LAUSR

2D perovskites are demonstrated as the growth template for the modulation of phase transition and crystallization of formamidinium lead triiodide (FAPbI3). However, it is challenging to obtain highly oriented α‐FAPbI3 and regulate crystallization at the bottom of the FAPbI3 film. Here, a 2D (BA)2PbI4 layer with preferred orientation is introduced to perform as the bottom template for the quasi‐epitaxial growth of FAPbI3, which triggers the α‐FAPbI3 perovskite formation. Owing to the assembly of [PbI6]4− octahedral on the 2D template, the highly (100) oriented FAPbI3 perovskites from the bottom to the top are attained. At the optimum concentration, 2D perovskites gradually vanish due to the extrusion of BA+ cation from the template, which guarantees the charge transport at the bottom. Consequently, the strategy effectively promotes film quality and suppresses non‐radiative recombination. The efficiency of perovskite solar cells (PSCs) is promoted from 23.06% to 24.78%.