Highly Transparent and Conductive ZnO:Al Thin Films for Photovoltaic Applications

Abstract

ZnO:Al films have been prepared by using reactive DC-Magnetron sputtering method from a Zn-target. Al-doping has been provided by cosputtering of Al which partially covers the Zn-target. A variation of doping from 1 to 5 atomic% has been achieved by changing the O₂-partial pressure from 4.10^-2 Pa to 110^-2Pa. The films show either hexagonal or wurtzite structure depending on preparation conditions. Enhanced light scattering in the films caused by large grain sizes which are in the order of the film thickness has been observed using elevated substrate temperatures. Transmissions of about 90% have been obtained in the visible region whereas free carrier absorption occurs at wavelengths exceeding 1000nm. At a preparation temperature of 350°C conductivities of the films are higher than 4000(Ωcm)^-1 at carrier concentration levels of about 7·10²⁰ cm^-3. Films prepared at 350°C show very good stability as proved in several ageing procedures. The applicability of these films in photovoltaic devices has been demonstrated by using the films as windows in Cu(Ga,In)Se₂ based solar cells and as TCO in a-SiGe:H-based solar cells.