Modulation and Direct Mapping of the Interfacial Band Alignment of an Eco-Friendly Zinc-Tin-Oxide Buffer Layer in SnS Solar Cells

In thin-film solar cells (TFSCs), the buffer layer plays a vital role in enhancing the power conversion efficiency (PCE) by improving the charge carrier dynamics. In the present study, a series of eco-friendly Zn1–xSnxOy (ZTO, 0 ≤ x ≤ 1) films grown by atomic layer deposition are used as buffer layers in the SnS-based TFSCs. We systematically investigate the effect of the Zn-to-Sn ratio on the band alignment at the SnS/ZTO heterojunction. The results revealed that by controlling the Zn-to-Sn ratio in the ZTO films, the energy band alignment changed from a “cliff-type” (9%Sn-ZTO41) to a “spike-type” (18%Sn-ZTO11). The spike-type band configuration with appropriate conduction band offset (CBO<0.4 eV) at the SnS/ZTO11 junction is visualized by cross-sectional scanning tunneling microscopy. This favorable CBO at the heterojunction reduces the interfacial recombination and enhances the charge collection probability in the ZTO11 device, ultimately significantly improving the open-circuit voltage (Voc) from 0.24 V to 0.34 V. The charge recombination mechanisms in the ZTO-based TFSCs is elaborated and compared with the CdS-based one, and the result confirms the charge recombination suppression at the SnS/ZTO11 interface. Moreover, the photocurrent in all ZTO-based TFSCs is significantly higher than CdS-based ones due to the broader band gap of ZTO compared to CdS. Improvements in Voc and Jsc significantly increase the PCE from 1.7% (CdS buffer layer) to 3% (ZTO11 with 18% Sn). This study highlights the potential of ZTO as an effective and environmentally friendly buffer layer in anisotropic photovoltaic materials.