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

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 CuxSe secondary phases and associated interfacial p+ defects (VCu-VSe 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 CuxSe 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.