Abstract
Tin-lead halide perovskites show promise as low-band gap absorbers to enable efficient all-perovskite tandem solar cells. While they have reached high efficiencies in single junction and in tandem solar cells, tin-containing perovskites face unique challenges - such as a tendency to degrade by oxidation of tin and sometimes very short carrier lifetimes that limit diffusion lengths. We map oxidation stability and important optoelectronic properties - band gap, carrier lifetime, and solar cell performance - across a wide range of compositions varying the A-site cation and the tin: lead ratio at the B-site. We identify mechanisms by which composition tunes the band gap in ways distinct from what has been reported for pure lead-based perovskites. We show that alloying tin with lead changes the mechanism by which tin is oxidized to a pathway that is relatively unfavourable. This significantly stabilizes the perovskite toward oxidation. Using a low band gap perovskite absorber, we demonstrate a 17.8% efficient single junction solar cell. Further, by employing a sputtered ITO top electrode, we demonstrate continuous operation of a solar cell in air with only a small drop in efficiency that is reversible upon storage in the dark. This work creates a set of guiding principles to intelligently choose the best compositions for making efficient and stable solar cells based on tin-containing low band gap perovskites.