Materials that can transport excitons (photo-generated electron-hole pairs) over distances of more than 100 nm are useful for a range of light-harvesting and optoelectronics devices, including solar cells and photodetectors. 100 nm and beyond is the optical absorption depth – a measure of how deep light can penetrate into a material. Such materials are difficult to make, but researchers in the UK say they have now developed polymer-based semiconducting nanofibres in which excitons can move extremely quickly and travel over distances as long as 200 nm.
“The ability to control the assembly of these relatively large and remarkably well-ordered nanofibre structures is really exciting and opens up a whole new landscape of possibilities in light-harvesting,” says Richard Friend of the Cavendish Laboratory at Cambridge University, who led this research effort together with George Whittell and Ian Manners at Bristol University.
The researchers made their nanofibres from a core of well-ordered short chains of an organic semiconductor polymer (crystalline poly(di-n-hexylfluorene), or PDHF). “The chains stack to create very good contacts between them. These contacts allow the excitons to move extremely quickly and travel over distances as large as 200 nm,” according to our measurements,” explains Friend. “The important point here is that 200 nm is thick enough to absorb all incident light, so we can envisage using these materials as light harvesters for solar cells and photodetectors,” he tells Physics World.
The nanofibres also contain a solvated, segmented corona consisting of polyethylene glycol (PEG) in the centre and quarternized polythiophene (QPT) at the ends. The excitons transfer with a diffusion coefficient as large as 0.5 cm2/s from the core to the lower-energy polythiophene corona in the end blocks.
A “real game-changer”
“We have been using polymer crystallization from solution as a method to make nanoparticles with complex structures and controlled size and shape for the past 12 years now,” add Whittell and Manners. “The fact that this same technique also allows us to produce particles with enhanced optoelectronic properties, compared to the same polymers processed in a different way, is a real game-changer for the field.”
Spurred on by their new result, the researchers say they are now planning to use their technique to prepare structures thicker than the optical absorption depth. “We would like to find out whether the large exciton diffusion lengths we observed in our present study can be preserved,” explains Friend. “The next step would be to construct bilayer junctions that may function as structurally simple but very efficient solar cell devices. We are also looking to prepare more complex structures, which will allow us to harvest the energy from light to promote chemical reactions.”
Full details of the research are reported in Science.