## 4 Conclusions  
This study aims to improve the efficiency of Sb\({}_{2}\)Se\({}_{3}\)-based solar cells by optimizing DC/RF sputtering for the ZnO/CdS-(Cd,Zn)S and back-contact layers, along with the CSS for Sb\({}_{2}\)Se\({}_{3}\), in order to improve film quality, interface engineering, and defect reduction. Compared to other Sb\({}_{2}\)Se\({}_{3}\) studies, this approach offers significant advantages. Unlike chemical bath deposition, electrodeposition, hydrothermal deposition, and thermal evaporation, it ensures superior crystallinity, lower defect density, and improved charge transport. Figure 14: \(ln(\omega_{0}/7^{2})\) (unit: s\({}^{-1}\cdot\)K\({}^{-2}\)) as a function of 1/KT, illustrating two distinct activation energies observed (**a**) under light and (**b**) in the dark. When considering alternative absorbers, like CdTe, CIGS, and perovskites, Sb\({}_{2}\)Se\({}_{3}\) stands out for its earth abundance, non-toxicity, and long-term stability. Although CdTe and CIGS offer higher efficiencies, they face material scarcity and toxicity concerns, while perovskites suffer from stability issues, particularly under prolonged exposure to moisture and heat, which hinder their large-scale deployment. Despite the actual lower efficiencies (-10-11%) of Sb\({}_{2}\)Se\({}_{3}\), advancements in passivation and heterojunction engineering can bridge this gap. This work contributes to the field by proposing novel scalable, reproducible deposition methods, implementing defect passivation strategies, and demonstrating industrial viability. With continued progress, Sb\({}_{2}\)Se\({}_{3}\) solar cells can achieve higher efficiencies and become a commercially viable and sustainable thin-film photovoltaic technology. In particular, we discussed the non-standard J-V characteristic of (Cd,Zn)S/Sb\({}_{2}\)Se\({}_{3}\) heterojunctions, particularly focusing on features such as kink, crossover, and photovoltage losses. The presence of a kink can be attributed to the accumulation of positive charges in the junction region, arising from a non-perfect alignment of the energy bands with the (Cd,Zn)S. On the other hand, the occurrence of a crossover is likely due to a small built-in voltage resulting from the limited density of holes in Sb\({}_{2}\)Se\({}_{3}\). Capacitance measurements suggest that the (Cd,Zn)S/Sb\({}_{2}\)Se\({}_{3}\) junction is not sharp but rather presents a linear charge distribution. Two deep levels sensitive to illumination conditions were identified through admittance measurements, and there are indications that V\({}_{\rm Sb}\) is involved as one of the traps. Higher power conversion efficiencies in Sb\({}_{2}\)Se\({}_{3}\)-based solar cells can be expected by properly addressing some major issues:  
1. Given the imperfect alignment of the energy bands with (Cd,Zn)S, it is imperative to explore alternative n-type partners, such as Zn(S,O), TiO\({}_{2}\), MgO\({}_{2}\), Cd\({}_{2}\)Sn0\({}_{4}\), and others, in an attempt to minimize the interface barrier. A possible benefit of such attempts could be the substitution of cadmium-based semiconductors with environment-friendly materials and, at the same time, enhanced transparency in the ETL. 2. Replacing cadmium-based materials in Sb\({}_{2}\)Se\({}_{3}\) solar cells offers significant environmental benefits by reducing toxicity and minimizing contamination risksEliminating cadmium simplifies manufacturing, enhances worker safety, and facilitates recycling by avoiding hazardous waste management. Alternative materials, such as Zn(O,S), ZnMgO, or low-Cd (Cd, Zn)S, can provide suitable band alignment while reducing environmental impact, though interface engineering remains crucial to maintaining performance. As sustainability becomes a key factor in solar energy adoption, cadmium-free Sb\({}_{2}\)Se\({}_{3}\) solar cells align with stricter environmental policies and market demands, enhancing their long-term commercial viability. 3.