Precise Modulation of Reorganization Energy through Methyl Substitution for High Performance Organic Solar Cells

Efficient charge transport and minimized energy loss are critical for advancing the performance of organic solar cells (OSCs). In this study, a series of quinoxaline-based electron acceptors, BQx-MeF, BQx-MeCl, and BQx-MeBr, featuring methyl and halogen substitutions is designed and synthesized to systematically modulate reorganization energy (λ) and film morphology. Quantum chemical calculations confirmed that methylation effectively reduces λ by limiting structural relaxation, leading to suppressed non-radiative recombination energy loss (ΔE_nr) and improved charge transport. Among the synthesized materials, BQx-MeCl exhibited the lowest energy loss and the most balanced electron and hole mobilities, resulting in a superior power conversion efficiency (PCE) of 19.2% in a binary device. In optimized ternary OSCs, BQx-MeCl further reached a remarkable PCE of 19.6%. This enhancement is attributed to optimized molecular stacking, improved film morphology, and reduced trap-assisted recombination. These findings highlight the pivotal role of molecular design in lowering reorganization energy to minimize energy losses and maximize charge collection, offering an effective strategy for the development of high-efficiency OSCs.