Photovoltaic Effect

Photovoltaic Effect

• The photovoltaic effect is defined as the generation of an electromotive force as a result of absorption of ionizing radiation.
•  Energy conversion devices which are used to convert sunlight to electricity by the use of photo voltaic effect are called solar cells.
• A single converter cell is called a solar cell or a photovoltaic cell; and combination of such cells used to increase the electric power output is called a solar module or solar energy.
• Photovoltaic cells are made of semiconductors that generate electricity when they absorb light As photons are received , free electrical charges are generated that can be collected on contacts applied to the surfaces of semiconductors.
• Solar cells are adopted to the weak energy flux of solar radiation operating at room temperatures . These devices have theoretical efficiencies of the order of 25%. Actual operating efficiencies are less than half of this value and it decreases rapidly with increasing temperatures.

Principle of Solar Cell :

• The photovoltaic effect can be observed in nature in a variety of materials, but the materials that have shown best performance in sunlight are the semiconductors.
• They create free electrons with higher energies than the electrons which provide the bonding In the base crystal.
• Once these electrons are created, there must be an electric field to induce these higher energy electrons to flow out of the semiconductor to do useful work. The electric field in most solar cells is provided by a Junction of materials which have different electrical characteristics.
• To obtain a useful power output from photon Interaction in a semiconductor, three processes are required :

(a) The photons have to be absorbed in the active part of the material resulting in electrons being excited to a higher energy potential.

(b) The electron-hole charge carrier created by the absorption must be physically separated and moved to the edge of the cell.

(c) The charge carriers must be removed from the cell and delivered to a useful load.

Completing the above processes, a solar cell consists of :

1. Semiconductor in which electron-hole pairs are created by absorption of incident solar radiation.
2. Region containing a drift field for charge separation,
3. Charge collecting front and back electrodes.

• The photovoltaic effect can be described for p-n junction in a semiconductor. In an instrinsic semiconductor such as silicon, each one of the four valence electrons of the material atom is tied in a chemical bond, and there are no free electrons at absolute zero.
• If a piece of such material is doped on one side by a five valence electron material, such as arsenic, or phosphorous, there will be an excess electron in that side, or it becomes an n-type semiconductor.
• The excess electron will be practically free to move in the semiconductor lattice. When the other side of same piece of material is doped by a three valence electron material, such as boron, there will be deficiency of electrons leading to a p-type semiconductor.
• This deficiency is expressed in terms of excess of holes free to move in the lattice. Such a piece of semiconductor with one side of p-type and the other of the n-type is called a p-n junction.
• In this junction after the photons are absorbed, the free electrons of the n – side will tend to flow to the p-side, and the holes of the p-side will tend to flow to the n-region to compensate for their respective deficiencies.
• This diffusion will create an electric field E_F, from the n-region to the p-region. This field will increase until it reaches equilibrium for V_e. The sum of diffusion potentials for holes and electrons. If electrical contacts are made with the two semiconductor materials and the contacts are connected through an external electrical conductor, the free electrons will flow from n-type material through the conductor to the p-type material.
• Here the free electrons will enter the holes and become bound electrons. Thus, both free electrons and holes will be removed. The flow of electrons through the external conductor constitutes an electric current which will continue as long as more free electrons and holes are being formed by the solar radiation. This is the basis of photovoltaic conversion i.e. the conversion of solar energy into electrical energy. All such cells generate direct current which can be converted into alternating current if desired.