Sensitised cells

An international group of researchers reported last month that it has developed and tested a new generation of photovoltaic cells in Lausanne, Switzerland.

For the first time, the team at Ecole Polytechnique Federale de Lausanne (EPFL) used two different dyes to make dye-sensitised solar cells, making them capable of reacting to a larger range of the light spectrum and thus more efficient.

Dye-sensitised solar cells date to the 1990s. But the phthalocyanine dyes used in them are only sensitive to a restricted range of the solar spectrum.

The new cells, developed by a team led by EPFL's Prof Michael Grätzel, allows that spectral sensitivity to extend to the red, green and blue ranges of visible light, by incorporating a second, perylene dye. With a larger range of light sensitivity, the cells' efficiency is thus improved.

Although the perylenes do not directly generate an electric charge, they react to the blue and green parts of the visible light spectrum. They transfer their energy to the phthalocyanines, which in turn transmit the electric charge. Without the new dyes, the phthalocyanines can react only to the red part of the spectrum.

Khaja Nazeeruddin, a researcher in Grätzel’s group at EPFL, said: 'It is not possible for a single dye to be sensitive to the entire spectrum of visible light, which is why we incorporated a second dye. This is the first time this has been done.'

Grätzel added: 'It is what we call energy transfer by dipolar interaction. Until now, the dyes in our cells had the unique role of directly generating the electric charge.'

The cells were tested in the EPFL laboratory by Grätzel and Stanford University researcher, Brian Hardin. The charge transfer improved by 26 per cent over cells with phthaolcyanine alone.

The discovery is the first result of a recent partnership that was created between EPFL, Stanford, Berkeley and Georgia Tech. Led by Stanford, the project, known as CAMP, aims to improve the efficiency and longevity of molecular solar cells, and to develop low-cost solar-cell production techniques.