Sodium‐Induced Process Window Expansion Enables High‐Performance Solution‐Processed CISSe Thin‐Film Solar Cells for Tandem Applications

ABSTRACT This work reports a sodium‐enabled breakthrough in air‐processable N,N‐dimethylformamide (DMF) molecular ink routes for CuIn(S,Se) 2 (CISSe) solar cells. Excess Na incorporation significantly expands the otherwise narrow air annealing temperature (AAT) window required for high‐efficiency device fabrication. Mechanistic investigations reveal that Na suppresses the formation of detrimental Cu x Se secondary phases and associated interfacial p + defects (V Cu –V Se complexes), which are highly sensitive to AAT under Na‐deficient conditions. This effect originates from a Na‐induced transition in growth kinetics from long‐range, diffusion‐limited processes to short‐range growth, effectively preventing Cu x Se surface segregation. Consequently, Na incorporation mitigates interfacial p + transport barriers at the absorber/buffer interface while simultaneously enhancing processing tolerance. As a result, high and reproducible device efficiencies are achieved over a significantly broadened AAT range, with a new certified record efficiency of 14.70% for solution‐processed CISSe solar cells. Furthermore, integration into a solution‐processed 4‐terminal (4T) perovskite/CISSe tandem device yields a power conversion efficiency of 25.80%. These findings establish a viable pathway toward scalable, reproducible, and high‐performance solution‐processed tandem photovoltaics.