Crystal Phases and Thermal Stability of Co-evaporated CsPbX₃ (X = I, Br) Thin Films

We present the growth, phase transitions, and thermal decomposition of CsPbX₃ (X = I, Br) thin films monitored by in situ X-ray diffraction (XRD). The perovskite films are prepared in vacuum via co-evaporation of PbX₂ and CsX (X = I, Br) onto glass substrates. In situ X-ray diffraction allows the observation of phase transitions and decomposition while the samples are heated with a linear temperature ramp. Our experiments reveal the decomposition route for the CsPbX₃ perovskites in high vacuum, with a much higher stability than their hybrid organic-inorganic MAPbX₃ counterparts. We also observe the response of a black CsPbI₃ thin film to exposure to ambient air at room temperature using the same XRD system. Exposing the black CsPbI₃ to ambient air leads to the formation of yellow orthorhombic δ-CsPbI₃, whose crystal structure could be identified by its X-ray diffraction pattern. Additionally, the linear coefficients of expansion are determined for δ-CsPbI₃ and the (020)-orientation of CsPbBr₃.