Using Orbital Symmetry to Minimize Charge Recombination in Dye-Sensitized Solar Cells

Maggio, E., Martsinovich, N. and Troisi, A. (2013), Angew. Chem. Int. Ed., 52: 973–975.
Contributed by Gemma Solomon 

Orbital symmetry can be used in dye design to retard charge recombination in dye-sensitized solar cells. 

Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Finding the balance between competing processes is a central challenge in optimizing the performance of dye-sensitized solar cells. One aspect of this problem is the dichotomy between the requirement for fast charge injection between the dye and a semiconductor surface (for photo-induced charge separation) and slow charge recombination between the two. There are a range of strategies available for optimizing one or other of these processes; but clever chemical design is required in order to optimize the two together. Minimizing unwanted charge recombination involves beating the Coulomb interaction, and this is a formidable opponent indeed.

In their recent paper Maggio, Martsinovich and Troisi have proposed an elegant solution: use orbital symmetry to retard charge recombination. Building on the ideas from studies of electron transfer, they show that it is possible to design dyes where the charge injection is symmetry allowed, while charge recombination is symmetry forbidden. In terms of molecular orbitals, charge recombination involves injection into the HOMO of the dye from the semiconductor, while charge injection involves the LUMO of the dye injecting into the surface. Depending on the symmetry of the dye, and the position at which it is substituted, the authors show that it is possible to have the LUMO interacting strongly with the surface (delocalizing into the binding arm) while the HOMO remains isolated, as illustrated above.

This represents a new strategy for dye design and an exciting prediction from computational chemistry. It will be interesting to see how dyes using this strategy perform in devices and should remind us all that we have a powerful tool for molecular design with symmetry at our disposal.