Frequency-domain time-resolved luminescence for in-operando study of efficiency roll-off in organic light-emitting diodes

Time-resolved luminescence measurements are invaluable for understanding light-emitting materials but are difficult to perform on operating organic light-emitting diodes (OLEDs) because the light generated by the OLED can disturb common time-resolved techniques. We show that frequency-domain luminescence can be used to overcome this problem, and apply it to study efficiency roll-off in OLEDs. We measure the exciton lifetime of a phosphorescent OLED based on the emitter bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate) iridium(III) (Ir(MDQ)2(acac)) whilst it is operating. By driving the OLED continuously at current densities of up to 128mA/cm2, we observe that the exciton lifetime decreases with increasing current density as a result of triplet-polaron annihilation (TPA). The measurement presented can guide the development of new materials and device structures with improved efficiency at high brightness.