Suppressed recombination loss in organic photovoltaics adopting a planar–mixed heterojunction architecture

At present, high-performance organic photovoltaics mostly adopt a bulk-heterojunction architecture, in which exciton dissociation is facilitated by charge-transfer states formed at numerous donor–acceptor (D-A) heterojunctions. However, the spin character of charge-transfer states originated from recombination of photocarriers allows relaxation to the lowest-energy triplet exciton (T₁) at these heterojunctions, causing photocurrent loss. Here we find that this loss pathway can be alleviated in sequentially processed planar–mixed heterojunction (PMHJ) devices, employing donor and acceptor with intrinsically weaker exciton binding strengths. The reduced D-A intermixing in PMHJ alleviates non-geminate recombination at D-A contacts, limiting the chance of relaxation, thus suppressing T₁ formation without sacrificing exciton dissociation efficiency. This resulted in devices with high power conversion efficiencies of >19%. We elucidate the working mechanisms for PMHJs and discuss the implications for material design, device engineering and photophysics, thus providing a comprehensive grounding for future organic photovoltaics to reach their full promise.