Microstructured Porous PbI 2 ‐Enabling 3D/1D Heterojunction Formation in Two‐Step Fabricated Perovskite Solar Cells

Despite the remarkable progress in perovskite solar cells (PSCs), challenges related to film quality and scalability continue to hinder their commercial viability. In particular, the widely used two‐step fabrication method often suffers from incomplete halide precursor infiltration and poor crystallization, leading to suboptimal device performance. In this work, we develop a practical approach that addresses these limitations by combining a microcrystalline porous PbI 2 scaffold with the spontaneous formation of 1D perovskite structures at the top interface. The microcrystalline porous PbI 2 film is prepared by introducing 1‐butyl‐3‐methylimidazolium (BMIM)‐based ionic liquids with different halide anions (I − , Br − , Cl − ), which significantly improve halide diffusion and film uniformity. Among them, BMIMCl stands out by promoting the growth of large, well‐defined 1D crystallites at the surface of the 3D FA/MA‐based perovskite, forming a 3D/1D heterojunction. This structure not only enhances charge extraction and energy level alignment with the transport layers but also improves moisture resistance due to the hydrophobic nature of the 1D overlayer. As a result, the efficiency increases from 21.89% to 24.38%, with improved stability under humid conditions. This study highlights a simple yet effective route to boost both performance and durability in scalable PSC fabrication.