An unambiguous experimental procedure is illustrated for characterizing the behavior of electrical contacts on unipolar transport active molecular solids devoid of deep traps. Such polymeric media, originally developed to serve as the transport layer in bilayer electrophotographic receptors, are now under wider investigation for use in organic electronic and photoelectronic devices. The technique is illustrated for two types of carbon contact on the trap-free hole transport polymer PTPB. First, a carbon-filled polymer is shown to be operationally ohmic for 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 space-charge limited current. Using the same procedure, glassy carbon is clearly 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 transport-active states in polymer, including those not immediately adjacent to the contact interface. The technique described has also been used to demonstrate time-dependent changes in contact behavior. An illustration of this contactforming phenomenon following thermal deposition of Au on PTPB is included.
Martin A. Abkowitz, "Dark Injection as a Probe of Interfacial Processes in Disordered Molecular Systems" in Journal of Imaging Science and Technology, 1996, pp 318 - 321, https://doi.org/10.2352/J.ImagingSci.Technol.1996.40.4.art00009