Hole-blocking Material Research Articles

Red Phosphorescent Dopant/Polymeric Host System as an Emitting Layer: Effect of Hole Blocking Materials

Phosphorescent OLEDs with the configuration of ITO/PEDOT/EML/HBL/Alq3/LiF/Al were fabricated and their electroluminescent performance was investigated. Synthesized red phosphorescent iridium (III) bis(2-phenyl-isoquinoine) (2-acetyl-cyclohexane) (Ir-PIQCH) dopant and p{[N-(2-ethylhexyl)-3,6-bis(styryl) carbazole]-alt-[2,5-(4-biphenyl)-1,3,4-oxadiazole]} [P(CZ-OXD)] host were used as emitting layer in OLEDs. The devices showed red emission (λmax.EL about 660 nm), similar with PL emission of Ir-PIQCH dopant. EL properties of the phosphorescent OLEDs were measured to investigate the effect of hole blocking layers such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), bathocuproine (BCP), and bis(2-methyl-8-quinolinate)-4-phenylphenolate (BAlq) on luminescence and color purity of the devices. It was found that Device 3 using BCP as HBL showed pure red emission with CIE coordinates value as (x = 0.6673, y = 0.3231), due to higher energy barrier (1.03 eV) between P(CZ-OXD) host polymer and hole blocking BCP.

Four-coordinate boron compounds derived from 2-(2-pyridyl)phenol ligand as novel hole-blocking materials for phosphorescent OLEDs

Four-coordinate boron compounds of Ph2B · 1 (2) and (C6F5)3B(1 · H) (3) were prepared from the reaction of 2-(2-pyridyl)phenol (1 · H) ligand with triarylborane starting materials, BPh3 and B(C6F5)3, respectively, and tested as hole-blocking layer (HBL) materials in phosphorescent OLEDs. While the crystal structure of 2 reveals the pseudo-tetrahedral geometry around the boron center with bidentate [N,O] chelation by 1, 3 is characterized as the zwitterionic four-coordinate system where the ligand 1 · H acts as monodentate [O] chelator with N-protonation. UV–Vis absorption and PL spectra of 2 and 3 are consistent with the ligand-centered, HOMO–LUMO electronic transitions with charge transfer from a phenoxide ring to a pyridine, which was further supported by time dependent DFT calculation for 2. Both compounds are found to possess the HOMO–LUMO energy gap of 3.1 eV appropriate for hole-blocking materials for phosphorescent OLEDs. The devices incorporating 2 and 3 as HBL materials displayed stable green phosphorescence of Ir(ppy)3 (ppy = 2-phenylpyridine) with low turn-on voltage of 3.2 and 3.4 V, respectively, indicating that 2 and 3 function as HBL materials. Although both devices show the short lifetime (<1 h) probably owing to the low thermal stability, the device based on 2 displays better performances in terms of luminance, power and luminance efficiency, and external quantum efficiency in a wide range of current densities (0.1–100 mA/cm2) than the reference device incorporating BAlq as HBL materials.

High efficiency red organic light-emitting diodes using a phosphorescent iridium complex doped into a hole-blocking material

We demonstrate high-efficiency red electrophosphorescent organic light-emitting devices (OLEDs) by doping a red-emitting iridium complex, Bis[7-methyl-1-p- tolyisoquinolinato-N,C 2′]-iridium(III)(acetylacetonate) [(7-mtiq) 2Ir(acac)], into a hole-blocking material, 4-biphenyloxolato aluminum(III)bis(2-methyl-8- quinolinato)4-phenylphenolate. Both the phosphorescent characteristics of (7-mtiq) 2Ir(acac) and the electroluminescence mechanisms of OLEDs are investigated in this study. The Commission Internationale de L'Eclairage coordinates of (0.66, 0.34) is very close to the National Television System Committee standard red point (0.66, 0.33). With a dopant concentration of about 4%, a maximum luminance of 31317 cd/m 2 and a luminous efficiency of 21.6 cd/A have been obtained.

Zinc Complex Based on 2-(5-Methyl-2-hydroxyphenyl)benzotriazole: Synthesis and Electroluminescence Characteristics

Zinc (II) [2-(5-methyl-2-hydroxyphenyl)benzotriazole] (Zn(MHB)2) was synthesized and characterized by FT-NMR, FT-IR, UV-Vis, XPS and elemental analysis. The photoluminescence of Zn(MHB)2 was measured at 516 nm from the DMF solution. The HOMO (6.6 eV) and LUMO (3.1 eV) energy levels of Zn(MHB)2 were estimated from the measurement of cyclic voltammetry. The devices with structure of ITO/NPB/Zn(MHB)2/Alq3/LiF/Al were constructed to investigate their electroluminescent (EL) performance. Zn(MHB)2 was found to be a good hole-blocking material in the EL devices.

New Amorphous Hole Blocking Materials for High Efficiency OLEDs

Two new amorphous molecular materials, 2,5-bis(2',5'-dimethyl-4-triphenylsilyl-phenyl)-[1,3,4]oxadiazole (BDTSO) and 2,5-bis(2',5'-dimethyl-4-triphenylsilyl-phenyl)-[1,3,4]thiadiazole (BDTST) were synthesized and investigated as hole blocking materials (HBM) for organic light-emitting diodes. The efficiency of electroluminescent device was improved by using BDTSO instead of BAlq. The current and power efficiency of the device using BDTSO as HBM is 39.6 cd/A and 13.1 lm/W at 10 mA/cm2, respectively, which is higher compared to the same values for devices using BDTST and BAq which are typically used as HBM.

Theoretical Characterization of a Typical Hole/Exciton-Blocking Material Bathocuproine and Its Analogues

The structural, electronic, and carrier transport properties of bathocuproine (BCP), which is a typical hole/exciton-blocking material applied in organic light-emitting diodes (OLEDs), have been investigated based on density functional theory (DFT) and ab initio HF method. The detail characterizations of frontier electronic structure and lowest-energy optical transitions have been studied by means of time-dependent density functional theory (TD-DFT). Five BCP analogues, o-phenanthroline (1), 2,9-dimethyl-1,10-phenanthroline (2), 2,9-diphenyl-1,10-phenanthroline (3), 4,7-diphenyl-1,10-phenanthroline (4), and 2,9-bis(trifluoromethyl)-1,10-phenanthroline (5) have also been studied in order to select more suitable candidates of efficient hole-blocking materials. The calculated results showed that rigid planar structures, conjugate degrees, and substitute groups play crucial roles in the hole/exciton-blocking and electron-transport properties of these materials. The calculated geometries, ionization energies (IP), and energy gap between the singlet ground state and triplet excited state (E(T1)) were well in agreement with the experimental results. On the basis of the incoherent transport model, the calculated electron mobility of BCP is 1.79 x 10(-2) cm(2)/(V s), which is comparable to experimental results of 1.1 x 10(-3) cm(2)/(V s). The electron mobilities for compounds 1, 4, and 5 are 3.45 x 10(-2), 2.90 x 10(-2), and 1.40 x 10(-2) cm(2)/(V s), respectively. The calculated results indicated that compounds 1, 4, and 5 may be more effective hole/exciton-blocking materials than BCP.

Influence of charge balance and exciton distribution on efficiency and lifetime of phosphorescent organic light-emitting devices

We discuss the importance of appropriate charge carrier confinement and exciton management for the realization of highly efficient and stable organic light-emitting diodes (OLEDs). As an example, we choose red p-i-n-type OLEDs based on the iridium-based electrophosphorescent emitter Ir(MDQ)2(acac) doped in α-NPD as host material. We show how an appropriate choice of the hole blocking layer material allows external quantum efficiencies as high as 20% for this emitter. At the same time, the display-relevant brightness of 100 cd/m2 is reached at an operation voltage of only 2.4 V, which is close to the thermodynamic limit. As a result, a high total power efficiency of 37.5 lm/W at 100 cd/m2 is reached. In a further step, we study the influence of the blocker materials on device lifetime. We investigate the chemical reactions causing the degradation process by use of matrix assisted laser desorption time-of-flight mass spectrometry. It can be shown that discovered degradation reactions can be suppressed by an appropriate choice of the hole blocking material.

Multilayer bipolar field-effect transistors

Field-effect transistors comprising a layer of regioregular poly(3-hexylthiophene) (rr-P3HT) separated from a parallel layer of the soluble fullerene,[6,6]-phenyl C61-butyric acid methyl ester (PCBM) by a layer of titanium suboxide (TiOx), are fabricated by solution processing. Because the TiOx is an electron transporting material and a hole blocking material, this multilayer architecture operates either in the p-channel mode with holes in the rr-P3HT layer or in the n-channel mode with electrons in the PCBM layer.

Control of Electroluminescence Spectra Using Hole-Blocking Layer for White Organic Light-Emitting Diodes

We reviewed white organic light-emitting diodes (WOLEDs) briefly in this paper and suggested the significance of a hole-blocking material for the fabrication of WOLEDs. And we studied the electroluminescence (EL) spectra of two kinds of WOLEDs using each different hole-blocking material. The WOLED with blue-emitting aluminum quinoline (BAlq) had a broad EL spectrum. The broad spectra were caused by overlapping three emissions of 3 kinds of emission materials due to wide recombination zone. The WOLED with bathocuproine (BCP) showed two separated emissions. The difference between two specimens was concluded by strong hole-blocking property of BCP. The introduction of BCP produced a recombination zone due to hole accumulation at the BCP interface.

Aluminium–salen luminophores as new hole-blocking materials for phosphorescent OLEDs

The organic light-emitting diodes (OLEDs) employing complex [salen(tBu)4Al(OC6H4-p-C6H5)] (4) as a hole-blocking layer produced stable green EL emission of Ir(ppy)3 irrespective of changing current density and showed higher brightness and device efficiency than the BAlq-based device.

Application of organic bathocuproine-based alloy film to organic light-emitting diodes

Bathocuproine (BCP) is well known as a hole-blocking material and is often used in organic light-emitting diodes and organic solar cells. However, BCP film is immediately polycrystallized after vapor deposition. We achieved stabilization of vacuum-deposited BCP-based thin film using the organic alloy method. Organic alloy method led not only to the suppression of crystallization but also to improvement of electroluminescence properties of devices using a hole-blocking layer.

A yellow-emitting iridium complex for use in phosphorescent multiple-emissive-layer white organic light-emitting diodes with high color quality and efficiency

A cyclometalated iridium(III) complex containing 2-(9,9-diethylfluoren-2-yl)pyridine [ Ir(Flpy) 3 ] was prepared and used in the fabrication of both yellow and white organic light-emitting diodes (OLEDs). A hole-blocking material has been used as a hole barrier layer in-between different emission layers, helping the formation of the hole limitation region. With the proper position of a hole barrier layer and the construction of a four-emission-layer structure involving the use of [ Ir(Flpy) 3 ], the resulting WOLED shows sound device performance as well as very stable color even at high luminances. Such WOLEDs have been demonstrated to reveal superior white light color stability/efficiency trade-off optimization. The Commission Internationale de L’Eclairage (CIE) coordinate differences Δ x and Δ y are confined to ±0.015 when the luminance increases from 13 to 14806 cd/m 2. The color rendering index (CRI) of the device is also very good, which varies only from 86 to 87 by changing from the normal direction to 80° off-normal at 12 V. The peak electrophosphorescence efficiency can reach as high as 24.6 cd/A at 168 cd/m 2and it can still be kept at 17.2 cd/A at 10834 cd/m 2. Such outstanding performance renders this all-phosphor WOLED very attractive as a white light source for illumination applications, which typically demand high efficiency, high CRI, and stable color in high brightness work conditions.

Bipyridyl substituted triazoles as hole-blocking and electron-transporting materials for organic light-emitting devices

We have demonstrated bipyridyl substituted triazole derivatives (Bpy-TAZs) as an electron-transporting material for organic light-emitting devices (OLEDs). Substitution of triazole with bipyridyl is a good way to improve electron-transporting ability of triazoles with keeping good hole-blocking ability, which is a useful property of triazole derivatives. A Bpy-TAZ has high electron mobility of above 10−4cm2/Vs. Moreover, by employing one of Bpy-TAZs as a hole-blocking and electron-transporting material for phosphorescent OLEDs, lower operation voltage was achieved with keeping the same external quantum efficiency of electroluminescence (almost 10%) as compared with the conventional hole-blocking and electron-transporting bilayer consisting of bathocuproine and tris (8-hydroxyquinolinato) aluminum.

New oxadiazole derivatives as promising electron transport materials: synthesis and characterization of thermal, optical and electrochemical properties

Abstract2,5-bis-(4-biphenyl)-yl-1,3,4-oxadiazole (1a), 2,5-bis-(4-(6,8-difluoro)-biphenyl)-yl-1,3,4-oxadiazole (1b) and 2,5-bis-(4-(spiro-fluorenyl)-phenyl)-yl-1,3,4-oxadiazole (1c) were designed, synthesized and characterized. 1a–c were easily obtained from Suzuki reactions between 2,5-bis-(4-bromo-phynyl)-[1,3,4]oxadiazole (2) and aromatic boronic acids (3). They were characterized by 1H-NMR, DSC, TGA, UV-Vis, photoluminescence (PL) spectrometry and CV. The melting temperatures (T m) of 1a–c are 237, 208 and 370 °C, respectively, much higher than that of 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD, T m = 136 °C). The oxidation potentials of 1a–c are 1.86, 1.94 and 1.18 V, and their reduction potentials are −2.31, −2.22 and −2.27 V, respectively, indicating that the introduction of electronegative oxadiazole unit lowers the electron density in molecules and enhances their stabilities. The LUMO/HOMO energy levels of 1a–c are as low as −2.39/−6.56, −2.48/−6.69 and −2.43/−5.88 eV, respectively. The good thermal stabilities and low orbital levels of 1a–c make them promising electron-transporting or hole-blocking materials for organic optoelectronic devices.

Silane- and triazine-containing hole and exciton blocking material for high-efficiency phosphorescent organic light emitting diodes

One of the important factors for high efficiency phosphorescent organic light-emitting devices is to confine triplet excitons within the emitting layer. We synthesized and characterized a new hole blocking material containing silane and triazine moieties, 2,4-diphenyl-6-(4′-triphenylsilanyl-biphenyl-4-yl)-1,3,5-triazine (DTBT). Electrophosphorescent devices fabricated using the material as the hole-blocking layer and N,N′-dicarbazolyl-4,4′-biphenyl (CBP) doped with fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] as the emitting layer showed a maximum external quantum efficiency (ηext) of 17.5% with a maximum power efficiency (ηp) of 47.8 lm W−1, which are much higher than those of devices using bathcuproine (BCP) (ηext = 14.5%, ηp = 40.0 lm W−1) and 4-biphenyloxolate aluminium(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate (BAlq) (ηext = 8.1%, ηp = 14.2 lm W−1) as hole-blocking layers.

New hole blocking material for green-emitting phosphorescent organic electroluminescent devices

The new amorphous molecular material, 2,5-bis(4-triphenylsilanyl-phenyl)-[1,3,4]oxadiazole, that functions as good hole blocker as well as electron transporting layer in the phosphorescent devices. The obtained material forms homogeneous and stable amorphous film. The new synthesized showed the reversible cathodic reduction for hole blocking material and the low reduction potential for electron transporting material in organic electroluminescent (EL) devices. The fabricated devices exhibited high performance with high current efficiency and power efficiency of 45 cd/A and 17.7 lm/W in 10 mA/cm2, which is superior to the result of the device using BAlq (current efficiency: 31.5 cd/A and power efficiency: 13.5 lm/W in 10 mA/cm2) as well-known hole blocker. The ITO/DNTPD/α-NPD/6% Ir(ppy)3 doped CBP/2,5-bis(4-triphenylsilanyl-phenyl)-[1,3,4]oxadiazole as both hole blocking and electron transporting layer/Al device showed efficiency of 45 cd/A and maximum brightness of 3000 cd/m2 in 10 mA/cm2.

Synthesis and characterization of fluorescent poly(oxadiazole)s containing 3,4-dialkoxythiophenes

We report the synthesis, optical and electrochemical details, and properties of three conjugated copolymers (P1–P3) consisting of alternate hole-transporting thiophene derivative and electron-transporting 1,3,4-oxadiazole units. The polymers are prepared using the precursor polyhydrazide route. The polymers have well defined structure and exhibit good thermal stability with the onset decomposition temperature in nitrogen at around 300°C. All the polymers are soluble in organic solvents such as DMF, DMSO, NMP and in strong organic acids like trifluoroacetic acid. The optical and charge-transporting properties of the polymers are investigated by UV–visible spectroscopy, fluorescence emission spectroscopy and cyclic voltammetry. The UV–visible absorption spectra of polymers in solution showed a maximum at around 400nm. The polymers depicted green fluorescence both in solution and as thin films. Cyclic voltammetry studies reveal that these copolymers have low-lying LUMO energy levels ranging from −3.28 to −3.36eV and high-lying HOMO energy levels ranging from −5.5 to −5.56eV, which indicated that they may be promising candidates for electron-transporting or hole-blocking materials in light-emitting diodes.

General method to solution-process multilayer polymer light-emitting diodes

An intermediate liquid buffer layer is introduced to overcome the dissolution problem of solution-processed multilayer conjugated polymer light-emitting diodes. This method can be applied to arbitrary combinations of polymers with no restriction on solvents. As an example, a hole-blocking layer is successfully spin coated on the common p-type emissive polymer layers. One green- and two blue-emitting polymers are chosen as the emissive layers. The electron-hole balance and efficiency are significantly improved by the addition of hole-blocking layer. The electroluminescence efficiency can be increased up to nine times, while the luminance up to seven times. In particular, 1.5cd∕A is obtained for deep blue emission from poly(9,9-dioctyl-fluorene) with 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene spin coated as the hole-blocking material.

Characteristics of Electroluminescence Based on Zn Complexes

Recently, organic light-emitting diodes (OLEDs) have widely been used as one of the information display techniques. We synthesized 2-(2-hydroxyphenyl)benzoxazole [Zn(HPB)2], (1,10-phenanthroline)(8-hydroxyquinoline) [Zn(phen)q] and (2-(2-hydroxyphenyl)benzoxazole)(8-hydoxyquinoline) [Zn(HPB)q]. The ionization potential (IP) and electron affinity (EA) of Zn complexes were investigated by cyclic voltammetry (CV). The EAs of Zn(HPB)2, Zn(HPB)q, and Zn(phen)q were determined to be 2.8, 3.5, and 3.2 eV, respectively. Moreover, the IPs of Zn(HPB)2, Zn(HPB)q, and Zn(phen)q are 6.5, 6.8, and 7.3 eV, respectively. The EL spectra of Zn(HPB)2, Zn(HPB)q, and Zn(phen)q were observed at the wavelengths of 455, 532, and 535 nm, respectively. We used Zn(HPB)2 and Zn(HPB)q as hole-blocking materials and Zn(phen)q as an electron-transporting material. The efficiency of the device with Zn(HPB)2 increased compared with that of the device without it. The device with Zn(phen)q exhibited an improved performance Alq3 as an electron-transporting material.

Synthesis and characteristics of bis(2,4-dimethyl-8-quinolinolato)(triphenylsilanolato)aluminum (III): A potential hole-blocking material for the organic light-emitting diodes

A new five-coordinated bis(2,4-dimethyl-8-quinolinolato)(triphenylsilanolato)aluminum (III) (24MeSAlq) material, having bulky substituents, was prepared in one-step reaction and was characterized. The photoluminescent (PL) spectrum of 24MeSAlq shows the largest hypsochromic shift exhibiting the maximum wavelength at the peak of 461 nm among the blue-emitting q 2AlOR-type complexes (q = 8-quinolinolato ligand and OR = aryloxy or alkoxy ligand) reported. The deep blue device composed of ITO/2-TNATA (60 nm)/NPB (15 nm)/24MeSAlq (20 nm)/Alq 3 (45 nm)/LiF (1 nm)/Al (100 nm), which uses 24MeSAlq as a hole-blocking layer and applies a principle efficiently confining an exciton recombination zone into a hole transporting layer, shows the maximum electroluminescent (EL) at the peak of 446 nm originating from the NPB emissive layer. This is attributed to an excellent hole-blocking property due to the high HOMO (highest occupied molecular orbital) energy level (6.14 eV).