Paul Armstrong, Tabitha Clem, Tom Holcombe, Dave Kavulak, Xiaoyong Zhao, Claire Woo, Barry Thompson

Solar power using traditional silicon-based solar cells costs ten times more than energy produced with fossil fuels. This is because silicon is expensive to process. Organic materials can also harness the sun's energy. They are inexpensive, tractable, and, given the tools of organic synthesis, completely customizable. Polymers with conjugated carbon-carbon double bonds across the backbone are ideal for organic photovoltaics due to their semiconducting nature. For example, poly(3-hexylthiophene) (P3HT) has been extensively studied due to its high solubility in common organic solvents, visible light absorption, and relatively good semiconducting properties.

The Fréchet Group is interested in understanding structure-function correlations in conjugated polymers and synthesizing new materials for use in organic photovoltaics. Our ultimate goal is to control bulk material properties, and manipulate the nanoscale environment in which these materials interact to create efficient organic solar cells.

Tools such as UV-Vis absorption, cyclic voltammetry and field effect transistors, allow the determination of band gap, oxidation potential and charge carrier mobility. Working solar cells are also created to determine the efficacy of our new materials, and to determine important parameters in device fabrication.

One of the major drawbacks with organic photovoltaics in the literature is the relatively high propensity of the materials to be degraded by molecular oxygen. Our recent work has addressed this problem by synthetically accessing both regiorandom and regioregular polythiophenes containing electron-withdrawing carboxylate substituents. These materials provide better oxidative doping stability than conventional polythiophenes due to the lowering of the HOMO energy levels by approximately 0.5 eV.

Published devices with the highest power conversion efficiency employ a blend of an electron donor, P3HT, and an electron acceptor, [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), in a bulk heterojunction. An instability issue in these devices is the compatibility of these two components in a blended device. Thermal treatment is necessary for polymer chain alignment, but drives the phase separation of the two components and destroys the bulk heterojunction. Our group has developed and tested diblock co-polymer compatibilzers that allow the blend to remain homogeneous during thermal treatment. We found that devices could be subject to elevated temperatures for at least 10 times longer using only 17 wt% of the compatibilzer integrating into the thin film.

Selected Publications

• Sivula, K.; Ball, Z. T.; Watanabe, N.; Fréchet, J. M. J., Amphiphilic Diblock Copolymer Compatibilizers and Their Effect on the Morphology and Performance of Polythiophene:Fullerene Solar Cells, Advanced Materials , 2006 , 18(2),206 - 210.
• Ball, Z. T.; Sivula, K.; Frechet, J. M. J., Well-Defined Fullerene-Containing Homopolymers and Diblock Copolymers with High Fullerene Content and Their Use for Solution-Phase and Bulk Organization, Macromolecules , 2006 , 39 (1), 70-72.
• Murphy, A. R.; Liu, J.; Luscombe, C.; Kavulak, D.; Frechet, J. M. J.; Kline, R. J.; McGehee, M. D., Synthesis, Characterization, and Field-Effect Transistor Performance of Carboxylate-Functionalized Polythiophenes with Increased Air Stability, Chem. Mater. , 2005 , 17(20), 4892-4899.
• Liu, J.; Tanaka, T.; Sivula, K.; Alivisatos, A. P.; Frechet, J. M. J., Employing End-Functional Polythiophene To Control the Morphology of Nanocrystal-Polymer Composites in Hybrid Solar Cells, J. Am. Chem. Soc., 2004 , 126 (21), 6550-6551.