Hole-blocking Material Research Articles

New Series of Blue-Emitting and Electron-Transporting Copolymers Based on Cyanostilbene

A novel series of conjugated copolymers having fluorene and binaphthyl moieties in the main chain and based on cyanostilbene were synthesized in good yields by a palladium-catalyzed Suzuki coupling reaction, a new approach different from the traditional Knoevenagel condensation polymerization. Through controllable modification of the main chain structures, not only were the thermal, electronic, and optical properties of the polymers tuned, but also the structure−property relationships were studied. All these polymers possess excellent thermal stability with glass-transition temperatures of 60−159 °C and onset decomposition temperatures of 411−417 °C. Cyclic voltammetry studies reveal that these copolymers have low-lying LUMO energy levels ranging from −2.92 to −3.08 eV and low-lying HOMO energy levels ranging from −6.01 to −6.13 eV, and they may be promising candidates for electron-transporting or hole-blocking materials in light-emitting diodes (LEDs). The polymers in thin films emit strong blue luminance around 457−489 nm with narrow bandwidth upon photoexcitation. The single-layer LED fabricated with a copolymer F−CN composed of fluorene and cyanostilbene units using an air-stable aluminum electrode emits blue light with an external quantum efficiency of 0.006%. A double-layer LED, fabricated using a blend of poly(N-vinylcarbazole) and F−CN as emitting layer and tris(8-hydroxyquinolinato)aluminum as electron transporting layer, shows an external quantum efficiency of 0.2%. Preliminary electroluminescent results show that these polymers are new candidates for blue emissive materials in polymeric LEDs.

Polybenzobisazoles Are Efficient Electron Transport Materials for Improving the Performance and Stability of Polymer Light-Emitting Diodes

A series of seven polybenzobisazoles were investigated as electron transport materials in arylene vinylene polymer-based electroluminescent devices. A large enhancement in performance and stability was observed in poly(p-phenylene vinylene) and poly(2-methoxy-5(2‘-ethyl-hexyloxy)-1,4-phenylene vinylene) light-emitting diodes by using polybenzobisthiazoles and poly(p-phenylene benzobisoxazole) as electron-transport materials. Devices using polybenzobisazole electron transport layers and aluminum cathodes had a turn-on voltage as low as 2.8 V, a luminance of up to 1400 cd/m2, and an external quantum efficiency of up to 2.5%. These polymer devices and their performances were stable under repeated testing over a period of 9−10 months storage in air. The superior performance of the polybenzobisazole thin films as electron-transport and hole-blocking materials in polymer light-emitting diodes is due to their high glass-transition temperature, environmental resistance, and photochemical/electrochemical stability...

High operational stability of electrophosphorescent devices

Electrophosphorescent devices with fac-tris(2-phenylpyridine)iridium as the green emitting dopant have been fabricated with a variety of hole and exciton blocking materials. A device with aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolate (BAlq) demonstrates an efficiency of 19 cd/A with a projected operational lifetime of 10 000 h, operated at an initial brightness of 500 cd/m2; or 50 000 h normalized to 100 cd/m2. An orange-red electrophosphorescent device with iridium(III) bis(2-phenylquinolyl-N,C2′)acetylacetonate as the dopant emitter and BAlq as the hole blocker demonstrates a maximum efficiency of 17.6 cd/A with a projected operational lifetime of 5000 h at an initial brightness of 300 cd/m2; or 15 000 h normalized to 100 cd/m2. The average voltage increase for both devices is <0.3 mV/h. The device operational lifetime is found to be inversely proportional to the initial brightness, typical of fluorescent organic light emitting devices.

New Series of Blue-Emitting and Electron-Transporting Copolymers Based on Fluorene

A novel series of conjugated copolymers having oxadiazole, quinoline, quinoxaline, and phenylenecyanovinylene moieties in the main chain based on fluorene were synthesized in good yields by palladium-catalyzed Suzuki coupling reaction, a new approach different from the traditional polyhydrazide precursor route (oxadiazole-containing polymers), acid-catalyzed Friedlander condensation reaction (polyquinolines), and Knoevenagel condensation polymerization (poly(phenylenecyanovinylene)). The thermal, electrochemical, and optical properties of these copolymers were examined. All these polymers possess excellent thermal stability with glass transition temperatures of 114−208 °C and onset decomposition temperatures of 387−415 °C. Cyclic voltammetry studies reveal that these copolymers possess low-lying LUMO energy levels ranging from −3.01 to −3.37 eV and low-lying HOMO energy levels ranging from −6.13 to −6.38 eV and may be promising candidates for electron-transporting or hole-blocking materials in light-emitt...

Synthesis, Properties, and Applications of Tetraphenylmethane-Based Molecular Materials for Light-Emitting Devices

Tetraphenylmethane-based compounds, tetrakis(4-(5-(3,5-di-tert-butylphenyl)-2-oxadiazolyl)phenyl)methane (TBUOXD), tetrakis(4-(5-(3,4-dimethoxyphenyl)-2-oxadiazolyl)phenyl)methane (OMEOXD), tetrakis(4-(5-(3-(α,α,α-trifluoromethylphenyl))-2-oxadiazolyl)phenyl)methane (CF3OXD), tetrakis(4-(5-(4-diphenylaminophenyl)-2-oxadiazolyl)phenyl)methane (p-TPAOXD), and tetrakis(3-(5-(4-diphenylaminophenyl)-2-oxadiazolyl)phenyl)methane (m-TPAOXD), were synthesized. These compounds were characterized by NMR, microanalysis, UV−visible spectroscopy, fluorescence spectroscopy, cyclic voltammetry, DSC, and TGA. TBUOXD, OMEOXD, and CF3OXD showed rather similar electrochemical and spectroscopic characteristics to those of the hole-blocking material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD). However, TBUOXD, OMEOXD, and CF3OXD all crystallize well above 200 °C, much higher than the 70−90 °C observed for PBD. In addition, high onset glass transition temperatures of 97−175 °C were detected for the compounds. Both CF3OXD and PBD were adopted as a hole-blocking layer in organic light-emitting devices (OLEDs). A comparable performance with an external quantum efficiency of ∼1% was obtained for devices based on both CF3OXD and PBD, although a much lower allowed current density was found for the CF3OXD-based device. The bipolar blue fluorescent isomers p-TPAOXD and m-TPAOXD are both amorphous, showing no melting and crystallization transition. Only glass transitions at 187 and 149 °C were observed respectively. Single-layer OLEDs based on these two compounds were also fabricated by varied fabrication conditions (solvent, cathode material, film thickness, polymer blending). A turn-on voltage as low as 4 V, blue electroluminescence with an intensity of 1700 cd/m2, and a photometric efficiency more than 0.8 cd/A can be achieved with p-TPAOXD-based devices.

Hole blocking in carbon nanotube–polymer composite organic light-emitting diodes based on poly (m-phenylene vinylene-co-2, 5-dioctoxy-p-phenylene vinylene)

In order to investigate the role of carbon nanotubes in a polymer matrix, organic light-emitting diodes were fabricated from a polymer composite composed of poly (m-phenylene vinylene-co-2,5-dioctoxy-p-phenylene) (PmPV) and dispersed single-wall carbon nanotubes (SWNTs). Tris-(8-hydroxyquinolinolato) aluminum (Alq3) doped by Nile Red was used as an emissive material between the polymer composite and cathode. The device fabricated without SWNTs dispersed in the PmPV shows a dominant emission near red at 600 nm, which is in the range of the characteristic emission of Nile Red-doped Alq3, with a small amount of green emission from the PmPV. However, the devices fabricated with the polymer composite show an increase in the oscillator strength of the green emission with a dominant emission peak near 500 nm, the characteristic emission of PmPV. This was observed for SWNT concentrations up to 0.1 wt %. The shift in the emission indicates that the SWNTs in the PmPV matrix act as a hole-blocking material that results in a shifting of the recombination region from the Nile Red-doped Alq3 layer to the PmPV composite layer.

Hole-blocking characteristics of 2,4,6-triphenyl-1,3,5-triazine as a non-transporting filler material

2,4,6-Triphenyl-1,3,5-triazine (TTA) has been used as a hole-blocking material for electroluminescent devices (ELD) [R. Fink, C. Frenz, M. Thelakkat, H.W. Schmidt, Macromolecules, 30, (1997) 8177]. The EL devices of the four layer structure, ITO/hole transporting layer(HTL)/emitting layer/Al, were fabricated. TTA was doped into the poly( n-vinylcarbazole) (PVK), which was spin-coated on indium tin oxide (ITO) film. The emitting layer, tris(8-hydroxyquinolinato) aluminum(Alq 3), was deposited on top of PVK film. The ELD emits characteristic green color generated from Alq 3 emitting layer. As the percentage of TTA was increased, the efficiency of devices increased. The devices, which contained higher concentration of blocking material, were driven at a lower current under the same voltage. The optimum concentration of hole blocking material was 10 wt.%, which produced the highest ELD luminescence.

Polymer light-emitting diode prepared with an ionomer and polyaniline

Light-emitting diodes of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) have been fabricated from sodium sulphonated polystyrene (SSPS) ionomer as an electron-injecting and hole-blocking material and polyaniline doped with camphor sulphonic acid (PANI-CSA) as a polymer anode. The presence of PANI in an ITO/PANI/MEH-PPV/AI device makes little difference to its optical output and quantum efficiency. SSPS shows good electron injection in an ITO/MEH-PPV/SSPS/AI device. When SSPS ionomer is introduced in an ITO/PANI/MEH-PPV/SSPS/A1 device, a larger enhancement in optical output is obtained and its quantum efficiency is also enhanced because of its balanced charge injection. The electroluminescent device associated with SSPS ionomer and PANI is a well-designed structure to improve stability and efficiency. Copyright © 2000 John Wiley & Sons, Ltd.

A novel diamine compound containing carbazole groups for organic electroluminescent devices

We have synthesized a novel diamine compound (DCA) containing carbazole groups. The compound has an electron-transport ability and a high thermal stability, and a high ionization potential ( Ip). Green-emitting electroluminescent (EL) devices with the compound as a hole-blocking material have been fabricated. The luminance of the device achieved 10000 cd/m 2 and the EL efficiency was improved. Devices capable of emitting bright green, blue, or red emissions with DCA as an electron-transport layer have been fabricated.

Synthesis and Application of Dimeric 1,3,5-Triazine Ethers as Hole-Blocking Materials in Electroluminescent Devices

A series of low molecular weight dimeric 1,3,5-triazine ethers with high glass transition temperatures is synthesized from 2-(4-fluorophenyl)-4,6-diphenyl-1,3,5-triazine and various bisphenols. The thermal and electrochemical properties of these materials are examined using differential scanning calorimetry and cyclic voltammetry, respectively. The glass transition temperatures (Tg) are in the range of 106−144 °C, and all of them have similar thermal stability and show no weight loss up to 380 °C in thermogravimetric measurements. Some of these dimers form stable glasses which do not recrystallize on annealing at or above the Tg. Cyclic voltammetry studies reveal that these compounds undergo reversible reduction between −2.09 and −2.27 V vs ferrocene/ferrocenium as internal standard which correspond to LUMO values between −2.5 and −2.7 eV, respectively. The application of one of these dimeric ethers as a hole-blocking/electron-transport material in organic light-emitting devices is demonstrated.

Use of ionomer as an electron injecting and hole blocking material for polymer light-emitting diode

Light-emitting diodes of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEHPPV) are fabricated using sodium sulfonated polystyrene (SSPS) ionomer containing 10 mol % ionic groups as an electron injecting and hole blocking material. When an electric field stronger than 2.7×107 V/m is applied, SSPS in the indium–tin oxide ITO/MEH-PPV/SSPS/Al system causes a bridging effect between sodium ions and the Al cathode at the SSPS/Al interface, leading to excellent electron injection. Furthermore, the ionomer has a high band gap energy of ∼5 eV, resulting in hole blocking. The operating voltage for the ITO/MEH-PPV/SSPS/Al is reduced by ∼60% and the relative quantum efficiency is enhanced by three orders of magnitude compared with those of the corresponding single-layer MEH-PPV device.

Polymer Light-Emitting Diode with Ionic Material as an Electron Injecting and Hole Blocking Layer

Light-emitting diodes (LEDs) of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) were fabricated using sodium sulfonated polystyrene (SSPS) ionomer containing 10 mol% ionic groups as an electron injecting and hole blocking material. When a device was structured as ITO/MEH-PPV/SSPS/Al, the operating voltage was reduced by ∼60 % and the relative quantum efficiency was thousand-fold enhanced compared with the corresponding single-layer MEH-PPV device