Highly-transparent and conductive CuI films obtained by a redirected low-cost and electroless two-step route: Chemical solution deposition of CuS2 and subsequent iodination

Abstract

A simple, low-cost, electroless, and industrially scalable solution/vapor two-step strategy for the synthesis of highly-transparent and conductive CuI thin films is presented. The process is based in the coupling of the versatile chemical solution deposition technique to a gas–solid reaction method. It consists of the chemical deposition of CuS2 thin films (which can be performed over any kind of substrate) and their subsequent halidization by a gas–solid reaction with iodine vapor. The CuI obtained by this route is as transparent and conductive as the ones obtained by physical techniques coupled to an iodination reaction. Particularly, we present a 60-nm CuI sample obtained by iodination of a chemical-solution-deposited 21-nm CuS2, which is practically similar to the best reported in the literature, in terms of absence of the common frosted-glass-like appearance. The integrated transmittance in the visible region of this CuI thin film is 0.85, compared with the 0.91 of the glass slide used as a substrate, which is an indication of the high transparence of the synthesized CuI material. Hall-Effect and optical characterization of a thicker CuI thin film (146 nm) obtained by the solution/vapor two-step process presented here resulted in a p-type conductivity, a resistivity equal to 1.1 Ω cm, a hole concentration of 1.3 × 1019 cm–3, a hole mobility of 0.43 cm2/V·s, an integrated transmittance in the visible region of 0.65, and, according to the Z1,2 exciton absorption, an energy band gap of 3.05 eV. Analyses by XPS and XRD, as well as a rigorous chemical analysis and an accurate Rietveld refinement, confirmed that the composition of the obtained material is Cu0.85±0.06I with a zincblende structure, thus presenting copper vacancies; this composition explains the p-type conductivity of the synthesized material. All these results demonstrate that the two-step process presented here is a promising alternative to those including conventional physical techniques. The CuI thin films present enough quality to be studied for both photovoltaics and transparent electronics applications.