INFORMATION ON PV CELLS
Basic structure of a silicon based solar cell and its working mechanism.
By far, the most prevalent bulk material for solar cells is crystalline silicon (abbreviated as a group as c-Si), also known as "solar grade silicon". Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer.
monocrystalline silicon (c-Si): often made using the Czochralski process. Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the corners of four cells.
Poly- or multicrystalline silicon (poly-Si or mc-Si): made from cast square ingots — large blocks of molten silicon carefully cooled and solidified. These cells are less expensive to produce than single crystal cells but are less efficient.
Ribbon silicon: formed by drawing flat thin films from molten silicon and having a multicrystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require sawing from ingots.
The various thin-film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell. This can lead to reduced processing costs from that of bulk materials (in the case of silicon thin films) but also tends to reduce energy conversion efficiency, although many multi-layer thin films have efficiencies above those of bulk silicon wafers.
Cadmium Telluride is an efficient light-absorbing material for thin-film solar cells. Compared to other thin-film materials, CdTe is easier to deposit and more suitable for large-scale production. Despite much discussion of the toxicity of CdTe-based solar cells, this is the only technology (apart from amorphous silicon) that can be delivered on a large scale. There is a 40 megawatt plant in Ohio (USA) and a 10 megawatt plant in Germany (2007).
The perception of the toxicity of CdTe is based on the toxicity of elemental cadmium, a heavy metal that is a cumulative poison. Scientific work, particularly by researchers of the National Renewable Energy Laboratories (NREL) in the USA, has shown that the release of cadmium to the atmosphere is lower with CdTe-based solar cells than with silicon photovoltaics and other thin-film solar cell technologies.
The materials based on CuInSe2 that are of interest for photovoltaic applications include several elements from groups I, III and VI in the periodic table. These semiconductors are especially attractive for thin film solar cell application because of their high optical absorption coefficients and versatile optical and electrical characteristics which can in principle be manipulated and tuned for a specific need in a given device. CIS is an abbreviation for general chalcopyrite films of copper indium selenide (CuInSe2), CIGS mentioned below is a variation of CIS. CIS films (no Ga) achieved greater than 14% efficiency. However, manufacturing costs of CIS solar cells at present are high when compared with amorphous silicon solar cells but continuing work is leading to more cost-effective production processes.
When gallium is substituted for some of the indium in CIS, the material is sometimes called CIGS , or copper indium/gallium diselenide, a solid mixture of the semiconductors CuInSe2 and CuGaSe2, often abbreviated by the chemical formula CuInxGa(1-x)Se2. Unlike the conventional silicon based solar cell, which can be modelled as a simple p-n junction, these cells are best described by a more complex heterojunction model. The best efficiency of a thin-film solar cell as of December 2005 was 19.5% with CIGS absorber layer. Higher efficiencies around 30% can be obtained by using optics to concentrate the incident light. The use of gallium increases the optical bandgap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage. In another point of view, gallium is added to replace as much indium as possible due to gallium’s relative availability to indium. Approximately 70% of indium currently produced is used by the flat-screen monitor industry. Some investors in solar technology worry that production of CIGS cells will be limited by the availability of indium. Producing 2 GW of CIGS cells (roughly the amount of silicon cells produced in 2006) would use about 10% of the indium produced in 2004. For comparison, silicon solar cells used up 33% of the world’s electronic grade silicon production in 2006. Conversion efficiency values on metallic flexible foils were reported by AbuShama et al in the proceedings of the IEEE 4th World Conference on Photovoltaic Energy Conversion 2006 in Hawaii, USA. Dr. Jehad AbuShama also reported total area world record efficiencies of 15.0% and 10.2% for CIS and modified CGS solar cells, respectively.
That being said, indium can easily be recycled from decommissioned PV modules. The recycling program in Germany would be is an example that highlights the regenerative industrial paradigm: "From cradle to cradle".
Selenium allows for better uniformity across the layer and so the number of recombination sites in the film are reduced which benefits the quantum efficiency and thus the conversion efficiency.
Nanosolar, a California-based company, will soon be producing over 400 megawatts worth of CIGS-based solar arrays per year. If this production is reached, they will be one of the world’s largest producer of solar cells.
Gallium arsenide (GaAs) multijunction
High-efficiency cells have been developed for special applications such as satellites and space exploration. These multijunction cells consist of multiple thin films produced using molecular beam epitaxy. A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GalnP2. Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible.
GaAs multijunction devices are the most efficient solar cells to date, reaching a record high of 40.7% efficiency under solar concentration and laboratory conditions. These devices use 20 to 30 different semiconductors layered in series. At the US National Renewable Energy Lab., a new cell of area 0.26685 cm² will generate a power of 2.6 W. They estimate that this technology could eventually produce electricity at a mere 8–10 cents/kWh. This is similar to the price of electricity today. Thus, this breakthrough could ultimately result in increased consumer use of solar cells.
This technology is being used right now on the Mars rover missions. The rovers have outlived their predicted life spans and have worked for over two years. Their success in the dust-ridden Martian environment is a strong testament to the durability and longevity of these types of solar cells.
Solar arrays made with a material which contains gallium arsenide GaAs and germanium Ge is seeing demand rapidly rise. In just the past 12 months (12/2006 - 12/2007), the cost of 4N gallium metal has risen from about $350 per kg to $680 per kg. Additionally, germanium metal prices have risen substantially to $1000-$1200 per kg this year. Although some Chinese producers of these materials may be able to offset some of the price increases with their lower labor costs. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry.
Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titaniu dioxide to greatly amplify the surface area (200-300 m²/g TiO2, as compared to approximately 10 m²/g of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2, and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and is typically manufactured by screen printing, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light, and the cell casing is difficult to seal due to the solvents used in assembly. In spite of the above, this is a popular emerging technology with some commercial impact forecast within this decade.
Organic/polymer solar cells
Organic solar cells and Polymer solar cells are built from thin films (typically 100 nm) of organic semiconductor such as polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes. Energy conversion efficiencies achieved to date using conductive polymers are low at 6% efficiency for the best cells to date. However, these cells could be beneficial for some applications where mechanical flexibility and disposability are important.
Silicon thin-films are mainly deposited by chemical vapor deposition (typically plasma-enhanced (PE-CVD)) from silane gas and hydrogen gas. Depending on the deposition’s parameters, this can yield:
Amorphous Silicon(a-Si or a-Si:H)
Protocrystalline Silicon or
Nanocrystalline Silicon(nc-Si or nc-Si:H).
These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower energy conversion efficiency than bulk silicon, but are also less expensive to produce. The quantum efficiency of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon.
Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the two material can be combined in thin layers, creating a layered cell called a 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.
Recently, solutions to overcome the limitations of thin-film crystalline silicon have been developed. Light trapping schemes where the incoming light is obliquely coupled into the silicon and the light traverses the film several times enhance the absorption of sunlight in the films. Thermal processing techniques enhance the crystallinity of the silicon and pacify electronic defects. The result is a new technology — thin-film Crystalline Silicon on Glass (CSG. CSG solar devices represent a balance between the low cost of thin films and the high efficiency of bulk silicon.
A silicon thin film technology is being developed for building integrated photovoltaics (BIPV) in the form of semi-transparent solar cells which can be applied as window glazing. These cells function as window tinting while generating electricity.