Magnetron sputtering for II-VI solar cells: thinning the CdTe
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1165-M09-01
Magnetron sputtering for II-VI solar cells: thinning the CdTe V. V. Plotnikov, A. C. Vasko, A. D. Compaan, X. Liu, K. A. Wieland, R. M. Zeller, J. Li, R. W. Collins Department of Physics and Astronomy, and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, OH 43606 ABSTRACT Magnetron sputtering (MS) of CdTe and related II-VI materials facilitates low energy ion and electron bombardment that promotes good film growth at substrate temperatures well below those needed for other physical vapor deposition methods. MS also provides good control of deposition rates while allowing scale-up to large areas. In this paper we review the use of MS for deposition of polycrystalline thin films of CdS, CdTe and related materials for solar cells with a focus on reducing the thickness. We relate the deposition conditions and plasma properties determined by Langmuir probe measurements to some of the materials properties of the films through spectroscopic ellipsometry and high resolution electron microscopy. For cells with CdTe layers from 0.35 to 2.5 µm, we have done a first-order optimization of chloride treatment conditions and back contact structure. We discuss the influence of CdTe thickness and post-deposition processing on the efficiency, open-circuit voltage, short-circuit current, and fill factor and show that 10% efficient cells can be fabricated with 0.5 µm of CdTe.
INTRODUCTION Reduction of absorber layer thickness in thin-film solar cells can speed the fabrication process, which is particularly important for lower-speed deposition methods, reduce the demand for source or feedstock materials, extend supplies and thereby enlarge the ultimate market penetration, lessen environmental concerns, and facilitate recovery of materials at the end of life. In addition, the use of extremely thin layers, while maintaining efficiency, can open new applications such as semitransparent photovoltaic coatings for architectural glazing. In this paper we describe our studies with sputter deposition for achieving the high quality absorber layers that can facilitate fabrication of cells with CdTe thickness well below one micron. Understanding the plasma conditions through the use of a Langmuir probe, in situ monitoring of the deposition process with spectroscopic ellipsometry and optical reflection, as well as optimization of the post-deposition treatments have been keys to the achievement of an efficient cell with less than 0.5 µm of CdTe.
EXPERIMENTAL DETAILS We have shown that rf sputtering of ZnO:Al, CdS, and CdTe layers can produce 14% efficient cells on 1 mm aluminosilicate glass (ASG).1 In addition, we have shown that ~12.5% efficient cells can be prepared by sputtering on commercially available 3 mm coated soda lime glass (Pilkington TEC-15).2, 3 In recent work we have exceeded 13% cells on TEC-15 if a high 1
resistivity transparent (HRT) layer is used between the SnO2:F and the CdS layer. In the case of ASG substrates, we have not yet used an HRT layer which facilitates
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