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.