Over the last two decades, experimental investigations of the mobility of photo-injected charges in a wide variety of molecularly doped, pendant-, and main-chain polymers, and vapor deposited molecular glasses have shown that the mobility at high electric fields is universally described by the Poole-Frenkel law, micrometers equals exp(- (Delta) /kT) exp((gamma) (root)E). With few exceptions, the activation energy (Delta) is about 0.5eV, and the Poole-Frenkel factor (gamma) is a function of temperature which follows the empirical relation (gamma) equals B(1/kT - 1/kT₀), where B approximately equals 4 X 10^-4(e²Vcm)^1/2. The remarkable similarity among experiments suggests a mechanism for these dependencies which is largely based on a feature these materials have in common--lack of a crystal structure. Each material consists of a highly disordered array of active molecules, which is believed in turn to give rise to large energetic fluctuations along the conduction pathways. Computer simulations of hopping transport on a disordered lattice have shown behavior in agreement with many aspects of experiment, lending support to the concept of a disorder-based mechanism. To increase our understanding of the high-field effects of disorder, we have developed a self-consistent algorithm with which the field dependence of the mobility in disordered systems may be calculated analytically. We examine the field dependence within this framework, and suggest that the (root)E- dependence may be understood when the theory of Scher and Montroll is applied to a length scale which the hopping matrix may be considered to be of a lower dimensionality.

Photoexcited charge conduction in molecularly-doped polymers can be described in terms of a biased random walk among a set of energetically and spatially disordered molecular centers. We describe a numerical renormalization procedure for studying hopping transport in disordered systems of this type. In this procedure, transport properties of cubic regions drawn randomly from the sample are calculated exactly using equation of motion methods. The original system is then replaced with a new one, defined on a larger length scale, but possessing the same macroscopic transport properties as the original. Fluctuations of the system are smaller at the new length scale, allowing for an accurate calculation of the conductivity using effective medium theory. The approach has been used to study the crossover between nearest-neighbor and variable-range hopping, where it has revealed that the exponential dependence of mobility of the mean interparticle spacing is substantially influenced by the magnitude of the energetic disorder in the system.

Hole mobilities in p-diethylaminobenzaldehyde diphenylhydrazone (DEH) doped polycarbonate films are determined using the time-of-flight transient photocurrent technique. Measurements of hole transport parameters are determined over a range of electric fields before and after the samples are deliberately irradiated with UV light. UV irradiation of the hole transport molecule DEH results in the creation of a photoproduct, 1-phenyl-3-(4- diethylamino-1-phenyl)-1, 3-indazole with moderately high efficiency. Once formed, this photoproduct has been shown to act as a barrier to hole conduction. We exploit this photochemical reaction to examine the hole transport properties in a molecularly doped polymer system containing DEH doped polycarbonate. We propose that the increase in concentration of the photoproduct modifies the intrinsic order of the system and provides a unique probe to distinguish between the disorder formalism of Baessler and coworkers and models which propose polaron formation.

A transport equation has been fitted to time-of-flight transient photocurrent signals of molecularly doped polymers in order to obtain both diffusion coefficient (D) and drift mobility ((mu) ) simultaneously. The logarithm of (mu) increased linearly with the square root of the electric field. The negative field dependence on the square root of the electric field at a low electric field that had been reported appeared to superimposing of drift and diffusion. The logarithm of D increased linearly with the square root of the electric field. The logarithm of D decreased linearly with T^-2.

We apply the techniques of energy band theory to the analysis of polaron structure in ordered materials. Our interest is in the fact that the local structure of polarons involves information drawn from a large number of delocalized eigenstates. Polaron Wannier states are proposed as localized functions in which polaron structure is manifest. Examples of polaron Wannier states are constructed explicitly and their basic properties explained.

Photogeneration efficiencies have been measured in dual layer aggregate photoreceptors over a wide range of fields and wavelengths. The results are described by a surface-enhanced exciton dissociation model. The model is based on a theory of geminate recombination, originally due to Onsager. The Onsager theory is based on the assumption that the absorption of a photon creates a bound electron-hole pair which then either dissociates into a free electron and a free hole, or undergoes recombination. The key parameters of the theory are the fraction of absorbed photons that create bound pairs, and the electron-hole separation distance of the bound pair. For aggragate materials, the yield of creating the bound pairs is 0.60. The separation distances are between 20 and 60 angstrom. The high photogeneration efficiencies in these materials are attributed to both the high yield of creating the bound pairs and the low electron-hole recombination probability because of the large pair separation distances.

Electron mobilities have been measured in a series of acceptor doped polymers over a range of fields and temperatures. The results are described within the framework of a formalism based on disorder, due to Baessler and coworkers. The formalism is premised on the assumption that charge propagation occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. A key parameter of the formalism is the energy width of the hopping site manifold. The width is in good agreement with predictions of a model based on dipolar disorder recently proposed by Young. The model assumes that the width is determined by a dipolar component due to the dopant molecule and a van der Waals component. These results, in conjunction with literature results for a wide range of donor doped polymers, show that for a given dopant concentration, the van der Waals component is essentially constant and independent of the donor or acceptor molecule. It is suggested that the van der Waals component is largely of geometrical or positional origin.

Two experimental studies of new photorefractive polymer materials are described. First, an examination of the fabrication, stability, and photorefractive behavior of a recently-reported high-efficiency polymeric composite 33%PVK:50%DMNPAA:16%ECZ:1%TNF (K. Merrholz et at., Nature 371, 497) shows that this material is metastable, crystallizing on time scales from hours to days depending upon preparation. The grating phase shift also varies considerably from sample to sample. In the second part of this paper, first measurements and analysis of the properties of a new photorefractive polymer composite which contains a liquid nonlinear optical chromophore are presented. This new composite, which is immune to crystallization even at high chromophore loading levels, shows a useful diffraction efficiency of several percent at high fields, but the index of refraction grating is shown to consist of two components; a smaller photorefractive index grating and a grating which is not photorefractive in origin as shown by grating translation measurements.

Modification of the trap density of the photorefractive polymer composite poly(N-vinyl carbazole), 2,4,7-trinitro-9-fluorenone, and N,N-diethyl-para-nitrozniline is achieved with the addition of 4-(diethylamino)bensaldehyde diphenylhydrazone (DEH). Measurements of the response time and the phase shift of the photorefractive gratings indicate that at low concentrations DEH acts as a trap, while at higher concentrations a new charge transport pathway through hopping between DEH molecules is established. The transient behavior of these gratings further supports this picture.

We have observed the electron paramagnetic resonance (EPR) signal in the photorefractive polymer BisA-NAS:DEH. The polymer is a mixture of the electro-optic polymer bisphernol A 4,4-nitroaminostilbene (BisA-NAS) with 29% by weight benzaldehyde diphenyl-hydrazone (DEH), a hole transport agent. The EPR signal was observed at 3431.590 G. Illumination of the photorefractive polymer by an Ar⁺ laser leads to an interesting phenomenon. With an aim to understand the photosensitivity of the photorefractive polymer, we have monitored the changes in the intensity of the EPR line. The measurements were done in situ at room temperature and liquid nitrogen temperature 77 K. The sample was loaded in an optical transmission cavity to facilitate the use of the laser. When the sample is illuminating with low laser intensity, illumination decreases the EPR signal while for higher laser intensity, the EPR signal grows. We provide a tentative explanation for this phenomenon. Low laser intensity introduces photoconductivity, which reduces the EPR signal, but does not create free radicals. For higher laser intensity, paramagnetic free radicals are formed contributing to the increase of the EPR signal. The relation between observed phenomenon and photosensitivity will be discussed. Also the EPR results at low temperature will be presented.

The photorefractive effect is a reversible mechanism of holographic grating formation in electro-optic materials, that has potential applications in integrated optics, optical data storage, optical computing, and several other areas. This effect was recently observed in photorefractive polymers doped with charge transport agents. The grating formation is initialized by the photoconductive response which includes charge generation, mobility, and trapping, occurring in successive order. Finally, the grating is formed as a result of modulation of the refractive index by the resulting space charge field, via the electro-optic (Pockels) effect. We present the results of photoconductive measurements as a function of temperature, applied electric field, and illuminated intensity. The investigation is focused on the nonlinear optical polymer bisphenol A 4-4-nitroaminostilbene mixed with 30 weight % of the hole transport agent diethylamino-benzaldehyde diphenyl hydrazone. We observed that the photoconductivity of the photorefractive polymer has the form exp[ aE^1/2 + bE^1/2/T² - c/T²] in agreement with the diorder theory of the well- known hopping model developed for charge-transport in molecularly doped polymers. We also observed that the apparent hopping distribution bandwidth increases with increased intensity, consistent with an increasing density of states.

Photorefractive materials have many potential photonic applications, including dynamic holographic storage and image processing. Recently, the new class of photorefractive polymers has emerged, offering wide structural flexibility, easy processability, and low cost. Progress in this field has led to nearly 100% diffraction efficiency for the readout of a hologram stored in the material and to extremely large net gain coefficients of more than 200 cm^-1. These excellent properties result from refractive index modulations as large as (Delta) n approximately equals 10^-2 which could be generated in these materials with low- power lasers, such as HeNe laser or laser diodes.

Several novel photorefractive polymers based on the charge transfer properties in photosynthetic model compounds were synthesized. These polymers contain a prophyrin moiety, a second order nonlinear optical chromophore covalently linked to a polyimide backbone. High thermal stability of the second order nonlinear effect were observed at elevated temperatures. An asymmetric optical energy exchange was detected in two beam coupling experiments, indicating the photorefractive nature of the polymers. Large optical gains were deduced at zero-external electric filed (20 cm^-1).

We report on the synthesis of two novel photorefractive polymers: one host-guest material based on the well known charge transport molecule TPD and one fully functionalized polymer with extended transparency over the visible. Preliminary results of photorefractive measurements are reported.

The band transport model of space-charge production has proved very successful in describing photorefractive phenomena in a wide range of inorganic crystals, ranging from high-mobility semiconductors like GaAs, to highly insulating ferroelectrics like BaTiO₃. This success is primarily due to the applicability of a generic picture of charge transport in extrinsic crystalline semiconductors, a picture that becomes cloudy in noncrystalline systems like the new photorefractive polymers. Photorefractive polymers exhibit the classic photorefractive behaviors such as photoconduction, electro-optic response, hologram formation and storage, and two-beam energy coupling, all without benefit of a crystalline lattice or well defined conduction band. In this report, I will outline how the band transport model can be adapted to describe photorefraction in noncrystalline materials by the dual expediencies of renaming certain embarrassing 'constants' and admitting that they are strongly dependent on the total electric field.

The efficiency of carrier generation by the zerographic discharge technique has been measured on four films; a molecularly C₆₀-doped polymer film, a molecularly C₇₀-doped polymer film, an aggregate C₆₀ film, and an aggregate C₇₀ film. The yield for molecularly C₇₀-doped polymer was three orders of magnitude larger than that for the film of molecularly C₆₀-doped polymer. The photon energy dependence of efficiency for the aggregate films differed from that for the molecularly doped polymer films. The carrier generation efficiency spectrum for the aggregate films showed peaks corresponding to the Frenkel type excitons. The sensitivity of a photoreceptor fabricated with C₇₀ was 0.5(cm²/(mu) J) at wavelengths in the 420nm-670nm region.

Aspects of dynamic disorder in charge transport in polymers are investigated. Basic issues such as polaron formation, the validity of semiclassical arguments and the memory function method of mobility calculation are studied.