High-efficiency and thermally stable FACsPbI3 perovskite photovoltaics

α-FA 1− x Cs x PbI 3 is a promising absorbent material for efficient and stable perovskite solar cells (PSCs) 1 , 2 . However, the most efficient α-FA 1− x Cs x PbI 3 PSCs require the inclusion of the additive methylammonium chloride 3 , 4 , which generates volatile organic residues (methylammonium) that limit device stability at elevated temperatures 5 . Previously, the highest certified power-conversion efficiency of α-FA 1− x Cs x PbI 3 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-FA 1− x Cs x PbI 3 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-FA 1− x Cs x PbI 3 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-FA 1− x Cs x PbI 3 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). Suppressing surface Cs + accumulation in methylammonium-free α-FA 1− x Cs x PbI 3 perovskite with an intermediate phase-assisted strategy enables high-efficiency and thermally stable photovoltaics.