Analytical Evaluation of Lead Iodide Precursor Impurities Affecting Halide Perovskite Device Performance

Mirroring established semiconductor technologies, using higher quality reagents to synthesize halide perovskite materials results in improved optoelectronic performance. In this study, we selected five different commercial PbI2 sources of various purities and fabricated solar cells in three different perovskite composition-device architecture combinations. In all cases, we observed similar device performance correlations to the PbI2 reagent source across the different processing recipes and architectures. We then employed a suite of analytical characterization techniques to determine the identity and concentration of impurities within the PbI2 reagents that affected the device performance trends. Many impurities were observed; some remain unidentified, but it was possible to single out acetate (OAc) and potassium (K) as key species displaying the largest variation in concentrations among the as-received PbI2. Acetate was identified as a detrimental impurity, while K impurities may be advantageous as suggested by the previous literature on alkali cation additives. A simple aqueous recrystallization successfully decreased the concentration of many impurities, and the results from devices fabricated from recrystallized PbI2 reagents are interpreted in terms of their new impurity profiles. This work significantly contributes to the list of known impurities in perovskite reagents that researchers should be aware of, and we propose improved purification methods for perovskite precursors will further benefit device performance, run-to-run, and batch-to-batch reproducibility.