New Insights into MAI Additives in 2D‐Assisted 3D Controlled Crystallization Toward High‐Quality α‐Phase FAPbI₃ Perovskites

Abstract The highly oriented 2D perovskite templates of n = 1 have typically been created to attain controllable and oriented crystallization of 3D α‐phase formamidinium lead triiodide (α‐FAPbI₃) perovskites. However, the role of methylammonium iodide (MAI), a widely used α‐FAPbI₃ phase stabilizer, in regulating the growth dynamics of 2D/3D perovskites is generally ignored. Herein, Ruddlesden–Popper type n = 1 2D octylammonium lead iodide (OA₂PbI₄) perovskites are added into FAPbI₃ precursor solution. The template of n = 2 (OA₂MAPb₂I₇), which is spontaneously constructed by the mixture of n = 1 2D and methylammonium chloride (MACl), acts as a skeleton to template the epitaxial growth of α‐FAPbI₃. However, the volatilization of MACl inevitably causes damage to the 2D structure during thermal annealing. This study reveals that small amounts of less volatile MAI additive enables the creation of stable 2D template, leading to more controlled vertical orientation crystallization. Consequently, the high‐quality mixed‐dimensional perovskite film delivers a high efficiency of 24.19% together with improved intrinsic stability. This work provides an in‐depth understanding of 2D‐assisted controlled epitaxial growth of α‐FAPbI₃.