Slow geminate-charge-pair recombination dynamics at polymer: Fullerene heterojunctions in efficient organic solar cells

We explore charge recombination dynamics at electron donor-acceptor heterojunctions, formed between a semiconductor polymer (PCDTBT) and a fullerene derivative (PC70BM), by means of combined time-resolved photoluminescence and transient absorption spectroscopies. Following prompt exciton dissociation across the heterojunction, a subset of bound electron-hole pairs recombines with a temperature-independent rate distribution spanning submicrosecond timescales to produce luminescent charge-transfer excitons (CTX). At 14 K, this slow mechanism is the dominant geminate charge recombination pathway, whereas we also observe CTX emission on subnanosecond timescales at 293 K. We thus find that at these temperatures, a fraction of the initial charge-pair population is trapped deeply such that they only recombine slowly over a broad distribution of timescales by quantum tunneling. We identify geminate polaron pairs (GPP) as a reservoir of long-lived localized states that repopulate the CTX up to microsecond timescales. The observation of such distributed geminate-charge recombination highlights the importance of the molecular nature of specific donoracceptor electronic interactions in defining the relaxation pathways of trapped GPP. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012