Other Photoconductive Polymers

The literature refers to several other apparently marginally photoconductive polymers that do not fall into the above groins of polymers. 

Gipstein and Hewett nthe ed the polymer and copolynffir of 5-vinyl-2,2 -bithiophene which becomes photoconductive when complexed with large 

Similarly, vinylferrocene copolymerized with a small amount of a film forming monomer displays photoconductivity when complexed with 2,4-dinitrophen-anthrolinequinone. 

Some proteins have been reported as photooonductors. Eley and Metcalfe measured nontransient photocurrents in hemc kAin, mitochondria and cytochrome C. The photocurrents were similar to those of DNA. 

As mentioned above, most of the discussed photoconductive polymers are p-t) e photoconductors. Even in those cases where the charge carriers were not identified, it is safe to assume that the m ority carriers are holes. All polynuclear aromatic and arylamine polymers are basically electron donors and therefore the transport involves equilibria between the neutral monomer units and positively 

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In general, the polymers with polyconjugated systems of double and triple bonds are photoconductive in the UV and at least part of the visible range. In some cases the photoresponse extends to the near infrared range. Although their usefulness in practical applications has been many times suggested, the results have been more or less disappointing. The main problems still remain difficult synthesis, in most cases poorly identified structure, and with few exceptions insolubility and intractability of the polymers. The direct comparison with poly(N-vinyl carbazole) and other photoconductive polymers is not possible for lack of comparative data. 

Other Photoconductive Polymers with Non-Conjugated Main Chain... 

Many other photoconductive polymers with extended polyconjugation of multiple bonds and complex structures have been described in the literature. In many cases, however, these polymers have low molecular weight, their structures are sometimes poorly characterized or they are insoluble. Moreover, direct comparison with well-studied polymers such as PVK is not possible owing to lack of data [2]. 

OTHER PHOTOCONDUCTIVE POLYMERS WITH NON-CONJUGATED MAIN CHAINS... 

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. 

The excellent agreement between the TSC and P1A results has two implications. First, since the TSC method probes the product of mobility and carrier density, while the P1A probes only the carrier density, there seems to be no dominant influence of temperature on the carrier mobility. This was also found in other conjugated polymers like /ra/ry-polyacetylene [19, 36]. Second, photoconductivity (observed via the thermal release of photoexcited and trapped earners) and photo-induced absorption probe the same charged entity [36, 37J. 

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

The presence of the inverted region, however, has not yet been demonstrated [29], With the appropriate design (see below) some of these ions can be considered as a new type of an ion pair [42], a penetrated ion pair (PIP) in which one of the ions is buried inside the other. Research into PET in organized media is very active [40-41] and encompasses a variety of topics from semiconductors [43] and zeolites [44], through various photoconductive polymers [45] to PET-initiated polymerizations and depolymerizations [46] that are generally outside of the scope of this review. 

The following photoconductive polymers can also be clarified as polymers of aromatic amines poly(N-vinylphenothiazine) and poly(N-vinylphenoxazine ° and poly(N-acrylodibenzazepine) ° Poly(N-vinylcarbazole) is basically a modified vinyldiphenylamine polymer . It has yet to be detemined if the transport characteristics of PVK with the diphenyl amino group forced into planarity are different from those of poly(N-vinyldiphenylamine) which would possess a greater freedom of rotation. The properties of PVK have been discussed in many articles and reviews [for example see Ref. ]. Several articles and patents have been published recently which deal with carbazole containing polymers other than PVK, and copolymers of N-vinylcarbazole with some other monomers. 

A number of other unsaturated poiyhydrocarbons have practical applications. These include poiy(phenyl acetylene) and poly((E,E)-[6.2]paracyclophane-1,5-diene), which have been studied as photoconducting polymers. The thermal decomposition of polyacetylenes and of poly((E,E)-[6.2]paracyclophane-1,5-diene) generates fragments summarized in Table 7.1.8 [19]. 

Some polymeric systems which open up other areas of general photophysical interest include an examination of the properties of photoconducting polymers with the two pendant chromophores,... 

Since 1992 when the first edition of the Handbook of Polymer Synthesis was published a number of new applications for photoconductive polymers or, to put it correct, charge transport materials, have appeared. The most successful development are organic light emitting diodes (OLEDs) whieh right now enter the market as bright displays for cellular phones and ear radios. Other imortant areas are organie field effect transistors, solar cells and lasers. 

A number of composite systems have been reported in which a polymer possessing one of the requisite functionalities, e.g., covalently attached NLO chromophores, is doped with the others, e.g., CG and CT dopants, or a photoconductive polymer, e.g., poly(N-vinylcarbazole), is doped with the sensitizer and NLO dopants (1, 4-7). The high dopant loading levels necessary (up to 50 wt%) result in severe limitations of doped systems, including diffusion, volatilization, and/or phase separation (crystallization) of the dopants. In addition, plasticizers and compatibilizers are often used to lower the glass transition temperature (Tg) of the polymer and increase solubility of the dopants in the host polymer, respectively. This, in turn, dilutes the effective concentration of CT, CG, and NLO moieties, diminishing the efficiency and sensitivity of the photorefractive polymer composite. 

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