Charge Separation from a Cold Charge-Transfer State Driven by a Nonuniform Electric Field in Polymer-Based Donor/Acceptor Heterojunctions

Charge separation at donor/acceptor (D/A) interfaces plays a crucial role in the operation of photovoltaic polymer solar cells (PSCs), but the underlying mechanism still remains debated. Here, we present a model study to demonstrate how charge separation is achieved from a cold charge-transfer (CCT) state in the presence of a nonuniform electric field along a continuous polymer-based D/A interface. We find that a CCT state experiences two kinds of evolution paths, which are determined by the field strength of positions where the CCT state is initially generated. In strong-field positions, the CCT state is directly dissociated into free charges, while in weak-field positions, before the desired charge separation, the CCT state experiences an ultrafast migration process along the D/A interface. In both cases, charge separation takes place in a time scale less than 200 fs, which might contribute to the ultrafast charge separation reported experimentally in numerous high-performance PSCs. Finally, the mechanism about the ultrafast migration of the CCT state is revealed, by which we also analyze the effect of the aggregation or crystallinity between acceptor molecules on the migration process.