LAUSR

Summary Inorganic cesium lead halide perovskites (CsPbI 3 ) are promising materials for efficient wide-bandgap perovskite solar cells, but they suffer from a phase transition from black α phase to yellow δ phase at room temperature. Here, we report a facile method to stabilize the α-phase CsPbI 3 films via a single-step film deposition process. A small amount (∼1.5 wt %) of sulfobetaine zwitterion mixed in CsPbI 3 precursor solution could facilitate the formation of black-phase CsPbI 3 films that show significantly improved phase stability in air. The black-phase stabilization can be explained by the formation of small CsPbI 3 grains with average size of ∼30 nm, which increased the grain surface area to stabilizes the α phase. The zwitterions were found to impede the crystallization of CsPbI 3 perovskite films via electrostatic interaction with the ions and colloids in the CsPbI 3 precursor solution. Solar cells using these zwitterion-stabilized perovskite films showed stabilized power conversion efficiency of 11.4% under 1-sun illumination.