Thin-film amorphous Silicon (a-Si) solar cells are gaining momentum in the market place. Amorphous Silicon cells use layers of a-Si only a few micrometers thick, attached to an inexpensive backing such as glass, flexible plastic, or stainless steel. This means that they use less than 1% of the raw material (silicon) compared standard crystalline Silicon (c-Si) cells, leading to a significant cost saving. Production of these cells is therefore less subject to the high prices of silicon caused by recent shortages. The flexible backing allows them to be formed to fit applications, allows for the bending inherent when used in building materials, such as roofing, and prevents breakage during shipping and handling at the installation site.
Amorphous solar cells absorbs light more efficiently than c-Si, but they do not convert sunlight quite as efficiently, they require considerably less energy to produce, and are superior to crystalline cells in terms of the time required to recover the energy cost of manufacture. Amorphous materials, by definition, lack a crystalline structure and can be created by melting and then rapid cooling a crystalline substance as is done with a plasma vapor deposition process.
Amorphous silicon is gradually degraded, by exposure to light, by a phenomena called the Staebler-Wronski Effect (SWE). SWE effects the power output of a-Si modules by as much as 10%. This light induced degradation is reduced by depositing the layers of the cell using high hydrogen dilution and by making combinations (alloys) of different types of cells. Because of SWE, a-Si cells are rated in the stabilized condition, which occurs after about 100 hours exposure to light.
Unlike crystal silicon, in which atomic arrangements are regular, amorphous silicon features irregular atomic arrangements. As a result, the reciprocal action between photons and silicon atoms occurs more frequently in amorphous silicon than in crystal silicon, allowing much more light to be absorbed. Thus, an ultra-thin amorphous silicon film of less than 1µm can be produced and used for power generation. This film, because it is not crystalline and is so thin, will not break when it is flexed, thus allowing it to be deposited on flexible substrates. Because of the flexability of the cell and the substrates a-Si producers are able to use automated "roll-to-roll" manufacturing processes in which the substrate and deposited material move through the production process as one continuous strip passing over several rolls in the process which maintain stability to the process as well as moving the product along its way.
Amorphous silicon films are fabricated using plasma vapor deposition techniques to apply silane (SiH4) to the substrate or other beneficial film, allowing large-area solar cells to be fabricated much more easily than with conventional c-Si. Three amorphous silicon layers — p-layer, i-layer, and n-layer — are formed consecutively on the substrate. This p-i-n junction corresponds to the p/n junction of a c-Si solar cell. Amorphous silicon can be deposited onto a many substrates Including glass and ceramics, metals such as stainless steel, and plastics.
Although, substantial advances have been made in the development of manufacturing technology for c-Si cells, the cost of crystalline PV modules is still high because of materials costs and numerous processing steps that are needed to manufacture the modules. Both of these factors are substantially improved with a-Si cells, which more than compensates for their lower efficiency. Amorphous cells have a disadvantage in some applications where their larger area may be a disadvantage.
Nano crystalline Si and a-Si can be combined by depositing two diodes on top of each other : the tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si.
Energy Photovoltaics makes a-Si/a-Si double-junction modules which may be the lowest price, $2.25 per watt (2003) module available. They are also developing the technology to develop a-Si/nc-Si cells to enhance the performance of their offerings.
SANYO takes this approach a step further in making its cells. Their thin layer solar cell is composed of a single thin layer of crystalline silicon surrounded by ultra-thin amorphous silicon layers. The solar cells improve boundary characteristics and reduce power generation losses by forming impurity-free i-type amorphous silicon layers between the crystalline base and p- and n-type amorphous silicon layers. Cell efficiencies of about 18% and module efficiencies of 16% or above are reported for commercial modules. These efficiencies are better than those achieved by c-Si cells.
United Solar Ovonics manufactures modules using a-Si/a-SiGe/a-SiGe triple junction cell. Amorphous cells with different light absorption properties deposited continuously, one on top of another, to capture the broad solar spectrum more effectively. This increases the energy conversion efficiency of the multi-cell device and improves performance stability. In other words their structure enhances the ability of the cell to absorb different wavelengths of light and helps stabilize the cell in regard to the SWE effect.
"Progress in Amorphous Silicon Based Solar Cell Technology", C. R. Wronski et al, RIO 02 - World Climate & Energy Event, 1/6-11/02
"Status of Amorphous and Crystalline Thin-Film Silicon Solar Cell Activities", Bolko von Roedern, National Renewable Energy Laboratory (NREL), NCPV and Solar Review Meeting 2003, March 2003
Manufacturers of a-Si solar cells, location, cell structure, substrate material, manufacturing method, annual capacity-MW/yr:
Energy Photovoltaics, Inc., Princeton, NJ, a-Si/a-Si double-junction, glass substrate, batch, 5MW/yr
Free Energy Europe Energy , Lens, France, a-Si, automated laminating line, 1.4 MW
Fuji Electric, Kanagawa, Japan, a-Si/a-SiGe tandem cell, plastic substrate, roll-to-roll, 3MW (early production)
ICP Solar Technologies, Montreal, Quebec, Canada, a-Si,3M
Iowa Thin Film Technologies, Ames, IA, a-Si, polymer substrate, roll-to-roll,
Kaneke Corporation, Osaka, Japan, a-Si, 20 MW, expanding to 40 MW
Mitsubishi Heavy Industries, Power Systems Division, Tokyo, Japan, a-Si, glass substrate
SANYO Electric Co.,LTD. Clean Energy Co, Japan, a-Si/c-Si/a-Si, 250 MW in 2006, expanding to 1,000 MW by 2010
Sinonor Corporation, Hsinchu, Taiwan, a-Si, 3MW
Solar Cells Cells, Split, Croatia, a-Si, glass substrate
TerraSolar Inc, Brooklyn, NY, a-Si Thin Film, 2.5 MW
Tianjin Jinneng Solar Cell Co., LTD., Tianjin, PR China, a-Si, multi-junction, glass or SS substrate
United Solar Ovonic Corp, Auburn Hills, MI, a-Si/a-SiGe/a-SiGe triple junction, SS substrate, roll-to-roll, 25MW expanding to 50 MW
VHF Technologies SA, Yverdon-les-Bains, Switzerland, a-Si, plastic substrate, roll-to-roll