Exploring Isovalent Substitution of Lead with Nickel in Methylammonium Lead Iodide Perovskites

We replaced lead ions with nickel ions in methylammonium lead triiodide (MAPbI3) perovskites and studied their electronic and photophysical properties. We synthesized thin films using solutions containing methylammonium iodide (MAI), PbI2, and NiI2 with varying Pb/Ni precursor ratios. We show that MAPbI3 retains its three-dimensional perovskite structure in the presence of Ni2+. We were able to incorporate up to 30% Ni2+ before we note the appearance of unconverted NiI2 via X-ray diffraction. Although the structures of NiI2 and PbI2 are isostructural and the metal ions have the same oxidation states, the bulk material did not behave as a solid solution. Furthermore, the addition of Ni2+ thoroughly quenched the emission of MAPbI3, suggesting that Ni2+ may act as a recombination center for excited charge carriers. Additionally, the materials displayed significant instability towards water. Thus, we conclude that there is a limited application for nickel and perhaps other transition metal ions as a replacement ion for lead in thin-film perovskite photovoltaic devices fabricated in these conditions.