Optimized Rear‐Interface Passivation of SnS Thin‐Film Solar Cells Using a Controlled Germanium Oxide Interlayer for Enhanced Photovoltaic Performance

Abstract Tin monosulfide (SnS) holds significant promise as a sustainable, earth‐abundant absorber for thin‐film solar cells (TFSCs), however, device efficiencies remain hindered by detrimental interfacial quality at the rear contact. Defect states, interfacial reactions, and uncontrolled alkali diffusion at the Mo/SnS interface introduce severe recombination losses, limiting photovoltaic (PV) performance. Here, a tailored back‐interface engineering strategy employing a thermally evaporated and controlled oxidized Germanium (germanium oxide GeO _x ) interlayer to passivate the Mo/SnS interface is reported. This compact, chemically stable GeO _x interlayer simultaneously improves absorber morphology, passivates deep‐level defects, suppresses sodium (Na ⁺ ) diffusion from the substrate, and inhibits MoS ₂ formation during thermal processing. Deep‐level transient spectroscopy (DLTS) analysis confirms a significant reduction in mid‐gap donor‐like traps associated with Na‐induced defects, thereby mitigating non‐radiative recombination pathways. These synergistic effects collectively lead to a substantial enhancement in power conversion efficiency, increasing from 3.71% in the control device to 4.81% in the GeO _x ‐modified device having device stack of SLG/Mo (800 nm)/GeO _x (7 nm)/SnS (1500 nm)/CdS (50 nm)/i‐ZnO (50 nm)/AZO (400 nm)/Al (1000 nm). Overall, this study highlights the potential of GeO _x as an effective interfacial modifier for SnS PVs, offering a practical strategy to overcome longstanding limitations in device performance.