41 Whiskers are very thin, 0 -0.3 wm, single-crystalline ceramic "fibers" with the highest known tensile strengths.have low efficiencies of energy conversion due to poor photoinduced charge separation and transport of photogenerated charges."] When the light is absorbed in these polymers, electron-hole pairs are created in the form of excitons. The characteristic length of the exciton distribution is given by the inverse absorption coefficient, and can be on the order of 20-1000 nm. These excitons must then be dissociated, and the different photogenerated charges must be transported to electrodes to produce an appreciable photocurrent. Because the life cycle of the exciton includes both radiative and non-radiative decay, which give a lifetime in the range of nanoseconds, and the diffusion length of the exciton in these polymers is in the range of 10 nm, it is necessary to move the excited state to a site for charge dissociation within this distance and time. The dissociation often occurs in the strong electric field in the depletion layer that is created at the polymer/metal interface by the difference of electron affinities or ionization potentials of these materials. Only a small fraction of the photogenerated pairs will be able to transit this region, and thus contribute to the photocurrent.In order to improve the exciton dissociation it is necessary to distribute the sites for photoseparation. This approach involves the formation of a composite of two phaseseparated materials with different electron affinities, where one is the donor and the other is the acceptor material, making what is called a "bulk D-A heterojunction material",[2-61 which provides the spatially distributed interface necessary to dissociate the excitons. The free charges created at these D-A heterojunctions are then transported separately by donor or acceptor material, and are collected at the contacting electrodes. The two contacting electrodes are chosen so as to create a built-in electric field in the device, due to the different work functions of the electrodes, and in this way to enhance the photoseparation and the discharge of photoinduced carriers. Conjugated polymer/ buckminsterfullerene (C,,) composites, as well as two different polymers with different electron affinities, have been proposed for use as the D-A materials.[241We have focused on the use of polythiophenes in combination with C6,; earlier studies have reported on this system."] Several reasons can be given for this choice. First, the solubility and/or fusibility of these polymers allows formation of thin films of optical quality; they also allow formation of complex morphologies due to phase separation and supramolecular organization.17] Second, it is easy to tailor their optical properties by modifying the polythiophenes via simple substitution on the main chain, at least for bandgaps varying from 1.7 to 3 eV.I8] The lowest bandgap allows the creation of polymer light-emitting diodes in the near infrared rangeLy1 and matches well with the ultimate requirement for sola...

Spectroelectrochemical techniques have been used to study doping-induced reactions in conjugated polymers. Here, we report results on reduction reactions (n-doping) of the conjugated ladder polymer polybenzimidazobenzophenanthroline (BBL). The spectra are recorded in situ during applied potential in a three-electrode spectroelectrochemical cell. The spectral and thus the structural changes during the reduction (n-doping) of the polymer film at different electrode potentials are discussed. In contrast to most of the other conjugated polymers, this polymer shows four reversible redox reactions during n-doping, corresponding to various insulating and conducting forms of BBL.

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