Donor-acceptor “bulk heterojunction concept

While in the early 1990s power conversion efficiencies in single layer, single component devices were still limited to less than 0.1% [22-25], improvements over the turn of the millennium are attributed to a great extent to the introduction of the donor-acceptor bulk heterojunction concept, which makes use of two electronic components that exhibit an energy offset in their molecular orbitals [26-34]. 

Several organics, e.g. pristine poly(3-octylthiophene), polyfluorene, bifunctional spiro compounds and polyphenyleneethynylene derivative, have been used for fabricating photOFETs. Responsivity as high as 0.5-1 A/W has been achieved in some of these transistors. We have already discussed the bulk heterojunction concept in Chapter 5. The bulk heterojunctions are fabricated using acceptor materials with high electron affinity (such as C<5o or soluble derivatives of C6o) mixed with conjugated polymers as electron donors. PhotOFETs based on conjugated polymer/fullerene blends are expected to show... 

Donor-acceptor diblock copolymers constitute a further interesting class of materials based on the bulk heterojunction concept being developed lately [38-43]. 

This limitation was finally overcome by the concept of the bulk heterojunction, where the donor and acceptor materials are intimately blended throughout the bulk [28-30]. In this way, excitons do not need to travel long distances to reach the donor/acceptor interface, and charge separation can take place throughout the whole depth of the photoactive layer. Thus the active zone extends throughout the volume, as illustrated in Fig. 11. Conse-... 

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