The PV sector currently employs three main technologies. By far, the most common one uses a superabundant material, called silicon. The second, a thin film technology, involves the deposit of a combination of various materials in ultra-thin layers onto a substrate. And the third, concentrated photovoltaics, or CPV, focuses sunlight up to 1,000 times its normal intensity on the cells.
SiliconSilicon crystals come from silica, the main compound in quartz and sand. Silicon is a semi-conducting material. technology is based on a semiconductor material, silicon, which can be produced from a virtually inexhaustible natural resource, quartz, found in granite, sand and sandstone. The solar panels made out of cells (see sidebar) also possess the major advantage of being usable in large solar farms and small, standalone systems alike. The industry has long been proficient in silicon technologies.
It is therefore not surprising that silicon technology now accounts for 90% of the global market.
Improving Carbon Footprint Once in place, photovoltaic systems generate electricity without emitting carbon dioxide. However, the production of PV panels uses energy and therefore emits greenhouse gases. A photovoltaic system has to operate for one to two-and-a-half years to produce the amount of energy used to manufacture the equipment. The high-temperature processes used to treat the cells are the reason. Researchers are looking into ways to reduce heat requirements and make the wafers thinner. Thin-film technology may improve the carbon footprint of the manufacturing processes.
There are two variations on the silicon process:
- The first comprises monocrystalline silicon wafers, sliced from a block (“ingot”) of very pure, single-crystal silicon. Although the industrial process is complicated and expensive, this solution yields cells with 20% efficiency (the ratioIn mathematics, the relationship between two numbers (the ratio of x to y is equal to x/y). Often expressed as a percentage... of the electrical output of a cell to the incident energy in the form of sunlight). The record for commercial-scale production, from a 125 x 125-square-millimeter panel, is held by SunPower, at 24.2%. With this solution, the electrical circuit is also placed on the back of the cell, boosting its performance. Such panels, used on the Solar Impulse solar-powered airplane, do not have the grooved surfaces characteristic of other types of panels.
- The second process involves polycrystalline silicon cells. Polycrystalline silicon is less pure and cheaper, but has an efficiency of around 15%. Chinese companies lead this segment. The cell's lower cost has propelled its strong global growth, but powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... conversion efficiency is, of course, a decisive factor in the final calculus determining the financial viability of PV systems (See Close-Up: Photovoltaic Solar Is Increasingly Competitive).
Slicing wafers from silicon ingots wastes around 30% of the material. Researchers have developed an approach based on thin films about 2 microns thick, deposited on a glass or flexible substrate. Thin-film stacks can harness the properties of the various materials they contain, in particular the capacity of each to use a slightly different light wavelength.
There are two types of thin film:
- The first uses cadmium telluride, which is cheaper than silicon but not very abundant and potentially damaging to the environment. The U.S. company First Solar is the world leader in this technology.
- The second uses a copper, indium and selenium alloy (CIS), sometimes adding gallium to the mix (CIGS). Both these techniques result in very good light absorption. However, gallium supplies are limited and indium is already widely used by flat-screen makers. The Japanese company Solar Frontier occupies the CIS niche.
Organic polymers can also be used (see sidebar).
Organic Photovoltaics (OPV) Researchers are working on the possibility of using organic polymers in thin film technology. Organic polymers are plastics made using renewable resources such as plants, algae or animal matter. Because their light absorption coefficient is very high, organic constituents can be spread into quite thin layers on flexible substrates. The technology has two main challenges to meet: boosting cell efficiency and extending cell life (around 1,000 hours).
Concentrated PV Solar
Another way to boost the power of photovoltaic cells is to concentrate sunlight onto them using a parabolic mirror or a Fresnel lens. Very high concentrations, up to 1,024 times the intensity of sunlight, can be achieved. The technique can potentially attain 30% efficiency rates, but the set-up is complicated. For one thing, residual heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... has to be channeled away to avoid damaging the cells. For another, the panels housing the cells need a tracking system to stay in optimal position in relation to the sun, which requires complex, expensive mechanisms that can only be installed in big solar farms. The German company Soitech is very active in this sector.
From Silicon to Panel
Silicon is obtained by heating a mixture of quartz, coalCoal is ranked by its degree of transformation or maturity, increasing in carbon content from... and wood to a very high temperature. The raw silicon must be rendered 99.999% pure to qualify as solar grade. The silicon ingot is sliced into thin wafers using silicon carbide or diamond cutting blades.
The wafers are then treated in various ways. To improve their light absorption, they are immersed in an etching solution, giving them their frequent blue color. They are treated with phosphorus or boronBoron is a metalloid, semi-conducting chemical element (symbol B)... in a furnace, to "dope" the semiconductor silicon.
This produces cells, or basic building blocks, that generate electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor... when exposed to light photons, through the photovoltaic effectCreation of electric current when a semiconductor material is struck by light photons.. An electrical circuit is printed on the surface, to transfer the current generated. Most cells consist of square sheets 160 to 200 microns thick and roughly one-twelfth of a centimeter long on each side.
Cells are then connected to one another, encapsulated between two sheets of resin and covered with a sheet of glass and an airtight sheet of film, to form panels. The resulting panels (or modules) are then assembled to build photovoltaic systems, whether big solar arrays or small systems.