Thermodynamically Stable Orthorhombic γ-CsPbI₃ Thin Films for High-Performance Photovoltaics

TLDR

All‑inorganic lead halide perovskites are more thermally stable than hybrid perovskites, yet the cubic α‑CsPbI₃, which has the ideal bandgap for light harvesting, is unstable at room temperature and converts to the undesired orthorhombic δ‑CsPbI₃. The study introduces orthorhombic γ‑CsPbI₃ thin films as a thermodynamically stable, black‑phase perovskite for high‑efficiency photovoltaics. γ‑CsPbI₃ films are stabilized by a simple solution process that uses trace H₂O to trigger proton‑transfer‑mediated phase control without organic ligands or mixed cations. The γ‑CsPbI₃ phase is thermodynamically favored over δ‑CsPbI₃ at surface areas >8600 m² mol⁻¹, shows optoelectronic properties comparable to α‑CsPbI₃, and enables thin‑film solar cells with a reproducible 11.3 % efficiency and long‑term ambient stability.

Abstract

All-inorganic lead halide perovskites demonstrate improved thermal stability over the organic–inorganic halide perovskites, but the cubic α-CsPbI3 with the most appropriate bandgap for light harvesting is not structurally stable at room temperature and spontaneously transforms into the undesired orthorhombic δ-CsPbI3. Here, we present a new member of black-phase thin films of all-inorganic perovskites for high-efficiency photovoltaics, the orthorhombic γ-CsPbI3 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-CsPbI3 thin films by a simple solution process in which a small amount of H2O manipulates the size-dependent phase formation through a proton transfer reaction. Theoretical calculations coupled with experiments show that γ-CsPbI3 with a lower surface free energy becomes thermodynamically preferred over δ-CsPbI3 at surface areas greater than 8600 m2/mol and exhibits comparable optoelectronic properties to α-CsPbI3. Consequently, γ-CsPbI3-based solar cells display a highly reproducible efficiency of 11.3%, among the highest records for CsPbI3 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.