Amorphous Silicon based Solar Cell Technologies: Status, Challenges, and Opportunities
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Amorphous Silicon based Solar Cell Technologies: Status, Challenges, and Opportunities Rajeewa R. Arya Arya International, Inc. P. O .Box 6852 Williamsburg, VA 23185 E-mail: [email protected] ABSTRACT Advances in amorphous silicon solar cell and module development over the past two decades has led to widespread commercial application in consumer and building integrated photovoltaic applications (BIPV). The technology has taken two pathways: (i) superstrate and (ii) substrate. Both pathways have unique advantages over crystalline modules and have demonstrated promising stability and reliability with continuous improvement in performance. Multi-junction modules with amorphous and microcrystalline silicon have demonstrated initial conversion efficiencies in the range of 13%-13.5%. INTRODUCTION Amorphous silicon solar cells and modules have been of scientific, technological, and commercial interest for over 25 years because they have held the best hope for low cost photovoltaic power generation. Research on amorphous silicon for solar cells started in the early seventies and continues to be a vibrant technology. The first commercial product (calculator modules) with amorphous silicon entered the marketplace in early eighties. In the past two decades the technology has made large strides in research, development, and manufacturing – today there are a whole gamut of products, from small calculator modules to large-area power modules which address consumer, architectural and power generation market segments. HISTORICAL DEVELOPMENT Amorphous silicon technology has developed with two basic configurations in the device structure: (1) superstrate configuration using glass as the superstrate, and (2) substrate configuration using stainless steel or Plastic foil/ribbon. In these two configurations light enters the solar cell device either through glass or through a transparent plastic like Tefzel. The two configurations are schematically depicted in Figure 1(a) and Figure 1(b) respectively. Both configurations employ optical enhancement to maximize absorption of incoming light. This is accomplished by light-scattering (either from front contact or from back contact) and reflection from the rear contact. While both configurations have a front contact which is a transparent conducting oxide (TCO) like tin oxide, indium-tin-oxide, or zinc oxide, in superstrate configuration the TCO is relatively thick (~600 nm) and textured while in substrate configuration the front contact is thin ( ~20 nm) and coats the textured substrate. The semiconductor thin-films layers within the two contacts can be a
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Figures 1 (a) & (b): Schematic of the superstrate and substrate device configurations respectively. single junction p-i-n (a-Si), a tandem junction p-i-np-in, or a triple junction p-i-np-i-np-in. The tandem or triple junction configurations employ either all a-Si: H intrinsic layers or a combination of a-Si: H layers and a-SiGe: H alloy layers and/or microcrystalline silicon layers. KEY TECHNOLOGY DEVELOPMENTS The first
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