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solar cell

solar cell, semiconductor devised to convert light to electric current. It is a specially constructed diode, usually made of silicon crystal. When light strikes the exposed active surface, it knocks electrons loose from their sites in the crystal. Some of the electrons have sufficient energy to cross the diode junction and, having done so, cannot return to positions on the other side of the junction without passing through an external circuit. Since the current obtained from these devices is small and the voltage is low, they must be connected in large series-parallel arrays (solar panels) if useful amounts of energy are to be converted. Practical devices of this kind are about 10% to 15% efficient and for many years were most commonly used to provide electric power for spacecraft. For large-scale power conversion solar cells offer a number of practical problems; one of the most serious of these is the wide variation of output voltage and current accompanying changes in the amount of incident light; this can be compensated for on smaller scales by storing energy produced during peak periods in batteries. Reductions in the cost of producing solar panels have made them a viable alternative to electrical generators for homes and villages in remote areas.
A solar cell or photovoltaic cell is a device that converts solar energy into electricity by the photovoltaic effect. Photovoltaics is the field of technology and research related to the application of solar cells as solar energy. 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.

Solar cells have many applications. Individual cells are used for powering small devices such as electronic calculators. 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. Photovoltaic electricity is also increasingly deployed in grid-tied electrical systems. Similar devices intended to capture energy radiated from other sources include thermophotovoltaic cells, betavoltaics cells, and optoelectric nuclear batteries.

History

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 unit of electrical potential, the volt, is named. The term "photo-voltaic" has been in use in English since 1849.

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 the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. Russell Ohl patented the modern solar cell in 1946 ("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.

This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6 percent. The first spacecraft to use solar panels was the US satellite Vanguard 1, launched in March 1958 with solar cells made by Hoffman Electronics. This milestone created interest in producing and launching a geostationary communications satellite, in which solar energy would provide a viable power supply. 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. 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 "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%. 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.

In 2007, two companies in the United States, Emcore Photovoltaics and Spectrolab, produce 95% of the world's Triple Junction solar cells which have a commercial efficiency of 38%[citation needed]. In 2006 Spectrolab's cells achieved 40.7% efficiency in lab testing.

Scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have set a world record in solar cell efficiency with a photovoltaic device that converts 40.8 percent of the light that hits it into electricity. This is the highest confirmed efficiency of any photovoltaic device to date .

Three generations of solar cells

Solar Cells are classified into three generations which indicates the order of which each became prominent. At present there is concurrent research into all three generations while the first generation technologies are most highly represented in commercial production, accounting for 89.6% of 2007 production.

First Generation

First generation cells consist of large-area, high quality and single junction devices. First Generation technologies involve high energy and labour inputs which prevent any significant progress in reducing production costs. Single junction silicon devices are approaching the theoretical limiting efficiency of 33% and achieve cost parity with fossil fuel energy generation after a payback period of 5-7 years.

Second Generation

Second generation materials have been developed to address energy requirements and production costs of solar cells. Alternative manufacturing techniques such as vapour deposition and electroplating are advantageous as they reduce high temperature processing significantly. It is commonly accepted that as manufacturing techniques evolve production costs will be dominated by constituent material requirements, whether this be a silicon substrate, or glass cover. Second generation technologies are expected to gain market share in 2008.

The most successful second generation materials have been cadmium telluride (CdTe), copper indium gallium selenide, amorphous silicon and micromorphous silicon. These materials are applied in a thin film to a supporting substrate such as glass or ceramics reducing material mass and therefore costs. These technologies do hold promise of higher conversion efficiencies, particularly CIGS-CIS, DSC and CdTe offers significantly cheaper production costs. Among major manufacturers there is certainly a trend toward second generation technologies however commercialisation of these technologies has proven difficult. In 2007 First Solar produced 200 MW of CdTe solar cells making it the fifth largest producer of solar cells in 2007 and the first ever to reach the top 10 from production of second generation technologies alone.. Wurth Solar commercialised its CIS technology in 2007 producing 15 MW. Nanosolar commercialised its CIGS technology in 2007 with a production capacity of 430 MW for 2008 in the USA and Germany. In 2007 CdTe production represented 4.7% of total market share, thin film silicon 5.2% and CIGS 0.5%.

Third Generation

Third generation technologies aim to enhance poor electrical performance of second generation thin film technologies while maintaining very low production costs. Current research is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques. There are a few approaches to achieving these high efficiencies:

Applications and implementations

Solar cells are often electrically connected and encapsulated as a module. 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.). 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.

The power output of a solar array is measured in watts or kilowatts. 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 4.8 kWh 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.
2. Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. The special composition of solar cells only allow the electrons to move in a single direction. 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.
3. An array of solar panels converts solar energy into a usable amount of direct current (DC) electricity.

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. 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. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. 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. 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.

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

The most commonly known solar cell is configured as a large-area p-n junction made from silicon. 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). 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. The electric field established across the p-n junction creates a diode that promotes current to flow in only one direction across the junction. Electrons may pass from the n-type side into the p-type side, and holes may pass from the p-type side to the n-type side, but not the other way around. This region where electrons have diffused across the junction is called the depletion region because it no longer contains any mobile charge carriers. It is also known as the "space charge region".

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. 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 are 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. 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. 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
 $I = I_L - I_o left\left(e^\left\{q left\left(V + I times R_s right\right) / nkT\right\} - 1 right\right)$ $I_L = I_\left\{L left\left(T_1 right\right)\right\} left\left(1 + K_o left\left(T - T_1 right\right) right\right)$ $I_\left\{L left\left(T_1 right\right)\right\} = G times I_\left\{sc left\left(T_1 right\right) \right\}$ $K_o = left\left(I_\left\{ sc left\left(T_2 right\right) \right\} - I_\left\{sc left\left(T_1 right\right) \right\} right\right) / left\left(T_2 - T_1 right\right)$ $I_o = I_\left\{ o left\left(T_1 right\right) \right\} times \left\{left\left(T / T_1 right\right)\right\}^\left\{3/n\right\} * e^\left\{ -q * V_g / nk * left\left(1/T - 1/T_1 right\right)\right\}$ $I_\left\{ o left\left(T_1 right\right) \right\} = I_\left\{ sc left\left(T_1 right\right) \right\}/ left\left(e^\left\{ q*V_\left\{ oc left\left(T_1 right\right) \right\} / nkT_1 \right\} - 1 right\right)$ $R_s = - \left\{ left\left(tfrac\left\{dV\right\}\left\{dI\right\} right\right) \right\}_\left\{ V_\left\{oc\right\} \right\} - 1 / X_v$ $X_v = I_\left\{ o left\left(T_1 right\right) \right\} times q / nkT_1 times$e^{ q*V_{ oc left(T_1 right) } / nkT_1 } k is Boltzmann's constant q is charge on an electron Vg is band gap voltage n is the ideality factor, or diode quality factor Rs is series resistance of cell Rsh is shunt resistance T1 and T2 are reference temperature in Kelvin T is working temperature of cell in Kelvin Voc is open circuit voltage IL is current produced by the cell, also known as Iph Isc is short circuit current Io is reverse saturation current of diode

Solar cell efficiency factors

Energy conversion efficiency

A solar cell's energy conversion efficiency ($eta$, "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, Pm, divided by the input light irradiance (E, in W/m²) under standard test conditions (STC) and the surface area of the solar cell (Ac in m²).

$eta = frac\left\{P_\left\{m\right\}\right\}\left\{E times A_c\right\}$

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. 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, VOC 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, 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.

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.

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.

Quantum efficiency refers to the percentage of photons that are converted to electric current (i.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. 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.

Maximum-power point

A solar cell may operate over a wide range of voltages (V) and currents (I). 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. (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. 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.

Fill factor

Another defining term in the overall behavior of a solar cell is the fill factor (FF). This is the ratio of the maximum power point divided by the open circuit voltage (Voc) and the short circuit current (Isc):

$FF = frac\left\{P_\left\{m\right\}\right\}\left\{V_\left\{oc\right\} times I_\left\{sc\right\}\right\} = frac\left\{eta times A_c times E\right\}\left\{V_\left\{oc\right\} times I_\left\{sc\right\}\right\}$

Comparison of energy conversion efficiencies

At this point, discussion of the different ways to calculate efficiency for space cells and terrestrial cells is necessary to alleviate confusion. 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 (AM0) 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. Solar cell energy conversion efficiencies for commercially available multicrystalline Si solar cells are around 14-19%. 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. It is the output power under these conditions known as STC:

1. insolation (solar irradiance) 1000 W/m²
2. solar reference spectrum AM (airmass) 1.5
3. cell temperature 25°C

Solar cells and energy payback

Energy payback is the recovery (period) of the energy spent for manufacturing of the respective technical energy systems, also called harvesting ratio (ISO 13602).

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. 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 depending on the type and where it is used (see net energy gain). 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.

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. 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.

There are many new alternatives to Silicon photocells. Proprietary nano-particle silicon printing processes promises many of the photovoltaic features that conventional silicon can never achieve. It can be printed reel to reel on stainless steel or other high temperature substrates . However, most of the work on the next generation of photovoltaics is directed at printing onto low cost flexible polymer film and ultimately on common packaging materials. The main contenders are currently CIGS, CdTe, DSSC and organic photovoltaics .

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

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.

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.
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

The various thin-film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell. 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.

A cadmium telluride solar cell is a solar cell based on cadmium telluride, an efficient light-absorbing material for thin-film cells. 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. 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.

Copper-Indium Selenide

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. 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 CuInxGa(1-x)Se2. 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. The best efficiency of a thin-film solar cell as of March 2008 was 19.9% with CIGS absorber layer. 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% 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. 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. 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". 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

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. 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, 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.

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.

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, and also by the Dutch solar cars Solutra (2005) and Twente One (2007).

The Dutch Radboud University Nijmegen set the record for thin film solar cell effiency 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 glas or plastic film.

Light-absorbing dyes (DSSC)

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). 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. Energy conversion efficiencies achieved to date using conductive polymers are low compared to inorganic materials, with the highest reported efficiency of 6.5% 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. 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 .

Silicon Thin Films

Silicon thin-films are mainly deposited by chemical vapor deposition (typically plasma-enhanced (PE-CVD)) from silane gas and hydrogen gas. 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).

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.

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 two materials 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

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). 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. 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. 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. This plant, at 154 MW, would be ten times larger than the largest current photovoltaic plant in the world.

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. 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. To make a solar cell from the wafer, a surface diffusion of n-type dopants is performed on the front side of the wafer. This forms a p-n junction a few hundred nanometers below the surface.

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. 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. 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. Tempered glass cannot be used with amorphous silicon cells because of the high temperatures during the deposition process.

Lifespan

Solar cells are expected to be capable of producing electricity for at least twenty years without a significant decrease in efficiency.

Costs

Slicing costs

University of Utah engineers devised a new way to slice thin wafers of the chemical element germanium for use in the most efficient type of solar power cells. The new method should lower the cost of such cells by reducing the waste and breakage of the brittle semiconductor .

Low Cost Solar Cell

Dye-sensitized solar cell is considered the low cost solar cell.

This cell is extremely promising because it is made of low-cost materials and does not need elaborate apparatus to manufacture, so it can be made in a DIY way allowing more players to produce it than any other type of solar cell. In bulk it should be significantly less expensive than older solid-state cell designs. It can be engineered into flexible sheets. 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.

Current research on materials and devices

There are currently many research groups active in the field of photovoltaics in universities and research institutions around the world. 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. 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. 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. 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). 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.

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. . 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). 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.

Metamorphic Multijunction Solar Cell

The National Renewable Energy Laboratory won a R&D Magazine's R&D 100 Awards for its Metamorphic Multijunction Solar Cell, an ultra-light and flexible cell that converts solar energy with record efficiency .

The ultra-light, highly efficient solar cell was developed at NREL and is being commercialized by Emcore Corp. of Albuquerque, N.M., in partnership with the Air Force Research Laboratories Space Vehicles Directorate at Kirtland Air Force Base in Albuquerque.

It represents a new class of solar cells with clear advantages in performance, engineering design, operation and cost. For decades, conventional cells have featured wafers of semiconducting materials with similar crystalline structure. Their performance and cost effectiveness is constrained by growing the cells in an upright configuration. Meanwhile, the cells are rigid, heavy and thick with a bottom layer made of germanium.

In the new method, the cell is grown upside down. These layers use high-energy materials with extremely high quality crystals, especially in the upper layers of the cell where most of the power is produced. Not all of the layers follow the lattice pattern of even atomic spacing. Instead, the cell includes a full range of atomic spacing, which allows for greater absorption and use of sunlight. The thick, rigid germanium layer is removed, reducing the cell's cost and 94% of its weight. By turning the conventional approach to cells on its head, the result is an ultra-light and flexible cell that also converts solar energy with record efficiency (40.8 percent under 326 suns concentration).

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. 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. The conjugated double bond systems in the polymers, which carry the charge, are always susceptible to breaking up when radiated with shorter wavelengths. 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. carbon nanotubes or quantum dots, embedded in conductive polymers or mesoporous metal oxides. 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).

Researchers located 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. It sounds similar to a project announced by a consortium of German universities, working in concert with Harvard University Science department.

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.g.: ZnO:Al), and indium tin oxide (abbreviated "ITO"). 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. 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.

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.

IBM has a semiconductor wafer reclamation process that uses a specialized pattern removal technique to repurpose scrap semiconductor wafers to a form used to manufacture silicon-based solar panels. The new process was recently awarded the “2007 Most Valuable Pollution Prevention Award” from The National Pollution Prevention Roundtable (NPPR).

Infrared solar cells

Researchers at Idaho National Laboratory, along with partners at Microcontinuum Inc. (Cambridge, MA) and Patrick Pinhero of the University of Missouri, have devised an inexpensive way to produce plastic sheets containing billions of nanoantennas that collect heat energy generated by the sun and other sources, which garnered two 2007 Nano50 awards. The technology is the first step toward a solar energy collector that could be mass-produced on flexible materials. While methods to convert the energy into usable electricity still need to be developed, the sheets could one day be manufactured as lightweight "skins" that power everything from hybrid cars to computers and iPods with higher efficiency than traditional solar cells. The nanoantennas also have the potential to act as cooling devices that draw waste heat from buildings or electronics without using electricity. The nanoantennas target mid-infrared rays, which the Earth continuously radiates as heat after absorbing energy from the sun during the day; also double-sided nanoantenna sheets can harvest energy from different parts of the sun's spectrum. In contrast, traditional solar cells can only use visible light, rendering them idle after dark .

Also Konarka is researching infrared light activated photovoltaics which would enable night-time power generation.

Manufacturers

Solar cells are manufactured primarily in Japan, China, Germany, Taiwan and the USA , 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.