Chang-Hao Chuen

Highly-bright white organic light-emitting diodes based on a single emission layer

A very bright white organic light-emitting diode (OLED) was fabricated with a thin layer of 4-{4-[N-(1-naphthyl)-N-phenylaminophenyl]}-1,7-diphenyl-3,5-dimethyl-1,7-dihydro-dipyrazolo [3,4-b;4′3′-e]pyridine (PAP-NPA) doped with rubrene as the source of the white emission. Thus, with a simple three-layer structure of ITO/NPB(40 nm)/PAP-NPA:0.5% rubrene(20 nm)/TPBI(40 nm)/Mg:Ag, a white light with Commission Internationale de l’Eclairage (CIE) coordinates of (0.31, 0.33) were generated. The device gave a maximum brightness of ∼42 000 cd/m2 at 14 V, and maximum luminance efficiencies of 2.92 lm/W at 6.5 V and 6.11 cd/A at 7.0 V. The CIE coordinates stayed virtually constant when the voltage increased from 8 to 12 V. Furthermore, with a two-layer structure of ITO/PAP-NPA:0.4%rubrene(40 nm)/TPBI(40 nm)/Mg:Ag, the device also reached a stable white color with maximum brightness of ∼37 000 cd/m2 and maximum luminance efficiencies of 2.51 lm/W at 6.5 V and 5.57 cd/A at 8.5 V. The stability of the white color is a...

Hexaphenylphenylene dendronised pyrenylamines for efficient organic light-emitting diodes

A series of blue-emitting triarylamines containing pyrene and hexaphenylbenzene dendrons were successfully synthesized by employing the Diels–Alder and palladium catalyzed C–N coupling reactions. They display high glass transition temperatures (>140 °C) in differential scanning calorimetry and facile reversible oxidation couple in cyclic voltammetry. They are also thermally stable exhibiting decomposition temperature above 385 °C. Efficient blue-emitting electroluminescent devices were fabricated using these novel amines as the hole transporting layer and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) as the electron transporting layer. Colour mixing or green emission was observed when tris(8-hydroxyquinoline)aluminium (Alq3) was used as the electron transporting layer. However insertion of a thin layer of TPBI or 1,3,5-tris(4-tert-butylphenyl-1,3,4-oxadiazolyl)benzene (TPOB) in between the HTL and Alq3 layers led to pure blue emission owing to the confinement of recombination inside the HTL layer containing the compounds.

White organic light-emitting diodes based on 2,7-bis(2,2-diphenylvinyl)-9,9′-spirobifluorene: Improvement in operational lifetime

Very bright white organic light-emitting diodes were fabricated using 2,7-bis(2,2-diphenylvinyl)-9,9′-spirobifluorene (DPVSBF) doped with [4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran)] (DCJTB) as the emission layer. With a device configuration of ITO∕NPB(40nm)∕DPVSBF:0.2%DCJTB(11nm)∕Alq(30nm)∕LiF(1nm)∕Al(150nm), a brightness of 1575cd∕m2 with an external quantum efficiency of 3.31% and luminous efficiency of 8.00cd∕A, a power efficiency of 5.35lm∕W was achieved at a driving current of 20mA∕cm2 (4.7V). The brightness reached 66000cd∕m2 at 15V. The Commission Internationale de l’Eclairage coordinates stayed nearly constant, changed from (0.35, 0.36) to (0.32, 0.34) when the voltage increased from 6to12V. The relative operational lifetime of the device increased by a factor of ∼20 compared with a similar device based on 4,4′-bis-(2,2-diphenylvinyl)-1,1′-biphenyl as the source of the blue emission. The much extended half-lifetime was attributed to the higher morphological s...

Electroluminescent bipolar compounds containing quinoxaline or pyridopyrazine and triarylamine segments

Dipolar compounds containing both quinoxaline/pyridopyrazine acceptors and arylamine/carbazole donors have been prepared by palladium catalyzed C–C coupling (Stille) reactions in good yields and have been characterized by spectroscopic, electrochemical and thermal methods. The charge transfer character of these molecules can be changed by modification of the acceptor or donor ends. They are green emitting with moderate quantum yields in toluene solutions. However, in dichloromethane the emission maximum is red shifted and the quantum yield diminishes appreciably. High glass transition temperatures were realized for these compounds (143–162 °C). The quinoxaline derivatives effectively function as a hole-transporting and emitting layer in yellow/orange emitting double layer electroluminescent devices. We also demonstrate the possibility of using these materials in single layer devices.