Dual Additives toward High-Performance Wide-Bandgap Perovskite Solar Cells.

Wide-bandgap (WBG) metal halide perovskites are key materials for constructing high-efficiency tandem solar cells by coupling with narrow-bandgap bottom subcells. However, their practical performance is severely limited by pronounced open-circuit voltage loss and instability, arising from accelerated crystallization, abundant defect states, and photo induced halide phase segregation induced in Br-rich compositions. These issues are closely associated with iodine-related redox imbalance and undercoordinated Pb2+ sites induced by high bromide incorporation, which intensify nonradiative recombination and ion migration. Herein, a dual-additive strategy based on cesium formate (CsFa) and formamidinium formate (FAFa) is proposed, in which the introduced formate ion (HCOO-) plays a dominant bifunctional role. Acting simultaneously as a mild reducing agent and a coordinating ligand, HCOO- also converts photothermally generated I2/I3- species into I-, thereby rebalancing the iodine redox chemistry, while coordinating with undercoordinated Pb2+ ions to modulate crystallization kinetics and promote grain growth, thereby suppressing defect formation, and improving energy-level alignment. As a result, high quality WBG perovskite films with enlarged grain size and reduced defect density are obtained, enabling P-I-N structured perovskite solar cells with a bandgap of ∼1.79 eV to achieve a power conversion efficiency of 17.83% along with enhanced operational stability, demonstrating an effective strategy for performance optimization of WBG top cells in tandem photovoltaic devices.