The most notable impurity we identified with TOF-SIMS was K, which also likely originates from synthesis. Figure shows the integrated K + intensity of the PbI2 materials as-received and after recrystallization (denoted by -RC suffix) highlighting large variation in relative K concentration by 3-4 orders of magnitude among the 10 different samples. The relative SIMS intensities for other alkali cations such as Li, Na, and Rb only varied by ~1 order of magnitude among the PbI2 (Supplementary Figure ), which justifies our focus on K in relation to changes in device performance. As-received, AA5 displays K + intensity 2 orders of magnitude higher than TCI4, the second-highest quality and most pure reagent (see below). Hydrocarbon/carbohydrate (Fig. ), oxide/hydroxide (Fig. ), and nitrogen oxide (Fig. ) SIMS fragment intensities suggest TCI4 and AA5 contain the lowest concentrations of oxygen-containing anion impurities (Fig. fragment intensities further reflect significant OAc impurity in SA2 ). Together, these two measurements on AA5 -low oxidic anion concentrations and I:Pb ratio greater than 2 -imply that KI is the main impurity in this commercial source. For all reagents except AA5, due to I:Pb ≤ 2, the K impurities can be regarded as KOH, KOAc, or KNO3. Potassium iodide is known as a beneficial processing additive for perovskite devices , and residual KI in AA5 plausibly explains the observation that AA5-derived perovskites slightly outperform those processed from TCI4 in all our device data sets (Fig. and Supplementary Fig. and). Finally, recrystallization reduced K + intensity in AA5 by nearly 3 orders of magnitude (Fig. ). The relative decrease in other alkali cations was a maximum of ~ 1 order of magnitude, again suggesting the large drop in K for AA5 is significant. Removing KI from a PbI2 reagent free of anionic impurities is consistent with the observed decrease in ICP-OES stoichiometry (Fig. )