Electrodeposition is widely studied in the fabrication of semiconductors for solar application, because of the low cost and easy scalability of this technique. For instance, in the last decades large effort has been dedicated to the electrodeposition of Cu₂ZnSnS₄ (CZTS) through different approaches, including either direct co-electrodeposition of the sulfide, the deposition of metallic alloy precursors or by stacked elemental layer, depositing Cu, Zn and Sn in sequence, then converting it into kesterite with thermal treatment in sulfur atmosphere.^1–4 While being a promising alternative to CIGS (CuInGaS₂), thanks to the non-toxicity and higher abundance of its constituting elements, CZTS absorbers still encounter some difficulties to their integration of high-performance solar devices. In fact, the electronic and optical properties of the material are affected by the narrow stoichiometric window acceptable for kesterite formation, inducing the segregation of secondary phases, along with the formation of lattice defects that act as recombination sites.⁵ A reported strategy to suppress the latter limitation is to introduce doping elements such as Ag⁶ and Cd^7,8, partially substituting metallic atoms in the semiconductor, Cu and Zn respectively. The larger atomic radii of these elements can prevent the formation of antisite defects such as Cu_Zn, largely increasing the semiconductor performance⁷. Here we propose a synthesis route of CZCTS through the co-electrodeposition of a thin film Cu-Sn-Zn-Cd alloy followed by sulfurization treatment. The deposition on SLG/Mo substrate is achieved with a sulfate-based electrolyte with citrate complexing agents and performed in three-electrode potentiostatic conditions. Proper bath formulation is designed to obtain a desirable Zn-rich, Cu-poor composition, varying the degree of Zn substitution with Cd. An alternative 2-step deposition of CuZnSn/Cd stacked precursor is also investigated, as an attempt to compensate the effect of the low H⁺ adsorption energy of cadmium leading to the formation of blisters in the quaternary alloy. Sulfurized samples characterization shows the formation of good quality of kesterite CZCTS, in spite of the segregation of secondary phases on the surface. Cd doping of CZTS is observed through the characteristic shift of the main kesterite XRD peak (112) and an increase of the average size of grains. The photocurrent of finalized photocathodes with structure SLG/Mo/CZCTS/CdS/Pt is measured under 10 mW/cm² AM 1.5 radiation reporting an improvement with respect to the undoped analogue, increasing the current density at 0 V vs RHE from -5.73 to -7.18 mA/cm² in the best case.

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