Charge transfer rate modulated energy loss in indoor organic photovoltaic cells

Abstract

Minimizing energy loss is pivotal for achieving high-performance indoor organic photovoltaic (IOPV) cells, where charge transfer (CT) rate critically governs exciton-to-charge conversion efficiency. Nevertheless, the fundamental correlation between CT rate and energy loss remains insufficiently understood. In this study, we systematically investigate how CT rate affects the radiative and non-radiative energy loss in IOPV devices employing PBDB-T donor and three wide-bandgap BTA3 series acceptors. A developed global fitting model for charge separation pathways demonstrates that the intra-moiety delocalized excitations and CT states show comparable contributions to exciton dissociation efficiency (each accounting for approximately 50%). Notably, our findings reveal that excessive driving forces can paradoxically reduce CT rate, resulting in prolonged existence of CT states at the donor:acceptor interfaces that amplifies radiative and non-radiative recombination. The lower CT rate-mediated energy loss becomes increasingly dominant under indoor weak illumination compared to standard solar conditions due to reduced density of CT states and intensified influence of trap states. This elucidates a CT rate-dependent energy loss mechanism that warrants greater consideration in the design of IOPVs. These results highlight the importance of CT rate modulation for suppressing energy loss, providing valuable insights for the rational design of photovoltaic materials for diverse indoor energy harvesting applications.