Carrier lifetimes of >1 ms in Sn-Pb perovskites enable efficient all-perovskite tandem solar cells

All-perovskite-based polycrystalline thin-film tandem solar cells have the potential to deliver efficiencies of >30%. However, the performance of all-perovskite-based tandem devices has been limited by the lack of high-efficiency, low-band gap tin-lead (Sn-Pb)mixed-perovskite solar cells (PSCs).We found that the addition of guanidinium thiocyanate (GuaSCN) resulted inmarked improvements in the structural and optoelectronic properties of Sn-Pb mixed, low-band gap (~1.25 electron volt) perovskite films.The films have defect densities that are lower by a factor of 10, leading to carrier lifetimes of greater than 1 microsecond and diffusion lengths of 2.5 micrometers.These improved properties enable our demonstration of >20% efficient low-band gap PSCs.When combined with wider-band gap PSCs, we achieve 25% efficient four-terminal and 23.1%efficient two-terminal all-perovskite-based polycrystalline thin-film tandemsolar cells.