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Qiao D, Chen G, Gong Y, Li K, Fan Y, Zhang B, Jia F, Abubakar Y, Jones A, Otung I, Copner N. Design and optical characterization of an efficient polarized organic light emitting diode based on refractive index modulation in the emitting layer. OPTICS EXPRESS 2020; 28:40131-40144. [PMID: 33379545 DOI: 10.1364/oe.412292] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/13/2020] [Accepted: 12/10/2020] [Indexed: 06/12/2023]
Abstract
Luminescent liquid Crystal (LC) material is regarded as the most promising material for polarized organic light emission due to their intrinsic characteristics including orderly alignment and luminescence. Nevertheless, the optical extraction efficiency of LC based organic light emitting diodes (OLEDs) devices still requires significant effort and innovation towards real-world applications. In this paper, we propose the design of a highly linearly polarized light-emission from OLEDs with integrated refractive index nanograting in the emissive layer (EML) based on photo aligned luminescent liquid crystal material. The simulation results indicate that the geometrically optimized polarized device yields an external quantum efficiency (EQE) up to 47% with a polarized ratio up to 28 dB at a 550 nm emission wavelength. This conceptual design offers a new opportunity to achieve efficient polarized organic luminescence, and it is (to the best of our knowledge) the first approach that enhances the light extraction of OLEDs based on luminescent liquid crystal via index grating in the EML.
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Schmid M, Harms K, Degitz C, Morgenstern T, Hofmann A, Friederich P, Johannes HH, Wenzel W, Kowalsky W, Brütting W. Optical and Electrical Measurements Reveal the Orientation Mechanism of Homoleptic Iridium-Carbene Complexes. ACS APPLIED MATERIALS & INTERFACES 2020; 12:51709-51718. [PMID: 33164497 DOI: 10.1021/acsami.0c14613] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
Understanding and controlling the driving forces for molecular alignment in optoelectronic thin-film devices is of crucial importance for improving their performance. In this context, the preferential orientation of organometallic iridium complexes is in the focus of research to benefit from their improved light-outcoupling efficiencies in organic light-emitting diodes (OLEDs). Although there has been great progress concerning the orientation behavior for heteroleptic Ir complexes, the mechanism behind the alignment of homoleptic complexes is still unclear yet. In this work, we present a sky-blue phosphorescent dye that shows variable alignment depending on systematic modifications of the ligands bound to the central iridium atom. From an optical study of the transition dipole moment orientation and the electrically accessible alignment of the permanent dipole moment, we conclude that the film morphology is related to both the aspect ratio of the dye and the local electrostatic interaction of the ligands with the film surface during growth. These results indicate a potential strategy to actively control the orientation of iridium-based emitters for the application in OLEDs.
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Affiliation(s)
- Markus Schmid
- Institute of Physics, University of Augsburg, 86135 Augsburg, Germany
| | - Kristoffer Harms
- Applied Organic Materials, Institute for High Frequency Technology, Technical University of Braunschweig, 38106 Braunschweig, Germany
| | - Carl Degitz
- Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
- Merck KGaA, Performance Materials - Display Solutions, Frankfurter Straße 250, 64293 Darmstadt, Germany
| | | | - Alexander Hofmann
- Institute of Physics, University of Augsburg, 86135 Augsburg, Germany
| | - Pascal Friederich
- Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
- Institute of Theoretical Informatics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
| | - Hans-Hermann Johannes
- Applied Organic Materials, Institute for High Frequency Technology, Technical University of Braunschweig, 38106 Braunschweig, Germany
| | - Wolfgang Wenzel
- Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
| | - Wolfgang Kowalsky
- Applied Organic Materials, Institute for High Frequency Technology, Technical University of Braunschweig, 38106 Braunschweig, Germany
| | - Wolfgang Brütting
- Institute of Physics, University of Augsburg, 86135 Augsburg, Germany
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Ke X, Gu H, Chen L, Zhao X, Tian J, Shi Y, Chen X, Zhang C, Jiang H, Liu S. Multi-objective collaborative optimization strategy for efficiency and chromaticity of stratified OLEDs based on an optical simulation method and sensitivity analysis. OPTICS EXPRESS 2020; 28:27532-27546. [PMID: 32988045 DOI: 10.1364/oe.398998] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/29/2020] [Accepted: 08/26/2020] [Indexed: 06/11/2023]
Abstract
The low efficiency and dissatisfactory chromaticity remain as important challenges on the road to the OLED commercialization. In this paper, we propose a multi-objective collaborative optimization strategy to simultaneously improve the efficiency and ameliorate the chromaticity of the stratified OLED devices. Based on the formulations derived for the current efficiency and the chromaticity Commission International de L'Eclairage (CIE) of OLEDs, an optical sensitivity model is presented to quantitatively analyze the influence of the layer thickness on the current efficiency and the CIE. Subsequently, an evaluation function is defined to effectively balance the current efficiency as well as the CIE, and a collaborative optimization strategy is further proposed to simultaneously improve both of them. Simulations are comprehensively performed on a typical top-emitting blue OLED to demonstrate the necessity and the effectivity of the proposed strategy. The influences of the layer thickness incorporated in the blue OLED are ranked based on the sensitivity analysis method, and by optimizing the relative sensitive layer thicknesses in the optical views, a 16% improvement can be achieved for the current efficiency of the OLED with desired CIE meantime. Hence, the proposed multi-objective collaborative optimization strategy can be well applied to design high-performance OLED devices by improving the efficiency without chromaticity quality degradation.
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Ke X, Gu H, Zhao X, Chen X, Shi Y, Zhang C, Jiang H, Liu S. Simulation method for study on outcoupling characteristics of stratified anisotropic OLEDs. OPTICS EXPRESS 2019; 27:A1014-A1029. [PMID: 31510487 DOI: 10.1364/oe.27.0a1014] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/12/2023]
Abstract
We derive explicit power dissipation functions for stratified anisotropic OLEDs based on a radiation model of dipole antennas inside anisotropic microcavity. The dipole field expressed by vector potential is expanded into plane waves whose coefficients are determined by scattering matrix method, and then an explicit expression is derived to calculate the energy flux through arbitrary interfaces. Taking advantage of the formulation, we can easily perform quantitative analysis on outcoupling characteristics of stratified anisotropic OLEDs, including outcoupling efficiency, normalized decay rate and angular emission profile. Simulations are carried out on a prototypic stratified OLED structure to verify the validity and capability of the proposed model. The dependencies of the outcoupling characteristics on various emission feature parameters, including dipole position, dipole orientation, and the intrinsic radiative quantum efficiency, are comprehensively evaluated and discussed. Results demonstrate that the optical anisotropy in different organic layers has nonnegligible influences on the far-field angular emission profile as well as outcoupling efficiency, and thereby highlight the necessity of our method. The proposed model can be expected to guide the optimal design of stratified anisotropic OLED devices, and help to solve the inverse outcoupling problem for determining the emission feature parameters.
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Affiliation(s)
- Tommaso Marcato
- Institute for Chemical and Bioengineering, Department of Chemistry and Applied BiosciencesETH Zürich Vladimir-Prelog-Weg 1–5/10 CH-8093 Zürich Switzerland
| | - Chih‐Jen Shih
- Institute for Chemical and Bioengineering, Department of Chemistry and Applied BiosciencesETH Zürich Vladimir-Prelog-Weg 1–5/10 CH-8093 Zürich Switzerland
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Kim KH, Kim JJ. Origin and Control of Orientation of Phosphorescent and TADF Dyes for High-Efficiency OLEDs. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2018; 30:e1705600. [PMID: 29707823 DOI: 10.1002/adma.201705600] [Citation(s) in RCA: 123] [Impact Index Per Article: 20.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/27/2017] [Revised: 12/29/2017] [Indexed: 06/08/2023]
Abstract
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.
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Affiliation(s)
- Kwon-Hyeon Kim
- Department of Materials Science and Engineering, RIAM, Seoul National University, Seoul, 151-742, South Korea
| | - Jang-Joo Kim
- Department of Materials Science and Engineering, RIAM, Seoul National University, Seoul, 151-742, South Korea
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Salehi A, Chen Y, Fu X, Peng C, So F. Manipulating Refractive Index in Organic Light-Emitting Diodes. ACS APPLIED MATERIALS & INTERFACES 2018; 10:9595-9601. [PMID: 29494123 DOI: 10.1021/acsami.7b18514] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
In a conventional organic light-emitting diode (OLED), only a fraction of light can escape to the glass substrate and air. Most radiation is lost to two major channels: waveguide modes and surface plasmon polaritons. It is known that reducing the refractive indices of the constituent layers in an OLED can enhance light extraction. Among all of the layers, the refractive index of the electron transport layer (ETL) has the largest impact on light extraction because it is the layer adjacent to the metallic cathode. Oblique angle deposition (OAD) provides a way to manipulate the refractive index of a thin film by creating an ordered columnar void structure. In this work, using OAD, the refractive index of tris(8-hydroxyquinoline)aluminum (Alq3) can be tuned from 1.75 to 1.45. With this low-index ETL deposited by OAD, the resulting phosphorescent OLED shows nearly 30% increase in light extraction efficiency.
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Affiliation(s)
| | | | - Xiangyu Fu
- Department of Material Science and Engineering , University of Florida , Gainesville , Florida 32611 , United States
| | - Cheng Peng
- Department of Material Science and Engineering , University of Florida , Gainesville , Florida 32611 , United States
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Keum CM, Liu S, Al-Shadeedi A, Kaphle V, Callens MK, Han L, Neyts K, Zhao H, Gather MC, Bunge SD, Twieg RJ, Jakli A, Lüssem B. Tuning charge carrier transport and optical birefringence in liquid-crystalline thin films: A new design space for organic light-emitting diodes. Sci Rep 2018; 8:699. [PMID: 29335503 PMCID: PMC5768873 DOI: 10.1038/s41598-018-19157-9] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2017] [Accepted: 12/12/2017] [Indexed: 11/09/2022] Open
Abstract
Liquid-crystalline organic semiconductors exhibit unique properties that make them highly interesting for organic optoelectronic applications. Their optical and electrical anisotropies and the possibility to control the alignment of the liquid-crystalline semiconductor allow not only to optimize charge carrier transport, but to tune the optical property of organic thin-film devices as well. In this study, the molecular orientation in a liquid-crystalline semiconductor film is tuned by a novel blading process as well as by different annealing protocols. The altered alignment is verified by cross-polarized optical microscopy and spectroscopic ellipsometry. It is shown that a change in alignment of the liquid-crystalline semiconductor improves charge transport in single charge carrier devices profoundly. Comparing the current-voltage characteristics of single charge carrier devices with simulations shows an excellent agreement and from this an in-depth understanding of single charge carrier transport in two-terminal devices is obtained. Finally, p-i-n type organic light-emitting diodes (OLEDs) compatible with vacuum processing techniques used in state-of-the-art OLEDs are demonstrated employing liquid-crystalline host matrix in the emission layer.
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Affiliation(s)
- Chang-Min Keum
- Department of Physics, Kent State University, Kent, OH, 44242, USA.
- Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, United Kingdom.
| | - Shiyi Liu
- Department of Physics, Kent State University, Kent, OH, 44242, USA
| | - Akram Al-Shadeedi
- Department of Physics, Kent State University, Kent, OH, 44242, USA
- Department of physics, University of Baghdad, Al-Jadriya, Baghdad, 10071, Iraq
| | - Vikash Kaphle
- Department of Physics, Kent State University, Kent, OH, 44242, USA
| | - Michiel Koen Callens
- Department of Electronics and Information Systems, Ghent University, Ghent, B-9000, Belgium
| | - Lu Han
- Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, OH, 44106, USA
| | - Kristiaan Neyts
- Department of Electronics and Information Systems, Ghent University, Ghent, B-9000, Belgium
| | - Hongping Zhao
- Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, OH, 44106, USA
| | - Malte C Gather
- Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, United Kingdom
| | - Scott D Bunge
- Department of Chemistry and Biochemistry, Kent State University, Kent, OH, 44242, USA
| | - Robert J Twieg
- Department of Chemistry and Biochemistry, Kent State University, Kent, OH, 44242, USA
| | - Antal Jakli
- Liquid Crystal Institute, Kent State University, Kent, OH, 44242, USA
| | - Björn Lüssem
- Department of Physics, Kent State University, Kent, OH, 44242, USA
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Sim B, Moon CK, Kim KH, Kim JJ. Quantitative Analysis of the Efficiency of OLEDs. ACS APPLIED MATERIALS & INTERFACES 2016; 8:33010-33018. [PMID: 27809477 DOI: 10.1021/acsami.6b10297] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
We present a comprehensive model for the quantitative analysis of factors influencing the efficiency of organic light-emitting diodes (OLEDs) as a function of the current density. The model takes into account the contribution made by the charge carrier imbalance, quenching processes, and optical design loss of the device arising from various optical effects including the cavity structure, location and profile of the excitons, effective radiative quantum efficiency, and out-coupling efficiency. Quantitative analysis of the efficiency can be performed with an optical simulation using material parameters and experimental measurements of the exciton profile in the emission layer and the lifetime of the exciton as a function of the current density. This method was applied to three phosphorescent OLEDs based on a single host, mixed host, and exciplex-forming cohost. The three factors (charge carrier imbalance, quenching processes, and optical design loss) were influential in different ways, depending on the device. The proposed model can potentially be used to optimize OLED configurations on the basis of an analysis of the underlying physical processes.
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Affiliation(s)
- Bomi Sim
- WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University , Seoul 151-744, South Korea
| | - Chang-Ki Moon
- Department of Materials Science and Engineering, Seoul National University , Seoul 151-744, South Korea
| | - Kwon-Hyeon Kim
- Department of Materials Science and Engineering, Seoul National University , Seoul 151-744, South Korea
| | - Jang-Joo Kim
- WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University , Seoul 151-744, South Korea
- Department of Materials Science and Engineering, Seoul National University , Seoul 151-744, South Korea
- Research Institute of Advanced Materials (RIAM), Seoul National University , Seoul 151-744, South Korea
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Shin H, Lee JH, Moon CK, Huh JS, Sim B, Kim JJ. Sky-Blue Phosphorescent OLEDs with 34.1% External Quantum Efficiency Using a Low Refractive Index Electron Transporting Layer. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2016; 28:4920-4925. [PMID: 27060851 DOI: 10.1002/adma.201506065] [Citation(s) in RCA: 66] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/07/2015] [Revised: 02/24/2016] [Indexed: 06/05/2023]
Abstract
Blue-phosphorescent organic light-emitting diodes (OLEDs) with 34.1% external quantum efficiency (EQE) and 79.6 lm W(-1) are demonstrated using a hole-transporting layer and electron-transporting layer with low refractive index values. Using optical simulations, it is predicted that outcoupling efficiencies with EQEs > 60% can be achieved if organic layers with a refractive index of 1.5 are used for OLEDs.
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Affiliation(s)
- Hyun Shin
- Department of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea
| | - Jeong-Hwan Lee
- Department of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea
| | - Chang-Ki Moon
- Department of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea
| | - Jin-Suk Huh
- Department of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea
| | - Bomi Sim
- Department of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea
| | - Jang-Joo Kim
- Department of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea
- Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul, 151-744, South Korea
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