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Inkjet printing a small-molecule binary emitting layer for organic light-emitting diodes
摘要： For inkjet-printed organic light-emitting diodes, it is critical to obtain a homogeneous small molecular-based emitting film. In contrast to the conventional strategy of utilizing mixed solvents, we have successfully inkjet printed a binary electrophosphorescent emitter with a single solvent. The binary emitter consists of a new host material of 3-(3,6-di-t-butylcarbazol-9-yl)-3'-(2-(4-t-butylphenyl)benzoimidazol-1-yl)-[1,1'-biphenyl], and an iridium complex dopant of bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)- iridium(III) (Ir(MDQ)2(acac)). Compatible with Ir(MDQ)2(acac), the newly developed host is highly soluble and amorphous with a glass transition temperature of 137 oC upon cooling from the melt. The inkjet printability and film morphology of the emitting layer are strongly dependent both on substrates and the solvents. With the o-dichlorobenzene (o-DCB) as the single solvent, and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4?-(N-(p-butylphenyl))diphenylamine)] (TFB) as the hole transporting layer, a uniform emitter film morphology without any coffee ring is obtained. The phosphorescent OLEDs with the inkjet-printed emitter yields a maximum current efficiency of 17.89 cd/A, comparable to that of the Ir(MDQ)2(acac)?based OLEDs with spin-cast emitter.
关键词： Dewetting,Coffee ring,Inkjet printing,Phosphorescent OLEDs,Small molecules
更新于2025-09-23 15:21:01
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A 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) transport layer with high electron mobility for thick organic light-emitting diodes
摘要： In our previous paper [T. Matsushima et al., Nature 572, 502 (2019)], current densities of organic light-emitting diodes (OLEDs) did not decrease significantly when the thicknesses of a 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) transport layer were increased from tens of nanometers to 1 μm. To make this mechanism clear, we carried out several experiments in terms of electron transfer with other organic layers and electron mobility of HAT-CN. Finally, we found that the vacuum-evaporated HAT-CN layers have very high electron mobility and, therefore, using a HAT-CN transport layer can suppress the decrease in current density even in thick OLEDs. The electron mobility of vacuum-deposited HAT-CN layers, which was measured using analysis with a space-charge-limited current model, was 0.1–1 cm2 V?1 s?1. This electron mobility is much higher than those of conventional organic transport layers used in OLEDs (<10?3 cm2 V?1 s?1) even though the HAT-CN layers are amorphous-like. We attributed one of the reasons for this extraordinarily high mobility to be a better overlap of π orbitals in the substrate normal, which is associated with horizontally oriented HAT-CN molecules on a substrate.
关键词： transport layer,thick organic light-emitting diodes,HAT-CN,OLEDs,electron mobility
更新于2025-09-23 15:21:01
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[SpringerBriefs in Applied Sciences and Technology] Air-Stable Inverted Organic Light-Emitting Diodes || Carrier Injection Mechanism
摘要： Both hole and electron injection layers are commonly used in recent OLEDs to reduce the injection barrier between electrodes and organic layers. This injection barrier originates from the energy difference between the work function (WF) of the electrode and the energy level of the organic layer. For instance, the hole injection barrier is defined as the energy difference between the Fermi level of the anode and the highest occupied molecular orbital (HOMO) level of the organic layer on the anode, as shown in Fig. 4.1a. Thus, an ideal hole injection material is the material that can make the surface WF of the anode larger (Fig. 4.1b). On the other hand, an ideal electron injection material is the material that can make the surface WF of the cathode smaller (Fig. 4.1c).
关键词： energy level alignment,OLEDs,electron injection,hole injection,work function
更新于2025-09-23 15:21:01
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The effects of chemical treatment on ITO properties and performance of OLED devices
摘要： In this work, chemical treatment was performed on the surfaces of two types of indium tin oxides (ITOs) with different physical properties. The effects of the chemical treatment on the physical parameters of the ITOs, and on the efficiency of the organic light emitting diodes (OLEDs) fabricated on the ITOs, were then examined. The ITO substrates were characterized via X-ray diffraction, atomic force microscopy, optical absorption, and electrical resistance measurement. The results showed that the chemical treatment exerted no significant changes on the structure and transparency of the wavelength of the light emitted from Alq3; however, the treatment improved the morphology and resistance of the ITOs examined. To serve the research purpose, the ITOs were first characterized, and then, the OLEDs were fabricated so that they would have a structure of Glass/ITO/PEDOT:PSS/Alq3/Al. The OLEDs were characterized current density, resistance, power, and irradiance as a function of the voltages. The results of the study showed decreases in the turn-on voltage (from 5.5 to 4 V for one of the OLEDs and 7 to 6 V for the other), resistance and power (for one of the ITOs), and irradiance (for one of the ITOs) due to the chemical treatment.
关键词： Chemical treatment,Organic light emitting diodes (OLEDs),ITO
更新于2025-09-23 15:21:01
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Carbazole/Benzimidazole-Based Bipolar Molecules as the Hosts for Phosphorescent and Thermally Activated Delayed Fluorescence Emitters for Efficient OLEDs
摘要： A series of carbazole/benzimidazole-based molecules, namely, o-CbzBiz, m-CbzBiz, and p-CbzBiz, were readily synthesized in three steps by integrating carbazole with benzimidazole via the ortho-, meta-, and para-positions of phenyl linked to N-phenyl carbazole. These bipolar molecules exhibited a maximum UV absorption band ranging from 310 to 327 nm and a maximum emission band ranging from 380 to 400 nm. Density functional theory calculations showed that the twist angles between the donor and acceptor moieties of these molecules were from 54.9 to 67.1°. Such a twisted structure hampered the π-electron conjugation within the molecule and resulted in high-lying LUMO levels and triplet energies, which make them suitable to be applied as host materials in OLED devices. Our results showed that a maximum external quantum efficiency (EQE) of OLED reached 21.8% when p-CbzBiz was applied as the host of a green phosphorescent emitter, i.e., Ir(ppy)2(acac). In addition, a maximum EQE of OLED reached 16.7% when o-CbzBiz with the host of a green TADF emitter, i.e., 4CzIPN. Moreover, these devices exhibited lower efficiency roll-off than the CBP-hosted device using the same emitters, which demonstrated the bipolar charge carrier property of carbazole/benzimidazole-based molecules.
关键词： thermally activated delayed fluorescence,bipolar molecules,phosphorescent,carbazole,OLEDs,benzimidazole
更新于2025-09-23 15:21:01
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Cyclometalated Ir(III) Complexes Towards Blue-Emissive Dopant for Organic Light-Emitting Diodes: Fundamentals of Photophysics and Designing Strategies
摘要： The main difficulties hindering development of a deep-blue phosphorescent cyclometalated Ir(III) complex are insufficient colour purity, i.e., failure to achieve ideal Commission Internationale de L’Eclairage (CIE) coordinates of (0.14, 0.09), and insufficient emission efficiency and stability. The latter problem is due to the highly energetic and hot excited states of these complexes, which yield faster decomposition. Therefore, control of the excited-state properties of cyclometalated Ir(III) complexes through systematic chemical modification of the ligands is being extensively investigated, with the aim of developing efficient and stable blue phosphorescent materials. The most common strategies towards achievement of a blue phosphorescent cyclometalated Ir(III) complex involve 1) substitution of electron-withdrawing F atoms at the cyclometalating ligands that stabilise the HOMO orbitals and 2) use of a heteropeltic system with electron-rich ancillary ligands bearing a 5-membered ring heterocycle to increase the LUMO energy level. However, the C–F bonds on the cyclometalating ligands have been found to be inherently unstable during device operation; thus, other types of electron-withdrawing groups (e.g., the cyano, trifluoromethyl, and sulfonyl groups) have been applied. Along with phosphorescence colour tuning to blue, the influence of the ligand structure on the photoluminescence quantum yield (PLQY) is also being intensively investigated. Two major PLQY lowering mechanisms for blue emissive Ir(III) complexes have been identified: 1) the vibronic-coupled non-radiative decay process and 2) crossing from the emissive state to an upper non-emissive 3MC excited state. To enhance the PLQY, mechanism 1) can be suppressed by employing rigid ligand frameworks to restrict intramolecular motion, whereas mechanism 2) can be prevented by destabilising the 3MC state using strong σ donor ligands such as N-heterocyclic carbenes. This review summarises the fundamental photophysics of cyclometalated Ir(III) complexes and surveys design strategies for efficient blue phosphorescent Ir(III) complexes, to provide a guide for future research in this field.
关键词： blue phosphorescent,3MC excited state,photoluminescence quantum yield,non-radiative decay,N-heterocyclic carbenes,cyclometalated Ir(III) complexes,OLEDs
更新于2025-09-23 15:21:01
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High External Quantum Efficiency in Fluorescent OLED by Cascade Singlet Harvesting Mechanism
摘要： The cascade singlet harvesting (CSH) organic light-emitting diodes (OLEDs) are devised to resolve the low quantum efficiency issue of fluorescent OLEDs by efficient singlet exciton harvesting of the fluorescent emitters. The CSH mechanism is realized by doping a fluorescent emitter in the singlet exciton harvesting matrix consisted of high energy exciplex and low energy exciplex. The high energy exciplex serves as the main component of the emitting layer and the low energy exciplex is a medium harvesting the singlet excitons of the fluorescent emitter. Both exciplexes are thermally activated delayed fluorescence type exciplexes to effectively harvest singlet excitons by reverse intersystem crossing process. The singlet excitons of the low energy exciplex are harvested by the high energy exciplex through F?rster energy transfer and then the singlet excitons of the fluorescent emitter are harvested by the low energy exciplex through the second F?rster energy transfer process. The CSH mechanism maximizes the singlet exciton formation in the fluorescent emitter, which significantly enhances the external quantum efficiency (EQE) of the fluorescent OLEDs. The optimization of the emitting layer structure provides high EQE of 19.9% in the fluorescent OLEDs compared with 10.4% of a conventional singlet harvesting fluorescent OLED.
关键词： energy transfer,external quantum efficiency,fluorescent OLEDs,thermally activated delayed fluorescence,cascade singlet harvesting,exciplex
更新于2025-09-23 15:19:57
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Novel adamantane-bridged phenanthroimidazole molecule for highly efficient full-color organic light-emitting diodes
摘要： Highly efficient deep blue phenanthroimidazole fluorescent material by incorporating an adamantane moiety (AD-BPI) is designed and synthesized. AD-BPI exhibits excellent photophysical properties including a deep blue emission, high triplet energy level and photoluminescence efficiency, fine morphological and thermal stability, and ambipolar nature. Non-doped deep blue organic light-emitting diodes (OLEDs) following the construction of ITO (Indium–Tin Oxide)/TAPC (Bis [4-[N, N-di (4-tolyl) amino] phenyl]-cyclohexane, 30 nm)/AD-BPI (100 nm)/TPBi (1, 3, 5-tris (2-N-phenylbenzimidazolyl) benzene, 50 nm)/Liq (8-hydroxyquinolatolithium, 2 nm)/Al (100 nm) utilizing AD-BPI as emissive layer achieve the peak external quantum efficiency (EQE) of 5.8% with the CIE coordinates of (0.15, 0.07). Using AD-BPI as universal host, the corresponding green, yellow and red phosphorescent devices are also fabricated and exhibit the maximum EQEs of 23.3%, 16.7% and 19.1% accompanying with negligible efficiency roll-off under ultrahigh luminance. These experimental performances are intensely competitive with the recently reported advanced results of full-color OLEDs. It is the first demonstration of adamantane-based phenanthroimidazole molecule featuring as both host and emitter.
关键词： Adamantane,Host,Deep blue,OLEDs,Phenanthroimidazole
更新于2025-09-23 15:19:57
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Efficient non-doped blue fluorescent OLEDs based on bipolar phenanthroimidazole-triphenylamine derivatives
摘要： Blue organic emissive materials are still the most important bottlenecks for the development of organic light-emitting diodes (OLEDs). To enrich the material library, herin, three bipolar phenanthroimidazole derivatives, namely MePPIM-TPA, ClPPIM- TPA and BuPPIM-TPA, are synthesized using triphenylamine as electron-donor and phenanthroimidazole as the electron-acceptor. The photophysical, thermal and electrochemical properties of three compounds are investigated with high decomposition temperature up to 350 °C, and strong blue emission. Single-carrier devices are fabricated to show that three compounds have good bipolar carrier transport characteristic. The non-doped fluorescent OLEDs devices using three compounds as emitting layers are fabricated among which the devices based on MePPIM-TPA achieved the maximum luminance of 1743 cd/m2, the maximum external quantum efficiency (EQE) of 2.99% which is relatively comparable to commonly used blue emitters.
关键词： Non-doped OLEDs,Bipolar characteristic,Triphenylamine,Fluorescent materials,Phenanthroimidazole
更新于2025-09-23 15:19:57
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Design of ultrathin OLEDs having oxide-based transparent electrodes and encapsulation with sub-mm bending radius
摘要： Highly flexible ultrathin organic light emitting diodes (OLEDs) hold vast potential as light sources particularly for wearable and imperceptible electronics. Most of the work demonstrated to date for highly flexible OLEDs, however, has relied on non-conventional transparent conductors such as conjugated polymers and has had no proper encapsulation or, if any, simple polymer-based encapsulation. We here demonstrate OLEDs that can be bent at a sub-mm radius even with conventional transparent conductive oxides (TCOs) and full encapsulation based on a multilayer gas barrier containing aluminum oxides, both of which are prone to strain-induced fracture. We realize such a small bending radius not only by adopting ultrathin substrates but also by exploiting the beneficial neutral plane shift toward the top of substrates identified in a system consisting of a ultrathin substrate and a multilayer device structure on its top. The proposed OLEDs exhibit stable performance after 1,000 bending iterations even at a bending radius far smaller than 1 mm and show similar reliability to that of glass-based reference devices even after two weeks in the acceleration test chamber.
关键词： Transparent conductive oxides,Ultrathin OLEDs,Multilayer encapsulation,Crack-onset strain,Neutral plane shift
更新于2025-09-23 15:19:57