Reducing Saturation-Current Density to Realize High-Efficiency Low-Bandgap Mixed Tin-Lead Halide Perovskite Solar Cells

The unsatisfactory performance of low-bandgap mixed tin (Sn)-lead (Pb) halide perovskite subcells has been one of the major obstacles hindering the progress of the power conversion efficiencies (PCEs) of all-perovskite tandem solar cells. By analyzing dark-current density and distribution, it is identified that charge recombination at grain boundaries is a key factor limiting the performance of low-bandgap mixed Sn-Pb halide perovskite subcells. It is further found that bromine (Br) incorporation can effectively passivate grain boundaries and lower the dark current density by two-three orders of magnitude. By optimizing the Br concentration, low-bandgap (1.272 eV) mixed Sn-Pb halide perovskite solar cells are fabricated with open-circuit voltage deficits as low as 0.384 V and fill factors as high as 75%. The best-performing device demonstrates a PCE of >19%. The results suggest an important direction for improving the performance of low-bandgap mixed Sn-Pb halide perovskite solar cells.