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U N I V E R S I TAT I S O U L U E N S I S
ACTAC
TECHNICA
U N I V E R S I TAT I S O U L U E N S I S
ACTAC
TECHNICA
OULU 2010
C 351
Hanna Haverinen
INKJET-PRINTED QUANTUM
DOT HYBRID LIGHT-EMITTING
DEVICESTOWARDS DISPLAY
APPLICATIONS
FACULTY OF TECHNOLOGY,
DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING,
OPTOELECTRONICS AND MEASUREMENT TECHNIQUES LABORATORY,UNIVERSITY OF OULU
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A CTA UNIVE RS ITA T I S O UL
C Techn i c a 35 1
HANNA HAVERINEN
INKJET-PRINTED QUANTUM
HYBRID LIGHT-EMITTING
DEVICESTOWARDS DISPLA
APPLICATIONS
Academic dissertation to be presented with
the Faculty of Technology of the Universitypublic defence in OP-sali (Auditorium L10), L
19 March 2010, at 12 noon
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Copyright 2010
Acta Univ. Oul. C 351, 2010
Supervised byProfessor Risto Myllyl
Professor Ghassan E. Jabbour
Reviewed by
Doctor Arto MaaninenDoctor Patrick J. Smith
ISBN 978-951-42-6126-8 (Paperback)ISBN 978-951-42-6127-5 (PDF)
http://herkules.oulu.fi/isbn9789514261275/ISSN 0355-3213 (Printed)
ISSN 1796-2226 (Online)http://herkules.oulu.fi/issn03553213/
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Haverinen, Hanna, Inkjet-printed quantum dot hybrid lig
devicestowards display applicationsFaculty of Technology, Department of Electrical and Information Engineering, Opand Measurement Techniques Laboratory, University of Oulu, P.O.Box 450
University of Oulu, Finland
Acta Univ. Oul. C 351, 2010
Oulu, Finland
Abstract
This thesis presents a novel method for fabricating quantum dot light-emitting devic
based on colloidal inorganic light-emitting nanoparticles incorporated into
semiconductor matrix. CdSe core/ZnS shell nanoparticles were inkjet-printe
sandwiched between organic hole and electron transport layers to produce effi
emissive media. The light-emitting devices fabricated here were tested as individu
integrated into a display setting, thus endorsing the capability of this method as a m
approach for full-colour high-definition displays.
By choosing inkjet printing as a deposition method for quantum dots, seve
currently inevitable with alternative methods are addressed. First, inkjet printing pr
patterning due to its drop-on-demand concept, thus overruling a need for com
laborious patterning methods. Secondly, manufacturing costs can be reduced sig
introducing this prudent fabrication step for very expensive nanoparticles.
Since there are no prior demonstrations of inkjet printing of electroluminescen
devices in the literature, this work dives into the basics of inkjet printing of l
relatively highly volatile quantum dot inks: piezo driver requirements, jetting par
dynamics in the cartridge and on the surface, nanoparticle assembly in a wet drople
of dots on the surface are main concerns in the experimental part. Device performan
discussed and plays an important role in this thesis. Several compositional QDLED
described. In addition, different pixel geometries are discussed. The last part of th
deals with the principles of QDLED displays and their basic components: RGB pixe
thin-film transistor (OTFT) drivers. Work related to transistors is intertwined with Q
ideas for surface treatments that enhance nanoparticle packing are carried ov
assembled monolayer (SAM) studies in the OTFT field. Moreover, all the work don
project was consolidated by one method, atomic force microscopy (AFM), which
throughout the entire thesis.
Keywords: atomic force microscopy (AFM), inkjet printing, inorganic n
organic thin-film transistors, quantum dots, RGB displays, self-assembled
surface treatment
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Acknowledgements
This study is based on research carried out in the Flexible Displa
Arizona State University, USA, and the Optoelectronics and M
Techniques Laboratory at the University of Oulu during the years 200
I thank the Graduate School of Modern Optics and Photon
(Finnish Funding Agency for Technology and Innovation),
Synjkangas Foundation and the Nokia Science Foundation for research.
I want to acknowledge and thank all my colleagues at Ar
University for their help, guidance, discussion and support during m
research program. My understanding in the field was strongly in
several people. Dr. Brigitte Wex introduced me to organic chemist
dots and several processing methods and encouraged me. Dr. Mike
Evan Williams and Dr. Parul Dhagat have given me technical su
grateful to Dr. Madhusudan Singh, Mrs. Tricia Youngbull and Dr. In
their fruitful discussions and friendship.
Not to forget my colleagues in the Optoelectronics and
Techniques Laboratory, I direct my thanks to Dr. Erkki Alarousu fo
ideas, experiences and dreams and Dr. Tapio Fabritius, Dr. Juha
Lic.Tech. Eija Vieri-Gashi for their discussions and friendship.I have been fortunate to have two supervisors during my years
Professor Risto Myllyl has my greatest gratitude for always believin
pushing me forward. Professor Ghassan E. Jabbour got me excit
topic, gave me a great deal of knowledge and encouraged me wh
doubt. He is acknowledged for being a major part of my success.
My family, my parents and my brother, have been my source without their support, love and guidance, I wouldnt have accom
dreams. I forever carry my gratitude for them with me.
My deepest gratitude goes to my fiance, partner in life and
Without him I would have never believed in myself and would nev
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Abbreviations and symbols
AC alternating current
AFM atomic force microscopy
AM active matrix
CIE commission internationale de l'clairage
CRT cathode ray tube
DOD drop-on-demandEQE external quantum efficiency
ETL electron transporting layer
FWHM full width at half maximum
HOMO highest occupied molecular orbital
HTL hole transporting layer
LCD liquid crystal display
LED light-emitting device/diode
LUMO lowest unoccupied molecular orbital
OLED organic light-emitting device/diode
OTFT organic thin-film transistor
PLED polymer light-emitting device/diode
PM passive matrix
QD quantum dotQDLED quantum dot light-emitting device/diode
QVGA quarter video graphics array
QY quantum yield
RGB red, green, blue
UV ultraviolet
Alq3 Tris(8-hydroxyquinolinato)aluminium
CdS Cadmium Sulfide
ITO Indium Tin Oxide
MeO-TPD N,N,N,N-tetrakis(4-methoxyphenyl)-benzidine
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PEDOT:PSS Poly(3,4-ethylenedioxythiophene) poly(styrenes
poly-TPD poly-(N, N-bis(4-butylphenyl-N,N-bis(phenyl)bePPV p-paraphenylene vinylene
PVK poly(N-vinylcarbazole)
SiO Silicon Monoxide
TOPO Trioctylohosphine oxide
TPBI 1,3,5-tri(phenyl-2-benzimidazole)-benzene
TPD N,N bis(3-methylphenyl) N,N bis(phenyl)ZnS Zinc Sulfide
II-VI refers to the periodic table classification; materi
group two and six materials
a.u. arbitrary unit
c speed of light, 2.99792458108 m/s
cd/m2 candela per square meter
cP centipoises (1 cP = 0.01 Pas)
eV electron volt
g/ml grams per milliliter
h Plancks constant, 4.135667331015 eVs
reduced Plancks constant, 6.582118991016 eVk wave vector
k wave vector, scalarm mass of an electron, 9.10938188 10-31 kg
mM millimole
mg/ml milligrams per milliliter
m/s meters per secondn nano, 10-9
nm nanometer
p momentum
rms root mean square
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I current
V voltI-V current-voltage
ngstrom, 10-10
wavelength
angular frequency
micro, 10-6
s microsecond
theta, contact angle
liquid free surface energy of a liquid
solid free surface energy of a solid
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List of original papers
This thesis is a summary of the following papers published in internat
journals:
I Haverinen HM, Myllyla RA & Jabbour GE (2009) Inkjet printing of
quantum dots. Applied Physics Letters94: 073108.
II Haverinen HM, Myllyla RA & Jabbour GE (2010) Inkjet printed RGB
hybrid LED. IEEE Journal of Display Technology 6(3).
III Singh M, Haverinen HM, Dhagat P & Jabbour GE (2009) Progress
printing process and applications. Advanced Materials 21: 113.
IV Dhagat P, Haverinen HM, Kline RJ, Jung Y, Fischer DA, DeLongc
Jabbour GE (2009) Influence of dielectric surface chemistry on the micr
carrier mobility of an n-type organic semiconductor. Advanced Functio
19: 23652372.
V Li FM, Dhagat P, Haverinen HM, McCulloch I, Heeney M, Jabbour GE
(2008) Polymer thin-film transistor without surface pretreatment on a gate dielectric. Applied Physics Letters 93: 073305.
Authors contribution to publications I-V:
I II The author defined the research plan and conducted the
with measurements. She analysed the research results
the literature review, wrote the manuscript and producpaper.
III The author discussed the topic with her co-authors and
structure of the review paper. She was responsible for t
review of all the nanoparticle applications, wrote the r
them and helped to organise the final version adjunct
authors.IVV The author helped define the structural analysis and ca
the atomic force microscopy measurements. She
measurement part of the publication and helped revie
paper.
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Contents
Abstract
Acknowledgements
Abbreviations and symbols
List of original papers
Contents
1 Introduction 1.1 Related work on quantum dot light-emitting devices ..............1.2 Organisation of the thesis ........................................................
2 Quantum dots 2.1 Propagation of photons and electrons .....................................
2.1.1 Free space ......................................................................2.1.2 Confinement effects ......................................................
2.2 Quantum-confined materials ...................................................2.2.1 Core-shell structure of fluorescent quantum dots .........2.2.2 Optical properties of quantum dots ..............................2.2.3 Colloidal processing method ........................................
3 Light-emitting devices 3.1 Organic light-emitting devices ................................................3.2 Quantum dot light-emitting devices .........................................3.3 Fundamentals of display technology ........................................
3.3.1 Engineering points to look at when designing a display3.3.2 Active vs. passive displays ............................................3.3.3 Active matrix OLED displays .......................................3.3.4 Why a QDLED display? ...............................................
4
Inkjet printing 4.1 Background of printing technology ........................................4.2 Piezo inkjet printing technology .............................................4.3 Fluid dynamics ........................................................................
4.3.1 Printhead dynamics ......................................................
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6 Results 6.1 Early steps of device fabrication ................................6.1.1 Spin-coated devices ..........................................
6.1.2 Printed devices .................................................6.2 Poly-TPD-based monochromatic devices ..................6.3 RGB devices ..............................................................
6.3.1 Monochromatic devices ...................................6.3.2
Patterned RGB devices ....................................6.4 Debate on the wetting properties of nanoparticle ink ..
7 Discussion 7.1 Accomplished results ..................................................7.2 Limitations .................................................................7.3 Future work ................................................................
8 Conclusion References
Original Papers
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1 Introduction
Based on a display market survey conducted by the DisplaySearch
2008, over three-billion-dollar markets were estimated for flat panel
the end of 2009 (DisplaySearch Company 2008). Moreover, another
the Freedonia Group indicated that the annual increase in display ma
be 13.2% until 2008 (Freedonia Group 2004). These numbers clearl
important a business display technology is worldwide. The mo
application areas include, in addition to television screens and comput
cellular phones and portable IT devices. Currently, liquid crystal dis
and plasma flat screen technologies lead the market, overshadowing
display technology based on cathode ray tubes (CRTs). However, ne
trying to step into the worldwide markets; one of the first displ
organic light-emitting diodes (OLEDs) were lately debuted by LG a
(www.oled-displays.net 2009, Samsung Electronics 2009) in additiTrends indicate favourability for very thin, fast-switching, high col
displays with affordable price tags.
Ever since the discovery of OLEDs in 1987 (Tang et al. 198
leading towards thin, flexible displays has been rapid. OLED displ
several benefits over their competitors, including a wide viewing a
excellent colour contrast and fast response. Furthermore, organic manaturally emissive, there is no need for backlights or colour filters,
very thin devices (Overwijk et al. 2004). Polymer-based OLED
attraction due to their suitability for solution processing, thu
manufacturing cost drastically. Now, inkjet printing technology beca
thus revolutionising the display manufacturing business. Regard
superior features of OLED TVs, they are still facing some challengemitters are used for red, green and blue pixels, and they are al
different compounds with several different properties, i.e. each of
different lifetime and has to be optimised separately. The bottle
lifetime properties of OLEDs has been the short lifetime (less than 2
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structure). By choosing inorganic lumophores, colour purity
the very narrow emission spectra (FWHM ~ 20 nm) the(Dabbousi et al. 1997). Quantum dot hybrid LEDs utilise th
and thermal properties of inorganic crystals: high temperature
defect toleration as well as virtually no optical quenching
2004). In addition, their applicability in creating flexible,
ensured by organic layers around the quantum dots. Thu
address the problems that both entirely organic and entirelwould individually have.
This study shed light on the following questions: can we
a deposition method to fabricate QDLEDs? What thin-film ch
be achieved in order to produce efficient and bright devices?
properties and ink properties need to be modified? What are t
approach? How do the devices behave? What are their e
properties? Can we use inkjet-printed QDLEDs in display ap
be commercialised? In a nutshell, a conclusion about the
printing technology as a fabrication method for QDLEDs is
this thesis.
This thesis is a compendium of five separate publications
with inkjet printing of nanoparticles. The latter part of this th
publications related to OTFTs, thus introducing the display thon RGB displays and integration of the proposed LED str
backplanes. The results are presented and discussed, taking in
main ambition of the thesis project.
1.1 Related work on quantum dot light-emitting de
One of the earliest studies of light emission from colloidal n
et al. 1982) goes back to the early 1980s, although the qu
effects of IIVI semiconductors were under investigation a
earlier (Bergstresser et al. 1967, Lawson et al. 1960). Howev
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luminance of 100cd/m2. Regardless of the inefficient device stru
results started a new era in the research on hybrid QDLEDs. (Colvin eSimilar to the work by Colvin et al., another report using t
structure was published, this time by Kumaret al. (Kumaret al. 1997
sulfide (CdS) quantum dots were spin coated onto the PPV laye
aluminium was used as the cathode as opposed to the magnesium use
In this particular device, quantum dot layer functioned as charge tra
emission was dominantly from the PPV layer, not from the nLuminance reported was 150 cd/m2 and maximum efficiency reached
A true breakthrough was made in 2002, when a 25-fold
luminescent power efficiency in QDLEDs was reported. S. Coe et al.
new approach, where quantum dots were dispersed in a N,N
methylphenyl) (1,1 biphenyl) -4,4 diamine (TPD) solution an
resulting in a monolayer formation through phase separation (Coe et a
electron transporter, Tris(8-hydroxyquinolinato)aluminium, (Alq3),
sandwich structure, which became the most common structure ov
Luminance was still close to a few hundred candelas per square
efficiency reached the highest value reported thus far, 0.52%.
improvement was explained by a double exciton-forming mechanis
energy transfer from (HTL) - (ETL) excitons to quantum dots and a d
injection into the dots.The same group mastered this approach later on. The need for
study played a big role, and by adjusting the quantum dot layer thickn
quantum efficiency was doubled; 1.1% was reported (Coe-Sullivan
Laudable device performance reported in 2005 set the bar for eff
brightness really high; 2% EQE and 7000 cd/m2 was by far a record i
of QDLED development (Coe-Sullivan et al. 2005). Interestingly, used the same phase separation approach, yet did not achieve sup
performance; currents of less than 1 mA were measured (Zhao et al. 2
Spin casting has been the most popular approach to fabricatin
mainly due to the simple method for achieving a nearly m
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polar solvents as the HTL and ETL and they spun QDs from
thus avoiding dissolving the first hole transport layenanoparticles, and later on protecting the QD layers when th
layer was deposited (Chaudhary et al. 2004). This polymer
0.22% EQE and 500 cd/m2 brightness. On the other han
Gingers groups approached this problem by introducing
linkable hole transport layer. The initial device, with nearly
cd/m2 brightness, was improved in a later study by optimisin
EQE was doubled and luminance peaked at 6000 cd/m2.(Zha
al. 2007.)
Char et al. recently showed how by attaching an anc
quantum dots, a hybrid material can be produced when
polymer surrounds the thiol-anchored QDs. Thus, an ea
resulted in devices where EQE increased threefold compare
unmodified dots (Zorn et al. 2009). Interestingly, highly ef
reported without a cross-linked HTL; CdSe/ZnS quantum d
spun directly onto a polymer TPD layer, resulting in green-e
maximum brightness above 10 000 cd/m2 and 1.4% EQE (B
improvement in efficiency was explained as being caused by
chemical composition gradient that increased the photolumine
above 80%. The same group also reported highly efficient bsimilar device structure, however emphasising QD processin
2009b). In addition to the studies reported here, various grou
casting as a primary deposition method for colloidal quant
(Son et al. 2004, Cheng et al. 2009), red (Stouwdam et al. 20
al. 2007) QDLEDs have been fabricated by spin casting, to
The common denominator of these is the similar device strucomposition structure altered by doping or material selection.
In attempting to permit patterning, the spin coating met
avoided by microcontact printing. Typically, polydimethyl
moulded to form a desired pattern, followed by spin casting
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varied between 0.52%, depending on the position of the QD layer in
i.e. in the HTL, the ETL or between both. Moreover, both groups same method for fabricating white QDLEDs: mixing red, green and
solution in the correct ratio and then transferring the spin-coated lay
PDMS stamp to the organic HTL surface. Furthermore, the dev
current densities in the same order of magnitude (Anikeeva et al. 20
al. 2008a), thus being fully comparable.
Other approaches to depositing QDs include mist fabrication
2008), dip coating (Lee et al. 2009) and inkjet printing (Haverinen
Haverinen et al. 2010, Tayloret al. 2007, Wood et al. 2009), the latte
suitable for low-cost fabrication with exquisite patterning capabiliti
demonstration of electroluminescent inkjet-printed quantum dots was
Haverinen et al. Lately, AC-driven hybrid QDLEDs in which the do
inkjet printed were reported. However, the basis of this thesis repres
attempts to fabricate monochromatic as well as RGB QDLEDs
printing technology.
1.2 Organisation of the thesis
This thesis is organised as follows:
Chapter 2 focuses on the theory of quantum dots: quantum
effects, optical properties and processing methods where th
structure is discussed and the properties of organic ligands are exp
Chapter 3 dives into the properties of organic LEDs and quantu
LEDs, thus explaining in principle how these devices work. Furt
fundamentals of display technology are introduced in this chapter
Chapter 4 concentrates on inkjet technology: the principles of in jetting properties and ink requirements. The second part di
dynamics in the cartridge and on the surface.
Chapter 5 introduces the materials and methods used and briefly d
QDLED f b i i
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Chapter 7 contains a discussion on the obtained results
contribution of this thesis on inkjet-printed QDLEDs. Alimitations of the method used, with some suggestions for
Chapter 8 provides a summary of the results.
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2 Quantum dots
This chapter briefly discusses electron and photon propagation in fre
in a confined system, thus shedding light on the definition of q
confinement. The manifestations of quantum confinement incl
properties; the effect of quantum dot size and structure on optical
described. Furthermore, processing approaches are introduced by ex
simple steps of core-shell fabrication.
2.1 Propagation of photons and electrons
The physical behaviour of photons and electrons is rather similar.
described as a particle and as a wave, known as dualism (Feynman
However, classical physics emphasises that a photon is mainly
transporting electromagnetic wave, while an electron is treated as a charged particle with the lowest mass of matter. However, quantum p
photons and electrons analogously (Prasad 2004). For example, w
described with de Broglies postulate, = h/p, where h is Plancks co
is particle momentum. Electrons possess greater momentum, thus ha
wavelengths, which also has another consequence: confinement
photons occur at larger scales (Atkins & dePaula et al. 2002)Photon propagation is typically described by the electromagnetic
of a medium, where the refractive index of matter resists propagati
propagation, on the other hand, is described by time-independent
equations, where kinetic energy and potential energy are taken into a
equation called the Hamiltonian operator postulates that electron pr
dependent on the kinetic energy of a charged particle and is retarded interactions, described as the potential energy of the particle. Sol
Schrdinger equation gives the allowed energy states of electrons.
The biggest difference between electrons and photons relates t
description: photons have a vector field, while electrons have a sca
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no spatial variance in their refractive index. In this situa
electromagnetic field behaves as a plane wave with a complexvectork implicates the direction of the wave; positive value
direction when a Cartesian plane is the reference plane. Th
direct relationship to momentum and an inverse relation to w
= /, wherep is reduced Plancks constant. Photon proportional to angular frequency; E = (Born & Wolf 1998
The description of free-space propagation of electrons in
the Schrdinger equations resulting in an oscillating sinu
similar to photon propagation. Moreover, wave vector k c
movement. The energy of an electron is postulated in the fol
2k2/2m, indicating all the energy values of electrons allowe
The same reflects to photons; all the frequency values allowed
Therefore, continuous values of E and form a band, thu
electrons and photons within the band. (Born & Wolf 1998.)
2.1.2 Confinement effects
Confinement simply indicates that propagation is dimension
interaction potential that causes reflection, thus inhibiting th
For photons, this can be introduced by trapping a light wavehigher refractive index (Saleh & Teich 1991).
For electrons, confinement is created by an internal ene
from increased potential energy. This traps electrons betw
creating a box where electrons can travel freely inside but w
potential energy of the particle is smaller than the barrier e
Merzbacher 1998). This barrier can be thought of as a barriatoms in a material. For quantum dots, the diameter of a
determines the length of the box. The most important
quantisation of allowed energies, which is visualised in
between atom orbitals forces band splitting to occur due to
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Fig. 1. Molecular orbitals create band splitting when more atoms are close proximity. Bulk crystal has smaller spacing between adjacent e
compared with e.g. quantum dots.
Typical semiconducting quantum dots have a few hundred to seve
atoms, thus their actual size ranges between 210 nm (Borovitsk
2002). Now, wave functions cannot extend fully inside the dots, whic
energy state. Intuitively, smaller dots have a higher energy state than
Therefore, smaller dots have a larger energy gap, thus they emit at a h
(Fig. 2). Size-dependent emission is one of the most intriguing featu
by changing only quantum dot size, emission colour can be tuned th
entire visible spectrum (Houtepen 2007, Borovitskaya & Shur 2002).
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Fig. 2. Quantum dot size relationship to emission wavelength.
2.2 Quantum-confined materials
2.2.1 Core-shell structure of fluorescent quantum d
The original quantum dots had only a core without any p
However, the optical properties of quantum dots can be dra
coating the light-emitting core with a monolayer or a few mo
band gap material shell (Wilson et al. 1993). By passivatingthe core surface with shell molecules, the charge-carrier prop
efficiency and band gap profile can be modified (Prasad200
material is chosen to allow emission from the core through t
absorption losses. Even though the most popular structure inv
layer, multiple shell layers have been demonstrated. A so-ca
has double or triple shell layers with alternating low and high(Schooss et al. 1994). This structure further modifies emissio
to red wavelengths, thus widening the possibilities of tuning e
quantum dots used in this thesis project were CdSe/ZnS dots
Dabboussi et al.
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drawback in this approach is that ligands function as a dielectric, th
charge transport efficiency.
2.2.2 Optical properties of quantum dots
Due to three-dimensional confinement, quantum dots are known to
narrow emission band. FWHM is dictated by the uniformity
distribution; 20 40 nm FWHM is typical nowadays. Size distrib
reason for spectrum broadening; quantum physical properties such a
spectral line width of quantum dots and temperature dependence o
structure also widen the spectra of quantum dots.
Interestingly, the onset of quantum dot photoluminescence is ind
the wavelength of excitation, as long as it is lower than the first abso
as seen in Fig. 3. It is worth mentioning that the peak of lowest energ
and the photoluminescence emission peak have a small shift. ThisStokes shift and is a typical phenomenon with quantum dots. Fur
decreases when dot size increases, reaching a point when a Stokes s
occur (Bagga et al. 2001)
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3 Light-emitting devices
By nature, organic compounds, like polymers for example, are in
should not be good conductors. However, a surprising discovery
Shirakawa and Heeger opened doors for organic electronics. In t
polyacetylene polymer was doped with iodine, thus rendering an ins
conductor (Chiang et al. 1977). The conduction mechanism
compounds is mainly explained by two mechanisms: 1) overlappin
orbitals with adjacent molecules create a band structure for charg
propagate and/or 2) hopping of carriers over a small barrier to
molecules, enabled by perturbation from surrounding molecules (
Brenner 1964). This is the basis for OLED and QDLED operation.
3.1 Organic light-emitting devices
An over-simplistic model of an OLED consists of two organic ma
transporting and electron-transporting, sandwiched between low an
function electrodes. In some cases, like in polymer light-emitt
(PLEDs), one layer functions as the hole and electron transporter
emitter (Burroughes et al. 1990). In Fig. 4, the operation of a two-lay
used as an example. A high work function material, Indium Tin Oxused as a transparent anode. Holes are injected into the hole transpo
highest occupied molecular orbital (HOMO) and transferred towards
transport layer. At the same time, electrons are injected from th
function cathode into the electron transport materials lowest
molecular orbital (LUMO) and transferred towards the ETL/HTL in
to Coulomb attractive interaction, the holes and electrons form a p
exciton. An exciton site can be formed between two different molecu
a single material, depending on material choices. After a recombina
photon is emitted. An exciton can also relax non-radiatively when ma
energy is released into the system.
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electron injection material (typically a fluoride-based com
between the cathode and the ETL to assist in the injection some devices an additional layer, an emissive layer, can be a
and HTL inject charge carriers into the emissive layer, which
them. In addition, the hole-blocking layer at the cathode site i
exciton-forming site away from the contacts to prevent n
quenching. Regardless of the device configuration, the final
(in the order of a hundred nanometers), and in the case of
virtually immune to bending and twisting.
Fig. 4. Energy level diagram of a typical two-layer OLED system
formed between the ETL and HTL.
3.2 Quantum dot light-emitting devices
A quantum dot light-emitting device has a similar structure
OLED. Here the emitter is a semiconductor nanoparticle
between the charge carrier layers. However, several s
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The reasoning behind the popularity of the sandwich structure
optimal exciton formation in the quantum dots. Fig. 5 a) depicts how dots transport electrons. In transferring an electron from the conduc
one dot to another, the electron faces a barrier due to a wider ban
which is heightened further by a barrier composed of ligands. N
quantum dot layer is very thin, in the order of a monolayer, surroun
layers can easily transport charge carriers into the dot, as seen in Fi
direct injection of charge carriers is assumed to be the most common p
for creating an exciton in the device.
An exciton can also be formed via an energy transfer route. An e
pair is formed in the organic molecule and transferred as a bound e
dots, where it relaxes radiatively. This Frster energy transfer can b
congenial material choice to assist in exciton formation, thus resultin
and more efficient devices (Coe-Sullivan 2005).
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3.3 Fundamentals of display technology
3.3.1 Engineering points to look at when designing
Regardless of display type - LCD, plasma or OLED
specifications that make a display exceptional. Brightness is
consumer estimates first. When RGB pixels have 23% diff
the human eye can sense it. It is a bit more tolerant of vid
change in brightness is acceptable due to the quickly switch
2001). The human eye is also most sensitive to waveleng
which has to be taken into account when designing the optim
of emitting pixels. Good contrast, meaning the boundary bet
regions, resolution, the size of a pixel and the viewing angle
viewers experience of the visualised data. From the engine
due to limitations of the driving circuitry, the driving power30 V if driven with direct current. Furthermore, the switching
be fast; microsecond speeds are achievable, especially w
However, the human eye cannot resolve anything faster th
known that high speed consumes more power, which d
Addressability and degradation mechanisms are both engine
need to be solved. Ultimately, the consumer is looking for ayears to come, which emphasises the importance of improv
display (Pankove 1980).
3.3.2 Active vs. passive displays
Displays can be divided into categories, not only by the emisalso by the pixel driving method. Typically, displays are e
(PM) or active matrix (AM) displays. The pixels of both disp
transistor circuitry. Moreover, a display is formed by rows an
When a passive matrix display is driven, a bias is given to a
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3.3.3 Active matrix OLED displays
Since an OLED display is closest to a QDLED display, it is used
example. An OLED display incorporates five main parts. An integ
with RGB pixels is positioned on a display component carrier. This sy
have a hermetically sealed transparent cover and a set of wire
encapsulated at the end (Pankove 1980). The integrated circuitry inclu
transistor that is responsible for charging and discharging a storage
drive transistor addresses the LED and is bigger in physical dimensiohigher current requirement. However, the bigger size decreases the a
and is the main limiting factor when designing the emissive area of
simplest AM LED display has two resistors and a capacitor to drive o
It is evident that transistors create a bottleneck for opti
performance; the higher the driving current provided by the transisto
the brightness. Most drivers of OLED displays, e.g. Sanyo and Kutilised polysilicon transistors. While the efficiencies of OLEDs hav
lower-mobility transistors have become possible; a-Si:H or organ
transistors (OTFTs) can be used. OTFTs would enable flexi
manufacturing, thus revolutionising display markets. However,
concern in using OTFTs is whether they can provide a steady curren
enough time. Dielectric integrity is also an important factor, which is
Papers 4 and 5 in this thesis. Furthermore, device passivation and TFT
are properties that need to be optimised (Klauk 2006).
3.3.4 Why a QDLED display?
To fabricate full-colour displays from OLEDs, several processin
required, which complicate the manufacturing process. One approac
white OLED as an emitter and colour filters to produce RGB images
pixels of each colour with shadow masks. OLEDs can also be stacke
conversion of blue light can be used to create red and green (Burrows
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QDLEDs one can combine the benefits of inorganic lumoph
the flexibility and simple fabrication routes provided by organ
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4 Inkjet printing
4.1 Background of printing technology
The history of printing technology is long, and most credit bel
Chinese, who first demonstrated printed text and images via woodbl
in the 2nd century (Hind 1935). It took 1700 years to come up with
non-contact printing. Prior to that, all printing methods used phys
parts to make an image. Moreover, most methods invented to date
physical phenomena to deposit material on a substrate. Flexography
use a relief method, offset lithography relies on surface energy, scr
require masking, embossing uses high force and pressure to replace m
ablation removes material, and laser transfer uses excessive force an
release material (Khan 2006). Inkjet technology is the only method
dispenses material, without contact, to the receiving material. Furthertechnology is the only method that uses remotely amount of material
potential to be a high-throughput, low-cost process.
Inkjet printing is also called a digital printing method, since
controlled by electronic devices. The dispensing method div
technologies into two groups: continuous and drop-on-demand (
bubble jet technique is a DOD technique, which differs from piezoein terms of droplet production (Gamota et al. 2004). Fig. 6 demo
principles of each method. The continuous method electrically defle
of droplets by using high-voltage deflection plates. Droplets that
deposited are collected by a gutter and re-circulated. Piezoelectric m
piezoelectric crystal chamber that expands or contracts when a bia
This material deformation creates a pressure wave that forces theexpand and exit through an orifice. Bubble jet is also called a ther
method. As the name describes, a vapour bubble is heated, creating
bubble that results in the formation of an acoustic wave, thus forcing
of the chamber (Le 1998)
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thermal competitors, and they can be used with a wider range
(Clymer & Asaba 2004). Piezoelectric inkjet is used in this wo
Fig. 6. Three main categories of inkjet technology. a) continuous
c) bubble jet.
Inkjet printing is a simple method in theory, but is rather invo
Some of the main things that need to be controlled during the
drop volume, drop shape and formation, printing distance an
quality, evaporation kinetics of the droplet, surface energy
surface tension of the droplet, penetration or spreading p
thickness. Overall, material printing differs considerably
printing; uniformity and excellent film quality are required in
while graphic arts printing accept discontinuous films,
resolution of the human eye (55 m or more). Furthermore,
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The active ingredients in the ink can be polymers, metal n
biological cells or carbon nanotubes, to name only a few. It can
material printing has created new challenges in the design of ink
Therefore, printing should be divided into conventional graphic art
functional material printing techniques.
4.2 Piezo inkjet printing technology
Regardless of the actual implementation of a piezoelectric inkjet syste
thermal inkjet system, the following features can be found in each o
print driver is the heart of the manipulation process. This digital imag
converts data into a location map of dots and also drives the electr
head, creating motion in the piezo crystal.
The print cartridge contains a reservoir for the ink and is atta
actual printhead. The printhead has a small chamber surrounded by a pand an electronic subsystem that is responsible for ejecting co
optimally shaped droplets. This small module is a technologica
system that has to control various functions during and after jetti
maintain thermal and fluid stability. The printhead also has a mainten
for cleaning between print cycles by jetting excessive material, thus
orifice and forcing dried ink out from the opening. Typical prinnozzles in one column; the number of nozzles varies from a few
(Clymer & Asaba 2004).
In addition to the main parts mentioned here, some printer
designed for functional materials have sophisticated piezo control fea
as camera systems in order to have ultimate control of the jetting prop
4.3 Fluid dynamics
4.3.1 Printhead dynamics
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Piezo actuation has several steps, as seen in Fig. 7. Pie
starts the formation of the pressure wave; negative pressure
the change in the shape of the channel. Shortly after the ch
shape of the channel returns to its original state. Meanwh
negative pressure wave is reflected from the bottom of th
become positive pressure. The most efficient way to
amplitude is to amplify the pressure wave after reflection. Jet
propagating pressure wave has overcome surface tension and
entire process typically takes 1020 s, resulting in droplet s
m/s inside the chamber (Wijshoff 2008).
Fig. 7. Piezo actuation and acoustic wave behaviour in the cha
crystal is actuated.
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Channel geometry has other effects, as well. A small noz
accelerates ink movement in the channel, thus decreasing the driv
Furthermore, a small opening together with short channel lengths
overfill effect due to faster pressure wave damping. Moreover, chan
not the only characteristic affecting damping time; higher-viscosity in
damping more efficiently. As a conclusion, optimal damping of the pr
for refilling is not optimal for avoiding overfill, thus piezo actuation
trade-off.
4.3.2 Drop dynamics
Drop shape is an important issue for excellent quality print. Ideally
needs to be fully spherical without following drops, called satellites, b
hits the substrate. However, if small droplets are created near the m
they tend to recombine prior landing onto the substrate thus causinFurthermore, in reality drop shape tends to be more tear drop shape th
yet resulting excellent quality films. In this chapter theoretical exp
drop formation is discussed.
Various parameters affect drop formation: nozzle area and shape,
waveform shape and duration and ink properties, such as viscosity
tension, which are explained here. Nozzle diameter and voltage bomost impact on the maximum volume of the droplet, even th
oscillating parameters modifies the drop volume. Nozzle quality
chips, grooves or dirt can alter the trajectory of the droplets, thus
registration.
Fig. 8 depicts how a drop is formed in an ideal case. Drop speed
as the meniscus speed before ejection from the nozzle. Right after a d
pinched off from the main meniscus, drop speed is reduced typically
initial speed. The tail of the liquid cone slows down the falling ink dr
tail is short enough, the effect of decreasing velocity is less (improved
accuracy due to higher speed), satellite drops cannot be formed and
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Fig. 8. Ideal jetting results in a perfect spherical drop prior t
substrate. In addition to jetting parameters such as piezo acproperties play an important role in drop formation.
Viscosity actually has the biggest impact on the length of th
estimated if in addition to viscosity; also density and surface
Tail length, also called viscous length, is directly proportio
inversely proportional to the product of density and surface
highly viscous inks have a longer tail, which is also symviscosity induces a smaller tail. In addition, higher concentrat
tails. Drop formation is complete when the tail merges w
droplet, creating a perfect spherical drop (Wijshoff 2008).
If the tail is too long, it breaks into smaller drops, calle
break-off in surface tension. These small drops may have diff
can end up around the main drop on the substrate, thus dectremendously. A typical satellite-forming mechanism is high
drop, which on the other hand is created by a high pie
Moreover, satellites can be produced if the pulse width of the
too short or too long Interestingly the tail is not affected b
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power are the main determinants of drop size formation. In dro
inkjet printing, drop diameter is similar to the nozzle diameter. (Wijsh
Drop formation can be easily disturbed by several factors, w
biggest challenges to overcome in order to increase printhead life. N
easily, especially when highly volatile solvents are involved in the in
The clogging problem can be partially solved by introducing clea
between runs and while the printer is idle. Furthermore, air entrap
channel disrupts ejection of droplets. However, the main challenge, a
the research presented in this thesis, is overfill, which means exceplate wetting with ink. The reasons for this include too high a repe
drops, an overfilled channel, wetting properties of the nozzle plate
tension, which allows merging of the ink blob on the nozzle pla
meniscus. Low-viscosity inks tend to overfill easier than higher-vi
Once overfill occurs, it cannot be reversed and the nozzle in that
(Wijshoff 2008, Shin & Smith 2008).
4.3.3 Fluid dynamics at the surface
Fluid dynamics at the surface are a rather complex process. Ink chemi
perfected to meet required properties, such as viscosity, printability, r
surface energy. Moreover, dry ink properties must also be taken iwhat is the desired structure of the film (in terms of physical s
toughness), the adhesion and cohesion attributes and the surface p
printed features (Gamota et al. 2004). Printability of an ink is domina
by surface tension and rheology. Rheology describes the deformation
materials and influences the processability of the ink; thus, film
uniformity and the final properties of the ink are important param
process. Moreover, by definition, surface energy/tension describes
between ink and substrate. Fig. 9 depicts the definition of Yangs equa
equation describes the relationship between surface energies and
which can be described as a physical measure called the contact angle
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Fig. 9. The contact angle is dependent on the free surface energ
and solid/liquid interface.
The contact angle can be derived if the surface energies of th
substrate/ink are known. The contact angle can be though
energy difference between the ink and the substrate. Yangs eq
solidsolid/liquid = liquid cos,
where gamma is the free surface energy at a given point an
between the substrate and the tangent of the ink/substrate
theoretical calculation is rather difficult in reality; it is easie
using a camera and a protractor. However, theoretical c
difficult in reality; it is easier to be measured by using a camer
A large contact angle refers to a pinned, round droplet, wlow surface energy of the substrate and high surface energy
contact angle prohibits spreading of the droplet, thus wetting
Inadequate wetting, however, decreases adhesion (Tekin et
high surface energy of the substrate and low surface energy o
small contact angle, thus enhancing wetting.
The final footprint of the ink is a result of the surfac
described in three steps. The droplets impact with the substr
on drop shape. In addition, gas pressure plays a big role in d
surface. After hitting the surface, the droplet starts expand
order of magnitude comparable with the drop diameter while
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Fig. 10. Marangoni effect on a pinned drop. At t1 the evaporation rate is h
circumference. Marangoni flow (red arrows) takes place at t2 and so
towards the edge. The result is highest particulate concentration at the bo
The solvent evaporating rate is higher at the edge of the droplet, cre
from the centre of the droplet towards the circumference. Since the
cannot retract, the solute will migrate towards the boundary until al
has evaporated (Crasteret al. 2009). This phenomenon can be slow
even avoided if the substrate is heated, thus allowing less time for th
flow to take place, or if the solute/solvent ratio is increased or if low
boiling-point solvents are mixed. The benefit of mixing solvents com
fact that a high-boiling-point solvent at the circumference will evap
which in turn increases the concentration of low-boiling-point solven
location, thus balancing evaporation rates throughout the whole drople
However, in the case of nanoparticles, a high contact angle mig
Ko, et al. recently showed that in the case of spherical nanoparticontact angle provides uniform gas pressure throughout the drople
well as longer drying time, which will assist in close packing of nano
et al. 2004). Furthermore, Perelaeret al. dove deeper into this topic
light into packing of nanoparticles in inkjet printed droplet (Perelaer
Similar results were seen during the research presented in this thesis
the conventional rules of surface properties may not be valid in nanoparticles.
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5 Materials and methods
5.1 Fabrication steps for QDLEDs
All the devices fabricated during this research had the same fabr
regardless of the material choices, e.g. in the charge carrier layers. F
the materials used were taken from the same batch to make a compari
each device set easier. Device preparation involved pre-cleaning of th
spin coating of the hole injection and hole transporting layers,
quantum dots and thermal evaporation of the electron transport
cathode. Testing occurred typically a few hours after device fabricatio
5.1.1 Device preparation
Substrate
Transparent Indium Tin Oxide (ITO) coated on glass was used a
contact of the device. When pixilated ITO was used, UV -cured pho
patterned to define the pixel area; no ITO was etched out. The sub
cleaned with a three solvent method in which the substrates were s
minutes in methanol, acetone and isopropyl alcohol and dried wnitrogen. Prior to any deposition step, the ITO/glass was also plasma
2 minutes with oxygen plasma. Based on Atomic Force Microscop
root mean square (rms) roughness was typically around 0.50.9 n
highest peaks around 4060 nm.
Hole injection and hole transport layers
Due to the roughness of the ITO surface, which causes high local e
that in the worst case could short the device, the hole injection layer, c
d i l PEDOT PSS (B P4083) i d i
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and less than 10 nm maximum peaks were measured. T
PEDOT:PSS film was 30 nm, measured with a Dektak 3030 s
The hole injection material in the device was mostly
poly-TPD [(poly-(N,N-bis(4-butylphenyl-N,N-bis(phenyl)b
thermally cross-linkable poly(N-vinylcarbazole) (PVK) de
Corning was also used. In the case of poly-TPD, a 6 mg/ml co
utilising chloroform as a solvent was prepared. PVK, howev
solution from the manufacturer, with 3 wt% PVK content
ketone (MIBK) solvent, and it was used as received, but wwhen dopants were added. The poly-TPD layer was cross-link
minutes, and the PVK, at 190 C for 5 minutes. After cross-li
was 40 nm thick (0.65 nm rms roughness) and the PVK (0.39
was 50 nm thick. Polymerisation strength was tested for bo
coating bare solvent on the cross-linked layer and checked
removal of un-polymerised material. Both materials seemedusing the given method.
Quantum dot layer
The quantum dots used in the first set of monochromatic dev
CdSe/ZnS with an octadecylamine ligand. These dots were Nanotech, LLC from Arkansas. However, the RGB devic
commercial quantum dots from Evident Technologies, als
core/shell structure. The organic ligand in this case was n
provider, but was most likely based on oleic acid or oleyl a
soluble in non-polar solvents. The quantum dot concentratio
to study between 2 mg/ml and 10 mg/ml. The dots were di
cyclohexylbenzene or a toluene:dichlorobenzene mixture.
The inkjet printer used in this study was a Fuji Dimat
(DMP2800 series), which has one row of 16 piezo driven no
nozzle opening and 254 m spacing between adjacent noz
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adjacent drops was set at 510 m. The printed films were annealed
after deposition at 180 C for 10 minutes to remove the organic lig
remaining solvent. The substrates were annealed again in a glove
thermal evaporation in order to remove adsorbed water.
Electron transport layer and cathode
The electron transport material, TPBi (1,3,5-tri(phenyl-2-ben
benzene), was thermally evaporated in a 10-6 Torr vacuum at the ratLayer thickness was 40 nm. The cathode consisted of a 1.5 nm laye
fluoride (LiF), which served as the electron injection layer, and 150
aluminium. The LiF was deposited at a rate of 0.04 /s and the alu
higher rate, typically a bit below 2 /s. The substrates were transfe
nitrogen-filled tester glove box immediately after deposition.
5.1.2 Testing and measurements
Various measurement tools were utilised throughout the experimen
absorption properties of the quantum dots were measured with a Car
Vis spectrophotometer at a range of 200800 nm. Photolumine
measured with a Jobin Yvon FL4-1057 Fluorolog setup. The surface and topography of the various films were analysed by Veeco DI3
Force Microscopy. The same system was also used for thickness mea
addition to a Dektak 3030 profilometer.
Electroluminescence spectra were measured with an Ocean Opt
spectrometer and I-V measurements were carried out using a Ke
source meter. Optical power was measured using a Newport 2832 C p
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6 Results
The results of this thesis project involve device performanceengineered structures along with a structural analysis of the deposited
AFM. The work presented in Papers IV and V is strongly bound to
of quantum dot layers, therefore making them an important part o
obtained in this thesis project. All of the major topics - device ch
AFM work and surface modification - are intertwined into the result
thesis and are not separated into individual topics.
6.1 Early steps of device fabrication
6.1.1 Spin-coated devices
Optimisation of the device architecture started with spin coating qonto various QDLED structures in order to find the most promising c
to be used in printed devices. Fig. 11 reveals the first set of devices te
energy band diagram associated with them. A device with only PED
the anode side was investigated to shed light on the exciton-forming
and to see how important a role the hole transport layer has in the ac
only injection of holes is possible in the proposed device. Moreoverlower HOMO level of PVK than of PEDOT:PSS, PVK was introduce
transport layer in order to achieve more efficient hole injection. The
PVK 3 wt% (roughly 50 nm) was thought to be enough to smooth
surface, therefore no PEDOT:PSS layer was spun onto the ITO
arrangement, the device was kept as simple as possible in order to ke
voltages at reasonable levels, but also to decrease the number of fabri
Now, by doping PVK with PBD [2-(4-tert-butylphenyl)-5-(4-biphe
oxadiazole], which has traditionally been used as an emitter in OLE
and possesses electron-transporting properties (Lee et al. 2000), the en
of the holes was further decreased (HOMO level at -6 2 eV) Howeve
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Fig. 11. First device structures and their corresponding energy in energy levels are electron volts [eV].
For these devices the quantum dots were dispersed in tolue
concentration, spin coated at 1000 rpm for 60 seconds and he
minutes. The quantum dots used in devices 1, 2 and 4 had
600 nm ((QY > 40% and FWHM 25 nm), emitting in the dee
visible spectrum. The quantum dots used in device 3 were demission peak at 620 nm ((QY > 40% and FWHM 23 nm). It
here that for dots with a 600 nm emission peak, their energ
Eg= hc/, was 2.06 eV, and for dots emitting at 620nm Eg, it
this small gap difference of 0.07 eV, comparisons between de
feasible; the PVK:PBD device still has the smallest energy ba
quantum dot boundary.Fig. 12 depicts the emission spectrum of each device. Em
the quantum dots, indicating good surface coverage by dots a
4. Pinholes in the quantum dot layers would show some e
l l th N b d th b d di
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nm. This, in turn, would suggest there is no physical hindrance
forming a closely packed layer.
Fig. 12. Normalised emission spectrum of each device. Insert: phoilluminated devices; top picture for 600 nm emission peak dots and low
620 nm emission peak dots.
As expected, the device with only PEDOT:PSS on the p-type side of
the lowest brightness and external quantum efficiency (EQE). Bright
a maximum of 125 cd/m2 at 10 V, while maximum EQE was 0.012%
to the small overall thickness of the device, the turn-on voltag
Regardless of the largest energy barrier of the holes at the QD sit
based device reached maximum brightness of 298 cd/m2 at 15.8V. T
of this structure was low EQE due to an unbalanced hole-electron co
only 0 058 % was measured By doping PVK with PBD thus l
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Fig. 13. I-V curves of each device. TPD-doped PVK (black
performance.
6.1.2 Printed devices
The same structures were used when the quantum dot layer w
this point, the dots were dispersed in toluene and th
concentration was chosen. Due to the really poor performa
based device, it was not used in the study. Therefore, only de
11 were fabricated by printing.All the devices had 10-micron drop spacing, which was su
surface completely. However, it turned out that the drop spaci
overlapping of adjacent rows created thick lines at the edg
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Fig. 14. Example of a printed device with dark non-emitting regions due
concentration and drop spacing (a). Optical interferometry measuremen
structure in more detail (b), scan area roughly 600 m x 400 m.
Furthermore, the devices had low brightness and low EQE. Device
PVK on the anode side, showed maximum brightness of 33 cd/m2
afor the same device was measured to be 0.03% at 18 V. The PBD-
device had the highest EQE in this batch of devices; 0.04% at 30 V w
Luminance, however, was only 20 cd/m2 at 35 V. The TPD-doped
had really low EQE, 0.024%, and luminance of 21 cd/m2 at 25 V.
It was evident, based on these first printed devices that much effo
be placed on optimising the QD ink. Concentration adjustment w
parameter to be changed. In addition, the solvent choices were recon
on. Fig. 15 shows the emission spectra of TPD-doped PVK device
printed QD ink had concentrations of 10 mg/ml, 7 mg/ml an
Interestingly, the highest-concentration QD ink resulted in the po
devices; exciplex emission was more prominent in these devices.
indicate either larger pinholes or a higher density of smaller uncovere
reason for this phenomenon is most likely explained by the solvent/so5 mg/ml ink droplet has more solvent per drop, therefore its drying
longer. Thus, quantum dots would get more time to reassemble on the
form a more uniform layer.
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Fig. 15. Concentration dependence between QD ink and the spec
doped PVK devices. Lower-concentration ink results in better QDoptimal solvent/solute ratio.
When the concentration was decreased further to 2 mg/ml, fu
achieved with the given drop spacing. Fig. 16 shows the A
quantum dots on a TPD-doped PVK surface. Fig. 16 a) is a he
nm scan area. Brighter colour indicates a higher surface. T
section of one scan line and clearly depicts monolayer islanFig. 16 b) is an amplitude image of the same scan showing i
vividly. The rms roughness of the measured area was 2.05 n
was ~5 nm. The dots had a total diameter of 46 nm
measurements. Therefore, the observed islands are monolayer
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Fig. 16. AFM data of QD (2 mg/ml) on a PVK:TPD surface. Full cover
obtained with the given concentration and drop spacing. Coverage is
Scan size is 500 nm in both figures.
6.2 Poly-TPD-based monochromatic devices
As mentioned earlier, solvent choice needed to be re-evaluated. Tolue
viscosity (0.590 cP), low-density (0.867 g/ml) solvent which also h
pressure of 2.9 kPa. A solvent with better jetting capability as well as
drying was needed. Chlorobenzene was the choice due to its higher v
cP) and lower vapour pressure (1.6 kPa), which both assist in unformation. Furthermore, chlorobenzene has a density of 1.1 g/ml, thu
favourably with toluene in this aspect, as well.
Fig. 17 shows how solvent choice impacted jetting parameters
shows jetting of toluene-based ink. As can be seen, satellite droplet
regardless of optimisation of piezo actuation. However, when the
switched to chlorobenzene, jetting was more controllable and individwere formed, as seen in Fig. 17 b). Therefore, small changes in visco
pressure and density affected jetting characteristics positively. N
nozzles are actuated simultaneously in Fig. 17 a), but only one nozz
b) Th i t k ith th i t
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Fig. 17. Drop watcher images of the jetting nozzles. Toluene (a) a
had different properties, where chlorobenzene had better con
higher viscosity and lower vapour pressure.
When the solvent was changed to chlorobenzene, a new
Contact angle measurement indicated that the QD ink drop
behaved hydrophobically, as seen in Fig. 18. The contact a
was 78.4. Quick oxygen plasma treatment of PVK:TPD
sufficiently, and the contact angle was close to zero a
However, regardless of the short ashing time (10 s), t
morphologically and ashing also introduced an extra step iwas not ideal when processing was designed to be kept as
Therefore, a new hole transport layer was investigated. Based
with spin-coated devices, TPD-based devices seemed
performance. Now, UV cross-linkable poly-TPD was desig
rigid HTL layer that also tolerated the solution process after
showed good compatibility with QD ink as far as wettabilityseen in Fig. 18 b). The contact angle was measured to be close
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Fig. 18. Contact angle measurement of a QD/chlorobenzene drop on PVK
a QD/chlorobenzene drop on poly-TPD (b).
Now, using poly-TPD as the HTL, the concentration study was partia
Since the previous results suggested that the most optimal concentr
be between 2 mg/ml and 5 mg/ml, two devices were fabricated with 4 mg/ml QD ink. In order to verify the best surface coverage, AFM m
was executed. The AFM scan was performed in air after printing th
tapping mode with an aluminium-coated tip was used for phase ima
the small size of the dots and the surrounding material, only ph
important information about the dots. Phase imaging is optimal f
smooth samples due to its excellent sensitivity to morphological chasurface. The exceptional properties of phase imaging are also describ
paper I in the supplements. Two phase images are shown in Fig. 1
shows the QD surface when the concentration was 3 mg/ml and
displays the printed QD surface with 4 mg/ml ink. Hard quantum d
brighter in the scan as opposed to soft organic material, thus
individual dots on the film. It is evident on the basis of the AFM
mg/ml shows better surface coverage than 3 mg/ml, therefore the d
prepared using 4 mg/ml ink.
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Fig. 19. AFM phase images of 3 mg/ml ink printed on poly-TPD (a
Scan size is 500 nm in both images.
However, printed QDs do not show a hexagonal closely
reported by other groups when dots were spin coated [19].
even though small areas of closely packed structure can be
this was sought from the underlying poly-TPD layer. A cros
test was conducted by introducing a solvent layer on a cured p
measurement was conducted prior to and after the solvent trea
a solvent layer was deposited on a poly-TPD layer via a spin
a printing process.
The poly-TPD layer was spin coated at 2000 rpm for 6cured at 254 nm for 10 minutes. AFM measurement was us
imaging and roughness measurement. The poly-TPD film
smooth after deposition and curing; the rms value was 0.248
valley value was 8.34 nm. The solvent used for this study was
in the case of spin coating, it was spun onto the cross-linked H
60 seconds and dried prior to the AFM scans. Furthermore, was used as the solvent deposition method, chlorobenzene w
TPD and dried in air post-deposition. Fig. 20 points out th
prior deposition (a), after spin coating (b) and after inkjet prin
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Fig. 20. AFM height images of cross-linked poly-TPD (a), the same lay
coating chlorobenzene (b) and after inkjet printing chlorobenzene (c). Sc
micrometers.
The rms roughness of the deposited poly-TPD was 0.248 nm an
coating, 0.514 nm. The AFM scan indicated that solvent treatmen
coating had virtually no effect on the topography of the HTL laye
when the solvent was jetted, apparent changes occurred. The rms roug
layer was 0.972 nm. The explanation for this difference between the t
solvent treatments arises from the solvent interaction time with th
layer. When the solvent is spin cast it has a relatively short time to
the underlying layer; within the first few seconds a majority of the
been removed and only a really thin layer remains throughout
process. Nevertheless, inkjet-printed solvent stays on the surfaccompletely dried, thus having more time to interact with the polym
linking is not complete, non-cured material can start to move around
dynamic laws and rearrange itself in lumps, since they are
favourable structures. The small material islands in Fig. 20 c) depict d
packing. Island height is roughly 5 nm.
Regardless of the less than optimal QD layer, the devices were obtain valuable information on QD performance in a device setting. T
in these devices was ITO/ poly-TPD/QDs/TPBi/LiF/Al. The HTL wa
thick, whereas the QD layer was in the order of a few monolayer
increase in device brightness only Furthermore EQE was 1
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increase in device brightness only. Furthermore, EQE was 1
compared with the TPD-doped PVK devices.
Fig. 21. EL properties of a poly-TPD-based device. The sp
dominant emission from the dots. The IV characteristics (b) s
cd/m2
brightness and ~ 0.2 maximum EQE (Paper I American In
At this point, attention focused on patterned devices. The fo
towards display applications, which meant printing onto pixi
first attempt to show the capability of inkjet-printed QD devi
substrates was demonstrated; refer to Paper I. Fig. 22 summ
amount of jetted material was optimised by analysing the spedevice (a). Fifteen drops per pixel (ca. 150 pL per pixel
exciplex emission significantly, indicating better film coverag
also had a coherent response to layer thickness (b); 15 dr
roughly 2.5 V higher turn-on bias compared with the 10 dro
pL per pixel).
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Fig. 22. Optimising ink volume per pixel. Fifteen drops per pixel had b
response than 10 drops/pixel (a). However, driving bias was increase
thicker quantum dot layer (b). (Paper I American Institute of Physics).
6.3 RGB devices
Studying monochromatic red, green and blue quantum dot LED str
naturally one step closer to RGB displays based on QDL
demonstration of these structures, integrated devices would follow
changed in these structures compared with the previous ones was usi
ITO as a substrate, adding a PEDOT:PSS layer under the HTL layer
solvent mixture in the QD ink. Now, a 1:1 ratio of toluene:dichfunctioned as the solvent and 2.5 mg/ml of quantum dots were dispe
Solvent mixture was essential when jetting was optimized since th
dispersed into toluene when purchased. Jetting properties were imp
dichlorobenzene was added into toluene. Due to the lower concentrat
of drops per pixel was increased. The typical number of jetted drops p
18 for red and green ink and 39 for blue ink. The volume of blue ink p
to be increased because it seemed to be harder to obtain dominant em
the dots only. The reason for this is unknown, as the company provid
did not reveal details about the ligand. If the ligand in blue dots were
i ht h h d i t tti ti
shoulder can be seen in red and green emission The spectra
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shoulder can be seen in red and green emission. The spectra
the red and blue devices and at 20 V for the green device. As
concerned, the red device had the greatest brightness; 352 cd/17.5 V and EQE at the same bias was 0.23%. Energetically
case since red dots have a smaller energy barrier from the H
which assists in more efficient hole injection. When the dot s
valence band also goes deeper, thus increasing the energy ba
layer is kept the same in these devices, the red devices have t
due to the most favourable structure of the device.Green devices had the best brightness, 270 cd/m2 at 18 V
at the same voltage. Furthermore, blue devices exhibited 122
corresponding EQE of 0.13%. More details about device
analysis can be found in Paper II in the supplements.
Fig. 23. Device characteristics of R, G and B devices. The EL s
emission from the dots (a). However, the red device was
energetically most favourable structure (b). (Paper II IEEE
Technology).
The purity of emission colour can be estimated by calculating
of the emission spectrum. The CIE (Commission Internatio
coordinates for the red device were (0.57, 0.37), for green
blue (0 12 0 13) as seen in Fig 24 The red and green dev
that emit in deep blue wavelengths. Regardless of quantum dot choi
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p g g q
the process remains unaltered.
Fig. 24. Photographs of monochromatic devices. 232 pixels are driven sim
The red device has CIE coordinates of (0.57, 0.37), the green device (band the blue device (c), (0.12, 0.56). (Paper II IEEE Journal of Display Te
6.3.2 Patterned RGB devices
Integrating RGB into one device was a rather simple task, since th
parameters of each QD ink were established. The biggest challenge w
a routine for position accuracy when the ink cartridges were changedone ink can be deposited at a time, each ink needs to be in an individu
Furthermore, the cartridges are manually changed between print
increases the position error of the following layer However a succes
balance. After all, each pixel is driven simultaneously with a
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, p y
increases the red emission ratio in the entire spectrum. E
driven individually for optimal colour balance.
Fig. 25. Integrated RGB device. The EL is dominated by emissioInsert: Photograph of 232 sub-pixels illuminated simultaneously
high brightness and EQE comparable with monochromatic de
IEEE Journal of Display Technology).
The integrated RGB device turned on at 5.2 V, which is optim
lower than in devices fabricated earlier. Maximum brightnes
at 17.5 V and EQE 0.14 at the same bias. Typical video brig
which was obtained at 9.3 V with 0.24% efficiency; see Fig. 2
6.4 Debate on the wetting properties of nanopartic
The conventional approach to printing functional materials is
energy of the substrate to allow complete wetting. Ideally, t
will be diminished, thus resulting in a more uniform layesupplements). However, some studies have indicated th
nanoparticles, hydrophobicity would be beneficial in nanopa
to the longer drying time and more uniformly distributed
the surface hydrophilic. Contact angle measurement gave a ~0 angl
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droplet on glass. Now, an octadecyltrichlorosilane (OTS-18) -treated
was altered to be hydrophobic, and had a contact angle of 95-100. Fan octanedecanethiol (ODT) -treated gold surface with a 110 c
functioned as a superhydrophobic surface. The OTS treatment
explained in Paper IV. Thiol treatment, however, included soaking o
substrates for 10 minutes in ethanol and then transferring them int
ODT solution for 18 hours. The impact of surface treatment, or on th
a non-treated surface, on OTFT performance was studied in Papers Ished light on packing of molecules onto the surface as well as the nec
self-assembled monolayers when the properties of the deposited lay
optimised. The results from those works were reflected and exploited
this study.
Fig. 26 shows three AFM scans of each prepared substrate. T
contains amplitude images of the scans and the second rocorresponding phase images. Two images per scan are included to cla
what was concluded. The first column (a) and d) images) shows how
dots occurred on bare glass, i.e. on a hydrophilic surface. The rms rou
2.02 nm and the peak-to-valley value was 13.6 nm. The amplitud
reveals this surface to be most disorganised. Moreover, the phase ima
same scan depicts voids in packing. In the second column, OTS trea
to enhance packing of dots. The rms roughness of this surface was
the peak-to-valley value 6.73 nm. However, the best packing visuall
the thiol-treated surface, seen in the third column. The rms value was
be 0.612 nm and peak-to-valley, 5.21 nm. All the scans are 500 nm.
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Fig. 26. AFM scans of printed dots on three different surfaces
either hydrophobic (middle and right column) or hydrophilic (l
row shows amplitude images of each surface (a, b, c) and th
corresponding phase images (d, e, f). Scan size in each image is
Based on these results, it seems that a hydrophobic surfac
favourably. However, it is hard to achieve a hydrophobic su
device setting, thus the question is how important this is for
This will be discussed further in Chapter 7.
7 Discussion
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Since the most important aim of this thesis project was to demapplicability of inkjet printing technology in the fabrication of QD
perspective dominates the following discussion on the results. T
providing plausible explanations for the obtained results and di
limitations of the applied methods.
7.1 Accomplished results
The results of the work introduced in this thesis clearly show the rele
method used to fabricate LEDs based on colloidal nanoparticle
Moreover, the described method has major benefits over other proc
make these results remarkable. The strongest argument supporting in
as a deposition method arises from the capability to pattern in innum
with virtually no material loss during the process. Colloidal q
chemistry seems rather simple; however, obtaining infinitely
distribution is a tremendous challenge. To achieve this is laborious
which also is the main cost factor of QDLED manufacturing. There
fabrication should be as simple as possible, where inkjet printing is
solution.
Even though the studied device structures were not fully ablesuper-brightness, it is justifiable to claim that inkjet-printed QDLED
potential. Since spin-coated devices showed results comparable
printed devices, there is not much difference in the devices as far as
is concerned. Thus, the limiting factor of devices with high brightne
EQE is not the inkjet process; lower brightness compared with ot
results is a consequence of something else.Plenty of effort has been put into this research field by other grou
hexagonally closely packed QD monolayers. This issue was also c
during this research, but interestingly, it seemed not to have much effe
similar performance, which questions the requirement of a p
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layer.
The most significant result of this thesis project is the dedevices. These results substantiate the great potential that in
have for display manufacturing, as well. Moreover, the devic
a level where the process could be easily transferred to a
fabrication. In addition, integration with OTFTs, studied in
feasible and could be achieved with reasonable effort.
The results achieved in this thesis project will also benefields as well, and are not limited to serving only display ma
sensor applications using nanoparticles in their process woul
applicable. More applications are mentioned in Paper III.
7.2 Limitations
A major limitation of using inkjet printing as a QD depositio
from ink/organic layer compatibility. The hole transport lay
cross-linked in order to withstand the harsh organic solvent
seen in chapter 6, spin coating tolerates solvents better due to
time compared with jetted solvents. Therefore, device c
dependent on HTL quality, which limits configurations sign
cross-linkable HTL materials other than the ones used in thi
allow successful device design.
Moreover, modification of QD ink is limited to solvent
conventional methods that alter, e.g. viscosity, with additives
be utilised. Thus, the material printers limitations in viscosi
an important role when designing QD inks. For example, the
in this study has a viscosity range of 120 cP, whereas threality is typically below 1 cP. Overcoming this dilemma
challenging part of the work, and much effort was needed
actuators successfully.
7.3 Future work
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Future work includes integrating RGB QDLEDs into an active matrdemonstrate a QVGA AM-QDLED display. In addition, more atten
placed on defining the best solvent choice or mixture that would be
with the cross-linked HTL. Furthermore, there is room for device opt
possibly emphasising charge balance by utilising hole blocking
blocking layers. Some studies also show that QDs should not be
between the HTL and ETL, but should be embedded into organic laye
White light QDLEDs produced by inkjet printing is also a pros
step and would be studied by printing mixtures of dots or by print
R,G and B inks onto pixels in an optimal ratio. Moreover, QDLED
sensor applications is also an interesting research area.
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8 Conclusion
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The main ambition of this thesis project was to investigate an inmethod used with low-viscosity nanoparticle inks to fabricate quantu
emitting devices. First, spin coating was used as a reference method
device configurations were made and tested. Based on these resu
attempts were made to print QDs and investigate device performance.
In order to improve device performance, fine tuning of
configuration became essential along with ink modification, whemphasis was on solvent mixtures. Atomic force microscopy measur
employed to assist in determining the optimal parameters of the
process itself.
Monochromatic printed QDLEDs were fabricated successfully, a
fully integrated RGB device was presented. The film quality requ
assessed on the basis of a study consisting of printing nanopartic
energetically altered surface. The results showed no significant impa
performance, regardless of whether dot packing was optimised or no
further studies are needed to fully confirm this statement.
The results of this thesis project clarify that inkjet printing is an a
method in QDLED fabrication, and this technique avoids various ob
with other methods. The most significant achievements would be eas
of RGB pixels in display fabrication, lowering the cost of the mprocess and making it an environmentally friendly process where to
are not wasted. Furthermore, up-scaling of the process is feasible an
by other technologies associated with display manufacturing, not th
manufacturing process. For example, backplane technology is a lim
when designing larger-scale displays.
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