A highly efficient light harvesting molecule could lead to cheaper solar cells, claim international scientists.

Conjugated ligands increase the amount of light absorbed by the ruthenium complex

Developed in the early 1990s, dye-sensitised solar cells are a class of low-cost solar cells which have a layer of titanium dioxide coated with a light-harvesting sensitiser (dye). Peng Wang from Changchun Institute of Applied Chemistry, China, and Shaik Zakeeruddin and Michael Grätzel from Swiss Federal Institute of Technology, Lausanne, Switzerland, and colleagues have made a new sensitiser with a high extinction coefficient - meaning it is excellent at absorbing light.

The new sensitiser is a ruthenium complex with highly conjugated ligands containing thiophene rings. Preliminary tests using this sensitiser in a solar cell obtained a power conversion efficiency of 10.53 per cent, which is comparable with the 11.1 per cent achieved by the most efficient dye-sensitised solar cells reported to date. Grätzel says that this 'looks very promising'. This conversion may be lower than for commercially available silicon-based solar cells - at 20-25 per cent efficiency - but dye-sensitised solar cells are still desirable as they are more robust and intrinsically more stable than silicon-based solar cells.

The addition of the thiophene rings to the ligands in the ruthenium complex increases the conjugation, and therefore improves the sensitiser's overall light absorbing capabilities. 'Less dye is required to extract the same amount of energy from the available light, which translates into immediate cost savings,' says Kevin Tabor, director of science and research at G24 Innovations, Cardiff, UK, a large scale manufacturer of solar cells using Grätzel's technology. Additionally the thiophene ring has shifted the absorption band of the sensitiser into the red region. 'This means further energy uptake over other sensitisers and from an aesthetics perspective, a deeper colored, almost black solar cell,' explains Tabor.

Developing a more efficient sensitiser, which is the heart of the dye-sensitised solar cells, will help towards increasing the - currently very small - market share of this type of solar panel, says Grätzel.

Emma Shiells