Controlling Preferred Grain Orientation in Vapor-Deposited Metal-Halide Perovskite Thin Films

Metal-halide perovskites are promising semiconductor materials for light-absorber layers in solar cells due to their optimized band gap, defect tolerance, and relative ease of synthesis. In this work, we demonstrate the viability of vapor transport deposition (VTD), a solvent-free processing method, to realize polycrystalline films of both lead iodide (PbI₂) and methylammonium lead iodide (MAPbI₃) perovskites showing preferential crystallographic orientation. Notable is the difference in perovskite film texturing observed for sequentially and co-deposited VTD films of MAPbI₃, as well as the lack of strong preferential orientation in solution-processed MAPbI₃ films. While sequential deposition of the perovskite precursors PbI₂ and methylammonium iodide (MAI) leads to a MAPbI₃ film with a limited preferential grain orientation, co-deposited MAPbI₃ films mirror the orientation of excess PbI₂ present in the film. The latter is confirmed by the similar dependence of preferential orientation for PbI₂ and co-deposited MAPbI₃ on underlayer roughness, as well as the emergence of preferential orientation in MAPbI₃ only when excess PbI₂ is present. Both MAPbI₃ and PbI₂ films show a strong preferred lattice orientation on smooth substrates, while on rougher substrates, this orientation is disrupted. These results are interpreted as excess PbI₂ frustrating the diffusion of MAI in the film, which would otherwise disrupt the preferred orientation, pointing to a notable difference in lattice orientation arising from processing technique and conditions.