Gradient Doping in Sn–Pb Perovskites by Barium Ions for Efficient Single-Junction and Tandem Solar Cells

Narrow-bandgap (NBG) tin (Sn)–lead (Pb) perovskites generally have a high density of unintentional p-type self-doping, which reduces the charge-carrier lifetimes, diffusion lengths, and device efficiencies. Here, a p–n homojunction across the Sn–Pb perovskite is demonstrated, which results from a gradient doping by barium ions (Ba²⁺). It is reported that 0.1 mol% Ba²⁺ can effectively compensate the p-doping of Sn–Pb perovskites or even turns it to n-type without changing its bandgap. Ba²⁺ cations are found to stay at the interstitial sites and work as shallow electron donor. In addition, Ba²⁺ cations show a unique heterogeneous distribution in perovskite film. Most of the barium ions stay in the top 600 nm region of the perovskite films and turn it into weakly n-type, while the bottom portion of the film remains as p-type. The gradient doping forms a homojunction from top to bottom of the perovskite films with a built-in field that facilitates extraction of photogenerated carriers, resulting in an increased carrier extraction length. This strategy enhances the efficiency of Sn–Pb perovskite single-junction solar cells to over 21.0% and boosts the efficiencies of monolithic perovskite–perovskite tandem solar cells to 25.3% and 24.1%, for active areas of 5.9 mm² and 0.94 cm², respectively.