[size=19pt]  [center]Solar cell   [/center] [/size]

A solar cell or photovoltaic cell is a device that converts sunlight directly into electricity  by the photovoltaic effect . 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 light source is unspecified.

Assemblies of cells are used to make solar panels

solar modules or photovoltaic arrays ,. Photovoltaics  is the field of technology and research related to the application of solar cells in producing electricity for practical use. The energy generated this way is an example of solar energy



A solar cell made from a monocrystalline

Timeline of Solar Energy

The term "photovoltaic" comes from the Greek  (ph?s) meaning "light", and "voltaic", meaning electric, from the name of the Italian physicist Volta after whom a unit of 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. 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.

Sven Ason Berglund had a number of patents concerning methods of increasing the capacity of these cells. Russell Ohl  patented the modern junction semiconductor solar cell in 1946[2], which was discovered while working on the series of advances that would lead to the transistor

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[citation needed ].Daryl Chapin, with Bell Labs colleagues Calvin Fuller and Gerald Pearson, invented the first practical device for converting sunlight into useful electrical power.

This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6 percent.The solar battery was first demonstrated on April 25, 1954.

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.

Recent world record claims of efficiency for multiple junction solar cells are discussed in the Records section.

Three generations of solar cells

Solar Cells are classified into three generations which indicates the order of which each became important. 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

Crystalline silicon  and Vacuum deposition 

First generation cells consist of large-area, high quality and single  devices. First Generation technologies involve high energy and labor 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

Thin-film cell

Second generation materials have been developed to address energy requirements and production costs of solar cells. Alternative manufacturing techniques such as vapour deposition , electroplating , and use of Ultrasonic Nozzles  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

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 CIGS  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.Honda , also began to commercialize their CIGS base solar panel in 2008.

In 2007, CdTe production represented 4.7% of total market share, thin-film silicon  5.2% and CIGS 0.5%.

Third GenerationThird 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.They can exceed the theoretical solar conversion efficiency limit for a single energy threshold material, that was calculated in 1961 by Shockley and Queisser as 31% under 1 sun  illumination and 40.8% under the maximal artificial concentration of sunlight (46,200 suns, which makes the latter limit more difficult to approach than the former

There are a few approaches to achieving these high efficiencies including the use of Multijunction photovoltaic cells , concentration of the incident spectrum, the use of thermal generation by UV light  to enhance voltage or carrier collection, or the use of the infrared spectrum for night-time operation

High efficiency cells

High efficiency solar cells are a class of solar cell that can generate electricity  at higher efficiencies than conventional solar cells. While high efficiency solar cells are more efficient in terms of electrical output per incident energy (watt/watt), much of the industry is focused on the most cost efficient  technologies, i.e.cost-per-watt .

Many businesses and academics are focused on increasing the electrical efficiency of cells, and much development is focused on high efficiency solar cells

Records

Multiple junction solar cells
The record for multiple junction solar cells is disputed. Teams led by the University of , the Fraunhofer Institute ,and NREL  all claim the world record title at 42.8, 41.1, and 40.8 percent, respectively.

NREL claims that the other implementations have not been put under standardized tests and, in the case of the University of Delaware project, represents only hypothetical efficiencies of a panel that has not been fully assembled. NREL claims it is one of only three laboratories in the world capable of conducting valid tests, although the Fraunhofer Institute is among those three facilities.


Thin film solar cells

In 2002, the highest reported efficiency for solar cells based on thin films of CdTe is 18%, which was achieved by research at Sheffield Hallam University  although this has not been confirmed by an external test laboratory[citation needed

The US national renewable energy research facility NREL achieved an efficiency of 19.9% for the solar cells based on copper indium gallium selenide  thin films, also known as CIGS.

These CIGS films have been grown by physical vapour deposition in a three-stage co-evaporation process. In this process In, Ga and Se are evaporated in the first step; in the second step it is followed by Cu and Se co-evaporation and in the last step terminated by In, Ga and Se evaporation again.

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 (24 hours x 1kW x 20% = 4.8 kWh)

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.

Solar cells can also be applied to other electronics devices to make it self power sustainable in the sun. There are solar cell phone chargers, solar bike light and solar camping lanterns that people can adopt for daily use.

Simple explanation

Photons  in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon
Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity .

Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. 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 array of solar cells 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:

The photon can pass straight through the silicon - this (generally) happens for lower energy photons,

The photon can reflect off the surface,

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 energ


Charge carrier separation

There are two main modes for charge carrier separation in a solar cell:
drift of carriers, driven by an electrostatic field established across the device
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 solar cell  research such as dye  and polymer 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 charge flow, known as drift current , that opposes and eventually balances out the diffusion of electron and holes.

This region where electrons and holes 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".


[size=16pt]Chosing Your Home Solar Electricity System[/size]

What You Should Know Before Purchasing a Solar Power System for your Home
Getting home solar electricity is much more affordable today than it was ten years ago.

In fact, many state governments are currently offering tax rebates to persons who choose to convert their systems to home solar electricity.

But, if installing a complete photovoltaic system to supply your home electricity is beyond your financial capabilities, there are also programs available which enable you to purchase your utilities from an accredited green energy supplier.
Green energy suppliers offer utilities which are fueled only by renewable resources, such as solar electricity, hydroelectricity, and wind-generated electricity.

If, however, you have decided to switch to utilizing a solar electricity system in your home, it's important to determine your electrical needs before you start shopping or planning.

How much power you need - The first thing to determine is how much power you will actually need for your home. Different arrays and solar panel set-ups offer different wattage outputs. Its important to take a the time to to determine what your average daily electricity usage will be.

You are also looking for a number that takes the entire year into account, and affords for the different amount of energy you will use during summer and winter months.

Most of us use more lights, heaters, air conditioners, etc., during different times of the year. So you will usually find that you use more electricity during some months, than you do during others.

The quality of the product - When shopping for the right photovoltaic system to supply your home solar electricity, make sure to research the quality of the product. A product which offers 13% efficiency, with a lifetime warranty, will be far more valuable than a panel offering 15% efficiency with a one-year warranty.

If you know of neighbors, friends, or even strangers who are currently using home solar electricity, ask them about it. Try to learn more about the differences amongst different types of systems.

You should also take some time to interview installers, and research different manufacturers as well.

Decide on your system – You will eventually need to make a final decision as to what type of system you will use, in order to supply your home with electricity. Generally three types of photovoltaic systems (which use solar panels) used to supply home solar power:

The first type is a grid-tied system. In this system, your solar panels are tied in with your existing power meter. You use both the electricity supplied by your panels, and, if and when needed, electricity from your electric company.
The second type of home solar power system is a grid-tie with battery backup. With this system you collect electricity from your solar panels, fill your batteries for emergencies, and send the excess back to the electric company through your power meter.

A stand-alone system – This is very similar to the grid-tied system with battery backup, except that you are not connected at all to any major power utility. This is the type of system which would be employed when living off-grid.
Once you have educated yourself on the different home solar power options that exist, you will probably be ready to start shopping.

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Remember, converting to home solar power is an investment. Financially, you will reap the rewards over time, 

You will also reduce the harmful impact of polluting fossil fuels on today's environment, which may be the most important benefit of all

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