Thin film role

A government agency has injected over £6m into UK research which aims to halve the cost of solar photovoltaic (PV) cells.

The funding renews the EPSRC's original four-year, £4.5m project — part of the Supergen initiative — to 2012. By then, researchers from a consortium of UK universities, including Durham, Southampton and Bangor, hope to have developed more efficient and cheaper PV cells that can be manufactured on a larger scale.

Industry partners include First Solar and Pilkington Technology, with further academic input coming from Bath, Cranfield, Northumbria and Edinburgh universities and Imperial College London.

In the first four years of the project, the scientists created platform technologies in crystalline silicon, thin film silicon, thin film cadmium telluride and thin film copper indium diselenide. Now they will narrow down the research.

'We decided we would concentrate on thin film for the renewable programme, and the crystalline silicon would go down another path. The facility being developed for crystalline silicon, Loughborough, is much more industry-standard type of stuff,' said programme director Prof Stuart Irvine, who is based at Bangor.

'That reflects the fact that currently most of the world's PV industry is based on crystalline silicon so it is much more near-term in terms of the impact that it can make,' he added.

While thin film is in production already, it only makes up about 10 per cent of the total world production. However, Irvine believes the production would expand rapidly because of its inherent low cost.

The researchers will not make modules as part of the programme, but have set a target of 50 per cent reduction in cost/w at the module level. A PV module presently costs around £2/w.

'Thin film is now looking cheaper per watt for two reasons —the production scale is much larger with thin film and cost per watt.' said Irvine.

'If you have a very inefficient panel, you have got to make a bigger area to generate the same amount of electricity, and that has been one of the problems of thin film. Its efficiency has improved over the past few years, and although it is nowhere near as efficient as crystalline silicon that is something that we can address,' said Irvine.

To increase the efficiency of PV, the researchers are looking at ways to make more efficient use of the light being collected. One aspect involves exploring ways to improve the collection of light throughout the day.

'Southampton is looking at moth eye structures, replicating with nanostructures what nature does in moths in not reflecting light back from their eyes — because predators would be able to catch them,' said Irvine.

Another method is to widen the spectrum of light captured by the cells using single-junction cells. A junction works like a one-way valve to direct unstable electrons out of the cell.

'There is also work on light harvesting. This is about changing the absorption of the materials; it is about creating layers that are fluorescent so they absorb one wavelength and re-emit at a wavelength which is efficient for the cell to capture that light.

'Take cadmium telluride cells. Because of their structure, the light is absorbed in the blue part, and the ultraviolet part of the spectrum is not captured because of a layer which absorbs that. If we had a layer above it that absorbed the UV in the blue, and then recycled the protons into the red, then they get through this layer which is at present absorbing but is necessary as part of the junction,' said Irvine.

In developing more producible cells, the researchers found that at higher production capacity, materials costs are the main issue.

'We can look for alternative and cheaper materials. A material's sustainability also comes into that, because if we have to use all the world's reserves of indium for copper indium diselenide, then that is not a sustainable process,' said Irvine.

Using less of the material would be another way to approach the problem. For instance, cadmium telluride cells are produced at a thickness of about eight microns because of the nature of the material and the cell structure.

'Most of the light you are going to absorb will absorb within one micron, so we can engineer the device in a way where we can actually get away with one micron,' said Irvine. While they could go thinner, the cells can become transparent and let light through, which is where the light capturing research comes into play.