Dependence of apparent emitting dipole orientation of an Ir(III) complex on doping concentration, film thickness, and excitation condition

Abstract

This paper outlines the effects of doping concentration, film thickness, and excitation condition on the apparent emitting dipole orientations (EDOs) of an Ir(III) complex, bis[2-(2-pyridinyl-N)phenyl-C](2,4-pentanedionato-O2,O4)iridium(III) (Ir(ppy)2acac), doped in organic thin films measured by the angle-dependent photoluminescence or angle-dependent electroluminescence (ADPL and ADEL, respectively) techniques. Under direct excitation conditions, EDO does not exhibit changes until 32 mol%. When Ir(ppy)2acac is excited via energy transfer from host molecules, however, the measured horizontal dipole ratio decreases with decreasing doping concentration at low doping levels. Similar behavior was observed in a bi-layer system, where the effects of polarization of excited host molecules were eliminated, and in organic light-emitting diodes. This finding indicates that apparent EDO values measured by ADPL and ADEL can be varied according to the excitation process of emitter molecules.