POLYMERIC PHOTOCONDUCTORS

A short review on photoconductivity of polymers is given. Whereas the intrinsic photocarrier generation in polyacetylene involves the direct band-to-band transitions, in other materials the exciton autoionization process dominates. Then, the charge carrier photogeneration can be depicted as a multistage process that involves photoexcitation to a neutral molecular electronic state, autoionization of the excited state resulting in the creation of a localized hole and a hot quasi-free electron; thermalization of the hot electron leading to the formation of a Coulomb field-bound geminate electron-bole pair to free carriers. An important feature of polymers is the ability to form long-lived electron-hole pairs through the mediation of soliton or polaron pairs, charge carrier trapping, coulombic glass formation, charge self-trapping and dimer or excimer formation. These processes reduce the efficiency of the geminate recombination and increase the photogeneration yield of the steady state photoconductivity.