Interfacial Defect Passivation for High-Performance n–i–p Less Lead Perovskite Solar Cells

Abstract

Metal halide perovskites are promising photovoltaic absorbers due to their strong optoelectronic properties and defect-tolerant electronic structure. However, n–i–p Sn–Pb perovskites exhibit pronounced interfacial instabilities that induce defect formation, hinder charge extraction, and accelerate Sn oxidation under thermal and environmental stress, ultimately limiting both efficiency and operational durability. To address these challenges, this dissertation proposes an interfacial engineering strategy comprising three complementary strategies: buried-interface stabilization, thermally resilient top surface passivation, and additive-driven surface modification for Sn–Pb and Sn-based perovskite systems. The first strategy focuses on controlling the buried SnO2/perovskite interface through thermally oriented fullerene passivation. Conventional phenyl-C61-butyric acid methyl ester (PCBM) layers suffer from disordered molecular packing and poor resistance to polar solvents, resulting in partial dissolution during perovskite deposition and the generation of structural voids. By annealing PCBM above its glass transition temperature, this work achieves a highly ordered PCBM interlayer that possesses enhanced solvent resistance and improved electronic percolation pathways. The oriented fullerene network effectively suppresses interfacial recombination by preventing direct chemical interactions between SnO2 and the perovskite precursor, thereby reducing oxygen-related defect formation at the buried interface. Devices incorporating high-temperature-annealed PCBM exhibit enlarged perovskite grain sizes, reduced trap-filled limit voltages, and more balanced charge transport, leading to improved operational reproducibility and significantly enhanced thermal durability. These results demonstrate that buried-interface stabilization is an essential prerequisite for overcoming the intrinsic instabilities of n–i–p Sn–Pb devices. The second strategy addresses the thermally vulnerable top perovskite surface by introducing an ionic-liquid PEAFo passivation layer. Unlike phenethylammonium iodide (PEAI), which induces quasi-2D surface crystallization, PEAFo forms a uniform, amorphous coordination layer without generating insulation 2D domains. The formate anion strongly binds to undercoordinated Sn2+ and Pb2+, effectively eliminating dominant trap states and markedly suppressing Sn oxidation that typically accelerates at elevated temperatures. Owing to its ionic-liquid characteristics, PEAFo undergoes mild softening and partial volatilization during thermal processing, enabling surface reconstruction that ensures complete coverage while preventing the accumulation of residues. The n–i–p Sn–Pb devices treated with PEAFo achieve a champion efficiency of 18.5% and retain 90% of their initial efficiency after 500 h at 85°C, establishing one of the highest intrinsic thermal-stability benchmarks reported for n–i–p Sn–Pb perovskite solar cells. The third strategy extends surface defect passivation to Sn-only perovskites by introducing a fluorinated benzylammonium iodide derivative (F3-BAI) that undergoes surface-selective crystallization due to its relatively low solubility. Rather than remaining dissolved in the precursor solution and distributing throughout the bulk, the additive rapidly reaches supersaturation during the antisolvent step and nucleates preferentially at the film surface. This crystallization sequence, combined with the tendency of the -CF3 group to orient toward the air interface, leads to its enrichment at the growing surface instead of incorporation into the bulk lattice. The resulting surface-localized molecular layer stabilizes surface Sn2+ against oxidation and provides hydrophobic protection against moisture. As a result, F3-BAI substantially improves resistance to moisture- and oxygen-induced degradation, demonstrating an interfacial passivation pathway that extends the overall interface engineering strategy to highly reactive Sn-based perovskites. Collectively, the three approaches presented in this dissertation demonstrate that interfacial defect passivation is a decisive factor governing efficiency, reproducibility, and long-term stability in Sn- containing perovskite solar cells. Buried-interface stabilization through thermal fullerene alignment, thermally resilient top surface reconstruction via ionic-liquid formate coordination, and solubility- governed surface passivation enabled by F3-BAI together establish a coherent and generalizable strategy for enhancing the durability of n–i–p Sn–Pb and Sn-only perovskite solar cells. These findings provide fundamental insights and practical design rules for advancing low-bandgap perovskite photovoltaics toward stable, high-performance device operation.