Determining emissive dipole orientation in organic light emitting devices by decay time measurement

Penninck, Lieven; Steinbacher, Frank; Krause, Ralf; Neyts, Kristiaan

doi:10.1016/j.orgel.2012.09.014

Cited by 48 publications

(45 citation statements)

References 21 publications

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“…[ [14][15][16]20,24,25] However, in the past, time-resolved measurements of transition dipole orientations have been limited to optical excitation, [14,16] under which location and width of the emission zone are different than under electrical excitation. Measuring orientation under electrical excitation more closely resembles the situation in the real device and ensures that one obtains the average orientation of exactly those emitter molecules that contribute to the electroluminescence generated by the device.…”

Section: Measurement Of the Emitter Lifetime And Modeling Of The Oriementioning

confidence: 99%

“…While the intrinsic triplet lifetime is a material parameter and denotes the lifetime the emitter molecule would have in free space, the effective lifetime takes the influence of the optical environment into account. [14][15][16] For instance, improved roll-off has been demonstrated for top-emitting OLEDs, where the presence of a stronger microcavity than in conventional bottom-emitting structures shortens the emitter lifetime. [17] Song et al have found some evidence that the roll-off also correlates with the distance between the emitter and the reflecting metal cathode.…”

Section: Introductionmentioning

confidence: 99%

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

Murawski

Liehm

Leo

et al. 2013

Adv Funct Materials

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This article describes the first systematic investigation of how the efficiency roll-off in organic light-emitting diodes (OLEDs) is influenced by the position and orientation of the emitter molecules within the OLED cavity. The efficiency roll-off is investigated for two OLED stacks containing either the phosphorescent emitter Ir(MDQ) 2 (acac) or Ir(ppy) 3 by varying the distance between emitter and metal cathode; a strong influence of emitter position and orientation on roll-off is observed. The measurements are modeled by triplet-tripletannihilation (TTA) theory yielding the critical current density and the TTA rate constant. We find that Ir(MDQ) 2 (acac) shows the lowest roll-off when the emitter is located in the first optical maximum of the electromagnetic field, whereas the roll-off of the Ir(ppy) 3 stack is lowest when the emitter is positioned closer to the metal cathode. Measurement and modeling of time-resolved electroluminescence show that the different roll-off behavior is due to the Submitted to 2 different orientation and the corresponding change of the decay rate of the emissive dipoles of Ir(MDQ) 2 (acac) and Ir(ppy) 3 . Finally, we develop design principles for optimal highbrightness performance by modeling the roll-off as a function of emitter-cathode distance, emissive dipole orientation, and radiative efficiency.

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Section: Measurement Of the Emitter Lifetime And Modeling Of The Oriementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Murawski

Liehm

Leo

et al. 2013

Adv Funct Materials

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“…Therefore, it is believed that the theoretical EQE limit of phosphorescent OLEDs is 30%. Recently, however, some heteroleptic iridium complexes (HICs) have been reported to have transition dipole moments oriented preferentially along the horizontal direction 5,[14][15][16][17][18] . This implies that it may be possible to exceed the efficiency limit of 30% under isotropic orientations of the emitting dipoles.…”

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confidence: 99%

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes

et al. 2014

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Organic light-emitting diodes (OLEDs) are among the most promising organic semiconductor devices. The recently reported external quantum efficiencies (EQEs) of 29-30% for green and blue phosphorescent OLEDs are considered to be near the limit for isotropically oriented iridium complexes. The preferred orientation of transition dipole moments has not been thoroughly considered for phosphorescent OLEDs because of the lack of an apparent driving force for a molecular arrangement in all but a few cases, even though horizontally oriented transition dipoles can result in efficiencies of over 30%. Here we use quantum chemical calculations to show that the preferred orientation of the transition dipole moments of heteroleptic iridium complexes (HICs) in OLEDs originates from the preferred direction of the HIC triplet transition dipole moments and the strong supramolecular arrangement within the co-host environment. We also demonstrate an unprecedentedly high EQE of 35.6% when using HICs with phosphorescent transition dipole moments oriented in the horizontal direction.

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“…3,14 Indeed, in the last couple of years numerous authors have presented methods to recover EZ properties from a variety of optical measurements, such as electroluminescence (EL) spectra, emission patterns, external efficiencies, and photoluminescence lifetimes. 12,[15][16][17][18][19][20][21] These methods heavily utilize fitting procedures, which may yield highly resolved evaluation, however, usually require extensive data sets, and naturally rely on advanced numerical techniques, which tend to obscure the underlying physical phenomena. 19,22,23 In recent work, we have presented a different approach to this problem, developing analytical closed-form formulae to extract the emission zone location from measured emission pattern extrema, assuming the excitons are concentrated in a very narrow region.…”

Section: Introductionmentioning

confidence: 99%

“…12,[15][16][17][18][19][20][21][22] First, for some OLED engineering tasks, the complexity involved in employing the numerical methods is not very cost effective. For initial design stages and routine verification processes, for instance, it seems that a more intuitive, computationally efficient, approach, as the one presented in this paper, would be a better choice.…”

Section: Introductionmentioning

confidence: 99%

Analytical estimation of emission zone mean position and width in organic light-emitting diodes from emission pattern image-source interference fringes

Epstein

Roberts²,

Tessler

et al. 2014

Journal of Applied Physics

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We present an analytical method for evaluating the first and second moments of the effective exciton spatial distribution in organic light-emitting diodes (OLED) from measured emission patterns. Specifically, the suggested algorithm estimates the emission zone mean position and width, respectively, from two distinct features of the pattern produced by interference between the emission sources and their images (induced by the reflective cathode): the angles in which interference extrema are observed, and the prominence of interference fringes. The relations between these parameters are derived rigorously for a general OLED structure, indicating that extrema angles are related to the mean position of the radiating excitons via Bragg's condition, and the spatial broadening is related to the attenuation of the image-source interference prominence due to an averaging effect. The method is applied successfully both on simulated emission patterns and on experimental data, exhibiting a very good agreement with the results obtained by numerical techniques. We investigate the method performance in detail, showing that it is capable of producing accurate estimations for a wide range of source-cathode separation distances, provided that the measured spectral interval is large enough; guidelines for achieving reliable evaluations are deduced from these results as well. As opposed to numerical fitting tools employed to perform similar tasks to date, our approximate method explicitly utilizes physical intuition and requires far less computational effort (no fitting is involved). Hence, applications that do not require highly resolved estimations, e.g., preliminary design and production-line verification, can benefit substantially from the analytical algorithm, when applicable. This introduces a novel set of efficient tools for OLED engineering, highly important in the view of the crucial role the exciton distribution plays in determining the device performance.

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Determining emissive dipole orientation in organic light emitting devices by decay time measurement

Cited by 48 publications

References 21 publications

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

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

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes

Analytical estimation of emission zone mean position and width in organic light-emitting diodes from emission pattern image-source interference fringes

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