Hybrid halide perovskites (HHPs) are being commercialized as solar cells and are being investigated for use in a wide variety of other optoelectronic devices. The most efficient solar cells rely on alloys of the cubic perovskite AMX3 structure, such as (CS.FA,MA)Pb(I,Br)3, where FA and MA stand for formamidinium and methylammonium. HHPs have incredibly versatile structure and consequently, emission color, making them attractive for light emitting diodes or lasers. In addition to the AMX3 structure, HHPs can be made as two-dimensional (2D) materials, whereby large bulky organic cations separate M-X (metal-halide) semiconducting sheets. Tuning the thickness of the M-X layer changes the color of the exciton emission, and thus much effort has been devoted to making optoelectronic devices from 2D perovskites. Generally, HHP-based devices rely on polycrystalline thin films, which poses challenges: polycrystalline thin films contain grain boundaries, exhibit film strain, and are prone to forming undesired crystalline phases.Here, the fundamental structural, ionic and optical properties of HHP thin films are investigated. The first section is dedicated to understanding how film strains, such as those imparted by commercial fabrication procedures, may change sub-grain structure and cause degradation of the HHP. The second section examines phase stability and halide interdiffusion in mixed-halide 3D alloys. The third and fourth sections report phase-pure 2D film fabrication, and examine how strain and residual solvent can turn off certain emission features intrinsic to the 2D phase in question. These results help extend the utility of HHPs for optoelectronic devices by providing design rules for how to grow films with targeted structural and optoelectronic properties
