, as experiments from various groups seem to be in agreement. It is thus a good

opportunity to compare experiments with theoretical models, and perhaps look ahead at what

molecules may be good for. <P> I will start by describing a unified theory of current flow

through any nano- channel, from small organic molecules, all the way to larger „molecules‟ such

as silicon nanowires, carbon nanotubes and graphene nanoribbons. By coupling this method with

quantitative band-structure theory, we can quantitatively explain a wide variety of observed

molecular current-voltage characteristics. For insulating alkanethiols, we can explain the

magnitude and robustness of the decay constant and derive a modification to Simmons‟ equation

that yields the current per molecule. For aromatic molecules, we can establish both the maximum

current and the zero-bias conductance - the latter requiring incorporation of self- interaction

correction to get the right barrier height. For hybrid devices, we can predict the influence of

bonding configuration and molecular dipole on the band- structure of the underlying

semiconducting substrate, capturing molecular resonant tunneling diodes, molecular sensing and

molecular rectifiers on silicon. For molecular quantum dots, we can accurately describe Coulomb

Blockade effects that require careful attention to many-body excitations. <P> While our model

provides excellent quantitative benchmarks with dozens of experiments, its prediction for

molecular transistors is grim - we expect moleFETs to underperform due to strong tunneling, low

mobility and poor electrostatics. The strength of molecules lies instead in its charging energy,

suggesting that they be used as scatterers rather than channel materials. In fact, molecular

chemistry and self-assembly could be utilized to engineer surface traps in a silicon transistor,

leading to a new class of devices that rely on the utilization rather than the elimination of

scattering at nano-micro interfaces, involving novel multi-trap correlation effects that are

observed experimentally.

9:45 AM B1.4 2D Molecular Networks for Organic and Molecular Electronics Dmitrii F. Perepichka,

Chemistry, McGill University, Montreal, Quebec, Canada.

The single greatest advantage of organic molecules vs inorganic materials for electronic

applications is their capacity to self-assemble into complex and, at times, functional

architectures. Yet, practical applications of this property in organic/molecular electronics is

essentially limited to self-assembled monolayers (SAMs, where the molecules are simply grafted

on a surface with very limited lateral order) or 3D crystals (where the molecular packing is

critical for charge transport phenomenon, but little or no rational control of this packing can yet

be achieved). I will present our achievements in synthesis and surface characterization (STM and

auxiliary techniques) of 2D pi-electron functional molecular networks, highlighting two major

approaches: (i) using weak interactions to create nanotemplates on which organic

semiconducting molecules can be assembled, and which symmetry and periodicity can be

rationally and predictably controlled by the structure of a building block,[1] and (ii) exploring

surface-confined reactivity to create pi-conjugated polymers through epitaxial polymerization

reactions.[2] The latest development of the second approach opens an opportunity for creating a

new class of electronic materials- two dimensional (2D) conjugated polymers. I will describe the

principles of molecular design, the templating and catalytic role of the surface in formation of

these ordered functional materials, and provide our view on implications for the future

development of molecular electronics. [1] K.Nath et al. JACS 2006, 128, 4212; K.Nath et al.

JPC-C 2007, 111, 16996; J.MacLeod et al. Nanotechn. 2007, 18, 424031; O.Ivasenko et al.,

submitted. [2] J.Lipton-Duffin et al. submitted; (see also L.Grill et al. Nature Nanotech. 2007, 2,

687)

10:30 AM *B1.5 Conductance of Single Molecule Circuits. Latha Venkataraman, Applied Physics, Columbia

University, New York, New York.

The field of molecular electronics involves probing, manipulation, and control of single

molecules as active elements in electrical circuits. The underlying focus is to fabricate single

molecule circuits, a molecule attached to two electrodes, with varied functionality, where the

circuit structure is potentially defined with atomic precision. An experimental prerequisite to

creating functional molecular devices is to fabricate single molecule junctions reliably and

understand their physical properties. This poses fundamental challenges to nanoscience. Building

single molecule junctions with atomic precision is beyond the capability of top-down fabrication

techniques; however, variations in the device anatomy at the single bond scale can significantly

affect the junction characteristics. A bottom-up approach based on synthetic chemical control is

required to enable reproducible junction formation with well defined metal-molecule bonding

motifs. From a fundamental physics perspective, we need to understand, both experimentally and

theoretically, the electronic properties of metal-molecule-metal junctions. This will be the

primary focus of my talk. In this talk, I will review the scanning tunneling microscope break-

juncion technique we use to measure electronic transport through single molecule junctions, and

focusing on electron transport in the linear response regime, I will show that junctions formed

with gold-thiol links have large variability in conductance which can be attributed to the atomic

structure of the single molecule junctions formed with Au-S bonds. I will then discuss our

measurements using using novel metal-molecule link chemistries including amine-gold, methyl

sulfide-gold and dimethyl phosphine-gold to overcome the inherent variability of the gold-thiol

bonds. I will show that low-bias conductance of single molecules can be measured reliably and

reproducibly using these link groups [1-3]. I will then discuss the influence of the intrinsic

molecular properties, including their length, conformation, the gap between the highest occupied

molecular orbital and the lowest unoccupied molecular orbital and the alignment of this gap to

the metal Fermi level on the measured conductance. For single molecule junctions, I will show

that conductance relates to molecular conformation a biphenyl, two benzene rings linked

together by a single C-C bond [2]. For substituted benzenes, the relation between measured

conductance and the calculated ionization potential will be discussed [4]. For fused benzene

rings, I will show that conductance depends on the location of the amine groups on the molecule

[5]. [1] L. Venkataraman, et al., Nano Lett. 6, 458 (2006). [2] L. Venkataraman, et al., Nature

442, 904 (2006). [3] Y. S. Park, et al., J. Am. Chem. Soc. 129, 15768 (2007). [4] L.

Venkataraman et al., Nano Letters 7, 502 (2007). [5] J. R. Quinn et al., J. Am. Chem. Soc. 129,

6714 (2007).

11:00 AM B1.6 Charge Transport and Vibronic Effect in Nanoscale Molecular Junctions. Hyunwook Song

and Takhee Lee; Department of Materials Science and Engineering, Gwangju Institute of

Science and Technology, Gwangju, Korea, South.

Study of charge transport in molecules has become an active area of experimental and theoretical

research with promise in a variety of applications including molecular electronics [1],

optoelectronics [2], and thermoelectric energy conversion [3]. In this presentation, we interrogate

critical aspects on charge transport and vibronic effect in molecular junctions (MJs) by using

three different prototypes of device test-beds where either individual molecules or an ensemble

of molecules form a bridge between metallic electrodes in nanometer-scale junction area to

measure charge transport characteristics and electron-vibration coupling in MJs. First, we report

on a statistical method for investigating charge transport through MJs exploiting mass-produced

nanowell device structures (~7,000 devices) [4], which provides an objective criterion to

determine the most probable transport characteristics including a dominant conduction

mechanism in MJs. In the second part of the presentation, we will discuss the observation of two

tunneling pathways possible in self-assembled alkanethiol monolayers, i.e., through σ-bond and

intermolecular chain-to-chain tunneling, using conducting atomic force microscopy [5]. Lastly,

we present a method to explore the energy band lineups in MJs using a single-molecule transistor

(SMT) technique where individual molecules are linked between source and drain electrodes

with a third gate electrode. Furthermore, inelastic electron tunneling spectroscopy (IETS) is used

to observe vibrational spectra in a SMT configuration as the position of electronic resonance

level shifts by adjusting the gate voltage. The IETS provides not only in situ characterization

technique to identify the molecules associated with charge transport, but also a valuable insight

into how the interaction of charge carriers with molecular vibrational modes influences the

overall charge transport characteristics in MJs. [1] M. Galperin, M. A. Ratner, A. Nitzan, and A.

Troisi, Science 319, 1056 (2008). [2] M. S. Gudiksen, K. N. Maher, L. Ouyang, and H. Park,

Nano Lett. 5, 2257 (2005). [3] P. Reddy, S. Y. Jang, R. A, Segalman, and A. Majumdar, Science,

315, 1568 (2007). [4] H. Song, T. Lee, N. -J. Choi, and H. Lee, Appl. Phys. Lett. 91, 253116

(2007). [5] H. Song, H. Lee, and T. Lee, J. Am. Chem. Soc. 129, 3806 (2007).

11:15 AM B1.7 Electron-Phonon Interactions in Single Molecule Junctions. Joshua Hihath

1, Carols R

Arroyo2, Gabino Rubio-Bollinger

2, Nongjian Tao

1 and Nicolas Agrait

2;

1Center for

Bioelectronics an Biosensors, Arizona State University, Tempe, Arizona; 2Departamento Fisica

de la Materia Condensada CIII, Universidad Autonoma de Madrid, Madrid, Spain.

Although significant advances have been made in molecular electronics in recent years, a

complete understanding of how a molecule interacts with its environment when connected to two

electrodes is still lacking. To help alleviate this issue, spectroscopy is becoming an increasingly

important aspect molecular electronics research. And, Inelastic Electron Tunneling Spectroscopy

(IETS), which provides information about the vibration modes of the molecular junction, has

become one of the more commonly used tools for characterizing molecular systems by helping to

detail electron-phonon interactions and tunneling pathways. However, even more information

can be gained about molecular transport by applying these same principles at the single molecule

level. We have studied the electron-phonon interactions in a single molecule junction where the

molecule is covalently bound to two electrodes. In this study, the vibration modes in a single

molecule junction are measured while the strain in the junction is changed by separating the two

electrodes. This unique approach allows changes in the conductance to be compared to changes

in the configuration of a single molecule junction by recognizing changes in the vibration energy

of the junction bonds. This approach opens a new door for characterizing single molecule

junctions and better understanding of the relationship between molecular conductance, electron-

phonon interactions, and junction configuration.

11:30 AM B1.8 Reversible Conductance Switching in Molecular Devices. Auke Jisk Kronemeijer

1, Hylke B

Akkerman1, Tibor Kudernac

2, Bart J van Wees

1, Ben L Feringa

2, Paul W.M. Blom

1 and Bert de

Boer1;

1Zernike Institute for Advanced Materials, University of Groningen, Groningen,

Netherlands; 2Stratingh Institute for Chemistry, University of Groningen, Groningen,

Netherlands.

Since the theoretical study of Aviram and Ratner in 1974 predicting a single molecule

functioning as a diode, the field of Molecular Electronics has received increasing interest.

Methodologies and processing techniques have been investigated to measure the electrical

properties of (single) molecules, including breakjunction techniques and scanning probe

microscopy techniques. Our approach has been to produce molecular junctions in vertical

interconnects, predefined in an insulating photoresist matrix [1]. In the vias, a self-assembled

monolayer (SAM) is created on the underlying gold patterned substrate and the junctions are

finished with a conducting polymeric top contact. The formation of this top contact is the key

step in the processing since it has been shown that direct evaporation of metals on top of SAMs

results in penetration of the metal through the monolayer. Instead, the conducting polymer

PEDOT:PSS is spincoated on top of the monolayer. Because of this extra layer, direct

evaporation of the auxiliary top contact is possible, penetrating to some extent the polymer layer

but leaving the SAM intact. Reliable molecular junctions based on alkane(di)thiol molecules

demonstrated that the electronic properties of the molecules in the SAM are indeed measured [2].

Therefore, functionalities can now be incorporated into the molecules. Photochromic switches

are molecules which are stable in two different states and can be driven from one state into

another by applying light of specific wavelength. An interesting subgroup of switches are the

diarylethenes, because during the transition between the two states, the π-conjugation between

both ends of the molecule is formed or disrupted. Supposedly, this change in conjugation will

change the conducting properties of the molecules in molecular junctions. Molecular devices

which show (non-volatile) conductance switching are interesting for memory applications.

Therefore, diarylethenes have been assembled in a monolayer in the molecular junctions. J-V

characteristics of the molecular junctions were obtained for the two different states of the

switches, resulting in a clear difference in conductance between both forms. In situ irradiation of

the devices results in reversible switching of the conductance of the devices [3]. Reference

devices exhibit no switching. Therefore, the bidirectional conductance switching can indeed be

attributed to the monolayer of molecular switches in the devices. [1] H.B. Akkerman, D.M. de

Leeuw, P.W.M. Blom and B. de Boer, Nature 441, 69-72 (2006) [2] P.A. van Hal, E.C.P. Smits,

T.C.T. Geuns, H.B. Akkerman, B.C. de Brito, S. Perissinotto, G. Lanzani, A.J. Kronemeijer, V.

Geskin, J. Cornil, P.W.M. Blom, B. de Boer and D.M. de Leeuw, Nature Nanotechnol.

Advanced Online Publication 2008 [3] A.J.Kronemeijer, H.B. Akkerman, T. Kudernac, B.J. van

Wees, B.L. Feringa, P.W.M. Blom and B. de Boer, Adv. Mat. 20, 1467-1473 (2008)

11:45 AM B1.9 Conjugated Molecular Junctions with Strong Electronic Coupling. Richard L McCreery

1,

Adam J Bergren1, Ken Harris

1 and Sergio Jimenez

2;

1National Institute for Nanotechnology,

University of Alberta, Edmonton, Alberta, Canada; 2Cinvestav Querétaro, Libramiento

Norponiente No 2000, Mexico.

A thin layer (1-5 nm thick) of organic molecules bonded covalently to a very flat (<0.5 nm rms)

graphitic carbon substrate comprise the active region of reproducible, large area molecular

electronic junctions. UV-Vis and Raman spectroscopy indicate strong electronic coupling

between the molecule and substrate, possibly due to the nearly symmetric C-C bond between

phenyl rings. Vapour deposition of copper as a top contact completes the molecular junction,

resulting in >90% device yield, and with current-voltage characteristics that strongly depend on

the structure of the molecular component(1,2). Current-voltage curves are repeatable for millions

of cycles, and absolute current densities vary by 10-20% for several devices made with a given

molecule. In some cases, a covalent bond between the Cu top contact and a functional group in

the molecule has been detected such that both molecular layer-contact interfaces involve

covalent bonds The observed current is dependent on temperature above ~250 K, but

independent of temperature between 200 K and 5 K. The linear (i.e. “ohmic”) current-voltage

behaviour observed at 5 K indicates that the molecular layer has orbitals at the Fermi energy of

the contacts, and that transport through the molecular layer is resonant. Although transport

appears to be of the “on resonance” type, it occurs through a small subset of the available

molecules in the devices examined to date. While the current junctions behave as nonlinear

conductors, related devices show promise as photonic interfaces(3) and non-volatile

memory(4,5). References (1) Bergren, A. J.; Harris, K. D.; Deng, F.; McCreery, R.; J. Phys.

Condens. Matter 2008, 20, 374117. (2) Anariba, F.; Steach, J.; McCreery, R.; J. Phys. Chem B

2005, 109, 11163. (3) Bonifas, A. P.; McCreery, R. L.; Chem. Mater. 2008, 20, 3849. (4)

Barman, S.; Deng, F.; McCreery, R.; J. Am Chem Soc 2008, 130, 11073. (5) Wu, J.; Mobley, K.;

McCreery, R.; J. Chem. Phys. 2007, 126, 24704.

SESSION B2: Molecular Scale Electronics II

Chairs: Saw-Wai Hla and Heike Riel

Monday Afternoon, April 13, 2009

Room 2001 (Moscone West)

1:30 PM *B2.1 Charge Transport through Single Molecules - Opportunities and Challenges. Emanuel

Loertscher, Raoul Scherwitzl, Marius Trouwborst and Heike Riel; IBM Research, Rueschlikon,

Switzerland.

As device dimensions continue to shrink into the nanometer length-scale regime, conventional

semiconductor technology will be approaching fundamental physical limits. New strategies,

including the use of novel materials, innovative device architectures, and smart integration

schemes need to be explored and assessed. In that respect, molecular electronics is aimed at the

use of individual or small ensembles of molecules as functional building blocks in electronic

circuits. However, in order for molecular electronics to become a viable technology a number of

fundamental scientific issues need to be addressed. We have investigated electronic transport

through individually contacted and addressed molecules in a two-terminal break-junction

configuration under ultra-high vacuum condition at various temperatures. In this talk I will

discuss in particular our investigations of the molecule-metal coupling which we studied using

an ideal molecular system by anchoring a phenyl ring either by thiol, isocyanide or alternative

linker groups to gold electrodes. Transport measurements have been performed by repeatedly

forming and breaking the molecular junction during simultaneously recording the conductance

(under a fixed bias) and the current-voltage characteristics. The molecule-metal coupling was

derived from the molecular level broadening in resonance on the single-molecule level and from

the off-resonance conductance averaged over many thousands of junctions. The level broadening

was found to be 50% larger in case of thiol compared with isocyanide coupling, which is in good

agreement with the off-resonant conductance determined to be 25% higher for thiol coupling.

Moreover, electrical transport studies using alternative linker groups with a larger number of

chemical bonds will be presented. The conductance of the metal-molecule-metal system is

compared with thiol and isocyanide linker groups in the in-resonance as well as in the off-

resonance case.

2:00 PM B2.2 A Bistable Poly[n]rotaxane-Based Solid-State Switch. Erica Deionno

1, Wenyu Zhang

2,

William Dichtel3 and J. F Stoddart

4;

1Electronics and Photonics Laboratory, The Aerospace

Corporation, El Segundo, California; 2Department of Chemistry and Biochemistry, University of

California, Los Angeles, Los Angeles, California; 3Department of Chemistry and Chemical

Biology, Cornell University, Ithaca, New York; 4Department of Chemistry, Northwestern

University, Evanston, Illinois.

Mechanically interlocked compounds, such as bistable [2]catenanes and [2]rotaxanes, have been

shown to exhibit switching behavior in both the solution phase and the solid-state. Bistable

poly[n]rotaxanes, a new class of materials containing mechanical bonds and donor-acceptor

interactions, have been synthesized, and these polymers might be capable of similar switching

behavior in the solid-state. We report in this paper the fabrication and electrical characterization

of solid-state devices incorporating a bistable poly[n]rotaxane. Preliminary device measurements

show that the electrical properties of the devices are dependent on the electrode materials.

Devices with metal electrodes did not exhibit any switching behavior, while devices with silicon

bottom electrodes displayed a hysteretic response with applied voltage, indicating that the

devices can be switched between two conductance states, on and off. The polymer films can be

spin-cast on the bottom electrodes and the thickness of the films can be easily tuned, providing

an advantage over other molecule switches in the ease of fabrication.

2:15 PM B2.3 Collective Reactivity of Molecular Chains Self-Assembled on a Surface Peter

Maksymovych1,2

, Dan C Sorescu3, Kenneth D Jordan

2 and John T Yates, Jr.

4,2;

1Center for

Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee; 2Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania;

3U. S.

Department of Energy, National Energy Technology Laboratory, Pittsburgh, Pennsylvania; 4Department of Chemistry, University of Virginia, Charlottesville, Virginia.

Molecular self-assembly on surfaces is a route toward not only creating structures, but also

engineering new chemical reactivity afforded by the intermolecular interactions. We will

describe a new type of chemical reaction which occurs in linear chain-like molecular structures

spontaneously self-assembled from CH3SSCH3 molecules on Au(111) and Au(100) surfaces [1]:

injecting low energy electrons from a scanning tunneling microscope tip into a molecule in the

chain initiates a chain reaction that propagates through as many as 10 molecules in a row before

being quenched. Nearly every aspect of the chain reaction, as revealed by scanning tunneling

microscopy (STM) at 5K, is unprecedented. From STM-kinetic measurements, the chain reaction

can be triggered by one or two electrons, depending on the energy of the tunneling electrons. In

conjunction with the DFT-calculations of the electronic structure of the adsorbed CH3SSCH3

molecule, this identifies electron attachment to a CH3SSCH3 molecule as the triggering stage of

the chain reaction. Subsequent dissociation of the CH3SSCH3 molecule produces two hot CH3S

fragments, one or both of which then collide with neighbor CH3SSCH3 molecules, causing their

S-S bond to dissociate with the ejection of another CH3S fragment, and so forth. The propagation

of the chain reaction is thus reminiscent of the operation of the Newton‟s cradle. The majority

reaction products turn out to be CH3SSCH3 molecules that relate as mirror images to the original

CH3SSCH3 molecules. This is a unique product, since exciting an isolated CH3SSCH3 molecule

leads only to its dissociation or diffusion as a whole, but never to simultaneous rotation of both

methyl groups about the S-S bond. Self-assembly therefore provides a new reaction coordinate

for the CH3SSCH3 molecules dissociating on gold surfaces. DFT calculations of the transition

states reveal a nearly barrierless sequence of elementary steps along the new reaction coordinate

involving CH3S intermediates stabilized by neighbor CH3SSCH3 molecules. [1] P.

Maksymovych, D. C. Sorescu, K. D. Jordan, and J. T. Yates, Jr., Science (2008) in press. We

thank D. B. Dougherty for fruitful discussions. P. M. and J.T.Y: Supported by the W. M. Keck

Foundation and by the Army Research Office. K.D.J acknowledges support from the National

Science Foundation through grant CHE0518253. A grant of computer time at the Pittsburgh

Supercomputer Center is gratefully acknowledged. P.M.: Research performed in part as a

Eugene P. Wigner Fellow and staff member at the Oak Ridge National Laboratory, managed by

UT-Battelle, LLC, for the U.S. Department of Energy under Contract DE-AC05-00OR22725.

2:30 PM B2.4

Self-Assembled Monolayers of Various Conjugated Macrocycles Grafted on Silicon Oxide

for Memory Applications Virginie Gadenne1, Lionel Patrone

1, Alexandre Merlen

2, Mireille

Mossoyan-Deneux3 and Louis Porte

3;

1Institut Supérieur de l‟Electronique et du Numérique,

IM2NP, CNRS, IM2NP (UMR 6242), Maison des Technologies, Place Georges Pompidou, F-

83000 Toulon, France; 2Université du Sud Toulon-Var, IM2NP, CNRS, IM2NP (UMR 6242),

Bâtiment R, BP 20132, F-83957 La Garde cedex, France; 3Aix-Marseille Université, IM2NP,

CNRS, IM2NP (UMR 6242), Faculté des Sciences et Techniques, Campus de Saint-Jérôme,

Avenue Escadrille Normandie Niemen - Case 142, F-13397 Marseille Cedex, France.

Today self-assembled monolayers (SAM) [1,2] constitute a promising strategy to build

molecular nano-devices. Beside many studied molecules, conjugated macrocycles such as

porphyrins and phthalocyanines are good candidates for charge storage, within molecular

memory cells [3]. However their correct operation strongly depends on both molecular order and

binding to the substrate. Thus it is very important to control the deposition of these macrocycles

to form spontaneously ordered monolayers grafted on silicon for applications compatible with

microelectronics technology. In this work, we studied SAMs of three conjugated macrocycles on

silicon oxide pre-functionalized by aminopropyltrimethoxysilane (APTMS) coupling agent.

Selected macrocycles are Zn and Fe protoporphyrins bearing two carboxylic acid functions,

ZnPP and FePP respectively, and a synthesized Zn phthalocyanine surrounded by height

carboxylic acid functions, ZnPc(COOH)8. Carboxylic functional groups on peripheral cycles

allow molecules anchoring on the surface by imino link formation with APTMS. The samples

were characterized by scanning probe microscopy, ellipsometry, contact angle measurements,

Fourier transform infrared (ATR-FTIR) and UV-visible spectroscopy. Using gold surfaces and

replacing APTMS by amino-thiol coupling moieties allowed us to perform complementary

analysis of the grafting and structure of macrocycles with scanning tunneling microscopy and

surface enhanced Raman spectroscopy on appropriate nanostructured substrates. From analysis

carried out all along ZnPc(COOH)8. SAM formation, we show UV-visible spectroscopy exhibits

both a blue shift suggesting a growing face to face orientation and a splitting of the Q-band (at

698 nm). These results suggest a dimerization, due to formation of hydrogen bonds between

peripheral carboxylic acid groups [4]. They show that a great number of carboxylic acid

functions surrounding the molecule allows promoting a “π-stacking” arrangement interesting to

obtain a charge delocalization over a small memory cell. On the contrary, for ZnPP SAM, Soret

band (at 400 nm) is red-shifted during the growth, which is typical for an edge to edge

organization [5]. However no shift is observed for FePP, indicating these molecules assemble in

a different way. Furthermore FTIR spectroscopy together with ellipsometry and scanning probe

microscopy experiments indicate that ZnPc(COOH)8 and ZnPP anchor perpendicular to the

surface via imino bonds whereas FePP is linked by axial ligand position owing to iron electronic

configuration, which is less favorable for memory applications. [1] A. Ulman, Chem. Rev. 1996,

96, 1533 [2] F. Schreiber, Progress in Surf. Sci. 2000, 65, 151 [3] C.M. Carcel, J.K. Laha, R.S.

Loewe, P. Thamyongkit, K.-H. Schweikart, V. Misra, D.F. Bocian, J.S. Lindsey, J.Org. Chem.

2004, 69, 6739 [4] H. Xia, M. Nogami, Opt. Mater. 2000, 15, 93 [5] S. Boeckl, A.L. Bramblett,

K.D. Hauch, T. Sasaki, B.D. Ratner, J.W. Rogers., Langmuir 2000, 16, 564.

2:45 PM B2.5 Three-terminal, Single Molecule Circuits with Carbon Nanotube Wiring. Philip G. Collins,

Vaikunth Khalap, Danny Wan and Alexander Kane; Physics and Astronomy, Univ. of

California, Irvine, Irvine, California.

The vision for molecular electronics extends well beyond miniaturation and scaling of digital

electronics to include new techniques for studying chemical bonding, biocatalysis, and molecular

recognition in real time. However, operational single molecule devices remain exceedingly

fragile and difficult to fabricate. We have demonstrated a promising new architecture for the

versatile study of single molecule interrogation based on "point functionalization" of single-

walled carbon nanotube circuits [1]. In this technique, single defects are created in the sidewall

of an electrically connected nanotube. The technique, free of precision lithography or mechanical

manipulation, produces single, carbon-based attachment sites in operational circuits. A wide

range of organic molecules and biomolecules may then be covalently attached with single

molecule precision. This presentation will overview these experimental techniques and

demonstrate the types of signals that have been observed. Various chemical processes including

oxidation, conjugation, recognition and binding have all been monitored in real time [2,3]. The

circuit conductance and its spectral power density provide a direct measure of these time-

dependent, dynamic processes. Besides this high amplification transduction, other advantageous

properties of this architecture include excellent electrical, mechanical, and chemical stabilities

and well-defined bonding to the molecules of interest. [1] B. R. Goldsmith et al, Science v315 77

(2007). [2] B. R. Goldsmith et al, Nano Lett. v8 189 (2008). [3] J. Coroneus et al,

ChemPhysChem v9 1053 (2008).

3:30 PM *B2.6

Self-assembled Monolayers on Graphene Surfaces: Prospects for Graphene-based

Molecular Electronics. Mark C. Hersam, Materials Science and Engineering, Northwestern

University, Evanston, Illinois.

Chemically functionalized semiconductor surfaces have been widely explored due to their

potential for enabling molecular electronic and sensing devices that are compatible with

conventional microelectronic technology. Thus far, the vast majority of work in this field has

focused on established semiconductors including silicon, germanium, and gallium arsenide.

Meanwhile, the condensed matter physics community has diverted substantial experimental and

theoretical effort to graphene, an emerging electronic material with superlative carrier mobility

and exotic charge transport phenomena such as the quantum Hall effect. In an attempt to unify

these two fields, this talk will describe our recent efforts to interrogate and exploit organic self-

assembled monolayers on graphene surfaces. In particular, we have recently demonstrated that

self-assembled monolayers of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA)

can be formed on graphene surfaces via gas-phase deposition in ultra-high vacuum (UHV)

environments. Molecular-scale resolution scanning tunneling microscopy (STM) images reveal

long-range order in the PTCDA monolayers, while scanning tunneling spectroscopy (STS)

measurements yield the interplay between the electronic structure of the PTCDA molecules and

the underlying graphene substrate. These fundamental studies are expected to provide insight

into the prospects and opportunities for graphene-based molecular electronics.

4:00 PM B2.7 Non Destructive Metallic Contacts for Molecular Electronic Devices. Oliver Seitz

1, Min Dai

2

and Yves J. Chabal1;

1University of Texas at Dallas, Dallas, Texas;

2Rutgers University,

Piscataway, New Jersey.

Making reliable metallic contacts to organic molecules probably remains one of the biggest

challenges. Various approaches for establishing electrical contacts on self assembled monolayers

(SAMs) have been investigated (non-metallic: conductive polymers; metallic: LOFO, PALO,

direct and indirect evaporation, liquid metal), but none of them can provide a reliable non-

destructive contact with atomic control necessary for microelectronic devices. Atomic Layer

Deposition (ALD) is being considered for such applications because it could in principle make it

possible to chemically bond metal atoms to organics in a non destructive fashion. Using a novel

liquid copper precursor--copper(I) di-sec-butylacetamidinate ([Cu(sBu-amd)]2), which can react

with molecular hydrogen at moderate temperatures (~180oC), we have successfully deposited

metallic copper on top of a carboxylic terminated SAM that is covalently attached to the silicon

surface (Si-C bond). The successful intimate contact between the molecules and the Cu as well

as the integrity of the monolayer quality (no interfacial oxide growth during the process) has

been verified by in-situ Fourier transformed infrared spectroscopy (FTIR). X-ray photoelectron

spectroscopy (XPS) confirm the metallic state of the copper and the non oxidation of the Si

interface. Atomic force microscopy (AFM) was used to quantify the homogeneity of these

contacts and the ability to create/control them at the microscale. We are now in the process of

electrical measurements, using a mercury contact as a reference electrode.

4:15 PM B2.8

Simultaneous Opto-electronic Measurement of Metal Filament Formation in Molecular

Electronic Devices. Nazanin Davani1, Ken T Shimizu

2 and Nicholas A Melosh

2;

1Chemical

Engineering, Stanford University, Stanford, California; 2Materials Science and Engineering,

Stanford University, Stanford, California.

The behavior of molecular devices is known to depend upon the electrodes, molecules, and

electrode-molecule contacts, and electrical switching in molecular junctions has been reported

for several molecules. However, it can be difficult to distinguish between molecular dependent

switching and metallic filament formation when investigating dynamic switching events, largely

because of the reliance on electrical characterization. Recently, we have demonstrated that it is

possible to measure the real-time optical absorption of the molecular layer in between metallic

contacts during voltage cycling using Surface Plasmon Resonance Spectroscopy (SPRS). SPRS

can provide detailed information about the optical variations in addition to electrical changes in

the junction. Here we report the opto-electronic properties of thin films of electrically inert poly

methylmethacrylate (PMMA). Surprisingly, PMMA films with Au electrodes show changes in

electrical conductivity and SPRS signal as a function of electrical bias. The observations are

consistent with electro-dissolution/deposition of Au ions within the junction. These phenomena

are characterized optically and electrically, and demonstrate that the number of filaments that

contributes to electrical conductivity is a small fraction of the total present in the junction.

Further, the optical and kinetic characteristics of the metal filaments are largely distinct from

molecular charges allowing the two processes to be separated. These results have significant

implications for stability and switching in thin molecular film devices.

4:30 PM B2.9 The Nature of Fluctuations in Molecular Heterojunction Electronics. Jonathan A Malen

1,5,

Kanhayalal Baheti3,5

, Peter Doak3,5

, T. Don Tilley3,5

, Rachel A Segalman2,5

and Arun

Majumdar1,4,5

; 1Mechanical Engineering, UC Berkeley, Berkeley, California;

2Chemical

Engineering, UC Berkeley, Berkeley, California; 3Chemistry, UC Berkeley, Berkeley,

California; 4Materials Science and Engineering, UC Berkeley, Berkeley, California;

5Materials

Science Division, Lawrence Berkeley Laboratory, Berkeley, California.

Single molecule circuits represent a lower limit on the scalability of electronic devices and are

hence an ultimate goal of nanotechnology. A deeper understanding of electronic transport

through single molecules is of critical importance to molecular electronics and bears added value

to the wider field of organic-inorganic heterostructured materials. While important experimental

and theoretical advances over the last decade have yielded some insight into electron transport in

metal-molecule-metal junctions, a number of fundamental questions remain unresolved. One

area of rich debate is the origin of fluctuations and stochastic switching of conductance. In

particular, (i) Compared to the energy barrier height, how large are the fluctuations in the charge

transmission function? (ii) Are thermal fluctuations more important than that of the metal-

molecule contact, or are both equally important? (iii) Do these values change with the molecule

or is there some universal behavior? By measuring the thermopower of a series of

pheneylenedithiol molecules we herein answer these questions. Junction thermopower S, an

alternative transport property to conductance, was determined by measuring the voltage

difference across molecules trapped between two gold contacts held at different temperatures (S

= V/ΔT). A modified scanning tunneling microscope (STM) was used to form a junction

between a gold STM tip and the molecule-coated gold substrate. An applied temperature bias ΔT

between the tip and substrate generated a proportional voltage V, related to S by V=S*ΔT. The

experiment was repeated 500-1000 times at each ΔT, resulting in a histogram of measured

voltages. Transport variations were quantified by ΔS/S, where ΔS is the observed spread in S.

Statistical analysis of data from repeated measurements shows that ΔS/S is dominated by

junction-to-junction differences rather than thermal fluctuations within a given junction. The

remarkably large Seebeck variation (ΔS/S~1) implies that variation in the molecular transmission

barrier is similar in magnitude to the barrier height itself. Measurements of 1,4-benzenedithiol,

4,4‟-dibenzenedithiol and 4,4”-tribenzenedithiol, show that increased degrees of freedom from

added benzene rings result in increased transport variations. These inherent sources of noise

represent a significant obstacle for single molecule electronics, but may be obscured when large

ensembles of molecules are present.

4:45 PM B2.10 Alignment of Molecular Energy Levels Between Two Biased Metallic Electrodes. Nikolai

Severin1, Daria Skuridina

1,2, Christian Seifert

1, Xie Dou

3, Ragnar Stoll

1, Stefan Hecht

1, Igor M

Sokolov1, Klaus Müllen

3 and Juergen P Rabe

1;

1Humboldt University, Berlin, Germany;

2Lomonosov University, Moscow, Russia;

3Max-Planck-Institute for Polymer Research, Mainz,

Germany.

While originally Aviram and Ratner had proposed a molecular recifyer based on tunneling

through donor and acceptor moieties linked by a spacer, it has been argued later that rectification

may be also achieved with a single physisorbed donor or acceptor molecule located

asymmetrically between two electrodes. The latter was attributed to the dependence of the

potential at the position of the molecular orbitals in the tunneling gap on their relative position

within the gap. However, it has been also claimed that the potential of a molecular adsorbate is

not dependent on the tip-sample distance but rather equal to the substrate potential. Here we

report in-situ STM and STS data on mono- and bilayers of conjugated molecules self-assembled

at the interface between an organic solution and the basal plane of graphite, in order to resolve

this issue. They confirm that the electron potential drops gradually across the molecular

adsorbate. In a first set of experiments, bias dependent STM-imaging of a molecular bi-layer

revealed a dependence of their visibility on the applied bias as predicted by the model. In a

second experiment, the relative position of a conjugated molecule within the tunneling gap was

varied by controlling the tip-surface distance. The dependence of the current rectification ratio on

the relative position of different molecules between substrate and tip is also consistent with the

model. The results indicate that resonance enhanced tunneling through physisorbed conjugated

molecules between two biased metallic electrodes depends sensitively on the gap width and the

relative position of the electronic orbitals within the gap, thereby providing a means to precisely

control current-voltage characteristics through the geometry of the gap.

SESSION B3: Molecular Scale Electronics III

Chairs: Avik Ghosh and Latha Venkataraman

Tuesday Morning, April 14, 2009

Room 2001 (Moscone West)

8:30 AM *B3.1

Imaging the Local Properties of Graphene: a New Platform for Molecular Electronics. Michael F. Crommie, Y. Zhang, V. Brar, C. Girit and A. Zettl; Physics Dept. and Materials

Science Division, LBNL, UC Berkeley, Berkeley, California.

Graphene, a single atomic layer of carbon, provides an exciting new platform for molecular

electronics due to its chemical and structural flexibility as well as its novel electrical,

mechanical, and magnetic properties. Scanning tunneling microscopy (STM) is an ideal tool to

study the properties of graphene at lengthscales necessary to evaluate its utility for molecule-

scale device applications. We have used STM to explore backgated graphene flakes, and we

observe surprisingly strong electron-phonon coupling in electronic tunneling spectra. We also

find that the graphene charge neutral point (the Dirac point) manifests itself as a clear feature in

tunnel spectra, and it can be shifted by applied gate voltage. By spatially mapping the Dirac point

we are able to map out electron density inhomogeneities in graphene with a spatial resolution at

the nm scale. Using this new technique we have observed molecule-induced charge

inhomogeneities that coexist with energy-dependent electronic interference patterns in graphene,

giving us new insight into the microscopic mechanisms that determine graphene electron

mobility.

9:00 AM B3.2 Metal/Insulator/Metal Thin Film Electrodes for Molecular Conduction. Bing Hu, Pawan

Tyagi and Bruce Jackson Hinds; Chemical and Materials Engineering, Univ. of Kentucky,

Lexington, Kentucky.

Producing reliable electrical contacts with gaps having the dimensions of molecular lengths is a

difficult challenge for molecular electronics. As a promising alternative to break-junctions, we

use conventional film deposition and photolithography to form an exposed edge of a thin film

multilayer structure (metal/insulator/metal). Molecules can self-assemble on the exposed edge

offering an alternative conduction path through the molecules with angstrom-scale dimensional

control. Critical to this approach is to have minimal background tunnel current through the

insulator layer sandwiched between metal layers. Robust electrodes with aluminum oxide

insulator layer are found on alloys of Al/Au and Ta/Au. The readily oxidized Al/Ta reduce

surface energy for a mechanically stable interface with oxide but allow molecular contact with

metallic gold at the pattern edge. Electrodes were successfully fabricated with this strategy with

current measured through a metal coordination compound cluster composed of a cube with

cyano linked Ni or Fe at the corners. Thiolacetate ligand tethers come off of the cluster core and

bind the complex to the metal leads, allowing the molecule to span the insulator gap on the

surface of the etched pattern. Molecules that do not bridge the gap are not electrically active.

Along the 10um pattern edge approximately 6000 molecules are involved in conduction. 10nA

per molecule is seen at 10mV bias. Tunnel current through the molecules is analyzed with

Simmons model and barrier height is found to be 1.1 eV and tunnel length of 1.2nm.

9:15 AM B3.3

Dipole Effects on Electron Transport through Helical Peptides Immobilized on Gold. Shunsaku Kimura and Tomoyuki Morita; Dept. of Material Chemistry, Kyoto University, Kyoto,

Japan.

Electron injection or extraction from helical peptides to gold was studied to evaluate the dipole

effects of Au-S linkages and helical peptides. The helical lengths of the helical peptides exceeds

over 3 nm, where the electron hopping mechanism is prevailing. The molecular terminal has a

ferrocene group as a redox species, and the electron transfer from the ferrocene unit to gold was

evaluated by electrochemical methods. The electron transfer process comprises three regions; the

hopping through the helical peptide under the peptide dipole, through the linker, and through the

covalent connection between Au and the peptide under the Au-S dipole. Not only the dipoles but

also the physical properties of the helical peptide SAMs were found to influence the electron

transfer processes at the interface. Eventually, we try to extract the dipole effects on the electron

transport through the helical peptide SAMs.

9:30 AM B3.4 Creating and Characterizing Robust, Large Area Molecular Electronic Junctions. Michael

Preiner1 and Nicholas Melosh

2;

1Applied Physics, Stanford University, Stanford, California;

2Materials Science and Engineering, Stanford University, Stanford, California.

While recent years have seen numerous advances in creating single-molecule electronic

junctions, practical molecular electronic devices will almost certainly require junctions with

large numbers of molecules. We demonstrate a technique for creating large area, electrically

stable molecular junctions. We use atomic layer deposition to create nanometer thick passivating

layers of aluminum oxide on top of self-assembled organic monolayers with hydrophilic terminal

groups. This layer acts as a protective barrier and allows simple vapor deposition of the top

electrode without short circuits or molecular damage. This method allows nonshorting molecular

junctions of up to 9 mm2 to be easily and reliably fabricated. The effect of passivation on

molecular monolayers is studied with Auger and x-ray spectroscopy, while electronic transport

measurements confirm molecular tunneling as the transport mechanism for these devices. Using

a similar processing method, we also demonstrate the ability to rapidly characterize (and

passivate) single defects on hydrophobic SAMs.

9:45 AM B3.5 Atomic Level Analysis of Polythiophene by the Scanning Atom Probe. Osamu - Nishikawa

1,

Masahiro - Taniguchi2, Hitoshi - Kato

3 and Satoru - Tanemura

4;

1Chemistry & Biology,

Kanazawa Institute of Technology, Nonoichi, Japan; 2Chemistry & Biology, Kanazawa Institute

of Technology, Nonoichi, Japan; 3Physics of Materials, Kanto Gakuin University, Yokohama,

Japan; 4Physics of Materials, Kanto Gakuin University, Yokohama, Japan.

The atom probe (AP) is known to be an ultimate micro mass analyzer that allows atom-by-atom

mass analysis. However, the analyzable area of the conventional AP is limited to a minute

hemispherical area at the apex of an extremely sharp and long tip because a high field required to

field evaporate surface atoms as positive ions can be generated by a relatively low voltage. The

fabrication of such a filamentary long tip is extraordinary difficult for many materials.

Accordingly, the preparation of the specimens, such as polymers, is another barrier for wide

range applications of the atom probe. In order to overcome this difficulty, a funnel-shaped micro-

extraction electrode is introduced in the conventional AP. Since this electrode scans over a

planar specimen surface, this atom probe is named as a scanning atom probe (SAP). The

extraction electrode confines the high field required for field evaporation of surface atoms into a

small space between an apex of a minute cusp on the planar surface and an open end of the

electrode. Thus, the SAP can analyze not only a sharp slender tip but also an apex area of the

cusp. Thin films of conductive polythiophene are fabricated by electrochemically polymerizing

thiophene monomers on an ITO substrate. Film thickness is about 10μm-30μm and the dopant is

BF4-. A small piece of the polythiophene films is peeled off from the substrate and inserted into

a narrow gap between two small Nichrome nails. Then the film is clipped by the nails. The

Nichrome holder is introduced in the SAP and placed in front of the electrode. DC voltages and

pulsed YAG laser beams are applied to the specimen. The pulse width of the pulsed laser is 5 ns

and its wavelength is 532 nm. The field evaporated ions pass through the open hole of the

electrode and enter the reflectron type mass analyzer. Mass resolution of the analyzer m/dm is

better than 1000. Mass spectra of the analyzed polythiophene exhibit a large mass peak of

SC4Hn2+, the radical of the polythiophene, and various singly charged fragment ions such as

C2H+ and C3Hn+. The doubly charged radical ions indicate that the radicals are strongly bound.

All sulphur atoms are detected as S-C clusters such as SC+, SC2+ and SC3Hn+. Although no

fluorine ions are detected, most boron atoms are detected as the cluster ions with S and C such as

SCB2+ and C4HB+. Presently the mass analysis of the polythiophene films containing C60

molecules are under progress. The distribution of C60 and dopants in the polythiophene films

will be discussed.

10:30 AM B3.6

Monitoring Dynamic Molecular Electronic Processes at Buried Interfaces with Ångstrom

Resolution using X-Ray Reflectivity. Jason D. Fabbri, Michael F Toney, Nazanin Davani, Ken

T Shimizu and Nicholas A Melosh; Stanford University, Stanford, California.

Molecular electronics has been proposed as a successor to conventional CMOS technology as

device dimensions continue to shrink. Interesting electrical behavior has been observed in

molecular junctions and structural and spectroscopic probes are needed to understand and

improve upon it. We demonstrate the utility of X-ray reflectivity as an in-situ probe of molecular

electronic junctions under electrical bias. In the present study, we have examined chlorophyll

monolayers deposited on silicon bottom electrodes using the Langmuir-Blodgett technique. The

spot size of the X-ray beam necessitates large area junctions (~ cm^2) posing a formidable

fabrication challenge. Using a soft top contact deposition technique we have developed we

obtain functioning molecular junctions. From the X-ray reflectivity curves, we obtain precise

structural characterization of the junction. In addition, by applying a pulsed voltage sequence we

detect changes in reflected intensity as a function of applied electric field. Control experiments

and modeling show that these changes can be attributed to polarization of the molecular layer.

Further modeling allows us to extract the voltage drop across the monolayer, an important

parameter for molecular electronic devices. Finally, we will discuss the general utility of this

technique as a highly spatially sensitive probe of electronic changes in thin film devices.

10:45 AM B3.7 Single Nanometric Memory Unit Based On a Protein-Nanoparticle Hybrid. Izhar Medalsy

1,

Arnon Heyman2, Or Dgany

2, Oded Shoseyov

2 and Danny Porath

1;

1Physical Chemistry, The

Hebrew University, Jerusalem, Israel; 2Agriculture, The Hebrew University, Jerusalem, Israel.

Protein as a versatile isolating template on one hand and a nanoparticle (NP) as an electric

storage component on the other hand have long been investigated as independent entities. The

ability to combine thsee two species to form a single addressable unit cell isolated from the

conductive surface and from adjacent NPs gives rise to a wide range of nanoelectronic device

applications. Here we demonstrate the means to achieve an ultra dense memory unit using

individual protein-NP hybrids by Conductive Atomic Force Microscopy (C-AFM). SP1 (Stable

Protein 1) is a boiling-stable (melting temperature, Tm~109 oC) ring-shaped protein complex, 11

nm in diameter. Mutants of SP1 were synthesized by means of genetic engineering, allowing its

selective attachment to gold or silica surfaces (SiO2). The SP1-gold affinity is controlled by the

cys-group at different positions on the protein structure. Furthermore a switchable silica binding

SP1 mutant was engineered. Thru solvent condition changes, silica-binding peptides (serving as

anchors) are gradually and controllably exposed, thus creating a tunable silica-binding scaffold

while significantly reducing nonspecific surface binding. The mutants and their corresponding

attachment to the surfaces were characterized by gel electrophoresis and AFM. In order to serve

as an addressable memory device, 2D arrays of the SP1 protein were formed using different

methods such as phospholipids trough and Langmuir Blodgett and characterized using TEM and

AFM. In addition to the capability of selectively attaching to different surfaces and forming

ordered 2D arrays, SP1 was connected to gold and Si-SiO2 NP. This setup of an isolating unit

connected to a chargeable NP over a conductive surface enables selective charging of the NP.

Each NP holds three charging states: natural, positive and negative. The charging of the

conjugated Si-SiO2 NP was induced and tested using C-AFM, revealing life times of the charged

states of 10 min in ambient and hours in vacuum. Using this setup, and the relative long charging

time, we were able to apply a read and write operations on individual 5nm Si-SiO2 NP

embedded in a stable protein. Having a stable and well ordered array of SP1-NP hybrids capable

of charge storage at a three state setup for long times will enable to implement ultra high density

memory array.

11:00 AM *B3.8 Transition from Tunneling to Hopping Transport in Long, Conjugated Molecular Wires C.

Daniel Frisbie, Chemical Eng & Materials Science, University of Minnesota, Minneapolis,

Minnesota.

In this talk I will describe a series of comprehensive electrical transport measurements on

conjugated molecular wires up to 10 nm in length, grown off of gold electrodes. The wires are

grown using stepwise imine-forming reactions; using this chemistry the length of the wires can

be precisely controlled from 1-10 nm. Electrical conduction of ~100 parallel wires is measured

using metal coated AFM tips to make the second contact. Both the temperature and length

dependence of the wire resistance indicates a transition from tunneling to hopping transport at

approximately 4 nm in wire length. In addition, the current-voltage characteristics reveal that

field emission can occur at higher bias voltages. An important benefit of the length dependent

transport measurements is that the role of contact resistance can be assessed directly and

separated from the wire resistance. Overall, these measurements combined with the synthesis

chemistry open unprecedented opportunities to probe the connection between tailored molecular

structure and charge conduction in small bundles of conjugated molecular wires.

11:30 AM B3.9

Formation of Conjugated Monolayers on Metallic Nanoparticles for Single-Molecule

Transport Studies. Alexander Benjamin Neuhausen1, David Goldhaber-Gordon

2, Chris

Chidsey3 and Zhenan Bao

4;

1Electrical Engineering, Stanford University, Palo Alto, California;

2Physics, Stanford University, Palo Alto, California;

3Chemistry, Stanford University, Palo Alto,

California; 4Chemical Engineering, Stanford University, Palo Alto, California.

We present studies of the formation of monolayers of conjugated organic molecules on metallic

nanoparticles. After monolayer formation, we have used click chemistry to link azide-terminated

monolayers with dialkyne bridge molecules. With appropriate tuning of azide density and the

concentration of linking molecules, we make the case for single-molecule linking of metallic

nanoparticles to form a dimer or "dumbbell" structure, and propose using nanoscale lithography

to fabricate leads to the nanoparticles, allowing for single-molecule transport and surface-

enhanced Raman spectroscopy measurements.

11:45 AM B3.10

Sub-10 nm Nanoimprint Lithography for the Application to Molecular and Organic

Electronic Devices. Andrew Paul Bonifas1,2

and Richard L McCreery2,3

; 1Materials Science and

Engineering, The Ohio State University, Columbus, Ohio; 2National Institute for

Nanotechnology, Edmonton, Alberta, Canada; 3Chemistry, University of Alberta, Edmonton,

Alberta, Canada.

As the fields of molecular and organic electronics mature, progression from fundamental

experiments to the incorporation of molecular/organic components into “real world” devices has

become of primary interest. Numerous methods have been proposed to facilitate this progression

which include, but are not limited to, top contact fabrication on a preassembled molecular layer

through direct or indirect metal evaporation, conducting polymer intermediate, and soft contact

methods. Although these methods incorporate standard semiconductor processes, the degree of

structural damage to the molecular layer is highly debatable. To circumnavigate damage to the

molecular layer, methods where both electronic contacts are fabricated prior to the molecular

layer formation can be employed. The limitation with this approach is the difficulty to fabricate

on the length scale of several aromatic molecules for molecular devices or the polaron

delocalization length in conducting polymer devices. This presentation will outline a novel

technique to fabricate sub-10 nm gaps with nanoimprint lithography (NIL) and its application to

molecular/organic electronic devices. The inherent 1:1 pattern transfer from the NIL mold to the

substrate limits NIL to the resolution of the mold fabrication process. A straight forward NIL

mold fabrication technique incorporates electron beam lithography (EBL) followed by reactive

ion etching (RIE) of the NIL mold. The resolution of this technique is limited by the EBL

process which is on the order of 15-20 nm. Our approach is an extension of the standard

EBL/RIE process where the RIE of a Si mold is followed by a controlled Si oxidation process.

The formation of SiO2 narrows the gap between the NIL mold‟s features and provides a surface

to allow the attachment of a silane based anti-adhesion layer. Since the mold‟s features are

fabricated from a Si single crystal, the oxidation process is highly uniform. This technique is

intended to make sub-10 nm nanoimprint lithography accessible to a large range of researchers.

To illustrate applications of this technique to molecular/organic electronic devices,

experimentally measured electronic properties of conduction polymers are presented as the gap

of the prefabricated contacts approaches the polaron delocalization length. Electronic differences

between spin-coated and direct-polymerized polymer layers are discussed as a function of the

contact gap dimension, fabrication method, and applied gate bias.

SESSION B4: Molecular Scale Electronics IV

Chairs: Michael Crommie and Daniel Frisbie

Tuesday Afternoon, April 14, 2009

Room 2001 (Moscone West)

1:30 PM B4.1 ATP-driven Ionic Transport in 1-D Lipid Bilayer Nano/bioelectronic Devices. Shih-Chieh

Huang1,2

, Nipun Misra1,3

, Alexander B Artyukhin1, Costas P Grigoropoulos

3, Jiann-Wen Ju

2 and

Aleksandr Noy1;

1Lawrence Livermore National Laboratory, Livermore, California;

2Civil and

Environmental Engineering, UCLA, Los Angeles, California; 3Mechanical Engineering, UC

Berkeley, Berkeley, California.

1-D lipid bilayers- protective lipid bilayer shells on the one-dimensional inorganic nanomaterials

such as nanotubes and nanowires could provide a versatile matrix for incorporating biological

molecules into nanoscale electronic devices. We have used this platform to show that a

biological ATP-driven ion pump can efficiently gate single carbon nanotube transistors. We

show that these hybrid bio/nanoelectronic devices respond to the specific ion pump inhibitors.

The device response to the ATP concentration increase is consistent with the Michaelis-Menten

kinetics and shows that the electrical signals observed in our experiments originate from the

activity of the membrane ion pump.

1:45 PM B4.2

Environmental Effects on the Single Molecule Conductance of

bis(thiahexyl)oligothiophenes. Simon J. Higgins1, Edmund Leary

1, Harm van Zalinge

1,

Wolfgang Haiss1, Richard J Nichols

1, Christopher M Finch

2, Iain Grace

2 and Colin Lambert

2;

1Department of Chemistry, University of Liverpool, LIVERPOOL, United Kingdom;

2Department of Physics, University of Lancaster, LANCASTER, United Kingdom.

It is now possible to measure the electrical conductance of nanoscale junctions in which a small

integer number of molecules bridges two metal contacts. This has further stimulated research

into single molecules as electronic components. There have been surprisingly few reports of

significant solvent effects upon single molecule conductance. Simple alkanedithiols exhibit the

same molecular conductance whether measured in air, under vacuum or under liquids of different

polarity. Here, using an STM-based method, we show that the presence of water „gates‟ the

conductance of a family of bis(thiahexyl)oligothiophenes, and that the longer the oligothiophene,

the larger is the effect; for the longest example studied, the molecular conductance is over two

orders of magnitude larger in the presence of water. This is an unprecedented result, suggesting

that, depending upon molecular structure, ambient water may need to be taken into account when

measuring single molecule conductances, or in the design of future single molecule electronic

devices. Theoretical investigation of electron transport through the molecules, using the ab initio

non-equilibrium Green‟s function (SMEAGOL) method, suggests that water molecules interact

directly with the thiophene rings, significantly shifting the transport resonances and greatly

increasing conductance.

2:00 PM *B4.3

Designing, Measuring and Controlling Molecular- and Supramolecular-Scale Properties

for Molecular Devices. Paul Weiss, Departments of Chemistry and Physics, Pennsylvania State

University, University Park, Pennsylvania.

We use molecular design, tailored syntheses, intermolecular interactions and selective chemistry

to direct molecules into desired positions to create nanostructures, to connect functional

molecules to the outside world, and to serve as test structures for measurements of single or

bundled molecules. Interactions within and between molecules can be designed, directed,

measured, understood and exploited at unprecedented scales. We look at how these interactions

influence the chemistry, dynamics, structure, electronic function and other properties. Such

interactions can be used to advantage to form precise molecular assemblies, nanostructures, and

patterns, and to control and to stabilize function. These nanostructures can be taken all the way

down to atomic-scale precision or can be used at larger scales. We select and tailor molecules to

choose the intermolecular interaction strengths and the structures formed within the film. We

selectively test hypothesized mechanisms for electronic switching by varying molecular design,

chemical environment, and measurement conditions to enable or to disable functions and control

of these molecules with predictive and testable means. Critical to understanding these variations

has been developing the means to make tens to hundreds of thousands of independent single-

molecule measurements in order to develop sufficiently significant statistical distributions,

comparable to those found in ensemble-averaging measurements, while retaining the

heterogeneity of the measurements. We quantitatively compare the conductances of molecule-

substrate junctions. We demonstrate the importance of these junctions in conductance switching

of single molecules. We find that the contacts and substrate play critical roles in the switching.

Switching of rigid, conjugated molecules is due to changes the molecule-substrate bonds, which

involves motion of the molecules and also motion of substrate atoms. We are able to measure the

coupling of the electrons of the molecules to those of the substrate by measuring the

polarizabilities of the connected functional molecules in high and low conductance states. These

polarizabilities are compared to those of other families of molecules and to detailed calculations.

2:30 PM B4.4

Single Molecular Dopants in Pentacene and Corresponding Spatial Electronic Effects

Observed by Scanning Tunneling Microscopy/spectroscopy. Sieu D Ha and Antoine Kahn;

Electrical Engineering, Princeton University, Princeton, New Jersey.

The physics of doping in inorganic semiconductors has been studied extensively in the past half

century and is generally well understood. In particular, the standard hydrogenic model

sufficiently describes dopant atoms, and the Coulomb potential therein specified has been

observed by scanning tunneling microscopy (STM). In contrast, analogous experimental and

theoretical research for organic doping is not nearly as developed. We present here the first (to

the best of our knowledge) observation by STM of a single molecular dopant in an organic host

matrix. These observations at the molecular level are key to better understand the mechanisms of

charge release and doping in van der Waals-bonded systems. The investigated organic system is

pentacene grown on Si(111)/Bi(001) and p-doped with tetrafluoro-tetracyanoquinodimethane

(F4-TCNQ). Pentacene has been shown to grow in a near-bulk upright phase on Si(111)/Bi(001)

and is thus an ideal candidate for epitaxial multilayer film deposition. Multilayer films are

necessary to decouple the surface electronic structure of the molecular film from the metallic

substrate. Pristine pentacene films three to four monolayer thick are surface-doped with F4-

TCNQ, and molecular resolution is obtained by STM. The topographic effects of the surface

dopants are predominantly localized and are directly dependent on sample bias. As the applied

bias is varied, in particular for imaging filled and empty states, the appearance of the acceptor

molecules changes reversibly from bright clusters to dark vacancies. This effect is purely

electronic and is thus strong evidence of dopant observation. Upon deposition of an additional

monolayer of pentacene on the surface-doped film, the dopants appear more delocalized. As the

bias is varied, the subsurface dopant appearance correspondingly varies from bright hillock to

dark depression. The dopant feature extends over several molecular lattice sites depending on the

sample bias and is reminiscent of the Coulomb potential as observed in inorganic semiconductor

systems. Moreover, at low bias conditions the subsurface dopant cannot be observed. The

localized electronic effects of surface dopants are compared with the delocalized effects of

subsurface dopants. Defects in the pentacene film are easily distinguishable from proposed

dopants, as they appear as vacancies for all tunneling conditions with minimal bias dependence.

As an additional check, scanning tunneling spectroscopy measurements show reproducible

differences between the local density of states measured near dopant centers and far from the

outer extent of the Coulomb potential.

2:45 PM B4.5

Towards Single-Molecule Electronics: Synthesis of Single Organic molecule-Bis(long DNA)

Structures and their Characterizations.JungKyu Lee1, Frank Jaeckel

1,2, Young Hwan Jung

3,

Jeffrey B.-H. Tok4, William E Moerner

1 and Zhenan Bao

5;

1Chemistry, Stanford University,

Stanford, California; 2Physics, Ludwig-Maximilians-Universität München, München, Germany;

3Bioinformatics, Korea Bio Polytechnic, Chungnam, Korea, South;

4Micropoint Biosciences

Inc., Sunnyvale, California; 5Chemical Engineering, Stanford University, Stanford, California.

Precise electrical contact of single organic molecules onto electrodes is a key step to study

single-molecule electronics and its applications, such as nanodevices. To realize a reliable

electric contact, we introduce DNA as a template in the field of nanoelectronics because DNA

can be easily tethered to a single organic molecule using coupling reactions as well as be used as

a conducting element after DNA metallization. Herein, we demonstrate that the reactivity

between ssDNA and single organic molecules using several coupling reactions to build triblock

molecules. Furthermore, we show that the DNA length of the triblock molecules can be readily

controlled using the PCR or DNA hybridization. Finally, We will discuss recent technique to

characterize a single organic molecule-bis(long DNA) triblock architecture using the gel

electrophoresis, atomic force microscope (AFM), and single-molecule fluorescence microscopy.

3:30 PM *B4.6 Spin Excitations in Single Molecule Electronics Ungdon Ham, Physics and Astronomy,

University of California, Irvine, Irvine, California.

Recent results with a high field, sub-Kelvin scanning tunneling microscope (STM) in ultrahigh

vacuum will be presented. Inelastic electron tunneling spectroscopy (IETS) through spin-flip

excitation makes it possible to study spin degrees of freedom within a single molecule on the

surface. The STM allows the determination of the relationship between spin excitation and the

electronic and geometric structures of single molecules. The high field, low temperature

capabilities of the STM enables it to probe the dependence of spin excitation on the coupling of

the molecule to the substrate for a wide range of molecules such as single atoms, organometallic

molecules, and molecular radicals. Similar to the electron-vibrational coupling, spin-flip

excitations should play a significant role in electron transport through molecules.

4:00 PM B4.7 Role of Labile Bonding in Stochastic Switching of Molecular Conductance. Lionel Patrone

1,

Jeremie Soullier1, Pascal Martin

1, Bruno Jousselme

2 and Fabrice Moggia

2;

1Institut Supérieur de

l‟Electronique et du Numérique, IM2NP, CNRS, IM2NP (UMR 6242), Maison des

Technologies, Place Georges Pompidou, F-83000 Toulon, France; 2CEA/DSM/IRAMIS/SPCSI,

Bât. 466, CEA/Saclay, F-91191 Gif-sur-Yvette cedex, France.

Due to the miniaturization of CMOS devices new memory cells with high density and low-power

consumption are investigated. Molecular switches are likely to be among the most basic and

important components of future molecule-based electronic devices. In the literature, on-off

switching effect has been reported with phenylene-ethynylene oligomers bonded to gold via S-

Au, using nanopore junctions [1] and scanning tunneling microscopy (STM). In these STM

studies, molecules of interest have been inserted in an alkylthiol self-assembled monolayer

(SAM). The observed stochastic conductance switching has been explained by conformational

changes through aromatic ring rotation [2] or molecular hybridization changes at the interface

with gold [3]. However, independent STM studies on SAMs on gold have reported similar

stochastic switching effects for thiol molecules which cannot exhibit any conformational changes

[4]. A thiol bond breaking/reforming mechanism was invoked, due to the labile nature of thiol

bonding. Potential use of such conductance switching for molecular memory cells requires

determining whether it is intrinsic to specific molecules or due to a bond fluctuation mechanism.

For this purpose, comparative experiments need to be carried out on molecules more strongly

bonded to the substrate. In this work, we compare the influence of S-Au labile bond versus C-Si

stronger covalent one in obtaining a stochastic conductance switching of terthiophene molecules

(3T). First we prepared binary SAMs of small bundles of 3T dispersed in dodecyl (DD) matrix,

both on Au(111) using thiol-ended molecules and on hydrogenated silicon surface H-Si(111)

with molecules bearing a vinyl reactive head. For this purpose, terthiophene-thiol and

terthiophene-allyl molecules were synthesized. If obtaining binary SAMs on Au(111) is well

known [5], growth of binary SAMs on H-Si(111) was studied using ellipsometry, contact angle

measurements and scanning probe microscopy in order to obtain the right conditions giving

isolated 3T molecules in DD. Then, we performed STM experiments on these binary SAMs,

using the apparent molecular height of 3T above DD matrix as a measure of electronic

conductance. We observed stochastic switching events for S-Au bond as reported in the

literature. A statistical analysis of molecular blinking was carried out. However we show that

stochastic switching is hindered in the case of C-Si bond. These results allow attributing the

origin of published stochastic switching observations to a bond fluctuation mechanism.

Moreover this work shows silicon is a suitable substrate for developing molecular memory cells

both avoiding stochastic switching and being compatible with microelectronics technology. [1] J.

Chen et al., Science 1999, 286, 1550 [2] Z.J. Donhauser et al., Science 2001, 292, 2303 [3] A.M.

Moore et al., J. Am. Chem. Soc. 2006, 128, 1959 [4] G.K. Ramachandran at al., Science 2003,

300, 1413 [5] L. Patrone et al., Chem. Phys. 2002, 281, 325

4:15 PM B4.8

Random Telegraph Signal in Macroscopic Silicon/Alkyl chain/Metal Molecular Tunnel

Junctions. Dominique Vuillaume1, Nicolas Clement

1, David Guerin

1, Stephane Pleutin

1 and

David Cahen2;

1IEMN-CNRS, Villeneuve d'Ascq, France;

2Weizmann Institute of Science,

Rehovot, Israel.

Monolayers of organic molecules present one of the main systems studied in molecular

electronics. Recently, very high quality alkyl monolayers on oxide-free silicon were reported to

be a basic system, very reproducible to study the electrical transport through these molecules

(1,2). In a previous report, we have studied the low frequency tunnel current noise in these self-

assembled monolayers (3). We report here the observation and study of a particular type of noise

called Random Telegraph Signal (RTS) in Si/SiO2/Alkyl chain/(Hg or Al) and Si/Alkyl

chain/(Hg or Al) junctions.The 2 levels of current can be clearly distinguished. With a

sufficiently long recording time (> 500 events), statistics can be performed on the current level

and on the upper and lower times. The RTS amplitude is usually few % of the average current

and the process follows poissonian statistics. This RTS signal is also modulated by another RTS

with a much longer time constant. This allowed us evaluation of the change of noise in the

frequency domain from 1/f noise to Lorentzian like spectrum. In inorganic tunnel junctions, such

signal can only be observed in sub-micrometric junctions whereas we observe it in almost

millimetric junctions. This precludes mechanisms involving electron trapping / detrapping in

single isolated trap. We propose several hypotheses leading to long-range fluctuations including

molecular dynamics and relaxation processes. (1) A.Salomon, T.Boeking, C.K.Chan, F.Amy,

O.Girshevitz, D.Cahen and A.Kahn, Phys.Rev.Lett. 95, 266807 (2005) (2) O.Seitz, T.Böcking,

A.Salomon, J.J.Gooding and D.Cahen, Langmuir 22, 6915 (2006) (3) N.Clement, S.Pleutin,

O.Seitz, S.Lenfant, D.Cahen and D.Vuillaume, Phys. Rev.B. 76, 205407 (2007) (4) N.Clement,

S.Pleutin, D.Guerin, D.Cahen and D.Vuillaume, in preparation.

SESSION B5: Poster Session I

Chairs: Saw-Wai Hla, Norbert Koch, Xiaoyang Zhu and Egbert Zojer

Tuesday Evening, April 14, 2009

8:00 PM

Exhibition Hall (Moscone West)

B5.1 Charge Transfer Excitons on Organic Semiconductor Surfaces. Qingxin Yang, Matthias

Muntwiler and Xiaoyang Zhu; Department of Chemistry, University of Minnesota, Minneapolis,

Minnesota.

Charge transfer (CT) excitons across donor/acceptor interfaces are believed to be barriers to

charge separation in organic solar cells, but little is known about their physical characteristics.

Here we probe CT excitons on crystalline pentacene and tetracene surfaces using time-resolved

two-photon photoemission spectroscopy. We observe a series of atomic-hydrogen like CT

exciton states, with the 1s CT exciton state at a binding energy of ~0.5 eV. The large binding

energy of the 1s CT exciton excludes its participation in photovoltaic. Efficient charge separation

in organic hetero-junction solar cells must involve a series of hot CT excitons.

B5.2

Optical and Structural Properties of rrP3HT on Highly Dense Vertically Aligned

CNTs.Wei-Chao Chen1,2

, Chien-Hung Lin3, Hsiang-Ting Lien

4, Kuei-Hsien Chen

1 and Li-

Chyong Chen2;

1Institute of Atomic and Molecular Science, Academia Sinica, Taipei, Taipei,

Taiwan; 2Center for Condensed Matter Science, National Taiwan University, Taipei, Taipei,

Taiwan; 3Graduate Institute of Photonics and Optoelectronics and Department of Electrical

Engineering, National Taiwan University, Taipei, Taipei, Taiwan; 4Institute of Organic and

Polymeric Materials, National Taipei University of Technology, Taipei, Taipei, Taiwan.

Influence of interchain-interactions in rrP3HT/VACNTs (vertically aligned carbon nanotubes)

composites have been investigated via optical and structural analyses. The composite was

synthesised through dip coating in different concentration or deposition time. The interchain

coupling can strongly influence the electronic properties of polymer chains. On the surface of

highly dense VACNTs, it was observed the pristine rrP3HT [regioregular poly(3-

hexylthiophene)] naturally induces the self-assembling organization with high-symmetry cofacial

configurations, particularly stronger edge-on orientation. From Raman spectrum, the outmost

surfaces of CNTs possessing electron-rich property strongly induce the non-specific covalent

interactions (π-π or CH-π) with rrP3HT. Via strong π-π interactions between polymer-backbone

and CNTs-surface, or the multiple CH-π interactions between the alkyl side chains of conjugated

polymer and outmost graphene of CNTs, the high-symmetry cofacial orientation can be induced.

The organized structure, as observed by XRD, exhibits a highly lamellae orientation of P3HT in

parallel with VACNTs template on a-face [100] Photoluminescence excitation (PLE) mapping of

this solid structure of rrP3HT/VACNTs also reveals a blue shift of radiative emission from 0-1 to

0-0 transition, (or this 2-dimentional conjugated orientation of rrP3HT leads to the blue shift of

radiative emission at 0-1 and 0-0 transitions. These results suggest decreasing in the polaron-

binding energy of rrP3HT/VACNTs. The self-assembling 2-dimentional structure of rrP3HT is

naturally achieved on VACNTs, via highly cofacial orientation, without any requirement of

surface-treatment.

B5.3

Semiconductor Organic-Inorganic Nanocomposites at the Air/Water Interface and Their

Performance in Thin Film Photovoltaic Devices. Zhiqun Lin, Matthew Goodman, Jun Xu and

Jun Wang; Iowa State University, Ames, Iowa.

Organic-inorganic nanocomposites consisting of electroactive conjugated polymer, poly(3-

hexylthiophene) (P3HT) intimately tethered on the surface of semiconductor CdSe quantum dot

(i.e., P3HT-CdSe nanocomposites) at the air/water interface formed via Langmuir isotherms

were explored for the first time. The P3HT-CdSe nanocomposites displayed a high pressure

plateau in the Langmuir isotherm, illustrating their complex packing at the air/water interface.

Furthermore, photovoltaic devices fabricated from the LB depositions of the P3HT-CdSe

nanocomposites exhibited a relatively high short circuit current, ISC, while maintaining a thin

film profile. These studies provide insights into the fundamental behaviors of semiconductor

organic-inorganic nanocomposites confined at the air/water interface as well as in the active

layer of an organic-based photovoltaic device.

B5.4

Co-deposited Films of Rod-like Conjugated Molecules: Mixing versus Phase Separation. Jörn-Oliver Vogel

1, Ingo Salzmann

1, Steffen Duhm

1, Bert Nickel

2, Juergen P Rabe

1 and Norbert

Koch1;

1Institut für Physik, Humboldt-Universität zu Berlin, Berlin, Berlin, Germany;

2Department für Physik, Ludwig-Maximilians-Universität, Munich, Bavaria, Germany.

The co-deposition of two conjugated molecular species can lead to novel materials with new

properties. The formation of well ordered mixed layers of two different molecules can be used to

continuously tune the material properties, e.g., similar to the doping of inorganic semiconductors

[1] On the other hand, phase separation of donor and acceptor material is desired for organic

hetero-junction solar cells. To assess the extent to which the molecular length determines either

phase separation or mixing in co-deposited thin films, we investigated pairs of five different rod-

like conjugated molecules: α-quaterthiophene (4T), α-sexithiophene (6T), α,ω-

dihexylsexithiophene (DH6T), p-sexiphenyl (6P), and pentacene (PEN). With these molecules

we realized pairs differing in length either via the size of the conjugated core (CC) alone, or

additionally through the end-termination by alkyl chains. Such films were characterized with

specular and in-plane X-Ray diffraction (XRD), infrared absorption spectroscopy (IR) and

scanning force microscopy (AFM). The co-deposition of molecules with similarly sized CC, i.e.,

{6T/6P} led to well ordered mixed structures. The co-deposition of molecules with different

molecular length but similarly sized CC, i.e., {6T/DH6T and 6P/DH6T} led also to ordered

mixed layers. A particularly appealing feature of these films is that the interlayer distance can be

controlled via the mixing ratio. The co-deposition of molecules with differently sized CC, e.g.,

{4T/6T} or {6P/PEN} led to pronounced phase separation of the two species in thin films. The

results of this work will be useful to tailor the morphology and structure of organic co-deposited

films, in particular for solar cell applications. [1] P. Cosseddu, J.-O. Vogel, B. Fraboni, J. P.

Rabe, N. Koch, A. Bonfiglio, Adv. Mater., in press.

B5.5

Dependence of Organic Thin-film Transistors with Polymer-blend Small Molecule

Semiconductors on Molecular Weights and Types of Polymers. Takahiro Ohe, Miki

Kuribayashi, Ryoichi Yasuda, Ami Tsuboi, Kotaro Satori, Masao Itabashi and Kazumasa

Nomoto; Advanced Materials Laboratories, sony, Atsugi, Kanagawa, Japan.

We will discuss how performance of solution-processed organic thin-film transistors (OTFTs)

with a polymer-blend 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) channel are

affected by molecular weight (MW) and type of the polymers. It has been reported that spin-

coating of a solution of TIPS-pentacene and poly(alpha-methylstyrene) (PaMS) induces phase

separation, and results in a trilayer film: a TIPS-pentacene layer, a mixed layer of TIPS-

pentacene/PaMS, and a TIPS-pentacene layer[1,2]. The experiments with two different MW of

PaMSs[2] and with various types of polymer dielectrics[3] have been reported. It, however, has

not been clear the specific dependence of phase separation phenomena, and performance of

OTFTs on MW and types of polymers. In order to understand the relations among them, we have

performed systematic study. First, we investigated the dependence of the filed-effect mobility on

MW of polymer dielectric. The results showed that when MW of PaMS is around 20 k, 60 k, 100

k, and 800 k, the OTFTs exhibits higher mobility around 0.1 cm2/Vs. The OTFTs with PaMS

with lower MW ~2 k showed lower mobility around 1×10-4

cm2/Vs. TOF-SIMS analysis with

sputtering the film revealed that when MW is ~100 k (~2 k), the active layer is the separated

trilayer film (a homogeneously mixed film). This structural difference can be attributed to the

difference in the performance of the OTFTs. Various types of polymers have been also

examined. When we use the solubility parameter of the polymer closer to that of TIPS-

pentacene, for example, poly(isobutyl methacrylate) is employed as a polymer, the OTFTs

showed only lower mobility and the active layer was a homogeneously distributed structure. In

order to explain our results, we applied the Flory-Huggins theory to the polymer-blend organic

semiconductor system. In this theory, the Gibbs free energy change ΔGm for mixing a polymer

with a small molecule is given by ΔGm = kNTT[(χ12φ1φ2) + (φ1lnφ1 + (φ2/x)lnφ2)] , which depends

on MW and type of polymer. The results have shown that experimentally obtained conditions for

the phase separation are reasonably explained in term of the Gibbs energy calculated with this

equation. This consideration will be useful to develop higher-performance OTFTs. [1] T. Ohe et

al., Appl. Phys. Lett. 93, 053303 (2008). [2] J. Kang et al., J. Am. Chem. Soc., 130, 12273

(2008). [3] M.-B. Mardec et al., J. Mater. Chem., 18, 3230 (2008).

B5.6

``Column-like" Structure of the Cross-Sectional Morphology of Bulk Heterojunction

Materials. Ji Sun Moon, Jae Kwan Lee and Alan J Heeger; Center for Polymers and Organic

Solids, University of California, Santa Barbara, Santa Barbara, California.

Transmission electron microscopy (TEM) through thin sections cut from films of the bulk

heterojunction (BHJ) material comprising rr-poly(3-hexylthiophene), rrP3HT, and [6,6]-Phenyl-

C61 butyric acid methyl ester (PCBM) provides information on the cross-sectional morphology.

Previous studies of the BHJ morphology mostly have been carried out on or through the top

surface of the film (with Atomic Force Microscopy or with TEM, respectively). However,

subsequent to photo-induced charge separation, the photogenerated carriers must move toward

the electrodes by traveling across the film thickness rather than parallel to the film surface. Thus,

there is limited information on direct correlation between the cross-sectional morphology of the

BHJ material and the solar cell device performance. We report here the observation of “column-

like” structures in the defocused cross-sectional TEM images. These “column-like” structures

provide the required pathways for charge transport across the film thickness. We calculate the

power spectral density and the autocorrelation function of the vertical pathways and thereby

obtain information on the length scale of the nanometer scale phase separation.

B5.7

Layer Cross-Fading at Organic/Organic Interfaces in OVPD-Processed Red

Phosphorescent Organic Light Emitting Diodes as a New Concept to Increase Current and

Luminous Efficacy. Florian Lindla1, Manuel Bosing

1, Christoph Zimmermann

1, Frank Jessen

1,

Philipp van Gemmern2, Dietrich Bertram

2, Dietmar Keiper

3, Nico Meyer

3, Michael Heuken

1,3,

Holger Kalisch1 and Rolf H. Jansen

1;

1Chair of Electromagnetic Theory, RWTH Aachen

University, Aachen, Germany; 2Philips Technologie GmbH, Aachen, Germany;

3AIXTRON AG,

Aachen, Germany.

Organic light emitting diodes (OLED) have the potential to play a dominant role in solid state

lighting. Most small molecule based OLEDs are at present processed either by vacuum thermal

evaporation or organic vapor phase deposition (OVPD), with the latter offering unique features.

Most important, several growth parameters (substrate temperature, deposition chamber pressure,

carrier gas flows) can be controlled individually and growth rates are stable over a long period of

time. These features make it possible to control the ratios in mixtures of organic materials during

the deposition process precisely by adjusting the carrier gas flows through different organic

sources. This is used to investigate the impact of an introduced layer cross-fading at

organic/organic interfaces on current and luminous efficacy of phosphorescent red OLEDs.

Layer cross-fading describes linearly decreasing the fraction in growth rate of an organic layer

during deposition over a certain period of time while increasing the fraction in growth rate of the

following layer. The result is a cross-fading zone of controlled thickness. The base structure of

the investigated OLED consists of an ITO anode with a 20 nm hole injection layer (HIL), a 20

nm hole transport layer (HTL), a 40 nm host/guest system as red emission layer (EL) and a 30

nm electron transport layer (ETL) followed by a LiF/Al cathode. Devices with cross-faded

interfaces are compared with the basic layer-by-layer processed structure with sharp interfaces

and OLEDs in which the cross-fading zone is replaced by a mixed interlayer with constant ratios

of organic materials. The layer-by-layer processed OLED shows a current and luminous efficacy

of 18.8 cd/A and 14.1 lm/W (at 1000 cd/m2). A cross-fading zone of 10 nm thickness from either

the HIL to HTL or the EL to ETL has no impact on the efficacies. Whereas with a rising cross-

fading zone thickness of 40 nm at the HTL to EL interface, 29.3 cd/A (+56%) and 25.9 lm/W

(+84%) can be measured, compared to a 40 nm mixed interlayer which shows 24.8 cd/A (+32%)

and 20.4 lm/W (+45%). Different cross-fading zone and mixed interlayer thicknesses are

compared. The efficacy of OLEDs with a layer cross-fading always exceeds the one with mixed

interlayers. As result, layer cross-fading significantly increases both efficacy figures. The

observation of a lower driving voltage will be discussed in terms of an interpenetrating network

created in the cross-fading zone, which might improve charge injection and transport. A better

mixing of charge carriers would broaden the recombination zone and increase the current

efficacy. With an OVPD-based process, it is possible to realize nearly any kind of cross-fading

profile. Different profiles will be studied in future. Furthermore, the lifetime of OLEDs with

cross-faded interfaces will be investigated. An improved charge injection and transport should

have a positive impact here as well.

B5.8 Solution Processed Low-Voltage Organic Field-Effect Transistors Paul H Woebkenberg,

James M Ball, Florian Colleaux, Donal D Bradley and Thomas D Anthopoulos; Blackett

Laboratory, Imperial College London, London, United Kingdom.

The field of organic microelectronics has evolved rapidly during the past twenty years and is

now producing the first commercial applications. While recent progress in the area has been

astonishing, some major technology bottlenecks still remain and hinder practical implementation

of organic microelectronics in large-volume, low-end applications. One such technology

bottleneck is the large operating voltages of state-of-the-art organic field-effect transistors

(OFETs) and the resulting high power consumption. For example, the majority of OFETs

reported in literature operate at voltages in excess of 20 V. This makes organic transistor

technology unsuitable for use in a wide range of future applications including portable, battery-

powered devices where organic microelectronics could potentially play a dominant role. A very

promising approach towards low-voltage operation is the use of self-assembled monolayer

(SAM) nanodielectrics. Unfortunately, the majority of SAM based organic transistors have so far

been restricted to device architectures incorporating evaporated organic semiconductors.

Demonstration of similar low-voltage devices fabricated via solution processing has also been

attempted with limited success. It is the aim of this work to address both low-voltage operation

and solution processing of OFETs that are suitable for use in integrated circuits. We present a

simple method for fabricating <5 nm thin gate dielectrics from solution at room temperature

utilising suitably designed phosphonic-acid SAMs. We previously demonstrated that solution

processing of organic semiconductors on methyl terminated octadecylphosphonic acid is

incompatible with most organic semiconductors due to the SAM's low surface energy

characteristics. By modifying the terminal group (i.e. end-group) of the alkyl chain in these

molecules we are able to tailor the SAM‟s surface properties, consequently enabling solution

processing of a much wider range of organic semiconductors. Based on this approach we have

successfully demonstrated solution-processed hole (p-channel) and electron transporting (n-

channel) OFETs operating at voltages below |1.5| V. To demonstrate the potential of the

technology for use in practical applications, the transistors are integrated to form low-voltage,

low-power logic circuits such as unipolar and complementary voltage inverters. Based on the

same approach we also realise ambipolar SAM transistors and complementary-like inverters

employing a single semiconductor material. The potential application of these low-voltage

ambipolar organic transistors in light-sensing applications is also discussed. This work is a

crucial step towards the production of solution processed low-voltage, low-power organic

circuits and sensor arrays utilising low manufacturing cost methodologies.

B5.9 Properties of Fluorenyl Silanes in Organic Light Emitting Diodes. Wei Wei, Peter Djurovich

and Mark E Thompson; Department of Chemistry, University of Southern California, Los

Angeles, California.

Fluorene derivatives have been developed as potential charge transporters for OLEDs because of

their high charge transport mobilities and thermal stabilities. Here, we have designed and

synthesized four different fluorene silicon derivatives (PhnSi(DMFL)4-n, DMFL= 9,9-

dimethylflouren-2-yl, n= 0, 1, 2 and 3) with increasing number of fluorene units, in order to

provide a systematic study for investigating the changes of the molecular as well as the device

properties when the fluorene ratio increases in the molecule. All of these compounds possess

high triplet energies, large HOMO-LUMO gaps and high glass transition temperatures. Both

glass transition and sublimation temperatures increase linearly as the fluorene ratio increases. In

contrast, there is no apparent change in their electrochemical or photophysical properties, which

indicates that fluorene moieties have been conjugatively isolated by the central silicon. These

molecules exhibit ambipolar transport characteristics in undoped OLED devices

(ITO/NPD/mCP/PhnSi(DMFL)4-n/Alq3/LiF/Al) and the conductivity of the device is enhanced

by the molecules with higher fluorene ratios. Hence, the Si(DMFL)4 was used as the host of

Ir(ppy)3 and PQIr phosphorescence devices, respectively, and high device external efficiencies

were achieved.

B5.10 Stabilizing Single Atom Contacts by Molecular Bridge Formation. Everardus Hendrik

Huisman1, Marius L Trouwborst

1, Frank L Bakker

1, Bert de Boer

1, Bart J van Wees

1 and Sense

Jan van der Molen2;

1Zernike Institute for Advanced Materials, University of Groningen,

Groningen, Netherlands; 2Kamerlingh Onnes Laboratory, Leiden University, Leiden,

Netherlands.

A single molecule forms a potential electronic component, offering the perspective of true

bottom up engineering of nanodevices. However, the field of molecular electronics as been

troubled by difficulties in making reliable and well-defined contacts to single molecules.

Fortunately, recent times have seen a significant growth of independent techniques to contact

single molecules or small ensembles of molecules [1,2]. A popular way to form a single metal-

molecule-metal bridge is to carefully break a gold nanowire in a solution containing dithiolated

molecules [3]. Surprisingly, there is little understanding on the mechanical details of the bridge

formation process and specifically on the role that the dithiol molecules play themselves. We

demonstrate that alkanedithiol molecules have already formed bridges between the gold

electrodes before the atomic gold-gold junction is broken [4]. This leads to stabilization of the

single atomic gold junction, as observed experimentally. Our data can be understood within a

simple spring model. [1] Chen, F.; Hihath, J.; Huang, Z.; Li, X.; Tao, N. J. Annu. Rev.

Phys.Chem. 2007, 58, 535-564. [2] Akkerman, H. B.; de Boer, B. J. Phys.: Condens. Matter

2008, 20, 013001. [3] Xu, B.; Tao, N. Science 2003, 301, 1221-1223. [4] Huisman, E.H.;

Trouwborst, M.L.; Bakker, F.L.; de Boer, B.; van Wees, B. J.; van der Molen, S.J. Nano Lett.

2008, 8, 3381-3385.

B5.11

Carbon Nanotube Enabled Vertical Organic Field Effect and Light Emitting

Transistors.Mitchell McCarthy1,2

, Bo Liu2, Youngki Yoon

3, Do Young Kim

1, Zhuangchun Wu

2,

Franky So1, Paul H Holloway

1, John R Reynolds

4, Jing Guo

3 and Andrew G Rinzler

2;

1Materials

Science and Engineering, University of Florida, Gainesville, Florida; 2Physics, University of

Florida, Gainesville, Florida; 3Electrical and Computer Engineering, University of Florida,

Gainesville, Florida; 4Chemistry, University of Florida, Gainesville, Florida.

Active matrix organic light emitting diode (AMOLED) displays are expected to compete with

liquid crystal displays in the coming years due to the promise of increased power efficiency,

larger contrast ratio, wider viewing angle and thinner design. Currently there are technological

limitations to the widespread use of AMOLED displays such as limited lifetime of the OLED as

well material limitations preventing further technological advances, such as making AMOLED

displays flexible. All-organic AMOLED displays are required for use on flexible substrates;

however, the low mobility of organic materials, while enabling flexibility, hinders the overall

device performance by the requirement of larger driving voltages and smaller aperture ratios

leading to reduced power efficiency and lifetime. In this study, nanoscale materials are shown to

provide a promising path forward in the realization of industrially feasible all-organic flexible

AMOLED displays. In this collaborative study between the Departments of Computer Science

and Engineering, Materials Science and Engineering, Chemistry and Physics at the University of

Florida, carbon nanotube enabled vertical organic light emitting transistors (VOLETs) are

fabricated and characterized. Single walled carbon nanotube (CNT) networks possess unique

properties which make the networks ideally suited for use in the VOLET architecture. The low

density of states of the CNTs allows significant modulation of their Fermi level, resulting in

transconductance due to modulation of the hole injection barrier between the CNTs and the

active layer. It is shown by experiment and simulation that the unique properties afforded by the

nanoscopic nature of carbon nanotubes are responsible for the transconductance observed.

Preliminary devices using CNT networks show superior performance when compared to

VOLETs fabricated from other materials.

B5.12

Atomic-Scale Scanning Tunneling Microscopy and Spectroscopy Studies of Nanometer-

Sized Graphene Flakes on Semiconducting Surfaces. Justin Charles Koepke1,2

, Kevin He1,2

and Joseph W Lyding1,2

; 1Electrical and Computer Engineering, University of Illinois at Urbana-

Champaign, Urbana, Illinois; 2Beckman Institute for Advanced Science and Technology,

Urbana, Illinois.

We have used ultrahigh vacuum scanning tunneling microscopy to perform atomic level studies

of graphene on semiconducting surfaces. We used a dry contact transfer technique (DCT)

developed by Albrecht and Lyding [1] to deposit mechanically exfoliated graphene in-situ onto

atomically clean semiconducting surfaces [2]. The DCT technique deposits predominantly single

and double layers of atomically clean graphene with lateral dimensions ranging from 2 - 60 nm.

We observe varying degrees of transparency of the graphene monolayers depending on the

substrate. This is most pronounced for graphene on the cleaved InAs(110), GaAs(110), and

Si(111)-7x7 surfaces, where the substrate atomic structure is clearly seen through the graphene.

On the Si(111)-7x7 surface, the substrate atomic structure can also be seen through graphene

bilayers. We believe that electronic structure of a graphene monolayer on the InAs(110) and

GaAs(110) surfaces leads to the transparency of monolayers and the opacity of bilayers similar

to the findings of Rutter, et al [3]. We suspect that the transparency of graphene bilayers on the

Si(111)-7x7 surface is due to a similar effect, while the transparency of graphene monolayers on

the same surface also has a topographic component. Room-temperature scanning-tunneling

spectroscopy (STS) measurements of the graphene monolayers and bilayers on the Si(111)-7x7

surface show predominantly metallic behavior. STS measurements of graphene features on the

InAs(110) and GaAs(110) surfaces also show predominantly metallic behavior, but

Semiconducting behavior is observed for the smaller features. [1] P.M. Albrecht and J.W.

Lyding, Appl. Phys. Lett. 83, 5029 (2003). [2] K.A. Ritter and J.W. Lyding, Nanotechnology 19,

015704 (2008). [3] G.M. Rutter, et al, Phys. Rev. B 76, 235416 (2007).

B5.13 Gold Work Function Reduction by 2.2 eV with a Molecular Donor Layer. Benjamin

Broeker1, Ralf-Peter Blum

1, Johannes Frisch

1, Antje Vollmer

2, Oliver T Hofmann

3, Ralph

Rieger4, Klaus Muellen

4, Juergen P Rabe

1, Egbert Zojer

3 and Norbert Koch

1;

1Institut für Physik,

Humboldt-Universität zu Berlin, Berlin, Germany; 2Berliner Elektronenspeicherring-Gesellschaft

für Synchrotronstrahlung mbH, Berlin, Germany; 3Institut of Solid State Physics, Graz

University of Technology, Graz, Austria; 4Max Planck Institut für Polymerforschung, Mainz,

Germany.

Ultraviolet photoelectron spectroscopy was used to investigate neutral methyl viologen (1,1'-

dimethyl-1H,1'H-[4,4']bipyridinylidene, MV0) deposited on Au(111) surfaces. As a result of

molecule-to-metal electron transfer, the work function of Au(111) was decreased from 5.5 eV to

3.3 eV. The energy levels of electron transport layers deposited on top of modified Au surfaces

were shifted to higher binding energy compared to layers on pristine Au, and the electron

injection barrier was reduced by 0.8 eV for tris(8-hydroxyquinoline)aluminum (Alq3) and by 0.7

eV for C60. The air-stable donor MV0 can thus be used to facilitate electron injection into

organic semiconductors even from high work function metals. This work is financially supported

by European Community project "IControl" (EC-STREP-033197).

B5.14 Electronic Properties at Gold/Conjugated Polyelectrolyte Interfaces. Jung Hwa Seo,

Renqiang Yang, Jasek Z. Brzezinski, Bright Walker, Thuc-Quyen Nguyen and Guillermo C.

Bazan; UC Santa Barbara, Santa Barbara, California.

The electronic properties of conjugated polyelectrolytes (CPEs) with poly(fluorene-co-

phenylene) backbones and different counterions and charges have been investigated using

absorption, x-ray photoemission (XPS) and ultraviolet photoelectron spectroscopy (UPS). The

optical energy band gap of CPEs depends mainly on their conjugated backbone and nearly

insensitive to the charges or counterions. XPS and UPS measurements reveal that electron

injection from Au to polymers with cationic groups is more efficient than for the neutral and

anionic counterparts. The vacuum levels of CPEs were also shifted toward higher or lower

binding energy, relative to that of Au depending on the charge and counterion presence, and

provide insight into the general alignment of dipoles at the metal/organic interface. This finding

shows that counterions and backbone charges enable control of the electronic and chemical

nature of critical device interfaces.

B5.15

Study of Solid/liquid Interfaces in Organic Field-effect Transistors with Ionic

Liquids.Shimpei Ono1, Kazumoto Miwa

1, Shiro Seki

1 and Jun Takeya

2;

1Materials Science

Research Laboratory, Central Research Institute of Electric Power Industry, Tokyo, Japan; 2Graduate School of Science, Osaka University, Osaka, Japan.

There has been significant interest to develop new kinds of organic transistors such as those

using electric double layers (EDLs) of electrolytes. It is reported that the EDL gating can be a

promising technology not only to realize high performance organic field-effect transistors

(OFETs) [1-4], but also to drive phase transitions in strongly correlated electron systems to a

metal [5] or a superconductor [6]. However, there is no detailed study of microscopic

mechanisms of the charge transport in the vicinity of the solid/liquid interfaces. In this work, we

report the two-dimensional charge transport at the interface with the use of various ionic liquids

as the electrolyte layer in OFETs. Since the use of ionic liquid electrolytes in OFETs enables

high-density carrier doping with minimum gate voltages without scarifying the carrier mobility

[7], higher performances are likely to emerge by elaborate search for compounds incorporated in

OFETs. We have formed a well structure of polydimethylsiloxan elastomer on which rubrene

single crystal is electrostatically attached and each ionic liquid is poured underneath a rubrene

single crystal by the capillary force, so the EDL in the ionic liquid can induce high-density

carriers at the surface of the crystal. The achieved reproducibility permits one to observe that the

mobility of the charge carriers systematically increases with decreasing the dielectric constant of

the ionic liquids and becomes as high as 9.5 cm2/Vs in 1-ethyl-3methylimidazolium

bis(fluorosulfonyl)imide, which is only half of the value of the air-gap transistors. These results

suggest that the mobility of carriers in OFETs is an intrinsic property of the solid/liquid interface

between organic semiconductors and the ionic liquids. [1] M. Panzer, C. D. Frisbie et al., Appl.

Phys. Lett. 88 203504 (2006). [2] J. Takeya et al., Appl. Phys. Lett. 88 112102 (2006). [3] H.

Shimotani et al., Appl. Phys. Lett. 89 203501 (2006). [4] J. Lee, C. D. Frisbie et al., J. Am.

Chem. Soc. 129 4532 (2007). [5] H. Shimotani, et al., Appl. Phys. Lett. 91 082106 (2007). [6] K.

Ueno et al., Nature Materials (2008) in press. [7] S. Ono et al., Appl. Phys. Lett. 92 103313

(2008).

B5.16

Liquid-crystalline Semiconducting Copolymers with Intramolecular Donor-acceptor

Building Blocks for High-stability Polymer Transistors. Do Hwan Kim1, Bang-Lin Lee

1,

Hyunsik Moon1, Eun-Jeong Jeong

1, Jeong-Il Park

1, Kuk-Min Han

1, Byung Wook Yoo

1, Bon

Won Koo1, Joo Young Kim

1, Wi Hyung Lee

2, Kilwon Cho

2, Hector A Becerril

3, Zhenan Bao

3

and Sangyoon Lee1;

1Display Laboratory, Samsung Advanced Institute of Technology, Samsung

Electronics Co., LTD., Yongin, Korea, South; 2Department of Chemical Engineering, Pohang

University of Science and Technology, Pohang, Korea, South; 3Department of Chemical

Engineering, Stanford University, Stanford, California.

Organic field-effect transistors (OFETs) based on π-conjugated polymers have recently attracted

significant attention because of their potential use as soft channel materials in organic/printed

optoelectronic devices such as active-matrix flat panel displays (AMFPDs), electronic paper,

RFID tags, and chemical/bio-sensors. The solution-processability of π-conjugated polymers has

also stimulated interest in their utilization in active electronic elements for low-cost, large-area,

and flexible active matrix display backplanes, with performances that are comparable to those of

hydrogenated amorphous silicon (a-Si:H)-based thin-film transistors. However, when OFETs

based on π-conjugated polymers are fabricated and tested under ambient conditions, their

electrical performance decreases remarkably, which is primarily due to the sensitivity of the

polymer chains to atmospheric O2/H2O and structural defects. In particular, OFETs composed of

solution-processed π-conjugated polymers commonly exhibit some electrical instability under

external bias stress due to the less-ordered molecular structure of their semiconductor films;

charge trapping instability under bias-stress and environmental stability are problems for these

materials. In order to be comparable to a-Si-based TFTs, OFETs should exhibit similar

performance with respect to electrical bias stress. Although there have been a few studies aimed

at enhancing the environmental/electrical stability of π-conjugated polymers under external bias

stress, an adequate understanding of the relationship between crystalline nanostructure and bias

stress driven electrical instability on the microscopic scale still eludes us. Here we report a novel

charge-transfer type liquid-crystalline semiconducting copolymer,

poly(didodecylquaterthiophene-alt-didodecylbithiazole), which contains both electron-donating

quaterthiophene and electron-accepting 5,5‟-bithiazole units, and exhibits unprecedented

electrical characteristics such as field-effect mobilities as high as 0.33 cm2/Vs and good bias-

stress driven electrical stability that is comparable to that of amorphous silicon (a-Si). Liquid-

crystalline thin films with structural anisotropy form spontaneously through the self-organization

of the individual polymer chains as a result of intermolecular interactions in the liquid-crystalline

mesophase, and adopt preferential well-ordered inter-molecular π-π stacking parallel to the

molecular surface. This bottom-up assembly of the liquid-crystalline semiconducting copolymer

enables the facile fabrication of highly-ordered soft channel layers with a minimal concentration

of charge traps, as well as unidirectional and delocalized transport with hitherto unreported

electrical stability.

B5.17

Reversible Electro-optical Switch of Self Assembled Monolayers of azobenzene-derivatized

Oligothiophenes Grafted on Gold. Dominique Vuillaume1, Kacem Smaali

1, Stephane Lenfant

1,

Dominique Deresmes1, Sandrine Karpe

2, Maitena Ocafrain

2, Philippe Blanchard

2 and Jean

Roncali2;

1IEMN-CNRS, Villeneuve d'Ascq, France;

2CIMA-CNRS, Angers, France.

The grafting of azobenzene-based molecules on surfaces is an attractive approach for optical

switches [1;2], molecular machines [3] and biosensors [4]. Here, we report the preparation of a

self-assembled monolayer (SAM) on gold surface obtained by grafting molecules based on an

azobenzene moiety associated to a bithiophene unit (see inset below). The structure of the SAM

was characterized using various techniques such as ellipsometry, X-ray Photoelectron-

spectroscopy, cyclic voltammetry and contact angle measurement. This azobenzene moiety can

switch between two isomeric configurations (trans and cis) when irradiated by visible (480 nm)

and UV (360 nm) light respectively. The electrical properties of the SAM in each configuration

were studied by conducting-AFM and by eutectic drop contact (GaIn). We clearly observed the

effect of this photo-isomerization on the current, at the nano-scale by conducting-AFM and at

macroscopic scale using GaIn. The current (ION) measured by C-AFM at 1.5V for the cis isomer

was higher by a factor ~2000 compared to the current (IOFF) for the trans isomer. By measuring

with GaIn contact, the value of this ION/ IOFF ratio was 223 at 0.5V (see graph). These results

represent significant improvement compared to those reported by Mayor and coworkers namely

~ 30 at 0.3V by mercury drop contact [2] and ~ 20 at 0.5V by C-AFM [3]. Various hypotheses to

explain this conductivity variation will be discussed, in particular an I-V analysis (Fowler-

Nordheim plot) that suggests an injection barrier reduction (lowering LUMO). [1] Kumar A.S. et

al., Nanoletters 8(6), 1644-1648 (2008) [2] Mativetsky et al., JACS 130(29), 9192-9193 (2008)

[3] Ferri V. et al., Angew. Chem. Int. Ed. 47, 3407-3409 (2008) [4] Dietrich P. et al., Appl. Phys.

A 93, 285-292 (2008)

B5.19

Extended Lifetime of Organic Field-Effect Transistors Encapsulated with Titanium Sub-

Oxide as an `Active’ Passivation/Barrier Layer. Shinuk Cho1, Kwanghee Lee

2 and Alan J

Heeger1;

1Center for Polymers and Organic Solids, University of California at Santa Barbara,

Santa Barbara, California; 2Department of Materials Science and Engineering, Gwangju Institute

of Science and Technology, Gwangju, Korea, South.

Despite significant improvements in the performance of organic electronic devices, the operating

lifetimes are limited by the intrusion of oxygen (O2) and water vapor (H2O). High performance

encapsulation with low-cost is therefore required to achieve lifetimes sufficiently long to enable

commercialization of plastic electronics technology. We report that a thin capping layer of

titanium sub-oxide (TiOx), prepared by sol-gel synthesis from titanium alkoxides, extends the

lifetime of organic FETs. The TiOx layer functions as an „active‟ passivation/barrier layer that

actually removes oxygen and water vapor from the organic semiconductor. The results

demonstrate a significant improvement in the lifetime of organic field effect transistors when

exposed to air.

B5.20 peri-Xanthenoxanthene Thin-Film Transistors. Norihito Kobayashi, Mari Sasaki, Noriyuki

Kawashima and Kazumasa Nomoto; Advanced Materials Laboratories, Sony Corporation,

Atsugi, Japan.

We have synthesized and characterized stable organic semiconductors (OSCs), 3,9-diphenyl-

peri-xanthenoxanthene (Ph-PXX) and its soluble derivatives of 3,9-bis(p-alkylphenyl)-peri-

xanthenoxanthene (CnPh-PXX) for organic thin-film transistors (OTFTs). A π-system is

stabilized against oxidation by introduction of hetero-atoms and phenyl groups into the reactive

sites in the π-system. This strategy for stabilization does not suffer from the conventional trade-

off between environmental stability and efficient carrier injection, which appears when OSCs

with deeper highest occupied molecular orbitals (HOMOs) are applied. UV-Vis spectra of an air-

saturated solution of Ph-PXX were unchanged over 120 hours, indicating that the molecule has

great environmental stability. A HOMO level of Ph-PXX molecule was estimated to be only 5.1

eV below vacuum level, achieving efficient carrier injection from Au electrodes. In fact, OTFTs

with Ph-PXX showed high apparent mobility over 0.4 cm2/Vs without demonstrating nonlinear

behavior of source-drain ohmic contacts, and have been stable over five months under ambient

conditions. In addition, the OTFTs showed great thermal stability at temperatures up to 150°C in

air. These characteristics have been also achieved with a solution-processed OTFT with a soluble

CnPh-PXX. In this presentation, we will talk about the molecular design with the passivation of

the reactive cites leading to stable molecules and the characteristics of its OTFTs with efficient

carrier injection between metal electrodes and OSCs.

B5.21

Self Assembly and Activation of Phosphonic Acid Monolayers on GaN and AlGaN for

Biosensing Applications. Soonwook Hong2, B. S Simpkins

1, R. Stine

1, M. A Mastro

1, C. R

Eddy Jr.1 and P. E Pehrsson

1;

1Naval Research Lab, Washington, District of Columbia;

2Thomas

Jefferson High School, Alexandria, Virginia.

Self assembled monolayers of 6-phosphonohexanoic acid and n-octadecylphosphonic acid

(ODPA) were formed on silicon dioxide, gallium nitride (GaN) and aluminum gallium nitride

(AlGaN) surfaces. Deposition and long-term stability of these phosphonic acids was verified

through water contact angle and X-ray photoelectron spectroscopy (XPS) measurements. The

ODPA monolayers reduced the conductivity of AlGaN/GaN HFET structures by ~30%. This

effect was evaluated in the context of the dipole contribution due to the PO3 end group found on

ODPA. We also report a novel method to activate ODPA monolayers for biological applications

by forming active carbonyl groups through exposure to microwave oxygen plasma. This

activation enables immobilization of 2-mercaptoethylamine (MEA) on functionalized silicon

oxide substrates. This research is of direct relevance to the operation of field effect transistor-

based biochemical sensors, and provides a fundamental basis for further applications of Si, GaN

and AlGaN in biosensing and microelectronics technologies.

B5.22

Anomalous Tunneling in Carbon/Alkane/TiO2/Au Molecular Electronic Junctions: Energy

Level Alignment at the Metal/Semiconductor Interface. Haijun Yan1,2

and Richard

McCreery2,3

; 1Department of Chemistry, The Ohio State University, Columbus, Ohio;

2National

Institute for Nanotechnology, Edmonton, Alberta, Canada; 3Department of Chemistry,

University of Alberta, Edmonton, Alberta, Canada.

Carbon/TiO2/Au electronic junctions show slightly asymmetric electronic behavior, with higher

current observed in current density (J)-voltage (V) curves when carbon is biased negative with

respect to the Au top contact. When a ~1 nm thick alkane film is deposited between the carbon

and TiO2, resulting in a carbon/alkane/TiO2/Au junction, the current increases significantly for

negative bias and decreases for positive bias, thus creating a much less symmetric J-V response.

Similar results were obtained when SiO2 was substituted for the alkane layer, but Al2O3 did not

produce the effect. The observation that by adding an insulating material between carbon and

TiO2, the junction becomes more conductive is unexpected and counterintuitive. Kelvin probe

measurements revealed that while the apparent work function of the PPF electrode is modulated

by surface dipoles of different surface-bound molecular layers, the anomalous effect is

independent of the direction of the surface dipole. We propose that by using a nanometer thick

film with a low dielectric constant as an insertion layer, most of the applied potential is dropped

across this thin film, thus permitting alignment between the carbon Fermi level and the TiO2

conduction band. Provided the alkane layer is sufficiently thin, electrons can directly tunnel from

carbon to the TiO2 conduction band. Therefore, the electron injection barrier at the carbon/TiO2

interface is effectively reduced by this energy level alignment, resulting in an increased current

when carbon is biased negative. The modulation of injection barriers by a low-κ molecular layer

should be generally applicable to a variety of materials used in micro- and nano-electronic

fabrication.

B5.23

Single and Mixed Self-Assembled Monolayers of Phenyl Species on Silicon with Various

Ring to Ring Interactions Virginie Gadenne, Simon Desbief and Lionel Patrone; Institut

Supérieur de l‟Electronique et du Numérique, IM2NP, CNRS, IM2NP (UMR 6242), Maison des

Technologies, Place Georges Pompidou, F-83000 Toulon, France.

Preparation of self-assembled monolayers (SAM) [1] of aromatic conjugated molecules is a key

point in molecular electronics [2]. Moreover, regarding potential applications, it is important to

be able to prepare nano-islands of such active molecules on silicon. Nevertheless few works

addressed this subject [3]. In a first part of our work, in order to control the formation of

conjugated molecular nano-domains on native oxide covered silicon, we studied how various tri-

functionalized silane molecules bearing a phenyl cycle, modified or not, interact during their

self-assembly [4]. For phenyl rings without alkyl chain, SAM growth is shown to occur in a

single step: chemisorption on the surface. This step is thermally activated and does not depend

on ring to ring interactions. We show that adding a short alkyl chain (3-4 carbon atoms) to the

phenyl ring gives the molecules enough flexibility to generate an additional second growth step.

The latter is independent from the deposition temperature and corresponds to the arrangement

between molecules. We found that this packing step is accelerated by replacing phenyl by

pentafluoro-phenyl rings, possibly due to quadrupolar interactions between fluorinated cycles.

Furthermore we demonstrate that mixing phenyl and pentafluoro-phenyl molecules leads to an

even faster packing step which is accounted for by hydrogen bonding CH...FC in a face to face

phenyl/pentafluoro-phenyl arrangement [5]. We believe these results allow improving charge

delocalization over conjugated molecular domains. In a second part, we studied the phase

separation between phenyl-alkylsilane and octadecyltrichlorosilane (OTS) molecules. Improving

the phase separation was studied using two parameters: ring to ring interactions afore-analyzed

and reactive heads with different grafting kinetics. Using the same trichlorosilane grafting

moiety for phenyl molecules as for OTS, we show that phase separation is improved and OTS

islands are smaller with phenyl species that involve stronger ring to ring interactions. The best

case is obtained with mixing phenyl and pentafluoro-phenyl rings using hydrogen bonds for

packing together the aromatic species of the SAM. Small phenyl species islands (40-100 nm in

diameter) could be obtained inside the OTS SAM using a less reactive grafting head for the

aromatic molecules. These two cases demonstrate an improved control of SAM composition and

morphology essential to further use the obtained islands for building molecular devices. [1] F.

Schreiber, Progress in Surf. Sci. 2000, 65, 151. [2] H.B. Akkerman et al., Nature 2006, 441, 69.

[3] F. Fan et al., Langmuir 2003, 19, 3254. [4] J. Moineau et al., Langmuir 2004, 20, 3202. [5]

V.R. Thalladi et al., J.Am.Chem.Soc. 1998, 120, 8702 ; J.D. Dunitz, ChemBioChem 2004, 5,

614; S. Zhu et al., Tetrahedr. Lett. 2005, 46, 2713.

B5.24

The Effects of Octanedithiol Additive on the Three-Dimensional Nanoscale Organization of

Highly Efficient Conjugated Polymer Bulk Heterojunction Solar Cells. Mark Dante, Andres

Garcia and Thuc-Quyen Nguyen; University of California, Santa Barbara, Santa Barbara,

California.

It was recently shown that the addition of a small percentage of 1,8-octanedithiol to the solution

from which polymer:fullerene bulk heterojunction films are spin-coated leads to solar cell power

conversion efficiencies of greater than 5%. This “additive” approach circumvents the need of

post-deposition processing that improve efficiencies, such as thermal or solvent annealing, and

thus has the potential to greatly simplify the fabrication methods; an important consideration

when comparing polymer solar cell devices versus their inorganic counterparts. This effect was

first observed in poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester

(P3HT:PCBM) bulk heterojunction solar cells. The increase in efficiency of these devices has

been attributed to an increase in the crystallinity of the polymer phase, resulting in a higher

charge carrier mobility. Although similar increases in efficiency have been observed when

incorporating additives into bulk heterojunction blends containing the amorphous conjugated

polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b‟]-dithiopene)-alt-4,7-(2,1,3-

bezothiadiazole)], and [6,6]-phenyl C71-butyric acid methyl ester (PCPDTBT:C71-PCBM), the

lack of crystallinity of the polymer phase with or without additive treatment and no increase in

the charge mobility indicate a different mechanism is responsible for the performance

improvement. Scanning probe examination of cross sections of PCPDTBT:C71-PCBM bulk

heterojunctions reveal a structural change in the internal morphology of the hole and electron

transporting networks when the film is cast from a solution containing 2% by volume 1,8-

octanedithiol. Phase separation of the nanoscalar domains becomes more defined and the average

sizes of hole and electron transporting networks double upon addition of the additive. The

increase in the size of the domains likely gives rise to less charge recombination.

B5.25

Maskless Patterning of Active Layer for Organic Thin-Film Transistor by Transfer-

Printing Based Lift-Off Technique. Wonsuk Choi, Min-Hoi Kim, Won-Ho Kim, Kyungmo

Koo and Sin-Doo Lee; School of Electrical Engineering, Seoul National University, Seoul,

Korea, South.

Organic thin film transistors (OTFTs) have great potential to replace conventional silicon-based

inorganic semiconductor devices as a mainstream of ubiquitous flexible electronic applications.

Although there has been a tremendous progress in the device performance of OTFTs during the

past several years, there still remains a critical issue on the methods of precisely patterning

organic active materials. For instance, accurate patterning of active semiconducting material in

the OTFTs is inevitably required for the enhanced device performances. Here, we present a high-

resolution patterning method of organic materials using transfer-printing based lift-off (TPLO)

technique. The patterning processes using the TPLO are described as: (i) a fluoro-polymer layer

is coated on a patterned polydimethylsiloxane (PDMS) stamp, (ii) the fluoro-polymer patterns

are transfer-printed onto a substrate as a sacrificial lift-off layer, (iii) an organic material is

deposited on the entire substrate having the lift-off layer, and (iv) the pattern of organic material

is formed by lifting-off the organic material remaining on the fluoro-polymer using a fluoro-

solvent. The TPLO technique has two distinctive advantages over existing approaches. First, the

TPLO is a maskless process which is inherently suitable for large-area and high-resolution

patterning so that a feature resolution down to 3 um (the resolution of conventional shadow mask

technique is about 30 um) was achieved for various organic materials. Second, the TPLO has the

capability of patterning organic materials onto various types of substrates including plastic

substrates due to the chemical compatibility of the fluoro-solvent. As a demonstration, we

fabricated pentacene patterns on the polymeric gate insulator layer using the TPLO technique to

fabricate OTFT device arrays. The surface morphology and the electrical characteristics of our

OTFTs are found to be comparable to those of the OTFT fabricated by a conventional shadow

mask process. In summary, we developed a transfer-printing based lift-off technique (TPLO) as a

general and versatile platform for precise patterning of organic materials in high resolution of a

few micrometers. Our pattering process is expected to be widely used for fabricating various

plastic electronic devices including high-resolution organic light emitting diode (OLED) displays

and top-contact OTFTs with a very short channel length.

B5.26

Electrical and Friction Properties of Stilbene Based Self Assembled Monolayers on Au

(111): The Role of Molecular Ordering. Yabing Qi1,2

, Bas Hendriksen2, Xiaosong Liu

3,

Violeta Navarro2, Jeong Y Park

2, Imma Ratera

2, John Klopp

4, Carine Edder

4, Franz J Himpsel

3,

Jean Frechet4,5

and Miquel Salmeron2,4,6

; 1Applied Science and Technology Graduate Group,

University of California Berkeley, Berkeley, California; 2Materials Sciences Division, Lawrence

Berkeley National Laboratory, Berkeley, California; 3Department of Physics, University of

Wisconsin, Madison, Madison, Wisconsin; 4The Molecular Foundry, Lawrence Berkeley

National Laboratory, Berkeley, California; 5Department of Chemistry, University of California

Berkeley, Berkeley, California; 6Department of Materials Science and Engineering, University of

California Berkeley, Berkeley, California.

We intend to understand charge transport mechanisms, responsible for organic/molecular

electronics, by studying the relation between the structure of self-assembled monolayers of

molecules that contain conjugated groups and their electrical conductivity. As an example, we

investigated the electrical and friction properties of ω-(trans-4-stilbene)alkylthiol self-assembled

monolayers (SAMs) on Au(111) using Atomic Force Microscopy (AFM). The sample surface

prior to heating was uniformly covered with a molecular film that comprises of very small

grains. Well-packed and flat islands were formed after the sample was heated in nitrogen at 120

°C for 1 h. While the lattice resolved AFM images revealed an enhanced ordering in islands, the

substrate area between islands was covered with disordered molecules. The islands exhibit

substantial reduction (50%) in friction supporting the existence of good ordering. The islands

were ∼ 8Å higher than the surrounding disordered phase in the AFM topographic image,

indicating that the molecules of islands stand more upright on the substrate. Near edge X-ray

absorption fine structure spectroscopy (NEXAFS) measurements revealed an almost upright

molecular orientation for samples both before and after heating, with substantial increase of

ordering after heating. Conductance-AFM measurements revealed a more than 2 orders of

magnitude higher conductivity on the large islands than that of the disordered phase on the

heated samples. The current level on the ordered islands is also much larger than that of the non-

annealed SAM. We propose that the conductance enhancement is a result of a better π-π stacking

between trans-stilbene units of neighboring molecules as a result of improved ordering in islands.

B5.27 High Mobility n-Channel Organic Field-Effect Transistors by Solution Process. Peng Wei,

Joon Hak Oh and Zhenan Bao; Chemical Engineering, Stanford University, Stanford, California.

The development of n-channel organic field-effect transistors (OFETs) by solution-process is

crucial for low-cost and flexible electronics. For n-channel OFETs, the moisture and hydroxyl

groups at the semiconductor-dielectric interface act as the trap sites during operation. In this

work, we used double-layer dielectric with hydroxyl-free divinyltetramethyldisiloxane-

bis(benzocyclobutene) (BCB) to eliminate the electron-trap. High mobility (0.1 cm2/Vs) n-

channel OFET was obtained based on [6,6]-phenyl-C61-butyric acid ester (PCBM) by solution

process, which is a commercial available n-channel semiconductor. In comparison with SiO2

dielectric, both the mobility and on/off ratio significantly increase with this double-layer

dielectric.

B5.28

Measurement and Modeling of Current in Hole-only Tandem Structures of N,N'-diphenyl-

N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPB) and 4,4′,4″-tris (3-

methylphenylphenylamino)-triphenylamine (m-MTDATA)Christoph Zimmermann1, Manuel

Bosing1, Florian Lindla

1, Frank Jessen

1, Hans-Peter Loebl

2, Dietrich Bertram

3, Michael

Heuken1,4

, Holger Kalisch1 and Rolf H Jansen

1;

1Chair of Electromagnetic Theory, RWTH

Aachen University, Aachen, Germany; 2Philips Technologie GmbH Forschungslaboratorien -

Philips Research Laboratories, Aachen, Germany; 3Philips Technologie GmbH, Aachen,

Germany; 4AIXTRON AG, Aachen, Germany.

Organic light emitting diodes (OLED) have the potential to replace present light sources. The

most advanced types of OLED are complex stacks of doped transport and emission layers

consisting of small molecules. One advantage of multilayer OLED is the possibility to confine

the recombination zone in an emission layer by proper choice of energy levels. Little effort has

been spent so far to examine the charge carrier transport over energy barriers between layers of

small organic molecules. We have examined the hole transport through the interface between

layers of 4,4′,4″-tris(3-methylphenylphenylamino)-triphenylamine (m-MTDATA) and N,N'-

diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPB), both common hole transport

materials used in OLED. Samples with different combinations of layer thicknesses were

produced by vacuum thermal evaporation. They were designed as hole-only diodes with a well

injecting anode of indium tin oxide and a non-injecting cathode of aluminium on top.

Temperature-dependent I-V curves were measured for all samples. Additionally, dark injection

measurements were performed on the diodes to analyze their transient behaviour. Comparison of

the I-V curves of the different samples showed clear evidence that the hole current is limited by

the energy barrier between the two layers. The propagation time of carriers from the anode to the

interface and their accumulation at the barrier could be recognized in the dark injection signals.

The transient and stationary results were modeled with a numerical algorithm solving the time-

dependent semiconductor device equations with arbitrary mobility models and boundary

conditions at internal interfaces. The empirical findings with respect to voltage and temperature

dependence of the current were compared to established theoretical assumptions about hopping

transport in amorphous organic materials. Qualitative agreements concerning the shape of the

transient currents and the temperature dependence of the I-V curves were found, quantitative

deviations will be discussed.

B5.29

Abstract Withdrawn

B5.30

Torsion Mode Conductive Atomic Force Microscopy Study on Polypyrrole Based

Polyelectrolyte.Ling Sun1, Jianjun Wang

1, Elmar Bonaccurso

1, Andreas Muehlebach

2, Hans-

Juergen Butt1 and Gerhard Wegner

1;

1Physics of polymers, Max-planck institute for polymer

research, Mainz, Germany; 2Group Research, Ciba Inc., Basel, Switzerland.

It has been commonly understood that the properties of organic functional materials and the

devices using these materials are determined by their local structure/morphological features .

Conductive atomic force microscopy (c-AFM) is usually used to study the morphological and

electronic properties of organic functional materials, especially conductive polymers. It measures

topography of the sample surface and current flow through the bulk material with a nanometer

resolution. However, conventional c-AFM has to be operated with a contact mode, which is

usually destructive. This is not favored for soft materials, i.e. polymers. As an alternative, we use

a torsion mode c-AFM to image the topography and current. In torsion mode c-AFM, the

cantilever is oscillated at the first torsional resonance frequency by two piezos. The lateral forces

that act on the tip cause a change in the torsional resonance frequency, amplitude and/or phase of

the cantilever. The oscillation of the cantilever is maintained in a near-field region where

electronic interaction still exists. Therefore torsion mode c-AFM could non-destructively

measure the topography and current flow through the sample. Here we present the electronic

property study of a newly synthesized Polypyrrole (PPy) based conductive polymer, by torsion c-

AFM and Kelvin probe force microscopy (KPFM). PPy has a high conductivity and of neutral

pH when doped with polystyrene sulfonate (PSS) . Thin films cast from PPy water suspension

are conductive and transparent as proved in our previous study . However, the microscopic

structure of the conductive species, i.e. the conductive grains, is elusive. We use torsion C-AFM

to visualize the conductive grain structure. The difference in conductivity implies a phase

separation which is also proved by the difference in Kelvin potential. The results suggest us the

mesoscopic structure of PPy/PSS and help to correlate the material structure and its property.

B5.31

Fabrication of Light-Emitting Transistor Combined with ZnO thin-film Transistors. Hiroshi Yamauchi

1, Yasuyuki Watanabe

2, Masaaki Iizuka

3, Masakazu Nakamura

4 and Kazuhiro

Kudo4;

1Faculty of Engineering, Chiba University, Chiba, Japan;

2Center of Frontier Science,

Chiba University, Chiba, Japan; 3Faculty of Education, Chiba University, Chiba, Japan;

4Graduate school of Engineering, Chiba University, Chiba, Japan.

Organic light emitting diodes (OLEDs) have much attention for flexible, low cost, and ease of

processing. In this work, as one of the method for expanding effective light-emitting area for

active matrix displays, a new type active light-emitting device combined with ZnO transistor is

proposed. An OLED is fabricated on the transparent FET. A transparent ZnO FET has an

indium-tin-oxide gate and a silicon nitride gate insulator fabricated by plasma-enhanced

chemical vapor deposition. These transparent materials are expected to be promising components

of high-efficiency light-emitting devices. This light-emitting device has an advantage to fabricate

without the damage to organic layer by the electrode sputtering and oxidation of emitting layer

with oxygen in the atmosphere. The light-emitting devices proposed here are suitable for the

display element because these ZnO layers between light-emitting layer work high-transparent

electron injection, electrode, and active layer of FET, therefore the aperture ratio increases and

light-emitting occurs efficiently. We describe the basic characteristics of transparent FET using

thin-film ZnO. The ZnO works as the active channel, and Al-doped ZnO (AZO) is used as the

source/drain electrode. These films were deposited by radio frequency (rf) magnetron sputtering.

In an attempt to reduce the plasma damage, we studied a method for fabricating the AZO

source/drain electrode. AZO layers are fabricated by the stepped rf power deposition method.

There is a correlation between rf power and resistivity and the AZO film deposited at low rf

power is low conductivity We compare the static characteristics of ZnO FETs with AZO

electrode deposited by uniform power deposition and those with AZO electrode deposited by

stepped power deposition. The current at Vds = 60 V saturated below 3.5μA. In this experiment,

Vg was varied from -10 V to +100 V with 10 V steps. Id of uniform power deposition FET

shows a current saturation at low Vds and the current remains at low value of around 20 nA. On

the other hand, Id of stepped power deposition increases two orders magnitude of around 3μA.

These improvements are derived from low-damage and high-conductivity AZO films obtained

by the new deposition method. We fabricated the active light-emitting device with ZnO

transparent FET and reported on the electrical properties of ZnO FET. The low-temperature and

low-damage sputtering method improves the on current in two orders magnitude. Luminance of

the active light-emitting device driven by ZnO transparent FET was obtained. These

experimental results indicate that the active light-emitting device using transparent ZnO thin

films on a plastic substrate should be realized by optimizing the device design and the fabrication

process.

B5.32

Abstract Withdrawn

B5.33

Vertical-channel Organic Transistors using Step-edge Structure and its Application to

Pixel Circuits. Makoto Shirakawa, Gaku Harada, Toru Ishikawa, Miwa Ogawa, Takeshi Sano

and Kenichi Shibata; Sanyo Electric Co., Ltd., Hirakata, Osaka, Japan.

One of the possible applications of organic thin-film transistors (OTFTs) is presumed to be a

backplane for an active matrix organic light emitting diode display (AM- OLED), however, high

driving voltage and low output current of OTFTs are the problems. In this work, we have

introduced a step-edge structure to form a vertical-channel OTFT in order to improve driving

voltage and output current for the use of driving transistor in a pixel circuit. By optimizing the

structure and the fabrication process of our vertical-channel OTFT, we have obtained sufficient

current for OLED which is around Id = 500 μA (@ W = 3 mm, Vd, Vg = -10 V) and high on/off

ratio of 105 at a test piece using pentacene for the organic semiconductor layer. Our vertical-

channel OTFT using a step-edge structure also has an advantage of a high-resolution patterning

due to its self-aligned process, which is different from other types of vertical-channel transistors

reported before. We successfully fabricated OTFT arrays which consist of a driving transistor (W

/ L = 240μm / 2μm) and a switching transistor (W / L = 120μm / 2μm) in the pixels (450μm x

450μm). A part of this work was performed under management of the OITDA supported by

NEDO.

B5.34 Logic Devices with p-channel Organic and n-channel Inorganic Transistors. Yasuyuki

Watanabe1, Hiroyuki Iechi

2, Hiroshi Yamauchi

3 and Kazuhiro Kudo

4;

1Center for Frontier

Science, Chiba University, Chiba, Japan; 2Tohoku R&D Center, Research and Development

Group, RICOH Co., Ltd, Miyagi, Japan; 3Faculty of Engineering, Chiba University, Chiba,

Japan; 4Graduate school of Engineering, Chiba University, Chiba, Japan.

Flexible electronic devices have been strongly attracted much attention to the potential

application for the flexible displays, radio-frequency identification cards (RFIDs), etc.

Especially, it is required that the high-performance organic field-effect transistors (OFETs)

which are indispensable for the more advancing flexible displays because it realized the flat-

panel television using organic light-emitting diodes (OLEDs) on last year. Therefore, it is

important to study the characteristics of logic circuits based on p-type and n-type

semiconductors. We have already reported on the characteristics of the logic devices using

pentacene and ZnO FETs [1] and pentacene organic static induction transistors (OSITs) [1, 2]. In

general, organic semiconductors such as pentacene show p-type properties with low-mobility and

high-resistivity compared with inorganic semiconductors. In particular, it is difficult to control

the electrical properties such as the mobility of organic semiconductors because the doping

technology has not been established in organic semiconductors. On the other hand, ZnO

materials show the wide-range properties from insulator to low conductive n-type

semiconductor. In this study, we focus on the organic semiconductors with p-type properties and

the inorganic semiconductors with n-type properties for the fabrication of flexible logic devices.

First, the charge transport characteristics both ZnO and pentacene thin films are investigated.

Next, the performances of the CMOS inverter with pentacene and ZnO FETs are controlled by

matching the characteristics of ZnO to those of pentacene FETs. Finally, to improve the

characteristics of ZnO/pentacene logic devices, we propose new device structures of step-edge-

vertical-channel OFETs (SEVC-OFETs). The obtained results demonstrate that the hybrid

complementary inverters described here have potential for use as advancing flexible displays and

RFIDs. References [1] H. Iechi, Y. Watanabe and K. Kudo, Jpn. J. Appl. Phys. 46,4B, 2645

(2007). [2] Y. Watanabe, H. Iechi and K. Kudo, Thin Solid Films, 516, 2729 (2008).

B5.35

Study on the Spinodal Decomposition of P3HT:PCBM Layer with Different P3HT:PCBM

Ratio for High Performance Organic Solar Cells. Woon-Hyuk Baek1, Tae-Sik Yoon

1, Hyun

Ho Lee2 and Yong-Sang Kim

1,3;

1Nano Science & Engineering, Myongji University, Yongin,

Gyeonggi, Korea, South; 2Chemical Engineering, Myongji University, Yongin, Gyeonggi,

Korea, South; 3Electrical Engineering, Myongji University, Yongin, Gyeonggi, Korea, South.

In organic solar cells, composites of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-

butyric acid methyl ester (PCBM) are most widely used as an active layer and recorded to have

the highest power conversion efficiency. Since typical range of exciton diffusion length is 4~20

nm in polymer, nanoscale interpenetrating network morphology of donor (P3HT) and acceptor

(PCBM) is indispensable for efficient charge separation with extended interface and highly

efficient polymer solar cell. The relation between optical absorption and chemical properties of

P3HT is already well established. When P3HT has more ordered structure with longer chain

length and higher crystallinity, its UV/visible absorption is shown to be red-shifted, higher,

broader and more vibronic. And electro-optical and morphological characteristics of photovoltaic

cells with different P3HT:PCBM ratio were also reported. In this study, it is also observed that

the absorption of PCBM increases while that of P3HT decreases with increasing concentration of

PCBM in active layer. Moreover, the crystallinity of P3HT characterized by peak intensity of

XRD diffraction increases with increasing P3HT concentration. However, further increasing

P3HT concentration results in the reduced peak intensity. This anomalous observation of

crystallinity of P3HT layer depending on the ratio of P3HT and PCBM is discussed with the

behavior of spinodal decomposition of P3HT and PCBM. The P3HT:PCBM layers with varied

ratio are prepared on polyethylenedioxythiophene:polystyrenesulphonate (PEDOT:PSS) coated

indium tin oxide (ITO) substrate by spin-coating and subsequent annealing. The behavior of

spinodal decomposition is investigated by using atomic force microscopy (AFM) and

transmission electron microscopy (TEM).

B5.36

Transport Properties of Graphene Field-effect Transistors with Different Metal Electrodes. Tatsuya Saito

1, Ryo Nouchi

2, Hiroki Watanabe

1 and Katsumi Tanigaki

1,2;

1Department of

Physics, Tohoku university, Sendai, Japan; 2WPI Advanced Institute for Materials Research,

Tohoku university, Sendai, Japan.

A single-layer carbon sheet, graphene, has many interesting physical properties resulting from a

zero-gap band structure with the linear energy dispersion around the Fermi energy. In addition,

the highest mobility at room temperature among all materials makes graphene a major candidate

for future high-speed electronic devices. On the other hand, graphene is easily affected by

environment; the influences of metal contact on the electronic transport have been pointed out to

be large [1]. In fact, with Co contacts, the conducting property of graphene shows an anomalous

behavior [2]. In this study, we investigate the contact effect by employing field-effect transistors

(FETs) with different metal electrode, where the material for the source electrode is different

from that for the drain electrode (Au/Ca etc). We acquired graphene by the mechanical

exfoliation method [3], and patterned electrodes onto it by using electron beam lithography and

lift-off techniques. The fabricated FETs were measured in an inert atmosphere to avoid oxidation

of low work function metals. In this presentation, we report on the contact doping to the

graphene FETs from the view point of work function. Especially, p-n junctions formed by the

contact doping are discussed.[1] G. Giovannetti et al., Phys. Rev. Lett. 101, 026803 (2008). [2]

R. Nouchi et al., Appl Phys Lett. 93, 152104 (2008). [3] K. S. Novoselov et al., Proc. Natl. Acad.

Sci. U.S.A. 102,10453 (2008).

B5.37

Formation of Composite Organic Thin Film Transistors with One-Dimensional

Nanomaterials Gen-Wen Hsieh1, Flora M Li

2, Sharvari Dalal

1, Husnu Emrah Unalan

1, Massimo

Spina1, Pritesh Hiralal

1, Arokia Nathan

3, Paul Beecher

4, James E Stott

3, Andrew J Flewitt

1,

Gehan Amaratunga1 and William I Milne

1;

1Engineering Department, University of Cambridge,

Cambridge, United Kingdom; 2Electrical and Computer Eng., University of Waterloo, Waterloo,

Ontario, Canada; 3London Centre for Nanotechnology, University College London, London,

United Kingdom; 4Nokia Research Centre, Cambridge, United Kingdom.

Large area and flexible electronics is a rapidly expanding research area, which opens our eyes

into a new version of future electronics and is expected to revolutionise the electronics industry.

In this area, organic semiconductors and organic thin film transistors (OTFTs) have attracted

much interest by virtue of their solution processibility, low temperature processing, and

potentially low cost fabrication, with the advantage of lightweight, bendable features. Despite

ongoing advancements in material functionality and processing technologies, OTFTs are

somewhat limited in terms of device lifetime and mobility, which hinder their adoption on a

wider scale. Here, the purpose of this study is to develop augmentative material and fabrication

systems in the quest for higher performance transistors for large area and flexible applications.

One-dimensional (1-D) nanomaterials, such as nanotubes and nanowires, are of interest in view

of their unique optoelectronics properties, high aspect-ratio structures, high mobility and

conductivity. These nanostructures that present feasible alternatives can be used as the sole

material in a device structure, or can be implemented as a complement to organic materials. To

combine the advantages of 1-D nanostructures with the ease of processibility offered by organic

semiconducting materials, we aim to investigate means of incorporating 1-D nanomaterials into

organic devices by different techniques, such as ink-jet printing, contact printing, and spin

coating. A variety of carbon nanotubes (CNTs), and metallic and semiconducting nanowires

(NWs) are utilised as a prospective booster to p-type or n-type organic semiconductors. The first

is the fabrication of p-type OTFTs comprised of CNTs and semiconducting polymer by means of

ink-jet printing. Results show that low density ink-jet printed CNT networks can enhance the

performance of polythiophene-based OTFTs. For instance, the field effect mobility exhibited by

composite CNT-OTFT devices is seven times higher than that of pristine organic devices.

Secondly, a facile technique for large-scale parallel assembly of 1-D nanostructures by a dry

shear-sliding concept is demonstrated. Parallel arrays of silicon (Si) nanowires are employed in

the fabrication of bilayer composite Si-OTFTs. The electrical measurements of these p-type

OTFTs show that the relative direction of aligned NWs corresponding to the current flow

direction influences the field effect mobility and ON/OFF current ratio. The third strand of this

work features the use of n-type semiconducting zinc oxide (ZnO) nanowires as the mobility

enhancer in n-type OTFTs which represents a remarkable increase in field effect mobility by as

much as a factor of 40 over OTFTs comprising pristine organics.

B5.38

Self Organization of Regioregular Poly(3-Hexyl Thiophene)-Poly(ε-Caprolactone) Block

Copolymers Obtained by a Novel Controlled Synthesis Method. Mathieu Surin1, Khoa Tran

2,

Olivier Coulembier2, Philippe Dubois

2, Roberto Lazzaroni

1 and Philippe Leclere

1;

1Chemistry

for Novel Materials, University of Mons-Hainaut, Mons, Belgium; 2Laboratory of Polymeric and

Composite Materials, University of Mons-Hainaut, Mons, Belgium.

Since the initial discovery of conductive organic polymers, tremendous efforts have been

devoted to the synthesis of processable compounds exhibiting high electrical conductivity or

charge mobility. Among these, polythiophene-based materials have received particular attention,

with applications ranging from field-effect transistors (FETs) to optical and electronic sensors,

photovoltaic diodes and non-linear optical materials. Substitution at the 3-and/or 4-position of

the thiophene ring not only improves the solubility and processability of poly(thiophene)s but

also strongly influences the macromolecular ordering by interdigitation of the alkyl side chains

between the lamellar structures of π-stacked conjugated backbones. Regioregular poly(3-

alkylthiophene)s (rr-P3ATs) have been initially obtained by Mc Cullough and Rieke, who

discovered that transition metal-catalyzed polycondensation lead to head-to-tail P3ATs. While

side-chain functionalization has been demonstrated to be an effective way to modulate the

optoelectronic properties of rr-P3ATs, the synthesis of block copolymers containing a π-

conjugated polymer segment can generate unique self-assembled electronic materials with

enhanced mechanical properties displaying phase separation and yielding discrete

microstructures. Here we propose an original, well-controlled synthesis method, based on the

association of the GRIM approach with the ring-opening polymerization (ROP) of lactones, for

the design of poly(3-hexyl thiophene) (P3HT) block copolymers with poly(ε-caprolactone)

(PCL) segments. This would lead to a new type of material, since PCL is: (i) crystallizable (in

contrast to other segments copolymerized with P3HT, such as poly(methacrylate)s).; (ii)

removable by hydrolysis, and ; (iii) biocompatible. The present communication reports on the

stepwise synthesis of new P3HT-PCL di- and triblock copolymers by controlled ROP of CL

from hydroxyl-terminated P3HT. Thin deposits of P3HT homopolymer compounds onto various

surfaces show a fibrillar morphology, as observed with Atomic Force Microscopy (AFM). Since

these deposits are formed from a good solvent for both blocks (i.e., tetrahydrofuran), the

observed morphology arises from the self-assembly of the P3HT segments into π-stacked

assemblies, as observed for other poly(thiophene)s and conjugated (co)polymers systems. For the

P3HT-PCL block copolymers, they also self-assemble into fibrillar structures, with

semiconducting nanoribbons separated by removable, insulating domains of PCL. We investigate

the electrical properties of the P3HT-PCL fibrillar assemblies at the local scale via conducting

probe AFM techniques. Finally, since P3HT can be chemically doped after self-assembly process

and the lateral dimensions can be tuned by controlled synthesis of the polymer segments, this

could lead to an elegant way for fabricating „nanowires‟ for organic electronics.

B5.39

Temperature and Coverage Dependent Dewetting of Ultra-thin Films of Discoid Organic

Molecules on Au(111) and Ag(111). Paul Frank1, Norbert Koch

2, Ralph Rieger

3, Klaus

Muellen3 and Adolf Winkler

1;

1Institute of Solid State Physics, Graz University of Technology,

Graz, Austria; 2Institut für Physik, Humboldt-Universität zu Berlin, Berlin, Germany;

3Max

Planck Institute of Polymer Research, Mainz, Germany.

Ultra-thin films of hexaazatriphenylene-hexacarbonitrile (HAT-CN) were prepared on a

gold(111) single crystal and on a silver(111) single crystal utilizing organic molecular beam

deposition (OMBD) under well defined ultra high vacuum (UHV) conditions. The thin films

were investigated by thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy

(XPS) and atomic force microscopy (AFM). XPS in combination with TDS was applied to reveal

the kinetics of the layer growth. Additionally, the HAT-CN growth characteristics were studied

as a function of substrate temperature. From TDS, the pre-exponential factors and the heats of

evaporation for the individual layers could be determined experimentally. Ex-situ AFM was used

to determine the film morphology. A Kelvin probe (KP) was used to follow the change in the

work function with increasing film thickness. The HAT-CN molecules exhibit a quite unusual

layer growth behaviour and thermal stability. On the gold single crystal, the first two layers of

flat lying molecules wet the surface and they are stable up to 400 K and 500 K, respectively.

With increasing coverage islands form on the wetting layer, which induce a destabilisation of

these layers. As a function of coverage and temperature first the 2nd and then the 1st monolayer

are incorporated into the islands, as determined by thermal desorption spectroscopy. On the

silver single crystal, HAT-CN shows a similar layer growth and desorption behaviour, except

that the molecules in the 1st monolayer are bound more strongly and dissociate upon heating.

B5.40 Surface Plasmon and Semiconducting Polymer Luminescent Solar Concentrators. Michael

Griffo and Sue A Carter; Physics, University of California, Santa Cruz, California.

Luminescent solar concentrators use fluorescing materials with large Stokes shifts to absorb

incident sunlight and waveguide the emitted light to photovoltaic cells. Although the idea behind

luminescent solar concentrators is not new, they have recently been of great interest. Advances in

polymer and other fluorescent organic materials have enabled some increases in efficiency;

however, further improvement in the outcoupling and PL efficiency, especially of near-IR

emitting materials, is needed. We use tuned surface plasmons on silver nanoparticles to enhance

the fluorescence of semiconducting polymers and to increase coupling between the incident light

and glass to waveguide light for luminescent solar concentrator applications. We have

demonstrated a 5.6 fold increase in photoluminescence efficiency with this device structure and

enhancement in collection efficiency. We analyze the collected emission spectra as a function of

distance from the point the incident light is absorbed to the point the light exits the concentrator.

Comparing this with a control, the incident spectra, and the absorption spectra we gain insight

into the factors affecting the optical efficiencies of the structure.

B5.41

Abstract Withdrawn

B5.43

The Use of Tailored Host and Hole Blocking Materials for Improved Power Efficiencies of

Blue OLEDs.Asanga B Padmaperuma, Philip K Koech, Eugene Polikarpov, Amber L Von

Ruden, James S Swensen, Lelia Cosimbescu and Daniel J Gaspar; Pacific Northwest National

Laboratory, Richland, Washington.

The development of stable and efficient blue OLEDs is essential in the development of organic

solid state white lighting with 50% power conversion efficiency. Nationwide adoption of high-

efficiency OLED lighting instead of incandescent lighting would result in significant energy

savings. The simplest and lowest cost method of generating white light is to convert part of the

emission from a blue light source to white light using a system of phosphors external to the light

generating region. State of the art OLEDs are based on organometallic phosphors doped into

wide bandgap host materials. However, poor hole injection in these materials has limited

measured efficiencies, and in most cases the emitter is implicated in charge trapping. A direct

consequence of emitter charge trapping is a decrease in the operating voltage with increased

dopant concentration, but also reduced device efficiency at higher current densities. We have

demonstrated high efficiencies at lower operational voltages by using lower dopant

concentrations (~ 2% wt) in an ambipolar host material. Here we discuss the use of ambipolar

charge-transporting PO host materials and hole blocking materials for blue OLEDs. Ambiploar

host materials were developed by combining hole-transporting moieties with PO-based electron-

transporting moieties (ETm). The use of the same ETm as in the host in designing the hole

blocking materials gave rise to hosts and blocking materials with improved energy matching.

Design principles, physical properties, and electronic properties of these materials will be

discussed. Furthermore, we show the correlation of OLED current density and emission

efficiency results to the structure of the materials.

B5.44

Solution Processable n-Channel Semiconductors for Thin-Film Transistors Based on the

Pyromellitic Diimide Core. Qingdong Zheng1, Jia Huang

1, Amy Sarjeant

2 and Howard E Katz

1;

1Department of Materials Science & Engineering, The Johns Hopkins University, Baltimore,

Maryland; 2Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland.

In the past decade, there has been extensive effort toward developing new organic

semiconductors for thin film transistors due to their applications in complementary circuits, flat-

panel displays and sensors. Pyromellitic dimides (benzenetetracarboxylic diimides) are best

known as segments of highly insulating polyimide dielectrics. However, the authors report here

that this simple structure can be a sufficiently conjugated core for the construction of n-channel

organic semiconductors. Several pyromellitic diimides with fluorinated side chains were

synthesized by a one-step reaction. The field effect devices can be easily fabricated from the

synthesized pyromellitic diimides through vacuum sublimation or solution deposition. For the

vacuum sublimed materials, the electron mobility is found to be as high as 0.136 cm2/Vs and the

on/off ratios can reach a high value of 1000000. The electron mobility is found to be ~0.01

cm2/Vs for the solution deposited materials. We also report on the effects of deposition

temperature, surface treatment and chemical structure on the electron mobility of the fabricated

devices. An attempt to synthesize soluble polymers based on some functionalized pyromellitic

dimides is made, and the electron carrying properties of the resulting polymers are investigated

and discussed. Pyromellitic dimides are more readily available than the higher rylenes (such as

naphthalene or perylene tetracarboxylic diimides) already investigated, are highly transparent

and chemically stable, and lead to processable and electronically tunable derivatives. These

derivatives offer great opportunity for both scientific exploration and utilization in organic and

polymer-based devices.

B5.45

N-type and P-type Controlled Molecular Doping of MEH-PPV with all Solution-processed

Method Yuan Zhang, Mingtao Lu, Bert de Boer and Paul Blom; Zernike Institute for Advanced

Materials, University of Groningen, Groningen, Netherlands.

In organic light-emitting diodes (OLEDs) based on evaporated small molecules it has been

demonstrated that doping of the hole- and electron transport layers strongly reduces their

operation voltage.1 Furthermore, the use of doped layers makes the OLEDs less sensitive to the

workfunctions of the anode and cathode. For solution-processed LEDs based on conjugated

polymers, however, doped charge transport layers are hardly applied. Here we present controlled

p-type and n-type doping of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]

(MEH-PPV) deposited from solution. Tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) and

bis(pentamethylcyclopentadienyl)cobalt(II) (DMCO) are used for the p- and n-type dopants

respectively. By choosing the appropriate dopant solvents and adjusting the polarity of the

solution aggregation can be prevented and doped films can be deposited with a controlled carrier

density. Upon the p-type doping the low voltage part of the J-V characteristics of MEH-PPV

based hole-only devices is increased by several orders of magnitude and a clear Ohmic behavior

appears. We find that a doping concentration of 1.0wt.% leads to a free carrier density of 2×1022

m-3

. For the n-type doping it is observed that the electron transport greatly improves. The

electrons from the DMCO donor fill the trap states lying below the LUMO of the MEH-PPV.

For sufficiently high n-type doping trap-free electron transport is observed in PPV-based diodes.

We for the first time demonstrate that in MEH-PPV the free electron mobility is equal to the hole

mobility. Finally, we demonstrate the first working example of light-emitting polymer-based p-n

junctions from solution processing. References: 1 M. Pfeiffer et al., Organic Electronics, 4, 89-

103 (2003)

B5.46 Exciton Diffusion in Organic Semiconductors. Paul E Shaw, Arvydas Ruseckas and Ifor D.

W. Samuel; School of Physics and Astronomy, University of St Andrews, St Andrews, United

Kingdom.

The transport of charge has been extensively and elegantly studied in organic semiconductors.

However, the transport of excitons has received much less attention even though it is important

in a wide range of organic optoelectronic devices. In OLEDs it determines whether confinement

layers are needed and the roll-off at high brightness. In lasers exciton-exciton annihilation can be

a major loss mechanism. In polymer solar cells, the need for bulk heterojunctions is due in large

part to the limited exciton diffusion length. Furthermore exciton diffusion determines the

distance over which interfaces will strongly affect materials photophysics and device operation.

Estimates of exciton diffusion lengths in organic semiconductors range from nanometres to

microns, making it urgent to develop improved measurements that can guide the development of

improved materials. We show that time-resolved photoluminescence (PL) measurements provide

a powerful way of studying exciton diffusion. We have explored three approaches. In the first,

organic semiconductor layers of a range of thicknesses are deposited on an inorganic quencher

such as TiO2. Faster decay of the PL is seen for thinner samples, and the results are fitted to a

one-dimensional diffusion equation. For the widely studied solar cell material, poly(3-

hexylthiophene) we find that PL decays for film thicknesses ranging from 6 to 32 nm can be

fitted by a single fitting parameter - the diffusion constant and we obtain a value (1.8±0.3)x10-3

cm2/s which corresponds to a diffusion length of 8.5±0.7 nm. The second method for

investigating exciton diffusivity involves studying exciton-exciton annihilation. At high exciton

densities the PL decays faster and this change in behaviour can be modelled as a bimolecular

process, linked to the exciton diffusivity. For P3HT the results can be combined with the first

method to give an estimate for the distance apart at which two excitons annihilate of 1.8±0.4 nm.

In the third method, the decay of fluorescence anisotropy is used to study exciton diffusion: as

excitons diffuse they move to conjugated segments of different orientation and polarisation

memory is lost. These powerful techniques enable us to study how structure and processing

influence exciton diffusion in organic semiconductors. For example, we have demonstrated a

large reduction in exciton diffusivity in dilute copolymers. We also find that exciton diffusivity

is higher in MEH-PPV than P3HT, underlining the differences between charge transport and

exciton transport.

B5.47 Reduced Graphene Oxide Electrodes For Organic Thin-film Transistors. Hector Alejandro

Becerril-Garcia1, Randall M Stoltenberg

2, Ming Lee Tang

2, Mark E Roberts

1, Zunfeng Liu

3,

Yosheng Chen3 and Zhenan Bao

1;

1Chemical Engineering, Stanford University, Stanford,

California; 2Chemistry, Stanford University, Stanford, California;

3Key Laboratory for

Functional Polymer Materials and Center for Nanoscale Science and Technology, Nankai

University, Nankai, Nankai, China.

Graphene oxide (GO) is a water-soluble nanomaterial that can be prepared in large quantities

from graphite and solution-processed into conformal thin-films on diverse substrates. GO thin-

films are electrically insulating, but become conductive upon removal of oxygenated

functionalities from the material. We previously reported the dependence of the conductivity of

reduced GO (RGO) on the reduction conditions, and the successful application of RGO films as

transparent electrodes for organic solar cells. We now present our evaluation of the use of

micropatterned RGO films as electrodes for organic thin-film transistors made with well

characterized p- and n-channel materials. Patterning techniques will be discussed. We show that

RGO films can inject electrons or holes into organic semiconductor layers. Furthermore, organic

semiconductors deposited on RGO show different morphology from that observed on metal

electrodes, leading to improved transistor performance. These findings illustrate the potential

benefits of using carbon electrodes for carbon-based electronics.

B5.48 Pendant Polymers in Light Emitting and Organic Photovoltaic Applications. Akhil Gupta

1,

Scott Watkins2, Katalin Hegedus

3, Melissa Skidmore

4, Lynn Rozanski

5, Gerard J Wilson

6 and

Richard A Evans7;

1Molecular and Health Technologies, CSIRO, Melbourne, Victoria, Australia;

2Molecular and Health Technologies, CSIRO, Melbourne, Victoria, Australia;

3Molecular and

Health Technologies, CSIRO, Melbourne, Victoria, Australia; 4Molecular and Health

Technologies, CSIRO, Melbourne, Victoria, Australia; 5Energy Technology, CSIRO,

NewCastle, New South Wales, Australia; 6Molecular and Health Technologies, CSIRO,

Melbourne, Victoria, Australia; 7Molecular and Health Technologies, CSIRO, Melbourne,

Victoria, Australia.

Plastic solar cells produced from organic semiconductors offer the potential to deliver efficient

solar energy conversion with low-cost fabrication. The challenge is to develop materials for

efficient charge separation and charge transport. We are examining the potential of using free

radically synthesized polymers to replace conventional conjugated polymers such as

poly(alkylthiophenes) or poly(dialkylfluorenes) in polymeric light emitting diode (PLED) and

organic photovoltaic (OPV) applications. The use of free radically synthesized polymers has the

potential of cheap synthesis together with the opportunity of controlling polymer structure living

radical polymerization techniques such as RAFT and ATRP in order to probe structure property

relationships. Our starting point was to examine conventional cyano-PPV type polymers and by

using retro-synthetic analysis to make free radically polymerizable monomers with Cyano-PPV

functionality as pendant substituents. A variety of analogues were prepared by the condensation

of the appropriate aldehyde with p-cyanomethylstyrene. They were polymerized (uncontrolled)

and their HOMO and LUMO energy levels were determined by cyclic voltametry and UV-vis

spectroscopy. It was found that the incorporation of a triaryl amine group in the molecule (AG-1-

78 monomer: AG1-80 polymer) allowed hole transport in PLED and OPV applications. The new

material AG1-80 was evaluated in a hetero-junction organic photovoltaic device where AG1-80

replaced poly(3-hexylthiophene) (P3HT). AG1-80 is colored due to the acceptor donor nature of

the molecule which induces an internal dipole. Thus AG1-80 acts as absorbing chromophore and

hole transport material. An optimized device of AG1-80 and PCBM provided efficiencies of

0.2% across the entire solar spectrum. This modest efficiency would be largely due to the limited

absorption of the visible spectrum of AG1-80 (λmax 406 nm). However efficiency at its

absorption maxima was 10.5%. This suggests that the pendant nature of the AG1-80 was not a

significant problem for actual charge transportation. Thus, we have shown free radical polymers

with pendant electro-active moieties that combine charge transport and chromophore

functionalities may compete with conjugated polymers although an increased absorption of the

solar spectrum is essential for photovoltaic devices. This presentation will provide an overview

of this research from the design, synthesis, characterization and device evaluation of these

cyano-PPV derived materials within the Electro-active Nanomaterials Theme of CSIRO‟s Niche

Manufacturing Flagship. Acknowledgements: Australian Department of Innovation, Industry,

Science and Research. International Science Linkage, CG100059 Victorian Department of

Primary Industries, Sustainable Energy Research and Development Grant and Merck Speciality

Chemicals for the generous donation of poly(3-hexylthiophene

B5.49 FET Characteristic of Chemically-Modified CNT Ryotaro Kumashiro

1, Yan Wang

1, Naoya

Komatsu1 and Katsumi Tanigaki

2,1;

1Department of Physics, Graduate School of Science,

Tohoku University, Sendai, Japan; 2World Premier International Research Center, Tohoku

University, Sendai, Japan.

Semiconducting carbon nanotubes (CNT) have shown promising applications as electronic

materials for nano-scale devices in the future. It is well known that the field effect transistors

(FET) fabricated by CNT show high performance. Semiconducting CNT have hole- and

electron-carrier type, therefore, CNT-FET usually exhibit ambipolar charge transport. In recent

years, the FET structure has attracted intense research interest as a light emitting device, and the

research related to organic light-emitting FET are of fundamental and practical significance. In

this study, we will report the light emission properties of CNT-FET with organic light emitter.

1,3,6,8-tetraphenylpyrene (TPPy) was used as a light-emitting organic-material. TPPy/CNT-FET

devices were fabricated by drop-cast method on SiO2/Si substrate. From the experimental

results, it was shown that the light emission in visible-light region can be observed in FET

operation and the state of light emission is changed by VG. The mechanism of light emission in

TPPy/CNT-FET will also be discussed.

B5.50

Controlling the Temperature Dependent Electronic Properties of Organic Thin Film

Transistors by Self Assembled Monolayers. Marco Marchl1, Peter Pacher

1, Andrej Golubkov

1,

Anja Haase3, Barbara Stadlober

3, Stefan Possaner

2, Ferdinand Schuerrer

2, Karin Zojer

2, Lucas

Hauser4, Christian Slugovc

4, Gregor Trimmel

4 and Egbert Zojer

1;

1Institute of Solid State

Physics, TU Graz, Graz, Austria; 2Institute of Theoretical Physics/Computational Physics, TU

Graz, Graz, Austria; 3Institut für Nanostrukturierte Materialien und Photonik, Joanneum

Research, Weiz, Austria; 4Institute for Chemistry and Technology of Materials, Tu Graz, Graz,

Austria.

Manipulating the properties of the interface between the gate dielectric and the active layer in

Organic Thin Film Transistors (OTFTs) is of particular interest as this allows controlling the

charge carrier mobility and the threshold voltage. One possibility to tune the properties of this

interface is the insertion of covalently linked functional layers. We demonstrated recently that [2-

[4-(chlorosulfonyl)phenyl]ethyl]trichlorosilane and its sulfonic acid analogue (T-SC/SA) in

combination with exposure to ammonia can be used to shift the threshold voltage in

poly(thiophene) (rr-P3HT) based devices over more than 60 V exploiting acid/base chemistry. In

this work, we compare the results for rr-P3HT based transistors with data on transistors with

vacuum deposited pentacene as the active layer. To gain deeper insight into the mechanisms of

carrier transport and the origin of the threshold voltage shift, the devices were studied at

temperatures down to 77 Kelvin. The temperature dependent mobility could be described on the

basis of the multiple trapping and release model (MTR). Furthermore a drift-diffusion based

modelling has been performed to understand temperature dependent threshold voltage shifts.

B5.51 Regio-regular Pentacene Containing Polymers for Organic Photovoltaic Cells. Sanghyun

Hong, Joon Hak Oh, Ying Jiang, Rajib Mondal, Hector A Becerril, Toshihiro Okamoto,

Nobuyuki Miyaki and Zhenan Bao; Chemical Engineering, Stanford University, Stanford,

California.

The development of solar energy sources is necessary to decrease our dependency on fossil fuels

which are contributing to climate change and the polluting our environments. To make solar

energy sources inexpensive, many research groups have studied polymer photovoltaic cells.

Pentacene can be a good moiety for polymer photovoltaic cells due to its good hole mobility and

low bandgap property. However, pentacene containing polymers have not been widely

investigated for photovoltaic cells. Recently, our group developed the synthetic methodology to

make regio-regular pentacene moiety which can be easily polymerized with other co-monomers.

Pentacene-dithiophene conjugated polymers and pentacene- cyclopenta[2,1-b:3,4-b']dithiophene

conjugated polymers were prepared by this method. Regio-regular pentacene containing

polymers showed the good hole mobility up to 10-3

cm2/Vs and below 1.7 eV bandgap properties

on solid state. Their power conversion efficiency will also be discussed in this presentation.

B5.52

Ion-induced Charge Screening and Significant Conductivity Enhancement of Conducting

Polymers.Yijie Xia and Jianyong Ouyang; Materials Science and Engineering, National

University of Singapore, Singapore, Singapore.

A novel approach will be reported to significantly enhance the conductivity of conducting

polymers. The approach is based on a new concept by considering conducting polymers as

complexes of polyions. Our novel approach is to reduce the Coulombic interaction between the

positive charge on the polymer chain and the counter anion by ion-induced charge screening.

The conducting polymer films by this approach can be used as the electrode of optoelectronic

devices.

B5.53

Determining the Optimum Pentacene Channel Thickness on Hydrophobic and Hydrophilic

Dielectric Surface.Sung-jin Mun, Seongil Im, Jeong-M. Choi, Kwang H. Lee and Kimoon Lee;

Physics, Yonsei University, Seoul, Korea, South.

Pentacene thin-film transistors (TFTs) are extensively studied for more than a decade due to its

potential advantages to replace amorphous Si TFTs: high mobility exceeding that of amorphous

Si TFTs, low channel deposition temperature, and accessibility to flexible plastic substrate.

Characterization of pentacene crystalline growth on various dielectric surfaces was one of the

important studies, as a fundamental intrinsic study to achieve high field effect mobility from

pentacene channel. Inorganic oxide dielectrics, self-assembled-monolayer (SAM)-coated oxide

dielectrics, and organic polymer dielectrics were studied for finding an enhanced mobility

pentacene TFT. As an extrinsic geometrical factor, optimum channel thickness in pentacene-

based TFTs was also discussed but only in adopting the case of inorganic dielectrics which have

generally hydrophilic surface. Now we report that the optimum pentacene channel thickness is

dependent on the surface energy state of its dielectric substrate. Pentacene TFT with hydrophobic

substrate displays a peak mobility at an optimum channel thickness of 50 nm, below or above

which the linear mobility decreases. In contrast, the linear mobility of the TFT with hydrophilic

substrate monotonically increases until the channel thickness decreases to 15 nm. According to

atomic force microscopy of 15 nm-thin pentacene grown on the SiO2 and PVP dielectrics, the

pentacene islands on poly-4-vinyphenol (PVP) are not perfectly interconnected unlike the case

on SiO2. We briefly conclude that the pentacene TFT with hydrophobic PVP dielectric clearly

has an optimum channel thickness of 50 nm for a peak mobility while the TFT with hydrophilic

SiO2 would show such peak mobility at even less than 15 nm of pentacene thickness. More

quantitative details on the channel thickness will be discussed in the conference.

B5.54 Epitaxially Grown Pentacene on h-BN Nanomesh. May Ling Ng

1,2, Alexei B Preobrajenski

2,

Alexei Zakharov2, Alexander S Vinogradov

3, Sergey A Krasnikov

4, John P Beggan

4, Anthony A

Cafolla4 and Nils Martensson

1,2;

1Department of Physics, Uppsala University, Uppsala, Sweden;

2MAX-lab, Lund University, Lund, Sweden;

3Institute of Physics, St.-Petersburg State

University, St. Petersburg, Russia; 4School of Physical Sciences, Dublin City University, Dublin,

Ireland.

The advancement of lithography and imprinting technology is gradually approaching saturation

while the needs for faster and smaller electronics continue to grow exponentially. Therefore, it is

interesting, if not necessary, to explore alternative opportunities in organic and hybrid organic-

inorganic electronics. Pentacene is an interesting organic molecule because it crystallizes in a

well-ordered structure, resulting in high charge carrier mobility (comparable to amorphous

silicon). However, the very first challenge in realizing the concept of pentacene organic thin film

transistors or photovoltaic cells is to find an insulating substrate for the long range epitaxial

growth of structurally perfect pentacene. Hexagonal boron nitride (h-BN) can be a good choice

for such substrate due to its 2D nature, i.e. high surface mobility, insulating property and ease of

preparation by CVD. In the present study, we have investigated the structure of pentacene grown

by thermal evaporation on h-BN/Rh(111), alternatively known as h-BN nanomesh, at RT. Each

deposition step (from 0.06 ML to 4 ML) is characterized by recording the C 1s spectra at grazing

(20°) and normal incidence (90°), and C 1s PE spectra at normal emission. In addition, we have

used LEEM and micro-LEED to reveal the influence of the h-BN monolayer on the pentacene

growth mode. We have also checked the structure of the multilayer pentacene/nanomesh with

STM. For a small amount of pentacene (up to 0.3 ML), the pentacene molecules first fill the

pores of the nanomesh because the pores act as physical traps for the molecules that have only

weak bonding to h-BN (PES C 1s profile resembles the one of free pentacene molecule). These

molecules are lying flat due to the lack of pentacene neighbours to form crystal structure. On the

contrary, for 0.8 ML and above, pentacene molecules start to re-arrange among the neighbouring

pentacene molecules with hydrogen bonding and become more vertically aligned, i.e. imitating

its natural herringbone crystalline structure. With LEEM we observe pentacene grows in large

2D islands on h-BN while on clean rhodium it grows randomly with high nucleation due to the

strong bonding between pentacene and Rh. The lateral profile of the 2D pentacene crystals on h-

BN/Rh is modulated by the nanomesh buckling, as reflected in the micro-LEED patterns.

B5.55

Tuning the Ionization Energy of Organic-semiconductor Films: The Role of Intramolecular

Polar Bonds.Ingo Salzmann1, Steffen Duhm

1, Georg Heimel

1, Martin Oehzelt

2, Rolf Kniprath

1,

Robert L Johnson3, Juergen P Rabe

1 and Norbert Koch

1;

1Institut für Physik, Humboldt-

Universität zu Berlin, Berlin, Germany; 2Institut für Experimentalphysik, Johannes Kepler

Universität, Linz, Austria; 3Institut für Experimentalphysik, Universität Hamburg, Hamburg,

Germany.

For the prototypical conjugated organic molecules pentacene and perfluoropentacene, we

demonstrate that the surface termination of ordered organic thin films with intramolecular polar

bonds (e.g., -H versus -F) can be used to tune the ionization energy. The collective electrostatics

of these oriented bonds also explains the pronounced orientation dependence of the ionization

energy. Furthermore, mixing of differently terminated molecules on a molecular length scale

allows continuously tuning the ionization energy of thin organic films between the limiting

values of the two pure materials. Our study shows that surface engineering of organic

semiconductors via adjusting the polarity of intramolecular bonds represents a generally viable

alternative to the surface modification of substrates to control the energetics at

organic/(in)organic interfaces.

B5.56

Metal-contact Induced Anomaly in Transfer Characteristics of Graphene Field-effect

Transistors. Ryo Nouchi1, Tatsuya Saito

2, Hiroki Watanabe

2 and Katsumi Tanigaki

1,2;

1WPI

Advanced Institute for Materials Research, Tohoku University, Sendai, Japan; 2Department of

Physics, Tohoku University, Sendai, Japan.

Graphene is a name given to a one-atomic carbon sheet which forms a honeycomb structure.

This material shows a charge carrier mobility of higher than 200,000 cm2 V-1 s-1 at room

temperature and is regarded as a major candidate for the future high-speed electronics. In

addition to the strikingly high mobility, a weak spin-orbit interaction in graphene makes it a

pivotal material in the field of molecular spin electronics. In order to construct such electronic

devices, metallic materials should make a contact with graphene. When a molecule comes into

contact with a metal, the electronic structure of the molecule should be affected by a chemical

interaction with the metal. Recently, we observed anomalous distorted transfer characteristics in

Co-contacted graphene devices [1]. The present study aims at clarifying the microscopic

mechanism of the anomaly in the carrier transport properties. Graphene layers were formed onto

a highly doped Si substrate with a 300 nm thick thermal oxide layer using conventional

mechanical exfoliation [2]. These layers were confirmed to be single layer graphene from the

optical contrast. Metal electrodes were fabricated onto the graphene layers by electron beam

lithography and lift-off techniques. While graphene devices with inert metals (e.g. Au) displayed

conventional transfer characteristics, those with more reactive metals (e.g. Ni) showed distorted

characteristics. The distortion appeared only in short channel devices, suggesting that it is a

consequence of the metal contact. Carrier conduction through short graphene channels can be

ballistic [3]. Possible mechanisms of the anomaly, including one based on ballistic conduction,

will be presented. [1] R. Nouchi et al.: Appl. Phys. Lett. 93 (2008) 152104. [2] K. S. Novoselov

et al.: Proc. Natl. Acad. Sci. U.S.A. 102 (2005) 10451. [3] F. Miao et al.: Science 317 (2007)

1530.

B5.57

Abstract Withdrawn

B5.58

Abstract Withdrawn

B5.59 Ultrasensitive Solution Processed Polymer Photodetectors Xiong Gong, Alan J Heeger,

Minghong Tong and Yong Cao; UCSB and CBrite, UCSB and CBrite, Santa Barbara, California.

Semiconducting polymeric optoelectronic and electric devices have evolved as a promising cost-

effective alternative to silicon-based devices. Organic photodetectors have been the subject due

to several inherent advantages. Some of the important advantages of these so-called “plastic”

electronics include large-area detection, low cost of fabrication, ease of processing, mechanical

flexibility and versatility of chemical structure due to the advancements in organic chemistry.

Full-color, fast-response and positive sensitivity organic photodetectors have been reported.

However, there are few reports on organic photodetectors whose performances are comparable

with inorganic countparts. will report ultrasensitive solution processed photodetectors fabricated

by different semiconducting polymers as the electron donors and various fullerences derivatives

and/or inorganic quantum dots as the electron acceptors. Polymer photodetectors with different

photo-response and detectivity were demonstrated. One example is that polymer photodetectors

have photo-response from 300nm to 1450nm, the detectivity larger than 1012 cm Hz1/2/W, and

linear dynamic range larger than 120 dB. All these values are comparable to or even better than

their inorganic counterparts.

B5.60

Photo-crosslinkable Polythiophenes for Efficient Thermally Stable Organic

Photovoltaics.Yoshikazu Miyamoto1,2

, Bumjoon J Kim3, Biwu Ma

2 and Jean Frechet

1,2;

1Department of Chemistry and Chemical Engineering, University of California, Berkeley,

California; 2The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley,

California; 3Department of Chemical and Biomolecular Engineering, Korea Advanced Institute

of Science and Technology, Daejeon, Korea, South.

Polymer based organic photovoltaics have attracted a great deal of attention due to their potential

as cost-effective, light-weight and flexible solar cells. Here we report a series of photo-

crosslinkable poly(3-hexylthiophene-co-3-(6-bromohexyl)thiophene) (P3HT-Br) for use in

solution processed organic photovoltaics. P3HT-Br copolymers were synthesized from two

different monomers, 2-bromo-3-hexylthiophene and 2-bromo-3-(6-bromohexyl)thiophene, where

the ratio of the two monomers was carefully controlled to achieve a UV photo-crosslinkable

layer while leaving the π-π stacking feature of conjugated polymers unchanged. We demonstrate

efficient thermally stable organic photovoltaics by using P3HT-Br copolymers as electron donors

in both bulk heterojunction (BHJ) and bilayer type devices. Our photo-crosslinking approach

stabilizes the nanophase separated morphology in the crosslinked P3HT-Br:PCBM BHJ with the

minimum disturbance in π-π stacking, which leads to photovoltaics with comparable power

conversion efficiency as compared to those of P3HT:PCBM and remarkably enhanced thermal

stability (over 2 days at 150°C). We have further applied these photo-crosslinkable P3HT-Br

copolymers to making efficient solution processed bilayer devices. Benefited from the little

disturbance in π-π stacking by crosslinkable units, P3HT-Br/PCBM bilayer devices show high

power conversion efficiency of over 2.2% and excellent thermal stability (over 3 days at 150°C),

which represents one of the highest performing bilayer devices fabricated by solution processing.

B5.61 Solution Processing of Small Molecules for Efficient Organic Photovoltaics Biwu Ma

1,

Claire Woo2, Yoshikazu Miyamoto

2,1 and Jean Frechet

2,1;

1The Molecular Foundry, Lawrence

Berkeley National Laboratory, Berkeley, California; 2Department of Chemistry and Chemical

Engineering, University of California, Berkeley, Berkeley, California.

Thin film organic photovoltaics (OPVs) based on polymeric materials or small molecules have

attracted tremendous research attention due to their potential application as low cost solar energy

conversion devices. Efficient OPVs have been fabricated by either solution processing of

polymeric materials or vapor deposition of thermally stable small molecules. To date, there has

not been much success in solution processing of small molecules for OPVs due to several

factors, such as limited solubility and poor film formation. Here I present our research efforts on

solution processing of small molecules for efficient OPVs. The materials of interest are soluble

conjugated organic subphthalocyanines. Due to their unique structural and photophysical

properties, including high solubility, low tendency of aggregation and strong light absorption, we

have been able to prepare amorphous thin films with high charge transporting and light

harvesting properties via simply solution casting. By using these materials as donor and

fullerenes as acceptor, we have demonstrated simple planar heterojunction OVPs with power

conversion efficiencies over 1.5 %, which represent the highest performing OPVs based on

solution processable small molecules to date. Our work clearly shows that solution processing of

light harvesting small molecules has great potential in low cost thin film organic photovoltaics.

B5.63

Energy Level Alignment of Model Molecular Electronic Systems on Silicon 111 7x7

Surfaces Conan Weiland1, Liu Yang

2, Dimitri Skliar

3, Brian Willis

3, Doug Doren

2 and Robert

Opila1;

1Department of Materials Science and Engineering, University of Delaware, Newark,

DE, Delaware; 2Department of Chemistry and Biochemistry, University of Delaware, Newark,

Delaware; 3Department of Chemical Engineering, University of Delaware, Newark, Delaware.

Molecular electronics promises great advances in device technology. With specialized chemical

functionalizations, molecular devices offer the prospect of novel device properties, while

promising reduced power consumption, inexpensive manufacturing and integration into everyday

products. Electron transport through single molecules has been well studied; however, the role of

the interface in charge transport in molecular electronic/substrate systems is not yet well

understood. This talk will focus on understanding the role of energy level alignment and hence

charge transfer at the molecule/silicon (111) 7x7 interface. Phenylacetylene and styrene were

used in this study as model molecular electronic systems as both molecules are conjugated over

the full molecule and are similar to the oligo-phenylene-vinylene moieties used in many

molecular electronics studies. Bonding between the molecule and surface was verified using X-

ray and UV photoelectron spectroscopy (XPS, UPS) as well as scanning tunneling microscopy

(STM). These measurements were also compared with density functional theory calculations.

Both molecules are seen to bind in a similar configuration - between the 2 carbon atoms of the

substituent and a silicon surface adatom-rest atom pair. Energy level alignment between

molecule and substrate was measured with UPS and bias dependent STM. The highest occupied

molecular orbital of both phenylacetylene and styrene were found to lie 0.7 eV below the silicon

Fermi level. The lowest unoccupied molecular orbital (LUMO) of phenylacetylene was found to

be about 0.7 eV above the silicon Fermi level, while the styrene LUMO was greater than 2 eV

above. This result shows that the specific chemical bond between molecule and substrate plays a

large role in the energy level alignment and hence charge transfer between molecule and

substrate.

B5.64

Enhanced Performance of Inverted Bulk-heterojunction Photovoltaic Cells by Interface

Modification with Self Assembled Monolayers Steven Hau1, Hin-Lap Yip

1,3, Kevin O'Malley

2,

Kung-shih Chen1, Jingyu Zou

1, Hong Ma

1,3 and Alex K.-Y. Jen

1,2,3;

1Materials Science and

Engineering, University of Washington, Seattle, Washington; 2Department of Chemistry,

University of Washington, Seattle, Washington; 3Institute of Advanced Materials and

Technology, University of Washington, Seattle, Washington.

Compared to the conventional photovoltaic architecture which uses low work function metals,

inverted bulk-heterojunction photovoltaic cells have the advantage of using a more air stable

high work function metal (Au, Ag) as the hole collecting back electrode and a thin n-type metal

oxide (ZnO, TiO2) layer at the ITO interface as the electron selective contact. However, inverted

structures have had lower photocurrent density and fill factor compared to the normal device

structures due to the poor electrical coherence between the inorganic metal oxide and organic

active layers. Self-assembled monolayers (SAMs) have been shown to significantly change the

interfacial properties of various oxide and metallic surfaces. The adhesion, compatibility,

morphology and electrical coherence at the inorganic/organic interface can be tuned by

assembling different functional SAMs. Modifying the interface of the electron selective ZnO

nanoparticles layer with a C60 functionalized SAM in inverted bulk-heterojunction photovoltaic

cells lead to an improvement in photocurrent density and fill factor. This monolayer can serve

multiple functions including the passivation of inorganic surface traps, enhancing interfacial

exciton dissociation efficiency, optimizing the upper organic layer morphology and stabilizing

the interface against dewetting.1 In addition, these unencapsulated inverted devices have

excellent ambient stability retaining over 80% of its original conversion efficiency after 40 days

of exposure.2 1. Hau, S., Yip, H.-L., Acton, O., Baek, N. S., Ma, H., Jen, A.K-Y., J. of Mater.

Chem. 18, 5113 (2008). 2. Hau, S., Yip, H.-L., Baek, N. S., Jen, A.K-Y., Appl. Phys. Lett. 92,

253301 (2008).

B5.65

Investigation on Optical and Optoelectronic properties of Blue Emission Conjugated

Polymer Optical Gain Media Cora E. C. Cheung, Boon Kar Yap, Ruidong Xia, Xuhua Wang,

Colin R. Belton, Paul N. Stavrinou and Donal D. C. Bradley; Department of Physics, Imperial

College London, London, United Kingdom.

Conjugated polymer semiconductors combine the processing and mechanical characteristics of

plastics with the desirable optical and electronic properties of semiconductors. A broad range of

devices including LEDs, field effect transistors, photodiodes and optical amplifiers and lasers are

under development with application potential in displays, lighting, RFID, display driver circuits,

imaging, solar energy conversion and data communications. Our focus here is on gain media,

where conjugated polymers are of interest for laser and optical amplifier development, with

visible spectrum emission, wavelength tunability and polymer optical fibre compatibility.

Extensive research has been undertaken on laser design and performance and it has become

evident that not all of the conjugated polymers optimised for LED operation are good gain

media. This presentation focuses on an investigation of the optical and optoelectronic properties

of a series of blue emission conjugated polymers, motivated by a desire to establish an effective

means to optimise polymer chemical structure for optical gain. Nine different polyfluorenes have

been selected for comprehensive characterisation of their optical properties, including amplified

spontaneous emission (ASE) measurements on asymmetric slab waveguides. The chosen

materials are from two groups of polyfluorenes coded as the SCB group (SCB3, SCB9, SCB11

and SCB18) and the SC group (SC005, SC006, SC007, SC008 and SC010), both from

Sumitomo Chemical. The S1 to S0 0-1 vibronic peaks (where gain is typically maximised) in the

polymer photoluminescence spectra range from 450nm (SCB11) to 463nm (SC006) and their

refractive indices range from 1.75 to 1.96. Three of the nine polymers (SCB3, SCB11 and

SC006) were unable to generate ASE at low pump energy (<30µJ per pulse, 10ns, 10Hz). All

three of these non-ASE polymers have long singlet excited state lifetimes (0.32-1.7ns) whilst the

polyfluorenes that do give ASE have significantly shorter lifetimes (134-250ps). In addition, the

highest photoluminescence quantum efficiency (PLQE) polymer is SC006 (96%) and lowest

SCB18 (22% with an ASE threshold of 1.7µJ/pulse). It is therefore evident that high steady state

PLQE and long excited state lifetime are insufficient for good optical gain properties. SC006 is,

moreover, the best LED material (highest efficiency) in the SC family whilst SC007 (which does

give ASE) is the worst. Similar trends apply to the SCB group, evincing an anti-correlation

between optimised LED and optical gain characteristics. The optimised LED materials have low

charge carrier mobility (10-8

cm2/Vs for holes in SC006 c.f. 10

-2cm

2/Vs for SC007) consistent

with charge trapping due to the incorporation of a fraction of relatively low ionisation potential

arylamine moieties within the polymer chain that also bestow an excited state charge transfer

character. The nature of the differences between LED and optical gain optimised polymers will

be discussed in detail.

B5.66

Ultrathin Self-Assembled Organophosphonic Acid Monolayers/Hafnium Oxide Hybrid

Dielectrics for Low-Voltage Organic Thin Film Transistors Orb Acton1, Hong Ma

1,2, Guy

Ting3, Hin-Lap Yip

1,2, Balaji Purushothaman

4, Itaru Osaka

5, Tomek Kowalewski

5, Richard D

McCullough5, John E Anthony

4 and Alex K.-Y. Jen

1,2,3;

1Materials Science & Engineering,

University of Washington, Seattle, Washington; 2Institute of Advanced Materials and

Technology, University of Washington, Seattle, Washington; 3Department of Chemistry,

University of Washington, Seattle, Washington; 4Department of Chemistry, University of

Kentucky, Lexington, Kentucky; 5Department of Chemistry, Carnegie Mellon University,

Pittsburgh, Pennsylvania.

Organic thin film transistors (OTFTs) based on pi-conjugated materials are envisioned for use in

ubiquitous low-cost flexible electronic devices, such as displays, sensors and electronic barcodes.

A prerequisite for realizing practical applications lies on the development of stable gate

dielectrics with low leakage current, low interface trap density, and high capacitance that afford

low-voltage OTFT operation with high performance. However, standard OTFTs still require

rather high operating voltages, often exceeding 20 V. By using organophosphonic acid self-

assembled monolayers (SAMs) on low-temperature solution processed hafnium oxide (HfOx) as

ultrathin hybrid gate dielectrics, we have realized low-voltage high performance OTFTs with

low leakage currents, and high charge carrier mobilities. In the demonstrated OTFTs, the

following device characteristics have been achieved: 1) low leakage current density - down to 1

nA/cm2; 2) large capacitance density - up to 630 nF/cm2; 3) low operating voltage (<2 V); 4)

high charge carrier mobility - up to 2.5 cm2/(Vs); 5) small subthreshold slope - down to 100

mV/decade; 6) low-hysteresis device operation. This is achieved by using well-packed and dense

organophosphonic acid SAMs (<4 nm) on HfOx (<5 nm) as ultrathin hybrid dielectrics and

through interfacial engineering using an appropriate SAM to control the chemical, electrical, and

morphological structure at the semiconductor/dielectric interface. Furthermore, this hybrid

dielectric system is shown to be generally applicable for solution processed organic

semiconductors by achieving stable device operation for polymer and soluble acene

semiconductors with mobilities >0.1 cm2/(Vs). This work represents a major advancement

towards developing large-area low-cost solution-processed/printed, flexible organic electronic

devices.

B5.67

Abstract Withdrawn

B5.68

Structured Interfaces in the Assembly and Performance of Organic Field-Effect

Transistors Gregory L Whiting, Rene J Kist, Tse Nga Ng, Sanjiv Sambandan, Beverly Russo,

Brent S Krusor and Ana C Arias; Electronic Materials and Devices Laboratory, Palo Alto

Research Center (PARC), Palo Alto, California.

Interfaces play an important role in the operation of organic thin-film field-effect transistors

(FETs). For example, many reports have shown that the composition and quality of the

dielectric/semiconductor or electrode/semiconductor interface has a significant effect on the

overall device performance. Control over the chemistry of surfaces can also greatly aide the

fabrication of solution-processed devices and the patterning of larger arrays of these devices to

form electronic components. This control is particularly useful when deposition methods like

ink-jet printing are used, as parameters such as the placement and drying of a drop can be

influenced. This report will focus on both n- and p-type solution processed FETs. Ink-jet printing

is used to pattern small molecule organic semiconductors, as well as silver source, drain and gate

contacts. By careful modification of surface energy, particularly through the use of self-

assembled monolayers, well-defined metal contacts can be produced, and semiconductor

morphology can be controlled. Device parameters can also be strongly affected by these

interfaces. For example, electrode work functions can be tailored to achieve good contact

between the printed silver and organic semiconductor. Both top- and bottom-gate architectures

have been used for this study, which present different benefits and challenges for additive

solution processing. In addition, ink-jet printed complementary circuits using these materials and

methods will be described.

B5.69

Impact of Interfacial Modification on Polymer Morphology, Photoexcitation Dynamics,

and Device Performance in P3HT/ZnO Solar Cells. Matthew Thomas Lloyd1, Rohit P

Prasankumar2, Michael B Sinclair

3, Alex C Mayer

4 and Julia W Hsu

1;

1Surface and Interface

Sciences, Sandia National Laboratories, Albuquerque, New Mexico; 2Center for Integrated

Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico; 3Sandia

National Laboratories, Albuquerque, New Mexico; 4Department of Materials Science and

Engineering, Stanford Univeristy, Stanford, California.

Short-circuit current, which is determined by interfacial charge separation and recombination, in

ZnO/poly 3-hexylthiophene (P3HT) solar cells is found to improve by intentional modification

of the heterojunction interface. To understand the origin of this enhancement, we investigate the

morphology of the interfacial polymer layer and decay dynamics of photoexcited species in

P3HT deposited on glass, bare ZnO, and ZnO modified with an alkanethiol monolayer. These

results are correlated with the characteristics of P3HT/ZnO and P3HT/alkanethiol-modified ZnO

bilayer photovoltaic devices. Synchrotron x-ray diffraction spectra of pristine P3HT and P3HT

on an alkanethiol-modified ZnO surface point to a more crystalline P3HT interfacial layer, while

an amorphous interfacial layer of P3HT is found on unmodified ZnO. To investigate the decay

dynamics of initial photoexcited states in the ZnO/P3HT system, the composite samples are

interrogated by pump-probe spectroscopy with sub-picosecond resolution. Transient

photoinduced absorption spectra are collected for photoexcited species in the range of 1090 nm

to 3034 nm after excitation with a 550 nm pump. Compared to P3HT/ZnO composite films, the

decay behavior for both polarons and excitons over a 500 ps time interval becomes significantly

slower with alkanethiol modification, indicating a reduction in of early-stage charge

recombination. Accompanying the decrease in recombination, we find an increase in the short-

circuit current in the alkanethiol-modified ZnO devices in spite of the electron tunneling barrier

presented by the alkanethiol monolayer. External quantum efficiency measurements in

alkanethiol-modified devices also exhibit a clear signature of crystalline P3HT within an exciton

diffusion length from the heterojunction interface. As these experiments demonstrate, charge

injection efficiency and device performance can be improved by control of the polymer

morphology at the heterojunction interface. Sandia is a multiprogram laboratory operated by

Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy‟s

National Nuclear Security Administration under contract DE-AC04-94AL85000.

B5.70

Shelf Lifetime Study of Unencapsulated Organic and Hybrid Photovoltaic Devices. Matthew Thomas Lloyd

1, Dana C Olson

2, Matthew O Reese

2, Erica Fang

1, Diana L Moore

3,

James A Voigt3 and Julia W Hsu

1;

1Surface and Interface Sciences, Sandia National

Laboratories, Albuquerque, New Mexico; 2National Renewable Energy Laboratory, Golden,

Colorado; 3Sandia National Laboratories, Albuquerque, New Mexico.

As organic photovoltaics (OPVs) approach commercially viable power conversion efficiencies,

in depth investigation of degradation mechanisms and methods for their mitigation becomes

increasingly important. To address this issue, we monitored the difference in degradation rates

for polymer:fullerene bulk heterojunctions (BHJ) and polymer/ZnO hybrid devices. The device

performance was measured over the course of 6 weeks, storing the devices in the dark in ambient

atmosphere between measurements. We found that the performance of BHJ devices employing

silver electron-extracting contacts degraded rapidly (with a half-life of three days). BHJ devices

with calcium/aluminum electron-extracting electrodes also degrade, but at a slower rate. These

results are in contrast with the behavior of “inverted” BHJ and hybrid polymer/ZnO devices,

where an ITO/ZnO electrode serves as the electron-extracting contact and silver serves to extract

holes. Initially, the efficiency of these inverted devices increased; notably, the inverted BHJ

device improved by more than 50% at day nine and maintained an efficiency greater than the

initial value for the duration of the degradation study. These results indicate that the cause of

OPV shelf life degradation is not, as commonly believed, the instability of the active organic

materials. We find that the changes in the contact are a more important cause. Specifically, as

silver oxidizes in air, its work function increases and loses selectivity as an electron-accepting

electrode. A larger work function is found to advantageously increase open-circuit voltage and

short-circuit current in both inverted BHJ and hybrid devices where Ag functions as the hole-

collecting electrode. In addition, we have measured the change of many polymer/ZnO bilayer

and nanorod devices over a longer period of time and found that, over 450 days, the devices still

perform at 50% of their initial efficiency. In summary, we found that changes in the metal

contact have a larger effect than changes in the active material on the degradation of organic and

hybrid solar cells. Sandia is a multiprogram laboratory operated by Sandia Corporation, a

Lockheed Martin Company for the United States Department of Energy‟s National Nuclear

Security Administration under contract DE-AC04-94AL85000.

B5.71

Abstract Withdrawn

B5.72 Nanotube Based High Current Transistor with Good on/off Ratios. Islamshah Amlani and

Rudy Emrick; Applied Research and Technology Center, Motorola, Tempe, Arizona.

Aligned and randomly networked ensembles of single-walled carbon nanotubes (SWNT)

represent a potentially powerful platform for developing thin film semiconductor technologies

such as flexible electronics, optoelectronics, RF electronics and sensing. Electrical characteristics

of SWNT based devices that contain ensemble of nanotubes in the channel exhibit a cumulative

effect of the characteristics due to varying chiralities of the constituent nanotubes. For instance,

in an aligned SNWT device the lower limit of the off state leakage current characteristics will

depend on the fraction of metallic SWNTs in the channel. One way to improve the on/off ratio

post-fabrication is by selective joule heating of metallic and high-leakage ambipolar

semiconducting nanotubes. A typical approach is to bias the semiconducting tubes in the off-

state by applying an appropriate gate bias and apply a sufficiently high drain current to

electrically stress and break the conducting nanotubes. If not properly configured however, on-

current can also diminish substantially in this process due to undesirable burning of

semiconducting nanotubes. Here, we present an automated electrical burning process that

preferentially breaks down both metallic and high-leakage semiconducting SWNTs in a

nanotube ensemble to obtain on/off ratios higher than three orders of magnitude. Typical on-state

current of our back gated devices with interdigitated source drain geometry is in the milliampere

range due to a large number of nanotubes in the channel and the reduction in on-state current at

the end of the electrical burning process is minimal (~ 30%) with on/off ratios of 1000 or greater.

These results are among the best that have been reported. However, we were not able to obtain

electrical burning results on top gated geometry with similar efficiency. We perform two

different experiments to elucidate that nanotubes break at the center when electrically stressed,

the main reason for only marginal success with top gated devices. In the first experiment, the

channel of the back gated devices are completely passivated by PMMA to purposely block off

oxygen at the nanotube interface. Electrical burning result in this case is rather poor as lack of

oxygen availability at the nanotube interface leads to non-preferential breakdown of both

metallic and semiconducting nanotubes. In the second experiment we expose the center portion

consisting of ~30-50% of the channel by performing electron beam lithography and developing

the PMMA resist. Electrical burning results on these devices yield similar to those without any

passivation. This indirectly suggests that breakdown indeed takes place in the center of the

nanotubes and good on/off ratios can be obtained using this approach as long as the middle of the

channel is exposed to air. This has important consequences for top-gated devices where the

center portion of the channel is passivated by gate dielectric and metal.

B5.73 Control of Pentacene Growth and Its Effects on Organic TFT Characteristics Jong Sun

Choi1, Jaehoon Park

1, Jong Won Lee

1, Dong Wook Kim

1, Hyung Tak Kim

1 and Dong Myung

Shin2;

1Dept. of Electrical, Information and Control Engineering, Hongik University, Seoul,

Korea, South; 2Dept. of Chemical Engineering, Hongik University, Seoul, Korea, South.

Recently, organic semiconductor devices have been extensively investigated and steady

progresses in device performances are continuously being obtained with ever increasing range of

applications. One of the most promising organic semiconductors is pentacene, mainly due to the

high hole mobility in pentacene-based TFTs. And many researchers continue to develop high-

performance pentacene thin-film transistors (TFTs), focusing on improving the electrical

conduction in the pentacene active layer. In this study, we have fabricated blends of two different

polyimides with varying the composition ratio and investigated the growth of pentacene grains

on each blend film. It is observed that the pentacene grain size pronouncedly reduced with

increasing the content of hydrophobic polyimide in the blend film. This result can be explained

by the interaction between the adsorbed pentacene molecules and the protrusion of hydrophobic

component in the blend film. Indeed, atomic force microscopy images show that the surface

diffusion of pentacene molecules on the blend film was limited by the hydrophobic protrusions.

Accordingly, it is confirmed that the fabricated blend films can be applied to control the

pentacene grain growth. We extended these results to investigate the effects of pentacene grain

size on the TFT characteristics. Organic TFTs with larger pentacene grains exhibited improved

device performances, which might be attributed to less grain boundaries. And experimental

results show that grain boundaries act as trap sites during device operation and the trapped

charges at grain boundaries can be activated by increasing the gate-source field. However, the

threshold voltage shift upon a gate-voltage sweep direction was more pronounced for the device

with larger pentacene grains, even strongly depending on the delay time of gate-voltage step.

These results are considered to be attributed to a strong interaction between the pentacene layer

and the concomitant blend film owing to its polar feature. Of note, grain boundaries can be

identified as a limiting factor to the charge transport in organic TFTs and the interface between

the organic semiconductor and insulator layer plays a significant role in the operational stability.

Further investigations are focused on evaluating the activation energies of trapped charges at

grain boundaries as well as the interface. These results will be presented.

B5.74

Effects of Alignment Layers on Pentacene Molecular Orientation and Thin-Film Transistor

Characteristics Jae-Hoon Kim1, Jongseung Kim

1, Hyunsuck Kim

1, Jaehoon Park

2, Jong Sun

Choi2 and Dong Myoung Shin

3;

1Department of Electronics and Computer Engineering,

Hanyang University, Seoul, Korea, South; 2Dept. of Electrical, Information and Control

Engineering, Hongik University, Seoul, Korea, South; 3Dept. of Chemical Engieering, Hongik

University, Seoul, Korea, South.

Pentacene, a fused-ring polycyclic aromatic hydrocarbon, is one of the most intensively

investigated systems among various organic semiconductors due to great mobility and good

semiconducting behavior. The electrical conductivity in this material strongly depends on the

direction of applied electrical field to its long molecular axis. And also, it is known that the

vertical alignment of pentacene molecules to the gate insulator surface provides a strong π-π*

overlap and increases the electrical conductivity in the direction of perpendicular to the long-

axis. These have motivated several studies of the effects of pentacene molecular orientations on

the performance of organic thin-film transistors (OTFTs). In the present work, we use different

alignment layers to investigate their effects on pentacene molecular orientation and the

concomitant performance of organic TFTs. For the fabrication of morphological alignment layer,

polyimide films were formed by spin-coating and then rubbed in the parallel and vertical

directions to conducting channel in OTFTs. And also, liquid crystal (LC) material was used for

the fabrication of molecular alignment layer. LC molecules were aligned in the parallel and

vertical directions to channel direction. Experimental results show that the transistor

characteristics were dependent on the directions of alignment layers. The OTFTs with the

morphological alignment layer exhibited an increase in the drain current compared to the device

without rubbing treatment, independent on the rubbing directions. Dichroic ratio of the drain

current was about 1.2, which is defined as the ratio of current for the device with the parallel

alignment layer to that with the vertical alignment layer. On the other hand, the OTFTs with the

molecular alignment layer showed a significant dependence of drain current on the direction of

alignment layer: the drain current in the parallel direction increased compared with that for the

device with unaligned LC layer, but the drain current in the vertical direction even deteriorated.

In this case, dichroic ratio was about 2.1. These results indicate that the morphological effect on

the pentacene molecular orientation is intrinsically different from that of a prior molecular

orientation. We will report the detailed growth mechanism of pentacene molecule on these

alignment layers, combining with the electrical characteristics of OTFTs.

B5.75

Nanostructure-Assisted Hole Injection in Schottky Diodes and Application in Organic

TFTs Hyunsuck Kim1, Jongseung Kim

1, Jae-Hoon Kim

1, Jaehoon Park

2 and Jong Sun Choi

2;

1Department of Electronics and Computer Engineering, Hanyang University, Seoul, Korea,

South; 2Dept. of Electrical, Information and Control Enigeering, Hongik University, Seoul,

Korea, South.

Organic thin-film transistors (OTFTs) have shown great promise for a variety of electronic

applications, including flexible displays, chemical sensors, and low-cost microelectronics. The

characteristics of OTFTs have been remarkably advanced and even surpass those of TFTs with

amorphous Si. Most of efforts for OTFTs have been mainly focused on the improvements of

electrical performance. But the basic study in the interfacial characteristics related with device

performance is not so much progressed, which is also necessary for understanding device physics

and further improvements in device characteristics. The interface between an organic material

and a metal is one of critical factors for the device performance. Recently, some groups have

investigated the importance of the interfacial characteristics and attempted to enhance the

interfacial characteristics. Therefore, the interfacial properties should be investigated and must

be improved in order to make OTFTs competitive with more conventional amorphous Si and

poly-Si TFTs. In this work, we have fabricated the conic-nanostructures on the Al-bottom

electrode and investigated the growth of pentacene molecules on the rough conic-nanostructures,

combing with the barrier heights for hole injection in Schottky diodes. For the fabrication of

conic-nanostructures, the H1 solution, in which polyurethane was dissolved into acetone solvent,

was spin-coated onto the Al-bottom electrode. X-ray diffraction results show that the conic-

nanostructures can contribute to the ordered growth pentacene molecules. And the barrier height

for hole injection from the top-Au electrode was calculated by Fowler-Nordheim theory and

found to be lowered for the Schottky diode with conic-nanostructures. This injection barrier

lowering can be explained by the ordered growth of pentacene molecules under the influence of

conic-nanostructures. We also introduced these structures into the interface bewteen the

pentacene layer and gate insulator of OTFTs. It is observed that the electrical characteristics of

the OTFT with conic-nanostructures were higher for the device without nanostructures. In

particular, the field-effect mobility was significantly improved by using conic-nanostructures,

calculated to be about 2.94 cm2/Vs. Consequently, we conclude that the nanostructure-assisted

hole injection facilitated the barrier lowering, thereby contributing to achieving high

performances in OTFTs. These results will be discussed.

B5.76

Structural Phase Transition and Molecular Electron Tunneling in Self-Assembled

Monolayers Kyoungja Seo and Hyoyoung Lee; ETRI, Daejeon, Korea, South.

The electrical and chemical nature of organic molecules in self-assembled monolayers (SAMs)

on metal or semiconductors have been studied for applications of molecular electronics[1]. As a

model system, alkanethiols have been studied extensively by current-voltage (I-V) characteristics

in molecular junctions (metal-molecule-metal). Electron transport across the alkanethiol

monolayers is influenced by a tunneling pathway through the σ-bonded alkyl chain (through-

bond tunneling) and through the intermolecular charge transport (through-space tunneling)[2].

Strong electronic coupling of a sulfur atom to a gold atom by chemisorption enhances the

electron transport via through-bond tunneling across the interfaces of alkanethiol and gold. A

molecular tilt in alkanethiol SAMs enhances contribution of through-space tunneling by the

intermolecular coupling, and molecular conductance across the SAM relatively decreases[3].

Molecular orientation on alkanethiol SAMs is changed with structural phase transition by

thermal annealing in the SAMs. Molecular electron transport characteristics will be varied by

different intermolecular coupling induced in each structural phase. However, tunneling

characteristics such as an electron tunneling barrier of molecular junctions has not been reported

with different structural phases, yet. In addition, it was reported that the thermal and electrostatic

effects influence conductance across molecular junctions, dependent of the local molecular

environment induced by neighboring molecules. Thus, we expect that the structural phase

transition-induced a difference in molecular electron tunneling characteristics will depend on

different intermolecular coupling effects created in a large molecular junction and an individual

molecular junction. In this study, we demonstrate structural phase dependency of conductance

across the thiolate (e.g., alkanethiol and alkanedithiol) SAMs, dependent of a junction area in

size. The molecular electron tunneling characteristics for each structural phase are compared

with I-V curves obtained in a scanning tunneling microscope (STM)-based individual molecular

junction and a micropore-based large molecular junction. A tunneling barrier is proved as a

measure of the intermolecular coupling on different structural phases of the thiolate SAMs. It is

the first examination of the molecular electron tunneling in different structural phases of the

thiolate SAMs and dependence of the molecular electron tunneling on a junction area in size. [1]

A. Salomon, D. Cahen, S. Lindsay, J. Tomfohr, V. B. Engelkes, C. D. Frisbie, Adv. Mater. 2003,

15, 1881-1890. [2] X. D. Cui, X. Zarate, J. Tomfohr, O. F. Sankey, A. Primak, A. L. Moore, T.

A. Moore, D. Gust, G. Harris, S. M. Lindsay, Nanotechnology 2002, 13, 5. [3] H. Song, H. Lee,

T. Lee, J. Am. Chem. Soc. 2007, 129, 3806-3807.

B5.77 Semi-transparent Flexible Photo-detector using Tetracene/ZnO hybrid p-n Junction. Aaron

Park, Seongil Im, Kimoon Lee and Kwang H. Lee; physics, Yonsei university, Seoul, Korea,

South.

Over the last few decades, organic thin-film devices, such as organic light-emitting diodes

(OLED), organic thin-film transistors (OTFT), solar cells, and photodetectors have made steady

progresses in their performances. Among organic materials polyacenes like tetracene, which is

one of the important organic molecules composed of four benzene rings, have been studied for

applying to optoelectronic devices due to their good optical properties. We report on the

fabrication of p-type organic/n-type ZnO hetero-junction diode and its applications. To construct

the organic/inorganic hetero-junction, 50 nm-thick ZnO was deposited on ITO-coated flexible

PET substrate at 100 °C and then 100 nm-thick tetracene as a p-type organic semiconductor was

evaporated on the ZnO layer. Not only to make an ohmic contact with p-type organic layer but

also to investigate the photo-response properties of the p-n junction, we adopt the semi-

transparent NiOx electrode with large work-function value of ~5.1 eV and transmittance of ~30

%. Organic/inorganic hetero-junction of p-type tetracene/n-type ZnO showed rectifying behavior

with current-voltage (I-V) characteristic and exhibited quite a high current density under forward

bias. Also the tetracene/ZnO p-n diode has excellent photo-response properties under green,

blue, and weak UV illuminations. We actually fabricated pentacene/ZnO p-n diode using the

similar method, to compare with the tetracene/ZnO diode. Although the pentacene/ZnO diode

displayed a little higher forward current than that of the tetracene/ZnO, we now regard that our

p-tetracene/n-ZnO diode has clear advantages over pentacene/ZnO as an optoelectronic device

due to its excellent photodetecting potentials. More details on the comparison will be discussed

in the conference meeting.

B5.78

Characteristics of Organic Thin-Film Transistors with Anodized Aluminum Oxide as a

Gate Insulator.Jong Won Lee1, Dong Wook Kim

1, Jaehoon Park

1, Hyoung Tak Kim

1, Dong

Myoung Shin2 and Jong Sun Choi

1;

1Dept. of Electrical, Information and Control Engineering,

Hongik University, Seoul, Korea, South; 2Dept. of Chemical Engineering, Hongik University,

Seoul, Korea, South.

Organic thin-film transistors (OTFTs) have been expected by switching devices for flexible

display. Aluminum gate electrode is usually adopted to reduce the delay time because of the

property of low resistivity. And anodized aluminum oxide film is considered as a good insulator

because it is resistant to various chemical solvent and has a high dielectric constant (high-k)

contributing to reduction of the threshold voltage. The reduction of threshold voltage can lead to

lowering supply voltage and thus resulting in lowering power dissipation. In general, anodized

oxide films form two types of morphology, i.e. barrier-type and porous-type films, by altering

the condition of electrolytes. Barrie-type films in the pH range of 5 to 7 are utilized as

electrolytic capacitors due to thin, compact, and non-porous structure, while porous-type films

are applied to nano-templates due to its thick and porous structure. In this work, we fabricated

two types of anodizing aluminum oxide films with varying the pH values of electrolyte. Atomic

force microscopy images of anodized aluminum oxide films show that the surface of film

forming the barrier-type structure grown at pH 6.3 is smoother than that for the porous-like

structure grown at pH 4.1. The difference in the height of these two films was about 10-30 nm.

And the OTFT with the barrier-type aluminum oxide insulator exhibited the mobility of 0.09

cm2/Vs, the subthreshold slope of 1.1 V/decade, and the on/off ratio of 10_4, which are superior

to those for the device with the porous-type insulator. It is thought that the smooth surface of the

barrier-type film might contribute to a long range hopping of charge carriers in the conducting

channel. Indeed, we found that the activation energy for device with the porous-like anodized

insulator was much larger than that for the barrier-type structure, which demonstrates that the

characteristic improvement in the device with the barrier-type structure was attributed to low

activation energy for carrier transport in the conducting channel. These results will be discussed.

B5.79 Scanning Probe Analysis of Poly(3-Hexylthiophene) Thin Films. Rajiv Giridharagopal and

Kevin Kelly; Electrical and Computer Engineering, Rice University, Houston, Texas.

The electronic behavior of conducting polymers at the polymer-electrode interface is of great

interest, both technologically and in terms of basic materials science. We have used scanning

tunneling microscopy (STM), including spectroscopic extensions such as work function imaging

and alternating current STM (ACSTM), to probe conducting polymer thin films and monolayers

with high spatial resolution in both ambient and ultrahigh vacuum environments. In our studies

we focus on poly(3-hexylthiophene) (P3HT) films, each approximately one to three monolayers

in thickness, deposited on highly-ordered pyrolytic graphite (HOPG) and molybdenum disulfide

(MoS2) substrates. P3HT is one of a number of conducting polymers, and STM reveals that

P3HT deposited on HOPG or MoS2 self-assembles to form a highly-ordered structure with

symmetry commensurate with that of the underlying lattice. Additionally, we have used ACSTM

to analyze spatial variations in the charge carrier density in such layers, thus shedding light on

substrate-dependent charge transfer in P3HT films on different materials. Understanding such

effects is of great importance for improving P3HT devices and conducting polymer-metal

interfaces in general.

B5.80

Hybrid Nanostructures «Semiconductor/Organic Dye J-Aggregate» In Reverse Micelles. Vladimir Razumov, Lubov Nikolenko and Sergey Brichkin; Institute of Problems of Chemical

Physics RAS, Chernogolovka, Russia.

Hybrid nanostructures such as «molecular aggregate/semiconductor nanoparticle» or core/shell

structures in which core is the semiconductor nanocrystal and shell is the organic dye are very

important for the charge separation at light absorption. These structures are useful for

development of light-emitting devices, thin-film transistors, and optical memory or solar cells.

Organic/inorganic semiconductor interfaces play an important role for effective charge

separation. There are different methods for design of hybrid nanostructures. One of these

methods is a self-assembly using of reverse micelles. The main procedure includes three stages.

The first is synthesis of semiconductor nanoparticles by chemical reaction controlled by

intermicellar exchange in «water in oil» microemulsions. The second stage is molecular dye

aggregation and the third stage is selfassembly of organic/inorganic nanostructures. Self-

assembly of hybrid “nanocrystal/J-aggregate” nanostructures after mixing two reverse micelle

AOT/water/hexane solutions: one containing J-aggregates of pyridinium salt of betaine 3,3'-di(γ-

sulfopropyl)-4,5,4',5'-dibenzo-9-ethylthiacarbocyanine and the other - AgI nanocrystals was

shown. During the hybrid nanostructure formation new dye absorption bands with λmax≈673

and 695 nm appeared, these belong to J-aggregates of different structures adsorbed on

nanocrystals. It was found that the excess of iodide ions during AgI nanocrystal synthesis is the

main factor influencing the hybrid nanostructures self-assembly, so AgI crystal lattice is of great

importance in this process. Taking into account that AgI nanocrystals synthesized in iodide

excess have hexagonal crystal lattice it can be concluded that J-aggregates effective absorb on β-

AgI and do not absorb on γ-AgI. There occurs a “symbiosis” of the hybrid structure components:

nanocrystals raise the photostability of J-aggregates, and adsorbed J-aggregates efficiently

stabilize the nanocrystal size. It was shown that the stable hybrid nanostructures may be

extracted from micellar solution without further aggregation. Spectral and structure

investigations of these hybrid systems were carried out depending on average size of reverse

micelles using of light absorption and TEM techniques.

B5.81

Morphological Stabilization of Polymer Photovoltaic Cells by Using Cross-linkable

PolythiopheneShoji Miyanishi1, Keisuke Tajima

1 and Kazuhito Hashimoto

1,2;

1Enginnering,

Graduate School of The Univerity of Tokyo, Tokyo, Japan; 2JST-ERATO, Tokyo, Japan.

Polymer photovoltaic cells draw considerable attention these days for their potential of low cost

fabrication of large area devices by simple means of painting or printing from the polymer

solutions. The most commonly used structure for the polymer photovoltaic cells now is a bulk

heterojunction, which is a physical mixture of donor and acceptor materials. Recent research has

suggested that control of the mixing morphology of the donor and the acceptor in films is of high

importance to achieve highly efficient charge separation and transport. Especially in the case of

the combination of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl

ester (PCBM), thermal annealing of the films has been used to control this phase separation.

During the thermal annealing, both the polymer and PCBM crystallize to form an

interpenetrating network in nanoscale, resulting in drastic enhancement of the device

performance. However, even if such phase-separated nanostructure is constructed, this

morphology is not thermally stable and may gradually undergo changes during the device

operation since the polymer and the PCBM thermodynamically prefer to segregate from each

other. In fact, a prolonged thermal treatment of the device induces the formation of large

aggregations of PCBM in the films that lowered the device performance significantly. Therefore,

morphology control that only depends on thermal annealing might be insufficient for achieving a

stable and reliable mixing morphology of the donor and the acceptor. In this work a new cross-

linkable regioregular poly(3-(5-hexenyl)thiophene) (P3HNT) was synthesized for the purpose of

stabilizing the film morphology in polymer photovoltaic cells. The vinyl group at the side chains

of P3HNT is expected to conduct cross-linking reaction by thermal treatment. After the process

of cross-linking is complete, the diffusion of PCBM into the film can be lowered to suppress the

formation of large aggregations. As a result, the thermal stability of the cells is expected to

improved in comparison to non-cross-linkable P3HT:PCBM bulk heterojunction. P3HNT was

characterized by NMR, GPC, UV-vis absorption spectra, XRD spectra and DSC. XRD and UV-

vis spectra showed that P3HNT has similar crystallinity to P3HT. As a result, the performance of

the P3HNT:PCBM bulk heterojunction devices showed comparably high efficiency over 3%

similar to P3HT:PCBM cells. Furthermore, P3HNT was certainly cross-linked during thermal

treatment, confirmed by the insolubility of the films in organic solvents. This cross-linkability

was also confirmed even in the mixture films with PCBM. As the result, the formation of large

aggregations of PCBM was prevented in P3HNT:PCBM films even after prolonged thermal

annealing. In addition, the deterioration of the photoconversion performance at a high

temperature was suppressed in the polymer solar cells compared to the control cells with

P3HT:PCBM.

B5.82 Low-Voltage Self-Supported Ion Conductive Membrane Based Transistors Nikolai J.

Kaihovirta1,3

, Carl-Johan Wikman2, Tapio Makela

1, Carl-Eric Wilen

2 and Ronald Osterbacka

1;

1Center for Functional Materials and Department of Physics, Åbo Akademi University, Turku,

Finland; 2Center for Functional Materials and Laboratory of Polymer Technology, Åbo Akademi

University, Turku, Finland; 3Graduate School of Materials Research, Turku Universities, Turku,

Finland.

Ion enhanced organic transistors are promising candidates for large-scale fabrication of low-

voltage applications [1-3]. In the ion enhanced organic transistor, an ionic insulator replaces the

traditional dielectric insulator. Two types of ion enhanced polymer transistors have been

presented in the literature: Electrochemical transistors [1, 4] and electric double layer (EDL)

gated transistors [2, 3, 5, 6]. We present a novel EDL-gated transistor using a thick (> 50 μm),

ion conducting membrane (MemFET) [7]. The membrane acts both as gate insulator and as

mechanical support. The fabrication of the membrane starts with a PVDF-film as base material.

The PVDF-film is functionalized by the roll-to-roll suitable electron beam irradiation induced

grafting technique [8]. For comparison, we have also used the commercially available, proton-

conducting Nafion®-membrane, as received. The MemFETs are fabricated by standard

laboratory fabrication techniques using soluble polymers. As semiconductor we apply the

regioregular P3HT. The highly conducting polymers PANI or PEDOT:PSS are used for the gate

electrode, while evaporated gold is chosen, for convenience, for the source and drain electrodes.

The MemFETs operate at low voltages (1 V) with a high current throughput. By using the

electron beam irradiation induced grafting technique we can fabricate membranes to conduct

different ions and/or simply tailor-make membranes to be locally ion conducting in whatever

pattern needed. MemFETs fabricated on different membranes will be presented. Furthermore, the

ion conducting membrane allows for fabrication of two (or more) devices on the same membrane

in a simple and cost-effective matter. This will be shown by driving an electrochromic display

pixel with a MemFET, both fabricated on the same membrane. [1] R. Mannerbro, M. Ranlöf, N.

Robinson, R. Forchheimer, Synth. Met. 158 (2008), 556-560. [2] D. Tobjörk, N. J. Kaihovirta, T.

Mäkelä, F. S. Pettersson, R. Österbacka, Org. Electron. 9 (2008), 931-935. [3] J. H. Cho, J. Lee,

Y. Xia, B. Kim, Y. He, M. J. Renn, T. P. Lodge, C. D. Frisbie, Nat. Mater. 7 (2008), 900-906.

[4] D. Nilsson, M. Chen, T. Kugler, T. Remonen, M. Armgarth, M. Berggren, Adv. Mater. 14

(2002), 51-54. [5] H. G. O. Sandberg, T. G. Bäcklund, R. Österbacka, H. Stubb, Adv. Mater. 19

(2004), 1112-1115. [6] M. J. Panzer, C. D. Frisbie, Adv. Funct. Mater. 16 (2006), 1051-1056. [7]

N. J. Kaihovirta, C, -J. Wikman, T. Mäkelä, C. -E. Wilén, R. Österbacka, Adv. Mater. (2008) in

press. [8] T. Lehtinen, G. Sundholm, S. Holmberg, F. Sundholm, P. Björnbom, M. Bursell,

Electrochim. Acta 43 (1998), 1881-1890.

B5.83

Understanding Aminated Silane Monolayer Formation Kinetics for Use in Organic

Electronics Justin Opatkiewicz, Melbs C LeMieux and Zhenan Bao; Chemical Engineering,

Stanford University, Stanford, California.

Aminated silane films are being used in a wide variety of fields: from biological sciences to

semiconductor research. Amines with varying degrees of substitution are common in nature and

hence can be used to modify surfaces to interface with biological systems. 3-

aminopropyltriethoxysilane (APTES) is a very common silane and has been characterized in

many studies. It is generally accepted that the amine catalyzes monolayer formation by

electrostatic attraction of the amine to a hydrophilic surface. Silanes with very similar structures,

however, have not been analyzed in as much depth. Similar molecules varying simply by methyl-

substitutions, such as N-methylaminopropyltrimethoxysilane (MAPS) and (N,N-

dimethylaminopropyl)trimethoxysilane (DMAPS), can potentially be used alongside APTES in a

variety of applications such as biological linkages and carbon nanotube separation. Here, we

compare the kinetics of MAPS and DMAPS to APTES surface reactions and determine the

influence of the methyl substitution on monolayer formation. After the kinetics of the three

silanes are understood, their abilitly to separate carbon nanotubes (CNTs) are analyzed.

B5.84 Enhanced Performance of Organic Light Emitting Diodes Using LiF Buffer Layer. Omkar

Vyavahare1 and Richard Hailstone

2;

1Materials Science and Engineering, Rochester Institute of

Technology, Rochester, New York; 2Imaging Science, Rochester Institute of Technology,

Rochester, New York.

Since the invention of organic electroluminescent devices a great deal of effort has been made to

improve their performance. Reducing the barrier and optimizing charge injection is crucial for

efficient and bright Organic Light Emitting Diodes (OLEDs). We report improvement in the

performance of OLEDs with ITO/TPD/Alq3/Al structure by inserting LiF both at electrode-

organic interface and organic-organic interface. In this paper, we elucidate the mechanism of LiF

buffer layer inserted at different interfaces. These devices show improved luminescence and

steeper IV characteristics.

B5.85

Transparent Photo-stable Complementary Inverter with Organic-inorganic Nano-hybrid

Dielectrics.Min Suk Oh1, Yong Hoon Kim

1, Sung Kyu Park

1, Jeong In Han

1, Byoung H. Lee

2,

Myung M. Sung2, Kimoon Lee

3, Kwang H Lee

3, Sung Hoon Cha

3 and Seongil Im

3;

1Flexible

Display Research Center, Korea Electronics Technology Institute, Seongnam, Korea, South; 2Department of Chemistry, Hanyang University, Seoul, Korea, South;

3Institute of Physics and

Applied Physics, Yonsei University, Seoul, Korea, South.

“Transparent” electronics has been one of the key terminologies forecasting the ubiquitous

technology era. Several researchers have thus extensively developed transparent oxide-based

thin-film transistors (TFTs) on glass and plastic substrates although in general high voltage

operating devices have been mainly studied considering transparent display drivers. However,

low voltage operating oxide TFTs with transparent electrodes are very necessary if we are

aiming at logic circuit applications, for which transparent complementary or one-type channel

inverters are required. The most effective and low power consuming inverter should be a form of

complementary p-channel and n-channel transistors but real application of those complementary

TFT inverters also requires electrical- and even photo-stabilities. Since p-type oxide TFTs have

not been developed yet, we previously adopted organic pentacene TFTs for the p-channel while

ZnO TFTs were chosen for n-channel on sputter-deposited AlOx film. As a result, decent

inverting behavior was achieved but some electrical gate instability was unavoidable at the

ZnO/AlOx channel interface. Here, considering such gate instability issues we have designed a

unique transparent complementary TFT (CTFTs) inverter structure with top n-ZnO channel and

bottom p-pentacene channel based on 12 nm-thin nano-oxide/self assembled monolayer

laminated dielectric, which has a large dielectric strength comparable to that of thin film

amorphous Al2O3. Our transparent CTFT inverter well operate under 3 V, demonstrating a

maximum voltage gain of ~20, good electrical and even photoelectric stabilities. The device

transmittance was over 60 % and this type of transparent inverter has never been reported, to the

best of our limited knowledge.

B5.86

Pentacene- and Anthradithiophene-Containing Conjugated Polymers for Organic

Photovoltaics. Ying Jiang1, Toshihiro Okamoto

2,1, Hector A Becerril

1, Sanghyun Hong

1 and

Zhenan Bao1;

1Chemical Engineering, Stanford University, Stanford, California;

2Functional Soft

Matter Eng. Laboratory, The Institute of Physical and Chemical Research (RIKEN), Saitama,

Japan.

Material innovation is a key component of the research towards enhancing organic solar cells

performance. We have taken the new approach of incorporating acenes such as pentacene and

anthradithiophene into conjugated copolymers, based on their superior thin-film transistor

performance. In this work, we extend our research to report two classes of novel acene-

containing polymers. The first class consists of the regioregular 2,9- and 2,10-pentacene-

diethynylbenzene copolymers. The two polymers are observed. to differ in absorption properties

owing to foreseeably different conformations. The second class of materials consisting

anthradithiophene-cyclopentadithiophene copolymers shows excellent film absorption properties

as well as solubility in common organic solvents. High field effect mobilities of up to 10-3

cm2/Vs are measured for anthradithiophene-4,4-bis(2-ethylhexyl)cyclopentadithiophene

deposited in top-contact geometry on a silane treated silicon oxide surface. The material also

achieves a power conversion efficiency of 0.59% in preliminary solar cell devices.

B5.87

Synthesis and Optical properties of Perylene Bisimide Incorporated Low Bandgap

Polymers for Photovoltaics. Sivamurugan Vajiravelu and Valiyaveettil Suresh; Department of

Chemistry, National University of Singapore, Singapore, Singapore.

Synthesis and development of a broad range light absorbing molecules with high extinction

coefficient as suitable metarials to improve the solar cell efficiency is exciting due to the great

demand in energy for our day to day life. Perylene diimide (PDI) derivatives are most attractive

molecule owing to high charge mobility, greater electron affinity and act as good n-type

materials. In this investigation, we focused on the synthesis of alternative donor-acceptor

conjugated systems prepared using Suzuki polymerization of N,N‟-didodecyl 1,7-

dibromoperylene diimide (PDI) with diboronic acids of flurene and dithiophene respectively.

The polymers were designed to achieve molecular heterojunction through charge transfer from

donor to acceptor. The polymers were characterised using GPC, 1H, 13C NMR and elemental

analysis. TGA and DSC techniques were used to identify thermal stability and phase changes of

the polymers. The absorption spectra of polymers covered whole range of visible absorption

region from 300 to 800 nm in solution and the absorption maximum shifts to higher wavelength

in solid state.

B5.88 Multifunctional Organic Field-effect Transistor Based on Polydiacetylene. Jaehui Ahn, Doo

Ho Yang, Chunzhi Cui, Jihyun Kim and Dong June Ahn; Chemical & Biological Eng., Korea

Univ., Seoul, Korea, South.

Recently, the interest of low cost and reliable organic materials for multifunctional device is

increased. Organic field effect transistors (OFETs) are suitable for multifunctional devices

because of their potential applications. In our research, PCDA based FET was used. Among a lot

of conjugated polymer, the hole mobility of single crystal polydiacetylene is founded to be 1-10

cm2/V*s by time of flight method[1], it is good property as candidate of OFETs material.

Another unique property of PDA is electrical property to be varied as change of PDA‟s phase (so

called blue and red phase), which can be controllable by UV and external stimuli. We

demonstrated that the single device performed as both UV and gas sensor which shows that

PCDA based FET has potential application in multifunctional device. In this research, we

investigated electrical property of both red phase and blue phase PDA. First, we made thin film

of 10,12-pentacosadiynoic acid (PCDA) by the Langmuir-Blodgett (LB) deposition on SiO2/Si

substrate that has predefined source and drain metal. And then, we performed polymerization to

obtain PDA thin film, followed by thermal treatment to obtain red phase PCDA. Then, we

measured specific contact resistance by the transmission line modeling (TLM) method and the

characterization of Field Effect Transistor. We found that the red phase of PCDA can be

obtained by specific species by the surface modification to increase the selectivity. In our

experiment, the surface of PCDA was modified to have selectivity for NH3 over N2. The details

about the fabrication process and sensing mechanism will be presented.

B5.89

Abstract Withdrawn

B5.90

Doping of Perylene Diimide Derivatives n-type Semiconductor Layer and Interface

Modification for Organic Thin Film Transistor. Heng-Wen Ting and Tri-Rung Yew;

Department of Materials Science and Engineering, Nationl Tsing Hua University, Hsinchu,

Taiwan.

Doping of air-stable solution-processed tetrachloroperylene tetracarboxyldiimide based n-type

semiconductor (TC-PDI-F) layer by dipolar molecules and ionic compounds is demonstrated to

enhance the electrical properties of organic thin film transistors (OTFT). Besides, the interface

modifications between semiconductor and insulator layers by dipolar molecules with phenyl,

alcohol and fluoro-functional groups were applied to improve the molecules packing order and

the electrical performance of doped TC-PDI-F OTFTs. The electronic structures were

characterized by cyclic voltammetry (CV), UV-visible optical absorption spectroscopy (UV-Vis)

and photoluminescence spectroscopy (PL). The surface properties were also inspected by atomic

force microscope (AFM), contact angle system and X-ray diffraction spectroscopy (XRD). The

electrical properties of OTFTs were also measured. All processes were fabricated and measured

in air.

B5.91 Self-Assembled Hydrophobin Protein Membranes on Silicon Platforms Jouni Ahopelto

1,

Markku Kainlauri1, Jani Kivioja

1, Paivi Laaksonen

2, Arja Paananen

2 and Markus Linder

2;

1Micro

and Nanoelectronics, VTT, Espoo, Finland; 2Nanobiomaterials, VTT, Espoo, Finland.

We report on formation and characterisation of ordered single layer protein crystals formed by

directed self-assembly on hydrophobic substrates, such as graphite and silicon platforms. The

hydrophobin proteins form an ordered two-dimensional crystal at air-water interface with the

protein molecules all having a well defined orientation and position [1]. From the air-water

interface the crystal membranes are transferred onto surface of highly oriented pyrolytic graphite

or on patterned silicon substrates. The thickness of the membrane is about 3 nm and it has a

hexagonal-like lattice with lattice constant of about 6 nm. On silicon substrates selectivity can be

obtained between hydrophilic oxide and hydrophobic silicon areas, providing means to integrate

protein membranes with silicon microelectronics [2]. By decorating the proteins with Au

nanoparticles, nanoelectrodes for electrical measurements and, on the other hand, plasmonic

devices can be envisaged. The effect of nanoelectrodes, as measured by conducting AFM, can be

seen as largely enhanced conductivity through the protein membrane. Also, on glass slides made

hydrophobic by silanization, clear plasmon band absorption arising from the Au nanoparticles

can be seen. [1] G. Szilvay, A. Paananen, K. Laurikainen, E. Vuorimaa, H. Lemmetyinen, J.

Peltonen, M. Linder, Self-assembled hydrophobin protein films at the air-water interface:

structural analysis and molecular engineering, Biochemistry . Vol. 46 (2007) 2345 - 2354. [2] P.

Laaksonen, J. Kivioja, A. Paananen, M. Kainlauri, K. Kontturi, J. Ahopelto, M. B. Linder,

Selective nanopatterning using citrate stabilized Au nanoparticles and NCysHFBI fusion protein,

submitted 2008.

B5.92 A Self-patterned Polymer Dielectric for Low Voltage Pentacene Thin Film Transistor. Hui-

Chen Huang, Ting-Hsiang Huang and Zingway Pei; Graduate Institute of Optoelectronic

Engineering, Department of Electrical Engineering, National Chung Hsing University, Taichung,

Taiwan.

Organic thin-film transistors (OTFTs) based on pentacene has attract much attention due to low

temperature and low cost fabrication process. Although mobility of pentacene is comparable to

the a-Si:H having potential to replace a-Si:H TFT in display applications, the high driving

voltage is still a problem. Self-assembled monolayer (SAM) or ultra thin film are general

methods used to reduce the operation voltage to about 2~ 5 volts. However, SAM methods

require unique coating technique and the thickness of the ultra thin film is hard to control during

the coating process limit theses method only demonstrated by some very specific group. In this

work, a random copolymer, PS-r-PMMA, is used as dielectric material for a pentacene OTFT.

The PS-r-PMMA dielectric with 10 nm thick can be uniform coated on the surface with hydroxyl

groups in a very simple spin coating process. The PS-r-PMMA is initially coated in roughly 50

nm thick. After thermal annealing, a layer of PS-r-PMMA will connect to the OH- group

forming a 10nm thick layer no matter how thick of the initial layer. Besides the thickness control,

the PS-r-PMMA only coated on the surface having OH- groups, the self-patterning is possible to

prevent the chemical etching process during the organic circuit fabrication. The designed OTFT

is bottom gate and top contact structure. The substrate is glass and gate material is aluminum.

After the deposition of the Al, the UV/ozone process is performed to convert the surface of Al

into Al2O3 having OH- groups. After this, PS-r-PMMA is spin coated and heated sequentially.

Finally, pentacene and gold contact electrodes are deposited sequentially by thermal evaporation

to finish the OTFT. By using this self-patterning thin dielectric, the pentacene OTFT exhibits

operation voltage as low as 5V and an on/off ratio large than 105.

B5.93

Abstract Withdrawn

B5.94

Controlling the Electrical Properties of Molecule-Terminated, Non-Oxidized Silicone by

the C-C Bond Nearby the Surface Sreenivasa R Puniredd1, Ilia Platzman

1 and Hossam

Haick1,2

; 1Chemical Engineering, Technion- Israel Institute of Technology, Haifa, Israel;

2Russell Berrie Nanotechnology Institute, Technion- Israel Institute of Technology, Haifa, Israel.

The ability to exert systematic control over the electronic properties of Si is an important factor

for the realization of nanoelectronic devices. Mostly, such controllability can be achieved by

placing molecules, whose dipole can be changed systematically, at the device surface and/or

interface. In this study, we present an approach for controlling the electrical properties of Si by

inducing deliberate interaction between the energy levels of organic molecules and Si, without

the need for a dipole modification. We illustrate this approach by functionalizing 50±2% of Si

atop sites of n- and p-type Si (111) oxide-free surfaces with various organic molecules having

similar (3C) backbone but different in their C-C bond close to the Si surface (i.e., C-C vs. C=C

vs. C≡C bonds). These molecules have nearly similar dipole moment (0.9-1.6 Debye). Electrical

characterizations of molecule/Si surfaces and Hg/molecule/Si junctions showed a systematic

control over the work function and Schottky barrier of the Si, respectively, in the range from 0.2

to 0.8 eV, as compared to H-terminated Si. Our results show that the control over the electrical

properties of n- and p-type Si is dominated by systematic electron transfer from the Si surface to

the organic monolayers. The results indicate that the extent of electron transfer depends on the

difference between the energy levels of the organic molecules and the Si surface. This finding,

for which we will present a detailed explanation, has very significant implications as it suggests

the ability to control the electrical properties of semiconductors with minimal depolarization

effects.

B5.95 Ultrathin and Printable Conjugated Films for Organic Electronics. Li Tan, Engineering

Mechanics and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln,

Lincoln, Nebraska.

Ultrathin organic films or self-assembled monolayers (SAMs) can provide molecular or organic

electronics unmatched processing convenience and some knowledge in device operation.

Conventional concepts often come from a head-tail molecular design, where the head carries the

function of charge transport and the tail is capable of self-assembly. Amorphous nature of the

monolayers and rather significant defects along grain boundaries, however, all inhibit their

device performances at high current density and high temperature conditions. Toward this end,

we are investigating a new family of ultrathin organic thin films, i.e., self-organized nanolayers.

This type of molecules enjoys a polymeric backbone, branched with an extended pi-pi system.

Rather rigid branches, plus the chain-chain cooperation allow the polymer form long-range

ordered lamellae in solution. Printed films have a super dense, multistack morphology over tens

of microns. Unprecedented thermal stability is also realized due to an enhanced molecular

interaction between the pi-pi units, plus a strong interlayer binding between the layers. Research

efforts with scientific and engineering importance are worth of reporting, including 1) strategy of

the molecular design and synthesis; 2) film casting and characterizations; and 3) molecular

modeling to interpret the structuring mechanism of the nanolayers. Merits of well-ordered

packing, plus easiness to chemically functionalize these nanolayers, all promise great

applications in molecular or organic electronics.

B5.96

Probing Nanoscale Electric Field Fluctuations: Towards a Local Measurement of Carrier

Mobility in Organic Semiconductors. Showkat Yazdanian1, Seppe Kuehn

2, Nikolas Hoepker

1,

Roger Loring1 and John Marohn

1;

1Cornell University, Ithaca, New York;

2Rockefeller

University, New York, New York.

A rich array of dynamic physical processes can be modelled as fluctuating electric fields. These

processes include electron transfer, enzyme kinetics, ionic motion in solution and the glass

transition. We have shown that cantilever friction can be used to study dielectric fluctuations in

the vicinity of the cantilever resonance frequency. Here we show that cantilever frequency

fluctuations (“jitter”) can be used to probe low frequency dielectric fluctuations over a much

broader frequency window. We present measurements of dielectric fluctuations in polymers in

tandem with a theory predicting the shape and magnitude of the effect. We also present our

progress towards employing this technique to measure the local carrier mobility in organic

semiconductor systems.

B5.97

Porphyrins and N-confused Porphyrins as Spectral Dopants in Organic Solar Cells. Warwick J Belcher, Nathan Cooling and Paul Christopher Dastoor; Physics, University of

Newcastle, Callaghan, New South Wales, Australia.

Organic photovoltaic cells show immense promise as a new alternative for renewable energy.

However, one major problem with these devices is that the polymers typically employed in their

design absorb light in only a limited part of the solar spectrum (usually <600nm). One way to

further extend the spectral response of these devices is via the addition of complimentary

chromophores. It has been shown that the absorption of light by the Q-bands of porphyrins

incorporated into MEH-PPV/PCBM blends contributes up to 20% of the total photocurrent

generated by the device by utilising light that would not normally be absorbed [1]. Furthermore,

N-confused porphyrins could be used to extend the absorption spectra of these devices even

further, since they have Q-band absorptions extending well beyond the range of a standard

porphyrin. Despite this promise porphyrin aggregation within these ternary devices has been

observed to lead to disruption of the crucial morphology of the active layer and lowered device

efficiency. Porphyrin aggregation can be controlled by controlling the steric bulk of peripheral

substituents on the porphyrin. A series of substituted tetraphenylporphyrins and N-confused

tertraphenylporphyrins have been prepared in which the steric bulk of the peripheral substituents,

and thus the degree to which aggregation occurs, was varied. These materials have been used to

prepare a series of MEH-PPV/Porphyrin/PCBM and PPV/N-confused Porphyrin/PCBM ternary

organic photovoltaic devices. Furthermore, we have observed that the porphyrin molecules act as

“hole traps” within the devices due to the basicity of the pyrollic nitrogens, lowering device

efficiency. Reduction of this basicity has been achieved by metallation and alkylation of these

sites. The effect that these structural changes have on device performance will be presented.

References [1] P.C. Dastoor, C. R. McNeill. H. Frohne, C, J, Foster, B. Dean, C.J. Fell, W. J.

Belcher, W. M. Campbell, D.L. Officer, I. M. Blake, P. Thordarson, M.J. Crossley, N.S. Hush,

R. Jeffrey, J. Phys. Chem. 111(42) (2007), 15415-15426.

B5.98 Near Infrared Fluorescent and Phosphorescent Organic Light-Emitting Devices. Yixing

Yang1, Richard T Farley

2, Timothy T Steckler

2, Jonathan Sommer

2, Sang-Hyun Eom

1, John R

Reynolds2, Kirk S Schanze

2 and Jiangeng Xue

1;

1Department of Materials Science and

Engineering, University of Florida, Gainesville, Florida; 2Department of Chemistry, Center for

Macromolecular Science and Engineering, University of Florida, Gainesville, Florida.

There has been a growing interest in the development of near-infrared (NIR) organic light-

emitting devices (OLEDs) due to their potential applications in defense, biomedical sectors, and

telecommunications. For example, NIR OLEDs can be used as illumination and signaling

sources for night vision and friend-foe identification, and have advantages in light weight, low

thermal signature, low power consumption, and compatibility with large area and flexible

substrates. Existing NIR OLEDs have been mostly based on lanthanide-based organometallic

complexes, which generally have low external quantum efficiencies (EQE) (<0.1%). Only

recently were high efficiency NIR OLEDs reported, in which peak emission at λ≈770 nm and a

maximum EQE up to 8.5% were achieved using a phosphorescent Pt-porphyrin complex.1

Nonetheless, alternative materials and devices that exhibit longer emission wavelengths (λ>800

nm) with high efficiencies are still needed. Here we report fluorescent and phosphorescent NIR

OLEDs with peak emission wavelengths up to 892 nm and EQE up to 3.8%. First, OLEDs based

on two NIR-emitting fluorescent donor-acceptor-donor (DAD) oligomers, BEDOT-TQMe2 and

BEDOT-BBT, are demonstrated. In these molecules, the energies of the highest occupied and

lowest unoccupied molecular orbitals are controlled by the donor and the acceptor portion,

respectively. A maximum EQE of ηEQE = 1.6% and a maximum power efficiency of ηP = 7.0

mW/W are achieved in devices based on BEDOT-TQMe2, with the electroluminescence (EL)

peaked at 692 nm but extending to well above 800 nm. BEDOT-BBT based OLEDs show red-

shifted emission peaking at 815 nm (and extending to as far as 950 nm), although the maximum

efficiencies were reduced to ηEQE = 0.51% and ηP = 2.1 mW/W due to the significantly lower

fluorescent quantum yield of the NIR emitter. The efficiencies of these fluorescent OLEDs were

further increased by two to three times by incorporating a phosphorescent sensitizer in the

emissive layer to funnel the triplet excitons formed on the host molecules to the fluorescent

emitters, which are not utilized in fluorescent devices. Using this sensitized fluorescence

structure, we achieved maximum efficiencies of ηEQE = 3.1% and ηP = 12 mW/W for BEDOT-

TQMe2 based devices, and ηEQE = 1.5% and ηP = 4.0 mW/W for BEDOT-BBT based devices.

Finally, a phosphorescent NIR emitter, platinum tetraphenyltetranaphtho[2,3]porphyrin (Pt-

TPTNP), was synthesized. The more extended conjugation on this molecule compared to the Pt-

porphyrin complex used in Ref. 1 leads to a red shift in the emissive wavelength by more than

100 nm. In the Pt-TPTNP-based OLEDs, we obtain peak emission at 892 nm, and maximum

efficiencies of ηEQE = 3.8% and ηP = 19 mW/W, much higher than those of the sensitized

fluorescence OLEDs based on DAD oligomers. 1 Y. Sun et al., Appl. Phys. Lett. 90, 213503

(2007)

B5.99

Non-Volatile Molecular Monolayer Memory. Sangkwan Kim1,2

, Hojong Chang1,2

, Junghyun

Lee1, Gyeong Sook Bang

1 and Hyoyoung Lee

1;

1Center for Smart Molecular Memory,

Electronics and Telecommunications Research Institute(ETRI), Deajeon, Korea, South; 2Next

Generation Device Engineering, University of Science and Technology, Deajeon, Korea, South.

Molecular monolayer memory is considered to be one of the best solutions to the scaling limit

problem in the silicon industry. Organic self-assembled monolayers (SAMs) as active memory

elements in electronic devices are capable of highly integrated density in metal-molecule-metal

(MMM) types of devices. Newly designed dialkylthiolate-tethered metal complex SAMs

(RuII(tpy(CH2)nS)2) are introduced. These SAMs using a conducting polymer, PEDOT:PSS, as

a soft part of top electrode provide a stable and reproducible molecular monolayer memory

device that shows hysteretic I-V characteristics, write-multiple read-erase-multiple read pulse

cycles and good retention time. As alkyl chain lengths of metal complexes increase from 7 to 13,

molecular monolayer device yield improves up to 81% at a micro-scale well and simultaneously

its retention time increases from 200 to 390 seconds. The non-volatile molecular monolayer

memory is non-destructive for more than 300 write-multiple read-erase-multiple read cycles. A

conduction mechanism is direct tunneling with little temperature dependence

B5.100

Light Absorption and Charge Generation in Polymer-Fullerene Organic Photovoltaic

Devices. Paul Christopher Dastoor, Centre for Organic Electronics, University of Newcastle,

Callaghan, New South Wales, Australia.

Solar cells based on semiconducting polymer blends offer great potential for the development of

low-cost printable photovoltaic arrays. Organic photovoltaic (OPV) devices fabricated from

polymer-fullerene blends are of particular interest since they provide the greatest power

conversion efficiency. The conventional view of device operation is that the light photons are

absorbed primarily by the polymer component resulting in the generation of excitons that in turn

are separated at the many polymer-fullerene interfaces that are formed during the spin-coating of

these blended materials. The main role of the fullerene component is to provide the percolation

network for charge conduction [1-3]. In this paper we show that for some polymer-fullerene

blends this traditional paradigm of device operation needs to be reconsidered. We demonstrate

that ternary blends incorporating porphyrins can be fabricated with less than 10% polymer

composition and yet still exhibit full device functionality. By analysing the proportion of light

absorbed by each individual component it can be shown that the majority of the light is absorbed

by the fullerene component with over 50% of the photocurrent produced under AM 1.5

conditions occurring subsequent to C60-fullerene absorption [4]. New multi-wavelength near-

field scanning photocurrent microscopy studies show distinct differences in the distribution of

light absorbed by these devices at different wavelengths. These results provide for a consistent

understanding of the origin of primary charge separation in general polymer/C60-fullerene

blends.

B5.101 Nano-structured ITO Electrode Applied in Polymer Solar Cell Ming-Shin Su

1, Kung-Hwa

Wei1, Chia-Hua Chang

2 and Prichen Yu

2;

1Department of Material Science and Engineering,

National Chiao Tung University, Hsinchu, Taiwan; 2Institute of Electro-Optical Engineering,

National Chiao Tung University, Hsinchu, Taiwan.

Organic photovoltaic has become an emerging research subject recently because of their easy

and low-cost fabrication process as compared to their counter parts such as silicon or compound

semiconductor solar cells. In particular, bulk hetero-junction (BHJ) polymer solar cells that

typically consist of an active layer of about 100nm-thick film blends constituted by P-type

conjugated polymers and N-type fullerene derivatives sandwiched between two electrodes

appear to attract large attentions since they can provide large surface area with limited amount of

effort. Since the power conversion efficiency (PCE) of BHJ polymer solar cells still remains an

area where improvements are greatly needed. Many researchers have tried to optimize the

interface structures of these polymer solar cells by modifying the work function of the electrodes

or adopting other transparent metal oxides as electrode, for enhancing the PCE. Nanotechnology

has provided many researchers a new way to achieving this goal by applying nano-structure to

enhance the performance of these cells. In the present study, we would like to use a nano-

structured ITO electrode to enhance internal reflection of the incident light as well as to facilitate

the charge transport in the devices In this paper, we report the performance of polymer solar cell

devices that used the ITO electrodes with nanorods structures produced by an oblique-

evaporation deposition method. These ITO nanorods have a uniform shape with diameter

ranging from 120nm to 160nm and the inter-rod spacing between them is about 150nm to

300nm. The structures of the polymer solar cell device are composed of ITO glass substrate with

or without ITO nanorods, PEDOT:PSS, P3HT/PC61BM, and top Al electrode. The PCE of the

devices with ITO nanorods on the ITO electrode is about 3.3% under AM1.5G (100mW/cm2)

standard testing condition, as compared to 3% for the devices without ITO nanorods.

Additionally, we measured these two types of devices under 5 suns (500mW/cm2) incident light

intensity condition. The PCE of the devices with ITO nanorods reached about 4% as compared to

3.3% for the ones without ITO nanorods under 5 suns condition. Then, we make lifetime

measurement under 5 suns condition and all the tested devices are maintained at the short circuit

state under illumination. It takes almost 110 minutes to decay to 80% of their original PCE value

for the devices with ITO nanorods, whereas the ones without ITO nanorods takes 55 minutes to

decay to the same extent. The ITO glass substrate with ITO nanorod is a proper choice to

enhance polymer solar cell PCE. Polymer solar cell containing substrate with ITO nanorod has

better lifetime and the electrode with nano structure can sustain such high photo current

generation or high incident light intensity with lower degradation rate. The two benefits imply

that the polymer solar cell with ITO nanorod electrode has the potential to be developed into a

concentrator type for polymer solar cells.

B5.102

Electrochemical Characterization of Tyrosinase Nanoparticles Protected by Organic-

inorganic Network in Nano-biosensor. Woo Jin Lee, Dong-Hwa Yun, Jun-Hyoung Chang,

Keum-Ju Lee and Suk-In Hong; Biochemical engineering, Korea University, Seoul, Korea,

South.

Semiconductor techologies have become essential in various field of biosensors and bio-

microelectromechaniical systems. They provide miniaturization and mass production of silicon-

based sensors. However, the smaller the area of sensing electrode, the more and more problems,

i.g., depreciation of the whole performances including low sensitivity and stability,

embarrassment of enzyme or any kinds of bio-molecular immobilization occur. Tyrosinase also

known as polyphenol oxidase(PPO), is a copper containing enzyme. Nowdays mushroom

tyrosinase has become important because it is readily available and useful in a number of

applications which environment, food industry, and medical science. Therefore, many research

groups have developed tyrosinase enzzyme. Tyrosinase has a pair of copper ions with six

conserved histidine residues in the active site. But Tyrosinase stability has a connected with of

oxidation of cooper ions in the air. To overcome this problem, we approach to improve the

enzyme stability in various nanostructures such as nanoparticles, mesoporous materials and

single enzyme nanoparticles. In this works, tyrosinase nanoparticles surrounded by a polymeric

organic-inorganic network resulting in stabilization of enzyme activity. They were observed by

transmission electron microscope (TEM) and analyzed electrochemical properties using cycle

voltametry(CV) by VMP40 Multi-potentiostat.

B5.103

The Newly Designed p-type Polymer Containing alkyl-substituted Thiophene for Organic

Thin Film Transistors (OTFTs) Pengtao Kang1, JongWon Park

1, SungJin Park

1, JinUk Ju

1,

Yunhi Kim2, DaeSung Jung

3, ChanEon Park

3 and SoonKi Kwon

1;

1School of Nano & Advanced

Materials Science and Engineering and ERI, Gyeongsang National University, Jinju, Korea,

South; 2Department of Chmistry, Gyeongsang National University, Jinju, Korea, South;

3Department of Chmical Engineering, Pohang University of Science and Technology, Pohang,

Korea, South.

A new p-type polymer was synthesized by the oxidation coupling reaction. The monomers were

prepared with highly overall yields and the obtained polymer is confirmed by elemental analysis,

1H NMR, 13C NMR, FT-IR, UV absorption, photoluminescence (PL), and cyclic voltammetry

(CV). The polymer is having alkyl-substituted thiophene and we are expected to have a high

solubility by introducing the long alkyl-chain. Furthermore, the introduction of the alkyl chain

can increase the degree of orderness by self-assembly. The thermal property of polymer was

confirmed by using the thermogravimetric analysis (TGA) and differential scanning calorimetry

(DSC). The polymer was showed 5% weight losing temperature at 438 oC and the curve of alkyl

chain was showed around 80 oC. The number-, weight-average molecular weight of the

polymers were determined to be 10,820 and 15,663 gel permeation chromatography (GPC) using

polystyrene standards for calibration in the eluent THF. The solubility of the polymer was

showed a poor solubility in common solvents, such as chloroform, tetrahydrofuran (THF),

toluene. On the other hand, the polymer was showed a good solubility in chlorobenzene at room

temperature with 0.7 wt%.

B5.104 Formation of Nanopatterned Polymer Blends in Photovoltaic Devices. Ximin He

1,2, David

Hasko3, Ullrich Steiner

3, Richard Friend

3, Sven Hunter

3 and Wilhelm Huck

1,2;

1Chemistry,

University of Cambridge, Cambridge, Canbridgeshire, United Kingdom; 2The Nanoscience

Centre, University of Cambridge, Cambridge, United Kingdom; 3Physics, University of

Cambridge, Cambridge, United Kingdom.

The ideal structure for polymer photovoltaic (PV) devices has been believed so far as an

interpenetrating nanoscale columnar architecture that maximizes the interface between electron

and hole transporting polymers, and thereby greatly improves exciton dissociation. Despite

attempts at achieving such structures via block copolymers, inorganic templates, columnar liquid

crystalline phases, no such structures have been achieved in efficient devices at the level of

practical applications. Instead, the best polymer photovoltaic devices, based on P3HT:PCBM,

rely on a complicated procedure of spincoating from the most suitable solvents and thermal or

solvent annealing steps to improve the nanoscale morphology and thereby the blend

performance. Here, we have developed a double nanoimprinting (NIL) process that allows the

formation of nanostructured polymer blends of any composition and morphology and

demonstrated how it approached the ideal polymer film architecture for PV devices. It has been

conceived that we could use a pre-patterned polymer film to imprint another polymer film

through NIL, with feature size down to 20 nm. The new and simple approach, which only

employ solution spin coating of polymer film and NIL, was basically established by successfully

obtaining very uniform and precise patterns, in quite complete large area, inside of a variety of

polymer blends, ranging from conventional and semiconductor polymers to organic-polymer

blend, as a photoactive layer in PV cell, including PS/PEO, PMMA/PEO, PVP/PS,

F8BT/PMMA and P3HT/PCBM. Evidenced by morphology studies through AFM and SEM, this

approach is able to create integrated polymer layer with controllably varied and precisely defined

interdigitated interface nano-geometric structures inside, which is the ideal structure of high-

efficiency PV cells, so called well-ordered bulk heterojunctions, benefiting high intersurface area

and straight carrier transport pathway. Conjugated polymer-based PV cells with both 1D and 2D

nanostructured donor-acceptor interface of well-ordered densely packed 20~200-nm-wide lines

and 25~200-nm-wide dots were fabricated by employing this double-NIL approach. Cell devices

performance enhancement was found in the nanostructured one over the conventional flat-

interface double-layer one, which was believed to result from well-ordered vertically oriented

heterojunction with precisely defined interdigitated nanostructured interface of electron donor

and acceptor components, which facilitated charge transport as well as charge separation,

ordering of conjugated polymer. Systematic study showed PV performance improved as domain

size decreased and interface area increased, especially pronouncedly with domain sizes closer to

carrier diffusion length, convincing the interdigitated structure as ideal polymer PV

configuration. This technique of producing nanostructured polymer or organic blends are

expected to be applied into more other device fabrications in the future.

B5.105

Numerical Modeling for Optical Property and Charge Transport in Bulk Heterojunction

Organic Solar Cells. Young Min Nam1, June Huh

2 and Won Ho Jo

1;

1Materials Science and

Engineering, Seoul National University, Seoul, Korea, South; 2Active Polymer Center for Pattern

Integration, Yonsei University, Seoul, Korea, South.

Organic photovoltaic cells based on bulk heterojunction concept has been a promising alternative

for conventional silicon-based solar cells, mainly due to their potential for low cost, ease of

fabrication, and mechanical flexibility. However, the power conversion efficiencies of such

devices remain too low for commercialization, and thus a number of researchers have devoted to

improve the efficiency. Despite growing research efforts, the fundamental processes governing

the performance of organic photovoltaic cells are still poorly understood and therefore a

theoretical model, which deals with a whole process of optical-to-electric energy conversion, is

highly demanded. Such a model should take into account not only dynamic characteristics of

exciton and electron/hole transfer in the active layer but also optical interference between layers

of organic solar cell so as to cover the overall conversion process in the multilayered structure of

solar cell device. In this study, a device model accounting for the effect of morphology of active

layer as well as the effect of layer configuration of multilayer-structured device is established to

predict the current-voltage characteristics of polymer/fullerene bulk heterojunction solar cells.

The model morphologies of active layers were generated by using Cahn-Hilliard-Cook diffusion

dynamics which models the dynamics of phase separation between polymers and fullerenes,

varying the thickness of active layer, characteristic dimension of phase-separated morphology,

and long range order of phase-separated domains. The steady-state behavior of electron, hole and

exciton concentrations in phase-separated morphology is captured by the drift-diffusion model

for the generated morphologies of active layer, while the optical transfer matrix theory is used

for optical carrier generation to consider the effects of active layer thickness and layer

configuration on the light absorption efficiency in the active layer which ultimately affects the

power conversion efficiency of solar cells. The model was validated by comparing the simulation

results with the experimental data, such as J-V characteristics of solar cell and the thickness of

each layer. Simulation results show that the power conversion efficiencies of solar cells with

ideal active layer morphology are much higher than those of solar cells with randomly oriented

active layer morphology. Furthermore, the performance of solar cells with randomly oriented

active layer morphology changes dramatically with variation of the characteristic dimension of

phase-separated morphology, while the performance of solar cells with oriented active layer

morphology is less dependent to the characteristic dimension of phase-separated morphology. It

is also found that the optimum thickness of active layer with oriented morphology is much larger

than the optimum thickness of active layer with randomly oriented morphology.

B5.106

Abstract Withdrawn

B5.107

Effect of the Surface Characteristics of TCO Thin Films on the Performance of OLED

Devices.Yu Lim Lee and Kyu-Mann Lee; Materials Engineering, Korea University of

Technology and Education, Cheonan-city, Chungnam, Korea, South.

OLED device is one of the most attractive and alternative display components, which stems

primarily from the self-emission, large intrinsic viewing angle, and fast switching speed.

However, because of its relatively short history of development, much remains to be studied in

terms of its basic device physics, manufacturing processes, and reliability etc. Especially among

several issues, it should be noted that the device characteristics are very sensitive to the surface

properties of transparent conducting oxide (TCO) electrode materials. Sn-doped In2O3 (ITO)

thin films have been extensively studied for OLED devices because of transparency, high electric

conductivity, and large work function. However, indium has some problems such as rare raw

material, high cost, low stability in plasma, and toxicity. On the other hand, Al-doped ZnO

(AZO) thin films have a lot of advantages, such as low cost, good stability in plasma, non-

toxicity, optical transparency, and good electrical conductivity. As a result, AZO thin films are

actively studied as an alternative candidate for ITO substrates. In this study, we have investigated

the performance of OLED devices as a function of sheet resistance and surface roughness of

TCO thin films. For this purpose, ITO and AZO thin films were deposited by r. f. magnetron

sputtering under various ambient gases (Ar, Ar+O2 and Ar+H2, respectively). The micro-

structural observation and crystal orientation of the TCO thin films were evaluated using X-ray

diffraction and FESEM (field emission scanning electron microscope), respectively. The optical

transmittance and the film thickness were measured using ultraviolet spectrophotometer (Varian,

cary-500) and surface profile measurement system, respectively. The electrical properties such as

sheet resistance, charge carrier concentration, and mobility of TCO films were measured using

four point probe system and hall effect measurement (HMS-3000), respectively. In order to

control the surface roughness of TCO films, we have proceeded with photo-lithography and

reactive ion etching processes. The micro-size patterned mask was used and the etching depth

was regulated by changing etching time. The surface morphology of the TCO films was

observed by FESEM and atomic force microscopy (AFM). And then, organic materials and

cathode electrode were sequentially deposited on the TCO thin films. Device structure was

TCO/α-NPD/DPVB/Alq3/LiF/Al. The DPVB was used as a blue emitting material. The

electrical characteristics such as current density vs. voltage and luminescence vs. voltage of

OLED devices were evaluated by using spectrometer (minolta CS-1000A).

B5.108

Substrate Effect on Energy Level Alignment at the CuPc-C60 Interface in Organic Solar

Cells.Zengtao Liu, City University of Hong Kong, Hong Kong, China.

The interface energy level alignment between copper phthalocyanine (CuPC) and fullerene

(C60) on indium-tin oxide (ITO) and Mg substrate was investigated. The CuPC/C60 interface

deposited on ITO shows a nearly common vacuum level, but a dipole and band-bending exist,

resulting in a 0.8 eV band offset at the same interface on Mg. Significantly, this observation

indicates that the energy difference between the highest occupied molecular orbital of CuPC and

the lowest unoccupied molecular orbital of C60, which dictates the open circuit voltage of the

CuPC/C60 OPV, can be tuned by the work function of the substrate. Furthermore, the substrate

effect on the energy alignment at the donor/acceptor interface can satisfactorily explain that a

device with an anode of a smaller work function would provide a higher open circuit voltage.

B5.109

New Multi-branched π-conjugated Molecules bearing Benzothiadiazole-based Peripheral

Moieties and Their Electrical Properties Ki Won Lee1, Dae Cheol Kim

1, Tae Wan Lee

1,

Kyung Hwan Kim1, Min Ju Cho

1, Dong Hoon Choi

1 and Jae-Woong Yu

2;

1chemistry, Korea

University, Seoul, Korea, South; 2Korea Institute of Science and Technology, Seoul, Korea,

South.

Photovoltaic cells based on polymer semiconductors are of great interest as a low cost approach

to solar energy conversion. Although numerous polymers have to date been explored as the

donor in polymers / fullerene bulk heterojunction solar cells, not many new functional materials

were demonstrated for the efficient photovoltaic devices. For the development of ideal donor

material, π-conjugated small molecules, dendrimers, oligomers, and polymers were employed

because of their strong potential applications to electronics and optoelectronics. We recently

reported the facile synthesis of new p-type multi-branched semiconducting molecules and their

organic photovoltaic cells applications including the device performance of organic thin film

transistors. For this presentation, new conjugated crystalline multi-branched molecules bearing

benzothiadiazole-based peripheral moieties have been synthesized through various cross-

coupling reactions. A good correlation between theoretical calculations performed on model

compounds and the experimental HOMO, LUMO, and band gap energies of the serial materials

has been obtained. These materials was characterized by UV-visible spectroscopy,

photoluminescence spectroscopy, cyclic voltammetry, thermogravimetric analysis and

differential scanning calorimetry. They also display a p-type semiconducting behavior and their

electrical properties are investigated in detail. We investigated the performance of bulk

heterojunction type photovoltaic device and capability of molecular heterojunction or

supramolecule through self-association in solution or in the bulk.

B5.110

Numerical Study for the Morphological Effect on the Performance of Organic Solar

Cells.Sungjun Kong and Dongchoul Kim; Mechanical Engineering, Sogang University, Seoul,

Korea, South.

The conversion of solar energy into electricity indicates the route to reduce the concern over

global warming and satisfy our global energy needs. Polymer solar cells have potential to be

alternative to silicon-based solar cells which are impractically expensive. To improve the

performance of polymer solar cells, significant efforts have been made to decrease energy loss

from the internal morphology in a bilayer system. Here, we present a three-dimensional model to

study the effect of device morphologies on photovoltaic performance. The diffuse interface

model is employed and incorporates photovoltaic mechanism. The semi-implicit Fourier spectral

method and preconditioned biconjugate-gradient method are applied for high efficiency and

numerical stability. The simulations demonstrate the improved performance of the organic solar

cells with designed morphologies.

B5.111

Conjugated Polymers Based on Pyridazine and Benzothiadiazole for Photovoltaic

Applications.Sangwon Ko1, Rajib Mondal

2, Sanghyun Hong

2, Hector A Becerril

2 and Zhenan

Bao2;

1Chemistry, Stanford University, Stanford, California;

2Chemical Engineering, Stanford

University, Stanford, California.

A new class of conjugated copolymers based on pyridazine and benzothiadiazole was designed

and synthesized for application in polymer solar cells (PSCs). The absorption spectra,

electrochemical, field effect carrier mobility, and photovoltaic properties of these polymers were

investigated. The broad absorptions (300-600 nm), and low band gaps (~ 2.0 eV), and high hole

mobility of the polymers make them promising for solar cell application. The changing

copolymerization units leaded further red-shifted λ(max) broadening of the wavelengths at which

high photoconversion efficiencies can be achieved. The results indicate that the polymers

consisting of further improved the absorption properties are promising polymer photovoltaic

materials.

B5.113 Predicting the Efficiency of Organic Photovoltaics Based on Optical Properties. Katherine

E. Hurst, Nathan A Tomlin and John H Lehman; National Institute of Standards and Technology,

Boulder, Colorado.

Improving the quantum efficiency of organic photovoltaics is a requirement for large-scale

commercial use. It has been established that doping of conjugated-polymer films with single

walled carbon nanotubes (SWCNTs) facilitates exciton dissociation and electron transport.

Accurate measurement of important intrinsic device properties such as absorbance with respect

to film thickness and SWCNT concentration is necessary for informing our expectations for

extrinsic photovoltaic efficiency. This is also the basis for modeling the role of the photoactive

layer and maximizing the efficiency of the device. In this work, we explicitly measured the

absolute absorbance of photoactive films by depositing them on a pyroelectric detector and

modeled the optical function of the doped films based on Kramers-Kronig analysis. Raman

spectroscopy, UV-VIS absorption and four-point probe measurements provide further

characterization of nanotube concentration and homogeneity of our films. Our study provides

direct quantitative measurements traceable to NIST standards, which demonstrates variations in

the intrinsic efficiency of the photoactive layer directly correlate to the extrinsic efficiency of the

SWCNT-polymer photovoltaics.

B5.114

Abstract Withdrawn

B5.115

N-type Semiconducting Organic Materials for High-Performance and Air-Stable Organic

Field-Effect Transistors and Circuits. Yoonyoung Chung1, Joon Hak Oh

2, Boris Murmann

1

and Zhenan Bao2;

1Electrical Engineering, Stanford University, Stanford, California;

2Chemical

Engineering, Stanford University, Stanford, California.

Unlike inorganic electronic devices which have higher electron mobility than hole mobility,

organic electronic devices show superior performance in p-type where holes dominate the

current. Although p-type organic field-effect transistors (OFET) have been widely investigated,

air-stable and high-performance n-type OFETs have been less developed. A major drawback of

n-type materials is the existence of oxygen and water molecule which have high electron affinity.

Considering many advantages of complementary circuit design, there is no doubt about the

necessity of fast and reliable n-type OFETs. We report the fabrication of simple electronic

circuits, such as inverters and ring oscillators, using high performance air-stable n-type OFETs,

based on Perylene Diimides (PTCDI) derivatives and Naphthalene Diimides (NTCDI)

derivatives, together with p-type pentacene OFETs. Their electrical characteristics such as bias-

stress effect and air-stability have also been investigated. The field-effect mobility of the n-type

OFETs was close to 1 cm2/Vs in air.

B5.116

A Crystalline Dielectric Surface-modification Layer for High Performance Organic Thin-

film Transistors. Ajay Virkar1, Stefan Mannsfeld

1, Yutaka Ito

1, Michael Toney

2 and Zhenan

Bao1;

1Stanford University, Stanford, California;

2Stanford Synchrotron Radiation Lab, Menlo

Park, California.

In organic thin film transistors (OTFTs),the vast majority of the current flows within the first few

monolayers of the semiconductor at the dielectric interface. To improve performance it is very

common to modify the SiO2 dielectric with an octadecylsilane (OTS) monolayer. We found that

the phase (order) of the underlying OTS is a critical device parameter which must be controlled

in order to optimize OTFTs. We found a crystalline, dense octadecylsilane (OTS) surface

modification layer promotes two-dimensional growth in a variety of organic semiconductors.

Higher mobility is achieved for OTFTs with a crystalline OTS layer compared to a disordered

one, with mobilities as high as 5.3 cm2/Vs and 2.2 cm2/Vs for C60 and pentacene, respectively.

Moreover, we developed a simple, scalable spin-coating method to produce crystalline OTS.

This work demonstrates a significant step towards a general approach for morphological control

of organic semiconductors which is directly linked to their performance in OTFTs. Finally,

nucleation and thin film growth of semiconductors on the different OTS surfaces will be

discussed with particular attention to the relevant energetics which must be considered to design

ideal dielectric surfaces.

B5.117 Energetics and Stability of Pentacene Thin Films on Methyl Terminated Surfaces Ajay

Virkar, Stefan Manssfeld and Zhenan Bao; Stanford University, Stanford, California.

Increasingly there has been interest in analyzing the nucleation, growth, and morphology of

pentacene thin films on methyl terminated surfaces. We recently discovered that pentacene has a

high nucleation density and a 2D growth mode on dense methyl terminated surfaces compared to

loosely packed methyl surfaces. The desired 2D growth gave rise to much better performance

pentacene transistors. In this work we discuss the relevant energetics of pentacene nucleation and

thin film growth on methyl surfaces. We also demonstrate how to design the dielectric surface so

that pentacene thin films are stable. These findings are important for monolayer transistors and

ultrathin organic transistor based sensors.

B5.118

Three-dimensional Anisotropic Electronic Properties of Solution Grown Organic Single

Crystals Measured by Space-Charge Limited Current (SCLC). Beatrice Fraboni1, Fabio

Brigidi1, Anna Cavallini

1 and Alessandro Fraleoni-Morgera

2;

1Physics, University of Bologna,

Bologna, Italy; 2Sincrotrone ScpA, Trieste, Italy.

Charge transport processes in organic materials are highly sensitive to the material deposition

and device fabrication conditions that, in turn, affect to a major extent the molecular packing and

interface defective states. In order to correlate the electrical properties of organic semiconductors

to their molecular functionality, it is necessary to control the effects of impurity and disorder

induced defects. It is has been shown that organic single crystals offer the possibility of studying

the intrinsic properties of organic molecules thanks to their high purity and molecular order, and

single crystal field effect transistors (SCFET) are a powerful tool in this respect. The observation

of anisotropic transport in single crystals, due to the anisotropic packing of the organic

molecules, provides an assessment of the occurrence of band-like transport processes.We studied

the three-dimensional anisotropic charge transport properties of solution-grown organic single

crystals based on a dipolar molecule 4HCB (4-hydroxy-cyanobenzene) by Space Charge Limited

Current (SCLC), by spectral photocurrent (PC) and by X-ray diffraction analyses. Most reports

on organic single crystals deal with crystals grown by vacuum deposition methods, while only a

few are available on high quality single crystals obtained by growth from solution [1,2]. We have

measured the charge carrier mobility along the three crystallographic axes of single crystals by

SCLC analyses obtaining highly reproducible and markedly anisotropic values (i.e. 5x10-2

cm2/Vs for the main axis a, 3x10-3 for the axis b and 3x10-6 cm2/Vs for the axis c, along the

crystal thickness). These results fully confirm values we have previously obtained by measuring

the carrier mobility using FET devices [2]. In addition, a modelization of the obtained data

allowed to evaluate the concentration and activation energy of the dominant deep traps along

each crystallographic direction. We observed how the exposure to visible light induced a

different effect on the transport properties along the two directions, that we have attributed to the

presence and alignment of the electron-attractor cyano group. We suggest that the presence of an

intrinsic molecular dipole differently affects the flow of charge carriers along the two main

planar crystal axes, thus altering the charge transport anisotropy induced by the molecular π-

orbitals stacking. A band of electrically active deep traps was identified by PC analyses and we

suggest that this lattice structure may be ascribed to the intrinsic 4HCB molecular electric dipole

that may originate trapping centers that behave like deep donors and that are more efficient along

one of the two main planar axes. [1] S. C. B. Mannsfeld, J. Locklin, C. Reese, M. E. Roberts, A.

J. Lovinger, Z. Bao, Adv. Funct. Mater., 17, 1617 (2007) [2] B. Fraboni, R. DiPietro, A.

Castaldini, A. Cavallini, A. Fraleoni-Morgera, L. Setti, I. Mencarelli, C. Femoni, Organic

Electronics, 9, 974 (2008)

B5.119 Photocurrent Studies of Sexythiophene-based OFETs. Beatrice Fraboni

1, Riccardo DiPietro

1,

Anna Cavallini1, Piero Cosseddu

2, Annalisa Bonfiglio

2, Jorg Vogel

3 and Jurgen Rabe

3;

1Physics,

University of Bologna, Bologna, Italy; 2Electrcal Engineering, University of Cagliari, Cagliari,

Italy; 3Physics, Humboldt University, Berlin, Germany.

Photocurrent (PC) spectroscopy is proposed as a reliable tool in the investigation of the transport

properties of organic thin film transistors (OFETs). We have applied PC analyses to the study of

the electronic density of states distribution (DOS) of the OFETs with crystalline mixed films of

two derivatives of a conjugated molecule [α-sexithiophene (6T), and its alkylated analogue α,ω-

dihexylsexithiophene (DH6T)] as the active semiconductor. The charge carrier mobility in

organic semiconducting materials depends on the effective charge carrier density, which can be

modulated, e.g., by gate voltage induced charge in a transistor geometry. We investigated the

modifications in the DOS distribution associated to variations in the carrier density in the OFET

channel and we detected the formation of deep electrically active states in the below-band-gap

region, associated to polaron states induced by a prolonged exposure of the device to

atmosphere. A clear correlation between the PC results and the electrical characteristics of the

corresponding FET devices has been observed

B5.120

Abstract Withdrawn

B5.121 Solution Processable Green Polymers for Use in Solar Cells Subbiah Jegadesan

1, Pierre

Beaujuge2, Kaushik Roy Choudhury

1, John Reynolds

2 and Franky So

1;

1Dept of Materials

Science and Engineering, University of Florida, Gainesville, Florida; 2Department of Chemistry,

University of Florida, Gainesville, Florida.

Polymer solar cells have attracted much attention in the last few years due to their promising

applications in clean and renewable energy sources. Over the years, the power conversion

efficiency of the polymer solar cells has steadily been improved with the use of new materials

and device architectures. Here, we report on bulk-heterojunction solar cells using a solution

processable neutral green conjugated polymer as the donor and PCBM as the acceptor. We have

found that the short-circuit current is very sensitive to the composition of the donor-acceptor

blend. The device with a donor-acceptor ratio of 1:8 gives the best performance with a power

efficiency of 1.9 %, a short-circuit current of 5.56 mA/cm2 and an open-circuit voltage of 0.77

V. The incident photon-to-current efficiency (IPCE) of the green solar cells shows two bands,

one with a maximum of 57% in the UV region corresponding to the absorption of PCBM and a

second one with a maximum of 42% in the visible-near IR region corresponding to the

absorption of the green polymer.

SESSION B6: Understanding Interfaces

Chairs: Antoine Kahn and Egbert Zojer

Wednesday Morning, April 15, 2009

Room 2001 (Moscone West)

8:30 AM *B6.1 Energetics of Organic-Electrode Interfaces: Polymer vs. Small Molecule FilmsAntoine

Kahn, Electrical Engineering, Princeton University, Princeton, New Jersey.

This talk reviews fundamental work on interfaces involving small molecule and polymer films,

which illuminates the commonality between the electronic structures, e.g. charge injection

barriers, of these two types of interfaces. The early work on small molecule-metal systems was

performed on ultra-clean interfaces, i.e. UHV deposition on ultra-clean substrate surfaces, in

order to identify intrinsic mechanisms that control the interface electronic structure, e.g.

chemistry or induced density of interface states. Results pointed out critical differences between

small molecule and polymer interfaces, the former displaying evidence of strong deviation from

the Schottky-Mott limit, and the latter following closely that limit. Recent work moved

increasingly toward “realistic” interfaces involving electrode surfaces exposed to ambient

conditions, much closer to future viable organic device fabrication conditions. This work show

that the nature of the interface is strongly related to the environmental conditions and processing

sequence used during interface formation. For example, the Schottky-Mott limit is (nearly)

achieved with small molecule interfaces formed on surfaces of contaminated (or otherwise

treated) or non-metallic electrodes, whereas a departure from the Schottky-Mott limit is observed

on polymer interfaces formed by vacuum evaporation of top contacts. The issue of Fermi level

pinning near the edges of the organic semiconductor transport gap, when using low or high work

function electrodes, is also discussed and results compared for polymer and small molecule

films.

9:00 AM B6.2

Polymer Solar Cells: Non-Conjugated Polymers and PESA Studies of Semiconductor

Energy LevelsScott E. Watkins, Ming Chen, Richard A Evans, Akhil Gupta, Matthias

Haeussler, Katalin Hegedus, Y. Phei Lok, Graeme Moad, Ezio Rizzardo, Gerard J Wilson and

Kevin Winzenberg; Molecular and Health Technologies, CSIRO, Melbourne, Victoria, Australia.

Plastic solar cells produced from organic semiconductors offer the potential to deliver efficient

solar energy conversion with low-cost fabrication. The challenge is to develop materials for

efficient charge separation and charge transport. Well-defined block-copolymers consisting of

organic conjugated chromophores are advantageous as their energy levels can be tuned relatively

easily through a structural engineering approach. In this contribution, we will discuss device and

characterisation results for new polymer building blocks including the use of bilayer devices to

study polymer fragments. Finally, we will present work on determining the energy levels of

known and novel materials by Photo Electron Spectroscopy in Air (PESA) and compare these

results with Ultraviolet Photoelectron Spectroscopy (UPS) and electrochemistry studies.

9:15 AM B6.3

Interface Engineering Concepts for Highly Efficient Inverted Organic Photovoltaics. Roland Steim

1,2, Stelios A Choulis

3, Pavel Schilinsky

1 and Christoph J Brabec

1;

1Konarka

Technologies GmbH, D-90443 Nürnberg, Germany; 2Light Technology Institute, Universität

Karlsruhe (TH), D-76131 Karlsruhe, Germany; 3Department of Mechanical Engineering and

Materials Science and Engineering, Cyprus University of Technology, 3603 Limassol, Cyprus.

For organic photovoltaic‟s interface materials with high selectivity are needed to prevent

recombination losses, limit leakage currents and enable carrier extraction of one sort. Thus

beside the transport properties of the active layer, interface materials determine the fill-factor and

thus the efficiency of organic solar cells. Especially for low light applications but also for the

stability of devices under reverse bias, which is of importance for module applications, low

leakage current devices are needed [1]. We present data that stacking of solution processed

organic and metal oxide interfacial layers give highly charge selective low ohmic cathodes. We

present data that the incorporation of a polyoxyethylene tridecyl ether (PTE) interfacial layer

between ITO and solution processed titanium oxide (TiOx) raised the fill factor and power

conversion efficiency of inverted polymer:fullerene bulk heterojunction solar cells to 3.6 % of

efficiency. This is an improvement of around 15% in their performance over devices without the

organic interfacial layer [2]. We investigated the origin of this improvement in detail and present

morphological and surface energetically measurements. [1] R.Steim, P. Schilinsky, S.A. Choulis,

U. Lemmer and C.J. Brabec, manuscript submitted to APL [2] R. Steim, S.A. Choulis, P.

Schilinsky and C.J. Brabec, Interface modification for highly efficient organic photovoltaics,

Appl. Phys. Lett. 92 (2008), p. 093303-1-3

9:30 AM B6.4

Surface-Segregated Monolayers: A New Type of Ordered Molecular Monolayer for

Surface Modification of Organic SemiconductorsKazuhito Hashimoto1,4

, Qingshuo Wei1,

Keisuke Tajima1, Yujin Tong

2 and Shen Ye

2,3;

1Department of Applied Chemistry, The

University of Tokyo, Tokyo, Japan; 2Catalysis Research Center, Hokkaido University,

Hokkaido, Japan; 3Precursory Research for Embryonic Science and Technology (PRESTO),

Japan Science and Technology Agency (JST), Hokkaido, Japan; 4HASHIMOTO Light Energy

Conversion Project, Exploratory Research for Advanced Technology (ERATO), Japan Science

Technology Agency (JST), Tokyo, Japan.

Well-ordered ultrathin films of organic molecules have been extensively investigated due to their

potential applications in various fields, such as the molecular electronics, nonlinear optics and

biological sensors. Langmuir-Blodgett (LB) films and self-assembled monolayers (SAMs) are

two important classes of such films and frequently used for surface modification of metals, metal

oxides and inorganic semiconductors to control their chemical and physical properties of the

surfaces. However, the surface modifications of the organic semiconductors by using LB films

and SAMs are challenging because of the instability of the organic substrates in solutions and the

lack of the specific interaction between the functional head groups of SAMs or LB molecules

and the organic surfaces. Recently, we have reported a self-organized layer formation on organic

semiconducting materials by utilizing the phenomenon of surface segregation.6 A novel

fullerene derivative with a fluorocarbon chain (FC7) was designed and synthesized. When a

small amount of FC7 was mixed in the solution of [6,6]-phenyl-C61-butyric acid methyl ester

(PCBM), it spontaneously migrated to the air/solvent interface during spin-coating owing to the

low surface energy of the fluorocarbon, and forms a very thin FC7 layer on the PCBM surface in

a single step. As a buffer layer, the top FC7 layer resulted in the improvement of the

performance in bulk-heterojunction photovoltaic devices. Interestingly, the FC7 layer on PCBM

film showed a large shift of the ionization potential, suggesting the alignment of the molecular

dipole moment at the surface. This observation gave us a notion that the layer of FC7 could have

a well-oriented, ordered structure on the surface, despite the easiness of the procedure. In this

contribution, we have synthesized a series of the fullerene derivative with different lengths of

fluorocarbon chains (FCn) and further studied the structure of the surface segregated FCn thin

layer by X-ray photoelectron spectroscopy (XPS) and sum-frequency generation (SFG) vibration

spectroscopy. SFG spectra clearly revealed the uniform monolayer formation with high

molecular orientations on the PCBM surface. Remarkably, the intensity of the SFG signals

showed a clear odd-even effect when the length of fluorocarbon chain was changed. It indicates

that these surface-segregated monolayers have a closely packed and well-ordered structure on the

surface. This new class of monolayer can be a facile and versatile approach to modify the surface

of the organic semiconductors and applicable to various organic devices. [1] Wei, Q.S.;

Nishizawa, T.; Tajima, K.; Hashimoto, K., Self-Organized Buffer Layers in Organic Solar Cells.

Advanced Materials 2008, 20, (11), 2211-2216.

9:45 AM B6.5 ''Soft" Metallic Contact to Isolated C60 Molecules. Hendrik Glowatzki

1, Benjamin Broeker

1,

Ralf-Peter Blum1, Oliver T Hofmann

2, Antje Vollmer

3, Ralph Rieger

4, Klaus Muellen

4, Egbert

Zojer2, Juergen P Rabe

1 and Norbert Koch

1;

1Institut für Physik, Humboldt Universität zu Berlin,

Berlin, Germany; 2Institut für Festkörperphysik, Technische Universität Graz, Graz, Austria;

3Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H., Berlin,

Germany; 4Max Planck Institute for Polymer Research, Mainz, Germany.

We investigated the electronic as well as the structural properties of Hexaazatriphenylene-

hexanitrile (HATCN) and C60 on Ag(111) by using ultraviolet photoelectron spectroscopy (UPS)

and scanning tunneling microscopy (STM) accordingly. HATCN was used as buffer layer to

decouple the test-molecule C60 from the metal substrate. Photoemission revealed metallic

behavior of HATCN on Ag(111) which calculations confirmed to be due to partial filling the

LUMO of chemisorbed HATCN. From STM a regular honeycomb-structure was found. Using

C60 as test-molecule on top of this layer the holes within the honeycombs were acting as

adsorption centers leading to laterally spacing between C60 molecules as observed by STM. At

the same time UPS revealed virtually bulk like electronic properties of C60 on metallic

HATCN/Ag(111) which is in strong contrast to the bare C60-Ag(111) interface being

significantly altered by charge transfer. This opens up new possibilities in probing the

undisturbed properties of individual molecules on metal substrates. This work is financially

supported by European Community project "IControl" (EC-STREP-033197).

10:30 AM B6.6

An Investigation of the Barrier Height, Morphology and Overlayer Lattice Structure of

Perylene Deposited on Au(111), Ag(111) and Cu(111). Kedar Manandhar1,2

, Justin Sambur3

and Bruce A Parkinson1,2

; 1Chemistry, University of Wyoming, Laramie, Wyoming;

2School of

Energy Resources, University of Wyoming, Laramie, Wyoming; 3Chemistry, Colorado State

University, Fort Collins, Colorado.

In situ deposited perylene films, with thicknesses from sub-monolayer to 1024 Å, deposited on

Au, Ag, Cu (111) surfaces were investigated using photoelectron-spectroscopy (UPS and XPS)

and scanning tunneling microscopy (STM). The analysis of the core level and the valence band

spectra showed no band bending at the perylene/metal interfaces. A Stranski-Krastanov type

growth mode of the films was revealed from the intensity analysis of metal core level and carbon

1s emission as a function of film thickness. A band energy diagram for these systems, the slope

of barrier height vs metal work function and the overlayer lattice structures and the morphology

of thin films will be presented and discussed.

10:45 AM B6.7 Formation and Electrical Interfacing of Nanocrystal-Molecule Nanostructures. Aidan

Quinn and Claire Barrett; Nanotechnology Group, Tyndall National Institute, Cork, Ireland.

Recent developments in the design and synthesis of nanoscale building blocks as active elements

for information storage, biosensing and photoconductive devices have the potential to

revolutionise several emerging technology markets across multiple sectors including healthcare,

printable electronics, security and energy conversion. Functional organic molecules and

biomolecules (~1-5 nm length scale) are attractive candidates as building blocks due to their

composition-, size- and structure-dependent electronic properties, the ability to design and

manipulate these properties via low-cost chemical synthesis, and finally the potential for

formation of ordered structures through (bio)-molecular recognition and self-assembly. Ligand-

stabilised inorganic nanocrystals (~1.6 - 30 nm core diameter) also represent attractive

candidates, both as building blocks with novel functionality (e.g., single charge tunnelling or

quantum confinement) and also as terminals or nodes to bridge the gap between length scales

accessible via top-down lithography (~ 25 nm) and molecular length scales. We present recent

results on solution-based formation of nanocrystal-molecule-nanocrystal “n-mer” nanostructures,

directed assembly of n-mers at contact nanoelectrodes and initial investigations of charge

transport in these “few-molecule” devices. For these studies, we have employed gold

nanocrystals with core diameters, d = 20 nm, and bifunctional organic linker molecules. The

novel plasmonic properties of these nanostructures can be used for in situ monitoring of the

nanostructure formation process. We have developed a rigorous nanovisulisation process based

on scanning electron microscopy to quantify the distributions of dimers, trimers and higher order

n-mers formed in solution, in order to investigate relationships between the distribution of

nanocrystal-molecule nanostructures formed and measured optical properties. Very recently, we

have demonstrated that UV-visible absorbance data reveals a characteristic response close to 600

nm upon formation of dimer nanostructures. Simulations based on the generalized multiparticle

Mie method are currently under development to gain further insight into this novel plasmonic

response. Directed assembly processes based on dielectrophoretic trapping have also been

developed for electrical interfacing of these nanostructures between top-down nanoelectrode

pairs (~25 nm interelectrode gaps). Initial current-voltage characterisation reveals contact

formation with ~10% yield, comparable to contacting yields for larger self-assembled networks

of metal nanocrystals. These contacting strategies are being iteratively optimized via a

combination of directed assembly and electrode functionalisation processes.

11:00 AM *B6.8 Theoretical Spectroscopy of Organic Semiconductors. Leeor Kronik, Materials and

Interfaces, Weizmann Institute of Science, Rehovoth, Israel.

The combination of density functional theory (DFT) with powerful spectroscopic tools, e.g.,

photoemission spectroscopy or absorption spectroscopy, is an important approach to elucidating

the electronic structure of materials. In recent years, it has become a particularly popular tool for

studying organic semiconductors and their interfaces with (metallic or semiconducting) inorganic

substrates - topics of great importance in organic electronics. In this talk, I review our recent

progress in understanding the strengths, limitations, and true predictive power of such analyses,

and our advances in accurate calculations of systems usually believed to be "too difficult for

DFT". In particular, the consequences of self-interaction and derivative discontinuity errors and

the role of dispersive interactions are addressed. The pros and cons of different functionals,

including standard and novel range-split hybrid functionals, as well as other orbital-dependent

approaches, are discussed. The role of post-DFT methods, especially many-body perturbation

theory, is also examined. Perhaps most importantly, general criteria for a priori selection of the

best-suited theoretical approach and evaluation of its accuracy are suggested. The new

approaches are demonstrated through a judicious comparison of DFT and related approaches

with concrete experimental results for a variety of prototypical semiconducting organic

molecules and organic/inorganic interfaces.

11:30 AM B6.9 Energy-Level Alignment at Metal/SAM/Organic Contacts. Ferdinand Rissner

1, Gerold M

Rangger1, Oliver T Hofmann

1, Anna M Track

1, Georg Heimel

2 and Egbert Zojer

1;

1Institut für

Festkörperphysik, Technische Universität Graz, Graz, Austria; 2Institut für Physik, Humboldt-

Universität zu Berlin, Berlin, Germany.

In the field of organic electronics, self-assembled monolayers (SAMs) are commonly applied to

enhance the performance of (opto)electronic devices. Over the past years considerable efforts

have been made to better understand the influence of SAMs on surface properties. In particular,

it has been shown that the deposition of SAMs on metals can cause significant work function

modifications. It is essential to understand to what extent a modification of the electrode work

function can be directly translated into a change of the electron and hole injection barriers into an

organic semiconductor (OSC) on top of the SAM, i.e., it needs to be tested under which

conditions the “simple” Schottky-Mott limit applies. In this contribution, density functional

theory based slab-type band-structure calculations are performed, adopting the VASP code. In

order to model metal/SAM/organic interfaces, we investigate three-component systems,

consisting of a metal surface, a SAM and a monolayer of a prototypical organic semiconductor

on top of the SAM. By varying the nature of the SAM interlayer, we can distinguish between

regions of Fermi level pinning and vacuum level alignment, where the former is found to be a

consequence of polarization of the SAM in addition to significant charge transfer at the

SAM/OSC interface. Support by the European Commission through the STREP project

ICONTROL (EC-STREP-033197) and by the FWF through project P20972-N20 is gratefully

acknowledged.

11:45 AM B6.10

Nanomagnetism at Organic/inorganic Interfaces.Jian-Bin Xu1,2

, Xiao-Qing Tian1,2

and Rong

Zhang3;

1Dept. of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong

SAR, China; 2Materials Science and Technology Research Center, The Chinese University of

Hong Kong, Hong Kong SAR, China; 3Physics Dept., Nanjing University, Nanjing, Jiang Su,

China.

Nanomagnetism at CoPc/Au(111) and CoPc/graphene interfaces are investigated by ab initio

calculations and Anderson localized magnetic impurity model. The strong coupling between Co

and Au(111) substrate causes the magnetic impurity d orbital redistribution. This results in

magnetic-nonmagnetic transition of the magnetic impurity. The graphene substrate has no

effective impact on the electronic structure of magnetic impurity, and thus there is no magnetic-

nonmagnetic transition at CoPc/graphene interface.

SESSION B7: Charges & Transport I

Chairs: Leeor Kronik and Xiaoyang Zhu

Wednesday Afternoon, April 15, 2009

Room 2001 (Moscone West)

1:30 PM B7.1 In Situ Optical Spectroscopy of Organic Electronics and Optoelectronics. Travis Mills,

Loren G Kaake and Xiaoyang Zhu; Chemistry, University of Minnesota, Minneapolis,

Minnesota.

Organic electronics and optoelectronics are commonly characterized from device perspectives,

such as I-V characteristics in organic thin film transistors (OTFTs) and photo-to-electric

conversion efficiencies in organic photovoltaics (OPVs). Little is known at the molecular and

quantitative levels regarding charge carriers in operating organic electronic and optoelectronic

devices. We apply attenuated total internal reflection Fourier transform infrared and near-

infrared spectroscopy (ATR-FTIR or FTNIR) to directly probe OTFTs and OPVs under

operational conditions. These measurements have been enabled by the fabrication of devices on

optical waveguides. We use this approach to quantify charge separation and collection

efficiencies in bilayer OPVs from molecular and polymeric semiconductors. This measure fills a

critical gap in current understanding of the efficiencies of OPVs. We also apply this

spectroscopic approach to quantify charge carriers in OTFTs gated with polymer electrolyte

dielectrics and show the limits in switching speed due to carrier and ion movements.

1:45 PM B7.2

Gating Conductance of Metal-organic Semiconductor Contacts: a Macro-scale Device

Reflecting Molecular-scale Switching. Jun Takeya1, K. Nakayama

1, T. Uemura

1, M.

Yamagishi1 and M. Uno

1,2;

1Osaka University, Toyonaka, Japan;

2TRI-Osaka, Izumi, Japan.

It is argued on one hand that an advantage of organic electronics is in large-scale application

utilizing a low-temperature roll-to-roll printing process whereas it is also addressed that organic

molecules can form ultimate nanoscale devices, on the other hand, when each molecular state is

independently controlled. It appears that the extreme multi-scalability is a pronounced feature of

organic materials that overwhelms that of inorganics. In fact, charge transport in organic

transistors, which are regarded as representative of the macro-scale devices, consists of

conductance with two very different length scales; i.e. carrier diffusion in the semiconductor

channels typically of μm scales and carrier injection from metal electrodes to the organic

semiconductors governed by nanometer-scale physics. In this presentation, we disclose a device

of controlling the conductance of metal-organic semiconductor contacts, in contrast to usual

organic transistors where that of the semiconductor channels is modified. The present device

possesses potential advantages in 1) effective integration of the nano-scale active components, 2)

higher current-amplification performance due to the mechanism of controlling chemical potential

of the interface instead of electrostatic charging, and 3) high-speed response as the result of very

small amount of charge necessary for the operation. Prototypical switching devices were

prepared based on macro-scale organic top-contact single crystal transistors, in which thin

platelets of rubrene single crystals with the typical thickness of 2 μm were laminated on SiO2 /

doped Si substrates and two metal electrodes are deposited from the top. At this stage, the

devices worked as top-contact organic transistors where the charge amount was controlled at the

bottom surface with the application of gate electric field in the SiO2 dielectrics. The top surfaces

of the single crystals were then coated with fluoro-silane molecular layers, which form a hole-

conducting layer at the top crystal surfaces as the result of mutual charge transfer between the

silane and rubrene molecules [1]. The structure ended up with normally-on transistors with the

sheet conductivity of 105 S which was little changed by the field effect. Interestingly, the device

suddenly switched to an off state with the application of positive gate voltage of a few volts,

which is far less than necessary to deplete the whole carriers in the top channel. Indeed, the result

of four-terminal-conductivity measurement did not show any sudden switching, meaning that

contacts are responsible for the effect. The conductance of the molecular-scale contacts was

tuned by the chemical potentials in the vicinity of the electrodes, modified through weak band-

bending in clean rubrene crystals. In other words, the observed performance of the macro-scale

devices reflects switching occurring in the molecular scale. [1] M. F. Calhoun et al., Nature

Mater. 7, 15259 (2008).

2:00 PM *B7.3

Electrostatic Modulation of Charge Density in Polymers and Molecular Crystals: an

Infrared Study. Dmitri N Basov, UCSD, La Jolla, California.

Recent advances in infrared spectroscopy enabled experimental studies of the electromagnetic

response of charges in a single molecular monolayer [Li et al. Nature-Physics 4 532 (2008)].

These advances paved the way towards a systematic exploration of electronic excitations

associated with high density of charges in the accumulation layer of organic field effect

transistors (OFET) with both polymers and molecular crystals as active semiconductors. Infrared

data for rubrene-based OFETs indicate that transport properties of these transistors at room

temperature are governed by light quasiparticles in molecular orbital bands with the effective

masses m* comparable to free electron mass [Li et al. PRL 99, 016403 (2007)]. Infrared

microscopy results have uncovered giant length scales characteristic of charge injection in

polymers with high electronic mobility including P3HT and PBTTT [Li et al. Nano Letters 6,

224-228 (2006)].

2:30 PM B7.4 Scanning Potentiometry of ribbon-phase pBTTT Thin Film Transistors Toby Hallam

1, Ni

Zhao1, Mi Jung Lee

1, Iris Nandhakumar

3, Martijn Kemerink

2 and Henning Sirringhaus

1;

1Physics, University of Cambridge, Cambridge, United Kingdom;

2Physics, Eindhoven

University of Technology, Eindhoven, Netherlands; 3Chemistry, University of Southampton,

Southampton, United Kingdom.

The polymer poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene), pBTTT has been shown

to exhibit a highly organized „terrace‟ structure with grain sizes several µm across after

annealing to a liquid crystalline phase at 180°C. Thin films of pBTTT in the terrace phase have

demonstrated large field effect mobilities comparable to that achieved with small molecules or

amorphous silicon. Upon annealing at 260°C, pBTTT adopts a highly crystalline „ribbon‟ phase

where well-defined, ~80nm wide crystalline ribbons are observed within which the polymer

adopts an extended chain conformation. The ribbons are separated by similarly well-defined

grain boundary regions with more disordered chain conformation. Until recently, Scanning

Kelvin Probe Microscopy (SKPM) has been limited to mapping surface potential in Organic

Thin Film Transistors with a resolution >100nm. We demonstrate here <50nm lateral potential

resolution achieved through use of AFM tips with attached carbon nanotubes. This has allowed

resolving potential variations associated with individual nanoribbons and grain boundary regions

in the ribbon phase of pBTTT. Using SKPM we have directly observed hole trapping at the inter-

grain regions in ribbon-phase pBTTT. For an unstressed pBTTT film the surface potential is

found to exhibit a characteristic, normal distribution. Following a prolonged negative gate bias,

the distribution of surface potentials measured across the surface exhibited an additional tail

towards more positive surface potential, which we interpret as being due to trapping of hole

carriers in the channel. We have been able to correlate the spatial locations of the sites that

exhibit the more positive surface potential after bias stress with the grain boundary regions of the

ribbon phase. This provides direct evidence that charge trapping occurs preferentially within the

grain boundary regions and that these provide bottlenecks for charge transport. Hole

delocalization in the pi-pi stacking direction also plays a crucial role in electronic transport. By

applying a source-drain voltage we can identify preferential current paths which appear to be

influenced by a combination of the alignment of lamellar packing for adjacent crystallites and the

direction of the applied lateral field. We have also investigated the temperature dependence of

the surface potential along the channel in the ribbon phase. We are able to resolve potential

variations across the ribbons and across the grain boundary regions between them and we have

observed a characteristic change in the nanoscale potential profiles around ~200K. We will

discuss these results in terms of the nanoscale charge transport properties of the polymer.

2:45 PM B7.5

Capacitance-Voltage Analysis for Interface Trap Density between Pentacene and

Insulators Seung-Hyeon Jeong and Chung-Kun Song; Electronics Engineering, Dong-A

University, Busan, Korea, South.

The study of interface trap density between semiconductor and dielectric is the important issue to

improve the performance of organic thin film transistors. In this study, we analyzed the interface

trap density between pentacene organic semiconductor and gate dielectrics such as poly-4-

vinylphenol (PVP) and SiO2 by using the combined high-low frequency C-V characteristics. In

capacitor consisting of organic semiconductor and organic dielectric, it is difficult to extract the

reliable low frequency C-V curve because of the slow responding interface traps. By varying

step delay time and frequency of small signal we extracted the measurement conditions for the

reliable low frequency C-V curve. From the reliable low and high frequency C-V curves we

obtained interface trap density between pentacene-PVP and pentacene-SiO2. The former

exhibited the large shallow trap density of 1013 [#/cm2 ev] and the small deep trap density of

109 [#/cm2 ev] meanwhile the later the large shallow trap density of 1013 [#/cm2 ev] and the

large deep trap of 1012 [#/cm2 ev] as well. The organic-organic interface produced the better

properties than the organic-inorganic interface in terms of trap density. *This research was

supported by a grant(F0004020-2008-31) from Information Display R&D Center, one of 21st

Century Frontier R&D Program funded by the Ministry of Knowledge Economy of Korean

government.

3:30 PM *B7.6 Organic Thin-Film Transistors and Circuits with Low Operating Voltage. Hagen Klauk,

Max Planck Institute for Solid State Research, Stuttgart, Germany.

Organic thin-film transistors (TFTs) often employ relatively thick gate dielectrics with small

capacitance (usually between 20 and 100 nF/cm2) und thus typically require operating voltages

in the range of 10 to 50 V. For certain applications, organic TFTs that can be operated with

voltages between 2 and 3 V will be beneficial. For example, state-of-the-art organic light-

emitting diodes (OLEDs) that employ doped transport layers require supply voltages between 2.5

and 3 V, reaching efficiencies between 10 cd/A (red and blue) and more than 70 cd/A (green)

[1]. To realize flexible active-matrix displays with such OLEDs, low-voltage organic TFTs will

be required. A second example for applications that will benefit from low-voltage organic TFTs

are hybrid systems in which flexible large-area TFT arrays share signals and power with silicon-

based processor units, which are typically designed to operate with a supply voltage of 2 or 3 V

[2]. To realize flexible organic TFTs that can be operated with voltages of 3 V or less, a low-

temperature-processed gate dielectric with a capacitance above about 0.5 µF/cm2 is required. A

promising approach are hybrid dielectrics based on a thin, plasma-grown metal oxide in

combination with an organic self-assembled monolayer (SAM). The metal oxide is obtained by

exposing the metal gate electrode to an oxygen plasma; in the case of aluminum gate electrodes

this forms an AlOx layer with a thickness of 3 to 4 nm, depending on the plasma power. This

oxide layer alone is a poor dielectric characterized by substantial leakage currents. However, the

plasma-grown oxide provides an excellent surface for the self-assembly of a molecular

monolayer of an aliphatic phosphonic acid. This monolayer is obtained either by immersing the

substrate in a solution of the phosphonic acid or by transferring the monolayer from an

elastomeric stamp. The self-assembled monolayer has a thickness of about 1.5 to 2 nm, giving a

total oxide/SAM dielectric thickness of about 5 to 6 nm and a gate dielectric capacitance of 0.7

to 1 μF/cm2. Despite its small thickness and the low process temperature, the oxide/SAM

dielectric provides leakage currents of less than 10 μA/cm2 at 3 V. This allows p-channel and n-

channel organic TFTs on glass and on flexible polymeric substrates to operate with voltages

between 1.5 and 3 V and with excellent static and dynamic characteristics, including large carrier

mobility, large on/off current ratio, steep subthreshold slope, small gate leakage currents, and

large cut-off frequency. [1] P. Wellmamm et al., J. Soc. Inf. Display, vol. 13, p. 393, 2005 (see

also: www.novaled.com). [2] T. Sekitani et al., Int. Solid-State Circuits Conf. (San Francisco,

February 2009).

4:00 PM B7.7 High Performance N-type Organic Thin-Film Transistors with Inert Contact Metals. Sarah

Schols1,2,5

, Lucas Van Willigenburg1, Robert Muller

1, Dieter Bode

1,2, Maarten Debucquoy

1,2, Jan

Genoe1, Paul Heremans

1,2, Shaofeng Lu

3 and Antonio Facchetti

3,4;

1SOLO/PME, IMEC, Leuven,

Belgium; 2Electrical Engineering Department, Katholieke Universiteit Leuven, Leuven,

Belgium; 3Polyera Corporation, Skokie, Illinois;

4Department of Chemistry and the Material

Research Center, Northwestern University, Evanston, Illinois; 5Aspirant at the FWO Vlaanderen,

Brussel, Belgium.

Driven by potential applications of complementary logic, the field of electron-channel (n-type)

organic thin-film transistors (n-OTFT) and corresponding n-type materials has gained a lot of

attention. 5,5'''-diperfluorohexylcarbonyl-2,2':5',2'':5'',2'''-quaterthiophene (DFHCO-4T) is an

example of such a promising electron-conducting organic semiconductor. Recently, high electron

field-effect mobilities were reported for DFHCO-4T.[1,2] While most n-OTFTs need low-

workfunction metal contacts such as Mg, Ca or LiF/Al for efficient electron injection into the

semiconductor lowest unoccupied molecular orbital (LUMO), DFHCO-4T-based n-OTFTs

function properly with high-workfunction source and drain Au contacts.[1] This fact is of high

technological relevance because for use in complementary logic it is preferable to use a single

type of source and drain metal for both the p-type and the n-type OTFTs. Here, we discuss the

optimization of DFHCO-4T growth and compare the performance of DFHCO-4T transistors

with different top contact metals. Thin film growth by high vacuum evaporation of the n-type

organic semiconductor DFHCO-4T on poly-(α-methylstyrene)-coated n++

-Si/SiO2 substrates is

investigated at various deposition fluxes and substrate temperatures. Film characterization by

atomic force microscopy reveals typical Stransky-Krastanov growth. Transistors with Au source-

drain top contacts and optimized DFHCO-4T deposition conditions exhibit an apparent

saturation field-effect mobility of 4.6 cm2/Vs, whereas this parameter is 100x lower for similar

transistors fabricated with LiF/Al or Yb top contacts. We explain this reduced performance of

transistors with easily oxidizable top-contact metals such as Al by the formation of a thin

interfacial layer with poor injection properties resulting from an electron-transfer reaction

between the metal and DFHCO-4T. [1] M. Yoon, C. Kim, A. Facchetti, and T. J. Marks, J. Am.

Chem. Soc. 128, 12851 (2006). [2] M. Yoon, S. A. DiBenedetto, M. T. Russell, A. Facchetti, and

T. J. Marks, Chem. Mater. 19, 4864 (2007).

4:15 PM B7.8 Self-Assembly in Organic LEDs and FETs Simon Mathijssen

1,2, Edsger Smits

3,2, Paul van

Hal2, Ton van den Biggelaar

2, Monja Kaiser

2, Bert de Boer

3, Sergei Ponomarenko

4, Martijn

Kemerink1, Rene Janssen

1 and Dago de Leeuw

2,3;

1Applied Physics, Eindhoven University of

Technology, Eindhoven, Netherlands; 2Philips Research Laboratories, Eindhoven, Netherlands;

3University of Groningen, Groningen, Netherlands;

4Enikolopov Institute of Synthetic Polymer

Materials of Russian Academy of Sciences, Moscow, Russia.

Self assembly, the autonomous organisation of components into patterns and structures without

human intervention is the ultimate technology for mass production of large area electronics. First

we demonstrate SAMFETs, field-effect transistors where the semiconductor is a monolayer

spontaneously formed on the gate dielectric. In order to form a conducting path in between the

source and drain electrode, the molecules in the self-assembled monolayer (SAM) should be

intimately connected. Any structural imperfection as voids or grain boundaries leads to potential

barriers and, hence, to a deteriorated charge carrier mobility. In addition the nature of the

electrical contact is crucial. Formation of an effective injecting electrode to a single layer of

molecules has proven to be a historical challenge. Here we show that the SAMFET consists of a

semiconducting monolayer. The electrical connectivity between the molecules is inferred from

electrical transport measurements as well as the local surface potential as determined from

scanning Kelvin probe microscopy (SKPM) measurements. To elucidate the efficient injection of

charge carriers in the SAM, the critical region where the SAM meets the edges of the electrode

was imaged with transmission electron microscopy. Morphological studies and SKPM

measurements substantiate that the prerequisites for efficient charge transport in a field-effect

transistor are fulfilled; we observe long-range connectivity together with an intimate contact

between the semiconductor and electrodes. The small parameter variation between transistors

allowed integration into a functional 15-bit code generator. Furthermore we applied self-

assembly to produce patterned OLEDs with micro-contact printed self-assembled monolayers.

Here SAMs with opposite dipole moments change the local work function and hence the

injection, which results in a patterned light emission. The local work function is analyzed using

scanning Kelvin probe microscopy. The scanning probe measurements together with optical

micrograph images of the patterned OLEDs demonstrate a direct correlation between the local

work function and light emission. In summary, we show the incorporation of SAMs in organic

field-effect transistors and light-emitting diodes. In the first case, the monolayer is acting as the

semiconductor, in the latter as an injection modifier.

4:30 PM B7.9 An Organic-nanoparticle Transistor Behaving as a Spiking Synapse.Dominique Vuillaume

1,

Fabien Alibart1, David Guerin

1, Stephane Pleutin

1, Kamal Lmimouni

1, Christophe Novembre

2

and Christian Gamrat2;

1IEMN-CNRS, Villeneuve d'Ascq, France;

2CEA-LIST, Gif-sur-Yvette,

France.

We demonstrate that an organic transistor, made of metal nanoparticles (NP) embedded into an

organic semiconductor channel, behaves as a spiking synapse (hereafter called an organic-

nanoparticle transistor-synapse - ONTS for short). We demonstrate that this ONTS device

exhibits the main behavior of a biological synapse. For instance, the ONTS can be programmed

to work as a facilitating or depressing synapse; it exhibits short-term plasticity as well as spike

timing dependent plasticity. This behavior is obtained by virtue of the combination of two

properties of the ONTS: the transconductance gain of the transistor and the memory effect due to

charges stored in the NP. We previously demonstrated that this type of device works as a non-

volatile memory (1) but with a “leaky” behavior. This behavior is used here to implement the

synapic weight wij with a possible dynamic behavior, a mandatory condition to obtain the

training/learning of a spiking neural network (2). The gold NP are immobilized into the source-

drain channel by using surface chemistry (self-assembled monolayers) and they were

subsequently covered by a thin film of pentacene. In a biological synapse, the facilitating

behavior means that an incoming signal with a given frequency and duty cycle induces a post-

synaptic signal having an increasing trend, whereas in the case of an depressing synapse, the

post-synaptic signal tends to decrease (3). This behavior is exactly what we demonstrated for the

ONTS. Our results also compare qualitatively well with a simulation model (4). (1) C.

Novembre, D. Guérin, K. Lmimouni, C. Gamrat, D. Vuillaume, Appl. Phys. Lett. 92, 103314

(2008). (2) S. Haykin, Neural networks. A comprehensive foundation., (Macmillian, NewYork,

1994). (3) H. Markram, Y. Wang, M. Tsodyks, Proc. Natl. Acad. Sci. USA 95, 5323 (1998). (4)

M. Tsodyks, K. Pawelzik, H. Markram, Neural Computation 10, 821 (1998)

4:45 PM B7.10 Light-emitting Ambipolar Field-effect Transistors using Organic Single Crystals. Hajime

Nakantani and Chihaya Adachi; Center for Future Chemistry, Kyushu University, Fukuoka,

Japan.

An ambipolar light-emitting organic field-effect transistor (LE-OFET) based on a 1,4-Bis(4-

methylstyryl)benzene (BSP-Me) single crystal was developed. The BSP-Me single crystal has

very high photoluminescence quantum efficiency (Fai(PL)) of 89%, while Fai(PL) of the BSP-

Me vapor-deposited film is limited to a much lower value of 54%. Ambipolar operation with

successive blue electroluminescence from the FETs based on the BSP-Me single crystals was

demonstrated by realizing nearly equal electron and hole mobilities (about 0.005 cm2/Vs) with

asymmetric gold-calcium contacts. Since BSP-Me single crystals can perform light

amplification, the BSP-Me-based ambipolar LE-OFET is a promising candidate for future

electrically driven organic blue-emitting solid-state lasers. We also mention organic/inorganic

hybrid FET structures for aiming efficient carrier recombination.

SESSION B8: Charges & Transport II

Chairs: Hagen Klauk and Nobuo Ueno

Thursday Morning, April 16, 2009

Room 2001 (Moscone West)

8:30 AM *B8.1

The first principles measurement of charge mobility of organic semiconductors with UPS. Satoshi Kera

1, Hiroyuki Yamane

2, Shunsuke Hosoumi

1, Shin-ichi Nagamatsu

1 and Nobuo

Ueno1;

1Graduate School of Advanced Integration Science, Chiba University, Chiba, Japan;

2Institute for Molecular Science, Okazaki, Japan.

Most of the over 86 million registered materials are organic materials, and many of them form

solid by weak intermolecular interaction. Organic semiconductor is the representative of

electronic function brought by the weak intermolecular interaction coupled with individual

molecular characteristics, and has been increasingly studied for device application. As organic

semiconductor films show various „faces‟ depending on molecular packing structure as well as

on individual molecular structure, many mysteries exist to be elusive. A key issue of organic

FET devises is how one can improve the carrier mobility (μ) of organic thin films. As the

electrical conductivity (σ) is given by σ=nqμ, where n is the carrier concentration and q is the

charge of the carrier concerned, one must know principal mechanism which dominates μ, namely

coherent band conduction or incoherent hopping conduction. In relation to organic transistors,

many electrical measurements have been performed to investigate the charge mobility.

Unfortunately, however, most of the works have directed to obtain phenomenological μ from I-V

measurements. The coherent conduction is dominated by the band dispersion with a mean free

path of the carrier much longer than intermolecular distance, and the hopping conduction is

specified by two physical parameters, the transfer integral (t) and the charge reorganization

energy (λ). t is the measure of the intermolecular interaction, and λ is related to charge-vibration

coupling. To go into the mobility, one needs to measure t and λ experimentally. t is given by

observing the energy band dispersion or the energy level splitting in finite molecular stacks, and

λ is obtained by measuring hole- or electron-vibration coupling in organic systems at low

temperature (namely not in gas phase but in a film). UPS can in principle measure these two

targets. At this conference we will present our recent challenges on UPS of the thin films that

give (1) the energy level splitting and the band dispersion, both of which offer the value of t, and

(2) λ obtained directly from the measurement of the HOMO hole-vibration coupling in various

organic semiconductor films. From these we can obtain the ultimate hole mobility for the

coherent band conduction and the hopping conduction.

9:00 AM B8.2

Charge Transport and Microstructure Correlations using Anisotropic Polythiophene Thin

Films Fabricated via Directional Crystallization. Leslie Hendrix Jimison and Alberto H

Salleo; Materials Science and Engineering, Stanford University, Stanford, California.

In recent years much research has been devoted to the improved fabrication and understanding

macroelectronics: a category of devices that include thin film transistors, photovoltaics and light

emitting diodes. Polymeric semiconductors have great potential as the active layer for such large

scale systems, with a key attribute being their ability to be dissolved in solvents to make

printable “semiconducting inks.” While polymer semiconductors are beginning to reach

performance levels that enable them to be competitive in low-cost electronics, what is lacking is

an understanding of the fundamental charge transport processes in relation to microstructure. We

have used a means of controlling the orientation and size of crystallites in the plane of the

substrate to produce films with well-known grain-boundary types. As a result we are able to

explore the relationship between trap density within grain boundaries and charge transport.

Regioregular poly-3-hexylthiophene (P3HT) is the material under investigation. We have

fabricated anisotropic films on glass and silicon substrates via directional solidification, using

1,3,5-trichlorobenzene first as a solvent and then as a substrate for epitaxy. The technique

enables us to create films consisting of large (mm2) domains of uniform extinction under crossed

polarizers, suggesting long range orientation of the polymer chain axis. Using a film lift-off

technique, atomic force microscopy reveals the lamellar microstructure at both the polymer/air

interface and polymer/substrate interface. Film microstructure was further characterized at the

Stanford Synchrotron Radiation Laboratory (SSRL). We have collected 2D diffraction patterns,

specular diffraction patterns, and grazing incidence diffraction patterns, confirming the unique

in-plane and out-of-plane texture of the polymer films. Charge transport in the directionally

crystallized films was probed by measuring temperature-dependent mobilities using thin film

transistors with the oriented film as the active layer. Devices were made with different relative

orientations between the channel and the polymer film. Transport measurements as a function of

charge density and temperature for different orientations of our film confirm mobility anisotropy

but no activation energy anisotropy. Temperature dependent transport measurements also reveal

increased effect of bias stress between 220-280°C. Furthermore, there is marked anisotropy of

the bias stress effect between the different device orientations. Because our films are anisotropic

in the type of grain boundary present, this strongly suggests that bias stress in semicrystalline

organic thin films is dependent on the microstructure at the grain boundaries.

9:15 AM B8.3

Scanning Kelvin Probe Measurements on Pentacene based Field-Effect Transistors with

UV-modified Gate Dielectric Christopher Siol, Niels Benson, Christian Melzer and Heinz von

Seggern; Institute of Material Science, Technische Universitaet Darmstadt, Darmstadt, Germany.

The use of organic field-effect transistors (OFETs) in organic electronics is often hampered by

the fact that solely unipolar logic is implemented. In view of a performance improvement, it is

aspired to use complementary metal oxide semiconductor (CMOS) -like techniques to benefit

from the efficiency of complementary logic circuits. In recent publications we have

demonstrated a CMOS-like inverter based on pentacene n- and p-type OFETs that comprise

identical device layouts thus facilitating the production of CMOS-like elements. Solely the

treatment of the used PMMA gate dielectric with UV light prior to the pentacene deposition

allowed for an inversion of the operation mode from the usual n-type to a unipolar p-type

behaviour. Even though the stable device performance of the thus produced inverter

demonstrates the potential of the proposed UV treatment the exact mechanism allowing for the

polarity change is not known in detail. In this contribution, we try to reveal this mechanism by

discussing the results of Scanning Kelvin Probe Measurements (SKPM) performed in the

channel of UV-treated OFETs in combination with the analysis of the OFET performance. From

the current-voltage characteristics it is supposed that the UV treatment itself results in the

formation of electronic trap states, in particular in electron traps. Since the establishment of the

p-type behaviour requires, besides the UV treatment, a precedent operation of the transistor in

electron accumulation, it is speculated that the electron traps have to be negatively charged.

Indeed, SKPM disclose that the accumulation of negative charge carriers at the PMMA /

pentacene interface results in a distinct and stable trapping of electrons, which proves the

existence of efficient electron traps in UV-modified PMMA. In what way this charging helps to

convert the n-type transistor to a p-type transistor, in particular to inject holes from the employed

low workfunction metal Ca, will be discussed in detail. The stably trapped negative areal charge

density leads to a gate-field enhancement at the contacts. It is supposed that this field

enhancement is sufficient to inject holes from Ca into the channel. Under current flow, the

channel in the vicinity of the source will again partly deplete from the compensating holes

allowing anew for a field-enhanced hole injection from Ca.

9:30 AM B8.4

Trap-Dominated Charge Transport in Organic Transistors as Investigated by Field-

Induced ESR Spectroscopy. Hiroyuki Matsui1,2

and Tatsuo Hasegawa1;

1PRI, AIST, Tsukuba,

Japan; 2Department of Advanced Materials Science, University of Tokyo, Tokyo, Japan.

Recently we reported that motional narrowing effect can be observed in field-induced electron

spin resonance (ESR) spectra of high-mobility pentacene thin-film transistors (TFTs). We found

that the methods are quite useful in elucidating the carrier dynamics in organic TFTs since the

narrowed linewidth allows us to estimate the average residence time of carriers at respective

sites. In particular the analyses afford a clear evidence for the considerably long trap residence

time or the trap-dominated conduction in pentacene TFTs. Meanwhile, motional narrowing

effect is directly evidenced by temperature-dependent and gate-field-dependent single-

Lorentzian ESR spectra. However, the temperature-dependent feature deviates from the simple

motional narrowing regime in high (T > 200 K) and low (T < 50 K) temperature ranges. Here we

discuss the whole picture of field-induced ESR spectra in the temperature range of 300 - 20 K on

the basis of continuous wave saturation experiments. The method enables us to check the

homogeneity of ESR spectra as well as to estimate spin-lattice relaxation time T1. It is found

from the temperature dependence of ESR linewidth that the linewidth increases as temperature

decreases with activation energy of about 15 meV at 50 K < T < 200 K, while it deviates from

the feature at higher and lower temperature ranges. First we examined the saturation behavior of

ESR signals, which allows us to separate the contribution of spin-lattice relaxation from

motionally-narrowed inhomogeneous linewidth. The motionally-narrowed and the spin-lattice

relaxation components compete with each other at around 200 K. From this we conclude that the

small increase of linewidth at higher than 200 K with increasing tempeature should be attributed

to the spin-lattice relaxation. At the temperature lower than 50 K, on the other hand, the

linewidth tends to converge at about 0.18 mT. The ESR spectra do not show broadening under

saturation at high microwave power, demonstrating the inhomogeneity of the ESR absorption. It

means that motional narrowing is no longer effective in the low temperature range because of

long residence time at trap states. Transport and localization of field-induced carriers will be

discussed on the basis of these experimental results.

9:45 AM B8.5 Electrochemical Transistors: New Platforms to Study Interfaces in Liquids. Fabio Cicoira

1,2,

Sang Yang Yoon1, DeFranco A John

1 and George G Malliaras

1;

1MSE, Cornell UNiversity,

Ithaca, New York; 2Institute of Photonics and Nanotechnology, CNR, Trento, Italy.

The considerable research efforts in organic electronics have led to the development of a number

of devices like organic light emitting diodes, solar cells and organic thin film transistors that are

nowadays in production or prototype stage. Along with these well-established fields, exciting

emerging applications are taking advantage of the mixed ionic/electronic transport in organic

electronics devices [1]. Along this line, the application of organic semiconductor devices to

chemical and biological sensors seems to be a great fit. A promising approach towards organic-

based sensors involves the use of organic electrochemical transistors (OECTs). OECTs consist of

source and drain electrodes, and a channel containing the organic active material in ionic contact

with a gate electrode via an electrolyte solution. These devices can be operated in aqueous

environment as efficient ion-to-electron converters, thus providing an interface between the

worlds of biology and electronics and also a unique platform for the study of organic/organic and

organic/metal interfaces in liquids. Although electrochemical transistors have been known since

long time [3], they received little attention in the scientific community until the recent resurgence

due to their application in biosensors. Therefore a great deal of work is needed to understand the

fundamental processes that take place in these devices, essential for their use in sensing

applications. In this presentation we intend to address this important issue. Using

photolithography, surface engineering and micro fluidics we have developed several technique to

fabricate OECTs having different geometries. This allows us to study the basic electronic

properties and the sensing response of devices in order to understand their mechanism of

operation [4] [5]. We studied how the dimensions of the transistors (in particular on the

gate/channel area ratio) and the gate electrode material (metal or polymer) can be used to tune

the device response. The effect of the electrolyte on device response was evaluated studying

transistors in aqueous electrolytes and ionic liquids. The detection limit of OECTs based sensors

having different geometry, was analyzed for glucose and hydrogen peroxide (a species involved

in glucose sensing). [1] J. M. Leger, Adv. Mater. 2008, 20, 837. [2] M. J. Panzer, C. D. Frisbie,

Adv. Mater. 2008, 20, 3177. [3] D. Vanmaekelberg, A. J. Houtepen, J. J. Kelly, Electrochem.

Acta 2007, 53, 1140. [4] D. A. Bernards, G. G. Malliaras, Adv. Funct. Mater. 2007, 17, 3538. [5]

D.A. Bernards, G. G., Malliaras, D. J. Macaya, M. Nikolu, J. A. DeFranco, S. Takamatsu, G. G.

Malliaras, J. Mater. Chem. 2008, 18, 116.

10:30 AM *B8.6 Electronic Structure of Organic Heterointerfaces. Henning Sirringhaus, University of

Cambridge, Cambridge, United Kingdom.

Charge injection at metal-semiconductor interfaces as well as transport in the accumulation layer

of organic FETs is critically determined by the electronic structure at these interfaces. In this

presentation we will review our current understanding of the factors that determine the molecular

structure, energetic disorder and polaronic relaxation processes at interfaces and discuss recent

experiments that have yielded information about such processes with high interfacial sensitivity.

A thorough understanding of interfacial electronic structure is needed in order to identify the

factors that limit performance of state-of-the-art organic FETs.

11:00 AM B8.7

Morphology and Energy Levels in Conjugated Polymers: A Theoretical View on P3HT. Georg Heimel and Juergen P Rabe; Insitut für Physik, Humboldt-Universität zu Berlin, Berlin,

Germany.

In the field of organic electronics, π-conjugated polymers bear great promise for solution-

processible, flexible applications such as organic light-emitting devices (OLEDs), organic field-

effect transistors (OFETs), or organic photovoltaics. In addition to the bulk material properties of

the active organic part, the interfaces between organic and inorganic components are well

acknowledged to be of paramount importance for device performance and functionality. Most

importantly, the energetic position of the conducting states in the organic material crucially

impacts the energy barriers for charge injection into the device. These barriers not only limit the

overall injection rates but also give rise to often undesirably high onset voltages below which the

device remains inactive. Preparation conditions are know to crucially impact the thin-film

morphology of the prototypical π-conjugated polymer region-regular poly(3-hexylthiophene)

and, consequently, also the device characteristics of, e.g., OFETs based on this material (rr-

P3HT). In such devices, it has been shown that an edge-on configuration of highly ordered

polymer chains favorably impacts transistor performance while a face-on morphology proved

detrimental; in the former, the preferential direction for charge transport in these systems, the π-

stacking direction in co-facial polymer chains, is aligned with the direction of current flow in an

OFET device geometry. In our contribution, we present density-functional theory (DFT) based

band-structure calculations on highly ordered rr-P3HT monolayers of different morphology, i.e.,

edge-on, face-on, and intermediate regimes. Supported by electrostatic modeling, we find that

the backbone orientation importantly influences the energy-level positions in these thin films due

to intra-molecular surface dipoles. As a consequence, also the hole-injection barriers (HIB) can

be expected to depend critically on thin-film morphology. Our calculations suggest that the HIB

into rr-P3Ht films with edge-on morphology can be up to 0.5 eV lower than into films of face-on

orientation. In addition to underlining the importance of morphology control in polymer-based

organic electronic devices, understanding the impact of intra-molecular surface dipoles also

paves the way towards novel strategies for material design. To that end, we extend our

investigations towards rr-P3HT with end-fluorinated alkyl side-chains. Our calculations reveal

that, due to the strongly negative surface termination in such films, the energy levels in the

polymer could be lowered by as much as 1.5 eV in the edge-on configuration compared to face-

on. This scenario would lead to significantly reduced electron injection-barriers and, thus, favor

device operation involving negative charge-carriers. Consequently, our results imply that such

materials, together with improved control over morphology and electron traps, could potentially

serve as active organic components in n-type polymer OFETs.

11:15 AM B8.8 Influence of Contact Effects on the Switching Behavior of Organic Transistors Arne

Hoppe1, Dietmar Knipp

2, Benedikt Gburek

1, Marko Marinkovic

2 and Veit Wagner

1;

1Molecular

and Nanoelectronics Laboratory, Jacobs University Bremen, Bremen, Germany; 2Electronic

Devices and Nanophotonics Laboratory, Jacobs University Bremen, Bremen, Germany.

The influence of contact effects on the switching behavior of organic transistors was studied.

High switching frequencies can be achieved by using short channel transistors with high charge

carrier mobilities. However, most of the short channel transistors exhibit a distinct drop of the

device charge carrier mobility. The reduced device mobility is caused by the influence of contact

effects. In this study the influence of the drain/source contacts and the device geometry on the

switching frequency of high mobility dihexyl-7-thiophene (DH7T) oligothiophene thin film

transistors was investigated. The transistors were realized with channel lengths ranging from 50

nm to 50 μm. The transistors exhibit high charge charier mobilities of 0.1 cm2/Vs and high

switching frequencies between 200 kHz and 2 MHz at low operating voltages of 5 V. The

maximum of the switching frequency is limited by the specific contact resistance and the overlap

capacitance between drain/source and gate electrodes. The normalized contact resistance of the

oligothiophene transistor was determined to be 3.5 kΩcm. An upper limit of the switching

frequency for organic thin film transistors was derived, which is determined by the specific

contact resistance between the drain and source electrodes and the organic channel material. The

charge carrier mobility does not affect the upper limit of the switching frequency. Different

strategies will be discussed to maximize the cut-off frequency.

11:30 AM B8.9

Polymer-Small Molecule Semiconductor Blends for Integrated Circuits with a 712 ns

Single Stage Delay. Jeremy Smith1, Richard Hamilton

2, Donal D Bradley

1, Iain McCulloch

2,

Martin Heeney3, Dago de Leeuw

4, John E Anthony

5 and Thomas D Anthopoulos

1;

1Physics,

Imperial College London, London, United Kingdom; 2Chemistry, Imperial College London,

London, United Kingdom; 3Materials, Queen Mary University of London, London, United

Kingdom; 4Philips High-Tech Campus, Eindhoven, Netherlands;

5Chemistry, University of

Kentucky, Lexington, Kentucky.

Solution processed semiconducting blends of acene based small molecules with amorphous

polymers provide a means of combining the high mobility of the former with the ease of

processing and thin-film uniformity of the latter. We have demonstrated organic field-effect

transistor (OFET) mobilities of up to 2.4 cm2/Vs with low device-to-device variation[1]. This

level of performance qualifies the system for use in integrated circuits with the possibility of

future applications such as organic radio frequency identification tags and electronic paper. High

mobility and optimal device architecture is required to attain a high operation frequency. In this

work logic inverters and ring oscillators have been used to evaluate the dynamic performance of

difluorinated triethylsilylethynyl anthradithiophene blended with a poly(triarylamine) and to

demonstrate that the technology can be transferred from single transistors to multi-transistor

circuits such as ring oscillators. The latter are often used as prototype circuits for the evaluation

of new semiconductors and deposition processes. They allow the determination of an inverter

stage delay, t (i.e. the switching time between a high and a low state) using the equation

t=1/(2nf), where n is the number of inverting stages and f the oscillation frequency. This delay is

proportional to the time taken for charging or discharging at the inverter input/output node and is

in turn determined by the charge carrier mobility, the supply voltage used and the transistor

channel length. Stage delay thus provides a useful indication of the speed of integrated circuit

operation. Ring oscillators were measured with a variety of design rules, specifically channel

lengths, and over a range of supply voltages resulting in the expected trends for oscillation

frequency. To obtain high performance circuits, an understanding of these effects and the ability

to control film morphology within the transistor channels are very important. The highest

mobility transistors employ a top-gate design due to vertical phase separation of the acene

molecule to the semiconductor-dielectric interface[1]. However, a bottom-gate, bottom-contact

circuit architecture is much easier to construct[2]. Hence surface energies and thin film formation

become critical to maintaining a high enough mobility for low t devices. By using short channel

transistors employing gold source-drain contacts functionalised with a self-assembled monolayer

(SAM) as hole injecting electrodes, we have fabricated 7-stage ring oscillators with a single

stage delay of less than 800 ns which, to our knowledge, is one of the fastest organic integrated

circuits to date[3]. This level of performance makes the polymer-small molecule blend an

excellent candidate for use in a wide range of low-end organic electronic applications. [1]

Hamilton, R. et al., Adv. Mater., (2008) in press. [2] Gelinck, G. H. et al., Nature Mater. 3 (2004)

106. [3] Smith, J. et al., (2008) submitted.

11:45 AM B8.10

Charge Transport in Organic Transistors Based on Interconnected Polythiophene

Nanofibrillar Network Embedded in Insulating Polymer Kilwon Cho1,2

, Longzhen Qiu1,

Jung Ah Lim1, Juhyun Kim

2 and Wi Hyoung Lee

1;

1Chemical Engineering, Pohang University of

Science and Technology, Pohang, Korea, South; 2School of Environmental Science and

Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk, Korea, South.

Semiconducting and insulating polymer blends has brought a new way to tune the electronic

properties of devices and combine the electronic properties of semiconducting polymers with the

low-cost and excellent mechanical characteristics of insulating polymers. However, the presence

of the insulating component tends to degrade the device performance by diluting the current

density of the film. In this study, we demonstrated that organic field-effect transistors (OFETs)

with excellent electronic performance can be achieved in blends of poly(3-hexylthiophene)

(P3HT) and amorphous polystyrene (a-PS) at very low P3HT content by controlling the

solubility of solvent using a marginal solvent or solvent mixture. The controlled solubility allows

the P3HT chain in blends to form highly crystalline, interconnected nanowire networks

embedded in PS matrix. In particular, the precise control of solvent composition of P3HT/PS

blend by using solvent mixture causes the inkjet-printed blend deposits to form a uniform film-

thickness and highly ordered P3HT nanowire structure in PS matrix. The transistors based on

inkjet-printed P3HT/PS blends exhibits the improved electrical performance even though a

single droplet deposit with super-low content of P3HT (tens of pictogram) is used as an active

material. This finding may offer a excellent route to the direct-write fabrication of OFETs with

low semiconductor cost, high environmental stability, and good mechanical properties.

Acknowledgement. This work was supported by a grant (F0004021-2008-31) from the

Information Display R&D Center under the 21st Century Frontier R&D Program, and Creative

Research Initiative(CRI)-Acceleration Research (R17-2008-029-01000-0).

SESSION B9: Photovoltaics

Chairs: Emil List and Henning Sirringhaus

Thursday Afternoon, April 16, 2009

Room 2001 (Moscone West)

1:30 PM B9.1

Charge Extraction in Planar Heterojunction Organic Photovoltaics. Cody W Schlenker and

Mark E Thompson; Chemistry, University of Southern California, Los Angeles, California.

The buffer layer between the acceptor and cathode in multi-heterojunction organic photovoltaic

(PV) devices has the potential to act as powerful light management tool. However, conventional

buffer materials rely on metal mediated charge transport that is generally limited to ~10 nm,

precluding their use as versatile optical spacers. The complex tris(β-diketonato)ruthenium(III)

has shown promising thickness tolerance resulting from reciprocal carrier transport in the

archetypical device anode/CuPC/C60/buffer/cathode. We examined the energetic constraints on

reciprocal carrier transport in the following series of 6 ruthenium diketonate complexes: 1,3-CH3

(acac); 1,3-C6H5; 1-CF2H,3-C6H5; 1-CF3,3-C4H3S; 1-CF3,3-C10H7; 1-CF3,3-C6H5. These

compounds were selected to adjust the carrier injection barriers as their oxidation and reduction

potentials vary by 680 mV and 830 mV respectively. The current density-voltage (J-V)

characteristics of PV devices were recorded in the dark, and under both white and

monochromatic illumination. The fill factor and maximum power output correlate with the

oxidation potential of the Ru complex. Due to concavity changes in the J-V trace, we attribute

this to an interfacial contact resistance arising from an increased hole injection barrier at the

buffer/cathode interface. Charge carrier injection, energy level alignment, and electrical contacts

to organic semiconductors will be discussed, as well as prospects for further study.

1:45 PM B9.2

Energy Level and Morphology Optimization of Small Band Gap Polymers for

Photovoltaics. Arjan Pieter Zoombelt1,2

, Jan Gilot1 and Rene Janssen

1,2;

1Eindhoven University

of Technology, Eindhoven, Netherlands; 2Dutch Polymer Institute, Eindhoven, Netherlands.

Many small band gap polymers have been synthesized over the past few years to be utilized in

bulk heterojunction solar cells. The small band gap, in the range of 1.8-1.3 eV, would lead to an

improved overlap of the polymer absorption with the solar emission spectrum, which peaks

around 700 nm (1.77 eV), leading to an increase of absorbed photons and therefore would

enhance the efficiency of solar cells. A successful and flexible strategy to achieve small band gap

conjugated polymers involves the alternation of electron-rich (Donor) and electron-deficient

(Acceptor) units in the polymer chain. The chemical nature of the building blocks and side

chains determines energy level positions. These need to be judiciously positioned with respect to

those of phenyl-C61 butyric acid methyl ester (PCBM), the most commonly used acceptor, to

enable efficient charge separation and maintain the highest possible open-circuit voltage (Voc).

Furthermore, the polymer‟s solubility, molecular weight, crystallization behavior, and miscibility

with the acceptor are key factors in determining photovoltaic performance. A series of new

conjugated polymers will be presented with band gaps ranging from 1.8 to 1.3 eV and energy

levels that are well matched for optimal performance. The focus will be on the effect of side

chain position, head-to-head and head-to-tail coupling, on Voc and short circuit currents (Jsc)

and the influence of solubility and aggregation behavior upon the processing conditions of the

active layer. We will show that a more planar backbone results in a lower Voc, but increases Jsc

due to a slightly improved absorption and hole mobility. In addition, we will present the

optimization of morphology of the active layer for a variety of polymers and show how different

polymers having a different solubility require distinct processing conditions.

2:00 PM B9.3

A Systematic Approach to the Design and Synthesis of New Acceptors for Organic

Photovoltaics. John E. Anthony1, Ying Shu

1, Sean Parkin

1, Yee-Fun Lim

2 and George G

Malliaras2;

1Chemistry, University of Kentucky, Lexington, Kentucky;

2Materials Science and

Engineering, Cornell University, Ithaca, New York.

High-performance organic photovoltaic reports are dominated by bulk-heterojunction structures

where semiconducting polymers are blended with the fullerene derivative PCBM (or higher-

order fullerene derivatives of this molecule). We wondered whether the fullerene-based acceptor

could be modified to yield improved open-circuit voltage and short-circuit current. Using an

approach similar to that used to engineer the solid-state order of pentacene, we have found a

simple route to a series of crystalline fullerene derivatives that can be prepared quickly and in

high yield. Our initial studies have shown that changes in the aromatic portion of the substituent

on the fullerene can yield significant changes in open-circuit voltage, arising from changes to the

fullerene LUMO arising from close contact with these pendant aromatic groups. Alternatively,

changes to the hydrocarbon portion of the substituent on the fullerene lead to alteration of the

crystal packing of the fullerene, which profoundly influences the short-circuit current of these

devices. The ability to independently tune both voltage and current is allowing us to explore

wide substitution space in preparing design rules for this class of fullerene-based acceptors. The

high energy requirements for fullerene synthesis may make these materials less desirable as

components of “green” energy sources. We are concurrently exploring alternative acceptors

based on nitrile-functionalized pentacenes. Here again, functionalization can be used to tune both

photovoltage and photocurrent, and our current best-performing material yields power

conversion efficiency > 1% in photovoltaic cells using poly(3-hexylthiophene) as donor. Our

progress in this material class will also be discussed.

2:15 PM B9.4 Morphological Model of Polymer:Fullerene Solar Cells. Klara Maturova

1, Martijn Kemerink

1,

Svetlana S van Bavel2 and Rene A.J. Janssen

1;

1Dept. of Applied Physics, Eindhoven University

of Technology, Eindhvoen, Netherlands; 2Laboratory of Polymer Technology, Eindhoven

University of Technology, Eindhoven, Netherlands.

Research on organic bulk heterojunction solar cells, which are promising candidates for low cost

energy harvesting, leads to yearly improvements in efficiency. Further improvement requires

better understanding of limiting factors of current devices. It is widely accepted that for

polymer:polymer or polymer:fullerene solar cells the morphology and phase separation are of

high importance. Most of the current device models treat the organic solar cells as homogeneous

devices. Moreover, it is commonly accepted that field dependent exciton dissociation is

responsible for features in high field regime of I-V curves. Here we present results from 1D as

well as 2D modeling of organic solar cells which are based on solving the coupled drift-

diffusion, continuity and Poisson equations. We are able to predict the I-V curves in the

operational and the high field regime. We have chosen three systems: MDMO-PPV:PCBM,

P3HT:PCBM and PF10TBT:PCBM, which represent the improvement in the field of organic

solar cells in going from an efficiency of 1% to 4%. Depending on spincoating conditions,

solvent and composition, and thus on morphology, two distinct situations can be distinguished:

one that can be described by a 1D model, i.e. one where phase separation is irrelevant and one

that requires a 2D model, i.e. inclusion of phase separation. A typical representative of a 2D

structure is MDMO-PPV:PCBM spincoated from toluene on glass/ITO/PEDOT:PSS substrates It

has a coarse phase separation consisting of 500 nm large PCBM clusters surrounded by a blend

of MDMO-PPV and PCBM. We have found that in this type of systems the short circuit current

Jsc and the characteristic linear part at intermediate bias in a double-log I-V curve are

determined by the length scale of the phase separation. The latter feature was previously

assigned to field-dependent exciton dissociation. The coarse phase separation and high electron

mobility in the PCBM cause that lateral charge transport at low fields remarkably contributes to

device current. Also the unannealed 1:1 (mass ratio) P3HT:PCBM and 1:4 PF10TBT:PCBM

cells fall in this 2D category. On the other hand, the annealed 1:1 P3HT:PCBM and the

PF10TBT:PCBM in weight ratio 4:1 (both spincoated from chlorobenzene) have very fine phase

separation and are found to behave mostly 1D-like. P3HT:PCBM after annealing at 130°C has

20 nm large nanocrystals of P3HT surrounded by the blend of P3HT and PCBM, and can be

modeled by a 1D model. However to be able to determine the Jsc 2D model has to applied.

2:30 PM B9.5

Polymer Solar Cells using Self-Assembled Monolayers Modified Metal oxide/Metal as

Electron Collecting Electrode Hin-Lap Yip, Steve K. Hau, Hong Ma and Alex K.-Y. Jen;

Materials Science and Engineering, University of Washington, Seattle, Washington.

For efficient electron collection in polymer solar cells, thin films of low work-function metals

such as Ca/Al and LiF/Al are commonly used as the cathode. However, the performance of those

devices is limited by their poor stability due to the vulnerable contact between the polymeric

layer and the reactive metals. Here a simple method was developed to tune the interface of the

cathode in polymer solar cells. This was achieved by inserting a layer of metal oxide/self-

assembled monolayer (SAM) between the active poly(3-hexylthiophene) (P3HT) : [6,6]-phenyl-

C61 butyric acid methyl ester (PCBM) bulk-heterojunction film and the top metal cathode. We

found that the device performance could be significantly altered depending on the magnitude and

direction of dipole, and chemical bonding between the SAM and metals. With appropriate choice

of SAMs, devices showed dramatically improved efficiencies and even high work-function

metals such as Ag and Au could be used as electron collecting electrodes. This finding provides

an efficient method for interface engineering in organic-based optoelectronic devices.

2:45 PM B9.6 Charge-transfer Excitons in Strongly Coupled Supramolecular Semiconductors. Carlos

Silva, Department of Physics, Université de Montréal, Montreal, Quebec, Canada.

Time-resolved and temperature-dependent photoluminescence measurements on one-

dimensional sexithiophene lattices reveal intrinsic branching of photoexcitations to two distinct

species: self-trapped excitons and dark charge-transfer excitons (CTX; 5% yield), with radii

spanning 2-3 sites. The significant CTX yield results from the strong charge-transfer character of

the Frenkel exciton band due to the large free exciton bandwidth (∼ 400 meV) in these

supramolecular nanostructures.

3:30 PM B9.7

Dye Sensitized Solar Cells Fabricated Using Transparent Vertically Aligned Titania

Nanotube Arrays up to 18 µm in Length Grown on FTO Coated Glass. Maggie Paulose1,

Oomman K Varghese1 and Craig A Grimes

2,1;

1Materials Research Institute, The Pennsylvania

State University, University Park, Pennsylvania; 2Department of Electrical Engineering, The

Pennsylvania State University, University Park, Pennsylvania.

Films comprised of a random interpenetrating network of titania nanoparticles, approximately

10-15 µm thick, have been the foundation of dye sensitized solar cells since the potential of such

films were revealed by Graetzel and co-workers. Our research efforts to obtain ordered nano-

architectures of titania with superior light transmission and charge transport characteristics

yielded highly ordered nanotube arrays on titanium foils up to several hundred microns in length.

However the opaque Ti substrate necessitates a reversal in the solar cell illumination geometry,

which limits the solar cell efficiency due to loss of light at the counter electrode and electrolyte.

Fabrication of transparent nanotube array films, several microns thick, on FTO-coated glass has

been difficult to achieve. Requirements include the necessity of a thick and well-adhered starting

Ti film on the FTO glass substrate, obtaining suitable transparency in the anodized film, and

achieving a film robust enough to withstand various thermal, physical and chemical processes

during the solar cell fabrication. We have successfully fabricated robust, highly ordered

vertically aligned transparent nanotube array films on FTO glass by anodizing titanium films of

up to ≈ 20 µm thickness sputter deposited onto FTO glass. The transparent films consist of

nanotube arrays up to about 18 µm in length. We will discuss the effects of anodization

chemistry as well as physical and chemical surface treatments on the performance of dye

sensitized solar cells fabricated using these transparent nanotube array films.

3:45 PM B9.8

Carbon Nanotube Based Near-Infrared Photodetectors with >1% External Quantum

Efficiency. Jeramy D. Zimmerman1, Michael S Arnold

2, Xin Xu

3, Christopher K Renshaw

4,

Christine M Austin1, Richard R Lunt

5 and Stephen R Forrest

1,2,6;

1Physics, University of

Michigan, Ann Arbor, Michigan; 2Electrical Engineering and Computer Science, University of

Michigan, Ann Arbor, Michigan; 3Electrical Engineering, Princeton, Princeton, New Jersey;

4Applied Physics, University of Michigan, Ann Arbor, Michigan;

5Chemical Engineering,

Princeton, Princeton, New Jersey; 6Materials Science and Engineering, University of Michigan,

Ann Arbor, Michigan.

Organic photodetectors have seen limited success in the near infrared (NIR) region where

detection has had limited success beyond 1000 nm.i Carbon nanotubes (CNT) hold great promise

for extending the NIR sensitivity because they can be readily produced with diameters that

absorb between 1000 and 2000 nm and have carrier mobilities exceeding 105 cm

2-V

-1s

-1.ii Due to

the inherent presence of metallic tubes and difficulties in processing, successful demonstrations

of photovoltaic action have been limited to single CNT devices.iii

The CNTs are wrapped in the

polymer MDMO-PPV. Films are then doctor bladed onto indium tin oxide coated glass, and

followed by deposition of a compound acceptor/cathode structure by vacuum thermal

evaporation. These devices show CNT based photovoltaic response up to wavelengths of 1450

nm with external quantum efficiencies of over 1.5% at 1155 nm and 1300nm. By adjusting the

processing conditions, dark current rectification ratios of over four orders of magnitude are

achieved. A spectrally resolved specific detectivity of >1010

cm-Hz1/2

W-1

from 400 nm to 1400

nm is realized. These devices significantly extend the detection capabilities of existing organics

into the NIR and indicate a promising new direction for CNT based optoelectronics. i R. Kroon

et al., Polym. Rev. 48, 531 (2008) ii T. Durkop et al., J. Phys.-Condes. Matter 16, R553 (2004)

iii

P. Avouris et al., Nat. Photonics 2, 341 (2008)

4:00 PM B9.9

Template-Assisted Fabrication of Free-Standing Nanorod Arrays of Semi-Conducting

Polymers for the Preparation of Organic Solar Cells. Niko Haberkorn and Patrick Theato;

Johannes Gutenberg University, Institute of Organic Chemistry, Mainz, Germany.

Arrays of free-standing nanorods are of great interest for the fabrication of high efficient bulk

heterojunction organic solar cells. The ideal structure that has been proposed for organic solar

cells is a bicontinuous and interpenetrating network of donor and acceptor phase, with the

interfacial distance being smaller than the exciton diffusion length in the polymer (~ 10-20 nm).

Such a nanostructured interdigitated network should result in an efficient exciton separation at

the interface between the donor and acceptor phase and the perculated pathways would ensure a

high mobility charge carrier transport to the electrodes. [1][2] In this study, we present a

template-assisted approach to pattern semi-conducting polymers to build up a photoactive layer

with a well-defined morphology that would fulfill the above mentioned requirements for

photovoltaic devices. Anodized aluminum oxide (AAO) membranes with a highly ordered

nanoporous structure were fabricated by a controlled anodization process and used as

templates.[3] [4] These membranes with pore diameters down to 20 nm and pore lengths of

several hundred nanometers were filled with cross-linkable triarylamines by a solution wetting

process. Thermal curing and selective etching of the AAO template resulted in free-standing

nanorod arrays of the cross-linked triarylamines attached to a conductive substrate. To overcome

aggregation and collapse of the nanorods after removal of the template, their aspect ratio was

optimized and a freeze-drying technique was applied after the etching step. Afterwards, an

electron-conducting material, e.g. perylene bisimide derivative, was used to fill the void between

the hole-conducting nanorods, which resulted in a bicontinuous photoactive layer with a large

donor-acceptor interface. The polymeric interpenetrating network was analyzed by cross-

sectional transmission electron microscopy (TEM). Further measurements concerning the

performance of organic photovoltaic devices that have been prepared by this template assisted

approach will be discussed. References [1] X. Yang, and J. Loos, Macromolecules, 2007, 40,

1354. [2] S. Günes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 2007, 10, 1324. [3] M.

Masuda, and K. Fukuda, Science, 1995, 268, 1466. [4] M. Steinhart, J. H. Wendorff, A. Greiner,

R. B. Wehrspohn, K. Nielsch, J. Schilling, J. Choi and U. Gosele, Science, 2002, 296, 1997.

4:15 PM B9.10

Vertical Phase-Separation Due to Differences in Surface Energies in Bulk Heterojunction

Polymer Solar Cells. Sarah R. Mednick2,1

, Anshuman Roy2, Ji Sun Moon

2,1, Sung Heum Park

2

and Alan J Heeger2;

1Materials Science & Engineering, University of California, Santa Barbara,

Santa Barbara, California; 2Center for Polymers and Organic Solids, University of California,

Santa Barbara, Santa Barbara, California.

The synthesis and testing of new photoactive polymers is steadily improving the light conversion

efficiencies of organic bulk heterojunction (BHJ) solar cells. Understanding the physical

interactions between the polymer donor material and the electron acceptor is critical in

controlling and optimizing the morphology of the blend. While interactions between the donor

and acceptor in the blend determine the scale and stability of lateral phase separation,

interactions between the constituents of the blend and the neighboring device layers are equally

important. In this work, we demonstrate that bulk heterojunction constituents in a polymer solar

cell, i.e. the electron donating material and the electron accepting material, tend to vertically

phase-separate due to differences in surface energies. Using a combination of cross-sectional

transmission electron microscopy (TEM), variable angle spectroscopic ellipsometry (VASE),

and a contact angle study, we probe the vertical phase separation in poly(3-hexylthiophene) :

[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and poly[N-9′-heptadecanyl-2,7-

carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] : [6,6]-phenyl-C71-butyric acid

methyl ester (PCDTBT:PC70BM). Finally, we demonstrate the relevance of vertical phase

separation in understanding the device level physics of bulk heterojunction polymer solar cells.

4:30 PM B9.11 Hybrid Organic/Quantum Dot Photodetector. Tim Osedach

1,3, Scott M Geyer

2, John C Ho

1,

Alexi C Arango1, Moungi G Bawendi

2 and Vladimir Bulovic

1;

1Department of Electrical and

Computer Engineering, MIT, Cambridge, Massachusetts; 2Department of Chemistry, MIT,

Cambridge, Massachusetts; 3School of Engineering and Applied Sciences, Harvard University,

Cambridge, Massachusetts.

We describe a heterojunction photodetector of lateral geometry that consists of molecular

organic and colloidal quantum dot (QD) films. The interface between the organic and QD films

creates a type-II heterojunction that dissociates photo-generated excitons. Carriers are collected

at lateral in-plane electrodes in the presence of an applied electric field on the order of 10^5

V/cm. In contrast to organic photodetectors (PDs) and photovoltaics (PVs) of the more familiar

sandwich geometry, this unique device structure allows for independent optimization of optical

absorption and charge transport through the selection of the heterojunction materials. Optical

sensitivity can be controlled via the selection and sizing of the colloidal QDs and electronic

transport is dominated by the choice of organic film. We present numerical modeling results to

elucidate device operation as well as experimental data characterizing performance for devices

consisting of CdSe QDs and a variety of organic hole-transporting materials including Spiro-

TPD, NPD and MEO-TPD. Current is found to follow Child's Law for space-charge-limited

conduction and high photon-to-electron conversion efficiencies are measured under illumination.

4:45 PM B9.12

Molecular Semiconductor Blends: Microstructure, Charge Carrier Transport and

Application in Photovoltaic Cells. Andreas Opitz1, Julia Wagner

1, Bernhard Ecker

1, Marcel

Goetzenbrugger1, Ulrich Hoermann

1, Markus Bronner

1, Michael Kraus

1, Wolfgang Bruetting

1,

Alexander Hinderhofer2 and Frank Schreiber

2;

1Institute of Physics, University of Augsburg,

Augsburg, Germany; 2Institute of Applied Physics, University of Tübingen, Tübingen, Germany.

Blends of organic electron and hole conductive materials are widely used for ambipolar charge

carrier transport and photovoltaic cells. Many investigations have reported an increase of the

solar cell efficiency by optimizing the balance between charge carrier transport in phase-

separated structures and exciton dissociation at the interface between these phases. Here we

show the implications of blending molecular materials for structural, optical and electrical

properties in two model systems. These are (i) fullerene C60 combined with copper-

phthalocyanine (CuPc) and (ii) CuPc in combination with fluorinated CuPc. The analysis of X-

ray diffraction measurements shows the formation of phase-separated nanocrystals for blends

from C60/CuPc and indicates the formation of mixed crystals for the CuPc/F16CuPc blends. The

absorption spectra for the nano-phase separated blends scale with the concentration of the

individual components whereas the absorption spectra for the mixed crystals show a decreasing

interaction between the F16CuPc molecules. The formation of mixed crystals is a new feature for

organic blends, which has not yet been explored in organic solar cells. Additionally the charge

carrier transport and the electronic structure were analyzed for the C60/CuPc blends. The

exponential decrease of the mobility by dilution of the respective transport material indicates that

percolation is a crucial feature in mixtures. Photoelectron spectroscopy measurements show that

mixing of the organic materials reduces the intermolecular gap between the highest occupied

molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor. The

C60/CuPc system was analyzed also in solar cells. The comparison between bilayered and

blended cells shows a higher open circuit voltage of bilayered cells, which is related to the higher

intermolecular gap in this system. Nevertheless, the blended solar cells reach higher short circuit

currents based on the larger donor/acceptor interface even though the mobility in the mixed

system is much lower. This indicates that other solar cell geometries might be required to

combine a high open circuit voltage and a high short circuit current.

SESSION B10: Poster Session II

Chairs: Saw-Wai Hla, Norbert Koch, Xiaoyang Zhu and Egbert Zojer

Thursday Evening, April 16, 2009

8:00 PM

Salon Level (Marriott)

B10.1

Micro-cavity Effect on Light Extraction Efficiency of Blue Phosphorescent Organic Light

Emitting Devices. Jaewon Lee, Neetu Chopra and Franky So; Dept of Materials Science and

Engineering, University of Florida, Gainesville, Florida.

Recently we reported high efficiency blue phosphorescent organic light emitting devices

(PHOLEDs) (49 cd/A, 23% EQE) by controlling the device charge balance. In this paper we

demonstrated a Iridium(III)bis [(4,6-di-fluorophenyl)-pyridinato-N,C2] picolinate (FIrpic) based

blue emitting micro-cavity PHOLEDs . Our results show that in addition to a slightly blue shift

with spectral narrowing resulting in a more saturated blue color, there is also an enhancement in

current efficiency. Micro-cavity blue PHOLEDs were fabricated on two different dielectric

mirror substrates: two-layer quarter wave stacks (2QWS) with reflectivity of 0.39, and 4-layer

quarter wave stacks (4QWS) with reflectivity of 0.7 at 475nm. The design of the dielectric stacks

is to maximize the micro-cavity effect at 475nm. The devices have the following structure: glass

substrate (1mm)/SiO2 (79nm)/TiO2 (48nm) or SiO2 (79nm)/TiO2 (48nm)/SiO2 (79nm)/TiO2

(48nm)/ITO (50nm)/ 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) (45nm)/mCP

(10nm)/ [1,4-bis(triphenylsilyl)benzene] (UGH-2) (20nm) doped with 20 wt % FIrpic/ tris[3-(3-

pyridyl)-mesityl]borane (3TPYMB) (40nm)/LiF (1nm)/Aluminum (100nm). Non-cavity devices

were also fabricated on ITO coated substrates without the SiO2/TiO2 dielectric stacks. We found

that the micro-cavity devices show enhancement in current efficiency (61 cd/A compared with

49 cd/A in non-cavity device) in addition to a more saturated blue color in micro-cavity devices

(CIE coordinates of x=0.12 and y=0.26 for the cavity device compared with x=0.15 and y=0.32

for non-cavity device). Furthermore, we also measured the amount of substrate guided modes

and found that the 4QWS cavity device has the strongest substrate modes compared with the

2QWS cavity device and the non-cavity device. Simulation results also confirmed the

experimental data and the substrate modes are coming from the secondary cavity mode at higher

angles.

B10.2

High Efficiency Blue Phosphorescent OLEDs by Tuning Charge Transport in the Emitting

Layer. Neetu Chopra, Jaewon Lee and Franky So; Dept of Materials Science and Engineering,

University of Florida, Gainesville, Florida.

Organic light-emitting diode (OLED) is a promising candidate for solid state lighting and display

applications. While high efficiency green phosphorescent OLEDs (PHOLEDs) have been

demonstrated, the efficiency of blue PHOLED is still low. Hereby, we demonstrate a high

efficiency iridium (III) bis[(4,6-di-fluorophenyl)-pyridinate-N,C2`]picolinate (FIrpic) PHOLED

by enhancing the carrier transport properties of the emitting layer. Charge transport and balance

in OLEDs are very important for achieving high device efficiency. Apart from charge transport

and balance in the charge transporting layers, charge transport in the emitting layer also plays an

important role determining the device performance. In this study we investigate and tune the

charge transport in the emitting layer to achieve high efficiency PHOLEDs. Wide gap materials

are required as the host in blue PHOLEDs. For some wide band gap materials such as p-bis

(triphenylsilyly) benzene (UGH2), they are poor carrier transporters and electro-

phosphorescence occurs by charge trapping on dopant molecules. Thus, the carrier transport

properties of the emitting layer can be tuned by controlling the dopant concentration in the

UGH2 host. In this work, we have fabricated PHOLED devices with different FIrpic

concentrations and found that the carrier transport property of the emitting layer is a strong

function of FIrpic concentration. As the dopant concentration is increased, the current density of

the devices increases and the turn-on voltage decreases. Furthermore, we also found that the

device efficiency increases with increasing FIrpic concentration. A maximum external quantum

efficiency of 25.6% (corresponding to 56 cd/A current efficiency and 40 lm/W luminous

efficiency) was achieved with 20% FIrpic doping concentration. This is one of the highest

efficiencies reported for blue PHOLEDs. Further increase in doping concentration decreases the

device efficiency due to concentration quenching. In addition to OLED devices, single carrier

devices with different FIrpic concentrations were also fabricated and the device data confirmed

the enhanced carrier transport due to doping.

B10.3

Enhanced Performance in Organic Solar Cells using Thermally-evaporated Tungsten

Oxide Interlayer and its Application to ITO-free Organic Solar Cells. Seungchan Han1, Won

Suk Shin2, Myungsoo Seo

1, Dipti Gupta

1 and Seunghyup Yoo

1;

1Department of Electrical

Engineering and Computer Science, KAIST, Daejeon, Korea, South; 2Energy Materials Research

Center, Korea Research Institute on Chemical Technology (KRICT), Daejeon, Korea, South.

In line with the recent trend that various metal oxide layers are used as an interfacial buffer layer

in organic solar cells to enhance their photovoltaic (PV) performance [1,2] we explore the effect

of insertion of tungsten oxide (WO3) films on the performance of organic solar cells based on

poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM-60).

Experimental results show that the insertion of thermally evaporated WO3 layer in between ITO

and photoactive layers renders fill factor (FF) and open-circuit voltage (VOC) of P3HT:PCBM

cells get significantly improved from 0.60 and 0.441 V of reference cells to 0.69 and 0.600 V

with the short-circuit current density (JSC) virtually unchanged, resulting in a net improvement

in power conversion efficiency by +67%. X-ray diffraction results consistently indicate that

P3HT films grown on WO3 layers have a higher degree of ordering than those grown on ITO or

on PEDOT:PSS layers. Such difference in growth behavior and PV performance improvement

are discussed in conjunction with the contact-angle measurement which revealed the relatively

low surface energy of WO3 films when compared to those of ITO and PEDOT:PSS surfaces.

Finally, we demonstrate the versatility of WO3 interlayers by presenting ITO-free organic solar

cells that employ a multilayer electrode in which WO3 layers play a key role in terms of both

optical and electrical properties. References [1] M. D. Irwin, B. Buchholz, A. W. Hains, R. P. H.

Chang, and T. J. Marks, PNAS 105, 0711990105 (2008) [2] V. Shrotriya, G. Li, C.-W. Chu, and

Y. Yang, Appl. Phys. Lett. 88, 073508 (2006)

B10.4

A New Physical and Electrical Model of Capacitor Consisting of Organic Semiconductor

and Organic Dielectric.Seung-Hyeon Jeong and Chung-Kun Song; Electronics Engineering,

Dong-A University, Busan, Korea, South.

In this study, we proposed a new physical and electrical model of capacitor consisting of

pentacene organic semiconductor and PVP dielectric. The measured capacitances in the

depletion region for low and high frequency mode as well as the capacitance in accumulation

region for high frequency were identical to the theoretical values. However, for low frequency

the measured capacitance in accumulation region was 40 nF/cm2 much larger than 8.6 nF/cm2 of

the theoretical one. For SiO2 dielectric with pentacene this was not true. The accumulation

capacitance for low frequency was identical to dielectric capacitance of 95 nF/cm2, which

satisfied the theory of MOS capacitor. The reasons of the large accumulation capacitance for low

frequency can be estimated from the two causes. The first one is due to the residual OH- ions in

PVP, and the other on is due to the surface roughness of PVP dielectric layer. Both factors

contributed to the parallel circuit effect of interface capacitance Cit to dielectric capacitance

CPVP instead of serial circuit in the case of pentacene-PVP capacitor. This model was confirmed

by the capacitor with PVP dielectric containing more OH- ions and rougher surface, which

exhibited 55 nF/cm2 in accumulation region for low frequency mode. *This research was

supported by a grant(F0004020-2008-31) from Information Display R&D Center, one of 21st

Century Frontier R&D Program funded by the Ministry of Knowledge Economy of Korean

government.

B10.6

Vertically Phase-separated Conjugated Molecules / Dielectric Bilayer Structure for High

Performance Organic Transistors. Wi Hyoung Lee, Jung Ah Lim, Donghoon Kwak and

Kilwon Cho; Chemical Engineering, POSTECH, Pohang, Kyungbuk, Korea, South.

We report on the structural development and phase separation behavior of spin-cast

triethylsilylethynyl anthradithiophene (TES-ADT) and poly(methyl methacrylate) (PMMA)

blends and their application in field-effect transistors (FETs). The difference of surface energy

between TES-ADT and PMMA causes TES-ADT-the phase with lower surface energy-

segregating at the air-film surface after spin-casting. Furthermore, TES-ADT molecules in

blended films can move toward the air-film surface after solvent-vapor annealing by the

minimization of surface energy, and large crystals of TES-ADT are formed at the surface. The

use of these phase-separated blended film as semiconducting (TES-ADT) and dielectric

(PMMA) layers leads to high performance FETs (i.e., field-effect mobility as high as 0.6 cm2V-

1s-1 and nearly zero hysteresis) because conducting channel is formed at the molecular interface

of phase-separated TES-ADT crystals and PMMA dielectric in a one step-process. In addition,

all organic FETs onto flexible substrates using phase-separated films of TES-ADT and PMMA

are successfully demonstrated using all solution process. Acknowledgement. This work was

supported by a grant (F0004021-2008-31) from the Information Display R&D Center under the

21st Century Frontier R&D Program, Creative Research Initiative(CRI)-Acceleration Research

(R17-2008-029-01000-0), and a Grant (RTI04-01-04) from the Regional Technology Innovation

Program of the MOCIE.

B10.7

Control of Morphology and Crystalline Microstructure of Inkjet-Printed Functionalized

Pentacene via Evaporation-Induced Flows in a Drying Droplet Jung Ah Lim, Wi Hyoung

Lee, Donghoon Kwak and Kilwon Cho; Chemical Engineering, POSTECH, Pohang, Korea,

South.

The evaporation of inkjet-printed droplets on solid surfaces has received special attention as a

key technology for controlling the morphology of dried deposits. The evaporation behavior of

droplets is a function of the liquid composition of the ink, the surface properties of substrate, and

the environmental conditions (i.e., temperature, moisture, and ambient pressure). Since the

surface properties of gate dielectrics in organic transistors significantly influence the

performance of the devices, understanding how the surface wettability of the dielectric layer

influences the evaporation behavior and the distribution of molecules is very important in inkjet

printing of organic semiconductors for the fabrication of organic transistors. In this study, we

systematically studied the self-organization phenomena of an inkjet-printed organic

semiconductor on a dielectric surface with controlled surface wettability through self-assembled

monolayers (SAMs). We used 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS_PEN), which

is a promising organic semiconductor due to its solution processability, and has a significantly

greater π-orbital overlap and a smaller interplanar spacing than un-substituted pentacene. As a

result, the different types of evaporation, induced by the various surface wettabilities of the

dielectric substrates, lead to significantly different morphologies and crystalline microstructures

of the deposits. Interestingly, self-aligned TIPS_PEN crystals with highly ordered crystalline

structure were successfully produced on the hydrophilic surface when the contact line was

pinned upon drying. Our study indicates that a control of both contact line dynamics and the

evaporation behavior of inkjet-printed droplets on dielectric surfaces could become a key

technology for inkjet printing of organic semiconductors. Acknowledgement. This work was

supported by a grant (F0004021-2008-31) from the Information Display R&D Center under the

21st Century Frontier R&D Program, Creative Research Initiative(CRI)-Acceleration Research

(R17-2008-029-01000-0), and the ERC Program of MOST/KOSEF (R11-2003-006-06004-0).

B10.8

Effect of the Phase States of Self-Assembled Monolayers on Pentacene Growth and Thin-

Film Transistor Characteristics Hwa Sung Lee, Do Hwan Kim, Jeong Ho Cho and Kilwon

Cho; Chemical Engineering, POSTEH, Pohang, Korea, South.

To investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers

(SAMs) on the growth mode of pentacene films and the performance of organic thin-film

transistors (OTFTs), we deposited pentacene molecules on SAMs of octadecyltrichlorosilane

(ODTS) with different alkyl-chain orientations at various substrate temperatures (30, 60, and

90°C). We found that the SAM phase state played an important role in both cases. Pentacene

films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a

better interconnectivity between the pentacene domains, which directly serves to enhance the

field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in

crystallinity and field-effect mobility between pentacene films grown on ordered and disordered

substrates increased with increasing substrate temperature. These results can be possibly

explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer,

and (2) the temperature-dependent alkyl chain mobility of the ODTS monolayers.

Acknowledgement. This work was supported by a grant (F0004021-2008-31) from the

Information Display R&D Center under the 21st Century Frontier R&D Program, Creative

Research Initiative(CRI)-Acceleration Research (R17-2008-029-01000-0), and a Grant (RTI04-

01-04) from the Regional Technology Innovation Program of the MOCIE.

B10.9

Tuning the Threshold Voltage in Organic Field Effect Transistors by Space Charge

Polarization of Gate Dielectrics Heisuke Sakai, Koudai Konno and Hideyuki Murata; Japan

Advanced Institute of Science and Technology, Nomi, Japan.

Studies of gate dielectrics in organic field effect transistors (OFETs) have been attractive

because the electric properties of OFETs are susceptibly affected by the choice of the gate

dielectrics. Here, we demonstrate a tunable threshold voltage in an organic field effect transistor

(OFET) using an ion-dispersed gate dielectrics. By applying external electric field (Vex) to the

gate dielectrics, the dispersed ions in the gate dielectrics are separated by electrophoresis and

form space charge polarization. The drain current of the OFET increased over 1.9 times and the

threshold voltage (Vth) decreased 22 V (from -35.1 V to -13.1 V). The shift direction of Vth was

easily tuned by the polarity of the external voltage. The dielectric permittivity of the gate

dielectrics and mobility of the active layer were unchanged after the polarization of the gate

dielectrics. The UV-VIS differential absorption spectra of the OFETs indicate that there is no

chemical doping in the active layer of the OFETs. These results indicated the shifts of threshold

voltages were originated from the polarization of gate dielectrics.

B10.10 Spin Transport and Magnetoresistance in Organic Semiconductors. Mohammad Yunus

1,

Paul P Ruden1 and Darryl L Smith

2;

1Electrical and Compueter Engineering, University of

Minnesota, Minneapolis, Minnesota; 2Los Alamos National Laboratory, Los Alamos, New

Mexico.

Small molecules and polymers of π-conjugated organic semiconductors are of interest for the

fabrication of spintronic devices, primarily because the charge carrier spin relaxation times are

expected to be long and integration with extremely spin polarized (half-metallic) materials is

feasible. Half-metallic contacts (or ferromagnetic contacts with spin selective tunnel barriers) can

inject spin polarized charge carriers into organic semiconductors. Detection of the resulting spin

polarization of the current is, in principle, possible through the measurement of the magneto-

resistance of a spin valve device. Indeed, evidence for large MR effects in organic spin valves

has been reported in the literature.[1,2] However, the absence of MR, spin injection, and

transport in organic semiconductors has also been reported.[3] In this work, we model spin

injection, transport, and magneto-resistance (MR) in structures consisting of an organic

semiconductor layer sandwiched between two ferromagnetic contacts. We explore the different

regimes of diffusion dominated and drift dominated current, the effects of contact polarization

ranging from that of conventional ferromagnetic metals to half-metals, and the effectiveness of

spin polarized tunnel injection and extraction for the observation of MR. Carrier transport in

organic semiconductors is modeled with spin dependent device transport equations in drift-

diffusion approximation. We find that half-metallic contacts can inject strongly polarized charge

carriers, but that the resulting spin current does not necessarily manifest itself in a measureable

MR. Small MR effects may be observable at low bias. Furthermore, even in the case of

conventional ferromagnetic metal contacts, spin injection can be greatly enhanced if (spin

dependent) tunneling is the limiting process for carrier injection. However, the spin current by

itself does not give rise to measurable MR. In order to obtain a strong MR effect, we find that a

second tunnel barrier at the extracting contact is necessary. * This work was supported in part by

NSF-ECCS. [1] Z. H. Xiong, Di Wu, Z. Valy Vardeny, and J. Shi, Nature 427, 821 (2004). [2] S.

Majumdar, H. Huhtinen, H. S. Majumdar, R. Laiho, and R. Österbacka, J. Appl. Phys. 104,

033910 (2008). [3] J. S. Jiang, J. E. Pearson, and S. D. Bader, Phys. Rev. B 77, 035303 (2008).

B10.11

Novel Nanocomposites of Covalently Bonded Multi-Wall Carbon Nanotubes to Conducting

Polymers Yeong Joon Kim1, Changhyun Park

1 and Jae H Song

2;

1Chemistry, Chungnam

National University, Daejeon, Korea, South; 2Chemistry, Sunchon National University, Sunchon,

Korea, South.

We describe here a synthesis of novel nanocomposites of covalently bonded multi-wall carbon

nanotubes (MW-CNT) to conducting polymers such as polypyrrole. The surface modification of

MW-CNT was done by the reaction with bis(2-picolyl)amine after oxidation. The Fe(III) ions

were able to coordinate to the surface functionalized MW-CNT and were used as in-situ catalysts

for the polymerization of pyrrole. According to TEM images, polymers cover the MW-CNT very

uniformly and the width of resulting nanocomposite can be controlled by the reaction conditions.

We will discuss the physical properties of novel nanocoposites.

B10.12 Self-assembled Nanostructured Molecular Memory Hyoyoung Lee, ETRI, Daejeon, Korea,

South.

For the realization of nano-structured molecular monolayer devices, it is essential to synthesize

and characterize active switching and memory materials that have hysteretic I-V characteristics

in the solid state, develop nano-sized conducting organic electrodes and patterning of active

memory materials on the nano-sized array electrodes. In this presentation, we like to introduce

thiol-terminated metal (Ru, Co, and Fe)-terpyridine, metal-to-ligand charge transfer (MLCT)

complexes for molecular switches and memory devices. Bis(terpyridine)-transition metal

complexes exhibit superior chemical and electronic stability toward redox reactions due to their

octahedral configuration in coordination.[1] We like to also introduce how to improve molecular

device yields using self-assembly technique.[2] For patterning-of organic electrodes, we like to

introduce a self-assembly process for an immobilization of the conducting polymer materials.[2]

Finally we like to present nano-patterning for molecular memory array devices using

nanoimprint lithography.[3] References 1. Seo, K., Konchenko, A. V., Lee, J., Bang, G. S., Lee,

H. Molecular Conductance Switch-On of Single Ruthenium Complex Molecules. Journal of the

American Chemical Society, 130, 2553-2559 (2008). 2. Bang, G. S., Chang, H., Koo, J.-R., Lee

T., Advincula R., Lee, H., The high-fidelity formation of molecular junction device using a

thickness controlled bilayer architecture, Small, 4, 1399-1405 (2008). 3. Jung, M.-H., Lee, H.,

Patterning of conducting polymers using charged self-assembled monolayers, Langmuir, 24,

9825-9831 (2008).

B10.13

Material Design of Triarylamine-Based Amorphous Polymers for Organic Field-effect

Transistors Takeshi Yasuda1,2

, Takao Suzuki3, Mitsuru Takahashi

3 and Tetsuo Tsutsui

4;

1Innovative Materials Engineering Lab, National Institute for Materials Science, Tsukuba,

Ibaraki, Japan; 2PRESTO-JST, Tokyo, Japan;

3Nanyo Research Laboratories, Tosoh

Corporation, Syunan, Yamaguchi, Japan; 4Institute for Materials Chemistry and Engineering,

Kyushu University, Kasuga, Fukuoka, Japan.

There has been a recent surge of interest in organic semiconductors with high carrier mobility

through ordering at the molecular level, as in single-crystalline, polycrystalline, and liquid-

crystalline films. However, thin films of organic semiconductors with an amorphous state have

some merits compared to those with the ordered state mentioned above. Most importantly,

morphological effects, which often lead to a poor reproducibility for the performance of organic

field-effect transistors (OFETs) using polycrystalline or liquid-crystalline films, are not a major

consideration. In this study, we have designed and synthesized twenty eight kinds of

polytriarylamine (PTAA)1)

-based amorphous semiconductors for OFETs. The triarylamine-based

polymers which we developed in this study have different side-alkyl chain systems and

conjugated main-chain each. The molecular weights with polydispersity index, the glass

transition temperature, HOMO energy levels and the optical bandgap were estimated as the

physical and optical properties of polymers. Effects of the molecular design on the performance

of OFETs were also investigated. We fabricated OFETs having a top contact geometry using

amorphous polymers. The polymers as organic semiconductors were spin coated from toluene

solution onto a gate insulator of poly-chloro-p-xylylene (Parylene-C). A shadow mask was

attached onto the film to form Au source-drain electrodes. The field-effect mobility was

calculated from the saturation drain currents. All triarylamine-based polymers in this study

exhibited p-channel behaviors in OFETs and filed-effect hole mobilities were ranging from 10-6

to 10-3

cm2/Vs. For example, OFETs using a polymer with anthracene unit in the conjugated

main-chain showed a field-effect hole mobility of 6.1×10-6

cm2/Vs. On the other hand, a

compound having naphthalene unit in the conjugated main-chain exhibited 1.6×10-4

cm2/Vs. This

study on the field-effect hole mobilities in synthesized twenty eight kinds of triarylamine-based

polymers provides a good start point for molecular design of amorphous organic semiconductors

for OFETs. 1) J. Veres, S. D. Ogier, S. W. Leeming, D. C. Cupertino and S. M. Khaffaf, Adv.

Funct. Mater. 13 (2003) 199

B10.14

Fabrication of Pentacene Organic Thin Film Transistor with a Trapezoid-Shaped Active

Layer for Large Driving Drain Current. Min-Hoi Kim, Jin-Hyuk Bae, Won-Ho Kim, Chang-

Min Keum and Sin-Doo Lee; Electrical Engineering, Seoul National University, Seoul, Korea,

South.

Over the last decade, organic thin-film transistors (OTFTs) have attracted much attention for a

variety of low cost and large area electronic applications including chemical sensors, flat panel

displays, and smart cards. Although a pentacene based top contact OTFT shows good carrier

mobility comparable to an amorphous silicon based TFT, a drain current is still low for realizing

practical applications. In order to enhance the drain current in the top contact OTFT, reduction of

the channel length is one of simple ways since the drain current is inversely proportional to the

channel length. In principle, for a top contact OTFT, it is known to be difficult to fabricate a

short channel without deteriorating the organic active layer during patterning the channel by

lithography. In this work, we fabricate a top contact OTFT with trapezoid-shaped active layer for

a large drain current by using a short channel. Our approach is to use a trapezoid-shaped

structure of the organic active layer tilted from the substrate. The channel length in our

trapezoid-shaped OTFT is reduced as short as 1.5 um, which is 30 times smaller than the channel

length (typically, about 50 um) in a shadow mask-processed top contact OTFT with the help of a

ladder support. Note that the channel in our trapezoid-shaped OTFT is self-patterned by slanted

deposition without a shadow mask. Due to the short channel, a drain current per channel width in

our trapezoid-shaped OTFT is found to reach at about 100 uA/mm, which is at least 10 times

larger than that of the conventional top contact OTFT. Furthermore, our trapezoid-shaped OTFT

is operated at a drain voltage as low as -3V. In conclusion, we fabricated a short channel

trapezoid-shaped OTFT for a large drain current. By slanted deposition, the short channel length

of our trapezoid-shaped OTFT was naturally produced. Our concept of using a ladder support in

the OTFT would be a viable platform for realizing active driving elements in a variety of organic

electronic systems that require large drain currents in a low voltage regime.

B10.15

In Depth Investigations of State of the Art Molecules used to Tune Surface Properties. Gerold M. Rangger

1, Oliver T Hofmann

1, Anna M Track

1, Ferdinant Rissner

1, Lorenz

Romaner1,2

, Benjamin Broeker3, Ralf-Peter Blum

3, Norbert Koch

3 and Egbert Zojer

1;

1Institute

of Solid State Physics, University of Technology Graz, Graz, Austria; 2Department of Materials

Physics, Montanuniversität Leoben, Leoben, Austria; 3Institut für Physik, Humboldt-Universität

zu Berlin, Berlin, Germany.

The deposition of organic (sub)monolayers on metals is of particularly interest for modifying

surface properties, such as carrier injection barriers, for applications like chemical sensing, and

in the field of molecular electronics. Here, we present a joint theoretical and experimental study

of a number of prototypical acceptor molecules, such as, for example, F4TCNQ. In the

experiments, pronounced changes in the UPS spectra and an increase of the surface work

functions are observed. Density-functional theory based band-structure calculations as well as X-

ray standing wave experiments reveal a heavily distorted adsorbate structure. The modeling

provides also additional information on the intrinsic properties of the interfaces. For example,

employing certain projection techniques reveals that the experimentally observed (partial) filling

of the molecular π-LUMO is to a significant extent compensated by back-transfer from localized

sigma-states. With the help of the crystal orbital overlap population (COOP) we identify

especially favored substituents that act as docking groups and, thus, are of particular relevance

when designing new molecules for surface treatments. In addition to varying the adsorbate

molecules, the role of the substrate metal as well as the implications of different packing

densities are discussed relying on the modeling and, whenever possible, also on experimental

data. Support by the European Commission through the STREP project ICONTROL (EC-

STREP-033197) is gratefully acknowledged.

B10.16

Structure and Electronic Properties of Anthraceneselenole SAMs - a Joint Theoretical and

Experimental Study. Anna Maria Track1,2

, Ferdinand Rissner1, Daniel Kaefer

3, Asif Bashir

3,

Georg Heimel4, Gerold M Rangger

1, Oliver T Hofmann

1, Tomas Bucko

5, Lorenz Romaner

6,

Gregor Witte7 and Egbert Zojer

1;

1Institut für Festkörperphysik, Technische Universität Graz,

Graz, Austria; 2Institut für Physik, Karl-Franzes Universität Graz, Graz, Austria;

3Lehrstuhl für

Physikalische Chemie I, Ruhr-Universität Bochum, Bochum, Germany; 4Insitut für Physik,

Humbold-Universität zu Berlin, Berlin, Germany; 5Insitut für Physik, Universität Wien, Wien,

Austria; 6Lehrstuhl für Atomistic Modelling and Design of Materials, Montanuniversität Leoben,

Leoben, Austria; 7Molekulare Festkörperphysik, Philipps-Universität Marburg, Marburg,

Germany.

Self-assembled monolayers (SAMs) provide a simple, flexible, highly ordered and convenient

system to tailor and functionalize surface and interface properties of metals, metal oxides and

semiconductors. In particular, SAMs of π-conjugated organic molecules have attracted

significant interest in the field of molecular and organic electronics because of their considerable

conductivity and there ability to change the substrate work function. We performed density-

functional theory (DFT) based slab-type band-structure calculations - including geometry

optimization in internal coordinates - to gain deeper insight into the energetic, chemical, and

physical properties of the interface between a metallic substrate and a covalently bound organic

semiconductor. In particular, we studied SAMs of anthracene-2-selenol on Au(111), which have

been characterized in detail with various experimental methods including Scanning Tunneling

Microscopy (STM), Ultraviolet Photoemission Spectroscopy (UPS), and Low Energy Electron

Diffraction (LEED) [1]. Only the combination of experiments and theoretical calculations allows

an accurate understanding of the structural ordering of the SAM on the surface. After identifying

the correct structure, the electronic properties (such as the work-function modification, the

interfacial charge rearrangements and the energy-level alignment) can be provided theoretically.

The obtained values agree very well with the experimental data which, in turn, allow a

benchmarking of the employed theoretical methods. [1] A. Bashir, D. Käfer, J. Müller, C. Wöll,

A. Terfort, G. Witte, Angew. Chem. Int. Ed. 2008, 47, 5250-5252

B10.17

Turning Gold into Lead - Reducing the Work Function by Charge Transfer Monolayers. Oliver T. Hofmann, Gerold M Rangger, Ferdinand Rissner and Egbert Zojer; Institut für

Festkörperphysik, Technical University of Graz, Graz, Austria.

Application of (sub)monolayers of organic molecules on metal electrodes in order to tune the

effective work function has become a field of significant interest. Particular electron poor or rich

molecules grow charge transfer layers, which form an infinite dipole layer across the electrode,

thus altering the vacuum potential above the surface and hence its effective work function. In this

contribution, we investigate the interaction between the electron donor viologen and an Au(111)

surface. Depending on the coverage of the monolayer, the work function of the metal can be

theoretically lowered to the value of pristine lead (ca. 4.0 eV) or even that of Mg (ca. 3.7 eV).

Using density functional theory, an in-depth analysis of the electronic structure at the interface is

presented and compared to that of the prototypical donor tetrathiafulvalene (TTF). The work

function modification in both systems is found to be determined by a subtle interplay between

adsorption induced geometric distortions and electron donation from the respective molecular

HOMO to the metal. Unlike carbon monoxide or strong electron acceptors, such as F4TCNQ, on

noble metal surfaces, no significant back donation from the metal into the molecule is observed.

Furthermore, we show that work function modification can also be tuned using spacer elements,

e.g. in the form of voluminous groups enforcing a larger distance between molecule and metal.

With increasing distance the transferred charge decreases. Interestingly, we find that the net

interface dipole increases with increasing distance as will be shown here on the example of

viologen. This work has been supported by the EC through the ICONTROL project.

B10.18

Controllable Shifts in Threshold Voltage of Top-Gate Polymer Field-Effect Transistors and

its Application to Organic Transistor Memory Kang-Jun Baeg1,3

, Yong-Young Noh3,

Henning Sirringhaus2 and Dong-Yu Kim

1;

1Dept. of Materials Science and Engineering,

Gwangju Institute of Science and Technology, Gwangju, Korea, South; 2Department of Physics,

University of Cambridge, Cambridge, United Kingdom; 3Convergence Components and

Materials Research Laboratory, Electronics and Telecommunications Research Institute,

Daejeon, Korea, South.

Organic materials are attractive for many components of electronic devices such as active

semiconductor layers, insulator layers, and electrodes, due to a lot of unique advantages over

their inorganic counterparts. Although the organic materials are not currently suitable for

applications requiring high-end performances, their low-cost and low-temperature fabrication

using solution-based processing make them ideal for large-area, flexible, transparent, and

disposable applications. Moreover, organic non-volatile memories are another emerging class of

research fields based on the advantages of organic materials. A variety of approaches have been

progressed including cross-point type organic bistable devices and organic transistor-based

memories. Here we report a solution-processed polymer FET memory device with top-gate and

bottom-contact device configuration. With incorporation of gold nanoparticles (NPs) inside

double-layered polymer gate dielectrics, the threshold voltage (VTh) of polymer FET devices

could be reversibly and systemically controlled by application of external gate fields. This

reversible shifts in VTh was originated from charge trapping in Au NPs, and might be used as

organic transistor based non-volatile memory devices.

B10.19

Ion Irradiation Effects on The Transport Properties and Degradation Mechanisms of

Organic Field-Effect Transistors. Beatrice Fraboni1, Anna Cavallini

1, Piero Cosseddu

2,

Annalisa Bonfiglio2, Yongquiang Wang

3 and Michael Nastasi

3;

1Physics, University of Bologna,

Bologna, Italy; 2Electronic Engineering, University of Cagliari, Cagliari, Italy;

3Los Alamos

National Laboratory, Los Alamos, New Mexico.

The remarkable advances recently made in the development of organic semiconductor field

effect devices (OFET) prospect challenging applications in the field of low-cost flexible,

lightweight, and conformal electronics One of the interesting features of organic active layers is

their capability to respond to the environment chemical composition, but this unfortunately

opens up the issue of long-term stability of devices based on organic materials, as oxidation is

believed to be a major reason for early device failure. The focus of our research is to investigate

the potentiality of low energy ion irradiation in the reduction and control of the degradation of

the organic material due to the exposure to atmosphere (i.e. oxygen and water). We have studied

the effects of N irradiation on pentacene and sexithiophene based OFETs. The damage induced

by the ions can induce a rearrangement of the molecular alignment by breaking covalent bonds,

crosslinking the neighboring polymer chains and forming a hydrogen-depleted three dimensional

carbon network The strong molecular structure modification affects the carrier mobility and the

threshold voltage of the device, but since the electrical transport in OFETs occurs in a few active

molecular layers at the organic/dielectric interface, we have observed that a controlled damage

depth distribution preserves the functionality of the organic active layer. We have studied the

variation of the transport parameters as a function of the irradiation energy and dose by

characterizing the optical and electrical properties of the material by means of electrical transport

analyses and photocurrent spectroscopy. We have monitored the effectiveness of the low energy

irradiation process in providing an hermetic protection to the organic active layer from the

ambient conditions.

B10.20 PN-junction Diodes made of p-type Pentacene and n-type SnO2 Nanowires. Sung Chan

Park1, Daeil Kim

1, Seongmin Yee

2, Junghwan Huh

2, Gyu Tae Kim

2 and Jeong Sook Ha

1;

1Chemical and Biological Engineering, Korea University, Seoul, Korea, South;

2School of

Electrical Engineering, Korea University, Seoul, Korea, South.

PN-junctions are of great importance in modern electronic applications for achieving the

integrated circuits and understanding the device characteristics. In our presentation, we will show

the rectifying current-voltage curves and the temperature dependent diode characteristics. Hybrid

pn-junction devices consisted of p-type organic semiconductor (pentacene) and n-type inorganic

semiconductor (SnO2 nanowires). SnO2 nanowires can be easily synthesized or grown by VLS

mechanism in CVD, showing n-type semiconducting properties with a high mobility. SnO2

nanowire networks were formed by a selective growth on the Au catalyst with various growth

conditions. Onto the SnO2 nanowire networks, pentacene was deposited by thermal evaporation

with various thicknesses under 7×10^-7 Torr. Ti (20 nm) or Cr (20 nm)/Au (300 nm) electrode

and the 300 nm thickness of Au electrode were deposited on the SnO2 channels and pentacene

channels for Ohmic contacts, respectively, which was confirmed by Ohmic current-voltage

characteristics. Microscopic and chemical analyses were investigated for characterizing each

component of the hybrid pn-junctions. As the temperature decreased, the current levels reduced

following the diode equation of I=I0(exp(ηkT)-1) with the big ideality factor reaching 420,

which indicate the big surface states at the junction parts. Considering the large ideality factor,

the soft-reverse characteristics were not so significant with a good on/off ratio of 10^3 at ±60V.

The possible application of organic/inorganic hybrid pn-diodes will be discussed from the point

of the solar-cell or photo-detectors.

B10.21

Synthesis and Characterization of High Efficiency Copolymers via Effective Energy-

Transfer for Polymer Light-Emitting Diodes (PLEDs) Qinghua Zhao1, Shuang Zhang

1, Jong-

Won Park1, Sung Ouk Jung

1, Yun-Hi Kim

2 and Soon-Ki Kwon

1;

1school of materials science and

engineering, engineering research institute (ERI),, gyeongsang national university, Jin ju, Korea,

South; 2department of chemistry, gyeongsang national university, jin ju, Korea, South.

Polymer light-emitting diodes (PLEDs) have been the subject of intense research interests

recently due to their applications in large-area flat panel displays.1 To achieve highly efficient

PLED devices, charge (holes and electrons) injection and transport from both the anode and the

cathode should be balanced at the junction of the emitting layer to yield the maximum exciton

formation.2 Although some polymer LEDs have shown high-enough efficiencies and long

lifetimes, they are mainly multilayer LEDs, which involve complicated and difficult device

fabrication processes, or single layer LEDs based on polymer blends. So, it is important to

achieve conjugated polymers which have both functions of hole/electron affinity in the single

layer LEDs. The bipolar transport characteristics of polymers are important for high EL

efficiency, because the polymers offer good recombination sites for hole and electron charge

carriers.3 In this communication, we designed and synthesized a series of polymers with hole

transporting and electron transporting ability. Their photophysical and thermal properties would

be investigated and further compared. The PLED devices would be fabricated and discussed

further. These polymers are expected to obtain high efficient from PLEDs due to effective

energy transfer from large bandgap site to narrow bandgap site. [1] J. H. Burroughes, D. D. C.

Bradley, A. R Brown, R. N. Marks, K Mackay, R. H. Friend, P. L Burn, A. B. Holmes, Nature

1990, 347, 539. [2] F. Garten, A Hilberer, F. Cacialli, E. Essenlink, Y. van Dam, B. Schlatmann,

R. H. Friend, T. M. Klapwijk, G.. Hadziioannou, Adv Mater 1997, 9, 127. [3] N. Tamoto, C.

Adachi, K. Nagai, Chem. Mater. 1997, 9, 1077.

B10.22

Abstract Withdrawn

B10.23

Para-sexiphenyl Based OLED Devices Grown on a Pre-patterned Polymeric Substrate. Gerardo Hernandez-Sosa

1, Clemens Simbrunner

1, Thomas Hoefler

2, Armin Moser

3, Oliver

Werzer3, Birgit Kunert

3, Gregor Trimmel

2, Wolfgang Kern

4, Roland Resel

3 and Helmut Sitter

1;

1Institute for Semiconductors and Solid State Physics, Johannes Kepler University Linz, Linz,

Austria; 2Institute for Chemistry and Technology of Materials, Graz University of Technology,

Graz, Austria; 3Institute of Solid State Physics, Graz University of Technology, Graz, Austria;

4Institute of Chemistry of Polymeric Materials, Montanuniversität Leoben, Leoben, Austria.

During the last years organic devices became of increasing interest in many fields of electronics.

A bright future for organic light emitting devices (OLED) is expected as organics provide a wide

spectrum of molecules emitting at various photon energies [1]. Para-sexiphenyl represents an

organic molecule which has been established as active material for blue emitting OLEDs [2, 3].

As a matter of fact, a defined control of the substrate surface properties is a good way to improve

the quality of organic films. Surface properties like polarity, hydrophilicity or hydrophobicity

can be tuned to change the way the film is growing. These changes can lead to an improvement

on the intrinsic properties of the film and consequently have an impact on the performance of the

final device. Para-sexiphenyl (PSP) has been deposited by Hot Wall Epitaxy (HWE) on

poly(diphenyl bicyclo(2.2.1)hept-5-ene-2,3-dicarboxylate) (PPNB), a photosensitive polymer,

which has been pre-treated by UV illumination leading to a change of the surface polarity [4]. A

detailed analysis of the growth morphology as a function of substrate temperature, growth time

and UV illumination of the substrate has been performed by Atomic Force Microscopy. The

surface morphology and growth kinetics of PSP were found to be different depending on which

surface it was deposited, as prepared or UV exposed. Furthermore, the crystallographic

properties has been analysed by X-ray diffraction and Grazing Incidence X-ray Diffraction

(GIXD). The crystal structure and the degree of order of the deposited PSP thin films were

determined by specular scans and reciprocal space maps. Again a clear change on the structural

properties between the films deposited on the as prepared and pre-treated surfaces was observed.

Besides the film characterization we also report on OLEDs based on Para-sexiphenyl (PSP),

which has been deposited by HWE on a UV pre-treated ITO/PEDOT:PSS/PPNB substrate. A

complete quenching of the PSP electroluminescence is observed on the UV pre- illuminated

regions while on the as-prepared one the characteristic PSP emission is observed. Consequently,

this method has proven to be a very effective pre-patterning tool which could be fully compatible

with the actual OLED production technology. [1] M. Muccini, Nature materials 5 (2006), 605 [2]

G. Kranzelbinder et. al.,Synthetic Metals 102 (1999), 1073-1074 [3] A. Niko et. al., J. Appl.

Phys. 82 (1997), 4177 [4] T. Höfler et al., Polymer 48 (2007), 1930-1939

B10.24

Chemical and Electronic Properties of Self-assembled Organic Monolayers on SiC

Surfaces. I. D. Sharp, M. Hoeb, S. J Schoell, C. Diaz Alvarez, J. Howgate, M. S Brandt and M.

Stutzmann; Walter Schottky Institut, Technische Universität München, 85748 Garching,

Germany.

Wide bandgap materials are useful systems for the study of electronic processes at the organic-

inorganic hybrid interface due to the large energetic tunability of the semiconductor Fermi level.

Here, the group IV compound semiconductor silicon carbide (SiC) is used as a model substrate

for organic monolayer self-assembly by reaction with organosilanes and alkenes on both C- and

Si-polar surfaces. Comparison of the properties of both types of films on surfaces of both

polarities reveals the important role of interfacial chemistry and the related binding dipole on the

electronic properties at the organic-inorganic interface. In the well-known case of Si, organic

functionalization is typically achieved via hydrosilylation of H-terminated surfaces or

silanization of an intermediate OH-terminated thin oxide. In contrast, SiC surfaces are OH-

terminated following HF etching and are thus ideally suited for silanization reactions. We

demonstrate that reaction of these surfaces with octadecyltrimethoxysilane (ODTMS) yields

stable, high-quality organic monolayers directly bound to the semiconductor surface without an

oxide interlayer. Furthermore, we show by x-ray photoelectron spectroscopy (XPS) and Fourier

transform infrared (FTIR) spectroscopy that reaction of OH-terminated SiC with 1-octadecene

also yields high quality monolayers but occurs over a bridging oxygen atom that is not present in

the case of hydrosilylated Si. Spectrally resolved photocurrent measurements on functionalized

surfaces reveal significant conductivity enhancements relative to oxidized surfaces, indicating a

reduction of interfacial defect concentrations. These enhancements are particularly pronounced

on C-polar SiC and are greater on alkene-reacted surfaces than on silanized surfaces. Transport

and impedance measurements with a mercury-drop-contact across monolayer-SiC structures

yield thermionic emission barriers and flat band potentials that are consistent with large dipolar

contributions to interfacial band alignment and that depend on the specific chemical binding of

the organic monolayers. Patterned arrays of covalently grafted, fluorescence-labeled proteins

onto SiC were fabricated using both functionalization routes, thus further demonstrating the

potential of the stable, biocompatible SiC surface in future biosensor and protein-integrated

electronics applications.

B10.25 Stability Issues of Organic Thin-Film Transistors. Andreas Klug

1, Raphael Pfattner

2, Matthias

Baumann2, Gerhild Wurzinger

1, Arno Meingast

1, Benjamin Souharce

3, Michael Forster

3, Ullrich

Scherf3 and Emil J.W. List

1,2;

1NanoTecCenter Weiz Forschungsgesellschaft mbH, Weiz,

Austria; 2Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State

Physics, Graz University of Technology and Institute of Nanostructured Materials and Photonics,

Joanneum Research, Graz/Weiz, Austria; 3Macromolecular Chemistry, University of Wuppertal,

Wuppertal, Germany.

For more than two decades organic thin-film transistors (OTFTs) have been investigated, being

key devices for large-area, low-cost electronics fabricated with cheap and easy processing

techniques such as spin-coating, inkjet-printing or soft lithography. Essential properties of the

applied organic semiconductors include solution-processability, high field-effect mobility,

compatibility with adjacent layers and - not to forget - stability with respect to ambient

conditions. Here we report on the ambient, operational and shelf-life stability of OTFTs with a

polytriphenylamine-(PTPA)-based polymer active material. The results are benchmarked against

the well-established transistor polymer poly(3-hexylthiophene) (P3HT), yielding comparable

mobility values around 10-4

cm2/Vs. However, upon air exposure P3HT-based devices exhibit

switch-on voltage shifts of more than 30 V and a distinct off-current increase due to

oxygen/moisture-induced doping. Stable air operation therefore involves expensive device

encapsulation or a top-gate architecture, where P3HT is shielded by an appropriate dielectric

material. The corresponding device parameters of PTPA-based OTFTs, on the contrary, remain

rather stable and make device encapsulation obsolete. Moreover, we show that PTPA-based

devices with polyvinylalcohol as dielectric yield improved operational stability and we present

flexible OTFTs based on the two active semiconductor materials.

B10.26

Para-sexiphenyl-CdSe Nanocrystals Hybrid Light Emitting Diodes with Optimized Layer

Thickness and Interfaces. Clemens Simbrunner1, Gerardo Hernandez-Sosa

1, Eugen

Baumgartner1, Juergen Roither

1, Guenter Hesser

2, Wolfgang Heiss

1 and Helmut Sitter

1;

1Institute

of Semiconductors and Solid State Physics, Johannes Kepler University Linz, Linz, Austria; 2ZONA (Zentrum f. Oberflächen- und Nanoanalytik), Johannes Kepler University Linz, Linz,

Austria.

In the variety of molecules used for Organic Light Emitting Diode (OLED) fabrication Para-

sexiphenyl (PSP) represents a well known candidate for the fabrication of high photon energy

emitting devices [1,2]. The high energy gap (3.1 eV) of PSP and consequently its blue

electroluminescence (EL) emission recommends PSP as a component for multi-color OLED

displays and white LEDs. Recent publications promote solution-based semiconducting

nanocrystals or quantum-dots (QD) integrated within the OLED structure leading to a high

efficient electroluminescence emission of organic-inorganic hybrid devices [3-7]. This approach

requires a very good control of the layer thickness and very well defined interface properties of

the multilayer to obtain a good carrier injection and an efficient electroluminescence (EL). In this

contribution we report on the optimization of the interface and thickness between PSP and CdSe

nanocrystals as well as on the role of PSP as electron transport layer on Hybrid-LEDs. The

devices, emitting at 422nm, 549nm and 610 nm, were fabricated by spin coating the CdSe NC

from solution onto (ITO/PEDOT:PSS) anode substrates and subsequently evaporating PSP under

vacuum conditions. Aluminum contacts provide electron injection in the device structure. The

optimal layer thickness and well defined interface was determined by cross sectional scanning

electron microscopy (SEM), while the carrier injection was studied by monitoring the EL

intensity as a function of emission wavelength and applied device current. The optimized values

were found to be less than 25 nm for the PSP layer and 2 monolayers for the CdSe nanocrystals,

resulting in low onset voltages (3-4 V). Homogeneous layers are achieved across the whole

device which is verified by the resulting homogeneous light emission. The obtained spectra of

the optimized devices indicate high color purity, mainly determined by well defined emission

lines, their low FWHM and the absence of parasitic emissions. Consequently the excellent

device properties achieved by the optimization promote the demonstrated material system for a

future candidate in the fabrication of QD-PSP based LED for displays and lighting applications

[8]. [1] S. Tasch, C. Brandstätter, F. Meghdadi, G. Leising, G. Froyer, L. Althoel, Adv.Mat. 9

(1), 33 (1997) [2] G. Kranzelbinder, F. Meghdadi, S. Tasch, G. Leising, L. Fasoli, M. Sampietro,

Syn. Metals 102, 1073 (1999) [3] S. Coe, W. Woo, M. Bawendi, V. Bulovic, Nature 420, 800

(2002) [4] J. Zhao, J. Bardecker, A. Munro, M. Liu, Y. Niu, I. Ding, J. Luo, B. Chen, A. Jen, D.

Ginger, Nano Lett. 6 (3), 463 (2006) [5] Q. Sun, Y. Wang, L. Li, D. Wang, T. Zhu, J. Xu, C.

Yang, Y. Li, Nature Photonics 1, 717 (2007)

B10.27 Patterning of Organic Monolayers on GaN via UV-induced Charge Transfer. S. J Schoell, J.

Howgate, I. D. Sharp, W. Steins, M. S Brandt, M. Eickhoff and M. Stutzmann; Walter Schottky

Institut, Technische Universität München, Garching, Germany.

Understanding charge transfer processes and their mechanisms at organic-inorganic interfaces is

of fundamental importance in the emerging field of organic electronics. Wide bandgap

semiconductors are particularly useful for studying interfacial electronic processes since the bulk

Fermi level can be systematically varied over a wide energy window. Here, the affinity of GaN

to form OH-terminated surfaces by wet chemical treatments is exploited to generate functional

organosilane monolayers. In particular, wet-chemically processed layers of

octadecyltrimethoxysilane (ODTMS) and aminopropyl-triethoxysilane (APTES) on n-type as

well as on p-type Ga-face GaN were formed. For comparative purposes, identical layers were

also formed on n- and p-type 6H-SiC surfaces. The structural and chemical properties of these

layers were studied by static water contact angle measurements, thermal desorption spectroscopy

(TDS), X-ray photoelectron spectroscopy (XPS), Fourier transform IR spectroscopy (FTIR), and

X-ray reflectivity measurements. The organic layers are smooth and change their wetting

properties depending on the molecules used. Thermal desorption temperatures in the range of

500°C indicate covalent bonding of the organic molecules to the GaN surfaces. UV-induced

charge transfer between the semiconductor substrate and organic monolayers was studied after

irradiation with a low pressure Hg lamp at a wavelength of 253.7 nm. On n-type GaN this results

in a decreased water contact angle and reduced thickness of the organic layer, saturating at an

illumination time of 30 min. Furthermore, analysis via capacitance-voltage and current-voltage

measurements revealed that this effect is accompanied by the accumulation of trapped charge

within the organic layer. XPS data prove a decreased hydrocarbon signal on UV-irradiated n-

type GaN and FTIR shows a complete loss of terminating endgroups. In contrast, the wetting

behavior, as well as the XPS and FTIR results, of silanized p-type GaN, as well as n- and p-type

6H-SiC, are largely unaffected by UV irradiation, suggesting a photoinduced charge transfer

through the organic/semiconductor interface which leads to the observed degradation for n-type

GaN. This effect was exploited to achieve laterally patterned organic monolayers on n-type GaN

by selective UV illumination through a shadow mask, followed by site-sensitive covalent

grafting of proteins. We thus show that GaN is a useful platform for fundamental studies of

charge transfer processes at the organic-inorganic interface as well as a promising material for

future biosensor applications.

B10.28

Field Effect Transistor as a Platform to Study Photoinduced Processes in a Donor-acceptor

System. K. S Narayan and Manohar Rao; JNCASR, Bangalore, India.

The existence of donor-type polymer FETs exhibiting p-type characteristics and acceptor-type

molecular FETs with n-type characteristics provide an interesting possibility of a combined

active bilayer system, especially under photoexcitation. We present studies of single layer p-

channel P3HT, single layer n-channel PCBM, and bilayer PCBM/P3HT based FETs in dark and

illuminated conditions. The single layer p type mobility was estimated to be ≈ 10-2

cm2/V-s and n

type mobility in PCBM was ≈ 10-5

cm2/V-s. The bilayer structure was fabricated by introducing

the P3HT film on a PCBM device using different methods. The underlying n-type characteristic

of the PCBM device alters significantly upon illuminating the bilayer structure. Variety of

photophysical processes arising from charge generation and interfacial processes can be followed

and studied by transistor characteristics. These results are compared to the switching and

relaxation effects in single layer P3HT device under photoexcitation.1,2

References 1)Light

Responsive Polymer Field Effect transistor. Applied Physics Letter, 79, 1891 (2001).

2)Nonexponential relaxation of photoinduced conductance in organic filed effect transistors.

Physical Review B, 68, 125208 (2003).

B10.29

Effect of Donor, Acceptor, and Hole/Exciton Blocking Layer Thickness on Power

Conversion Efficiency for Small-molecular Organic Solar Cells Su-Hwan Lee, Dal-Ho Kim,

Ji-Heon Kim, Tae-Hun Shim and Jea-Gun Park; Nano SOI Process Lab., Hanyang university,

Seoul, Korea, South.

We investigated the dependency of the power conversion efficiency on the thickness of donor

(copper phthalocyanine; CuPc), acceptor (fullerene; C60), and hole/exciton blocking (2,9-

Dimethyl-4,7-diphenyl-1,10-phenanthroline; BCP) layers for the organic photovoltaic (OPV)

devices fabricated with double small-molecular layers. The power conversion efficiency peaked

at a specific layer thickness, ~12.7 nm for the donor layer, ~17.5 nm for the acceptor layer, and

~8.0 nm for the hole/exciton blocking layer. This trend is associated with the light absorption

and carrier transport resistance of the small-molecular donor layer, both of which strongly

depend on the layer thickness. Experimental and calculated results showed that the short-circuit

current (Jsc) due to light absorption increased with the donor layer thickness, while that due to

current through the donor layer decreased with 1/R3. Since the total short-circuit current is the

product of the light absorption current and current through the donor layer, there is a trade-off,

and the maximum power conversion efficiency occurs at a specific small-molecular donor layer

thickness (e.g., ~12.7 nm in this experiment). In addition, power conversion efficiency was

determined by short-circuit-current rather than open-circuit-voltage after light absorption.

Furthermore, the donor layer thickness was more sensitive than the thickness of the acceptor or

hole/exciton blocking layers in improving power conversion efficiency; i.e., ~130% for the

donor layer, ~118% for the acceptor layer, and ~112% for the hole/exciton blocking layers.

Acknowledgement * This research was supported by "The National Research Program for

Terabit Nonvolatile Memory Development” sponsored by the Korean Ministry of Knowledge

Economy.

B10.30 Multi-Functional Optical Thin Film Elements by an Imprinting Technique Yong-Woon

Lim, Wonsuk Choi and Sin-Doo Lee; Electrical Engineering and Computer Science, Seoul

National University, Seoul, Korea, South.

In recent years, organic electro-optical materials have great interest for applications in optical

components and liquid crystal displays (LCDs). However, a reliable micro-patterning technology

of organic materials is not well-established so far. The imprinting lithography (IL) is a very

useful method of fabricating such organic-based microstructure devices for use in electronics,

optics, microfluidics, biology, and related areas. In this work, we demonstrate two optical

elements of a liquid crystalline polymer (LCP) and dichroic dyes-doped LCP using an IL

technique combined with the exposure of ultraviolet light. The imprinted optical elements (IOEs)

can be used as both a functional optical film and an alignment layer of the liquid crystal (LC) for

the LC-based optical devices. The multi-functional optical films can be used as an in-cell

retarder or a viewing angle enhancement film and in-cell dye-polarizer. In addition, the

microgrooves produced on the LCP through an IL process are well-defined in shape and so that

they provide the uniform alignment of the LC molecules onto the IOEs without extra-coated

alignment layer by taking the Berreman‟s effect. One IOE with function of in-cell retarder (A-

plate) was fabricated using a LCP with positive dielectric anisotropy such as RMS 03-001C (E.

Merck) by an IL process. The phase retardation of the A-plate measured as a function of

azimuthal angle by a photo-elastic modulation technique is about 1.6. The value was about π/2

corresponds approximately to λ/4 of the wavelength, λ=632.8 nm, used. On the other hand,

another IOE was fabricated by a same IL process of fabricating the above mentioned A-plate

using another LCP with negative dielectric anisotropy such as RMS 03-015 (E. Merck) instead

of LCP with positive dielectric anisotropy. The hydrophobic surface wettability of used mold

was induced the homeotropic aligned (HA) LCP molecules and the microstructure was

transferred on the LCP layer from the mold. The HA IOE shows the dark state in any direction of

optic axis. And thus, the IOE achieved wider viewing characteristics preserved microstructure on

the surface of the LCP film. The other IOE with function of in-cell dye-polarizer with unique axe

or multi-axes was fabricated using dichroic dyes-doped LCP by an IL process. This thin film

polarizer shows excellent optical performances that can be expressed by the polarization

efficiency and the extinction ratio in combination with single-piece transmittance. For using

above mentioned multi-functional IOEs, we proposed two types of high-efficient LC devices.

Consequently, it possesses high contrast and wider viewing characteristics due to employ two

imprinted multi-functional optical films.

B10.31

Comparative Study of Ni or NiO Treated Hole Injection Property for Organic Light

Emitting Diodes.Sung-Ho Woo1, Youngkyoo Kim

2, Gwijeong Cho

2, Kangpil Kim

1, Hongkeun

Lyu1 and Jaehyun Kim

1;

1Division of Nano&Bio Technology, DGIST, Daegu, Korea, South;

2Department of chemical engineering, Kyungpook National University, Daegu, Korea, South.

In this letter, we compared hole injection enhancing effect for Ni and NiO. Deposition of NiO

layer on ITO showed good property for both hole injection and device efficiency, while Ni was

deviated from expectation, though its work function is adequately high to match with that of ITO

electrode, due to dark-spot formation caused by its unstableness with respect to circumstances

and adjacent layer. We obtained about 50% in-crease of device efficiency as well as 3 times

increase of maximum brightness by insertion of NiO. From energy level diagram, we can suggest

a bifunctional property of NiO layer to facilitate hole injection and block the leakage electron to

ITO, which are come from high work function and wide band gap property, respectively. This

results proposed simple method for improving efficiency of OLEDs device by insertion of

thermal evaporated NiO thin layer without any addition of process facility. It is also widely

applicable to other organic electronic devices such as polymer light emitting diodes, and organic

photovoltaics.

B10.32

Effect of Small-molecular Blocking Layer Thickness on Power Conversion Efficiency in

Small-molecular Blended Polymer Solar Cells Su-Hwan Lee, Dal-Ho Kim, Ji-Heon Kim, Tae-

Hun Shim and Jea-Gun Park; Nano SOI Process Lab., Hanyang university, Seoul, Korea, South.

We investigated the dependency of the power conversion efficiency throughout the blended

donor and acceptor layer thickness variation using poly(3-hexylthiophene (P3HT) and [6,6]-

phenyl-C61 butyric acid methyl ester (PCBM), and small-molecular hole/exciton blocking (2,9-

Dimethyl-4,7-diphenyl-1,10-phenanthroline; BCP) layers for organic solar cells. The power

conversion efficiency peaked at a specific layer thickness, determined by ~1000 rpm for the

blending layer (P3HT and PCBM). This trend is associated with the light absorption and carrier

transport resistance of the blending layer, both of which strongly depends on the blending layer

thickness and shows a trade-off for power conversion efficiency. Also, the blocking efficiency

for the hole/exciton of the BCP layer peaked at a specific thickness (~0.5 nm) of the BCP layer.

This trend is associated with the blocking efficiency and carrier transport resistance, both of

which strongly depends on the blocking layer thickness and shows a trade-off for power

conversion efficiency. In our work, we achieved the maximum power conversion efficiency of

~6.74% for our development small-molecular blended polymer solar cell. Acknowledgement *

This research was supported by "The National Research Program for Terabit Nonvolatile

Memory Development” sponsored by the Korean Ministry of Knowledge Economy.

B10.33

Synthesis and Application of New Semiconducting Copolymer for Organic Devices using

N-9-heptadecanyl-dithienopyrrole. Hyung-Gu Jeong, Bogyu Lim, Seok-In Na, Kang-Jun

Beag, Jin-Mun Yun, Juhwan Kim and Dong-Yu Kim; Dept. of Materials Science and

Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju, Korea, South.

The optical and electronic properties of conjugated organic polymers have led to their use in a

variety of applications including organic light-emitting diodes (OLEDs), organic field-effect

transistors (OFETs) and organic photovoltaic cells (OPVs). N-functionalized dithienopyrroles

(DTPs) introduce rigidity to the backbone, resulting in increased planarity and longer

conjugation length. However, some DTPs have low solubilties and low molecular weight, which

greatly limits their use in devices. To make soluble polymers, we incorporated a secondary alkyl

side chains. So we have synthesized N-9-heptadecanyl-dithienopyrrole based copolymers via

stille coupling reaction. The polymer can be easily dissolved in common organic solvents, such

as chloroform, tetrahyrofuran(THF) and chlorobenzene. The good solubility can be attributed to

the bulky side chain. No obvious DSC peak was detected in the trace, suggesting the amorphous

nature of polymer. In this presentation, we will discuss on the performance of OTFTs, OPVs

fabricated using a new semiconducting polymer.

B10.34

Hybrid Organic-Inorganic Heterojunctions made of Doped Crystalline Silicon and a Hole

Conducting Polymer. Roland Dietmueller, Sabrina Niesar, Helmut Nesswetter and Martin

Stutzmann; Walter Schottky Institut, Technische Universität München, Garching, Germany.

There is an increasing research activity in hybrid heterojunctions made of organic

semiconductors and inorganic semiconductors, due to their possible applications in new types of

photovoltaic and optoelectronic devices. The use of conjugated polymers for the organic side of

the hybrid heterojunction is especially interesting due to the low-cost fabrication of organic

layers from solution. The inorganic semiconductor, on the other hand, can be easily doped as an

n-type or p-type material at different concentrations, which opens additional possibilities for the

application of hybrid heterostructures. We have studied hybrid heterojunctions as a model

system made of crystalline Silicon (Si) and the organic hole conductor poly(3-hexylthiophene-

2,5-diyl) (P3HT). The heterojunctions were fabricated by spin coating a P3HT layer from

solution on Si wafers with an ohmic metal back contact. Afterwards, a semitransparent

Aluminum front contact was applied on top of the P3HT layer. These devices were characterized

via current-voltage measurements and spectrally resolved photocurrent measurements. To

investigate the influence of doping on the hybrid heterojunction, we performed experiments as

well with n-type Si with a Phosphorus concentration of 1.5 * 1016

- 1 * 1017

cm-3

as with p-type

Si with a Boron concentration of 3 * 1015

- 2 * 1016

cm-3

. Depending on the use of n-type or p-

type Si, we have observed strongly different current-voltage characteristics for the P3HT/Si

heterojunctions. The diode behavior of the p-type Si/P3HT heterojunction can be explained by

the interplay of the p-type Si/P3HT contact and a Schottky contact at the P3HT/Al interface,

which dominates the electrical characteristics of the whole structure. This finding is supported by

the current-voltage characteristics under illumination with white light, when the p-type Si/P3HT

heterostructure works as a solar cell with similar characteristics as a P3HT/Al Schottky solar

cell. However, for the n-type Si/P3HT heterojunction a more complex current-voltage

characteristic is observed, where under illumination of the heterostructure the forward direction

of the diode is inverted. To explain this behavior, we take into account two rectifying interfaces

with different forward directions, namely the Si/P3HT heterojunction and the P3HT/Al Schottky

contact, and also the strong influence of photo-generated charge carriers on the Si/P3HT

heterojunction. These investigations are important for the understanding of the fundamentals of

the organic-inorganic interface in semiconductors.

B10.35

Novel Water-Soluble Polyfluorenes as an Interfacial Layer in Polymer Solar Cells with

High Work-Function Metal Cathodes. Seung-Hwan Oh1, Seok-In Na

1, Jang Jo

1, Yung-Eun

Sung2 and Dong-Yu Kim

1;

1Department of Materials Science and Engineering, Gwangju

Institute of Science and Technology (GIST), Gwangju, Korea, South; 2School of Chemical &

Biological Engineering, Seoul National University, Seoul, Korea, South.

Water solubility of conjugated polymers may offer many applications. Potential applications of

water-soluble conjugated polymers include the polymer light-emitting diode and new materials

for nano and micro hollow-capsules, and bio- or chemo-sensors. We synthesized neutral

polyfluorenes containing bromo-alkyl groups by the palladium catalyzed Suzuki coupling

reaction. Bromo-alkyl side groups in neutral polyfluorenes were quaternized by tri-methyl amine

solution. The electrochemical and optical properties of water-soluble conjugated polymers are

discussed. This novel synthesized water-soluble conjugated polymers were used as a dipole layer

between active layer and metal cathode in polymer solar cell for enhancement of open-circuit

voltage (Voc), which is one of the most critical factors in determining device characteristics. We

also investigated the device performance of polymer solar cell with different metal cathode such

as Al, Ag, Au and Cu. In polymer solar cell, novel cationic water-soluble conjugated polymers

were inserted between active layer and high-work function cathode (Al, Ag, Au and Cu).

B10.36

Crystallization of Organic Molecules on the Monolayers of Binaphthyl Derivatives for

Thin-Film TransistorsSang-Mi Jeong1,2

and Ji-Woong Park1,2

; 1Materials Science &

Engineering, Gwangju Institute of Science and Technology, Gwangju, Korea, South; 2Program

for integrated Molecular Systems, Gwangju, Korea, South.

Monolayers of 1,1‟-bi-2-naphthol (BN) derivatives, of which the two naphthalene rings are

twisted along the carbon(1)-carbon(1‟) single bond, were studied for their conformational effect

on the growth of the organic molecules on their monolayer surface. We demonstrate that when

pentacene was thermally evaporated on the BN monolayer, the crystallization of the pentacene

molecules occurred at a low nucleation density and a high growth rate because the amorphous

nature of the BN monolayer hindered the intermolecular packing between neighboring

absorbates in the diffusion regime of the crystallization. We also found that the BN monolayer

caused the similar effect in the solution phase crystallization of anthracenes. We fabricated

organic thin film transistors (OTFTs) with the BN monolayers coated between the insulator and

semiconductor layers. The electrical properties of these devices were compared with those

constructed on the bare silica surface. The field-effect mobility (μ) of the device with the BN

monolayer appeared 50% higher than those of silica. The morphology of the semiconductor layer

was investigated using atomic force microscopy (AFM) and X-ray diffraction (XRD) method.

AFM image showed that the pentacene layer on the BN monolayer consisted of larger and more

interconnected grains than those on the silica surface. XRD spectra of the 50 nm thick pentacene

film exhibited that the pentacene crystals on the BN monolayer consisted of mostly thin film

phase (d001 = ~15.5 Å) while those on the silica surface contained both the thin film phase (d001 =

~15.5 Å) and bulk phase (d001 = ~14.5 Å).

B10.37

Synthesis of Polymeric Self-Assembled Monolayer Using a Surface-Reactive Rod-Coil

Diblock Copolymer. Mingu Han and Ji-Woong Park; Gwangju Institute of Science and

Technology, Gwangju, Korea, South.

Rodlike polymers which are composed of a rigid backbone with alkyl side groups can be

consider as one-dimensional self-assembled monolayers (SAMs). If the rods with alkyl side

groups are aligned on the surface with thickness of a single chain diameter, this SAMs become

two-dimensional. Attaching a short surface-reactive coil to the end of rod, the rigid rodlike

polymer can be anchored onto the surface via covalent bonding. The resulting surface tethered

rodlike chains form a polymeric self-assembled monolayer (PSAM). Because the rodlike blocks

of the PSAM are aligned to rotate in their azimuthal and polar anchoring angle, the PSAM may

be stimuli-responsive. In our previous study, we showed that an amphiphilic rod-coil block

copolymer was strongly adsorbed by the hydrogen bonding of coil blocks onto mica surface and

that the rodlike PHIC blocks oriented planar to the surface to form liquid crystalline SAM[1,2].

Here we synthesize, poly(n-hexylisocyanate)-b-poly[(3-trimethoxysilyl)propyl methacrylate]

(PHIC-b-PTMSPMA), a new rod-coil block copolymer which can form PSAM via covalent

bonding of the alkoxysilyl containing PTMSPMA coil block to the silica substrate. The PSAM

was simply produced by the immersion coating method. It exhibited uniform thickness of about

1.6 nm with low RMS roughness of below 0.4 nm. The water contact angle of the PSAM was

about 100°. The PSAM showed an anisotropic morphology resulting from local ordering of

grafted rods on annealing with tetrahydrofuran (THF) vapor. P3HT-based Field-Effect

Transistors (FETs), which were fabricated using a PSAM-coated SiO2/Si substrate, exhibited

about one order of magnitude higher charge carrier mobility than those using untreated silica

dielectric layer. We also demonstrate that PSAM is micro-patternable using UV-induced

photochemical cleavage of the methacrylate moieties of the anchoring blocks (PTMSPMA).

Acknowlegdment : This work was supported by the Korea Science and Engineering Foundation

(KOSEF) grant [R01-2008-000-12246-0] funded by the Korean goverment (MEST) and the

Program for Integrated Molecular System at GIST, Korea Reference : [1] J. H. Kim, M. S.

Rahman, J. S. Lee, J. -W. Park, J. Am. Chem. Soc. 2007, 129, 7756-7757 [2] J. H. Kim, M. S.

Rahman, J. S. Lee, J. -W. Park, Macromolecules 2008, 41, 3181-3189

B10.38 Degradation of Ir(ppy)2(dtb-bpy)PF6 iTMC OLEDs Velda Goldberg

1, Michael D Kaplan

1,2,

Leonard Soltzberg2, Dolly Armira

2, Megan Bigelow

1, Rachel Brady

2, Shannon Browne

1, Bianca

Dichiaro1, Heather Foley

1, Lauren Hutchinson

2, Alison Inglis

2, Nicole Kawamoto

2, Amanda

McLaughlin2, Caitlin Millett

2, Hanah Nasri

1, Sarah Newsky

2, Tram Pham

2, Cassandra Saikin

2,

Mary Scharpf2, Melissa Trieu

2, George G Malliaras

3 and Stefan Bernhard

4;

1Physics, Simmons

College, Boston, Massachusetts; 2Chemistry, Simmons College, Boston, Massachusetts;

3Materials Science & Engineering, Cornell University, Ithaca, New York;

4Chemistry, Princeton

University, Princeton, New Jersey.

Simplicity of construction and operation are advantages of iTMC (ionic transition metal

complex) OLEDs compared with multi-layer OLED devices. Unfortunately, lifetimes do not

compare favorably with the best multi-layer devices. We have previously shown for

Ru(bpy)3(PF6)2 based iTMC OLEDs that electrical drive produces emission-quenching dimers of

the active species. We report evidence here that a chemical process may also be implicated in

degradation of devices based on Ir(ppy)2(dtb-bpy)PF6 albeit by a very different mechanism. It

appears that degradation of operating devices made with this Ir-based complex is related to

current-induced heating of the organic layer, resulting in loss of the dtb-bpy ligand. (The neutral

dtb-bpy ligand is labile compared with the cyclometallated ppy ligands.) Morphological changes

observed in electrically driven Ir(ppy)2(dtb-bpy)PF6 OLEDs provide evidence of substantial

heating during device operation. Evidence from UV-vis spectra in the presence of an electric

field as well as MALDI-TOF mass spectra of the OLED materials before and after electrical

drive add support for this model of the degradation process.

B10.39

Industrial Applicability of Molecular Acceptors in Organic Electronic Devices using Wet-

chemical Deposition. Sibe Mennema1, Stephan Harkema

2, Ralph Rieger

3, Walter Stals

1, Jorgen

Sweelssen1, Hans Joachim Raeder

3, Klaus Muellen

3 and Herman Schoo

2;

1TNO Science and

Industry, Eindhoven, Netherlands; 2Holst Centre, Eindhoven, Netherlands;

3Max Planck Institute

for Polymer Research, Mainz, Germany.

The relative position of the energy levels of the various materials used in organic electronic

devices is of critical importance for their efficient operation. Molecular acceptors, e.g.

tetracyanoquinodimethane (F4TCNQ), are molecules that have the capacity to raise the work

function of a metal surface upon chemisorption. The chemisorption of such acceptors is

accompanied by an electron transfer from the metal to the molecule, thus introducing local

dipoles with their negative ends oriented away from the surface, and increasing the work

function. The area-averaged work function of a metal surface can be adjusted by controlling the

area density of such dipoles. This principle has previously been demonstrated through UHV

deposition and XPS / UPS analysis [1]. Here the transfer of this concept to a solution-based

process, which was carried out in ambient conditions, is presented. Apart from results on

F4TCNQ, also results are discussed of the purposely synthesised hexacyanohexaazatriphenylene

(HATCN). HATCN is also a very strong acceptor, but is expected to persist better at the

interface due its planar configuration, larger size and six instead of four binding sites. Spin-

coating and dip-coating were used to deposit the electron acceptors onto the metal(oxide)

surface. The area density of acceptors was controlled through the concentration of molecules in

solution. Following deposition, Kelvin Probe Force Microscopy (KPFM) was used to investigate

the induced work function changes. An increase of up to 0.9 eV was observed for various

molecular acceptors on ITO, Ag and Au surfaces, and lower concentrations lead to smaller work

function changes. Furthermore, results of devices in which molecular acceptors were

incorporated are presented. Using wet-chemical processing in ambient conditions, Organic

Light-Emitting Diodes (OLEDs) and Organic Photo-Voltaic Cells (OPVCs) were fabricated on

lab-size 3×3 cm2 wafers as well as industrial size 15×15 cm

2 wafers. The concentration of

molecular acceptors had a substantial effect on the electronic and optical properties of the

devices, clearly indicating that the work function changes observed by KPFM can be related to

device characteristics. In the case of OLEDs, the onset voltage has been varied by a factor 2, and

the luminance at a fixed voltage by an even larger factor. In the case of OPVCs, the open-circuit

voltage can be controlled in a similar way. Routes towards exploiting these results in practical

applications will be discussed. [1] N. Koch et al., Phys. Rev. Lett. 95 (2005) 237601

B10.40

Comparison of Molecular Monolayer Interface Treatments in Organic-inorganic

Photovoltaic Devices. Jamie Albin1, Darick J Baker

1, Cary G Allen

1, Tom E Furtak

1, Reuben T

Collins1 and David S Ginley

2;

1Physics, Colorado School of Mines, Golden, Colorado;

2National

Renewable Energy Laboratory, Golden, Colorado.

Excitonic hybrid organic-inorganic solar cells are gaining viability as alternatives to p-n junction

photovoltaics. Although hybrid cells typically have lower efficiencies than their inorganic

counterparts, they are more compatible with inexpensive manufacturing techniques such as spray

deposition and roll-to-roll processing, which can reduce the fabrication cost per photovoltaic

watt. Polymer devices with nanostructured ZnO as the electron-accepting layer have the potential

to improve carrier collection and power conversion efficiency in the bulk heterojunction

approach to organic solar cells. The ZnO/polymer interface, however, is not optimal and

properties such as polymer ordering and wetting at the interface need improvement.

Functionalization of the ZnO surface with molecular monolayers has the potential to resolve

these issues. In this study, we compare the performance of inverted planar ZnO/P3HT

photovoltaic cells made from sol gel-derived ZnO that was functionalized using thiol and silane

based attachment chemistries. Differences in the attachment scheme were explored using

molecules with the same end group. For example, octadecyltriethoxysilane (OTES) and

octadecylthiol (ODT) both yield surfaces with an 18 carbon alkyl chain termination. Both

showed improved polymer ordering relative to control samples. The ODT-modified devices had

higher efficiencies than OTES-treated devices, however, both treatments led to decreased short

circuit current compared to optimized control devices. Similarly, the effect of the end group was

explored using molecules that attach with the same chemistry but leave different, exposed,

terminal groups. Phenyltriethoxysilane (PTES) treated ZnO, for example, shows significantly

improved polymer wetting relative to OTES treatment and untreated surfaces. We discuss these

observations in terms of the nature of the terminal group, differences in the attachment scheme

(silicon vs. sulfur), and differences in surface coverage of the molecular layers. This work was

supported by the National Science Foundation under Grants DMR-0606054 and DMR- 0820518.

B10.41

Prediction of Dynamical Properties of Organic Field-Effect Transistors from DC

Transistor Parameters. Benedikt Gburek and Veit Wagner; School of Engineering and Science,

Jacobs University Bremen, Bremen, Germany.

Dynamical properties of Organic Field-Effect Transistors (OFETs) are of crucial importance for

almost any application. However, not direct AC data but DC measurements are usually used to

optimize transistor performance. Here we present a systematic study to which extent DC

parameters can actually be used to predict AC performance and AC limits of transistors. The

standard FET theory for long channel devices predicts a maximum device bandwidth of ωB = µ

V / L2. However, this holds only for ideal situations, e.g. without parasitic capacitances, and not

too high frequencies. Furthermore, as was recently shown [1], contact resistances can pose

severe additional high-frequency limits to the AC performance. To check the AC limit of a given

transistor directly we perform a frequency scan up to the frequency where the gate current equals

the drain current, which defines the true bandwidth of the device. The frequency-scanned AC

voltage is applied at the gate while the drain contact is kept at a constant voltage. The

dependence of the bandwidth on the drain-source voltage as well as on the DC offset of the gate-

source voltage is analyzed in detail. Among others, this analysis allows for a more accurate

determination of the threshold voltage, often difficult to be determined from standard DC

transistor characteristics. In addition, it offers better insight into carrier mobility dependencies

and distribution of energy states. A model which defines a correction factor to the ideal ωB = µ

V / L2 behavior is proposed in order to predict the measured bandwidth from DC transistor

parameters correctly. The model takes into account parasitic capacitances which occur from the

source and drain contacts. Furthermore, limitations of the bandwidth due to contact resistance are

presented and included in the model. [1] V. Wagner, P. Wöbkenberg, A. Hoppe, J. Seekamp,

Appl. Phys. Lett. 89 (2006) 243515

B10.42 Ambipolarity and Light Emission from Acene Based Transistors. Martin Schidleja, Christian

Melzer and Heinz von Seggern; Institute of Material Science, Technische Universität Darmstadt,

Darmstadt, Germany.

Polycrystalline ambipolar light-emitting organic field-effect transistors (LEOFETs) offer new

possibilities for the characterization of ambipolar devices. The main obstacles for the further

development of LEOFETs is the lack of efficiency, brightness and the limited choice of materials

allowing for high injection efficiency, charge carrier mobility and fluorescence yield. In our

contribution the realization of low injection barrier LEOFETs will be investigated using different

acenes as active layer and its relevance for organic light-emitting diodes (OLEDs) will be

discussed. Devices based on F8BT (poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)) as

semiconductor and gold as source/drain metal show rather efficient light emission. Recent results

disclose a contact dominated device behavior due to high injection barriers for both charge

carrier types. In order to reduce the injection barriers Ca and Au as source and drain contacts in

combination with organic materials such as pentacene, tetracene and ditetracene with electron

affinities (EA) and ionization potentials (IP) matching the metal workfunctions are used. It will

be demonstrated that all these device structures exhibit light emission, meaning ambipolar

conduction, and how that can be utilized to investigate the injection efficiency of the individual

material combinations. Although light emission verifies the presence of electrons and holes in all

devices, the electron conduction in the channel deteriorates for decreasing EA of the organic

semiconductor. Polycrystalline pentacene allows for rather balanced electron and hole

conduction whereas the electron mobility worsens for ditetracene and tetracene. In all acene

based devices the highest intensity of the emitted light is observed in the hole dominated regime,

next to the Ca source electrode, indicating highly efficient electron injection from Ca into the

respective acene. This behavior cannot be observed in F8BT based devices due to the high

injection barriers for both charge carriers from the utilized gold electrodes. In the presented

contribution the relative impact of injection compared to transport will be highlighted and its

applicability to OLEDs will be discussed.

B10.43 Interface Engineering in Organic Thin Film Transistors. Philipp Stadler

1, Anna Track

3,

Mujeeb Ullah2, Thockchom B Singh

1, Gebhard J Matt

1,2, Helmut Sitter

2, Michael G Ramsey

3

and N. Serdar Sariciftci1;

1Institute for Organic Solarcells, Johannes Kepler University, Linz,

Austria; 2Institute for Solid State Physics, Johannes Kepler University, Linz, Austria;

3Institute

for Physics, Karl Franzens University, Graz, Austria.

Interface engineering in organic thin film transistors (oTFT) has become the key issue for

optimizing the device operation. Recent effort has been made by using passivation layers on top

of silicon dioxide (SiOx) and alumina (Al2O3) [1]. In this work we present a oTFT with C60 as

semiconductor in bottom gate and top contact structure. We compared silicon dioxide,

electrochemically grown alumina and divinyltetramethyldisiloxane-bis(benzocyclobutene)

(BCB) on alumina as gate insulator [2,3,4]. The nature of the interfaces between C60 and the

BCB and between C60 and the oxide insulators is studied in detail by X-ray and ultraviolet

photoemission spectroscopy (XPS and UPS). The interface system fullerene and oxide is

displaying large positive threshold voltages and lower drain/source currents in

accumulation/depletion regime as compared to oTFT's using organic gate insulators [5]. Here we

experimentally show that by introducing a thin polymeric buffer layer (BCB) between the oxide

and the fullerene the threshold voltage of the transistor characteristics can be shifted from

positive values in the oxide case to negative values in the polymer case [6]. The transistor is

operating within one volt and we observe an enhanced on/off ratio (4 decades) and an electron

mobility of approx. 1 cm2 V-1 s-1. We correlate this different transport performance to the

change in the work function of the semiconductor seen in the UPS specta and the shift in binding

energy of the C1s peak of the C60 in the XPS spectra respectively. [1] M. Halik, H. Klauk, U.

Zschieschang, G. Schmied, C. Dehm, M. Schütz; S. Maisch, F. Effenberger, M. Brunnbauer, F.

Stellacci, Nature 431, 963-966 (2004) [2] T. D. Anthopoulos, T. B. Singh, N. Marjanovic, N.S.

Sariciftci, A. Ramil, H. Sitter, M. Cölle, D. de Leeuw, Appl. Phys. Lett. 89 213504-1 (2006) [3]

T. B. Singh, N. S. Sariciftci, H. Yang, L. Yang, B. Plochberger, H.Sitter, Appl. Phys. Lett., 90

(2007) [4] R. Schroeder, L. A. Majewski, M. Grell, Adv. Mater. 16, 633 (2004) [5] G. Horowitz,

R. Hajlaoui, R. Bourguiga, M. Hajlaoui, Syn. Met. 101, 401-404 (1999) [6] X. H. Zhang and

Kippelen, Appl. Phys. Lett. 93, 133305 (2008)

B10.44

New Air-Stable Organic Semiconductor for p-Channel Transistors with Large Mobility

and Low-Voltage Integrated Circuits on Flexible Substrates. Ute Zschieschang1, Tatsuya

Yamamoto2, Kazuo Takimiya

2, Tsuyoshi Sekitani

3, Takao Someya

3 and Hagen Klauk

1;

1Max

Planck Institute for Solid State Research, Stuttgart, Germany; 2Hiroshima University, Higashi-

Hiroshima, Japan; 3University of Tokyo, Tokyo, Japan.

The performance of organic thin-film transistors (TFTs) often degrades upon exposure to air.

This is due to the generation of charge traps as a result of the oxidation of the conjugated

molecules. One strategy to improve the air stability of organic TFTs is thus the synthesis of

organic semiconductors with reduced susceptibility to oxidation. For p-channel TFTs this implies

a large ionization potential. However, most organic TFTs using semiconductors with large

ionization potential show rather small hole mobilities, usually below 0.5 cm2/Vs [1-3]. This has

been attributed to poor molecular ordering and hence poor overlap of the molecular orbitals.

Recently, a six-ring fused heteroarene, dinaphtho-[2,3-b:2‟,3‟-f]thieno[3,2-b]thiophene (DNTT),

was synthesized that has a large ionization potential (5.4 eV) and forms well-ordered films, with

hole mobilities as large as 2 cm2/Vs for TFTs made on doped silicon wafers (serving as a global

gate electrode) with a thermally grown SiO2 gate dielectric [4]. Here we report on the

performance and stability of DNTT transistors, inverters and ring oscillators on glass and flexible

polyethylene naphthalate (PEN) substrates. The TFTs and circuits use an inverted staggered

(bottom-gate, top-contact) device structure with patterned aluminum gates, a thin gate dielectric

based on an oxygen-plasma-grown aluminum oxide layer (3.6 nm thick) in combination with a

self-assembled monolayer (SAM) of an aliphatic phosphonic acid (1.7 nm thick), a thermally

evaporated DNTT layer and gold source/drain contacts. Gates, semiconductor, and source/drain

contacts were patterned using shadow masks. Owing to the large capacitance of the AlOx/SAM

gate dielectric (700 nF/cm2), the TFTs and circuits can be operated with low voltages of about 3

V. DNTT TFTs on glass have a mobility of 1.5 cm2/Vs, an on/off ratio of 1e7 and a subthreshold

swing of 80 mV/decade. On flexible PEN, the TFTs have a mobility of 0.6 cm2/Vs, an on/off

ratio of 1e7 and a subthreshold swing of 110 mV/decade. These mobilities are a factor of 2 larger

than those of pentacene TFTs manufactured with the same technology [5]. Because the

ionization potential of DNTT (5.4 eV) is much larger than that of pentacene (5 eV), the DNTT

TFTs have significantly better air stability, showing no degradation while stored in ambient air

for 3 months. Five-stage ring oscillators based on unipolar inverters with saturated load show

stable oscillations for supply voltages between 2.2 and 5 V, with a signal delay of 37 µsec per

stage (27 kHz) at 3 V and 18 µsec per stage (55 kHz) at 5 V. This is the highest frequency

reported for flexible organic circuits at low supply voltage. [1] H. Meng et al., J. Am. Chem.

Soc., vol. 123, p. 9214, 2001. [2] J. A. Merlo et al., J. Am. Chem. Soc., vol. 127, p. 3997, 2005.

[3] J. Locklin et al., Adv. Mater., vol. 18, p. 2989, 2006. [4] T. Yamamoto et al., J. Am. Chem.

Soc., vol. 129, p. 2224, 2007. [5] H. Klauk et al., Nature, vol. 445, p. 745, 2007.

B10.45

Optical Stability of Small-molecule Thin-films Determined by Photothermal Deflection

Spectroscopy. Marco Stella2, Monica Beatriz Della Pirriera

1, Joaquim Puigdollers

1, Cristobal

Voz1, Jordi Andreu

2, Ramon Alcubilla

1 and Joan Bertomeu

2;

1Electronic Engineering,

Universidad Politécnica de Cataluña, Barcelona, Spain; 2Fisica Aplicada y Optica, University de

Barcelona, Barcelona, Spain.

Organic semiconductors represent a new interesting class of materials for several electronic

applications. Organic solar cells performance have improved significantly in the last few years

thanks to the optimization of the solar cell structure and, specially, to the ability to process new

organic semiconductors with optimised properties. Among different deposition techniques,

thermal evaporation in high-vacuum is the more suitable process to obtain small-molecule

organic thin-films with well organize molecular structure. In this paper the optical absorption

properties of n-type (C60 and PTCDA) and p-type (CuPc) small-molecule semiconductors are

investigated by optical transmission and Photothermal Deflection Spectroscopy (PDS). Results

show the usual absorption bands related to HOMO-LUMO transitions in the high absorption

region of transmission spectra. PDS measurements also evidences exponential absorption

shoulders with different characteristic energies (47 meV for CuPc, 50 meV for PTCDA and 87

meV for C60). In addition, broad bands in the low absorption level are observed for C60 and

PTCDA thin-films. These bands have been attributed to contamination due to air exposure (1). In

order to get deeper understanding of the degradation mechanisms single, bilayer and co-

evaporated thin-films have been characterized by PDS. Dependence of the optical coefficient on

light illumination and air exposure have been studied and correlated to the structural properties

of the films (as measured by X-Ray Diffraction Spectroscopy). Results show that CuPc:PTCDA

and CuPc:C60 co-evaporated films are stable after light and air exposition. However, single

layers of C60 shows significant increase of the low level optical absorption coefficient.

B10.46

Non-Volatile Organic Memory based on Electrically Doped Organic Heterostructures. Frank Lindner, Phillip Sebastian, Bjoern Lussem and Karl Leo; Institut für Angewandte

Photophysik, TU Dresden, Dresden, Germany.

Within the last few years organic memory devices have attracted considerable attention. Several

different approaches for organic memory devices have been reported in literature that show

bistable memory behaviour with high switching speeds and high ON/OFF ratios [1]. The

drawbacks of most of these devices are a rather low stability and reproducibility and a lack of

knowledge about he precise switching mechanism. Here, we report on a novel approach based on

charge trapping in a two-well heterostructure device. Consisting of materials which are well

known from organic light emitting devices, we obtain reproducible bistable electrical switching

and memory phenomena. Depending on the number of charges stored in the wells the resistance

of the device changes, i.e. the measured current-voltage characteristic shows two states of

different conductivity at the same applied voltage. The ratio between the resistance in the OFF

state and in the ON state can be varied by the write and erase voltage. More than 2000 Write-

Read-Erase cycles are obtained without degradation. The memory state is retained for several

days before reading the devices. In conclusion, the memory concept we present shows higher

reproducibility and stability compared to other organic memories. Device performance tests

show that the heterostructure devices are a promising candidate for low-cost, electrically

addressable data storage applications. [1] J.C. Scott, L.D. Bozano, “Nonvolatile Memory

Elements Based on Organic Materials”, Advanced Materials 2007, 19, 1452-1463.

B10.47

Synthesis of Monofunctionalized Electroactive Molecules and their Application in

Electrode Modification in Organic Devices. Gabriele Kremser1, Thomas Rath

1, Thomas

Griesser2 and Gregor Trimmel

1;

1Institute for Chemistry and Technology of Materials, Graz

Universitiy of Technology, Graz, Austria; 2Institute of Chemistry of Polymeric Materials,

Montanuniversität Leoben, Leoben, Austria.

The control of the interface of organic semiconducting materials and inorganic electrodes or

semiconductors is one of the crucial points for high performing organic electronic devices.

Therefore, different surface treatments and/or interfacial layers are often used in the device

assembly. Self assembled monolayers e.g. organosilanes, thiols or phosphonates have been

employed in the modification of indium tin oxide electrodes and gate oxides in organic field

effect transistors. In this contribution we present the syntheses of novel functionalized

electroactive organic compounds which are capable to interact with inorganic oxide materials.

These molecules consists of an electroactive unit of a stilbene, a quarterthiophene or a

biphenylterthiophene moiety which is asymmetrically capped with a polar group e.g. phosphonic

esters, carboxylic acid and hydroxy functionality. The PPV-compounds have been synthesized

by Wittig reaction or Horner-Emmons, whereas the thiophene-based molecules have been

prepared by Suzuki and Stille cross coupling reactions. All materials were identified by nuclear

magnetic resonance (NMR) and infrared (IR) spectroscopy. The optical properties were

investigated with UV-vis and photoluminescence spectroscopy. First result on the use of these

molecules for the modification of indium tin oxide electrodes in organic electronic devices will

be presented.

B10.48

Bipolar Transport in Organic Field-effect Transistors: Organic Semiconductor Blends

versus Contact Modification. Andreas Opitz, Michael Kraus, Markus Bronner, Julia Wagner

and Wolfgang Bruetting; Institute of Physics, University of Augsburg, Augsburg, Germany.

The achievement of bipolar transport is an important feature of organic semiconductors, both for

a fundamental understanding of transport properties and for applications such as complementary

electronic devices. We have investigated two routes towards organic field-effect transistors

exhibiting bipolar transport characteristics. As a first step, mixtures of p-conducting copper-

phthalocyanine (CuPc) and n-conducting buckminsterfullerene (C60) were used to realize

ambipolar field-effect transistors. As a second step, bipolar transport in copper-phthalocyanine

was achieved by a modification of the gate dielectric in combination with a controlled variation

of the electrode materials used for carrier injection. Although both routes are in principle

successful, there are some characteristic differences. In molecular blends, the charge carrier

mobilities decrease exponentially by dilution of the respective transport material. This indicates

that percolation is a crucial feature in mixtures of both materials to achieve ambipolar carrier

flow. In this case, balanced mobilities can be achieved by adjusting the mixing ratio. This is a

necessity to design ambipolar inverters with symmetric transfer characteristics. In neat films of

one single material, suitable contact modification allows for bipolar charge-carrier transport

together with the prevention of charge carrier traps at the insulator/semiconductor interface. Here

the obtained electron and hole mobilities differ by less than one order of magnitude.

B10.49 Hysteresis-free Electron Currents in Conjugated Polymers. Irina Craciun, Yuan Zhang and

Paul Blom; Molecular Electronics, University of Groningen, Groningen, Netherlands.

Charge transport is an important issue with regard to the understanding and optimization of

electronic devices made from conjugated polymers. In the last two decades a large effort has

been put on the characterization of the transport of holes, which is the dominant charge carrier. It

has been demonstrated that the hole transport is governed by hopping between localized states,

characterized by a mobility that depends on density, electric field and temperature.[1] The

transport of electrons is far less well characterized, and the strongly reduced electron currents are

attributed to trapping of electrons.[2] A major problem with the investigation of the electron

transport is the construction of so-called electron only devices, where hole blocking electrodes

are required that are usually reactive. The resulting J-V characteristics of these devices often

exhibit, next to low currents, strong hysteresis effects that strongly hinder the interpretation of,

for example, a temperature scan. Major candidates responsible for the strong hysteresis are

electrons trapped either in the bulk of the polymer or at the hole blocking electrode/ polymer

interface. The transport of electrons in electron-only devices based on poly (2-methoxy, 5- (2‟

ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) is investigated for various hole-blocking

bottom electrodes as well as purification of the polymer. Using a variety of metallic electrodes as

a bottom contact no improvement in the observed hysteresis is observed. As a next step a n-type

doped PPV-based polymer was used as bottom electrode for the electron-only devices. Due to

the n-type doping the J-V characteristics of the doped electron-only device are hysteresis free.

However, addition of an undoped MEH-PPV layer on top directly resulted in large hysteresis,

showing that it originates from trapping in the bulk of the undoped polymer. As a final step we

demonstrate that by proper purification of the MEH-PPV hysteresis free electron-only currents

can be obtained, enabling a further quantitative characterization of the electron transport

mechanisms in this class of materials. [1] C. Tanase et al., Phys. Rev. B 70, 193202 (2004) [2]

M. M. Mandoc et al., Phys. Rev. B 73, 155205 (2006)

B10.50

Abstract Withdrawn

B10.51

Hole and Electron Polaron Absorptions Measured using Charge Modulation Spectroscopy

on Ambipolar Polyfluorene-based FETs. Matt Bird, Ni Zhao and Henning Sirringhaus;

Optoelectronics Group, Cambridge University, Cambridge, United Kingdom.

When a charge resides on a polymer it couples strongly to a local distortion in the molecular

structure; the combination is known as a polaron. The polaronic state induces new optical

transitions with sub band gap energies. In combination with quantum chemical modeling, the in-

situ measurement of these new transitions using charge modulation spectroscopy on operating

devices can offer a unique insight into the molecular packing, the degree of polaron localization

and the origin of energetic disorder [1, 2] in semiconducting polymers. We present here for the

first time the charge-induced absorption spectra for both the hole and electron polarons in a

range of polyfluorene-based conjugated polymers. We perform these measurements on polymer

transistor structures with ambipolar charge injection and transport [3] in which one can induce a

hole accumulation layer for negative gate voltage and an electron accumulation layer for positive

gate voltage. By comparing the induced absorption spectra for electron and hole polarons we

obtain information about fundamental electron-hole symmetries in conjugated polymers [4]. In

particular, over the range of polymers tested, we can correlate shifts in the spectra with the

different degrees of localization of electrons and holes on particular units of the backbone and

different degrees of interchain interaction. [1] J-F. Chang, M. Giles, M. Heeney, I. McCulloch

and H. Sirringhaus Phy. Rev. B 76, 205204 (2007) [2] D. Beljonne, J. Cornil, H. Sirringhaus, P.

J. Brown, M. Shkunov, R. H. Friend, J.-L. Brédas, Func. Mater. 11, 229-234 (2001) [3] J.

Zaumseil, C. L. Donley, J.-S. Kim, R. H. Friend, H. Sirringhaus, Adv. Mater. 18, 2708-2712

(2006) [4] K. Fesser, A. R. Bishop, and D. K. Campbell Phys. Rev. B 27, 4804 - 4825 (1983)

B10.52 Fully Solution-Processed Organic Solar Cells on Metal Foil Substrates. Whitney Gaynor

1,

Jung-Yong Lee2 and Peter Peumans

2;

1Materials Science and Engineering, Stanford University,

Stanford, California; 2Electrical Engineering, Stanford University, Stanford, California.

Polymer bulk heterojunction photovoltaic cells are an area of intense research because they show

promise as a lower-cost alternative to their inorganic counterparts. Much of this cost reduction

relies on the ability to use high-throughput processing techniques such as roll-to-roll coating and

depositing materials from solution. However, the highest performance polymer bulk

heterojunction cells require a vacuum-deposited metal contact. In this talk, we report polymer

bulk heterojunction cells, based on the poly-3-hexylthiophene and phenyl-C61-butyric acid

methyl ester (P3HT:PCBM) system, in which each layer in the cell, including the top transparent

electrode, is processed from solution onto metal foil substrates. Silver-coated metal foil is used

as the cathode, followed by a solution-processed cesium carbonate interface layer to enhance

electron transport. The active layer is the bulk heterojunction P3HT:PCBM, followed by a

poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer for hole

collection, and a laminated transparent solution-processed silver nanowire film as the anode.

Power conversion efficiencies of 1.6% were obtained by optimization of the cathode and anode

interfaces. With continued optimization, efficiencies approaching those of conventionally

fabricated organic PV cells may be reached. The metal foil substrate offers advantages such as

low cost and superior barrier properties. Moreover, the metal foil can be replaced by metal fibers

enabling fiber-shaped solar cells. The metal foil also offers an easy and low-cost way to obtain

substrate shaping to enhance light trapping in the cell, further increasing the power conversion

efficiency.

B10.53

Chemical Modification of Self-assembled Monolayers: Tailor-made and Reversible Surface

Functionalization Schemes. Stephanie Hoeppener1, Claudia Haensch

1, Manuela Chiper

1 and

Ulrich S. Schubert1,2

; 1Lab. of Macromolecular Chemistry and Nanoscience, Eindhoven

University of Technology, Eindhoven, Netherlands; 2Lab. of Macromolecular and Organic

Chemistry, Friedrich-Schiller-University, Jena, Germany.

Chemical surface reactions performed on self-assembled monolayers represent a versatile

method to tailor the properties of surfaces. As a suitable precursor molecule 11-

bromoundecyltrichlorosilane can be used, which forms reasonable well ordered monolayers on

glass and silicon substrates. The bromine functionalities of such monolayers can be employed in

a number of derivatization reactions, leading to a variety of functional groups, i.e. thiols, azides,

or amines. Besides of the direct conversion of the bromine functions, reaction schemes, i.e. click

chemistry, can be employed to further increase the variety of available surface functions and/or

to covalently bind functional molecules. Examples include here the covalent binding of dye

molecules or the introduction of supramolecular precursors, which are able to tailor the

photochemical properties of surfaces. Terpyridine motifs represent here an example that allows

to reversibly modify the optical properties of surfaces, by external stimuli. In combination with

electrochemical structuring techniques it is demonstrated that different functional groups can be

locally introduced in a sequential derivatization approach, which leads to the fabrication of

complex structures, that can be, e.g., used to fabricate nanoelectronic device components.

B10.54

Temperature Modulation Spectroscopy for Probing Excitonic Properties of Organic

Semiconductors.Abhishek Yadav1, Kwok L Chan

3, Max Shtein

2 and Kevin P Pipe

1;

1Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan;

2Department of Materials Science and Engineering, University of Michigan, Ann Arbor,

Michigan; 3Department of Mechanical engineering, Hong Kong Polytechnic University, Hong

Kong, China.

The performance of organic optoelectronic devices is dependent on exciton generation and

transport properties that govern optical absorption and emission. Excitons in organic thin films

fall into two categories: 1) Frenkel excitons in which both the hole and electron are bound on the

same molecule, and 2) charge transfer (CT) excitons in which the hole and electron are bound to

each other but exist on neighboring molecules. Due to disorder and self-polarization, Frenkel

excitons are more common, although CT can also play an important role (e.g. in mediating

charge separation in photovoltaic devices). Optical spectroscopy is widely used to study exciton

transitions, spectral broadening, and oscillator strength. Simple absorption spectroscopy is better

suited for Frenkel excitons, because of their larger oscillator strength. Modulation spectroscopy

can be used to study CT excitons; for example, electric field modulation spectroscopy utilizes the

Stark effect to provide the derivative of the absorption spectrum with respect to exciton transition

level, and can be used to identify weak CT transitions. Unfortunately, the application of electric

fields can create parasitic effects, including strong electric fields at interfaces and destruction of

crystal symmetry, reducing the method‟s utility for thin films and nanostructured samples. Here,

we introduce temperature modulation spectroscopy as an alternative method to probe excitonic

properties of organic semiconductors. The oscillator strength of CT excitons decreases with

temperature, due to a reduced overlap of wavefunctions between neighboring molecules caused

by lattice thermal expansion [1]. For molecular crystals, with a small intermolecular overlap of

electronic wavefunctions, the oscillator strength of CT excitons is balanced by Frenkel exciton

states [2]. Hence, the oscillator strength of Frenkel excitons is expected to increase with a

decrease in oscillator strength of CT excitons [1]. We use this difference in behavior to

distinguish CT and Frenkel excitons in device-relevant organic thin films, including pentacene

and CuPc deposited on glass. The temperature of the film is modulated in a lock-in amplification

technique by passing a sinusoidal current through a thin silver film heater deposited on top of

organic film, while transmittance is measured using a photomultiplier tube. A Lorentz-Lorenz

dielectric model is used to fit the spectroscopic photometry data, yielding information about

excitonic transitions, oscillator strength, and broadening. To obtain the temperature dependencies

of these exciton properties, we fit the derivative of the Lorentz-Lorenz model to the modulated

transmission data. We show that temperature modulation spectroscopy is useful for studying the

excitonic properties of nanostructured thin films, without many of the parasitic effects of other

modulation spectroscopies. [1] Tanaka et al, J Chem. Phys 95: 2371, 1991 [2] Hernandez et al, J

Chem Phys 50: 1524, 1966

B10.55

3-dimensional Organic Field Effect Transistors: Charge Accumulation in the Vertical

Semiconductor Channels.M. Uno1,2

, I. Doi3, K. Takimiya

3 and Jun Takeya

1;

1Osaka University,

Toyonaka, Japan; 2TRI-Osaka, Izumi, Japan;

3Hiroshima University, Higashi-hiroshima, Japan.

Organic electronics have attracted much attention as “post-silicone electronics” due to the

mechanical flexibility, as well as low-cost and energy-saving fabrication processes of their

devices. Among them, organic field-effect transistors (OFETs) are the key devices in which

further development is necessary for such applications as full-flexible displays, incorporated in

their matrix-controlling elements. So far, though much effort have been devoted to material

development, even the best value of the carrier mobility μ of organic semiconductors remain in

the order of 1 cm2/Vs except for single-crystal devices, resulting in unsatisfactory current

amplification per pixcel. Though decreasing the channel length L and increasing the width W can

be another approach, this effect is limited in the condition that the channels and the electrodes

are confined in the same plane as the conventional OFETs. In this presentation, we propose a

three-dimensional organic field-effect transistor (3D-OFET) to accumulate charge in its vertical

semiconductor channel, so that space availability for the field-induced carriers is essentially

enlarged [1]. A multi-columnar structure is built to multiply the channel area, partially dry-

etching Si substrates and thermally oxidizing their surfaces to form gate dielectric SiO2 layer. On

the sidewall of the column, we deposit a vertical semiconductor layer of dinaphtho[2,3-b:2',3'-f]

thieno[3,2-b]thiophene (DNTT), which was reported to have high μ (~ 1 cm2/Vs) and excellent

air stability. Since W corresponds to the total length of all the column edges and L equals to the

height of the columns, one can design the devices without constraints of the lateral space, which

results in much higher ratio of W/L. Indeed, pronounced field-effect amplification is realized

with 500-nm thick SiO2 gate insulator with the on-off ration of 106, in which current

amplification up to 100 μA in an area of 500 μm square pixel is achieved with VG of 5 V and

drain voltage VD of 5 V. Carrier mobility of the DNTT film in the vertical channel is estimated to

be around 0.7 cm2/Vs. On the way for the organic flexible displays, the next challenge is to build

the 3D-OFETs on plastic substrates; however it is not welcomed to complicate the fabrication

process with additional deposition of dielectrics. Therefore, we propose to use an electric double-

layer (EDL) capacitance CEDL of ionic liquid, of which we reported outstanding gating

performance recently. Similar current-amplification capability is maintained for the ionic-liquid

gated 3D-OFETs as compared to that of the above SiO2-gated devices; 0.5 V is enough for both

VG and VD to obtain ID of 15 μA, resulted from much larger CEDL but smaller μ. The results

already demonstrate usefulness of the 3D structure in achieving sufficient current per pixel for

matrix-controlling elements, which can drive industrial development of organic flexible displays.

[1] M. Uno et al., Appl. Phys. Lett. 93, 173301 (2008).

B10.56

Negligible Contact Resistances in Organic Single-crystal Transistors with Secondary Gates

on Source and Drain Electrodes. K. Nakayama1, T. Uemura

1, M. Uno

1,2 and Jun Takeya

1;

1Osaka University, Toyonaka, Japan;

2TRI-Osaka, Izumi, Japan.

To realize the maximum device performance of organic field-effect transistors (OFETs), a

significant challenge is to achieve efficient carrier injection at the electrodes. Such contact

performance is the most seriously concerned for short-channel devices with the channel length

typically less than sub-micrometers, though they are highly attractive because of their high-

frequency response and capability of high-density integration. As compared to common silicon

metal-oxide-semiconductor field-effect transistors where heavily doped carrier-rich region is

incorporated next to the channel, all the reported organic field-effect transistors suffer from the

carrier injection from different materials such as metals. In this presentation, we report that the

influence of injection barriers can be negligible with a construction of a contact between a

carrier-rich region and the channel region to be gated using the same semiconductor material,

even for devices with low channel resistances such as rubrene single crystal transistors with

micron-scale channel lengths. Previously, we reported a device structure with “split gates” on the

source and drain electrodes buried in the gate-insulating layers, so that the carrier density in the

organic semiconductors in the vicinity of the source and drain electrodes can be varied

independently of the primary gate electric fields applied to the central channel in the

semiconductors. Thereby, the carrier reservoirs are formed in the semiconductor locally near the

electrodes by the secondary gate voltage. To fabricate the bottom contact OFETs, we first

deposit the split-gate electrodes of gold on 500-nm thick SiO2 / doped silicon substrates by

evaporation. An amorphous fluoropolymer (CYTOP, Asahi Glass Co.) dielectric is prepared to

the thickness of 1 μm for the second gate dielectric layer, and source and drain electrodes are

formed to the size slightly smaller than the split-gate electrodes, so that the split gate extends to

the semiconductor channel in the vicinity of the edges of the source and drain electrodes. Finally,

a thin platelet of rubrene crystal, which was independently grown by physical vapor transport

technique, is laminated by natural electrostatic force. Transfer characteristics of devices with five

different channel lengths are measured sweeping the primary gate voltage with the application of

the maximum secondary split-gate voltages to prepare the “hole-rich” region in the rubrene

crystals. As the result of the gradual channel plot, the normalized contact resistance RCW was

immeasurably small, where RC and W represent contact resistance estimated from the intercept of

the plot and the channel width, respectively. The result means that the value is smaller than 100

Ωcm in our present setup. At least being comparable to the smallest contact resistance reported

so far, it is required to be measured with improved measurement systems. [1] K. Nakayama et

al., Appl. Phys. Lett. 93, 153302 (2008).

B10.57

Effect of Monochromatic Wavelength and Intensity on P3HT/PCBM Device

Characteristics. Harold T Evensen1, Garth A Berriman

2, Warwick J Belcher

2, John L

Holdsworth2 and Paul C Dastoor

2;

1Chemistry & Engineering Physics, University of Wisconsin-

Platteville, Platteville, Wisconsin; 2Centre for Organic Electronics, University of Newcastle,

Callaghan, New South Wales, Australia.

In order to probe photon energy-dependent mechanisms in bulk heterojunction solar cells, high

intensity LEDs were used to excite poly(3-hexylthiophene) (P3HT)/ [6,6]-phenyl C61-butyric

acid methyl ester (PCBM) devices. The light sources, ranging from 450 nm to 640 nm, delivered

a variable, monochromatic intensity from less than 0.5 mW/cm2 to over 50 mW/cm

2, or up to

eight to twenty times the equivalent solar irradiance within their bandwidths. The open circuit

voltage and short circuit current varied with intensity, and the open circuit voltage exhibited

distinctly different behavior under red light as compared to the other wavelengths. The results

are compared with the predictions of several device models.

B10.58

Permanent and Pattern-resolved Adjustment of the Surface Potential of Graphene-like

Carbon Through Chemical Functionalization. Fabian M. Koehler1, Norman A Luechinger

1,

Dominik Ziegler2, Evagelos K Athanassiou

1, Robert N Grass

1, Antonella Rossi

3,4, Christofer

Hierold2, Andreas Stemmer

2 and Wendelin J Stark

1;

1Chemistry and Applied Biosciences, ETH

Zurich, Zurich, Switzerland; 2Mechanical and Process Engineering, ETH Zurich, Zurich,

Switzerland; 3Materials, ETH Zurich, Zurich, Switzerland;

4Chimica Inorganica ed Analitica,

Universita degli Studi di Cagliari, Cagliari, Italy.

The potential use of graphene in electronics requires reliable and pattern-controlled methods to

inject or remove electron density from the two-dimensional carbon honeycomb lattices.

Combining well established radical chemistry at ambient conditions and classical lithography,

we find that the structure of graphene layers can be permanently altered through covalent

chemical functionalization. We further demonstrate how the classic Hammett concept and the

Linear Free Enthalpy Relationship from organic chemistry can be used to predict the surface

potential shifts in graphene-like carbon surfaces. Our study (1) reveals an astonishingly simple

yet accurate method to adjust the surface potential in graphene layers. Next to the direct

application of such patterning in device fabrication, the here shown covalent attachment

introduces a third dimension in the two-dimensional graphene base. Such functionalization can

ultimately lead to covalent attachment of molecular electronics, circuitry incorporated chemical

sensors or actuators and offers an alternative approach to address the contacting problem in the

nm range. The possibility to print a potential pattern onto a graphene sheet was only conceptually

shown at present, but this approach offers a permanent polarization of the graphene sheets as in

the case of gate-induced changes in conductivity. Latter will be of crucial importance when using

a larger graphene sheet as a starting material in device fabrication (3D analogy: native Si wafer),

which is then doped and partially oxidized (3D analogy: insulating silicon oxide). One might

envision that for graphene, doping or oxidation to an insulating state could therefore be achieved

through simple chemical processing and classical or dip-pen lithography. [1] F.M. Koehler, N.A.

Luechinger, D. Ziegler, E.K. Athanassiou, R.N. Grass, A. Rossi, C. Hierold, A. Stemmer, W.J.

Stark, Angew. Chem. Int. Ed., 2008, in print

B10.59 Low Temperature Thermal Conductivity of Rubrene Single Crystals. Y. Okada

1, M. Uno

1,2,

M. Yamagishi1 and Jun Takeya

1;

1Osaka University, Toyonaka, Japan;

2TRI-Osaka, Izumi,

Japan.

Recently, organic crystals are gaining considerable interest linked to such practical applications

as electronic semiconductor devices and nonlinear optical components; rubrene single crystal

field-effect transistors exhibit one-order higher performance as compared to popular

polycrystalline thin-film organic transistors and such materials as DAST shows excellent

nonlinearity in response to light irradiation because of vibronic coupling to molecular

polarization. However, it is concerned that density of dilutely distributed defects in these crystals

is generally difficult to define, though it is recognized as crucial factor for transport or optical

performances of the above devices. In this work, we measured low-temperature thermal

conductivity of phonons to estimate their mean-free paths and density of crystalline defects

responsible for scattering the phonons. It is known that temperature dependence of thermal

conductivity in high-quality single crystals shows a pronounced peak in the temperature range

below typically 30 K, below which phonons are predominantly scattered by defects. At higher

temperatures, phonon-phonon Umklapp process dominates their scattering events. Thermal

conductivity is measured from 0.5 K to room temperature using a steady-state one heater two

thermometer technique. A set of mm-size chip resistors are mechanically attached for stand-

alone crystals in case they have the length of subcentimeter. In addition, we have developed a

measurement device incorporating resistive thin films of ZrN with the dimension of 100 μm,

which work both for the heater and the thermometers. Employing MEMS (Micro Electro-

Mechanical Systems) technique, these resistive thin films are sustained only by 1-μm thick

membrane, so that the heat current is essentially restricted to the crystal. Using the latter

technique, we can measure crystals of submillimeter sizes. Temperature profiles of the thermal

conductivity of rubrene single crystals grown by physical vapor transport showed a well-defined

peak, indicating that the crystal is indeed of exceptional quality as compared to literature for

typical organic charge-transfer complex. The crystals grown from solution have smaller peaks in

general, which suggests more inclusion of defects in the crystals. It turned out that it grows in

proportion to T2 at low temperatures, meaning that the dominant scattering source is strain

dislocations distributed in the crystals. Moreover, as the result of further quantitative estimation,

typical density of the crystal dislocations is estimated to be in the order of 1016

cm-3

, which is

consistent with the performance of double-sided rubrene single crystal transistors interacting

with each other over 1-μm distance, if we assume the dislocations themselves induce deep hole-

trap levels in the bulk of the rubrene crystals.

B10.60

Low and Intermediate Coverage Self-assembled Structures of Alkanethiolates and

Phenylthiolate on Au(111) Incorporating Au-adatoms Peter Maksymovych1,2

and John T

Yates, Jr.3,2

; 1Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak

Ridge, Tennessee; 2Department of Chemistry, University of Pittsburgh, Pittsburgh,

Pennsylvania; 3Department of Chemistry, University of Virginia, Charlottesville, Virginia.

Molecular self-assembly is a quickly developing field of nanoscience that aims to tailor self-

recognizing and self-organizing properties of molecules for the bottom-up construction of

complex molecular systems and implementation of designer molecular functionality. Self-

assembled monolayers of alkanethiols on Au(111) are intensely studied both as a model system

yielding three-dimensional, crystalline monolayers and in practical applications involving

surface functionalization. Despite numerous previous attempts to understand the atomistic

picture of self-assembly, most of its aspects have remained controversial. Until only recently, the

gold surface has been considered a passive, unreconstructed template which provides a series of

high-symmetry adsorption sites for the alkylthiolate molecules. Several recent experiments have,

however, challenged this conservative view [1-4]. In this talk we will present self-assembled

structures of the methylthiolate (CH3S) from the lowest up to its saturation coverage of 1/3 ML,

developed based on high resolution STM images and DFT modeling. All these structures are

constructed using essentially the same building block of two CH3S-species joined by a Au-

adatom. Contrary to a common view that methylthiolate does not form a two-dimensional stripe-

phase monolayer, we show compelling STM evidence that methylthiolate-adatom complexes do

arrange into a stripe-like structure, albeit with preferentially 1D ordering. We compare the

“stripe-phases” of methylthiolate, propylthiolate and phenylthiolate [4], each of which is distinct

despite being constructured from constitutionally identical building blocks. The case of

phenylthiolate is intriguing as the steric hindrance of the phenyl groups prevents the

phenylthiolate-adatom complexes from forming well-ordered stripes as in methylthiolate.

However, weak C-H…S hydrogen bonding does arrange these complexes in a locally-ordered

fashion. Phenylthiolate self-assembly on the Au(111) surface is therefore strikingly similar to its

collective behavior on nanoparticles comprising as few as 100 atoms. Thorough understanding of

the low-coverage phases will enable resolving the self-assembled structures at higher coverages,

with the symmetry of (3x4) and the √3x√3R30o for methylthiolate and c(4x2) for long-chain

alkanethiolates. [1] P. Maksymovych, D. C. Sorescu, and J. T. Yates, Jr., Phys. Rev. Lett. 97

(2006) 146103. [2] M. Yu, N. Bovet, C. J. Satterley, et. al., Phys. Rev. Lett. 97 (2006) 166102.

[3] A. Cossaro, R. Mazaarello, R. Rousseau, et. al., Science 321 (2008) 943. [4] P.

Maksymovych, J. T. Yates, Jr., J. Am. Chem. Soc. 130 (2008) 7518. P. M. and J.T.Y: Supported

by the W. M. Keck Foundation and by the Army Research Office. P.M.: Research performed in

part as a Eugene P. Wigner Fellow and staff member at the Oak Ridge National Laboratory,

managed by UT-Battelle, LLC, for the U.S. Department of Energy under Contract DE-AC05-

00OR22725.

B10.61 The Self-assembly of Thiol-modified Diamondoids on Silver and Gold Trevor M Willey

1,

Jonathan R I Lee1, Jason D Fabbri

2, Peter R Schreiner

3, Andrey A Fokin

3, Boryslav A

Tkachenko3, Natalie A Fokina

3, Jeremy E P Dahl

4, Robert M K Carlson

4, Louis J Terminello

4,

Nick A Melosh2 and Tony van Buuren

1;

1Lawrence Livermore National Laboratory, Livermore,

California; 2Stanford University, Stanford, California;

3Justus-Liebig University, Giessen,

Germany; 4MolecularDiamond Technologies, Chevron, Richmond, California.

Higher diamondoids, hydrocarbon cages with a diamond-like structure, have largely evaded

laboratory synthesis but can be purified from petroleum sources. This new class of nanometer-

sized rigid hydrocarbon molecules shows promise in various areas of nanotechnology.

Particularly, computation indicates individual diamondoids possess negative electron affinity,

and diamondoid monolayers exhibit incredibly intense, monochromatic photoemission. Such

surface-attached diamondoids have technological possibilities as high-efficiency field emitters in

molecular electronics, as well as other nanotechnological applications, and fundamental studies

of the properties of these monolayers are a necessary precursor. Methods to site-selectively

functionalize diamondoids enable formation of controllable diamondoid self-assembled

monolayers on surfaces. We have investigated a number of thiol-modified diamantanes,

triamantanes, tetramantanes, and pentamantanes adsorbed on silver and gold, using near-edge x-

ray absorption fine structure spectroscopy (NEXAFS) and x-ray photoelectron spectroscopy

(XPS). The results illustrate how thiol position, diamondoid size, and diamondoid shape affect

the diamondoid self-assembled monolayer structure.

B10.62

Abstract Withdrawn

B10.63

Oligothiophene Derivatives Functionalized with a Diketopyrrolopyrrolo Core for Solution-

Processed Field-Effect Transistors: Effect of Alkyl Substituents and Thermal Annealing. Mananya Tantiwiwat

1,2,3, Arnold Tamayo

2,3, Ngoc Luu

2,3, Xuan-Dung Dang

2,3 and Thuc-Quyen

Nguyen2,3

; 1Physics, University of California, Santa Barbara, Goleta, California;

2Chemistry and

Biochemistry, University of California, Santa Barbara, Goleta, California; 3Center of Polymers

and Organic Solids, University of California, Santa Barbara, Goleta, California.

Two new oligothiophene derivatives bearing a diketopyrrolopyrrole core, 2,5-di-n-hexyl-3,6-bis-

(5''-n-hexyl-[2,2';5',2'']terthiophen-5-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione (DHT6DPPC6) and 2,5-

di-n-dodecyl-3,6-bis-(5''-n-hexyl-[2,2';5',2'']terthiophen-5-yl)pyrrolo[3,4-c]pyrrole-1,4-dione

(DHT6DPPC12), and their use in solution processed organic field effect transistors are reported.

Depending on the type of alkyl substituent and film annealing temperature, the crystal grain sizes

and interlayer spacing vary as observed using atomic force microscopy and X-Ray

diffractometry, respectively. These changes in film morphology and interlayer spacing lead to

one order of magnitude difference in the field effect mobilities. The field effect mobilities for

annealed DHT6DPPC6 and DHT6DPPC12 films are 0.02 cm2/Vs and 0.01 cm2/Vs,

respectively.

B10.64

Potential Mapping in the Channel of Organic Field Effect Transistors by Additional Sense

Contacts. Risha Sharma, Benedikt Gburek, Torsten Balster and Veit Wagner; Jacobs University

Bremen, Bremen, Germany.

In the past few years organic semiconductors have attracted considerable interest in research as

well as in industrial applications. In particular, they are used in thin film transistors offering the

option for cheap and large area production of electronic circuits. However, organic

semiconductors exhibit a wide range of transport properties with often rather complex behaviors,

which are not well understood compared to their inorganic counterparts. The analysis and

understanding of these properties is often difficult if only the integral IV measurements are

available. A very helpful additional information in this context is the potential distribution in the

channel of the organic field-effect transistor (OFET). One very valuable option for this purpose

is the Scanning Kelvin Probe method, which, however, is often restricted to vacuum environment

and cannot access directly buried interfaces. To overcome these restrictions in our approach we

pattern sense fingers into the channel by lithographic means in addition to the source and drain

electrodes of the transistor. These sense electrodes are in direct contact with the conducting

channel and their potentials can be directly accessed by high impedance electrometers. This

approach works in atmospheric conditions and even for buried channels, i.e. in top gate

geometry. As organic semiconductor thiophenes in form of small molecules, thiophene

oligomers, and as polymers, regio-regular poly-(3-hexylthiophene) (rr-P3HT), are investigated.

These materials are known to show rather differing electronic transport properties. For these

systems the additional sense fingers allow to analyse contact properties to the gold contacts as

well as the charge carrier density dependent mobility in detail. For the consistent interpretation

of the measured current and potential data a combined model of contact resistance, parallel bulk

resistance, and charge carrier density dependent mobility on the basis of the Vissenberg-Matters

model is required. This theoretical model was found to be suitable to describe the various

semiconducting materials investigated as well as different device designs, as there are top and

bottom gate structures or the use of silicon or plastic substrates. The presented method has

proved to provide valuable additional information of the internal properties of the conducting

channel and the contacts and is compatible to standard production techniques and operation

environments of organic field-effect transistors.

B10.65

Semiconducting Polythiophenes at High Carrier Densities: Nonlinear Transport in

Agreement with Luttinger Liquid Theory. Jonathan Dsu-Bei Yuen1, Reghu Menon

2, Nelson E

Coates1, Ebinazar B Namdas

1, Shinuk Cho

1, Scott T Hannahs

3, Daniel Moses

1 and Alan J

Heeger1;

1Center for Polymers and Organic Solids (CPOS), University of California, Santa

Barbara, Santa Barbara, California; 2Department of Physics, Indian Institute of Science,

Bangalore, India; 3National High Magnetic Field Laboratory, Tallahassee, Florida.

We present strong evidence that the nonlinear temperature-dependent transport data in field

effect transistors (FETs), with poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)

(PBTTT) as the electronically active semiconductor, and doped PBTTT films agrees with the

predictions of the Luttinger Liquid (LL) theory. Regiosymmetric PBTTT is a planar, rigid rod

conjugated polymer which exhibits a liquid crystalline phase occurring below the melting point.

Thus, thin films can be fabricated with large crystalline domains ranging from hundreds of

nanometers up to a few microns, exhibiting a high degree of crystallinity (90% and above).

Within the domains, the PBTTT chains are straight and aligned parallel to each other, and while

there is possibility for finite overlap of the π-orbitals between chains due to π- π stacking, the

intrachain π-electron overlap is similar to that of cis-polyacetylene, exceeding the interchain

overlap. Therefore the bandwidth that determines delocalization and, thus, the transport along the

chain (W⊥) is several eV, whereas the corresponding bandwidth for interchain transport (W∥) is

comparable to that of molecular crystals and therefore at least an order of magnitude smaller.

Thus, based upon the structure and assuming W⊥/W∥ >> 1, PBTTT would be properly classified

as a quasi-one dimensional (1D) system.

B10.66

Electron Transfer/transport in 2D Nanoparticle Array/polymer Composite Thin Layers. Shisheng Xiong

1, Yongqian Gao

2, John K Grey

2 and Jeffrey C Brinker

1,3;

1Department of

Chemical Engineering, University of New Mexico, Albuquerque, New Mexico; 2Department of

Chemistry, University of New Mexico, Albuquerque, New Mexico; 3Advanced Material

Laboratory, Sandia National Laboratories, Albuquerque, New Mexico.

The mechanisms of charge carrier transfer across the metal/organic interfaces and the charge

transport at the nanoscale are crucial for molecular electronics and are under extensive

investigation for photovoltaics, light-emitting diodes and sensors. A universal, fast and facile

method to prepare robust, freestanding and patternable NP/polymer composites by evaporation-

induced self-assembly on a fluid interface has been developed (JACS 2008). The thiol-capped

nanoparticles (about 5.5nm in diameter) embedded within the polymer matrix are 2D monolayer

arrays, ordered in a hexagonal close-packed (hcp) arrangement, enabling us to study the charge

transfer/transport characteristics in a 2D configuration. Using a "shadow deposition" technique,

the gold nanoparticle (GNP)/conjugated polymer films are then transferred onto CINT*

Electrical Transport and Optical Spectroscopy Discovery Platforms™ with prefabricated

interdigitated fingers (grid spacing around 200 microns). The absorption spectra of composite

films are almost identical to the pure P3HT films. AFM scanning has verified that the composite

films are conformal with the electrodes. Sheet resistance of the composite films are measured at

both the ground state and excited state by charge injection through the source-drain setup using a

van der Pauw four probe station. Combining both the intensity and frequency resolved spatial

Raman spectra, acquired by Raman scanning spectroscopy, in terms of the C=C stretching mode

of P3HT with a potential bias applied across the interdigitated electrodes, charge carrier

migration and photophysical processes are interrogated. Experimental results show that the

Raman intensity of the composite films transferred to the Discovery Platform is greater than

films transferred to bare silicon because of possible electrochemical interactions between the

film and gold fingers. Increasing the potential leads to a decrease of the intensity difference due

to charge injection. Both thermal annealing and potential increase result in a downshift of the

Raman frequency of the C=C stretching mode, suggesting that charge transport has been

facilitated. *(Center for Integrated Nanotechnology, Sandia National Laboratories and Los

Alamos National Laboratories)

B10.67

Abstract Withdrawn

B10.68

Investigating the Role of Interfaces in Nanotube Polymer Composite Films by Resonant

Photoconductive Decay. Katherine E. Hurst1, Richard K Ahrenkiel

2 and John H Lehman

1;

1National Institute of Standards and Technology, Boulder, Colorado;

2Colorado School of Mines,

Golden, Colorado.

Incorporating single-walled carbon nanotubes into conjugated polymers composite thin films

facilitates the transport of electrons and increases the overall efficiency of organic

photoconductive films. The role of the interface between the nanotube and polymer is

fundamental to the dissociation of photoexcited excitons, and understanding this mechanism is

needed to increase the overall efficiency of devices. In this work, we apply a resonant-coupled

photoconductive decay (RCPCD) method to determine the recombination lifetime of nanotube

polymer composite films. The carrier recombination lifetime of carbon nanotubes is typically

determined by contact-based techniques or spectroscopic methods. RCPCD is a non-contact

measurement that is based on a pump-probe technique in which an optical pump and a low-

frequency microwave probe are employed. This method is well suited for characterization of

bulk and extrinsic material properties. Our results demonstrate the role of interfaces in regards to

nanotube purity, concentration and extent of dispersion on the recombination lifetime. The

mechanisms describing the interaction of photoexcited carriers and the composite material will

be discussed. Raman spectroscopy, UV-VIS absorption and four-point probe measurements

provide further identification and characterization of composite thin films. Finally, we report the

wavelength dependence of photoconductive lifetimes.

B10.69 Molecular Gating of Silicon-on-Insulator Surfaces. Girjesh Dubey

1,2, Gregory P Lopinski

1

and Federico Rosei2;

1Steacie Institute for Molecular Sciences, National Research Council,

Ottawa, Ontario, Canada; 2INRS-EMT (Energie, Materiaux, et Telecommunications), Universite

du Quebec, Varennes, Quebec, Canada.

The electronic properties of semiconductors respond strongly to transverse electric fields, which

penetrate susbstantially over macroscopic lengths. Nanometer-scale crystalline films are

therefore attractive systems for detecting charge state occupation at molecular interfaces,

inducing surface band-bending. Silicon-on-insulator (SOI) platforms are especially sensitive to

this, since the thickness of the top silicon layer is comparable to the Debye length of the material.

Therefore conductivity is ideally suited as a dynamic probe of events such as molecular

physisorption1,chemisorption

2, surface dipoles, and occupation of electrically active gap states.

In this work, sheet resistance (Rs) and Hall effect measurements (VH) in high vacuum

environments have been used to monitor adsorption (desorption) events on hydrogen terminated

silicon-on-insulator films (H-SOI). Complementary measurements of pseudo-MOS transistor

characteristics have been used to monitor shifts in threshold voltage (VT). Effects from gases in

the torr range are discussed for n and p doping at room temperature. “Hole-trapping” species,

such as (C5H5N:) and ammonia (:NH3) are found to mimic gate action analagous to a field effect

transistor, biasing p-type surfaces into inversion. Their adsorption on n-type SOI produces a high

concentration electron accumulation layer. These effects are reversible by desorption.

Surprisingly, minority channels have also been formed from water vapor (H2O) isotherms. High

electron affinity “electron-trapping” species, such as tetracyanoethylene (C2(CN)4), have been

found to severely deplete n-type SOI even at 10 ppb exposure levels. These results demonstrate

the efficacy of dc transport on SOI platforms for studies of molecular adsorption and charge

transfer effects at semiconductor surfaces. References: 1G. Dubey, G. P. Lopinski, and F. Rosei,

Applied Physics Letters 91, 232111 (2007). 2 G. P. Lopinski, B. J. Eves, O. Hul'ko, et al.,

Physical Review B (Condensed Matter and Materials Physics) 71, 125308 (2005).

B10.70

Understanding the Modulation of Semiconductor Band-bending through Transport

Studies of Metal-molecule-semiconductor Device Junctions. Archana Bahuguna, Fernanda

Alanis-Camacho, Riya Shergill, Avik Ghosh and Nathan Swami; Electrical Engineering,

University of Virginia, Charlottesville, Virginia.

An understanding of the effects of molecular adsorption on the modulation of semiconductor

band-bending is of great significance for the characterization of electronic coupling in a variety

of device paradigms such as contacts for molecular electronics, [1], probing the passivation of

semiconductor surface states,[2], and probing signal transduction for sensor schemes based on

the controlled introduction of surface charge using molecular dipoles on self-assembled

monolayers (SAMs) on semiconductor substrates. [3]. The fitting of experimental transport data

at metal-molecule-semiconductor junctions to models on electronic coupling,[4], can enable the

extraction of significant device parameters that can eventually guide the development of novel

chemistries and device structures that are based on modulated semiconductor band bending.

Much of the prior models do not account for tunnel barrier shape and asymmetry of the Schottky

device junctions. In this study, the Non-equilibrium Green‟s Function (NEGF) formalism was

used to examine electronic transport and band bending at metal - molecule - semiconductor

device junctions, and correlate transport characteristics to experimental I-V data. COOH-

terminated SAMs of varying length (SH-CnCOOH, n=7, 10, 15) were deposited on patterned

areas (7 μm) on GaAs (001) substrates; and copper complexation and electroless deposition

methods were used to ensure the deposition top metal contacts, without any penetration into the

underlying SAM layer [5]. In this manner, the molecular device length was varied from 0.7 nm

to 2 nm, and substrate doping was varied from n+ doping (1018/cm3) to p+ doping (1019/cm3).

Based on this, the effects of varying SAM length, substrate doping, and top-contact work

function on electronic transport mechanisms were interpreted by fitting the model to the

experimental I-V characteristics to enunciate the role of the tunnel barrier shape, semiconductor

band bending, depletion width and surface charge on transport. [1] Lodha S, Carpenter P, Janes

DB, J. Appl. Phys. 2006, 99. [2] Y. Liu, H. Yu, J. Phys. Chem. B 2003, 107, 7803-7811. [3]

Ashkenasy, G.; Cahen, D.; Cohen, R.; Shanzer, A.; Vilan, A. Acc. Chem. Res. 2002, 35, 121. [4]

Nesher G., Vilan A., Hwang J., Cohen H., Cahen D., Amy. F., Chan C., Hwan H., Kahn A., J.

Phys. Chem. B 2006, 110, 14363-14371. [5] Camacho-Alanis F., Wu L., Zangari G., Swami N.

Journal of Mat. Chem. DOI/10.1039/b800001a. In press.

B10.71

Effects of Molecular Packing and Film Thickness on the Performance of N-Channel

Organic Thin Film Transistors. Joon Hak Oh1, Ya-Sen Sun

1, Ruediger Schmidt

2, Frank

Wuerthner2 and Zhenan Bao

1;

1Department of Chemical Engineering, Stanford University,

Stanford, California; 2Institut für Organische Chemie, Universität Würzburg, Würzburg,

Germany.

The availability of high performance n-channel organic semiconductors is indispensable to

practical applications such as p-n junctions, bipolar transistors and complementary circuits. Two

main issues in n-channel organic thin film transistors (OTFTs) are i) high performance and ii)

air-stability. To illuminate the interplays between molecular structures, electronic properties,

solid-state packings, and field-effect mobilities, we have prepared OTFT devices based on a

series of core-halogenated PBIs with varying fluorinated imide substituents. We present a

detailed evaluation on the influence of bay and imide substituents of perylene diimide (PDI)

derivatives on the crystal packing and their electrical performance in n-channel OTFTs.

Furthermore, the air-stability mechanisms have been investigated in depth with PDI thin films as

functions of the lowest unoccupied molecular orbital (LUMO) energy levels and the thin film

thickness. Our findings provide a new insight on the performance and air-stability of n-channel

OTFTs.

B10.72

Abstract Withdrawn

B10.73

Increase in Open-circuit Voltage and Improved Stability of Organic Solar Cells by

Inserting a Molybdenum Trioxide Buffer Layer. Hideyuki Murata, Yoshiki Kinoshita,

Yoshihiro Kanai and Toshinori Matsushima; School of Materials Science, Japan Advanced

Institute of Science and Technology (JAIST), Nomi, Ishikawa, Japan.

Power conversion efficiency (ηP) of organic solar cells has steadily improved through the use of

new materials and device structures. In particular, great efforts have been made for the

enhancement of short-circuit current density (Jsc). The use of bulk heterojunctions (e.g., the

composite of p-type and n-type materials) as an active layer is very effective to increase in Jsc in

both polymer and small molecule-based solar cells. However, in the bulk heterojunction solar

cells, it is quite challenging to precisely control the formation of interpenetrating network by

fabrication process such as annealing condition. Furthermore, there is no enhancement effect of

open-circuit voltage (Voc) due to the formation of interpenetrating network. For the further

improvement of ηP, it is essential to enhance Voc, with maintaining the corresponding Jsc. In

this study, we found that a modification of ITO surface by a high work function metal oxide

(molybdenum trioxide MoO3) is very effective to increase in Voc. We demonstrate the

systematic control of Voc as a function of the film thickness of MoO3 buffer layer in the organic

solar cells. The open-circuit voltage increased from 0.57 to 0.97 V as the thickness of MoO3 film

is increased from 0 to 50 nm in the device structure of indium-tin-oxide ITO/ MoO3 (x nm) /

5,10,15,20-tetraphenylporphine (H2TPP, 10 nm) /C60 (40 nm)/bathocuproine (10 nm) /Ag (100

nm). The values between Voc and the ionization potential of MoO3 (x nm) on ITO exhibit linear

relationship, where the Ip values change from 4.92 to 5.92 eV as they increase from 0 to 50 nm.

Interestingly, the enhancement of Voc was achieved without affecting the Jsc and the fill factor.

Consequently, the power conversion efficiency of the device increases from 1.24% to 1.88%

primarily due to the increase in Voc. We also found that a MoO3 buffer layer enhances the

stability of organic solar cells (OSCs) under photo-irradiation. We have investigated the OSCs,

where the structure is ITO/ MoO3 (0 or 20 nm)/ p-type layer/ C60 (40 nm)/ Bathocuproine

(BCP) (10 nm)/ Ag (100). As a p-type layer, we use H2TPP and N,N‟-di(1-naphthyl)-N,N‟-

diphenylbenzidine (α-NPD). Without MoO3 layer, the devices showed a dramatic decrease in ηp

as increasing light exposure time (18 % for H2TPP and 45 % for α-NPD in initial ηp after 30

min). On contrary, the both devices with MoO3 layer showed good stability maintaining 72 %

for H2TPP and 95 % for α-NPD in initial ηp at the same measurement conditions. These results

clearly suggest that the ITO/p-type layer interface affects the device stability. We investigated

the enhancement mechanism of the stability and found that the reaction at ITO/p-type layer was

prevented by inserting MoO3 buffer layer.

B10.74

Structural and Electronic Properties of Thin Films of Organic Charge-transfer

Compounds.Michael Kroeger and Antoine Kahn; Department of Electrical Engineering,

Princeton University, Princeton, New Jersey.

So-called organic charge transfer complexes usually consist of a mixture of one species of

molecules with very strong electron-accepting properties and one species which acts as an

electron donor. In single-crystal form, these materials have been extensively studied in the 1970s

and 1980s. It was shown, that in CT-compounds, donor and acceptor molecules form segregated

stacks and very often exhibit one- or two dimensional charge transport along donor and acceptor

planes. Further, the band gap energy of organic CT-compounds and therefore their electric

properties strongly depend on the degree of charge transfer between donor and acceptor

molecules. Despite offering the possibility to tailor-design electric properties by substitution or

chemical modification of the constituting species, organic CT-compounds have not played a

significant role as materials for organic electronic devices. This is due to the technical difficulties

which arise when handling organic single crystals, and which possibly exclude manufacturing of

devices based on organic single crystals. In contrast, most applications for organic

semiconductors, e.g. organic light emitting diodes, organic field-effect transistors and organic

photovoltaic cells, rely on thin-film deposition techniques. Therefore, we studied the thin-film

growth of organic CT-compounds deposited by thermal evaporation and their electronic

structure. Materials which have been used for this study are combinations of the donor molecules

dibenzo-tetrathiafulvalene (DB-TTF) and bis-pentamethylene-tetrathiafulvalene (BPM-TTF) and

acceptor molecules tetracyanoquinodimethane (TCNQ) and tetrafluorotetracyano-

quinodimethane (F4-TCNQ). The film morphology was studied with atomic force microscopy

(AFM) and x-ray diffraction (XRD). For resolving the electronic structure, ultra-violet

photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES) were used.

Electrical characterization of two-terminal thin-film devices was carried out in-vacuo at

temperatures between 60 K and 340 K. On glass substrates (quartz), DB-TTF/TCNQ exhibits a

pronounced 3-D growth and continuous thin films could not be achieved. Replacing TCNQ by

F4-TCNQ yields polycrystalline but continuous thin films with grain sizes in the micron-range.

A very similar thin film growth is observed for BPM-TTF/TCNQ deposits on quartz. When

applying ternary CT-compounds the electrical conductivity increases by nearly two orders of

magnitude, for example, comparing DB-TTF/F4-TCNQ(0.9)/TCNQ(0.1) to DB-TTF/F4-TCNQ.

Whether this effect is related to a change in the electronic band structure and/or to a change in

morphology will be discussed.

B10.75

Structure Function Relationships of Conjugated Polyelectrolyte Electron

Injection/Transport Layers in Polymer Light Emitting Diodes. Andres Garcia1,2

, Jacek Z

Brzezinski1,2

, Youngeup Jin3 and Thuc-Quyen Nguyen

1,2;

1Department of Chemistry and

Biochemistry, University of California, Santa Barbara, California; 2Center for Polymers and

Organic Solids, University of California, Santa Barbara, California; 3Chemistry, Pusan National

University, Busan, Korea, South.

Charge injection and transport play an important role in organic light emitting diodes (OLEDs),

in which holes are injected from the anode into the highest occupied molecular orbital (HOMO)

and electrons are injected from the cathode into the lowest unoccupied molecular orbital

(LUMO) of the organic semiconductor. In the absence of interfacial effects, one needs to match

the energies of the HOMO and the LUMO with the work function of the anode and cathode,

respectively, so to minimize charge injection barriers. Stable metals with high work functions

thus typically give rise to larger electron injection barriers when used as cathodes and hence low

device performance. Recently, excellent device performance have been observed in OLEDs with

high work function cathodes such as Al, Ag, Cu and Au by deposition of a conjugated

polyelectrolyte (CPE) electron injection/transport layer (EIL/ETL). Some CPEs in OLEDs with

high work functions cathodes have been shown to lead to similar or higher performances than

identical OLEDs with no or low electron injection barrier low work function cathodes, allowing

the fabrication of air stable devices. While others CPEs have exhibited much lower

performances. How the molecular features of these materials modify the function of the

multilayer OLED performances remains to be fully understood. Here we present structure

function relationship studies of CPE EILs/ETLs in polymer light emitting diodes (PLEDs) in

which the influence of the conjugated backbone, appended ionic functionality and counter-ion

electronic properties on electron transport and PLED device performance are investigated. In

CPEs with identical appended cationic units and counter-ions but different conjugated backbone

a difference of ~ 10 in electron mobility and PLED luminous efficiency is observed, with the

higher electron mobility CPE exhibiting the higher device efficiency. While CPEs with identical

conjugated backbones but bearing anionic units rather than cationic units, exhibit higher device

efficiencies. The electronic properties of halide counter-anions in cationic CPE are also found to

influence the device efficiency of PLEDs, with a correlation between the oxidative properties of

the counter-ion and device efficiencies. The attractive properties of CPEs with respect to device

improvement together with the uncertainties in mechanistic function argue in favor of systematic

studies with the long term goal of making concrete structure-function predictions.

B10.76

Fabrication of Long-Chain Organic Thin films by Physical Vapor Deposition Process for

Gas Sensor Applications Nilima V Hullavarad and Shiva S Hullavarad; Office of Electronic

Miniaturization, University of Alaska Fairbanks, Fairbanks, Alaska.

There is ever growing need for gas detectors characterized by 4 S‟s - viz., Sensitivity,

Selectivity, Speed and Stability. Organic based thin films are being investigated for detector

fabrication due to potential of scale up processing for commercial interest . The organic based

thin films have the advantages of higher selectivity because of ease of tailoring the chemical

composition of organic thin films that in turn affect the electronic properties. There have been

various techniques evolved in last two decades to grow organic thin films, predominantly by

solution based chemical techniques allowing to form detectors from bottom-up approach. The

technique described in this paper is CMOS compatible used in standard semiconductor

fabrication. In this paper, we demonstrate the simple physical deposition technique to form

organic thin films. This technique is shown to be useful to deposit long chain molecules with

amphiphilic nature. The long chain molecules such as stearic acid and calcium stearate are

deposited on Si, quartz and alumina substrates. The morphology of thin films is found to depend

on the crystalline nature of the substrates. The FTIR spectroscopy and XRD characterization

reveal that the bonding and the crystalline properties change with deposition temperatures within

20C (+/- 2 C). In conclusion, the feasibility of physical vapor deposition is demonstrated for

fabricating long chain organic molecules for organic electronic applications.

B10.77

Precise Structure of Pentacene Monolayers on Amorphous Silicon Oxide and Relation to

Charge Transport. Stefan C.B. Mannsfeld1, Ajay Virkar

1, Colin Reese

1, Michael F Toney

2 and

Zhenan Bao1;

1Department of Chemical Engineering, Stanford University, Stanford, California;

2Stanford synchrotron radiation laboratory (SSRL), Stanford, California.

<p>In the field of organic semiconductors, pentacene has developed into a benchmark material

because it easily and robustly yields high-performance thin film transistor (TFT) devices. The

charge transport in TFT devices occurs predominantly in a few monolayers above the dielectric

interface.[1] Therefore, knowledge of the precise packing in the first monolayer is important to

understand the charge transport properties of pentacene TFTs. Grazing incidence X-ray

diffraction (GIXD) using a synchrotron light source provides structural information on ultra-thin

films down to a single monolayer. Here we present the first direct determination of the molecular

packing in a pentacene monolayer on silicon oxide. Using crystallographic refinement

techniques, it is found that in pentacene monolayers on silicon oxide, the pentacene molecules

pack in a completely tilt-free herringbone motif, unlike in the commonly cited thin-film phase or

in the bulk crystal. The charge transport in the first monolayer is discussed on the basis of

density functional theory calculations. The results explain the high performance of pentacene

TFTs relative to that of pentacene single crystals on the silicon oxide substrates. [1] A.

Dodapalapur, L. Torsi, and H. E. Katz, Organic Transistors: Two-dimensional transport and

improved electrical characteristics, Science 268, 270-917 (1995).

B10.78 Probing the Nanomorphology of Annealed Conjugated Polymer Blends. Sufal Swaraj

1,

Cheng Wang2, Chris R McNeill

3, Benjamin Watts

4 and Harald Ade

1;

1Department of Physics,

North Carolina State University, Raleigh, North Carolina; 2Advanced Light Source, Lawrence

Berkeley National Laboratory, Berkeley, California; 3Cavendish Laboratory, Department of

Physics,, University of Cambridge, J J Thomson Ave,, Cambridge, CB3 0HE, United Kingdom; 4Paul Scherrer Institute, 5232 Villigen PSI, Switzerland.

We investigate the influence of annealing on initially intimately mixed polymer blends of the

conjugated polymers poly(9,9'-dioctylfluorene-co-bis-N,N'(4,butylphenyl)-bis-N,N'-phenyl-1,4-

phenylene-diamine) (PFB) and poly(9,9'-dioctylfluorene-co benzothiadiazole) (F8BT). Scanning

Transmission X-ray Microscopy (STXM) imaging with sub-100 nm resolution was used for

investigating the evolution of the morphology. An increase in domain size is observed with

annealing due to phase separation. Photoluminescence (PL) quantum efficiency studies of these

films indicate a direct correspondence with the domain size for blends annealed at temperatures

above 180 degC. The phase separation first evolves with the evolution of relatively pure phases

at length scales beyond the resolution of STXM. At higher temperatures (>=180degC) the

hierarchy of phase separation is lost and the length scales can be readily imaged by STXM.

Resonant Soft X-ray Scattering (RSoXS) studies show an evolution towards purer domains along

with increase in domains size with annealing. At annealing temperatures below 180oC, the

correlation function analysis of the RSoXS data shows the evolution and distribution of

subdomains that span a range of length scales (4 - 100nm).The correlation function complements

the average domain size obtained from the PL data and a Monte-Carlo model that takes the

excition diffusion into account.

B10.79 Probing the Buried Interface in Annealed Conjugated Polymer Bilayers. Sufal Swaraj

1,

Cheng Wang2, Hongping Yan

1, Chris R McNeill

3 and Harald Ade

1;

1Department of Physics,

North Carolina State University, Raleight, North Carolina; 2Advanced Light Sounce, Lawrence

Berkeley National Laboratory, Berkeley, California; 3Cavendish Laboratory, Department of

Physics,, University of Cambridge, J J Thomson Ave,, Cambridge, CB3 0HE, United Kingdom.

The properties of polymer/polymer interfaces in conducting polymeric devises critically

influences device performance, yet relatively few studies of such interfaces have been

performed. We show that Resonant Soft X-ray Reflectivity (RSoXR) as a powerful tool for the

characterization of bilayers of conducting polymers, a material class that had mostly been

investigated with neutron reflectivity. The rapid changes of optical properties near the Carbon

absorption edge provides selectivity to specific chemical moieties and high contrast for

investigated materials. We exemplify the use of this technique for conducting polymers by

characterizing the buried interface in bilayers of poly(9,9'-dioctylfluorene-co-bis-

N,N'(4,butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) (PFB) and poly(9,9'-

dioctylfluorene-co benzothiadiazole). We investigate the influence of annealing on the

polymer/polymer interface and the surface and quantify the surface and interfacial widths.

RSoXR results point to an interesting strategy that will allow the interdiffusion and physical

roughness at a buried polymer/polymer interface to be determined separately by diffuse

scattering at an angle and photon energy where the top surface exhibits little scattering, yet the

polymer/polymer interfaces will exhibit total internal reflection.

B10.80 Novel Micro and Nano Patterning Techniques for Organic Electronic Systems. Alexander

Zakhidov, Jin-Kyun Lee, John DeFranco, Hon Hang Fong, Priscilla G Taylor, Christopher K

Ober and George G Malliaras; Cornell University, Ithaca, New York.

Organic electronics and optoelectronics are fast developing branches of modern science and

technology that are aiming to replace conventional inorganic materials with light, inexpensive,

flexible organic materials. One of the main issues to be solved on the route to real word

application is patterning and processing of thin layer active organic materials. In this report we

present new reliable photolithography micro and nano patterning techniques for high resolution,

high throughput patterning of solution processable organic materials [1,2]. We also demonstrate

that proposed approach is completely benign [3] to majority of organic electronic materials as

well as environmental friendly and can be easily adopted by industry. In order to demonstrate

potential application of developed patterning technique we fabricate micropatterned top-contact

organic thin film transistors and prototypes of organic light emitting displays. [1] H. S. Hwang,

Al. A. Zakhidov, J.-K. Lee, X. Andre, J. A. DeFranco, H. H. Fong, A. B. Holmes, G. G.

Malliaras, C. K. Ober, Journal of Materials Chemistry, 2008, 18, 3087. [2] J.-K. Lee, M.

Chatzichristidi, Al. A. Zakhidov, P. G. Taylor, J. A. DeFranco, H. S. Hwang, H. H. Fong, A. B.

Holmes, G. G. Malliaras, C. K. Ober, J. Am. Chem. Soc., 2008, 130, 11564. [3] Al. A. Zakhidov,

J.-K. Lee, H. H. Fong, J. A. DeFranco, M. Chatzichristidi, P. G. Taylor, C. K. Ober, G. G.

Malliaras, Advanced Materials, 2008, 20, 3481.

B10.81

Factors Determining the Efficacy of Optical Spacers in Polymer Solar Cells: The Role of

Active Layer Morphology. Anshuman Roy, Sarah Mednick, Sung Heum Park, Ji Sun Moon

and Alan J Heeger; University of California, Santa Barbara, California.

Polymer photovoltaic devices stand at the cusp of rapid commercialization today, with the

maximum power conversion efficiency reported to be over 6% [J. Y. Kim et al., Science, 317

(2007) 222-225]. The photo-active layer in these devices consists of conjugated polymers and

modified fullerenes that form a bulk heterojunction (BHJ) composite film about 100 nm in

thickness. Additionally, a layer of TiOx (Titanium Oxide) approximately 10 nm in thickness is

spin cast on top of the active layer to act as an optical spacer that maximizes the incident light

intensity in the photo-active layer [Kim et al., Adv. Mater., 18 (2006) 572-576]. Using a

combination of numerical modeling and experiments, including ellipsometry, transmission

electron microscopy and cell efficiency, we show that the efficacy of the TiOx layer as an optical

spacer is dependent not only on the thickness of the BHJ layer but also on the nano-scale

structure of the BHJ layer, which in turn depends on a host of processing conditions.

B10.82

Transport Anisotropy in Films of Organic n-type Semiconductor with Controlled In-plane

Grain Boundary Orientation. Jonathan Rivnay1, Antonio Facchetti

2,3 and Alberto Salleo

1;

1Dept. of Materials Science and Engineering, Stanford University, Stanford, California;

2Polyera

Corporation, Skokie, Illinois; 3Dept. of Chemistry, Northwestern University, Evanston, Illinois.

Solution processable small molecule organic semiconductors have gained interest due to their

potential low cost processing and field effect mobilities nearing that of their vapor deposited,

non-soluble counterparts. Unfortunately, pristine films of soluble small molecules suffer from

poor thin-film transistor device-to-device reproducibility (mobility, VT and stability) due to the

existence of grain-boundaries that are not uniformly distributed throughout the film. The effect

of grain boundaries in thin films of small molecule semiconductors is relatively large compared

to that of semicrystalline polymer devices for two reasons: the grain size is often on the order of

the channel length preventing averaging effects and boundary regions between grains are more

abrupt. Indeed, in semicrystalline polymer films, a single chain can bridge two or more adjacent

crystallites, facilitating transport through amorphous-like regions/grain boundaries. Research

should thus focus on understanding of the relationship between microstructure and charge

transport in order to design devices that do not necessarily eliminate grain boundaries, but limit

their penalty on electrical performance, while also allowing for lower device-to-device

variability. To this end, in this work we use anisotropic films of the n-type small molecule N,N‟-

bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide), (PDI8-CN2) to explore the

effect of grain boundaries on field effect mobility, and understand their implication in charge

transport in thin films. To fabricate samples, we use an inclined drop casting method with a

heating stage. In-plane orientation and morphology are characterized with x-ray diffraction,

polarized light microscopy, and AFM. Thin film transistors (TFTs) were made to probe transport

as a function of charge density and temperature across two types of grain-boundaries that are

formed. These PDI8-CN2-based TFTs exhibit a mobility anisotropy of two orders of magnitude

depending on the relative orientation of the grains with the current flow. Parallel devices, with

charge transport presumably parallel to the fast growth direction of the crystallites show room

temperature mobilities above ~0.01 cm2/Vs (EA=120meV), near that of isotropic solution cast

films. Perpendicular devices, on the other hand, show low mobilities of ~10-4

cm2/Vs

(EA=340meV). The difference between the two orientations is much larger than a 2-3 fold

difference associated with crystalline anisotropy. The similar mobilities measured in parallel and

isotropic films suggest defects and grain boundaries of this device are similar to those present in

„typical‟ devices. Though the morphology and exact nature of grain boundaries in the orthogonal

direction are unknown we postulate that they are host to large energetic barriers, in agreement

with the larger EA, and may explain device-to-device non-uniformity in isotropic films, due to

the percolative nature of charge transport.

B10.83

Synthesis and Characterization of Soluble Copolymers Containing Dialkyl

Quarterthiophene. Jun Chen1, Sung J Park

1, Jae W Jang

1, Yun H Kim

2 and Soon K Kwon

1;

1School of Nano and Advanced Material Science & Engineering and ERI, Gyeongsang National

University, Jinju 660-701, Gyeonsangnam-do, Korea, South; 2Department of Chemistry and

RINS, Gyeongsang National University, Jinju 660-701, Gyeonsangnam-do, Korea, South.

Soluble conjugated 4ThFlu and 4ThNa copolymers were synthesized by the Suzuki coupling

reaction. The weight-average molecular weight of the 4ThFlu and 4ThNa copolymer were

determined to be 16848 and 13505, respectively. And the polydispersity indexes obtained by gel

permeation chromatography (GPC) using polystyrene standards for calibration in the eluent THF

is 1.55 and 1.59, respectively. The thermal, optical and electronic properties of copolymers were

investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV-

vis absorption, photoluminescence spectroscopies, cyclic voltammetry. The absorption

maximum spectra of copolymers were observed 452, 453 nm in solution state and 563, 563 nm

in film state. The PL maximum spectra of copolymers were observed at 467, 503 nm in solution

state and 585, 624 nm in film state, respectively. The copolymers were showed highly thermal

stability of decomposition temperature over 300 oC. Especially, the copolymers were easy to be

soluble in common solvents (THF, CHCl3, Toluene etc) due to the introduction of long alky

chains.

B10.84 Blue Organic Light Emitting Diodes from Solution Processed Small Molecules. Bright

Walker, Arnold B Tamayo, Wesley Walker, Jihua Yang, Fred Wudl and Thuc-Quyen Nguyen;

Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara,

California.

Solution-processed organic light-emitting devices provide the potential for cheap, ultrathin, light-

weight, and large-area illumination sources. In comparison with red or green light-emitting

devices, efficient blue emitting diodes are typically more difficult to obtain because the emitting

material requires a wider band gap for radiative recombination. Organic blue light-emitting

diodes were studied using the solution processed small molecules 2,7-dipyrenyl-9 ,9[|#1#|]-

dioctyl-fluorene as well as 1,3,5-tris(7,10-diphenylfluoranthene-8-yl)benzene as emissive layers

(EMLs). Devices were fabricated using the conjugated polyelectrolyte poly(9,9'-bis[6"-(N,N,N-

trimethylammonium) hexyl]fluorene-alt-co-phenylene) with tetrakis(imidazolyl)borate

counterions (PFN-BIm4) as an electron injecting layer and poly(9-vinylcarbazole) (PVK) as an

electron blocking layer. Efficient blue light emission was observed for devices with the

architecture ITO/PEDOT:PSS/PVK/EML/PFNBIm4/Al in which all organic layers were

deposited by solution processing. Using PVK and PFN-BIm4 layers results in a significant

improvement in device performance compared to devices without the layers or with common

electron injecting layers such as lithium fluoride or barium.

B10.85

Meta-stable Interfaces between Soft-contact and Closed-shell Semiconductor Surfaces. Yang Li, Wei Long and Raymond T. Tung; Physics Department, Brooklyn College, the City

University of New York, Brooklyn, New York.

The use of a self-assembled monolayer (SAM) of molecules to modify and tailor interface

electronic properties is an attractive approach for sensors and semiconductor electronic

applications, because of the molecules‟ functional variety and flexibility. In studies involving

SAM attached semiconductor surfaces, the deposition/application of metallic contacts is geared

toward minimizing disturbance to the molecular layer, i.e. for a preservation of meta-stable non-

interacting molecule-metal interfaces. However, interactions between the metal and the

molecular layer are often found to influence and even dominate the dipolar effect of the

molecular layer. Significant barrier height inhomogeneity has also been reported. In this work,

we attempt to establish a baseline for studies involving meta-stable metal-molecule interfaces. A

dual-station UHV chamber was constructed for this study. Silicon surfaces terminated with

different types of stable closed-shell configurations (Cl-, S- and H-) are used to simulate the

stable molecular termination on SAM/Si surfaces. Meta-stable metal-Si structures are fabricated

using soft-landing deposition of metal contacts (Au, Ag, and Al) with indirect deposition in an

inert gas ambient or direct deposition in vacuum, and at variable temperatures. In situ

characterization by surface spectroscopies and Kelvin probe techniques showed the varied levels

of intermixing under different deposition conditions. Electrical measurements, by variable

temperature I-V and C-V method, of Au/Si and Ag/Si diodes fabricated on n- and p-type <100>

and <111> substrates also showed a significant dependence on fabrication conditions of the

Schottky barrier. Lower deposition temperature led to more uniform contact between metal and

semiconductor, which then led to higher SBH on n-type Si, with the expected, opposite

dependence observed on p-type Si. Also, the orientation of the semiconductor surface was shown

to have a significant influence on the formation of the SBH. Electrical results and results

obtained from surface chemical analysis and microscopic techniques are presented with special

attention paid to the possible electrical inhomogeneity in the systems. These results are compared

with results involving molecular layers obtained previously in our and other groups.

B10.86

Abstract Withdrawn

B10.87

Picosecond Photoexcitation Dynamics in the Poly(2,7-Carbazole) Copolymer, PCDTBT,and

in Bulk Heterojunction Composites with PC70BM. Minghong Tong1, Nelson E. Coates

1,

Daniel Moses1, Alan J Heeger

1, Serge Beaupre

2, Mario Leclerc

2 and Russell Gaudiana

3;

1Center

for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara,

California; 2Departement de Chimie, Université Laval, Quebec City, Quebec, Canada;

3Konarka

Technologies, Inc, Lowell, Massachusetts.

We have studied the nature of ultrafast photoexcitations and their recombination dynamics in an

alternating donor-acceptor low-bandgap Poly(2,7-Carbazole) copolymer (PCDTBT; see Blouin,

N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295 - 2300) and in composites of that

polymer with the fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester (PC70BM); This

class of alternating donor-acceptor copolymers (with different acceptor units) holds promise for

photovoltaic applications because of the ability to tune the electronic energy levels by changing

acceptor units. The implied flexibility in the synthesis can lead to both a lower bandgap that

better uses the solar radiation spectrum, and a lower HOMO level that increases the open circuit

voltage of photovoltaic devices. We have used transient photinduced absorption spectroscopy,

over a wide spectral range in order to study the nature of the photoexcitations, and in particular

the carrier generation and recombination dynamics at short time scales. A very long carrier

lifetime (>>1ns) is observed in the PCDTBT-Fullerene composite.

B10.88

Steady-State and Transient Photoconductivity in the Poly(2,7-Carbazole) Copolymer

PCDTBT, and in Bulk Heterojunction Composites with PC70BM. Nelson Coates1, Minghong

Tong1, Daniel Moses

1, Alan J Heeger

1, Serge Beaupre

2, Mario Leclerc

2 and Russell Gaudiana

3;

1Physics, University of California, Santa Barbara, Goleta, California;

2Chimie, Université Laval,

Quebec City, Quebec, Canada; 3Konarka Technologies Inc., Lowell, Massachusetts.

We have studied the nature of carrier generation using steady-state and transient

photoconductivity in an alternating donor-acceptor low-bandgap Poly(2,7-Carbazole) copolymer

(PCDTBT; see Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295 - 2300) and in

composites of that polymer with the fullerene derivative [6,6]-phenyl C70-butyric acid methyl

ester (PC70BM). This class of alternating donor-acceptor copolymers (with different acceptor

units, X) holds promise for photovoltaic applications because of the ability to tune the electronic

energy levels by changing X. The implied flexibility in the synthesis can lead to both a lower

bandgap that better uses the solar radiation spectrum, and a lower HOMO level that increases the

open circuit voltage of photovoltaic devices. In PCDTBT, the absorption band extends out to ~

700 nm, with two distinct but broad absorption bands that are centered at ~ 400 nm and ~ 600

nm. Higher solar cell power conversion efficiency is achieved in PCDTBT devices than in P3HT

based devices because of the improved light harvesting and larger open circuit voltage. We have

used steady-state and transient photoconductivity to investigate the carrier generation and

collection efficiency of PCDTBT and its composite with the soluble fullerene, PC70BM, after

photoexcitation at each of its distinct absorption bands. In pristine PCDTBT, higher carrier

quantum efficiency is observed with excitation at the high energy absorption band, but in the

PCDTBT-fullerene composite, this efficiency is an order of magnitude greater and relatively

wavelength independent.

B10.89 High-Mobility n-Channel Organic Thin-Film Transistors Henry Yan and Antonio Facchetti;

Polyera Corporation, Skokie, Illinois.

We report here our recent progress enabling high-performance top-gate organic thin-film

transistors (OTFT). Electron mobility of ~ 1.0 cm2/Vs was achieved for top-gate bottom-contact

devices tested in ambient using both the semiconductor and dielectric layers fabricated by spin-

coating. To the best of our knowledge, this is first report of solution-processed top-gate n-

channel TFTs with mobility higher than 1 cm2/Vs. Furthermore, we will also present updated

performance of bottom-gate bottom-contact transistors using Polyera materials. Electron

mobility of ~ 0.1 cm2/Vs was achieved in a bottom-gate bottom-contact structure with both the

semiconductor and dielectric layers fabricated by spin-coating process. In this case, the dielectric

material is an UV crosslinkable dielectric polymer having good compatibility with n-type

organic semiconductors.

B10.90 Effects of Metal and Organic Impurities on Pentacene Electronic Structures Jing Xue

1,

Geunsik Lee1 and Kyeongjae Cho

1,2;

1Physics, University of Texas at Dallas, Richardson, Texas;

2Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas.

Organic semiconductors have received much attention due to their successful application in

optical and electronic areas. Pentacene is one of the most promising materials in the organic

semiconductors because of its highest hole mobility. In this work, we study the effects of metal

contact and organic impurities on pentacene‟s electronic structure by using ab initio density

functional theory (DFT) methods. Pentacene in the electronic devices directly contacts with

metal electrode, and metal atoms may diffuse into pentacene and form impurities interacting with

pentacene. Therefore, the interface between metal and pentacene can be modified by the metal

atomic diffusion. We calculate electronic structure of pentacene adsorbed with different metallic

elements (Au, Pd, Ni) and study their effects on the carrier injection barrier. We found that the

calculated energy gap for isolated pentacene molecule is 1.13 eV. The main contribution of

HOMO and LUMO come from pi orbital of C-atoms. For metal adsorption, the energy gap will

be decreased. Compared the binding energy and the bulk cohesive energy, Ni and Pd are easier

to diffuse into the pentacene than Au. Depending on the valence electron structure of the metal

atom to be adsorbed, the electronic structure of pentacene near the Fermi level has changed

differently. For Au and Ni, there will form new mid gap states and the new mid-gap states

decrease the contact barriers to improve the charge injection from the electrode. Organic

semiconductor device characteristics is also sensitive to organic impurities which may be

introduced during synthesis or by air exposure. It is worth to focus that dissociation of H2O into

H and OH may form some organic defect. We consider the defect in pentacene in forms of C-H2,

C=O and OH. All these organic impurities will give rise to gap states. For C-H2 or C=O defect,

the 6-top site is mostly favored energetically by an additional H adsorption (C22H15)or H

replacement by O (C22H13O), respectively. In both systems, the pz orbital on the perturbed C

atom no longer participates in the π bonding once the defect is introduced. For H replacement by

an OH, it seems that, there are no much differences between C22H14O and C22H14 because all

C atoms are still participates in their π bonding. However, for additional adsorption of OH,

C22H15O, the electronic structure is different from that of pentacene molecule. In addition, it is

shows that the electronic structure of pentacene with the same type of defect is affected by the

defect position. These calculations show the role of metal or organic impurities in generating the

defect states with the HOMO-LUMO gap of the pentacene molecules. These gap states may play

the critical role in limiting the mobility of the organic semiconductor channel and the efficiency

of the optical device applications. Our detailed electronic structure study provides a fundamental

insight on the effects of impurities in the pentacene device performance.

B10.91 Interface Reaction of Aluminum and 8-hydroxyquinolatolithium. Young Mi Lee

1, Yeonjin

Yi2, Jeong Won Kim

2 and Yongsup Park

1;

1Dept.of Physics, Kyung Hee University, Seoul,

Korea, South; 2Korea Research Institute of Standards and Science, Daejeon, Korea, South.

One of organic electron injection layer materials, Liq (8-hydroxyquinolatolithium) shows a few

advantages over other inorganic materials for organic light emitting device (OLEDs). As the Liq

possesses the similar structure to Alq3, the most common light emission layer material, it is

believed to provide a smooth interface and be more compatible to flexible displays. The energy

alignment and device performance including the Liq between Al and Alq3 have recently been

demonstrated. Here the interface chemical reaction at the Liq/Al interfaces was investigated by

using high resolution synchrotron radiation photoelectron spectroscopy. The different deposition

sequence gives different reactions. While strong reactions are observed throughout the Liq film

when Al is deposited on Liq layer, an interface localized reaction occurs just at the interface

upon the Liq deposition onto Al surface. Either sequence of film stacks, Liq/Al and Al/Liq

produce an interface gap state respectively at 2.1 eV and 2.8 eV below the Fermi level. Both of

the highest occupied molecular orbital (HOMO) and N 1s core level peaks are shifted to the high

binding energy side by 0.35 eV on Al/Liq whereas it is not the case on Al/Liq. Based on these

observations, the differences in electron injection barrier and interface dipole between the two

opposite deposition sequences could be drawn.

B10.92

Formation of Ohmic Carrier Injection at Anode/organic Interfaces and Carrier Transport

Mechanisms of Organic Thin Films. Toshinori Matsushima, Guang-He Jin, Yoshihiro Kanai,

Tomoyuki Yokota, Seiki Kitada, Toshiyuki Kishi and Hideyuki Murata; School of Materials

Science, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan.

Organic light-emitting diodes (OLEDs) are being developed due to their high potentials for use

in low-cost, mechanically flexible, and lightweight display, and lighting applications. In typical

multilayer OLEDs, a hole-injection barrier is present at the interface of an indium tin oxide

(ITO) anode and a hole-transport layer (HTL) and causes an increase in driving voltage of

OLEDs. Various organic and inorganic hole-injection layers (HILs) have been inserted between

ITO and HTL to reduce the driving voltages. However, Ohmic contacts have never been

achieved at the ITO/HIL/HTL interfaces to date. If Ohmic contacts can be formed, further

improvements of driving voltages, power conversion efficiencies, and stability of OLEDs are

possible. Besides the improvements in OLED performance, understanding carrier transport

mechanisms in organic films is very crucial to developing fundamental science. However, the

carrier transport mechanisms have not yet been elucidated, and must be further clarified to bring

about maximum device performance. Large charge carrier injection barriers at anode/HTL

interfaces would make clarifying the carrier transport mechanisms difficult because observed

currents are governed by both carrier injection and transport. Therefore, the formation of Ohmic

contacts at the interfaces is indispensable to clarifying the carrier transport mechanisms. In this

study, we found that insertion of an ultrathin molybdenum oxide (MoO3) HIL between ITO and

HTL provides Ohmic hole injection at the interfaces. We fabricated the hole-only devices with a

glass substrate/ITO anode (150 nm)/MoO3 HIL (X nm)/organic HTL (100 nm)/MoO3 electron-

blocking layer (10 nm)/Al cathode (100 nm) structure. As the HTL, we used pentacene, alpha-

sexithiophene (a-6T), copper phthalocyanine (CuPc), 4',4"-tris(N-3-methylphenyl-N-phenyl-

amino)triphenylamine (m-MTDATA), 4,4',4"-tris(N-2-naphthyl-N-phenyl-amino)triphenylamine

(2-TNATA), N,N-di(m-tolyl)-N,N-diphenylbenzidine (TPD), rubrene, or N,N-diphenyl-N,N-

bis(1-naphthyl)-1,1-biphenyl-4,4-diamine (a-NPD). Although the thickness of MoO3 HILs

previously used is in the range between 5 and 50 nm, the results in the present study clearly

indicated that such thick MoO3 HILs do not provide Ohmic hole injection. We found that the

optimized X is smaller than 1 nm. The hole-only devices with the optimized X exhibited

completely transport-limited currents, indicating that an Ohmic contact is formed at the

interfaces. By analyzing the current density-voltage characteristics of the devices with a space-

charge-limited current theory, we clarified the carrier transport mechanisms of the above-

mentioned organic HTLs.

B10.93

High Efficiency Bulk Heterojunction Solar Cells with Internal Quantum Efficiency (IQE)

approaching 100% fabricated with the Poly(2,7-Carbazole) Copolymer, PCDTBT Sung

Heum Park1,2

, Anshuman Roy1, Shinuk Cho

1, Ji Sun Moon

1, Nelson E Coates

1, Daniel Moses

1,

Kwanghee Lee1,2

, Alan J Heeger1,2

, Serge Beaupre3, Mario Leclerc

3 and R. Gaudiana

4;

1Center

for Polymer and Organic Solids, University of California at Santa Barbara, Santa Barbara,

California; 2Heeger Center for Advanced materials, Gwangju Institue of Science and

Technology, Gwangju, Korea, South; 3Departement de Chimie, Université Laval, Quebec,

Quebec, Canada; 4Konarka Technologies, Inc, Lowell, Massachusetts.

We report here that we have successfully fabricated a single polymer solar cell with 6% power

conversion efficiency using an alternating donor-acceptor low-bandgap Poly(2,7-Carbazole)

copolymer (PCDTBT; see Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295 -

2300) in composite with the fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester

(PC70BM). This class of alternating donor-acceptor copolymers (with different acceptor units,

X) holds promise for photovoltaic applications because of the ability to tune the electronic

energy levels by changing X. The implied flexibility in the synthesis can lead to both a lower

bandgap that better uses the solar radiation spectrum, and a lower HOMO level that increases the

open circuit voltage of photovoltaic devices. The PCDTBT device performance is as follows: Jsc

= 10.5 mA/cm2, Voc = 0.88 V, FF = 0.66 and ηe = 6.1% under air mass 1.5 global (AM 1.5G)

illumination from a calibrated solar simulator with irradiation intensity of 100 mW/cm2. The

IQE approaches 100%, implying that essentially every photon absorbed leads to a charge

separated pair and that every photogenerated carrier is collected at the electrodes.

B10.94 Self Sorted Carbon Nanotube Transistors and Conductive Films Soumendra Barman,

Melburne C Lemieux and Zhenan Bao; Department of Chemical Engineering, Stanford

University, Stanford, California.

Single-walled carbon nanotubes (SWNTs) possesses great potential to be the next breakthrough

for sensors, flexible computing networks and next generation electronics. The manufacture of

carbon nanotubes involves the decomposition of carbon into graphitic tubes of different

diameters and chiralities. Depending on the chirality of the nanotube, it behaves as a

semiconductor or a metal. One of the greatest obstacles for creating the next generation of

devices is the separation of nanotubes by chirality. Here, we refine the surface sorting separation

technique we have developed by modifying the solution processing conditions and self

assembled monolayer functionalities in an effort to understand the interactions leading to

chirality selectivity. Using self sorting we have fabricated high performance SWNT thin film

transistors (TFTs) and transparent conductive electrodes. UV-vis-NIR spectroscopy, Raman

spectroscopy, AFM and semiconductor parameter analysis were used to characterize the results.

The knowledge gained from understanding the mechanism for surface self sorting is critical for

the fabrication of devices and could be important for the integration of carbon nanotubes into

large scale electronics.

B10.95

Accurate and Simultaneous Determination of Carrier Density and Mobility in Organic

Semi-conducting Materials. Kai Shum1 and Jim Shi

2;

1Physics, Brooklyn College - CUNY,

Brooklyn, New York; 2Chemistry, College of Staten Island - CUNY, Staten Island, New York.

Accurate and simultaneous determination of carrier mobility and density in organic

semiconducting materials is very important for their optoelectronic applications including light-

emitting diodes (LEDs), solar cells, and thin film field-effect transistors. In this work, we report

on a unique data analysis procedure for space-charge limited currents to obtain the carrier density

and carrier mobility for our newly synthesized perylene tetracarboxylic diimides with finely

tuned pi-stack structures without altering the electronic characteristic of individual molecules.

How pi-stack structural variations affect charge transport performance will be discussed.

B10.96

High Efficiency, Solution-Processed Bulk Heterojunction Solar Cells Using

Diketopyrrolopyrrole-Containing Thiophene Oligomers Arnold Tamayo, Xuan-Dung Dang,

Bright Walker and Tyler Kent; Center for Polymers and Organic Solids, University of California,

Santa Barbara, California.

Bulk heterojunction (BHJ) solar cells are fabricated from blends of 2,5-di-(2-ethylhexyl)-3,6 bis-

(5''-n-hexyl-[2,2';5',2'']-terthiophen-5-yl)pyrrolo[3,4-c]pyrrole-1,4-dione:[6,6]-phenyl C71

butyric acid methyl ester (SMDPPEH:C71-PCBM). Absorption and photocurrent of the blend

films extend to 800 nm. A power conversion efficiency of 3.0% with a 50:50 donor-acceptor

ratio is obtained under simulated 100 mW/cm2 AM 1.5 illumination with a 9.2 mA/cm<2>

short-circuit current density and an open-circuit voltage of 0.72 V. The hole and electron

mobilities in the 50:50 blend are fairly balanced, 1.0 × 10<-4> cm<2>/V-s and 4.3 × 10<-4>

cm2/V-s, respectively. This is the highest PCE reported to date for BHJ solar cells using solution

processable small molecules.

B10.97

Abstract Withdrawn

B10.98

Harvesting Lost Photons: Minimizing Sub-Bandgap Losses in Organic Photovoltaic

Devices by Up-conversion Sebastien Loranger1, David Banville

1, Jennifer McLeod

3, Rosei

Federico3, Dmytro F Perepichka

2 and Clara Santato

1;

1Genie Physique, Ecole Polytechnique de

Montreal, Montreal, Quebec, Canada; 2Chemistry, McGill University, Montreal, Quebec,

Canada; 3INRS, Varennes, Quebec, Canada.

Organic semiconductors (OSC) for photovoltaic applications offer advantages that include

significantly lowered manufacturing costs (solution processing), flexibility of the device (roll-up

PV panels), and the possibility to easily tailor the optoelectronic properties through chemical

synthesis. Unfortunately, the maximum conversion efficiency obtained so far in Organic PV

(OPV) devices is ~5 %. Among other reasons, this is due to a large mismatch between the

absorption characteristics of organic semiconductors and the solar spectrum. A significant

portion of the solar photons are “lost” (can not be converted). To increase the power efficiency

of OPV devices the material‟s properties need to be tuned to increase light absorption. Our

approach is based on the use of the well known phenomenon of up-conversion of rare earth-

doped nanocrystals. We investigate conventional OPV devices where rare earth-doped

nanoparticles, dispersed in the organic active layer, can transfer the up-converted energy of NIR

photons to the polymer semiconductor, improving the overall efficiency. The key physical

process in the proposed hybrid PV system is the energy transfer from the excited up-converting

material to the organic semiconductor of the PV cell. We focus on a hybrid organic/inorganic

system prepared by blending organic and up-converting nanoparticles (UCNPs) components. [1]

Specifically, polythiophenes are considered as the p-type materials and functionalized fullerenes

as the n-type. Systematic investigations of the energy transfer between UCNPs and conjugated

polymers have been performed by means of steady-state NIR spectroscopy (UCNP-induced

emission of the polymers). Interestingly, the photoluminescence of UCNP decreases with the

increase of the polythiophene component in the blend. This suggests the possibility of significant

energy transfer between the UCNP and the organic semiconductors. On the other hand, the

fullerene has no significant effect on the UCNP emission. UCNP of different qualities are being

investigated, with different crystallographic structures (cubic and hexagonal) and stabilized with

different capping agents (oleic acid versus oleilamine). Time resolved spectroscopy is going to

be performed to measure life-times of the UCNP excited states in presence of the organic

semiconductors. To assess the effect of the UCNP on the electrical properties of the active layer

of the OPV device, we performed charge transport measurements in field-effect transistor

configuration, irradiating the layer with a low power NIR laser (phototransistor configuration).

This approach permits to identify the optimum size of the UCNP to be incorporated in the active

layer, together with their optimum concentration in the layer. [1] J.-C. Boyer,F. Vetrone, L. A.

Cuccia, J. A. Capobianco J. Am. Chem. Soc. 2006, 128, 7444.

B10.99 Ordered Titanium Dioxide Films Grown on Self-Assembled Monolayers. Shirin M Usmani,

Diana Mars and Andrew S Ichimura; Chemistry & Biochemistry, San Francisco State University,

San Francisco, California.

Titanium dioxide finds extensive applications as pigments, in medicine, wastewater remediation,

oxidative photocatalysis, and in dye-sensitized solar cells. Applications such as hybrid solar cells

utilize thin films of titanium dioxide as the electron transport material. Typically, the films are

prepared from TiO2 nanoparticle containing sols that are spin-coated onto substrates and

subsequently sintered to induce phase transformation and interparticle contact. We have pursued

a strategy of thin film preparation that involves growth of crystalline TiO2 directly onto a

functionalized surface from homogeneous solutions. In this approach, a densely packed self-

assembled monolayer (SAM) with a terminal Ti-OH functional group is used to chemically bond

the film to the underlying gold substrate. The advantage of this method is that resultant films are

highly ordered polycrystalline arrays in which monolithic crystals span the film from substrate to

external surface. This arrangement may facilitate charge transport across the layer and thus

decrease the probability of electron-hole recombination. In a larger sense, SAM chemistry allows

us to explore avenues for controlling crystal growth through a tailoring of the surface of the

substrate. In this work, anatase and rutile films grown on Ti-OH terminated SAMs from

homogeneous solution will be described. Characterization methods include powder X-ray

diffraction, HR-SEM, IR, UV-vis-NIR spectroscopy, and 4-probe conductivity studies.

B10.100 Self-Assembled Monolayers to Support the Growth of Inorganic Films Diana Mars, Shirin

M Usmani and Andrew S Ichimura; Chemistry & Biochemistry, San Francisco State University,

San Francisco, California.

Self-assembled monolayers (SAMs) provide a direct route to modify the interfacial chemistry

between a metal surface such as gold to organic or inorganic films. SAM chemistry and

formation on gold substrates is well-known and organo-sulfur compounds such as

hexadecanethiol self-assemble to form densely packed quasicrystalline 2D arrays. Depending on

the terminal functional groups, SAMs can be used to passivate a surface, control macroscopic

surface properties such as wetting and friction, and block or permit charge transport across the

film. Despite advances in SAM chemistry in recent years, considerable opportunities to develop

new monolayers remain. For example, increasing the thermal stability of SAMs and tailoring the

terminal group to specific add-layers would extend the range of applications. In this work, we

have prepared SAMs through the reaction of SiCl4, TiCl4, P(O)Cl3, and P(S)Cl3 with densely

packed -OH terminated monolayers. The result is a trithiolate or tripod SAM that has three

thiolate bonds to the gold surface. The tripod SAM proves to have a higher thermal stability than

single thiolate-Au bonds as measured by temperature programmed desorption (TPD) even for

very short alkyl chains. By terminating the tripod SAM with a SiOH, TiOH, P=O, or P=S

functional group, these films can be used as supports for the growth of inorganic films such as

silica zeolites, TiO2, or other chalcogonide based films. The structures and properties of the

monolayers were elucidated by fourier transform infrared (FTIR) spectroscopy, single

wavelength ellipsometry (SWE), TPD, and density functional theory (DFT) and will be reported

in this paper.

B10.101

Solvent Induced Structural Transition in Continuous and Nanopatterned Pentacene Thin

Films. Aram Amassian1, Vladimir A Pozdin

1, Alexander Zakhidov

1, Detlef M Smilgies

2 and

George G Malliaras1;

1Materials Science and Engineering, Cornell University, Ithaca, New York;

2Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York.

Solvent vapor annealing is increasingly used to induce structural and morphological changes in

polymer systems with the goal of increasing intermediate and long range order. In this paper, we

demonstrate that solvent vapor annealing can be used in small-molecule polycrystalline thin

films of pentacene to control its structure and polymorphism. Using acetone solvent vapor we

have transformed pentacene films which form in the “thin film” polymorph into the bulk phase.

In situ time-resolved grazing incidence X-ray diffraction measurements suggest that the phase

transformation occurs throughout the depth of the film. Closer inspection of diffraction patterns

reveals smearing and “dissolution” of the lattice followed by formation of the bulk lattice,

indicating a first-order phase transition. This enables fabrication of OTFTs with the bulk phase

films of pentacene and makes it possible to compare charge transport measurement from the thin

film and bulk polymorphs. This was previously impossible due to the fact that as-deposited

pentacene films are dominated by the “thin film” phase. We have gone a step further and

performed solvent-vapor annealing on nanopatterned pentacene films to investigate size effects

and to produce arrays of well-ordered crystalline microdomains for transistor fabrication.

B10.102

Novel Morphological Relationship between Field-Effect Mobility and Molecular Layer

Population at the Semiconductor-Dielectric Interface. Aram Amassian1, Tushar V Desai

2,

Vladimir A Pozdin1, Stefan Kowarik

4, Sukwon Hong

2, Arthur R Woll

3, Detlef M Smilgies

3,

Frank Schreiber4, George G Malliaras

1 and James R Engstrom

2;

1Materials Science and

Engineering, Cornell University, Ithaca, New York; 2School of Chemical and Biomolecular

Engineering, Cornell University, Ithaca, New York; 3Cornell High Energy Synchrotron Source,

Cornell University, Ithaca, New York; 4Institute für Angewandte Physik, Universität Tübingen,

Tübingen, Germany.

We report a novel relationship between the morphology of the semiconductor-dielectric interface

and the field-effect mobility of top-contact organic thin film transistors (OTFTs). We have

achieved a variation of the field effect mobility by 4 orders of magnitude (from 10 -5

to 10-1

cm2-

V-1

-s-1

) in small-molecule thin films of diindenoperylene (DIP) by controlling the morphology

and molecular layer population of the DIP films near the semiconductor-dielectric interface. The

morphological differences appear to be related to the degree to which molecular layers near the

semiconductor-dielectric interface are populated during deposition of the molecular

semiconductor. Control of layer population at the semiconductor-dielectric interface was

achieved by depositing DIP films in a variety of processing conditions, including by vacuum

sublimation, by supersonic molecular beam deposition, on bare SiO2 and on SiO2 treated with

hexamethyldisilazane (HMDS). To measure variations in layer population, we have used a

combination of in situ time-resolved X-ray reflectivity and ex situ non-contact atomic force

microscopy. The histogram of surface height obtained independently from these two methods

hold crucial clues about the morphology of the interfacial region and can be directly linked to the

performance of top-contact OTFTs. In particular, the asymmetry or skewness of these

distributions can be related directly to the transport properties of OTFTs. When the histograms

are strongly skewed toward the interface (layer population at the interface is deficient), mobility

is substantially lower than when the histogram is symmetric or skewed toward the surface (layer

population at the interface is adequate). Remarkably, the lateral size of crystalline and

morphological features does not appear to have a strong bearing on the performance of devices.

Our study suggests that layer population near the semiconductor-dielectric interface may be

influencing the degree of interconnectivity between crystallites, i.e., at grain boundaries.

B10.103

Highly Soluble Polymer Containing Thiophene and Alkyl-substituted Fluorene for Organic

Thin Film Transistors (OTFTs) Jae Wan Jang, Jin Uk Ju, Peng Tao Kang, Qinghua Zhao and

Soon-Ki Kwon; Polymer Engineering, Gyeongsang National University, JinJu, Korea, South.

Highly soluble conjugated a new p-type copolymers (PBTADF and PD5TADF) were

synthesized by the Suzuki coupling reaction. The newly designed copolymers are expected to

have high mobility because long-linked thiophene and long alkyl chains can give a high charge

density, π-stacking, and orderness. The monomers were prepared with highly overall yields.

Chemical structures and optoelectronic properties of the copolymers were characterized by

elemental analysis, 1H NMR, FT-IR, UV absorption, cyclic voltammetry (CV), and

photoluminescence (PL). In the case of the PBTADF, the absorption maximum was observed at

455 nm for solution and 520 nm for film, respectively. The PL maximum was observed at 561

nm for solution and 610 nm for film, respectively. The thermal properties of copolymers were

characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).

The copolymers were showed 5% weight losing temperature at 407 oC for PBTADF and 419 oC

for PD5TADF, respectively. The weight-average molecular weight of the copolymers were

determined to be 41,202 for PBTADF and 29,322 for PD5TADF by gel permeation

chromatography (GPC) using polystyrene standards for calibration in the eluent THF with

polydispersity indexes of 1.66 for PBTADF and 1.29 for PD5TADF, respectively. The solubility

of the copolymers were important property for conjugated polymers used in OTFTs. Especially,

the copolymers were easy to be soluble in common solvents, such as chloroform, tetrahydrofuran

(THF), toluene, chlorobenzene, and dichlorobenzene., this is due to the introduction of long alky

chains.

B10.104

Synthesis and Characterization of High-Performance Semiconductors based on

Alkoxylnaphthyl for Organic Thin Film Transistors Dong Hee Lee1, Qinghua Zhao

1, Jong-

Won Park1, Sung Ouk Jung

1, Yun-Hi Kim

2 and Soon-Ki Kwon

1;

1GNU, Jinju, Korea, South;

2department of chemistry, Gyeongsang national university, Jinju, Korea, South.

Organic semiconductors that consist of conjugated oligomers or polymers have attracted much

interest in recent years, due to their potential applications in organic thin-film transistors

(OTFTs). OTFTs can be fabricated at a low cost, over large areas, using flexible substrates. Over

the past decade, many researchers have made great progress in increasing the performances

(charge-carrier mobility, on/off ratio) of organic semiconductors for OTFTs, by designing new

fused aromatic compounds. Suzuki et al. reported oligo(2,6-anthrylene) derivatives with a

mobility of 0.18 cm2V-1s-1. Frisbie et al. demonstrated high mobility thiophene/acene hybrid

semiconductors, showing anthracene end-capped oligomers with stable device performance with

a mobility of up to 0.12 cm2V-1s-1. Moreover, µTFT values of up to 0.40 cm2V-1s-1 were

reported for naphthyl end-capped quarterthiophene, 5,5‟‟‟- bis(naphth-2-yl)-2,2‟:5, 2‟‟:5‟‟, 2‟‟‟-

quaterthiophene (NaT4), by Tian et al. Recently, we reported thienothiophene end-capped new

oligo acene derivatives, 2,6-bis(5‟-hexyl-thieno[3,2-b]thiophen-2‟-yl)anthracene (DH-TAT),

which showed a hole mobility of 0.14 cm2V-1s-1 , and an on/off current ratio of 6.3 x 106. In

this paper, we synthesized a series of new organic semiconductors, one of them showed a field

effect mobility value higher than 0.50 cm2V-1s-1, on/off ratios greater than 7.5 x 105 along with

a low threshold voltage of 4.4 V and subthreshold slope of 0.8 V/dec. Their properties of optical,

thermal stability and device performance would be discussed further.

B10.105

A Nanoengineering Path Towards All-printed DH4T-based Thin Film Transistors by Soft

Lithography.Dana Alina Serban1, Alexandru Vlad

1, Constantin A Dutu

1, Sorin Melinte

1,

Pierpaolo Greco2, Massimiliano Cavallini

2, Fabio Biscarini

2, Stefano Zacchini

3 and Maria C

Iapalucci3;

1Electrical Engineering, Universite catholique de Louvain, Louvain-la-Neuve,

Belgium; 2Istituto per lo Studio dei Materiali Nanostrutturati, Bologna, Italy;

3Dipartimento di

Chimica Fisica e Inorganica, Universita di Bologna, Bologna, Italy.

Organic semiconductors have received utmost attention recently due to their increasing role in

emergent electronic devices and optoelectronics. As the deposition method dictates key physical

characteristics of the semiconductor such as molecular packing and charge carrier mobility,

various techniques have flourished in the quest of fulfilling various performance requirements.

Yet, the need of innovative approaches should not be limited to the semiconductor deposition,

but ought to also touch other transistor components. Here we report on a simple and versatile

method of fabricating dihexylquaterthiophene (DH4T)-based coplanar bottom-gate thin film

transistors by means of soft lithography. Namely, microinject molding in capillaries (MIMIC) [1]

has been used by turns to define the electrodes and the active layer. In a first set of experiments,

metallic stripes - 3 mm long spaced by 200 µm - acting as source and drain have been deposited

from Pt salts [2] on heavily-doped Si wafers bearing 200-nm-thick silicon dioxyde dielectrics.

Electrical conductivity measurements of the Pt MIMIC stripes have been consistently reproduced

upon several voltage-sweep cycles. Finally, the devices were completed by drop-casting a

solution of DH4T diluted at 0.2% wt. in tetrahydrofuran. On the other hand, we have employed

the MIMIC approach to pattern the DH4T on top of conventional transistors [3]. These devices

have electron-beam evaporated Pd source and drain electrodes, channel width of 1 mm and

length of 10 µm. We formed the active layer by depositing the DH4T using a

poly(dimethylsiloxane) stamp accommodating 800-nm-wide channels. Samples have been

analyzed by Polarized Optical Microscopy, Tapping-Mode Atomic Force Microscopy and X-Ray

Reflectometry. They uncovered the formation of multidomains with preferential orientation

along the long axis of the stripes, as well as the organization in (32Å-high) monomolecular

terraces [3]. Electrical characterisation of the transistors revealed the key role of the

nanostructuration: higher field effect mobilities are achievable when the semiconductor is

deposited by MIMIC with respect to homogeneous films. We explain the improved mobility by a

preferential charge transport taking place inside the stripes, as their increased internal order

provides a better pathway for the hole carriers. Our findings that nanoengineered transistors

exceed in performance those fabricated by standard approaches are very encouraging in the light

of fabricating fully-patterned devices from solution processable materials. [1] E. Kim, Y. Xia, G.

M. Whitesides, Nature 1995, 376, 581; M. Cavallini and F. Biscarini, Nano Lett. 2003, 3, 1269.

[2] P. Greco et al., J. Am. Chem. Soc. 2008, 130, 1177. [3] D. A. Serban et al., Appl. Phys. Lett.

2008, 92, 143503.

B10.106 Ultrathin High Efficiency Solar Cell Based on Vapor Deposited Squaraine Dye. Siyi Wang

1,

Elizabeth Mayo2, Dolores Perez

1, Laurent Griffe

1, Guodan Wei

2, Brian Lassiter

2, Peter

Djurovich1, Stephen Forrest

2 and Mark Thompson

1;

1Chemistry, university of southern

california, Los angeles, California; 2Electrical Engineering and Computer Science, Physics,

Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan.

Squarains are a family of dyes with sharp and intense absorption in solution, which exhibit

panchromatic absorption in the solid state. Squaraines have highest occupied molecular orbital

(HOMO), lowest unoccupied molecular orbital (LUMO) energy levels comparable to the

common OPV donor material copper phthalocyanine (CuPc). 2,4-Bis [4-(N,N-diisobutylamino)-

2,6-dihydroxyphenyl] squaraine (SQ1) is demonstrated to be a successful donor like material in

organic heterojunction photovoltaics. Optimal SQ1 device is fabricated as ITO/SQ1 /C60 /BCP

/Al, which generates high short circuit current Jsc of 6.54 mA/cm2, and open circuit voltage Voc

of 0.80 V , resulting in power conversion efficiency η of 3.4 %, at 1 sun incident irradiation

(AM1.5). Compared with the 2% power conversion efficiency of our conventional CuPc device

[ITO/CuPC (400 Å)/C60 (400 Å)/BCP (100 Å)/Al (1000 Å)]. It is possible to tune the absorption

spectra of squaraines to near infrared region by varying the substituent groups. The

demonstration of sublimable SQ1 as a donor material opens up a new family of compounds for

small molecular heterojunction photovoltaics.

B10.107

Abstract Withdrawn

B10.108 Towards Greatly Improved Efficiency of Polymer LED. Zhang-Lin Zhou

1, Lihua Zhao

1,2,

James Brug1, Xia Sheng

1, Gary Gibson

1, Sity Lam

1 and K. Nauka

1;

1Hewlett Packard Labs, Palo

Alto, California; 2University of California, Berkeley, California.

Organic/polymer light-emitting diodes (OLEDs) show great promise of revolutionizing display

technologies. Hence, these devices and the materials that render them functional are the focus of

intense scientific and technological interest. The archetypical multilayer OLED heterostructure

introduces numerous chemical and physical challenges to the develoment of efficient and robust

devices. This requires placing a nanoscale feature at desired sites within the thin film stack.

These layered structures are formed from solution based spin-casting or printing with

subsequesnt removal of the solvent. However, solvent from the freshly deposited film can

dissolve or partially dissolve the underlying layer resulting in loss of the desired structure and

corresponding device functionality. Undesirable redistribution of the nanoscale features within

the polymer is another detrimental effect associated with solvent removal. In this paper, we will

describe a new solution to the above problem by embedding hole transporting materials (HTLs)

in a cross-linked polymer matrix. We have successfully demonstrated this technique to deposit

the HTL in our OLED device. In this device, the HTL is deposited together with cross-linkable

monomers on the HIL that is pre-deposited over the patterned ITO anode. The next layer for the

stacking process is EML (PFO) that is followed by the deposition of the ETL. Since solvents that

are used for EML polymer are commonly shared by the under-layer HTL polymer, embedding

the HTL molecules into the inert cross-linked polymer network is a good way to minimize the

undesirable impact from solvents that are used by the EML polymer. There are many options that

could be employed to form the cross-linked inert polymer network: a mixture of cross-linkable

monomer, oligomers, and polymers, in addition to cross-linking agent and an initiator. The cross-

linking agent could be a 2-branch, 3-branch, or 4-branch cross-linker. Cross-linking could be

activated using appropriate energy sources such as UV-exposure or thermal process. Device

fabrication together with the total luminance and external quantum efficiency of such devices

will be presented.

B10.109

Hysteresis Mechanism and Reduction in the Bis(triisopropylsilylethynyl)Pentacene Thin

Film Transistors with Polymer Blend, Cross-linked Poly(vinyl phenol). Jin Young Oh1,

Young Bum Yoo1, Man Hyeop Han

1, Byeong har Hwang

1, Hyeon Seok Hwang

1, Joo Hee Kim

2

and Hong Ku Baik1;

1material science, Yonsei university, seoul, Korea, South;

2chemistry, Iwha

woman university, seoul, Korea, South.

We have studied the electrical stability of cross-linked Poly(Vinyl Phenol) dielectric and active

layer with polymer blend for the Bis(triisopropylsilylethynyl)Pentacene[tips-pentacene] thin film

transistors. the hysteresis occurs mainly due to the dielectric surface -OH group and the rough

surfaces which are induced by trap sites at the interface in OTFTs. In order to reduce hysteresis,

we used an OH free and smooth dielectric and active layer with Polymer blend(PaMS). As a

result, Increased mobility and decreased hysteresis are observed in Tips-pentacene thin film

transistors with polymer blend(PaMS) and PVP dielectric.

B10.110

π-Conjugated Molecules based on Thiophene Derivatives as Organic Semiconductors for

Thin Film Transistor Applications. Ki Hwa Jung, kyung Hwan Kim, Min Ju Cho and Dong

Hoon Choi; Korea university, Seoul, Korea, South.

Organic semiconductor materials based on extended linear-conjugated systems have been very

intriguing and significant development has been achieved in these materials over the one decade.

In the exploration of the application as organic semiconductors in organic field effect transistor

(OFET) is an important component for developing future flexible displays. Accordingly, a

number of researchers have attempted to synthesize π-conjugated small molecules, dendrimers,

oligomers, and polymers because of their strong potential applications to electronics and

optoelectronics such as in organic light-emitting diodes (OLEDs), organic field effect transistors

(OFETs), and photovoltaic cells. In organic semiconductors, the intrinsic carrier mobility

depends critically on the degree of molecular orientation and on the extent of the intermolecular

interaction. In this study, new conjugated crystalline cruciform molecules have been synthesized

through the Sonogashira coupling reaction. Introduction of thiophene-based peripheral moiety

into an aromatic core improve the solubility in organic solvent for facilitating the solution device

fabrication. They display a p-type semiconducting behaviors and their electrical properties are

investigated in detail. We investigated the UV-visible spectroscopy, photoluminescence

spectroscopy, cyclic voltammetry, thermogravimetric analysis and differential scanning

calorimetry properties of the new π-conjugated molecules. Finally, we fabricated thin film

transistor to investigate the carrier mobility and its photosensitivity under light illumination.

B10.111

Abstract Withdrawn

B10.112

Fused Aromatic Thienopyrazine Copolymers for Photovoltaic and Thin Film Transistor

Applications. Rajib Mondal1, Nobuyuki Miyaki

1, Hector A Becerril

1, Jack Parmer

2, Michael D

McGehee2 and Zhenan Bao

1;

1Chemical Engineering, Stanford University, Stanford, California;

2Materials Science and Engineering, Stanford University, Stanford, California.

Development and tuning the fundamental properties of polymeric semiconducting materials have

become an active area of research in recent years due to their potential uses in light weight and

flexible optoelectronic devices, such as organic photovoltaics (OPVs) and organic thin film

transistors (OTFTs). In this regard, several fused aromatic thienopyrazine (TP) copolymers were

synthesized and investigated. The fused aromatic TP unit provides a planar and rich π-face,

which promotes the π-π stacking between the polymeric chains and enhances the device

efficiencies. The electronic properties of these polymers can also be tuned significantly using

different aromatic group used in TP unit. For example, band gap of TP-fluorene copolymer was

reduced to ~1.4 eV from ~1.7 eV by replacing phenanthrene from acenaphthyl. Various alkyl

chains were used at different positions of the polymers to achieve the morphological control.

Power conversion efficiency more than 1% in OPV devices and hole carrier mobility of ~0.2

cm2/Vs in OTFT devices have already been achieved using some of these polymers. Further

optimization of the newer polymers is currently underway.

B10.113

Abstract Withdrawn

B10.114

Synthesis of Donor-Acceptor Diblock Copolymer Based on Regioregular Poly(3-

hexylthiophene) and Fullerene and Its Use as a Compatibilizer in P3HT/PCBM Bulk

Heterojunction Solar Cells. Jea Uk Lee, Jae Woong Jung and Won Ho Jo; Materials Science

and Engineering, Seoul National University, Seoul, Korea, South.

Among various organic photovoltaics, the conjugated polymer/fullerene bulk heterojunction

system is one of the most promising ones for achieving high performance of polymer

photovoltaics. Recently, it has been reported that thin-film bulk heterojunction solar cells

fabricated by simple blending of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-

C61-butyric acid methyl ester (PCBM) show high power conversion efficiency. Despite

remarkable recent progress, the bulk heterojunction system still has some problems to be solved

for commercialization. The most serious problem is that this approach is largely dependent upon

the solid state phase morphology of the two components (donor/acceptor) in the photoactive

layer because the exiton diffusion length is limited within 10 nm. Although nanometer scale

phase separation of donor/acceptor blend can be attained through optimized device conditions in

laboratory (specific blending ratio, very short thermal annealing time), exposure to sun light for

long time will cause macrophase separation in the blend of conjugated polymer and fullerene

derivatives. The macrophase separation may limit charge separation and thus lower the power

conversion efficiency in the photovoltaic device. For the purpose to solve this problem, in this

study a novel diblock copolymer composed of regioregular poly(3-hexylthiophene) and fullerene

derivates (P3HT-b-C60) was synthesized and used as a compatibilizer for P3HT/PCBM bulk

heterojunction system. It has been well known that a proper compatibilizer can reduce the

domain size of phase separation and also retard the rate of phase separation in the blend system.

With varying the fabrication conditions, the change of domain size and the power conversion

efficiencies of P3HT/PCBM/P3HT-b-C60 blends were measured. First, to examine the effect of

compatibilizer on the performance of heterojunction solar cells, P3HT/PCBM bulk

heterojunction devices were fabricated with varying the amount of P3HT-b-C60 compatibilizer.

Second, the effect of the thermal annealing time and temperature on the performance of

P3HT/PCBM/P3HT-b-C60 bulk heterojunction solar cells was investigated. Third, the solvent

annealing (slow drying) method was also applied to the P3HT/PCBM/P3HT-b-C60 blend in

order to obtain the desirable morphology. AFM and TEM images of blend films with

compatibilizer exhibited uniform morphology with finer domain size after annealing as

compared with the blends without compatibilizer, when a small amount of compatibilizer was

added to the blend. The power conversion efficiency of blend with compatibilizer was improved

as compared with the blend without compatibilizer. Particularly, the short circuit current (Jsc) of

the blend with the compatibilizer was higher than the blend without the compatibilizer due to

improvement of phase morphology of P3HT/PCBM/P3HT-b-C60 blend.

B10.115 High Electron Carrier Mobility Electron Transport Materials for OLED Devices Heh-Lung

Huang, Teng-Chih Chao and Mei-Rurng Tseng; Industrial Technology Research Institute,

Chutung, Hsinchu, Taiwan.

We developed a series of new electron transport materials. These materials contain heterocyclic

related core structure and with other electron withdrawing group. The electron mobility of

thermal deposition for these materials are greater than 10-4 cm2/Vs. We are now investigating

the effect of new electron transport materials for the OLED devices and lifetime. Currently, the

current efficiency and brightness of green phosphorescent OLED devices with new electron

transport materials are 46.0 cd/A and 82123 cd/m2 respectively. We are still investigating and

optimize the OLED devices with those new electron transport materials. With the increasing

mobility of the new electron transport materials, we can apply on the flexible plastic substrate to

fabricate the flexible OLED device.

B10.116

Multilayer Formation Using a Lamination Process Toward Efficient Polymer Solar Cells Keisuke Tajima

1, Motoshi Nakamura

1, Erjun Zhou

2, Chuhe Yang

2 and Kazuhito Hashimoto

1,2;

1Department of Applied Chemistry, The University of Tokyo, Tokyo, Japan;

2HASHIMOTO

Light Energy Conversion Project, Exploratory Research for Advanced Technology (ERATO),

Japan Science and Technology Agency (JST), Tokyo, Japan.

Polymer solar cells (PSCs) are drawing much attention these days because of their potentials for

the production of flexible and large-area solar cells at dramatically low costs. In the fabrication

method commonly used now, however, an metal electrode is deposited on the polymer layer by

vacuum evaporation, which is both costly and time-consuming process. Furthermore, it is

suggested that the device performance and stability is detracted by physical or chemical damage

to the organic active layer form the evaporated metals. In this presentation, first we report the use

of a lamination method to fabricate the PSCs. Two different polymer layers are coated on

conductive substrates (ITO covered with TiO2 and Au with PEDOT:PSS) and the substrates

were simply laminated by applying pressure (2-6 MPa) and heat (100-150 °C) to fabricate the

devices. By this simple method, we obtained physically and electronically connected interface

between the polymer layers. The photovoltaic performances of the devices with P3HT:PCBM

bulk heterojunction are compared to those of the conventional devices with the electrode

deposited by vacuum evaporation. As the result, the two devices showed almost identical J-V

curves, and the PCE reached 3.3% in both devices under the AM1.5 irradiation. This result opens

up the possibility of low-cost mass production of the PSC through a roll-to-roll lamination

system. Since we can apply this method to the heterointerface of the semiconducting polymers, it

is possible to enhance the light absorption of the PSCs by combining the two polymer layers with

different absorption ranges. For this purpose, a novel low band gap polymer, PDTPDTBT was

synthesized with an alternating structure of N-substituted dithieno[3,2-b:2‟,3‟-d]pyrrole (DTP) as

a new donor, and 4,7-dithien-2-yl-2,1,3-benzothiadiazole as an acceptor group. The new polymer

has a low optical band gap (1.46 eV) and broad absorption with peaks at 451 and 697 nm in the

film, which is complementary to the absorption of P3HT. Preliminary results showed that the

power conversion efficiency of bulk heterojunction type PSC with PDTPDTBT reached 2.18%

in combination with PCBM. The combination of P3HT and PDTPDTBT layers using the

lamination method gave the devices that respond to the broad range of the solar spectrum.

Further optimization of the layer components would give us a possibility to reach higher

conversion efficiency. [1] Nakamura, M.; Tajima, K.; Yang, C.-H.; Hashimoto, K. Control of the

Active Layer/Metal Electrode Interface Using a Lamination Process toward Efficient Polymer

Solar Cells. Submitted. [2] Zhou, E.J.; Nakamura, M.; Nishizawa, T.; Zhang, Y.; Wei, Q.S.;

Tajima, K.; Yang, C.H.; Hashimoto, K. Synthesis and Photovoltaic Properties of A Novel Low

Band Gap Polymer Based on N-substituted dithieno[3,2-b:2‟,3‟-d]pyrrole. Macromolecules in

press.

B10.117

Synthesis and Molecular Orientation Control of Oligo(p-phenylenevinylene) Derivatives

for the Application to Organic Photovoltaic Devices. Keisuke Tajima1, Takeshi Nishizawa

1,

Hady Kesuma Lim1 and Kazuhito Hashimoto

1,2;

1Department of Applied Chemistry, The

University of Tokyo, Tokyo, Japan; 2HASHIMOTO Light Energy Conversion Project,

Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology

Agency (JST), Tokyo, Japan.

Nanoscale morphology of the donor and acceptor is of high significance to achieve a high

efficiency in the organic bulk heterojunction (BHJ) solar cells. Namely, large interface and

interpenetrating network of the donor and acceptor realize the efficient charge separation and

transport in the organic film, respectively. In the solution-processed BHJ solar cells, formation of

such a nanostructure is simply achieved by spin-coating and thermal annealing and it is affected

by many factors such as solvent, donor/acceptor mixing ratio, character of the materials used,

etc. Important characters of the materials are crystallinity and molecular orientation that facilitate

the formation of the network to achieve the efficient charge transport. In this presentation, we

first report the synthesis of novel oligo(p-phenylenevinylene)s (OPV) to demonstrate the effect

of the crystallinity of the donor on the morphology and solar cell efficiency in the OPV and

fullerene derivative (PCBM) BHJ solar cells. The crystallinity of the OPVs is tuned by changing

the alkyl side chain length. AFM images revealed that the film with highly crystalline OPV

showed a significantly different morphology and prevented the large PCBM aggregation that was

observed in the films with the lower crystalline OPVs. The solar cell with the highly crystalline

OPV showed the improvement in the short circuit current and the fill factor. These results

suggest that the highly crystalline OPV formed the donor network and prevented the PCBM

aggregation, resulting in the improved efficiency in the solar cells. Similarly, the crystallinity of

the donor group in the donor-acceptor dyad is also of high importance to achieve a high

efficiency in the solar cells [1, 2]. We also report the synthesis of novel oligo(p-

phenylenevinylene)-fullerene dyads and their solar cell efficiency. The strong pi-pi interaction in

the highly crystalline donor groups enhances the intermolecular charge hopping, resulting in the

improved fill factor and efficiencies over 1%. Finally, highly uniaxial alignment of the OPVs

were demonstrated in the as-cast films spin-coated from the solutions on mechanically rubbed

polymer alignment layers such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid)

(PEDOT:PSS) films. High dichroic ratio (Ap / An) of 41.0 and order parameter ([An - Ap] / [An

+ 2Ap]) of 0.93 were estimated from the polarized absorption spectra of the films. The molecular

orientation was also investigated by in-plane X-ray diffraction measurements, revealing the

highly uniaxial alignment of the molecules with its long axis parallel to the rubbing direction.

This spontaneous alignment could be a new method useful to control the

electronic/optoelectronic properties of the semiconducting materials in the thin films. [1] T.

Nishizawa, K. Tajima, K. Hashimoto, J. Mater. Chem. 2007, 17, 2440-2445. [2] T. Nishizawa,

K. Tajima, K. Hashimoto, Nanotechnology, 2008, 19, 424017.

B10.118

Solution Processed High Mobility Tetrathienoacene Polymer Semiconductors with a Long

Shelf Life in Ambient Environment. H. H. Fong1, Vladimir Pozdin

1, Aram Ammassian

1,

George Malliaras1, Mingqian He

2, Susan Gasper

2, Feixia Zhang

2 and Michael Sorensen

2;

1Materials Science and Engineering, Cornell University, Ithaca, New York;

2Corning

Incorporated, Corning, New York.

We demonstrate that high performance, soluble polymeric semiconductors can be achieved by

increasing the rigidity of the thiophene monomer through the use of an alkyl-substituted core that

consists of four fused thiophene rings. We report on a member of the family of di-alkylated

tetrathienoacene copolymers, namely poly(2,5-bis(thiophene-2-yl)-(3,7-

ditridecanyltetrathienoacene) (P2TDC13FT4), that can be deposited from a 1,2-dichlorobenzene

solution into highly ordered films with a field-effect hole mobility exceeding 0.3 cm2/V/s.

Devices prepared on vapor phase deposition of silane based oxide gate oxide show a remarkably

long life-time of one year in ambients, with no significant change on the performance. We show

that di-alkylated tetrathienoacene copolymers represent a new class of organic semiconductors

that can be easily processed from solution into ordered, high mobility thin films. The

straightforward processing and the relatively low temperatures involved make them particularly

attractive for the development of electronics on plastic substrates.

B10.119 Photolithographic Micropatterning of Organic Electronic Materials. H. H. Fong, Jin-Kyun

Lee, Alexander Zakhidov, Priscilla Taylor, John DeFranco, Christopher Ober and George

Malliaras; Materials Science and Engineering, Cornell University, Ithaca, New York.

We developed a class of organic electronic materials that can be processed using fluorinated

solvents. These materials can function as imaging materials for photolithographic applications

and light-emitting applications for full color display. We demonstrate a critical technique of

patterning multi-layer organic devices by different orthogonal development media including

hydrofluoroethers and supercritical carbon dioxide. Small molecules and polymer materials can

be patterned on top of different substrates such as metal, transparent conducting oxide, and even

polymer, with a spatial resolution down to a micron. Using supercritical carbon dioxide, we

fabricated a patterned polymer light-emitting diode. A molecularly-doped polymer light emitting

diode is patterned on top of Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)

(PEDOT:PSS). This green-emitting device exhibits an efficiency of 22 cd/A and 8 lm/W at 100

cd/m2, which is compatible to the typical device performance. Using hydrofluoroethers as

development media, we developed a novel photosensitive material that allows us to pattern

traditional organic electronic materials without any damage. A stack structure of patterned multi-

layers is demonstrated. Furthermore, P3HT and pentacene transistors with patterned Au top

contacts using lift-off method are demonstrated with decent performance. We also fabricate a

class of highly fluorinated red, green, and blue light-emitting materials that can be only

processed using fluorinated solvents. These materials show superior stability in common organic

solvents including xylene, toluene, acetone, isoproponal and chlorinated solvents. Preliminary

results show a current efficiency of >5 cd/A. We can therefore pattern these solution processed

light-emitting materials using traditional photolithography under aqueous based media and

further aim for fabricating RGB pixels.

B10.120

Charge Injection in Polyfluorene Films using High Conductivity Thiophene Copolymers.

H. H. Fong, Arunabh Batra and George Malliaras; Materials Science and Engineering, Cornell

University, Ithaca, New York.

Fluorene-based copolymer is considered to be one of the most promising hole transporting and

blue light-emitting conjugated polymers used in polymeric light-emitting diodes (PLEDs) due to

its high-lying highest occupied molecular orbital (HOMO) of arylamine moiety and high carrier

mobilities. It is therefore desirable to boost up the charge injection. Our work attempts to provide

a comprehensive understanding on charge transport of fluorine-arylamine copolymers and

examine new approaches to improve carrier injection. Time-of-flight (TOF) technique has been

employed to evaluate the charge drift mobility at the thick film regime (1-10 micron) and shows

the polymer possesses superior mobility of > 0.01cm2V-1s-1. We examine the hole injection in

the polyfluorenes using chemically doped thiophene copolymers. Furthermore, the effect using

molecular additives on tuning the conductivity of these thiophene polymers is also investigated.

Results show a significant improvement of charge injection efficiency by tuning the conductivity

of the doped thiophene hole injecting layer. Moreover, the charge injection in this polyfluorene

with high-lying HOMO can be further enhanced by using vapor phase polymerization of

thiophene contained monomers that increases the injection efficiency to ~ 0.1 for a 100nm

polyfluorene film. Results will shed light on the enhancement of device efficiency and stability

in the future polymer electronic devices.

B10.121

Spin-Cast High-κ Non-Hysteresis Barium Titanate Nanoparticle Thin Film as Gate

Dielectric in Organic Thin Film Transistors. Zhang Jia1, Limin Huang

1, Stephen O'Brien

1 and

Ioannis Kymissis2;

1Applied Physics and Applied Mathematics, Columbia University, New

York, New York; 2Electrical Engineering, Columbia University, New York, New York.

We demonstrate the usage of spin-cast 8nm-diameter high-κ BaTiO3 nanocrystals as gate

dielectric with parylene C coating as surface treatment in the fabrication of high-performance

pentacene organic field effect transistors (OFETs). Thanks to the extremely small size of BaTiO3

nanocrystals, the hysteresis seen in bulk material is quenched. This structure enhances the

OFETs performance while retaining all the processing advantages of organic semiconductors:

low temperature processing (<60 Celsius), low cost, applicability for large area casting, spraying

and printing on various substrates, etc. Pentacene mobility in the linear region was enhanced

from ~0.03 cm2/(Vs) at gate bias -20V in Parylene OFETs to ~0.35 cm2/(Vs) at gate bias -20V

in BaTiO3/Parylene OFETs under identical conditions, resulted from increased concentration of

accumulated carriers in the latter.

B10.122 Photocurrent Study of Traps States in Pentacene Thin Film Transistors. Zhang Jia

1, Laura

Banu2 and Ioannis Kymissis

2;

1Applied Physics and Applied Mathematics, Columbia University,

New York, New York; 2Electrical Engineering, Columbia University, New York, New York.

We demonstrate the gate-bias dependent and wavelength dependent photocurrent study of both

UV-ozone treated and air-free pentacene transistors with bottom gate top contact structure. Our

unique facility enables us to fabricate the pentacene transistors without the exposure to air in any

phase of the fabrication process. Traps states are intentionally introduced by treating the

dielectric surface with UV ozone before the pentacene deposition. The photocurrent

measurement is carried out utilizing chopped excitation with wavelength between 300~800nm

and a lock-in amplifier with the frequency locked to the chopping frequency. The wavelength

dependent photocurrent spectrum shows extra peaks from the UV-ozone sample in the range of

350nm to 470nm, which correspond to energy transitions larger than the pentacene HOMO-

LUMO gap. This indicates extra trap states are introduced by UV-ozone treatment. The gate

dependent photocurrent spectrum shows the photocurrent intensity is proportional to the square

root of source-drain current at various gate bias in the saturation region. It also shows that the

UV-ozone treated samples have a positively shifted threshold voltage, which is confirmed by

direct electrical measurements. A more detailed transport model is proposed based on the trap

and release model and these photocurrent studies.

SESSION B11: Processing, Sensing & Memory

Chairs: Norbert Koch and George Malliaras

Friday Morning, April 17, 2009

Room 2001 (Moscone West)

8:30 AM B11.1

Organic Device Fabrication by Nanoskiving: Conjugated Polymer Photovoltaic Cells,

Nanowires, and Templates for Electropolymerization. Darren John Lipomi1, Ryan C

Chiechi1, Michael D Dickey

2, William F Reus

1 and George M Whitesides

1;

1Department of

Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts; 2Chemical

and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina.

This paper describes a new technique for the fabrication of conjugated polymer nanostructures

by sectioning thin films with an ultramicrotome. In the first application, a laterally ordered bulk

heterojunction of two conjugated polymers is fabricated by a three-step process: i) spin-coating a

multilayered film of the two polymers, ii) rolling the film into a cylinder (a “jelly roll”) and iii)

sectioning the film perpendicular to the axis of the roll with an ultramicrotome (nanoskiving).

The conjugated polymers are poly(benzimidazobenzophenanthroline ladder) (BBL, e-acceptor)

and poly(2-methoxy-5-(2‟-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV, e-donor). The

procedure produces sections with an interdigitated junction of the two polymers. The spacing

between the phases is determined by spin-coating (~15 nm to 100 nm) and the thickness of each

section is determined by the ultramicrotome (100 to 1000 nm). The minimum width of the MEH-

PPV layers accessible with this technique (~15 nm) is close to reported exciton diffusion lengths

for the polymer. When placed in a junction between two electrodes with asymmetric work

functions (tin-doped indium oxide (ITO) coated with poly(3,4-

ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS), and eutectic gallium-indium,

EGaIn) the heterostructures exhibit a photovoltaic response under white light, although the

efficiency of conversion of optical to electrical energy is low. Selective excitation of BBL with

red light confirms that the photovoltaic effect is the result of photoinduced charge transfer

between BBL and MEH-PPV. In the second application, a similar process is used to generate

uniaxial collections of nanowires of BBL and MEH-PPV. These nanowires function as high-

surface-area chemical sensors. In the third application, gold nanowire electrodes are prepared

with 30-nm spacing using micromolding in capillaries and nanoskiving and addressed

individually. These nanowires serve as a template for the electrochemical deposition of

polyaniline.

8:45 AM B11.2

Abstract Withdrawn

9:00 AM *B11.3 Photolithographic Patterning of Organic Semiconductors George Malliaras, Materials

Science, Cornell University, Ithaca, New York.

A critical step for the realization of organic electronics is the availability of patterning techniques

that are compatible with these materials. Although great strides have been achieved in our ability

to pattern organics, the techniques used in the mature and entrenched industry of silicon

processing have made little impact in this field. This is primarily due to incompatibilities

between chemicals used in photolithography and the vast majority of organics. Overcoming

these incompatibilities promises a breakthrough in the manufacturing of organic electronics since

it would provide for massively parallel output along with process knowledge and equipment

already available from a very successful industry. We report on a few generic approaches for the

photolithographic patterning of organic materials using sacrificial layers as well as photoresists

that can be processed with solvents that are orthogonal to organics. We demonstrate the

applicability of these approaches to the additive and subtractive patterning of several organic

semiconductors, including polymers and small molecules. The application of photolithography to

pattern various organic devices with micron-scale features is demonstrated.

9:30 AM B11.4

Printable Organic-Metal Electrodes for Controlled Carrier Injections in Organic

Transistors Tatsuo Hasegawa, PRI, AIST, Tsukuba, Japan.

In this talk we report our recent studies on contact engineering of organic thin-film transistors

(TFTs) with use of conductive charge-transfer (CT) complex films as source/drain electrodes.

We successfully produced metallic CT compound films with predefined structures by "double-

shot" inkjet printing (DS-IJP) technique, in which soluble donor and acceptor components are

printed individually and combine on the substrates to form barely-soluble compound films. For

predefining shapes of the electrodes, we used hydrophobic/hydrophilic surface modifications on

top of the substrates. We found that the microscale intermixing of the printed droplets results in

the instantaneous formation of high-quality polycrystalline CT compound films. We consider

that this new type of solution process should allow us to make full use of highly-functional

organic CT compound materials in organic electronics devices. It is fount that fairly good

electrical contacts are obtained in the pentacene TFTs with the printed tetrathiafulvalene-

tetracyanoquinodimethane (TTF-TCNQ) films as bottom-contact source/drain electrodes that

show high mobility and sharp on/off switching at low gate voltages. We will present and discuss

various interesting aspects of the functional organic-metal electrodes. 1) Y. Takahashi et al. APL

86, 63504 (2005), 2) Y. Takahashi et al. APL 88, 073504 (2006), 3) Y. Takahashi et al. Chem.

Mater. 19, 6382 (2007), 4) M. Hiraoka et al. APL 89, 173504 (2006), 5) M. Hiraoka et al. Adv.

Mater. 19, 3248 (2007).

9:45 AM B11.5

Introduction of Innovative Dopant Concentration Profiles to Broaden the Recombination

Zone of Phosphorescent OVPD-Processed Organic Light Emitting Diodes. Manuel Bosing1,

Christoph Zimmermann1, Florian Lindla

1, Frank Jessen

1, Philip van Gemmern

2, Dietrich

Bertram2, Dietmar Keiper

3, Nico Meyer

3, Michael Heuken

1,3, Holger Kalisch

1 and Rolf Jansen

1;

11Chair of Electromagnetic Theory, RWTH Aachen University, Aachen, NRW, Germany;

2Philips Technologie GmbH, Aachen, Germany;

3AIXTRON AG, Aachen, Germany.

The introduction of phosphorescent emitters for OLED has established a basis for the

development of OLED with impressive luminous efficiencies. However, to exploit the full

potential of phosphorescent light emission, it is crucial to develop device structures which lead to

a relatively broad exciton recombination zone in order to avoid triplet-triplet-annihilation (TTA).

In many OLED, matrix and emitter contribute differently to the mobility of holes and electrons

within the emissive layer, so the concentration of the dopant can be used to locate and/or broaden

the recombination zone. In contrast to VTE (Vacuum Thermal Evaporation), OVPD (Organic

Vaporphase Deposition) offers the opportunity to vary the concentration of the dopant during the

deposition of the emissive layer, so that complex concentration profiles can be realized. In this

work, the concept of non-constant dopant concentration profiles was tested on a green

phosphorescent OLED. All samples have been deposited employing AIXTRON OVPD systems.

The investigated OLED consist of an ITO anode, a hole injection layer (HIL), NPB (N,N'-

diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine) as hole transporting layer, an

emissive layer of Ir(ppy)3 doped in a suitable host material (PH1), Alq3 (tris-(8-

hydroxyquinoline) aluminum) as electron transporting layer and finally a LiF/Al cathode. PH1 is

a good electron conductor, but conducts holes poorly. Therefore, the hole transport in the

emissive layer is sustained mainly by the dopant Ir(ppy)3 and consequently depends strongly on

the Ir(ppy)3 concentration. OLED with various dopant concentration profiles have been

processed and compared with regard to their luminous efficacy and I-V characteristics. When

processed with a constant dopant concentration, the OLED described above reached a luminous

efficacy of up to 18 cd/A and 11 lm/W (at 1000 cd/m2). By means of more complex

concentration profiles, the luminous efficacy could be increased to 24 cd/A and 19 lm/W (at

1000 cd/m2). All OLED with a dopant concentration profile starting with a rather high dopant

concentration on the anode side of the emissive layer, showed steeper I-V-curves and therefore

needed lower driving voltages than their conventional counterparts. So apparently, a higher

initial dopant concentration improves the hole injection into the emissive layer. This approves

that the balancing act of high dopant concentrations (to improve hole mobility) versus low

dopant concentrations (to avoid triplet-triplet-annihilation) can be embraced with this concept.

Parallel to these experiments, the width and location of the recombination zone have been

simulated for all investigated concentration profiles by numerical solution of the semiconductor

device equations using experimentally determined doping-dependent charge carrier mobilities.

The obtained theoretical results are discussed with regard to the accomplished experiments.

10:30 AM B11.6

Organic Thin-Film Transistor based Sensors used for Low-Concentration Ammonia

Detection. Andreas Klug1,2

, Martin Denk2, Martina Sandholzer

3, Thomas Bauer

3, Christian

Slugovc3, Ullrich Scherf

4 and Emil J.W. List

1,2;

1NanoTecCenter Weiz Forschungsgesellschaft

mbH, Weiz, Austria; 2Institute of Solid State Physics, Graz University of Technology, Graz,

Austria; 3Institute for Chemistry and Technology of Organic Materials, Graz University of

Technology, Graz, Austria; 4Macromolecular Chemistry, University of Wuppertal, Wuppertal,

Germany.

Organic materials can be easily tailored to enable, increase or optimize sensitivity and selectivity

with respect to an analyte of interest and therefore organic-based devices such as organic thin-

film transistors (OTFTs) are also believed to find their application as smart disposable sensor

devices in health-, food- and environmental monitoring, diagnostics and control, ranging from

light- and chemical vapor sensors to transducers for ions and biological substances. Compared to

resistor-based sensors, OTFTs generally have the advantage of being more sensitive due to the

inherent amplification of the sensing event. Moreover, they usually exhibit higher response

times, low power consumption, the ability to implement multi-parameter sensing as well as the

possibility to be integrated in complex circuits including signal processing. Here we present an

electrical sensor based on an OTFT, which is used for detecting ammonia as gaseous analyte via

functionalized ring-opening methathesis polymerized materials, strongly changing the electrical

properties when exposed to NH3. In detail, a source-to-drain current increase of 500% and more

is observed upon exposure to ammonia gas with a concentration <=1%. Besides current-voltage

OTFT analysis, UV/Vis spectroscopy measurements, Fourier transform infrared spectroscopy, C-

V/C-f measurements and atomic force microscopy are used to investigate the NH3-sensitive

materials constituting the sensor device.

10:45 AM B11.7

Organic Transistors for Sensor Applications under Gaseous and Aqueous Conditions Anatoliy N Sokolov, Mark E Roberts and Zhenan Bao; Stanford University, Stanford, California.

Organic field-effect transistors (OFETs) are ideal for inexpensive, chemical sensors owing to

their compatibility with flexible, large-area substrates, simple processing, and tunable active

layer materials. Specifically, the use of small molecule-based organic semiconductors or single-

walled carbon nanotubes allows specific recognition site incorporation to achieve selectivity.

Previously, the use of OTFTs as sensors has been limited to vapor phase systems. However,

while chemical detection for comprehensive monitoring will require the sensors to operate under

both aqueous and ambient air conditions, the OTFTs reported to-date have not been suitable for

applications in aqueous media owing to high operating voltages, degradation and delamination.

We now introduce novel low-temperature cross-linkable gate dielectric films compatible with

vapor and solution deposited semiconductors for aqueous sensors. The polymer matrix for the

gate dielectric layer in this study is poly(4-vinylphenol), selected for its dielectric characteristics

and compatibility with various organic semiconductors. These materials exhibit stability in

aqueous operation for over 10^4 electrical cycles. The work has led to the observation of a

significant drain current response for solutions with concentrations as low as parts per billion of

trinitrobenzene and methylphosphonic acid. We now describe the initial investigation of

semiconductor materials to achieve air- and water-stable sensors, as well as the introduction of

specific recognition elements to enhance sensitivity. The investigations will focus on the use of

functional groups as binding sites for the semiconductor:analyte interaction both under aqueous

and ambient air conditions, with the goal of developing environmentally stable sensors. The

mechanism for OTFT sensing is investigated by varying device parameters including

semiconductor thickness and operating conditions, such as analyte concentration and gate bias.

11:00 AM B11.8 Organic versus Inorganic Thin-Film Transistor Chemical Vapor Sensors Soumya Dutta and

Ananth Dodabalapur; Microelectronics Research Centre, University of Texas at Austin, Austin,

Texas.

This presentation will highlight the differences between responses of thin-film transistor (TFT)

based chemical vapor sensors with organic semiconductors such as pentacene and inorganic

semiconductors such as Zinc Tin Oxide (ZTO). In organic TFT chemical vapor sensors,

numerous studies have shown that trapping of charges by polar analytes at grain boundaries and

other device interfaces often results in a decrease in drain current (Id) [1-2]. In ZTO TFT

sensors, the drain current increases substantially upon exposure to such polar analytes. For

example, the drain current of a ZTO TFT sensor exhibits a substantial (~ 6-7 fold) increase of Id

upon exposure to IPA. The differences in response between the organic and inorganic TFT

sensors to the same analytes is due to the strong effects that polar molecules/materials have in

trapping charges in organic semiconductors. Increase in Id is also sometimes seen in organic

TFT sensors upon exposure to polar analytes. This happens when the molecular ordering is not

good and the mobilities are very low. Under these conditions, the extra charge density in the

channel induced by the polar analytes overwhelms the drain current reduction due to the trapping

of charges. We point out that this trapping of charges in the presence of polar molecules is also

seen when water is present in the vicinity of the transistor channel. A possible reason for the

trapping and mobility reduction in organic TFTs is the formation of bipolarons which are

stabilized by the polar analyte molecules. Since bipolarons have much lower mobilities, we have

a decrease in current. Activation energy measurements of charge carriers in organic TFT sensors

indicates that polar analyte molecules result in increased activation energies, supporting this

physical model. We further investigate the differences between OTFT sensors and inorganic TFT

sensors with the help of a novel four-terminal device configuration which allows us to extract

important information about trapping and threshold voltage shifts. In this four-terminal sensor

device, there are two coupled channels, one in an organic semiconductor and the second in an

inorganic semiconductor. These channels are electrostatically coupled, permitting both

capacitance-voltage and current voltage measurements. References: [1] L. Torsi and A.

Dodabalapur, Anal. Chem. 77, 380A (2005). [2] B. Crone, A. Dodabalapur, A. Gelperin, L.

Torsi, H. E. Katz, A. J. Lovinger, and Z. Bao, Appl. Phys. Lett. 87, 2229 (2001).

11:15 AM B11.9 The Development of Biosensors Based on Organic Thin Film Transistors Warwick J

Belcher, Xiaojing Zhou, Kathleen Sirois, Daniel Elkington and Paul Christopher Dastoor; Centre

for Organic Electronics, University of Newcastle, Callaghan, New South Wales, Australia.

Field effect transistors (FETs) are commonly used as the basis of sensor devices because of their

attractive combination of transduction and amplification properties. In particular, organic thin

film transistors (OTFTs) have received increased recent interest because they also offer

improved material compatibility with enzymes and biomolecules in general. This overcomes, in

part, the traditional issue of forming an effective recognition element - transducer interface. The

possibility of these also being “all plastic” devices means that biosensors based on OTFTs have

the potential to be easy and cheap to fabricate. A relatively new class of OTFTs are based on a

hygroscopic insulator layer. These function via a mechanism that involves ionic drift within the

insulator layer, resulting in electrochemical doping of the channel, and the formation of charged

layers at the insulator-gate and insulator-channel interfaces. The result of this dual mechanism is

a device that has remarkably low turn on voltages compared to a traditional OTFT or metal-

based FETs. This makes this class of OTFT particularly attractive for use with biological

molecules which may not be compatible with the high operating voltages required in most FETs.

We have fabricated and characterised all-polymer devices based on a variety of hygroscopic

insulators. Using glucose oxidase as a model system we have investigated interfacing enzymes

with these devices to create working biosensors. The results of this research, along with recent

developments, will be presented.

11:30 AM B11.10

Interfacial Effects Affecting Data Retention in Organic Non-volatile Memory Elements

Based on Ferroelectric Field-effect Transistors. Tse Nga Ng, Sanjiv Sambandan, Robert Street

and Ana Claudia Arias; Electronic Materials Lab, Palo Alto Research Center, Palo Alto,

California.

Non-volatile memory elements for mechanically flexible electronics are being developed using

organic materials to integrate data-sensing and storage. Ferroelectric field effect transistors

(feFETs) offer the advantages of non-destructive read-out and smaller cell size when compared

to capacitor structures. Unlike resistive switches for which the memory mechanism is still

unclear, it is well-established that ferroelectric devices switch states by dipole alignment and

allow faster switching speed than memories based on charge trapping in floating-gate transistors.

However, the retention time of ferroelectric memory is merely on the order of days, and

systematic studies of degradation mechanisms are needed to improve the retention time. In this

paper, factors affecting the current output of feFETs are examined, including changes in

semiconductor mobility, threshold voltage, and dielectric capacitance with time. Degradation in

the dielectric and in the semiconductor is separated for understanding the origins of instability.

Measurement results indicate that the semiconductor-dielectric interface is critical for

polarization retention in feFETs. The current decay of feFET hysteresis was partly caused by

charge trapping due to ferroelectric polarization. Another contribution to hysteresis loss is the

depolarization field at the channel interface; the depolarization field is not completely

compensated because of low carrier density in organic semiconductor. With better understanding

of the interfacial effects, memory cell structures are improved to retain 50% of output current

over 7 days. Due to the gradual decrease in feFET current, calibration method for extracting the

original input voltage is demonstrated, enabling feFETs to be used as analog memory. Finally,

ferroelectric transistor dimensions will be discussed with regards to improving data retention

time.

11:45 AM B11.11

Direct Tunneling into the Gate Dielectric by both Holes and Electrons in an Ambipolar

Organic Memory Transistor. Maarten Debucquoy1,2

, M. Rockele1, Jan Genoe

1, Gerwin

Gelinck3 and Paul Heremans

1,2;

1SOLO/PME, IMEC vzw, Leuven, Belgium;

2ESAT/INSYS,

Katholieke Universiteit Leuven, Leuven, Belgium; 3TNO, Eindhoven, Belgium.

The flash memory transistor is the standard non-volatile memory device in silicon industry, and

responsible for the omnipresence of portable data storage media in our electronic environment.

For organic semiconductor applications to become fully mature, a non-volatile organic memory

device, which can be as easily integrated into existing circuit technology, would be desirable.

Different structures for organic memory transistors have been proposed in literature. The

mechanisms identified to change the threshold voltage of the memory transistor and in this way

to switch between the different memory states, include polarization of the gate dielectric,[1]

charge carrier trapping in the semiconductor[2] and charge carrier trapping in the gate

dielectric.[3-5] This last mechanism, where a floating gate is suggested to charge and discharge

comparable to the operation of the standard flash memory transistor, has shown promising results

in terms of switching speed and retention.[4] In our work we studied a top contact pentacene

memory transistor with a structure similar to the structure used by Baeg et al. We show that the

dependence of the threshold voltage shift on the length of the programming pulse indeed accords

to the direct tunneling of charge carriers from the channel into the gate dielectric, as known from

inorganic flash memory transistors.[6] Even more, we prove that not one type of charge carrier

tunnels, but both holes and electrons do, depending on the sign of the programming voltage. The

result is a nearly perfect symmetrical switching of the transfer characteristics around the initial

threshold voltage of the memory transistor. The limiting factor for this tunneling is the supply of

these charge carriers in the channel of the transistor: exposure to air kills the tunneling of the

electrons, while with a purely n-type semiconductor no tunneling of holes is observed. Based on

these experiments, we pinpoint the important role of the polymer used as the gate dielectric in an

organic memory transistor. It does not only act as the floating gate but as this polymer controls

the interface with the organic semiconductor,[7] it also controls the supply of charge carriers in

the channel and in that way the tunneling of these charge carriers and the behavior of the organic

flash memory transistor. [1] R. C. G. Naber et al. Nat. Mater. 4, 243 (2005) [2] A. R. Völkel et

al. Phys. Rev. B 66, 195336 (2002) [3] H. E. Katz et al. J. Appl. Phys. 91, 1572 (2002) [4] K.-J.

Baeg et al. Adv. Mater. 18, 3179 (2006) [5] Wu et al. Adv. Func. Mater. 18, 2593 (2008) [6]

Ferris-Prabhu A. V. IEEE Trans. Electron Dev. ED-24, 524 (1977) [7] L.-L. Chua et al. Nature

434 194 (2005)

SESSION B12: Light Emission

Chair: Egbert Zojer

Friday Afternoon, April 17, 2009

Room 2001 (Moscone West)

1:30 PM B12.1

Solution-processible Phosphorescent Polymers for Electroluminescence and Light-

harvesting Andrew B Holmes1,2

, Robert J Borthwick1, Inam Ul Haq Raja

1, David J Jones

1, Tae-

Hyuk Kwon1, Georgia E McCluskey

1, Weihua Tang

1, Doojin Vak

1, Scott E Watkins

2 and

Wallace W H Wong1;

1Bio21 Institute, University of Melbourne, Parkville, Victoria, Australia;

2Division of Molecular and Health Technologies, CSIRO, Clayton, Victoria, Australia.

In this paper we report the synthesis of solution-processible conjugated polymers in which a

phosphorescent organometallic iridium complex is either conjugatively or non-conjugatively

linked to the polymer host. Phosphorescent emission is dependent on the appropriate design of

the triplet energy of the polymer host and the energy transfer to the emissive organometallic

ligand. Examples of high triplet energy hosts with the potential to support blue emissive guests

will be reported. In a parallel investigation we report the efficacy of using low band gap solution-

processible phosphorescent polymers as potential light harvesting agents for organic photovoltaic

cells. Performance will be compared with conventional excitonic heterojunction cells.

1:45 PM B12.2 White-Light Emitting Supramolecular Polymers.Philippe Leclere

1,2, Robert Abbel

2, Mathieu

Surin1, Wojciech Pisula

3, Roberto Lazzaroni

1, W. Stouwdam

2, Christophe Grenier

2, Bert Meijer

2

and Albertus P.H.J. Schenning2;

1University of Mons-Hainaut, Mons, Belgium;

2Eindhoven

University of Technology, Eindhoven, Netherlands; 3Max-Planck Institute for Polymer Research,

Mainz, Germany.

White-light emitting plastic materials are attractive candidates for applications in flexible organic

based lighting applications and in backlights for liquid crystalline displays. Common approaches

to obtain emission over the whole visible spectrum include energy transfer from donor polymers

mixed with luminescent acceptor chromophores, or copolymers containing moieties emitting at

different wavelengths. Multicomponent pi-conjugated systems based on supramolecular

interactions combine the advantages of small molecules with those of polymers and therefore can

facilitate energy transfer processes due to a spatial proximity and controlled orientation between

the different species. In this work, this concept is developed to create white emitting

supramolecular polymers by using two novel different approaches based on (i) the self

complementarity of multiple hydrogen bonding moieties or (ii) polycatenars (rigid molecules

equipped with flexible wedges at the ends). The first approach makes use of 2-ureido-4[H]-

pyrimidinone (UPy), as a strongly self-complementary H-bonding moiety, for the formation of

noncovalent polymers with high molecular weight. Difunctionalized UPy oligomers (fluorene as

blue emitter, para-phenylenevinylene as green emitter, and perylene as red emitter) have been

synthesized and mixed together to create white photoluminescent supramolecular polymers. In

the second approach, fluorene-based co-oligomers have been prepared in order to study

differences in their aggregation behavior upon subtle modifications of their chemical structures.

They consist of two fluorene units linked by a central aromatic moiety (fluorene, naphthalene,

quinoxaline, benzothiadiazole or thienopyrazine) that has been chosen such that the emission

color covers the entire visible range. Amide linkers between the chromophores and the wedges

were chosen in order to introduce hydrogen bonding as a noncovalent ordering interaction. These

chromophores form organogels in which the emission wavelength can be tuned by partial

energry transfer yielded blue, red and white fluorescence systems. For the two approaches, the

degree of polymerization is sufficiently high to fabricate white photoluminescent supramolecular

polymers. The morphology of these organogels has been studied by Scanning Probe Microscopy

and Wide Angle X-ray Scattering. In order to understand the results in terms of supramolecular

organization within the fibrillar morphology, comparisons with molecular modeling simulations

are performed. LEDs based on these noncovalent polymers have been successfully prepared

from all three types of pure materials, yielding blue, green and red devices, respectively. At an

appropriate mixing ratio of the three compounds, white electroluminescence is observed. This

modular approach yields a toolbox of molecules that can be easily used to construct pi-

conjugated electroactive supramolecular polymers with a variety of compositions and tunable

emission colors.

2:00 PM B12.3

Efficient Electron Injection via Organic Interlayers for Inverted Organic Light-emitting

Diodes Free of Dopants and Reactive Metals. Seunghyup Yoo, Chang-Hun Yoon and Hyunsu

Cho; Electrical Engineering, KAIST, Daejeon, Korea, South.

Inverted organic light-emitting diodes (OLEDs) are highly desired from both fabrication and

driving standpoint in active-matrix OLED (AMOLED) displays based on n-type thin-film

transistors (TFTs), which include a-Si TFTs as well as a variety of emerging ZnO-based TFTs.

However, the actual application of the inverted OLED technologies has been rather slow due to

the lack of reliable means for electron injection from the bottom cathodes to typical electron

transport or emission layers. Moreover, from the point of view of practicality in fabrication and

operational stability, it is considered that one should avoid or minimize, if possible, use of low

work function metals like Mg as cathodes or alkali-based doping for electron transport layers[1],

rendering the realization of inverted OLEDs even more challenging. Here we present our study

of using organic electron injection layers (OEIL) to realize efficient inverted organic light-

emitting diodes (IOLEDs) via cascaded electron injection from bottom aluminum cathodes.

These IOLED devices exhibit luminescence of 1,000 cd/m2 at 5 V with a maximum luminous

efficacy of 5 cd/A in a typical OLED using Alq3 as a light emitter. Our study of Al/OEIL/Al

devices shows that electrons can be efficiently injected to the OEIL from both top and bottom

electrodes depending on the polarity of the applied bias. Further analysis indicates that the

successful operation of these IOLEDs can be attributed to (i) the relatively high electron mobility

of OEIL films studied and (ii) its ideal LUMO energy level positioned between that of Alq3 and

the workfunction of Al. Finally, we demonstrate the versatility of the OEIL under study by

presenting ITO-free, transparent inverted OLEDs in which the OEIL plays a critical role for

enhancement in both electrical injection and optical transmission. Reference [1] C.-C. Wu, C.-W.

Chen, C.-L. Lin, and C.-J. Yang, J. of Display Tech. 1 (2), 248 (2005)

2:15 PM B12.4

The Mechanism for Improved Charge Injection in Polymer Light Emitting Diodes with

Conjugated Polyelectrolyte Electron Transporting Layers. Corey V. Hoven1,3

, Rengiang

Yang2,3

, Andres Garcia2,3

, Jeffrey Peet1,3

, Alan J Heeger1,2,3

, Thuc-Quyen Nguyen2,3

and

Guillermo C Bazan1,2,3

; 1Department of Materials, University of California, Santa Barbara,

California; 2Department of Chemistry and Biochemistry, University of California, Santa Barbara,

California; 3Institute for Polymers and Organic Solids, University of California, Santa Barbara,

California.

Polymer light emitting diodes (PLEDs) with conjugated polyelectrolyte (CPE) electron

transporting layers (ETLs) and high work function cathodes exhibit excellent efficiencies,

despite apparently large electron injection barriers. The improvement is found to be due an

electric field redistribution within the PLED through a combination of hole accumulation at the

emitting layer (EML)/ETL interface and ionic rearrangement within the ETL layer. Efficient

PLEDs typically require low work function metal cathodes to provide balanced charge injection.

However, low work function metals, such as Ba and Ca, are unstable and decrease device

operational lifetimes. Although the improved electron injection from high work function metals

by the insertion of a CPE ETL is quite valuable, the mechanism behind the improved injection

has been poorly understood. The improved electron injection has typically been attributed to a

permanent interfacial dipole at the CPE/cathode interface. However, a correlation of electrical

measurements (current density (J)-voltage (V), luminance (L)-V, time response of J and L under

constant V, electroabsorption, and impedance measurements) with device structure (with or

without the ETL, choice of cathode, choice of counterion and layer thickness) provided evidence

that a combination of hole accumulation and ion redistribution leads to screening of the internal

electric field and an effective lowering of the electron injection barrier. As holes are injected they

accumulate at the EML/ETL interface and screen the electric field in the EML. The electric field

is thereby redistributed so that voltage drops predominantly across the ETL producing a large

internal field in that layer. The electric field across the ETL is redistributed by the mobile ions

equally into two double layers at the EML/ETL interface and the ETL/cathode interface. The end

result is that the electric fields in both layers are screened with the entire applied voltage

localized across the double layers. Efficient electron injection occurs via tunneling through the

ultra-thin double layer at the ETL/cathode interface. The hole and electron currents are diffusion

currents rather than drift currents. The diffusion currents in each layer are space charge limited,

the EML conducts only holes, the ETL conducts only electrons, and recombination takes place at

the interface.

2:30 PM B12.5 Multi-color High Performance Organic Light Emitting Transistors. Ebinazar B Namdas

1,

Ifor D. W. Samuel1,2

and Alan J Heeger1;

1Center for Polymers and Organic Solids, University of

California, Santa Barbara, California; 2School of Physics and Astronomy, University of St

Andrews, St Andrews, United Kingdom.

Recently organic light emitting field effect transistors (LEFET) are of interest as a new

architecture with potential for new applications such as simplified pixels in flat panel displays, as

optoelectronic devices in communications, and potentially as electrically driven organic lasers. A

LEFET combines the emission properties of a diode with the switching properties of a transistor

in a single device structure. The brightness and light emission performance reported for LEFETs

in the literature are low. Here we present and discuss our latest results on the design and

fabrication of high performance LEFETs. In particular we have demonstrated red, green and blue

light-emission, as would be required for display applications of these devices. The

semiconducting polymer layers were deposited from solution by spin-coating. The interface with

the dielectric is an important issue in such devices, and we find that the best performance can be

obtained by using a device structure with two organic layers. The first is a hole transporting

polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b] thiophene) and the second is a

light-emitting polymer. The devices were fabricated in the bottom gate architecture (Si/SiO2)

with top-contact Ca/Ag as source/ drain electrodes. Our results show the interplay between

charge injection, charge transport and photo-physical properties in the operation of LEFETs,

enabling their efficiency to be improved.

3:15 PM B12.6 Trap-assisted Recombination in Polymer Light-emitting Diodes. Martijn Kuik, Martijn

Lenes, Herman Nicolai and Paul W. M. Blom; Zernike Institute for Advanced Materials,

University of Groningen, Groningen, Groningen, Netherlands.

Polymer light-emitting diodes (PLEDs) are promising candidates for displays and lighting

applications. To further improve the light output efficiency of these devices a fundamental

understanding of the charge recombination mechanisms is required. Until now the recombination

process is assumed to be bimolecular between free electrons and free holes (Langevin

recombination). However, measurements on electron-only devices demonstrate that a large part

of the injected electrons in a PLED are being trapped. The fundamental question arises whether

these trapped electrons recombine with free holes and as a result limit the performance of a

PLED, since such a trap-assisted recombination mechanism is often of non-radiative nature. To

unravel the various recombination processes in PLEDs we investigate the photocurrent that can

be generated in PLED devices. Earlier work on the photoconduction in organic solar cells has

shown that a linear dependence of the open-circuit voltage (Voc) on the light intensity only

occurs when both the electron and hole transport is trap free, leading to a dominance of

bimolecular Langevin recombination [1]. Furthermore, it has been demonstrated that for

polymer/polymer solar cells [2], where the electron transport is limited by traps in the polymer,

or by deliberately adding electron traps in a trap-free bulk heterojunction solar cell [3], the

dependency of Voc on the light intensity becomes superlinear. This can be quantitatively

explained by the recombination of trapped with free charges (Shockley Read Hall

recombination), in addition to Langevin recombination. As a result the light intensity dependence

of the Voc in a PLED is a measure for the occurrence of trap-assisted recombination. We have

investigated the dependence of the open-circuit voltage on the light intensity in pristine MEH-

PPV and MDMO-PPV based LEDs: Our measurements revealed that the Voc dependence on the

light intensity turned out to be even stronger then quadratic, indicating a strong trap-assisted

recombination mechanism in the PLEDs. We present a novel device model for PLEDs based on

these observations, which includes Langevin as well as Shockley Read Hall recombination. In

contrary to common beliefs we show for PPV-based PLEDs the trap assisted recombination rate

is at least equal to the free carrier recombination rate, demonstrating the significant contribution

of the trap assisted recombination in a PLED. [1] L. J. A. Koster, V. D. Mihailetchi, R. Ramaker

and P. W. M. Blom, Appl. Phys. Lett. 86, 123509 (2005). [2] M. M. Mandoc, W. Veurman, L. J.

A. Koster, B. de Boer and P. W. M. Blom, Adv. Func. Mater. 17, 2167-2173 (2007). [3] M. M.

Mandoc, F. B. Kooistra, J. C. Hummelen, B. de Boer and P. W. M. Blom, Appl. Phys. Lett. 91,

263505 (2007).

3:30 PM B12.7 Silicon Based Organic Materials for Light Emitting Diodes. Colin Keyworth

1,3,2, Andrew B

Holmes3,1

and Charlotte K Williams1;

1Chemistry, Imperial College London, London, United

Kingdom; 2Cambridge Display Technology (CDT), Cambridge, United Kingdom;

3Bio21

Laboratory, The University of Melbourne, Melbourne, Victoria, Australia.

A range of organic conjugated materials have been developed, utilising Suzuki methods and

some novel synthetic methodologies, for the practical application of blue light emitting diodes.

The use of silicon as a heteroatom within the organic framework has been explored and the

effects on the electroluminescence of the prepared materials has been studied in order to gain

insight into the role of silicon within organic fluorescent polymers and small molecule systems.

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