Utilizing hybridization effects to tune morphology and electron mobility of Y6 through asymmetric small- and large-scale modifications of terminal groups

While exploring molecular modifications of the high-performance acceptor Y6 with an A-DA'D-A framework, researchers have discovered that asymmetric modification of terminal groups (TGs) appears to be a promising approach as it frequently enhances the photovoltaic performance of organic solar cells (OSCs) effectively. However, the underlying mechanism about how asymmetric TG modifications influence morphology and charge carrier mobility remains unclear. We have conducted a systematic study in this work to investigate the morphology and electron mobility of two asymmetric Y6 derivatives with the A1-DA'D-A2 framework: Y6-asym-IM2O (A1 = IM-2F and A2 = IM2O, representing small-scale TG modification) and Y6-asym-BR (A1 = IM-2F and A2 = BR, representing large-scale TG modification), along with their symmetric counterparts (A1 = A2 = IM-2F/BR/IM2O). The results demonstrate that small-scale asymmetric TG modifications such as Y6-asym-IM2O fine-tune molecular packing, while large-scale modifications such as Y6-asym-BR drastically alter stacking patterns. In addition, hybridization effects are found in the frontier molecular orbital energy, electrostatic potential, and electron mobility of the asymmetric molecules, which fall between the values of their symmetric counterparts. In particular, the results of small-scale asymmetric modification of Y6 reveal that the introduction of promising TGs in an asymmetric manner can further improve electron mobility by tuning reorganization energy and morphology, and vice versa. While previous studies focused on symmetric modifications, this work systematically investigates asymmetric substitution patterns and further elucidates the impact of these methods on charge transfer for the first time. These discoveries underscore the potential of utilizing asymmetric modification of TGs as a quantitative means to regulate electron mobility in Y6-based OSCs.