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14 février 2011 1 14 /02 /février /2011 14:45

A solar cell (also called photovoltaic cell) is a solid state device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Assemblies of cells are used to make solar modules, also known as solar panels  (Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

The energy generated from these solar modules, referred to as solar power, is an example of solar energy.

Photovoltaics is the field of technology and research related to the practical application of photovoltaic cells in producing electricity from light, though it is often used specifically to refer to the generation of electricity from sunlight  Dell Studio XPS 1640 Battery .

Cells are described as photovoltaic cells when the light source is not necessarily sunlight. These are used for detecting light or other electromagnetic radiation near the visible range, for example infrared detectors, or measurement of light intensity  Dell Vostro 1710 Battery .

History of solar cells

The term "photovoltaic" comes from the Greek ??? (ph?s) meaning "light", and "voltaic", meaning electric, from the name of the Italian physicist Volta, after whom a unit of electro-motive force, the volt, is named. The term "photo-voltaic" has been in use in English since 1849     Dell KM958 battery .

The photovoltaic effect was first recognized in 1839 by French physicist A. E. Becquerel. However, it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated thesemiconductor selenium with an extremely thin layer of gold to form the junctions   Dell Studio 1555 battery .

The device was only around 1% efficient. In 1888 Russian physicist Aleksandr Stoletov built the first photoelectric cell(based on the outer photoelectric effect discovered by Heinrich Hertz earlier in 1887). Albert Einstein explained the photoelectric effect in 1905 for which he received the Nobel prize in Physics in 1921   Sony VGP-BPS13 battery .

Russell Ohl patented the modern junction semiconductor solar cell in 1946, which was discovered while working on the series of advances that would lead to the transistor.

Bell produces the first practical cell

The modern photovoltaic cell was developed in 1954 at Bell Laboratories Sony VGP-BPS13/B battery .

The highly efficient solar cell was first developed by Daryl Chapin, Calvin Souther Fuller and Gerald Pearson in 1954 using a diffused silicon p-n junction. At first, cells were developed for toys and other minor uses, as the cost of the electricity they produced was very high - in relative terms, a cell that produced 1 watt of electrical power in bright sunlight cost about $250, comparing to 2 to $3 for a coal plant   Sony VGP-BPS13/S battery .

Solar cells were rescued from obscurity by the suggestion to add them to the Vanguard I satellite. In the original plans, the satellite would be powered only by battery, and last a short time while this ran down. By adding cells to the outside of the fuselage, the mission time could be extended with no major changes to the spacecraft or its power systems   Sony VGP-BPS13A/B battery .

There was some skepticism at first, but in practice the cells proved to be a huge success, and solar cells were quickly designed into many new satellites, notably Bell's own Telstar.

Improvements were slow over the next two decades, and the only widespread use was in space applications where their power-to-weight ratio was higher than any competing technology   Sony VGP-BPS13B/B battery .

However, this success was also the reason for slow progress; space users were willing to pay anything for the best possible cells, there was no reason to invest in lower-cost solutions if this would reduce efficiency. Instead, the price of cells was determined largely by the semiconductor industry     Sony VGP-BPL9 battery ;

their move to integrated circuits in the 1960s led to the availability of larger boules at lower relative prices. As their price fell, the price of the resulting cells did as well. However these effects were limited, and by 1971 cell costs were estimated to be $100 a wattSony VGP-BPS13B/B battery .

Berman's price reductions

In the late 1960s, Elliot Berman was investigating a new method for producing the silicon feedstock in a ribbon process. However, he found little interest in the project and was unable to gain the funding needed to develop it   Sony VGP-BPL15 battery .

In a chance encounter, he was later introduced to a team at Exxon who were looking for projects 30 years in the future. The group had concluded that electrical power would be much more expensive by 2000, and felt that this increase in price would make new alternative energy sources more attractive, and solar was the most interesting among these   Dell Inspiron E1505 battery .

In 1969, Berman joined the Linden, New Jersey Exxon lab, Solar Power Corporation (SPC).

His first major effort was to canvas the potential market to see what possible uses for a new product were, and they quickly found that if the dollars per watt was reduced from then-current $100/watt to about $20/watt there was significant demand  Dell Latitude E6400 battery .

Knowing that his ribbon concept would take years to develop, the team started looking for ways to hit the $20 price point using existing materials.

The first improvement was the realization that the existing cells were based on standard semiconductor manufacturing process, even though that was not ideal      HP Pavilion dv6000 Battery .

This started with the boule, cutting it into disks called wafers, polishing the wafers, and then, for cell use, coating them with an anti-reflective layer. Berman noted that the rough-sawn wafers already had a perfectly suitable anti-reflective front surface, and by printing the electrodes directly on this surface, two major steps in the cell processing were eliminated   SONY VAIO VGN-FZ Battery .

The team also explored ways to improve the mounting of the cells into arrays, eliminating the expensive materials and hand wiring used in space applications with a printed circuit board on the back, acrylic plastic on the front, and silicone based glue between the two potting the cells    SONY VAIO VGN-FZ18 Battery .

But the largest improvement in price point was Berman's realization that existing silicon was effectively "too good" for solar cell use; the minor imperfections that would ruin a boule (or individual wafer) for electronics would have little effect in the solar applicationSONY VAIO VGN-FZ21E Battery .

Putting all of these changes into practice, the company started buying up "reject" silicon from existing manufacturers at very low cost. By using the largest wafers available, thereby reducing the amount of wiring for a given panel area, and packaging them into panels using their new methods      SONY VAIO VGN-FW21E Battery ,

by 1973 SPC was producing panels at $10 and selling them at $20, a fivefold decrease in prices in two years.

Navigation market

SPC approached companies making buoys as a natural market for their products, but found a curious situation  SONY VAIO VGN-NR11S/S Battery .

The primary company in the business was Automatic Power, a battery manufacturer. Realizing that solar cells might eat into their battery profits, Automatic purchased the rights to earlier solar cell designs and suppressed them. Seeing there was no interest there, SPC turned toTideland Signal, another battery company formed by ex-Automatic managers   SONY VAIO VGN-NR11M/S Battery .

Tideland introduced a solar-powered buoy and was soon ruining Automatic's business.

The timing could not be better; the rapid increase in the number of offshore oil platforms and loading facilities produced an enormous market among the oil companies       SONY VAIO VGN-NR11Z/S Battery .

As Tideland's fortunes improved, Automatic started looking for their own supply of solar panels. They found Bill Yerks of Solar Power International (SPI) in California, who was looking for a market. SPI was soon bought out by one of its largest customers, the ARCO oil giant, forming ARCO Solar       SONY VAIO VGN-NR11Z/T Battery .

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 this day.

This market, combined with the 1973 oil crisis, led to a curious situation     Sony VAIO VGN-FZ21E Battery .

Oil companies were now cash-flush due to their huge profits during the crisis, but were also acutely aware that their future success would depend on some other form of power. Over the next few years, the major oil companies started a number of solar firms, and were for decades the largest producers of solar panels      Sony VAIO VGN-FW21E Battery .

Exxon, ARCO, Shell, Amoco (later purchased by 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  Sony VAIO VGN-NR11S/S Battery .

Further improvements

In the time since Berman's work, improvements have brought production costs down under $1 a watt, with wholesale costs on the order of $2. "Balance of system" costs are now more than the panels themselves, with large commercial arrays falling to around $5 a watt, fully commissioned, in 2010       Sony VAIO VGN-NR11Z/S Battery .

As the semiconductor industry moved to ever-larger boules, older equipment became available at fire-sale prices. Cells have grown in size as older equipment became available on the surplus market; ARCO Solar's original panels used cells with 2 to 4 inch diameter  Sony VAIO VGN-NR11M/S Battery .

Panels in the 1990s and early 2000s generally used 5 inch wafers, and since 2008 almost all new panels use 6 inch cells. Another major change was the move to polycrystalline silicon. This material has less efficiency, but is less expensive to produce in bulk  Sony VAIO VGN-NR11Z/T Battery .

The widespread introduction of flat screen televisions in the late 1990s and early 2000s led to the wide availability of large sheets of high-quality glass, used on the front of the panels.

Other technologies have also come to market   SONY VAIO VGN-FZ180E Battery .

First Solar has grown to become the largest panel manufacturer, in terms of yearly power produced, using a thin-film cell sandwiched between two layers of glass. This was the first product to beat $1 a watt for production costs.  Since then a glut of polycrystalline silicon has pushed prices of conventional panels into the same range   SONY VAIO VGN-FZ220E Battery .

Applications

Solar cells 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-driven debris, rain, hail, et cetera   SONY VAIO VGN-FZ340E Battery .

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 then interconnected, in series or parallel, or both, to create an array with the desired peak DC voltage and current   SONY VAIO VGN-FZ430E Battery .

To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected photovoltaic systems); in stand-alone systems, batteries are used to store the energy that is not needed immediately. Solar panels can be used to power or recharge portable devices  SONY VAIO VGN-FZ460E Battery .

Theory

The solar cell works in three steps:

  1. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon  SONY VAIO VGN-FZ480E Battery .
  2. 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.
  3. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity  SONY VAIO VGN-FZ4000 Battery .

Efficiency

The efficiency of a solar cell may be broken down into reflectance efficiency, thermodynamic efficiency, charge carrier separation efficiency and conductive efficiency. The overall efficiency is the product of each of these individual efficiencies  SONY VAIO VGN-FZ31E Battery .

Due to the difficulty in measuring these parameters directly, other parameters are measured instead: thermodynamic efficiency, quantum efficiency, VOCratio, and fill factor. Reflectance losses are a portion of the quantum efficiency under "external quantum efficiency"   SONY VAIO VGN-FZ31B Battery .

Recombination losses make up a portion of the quantum efficiency, VOC ratio, and fill factor. Resistive losses are predominantly categorized under fill factor, but also make up minor portions of the quantum efficiency, VOC ratio.

Crystalline silicon devices are now approaching the theoretical limiting efficiency of 29%  SONY VAIO VGN-FZ31J Battery .

Cost

The cost of a solar cell is given per unit of peak electrical power. Manufacturing costs necessarily include the cost of energy required for manufacture. Solar-specific feed in tariffs vary worldwide, and even state by state within various countries   SONY VAIO VGN-FZ31M Battery .

Such feed-in tariffs can be highly effective in encouraging the development of solar power projects.

High-efficiency solar cells are of interest to decrease the cost of solar energy. Many of the costs of a solar power plant are proportional to the area of the plant   SONY VAIO VGN-FZ31Z Battery ;

a higher efficiency cell may reduce area and plant cost, even if the cells themselves are more costly. Efficiencies of bare cells, to be useful in evaluating solar power plant economics, must be evaluated under realistic conditions. The basic parameters that need to be evaluated are the short circuit current, open circuit voltage  SONY VAIO VGN-FZ38M Battery .

The chart at the right illustrates the best laboratory efficiencies obtained for various materials and technologies, generally this is done on very small, i.e. one square cm, cells. Commercial efficiencies are significantly lower   SONY VGP-BPS8 Battery .

A low-cost photovoltaic cell is a thin-film cell intended to produce electrical energy at a price competitive with traditional (fossil fuels and nuclear power) energy sources. This includes second and third generation photovoltaic cells, that is cheaper than first generation (crystalline silicon cells, also called wafer or bulk cells)  SONY VGP-BPS13/S Battery .

Grid parity, the point at which photovoltaic electricity is equal to or cheaper than grid power, can be reached using low cost solar cells. It is achieved first in areas with abundant sun and high costs for electricity such as in California and Japan. Grid parity has been reached in Hawaii and other islands that otherwise use diesel fuel to produce electricity   SONY VGP-BPS13A/B Battery .

George W. Bush had set 2015 as the date for grid parity in the USA.Speaking at a conference in 2007, General Electric's Chief Engineer predicted grid parity without subsidies in sunny parts of the United States by around 2015      SONY VGP-BPS13B/B Battery .

The price of solar panels fell steadily for 40 years, until 2004 when high subsidies in Germany drastically increased demand there and greatly increased the price of purified silicon (which is used in computer chips as well as solar panels)   SONY VGP-BPS13A/S Battery .

One research firm predicted that new manufacturing capacity began coming on-line in 2008 (projected to double by 2009) which was expected to lower prices by 70% in 2015. Other analysts warned that capacity may be slowed by economic issues, but that demand may fall because of lessening subsidies   SONY VGP-BPS13AS Battery .

Other potential bottlenecks which have been suggested are the capacity of ingot shaping and wafer slicing industries, and the supply of specialist chemicals used to coat the cells.

Materials

The Shockley-Queisser limit for the theoretical maximum efficiency of a solar cell. Semiconductors with bandgap between 1 and 1.5eV have the greatest potential to form an efficient cell   Dell Inspiron 1320n Battery .

(The efficiency "limit" shown here can be exceeded bymultijunction solar cells.)

Different materials display different efficiencies and have different costs. Materials for efficient solar cells must have characteristics matched to the spectrum of available light       Dell Inspiron 1464 Battery .

Some cells are designed to efficiently convert wavelengths of solar light that reach the Earth surface. However, some solar cells are optimized for light absorption beyond Earth's atmosphere as well. Light absorbing materials can often be used in multiple physical configurations to take advantage of different light absorption and charge separation mechanisms  Dell Inspiron 1564 Battery .

Materials presently used for photovoltaic solar cells include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium selenide/sulfide  Dell Inspiron 1764 Battery .

Many currently available solar cells are made from bulk materials that are cut into wafers between 180 to 240 micrometers thick that are then processed like other semiconductors      Dell Studio 1450 Battery .

Other materials are made as thin-films layers, organic dyes, and organic polymers that are deposited on supporting substrates. A third group are made fromnanocrystals and used as quantum dots (electron-confined nanoparticles). Silicon remains the only material that is well-researched in both bulk and thin-filmforms     Dell Studio 1457 Battery .

Crystalline silicon

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  Dell Latitude D610 Battery .

  1. 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  Toshiba NB100 Battery .
  2. 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  Toshiba Satellite M65 battery .
  3. Ribbon silicon 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   Toshiba Satellite M60 battery .

Analysts have predicted that prices of polycrystalline silicon will drop as companies build additional polysilicon capacity quicker than the industry’s projected demand. On the other hand, the cost of producing upgraded metallurgical-grade silicon, also known as UMG Si, can potentially be one-sixth that of making polysilicon   Dell Latitude D830 Battery .

Manufacturers of wafer-based cells have responded to thin-film lower prices with rapid reductions in silicon consumption. According to Jef Poortmans, director of IMEC's organic and solar department, current cells use between eight and nine grams of silicon per watt of power generation, with wafer thicknesses in the neighborhood of 0.200 mm   Dell Latitude D620 Battery .

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 thick wafers and is testing processes for 0.16 mm wafers cut with 0.1 mm wire  Dell Inspiron Mini 10 Battery .

IMEC's roadmap, presented at the organization's recent annual research review meeting, envisions use of 0.08 mm wafers by 2015.

Thin films

Thin-film technologies reduce the amount of material required in creating a solar cell   Sony VGN-FW11S Battery .

Though this reduces material cost, it may also reduce energy conversion efficiency. Thin-film silicon cells have become popular due to cost, flexibility, lighter weight, and ease of integration, compared to wafer silicon cells   Sony VGN-FW11M Battery .

Cadmium telluride solar cell

A cadmium telluride solar cell use a cadmium telluride (CdTe) thin film, a semiconductor layer to absorb and convert sunlight into electricity. Solarbuzz  has reported that the lowest quoted thin-film module price stands at US$1.76 per watt-peak, with the lowest crystalline silicon (c-Si) module at $2.48 per watt-peak   Sony VGN-FW139E/H battery .

The cadmium present in the cells would be toxic if released. However, release is impossible during normal operation of the cells and is unlikely during ?res in residential roofs. A square meter of CdTe contains approximately the same amount of Cd as a single C cell Nickel-cadmium battery, in a more stable and less soluble form   Dell Latitude E5400 Battery .

Copper-Indium Selenide

Copper indium gallium selenide (CIGS) is a direct-bandgap material. It has the highest efficiency (~20%) among thin film materials (see CIGS solar cells). Traditional methods of fabrication involve vacuum processes including co-evaporation and sputtering   Dell Latitude E4200 Battery .

Recent developments at IBM and Nanosolar have been targeting to lower the cost by using non-vacuum solution processes.

Gallium arsenide multijunction

High-efficiency multijunction cells were originally developed for special applications such as satellites and space exploration, but at present, their use in terrestrial concentrators might be the lowest cost alternative in terms of $/kWh and $/W  Dell Inspiron 300M Battery .

These multijunction cells consist of multiple thin films produced using metalorganic vapour phase epitaxy. A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP2.Each type of semiconductor will have a characteristic band gap energy which   Dell Vostro A840 Battery ,

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   Dell Studio 1737 battery .

GaAs based multijunction devices are the most efficient solar cells to date. In October 2010, triple junction metamorphic cell reached a record high of 42.3%.

This technology is currently being utilized in the Mars Exploration Rover missions which have run far past their 90 day design life  Dell Inspiron E1505 battery .

Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge pn junctions, are 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   Dell RM791 battery .

Additionally, germanium metal prices have risen substantially to $1000–$1200 per kg this year. 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  Dell XPS M1530 battery .

Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time World Solar Challenge winners Nuna in 2003, 2005 and 2007, and also by the Dutch solar cars Solutra (2005), Twente One (2007) and 21Revolution (2009)  Dell XPS M2010 battery .

The Dutch Radboud University Nijmegen set the record for thin film solar cell efficiency using a single junction GaAs to 25.8% in August 2008 using only 4 µm thick GaAs layer which can be transferred from a wafer base to glass or plastic film   Acer Aspire One battery .

Light-absorbing dyes (DSSC)

Dye-sensitized solar cells (DSSCs) are made of low-cost materials and do not need elaborate equipment to manufacture, so they can be made in a DIY fashion, possibly allowing players to produce more of this type of solar cell than others   Toshiba Satellite P10 Battery .

In bulk it should be significantly less expensive than older solid-state cell designs. DSSC's can be engineered into flexible sheets, and although its conversion efficiency is less than the best thin film cells, its price/performance ratio should be high enough to allow them to compete with fossil fuel electrical generation   SONY VGN-FZ210CE Battery .

The DSSC has been developed by Prof.Michael Grätzel in 1991 at the Swiss Federal Institute of Technology (EPFL) in Lausanne (CH).

Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material  Dell Precision M70 Battery.

The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium 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 absorbed by an electrolyte on the other side of the dye  Toshiba Satellite L305 Battery .

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 and/or use of Ultrasonic Nozzles, with the potential for lower processing costs than those used for bulk solar cells   Toshiba Satellite T4900 Battery .

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     Toshiba PA3399U-2BRS battery .

The first commercial shipment of DSSC solar modules occurred in July 2009 from G24i Innovations (www.g24i.com).

Organic/polymer solar cells

Organic solar cells are a relatively novel technology, yet hold the promise of a substantial price reduction (over thin-film silicon) and a faster return on investment    Toshiba Satellite A200 Battery .

These cells can be processed from solution, hence the possibility of a simple roll-to-roll printing process, leading to inexpensive, large scale production.

Organic solar cells and polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors including polymers    Toshiba Satellite 1200 Battery ,

such as polyphenylene vinylene and small-molecule compounds like copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes and fullerene derivatives such as PCBM. Energy conversion efficiencies achieved to date using conductive polymers are low compared to inorganic materials     Toshiba Satellite M300 Battery .

However, it has improved quickly in the last few years and the highest NREL (National Renewable Energy Laboratory) certified efficiency has reached 6.77%. In addition, these cells could be beneficial for some applications where mechanical flexibility and disposability are important      SONY VGP-BPS13A/Q Battery .

These devices differ from inorganic semiconductor solar cells in that they do not rely on the large built-in electric field of a PN junction to separate the electrons and holes created when photons are absorbed. The active region of an organic device consists of two materials, one which acts as an electron donor and the other as an acceptor   SONY VGP-BPS13A/Q Battery .

When a photon is converted into an electron hole pair, typically in the donor material, the charges tend to remain bound in the form of an exciton, and are separated when the exciton diffuses to the donor-acceptor interface. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices   SONY VGP-BPS13B/Q Battery .

Nanostructured interfaces, sometimes in the form of bulk heterojunctions, can improve performance.

Silicon thin films

Silicon thin-film cells are mainly deposited by chemical vapor deposition (typically plasma-enhanced (PE-CVD)) from silane gas and hydrogen gas      SONY VGP-BPS13B/Q Battery .

Depending on the deposition parameters, this can yield:

  1. Amorphous silicon (a-Si or a-Si:H)
  2. Protocrystalline silicon or
  3. Nanocrystalline silicon (nc-Si or nc-Si:H), also called microcrystalline silicon   SONY VGP-BPS13/Q Battery .

It has been found that protocrystalline silicon with a low volume fraction of nanocrystalline silicon is optimal for high open circuit voltage.[30] 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)   SONY VGP-BPS13/Q Battery .

The solar cells made from these materials tend to have lowerenergy 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   SONY VGP-BPS21A/B Battery .

An amorphous silicon (a-Si) solar cell is made of amorphous or microcrystalline silicon and its basic electronic structure is the p-i-n junction. As the amorphous structure has a higher absorption rate of light than crystalline cells, the complete light spectrum can be absorbed with a very thin layer of photo-electrically active material   SONY VGP-BPS21/S Battery .

A film only 1 micron thick can absorb 90% of the usable solar energy.The production of a-Si thin film solar cells uses glass as a substrate and deposits a very thin layer of silicon by plasma-enhanced chemical vapor deposition (PECVD)   SONY VGP-BPS21B Battery .

A-Si manufacturers are working towards lower costs per watt and higher conversion efficiency with continuous research and development on Multijunction solar cells for solar panels. Anwell Technologies Limited recently announced its target for multi-substrate-multi-chamber PECVD, to lower the cost to USD0.5 per watt  SONY VGP-BPS21A Battery .

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 nc-Si and a-Si can advantageously be combined in thin layers, creating a layered cell called a tandem cell SONY VGP-BPS21 Battery .

The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nc-Si.

Recently, solutions to overcome the limitations of thin-film crystalline silicon have been developed   Sony VGP-BPS21A/B Battery .

Light trapping schemes where the weakly absorbed long wavelength light is obliquely coupled into the silicon and traverses the film several times can significantly enhance the absorption of sunlight in the thin silicon films. Thermal processing techniques can significantly enhance the crystal quality of the silicon and thereby lead to higher efficiencies of the final solar cells  Sony VGP-BPS21/S Battery .

Manufacture

Because solar cells are semiconductor devices, they share many of the same processing and manufacturing techniques as other semiconductor devices such as computer and memory chips   Sony VGP-BPS21A Battery .

However, the stringent requirements for cleanliness and quality control of semiconductor fabrication are a little more relaxed for solar cells. Most large-scale commercial solar cell factories today make screen printed poly-crystalline silicon solar cells   Sony VGP-BPS21 Battery .

Single crystalline wafers which are used in the semiconductor industry can be made into excellent high efficiency solar cells, but they are generally considered to be too expensive for large-scale mass production    Sony Vaio PCG-5G2L Battery .

Poly-crystalline silicon 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, a surface diffusion of n-type dopants is performed on the front side of the wafer     Sony Vaio PCG-5G3L Battery .

This forms a p-n junction a few hundred nanometers below the surface.

Antireflection coatings, to increase the amount of light coupled into the solar cell, are typically next applied   Sony Vaio PCG-5J1L Battery .

Silicon nitride has gradually replaced titanium dioxide as the antireflection coating because of its excellent surface passivation qualities. It prevents carrier recombination at the surface of the solar cell. It is typically applied in a layer several hundred nanometers thick using plasma-enhanced chemical vapor deposition (PECVD)   Sony Vaio PCG-5K2L Battery .

Some solar cells have textured front surfaces that, like antireflection coatings, serve to increase the amount of light coupled into the cell. Such surfaces can usually only be formed on single-crystal silicon, though in recent years methods of forming them on multicrystalline silicon have been developed       Sony Vaio PCG-5J2L Battery .

The wafer then has a full area metal contact made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "busbars" are screen-printed onto the front surface using a silverpaste. The rear contact is also formed by screen-printing a metal paste, typically aluminium       Sony Vaio PCG-5K1L Battery .

Usually this contact covers the entire rear side of the cell, though in some cell designs it is printed in a grid pattern. The paste is then fired at several hundred degrees celsius to form metal electrodes in ohmic contact with the silicon. Some companies use an additional electro-plating step to increase the cell efficiency      Sony Vaio PCG-5L1L Battery .

After the metal contacts are made, the solar cells are interconnected in series (and/or parallel) by flat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back       Sony Vaio PCG-6S2L Battery .

Lifespan

Most commercially available solar cells are capable of producing electricity for at least twenty years without a significant decrease in efficiency. The typical warranty given by panel manufacturers is for a period of 25 – 30 years, wherein the output shall not fall below a specified percentage (around 80%) of the rated capacity       Sony Vaio PCG-6S3L Battery .

Research topics

There are currently many research groups active in the field of photovoltaics in universities and research institutions around the world        Sony Vaio PCG-6V1L Battery .

This research can be divided into three areas: making current technology solar cells cheaper and/or more efficient to effectively compete with other energy sources; developing new technologies based on new solar cell architectural designs; and developing new materials to serve as light absorbers and charge carriers       Sony Vaio PCG-6W1L Battery .

Manufacturers and certification

National Renewable Energy Laboratory tests and validates solar technologies. There are three reliable certifications of solar equipment: UL and IEEE (both U.S. standards) and IEC       Sony Vaio PCG-6W2L Battery .

Solar cells are manufactured primarily in Japan, Germany, Mainland China, Taiwan and United States, though numerous other nations have or are acquiring significant solar cell production capacity. While technologies are constantly evolving toward higher efficiencies      Sony Vaio PCG-6W3L Battery ,

the most effective cells for low cost electrical production are not necessarily those with the highest efficiency, but those with a balance between low-cost production and efficiency high enough to minimize area-related balance of systems cost     Sony Vaio PCG-7111L Battery .

Those companies with large scale manufacturing technology for coating inexpensive substrates may, in fact, ultimately be the lowest cost net electricity producers, even with cell efficiencies that are lower than those of single-crystal technologies  Sony Vaio PCG-7112L Battery .

China

Backed by Chinese government's unprecedented plan to offer subsidies for utility-scale solar power projects that is likely to spark a new round of investment from Chinese solar panel makers. Chinese companies have already played a more important role in solar panels manufacturing in recent years      Sony Vaio PCG-7113L Battery .

China produced solar cells/modules with an output of 1,180 MW in 2007 making it the largest producer in the world, according to statistics from China Photovoltaic Association.Some Chinese companies such as Suntech Power    Sony Vaio PCG-7133L Battery ,

Yingli, LDK Solar Co, JA Solar and ReneSola have already announced projects in cooperation with regional governments with hundreds of megawatts each after the ‘Golden Sun’ incentive program was announced by the government   Sony Vaio PCG-7Z2L Battery .

The new development of solar module manufacturers with thin-film technology such as Veeco and Anwell Technologies Limited will further help to boost the domestic solar industry        Sony Vaio PCG-8Y1L Battery .

United States

New manufacturing facilities for solar cells and modules in Massachusetts, Michigan, New York, Ohio, Oregon, and Texas promise to add enough capacity to produce thousands of megawatts of solar devices per year within the next few years from 2008   Sony Vaio PCG-8Y2L Battery .

In late September 2008, Sanyo Electric Company, Ltd. announced its decision to build a manufacturing plant for solar ingots and wafers in Salem, Oregon. The plant will begin operating in October 2009 and will reach its full production capacity of 70 megawatts (MW) of solar wafers per year by April 2010      Sony Vaio PCG-8Z1L Battery .

In early October 2008, First Solar, Inc. broke ground on an expansion of its Perrysburg, Ohio, facility that will add enough capacity to produce another 57 MW per year of solar modules at the facility, bringing its total capacity to roughly 192 MW per year. The company expects to complete construction early next year and reach full production by mid-2010  Sony Vaio PCG-8Z2L Battery .

In mid-October 2008, SolarWorld AG opened a manufacturing plant in Hillsboro, Oregon, that is expected to produce 500 MW of solar cells per year when it reaches full production in 2011.

Solyndra has a manufacturing facility for its unique tubular CIGS technology in California       Sony VAIO PCG-5G2L Battery .

In March 2010, SpectraWatt, Inc. began production at its manufacturing plant in Hopewell Junction, NY, which was expected to produce 120 MW of solar cells per year when it reaches full production in 2011. However, the closure of this plant was announced in late 2010      Sony VAIO PCG-5G3L Battery .

Multi-junction solar cells are solar cells containing several p-n junctions. This allows to cover a wider part of the light spectrum, but increases the complexicity of the cell design and manufacture.

Much effort has been spent to improve the production of renewable energy       Sony VAIO PCG-5J1L Battery .

One of the most promising technologies is photovoltaic solar panels which convert solar energy into electrical energy. The first solar cell was a c-Si solar cell realised in 1954 by researchers at Bell Labs had an efficiency of 6%. Within fifty years, considerable progress has been made in the field of photovoltaic technology and many different technologies have emerged   Sony VAIO PCG-5K2L Battery.

Nowadays, common c-Si solar cells have a maximum efficiency of 24.7% but new devices like multi-junction (MJ) solar cells are more promising. With the best efficiency of 42.3% as of October 2010 (under a concentration of 406 suns)  they are well-suited for space applications       Sony VAIO PCG-5J2L Battery .

They also have a significant development potential because theoretical efficiency for an infinite number of pn junctions is 86.8%. However, the structure of MJ solar cells is more complex as compared to other photovoltaic devices.

ndustry sells photovoltaic panels that are composed of several modules       Sony VAIO PCG-5K1L Battery .

All modules are connected in series or in parallel and include numerous solar cells. These are composed of a pn junction made of doped silicon and some metallic contacts to guarantee an electrical connection. When photons hit the top surface of the solar cell, they are either reflected or transmitted       Sony VAIO PCG-5L1L Battery .

Those who are transmitted, give their energy h? to an electron. If h? ? Eg, there is direct electron/hole generation. As shown in Figure A, electrons move from the valence band to the conduction band and that holes appear in the valence band   Sony VAIO PCG-6S2L Battery .

In the depletion region, the drift electric field Edrift accelerates both electrons and holes towards their respective n-doped and p-doped regions. The resulting current Ig is called the generation photocurrent. In the quasi-neutral region, the scattering electric field Escatt accelerates holes (electrons) towards p-doped (n-doped) region  Sony VAIO PCG-6S3L Battery ,

which gives a scattering photocurrent Ipscatt (Inscatt). Consequently, due to the accumulation of charges, a potential V and a photocurrent Iph appear. The expression for this photocurrent is obtained by adding generation and scattering photocurrents: Iph = Ig + Inscatt + Ipscatt Sony VAIO PCG-6V1L Battery .

The J-V characteristics (J is current density, i.e. current per area of pn junction) of a solar cell under illumination are simply obtained by shifting the J-V characteristics of a diode in the dark downward by Iph, as shown in Figure B   Sony VAIO PCG-6W1L Battery .

Since solar cells are designed to supply power and not absorb it, the power P = V·Iph must be negative. Hence, the operating point (Vm, Jm) is located in the region where V>0 andIph<0 are chosen so that they maximize the absolute value of the power |P|  Sony VAIO PCG-6W2L Battery .

The fundamental difference between multi-junction solar cells and c-Si solar cells is that there are several pn junctions connected in series instead of one, as illustrated on figure C(a). In order to better cover the solar spectrum AM1.5 (terrestrial reference spectrum for photovoltaic performance evaluation), suitable materials must be chosen for each pn junction  Sony VAIO PCG-6W3L Battery .

First, their band gaps Eg have to be significantly different so that photons of different wavelength ? =hc/(eEg) ? 1.24×10?6/Eg are absorbed. The figure C(b) illustrates this fact by plotting spectral irradiance G(?), which is the source power density at a given wavelength ?       Sony VAIO PCG-7111L Battery .

Secondly, Eg must decrease from top cell to bottom cell so that photons would not all be absorbed by the top cell. Thirdly, the layers have to be electrically optimal for high performances and be stacked on each other         Sony VAIO PCG-7112L Battery .

This necessitates finding materials with better absorption coefficients ?(?), higher minority carrier lifetimes ?minority, higher mobilities µ and similar lattice constants a. The values in Table below justifie the choice of the usual materials used for multi-junction solar cells  Sony VAIO PCG-7113L Battery :

the top cell is generally made in InGaP (Eg = 1.86 eV), the middle cell in InGaAs (Eg = 1.4 eV) and the bottom cell in Ge (Eg = 0.65 eV). The use of germanium is mainly due to its robustness, its low cost and its facility in production      Sony VAIO PCG-7Z1L Battery .

In addition to the benefits listed above, the use of GaAs instead of Si allows to reach higher efficiencies because the efficiency of GaAs cells is 25.1% while the efficiency of c-Si cells is 24.7%      Sony VAIO PCG-7Z2L Battery .

Because the different layers are closely lattice-matched, the fabrication of the device is usually obtained by using the metal-organic chemical vapor deposition (MOCVD). This technique is preferable to the molecular beam epitaxy (MBE) because it ensures high crystal quality and large scale production       Sony VAIO PCG-8Y1L Battery .

Metallic contacts

The metallic contacts in aluminium are low-resistivity electrodes that make contact with the semiconductor layer in GaAs. They are disposed on the two sides of the structure but mainly on the backwards face so that the shadowing on the lightning surface is reduced   WD passport essential (500GB/640GB) .

Anti-reflective coating

Anti-reflective (AR) coating is generally composed of several layers in the case of MJ solar cells. The top AR layer has usually a NaOH surface texturation with several pyramids in order to increase the transmission coefficient T WD passport essential (250GB/320GB) ,

the trapping of the light in the material (because photons cannot easily get out the MJ structure due to pyramids) and therefore, the path length of photons in the material  . On the one hand, the thickness of each AR layer is chosen to get destructive i

nterferences. Therefore, the reflection coefficient R decreases to 1%       WD  passport essential SE (750GB/1TB .

In the case of two AR layers L1 (the top layer, usually SiO2) and L2 (usually TiO2), there must be  to have the same amplitudes for reflected fields and nL1dL1 = 4?min, nL2dL2 = ?min/4 to have opposite phase for reflected fields. On the other hand, the thickness of each AR layer is also chosen to minimize the reflectance at wavelengths for which the photocurrent is the lowest  WD  passport elite(250GB/320GB) .

Consequently, this maximizes JSC by matching currents of the three subcells. As example, because the current generated by the bottom cell is greater than the currents generated by the other cells, the thickness of AR layers is adjusted so that the infrared (IR) transmission (which corresponds to the bottom cell)       WD passport elite(500GB/640GB)

is degraded while the UV transmission (which corresponds to the top cell) is upgraded. Particularly, an AR coating is very important at low wavelengths because, without it, T would be strongly reduced to 70%        WD  passport studio for Mac(320GB/500GB) .

Tunnel junctions

The main goal of tunnel junctions is to provide a low electrical resistance and optically low-loss connection between two subcells.  Without it, the p-doped region of the top cell would be directly connected with the n-doped region of the middle cell  WD  passport studio for Mac(500GB/640GB) .

Hence, a pn junction with opposite direction to the others would appear between the top cell and the middle cell. Consequently, the photovoltage would be lower than if there would be no parasitic diode. In order to decrease this effect, a tunnel junction is used. It is simply a wide band gap, highly doped diode     WD  Elements series(250GB/320GB) .

The high doping reduces the length of the depletion region because

Hence, electrons can easily tunnel through the depletion region. The J-V characteristic of the tunnel junction is very important because it explains why tunnel junctions can be used to have a low electrical resistance connection between two pn junctions  WD  Elements SE(500GB/640GB) .

Figure D shows three different regions: the tunneling region, the negative differential resistance region and the thermal diffusion region. The region where electrons can tunnel through the barrier is called the tunneling region   WD Elements SE(750GB/1TB) .

There, the voltage must be low enough so that energy of some electrons who are tunneling is equal to energy states available on the other side of the barrier. Consequently, current density through the tunnel junction is high (with maximum value of JP, the peak current density) and the slope near the origin is therefore steep  WD  Elements desktop(500GB/640GB) .

Then, the resistance is extremely low and consequently, the voltage too. This is why tunnel junctions are ideal for connecting two pn junctions without having a voltage drop. When voltage is higher, electrons cannot cross the barrier because energy states are no longer available for electrons      WD  Elements desktop(750GB/1TB) .

Therefore, the current density decreases and the differential resistance is negative. The last region, called thermal diffusion region, corresponds to the J-V characteristic of the usual diode               WD  Elements desktop(1.5 TB/2TB) :

In order to avoid the reduction of the MJ solar cell performances, tunnel junctions must be transparent to wavelengths absorbed by the next photovoltaic cell, the middle cell, i.e. EgTunnel > EgMiddleCell          WD  passport essential SE (750GB/1TB)--USB 3.0) .

Window layer and back-surface field

Figure E: (a) Layers and band structure of a window layer. The surface recombination is reduced. (b) Layers and band structure of a BSF layer. The scattering of carriers is reduced        WD  passport essential (500GB/640GB) .

A window layer is used in order to reduce the surface recombination velocity S. Similarly, a back-surface field (BSF) layer reduces the scattering of carriers towards the tunnel junction. The structure of these two layers is the same: it is a heterojunction which catches electrons (holes)      WD  passport for Mac(320GB/500GB) .

Indeed, despite the electric field Ed, these one cannot jump above the barrier formed by the heterojunction because they don't have enough energy, as illustrated in figure E. Hence, electrons (holes) cannot recombine with holes (electrons) and cannot diffuse through the barrier        WD  passport for Mac(640GB/1TB) .

By the way, window and BSF layers must be transparent to wavelengths absorbed by the next pn junction i.e. EgWindow > EgEmitterand EgBSF > EgEmitter. Furthermore, the lattice constant must be close to the one of InGaP and the layer must be highly doped (n ? 1018 cm?3)     My book essential 4 generation  (640GB/1TB) .

J-V characteristic

For maximum efficiency, each subcell should be operated at its optimal J-V parameters, which are not necessarily equal for each subcell. If they are different, the total current through the solar cell is the lowest of the three      WD  My book essential 4 generation( 1.5TB/2TB) .

By approximation,  it results in the same relationship for the short-circuit current of the MJ solar cell: JSC = min (JSC1, JSC2, JSC3)where JSCi(?) is the short-circuit current density at a given wavelength ? for the subcell i WD  My book elite( 1TB/1.5TB) .

Because of the impossibility to obtain JSC1, JSC2, JSC3 directly from the total J-V characteristic, the quantum efficiency QE(?) is utilized. It measures the ratio between the amount of electron-hole pairs created and the incident photons at a given wavelength ?. Let ?i(?) be the photon flux of corresponding incident light in subcell i and QEi(?) be the quantum efficiency of the subcell i WD  My book studio(1TB/2TB) .

By definition, this equates to:

The value of QEi(?) is obtained by linking it with the absorption coefficient ?(?), i.e. the number of photons absorbed per unit of length by a material        WD  My book essential 4 generation( 1.5TB/2TB) .

If it is assumed that each photon absorbed by a subcell creates an electron/hole pair (which is a good approximation), this leads to :

The performances of MJ solar cells can be improved as follows    WD  My book elite(640GB/2TB) .

Firstly, GaAsSb-based heterojunction tunnel diodes, instead of conventional InGaP highly doped tunnel diodes described above, have a lower tunneling distance. Indeed, in the heterostructure formed by GaAsSb and InGaAs, the valence band of GaAsSb is higher than the valence band of the adjoining p-doped layer  Seagate  expansion portable (320GB/500GB) .

Consequently, the tunneling distance dtunnel is reduced and so the tunneling current, which exponentially depends of dtunnel, is increased. Hence, the voltage is lower than that of the InGaP tunnel junction. There are other advantages to use GaAsSb heterojunction tunnel diode         Seagate  expansion (1.5TB/2TB) .

Firstly, the same current can be achieved by using a lower doping.  Secondly, because the lattice constant is larger for GaAsSb than Ge, one can use a wider range of materials for the bottom cell because more materials are lattice-matched to GaAsSb than to Ge  Seagate  Freeagent Desktop (500GB/1TB) .

Secondly, chemical components can be added to some layers. On the one hand, adding about one percent of Indium in each layer better matches lattice constants of the different layers  . Without it, there is about 0.08 percent of mismatching between layers, which contributes to deteriorate the performances of MJ solar cells     Seagate  Freeagent Go(250GB/320GB) .

On the other hand, adding aluminium to the top cell increases its band gap to 1.96 eV. It allows to cover a larger part of the solar spectrum and obtain a higher open-circuit voltage VOC.

Thirdly, the number of pn junctions in the cell can be increased        Seagate  Freeagent Go(500GB/640GB) .

Indeed, the theoretical efficiency of MJ solar cells is 86.8% for an infinite number of pn junctions. However, increasing the number of layers is very difficult in practice. The maximum theoretical efficiency is 37, 50, 56, 72% for 1, 2, 3, 36 pn junctions, respectivelySeagate  Freeagent Go(750GB/1TB) .

Thus with the increasing number of pn junctions, the effiency gain is small but the complexity of production and matching the individual junctions increases dramatically. Instead of increasing the number of subcells, one can optimize their thickness. Particularly, decreasing the thickness of the top cell will increase the transmission coefficient T Seagate  Freeagent Goflex(250GB/320GB) .

Finally, an InGaP hetero-layer between the p-Ge layer and the InGaAs layer can be added in order to create automatically the n-Ge layer by scattering during MOCVD growth and increase significantly the quantum efficiency QE(?) of the bottom cell. InGaP is advantageous because of its high scattering coefficient and low solubility in Ge      Seagate Freeagent Goflex(500GB/640GB) .

Spectral variations

Solar spectrum at the Earth surface changes constantly depending on the weather and sun position. This results in the variation of ?(?), QE(?), ?(?) and thus the short-circuit currents JSCi Seagate  Freeagent Goflex(750GB/1TB) .

As a result, the current densities Ji are not necessarily matched and the total current becomes lower. Spectral variations of the light can be quantified using the average photon energy (APE) which is the ratio between the spectral irradiance G(?) (the power density of the light source in a specific wavelength ?) and the total photon flux density        Seagate  Freeagent Goflex Pro(500GB/750GB) .

It can be shown that a high (low) value for APE means low (high) wavelengths spectral conditions and higher (lower) efficiencies. Thus APE is a good indicator for quantifying the effects of the solar spectrum variations on MJ solar cell performances and has the added advantage of being independent of the device structure and the absorption profile of the device  Seagate  Freeagent Goflex desktop(1TB/2TB) .

Use of light concentrators

Light concentrators allow to increase efficiencies and reduce the cost efficiency ratio. There are two types of light concentrators: refractive lenses like Fresnel lenses and reflective dishes        Seagate  Freeagent go for Mac(320GB/640GB) .

Thanks to these devices, light arriving on a large surface can be concentrated on a small surface, the MJ solar cell. The intensity concentration ratio (or “suns”) is the average intensity of the focused light divided by 0.1 W/m². If its value is X then the MJ current becomes X higher under concentrated illumination       Samsung  G2 protable (250gb/320GB) .

Comparison with other technologies

There are four main categories of photovoltaic cells: c-Si solar cells, thin film solar cells, MJ solar cells and new technologies (including organic solar cells)        Samsung  G2 protable (500GB/640GB) .

Big differences between multi-junction solar cells and other photovoltaic devices can be observed (see the table above). Physically, the main property of a MJ solar cell is to have more than one pn junction in order to catch a larger photon energy spectrum while the main property of the thin film solar cell is to use thin films instead of thick layers in order to decrease the cost efficiency ratioSamsung  S2 protable (320GB/500GB) .

Economically, there are also some differences: MJ solar panels are more expensive than the others. These differences involve different applications for each ones: MJ solar cells are rather used in space while c-Si solar cells are rather used for terrestrian applications      Samsung S1 Mini (120GB/160GB) .

There are also differences in the evolution of efficiency for different types of solar technology. The efficiencies of solar cells and Si solar technology are relatively stable, while the efficiency of solar modules and multi-junction technology are progressing  Samsung  S1 Mini (250GB/320GB) .

It is important to keep in mind that measurements on MJ solar cells are usually made in laboratory, using light concentrators (this is often not the case for the other cells) and under standard test conditions (STCs). STCs prescribe, for the terrestrian application, the AM1.5 spectrum as reference          Samsung story station (1TB/1.5TB) .

This air mass (AM) corresponds to a fix position of the sun in the sky of 48° and a fix power of 833 W/m². Therefore, spectral variations of incident light and environmental parameters are not taken into account under STC. Consequently, performance of MJ solar cells in terrestrial environment is inferior to that achieved in laboratory      Samsung Story station (1.5TB/2TB) .

Moreover, MJ solar cells are created such that currents are matched under STC but not necessarily under field conditions. One can use QE(?) to compare performances of different technologies, butQE(?) contains no information on the matching of currents of subcells        Samsung story station Esata(1TB/1.5TB) .

An important comparison point is rather the output power per unit area generated with the same incident light.

Applications

The cost of MJ solar cells is currently too high to allow a high scale use of it for individuals        Samsung  G3 station (1TB/1.5TB) .

The high cost is mainly due to the complex structure and the high price of materials. Nevertheless, with light concentrators under illumination of at least 400 suns, the use of MJ solar panels become possible and profitable.

The environment in space is quite different from the terrestrian one        Maxtor  one touch 4 plus (500GB/750GB) .

Because there is no more atmosphere in space, the solar spectrum is different (AM0) and there is some damage due to radiation particles who are no longer filtered. They are two kinds of damage: ionisation and atomics displacement       Maxtor one touch 4 plus (1TB/1.5TB) .

They should be considered when sending solar panels to space. The comparison with other technologies shows that it is more convenient to use multi-junction solar cells in space. Indeed, it has a higher radiation resistance, a higher efficiency and a lower temperature coefficient   Maxtor cool black(640GB/1TB) .

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