Homogenous Alloys of Formamidinium Lead Triiodide and Cesium Tin Triiodide for Efficient Ideal‐Bandgap Perovskite Solar Cells

Abstract The alloying behavior between FAPbI 3 and CsSnI 3 perovskites is studied carefully for the first time, which has led to the realization of single‐phase hybrid perovskites of (FAPbI 3 ) 1− x (CsSnI 3 ) x (0< x <1) compositions with anomalous bandgaps. (FAPbI 3 ) 1− x (CsSnI 3 ) x perovskites exhibit stable, homogenous composition/microstructure at the nanoscale, as confirmed by microscopic characterization. The ideal bandgap of 1.3 eV for single‐junction solar cell operation is achieved in the rationally‐tailored (FAPbI 3 ) 0.7 (CsSnI 3 ) 0.3 ‐composition perovskite. Solar cells based on this new perovskite show power conversion efficiency up to 14.6 %.