Solvent Dependent Iodide Oxidation in Metal-Halide Perovskite Precursor Solutions

Solar cell absorbing layers made of metal-halide perovskites (MHPs) are usually deposited from solution phase precursors, which is one of the reasons why these materials received huge research attention in the last few years. A detailed knowledge of the solution chemistry is critical to understand the formation of MHP thin films and thus to control their optoelectronic properties and the reproducibility issues that usually affect their synthesis. In this regard, the concentration of triiodide, I3-, is one factor known to have an influence on regulating important aspects such as the particle size in the solution and the defect concentration in the film. In this study, we highlight an underestimated source of I3-, namely the iodide salt solutions ubiquitously employed in MHP synthetic routes, which not only lead to the formation of I3- but also detracts available I- for the MHP synthesis, thus establishing under-stoichiometric conditions. Particularly, we show how the oxidation of I- to I3- changes in time with both the iodide salt counter-cation (K+, CH3NH3+) and the used solvent, meaning that variable quantities of I3- are found depending on the synthesis conditions, with enhanced oxidation found in the γ-butyrolactone (GBL) solvent. Though these differences are generally small, we shed light on a hidden and ever-present reaction which is likely to be related to the overall processing quality of MHP thin films.