18 March 2011
Researchers at TU Delft in the Netherlands have shown that the speed of amorphous silicon solar cells production could be increased by a factor of ten without any detriment to the energy yield of the cells. Producing the solar cells faster will help reduce their cost.
"An interesting alternative to crystalline silicon is amorphous silicon," says Professor Miro Zeman of TU Delft, who supervised PhD student Michael Wank's research on amorphous silicon production. "Although this material has a lower energy yield than crystalline silicon, these solar cells can be produced far more cheaply. The nature of the material means that much thinner layers can be used - around 250 nm thick, compared with the 200-micron thickness in the case of crystalline silicon."
Amorphous silicon solar cells are already being commercially produced, but the typical production technique (vaporizing layers of silane gas) is too slow. It takes about one second to apply a 0.1-nm layer, so the complete 250-nm thick layer takes about 40 minutes, explains Zeman. "That is really too long, and is reflected in too high a cost-price."
Thus Wank looked at how a new technique called expanding thermal plasma chemical vapour deposition (ETP-CVD) production could be used with amporphous silicon solar cells. The production technique was developed by Eindhoven University of Technology and the project was carried out in collaboration with the Plasma and Material Processing group of Professor Richard van de Sanden. They were increase production speed by a factor of ten, which corresponds to a layer 1-nm thick per second, while maintaining a energy yield of around 7%, which is good for amorphous silicon.
There was one remaining obstacle – the ETP-CVD technique requires a temperature of around 350 °C to produce amorphous silicon of the required quality. That high production temperature causes damage to the solar cells, which affects their energy yield. To circumvent this, Wank applied ion bombardment during the production process. The ions provide the developing surface with sufficient energy for production to take place at a much lower and therefore non-harmful temperature of around 200 °C. Wank recently defended his thesis on this subject at TU Delft.
"The results of the research are of great interest to industry, which can use the method to make solar cells quickly and inexpensively," says Zeman. "As well as the greater speed, another benefit is that the machines needed for this technology are smaller. All in all, this technique promises to cut the production costs of this type of solar cell considerably."
Written by Nancy Lamontagne, Contributing Editor - US