Understanding the Role of Solvent-Solute Interactions on Hybrid Organic-Inorganic Perovskite Formation

Hybrid organic-inorganic perovskites (HOIPs) formed from organoammonium iodide and lead iodide precursor solutions are promising materials for photovoltaic applications. While lead polyiodide and lead-solvent complexes formed in solution are intermediates for HOIP crystallization, the influence of solvent choice upon the formation of such intermediates is not well understood. We show that the formation of lead polyiodides in the precursor solutions is correlated with the basicity of the processing solvent (quantified by Gutmann’s donor number, DN). Solvents with low DN exhibit a strong propensity to allow the formation of lead polyhalides. We infer that such solvents interact weakly with lead iodide, which favors the precipitation of HOIP single crystals from solution. Conversely, high-DN solvents suppress the formation of lead polyiodides, indicative of strong lead-solvent coordination. Such solvents support the formation of stable precursor solutions for HOIP thin-film processing and may be added in fractional quantities to tune the basicity of the processing solvent. The tunabilty introduced by high-DN additives provides finer control over perovskite crystallization, post-deposition processability, and the morphology of HOIP active layers for photovoltaic applications.