Crystallinity and Phase Control in Formamidinium‐Based Dion–Jacobson 2D Perovskite via Seed‐Induced Growth for Efficient Photovoltaics

2D perovskites based on Formamidinium (FA) hold the potential for excellent stability and a broad absorption range, making them attractive materials for solar cells. However, FA‐based 2D perovskites produced via one‐step processing exhibit poor crystallinity and random quasi‐quantum wells (QWs), leading to subpar photovoltaic performance. In this study, a seed‐induced growth approach is introduced employing MAPbCl3 and BDAPbI4 in the deposition of FA‐based Dion‐Jacobson 2D perovskite films. This method yields high‐quality perovskite films as the seeds preferentially precipitate and serve as templates for the epitaxial growth of FA‐based counterparts, effectively suppressing the δ phase. Moreover, the epitaxial growth facilitated by uniformly dispersed seeds results in simultaneous crystallization from top to bottom, efficiently mitigating random phases (n = 2, 3, 4…) induced by the diffusion of organic cations and, in turn, minimizing energy loss. The impact of seed‐induced growth on the crystallization and phase distribution of FA‐based 2D perovskites is systematically investigated. As a result, the optimized FA‐based 2D perovskite solar cell delivers an outstanding efficiency of 20.0%, accompanied by a remarkable fill factor of 0.823. Additionally, the unencapsulated device demonstrates exceptional stability, maintaining 98% of its initial efficiency after 1344 h of storage. This study presents a seed‐induced growth approach employing MAPbCl3 and BDAPbI4 in the deposition of FA‐based Dion‐Jacobson 2D perovskite films. This method yields high‐quality perovskite films as the seeds serve as templates for the epitaxial growth of FA‐based counterparts, effectively suppressing the δ phaseand mitigating random phases. As a result, the optimized device delivers an outstanding efficiency of 20.0%.