Exciton dissociation in conjugated polymers

In conjugated polymers, a majority of photogenerated charges form metastable geminate pairs (GPs), of which only some fraction can dissociate completely. Both the yield of GP photogeneration and the probability of further dissociation of GPs into free charges depend upon an external electric field. In the present article we discuss several experimental methods to detect the existence of geminate pairs such as delayed field collection of charges, field quenching of fluorescence, and field-assisted photoinduced optical absorption. It is shown that the field dependences of the exciton dissociation into GPs and of the free carrier photogeneration yield are rather similar. This is in contrast with the traditional Onsager theory, which assumes field-independent yield of primary photoionization and disregards the field dependence of the initial separations between carriers in GPs.