High transmittance and low resistive ZnO:Al films for thin film solar cells

Abstract

Transparent conductive oxides (TCO) are indispensable as front electrode for most of the thin film solar cells. Thin films of aluminum-doped zinc oxide (ZnO:Al), one of the promising TCOs, were prepared by radio frequency (rf) magnetron sputtering on glass (Corning 1737) substrates as a function of the deposition condition. Argon gas pressure during deposition was kept in the range 0.04–1.33 Pa, and the temperature was maintained in-between 300 and 673 K. The surface roughness was found to increase with the increase in argon pressure, whereas the deposition rate was observed to decrease. The prepared films had excellent electrical properties (ρ=1.9×10⁻⁴ Ω cm) and high transmittance (>80%) in the wavelength range of 400–800 nm. Also, a maximum of 70% spectral haze (T_diffuse/T_total), an indicator for scattering properties of the etched ZnO:Al films, was achieved in the wavelength range of 400–800 nm. The surface morphology and thereby the light scattering properties of the films varied over a wide range with the change in substrate temperature and gas pressure. At low pressures (≤0.27 Pa) and high-substrate temperatures (573 K), the surface morphology of the films exhibited a denser and compact film structure with effective light-trapping to apply for the fabrication of thin film silicon solar cells.