A solar cell, a form of photovoltaic cell, is a device that uses the photoelectric effect to generate electricity from light, thus generating solar power (energy). Solar cells are used to power many kinds of equipment, including satellites, calculators, remote radiotelephones, and advertising signs. Most often, many cells are linked together to form a solar panel with increased voltage and/or current. Solar cells produce direct current (DC) which can be used directly, stored in a battery or converted from DC to AC to directly power common household devices or to feed into the utility grid. This DC to AC conversion is done by means of an inverter. Since the solar cell, grid feeding and anti-islanding requires special handling, so called Photovoltaic Inverters are used.
The main component of a solar cell is silicon 'doped' with trace amounts of impurities. In pure silicon, each atom is fixed in a crystal lattice and bonded to other silicon atoms covalently, sharing the 4 valence electrons in their outer shells with them. There are thus few free electrons or positive charge carriers to carry charge, and pure silicon is thus a bad conductor.
In doped silicon, atoms with 3 or 5 valence electrons are introduced to the lattice. Take arsenic or phosphorus for example, with 5 valence electrons. Since silicon atoms require only 4 of those electrons to form stable bonds, there will be one free electron which can move and thus carry charge. Since there are so many free electrons in silicon doped with arsenic or phosphorus (compared to pure silicon), this sort of silicon is called "n-type silicon".
If the silicon is doped with boron, which has 3 valence electrons, when it bonds with silicon it will be short of one electron. This 'hole' is also free to move. Since there are so many positively-charged holes in silicon doped with boron, this sort of silicon is called "p-type silicon".
The p-n junction
In a solar cell, a plate of p-type silicon is placed next to a plate of n-type silicon. At the junction between the two, electrons in the n-type plate will migrate to the p-type plate, and vice-versa for the holes in the p-type plate. After a while, enough holes and electrons would have combined to form a barrier at the junction, preventing further flow of holes and electrons.
There is now an electric field across the p-n junction — positive on the n-type silicon plate, since the electrons have crossed over and there are excess protons which do not have corresponding electrons and negative on the p-type silicon plate, because the reverse occurs.
The electric field and the barrier act as a diode — electrons can move from the n-type plate to the p-type plate easily due to the electric field, but the reverse is difficult.
Light energy is transmitted by photons, and its quantity is given by the formula: E = hν — the energy of a photon equals its frequency multiplied by Planck's constant (6.626 × 10−34 m²kg/s).
When photons hit the silicon plates, electron-hole pairs are created (with a probability depending on the quantum efficiency) and separated. The electric field across the p-n junction draws the electrons and holes in opposite directions, and they then diffuse to the front and back contacts. If the 2 silicon plates are connected across a load, the electrons and holes can be extracted, driving the load in the process. If nothing is connected, electrons and holes, each being in minority in their respective zone, recombine with the majority carriers. It is the open-circuit voltage (Voc) condition.
Practical Solar cells
Because the current and voltage supplied by any one solar cell is small, many cells are typically coupled in series and parallel to produce the desired level of output.
The simplest type of solar cell is a silicon diode, but research is continuing into more exotic materials (see below) with greater efficiencies. Modern solar cells are encapsulated in glass-fronted plastic sheets. They have design lifetimes that exceed forty years. Sunlight provides about 1 kilowatt per square meter at the Earth's surface, and most solar cells are between 8 and 12 percent efficient. In desert areas, they can operate for an average of 6 hours per day when mounted in nonrotating brackets.
Solar panels come in four varieties. Most common are rigid monocrystalline and polycrystalline silicon sheets. Monocrystalline silicon provides the highest efficiency of all four types but is also the most expensive. Polycrystalline silicon is lower cost but lower in efficiency. Also available are amorphous silicon solar cells which can be applied to a variety of substrates including flexible ones, like metal foil or plastic foil. The main advantage of amorphous silicon solar cells is that they should eventually be able to be processed at a much lower cost than crystalline solar cells. Nanocrystalline silicon has also been used, in the same processing systems as for amorphous silicon, and it shows slightly higher efficiency due to the increased absorption in the longer wavelengths. Experimental non-silicon solar panels are made of carbon nanotubes embedded in 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.
In late 2001, with batteries to provide power at night, desert climates can get power for about 8 cents per kilowatt hour using solar cells, batteries and electronic inverters. By contrast, nuclear and hydroelectric power plants can provide power at 1.5 to 3 cents per kilowatt hour. Solar power is already cheaper than internal combustion generators that use natural gas, diesel or gasoline, and is becoming competitive with the costs of coal power in some areas. Homes in the U.S. use between 5 and 20 kilowatt hours per day, depending on whether they use electricity for lighting, or heating, cooling and cooking as well.
If a roof is required for other reasons, and the solar cells are chosen and fabricated to form a weather-resistant roof, the value of the roof reduces solar costs considerably. A well-designed solar cell roof will simply be constructed at the optimal angle to collect power. This saves the cost of mounting brackets to raise the cells to an optimal angle, saving quite a bit of money. Solar panel shingles which replace ordinary shingles have recently become available.
The least expensive way to buy a solar power system for a home or small business is as part of a cooperative. Periodically, a group will form on the internet to purchase solar equipment cooperatively.
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 solar cell as of April 2003 was 16.5% [Dr IM Dharmadasa, Sheffield Hallam University, UK]. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light.
Polymer or organic solar cells are built from thin layers 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. The technology has however already successfully been commercialised in organic LED's and organic displays, also called polymer displays
Graetzel cells (sometimes called photoelectrochemical cells) have been around for two decades or so. A p/n junction is used here too in the form of a doped solid (normally titanium dioxide) in contact with a solid or liquid electrolyte (for example CuI). In contrast to the classical solar cell not the semiconductor but a dye placed at the p/n interface is used for absorption of radiation, mimicking the process of photosynthesis. As a result, this type of cell allows a more flexible use of materials. Like organic solar cells, Graetzel cells can be manufactured under "dirty" conditions. Commercial applications have failed to appear due to the fast degradation occurring in Graetzel cells.
- Pennicott, Katie, "Solar cell edges towards endless energy (http://physicsweb.org/article/news/5/12/2)". December 7, 2001. PhysicsWeb.
- Dye Sensitized Solar Cells (http://dcwww.epfl.ch/lpi/solarcellE.html) (DYSC) based on Nanocrystalline Oxide Semiconductor Films
- News searching: Solar Cell (http://news.google.com/news?hl=da&q=%22Solar+Cell%22), Photovoltaic (http://news.google.com/news?hl=da&q=Photovoltaic)
- Historical: Photovoltaic Solar Energy Conversion: An Update (http://www.atse.org.au/publications/focus/focus-green.htm)
- Wladek Walukiewicz, Materials Sciences Division, Berkeley Lab.: Full Solar Spectrum Photovoltaic Materials Identified. (http://www.lbl.gov/msd/PIs/Walukiewicz/02/02_8_Full_Solar_Spectrum.html) Quote: "... Maximum, theoretically predicted efficiencies increase to 50%, 56%, and 72% for stacks of 2, 3, and 36 junctions with appropriately optimized energy gaps, respectively...."
- CNET: 5/12/03 SunPower Announces World's Most Efficient, Low-Cost Silicon Solar Cell (http://news.cnet.com/investor/news/newsitem/0-9900-1028-21199489-0.html) Quote: "...The National Renewable Energy Laboratory (NREL) (http://www.nrel.gov/) has verified 20.4 percent conversion efficiency for the A-300...."
- SunPower A-300 (pdf) (http://www.sunpowercorp.com/html/Products/Datasheets/A-300/A-300.pdf), SunPower (http://www.sunpowercorp.com/)
- March 29, 2002, Scientists Create New Solar Cell (http://www.sciam.com/article.cfm?chanID=sa003&articleID=0004C094-02CC-1CD0-B4A8809EC588EEDF) Quote: "...semiconducting plastic material known as P3HT... 1.7 percent for sunlight..."
- 15 February 03, 'Denim' solar panels to clothe future buildings (http://www.newscientist.com/news/news.jsp?id=ns99993380) Quote: "... Unlike conventional solar cells, the new, cheap material has no rigid silicon base..."
- Examples of Photovoltaic Systems (http://www.sma-america.com/installations.html)
- How Solar Cells Work (http://science.howstuffworks.com/solar-cell.htm)
- http://www.tectosol.staticip.de/index_en.htm electricity yield of a solar power system
- http://www.sunny-portal.de Yield Portal for solar power system users
- National Renewable Energy Laboratory (NREL): Photovoltaics for Buildings: PV Technology for the Home Factsheets (http://www.nrel.gov/buildings/pv/factsheets.html)
- 1993, National Renewable Energy Laboratory (NREL): Photovoltaics: Unlimited Electrical Energy From the Sun (http://www.nrel.gov/research/pv/docs/pvpaper.html) BrokenLink
PEC (Photo Electro Chromic)
Cuprous oxide solar cells
- Make a solar cell in your kitchen (http://www.scitoys.com/scitoys/scitoys/echem/echem2.html#solarcell), A flat panel solar battery (http://www.scitoys.com/scitoys/scitoys/echem/echem3.html#sflatpanel)
- From: How to Build a Solar Cell That Really Works by Walt Noon (http://www.zetatalk.com/energy/tengy17f.htm)
- Home Made PV Cells (http://www.angelfire.com/ak/egel/flatcell.html)