By Renewable Energy Focus staff

According to PhD student Michael Wank's research, which focuses on amorphous silicon thin film, this will almost certainly "result in a further reduction in the price of the a-Si cells".

"Amorphous silicon (a-Si) is an interesting alternative to crystalline silicon," begins Professor Miro Zeman of TU Delft, Michael Wank’s PhD supervisor. "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 nanometres thick, compared with the 200-micrometre thickness in the case of crystalline silicon".

But despite a-Si already being used commercially, one problem for the industry is that the usual production technique (vapourising layers of silane gas) is too slow. "It takes about one second to apply a 0.1-nanometre layer, so to apply a complete 250-nanometre layer requires about 40 minutes," explains Professor Zeman. "That is really too long, and is reflected in too high a cost-price".

Energy yield

It was for this reason that PhD student Wank concentrated on the new ETP-CVD (expanding thermal plasma chemical vapour deposition) production technique, developed by Eindhoven University of Technology; using this the speed of production was eventually increased by a factor of 10, to one nanometre per second, while maintaining an energy yield (for amorphous silicon) of around 7 per cent.

Obstacle

There was one remaining obstacle Wank had to overcome – the fact that the ETP-CVD technique requires a temperature of around 350 degree Celsius in order to make amorphous silicon of the required quality. A production temperature of that level causes damage to the solar cells, which affects their energy yield.

In order to circumvent this, Wank applied an ion bombardment (charged particles) during the production process. The ions provide the developing surface with sufficient energy, so that production can take place at a much lower and therefore non-harmful temperature of around 200 degrees Celsius.

Industry

"The results of the research are of great interest to industry, which can use the method to make solar cells quickly and inexpensively," says Professor 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."