Interface engineering of cadmium-free Sn1-xGexO buffer layer for enhanced open-circuit voltage in Sb2Se3 solar cells

• Sn 1-x Ge x O proposed as a novel buffer for Cd-free Sb 2 Se 3 solar cells. • Optimal Ge/(Sn + Ge) = 0.2 yields superior band alignment with Sb 2 Se 3 absorber. • Wide bandgap of Sn 1-x Ge x O enhances transparency and boosts J sc . • Flat CBO (ΔE c ≈ +0.01 eV) reduces recombination at the interface. • Simulated efficiency improved by 85% compared to the baseline Sb 2 Se 3 /CdS cell. Antimony selenide (Sb 2 Se 3 ) has attracted increasing attention as a promising absorber material for thin-film photovoltaic technologies owing to its earth abundance, suitable bandgap, strong optical absorption, and anisotropic charge transport. However, device performance is strongly limited by unfavorable energy band alignment at the absorber/buffer interface, which governs carrier extraction and interfacial recombination. Conventional Sb 2 Se 3 solar cells employing cadmium sulfide (CdS) as the buffer layer face two critical drawbacks: parasitic light absorption due to its relatively narrow bandgap and environmental concerns associated with cadmium toxicity. To overcome these limitations, we propose tin germanium oxide (Sn 1-x Ge x O) as a cadmium-free buffer layer with a tunable band structure and a wide bandgap. By adjusting the Sn/Ge ratio, the conduction band alignment with Sb 2 Se 3 can be precisely engineered. The results indicate that Sb 2 Se 3 /Sn 0.8 Ge 0.2 O yields a nearly ideal conduction band offset (CBO) of + 0.01 eV, which effectively suppresses electron backflow and minimizes interfacial recombination. In contrast, the Sb 2 Se 3 /CdS junction exhibits a large cliff-type offset of −0.4 eV, resulting in substantial V oc losses. The optimized Sb 2 Se 3 /Sn 0.8 Ge 0.2 O device achieves a power conversion efficiency of 18.68%, surpassing CdS-based counterparts. These findings highlight Sn 1-x Ge x O as a non-toxic, wide-bandgap buffer and underscore the importance of interfacial band engineering for advancing Sb 2 Se 3 solar cells.