Photoinduced Current in Polyvinylcarbazole

Measurements of the electrical current associated with the return to thermal equilibrium after photoexcitation of polyvinylcarbazole (PVK), a photoconducting organic polymer, are presented and discussed. Thin sandwich layers of PVK were excited with weakly absorbed light with the electrodes shorted. After removal of the light, a voltage was applied and the current measured. The current decayed as ${\mathrm{}\left(\mathrm{time}\right)\mathrm{}}^{-1\mathrm{}}$ and was proportional to the electric field and dependent upon the light exposure. Discussion is presented to support the following conclusions: (a) Al and Au form blocking contacts to PVK; (b) the hole range in PVK is at least 4×${10}^{-6\mathrm{}}$ ${\mathrm{cm}}^2$/ V; and (c) the charge carriers observed in this experiment are stored only at the surface and either recombine there or transit completely across the PVK when released. No bulk recombination or deep trapping of holes occurs. Two alternative models are proposed which produce the observed hyperbolic time dependence. One involves thermal release of charge carriers from a distribution of traps uniform in energy. A second model postulates that the current is proportional to the concentration of a bimolecularly decaying species. The existence of excitons as an intermediate state in the photoconduction process is strongly suggested.