Ethylamine Iodide Additive Enables Solid‐to‐Solid Transformed Highly Oriented Perovskite for Excellent Photodetectors

Perovskite has been widely applied in the optoelectronic field due to its strong light absorption and high carrier mobility. Maintaining high crystallization is critical to fabricate high‐performance perovskite devices, where many methods have been reported, such as the use of additives in precursor solutions. However, there are few reports for the working mechanism of these additives. Herein, a new method to obtain highly crystalline formamidinium‐lead triiodide (FAPbI3) perovskite films by introducing ethylamine iodide (EAI) into perovskite precursors is reported and a novel working mechanism for such alkyl amine additives in the crystallization process of perovskites is proposed. Unlike traditional liquid‐to‐solid (L–S) crystallization theory, the research results indicate that EAI affects the solid‐to‐solid (S–S) transition process from the intermediate yellow phase to the final black phase, and this mechanism is further verified to be universal using other common alkyl amines. A self‐powered photodetector based on an as‐fabricated FAPbI3 film is fabricated with an external quantum efficiency of over 90%. This work provides a deeper understanding of the perovskite crystallization process. Ethylamine iodide (EAI) is employed to modify pure FAPbI3 film, and a novel solid‐to‐solid transition theory is raised to depict the effect of EAI on the perovskite crystallization. The photodetector based on the modified film shows high performance.