Photocell or Photoelectric cell
Photoelectric cell is a device for converting light energy into electrical energy. It is based on photoelectric effect.
Photoelectric cells are mainly of three types.
I. Photoemissive cell
II. Photovoltaic cell
III. Photoconductive cell
I. Photoemissive cell
Photoemissive cells are of two types:
(a) Vacuum type
(b) Gas filled type.
(а) Vacuum type Cell
Principle: Photoemissive is based on the principle that electrons are emitted from the cathode when illuminated by radiations of suitable frequency.
Construction: It consists of an evacuated glass bulb fitted with two metallic electrodes i.e. a cathode and an anode as shown in Fig. (a). Cathode C is semi-cylindrical metal coated with a photosensitive material and connected to the negative terminal of the battery B. Anode A is in the form of a straight wire so placed that it does not obstruct the light falling on cathode.
Working: When visible U.V. light is made to fall on the photosensitive cathode, electrons are emitted which are attracted by the positive anode. The magnitude of current is directly proportional to the intensity of incident radiations though the magnitude of current is small.
(b) Gas filled type cell: To increase the magnitude of the current the cell is filled with an inert gas like argon or neon. When cathode C is illuminated by radiations of suitable frequency, the emitted electrons from the cathode ionise the gas, hence large number of electrons are produced. These electrons are attracted by the anode, so a large current is produced by this cell. In these cells, the magnitude of the photoelectric current is not proportional to the intensity of incident radiations.
Uses
- They are used for sound reproduction from a motion-picture film.
- In television
- As burglar’s alarm.
- In photometry
- To control the temperature of furnaces.
II. Photovoltaic cell
It is self-generating cell which employs semiconductor contacts against metals. When light is incident on the semiconductor, it generates an e.m.f. and can therefore, provide a current without a battery.
Principle: It is based on the principle that when light or any radiations are ms e.m.f. which is proportional to the radiant energy incident on it.
Construction: The most commonly used photovoltaic cells are of the barrier layer type iron-selenium cells. A thin layer of selenium (semiconducting material) is placed on an iron disc. The surface film is a translucent layer of silver or platinum to act as a front electrode. The barrier layer is formed by cathode sputtering process. A contact ring on the silver layer acts as one electrode and the iron base as the other.
Working: When light is made to fall on the semiconductor i.e. selenium, it ejects electrons which travel from selenium to the front silver electrode through the barrier layer. The barrier layer does not allow the electrons to go in opposite direction because it acts as a rectifier. The e.m.f. generated between silver electrode and selenium is almost proportional to the intensity of incident light. The main advantage of this cell is that it requires no external battery for its own operation i.e. it is self-generating.
Uses: Since the internal e.m.f. and hence current generated by it is large enough to be measured by a pointer type galvanometer, so they are used in photographic exposure meters, direct reading illumination meters and photometers.
III. Photoconductive Cell
Principle: It is based on the principle that the resistance of semiconductor like selenium (Se), Cd S, Pb etc. decreases when radiations are made to fall on them.
Construction: The simplest form of photo-conductive cell using selenium is as shown in Fig. (c). It consists of a pair of interlocking comb-like electrodes made of gold deposited on glass. Over these a thin film of selenium is deposited. Actually the selenium forms a large number of strips, electrically in parallel because Se has very high resistivity (700 Qm in the dark). The commonly used Cd S cell is shown in Fig. (d). It has a light sensitivity than Se.
Working: When no light is falling on the cell (dark), its resistance is very high hence current is low. When the cell is illuminated, its resistance decreases and circuit current becomes large. The resistance between their terminals XY, falls from about 107 Ω in the dark to about 106 Ω in bright light.
Uses: Photoconductive cells have been used for detecting ships and aircrafts by the radiations given out by their exhausts or funnels and for telephony by modulated infrared light.
