Direct Detection of Photoinduced Charge Transfer Complexes in Polymer:Fullerene Blends

TL;DR

This study uses transient electron paramagnetic resonance to directly observe and characterize photoinduced charge transfer complexes in a polymer:fullerene blend, revealing their role as intermediates in charge separation.

Contribution

The paper provides the first direct detection of geminate polaron pairs as charge transfer complexes in polymer:fullerene blends using trEPR.

Findings

Identification of spin-correlated polaron pairs with characteristic g-values.

Evidence of charge transfer complexes persisting microseconds after excitation.

Confirmation that these complexes mediate exciton-to-charge conversion.

Abstract

We report transient electron paramagnetic resonance (trEPR) measurements with sub-microsecond time resolution performed on a P3HT:PCBM blend at low temperature. The trEPR spectrum immediately following photoexcitation reveals signatures of spin-correlated polaron pairs. The pair partners (positive polarons in P3HT and negative polarons in PCBM) can be identified by their characteristic g-values. The fact that the polaron pair states exhibit strong non-Boltzmann population unambiguously shows that the constituents of each pair are geminate, i.e. originate from one exciton. We demonstrate that coupled polaron pairs are present even several microseconds after charge transfer and suggest that they embody the intermediate charge transfer complexes which form at the donor/acceptor interface and mediate the conversion from excitons into free charge carriers.