Tin‐Lead Perovskite Fabricated via Ethylenediamine Interlayer Guides to the Solar Cell Efficiency of 21.74

Tin‐lead perovskite solar cells (PSCs) show inferior power conversion efficiency (PCE) than their Pb counterparts mainly because of the higher open‐circuit voltage (Voc) loss. Here, it is revealed that the p‐type surface of perovskite transforms to n‐type, based on post‐treatment by a Lewis base, ethylenediamine. This approach forms a graded band structure owing to the rise of the Fermi‐energy level at the surface of the perovskite layer, and increases the built‐in potential from 0.56 to 0.76 V, which increases the Voc by more than 100 mV. It is demonstrated that EDA can lower the defect density (Sn4+ amount) by screening perovskite against oxygen, and by bonding with undercoordinated Sn on the surface. This study further explores the role of Br anion inclusion in the perovskite lattice from the viewpoint of reducing the lattice strain and Urbach energy. Finally, a high Voc of 0.86 V is obtained, corresponding to a voltage deficit of 0.39 V, using a perovskite absorber with a bandgap of 1.25 eV and the highest PCE (21.74%) reported so far for Sn‐Pb PSCs is achieved. Ethylenediamine (EDA) coating changes the p‐type tin‐lead perovskite to n‐type, increases the built‐in potential, and decreases the open‐circuit voltage (Voc) loss in perovskite solar cells. With Br inclusion into the lattice and passivation by EDA, the highest power conversion efficiency of 21.74% and Voc of 0.86 V is achieved using Cs0.025FA0.475MA0.5Sn0.5Pb0.5I2.975Br0.025 perovskite film with a bandgap of 1.25 eV.