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.