Improving Molecular Planarity by Changing Alky Chain Position Enables 12.3% Efficiency All‐Small‐Molecule Organic Solar Cells with Enhanced Carrier Lifetime and Reduced Recombination

Molecular stacking plays an important role in defining the active layer morphology in all‐small‐molecule organic solar cells (ASM OSCs). However, the precise control of donor/acceptor stacking to afford optimal phase separation remains challenging. Herein, the molecular stacking of a small‐molecule donor is tuned by changing the alky chain position to match a high‐performance small‐molecule nonfullerene acceptor (NFA), Y6. The alky chain engineering not only affects the planarity of the small‐molecule donor, but also the molecular aggregation and the active layer morphology, and thus the photovoltaic performance. Notably, single‐junction ASM OSCs with 12.3% power conversion efficiency (PCE) are achieved. The PCE of 12.3% is among the top efficiencies of single‐junction ASM OSCs reported in the literature to date. The results highlight the importance of fine‐tuning the molecular structure to achieve high‐performance ASM OSCs.