Thermal Control of PbI2 Film Growth for Two-Step Planar Perovskite Solar Cells

We used a two-step process to fabricate planar perovskite solar cells (PSCs) in which CH3NH3I (MAI) molecules intercalated into a PbI2 film. The crystal growth of PbI2 is particularly important to achieve perovskite films with high PSC performance. Herein, we attempted to control the crystal growth of PbI2 films through a simple spin-coating method at annealing temperatures of 70, 150, 250, and 350 °C and times of 1, 5, 10, and 30 min. We investigate the effects of the high-crystallite host PbI2 film on the crystallinity, film quality, and absorption of the resulting MAPbI3 perovskite film as well as the resulting performance of the planar PSCs. Under the optimal annealing conditions, a highly crystalline PbI2 film was reproducibly achieved at 250 °C for 30 min. Precise intercalation of MAI molecules into the PbI2 film contributed to a smooth perovskite film and good PSC performance. This work develops a fundamental understanding of the thermally induced controlled crystal growth of the parent PbI2 film, which influenced the bridging and intercalation with MAI, resulting in a high-quality perovskite film. This study will guide further development based on two-step processes of perovskite films that have optimal crystallinities and morphologies that contribute to high PSC performance.