solar cell

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solar cell,

solid material whose electrical conductivity at room temperature is between that of a conductor and that of an insulator (see conduction; insulation). At high temperatures its conductivity approaches that of a metal, and at low temperatures it acts as an insulator.
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 devised to convert light to electric current. It is a specially constructed diodediode
, two-terminal electronic device that permits current flow predominantly in only one direction. Most diodes are semiconductor devices; diode electron tubes are now used only for a few specialized applications.
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, usually made of forms of crystalline silicon or of thin films (as of copper indium gallium selenide or amorphous silicon). When light strikes the exposed active surface, it knocks electrons loose from their sites in the semiconductor. Some of the electrons have sufficient energy to cross the diode junction and, having done so, cannot return to positions on the other side of the junction without passing through an external circuit. Since the current obtained from these devices is small and the voltage is low, they must be connected in large series-parallel arrays (solar panels) if useful amounts of energy are to be converted. Practical devices of this kind are about 10% to 15% efficient and for many years were most commonly used to provide electric power for spacecraft. For large-scale power conversion solar cells offer a number of practical problems; one of the most serious of these is the wide variation of output voltage and current accompanying changes in the amount of incident light; this can be compensated for on smaller scales by storing energy produced during peak periods in batteries. Reductions in the cost of producing solar panels have made them a viable alternative to electrical generators for homes and villages in remote areas, and solar panels have become more common as an alternative energy source for residences and businesses.

solar cell

A device by which incident solar radiation is converted directly into electrical energy. It is a semiconductor device that is identical in principle to the photovoltaic detector and has a p-n junction with a large surface area. Solar radiation falling on or near the junction produces an external voltage. A variety of different semiconductors, dopants, and fabrication techniques have been used to increase the conversion efficiency and the power delivered. The conversion efficiency can exceed 30%.

Solar cells form the main power supply in satellites, space stations, and short-range planetary probes. The cells are arranged on flat solar panels outside the craft to receive the maximum amount of radiation from the Sun. On probes traveling beyond Mars the radiation flux is insufficient to power the instruments: the solar constant at Jupiter's orbit is only about 4% of the value at the Earth's orbit. Power must then be obtained from other sources, such as thermoelectric generators.

solar cell

[′sō·lər ′sel]
A pn-junction device which converts the radiant energy of sunlight directly and efficiently into electrical energy.

Solar cell

A semiconductor electrical junction device which absorbs and converts the radiant energy of sunlight directly and efficiently into electrical energy. Solar cells may be used individually as light detectors, for example in cameras, or connected in series and parallel to obtain the required values of current and voltage for electric power generation.

Most solar cells are made from single-crystal silicon and have been very expensive for generating electricity, but have found application in space satellites and remote areas where low-cost conventional power sources have been unavailable.

The conversion of sunlight into electrical energy in a solar cell involves three major processes: absorption of the sunlight in the semiconductor material; generation and separation of free positive and negative charges to different regions of the solar cell, creating a voltage in the solar cell; and transfer of these separated charges through electrical terminals to the outside application in the form of electric current.

When light is absorbed in the semiconductor, a negatively charged electron and positively charged hole are created. The heart of the solar cell is the electrical junction which separates these electrons and holes from one another after they are created by the light. An electrical junction may be formed by the contact of: a metal to a semiconductor (this junction is called a Schottky barrier); a liquid to a semiconductor to form a photo-electrochemical cell; or two semiconductor regions (called a pn junction).

The fundamental principles of the electrical junction can be illustrated with the silicon pn junction. Pure silicon to which a trace amount of a group V element (in the periodic table) such as phosphorus has been added is an n-type semiconductor, where electric current is carried by free electrons. Each phosphorus atom contributes one free electron, leaving behind the phosphorus atom bound to the crystal structure with a unit positive charge. Similarly, pure silicon to which a trace amount of a group III element such as boron has been added is a p-type semiconductor, where the electric current is carried by free holes. The interface between the p- and n-type silicon is called the pn junction. The fixed charges at the interface due to the bound boron and phosphorus atoms create a permanent dipole charge layer with a high electric field. When photons of light energy from the Sun produce electron-hole pairs near the junction, the built-in electric field forces the holes to the p side and the electrons to the n side. This displacement of free charges results in a voltage difference between the two regions of the crystal. When a load is connected at the terminals, an electron current flows and useful electrical power is available at the load. See Semiconductor, Solar energy

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