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Cakaj A, Schmid M, Hofmann A, Brütting W. Controlling Spontaneous Orientation Polarization in Organic Semiconductors─The Case of Phosphine Oxides. ACS APPLIED MATERIALS & INTERFACES 2023; 15:54721-54731. [PMID: 37970727 DOI: 10.1021/acsami.3c13049] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/17/2023]
Abstract
Upon film growth by physical vapor deposition, the preferential orientation of polar organic molecules can result in a nonzero permanent dipole moment (PDM) alignment, causing a macroscopic film polarization. This effect, known as spontaneous orientation polarization (SOP), was studied in the case of different phosphine oxides (POs). We investigate the control of SOP by molecular design and film-growth conditions. Our results show that using less polar POs with just one phosphor-oxygen bond yields an exceptionally high degree of SOP with the so-called giant surface potential (slope), reaching more than 150 mV nm-1 in a neat bis-4-(N-carbazol(yl)phenyl)phenyl phosphine oxide (BCPO) film grown at room temperature. Additionally, by altering the evaporation rate and substrate temperature, we are able to control the SOP magnitude over a broad range from 0 to almost 300 mV nm-1. Diluting BCPO in a nonpolar host enhances the PDM alignment only marginally, but combining temperature control with dipolar doping can result in highly aligned molecules with more than 80% of their PDMs standing upright on the substrate on average.
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Affiliation(s)
- Albin Cakaj
- Institute of Physics, University of Augsburg, Augsburg 86135, Germany
| | - Markus Schmid
- Institute of Physics, University of Augsburg, Augsburg 86135, Germany
| | - Alexander Hofmann
- Institute of Physics, University of Augsburg, Augsburg 86135, Germany
| | - Wolfgang Brütting
- Institute of Physics, University of Augsburg, Augsburg 86135, Germany
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2
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Jayabharathi J, Thanikachalam V, Thilagavathy S. Phosphorescent organic light-emitting devices: Iridium based emitter materials – An overview. Coord Chem Rev 2023. [DOI: 10.1016/j.ccr.2023.215100] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/12/2023]
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3
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Wang WC, Nakano K, Hsu CS, Tajima K. Synthesis of 2,5,8-Tris(1-phenyl-1 H-benzo[ d]imidazol-2-yl)benzo[1,2- b:3,4- b':5,6- b″] Trithiophenes and Their Spontaneous Orientation Polarization in Thin Films. ACS APPLIED MATERIALS & INTERFACES 2023; 15:20294-20301. [PMID: 37058452 DOI: 10.1021/acsami.3c02785] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/19/2023]
Abstract
To investigate the relationship between molecular structures and spontaneous orientation polarization (SOP) in organic thin films, 2,5,8-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b':5,6-b″] trithiophene (TPBTT) and its ethyl derivative (m-ethyl-TPBTT) were synthesized. Variable angle spectroscopic ellipsometry and two-dimensional grazing-incidence wide-angle X-ray scattering showed that the vacuum-deposited films of TPBTT and m-ethyl-TPBTT had a higher degree of molecular orientation parallel to the substrate compared with that of prototypical 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) due to the larger π-conjugated benzotrithiophene core. However, TPBTT films showed a lower SOP of +54.4 mV/nm than did the TPBi film (+77.3 mV/nm), indicating that the molecular orientation alone did not determine the SOP. In contrast, m-ethyl-TPBTT showed a larger SOP of +104.0 mV/nm in the film. Quantum chemical calculations based on density functional theory suggested that the differences in the stable molecular conformation and the permanent dipole moments between TPBTT and m-ethyl-TPBTT caused the differences in SOP. These results suggest that the simultaneous control of the orientational order and conformation of the molecules is important to achieving a large SOP in films.
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Affiliation(s)
- Wei-Chih Wang
- RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
- Department of Applied Chemistry, National Yang Ming Chiao Tung University, 1001 Daxue Road, Hsinchu 300093, Taiwan
- Center for Emergent Functional Matter Science, National Yang Ming Chiao Tung University, 1001 Daxue Road, Hsinchu 300093, Taiwan
| | - Kyohei Nakano
- RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
| | - Chain-Shu Hsu
- Department of Applied Chemistry, National Yang Ming Chiao Tung University, 1001 Daxue Road, Hsinchu 300093, Taiwan
- Center for Emergent Functional Matter Science, National Yang Ming Chiao Tung University, 1001 Daxue Road, Hsinchu 300093, Taiwan
| | - Keisuke Tajima
- RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
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4
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He S, Pakhomenko E, Holmes RJ. Process Engineered Spontaneous Orientation Polarization in Organic Light-Emitting Devices. ACS APPLIED MATERIALS & INTERFACES 2023; 15:1652-1660. [PMID: 36548807 DOI: 10.1021/acsami.2c17960] [Citation(s) in RCA: 4] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/17/2023]
Abstract
Polar molecules with appreciable permanent dipole moments (PDMs) are widely used as the electron transport layer (ETL) in organic light-emitting devices (OLEDs). When the PDMs spontaneously align, a macroscopic polarization field can be observed, a phenomenon known as spontaneous orientation polarization (SOP). The presence of SOP in the ETL induces considerable surface potential and charge accumulation that is capable of quenching excitons and reducing device efficiency. While prior work has shown that the degree of SOP is sensitive to film processing conditions, this work considers SOP formation by quantitatively treating the vapor-deposited film as a supercooled glass, in analogy to prior work on birefringence in organic thin films. Importantly, the impact of varying thin-film deposition rate and relative temperature is unified into a single framework, providing a useful tool to predict the SOP formation efficiency for a polar material, as well as in blends of polar materials. Finally, in situ photoluminescence characterization and efficiency measurements reveal that SOP-induced exciton-polaron quenching can be reduced through an appropriate choice of processing conditions, leading to enhanced OLED efficiency.
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Affiliation(s)
- Siliang He
- Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota55455, United States
| | - Evgeny Pakhomenko
- Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota55455, United States
| | - Russell J Holmes
- Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota55455, United States
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5
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Tanaka M, Auffray M, Nakanotani H, Adachi C. Spontaneous formation of metastable orientation with well-organized permanent dipole moment in organic glassy films. NATURE MATERIALS 2022; 21:819-825. [PMID: 35637340 DOI: 10.1038/s41563-022-01265-7] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/21/2021] [Accepted: 04/20/2022] [Indexed: 06/15/2023]
Abstract
The performance of organic optoelectronic and energy-harvesting devices is largely determined by the molecular orientation and resultant permanent dipole moment, yet this property is difficult to control during film preparation. Here, we demonstrate the active control of dipole direction-that is, vector direction and magnitude-in organic glassy films by physical vapour deposition. An organic glassy film with metastable permanent dipole moment orientation can be obtained by utilizing the small surface free energy of a trifluoromethyl unit and intramolecular permanent dipole moment induced by functional groups. The proposed molecular design rule could pave a way toward the formation of spontaneously polarized organic glassy films, leading to improvement in the performance of organic molecular devices.
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Affiliation(s)
- Masaki Tanaka
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Nishi-ku, Fukuoka, Japan.
- Department of Biotechnology and Life Science, Tokyo University of Agriculture and Technology, Tokyo, Japan.
| | - Morgan Auffray
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Nishi-ku, Fukuoka, Japan
| | - Hajime Nakanotani
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Nishi-ku, Fukuoka, Japan.
- International Institute for Carbon Neutral Energy Research (I2CNER), Kyushu University, Nishi-ku, Fukuoka, Japan.
| | - Chihaya Adachi
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Nishi-ku, Fukuoka, Japan.
- International Institute for Carbon Neutral Energy Research (I2CNER), Kyushu University, Nishi-ku, Fukuoka, Japan.
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6
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Tenopala-Carmona F, Lee OS, Crovini E, Neferu AM, Murawski C, Olivier Y, Zysman-Colman E, Gather MC. Identification of the Key Parameters for Horizontal Transition Dipole Orientation in Fluorescent and TADF Organic Light-Emitting Diodes. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2021; 33:e2100677. [PMID: 34338351 DOI: 10.1002/adma.202100677] [Citation(s) in RCA: 38] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/26/2021] [Revised: 04/21/2021] [Indexed: 06/13/2023]
Abstract
In organic light-emitting diodes (OLEDs), horizontal orientation of the emissive transition dipole moment (TDM) can improve light outcoupling efficiency by up to 50% relative to random orientation. Therefore, there have been extensive efforts to identify drivers of horizontal orientation. The aspect ratio of the emitter molecule and the glass-transition temperature (Tg ) of the films are currently regarded as particularly important. However, there remains a paucity of systematic studies that establish the extent to which these and other parameters control orientation in the wide range of emitter systems relevant for state-of-the-art OLEDs. Here, recent work on molecular orientation of fluorescent and thermally activated delayed fluorescent emitters in vacuum-processed OLEDs is reviewed. Additionally, to identify parameters linked to TDM orientation, a meta-analysis of 203 published emitter systems is conducted and combined with density-functional theory calculations. Molecular weight (MW) and linearity are identified as key parameters in neat systems. In host-guest systems with low-MW emitters, orientation is mostly influenced by the host Tg , whereas the length and MW of the emitter become more relevant for systems involving higher-MW emitters. To close, a perspective of where the field must advance to establish a comprehensive model of molecular orientation is given.
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Affiliation(s)
- Francisco Tenopala-Carmona
- Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK
- Humboldt Centre for Nano- and Biophotonics, Department of Chemistry, University of Cologne, Greinstr. 4-6, 50939, Köln, Germany
| | - Oliver S Lee
- Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK
- Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, UK
| | - Ettore Crovini
- Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, UK
| | - Ana M Neferu
- Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK
- Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, UK
| | - Caroline Murawski
- Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK
| | - Yoann Olivier
- Unité de Chimie Physique Théorique et Structurale & Laboratoire de Physique du Solide, Namur Institute of Structured Matter, Université de Namur, Rue de Bruxelles, 61, Namur, 5000, Belgium
| | - Eli Zysman-Colman
- Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, UK
| | - Malte C Gather
- Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK
- Humboldt Centre for Nano- and Biophotonics, Department of Chemistry, University of Cologne, Greinstr. 4-6, 50939, Köln, Germany
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7
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Gunawardana HDCN, Osada K, Koswattage KR, Noguchi Y. Enhancement of the molecular orientation of TPBi in coevaporated films of UGH‐2 host molecules. SURF INTERFACE ANAL 2021. [DOI: 10.1002/sia.6933] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Affiliation(s)
| | - Kohei Osada
- School of Science & Technology Meiji University Kawasaki Japan
| | | | - Yutaka Noguchi
- School of Science & Technology Meiji University Kawasaki Japan
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8
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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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9
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Bangsund JS, Van Sambeek JR, Concannon NM, Holmes RJ. Sub-turn-on exciton quenching due to molecular orientation and polarization in organic light-emitting devices. SCIENCE ADVANCES 2020; 6:eabb2659. [PMID: 32821834 PMCID: PMC7413727 DOI: 10.1126/sciadv.abb2659] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/13/2020] [Accepted: 06/25/2020] [Indexed: 05/20/2023]
Abstract
The efficiency of organic light-emitting devices (OLEDs) is often limited by roll-off, where efficiency decreases with increasing bias. In most OLEDs, roll-off primarily occurs due to exciton quenching, which is commonly assumed to be active only above device turn-on. Below turn-on, exciton and charge carrier densities are often presumed to be too small to cause quenching. Using lock-in detection of photoluminescence, we find that this assumption is not generally valid; luminescence can be quenched by >20% at biases below turn-on. We show that this low-bias quenching is due to hole accumulation induced by intrinsic polarization of the electron transport layer (ETL). Further, we demonstrate that selection of nonpolar ETLs or heating during deposition minimizes these losses, leading to efficiency enhancements of >15%. These results reveal design rules to optimize efficiency, clarify how ultrastable glasses improve OLED performance, and demonstrate the importance of quantifying exciton quenching at low bias.
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10
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Tanaka Y, Matsuura N, Ishii H. Self-Assembled Electret for Vibration-Based Power Generator. Sci Rep 2020; 10:6648. [PMID: 32313082 PMCID: PMC7170925 DOI: 10.1038/s41598-020-63484-9] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2019] [Accepted: 04/01/2020] [Indexed: 11/11/2022] Open
Abstract
The vibration-based electret generators (EGs) for energy harvesting have been extensively studied because they can obtain electrical energy from ambient vibrations. EGs exhibit a sandwich structure of electrodes surrounding an air gap and an electret, which is a dielectric material with a quasi-permanent electrical charge or dipole polarisation. Various charging processes have been developed because the surface charge density (σ) of the electret determines the output power of the device. However, such processes are considered to constitute a key productivity-limiting factor from the mass production viewpoint, making their simplification or elimination a highly desired objective. Herein, a model EG that does not require any charging process by utilising the spontaneous orientation polarisation of 1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene (TPBi) is demonstrated. The surface potential (Vsp) of an evaporated TPBi film has reached 30.2 V at a film thickness of 500 nm without using a charging process. The estimated σ of 1.7 mC m−2 is comparable with that obtained using a conventional polymer-based electret after charging. Furthermore, Vsp is considerably stable in environmental conditions; thus, TPBi can be considered to be “self-assembled” electret (SAE). Application of SAE leads to developing an EG without requiring the charging process.
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Affiliation(s)
- Yuya Tanaka
- Center for Frontier Science, Chiba University, Chiba, 263-8522, Japan. .,Graduate School of Science and Engineering, Chiba University, Chiba, 263-8522, Japan. .,Japan Science and Technology Agency, PRESTO, Saitama, 332-0012, Japan.
| | - Noritaka Matsuura
- Graduate School of Science and Engineering, Chiba University, Chiba, 263-8522, Japan
| | - Hisao Ishii
- Center for Frontier Science, Chiba University, Chiba, 263-8522, Japan.,Graduate School of Science and Engineering, Chiba University, Chiba, 263-8522, Japan.,Molecular Chirality Research Center, Chiba University, Chiba, 263-8522, Japan
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11
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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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12
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Su N, Yang HQ, Zheng YX, Chen ZX. Sulfur atom containing ligands induced rapid room temperature synthesis of red iridium(iii) complexes with Ir–S–P–S structures for OLEDs. NEW J CHEM 2019. [DOI: 10.1039/c9nj01599j] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Two red iridium(iii) complexes containing a four-membered ring Ir–S–P–S backbone were synthesized at room temperature in 5 min, and their OLEDs exhibit an EQEmax of 19.90%.
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Affiliation(s)
- Ning Su
- State Key Laboratory of Coordination Chemistry
- Jiangsu Key Laboratory of Advanced Organic Materials
- Collaborative Innovation Center of Advanced Microstructures
- School of Chemistry and Chemical Engineering
- Nanjing University
| | - Hui-Qing Yang
- State Key Laboratory of Coordination Chemistry
- Jiangsu Key Laboratory of Advanced Organic Materials
- Collaborative Innovation Center of Advanced Microstructures
- School of Chemistry and Chemical Engineering
- Nanjing University
| | - You-Xuan Zheng
- State Key Laboratory of Coordination Chemistry
- Jiangsu Key Laboratory of Advanced Organic Materials
- Collaborative Innovation Center of Advanced Microstructures
- School of Chemistry and Chemical Engineering
- Nanjing University
| | - Zhao-Xu Chen
- State Key Laboratory of Coordination Chemistry
- Jiangsu Key Laboratory of Advanced Organic Materials
- Collaborative Innovation Center of Advanced Microstructures
- School of Chemistry and Chemical Engineering
- Nanjing University
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13
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Babazadeh M, Burn PL, Huang DM. Calculating transition dipole moments of phosphorescent emitters for efficient organic light-emitting diodes. Phys Chem Chem Phys 2019; 21:9740-9746. [DOI: 10.1039/c9cp01045a] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Quantum-chemical calculations show that the direction of the transition dipole moment of organometallic phosphorescent emitters is sensitive to molecular geometry.
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Affiliation(s)
- Mohammad Babazadeh
- Centre for Organic Photonics & Electronics
- School of Chemistry and Molecular Biosciences
- The University of Queensland
- Australia
| | - Paul L. Burn
- Centre for Organic Photonics & Electronics
- School of Chemistry and Molecular Biosciences
- The University of Queensland
- Australia
| | - David M. Huang
- Department of Chemistry
- School of Physical Sciences
- The University of Adelaide
- Adelaide 5005
- Australia
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