Charge Transfer State, Energy Losses, and Spatial Molecular Distribution in the Nonfullerene-Based Organic Solar Cells

Abstract

Energy losses significantly reduce the open-circuit voltage even 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) were found to display a tradeoff behavior between radiative energy losses, i.e., charge transfer state (CTS) radiative energy loss and the loss associated with CTS formation from acceptor singlet excitons (SE). In addition, the efficiency of CTS radiative decay is found to demonstrate competition with the nonradiative decay which extends even on thermally degraded devices. Accordingly, this work provides further understandings of energy losses relevant to overcome the current limitations concerning OSCs developments. Further, due to the high similarity of molecular fragments and functionalities between polymer donors and non-fullerene acceptors, the spatial molecular distributions are commonly not given enough considerations among other studies. This work demonstrates how vertical phase segregations and microscopic characteristics can influence the device performance and properties of NFA-based OSCs using time-of-flight secondary ion mass spectrometry (TOF-SIMS) molecular depth profiling and other optical and electronic characterizations. This study then also provides enlightenments to enable more precise and consistent data interpretations among future related studies.