The Role of Terminal Fluorination on Energy Inversion in Organic Solar Cells

Suppressing non-radiative energy loss (ΔE₃) mediated by the triplet charge transfer state is crucial for high-performance organic solar cells (OSCs). Here, we decode the energy inversion through multi-scale theoretical simulations, which inhibit the formation of non-emissive triplet (T₁) state. However, it is mystified by the system dependence. We first demonstrate a direct relationship of "the probability of Face-on orientation (P_Face-on) is proportional to the probability of energy inversion (P_EI)", which is related to the function of terminal fluorination. Through Pearson's correlation coefficient and machine learning model, the useful stacking structural parameters were obtained to clarify the effect of π-bridge group on the function of terminal fluorination. Based on the molecular descriptors established, we explain that the fluorination effect is beneficial to Face-on orientation and thus energy inversion due to the enhanced intermolecular coupling. But the π-bridge inhibits this coupling with the interfacial stacking configuration appearing more "TT_IC". This work provides a directional standard for promoting energy inversion to reduce ΔE₃ for the high-performance OSCs.