Cathodic electrochemical deposition of CuI from room temperature ionic liquid-based electrolytes

• Novel route to obtain CuI films
• The cathodic electrodeposition of CuI, from the reduction of I2, is viable in different solvents with low dielectric constant.
• The solvent nature affects significantly the packing density degree of the films, providing large versatility for the tailoring of the microstructural properties
• The present results open wide possibilities of the electrodeposition of other metal iodides and more complex halide compounds such as perovskites
• The competitive optoelectronic properties of the obtained CuI films have been proved in ZnO/CuI heterostructures.

An innovative electrochemical route is proposed to obtain CuI films. The approach is based on the electrochemical reduction of I2 in a solution of copper bis(trifluoromethanesulfonyl)imide salt in 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide room temperature ionic liquid. The mechanisms involved in the deposition process are analyzed by cyclic voltammetry, pointing out that the CuI formation occurs from the chemical reaction between the Cu2 + and I- generated from the I2 reduction. Homogenous films constituted of open-packed primary CuI nanocrystals (~ 100 nm) are obtained. The electrodeposition from organic solvent-based media is also investigated, finding that the solvent nature affects strongly to the nanocrystal packing density. Compact CuI thin films are electrodeposited from the isopropanol-based electrolytes. The electrodeposition of ZnO/CuI heterostructures, with high transmittance in the visible range (i.e. > 75%), is also reported. The current density-voltage characteristic of the resulting device exhibits clear rectifying behavior, with a rectification of ~ 2x103 at V = ± 1.5 V. This result anticipates a significant potential of the present electrochemical route to obtain CuI films with competitive optoelectronic functionalities.