Zipping photovoltaics up straight

Phillip Broadwith/Glasgow, UK

Straightening out the backbone of supramolecular self-assembling photovoltaic devices leads to dramatic improvements in device performance, Swiss scientists revealed this week at the Iupac Congress in Glasgow, UK.

Stefan Matile from the University of Geneva has pioneered what he calls a ’3D Tetris’ approach to building materials that can harvest light energy and convert it to electricity. The key feature of such materials is the ability to use energy from light to separate charges inside the structure to create negatively charged electrons and positively charged ’holes’, but one of the major challenges has been to get these electrons and holes to move rapidly through the material in opposite directions without recombining, thereby generating an electric current.

By using supramolecular assembly, Matile explains that interpenetrating hole- and electron-conducting pathways can be constructed very precisely using interlocking ’zipper’ molecules. The hole-conducting part comprises rod-like chains of aromatic rings with charged side groups, whereas the electrons are passed through stacks of naphthalene diimide (NDI) dyes which bear complementary charged groups that mesh together with the side chains of the hole-conducting rod molecules like a zip.

Initially, Matile used simple rod molecules with adjacent aromatic rings, but these made bent zipper structures because of a size mismatch with the electron-conducting NDI stacks. By adding a carbon-carbon triple bond between consecutive rings, he explains that the size match is almost perfect and so the structures stand up straight and conduct much more effectively. ’The difference between the two molecules is so small, but the difference in the final structure is clearly visible - it’s quite amazing,’ he adds.

Anthony Harriman from the University of Newcastle, UK, was very impressed by the work. ’It’s a very imaginative approach,’ he says, ’and I think it could also be very relevant to molecular electronics because of the high degree of control over the organisation and orientation of the charge carrying molecules.’

Matile says that the next step is to build redox gradients into the structures by using cascades of subtly different electron- and hole-conducting materials in sequential layers. ’At the moment the diffusion of electrons and holes is still passive. By introducing gradients we can actively funnel the electrons in one direction and the holes in the opposite.’