Silicon - Solar Grade

Although solar platforms are evolving, silicon is still the primary energy producing material. Not just any silicon will do. It has to be silicon of solar grade.

Silicon - Solar Grade

Solar energy is one of the most promising and secure sources of alternative energy. It could reduce our reliance on fossil fuels and reduce pollution. Currently, the solar energy market share is small, but it is growing at a rate of 25% to 45% annually. The cost of a residential solar cell system could cost $25,000, which is prohibitive to the average family.

Silicon is the second most abundant element on Earth. One would think that since is so common, the silicon necessary for making solar cells would be inexpensive. Although the element is common, creating a purity useful for photovoltaic cells is expensive. Traditionally, solar cells have been made of electronic-grade polycrystalline silicon, which is also used for making silicon computer chips. So, solar energy has to compete with the computer industry for a limited, difficult to produce and therefore expensive resource. Electronic-grade silicon is very pure, more pure than is necessary for solar cells. Solar grade silicon is an intermediate grade, and therefore less expensive.

Solar Grade Silicon is metallurgical grade silicon, which is about 97 percent pure, which has been refined to over ninety-nine percent pure. The grade of the silicon is defined by its transition metal content. The most common means of removing the contaminants is by dissolving them atomically. This is highly effective, but expensive. Gettering and hydrogen passivation are complimentary techniques that can increase the performance of relatively higher quality metallurgical silicon to a grade that is usable in solar cells, but won’t be sufficient if lower grade silicon is used.

Another means of forming solar grade silicon from metallurgical grade is by manipulating the impurities in a way that they have less impact on the solar cell efficiency. This technique is called “Defect Engineering of Metal Nanodefects.” It can affect changes such as causing a redistribution of metal clusters in the silicon from a high density of small clusters and complexes to a lower density of precipitates that are larger and more spread out. This method could lead to new, lower cost solar cells.

Research is also in the works to develop solar grade silicon by decomposing high purity silane (SiH4) with a swirl-flow reactor. Silane is the silicon analog of an alkane hydrocarbon. The swirl-flow reactor provides a way for the transport of silicon particles formed in the decomposition, so the resulting transport is quick and efficient. This process is not proven.

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