Non-Cadmium TiO₂/Sb_<b>2</b>(Se, S)_<b>3</b> Heterojunction Solar Cells with Improved Efficiency by NaCl-Treated Interface Engineering

Cadmium sulfide (CdS) is widely employed as the electron transport layer due to its ability to form dense films in the fabrication of antimony selenosulfide (Sb₂(S, Se)₃) solar cells. However, it presents significant drawbacks: its toxicity poses environmental risks, and its narrow bandgap restricts the collection of higher-energy carriers. Titanium dioxide (TiO₂) stands out as a viable and environmentally friendly alternative, offering features, such as high optical transparency, excellent stability, and nontoxic characteristics, making it highly suitable for application in Sb₂(S, Se)₃ thin-film solar cells. In our study, we employed a sodium chloride (NaCl) solution treatment to enhance the quality of TiO₂ films grown via the chemical bath deposition (CBD) method. The Na ions introduced during postannealing play a pivotal role in optimizing the interface between the TiO₂ and Sb₂(S, Se)₃ layers. This treatment enhances the bandgap of the TiO₂ layer, improving electronic coupling at the p-n junction. This process significantly boosts device performance, including the short-circuit current density (<i>J</i>_SC) and open-circuit voltage (<i>V</i>_OC). As a result, the power conversion efficiency (PCE) of the TiO₂/Sb₂(S, Se)₃ heterojunction solar cells improved remarkably from 2.3% to 5.5%. The novel approach highlights the effectiveness of wide-bandgap TiO₂ buffer layers in advancing Sb₂(S, Se)₃ solar cells. By overcoming the limitations of traditional CdS layers and integrating Na ion-enhanced TiO₂ films, this study demonstrates a promising route for achieving high-efficiency and environmentally sustainable solar cells.