Solar cell

SOLAR CELL

It is a P-N junction diode which converts solar energy (light energy) into electrical energy.

Common materials for solar cells include silicon (Si), Gallium Aresnide (GaAs), Indium Arsenide (InAs) and Cadmium Arsenide (CdAs). The most common is silicon.

For silicon, the band gap (the energy necessary to transfer an electron from upper valence level to conduction band) is 1.12 eV.

Construction

It consists of P-N junction diode made of Silicon (fig. 4.16 (a)). The P-N diode is packed in a can with glass window on top such that light may fall upon P and N type materials. The symbol of the solar cell is shown in fig. 4.16 (b). The inward arrow indicates the incoming light.

The thickness of the P-region is kept very small. Therefore, electrons generated in P region can diffuse to the junction before recombination takes place.

The thickness of N-region is also kept small to allow holes generated near the surface to diffuse to the junction before they recombine.

A nickel ring is provided around the P-layer which acts as the positive output terminal. A metal contact at the bottom serves as the negative output terminal.

Working

When light radiation from sun falls on the P-N junction diode, the photon energy is sufficient to break the covalent-bond and produce electron-hole pair. Thus, electron-hole pairs are generated in both P and N sides of the junction.

These electrons and holes reach the depletion region by diffusion (fig.4.17 (a)) and they are separated by the strong barrier electrical field existing there.

The minority carrier electrons in the P-side cross the barrier potential to reach N-side and the holes in N-side move to the P-side (fig. 4.17 (b)).

Their flow constitutes the minority current which is directly proportional to the illumination of light and the surface area being exposed to light.

The electrons and holes are accumulated on the two sides of the junction. This leads to an open circuit voltage V∝ which is a function of illumination.

The open-circuit voltage produced for a silicon solar cell is typically 0.6 volt and the short-circuit current is about 40 mA/cm² in bright noon day sun light.

V-I Characteristics

The V-I characteristics of the solar cell, corresponding to different levels of illumination is shown in fig. 4.18.

The maximum power output is obtained when the solar cell is opened at the knee of the curve.

Efficiency of a Solar Cell

Efficiency of a solar cell is defined as the ratio of the total power converted by the solar cell to the total power available for energy conversion.

Ƞ = Maximum output electrical power / Input optical power

Ƞ = P_max / Light intensity × Area of solar cell × 100

Advantages

1. The solar cell operates with fair efficiency.

2. It has unlimited life.

3. It can be mass produced.

4. It has a high power capacity per weight.

5. Its size is small and compact.

Disadvantages

Solar energy is not available round the clock. It cannot be obtained during night time.

Uses

• Solar cells are used in satellites and space vehicles to supply power to electronic and other equipments and to charge storage batteries.

• They are used to give power to the calculators and watches.

• They are used to provide commercial electricity.