Cubic silicon carbide could be twice as good as silicon for solar cells
June 22, 2012 // Julien Happich
According to Swedish researchers at the Linköping University, cubic silicon carbide may be capable of collecting more than twice the solar energy than silicon based solar cells.
The cubic silicon carbide is considered a perfect material for impurity (intermediate bandgap) solar cell. For boron doped cubic SiC, the dopant band of B in the bandgap of 3C‐SiC leads to an efficient use of sun light so that an efficiency up to 48‐60% could be achieved depending on theoretical models. But so far, cubic silicon carbide has shown to be the black sheep of the silicon carbide family, explains Mikael Syväjärvi, associate professor at the Linköping University. While hexagonal silicon carbide types have been commercialized for many years, cubic silicon carbide has faced too many challenges, such as being metastable, meaning that it does not really want to form. One has to decrease the growth temperature to make it form, but at the same time the growth rate is decreased. A common approach is to use silicon as a substrate, but the lattice and thermal mismatch causes defects and stress.
The Swedish researchers have applied a bulk growth approach like used in production of hexagonal silicon carbide. According to Syväjärvi, the trick is to lower the growth temperature while adjusting other parameters to maintain a high growth rate, at 1mm/hour. The group applies hexagonal silicon carbide as substrate, and the material is transformed to the cubic structure during initial stage of growth. The advantage of using hexagonal substrate is the perfect matching. Structural measurements have showed a similar quality like in hexagonal commercial material. The key parameter in showing off the quality is the carrier lifetime. Previously this had a lifetime of about 0.1 µs, while the new record value is 8.2µs in as‐grown material, an increase of almost two orders of magnitude. In comparison, this is even slightly better than that in hexagonal silicon carbide.
Today's silicon solar cells have an efficiency of 20%. In order to increase the efficiency of solar cells, multi-junction (thin film) solar cells with different bandgaps is one of the most promising approaches. The best efficiency of such solar cells demonstrated on the research scale is 43.5%. However, the challenges in fabrication of multi-junction solar cells lie in the growth of multi‐stacked material and balance of junction currents. Cubic silicon carbide in a single material which is doped during growth, having a high growth rate such as 1 mm/hour, could pave the way for more efficient solar cell concepts. These results on the growth of cubic silicon carbide were published in the Applied Physics Letter of June 18.
Visit the Linköping University at www.liu.se
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