Photorefractive polymers with low intrinsic trap density

Two novel photorefractive polymers are presented, based on the charge transport molecule N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine (TPD). In one polymer the TPD unit is chemically modified so that it can function both as charge transport and as electro-optic molecule. In the other polymer the TPD is incorporated into the polymeric backbone and provides the charge transport and functions as a host for the dispersed electro-optic molecules. In both types of polymers the trap density is very low, which causing a 90 degrees phase shift between the refractive index grating and the illumination pattern and a rather small photorefractive performance. The trap density can in the case of the bifunctional molecule based polymer be increased by adding small amounts of N,N,N',N'-tetramethyl-paraphenylene diamine (TMPD) which has a lower ionization energy than the bifunctional molecule. This results in a strong increase of photorefractive performance and simultaneously in a lowering of the phase-shift. The ionization energy of some molecules used in photorefractive polymers is determined using gas-phase ultraviolet photoelectron spectroscopy and cyclovoltammetry. Using these ionization energies the process of the space-charge field formation can be understood on the basis of the components of the photorefractive polymer