Silicon nanocrystals could lead to improved solar cells
Press release Technology Foundation STW
Tightly packed silicon nanocrystals can convert incoming light particles into at least twice as many electrical charge carriers and that process proceeds with extreme efficiency. A group of physicists from the University of Amsterdam, together with a colleague from the Charles University in Prague, have demonstrated this experimentally. The conversion of light particles into more charge carriers was an indirectly known phenomenon for many different types of semiconductors. However, the efficiency of the process was heavily disputed. The researchers have shown that unlike in other semiconductors, the multiplying of charge carriers in silicon nanocrystals is highly efficient (up to 100%), and occurs at energy levels present in the sun's spectrum. Tightly packed silicon nanocrystals could therefore be used to produce types of solar cells with a far higher yield. Subsequent experiments will need to demonstrate the feasibility of this. The researchers published their findings in an Advance Online Publication for Nature Nanotechnology on 9 October 2011.
The researchers in Amsterdam used two types of silicon crystals for their experiments. They produced one type by condensing a thin layer of silicon and silicon dioxide on a substrate of quartz. They then annealed this material at a high temperature. What remained were silicon nanocrystals with dimensions of 3 to 4 nm, embedded in silicon dioxide. They made the other type by electrochemically etching crystalline silicon. The resultant porous silicon grains left after that process consisted of many nanocrystals (1 to 3 nanometres in diameter). Samples from both types were subsequently illuminated with light of consistently one colour. The energy of the photons from that light was absorbed by the crystals and subsequently re-emitted as photoluminescence. The yield this gives is a measure for the generation of charge carriers in the samples. Unlike what normally happens in semiconductor nanocrystals, the nanocrystals investigated showed an increase in yield with increasing photon energy. As the concentration of nanocrystals increased this process proceeded far better. The process of multiplying charge carriers already begins within the wavelength range of visible sunlight. Applying high concentrations of silicon nanocrystals would therefore seem to be a highly promising way of increasing the yield of ordinary solar cells produced from silicon. This idea will now have to be tested in photovoltaic devices to see if an increased production of charge carriers still occurs.
Further information
MSc. Dolf Timmerman, tel. +31 (0)20 525 63 39
Prof.dr. Tom Gregorkiewicz, tel. +31 (0)20 525 56 43
Reference
Step-like enhancement of luminescence quantum yield of Si nanocrystals,
Dolf Timmerman, Jan Valenta, Katerina Dohnalová, Wieteke de Boer and Tom Gregorkiewicz, AOP, DOI 10.1038/NNANO.2011.167, Nature Nanotechnology, 9 October 2011.
All of the authors are working at the Van der Waals-Zeeman Institute of the University of Amsterdam with the exception of J. Valenta, Charles University, Prague.
The research was funded by NWO, Technology Foundation STW and the FOM Foundation.