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Solar cell - Current research

Solar cell - Current research: Encyclopedia II - Solar cell - Current research

There are currently many research groups active in the field of photovoltaics at universities and research institutions around the world. Much of the research is focused on making solar cells cheaper and/or more efficient, so that they can more effectively compete with other energy sources, including fossil energy. One way of doing this is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common ...

Solar cell: Encyclopedia II - Solar cell - Current research

Solar cell - Current research

There are currently many research groups active in the field of photovoltaics at universities and research institutions around the world.

Much of the research is focused on making solar cells cheaper and/or more efficient, so that they can more effectively compete with other energy sources, including fossil energy. One way of doing this is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica sand.

The current industrial production of silicon is via the reaction between carbon (charcoal) and silica at a temperature around 1700 degrees Celsius. In this process, known as carbothermic reduction, each tonne of silicon (metallurgical grade, about 98% in purity) is produced with the emission of about one and half tonnes of carbon dioxide.

It is recently reported that solid silica can be directed converted (reduced) to pure silicon by electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 degrees Celsius). [refs. T. Nohira et al, ‘Pinpoint and bulk electrochemical reduction of insulating silicon dioxide to silicon’, Nat. Mater., 2 (2003) 397. X. B. Jin et al, Electrochemical preparation of silicon and its alloys from solid oxides in molten calcium chloride’, Angew. Chem. Int. Ed., 43 (2004) 733.] While this new process is in principle the same as the FFC Cambridge Process which was first discovered in late 1996, the interesting laboratory finding is that such produced electrolytic silicon is in the form of porous silicon which turns readily into a fine powder (particle size: a few micrometers), and may therefore offer new opportunities for development of solar cell technologies.

Another approach is to significantly reduce the amount of raw material used in the manufacture of solar cells. The various thin-film technologies currently being developed make use of this approach to reducing the cost of electricity from solar cells.

One approach to reduce the amount of silicon used and thus cost, as done by Australian National University in production of their "Sliver" cells, is to micromachine wafers into very thin, practically transparent layers, that could be used as transparent architechtural coverings. Using this technique, two silicon wafers are enough to build a 140 W panel, compared to about 60 wafers needed for conventional modules of same power output.

The invention of conductive polymers, (for which Alan Heeger was awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics, rather than semiconductor grade silicon. However, all organic solar cells made to date suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable.

Because photovoltaic panels convert a small fraction of the received light energy to electricity, there has been continued interest in laminating photovoltaic cells onto solar thermal panels to make PVT panels. Research in this area has found many difficulties and not much success (see References), for four reasons:

Photovoltaic panels are usually designed to reflect light not used, rather than absorb it, and the difference requires quite a bit of development.

Solar thermal panels for domestic hot water or space heating are usually glazed, and the glass tends to reflect some of the light that the PV cell might absorb.

Because the panels convert less light to heat, they require more area for the same heat and suffer larger radiative and convective losses, all of which reduces the overall cost effectiveness of the solar thermal system.

Because the manufacturing process is less developed, costs are higher, which is crippling in the extremely cost-sensitive solar market.

Solar cell - Thin-film solar cells

The next step in reducing the cost of solar cells and panels seems certain to come from thin-film technology. Thin-film solar cells use less than 1% of the raw material (silicon) compared to wafer based solar cells, leading to a significant price drop per kWh. There are many research groups around the world actively researching different thin-film approaches and/or materials.

Thin Film solar cells are mainly deposited by PECVD from silane gas and hydrogen. This process produces a material without crystalline orientation : amorphous silicon. Depending on the deposition's parameters both protocrystalline silicon, which has been shown to exhibit the most stability, and nanocrystalline silicon can also be obtained. These types of silicon present dandling and twisted bonds, which results in the aparition of deep defects (energy levels in the bandgap) as well as in the deformation of the valence and conduction bands (band tails). This contributes to reduce the efficiency of Thin-Film solar cells by reducing the number of collected electron-hole pair by incident photon.

Amorphous silicon (a-Si) has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it is more efficient to absorb the visible part of the solar spectrum, but it fails to collect an important part of the spectrum : the infrared. As nano crystalline Si has about the same bandgap as c-Si, the two material 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.

One particularly promising technology is crystalline silicon thin-films on glass substrates. This technology makes use of the advantages of crystalline silicon as a solar cell material, with the cost savings of using a thin-film approach.

Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind of materials, including flexible substrates (PET for example), which opens a new dimension for new applications.

Solar cell - Emerging Materials

For special applications, such as Deep Space 1, high-efficiency cells can be made from gallium arsenide by molecular beam epitaxy. Such cells have many diodes in series, each with a different band gap energy so that it absorbs its share of the electromagnetic spectrum with very high efficiency. Triple junction solar cell have (as the name suggest) 3 diodes layered on top of each other, each absorbing a different spectrum of light, efficiency as high as 35.2% have been achieved. The multiple junction solar cells may be very efficient, but are prohibitively expensive to make. Cost-effective use of these cells could be achieved with concentrating optics so that less of the array consists of actual semiconductor devices.

Experimental non-silicon solar panels can be made of quantum heterostructures, eg. carbon nanotubes or quantum dots, embedded in a special plastic. These have only one-tenth the efficiency of silicon panels but could be manufactured in ordinary factories, not clean rooms which should lower the cost. While conventional solar cells only generate electricity from the visible light spectrum, experimental cells have been made that use the infrared spectrum. By varying the size of the quantum dots, the cells can be tuned to absorb different wavelengths. If panels that absorb both visible and infrared spectrums are able to be manufactured, the panels may be able to achieve up to 30 percent efficiency. (McDonald, et al., 2005)

Some of the most efficient solar cell materials are cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). Unlike the basic silicon solar cell, which can be modelled as a simple p-n junction (see under semiconductor), these cells are best described by a more complex heterojunction model. The best efficiency of a bare thin-film solar cell as of December 2005 was 19.5% (CIGS). Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light.

Polymer solar cells, also called organic solar cells, are built from ultra thin layers (typically 100 nm) of organic semiconductors such as polyphenylene vinylene and fullerene. The p/n junction model is only a crude description of the functioning of such cells, as electron hopping and other processes also play a crucial role. They are potentially cheaper to manufacture than silicon or inorganic cells, but efficiencies achieved to date are low and cells are highly sensitive to air and moisture, making commercial applications difficult. In the reverse mode, the technology has however already successfully been commercialised in organic LEDs and organic displays, also called polymer displays.

Graetzel cells (sometimes called photoelectrochemical cells or dye-sensitized solar cells) were first announced in Nature in October 1991. The cell depends on a layer of nanoparticulate titanium dioxide, sensitised by a dye. In contrast to the classical solar cell the dye absorbs the radiation, mimicking the process of photosynthesis. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. As a result, this type of cell allows a more flexible use of materials, and typically are manufactured by screen printing, with cost advantages over the more expensive manufacturing techniques and equipments used for traditional and thin film solar cells, and significantly less embodied energy. This is an emerging technology with commercial impact forecast within this decade.