Electrodeposition has been always considered a valuable fabrication technique for semiconductor materials, especially because of its relatively low cost and scalability [1-2]. However, some intrinsic limitations are affecting such a technique, mainly due to the interaction between the substrate and the plating solution. In this view, the employment of organic solution may be a valid option to plate a metallic layer on top of less noble ones. In the framework of Cu₂ZnSnS₄ (CZTS) synthesis, the typical route consists indeed in the electrodeposition of a stacked layer from the noblest metal to the least one, i.e. Mo/Cu/Sn/Zn [1-2]. This is limited to the high reactivity of zinc when immersed in a copper or tin-based solution resulting in fast displacement reactions occurring due to the huge difference in the reduction potential of the two elements. However, since the metallic precursor layer needs to be reactively annealed at significantly high temperatures >500°C, zinc loss may be significant being the outermost layer, with consequent modification of the film stoichiometry. Moreover, few studies based on vacuum fabrication techniques demonstrated the beneficial effect of zinc when directly in contact with Mo substrate having copper and tin as outermost layers [3].

Based on these premises, the design of an organic solution, overcoming the intrinsic limitation water-based ones [4-5], is proposed and successfully employed for the electrodeposition of copper onto zinc. Copper displacement reaction is assessed and investigated, paying attention to how the metal salt concentration and the introduction of a copper complexing agent affected the reactivity towards zinc. Once minimized displacement reaction and electrochemically characterized the electrolyte through cyclic voltammetry, the galvanostatic deposition was carried out to obtain the desired film thickness and morphology. The precursor stack (Mo/Zn/Cu/Sn) was then soft annealed at 300 °C and converted into CZTS kesterite through reactive annealing in a sulfur atmosphere at 560-580 °C. The film was characterized by means of XRD, SEM and Raman spectroscopy. At last, the CdS buffer layer and the Pt catalyst was deposited in the view of photoelectrochemical investigation (100 mW/cm² AM 1.5 G) to assess the semiconductor photoactivity

References

[1] Colombara, Diego, et al. "Electrodeposition of kesterite thin films for photovoltaic applications: Quo vadis?." physica status solidi (a) 212.1 (2015): 88-102.

[2] Ahmed, Shafaat, et al. "A high efficiency electrodeposited Cu2ZnSnS4 solar cell." Advanced Energy Materials 2.2 (2012): 253-259.

[3] Araki, Hideaki, et al. "Preparation of Cu2ZnSnS4 thin films by sulfurization of stacked metallic layers." Thin Solid Films 517.4 (2008): 1457-1460.

[4] Welton, Thomas, et al. "Room-temperature ionic liquids. Solvents for synthesis and catalysis." Chemical reviews 99.8 (1999): 2071-2084.

[5] Smith, Emma L., et al. "Deep eutectic solvents (DESs) and their applications." Chemical reviews 114.21 (2014): 11060-11082.