Charge transfer polymers, applications

Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole (PVK) and other selected polymers, such as poly(paraphcnylene-vinylene) (PPV) and phenylmethylpolysilane (PMPS), doped with a mixture of Cgo and C70 have been reported to exhibit exceptionally good photoconductive properties [206, 207, 208] which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes (e.g., by weight) lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes, thereby promoting the conduction of mobile holes in the polymer [209]. Fullerene-doped polymers also have significant potential for use in applications, such as photo-diodes, photo-voltaic devices and as photo-refractive materials. 

For photovoltaic cells made with pure conjugated polymers, eneigy conversion efficiencies were typically I0 3-I0 1%, loo low to be used in practical applications [48, 63, 67]. Thus, pholoinduced charge transfer across a donor/acceptor... 

The significance of the semiconductivity of organic dyes and other classes of organic solids, such as charge-transfer complexes, which behave in some ways like metals 7> or polymers 8>9>, is to day beyond doubt. The possible industrial applications of this new class of semiconductors are, however, still in the discussion stage. 

When two polymeric systems are mixed together in a solvent and are spin-coated onto a substrate, phase separation sometimes occurs, as described for the application of poly (2-methyl-1-pentene sulfone) as a dissolution inhibitor for a Novolak resin (4). There are two ways to improve the compatibility of polymer mixtures in addition to using a proper solvent modification of one or both components. The miscibility of poly(olefin sulfones) with Novolak resins is reported to be marginal. To improve miscibility, Fahrenholtz and Kwei prepared several alkyl-substituted phenol-formaldehyde Novolak resins (including 2-n-propylphenol, 2-r-butylphenol, 2-sec-butylphenol, and 2-phenylphenol). They discussed the compatibility in terms of increased specific interactions such as formation of hydrogen bonds between unlike polymers and decreased specific interactions by a bulky substituent, and also in terms of "polarity matches" (18). In these studies, 2-ethoxyethyl acetate was used as a solvent (4,18). Formation of charge transfer complexes between the Novolak resins and the poly (olefin sulfones) is also reported (6). 

Intrinsically carbazole containing polymers are photosensitive in the UV range of spectra. The applications of such polymers in electrophotography and related processes need sensitization to the visual wavelengths. The most acceptable method is charge transfer formation between polymer donor and acceptor molecules. Hoegl pointed out that 0.1-2% of acceptor molecules inserted in the polymer matrix lead to a substantial increase in the photoconductivity especially in the CT bands. Subsequently, a lot of paper were published for CT-carbazole-containing complexes and such materials were used in photosensitive processes. Various types of molecules were used as a photosensitizers. 

Photoeffect sensitization in polymers is realized by means of charge-transfer complexes, dyes, polymers, heterogenous and multilayer structures. Every method permits us to obtain some information about photogeneration, charge transfer processes and can be used for concrete practical applications. 

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