The effect of substrate temperature on optoelectronic characteristics of surface-textured ZnO:Al films for micromorph silicon tandem solar cells

Abstract

ZnO:Al films were deposited on the low cost soda lime glass substrates by d.c. magnetron sputtering from ZnO:Al (2 wt.%) ceramic target. The electrical and optical properties of surface textured ZnO:Al film deposited at the substrate temperatures of room temperature, 200 °C, 300 °C, and 400 °C were studied. The deposition rate and etch rate of ZnO:Al films was decreased with an increase of substrate temperature, implying that the film density was increased. The electrical resistivity was also decreased. The ZnO:Al film deposited at 300 °C showed low electrical resistivity of about 2×10-4 ohm cm. The refractive index at 550 nm was increased with substrate temperature. It is attributed to Al content increase in the film measured by energy dispersive x-ray spectroscopy. The surface morphologies, wet-chemical etching profiles, and haze characteristics of surface textured ZnO:Al films were analyzed as a function of substrate temperature. The ZnO:Al film deposited at 200 °C showed the largest craters in ZnO:Al film surface and highest haze values in any other sample although the ZnO:Al films deposited at over 200°C showed higher transmission and lower resistivity than ZnO:Al film deposited at 200°C. In a further increase of substrate temperature over 200°C, the film density was increased and wet-chemical etching was not efficient. So, small craters or pinholes which cause low haze values were formed in the ZnO:Al film. The ZnO:Al film deposited at optimum substrate temperature of 200 °C showed high transmittance of 90.3 % and 86.2% at the wavelength of 550 nm and 800 nm, respectively. It also showed high haze values of about 70 % and 36 % at 550 nm and 800 nm, respectively. The Al content in the film was 1.59 (at. %), and refractive index at 550 nm was 2.1. The ZnO:Al film deposited at 200 °C showed low resistivity of about 2.3×10-4ohm cm. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.