Triplet exciton recycling in organic solar cells enabled by an extrinsic triplet-triplet annihilation dopant.

Enabled by the optimisation of non-fullerene electron acceptors (NFAs), organic solar cells (OSCs) now exhibit power conversion efficiencies exceeding 20%. However, to further increase OSC performance, it is essential to target the areas of device function where significant losses remain. One key factor now limiting OSCs is charge recombination via the spin-triplet exciton state on the NFA component. Here, we show that by doping the archetypal triplet-triplet annihilation (TTA) material rubrene into a model PM6:Y6 blend, the triplet excitons formed on Y6 via non-geminate charge recombination can be transferred to rubrene. The rubrene triplets then undergo TTA to form singlet excitons followed by recycling into free charge carriers. We further reveal a complex evolution in the bulk heterojunction morphology upon the addition of rubrene, including the formation of mixed domains containing all three materials. We propose that the presence of rubrene in this mixed region near the PM6:Y6 interface enables it to rapidly accept the triplets generated on Y6. Our work therefore provides a proof of concept for triplet recycling by an extrinsic TTA dopant in OSCs, which can inspire further innovative strategies for triplet exciton management in both OSCs and photon upconversion systems.