Charge Separation and Recombination at Polymer-Fullerene Heterojunctions: Delocalization and Hybridization Effects

We address charge separation and recombination in polymer/fullerene solar cells with a multiscale modeling built from accurate atomistic inputs and accounting for disorder, interface electrostatics and genuine quantum effects on equal footings. Our results show that bound localized charge transfer states at the interface coexist with a large majority of thermally accessible delocalized space-separated states that can be also reached by direct photoexcitation, thanks to their strong hybridization with singlet polymer excitons. These findings reconcile the recent experimental reports of ultrafast exciton separation ("hot" process) with the evidence that high quantum yields do not require excess electronic or vibrational energy ("cold process), and show that delocalization, by shifting the density of charge transfer states toward larger effective electron-hole radii, may reduce energy losses through charge recombination.