Revisiting the Double-Cable Approach

The fundamentals and the various factors affecting the physics of bulk heterojunction solar cells have already been studied and modeled in several original research papers [1-3, 28, 30, 57-59]. However, diffusion processes driven by photoinduced chemical potential gradients appear to play a very important role in D-A solar cells, a fact that, at least from a molecular point of view, must be taken into account when designing intrinsic D-A ambipolar materials. As discussed below, this impUes that a too intimate, overly homogenous D-A distribution is counterproductive for the operation of bulk heterojunction solar cells. 

The open circuit voltage (Vqc) of bilayer heterojunction solar cells is governed by the energetics at the D/A interface rather than that involving the electrodes, as pointed out already by Tang in his seminal paper [26]. Conversely, bulk heterojunc- 

Therefore, no photoinduced chemical-potential energy gradient Vght) (represented by arrows) is established. For the cell functioning, an electric field is required, in D-A cells electrons are photogenerated In one phase while holes are generated in the other via interfacial exciton dissociation Vgto drives 

hole conducting, conjugated backbones with tethered fullerene moieties have been reviewed. The experimental results obtained so far evidence that in this class of polyfuUerenes a photoinduced electron transfer occurs, resulting in long-lived, mobile holes and electrons. Although the first reports of double-cable poly-fullerenes based solar cells and photodetectors were published almost a decade ago, examples of appUcation of these materials in real devices remain scarce. Since in double-cable polyfuUerenes the degree of D-A phase separahon can be dictated by their primary structure, they could in principle lead to the simultaneous control of the electronic properties and the meso- or nano-structure of the photoachve layer of plastic solar cells. 

Finally, double-cable polymers could be ambipolar materials for photomodu-lated p-n type organic field effect transistors (OFETs). 

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