Polycrystalline sputtered Cd(Zn, Mn)Te films for top cells in PV tandem structures

The CdTe-ZnTe or CdTe-MnTe alloy system is evaluated for application in high efficiency polycrystalline thin film multijunction solar cells. CdZnTe alloy films with 1.6 and 1.7 eV band gap and CdMnTe with 1.6 and 1.8 eV were deposited by RF magnetron sputtering from targets made of mixed powders of CdTe and ZnTe (25% and 40%) or MnTe (13% and 25%), respectively. Without postdeposition treatment, however, both of these alloy films exhibit quite low photovoltaic performance when used to make cells with CdS as the heterojunction partner. Therefore, we have searched for an appropriate postdeposition treatment to improve the efficiency of these alloy films. As-grown films have been postdeposition treated with vapors of chlorine-containing materials such as HCl gas, CdCl2, ZnCl2 or MnCl2 in dry air or H-2 + Ar atmosphere. When the CdCl2 + ZnCl2 or MnCl2 vapor was used, the ratio of CdCl2 to ZnCl2 or MnCl2 was adjusted to match the stoichiometry of alloy films. The temperature of the treatment was chosen to represent conditions employed for those alloy films/CdS structures. As a result, the postdeposition treated CdZnTe and CdMnTe cell efficiency can be improved by a factor of over 100. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.