Controlling PbI2 stoichiometry during synthesis to improve the performance of perovskite photovoltaics

Over the past decade, remarkable progress has advanced the field of perovskite solar cells to the forefront of thin film solar technologies. The stoichiometry of the perovskite material is of paramount importance as it determines the optoelectronic properties of the absorber and hence the device performance. However, little published work has focused on the synthesis of fully stoichiometric precursor materials of high purity and at high yield. Here, we report a low-cost, energy-efficient, and solvent-free synthesis of the lead iodide precursor by planetary ball milling. With our synthetic approach, we produce low-oxygen, single or multiple polytypic phase PbI2 with tunable stoichiometry. We determine the stoichiometry and the polytypes present in our synthesized materials and further compare them to commercially available materials, using X-ray diffraction, X-ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy. Both the stoichiometric PbI2 we synthesized and a substoichiometric commercially available PbI2 (where the iodide content is below the optimum Pb:I atomic ratio of 1:2) were used to grow methylammonium lead iodide microcrystals (which corrects the iodide content). Perovskite solar cells were then produced using stoichiometric and substoichiometric PbI2 mixed with an equimolar amount of methylammonium iodide and compared to devices produced from re-dissolved microcrystals. The photoactive perovskite layer deposition was processed in air, enabled by the use of a single low-toxicity solvent (dimethyl sulfoxide) combined with vacuum-assisted solvent evaporation. We find that the device performance is strongly dependent upon the stoichiometry of the lead iodide precursor, reaching champion efficiencies over 17%, with no obvious correlation with its polytypic phases. This work highlights the critical role of PbI2 stoichiometry in hybrid perovskites as well as demonstrating synthesis methods and perovskite layer fabrication protocols suitable for low-cost solar energy harvesting.