13 Photoconductive Polymers P. Strohriegl Universita ¨ t Bayreuth, Makromolekulare Chemie I, and Bayreuther Institut fu ¨ r Makromoleku ¨ lforschung (BIMF), Bayreuth, Germany J. V. Grazulevicius Kaunas University of Technology, Kaunas, Lithuania I. FOREWORD Since 1992 when the first edition of the Handbook of Polymer Synthesis was published a number of new applications for photoconductive polymers or, to put it correct, charge transport materials, have appeared. The most successful development are organic light emitting diodes (OLEDs) which right now enter the market as bright displays for cellular phones and car radios. Other imortant areas are organic field effect transistors, solar cells and lasers. For this reason the review has been thoroughly updated mainly in the Sections V.B and V.C which deal with conjugated polymers, a very active research area in which A. Heeger, A. McDiarmid and H. Shirakawa received the Nobel Prize in 2000. A large number of new polyme rs and up-to-date references have been included. II. INTRODUCTION Photoconductivity is defined as an increase of electrical conductivity upon irradiation. According to this definition photoconductive polymers are insulators in the dark and become semiconductors if illuminated. In contrast to electrically conductive polymers photoconductors do not have free carriers of charge. In photoconductors these carriers, electrons or holes, are generated by the action of light. The carriers of electricity can also be photogenerated extrinsically in an adjacent charge generation layer, and injected into the polymer which in this case acts as a charge transporting material. Only polymers capable of both producing charge carriers upon exposure to light and transporting them through the bulk are true photoconductors. Polymers that do not absorb the incident light but accept charges generated in an adjacent material are merely charge transport materials. Copyright 2005 by Marcel Dekker. All Rights Reserved. The discovery of photoconductivity dates back to 1873 when W. Smith found the effect in selenium. Based on this discovery C. F. Carlson developed the princip les of the xerographic process already in 1938. Photoconductivity in polymers was first discovered in 1957 by H. Hoegl [1,2]. He found that poly(N-vinylcarbazole) (PVK) sensitized with suitable electron acceptors showed high enough levels of photoconductivity to be useful in practical applications like electrophotography. As a result of the following activities IBM introduced its Copier I series in 1970, in which an organic photoconductor, the charge transfer complex of PVK with 2,4,7-trinitrofluorenone (TNF), was used for the first time [3]. The photoconductor was a 13 mm single-layer device. It was prepared by casting a tetrahydrofuran solut ion containing PVK and TNF onto an aluminum substrate [4]. Since then numerous photoconductive polymers have been described in literature and specially in patents. The ongoing interest in photoconducting polymers is connected with an increasing need for low cost, easy to process and easy to form large area materials. The polymeric photocond uctors used in practice are based on two types of systems. The first one are polymers in which the photoconductive moiety is part of the polymer, for example a pendant or in-chain group. The second group involves low molecular weight chromophores imbedded in a polymer matrix. These so called molecularly doped polymers are widely used today. Almost 100% of all xerographic photoreceptors at present are made of organic photoconductors [5]. The main area of application of polymeric photo- conductors is electrophotography [6]. Photoconductive polymers are used in photocopiers, laser printers, electrophotographic printing plates, and electrophotographic microfilming. During the last decade, photoconductive or more precisely charge transporting polymers have been widely used in photorefractive composites [7] and in organic light emitting diodes (OLEDs) [8,9]. An upcoming field for the application of charge-transporting polymers are photovoltaic devices [10,11]. The process of electrophotography is schematically shown in Figure 1. It is a complex process involving at least five steps [12]. 1. Charge. In the first step the surface of the photoconductor drum is uniformly charged by a corona discharge. 2. Expose. Parts of the photo conductor are discharged by light reflected from an image. So the information is transferred into a latent, electrostatic image on the surface of the photoconductor. 3. Develop. Electrostatically charged and pigmented polymer particles, the toner, are brought into the vicinity of the oppositely charged latent image transforming it into a real image. 4. Transfer. The toner particles are transferred from the surface to a sheet of paper by giving the back side of the paper a charge opposite to the toner particles. 5. Fuse. In the last step the image is permanently fixed by melting the toner particles to the paper between two heated rolers. The photoconductor drum is cleaned from any residual toner and is ready for the next copy. Organic electrophotographic photorecept ors are also widely used in laser printers [13,14]. The principal of these printers is almost the same as in a photocopier except the direct generation of the image by a laser instead of the optical system in a copier. Photoreceptors of the laser printers have to absorb in the near infrared range of spectrum. The third area in which photoconductive polymers or polymer composites are applied are electrophotographic printing plates. Copyright 2005 by Marcel Dekker. All Rights Reserved. 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 propert ies of photoconductive polymers. In their comprehensive book, Borsenberger and Weiss described all aspects of photoconductive materials [6]. Photoconductive polymers can be p-type (hole-transporting), n-type (electron- transporting), or bipolar (capable of transporting both holes and electrons). Typically, bipolarity can be accomplished by adding electron-transporting molecules such as TNF to a donorlike, hole-transporting polymer such as PVK. Most of practical photoconductive polymers are p-type, however recently much attention is paid to electron-transporting and bipolar polymers [22]. III. BASIC PRINCIPLES OF PHOTOCONDUCTIVITY Since the major goal of this chapter is the description of the different classes of photoconductive polymers, the underlying physical principles will be only briefly discussed. For more detailed reviews dealing with photoconductor physics the reader is referred to the literature [21–24]. The process of photoconduction involves several steps [15]. Figure 1 Principles of the xerographic process (for explanations see text). Copyright 2005 by Marcel Dekker. All Rights Reserved. A. Absorption of Radiation The first step to a charge carrier generation is the absorption of radiation. Photo- conductive materials are truly photoconductive only in the range of wavelength of absorption. Thus PVK is a photoconductor only in the UV range. To produce carriers by visible light sensitizin g dyes or electron acceptors forming coloured charge transfer complexes must be added. B. Generation of Charge Carriers By the absorption of light the active groups are excited and form closely bound electron– hole pairs. The key process that determines the overall photogeneration efficiency is the following field induced separation into free charge carriers. This process competes with the geminate recombination of the electron–hole pair. A theoretical description of this process is provided by Onsager’s [25] theory for the dissociation of ion pairs in weak electrolytes in the presence of an electric field. The model has been successfully applied to amorphous photoconductors [26]. It was found that the photogeneration efficien cy, in other words quantum yield of the process, is a complicated function of several variables such as electric field strength, temperature, and separation distance. The predicted relationship is in good agreement with experimental data for doped polymers like N-isopropylcarbazole in polycarbonate [27], triphenylamine doped polycarbonate [28] and PVK [29,30]. The quantum yields in ‘pure’ photoconductors absorbing in the UV range are usually low and strongly field dependent. So at room temperature and an excitation wavelength of 345 nm the quantum yield È for PVK rises from 0.01% at 10 4 V/cm to about 6% at 10 6 V/cm [28]. Substantially higher values for È are obtained in the presence of complexing additives like dimethyl terephthalat e [31,32]. The addition of suitable electron acceptors which form colored charge-transfer complexes is a proven way to increase the photogeneration efficiency. 2,4,7-Trinitrofluorenone (TNF) in combination with PVK is so effective that the combination was used in the IBM copier I, the first commercial copier with an organic photoconductor. C. Injection of Carriers An injection of carriers only occurs if an extrinsic photogenerator is used together with a charge transporting material. Usually dye particles are dispersed in a polymer matrix or evaporated on top of a conductive substrate and then covered with the charge transporting polymer. The carriers are generated in the visible light-absorbing material and injected into the polymer. D. Carrier Transport The photogenerated or injected charge carriers move within the polymer unde r the influence of the electric field. In this process the photoconductive species, for example carbazole groups in PVK, pass electrons to the electrode in the first step and thereby become cation radicals. The transport of carriers can now be regarded as a thermally activated hopping process [33–37], in which the hole hops from one localized site to another in the general direction of the electric field (Figure 2). The moving cation radical can accept an electron from the neighboring neutral carbazole group which in turn becomes a hole, and so on. Effectively the hole moves within the material while electrons Copyright 2005 by Marcel Dekker. All Rights Reserved. only jump among neighboring species. Hole transport can therefore be described as a series of redox reactions among equivalent groups. During transit, the carriers do not move with uniform velocity but reside most of the time in localized states (traps) and only occasionally are released from these traps to move in field direction. This trapping process is responsible for the extremely low hole mobilities in photoconductiv e polymers. For PVK room temperature mobilities from 3 Â 10 À8 to 10 À6 cm 2 /Vs (E ¼ 10 5 V/cm) have been reported [6]. Since the transport of holes can be described as a series of electron transfer reactions with a certain activation energy it is not surprising that the carrier mobility is temperature- and field-dependent. IV. EXPERIMENTAL TECHNIQUES For the characterization of polymeric photoconductors two established methods exist: the Time of Flight (TOF) and the xerographic method. Both methods provide information about the two fundamental parameters that characterize a photoconductive material: carrier mobility m an d quantum yield È. The principle of TOF method is shown in Figure 3. A thin film of photoconductive material is sandwiched between a conductive substrate, for example an aluminized Figure 2 Principles of carrier transport (for explanations see text). Copyright 2005 by Marcel Dekker. All Rights Reserved. mylar film, and a semitransparent top electrode and connected to a voltage source and a resistor R. Because of the blocking electrodes the source voltage appears across the film. A thin sheet of charge carriers is generated near the top electrode by a short pulse of strongly absorbed light. Due to the influence of the applied field the carriers drift across the sample towards the bottom electrode. The resulting current is measured in the external circuit at the resistor R. A typical experimental photocurrent for the polysiloxane 11c (m ¼ 3) with pendant carbazolyl groups is shown in Figure 4 [38]. In the double logarithmic plot of photocurrent versus time the bend at the transit time t t is clearly detect able. The effective carrier mobility m is calculated from the transit time according to Equation (1) m ¼ d=t t E ð1Þ Figure 3 Typical time-of-flight (TOF) setup for measuring hole mobilities in polymers. Figure 4 Typical experimental photocurrent of polysiloxane 13 (m ¼ 3) at an electric field of 3 Â 10 5 V/cm (T ¼ 293K). The arrow marks the transit time. Copyright 2005 by Marcel Dekker. All Rights Reserved. where d denotes the sample thickness and E is the electric field strength. With d ¼ 6.7 mm, E ¼ 4.6 Â 10 5 V/cm and a transit time t t of 2.8 Â 10 À5 ms an effe ctive carrier mobility of 1 Â 10 À4 cm 2 /Vs is calculated from Figure 4. Note that for the conjugated trimer (74) with its high mobility the transit time can be seen even in a linear plot of I photo vs. time (inset). The carrier mobility m is temperature- and field-dependent. M any theories have been developed to explain the temperatur e dependence, but no comprehensive model is yet available. It is still not clear whether the charge carrier mobility follows a simple Arrhenius relationship (log m ffi 1/T ) as predicted by Gill [33] or if the more complex relationship log m ffi 1/T 2 proposed by Ba ¨ ssler [39] is valid. The relationship between the mobility m and the electrical field strength E is equally unclear. Here Gill’s model predicts a log m ffi E 1/2 dependence which is consistent with a Pool–Frenkel formalism, whereas Ba ¨ ssler’s calculations lead to a log m ffi E dependence. A detailed description of the different models and results obtained by fitting experimental mobility data to those models is beyond the scope of this chapter. It shall only be pointed out here that the main difficulty is the limited range of temperature and electric field in which carrier mobilities can be measured [38]. Additional experiments are necessary to understand the mechanism of carrier transport in photoconductive polymers in detail. V. CLASSES OF PHOTOCONDUCTIVE POLYMERS Several polymer types and classes are known to exhibit photoconductivity. Consequently no preferred method of synthesis exists. The known photoconductive polymers are prepared by almost all common methods like free-radical, cationic, anionic, coordina- tion, and ring-opening polymerization, step-growth polymerization, and polymeranalo- gous reactions. The only common requirement for all photoconductive materials is that they have to be of extreme purity. It is well known [40–42] that even traces of impurities act as traps and have drastic influence on both quantum yield and carrier mobility. From the structural point of view the photoconductive polymers described in this chapter can be divided into three groups (Figure 5):  Polymers with pendant or in-chain electronically isolated photoactive groups with large p-ele ctron systems, for example, aromatic amino groups, like carba- zole or condensed aromatic rings, like anthracene  Polymers with p-conjugated main chain like polyacetylene and poly(1,4- phenylenevinylene)  Polymers with s-con jugated backbone, like organopolysilanes A. Polymers with Pendant or in-Chain Electronically Isolated Photoactive Groups An aromatic amino group is a common building block of many known photoconductive or charge transporting materials. Many practical systems used in electrophotography belong to this category. The active groups in these materials are either part of the polymer structure or low-molecular dopants imbedded in a polymer matrix. The later group of Copyright 2005 by Marcel Dekker. All Rights Reserved. materials of which numerous examples exist especially in the patent literature will not be discussed here. 1. Carbazole-Containing Polymers 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-vin ylcarbazole (2a). A detailed description of the PVK synthesis is given in Chapter 2 of this handbook. ð2Þ Poly(N-ethyl-2-vinylcarbazole) (Structure 3a) has been prepared by free-radical polymerization, whereas poly(N-ethyl-3-vinylcarbazole) (3b) was synthesized by cationic polymerization with a boron trifluoride initiator [43]. The 2-isomer is reported to exhibit Figure 5 Different types of photoconductive polymers. Copyright 2005 by Marcel Dekker. All Rights Reserved. higher carrier mobility than PVK, while that of the 3-isomer is lower [44]. ð3Þ Tazuke and Inoue [45] reported on the synthesis of a polyvinyl derivative having a pendant dimeric carbazole unit, 1,2-trans-bis(9H-carbazol-9-yl)cyclobutane (DCZB). Poly(trans-1-(3-vinyl-)-carbazolyl)-2-(9-carbazolyl)cyclobutane) (4a) was prepared by cationic polymerization of the corresponding monomer with boron trifluoride. The reaction yielded a polymer of relatively high molecular weight (M n ¼ 2.5 Â 10 5 , M w ¼ 5.8 Â 10 5 ). Copolymers of the vinyl derivative of DCZB with N-ethyl-3-vinylcarbazole were also obtained. Fluorescence spectroscopy data have indicated that the polymer (4a) does not form excimers. The photoconductive properties of polymer (4a) as well as of its copolymers have been studied by the xerographic technique, both in the presence and in the absence of the sensitizer TNF [46,47]. The photoconductivity of (4a) is increased compared to PVK when the charge transfer band of the complex is irradiated. Better photoconductive properties of (6a) correlate with its photophysical properties. Excimer formation is sterically hindered by DCZB groups whereas energy migration occurs effi- ciently in it. Charge transfer interaction with TNF is also stronger for (4a) than for PVK. Several polyacrylates and polymethacry lates with pendant carbazole groups have been described. Poly(2-(N-carbazolyl)ethyl acrylate) (formula 4b) has been prepared by free radical polymerization of the corresponding monomer [48]. ð4Þ The polymer exhibits a charge carrier mobility of 7 Â 10 À6 cm 2 /Vs (20  C, 5 Â 10 5 V/cm) which is higher than in PVK. The enhanced carrier mobility in the carbazole containing polyacrylate is apparently due to the lack of excimer-forming sites in it. Polymer (4b) has also been prepared anionically with ethyl magnesium chloride/benzalaceto- phenone as catalyst [49,50] to yield an almost exclusively isotactic product. Due to the insolubility of the polymer in the toluene/diethyl ether mixture in which the polymerization was carried out the molecular weight is low and the product shows a broad molecular weight distribution. Nevertheless time of flight measurements show that the carrier mobility Copyright 2005 by Marcel Dekker. All Rights Reserved. of the isotactic material (1.7 Â 10 À5 cm 2 /Vs, 20  C, 2 Â 10 5 V/cm) is about six times higher than the mobility of the atactic polymer. The authors concluded that stereoregular structures enhance the hole drift mobility of pendant-type photoconductive polymers. However, the relatively small increase of the measured mobilities should be interpreted with caution because it is well known that even traces of impurities may have a drastic influence on the carrier mobility. A series of polyacrylates and polymethacrylates (5a) in which the carbazolyl groups are separated from the polymer backbone by alkyl spacers of variable length have been prepared by different methods as shown in the Scheme 5 [51]. The molecular weigh ts of the polymers obtained by free-radical polymerization with AIBN in toluene solution are rather low and all polymers exhibit a broad molecular weight distribution. The reason is the low solubility of the polymers in the polymerization solvent toluene. In more polar solvents like tetrahydrofuran the molecular weight is limited by chain transfer reactions. High-molecular weight poly(meth)acrylates (M w ¼ 100,000–150,000, M n ¼ 50,000–70,000) were obtained by polyme ranalogous reaction of o-hydroxyalkylcarbazoles with poly(meth)-acryloylc hloride. IR and 1 H NMR spectroscopy as well as elemental analysis show that the reaction yields poly(meth)acrylates with an almost quantitative degree of substitution. ð5Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. [...]... [134 ] or chloride (Scheme 28) [130 133 ] ð28Þ The latter is polymerized to yield a water soluble sulfonium salt polyelectrolyte (28d) which is then purified by dialysis [135 ] The precursor polymer is converted to PPV (28e) by the thermal elimination of dimethyl sulfide and HCl The method has been later developed by Horhold et al [136 ], Lenz et al [137 ,138 ], Murase et al [139 ] and Bradley ¨ [140] One of. .. cationic polymerization of the corresponding monomer (20a) [96] Cationic polymerization is favored ð20Þ At a field of 4 Â 105 V cmÀ1 pure polymer (20b) shows a mobility of 1.2 Â 10À6 cm2 VÀ1 sÀ1 and that doped with 4% of TCNE exhibits a mobility of 3.6 Â 10À5 cm2 VÀ1 cmÀ1 [97] Polymers containing triphenyldiamine (TPD) moieties in the main chain, obtained by step growth polymerization are of increasing... free-radical polymerization of the corresponding acrylate [53] The molecular weight of the polymer (6) established by vapour pressure osmometry is 46,000 The hole drift mobility of polymer (6) is more than ten times higher than that of PVK or poly(9-ethyl-3-vinylcarbazole) ð6Þ It was established that the elevated hole drift mobility of DCZB polymers is due to the reduced concentration of trapping sites... improvements was the use of tetrahydrothiophene instead of dimethyl sulphide in the synthesis of the precursor polymer [141] The use of the cyclic leaving group facilitates the elimination when the precursor polymer is heated at 230–300  C and leads to PPV with reduced amounts of defect structures in the polymer chain Copyright 2005 by Marcel Dekker All Rights Reserved The photoconductivity of PPV prepared... initiator [120] Polymer (24a) was only partly soluble in some solvents like tetrahydrofuran, chloroform, nitrobenzene, and p-dichlorobenzene In contrast to Ti(OBu)/Et3Al initiation polymerization of 3-(Ncarbazolyl)-1-propyne with MoCl5 and WCl6 based catalysts gave high yields of yellow Copyright 2005 by Marcel Dekker All Rights Reserved polymer insoluble in any solvent [121] ð24Þ Copolymerization of 3-(N-carbazolyl)-1-propyne... for application Another series of soluble hole-transporting polymers containing pendant arylamine groups were prepared by anionic polymerisation of newly synthesized vinylarylamines [94] The general structure of the poly(vinylarylamines) reported is shown in Scheme (19) n-Buthyllithium was used for the initiation of the anionic polymerization The molecular weight of the polymers obtained is not high... as photoreceptors and for light emitting diodes A series of TPD-containing condensation polymers is described in the patent [98] The structure of one such polymer is shown in Scheme (21) The application of the hole-transporting polymers instead of the low-molar-mass compounds for the charge transport layers of photoreceptors prevents penetration of the small Copyright 2005 by Marcel Dekker All Rights... anionic initiators Polymerization with potassium hydride yields polymers of a degree of polymerization up to 62 Since the carbazole units in (10b) are removed from the main chain compared to PEPK it has a lower glass transition temperature and exhibits good film-forming properties in a wide range of molecular weights Xerographic photosensitivity of its layers doped with TNF is lower than that of the corresponding... (12) a carbazole group is incorporated into a mesogenic unit The polymers are prepared by a multistep synthesis the last step of which is the polymer analogous reaction of the mesogenic unit with an alkenyl-terminated spacer and poly(hydrogenmethylsiloxane) [77] The polymers exhibit broad mesophases, for example polymer (12) with a spacer of three methylene units (m ¼ 3) has a glass transition at 69 ... Incorporation of the DCZB moieties into the copolymers resulted in homogeneous dispersion of carrier groups, but a great extent of destabilization of the liquid crystalline Copyright 2005 by Marcel Dekker All Rights Reserved phase was observed Nevertheless the hole drift mobility was found to be enhanced in copolymer films with more ordered structure of the DCZB moieties, indicating that orientation of the . University of Technology, Kaunas, Lithuania I. FOREWORD Since 1992 when the first edition of the Handbook of Polymer Synthesis was published a number of new applications for photoconductive polymers. molecularly doped polymers are widely used today. Almost 100% of all xerographic photoreceptors at present are made of organic photoconductors [5]. The main area of application of polymeric photo- conductors. ð1Þ Figure 3 Typical time -of- flight (TOF) setup for measuring hole mobilities in polymers. Figure 4 Typical experimental photocurrent of polysiloxane 13 (m ¼ 3) at an electric field of 3 Â 10 5 V/cm (T