Preparation And Properties Of Surface Textured AZO Transparent Conducting Films

As standard front contacts, ZnO:Al (AZO) films have received great interest in the applications of silicon-based thin film solar cells, owing to their good optoelectronic properties, as well as their low cost, abundant resource and stability under hydrogen plasma environment. Moreover, AZO film with rough surface can realize light trapping effect and increase the absorption of light in the absortion layer, which can achieve a high efficiency in thin film solar cells. Therefore, it is very important to study the preparation and properties of textured AZO films with high transparency in the visible wavelength range, low resistivity and good light trapping ability.At present, surface textured AZO films were prepared by wet chemical etching after sputtering deposition, and HCl solution was used in most research. However, it is difficult for HCl solution to control the etching process and surface morphology of AZO films because of the high etching rate of HCl solution. In this thesis, through the wet chemical etching process after sputtering deposition, surface textured AZO films with light trapping ability were obtained in NaOH solution in order to control the etching process and surface morphology. The structural, optoelectronic properties, surface morphology and light trapping ability of smooth and textured AZO films were investigated in detail. And light trapping ability of textured AZO films deposited by re-deposition after etching was studied. The main research contents are as follows:The smooth AZO films were prepared by direct current pulse magnetron sputtering from a ceramic ZnO:Al2O3(98wt.%:2wt.%) target. The influence of substrate temperature, Ar flow rate, working pressure, annealing temperature and film thickness on the smooth AZO films was studied. All the sputtering AZO films had a hexagonal wurtzite structure with (002) preferred orientation. And all the AZO films had an average transmittance of above90%throughout the visible wavelength range. The deposition conditions had important effects on the absorption edge of the transmittance spectra and the electrical properies of AZO films. From the research results, the optimum deposition conditions for AZO films with good optoelectronic properties were obtained:substrate temperature (270℃), working pressure (0.8Pa), Ar flow rate (50sccm) and sputtering power (350W). AZO films deposited under this condition had an average transmittance of above90%throughout the visible wavelength range, and the resistivity was1.3×10-3Ω·cm.NaOH solution was used to etch the smooth AZO films, and effective surface textured structures were obtained successfully. The surface of AZO film was almost uniformly covered by the craters with diameters ranging from0.5to1μm, which is an effective feature for light trapping. Moreover, the resistivity of the textured AZO film is3.2×10-3Ω·cm, and the high transparency and light trapping ablity were also obtained. The influence of deposition and etching conditions on the properties of textured AZO films etched with NaOH solution was studied. All the AZO films had high transparency throughout the visible wavelength range, and the average transmittance reached above90%. When substrate temperature was higher than270℃, the crater size and haze became smaller with the increasing substrate temperature. With the increasing of Ar flow rate, different surface features ranging from honey-comb-like to crater-like structures were observed and the haze was increased. The higher working pressure was not fit for the formation of textured structure. With the increasing etching time and concentration, the crater size was increased while the crater density was decreased, which improved the light trapping ability of textued AZO films. But textured structure was not obtained by increasing the etching temperature. The formation mechanism of textured structure of AZO films was investigated, and the relation between the textured morphology control, light trapping ability improving and preparation condition was discussed. Furthermore, modified Thornton model was supplemented.Textured AZO films were prepared by re-deposition after etching. During the deposition+etching (NaOH solution)+deposition process, the influence of sputtering deposition conditions before and after etching on the light trapping prperties of textured AZO films was studied. When the deposition and the re-deposition condition are different, light trapping ability of textured AZO films with high transparency throughout the visible wavelength range was improved obviously.