A solar cell or photovoltaic cell is a device that converts solar energy into electricity by the photovoltaic effect. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 A wafer is a thin slice of Semiconductor material such as a Silicon crystal used in the fabrication of Integrated circuit and other microdevices Solar energy is the Light and radiant heat from the Sun that powers Earth 's Climate and Weather and sustains Life Photovoltaics is the field of technology and research related to the application of solar cells as solar energy. Photovoltaics ( PV) is the field of technology and research related to the application of Solar cells for Energy by converting Sunlight directly Solar energy is the Light and radiant heat from the Sun that powers Earth 's Climate and Weather and sustains Life Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the source is unspecified.

Assemblies of cells are used to make solar modules, which may in turn be linked in photovoltaic arrays. In the field of Photovoltaics, a photovoltaic module is a packaged interconnected assembly of photovoltaic cells also known as Solar cells An installation of A photovoltaic array is a linked collection of Photovoltaic modules which are in turn made of multiple interconnected Solar cells The cells convert

Solar cells have many applications. Individual cells are used for powering small devices such as electronic calculators. A calculator is device for performing mathematical calculations distinguished from a Computer by having a limited problem solving ability and an interface optimized for interactive Photovoltaic arrays generate a form of renewable electricity, particularly useful in situations where electrical power from the grid is unavailable such as in remote area power systems, Earth-orbiting satellites and space probes, remote radiotelephones and water pumping applications. Renewable energy is Energy generated from Natural resources mdashsuch as Sunlight, Wind, Rain, tides and geothermal Electric power transmission, a process in the delivery of Electricity to consumers is the bulk transfer of electrical power EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 This article is about artificial satellites For natural satellites also known as moons see Natural satellite. A radiotelephone is a communications device that allows two or more people to talk using Radio. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Photovoltaic electricity is also increasingly deployed in grid-tied electrical systems. A grid-tied electrical system, also called Tied to grid or Grid tie system is a semi-autonomous electrical generation or Grid energy storage system which links to the mains

Contents 1 History 1.1 Three generations of solar cells 2 Applications and implementations 3 Theory 3.1 Simple explanation 3.2 Photogeneration of charge carriers 3.3 Charge carrier separation 3.4 The p-n junction 3.5 Connection to an external load 3.6 Equivalent circuit of a solar cell 3.7 Circuit Equations defining solar cell 4 Solar cell efficiency factors 4.1 Energy conversion efficiency 4.2 Thermodynamic Efficiency Limit 4.3 Quantum efficiency 4.4 VOC ratio 4.5 Maximum-power point 4.6 Fill factor 4.7 Comparison of energy conversion efficiencies 4.7.1 Watts peak 4.7.2 Solar cells and energy payback 5 Light-absorbing materials 5.1 Bulk 5.1.1 Silicon 5.2 Thin films 5.2.1 CdTe 5.2.2 Copper-Indium Selenide 5.2.3 Gallium arsenide (GaAs) multijunction 5.2.4 Light-absorbing dyes 5.2.5 Organic/polymer solar cells 5.2.6 Silicon 5.3 Nanocrystalline solar cells 6 Concentrating photovoltaics (CPV) 7 Silicon solar cell device manufacture 8 Current research on materials and devices 8.1 Silicon processing 8.2 Thin-film processing 8.3 Polymer processing 8.4 Nanoparticle processing 8.5 Transparent conductors 8.6 Silicon wafer based solar cells 9 Manufacturers 10 See also 11 References 12 External links

History

Main article: Timeline of solar cells

The term "photovoltaic" comes from the Greek φώς:phos meaning "light", and "voltaic", meaning electrical, from the name of the Italian physicist Volta, after whom the measurement unit volt is named. The Timeline of Solar cells begins in the 1800s when it is observed that the presence of sunlight is capable of generating usable electrical energy Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly The' Italian people' are a Southern European Ethnic group located primarily in Italy, Switzerland, France and by virtue of a wide-ranging Count Alessandro Giuseppe Antonio Anastasio Volta was a Lombard physicist known especially for the development of the first electric cell in The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. The term "photo-voltaic" has been in use in English since 1849. ^[1]

The photovoltaic effect was first recognized in 1839 by French physicist Alexandre-Edmond Becquerel. Year 1839 ( MDCCCXXXIX) was a Common year starting on Tuesday (link will display the full calendar of the Gregorian Calendar (or a Common Alexandre-Edmond Becquerel ( March 24, 1820 - May 11, 1891) was a French Physicist who studied the Solar spectrum However, it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. Year 1883 ( MDCCCLXXXIII) was a Common year starting on Monday (link will display the full calendar of the Gregorian calendar (or a Common Charles Fritts was an American Inventor credited with creating the first working Solar cell in 1884 A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that Selenium (səˈliniəm is a Chemical element with the Atomic number 34 represented by the chemical symbol Se, an atomic mass of 78 Gold (ˈɡoʊld is a Chemical element with the symbol Au (from its Latin name aurum) and Atomic number 79 The device was only around 1% efficient. Russell Ohl patented the modern solar cell in 1946 (U.S. Patent 2,402,662 , "Light sensitive device"). Russell Ohl (1898 - 1987 was an American engineer who is generally recognized for Patenting the modern Solar cell (US Patent 2402662 "Light sensitive device" Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells. The modern age of solar power technology arrived in 1954 when Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light. Bell Laboratories (also known as Bell Labs and formerly known as AT&T Bell Laboratories and Bell Telephone Laboratories) is the Research organization

This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6 percent. Russia launched the first artificial satellite in 1957, and the United States' first artificial satellite was launched in 1958 using solar cells created by Peter Iles in an effort spearheaded by Hoffman Electronics. The first spacecraft to use solar panels was the US satellite Explorer 1 in January 1958. Explorer 1 (officially titled at NASA as satellite 1958 Alpha) was the first Earth Satellite of the United States, launched on February 1 This milestone created interest in producing and launching a geostationary communications satellite, in which solar energy would provide a viable power supply. A geostationary orbit (GEO is a Geosynchronous orbit directly above the Earth 's Equator (0° Latitude) with a period equal to the Earth's This article is about artificial satellites For natural satellites also known as moons see Natural satellite. This was a crucial development which stimulated funding from several governments into research for improved solar cells.

In 1970 the first highly effective GaAs heterostructure solar cells were created by Zhores Alferov and his team in the USSR. Year 1970 ( MCMLXX) was a Common year starting on Thursday (link shows full calendar of the Gregorian calendar. Gallium arsenide ( GaAs) is a compound of two elements Gallium and Arsenic. A heterojunction is the interface that occurs between two layers or regions of dissimilar Crystalline Semiconductors These semiconducting materials have unequal Zhores Ivanovich Alferov (Жоре́с Ива́нович Алфёров ʐɐˈrʲɛs ɪˈvanəvʲɪtɕ ɐlˈfʲorəf (born March 15 1930) is a Russian The Union of Soviet Socialist Republics (USSR was a constitutionally Socialist state that existed in Eurasia from 1922 to 1991 ^[2][3][4] Metal Organic Chemical Vapor Deposition (MOCVD, or OMCVD) production equipment was not developed until the early 1980s, limiting the ability of companies to manufacture the GaAs solar cell. In the United States, the first 17% efficient air mass zero (AM0) single-junction GaAs solar cells were manufactured in production quantities in 1988 by Applied Solar Energy Corporation (ASEC). The air mass coefficient characterizes the solar spectrum after the Solar radiation has traveled through the atmosphere The "dual junction" cell was accidentally produced in quantity by ASEC in 1989 as a result of the change from GaAs on GaAs substrates to GaAs on Germanium (Ge) substrates. The accidental doping of Ge with the GaAs buffer layer created higher open circuit voltages, demonstrating the potential of using the Ge substrate as another cell. As GaAs single-junction cells topped 19% AM0 production efficiency in 1993, ASEC developed the first dual junction cells for spacecraft use in the United States, with a starting efficiency of approximately 20%. The air mass coefficient characterizes the solar spectrum after the Solar radiation has traveled through the atmosphere These cells did not utilize the Ge as a second cell, but used another GaAs-based cell with different doping. Eventually GaAs dual junction cells reached production efficiencies of about 22%. Triple Junction solar cells began with AM0 efficiencies of approximately 24% in 2000, 26% in 2002, 28% in 2005, and in 2007 have evolved to a 30% AM0 production efficiency, currently in qualification. The air mass coefficient characterizes the solar spectrum after the Solar radiation has traveled through the atmosphere The air mass coefficient characterizes the solar spectrum after the Solar radiation has traveled through the atmosphere In 2007, two companies in the United States, Emcore Photovoltaics and Spectrolab, produce 95% of the world's 28% efficient solar cells. Spectrolab, headquartered in Sylmar California, is a subsidiary of The Boeing Company that manufacturers space Solar cells and panels

Three generations of solar cells

Possible reference please confirm. Retrieved on 2008-05-22. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 334 BC - The Greek army of Alexander the Great defeats Darius III of Persia in the Battle of the Granicus.

The first generation photovoltaic cell consists of a large-area, single-crystal, single layer p-n junction diode, capable of generating usable electrical energy from light sources with the wavelengths of sunlight. A p-n junction is a junction formed by combining P-type and N-type Semiconductors together in very close contact Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. These cells are typically made using a diffusion process with silicon wafers. Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 A wafer is a thin slice of Semiconductor material such as a Silicon crystal used in the fabrication of Integrated circuit and other microdevices First-generation photovoltaic cells (also known as silicon wafer-based solar cells) are the dominant technology in the commercial production of solar cells, accounting for more than 86% of the terrestrial solar cell market.

The second generation of photovoltaic materials is based on the use of thin epitaxial deposits of semiconductors on lattice-matched wafers. There are two classes of epitaxial photovoltaics - space and terrestrial. Space cells typically have higher AM0 efficiencies (28-30%) in production, but have a higher cost per watt. Their "thin-film" cousins have been developed using lower-cost processes, but have lower AM0 efficiencies (7-10%) in production and are questionable for space applications. The advent of thin-film technology contributed to a prediction of greatly reduced costs for thin film solar cells that has yet to be achieved. There are currently (2007) a number of technologies/semiconductor materials under investigation or in mass production. Examples include amorphous silicon, polycrystalline silicon, micro-crystalline silicon, cadmium telluride, copper indium selenide/sulfide. Amorphous silicon (a-Si is the non-crystalline Allotropic form of Silicon. Cadmium telluride (CdTe is a Crystalline compound formed from Cadmium and Tellurium with a zinc blende (cubic crystal structure An advantage of thin-film technology theoretically results in reduced mass so it allows fitting panels on light or flexible materials, even textiles. The advent of thin GaAs-based films for space applications (so-called "thin cells") with potential AM0 efficiencies of up to 37% are currently in the development stage for high specific power applications. Second generation solar cells now comprise a small segment of the terrestrial photovoltaic market, and approximately 90% of the space market.

Third-generation photovoltaics are proposed to be very different from the previous semiconductor devices as they do not rely on a traditional p-n junction to separate photogenerated charge carriers. For space applications, quantum well devices (quantum dots, quantum ropes, etc. ) and devices incorporating carbon nanotubes are being studied - with a potential for up to 45% AM0 production efficiency. For terrestrial applications, these new devices include photoelectrochemical cells, polymer solar cells, nanocrystal solar cells, Dye-sensitized solar cells and are still in the research phase. Photoelectrochemical cells or PECs are Solar cells and extract electrical energy from light including Visible light. Polymer solar cells are a type of Organic solar cell: they produce Electricity from Sunlight. Nanocrystal solar cells or Quantum dot solar cells are Solar cells based on a Silicon Substrate with a Coating of Nanocrystals

Applications and implementations

Main article: photovoltaic array

Solar cells are often electrically connected and encapsulated as a module. A photovoltaic array is a linked collection of Photovoltaic modules which are in turn made of multiple interconnected Solar cells The cells convert PV modules often have a sheet of glass on the front (sun up) side , allowing light to pass while protecting the semiconductor wafers from the elements (rain, hail, etc. A wafer is a thin slice of Semiconductor material such as a Silicon crystal used in the fabrication of Integrated circuit and other microdevices Rain is Liquid precipitation. On Earth it is the condensation of atmospheric Water vapor into drops heavy enough to fall often making it to Hail is a form of precipitation which consists of balls or irregular lumps of ice (hailstones ). Solar cells are also usually connected in series in modules, creating an additive voltage. If two or more circuit components are connected end to end like a daisy chain it is said they are connected in series. Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical 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.

The power output of a solar array is measured in watts or kilowatts. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. In order to calculate the typical energy needs of the application, a measurement in watt-hours, kilowatt-hours or kilowatt-hours per day is often used. A common rule of thumb is that average power is equal to 20% of peak power, so that each peak kilowatt of solar array output power corresponds to energy production of 4. A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation 8 kW·h per day.

To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected PV systems); in stand alone systems, batteries are used to store the energy that is not needed immediately.

Theory

Simple explanation

1. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Sunlight, in the broad sense is the total spectrum of the Electromagnetic radiation given off by the Sun. Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14
2. Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J The complementary positive charges that are also created (like bubbles) are called holes and flow in the direction opposite of the electrons in a silicon solar panel. An electron hole is the conceptual and mathematical Opposite of an Electron, useful in the study of Physics and Chemistry.
3. An array of solar panels converts solar energy into a usable amount of direct current (DC) electricity. Direct current ( DC) is the unidirectional flow of Electric charge.

Optionally:

1. The DC current enters an inverter. An inverter is an electrical or electro-mechanical device that converts Direct current (DC to Alternating current (AC the resulting AC can be at any required voltage
2. The inverter turns DC electricity into 120 or 240-volt AC (alternating current) electricity needed for home appliances.
3. The AC power enters the utility panel in the house.
4. The electricity is then distributed to appliances or lights in the house.
5. The electricity that is not used will be re-routed and used in other facilities.

Photogeneration of charge carriers

When a photon hits a piece of silicon, one of three things can happen:

1. the photon can pass straight through the silicon — this (generally) happens for lower energy photons,
2. the photon can reflect off the surface,
3. the photon can be absorbed by the silicon, if the photon energy is higher than the silicon band gap value. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena In Solid state physics and related applied fields a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states This generates an electron-hole pair and sometimes heat, depending on the band structure.

When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far. In Solids the valence band is the highest range of Electron energies where electrons are normally present at Absolute zero. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. In the Physics field of Semiconductors and insulators the conduction band is the range of Electron Energy, higher than that of the The covalent bond that the electron was previously a part of now has one fewer electron — this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs.

A photon need only have greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. Familiar concepts associated with a Frequency are colors musical notes radio/TV channels and even the regular rotation of the earth In Physics, a black body is an object that absorbs all light that falls on it EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations — called phonons) rather than into usable electrical energy. In Physics, a phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the Atomic lattice of a Solid

Charge carrier separation

There are two main modes for charge carrier separation in a solar cell:

1. drift of carriers, driven by an electrostatic field established across the device
2. diffusion of carriers from zones of high carrier concentration to zones of low carrier concentration (following a gradient of electrochemical potential).

In the widely used p-n junction solar cells, the dominant mode of charge carrier separation is by drift. However, in non-p-n-junction solar cells (typical of the third generation of solar cell research such as dye and polymer thin-film solar cells), a general electrostatic field has been confirmed to be absent, and the dominant mode of separation is via charge carrier diffusion.

The p-n junction

Main article: semiconductor

The most commonly known solar cell is configured as a large-area p-n junction made from silicon. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that A p-n junction is a junction formed by combining P-type and N-type Semiconductors together in very close contact As a simplification, one can imagine bringing a layer of n-type silicon into direct contact with a layer of p-type silicon. In practice, p-n junctions of silicon solar cells are not made in this way, but rather, by diffusing an n-type dopant into one side of a p-type wafer (or vice versa).

If a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (p-type side of the junction). Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. The diffusion of carriers does not happen indefinitely however, because of an electric field which is created by the imbalance of charge immediately on either side of the junction which this diffusion creates. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can The electric field established across the p-n junction creates a diode that promotes current to flow in only one direction across the junction. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. Electrons may pass from the p-type side into the n-type side, and holes may pass from the n-type side to the p-type side, but not the other way around^[5]. This region where electrons have diffused across the junction is called the depletion region because it no longer contains any mobile charge carriers. In Semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region It is also known as the "space charge region".

Connection to an external load

Ohmic metal-semiconductor contacts are made to both the n-type and p-type sides of the solar cell, and the electrodes connected to an external load. An ohmic contact is a region on a Semiconductor device that has been prepared so that the current-voltage (I-V curve of the device is linear and symmetric The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across Electrons that are created on the n-type side, or have been "collected" by the junction and swept onto the n-type side, may travel through the wire, power the load, and continue through the wire until they reach the p-type semiconductor-metal contact. Here, they recombine with a hole that was either created as an electron-hole pair on the p-type side of the solar cell, or swept across the junction from the n-type side after being created there.

Equivalent circuit of a solar cell

To understand the electronic behavior of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behavior is well known. See also Scale model A physical model is a smaller or larger physical copy of an object An ideal solar cell may be modelled by a current source in parallel with a diode; in practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes ^[6] The resulting equivalent circuit of a solar cell is shown on the left. Also shown, on the right, is the schematic representation of a solar cell for use in circuit diagrams.

Circuit Equations defining solar cell

The equations which describe the I-V characteristics of the cell are

1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 

where

• k is Boltzmann's constant
• q is charge on an electron
• V_g is band gap voltage
• n is diode quality factor
• R_s is series resistance of cell
• R_sh is shunt resistance
• T₁ and T₂ are reference temperature in Kelvin
• T is working temperature of cell in Kelvin
• V_oc is open circuit voltage
• I_sc is short circuit current
• I_o is reverse saturation current of diode
• I is the current through Rs
• I_L is current produced by the cell

in many sources I_L is also known as I_ph. Bridge from macroscopic to microscopic physics Boltzmann's constant k is a bridge between Macroscopic and microscopic physics

Solar cell efficiency factors

Energy conversion efficiency

A solar cell's energy conversion efficiency (η, "eta"), is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of the maximum power point, P_m, divided by the input light irradiance (E, in W/m²) under standard test conditions (STC) and the surface area of the solar cell (A_c in m²). Irradiance, radiant emittance, and radiant exitance are Radiometry terms for the power of Electromagnetic radiation at a surface per unit

STC specifies a temperature of 25°C and an irradiance of 1000 W/m² with an air mass 1. 5 (AM1. 5) spectrum. These correspond to the irradiance and spectrum of sunlight incident on a clear day upon a sun-facing 37°-tilted surface with the sun at an angle of 41. 81° above the horizon. ^[7][8] This condition approximately represents solar noon near the spring and autumn equinoxes in the continental United States with surface of the cell aimed directly at the sun. Thus, under these conditions a solar cell of 12% efficiency with a 100 cm² (0. 01 m²) surface area can be expected to produce approximately 1. 2 watts of power.

The losses of a solar cell may be broken down into reflectance losses, thermodynamic efficiency, recombination losses and resistive electrical loss. The overall efficiency is the product of each of these individual losses.

Due to the difficulty in measuring these parameters directly, other parameters are measured instead: Thermodynamic Efficiency, Quantum Efficiency, V_OC ratio, and Fill Factor. Reflectance losses are a portion of the Quantum Efficiency under "External Quantum Efficiency". Recombination losses make up a portion of the Quantum Efficiency, V_OC ratio, and Fill Factor. Resistive losses are predominantly categorized under Fill Factor, but also make up minor portions of the Quantum Efficiency, V_OC ratio.

Thermodynamic Efficiency Limit

Solar cells operate as quantum energy conversion devices, and are therefore subject to the "Thermodynamic Efficiency Limit". Photons with an energy below the band gap of the absorber material cannot generate a hole-electron pair, and so their energy is not converted to useful output and only generates heat if absorbed. For photons with an energy above the band gap energy, only a fraction of the energy above the band gap can be converted to useful output. When a photon of greater energy is absorbed, the excess energy above the band gap is converted to kinetic energy of the carrier combination. The excess kinetic energy is converted to heat through phonon interactions as the kinetic energy of the carriers slows to equilibrium velocity.

Solar cells with multiple band gap absorber materials are able to more efficiently convert the solar spectrum. By using multiple band gaps, the solar spectrum may be broken down into smaller bins where the thermodynamic efficiency limit is higher for each bin. ^[9]

Quantum efficiency

As described above, when a photon is absorbed by a solar cell it is converted to an electron-hole pair. This electron-hole pair may then travel to the surface of the solar cell and contribute to the current produced by the cell; such a carrier is said to be collected. Alternatively, the carrier may give up its energy and once again become bound to an atom within the solar cell without reaching the surface; this is called recombination, and carriers that recombine do not contribute to the production of electrical current. In the Solid state physics of Semiconductors carrier generation and recombination are processes by which mobile Electrons and Electron holes

Quantum efficiency refers to the percentage of photons that are converted to electric current (i. Quantum efficiency (QE is a quantity defined for a photosensitive device such as Photographic film or a Charge-coupled device (CCD as the percentage of Photons e. , collected carriers) when the cell is operated under short circuit conditions. External quantum efficiency is the fraction of incident photons that are converted to electrical current, while internal quantum efficiency is the fraction of absorbed photons that are converted to electrical current. Mathematically, internal quantum efficiency is related to external quantum efficiency by the reflectance of the solar cell; given a perfect anti-reflection coating, they are the same.

Quantum efficiency should not be confused with energy conversion efficiency, as it does not convey information about the power collected from the solar cell. Energy conversion efficiency is the Ratio between the useful output of an Energy conversion machine and the input in Energy terms Furthermore, quantum efficiency is most usefully expressed as a spectral measurement (that is, as a function of photon wavelength or energy). Since some wavelengths are absorbed more effectively than others in most semiconductors, spectral measurements of quantum efficiency can yield information about which parts of a particular solar cell design are most in need of improvement.

V_OC ratio

Due to recombination, the open circuit voltage (V_OC) of the cell will be below the band gap voltage of the cell. Since the energy of the photons must be at or above the band gap to generate a carrier pair, cell voltage below the band gap voltage represents a loss. This loss is represented by the ratio of V_OC divided by V_G

Maximum-power point

A solar cell may operate over a wide range of voltages (V) and currents (I). Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. By increasing the resistive load on an irradiated cell continuously from zero (a short circuit) to a very high value (an open circuit) one can determine the maximum-power point, the point that maximizes V×I; that is, the load for which the cell can deliver maximum electrical power at that level of irradiation. Short Circuit is a 1986 comedy Science fiction film starring Ally Sheedy and Steve Guttenberg and directed by In Electrical engineering, the maximum power (transfer theorem states that to obtain maximum external power from a source with a finite internal resistance (The output power is zero in both the short circuit and open circuit extremes).

A high quality, monocrystalline silicon solar cell, at 25 °C cell temperature, may produce 0. 60 volts open-circuit (Voc). The cell temperature in full sunlight, even with 25 °C air temperature, will probably be close to 45 °C, reducing the open-circuit voltage to 0. 55 volts per cell. The voltage drops modestly, with this type of cell, until the short-circuit current is approached (Isc). Maximum power (with 45 °C cell temperature) is typically produced with 75% to 80% of the open-circuit voltage (0. 43 volts in this case) and 90% of the short-circuit current. This output can be up to 70% of the Voc x Isc product. The short-circuit current (Isc) from a cell is nearly proportional to the illumination, while the open-circuit voltage (Voc) may drop only 10% with a 80% drop in illumination. Lower-quality cells have a more rapid drop in voltage with increasing current and could produce only 1/2 Voc at 1/2 Isc. The usable power output could thus drop from 70% of the Voc x Isc product to 50% or even as little as 25%. Vendors who rate their solar cell "power" only as Voc x Isc, without giving load curves, can be seriously distorting their actual performance.

The maximum power point of a photovoltaic varies with incident illumination. Photovoltaics ( PV) is the field of technology and research related to the application of Solar cells for Energy by converting Sunlight directly For systems large enough to justify the extra expense, a maximum power point tracker tracks the instantaneous power by continually measuring the voltage and current (and hence, power transfer), and uses this information to dynamically adjust the load so the maximum power is always transferred, regardless of the variation in lighting. A maximum power point tracker (or MPPT) is a high Efficiency DC to DC converter which functions as an optimal electrical load for a photovoltaic Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere.

Fill factor

Another defining term in the overall behavior of a solar cell is the fill factor (FF). Fill factor in the context of Solar cell technology it is defined as the ratio (given as percent of the actual maximum obtainable power, (Vmp x Jmp This is the ratio of the maximum power point divided by the open circuit voltage (V_oc) and the short circuit current (I_sc):

Comparison of energy conversion efficiencies

Main article: Photovoltaics

At this point, discussion of the different ways to calculate efficiency for space cells and terrestrial cells is necessary to alleviate confusion. Photovoltaics ( PV) is the field of technology and research related to the application of Solar cells for Energy by converting Sunlight directly In space, where there is no atmosphere, the spectrum of the sun is relatively unfiltered. However on earth, with air filtering the incoming light, the solar spectrum changes. To account for the spectral differences, a system was devised to calculate this filtering effect. Simply, the filtering effect ranges from Air Mass 0 in space, to approximately Air Mass 1. 5 on earth. Multiplying the spectral differences by the quantum efficiency of the solar cell in question will yield the efficiency of the device. For example, a Silicon solar cell in space might have an efficiency of 14% at AM0, but have an efficiency of 16% on earth at AM 1. 5. Terrestrial efficiencies typically are greater than space efficiencies.

Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 40. 7% with multiple-junction research lab cells and 42. 8% with multiple dies assembled into a hybrid package. ^[10] Solar cell energy conversion efficiencies for commercially available multicrystalline Si solar cells are around 14-19%^[11]. The highest efficiency cells have not always been the most economical — for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide and produced in low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while only delivering about four times the electrical power.

However, there is a way to "boost" solar power. By increasing the light intensity, typically photogenerated carriers are increased, resulting in increased efficiency by up to 15%. These so-called "concentrator systems" have only begun to become cost-competitive as a result of the development of high efficiency GaAs cells. The increase in intensity is typically accomplished by using concentrating optics. A typical concentrator system may use a light intensity 6-400 times the sun, and increase the efficiency of a one sun GaAs cell from 31% at AM 1. 5 to 35%.

A common method used to express economic costs of electricity-generating systems is to calculate a price per delivered kilowatt-hour (kWh). The solar cell efficiency in combination with the available irradiation has a major influence on the costs, but generally speaking the overall system efficiency is important. Using the commercially available solar cells (as of 2006) and system technology leads to system efficiencies between 5 and 19%. As of 2005, photovoltaic electricity generation costs ranged from ~0. 60 US$/kWh (0. 50 €/kWh) (central Europe) down to ~0. 30 US$/kWh (0. 25 €/kWh) in regions of high solar irradiation. This electricity is generally fed into the electrical grid on the customer's side of the meter. The cost can be compared to prevailing retail electric pricing (as of 2005), which varied from between 0. 04 and 0. 50 US$/kWh worldwide. (Note: in addition to solar irradiance profiles, these costs/kwh calculations will vary depending on assumptions for years of useful life of a system. Most c-Si panels are warrantied for 25 years and should see 35+ years of useful life. )

The chart at the right illustrates the various commercial large-area module energy conversion efficiencies and the best laboratory efficiencies obtained for various materials and technologies.

Watts peak

Since solar cell output power depends on multiple factors, such as the sun's incidence angle, for comparison purposes between different cells and panels, the measure of watts peak (Wp) is used. The Sun (Sol is the Star at the center of the Solar System. Angle of incidence is a measure of deviation of something from "straight on" for example in the approach of a ray to a surface or the angle It is the output power under these conditions known as STC:^[12][13]

1. insolation (solar irradiance) 1000 W/m²
2. solar reference spectrum AM (airmass) 1. Insolation is a measure of Solar radiation energy received on a given surface area in a given time Irradiance, radiant emittance, and radiant exitance are Radiometry terms for the power of Electromagnetic radiation at a surface per unit For air mass in Meteorology, see Air mass. In Astronomy, airmass is the optical path length through Earth's 5
3. cell temperature 25°C

Solar cells and energy payback

In the 1990s, when silicon cells were twice as thick, efficiencies were 30% lower than today and lifetimes were shorter, it may well have cost more energy to make a cell than it could generate in a lifetime. The Celsius Temperature scale was previously known as the centigrade scale. In the meantime, the technology has progressed significantly, and the energy payback time of a modern photovoltaic module is typically from 1 to 4 years ^[14] depending on the type and where it is used (see net energy gain). See also EROEI (Energy Return on Energy Invested Net Energy Gain ( NEG) is a concept important in Energy economics, referring to a With a typical lifetime of 20 to 30 years, this means that modern solar cells are net energy producers, i. e they generate much more energy over their lifetime than the energy expended in producing them. ^[15][14][16]

Light-absorbing materials

All solar cells require a light absorbing material contained within the cell structure to absorb photons and generate electrons via the photovoltaic effect. In Physics, absorption of electromagnetic radiation is the process by which the Energy of a Photon is taken up by matter typically the electrons of an Photovoltaics ( PV) is the field of technology and research related to the application of Solar cells for Energy by converting Sunlight directly The materials used in solar cells tend to have the property of preferentially absorbing the 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. Many currently available solar cells are configured as bulk materials that are subsequently cut into wafers and treated in a "top-down" method of synthesis (silicon being the most prevalent bulk material). Other materials are configured as thin-films (inorganic layers, organic dyes, and organic polymers) that are deposited on supporting substrates, while a third group are configured as nanocrystals and used as quantum dots (electron-confined nanoparticles) embedded in a supporting matrix in a "bottom-up" approach. Silicon remains the only material that is well-researched in both bulk and thin-film configurations. The following is a current list of light absorbing materials, listed by configuration and substance-name:

Bulk

These bulk technologies are often referred to as wafer-based manufacturing. In other words, in each of these approaches, self-supporting wafers between 180 to 240 micrometers thick are processed and then soldered together to form a solar cell module. A general description of silicon wafer processing is provided in Manufacture and Devices.

Silicon

Main articles: Polysilicon, Silicon, and list of silicon producers

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". In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer. An ingot is a material usually metal that is cast into a shape suitable for further processing A ribbon or riband is a thin band of flexible material typically Cloth but also Plastic or sometimes Metal, used primarily for binding and tying In cooking a wafer is a crisp often sweet very thin flat and dry Cake, often used to decorate Ice cream.

1. monocrystalline silicon (c-Si): often made using the Czochralski process. The Czochralski process is a method of Crystal growth used to obtain Single crystals of Semiconductors (e 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.
2. Poly- or multicrystalline silicon (poly-Si or mc-Si): made from cast square ingots — large blocks of molten silicon carefully cooled and solidified. These cells are less expensive to produce than single crystal cells but are less efficient.
3. Ribbon silicon: formed by drawing flat thin films from molten silicon and having 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.

Thin films

Main article: Thin film

The various thin-film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell. Thin films are thin material layers ranging from fractions of a Nanometre to several Micrometres in thickness This can lead to reduced processing costs from that of bulk materials (in the case of silicon thin films) but also tends to reduce energy conversion efficiency, although many multi-layer thin films have efficiencies above those of bulk silicon wafers.

CdTe

Cadmium telluride is an efficient light-absorbing material for thin-film solar cells. Cadmium telluride (CdTe is a Crystalline compound formed from Cadmium and Tellurium with a zinc blende (cubic crystal structure Compared to other thin-film materials, CdTe is easier to deposit and more suitable for large-scale production. Despite much discussion of the toxicity of CdTe-based solar cells, this is the only technology (apart from amorphous silicon) that can be delivered on a large scale. The perception of the toxicity of CdTe is based on the toxicity of elemental cadmium, a heavy metal that is a cumulative poison. Cadmium (ˈkædmiəm is a Chemical element with the symbol Cd and Atomic number 48 Bioaccumulation occurs when an organism absorbs a toxic substance at a rate greater than that at ftudruinsubstance is lost However it has been shown that the release of cadmium to the atmosphere is lower with CdTe-based solar cells than with silicon photovoltaics and other thin-film solar cell technologies. ^[17]

Copper-Indium Selenide

Possible combinations of I III VI elements in the periodic table that have photovoltaic effect

The materials based on CuInSe₂ that are of interest for photovoltaic applications include several elements from groups I, III and VI in the periodic table. These semiconductors are especially attractive for thin film solar cell application because of their high optical absorption coefficients and versatile optical and electrical characteristics which can in principle be manipulated and tuned for a specific need in a given device. CIS is an abbreviation for general chalcopyrite films of copper indium selenide (CuInSe₂), CIGS mentioned below is a variation of CIS. CIS films (no Ga) achieved greater than 14% efficiency. ^[18] However, manufacturing costs of CIS solar cells at present are high when compared with amorphous silicon solar cells but continuing work is leading to more cost-effective production processes. The first large-scale production of CIS modules was started in 2006 in Germany by Wuerth Solar.

When gallium is substituted for some of the indium in CIS, the material is sometimes called CIGS , or copper indium/gallium diselenide, a solid mixture of the semiconductors CuInSe₂ and CuGaSe₂, often abbreviated by the chemical formula CuIn_xGa_(1-x)Se₂. Copper indium gallium selenide ( CIGS) is a new Semiconductor material composed of Copper, Indium, Gallium, and Selenium Unlike the conventional silicon based solar cell, which can be modelled as a simple p-n junction (see under semiconductor), these cells are best described by a more complex heterojunction model. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that The best efficiency of a thin-film solar cell as of March 2008 was 19. 9% with CIGS absorber layer. ^[19] Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. The use of gallium increases the optical bandgap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage. In another point of view, gallium is added to replace as much indium as possible due to gallium's relative availability to indium. Approximately 70%^[20] of indium currently produced is used by the flat-screen monitor industry. Some investors in solar technology worry that production of CIGS cells will be limited by the availability of indium. Producing 2 GW of CIGS cells (roughly the amount of silicon cells produced in 2006) would use about 10% of the indium produced in 2004. ^[21] For comparison, silicon solar cells used up 33% of the world's electronic grade silicon production in 2006. Nanosolar claims to waste only 5% of the indium it uses. Nanosolar is a developer of Solar power technology Based in San Jose, CA Nanosolar has developed and commercialized an extremely low-cost printable Solar As of 2006, the best conversion efficiency for flexible CIGS cells on polyimide is 14. 1% by Tiwari et al, at the ETH, Switzerland.

That being said, indium can easily be recycled from decommissioned PV modules. The recycling program in Germany is an example that highlights the regenerative industrial paradigm: "From cradle to cradle". Cradle to Cradle Design (sometimes abbreviated to C2C) is a Biomimetic approach to the design of systems

Selenium allows for better uniformity across the layer and so the number of recombination sites in the film are reduced which benefits the quantum efficiency and thus the conversion efficiency.

Gallium arsenide (GaAs) multijunction

Main article: Multijunction photovoltaic cell

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. Multijunction photovoltaic cells are a sub-class of Solar cell or photovoltaic cell developed for higher efficiency This article is about artificial satellites For natural satellites also known as moons see Natural satellite. History First orbital flights The first successful orbital launch was of the Soviet unmanned Sputnik ^[22] These multijunction cells consist of multiple thin films produced using Metalorganic vapour phase epitaxy. Metalorganic vapour phase epitaxy (MOVPE is a Chemical vapour deposition method of epitaxial growth of materials especially Compound semiconductors from A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP₂. Gallium arsenide ( GaAs) is a compound of two elements Gallium and Arsenic. Germanium (dʒɚˈmeɪniəm is a Chemical element with the symbol Ge and Atomic number 32 Indium gallium phosphide ( InGaP) is a Semiconductor composed of Indium, Gallium and Phosphorus. ^[23] Each type of semiconductor will have a characteristic band gap energy which, 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. In Solid state physics and related applied fields a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. 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.

GaAs based multijunction devices are the most efficient solar cells to date, reaching a record high of 40. 7% efficiency under solar concentration and laboratory conditions. ^[24]

This technology is currently being utilized in the Mars rover missions.

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. 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.

Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time World Solar Challenge winners Nuna in 2005 and 2007. The World Solar Challenge is a solar-powered car race which covers 3021 km (1877 miles through the Australian Outback, from Darwin The Nuna4 is a solar car developed by the Delft University of Technology in 2006-2007 for the 2007 World Solar Challenge.

Light-absorbing dyes

Main article: Dye-sensitized solar cells

Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material. 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). In Nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties Titanium dioxide, also known as titanium(IV oxide or titania, is the naturally occurring Oxide of Titanium, chemical formula Ti[[oxygen The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO₂, and the holes are passed to an electrolyte on the other side of the dye. 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, with the potential for lower processing costs than those used for bulk solar cells. 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.

Organic/polymer solar cells

Organic solar cells and Polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors such as polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes. Polymer solar cells are a type of Organic solar cell: they produce Electricity from Sunlight. An organic semiconductor is any Organic material that has Semiconductor properties Poly(p-phenylene vinylene ( PPV, or polyphenylene vinylene) is a conducting polymer of the Rigid-rod polymer host family "C60" and "C-60" redirect here For other uses see C60 (disambiguation. Energy conversion efficiencies achieved to date using conductive polymers are low compared to inorganic materials, with the highest reported efficiency of 6. 5% ^[25] for a tandem cell architecture. However, these cells could be beneficial for some applications where mechanical flexibility and disposability are important.

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. 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. This page is about the Quasiparticle. Exciton is also the title of a single by IDM composer Squarepusher. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices. Nanostructured interfaces, sometimes in the form of bulk heterojunctions, can improve performance ^[26].

Silicon

Silicon thin-films are mainly deposited by chemical vapor deposition (typically plasma-enhanced (PE-CVD)) from silane gas and hydrogen gas. Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 Chemical vapor deposition (CVD is a Chemical process used to produce high-purity high-performance solid materials Silane is a Chemical compound with Chemical formula Si[[hydrogen H]]4 Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Depending on the deposition's 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). Amorphous silicon (a-Si is the non-crystalline Allotropic form of Silicon. A distinct phase occurring during Crystal growth which evolves into a Microcrystalline form Nanocrystalline silicon ( nc-Si)—an allotropic form of Silicon with Paracrystalline structure—is similar to Amorphous silicon

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). The solar cells made from these materials tend to have lower energy 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. Quantum efficiency (QE is a quantity defined for a photosensitive device such as Photographic film or a Charge-coupled device (CCD as the percentage of Photons

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. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of As nc-Si has about the same bandgap as c-Si, the two material can be combined in thin layers, creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si.

Recently, solutions to overcome the limitations of thin-film crystalline silicon have been developed. Light trapping schemes where the incoming light is obliquely coupled into the silicon and the light traverses the film several times enhance the absorption of sunlight in the films. Thermal processing techniques enhance the crystallinity of the silicon and pacify electronic defects.

A silicon thin film technology is being developed for building integrated photovoltaics (BIPV) in the form of semi-transparent solar cells which can be applied as window glazing. These cells function as window tinting while generating electricity.

Nanocrystalline solar cells

Main article: Nanocrystal solar cell

These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption). Nanocrystal solar cells or Quantum dot solar cells are Solar cells based on a Silicon Substrate with a Coating of Nanocrystals Using nanocrystals allows one to design architectures on the length scale of nanometers, the typical exciton diffusion length. In particular, single-nanocrystal ('channel') devices, an array of single p-n junctions between the electrodes and separated by a period of about a diffusion length, represent a new architecture for solar cells and potentially high efficiency.

Concentrating photovoltaics (CPV)

Concentrating photovoltaic systems use a large area of lenses or mirrors to focus sunlight on a small area of photovoltaic cells. ^[27] If these systems use single or dual-axis tracking to improve performance, they may be referred to as Heliostat Concentrator Photovoltaics (HCPV). The primary attraction of CPV systems is their reduced usage of semiconducting material which is expensive and currently in short supply. Additionally, increasing the concentration ratio improves the performance of general photovoltaic materials^[28]. Despite the advantages of CPV technologies their application has been limited by the costs of focusing, tracking and cooling equipment. On October 25, 2006, the Australian federal government and the Victorian state government together with photovoltaic technology company Solar Systems announced a project using this technology, Solar power station in Victoria, planned to come online in 2008 and be completed by 2013. Events 1147 - The Portuguese, under Afonso I, and Crusaders from England and Flanders conquer Lisbon after a Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. For a topic outline on this subject see List of basic Australia topics. Solar Systems is a leader in high concentration solar Photovoltaic applications and the company is preparing to build the world's largest heliostat concentrator photovoltaic A large new solar power station in Victoria is planned Solar Systems is to build the world’s most advanced Photovoltaic (PV Heliostat solar concentrator This plant, at 154 MW, would be ten times larger than the largest current photovoltaic plant in the world. ^[29]

Silicon solar cell device 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. A computer is a Machine that manipulates data according to a list of instructions. Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside 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. 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.

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. A P-type semiconductor (P for Positive) is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order To make a solar cell from the wafer, a surface diffusion of n-type dopants is performed on the front side of the wafer. An N-type semiconductor (N for Negative) is obtained by carrying out a process of doping, that is by adding an impurity of valence -five elements to This forms a p-n junction a few hundred nanometers below the surface. A p-n junction is a junction formed by combining P-type and N-type Semiconductors together in very close contact

Antireflection coatings, which increase the amount of light coupled into the solar cell, are typically next applied. Over the past decade, silicon nitride has gradually replaced titanium dioxide as the antireflection coating of choice because of its excellent surface passivation qualities (i. e. , 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). 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.

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 silver paste. Silver (ˈsɪlvɚ is a Chemical element with the symbol " Ag " (argentum from the Ancient Greek: ἀργήντος - argēntos gen The rear contact is also formed by screen-printing a metal paste, typically aluminium. 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. An ohmic contact is a region on a Semiconductor device that has been prepared so that the current-voltage (I-V curve of the device is linear and symmetric 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". In the field of Photovoltaics, a photovoltaic module is a packaged interconnected assembly of photovoltaic cells also known as Solar cells An installation of Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back. Toughened or tempered glass is Glass that has been processed by controlled thermal or chemical treatments to increase its strength compared with normal glass A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds Tempered glass cannot be used with amorphous silicon cells because of the high temperatures during the deposition process.

Current research on materials and devices

See also: Timeline of solar cells

There are currently many research groups active in the field of photovoltaics in universities and research institutions around the world. The Timeline of Solar cells begins in the 1800s when it is observed that the presence of sunlight is capable of generating usable electrical energy Photovoltaics ( PV) is the field of technology and research related to the application of Solar cells for Energy by converting Sunlight directly A university is an institution of Higher education and Research, which grants Academic degrees in a variety of subjects 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.

Silicon processing

One way of reducing the cost is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica, or silica sand. Sand is a naturally occurring Granular material composed of finely divided rock and Mineral particles Processing silica (SiO₂) to produce silicon is a very high energy process - at current efficiencies, it takes over two years for a conventional solar cell to generate as much energy as was used to make the silicon it contains. ^[30] More energy efficient methods of synthesis are not only beneficial to the solar industry, but also to industries surrounding silicon technology as a whole.

The current industrial production of silicon is via the reaction between carbon (charcoal) and silica at a temperature around 1700 degrees Celsius. The Celsius Temperature scale was previously known as the centigrade scale. In this process, known as carbothermic reduction, each tonne of silicon (metallurgical grade, about 98% pure) is produced with the emission of about 1. 5 tonnes of carbon dioxide.

Solid silica can be directly converted (reduced) to pure silicon by electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 degrees Celsius). ^[31][32] While this new process is in principle the same as the FFC Cambridge Process which was first discovered in late 1996, the interesting laboratory finding is that such electrolytic silicon is in the form of porous silicon which turns readily into a fine powder, (with a particle size of a few micrometres), and may therefore offer new opportunities for development of solar cell technologies. The FFC Cambridge Process is an Electrochemical method in which solid metal compounds particularly Oxides are cathodically reduced to the respective metals

Another approach is also to reduce the amount of silicon used and thus cost, is by micromachining wafers into very thin, virtually transparent layers that could be used as transparent architectural coverings. ^[33] . The technique involves taking a silicon wafer, typically 1 to 2 mm thick, and making a multitude of parallel, transverse slices across the wafer, creating a large number of slivers that have a thickness of 50 micrometres and a width equal to the thickness of the original wafer. These slices are rotated 90 degrees, so that the surfaces corresponding to the faces of the original wafer become the edges of the slivers. The result is to convert, for example, a 150 mm diameter, 2 mm-thick wafer having an exposed silicon surface area of about 175 cm² per side into about 1000 slivers having dimensions of 100 mm x 2 mm x 0. 1 mm, yielding a total exposed silicon surface area of about 2000 cm² per side. As a result of this rotation, the electrical doping and contacts that were on the face of the wafer are located the edges of the sliver, rather than the front and rear as is the case with conventional wafer cells. This has the interesting effect of making the cell sensitive from both the front and rear of the cell (a property known as bifaciality). ^[34] Using this technique, one silicon wafer is enough to build a 140 watt panel, compared to about 60 wafers needed for conventional modules of same power output.

Thin-film processing

Thin-film solar cells use less than 1% of the raw material (silicon or other light absorbers) compared to wafer based solar cells, leading to a significant price drop per kWh. There are many research groups around the world actively researching different thin-film approaches and/or materials, however it remains to be seen if these solutions can generate the same space-efficiency as traditional silicon processing.

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

Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind of materials, including flexible substrates (PET for example), which opens a new dimension for new applications. Uses PET can be semi-rigid to rigid depending on its thickness and is very lightweight

Polymer processing

The invention of conductive polymers (for which Alan Heeger, Alan G. MacDiarmid and Hideki Shirakawa were awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics. Semiconducting and metallic “organic” polymers are based on Sp2 Hybridized linear carbon chains Alan Jay Heeger (born January 22, 1936) is an American Physicist, Academic and Nobel Prize laureate in chemistry Alan Graham MacDiarmid ONZ ( April 14 1927 - February 7 2007) was a chemist and one of three recipients of the Nobel Prize Professor Hideki Shirakawa 白川 英樹 Shirakawa Hideki, born in Tokyo on August 20, 1936) is a Japanese Chemist and winner of The Nobel Prize (Nobelpriset (Nobelprisen is a Swedish prize established in the 1895 will of Swedish chemist Alfred Nobel; it was first awarded in Peace, Literature However, all organic solar cells made to date suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable. Polymer solar cells are a type of Organic solar cell: they produce Electricity from Sunlight. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays The conjugated double bond systems in the polymers, which carry the charge, are always susceptible to breaking up when radiated with shorter wavelengths. A chemically conjugated system is a system of atoms Covalently bonded with alternating single and multiple (e Additionally, most conductive polymers, being highly unsaturated and reactive, are highly sensitive to atmospheric moisture and oxidation, making commercial applications difficult.

Nanoparticle processing

Experimental non-silicon solar panels can be made of quantum heterostructures, eg. Quantum heterostructure is a Heterostructure in a substrate (usually a Semiconductor material) with size restricting the movements of the Charge carriers carbon nanotubes or quantum dots, embedded in conductive polymers or mesoporous metal oxides. See also Graphene, Buckypaper Carbon nanotubes (CNTs are Allotropes of carbon with a nanostructure that can have a length-to-diameter A quantum dot is a Semiconductor whose Excitons are confined in all three Spatial dimensions. Semiconducting and metallic “organic” polymers are based on Sp2 Hybridized linear carbon chains In addition, thin films of many of these materials on conventional silicon solar cells can increase the optical coupling efficiency into the silicon cell, thus boosting the overall efficiency. By varying the size of the quantum dots, the cells can be tuned to absorb different wavelengths. Although the research is still in its infancy, quantum dot-modified photovoltaics may be able to achieve up to 42 percent energy conversion efficiency due to multiple exciton generation(MEG). A quantum dot is a Semiconductor whose Excitons are confined in all three Spatial dimensions. ^[35]

Researchers at the University of California, San Diego have come up with a way of making solar photovoltaic cells more efficient by making them fuzzy with indium phosphide nanowires. The University of California San Diego (popularly known as UC San Diego or UCSD) is a public Research university in San Diego, California Indium phosphide ( is a binary Semiconductor composed of Indium and Phosphorus. A nanowire is a wire of diameter of the order of a nanometer (10−9 meters It sounds similar to a project announced last week by a consortium of German universities, working on concert with Harvard’s Science department^[36]

Transparent conductors

Many new solar cells use transparent thin films that are also conductors of electrical charge. The dominant conductive thin films used in research now are transparent conductive oxides (abbreviated "TCO"), and include fluorine-doped tin oxide (SnO₂:F, or "FTO"), doped zinc oxide (e. Zinc oxide is a Chemical compound with the formula ZnO It is nearly insoluble in water but soluble in Acids and Bases It occurs g. : ZnO:Al), and indium tin oxide (abbreviated "ITO"). Uses ITO is mainly used to make transparent conductive coatings for Liquid crystal displays Flat panel displays Plasma displays touch panels These conductive films are also used in the LCD industry for flat panel displays. The dual function of a TCO allows light to pass through a substrate window to the active light absorbing material beneath, and also serves as an ohmic contact to transport photogenerated charge carriers away from that light absorbing material. The present TCO materials are effective for research, but perhaps are not yet optimized for large-scale photovoltaic production. They require very special deposition conditions at high vacuum, they can sometimes suffer from poor mechanical strength, and most have poor transmittance in the infrared portion of the spectrum (e. g. : ITO thin films can also be used as infrared filters in airplane windows). These factors make large-scale manufacturing more costly.

A relatively new area has emerged using carbon nanotube networks as a transparent conductor for organic solar cells. See also Graphene, Buckypaper Carbon nanotubes (CNTs are Allotropes of carbon with a nanostructure that can have a length-to-diameter Polymer solar cells are a type of Organic solar cell: they produce Electricity from Sunlight. Nanotube networks are flexible and can be deposited on surfaces a variety of ways. With some treatment, nanotube films can be highly transparent in the infrared, possibly enabling efficient low bandgap solar cells. Nanotube networks are p-type conductors, whereas traditional transparent conductors are exclusively n-type. The availability of a p-type transparent conductor could lead to new cell designs that simplify manufacturing and improve efficiency. A P-type semiconductor (P for Positive) is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order

Silicon wafer based solar cells

Despite the numerous attempts at making better solar cells by using new and exotic materials, the reality is that the photovoltaics market is still dominated by silicon wafer-based solar cells (first-generation solar cells). This means that most solar cell manufacturers are equipped to produce these type of solar cells. Therefore, a large body of research is currently being done all over the world to create silicon wafer-based solar cells that can achieve higher conversion efficiency without an exorbitant increase in production cost. The aim of the research is to achieve the lowest cost per watt solar cell design that is suitable for commercial production.

Manufacturers

Solar cells are manufactured primarily in Japan, Germany, USA, and China, though numerous other nations have or are acquiring significant solar cell production capacity. While technologies are constantly evolving toward higher efficiencies, 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. 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.

See also

• Autonomous building
• Energy development
• Fluorescent solar collector
• Fuzzy
• Green technology
• Helianthos
• Microgeneration
• Maximum Power Point Tracking
• Nanowire
• Photodiode
• Anomalous photovoltaic effect
• Photovore - (robot -- light-seeking, often light-powered)
• Renewable energy
• Solar Engine
• Solar panel
• Solar roof
• Solar shingles
• Solar tracker
• Timeline of solar energy

References

• McDonald SA, Konstantatos G, Zhang S, Cyr PW, Klem EJ, Levina L, Sargent EH (2005). "Solution-processed PbS quantum dot infrared photodetectors and photovoltaics". Nature Materials 4 (2): 138-42. doi:10.1038/nmat1299. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 15640806.  
• PVNET European Roadmap for PV R&D Ed Arnulf Jager-Waldan Office for Publications of the European Union 2004

External links

• Howstuffworks.com: How Solar Cells Work
• Solar cell manufacturing techniques
• Renewable Energy: Solar at the Open Directory Project
• Solar Energy Laboratory at University of Southampton
• NASA's Photovoltaic Info

The Open Directory Project ( ODP) also known as dmoz (from directory The University of Southampton is a university situated in the city of Southampton, on the south coast of England.

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