The spatial heterogeneity of halogen distribution in perovskite thin films represents a critical factor currently limiting both the power conversion efficiency and stability of solar cells. We identified pronounced through-film inhomogeneity in chlorine distribution in formamidinium lead iodide films, with the generally used additive methylammonium chloride. We demonstrated that incorporating alkali metal oxalates could effectively homogenize the chlorine distribution. These compounds underwent thermal dissociation, releasing alkali metal cations that selectively bound chloride ions, which considerably suppressed surface defects and eliminated interfacial barriers. A certified steady-state power conversion efficiency (PCE) of 27.2% (device area and measured mask area: 0.108 square cm and 0.074 square cm, respectively) in perovskite solar cells was achieved, and devices retained 86.3% of their initial PCE after 1529 hours of continuous maximum power point tracking (MPPT) under 1 Sun condition. Moreover, the unpassivated device maintained 82.8% of its original PCE under MPPT at 85°C aging under 1 Sun illumination after 1000 hours.

The development of highly stable and efficient wide-bandgap (WBG) perovskite solar cells (PSCs) based on bromine-iodine (Br–I) mixed-halide perovskite (with Br greater than 20%) is critical to create tandem solar cells. However, issues with Br–I phase segregation under solar cell operational conditions (such as light and heat) limit the device voltage and operational stability. This challenge is often exacerbated by the ready defect formation associated with the rapid crystallization of Br-rich perovskite chemistry with antisolvent processes. We combined the rapid Br crystallization with a gentle gas-quench method to prepare highly textured columnar 1.75–electron volt Br–I mixed WBG perovskite films with reduced defect density. With this approach, we obtained 1.75–electron volt WBG PSCs with greater than 20% power conversion efficiency, approximately 1.33-volt open-circuit voltage ( V _oc ), and excellent operational stability (less than 5% degradation over 1100 hours of operation under 1.2 sun at 65°C). When further integrated with 1.25–electron volt narrow-bandgap PSC, we obtained a 27.1% efficient, all-perovskite, two-terminal tandem device with a high V _oc of 2.2 volts.