Vapor Deposition of Hybrid Halide Perovskites in Ultra-High Vacuum: Challenges Revealed by Probing the Organization Dynamics in Real Time

With record efficiencies and reduced costs, hybrid halide perovskite solar cells could revolutionize photovoltaics, provided that the crippling instability problem is resolved. To reveal the relationship between structural defects and instability, vacuum deposition emerges as a promising alternative to the prevalent solution method. In addition to responding to the major upscaling issue, the vacuum approach offers greater control over the film growth. Vacuum evaporation could also help elucidate the puzzle regarding the link between morphology and optoelectronic properties. Paradoxically, layers prepared in solution, polycrystalline by nature and containing numerous defects, have so far achieved better performances than layers prepared in a vacuum, although the latter show better crystalline quality. Here, we describe an original approach, setting the conditions for the best possible control of layer growth. Deposition is performed in ultrahigh vacuum (UHV) and the growth dynamics characterized in real-time by grazing incidence fast atom diffraction. Applied to the prototypical system CH₃NH₃PbI₃, we demonstrate that chemical reaction between the coevaporated precursors, CH₃NH₃I and PbI₂, cannot take place at room temperature. In sequential mode, pure PbI₂ layers of high crystalline quality cannot be converted to CH₃NH₃PbI₃ by subsequent evaporation of CH₃NH₃I. These findings raise fundamental questions regarding the role of contaminants from the residual gas when CH₃NH₃PbI₃ layers are deposited in high vacuum conditions with unbaked vessels. These results, therefore, also question recent molecular dynamics models describing the reaction steps between PbI₂ layers and CH₃NH₃I molecules that intercalate in the van der Waals space of the inorganic network.