New Dopant and Host Materials for Blue‐Light‐Emitting Phosphorescent Organic Electroluminescent Devices

Abstract

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...

Recent Development of Blue Fluorescent OLED Materials and Devices

“…To date, the best triplet blue emitter reported with limited lifetime is due to UDC [7] which claimed to have achieved a luminous efficiency of about 30 cd/A at 100 cd/m with a sky blue (0.16, 0.38) and a lifetime of only 2000 h. A slightly deeper blue emitter of (0.14, 0.21) disclosed recently fairs no better with 19 cd/A at 100 cd/m and a similar dismal lifetime. Very recently, at the International Display Manufacturing Conference (IDMC) 2005, held in Taipei, Taiwan, Samsung SDI managed to disclose for the first time [8] a deep blue phosphorescent emitter with (0.15, 0.15) but only can last for 150 h. Part of the problems in producing a decent saturated blue phosphorescent emitter with lies in the fact that there is simply no good host material available that possesses sufficiently large triplet bandgap energy ( 3 eV) to effectively prevent triplet exciton of the dopant from quenching by back energy transfer, and the limitation of short-conjugated ligand selection for synthesizing transition metal-based metal-ligand charge-transfer (MLCT) complexes serves only to compound the already grave and aggravated situation [9]. All things considered, it suffices to predict that, in the foreseeable future, the advent of a robust phosphorescent blue emitter with all of the necessary attributes for high efficiency, saturated color, as well as device stability would be extremely difficult.…”

Materials for Organic Light Emitting Diode (OLED)

“…The National Television Standards Committee (NTSC) determined that the Commission Internationale de l'Eclairage (CIE) coordinates for blue are (x, y ¼ 0.14, 0.08). However, in typical blue phosphorescent complexes, such as iridium (III) bis(4,6-difluorophenylpyridinato)picolinate (FIrpic) [40], iridium (III) bis(4,6-difluorophenylpyridinato)tetrakis(1-pyrazolyl) borate (FIr6) [41], and iridium (III) bis(4,6-di-fluorophenylpyridinato)-5-(pyridine-2-yl)-1H-tetrazolate (FIrN4) [42], the sum of the x, y coordinates is more than 0.3, and the color is called sky blue. The second difficulty is insufficient emission efficiency (Fig.…”

Organic Semiconductor Electroluminescent Materials

“…The peak wavelength of the FIr6 is 458 nm compared with the 470 nm of FIrpic. Similarly, triazole (FIrtaz) [212] and tetrazole (FIrN4) [212] derivatives were adopted as the ancillary ligands. The emission wavelengths of FIrtaz and FIrN4 were 459 and 460 nm, respectively.…”