Efficient Narrow-Bandgap Mixed Tin-Lead Perovskite Solar Cells via Natural Tin Oxide Doping

Narrow-bandgap (NBG) mixed tin/lead-based (Sn-Pb) perovskite solar cells (PSCs) have attracted extensive attention for use in tandem solar cells. However, they are still plagued by serious carrier recombination due to inferior film properties resulting from the alloying of Sn with Pb elements, which leads to p-type self-doping behaviors. This work reports an effective tin oxide (SnO_x ) doping strategy to produce high-quality Sn-Pb perovskite films for utilization in efficient single-junction and tandem PSCs. SnO_x can be naturally oxidized from tin diiodide raw powders and successfully incorporated into Sn-Pb perovskite films. Consequently, Sn-Pb perovskite films doped with SnO_x exhibit dramatically improved morphology, crystallization, absorption, and more interestingly, upward-shifted Fermi levels. The resulting narrow-bandgap Sn-Pb PSCs with natural SnO_x doping have considerably reduced carrier recombination, therefore delivering a maximum power conversion efficiency (PCE) of 22.16% for single-junction cells and a remarkable PCE of 26.01% (with a steady-state efficiency of 25.33%) for two-terminal all-perovskite tandem cells. This work introduces a facile doping strategy for the manufacture of efficient single-junction narrow-bandgap PSCs and their tandem solar cells.