What is basically an electron blocking layer and a hole blocking layer in contrast to an electron transport layer or a hole transport layer? In terms of the bandgap, I think if the case is one of a staggered heterojunction with the material right of the junction having a lower conduction band edge, then the material on the left becomes electron blocking and hole transport layer while the material on the right becomes electron conducting and hole blocking layer. Am I correct or am I messing it up? In my diagram, if I am creating excitons in the material2 region, then I would assume material1 to be the electron blocking region and hole conduction region while material 3 would be the hole blocking region and electron transport region simultaneously. Is that the case?
[Physics] Electron blocking layer and hole blocking layer
electronic-band-theorysemiconductor-physicssolid-state-physics
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Maybe I know the origin of your question. I had the same doubt.
Depletion layer is said to be free of carriers upon established and equilibrium reached. This approach is widely used in text books to solve the depletion layer zone in pn junctions. However, the depletion layer is still able to produce new electron-hole pairs if depletion layer is excited with higher than band gap energy photons.
On other words, electrons from n-dopings centers in the n doped material were removed from this depletion layer side. But this depletion layer still contains intrinsic material able to absorb photons promoting electrons to conduction band and holes to valence band.
In fact, photovoltaic devices are fabricated looking to maximize this depletion layer width. A trick is to assemble p-i-n junctions where the intrinsic part is added to extend the depletion layer further. Inside depletion layer, drift transport has a relevant role (diffusion is unavoidable always anywhere). Probably, in photovoltaics using indirect band gap light absorbers, it is good to extend the depletion layer because it favours that carriers reach the frontier electrodes. However, I think that new light harvesters with direct band gap (like hybrid perovskite) generating almost free electron/holes (no excitons or weakly bounded excitons), the role of the depletion layer is no so relevant and it is prefered good transport properties.
When light is absorbed in a semiconductor, the excited electrons will quickly relax to the conduction band edge. The extra energy of the absorbed photon compared to the bandgap of the semiconductor is lost to heat. Materials with 0.4 eV bandgap like PbS are not used in solar cells because most of the solar spectrum is made of photons with greater than 0.4 eV energy, and therefore there is a lot of thermal energy loss upon absorption of those photons. You don't want too high of a bandgap either because then you won't absorb part of the solar spectrum. One can in fact calculate the optimum bandgap for the solar spectrum and obtain around 1.4 eV, giving a maximum power conversion efficiency of ~33% known as the Schockley-Queisser limit. Note that the maximum possible efficiency for a material with bandgap ~0.4 eV is only 10%.
The short penetration depth of high energy light basically arises from the fact that the density of states grows away from the band edge. For example, In a simple parabolic band model the density of states goes as $\sqrt{E-E_g}$. More states means more absorption and a shorter penetration depth. This fact also affects the design of Silicon solar cells, check out the last demonstration on this page.
Best Answer
If you are taking about p and n junction diode. In this section p type has electron deficient and it has positive charges and n type have negative charges. When you joint p and n type material than process of neutralization starts between holes from p type and electron from n type material. Positive charges from p type and negative charges from n type comes near to each other to become neutralized but they could not do so. And block further neutralization process of electron and holes. This is the depletion region or potential barrier of Diode. Hope it will help you.