Chemists in Switzerland and China have used a liquid electrolyte to make flexible solar cells that are better than current devices at withstanding heat from the sun's rays. The new type of dye-sensitised solar cell (DSC) is the latest product from Michael Grätzel, who was the first to make DSCs in 1991.
'This cell will make an important contribution to the market for power generation in the future,' says Grätzel, director of the laboratory of photonics and interfaces at the Swiss Federal Institute of Technology in Lausanne (EPFL). In contrast to traditional solid silicon semiconductor-based cells, the DSC uses light-sensitive dye molecules in a liquid electrolyte.
The liquid electrolyte is in contact with all parts of the cell at once, making it a highly efficient conductor. This also makes the cell flexible - so cells could be used in portable electronic devices. But using DSCs in large outdoor arrays has been difficult because the liquid electrolyte - often a volatile solvent - can evaporate at the high temperatures cells reach in direct sunlight.
Recent designs have made use of ionic liquids: salts with a low melting point that are liquid at room temperature. Although these solvent-free DSCs are more stable to heat, they can be inefficient.
Grätzel's team has solved this problem by using a 'eutectic melt' - a mixture of several solids that form a liquid when combined. One example of a eutectic melt is seen when salt is used to de-ice roads. Solid sodium chloride and solid water (ice) mix to form liquid saltwater.
'We mixed three solid imidazolium iodides to form a highly conductive, ambient-temperature liquid,' says Peng Wang, who worked on the project at the Chinese Academy of Sciences in Changchun, China. 'In contrast, the individual ingredients are solid and non-conductive at room-temperature.'
The advantage of using these eutectic mixtures, Wang explains, is that they have much higher conductivities than ordinary ionic liquids but remain extremely stable at high temperatures.
The solar cells can turn about 8 per cent of the sunlight that falls on them into electricity but Grätzel believes that their efficiency could be improved further. 'The maximum power conversion efficiency that can be reached by this kind of solar cell is around 31 per cent,' he says.
Commenting on the work, Juan Bisquert, who works on dye-sensitised solar cells at Jaume I University in Castellón de la Plana, Spain, says, 'The 8 per cent efficiency is comparatively high, as this is approaching the amorphous silicon technologies that are on the market.'