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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.