α-FA1−xCsxPbI3 is a promising absorbent material for efficient and stable perovskite solar cells (PSCs)1,2. However, the most efficient α-FA1−xCsxPbI3 PSCs require the inclusion of the additive methylammonium chloride3,4, which generates volatile organic residues (methylammonium) that limit device stability at elevated temperatures5. Previously, the highest certified power-conversion efficiency of α-FA1−xCsxPbI3 PSCs without methylammonium chloride was only approximately 24% (refs. 6,7), and these PSCs have yet to exhibit any stability advantages. Here we identify interfacial contact loss caused by the accumulation of Cs+ in conventional α-FA1−xCsxPbI3 PSCs, which deteriorates device performance and stability. Through in situ grazing-incidence wide-angle X-ray scattering analysis and density functional theory calculations, we demonstrate an intermediate-phase-assisted crystallization pathway enabled by acetate surface coordination to fabricate high-quality α-FA1−xCsxPbI3 films, without using the methylammonium additive. We herein report a certified stabilized power output efficiency of 25.94% and a reverse-scanning power-conversion efficiency of 26.64% for α-FA1−xCsxPbI3 PSCs. Moreover, the devices exhibited negligible contact losses and enhanced operational stability. They retained over 95% of their initial power-conversion efficiency after operating for over 2,000 h at the maximum power point under 1 sun, 85 °C and 60% relative humidity (ISOS-L-3).