8 c]Explain the construction and working of photovoltaic cells.
Photovoltaic cell: “These are the semiconductor device, which convert light energy directly to electrical energy”.
The most common material used for solar cells is crystalline silicon, which is the second member in the group IV A in the periodic table. It never occurs free in the nature, but occurs as oxides and silicates. Silicon is the second abundant element found in world.
Construction: PV cells mainly consists of a p-n junction diode made up of Si semiconductor as shown in fig 3.6. It is constructed by two electrical contacts, one is a metallic grid over n-type and second layer of sodium nitride or silver metal at back of the p-type semiconductor. An antireflective layer (silver nitride) or TiO2 is coated between the metal grids to prevent reflection of solar light.
Working: when sunlight with (photon of energy E=hv) required energy falls on p-n junction, electrons are ejected from electron hole pair in the p-n junction. The ejected electrons move to n-type since the movement in n- type is one way, these ejected electrons must flow through external circuit to combine with holes. These movement of electrons from n side to p side generates electric current.
Electrical energy is directly proportional to area of cell exposed to light radiation and intensity.