Surface CH₃NH₃⁺ to CH₃⁺ Ratio Impacts the Work Function of Solution‐Processed and Vacuum‐Sublimed CH₃NH₃PbI₃ Thin Films

Abstract CH₃NH₃PbI₃ thin films are fabricated using several representative synthesis methods such as spin‐coating, evaporation, and a combination of the two. These methods, which frequently occur in reported literatures, use the same precursors PbI₂ and CH₃NH₃I but differ in how the two are mixed. It is found that the latter plays a vital role in determining the surface morphology, composition, and grain size of the films, even when the same stoichiometric ratio of the precursors is used. X‐ray photoelectron spectroscopy reveals that the amount of CH₃⁺‐type defects, which results from CH₃NH₃I dissociation, is sensitive to both the physical state of CH₃NH₃I and the order of mixing sequence. The variation of the CH₃NH₃⁺:CH₃⁺ ratio also affects the valence band and the work function of the corresponding films, as revealed by ultraviolet photoelectron spectroscopy. Furthermore, the energy‐level alignment between the perovskite film and a model hole transport layer, N,N′‐di(1‐naphthyl)‐N,N′‐diphenylbenzidine (NPB) is examined. It is found that the CH₃NH₃⁺:CH₃⁺ ratio correlates with the offsets between the valence band maximum of perovskite film and the highest occupied molecular orbital of NPB as well, and the energy‐level alignment with the dual‐source, coevaporated CH₃NH₃PbI₃ film is most suitable for efficient hole transport.