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

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 MoS₂ 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.