Storage of charge carriers on emitter molecules in organic light-emitting diodes

Abstract

Organic light-emitting diodes (OLEDs) using the red phosphorescent emitter iridium(III)bis(2methyldibenzo[f,h]quinoxaline) (acetylacetonate) [Ir(MDQ) 2 (acac)] are studied by time-resolved electroluminescence measurements. A transient overshoot after voltage turn-off is found, which is attributed to electron accumulation on Ir(MDQ) 2 (acac) molecules. The mechanism is verified via impedance spectroscopy and by application of positive and negative off-voltages. We calculate the density of accumulated electrons and find that it scales linearly with the doping concentration of the emitter. Using thin quenching layers, we locate the position of the emission zone during normal OLED operation and after voltage turn-off. In addition, the transient overshoot is also observed in three-color white-emitting OLEDs. By time-and spectrally resolved measurements using a streak camera, we directly attribute the overshoot to electron accumulation on Ir(MDQ) 2 (acac). We propose that similar processes are present in many state-of-the-art OLEDs and believe that the quantification of charge carrier storage will help to improve the efficiency of OLEDs.

Influence of Cavity Thickness and Emitter Orientation on the Efficiency Roll‐Off of Phosphorescent Organic Light‐Emitting Diodes

“…Figure 5 shows two typical transients for the red-emitting OLEDs with ETLs of different thicknesses. All devices showed a mono-exponential decay; non-exponential processes such as TTA [4] , tripletpolaron-annihilation [5] , or delayed exciton generation [23] are not observed. The triplet lifetime of the device was extracted by fitting a mono-exponential decay function.…”

Doping-free hybrid white organic light-emitting diodes with fluorescent blue, phosphorescent green and red emission layers

“…5 solid line), the green EML is situated at the interface of the TAPC/Bepp 2 layer, and the red emission from Ir(MDQ) 2 (acac) is virtually absent. This can be attributed to the ability of the Ir(MDQ) 2 (acac) molecule to strongly trap electrons, which usually recombine with holes to emit red emission through a direct carrier trapping mechanism [33,34]. The absence of red emission in this case implies that the number of holes from TAPC to the red EML in Bepp 2 layer is quite few despite the almost aligned HOMO energy level of the two transport materials.…”

High performance flexible top-emitting warm-white organic light-emitting devices and chromaticity shift mechanism

“…With the charge carrier density variation, the triplet-polaron quenching shows a current dependence as the charge carrier trapping ratio R shows a voltage dependence. 20,25 The trapping rate decrease as well as the triplet-polaron quenching for orange-red dopant bring a distinct enhancement of blue emission via increasing voltage. Furthermore, the spectra in Fig.…”

“…The LUMO and HOMO of Ir(MDQ) 2 2 (acac) and TCTA. 20,21 The ratio R of the charge carrier trapping rate τ −1 trap to unperturbed carrier transport τ −1 with the driving voltage is expressed as following, 22, 23…”