Charge Photogeneration and Recombination in Organic Semiconductors, Lewis Rothberg, University of Rochester, DMR-

0309444 (with M. Rubner MIT MRSEC, T. Swager DMR-0314421)

(a)

(b)

Structure IStructure II

IG-II-23(C17H18K2O6)

Silver nanoparticles

Aminoalkylsilane

PSS-PAH mutilayed polyelectrolyte

O O

OK

O

KO

O

n

1 2 3 4 5 6 7 8 9 10 11 12 130

1x105

2x105

3x105

4x105

5x105

6x105

Number of PAH-PSS bilayers

Flu

ore

scen

ce (

CP

S)

with silver nanoparticle no silver nanoparticle

For optical radiation near the plasmon resonance, stronglyenhanced electromagnetic fields close to the metal surfacescan be achieved, and the field intensity can exceed that ofthe incident light by 3 orders of magnitude. Surface plasmonresonance can therefore increase fluorescent emission fromnearby fluorophores if charge and energy transfer quenchingof the molecular excited state can be avoided. Precisedistance control between emitters and metallic nanoparticleassemblies is therefore an important factor in realizingsubstantial enhancement. We have investigated the use of self-assembled polyelectrolyte multilayers to optimize fluorophore-to-particle distance for enhancement of conjugated polymer emission as is important in organiclight emitting diodes. Increases in luminescence of as much as 50 times were observed and most of this can be ascribed to plasmon enhancement. Spacer studies indicate optimum spacing of the chromophore from the nanoparticle of around 3-5 nm while spectroscopic work shows that the molecular resonance is distorted by interaction with the plasmon and “pulled” towards the plasmon resonance. We find that the absorption enhancement has about the same as emissive rate enhancement on the overall increase for IG-II-23.

Figure 1. (a) Geometries for the study of plasmon enhancedconjugated polymer photoluminescence. (b) Enhancement results.

Charge Photogeneration and Recombination in Organic Semiconductors, Lewis Rothberg, University of Rochester, DMR-

0309444Development of efficient organic photovoltaic devices for solar energy conversion depends strongly on control of the morphology at the nanometer level. Templating methods have been used to make nanopatterned semiconducting oxides such as TiO2 that can function as n-type semiconductors and one promising strategy is to infuse these structures with absorbing p-type conjugated polymers. This geometry can simultaneously satisfy the requirements for efficient charge photogeneration and charge collection via bicontinuous interpenetrated networks of n-type and p-type materials. We have made a major step along that road by demonstrating growth of polythiophene (PT) from small molecule “seeds” covalently attached to oxides. The scheme by which this is done is illustrated at right and the result is that PT photoluminescence is quenched more than twice as efficiently when grown directly on the TiO2 (red) than when we try to infuse polythiophene derivatives into the TiO2 matrix (black). The covalent attachment route overcomes the natural hydrophilicity of oxide surfaces leading to higher coverage and better charge photogeneration efficiency.

O O O O

Si Si Si SiO O O O O

C3H6 C3H6 C3H6 C3H6

Al2O3 Or TiO2

S

S

n

S

S

n

S

S

n

S

S

n

O O O O

Si Si Si SiO O O O O

C3H6 C3H6 C3H6 C3H6

S S SS

Al2O3 Or TiO2

OH OH OH OH

Al2O3 Or TiO2

S

S C3H6 Si (OEt)3

Figure 1. (a) Chemical approach to covalent attachment ofpolythiophene to oxide surfaces. (b) Luminescence quenchingof the PT when covalently grown on Al2O3 and TiO2.

(a)

(b)

400 450 500 550 600 650 700 750 800 8500.0

2.0x106

4.0x106

6.0x106

8.0x106

1.0x107

in-situ grown (covalently bound)

vs.

spin cast (physisorbed) P3HT/TiO2/AAO

P3HT/AAO

Em

issi

on

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

n p

Cathode

Anode (semitransparent)

n p n p n p

Light

Ideal photovoltaic morphology and energetics

10 nm10 nm

Energy

p-type n-type

Spatial layout

~100 nm

Energy level diagram

LUMO

HOMO

Education and Outreach:This past summer, two NSF-RET high school teachers, Phelesia Jones-Cooper and Robert Winston (pictures on the right) did research in the Rothberg lab. Rothberg’s group has hosted seventeen RET, REU and CIRE (through CCNY) participants from over the last seven summers. Rothberg leads a materials cluster consisting of five faculty groups within the Chemistry department to broaden student education and foster interdisciplinary interactions. He is also co-teaching a new credit course to help graduate students to become more effective teachers and learners. Rothberg is advisor to the undergraduate ACS council who put on a show for high schools in the community where over 100 students attend annually. The PI participates in a wide variety of community activities including science Saturdays at the Rochester Science museum and judging at high school science fairs. Local Webster High School student Daniel Chao worked in our lab in summer 2004 and was awarded runner-up honors in the Intel High School Science competition on the basis of his project to invent a method to separate single-stranded and double-stranded DNA from a mixture.

Charge Photogeneration and Recombination in Organic Semiconductors, Lewis Rothberg, University of Rochester,

DMR-0309444