Photoconduction and photorefraction in molecularly doped polymers

During the past 20 years organic photoconductors have found increasing application in electrophotographic copying and printing. Charge carrier mobility in these materials is achieved by doping a polymer matrix with up to 50% by weight of a molecular donor (acceptor) which provides sites for the hopping of holes (electrons). Sensitization to the desired wavelength of light is obtained via a suitable charge generation dye or pigment, frequently in the form of a second layer distinct from the transport layer. More recently, optically nonlinear polymers have been synthesized by the incorporation of conjugated donor-acceptor species into macromolecular chains and/or networks. When the donor-acceptor chromophores are aligned, for example, by poling in an electric field, second-order optical nonlinearity, including an electro-optic effect, is observed. Photoconductivity and nonlinearity can be combined for example, by doping a nonlinear polymer with a molecular charge transport agent, to yield a potentially photorefractive material. The photorefractive effect in several such formulations has been confirmed and characterized by a combination of absorption, photoconductivity, mobility, electro-optic, four-wave mixing (holography) and grating phase measurements. The results are discussed in terms of a mechanism involving (i) charge generation on the nonlinear chromophore, (ii) transport of holes by hoping among molecular donor sites, (iii) charge trapping to develop a space-charge electric field and (iv) modulation of the refractive index via the electro-optic effect. © 1993.