Excitons and free charges

In the optical and electrical activity of conjugated polymers several phenomena occur  

The optoelectronic properties of dilute solutions of oligomeric and (broken-conjugation] polymeric PPV chains were studied using optical absorption and (time-resolved] emission spectrophotometry. The following properties were determined absorption and emission spectra, fluorescence quantum yields and decay times, exciton polarizabilities and dissociation probabilities, charge mobilities, and RC absorption spectra. The experimental results are compared with theoretical calculations of exciton polarizabilities, charge mobilities, and RC absorption spectra.  

Some authors have proposed several modified process approaches to obtain electroactive electrospun nanofibers. One first practical and easy way is to spin a nonconductive polymeric web and after polymerize conductive polymers onto the fiber surface. For example, conductive polyamide-6 (PA-6] nanofibers were prepared by polymerizing pyrrole (Py] molecules directly on the fiber surface of PA-6. First, a solution of PA-6 added with ferric chloride in formic acid was electrospun with average diameter values around 260 nm. 

Fibers were then exposed to Py vapor and a compact coating of PPy was formed on the fiber surface, the PPy coating on the fibers turned out to be conductive. 

A similar treatment was used to electrospin poly(vinylidene fluoride] (PVDF]/PPy composites, which were prepared by spinning a nonwoven web from a solution of PVDF and CUCI2.2H2O in dimethylacetamide (DMAc) and then exposing the spun fibers to Py vapors in order to produce the conductive composites. The electrical conductivity of the PPy composites was affected by the fabrication method and oxidant content in the nonwoven web. 

In a perfectly ordered solid or a perfectly ordered macromolecule excitons move according to quantum-mechanical kinetics, i.e., like wave packets. Because this requires strict phase relationships in space and time, this mode of motion, addressed in the literature as coherent motion of excitons, is perturbed by all deviations from regularity. Chains of conjugated polymers always include various defects such as kinks and torsions. These break the conjugation. Typically, regular sequences extend only over five to ten repeat units. The results of measurements for a series of oligomers like the ones displayed in Fig. 7.4 can be used for the estimate. The red shift of the exciton frequency to with increasing monomer number, n, can be described by the equation 

Due to the coupling of the excited electron to the hole left back on the HOMO level, excitons are non-charged particles that cannot contribute to an electrical current. A current requests the motion of free charges, either electrons or holes. These can indeed exist in conjugated pol3miers. They are found if 

The experiments shown in Figs. 7.8 and 7.9 provide examples for the first two processes. 

The second example, which deals with poly(phenylene vinylene), shows that this difference can also be very small and virtually vanish. Here, the onset of the absorption is practically identical with the onset of the photocurrent. The reason for this peculiar property of poly(phenylene vinylene) is still under discussion. There might exist sites (in particular places located at the film-electrode interface) where the additional energy necessary to dissociate the exciton is spontaneously provided. 

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