US scientists say they can cut the effective cost of solar cells 10-fold by using a new way to capture and concentrate sunlight.

The system uses two different organic dyes coated on the surface of a sheet of glass - called the collector. Sunlight hits the collector and is absorbed by one kind of dye, which passes the energy to a second dye. The second dye then re-emits the light at longer wavelengths - which photovoltaic cells are able to convert to electricity more easily. The longer wavelength light then bounces around within the collector until it reaches one of the edges, where it is picked up by photovoltaic cells. Little of the light escapes on the way because it is reflected by the top and bottom surfaces of the collector.

'This idea has been around since the 1970s,' says Marc Baldo of MIT, who led the research team. 'But then the big problem was loss of light in the collectors. The light must travel a relatively long way to get to the edges. With the old dye systems the light was reabsorbed on its way and gradually petered out before it reached the cells at the edge.'

Organic solar concentrators collect and focus different colours of sunlight © D Coveney, MIT

The key aspect of the new system are that there is a lot more of the first, absorptive dye than the second, emissive one, says Baldo. That means long wavelengths of light are 'ignored' by the bulk of the dye molecules which are interested only in the shorter wavelengths. 

The same system has been used in laser systems for many years, says Baldo. One of the dyes that has proved effective in the MIT solar concentrator, for example, is DCJTB, which belongs to the dicyanomethlene class of laser dyes.

Because the collectors are transparent, Baldo says it should be possible to fix collectors on top of existing photovoltaic arrays. The collector would capture the shorter wavelength light while longer wavelengths would pass through to the cells below. 'In this way we calculate that we could get an overall increase of 20 to 30 per cent efficiency out of the system,' Baldo says. 'Overall we believe organic solar concentrators could reduce the effective cost of solar cells by an order of magnitude.'

Martyn Berry, a senior research engineer with the UK solar power company Solarcentury says, 'The notion that we could see a ten-fold reduction on the cost of photovoltaic technologies through the use of organic solar concentrators is certainly exciting.'

John Wilson, a photovoltaics and thin-film expert at Heriot-Watt University in the UK says that the work represents a significant advance. 'The system lets you shift the solar spectrum to a point where it will be more strongly absorbed, and they seem to have overcome the problem of losses, which previously severely limited the size of the collectors.'

Simon Hadlington

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