Angew. Chem. Int. Ed. http://doi.org/fz9jpn (2012)

In a dye-sensitized solar cell, a dye molecule is adsorbed on a semiconductor nanoparticle and injects an electron into the nanoparticle when exposed to sunlight. To improve the efficiency of such solar cells, researchers have tried to increase the rate of the charge-injection process while minimizing charge recombination, and it is known that both events are determined by energetic and kinetic factors. Alessandro Troisi and colleagues at the University of Warwick have now suggested that molecular symmetry should also be taken into account.

In an ideal system, charge injection would be allowed by symmetry and charge recombination forbidden by it. However, this is almost impossible when either the charge donor or acceptor is a semiconductor because there will always be a state in the semiconductor that matches the symmetry of the dye orbital involved in the process. The trick then is to use a bridging group that physically separates the dye from the semiconductor so that the through-bond channel is open only in one direction. Using simple quantum-mechanical calculations, Troisi and colleagues show that this is possible for molecules possessing a certain type of symmetry. In such systems, the dye localizes a high molecular-orbital density on the bridge and promotes charge injection, but when oxidized the dye molecule remains almost completely isolated from the bridge and prevents charge recombination.

With this approach, charge recombination rates could be reduced by two to three orders of magnitude.