Giant Small‐Molecule Donors With Controlled Backbone Planarity Afford High‐Performance and Photostable Organic Solar Cells

High power conversion efficiency (PCE) and long-term stability are critical requirements for the commercialization of organic solar cells (OSCs). Small-molecule donors (SMDs) are promising due to easy purification and excellent batch-to-batch reproducibility, but they suffer from poor morphological stability associated with their fast diffusion kinetics. Here, new giant small-molecule donors (GSMDs) are designed that combine increased molecular sizes with excellent optoelectronic properties, enabling both efficient and photostable OSCs. To optimize their thermal and electrical properties, we tune the backbone planarity of GSMDs by controlling the orientation of alkyl side chains. Specifically, two GSMDs with different molecular configurations are synthesized: 1) GSMD-syn, where alkyl side chains are aligned in the same direction, and 2) GSMD-anti, with oppositely oriented side chains. The anti-configuration reduces steric hindrance between backbone units, thereby enhancing crystallinity and charge transport. Consequently, GSMD-anti:Y6-based OSCs achieve a high PCE of 15.4%, significantly outperforming those based on conventional SMD (BTR-Cl:Y6, PCE = 13.4%) and GSMD-syn:Y6 (PCE = 11.9%). Furthermore, ternary devices incorporating GSMD-anti reach a PCE of 16.5%, which, to the best of our knowledge, is among the highest reported values for GSMD-based OSCs. Importantly, GSMD-anti:Y6 OSCs exhibit significantly enhanced photostability (t_80% = 1510 h), compared to BTR-Cl:Y6 OSCs (t_80% = 60 h), attributed to suppressed molecular diffusion resulting from the larger molecular size of GSMD-anti.