Electricity from photovoltaic solar cells: Flat-Plate Solar Array Project final Report. Volume II: Silicon material

Abstract

The Flat-Plate Solar Array (FSA) Project, funded by the U.S. Government and managed by the Jet Propulsion Laboratory, was formed in 1975 to develop the module/array technology needed to attain widespread terrestrial use of photovoltaics by 1985. To accomplish this, the FSA Project established and managed an Industry, University, and Federal Government Team to perform the needed research and development. The goal of the Silicon Material Task, a part of the FSA Project, was to develop and demonstrate the technology for the low-cost production of silicon of suitable purity to be used as the basic material for the manufacture of terrestrial photovoltaic solar cells. To be compatible with the price goals of the FSA Project, the price of the produced silicon was to be less than $1 O/kg (in 1975 dollars). Summarized in this document are 11 different processes for the production of silicon that were investigated and developed to varying extent by industrial, university, and Government researchers. The silane-production section of the Union Carbide Corp. (UCC) silane process was developed completely in this program. Coupled with Siemens-type chemical vapor deposition reactors, the process was carried through the pilot plant stage. The overall UCC process involves the conversion of metallurgical-grade silicon to silane followed by decomposition of the silane to purified silicon. Production of very high-purity silane and silicon was demonstrated. Although it has as yet not achieved commercial application, the development of fluidized-bed technology for the low-cost, high-throughput conversion of silane-to-silicon has been demonstrated in the research laboratory and now is in engineering development. A 100-MT /year pilot plant has been in operation since 1983, a 1200-MT /year commercial silane production plant started operation in 1985, and a second 1200-MT /year plant is being checked out and will start up in 1987. A third, larger plant with fluidized-bed reactors (FBRs) for silicon production is scheduled to be operating before the end of this decade. The semiconductor-grade silicon produced in these three plants, all funded by UCC, will constitute about one-third of the world production of silicon for all semiconductor devices. An economic estimate of the cost of producing silicon by the complete UCC process incorporating FBR technology is $16.05/kg (1985 dollars). This results in a price of $25.13/kg that includes a 20% return-on-investment. The goal of the Task was a price of $18.62/kg. The estimate was made by combining a calculation for the conversion of silane-to-silicon using FBRs with the Lamar analysis (see Section V.D .). The Lamar analysis was modified by deletion of the production section for the use of free-space reactors and melters. It must be kept in mind that these figures are not exact, but are the most recent preliminary chemical engineering estimates. The other process developments are described to the extent that they were supported by the Project. Some process developments have continued to be developed under private sponsorship. Studies are reported on .the effects of impurities in silicon on both silicon-material properties and on solar cell performance. These studies yielded extensive information and models for relating specific elemental concentrations to levels of deleterious effects.

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