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thumb|Photovoltaic SUDI shade is an autonomous and mobile station in France that provides energy for electric cars using solar energy. Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially utilized for electricity generation and as photosensors. A photovoltaic system employs solar modules, each comprising a number of solar cells, which generate electrical power. PV installations may be ground-mounted, rooftop-mounted, wall-mounted or floating. The mount may be fixed or use a solar tracker to follow the sun across the sky. PV has become the cheapest source of electrical power in regions with a high solar potential, with a bid for pricing as low as 0.01567 US$/kWh in Qatar in 2020. Panel prices have dropped by the factor of 4 between 2004 and 2011. This competitiveness opens the path to a global transition to sustainable energy, which would be required to help mitigate global warming. The emissions budget for to meet the 1.5 degree target would be used up in 2028 if emissions remain on the current level. However, the use of PV as a main source requires energy storage systems or global distribution by high-voltage direct current power lines causing additional costs, as well as a number of other specific disadvantages such as unstable power generation and the requirement for power companies to compensate for too much solar power in the supply mix by having more reliable conventional power supplies in order to regulate demand peaks and potential undersupply. Solar PV has specific advantages as an energy source: once installed, its operation generates no pollution and no greenhouse gas emissions, it shows simple scalability in respect of power needs and silicon has large availability in the Earth's crust, but other materials required in PV system manufacture such as silver will eventually constrain further growth in the technology. Other major constraints identified are competition for land use and lack of labor in making funding applications. Production and installation does cause pollution and greenhouse gas emissions and there are no viable systems for recycling the panels once they are at the end of their lifespan after 10 to 30 years. Photovoltaic systems have long been used in specialized applications as stand-alone installations and grid-connected PV systems have been in use since the 1990s. Photovoltaic modules were first mass-produced in 2000, when German environmentalists and the Eurosolar organization received government funding for a ten thousand roof program. Advances in technology and increased manufacturing scale have in any case reduced the cost, increased the reliability, and increased the efficiency of photovoltaic installations. Net metering and financial incentives, such as preferential feed-in tariffs for solar-generated electricity, have supported solar PV installations in many countries. More than 100 countries now use solar PV. In 2019, worldwide installed PV capacity increased to more than 635 gigawatts (GW) covering approximately two percent of global electricity demand. After hydro and wind powers, PV is the third renewable energy source in terms of global capacity. The International Energy Agency expects a growth by 700 - 880 GW from 2019 to 2024. In 2020, a rooftop photovoltaic system recoups the energy needed to manufacture it in 1.28 years in Ottawa, Canada, 0.97 years in Catania, Italy, and 0.4 years in Jaipur, India. PV has grown as an energy source primarily as a result of technological development delivering decreasing costs. Etymology The term "photovoltaic" comes from the Greek φῶς (''phōs'') meaning "light", and from "volt", the unit of electromotive force, the volt, which in turn comes from the last name of the Italian physicist Alessandro Volta, inventor of the battery (electrochemical cell). The term "photovoltaic" has been in use in English since 1849. Solar cells Photovoltaics are best known as a method for generating electric power by using solar cells to convert energy from the sun into a flow of electrons by the photovoltaic effect. Solar cells produce direct current electricity from sunlight which can be used to power equipment or to recharge a battery. The first practical application of photovoltaics was to power orbiting satellites and other spacecraft, but today the majority of photovoltaic modules are used for grid-connected systems for power generation. In this case an inverter is required to convert the DC to AC. There is still a smaller market for stand alone systems for remote dwellings, boats, recreational vehicles, electric cars, roadside emergency telephones, remote sensing, and cathodic protection of pipelines. Photovoltaic power generation employs solar modules composed of a number of solar cells containing a semiconductor material. Copper solar cables connect modules (module cable), arrays (array cable), and sub-fields. Because of the growing demand for renewable energy sources, the manufacturing of solar cells and photovoltaic arrays has advanced considerably in recent years.German PV market Solarbuzz.com. Retrieved on 3 June 2012. BP Solar to Expand Its Solar Cell Plants in Spain and India . Renewableenergyaccess.com. 23 March 2007. Retrieved on 3 June 2012. Bullis, Kevin (23 June 2006) Large-Scale, Cheap Solar Electricity Technologyreview.com. Retrieved on 3 June 2012. Cells require protection from the environment and are usually packaged tightly in solar modules. Photovoltaic module power is measured under standard test conditions (STC) in "Wp" (watts peak). The actual power output at a particular place may be less than or greater than this rated value, depending on geographical location, time of day, weather conditions, and other factors. Solar photovoltaic array capacity factors are typically under 25%, which is lower than many other industrial sources of electricity. Manufacturing Overall the manufacturing process of creating solar photovoltaics is simple in that it does not require the culmination of many complex or moving parts. Because of the solid state nature of PV systems they often have relatively long lifetimes, anywhere from 10 to 30 years. To increase electrical output of a PV system, the manufacturer must simply add more photovoltaic components and because of this economies of scale are important for manufacturers as costs decrease with increasing output. While there are many types of PV systems known to be effective, crystalline silicon PV accounted for around 90% of the worldwide production of PV in 2013. Manufacturing silicon PV systems has several steps. First, polysilicon is processed from mined quartz until it is very pure (semi-conductor grade). This is melted down when small amounts of boron, a group III element, are added to make a p-type semiconductor rich in electron holes. Typically using a seed crystal, an ingot of this solution is grown from the liquid polycrystalline. The ingot may also be cast in a mold. Wafers of this semiconductor material are cut from the bulk material with wire saws, and then go through surface etching before being cleaned. Next, the wafers are placed into a phosphorus vapor deposition furnace which lays a very thin layer of phosphorus, a group V element, which creates an n-type semiconducting surface. To reduce energy losses, an anti-reflective coating is added to the surface, along with electrical contacts. After finishing the cell, cells are connected via electrical circuit according to the specific application and prepared for shipping and installation. Crystalline silicon photovoltaics are only one type of PV, and while they represent the majority of solar cells produced currently there are many new and promising technologies that have the potential to be scaled up to meet future energy needs. As of 2018, crystalline silicon cell technology serves as the basis for several PV module types, including monocrystalline, multicrystalline, mono PERC, and bifacial. Another newer technology, thin-film PV, are manufactured by depositing semiconducting layers of perovskite, a mineral with semiconductor properties, on a substrate in vacuum. The substrate is often glass or stainless-steel, and these semiconducting layers are made of many types of materials including cadmium telluride (CdTe), copper indium diselenide (CIS), copper indium gallium diselenide (CIGS), and amorphous silicon (a-Si). After being deposited onto the substrate the semiconducting layers are separated and connected by electrical circuit by laser scribing. Perovskite solar cells are a very efficient solar energy converter and have excellent optoelectronic properties for photovoltaic purposes, but their upscaling from lab-sized cells to large-area modules is still under research. Thin-film photovoltaic materials may possibly become attractive in the future, because of the reduced materials requirements and cost to manufacture modules consisting of thin-films as compared to silicon-based wafers. In 2019 university labs at Oxford, Stanford and elsewhere reported perovskite solar cells with efficiencies of 20-25%.Best Research Cell Efficiences nrel.gov (16 September 2019). Retrieved on 31 October 2019. Other possible future PV technologies include organic, dye-sensitized and quantum-dot photovoltaics. Organic photovoltaics (OPVs) fall into the thin-film category of manufacturing, and typically operate around the 12% efficiency range which is lower than the 12–21% typically seen by silicon based PVs. Because organic photovoltaics require very high purity and are relatively reactive they must be encapsulated which vastly increases cost of manufacturing and meaning that they are not feasible for large scale up. Dye-sensitized PVs are similar in efficiency to OPVs but are significantly easier to manufacture. However these dye-sensitized photovoltaics present storage problems because the liquid electrolyte is toxic and can potentially permeate the plastics used in the cell. Quantum dot solar cells are solution processed, meaning they are potentially scalable, but currently they peak at 12% efficiency. Economics There have been major changes in the underlying costs, industry structure and market prices of solar photovoltaics technology, over the years, and gaining a coherent picture of the shifts occurring across the industry value chain globally is a challenge. This is due to: "the rapidity of cost and price changes, the complexity of the PV supply chain, which involves a large number of manufacturing processes, the balance of system (BOS) and installation costs associated with complete PV systems, the choice of different distribution channels, and differences between regional markets within which PV is being deployed". Further complexities result from the many different policy support initiatives that have been put in place to facilitate photovoltaics commercialisation in various countries. Renewable energy technologies have generally gotten cheaper since their invention. Hardware costs In 1977 crystalline silicon solar cell prices were at$76.67/W. Although wholesale module prices remained flat at around $3.50 to$4.00/W in the early 2000s due to high demand in Germany and Spain afforded by generous subsidies and shortage of polysilicon, demand crashed with the abrupt ending of Spanish subsidies after the market crash of 2008, and the price dropped rapidly to $2.00/W. Manufacturers were able to maintain a positive operating margin despite a 50% drop in income due to innovation and reductions in costs. In late 2011, factory-gate prices for crystalline-silicon photovoltaic modules suddenly dropped below the$1.00/W mark, taking many in the industry by surprise, and has caused a number of solar manufacturing companies to go bankrupt throughout the world. The $1.00/W cost is often regarded in the PV industry as marking the achievement of grid parity for PV, but most experts do not believe this price point is sustainable. Technological advancements, manufacturing process improvements, and industry re-structuring, may mean that further price reductions are possible. The average retail price of solar cells as monitored by the Solarbuzz group fell from$3.50/watt to $2.43/watt over the course of 2011. In 2013 wholesale prices had fallen to$0.74/W. This has been cited as evidence supporting Swanson's law, an observation similar to the famous Moore's Law that states that solar cell prices fall 20% for every doubling of industry capacity. Note that the prices mentioned above are for bare modules, another way of looking at module prices is to include installation costs. In the US, according to the Solar Energy Industries Association, the price of installed rooftop PV modules for homeowners fell from $9.00/W in 2006 to$5.46/W in 2011. Including the prices paid by industrial installations, the national installed price drops to $3.45/W. This is markedly higher than elsewhere in the world, in Germany homeowner rooftop installations averaged at$2.24/W. The cost differences are thought to be primarily based on the higher regulatory burden and lack of a national solar policy in the USA. By the end of 2012 Chinese manufacturers had production costs of $0.50/W in the cheapest modules. In some markets distributors of these modules can earn a considerable margin, buying at factory-gate price and selling at the highest price the market can support ('value-based pricing'). Levelised cost of electricity The levelised cost of electricity (LCOE) is the cost per kWh based on the costs distributed over the project lifetime, and is thought to be a better metric for calculating viability than price per wattage. LCOEs vary dramatically depending on the location. The LCOE can be considered the minimum price customers will have to pay the utility company in order for it to break even on the investment in a new power station. Grid parity is roughly achieved when the LCOE falls to a similar price as conventional local grid prices, although in actuality the calculations are not directly comparable. Large industrial PV installations had reached grid parity in California in 2011. Grid parity for rooftop systems was still believed to be much farther away at this time. Many LCOE calculations are not thought to be accurate, and a large amount of assumptions are required. Module prices may drop further, and the LCOE for solar may correspondingly drop in the future. Because energy demands rise and fall over the course of the day, and solar power is limited by the fact that the sun sets, solar power companies must also factor in the additional costs of supplying a more stable alternative energy supplies to the grid in order to stabilize the system, or storing the energy somehow (current battery technology cannot store enough power). These costs are not factored into LCOE calculations, nor are special subsidies or premiums that may make buying solar power more attractive. The unreliability and temporal variation in generation of solar and wind power is a major problem. Too much of these volatile power sources can cause instability of the entire grid. As of 2017 power-purchase agreement prices for solar farms below$0.05/kWh are common in the United States, and the lowest bids in some Persian Gulf countries were about $0.03/kWh. The goal of the United States Department of Energy is to achieve a levelised cost of energy for solar PV of$0.03/kWh for utility companies.

Subsidies and financing

Financial incentives for photovoltaics, such as feed-in tariffs (FITs), are often been offered to electricity consumers to install and operate solar-electric generating systems, and in some countries such subsidies are the only way photovoltaics can remain economically profitable. In Germany FIT subsidies are generally around €0.13 above the normal retail price of a kWh (€0.05). PV FITs have been crucial for the adoption of the industry, and are available to consumers in over 50 countries as of 2011. Germany and Spain have been the most important countries regarding offering subsidies for PV, and the policies of these countries have driven demand in the past. Some US solar cell manufacturing companies have repeatedly complained that the dropping prices of PV module costs have been achieved due to subsidies by the government of China, and the dumping of these products below fair market prices. US manufacturers generally recommend high tariffs on foreign supplies to allow them remain profitable. In response to these concerns, the Obama administration began to levy tariffs on US consumers of these products in 2012 to raise prices for domestic manufacturers. Under the Trump administration the US government imposed further tariffs on US consumers to restrict trade in solar modules. The USA, however, also subsidies the industry, offering consumers a 30% federal tax credit to purchase modules. In Hawaii federal and state subsidies chop off up to two thirds of the installation costs. Some environmentalists have promoted the idea that government incentives should be used in order to expand the PV manufacturing industry to reduce costs of PV-generated electricity much more rapidly to a level where it is able to compete with fossil fuels in a free market. This is based on the theory that when the manufacturing capacity doubles, economies of scale will cause the prices of the solar products to halve. In California PV reached grid parity in 2011, which is usually defined as PV production costs at or below retail electricity prices (though often still above the power station prices for coal or gas-fired generation without their distribution and other costs). However, in many countries there is still a need for more access to capital to develop PV projects. To solve this problem securitization has been proposed and used to accelerate development of solar photovoltaic projects. For example, SolarCity offered the first U.S. asset-backed security in the solar industry in 2013.

Solar cell efficiencies

Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 44.0% with experimental multiple-junction concentrated photovoltaics. Solar cell energy conversion efficiencies for commercially available photovoltaics are around 14–22%.

Other

Photovoltaic power is also generated during a time of day that is close to peak demand (precedes it) in electricity systems with high use of air conditioning. Since large-scale PV operation requires back-up in the form of spinning reserves, its marginal cost of generation in the middle of the day is typically lowest, but not zero, when PV is generating electricity. This can be seen in Figure 1 of this paper:. For residential properties with private PV facilities networked to the grid, the owner may be able earn extra money when the time of generation is included, as electricity is worth more during the day than at night. One journalist theorised in 2012 that if the energy bills of Americans were forced upwards by imposing an extra tax of \$50/ton on carbon dioxide emissions from coal-fired power, this could have allowed solar PV to appear more cost-competitive to consumers in most locations. Conventionally, direct current (DC) generated electricity from solar PV must be converted to alternating current (AC) used in the power grid, at an average 10% loss during the conversion. Because the batteries used in electric vehicles use DC, the owner of such products may be able to achieve some higher efficiencies if he/she is somehow able to plug the battery directly into the solar cell. An additional efficiency loss occurs in the transition back to DC for battery driven devices and vehicles.

Current developments

For best performance, terrestrial PV systems aim to maximize the time they face the sun. Solar trackers achieve this by moving PV modules to follow the sun.. Static mounted systems can be optimized by analysis of the sun path. PV modules are often set to latitude tilt, an angle equal to the latitude, but performance can be improved by adjusting the angle for summer or winter. Generally, as with other semiconductor devices, temperatures above room temperature reduce the performance of photovoltaic modules. A number of solar modules may also be mounted vertically above each other in a tower, if the zenith distance of the Sun is greater than zero, and the tower can be turned horizontally as a whole and each module additionally around a horizontal axis. In such a tower the modules can follow the Sun exactly. Such a device may be described as a ladder mounted on a turnable disk. Each step of that ladder is the middle axis of a rectangular solar panel. In case the zenith distance of the Sun reaches zero, the "ladder" may be rotated to the north or the south to avoid a solar module producing a shadow on a lower one. Instead of an exactly vertical tower one can choose a tower with an axis directed to the polar star, meaning that it is parallel to the rotation axis of the Earth. In this case the angle between the axis and the Sun is always larger than 66 degrees. During a day it is only necessary to turn the panels around this axis to follow the Sun. Installations may be ground-mounted (and sometimes integrated with farming and grazing)GE Invests, Delivers One of World's Largest Solar Power Plants
Huliq.com (12 April 2007). Retrieved on 3 June 2012.
or built into the roof or walls of a building (building-integrated photovoltaics).

Efficiency

The electrical efficiency of a PV cell is a physical property which represents how much electrical power a cell can produce for a given Solar irradiance. The basic expression for maximum efficiency of a photovoltaic cell is given by the ratio of output power to the incident solar power (radiation flux times area) :$\eta = \frac.$ The efficiency is measured under ideal laboratory conditions and represents the maximum achievable efficiency of the PV cell or module. Actual efficiency is influenced by temperature, irradiance and spectrum. The most efficient type of solar cell to date is a multi-junction concentrator solar cell with an efficiency of 46.0% produced by Fraunhofer ISE in December 2014. The highest efficiencies achieved without concentration include a material by Sharp Corporation at 35.8% using a proprietary triple-junction manufacturing technology in 2009, and Boeing Spectrolab (40.7% also using a triple-layer design). The US-based specialty gallium arsenide (GaAs) PV manufacturer Alta Devices produces commercial cells with 26% efficiency claiming to have "the world's most efficient solar" single-junction cell dedicated to flexible and lightweight applications. For Silicon solar cell, the US company SunPower remains the leader with a certified module efficiency of 22.8%, well above the market average of 15–18%. However, competitor companies are catching up like the South Korean conglomerate LG (21.7% efficiency) or the Norwegian REC Group (21.7% efficiency). There is an ongoing effort to increase the conversion efficiency of PV cells and modules, primarily for competitive advantage. In order to increase the efficiency of solar cells, it is important to choose a semiconductor material with an appropriate band gap that matches the solar spectrum. This will enhance the electrical and optical properties. Improving the method of charge collection is also useful for increasing the efficiency. There are several groups of materials that are being developed. Ultrahigh-efficiency devices (η>30%) are made by using GaAs and GaInP2 semiconductors with multijunction tandem cells. High-quality, single-crystal silicon materials are used to achieve high-efficiency, low cost cells (η>20%). Recent developments in organic photovoltaic cells (OPVs) have made significant advancements in power conversion efficiency from 3% to over 15% since their introduction in the 1980s. To date, the highest reported power conversion efficiency ranges from 6.7% to 8.94% for small molecule, 8.4%–10.6% for polymer OPVs, and 7% to 21% for perovskite OPVs. OPVs are expected to play a major role in the PV market. Recent improvements have increased the efficiency and lowered cost, while remaining environmentally-benign and renewable. Several companies have begun embedding power optimizers into PV modules called smart modules. These modules perform maximum power point tracking (MPPT) for each module individually, measure performance data for monitoring, and provide additional safety features. Such modules can also compensate for shading effects, wherein a shadow falling across a section of a module causes the electrical output of one or more strings of cells in the module to decrease. One of the major causes for the decreased performance of cells is overheating. The efficiency of a solar cell declines by about 0.5% for every 1 degree Celsius increase in temperature. This means that a 100 degree increase in surface temperature could decrease the efficiency of a solar cell by about half. Self-cooling solar cells are one solution to this problem. Rather than using energy to cool the surface, pyramid and cone shapes can be formed from silica, and attached to the surface of a solar panel. Doing so allows visible light to reach the solar cells, but reflects infrared rays (which carry heat).

Effect of the temperature

The performance of a photovoltaic (PV) module depends on the environmental conditions, mainly on the global incident irradiance G on the module plane. However, the temperature T of the p–n junction also influences the main electrical parameters: the short‐circuit current ISC, the open‐circuit voltage VOC, and the maximum power Pmax. The first studies about the behavior of PV cells under varying conditions of G and T date back several decades ago.1-4 In general, it is known that VOC shows a significant inverse correlation with T, whereas for ISC that correlation is direct, but weaker, so that this increment does not compensate for the decrease of VOC. As a consequence, Pmax reduces when T increases. This correlation between the output power of a solar cell and its junction working temperature depends on the semiconductor material,2 and it is due to the influence of T on the concentration, lifetime, and mobility of the intrinsic carriers, that is, electrons and holes, inside the PV cell. The temperature sensitivity is usually described by some temperature coefficients, each one expressing the derivative of the parameter it refers to with respect to the junction temperature. The values of these parameters can be found in any PV module data sheet; they are the following: – β Coefficient of variation of VOC with respect to T, given by ∂VOC/∂T. – α Coefficient of variation of ISC with respect to T, given by ∂ISC/∂T. – δ Coefficient of variation of Pmax with respect to T, given by ∂Pmax/∂T. Techniques for estimating these coefficients from experimental data can be found in the literature. Few studies analyse the variation of the series resistance with respect to the cell or module temperature. This dependency is studied by suitably processing the current–voltage curve. The temperature coefficient of the series resistance is estimated by using the single diode model or the double diode one.

Growth

Solar photovoltaics is growing rapidly and worldwide installed capacity reached about 515 gigawatts (GW) by 2018. The total power output of the world's PV capacity in a calendar year is now beyond 500 TWh of electricity. This represents 2% of worldwide electricity demand. More than 100 countries use solar PV. China is followed by the United States and Japan, while installations in Germany, once the world's largest producer, have been slowing down. In 2017 a study in Science estimated that by 2030 global PV installed capacities will be between 3,000 and 10,000 GW. The EPIA/Greenpeace Solar Generation Paradigm Shift Scenario (formerly called Advanced Scenario) from 2010 shows that by the year 2030, 1,845 GW of PV systems could be generating approximately 2,646 TWh/year of electricity around the world. Combined with energy use efficiency improvements, this would represent the electricity needs of more than 9% of the world's population. By 2050, over 20% of all electricity could be provided by photovoltaics.Solar Photovoltaic Electricity Empowering the World
. Epia.org (22 September 2012). Retrieved on 31 May 2013.

Environmental impacts of photovoltaic technologies

Applications

Photovoltaic systems

Basics of Space Flight
, Chapter 11. Typical Onboard Systems, Propulsion Subsystems
Spacecraft were one of the earliest applications of photovoltaics, starting with the silicon solar cells used on the Vanguard 1 satellite, launched by the US in 1958. Since then, solar power has been used on missions ranging from the MESSENGER probe to Mercury, to as far out in the solar system as the Juno probe to Jupiter. The largest solar power system flown in space is the electrical system of the International Space Station. To increase the power generated per kilogram, typical spacecraft solar panels use high-cost, high-efficiency, and close-packed rectangular multi-junction solar cells made of gallium arsenide (GaAs) and other semiconductor materials. *;Specialty Power Systems : Photovoltaics may also be incorporated as energy conversion devices for objects at elevated temperatures and with preferable radiative emissivities such as heterogeneous combustors. *;Indoor Photovoltaics (IPV) : Indoor photovoltaics have the potential to supply power to the Internet of Things, such as smart sensors and communication devices, providing a solution to the battery limitations such as power consumption, toxicity, and maintenance. Ambient indoor lighting, such as LEDs and fluorescent lights, emit enough radiation to power small electronic devices or devices with low-power demand. In these applications, indoor photovoltaics will be able to improve reliability and increase lifetimes of wireless networks, especially important with the significant number of wireless sensors that will be installed in the coming years. : Due to the lack of access to solar radiation, the intensity of energy harvested by indoor photovoltaics is usually three orders of magnitude smaller than sunlight, which will affect the efficiencies of the photovoltaic cells. The optimal band gap for indoor light harvesting is around 1.9-2 eV, compared to the optimum of 1.4 eV for outdoor light harvesting. The increase in optimal band gap also results in a larger open-circuit voltage (VOC), which affects the efficiency as well. Silicon photovoltaics, the most common type of photovoltaic cell in the market, is only able to reach an efficiency of around 8% when harvesting ambient indoor light, compared to its 26% efficiency in sunlight. One possible alternative is to use amorphous silicon, a-Si, as it has a wider band gap of 1.6 eV compared to its crystalline counterpart, causing it to be more suitable to capture the indoor light spectra. : Other promising materials and technologies for indoor photovoltaics include thin-film materials, III-V light harvesters, organic photovoltaics (OPV), and perovskite solar cells. :* Thin-film materials, specifically CdTe, have displayed good performance under low light and diffuse conditions, with a band gap of 1.5 eV. :* Some single junction III-V cells have band gaps in the range of 1.8 to 1.9 eV, which have been shown to maintain good performances under indoor lighting, with an efficiency of over 20%. :* There has been various organic photovoltaics that have demonstrated efficiencies of over 16% from indoor lighting, despite having low efficiencies in energy harvesting under sunlight. This is due to the fact that OPVs have a large absorption coefficient, adjustable absorptions ranges, as well as small leakage currents in dim light, allowing them to convert indoor lighting more efficiently compared to inorganic PVs. :* Perovskite solar cells have been tested to display efficiencies over 25% in low light levels. While perovskite solar cells often contain lead, raising the concern of toxicity, lead-free perovskite inspired materials also show promise as indoor photovoltaics. While plenty of research is being conducted on perovskite cells, further research is needed to explore its possibilities for IPVs and developing products that can be used to power the internet of things.

Photo sensors

Photosensors are sensors of light or other electromagnetic radiation. A photo detector has a p–n junction that converts light photons into current. The absorbed photons make electron–hole pairs in the depletion region. Photodiodes and photo transistors are a few examples of photo detectors. Solar cells convert some of the light energy absorbed into electrical energy.

The 122 PW of sunlight reaching the Earth's surface is plentiful—almost 10,000 times more than the 13 TW equivalent of average power consumed in 2005 by humans.Smil, Vaclav (2006
oecd.org. Retrieved on 3 June 2012.
This abundance leads to the suggestion that it will not be long before solar energy will become the world's primary energy source. Additionally, solar electric generation has the highest power density (global mean of 170 W/m2) among renewable energies. Solar power is pollution-free during use, which enables it to cut down on pollution when it is substituted for other energy sources. For example, MIT estimated that 52,000 people per year die prematurely in the U.S. from coal-fired power plant pollution and all but one of these deaths could be prevented from using PV to replace coal. Production end-wastes and emissions are manageable using existing pollution controls. End-of-use recycling technologies are under development and policies are being produced that encourage recycling from producers. PV installations could ideally operate for 100 years or even more with little maintenance or intervention after their initial set-up, so after the initial capital cost of building any solar power plant, operating costs are extremely low compared to existing power technologies. Grid-connected solar electricity can be used locally thus reducing transmission/distribution losses (transmission losses in the US were approximately 7.2% in 1995). Compared to fossil and nuclear energy sources, very little research money has been invested in the development of solar cells, so there is considerable room for improvement. Nevertheless, experimental high efficiency solar cells already have efficiencies of over 40% in case of concentrating photovoltaic cells and efficiencies are rapidly rising while mass-production costs are rapidly falling. In some states of the United States, much of the investment in a home-mounted system may be lost if the homeowner moves and the buyer puts less value on the system than the seller. The city of Berkeley developed an innovative financing method to remove this limitation, by adding a tax assessment that is transferred with the home to pay for the solar panels. Now known as PACE, Property Assessed Clean Energy, 30 U.S. states have duplicated this solution. There is evidence, at least in California, that the presence of a home-mounted solar system can actually increase the value of a home. According to a paper published in April 2011 by the Ernest Orlando Lawrence Berkeley National Laboratory titled An Analysis of the Effects of Residential Photovoltaic Energy Systems on Home Sales Prices in California:

*;Pollution and Energy in Production PV has been a well-known method of generating clean, emission-free electricity. PV systems are often made of PV modules and inverter (changing DC to AC). PV modules are mainly made of PV cells, which has no fundamental difference from the material used for making computer chips. The process of producing PV cells is energy-intensive and involves highly poisonous and environmentally toxic chemicals. There are a few PV manufacturing plants around the world producing PV modules with energy produced from PV. This counteractive measure considerably reduces the carbon footprint of the manufacturing process of PV cells. Management of the chemicals used and produced during the manufacturing process is subject to the factories' local laws and regulations. *; Impact on Electricity Network For behind-the-meter rooftop photovoltaic systems, the energy flow becomes two-way. When there is more local generation than consumption, electricity is exported to the grid, allowing for net metering. However, electricity networks traditionally are not designed to deal with two-way energy transfer, which may introduce technical issues. An over-voltage issue may come out as the electricity flows from these PV households back to the network. There are solutions to manage the over-voltage issue, such as regulating PV inverter power factor, new voltage and energy control equipment at electricity distributor level, re-conductor the electricity wires, demand side management, etc. There are often limitations and costs related to these solutions. High generation during the middle of the day reduces the net generation demand, but higher peak net demand as the sun goes down can require rapid ramping of utility generating stations, producing a load profile called the duck curve. *; Implications for Electricity Bill Management and Energy Investment There is no silver bullet in electricity or energy demand and bill management, because customers (sites) have different specific situations, e.g. different comfort/convenience needs, different electricity tariffs, or different usage patterns. Electricity tariff may have a few elements, such as daily access and metering charge, energy charge (based on kWh, MWh) or peak demand charge (e.g. a price for the highest 30min energy consumption in a month). PV is a promising option for reducing energy charges when electricity prices are reasonably high and continuously increasing, such as in Australia and Germany. However, for sites with peak demand charge in place, PV may be less attractive if peak demands mostly occur in the late afternoon to early evening, for example in residential communities. Overall, energy investment is largely an economic decision and it is better to make investment decisions based on systematic evaluation of options in operational improvement, energy efficiency, onsite generation and energy storage.

* Active solar * Agrivoltaic * American Solar Energy Society * Anomalous photovoltaic effect * * Cost of electricity by source * Energy demand management * * * List of photovoltaics companies * List of solar cell manufacturers * Photoelectrochemical cell * * Renewable energy commercialization * Solar cell fabric * Solar module quality assurance * Solar photovoltaic monitoring * Solar thermal energy * Theory of solar cell

References