In this report, we demonstrate the effect of the dipole moment of the charge transport agent on carrier mobilities in photorefractive polymer systems. The charge carrier mobility measurements are presented as a function of applied field and temperature in several model systems. In these model systems, 30 percent by weight of a hole or an electron transport agent covering a range of dipole moments form 0.8 to 4 Debye, is doped into a polystyrene (PS) polymer matrix containing 25 percent by weight of the nonlinear optical chromophore 2,5-dimethyl-4-(p-nitrophenylazo)phenol having a dipole moment of 7.6 Debye. The results are described by the Gaussian disorder model based on hopping through a manifold of states with superimposed energetic and positional disorder, with the addition of a dipolar disorder contribution. We conclude from the results that the dipole moment of the charge transport agent has a strong influence on carrier mobility even in the presence of strongly polar nonlinear optical chromophores. Transport agents with low dipole moments significantly increase the carrier mobility and thus provides a useful alternative approach to improve photorefractive speeds.

Saiwai-ku, Kawasaki 210-8582, JAPAN The drift mobility and the diffusion coefficient of molecularly doped polymers (MDPs) are measured by fitting the theoretical equation to the observed time—of—flight photocurrent transient. The 70% mobility enhancement is observed when 0.2 wt.% fullerene C70 is added to the diphenylamino— benzaldehyde diphenyl hydrazone (DPH) doped polystyrene. We consider this is due to the interaction between C0 and DPH which form the charge transfer complex.

Keywords: molecularly doped polymer, diffusion, drift mobility, time—of—flight experiment.

The refractive index modulation in photorefractive polymers with a low glass transition temperature is dominated by orientational birefringence effects. To take advantage of these effects we developed several photorefractive polymers that contain: (i) chromophores designed to have simultaneously a large dipole moment and a high linear polarizability anisotropy, (ii) nematic phase liquid crystal droplets, (iii) transparent molecules generally used for liquid crystal applications. We discuss recent advances in these three different classes materials and emphasize their merits and trade-off.

Seven novel photorefractive glasses have been prepared in an effective four-step synthesis. The low molar mass glasses consist of a triphenylamine-core substituted with two carbazole moieties and a NLO-chromophore (NLO-DCTAs). In a second series of compounds (NLO-DATAs) the carbazole groups are replaced by diphenylamine. The compounds from stable glasses with T_gs of about 120 degrees C and 85 degrees C. The photorefractive properties such as the diffraction efficiency (eta) , the modulation amplitude of the refractive index (Delta) n, and the gain coefficient (Gamma) were determined by means of degenerate four-wave mixing and two- beam coupling experiments. We observed diffraction efficiencies of (eta) equals 90 cm^-1. The samples have excellent properties with respect to optical quality and shelf-lifetime.

Photorefractive (PR) figures-of-merit (FOMs) were determined for a series of donor-acceptor substituted methine dyes by electrooptical absorption measurements (EOAM) in dioxane solution. EOAM yield the transition dipole moment (mu) ag, the ground state dipole moment (mu) ag and the change of the dipole moment upon optical excitation (Delta) (mu) within the intense charge transfer (CT) band of the dyes. These results allow us to estimate the PR FOMs of the dyes by using a two- level model. The dye structures were based on the strong acceptor unit 1-butyl-4-methyl-2, 6-dioxo-1,2,5,6- tetrahydro-pyridine-3-carbonitrile which was combined with donor units of increasing strength. The neutrocyanine dyes obtained cover the whole range of possible CT behavior: dyes with weaker donors exhibit large positive dipole difference, dyes with stronger donors reach the cyanine limit, while the strongest donors yield dyes beyond the cyanine limit. Optimized PR FOMs were obtained near or somewhat beyond the cyanine limit. PR polymers based upon optimized structures exhibited unprecedented refractive index modulations and gain coefficients.

Photorefractive guest-host systems based on photoconducting polymers such as poly(N-vinylcabazol) or polysiloxane generally exhibit large diffraction efficiencies and photorefractive gain coefficients. Their response times however, are limited by the orientation of the nonlinear optical chromophores and by the photoconducting properties of the polymer. Rise times down to 50 ms have been observed until now.

Recently, donor-acceptor substituted methine chromophores with a large figure-of-merit for incorporation into photorefractive polymers with low glass-transition temperature have become available. However, their rather polar nature lead to aggregate formation and dipole cancellation, thus limiting their use in typical non-polar photoconducting matrices. By increasing the polarity of the matrix we were able to overcome this problem. The material we report shows unprecedented performance compared to previously known materials, is sufficiently resistant against phase separation, and operates in the NIR at typical wavelengths of commercially available high-power laser diodes.

A new and simple spectroscopic method for the determination of the trap density in C₆₀-sensitized photorefractive polymers is presented. This method is compared with results obtained from traditional trap density measurements which involve two-wave mixing, and good correlation is observed. This technique is used to identify the inactive traps as the nonlinear optical chromophore. Also, composites with very low response times, high gain coefficients and low absorption are presented.

Multiple grating phenomena have been observed in a number of photorefractive systems, and interaction between such gratings has been proposed as the basis for the demonstration of interesting optical effects such as velocity filtering and image deafferentation. We are investigation grating interactions in azo-dye doped photorefractive polymers with a view to assessing their suitability for use in such information processing applications. We report the result of wave-mixing investigations on two photorefractive polymer systems. The first system is a conventional guest-host photorefractive polymer based on the photoconducting host PVK:ECZ:TNF and the nonlinear chromophore trans-4-N,N-diethylamino-(E)- cinnamonitrile (DEACST). The results of degenerate four wave mixing and two beam coupling experiments in this composite are described well by the orientational enhancement model. The second system is a hybrid composite consisting of PVK:DEACST:ECZ:TNF and the well known azo-dye disperse red 1. In this system, gratings may be written via the photochemical mechanism of photoisomerization as well as the photorefractive effect. We suggest that the diffraction behavior of the hybrid composite involves a disruption of the light-induced azo-dye molecular orientation by the space charge field.

Two-beam coupling gain obtained from an orientational index grating was observed without applying an external field in a photorefractive polymeric material with ow glass transition temperature. A novel carbazole derivative with a photorefractive function was used as the material doped with a sensitizer for photocarrier generation. The mechanism to form the grating and to give the gain was investigated by two-beam coupling measurements.

Push-pull molecules are key components for doped low glass transition temperature photorefractive polymers. In this paper we report on the optimization of the factor of merit of photorefractive polymers doped by molecules belonging two different classes of push-pull molecules, on one hand molecules with a dominant neutral resonance form and, on the other hand, zwitterionic or charge separated molecules. The theoretical analysis is illustrated by experimental result obtained both on a molecular level and on polymeric materials of the interest of the analysis developed.

Polymer-stabilized liquid crystals, consisting of low concentrations of a polymeric electron acceptor, are shown to exhibit significantly enhanced photorefractive properties. The charge generation and transport properties of these composite systems are strongly modified from nematic liquid crystals doped with electron donors and acceptors. The new composites are produced by polymerizing a small quantity of a 1,4:5,8-naphthalenediimide electron acceptor functionalized with an acrylate group in an aligned nematic liquid crystal. Photopolymerization creates an anisotropic gel-like medium in which the liquid crystal is free to reorient in the presence of a space charge field, while maintaining charge trapping sites in the polymerized regions of the material. The presence of these trapping sites results in the observation of longer lived, higher resolution holographic gratings in the polymer-stabilized liquid crystals than observed in nematic liquid crystals alone. These gratings display Bragg regime diffraction. Asymmetric beam coupling, photo-conductivity, and four-wave mixing experiments are performed to characterize the photophysics of these novel materials.

We have studied the photorefractive properties of a composite consisting of the polymer PVK doped with 4,4'- pentylcyanobiphenyl and C₆₀. The material is homogeneous, stable, and does not require a plasticizer agent. Net gain coefficients over 100 cm^-1 are observed. Measurements of orientational and electro-optic responses indicate that the orientational response is large, but markedly inhibited. We also report dramatic gain enhancement in two-beam coupling and novel angle and electric field dependencies in this composite.

The time-of-flight electron mobility measurements were carried out for studying the influence of applied electric field and temperature on the transport of charge carriers in ferroelectric vinylidene fluoride-trifluoroethylene copolymers, doped with ortho-dinitrobenzene. The value of electron mobility (mu) equals 3 by 10^-10 m²/(Vs) at E equals 1.4 by 10⁸ V/m and T equals 20 degrees C, and was independent of temperature from 20 degrees C to 110 degrees C.

The conducting polymer poly(3n-decylprrole) (P3DP) has a promising chemical stability and processability. The long alkylic chains makes P3DP soluble in common organic solvents even if they partially hinder a regular molecular arrangement. On account of structural disorder, the materials has a moderate conductivity despite the high doping level. The d.c. conductivity and the dielectric response of P3DP films with different dopants and synthesis conditions were measured at temperatures between 80 and 300 K. The d.c. conductivity of all the samples was well described by the variable range hopping model; the hopping parameters were found to be much affected by the synthesis and doping conditions. The electrical response exhibited a well-defined relaxation peak, visible only after deducting the d.c. conductivity contribution from the loss factor. The temperature behavior of the loss peak frequency parallelled that of the d.c. conductivity. The Barton-Nakajima-Namikawa equation, relating d.c. conductivity, relaxation time and relaxation strength, was verified. The relaxation strength, too large for being connected with a dipolar relaxation, was ascribed to the displacement of hoping charges. The conclusion of the analysis has been that the electrical response of the system was dominated by the hopping charge transport.

Hole trapping has been investigated in a series of donor doped polymers containing different concentrations of traps with different depths. The mobilities decrease with increasing trap concentration and trap depth while the general features of the field and temperature dependencies remain unchanged. The results are discussed within the framework of the recent simulations of Wolf et al. and Borsenberger et al. and an early model due to Hoesterey and Letson. The results are in agreement with the simulations of Wolf et al. and Borsenberger et al. and confirm the argument that for shallow trapping the basic phenomenology of transport, as revealed by the field and temperature dependencies of the mobility, remain unchanged. Quantitatively, the effects of traps can be accounted for by the replacement of the energy width of the hopping site manifold (sigma) by an effective width (sigma) _eff. However, the use of the Hoesterey-Letson formalism leads to some discrepancies with respect to different trap depths.

The reason for the Poole-Frenkel (PF) mobility field dependence in dependence in disordered organic materials is believed to be a long range spatial correlation in the distribution of enough levels of transport sites. This correlation is produced by molecules with significant permanent dipole moments. However, experimental data offer a strong evidence that in transport layers containing no molecules with high dipole moment essentially the same PF dependence of transport parameters on the mean intersite separation. By means of computer simulation and analytic calculation we consider the influence of deep traps on the charge carrier mobility in organic materials.

Hole mobilities have been measured in molecularly doped polymers and polystyrene (PS). Both polymers were doped with a series of 1,1-bis(4-(N-styryl- phenyl)amino)phenyl)cyclohexane (BTAS) derivatives that have different dipole moments. The measured results are described within the framework of a formalism based on disorder in which it is assumed that charge transport occurs by hopping through a manifold of hopping sites subject to energetic and positional disorder. For each polymer doped with BTAS derivatives, the energy width of its hopping site manifold increased with increasing dipole moments of BTAS derivatives. The BTAS doped PC, which is a highly polar polymer, has a larger energy width than does the BTAS doped PS, and its energy width is also less dependent on the dipole moment. We measured the energy width dependence on BTAS concentration, and found that the energy width decreased with increasing BTAS concentration in the PC systems, but increased in the PS systems. These dependencies may have been attributed to the dipolar fields associated with the dipole moments of not only the BTAS derivatives but also the respective polymers. To explain these dependencies, we proposed a new model which dealt with the effects of the dipole moments of PC on energy width. By using this model, we estimated the dipole moment of a monomer unit of PC to be approximately 2.1 D, which corresponds roughly to that of the carbonyl group within a monomer unit.

High field charge transport in molecularly-doped polymers and amorphous molecular glasses occurs through the hopping motion of photoexcited carriers among localized molecular transport sites characterized by considerable energetic and spatial disorder. A large number of numerical simulations of this process have been performed using models in which transport sties are located on the sties of a regular lattice, with spatial disorder included in a phenomenological fashion. We present calculations that explore drawbacks of this approach, and present arguments to show that the magnitude of the energetic disorder assumed. In particular, we show using a simple model of randomly- placed transport sites with no underlying lattice that such analyses have the potential for attributing to the energetic disorder an artificial dependence on dopant density similar to that reported in the experimental literature.

Photoinduced electron transfer (ET) and geminate hole- electron pair dissociation are the most essential processes for carrier photogeneration in organic materials In azo- compound-based layered photoreceptors, carrier photogeneration is sensitized by hole transport material (HTM) incorporation. We have investigated these two processes to elucidate the highly sensitizing mechanism. First, the photoinduced ET efficiency and overall quantum efficiency were measured and compared for the layered photoreceptor and carrier generation layer. The result that the HTM enhances the photoinduced ET implies that the HTM works, catalytically to diminish the activation energy. Although such extrinsic ET takes place independently of an electric field, the subsequent geminate pair dissociation depends on an electric field. Next, the energy gap dependence of the ET was investigated employing more than 50 photoreceptors to vary the energy gap over a wide range. The efficiency measured was plotted against the energy gap, in which the inverted region was not observed. This result was explained by the small change in ET activation energy even when the energy gap is large, which is a feature of charge separation. The Marcus inverted region reported for titanyl- phthalocyanine-based photoreceptors was compared with the azo-based system. Finally, the dissociation process of geminate pairs on azo and HTM was investigated and compared employing five azo compounds. The result that the dissociation efficiency strongly depends on the azo compound revealed that the reaction consists of azo + azo yields azo + azo. The dissociation efficiency exhibited da strong relationship with the polarizability of photoexcited azo compound. It is concluded that the strong polarizability azo compound enhances geminate pair dissociation.

Two-layer organic photoreceptors consisting of a dye sensitizer acting as the charge generation layer (CGL) and a hole transport layer (HTL) have been investigated by steady- state photoconduction measurements and time-of-flight experiments. With these techniques effects of exciton diffusion, charge-carrier generation, charge injection and charge transport can be studied. The measurements were carried out with (i) single generation layers consisting of azo-pigment dispersion and titanylphthalocyanine pigment dispersion and (ii) with two-layered photoreceptors using azo and TiOPc as charge generation layer and TDAPB and hexa(hexylthio)triphenylene (HHTT) as charge transport materials. We found that in the Phthalocyanine/HTL-system the efficiency for charge injection into the HTL is transport-limited due to the hole mobility of the Titanylphthalocyanine dispersion, whereas the efficiency in the azo/HTL receptor is limited by the diffusion of excitons and (or) holes towards the CGL/HTL-interface. We draw to the conclusion that illumination of the Phthalocyanine sensitizer leads to the formation of charge-transfer excitons which subsequently dissociate into free charges. In contrast to this, the azo-pigment as a sensitizer seems to form strongly bound excitons. Hence no dissociation in free charge-carriers occurs except on the CGL/HTL-interface where the hole transport molecules act as electron donors. From steady-state photocurrent measurements we calculate diffusion lengths belonging to the diffusion of excitons and monitory charge-carriers respectively.

We report free carrier photogeneration yields for an electron-donor, an electron-acceptor, and an electron-donor- acceptor molecular glasses. The donor molecule is 1,1-bis- (tri-p-tolylamine) cyclohexane (TAPC), the acceptor N,N'- bis(1,2-dimethylpropyl)-1,4,5,8-naphthalenetetracarboxylic diimide (NTDI), and the bifunctional molecule N-(p-(di-p- totylamino)phenyl)-N'-(1,2-dimethylpropyl)-1,4,5,8- naphthalenetetracaboxylic diimide (TAND). The functional moiety of TAND is derived from the donor and acceptor molecules. The photogeneration yields are obtained by xerographic photoinduced discharge with a positive bias or a negative bias on the free surface at room temperature. The primary quantum yields and thermalization distances derived from fitting the experimental data to a model due to Onsager are similar for these amorphous materials, suggesting that there is no apparent correlation between the photogeneration of free carriers and their morphology. Action spectra of the free carrier photogeneration of TAND are obtained at a constant field. The spectral range encompasses the localized and charge-transfer-type electronic state transitions. The action spectra show that the free carrier photogeneration is predominately due to the absorption derived from the charge transfer transition and the acceptor moiety, with the latter having higher yields by about a factor of two. An abrupt decrease of the yield in the donor moiety absorption region is also observed. We attribute the drastic change in the spectral response of the photogeneration yield to hole trapping and recombination.

Computational chemistry is playing an increasingly important role in the study of molecular materials in general and Charge Generation Materials in particular. In this paper the basic theory behind lattice energy calculations is described along with the application of such procedures to crystal structure refinement in the case of oxotitanium phthalocyanines. The application of various computational tools to aid in structure solution from high resolution x- ray powder diffraction data is demonstrated for the x-form of metal free phthalocyanine. The potential for ab-initio prediction of crystal structures and potential polymorphs prior to synthesis and development is highlighted.

Sensitization of various structured polyimides in the wavelength range 400-750 nm has been studied. Injection sensitization with thin films of nanocrystal CdSe, structural-chemical sensitization with active acceptors, as well as spectral sensitization with various dyes have been investigated. It has been concluded that the application of polyimides as a photosensitive medium allows characteristics of numerous devices to be improved.

A new fully functionalized photorefractive polyimine was prepared by the condensation polymerization between a photoconducting carbazole derivative, 9-(2- ethylhexyl)carbazole-3,6-dicarbaldehyde, and a nonlinear optical stilbene chromophore, 4-[N,N-bis(2- aminoethyl)amino]-4'-nitrostilbene. It showed excellent solubility in common organic solvents such as chloroform, cyclohexanone, tetrahydrofuran, etc. and high optical quality films were obtained by free standing film casting. Very high second order optical nonlinearity with d₃₃ equals 120 pm/V was observed by second harmonic generation at the fundamental 1064 nm wavelength. In demonstrated good thermal stability of the aligned dipoles by electric field up to ca. 125 degrees C. The diffraction grating was formed by the interference of two coherent Ar-ion laser beam at the wavelength of 488 nm. A holographic diffraction efficiency of about 15 percent has been achieve din a 10 micrometers -thick film. Storage state of our film shows remarkably long stability at room temperature.

The dual nature of Lemke-e allow us to write both permanent and erasable holographic gratings in the same storage volume. In the presence of a triplet excited sensitizer, Lemke-e undergoes a photochemical reaction allowing the storage of permanent photochromic holographic gratings. At wavelengths longer than the triplet state of the sensitizer, the composite behaves photorefractively, allowing the storage of erasable holograms in the same location as previously stored permanent photochromic holograms. Photochromic gratings can be written in less than one minute while photorefractive gratings can be written in less than one second. The photochromic gratings have a dark lifetime of several days. Storage times, write times, read cycles and multiplexing results are also presented.

Polymers in which photoconducting as well as nonlinear- optical moieties are attached to the polymer backbone have significant advantages over common guest-host photorefractive polymer systems. The main reasons for this are that such fully-functionalized polymers can be permanently poled and do not show chromophore recrystallization.