How does the nature of the band gap (direct or indirect) affect the efficiency of a solar? Recombination processes must be different for solar cells made from these solar cells, how does this relate to the absorption properties of the absorber material?
The more relevant attribute of an efficient solar cell is that the minority carrier diffusion length exceeds the absorption depth of the material used. Although an indirect semiconductor absorbs light poorly, is typically has a large recombination lifetime and therefore a large diffusion length. Since the diffusion length roughly scales inversely to the absorption strength of a pure semiconductor, there is no real disadvantage in using an indirect gap semiconductor, just as long as the absorbing layer is thick enough.
Is there some AC cell ?
Today's solar cels are passiv, a diode. Why not active?
transport mechanism will be different? other lengths?
of course it may cost energy.. but somehow?
Thank you all for the answers. Besides Si, which other absorber materials used in solar cells are indirect semiconductors?
Germanium is indirect bandgap semiconductor too like silicon. And it can also used as solar cell.
Si is used as a Solar cell material even though it is an indirect band gap semiconductor. An indirect bandgap semiconductor requires a photon and a phonon to go form the valence band to the conduction band. This involves a change in energy( due to a photon) and a change in momentum (due to a phonon). There are other semiconductors like GaAs which are direct bandgap semiconductors, ie, they involve only a photon. This might seem like a more viable option as a solar cell material as compared to Si. However, Si has its advantages which as a whole works better as a solar cell material.
1. Si is abundantly available. It is the second most abundant element in the Earth’s crust (about 28% by mass) after oxygen. So, it is very cheap.
2. Ge is also an indirect band gap semiconductor, however, the surface passivation is easier for Si as compared to Ge.
3. GaAs is very very expensive. So, it is generally used in multilayer concentrated solar cells where mirrors and reflectors are used to concentrate light on the cell of smaller area. The efficiency of this cell is maximized by the use of mirrors which add to the cost of the cell. These cells are used as space solar cells.
Keeping these reasons in mind, Si is used as a solar cell material.
I would say that except for silicon, indirect gap semiconductors are generally not used as absorbers. If one examines the the best research cell efficiencies reported over time (https://www.nrel.gov/pv/) one can readily see that (aside from silicon) the only materials that have really garnered serious attention in solar cell devices are direct gap semiconductors.
Mrinalini Padmanabhan makes a good point explaining why silicon is the only exception - because it is overwhelmingly abundant and economical to produce at high purity in large quantities. Also it has nice features like self-passivation.
Magnus C. Wagener makes an excellent point that as long as the minority carrier diffusion length is longer than the absorption length, indirect gap materials can in principal be just as efficient as direct gap materials. However, even if an indirect-gap device is just as efficient as a direct-gap device, you will still need to use 100 to 1000 times as much absorber material to make a functioning device. So you get more bang for your buck with direct gap absorbers. There's also the issue of convenience. With indirect gap absorbers, you need to make solar modules out of 3-dimensional wafers (basically thin bricks) rather than lightweight, flexible sheets in the case of direct gap semiconductors.
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