Solvent Dependent Iodide Oxidation in Metal-Halide Perovskite Precursor Solutions

Solar cells 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 a huge research boost in the last years. A detailed knowledge of the solution chemistry is critical to understand the formation of MHPs thin films and thus to control their optoelectronic properties, as well as the reproducibility issues that usually affect their synthesis. In this regard, the concentration of triiodide, I3-, is one factor known to have an influence, regulating important aspects such as particles size in the solution and defects concentration in the film. In this study, we highlight an underestimated source of I3-, namely the iodide salts solutions ubiquitously employed in MHPs synthetic routes, which not only bring to formation of I3- but also subtracts available I- for the MHPs 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 with the overall processing quality of MHPs thin films.