How do Photovoltaics Work?by Gil Knier

back to the Science@NASA story "The Edge of Sunshine"

Photovoltaics is the direct conversion of light into electricity at the atomic level. Some materials exhibit a property known as the photoelectric effect that causes them to absorb photons of light and release electrons. When these free electrons are captured, an electric current results that can be used as electricity.

The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in 1839, who found that certain materials would produce small amounts of electric current when exposed to light. In 1905, Albert Einstein described the nature of light and the photoelectric effect on which photovoltaic technology is based, for which he later won a Nobel prize in physics. The first photovoltaic module was built by Bell Laboratories in 1954. It was billed as a solar battery and was mostly just a curiosity as it was too expensive to gain widespread use. In the 1960s, the space industry began to make the first serious use of the technology to provide power aboard spacecraft. Through the space programs, the technology advanced, its reliability was established, and the cost began to decline. During the energy crisis in the 1970s, photovoltaic technology gained recognition as a source of power for non-space applications.

The diagram above illustrates the operation of a basic photovoltaic cell, also called a solar cell. Solar cells are made of the same kinds of semiconductor materials, such as silicon, used in the microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light energy strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current -- that is, electricity. This electricity can then be used to power a load, such as a

light or a tool.

A number of solar cells electrically connected to each other and mounted in a support structure or frame is called a photovoltaic module. Modules are designed to supply electricity at a certain voltage, such as a common 12 volts system. The current produced is directly dependent on how much light strikes the module.

Multiple modules can be wired together to form an array. In general, the larger the area of a module or array, the more electricity that will be produced. Photovoltaic modules and arrays produce direct-current (dc) electricity. They can be connected in both series and parallel electrical arrangements to produce any required voltage and current combination.

Today's most common PV devices use a single junction, or interface, to create an electric field within a semiconductor such as a PV cell. In a single-junction PV cell, only photons whose energy is equal to or greater than the band gap of the cell material can free an electron for an electric circuit. In other words, the photovoltaic response of single-junction cells is limited to the portion of the sun's spectrum whose energy is above the band gap of the absorbing material, and lower-energy photons are not used.

One way to get around this limitation is to use two (or more) different cells, with more than one band gap and more than one junction, to generate a voltage. These are referred to as "multijunction" cells (also called "cascade" or "tandem" cells). Multijunction devices can

achieve a higher total conversion efficiency because they can convert more of the energy spectrum of light to electricity.

As shown below, a multijunction device is a stack of individual single-junction cells in descending order of band gap (Eg). The top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower-band-gap cells.

Much of today's research in multijunction cells focuses on gallium arsenide as one (or all) of the component cells. Such cells have reached efficiencies of around 35% under

concentrated sunlight. Other materials studied for multijunction devices have been amorphous silicon and copper indium diselenide.

As an example, the multijunction device below uses a top cell of gallium indium phosphide, "a tunnel junction," to aid the flow of electrons between the cells, and a bottom cell of gallium arsenide.

 

Where Photovoltaics are Used Today

As a versatile, scalable, and independent source of electricity, photovoltaic systems are used to power a broad variety of technologies, buildings, and systems that need energy. Here we look at some of the ways photovoltaics are used today.

Residential Buildings

Installations of solar panels on homes make up one of the broadest current markets for photovoltaics in the United States . Because they can be sized to fit any building and any electrical load, solar panels are a practical choice for powering homes.

These installations have grown significantly and there are many installation companies that specialize in the residential market. For more on current residential installations in Massachusetts , see our section on Solar Energy in Massachusetts . We also provide more detailed information on solar installations for homeowners and homebuilders in our catalog of How To Guides.

Commercial, Institutional and Industrial Buildings

Larger-scale photovoltaic installations are also being seen on downtown office buildings, institutions like museums and community centers, shopping centers, and even some industrial buildings. Photovoltaics offer particular benefits to large scale buildings. In high-density areas like downtown Boston where space is at a premium, rooftop installations of solar panels can be an attractive power option. Other buildings like shopping malls and some industrial facilities have large roof areas which can accommodate many panels to power their high electricity needs. A certain subset of commercial buildings, hospitals, and high-tech facilities can benefit in another way, as they often need reliable backup power as insurance against blackouts and other power failures. As an on-site technology, photovoltaics are a viable source for this emergency power.

Though there are fewer large-scale buildings than residential ones, photovoltaic installations on these buildings can have significant impact on reducing emissions from electricity production and can provide much benefit to their owners. For more on current commercial, industrial, and institutional installations in Massachusetts , see our section on Solar Energy in Massachusetts and our How To Guide for large-scale building owners and developers.

Public Buildings and Municipal Installations

Another area where photovoltaics are becoming widely used is in public applications, largely on individual town, city and state buildings though some installations are in development to serve municipalities as a whole. The benefits of municipal installations are similar to those for large-scale commercial, institutional and industrial buildings. Additional benefit is possible when a city or town has developed a climate change action plan to reduce emissions, or has set a goal of meeting a certain percentage of its electricity needs with renewable energy. In each of these situations, photovoltaics can play a significant role in reducing environmental impacts.

In Massachusetts , school buildings have been the pioneering examples of municipal installations, with 16 schools developing or completing photovoltaic installations to date. For more on current municipal installations in Massachusetts , see our section on Solar Energy in Massachusetts . We also provide information on municipal planning efforts and setting goals for photovoltaic installations in our How To Guide for municipalities.

Remote Location Uses

New markets for photovoltaics have emerged in recent years that could have significant impact on people's access to electricity. These markets exist in the many locations throughout the world which do not have access to the electric grid. In many of these locations, photovoltaics have stepped in as a lower-cost alternative to expanding electric grids, and a large percentage of United States photovoltaic exports are now shipped to companies working in this market.

 

Other remote location applications exist throughout the United States , based on the same principle that when the electric grid cannot be expanded to reach a critical site, photovoltaics are often the least costly source of power. These applications range from remote telecommunication stations to the less obvious application of powering road signs and call boxes on highways. This last application is already used on most highways throughout the country.

There have been many interesting developments in remote photovoltaic applications. Photon International, a photovoltaic trade magazine with past articles online, is a good place to start in learning about these uses.

Space Applications

The first market for photovoltaics was in the space industry, starting in the 1950s and 60s, where remote power that did not require fuel was essential for successful early missions. The use of photovoltaics in space is a unique version of remote power, and the space industry is still a significant user of photovoltaics. NASA offers more information on this longstanding use of photovoltaics at its website.

 

 

 

 

Consumer Products

At the other extreme from space stations, photovoltaics are already used in a variety of consumer products, most notably the standard desk calculators used in offices everywhere. Though barely noticeable, these calculators have a thin strip of photovoltaic cells at their top which can be activated by either sunlight or artificial light. These tiny photovoltaic systems provide constant, reliable power and displace the use of batteries which the calculator would otherwise need to work.

Other applications in consumer products have also been developed, though they are not yet as widely used. X site introduces some of the emerging uses of photovoltaics in consumer products.

How Photovoltaic Cells Work

Energy From Nuclear Reaction

Initially, the energy a photovoltaic cell uses comes from the sun. There, hydrogen nuclei fuse with each other to form helium nuclei and energy. It takes four hydrogen nuclei to form one helium nucleus.

Photons

Photons are the energy byproducts of the nuclear reaction in the sun. They are essentially "packets of energy."

Electrons Absorb Photons

When photons from the sun hit a photovoltaic cell, they may be absorbed by an electron. With this extra energy, the electron may become excited and break off its atom, and eventually begin an electric current.

Silicon Wafer

The silicon wafer is the basic starting material of photovoltaic cells. As other materials are added to both sides of the cell, the silicon remains neutral, and acts as a barrier layer. This is because of its four valence electrons.

Positive Layer

In a photovoltaic cell, boron is found on the bottom. This forms a positive layer. Often, silicon wafers are obtained with boron already infused into one

side.

Negative Layer

In a photovoltaic cell, phosphorus is found on the top, directly on top of the silicon. The phosphorus is a dopant, which forms a negative layer on the side of the cell facing the sun.

Electrons Move Into Phosphorus

The phosphorus layer, with a negative charge, can produce an electric current when the absorbed charge attempts to "spread out." Shown here is a picture of phosphorus dopant: a clear, odorous fluid.

Photoelectric Effect

The photoelectric effect explains the movement of electrons in the presence of photons. This was first hypothesized in 1839, but was not fully explained until 1921, when Albert Einstein won the Nobel Prize for it.

Professional Corporations

©2007 Bot Productions. All rights reserved.

Last Updated: December 30, 1999

What are Photovoltaics?

Definition of Photovoltaics Cells

Photovoltaic cells produce usable electric current, using energy in the form of photons. These cells are commonly known as solar cells or PV.

Photovoltaic Cells Produce Direct Current

These cells form an electric current in which the electrons go in a circular pattern, and do not alternate direction. This is simlar to battery power, as opposed to a power plant. This direct current can be converted to alternating current for home or industrial use.

Solar Arraying

Often, cells are not found alone but are grouped together in series, to form what is called a solar array. These are generally placed under glass or plastic for protection from the weather. When hooked in series, a lot more electricity is generated.

Sun Energy - Photovoltics and Photovoltaic Systems

From Mary Bellis,Your Guide to Inventors.FREE Newsletter. Sign Up Now!

Photovoltaic systems convert light energy into electricity.Solar panels are devices that convert light into electricity. They are called solar after the sun or "Sol" because the sun is the most powerful source of the light to use. They are sometimes called photovoltaics which means "light-electricity". Solar cells or PV cells rely on the photovoltaic effect to absorb the energy of the sun and cause current to flow between two oppositely charge layers.

PhotovoltaicsPhotovoltaic (or PV) systems convert light energy into electricity. The term "photo" is a stem from the Greek "phos," which means "light." "Volt" is named for Alessandro Volta (1745-1827), a pioneer in the study of electricity. Photovoltaics literally means light-electricity.

Most commonly known as "solar cells," PV systems are already an important part of our lives.

The simplest systems power many of the small calculators and wrist watches we use every day. More complicated systems provide electricity for pumping water, powering communications equipment, and even lighting our homes and running our appliances. In a surprising number of cases, PV power is the cheapest form of electricity for performing these tasks.

History of Photovoltaic CellsPhotovoltaic cells convert light energy into electricity at the atomic level. French physicist Edmond Becquerel first described the photovoltaic effect in 1839, but it remained a curiosity of science for the next three quarters of a century. At only nineteen, Becquerel found that certain materials would produce small amounts of electric current when exposed to light.

The effect was first studied in solids, such as selenium, by Heinrich Hertz in the 1870s. Soon afterward, selenium photovoltaic cells were converting light to electricity at one percent to two percent efficiency. As a result, selenium was quickly adopted in the emerging field of photography for use in light-measuring devices.

Major steps toward commercializing photovoltaic cells began in the 1940s and early 1950s, when the Czochralski process was developed for producing highly pure crystalline silicon. In 1954, scientists at Bell Laboratories depended on the Czochralski process to develop a crystalline silicon photovoltaic cell, with an efficiency of six percent.

Anatomy of a Solar CellBefore now, our silicon was all electrically neutral. Our extra electrons were balanced out by the extra protons in the phosphorous. Our missing electrons (holes) were balanced out by the missing protons in the boron. When the holes and electrons mix at the junction between N-type and P-type silicon, however, that neutrality is disrupted. Do all the free electrons fill all the free holes? No. If they did, then the whole arrangement wouldn't be very useful. Right at the junction, however, they do mix and form a barrier, making it harder and harder for electrons on the N side to cross to the P side. Eventually, equilibrium is reached, and we have an electric field separating the two sides.

The effect of the electric field in a PV cell

This electric field acts as a diode, allowing (and even pushing) electrons to flow from the P side to the N side, but not the other way around. It's like a hill -- electrons can easily go down the hill (to the N side), but can't climb it (to the P side).

So we've got an electric field acting as a diode in which electrons can only move in one direction.

When light, in the form of photons, hits our solar cell, its energy frees electron-hole pairs.

Each photon with enough energy will normally free exactly one electron, and result in a free hole as well. If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to their original side (the P side) to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage, we have power, which is the product of the two.

Operation of a PV cell

There are a few more steps left before we can really use our cell. Silicon happens to be a very shiny material, which means that it is very reflective. Photons that are reflected can't be used by the cell. For that reason, an antireflective coating is applied to the top of the cell to reduce reflection losses to less than 5 percent.

The final step is the glass cover plate that protects the cell from the elements. PV modules are made by connecting several cells (usually 36) in series and parallel to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with a glass cover and positive and negative terminals on the back.

Basic structure of a generic silicon PV cell

How much sunlight energy does our PV cell absorb? Unfortunately, the most that our simple cell could absorb is around 25 percent, and more likely is 15 percent or less. Why so little?

Besides Single-crystal Silicon...Single-crystal silicon isn't the only material used in PV cells. Polycrystalline silicon is also used in an attempt to cut manufacturing costs, although resulting cells aren't as efficient as single crystal silicon. Amorphous silicon, which has no crystalline structure, is also used, again in an attempt to reduce production costs. Other materials used include gallium arsenide, copper indium diselenide and cadmium telluride. Since different materials have different band gaps, they seem to be "tuned" to different wavelengths, or photons of different energies. One way efficiency has been improved is to use two or more layers of different materials with different band gaps. The higher band gap material is on the surface, absorbing high-energy photons while allowing lower-energy photons to be absorbed by the lower band gap material beneath. This technique can result in much higher efficiencies. Such cells, called multi-junction cells, can have more than one electric field.

A walk trough the time

"The essentials of a phenomenom are best understood if one tries to explore their rise from the very beginnings."

(Aristotle)

Vanguard I satellite before launching, Apollo solar array, solar module used in space applications in 1983

(source: NASA NIX)

Where are you: Home > History

The story of photovoltaics and how it all began in 1839 as a coincidence, just like many other discoveries in the past, such as penicillin, is a very interesting reading. The story will take you through some facts, persons and events, which have marked the history of photovoltaics.

"Being desirous of obtaining a more suitable high resistance for use at the Shore Station in connection with my system of testing and signalling during the submersion of long submarine cables, I was induced to experiment with bars of selenium - a known metal of very high resistance. I obtained several bars, varying in length from 5 cm to 10 cm,

and of a diameter from 1.0 mm to 1.5 mm. Each bar was hermetically sealed in a glass tube, and a platinum wire projected from each end for the purpose of connection..."

(Willoughby Smith: Letter to Latimer Clark, Wharf Road, 4th February 1873)

Did you know? If you will follow the link you will find some amazing historical facts about solar energy use.

Other topics: Historical facts

  1839 - 1899: Discovery of basic phenomena and properties of PV materials

Discovery of photovoltaic effect

A physical phenomenon allowing light-electricity conversion - photovoltaic effect, was discovered in 1839 by the French physicist Alexandre Edmond Becquerel. Experimenting with metal electrodes and electrolyte he discovered that conductance rises with illumination.

First solar cells

Willoughby Smith discovered photovoltaic effect in selenium in 1873. In 1876, with his student R. E. Day, William G. Adams discovered that illuminating a junction between selenium and platinum also has a photovoltaic effect. These two discoveries were a foundation for the first selenium solar cell construction, which was built in 1877. Charles Fritts first described them in detail in 1883.

Other interesting phenomena

In 1887, Heinrich Hertz discovered that ultraviolet light changes the voltage at which sparks between two metal electrodes would be initiated.

1900 - 1949: Theoretical explanation of the photovoltaic effect and the first solar cells

Theoretical explanation of the photovoltaic effect

The author of the most comprehensive theoretical work about the photovoltaic effect was Albert Einstein, who described the phenomenon in 1904. For his theoretical explanation he was awarded a Nobel Prize in 1921. Einstein's theoretical explanation was practically proved by Robert Millikan's experiment in 1916.

The first silicon solar cells

In 1918, a Polish scientist Jan Czochralski discovered a method for monocrystalline silicon production, which enabled monocrystalline solar cells production. The first silicon monocrystalline solar cell was constructed in 1941.

The photovoltaic effect in other materials

In 1932, the photovoltaic effect in cadmium-selenide was observed. Nowadays, CdS belongs among important materials for solar cells production.

1950 - 1969: Intensive space research

Intensive laboratory researches

In 1951, the first germanium solar cells have been made. Dr. Dan Trivich of Wayne State University has made some theoretical calculation on solar cell efficiency with different materials, and on solar spectrum wavelengths in 1953. In 1954, the RCA Laboratories published a report on CdS photovoltaic effect. AT&T organized several demonstrations on solar cells functioning the same year. The Bell's Laboratories published the results of the solar cells operation with 4.5 % efficiency. The efficiency was increased to 6 % within a few months.

The first satellites and solar powered cars

In 1955, the preparation on satellite energy supply by solar cells began. Western Electric put for sale commercial licenses for solar cells production. Hoffman Electronics-Semiconductor Division introduced a commercial photovoltaic product with 2 % efficiency for US$ 25 per cell with 14 mW peak power. The energy cost was US$ 1,785 per W. In 1957, Hoffman Electronics introduced a solar cell with 8 % efficiency. A year later, in 1958, the same company introduced a solar cell with 9 % efficiency. The first radiation proof silicon solar cell was produced for the purposes of space technology the same year. On 17th March, the first satellite powered by solar cells, Vanguard I, was launched. The system ran continuously for 8 years. Two other satellites, Explorer III and Vanguard II, were launched by Americans, and Sputnik III by Russians. The first telephone repeater powered by solar cells was built In Americus, Georgia. In 1959, Hoffman Electronics introduced commercially available solar cells with 10 % efficiency. Americans launched the satellites Explorer VI with photovoltaic field of 9,600 cells and Explorer VII. In 1960, Hoffman Electronics introduced yet another solar cell with 14 % efficiency. The first sun-powered automobile was demonstrated in Chicago, Illinois on August 31st, 1955.

The first photovoltaic conferences

A United Nation's conference on solar energy application in developing

countries took place in 1961. The Defence Studies Institute organized the first photovoltaic conference the same year in Washington. In 1962, the first commercial telecommunications satellite Telstar, developed by Bell Laboratories, was launched. The photovoltaic system peak power for satellite power supply was 14 W. The second photovoltaic conference took place in Washington.

The first solar modules

In 1963, Sharp Corporation developed the first usable photovoltaic module from silicon solar cells. The biggest photovoltaic system at the time, the 242 W module field was set up in Japan. A year later, in 1964, Americans applied a 470 W photovoltaic field in the Nimbus space project.

Intensive research on photovoltaic technologies for extraterrestrial application

In 1965, the Japanese scientific programme for Japanese satellite launch commenced. The following year, in 1966, an astronomic observatory with 1 kW peak power photovoltaic module field was tracked in the earthly orbit. In 1968, the OVI-13 satellite with two CdS panels was launched. 

Skylab (source: NASA NIX)

The first bigger company

In 1969, Roger Little established Spire Corporation, which became and still is an important producer of solar cells production equipment.

1970 - 1979: Establishment of the biggest photovoltaic companies

The first application of photovoltaic technologies on Earth

In 1970, Solar Power Corporation was established. The French implemented a CdS photovoltaic system enabling educational TV programme broadcast in the province of Niger in 1972. A year after, in 1973, Solarex Corporation was established. At the Delaware University a photovoltaic-thermal hybrid system Solar one, one of the first photovoltaic systems for domestic application, was developed. Besides the photovoltaic system, the system incorporated also a warmth keeper of phase changeable materials. A silicon solar cell of US$ 30 per W was produced. In 1974, the Japanese Sunshine project commenced. A year later, in 1975, Solec International and Solar Technology International were established. The American government encouraged JPL Laboratories research in the field of photovoltaic systems for application on Earth the same year.

The first photovoltaic systems for the third world rural areas

In 1976, under NASA protection LeRC commenced photovoltaic system installations for application on Earth, which continued until 1985 and later from 1992 until 1995. The systems were meant for refrigerator, telecommunication equipment, medical equipment, lighting and water pumping power supply as well as for other applications. NASA LeRC introduced several demonstration projects. The first amorphous silicon solar cell was developed by RCA Laboratories the same year. Solec International was established.

The first photovoltaic applications for supply of technologically sophisticated devices on Earth

In 1977, the world production of photovoltaic modules exceeded 500 kW. NASA LeRC commenced implementing photovoltaic systems in six meteorological stations in different locations within USA. NASA LeRC introduced additional trial demonstration projects. Solar Energy Research Institute located in Golden, Colorado launched its operation. In an American Indians reservation NASA LeRC set up a 3.5 kW system - the first system ever to satisfy the demands of the entire village. It was used for water pumping and power supply of 15 households. In 1979, ARCO Solar of Camarillo, California, built the biggest solar cells and photovoltaic systems production plant premises at that time. NASA LeRC built a 1.8 kW water pumping photovoltaic system in Burkina Faso. The system peak power was enlarged to 3.6 kW the same year. In Mt. Laguna, California, a trial 60 kW hybrid diesel-photovoltaic system was built for radar station power supply.

1980 - 1989: Large standalone systems installations

Large standalone photovoltaic systems

Many important events in the field of photovoltaics appeared in 1980. ARCO Solar was the first to produce photovoltaic modules with peak

power of over 1 MW per year. A trial photovoltaic system installation was made in the centre of the volcano observatory in Hawaii. A new company BP appeared in the market. ARCO Solar built a 105.6 kW system in the State of Utah. The modules integrated in the system were produced by Motorola, ARCO Solar and Spectrolab. A year later, in 1981, NASA LeRC began to build systems for vaccine refrigerators power supply on 30 locations around the globe (the project was closed in 1984). Solar Challenger, the first plane ever powered by solar energy, took off. A system with peak power of 90.4 kW with modules produced by Solar Power Corporation was built in Square Shopping Center in Lovington, New Mexico. A similar system was built for Beverly High School in Beverly, Massachusetts. A seawater desalination system with 10.8 kW peak power was built in Jeddah, Saudi Arabia the same year. Helios Technology, the oldest European solar cells producer, was established. The world production of photovoltaic modules exceeded 9.3 MW in 1982. Solarex established Solarex Aerospace division the same year. At the Vienna conference NASA LeRC introduced a trial case of terrestrial satellite reception station and public lighting electricity supply. Volkswagen began testing photovoltaic systems placed on vehicle roofs with 160 W peak power for vehicle start up. Solarex production premises rooftops in Frederick, Maryland, were equipped with photovoltaic systems with 200 kW peak power. ARCO Solar built a 1 MW PV power plant with modules on over 108 double-axis trackers in Hesperia, California.

Solar cars

A year later, in 1983, the world production of photovoltaic modules exceeded 21.3 MW peak power, with product worth of US$ 250 millions. Solar Trek vehicle with photovoltaic system of 1 kW drove 4,000 km in twenty days of Australia Race. The maximum speed was 72 km/h, and the average speed was 24 km/h. The same year the vehicle surpassed the distance of 4,000 km between Long Beach, California, and Daytona Beach, Florida, in 18 days. Solarex Corporation bought an amorphous cells production technology from cells producer RCA and built its own trial power plant in Newtown, Pennsylvania. ARCO Solar built a 6 MW photovoltaic power plant as a subsystem of the public electricity grid for Pacific Gas and Electric Company application in California. The system satisfied the demand of 2,000 to 2,500 households. Solar Power Corporation built four standalone photovoltaic systems for the needs of a village in Tunisia with total peak power of 31 kW per system. A 1.8 kW photovoltaic system was built to satisfy the needs of the local hospital in Guyana. The applications, such as vaccine refrigerator, indoor lighting, ordination lighting and radio appliance were powered by the system. The system was planned and built by NASA Lewis Research Center and Solarex. A similar yet more powerful photovoltaic system of 4 kW was set up in Ecuador. A 1.8 kW photovoltaic system was set up in Zimbabwe for the same purpose. Solarex Corporation merged with Amoco Solar Company, owned by Standard Oil Company.

The first amorphous solar module

In 1984, a 1 MW photovoltaic power plant began to operate in Sacramento, California. ARCO Solar introduced the first amorphous modules. NASA LeRC placed 17 photovoltaic systems to satisfy the demands of the local schools, lighting, medical equipment and water pumping in Gabon. BP Solar Systems with EGS donations built a 30 kW photovoltaic system connected to public electric grid nearby Southampton, Great Britain. Solarex Corporation closed the equipment supply for photovoltaic system for Georgetown University Intercultural Center demands with total peak power of 337 kW and 4,464 modules. BP Solar bought Monosolar thin film division, Nortek, Inc.

High efficient silicon solar cells and thin film solar module

In 1985, researches of University of New South Wales in Australia have constructed a solar cell with more than 20 % efficiency. BP built a power plant in Sydney, Australia and shortly after another one nearby Madrid. A photovoltaic system was built in Sulawesi, Indonesia for the purposes of a terrestrial satellite station. In 1986, ARCO Solar introduced a G-4000, the first commercial thin film photovoltaic module.

Solar car races - a new challenge for research labs

In the Pentax World Solar Challenge 1997 race through Australia a General Motors Sunracer vehicle won with average speed of 71 km/h. In 1988, the fourth Tour de Sol race of 350 km in Switzerland and Austria took place. The vehicle categories included photovoltaic supplied vehicles, additional pedals vehicles, commercial photovoltaic supplied vehicles and electric vehicles without photovoltaic supply. The overall award fond was worth SFR 140,000.

Third world projects and new production capacities

Solarex has received the United Nations tender to supply a 50kW system for UN research projects needs in Pakistan. ARCO Solar increased the thin film system production capacities in Camarillo, California to 7 MW per year. ARCO Solar opened production in Japan and Germany. BP Solar got a thin film technology patent for a solar cells production in 1989.

1990 - 1999: Large photovoltaic companies co-operation

The world biggest producers

In 1990, Energy Conversion Devices Inc. (ECD) and Canon Inc. established a joint company United Solar Systems Corporation for solar cells production. Siemens bought ARCO Solar and established Siemens Solar Industries, which is nowadays one of the biggest photovoltaic companies in the world. Solar Energy Research Institute (SERI) renamed

to National Renewable Energy Laboratory (NREL). A year later, in 1991, BP Solar Systems renamed to BP Solar International (BPSI), and became an independent unit within British Petroleum concern. In 1992, a photovoltaic system of 0.5 kW was placed in Antarctica for the laboratory, lighting, personal computers and microwave ovens needs. A silicon solar cell with 20 % efficiency was patented. In 1994, the National Renewable Energy Laboratory's (NREL), and important institution in the field of renewable energy sources in USA, launched its web site on the Internet. DOE built several trial systems for the need of agriculture, hospitals, lighting, water pumping and so on in Brazil. ASE GmbH from Germany purchased Mobil Solar Energy Corporation technology and established ASE Americas, Inc. A year later, in 1995, the first international fond for promotion of photovoltaic system commercialisation was established, which supported projects in India. The World Bank and the Indian Renewable Energy Sources Agency sponsored projects in co-operation with Siemens Solar. In 1996, BP Solar purchased APS production premises in California, and announced a commercial CIS solar cells production. Icar the plane, powered by solar energy, with 3,000 solar cells in total surface of 21 m2 flew over Germany.

The world biggest photovoltaic system plans

In 1997, Greece agreed to sponsor the first 5 MW of total planned 50 MW photovoltaic system on Crete. Due to misunderstanding among investors system probably won't be realised. The activities, which will result in 36,400 50 W systems within the next three years, started in Indonesia. In 1999, Solar Cells, Inc. (SCI), True North Partners, and LLC of Phoenix, Arizona merged to First Solar, LLC.

2000 - : Renewable energy and the Stock exchange

Photovoltaics and stock exchange in Europe

Mostly in Germany, some photovoltaic and renewable energy resources companies have shares listed at the stock exchange. Capital mergers in Germany led to large photovoltaic corporation establishments. During 2000 and 2001 production of Japanese producers increased significantly. Sharp and Kyocera each produce modules with peak power equivalent to the annual consumption in Germany, the most demanding European market. Sanyo is close as well.

HELIOS flight

After many years of research and trial flights HELIOS solar powered plane, developed by NASA and AeroVironment Inc., has broken the height record on 13th August 2001. HELIOS reached the height of almost 30,000 meters.

Large photovoltaic plants in Germany

In period 2002 - 2003 several large power plants were built in Germany. On April 29th 2003 at that time the world's largest photovoltaic plant was connected to the public grid in Hemau near Regensburg (Bavaria), Germany. The peak power of the "Solarpark Hemau" plant is 4 MW. Due to renewable energy law "EEG" many other large systems up to 5 MWp were built in Germany in year 2004. Some of them are Solarparks Geiseltalsee, Leipzig, Bürstadt, Göttelborn and others.

History: Photovoltaics Timeline

From Mary Bellis,Your Guide to Inventors.FREE Newsletter. Sign Up Now!

Photovoltaics literally means light-electricity.Today's photovoltaic systems are used to generate electricity to pump water, light up the night, activate switches, charge batteries, supply power to the utility grid, and much more.

1839: Nineteen-year-old Edmund Becquerel, a French experimental physicist, discovered the photovoltaic effect while experimenting with an electrolytic cell made up of two metal electrodes. 1873: Willoughby Smith discovered the photoconductivity of selenium.

1876:Adams and Day observed the photovoltaic effect in solid selenium.

1883:Charles Fritts, an American inventor, described the first solar cells made from selenium wafers.

1887:Heinrich Hertz discovered that ultraviolet light altered the lowest voltage capable of causing a spark to jump between two metal electrodes.

1904:Hallwachs discovered that a combination of copper and cuprous oxide was photosensitive. Einstein published his paper on the photoelectric effect.

1914:The existence of a barrier layer in PV devices was reported.

1916:Millikan provided experimental proof of the photoelectric effect.

1918:Polish scientist Czochralski developed a way to grow single-crystal silicon.

1923:Albert Einstein received the Nobel Prize for his theories explaining the photoelectric effect.

1951:A grown p-n junction enabled the production of a single-crystal cell of germanium.

1954:The PV effect in Cd was reported; primary work was performed by Rappaport, Loferski and Jenny at RCA. Bell Labs researchers Pearson, Chapin, and Fuller reported their discovery of 4.5% efficient silicon solar cells; this was raised to 6% only a few months later (by a work team including Mort Prince). Chapin, Fuller, Pearson (AT&T) submitted their results to the Journal of Applied Physics. AT&T demonstrated solar cells in Murray Hill, New Jersey, then at the National Academy of Science Meeting in Washington, DC.

1955:Western Electric began to sell commercial licenses for silicon PV technologies; early successful products included PV-powered dollar bill changers and devices that decoded computer punch cards and tape. Bell System's demonstration of the type P rural carrier system began in Americus, Georgia. Hoffman Electronics's Semiconductor Division announced a commercial PV product at 2% efficiency; priced at $25/cell and at 14 mW each, the cost of energy was $1500/W.

1956:Bell System's demonstration of the type P rural carrier system was terminated after five months.

1957:Hoffman Electronics achieved 8% efficient cells. "Solar Energy Converting Apparatus," patent #2,780,765, was issued to Chapin, Fuller, and Pearson, AT&T.

1958:Hoffman Electronics achieved 9% efficient PV cells. Vanguard I, the first PV-powered satellite, was launched in cooperation with the U.S. Signal Corp. The satellite power system operated for 8 years.

1959:Hoffman Electronics achieved 10% efficient, commercially available PV cells and demonstrated the use of a grid contact to significantly reduce series resistance. Explorer-6 was launched with a PV array of 9600 cells, each only 1 cm x 2 cm.

1960:Hoffman Electronics achieved 14% efficient PV cells.

1960 to present

1961:The UN conference on Solar Energy in the Developing World was held. The precursor to the PV Specialists Conference, the Meeting of the Solar Working Group (SWG) of the Interservice Group for Flight Vehicle Power, was held in Philadelphia, Pennsylvania. The first PV Specialists Conference was held in Washington, DC.

1963:Japan installed a 242-W PV array on a lighthouse, the world's largest array at that time.

1964:The Nimbus spacecraft was launched with a 470-W PV array.

1965:Peter Glaser, A.D. Little, conceived the idea of a satellite solar power station. Tyco Labs developed the edge-defined, film-fed growth (EFG) process, first to grow crystal sapphire ribbons and then silicon.

1966:The Orbiting Astronomical Observatory was launched with a 1-kW PV array.

1968:The OVI-13 satellite was launched with two CdS panels.

1972:The French install a CdS PV system in a village school in Niger to run an educational TV.

1973:The Cherry Hill Conference was held in Cherry Hill, New Jersey.

1974:Japan formulated Project Sunshine. Tyco Labs grew the first EFG, 1-inch-wide ribbon by an endless-belt process.

1975:The U.S. government began a terrestrial PV research and development project, assigned to the Jet Propulsion Laboratory (JPL), as a result of recommendations of the Cherry Hill Conference. Bill Yerkes opened Solar Technology International. Exxon opened Solar Power Corporation. JPL instituted the Block I procurement by the U.S. government.

1977:The Solar Energy Research Institute (SERI), later to become the National Renewable Energy Laboratory (NREL), opened in Golden, Colorado. Total PV manufacturing production exceeded 500 kW.

1979:Solenergy was founded. NASA's Lewis Research Center (LeRC) completed a 3.5-kW system on the Papago Indian Reservation in Schuchuli, Arizona; this was the world's first village PV system. NASA's LeRC completed an 1.8-kW array for AID, in Tangaye, Upper Volta, and later increased power output to 3.6 kW.

1980:The first William R. Cherry Award was given to Paul Rappaport, SERI's founding director. New Mexico State University, Las Cruces, was selected to establish and operate the Southwest Residential Experimental Station (SW RES). A 105.6-kW system was dedicated at Natural Bridges National Monument in Utah; the system used Motorola, ARCO Solar, and Spectrolab PV modules.

1981:A 90.4-kW PV system was dedicated at Lovington Square Shopping Center (New Mexico) using Solar Power Corp. modules. A 97.6-kW PV system was dedicated at Beverly High School in Beverly, Massachusetts, using Solar Power Corp. modules. An 8-kW PV-powered (Mobil Solar), reverse-osmosis desalination facility was dedicated in Jeddah, Saudi Arabia.

1982:Worldwide PV production exceeded 9.3 MW. Solarex dedicated its 'PV Breeder' production facility in Frederick, Maryland, with its roof-integrated 200-kW array. ARCO Solar's Hisperia, California, 1-MW PV plant went on line with modules on 108 dual-axis trackers.

1983:The JPL Block V procurement was begun. Solar Power Corporation completed the design and installation of four stand-alone PV village power systems in Hammam Biadha, Tunesia (a 29-kW village power system, a 1.5-kW residential system, and two 1.5-kW irrigation/pumping systems). Solar Design Associates completed the stand-alone, 4-kW (Mobil Solar), Hudson River Valley home. Worldwide PV production exceeded 21.3 MW, and sales exceeded $250 million.

1984:The IEEE Morris N. Liebmann Award was presented to Drs. David Carlson and Christopher Wronski at the 17th Photovoltaic Specialists Conference, "for crucial contributions to the use of amorphous silicon in low-cost, high-performance photovoltaic solar cells."

1991:The Solar Energy Research Institute was redesignated as the U.S. Department of Energy's National Renewable Energy Laboratory by President George Bush.

1993:The National Renewable Energy Laboratory's Solar Energy Research Facility (SERF), opened in Golden, Colorado.

1996:The U.S. Department of Energy announces the National Center for Photovoltaics, headquartered in Golden, Colorado.

Photovoltaic systems convert light energy into electricity.Solar panels are devices that convert light into electricity. They are called solar after the sun or "Sol" because the sun is the most powerful source of the light to use. They are sometimes called photovoltaics which means "light-electricity". Solar cells or PV cells rely on the photovoltaic effect to absorb the energy of the sun and cause current to flow between two oppositely charge layers.

PhotovoltaicsPhotovoltaic (or PV) systems convert light energy into electricity. The term "photo" is a stem from the Greek "phos," which means "light." "Volt" is named for Alessandro Volta (1745-1827), a pioneer in the study of electricity. Photovoltaics literally means light-electricity.

Most commonly known as "solar cells," PV systems are already an important part of our lives.

The simplest systems power many of the small calculators and wrist watches we use every day. More complicated systems provide electricity for pumping water, powering communications equipment, and even lighting our homes and running our appliances. In a surprising number of cases, PV power is the cheapest form of electricity for performing these tasks.

History of Photovoltaic CellsPhotovoltaic cells convert light energy into electricity at the atomic level. French physicist Edmond Becquerel first described the photovoltaic effect in 1839, but it remained a curiosity of science for the next three quarters of a century. At only nineteen, Becquerel found that certain materials would produce small amounts of electric current when exposed to light.

The effect was first studied in solids, such as selenium, by Heinrich Hertz in the 1870s. Soon afterward, selenium photovoltaic cells were converting light to electricity at one percent to two percent efficiency. As a result, selenium was quickly adopted in the emerging field of photography for use in light-measuring devices.

Major steps toward commercializing photovoltaic cells began in the 1940s and early 1950s, when the Czochralski process was developed for producing highly pure crystalline silicon. In 1954, scientists at Bell Laboratories depended on the Czochralski process to develop a crystalline silicon photovoltaic cell, with an efficiency of six percent.