Photoconductors, polysilanes

The history and development of polysilane chemistry is described. The polysilanes (polysilylenes) are linear polymers based on chains of silicon atoms, which show unique properties resulting from easy delocalization of sigma electrons in the silicon-silicon bonds. Polysilanes may be useful as precursors to silicon carbide ceramics, as photoresists in microelectronics, as photoinitiators for radical reactions, and as photoconductors. 

A rapidly increasing number of publications on polysilanes documents current interest in these polymers (JJ. Polysilanes are potentially applicable in microlithography as high resolution UV-resists (2J, imageable etch barriers ), or contrast enhancement layers (4). They have been successfully used as precursors to Si-C fibers (5J and ceramic reinforcing agents ((L). Polysilanes have also initiated polymerization of vinyl monomers (J ). Doping of polysilanes have increased their conductivity to the level of semiconductors (8). Very recently polysilanes were used as photoconductors (9) and non-linear optical materials (10b... 

The chemical modification of fullerenes has received considerable attention in the last decade in order to achieve new applications to material sciences [27]. Fuller-ene-bonded polysilane derivatives might be expected to show high conductivity since Ceo-doped polysilane is found to be a good photoconductor [28]. Therefore, a variety of silylated derivatives have been obtained to date, although the available methods are limited to the photoinduced addition of various silanes to Ceo-... 

Polysilane high polymers possessing fully saturated all-silicon backbone have attracted remarkable attention recently because of their unique optoelectronic properties and their importance in possible applications as photoresists, photoconductors, polymerization initiators, nonlinear optical materials etc. A number of review articles have been published on this topic4-9. The studies in this field have stimulated both experimental and theoretical chemists to elaborate on understanding the excited state nature of polysilanes and oligosilanes and of their mechanistic photochemistry. 

Tacticity is required for the synthesis of crystalline thin polysilane films used for optical and semiconductor devices. Modern synthetic routes allow control over the conformation and tacticity of polysilane molecules used as precursors for thin layers of photoresists, photoconductors and nonlinear optical phases in complex semiconductor and (opto)electronic devices. These properties can be exploited only if the synthesis method ensures a minimal level of contamination, especially with oxygen and metals, and special care is taken to limit electronic-grade polysilanes to a level of contamination on the order of a few ppm in the case of oxygen and in the ppb range for metals. The reactivity of polysilane toward oxygen has forced placing the devices in a helium environment during measurement procedures.36... 

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. 

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). 

Examples of polymers of the type -(M-M-) containing a bond in the main chain are known with metals or semimetals like B, Si, Ge, Sn, As, Sb, Th and Po [1]. Examples are shown in 66. Well-described are polydiorganosilicones (polysilanes) which are investigated for use as precursors of B-silicon carbide, as photoconductors, in nonlinear optics and microlithography. Preparations and properties are reviewed in [1,42,271,272]. 

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