Photoconductivity and Charge-Carrier Photogeneration in Photorefractive Polymers

We have studied photogeneration, transport, trapping and recombination as the governing mechanisms for the saturation field strength and the time response of the photorefractive (PR) effect in PVK-based PR materials, utilizing xerographic discharge and photoconductivity experiments. Both the charge carrier photogeneration efficiency and the photocurrent efficiency were found to be independent of chromophore content, suggesting that the chromophore does not participate in carrier generation and trapping. The photoconductivity gain factor G defined as the number of charge carriers measured in photoconductivity in relation to the number of carriers initially photogenerated as determined by the xerographic experiments is found to be much smaller than unity, which indicates that the mean free path of the photogenerated charge carriers is less than the grating period. Photoconductivity data can be explained over 3 orders of magnitude in field, assuming a field-independent trap density. Based on the photoelectric data, PR response times have been predicted by Yeh's model for the build-up of space or by calculating the time, which is necessary to fill all traps by photogenerated holes. Only the latter model can reasonably well explain the observed field dependence of the PR growth time, suggesting that trap-filling essentially controls the PR onset behaviour.