High-energy photons create multiple electron-hole pairs in quantumdots. (Illustration: Wieteke de Boer)
Surprising behaviour of electrons in quantum dots may raise the maximum efficiency of solar cells to 44 percent by producing two electrons for one photon.
Researchers in the Chemical Engineering (Applied Sciences) department are quite familiar with quantum nanodots, or q-dots. These tiny lumps of semiconductor material may be tuned to absorb specific wavelengths of light, just be adapting the size (or the material). The silicon q-dots in the most recent study for example measured 3.5 nanometers across and were tuned to infrared radiation (wavelength 800 nm).
But in an STW-funded study, published in ‘Nature Photonics’, the q-dots surprised the researchers by spreading excess energy to neighbouring q-dots, where it may create an extra electron-hole pair. When an incoming photon has more energy than the 1.5 eV it takes to create an electron-hole pair, the extra energy usually gets transferred to crystal vibrations and hence heat. When an incoming photon has twice or more the energy it takes to liberate an electron (the ‘bandgap’), it can theoretically create two electrons for one photon. This seldom happens however because the excess energy usually quickly decays into heat immediately.
Enter the closely packed array of silicon q-dots that Professor Tom Gregorkiewicz’s team from the University of Amsterdam has made by allowing vaporised silicon to condensate on a substrate, like nanoscaled drops of dew. At a distance of 1 nanometer apart, the dots are packed so close that an electron that is hit by a photon “spreads like a tiny cloud and excites an electron in one of the neighbouring dots,” Professor Laurens Siebbeles (Applied Science) explains. The advantage of this quantum mechanical energy transfer is that a high-energy photon creates two electron-hole pairs in two q-dots, thus reducing the chance of energy loss through recombination electrons and positive charges. “Allowing for some energy loss, photons with 2.5 to 3 times the bandgap create two electron-hole pairs,” Prof. Siebbeles explains, adding that this only applies for the high-energy part of the spectrum. Overall, the theoretical efficiency for such charge multiplication solar cells is 44 percent, instead of the current maximum of 33 percent for a conventional solar cell.
There is a snag, however. The created electron-hole pairs need to be separated and drawn from the q-dot to create an external current (which is what solar cells are all about). Prof. Siebbeles is hopeful his group may achieve this one-day by depositing q-dots as thin layers of highly efficient ‘solar’ paint.
Nature Photonics, 18 March 2012, DOI 10.1038/NPHOTON.2012.36
