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

The alloying behavior between FAPbI3 and CsSnI3 perovskites is studied carefully for the first time, which has led to the realization of single-phase hybrid perovskites of (FAPbI3 )1-x (CsSnI3 )x (0<x<1) compositions with anomalous bandgaps. (FAPbI3 )1-x (CsSnI3 )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 (FAPbI3 )0.7 (CsSnI3 )0.3 -composition perovskite. Solar cells based on this new perovskite show power conversion efficiency up to 14.6 %.