Researchers from the University of Chinese Academy of Sciences, Beijing, P.R. China reported spatial heterogeneity in halogen distribution in perovskite thin films. The issue had limited device efficiency and stability. The study identified strong through-film inhomogeneity of chlorine in formamidinium lead iodide films when methylammonium chloride was used as an additive. The team showed that alkali metal oxalates homogenized chlorine distribution by thermally dissociating and releasing cations that selectively bound chloride ions. This process reduced surface defects and removed interfacial barriers. The work achieved a certified steady-state PCE of 27.2% with a 0.108 square cm device area and 0.074 square cm mask area. Devices retained 86.3% of initial PCE after 1529 hours of MPPT under 1 Sun, while unpassivated devices maintained 82.8% after 1000 hours at 85 °C under 1 Sun.