Polymers photoconductive

Photoconductive polymers are widely used in the imaging industry as either photosensitive receptors or carrier (electron or hole) transporting materials in copy machines and laser printers. This is still the only area in which the photoelectronic properties of polymers are exploited on a large-scale industrial basis. It is also one electronic appHcation where polymers are superior to inorganic semiconductors. 

There are many excellent reviews of photoconductive polymers (4—10). This article emphasizes results obtained after 1980, up to early 1994. 

Photoconductive polymers can be convenientiy classified into five categories based on thek stmctures and modes of photoconduction. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

From the structural point of view the known photoconductive (or trans ing) polymers can be divided into three groups  

1) Polymers with a h degree of conjugation in the main diain. In these polymers the absorption extends well into the visible and in some cases into the near infrared region, especially when certain electron withdrawing groups are present. They are photoconductors in the visible region of the spectrum. 

2) Polymers with pendant or inchain large polynuclear aromatic groups with large rr electron systems. These polymers usually absorb only UV light below 400 nm 

Polymers and oligomers with conjugated C=C, C=N- and -N=N-doid)le and -C=C— triple bonds in the main chain received recently some attention for their photoconductive properties. Although most of these polymers are infusible and intractable, some of them are reported to possess good solubility and film-forming properties and outstanding thermal stability. 

Among the first poly conjugated polymers found to display an internal photoeffect were polyquinazones such as polyacenequinone radicals and aniline black  

Photoconductivity is the increase in electrical conductivity of a material caused by absorption of electromagnetic radiation. Photoconductive polymers are typically very good insulators in the dark, when the carrier(s) of electricity, free electrons and/or holes, are virtually absent. They become more conductive when exposed to light. In order to become a photoconductor, the polymer must satisfy two major requirements  

It must absorb light and thus allow photoexcitation of electrons from the ground state. The vacancy left behind (a hole) and the photo-excited electron form a pair of charge carriers, which can be separated by the action of an electric field. 

It must allow migration of either photoexcited electrons or holes, or both, through the polymer in the electric field towards the appropriate electrodes. Since no ions move and matter is not displaced, the conduction process is wholly electronic, with no ionic contribution. 

Photoconductive and charge-transporting polymers can be either p-type or n-type, depending on the majority carrier. Only in the special cases discussed below can polymers be bipolar, i.e. capable of trans- 

For each known photoconductive polymer, there exists a whole spectrum of electrodes ranging from perfectly Ohmic to perfectly blocking. Therefore in studies of steady-state photoconductivity one has to determine the role of electrodes before making any judgement about the quality of the photoconductor. 

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FURNACES,ELECTRIC - INDUCHON FURNACES] (Vol 12) -photoconductivity [PHOTOCONDUCTIVE POLYMERS] (Vol 18) for papermaking [PAPER] (Vol 18)... 

Photochemical technology Photoconductive polymers Photography Printing processes Radioactive tracers Radiopaques... 

Fig. 2. Molecular structures of selected photoconductive polymers with pendent groups (1) poly(A/-vinylcarba2ole) [25067-59-8] (PVK), (2) A/-polysiloxane carbazole, (3) bisphenol A polycarbonate [24936-68-3] (4) polystyrene [9003-53-6] (5) polyvin5i(l,2-/n7 j -bis(9H-carba2ol-9-yl)cyclobutane) [80218-52-6]...

Liquid Crystalline Systems. Conventional photoconductive polymers are amphorous or systems withlow order. In the case of PVK, the hole moves by hoppiag between the pendent carba2ole groups. The hole mobilities are usuaUylow, cm /Vs, due to a trap-dominated hoppiag... 

The most important industrial appHcation of photoconductive polymers is electrophotography (qv). This is a biUion doUar industry and one of the few electronic areas where polymeric material excels. The principles and practices of electrophotography have been reviewed in detail elsewhere (9,85) and are not repeated here. 

The avaHabihty of photoconductive polymers opens up many areas for research, in addition to electrophotography. These are relatively unexplored areas and represent promising future directions. 

Recently photorefractivity in photoconductive polymers has been demonstrated (92—94). The second-order nonlinearity is obtained by poling the polymer doped with a nonlinear chromophore. Such a polymer may or may not be a good photoconductor. Usually sensitizers have to be added to enhance the charge-generation efficiency. The sensitizer function of fuUerene in a photorefractive polymer has been demonstrated (93). 

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