Mapping the polymorphic phase transformations of CsPbI3 perovskite thin films

Inorganic perovskite CsPbI₃ has a bandgap of 1.7 eV, making it an ideal complementary absorber to Si for integration into tandem solar cells. However, the black, photoactive CsPbI₃ phases are metastable and readily transform into a yellow non-perovskite δ-CsPbI₃ phase at room temperature, posing a significant challenge to long-term device stability. In this study, we investigate the temperature-dependent dynamics of these phase transitions in CsPbI₃ thin films using a combination of in situ X-ray diffraction and time-resolved optical microscopy. We find the transformation rate to be highly temperature-dependent, with the fastest conversion occurring at 225 °C, where 50% of the film transformed to δ-CsPbI₃ within 17 minutes. To identify processing temperatures with longer phase-stability windows, we used the time- and temperature-dependent phase dynamics data to generate a time-temperature-transformation diagram for thin film CsPbI₃. Processing near the peak conversion temperature must be completed within two minutes to retain black-phase purity, while processing above 280 °C or below 150 °C provides a much wider processing window with <1% conversion to δ-CsPbI₃ occurring after 10 minutes. Conversely, it may be useful to hold CsPbI₃ solar cells or thin films with phase-stabilizing modifications near 225 °C to accelerate potential phase transitions and maximally stress their stability.