Composition Control of Cu2ZnSnS4 Thin Films in Electrodeposition
Authors: Tang Aiyue , Wang Feng , Li Zhilin 
Published: February 2020

Cu2ZnSnS4 (CZTS) is a promising quaternary kesterite compound, which has an optimal direct band gap and high absorption coefficient. Due to its earth abundant constituent elements, CZTS has drawn much attention recently. Electrodeposition was regarded as a feasible method for synthesizing various functional nanostructured materials in industry scale. However, the composition control of CZTS is difficult in electrodeposition. Composition control of the CZTS thin films is the primary work because the crystal structure of the CZTS thin films was determined by the chemical composition and the post heat treatment. One-step electrodeposition is simple in the application of industry scale, but it is difficult to maintain stoichiometry in the circumstance of increasing number of elements. The difficulty stemmed from too large interval of standard reduction potentials and the unclear understanding of the reduction processes of various metal ions. Herein, we report composition control of Cu2ZnSnS4 thin films in electrodeposition.

Electrodeposition was carried out potentiostatically in a conventional three electrode electrochemical cell at room temperature. The electrolyte was comprised of CuSO4·5H2O, ZnSO4·7H2O, SnSO4, Na2S2O3·5H2O, trisodium citrate di-hydrate (C6H5Na3O7·2H2O), and L-(+)-tartaric acid (C4H6O6). The trisodium citrate and tartaric acid were used as special additives to narrow the potential gaps among the metallic ions. The CZTS thin film was deposited in potentiostatic mode. The chemical composition were characterized by EDX and all the chemical composition (at. %) were measured on the average of 3 different areas of every sample.

Figure (A) shows the effect of potential on the atomic ratio of the as-deposit thin films. All the films obtained in this section show a severe deviation from stoichiometry and the chemical composition was Cu rich and Zn, S poor. However, it is obvious that Zn content increased and Cu content decreased with a negative shift of the potential. It can be deduced that higher over potential promoted the reduction of Zn, whereas inhibit the cathode process of Cu. Then, the concentration of tartaric acid and the pH value of the electrolyte were adjusted to obtain the thin films with appropriate chemical composition. In our present work, the stirring speed (Figure (B)) and the distance between working electrode and auxiliary electrode (Figure (C)) were other two factors that affect the chemical composition. The stirring caused the Cu content increased and S content decreased because Cu2+ reduction is easier than other metallic ions and the reduction of S2O3
2- depended on the diffusion in the electrolyte. The concentration gradient was disturbed in the electrolyte due to the stirring. The chemical composition fluctuated with the distance between working electrode and auxiliary electrode. The SEM image of the CZTS thin films was shown in Figure (D). The thin films, which were consist of agglomerated particles, showed a uniform surface.

Therefore, the concentration of tartaric acid and the pH value of the electrolyte are vital factors of the composition control of the CZTS thin films in electrodeposition. The optimization of chemical composition as well as the detail investigation of electrodeposition mechanism is undergoing.

Figure caption: 

Figure  Effects of potential (A), stirring speed (B), and the distance between working electrode and auxiliary electrode (C) on the chemical composition of the CZTS thin films. The SEM image (D) of the CZTS thin films.

Figure 1