Trace Water in Solvent Precisely Influences the Formation Process of the Active Layer for Organic Solar Cells.

Organic solar cells (OSCs) are attracting widespread attention as a promising renewable energy technology. The solvent purity exhibits a great impact on the photovoltaic performance of OSCs, but how trace water in solvent influences the formation process of the active layer and the device performance remains unclear. Here, we systematically investigate the impact of trace water in chloroform (CF) on film formation and performance of PM6:L8-BO OSCs. It demonstrates that lower trace water content significantly accelerates the liquid-to-solid transition and nucleation time during film formation, facilitating ordered molecular aggregation and crystallization. Thus, it results in producing efficient exciton dissociation, charge transport and collection, and suppressing charge recombination. By precisely controlling the water content in CF (200, 70, and 30 ppm), devices fabricated with CF-30 (30 ppm) exhibit a remarkable power conversion efficiency (PCE) of 18.49%, surpassing efficiencies of 16.12% and 13.15% from CF-70 and CF-200, respectively. Using 2PACz and Ph-4PACz as hole transport layers further raises PCEs to 19.57% and 19.92%. Furthermore, the strategy is validated across other binary systems, underscoring its general applicability. This work highlights the critical role of solvent purity in enhancing the efficiency and reproducibility of OSCs, offering valuable insights to guide future device optimization strategies.