Electronic Trap States in Methanofullerenes

The trap states in three fullerene derivatives, namely PC61BM ([6,6]-phenyl C-61 butyric acid methyl ester), bisPC(61)BM (bis[6,6]-phenyl C(61)butyric acid methyl ester) and PC71BM ([6,6]-phenyl C-71 butyric acid methyl ester), are investigated by means of thermally stimulated current measurements (TSC). Thereby, the lower limit of the trap densities for all studied methanofullerenes is on the order of 10(22) m(-3), with the highest trap density in bisPC(61)BM and the lowest in PC61BM. Fractional TSC measurements on PC61BM reveal a broad trap distribution instead of discrete trap levels, with activation energies ranging from 15 meV to 270 meV and the maximum at about 75 meV. The activation energies of the most prominent traps in the other two fullerene derivatives are significantly higher, at 96 meV and 223 meV for PC71BM and 184 meV for bisPC(61)BM, respectively. The influence of these findings on the performance of organic solar cells is discussed.