Abstract
The electroluminescence quantum efficiency roll-off in iridium(III)-based complexes, namely Ir(iqbt)2(dpm) and Ir(iqbt)3 (iqbt = 1 (benzo[b]thiophen-2-yl)-isoquinolinate, dpm = 2,2,6,6-tetramethyl-3,5-heptanedionate) utilized as near-infrared emitters in organic light emitting diodes with remarkable external quantum efficiencies, up to circa 3%, 1.5% and 1%, are measured and analyzed. With a 5–6 weight% of emitters embedded in a host matrix, the double-layer solution-processed structure as well as analogous three-layer one extended by a hole-conducting film are investigated. The triplet-polaron, the Onsager electron-hole pair dissociation and the triplet-triplet annihilation approaches were used to reproduce the experimental data. The mutual annihilation of triplets in iridium emitters was identified as prevailingly controlling the moderate roll-off, with the interaction between those of iridium emitters and host matrixes found as being less probable. Following the fitting procedure, the relevant rate constant was estimated to be cm3/s, values considered to be rather too high for disordered organic systems, which was assigned to the simplicity of the applied model. A coexistence of some other mechanisms is therefore inferred, ones, however, with a less significant contribution to the overall emission quenching.
Highlights
Near-infrared (NIR) organic light emitting diodes (OLEDs) have gained wide scientific attention during the last decades since they are expected to be successfully applied in medical, security and telecommunication sectors, e.g., for intracellular imaging, night-vision and fiber optic networks [1].The electroluminescence (EL) quantum efficiency (QE) of organic devices has, been progressively and significantly reduced due to the narrowing energy gap of the emitters from the visible to the NIR-region of the electromagnetic spectrum as a result of the energy-gap-law [2]
In what follows we discuss the influence of the current density on the values of external quantum efficiency (EQE) for all investigated devices
To rationalize the EQE experimental outcomes, it was assumed that the triplet exciton recombination processes in the devices do prevailingly occur with the participation of the emissive iridium complex molecules
Summary
Near-infrared (NIR) organic light emitting diodes (OLEDs) have gained wide scientific attention during the last decades since they are expected to be successfully applied in medical, security and telecommunication sectors, e.g., for intracellular imaging, night-vision and fiber optic networks [1]. [Ir(pbq-g) (Bphen)]+ PF6− and [Ir(mpbqx-g) (Bphen)]+ PF6 − (pbq-g = phenylbenzo[g]quinoline, mpbqx-g = 2-methyl-3-phenylbenzo(g)quinoxaline, Bphen = 4,7-diphenyl-1,10-phenanthroline) were synthesized and applied to fabricate the solution-processed OLEDs Those devices, emitting in the wavelength range of 690 − 850 nm, exhibited QEs of 0.67% and 0.61% accompanied by a comparatively small roll-off effect [6]. For OLEDs based on solution-processable, emitting complexes with fluorenyl or thienyl appendices at the peripheral side of the diphenylquinoxaline (dpqx) ligands, Ir(fldpqx) (acac) and Ir(thdpqx) (acac), QEs as high as 5.7% (with an emission peak at 690 nm) and 3.4% (with an emission peak at 702 nm) were respectively achieved [8] In these two papers the minor efficiency roll-off was detected. The current density-voltage characteristics, as well as the current density-external quantum efficiency (EQE) dependencies of the devices, were recorded, the latter ones fitted using the curves representing the TTA, Tq and the Onsager models in order to determine the values of the relevant rate constants
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