Charge transport in photorefractive polymers

This dissertation describes the experimental investigation of photoconductivity and charge carrier mobility in photorefractive polymers. The photorefractive polymer composite containing an electro-optic polymer, bisphenol A 4,4\sp\prime-nitrostilbene (bisA-NAS), and 30% weight of a hole transport agent, benzaldehyde-diphenyl hydrazone (DEH) was used extensively for these studies. The results show how to improve the response times in photorefractive polymers. We measured the photoconductivity and photorefractive response in the photorefractive composite as a function of applied electric field, temperature, light intensity and wavelength. The results show that photoconductivity is strongly dependent on temperature and applied electric field. The intensity-dependent photoconductivity measurements indicate a transition from unfilled to filled trapping states, giving an estimate of the shallow trap density. Two-beam coupling experiments (2BC), which measure the steady state photorefractive grating strength, and degenerate four-wave mixing experiments, which determine the transient photorefractive properties, were performed to characterize the photorefractive properties of the polymer composite. We measured the hole mobilities in the photorefractive polymer composite bisA-NAS:DEH as a function of temperature and applied electric field, using a conventional time-of-flight (TOF) technique. The results are described by the Gaussian disorder model (GDM) based on hopping through a manifold of states with superimposed energetic and positional disorder. The results reveal that the hole mobility is very low and is strongly dependent on temperature and applied electric field. Using several model systems, we investigated the effect of the polar additives on the carrier mobility in photorefractive polymers. Two different electro-optic chromophores having large dipole moments of order 7 Debye, two polymer binders with repeat unit dipole moments of 0.1 and 1 Debye, and several hole and electron transport agents covering a range of dipole moments from 0.8 to 4 Debye were used for this study. The results revealed that the strong dipolar chromophores required in photorefractive polymers significantly decrease the carrier mobility but low dipolar transport agents improve the carrier mobility and hence the speed of response

Effect of dipolar molecules on carrier mobilities in photorefractive polymers

The grating formation speed in photorefractive polymers is greatly reduced by highly polar molecules incorporated by necessity in large concentrations to produce large diffraction efficiency and two-beam energy coupling gain. The random electric fields generated by these dipoles interfere with charge transport by increasing the width of the hopping site energy distribution and thus greatly reducing the carrier mobility and the photorefractive speed. We conducted transport studies of several model systems consisting of combinations of two polymer binders, six charge transport agents (four for holes and two for electrons), and varying concentrations of two highly polar electro-optic chromophores. The results confirm that carrier mobility is greatly reduced in the presence of polar molecules in accordance with the predictions of models of hopping transport in the presence of dipolar disorder. The randomly positioned and oriented dipoles increase the width of the hopping site energy distribution by an amount proportional to the square root of the dipole concentration and to the strength of the dipole moment. The results also show that transport agents with smaller dipole moments reduce the sensitivity to the dipolar effect. The photorefractive speed may therefore be increased by using transport agents with small dipole moments

Low-field hole mobility in a photorefractive polymer

We present the time-of-flight measurements of hole mobility in a photorefractive polymer composite as a function of temperature and applied electric field. The analysis shows that the temperature dependence of the low-field mobility is in apparent disagreement with the predictions of the Gaussian disorder model and also with polaron models

Effect of beam attenuation on photorefractive grating erasure

We investigate the influence of attenuation on the speed of erasure of photorefractive gratings by solving the coupled-wave equations in the undepleted pump approximation and by taking into account the attenuation and Gaussian intensity profile of all the beams. The extrinsic grating decay rate is significantly lower than the intrinsic photorefractive decay rate in samples with overall attenuation as low as 10%. The Gaussian beam profiles of the readout and the erasing beams result in a further reduction of the extrinsic decay rate. The results of these calculations are used to determine the spectrum of intrinsic decay rates in a photorefractive polymer

Photoconductivity and grating response time of a photorefractive polymer

We report the photoconductive properties and photorefractive grating response time of a polymer mixture composed of 40-wt. % dissolved diethylamino-benzaldehyde diphenyl hydrazone (DEH) and the non-cross-linking epoxy polymer Bisphenol A 4,4\u27-nitroaminostilbene. The films have improved photoconductive sensitivities as high as 2.1 × 10–10 cm/(W Ω) at a wavelength of 650 nm with a corresponding reduction of the grating response time constant to 0.11 ± 0.02 s at an intensity of 1 W/cm2. The nitro-aminostilbene chromophore is deduced to be the source of photogenerated charge carriers on the basis of a comparison of the wavelength dependence of the photoconductivity and absorption coefficient. Degradation of the photoconductivity and the dark conductivity as well as of the photorefractive speed with sample age is attributed to precipitation of the DEH; this explanation is supported by x-ray diffraction observations of crystal growth in the polymer