Element substitution of kesterite Cu₂ZnSnS₄ for efficient counter electrode of dye-sensitized solar cells

Development of cost-effective counter electrode (CE) materials is a key issue for practical applications of photoelectrochemical solar energy conversion. Kesterite Cu₂ZnSnS₄ (CZTS) has been recognized as a potential CE material, but its electrocatalytic activity is still insufficient for the recovery of I^-/I₃^- electrolyte in dye-sensitized solar cells (DSSCs). Herein, we attempt to enhance the electrocatalytic activity of kesterite CZTS through element substitution of Zn²⁺ by Co²⁺ and Ni²⁺ cations, considering their high catalytic activity, as well as their similar atomic radius and electron configuration with Zn²⁺. The Cu₂CoSnS₄ (CCTS) and Cu₂NiSnS₄ (CNTS) CEs exhibit smaller charge-transfer resistance and reasonable power conversion efficiency (PCE) (CCTS, 8.3%; CNTS, 8.2%), comparable to that of Pt (8.3%). In contrast, the CZTS-based DSSCs only generate a PCE of 7.9%. Density functional theory calculation indicate that the enhanced catalytic performance is associated to the adsorption and desorption energy of iodine atom on the Co²⁺ and Ni²⁺. In addition, the stability of CCTS and CNTS CEs toward electrolyte is also significantly improved as evidenced by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy characterizations. These results thus suggest the effectiveness of the element substitution strategy for developing high-performance CE from the developed materials, particularly for multicomponent compounds.