Handbook of Polymer Synthesis Second Edition Episode 13 pdf

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
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The polyacrylate (6) with a pendant dimeric carbazole unit, 1,2-trans-bis(9H-
carbazol-9-yl)cyclobutane (DCZB), does not show excimer fluorescence and exhibits
improved hole drift mobility [52]. It is obtained by 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 which are in fact excimer-forming sites.
This was confirmed by the temperature and electric field dependencies of the hole mobility.
These observations su pport the idea that charge transport and exciton transport have
many features in common [54].
The cationic polymerization of 2-(N-carbazolyl)ethyl vinyl ether with boron
trifluoride etherate or ethylaluminum dichloride as initiator has been described by several
authors [55–58] (Scheme 7). Low-molar-mass polymers were obtained with both initiators
[56]. In the case of boron trifluoride etherate the molecular weight (M
n
) was 3160, and
the ethyl–aluminum dichloride initiated polymerization yielded poly(2-(N-carbazolyl)ethyl
vinyl ether) (11b) with M
n
¼ 24,500. At longer reaction times with ethylaluminum
dichloride, considerable amounts of insoluble material were formed by cross-linking
reactions. The data on the photoconductivity of the polymer (7b) are contradicting.
In a steady state measurement Okamoto et al. [55] found that the photocurrent in the
polymer (7b) is much lower than that in PVK. However xerographic discharge meas-
urements carried out by Turner and Pai showed that the samples of the polymer (7b)
prepared with boron trifluoride as initiator had carrier mobilities only slightly lower than
that of PVK [56]. The samples of (7b) prepared with ethylaluminum dichloride showed a
high level of charge trapping that stems from impurities in the polymer film.
ð7Þ
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Again, it becomes evident that it is almost impossible to compare the results of
photoconductivity measurements from different authors because of the different methods

of polymer synthesis, purification, and the varying measurement techniques.
Gaidelis et al. [59] reported that the carrier mobilities of poly-(N-epoxypropyl-
carbazole) (PEPK) (8b) are more than an order of magni tude higher than the values
reported for PVK. This observation later was confirmed by the data of Wada [60]. Because
of this property PEPK can be used as a charge transporting material in xerocopier drums
[61,62]. It was also used in electrophotographic microfilming [63]. High-molecular-weight
PEPK is prepared by substituting halogen atoms of epihalohydrin polymers with
carbazole in organic solvents in the presence of inorganic bases and phenol radical chain
inhibitors, like 2,6-di-tert-butyl-p-cresol [64] (Scheme 8).
ð8Þ
The weight average molecular weight (M
w
) of PEPK synthesized by such a method
is 440,000.
Oligomeric PEPK was produced industrially according to Scheme (9) [65] .
ð9Þ
Apart from hydroxy end groups PEPK (9b) contains also unsaturated end groups [66].
Propenylcarbazole groups appear in the oligomer during anionic polymerization of the
monomer (9a) as the result of a chain transfer reaction [67]. PEPK exhibits the best film
forming properties when its molecular weight (M
w
) is in the ran ge from 1000 to 1500. The
glass transition temperature of an oligomer of such molecular weight is 65–75

C.
Brominated analogues of PEPK enable to obtain electrophotographic layers of
enhanced electrophotographic photosensitivity [68]. The most promising from the point
Copyright 2005 by Marcel Dekker. All Rights Reserved.
of view of convenience of synthesis and photoactivity among the brominated
poly(carbazolyloxiran es) is poly(3,6-dibromo-9-(2,3-epoxypropyl)carbazole) (10a). It is

synthesized mainly by cationic ring-opening polymerization of the corresponding oxirane
monomer using Lewis acids [69] or triphenylcarbenium salts [70] as initiators. The
molecular weight of the oligom ers (10a) usually does not exceed 2000. Because of the
presence of heavy bromine atoms, the glass transition temperature of these oligomers
is higher than that of unbrominated PEPK. Their film-forming properties are usually
inferior to those of PEPK. Polymerization via activated monomer mechanism in the
presence of diols allows to prepare bifunctional oligomers of 3,6-dibromo-9-(2,3-
epoxypropyl)carbazole having hydroxyl end-groups and a flexible oxyalkylene fragment
in the main chain [71]. They show high electrophotographic photosensitivity when
sensitized and good film-forming properties [72].
Poly((2-(9-carbazolyl)et hoxymethyl)oxirane) (10b) has been synthesized both by
cationic polymerization of the corresponding epoxy monomer with Lewis acids [73],
triphenylcarbenium salts [74] and by anionic polymerization init iated with KOH [75] or
by potassium alkalide, potassium hydride, and potassium tert-butoxide [76]. Since chain
transfer reactions to (2-(9-c arbazolyl)ethoxymethyl)oxirane are not as intense as in the
case of EPK polymerization oligomers (10b) of higher molecular weight can be prepared
using both cationic and 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 layers of PEPK.
ð10Þ
A series of polysiloxanes with pendant carbazolyl groups (11c) have been synthesized
by the reaction of poly(hydrogenmethylsiloxane) with various o-alkenylcarbazoles [77].
ð11Þ
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Detailed time of flight measurements [78] have shown that the polysiloxane (11c)
with the shortest spacer (m ¼ 3) exhibits a carrier mobility which is about one order of
magnitude higher than that for PVK. The data of Goldie et al. [79] corroborate this

observation. The activation energy for carrier transport derived from the temperature
dependence of the carrier mobility is 0.6 eV for all the polysiloxanes and for both PVK
and N-isopropylcarbazole in a polycarbonate matrix. The fluorescence spectra [80] of the
extremely pure polysiloxanes prepared starting from synthetic carbazole show that these
polymers, due to the conformational freedom of the carbazole groups, are free of excimer
forming sites.
Thermotropic liquid crystalline side group polymers with carbazolyl groups have
been reported by Lux et al. [81]. The idea behind this work was to make a liquid crystalline
polymer with a photoconductive mesogenic unit. It should be possible to orient such
a polyme r by means of an electric or magnetic field at elevated temperatures where it
exhibits a mesophase and to freeze this orientation by cooling down below the glass
transition temperature. In the polysiloxanes (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 polyme rs exhibit broad mesophases,
for example polymer (12) with a spacer of three methylene units (m ¼ 3) has a glass
transition at 69

C and a smectic mesophase up to the clearing point at 215

C.
Unfortunately, the polysiloxanes show almost no photoconductivity.
ð12Þ
The influence of liquid crystalline media on the hole transport of organic
photoconductors has been demonstrated by Ikeda et al. [82]. They have established that
DCZB dissolved in polymer liquid crystals showed improved hole drift mobility owing
to the orientation of the carrier molecules. The same research group [83] has prepared
copolymers of acrylates with side chain mesogens and dimeric carbazoles (13).
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 photoconductive groups is favourable for the charge carrier transport.
ð13Þ
Apart from polymers containing both photoconductive and liquid crystalline side
groups a lot of attention has been paid to the synthesis of polymers in which both
photoconductive and nonlinear optical ch romophores are present. Polymers showing both
second-order nonlinear optical and photoconductive properties are photorefractive and
have potential application in data storage and image processing as well as in medicine [84].
Carbazolyl-containing photorefractive polymers have been reviewed [85]. An example of
such functional polymer is given in the Scheme 14. Tamura et al. [86,87] have synthesized
polyacrylates and polymethacrylates having carbazole and tricyanovinylcarbazole side
groups. 5-(N-carbazolyl)penty l methacrylate and acrylate were polymerized using AIBN
as an initiator. The resulting polymer was reacted with tetracyanoethylene to tetra-
cyanovinylate ca. 20% of the carbazole units.
ð14Þ
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All polymers discussed above have pendant carbazolyl groups. Only few poly-
condensates in which the carbazolyl group is part of the main chain have been reported.
Tazuke et al. [88–90] have synthesized polyurethanes, poly-Schiff bases and polyamides
containing DCZB moieties in the main chain. Polyurethanes containing DCZB moieties
in the main chain (15) were prepared by treating trans-1,2-bis(3-hydroxy-methyl-9-
carbazolyl)cyclobutane with the corresponding diisocyanate in the presence of dibutyltin
dilaurate [88]. The molecular weight of the polymer synthesized using hexamethylene
diisocyanate as a linking agent was 2700, and that of the polymer prepared with toluylene
diisocyanate was 16,000. Polymers (15) exhibit almost exclusively monomer fluorescence
in dilute solution, i.e., they practically have no intramolecular excimer-forming sites. Their
complexes with TN F show better photoconduc tive properties than PVK-TNF.
ð15Þ

Polyimines (16) containing DCZB moieties and a spacer of variable number of
methylene groups have been synthesized by Natansohn et al. [91] from trans-1,2-
bis(3-formyl-9-carbazolyl)cyclobutane and the corresponding aliphatic diamine. The
charge transfer complex es of the polyamines (16) with tetracyanoethylene and TNF
have been analyzed both in solution and in solid state. These polyimines do form charge
transfer complexes with both TNF and tetracyanoethylene, but these complexes have a
solution like behavior, i.e., the components are relatively free to move around. Charge
carrier transport in the polyimines (16) has been studied by the time-of-flight technique
[92]. The hole mobility in polyimines (16) is higher than that in PVK.
ð16Þ
2. Other Photoconductive Polymers with Non-Conjugated Main Chain
Besides polymers with a carbazole moiety a number of polymers with various pendant
aromatic amino groups have been reported. Poly(N-vinyldiphenylamine) (17a) and
Copyright 2005 by Marcel Dekker. All Rights Reserved.
poly(4-diphenyl-aminostyrene) (17b) have been reported in early patents reviewed by
Stolka and Pai [16]. The polymers were claimed to be useful in electrophotography.
ð17Þ
A detailed photoconductivity study has been carried out with a number of
polymethacrylates with pendant aromatic amino groups [93]. Among seven polymeth-
acrylates that have been synthesized from the corresponding methacrylate monomers
by free-radical polymerization, poly(2-(N-ethyl-N-3-tolylamino)ethyl methacrylate (18a)
and poly((4-diphenylamino)phenylmethylmethacrylate) (18b) exhibit carrier mobilities
that exceed the values of PVK by about one order of magnitude at all electric fields.
ð18Þ
Aromatic amino group-containing polymethacrylates alone only exhibit charge
carrier generation when irradiated with UV light in the range of absorption. The charge-
transfer complex of polymer (18a) with TNF (2 : 1 mol ratio) displays photoconductivity
in visible light. Xero-graphic discharge experiments of these polymers in combination with
a thin selenium layer proved aromatic amino group-containing polymethacrylates to be
useful 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. M
w
varies from 5000 to 15,700. The glass
Copyright 2005 by Marcel Dekker. All Rights Reserved.
transition temperature is in the range of 130–150

C. Poly(vinylarylamines) (19) have
been used as hole transport materials in two-layer light-emitting diodes with tris(8-
quinolinato)aluminium as electron transporting and emitting layer.
ð19Þ
Ulanski et al. [95] have reported that poly((E,E-[6,2]-paracyclophane-1,5-diene)
(20b) shows relatively high photoconductivity especially when it is doped with
tetracyanoethylene (TCNE). The polymer (20b) is obtained either by free-radical or
cationic polymerization of the corresponding monomer (20a) [96]. Cationic polymeriza-
tion is favored.
ð20Þ
At a field of 4 Â 10
5
Vcm
À1
pure polymer (20b) shows a mobility of 1.2 Â 10
À6
cm
2
V
À1

s
À1
and that doped with 4% of TCNE exhibits a mobil ity of 3.6 Â 10
À5
cm
2
V
À1
cm
À1
[97].
Polymers containing triphenyldiamine (TPD) moieties in the main chain, obtained
by step growth polymerization are of increasing interest both 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 inst ead 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 Reserved.
molecules from the charge transport to the charge generation layer.
ð21Þ
A poly(arylene ether sulfone) (22) containing TPD moiet ies was synthesized by the
reaction of the correspondi ng bisphenol with 4,4
0
-difluorodiphenylsulfone [99]. The weight
average molecular weight of the polymer (22) was determined to be 9300. Its thermal
properties are excellent for the application in electroluminescent devices as hole transport
layer. The glass transition temperature of the polymer (22) is 190

C.
ð22Þ

Polycarbonates [100] and polyethers [101] containing triphenylamine moieties in the
polymer backbone have also been synthesized and used as hole transport materials in
light-emitting diodes.
Crosslinkable charge transport materials recently attract much attention since they
allow to prepare multilayer devices by low cost techniques, i.e., the combination of
spin coating and crosslinking. Nuyken et al. [102] have reported the synthesis of
photocrosslinkable derivatives of TPD containing oxetane functionalities. One example
of a photocrosslinkable TPD is shown in Scheme (23). The photocrosslinking was carried
out by a cationic mechanism. The resulting films are resistant against solvents to use
in subsequent spin coating. The performance of single- and two-layer electroluminescent
devices based on the crosslinked polymers is reported to be greatly enhanced relative to
those containing the non-crosslinked compound (23) what is explained by the improved
stability of the crossl inked layer.
ð23Þ
B. Polymers with n-Conjugated Main Chain
A number of photoconductive polymers and oligomers with conjugated double bonds
along the polymer chain have been reported in the literature. Among p-conjugated
Copyright 2005 by Marcel Dekker. All Rights Reserved.
polymers are polyacetylene and its derivatives, polydiacetylenes, polyarylenes like
poly(phenylenevinylene) or poly(phenylenesulfide), polythiophene and poly(3-alkylthio-
phenes), polybenzothiazoles and others. These polymers are insulators in the dark and
exhibit photoconductivity when illuminated. After chemical or electrochemical ox idation
or reduction these p-conjugated polymers become conductive. In this section we are going
to describe only those p-conjugated polymers which have attracted much attention as
photoconductors.
The photoconductivity in polyacetylene, the simplest conjugated polymer, has been
the subject of intense investigations [103–106]. Transient photoconductivity measurements
on a picosecond time scale have been carried out [107–112]. These ultrafast methods are a
powerful tools to investigate the transport properties as well as the recombination kinetics
of charged excitations. It was found [107] that the photocurrent in trans-polyacetylene

consists of two components: a fast component which relaxes on a picosecond time scale
and for which a carrier mobility of about 1 cm
2
V
À1
s
À1
was report ed [110,111] and a slow
component with carrier lifetimes up to secon ds.
Some polyacetylene derivatives have also been thoroughly investigated. Kang et al.
[113,114] reported on photoconductivity measurements in trans-poly(phenylacetylene)
and its charge transfer complexes. Trans-poly(phenylacetylene) was prepared by the
polymerization of the corresponding monomer with W(CO)
6
in carbon tetrachlo-
ride solution under UV irradiation [115]. The reaction yiel ded a room-temperature-
soluble polymer of molecular weight (M
n
) 80,000. Poly(2-chloro-1-phenylacetylene)
was synthesized by a similar procedure. M
n
of the polymer was 400,000. Steady-state
and pulsed photoconductivities were explored in amorphous films of poly(phenylacetyl-
ene) and of that doped with inorganic and organic electron accepting compounds
like iodine and 2,3-dichloro-5,6-dicyano-p-benzoquinone and dyes like pyronin Y and
methylene blue [114,115]. It was concluded that the transport mechanism in these
systems is significantly different from the hopping transport which occurs in PVK and
its charge-transfer complexes. Cis-poly(phenylacetylene) can also be converted to a
photoconductive material. It has been done by irradiating with
60

Co and electron beam,
doping with iodine and ferric chlori de an d sensitizing with 4-isothiocyanatofluorescein
or TNF [116–118]. Cis-poly(phenylacetylen e) was prepared by a direct method of poly-
merization of phenylacetylene into a polymer film with a rare-earth coordination
catalyst. The cis-content of poly(phenylacetylene) obtained by this method was more
than 90%. The molecular weight (M
n
) was about 10
5
as measured by gel permeation
chromatography.
Pfleger et al. [119] have studied photoconduction in undoped poly(phenylacetylene)
which they prepared by coordination polymerization of phenylacetylene using the
methatesis catalyst WOCl
4
/Ph
4
Sn. The polymer thus obtaine d was predominantly in the
cis-transoidal form, as demonstrated by IR spectra, and had a molecular weight of (M
n
)
91,000. The photoconduction threshold has been detected at 410 nm, although absorption
of the film extends up to 550 nm. It is suggested that the mechanism of photogeneration
is intrinsic by nature. The formation of initial charge carrier pairs occurs by an exciton
autoionization process [42].
Poly(N-2-propynylcarbazole) (24a) and poly(N-2-propynylphenothiazine) (24b)
have been prepared with Ti(OBu)
4
/Et
3

Al as initiator [120]. Polymer (24a) was only
partly soluble in some solvents like tetrahydrofuran, chloroform, nitrobenzene, and
p-dichlorobenzene. In contrast to Ti(OBu)/Et
3
Al initiation polymerization of 3-(N-
carbazolyl)-1-propyne with MoCl
5
and WCl
6
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 with tert-butylacetylene initiated
by MoCl
5
/(C
4
H
9
)
4
Sn yielded copolymers of high molecular weight (M
w
¼ 350,000)
completely soluble in toluene and chloroform [121]. Polymers (24a) and (24b) were found
to show photoconductivity. Charge carrier photogeneration in these polymers and some
related copolymers has been studied in detail [122,123].
Poly(1,6-heptadiyne) derivatives containing a carbazole moiety (25b) were synthe-
sized by metathesis cyclopolymerization of bis(N-carbazolyl)-n-hexyl dipropargylmalo-

nate (25a) [124]. The resulting polymer exhibited good solubility in common organic
solvents and could easily be cast on a glass plate to give violet, shiny thin films. The
number-average molecular weight values of the polymer were in the range from 3.2 Â 10
4
to 8.9 Â 10
4
. Polymer (25b) shows two maximum values of the photocurrent around
350 nm and around 700 nm. The photo- to dark-conductivity ratio without doping was
found to be in the range of 30–50 at 10
3
–10
4
V/cm.
ð25Þ
Polydiacetylenes like poly(2,4-hexadiyne-1,6-diol bis( p-toluenesulfonate)) (26) have
been studied by several authors [111,112,125–128].
ð26Þ
They are unique in that that they can be obtained as polymer single crystals and therefore
they have found a considerable interest in fundamental studies. A carrier mobility of
5cm
2
V
À1
s
À1
has been reported for polymer (26) [112]. The field and temperature
dependencies of the mobility have been investigated in detail [128].
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Many years ago photoconductivity has been reported in a number of polyaryl-
enes like poly(phenylenevinylene) (PPV) (27a), poly(phenyleneazomethine) (27b) and

poly(phenylene sulfurdiimide) (27c) [16].
ð27Þ
These and a number of related polymers like poly(styrylpyrimidines), poly(quina-
zones), poly(pyrrones), and poly(benzoxazoles) have already been reviewed by Stolka and
Pai in 1978 [16]. They stat ed that there were some major problems with these polymers:
complicated synthesis, in many cases poorly identified structures, and with a few
exceptions insolubility and intractability. Large efforts have been made since then to
overcome these difficulties. Two major pathways have been established which lead to
tractable materials. Proper substitution of a rigid conjugated polymer leads to a soluble
and fusible material. A second approach to improve the processability of conjugated
polymers is to adopt a two step synthesis. In this case a nonconjugated polymer which can
be readily converted to the desired material by heat treatment and which has good stability
and processing propert ies is used as a precursor.
PPV (27a) has be en prepared by a number of different methods which were studied
in detail by Ho
¨
rhold and Opfermann [129]. It can be synthesized by bifunctional carbonyl
olefination of terephthalaldehyde according to Wittig’s reaction and from p-xylylene-bis-
(diethyl phosphonate) as well as by dehydrochlorination of p -xylylene dichloride with
sodium hydride in N,N-dimethylformamide and with potassium amide in liquid ammonia.
Another route to PPV used today is the precursor ro ute, first described by Wessling
[130–133] and Kanabe [134], starting from the monomers p-xylylene-bis(dimethylsulfo-
nium tetrafluoroborate) [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 dialy sis [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 Ho
¨
rhold et al. [136], Lenz et al. [137,138], Murase et al. [139] and Bradley

[140]. One of the major 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 by the precursor route has been studied by
several groups [142–145]. The polymer has a photoconductivity threshold at 506 nm that
coincides well with the absorption edge [145]. Measurements of the transient photocurrent
indicate a dispersive type of transport. The current is predominantly carried by holes
with mobilities in the range from 10
À3
to 10
À4
cm
2
V
À1
s
À1
. PPV was the first p-conjugated
polymer in which the phenomenon of electroluminescence was demonstrated and from
which light-emitting diodes were fabricated [146].
Soluble analogues of PPV with variety of substituents have been synthesized by
different methods in Ho
¨
rholds laboratories [147–154] and in other groups [155–159].
The synthetic routes to PPV have been recently reviewed by Holmes [9].

p-Conjugated polymers [160,161] and copolymers [162–164] of 9,9-dialkylfluorenes
now attract strong interest as blue-emitting polymers showing high hole mobilities and
having good prospects of commercial application in light-emitting diodes. Poly(2,7-
fluorenes) are prepared via Suzuki coupling [160,161,165] and nickel(0) catalyzed redu ctive
coupling [166] while the copolymers are also prepared by Wittig reaction [163] and
Heck reaction [164]. The most widely studied among the poly(fluorenes) is poly(9,9
0
-
dioctylfluorene) (29) [167–169]. This polymer forms a well defined thermotropic liquid
crystalline state that can be aligned on rubbed substrates and can be either quenched into
a glass or crystallized [161]. Polarized absorption and emission spectra of the polymer
show a high degree of orientation, indicating strong potential for use in polarized
electroluminescent devices. Poly(9,9
0
-dioctylfluorene) exhibits relatively high hole
mobility, which is necessary, since in order to ensure an acceptable power efficiency
high brightness of electroluminescent devices should be reached at low bias voltages. The
as-spin coated (‘isotropic’) polymer shows hole mobility of 3 Â 10
À4
cm
2
V
À1
s
À1
[168]. In
addition, hole transport is nondispersive, which points to a high degree of chemical purity
and regularity. Homogeneous nematic alignment of poly(9,9
0
-dioctylfluorene) films on

rubbed polyimide results in more than one order of magnitude increase in Time of Flight
hole mobility normal to the alignment direction. A hole mobility of 8.5 Â 10
À3
cm
2
V
À1
s
À1
at an electric field of 10
4
Vcm
À1
is reported for the aligned quenched film of poly(9,9
0
-
dioctylfluorene) [169].
ð29Þ
Conjugated triphenyldiamine (TPD) based oligomers (30) have been prepared by
polycondensation of the corresponding bis(sec-amines) and diodides [170]. The number
average molecular weight of the oligomers (30) ranges from 1400 to 1800. Their glass
transition temperatures are ca. 130

C.
ð30Þ
Copyright 2005 by Marcel Dekker. All Rights Reserved.
A polymer, incorporating both TPD and phenylenevinylene segments (31) has by
recently reported [171]. This polymer possesses excellent film-forming properties, good
thermal stability, and high electrochemical reversibility. It was prepared by the Wittig–
Horner polycondensation reaction between a TPD-based dialdehyde and 1,4-xylylene

diphosphate.
n
ð31Þ
Poly(9-hexyl-3,6-carbazolyleneethynylene) (32c) has been prepared by palladium
catalyzed polycondensation of 3,6-diiodo-9-hexylcarbazole (32a) and 3,6-diethynyl-9-
hexyl-carbazole (32b) [172]. The polymer has a number average molecular weigh t M
n
of
3000. By fractionation a polymer with M
n
of 6400 has been obtained.
n
ð32Þ
Polymer (32c) is soluble in common organic solvents. The trimer model compound of the
polymer (32c) 3,6-bis((9-hexyl-3-car bazolyl)ethynyl)-9-hexylcarbazole (33) forms a stable
glass with a glass transition at 41

C. The trimer as well as the dimer were synthesized
by stepwise reactions of the derivatives of 9-hexyl-carbazole [172]. Time-of-flight experi-
ments have revealed carrier mob ilities up to 2 Â 10
À4
cm
2
V
À1
s
À1
at an electric field of
6 Â 10
5

V/cm in the trimer (33).
ð33Þ
Copyright 2005 by Marcel Dekker. All Rights Reserved.
C. Polymers with p-Conjugated Main Chain
Polysilylenes (polysilanes) (34b) have received widespread interest. Their electronic pro-
perties are associated with s-electron conjugation in the silicon backbone which
allows a significant delocalization of electrons along the chain. In the usual synthesis of
polysilylenes, diorganodichlorsilanes (34a) are treated with sodium metal in a hydro-
carbon diluent [173]. In order to recreate the surfa ce of the sodium metal permanently
ultrasound is used in these reactions [174,175].
ð34Þ
Poly(methylphenylsilylene) (PMPS) obtained by this method has a high molecular
weight and a narrow molecular-weight distribution (M
n
¼ 184,000, M
w
/M
n
¼ 1.4) [175].
PMPS is the most thoroughly studied polysilylene. Photoconductivity measurements of
this polymer have been carried out by several groups [175–192]. The quantum yield of the
charge carrier generation È in PMPS is rather low (3 Â 10
À3
charges per photon at an
electric field of 3 Â 10
5
V/cm) [181], while the hole drift mo bility is rather high. Most of
the authors report room temperature mobilities of about 10
À4
cm

2
V
À1
s
À1
at an electric
field of 10
5
V/cm [175,179]. Higher hole mobilities exceeding 10
À3
cm
2
V
À1
s
À1
at room
temperature have been recently observed in self-organized individual oligomerhomo-
logues of poly(dimethylsilylene) [193]. Since there is no apparent difference in charge
carrier mobility in PMPS and poly(dialkylsilylenes) [183] it can be assumed that the
charge-carrier transport proceeds predominantly along the s-delocalized Si backbone.
The temperature and field dependencies of the carrier mobility in PMPS have been
studied in great detail and were discussed in relation with different theoretical
models [179].
In order to increase the quantum yield of charge carrier generation doping of PMPS
with different additives has been studied [184–1 87]. Doping of the polymer with electron
scavenging compounds generally resulted in soaring of the È values and plummeting of
the m values.
The influence of hole trapping substances, which at the same time are transport-
active, on the photoconductivity of PMPS has also been investigated [186,188,189].

Aromatic amines with different ionization potenti als have been examined. It turned out
that a small amount (1%) of N,N
0
-diphenyl-N,N
0
-bis(3-methylphenyl)-(1,1
0
-biphenyl)-
4,4
0
-diamine (TPD) (36a) did not exert any influence on the carrier mobility, while
other amines strongly diminished it. This observation was explained by the fact that the
ionization potential of TPD is equal to that of PMPS while the ionization potentials of
the other amines studied are lower.
A decrease of the hole drift mob ility was also observed in carbazole containing
polysilylenes relative to PMPS [194]. The polysilylenes of which the repeat units are shown
Copyright 2005 by Marcel Dekker. All Rights Reserved.