Red Phosphorescent Materials Research Articles

Novel bipolar host materials including carbazole and dioxy[2,3-b] pyrazine units for red phosphorescent organic light-emitting diodes

In this work, two novel bipolar host materials, 3,5-bis(9H-carbazol-9-yl) phenyl) benzo [1,4,5,6] dioxy [2,3-b] pyrazine (a-CzDp) and 7-(9-phenyl-9H-[3,9′- bicarbazole]-3′-yl) benzo [1,4,5,6] dioxy [2,3-b] pyrazine (b-CzDp) were designed and synthesized. In these two host materials, carbazole and dioxy [2,3-b] pyrazine act as the hole and electron transport unit, respectively. The two dipolar hosts both exhibit good thermal stability and deep highest occupied molecular orbital (HOMO) energy levels of -5.31 eV and −5.39 eV. The phosphorescent organic light-emitting diodes (PhOLEDs) including a-CzDp and b-CzDp as host and red phosphorescent material bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2acac) as guest in the emitting layer. The corresponding PhOLED devices with a-CzDp and b-CzDp as host exhibit the maximum brightness of 8932 cd m−2 and 12782 cd m−2 at 624 nm, together with the maximum external quantum efficiency (EQE) of 10.32% and 13.82%, respectively. B-CzDp based red PhOLED device shows higher EQE and lower turn-on voltage due to the more balanced hole and electron transport. The results demonstrate the potential of a-CzDp and b-CzDp as efficient host for red PhOLEDs.

Solution-processable red phosphorescent OLEDs based on Ir(dmpq)2(acac) doped in small molecules as emitting layer

Stable red phosphorescent materials are of high importance in OLED applications and a promising scenario relies on the organometallic emitters, such as phosphorescent dopants, constituting an effective emissive layer of OLEDs, which is usually embedded in an appropriate host matrix. Iridium (III) complexes are the most widely used dopants in Phosphorescent OLEDs because of their high internal quantum efficiency. In this work, Bis(2-(3,5-dimethylphenyl)quinoline-C,N)(acetylacetonato)iridium(III), (Ir(dmpq)2(acac)), was doped in four different small molecule host materials, such as 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP), 1,3-Bis(N-carbazolyl)benzene (mCP), 1,1-Bis[(di-4-tolylamino) phenyl]cyclohexane (TΑPC) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA), with doping concentrations of 4%, 6%, and 8%. Τhe net host materials and the Ir(dmpq)2(acac) were first characterized in respect of their optical and photophysical properties. Following, the doped thin films were implemented as the active layers in OLED devices, formed via the solution deposition method. It was found that OLED devices fabricated for all studied cases emit red light, a characteristic of Ir(dmpq)2(acac), with a maximum wavelength of approximately 620 nm. By exploring the different combinations between the host and the dopant, as well as the different doping concentrations, we aim to provide insights into the selection of the best performing host materials for PhOLEDs.

Interfacial Exciplex Host to Release Interfacial Accumulated Charges for Highly Efficient and Bright Solution‐Processed White Organic Light‐Emitting Diodes

AbstractWhite organic light‐emitting diodes (WOLEDs) have attracted extensive attention and are considered as an ideal next‐generation lighting source. However, maintaining high efficiency and high color rendering index (CRI) in WOLEDs still faces great challenges. The charges accumulation at the interface leads to nonradiative recombination and further Joule heat, which is a source of efficiency decrease and device degradation. In this paper, the interfacial exciplex is designed as the host for high efficiency; the blue thermally activated delayed fluorescence (TADF) material and the red phosphorescent material are employed as guests for high CRI. Due to the interfacial exciplex host, electrons don't need to overcome the potential barrier to inject into the light‐emitting layer, but can directly compound with holes at the interface, so as to release the interfacial accumulated charges and improve the recombination efficiency of carriers. Also, a suitable energy level gradient for hole injection transmission is constructed. Finally, WOLEDs with a current efficiency of 52.8 cd A−1, an external quantum efficiency of 22.9 %, the Commission Internationale de I'Eclairage (CIE) coordinate of (0.35,0.39), and the high CRI of 80 are obtained, which are the highest values ever achieved by the solution processed WOLEDs based on blue TADF emitters.

The Effect of Recombination Zone Positon on the Color Temperature Performance in the Color-Tunable White Organic Light-Emitting Diode

Using a color-tunable organic light-emitting diode (CT-OLED) can accord with the circadian cycle of humans and realize healthy lighting. The variation range of the correlated color temperature (CCT) is an important parameter to measure the performance of CT-OLEDs. In this paper, the effect of changing the utilization of phosphorescent materials and the position of the recombination zone (RZ) in the device are investigated by changing the thickness of the emissive layer (EML) and the doping ratio of the host and guest materials. The results show that reducing the red phosphorescent material and improving the blue phosphorescent material can affect the change direction of CCT, but it is not enough to expand the span of CCT (ΔCCT). It is more conducive to improving ΔCCT by more reasonable regulation of the position of the main RZ in EML and the energy transfer from the blue sub-EML to the red sub-EML. Device D obtains the best electro-optic and spectral characteristics, in which the maximum ΔCCT is 5746 K (2661 - 8407 K) as the voltage changes from 3.75 V to 9.75 V, the maximum current efficiency and luminance reach 18.34 cd·A-1 and 12,100 cd·m-2, respectively.

Efficient Sky‐Blue Bipolar Delayed Fluorescence Luminogen for High‐Performance Single Emissive Layer WOLEDs

AbstractLuminogens with high photoluminescence efficiencies and high triplet (T1) energy levels are desired to achieve high‐performance white organic light‐emitting diodes (WOLEDs). In this work, a sky‐blue luminogen (DCB‐BP‐SFAC) with enhanced delayed fluorescence in aggregated state, bipolar carrier transport ability and high T1 energy level is developed. DCB‐BP‐SFAC exhibits bright sky‐blue electroluminescence (EL) in both non‐doped and doped OLEDs with maximum external quantum efficiencies of 14.8% and 23.5%. DCB‐BP‐SFAC can function as efficient host for green, orange and red phosphorescent materials as well as thermally activated delayed fluorescence emitters with excellent EL performance. In addition, fluorescence/phosphorescence hybrid and all‐fluorescence WOLEDs with a single emissive layer are fabricated by adopting DCB‐BP‐SFAC as emitter and host simultaneously. The all‐fluorescence WOLED shows two‐color warm‐white light with good EL efficiencies of 47.1 cd A−1, 46.2 lm W−1, and 19.1%. The excellent performance of these OLEDs demonstrates the great application potential of DCB‐BP‐SFAC in practical display and lighting devices.

Synthesis and Characterization of High-Efficiency Red Phosphorescent Iridium(III) Complexes with 1-(4-(Trifluoromethyl)phenyl)isoquinoline Ligand

Two new tfmpiq-based bis-cyclometalated iridium(III) complexes, [(tfmpiq)2Ir(imdzppo)] (2a) and [(tfmpiq)2Ir(idzpo)] (2b) (where tfmpiq = 1-(4-(trifluoromethyl)phenyl)isoquinoline, imdzppo = 2-(imidazo[1,2-a]pyridin-2-yl)phenol, idzpo = 2-(2H-indazol-2-yl)phenol), have been synthesized and fully characterized. The single crystal structure of 2b has been determined. The relationship between the structures and photophysical properties of both complexes are considered, and the DFT calculations have been used to further support the deduction. These Ir(III) complexes emit red light with quantum yields of 39.9-51.9% in degassed CH2Cl2 solution at room temperature. Also, their emission originates from a hybrid 3MLCT/3LLCT/3LC excited state. All these results show that iridium(III) complexes 2a-2b are suitable for red-phosphorescent materials in OLEDs.

Efficient hybrid white organic light-emitting diodes for application of triplet harvesting with simple structure

In this study, we fabricated hybrid white organic light-emitting diodes (WOLEDs) based on triplet harvesting with simple structure. All the hole transporting material and host in emitting layer (EML) of devices were utilized with same material by using N,N'-di-1-naphthalenyl-N,N'-diphenyl-[1,1':4',1":4",1"'-quaterphenyl]-4,4"'-diamine (4P-NPD) which were known to be blue fluorescent material. Simple hybrid WOLEDs were fabricated three color with blue fluorescent and green, red phosphorescent materials. We was investigated the effect of triplet harvesting (TH) by exciton generation zone on simple hybrid WOLEDs. Characteristic of simple hybrid WOLEDs were dominant hole mobility, therefore exciton generation zone was expected in EML. Additionally, we was optimization thickness of hole transporting layer and electron transporting layer was fabricated a simple hybrid WOLEDs. Simple hybrid WOLED exhibits maximum luminous efficiency of 29.3 cd/A and maximum external quantum efficiency of 11.2%. Commission Internationale de l'Eclairage (International Commission on Illumination) coordinates of (0.45, 0.43) at about 10,000 cd/m^2.

Solution-processable iridium phosphors for efficient red and white organic light-emitting diodes with low roll-off

A new red phosphorescent material Ir(dmppm)2(dmd) is synthesized and successfully used to fabricate solution-processed red and white organic light-emitting diodes.

청색 형광과 적색 인광 물질을 사용한 백색 OLED의 발광 특성

We studied white organic light-emitting diodes using blue fluorescent and red phosphorescent materials. White single OLEDs were fabricated using SH-1 : BD-2 (3 vol.%) and CBP : (2 vol.%) as emitting layer (EML). The white single OLED using SH-1 : BD-2 (3 vol.% 8 nm) / CBP : (2 vol.% 22 nm) as emitting layer showed maximum current efficiency of 8.8 cd/A, Commission Internationale de l'Eclairage (CIE) coordinates of (0.403, 0.351) at , and variation of CIE coordinates with (, ) from 500 to . The white tandem OLED using SH-1 : BD-2 (3 vol.% 12 nm) / CBP : (2 vol.% 18 nm) showed maximum efficiency of 19.6 cd/A, CIE coordinates of (0.354, 0.365) at , and variation of CIE coordinates with (, ) from 500 to . Maximum current efficiency of the white tandem OLED was more twice as high as the single OLED. Our findings suggest that tandem OLED was possible to produce improved efficiency and excellent color stability.

Synthesis and Photophysical Properties of Iridium(III) Complex with Cyclometalated Styrylbenzothiazole Ligand

New red phosphorescent material, (E)-2-(4-fluorostyryl) benzo [d] thiazole (4-FSBT) iridium (III) dibenzoylmethane (DBM) complex [(4-FSBT)2Ir (DBM)] is synthesized from 2-methyl benzothiazole, 4-fluorobenzaldehyde, iridium trichloride anhydrate and DBM. The complex is characterized by MS and 1H-NMR. (4-FSBT)2Ir (DBM) gives a photoluminescence at 625.8nm, phosphorescent quantum yield: Φ(4-FSBT)2Ir (DBM) = 0.2 and EHOMO = -4.7 eV, ELUMO = -2.5eV, respectively. The complex may be a promising electrophosphorescent material.

Performance of blue fluorescence and red phosphorescent organic light‐emitting diodes using a molecular material with high hole drift mobility

AbstractThis study focuses on the carrier balance factor and transient characteristics of organic light‐emitting diodes (OLEDs) utilizing tris[4‐(5‐phenylthiophen‐2‐yl)phenyl] amine (TPTPA) as a blue emissive material or a host material of a red phosphorescent dye, which exhibits the highest level of hole drift mobility (µ = 1.0×10‐2 cm2V‐1s‐1) among those reported for organic disordered systems. We investigated the doping effect of a red phos‐phorescent material in the TPTPA. The peak wavelengths and the shape of the electroluminescent (EL) spectra agree with those of the photoluminescent spectra, which indicate that the EL originates from the excited state of the TPTPA and dopant. The maximum luminance of approximately 11,000 and 2,000 cd/m2 were achieved for devices without and with dopant. While TPTPA device achieved good carrier balance, the hole current was decreased by doping a red phosphorescent material. Compared with the short response times of TPTPA device, relatively long response times of doped device was observed owing to phosphorescent recombination lifetime of a dopant. The TPTPA device exhibited the relatively high light outcoupling efficiency of approximately 34% (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic field effects on electroluminescence emanated simultaneously from blue fluorescent and red phosphorescent emissive layers of an organic light-emitting diode

A simple method by employing white organic light-emitting diode (OLED) to simultaneously study the effects of magnetic fields (MFEs) on the fluorescent and phosphorescent emission was presented. The white OLED consisting of the blue fluorescent and red phosphorescent materials was fabricated. The low-pass filter chopped at 500nm and the high-pass filter chopped at 550nm were used to separate the MFEs on the blue fluorescence and red phosphorescence, separately. The magnetoelectroluminescences (MELs) on the fluorescence sharply increased at low magnetic field then tended to saturation at high magnetic field. While the MELs on the phosphorescence sharply increased at low magnetic field then slowly monotonically increased as the magnetic field further increased. Our results indicated that the magnetic-field modulated singlet-to-triplet electron–hole pair conversion combining with the magnetic-field modulated triplet–triplet annihilation process are the origins of the MFEs on the electroluminescence.

Application of Fused-Heterocyclic Compounds in Red Phosphorescent Iridium(III) Complexes

Organic light-emitting diodes (OLEDs) have attracted considerable attention during the last decade because of the potential advantages associated with their usage in full-color displays. Electroluminescent phosphorescent materials have also enjoyed similar levels of attention because of their excellent luminescent properties. The three primary colors, blue, green, and red, are essential for the practical application of phosphorescent complexes in flat-panel displays. Relative to the efficient green phosphorescent materials, red phosphorescent materials suffer many problems, including poor color purity, low efficiency, and low luminance, and the design of appropriate red colored materials therefore represents a significant challenge. Fused-heterocyclic compounds have been widely used in red phosphorescent iridium(III) complexes because of their high quantum efficiency, color adjustability, and balanced charge injection and migration. This review summarizes the development and application of fused-heterocyclic compounds in recent years in small molecule, dendrimer and polymeric red phosphorescent iridium(III) complexes. The influence of the molecular structures of iridium(III) complexes on the optical and electrical properties and device performances has also been described. Finally, a discussion of the prospect of developing fused-heterocyclic compounds in the red phosphorescent materials has also been provided.

Synthesis and Photophysical Properties of Red-Emitting Iridium （Ⅲ） Complex Bearing Benzothiazole

New red phosphorescent material, (E)-2-(2-(thiophen-2-yl) vinyl)benzothiazole(2THSBT) iridium(III) acetylacetone(acac) complex [(2THSBT)2Ir(acac)] is synthesized from 2-methyl benzothiazole, thiophene-2-carbaldehyde and iridium trichloride anhydrate. The complex is characterized by MS and 1H-NMR. (2THSBT)2Ir(acac) gives a photoluminescence at 687nm, phosphorescent quantum yield: Φ(2THSBT)2Ir(acac)=0.2 and EHOMO = -4.8 eV, ELUMO = -2.6eV, respectively. The complex may be a promising electrophosphorescent material.

Pure red electrophosphorescence from polymer light-emitting diodes doped with highly emissive bis-cyclometalated iridium(III) complexes

In order to develop highly emissive red phosphorescent materials for OLED application, novel bis-cyclometalated iridium(III) complexes were developed using the 1-(dibenzo[ b, d]furan-4-yl)isoquinolinato- N, C 3′ ( dbfiq) cyclometalating ligand. When 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate ( bdbp) is employed as an ancillary ligand, Ir( dbfiq) 2( bdbp) 1 exhibits red photoluminescence (PL) at 640 nm with a quantum yield ( Φ PL) of 0.61 (in toluene, 298 K). Replacement of bdbp to dipivaloylmethanate ( dpm) and acetylacetonate ( acac) (Ir( dbfiq) 2( dpm) 2 and Ir( dbfiq) 2( acac) 3, respectively) does not affect the PL spectrum, but reduces Φ PL to 0.55 and 0.49 for 2 and 3, respectively. Similar tendency is also found in the doped poly(methyl methacrylate) (PMMA) film, and 1 is more emissive ( Φ PL = 0.17) than 2 and 3 ( Φ PL = 0.08 and 0.06, respectively). Using 1 as a phosphorescent dopant, polymer light-emitting diodes (PLEDs) were fabricated, of which structure was ITO/PEDOT:PSS (40 nm)/PVCz: 1:PBD (100 nm)/CsF (1 nm)/Al (250 nm). Pure red electroluminescence (EL) is obtained from the fabricated PLEDs, affording a CIE chromaticity coordinate of (0.68, 0.31). When 0.51 mol% of 1 is incorporated in the PVCz-based emitting layer, the PLED shows maximum luminance of 7270 cd m −2 at 16.5 V, power efficiency of 1.4 lm W −1 at 7.5 V, and external quantum efficiency of 6.4% at 9.0 V. PLEDs with the same structure and components were also fabricated using 2 and 3, and their device characteristics were investigated. In proportion to the PL quantum yields, 1 affords better device performance than 2 and 3. Owing to four butoxy groups introduced to the bdbp ligand, 1 exhibits high solubility in organic solvents such as chloroform and toluene, and thus, is an excellent red phosphorescent dopant for solution-processed OLEDs.

White Organic Light Emitting Diodes using Red and Blue Phosphorescent Materials with Blocking Layer

High-efficiency white organic light-emitting diodes(WOLEDs) were fabricated with two emissive layers and an blocking layer was sandwiched between two phosphorescent dopants, bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III(FIrpic) as the blue emission and a newly synthesized red phosphorescent material guest, bis(5-acetyl-2-phenylpyridinato-N,C2`) acetylacetonate(). This blocking layer prevented a T-T annihilation in a red emissive layer, and balanced with blue and red emission as blocking of hole carriers. The white device showed Commission Internationale d`Eclairage() coordinates of (0.317, 0.425) at 22400 , a maximum luminance of 27300 at 268 , a maximum luminous efficiency and power efficiency of 26.9 cd/A and 18.6 lm/W.

Electroluminescent characteristics of novel platinum–porphyrin complex

Novel platinum complex of porphyrin with methyl and cyclohexyl substituents, PtMECHP, was designed and synthesized for use as a red phosphorescent material. PtMECHP was employed as an emission dopant in the OLED and the elctro-optical characteristics were investigated. The OLED was fabricated by vacuum deposition, and the layer-structure of device was ITO/NPD/PtMECHP(10%) in CBP/BCP/Alq/LiF/Al to exhibit brightness of 53cd/m2 at 11mA/cm2 and the external quantum efficiency of 1.2% at the same current density. The maximum EL band was detected at 650nm with red phosphorescence.

Theoretical Study of Ir(III) Complexes of Fluorinated Phenylbenzoquinoline as Red Phosphorescent Material

In this study, Ir(III) complexes with fluorinated 4-benzoquinoline (pbq) ligands were designed and characterized theoretically. The Hartree–Fock (HF) method with the 3-21G(d) basis set and density functional theory (DFT) utilizing the B3LYP functional with the 6-31G(d) basis set were used for the geometry optimization and the energy level calculation of the ground state of these complexes, respectively. Excited triplet and singlet states are examined using the time-dependent density functional theory (TD-DFT). As a result, it was found that these complexes produced a pure red emission due to the elongated conjugation length. In particular, it is concluded that the Ir(III) complex with pbq-CF3 ligands exhibits the largest emission efficiency and emits light of the purest red wavelength.

Color tuning of iridium complexes – Part I: Substituted phenylisoquinoline-based iridium complexes as the triplet emitter

A series of new iridium complexes with isoquinoline derivative ligands were synthesized for application in organic light-emitting diodes (OLEDs). It is demonstrated that varying the substituents at the 2′- or 4′-positions of the isoquinoline ligand makes the color tuning possible. Because of the steric effect, the 6′-substituted complexes: bis[1-(6′-methyl)phenylisoquinolinato-N,C 2′] iridium(III) (acetylacetonate) ( 6b1), bis[1-(6′-trifluoromethyl)phenylisoquinolinato-N,C 2′] iridium(III) (acetylacetonate) ( 6b2), and bis[1-(6′-methoxy)phenylisoquinolinato-N,C 2′] iridium(III) (acetylacetonate) ( 6b3) show red-shift effect with respect to the 4′-substituted complexes: bis[1-(4′-methyl)phenylisoquinolinato-N,C 2′] iridium(III) (acetylacetonate) ( 6a1), bis[1-(4′-trifluoromethyl)phenylisoquinolinato-N,C 2′] iridium(III) (acetylacetonate) ( 6a2), and bis[1-(4′-methoxy)phenylisoquinolinato-N,C 2′] iridium(III) (acetylacetonate) ( 6a3). All of these complexes are suitable for the red phosphorescent materials in OLEDs.

High-efficiency white phosphorescent organic light-emitting devices with greenish-blue and red-emitting layers

We demonstrate high-efficiency white light emission from a phosphorescent organic light-emitting device with greenish-blue and red emissive layers. Bis{2-[3,5-bis(trifluoromethyl)phenyl]-pyridinato-N,C2′}iridium(III)picolinate [(CF3ppy)2Ir(pic)] and bis{2-[2′-benzo(4,5-a)thienyl]pyridinato-N,C3′}iridium(III)acetylacetonate were employed for the greenish-blue and red phosphorescent materials for the emissive layers. To enhance the emission efficiency of (CF3ppy)2Ir(pic), 4,4′-bis(9-carbazolyl)-2,2′-dimethyl-biphenyl was used as the charge-transporting host. White light emission was obtained by controlling the layer thickness and the concentration of each phosphorescent material in the emissive layers. A maximum external quantum efficiency of 12%, a luminance efficiency of 18 cd/A, and a maximum power efficiency of 10 lm/W were achieved at a current density of 0.01 mA/cm2. This high efficiency is primarily attributed to the improvement of greenish-blue emission efficiency due to the strong confinement of triplet excitons on the (CF3ppy)2Ir(pic) molecules.