Polyaniline (PAni) dispersions can be efficiently used as hole injection layers (HIL) for passive and active matrix display applications. In earlier work the influence of conductivity and work function of HILs spin coated from water based PAni/PSS dispersions on device performance had already been presented. Recent investigations on hole transport mechanism in polyaniline systems now show the necessity of a minimum conductivity and an optimum work function for hole injection. Electrochemical Impedance Spectroscopy measurements combined with luminescence investigations showed that the lateral conductivity in the PAni films must be >10-6 S/cm. Otherwise, a decrease in maximum efficiency and an increase in driving voltage in dependence on coating thickness occurs. Work function investigations on water-free, highly conductive polyaniline dispersions emphasize the theory of an optimum range for hole injection from the anode into the light emitting polymer. The work function of highly conductive, non-aqueous PAni dispersion (0.1-5 S/cm) was determined by Scanning Kelvin Probe method to be 4.5 - 4.7 eV, which is outside of the optimum range at about 4.95 - 5.05 eV for polymeric light emitting diodes, resulting in poor efficiency values (max. 30 - 50% compared to PAni/PSS standard).
Although polyaniline (PAni) has been proposed for use as a hole injection layer (HIL) in organic light emitting diodes (OLEDs) and polymeric light emitting diodes (PLEDs) from very early on, the material does not seem to have found widespread use on a (pre)commercial scale. Recent results will be presented showing that PAni can be efficiently used as HIL, and that it even has some advantages over the often preferred poly[ethylenedioxy-thiophene] (PEDT). Intensive investigations on the influence of conductivity, morphology and especially the work function onto device performance have led to a commercially available water-borne PAni dispersion. The stable, nanoscaled system for HILs has a particle size of about 35 nm and a lateral conductivity (when deposited and dried) of around 10-6 S/cm. Using PAni dispersions for the generation of HILs the final device performance in OLEDs and PLEDs could be significantly improved. Depending on the used light emitting polymer (LEP), luminescence data were up to 30% more efficient compared to devices made with the widespread used PEDT.