Unveiling Crystal Orientation in Quasi-2D Perovskite Films by In Situ GIWAXS for High-Performance Photovoltaics.

Quasi-2D perovskites are enchanting alternative materials for solar cells due to their intrinsic stability. The manipulation of crystal orientation of quasi-2D perovskites is indispensable to target efficient devices, however, the origin of orientation during the film fabrication process still lacks in-depth understanding and convincing evidence yet, which hinders further boosting the performance of photovoltaic devices. Herein, the crystallizing processes during spin-coating and annealing are probed by in situ grazing-incidence wide-angle X-ray scattering (GIWAXS), and the incident-angle-dependent GIWAXS is conducted to unveil the phase distribution in the films. It is found that undesirable lead iodide sol-gel formed intermediate phase would disturb oriented crystalline growth, resulting in random crystal orientation in poor quasi-2D films. A general strategy is developed via simple additive agent incorporation to suppress the formation of the intermediate phase. Accordingly, highly oriented perovskite films with reduced trap density and higher carrier mobility are obtained, which enables the demonstration of optimized quasi-2D perovskite solar cells with a power conversion efficiency of 15.2% as well as improved stability. This work paves a promising way to manipulate the quasi-2D perovskites nucleation and crystallization processes via tuning nucleation stage.