Development of Cu-doped ZnTe as a back-contact interface layer for thin-film CdS/CdTe solar cells

Abstract

The full potential of thin-film, CdS/CdTe photovoltaic solar cells will not be realized until issues relating to the fabrication of environmentally stable, low-resistance, and easily manufactured contacts to the p-CdTe layer are addressed. One alternative that provides the required contact parameters employs a Cu-doped ZnTe(ZnTe:Cu) interface layer between the p-CdTe and the outer metal contact. Thin films of ZnTe:Cu containing various concentrations of metallic Cu are produced by rf-magnetron sputtering. Additionally, the effect of incorporating small amounts of excess Zn into the sputtering target is studied. We find that the electrical resistivity of ZnTe:Cu films deposited at 300 °C, and prepared with Cu concentrations of ∼0.45 at. %, is much higher than would be expected from studies of films doped with higher Cu concentrations (∼6 at. % Cu). We also find that postdeposition heat treatment significantly reduces the electrical resistivity of the films containing ∼0.45 at. % Cu. However, compositional analysis indicates that the surface of the films become increasingly enriched in Zn at annealing temperatures ≳350 °C. Analysis of the hole effective mass (mh) for films containing ∼6 at. % Cu indicates a value for mh of 0.35 me, and a high-frequency dielectric constant (ε∞) of 8.2.