Morphological and Composition Variations of Cu-Se Thick Films Electrodeposited on Au Substrate
Authors: Takaza Collen , Allongue Philippe , Fukunaka Yasuhiro , Homma Takayuki 
Published: December 2024

The electrodeposition of copper selenide (Cu2Se) has been demonstrated to have various applications, such as in photovoltaics and thermoelectric devices1. The stoichiometry of electrodeposited CuxSe is found to be rather complex and sensitive to deposition conditions. Several studies have focused on the influence of the effect of bath temperature, electrolyte composition, the applied potential, and current density2–5. Although these parameters can control the thickness and the morphology of the deposited film, thick film fabrication research has not been frequently reported despite the necessity of thermo-device application6. The long length of the thermo-legs in a thermo-device is important, tenths to hundreds of microns. Thick thermo-legs minimize the interface thermal resistance and maximize the temperature drop, ∆T TEG
. It is, however, noticed that the morphology and stoichiometry of thermos-leg often vary with film growth, posing many challenges in the fabrication of thick films7. Understanding the nucleation and growth mechanisms of Cu2Se during electrodeposition is essential for optimizing its properties.

This work examines the growth process of thick Cu2Se films by electrodeposition on a gold substrate. Films were fabricated by potentiostatic and pulsed electrodeposition on a gold working electrode in an acidic electrolyte, H2SO4, containing 10 mM CuSO4 and 5 mM H2SeO3 at 55oC. A platinum mesh served as the counter electrode and all potentials were referred to Ag/AgCl/sat reference electrodes. Atomic composition and morphology were investigated as a function of applied potential and film thickness direction. About 12µm thick and dense Cu2Se films were grown by optimizing the deposition parameters. SEM observations showed that the deposits, obtained at potentials ranging between -130 mV and -450 mV, were non-uniform, accompanying many clusters of mossy morphology with composition variations. EDS analysis indicates that films are initially Se-rich films, possibly an admixture of Cu2Se and Cu3Se2 transitioning to a near stoichiometry composition of Cu2Se with increasing thickness of about 1µm, at potentials of -130mV and -250mV. Increasing the copper electrolyte concentration and employing pulsed electrodeposition did not significantly influence the film composition in the initial stages. These parameter changes may avoid the formation of mossy deposits to provide a smoother surface.

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