Photoelectric cell
A photoelectric cell, also called a solar cell, solar cell, photocell or photovoltaic cell, is an electronic device that transforms light energy (photons) into electrical energy (flow of free electrons) through the photoelectric effect, generating photovoltaic solar energy. Composed of a material that has a photoelectric effect: it absorbs photons of light and emits electrons. When these free electrons are captured, the result is an electrical current that can be used as electricity.
The average conversion efficiency obtained by commercially available cells (produced from monocrystalline silicon) is around 16%, but depending on the technology used, it varies from 6% for amorphous silicon cells to 22% for amorphous silicon cells. monocrystalline silicon cells. There are also multilayer cells, usually made of gallium arsenide, which reach an efficiency of 30%. In the laboratory, 46% has been exceeded with experimental cells.
The average useful life at maximum performance is around 25 years, a period from which the delivered power decreases below a considerable value.
The group of photoelectric cells for solar energy is known as a photovoltaic panel. Photovoltaic panels are a network of solar cells connected as a series circuit to increase the output voltage to the desired value (usually 12 V or 24 V are used) while several networks are connected as a parallel circuit to increase the electric current that the device is capable of providing.
The type of electric current they provide is direct current, but you can use an inverter if you need alternating current, and a power converter if you want to increase its voltage.
History
The photovoltaic effect was first demonstrated experimentally by the French physicist Edmond Becquerel. In 1839, at the age of 19, he built the world's first photovoltaic cell in his father's laboratory. Willoughby Smith first described the "Effect of Light on Selenium during the passage of an Electric Current" in the February 20, 1873 issue of Nature. In 1883 Charles Fritts built the first solid-state photovoltaic cell by coating the semiconductor selenium with a thin layer of gold to form the junctions; the device was only about 1% efficient.
In 1888 the Russian physicist Aleksandr Stoletov built the first cell based on the external photoelectric effect discovered by Heinrich Hertz in 1887.
In 1905 Albert Einstein proposed a new quantum theory of light and explained the photoelectric effect in a landmark paper, for which he received the Nobel Prize in Physics in 1921.
Vadim Lashkaryov discovered unions p-n in Cu2{displaystyle}Or and silver sulfur protocells in 1941.
Russell Ohl patented the modern semiconductor junction solar cell in 1946 while working on the series of advances that would lead to the transistor.
The first practical photovoltaic cell was publicly displayed on April 25, 1954 at Bell Laboratories. The inventors were Daryl Chapin, Calvin Souther Fuller, and Gerald Pearson.
Solar cells gained notoriety with their incorporation into the Vanguard I artificial satellite in 1958, and their subsequent use in more advanced satellites, during the 1960s.
Improvements were gradual over the next two decades. However, this success was also the reason that costs remained high, because users of space applications were willing to pay for the best possible cells, without having any reason to invest in lower cost, less efficient solutions. The price was largely determined by the semiconductor industry; their move to integrated circuits in the 1960s led to the availability of larger ingots at relatively lower prices. As its price fell, the price of the resulting cells did too. These effects brought costs down in 1971 to about $100 per watt.
Principle of operation
In a semiconductor exposed to light, a photon of energy knocks out an electron, simultaneously creating a "hole" in the excited atom. Normally, the electron quickly finds another hole to refill, and the energy provided by the photon is therefore dissipated as heat. The principle of a photovoltaic cell is to force the electrons and the "holes" to move towards the opposite side of the material instead of simply recombining in it: thus, a potential difference and therefore voltage will be produced between the two parts of the material, as occurs in a pile.
To do this, a permanent electric field is created, through a pn junction, between two doped layers respectively, p and n. In silicon cells, which are mostly used, there are therefore:
- The upper layer of the cell, which consists of silicon doped type n. In this layer, there is a number of free electrons greater than in a layer of pure silicon, hence the name of doping n, negative. The material remains electrically neutral, since both silicon atoms and doping material are neutral: but the crystalline network has a global greater presence of electrons than in a pure silicon network.
- The lower layer of the cell, which is composed of type p doped silicon. This layer therefore has an average amount of free electrons less than a layer of pure silicon. The electrons are linked to the crystalline network which, consequently, is electrically neutral but presents hollowspositive (p). The electrical conduction is ensured by these load carriers, which move throughout the material.
At the time of creation of the pn junction, free electrons from the n shell instantly enter the p shell and recombine with holes in the p region. Thus, during the whole life of the union, there will be a positive charge in the region n along the union (because there are missing electrons) and a negative charge in the region at p a along the join (because the gaps have disappeared); the set forms the «Space Charge Zone» (ZCE) or «barrier zone» and there is an electric field between the two, from n to p. This electric field makes the ZCE a diode, which only allows the flow of carriers in one direction: in the absence of an external current source and under the sole influence of the field generated in the ZCE, electrons can only move from the p region to the p region. n, but not in the opposite direction and on the contrary the holes do not pass more than from n to p.
In operation, when a photon tears an electron from the matrix, creating a free electron and a hole, under the effect of this electric field each one goes in the opposite direction: the electrons accumulate in the n region (to become the negative pole), while the holes accumulate in the p-doped region (which becomes the positive pole). This phenomenon is most effective in the ZCE, where there are almost no charge carriers (electrons or holes), since they are cancelled, or in the immediate vicinity of the ZCE: when a photon creates an electron pair -hollow, they split apart and are unlikely to find their opposite, but if creation takes place at a site further from the junction, the electron (turned hole) maintains a great opportunity to recombine sooner to reach zone no. But the ZCE is necessarily very thin, so it is not useful to give a large thickness to the cell. Indeed, the thickness of the n layer is very small, since this layer is only basically needed to create the ZCE that runs the cell. cell. On the other hand, the thickness of the p layer is greater: it depends on a compromise between the need to minimize electron-hole recombinations, and, on the contrary, allow the capture of the greatest possible number of photons, for which a certain minimum thickness is required.
In short, a photovoltaic cell is the equivalent of a power generator to which a diode has been added. To achieve a practical solar cell, it is also necessary to add electrical contacts (which allow the energy generated to be extracted), a layer that protects the cell but allows light to pass through, an anti-reflective layer to guarantee the correct absorption of photons, and other elements that increase its efficiency.
Manufacturing technique
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Silicon is currently the most commonly used material for the manufacture of photovoltaic cells. It is obtained by reduction of silica, the most abundant compound in the Earth's crust, particularly in sand or quartz.
The first step is the production of metallurgical silicon, 98% pure, obtained from quartz stones from a mineral vein (the industrial production technique does not start from sand). Silicon is purified by chemical procedures (Washing + Pickling) frequently using distillations of chlorinated silicon compounds, until the concentration of impurities is less than 0.2 parts per million. This is how semiconductor silicon is obtained with a degree of purity higher than that required for the generation of photovoltaic solar energy. This has constituted the base of the supply of raw material for solar applications to date, currently representing almost three quarters of the supply of the industries.
However, for specifically solar uses, concentrations of impurities of the order of one part per million are sufficient (depending on the type of impurity and the crystallization technique). Material of this concentration is often referred to as solar-grade silicon.
With the molten silicon, a crystalline growth process is carried out that consists of forming monomolecular layers around a seed of crystallization or an initial crystallite. New molecules preferentially adhere to the face where their adhesion releases more energy. The energetic differences are usually small and can be modified by the presence of said impurities or by changing the crystallization conditions. The seed or germ of crystallization that causes this phenomenon is extracted from the molten silicon, which solidifies in a crystalline form, resulting, if time is sufficient, a monocrystal and if less, a polycrystal. The temperature at which this process is carried out is higher than 1500 °C.
The procedure most widely used today is the Czochralski Process, and casting techniques can also be used. The crystalline silicon thus obtained is in the form of ingots.
These ingots are then cut into thin square sheets (if necessary) 200 micrometers thick, which are called "wafers." After the treatment for the injection of the enriched with dopant (P, As, Sb or B) and thus obtain the P or N type silicon semiconductors.
After cutting the wafers, they show surface irregularities and cutting defects, as well as the possibility that they are dirty with dust or shavings from the manufacturing process. This situation can considerably reduce the performance of the photovoltaic panel, so a set of processes are carried out to improve the surface conditions of the wafers, such as preliminary washing, ultrasonic defect removal, pickling, polishing or cleaning with chemical products.. For the higher quality cells (single crystal) a texturing treatment is carried out to make the wafer absorb incident solar radiation more efficiently.
Subsequently, the wafers are "metalized", a process that consists of placing metal strips embedded in the surface connected to electrical contacts that absorb the electrical energy generated by the P/N junctions due to solar irradiation and transmit it.
The production of photovoltaic cells requires energy, and it is estimated that a photovoltaic module must work around 2 to 3 years according to its technology to produce the energy that was necessary for its production (energy return module).
The manufacturing techniques and characteristics of the main cell types are described in the following 3 paragraphs. There are other types of cells that are being studied, but their use is almost negligible.
Materials and manufacturing processes are the subject of ambitious research programs to reduce the cost and recycling of photovoltaic cells. Thin film technologies on unmarked substrates have received more modern industry acceptance. In 2006 and 2007, growth in global solar panel production has been hampered by a lack of silicon cells, and prices have not fallen as much as expected. The industry seeks to reduce the amount of silicon used. Monocrystalline cells have gone from 300 microns thick to 200 and it is thought that they will quickly reach 180 and 150 microns, reducing the amount of silicon and energy required, as well as the price.
Amorphous silicon cells
Silicon during its transformation produces a gas that is projected onto a glass sheet. The cell is very dark gray. It is the cell of calculators and watches called "solar".
These cells were the first to be manufactured, since they could use the same manufacturing methods as diodes.
- Advantages:
- It works with a low diffuse light (even in cloudy days),
- A little less expensive than other technologies,
- Integration on flexible or rigid support.
- Inconvenient:
- Low sun performance, 5 % to 7 %,
- Decreasing performance over time (~7 %).
Monocrystalline silicon cell
When the molten silicon cools, it solidifies into a single large crystal. The glass is then cut into thin layers that give rise to cells. These cells are usually a uniform blue.
- Advantages:
- Good performance from 14 % to 16 %
- Good power-surface ratio (~150 Wp/m2, which saves space if necessary)
- Number of high manufacturers.
- Inconvenient:
- Higher cost
Polycrystalline silicon cells
During the cooling of silicon in a mold, several crystals are formed. The photocell has a bluish appearance, but it is not uniform, different colors created by the different crystals can be distinguished.
- Advantages:
- Square cells (with rounded edges in the case of Si monocristalino) that allows better operation in a module,
- Optimal conversion efficiency, about 100 Wp/m2, but slightly lower than in the monocrystalline
- Cheaper to produce than the monocrystalline.
- Performance:14 %
- Inconvenient
- Low performance in low lighting conditions.
Polycrystalline or multicrystalline? We are talking here about multicrystalline silicon (ref. IEC TS 61836, international photovoltaic vocabulary). The term polycrystalline is used for the layers deposited on a substrate (small grains).
Tandem cell
Monolithic stacking of two individual cells. By combining two cells (amorphous silicon thin layer on crystalline silicon, for example) that absorb in the spectrum at the same time overlap, improving performance compared to separate individual cells, whether amorphous, crystalline or microcrystalline.
- Advantages
- High sensitivity in a wide range of wavelengths. Excellent performance.
- Inconvenient
- The cost is high due to the overlap of two cells.
Multijunction cell
These cells are highly efficient and have been developed for space applications. Multijunction cells are composed of several thin layers using molecular beam epitaxy.
A triple junction cell, for example, is made up of GaAs, Ge and GaInP2 semiconductors. Each type of semiconductor is characterized by a maximum wavelength beyond which it is not capable of converting photons into electrical energy (see bandgap). On the other hand, below this wavelength, the excess energy carried by the photon is lost. Hence the value of selecting materials with wavelengths as close to each other as possible, so that they absorb most of the solar spectrum, generating a maximum of electricity from the solar flux. The use of quantum box composite materials will allow reaching 65% in the future (with a theoretical maximum of 87%). GaAs multi-junction cell devices are more efficient. Spectrolab has achieved 40.7% efficiency (Dec 2006) and a consortium (led by University of Delaware researchers) has achieved 42.8% efficiency(Sep 2007). The cost of these cells is approximately $40/cm².
The FBI Semiconductor
The technique consists of depositing a semiconductor material containing copper, gallium, indium and selenium on a support.
One concern, however: raw material resources. These new techniques use rare metals, such as indium, whose world production is 25 tons per year and the price as of April 2007 is $1,000 per kg; tellurium, whose world production is 250 tons per year; gallium with a production of 55 tons per year and germanium with a production of 90 tons per year. Although the amounts of these raw materials required for the manufacture of solar cells are infinitesimal, a massive development of solar photovoltaic panels should take into account this limited availability.
Use
Photovoltaic cells are sometimes used alone (garden lighting, calculators, etc.) or grouped together in photovoltaic solar panels.
Used to replace batteries (whose energy is by far the most expensive for the user), cells have invaded calculators, watches, gadgets, etc.
It is possible to increase its range of use by storing it with a capacitor or batteries. When used with a device to store energy, it is necessary to place a diode in series to avoid discharge of the system during the night.
They are used to produce electricity for many applications (satellites, parking meters, etc.) and to power homes or in a public network in the case of a photovoltaic solar power plant.
The three generations of photoelectric cells
Photoelectric cells are classified into three generations that indicate the order of importance and relevance they have had historically. Currently there is research in all three generations while first generation technologies are the most represented in commercial production with 89.6% of production in 2007.
First generation
First generation cells have large surface area, high quality, and can be easily joined together. The technologies of the first generation no longer allow significant advances in the reduction of production costs. Devices formed by the union of silicon cells are approaching the limit of theoretical efficiency which is 31% and have a payback period of 5-7 years.
Second generation
Second generation materials have been developed to meet the needs of power supply and maintenance of production costs of solar cells. Alternative manufacturing techniques, such as chemical vapor deposition, and electroplating have further advantages, as they reduce the process temperature significantly.
One of the most successful materials in the second generation has been cadmium telluride (CdTe), CIGS, amorphous silicon and microamorphous silicon thin films (the latter consisting of a layer of amorphous and microcrystalline silicon). These materials are applied in a thin film to a supporting substrate such as glass or ceramic, reducing material and therefore cost significantly. These technologies promise to increase conversion efficiencies, in particular CIGS-CIS, DSC and CdTe offering significantly cheaper production costs. These technologies can have higher conversion efficiencies combined with much cheaper production costs.
Among manufacturers, there is a trend toward second generation technologies, but commercializing these technologies has been difficult. In 2007, First Solar produced 200 MW of CdTe photoelectric cells, the fifth largest manufacturer of cells in 2007. Wurth Solar commercialized its CIGS technology in 2007 producing 15 MW. Nanosolar commercialized its CIGS technology in 2007 and with a production capacity of 430 MW by 2008 in the US and Germany. Honda Soltec also began commercializing its CIGS solar panel base in 2008.
In 2007, CdTe production accounted for 4.7% of the market, thin film silicon 5.2%, and CIGS 0.5%.
Third generation
Third-generation solar cells are those that allow theoretical electrical conversion efficiencies much higher than current ones and at a much lower production price. Current research is directed at conversion efficiencies of 30–60%, while keeping materials and manufacturing techniques low cost. The theoretical limit of solar energy conversion efficiency for a single material, which was calculated, may be exceeded. in 1961 by Shockley and Queisser at 31%. Various methods exist to achieve this high efficiency including the use of multijunction photovoltaic cells, incident spectrum concentration, the use of thermal generation by ultraviolet light to increase voltage, or the use of the infrared spectrum for nocturnal activity.
Efficiency
The record for efficiency, without concentrating solar, is currently set at 45%. In concentrating, the Massachusetts Institute of Technology is testing solar cells that can exceed 80% efficiency and are composed of a layer of nanotubes of carbon with photonic crystals, to create an "absorber-emitter".