Photoconductivity of poly-N-vinylcarbazole

Another important problem of the photoconductivity of poly-N-vinylcarbazole is the carrier transport. With the so-called time of flight method this problem is well investigated. 

Thermally stimulated hole current in poly-N-vinylcarbazole shows distinct maximum at around 5°C and another large current above 100°C. This 5°C maximum is due to 0.56 eV hole traps of 7 x 1015 cm 3 density. Photoconductivity in poly-N-vinylcarbazole increases appreciably when irradiated with UV-light in air at room temperature and this increase accompanies the formation of 0.56 eV hole traps. The nature of this traps has been discussed. 

Since the discovery of photoconductivity in poly(N-vinylcarbazole) (PVK) [1,2] a variety of polymers with carbazole groups have been synthesized and their photophysical properties have been investigated. The main topic of this article is the synthesis of photoconductive polymers, so minor attention is given to their photophysical properties. PVK (2b) can be synthesized by free-radical, cationic, or charge-transfer initiated polymerization of N-vinylcarbazole (2a). A detailed description of the PVK synthesis is given in Chapter 2 of this handbook. 

In the case of undoped PVCa films, impurities and surface states dominate the photoconduction mechanism (6) leading one to question any study of intrinsic pKotoconduction in organic polymers. Poly(N-vinylcarbazole) films yellow under ambient laboratory conditions. Work in our laboratory (7) has shown that ageing of a purified sample of PVCa leads to an increase in photoresponse in the 350-450 nm region while there is an initial drop in photoresponse in the 250-... 

From the structural point of view, poly(N-vinylcarbazole)(PVK) is an example of many photoconductive polymers vdiich have in common a substituted arylamino poup ... 

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. 

As regards photoconducting polymers, typical work has been carried out with poly(N-vinylcarbazole), PVK, and polysilylenes. The first commercial photoconductor was based on a 1 1 charge-transfer (CT) complex between PVK and trini-trofluorenone (TNF) [11]. Similar photoconductor properties were found with a 1 1 CT complex of TNF with poly[bis(2-naphthoxy)phosphazene] (see Chart 2.7), which is an insulator if dopant-free [54]. 

For several decades, the fields of photoconducting (75) and purely electrooptic polymers (74) have been very active but had almost no direct overlap. With the development of photorefractive polymers in the early nineties, the knowledge of these two research areas could be combined and has led to a rapid improvement of the performance of existing photorefractive polymers. The photorefractive polymer composite DMNPAA PVK ECZ TNF (DMNPAA 2,5 -dimethyl-4-(p-nitrophenyl-azo)anisole PVK poly(N-vinylcarbazole) ECZ N-ethylcarbazole TNF 2,4,7-trinitrofluorenone) we developed recently (P) has reached a level of performance that competes with that of the best inorganic photorefractive crystals (77,72). With the recent progress achieved in the development of new chromophores for electro-optic applications (75), the efficiency of these new materials is expected to be significantly further improved. 

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. 

In the past 30 years, since H. Hoegl discovered photoconductivity in the organic polymer poly(N-vinylcarbazole) (PVK) (Fig. 1.2) [5, 6], organic materials have almost completely replaced their inorganic counterparts, although the effective mobility in these systems is typically 5 orders of magnitude lower as compared to a-Se. 

Photoconductivity in organic polymers was first discovered in 1957 by H. Hoegl, who found that poly(N-vinylcarbazole) (PVK) and charge transfer complexes of PVK with electron acceptors like 2,4,7-trinitrofluorenone act as photoconductors [1],... 

Photoconductive -ksn- ksk-tM adj (1929) Becoming electrically conductive when irradiated by light or ultraviolet light. (Optical and electrical properties of polymers Materials research society symposium proceedings, vol 24. Emerson JA, Torkelson JM (eds). Materials Research Society, 1991) See Poly (N-Vinylcarbazole). 

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. 

Xerography has driven much of the research into conjugated-side-chain semiconducting polymers. The most studied photoconductive polymer is poly(N-vinylcarbazole) (1 PNVC). " PNVC in its pure form is a hole conductor with a dark conductivity of the order of 10" Scm". The photoresponse is limited to the absorption profile of the carbazole moiety. Both properties are modified when molecular species are added, so that impurities can mask the intrinsic properties of PNVC, e.g. purification leads to lower quantum efficiency. 

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