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Polycrystalline II–VI semiconductor materials show great promise for efficient, low-cost photovoltaics. Large-area deposition of the 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 accrue 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 fabrication of tandem cells with efficiencies over 25%. Our best results have been obtained with sputtered ZnO:Al to achieve a CdTe solar cell having 14.0% efficiency at one sun with an air mass 1.5 global spectrum. We have also studied reactive sputtering of ZnTe:N which shows promise for use as a transparent back contact or as a recombination junction with ZnO:Al for II–VI based alloy top cells in a tandem solar-cell configuration. Finally, some advances have been made in substrate-configuration CdTe cells on Mo using sputter deposition that hold promise for flexible CdTe-based PV.

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.

Radio-frequency (rf)-sputtered Al-doped ZnO was used as the transparent front contact in the fabrication of high efficiency superstrate configuration CdS∕CdTe thin-film solar cells. These cells had CdS and CdTe layers also deposited by rf sputtering at 250°C with the highest processing temperature of 387°C reached during a post-deposition treatment. The devices were tested at National Renewable Energy Laboratory and yielded an efficiency of 14.0%, which is excellent for a CdTe cell using ZnO and also for any sputtered CdTe solar cell. The low-temperature deposition process using sputtering for all semiconductor layers facilitates the use of ZnO and conveys significant advantages for the fabrication of more complex multiple layers needed for the fabrication of tandem polycrystalline solar cells and for cells on polymer materials.

The present work focuses on the optimization of an interconnect junction for II-VI tandem solar cells. We have used ZnTe:N/ZnO:Al and ZnTe:Cu/ZnO:AI as interconnect junctions between a CdS/CdTe top cell and a CdS/HgCdTe bottom cell. Separate annealing studies of ZnTe:Cu show electrical and optical properties that are stable to 387/spl deg/C when the ZnTe:Cu is sputter deposited at /spl sim/320/spl deg/C. J-V measurements show that the V/sub OC/ of the tandem cell with ZnTe:Cu is much lower (/spl sim/820 mV) than the V/sub OC/ of the tandem cell (/spl sim/1031 mV) with ZnTe:N. One of the possible reasons is the interdiffusion of Cu into CdS, probably making it intrinsic and changing the junction properties of the top cell. Quantum efficiency measurements of the tandem cell with ZnTe:N show that the current is limited by the bottom cell, while the current seems to be limited by both the top cell and the bottom cell in the case of the tandem cell with ZnTe:Cu/ZnO:AI as the interconnect junction.

Large scale (gigawatts/yr) manufacturing of CdTe PV modules may be constrained due to the limited availability of the relatively rare (Te) element and the volume of potentially hazardous (Cd) material being used in 3-8 /spl mu/m thick CdTe absorber layer. This work is focused on cells with much thinner CdTe layers. Magnetron sputtering was used to prepare a series of CdS/CdTe cells with CdTe thickness from 0.5 to 1.28 /spl mu/m. Optimization of the post-deposition CdCl/sub 2/ treatment and back contact processing for the thinner layers improves the efficiency to very close to thicker CdTe cells. An efficiency of 11.8% has been achieved using only 0.87 /spl mu/m of CdTe. This cell showed V/sub OC/ of 772 mV, J/sub SC/ of 21.98 mA/cm/sup 2/ and FF of 69.7%.

We describe the application of sputter deposition processes to the fabrication of CdS/CdTe solar cells on light weight, flexible substrates in both the substrate and superstrate configurations. We have fabricated CdS/CdTe cells on molybdenum foil and Kapton polyimide. The Mo-based devices are among the best reported for substrate configuration CdTe solar cells, with efficiencies of /spl sim/7% at AM1.5.

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 Hg1−xCdxTe film as the bottom absorption layer was deposited by RF sputtering with 70% or 85% Cd content in the Hg1−xCdxTe magnetron target. Hg1−xCdxTe films with band gap from 0.98 eV to 1.45 eV were obtained by controlling the deposition temperature. CdCl2 thermal treatments were used to improve the Hg1−xCdxTe film electrical properties. A nitrogen-doped ZnTe film combined with an aluminium (Al) doped ZnO film formed a good interconnect junction. Results of Voc = 0.99 V and Jsc = 2.1 mA/cm2 were obtained in the best such tandem cell at one sun (AM1.5).

Reactively sputtered ZnTe:N is a close valence-band match to CdTe, transparent below 2.2 eV and therefore an attractive candidate for a back contact/tunnel junction in tandem cells using CdTe or CdZnTe top cells. We report on measurements of the optical emission spectra of N/sub 2/ during reactive sputtering as part of the doping optimization. For materials characterization, a series of films produced with various N/sub 2//Ar gas ratios were studied by x-ray diffraction (XRD), atomic force microscopy (AFM), Raman spectroscopy, optical absorption, variable angle spectroscopic ellipsometry (VASE), and the Hall effect. For transparent back contact fabrication, we used a ZnTe:N/ZnO:Al recombination junction bilayer.

An Al-doped ZnO front contact was successfully used for the first time for the fabrication of high efficiency CdS/CdTe thin-film solar cells. The ZnO:Al films were deposited on aluminosilicate glass by RF sputtering from a ZnO:Al2O3 target. The ZnO:Al film has ∼95% average transmission in the visible spectrum with ∼3 ohm/square sheet resistance. The CdS and CdTe thin films were also deposited by RF sputtering and devices were completed with a vapor CdCl2 treatment and evaporated Cu/Au back contacts. The highest processing temperature was 387°C, reached during the vapor CdCl2 treatment. The devices were tested at NREL with efficiency of 14.0% which is a record for an all-sputtered CdS/CdTe solar cell. The ZnO-based cell had JSC of 23.6 mA/cm2 compared to 20.7 mA/cm2 for our recent NREL-tested 12.6% cell on a commercial soda-lime-glass/SnO2:F substrate. Other parameters of the 14% ZnO based cell are: FF = 73.25% and VOC = 814 mV. The improved performance is almost entirely due to higher current because of better optical and electrical properties of ZnO:Al TCO. We report also on relative stability between devices on SnO2:F and ZnO:Al TCO, under one-sun light soak at VOC.

Most of the low resistance back contacts formed on high efficiency CdS/CdTe solar cells involve copper either in elemental form (such as Cu/Au back contacts) or as dopant in other material (such as Cu-doped ZnTe). But copper is also suspected to be a cause of degradation of devices in long-term stability tests due to its high diffusion coefficient in polycrystalline CdTe. In this paper, we present results on the development of nitrogen-doped ZnTe back contacts for CdS/CdTe solar cells. Reproducible N-doped p-ZnTe films were prepared using reactive RF magnetron sputtering with Ar/N2 gas mixtures. The conductivity of the doped ZnTe films was more than five orders of magnitude higher than that of intrinsic films. We find that annealing in air can further increase the conductivity. Efficiencies near 10% have been achieved with a ZnTe:N/Ni back contact.

High efficiency CdTe solar cells are typically grown with CdTe thicknesses from 3 to 15 μm, although the thickness required for 90% absorption of the incident irradiation at 800 nm is only ∼1 μm. In this paper, we present the effect of CdTe thickness reduction on the performance of CdS/CdTe solar cells in which both the CdS and CdTe films were grown by sputtering. We produced a series of cells with different CdTe thickness (from 0.5 to 3.0 μm), and held the CdS thickness and back-contact-processing constant. The effect of CdTe thickness reduction on the diffusion of CdS into CdTe was studied using optical absorption and x-ray diffraction techniques. Only slight decreases occur in open-circuit voltage, short-circuit current, and fill factor with decrease in CdTe film thickness to 1.0 μm. Almost 10% efficient cells were obtained with 1 μm CdTe. Below 1 μm, all cell parameters decrease more rapidly, including the red quantum efficiency.

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.