Thermal Annealing-Induced Phase Conversion in N-type Triple-Cation Lead-Based Perovskite Field Effect Transistors.

The field of perovskite optoelectronics and electronics has rapidly advanced, driven by excellent material properties and a diverse range of fabrication methods available. Among them, triple-cation perovskites such as CsFAMAPbI₃ offer enhanced stability and superior performance, making them ideal candidates for advanced applications. However, the multicomponent nature of these perovskites introduces complexity, particularly in how their structural, optical, and electrical properties are influenced by thermal annealing─a critical step for achieving high-quality thin films. Here, we propose a comprehensive mechanistic picture of the thin film formation process of CsFAMAPbI₃ during the thermal annealing step through systematic and comparative analyses, identifying two key thermally induced phase transitions: the crystallization of the perovskite phase facilitated by solvent evaporation and the formation of the PbI₂ phase due to thermal decomposition. Our results reveal that the crystallization process during annealing proceeds from the surface to the bulk of the films, with a significant impact on the film's morphology and optical characteristics. Controlled annealing enhances field-effect transistor device performance by promoting defect passivation and complete perovskite crystallization, while prolonged annealing leads to excessive PbI₂ formation, accelerating ion migration and ultimately degrading device performance. These insights offer valuable guidance for optimizing the design and performance of perovskite-based electronic and optoelectronic devices.