A prototype monolithic HgCdTe/CdTe superstrate tandem cell has been fabricated by RF sputtering, comprising a CdTe/CdS top cell, a ZnTe:N/ZnO:Al interconnect junction and a HgCdTe/CdS bottom cell. The Hg 1-x Cd x Te film as the bottom absorption layer was deposited by RF sputtering with 70% or 85% Cd content in the Hg 1-x Cd x Te magnetron target. Hg 1-x Cd x Te films with band gap from 0.98 eV to 1.45 eV were obtained by controlling the deposition temperature. CdCl 2 thermal treatments were used to improve the Hg 1-x Cd x Te film electrical properties. A nitrogen-doped ZnTe film combined with an aluminium (Al) doped ZnO film formed a good interconnect junction. Results of V oc = 0.99 V and J sc = 2.1 mA /cm 2 were obtained in the best such tandem cell at one sun (AM1.5).
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.
Real time spectroscopic ellipsometry (RTSE) based on rotating-compensator modulation and multichannel detection has been implemented to characterize polycrystalline thin film CdTe deposition for photovoltaic applications. RTSE is capable of providing routine deposition information on substrate temperature T and deposition rate. It is also capable of providing detailed information on the thickness evolution of microstructure and optical properties. In this study, we highlight the differences in nucleation that occur under different CdTe deposition conditions on smooth crystalline Si wafer substrates. Differing behavior in the initial stages of deposition has been observed, ranging from layer-by-layer growth to nucleation and coalescence of 45 Å thick clusters. We also consider the thickness and substrate dependence of the microstructure, comparing depositions on smooth Si wafer and rough thin film Mo substrates.
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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