Comparative study of molecular beam epitaxy-deposited ZnS:O and CdS:O as electron-transporting materials in Sb2(S,Se)3 solar cells†

Parasitic absorption and toxicity of cadmium sulfide (CdS) limit the efficiency improvement and future commercialization of thin-film solar cells, making it imperative to develop Cd-free electron-transporting materials. Among the alternatives, oxygen-doped zinc sulfide (ZnS:O) emerges as a promising candidate material for transporting electrons due to its non-toxicity and wide bandgap. However, ZnS:O films prepared by the common chemical bath deposition method have poor crystallinity, which is unfavorable for electron carrier transport. In this work, we develop a molecular beam epitaxy deposition method to prepare crystallinity-enhanced ZnS:O films. We further explore the impact of O content on the energy levels of ZnS:O films, as well as the crystal orientation of the subsequently deposited antimony selenosulfide (Sb₂(S,Se)₃) films, and achieve a power conversion efficiency of 5.15% for Sb₂(S,Se)₃ solar cells, which is a top value for single-layer ZnS:O applied to Sb₂(S,Se)₃ solar cells. Finally, to reveal the mechanism of performance difference between ZnS:O and oxygen-doped CdS (CdS:O) on Sb₂(S,Se)₃ solar cells, we conduct a comparative study focusing on their electrical properties, band alignment, interfacial properties, and carrier kinetics. The results reveal that a significant lattice mismatch and unfavorable band alignment between ZnS:O and Sb₂(S,Se)₃ severely impede the extraction and transport of photogenerated electrons, thereby limiting further improvement in the device efficiency. Overall, this study provides valuable guidance for the development of Cd-free thin-film solar cells.