Electrical Engineering ⇒ Topic : Solar Cells

Solar Cells

A solar cell is a device that converts light energy (e.g. sunlight) directly to electrical energy. Fig. (a) shows the construction of a simple solar cell. A wafer consisting of a pure silicon (semiconductor) is "*doped" with a specific amount of arsenic (donor impurity). This makes it N-type semiconductor. As a result of this, the wafer contains an excess of free electrons. This wafer is coated at its top with a very thin layer of silicon doped with appropriate amount of boron (acceptor impurity). This makes the top layer a P-type semiconductor and the contact surface becomes a P-Njunction. When sunlight shines on top P-type layer and penetrates into the N-type material just below it, the free electrons in N-type material receive energy and move across the P-N junction into P-type material. This movement of charge earners (i.e. holes from P-type and free electrons from N-type) constitutes electric current. The bottom of the wafer (N-type material) and a spot on the P-type layer are tinned for the connection leads.

                                                                                figure (a)

The operating voltage of one solar cell is about 0.39 V and the current may be between 30 and 40 mA. The output power of the cell depends upon the exposed area of the cell and the intensity of light falling upon it. The maximum output with the sun directly overhead on a clear day is 8 or 9 mW/cm². The *area of the cell cannot be made large because it is difficult to have large silicon crystals. The operating efficiency is about 10%. The life of a solar cell is estimated to be thousands of years. They do not deteriorate when not in use. The chief uses of solar cells are : to provide power for transistor portable radios, to charge nickel-cadmium batteries in satellites, to provide power for clocks and other devices such as aperture control for movie cameras, microwave relay stations etc