Spin Signatures of Photogenerated Radical Anions in Polymer-[70]Fullerene Bulk Heterojunctions: High Frequency Pulsed EPR Spectroscopy

TL;DR

This study uses high-frequency pulsed EPR spectroscopy to identify and distinguish the spin signatures of photogenerated radical anions and cations in polymer-fullerene bulk heterojunctions, revealing the localization of polarons.

Contribution

First experimental identification of the negative polaron on the C70 cage in polymer-fullerene blends using high-frequency EPR spectroscopy.

Findings

High-frequency EPR separates P+ and P- signals.

Polaron is localized on the C70 cage.

Spectral resolution improves identification of radical species.

Abstract

Charged polarons in thin films of polymer-fullerene composites are investigated by light-induced electron paramagnetic resonance (EPR) at 9.5 GHz (X-band) and 130 GHz (D-band). The materials studied were poly(3-hexylthiophene) (PHT), [6,6]-phenyl-C61-butyric acid methyl ester (C60-PCBM), and two different soluble C70-derivates: C70-PCBM and diphenylmethano[70]fullerene oligoether (C70-DPM-OE). The first experimental identification of the negative polaron localized on the C70-cage in polymer-fullerene bulk heterojunctions has been obtained. When recorded at conventional X-band EPR, this signal is overlapping with the signal of the positive polaron, which does not allow for its direct experimental identification. Owing to the superior spectral resolution of the high frequency D-band EPR, we were able to separate light-induced signals from P+ and P- in PHT-C70 bulk heterojunctions. Comparing signals from C70-derivatives with different side-chains, we have obtained experimental proof that the polaron is localized on the cage of the C70 molecule.