Fabrication of Cu₂ZnSnS₄ thin films using a Cu-Zn-Sn-O amorphous precursor and supercritical fluid sulfurization

A supercritical ethanol (scEtOH) environment, in which sulfur is highly soluble, can lead to safety improvements and cost reductions in the sulfurization process. In this study, the feasibility of the sulfurization process in scEtOH for the preparation of Cu₂ZnSnS₄ (CZTS) thin films at low temperature was verified with the purpose of creating a sustainable and cost-effective process for fabricating metal-sulfide solar cells. We found that to promote atomic diffusion of sulfur and achieve sulfurization at a low temperature, the presence of defects in the amorphous oxide thin film was preferable. Moreover, in thin film fabricated by sulfurization under ethanol, the crystal size was strongly affected by ethanol density. The grain size increased up to about 1 μm as the ethanol density increased, and grain growth was remarkable, particularly in the high-density conditions of over 3.0 mol/L. Finally, we fabricated a crystalline CZTS thin film, which exhibited structural and optical properties comparable to those of a film fabricated using conventional vapor-phase sulfurization. For the prepared disordered-kesterite CZTS thin film with a Cu-poor and Zn-rich composition, photoluminescence measurements confirmed that the donor or acceptor defects engage in the emission of about 1.24 eV at 5 K, and UV–Vis measurement revealed a bandgap of 1.38 eV at room temperature. - Highlights: • An amorphous metal-oxide precursor is utilized for fabrication of Cu₂ZnSnS₄ film. • Sulfurization is conducted under supercritical ethanol having reducibility. • The crystal growth proceeds in a high ethanol density region. • The structural and optical properties derived from Cu₂ZnSnS₄ are shown.