Polarons polymer heterojunctions

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. 

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... 

Heterojunctions formed by electrodepositing polypyrrole or polyaniline on retype silicon were studied with PVS [756]. Experimental observations were explained invoking photosensitization by electrochemically generated polarons in the polymer film. The effects of nano-sized particles of Fe203 that were incorporated into films of polypyrrole were investigated with PVS [757]. 

After splitting of the exciton at the heterojunction and further separation of the charge-transfer state, the charges (or polarons) need to be transported within the semiconductor materials network to the respective electrodes. Despite the recognition that it is mainly the separation of the charge-transfer states that limits current polymer polymer solar-cell devices, charge transport also has to be considered, because it is another potential loss mechanism. We will briefly cover this topic the interested reader is referred to a recent review by Blom et al. [7]. 

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