Electronic and optical properties of mixed Sn-Pb organohalide perovskites: a first principles investigation

Organohalide lead perovskites have attracted considerable interest among emerging photovoltaic technologies, delivering highly efficient solid-state solar cells. Despite the huge potential of this class of materials, the use of Pb-containing systems will likely hamper the wide spread take off of perovskite solar cells. The development of lead-free hybrid perovskites represents thus an important step in the exploitation of this promising technology. Very recently, the use of new mixed Pb-Sn MASn(x)Pb((1-x))I(3) perovskites has been reported, with a considerable increase in the extension of the solar spectrum absorption with respect to lead-halide perovskites, shifting the absorption onset down to the near-IR. In light of the anticipated potential of Sn-based organohalide perovskites in replacing lead-based materials, here we apply a recently developed computational approach to the description of mixed Sn-Pb compounds showing a range of compositions comparable to experimentally characterized compounds. For the investigated series of MASn(x)Pb((1-x))I(3) perovskites we find a continuous and monotonic variation of the energy levels, shifting at lower potentials; and band-gaps, which shift towards the near-IR, as the Sn content in the perovskite is increased. Notably, while we find slightly unbalanced electron/hole transport in the pure phases, Pb (Sn) materials being better electron (hole) transporters, for intermediate compositions an almost perfectly balanced charge carrier transport can be achieved, in line with recent experimental observations.