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

Abstract 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.