All films with Ni 2+ content between 0% and 50% showed evidence of 3D perovskite in the tetragonal phase, as seen in the powder X-ray diffraction patterns (Figure ). We observed a broad, amorphous peak in all samples between 21° -27° 2θ resulting from the air-free mylar sample holder. Upon addition of ≥40% Ni 2+ , we observe a new peak at 13.5° 2θ, which corresponds to the (330) plane of NiI2 present as a secondary phase (Figure ). This peak shifts to lower 2θ values upon the incorporation of 50% Ni 2+ may arise due to the poor signal-to-noise of this peak. Films containing ≥40% Ni 2+ displays an additional peak at ~11° 2θ, which is typically attributed to lower dimensional perovskite (1D) phases. Thus, we conclude that the upper limit of Ni 2+ incorporation into the 3D perovskite structure under these conditions is 30%. We note that NiI2 solubility in the precursor solution was relatively low, even at high temperatures. We hypothesize that the 40% and 50% Ni 2+ concentrations likely exceed the solid solubility limit of Ni 2+ substitution in the MAPbI3 films, thus resulting in the formation of NiI2.