No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Development of new emitting amorphous molecular materials for organic light-emitting diodes
Abstract
Molecular design concepts, synthesis, and properties of a few novel classes of emitting amorphous molecular materials with desired bipolar character are described. They include alpha,omega-bis{4-[bis(4-methylphenyl)amino]- phenyl}oligothiophene (BMA-nT), pi-electron systems end-capped with 4-[bis(9,9-dimethylfluoren-2-yl)amino]-phenyl group (BFA), and alpha-{4-[bis(9,9-dimethyl-fluoren-2-yl)amino]phenyl}-omega-(dimesitylboryl)oligothiophene (FlAMB-nT) families. Fabrication and performance of organic light-emitting diodes using these emitting materials are discussed.
... Therefore, aqueous solubility data of the individual compounds involved are of increasing interest and significance. The development of OLED displays has involved a large number of diverse compounds (Shinar and Savvateev, 2004; Shirota, 2006). An important class of chemicals employed in OLED production are aromatic amines used as hole transport materials (Strohriegl and Grazulevicius, 2002). ...
Two series of thin films of oligothiophene derivatives grown on Ag substrates have been studied with photoelectron spectroscopy. The oligothiophenes were end-capped with either electron-deficient dismesitylboryl or electron-rich diphenyltolylamine moieties to create molecules with electron-accepting or -donating properties, respectively. The position of the highest occupied molecular orbital HOMO at the metal/organic interface is found to be strongly dependent on the effective -conjugation length of the oligothiophenes capped with dimesitylboryl groups, whereas in the oligothiophenes capped with diphenyltolylamine, the position of the HOMO is independent of the molecular length. The difference in the observed HOMO characteristics is attributed to the different make-up of the frontier orbitals in the two molecular series. This will particularly affect the overall energy barrier for charge injection at the conductor/organic interface in a device structure, such as an organic light-emitting diode, utilizing the investigated molecules for carrier injection and transport. © 2002 American Institute of Physics.
Charge transport in the glassy state of a variety of low
molecular-weight organic compounds has ben studied. The hole drift
mobility of the molecular glass was found to be of the order of
10-6 approximately 10-2
cm2V-1s-1 at an electric field of 1.0
by 105 Vcm-1 at room temperature, being higher
than those of polymers and molecularly-doped polymer systems. The
electric-field and temperature dependencies of charge- carrier drift
mobility were analyzed in terms of the disorder formalism. The negative
electric-field dependence of charge-carrier drift mobility was observed
for the first time for the molecular glass. The relationship between
molecular structure and charge-transport properties is discussed.
A novel class of amorphous molecular materials, 1,3,5-
tris(4-biphenylyl)benzene (TBB), 1.3.5-tris(4-
fluorobiphenyl-4'-yl)benzene(F-TBB), 1,3,5-tris(9,9-
dimethylfluoren-2-yl)benzene(TFB), and 1,3,5-tris[4-(9,9-
dimethylfluoren-2-yl)phenyl]benzene(TFPB), were found to function as
hole-blocking materials in organic electroluminescent (EL) devices.
1.3.5-Tris[5- (dimesitylboryl)thiophen-2-yl]benzene(TMB-TB) was also
found to function as an electron transporter with better hole- blocking
properties relative to tris(8- quinolinolato)aluminum. These materials,
which readily form stable amorphous glasses with well-defined
glass-transition temperatures, are characterized by relatively high
oxidation potentials and large HOMO-LUMO energy gaps. The use of these
materials as hole blockers in multilayer organic EL devices permitted
efficient blue-violet emission from emitters with hole transporting
properties, e.g., N,N'bis(e-
methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (TPD),
N,N'-bis(4-biphenylyl)-N,N'-diphenyl-[1,1'-biphenyl]-
4,4'-diamine(p-BPD), N,N-bis(9,9-dimethylfluorene-2- yl)aniline
(F2PA), N,N'-bis[9,9-dimethylfluoren-2-yl]-
N,N'-diphenyl-9,9-dimethylfluorene-2,7-diamine (PFFA), and
N,N,N',N'-tetrakis(9,9-dimethylfluoren-2-yl)-[1,1'-
biphenyl]-4,4'-diamine(FFD).
Recent results on the creation of novel emitting amorphous molecular
materials and fabrication of blue or multi-color emitting organic
light-emitting diodes (OLEDs) are described. It is shown that
tri(p-terphenyl-4-yl)amine functions not only as a blue-emitting
material with hole-transporting properties but also as a good host
matrix for fluorescent dopants such as perylene.
5,5'-Bis(dimesitylboryl)-2,2'-bithiophene (BMB-2T) and
bis{4-[bis(4-methylphenyl)amino]phenyl}- (alpha) -oligothiophene
(BMA-nT) [n equals 1 to approximately 4)] are found to be a good
blue-emitting amorphous molecular material with electron-transporting
properties and good multi-color emitting amorphous molecular materials
with hole-transporting properties, respectively, for OLEDs. Exciplex
formation at the organic solid interface between the hole- and
electron-transporting materials and its potential application for color
tuning are also described.
It is shown here that poly(p-phenylene vinylene), prepared by way of a
solution-processable precursor, can be used as the active element in a
large-area LED. The combination of good structural properties of this
polymer, its ease of fabrication, and light emission in the green-yellow
part of the spectrum with reasonably high efficiency suggest that the
polymer can be used for the development of large-area light-emitting
displays.
Several novel families of amorphous molecular materials with high glass-transition temperatures (Tg) that function as charge-transport or emitting materials for organic LEDs have been deigned and synthesized. Double-layer and multilayer devices using these novel amorphous molecular materials were fabricated and their performance investigated. The use of the novel amorphous molecular materials with high Tgs enabled the fabrication of thermally stable organic LEDs; one of the devices was found to operate even at 170 degrees C. The multilayer device consisting of double hole-transport layers and an emitting layer was found to enhance significantly the durability of the device. Exciplex formation at the organic/organic solid interface in organic LEDs has also been investigated.
Effects of the method of preparation of organic thin films and chemical doping on charge injection from electrodes were investigated. It was found that charge injection from electrodes and the performance of organic electroluminescent (EL) devices are affected by the method of preparation of organic thin films, depending on the kind of materials. Organic EL devices using spin-coated films of N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (TPD) or N,N'-di(p-biphenyl-4-yl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (p-BPD) as a hole-transport layer and tris(8-quinolinolato)aluminum (Alq3) as an emitting layer exhibited higher injected current density and luminance than the corresponding devices using vacuum-deposited films of TPD or p-BPD. On the other hand, almost no difference was observed in the current density -- applied voltage and luminance -- applied voltage characteristics between Alq3-based triple-layer organic EL devices fabricated with the vacuum-deposited and spin-coated films of 4,4',4"-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) or 4,4',4"-tris(2-naphthylphenylamino)triphenylamine (2-TNATA) as a hole-injection layer together with the vacuum-deposited thin film of TPD as a hole-transport layer. An organic EL device using iodine-doped m-MTDATA as a hole-injection layer, TPD as a hole-transport layer, and Alq3 as an emitting layer operated at a lower drive voltage and exhibited higher external quantum efficiency relative to the corresponding device using undoped m-MTDATA. It is indicated that the use of iodine-doped m-MTDATA enhances not only hole injection from the ITO electrode but also electron injection from the cathode into Alq3.
New hole-transporting amorphous molecular materials with high glass-transition temperatures (Tgs) for organic electroluminescent (EL) devices, N,N,N′,N′-tetrakis(9,9-dimethyl-2-fluorenyl)-[1,1′-biphenyl]-4,4′-diamine (FFD) and N,N′-bis[9,9-dimethyl-2-fluorenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine (PFFA), have been developed and their charge-transport properties investigated. These materials with Tgs of 165 and 135°C permitted the fabrication of thermally stable organic EL devices. It was found that both FFD and PFFA glasses exhibit high hole drift mobilities of 4.1×10−3 cm2 V−1 s−1 and 1.1×10−3 cm2 V−1 s−1 at an electric field of 1.0×105 V cm−1 at 293 K. The electric-field and temperature dependencies of the drift mobilities were analyzed in terms of the disorder formalism.
The color of light emitted by organic electroluminescent devices can be tuned between light blue and orange in the family of materials presented here simply by varying the conjugation length (n = 1 to 4, see Figure) of the oligothiophene moiety. The synthesis of the amorphous molecular materials and the optical behavior of single-layer electroluminescent devices based on them—see also the cover—are described.
A novel emitting amorphous molecular material with a high glass-transition temperature of 158 degreesC, 2,5-bis (4-[bis-(9,9-dimethyl-2-fluorenyl)amino]phenyl)thiophene (BFA-1T), has been developed. BFA-1T emits greenish blue light with a relatively high fluorescence quantum yield of 0.48 in solution. A multilayer organic electroluminescent device using BFA-1T emitted greenish blue light with a maximum luminance of 12350 cd m(-2) at 11 V with a luminous efficiency of 1.1 lm W-1 at a luminance of 300 cd m(-2). The device worked even at 170 degreesC.
A novel family of amorphous molecular materials containing an oligothiophene moiety with varying conjugation length function as thermally and morphologically stable, colour-tunable emitting materials for organic light-emitting diodes (LEDs). Tuning of the emission colour from light blue to orange is achieved by varying the conjugation length of the oligothiophene moiety. Double-layer organic LEDs that use this novel class of amorphous molecular materials as an emitting layer and tris(quinolin-8-olato)aluminium or 1,3,5-tris[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene as an electron-transport layer exhibit high performances.
For the purposes of clarifying the properties of a molecular glass as a novel host matrix and gaining information on the microstructure of the molecular glass, the photochromic behavior of 4-dimethylaminoazobenzene (DAAB) in a novel molecular glass of 4,4′,4′′-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) was investigated, and compared with its behavior in a polystyrene glass matrix and a benzene solution. It was found that the fraction of the photoisomerized cis-isomer of DAAB at the photostationary state is smaller in the m-MTDATA glass matrix than in the polystyrene matrix and the benzene solution, and that the apparent initial rate constant for the backward cis→trans thermal isomerization of DAAB is much larger in the m-MTDATA glass than in the polystyrene matrix and the benzene solution. These results suggest that the average size of local free volume in the molecular glass of m-MTDATA is smaller than that in the polystyrene glass.
A novel amorphous molecular material, 5,5′′-bis{4-[bis(4-methylphenyl)amino]phenyl}2,2′:5′,2′′-terthiophene (BMA-3T), has been found to function as a yellow-emitting material in organic electroluminescent (EL) devices. Both the single layer EL device using BMA-3T alone and the double layer EL device consisting of an emitting layer of BMA-3T and an electron-transport layer of tris(8-quinolinolato)aluminum sandwiched between indium-tin-oxide (ITO) and an alloy of magnesium and silver (∼10:1) electrodes emitted a bright yellow light resulting from BMA-3T. The double layer EL device showed much better performances than the single layer EL device, exhibiting a maximum luminance of ∼13 000 cd m−2 at a driving voltage of 18 V, and a luminous efficiency of 1.1 lm W−1 at a luminance of 300 cd m−2. © 1997 American Institute of Physics.
A novel class of color-tunable emitting amorphous molecular materials with desired bipolar character, 4-dimesitylboryl-N,N-bis(9,9-dimethylfluoren-2-yl)aniline (FlAMB-0T), 2-{4-[bis(9,9-dimethylfluoren-2-yl)amino]phenyl}-5-(dimesitylboryl)thiophene (FlAMB-1T), 2-{4-[bis(9,9-dimethylfluoren-2-yl)amino]phenyl}-2‘-dimesitylboryl-5,5‘-bithiophene (FlAMB-2T), and 5-{4-[bis(9,9-dimethylfluoren-2-yl)amino]phenyl}-5‘ ‘-dimesitylboryl-2,2‘:5‘,2‘ ‘-terthiophene (FlAMB-3T), have been designed and synthesized. This novel class of compounds is characterized by reversible anodic oxidation and cathodic reduction, intense fluorescence emission, and ready formation of stable amorphous glasses with high glass-transition temperatures above 120 °C. They function as excellent emitting materials for organic electroluminescent (EL) devices, emitting multicolor light including white. They also serve as good host materials for emissive dopants in organic EL devices, permitting color tuning and leading to higher performance.
A novel family of photo- and electroactive amorphous molecular materials containing an oligothiophene moiety linked to two triphenylamine moieties has been designed and synthesized (see Figure). Their glass-forming properties, morphological changes, and molecular and solid-state properties are described. They are reported to exhibit multiredox behavior on electrochemical oxidation.
A novel electroluminescent device is constructed using organic materials as the emitting elements. The diode has a double‐layer structure of organic thin films, prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium‐tin‐oxide anode and an alloyed Mg:Ag cathode. Electron‐hole recombination and green electroluminescent emission are confined near the organic interface region. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W), and brightness (≫1000 cd/m<sup>2</sup>) are achievable at a driving voltage below 10 V.