Organic triplet-state light-emitting materials (organic phosphorophores) have been one of the most important recent developments in the field of organic light-emitting diodes (OLEDs).[1±4] Organic electrophosphorescent materials provided one of the major breakthroughs in electroluminescence efficiency, which is usually limited to an external quantum efficiency (EQE) of around 5 % for devices based on singletstate fluorescent materials. Owing to its thin-film, lightweight, fast-response, wide-viewing-angle, high-contrast, and low-power attributes, OLEDs promise to be one of the major flat- [16±20]However, the development of highly efficient blue-light-emitting phosphorescent emitters in OLEDs, indispensable for the realization of RGB full-color displays and WOLEDs, is still in its infancy, and blue-light-emitting phosphorescent EL performance lags far behind that of the green-or red-light emitters.One of the best known triplet-state blue-light emitters is iridium(III) bis(4,6-difluorophenylpyridinato)picolate (FIrpic, Scheme 1).[21±23] Although a reasonably good EQE of about 10 % (or 10 lm W ±1 ) has been reported, its blue-light emission was far from saturated, with 1931 Commission Internationale de L'Eclairage x,y coordinates (CIE x,y ) of (0.17, 0.34) [23] thatwere best characterized as cyan in color. The most recent, and probably only the second phosphorescent blue emitter of practical use, was iridium(III) bis(4¢,6¢-difluorophenylpyridinato)tetrakis(1-pyrazolyl) borate) (FIr6), [24] whose blue OLED showed EQEs of 9±10 % (or 11±14 lm W ±1 ), and whose bluecolor chromaticity had been considerably improved to CIE x,y = 0.16, 0.26. There are a couple of limitations in the usage of phosphorescence-based materials for OLEDs. Compared with the short emission lifetime (scale of nanoseconds) of fluorescent materials, the relatively long phosphorescence lifetimes (microseconds scale) of the iridium complexes may lead to dominant triplet±triplet (T 1 ±T 1 ) annihilation at high currents. Long emission lifetimes also cause a long range of exciton diffusion (> 100 nm) that could get quenched in the adjacent layers of materials in the OLED. Consequently, organic phosphorescent materials are often adopted as dopants dispersed in a suitable host material, usually of high bandgap energies and good carrier transport properties. Arylamino-containing organic substances are usually the host materials of choice to alleviate this situation. This has worked reasonably well for phosphorescent green-or red-light-emitting materials. However, it has been demonstrated that the energy differences in the triplet energies of host and guest materials are very important for the confinement of electro-generated triplet excitons on the dopant molecules.[22±28] In cases of triplet-state bluelight emitters, common arylamino-containing substances, such as 4,4¢-bis(9-carbazolyl)-2,2¢-biphenyl (CBP) simply do not have sufficient triplet-state energy for effective T 1 ±T 1 energy transfer; later, a structurally modified host molecule, 1,3-bis(9-carba...
In recent years, there has been considerable interest in developing blue organic light-emitting devices (OLEDs) with high efficiency, deep-blue color, and long operational lifetime. The deep-blue color is defined arbitrarily as having blue electroluminescent (EL) emission with a Commission Internationale de l'Eclairage y coordinate value (CIE y ) of < 0.15. Such emitters can effectively reduce the power consumption of a full-color OLED and can also be utilized to generate light of other colors by energy cascade to a suitable emissive dopant.It is well known that a guest-host doped emitter system can significantly improve device performance in terms of EL efficiency and emissive color, as well as operational lifetime.[1] Although many blue host materials have been reported, such as anthracene, [2] di(styryl)arylene, [3] tetra(phenyl)pyrene, [4] terfluorenes, [5] and tetra(phenyl)silyl derivatives, [6] blue-doped emitter systems having all the attributes of high EL efficiency, long operational lifetime, and deep-blue color, are rare. [7] This is because designing a fluorescent, deep-blue dopant capable of forming an amorphous glassy state upon thermal evaporation with a much shortened p-conjugation length is a rather daunting task. In addition, finding a deep-blue dopant with a small Stokes shift is essential for efficient Förster energy transfer from the host to dopant molecules, since the energy transfer efficiency is highly dependent on the spectral overlap between the emission of the host and the absorption of the dopant.Recently, we have successfully demonstrated an anthracene-based blue host material, 2-methyl-9,10-di(2-naphthyl)-anthracene (MADN), which possesses a wide energy bandgap of 3.0 eV and can also form a stable thin-film morphology upon thermal evaporation. When doped with a di(styryl)-amine-based blue dopant, p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph), it achieved a very high EL efficiency of 9.7 cd A -1 , with a greenish-blue color of CIE x,y (0.16, 0.32) and a long operational lifetime of 46 000 h at a normalized initial brightness of 100 cd m -2 .[8] However, the color saturation of the blue-doped emitter system (DSA-Ph@MADN) is far from adequate for application in full-color OLED displays. On the other hand, the symmetrical di(styryl)amine-based organic molecule is well known to possess a high fluorescent quantum yield, [9] and its emission wavelength (k max = 450-480 nm) is dependent on the p-conjugation length, which encompasses the two strongly donating arylamine moieties. However, the molecular engineering of symmetrical di(styryl)amine-based fluorescent dyes to cause hypsochromic shift has its limitations as both donors are already present as part of the styryl p-conjugation.[10]In this paper, we report a series of novel, deep-blue dopants based on unsymmetrical mono(styryl)amine derivatives, which provide us with a basic structure for color tuning within the 430-450 nm spectral region. We show that when doped in the MADN host as an OLED, some of these deep-blue dopants exhibit a...
Two types of tandem organic light-emitting diodes (OLEDs) with white-light emission have been developed by using Mg:Alq3∕WO3 as the interconnecting layer. While the Commission Internationale d’Eclairage (CIE) coordinates of the tandem device with individual blue- and yellow-emitting OLEDs was sensitive to the viewing angle and the operating time, the tandem device connecting two white-emitting OLEDs was considerably less. At an optimal WO3 thickness of 5nm, the tandem two-unit device produced three higher luminance efficiency than that expected of a single-unit device. A maximum efficiency of 22cd∕A was achieved by the tandem device comprised of two white-fluorescent OLEDs, and the projected half-life under the initial luminance of 100cd∕m2 was over 80000h.
We have developed a highly efficient and stable blue organic electroluminescent (EL) device based on a blue fluorescent styrylamine dopant, p-bis(p-N,N-diphenyl-aminostyryl)benzene, in a morphologically stable high band-gap host material, 2-methyl-9,10-di(2-naphthyl)anthracene, which achieved an EL efficiency of 9.7cd∕A and 5.5lm∕W at 20mA∕cm2 and 5.7 V, with Commission Internationale d’Eclairage coordinates of (x=0.16,y=0.32). The blue-doped device achieved a half-decay lifetime (t1∕2) of 46 000 h at an initial brightness of 100cd∕m2.
To overcome the thermal instability of a p-doped organic hole transporting layer using the state-of-the-art p-type dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, a potent electron accepter, 3,6-difluoro-2,5,7,7,8,8-hexacyanoquinodimethane, has been found to possess superior thermal stability and proved to be an excellent p-type dopant.
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