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

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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 ~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:PC 61 BM, a system with a very low driving energy for initial charge separation ( E g − E CT ~ 50 meV) and a high internal quantum efficiency ( η IQE ~ 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 photogeneration. In PIPCP:PC 61 BM, 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.