Grain Boundary Encapsulation for Efficient Perovskite Solar Cells via PbI ₂ ‐Derived Low‐Dimensional Structure

ABSTRACT The excess of residual PbI 2 and the grain boundary defects in perovskite films limit the performance and stability of perovskite solar cells (PSCs). This study proposes a strategy for secondary phase transformation of residual PbI 2 and grain boundary encapsulation in perovskite films based on the 4‐pyridinecarboximidamide hydrochloride (4‐FAPyCl 2 ) additive. By introducing 4‐FAPyCl 2 into the PbI 2 precursor solution, a quasi‐2D perovskite coating of the grain boundaries was successfully constructed, achieving effective protection of the 3D α‐FAPbI 3 perovskite. This strategy not only significantly increases the perovskite grain size and eliminates pinholes but also promotes the formation of a 2D/3D heterojunction encapsulation structure at the grain boundaries, effectively passivating defects, suppressing non‐radiative recombination, and relieving residual stress. The optimized n‐i‐p planar PSCs achieved a champion power conversion efficiency (PCE) of 25.32 %. Besides, PSCs prepared under natural humid air conditions exhibit a PCE loss < 3 %. Meanwhile, due to the enhanced hydrophobicity and phase stability of optimized perovskite films, the PSCs exhibited excellent long‐term humidity environment storage, thermal, and light stability. This work provides an innovative idea for synergistically regulating the photoactive secondary phase of PbI 2 and constructing high‐performance 2D/3D perovskite structures, promoting the development of efficient and stable PSCs.