Crystallization control of CH3NH3PbI3 film in inverted perovskite solar cells via atmospheric vapor-assisted deposition

The performance of halide perovskite solar cells (PSCs) is highly dependent on the halide perovskite film quality, i.e. crystallinity, surface coverage, and morphology. Conventionally, the antisolvent treatment is used to form high-quality halide perovskite films, but it is not a scale-up-friendly method. Antisolvent-free methods usually have the challenge of proper crystallization, as well as pinhole or coverage issues. Here, we explore a vapor-assisted deposition to study how we can approach a high-quality and pinhole-free CH₃NH₃PbI₃ (MAPbI₃) perovskite film formation process in inverted PSCs. Furthermore, the MAPbI₃ perovskite films are formed by exposing PbI₂ thin film to CH₃NH₃I (MAI) vapor in air. To reach a crystalline and uniform MAPbI₃ perovskite film, the effect of reaction temperature (130 – 170 °C) and reaction time (0.5 – 2.5 h) between PbI₂-coated films and MAI vapor on the MAPbI₃ perovskite film formation and resultant device performance has been explored. The detailed surface morphological, optical, and structural characterizations explicitly reveal that the pinhole-free, fully crystalline, and high-quality MAPbI₃ perovskite film forms at the reaction temperature and reaction time of 150 °C and 2.0 h, respectively. In addition, the device employing MAPbI₃ perovskite films prepared at the optimal reaction temperature and reaction time attains a champion power conversion efficiency of 14.3 %. Besides, the long-term stability test demonstrates that the inverted PSCs keep 80 % of their initial performance after 20 days of storage under dark ambient conditions (∼ 25 °C; 25 – 40 % humidity).