TIME-RESOLVED SPACE CHARGE-LIMITED INJECTION IN A TRAP-FREE GLASSY POLYMER

Time-resolved measurements of the dark current excited by application of a voltage step to the tetraphenylbenzidine containing condensation polymer PTPB fitted with ohmic contacts have been carried out as a function of field and temperature. At relatively high temperature, the experimental results are essentially in self-consistent conformity both with the predictions of trap-free space charge-limited (TFSCL) transport theory and with independent TOF measurements. Under trap-free transport conditions, the step field response of the dark current provides a unique means of following the temporal evolution of the drift mobility from the time scale defined by the average injected carrier transit time to a final steady state condition in which every state available to the transiting carriers has been statistically sampled. As the specimen temperature is lowered, increasing and systematic deviations from conventional TFSCLC theory are observed. The extent to which these deviations can be accounted for solely by modification of the theory to explicitly take into account the field dependence of the drift mobility or whether they are indicative of incomplete thermalization on the time scale of a transit time is analyzed.