Self‐Driven Prenucleation‐Induced Perovskite Crystallization Enables Efficient Perovskite Solar Cells

Abstract Perovskite film with high crystal quality is fundamental to achieving high‐performance solar cells. A fast nucleation process is crucial to improving the crystallization quality. Here, we propose a self‐driven prenucleation strategy to achieve fast nucleation. This is realized through rational solvent design. The key characteristics of different solvents are systematically evaluated. Among them, formamide, with ultra‐high dielectric constant, low Gutman donor number, and a high boiling point, is selected as the co‐solvent. These unique characteristics render formamide a double‐face solvent that is a good solvent for formamidinium iodide (FAI) and CsI while a poor solvent for PbI 2 . As a result, formamide induces the self‐driven prenucleation of PbI 2 ‐DMSO seeding crystals and accelerates the nucleation, improving the crystalline quality of perovskite film. The efficiency of the hole transport layer‐free carbon‐based perovskite solar cells is boosted beyond 19 % for the first time.