By Katrice Jalbuena
Though silicon is the second most abundant element on earth, a silicon shortage sometime in 2005 sent a fissure of fear in the solar market. The shortage pushed up the price of solar power from about US$4 a watt in the early 2000? to more than US$4.80 per watt, according to industry tracking firm Solarbuzz.
The growth of the solar market was too fast for silicon production, and the rather complicated production process for silicon solar cells was not helping keep the prices down. This led to the development of thin-film technologies which either used less silicon or simply other light-absorbing materials.
While reports state that the silicon shortage was set to end in 2008 with added production capacity and silicon prices falling to meet the demand, the interest in thin film and other solar cells remains great.
According to REN21¡®s 2009 report, solar PV in 2008 showed three trends: increase in utility-scale innovations, more thin-film PV installations and renewed in interest in building integrated PV.
The Rise of Thin-film Cells
The last two trends are especially interesting because they imply that the future really does lie in the use of less silicon for PV. Building integrated PV usually employs thin-film solar cells for their flexibility, opacity and because they can be applied to cheap substrates such as glass, stainless steel or plastic--materials already being employed in construction.
According to REN 21, of the 6.9 GW of global production of solar cells in 2008, 950 MW came from thin film, marking a 120% increase.
Based on Solarbuzz¡®s report, the world¡®s solar cell production reached 9.34 GW in 2009, with thin film accounting for 18% of the total. The report also notes that the price of crystalline silicon modules went down by 38% from the previous year.
As it stands, silicon solar cells have the advantage over cells using other materials in terms of efficiency or how well and how much power they can convert and store from light absorbed.
The average silicon cell solar module has an efficiency of around 15% while, in contrast, an average thin-film cell solar module has an efficiency of 6%.
The University of New South Wales reportedly has the highest efficiency silicon solar cells, achieving 25% efficiency in 2008. For silicon PV cells, 29% efficiency is the theoretical maximum efficiency.
SunPower reportedly owns the highest commercial efficiency with silicon solar cells, with an efficiency of 22% in mass production since 2007 and an achievement of 23.4% from a prototype solar cell in 2008.
Thin-film solar cells refer to low-cost photovoltaic cells that use little?bout 1%?to no silicon. According to the United States Department of Energy¡®s solar program, the name thin film comes from the method used to deposit the film of semiconducting materials.
Thin-film cells are deposited on very thin, consecutive layers of atoms, molecules or ions. They have certain advantages over ¡®thick-film¡® silicon cells: they use less material (1 micrometer to 10 micrometers thick compared to 100 micrometer- to 200 micrometer-thick silicon); they can be manufactured in a large-area, automated, continuous production process; and they can be deposited on flexible substrate materials.
Amorphous Silicon--The Forerunner
The amorphous silicon solar cell--one of the first and most common types of thin-film cells--is actually a ¡®low¡® silicon cell. It was in 1974 when the industry realized that amorphous silicon can be used in PV devices. It is now commonly utilized in low-power gadgets such as watches and calculators.
Amorphous silicon absorbs solar radiation 40 times more efficiently then crystalline silicon, so a film of only about 1 micrometer thick can absorb 90% of the light energy signing on it.
However, it only has a typical energy conversion efficiency of 6% to 8% and a shorter lifetime than silicon cells as amorphous silicon suffers from the Staebler-Wronski effect, which involves decreasing electrical output over a period of time after its first exposure to sunlight.
One economic advantage of amorphous silicon is it can be deposited easily--via roll-to-roll processing, inkjet printing or chemical vapor deposition--on low-cost substrates such as plastic, glass and metal. Not only does this keep costs down; it makes amorphous silicon ideal for building integrated PV.
In February 2010, Anwell Technologies claimed an efficiency of 8.58% for amorphous silicon. The company planed to ramp up volume production by the end of March 2010. Other companies that still work with amorphous silicon include Sharp Corporation, Mitsubishi Group, Applied Materials and Oerlikon Solar.
There are three relatively popular thin-film technologies that eliminate silicon all-together: Cadmium Telluride (CdTe); Copper Indium Diselenide (CIS) or Copper Indium Gallium Diselenide (CIGS), and galluim arsenide. Among these, cadmium telluride is said to have the lowest cost per megawatt of all thin film while copper indium gallium diselenide has the highest average efficiency among thin films at 20%.
Commercially produced CIS films have an efficiency of around 11% to 12%. German Wrth Solar began in 2006 the first large-scale production of CIS modules with an average efficiency of about 12%. German research institute ZSW was able to achieve 19.6% efficiency with a multistage, pilot production line in 2009.
Adding a small amount of gallium to the mix of a CIS solar cell produces a CIGS solar cell. A team at the National Renewable Energy Laboratory (NREL) was able to achieve 19.9% efficiency with CIGS.
The largest CIGS array became fully operational in December of 2008. Located at the manufacturing facility of Global Solar Energy in Tucson, Arizona, the 750-kW system provides power to the facility.
Cadmium telluride is another prominent thin-film material. NREL holds the world record for conversion efficiency of CdTe, attaining 16.5% in 2001. First Solar Inc., the largest thin-film manufacturer in the world, uses CdTe with an average conversion efficiency of 11%.
Cadmium telluride is cheaper than silicon and possesses the easier and more economical process advantage of thin film, but issues about elements used in CdTe remain. Cadium is an extremely toxic metal, and using it raises concerns on recycling and disposal. Tellurium, on the other hand, is not a common element, mostly coming as a byproduct from copper, lead and gold.
GE recently announced that it was partnering with PrimeStar Solar Inc. on CdTe technology in China where most of the CdTe raw materials are found.
Efficiency is crucial to making solar energy cheap and practical for widespread use. A solar power system must not cost more to install than it saves, or must at least recover its installation cost in savings in a short time.
While there are other ways of increasing the overall efficiency of a solar power system--multiple junction cells, solar trackers, concentrators, solar thermal, organic electronics--the end result is the same: a low-cost but effective solar cell is still key. To that extent, developing an efficient solar cell that uses less or no silicon will continue to command immense interest.
Katrice R. Jalbuena, a writer for EcoSeed, has written extensively about the renewable energy markets in the United States and Europe, and is particularly interested in technology-related news (http://www.ecoseed.org/).
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