A team of European scientists have synthesised a conjugated copolymer that behaves remarkably differently compared to related copolymers when light is shined on it. The result has considerable importance for the design of organic solar cells, they say. 

Conjugated copolymers containing both electron-rich (donor) and electron-poor (acceptor) units can act as semiconductors. When light shines on the copolymer, it excites an electron from one energy level to another. The electron and the resulting hole it leaves behind are known as an exciton. The interfaces in the polymer between the donor and acceptor units allow the exciton to split, separating the charges and generating a photocurrent. This property makes conjugated copolymers potentially useful materials for organic solar cells.

The researchers, led by Ullrich Scherf, at the University of Wuppertal, Germany, made two series of copolymers from quinoxaline (acceptor) and oligothiophene (donor) units. When they varied the length of the oligothiophene segment, they saw no effect on the UV/Vis absorption spectra of the copolymers. 

'This is an unexpected finding,' said Scherf. An increase in the length of the oligothiophene segment has been shown in related polymers to shift the absorption band of the copolymer towards lower energies. It is unclear why this is not seen with the quinoxaline-oligothiophene copolymer and it cannot be explained by current models. 

Hugh Burrow, an expert in the photophysics of conjugated polymers from the University of Coimbra, Portugal, was also surprised by the finding. 'As well as being important for the application of these copolymers in molecular electronic devices, the observation poses an interesting challenge for those involved in theoretical calculations on these systems,' he said.  

Joanne Thomson