Draw a schematic diagram of photovoltaic cell. Explain its principle and working.
When light is absorbed by a photovoltaic cell, photons of light can transfer their energy to electrons, allowing the electrons to flow through the cell as electrical current. This current flows out of the cell to metal contacts as electricity.
Construction and Working:
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.