Thermodynamically Stable Orthorhombic γ-CsPbI Thin Films for High-Performance Photovoltaics.

All-inorganic lead halide perovskites demonstrate improved thermal stability over the organic-inorganic halide perovskites, but the cubic α-CsPbI with the most appropriate bandgap for light harvesting is not structurally stable at room temperature and spontaneously transforms into the undesired orthorhombic δ-CsPbI. Here, we present a new member of black-phase thin films of all-inorganic perovskites for high-efficiency photovoltaics, the orthorhombic γ-CsPbI thin films with intrinsic thermodynamic stability and ideal electronic structure. Exempt from introducing organic ligands or incorporating mixed cations/anions into the crystal lattice, we stabilize the γ-CsPbI thin films by a simple solution process in which a small amount of HO manipulates the size-dependent phase formation through a proton transfer reaction. Theoretical calculations coupled with experiments show that γ-CsPbI with a lower surface free energy becomes thermodynamically preferred over δ-CsPbI at surface areas greater than 8600 m/mol and exhibits comparable optoelectronic properties to α-CsPbI. Consequently, γ-CsPbI-based solar cells display a highly reproducible efficiency of 11.3%, among the highest records for CsPbI thin-film solar cells, with robust stability in ambient atmosphere for months and continuous operating conditions for hours. Our study provides a novel and fundamental perspective to overcome the Achilles' heel of the inorganic lead iodide perovskite and opens it up for high-performance optoelectronic devices.