Comparison of the drift mobility measured under transient and steady-state conditions in a prototypical hopping system

It is demonstrated that Au, Ag and carbon form ohmic contacts for hole injection in polycarbonate films doped with the hole-transporting molecule tetraphenyldiamine (TPD) and that these contacts are also ohmic on amorphous TPD films. At fields which make the injected hole transit time less than the bulk dielectric relaxation time, the response of these films, in a sandwich cell configuration with ohmic contacts, to step-field excitation is demonstrated to be a trap-free space-charge-limited (TFSCLC) current. The TFSCLC depends on the bulk drift mobility and the specimen dimensions only. These films provide a unique opportunity to examine simultaneously, in a dispersive transport medium, time-of-flight transit times, space-charge injection transients and finally steady-state SCLC. The latter collectively provide a measure of the drift mobility at different times in the same applied field. Thus the time-dependent character of the drift mobility can be fully delineated.