DARK INJECTION AS A PROBE OF INTERFACIAL PROCESSES IN DISORDERED MOLECULAR-SYSTEMS

An unambiguous experimental procedure is illustrated for characterizin g the behavior of electrical contacts on unipolar transport active mol ecular solids devoid of deep traps. Such polymeric media, originally d eveloped to serve as the transport layer in bilayer electrophotographi c receptors, are now under wider investigation for use in organic elec tronic and photoelectronic devices. The technique is illustrated for t wo types of carbon contact on the trap-free hole transport polymer PTP B. First, a carbon-filled polymer is shown to be operationally ohmic f or hole injection. To account for current-voltage characteristics over a wide temperature range, it is necessary to consider explicitly the field dependence of the drift mobility in computing the trap-free spac e-charge limited current. Using the same procedure, glassy carbon is c learly demonstrated to be emission limiting. Injection in this case is shown to conform to a model in which carrier supply to the film bulk is controlled by thermally assisted tunneling from metal to discrete t ransport-active states in polymer, including those not immediately adj acent to the contact interface. The technique described has also been used to demonstrate time-dependent changes in contact behavior. An ill ustration of this contact-forming phenomenon following thermal deposit ion of Au on PTPB is included.