Precursor Solutions. In a nitrogen-filled glove box, methylammonium iodide (158 mg, 1 mmol) and PbI2 (461 mg, 1 mmol) were added to a vial followed by 1 mL of a co-solvent containing anhydrous dimethylformamide (DMF) and dimethylsulfoxide (DMSO) (v:v 8:2). For mixtures of ions, up to 0.5 mmol NiI2 (50% Ni 2+ ) was added to the PbI2 solution in various ratios while maintaining the total metal halide salt content at 1 mmol. For example, the 30% Ni 2+ mixture was prepared by adding 0.7 mmol of PbI2 and 0.3 mmol of NiI2 to total 1 mmol. We also experimented with 2-fold decrease in precursor concentrations (0.5 mmol MAI and 0.5 mmol NiI2/PbI2), and we observed similar structure and optical properties in these films. The solutions were prepared based on the procedure reported by Siegler et.al. The solutions were stirred on a hot plate at 110 °C for 2 h. DMSO solutions containing NiI2 appeared light green in color, consistent with solvated Ni 2+ ions. The solutions were kept hot until just before film preparation, since removing them from the heat-induced precipitation of the NiI2 salt. Solutions were prepared directly before spin coating, as old solutions appeared to degrade over time resulting in X-ray diffraction patterns with impurity phases.