Pathway-priming of intermediate phases in FACs-based wide-bandgap perovskites for Si tandem solar cells

Wide-bandgap (WBG) perovskite solar cells (PSCs) have emerged as promising candidates for tandem applications, despite potential instability induced by halogen components. Formamidinium-cesium (FACs)-based perovskites represent as an intrinsically stable WBG system due to their low bromine-to-lead ratio and the absence of thermally unstable methylammonium. However, the performance of FACs-based WBG perovskites is highly dependent on achieving precise control over the formation dynamics. Here, we employ in-situ characterization to reveal that the key to this control lies in the evolution of the intermediate phase and propose a pathway-priming strategy. By strategically guiding the early formation of the CsPbX3 intermediate phases, the strategy in turn templates and accelerates the subsequent transformation to the desired α-phase. The primed pathway yields a more uniform and complete phase transition, leading to enhanced film homogeneity. As a result, we demonstrate highly efficient 1.67 eV WBG PSCs with an efficiency exceeding 23%, along with excellent long-term operating stability maintaining 90% initial performance for 800 hours under continuous light stress. Furthermore, semitransparent WBG PSC achieve efficiencies of 20.91% (0.1 cm2) and 19.67% (1.0 cm2), corresponding to a four-terminal perovskite/silicon tandem solar cells with an efficiency of 31.04% (1.0 cm2), highlighting their potential for high-performance tandem photovoltaics. Wide-bandgap perovskites offer promise for tandems but suffer from formation-pathway instabilities. Huang et al. use in-situ analysis to guide CsPbX₃ intermediate evolution, enabling uniform α-phase formation and high-efficiency, stable devices.