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Thursday, October 23, 2008
The thin-film variants heading for the photovoltaic market
Ever new thin-film technologies are moving out of the laboratories into factories. But which one has the most promising future?
- The amorphous silicon cell, which as yet has by far the highest market share of all thin-film cells?
- Or the CIS cell of cadmium indium diselenide, favoured by the media in the past ten years?
- Or maybe the CIGS cell of copper, indium, gallium and selenide, whose conversion efficiency promises to be the highest?
- Optionally also in the CIGSSe variant with sulphide?
- Or the micromorphous silicon cell, which is claimed to better the amorphous silicon in efficiency, but is based on the practically infinitely available silicon?
At least one thing is clear, and that is that in the thin-film sector, too, silicon will play a large part as a raw material. But with the small quantities of silicon thin-film modules need, the present scarcity is no serious problem. And the fact that silicon is a harmless substance makes the metalloid an attractive option.
Hence a number of companies and researchers are seeking optimal forms for the silicon thin-film cell.
- CSG Solar AG in Thalheim will use one process. The abbreviation CSG stands for crystalline silicon on glass. Unlike the classic silicon wafer technology, in which sawn silicon slices are used, in this method silicon comes from silane gas. An extremely thin layer of silicon, less than two microns thick, is deposited onto the textured surface of a glass sheet. The silicon is then crystallised by heating. “The resulting layer is processed using patented laser and ink-jet printing techniques to form the electrical contacts,” CSG say.
- The modules show “practically no degradation” even after 30 years, the company claims, citing proof by an accelerated aging processes. The company puts the high durability down to the silicon and aluminium used, which are very stable in the environment.
- Correspondingly CSG gives a performance warranty of the type one knows from classic silicon modules: 90% after 12 yrs, 80% after 25 years.
- The company states the conversion efficiency at module level as 7.5% in the starting phase and hopes for 9.5% “by 2010 at the latest”. For now the company doesn’t want to comment on future prices.
Meanwhile, micromorphous solar cells were developed at the Institute of Microtechnology, University of Neuchâtel, Switzerland; they combine amorphous and microcrystalline silicon. These are so-called tandem cells of two ultra-thin silicon layers on top of each other. Because both silicon structures absorb different light spectrums, the micromorphous cells are claimed to achieve markedly higher efficiency than the conventional amorphous solar cell. Moreover, the natural initial degradation, i.e. the loss of some efficiency that is unavoidable with amorphous silicon because of the production technology used, is markedly less in this structure.
The IMT reports having achieved more than 11% efficiency in the laboratory. Swiss company Unaxis AG intends to use the technology in future.
Until the process is marketable in perhaps two years, Unaxis AG will use amorphous thin-film silicon. The enterprise is just building a production facility of this type for Ersol Solar Energy AG in Erfurt, Germany.
There the modules are to be produced in a vacuum process called plasma enhanced chemical vapour deposition, PECVD for short. It works like this: the amorphous silicon is deposited on a low-cost base material (for example glass).
In further production steps a back contact is applied and the layers are structured by laser. The module surface is processed to trap the entering light in the solar cell so that it can be absorbed by the photoactive layers.
Meanwhile, other companies are pursuing further development of the CIS technology originally based on cadmium indium diselenide.
At Berlin-based Sulfurcell, for example, CIS now stands for copper indium sulphide. The company wants to apply the layers by so-called sputter deposition. In this process, which is also known as cathode deposition, atoms of a solid material are ejected into the gas phase by bombardment with energetic ions before they are deposited on the carrier substance. As usual with thin-film cells, this is followed by structuration by laser.
Another advance in CIS technology are cells of copper, indium, gallium and selenide (CIGS), as well as copper, indium, gallium, sulphide and selenide (CIGSSe). The Oldenburg company Aleo Solar GmbH plans to market this variant of five substances by the middle of next year. Company head Jakobus Smit enthuses about module conversion efficiency “of up to 15%”, making CIGSSe the “currently most efficient thin-film technology”.
The price per watt will be about the same as that of the conventional module based on silicon wafers, he says. Because of its especially good responsiveness to weak light – the module works highly efficiently even under cloud cover – as well as only a small efficiency loss at high temperatures, the CIGSSe solar cell will yield five to six per cent more on annual average, he adds.
He also claims CIGSSe advantages not only over crystalline modules, but also over the other thin-film technologies. The reason for this, says Smit, is that five different elements can better use the light spectrum – from the ultraviolet to the red range – than “only” three or four elements.
On the other hand, the production process is more difficult because of the many layers. Smit is confident, though, that production problems will be solved. The process will be based on a licence of PTIP Ltd., a spin-off enterprise of the University of Johannesburg (South Africa). Aleo also sees attractive new design elements in the modules. “They’re completely black, which makes them look classier than conventional solar modules.”
Another substance already used in the thin-film sector is cadmium telluride. Antec Solar Energy AG (Frankfurt am Main) already sells the modules and First Solar LLC is planning to build a factory for them in Frankfurt (Oder). But there is some dispute about these modules because per kilowatt they contain about 100 grams of the heavy metal cadmium.
There is still disagreement among experts over whether thin-film can gain ground long-term on wafer-based silicon cells. Gerhard Willeke, solar cell department head at the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, rates the chances reservedly. For one thing, he points out, some processes bank on rare substances; indium and tellurium could become scarce with strong thin-film growth. For another thing, he isn’t convinced of the stability and robustness of thin-film modules. “Personally I wouldn’t now put thin-film modules on my roof.”
But the majority of experts is sure that despite reservations about some semiconductor materials or comparatively lower conversion efficiencies, thin-film technology will grow. “We have no choice but
to move ahead fast in the thin-film segment,” notes Murray Cameron, president of the European Photovoltaic Industry Association (EPIA).
Fulll Article
New Energy.Info By Bernward Janzing
