Growth of silicon sheets for photovoltaic applications
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Growth of silicon sheets for photovoltaic applications R. O. Bella) and J. P. Kalejs ASE Americas Inc., 4 Suburban Park Drive, Billerica, Massachusetts 01821 (Received 2 March 1998; accepted 2 March 1998)
Several techniques for the sheet growth of silicon for solar cell substrates are reviewed here. These techniques usually offer an economic advantage over growth in the form of bulk crystals. At least 16 different sheet growth systems have been proposed but only five, that are actively being pursued for commercialization, are discussed here. These include dendritic web, string ribbon, edge-defined film-fed growth (EFG), ribbon growth on a substrate (RGS), and Silicon-FilmTM . The growth systems and the characteristics of the resulting solar cells are briefly described. A discussion of their current status concludes the review.
I. INTRODUCTION
Many silicon sheet growth techniques have been developed over the years, mainly to produce wafers for substrates for solar cells. The advantages of such techniques are the efficient use of silicon feed stock with no kerf loss, and the elimination of mechanical sawing steps including etching to remove surface damage. Often continuous growth is possible. These advantages translate directly into improved economics. The trade-offs are mainly the difficulty of maintaining the intrinsic qualities of the silicon, i.e., single crystallinity, low defect density, long minority carrier lifetime, low stress, and mechanical integrity including flatness, uniform thickness, and surface roughness. Over the years a number of sheet growth systems have been proposed. A partial list of these techniques in roughly chronological order of when they were suggested is dendritic web,1 EFG (edge-defined, film-fed growth),2 RAD (ribbon against drop),3 SOC (silicon on ceramic),4 RTR (ribbon to ribbon),5 LASS (low angle silicon sheet growth),6 ESP (edge supported growth)7 and the similar ESR (edge stabilized ribbon)8 now called string ribbon,9 S-Web (supported web),10 RAFT (ramp assisted foil-casting technique),11 Spinning Method,12 the Silicon-FilmTM process,13 RGS (ribbon growth on a substrate),14 SSP (silicon sheets from powder),15 and CRP (cast ribbon process).16 In this review we will concentrate on several of these techniques that are being actively pursued with commercialization either realized or as a goal. These are dendritic web which is being developed at EBARA Solar, EFG at ASE Americas, string ribbon at Evergreen Solar, RGS at Bayer AG, and the Silicon-FilmTM process at AstroPower. We will not include bulk growth that requires sawing such as cast multi-crystalline silicon.
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J. Mater. Res., Vol. 13, No. 10, Oct 1998
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These five techniques are not at the same stage of development. EFG and dendritic web are in full scale production. Silicon-FilmTM is in pre-production with full scale production planned. String silicon is
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