How do solar cells work?

How do solar cells work?

The solar cell is an important candidate for an alternative terrestrial energy source because it can convert sunlight directly to electricity with good conversion efficiency, can provide nearly permanent power at low operating cost, and is virtually non-polluting. Solar cell also called as photovoltaic cell and are building blocks of solar panels. You must have seen these panels (large collection of solar cells) in green energy campaigns and also in developed cities in large arrays. It is used as a primary source of energy in space applications. ISS has large solar panels! Let’s understand the basics of solar cells.

 

The most commonly known solar cell is configured as a large-area p-n junction made from silicon. p-n junctions of silicon solar cells are made by diffusing an n-type dopant into one side of a p-type wafer (or vice versa). To get an idea about p-n junction click here. Now when light or photon hits the p-n junction it dislodges an electron and creates a hole in its place. Now the dislodged electron and hole are free to move in silicon crystal. Due to the electric field present, the electron moves to the n-type region and hole moves to the p-type region. The mobile electrons created in n-type are collected by thin metal fingers on the top of n-type region. Photons having energy equal to the band gap of silicon crystals are the only ones contributing in cells electrical output. Energy greater than band gap is lost as heat. We might also lose energy if electrons and holes recombine as soon as they are formed. So basically a solar cell works by knocking off electron of same energy as band gap of crystal material used and essentially converting light energy into electrical energy.

Schematic representation of silicon p-n junction Solar cell.

 

The radiative energy output from the sun derives from a nuclear fusion reaction. In every second, about 6 x 10^11 kg hydrogen is converted to helium, with a net mass loss of about 4 x 10^3 kg. We get a lot of energy from sun and if there is a way to harness this energy what’s stopping us from being completely reliant on solar power? There are various factors at play.

Terrestrially the sunlight is attenuated by clouds and by atmospheric scattering and absorption. Also we don’t receive sunlight during night time and during bad weather conditions. This is something we can’t do anything about. As mentioned above, only the photon having energy equal to band gap contributes in electrical output. Photons having energies other than band gap of semiconductor crystals either reflects back or goes through or the energy is just lost as heat energy. Antireflective coating is done to avoid reflection of photons. To deal with photons of energies other than band gap of that semiconductor crystal there is a different approach.

Spectrum Splitting : 

Spectrum splitting is a very good way of increasing efficiency of solar cells by splitting sunlight into narrow wavelength bands and directing each band to a cell that has a band gap optimally chosen to convert just this wavelength band of light. There is one more way, by simply stacking cell on top of one another with the highest band gap cell at the top which automatically achieves an identical spectral-splitting effect, making this “tandem” cell approach a reasonably practical way of increasing cell efficiency.

This is an area of active research where scientists all over the world are trying to increase the efficiency of solar cells. The most efficient solar cell yet still converts only 46% of solar energy into electricity. And most commercial systems convert only 15-20% of solar energy. Solar energy is a source of green and sustainable energy and probably a candidate for future source of energy.

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