Reductive 2D Capping Layers through Dopamine Salt Incorporation for Pb–Sn Mixed Perovskite Solar Cells

Pb–Sn mixed perovskite is a crucial component in all-perovskite tandem cells. However, it remains a challenging task to fabricate Pb–Sn perovskite solar cells (PSCs) due to the facile oxidation of Sn2+. In this work, we incorporate dopamine cation (DAH+) to create a reductive 2D capping layer on a Pb–Sn perovskite film, simultaneously enhancing the efficiency and stability of Pb–Sn PSCs. Dopamine acts as a strong reducing agent that effectively reduces Sn4+ to Sn2+. The 2D/3D heterojunction induces downward band bending at the perovskite top surface, facilitating carrier separation and extraction. Consequently, the champion PSC achieved a power conversion efficiency (PCE) of 22.31%. Benefiting from the protection of the reductive 2D capping layer, the device's stability is notably enhanced with 86% of the initial PCE retaining after 1000 h operation in N2. This work demonstrates a promising strategy of constructing a multifunctional 2D passivation layer toward high quality Pb–Sn mixed perovskite devices.