Numerical analysis of the carrier behavior of organic light-emitting diode: comparing a hopping conduction model with a SCLC model

To clarify conduction mechanisms of organic light-emitting diodes (OLEDs), transient behavior of carriers and exciton distributions and field distortion in OLED are calculated using an one-dimensional discontinuous conduction model proposed in our previous paper. This study compares our hopping model with a bilayer-type of space charge limited current (SCLC) model. When applied voltage (V-a) was so high that luminescence exceeded 1000 cdm(-2), calculated current density by the SCLC model was two orders of magnitude higher than experimental values; the internal electric field was discontinuous at the TPD/Alq(3) interface. Therefore, charge carrier density due to field discontinuity was approximately I X 10(16) m(-2). Density is so high that more than 1% of molecules take charge carriers in an interface layer. Voltage dependence of current density by the SCLC model differed with the experimental one at all voltages. Values of current density by the hopping model, which was fitted to an experimental value at V-a = 10 V, agreed with the experimental one for all applied fields. Luminance-voltage characteristics by the hopping model also agreed with experimental values. However, very large field distortion through the Alq3 layer from the TPD/Alq3 interface to the electrode existed by the SCLC model; this phenomenon did not exist in the hopping model. We concluded that electrical conduction in OLEDs could not be explained by the SCLC model. (C) 2003 Elsevier Science B.V. All rights reserved.