Characterization of Thin Film CdTe photovoltaic materials deposited by high plasma density magnetron sputtering
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Characterization of Thin Film CdTe photovoltaic materials deposited by high plasma density magnetron sputtering A. Abbas¹ ², J.W. Bowers², B. Maniscalco², S. Moh², G.D West¹, P.N. Rowley², H.M Upadhyaya² and J. M. Walls² 1
CREST (Centre for Renewable Energy Systems and Technology), Loughborough University, Loughborough, LE11 4TU, UK 2
Department of Materials, Loughborough University, Loughborough, LE11 3TU, UK,
ABSTRACT A new magnetron sputtering strategy is introduced that utilizes high plasma density (~5mA.cm-2) to avoid or reduce high temperature processing. The technique uses magnetrons of opposing magnetic polarity to create a “closed field” in which the plasma density is enhanced without the need for high applied Voltages. A batch system has been used which employs a rotating vertical drum as the substrate carrier and a symmetrical array of linear magnetrons. The magnetrons are fitted with target materials for each of the thin films required in the photovoltaic (PV) stack including the CdTe absorber layer, CdS window layer, metal contact using the conventional superstrate configuration. The “closed field” sputtering technology allows scale up not only for larger batch system designs but it is also configurable for “in-line” or “roll to roll” formats for large scale production. The morphology of each of the layers is characterized using a variety of structural and optical techniques including Field Emission Gun SEM and X-ray diffraction (XRD). INTRODUCTION Thin film CdTe photovoltaics is currently the leading thin film technology in commercial scale production. Its advantage lies in the simplicity of the design, its low cost of production and its stable long term performance. However, although the devices are in production there is still considerable scope for improvements in performance and in reducing cost still further by developing more efficient manufacturing techniques. In particular there is scope for reducing the thickness and cost of the CdTe absorber layer. In this paper, we present results using magnetron sputtering using a configuration that leads to the growing thin film being exposed to high plasma densities. The sputter-deposition technique is already well established in a number of large scale, high throughput coating applications such as on window glass and displays. Magnetron sputtering has the advantage of producing uniform coatings up to +/-1% over large areas. This is essential if the absorber thickness is to be optimised. The process is also controllable and cost effective.
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Magnetron Sputtering The magnetron sputtering configuration used in this study was initially developed for wear resistant coatings by Teer [1] and more recently for precision optical coatings [2]. The configuration links magnetic field lines between the magnetrons by reversing the polarity of adjacent magnetrons. The magnetrons are also unbalanced to further intensify the field and increase the ion current density in the plasma. Using this arrangement, the deposition volume in which the substrates are located
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