Yu-Tai Tao

The synthesis, crystal structure and charge-transport properties of hexacene

Acenes can be thought of as one-dimensional strips of graphene and they have the potential to be used in the next generation of electronic devices. However, because acenes larger than pentacene have been found to be unstable, it was generally accepted that they would not be particularly useful materials under normal conditions. Here, we show that, by using a physical vapour-transport method, platelet-shaped crystals of hexacene can be prepared from a monoketone precursor. These crystals are stable in the dark for a long period of time under ambient conditions. In the crystal, the molecules are arranged in herringbone arrays, quite similar to that observed for pentacene. A field-effect transistor made using a single crystal of hexacene displayed a hole mobility significantly higher than that of pentacene. This result suggests that it might be instructive to further explore the potential of other higher acenes.

Bright white organic light-emitting diode

A very bright white organic light-emitting diode (OLED) was realized by using a bright blue-emitting layer, 1,7-diphenyl-4-biphenyl-3,5-dimethyl-1,7-dihydrodipyrazolo[3,4-b;4′,3′-e]pyridine (PAP-Ph), together with a 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyry)-4H-pyran (DCM)-doped Alq [tris(8-hydroxyquinolinato) aluminum (III)] layer to provide blue, red, and green emission for color mixing. By appropriately controlling the layer thickness, the white light OLED achieved good performance of 24 700 cd/m2 at 15 V, 1.93 lm/W at 6.5 V, and >300 cd/m2 at 7.7 mA/cm2. The Commission Internationale de l’Eclairage coordinates of the emitted light are quite stable at voltages from 6 to 12 V, ranging from (0.35, 0.34) to (0.34, 0.35).

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...

Organic light-emitting diodes based on charge-neutral Os(II) emitters: generation of saturated red emission with very high external quantum efficiency

The OLED device using 6% of Os(fptz)2(PPh2Me)2 as the dopant emitter in a CBP host and BPAPF as hole transporting material shows an external quantum efficiency of 15.3% and luminous efficiency of 21.3 cd A−1, power efficiency of 6.3 lm W−1 at 20 mA cm−2. An even higher external quantum efficiency of ∼20% was achieved at a low current density of ∼1 mA cm−2.

High Tg blue emitting materials for electroluminescent devices

A series of 2′,7′-di-tert-butyl-9,9′-spirobifluorene derivatives (flu) incorporating arylamines at the 2- and/or 7-positions were synthesized. These compounds possess a high glass transition temperature ranging from 135 to 215 °C. Incorporation of spirobifluorene was found to be beneficial for raising the glass transition temperature (Tg) of the molecules. Most of the compounds are blue emitting with good solution quantum yields. Blue-emitting double-layer devices with narrow full width at half-maximum (fwhm < 68 nm) were constructed using these materials as the hole-transporting and emitting layer, and TPBI (1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene) as the electron-transporting layer (device I). In double-layer devices of the configuration ITO/flu (40 nm)/Alq3 (40 nm)/Mg : Ag (Alq3 = tris(8-hydroxyquinoline)aluminium) (device II), flu functioned mainly as a hole-transporting layer, and the green light characteristic of Alq3 was detected. Two of the devices of type I have very promising performance, and one of them emits pure blue light (CIE (x,y) = (0.15, 0.09)). One of the compounds was found to be an efficient hole-injection material.

Sharp green electroluminescence from 1H-pyrazolo[3,4-b]quinoline-based light-emitting diodes

A multilayer organic light-emitting diode was fabricated using a fluorescent compound {6-N,N-diethylamino-1-methyl-3-phenyl-1H-pyrazolo[3,4-b]quinoline} (PAQ–NEt2) doped into the hole-transporting layer of NPB {4,4′-bis[N-(1-naphthyl-1-)-N-phenyl-amino]-biphenyl}, with the TPBI {2,2′,2″-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole]} as an electrontransporting material. At 16% PAQ–NEt2 doping concentration, the device gave a sharp, bright, and efficient green electroluminescence (EL) peaked at around 530 nm. The full width at half maximum of the EL is 60 nm, which is 60% of the green emission from typical NPB/AlQ [where AlQ=tris(8-hydroxyquinoline) aluminum] device. For the same concentration, a maximum luminance of 37 000 cd/m2 was obtained at 10.0 V and the maximum power, luminescence, and external quantum efficiencies were obtained 4.2 lm/W, 6.0 cd/A, and 1.6%, respectively, at 5.0 V.

Dipyrazolopyridine derivatives as bright blue electroluminescent materials

Very bright blue organic light emitting diodes were fabricated using highly fluorescent dipyrazolopyridine derivatives, 4-(4-substituted phenyl)-1,7-diphenyl-3,5-dimethyl-1,7dihydrodipyrazolo[3,4-b,4′,3′-e]pyridine (PAP–X, X=CN, Ph, and OMe), as emitter by doping the dye in an electron-transporting host, 2,2′,2″-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzimidazole] (TPBI). Two hole-transporting layers, 4,4′-bis[N-(1-naphthyl-1-)-N-phenyl-amino]-biphenyl (NPB) and 4,4′-dicarbazolyl-1,1′-biphenyl (CBP) were used to achieve the emission from PAP–X. The devices with a general configuration of indium tin oxide/NPB/CBP/TPBI:PAP(2%)/Mg:Ag showed a bright blue emission. The PAP–CN-based device is exceptionally good, with a brightness of 11 200 cd/m2 at 14.2 V and the peak external quantum efficiency of 3.2%. The efficiency is the highest for the blue emission.

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.

Continuous modulation of electrode work function with mixed self-assembled monolayers and its effect in charge injection.

Self-assembled monolayers (SAMs) of binary mixtures of n-decanethiol and the fluorinated analogue (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decanethiol) were formed on silver surface. The film structure was characterized by reflection absorption IR and XPS to be a homogeneous mixture of the two components. The mixed monolayers serve to tune the work function of silver over a wide range by varying the surface composition of the mixed monolayer from 4.1 to 5.8 eV. The mixed SAM-modified Ag surfaces were used as the anode in the fabrication of hole-only devices with the device structure Ag/SAM/HTL/Ag, where HTL represents a hole-transporting layer. It is shown that depending on the HTL used and thus the HOMO level involved, the maximum current injection into the device occurred with differently modified Ag. Top-emitting organic light-emitting diodes fabricated with differently modified silver electrodes showed that the maximum current and maximum luminance efficiency occur at anodes of different modifications due to a change in the hole-electron charge balance.

Diazapentacene Derivatives as Thin-Film Transistor Materials: Morphology Control in Realizing High-Field-Effect Mobility

5,7,12,14-Tetrachloro-6,13-diaza-6,13-dihydropentacene (TCDAHP) and 5,7,12,14-tetrachloro-6,13-diazapentacene (TCDAP) were synthesized and assessed as the active channel materials for thin-film transistor applications. Analyses of the crystal structures of these molecules revealed that both exhibited slipped pi-pi stacking of the long and fused aromatic moiety. Although the packing features of the two compounds are basically identical, their highest occupied molecular orbitals, which are relevant to hole transport, are very different. Better mobility was predicted for TCDAHP over TCDAP based on the dimeric structure in the X-ray coordinates. The morphologies of thin films of TCDAHP and TCDAP prepared by thermal evaporation depend critically on the substrate on which the molecules were deposited: from the amorphous state on a SiO(2)/Si surface to the crystalline state on a pentacene buffer layer surface. The performance of thin-film transistors prepared on various substrate surfaces was studied. While no field-effect mobility was observed for these films deposited on SiO(2)/Si, a high mobility of 1.4 cm(2)/(V s) for the TCDAHP film was achieved when deposited on a pentacene buffer layer prepared on a rubbed monolayer of n-nonyltrichlorosilane on a SiO(2)/Si surface. A similar device prepared from TCDAP gave a mobility of 0.13 cm(2)/(V s).