Steric Hindrance Effects from External Alkyl Chains of Small Molecule Acceptors on Photovoltaic Performance of Organic Solar Cells†

Comprehensive Summary Herein, we designed and synthesized five small molecule acceptors (SMAs), Se‐1 to Se‐5, by systematically varying the bifurcation sites of branched alkyl chains on selenophene[3,2‐ b ]thiophene unit to investigate the steric hindrance effects of external alkyl chain in Y6‐type SMAs on molecular stacking, active layer morphology, and photovoltaic performance. As the steric hindrance of branched alkyl chains decreased, the bandgap of SMAs narrowed with π–π stacking interactions between adjacent molecules enhanced, and the reorganization energy with D18 decreased. In particular, the strong steric hindrance from 1‐position branched alkyl chain significantly suppressed π–π stacking, resulting in reduced carrier mobility within the active layer. Conversely, the weaker steric hindrance from 5‐position branched alkyl chain led to excessive crystallinity of the acceptor, causing an uneven donor–acceptor distribution and imbalanced charge transport. Notably, the 2‐position branched alkyl chain endowed Se‐2 with optimal crystallization behavior, enabling a uniform phase distribution and balanced charge mobility. As a result, binary device based on D18/Se‐2 achieved an efficiency of 19.65%, representing the highest reported efficiency for selenium‐containing SMAs. This work underscores the critical role of exo‐alkyl chain steric hindrance regulation, offering valuable insights for the rational design of high‐performance SMAs.