SOLAR CELL – The first solar cell was build by Charles Fritts in 1883, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. Modern solar cells are based on semiconductor physics.
A solar cell is a semiconductor device that that coverts photons of solar light into electricity. In general a solar cell is termed a photovoltaic cell.
A solar cell is based on the photovoltaic effect I.e . to convert light directly into electrical energy.
A solar cell is basically a silicon p – n junction photodiode with a very large large light sensitive area . The junction diode is made from extremely thin ( 180 to 350 micrometre) silicon slices or wafers are used doped lightly with a p- type impurity, on which a surface diffusion on N-S type doping materials is performed on the front side of the wafer. This forms a p – n junction a few hundred nanometers below the surface. To increase the light gathering power of the cell, a coating of titanium dioxide ( anti reflection material ) is applied. However, silicon nitride has proved to be more useful than titanium dioxide as the antireflection coating. It prevents charge carriers recombination at the surface of the solar cell. In some solar cells , it’s front surface is textured to further increase the light gathering power of the cell. As such the solar cell consists of three layers. The top junction layer ( made of n – type semiconductor) and the bottom junction layer ( made of p type semiconductor) are necessary for the energy conversion in a solar cell. The middle layer in the cell is the core of the device and is the absorber layer ( p – n junction ) . The silicon wafer is then provided a grid – like metal coming the front surface by using silver paste and a full area metal contact on its back surface using aluminum paste . After the metal contacts are made , the solar cells are interconnected in series ( and / or parallel) by flat wires or metal ribbons and assembled into solar panels . Solar panels have a sheet of tampered glass on the front and a polymer ( or resin ) cover on the back as in the figure.
However, to improve the photo conversion efficiency, the p – n junction diode having multiple layers are used. Each layer is designed to absorb a successively longer wavelength of light (lower energy) , thus absorbing more of the solar spectrum and increasing the amount of electrical energy produced.
When light falls on the N -type layer , which is on top of the solar cell , it gets absorbed. The absorbed energy knocks out electrons , which flow across the p – n junction to create a current. The current flow through the pp- layer . A load resistance is connected across the metal contacts of n – type and p – type layers to store the electricity. After flowing through the load resistance, the current flows back into the n- layer . Thus , a current is generated without any mechanical input energy. Obviously, the output of a solar cell is d.c. To run house hold or other appliances , d.c. is converted in to a.c. by using an alternator.
At solar noon on a clear March or September day , the solar radiation at the equator is about 1,000 W /m² .Thus , a 10% efficiency solar cell having 1 m² of surface area in full sunlight at solar noon at the equator during either March or September will produce approx
Following are a few applications of solar cells:
1. Solar cells are used to produce electrical power in the remote areas, where power from the power stations is not available.
2. Solar cells are used to supply power to the earth’s satellites.
3. Solar cells are used to power traffic signs.
4. Solar cells are used in handheld calculators in place of conventional batteries.
5. Solar cells are used in remote radiotelephones.
6. Solar cells are used for pumping water.
7. By mounting solar panels ( the assemblies of solar cells ) on the building roofs, the homes and offices can be powdered.
8. The silicon thin film in the form of semi-transparent solar cells are used as window glazings . These cells function as window tinting, while generating Electricity.