Surface Template Realizing Oriented Perovskites for Highly Efficient Solar Cells

Abstract Formamidinium lead iodide (FAPbI 3 ) perovskite films, ensuring optically active phase purity with uniform crystal orientation, are ideal for photovoltaic applications. However, the optically active α‐FAPbI 3 phase is easy to degrade into δ‐phase due to numerous defects within randomly oriented films. Here, a “quasi‐2D” perovskite template is pre‐deposited on the film surface within the crystallization process based on the two‐step preparation technology, which directly induced pure and highly orientated crystallization of α‐FAPbI 3 across the downward growth process. Furthermore, the enlarged interaction between 2D components with colloidal properties and lead iodide delayed the crystallization process effectively, yielding high crystallinity with low trap state density. The resulting perovskite photovoltaic devices exhibited a champion efficiency as high as 25.79% with comprehensively improved device stability. This work provides new insights into the utilization of 2D components and the formation mechanism behind 2D perovskites.