Tailoring metal halide perovskites through metal substitution: influence on photovoltaic and material propertiesElectronic supplementary information (ESI) available: Contains details of materials and experimental methods, photovoltaic performance metrics for mixed-metal perovskite solar cells, champion current-voltage characteristics from each mixed-metal composition considered in the screening study, representative perovskite film thicknesses, scanning electron micrographs of representative mix

We present herein an experimental screening study that assesses how partially replacing Pb in methylammonium lead triiodide perovskite films with nine different alternative, divalent metal species, B′ = {Co, Cu, Fe, Mg, Mn, Ni, Sn, Sr, and Zn}, influences photovoltaic performance and optical properties. Our findings indicate the perovskite film is tolerant to most of the considered homovalent metal species with lead-cobalt compositions yielding the highest power conversion efficiencies when less than 6% of the Pb 2+ ions are replaced. Through subsequent materials characterisation, we demonstrate for the first time that partially substituting Pb 2+ at the B-sites of the perovskite lattice is not restricted to Group IV elements but is also possible with at least Co 2+ . Moreover, adjusting the molar ratio of Pb:Co in the mixed-metal perovskite affords new opportunities to tailor the material properties while maintaining stabilised device efficiencies above 16% in optimised solar cells. Specifically, crystallographic analysis reveals that Co 2+ incorporates into the perovskite lattice and increasing its concentration can mediate a crystal structure transition from the cubic to tetragonal phase at room-temperature. Likewise, Co 2+ substitution continually modifies the perovskite work function and band edge energies without either changing the band gap or electronically doping the intrinsic material. By leveraging this orthogonal dimension of electronic tunability, we achieve remarkably high open-circuit voltages up to 1.08 V with an inverted device architecture by shifting the perovskite into a more favourable energetic alignment with the PEDOT:PSS hole transport material. Mixed-metal compositional screening identifies Co 2+ as capable of partial B-site substitution in CH 3 NH 3 PbI 3 , which supplies new dimensions of material tunability.