Low-bandgap mixed tin-lead iodide perovskite absorbers with long carrier lifetimes for all-perovskite tandem solar cells

Tandem solar cells using only metal-halide perovskite sub-cells are an attractive choice for next-generation solar cells. However, the progress in developing efficient all-perovskite tandem solar cells has been hindered by the lack of high-performance low-bandgap perovskite solar cells. Here, we report efficient mixed tin-lead iodide low-bandgap (similar to 1.25 eV) perovskite solar cells with open-circuit voltages up to 0.85V and over 70% external quantum efficiencies in the infrared wavelength range of 700-900 nm, delivering a short-circuit current density of over 29 mAcm(-2) and demonstrating suitability for bottom-cell applications in all-perovskite tandem solar cells. Our low-bandgap perovskite solar cells achieve a maximum power conversion efficiency of 17.6% and a certified efficiency of 17.01% with a negligible current-voltage hysteresis. When mechanically stacked with a similar to 1.58 eV bandgap perovskite top cell, our best all-perovskite 4-terminal tandem solar cell shows a steady-state efficiency of 21.0%.