Compositional engineering of tin-lead halide perovskites for efficient and stable low band gap solar cells

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.