Ohmic hole injection into a polyfluorene homopolymer

Here we report the observation of ohmic hole injection from a conducting polymer anode into poly(9,9-dioctylfluorene) (PFO) in a polymer light-emitting diode (LED) structure. Although initially non ohmic, the contact can be made locally ohmic by electrically conditioning the device at voltages higher than the electroluminescence (EL) onset voltage. The ohmic nature of the contact in selected regions is confirmed by the appearance of dark injection space-charge-limited transient currents, which yield hole mobilities in good agreement with those measured by the time-of-flight method. The appearance of ohmic injection is discussed within a model that assumes the existence of electron traps near the anode interface. When the sample is conditioned electrons are injected from the cathode and are trapped near the anode inducing an interfacial dipole that reduces the barrier for hole injection.