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Charge transport in photorefractive polymers
Abstract
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.
Subject Area
Condensation|Optics|Polymers|Electrical engineering
Recommended Citation
Goonesekera, Arosha Wimala, "Charge transport in photorefractive polymers" (1998). ETD collection for University of Nebraska-Lincoln. AAI9908471.
https://digitalcommons.unl.edu/dissertations/AAI9908471