Lewis Brindley /Turin, Italy

Rare ruthenium complexes that are a key component of dye-sensitised solar cells could in future be replaced by molecules based on copper, say Swiss researchers.

Dye-sensitised solar cells (which use absorbent dyes to convert sunlight into photoelectrons and a separate semiconductor to generate a current) are promising candidates to replace conventional silicon solar panels, since they are cheaper and more flexible - though not yet as efficient. 

Though only small amounts of a ruthenium-based dye are required in the solar cell, ruthenium is one of the rarest metals on Earth, so the team at the University of Basel are hoping to switch to complexes of a more common metal. 

Presenting their work at the 2nd EuCheMS chemistry congress in Turin, Italy, Ana Hernandez Redondo explained that the team have showed that complexes of copper(I) can interact with light in a similar way to ruthenium. The key, they revealed, is to keep copper in the ⁺1 oxidation state, which is achieved by 6,6'-disubstituted 2,2'-bipyridine ligands that grip the copper ion tightly, preventing it from oxidising into copper(II). 

'Our first-generation copper-based cells showed light conversion efficiencies of 2.3 per cent, which is around four times lower than ruthenium complexes currently on the market,' said Redondo. 'But our research indicates that copper polypyridine complexes are a candidate to be the sensitisers [dyes] of the future,' she added, pointing out that the efficiency of the new complexes is comparable to early ruthenium compounds first made during the 1980s. In a recently-published paper, the researchers estimate that the cost of the copper complex is 'an order of magnitude lower' than that of the ruthenium sensitiser. 

Redondo says the team are confident that small modifications to the ligand structure, based on past experience with ruthenium complexes, should boost light conversion efficiencies considerably. They have already made progress, she announced at the conference, by integrating an additional phenyl group in the polypyridine ligand. This gave greater light absorption over the visible spectrum and the team expected to see a jump in conversion efficiency. 

Dye-sensitised solar cell inventor Michael Grätzel, also speaking at the Turin conference, praised the work, saying that 'there is a great need to tailor the best sensitisers in order to continue optimising the efficiency and cost of these cells.' 

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