Charge Photogeneration and Recombination in Organic Semiconductors, Lewis Rothberg, University of Rochester, DMR (with M. Rubner MIT MRSEC, T. Swager DMR ) (a) (b) For optical radiation near the plasmon resonance, strongly enhanced electromagnetic fields close to the metal surfaces can be achieved, and the field intensity can exceed that of the incident light by 3 orders of magnitude. Surface plasmon resonance can therefore increase fluorescent emission from nearby fluorophores if charge and energy transfer quenching of the molecular excited state can be avoided. Precise distance control between emitters and metallic nanoparticle assemblies is therefore an important factor in realizing substantial 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 organic light 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 enhanced conjugated polymer photoluminescence. (b) Enhancement results.

Charge Photogeneration and Recombination in Organic Semiconductors, Lewis Rothberg, University of Rochester, DMR Development 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 TiO 2 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 TiO 2 (red) than when we try to infuse polythiophene derivatives into the TiO 2 matrix (black). The covalent attachment route overcomes the natural hydrophilicity of oxide surfaces leading to higher coverage and better charge photogeneration efficiency. Figure 1. (a) Chemical approach to covalent attachment of polythiophene to oxide surfaces. (b) Luminescence quenching of the PT when covalently grown on Al 2 O3 and TiO 2. (a) (b)

n p Cathode Anode (semitransparent) n p n p n p Light Ideal photovoltaic morphology and energetics 10 nm Energy p-typen-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