Solar cell

Solar cell

Overview

A solar cell is a solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 electrical device that converts the energy of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 directly into electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 by the photovoltaic effect
Photovoltaic effect
The photovoltaic effect is the creation of voltage or electric current in a material upon exposure to light. Though the photovoltaic effect is directly related to the photoelectric effect, they are different processes. In the photoelectric effect, electrons are ejected from a material's surface...

.

Assemblies of solar cells are used to make solar modules which are used to capture energy from sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

. When multiple modules are assembled together (such as prior to installation on a pole-mounted tracker system), the resulting integrated group of modules all oriented in one plane is referred to in the solar industry as a solar panel.
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Encyclopedia

A solar cell is a solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 electrical device that converts the energy of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 directly into electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 by the photovoltaic effect
Photovoltaic effect
The photovoltaic effect is the creation of voltage or electric current in a material upon exposure to light. Though the photovoltaic effect is directly related to the photoelectric effect, they are different processes. In the photoelectric effect, electrons are ejected from a material's surface...

.

Assemblies of solar cells are used to make solar modules which are used to capture energy from sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

. When multiple modules are assembled together (such as prior to installation on a pole-mounted tracker system), the resulting integrated group of modules all oriented in one plane is referred to in the solar industry as a solar panel. The general public and some casual writers often refer to solar modules incorrectly as solar panels; technically this is not the correct usage of terminology. Nevertheless, both designations are seen in regular use, in reference to what are actually solar modules. The distinction between a module and a panel is that a module cannot be disassembled into smaller re-usable components in the field, whereas a solar panel is assembled from, and can be disassembled back into a stack of solar modules. The electrical energy generated from solar modules, referred to as solar power
Solar power
Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar-powered resources such as wind and wave power, hydroelectricity and biomass, account for most of the available...

, is an example of solar energy.

Photovoltaics
Photovoltaics
Photovoltaics is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material...

 is the field of technology and research related to the practical application of photovoltaic cells in producing electricity from light, though it is often used specifically to refer to the generation of electricity from sunlight.

Cells are described as photovoltaic cells when the light source is not necessarily sunlight. These are used for detecting light or other electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

 near the visible range, for example infrared detector
Infrared detector
An infrared detector is a photodetector that reacts to infrared radiation. The two main types of detectors are thermal and photonic.The thermal effects of the incident IR radiation can be followed through many temperature dependent phenomena....

s, or measurement of light intensity.

History of solar cells


The term "photovoltaic" comes from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 φῶς (phōs) meaning "light", and "voltaic", from the name of the Italian
Italian people
The Italian people are an ethnic group that share a common Italian culture, ancestry and speak the Italian language as a mother tongue. Within Italy, Italians are defined by citizenship, regardless of ancestry or country of residence , and are distinguished from people...

 physicist Volta
Alessandro Volta
Count Alessandro Giuseppe Antonio Anastasio Gerolamo Umberto Volta was a Lombard physicist known especially for the invention of the battery in 1800.-Early life and works:...

, after whom a unit of electro-motive force, the volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

, is named. The term "photo-voltaic" has been in use in English since 1849.

The photovoltaic effect
Photovoltaic effect
The photovoltaic effect is the creation of voltage or electric current in a material upon exposure to light. Though the photovoltaic effect is directly related to the photoelectric effect, they are different processes. In the photoelectric effect, electrons are ejected from a material's surface...

 was first recognized in 1839 by French physicist A. E. Becquerel
A. E. Becquerel
Alexandre-Edmond Becquerel , known as Edmond Becquerel, was a French physicist who studied the solar spectrum, magnetism, electricity, and optics. He is known for his work in luminescence and phosphorescence. He is credited with the discovery of the photovoltaic effect, the operating principle of...

. However, it was not until 1883 that the first photovoltaic cell was built, by Charles Fritts
Charles Fritts
Charles Fritts was an American inventor credited with creating the first working solar cell in 1883.Fritts coated the semiconductor material selenium with an extremely thin layer of gold...

, who coated the semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...

 with an extremely thin layer of gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 to form the junctions. The device was only around 1% efficient. In 1888 Russian physicist Aleksandr Stoletov
Aleksandr Stoletov
Aleksandr Grigorievich Stoletov was a Russian physicist, founder of electrical engineering, and professor in Moscow University. He was the brother of general Nikolai Stoletov.-Biography:...

 built the first photoelectric cell based on the outer photoelectric effect
Photoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...

 discovered by Heinrich Hertz earlier in 1887.

Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

 explained the photoelectric effect in 1905 for which he received the Nobel prize in Physics in 1921. Russell Ohl
Russell Ohl
Russell Shoemaker Ohl was an American engineer who is generally recognized for patenting the modern solar cell ....

 patented the modern junction semiconductor solar cell in 1946, which was discovered while working on the series of advances that would lead to the transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

.

Bell produces the first practical cell


The modern photovoltaic cell was developed in 1954 at Bell Laboratories. The highly efficient solar cell was first developed by Daryl Chapin, Calvin Souther Fuller
Calvin Souther Fuller
Calvin Souther Fuller was a physical chemist at AT&T Bell Laboratories where he worked for 37 years from 1930 to 1967. Fuller was part of a team in basic research that found answers to physical challenges...

 and Gerald Pearson in 1954 using a diffused silicon p-n junction. At first, cells were developed for toys and other minor uses, as the cost of the electricity they produced was very high; in relative terms, a cell that produced 1 watt of electrical power in bright sunlight cost about $250, comparing to $2 to $3 for a coal plant.

Solar cells were rescued from obscurity by the suggestion to add them to the Vanguard I satellite, launched in 1958. In the original plans, the satellite would be powered only by battery, and last a short time while this ran down. By adding cells to the outside of the body, the mission time could be extended with no major changes to the spacecraft or its power systems. There was some scepticism at first, but in practice the cells proved to be a huge success, and solar cells were quickly designed into many new satellites, notably Bell's own Telstar
Telstar
Telstar is the name of various communications satellites, including the first such satellite to relay television signals.The first two Telstar satellites were experimental and nearly identical. Telstar 1 was launched on top of a Thor-Delta rocket on July 10, 1962...

.

Improvements were slow over the next two decades, and the only widespread use was in space applications where their power-to-weight ratio
Power-to-weight ratio
Power-to-weight ratio is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement of actual performance of any engine or power sources...

 was higher than any competing technology. However, this success was also the reason for slow progress; space users were willing to pay anything for the best possible cells, there was no reason to invest in lower-cost solutions if this would reduce efficiency. Instead, the price of cells was determined largely by the semiconductor industry; their move to integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

s in the 1960s led to the availability of larger boules
Boule (crystal)
A boule is a single-crystal ingot produced by synthetic means. A boule of silicon is the starting material for most of the integrated circuits used today....

 at lower relative prices. As their price fell, the price of the resulting cells did as well. However these effects were limited, and by 1971 cell costs were estimated to be $100 per watt.

Berman's price reductions


In the late 1960s, Elliot Berman was investigating a new method for producing the silicon feedstock in a ribbon process. However, he found little interest in the project and was unable to gain the funding needed to develop it. In a chance encounter, he was later introduced to a team at Exxon
Exxon
Exxon is a chain of gas stations as well as a brand of motor fuel and related products by ExxonMobil. From 1972 to 1999, Exxon was the corporate name of the company previously known as Standard Oil Company of New Jersey or Jersey Standard....

 who were looking for projects 30 years in the future. The group had concluded that electrical power would be much more expensive by 2000, and felt that this increase in price would make new alternative energy sources more attractive, and solar was the most interesting among these. In 1969, Berman joined the Linden, New Jersey Exxon lab, Solar Power Corporation (SPC).

His first major effort was to canvas the potential market to see what possible uses for a new product were, and they quickly found that if the price per watt
Price per watt
Price per watt, or $/W is a common way to compare the capital costs of various forms of electricity generation. It refers to the number of dollars one would have to spend to buy a machine capable of producing one watt of electricity...

 were reduced from then-current $100/watt to about $20/watt there would be significant demand. Knowing that his ribbon concept would take years to develop, the team started looking for ways to hit the $20 price point using existing materials.

The first improvement was the realization that the existing cells were based on standard semiconductor manufacturing process, even though that was not ideal. This started with the boule, cutting it into disks called wafers, polishing the wafers, and then, for cell use, coating them with an anti-reflective layer. Berman noted that the rough-sawn wafers already had a perfectly suitable anti-reflective front surface, and by printing the electrodes directly on this surface, two major steps in the cell processing were eliminated. The team also explored ways to improve the mounting of the cells into arrays, eliminating the expensive materials and hand wiring used in space applications. Their solution was to use a printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...

 on the back, acrylic plastic on the front, and silicone glue between the two, potting the cells. The largest improvement in price point was Berman's realization that existing silicon was effectively "too good" for solar cell use; the minor imperfections that would ruin a boule (or individual wafer) for electronics would have little effect in the solar application. Solar cells could be made using cast-off material from the electronics market.

Putting all of these changes into practice, the company started buying up "reject" silicon from existing manufacturers at very low cost. By using the largest wafers available, thereby reducing the amount of wiring for a given panel area, and packaging them into panels using their new methods, by 1973 SPC was producing panels at $10 per watt and selling them at $20 per watt, a fivefold decrease in prices in two years.

Navigation market


SPC approached companies making navigational buoys as a natural market for their products, but found a curious situation. The primary company in the business was Automatic Power, a battery manufacturer. Realizing that solar cells might eat into their battery profits, Automatic purchased the rights to earlier solar cell designs and suppressed them. Seeing there was no interest at Automatic Power, SPC turned to Tideland Signal
Tideland Signal
Tideland Signal, sometimes referred to as Tidelands, is a privately held, Houston, Texas based manufacturer of marine aids to navigation, with main offices in Lafayette, Louisiana, Vancouver, Canada, Burgess Hill, UK, Dubai, United Arab Emirates and Singapore...

, another battery company formed by ex-Automatic managers. Tideland introduced a solar-powered buoy and was soon ruining Automatic's business.

The timing could not be better; the rapid increase in the number of offshore oil platform
Oil platform
An oil platform, also referred to as an offshore platform or, somewhat incorrectly, oil rig, is a lаrge structure with facilities to drill wells, to extract and process oil and natural gas, and to temporarily store product until it can be brought to shore for refining and marketing...

s and loading facilities produced an enormous market among the oil companies. As Tideland's fortunes improved, Automatic Power started looking for their own supply of solar panels. They found Bill Yerks of Solar Power International (SPI) in California, who was looking for a market. SPI was soon bought out by one of its largest customers, the ARCO
ARCO
Atlantic Richfield Company is an oil company with operations in the United States as well as in Indonesia, the North Sea, and the South China Sea. It has more than 1,300 gas stations in the western part of the United States. ARCO was originally formed by the merger of East Coast-based Atlantic...

 oil giant, forming ARCO Solar. ARCO Solar's factory in Camarillo, California
Camarillo, California
Camarillo is a city in Ventura County, California, United States. The population was 65,201 at the 2010 census, up from 57,084 at the 2000 census. The Ventura Freeway Camarillo is a city in Ventura County, California, United States. The population was 65,201 at the 2010 census, up from 57,084 at...

 was the first dedicated to building solar panels, and has been in continual operation from its purchase by ARCO in 1977 to this day.

This market, combined with the 1973 oil crisis
1973 oil crisis
The 1973 oil crisis started in October 1973, when the members of Organization of Arab Petroleum Exporting Countries or the OAPEC proclaimed an oil embargo. This was "in response to the U.S. decision to re-supply the Israeli military" during the Yom Kippur war. It lasted until March 1974. With the...

, led to a curious situation. Oil companies were now cash-flush due to their huge profits during the crisis, but were also acutely aware that their future success would depend on some other form of power. Over the next few years, major oil companies started a number of solar firms, and were for decades the largest producers of solar panels. Exxon, ARCO, Shell, Amoco (later purchased by BP) and Mobil all had major solar divisions during the 1970s and 1980s. Technology companies also had some investment, including General Electric, Motorola, IBM, Tyco and RCA.

Further improvements


In the time since Berman's work, improvements have brought production costs down under $1 a watt, with wholesale costs well under $2. "Balance of system
Balance of system
The balance of system or BOS encompasses all components of a photovoltaic system other than the photovoltaic panels. This includes wiring, switches, support racks, an inverter, and batteries in the case of off-grid systems. In the case of free-standing systems, land is sometimes included as part of...

" costs are now more than the panels themselves. Large commercial arrays can be built at below $3.40 a watt, fully commissioned.

As the semiconductor industry moved to ever-larger boules, older equipment became available at fire-sale prices. Cells have grown in size as older equipment became available on the surplus market; ARCO Solar's original panels used cells with 2 to 4 inch (51 to 100 mm) diameter. Panels in the 1990s and early 2000s generally used 5 inch (125 mm) wafers, and since 2008 almost all new panels use 6 inch (150 mm) cells. Another major change was the move to polycrystalline silicon. This material has less efficiency, but is less expensive to produce in bulk. The widespread introduction of flat screen televisions in the late 1990s and early 2000s led to the wide availability of large sheets of high-quality glass, used on the front of the panels.

Current events


Other technologies have tried to enter the market. First Solar
First Solar
First Solar, Inc. is an American manufacturer of thin film photovoltaic modules, or solar panels, and a provider to PV power plants of supporting services that include finance, construction, maintenance and end-of-life panel recycling...

 was briefly the largest panel manufacturer in 2009, in terms of yearly power produced, using a thin-film cell sandwiched between two layers of glass. Since then Silicon panels reasserted their dominant position both in terms of lower prices and the rapid rise of Chinese manufacturing, resulting in the top producers being Chinese. By late 2011, efficient production in China, coupled with a drop in European demand due to budgetary turmoil had dropped prices for crystalline solar-based modules further, to about $1.09 per watt in October 2011, down sharply from from the price per watt in 2010.

Applications





Solar cells are often electrically connected and encapsulated as a module. Photovoltaic modules often have a sheet of glass on the front (sun up) side, allowing light to pass while protecting the semiconductor wafers
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...

 from abrasion and impact due to wind-driven debris, rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...

, hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...

, etc. Solar cells are also usually connected in series
Series and parallel circuits
Components of an electrical circuit or electronic circuit can be connected in many different ways. The two simplest of these are called series and parallel and occur very frequently. Components connected in series are connected along a single path, so the same current flows through all of the...

 in modules, creating an additive voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

. Connecting cells in parallel will yield a higher current; however, very significant problems exist with parallel connections. For example, shadow effects can shut down the weaker (less illuminated) parallel string (a number of series connected cells) causing substantial power loss and even damaging excessive reverse bias applied to the shadowed cells by their illuminated partners. As far as possible, strings of series cells should be handled independently and not connected in parallel, save using special paralleling circuits. Although modules can be interconnected in series and/or parallel to create an array with the desired peak DC voltage and loading current capacity, using independent MPPTs (maximum power point trackers) provides a better solution. In the absence of paralleling circuits, shunt diodes can be used to reduce the power loss due to shadowing in arrays with series/parallel connected cells.

To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected photovoltaic system
Photovoltaic system
A photovoltaic system is a system which uses one or more solar panels to convert sunlight into electricity. It consists of multiple components, including the photovoltaic modules, mechanical and electrical connections and mountings and means of regulating and/or modifying the electrical...

s); in stand-alone systems, batteries are used to store the energy that is not needed immediately. Solar panels can be used to power or recharge portable devices.

Theory


The solar cell works in three steps:
  1. Photon
    Photon
    In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

    s in sunlight
    Sunlight
    Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

     hit the solar panel and are absorbed by semiconducting materials, such as silicon.
  2. Electrons (negatively charged) are knocked loose from their atoms, causing an electric potential difference. Current starts flowing through the material to cancel the potential and this electricity is captured. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction.
  3. An array of solar cells converts solar energy into a usable amount of direct current
    Direct current
    Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

     (DC) electricity.

Efficiency


The efficiency of a solar cell may be broken down into reflectance efficiency, thermodynamic efficiency, charge carrier separation efficiency and conductive efficiency. The overall efficiency is the product of each of these individual efficiencies.

Due to the difficulty in measuring these parameters directly, other parameters are measured instead: thermodynamic efficiency, quantum efficiency
Quantum efficiency of a solar cell
Quantum efficiency is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the solar cell. QE therefore relates to the response of a solar cell to the various wavelengths in the spectrum of light shining on the cell. The QE is...

, integrated quantum efficiency
Quantum efficiency
Quantum efficiency is a quantity defined for a photosensitive device such as photographic film or a charge-coupled device as the percentage of photons hitting the photoreactive surface that will produce an electron–hole pair. It is an accurate measurement of the device's electrical sensitivity to...

, VOC ratio, and fill factor. Reflectance losses are a portion of the quantum efficiency under "external quantum efficiency". Recombination losses make up a portion of the quantum efficiency, VOC ratio, and fill factor. Resistive losses are predominantly categorized under fill factor, but also make up minor portions of the quantum efficiency, VOC ratio.

The fill factor is defined as the ratio of the actual maximum obtainable power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

, to the product of the open circuit voltage and short circuit current. This is a key parameter in evaluating the performance of solar cells. Typical commercial solar cells have a fill factor > 0.70. Grade B cells have a fill factor usually between 0.4 to 0.7. The fill factor is, besides efficiency, one of the most significant parameters for the energy yield of a photovoltaic cell. Cells with a high fill factor have a low equivalent series resistance and a high equivalent shunt resistance, so less of the current produced by light is dissipated in internal losses.

Single p-n junction crystalline silicon devices are now approaching the theoretical limiting efficiency of 37.7% as stated by Shockley-Quissler in 1961. However multiple layer solar cells have a theoretical limit of 86%.

Cost


The cost of a solar cell is given per unit of peak electrical power. Manufacturing costs necessarily include the cost of energy required for manufacture. Solar-specific feed in tariffs vary worldwide, and even state by state within various countries. Such feed-in tariffs can be highly effective in encouraging the development of solar power projects.

High-efficiency solar cells are of interest to decrease the cost of solar energy. Many of the costs of a solar power plant are proportional to the area of the plant; a higher efficiency cell may reduce area and plant cost, even if the cells themselves are more costly. Efficiencies of bare cells, to be useful in evaluating solar power plant economics, must be evaluated under realistic conditions. The basic parameters that need to be evaluated are the short circuit current, open circuit voltage.

The chart below illustrates the best laboratory efficiencies obtained for various materials and technologies, generally this is done on very small, i.e., one square cm, cells. Commercial efficiencies are significantly lower.
Grid parity
Grid parity
Grid parity is the point at which alternative means of generating electricity is at least as cheap as grid power.For solar energy, it is achieved first in areas with abundant sun and high costs for electricity such as in California, Hawaii, Spain and Japan. Many solar power advocates predict that...

, the point at which photovoltaic electricity is equal to or cheaper than grid power
Mains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...

, can be reached using low cost solar cells. It is achieved first in areas with abundant sun and high costs for electricity such as in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

 and Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

. Grid parity has been reached in Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

 and other islands that otherwise use diesel fuel to produce electricity. George W. Bush
George W. Bush
George Walker Bush is an American politician who served as the 43rd President of the United States, from 2001 to 2009. Before that, he was the 46th Governor of Texas, having served from 1995 to 2000....

 had set 2015 as the date for grid parity in the USA. Speaking at a conference in 2007, General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...

's Chief Engineer predicted grid parity without subsidies in sunny parts of the United States by around 2015.

The price of solar panels fell steadily for 40 years, until 2004 when high subsidies in Germany drastically increased demand there and greatly increased the price of purified silicon (which is used in computer chips as well as solar panels). The great recession of 2008 and the onset of Chinese manufacturing caused prices to resume their decline with vehemence. In the four years after January 2008 prices for solar modules in Germany dropped from €3 to €1 per peak watt. During that same times production capacity surged with an annual growth of more than 50%. China increased market share from 8% in 2008 to over 55% in the last quarter of 2010. Recently, since the middle of 2010, the price has been dropped to $1.2-$1.5/Wp (crystalline modules).

Materials



Different materials display different efficiencies and have different costs.
Materials for efficient solar cells must have characteristics matched to the spectrum of available light. Some cells are designed to efficiently convert wavelengths of solar light that reach the Earth surface. However, some solar cells are optimized for light absorption beyond Earth's atmosphere as well. Light absorbing materials can often be used in multiple physical configurations to take advantage of different light absorption and charge separation mechanisms.

Materials presently used for photovoltaic solar cells include monocrystalline silicon
Monocrystalline silicon
Monocrystalline silicon or single-crystal Si, or mono-Si is the base material of the electronic industry. It consists of silicon in which the crystal lattice of the entire solid is continuous, unbroken to its edges. It can be prepared intrinsic, i.e...

, polycrystalline silicon
Polycrystalline silicon
Polycrystalline silicon, also called polysilicon, is a material consisting of small silicon crystals. It differs from single-crystal silicon, used for electronics and solar cells, and from amorphous silicon, used for thin film devices and solar cells....

, amorphous silicon
Amorphous silicon
Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...

, cadmium telluride
Cadmium telluride
Cadmium telluride is a crystalline compound formed from cadmium and tellurium. It is used as an infrared optical window and a solar cell material. It is usually sandwiched with cadmium sulfide to form a p-n junction photovoltaic solar cell...

, and copper indium selenide/sulfide.

Many currently available solar cells are made from bulk materials that are cut into wafer
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...

s between 180 to 240 micrometers thick that are then processed like other semiconductors.

Other materials are made as thin-films layers, organic dye
Dye
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

s, and organic polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s that are deposited on supporting substrates. A third group are made from nanocrystals and used as quantum dot
Quantum dot
A quantum dot is a portion of matter whose excitons are confined in all three spatial dimensions. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. They were discovered at the beginning of the 1980s by Alexei...

s (electron-confined nanoparticle
Nanoparticle
In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. Particles are further classified according to size : in terms of diameter, coarse particles cover a range between 10,000 and 2,500 nanometers. Fine particles are sized...

s). Silicon remains the only material that is well-researched in both bulk and thin-film forms.

Crystalline silicon



By far, the most prevalent bulk material for solar cells is crystalline silicon (abbreviated as a group as c-Si), also known as "solar grade silicon". Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot
Ingot
An ingot is a material, usually metal, that is cast into a shape suitable for further processing. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.-Uses:...

, ribbon
Ribbon
A ribbon or riband is a thin band of material, typically cloth but also plastic or sometimes metal, used primarily for binding and tying. Cloth ribbons, most commonly silk, are often used in connection with clothing, but are also applied for innumerable useful, ornamental and symbolic purposes...

, or wafer
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...

.
  1. monocrystalline silicon (c-Si): often made using the Czochralski process
    Czochralski process
    The Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors , metals , salts, and synthetic gemstones...

    . Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the four corners of the cells.
  2. polycrystalline silicon, or multicrystalline silicon, (poly-Si or mc-Si): made from cast square ingots — large blocks of molten silicon carefully cooled and solidified. Poly-Si cells are less expensive to produce than single crystal silicon cells, but are less efficient. United States Department of Energy
    United States Department of Energy
    The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...

     data show that there were a higher number of polycrystalline sales than monocrystalline silicon sales.
  3. ribbon silicon is a type of polycrystalline silicon: it is formed by drawing flat thin films from molten
    Mölten
    Mölten is a comune in South Tyrol in the Italian region Trentino-Alto Adige/Südtirol, located about 60 km north of Trento and about 12 km northwest of Bolzano .-Geography:...

     silicon and results in a polycrystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require saw
    Saw
    A saw is a tool that uses a hard blade or wire with an abrasive edge to cut through softer materials. The cutting edge of a saw is either a serrated blade or an abrasive...

    ing from ingot
    Ingot
    An ingot is a material, usually metal, that is cast into a shape suitable for further processing. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.-Uses:...

    s.


Analysts have predicted that prices of polycrystalline silicon will drop as companies build additional polysilicon capacity quicker than the industry's projected demand. On the other hand, the cost of producing upgraded metallurgical-grade silicon, also known as UMG Si, can potentially be one-sixth that of making polysilicon.

Manufacturers of wafer-based cells have responded to high silicon prices in 2004-2008 prices with rapid reductions in silicon consumption. According to Jef Poortmans, director of IMEC
Imec
Imec is a micro- and nanoelectronics research center headquartered in Leuven, Belgium, with offices in Belgium, the Netherlands, Taiwan, USA, China and Japan. Its staff of about 1,900 people includes more than 500 industrial residents and guest researchers...

's organic and solar department, current cells use between eight and nine grams of silicon per watt of power generation, with wafer thicknesses in the neighborhood of 0.200 mm. At 2008 spring's IEEE Photovoltaic Specialists' Conference (PVS'08), John Wohlgemuth, staff scientist at BP Solar
BP Solar
BP has been involved in solar power since 1973 and its subsidiary, BP Solar, is a solar power company with production facilities in India and the People's Republic of China, employing a workforce of over 2,000 people worldwide. BP Solar is worldwide manufacturer and installer of photovoltaic solar...

, reported that his company has qualified modules based on 0.180 mm thick wafers and is testing processes for 0.16 mm wafers cut with 0.1 mm wire. IMEC's road map, presented at the organization's recent annual research review meeting, envisions use of 0.08 mm wafers by 2015.

Thin films


Thin-film technologies reduce the amount of material required in creating the active material of solar cell. Most thin film solar cells are sandwiched between two panes of glass to make a module. Since silicon solar panels only use one pane of glass, thin film panels are approximately twice as heavy as crystalline silicon panels. The majority of film panels have significantly lower conversion efficiencies, lagging silicon by two to three percentage points. Thin-film solar technologies have enjoyed large investment due to the success of First Solar and the largely unfulfilled promise of lower cost and flexibility compared to wafer silicon cells, but they have not become mainstream solar products due to their lower efficiency and corresponding larger area consumption per watt production. Cadmium telluride
Cadmium telluride
Cadmium telluride is a crystalline compound formed from cadmium and tellurium. It is used as an infrared optical window and a solar cell material. It is usually sandwiched with cadmium sulfide to form a p-n junction photovoltaic solar cell...

 (CdTe), copper indium gallium selenide
Copper indium gallium selenide
Copper indium gallium selenide is a I-III-VI2 semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide and copper gallium selenide...

 (CIGS) and amorphous silicon
Amorphous silicon
Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...

 (A-Si) are three thin-film technologies often used as outdoor photovoltaic solar power production. CdTe technology is most cost competitive among them. CdTe technology costs about 30% less than CIGS technology and 40% less than A-Si technology in 2011.

Cadmium telluride solar cell


A cadmium telluride solar cell uses a cadmium telluride (CdTe) thin film, a semiconductor layer to absorb and convert sunlight into electricity. Solarbuzz has reported that the lowest quoted thin-film module price stands at US$1.76 per watt-peak
Watt-peak
Watt-peak is a measure of the nominal power of a photovoltaic solar energy device under laboratory illumination conditions. Related units such as kilowatt-peak or kilowatts-peak and megawatt-peak are also used, and in the context of domestic installations kWp is the most common unit encountered...

, with the lowest crystalline silicon (c-Si) module at $2.48 per watt-peak.

The cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...

 present in the cells would be toxic if released. However, release is impossible during normal operation of the cells and is unlikely during fires in residential roofs. A square meter of CdTe contains approximately the same amount of Cd as a single C cell Nickel-cadmium battery
Nickel-cadmium battery
The nickel–cadmium battery ' is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes....

, in a more stable and less soluble form.

Copper indium gallium selenide


Copper indium gallium selenide (CIGS) is a direct band gap material. It has the highest efficiency (~20%) among thin film materials (see CIGS solar cell). Traditional methods of fabrication involve vacuum processes including co-evaporation and sputtering. Recent developments at IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

 and Nanosolar
Nanosolar
Nanosolar is a developer of solar power technology. Based in San Jose, CA, Nanosolar has developed and commercialized a low-cost printable solar cell manufacturing process. The company started selling panels mid-December 2007, and plans to sell them at around $1 per watt...

 attempt to lower the cost by using non-vacuum solution processes.

Gallium arsenide multijunction


High-efficiency multijunction cells were originally developed for special applications such as satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

s and space exploration
Space exploration
Space exploration is the use of space technology to explore outer space. Physical exploration of space is conducted both by human spaceflights and by robotic spacecraft....

, but at present, their use in terrestrial concentrators might be the lowest cost alternative in terms of $/kWh and $/W. These multijunction cells consist of multiple thin films produced using metalorganic vapour phase epitaxy
Metalorganic vapour phase epitaxy
Metalorganic vapour phase epitaxy , also known as organometallic vapour phase epitaxy or metalorganic chemical vapour deposition , is a chemical vapour deposition method of epitaxial growth of materials, especially compound semiconductors, from the surface reaction of organic compounds or...

. A triple-junction cell, for example, may consist of the semiconductors: GaAs
Gallium(III) arsenide
Gallium arsenide is a compound of the elements gallium and arsenic. It is a III/V semiconductor, and is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated circuits, infrared light-emitting diodes, laser diodes, solar cells and...

, Ge
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....

, and GaInP2
Indium gallium phosphide
Indium gallium phosphide , also called gallium indium phosphide , is a semiconductor composed of indium, gallium and phosphorus...

. Each type of semiconductor will have a characteristic band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...

 energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

 over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible.

GaAs based multijunction devices are the most efficient solar cells to date. In October 2010, triple junction metamorphic cell reached a record high of 42.3%. This technology is currently being utilized in the Mars Exploration Rover
Mars Exploration Rover
NASA's Mars Exploration Rover Mission is an ongoing robotic space mission involving two rovers, Spirit and Opportunity, exploring the planet Mars...

 missions, which have run far past their 90 day design life.

Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge p-n junctions, are seeing demand rapidly rise. Between December 2006 and December 2007, the cost of 4N gallium metal rose from about $350 per kg to $680 per kg. Additionally, germanium metal prices have risen substantially to $1000–$1200 per kg this year. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry.

Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time World Solar Challenge
World Solar Challenge
The World Solar Challenge is a solar-powered car race which covers through the Australian Outback, from Darwin to Adelaide.The race attracts teams from around the world, most of which are fielded by universities or corporations although some are fielded by high schools...

 winners Nuna in 2003, 2005 and 2007, and also by the Dutch solar cars Solutra (2005)
SolUTra
The Solutra is the first solar car designed by a group of students, the Raedthuys Solar Team, of the University of Twente. It participated in the 8th World Solar Challenge in 2005 and finished 9th place. Another Dutch team won the race with their car Nuna 3....

, Twente One (2007)
Twente One
The Twente One is the second solar car developed by the University of Twente and Saxion University in 2007, and succeeds the SolUTra...

 and 21Revolution (2009).

The Dutch Radboud University Nijmegen
Radboud University Nijmegen
Radboud University Nijmegen is a public university with a strong focus on research in Nijmegen, the Netherlands...

 set the record for thin film solar cell efficiency using a single junction GaAs to 25.8% in August 2008 using only 4 µm thick GaAs layer which can be transferred from a wafer base to glass or plastic film.

Light-absorbing dyes (DSSC)


Dye-sensitized solar cells (DSSCs) are made of low-cost materials and do not need elaborate equipment to manufacture, so they can be made in a DIY
Do it yourself
Do it yourself is a term used to describe building, modifying, or repairing of something without the aid of experts or professionals...

 fashion, possibly allowing players to produce more of this type of solar cell than others. In bulk it should be significantly less expensive than older solid-state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 cell designs. DSSC's can be engineered into flexible sheets, and although its conversion efficiency is less than the best thin film cells, its price/performance ratio
Price/performance ratio
In economics and engineering, the price/performance ratio refers to a product's ability to deliver performance, of any sort, for its price. Generally speaking, products with a higher price/performance ratio are more desirable, excluding other factors....

 should be high enough to allow them to compete with fossil fuel electrical generation. The DSSC has been developed by Prof. Michael Grätzel
Michael Grätzel
Michael Grätzel is a professor at the École Polytechnique Fédérale de Lausanne where he directs the Laboratory of Photonics and Interfaces. He pioneered research on energy and electron transfer reactions in mesoscopic-materials and their optoelectronic applications...

 in 1991 at the Swiss Federal Institute of Technology
École polytechnique fédérale de Lausanne
The École polytechnique fédérale de Lausanne is one of the two Swiss Federal Institutes of Technology and is located in Lausanne, Switzerland.The school was founded by the Swiss Federal Government with the stated mission to:...

 (EPFL) in Lausanne (CH).

Typically a ruthenium
Ruthenium
Ruthenium is a chemical element with symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most chemicals. The Russian scientist Karl Ernst Claus discovered the element...

 metalorganic
Metalorganics
Metalorganic compounds are a class of chemical compounds that contain metals and organic ligands. Metalorganic compounds are used extensively in materials science in applications such as metalorganic vapour phase epitaxy or sol-gel processing using alkoxides...

 dye
Dye
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

 (Ru-centered) is used as a monolayer
Monolayer
- Chemistry :A Langmuir monolayer or insoluble monolayer is a one-molecule thick layer of an insoluble organic material spread onto an aqueous subphase. Traditional compounds used to prepare Langmuir monolayers are amphiphilic materials that possess a hydrophilic headgroup and a hydrophobic tail...

 of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...

 to greatly amplify the surface area (200–300 m2/g TiO2, as compared to approximately 10 m2/g of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2, and the holes are absorbed by an electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

 on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and is typically manufactured by screen printing and/or use of Ultrasonic Nozzle
Ultrasonic Nozzle
This type of spray nozzle uses high frequency vibration to produce very narrow drop size distribution and low velocity spray from a low viscosity liquid. The vibration of a piezoelectric crystal causes capillary waves on the nozzle surface liquid film...

s, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation
Chemical decomposition
Chemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...

 under heat and UV light, and the cell casing is difficult to seal
Seal (mechanical)
A mechanical seal is a device which helps join systems or mechanisms together by preventing leakage , containing pressure, or excluding contamination...

 due to the solvents used in assembly. In spite of the above, this is a popular emerging technology with some commercial impact forecast within this decade. The first commercial shipment of DSSC solar modules occurred in July 2009 from G24i Innovations.

Organic/polymer solar cells


Organic solar cell
Organic solar cell
An organic photovoltaic cell is a photovoltaic cell that uses organic electronics--a branch of electronics that deals with conductive organic polymers or small organic molecules for light absorption and charge transport....

s are a relatively novel technology, yet hold the promise of a substantial price reduction (over thin-film silicon) and a faster return on investment. These cells can be processed from solution, hence the possibility of a simple roll-to-roll printing process, leading to inexpensive, large scale production.

Organic solar cells and polymer solar cell
Polymer solar cell
Polymer solar cells are a type of flexible solar cell. They can come in many forms including: organic solar cell , or organic chemistry photovoltaic cell that produce electricity from sunlight using polymers. There are also other types of more stable thin-film semiconductors that can be deposited...

s are built from thin films (typically 100 nm) of organic semiconductor
Organic semiconductor
An organic semiconductor is an organic material with semiconductor properties. Single molecules, short chain and organic polymers can be semiconductive. Semiconducting small molecules include the polycyclic aromatic compounds pentacene, anthracene, and rubrene...

s including polymers, such as polyphenylene vinylene and small-molecule compounds like copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes
Fullerene
A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...

 and fullerene derivatives such as PCBM
PCBM
PCBM is the common abbreviation for the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester. It is being investigated in organic solar cells.PCBM is a fullerene derivative of the C60 buckyball that was first synthesized in the 1990s...

. Energy conversion efficiencies achieved to date using conductive polymers are low compared to inorganic materials. However, it has improved quickly in the last few years and the highest NREL (National Renewable Energy Laboratory) certified efficiency has reached 8.3% for the Konarka Power Plastic. In addition, these cells could be beneficial for some applications where mechanical flexibility and disposability are important.

These devices differ from inorganic semiconductor solar cells in that they do not rely on the large built-in electric field of a PN junction to separate the electrons and holes created when photons are absorbed. The active region of an organic device consists of two materials, one which acts as an electron donor and the other as an acceptor. When a photon is converted into an electron hole pair, typically in the donor material, the charges tend to remain bound in the form of an exciton
Exciton
An exciton is a bound state of an electron and hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids...

, and are separated when the exciton diffuses to the donor-acceptor interface. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices. Nanostructured interfaces, sometimes in the form of bulk heterojunctions, can improve performance.

Silicon thin films


Silicon thin-film cells are mainly deposited by chemical vapor deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...

 (typically plasma-enhanced, PE-CVD) from silane
Silane
Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...

 gas and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 gas. Depending on the deposition parameters, this can yield:
  1. Amorphous silicon
    Amorphous silicon
    Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...

     (a-Si or a-Si:H)
  2. Protocrystalline
    Protocrystalline
    A protocrystalline phase is a distinct phase occurring during crystal growth which evolves into a microcrystalline form. The term is typically associated with silicon films in optical applications such as solar cells.-Silicon solar cells:...

     silicon or
  3. Nanocrystalline silicon
    Nanocrystalline silicon
    Nanocrystalline silicon , sometimes also known as microcrystalline silicon , is a form of porous silicon. It is an allotropic form of silicon with paracrystalline structure—is similar to amorphous silicon , in that it has an amorphous phase...

     (nc-Si or nc-Si:H), also called microcrystalline silicon.


It has been found that protocrystalline silicon with a low volume fraction of nanocrystalline silicon is optimal for high open circuit voltage. These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower energy conversion efficiency than bulk silicon, but are also less expensive to produce. The quantum efficiency
Quantum efficiency
Quantum efficiency is a quantity defined for a photosensitive device such as photographic film or a charge-coupled device as the percentage of photons hitting the photoreactive surface that will produce an electron–hole pair. It is an accurate measurement of the device's electrical sensitivity to...

 of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon.

An amorphous silicon (a-Si) solar cell is made of amorphous or microcrystalline silicon and its basic electronic structure is the p-i-n junction. a-Si is attractive as a solar cell material because it is abundant and non-toxic (unlike its CdTe counterpart) and requires a low processing temperature, enabling production of devices to occur on flexible and low-cost substrates. As the amorphous structure has a higher absorption rate of light than crystalline cells, the complete light spectrum can be absorbed with a very thin layer of photo-electrically active material. A film only 1 micron thick can absorb 90% of the usable solar energy. This reduced material requirement along with current technologies being capable of large-area deposition of a-Si, the scalability of this type of cell is high. However, because it is amorphous, it has high inherent disorder and dangling bonds, making it a bad conductor for charge carriers. These dangling bonds act as recombination centers that severely reduce the carrier lifetime and pin the Fermi energy level so that doping the material to n- or p- type is not possible. Amorphous Silicon also suffers from the Staebler-Wronski effect, which results in the efficiency of devices utilizing amorphous silicon dropping as the cell is exposed to light. The production of a-Si thin film solar cells uses glass as a substrate and deposits a very thin layer of silicon by plasma-enhanced chemical vapor deposition
Plasma-enhanced chemical vapor deposition
Plasma-enhanced chemical vapor deposition is a process used to deposit thin films from a gas state to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases...

 (PECVD). A-Si manufacturers are working towards lower costs per watt and higher conversion efficiency with continuous research and development on Multijunction solar cells for solar panels. Anwell Technologies Limited
Anwell Technologies Limited
Anwell Technologies Limited is a supplier of manufacturing equipment with trading and manufacturing business in optical disc and solar cell. They vertically integrated into manufacturing business of optical disc and solar cell in 2007 and 2008...

 recently announced its target for multi-substrate-multi-chamber PECVD, to lower the cost to US$0.5 per watt.

Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more strongly than the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the nc-Si and a-Si can advantageously be combined in thin layers, creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nc-Si.

Recently, solutions to overcome the limitations of thin-film crystalline silicon have been developed. Light trapping schemes where the weakly absorbed long wavelength light is obliquely coupled into the silicon and traverses the film several times can significantly enhance the absorption of sunlight in the thin silicon films. Minimizing the top contact coverage of the cell surface is another method for reducing optical losses; this approach simply aims at reducing the area that is covered over the cell to allow for maximum light input into the cell. Anti-reflective coatings can also be applied to create destructive interference within the cell. This can by done by modulating the Refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 of the surface coating; if destructive interference is achieved, there will be no reflective wave and thus all light will be transmitted into the semiconductor cell. Surface texturing is another option, but may be less viable because it also increases the manufacturing price. By applying a texture to the surface of the solar cell, the reflected light can be refracted into striking the surface again, thus reducing the overall light reflected out. Light trapping as another method allows for a decrease in overall thickness of the device; the path length that the light will travel is several times the actual device thickness. This can be achieved by adding a textured backreflector to the device as well as texturing the surface. If both front and rear surfaces of the device meet this criterion, the light will be 'trapped' by not having an immediate pathway out of the device due to internal reflections. Thermal processing techniques can significantly enhance the crystal quality of the silicon and thereby lead to higher efficiencies of the final solar cells. Further advancement into geometric considerations of building devices can exploit the dimensionality of nanomaterials. Creating large, parallel nanowire arrays enables long absorption lengths along the length of the wire while still maintaining short minority carrier diffusion lengths along the radial direction. Adding nanoparticles between the nanowires will allow for conduction through the device. Because of the natural geometry of these arrays, a textured surface will naturally form which allows for even more light to be trapped. A further advantage of this geometry is that these types of devices require about 100 times less material than conventional wafer-based devices.

Manufacture



Because solar cells are semiconductor devices, they share some of the same processing and manufacturing techniques as other semiconductor devices such as computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

 and memory
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....

 chips
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

. However, the stringent requirements for cleanliness and quality control of semiconductor fabrication are more relaxed for solar cells. Most large-scale commercial solar cell factories today make screen printed poly-crystalline or single crystalline silicon solar cells.

Poly-crystalline silicon wafers are made by wire-sawing block-cast silicon ingots into very thin (180 to 350 micrometer) slices or wafers. The wafers are usually lightly p-type
P-type semiconductor
A P-type semiconductor is obtained by carrying out a process of doping: that is, adding a certain type of atoms to the semiconductor in order to increase the number of free charge carriers ....

 doped. To make a solar cell from the wafer, a surface diffusion of n-type
N-type semiconductor
N-type semiconductors are a type of extrinsic semiconductor where the dopant atoms are capable of providing extra conduction electrons to the host material . This creates an excess of negative electron charge carriers....

 dopants is performed on the front side of the wafer. This forms a p-n junction a few hundred nanometers below the surface.

Anti-reflection coatings, to increase the amount of light coupled into the solar cell, are typically next applied. Silicon nitride has gradually replaced titanium dioxide as the anti-reflection coating, because of its excellent surface passivation qualities. It prevents carrier recombination at the surface of the solar cell. It is typically applied in a layer several hundred nanometers thick using plasma-enhanced chemical vapor deposition (PECVD). Some solar cells have textured front surfaces that, like anti-reflection coatings, serve to increase the amount of light coupled into the cell. Such surfaces can usually only be formed on single-crystal silicon, though in recent years methods of forming them on multicrystalline silicon have been developed.

The wafer then has a full area metal contact made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "bus bars" are screen-printed onto the front surface using a silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

 paste. The rear contact is also formed by screen-printing a metal paste, typically aluminium. Usually this contact covers the entire rear side of the cell, though in some cell designs it is printed in a grid pattern. The paste is then fired at several hundred degrees Celsius to form metal electrodes in ohmic contact
Ohmic contact
An ohmic contact is a region on a semiconductor device that has been prepared so that the current-voltage curve of the device is linear and symmetric. If the I-V characteristic is non-linear and asymmetric, the contact is not ohmic, but is a blocking or Schottky contact...

 with the silicon. Some companies use an additional electro-plating step to increase the cell efficiency. 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
Photovoltaic module
A solar panel is a packaged, connected assembly of solar cells, also known as photovoltaic cells...

 or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 encapsulation on the back.

Lifespan


Most commercially available solar panels are capable of producing electricity for at least twenty years. The typical warranty given by panel manufacturers is over 90% of rated output for the first 10 years, and over 80% for the second 10 years. Panels are expected to function for a period of 30 to 35 years.

Research topics



There are currently many research groups active in the field of photovoltaics in universities and research institutions around the world. This research can be divided into three areas: making current technology solar cells cheaper and/or more efficient to effectively compete with other energy sources; developing new technologies based on new solar cell architectural designs; and developing new materials to serve as light absorbers and charge carriers.

Manufacturers and certification


National Renewable Energy Laboratory
National Renewable Energy Laboratory
The National Renewable Energy Laboratory , located in Golden, Colorado, is the United States' primary laboratory for renewable energy and energy efficiency research and development. The National Renewable Energy Laboratory is a government-owned, contractor-operated facility; it is funded through...

 tests and validates solar technologies. There are three reliable certifications of solar equipment: UL
Underwriters Laboratories
Underwriters Laboratories Inc. is an independent product safety certification organization. Established in 1894, the company has its headquarters in Northbrook, Illinois. UL develops standards and test procedures for products, materials, components, assemblies, tools and equipment, chiefly dealing...

 and IEEE (both U.S. standards) and IEC
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...

.

Solar cells are manufactured primarily in Japan, Germany, mainland China, Taiwan and the United States, though numerous other nations have or are acquiring significant solar cell production capacity. While technologies are constantly evolving toward higher efficiencies, the most effective cells for low cost electrical production are not necessarily those with the highest efficiency, but those with a balance between low-cost production and efficiency high enough to minimize area-related balance of systems cost. Those companies with large scale manufacturing technology for coating inexpensive substrates may, in fact, ultimately be the lowest cost net electricity producers, even with cell efficiencies that are lower than those of single-crystal technologies.

China


Chinese companies have played an important role in solar panels manufacturing in recent years. China produced solar cells/modules with an output of 13 GW in 2010 which represents about half of the global production and makes China the largest producer in the world. Some Chinese companies such as Suntech Power
Suntech Power
Suntech Power Holdings Co., Ltd. is the world's largest producer of solar panels, with 1,800MW of annual production capacity by the end of 2010. With offices or production facilities in every major market, Suntech has delivered more than 13,000,000 solar panels to thousands of companies in more...

, Yingli
Yingli
Yingli also known as Yingli Green Energy Holding Company Limited , which holds the brand Yingli Solar, is a solar energy company and one of the largest vertically integrated manufacturers of photovoltaic solar modules...

, LDK Solar Co
LDK Solar Co
LDK Solar Co. Ltd. , located in Xinyu City, Jiangxi province in the People's Republic of China, manufactures multicrystalline solar wafers used in solar cells. Additionally, LDK provides wafering services for both monocrystalline and multicrystalline wafers to companies who provide their own ingot...

, JA Solar and ReneSola
ReneSola
ReneSola Ltd. is a Jiashan-headquartered manufacturer of solar wafers, which are thin sheets of crystalline silicon material primarily used in the production of solar cells....

 have already announced projects in cooperation with regional governments with hundreds of megawatts each after the "Golden Sun" incentive program was announced by the government.

United States


New manufacturing facilities for solar cells and modules in Massachusetts, Michigan, New York, Ohio, Oregon, and Texas promise to add enough capacity to produce thousands of megawatts of solar devices per year within the next few years from 2008.

In late September 2008, Sanyo
Sanyo
is a major electronics company and member of the Fortune 500 whose headquarters is located in Moriguchi, Osaka prefecture, Japan. Sanyo targets the middle of the market and has over 230 Subsidiaries and Affiliates....

 Electric Company, Ltd. announced its decision to build a manufacturing plant for solar ingot
Ingot
An ingot is a material, usually metal, that is cast into a shape suitable for further processing. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.-Uses:...

s and wafer
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...

s in Salem, Oregon. The plant began operating in October 2009 and reached its full production capacity of 70 megawatts (MW) of solar wafers per year in April 2010.

In early October 2008, First Solar, Inc. broke ground on an expansion of its Perrysburg, Ohio
Perrysburg, Ohio
As of the census of 2000, there were 16,945 people, 6,592 households, and 4,561 families residing in the city. The population density was 1,899.2 people per square mile . There were 6,964 housing units at an average density of 780.5 per square mile...

, facility that will add enough capacity to produce another 57 MW per year of solar modules at the facility, bringing its total capacity to roughly 192 MW per year. The company expects to complete construction early next year and reach full production by mid-2010.

In mid-October 2008, SolarWorld
SolarWorld
SolarWorld is a German company dedicated to the manufacture and marketing photovoltaic products worldwide by integrating all components of the solar value chain, from feedstock to module production, from trade with solar panels to the promotion and construction of turn-key solar power plants...

 AG opened a manufacturing plant in Hillsboro, Oregon
Hillsboro, Oregon
Hillsboro is the fifth-largest city in the U.S. state of Oregon and is the county seat of Washington County. Lying in the Tualatin Valley on the west side of the Portland metropolitan area, the city is home to many high-technology companies, such as Intel, that compose what has become known as the...

, that is producing 500 MW of solar cells per year as of 2011.

Solyndra
Solyndra
Solyndra was a manufacturer of cylindrical panels of CIGS thin-film solar cells based in Fremont, California. Although the company was once touted for its unusual technology, plummeting silicon prices led to the company being unable to compete with more conventional solar panels...

 had a manufacturing facility for its unique tubular CIGS technology in California. Solyndra closed its factory on August 31, 2011 and announced it would file for bankruptcy.

In March 2010, SpectraWatt, Inc. began production at its manufacturing plant in Hopewell Junction, NY, which was expected to produce 120 MW of solar cells per year when it reached full production in 2011. However, the closure of this plant was announced in late 2010 due to deteriorating market conditions coupled with demand drops from Europe. SpectraWatt filed for bankruptcy on August 24, 2011.

See also


  • Anomalous photovoltaic effect
    Anomalous photovoltaic effect
    The anomalous photovoltaic effect is a type of a photovoltaic effect which occurs in semiconducting materials. The "anomalous" refers to those cases where the photovoltage is larger than the band gap of the corresponding semiconductor....

  • Autonomous building
    Autonomous building
    An autonomous building is a building designed to be operated independently from infrastructural support services such as the electric power grid, gas grid, municipal water systems, sewage treatment systems, storm drains, communication services, and in some cases, public roads.Advocates of...

  • Black silicon
    Black silicon
    Black silicon is a semiconductor material, a surface modification of silicon with very low reflectivity and correspondingly high absorption of visible light. The modification was discovered in the 1980s as an unwanted side effect of reactive ion etching...

  • Energy development
    Energy development
    Energy development is the effort to provide sufficient primary energy sources and secondary energy forms for supply, cost, impact on air pollution and water pollution, mitigation of climate change with renewable energy....

  • Electromotive force (Solar cell)
  • Flexible substrate
  • Green technology
  • List of solar cell manufacturers
  • Metallurgical grade silicon
  • Microgeneration
    Microgeneration
    Microgeneration is the small-scale generation of heat and power by individuals, small businesses and communities to meet their own needs, as alternatives or supplements to traditional centralized grid-connected power...

  • Maximum power point tracking
  • Multijunction solar cell
  • Nanoflake
  • Photovoltaics
    Photovoltaics
    Photovoltaics is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material...

  • P-N junction
    P-n junction
    A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...

  • Plasmonic solar cell
    Plasmonic solar cell
    Plasmonic solar cells are a class of photovoltaic devices that convert light into electricity by using plasmons. PSCs are a type of thin-film SC which are typically 1-2μm thick. They can use substrates which are cheaper than silicon, such as glass, plastic or steel. The biggest problem for thin...

  • Printed electronics
    Printed electronics
    Printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment or other low-cost equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography and...



  • Quantum efficiency
    Quantum efficiency
    Quantum efficiency is a quantity defined for a photosensitive device such as photographic film or a charge-coupled device as the percentage of photons hitting the photoreactive surface that will produce an electron–hole pair. It is an accurate measurement of the device's electrical sensitivity to...

  • Quantum efficiency of a solar cell
    Quantum efficiency of a solar cell
    Quantum efficiency is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the solar cell. QE therefore relates to the response of a solar cell to the various wavelengths in the spectrum of light shining on the cell. The QE is...

  • Renewable energy
    Renewable energy
    Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable . About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from...

  • Roll-to-roll processing
    Roll-to-roll processing
    In the field of electronic devices, Roll-to-roll processing, also known as web processing, reel-to-reel processing or R2R, is the process of creating electronic devices on a roll of flexible plastic or metal foil...

  • Shockley-Queisser limit
    Shockley-Queisser limit
    In physics, the Shockley–Queisser limit or detailed balance limit refers to the maximum theoretical efficiency of a solar cell using a p-n junction to collect power from the cell. It was first calculated by William Shockley and Hans Queisser at Shockley Semiconductor in 1961...

  • Solar module quality assurance
    Solar module quality assurance
    Solar module quality assurance involves testing and evaluating solar cells to ensure the quality requirements of them are met. Solar modules are expected to have a long service life between 20 and 40 years. They must constantly and consistently deliver the power expected. In addition, modules are...

  • Solar roof
  • Solar shingles
    Solar shingles
    Solar shingles, also called photovoltaic shingles, are solar cells designed to look like conventional asphalt shingles. There are several varieties of solar shingles, including shingle-sized solid panels that take the place of a number of conventional shingles in a strip, semi-rigid designs...

  • Solar tracker
    Solar tracker
    A solar tracker is a generic term used to describe devices that orient various payloads toward the sun. Payloads can be photovoltaic panels, reflectors, lenses or other optical devices....

  • Spectrophotometry
    Spectrophotometry
    In chemistry, spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength...

  • Silicon thin-film cell
  • Theory of solar cell
    Theory of solar cell
    The theory of solar cells explains the physical processes by which light is converted into electrical current when striking a suitable semiconductor device...

  • Thermophotovoltaics
  • Thin film solar cell
    Thin film solar cell
    A thin-film solar cell , also called a thin-film photovoltaic cell , is a solar cell that is made by depositing one or more thin layers of photovoltaic material on a substrate...



External links