Enhancing Perovskite/Organic Tandem Solar Cell Efficiencies via Gradient‐Regulated Crystallization of Mixed‐Halide Perovskites

Mixed‐halide wide‐bandgap (WBG) perovskites suffer from inhomogeneous crystallization and vertical halide segregation during film formation and device operation. Here, we report a gradient‐controlled crystallization strategy to regulate crystallization kinetics and halide distribution in mixed‐halide perovskite films. Enabled by selective interactions with halide species and surface‐directed accumulation, this approach establishes a surface‐to‐bulk additive concentration gradient that synchronizes halide incorporation and suppresses the formation of Br‐rich regions. The strategy alleviates compositional heterogeneity, and the resulting surface‐enriched layer further retards the penetration of moisture and oxygen. Importantly, this strategy is broadly effective for mixed‐halide compositions spanning diverse halide mixing ratios. Consequently, WBG perovskite devices deliver superior photovoltaic performance, achieving a high V OC of 1.385 V and a PCE of 17.78%. Integration of the WBG subcells into perovskite/organic tandem devices yields a champion efficiency of 25.91% with a V OC of 2.191 V, while retaining more than 80% of the initial PCE after 600 h of continuous operation.