Challenging Uncooperating Precursors: Adapting an Industrially Attractive Colloidal Approach to Hybrid Perovskites for Photovoltaics

Synthetic hybrid perovskites of general formula APbI3 are taking the stage in the solar energy field for their outstanding optoelectronic properties, solution processability, limited costs of precursors and straightforward synthesis. However, the fabrication of high-quality materials still suffers from reduced outputs, the use of non-benign chemicals and the need for well-controlled conditions, limiting the attractiveness and feasibility for the industry. Our group designed a colloidal approach, carried at room temperature and in laboratory atmosphere, optimizing the procedure for all-inorganic CsPbBr3. The method proved to be reliable on a multigram scale thanks to an industrial dispersing tool and opened to the recycling of waste while providing idoneous nanocrystals for ink formulation. Extension of the protocol to the iodine containing APbI3 phases having applications for outdoor photovoltaic, requires extensive optimization of the method due to different solubility of the precursors as well as the inherent reduced stability of iodine rich phases with respect to CsPbBr3. The two most relevant challenges are the incorporation of methylammonium or formamidinium ions in the place of cesium into the perovskite crystals and the limited solubility of lead(II) iodide in the low boiling point solvents (heptane and 2-PrOH) we previously identified as ideal for the colloidal synthesis approach and recycle. Tuning the concentration of the cation containing precursor solutions as well as carefully controlling the halide stoichiometry are both key to develop viable syntheses. The limited solubility of the Lead(II) precursors is a more challenging issue. On following up with recent literature reports, we identified three strategies: a) replacing PbX2 with salts containing the Pb(II) and the X- species separately and along with spectator counterions; b) introducing complexation agents capable of enhancing the PbI2 solubility in the target solvents and c) operating in excess of X-, aging to increase the solubility of the lead(II) containing species by coordination. All strategies present pluses and minuses and we are also exploring combination of them to exploit possible cooperative effects. Once formed, iodine rich nanocrystals require suitable stabilization. As the game rules about the relation between the nature and solubility of involved actors and the direction toward the target phase are starting to get clearer, we are also exploring stabilizing agents capable of capping the nanocrystal surface, preventing evolution towards lead rich, non-luminescent and generally poorly performing phases.