Order enables efficient electron-hole separation at an organic heterojunction with a small energy loss

Donor-acceptor organic solar cells often show low open-circuit voltages (V-OC) relative to their optical energy gap (E-g) that limit power conversion efficiencies to similar to 12%. This energy loss is partly attributed to the offset between E-g and that of intermolecular charge transfer (CT) states at the donor-acceptor interface. Here we study charge generation occurring in PIPCP: PC61BM, a system with a very low driving energy for initial charge separation (E-g-E-CT similar to 50 meV) and a high internal quantum efficiency (eta(IQE) similar to 80%). We track the strength of the electric field generated between the separating electron-hole pair by following the transient electroabsorption optical response, and find that while localised CT states are formed rapidly (<100 fs) after photoexcitation, free charges are not generated until 5 ps after photo-generation. In PIPCP:PC61BM, electronic disorder is low (Urbach energy <27 meV) and we consider that free charge separation is able to outcompete trap-assisted non-radiative recombination of the CT state.