the basic elements of photovoltaics-the individual electricity-producing cell. The reader is told why PV cells work, and how they are made. There is also a chapter . into electricity. The solar cell is the elementary building block of the photovoltaic technology. Solar cells are made of semiconductor materials, such as silicon. Photovoltaic comes from the words photo meaning “light” and volt, a measurement of electricity. A slab (or wafer) of pure silicon is used to make a PV cell.

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Photovoltaic cell and module physics and technology. Vitezslav Benda, Prof. Czech Technical University in Prague [email protected] 1. PV Cell – Working Principle and Applications. 1/Dec/ Ir Dr Edward LO. Associate Professor. Department of Electrical Engineering. The Hong Kong. How a Photovoltaic Cell Works ƒ Step 1. A slab (or wafer) of pure silicon is used to make a PV cell. The top of the slab is very thinly diffused with an “n” dopant.

Proponents of solar hope to achieve grid parity first in areas with abundant sun and high electricity costs such as in California and Japan. George W. Bush set as the date for grid parity in the US. The recession of and the onset of Chinese manufacturing caused prices to resume their decline.

The second largest supplier, Canadian Solar Inc. Light transmits through transparent conducting electrode creating electron hole pairs, which are collected by both the electrodes. Once excited an electron can either dissipate the energy as heat and return to its orbital or travel through the cell until it reaches an electrode.

Current flows through the material to cancel the potential and this electricity is captured. The chemical bonds of the material are vital for this process to work, and usually silicon is used in two layers, one layer being doped with boron , the other phosphorus. These layers have different chemical electric charges and subsequently both drive and direct the current of electrons.

An inverter can convert the power to alternating current AC. The most commonly known solar cell is configured as a large-area p—n junction made from silicon. Other possible solar cell types are organic solar cells, dye sensitized solar cells, perovskite solar cells, quantum dot solar cells etc.

The illuminated side of a solar cell generally has a transparent conducting film for allowing light to enter into active material and to collect the generated charge carriers. Typically, films with high transmittance and high electrical conductance such as indium tin oxide , conducting polymers or conducting nanowire networks are used for the purpose.

Semiconductors with band gap between 1 and 1. The efficiency "limit" shown here can be exceeded by multijunction solar cells. Main article: Solar cell efficiency Solar cell efficiency may be broken down into reflectance efficiency, thermodynamic efficiency, charge carrier separation efficiency and conductive efficiency. The overall efficiency is the product of these individual metrics.

The power conversion efficiency of a solar cell is a parameter which is defined by the fraction of incident power converted into electricity. Due to the difficulty in measuring these parameters directly, other parameters are substituted: thermodynamic efficiency, quantum efficiency , integrated quantum efficiency , VOC ratio, and fill factor.

Reflectance losses are a portion of quantum efficiency under " external quantum efficiency ". Recombination losses make up another portion of quantum efficiency, VOC ratio, and fill factor. Resistive losses are predominantly categorized under fill factor, but also make up minor portions of quantum efficiency, VOC ratio. The fill factor is the ratio of the actual maximum obtainable power to the product of the open circuit voltage and short circuit current. This is a key parameter in evaluating performance.

Grade B cells were usually between 0. Single p—n junction crystalline silicon devices are now approaching the theoretical limiting power efficiency of Panasonic's was the most efficient.

Most designs sandwich active material between two panes of glass. Since silicon solar panels only use one pane of glass, thin film panels are approximately twice as heavy as crystalline silicon panels, although they have a smaller ecological impact determined from life cycle analysis. However cadmium is highly toxic and tellurium anion: The cadmium 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.

Copper indium gallium selenide CIGS is a direct band gap material. Traditional methods of fabrication involve vacuum processes including co-evaporation and sputtering. Recent developments at IBM and Nanosolar attempt to lower the cost by using non-vacuum solution processes.

Silicon thin-film cells are mainly deposited by chemical vapor deposition typically plasma-enhanced, PE-CVD from silane gas and hydrogen gas. Depending on the deposition parameters, this can yield amorphous silicon a-Si or a-Si: H , protocrystalline silicon or nanocrystalline silicon nc-Si or nc-Si: H , also called microcrystalline silicon. Amorphous silicon is the most well-developed thin film technology to-date.

Solar cell

An amorphous silicon a-Si solar cell is made of non-crystalline or microcrystalline silicon. Amorphous silicon has a higher bandgap 1. 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 PECVD. Protocrystalline silicon with a low volume fraction of nanocrystalline silicon is optimal for high open circuit voltage.

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. The semiconductor material Gallium arsenide GaAs is also used for single-crystalline thin film solar cells.

Solar cell

Although GaAs cells are very expensive, they hold the world's record in efficiency for a single-junction solar cell at Based on the previous literature and some theoretical analysis, there are several reasons why GaAs has such high power conversion efficiency. First, GaAs bandgap is 1. Second, because Gallium is a by-product of the smelting of other metals, GaAs cells are relatively insensitive to heat and it can keep high efficiency when temperature is quite high.

Third, GaAs has the wide range of design options. Using GaAs as active layer in solar cell, engineers can have multiple choices of other layers which can better generate electrons and holes in GaAs. Multi-junction cells consist of multiple thin films, each essentially a solar cell grown on top of another, typically using metalorganic vapour phase epitaxy.

Each layer has a different band gap energy to allow it to absorb electromagnetic radiation over a different portion of the spectrum. Multi-junction cells were originally developed for special applications such as satellites and space exploration , but are now used increasingly in terrestrial concentrator photovoltaics CPV , an emerging technology that uses lenses and curved mirrors to concentrate sunlight onto small, highly efficient multi-junction solar cells.

By concentrating sunlight up to a thousand times, High concentrated photovoltaics HCPV has the potential to outcompete conventional solar PV in the future.

Tandem solar cells based on monolithic, series connected, gallium indium phosphide GaInP , gallium arsenide GaAs , and germanium Ge p—n junctions, are increasing sales, despite cost pressures.

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. A triple-junction cell, for example, may consist of the semiconductors: In , a new approach was described for producing hybrid photovoltaic wafers combining the high efficiency of III-V multi-junction solar cells with the economies and wealth of experience associated with silicon.

The technical complications involved in growing the III-V material on silicon at the required high temperatures, a subject of study for some 30 years, are avoided by epitaxial growth of silicon on GaAs at low temperature by plasma-enhanced chemical vapor deposition PECVD. A dual-junction solar cell with a band gap of 1.

The two cells therefore are separated by a transparent glass slide so the lattice mismatch does not cause strain to the system. This creates a cell with four electrical contacts and two junctions that demonstrated an efficiency of However, using a GaAs substrate is expensive and not practical.

Hence researchers try to make a cell with two electrical contact points and one junction, which does not need a GaAs substrate. This means there will be direct integration of GaInP and Si. Perovskite solar cells are solar cells that include a perovskite -structured material as the active layer. Most commonly, this is a solution-processed hybrid organic-inorganic tin or lead halide based material.

So far most types of perovskite solar cells have not reached sufficient operational stability to be commercialised, although many research groups are investigating ways to solve this. With a transparent rear side, bifacial solar cells can absorb light from both the front and rear sides. Hence, they can produce more electricity than conventional monofacial solar cells. The first patent of bifacial solar cells was filed by Japanese researcher Hiroshi Mori, in Antonio Luque.

Based on US and Spanish patents by Luque, a practical bifacial cell was proposed with a front face as anode and a rear face as cathode; in previously reported proposals and attempts both faces were anodic and interconnection between cells was complicated and expensive. Due to the reduced manufacturing cost, companies have again started to produce commercial bifacial modules since By , there were at least eight certified PV manufacturers providing bifacial modules in North America.

Due to the significant interest in the bifacial technology, a recent study has investigated the performance and optimization of bifacial solar modules worldwide. Sun et al. An online simulation tool is available to model the performance of bifacial modules in any arbitrary location across the entire world. It can also optimize bifacial modules as a function of tilt angle, azimuth angle, and elevation above the ground. Intermediate band photovoltaics in solar cell research provides methods for exceeding the Shockley—Queisser limit on the efficiency of a cell.

It introduces an intermediate band IB energy level in between the valence and conduction bands. Theoretically, introducing an IB allows two photons with energy less than the bandgap to excite an electron from the valence band to the conduction band. This increases the induced photocurrent and thereby efficiency. Luque and Marti first derived a theoretical limit for an IB device with one midgap energy level using detailed balance.

They assumed no carriers were collected at the IB and that the device was under full concentration. They found the maximum efficiency to be In , researchers at California NanoSystems Institute discovered using kesterite and perovskite improved electric power conversion efficiency for solar cells. Photon upconversion is the process of using two low-energy e.

Either of these techniques could be used to produce higher efficiency solar cells by allowing solar photons to be more efficiently used. The difficulty, however, is that the conversion efficiency of existing phosphors exhibiting up- or down-conversion is low, and is typically narrow band.

Upconversion process occurs when two infrared photons are absorbed by rare-earth ions to generate a high-energy absorbable photon. As example, the energy transfer upconversion process ETU , consists in successive transfer processes between excited ions in the near infrared.

The upconverter material could be placed below the solar cell to absorb the infrared light that passes through the silicon. Useful ions are most commonly found in the trivalent state. The excited ion emits light above the Si bandgap that is absorbed by the solar cell and creates an additional electron—hole pair that can generate current. However, the increased efficiency was small.

Dye-sensitized solar cells DSSCs are made of low-cost materials and do not need elaborate manufacturing equipment, so they can be made in a DIY fashion.

In bulk it should be significantly less expensive than older solid-state cell designs.

Typically a ruthenium metalorganic dye Ru-centered is used as a monolayer of light-absorbing material. The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO 2 and the holes are absorbed by an electrolyte 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 more flexible use of materials and is typically manufactured by screen printing or ultrasonic nozzles , with the potential for lower processing costs than those used for bulk solar cells.

However, the dyes in these cells also suffer from degradation under heat and UV light and the cell casing is difficult to seal due to the solvents used in assembly. Quantum dot solar cells QDSCs are based on the Gratzel cell, or dye-sensitized solar cell architecture, but employ low band gap semiconductor nanoparticles , fabricated with crystallite sizes small enough to form quantum dots such as CdS , CdSe , Sb 2 S 3 , PbS , etc.

They also have high extinction coefficients and have shown the possibility of multiple exciton generation. This TiO 2 layer can then be made photoactive by coating with semiconductor quantum dots using chemical bath deposition , electrophoretic deposition or successive ionic layer adsorption and reaction. The electrical circuit is then completed through the use of a liquid or solid redox couple.

They can be processed from liquid solution, offering the possibility of a simple roll-to-roll printing process, potentially leading to inexpensive, large-scale production. In addition, these cells could be beneficial for some applications where mechanical flexibility and disposability are important. Current cell efficiencies are, however, very low, and practical devices are essentially non-existent. Energy conversion efficiencies achieved to date using conductive polymers are very low compared to inorganic materials.

However, Konarka Power Plastic reached efficiency of 8. The active region of an organic device consists of two materials, one electron donor and one electron 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 , separating when the exciton diffuses to the donor-acceptor interface, unlike most other solar cell types. 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.

They used block copolymers , self-assembling organic materials that arrange themselves into distinct layers. An adaptive material responds to the intensity and angle of incident light. At the part of the cell where the light is most intense, the cell surface changes from reflective to adaptive, allowing the light to penetrate the cell. The other parts of the cell remain reflective increasing the retention of the absorbed light within the cell. In , a system was developed that combined an adaptive surface with a glass substrate that redirect the absorbed to a light absorber on the edges of the sheet.

As the day continues, the concentrated light moves along the surface of the cell. That surface switches from reflective to adaptive when the light is most concentrated and back to reflective after the light moves along.

For the past years, researchers have been trying to reduce the price of solar cells while maximizing efficiency.

Thin-film solar cell is a cost-effective second generation solar cell with much reduced thickness at the expense of light absorption efficiency. Efforts to maximize light absorption efficiency with reduced thickness have been made. Surface texturing is one of techniques used to reduce optical losses to maximize light absorbed.

Currently, surface texturing techniques on silicon photovoltaics are drawing much attention. Surface texturing could be done in multiple ways. Etching single crystalline silicon substrate can produce randomly distributed square based pyramids on the surface using anisotropic etchants.

Multicrystalline silicon solar cells, due to poorer crystallographic quality, are less effective than single crystal solar cells, but mc-Si solar cells are still being used widely due to less manufacturing difficulties. It is reported that multicrystalline solar cells can be surface-textured to yield solar energy conversion efficiency comparable to that of monocrystalline silicon cells, through isotropic etching or photolithography techniques.

Rather some light rays are bounced back onto the other surface again due to the geometry of the surface. This process significantly improves light to electricity conversion efficiency, due to increased light absorption. This texture effect as well as the interaction with other interfaces in the PV module is a challenging optical simulation task.

Consequently, required thickness for solar cells decreases with the increased absorption of light rays. Solar cells are commonly encapsulated in a transparent polymeric resin to protect the delicate solar cell regions for coming into contact with moisture, dirt, ice, and other conditions expected either during operation or when used outdoors.

The encapsulants are commonly made from polyvinyl acetate or glass. Most encapsulants are uniform in structure and composition, which increases light collection owing to light trapping from total internal reflection of light within the resin. Research has been conducted into structuring the encapsulant to provide further collection of light.

Such encapsulants have included roughened glass surfaces, [] diffractive elements, [] prism arrays, [] air prisms, [] v-grooves, [] diffuse elements, as well as multi-directional waveguide arrays. Optical structures have also been created in encapsulation materials to effectively "cloak" the metallic front contacts. Solar cells share some of the same processing and manufacturing techniques as other semiconductor devices.

However, the stringent requirements for cleanliness and quality control of semiconductor fabrication are more relaxed for solar cells, lowering costs. Polycrystalline silicon wafers are made by wire-sawing block-cast silicon ingots into to micrometer wafers. The wafers are usually lightly p-type -doped.


A surface diffusion of n-type 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 are then typically applied to increase the amount of light coupled into the solar cell. Silicon nitride has gradually replaced titanium dioxide as the preferred material, because of its excellent surface passivation qualities. It prevents carrier recombination at the cell surface. Some solar cells have textured front surfaces that, like anti-reflection coatings, increase the amount of light reaching the wafer. Such surfaces were first applied to single-crystal silicon, followed by multicrystalline silicon somewhat later.

A full area metal contact is 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 paste. This is an evolution of the so-called "wet" process for applying electrodes, first described in a US patent filed in by Bayer AG. Usually this contact covers the entire rear, though some designs employ a grid pattern. The paste is then fired at several hundred degrees Celsius to form metal electrodes in ohmic contact with the silicon.

Some companies use an additional electro-plating step to increase efficiency. After the metal contacts are made, the solar cells are interconnected by flat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back. National Renewable Energy Laboratory tests and validates solar technologies. Three reliable groups certify solar equipment: Between and cell production has quadrupled.

Due to heavy government investment, China has become the dominant force in solar cell manufacturing. In , Malaysia was the world's third largest manufacturer of photovoltaics equipment, behind China and the European Union. Solar cell production in the U. Solar cells degrade over time and lose their efficiency. Solar cells in extreme climates, such as desert or polar, are more prone to degradation due to exposure to harsh UV light and snow loads respectively. The International Renewable Energy Agency estimated that the amount of solar panel waste generated in was 43,—, metric tons.

This number is estimated to increase substantially by , reaching an estimated waste volume of 60—78 million metric tons in In , most decommissioned solar panels were sent to landfills.

Recycling is limited because it is too expensive to process the low volume of solar panel waste. With the volume of solar panel waste set to increase, the safety of disposing solar panels in landfills is becoming a big concern. Many manufacturers are turning to recycling solar panels instead. The first solar panel recycling plant opened in Rousset, France in It was set to recycle tonnes of solar panel waste a year, and can increase its capacity to tonnes.

From Wikipedia, the free encyclopedia. For convection cells on the sun's surface, see Granule solar physics. See also: Main article: Photovoltaic system. Timeline of solar cells. Price per watt history for conventional c-Si solar cells since Swanson's law — the learning curve of solar PV. Growth of photovoltaics — Worldwide total installed PV capacity. Theory of solar cells. Solar cell efficiency.

Crystalline silicon. Monocrystalline silicon. Polycrystalline silicon. Thin-film solar cell. Cadmium telluride photovoltaics. Copper indium gallium selenide solar cell. Multi-junction solar cell. Perovskite solar cell. Intermediate band photovoltaics. Dye-sensitized solar cells. Quantum dot solar cell. Main articles: Organic solar cell and Polymer solar cell.

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Archived PDF from the original on 7 October Retrieved 7 October Archived PDF from the original on 29 March Sustainable energy systems engineering: McGraw Hill Professional. Patent 2,, Issue date: APS News. American Physical Society. April Cambridge University Press. Chasing the Sun: Solar Adventures Around the World. New Society Publishers. Harvard Business School. Background events and photovoltaic technology status". AIP Conference Proceedings. The multinational connections-who does what where.

Reed Business Information. Let the fun begin. Alternative energy will no longer be alternative". The Economist. Retrieved 28 December Does the Punishment Fit the Crime? Retrieved 3 January Clean Technica. Retrieved 18 May Archived from the original on 30 March Nano Today. Progress in Photovoltaics: Research and Applications. Archived from the original on 8 June Retrieved 4 August Retrieved 19 January August Challenges and Opportunities.

Retrieved 1 June Retrieved 20 April Bush set as the date for grid parity in the US. Following the oil crisis , oil companies used their higher profits to start or download solar firms, and were for decades the largest producers. Tandem solar cells based on monolithic, series connected, gallium indium phosphide GaInP , gallium arsenide GaAs , and germanium Ge p—n junctions, are increasing sales, despite cost pressures.

Some cells are designed to handle sunlight that reaches the Earth's surface, while others are optimized for use in space. In , a new approach was described for producing hybrid photovoltaic wafers combining the high efficiency of III-V multi-junction solar cells with the economies and wealth of experience associated with silicon. Retrieved 17 May Efforts to maximize light absorption efficiency with reduced thickness have been made.