Let Us Make PV-Grade Silicon And Pass The Test, Today
Several characteristics serve to differentiate ordinary silicon from the silicon in a photovoltaic cell. First, photovoltaic or other semi-conductive reactions are accomplished most efficiently in a material having a crystalline or polycrystalline structure. In a crystalline structure, all of the atoms and molecules are arranged in a definite, constantly repeated pattern. A polycrystalline structure is one in which several crystal patterns occur with the lines of demarcation between them being known as grain boundaries. Most of what are commonly called crystals are actually polycrystalline structures; the few exceptions are naturally occurring gemstones and the carefully fabricated crystal semiconductors required by the electronics and computer industries.
In the past, scientists thought that photovoltaic performance fell off significantly when polycrystalline material was substituted for the single- crystal variety, but this now appears not to be the case. A perfect single crystal is a good deal more difficult and expensive to produce than one that exhibits grain boundaries. Improved performance must be balanced against increased expense. Since the degree of incremental improvement in cell performance tapers off sharply as the single-crystal state is neared, the most attractive cost-efficiency ratios are currently achieved with silicon that is somewhat less crystalline perfect than that used by the computer semiconductor industry.
There is even a place in photovoltaics for amorphous silicon that is, silicon whose atomic or molecular structure makes no pretense of regularity. Such non-crystalline silicon responds photo-electrically to sunlight, although its efficiency is markedly lower and a larger cell area is required to produce the same amount of electricity as an array of crystalline or high-grade polycrystalline cells. Thin films of amorphous silicon are currently used in solar-powered watches, calculators and other small, low-wattage consumer products as well as building materials including batten-seam rooftops and shingles.
Since photovoltaic-grade, polycrystalline silicon requires less stringent manufacturing procedures than the single-crystal variety, you might think that producing an adequate supply would be relatively easy. Theoretically, it is. But -since the computer industry and others that use semiconductors grew to maturity while solar electric technologies were still in their experimental stages, existing silicon-processing facilities are geared toward single-crystal production of semiconductor-grade silicon. Capital investments must be directed into new manufacturing facilities needed for polycrystalline silicon to be produced in the quantity needed to create a cheap and abundant supply of solar-grade silicon. Fortunately, there is much evidence that indicates such investment is now under way.
Solar Development Cooperative's Comprehensive BI-PV Deployment Plan
In order for the BI-PV industry to flourish and grow; capital investment is needed for new manufacturing facilities to keep up with market demand. A deployment strategy that embraces PV-grade silicon development at its center involved in all phases of production from the mining of silica to the completed product of aesthtetic fully-integratable construction-industry standardized solar electric building materials will develop cooperative alliances as new mines and manufacturing businesses are opened in the near future. If we are going to invest the resources to evolve the photovoltaic industry by installing One Million Solar Rooftops In The USA 1997-2010, it is wise to build partnering alliances at every stage of product development, system design and installation to assure quality BI-PV products are developed for the end user of photovoltaics -the building owners. Through the intensive product development process suggested here, a quality service industry will naturally evolve as we will be very interested in how these new products perform and integrate aesthetically in a variety of situations presented by the many clients to be served. BI-PV is a product-driven market, not a fuel-driven market. This is one of several reasons direct economic and cost-efficiency comparisons between coal and BI-PV electricity are not sound. Certainly, these comparisons must involve at least a production level ratio in the formula to reflect rational valuations of the true cost of electricity being compared.
Manufacturing Solar-Grade SiliconThe first step in putting together a silicon photovoltaic cell is the processing of raw silicon. Two objectives dominate this phase of the operation: first, the silicon must be virtually free from impurities, except for the deliberately added dopants; and second, it must be delivered in a crystalline or acceptable polycrystalline state. As noted earlier, crystalline perfection is not nearly as imperative in solar-grade silicon as it is in the silicon use by the electronics industry. Manufacturers of solar silicon also have more latitude when it comes to purity. Instead of the - one part per billion impurity - ceiling mandated for computer-chip silicon, impurities in the base PV material need only be held to - one part per million. That may still seem impossibly finicky, but it translates into a considerable reduction in processing costs, which help make photovoltaics affordable.
Czochralski process--a procedure for producing large single crystals of silicon by slowly drawing a seed crystal from a crucible of melted silicon.
Wafer--a slice of silicon or other semiconductor from which a photovotlaic cell is made.
Boule--a cylindrical, single-crystal silicon ingot formed synthetically in a special furnace.
Single-Crystal Silicon
Polycrystalline Silicon
Mobil Solar Energy Corporation developed the edge-defined film-fed growth (EFG)
Dendric Web Ribbon
Dendrites--elongated starter crystals used in the formation of thin, web-like sheets of crystal silicon.
Amorphous Silicon
ARCO Solar's thin-film, amorphous silicon photovoltaic cells were available in many shapes and sizes to satisfy a variety of small power requirements.
From Wafer to Cell and from Cell to Module
We have looked at some of the ways photovoltaic-grade silicon can be produced. Let's now turn to the procedures involved in producing solar cells from this material and assembling these cells not into modules, but into standardized solar electric building materials for the construction industry. Electri-City® Solar Electric Building Materials is the mining and manufacturing branch of the Solar Development Cooperative -Lighting the Way With Creation’s Original Remedy. STRATEGIC INVESTMENTS FOR THE MILLION SOLAR ROOFTOPS USA 1997-2010 PROGRAM If we are to utilize this important and timely national initiative cost-effectively, we must be involved in building the solid foundations of an industry in lieu of merely installing one million solar rooftops in the next ten years. Silica mining for the express purpose of evolving PV-Grade Silicon that is used to produce aesthetic solar electric building materials in standardized sizes for the construction industry is an industry deployment plan that will protect national security while we assure a progressive and competitive role for the United States within the energy, construction and defense industries far into the next millennium. Consider the fact that the present semi-conductor modules are similar to the first computers. Acknowledge the advancements made in the computer industry in the last fifteen years. Imagine the potential of having not only aesthetic electricity-producing building envelopes, but digital computer motherboards the size of building walls covering an interior wall. The marriage of architectural building materials and the semi-conductor industry has far-reaching implications with the potential to effect the quality and style of human shelter, communications, transportation and commerce well into the 3rd Millennium and beyond.
The Solar Electric House -Energy for the Environmentally Responsive, Energy-Independent Home
-which I highly recommend for any who has a serious interest in either buying a solar system or going into business within the BI-PV Industry.
Intercultural Center at Georgetown University
Washington, DC Installed 1984 300 kilowatt peak
Dr. Lindmayer founder of Solarex facilitated project