Copper oxy-sulfide and copper sulfate thin films as transparent p-type conductive electrodes

Abstract

Starting from evaporated Cu layers, oxidation and sulfurization processes were developed by reaction at 250 °C with O2 or elemental sulfur to obtain Cu2O and CuS thin films, respectively, with p-type conductivity and visible transparency. With the aim of achieving mixed compounds that combine the best properties of oxides (transparency) and sulfides (metal-like conductivity), different methods were tested, including Cu2O sulfurization, CuS oxidation and direct Cu sulfoxidation. The crystalline structure, morphology and optical and electrical characteristics of the various samples were studied comparatively. CuSO4 crystals with a mean size above 40 nm were obtained by sulfoxidation, showing higher transmittance and lower resistance than the Cu2O film shaped by crystallites of ∼13 nm. Moreover, a superior electrode quality was achieved after CuS oxidation, which produces a mixture of small Cu2O and larger Cu2S nanocrystals with visible transmittance as high as the pure oxide and a hole density close to the sulfide.