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Copper indium gallium selenide (CIGS)

CIGS panels are one of the more complex alternatives to silicon. The basis of these solar panels is a tetrahedrally bonded semiconductor with a very complex crystal structure. Like cadmium-telluride, CIGS panels have a band gap that is well matched to the energy that photons deliver. These panels have reached an efficiency of 20%, making them the most efficient thin film panels to date.

Copper is an abundant, non-toxic material that has been used for thousands of years. As a metal, copper is a superior conductor and is the primary component of nearly all electrical wiring. At current rates of consumption, it is estimated that there is enough copper to be mined from the Earth’s crust to last 5 million years.

Unlike copper, indium is relatively rare element. It has the atomic number 49, which makes it a post-transition metal. Though not as rare as tellurim, the estimated reserve size of indium is 6,000 metric tons. This is, at current rates of consumption, only 13 years worth of indium. There is some debate over just how “recoverable” indium is because it is approximately three times more abundant than silver, which is mined at a rate of over 18,000 metric tons per year. It seems that the major problem with indium is that it is widely scattered throughout the crust and is not found in large deposits. This makes it slow to mine and also means that it must be heavily refined before use.

Gallium does not occur naturally in the Earth’s crust as an element. Rather, it is found only as a salt that occurs in aluminum and zinc ores. Gallium’s melting point is so low that it melts when held in the hand. Gallium is relatively abundant, with reserves in the U.S. alone exceeding one million metric tons.

Selenium, like gallium, rarely occurs in nature in its elemental form. It is generally found along with copper, silver, and lead ore and is processed most commonly from copper. As an interesting aside, selenium is necessary in small doses for many organisms, but is toxic in large doses.

CIGS Photovoltaic Panels

CIGS is a very dense material and strongly absorbs sunlight. As a result of these properties, CIGS solar panels must be very thin. In fact, their width is less than 50 micrometers when the glass backing and support is not considered. CIGS is used in the polycrystalline form to produce solar panels.

Though standard silicon panels are the most efficient, CIGS panels are the most efficient thin panels currently available. Their light weight and high efficiency makes them extremely useful in certain applications, such as on space vehicles and planetary rovers.
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