Insights into OLED functioning through coordinated experimental measurements and numerical model simulations - Review article

We present some typical applications of our device simulation model "MOLED" which has proven to be a very useful tool for obtaining insight into the functioning of multi-layer organic light-emitting devices (OLED's). Our general approach consists of combining experimental measurement and numerical device modelling in a deliberate and coordinated way. Taking a step-by-step, evolutive approach, starting with simple single layer devices and gradually moving up to more complex multi-layer architectures with a partially doped emission layer, we could progressively determine unknown parameters in the model and at the same time shed light on specific aspects of OLED functioning. For example, electrode characterization was done by measuring and modeling single layer devices with different electrode materials, while the simulation of time of flight signals gave us insight into the transient state and the effects of correlated disorder on the macroscopic mobility. The internal electric field in multi-layer OLED's could also be investigated by varying the thicknesses of the hole transport and electron transport layers. Applying this method to the standard OLED device structure that has received broad attention in the literature, we have found a number of surprising results. From our experiments, we have demonstrated that the average electric field inside the hole transport layer is larger than or equal to the average field in the emission layer over the entire current range. The device simulations fully clarify the situation, giving insight into the space charge effects as well as the hole and the electron current distributions in the device. In particular, we found that there is a leakage of unrecombined holes towards the cathode at low voltages. We also found a strong variation of the electric field in the Alq(3), layer due to space charge effects. By using the laser dye derivatives DCM-TPA with electron trapping capabilities and DCM-II with both electron and hole trapping capabilities as dopants in a standard OLED architecture, we could study the effect on transport and emission characteristics. In the case of the exclusively electron trapping dopant, a blue-shift of the emission color with increasing bias is observed which we find is due to a splitting of the recombination zone. (C) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.