Reshared post from +Kurt Tomlinson
Solar Cell Efficiency Record Broken!
Sharp claims to have made solar cells with an efficiency of 36.9%! You might be wondering, "How is that possible, Kurt? You just said a couple days ago that the maximum possible efficiency for a solar cell was around 25%. What gives?"
I'll tell you what gives. Those guys at Sharp are really…sharp! They took the took two biggest loss mechanisms, and used them as an opportunity to improve their cells. By using a triple-junction cell, they are able to capture more photons and use the energy of each photon more effectively.
How did they do that? Pretty easy really. They took three solar cells and stacked them one on top of the other. The genius in this design is that each of those three solar cells is designed to capture different wavelengths of light. The topmost solar cell captures blue light, the middle cell captures green light, and the bottom cell captures red light. Each cell allows the light it's not good at capturing to pass on through to the next cell.
Longer wavelength light has less energy and has to travel further in a material to be absorbed. Shorter wavelength light has more energy and gets absorbed in the top layers of whatever material it strikes. The guys at Sharp took advantage of this by putting a material with a large bandgap energy on the top layer, a slightly smaller bandgap material in the middle, and an even lower bandgap material at the bottom.
This allows them to capture more of each photon's energy because the high-energy photons get absorbed in the top cell and each photon gives rise to an electron with energy equal to the large bandgap. Since the difference in energy between the photon and the bandgap is smaller, the energy in these photons is collected more efficiently.
Any material with a bandgap greater than the energy of the photon looks transparent to that photon. So the photons cascade through each cell until they reach a cell that can absorb them. In this way, the scientists were able to reduce the losses due to excess photon energy (by my estimates) from 38% to somewhere around 10%. Although it might not be that low because they could have made up some of their efficiency by using a material with a bandgap smaller than silicon to absorb even longer wavelength photons.
Either way, this is a great achievement. Hopefully they will find a way to make these cells cheap enough that they can displace coal as America's main source of electricity!
Sharp claims to have developed the world’s most efficient solar cell. The company said its triple-layer cell achieved a conversion efficiency of 36.9 percent.