Photoconduction photoconductive polysilanes

In addition to photoconductivity, polysilanes have been found to exhibit marked nonlinear optical properties,95-97 suggesting that they may eventually be useful in laser and other optical technology. The third-order non-linear susceptibility, X3, is a measure of the strength of this effect. The non-linear properties of polysilanes, like the absorption spectra, seem to be dependent on chain conformation and are enhanced for polymers having an extended, near anti conformation (Table 5.5). The value of 11 x 10 12 esu observed for (n-Hex2Si) below its transition temperature is the largest ever observed for a polymer which is transparent in the visible region. 

Keywords Oligothiophene / FuIIerene / Photoconductivity / Polysilane / Electrophotography... 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

Possible ways in which polysilanes may be useful include, 1. As precursors to silicon carbide ceramics 2. As photoinitiators in radical reactions 3. As photoconductive materials, and 4. As photoresists in microelectronics. The last of these uses will be treated in the chapter by Miller,(31) and so will not be covered here. 

Polysilanes can be regarded as one-dimensional analogues to elemental silicon, on which nearly all of modern electronics is based. They have enormous potential for technological uses [1-3]. Nonlinear optical and semiconductive properties, such as high hole mobility, photoconductivity, and electrical conductivity, have been investigated in some detail. However, their most important commercial use, at present, is as precursors to silicon carbide ceramics, an application which takes no advantage of their electronic properties. 

An important application of polydimethylsilane is as a source of silicon carbide (SiC) fibres, which are manufactured under the trade-name Nicalon by Nippon Carbon in Japan. Heating in an autoclave under pressure converts polydimethylsilane to spinnable polycarbosilane (-Me2Si-CH2-) with elimination of methane. The spun fibres are then subjected to temperatures of 1200-1400 °C to produce silicon carbide fibres with very high tensile strengths and elastic moduli." As a result of their conductivity, polysilanes have also been used as hole transport layers in electroluminescent devices. In addition, the photoconductivity of polymethylphenylsilane doped with Cgo has been found to be particularly impressive. ... 

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 photoconductivity of polysilanes was described in Section 5.8, and their electroluminescence is covered in Section 5.9.2. These properties make polysilanes possible components of polymer light-emitting diodes, either as charge transport layers or as the actual emissive materials.146 A drawback of the polysilanes is their photodegradation under ultraviolet irradiation, a problem which must be overcome if polysilanes are to become commercially useful. 

A number of reviews have appeared of topical interest. These include photoisomerism of azo dyes, ° photofunctional polysilanes,photochromic pigments, rare earth complexes, pressure sensitive paints, electron-transfer processes,electroactive dendrimers, chiral polyisocyanates, photodefin-able benzocyclobutene, photoconductive polymers,excited states in conjugated polyenes, photosensitive materials and polyazomethanes. ... 

Research in several laboratories has shown that polysilane films have photoconductive properties129-131. Thus polysilanes may be useful in photographic processes based on photoconduction, such as Xerography. 

The hole mobility of 10 4 cm2 V"1 s -1 is quite high for an organic material. Alkyl as well as aryl polysilanes show photoconductivity, which is nearly independent of the polysilane structure and molecular weight. The transport states are therefore associated with the polysilane chain, and not with the pendant groups as in many other carbon-based photoconductors. 

In the second part of this paper we describe the photoconductive properties of a polysilane and the realization and performances of an organic spatial light modulator for optical correlation using this polymer. 

This enhancement of the photoconductivity caused by Ceo doping may be ascribed the photoinduced electron transfer. We have reported that while the photoinduced electron transfer from polysilanes with alkyl substituents to C6o does not occur, the electron transfer from polysilanes with aromatic side groups (e.g. PMPS) to Ceo is effective [13]. Yoshino et al. have also reported that the photoconduction of poly(3-alkylthiophene) is remarkably enhanced upon doping [14]. Sariciftci et al. reported the similar photoinduced electron transfer in oligothiophene/C6o composite films [15]. Therefore, we believe that the photoinduced electron transfer occurs between ji-orbitals of the thienylene rings and those of C6o-... 

USGS. The delocalization of a electrons in polysilanes gives rise to unique electronic and optical properties. Also, several polysilanes have been foimd to function as useful thermal precursors to silicon carbide fibers and these materials have attracted attention with respect to microlithographic applications and as polymerization initiators (1,27,28). The use of these materials as hole transport layers in electroluminescent devices has also been explored (42). Indeed, the photoconductivity of poly(methylphenylsilane) doped with Geo has been studied and has been found to be comparable with the best materials available (43). 

Most polysilanes show high photoconductivity, which has stimulated research of applications as organic photoconductors (OPC) in zerography. Carrier generation is, however, poor. Time-of-flight (TOF) studies revealed that holes are almost the sole carrier for most polysilanes (89) and the mobility (>= 10 cm W s) is 2-3 order higher than most carbon polymers (RilO cm AT s) (Table 4). 

Polysilanes (Fig. 1) [1] have attracted considerable attention due to their usefulness as precursors for thermally stable ceramics [2, 3] or a material for microlithography [4, 5] and also due to their potentiality in preparation of new types of material showing semiconducting, photoconducting, or nonlinear optical property [6-8]. 

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