Electrodeposition mechanism of quaternary compounds Cu2ZnSnS4: Effect of the additives

Highlights: • Synergetic effect between trisodium citrate and tartaric acid was verified. • Trisodium citrate was proved having strong complex effects with Cu²⁺ and Sn²⁺. • Tartaric acid was proved promoting Zn²⁺ reduction by providing hydrogen bond. • Mechanism of one-step electrodeposition of CZTS was proposed. The electrodeposition mechanism of pure phase Cu₂ZnSnS₄ (CZTS) thin film with subsequent annealing was investigated in detail. An electrolyte design principle of quaternary compounds was proposed. The complex ions of Cu(H₂C₆H₅O₇)⁺, Cu₂(C₆H₅O₇)⁺, Zn(C₄H₅O₆)⁺, Sn(H₂C₆H₅O₇)⁺ and Sn₂(C₆H₅O₇)⁺, which influenced the reduction process and played important roles in co-deposition, were identified by UV spectra. Electrochemical studies indicated that trisodium citrate and tartaric acid could narrow the co-deposition potential range of the four elements to −0.8 V to −1.2 V (vs. SCE). The cause was the synergetic effect that trisodium citrate inhibited the reduction of Cu²⁺ and Sn²⁺ and tartaric acid promoted the reduction of Zn²⁺. The reduction of S₂O₃²⁻ was mainly attributed to the induction effect of the metallic ions, and the H⁺ dissociated from tartaric acid could also promote the cathode process of S₂O₃²⁻. The reaction mechanism could be summarized as the following steps: (I) Cu(H₂C₆H₅O₇)⁺, Cu₂(C₆H₅O₇)⁺ → Cu, Sn(H₂C₆H₅O₇)⁺, Sn₂(C₆H₅O₇)⁺ → Sn, Zn(C₄H₅O₆)⁺ → Zn; (II) the desorption of (H₂C₆H₅O₇)⁻ and (C₆H₅O₇)⁻, and the reduction of S₂O₃²⁻ induced by metallic ions and H⁺. The mechanism studies provided a path of electrolyte design for multicomponent compounds.