Carbazol-9-yl ) Biphenyl Research Articles

Highly thermal stable and efficient carbazole/pyridine/dibenzothiophene based bipolar host material for red phosphorescent light-emitting diodes

In this work, we designed and synthesized two bipolar host materials, DBTPC1 and DBTPC2 through different linkage ways between carbazole, pyridine and dibenzothiophene. The photophysical properties and thermal properties of these hosts were systematically investigated. Their rigid molecular structures result in high glass transition temperatures, contributing to excellent thermal stability of molecules. Employing DBTPC1 and DBTPC2 as hosts, red phosphorescent organic light-emitting devices (PHOLEDs) with Ir(piq)2acac as an emitter display maximum external quantum efficiencies (EQEs) of 18.8 and 13.2%, respectively. The experimental results reveal that DBTPC1 shows better performance than commercial host 4,4’-bis(carbazol-9-yl)biphenyl (CBP), thus, would be a promising host material for practical application in red PHOLEDs and for the future exploration in multi-color PHOLEDs.

Rational strategy of exciplex-type thermally activated delayed fluorescent (TADF) emitters: Stacking of donor and acceptor units of the intramolecular TADF molecule

• Rational strategy to construct high efficiency exciplex-type chromophores is put forward. • The exciplex PL decay lifetimes can be tuned from 20 μs to 800 μs by the donor structures. • Exciplex-OLEDs show prominent efficiency with EQE of 16.9% and low efficiency roll-off. The lack of the exciplex-type chromophores with high quantum yields hinders their wide applications. Here, a rational strategy to construct exciplexes with thermally activated delayed fluorescence (TADF) were designed by stacking of the donor and acceptor units of an intramolecular charge transfer (CT) chromophore (TXO-PhCz). The intermolecular CT interaction can be anticipated in DFT calculations between the TXO unit and PhCz units. By mixing TXO-P-Si with four donor molecules 1,3-Bis(carbazol-9-yl)benzene (mCP), 4,4′-Bis(carbazol-9-yl)biphenyl (CBP), 3,5-Bis(3-(9H -carbazol-9-yl)phenyl)pyridine (3,5-DCzPPy) and 3-(Diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H -carbazole (PPO21), emission from these exciplexes can be easily observed. And the photophysical properties of these formed exciplexes can be tuned by the structure of the carbazole-based molecules. The exciplexes eliminate the high driving voltage issue, realizing highly efficient green exciplex OLED with a low driving voltage of nearly 3.5, 3.8, and 3.6 V and maximum external quantum efficiency (EQE) up to 16.9% (53.8 cd A −1 , and 48.3 lm W −1 ), 13.7% (39.9 cd A −1 , and 36.2 lm W −1 ), 16.1% (51.0 cd A −1 , and 45.1 lm W −1 ) for mCP:TXO-P-Si, CBP:TXO-P-Si, and 3,5-DCzPPy:TXO-P-Si, respectively. Meanwhile, the efficiency roll off is small, which is comparable with the device with low efficiency roll off. The ideal OLED device based on our strategy with high efficiency and low roll-off has been realized at the same time.

Highly Stable Inverted CdSe/ZnS-Based Light-Emitting Diodes by Nonvacuum Technique ZTO as the Electron-Transport Layer

CdSe/ZnS quantum dots (QDs) have attracted great consideration from investigators owing to their excellent photo-physical characteristics and application in quantum dot light-emitting diodes (QD-LEDs). The CdSe/ZnS-based inverted QD-LEDs structure uses high-quality semiconductors electron transport layers (ETLs), a multilayered hole transporting layers (HTLs). In QD-LED, designing a device structure with a minimum energy barrier between adjacent layers is very important to achieve high efficiency. A high mobility polymer of poly (9,9-dioctylfluorene-co-N-(4-(3-methylpropyl)) diphenylamine (TFB) was doped with 4,4′-bis-(carbazole-9-yl) biphenyl (CBP) with deep energy level to produce composite TFB:CBP holes to solve energy mismatch (HTL). In addition, we also improved the QD-LED device structure by using zinc tin oxide (ZTO) as ETL to improve device efficiency. The device turn-on voltage Vt (1 cd m−2) with ZTO ETL reduced from 2.4 V to 1.9 V significantly. Furthermore, invert structure devices exhibit luminance of 4296 cd m−2, current-efficiency (CE) of 7.36 cd A−1, and external-quantum efficiency (EQE) of 3.97%. For the QD-LED based on ZTO, the device efficiency is improved by 1.7 times.

Improved green thermal activated delayed fluorescence OLEDs based on thermally evaporated distributed Bragg reflector (DBR) of MgF2/ZnS

Unlike the traditional fabrication of distributed Bragg reflector (DBR) structure via atomic layer deposition or spin-coating, here the 1–6 pairs of magnesium fluoride (MgF2)/zinc sulfide (ZnS) alternative dielectric layers were grown via thermal evaporation. The absorption, transmission, reflection, and photoluminescence (PL) spectra were evaluated. 5 pair MgF2/ZnS denotes the largest reflectance (88.5% at 535 nm) together with a stopband at 450–650 nm among the 1– 6 pair dielectric layers, exhibiting the potential for using as DBR. Relative to the bare 4,4’-bis(carbazol-9-yl)biphenyl(CBP):(4s,6s)−2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN) film, the PL intensity of CBP:4CzIPN/5 pair MgF2/ZnS DBR is enhanced and splitted into two peaks. The 5 pair alternative dielectric film presents more uniform aggregation over 4 pair MgF2/ZnS. The cross-sectional scanning electron microscopic image denotes explicit layering for the MgF2 and ZnS. The organic light-emitting diode (OLED) incorporating 5 pair MgF2/ZnS DBR layers illustrates significantly improved electroluminescent (EL) performance due to the photons concentrated in the direction perpendicular to the DBR. The slightly narrowed EL spectrum is originated from the microcavity effect between the two Al electrodes. Here we develop a universal method for the DBR fabrication suitable to most of OLEDs.

Effects of rotational conformation on electronic properties of 4,4′-bis(carbazol-9-yl)biphenyl (CBP): the single-molecule picture and beyond

ABSTRACT 4,4-bis(carbazol-9-yl)biphenyl (CBP) is a commonly used hole-transport host material in phosphorescent organic light-emitting devices (PhOLEDs). Despite its widespread use, device degradation is observed that most likely involves excited states and ionic species. The present work aims at providing a first step towards a detailed CBP characterisation by computing excited-state properties and ionised species of CBP using time-dependent density functional theory (TD-DFT) and the Bethe-Salpeter equation (BSE) based on the GW method. The investigation reveals a strong dependence of the absorption spectrum on the interactions of the phenyl and carbazole units, which is computed for 1.6 k ground-state rotamers using the efficient GW-BSE method. A similar approach using a set of 1.6 k excited-state rotamers shows a significantly smaller dependence on fragment orientation for emission. In order to model ensembles of gas-phase CBP molecules, spectra are simulated employing a Boltzmann distribution for different temperatures based on the relative ground-state energies of the rotamers. This approach reveals that certain absorption bands experience almost no change in the ensemble picture while others vanish, which can be understood in terms of the geometry-dependence of the excited states due to the phenyl-carbazole interactions.

Solution-Processed Efficient Perovskite Nanocrystal Light-Emitting Device Utilizing Doped Hole Transport Layer.

Light-emitting devices (LEDs) with inorganic perovskite nanocrystals (PNCs) fabricated through the all-solution process have tremendous potential for new-generation illumination and displays on account of their large area and cost-effective manufacturing. However, the development of efficient solution-processed PNC LEDs remains challenge, which mainly results from the fact that only a few types of charge transport layers can be employed for the subsequent deposition steps, thus leading to injection barriers and charge injection imbalance inside these LEDs. Herein 4,4'-bis(carbazole-9-yl) biphenyl (CBP) is introduced as a dopant into the poly(9,9-dioctylfluorene-co-N-(4-(3-methylpropyl)) diphenylamine) (TFB) hole transport layer (HTL), which efficiently modulates the mobility of charge carrier as well as the energy level of the HTL, resulting in the barrier-free injection of the charge carrier in the as-fabricated solution-processed PNC LEDs. Consequently, the luminance of red LEDs (688 nm) reaches 2990 cd m-2, and the external quantum efficiency achieves 8.1%, which is the optimal performance for solution-processed PNC LEDs to date. Additionally, the turn-on voltage and roll-off have also been improved by the more balanced charge injection.

Investigation of bipolar matrix based mixed host in highly efficient green delayed fluorescence organic light-emitting diodes

The mixed cohosts of electron transport host (E-host): 4,40-bis(carbazol-9-yl)biphenyl (CBP) have been comparatively investigated for an efficient green fluorescent organic light emitting diode (OLED) doped with a thermally activated delayed fluorescence (TADF) emitter (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN). The E-host:CBP systems significantly enhance the electroluminescent (EL) properties. After doping E-host, the lifetime of the emissive layer decreases and the surface becomes smoother, together with the impedance decreases for one magnitude and the hole-injection depresses. The charge balance and improved interface both contribute to the EL performance enhancement. Here we develop a universal mixed host system suitable to most of emitters.

Solvent Resistant Hole-Transporting Thin Films via Diacetylene Cross-Linking and Their Applications in Solution-Processed QLEDs

A diacetylene based thermal cross-linking method for hole transporting layers (HTLs) has been investigated. The thin-films of 4,4-bis(carbazol-9-yl)biphenyl (CBP) oligomers functionalized with diac...

Radiative and Nonradiative Recombinations in Organic Radical Emitters: The Effect of Guest–Host Interactions

AbstractRadical‐carrying organic molecules have received significant attention to bypass the issue related to harvesting triplet excitons in current light‐emitting materials. While the computational efforts conducted so far have treated these radical emitters as isolated entities, in actual devices, they are embedded in a host matrix and subject to emitter–host interactions. Here, by combining molecular dynamics simulations and density functional theory calculations, the impact of the host matrix on the optoelectronic performance of radical emitters is evaluated, taking as a representative example the (4‐ncarbazolyl‐2,6‐dichlorophenyl)bis(2,4,6‐trichlorophenyl)‐methyl (TTM‐3NCz) radical emitter dispersed in a 4,4‐bis(carbazol‐9‐yl)biphenyl (CBP) host. A morphological analysis shows that steric effects around the radical centers, carried by the TTM electron‐poor moieties of the emitters, disfavor π–π interactions with the host molecules, which leads to random intermolecular orientations around the TTM moieties. The 3NCz electron‐rich moieties of the emitters, however, have much lesser spatial hindrance for intermolecular π–π stacking, which modulates the structural and electronic properties of the emitters in the host matrix. The influence of dynamic and static disorders on the radiative and nonradiative recombination processes is also investigated and it is found that the rates of nonradiative recombination are small, which opens the way to 100% internal quantum efficiency for the doublet‐based emission process.

A red emissive poly(dendrimer) for solution processed organic light-emitting diodes

Abstract A red-emissive poly(dendrimer) comprised of a norbornenyl-derived polymer backbone with a phosphorescent dendrimer side-chain has been prepared by ring-opening methathesis polymerization with a number average molecular weight of 164 kDa and a dispersity of 2.4. The dendrimer side-chain had a heteroleptic iridium(III) complex core containing two 2-phenylpyridyl ligands and one 2-(thiophen-2-yl)quinoline ligand. The 2-phenylpyridyl ligands had first generation fluorenylcarbazole dendrons with n-propyl surface groups attached and the 2-(thiophen-2-yl)quinoline ligand was linked to the polymer backbone. In spite of the presence of the two 2-phenylpyridine ligands that would normally give rise to green emission when complexed with iridium(III), the 2-(thiophen-2-yl)quinoline was responsible for the observed saturated red emission with CIE coordinates of (0.67, 0.33). The poly(dendrimer) had a high photoluminescence quantum yield (PLQY) for a red emissive species at 72 ± 7%, which was maintained when blended with 4,4′-bis(carbazol-9-yl)biphenyl (CBP) at a concentration of 10 wt % (PLQY = 74 ± 8%). Simple two-layer devices containing the blend emissive layer and an electron transporting hole blocking layer had a maximum external quantum efficiency of almost 11%.

Composite Hole Transport Layer Consisting of High-Mobility Polymer and Small Molecule With Deep-Lying HOMO Level for Efficient Quantum Dot Light-Emitting Diodes

Improving hole injection is a critical issue for further boosting the brightness and efficiency of quantum dot light-emitting diodes (QLEDs) with an organic-inorganic hybrid structure. In this work, a small molecule of 4,4’-bis-(carbazole-9-yl)biphenyl (CBP) with a deep-lying HOMO level was doped into high mobility polymer of poly(9,9-dioctylfluorene-co-N-(4-(3-methyl- propyl)) diphenylamine) (TFB) to construct an efficient composite TFB:CBP hole transport layer (HTL). Green QLEDs based on such solution-processed composite HTLs exhibited a maximum luminance of 90159 cd/m2, with a peak current efficiency of 21.6 cd/A and power efficiency of 12.3 lm/W. These values were over one-order of magnitude larger than those obtained in the CBP-only devices and also much larger than those in the TFB-only devices.

Stepwise Bi-Layer Hole-Transport Interlayers With Deep Highest Occupied Molecular Orbital Level for Efficient Green Quantum Dot Light-Emitting Diodes

The imbalanced charge transport/injection in quantum dot (QD) light-emitting diodes (QLEDs) due to the existing excess electrons has greatly restricted the device performance. Here, we designed a stepwise bi-layer structured hole-transport layer (HTL) containing 4,4'-bis-(carbazole-9-yl)biphenyl (CBP)/1,3-bis(9H-pyrido[2,3-b]indol-9-yl) benzene (mCaP) with deep highest occupied molecular orbital (HOMO) level to realize more efficient hole injection into QD emissive layer for balancing charge in the QLED devices. The resulting green QLEDs show a maximum current efficiency (CE) of 41.2 cd A <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> corresponding to an external quantum efficiency (EQE) of 12.6%, which is a significant efficiency enhancement compared with the devices with only single-layer HTL. In addition, the device turn-on voltage is also reduced from 5 to 3 V, and meanwhile the operational lifetime is increased by more than twofold.

Investigation of charge-transporting layers for high-efficiency organic light-emitting diode

In the present work, we study a comprehensive model to quantitatively investigate the role of charge-transporting materials on exciton recombination and electric field distribution across the emissive and electron-transporting layers in organic light-emitting diode (OLED) devices via electrical simulation. The devices are composed of 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA) and 4,4′-bis(carbazol-9-yl) biphenyl (CBP) hosts, bathophenanthroline (Bphen) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) as electron-transporting materials (ETMs), and 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as a hole-transporting material. The outcomes reveal that the recombination rate across the emissive layer (EML) is highly influenced by the charge carrier mobility of ETMs. The Bphen-based device exhibits a maximum recombination rate of 3.5 × 106 cm−3 s−1, which is 105 times higher than that of the TPBi-composed device. The recombination rate is increased by two orders of magnitude by incorporating a TAPC for the hole transport layer. It is increased from 1.9 × 106 to 3.5 × 106 cm−3 s−1 as the host is changed from TCTA to CBP. Meanwhile, the electric field distribution is decreased from 0.28 to 0.17 MV cm−1 across the TCTA EML in the presence of the hole transport layer, but is increased 0.36 MV cm−1 when TCTA EML is replaced with CBP. Experimentally, the Bphen-based device exhibits a power efficiency of 12.5 lm W−1 and an external quantum efficiency of 5.1%, which are higher than those of the TPBi counterpart.

Efficient green phosphorescent organic light-emitting diodes enabled with new and thermally stable carbazole/pyridine derivatives as hosts

Two host materials H1 and H2 by introducing dibenzothiophene and dibezofuran moieties into the carbazole and pyridine derivative, respectively, are reported. Their structural and physical properties were fully characterized. The thermal analysis demonstrated that both H1 and H2 are thermally stable. The triplet energy levels of H1 and H2 were determined to be 2.66 and 2.51 eV, respectively. Green phosphorescent OLEDs with H1 and H2 acting as host materials were fabricated to investigate their performances in PHOLEDs. Green PHOLED based on H1 shows the best performance with the maximum power efficiency of 50.4 lm/W and the lowest driving voltage of 2.5 eV. The characteristic measurements of OLED devices reveal that both H1 and H2 are promising host materials for practical application. By comparing with the commercial host material of 4,4′-bis(carbazol-9-yl)biphenyl (CBP), the power efficiency of green PHOLED was enhanced by 31.6%.

Assistant dopant system in red phosphorescent OLEDs and its mechanism reveal

We have constructed an assistant dopant system in red phosphorescent OLEDs. In the system, we chose the 4,4′-Bis(carbazol-9-yl)biphenyl (CBP) as the host material, a thermally activated delayed fluorescence (TADF) 4CzIPN as the assistant dopant and a red phosphor Hex-Ir(phq)2(acac) as the guest. The assistant dopant system can remarkably reduce the turn-on voltage and increase device brightness. Meanwhile, devices with assist dopant system show negligible efficiency roll-off even in high luminance and current density. The current efficiency of optimized device with 15 wt% 4CzIPN only drops to 12.7 cd/A at 74,532 cd/m2 from maximum 13.7 cd/A. By further investigation on the exciton generation mechanism and the energy transfer process in the system, the main cause of the improvement has been unveiled.

Exciton dynamics of luminescent defects in aging organic light-emitting diodes

Fundamental device physics of exciton dynamics is crucial to the design and fabrication of organic light-emitting diodes (OLEDs) with a long lifetime at high brightness. In this paper, we report a set of analytical equations which describe how and where defects form during exciton-driven degradation of an OLED and their impact on device operation. This set of equations allows us to quantify changes in the exciton and defect populations as a function of time in neat layers of 4,4′-Bis(carbazol-9-yl)biphenyl (CBP) in simple bilayer OLEDs. CBP produces luminescent defects which present a unique opportunity to quantify the exciton capturing dynamics of the defects. Through modeling of the time and current density dependence of both the CBP and defect emission, we clearly identify CBP singlet excitons as the source of OLED degradation. Further analysis of experimental data on devices with precisely positioned exciton capturing layers suggests that defects are formed near organic heterojunctions.

Assistant dopant system in solution processed phosphorescent OLEDs and its mechanism reveal

We have developed an assistant dopant system in phosphorescent OLEDs. The system is consisted of a conventional host material 4,4′-Bis(carbazol-9-yl)biphenyl (CBP), a thermally activated delayed fluorescence (TADF) 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyanobenzene (4CzIPN) and a red phosphor Bis[2-(4-n-hexylphenyl)quinoline](acetylacetonate)iridium(III) (Hex-Ir(phq)2(acac)), it has achieved maximum current efficiency and external quantum efficiency of 21.52 cd/A and 9.22%, respectively. The intrinsic mechanism and energy transfer process in the system have been discussed in detail. Moreover, a satisfactory critical current density J0 of 106.1 mA/cm2 has been obtained when the assistant dopant concentration ascends to a suitable level, which indicated low efficiency roll-off of the device. By investigating the exciton generation mechanism and the transient electroluminescence (EL), the main cause of the improvement has been revealed.

Tunneling Injection and Exciton Diffusion of White Organic Light-Emitting Diodes with Composed Buffer Layers

Four configurations of buffer layers were inserted into the structure of a white organic light emitting diode, and their impacts on the hole tunneling-injection and exciton diffusion processes were investigated. The insertion of a single buffer layer of 4,4′-bis(carbazol-9-yl)biphenyl (CBP) resulted in a balanced carrier concentration and excellent color stability with insignificant chromaticity coordinate variations of Δx < 0.023 and Δy < 0.023. A device with a 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) buffer layer was beneficial for hole tunneling to the emission layer, resulting in a 1.45-fold increase in current density. The tunneling of holes and the diffusion of excitons were confirmed by the preparation of a dual buffer layer of CBP:tris-(phenylpyridine)-iridine (Ir(ppy)3)/BCP. A maximum current efficiency of 12.61 cd/A with a luminance of 13,850 cd/m2 was obtained at 8 V when a device with a dual-buffer layer of CBP:6 wt.% Ir(ppy)3/BCP was prepared.

Novel carbazole/indole/thiazole-based host materials with high thermal stability for efficient phosphorescent organic light-emitting diodes

Two host materials TIC1 (N-linked carbazole) and TIC2 (C-3-linked carbazole) via changing the substitution position of carbazole, indole and thiazole moieties are reported. Their structural and physical properties were fully characterized. The thermal analysis demonstrated that both TIC1 and TIC2 are thermally stable. The triplet energy levels of TIC1 and TIC2 were determined to be 2.54 and 2.58 eV, respectively. Green and blue OLEDs with TIC1 and TIC2 acting as host materials were fabricated to investigate their performances in PHOLED. Both green and blue PHOLEDs based on TIC2 show the best performance with the maximum current efficiency and maximum power efficiency of 60.0 cd/A and 48.0 lm/W (for green), 18.4 cd/A and 15.2 lm/W (for blue), respectively. The characteristic measurements of OLED devices reveal that both TIC1 and TIC2 are promising host materials for practical application. By comparing with the commercial host material of 4,4′-bis(carbazol-9-yl)biphenyl (CBP) and N,N-dicarbazoyl-3,5-benzene (mCP), the power efficiency of green and blue PHOLED was enhanced by 55.3% and 245%, respectively.

Synthesis, characterization, and systematic structure–property investigation of a series of carbazole–thiophene derivatives

A series of carbazole–thiophene oligomers linked at the 3,6-positions of the carbazole fragment of 4,4′-bis(carbazol-9-yl)biphenyl (CBP) and 4,4′-bis(carbazol-9-yl)-2,2′-dimethylbiphenyl (CDBP) with systematically elongated molecular lengths were synthesized via the Suzuki–Miyaura and Ullmann coupling reactions. Their electronic properties were studied by UV-Vis, cyclic voltammetry, and theoretical calculations. The coupling of CBP and CDBP with thiophene and bi- and terthiophene residues stabilized the HOMO and LUMO energy levels. The absorption and emission spectra exhibited a gradual red shift. The compounds with oligothiophene units had greatly decreased band gaps compared with CBP and CDBP. Therefore, these units may be introduced into the backbone of π-conjugated small molecules to develop new materials with low band gaps that may have potential applications in optoelectronics.