Co-Electrodeposition of Metallic Precursors for Cd-Doped Cu2ZnSnS4 (CZCTS) Kesterite Absorber for Photoelectrochemical Water Splitting
Authors: Dell'Oro Ruben , Della Vedova Roberto , Marchionna Stefano , Magagnin Luca 
Published: November 2022

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 Cd7,8, 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 CuZn, 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.
Bibliography

Colombara, D. et al. Electrodeposition of kesterite thin films for photovoltaic applications: Quo vadis? Phys. Status Solidi Appl. Mater. Sci.
212, 88–102 (2015).

Ge, J. & Yan, Y. Controllable Multinary Alloy Electrodeposition for Thin-Film Solar Cell Fabrication: A Case Study of Kesterite Cu2ZnSnS4. iScience
1, 55–71 (2018).

Clauwaert, K., Binnemans, K., Matthijs, E. & Fransaer, J. Electrochemical studies of the electrodeposition of copper-zinc-tin alloys from pyrophosphate electrolytes followed by selenization for CZTSe photovoltaic cells. Electrochim. Acta
188, 344–355 (2016).

Jiang, F. et al. Pt/In2S3/CdS/Cu2ZnSnS4 Thin Film as an Efficient and Stable Photocathode for Water Reduction under Sunlight Radiation. J. Am. Chem. Soc.
137, 13691–13697 (2015).

Polizzotti, A., Repins, I. L., Noufi, R., Wei, S. H. & Mitzi, D. B. The state and future prospects of kesterite photovoltaics. Energy Environ. Sci.
6, 3171–3182 (2013).

Yuan, Z. K. et al. Engineering Solar Cell Absorbers by Exploring the Band Alignment and Defect Disparity: The Case of Cu- and Ag-Based Kesterite Compounds. Adv. Funct. Mater.
25, 6733–6743 (2015).

Tay, Y. F. et al. Solution-Processed Cd-Substituted CZTS Photocathode for Efficient Solar Hydrogen Evolution from Neutral Water. Joule
2, 537–548 (2018).

Su, Z. et al. Device Postannealing Enabling over 12% Efficient Solution-Processed Cu2ZnSnS4 Solar Cells with Cd2+ Substitution. Adv. Mater.
32, 1–12 (2020).