Electrodeposition Processes for Controlling Surface Morphology of Thick Cu2Se Films As Thermoelectric Conversion Materials
Authors: Takaza Collen , Allongue Philippe , Fukunaka Yasuhiro , Wang Tanyanyu , Homma Takayuki 
Published: August 2024

Copper selenides (Cu2Se) are a class of chalcogenides found in various applications owing to their semiconductor material properties. Cu2Se has shown great potential for thermoelectric (TE) device applications, and it has also demonstrated significant room for improvement at high temperatures owing to its phonon glass electron crystal (PGEC), an intrinsic property in potential thermoelectric materials1. Compact morphology and smooth surface are essential parameters in reducing the contact resistance between a thermoleg and the metal electrode2,3. Other studies have focused on further lowering thermal conductivity by increasing phonon drag through nano-structuring and introducing nano-inclusion4,5,6.
Among synthesis approaches, the electrodeposition of TE materials appears promising. Morphology control in electrochemical deposition has been demonstrated and has shown outstanding results7. In a Bi2Se3 growth study, the films showed a simultaneous change in morphology, smooth to rough films, and phase, rhombohedral to orthorhombic, with film thickness. These phases exhibit different crystallite sizes and Seebeck coefficients8. It has been demonstrated that film growth and thickness by electrodeposition mainly depend on electrolyte composition and the applied potential to control morphology and stoichiometry. Most studies have focused on tuning these parameters,resulting in films with well-defined stoichiometry but with porous, mossy, or low compactness 9,10,11. There is, therefore, room for improvement, and a better understanding of the deposition processes is key to controlling the growth modes, morphology, and composition.
In this work, we study the early stages of Cu2Se electrodeposition for TE application. Our goal is to optimize the electrodeposition conditions to obtain compact, well-crystallized films with Cu2Se stoichiometry. The films were obtained by potentiostatic deposition on a gold electrode from an acidic electrolyte, H2SO4 (pH 0.9 to 1.9), containing 10 mM CuSO4, (1 to 10 mM) H2SeO3, and the bath temperature varied from 25oC to 55oC. All potentials were measured and referred to Ag/Agl/sat, with a platinum mesh serving as the counter electrode. Results showed that at elevated temperature (55oC), pH 1.5, and a precursor concentration ratio [Cu(II) 10mM]/[Se(IV) 5mM] are suitable for Cu2Se film deposition. For films deposited in the potential range -500 mV ≤ V ≤ -50 mV, EDS analysis indicated that atomic composition is relatively constant and close to the desired Cu2Se stoichiometry (66.7 % Cu, 33.3 % Se). XRD spectra present sharp peaks corresponding to Cu2Se, indicating films are well crystallized, with an average crystallite size of 18 nm. SEM showed relatively smooth and compact films with, however, a high density of bulges. For films obtained at potentials more positive than -50 mV and more negative than -500 mV, notable changes in composition to Cu 60.8%, morphology, and the appearance of Cu3Se2 XRD peaks were observed, denoting the existence of mixed phases of Cu3Se2 and Cu2Se. Progress in the investigation of the growth mechanism/processes of Cu2Se films at the early deposition stages will also be presented.
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