Amorphous and Microcrystalline Silicon-based Photovoltaic
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Amorphous and Microcrystalline Silicon-based Photovoltaic Subhendu Guha United Solar Ovonic Corporation 3800 Lapeer Road Auburn Hills, MI 48326, U.S.A. ABSTRACT The last two decades have witnessed tremendous progress in the science and technology of amorphous and microcrystalline silicon-based photovoltaic. Advances in the understanding of materials and devices have led manufacturers to expand their production capacity; the production of solar panels based on this technology exceeded 25 MW in 2003. Hydrogen dilution in the active gas mixture during deposition has played a key role in improving the quality of the materials and the performance of the devices. In this paper I shall review the properties of the optimum material for device application, and discuss the production status. I shall also report on the new opportunities that are opening up for these products for space and stratospheric applications. INTRODUCTION Significant progress has been made in the field of science and technology of amorphous silicon (a-Si) alloys in the last two decades. When the first MRS symposium on a-Si alloy took place in 1985, the focus rightfully was on material aspects. Our understanding of multi-junction devices was at its infancy; manufacturing also was in very small volume for consumer products only. The field has matured significantly with contribution from a very active international scientific community; the industry is also thriving with production in 2003 exceeding 25 MW with the products used in applications ranging from battery charging for small appliances to large-scale grid-connected systems. While the industrial focus has been mainly on multi-junction cells using a-Si and amorphous silicon germanium (a-SiGe) alloys [1], research on microcrystalline silicon has established it as a potential material for use in the bottom cells of such devices [2]. a-Si alloy is usually prepared by glow-discharge decomposition of silane. It was first shown by Guha et al. [3] that superior film properties could be obtained by using a dilute mixture of silane in hydrogen. As the hydrogen dilution is increased, the transition from amorphous to microcrystalline phase takes place. Recent work has shown that the best amorphous silicon is grown at a dilution just below the edge of amorphous to microcrystalline transition [4]. On the other hand, the highest efficiency microcrystalline solar cells are made when the hydrogen dilution pushes the material just above the edge [5]. The material near the edge thus has received a great deal of attention. In this paper, we discuss material and cell properties of thin-film silicon in this deposition regime. We shall also describe the manufacturing technology of solar cells using this material, and some new applications.
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MATERIAL BELOW THE EDGE (AMORPHOUS PHASE) Plasma Chemistry The basic plasma chemistry prevalent in a discharge of silane with or without hydrogen dilution has been investigated extensively [6,7]. Electron-molecule collisions dissociate the molecules into ions and neutr
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