Determining emissive dipole orientation in organic light emitting devices by decay time measurement

We present a method to detect anisotropy in the distribution of the transition dipole moment in organic light emitting diodes (OLEDs). The method is based on the dependency of the exciton decay rate on the optical environment and the orientation of the dipole transition moment, also called the Purcell effect. We use this method to demonstrate a preferential orientation of the small molecule emitter Ir(MDQ)(2)(acac) in a TPBi matrix. The outcoupling improvement for OLEDs that could be obtained with perfectly oriented transition dipoles is estimated by simulation. For perfectly planar structures this shows an EQE in air of up to 34%. (C) 2012 Elsevier B.V. All rights reserved.