Blue Host Materials Research Articles

Unveiling the role of external electric field on the charge transport and excited-state properties of high T1 host for blue OLED devices

Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed to understand the effect of the external electric field (EEF) on the reorganization (λ) and the lowest triplet excited-state (T1) energies of high T1 blue host materials. Depending on the direction and the strength of the external electric field (EEF), the positive and negative changes of hole and electron λ(h and λ e) values were found in these materials. More importantly, λ e seems to be more sensitive than λ h values under the EEF. It is also noticed that the calculated T1 energies are meaningfully changed in the application of EEFx and EEFy. In contrast, the effect of EEFz on the T1 energies can be negligible. From the results of theoretical investigation, the obvious evidence related to the influence of EEF on the charge transport and excited-state properties of high T1 blue host materials were obtained. In the present work, we expect that our theoretical study will provide new insight into understanding the influence of EEF as a key player in manipulating essential properties of the high T1 blue host material during the electrical operation.

Highly Efficient Blue Host Compound Based on an Anthracene Moiety for OLEDs

We designed and successfully synthesized a novel anthracene-based host material, 10-(4-(naphthalen-1-yl)phenyl)-9-(naphthalen-3-yl)anthracene (2-NaAn-1-PNa). The 2-NaAn-1-PNa exhibited a PL max at 440 nm in film state and demonstrated a wide band gap of 2.95 eV, indicating its suitability as a blue host material. In an OLED device using 2-NaAn-1-PNa as the host and 3Me-1Bu-TPPDA as the dopant, real blue emission was achieved with an electroluminescence maximum (ELmax) at 460 nm and Commission Internationale de L’Eclairage coordinates of (0.133, 0.141). The device exhibited excellent EL characteristics, with an external quantum efficiency of 8.3% and a current efficiency of 9.3 cd/A at 10 mA/cm2. Furthermore, the efficiency roll-off was limited to only 1.9% up to 4000 nits, demonstrating exceptional performance.

Recent Advances in Triplet–Triplet Annihilation-Based Materials and Their Applications in Electroluminescence

Efficient and stable blue organic light-emitting devices (OLEDs) represent one of the most important components in future solid-state lighting and full color displays. Although red and green OLEDs have been successfully commercialized, it remains challenging to realize blue OLEDs showing high efficiency and high stability simultaneously, restricting their commercialization. Triplet–triplet annihilation (TTA)-based materials have been emerging as a new category of electroluminescent or host materials with promising potential for highly efficient and stable blue devices due to their appealing photophysical properties (e.g., ability to generate emissive singlet excited states through TTA process, high photoluminescence quantum yield, ease of molecular design, etc.). Recently, research on TTA-based materials toward high-performance blue OLEDs has received an increasing attention. In this review, recent research advances on the molecular design strategies, photophysical properties of TTA-based materials (both blue emitters and host materials), mechanism of TTA-based electroluminescence, as well as their OLED applications are presented comprehensively. Moreover, potential perspectives with future research opportunities in this field are also described. We believe that the emerging TTA-based materials will bring more opportunities for developing a large family of novel photofunctional materials showing appealing and tunable photophysical properties, thus providing new opportunities for high-efficiency and stable blue electroluminescence in the future.

A Bipolar Delayed Fluorescence Luminogen with Fast Reverse Intersystem Crossing and High Horizontal Dipole Orientation for High‐Performance Sky‐Blue and White OLEDs

AbstractThe development of robust luminescent materials plays a key role for organic light‐emitting diodes (OLEDs). Herein, a tailor‐made sky‐blue delayed fluorescence luminogen (DCPC‐BP‐SFAC) containing a carbonyl acceptor and spiro[acridine‐9,9″‐fluorene] and 9,9″‐(cyclohexane‐1,1‐diylbis(4,1‐phenylene))bis(9H‐carbazole) donors is explored, which holds merits of excellent thermal stability, strong photoluminescence, balanced carrier transport, and horizontal dipole orientation. Owing to a charge‐transfer dominated singlet state and a charge‐transfer and local‐excitation hybrid triplet state, DCPC‐BP‐SFAC has a small energy gap between singlet and triplet as well as a large spin–orbit coupling constant, which result in a fast reverse intersystem crossing process and thus efficient delayed fluorescence. High‐performance sky‐blue OLEDs with outstanding external quantum efficiencies (ηext) reaching 32.6% are achieved based on DCPC‐BP‐SFAC, benefiting from high exciton utilization and large light out‐coupling efficiency. Moreover, using DCPC‐BP‐SFAC as blue emitter and host material simultaneously, efficient all‐fluorescence out‐of‐phase sensitizing white OLEDs without interlayer are fabricated, providing high ηext of up to 21.6% and stable warm‐white light with a high color rendering index of 84. The outstanding electroluminescence performances demonstrate the great potentials of DCPC‐BP‐SFAC in practical display and lighting devices.

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.

Multifunctional pyridine styrylphenanthroimidazoles: electron transport materials for blue FOLEDs with low efficiency roll-off and hosts for PHOLEDs with low turn-on voltage

We have reported pyridine styrylphenanthroimidazole derivatives, SPPI-C2Py, SPPI-N1Py and SPPI-C2N1Py, as efficient electron transport materials (ETMs) for blue fluorescent OLEDs and host materials for phosphorescent organic light emitting diodes.

Highly-twisted pyrene derivative for pure-blue organic light emitting diodes

The new fluorescent blue host material, 1,3,6,8-tetrakis(2,5-dimethylbiphenyl-4-yl)pyrene (BD4PP), was synthesized for pure-blue organic light-emitting diodes (OLEDs). BD4PP has highly twisted and steric structure for efficient fluorescence, and has high thermal stability of 492°C. BD4PP host doped with 5wt.% 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi) or 5wt. % 1,4-bis[2-(3-N-ethylcarbazoryl)-vinyl]benzene (BCzVB) showed high device efficiencies due to wide bandgap (˜ 3.1eV) and electron-rich host. An OLED with the BD4PP:BCzVB emitter showed higher current and external quantum efficiencies of 4.41cd/A and 3.80% with color coordinates (0.16, 0.13) compared to those of the BD4PP:BCzVBi (3.63cd/A and 3.62%) with color coordinates (0.16, 0.11) and non-doped BD4PP (2.01cd/A and 1.61%) with color coordinates (0.18, 0.13).

12‐1: Analysis of Key Factors Affecting the Lifetime of Blue Phosphorescent OLED Using CN Modified Blue Host Materials

Three host materials were synthesized for blue phosphorescent organic light emitting diodes (OLEDs) by combining carbazolylcarbazole and benzonitrile. Each material has different CN positions relative to the carbazolylcarbazole donor as ortho,‐meta, and para. These materials were used for finding fundamental factors of lifetime degradation of blue OLEDs. By comparing the characteristics of the three hosts with the CN unit at ortho, meta and para positions, we studied the effects of the CN position on the lifetime of the blue phosphorescent OLEDs. We calculated bond dissociation energy of the three hosts and energy transfer to find key factors about degradation. We calculated Forster energy transfer rate from host to dopant by measuring parameters for the calculation. The correlation of the device lifetime with the parameters revealed that the bond dissociation energy and Forster energy transfer rate are critical to the device lifetime of the blue phosphorescent OLEDs.

Easily available, low-cost 9,9′-bianthracene derivatives as efficient blue hosts and deep-blue emitters in OLEDs

Abstract A series of alkyl substituted 9,9′-bianthracene derivatives (BAs) with easy availability and low cost have been applied as deep-blue emitters and blue host materials in fluorescent organic light-emitting diodes (OLEDs). The nearly perpendicular geometry configuration of 9,9′-bianthracene and the additional alkyl units with large steric hindrance in methyl substituted MBA and tert-butyl substituted TBBA would reduce the aggregation formation emitting at longer wavelength by intramolecular and intermolecular interaction, which endows both of the compounds with good thermal properties, and film-forming abilities as well as excellent electroluminescence performance. Nondoped pure deep-blue OLED using TBBA as the emitter showed good performance with EQE of 3.20% and high color purity with CIE (0.15, 0.06), which matched the EBU standard blue very well. Furthermore, these BAs worked as excellent host materials for p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSAph) dopant to get high-performance OLEDs with low turn-on voltages and high efficiencies, especially for MBA bearing two methyl substituents at 3- and 3′-positions of the 9,9′-bianthracene as the fluorescent host with a low turn-on voltage of 3.0 V, a high current efficiency of 16.54 cd/A and a high EQE of 9.47% with unobservable efficiency roll-off. Particularly, EQE can be maintained over 9.20% with very little variation of 0.54% in the range of luminance value of 1.54–13800 cd/m2 for the MBA-based device. The improved EQEs are due to the enhanced occurrence of singlet excitons in BAs-based devices.

Meltdown! Local Heating by Decaying Excited Host Positive Polarons Triggers Aggregation Quenching in Blue PhOLEDs.

Exciton-polaron induced aggregation (EPIA) in organic host materials for blue Phosphorescent Organic Light Emitting Diodes (PhOLEDs) is driven by a non-radiative decay of electronically excited positive polarons resulting in a local heating of the amourphous host matrix. The released heat triggers morphological changes, i. e. molecular aggregation between neighboring host molecules. The resulting aggregates, which our calculations identify as carbazolyl dimers, lead to decreased PhOLED efficiency. Statistical assessment of some host-only morphologies reveals a structure-dependent propensity for molecular aggregation corroborating the identified EPIA mechanism. Our findings provide a fresh look at established molecular design rules and will help to improve blue PhOLED host materials to enhance blue PhOLED device lifetimes.

Chromenopyrazole-Based Bipolar Blue Host Materials for Highly Efficient Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

New electron-acceptor cores are necessary for developing highly efficient bipolar hosts, particularly for blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). Herein, chromenopyrazole (CP) was used for the first time as an electron-acceptor core to design and synthesize two novel blue bipolar hosts, viz., 8-(9H-carbazol-9-yl)-3-methyl-1-phenylchromeno[4,3-c]pyrazol-4(1H)-one (CzCP) and 8-(9H-[3,9′-bicarbazol]-9-yl)-3-methyl-1-phenylchromeno[4,3-c]pyrazol-4(1H)-one (2CzCP). The influence of donor strength on the photophysical, electrochemical, and electroluminescent performances was systematically investigated. CzCP and 2CzCP both maintain high triplet energy (∼3.0 eV), appropriate highest occupied and lowest unoccupied energy levels (HOMO/LUMO), and bipolar nature. Consequently, OLEDs containing CzCP as a host in the emissive layer exhibited state-of-the-art performance with external quantum efficiency of 27.9% and CIE color coordinates of (0.15, 0.21), thus achievin...

Two novel blue phosphorescent host materials containing phenothiazine-5,5-dioxide structure derivatives.

Two novel D–A bipolar blue phosphorescent host materials based on phenothiazine-5,5-dioxide: 3-(9H-carbazol-9-yl)-10-ethyl-10H-phenothiazine-5,5-dioxide (CEPDO) and 10-butyl-3-(9H-carbazol-9-yl)-10H-phenothiazine-5,5-dioxide (CBPDO) were synthesized and characterized. The photophysical, electrochemical and thermal properties were systematically investigated. CEPDO and CBPDO not only have a high triplet energy but also show a bipolar behavior. Moreover, their fluorescence emission peaks are in the blue fluorescence region at 408 nm and the fluorescence quantum efficiency (Φ) of CEPDO and CBPDO were 62.5% and 59.7%, respectively. Both CEPDO and CBPDO showed very high thermal stability with decomposition temperatures (Td) of 409 and 396 °C as well as suitable HOMO and LUMO energy levels. This preferable performance suggests that CEPDO and CBPDO are alternative bipolar host materials for the PhOLEDs.

An ambipolar 3,3′-dimethyl-9,9′-bianthracene derivative as a blue host material for high-performance OLEDs

A new ambipolar 3,3′-dimethyl-9,9′-bianthracene derivative (MBAn-(4)-tBu) was designed as a deep blue fluorophore and blue host material for high-performance OLEDs.

Synthesis and device properties of mCP analogues based on fused-ring carbazole moiety

Novel mCP analogues consisting of blue phosphorescent host materials with fused-ring, 1,3-bis(5H-benzofuro[3,2-c]carbazol-5-yl)benzene (BFCz) and 1,3-bis(5H-benzo[4,5]thieno[3,2-c]carbazol-5-yl)benzene (BTCz) were designed and synthesized using benzofurocarbazole and benzothienocarbazole donor moieties. BFCz and BTCz exhibit high glass transition temperatures of 147 and 157 °C, respectively, and high triplet bandgaps of 2.94 and 2.93 eV, respectively. To explore the electroluminescence properties of these materials, multilayer blue phosphorescent organic light-emitting diodes (PHOLEDs) were fabricated in the following device structure: indium–tin-oxide (ITO)/PEDOT:PSS/4,4’-cyclohexylidene bis[N,N-bis(4-methylphenyl)aniline] (TAPC)/1,3-bis(N-carbazolyl) benzene (mCP)/host:FIrpic/diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1)/LiF)/Al. The PHOLEDs with BTCz exhibited efficient blue emission with luminous and quantum efficiencies of 30.9 cd/A and 15.5% at 1000 cd/m2, respectively.

A Theoretical Guideline for Designing Effective Host Materials Based on 4,4′-Bis(9-carbazolyl)-1,1′-biphenyl Derivatives for Blue Phosphorescent Devices

Abstract Development of host materials is crucial for organic electroluminescent devices. In order to generalize the way to molecular design the host materials, we here present a guideline to design effective host materials for blue phosphorescent devices based on 4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP) derivatives, comparing some theoretical parameters of seven CBP derivatives with experimental ones in terms of thermal stability, T1 energy level, and band gap. Although our computation at the density functional theory level could not reproduce the absolute values and phenomena that were experimentally obtained, theoretical relative correlations between different molecules agreed with experimental ones. Indeed, both experimental and theoretical approaches predicted that the same molecule (1,1-bis(4-(9-carbazolyl)-phenyl)cyclohexane) was viable as blue host material among the target molecules in terms of thermal stability, the T1 state with the LE character within carbazole moieties and T1 energy 3.00 eV higher than that of blue emitters. The qualitative guideline we proposed would be potentially helpful to save experimental work to design other hole- or electron-transporting materials.

Synthesis of novel benzothiophene derivative as a host material for blue phosphorescent organic light-emitting diodes

An organic compound having carbazole, benzothiophene, and phosphine oxide moieties, (5-(9H-carbazol-9-yl)benzo[b]thiophen-2-yl)diphenylphosphine oxide (CBTPO), was synthesized and employed as a blue host material to harvest triplet excitons from blue triplet emitters. The CBTPO material is a bipolar host material because it includes carbazole and diphenylphosphine oxide charge transport units with both a high triplet energy of 2.76 eV and a high quantum efficiency of 19.1% in blue phosphorescent device.

Di(biphenyl)silane and carbazole based bipolar host materials for highly efficient blue phosphorescent OLEDs

In this paper, a series of new bipolar blue host materials has been designed and synthesized by incorporating dimethyl or diphenyl silane core with connection of carbazole di(biphenyl) groups at different linking positions. The triplet energies and charge-transporting properties of these bipolar host compounds are controlled by restricting or enhancing the conjugation length among silane, biphenyl, and carbazole units. All materials exhibit good thermal/morphological stabilities. Among these host molecules diphenyldi(4-(3,5-bis-carbazol-9-yl)phenyl)phenyl silane (DM4CzDBPSi) showed the highest triplet energy (2.82 eV) and glass transition temperature (197 °C) along with optimal bipolar charge-transporting property. A phosphorescent blue organic light-emitting diode with DM4CzDBPSi silane host and FIrpic dopant system demonstrated best device performances with maximum external quantum efficiency of 20.9% and a power efficiency of 41.28 lm/W.

Effect of Substituents on the Electronic Structure and Degradation Process in Carbazole Derivatives for Blue OLED Host Materials

We investigate the dissociation mechanism of the C–N bond between carbazole and dibenzothiophene in carbazole-dibenzothiophene (Cz-DBT) positional isomers, selected as representative systems for blue host materials in organic light-emitting diodes (OLEDs). The C–N bond dissociation energies, calculated at the density functional theory level, are found to depend strongly on the charge states of the parental molecules. In particular, the anionic C–N bond dissociations resulting in a carbazole anion can have low dissociation energies (∼1.6 eV) with respect to blue emission energy. These low values are attributed to the large electron affinity of the carbazole radical, a feature that importantly can be modulated via substitution. Substitution also impacts the energies of the first excited electronic states of the Cz-DBT molecules since these states have an intramolecular charge-transfer nature due to the spatially localized character of the frontier molecular orbitals within the carbazole moiety (for the HOMO...

Control of a White Organic Light Emitting Diode emission parameters using a single doped RGB active layer

Solid State Lighting technologies based on Organic Light Emitting Diodes, became an interesting focus due to their unique properties. The use of a unique RGB active layer for white emission, although simple in theory, shows difficulty to stabilize both CIE coordinates and color modulation. In this work, a WOLED using a simple RGB layer, was developed achieving a high color stability and shade modulation. The RGB matrix comprises a blue host material NPB, doped with two guests, a green (Coumarin 153) and a red (DCM1) in low concentrations. The RGB layer carrier dynamics allows for the white emission in low device complexity and high stability. This was also shown independent of the white shade, obtained through small changes in the red dopant resulting in devices ranging from warm to cool white i.e. an easy color tuning. A detailed analysis of the opto-electrical behavior is made.

Selective Introduction of Carbazole and Diphenylamine into Spirofluorenexanthene Core for Different Phosphorescent Hosts

AbstractBuilt on the spiro[fluorene‐9,9′‐xanthene] (SFX) core and two frequently‐used hole‐transporting groups such as carbazole and diphenylamine, two SFX derivatives, namely SFXCz and SFXDPA, have been synthesized by one‐step reaction for red, green and blue phosphorescent organic light‐emitting devices (PHOLEDs). Though the properties of these two groups are very similar, the devices based on SFXCz and SFXDPA exhibit distinct performances. In blue PHOLEDs, the device based on SFXCz exhibited much better performances than that based on SFXDPA. However, the latter was superior to the former in green and red PHOLEDs. And the red PHOLED based on SFXDPA exhibited maximum current efficiency (CE) of 27.1 cd·A−1, power efficiency (PE) of 25.0 lm·W−1, and external quantum efficiency (EQE) of 15.0%. The results show that the introduction of diphenylamine group is suitable for constructing green and red host materials, whereas the introduction of carbazole group is suitable for constructing blue host materials.