Zinc oxide based transparent conductive oxide thin films deposited by R.F. magnetron sputtering for photovoltaic applications

Transparent conductive oxides (TCO) are photovoltaic materials with high optical transmittance and low resistivity have a common area of use in energy efficient, lowemission windows applications, touch screens and control panels, etc. Due to their electrical conductivity and transparency properties they are used for solar cells and flat panel displays as front-surface electrode. Nowadays, the usage of zinc oxide(ZnO) and aluminum doped zinc oxide (ZnO:Al) as a new generation TCO materials are increasing rapidly. They have enhanced properties as high optical transmission, lower electrical resistivity, cheaperraw material cost and absence of toxicity. Also,they are appropriate for various deposition techniques, and magnetron sputtering is one of them. According to many investigations, deposition parameters of sputtering such as deposition rate, r.f. power, and target-to-substrate distance (DTS), heating, process pressure etc. have a critical importance on the quality of thin film properties. To identify the influence of these parameters both ZnO and standard 2%Al doped ZnO thin films were deposited on glass by using r.f. magnetron sputtering without intentional heating. All the depositions were carried non-reactively. The r.f. power was varied in the range of 145 W to 175 W while, DTS was changed from 35 mm to 65mm. The process gas; argon pressure was changed from 0.15 to 0.60 Pa. The investigations were carried on in small variation ranges. Moreover, characterization of the growth thin films carried on using various techniques. These are; X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), four point probe and ultraviolet–visible (UV–VIS) spectrophotometer. According to the obtained results, for un-doped ZnO thin films with higher qualities as lowest resistivity (1.16×10-3Ω.cm) and highest transparency (80%) were achieved at 165W r.f. power, 45mm DTS and 0.30 Pa. However, for ZnO:Al thin films better results were obtained at around 165 W, 45mm DTS and 0.30 Pa process gas pressure. All the thin films were oriented with the crystallographic c-(002) axis perpendicular to the substrate surface. Also, ZnO:Al thin films had lower resistivity (1.1 ×10-3Ω.cm) and higher transmittance values (86%) than ZnO thin films. The electrical band gap was changed from 3.45eV to 3.87eV for ZnO:Al thin films. Surface roughness (RMS) values were obtained as 5.44nm for ZnO and as 2.36nm for ZnO:Al. In addition to the studies, deposition parameters of ZnO:Althin film was applied as TCO layer in a-Si: H/c-Si heterojunction solar cells and the achieved results were thermodynamically analyzed.