An in situ GIWAXS study of dual-source co-evaporation of CsPbI<sub>2</sub>Br thin films

<bold>Background</bold>Perovskite film fabrication techniques have a significant impact on film quality and device performance. While in situ synchrotron-based Grazing-Incidence Wide Angle X-ray Scattering (GIWAXS) has made significant progress in solution-processed perovskite films, the study of vacuum evaporation techniques still lacks a dedicated in situ characterization system.<bold>Purpose</bold>This study aims to investigate the crystallization process of dual-source co-evaporated CsPbI₂Br perovskite films using in situ GIWAXS.<bold>Methods</bold>Firstly, a customized dual-source co-evaporation vacuum system was developed for in situ GIWAXS experiments at synchrotron beamlines with vacuum level (6.7×10^-7 Pa), motion precision (5 μm) and angular precision (0.01°). Then, this system was installed at the X-ray diffraction beamline of Shanghai Synchrotron Radiation Facility (SSRF) to investigate the CsPbI₂Br perovskite film during its deposition at substrate temperatures of 25 ℃ or 85 ℃ followed by the annealing process. Finally, the fabricated films were characterized using in situ X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS) after the evaporation vacuum system interconnected with the photoemission vacuum system for analyzing their electronic structures.<bold>Results</bold>The GIWAXS measurements results reveal that preheating the substrate to 85 ℃ facilitated the formation of highly crystallized PbI₂ during deposition, with its diffraction intensity approximately twice that of the perovskite black-phase. After deposition on substrates at 85 ℃, an annealing at 380 ℃ successfully converts intermediate phases into pure black-phase CsPbI₂Br with high preferential orientations. The perovskite films deposited on substrates at 85 ℃ show significantly enhanced perovskite diffraction peak intensity compared to those on substrates at 25 ℃.<bold>Conclusions</bold>The dual-source co-evaporation system developed in this study enables comprehensive in situ GIWAXS study of CsPbI₂Br crystallization dynamics. Results of this study provide important guidance for optimizing the evaporation deposition process of large-area perovskite films for photovoltaic applications.