Controlled electrodeposition of Cu–Ga from a deep eutectic solvent for low cost fabrication of CuGaSe2 thin film solar cells

Abstract

The electrochemical deposition of Ga and Cu–Ga alloys from the deep eutectic solvent choline chloride/urea (Reline) is investigated to prepare CuGaSe₂ (CGS) semiconductors for their use in thin film solar cells. Ga electrodeposition is difficult from aqueous solution due to its low standard potential and the interfering hydrogen evolution reaction (HER). Ionic liquid electrolytes offer a better thermal stability and larger potential window and thus eliminate the interference of solvent breakdown reactions during Ga deposition. We demonstrate that metallic Ga can be electrodeposited from Reline without HER interference with high plating efficiency on Mo and Cu electrodes. A new low cost synthetic route for the preparation of CuGaSe₂ absorber thin films is presented and involves the one-step electrodeposition of Cu–Ga precursors from Reline followed by thermal annealing. Rotating disk electrode (RDE) cyclic voltammetry (CV) is used in combination with viscosity measurements to determine the diffusion coefficients of gallium and copper ions in Reline. The composition of the codeposited Cu–Ga precursor layers can be controlled to form Cu/Ga thin films with precise stoichiometry, which is important for achieving good optoelectronic properties of the final CuGaSe₂ absorbers. The morphology, the chemical composition and the crystal structure of the deposited thin films are analysed by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffraction (XRD). Annealing of the Cu–Ga films in a selenium atmosphere allowed the formation of high quality CuGaSe₂ absorber layers. Completed CGS solar cells achieved a 4.1% total area power conversion efficiency.