Etching process optimization of non-vacuum fabricated Cu₂ZnSnS₄ solar cell

Thin-film solar cell based on kesterite material Cu₂ZnSnS₄ (CZTS) is one of the third generation prospective solar cells replacing commercial Cu(In_x, Ga_1−x)S₂ (CIGS) material. However, the substitution of rare In and Ga material by Zn and Sn still requires further fabrication optimization. The secondary phases that formed during the fabrication process might have prohibited cell performance. Therefore, this work is essential to focus on introducing the etching process using 5% HCl with an etching time of 0, 100, 300, 480, and 600 s in order to minimize the secondary phases of Cu₂S, SnS. SnS₂, and ZnS during non-vacuum fabrication of Cu₂ZnSnS₄ kesterite. This Cu₂ZnSnS₄ active material works as a p-type semiconductor in thin layer solar cells. The film was deposited by the spin coating method with the standard structure of Mo/Cu₂ZnSnS₄/CdS/AZO/Ag. Characterization was carried out by X-ray diffraction (XRD) testing, scanning electron microscope–energy dispersive X-ray spectroscopy (SEM–EDX), UV–Vis spectroscopy, Tauc Plot analysis, and light-harvesting efficiency. The best results showed that the introduction of etching treatment using 5% HCl for 300 s in the CZTS layer has successfully reduced the secondary phases of ZnS, Cu₂S, and SnS₂ by 28.9, 5.8, and 0.3%, respectively. This absorber layer has contributed to achieving the maximum light-harvesting efficiency (LHE) of 95.2% with a bandgap of 1.76 eV. Interestingly, the treatment does not significantly contribute to changing the surface morphology and the grain size of Cu₂ZnSnS₄ kesterite film, but it affects the pores on the surface of the absorber layer due to the ZnS secondary phase reduction.