Question

Explain principle involved in working of photovoltaic cell.

Answer 1

A photovoltaic cell is made up of :

• a thin highly pure silicon semiconductor wafer made of two layers
• the layers have been doped with boron on one side and phosphorous on the other side, producing surplus of electrons on one side and a deficit of electrons on the other side.
•  metallic contacts on either side of the semiconductor
•  anti-reflective coating to reduce reflection and increase light absorption (silicon nitride or titanium oxide)

The schematic of photovoltaic cell is given here.

• When light falls on the cell, photons in the sunlight knock off some of excess electrons, this makes a voltage difference between the two sides as the excess electrons try to move to the deficit side.
•  With an external circuit attached to the metallic contacts, the electrons can get back to where they came from and a current flows through the circuit.
• The amount of current is determined by the number of electrons that the light photons knock off. Bigger cells, more efficient cells, or cells exposed to more intense sunlight will deliver more electrons.
• In practice, the typical photovoltaic cell has an overall thickness of between 0.25 and 0.35 mm and is made of mono or multi-crystalline silicon. Generally, it has a surface measuring somewhere between 100 and 225 m² and, with an irradiation of 1 KW/m² at a temperature of 25°C, produces a voltage of approximately 0.5V.
• The photovoltaic cell has no storage capacity, it simply acts as an electron pump.
• A number of photovoltaic cells (36 cells in series) electrically connected to each other and mounted in a support structure or frame is called a module. Modules are designed to supply electricity at a certain voltage, such as a common 12 volts system.
• Multiple modules can be wired together to form an array. In general, the larger the area of a module or array, the more electricity that will be produced.
• Energy produced by photovoltaic modules is stored in batteries to make it accessible when there is a shortage of light. This is the most critical element of the whole system and the only one which requires maintenance.