Understanding the Charge Transfer State and Energy Loss Trade-offs in Non-fullerene-Based Organic Solar Cells

Abstract

Energy losses significantly reduce the open-circuit voltage among current state-of-the-art organic solar cells (OSCs), which limits the further enhancement of their power conversion efficiencies (PCEs). In this study, the bulk heterojunction blends of PM6 donor and halogenated nonfullerene acceptors (NFAs) display a trade-off between radiative energy losses, i.e., charge transfer state (CTS) radiative energy loss (ΔErad) and the loss associated with CTS formation from acceptor singlet excitons (ΔECTEL). Similarly, a trade-off between ΔErad and the nonradiative energy loss (ΔEnr) is found, reflecting a competition in radiative and nonradiative charge recombination pathways. Further, the energy levels of relaxed CTS (ECTEL) are shown to exhibit dependence on morphologically induced energetic traps, suggesting that it should not be associated merely to blend constituents. Interestingly, these correlations extend even to thermally degraded devices considered herein. Accordingly, this work provides further understandings of energy losses relevant to overcome the current limitations concerning NFA-based OSC developments.