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Solar cell

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(Redirected from Photovoltaic)

A solar cell, or photovoltaic cell, is a semiconductor device consisting of a large-area p-n junction diode, which, in the presence of sunlight is capable of generating usable electrical energy. This conversion is called the photovoltaic effect. The field of research related to solar cells is known as photovoltaics.

Solar cells have many applications. They are particularly well suited to, and historically used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth orbiting satellites, handheld calculators, remote radiotelephones, water pumping applications, etc. Solar cells (in the form of modules or solar panels) are appearing on building roofs where they are connected through an inverter to the electricity grid in a net metering arrangement.

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A solar cell, made from a poly-crystalline silicon wafer
Contents

Introduction

Etymology

The etymology of the term "photovoltaic" comes from the Greek photos meaning light and the name of the Italian physicist Volta, after whom the volt (and consequently voltage) are named. It means literally of light and electricity.

History

Main article: Timeline of solar cells

Russell Ohl is generally recognized for patenting the modern solar cell in 1946 (US2402662 (http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=2402662.WKU.&OS=PN/2402662&RS=PN/2402662), "Light sensitive device"). Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells.

Materials and efficiency

Various materials have been investigated for solar cells. There are two main criteria - efficiency and cost. Efficiency is a ratio of the electric power output to the light power input. Ideally, near the equator at noon on a clear day, the solar radiation is approximately 1000 W/m². So a 10% efficient module of 1 square meter can power a 100 W light bulb. Costs and efficiencies of the materials vary greatly.

By far the most common material for solar cells (and all other semiconductor devices) is crystalline silicon. Crystalline silicon solar cells come in three primary categories:

  • Single crystal or monocrystalline wafers made using the Czochralski process. See Sunpower (http://www.sunpowercorp.com/html/) and Shell Solar (http://www.shell.com/). Most commercial monocrystalline cells have efficiencies on the order of 14%; the SunPower cells have high efficiencies around 20%. Single crystal cells tend to be expensive, and because they are cut from cylindrical ingots, they cannot completely cover a module without a substantial waste of refined silicon. Most monocrystalline panels have uncovered gaps at the corners of four cells.
  • Poly or multi crystalline made from cast ingots - large crucibles of molten silicon carefully cooled and solidified. See GT Solar HEM Furnace (http://www.gtsolar.com/products/hem.php), BP Solar (http://www.bp.com/modularhome.do?categoryId=4320&contentId=7004540), Sharp Solar (http://solar.sharpusa.com/solar/home/0,2462,,00.html) and Kyocera Solar (http://www.kyocerasolar.com). These cells are cheaper than single crystal cells, but also somewhat less efficient. However, they can easily be formed into square shapes that cover a greater fraction of a panel than monocrystalline cells, and this compensates for their lower efficiencies.
  • Ribbon silicon formed by drawing flat thin films from molten silicon and has a multicrystalline structure. See Evergreen Solar (http://www.evergreensolar.com/), and RWE Schott Solar (http://www.rweschottsolar.com/). These cells are typically the least efficient, but there is a cost savings since there is very little silicon waste since this approach does not require sawing from ingots.

These technologies are wafer based manufacturing. In other words, in each of the above approaches, self supporting wafers of ~300 micrometres thick are fabricated and then soldered together to form a module.

Thin film approaches are module based. The entire module substrate is coated with the desired layers and a laser scribe is then used to delineate individual cells. Two main thin film approaches are amorphous silicon and CIS:

  • Amorphous silicon films are fabricated using chemical vapor deposition techniques, typically plasma enhanced (PE-CVD). These cells have low efficiencies around 8%.
  • CIS stands for general chalcogenide films of Cu(InxGa1-x)(SexS1-x)2. While these films can achieve 11% efficiency, their costs are still too high.

There are additional materials and approaches. For example, Sanyo has pioneered the HIT cell. In this technology, amorphous silicon films are deposited onto crystalline silicon wafers.

The chart below illustrates the various commercial large area module efficiencies and the best laboratory efficiencies obtained for various materials and technologies.

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Interconnection and modules

Usually, solar cells are electrically connected, and combined into "modules", or solar panels. Solar panels, have a sheet of glass on the front, and a resin encapsulation behind to keep the semiconductor wafers safe from the elements (rain, hail, etc). Solar cells are usually connected in series in modules, so that their voltages add.

Theory

Background

In order to understand how a solar cell works, a little background theory in semiconductor physics is required. For simplicity, the description here will be limited to describing the workings of single crystalline silicon solar cells.

Silicon is a group 14 (formerly, group IV) atom. This means that each Si atom has 4 valence electrons in its outer shell. Silicon atoms can covalently bond to other silicon atoms to form a solid. There are two basic types of solid silicon, amorphous (having no long range order) and crystalline (where the atoms are arranged in an ordered three dimensional array). There are various other terms for the crystalline structure of silicon; poly-crystalline, micro-crystalline, nano-crystalline etc, and these refer to the size of the crystal "grains" which make up the solid. Solar cells can be, and are made from each of these types of silicon, the most common being poly-crystalline.

Silicon is a semiconductor. This means that in solid silicon, there are certain bands of energies which the electrons are allowed to have, and other energies between these bands which are forbidden. These forbidden energies are called the "band gap". The allowed and forbidden bands of energy are explained by the theory of quantum mechanics.

At room temperature, pure silicon is a poor electrical conductor. In quantum mechanics, this is explained by the fact that the Fermi level lies in the forbidden band-gap. To make silicon a better conductor, it is "doped" with very small amounts of atoms from either group 13 (III) or group 15 (V) of the periodic table. These "dopant" atoms take the place of the silicon atoms in the crystal lattice, and bond with their neighbouring Si atoms in almost the same way as other Si atoms do. However, because group 13 atoms have only 3 valence electrons, and group 15 atoms have 5 valence electrons, there is either one too few, or one too many electrons to satisfy the four covalent bonds around each atom. Since these extra electrons, or lack of electrons (known as "holes") are not involved in the covalent bonds of the crystal lattice, they are free to move around within the solid. Silicon which is doped with group 13 atoms (aluminium, gallium) is known as p-type silicon because the majority charge carriers (holes) carry a positive charge, whilst silicon doped with group 15 atoms (phosphorus, arsenic) is known as n-type silicon because the majority charge carriers (electrons) are negative. It should be noted that both n-type and p-type silcion are electrically neutral, i.e. they have the same numbers of positive and negative charges, it is just that in n-type silicon, some of the negative charges are free to move around, while the converse is true for p-type silicon.

Light generation of carriers

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The absorption of photons creates electron-hole pairs, which diffuse to the electrical contacts and can be extracted to power electrical devices

When a photon of light hits a piece of silicon, one of two things can happen. The first is that the photon can pass straight through the silicon. This (generally) happens when the energy of the photon is lower than the bandgap energy of the silicon semiconductor. The second thing that can happen is that the photon is absorbed by the silicon. This (generally) happens if the photon energy is greater than the bandgap energy of silicon. 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 neighbouring 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 less 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 only needs to have energy greater than the band gap energy 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.

The p-n junction

A solar cell is a large-area semiconductor p-n junction. To understand the workings of a p-n junction it is convenient to imagine what happens when a piece of n-type silicon is brought into contact with a piece of p-type silicon. In practice, however, the p-n junctions of solar cells are not made in this way, but rather, usually, by diffusing an n-type dopant into one side of a p-type wafer.

If we imagine what happens when a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then what occurs is a diffusion of electrons 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. This diffusion of carriers does not happen indefinitely however, because of the electric field which is created by the imbalance of charge immediately either side of the junction which this diffusion creates. Electrons from donor atoms on the n-type side of the junction are crossing into the p-type side, leaving behind the (extra) positively charged nuclei of the group 15 donor atoms, leaving an excess of positive charge on the n-type side of the junction. At the same time, these electrons are filling in holes on the p-type side of the junction, becoming involved in covalent bonds around the group 13 acceptor atoms, making an excess of negative charge on the p-type side of the junction. This imbalance of charge across the p-n junction sets up an electric field which opposes further diffusion of charge carriers across the junction.

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".

The electric field which is set up across the p-n junction creates a diode, allowing current to flow in only one direction across the junction. 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 since the sign of the charge on electrons and holes is opposite, conventional current may only flow in one direction.

Separation of carriers by the p-n junction

Once the electron-hole pair has been created by the absorption of a photon, the electron and hole are both free to move off independently within the silicon latttice. If they are created within a minority carrier diffusion length of the junction, then, depending on which side of the junction the electron-hole pair is created, the electric field at the junction will either sweep the electron to the n-type side, or the hole to the p-type side.

Connection to an external load

If 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, then electrons which 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 where they recombine with a hole which 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

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The equivalent circuit of a solar cell
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The schematic symbol of a solar cell

To understand the electronic behaviour of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behaviour 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 result is the "equivalent circuit of a solar cell" shown on the left. Also shown on the right, is the schematic representation of a solar cell for use in circuit diagrams.

Manufacture and devices

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 in to 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 (250 to 350 micrometre) slices or wafers. The wafers are usually lightly p-type doped.

To make a solar cell from the wafer, an n-type diffusion is performed on the front side of the wafer, forming a p-n junction a few hundred nanometres below the surface.

Antireflection coatings, which increase the amount of light coupled into the solar cell, are typically applied next. 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).

The wafer is then metallised, whereby a full area metal contact is made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "busbars" is screen-printed onto the front surface using a silver paste. The rear contact is also formed by screen-printing a metal paste, typically aluminum. Usually this contact covers the entire rear side of the cell, though in some cell designs it is printed in a grid pattern. The metal electrodes will then require some kind of heat treatment or "sintering" to make 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.

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.

Energy conversion efficiency

Typical module efficiencies for commercially available screen printed multicrystalline solar cells are around 12%. A solar module's energy conversion efficiency, (or just efficiency) is the ratio of the maximum output electrical power divided by the input light power under "standard" test conditions. The "standard" solar radiation (known as the "air mass 1.5 spectrum") has a power density of 1000 watts per square metre. Thus, a typical 1 m� solar panel in direct sunlight will produce approximately 120 watts of peak power.

Applications and implementations

See the article solar panel for information about applications and implementations of solar cells and panels.

Cost analysis

The US retail module costs are in the $3.50 to $5.00/Wp range (see SolarBuzz (http://www.solarbuzz.com/)). Additional installation costs for a residential rooftop retrofit in California (CA) is around $3.50/Wp or more. So on the low side, installed system costs are about $7.00/Wp in CA, and probably higher in places with less experience. Federal, state, utility, and other subsidies combined pay about half the cost. So CA rule of thumb is that the installed system PV will cost you at the low end, $3.50/Wp.

Under net metering, one offsets regular retail utility rate which for CA is about 11 cents/kWh. Knowing installed system costs, amount of sunshine, and the utility rates, one can figure out the years till payback with or without financing costs. Assuming no financing costs and a $6/Wp installed system cost (lower than current $7), one can take sunshine and utility rate information from around the globe and come up with a payback graph such as shown below. The addition of subsidies brings down the years to payback proportionately. For example, if the years to payback were 24 years at $6/Wp, and subsidies brought that down to $3/Wp, the years to payback would be 12.

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Current research

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

Much of the research is focussed on making solar cells cheaper and/or more efficient, so that they can more effectively compete with other energy sources, including fossil energy. One way of doing this is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica sand. Another approach is to significantly reduce the amount of raw material used in the manufacture of solar cells. The various thin-film technologies currently being developed make use of this approach to reducing the cost of electricity from solar cells.

The invention of conductive polymers, (for which Alan Heeger was awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics, rather than semiconductor grade silicon. 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.

Thin-film solar cells

The next step in reducing the cost of solar cells and panels seems certain to come from thin-film technology. Thin-film solar cells use less than 1% of the raw material (silicon) 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.

Thin Film solar cells are mainly deposited by PECVD from silane gas and hydrogen. This process produces a material without crystalline orientation : amorphous silicon. Depending on the deposition's parameters nanocrystalline silicon can also be obtained. These types of silicon present dandling and twisted bonds, which results in the aparition of deep defects (energy levels in the bandgap) as well as in the deformation of the valence and conduction bands (band tails). This contributes to reduce the efficiency of Thin-Film solar cells by reducing the number of collected electron-hole pair by incident photon.

Amorphous silicon (a-Si) has a higher bandgap (1.7 eV) than crystalline Silicon (c-Si) (1.1 eV), which means it is more efficient to absorb the visible part of the solar spectrum, but it fails to collect an important part of the spectrum : the infrared. As nano crystalline Si has about the same bandgap as c-Si, the two material can be combined by depositing to diodes on top of each other : the 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.

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. From the Pacific Solar (http://www.pacificsolar.com.au/) website:

"Crystalline Silicon on Glass (CSG) [is] the photovoltaic technology developed by Pacific Solar that is now being commercialised by CSG Solar. A very thin layer of silicon, less than two micrometres thick, is deposited directly onto a glass sheet whose surface has been roughened by applying a layer of tiny glass beads. The silicon is not crystalline when first deposited, but becomes so after heat treatment in an oven. The resulting layer is processed using lasers and ink-jet printing techniques to form the electrical contacts needed to get the solar-produced electricity out of the thin silicon film."

In 2005, a full-scale production factory is being built in Germany to commercialise this technology. CSG Solar expects to release its first product for sale in 2006. Each solar module will have a rated power exceeding 100 watts and will be cheaper than competing solar panels.

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.

Exotic materials

For special applications, such as Deep Space 1, high-efficiency cells can be made from gallium arsenide by molecular beam epitaxy. Such cells have many diodes in series, each with a different band gap energy so that it absorbs its share of the electromagnetic spectrum with very high efficiency. Triple junction solar cell have (as the name suggest) 3 diodes layered on top of each other, each absorbing a different spectrum of light, efficiency as high as 28% have been achieved. The multiple junction solar cells may be very efficient, but are prohibitively expensive to make. Cost-effective use of these cells could be achieved with concentrating optics so that less of the array consists of actual semiconductor devices.

Experimental non-silicon solar panels can be made of carbon nanotubes or quantum dots embedded in a special plastic. These have only one-tenth the efficiency of silicon panels but could be manufactured in ordinary factories, not clean rooms which should lower the cost.

Some of the most efficient solar cell materials are cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). Unlike the basic silicon solar cell, which can be modelled as a simple p-n junction (see under semiconductor), these cells are best described by a more complex heterojunction model. The best efficiency of a bare solar cell as of April 2003 was 16.5% [Dr IM Dharmadasa, Sheffield Hallam University, UK]. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light.

Polymer or organic solar cells are built from ultra thin layers (typically 100 nm) of organic semiconductors such as polyphenylene vinylene and fullerene. The p/n junction model is only a crude description of the functioning of such cells, as electron hopping and other processes also play a crucial role. They are potentially cheaper to manufacture than silicon or inorganic cells, but efficiencies achieved to date are low and cells are highly sensitive to air and moisture, making commercial applications difficult. In the reverse mode, the technology has however already successfully been commercialised in organic LEDs and organic displays, also called polymer displays.

Graetzel cells (sometimes called photoelectrochemical cells) have been around for two decades or so. A p/n junction is used here too in the form of a doped solid (normally titanium dioxide) in contact with a solid or liquid electrolyte (for example CuI). In contrast to the classical solar cell not the semiconductor but a dye placed at the p/n interface is used for absorption of radiation, mimicking the process of photosynthesis. As a result, this type of cell allows a more flexible use of materials. Like organic solar cells, Graetzel cells can be manufactured under "dirty" conditions. Commercial applications have failed to appear due to the fast degradation occurring in Graetzel cells.

Solar cells and energy payback

There is a common but mistaken notion that solar cells never produce more energy than it takes to make them. While the expected working lifetime is on the order of 40 years, the energy payback time of a solar panel is anywhere on the order of 2 to 30 years depending on the type and where it is used, see Net energy gain.

See also

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