Decoupling Nucleation and Growth of Crystal for DMSO‐Free Self‐Assembly Tin Perovskite Films and Their Optoelectronic Application

Abstract Tin (Sn)‐based perovskites show significant potential in lead‐free perovskite optoelectronics. Currently, the spin‐coating method combining DMSO co‐solvent and antisolvent‐dropping has been adopted to produce Sn‐based perovskite films. However, DMSO intrinsically oxidizes Sn ²⁺ while fast antisolvent‐dropping causes serious coupling between crystal nucleation and growth, leading to the easy formation of defects and poor stability of Sn‐based perovskite films. Herein, hydrazine acetate (HAAc) ionic salt, possessing strong coordination ability with Sn ²⁺ , is developed to stabilize Sn ²⁺ and decouple the crystallization by promoting pre‐formed crystals (PFCs) in precursor solution and enabling the self‐assembly of PFCs during spin‐coating. The HA _x FA _1‐x SnI ₃ films fabricated by this PFCs self‐assembly technique show tunable bandgap, low defect density, and oriented crystals, producing optoelectronic devices with decent photovoltaic and electroluminescence performance. The DMSO‐free one‐step film‐forming enabled by HAAc‐assisted crystallization decoupling can open up new avenues for facile and low‐cost manufacturing of efficient and stable optoelectronic devices adopting Sn‐based perovskite films.