Activation energies in organic light emitting diodes comprising ohmic contacts both for electron and hole injection

The lowest obtainable operating voltage for organic light emitting diodes (OLEDs) utilising a predefined organic layer setup can only be achieved with ohmic contacts both for electron and hole injection. We have investigated dark current transients of unipolar single-layer samples, and we have found ohmic contacts both for hole injection at indium tin oxide (ITO)/4,4′,4″-tris{N-(1-naphtyl)-N-phenylamino}-triphenylamine (1-Naphdata) interfaces and for electron injection at 8-hydroxyquinoline aluminum (Alq3)/LiF/Al interfaces. Therefore, the properties of OLEDs comprising these two interfaces are governed only by bulk material properties and internal organic/organic interfaces. In order to identify the dominating mechanisms concerning the temperature-dependent behaviour of prototypical double layer OLEDs, we have measured (with respect to the applied electric field) the activation energies of the charge carrier mobility and of the steady state current density in 1-Naphdata (holes) and Alq3 (electrons), the activation energies of the steady state current density and of the luminance in OLEDs comprising an 1-Naphdata/Alq3 heterojunction, plus the activation energy of the luminance onset. These experimentally activation energies are discussed with respect to device performance in the typical operating temperature range of flat panel displays including implications for further device optimisation.