Photovoltaic cells are also known more commonly as solar cells. A photovoltaic cell is a device that converts light energy to electrical energy. It generates electricity based on the photovoltaic effect, which will be explained in greater detail.
Photovoltaic cells are made of semiconductors, most commonly silicon. This is because silicon has four electrons in its outermost shell and thus forms covalent bonds with other atoms and forms a crystalline structure. Silicon is a poor conductor of electricity because its electrons are not free to move around like those in conductors such as copper. So impurities are added to the silicon to increase its electrical conductivity. When silicon is mixed with phosphorus, there will be free electrons available because phosphorous has five electrons in the outer shell; when it forms bond with silicon, only four of the five electrons will be used for bonding, therefore for every phosphorous atom added, there will be one electron that is free to move about (though the charge from the proton is holding it in place).
Some ways of mixing phosphorous with silicon include coating a layer of silicon material with phosphorous and then heating the surface. This allows the phosphorous atoms to diffuse into the silicon; the temperature is then reduced. This whole process of purposefully making silicon impure is called doping. The silicon mixed with phosphorous is called the N-Type semiconductor (N stands for negative) because there are free electrons in the compound. Phosphorous is one of the two dopants used in making photovoltaic cells; the other one is boron. Boron only has three electrons in its outer shell. When it bonds with silicon, there will be “openings” for electrons since one more electron is needed to make a completely neutral covalent bond. The silicon mixed with boron is therefore called the P-Type semiconductor (P stands for positive).
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When the N-Type and P-Type semiconductors are put together, and come into contact, the electrons in the N-Type semiconductor will start to flow to the P-Type semiconductor. The free electrons and the “openings”, or holes, mix and form pairs at a region called the “ N-P junction”. As the number of formed pairs increases, an electric field will be built up and it prevents more electrons from moving to fill the holes. This is because the direction of an electric field is the direction in which a positive particle travels in it. So, if the direction is to the left, the free electrons will be at the right of the junction, and the holes, being positive, will be at the left.
When photons (of sufficient energy) from the light land on the junction, the energy they carry will liberate electrons from the electron-hole pairs in the junction. Then, the electric field will immediately “push” the hole and the liberated electron to either side of the junction, respectively. This process is known as the photovoltaic effect because energy from photons causes a potential difference in the semiconductors. If the joined semiconductors are connected in an external circuit, the electrons will be moving through the wire to fill the holes in the other semiconductor. A direct current is thus produced.
Since photovoltaic cells are lasting and require little maintenance, there are many areas in which they can be used. Photovoltaic cells in large numbers can be used to generate electricity. This is the best way to generate electricity because compared to other non-renewable resources, the sun is an almost never perishable resource of energy. However, the cost for mass-producing photovoltaic cells remains too high for this technology to be widely applied. Because of the reliability of photovoltaic cells, they are also used as a secondary, or even primary source of energy to power electronics such as watches, out door thermometers and calculators. Since photovoltaic cells can provide electricity whenever there is enough sunlight, they can be used as back-up power sources for important equipment that cannot afford a power failure. They can store energy during a sunny day and provide energy to power the equipment on rainy or cloudy days, or when the main source of power fails. Photovoltaic cells can be used for such function on motor vehicles. The stored electricity can be used to start a car in case the car battery fails. Since they do not require much maintenance at all, photovoltaic cells can also be used on devices that come in large numbers, making it a hassle to check their power one by nee. Some examples are vending machines and parking meters by the road.
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