Multifunctional lignin for defect passivation and energy-level modulation of SnO2 electron transport layers in perovskite solar cells

SnO 2 is the preferred choice for the electron transport layer in perovskite solar cells (PSCs)due to its high electron mobility, high transparency, excellent chemical stability, and low-temperature fabrication process. However, its commercial implementation is frequently plagued by detrimental interface traps, unfavorable energy level alignment, and sluggish nucleation kinetics, which collectively hinder the realization of high-performance, durable PSC devices. In this study, a functionalized lignin derivative, 5-sulfosalicylic acid-modified lignin (SAL), as an efficient interfacial modifier for the SnO 2 to regulate the colloidal kinetics, passivate interfacial defects, and optimize the band-level alignment for efficient charge extraction. The introduction of abundant sulfonic and carboxyl groups significantly enhances the colloidal stability of SnO 2 through strengthened electrostatic repulsion. These functional groups passivate oxygen vacancies not only by coordinating with Sn 4+ on the SnO 2 surface but also by coordinating with under-coordinated Pb 2+ and halide vacancies at the perovskite/SnO 2 interface, thereby effectively suppressing non-radiative recombination. Consequently, the SnO 2 -SAL-based devices exhibit a superior power conversion efficiency (PCE) of 23.25% with negligible hysteresis under ambient conditions. Furthermore, owing to SAL-induced interfacial passivation and increased hydrophobicity, the unencapsulated devices maintained 75.25% and 90.27% of their initial PCE, respectively, after 600 h of aging under ISOS-D-3 (65°C, 85% RH, dark) and ISOS-L-1 (continuous 1 sun illumination, 25°C, 35% RH). This work provides a sustainable and effective strategy for developing high-performance, stable PSCs through molecular engineering of biomass-derived materials.