Dopant-Free, Amorphous–Crystalline Heterophase SnO₂ Electron Transport Bilayer Enables >20% Efficiency in Triple-Cation Perovskite Solar Cells

Improving the ohmic contact and interfacial morphology between an electron transport layer (ETL) and perovskite film is the key to boost the efficiency of planar perovskite solar cells (PSCs). In the current work, an amorphous–crystalline heterophase tin oxide bilayer (Bi-SnO₂) ETL is prepared via a low-temperature solution process. Compared with the amorphous SnO₂ sol–gel film (SG-SnO₂) or the crystalline SnO₂ nanoparticle (NP-SnO₂) counterparts, the heterophase Bi-SnO₂ ETL exhibits improved surface morphology, considerably fewer oxygen defects, and better energy band alignment with the perovskite without sacrificing the optical transmittance. The best PSC device (active area ≈ 0.09 cm²) based on a Bi-SnO₂ ETL is hysteresis-less and achieves an outstanding power conversion efficiency of ≈20.39%, which is one of the highest efficiencies reported for SnO₂-triple cation perovskite system based on green antisolvent. More fascinatingly, large-area PSCs (active areas of ≈3.55 cm²) based on the Bi-SnO₂ ETL also achieves an extraordinarily high efficiency of ≈14.93% with negligible hysteresis. The improved device performance of the Bi-SnO₂-based PSC arises predominantly from the improved ohmic contact and suppressed bimolecular recombination at the ETL/perovskite interface. The tailored morphology and energy band structure of the Bi-SnO₂ has enabled the scalable fabrication of highly efficient, hysteresis-less PSCs.