Solar cells reach into the infrared

Mark Peplow/San Francisco, US

A dye molecule that efficiently harvests the energy of near infrared light could boost the output from the next generation of solar cells.

The dye, a zinc phthalocyanine compound, has been developed by Michael Gr?tzel of the Swiss Federal Institute of Technology, Lausanne.

’This is a breakthrough in the design of infrared sensitisers,’ said Gr?tzel. Since about half the Sun’s energy arrives at near infrared frequencies, the dye will increase the amount of sunlight that can be harvested by panels mounted on the roofs of houses.

Gr?tzel was instrumental in developing the dye-sensitised solar cell (DSC) some fifteen years ago. Whereas conventional silicon-based photovoltaics use the same material to harvest light and turn it into an electrical current, DSCs rely on two different compounds for these tasks, just like photosynthetic plants.

While ruthenium-based dye molecules are responsible for capturing visible light, a thin layer of nanocrystalline titanium dioxide produces the electrical current. Titanium dioxide is a common mineral found in paints, toothpaste and sunscreen. 

This makes the DSCs relatively cheap, and Gr?tzel believes that panels built using this technology should be able to pay for themselves in less than a year - a sharp contrast with silicon photovoltaic panels that sometimes take 20 years to become financially worthwhile for homeowners. 

DSCs can currently turn about 11 per cent of the light energy that falls on them into electricity, about half that of conventional silicon photovoltaics, but Gr?tzel says there is much room for improvement. 

Gr?tzel suggests that the near infrared dyes could be used as part of a cocktail with the ruthenium dye, boosting the overall efficiency of the cell by around three per cent. ’It’s one of the most exciting developments in solar cells,’ commented Zakya Kafafi, an optoelectronics researcher at the US Naval Research Laboratory in Washington DC, US.

Gr?tzel also reported that his team has recently managed to remove the volatile solvents currently used as electrolytes in the DSCs without reducing their efficiency. The solvents have been replaced with ionic liquids - salts that are fluids at room temperature - which are less prone to escaping from the photovoltaic cell by evaporation. Using these liquid imidazolium salts should prolong the lifetime of the cells substantially, Gr?tzel predicted. He presented the research on 12 September at the American Chemical Society conference here in San Francisco.

Konarka Technologies of Lowell, Massachusetts, US, is working to commercialise Gr?tzel’s technology, and in July 2006 announced that they were setting up the first factory to produce DSCs on a large scale. ’The dye cell is now a serious competitor,’ said Gr?tzel, adding that the plant will be based in the Midlands region of the UK.

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