Solar cells that we see on calculators and satellites are photovoltaic cells or modules (modules are simply a group of cells electrically connected and packaged in Photovoltaic, as the word implies (photo = light, voltaic = electricity), convert directly into electricity. When we consider that the power density received from at sea level is about 100 mW/cm (1kW/rn it is certainly an energy source that requires further research and development to maximize the conversion efficiency from solar
to electrical energy.
The Fig shows the basic construction and cross section of solar cell. As shown
the top view, every effort is made to ensure that the surface area perpendicular to
the sun is maximum. The surface layer of p-type material is extremely thin so that
light can; penetrate to the junction. The nickel-plated ring around the p-type material is the positive output terminal, and the plating at the bottom of the n-type material is the negative output terminal. Usually, silicon is used as a semiconductor material.
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.
The Fig 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 Isc. Under open circuit condition I = 0 and photovoltaic voltage V will result. This is a logarithmic function of the illumination, as shown in Fig.
Selenium and silicon are the most widely used materials for solar cells, although selenium 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 withstands extreme high or low temperatures without a significant drop-off in efficiency. The efficiency of operation of a solar cell is determined by the electrical power out divided by the power provided by the light source. It is given by
Typical levels of efficiency range from 10 to 40%.
Application of solar cell
Solar cells find many applications such as
- Automated street light
- Emergency light