Unraveling All‐Inorganic CsPbI3 and CsPbI2Br Perovskite Thin Films Formation – Black Phase Stabilization by Cs2PbCl2I2 Addition and Flash‐Annealing

Abstract

AbstractPhase formation and transformation of 3D inorganic halide perovskites CsPbI3 and CsPbI2Br were investigated by in‐situ X‐ray diffraction (XRD) in nitrogen or air from 30 °C up to 320 °C. Thin films were obtained by spin‐coating of a liquid solution of salt precursors dissolved in DMSO under normal conditions. XRD data analysis, by using the Le Bail method, evidenced for the first time, to our knowledge, the spontaneous crystallization of black orthorhombic γ phase at ambient temperature, for both 3D perovskite phases. For once, this black γ phase could be studied upon heating. It transforms to the black tetragonal β and cubic α phases. The reverse transitions were also evidenced upon cooling. The influence of the all‐inorganic 2D Cs2PbI2Cl2 material addition over the black phase stability was unprecedently analyzed by in‐situ XRD. The mixture of the 3D and 2D phases, or mix [3D+2D], exhibits an improved stability of the black phase compared to the single 3D phase, and tends to induce highly oriented thin films and increased compactness, probably due to chemical insertion or anchoring. Finally, flash‐annealed thin films at various temperatures (RT<T<325 °C) of CsPbI2Br in air, alone and in the presence of the 2D phase, were studied. A more textured layer is evidenced for the mix [3D+2D] thin film, showing larger grains and more compact films above 200 °C. The mix [3D+2D] film flash‐annealed at 300 °C remained black for four days after air exposure and 50 % relative humidity.