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

TL;DR

This study investigates the spectroscopic signatures of radical anions generated in polymer-fullerene blends, revealing distinct features for C70 derivatives using PL, PIA, and ESR techniques, advancing understanding of charge transfer in organic photovoltaics.

Contribution

It provides new spectroscopic evidence of C70 radical anions in polymer-fullerene blends, highlighting differences from C60 derivatives and enhancing knowledge of photoinduced charge transfer mechanisms.

Findings

C70 derivatives show unique ESR signals at g>=2.005

A new absorption band at 0.92 eV indicates C70 radical anions

Distinct spectroscopic signatures differentiate C70 from C60 acceptors

Abstract

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.