Photon splitting as a means for better efficiency of solar cells
Researchers from the University of Amsterdam and the Foundation FOM have succeeded in splitting high energy photons in multiple photons with lower energies. They demonstrated this phenomenon in a system containing silicon nanocrystals. This splitting of photons can be used to considerably improve the efficiency of solar cells (up to 50%). The researchers (Tom Gregorkiewicz, Peter Stallinga, Ignacio Izeddin and Dolf Timmerman) publish their findings in the February issue of Nature Photonics. The article has been published online on the 20th of January.
In standard semiconductor solar cells a single photon will excite one electron, if the energy of the photon is large enough to overcome the bandgap of the semiconductor. This temporarily excited electron, and corresponding hole, will then cool down until their energy matches the bandgap after which this energy can be harvested as electricity. In this way the bandgap limits the efficiency in two ways. Photons with small energy will not be absorbed, because their energy is too small to overcome the bandgap, while for photons with large energy a lot of the energy is lost in cooling of charges. The value of the bandgap of the semiconductor is thus very important for the efficiency. The so-called Shockley-Queisser Limit shows that the maximum efficiency is obtained with a value of the bandgap of 1.1 electronvolt. In this case there is an optimum in the balance between losses on the short- and long wavelength part of the solar spectrum. This maximum efficiency is approximately 30%, which means that about 70% of the incident energy is lost in the conversion.
The bandgap of silicon closely resembles this optimum value of the solar spectrum and because of this most solar cells are made from this material. Furthermore, the level of technology of silicon is much higher than that of other materials.
The realization of quantum cutting
A large improvement of conversion efficiency can be realized if the energy of excited 'hot' electrons can be split before they cool down. This is exactly what the group of prof. Tom Gregorkiewicz of the Van der Waals–Zeeman Institute of the University of Amsterdam has realized and this in the technologically most interesting material silicon. In specially prepared systems based on silicon nanocrystals the researchers saw the quantum cutting, 'one photon in, two photons out', with the emerging photons having a lower energy than the incident photons. Moreover, it appeared that the photons where not necessarily emitted from the same object that absorbed a photon. Therefore next to splitting also transmission of energy has been realized.
The researchers describe their findings in the February issue of Nature Photonics.
A way to improve solar cells
The phenomenon, which is named SSQC (space separated quantum cutting) by the researchers, can have significant consequences for photovoltaic solar energy. Theoretic considerations using this phenomenon predicted an increase of the maximum efficiency from 30% to approximately 44%, in other words almost 50% more Watt per square meter! This is a significant increase and a serious development, in view of the current energy supply and climate change concerns.