SUPHANNEEPONGKITWITOON

Photoconducting Polymers

Presented by SUPHANNEE PONGKITWITOON

MATERIALS SCIENCE AND ENGINEERING

PENNSYLVANIA STATE UNIVERSITY
Content

1          Introduction

Polymers are typically utilized in electrical and electronic applications as insulators where advantage is taken of their very high resistivities.  Typical properties of polymeric materials are high strength, good flexibility, excellent elasticity, stability, mouldability, ease of handling, etc.  Polymers are capable of conducting when a structure of the polymers is conjugated p-electron backbones along which conduction can be induced by way of their delocalized orbitals [Winiarz et al, 1999].

Photoconductive polymers exhibit an increase in electrical conductivity upon illumination.  The important commercial applications of photoconductive polymers are using as transistors and detectors.  The photogeneration processes simply involves the direct excitation of an electron from the valence band to the conduction band after provided that hn exceeds the band gap energy [Suh et al, 1998].  Studies of organic system, especially in polymeric system are more attractive due to their ease of processabilities [Zhong, 2003].

1.2         Conduction Mechanisms

The outermost shell of electrons in a material contains the valence electrons and these can be placed in the valence band that defines their lowest energy states.  In order to conduction, an electron must obtain sufficient energy to promote it to the conduction band.  The energy difference between the valence band and the conduction band is known as the forbidden gap.  For a conductor, the highest energy level of the valence band and the lowest energy level of the conduction band are similar, meaning that the forbidden gap does not exist and electrons pass easily into the conduction band.  For an insulator, the separation between the two bands is large and promotion of an electron to the conduction band is not possible, while in semiconductors, the forbidden gap is moderate and limited conduction occurs [Zhong, 2003].  Electrical conduction in organic materials may occur through movement of either electrons or ions.

The conductivity, s, is equal to the product of the carrier mobility, m, its charge, q, and the number of carrier or the concentration, n.

s = m q n

Conducting polymers are usually polyconjugated structures, which are insulators in the pure state but when treated with an oxidizing or a reducing agent can be converted into polymer salts with electrical conductivities comparable to metals [Yoshida et al, 1999].

In general there are 2 types of conducting mechanism [Zhong, 2003].  The p-type doping is a removal of electrons from the valence band by the oxidizing agent, leaving the polymer with a positive charge, while the n-type doping is a donation of an electron to the empty conduction band by a reducing agent.  The addition of a donor or an acceptor molecule to the polymer is called "doping ", while the reaction that takes place is actually a redox reaction.

The proposed theory for conducting mechanism by Zhong, 2003 is following steps.  The first step is the formation of a cation (or anion) radical, which is called a soliton or a polaron.

P_n         Û      [P_n⁺ A^–]

(reduction)                  (oxidation)                                              (1.1)

This step may then be followed by a second electron transfer with the formation of a di-cation (or di-anion) known as a bipolaron.

[P_n⁺ A^–]    Û     [P_n²⁺ 2A^–]

 (reduction)                 (oxidation)                                              (1.2)

Alternatively after the first redox reaction, charge transfer complexes may from between charged and neutral segments of the polymer when possible.

 [P_n^·+ A^–]  +  P_m  ®  [(P_n P_m)^·+ A^–]                                         (1.3)

Other theories explain the conducting mechanism is delocalization [Zhong, 2003], for instant in polyacetylene [Shirakawa], the mechanism of conduction occurs when delocalization led to each bond having equal partial double bond character if all the bond lengths were equal.  Therefore the bonds would overlap and the polymer would behave like a quasi-one-dimensional metal having good conductive properties.

This is an unstable system and will undergo lattice distortion by alternative compression and extension of the chain, leading to alternating atom pairs with long and short interatomic distances found along the chain.  Single and double bond alternation persists in the polyacetylene chain leading to an energy gap between the valence and conduction bands.  The electron has an unpaired spin and is located in a non-bonding state in the energy gap, midway between the two bands.

Controlled addition of an acceptor or p-doping agent such as AsF₅, Br₂, I₂, or HClO₄, removes an electron and creates a positive soliton or removes a neutral one if the electron removed is not the free electron.  The resulted carbenium ion is stabilized by having the charge spread over several monomer units.  A negative soliton can be formed by treating the polymer with a donor or n-doping agent that adds an electron to the mid-gap energy level.  This can be done by dipping the film in the THF solution of an alkali metal naphthalide or by an electrochemical method.  At high doping level, the solution regions tend to overlap and create new mid-gap energy bands that may merge with the valence and conduction bands allowing freedom for extensive electron flow.  The charged solitons are responsible for making polyacetylene a conductor.

The last theory have been used to explain the conducting mechanism is band theory such as conduction in poly(p-phenylene) [Zhong, 2003].  The band theory model is that conduction occurs because the mean free path of a charge carrier extends over a large number of lattice sites.  The residence time on any one site is small compared with the time it would take for a carrier to become localized.  If a carrier is trapped it tends to polarize the local environment that relaxes into a new equilibrium position.  This deformed section of the lattice and the charge carrier then form a species called a polaron.  Unlike the soliton, the polaron cannot move without first overcoming an energy barrier so movement is by a hopping motion.  On poly(p-phenylene) the solitons are trapped by the charges in polymer structure because of the difference in energy and so a polaron is created which is an isolated charge carrier.  The chemical equivalents for polaron and bipolaron are the radical ion and di-ion respectively.  On poly(p-phenylene) and most other conjugated conducting polymers, the conduction occurs via the polaron or bipolaron.  Poly(p-phenylene) can be doped by p-dopants such as AsF₅ vapor (10^-2 S cm^-1) and n-dopant such as metal naphtalide in THF (~75 S cm^-1).  Films prepared by the precursor route can be stretched to give a structure with a high degree of uniform molecular orientation.  After doping, the conductivity in the direction parallel to the orientation can reach a value as high as ~2500 S cm^-1.

The soliton defect cannot be supported in poly (p-phenylene) as there is no degenerate ground state [Zhong, 2003].

2  Photoconducting Polymers

Photoconductive polymer is a polymer, which exhibits a relatively high electrical conductivity when irradiated with visible or ultraviolet light.  Such polymers are of interest as forming the basis of electro imaging processes such as xerography, and in other photoelectric devices.  The most widely studied polymer is Poly (N-vinyl carbazole) or PVK and its charge-transfer complexes, especially with 2, 4, 7 – trinitrofluorenone [Alger, Mark S.M., 1989].

Figure 2-1 Structure of Poly(N-vinylcarbazole), PVK

2.1  Photoconducting Mechanism

Photoconductivity is defined as an increase of conductivity caused by irradiation.  On the other word, photoconductive polymers are insulating or poorly conductive in the darkness and more conductive when illuminated [Zhong, Ben Tang, 2003].

The simplest process of photoconduction occurs when a photon is absorbed and an electron is promoted from the valence band to the conduction band.

Figure 2-2 : The simplest process of photoconduction occurs when photons is absorbed.  Consequently an electron is promoted from the valence band to the conduction band.

This process generates an electron hole pair, augmenting the concentration of intrinsic carriers.  The photon energy must exceed the band gap and consequently there is a threshold wavelength-energy associated with the process.  Carriers may also be generated from photon absorption in the generation of excitons.  The excitons are themselves unable to transport charge but can produce an electron hole pair if two excitons collide.

On many common polymers it is very difficult to observe photoconduction even when the samples are irradiated by photons with energies well in excess of their band gaps.  This is largely because the carriers have very short lifetime (<1 ns) due to rapid recombination and deep hole trapping.

Photoconductive polymers can be p-type (hole-transporting), n-type (electron- transporting), or bipolar (capable of transporting both holes and electrons).  All practical photoconductive charge-transporting polymers are p-type.

Poly(N-vinylcarbazole) (PVK) and other vinyl derivatives of polynuclear aromatic polymers such as poly(vinylpyrene) and poly(2-vinylcarbazole) have high photoconductive efficiencies.  These materials may take up a helical conformation with successive aromatic side chains having parallel to each other in a stack along which electron transfer is relatively easy.  PVK absorbs ultraviolet light in the 360-nm region and forms an exciton that ionizes in an electric field.  PVK is an insulator in visible light with a conductivity of around 10^-16 S cm^-1 at 550 nm.  The addition of an equimolar amount of the electron acceptor 2,4,7-trinitrofluorenone (TNF) shifts the absorption of PVK into the visible by the formation of a charge transfer state, rendering it photoconductive at 550 nm.

2.2    Electrooptic Properties based on Photorefractivity of Photoconducting Polymers

Using the simplest case of the band transport model, the steady state and the dynamics of the space charge field are surveyed.  The theory of two beam coupling in photorefractive media is briefly outlined.

2.2.1. The photorefractive effect

The photorefractive effect was accidentally discovered in 1966 in LiNbO3 and LiTaO3 as detrimental optically induced refractive index inhomogeneities.  It was referred to as "optical damage" because it caused a degradation of the performance of nonlinear optical devices based on these materials.  Two years later, holographic optical storage has been demonstrated in LiNbO3 using this newly discovered effect [Chen, 1969]. In 1969, Chen proposed a model based on the migration of photoexcited electrons which explained the main experimental observations and set the basis for future experimental and theoretical work [Chen, 1969]. The term photorefractive, which literally means light induced change of the refractive index, was introduced later on and since then has been reserved for this particular mechanism.  In 1976, Kukhtarev et al. derived the dependence of the refractive index change on light intensity and material parameters and described the coupling of beams in thick photorefractive gratings. Today, almost 30 years after its first discovery, photorefractivity is a blooming field of interdisciplinary research [Kukhtarev, 1976].

Numerous applications in optical data storage, image processing and amplification, self and mutually pumped phase conjugation, photorefractive resonators, programmable optical interconnects, simulation of neural networks etc. have been proposed and demonstrated on a laboratory scale.  Apart from potential applications, intensive research has been triggered for the understanding of the microscopic origin of the photorefractive effect, resulting in the discovery of new phenomena, such as the bulk photovoltaic effect and the excited state polarization.  Today, the mechanism of photorefractivity, although not fully, is quite well understood.  This led to the recent observation of photorefractivity in new classes of materials such as organic crystals, polymers and liquid crystals [Zheng et al, 1999].

In Figure 2.2.1.1, the basic mechanism is explained.  The photorefractive effect is observed in materials which are both electrooptic and photoconducting.  If such a sample is illuminated with a non-uniform light intensity pattern resulting from the interference of two mutually coherent beams, charge generation will take place at the bright areas of the fringes.  These photogenerated charges will migrate and eventually get trapped at the dark areas, a process which can take place through several circles of photogeneration, diffusion and trapping.  The resulting charge redistribution creates an internal electric field, the space charge field ESC, which changes the refractive index via the electrooptic effect.  The space charge field forces the charges to drift in the opposite direction than diffusion and a dynamic equilibrium is reached when it has grown strong enough to cause a drift current which totally compensates the diffusion current.  Application of an external electric field assists charge separation through drift and generally a higher space charge field can be produced in this way.

Figure 2.2.1.1 : Mechanism of the photorefractive effect.  A sinusoidal distribution of light intensity causes spatially modulating charge generation. The mobile charges diffuse and get trapped at the dark areas. A space charge field is established which changes the refractive index via the electrooptic effect.

From the above figure it is clear that the photorefractive effect provides a way to replicate light intensity patterns into refractive index gratings, with obvious potential applications in optical data storage. Several other mechanisms can do the same thing though: photochemical reactions, thermorefraction, formation of excited states, conventional c(3) etc. can change the refractive index in the illuminated parts of a sample.  The photorefractive effect however processes a combination of characteristics which make it unique: Very high nonlinearities can be achieved even with weak laser beams, as a result of the integrating nature of the effect.  The resulting refractive index gratings are reversible, as uniform illumination will erase the space charge field.  Another very important characteristic is the existence of a spatial phase shift between the illumination pattern and the refractive index grating.  This is the genuine signature of the photorefractive effect: No other mechanism can produce a phase shifted refractive index grating.  As will be discussed below, the existence of this phase shift gives rise to steady state asymmetric energy exchange between two laser beams, which is the basis for several specific applications.

Apart from applications, the photorefractive effect provides a means to investigate materials properties such as charge transport and trapping, with "clean" optical techniques.  Steady state and transient holographic techniques can be employed to measure small photocurrents optically, expelling the need for sensitive electronic equipment. Moreover, the bulk of the sample is directly probed, eliminating electrode problems.  Parameters like charge diffusion lengths, mobilities, trap densities and cross sections etc. are measured in this way [Zhong, 2003].

2.2.2. Standard model for photorefractivity

The model of Kukhtarev et al. has been very useful in helping to understand the microscopic origin of photorefractivity in inorganic crystals.  To some extent, photorefractivity in polymers can also be understood along the same lines.  For this reason a basic description of this model is presented here.  In figure 2.2.2.1, the basic idea for the space charge field formation is illustrated, for the case of electron transport and a single participating impurity level.  Depicted terms are the valence (EV) and the conduction (EC) band, together with donor (D) and acceptor (A) impurity levels.  The only role of the acceptors is to deprive some of the donors from their charge, creating an initial concentration of empty traps.  Let ND be the total donor density and ND + the density of the ionized ones which act as traps.  In the dark, electrical neutrality demands ND+=NA, where NA is the density of acceptors.  Let the crystal be illuminated with a sinusoidal intensity pattern: I=I0 (1+mcos(KGx)), where m is the modulation index and KG is the grating wave vector.  According to this model, the space charge field is created through the steps of photoionization of a donor in the bright areas of the fringes as step 1 in Figure 2.2.2.1, transport of the electron in the conduction band as step 2 in Figure 2.2.2.1 and subsequent trapping at an ionized donor level as step 3 in Figure 2.2.2.1.  The rate of formation of ionized donors has a generation term, proportional to the light intensity and the density of donors that can be ionized, plus an annihilation term proportional to the available density of electrons in the conduction band and the trap density: ¶ND

Figure 2.2.2.1 : Band transport model for the photorefractive effect.  Electrons are photoexcited (1) from donor states (D) to the conduction band (EC), where they migrate (2), until they get trapped at ionized donor sites (3).  Acceptors (A) are present to create a few initially empty traps.

Electrons are mobile once in the conduction band and their density changes not only due to photogeneration and trapping, but also due to transport.

2.2.3  Photorefractivity in polymers

Despite the numerous potential applications that have been proposed for photorefractive materials, none of them has ever been realized on a broad commercial scale.  One reason may be that optical processing schemes are highly task specific, meaning that a certain layout of optical components is only used to perform a unique operation.  In addition to that, they are bulky and rather complicated, demanding trained personnel for their operation and maintenance.  Moreover, existing electronic and hybrid technologies advanced very fast, making possible their application in tasks that were previously feasible only with optical processing schemes.  However, the notorious difficulty in the preparation of high quality photorefractive crystals certainly played a major role in that too.  The tunability of the properties of these materials is rather limited, and generally not a single crystal exists that combines all the desired characteristics for applications.

The observation of photorefractivity in polymers in 1991 [Zhong, 2003], created an alternative class of materials which show the effect.  Processability is one of their main advantages, while tunability is inherent in the way they are fabricated.  Moreover, potentially better performance is foreseen, as a result of the different nature of the electrooptic response [Sitch et al, 2001].  As discussed, the diffraction efficiency depends on the quantity of the electrooptic effect [Zhong, 2003]: large reff is always associated with large e.  The electronic nature of the electrooptic effect in polymers, plus the newly discovered mechanism of orientational enhancement [Zhong, 2003] gives a promise for improved performance.  The necessary properties that a material should have in order to be photorefractive are charge generation, transport and trapping and an electrooptic response.  These functionalities are given in a polymer with the addition of specific molecules or monomers which can be placed in three different positions: Incorporated on the polymer backbone, attached as a pendant side group, or simply doped into the polymer as Figure 2.2.3.1.  On the basis of this picture, one can classify the photorefractive polymers into two main groups, fully functionalized, where all the components are attached into the polymer backbone and composites, where low molecular weight dopants are present.

The processes of charge generation and transport are rather well studied in polymers, due to their application in xerography [Zhong, 2003].  The main requirements for the photorefractive effect are a high quantum yield of photogeneration and a high drift-mobility.  The most popular approach towards polymers with a high drift-mobility has been to disperse donor molecules referred to as charge transport molecules like aromatic hydrazones or amines into inert polymers like polycarbonate or polystyrene.  Hole drift-mobilities as high as 10-3 cm2/Vsec have been observed in these materials [Zhong, 2003].  Electron transport can be achieved through doping with acceptors like diphenoquinones, however, the drift mobilities that have been achieved are much lower.  Charge transport in these materials takes place via hopping among the dopant molecules.  The drift mobility increases exponentially with decreasing distance between hopping sites, thus as high as possible loading is preferred.  Phase separation, which (with a few exceptions) typically occurs at dopant concentrations in excess of 30% wt., is the main limiting factor.  Poly(N-vinylcarbazole) is one example where this problem has been tackled by linking the donor units (carbazoles) on an inert polymer backbone [Gill, 1976].  TPD1 is a second example, where very high concentrations have been achieved due to the inherent difficulty of this molecule to crystallize.  The drift mobilities in molecularly doped polymers with a few exceptions show a very strong electric field and temperature dependence, which has been a subject of intense experimental and theoretical interest [Reucroft, 1976].

Figure 2.2.3.1 : The required functionalities for the photorefractive effect are given by specific chemical units, which can be placed in three different positions: Incorporated on the polymer backbone, attached as a pendant side group, or simply doped into the polymer.

Molecularly doped polymers are usually photoconducting in the ultraviolet.  In order to extend their photoconductivity in the visible, sensitization is necessary.  This is achieved by the addition of small amounts of molecules which either absorb directly in the wavelength of interest, so called optical sensitization, or do not absorb themselves but form charge transfer complexes which do chemical sensitization [Reucroft, 1976]. The quantum yield of photogeneration in these materials has been shown to be highly electric field dependent, as the separation of the photogenerated electron-hole pairs has to compete with geminate recombination.  In most cases, this field dependence is explained within the framework of the Onsager theory of geminate recombination.  On the other hand, charge trapping in polymers is not well studied and it remains a vaguely understood process.  Intuitively, it is clear that hole-trapping should take place at sites with ionization potential lower than that of the hopping sites.  However, in almost all the photorefractive polymers that have been reported until today, 1-tetraphenylenediamine no attempt was made to deliberately introduce traps.  Trapping is assumed to take place at impurities accidentally present in the material [Reucroft, 1976], defects of the polymer backbone [Gill, 1976] etc.  Identification of the trapping sites is still a subject of investigation even in the case of well studied inorganic photorefractive crystals.  The electrooptic effect is a second order (c(2)) optical nonlinearity which requires the lack of inversion symmetry.  In polymers this is achieved by adding dipolar molecules with a large hyperpolarizability referred to as nonlinear optical (NLO) molecules and partially aligning them by poling close to the glass transition temperature (Tg) [Gill, 1976].

In the case where the Tg of the polymer is high enough, for example at 150 degrees, the orientational mobility of the nonlinear optical chromophores at room temperature is rather low and the non-centrosymmetry is maintained for long time.  Attaching the chromophores on the polymer backbone and/or crosslinking the polymer matrix further slows down the relaxation of the polar order [Gill, 1976].  On the other hand, if the Tg of the polymer is low enough, poling can be achieved at room temperature, but no orientation is maintained after the electric field is switched off.  In Figure 2.2.3.2 a schematic representation of the space charge field formation process in a photorefractive polymer is shown.  Indicated that are the highest occupied and the lowest unoccupied molecular orbitals or HOMO and LUMO respectively of the inert polymer backbone, the sensitizer called charge generating (CG) site and the nonlinear optical chromophore (EO), together with the HOMO's of the charge transport (CT) and trapping (TS) sites.  In this picture, photoionization of the charge transfer complex between the sensitizer and the charge transport species is depicted and a hole is transferred to a charge transport site from where it migrates until it gets trapped at a low ionization potential site.  Presented by this picture, the nonlinear optical chromophore does not contribute to the space charge field formation process.  Although it can act as a sensitizer at shorter wavelengths, it is preferable to have the functionalities of charge generation and electrooptic effect well separated.  Although a polymer which possessed all the necessary functionalities has been described earlier [Gill, 1976], the first unambiguous observation of the photorefractive effect in a polymer was demonstrated in 1991 [Chen et al, 1999], in an electrooptic epoxy doped with the charge transport molecule DEH2.  The diffraction efficiency was 2×10-5 and the typical response time several minutes.  Observation of net gain in a PVK based polymer doped with electrooptic molecules followed shortly after that.  Recently, a gain coefficient of more than 200 cm-1 and diffraction efficiency of almost 90% has been observed in a PVK based polymer [Hendrickx et al, 1999], setting photorefractive polymers at new 2, 4-(N,N- diethylamino) benzaldehyde diphenylhydrazone heights.  Today, several different structures have been synthesized that exhibit the photorefractive effect, indicating the versatility of polymeric materials.

Figure 2.2.3.2 : A schematic representation of the space charge field formation process in a photorefractive polymer. Indicated are the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO respectively) of the inert polymer backbone, the sensitizer (charge generating (CG) site) and the nonlinear optical chromophore (EO), together with the HOMO's of the charge transport (CT) and trapping (TS) sites. LUMO's of the latter are not shown for clarity. A hole which is created by photoionization of the charge transfer complex between the sensitizer and the charge transport species, migrates through hopping among charge transport species, until it gets immobilized at a cite of a trap.

2.3  Application in Photocopying Process

Photoconduction forms the basis of the electroreprographic industry.  On the photocopying process, a photoconductive material is coated onto a metal drum and uniformally charged in the dark by a corona discharge.  The drum is then exposed to the bright image of the item to be copied and the illuminated areas of the photoconductive material become conductive and dissipate their charge to the earthed metal drum.  The remaining photoconductor in the dark areas is still charged and able to attract a black positively charged resin coated toner powder forming a latent image.  The latent image is then transferred to a negatively charged piece of paper that is heated to fuse the resin, making the image permanent [Zhong, 2003].

Some applications are use of electronic devices, transistors, detectors, solar cell, batteries, photointegrated circuit [Bing t al, 2000] space applications, etc [Guo et al, 2000, Schamidt et al, 1995, Grote et al, 2002].

3. Conclusion and Discussion

Polymers having conducting properties are usually polyconjugated structures, which are insulators in the pure state but when treated with an oxidizing or a reducing agent can be converted into polymer salts with electrical conductivities comparable to metals [Yoshida et al, 1999].  In general there are 2 types of conducting mechanism [Zhong, 2003].  The p-type doping is a removal of electrons from the valence band by the oxidizing agent, leaving the polymer with a positive charge, while the n-type doping is a donation of an electron to the empty conduction band by a reducing agent.  The addition of a donor or an acceptor molecule to the polymer is called "doping ", while the reaction that takes place is actually a redox reaction.  The mechanisms proposed to explain the conduction are normally of band theory and delocalization.

Photoconductivity is defined as an increase of conductivity caused by irradiation.  On the other word, photoconductive polymers are insulating or poorly conductive in the darkness and more conductive when illuminated [Zhong, 2003].  The simplest process of photoconduction occurs when a photon is absorbed and an electron is promoted from the valence band to the conduction band.  This process generates an electron hole pair, augmenting the concentration of intrinsic carriers.  The photon energy must exceed the band gap and consequently there is a threshold wavelength-energy associated with the process.  Carriers may also be generated from photon absorption in the generation of excitons.  The excitons are themselves unable to transport charge but can produce an electron hole pair if two excitons collide.

The studies of photorefractivity in photoconducting polymers are used as a key to understand the electro-optic properties related to their applications.  Recently almost all applications of photoconducting polymers are widely used in electrooptic and electronic field.

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