References – Publications

Light‐Emitting Diodes: Organic Light‐Emitting Diodes with 30% External Quantum Efficiency Based on a Horizontally Oriented Emitter (Adv. Funct. Mater. 31/2013)

Sei‐Yong Kim Won‐Ik Jeong Christian Mayr Young‐Seo Park Kwon‐Hyeon Kim Jeong‐Hwan Lee Chang‐Ki Moon Wolfgang Brütting Jang‐Joo Kim

https://doi.org/10.1002/adfm.201370153

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Green phosphorescent organic light‐emitting diodes (OLEDs) based on a horizontally oriented emitter are analysed by J.‐J. Kim and co‐workers on page 3896. They demonstrate that theoretical analysis based on the orientation factor (the ratio of horizontal dipoles to total dipoles) and the quantum yield of the emitter predicts that the maximum EQE of OLEDs with this emitter is about 30%, matching experimental data, indicating that the electrical loss of the OLEDs is negligible and the device structure can be utilized as a platform to demonstrate the validity of optical modeling.

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes Kwon-Hyeon Kim, Sunghun Lee, Chang-Ki Moon, Sei-Yong Kim, Young-Seo Park, Jeong-Hwan Lee, Jin Woo Lee, June Huh, Youngmin You & Jang-Joo Kim Nature Communications volume 5, Article number: 4769 (2014)

https://www.nature.com/articles/ncomms5769

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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.

Unraveling the orientation of phosphors doped in organic semiconducting layers Chang-Ki Moon, Kwon-Hyeon Kim & Jang-Joo Kim Nature Communications volume 8, Article number: 791 (2017)

https://www.nature.com/articles/s41467-017-00804-0

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Emitting dipole orientation is an important issue of emitting materials in organic light-emitting diodes for an increase of outcoupling efficiency of light. The origin of preferred orientation of emitting dipole of iridium-based heteroleptic phosphorescent dyes doped in organic layers is revealed by simulation of vacuum deposition using molecular dynamics along with quantum mechanical characterization of the phosphors. Consideration of both the electronic transitions in a molecular frame and the orientation of the molecules at the vacuum/molecular film interface allows quantitative analyses of the emitting dipole orientation depending on host molecules and dopant structures. Interactions between the phosphor and nearest host molecules on the surface, minimizing the non-bonded van der Waals and electrostatic interaction energies determines the molecular alignment during the vacuum deposition. Parallel alignment of the main cyclometalating ligands in the molecular complex due to host interactions rather than the ancillary ligand orienting to vacuum leads to the horizontal emitting dipole orientation.

Exciplex‐Forming Co‐host for Organic Light‐Emitting Diodes with Ultimate Efficiency (Adv. Funct. Mater. 23/2013) Young‐Seo Park Sunghun Lee Kwon‐Hyeon Kim Sei‐Yong Kim Jeong‐Hwan Lee Jang‐Joo Kim

https://doi.org/10.1002/adfm.201300547

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Phosphorescent organic light‐emitting diodes (OLEDs) with ultimate efficiency in terms of the external quantum efficiency (EQE), driving voltage, and efficiency roll‐off are reported, making use of an exciplex‐forming co‐host. This exciplex‐forming co‐host system enables efficient singlet and triplet energy transfers from the host exciplex to the phosphorescent dopant because the singlet and triplet energies of the exciplex are almost identical. In addition, the system has low probability of direct trapping of charges at the dopant molecules and no charge‐injection barrier from the charge‐transport layers to the emitting layer. By combining all these factors, the OLEDs achieve a low turn‐on voltage of 2.4 V, a very high EQE of 29.1% and a very power efficiency of 124 lm W-1. In addition, the OLEDs achieve an extremely low efficiency roll‐off. The EQE of the optimized OLED is maintained at more than 27.8%, up to 10 000 cd m-2.

Highly Efficient Organic Light‐Emitting Diodes with Phosphorescent Emitters Having High Quantum Yield and Horizontal Orientation of Transition Dipole Moments ADVANCED MATERIALS 26, 3844-3847, (2014)

Kwon‐Hyeon Kim Chang‐Ki Moon Jeong‐Hwan Lee Sei‐Yong Kim Jang‐Joo Kim

https://doi.org/10.1002/adma.201305733

Ancillary ligands in heteroleptic iridium complexes significantly influence the orientation of the transition dipole moments. Ir(ppy)3, a homoleptic iridium complex, exhibits isotropic dipole orientation, whereas the heteroleptic Ir complexes of Ir(ppy)2tmd show a highly preferred dipole orientation (78%) in the horizontal direction. In addition, we demonstrate an unprecedented highly efficient green OLED exhibiting an EQE of 32.3% and a power efficiency of 142.5 lm/W by using an emitter with high quantum yield and horizontally oriented dipoles.

Crystal Organic Light‐Emitting Diodes with Perfectly Oriented Non‐Doped Pt‐Based Emitting Layer Kwon‐Hyeon Kim Jia‐Ling Liao Si Woo Lee Bomi Sim Chang‐Ki Moon Gene‐Hsiang Lee Hyo Jung Kim Yun Chi Jang‐Joo Kim

ADVANCED MATERIALS 28, 2526, (2016)

https://doi.org/10.1002/adma.201504451

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Organic light‐emitting diodes with external quantum efficiency of 38.8% are realized using a Pt‐based thin‐film emitting layer with photoluminescence quantum yield of 96% and transition dipole ratio of 93%. The emitting dipole orientation of the thin films fabricated using Pt complexes is investigated and the structural relationship between X‐ray structural analysis and the structures in thin films are discussed based on quantum chemical calculations.

Highly Enhanced Light Extraction from Surface Plasmonic Loss Minimized Organic Light‐Emitting Diodes ADVANCED MATERIALS 25, 3571-3577, (2013)

Jung‐Bum Kim Jeong‐Hwan Lee Chang‐Ki Moon Sei‐Yong Kim Jang‐Joo Kim

https://doi.org/10.1002/adma.201205233

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Extremely high light out‐coupling efficiency from a transparent organic light‐emitting diode integrated with microstructures on both sides of the device is reported. The metal free device offers dramatically reduced surface plasmonic and intrinsic absorption losses. Moreover, high refractive index micropatterns with optimal light extraction condition are fabricated based on the well‐matched analysis of optical simulations.

Origin and Control of Orientation of Phosphorescent and TADF Dyes for High‐Efficiency OLEDs

Adv. Mater. 30, 1705600 (2018)

Kwon‐Hyeon Kim Jang‐Joo Kim

https://doi.org/10.1002/adma.201705600

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It has been known for decades that the emitting dipole orientation (EDO) of emitting dyes influences the outcoupling efficiency of organic light‐emitting diodes (OLEDs). However, the EDO of dopants, especially phosphorescent dopants, has been studied less than that of neat films and polymer emitting layers (EMLs) due to the lack of an apparent driving force for aligning the dopants in amorphous host films. Recently, however, even globular‐shaped Ir complexes have been reported to have a preferred orientation in doped films and OLEDs. External quantum efficiencies (EQEs) higher than 30% have also been demonstrated using phosphorescent and thermally activated delayed fluorescent dyes (TADF) doped in EMLs. Here, recent results on the EDO of phosphorescent and TADF dyes doped in host films, and highly efficient OLEDs using these dyes are reviewed. The origin and control of the orientation of phosphors are discussed, followed by a discussion of future strategies to achieve EQEs of over 60% without a light extraction layer, from the material point of view.