Ye Zhang, Department of Materials Science and Engineering, University of Utah, 122 S. Central Campus Drive, Room 304, Salt Lake City, UT 84112 and Debra J. Mascaro, Department of Mechanical Engineering, University of Utah, 50 S. Central Campus Drive, Room 2110, Salt Lake City, UT 84112.
We use regioregular poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to fabricate bulk heterojunction organic solar cells. Thin films of the organic materials are deposited by one of two methods: (1) spin coating a blend of P3HT and PCBM or (2) spin coating a P3HT layer followed by thermal evaporation of a PCBM layer. Both deposition methods are followed by exposure of the films to solvent vapor. In the case of the blended film, the vapor exposure is used to optimize the phase segregation of the P3HT and PCBM, and to improve the crystallinity of the P3HT. For the layered device, the vapor exposure lowers the glass transition temperature of the P3HT, enabling PCBM from the top layer to penetrate into the P3HT layer to form a graded bulk heterojunction. In this presentation, we will compare and discuss the morphologies and device performance of the two heterojunction layers. In addition, we use a bond-fluctuation Monte Carlo simulation to examine: (1) P3HT crystallization and PCBM aggregation, and (2) diffusion of the PCBM into the P3HT layer above the P3HT glass transition temperature. Simulation results will be compared with solvent vapor exposure and thermal annealing results in order to elucidate the annealing mechanisms.