Stabilization of the response time in photorefractive polymers

Abstract

The optical and photoconductive fatigue of fast photorefractive polymers have been studied in a family of C 60-sensitized polymer composites containing styrene-based chromophores with varying ionization potential. Changes in response time and in photoconductivity were studied for exposures up to 10 4 J/cm 2. Increasing the chromophore ionization potential beyond that of the polyvinylcarbazole host was found to stabilize the response time. Studies of the electric-field dependence of the steady-state diffraction efficiency in various samples confirm the role of C 60 anions as possible traps.

Photophysical and Redox NIR‐Sensitivity Enhancement in Photorefractive Polymer Composites

“…In this context, it is important that the EO chromophores do not constitute hole traps, as is the case here. While an improved Dn upon pre-illumination have been observed before (reference [22]), so far only a deceleration [22,26] of the response speed or a modest acceleration accompanied by a high loss in diffraction efficiency [27] upon preillumination has been reported by other groups.…”

Organic Photorefractive Materials and Applications

“…Upon preillumination, the actual trap-depth and density as well as the nature of the incorporated sensitizer affect the response of the material to the subsequent holographic recording. Herlocker et al [ 61 ] and Ostroverkhova et al [ 22 ] studied the infl uence of the chromophore oxidation potential with regard to the CTA. Both found a more pronounced infl uence of pre-illumination on the behavior of the materials with the lower chromophore I p .…”

“…Since sensitizer radical anions constitute recombination traps for holes, this effect is denoted as "optical trap activation", [ 209 ] which led to a signifi cant reduction of PR performance in Ref. [61] under prolonged exposure.…”

“…Also, the relative position of the polymer HOMO with respect to the ionization potential of the other components of the blends determines the extent of extrinsic hole traps in the material, which was found to limit the performance of the composites under prolonged optical exposure. [ 61 ] Thus, besides the mobility, the knowledge about the ionization potential ( I p ) of the polymer and the respective dopants is of fundamental importance.…”

“…These chromophores are identifi ed as compensator sites for growing concentrations of sensitizer radical anions upon prolonged illumination, which is generally accompanied by a detrimental drop in performance and an undesired illumination history of the materials. [ 98 ] Besides adjusting the ionization potential of the chromophore, [ 61 ] the generally lower oxidation potential of TPD-compounds in comparison to PVK are considered to evade this problem [ 72 , 75 , 76 ] (see Figure 6 ). Materials with charge transporters as compounds of lowest ionization potential have thus featured stable material performance under prolonged exposure, manifested through 160,000 write-erase cycles without degradation of performance.…”