SnO2/Mg-Doped SnO2 bilayer ETL layer for improving interfacial performance in CsPbBr3 perovskite solar cells

Incomplete surface coverage, localized exposure of ITO protrusions, and interfacial nonuniformity in solution-processed SnO 2 electron transport layers (ETLs) restrict the performance of hole-transport-layer-free (HTL-free) CsPbBr 3 perovskite solar cells (PSCs). Herein, a planar homojunction SnO 2 /Mg-doped SnO 2 bilayer ETL was fabricated by depositing an ultrathin Mg-doped SnO 2 layer on the SnO 2 surface to regulate the ETL/perovskite interface in an ITO/SnO 2 /CsPbBr 3 /carbon device architecture. This bilayer configuration improves surface coverage, interfacial contact, and energy-level alignment while preserving lattice compatibility within the SnO 2 -based system. The Mg-doped SnO 2 top layer reduces surface cracks and pits, limits local ITO exposure, decreases roughness, and improves precursor wettability. These changes promote the formation of CsPbBr 3 films with larger grains, fewer pinholes, better vertical morphology, higher crystallinity, and a near-stoichiometric composition. The PL, TRPL, EIS, dark J–V, XPS, and UPS results collectively indicate improved interfacial charge extraction, reduced defect-assisted non-radiative recombination, and more improved energy-level alignment at the ETL/perovskite interface. Consequently, the optimized HTL-free device achieved a champion PCE of 8.74% with a Voc of 1.520 V and retained 90.69% of its initial efficiency after 50 days of unencapsulated storage in air. This work indicates that introducing an ultrathin Mg-doped SnO 2 overlayer effectively reconstructs the ETL surface and regulates the ETL/perovskite interface, providing an effective interfacial regulation strategy for developing efficient and stable HTL-free all-inorganic PSCs.