Comparative studies on the properties of magnetron sputtered transparent conductive oxide thin films for the application in solar cell

Abstract

Transparent conductive ZnO:Al and ZnO:Ga ultrathin films have been developed on glass substrate at room temperature by non-reactive RF magnetron sputtering using sintered ceramic disc of ZnO:Al target (ZnO 98%, Al 2%), and ZnO:Ga target (ZnO 97%, Ga 3%). The thin films (90–250 nm) of ZnO:Al and ZnO:Ga show low resistivity of 8.6 × 10−5 and 4.5 × 10− 4 Ω cm, respectively. The ZnO:Al thin film exhibits highest transparency of 92%, haze factor of 54% in the visible region and electrical mobility of 12 cm2/V s. Similar kinds of results (highest electrical mobility is 10 cm2/V s, haze factor varies from 28 to 44%) are observed for ZnO:Ga thin films. The moderate improvement on thickness-dependent electrical mobility for very thin ZnO:Ga and ZnO:Al films are due to improved crystallinity, increased crystallite sizes but negative effect comes from grain boundary scattering and higher surface roughness. X-ray diffraction spectra reveal polycrystalline nature of ZnO:Ga and ZnO:Al thin films with grain size 22 and 19 nm, respectively. Variation of thickness and composition-dependent structural, morphological and optical haze properties of Al- and Ga-doped ZnO thin films have been compared with that of conventional ITO and SnO2:F thin films.