Synthesis and mixing of complex halide perovskites by solvent-free solid-state methods

Halide perovskites are exciting photoactive semiconductors with exceptional photovoltaic and optoelectronic properties. Literature in this area focuses on solution phase aspects of these materials, for example in inks for solar cells, or the growth of bulk or nanosized crystals. Critically, varying solute-solvent interactions often cause heavily mixed perovskites to have compositions that strongly deviate from their synthetic loading. In contrast, complex halide perovskites prepared by solid-state methods in the absence of solvents display much more predictable compositions and significantly suppressed phase segregation. Further, because they generate less waste, solvent-free methods are often ‘greener’ and more industrially scalable. Herein, we review the solvent-free methods used to synthesize single composition ‘parent’ and heavily mixed perovskites in the solid-state. We discuss the known mechanisms for ion diffusion involved in these transformations, summarize and contrast their main benefits and features, and review their use in the preparation of mixed-cation and/or mixed-halide perovskites.