Carbazole-based Host Materials Research Articles

Fundamental guidelines for the active control of the molecular orientation of heteroleptic iridium complexes enabled by carbazole-based host materials

The transition dipole moments of a heteroleptic iridium complex are actively controlled by using cyano-containing carbazole-based hosts. The orientation ratio (Θ) is improved from 65% to 78%. The Θ value is correlated with the Tg value of host material.

Phenylpyridine and carbazole based host materials for highly efficient blue TADF OLEDs

In this work, two host materials for blue TADF (thermally activated delayed fluorescence) OLEDs (organic light-emitting diodes) were designed and synthesized for improving the lifetime by incorporating pyridine core carbazole connected at different liking positions. These materials exhibit good thermal stability, high bond dissociation energy (BDE), and longer lifetime than the reference host. In particular, a blue TADF OLEDs system consisting of 4N-oCBP (9-(3-(2-(9 H -carbazol-9-yl)phenyl)pyridin-4-yl)-9 H -carbazole host and TCz-Trz (2,4,6-tris(2-(9 H -carbazol-9-yl)phenyl)-1,3,5-triazine) dopant exhibited the best device performances with a maximum external quantum efficiency (EQE) of 16.2% and an improvement of 88% in operational lifetime (initial luminance of 200 cd/m 2 ). This improved lifetime could be explained by the C–N BDE in the host molecular structure. The carbazole-based host molecule with different linking positions to the pyridine core developed in this study has considerable potential for improving the efficiency and lifetime of the TADF OLEDs device. • Two host materials for blue TADF OLEDs were designed and synthesized for improving the lifetime by incorporating pyridine core carbazole connected at different liking positions. • This improved lifetime could be explained by the C–N BDE in the host molecular structure.

Molecular Orientations of Delayed Fluorescent Emitters in a Series of Carbazole-Based Host Materials.

Molecular orientation is one of the most crucial factors to boost the efficiency of organic light-emitting devices. However, active control of molecular orientation of the emitter molecule by the host molecule is rarely realized so far, and the underlying mechanism is under discussion. Here, we systematically investigated the molecular orientations of thermally activated delayed fluorescence (TADF) emitters in a series of carbazole-based host materials. Enhanced horizontal orientation of the TADF emitters was achieved. The degree of enhancement observed was dependent on the host material used. Consequently, our results indicate that π-π stacking, CH/n (n = O, N) weak hydrogen bonds, and multiple CH/π contacts greatly induce horizontal orientation of the TADF emitters in addition to the molecular shape anisotropy. Finally, we fabricated TADF-based organic light-emitting devices with an external quantum efficiency (ηext) of 26% using an emission layer with horizontal orientation ratio (Θ) of 79%, which is higher than that of an almost randomly oriented emission layer with Θ of 62% (ηext = 22%).

Novel aromatic extended carbazoles as a chemical platform of bipolar hosts for improved lifetime in phosphorescent organic light-emitting diodes

Novel aromatic extended carbazoles, 9H-fluoreno[9,1-bc]carbazole, 9H-dibenzo[a,c]carbazole, and 15H-phenanthro[9,10-a]carbazole, were developed as donor moieties constructing bipolar host materials for phosphorescent organic light-emitting diodes. The 9H-fluoreno[9,1-bc]carbazole, 9H-dibenzo[a,c]carbazole, and 15H-phenanthro[9,10-a]carbazole implemented hosts showed high glass transition temperature, bipolar charge transport character, and triplet energy for energy transfer to red phosphorescent emitters. The hosts built on the aromatic extended carbazoles and quinazoline greatly improved the lifetime of red phosphorescent organic light-emitting diodes while enhancing the quantum efficiency. The material characterization data and device analysis results confirmed that the aromatic extended carbazole based host materials are effective to extend the lifetime of the red phosphorescent devices by thermal stability and polaron stability.

Dimethyl modified terphenyl core based compounds as hosts of blue phosphorescent emitters

High triplet energy materials were derived from dimethyl modified terphenyl core structure in order to apply them as the host materials of blue-emitting phosphors. The dimethyl modification was effective to increase the triplet energy of the terphenyl and carbazole based host materials and enabled application of the host materials in blue phosphorescent organic light-emitting diodes. Four hosts based on the dimethyl modified terphenyl and carbazole derived charge transport units were synthesized and the hosts with the carbazole functionalized backbone structure performed effectively as the host materials of phenylpyridine type Ir emitter. An external quantum efficiency of 18.2% was achieved in the blue phosphorescent organic light-emitting diodes using the new host developed in this work.

Controlling excimer formation in indolo[3,2,1-jk]carbazole/9H-carbazole based host materials for RGB PhOLEDs.

Three novel materials (5CzICz, Cz2ICz and Cz3ICz), based on the indolo[3,2,1-jk]carbazole and 9H-carbazole building blocks, with high triplet energies (E T > 2.80 eV) and good thermal stability (T g > 101 °C) were synthesized, characterized and applied as host materials in PhOLED devices. In course of the preparation of the materials, an improved protocol for the synthesis of the indolo[3,2,1-jk]carbazole moiety has been developed. The careful molecular design of the title compounds allowed to avoid excimer formation of the indolo[3,2,1-jk]carbazole subunits in thin films. Therefore, the improved molecular design broadened the applicability of indolo[3,2,1-jk]carbazole host materials and significantly enhanced the efficiency of PhOLED devices based on these derivatives compared to previously reported indolo[3,2,1-jk]carbazole based compounds. Accordingly, employing the newly developed materials red (CEmax: 32.7 cd A-1, PEmax: 31.0 lm W-1, EQEmax: 20.4%), green (CEmax: 81.0 cd A-1, PEmax: 87.4 lm W-1, EQEmax: 21.5%) and blue (CEmax: 35.5 cd A-1, PEmax: 39.9 lm W-1, EQEmax: 18.0%) PhOLED devices with a remarkably low efficiency roll-off at 1000 cd m-2 (Cz2ICz - red: 5%; green: 0%; blue: 6%) were fabricated.

Exciton-Induced Degradation of Carbazole-Based Host Materials and Its Role in the Electroluminescence Spectral Changes in Phosphorescent Organic Light Emitting Devices with Electrical Aging.

We investigate the origins of the long-wavelength bands that appear in the emission spectra of carbazole-based host materials and play a role in the electroluminescence (EL) spectral changes of phosphorescent organic light emitting devices (PhOLEDs) with electrical aging. 4,4'-Bis(carbazol-9-yl)biphenyl (CBP) is used as a model carbazole host material and is studied using photoluminescence, EL, and atomic force microscopy measurements under various stress scenarios in both single and bilayer devices and in combination with various electron transport layer (ETL) materials. Results show that exciton-induced morphological aggregation of CBP is behind the appearance of those long-wavelength bands and that complexation between the aggregated CBP molecules and ETL molecules plays a role in this phenomenon. Comparisons between the effects of exciton and thermal stress suggest that exciton-induced aggregation may be limited to short-range molecular ordering or pairing (e.g., dimer or trimer species formation) versus longer-range ordering (crystallization) in the case of thermal stress. The findings provide new insights into exciton-induced degradation in wide band gap host materials and its role in limiting the stability of PhOLEDs.

Bipolar host materials with carbazole and dipyridylamine groups showing high triplet energy for blue phosphorescent organic light emitting diodes

A series of new bipolar host materials with carbazole and different number of dipyridyl amine groups were synthesized for blue phosphorescent organic light-emitting diodes (PHOLEDs). Especially, we applied dipyridyl amine to control the polarity as well as electron transporting property of the carbazole based host materials for comparison with the properties of 1,3-di(9H-carbazol-9-yl)benzene (mCP). As a result, all the carbazole based host materials with dipyridyl amine moieties showed very high triplet energies (>2.9 eV) as well as wide band gap (>3.5 eV). Thus, the blue phosphorescent devices employing those materials with bis(4,6-difluorophenyl pyridinato-N,C2)picolinato-iridium (III) (FIrpic) as a dopant and we obtained moderately high external quantum efficiency (EQE) up to 21.6% with N-(3,5-di(9H-carbazol-9-yl)phenyl)-N-(pyridin-2-yl)pyridin-2-amine (DCPPy).

Three carbazole-based host materials: facile synthesis, photophysical properties and performances in PhOLED

Three host materials based on the carbazole were designed and facilely synthesized, i. e., 9-(6-(9-carbazolyl)hexyl)carbazole (hCP), 9-(4-(2-thiophene-based)phenyl) carbazole (sCP) and 9-(4-(9-carbazolyl)phenyl)carbazole(pCP). The relationships between the structures and photophysical, electrochemical and electrophosphorescent performances were investigated. Their emission peaks are located in UV region and the optical band gap are 3.51, 3.57, and 3.89 eV, respectively. Hence, they can be used as hosts in phosphorescent organic light-emitting devices (PhOLEDs). The PHOLEDs were fabricated by doping tris(2-phenylpyridine)iridium (Ir(ppy)3) in host as light-emitting materials. The maximum current efficiency of the PhOLEDs employed sCP as host was 44.88 cd/A.

Acridine derived stable host material for long lifetime blue phosphorescent organic light-emitting diodes

A host material having acridine as a hole transport moiety, 10-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-9,9-dimethyl-9,10-dihydroacridine (CZBPAC), was explored as the host material of phenylimidazole type Ir triplet emitter to realize both high quantum efficiency and stable operational lifetime. The acridine containing CZBPAC was superior to carbazole based host material with the same backbone structure in that it can improve driving voltage, quantum efficiency and lifetime of the blue phosphorescent organic light-emitting diodes simultaneously.

Effect of substituents in a series of carbazole-based host-materials toward high-efficiency carbene-based blue OLEDs

In this study, we systematically investigated the effect of substituents in a series of carbazole-based host material toward blue OLED performances using a carbene-based phosphorescent emitter, mer-tris(N-dibenzofuranyl-N′-methylimidazole)iridium(iii) [Ir(dbfmi)].

Molecular design of triazine and carbazole based host materials for blue phosphorescent organic emitting diodes.

We synthesized 3-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-methylphenyl)-9-phenyl-9H-carbazole (TrzmPCz) as a new bipolar host material for blue phosphorescent organic light-emitting devices and investigated the electro-optical properties of the blue devices fabricated using the TrzmPCz host. We managed the triplet energy of the host by inserting a methyl substituent in the phenyl linkage between triazine and carbazole. The methyl substituent distorted the backbone structure of TrzmPCz and lead to high triplet energy of 2.79 eV. After optimization of the device structure, the TrzmPCz based organic light-emitting diodes achieved the maximum quantum efficiency of 16.4%, a current efficiency of 32 cd A(-1), and a power efficiency of 21.5 lm W(-1).

A rational design of carbazole-based host materials with suitable spacer group towards highly-efficient blue phosphorescence

Two novel carbazole derivatives, 3,3′-(2,6-dimethyl-1,4-phenylene)bis(9-phenyl-9H-carbazole) (DMCz) and 3,3′-(2,3,5,6-tetramethyl-1,4-phenylene)bis(9-phenyl-9H-carbazole) (TMCz), are designed and synthesized as host materials for blue phosphorescent organic light emitting diodes (PHOLEDs). Both the materials possess sufficiently high glass transition temperatures and triplet energies for the blue dopant Ir-complex iridium(III) bis(4,6-(difluorophenyl)-pyridinato-N,C′)picolinate (FIrpic, 2.65 eV). The device using DMCz as the host material showed a better performance than that of TMCz with a maximum power efficiency of 36.0 ± 0.7 lm per W, a maximum current efficiency of 43.7 ± 0.6 cd per A and a maximum external quantum efficiency (EQE) of 18.5 ± 0.1%.

Synthesis of 2- and 4-substituted carbazole derivatives and correlation of substitution position with photophysical properties and device performances of host materials

A synthetic method to synthesize 4-substituted carbazole derivative was developed to study the effect of substitution position of carbazole on photophysical properties and device performances of host materials. Two high triplet energy host materials with substituents at 2- and 4-positions of carbazole were synthesized by the new synthetic approach. Substitution of electron withdrawing group at 2-position was better than the substitution at 4-position to stabilize energy levels and to improve hole transport properties. However, high glass transition temperature and high quantum efficiency were obtained in the carbazole based host material with substituent at 4-position.

Solution-processable carbazole-based host materials for phosphorescent organic light-emitting devices

In this study, solution-processable carbazole-type host materials, 1,3-bis(3-(3,6-di-n-butylcarbazol-9-yl)phenyl)benzene (BCzPPh) and 4,6-bis(3-(3,6-di-n-butylcarbazol-9-yl)phenyl)pyrimidine (BCzPPm), were synthesized for use in phosphorescent organic light-emitting devices (OLEDs). Both host materials possess a high solubility in common organic solvents and high triplet energy to confine excitons to the phosphorescent emitter. The two nitrogen atoms in the central pyrimidine ring of BCzPPm have a profound effect on the photoluminescence properties and the electron-accepting capability. When doped with the green phosphorescent emitter tris(2-(4-tolyl)phenylpyridine)iridium (III), BCzPPh exhibited power efficiencies and external quantum efficiencies above 30lm/W and 13%, respectively, in a simple bilayer OLED.

N‐Heterocyclic Carbazole‐Based Hosts for Simplified Single‐Layer Phosphorescent OLEDs with High Efficiencies

Highly efficient single-layer organic light-emitting diodes (OLEDs) are demonstrated using new N-heterocyclic carbazole-based host materials. Phosphorescent OLEDs with a structure of ITO/MoO(3) /host/host:dopant/host/Cs(2) CO(3) /Al are fabricated in which the new materials act simultaneously as electron-transport, hole-transport, and host layer. Devices with maximum current and external quantum efficiencies of 92.2 cd A(-1) and 26.8% are achieved, the highest reported to date for a single-layer OLED.

New carbazole-based host material for low-voltage and highly efficient red phosphorescent organic light-emitting diodes

A new carbazole-based host material for red emitters, BBTC, was designed, synthesized and characterized with the phosphorescent organic light-emitting diodes (PHOLEDs). With the molecular design strategy of maintaining the large triplet energy and a good hole transporting ability of carbazole while increasing the morphological and electrochemical stability, the C3 and C6 positions of carbazole are blocked with biphenyl groups and the C9 position is terminated with a terphenyl group. Red PHOLEDs employing a conventional dopant material, bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) ((piq)2Ir(acac)), in the emissive layer showed nearly 100% internal quantum efficiency (corresponding to the external quantum efficiency of 19.3%) with reduced efficiency roll-off. We attribute these results to the good electron–hole balance resulting from the good hole mobility of BBTC and low hole injection barrier from the hole transport layer to BBTC host in the emission layer. In addition, owing to its good hole transporting property, it can be utilized as a hole transport layer in organic light-emitting devices enabling low voltage operation of the devices.

High-triplet-energy tri-carbazole derivatives as host materials for efficient solution-processed blue phosphorescent devices

A novel series of solution-processible carbazole-based host materials, 3,6-bis(N-carbazolyl)-N-phenylcarbazole (BCC-36), 3,6-bis(3,6-di-tert-butyl-9-carbazolyl)-N-phenylcarbazole (BTCC-36), 2,7-bis(N-carbazolyl)-N-phenylcarbazole (BCC-27), and 2,7-bis(3,6-di-tert-butyl-9-carbazolyl)-N-phenylcarbazole (BTCC-27), is designed and synthesized. Owing to the highly twisted configuration, these hosts exhibit high triplet energy levels (2.90–3.02 eV) and high glass transition temperatures (147–210 °C). They also exhibit appropriate HOMO energy levels (−5.21–−5.36 eV), resulting in an improved hole-injection property. These novel compounds are employed to fabricate phosphorescent organic lighting-emitting diodes (OLEDs) as the host materials doped with the guests of iridium(III) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic) and iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) by spin coating. The best device performance of FIrpic based blue-emitting devices has a rather low turn-on voltage of 3.9 V, a maximum efficiency of 27.2 cd A−1 (11.8 lm W−1), and a maximum external quantum efficiency of 14.0%. Moreover, the best device performance of FIr6 based deep-blue-emitting devices exhibits a turn-on voltage of 4.9 V, a maximum efficiency of 11.5 cd A−1 (4.9 lm W−1), and a maximum external quantum efficiency 6.8%. The performance data are outstanding for solution-processed blue phosphorescent OLEDs.

Triphenylsilyl- and Trityl-Substituted Carbazole-Based Host Materials for Blue Electrophosphorescence

Carbazole-based materials adopting the nonconjugated substitution of triphenylsilyl (-SiPh(3)) and trityl (-CPh(3)) side groups are studied as high-triplet-energy, morphologically, and electrochemically stable host materials with tunable carrier-transport properties for organic blue electrophosphorescence. The developed host materials 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi), 9-(4-tert-butylphenyl)-3,6-ditrityl-9H-carbazole (CzC), and 9-(4-tert-butylphenyl)-3-(triphenylsilyl)-6-trityl-9H-carbazole (CzCSi) all show high triplet energies of 2.97-3.02 eV, along with high glass transition temperatures of 131-163 degrees C and superior electrochemical stability. Nevertheless, the carrier-transport properties show rather significant dependence on different substitutions. Although three different host materials give similar peak electroluminescence efficiencies at low driving currents, the CzSi host, which has more suitable carrier-transport properties, renders broadened distributions of the triplet excitons in phosphorescent devices, reducing the quenching associated with triplet-triplet annihilation and giving larger resistance against efficiency roll-off at higher brightnesses.

A morphologically stable host material for efficient phosphorescent green and red organic light emitting devices

A carbazole-based host material, 4,4′- N, N′-[di(3, 6-di( t-butyl)carbazole]biphenyl (ttbCBP), with four t-butyl steric spacers on peripheral carbazole moieties was effectively synthesized through Friedel–Crafts Alkylation. Owing to the presence of sterically hindered t-butyl groups, ttbCBP exhibits a high glass transition temperature (175 °C) and is morphologically stable. This host material also preserves the characteristics of wide band-gap of 3.2 eV and high triplet energy of 2.64 eV. In addition, ttbCBP has been shown to be effective hosts for green and red phosphorescent emitters in organic electrophosphorescent devices.