We present the growth, phase transitions, and thermal decomposition of CsPbX (X = I, Br) thin films monitored by in situ X-ray diffraction (XRD). The perovskite films are prepared in vacuum via co-evaporation of PbX 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 CsPbX perovskites in high vacuum, with a much higher stability than their hybrid organic-inorganic MAPbX counterparts. We also observe the response of a black CsPbI thin film to exposure to ambient air at room temperature using the same XRD system. Exposing the black CsPbI to ambient air leads to the formation of yellow orthorhombic δ-CsPbI, whose crystal structure could be identified by its X-ray diffraction pattern. Additionally, the linear coefficients of expansion are determined for δ-CsPbI and the (020)-orientation of CsPbBr.

We present the identification of crystalline phases by in situ X-ray diffraction during growth and monitor the phase evolution during subsequent thermal treatment of CH3NH3PbX3 (X = I, Br, Cl) perovskite thin films.

Vacuum-based co-evaporation promises to bring perovskite solar cells to larger scales, but details of the film formation from the physical vapor phase are still underexplored. In this work, we investigate the growth of methylammonium lead iodide (MAPbI$$_3$$ 3 ) absorbers prepared by co-evaporation of methylammonium iodide (MAI) and lead iodide (PbI$$_2$$ 2 ) using an in situ X-ray diffraction setup. This setup allows us to characterize crystallization and phase evolution of the growing thin film. The total chamber pressure strongly increases during MAI evaporation. We therefore assume the total chamber pressure to be mainly built up by an MAI atmosphere during deposition and use it to control the MAI evaporation. At first, we optimize the MAI to PbI$$_2$$ 2 impingement ratios by varying the MAI pressure at a constant PbI$$_2$$ 2 flux rate. We find a strong dependence of the solar cell device performance on the chamber pressure achieving efficiencies > 14$$\%$$ % in a simple n-i-p structure. On the road to further optimizing the processing conditions we vary the onset time of the PbI$$_2$$ 2 and MAI deposition by delaying the start of the MAI evaporation by t = 0/8/16 min. This way, PbI$$_2$$ 2 nucleates as a seed layer with a thickness of up to approximately 20 nm during this initial stage. Device performance benefits from these PbI$$_2$$ 2 seed layers, which also induce strong preferential thin film orientation as evidenced by grazing incidence wide angle X-ray scattering (GIWAXS) measurements. Our insights into the growth of MAPbI$$_3$$ 3 thin films from the physical vapor phase help to understand the film formation mechanisms and contribute to the further development of MAPbI$$_3$$ 3 and related perovskite absorbers.

Lead-free, highly stable, inorganic double perovskites such as Cs2AgBiBr6 promise to solve two of the main issues that currently hold back the wide-scale application of perovskite-based opto-electronic devices: a lack of stability and the use of toxic lead. Recently, Cs2AgBiBr6 has been shown to be a very promising material for the use in UV and X-ray detectors. In this work, we analyze the film formation of Cs2AgBiBr6 thin films with time-resolved crystal structure analysis using in situ X-ray diffraction (XRD). We present an all-vacuum process for the preparation of Cs2AgBiBr6 thin films via multisource co-evaporation followed by thermal annealing. The successful synthesis of the double perovskite phase was confirmed by analysis with XRD and Raman spectroscopy. Our in situ XRD setup allows us to conduct an extensive temperature-dependent analysis of the phase evolution during annealing of the films during synthesis and also during thermal decomposition. We find a decomposition temperature for the Cs2AgBiBr6 thin films at roughly 300 °C, in between that of MAPbBr3 and CsPbBr3.

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