Comparing the emissive dipole orientation of two similar phosphorescent green emitter molecules in highly efficient organic light-emitting diodes

We investigate the average orientation of the transition dipole moments of two green phosphorescent emitters Ir(ppy)3 and Ir(ppy)2(acac) embedded in a CBP matrix, using in-situ angle resolved electroluminescence spectroscopy and optical simulations. The dipole orientation of Ir(ppy)3 is nearly isotropic while 77% of the dipoles are horizontally aligned for Ir(ppy)2(acac). Optimized organic light-emitting diodes based on these emitters achieve external quantum efficiencies of 18.3% (Ir(ppy)3) and 21.7% (Ir(ppy)2(acac)). This difference is partially explained by the different dipole orientations.