Quantum process in silicon nanocrystals obtains more energy from sunlight
The yield of solar cells can be vastly improved by reducing energy losses. Before the incident sunlight has managed to generate electricity a large part of that light energy has been lost to heating up the solar cell. Researchers from Technology Foundation STW, NanoNextNL, the University of Amsterdam and Delft University of Technology - including FOM workgroup leaders Prof.dr. Tom Gregorkiewicz and Prof.dr Laurens Siebbeles and former FOM PhD Dr. Tuan Trinh – have found a possible solution to this problem. They have obtained experimental insights into a quantum process in silicon nanocrystals that transfers excess energy from light to adjacent crystals so that energy loss in the form of heat is avoided. This mechanism has only recently been proposed. The researchers' measurements reveal that in practice the mechanism behaves just as the theory predicts. This opens up the way to solar cells with a considerably higher yield than the 25 percent currently achieved in practice. The researchers published their findings on 18 March in an Advance Online Publication on the website of Nature Photonics.
A solar cell consists of a semiconductor. That material absorbs light particles (photons) because electrons in the valency band absorb the energy from photons and consequently enter the conduction band, leaving behind a hole in the valency band. In the conduction band electrons have considerable freedom of movement. Electron–hole pairs can therefore be pulled out of the material and this generates an electric current. All of the energy that is greater than the so-called band gap (the distance, in energy, between the valency and the conduction band) is lost within several femtoseconds (several millionths of a billionth of a second) to lattice vibrations in the material that cause the solar cell to warm up. For silicon, the most used semiconductor in solar cells, this means that at most 30 percent of the energy in the Sun's spectrum can be converted into electricity. However only yields up to 25 percent are achieved in practice. The rest of the incident energy is lost.
Increasing the energy yield
This problem can be tackled with the help of silicon in the crystals in the material. Due to their dimensions of just several nanometres, quantum effects play a role: a process develops that produces far more charges than is the case with the simple absorption of photons. The technical term for this is Space-Separated Carrier Multiplication (SSCM). It ensures that the energy from light which is greater than the previously stated band gap is rapidly spread to other electron-hole pairs in adjacent nanocrystals before heating up takes place. In this way a single high-energy photon gives rise to several single electron-hole pairs in adjacent nanocrystals. In the ideal situation this process can increase the efficiency of silicon solar cells towards 50 percent. As current efforts to improve efficiency are only yielding a few fractions of a percent then this this represents a considerable advance.
Stunning insight
The SSCM process was a known phenomenon but until recently it had only been demonstrated indirectly, namely by measuring how many photons the nanocrystals in the material emitted. That is a measure for the energy previously absorbed. The researchers have now observed this process directly. To do this they irradiated a sample of nanocrystals using femtosecond laser pulses and they then immediately used the same laser to examine the consequences of this. As these pulses are shorter than the time period in which the heating up takes place the researchers could 'watch' the absorption process with their 'pump and probe' set-up and therefore see the moment when new energy carriers arose. This yielded an amazing insight: SSCM was found to take place in a direct form. In other words the absorption of a high-energy photon was immediately followed by the splitting in energy and space that resulted in the formation of different single electron-hole pairs in adjacent nanocrystals. This process is particularly attractive because it makes the duration of incoming energy from photons much longer than the initial absorption process, thereby providing more opportunities for energy conversion.
Towards a considerably higher yield
Silicon nanocrystals in solar cell material absorb photons and partially re-emit these at a different wavelength. This spectrum-deforming effect of the nanocrystals is already being extensively investigated to increase the yield of solar cells in an indirect manner. The new insight into the SSCM makes it clear that the nanocrystals can also contribute directly to the more efficient conversion of sunlight. This new insight is therefore a vital addition to previously gathered knowledge and appears to open up the way for solar cells constructed from silicon nanocrystals, which can achieve a substantially higher yield than existing cells.
Reference
"Direct generation of multiple excitons in adjacent silicon nanocrystals revealed by induced absorption", Tuan Trinh, Rens Limpens, Wieteke de Boer, Juleon Schins, Laurens Siebbeles and Tom Gregorkiewicz, AOP, Nature Photonics, 18 March 2012, http://dx.doi.org DOI 10.1038/NPHOTON.2012.36.
Trinh, Limpkens, De Boer and Gregorkiewicz work at the Van der Waals-Zeeman Institute of the University of Amsterdam. Schins and Siebbeles work at the Optoelectronics Materials Section of the Department of Chemical Engineering of Delft University of Technology. Trinh and De Boer are working on a Technology Foundation STW project for this research, together with Limpens (NanoNextNL).
Contact
For further information please contact Prof. Tom Gregorkiewicz +31 (o)20 525 56 43.
This is a press release from Technology Foundation STW.