Electrochemical characterization of transparent conducting IZO:Ga thin films

Abstract

Thin films of IZO:Ga transparent conducting oxide (TCO) material have been deposited onto glass substrates, using RF magnetron sputtering under different conditions. Film thicknesses varying from 179 to 472 nm, an average Ga/(Ga + Zn + In) atomic ratio value of 1.3%, and In/Zn atomic ratios from 0.55 to 1.05, were obtained. It has been determined that as the In/Zn atomic ratio increases, the resistivity of the films decreases from 1.30 to 0.83 mΩ cm. A single hexagonal c-axis oriented IZO:Ga pure phase has been obtained. The average transmittance of the IZO:Ga film is ca. 80% for all position in the 400–700 nm wavelength region. FE-SEM images show that the IZO:Ga film exhibit a void-free surface with a rough morphology, containing round shape hexagons nanostructures with sizes between 100 and 200 nm. In this work, the electrochemical behavior (stability and electron transfer properties), and photoelectrochemical characteristics of IZO:Ga thin film samples have been examined. The electrochemical stability behavior has been examined by using electrochemical cyclic voltammetry (CV) technique in an acidic aqueous electrolyte, and compared to the behavior of two commercial TCOs on glass substrates (i.e.: ITO and FTO). The effect of IZO:Ga resistivity on the electrochemical performance was also examined by CV. It has been found that the standard electron transfer rate constant of hexacyanoferrate ion diminished at the less electrical conductive IZO:Ga sample (ko = 6.27 × 10−5 cm s−1) compared to the more conductive one (ko = 1.43 × 10−4 cm s−1). Photoresponse and photoelectrochemical properties IZO:Ga thin films have been evaluated in 0.1 M Na2S + 0.1 M Na2SO3 aqueous electrolyte, and under AM1.5G (100 mW cm−2) illumination. All of the IZO:Ga thin film samples demonstrate a typical anodic photocurrent response confirming their n-type conductivity behavior, with photocurrent values as high as 4 μA/cm2 at anodic potential bias. These results provide useful insights into the behavior of this TCO material and serve as a starting point for the selection of appropriate substrate materials for evaluating novel electrocatalysts, photoelectrodes and optoelectronic devices. As a case study, ZnO nanorods arrays vertically oriented to the substrate plane, exhibiting 200 nm of average diameter, were electrochemically grown onto these synthesized substrates with encouraging novel results.