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Photo-Electric Cells

Photoelectric cell, a device whose electrical characteristics (e.g., current, voltage, or resistance) vary when light is incident upon it. The most common type consists of two electrodes separated by a light-sensitive semiconductor material. A battery or other voltage source connected to the electrodes sets up a current even in the absence of light; when light strikes the semiconductor section of the photocell, the current in the circuit increases by an amount proportional to the intensity of the light.

In the phototube, an older type of photocell, two electrodes are enclosed in a glass tube—an anode and a light-sensitive cathode, i.e., a metal that emits electrons in accordance with the photoelectric effect. The functioning of a photo-electric cell which is for e.g., an important component of an exposure meter is based on photo electric effects.

During the latter half of the nineteenth century many scientists and engineers were simultaneously observing a strange phenomenon: electrical devices constructed from certain metals seemed to conduct electricity more efficiently in the daytime than at night. This phenomenon, called the photoelectric effect, had been noted years earlier by the French physicist A.E.Becquerel (1820-1891), who had invented a very primitive device for measuring the intensity of light by measuring the electrical current produced by photochemical reactions.

A number of them succeeded. In 1883 the American inventor Charles Fritts created a working photoelectric cell; that same year a German engineer, Paul Nipkow, used a photoelectric cell in his "Nipkow's disk"—a device which could take a picture by measuring the lighter and darker areas on an object and translate them into electrical impulses. The precursor to the modern photoelectric cell was invented by the German physicists Hans Geitel (1855-1923) and Julius Elster (1859-1920) by modifying a cathode-ray tube.

Strangely, the explanation for why selenium and other metals produced electrical current did not come until 1902, when Phillip Lenard showed that radiation within the visible spectrum caused these metals to release electrons. This was not particularly surprising, since it had been known that both longer radio waves and shorter x rays affected electrons. In 1905 Albert Einstein (1879-1955) applied the quantum theory to show that the current produced in photoelectric cells depended upon the intensity of light, not the wavelength; this proved the cell to be an ideal tool for measuring light.

The electrons in a metal can have energy supplied to them by radiation, e.g., light rays. This is known as photo-electric effect. The energy of a light quantum (photon) is imparted to the most loosely bound electron of an atom (Fig.1a).

This energy may be sufficient to liberate the electron but not enough to eject it entirely from the metal (Fig.1b) (photo conductive effect); alternatively, it may be sufficient not only to liberate the electron but also to cause it to be ejected into the vacuum (Fig.1c) (normal photo-electric effect). The energy balance of the elementary process involved is given by Einstein’s equation: hv = A + ½ me V2 where hv denotes the energy of the photon, in which v is the frequency of the light radiation and h is Planck’s constant (h=6.625 x 10 –27 erg-seconds), A denotes the photo-electric work function (i.e., the energy required by a photon to eject an electron from a metal), me denotes the mass of the electron, and V its velocity in vacuum.

The normal photo-electric effect is applied in the photo-electric cell (Fig.2a) The light-sensitive photo-cathode, which is usually installed in an evacuated glass tube, may consist of a very thin film of cesium deposited by vaporization on to an oxidized silver base. For greater sensitivity the glass tube may be filled with an inert gas at low pressure. A battery in the external circuit serves to amplify the current by ionization of the gas filling.

The photo-conductive effect is utilized in the photo-conductive cell (Fig.2b). The sensitive material usually employed in this case is cadmium sulphide or cadmium selenide. These substances undergo changes in resistance in the ratio of 109 :1 between the extremes of darkness and maximum exposure to light.

When the photo-conductive effect appears at the P-N boundary of semi-conductors or at the boundary between a semiconductor and a metal e.g., cuprous oxide and copper, a potential difference will develop: this is known as the photo-voltaic effect, and a cell of this kind is called a photo-voltaic cell (Fig.2c).

The cells represented in Figs. 2a and 2c generate an electromotive force on their own account, causing a current to flow in the circuit even if no battery is included in the circuit, whereas the photo-conductive cell (Fig 2b) requires an auxiliary voltage provided by a battery.

Photo-electric cells are used for a wide variety of purposes in control engineering, for precision measuring devices, in exposure meters used in photography etc. They are also used in solar batteries as source of electric power for rockets and satellites used in space research. For this purpose silicon photo-electric cells are used; about 10% of the radiation energy which they absorb is converted into electric energy.