Controlled Crystallization and Enhanced Performance of γ-CsPbI Perovskite Through Methylammonium Iodide-Assisted Coevaporation.

Cesium lead triiodide (CsPbI) perovskites have garnered significant attention owing to their suitable bandgap for tandem silicon substrates and excellent chemical stability. However, γ-CsPbI prepared via low-temperature co-evaporation is limited by a narrow black phase processing window and random crystal orientation, hindering its optoelectronic performance and industrial applications. This study introduced trace amounts of methylammonium iodide (MAI) into the co-evaporation system, enhancing the crystallization process, promoting columnar grain growth, and stabilizing the γ-phase perovskite, resulting in films with improved structural integrity and reduced defect density. The optimal Pb/Cs ratio for achieving the best photoelectric performance shifted from 1:1 to 1.1:1 in the presence of MAI. Additionally, the incorporation of MAI allowed for more efficient longitudinal carrier transport, as evidenced by the enhanced photoluminescence (PL) intensity. The bandgap of CsPbI remained approximately at 1.7 eV before the δ-phase transition, ensuring suitability for photovoltaic applications. Ultimately, a photovoltaic device with 12% efficiency is achieved in the p-i-n structure without additional post-annealing of the CsPbI perovskite films, demonstrating the practical benefits of MAI incorporation.