A solarcell (also called photovoltaic cell or photoelectric cell) isa solid state electrical device that converts the energy of light directlyinto electricity by the photovoltaiceffect.
Assembliesof cells used to make solar modules whichare used to capture energy from sunlight, areknown as solar panels. The energy generated from these solar modules,referred to as solar power, is anexample of solar energy.
Photovoltaics is thefield of technology and research related to the practical application of photovoltaiccells in producing electricity from light, though it is often used specificallyto refer to the generation of electricity from sunlight.
Cells aredescribed as photovoltaic cells when the light source is not necessarilysunlight. These are used for detecting light or other electromagnetic radiation near the visible range, forexample infrared detectors, ormeasurement of light intensity.
[edit] History of solar cells
Mainarticle: Timelineof solar cells
The term"photovoltaic" comes from the Greek φῶς (phōs)meaning "light", and "voltaic", from the name of the Italianphysicist Volta, afterwhom a unit of electro-motive force, the volt, is named. The term"photo-voltaic" has been in use in English since 1849.[1]
The photovoltaiceffect was first recognized in 1839 by French physicist A. E.Becquerel. However, it was not until 1883 that the first photovoltaic cell wasbuilt, by Charles Fritts, whocoated the semiconductor selenium with anextremely thin layer of gold to form the junctions. Thedevice was only around 1% efficient. In 1888 Russian physicist AleksandrStoletov built the first photoelectric cell (based on the outer photoelectriceffect discovered by Heinrich Hertz earlierin 1887). Albert Einsteinexplained the photoelectric effect in 1905 for which he received the Nobelprize in Physics in 1921.[2] RussellOhl patentedthe modern junction semiconductor solar cell in 1946,[3] whichwas discovered while working on the series of advances that would lead to the transistor.
[edit] Bellproduces the first practical cell
Themodern photovoltaic cell was developed in 1954 at Bell Laboratories.[4] Thehighly efficient solar cell was first developed by Daryl Chapin, CalvinSouther Fuller and Gerald Pearson in 1954using a diffused silicon p-n junction.[5] Atfirst, cells were developed for toys and other minor uses, as the cost of theelectricity they produced was very high - in relative terms, a cell thatproduced 1 watt of electrical power in bright sunlight cost about $250,comparing to $2 to $3 for a coal plant.
Solarcells were rescued from obscurity by the suggestion to add them to the Vanguard Isatellite. In the original plans, the satellite would be powered only bybattery, and last a short time while this ran down. By adding cells to theoutside of the fuselage, the mission time could be extended with no majorchanges to the spacecraft or its power systems. There was some skepticism atfirst, but in practice the cells proved to be a huge success, and solar cellswere quickly designed into many new satellites, notably Bell's own Telstar.
Improvementswere slow over the next two decades, and the only widespread use was in spaceapplications where their power-to-weightratio was higher than any competing technology. However, this success wasalso the reason for slow progress; space users were willing to pay anything forthe best possible cells, there was no reason to invest in lower-cost solutionsif this would reduce efficiency. Instead, the price of cells was determinedlargely by the semiconductor industry; their move to integratedcircuits in the 1960s led to the availability of larger boules at lowerrelative prices. As their price fell, the price of the resulting cells did aswell. However these effects were limited, and by 1971 cell costs were estimatedto be $100 a watt.[6]
[edit] Berman's price reductions
In thelate 1960s, Elliot Berman was investigating a new method for producing thesilicon feedstock in a ribbon process. However, he found little interest in theproject and was unable to gain the funding needed to develop it. In a chanceencounter, he was later introduced to a team at Exxon who were looking for projects 30years in the future. The group had concluded that electrical power would bemuch more expensive by 2000, and felt that this increase in price would makenew alternative energy sources more attractive, and solar was the mostinteresting among these. In 1969, Berman joined the Linden, New Jersey Exxonlab, Solar Power Corporation (SPC).[7]
His firstmajor effort was to canvas the potential market to see what possible uses for anew product were, and they quickly found that if the dollars per watt wasreduced from then-current $100/watt to about $20/watt there was significantdemand. Knowing that his ribbon concept would take years to develop, the teamstarted looking for ways to hit the $20 price point using existing materials.[7]
The firstimprovement was the realization that the existing cells were based on standardsemiconductor manufacturing process, even though that was not ideal. Thisstarted with the boule, cutting it into disks called wafers, polishing thewafers, and then, for cell use, coating them with an anti-reflective layer.Berman noted that the rough-sawn wafers already had a perfectly suitableanti-reflective front surface, and by printing the electrodes directly on thissurface, two major steps in the cell processing were eliminated. The team alsoexplored ways to improve the mounting of the cells into arrays, eliminating theexpensive materials and hand wiring used in space applications. Their solutionwas to use a printed circuit board on the back, acrylic plastic onthe front, and silicone based glue between the two, potting the cells. But thelargest improvement in price point was Berman's realization that existingsilicon was effectively "too good" for solar cell use; the minorimperfections that would ruin a boule (or individual wafer) for electronicswould have little effect in the solar application.[8] Solarcells could be made using cast-off material from the electronics market.
Puttingall of these changes into practice, the company started buying up"reject" silicon from existing manufacturers at very low cost. Byusing the largest wafers available, thereby reducing the amount of wiring for agiven panel area, and packaging them into panels using their new methods, by1973 SPC was producing panels at $10 per Watt and selling them at $20 per Watt,a fivefold decrease in prices in two years.
[edit] Navigation market
SPCapproached companies making navigational buoys as a natural market for theirproducts, but found a curious situation. The primary company in the businesswas Automatic Power, a battery manufacturer. Realizing that solar cells mighteat into their battery profits, Automatic purchased the rights to earlier solarcell designs and suppressed them. Seeing there was no interest at Automatic,SPC turned to Tideland Signal, anotherbattery company formed by ex-Automatic managers. Tideland introduced asolar-powered buoy and was soon ruining Automatic's business.
Thetiming could not be better; the rapid increase in the number of offshoreoil platforms and loading facilities produced an enormous marketamong the oil companies. As Tideland's fortunes improved, Automatic startedlooking for their own supply of solar panels. They found Bill Yerks of SolarPower International (SPI) in California, who was looking for a market. SPI wassoon bought out by one of its largest customers, the ARCO oil giant, forming ARCO Solar.ARCO Solar's factory in Camarillo,California was the first dedicated to building solar panels,and has been in continual operation from its purchase by ARCO in 1977 to thisday.
Thismarket, combined with the 1973 oil crisis, led toa curious situation. Oil companies were now cash-flush due to their hugeprofits during the crisis, but were also acutely aware that their futuresuccess would depend on some other form of power. Over the next few years,major oil companies started a number of solar firms, and were for decades thelargest producers of solar panels. Exxon, ARCO, Shell, Amoco (later purchasedby BP) and Mobil all had major solar divisions during the 1970s and 80s.Technology companies also had some investment, including General Electric,Motorola, IBM, Tyco and RCA.[9]
[edit] Further improvements
In thetime since Berman's work, improvements have brought production costs down under$1 a watt, with wholesale costs on the order of $2. "Balanceof system" costs are now more than the panels themselves, with largecommercial arrays falling to $3.40 a watt,[10] fullycommissioned, in 2010.
As thesemiconductor industry moved to ever-larger boules, older equipment becameavailable at fire-sale prices. Cells have grown in size as older equipmentbecame available on the surplus market; ARCO Solar's original panels used cellswith 2 to 4 inch diameter. Panels in the 1990s and early 2000s generallyused 5 inch wafers, and since 2008 almost all new panels use 6 inchcells. Another major change was the move to polycrystalline silicon. Thismaterial has less efficiency, but is less expensive to produce in bulk. Thewidespread introduction of flat screen televisions in the late 1990s and early2000s led to the wide availability of large sheets of high-quality glass, usedon the front of the panels.
Othertechnologies have also come to market. First Solar hasgrown to become the largest panel manufacturer, in terms of yearly powerproduced, using a thin-film cell sandwiched between two layers of glass. Thiswas the first product to beat $1 a watt for production costs.[11] Sincethen the rapid rise of Chinese manufacturing and the commensurate continuousimprovement have pushed prices of the higher efficient crystalline siliconpanels into the same range.
[edit] Applications
Polycrystallinephotovoltaic cells laminated to backing material in a module
Polycrystallinephotovoltaic cells
Mainarticle: photovoltaicsystem
Solarcells are often electrically connected and encapsulated as a module.Photovoltaic modules often have a sheet of glass on the front (sun up) side,allowing light to pass while protecting the semiconductor wafers from abrasion and impact due to wind-drivendebris, rain, hail, etc.Solar cells are also usually connected in series in modules, creating an additive voltage.Connecting cells in parallel will yield a higher current. Modules are theninterconnected, in series or parallel, or both, to create an array withthe desired peak DC voltage and current.
To makepractical use of the solar-generated energy, the electricity is most often fedinto the electricity grid using inverters (grid-connected photovoltaicsystems); in stand-alone systems, batteries are used to store the energy thatis not needed immediately. Solar panels can be used to power or rechargeportable devices.
[edit] Theory
Mainarticle: Theory of solar cell
The solarcell works in three steps:
- Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon.
- Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction.
- An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity.
[edit] Efficiency
 | It has been suggested that Fill factor be merged into this article or section. (Discuss) Proposed since June 2011. |
Mainarticle: Solar cell efficiency
Theefficiency of a solar cell may be broken down into reflectance efficiency,thermodynamic efficiency, charge carrier separation efficiency and conductiveefficiency. The overall efficiency is the product of each of these individualefficiencies.
Due tothe difficulty in measuring these parameters directly, other parameters aremeasured instead: thermodynamic efficiency, quantum efficiency, integratedquantum efficiency, VOC ratio, and fill factor.Reflectance losses are a portion of the quantum efficiency under "externalquantum efficiency". Recombination losses make up a portion of the quantumefficiency, VOC ratio, and fill factor. Resistive losses arepredominantly categorized under fill factor, but also make up minor portions ofthe quantum efficiency, VOC ratio.
Crystallinesilicon devices are now approaching the theoretical limiting efficiency of 29%.
[edit] Cost
The costof a solar cell is given per unit of peak electrical power. Manufacturing costsnecessarily include the cost of energy required for manufacture. Solar-specificfeed in tariffs vary worldwide, and even state by state within variouscountries.[12] Suchfeed-in tariffs can be highly effective in encouraging the development of solarpower projects.
High-efficiencysolar cells are of interest to decrease the cost of solar energy. Many of thecosts of a solar power plant are proportional to the area of the plant; ahigher efficiency cell may reduce area and plant cost, even if the cellsthemselves are more costly. Efficiencies of bare cells, to be useful inevaluating solar power plant economics, must be evaluated under realisticconditions. The basic parameters that need to be evaluated are the shortcircuit current, open circuit voltage.[13]
The chartat the right illustrates the best laboratory efficiencies obtained for variousmaterials and technologies, generally this is done on very small, i.e. onesquare cm, cells. Commercial efficiencies are significantly lower.
Grid parity, thepoint at which photovoltaic electricity is equal to or cheaper than gridpower, can be reached using low cost solar cells. It is achieved first inareas with abundant sun and high costs for electricity such as in California and Japan.[14] Gridparity has been reached in Hawaii and other islands that otherwiseuse dieselfuel to produce electricity. George W. Bush had set2015 as the date for grid parity in the USA.[15][16] Speakingat a conference in 2007, General Electric's ChiefEngineer predicted grid parity without subsidies in sunny parts of the UnitedStates by around 2015.[17]
The priceof solar panels fell steadily for 40 years, until 2004 when high subsidies inGermany drastically increased demand there and greatly increased the price ofpurified silicon (which is used in computer chips as well as solar panels). Oneresearch firm predicted that new manufacturing capacity began coming on-line in2008 (projected to double by 2009) which was expected to lower prices by 70% in2015. Other analysts warned that capacity may be slowed by economic issues, butthat demand may fall because of lessening subsidies. Other potentialbottlenecks which have been suggested are the capacity of ingot shaping andwafer slicing industries, and the supply of specialist chemicals used to coatthe cells.[18]
[edit] Materials
The Shockley-Queisserlimit for the theoretical maximum efficiency of a solar cell. Semiconductorswith bandgap between1 and 1.5eV have thegreatest potential to form an efficient cell. (The efficiency "limit"shown here can be exceeded by multijunction solar cells.)
Differentmaterials display different efficiencies and have different costs. Materialsfor efficient solar cells must have characteristics matched to the spectrum ofavailable light. Some cells are designed to efficiently convert wavelengths ofsolar light that reach the Earth surface. However, some solar cells areoptimized for light absorption beyond Earth's atmosphere as well. Lightabsorbing materials can often be used in multiple physical configurationsto take advantage of different light absorption and charge separationmechanisms.
Materialspresently used for photovoltaic solar cells include monocrystallinesilicon, polycrystallinesilicon, amorphous silicon, cadmiumtelluride, and copper indium selenide/sulfide.[19]
Manycurrently available solar cells are made from bulk materials that are cut into wafers between 180 to 240 micrometers thick that arethen processed like other semiconductors.
Othermaterials are made as thin-films layers,organic dyes, andorganic polymers that aredeposited on supportingsubstrates. A third group are made from nanocrystals and used as quantum dots(electron-confined nanoparticles).Silicon remains the only material that is well-researched in both bulkand thin-film forms.
[edit] Crystalline silicon
Mainarticles: Monocrystallinesilicon, Polycrystallinesilicon, Silicon, and list of silicon producers
Basicstructure 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 gradesilicon". Bulk silicon is separated into multiple categories according tocrystallinity 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 four corners of the cells.
- Poly- or multicrystalline silicon (poly-Si or mc-Si): made from cast square ingots — large blocks of molten silicon carefully cooled and solidified. Poly-Si cells are less expensive to produce than single crystal silicon cells, but are less efficient. US DOE data shows that there were a higher number of multicrystalline sales than monocrystalline silicon sales.
- Ribbon silicon[20] is a type of multicrystalline silicon: it is formed by drawing flat thin films from molten silicon and results in 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.
Analystshave predicted that prices of polycrystalline silicon will drop as companiesbuild additional polysilicon capacity quicker than the industry’s projecteddemand. On the other hand, the cost of producing upgraded metallurgical-gradesilicon, also known as UMG Si, canpotentially be one-sixth that of making polysilicon.[21]
Manufacturersof wafer-based cells have responded to high silicon prices in 2004-2008 priceswith rapid reductions in silicon consumption. According to Jef Poortmans,director of IMEC'sorganic and solar department, current cells use between eight and nine grams ofsilicon per watt of power generation, with wafer thicknesses in theneighborhood of 0.200 mm. At 2008 spring's IEEE Photovoltaic Specialists' Conference (PVS'08),John Wohlgemuth, staff scientist at BP Solar,reported that his company has qualified modules based on 0.180 mm thickwafers and is testing processes for 0.16 mm wafers cut with 0.1 mmwire. IMEC's roadmap, presented at the organization's recent annual researchreview meeting, envisions use of 0.08 mm wafers by 2015.[22]
[edit] Thin films
Mainarticle: Thin film solar cell
Marketshareof the differrent PV technologies In 2010 the marketshare of thin film declined by30% as thin film technology was displaced by more efficient crystalline siliconsolar panels (the light and dark blue bars).
Thin-filmtechnologies reduce the amount of material required in creating a solar cell.Though this reduces material cost, it may also reduce energy conversionefficiency. Thin-film solar technologies have enjoyed large investment due tothe success of First Solar and the, large unfulfilled, promise of lower costand flexibility compared to wafer silicon cells, but they have not becomemainstream solar products due to their lower efficiency and correspondinglarger area consumption per watt production. Cadmium telluride (CdTe), copper indium gallium selenide (CIGS)and amorphous silicon (A-Si)are three thin-film techologies often used as outdoor photovoltaic solar powerproduction. CdTe technology is most cost competitive among them.[23] CdTetechnology costs about 30% less than CIGS technology and 40% less than A-Sitechnology in 2011.
[edit] Cadmium telluride solar cell
Mainarticle: Cadmium telluride photovoltaics
A cadmiumtelluride solar cell uses a cadmium telluride (CdTe) thin film, a semiconductor layer toabsorb and convert sunlight into electricity. Solarbuzz[24] hasreported that the lowest quoted thin-film module price stands at US$1.76 per watt-peak, withthe lowest crystalline silicon (c-Si) module at $2.48 per watt-peak.
The cadmium presentin the cells would be toxic if released. However, release is impossible duringnormal operation of the cells and is unlikely during fires in residential roofs.[25] A squaremeter of CdTe contains approximately the same amount of Cd as a single C cell Nickel-cadmiumbattery, in a more stable and less soluble form.[25]
[edit] Copper indium galium selenide
Mainarticle: Copper indium gallium selenide solar cell
Copperindium gallium selenide (CIGS) is a direct-bandgapmaterial. It has the highest efficiency (~20%) among thin film materials (see CIGS solar cell). Traditional methods offabrication involve vacuum processes including co-evaporation and sputtering.Recent developments at IBM and Nanosolar attemptto lower the cost by using non-vacuum solution processes.
[edit] Gallium arsenide multijunction
Mainarticle: Multijunction photovoltaic cell
High-efficiencymultijunction cells were originally developed for special applications such as satellites and spaceexploration, but at present, their use in terrestrialconcentrators might be the lowest cost alternative in terms of $/kWh and $/W.[26] Thesemultijunction cells consist of multiple thin films produced using metalorganic vapour phase epitaxy. Atriple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP2.[27] Eachtype of semiconductor will have a characteristic band gap energywhich, loosely speaking, causes it to absorb light most efficiently at acertain 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 solarspectrum, thus generating electricity from as much of the solar energy aspossible.
GaAsbased multijunction devices are the most efficient solar cells to date. InOctober 2010, triple junction metamorphic cell reached a record high of 42.3%.[28]
Thistechnology is currently being utilized in the Mars Exploration Rover missions which have run far pasttheir 90 day design life.
Tandemsolar cells based on monolithic, series connected, gallium indium phosphide(GaInP), gallium arsenide GaAs, and germanium Ge pn junctions, are seeingdemand rapidly rise. Between December 2006 and December 2007, the cost of 4Ngallium metal rose from about $350 per kg to $680 per kg. Additionally,germanium metal prices have risen substantially to $1000–$1200 per kg thisyear. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growingcrystals, and boron oxide, these products are critical to the entire substratemanufacturing industry.
Triple-junctionGaAs solar cells were also being used as the power source of the Dutchfour-time World Solar Challenge winners Nuna in 2003,2005 and 2007, and also by the Dutch solar cars Solutra (2005), Twente One (2007) and21Revolution (2009).
The DutchRadboud University Nijmegen set the record for thin filmsolar cell efficiency using a single junction GaAs to 25.8% in August 2008using only 4 µm thick GaAs layer which can be transferred from a waferbase to glass or plastic film.[29]
[edit] Light-absorbing dyes (DSSC)
Mainarticle: Dye-sensitized solar cells
Dye-sensitized solar cells (DSSCs) are made of low-costmaterials and do not need elaborate equipment to manufacture, so they can bemade in a DIY fashion,possibly allowing players to produce more of this type of solar cell thanothers. In bulk it should be significantly less expensive than older solid-state cell designs. DSSC's can be engineered intoflexible sheets, and although its conversionefficiency is less than the best thin film cells, its price/performanceratio should be high enough to allow them to compete with fossil fuel electrical generation. TheDSSC has been developed by Prof. MichaelGrätzel in 1991 at the Swiss Federal Institute of Technology (EPFL)in Lausanne (CH).
Typicallya ruthenium metalorganic dye (Ru-centered) is used as a monolayer oflight-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer ofnanoparticulate titaniumdioxide to greatly amplify the surface area (200–300 m2/g TiO2,as compared to approximately 10 m2/g of flat single crystal). Thephotogenerated electrons from the light absorbing dye are passed on tothe n-type TiO2, and the holes are absorbed by an electrolyte on theother side of the dye. The circuit is completed by a redox couple in theelectrolyte, which can be liquid or solid. This type of cell allows a moreflexible use of materials, and is typically manufactured by screenprinting and/or use of Ultrasonic Nozzles, withthe potential for lower processing costs than those used for bulk solarcells. However, the dyes in these cells also suffer from degradation underheat and UV light,and the cell casing is difficult to seal due tothe solvents used in assembly. In spite of the above, this is a popularemerging technology with some commercial impact forecast within this decade.The first commercial shipment of DSSC solar modules occurred in July 2009 fromG24i Innovations.[30]
[edit] Organic/polymer solar cells
Organicsolar cells are a relatively novel technology, yet hold thepromise of a substantial price reduction (over thin-film silicon) and a fasterreturn on investment. These cells can be processed from solution, hence thepossibility of a simple roll-to-roll printing process, leading to inexpensive,large scale production.
Organicsolar cells and polymersolar cells are built from thin films (typically 100 nm)of organicsemiconductors including polymers, such as polyphenylenevinylene and small-molecule compounds like copper phthalocyanine (a blue orgreen organic pigment) and carbon fullerenes andfullerene derivatives such as PCBM. Energy conversion efficienciesachieved to date using conductive polymers are low compared to inorganicmaterials. However, it has improved quickly in the last few years and thehighest NREL(National Renewable Energy Laboratory) certified efficiency has reached 8.3%for the Konarka PowerPlastic.[31] Inaddition, these cells could be beneficial for some applications wheremechanical flexibility and disposability are important.
Thesedevices differ from inorganic semiconductor solar cells in that they do notrely on the large built-in electric field of a PN junction to separate theelectrons and holes created when photons are absorbed. The active region of anorganic device consists of two materials, one which acts as an electron donorand the other as an acceptor. When a photon is converted into an electron holepair, typically in the donor material, the charges tend to remain bound in theform of an exciton, and areseparated when the exciton diffuses to the donor-acceptor interface. The shortexciton diffusion lengths of most polymer systems tend to limit the efficiencyof such devices. Nanostructured interfaces, sometimes in the form of bulkheterojunctions, can improve performance.[32]
[edit] Silicon thin films
Siliconthin-film cells are mainly deposited by chemical vapor deposition (typically plasma-enhanced,PE-CVD) from silane gas and hydrogen gas.Depending on the deposition parameters, this can yield:[33]
- Amorphous silicon (a-Si or a-Si:H)
- Protocrystalline silicon or
- Nanocrystalline silicon (nc-Si or nc-Si:H), also called microcrystalline silicon.
It hasbeen found that protocrystalline silicon with a low volume fraction of nanocrystallinesilicon is optimal for high open circuit voltage.[34] Thesetypes of silicon present dangling and twisted bonds, which results in deepdefects (energy levels in the bandgap) as well as deformation of the valenceand conduction bands (band tails). The solar cells made from these materialstend to have lower energy conversion efficiency than bulksilicon, but are also less expensive to produce. The quantumefficiency of thin film solar cells is also lower due toreduced number of collected charge carriers per incident photon.
Anamorphous silicon (a-Si) solar cell is made of amorphous or microcrystallinesilicon and its basic electronic structure is the p-i-njunction. a-Si is attractive as a solar cell material because it is abundantand non-toxic (unlike its CdTe counterpart) and requires a low processingtemperature, enabling production of devices to occur on flexible and low-costsubstrates. As the amorphous structure has a higher absorption rate of lightthan crystalline cells, the complete light spectrum can be absorbed with a verythin layer of photo-electrically active material. A film only 1 micron thickcan absorb 90% of the usable solar energy.[35] Thisreduced material requirement along with current technologies being capable oflarge-area deposition of a-Si, the scalability of this type of cell is high.However, because it is amorphous, it has high inherent disorder and danglingbonds, making it a bad conductor for charge carriers. These dangling bonds actas recombination centers that severely reduce the carrier lifetime and pin theFermi energy level so that doping the material to n- or p- type is notpossible. Amorphous Silicon also suffers from the Staebler-Wronski effect, whichresults in the efficiency of devices utilizing amorphous silicon dropping asthe cell is exposed to light. The production of a-Si thin film solar cells usesglass as a substrate and deposits a very thin layer of silicon by plasma-enhanced chemical vapor deposition (PECVD).A-Si manufacturers are working towards lower costs per watt and higherconversion efficiency with continuous research and development on Multijunction solar cells for solar panels. Anwell Technologies Limited recently announced its targetfor multi-substrate-multi-chamber PECVD, to lower the cost to USD0.5 per watt.[36]
Amorphoussilicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1eV), which means it absorbs the visible part of the solar spectrum morestrongly than the infrared portionof the spectrum. As nc-Si has about the same bandgap as c-Si, the nc-Siand a-Si can advantageously be combined in thin layers, creating a layered cellcalled a tandem cell. The top cell in a-Si absorbs the visible light andleaves the infrared part of the spectrum for the bottom cell in nc-Si.
Recently,solutions to overcome the limitations of thin-film crystalline silicon havebeen developed. Light trapping schemes where the weakly absorbed longwavelength light is obliquely coupled into the silicon and traverses the filmseveral times can significantly enhance the absorption of sunlight in the thinsilicon films.[37] Minimizingthe top contact coverage of the cell surface is another method for reducingoptical losses; this approach simply aims at reducing the area that is coveredover the cell to allow for maximum light input into the cell. Anti-reflectivecoatings can also be applied to create destructive interference within thecell. This can by done by modulating the Refractive index of thesurface coating; if destructive interference is achieved, there will be noreflective wave and thus all light will be transmitted into the semiconductorcell. Surface texturing is another option, but may be less viable because italso increases the manufacturing price. By applying a texture to the surface ofthe solar cell, the reflected light can be refracted into striking the surfaceagain, thus reducing the overall light reflected out. Light trapping as anothermethod allows for a decrease in overall thickness of the device; the pathlength that the light will travel is several times the actual device thickness.This can be achieved by adding a textured backreflector to the device as wellas texturing the surface. If both front and rear surfaces of the device meetthis criteria, the light will be 'trapped' by not having an immediate pathwayout of the device due to internal reflections. Thermal processing techniquescan significantly enhance the crystal quality of the silicon and thereby leadto higher efficiencies of the final solar cells.[38] Furtheradvancement into geometric considerations of building devices can exploit thedimensionality of nanomaterials. Creating large, parallel nanowire arraysenables long absoprtion lengths along the length of the wire while stillmaintaining short minority carrier diffusion lengths along the radialdirection. Adding nanoparticles between the nanowires will allow for conductionthrough the device. Because of the natural geometry of these arrays, a texturedsurface will naturally form which allows for even more light to be trapped. Afurther advantage of this geometry is that these types of devices require about100 times less material than conventional wafer-based devices.
[edit] Manufacture
Earlycalculator solar battery
Becausesolar cells are semiconductor devices, they share some of the same processingand manufacturing techniques as other semiconductor devices such as computer and memory chips.However, the stringent requirements for cleanliness and quality control ofsemiconductor fabrication are more relaxed for solar cells. Most large-scalecommercial solar cell factories today make screen printed poly-crystalline orsingle crystalline silicon solar cells.
Poly-crystallinesilicon wafers are made by wire-sawing block-cast silicon ingots into very thin(180 to 350 micrometer) slices or wafers. The wafers are usually lightly p-type doped. To make a solar cell from the wafer, asurface diffusion of n-type dopants is performed on the front side of thewafer. This forms a p-n junction a few hundred nanometers below the surface.
Antireflectioncoatings, to increase the amount of light coupled into the solar cell, aretypically next applied. Silicon nitride has gradually replaced titanium dioxideas the antireflection coating because of its excellent surface passivationqualities. It prevents carrier recombination at the surface of the solar cell.It is typically applied in a layer several hundred nanometers thick usingplasma-enhanced chemical vapor deposition (PECVD). Some solar cells havetextured front surfaces that, like antireflection coatings, serve to increasethe amount of light coupled into the cell. Such surfaces can usually only beformed on single-crystal silicon, though in recent years methods of formingthem on multicrystalline silicon have been developed.
The waferthen has a full area metal contact made on the back surface, and a grid-likemetal contact made up of fine "fingers" and larger"busbars" are screen-printed onto the front surface using a silver paste.The rear contact is also formed by screen-printing a metal paste, typicallyaluminium. Usually this contact covers the entire rear side of the cell, thoughin some cell designs it is printed in a grid pattern. The paste is then firedat several hundred degrees celsius to form metal electrodes in ohmiccontact with the silicon. Some companies use an additional electro-plating stepto increase the cell efficiency. After the metal contacts are made, the solarcells are interconnected in series (and/or parallel) by flat wires or metalribbons, and assembled into modules or"solar panels". Solar panels have a sheet of tempered glass on thefront, and a polymerencapsulation on the back.
[edit] Lifespan
Mostcommercially available solar panels are capable of producing electricity for atleast twenty years. The typical warranty given by panel manufacturers is over90% of rated output for the first 10 years, and over 80% for the second 10years. Panels are expected to function for a period of 30 – 35 years.[39]
[edit] Research topics
Mainarticle: Solar cell research
There arecurrently many research groups active in the field of photovoltaics in universities andresearch institutions around the world. This research can be divided into threeareas: making current technology solar cells cheaper and/or more efficient toeffectively compete with other energy sources; developing new technologiesbased on new solar cell architectural designs; and developing new materials toserve as light absorbers and charge carriers.
[edit]Manufacturers and certification
National Renewable Energy Laboratory testsand validates solar technologies. There are three reliable certifications ofsolar equipment: UL and IEEE (bothU.S. standards) and IEC.
Solarcells are manufactured primarily in Japan, Germany, Mainland China, Taiwan andthe United States,[40] thoughnumerous other nations have or are acquiring significant solar cell productioncapacity. While technologies are constantly evolving toward higherefficiencies, the most effective cells for low cost electrical production arenot necessarily those with the highest efficiency, but those with a balancebetween low-cost production and efficiency high enough to minimize area-relatedbalance of systems cost. Those companies with large scale manufacturingtechnology for coating inexpensive substrates may, in fact, ultimately be thelowest cost net electricity producers, even with cell efficiencies that arelower than those of single-crystaltechnologies.
[edit] China
Backed byChinese government's unprecedented plan to offer subsidies for utility-scalesolar power projects that is likely to spark a new round of investment fromChinese solar panel makers. Chinese companies have already played a moreimportant role in solar panels manufacturing in recent years. China producedsolar cells/modules with an output of 13 GW in 2010 which represents about halfof the global production and makes China the largest producer in the world.[41] SomeChinese companies such as Suntech Power, Yingli, LDK SolarCo, JA Solar and ReneSola havealready announced projects in cooperation with regional governments withhundreds of megawatts each after the ‘Golden Sun’ incentive program wasannounced by the government.[42] Therapid expansion of silicon and wafer production by GCL, China's largest privatepower producer, will further fuel China's growth as the world's solarmanufacturer.
[edit] United States
Mainarticle: Photovoltaics in the United States
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Newmanufacturing facilities for solar cells and modules in Massachusetts,Michigan, New York, Ohio, Oregon, and Texas promise to add enough capacity toproduce thousands of megawatts of solar devices per yearwithin the next few years from 2008.[43]
In lateSeptember 2008, Sanyo Electric Company, Ltd. announcedits decision to build a manufacturing plant for solar ingots and wafers inSalem, Oregon. The plant began operating in October 2009 and reached its fullproduction capacity of 70 megawatts (MW) of solar wafers per yearin April 2010.
In earlyOctober 2008, First Solar, Inc. broke ground on an expansion of its Perrysburg,Ohio, facility that will add enough capacity to produce another 57 MW peryear of solar modules at the facility, bringing its total capacity to roughly192 MW per year. The company expects to complete construction early next yearand reach full production by mid-2010.
Inmid-October 2008, SolarWorld AGopened a manufacturing plant in Hillsboro, Oregon, that iscurrently producing 500 MW of solar cells per year in 2011.
Solyndra has amanufacturing facility for its unique tubular CIGS technology in California.
In March2010, SpectraWatt, Inc. began production at its manufacturing plant in HopewellJunction, NY, which was expected to produce 120 MW of solarcells per year when it reached full production in 2011. However, the closure ofthis plant was announced in late 2010 due to deteriorating market conditionscoupled with demand drops from Europe.[44]
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