Measuring electric field dependent photodegradation and recovery of disperse orange 11 dye doped polymer thin films using photoconductivity and digital imaging microscopy Benjamin Anderson, Sheng Ting Hung, and Mark G. Kuzyk. a Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814 ABSTRACT One of the limiting factors of optical devices for space applications is photodamage from high intensity light and radiation. Dye doped polymers offer many advantages in device design but are susceptible to photodamage, especially due to high intensity UV radiation. Several organic dyes have been observed to self heal after pho- todegradation. We seek to understand the underlying mechanism with the goal of designing materials that are more robust to photodegradation. We test the hypothesis that photodegradation is due to charge injection into the polymer and healing due to recombination using photoconductivity and imaging measurements of disperse orange 11(DO11) doped into PMMA. Keywords: Photocharge ejection and recombination, photo damage, self healing, organic dyes, photoconduc- tivity 1. INTRODUCTION Organic dyes and polymers find wide uses in many different fields including photolithography, dye lasers, fiber optics, organic light emitting diodes, optical data storage, etc. 1–5 One problem with using organic dye-doped polymers in optical devices is photodegradation. Much research has been performed to understand photodegra- dation in dye-doped polymers. 6, 7 Furthermore, over the past two decades, an exciting effect has been discovered in which a wide variety of organic-dye doped polymers show self healing after being photodegraded. 8–16 Currently the actual mechanism of self healing is unknown, but several mechanisms have been proposed. One of the proposed mechanisms of photo decay and self healing in dye-doped polymers is that of charge ejection and recombination. 17 The proposed mechanism assumes when a dye molecule is damaged by intense light it ejects a charge into the polymer; the charge is free to move and possibly get trapped. Self healing occurs when charges eventually migrate back and recombines with a damaged molecule, returning it to its original state. This is consistent with our observation that the organic dyes of interest degrade irreversibly in solution, but recover in a polymer matrix. 9, 10 Without the polymer and the trap sites, the photo ejected charge is free to leave and will not recombine with the damaged molecule. An obvious consequence of the model of charge ejection and recombination is that the recovery will be affected by an applied electric field. To test the charge ejection and recombination mechanism, we developed an experiment with two probes of decay and recovery. The first builds on our previous work with digital imaging microscopy 15, 16 by using the same apparatus, now applying an electric field while imaging; the second method is a more direct electrical measurement utilizing photoconductivity as a measure of decay and recovery if charged products are indeed formed. 2. PHOTOCONDUCTIVITY THEORY The process of photoconductivity in dye-doped polymers is a far more complicated matter than the usual exem- plar of semiconductors. Semiconductors have a periodic structure, which leads to a predictable band structure used to describe their conductivity and photoconductivity. 18, 19 Dye-doped polymers have varying degrees of conductivity and photoconductivity, but one cannot simply describe the processes using a simple periodic band structure. Although dye-doped polymers do have a band structure, due to their random nature the bands are Invited Paper Nanophotonics and Macrophotonics for Space Environments VI, edited by Edward W. Taylor, David A. Cardimona, Proc. of SPIE Vol. 8519, 85190H · © 2012 SPIE CCC code: 0277-786/12/$18 · doi: 10.1117/12.929776 Proc. of SPIE Vol. 8519 85190H-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 01/16/2013 Terms of Use: http://spiedl.org/terms