Polaron pairs, geminate

Recently, geminate polaron pairs have been proposed for polymer-polymer [35,56,57] and polymer-fullerene [58,59] blends as photoinduced intermediates. Here the hole and electron remain coulombically bound across the interface of the donor-acceptor heterojunction. Only via an electric field and/or a temperature-assisted secondary process, these geminate polaron pairs are dissociated, leading to free charge carriers. This can have a considerable effect on the achievable charge separation efficiencies, since the geminate... 

Morteani et al. demonstrated that after photoexcitation and subsequent dissociation of an exciton at the polymer-polymer heterojunction, an intermediate bound geminate polaron pair is formed across the interface [56,57]. These geminate pairs may either dissociate into free charge carriers or collapse into an exciplex state, and either contribute to red-shifted photoliuni-nescence or may be endothermically back-transferred to form a bulk exciton again [57]. In photovoltaic operation the first route is desired, whereas the second route is an imwanted loss channel. Figure 54 displays the potential energy ciu ves for the different states. 

At the donor/acceptor interface (step III in Figure 5), exciton D is quenched via electron transfer to the lowest unoccupied molecular orbital (LUMO) level of the acceptor molecule (A°). On the contrary, exciton A is quenched via hole transfer to the highest occupied molecular orbital (HOMO) level of the donor molecule (D ). Both pathways result in the formation of the same charge separated state D+ A . Positive and negative charges in this ion pair are bond by Coulomb attraction forces and also denoted as geminate polaron pair. This pair can dissociate in the electric field induced by the potential jump at the heterojunction and/or by the difference in the electrode work functions. At the same time, the energy difference... 

Thus, the relevant question is whether the photoconductive response in PPV results principally from secondary processes following the photogeneration of neutral geminate pairs, in agreement with the exciton model, or from separated, mobile, positive and negative charged polarons. 

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