Triplet vs Singlet Energy Transfer in Organic Semiconductors: The Tortoise and the Hare

Current bilayer organic photovoltaics cannot be made thick enough to absorb all incident solar radiation because of the short diffusion lengths (approximate to 10 nm) of singlet excitons. Thus, the diffusion length sets an upper bound on the efficiency of these devices. By contrast, triplet excitons can have very long diffusion lengths (as large as 10 mu m) in organic solids, leading some to speculate that triplet excitonic solar cells could be more efficient than their singlet counterparts. In this paper, we examine the nature of singlet and triplet exciton diffusion. We demonstrate that although there are fundamental physical upper bounds on the distance singlet excitons can travel by hopping, there are no corresponding limits on triplet diffusion lengths. This conclusion strongly supports the idea that triplet diffusion should be more controllable than singlet diffusion in organic photovoltaics. To validate our predictions, we model triplet diffusion by purely ab inito means in various crystals, achieving good agreement with experimental values. We further show that in at least one example (tetracene), triplet diffusion is fairly robust to disorder in thin films, as a result of the formation of semicrystalline domains and the high internal: reorganization energy for triplet hopping. These results support the potential usefulness of triplet excitons in achieving maximum organic photovoltaic device efficiency.