We subsequently performed an aqueous recrystallization of all five PbI2 sources to remove impurities that might alter the measured stoichiometry. As shown in Fig. , the variation in I:Pb ratio was successfully reduced among the newly recrystallized (RC) PbI2 reagents. Most significant, the I:Pb ratio of the two outliers, SA2 and AA5, became statistically equivalent and much closer to two. Devices were fabricated from the recrystallized PbI2, and the measured SPO in Fig. shows efficiency no longer correlates to the new stoichiometry. However, though it is significantly less obvious, the performance trend resembles that observed for the as-received materials. Note the recrystallized device data includes SA5 that became soluble after recrystallization, and the solubility of AA4 also improved slightly, which is a consistent improvement observed by several groups. This experiment illustrates that I:Pb ratio of the PbI2 reagent has some influence on the perovskite quality, but it is not a reliable indicator of device efficiency. Device level metrics are likely dependent on the specific impurities that alter I:Pb ratio, so we turned to more in-depth characterization techniques to reveal the identity of the dominant impurity species. Using 1 H-NMR, we identified OAc as a major anionic impurity in as-received SA2 (Fig. ) which we confirmed by spiking with lead acetate trihydrate and additional C-NMR (inset).