Silicon Ribbon for Space Solar Cells CHRISTIAN BELOUET Summary The classical approach based on flat solar arrays using crystalline silicon solar cells 50 pm thick is one of the approaches which has been considered for the achievement of space photovoltaic systems. The preparation of thin silicon sheets in the 50 pm range is a difficult operation which can be obviated by means of ribbon growth technologies, such as the RAD process developed at the Laboratoires de Marcoussis. The purpose of this article is to present the RAD process and to discuss its ability to produce a material appropriate for space solar cells. It is shown that the RAD material may closely approach the performances of single crystalline silicon, if the growth process and the cell fabrication are optimized for space cell applications. A 15.5% AMI conversion efficiency has always been demonstrated in the laboratory, despite the polycrystalline texture of the material, with a conventional n+/p/p+ structure. As the power of space photovoltaic systems has grown from a few watts (W) in the 1950s to a few kW at present, the supremacy of silicon as the photovoltaic basematerial has been increasingly challenged by gallium arsenide which offers a better power-to-weight ratio. This follows from the projected superiority of gallium arsenide with respect to conversion efficiency (18.4% for 4x4 cm2 cells against 14% for silicon under AMO illumination [1]), radiation resistance and thermal properties [2]. A further growth to about 100 kW will probably occur during the next decade with the permanent Earth-orbiting stations in preparation, not to mention the large space power stations envisaged in the far future [3]. The trend towards the use of high-efficiency cells initiated with the introduction <jf gallium arsenide may be pursued if the studies on concentrator arrays using multibandgap cells were successful in all respects [4, 5]. However, the concept of large stations will be increasingly influenced by the cost per watt as their size grows and this aspect may eventually retain the overall competitiveness of silicon-based technology. Present silicon technology for space applications still uses relatively thick singlecrystalline wafers, although the advantages of thin cells - typically 50 pm thick-have been demonstrated in recent years [6]. Thus far, one of the major difficulties encountered in thin cell technology was in the preparation of the base material. This difficulty may be alleviated by the development of ribbon growth techniques, stimulated by terrestrial photovoltaics, which are able to produce high-quality, thin sheets. The purpose of this paper is to present these new technological developments, the emphasis being made on the RAD growth process which can deliver polycrystalline Christian Belouet, Laboratoires de Marcoussis, CGE Research Center, Route de Nozay, 91460 Marcoussis, France.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==