Polycrystalline II-VI semiconductor materials show great promise for thin-film photovoltaic cells and modules. Large-area deposition of these II -VI semiconductors such as CdTe is possible by a variety of methods but the use of a plasma-based method such as magnetron sputtering can have significant advantages. Here we present recent results in the fabrication of CdS/CdTe cells using rf magnetron sputtering and discuss some of the advantages that appear possible from the use of sputtering methods in this class of materials. Some of these advantages are particularly relevant as the polycrystalline thin-film PV community addresses issues related to the challenges of fabricating high efficiency tandem cells with efficiencies over 25%. Our best results have been obtained with sputtered ZnO:Al to achieve a CdTe solar cell with 14.0% efficiency at one sun for an air-mass-1.5 global spectrum. In addition, we have studied reactive sputtering of ZnTe:N which shows promise for use as a transparent back contact or recombination junction for alloyed II -VI-based top cells in a tandem solar-cell configuration.
Polycrystalline Hg 1-x Cd x Te films were investigated for their potential as bottom cells of a CdTe-based tandem solar cell. The films were deposited by RF sputtering from a cold pressed target containing 30% HgTe + 70% CdTe. The as-deposited films were highly resistive with (111) preferred orientation and a bandgap of ~1.0 eV. Various thermal treatment schemes were investigated under different conditions of ambient and temperature to reduce the resistivity. The film properties were analyzed using infrared transmission spectra, energy dispersive X-ray spectra and X-ray diffraction. N doped p-HgCdTe films were also prepared by reactive sputtering in a N 2 /Ar ambient. P-n junction solar cells were fabricated with CdS films as the heterojunction partner.
Cd1-xMnxTe alloy films with band gaps of 1.6 ∼ 1.8 eV have been deposited by RF magnetron sputtering for solar-cell applications. The films have been treated by chloride vapors to improve the photovoltaic performance. These as-deposited and chloride-treated CdMnTe films have been investigated by Raman spectroscopy, x-ray diffraction (XRD) and scanning electron microscopy (SEM). Raman results indicate that Te and/or TeO2 exists in the annealed samples depending on anneal conditions.
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