van der Waals Mixed Valence Tin Oxides for Perovskite Solar Cells as UV-Stable Electron Transport Materials

Stable electron transport materials (ETMs) with fewer surface defects and proper energy level alignments with halide perovskite active layers are required for efficient perovskite solar cells (PSCs) with long-term durability. Here, two-dimensional van der Waals mixed valence tin oxides Sn₂O₃ and Sn₃O₄ are controllably synthesized and applied as ETMs for planar PSCs. The synthesized Sn₂O₃ and Sn₃O₄ have size of 5-20 nm and disperse well in water as stable colloids for months. Both Sn₂O₃ and Sn₃O₄ exhibit typical n-type semiconductor energy band structures, low trap density, and suitable energy level alignments with halide perovskites. Steady-state power conversion efficiencies (PCEs) of 22.36% and 21.83% are obtained for Sn₂O₃-based and Sn₃O₄-based planar PSCs. In addition, the half cells without hole transport materials and back electrodes show good UV-stability with average PCE of 99.0% and 95.7% for Sn₂O₃-based and Sn₃O₄-based devices remaining after 1000 h of ultraviolet soaking with an intensity of 70 mW cm^-2.