Hot exciton dissociation in polymer solar cells

Abstract

The standard picture of photovoltaic conversion in all-organic bulk heterojunction solar cells predicts that the initial excitation dissociates at the donor/acceptor interface after thermalization^{1,2,3,4,5,6,7,8,9,10,11,12}. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation^{2,3,11,13,14,15,16}. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC₆₀BM system. Exciton splitting occurs within the first 50 fs, creating both interfacial charge transfer states (CTSs) and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial CTSs that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot CTSs with respect to the relaxed ones, which enhances the probability of charge dissociation in the first 200 fs. Thus, the hot CTS dissociation produces an overall increase in the charge generation yield.