Electricity photovoltaic cells

Solar cells (photovoltaic cells) convert simlight to electricity. Photovoltaic cells are made of semiconductor materials such as silicon and gallium arsenide. When light strikes the cell, photons are absorbed within the semiconductor and create electron-hole pairs that move within the cell. This generates the energy that is used to power space vehicles. 

A more promising approach to the use of solar energy is the development of photovoltaic cells, such as those shown in Figure 15.4. These devices convert solar radiation directly to electricity. Photovoltaic cells supply power for astronauts in space, but they are not used extensively for ordinary energy needs. That is because the cost of supplying electricity by means of photovoltaic cells is high compared to the cost of burning coal or oil. 

There are a variety of ultraviolet detection devices. They convert radiation arriving at a sensing device or medium to some form of display. Conversion methods include electrical (photovoltaic cells and phototubes), thermal (thermopile), and chemical (photographic plates). For most devices, selective filters determine what wavelengths arrive at the sensor. 

Devices for generating electricity, photovoltaic cells based on Ti02 (dye-sensitized solar cells). 

Another growing apphcation that overlaps the electrically functional area is the use of transparent conductive coatings or tin oxide, indium—tin oxide, and similar materials in photovoltaic solar ceUs and various optic electronic apphcations (see Photovoltaic cells). These coatings are deposited by PVD techniques as weU as by spray pyrolysis, which is a CVD process. 

Photovoltaic cells. The selenium photographic exposure meter has already been mentioned it goes back to Adams and Day s (1877) study of selenium, was further developed by Charles Fritt in 1885 and finally became a commercial product in the 1930s, in competition with a device based on cuprous oxide. This meter was efficient enough for photographic purposes but would not have been acceptable as an electric generator. 

A photovoltaic cell (often called a solar cell) consists of layers of semiconductor materials with different electronic properties. In most of today s solar cells the semiconductor is silicon, an abundant element in the earth s crust. By doping (i.e., chemically introducing impurity elements) most of the silicon with boron to give it a positive or p-type electrical character, and doping a thin layer on the front of the cell with phosphorus to give it a negative or n-type character, a transition region between the two types... 

In the electric power industry, batteries are used in conjunction with other ways of generating electricity. For example, a home may employ photovoltaic cells or a wind turbine for electricity. However, photovoltaic cells cannot operate without sunlight and wind turbines cannot produce electricity without wind. Batteries charged by photovoltaic cells or wind turbines in off-use times or by the electric utility at hand can provide electricity when the local system is inoperative. The electric power delivered in this mode tends to be in the range of 1 to 4 kilowatts. Batteries have been used for this puiyiose since the 1970s. 

Photovoltaic cells are independent of an electrical supply but, in general, they lack sensitivity as compared with photoemissive units. 

A photovoltaic cell is basically a semiconductor diode consisting of a junction similar to the junction of a transistor. An electrical potential is formed by n-type doping on one side and p-type on the other. Under the impact of light (photons), such as in sunlight, electrons move from the p side, across the junction to the n side, and, through electrical contacts, can be drawn as a usable current (Fig. 15.4). 

The great energy consumption, limited recources of traditional fuels and environmental problems have lead to intensive research on the conversion of solar energy during the last fifteen years. Conversion into electrical energy has been realized in technical devices consisting of pn-junction photovoltaic cells. Efficiencies of up to 20 % have been obtained with single crystal devices and around 9 % with polycrystalline or amorphous layers. 

Electrical cells based on semiconductors that produce electricity from sunlight and deliver the electrical energy to an external load are known as photovoltaic cells. At present most commercial solar cells consist of silicon doped with small levels of controlled impurity elements, which increase the conductivity because either the CB is partly filled with electrons (n-type doping) or the VB is partly filled with holes (p-type doping). The electrons have, on average, a potential energy known as the Fermi level, which is just below that of the CB in n-type semiconductors and just above that of the VB in p-type semiconductors (Figure 11.2). 

At present, photovoltaic cells are made of thin, rigid sheets of silicon with other semiconducting materials. Standard commercial panels convert about one tenth of the sunlight into electricity, that is, an efficiency of about 10%. The highest performance panels have an efficiency of roughly 20%. 

Photoelectrochemical semiconductor cells are used to convert photon energy into chemical substances or into electricity, the former is a photodectrolytic cell and the latter is a photovoltaic cell. A photoelectrochemical semiconductor cell consists of either a pair of metal and semiconductor electrodes or a pair of two semiconductor electrodes. 

Here, no net chemical change occurs. In the photovoltaic cell, almost all the photopotential generated exists between the two electrodes and can be used to produce electric energy. 

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