Spectroscopic Signatures of Photogenerated Radical Anions in Polymer-[C₇₀]Fullerene Bulk Heterojunctions

Light induced polarons in solid films of polymer-fullerene blends were studied by applying photoluminescence (PL), photo induced absorption (PIA) techniques as well as electron spin resonance (ESR). The materials used were poly(3-hexylthiophene) (P3HT) and poly-[2-methoxy, 5-(2'-ethyl-hexyloxy) phenylene vinylene] (MEH-PPV) as donors. As acceptors we used [6,6]-phenyl-C61-butyric acid methyl ester ([C60]PCBM) and various soluble C70-derivates: [C70]PCBM, diphenylmethano[70]fullerene oligoether (C70-DPM-OE), C70-DPM-OE2, and two fullerene dimers, C70-C70 and C60-C70 (all shown in figure 1). In all blends containing C70 we found typical signatures which were absent if [C60]PCBM was used as acceptor. Light-induced ESR revealed signals at g>2.005, which we previously assigned to an electron localized on the C70 cage, the PIA measurements showed a new sub-bandgap absorption band at 0.92 eV, which we correspondingly ascribe to C70 radical anions formed in the course of photoinduced electron transfer from donor to acceptor.