Question

How do solar cell and LED work? Explain de differences and draw out an example of both.

Answer 1

Working of solar cell :

A solar cell is a simple pn junction semiconductor device. The schematic diagram of a solar cell is shown below.

The n region of the semiconductor is heavily doped and thin (as you can see in the fig.) so that the light entering into can penetrate through it easily whereas the p region is lightly doped. The penetration of the light depends on the wavelength and the absorption coefficient increases as the wavelength decreases. Radiation is absorbed in the depletion region and produces electrons and holes. These are separated by the built-in potential. Depending on the wavelength and the thickness different parts of the device can absorb different regions of the solar spectrum. Electrons and holes are also generated with the p and n regions, as seen from figure. The shorter wavelengths (higher absorption coefficient) are absorbed in the n region and the longer wavelengths are absorbed in the bulk of the p region. Some of the EHPs generated in these regions can also contribute to the current. So there is an optimum range of wavelengths where EHPs can contribute to photocurrent.

Working of LED :

A light emitting diode (LED) is a device which converts electrical energy to light energy. An LED is also a p-n junction with a heavily doped n-type semiconductor and a lightly doped p-type. The device works if it is forward biased. When p- side of the junction is connected to the positive terminal of a battery and the n side to the negative terminal, the barrier height gets reduced and the carriers diffuse to the other side of the junction. As the p- side is lightly doped, much fewer number of holes cross over to the n-side than the number of electrons which diffuse to the n-side. Recombination takes place in the depletion region (also called the active layer) and light is emitted. The emission takes place in random direction. If the width of the p-side is made much thinner than the width of the n-side, the emitted light can escape from the device without getting reabsorbed by the material. A schematic diagram of the LED is shown below.