Electron Transport Bilayer with Cascade Energy Alignment for Efficient Perovskite Solar Cells

Energy alignment between electron transport layers (ETLs) and perovskite has a strong influence on the device performance of perovskite solar cells (PSCs). Two approaches are deployed to tune the energy level of ETLs: 1) doping ETLs with aliovalent metal cations and 2) constructing heterojunction bilayers with different materials. However, the abrupt interfaces in the heterojunction bilayers introduce undesirable carrier recombination. Herein, a homojunction bilayer ETL is developed by stacking Sb-doped SnO₂ (Sb-SnO₂) and SnO₂ ETLs via low-temperature spin-coating processes. The energy levels of ETLs are tuned by the incorporation of Sb and altering stacking orders. Bilayer ETL of Sb-SnO₂/SnO₂ with cascade energy alignment promotes the best power conversion efficiency of 20.73%, surpassing single-layer ETLs of SnO₂ (18.23%) and Sb-SnO₂ (19.15%), whereas the SnO₂/Sb-SnO₂ bilayer with barricade energy alignment receives the poorest device performance. The cascade bilayer ETL facilitates charge separation and suppresses carrier recombination in PSCs, which is verified by photoluminescence, conductivity, and impedance characterizations. The homojunction bilayer ETLs with adjustable energy levels open a new direction for interface engineering toward efficient PSCs.