10min:

HUI LIU, JINJUN LIU, Department of Chemistry, University of Louisville, Louisville, KY 40292; HEMANT M. SHAH AND BRUCE W. ALPHENAAR, Department of Electrical & Computer Engineering, University of Louisville, Louisville, KY 40292.

Fullerene derivatives such as [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₀BM) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) are promising electron acceptors for use in efficient organic solar cells. Capacitive photocurrent spectra of both PC₆₀BM and PC₇₀BM in conjunction with indium tin oxide (ITO) reveal peaks with wavelengths longer than the S₁ S₀ transitions. The energies of low-lying triplet states of both molecules calculated using the ZINDO/S method agree with the experimentally observed transition frequencies. An excitation mechanism that involves collisions between the photoinduced free electrons in ITO and the organic molecules on the interface is proposed to explain the experimental observation. Tests on other organic solar cells are in process. Possibilities of improving the conversion efficiency of organic solar cells utilizing this mechanism will be discussed.