Molecule and Charge Carrier Diffusion in Optical and Electro-Optical Devices
- PDF / 400,986 Bytes
- 7 Pages / 420.48 x 639 pts Page_size
- 52 Downloads / 154 Views
89 Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
by photoinduced separated electric charges. There is usually a global field applied as well, but this field acts to drive the photoinduced charges in a particular direction and maintain the net molecular orientation, and is not intended to modulate the overall refractive index. Thus, the degree to which charges are induced to migrate following photoexcitation, and the depth at which they are trapped, are major influences on the signal strength (e.g. diffraction efficiency and net gain) and lifetime of photorefractives. This is in marked contrast to the ideal electro-optic modulating material, where charge migration mostly contributes deleterious effects such as dielectric loss and chemical degradation. A schematic photorefractive material is shown in Figure 1. Irradiation and application of an electric field causes dipole orientation and generation of charged species from the photoreceptors.
Figure 1. Schematic of photorefractive polymer. Arrows are dipolar chromophores, small stars are photoactive sites, and diamonds and circles are charge centers. In order to maximize poling efficiency and charge migration, it has become desirable to employ relatively soft materials as photorefractive composites, and large enhancements in photorefractive effects have been obtained.[6] Careful examination of the time scales and field dependencies of these effects has identified a second mode of activity in these materials that arises from the increased molecular motions available relative to glassy electro-optic materials. In addition to the polarization of the electrons within the conjugated system of one molecule, the locally produced electric field is capable of additional poling, above and beyond that which is accomplished by the global field. This added orientation enhances the refractive index contrast by increasing the local polarity of the molecules on which the photogenerated field acts. There is yet another contribution simply from the dielectric anisotropy of the molecules, just as in ordinary
90
liquid crystal devices, where simply changing the average pi-system plane changes the molecular contribution to the refractive index even in the absence of chromophore polarization. While attention has been focussed on molecular orientation and electro-optic effects, there has been relatively little design effort devoted to the charge migration aspect of these materials. The rising importance of organic-based light-emitting diodes and fieldeffect transistors has led to greatly increased understanding of intermolecular charge injection and mobility.[7] Certain classes of chromophores have been identified as primarily electron or hole transporting, mobilities can range from I cm2 /Vs to many orders of magnitude less, and the propensity to form traps varies widely.[8] While the highest mobilities measured in nonphotoexcited organic materials have been on crystalline compounds in field-effect transistor geometries, significant charge migration occurs in
Data Loading...
