Photoconductivity of polymers

Photoconductivity of polymers will be reviewed within the framework of semiconductor physics. The focus of attention will be the photogeneration and transport of the charge carriers, the relation between chemical structure and photoelectrical properties, and sensitization processes involving dyes and dopant molecules. 

Reucroft et al. 37>38 reported photoconductivity of polymers with polyconjugation in the chain, prepared by polycondensation of 3,3 -diaminobenzidine and 3,3, 4,4 -benzophenone tetracarboxylic acid dianhydride39, pyrrone polymers ... 

Reucroft et al. reported photoconductivity of polymers with polycon-... 

On the other hand, even in weak CTCs, the completely charge-separated state can be formed at the excited state. Accordingly, photoconductivity could be a first candidate among physical properties influenced strongly by the CT character in wholly aromatic Pis. Photoconductivity of polymers has been widely reported, in particular, with great interest on low molecular weight electron acceptors-loaded poly (vinyl carbazole) for xerographic applications [117,118]. 

The first comprehensive reviews on photoconductive polymers were published by Stolka alone [15] and in co-authorship with Pai [16]. Chemical aspects of the topic were later reviewed by several authors [17-19]. In the work of Mylnikov photoconductivity of polymers was reviewed within the framework of semiconductor physics [20], whereas Haarer [21] has concentrated mainly on the transport properties of photoconductive polymers. In their comprehensive book, Borsenberger and Weiss described all aspects of photoconductive materials [6]. 

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. 

Other polymers ia this category iaclude CJ-conjugated polygermylenes (20) and TT-conjugated poly acetylene, polythiophene, and poly(p-phenylenevinylene). The photoconductivity of many TT-conjugated polymers can be enhanced by dopiag with fuUerenes (21). 

The use of interpenetrating donor-acceptor heterojunctions, such as PPVs/C60 composites, polymer/CdS composites, and interpenetrating polymer networks, substantially improves photoconductivity, and thus the quantum efficiency, of polymer-based photo-voltaics. In these devices, an exciton is photogenerated in the active material, diffuses toward the donor-acceptor interface, and dissociates via charge transfer across the interface. The internal electric field set up by the difference between the electrode energy levels, along with the donor-acceptor morphology, controls the quantum efficiency of the PV cell (Fig. 51). 

The surface oxidation products dete ted by the decrease in contact angle upon photolysis of PVCa films may dominate the photoconductivity of t. is polymer. Work is underway to confirm this relatio. ship and measure surface conductivity simultaneously with bulk conductivity as a function of photodegradation. 

Y Wang, Photoconductivity of fullerene-doped polymers, Nature, 356 585-587, 1992. 

Various types of the photoconductive polymers are available now. The photoconductivity of such materials may be essentially increased by means of the chemical and spectral sensitization [12-14]. Spectral sensitization is connected with the appearance of the photosensitivity in the new spectral bands and the chemical sensitization with the increase of the proper sensitivity. As a rule both types of sensitisation may take place in the photoconductor at the same turn. The first data about chemical and spectral sensitization in organic photoconductors appeared in [19, 20]. The example of the chemical and spectral sensitization of the photoconductivity by dyes in polymeric copper-phenyl-acetylenide is presented in Fig. 2. Later on it was proposed that not only low molecular weight compounds but polyconjugated polymers could also be used as sensitizers [21] having broad absorption tends and high thermostability compared with dyes. Now it is clear that various types of molecules may be used as a photosensitizers. 

Some types of the polymers were investigated in detail. The photoconductivity of polyethylene with quantum efficiency 10 5-10 10 is caused by impurities, Schottky type contact injection, and hole transport [82,83], The crystallinity increase is accompanied by a photocurrent increase. There is no clear correlation between the chemical structure and the photocurrent. 

The photoconductivity spectrum after preliminary irradiation of polymer is given by curve 2. The observed redistribution of the peaks is partly reversed on prolonged exposure to air. The bathochromic shift of the shorter wavelength peak depends on the exposure time. Ultraviolet irradiation produces a slight change in the polymer colour. Such irradiation increases, likewise, the photo-electromotive force a 1.5 h irradiation increases it 10 times. The photoconductivity spectrum is situated at longer wavelengths than the photo-electromotive force spectrum. 

The photoconductivity of poly-p-xylylenes were investigated under various conditions [218-221]. The fundamental edge absorption of the polymers is at 300 nm. The photocurrents in the visual range of the spectra depended on the electrode nature. So it was interpreted as photoinjection of the charges from electrodes and separation of them at a Schottky type barrier. Suppression of hole injection for the plasma-treated polymer is related to the existence of an oxidized surface layer. 

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]. 

Interest in the photoconductive properties of the carbazole nucleus has also prompted studies concerned with its incorporation into condensation polymers. Examples of polymers prepared include polyamides (34), polyesters (35) and polyurethanes (36) (80MI11105). Thorough studies on the CT interactions of these polymers with the monomeric acceptor 2,4,7-trinitrofluorenone have been done. In all cases, the formation constant for the CT complex was higher with polymers than for monomeric models. At least two polymer... 

---