Blue Phosphorescent Devices Research Articles

Highly efficient co-doped blue phosphorescent organic electroluminescent devices due to improved carriers balance and broadening recombination zone

High performance blue phosphorescent organic electroluminescent devices were realized by utilizing trivalent terbium complex and iridium complex as co-dopant and emitter, respectively. In this case, the co-doped trivalent terbium complex molecules within light-emitting layer function as deeper electron trappers due to its low-lying energy levels. Compared with reference device, co-doped devices displayed relatively higher electroluminescent performances due to improved carriers’ balance as well as broadening recombination zone. Finally, the optimal device with main emission peak at 462 nm obtained the maximum brightness, external quantum efficiency, current efficiency and power efficiency up to 24396 cd/m2, 26.3%, 46.93 cd/A and 50.82 lm/W, respectively. Meanwhile, even at the practical brightness of 1000 cd/m2 (4.3 V), prominent external quantum efficiency and current efficiency as high as 24.0% and 40.73 cd/A, respectively, can still be retained because the broadening recombination zone helps to decrease exciton density thus suppress exciton quenching.

Unlocking Structurally Nontraditional Naphthyridine-Based Electron-Transporting Materials with C-H Activation-Annulation.

The inherent benefits of C-H activation have given rise to innovative approaches in designing organic optoelectronic molecules that depart from conventional methods. While theoretical calculations have suggested the suitability of the 2,6-naphthyridine scaffold for electron transport materials (ETMs) in organic light-emitting diodes (OLEDs), the existing synthetic methodologies have proven to be insufficient for the construction of multiple arylated and fully aryl-substituted molecules. Herein, we present a solution for the synthesis of 2,6-naphthyridine derivatives, with the rhodium-catalyzed consecutive C-H activation-annulation process of fumaric acid with alkynes standing as the pivotal step within this strategy. The ETMs, purposefully designed and synthesized based on the 2,6-naphthyridine framework, exhibit an impressively high glass-transition temperature (Tg) of 282 °C and high electron mobility (μe), setting a new benchmark for ETMs in OLEDs with a μe exceeding 10-2 cm2 V-1 s-1. These materials prove to be versatile ETM candidates suitable for red, green, and blue phosphorescent OLED devices.

Management of Exciton Distribution for High-Performance Organic Light-Emitting Diodes Based on Interfacial Exciplex Architecture.

Interfacial exciplex has recently been adopted as an effective host to achieve phosphorescent organic light-emitting diodes (OLEDs) with high efficiencies and low driving voltages. However, a systematic understanding of exciton recombination behavior in either host of interfacial exciplex is still deficient. Herein, the strategic design rule of interfacial exciplex host is proposed to overcome the negative effects of direct trapping recombination by systematically investigating exciton recombination behavior in interfacial exciplex hosts. As a result, blue and orange phosphorescent devices acquire peak external quantum efficiencies of 23.5% and 29.2% with low turn-on voltages. These results provide a simple method to realize highly efficient OLEDs aiming for general lighting and display applications.

C1,C8-modified carbazole-based bipolar host materials for blue phosphorescent electroluminescent devices

Herein, we report a new molecular design based on carbazole for the development of high triplet energy bipolar host materials for blue phosphorescent organic light emitting diodes (PhOLEDs). Two new isomeric host materials named, 11CzCzCN and 14CzCzCN were designed and synthesized by modifying the carbazole core with carbazole donor at C1 and cyano acceptor at C8-position. The effective interchromophoric electronic communications between the attached donors and central carbazole were interrupted by virtue of large dihedral angle at C1 and short-axis linkage. Also, the strong charge transfer interactions between the donor at C1 and acceptor at C8 was hampered due to the broken conjugation. As a result, the compounds showed high triplet energy (ET) over 2.75 eV without compromising the bipolar charge transporting property. Interestingly, computations and single carrier device analysis evidenced good bipolar charge transporting ability for 11CzCzCN over its structural isomer 14CzCzCN. The applicability of these compounds as hosts for blue PhOLEDs was explored by using a well-known Flrpic dopant. The 11CzCzCN hosted device showed higher performance with maximum EQE of 20.6%, current efficiency of 39.6 cd/A and efficiency roll off of 8.2% at 1000 cd/m2 over its analogue 14CzCzCN hosted device attributed to its high ET and balanced charge transporting property. We believe that this study would provide valuable insights for executing new molecular designs on carbazole for the development of efficient host materials for blue PhOLEDs.

Symmetric “Double Spiro” Wide Energy Gap Hosts for Blue Phosphorescent OLED Devices

AbstractWide energy gap materials dispiro[fluorene‐9,9′‐anthracene‐10′,9″‐fluorene] (SAS) and dispiro[xanthene‐9,9′‐anthracene‐10′,9″‐xanthene] (XAX) containing double spiro–carbons, are introduced as hosts for blue phosphorescent organic light‐emitting diodes (PHOLEDs). Both SAS and XAX are free of heteroatomic exocyclic bonds, which are implicated in limiting the stability of blue PHOLEDs. The materials are synthesized in gram‐scale quantities through short and efficient paths. They have large energy gaps (≥5.0 eV) between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and correspondingly have high triplet energies in solid state (ET ≈ 3.0 eV). Analysis of devices using SAS and XAX as host materials with the blue phosphorescent dopant fac‐tris(N,N‐di‐p‐tolyl‐pyrizinoimidazol‐2‐yl)iridium(III) (Ir(tpz)3), shows that charges are transported and trapped by the dopant, which subsequently forms excitons directly on the phosphor. As a result, luminescence quenching pathways are suppressed which leads to blue phosphorescent devices with high (≈18%) external quantum efficiency. Thus, SAS and XAX serve as promising host materials, with high triplet energies suitable for blue PHOLEDs.

Rational Management of Charge Transfer Character of Benzothienopyrimidine‐Based N‐Type Host for Blue Phosphorescent Organic Light‐Emitting Diodes

AbstractTwo materials, 3‐(4‐(9H‐carbazol‐9‐yl)benzo[4,5]thieno[3,2‐d]pyrimidin‐2‐yl)‐5‐(triphenylsilyl)benzonitrile (CzBTPCNmSi) and 9‐(2‐(3‐(triphenylsilyl)phenyl)benzo[4,5]thieno[3,2‐d]pyrimidin‐4‐yl)‐9H‐carbazole‐3‐carbonitrile (CNCzBTPmSi), are synthesized as the N‐type host (electron‐transport type host) of a mixed host, harvesting triplet excitons of the blue phosphor by engineering the charge transfer character using a CN unit. The two host materials are based on benzothienopyrmidine modified using CN, tetraphenylsilyl, and carbazole groups to control the lowest unoccupied molecular orbital level, manage their intermolecular packing, and stabilize the molecule against holes, respectively. The two hosts are mixed with a P‐type 3,3‐di(9H‐carbazol‐9‐yl)biphenyl (mCBP) to offer a mixed host for sky and deep blue‐emitting phosphors. In the device applications, CzBTPCNmSi and CNCzBTPmSi show high external quantum efficiency (EQE) (around 24% in the sky‐blue devices), whereas only CNCzBTPmSi exhibited high EQE (above 20% in the deep blue devices) due to high triplet energy. Moreover, the CNCzBTPmSi demonstrates a longer device lifetime than the control host without the CN unit. Thus, CN engineering can manage the triplet energy, EQE, and device lifetime of blue phosphorescent devices by controlling the charge transfer character.

Cyanocarbazole-substituted di(9H-carbazol-9-yl)-1,1'-biphenyl derivatives for efficient blue phosphorescent host materials for organic light emitting diodes

To develop the efficient blue phosphorescent host materials for Organic light-emitting Diodes (OLEDs), cyanocarbazole-substituted 3',5'-di(9H-carbazol-9-yl)-1,1'-biphenyl derivatives are designed and synthesized. Multilayer devices were fabricated in the following sequence; ITO (180 nm)/Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) (75 nm)/Blue phosphorescent host materials (15 nm)/Bis[2-(4,6-difluorophenyl)pyridinato-C 2,N](picolinato)iridium(III) (Firpic) (15 nm)/1,3,5-Tri(m-pyridine-3-ylphenyl)benzene (TmPyPB) (40 nm)/Liq (2 nm)/Al (100 nm). In particularly, blue phosphorescent device using 9-(3',5'-di(9H-carbazol-9-yl)-[1,1'-biphenyl]-4-yl)-9H-carbazole-3-carbonitrile as a host material exhibited efficient emission with a luminous efficiency, a power efficiency, an external quantum efficiency, and CIE coordinates of 11.6 cd/A, 4.10 lm/W, 5.62% at 20 mA/cm2, and (0.15, 0.33) at 8 V, respectively.

Highly efficient tandem PHOLEDs with lithium-doped BPhen/NDP-9-doped TAPC as a charge generation layer

The development of large-area organic light-emitting diode (OLED) displays requires a highly efficient tandem device architecture and an easily processable charge generation layer (CGL) with a low voltage drop and high optical transparency. In this study, we investigated and applied a doped organic n-CGL/p-CGL using thermal vacuum deposition in tandem OLED devices. A doping concentration of 1.0 wt.% for Li in 4, 7-Diphenyl-1, 10-phenanthroline (BPhen) was optimal for the n-CGL with 8 wt.% for 2-(7-dicyanomethylene-1,3,4,5,6,8,9,10-octafluoro-7H-pyrene-2-ylidene)-malononitrile (NDP-9)-doped N,N-bis(4-methylphenyl)benzenamine (TAPC) as a p-CGL. Maximum luminous efficiencies of 42.5 and 63.4 cd/A and a 4,000 cd/m2 current density for the target luminance values of 11.2 and 6.5 mA/cm2 were demonstrated for double-stack and triple-stack tandem blue phosphorescent OLED devices, respectively. Implementing these highly efficient tandem device structures will improve the overall lifetime of OLED displays by lowering their operating current density at the target luminance.

Effects of Substitution Position of Carbazole-Dibenzofuran Based High Triplet Energy Hosts to Device Stability of Blue Phosphorescent Organic Light-Emitting Diodes.

High triplet energy hosts were developed through the modification of the substitution position of carbazole units. Two carbazole-dibenzofuran-derived compounds, 9,9′-(dibenzo[b,d]furan-2,6-diyl)bis(9H-carbazole) (26CzDBF) and 4,6-di(9H-carbazol-9-yl)dibenzo[b,d]furan (46CzDBF), were synthesized for achieving high triplet energy hosts. In comparison with the reported hole transport type host, 2,8-di(9H-carbazol-9-yl)dibenzo[b,d]furan (28CzDBF), 26CzDBF and 46CzDBF maintained high triplet energy over 2.95 eV. The device performances of the hosts were evaluated with electron transport type host, 2-phenyl-4, 6-bis(3-(triphenylsilyl)phenyl)-1,3,5-triazine (mSiTrz), to comprise a mixed host system. The deep blue phosphorescent device of 26CzDBF:mSiTrz with [[5-(1,1-dimethylethyl)-3-phenyl-1H-imidazo[4,5-b]pyrazin-1-yl-2(3H)-ylidene]-1,2-phenylene]bis[[6-(1,1-dimethylethyl)-3-phenyl-1H-imidazo[4,5-b]pyrazin-1-yl-2(3H)-ylidene]-1,2-phenylene]iridium (Ir(cb)3) dopant exhibited high external quantum efficiency of 22.9% with a color coordinate of (0.14, 0.16) and device lifetime of 1400 h at 100 cd m−2. The device lifetime was extended by 75% compared to the device lifetime of 28CzDBF:mSiTrz (800 h). These results demonstrated that the asymmetric and symmetric substitution of carbazole can make differences in the device performance of the carbazole- and dibenzofuran- derived hosts.

Efficiency enhancement in blue phosphorescent organic light emitting diode with silver nanoparticles prepared by plasma-assisted hot-filament evaporation as an external light-extraction layer

The uniform growth of silver (Ag) nanoparticles in large areas is important for improving the flat panel devices, mainly organic light emitting diodes (OLED). In this work, two dimensional (2D) arrays of Ag nanoparticles were grown on glass surface substrates via plasma-assisted hot-filament evaporation technique at different plasma environments: no plasma, plasma cleaning (PC), plasma deposition (PD) and PC&PD, which were utilized as an external light-extraction layer for blue phosphorescent OLED device. The surface morphology of the Ag nanoparticles revealed the formation of spherical-like shapes of uniform size which ranged from 7 to 14 nm at different plasma conditions. X-ray diffraction showed that the nanoparticles were mainly single crystals and their crystallinity was significantly enhanced by employing plasma process in PC&PD. The localized surface plasmon resonance peaks of Ag nanoparticles were located at around 380–430 nm which suggests a dependency on diameter and interparticle spacing. The variation of refractive index of Ag nanoparticles 2D array in the visible wavelength from 1.9 to 2.4 plays a key role in high performance OLED. The light-extraction of the OLED device with Ag nanoparticles at PC&PD condition appeared to increase by 82.5% the luminance efficiency compared to that of a reference device without the Ag nanoparticle layer. The effects of the plasma environment on the morphological, structural and optical properties of Ag nanoparticles layer are discussed in the following sections by taking into consideration the effects of glass modification by these Ag nanoparticles on OLED performance.

Comprehensive Model of the Degradation of Organic Light-Emitting Diodes and Application for Efficient, Stable Blue Phosphorescent Devices with Reduced Influence of Polarons

We present a comprehensive model to analyze, quantitatively, and predict the process of degradation of organic light-emitting diodes (OLEDs) considering all possible degradation mechanisms, i.e., polaron, exciton, exciton-polaron interactions, exciton-exciton interactions, and a newly proposed impurity effect. The loss of efficiency during degradation is presented as a function of quencher density, the density and generation mechanisms of which were extracted using a voltage rise model. The comprehensive model was applied to stable blue phosphorescent OLEDs (PhOLEDs), and the results showed that the model described the voltage rise and external quantum efficiency (EQE) loss very well, and that the quenchers in emitting layer (EML) were mainly generated by dopant polarons. Quencher formation was confirmed from a mass spectrometry. The polaron density per dopant molecule in EML was reduced by controlling the emitter doping ratio, resulting in the highest reported LT50 of 431 hours at an initial brightness of 500 cd/m2 with CIEy<0.25 and high external quantum efficiency (EQE) >18%.

6‐4: Late‐News Paper: Realizing Deep Blue Emission in Blue Phosphorescent Organic Light‐Emitting Diodes

The origin of spectrum broadening in blue phosphorescent organic light‐emitting diodes (OLEDs) was investigated in order to improve color purity of the OLED device aiming to realize efficient blue phosphorescent OLED with deep blue emission. Assuming the formation of exciplex between host and guest (H‐G) material from intermolecular charge transfer (CT) state of the two, the experiment was performed controlling the distance between H‐G for the verification. The exciplex formation was observed dependent on the distance of H‐G, indicating that the spectrum broadening occurred due to H‐G CT state. The critical distance to avoid H‐G CT state was found which can be further utilized in designing host and dopant materials for blue phosphorescent OLEDs. The EL efficiency and the stability of the blue phosphorescent device were enhanced by 1.2 and 5 times, respectively, by minimizing H‐G CT state.

Suppression of device degradation mechanism by triphenylsilyl group substitution of the host for blue phosphorescent organic light-emitting diodes

The effect of triphenylsilyl group substitution in the host materials for blue phosphorescent organic light-emitting diodes was studied by synthesizing two hole transport type hosts with and without the triphenylsilyl group in the identical backbone structure. The two host materials were applied as the hosts of blue phosphorescent emitter and the comparison of the emission behavior of the two hosts revealed that the triphenylsilyl group suppressed triplet exciton quenching of the blue phosphorescent emitters. As a result, the blue host with the triphenylsilyl group showed improved device efficiency and extended device lifetime in the blue phosphorescent devices. The blue phosphorescent device with the triphenylsilyl group substituted host showed improved external quantum efficiency and elongated device lifetime compared to the triphenylsilyl free host based device. The bulky triphenylsilyl group was effective to extend the device lifetime of the blue phosphorescent device through reduced triplet exciton induced annihilation.

Soluble host materials with ortho-phenylene group for blue phosphorescent devices

Soluble host materials with ortho-phenylene group for blue phosphorescent devices

Soluble host materials with ortho-phenylene group for blue phosphorescent devices

Soluble host materials with ortho-phenylene group for blue phosphorescent devices

An effective thermally activated delayed fluorescence host material for highly efficient blue phosphorescent organic light-emitting diodes with low doping concentration

A thermally activated delayed fluorescence (TADF) material BCz-2SO has been designed and synthesized as host for blue phosphorescent organic light-emitting diodes (OLEDs). Photophysical studies and theoretical calculations show that the molecule has a small singlet-triplet energy gap (ΔEST) of 0.345 eV, which is beneficial to the reverse energy transferring between the singlet and triplet state. Thanks to the TADF property, the triplet energy can be transmitted to the singlet state through reverse intersystem crossing (RISC), and then transmitted to the guest through the Förster energy transfer (FET) to achieve 100% utilization of energy. Thus, the triplet–triplet annihilation (TTA) of the blue phosphor can be avoided by the extremely low doping concentration of 1%. By using BCz-2SO as the host of FIrpic, the solution-processed blue phosphorescent device achieves the maximum current efficiency (CE), power efficiency (PE), external quantum efficiency (EQE) and highest brightness of 16.38 cd A−1, 9.04 lm W−1, 7.8% and 16,537 cd m-2, respectively. It demonstrates that one can employ the solution-processed method to prepare the high performance phosphorescent OLEDs using the TADF host material we have developed.

Improved carrier injection and balance in solution-processed blue phosphorescent organic light emitting diodes based on mixed host system and their transient electroluminescence

Solution processed blue phosphorescent organic devices based on mixed host system were fabricated in this paper. A set of blue phosphorescent organic light emitting diodes (PhOLEDs) utilizing two small molecular materials of 3,3-Di(9H-carbazol-9-yl) biphenyl (mCBP) and 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) as mixed host doped with phosphor bis[(4,6-difluorophenyl)-pyridina-to-N,C2](picolinate) iridium(III) (FIrpic) are fabricated. The results show that the performance of mixed host system devices is improved compared with two single host devices. The max luminance of 23,340 cd/m2 is achieved in the mCBP:TCTA (2:1) mixed host device while the 14,400 cd/m2 in mCBP single host devices and the 12,968 cd/m2 in TCTA, and the current efficiency is improved from 8.17 cd/A(mCBP), 9.61 cd/A (TCTA) to 13.62 cd/A at the luminance of 1000 cd/m2. The transient electroluminescence measurement was utilized to detect the carrier injection between two kinds of PhOLEDs and penetrate how the trap charges works in the mixed host devices. This presents that the mixed host system contributes to the hole injection and the equilibrium of the carrier transport, reduces trapped charges, thus brings about high efficiency devices.

Design approach of exciplexes enhancing the singlet and triplet energy by managing electron transport type host

Abstract A high singlet energy and triplet energy exciplex host was developed by synthesizing an electron transport type host with a controlled lowest unoccupied molecular orbital (LUMO) level and intermolecular interaction. A diphenyltriazine derived host was designed by substituting t-butyl functional moieties to the electron transporting core structure to make the LUMO shallow for high exciplex emission energy. An exciplex host with a singlet energy of 3.49 eV and a triplet energy of 3.02 eV was derived by mixing the t-butyl modified diphenyltriazine type host with a hole transport type host. The comparison of the newly developed t-butyl modified host with a previous host without the t-butyl group clarified that the t-butyl group had a singlet and triplet energy boosting effect. A high external quantum efficiency of 17.3% was demonstrated in the blue phosphorescent device using the high triplet energy exciplex host.

P‐171: Modulation of Dibenzothiophene and Carbazole Moieties in Host Material towards High Performance Blue Phosphorescent OLEDs

A new host material was synthesized for blue phosphorescent organic light emitting diodes (PHOLEDs) by modulating carbazolyl‐carbazole and dibenzothiophene groups. In particular, by comparing the characteristics of the two hosts with and without a biphenyl linker, we studied the effects of the biphenyl linker on photophysical properties and on the device performances of a blue emitting phosphorescent dopant. As a result, the newly synthesized host materials were functioned as high efficiency and long lifetime host materials in blue phosphorescent devices and they prolonged life span of the blue devices by 5 times compared to a common mCBP blue host.

Efficient blue and green phosphorescent OLEDs with host material containing electronically isolated carbazolyl fragments

Dry process-able host materials are well suited to realize high performance phosphorescent organic light-emitting diodes (OLED) with precise deposition of organic layers. We demonstrate in this study high efficiency green and blue phosphorescent OLED devices by employing 3-[bis(9-ethylcarbazol-3-yl)methyl]-9-hexylcarbazole based host material. By doping a typical green emitter of fac tris(2-phenylpyridine)iridium (Ir (ppy)3) in the compound the resultant dry-processed green device exhibited superior performance with low turn on voltage of 3.0 V and with peak efficiencies of 11.4%, 39.9 cd/A and 41.8 lm/W. When blue emitter of bis [2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium (III) was used, the resultant blue device showed turn on voltage of 2.9 V and peak efficiencies of 9.4%, 21.4 cd/A and 21.7 lm/W. The high efficiencies may be attributed to the host possessing high triplet energy level, effective host-to-guest energy transfer and effective carrier injection balance.