Cadmium zinc telluride films for wide band gap solar cells

The CdTe-ZnTe alloy system is evaluated for application in high efficiency polycrystalline thin-film multijunction solar cells. Single phase Cd_1-xZn_xTe alloy films 3-4 μm thick, with x from 0 to 1 and band gap from 1.5 to 2.25 eV, were deposited by co-evaporation from binary CdTe and ZnTe sources. Film composition, x in Cd_1-xZn_xTe varied linearly with source effusion rate ratio. Lattice parameter varied linearly with composition, while optical band gap exhibited a bowing parameter, b = 0.3. Sub-band gap transmission, T/(1-R), is ∼80%. Post-deposition treatment in argon at 400°C to 600°C does not measurably after the alloy composition. Treatment in CdCl₂:O₂:Ar vapor at 400°C preferentially removes Zn from the Cd_1-xZn_xTe alloy. Treatment in ZnCl₂:O₂:Ar vapor at ∼400°C results in slight compositional change due to formation of ZnO and Te at the surface. The photovoltaic operation of Cd_1-xZn_xTe/CdS devices is presented.