Paper TF-TuP3
Influence of Target Type on Surface Texture-etched AZO Films Prepared by Magnetron Sputtering for Solar Cell Transparent Electrode Applications

Tuesday, November 1, 2011, 6:00 pm, Room East Exhibit Hall

This paper describes the influence of sintered oxide targets on the surface texture formation as well as light management obtainable by wet-chemically etching transparent conducting Al-doped ZnO (AZO) thin films prepared by different types of magnetron sputtering depositions (MSD). For transparent electrode applications in thin-film solar cells based on CuIn_1-XGa_XSe₂ and Si, transparent conducting AZO thin films require an appropriate surface texture to improve photovoltaic properties through induced light scattering and subsequent light trapping. It was found that the suitability of the light scattering characteristics for thin-film solar cell applications was considerably dependent on both the type of AZO target and the type of MS deposition used.

The surface texture-etched AZO thin films were prepared by a d.c.- or r.f. (13.56 MHz) power-superimposed d.c.- magnetron sputtering deposition (dc-MSD or rf+dc- MSD) with sintered AZO targets that was followed by a heat treatment with rapid thermal annealing (RTA) and a subsequent wet-chemical etch. Five types of commercially available high-density-sintered rectangular AZO targets were used. The suitability of the light scattering characteristics for thin-film solar cell applications was evaluated by carrying out surface texturing of the samples with wet-chemical etching in a 0.1% HCl solution conducted after the heat treatment with RTA (at a temperature of 300-500^oC for 1-5 min in air). It was found that the obtainable surface texture as well as the haze value in the range from visible to near infrared were considerably affected by not only the AZO target used but also the type of MSD; the dc- and rf+dc-MSD were conducted using the five types of targets with properties that depended on the supplier. The surface-textured AZO thin films prepared by rf+dc-MSD and etched to an appropriate depth after RTA were found to exhibit a higher haze value and a lower resistivity than the values exhibited by equivalent films prepared by dc-MSD. It was also found from X-ray diffraction analyses that the as-deposited AZO thin films prepared by rf+dc-MSD exhibited better crystallinity than those prepared by dc-MSD; also, the crystallinity of AZO thin films, such as the c-axis orientation, was slightly improved by the RTA treatment. A high haze value generally above 70% in the range from visible to near infrared (at wavelengths up to 1200 nm), which is suitable for thin-film solar cell applications, was obtained in the best surface-textured AZO thin films: film thickness of 2 μm prepared by rf+dc-MSD with an appropriate AZO target and etched to a depth of approximately 300 nm after RTA at approximately 500^oC for 5 min.