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Thursday, 17 October 2019

Photovoltaic cell and its Application

Photovoltaic cells:

It is a device by which e.m.f. is generated under the illumination of barrier layer. They convert light energy into electrical energy. To produce current flow no outside source of electrical energy is required. The materials are compounds of Sulphur, Selenium, and Tellurium with different metals such as lead, copper, cadmium. A very widely used cell is Selenium Cell. It consists of a metal electrode on which a layer of Selenium is deposited. On top of this barrier layer is formed which is coated with a very thin layer of gold to serve as a translucent electrode through which light can fall on the layer below. When light falls on the semi conductor it absorbs energy and frees electrons or holes which diffuses across the barrier layer. An e.m.f. called photo e.m.f. is developed across the electrodes of the cell. A negative charge is built up on the gold electrode and a positive charge on the bottom electrode. Selenium cells have a sensitivity of 500 micro A/lumen. 

Now-a-days solar radiant energy is converted into electrical energy for space craft power supply by Silicon Voltaic Cell commonly known as Solar Cells. Radiation incident on a shallow P-N junction is absorbed by the creation of hole-electron pairs, the minority carrier drifts towards the junction and is swept rapidly across to the other side. There it contributes to the load current as shown in Figure. If a load resistor RL is connected across the solar cell terminals and is increased in value from 0 ohms to infinity, the load current will vary from infinity to short circuit current zero. The power delivered to the load will be IN and there will be an optimum load at which maximum power is delivered. These can develop an e.m.f. of 0.5 V and one cell provides an output of 6 mw/cm2 at about 6 to 10% efficiency.

The Photo Voltaic Cells finds its application in Photographic exposure meters, foot candle meters, lighting control, automobile aperture control in cameras, television circuits, sound motion Picture recording and reproduction equipments etc. Figure shows the construction of solar cell.

Photoconductive Cells :

The resistance of semi conductors is low under light but increases in darkness. The property is utilised in photo conductive cells. The semi conductor materials are connected in series with the source of voltage. The resistance of the material will change with light and hence the current in the circuit. Photo conductive cells can be used in application which requires the control of a certain function according to intensity of light. Some of the applications are door operators, control for street lights, flame detectors, smoke detectors, burglar alarms.

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