Whenever we think of solar energy, we assume that it is just simple sunlight, ignoring the fact that visible light is just part of the complete electromagnetic spectrum.
It is important to keep in mind that electromagnetic radiation is not monochromatic, it’s made up of a wide range of different wavelengths, and therefore different energy levels.
It is possible to separate light into different wavelengths, which can be seen in the form of a rainbow. And as the light that falls on our solar cell has multiple photons carrying different ranges of energies, some of these photons don’t have enough energy to alter an electron-hole pair.
This means that they’ll simply pass through the cell as if it were transparent. However, other photons may have too much energy. Only a specific amount of energy, which can be measured in electron volts (eV), and is about 1.1 eV for crystalline silicon, is needed to loosen up an electron. This is called the band gap energy of a material.
In case the photon has more energy than the required amount, then this extra energy is lost. Thus if the photon has less energy, or has too much energy, in both the cases energy will be lost. These losses can account for about 70 percent of the radiation energy incident on our cell.
Question arises that why can’t we use a material that has a really low band gap, so we can use more photons? But this is not possible as unfortunately, our band gap also decides the strength (voltage) of our electric field, and if it’s too low, then whatever we make up in extra current through absorbing more photons, we will loose by having a small voltage.
Balancing both these effects, the optimal band gap can be found around 1.4 eV for a cell made from a single material.
However, these aren’t the only losses that we face. The electrons have to flow from one side of the cell to the other through an external circuit. The bottom can be made with a metal which allows good conduction, but if the top is completely covered, then photons can’t get through the opaque conductor and we lose all of our current.
Also, as silicon is a semiconductor, its internal resistance is fairly high, leading to high losses. To minimize these losses, cells are typically covered by a metallic contact grid that shortens the distance that electrons have to travel while covering only a small part of the cell surface. Even after this, some of the photons are blocked by the grid.
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