Polymeric photoconductors

Table 3. Charge-Generation Efficiency of Selected Polymeric Photoconductors ...

Experimental Values of Charge-Generation Efficiencies. In this section the charge-generation efficiencies of many polymeric photoconductors are compared (Table 3). When the experimental data has been fitted to the Onsager model, the initial electron—hole separation distance,... 

Polymers with saturated bonds, heteroatoms, heterostructures and poly-conjugated ones are available now as photosensitive materials. Really one cannot expect a single mechanism to be reponsible for photoconductivity in so many diverse systems. However, there are a lot of verified factors which permit us to explain the main features of the photoconductive processes in polymers. The status and prospects of the application of polymeric photoconductors as prospective new electronic materials will be also analyzed for various types of photoconductors. 

As already shown, the sensitized spectra follow the dye absorption. Most of the dyes have sufficiently narrow absorption bands. This does not permit us to obtain the panchromatic sensitivity in the sufficiently broad spectral range. It was proposed to use the polymers with conjugated bonds as sensitizers [21]. The broad diffuse absorption spectra are inherent to such compounds. One can expect higher thermal stability from such sensitizers. In addition the application of binder may be omitted from the preparation of the photosensitive layers, for example, in electrophotography. Polymers with triple bonds, polyphenylenes and polyoxiphenylenes were used as sensitizers [10, 14, 278-280]. The typical results are shown in Fig. 47. The main rules for photoconductivity sensitized by polymers were the same as for the dyes. Optimum sensitization was obtained at the concentration of the sensitizer of 10 1-10-2 g/cm3 relative to the polymeric photoconductor weight. 

Conclusions. The Status and Prospects of Application of Polymeric Photoconductors... 

Nowadays, polymeric photoconductors may be used in electrophotography, microfilms, photothermoplastic recording, spatial light modulators, and nonlinear elements. The combination of photosensitivity with high quality electrical and mechanical properties permits the use of such materials in optoelectronics, holography, laser recording and information processes. The applications of the various types of polymers were reported in the final parts of the relevant items in the earlier sections. Here, we will briefly analyze the common features of photoconductive polymer applications. The separate questions of each type have been dealt with in some books and papers [3, 11, 14, 329]. 

The most commonly used polymers in this field are the ones containing carbazole. Prospective uses may be connected with heterocycle heterogeneous and multilayer systems with sensitivities of up to 102m2 J"1 and resolutions 103 mm-1. The partial transparency permits the use of polymeric photoconductors in microfilms and related fields. The sensitivity of such materials is of the order 1 m2 J 1, charge potential 250-400 V, resolution 70-500mm-1, and transparency 65-70%. 

Fullerene-Doped Polyvinylcarbazole. Fullerenes are known to be good electron acceptors. In the presence of electron donors such as aromatic amines, weakly bonded charge-transfer complexes can be formed [115]. Through virtual excitation, the existence of charge-transfer interaction can enhance the second-order optical nonlinearity of fullerenes [116], With direct excitation, excited state electron transfer between fullerenes and various electron donors such as aromatic amines [115,117], semiconductor colloids [118], porphyrin [119], and polymers [101, 103, 120] can occur. This electron-accepting property led to the development of fullerene-doped polymeric photoconductors [101,103]. 

PVK is the oldest known and most widely characterized polymeric photoconductor. Classically it is used in combination with TNF in photocopiers. New materials have been proposed, because the PVK/TNF system has a comparatively low photosensitivity. TNF has a high toxicity and the films of PVK have a poor mechanical strength. 

Fig. 6.8 Polymeric photoconductors, including the charge-carrier mobility achieved so far...

For the characterization of polymeric photoconductors two established methods exist the Time of Fhght (TOF) and the xerographic method. Both methods provide information about the two fundamental parameters that characterize a photoconductive material carrier mobility p. and quantum yield d>. 

The development of organic polymeric photoconductors was stimulated by the discovery that poly(N-vinylcarbazole) (PVK, Fig. 8.1), sensitized by certain dyes and pigments, displays high enough photoconductivity [1] to be usable in electrophotographic photoreceptors. 

The most important characteristic parameters of each photoconductor are the quantum efficiency of photogeneration 0 and the carrier mobililv //. In most examined unsensitized polymeric photoconductors 0 < 10 , even at high electric fields. 

The polymeric photoconductors used in practice are based on two types of system. The first one is based on polymers in which the photoconductive moiety is part of the polymer, for example a pendant or in-chain group. The second system involves low molecular weight chromophores imbedded in a polymer matrix. These so-called molecularly doped polymers are widely used today. They are described in Chapter 10 in this handbook by Law. Almost 100% of all xerognq>hic photoreceptors are made from organic photoconductors [5]. The main areas of application of polymeric photoconductors are as follows ... 

In contrast to PVK, PEPK shows no signs of crystallinity [244]. It was assumed on this basis that molecules of PEPK are atactic and it was concluded that crystallinity or potential ability to crystallize are not obligatory conditions for good photoconducting properties in carbazole-containing polymers and polymeric photoconductors on the whole [244]. 

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