Photovoltaic Cell or Solar Cell

Photovoltaic Cell or Solar Cell

• The solar cell is nothing but a photovoltaic cell which works on the principle of photovoltaic effect. The solar cell generates voltage proportional to the light incident on it.

• A solid state device which converts the light energy into an electrical energy based on photovoltaic effect is called a solar cell.

1. Construction and Working

• Fig. 6.11.1 (a) shows the construction of a solar cell while the Fig. 6.11.1 (b) shows the symbol of a solar cell.

• The surface layer of p‒type material is extremely thin so that light can penetrate to the junction.

• The nickel plated ring around p‒type is the positive output terminal. The nickel plated ring around n‒type material acts as the negative output terminal.

• When the light strikes the cell, the photon light energy is absorbed by the semiconductor material and number of electron‒hole pairs are generated. This happens on both the sides of the junction.

• The holes are directed towards the p‒region while the electrons are directed towards the n‒region due to the electric field present. Due to this movement the minority current is set up across the junction due to which the voltage is developed across the p and n regions. This voltage is taken out as the output voltage.

• The selenium and silicon are most widely used materials for the solar cells. The gallium arsenide, indium arsenide nd cadmium sulphide are also used for the solar cells.

• The efficiency of solar cell is the ratio of electrical power output to the power input by the light source. It is typically between 15 to 40 %.

 %η = [ P₀(electrical) / Pi(light energy) ] × 100

2. Characteristics

• When light strikes the cell, certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor.

• This photon light energy collides with a valence electron and imparts to it sufficient energy to leave the parent atom. This results in the generation of free electrons and holes. This phenomenon will occur on each side of the junction.

• In the p‒type material, the newly generated electrons are minority carriers and will move rather freely across the junction as explained for the basic p‒n junction.

• A similar discussion is true for the holes generated in the n‒type material.

• The result is an increase in the minority carrier flow which is opposite in direction to the conventional forward current of a p‒n junction.

• Fig. 6.11.2 shows the characteristics of solar cell.

• At vertical axis, V = 0 and it is short circuit condition. The short circuit current is represented by notation I_SC.

• Under open circuit condition I = 0 and photovoltaic voltage Voc will result.

3. Materials

• Selenium and silicon are the most widely used materials for solar cells, although gallium arsenide, indium arsenide, and cadmium sulphide, among others, are also used.

• However, silicon has a higher conversion efficiency and greater stability and is less subjected to fatigue. It also has excellent temperature characteristics. That is, it withstand can extreme high high or low temperatures without a significant drop‒off in efficiency.

4. Applications

• The various applications of solar cell are,

1. Satellites and space vehicles.

2. Automated street lights.

3. Power supply to calculators.

4. Emergency lights.

5. For charging the batteries.

6. Solar panels for domestic purposes, water pumps etc.

• In practice to get desired output power, the solar cells are connected in series and such groups are connected in parallel. This is called array of solar cell and shown in Fig. 6.11.4.

• The series solar cells provide necessary output voltage while parallel groups provide necessary output current.

Review Questions

1. Explain the construction, symbol and characteristics of photovoltaic cell.

2. List the applications of photovoltaic cell.

3. Write a note on solar cell.