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Evidence for non-isotropic emitter orientation in a red phosphorescent organic light-emitting diode and its implications for determining the emitter’s radiative quantum efficiency
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FIG. 1.

(Color online) Layer stack of the investigated devices together with an exemplary phosphorescence decay (blue curve) and a monoexponential fit (dashed red line) yielding τ = 1.36 μs for an ETL thickness of 249 nm. The inset shows the intrinsic emitter spectrum used for optical simulations.

Image of FIG. 2.

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FIG. 2.

(Color online) Measured phosphorescence lifetimes normalized to the intrinsic value τ 0 = 1.37 μs together with simulated curves for different emitter orientations as a function of the ETL thickness: isotropic with Θ = 1/3 and q = 0.8 (dashed black line) and oriented with Θ = 0.24 and q = 0.7 (solid red line).

Image of FIG. 3.

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FIG. 3.

(Color online) Measurements at a current density of j = 1 mA/cm2 of the external quantum efficiency of OLEDs with different ETL thicknesses (without and with macroextracor) together with simulations assuming different emitter orientation: isotropic with Θ = 1/3 and q = 0.8 (dashed black line) and oriented with Θ = 0.24 and q = 0.7 (solid red line).

Image of FIG. 4.

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FIG. 4.

(Color online) Calculated power distribution between different modes for the OLED stack depicted in Fig. 1 as a function of the ETL thickness for q = 1. (a) isotropic (Θ = 1/3) and (b) completely horizontal (Θ = 0) emitter orientation.

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/content/aip/journal/apl/99/16/10.1063/1.3653475
2011-10-18
2014-04-18

Abstract

The efficiency of organic light-emitting diodes is limited as only a fraction of the consumed electrical power is converted into light that is finally extracted to air. Especially, the radiative quantum efficiency of the guest-host system is of interest and should be close to unity to achieve highly efficient devices. We show that the red phosphorescent emitter Ir(MDQ)2(acac) doped in an α-NPD matrix exhibits a profound non-isotropic dipole orientation. Ignoring this feature leads to a significant overestimation of the emitter efficiency. Furthermore, we demonstrate the huge potential for efficiency enhancement of mainly parallel emitter orientation in phosphorescent organic light-emitting diodes.

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Scitation: Evidence for non-isotropic emitter orientation in a red phosphorescent organic light-emitting diode and its implications for determining the emitter’s radiative quantum efficiency
http://aip.metastore.ingenta.com/content/aip/journal/apl/99/16/10.1063/1.3653475
10.1063/1.3653475
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