Electronic Trap States in Methanofullerenes

The trap states in three fullerene derivatives, namely PC 61 BM ([6,6]‐phenyl C 61 butyric acid methyl ester), bisPC 61 BM (bis[6,6]‐phenyl C 61 butyric acid methyl ester) and PC 71 BM ([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 PC 61 BM. Fractional TSC measurements on PC 61 BM 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 PC 71 BM and 184 meV for bisPC 61 BM, respectively. The influence of these findings on the performance of organic solar cells is discussed.