High external quantum efficiency in deep blue phosphorescent organic light emitting diodes using a simple device structure

Simple high efficiency deep blue phosphorescent organic light-emitting diodes were developed using a mixed host of high triplet energy host materials. A hole transport type host was used both as the hole transport layer and host in the mixed host emitting layer and an electron transport type host was mixed with the hole transport type host in the emitting layer. A three organic layer device structure of the hole transport layer/emitting layer/electron transport layer gave high external quantum efficiency of 26.4% with a color coordinate of (0.14, 0.19).     

High quantum efficiency in simple blue phosphorescent organic light-emitting diodes without any electron injection layer

High efficiency simple blue phosphorescent organic light-emitting diodes (PHOLEDs) without any electron injection layer were developed using a spirobifluorene-based phosphine oxide (SPPO13) as a host material in the emitting layer. A high quantum efficiency of 20.3% was obtained from the SPPO13 device, with a common device structure and quantum efficiency of 19% achieved in the simple blue device without any LiF electron injection layer. Efficient electron injection from the Al cathode to the SPPO13, without any electron injection layer, was responsible for the high quantum efficiency in the blue PHOLEDs.     

Confinement of holes and electrons in blue organic light-emitting diodes with additional red emissive layers

We used various emissive layer (EML) structures with ultrathin red EMLs to enhance the charge carrier balance and carrier recombination rate in blue PHOLED devices. These EML materials have different energy gaps between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The ultrathin red EMLs, which were inserted in between the blue EMLs, effectively confined the charge carriers in EML, and increased the carrier recombination rate. The thickness of the individual EML was optimized, under 30 nm of the total thickness of EML. The blue PHOLEDs with ultrathin red EMLs achieved a luminous efficiency of 19.24 cd/A, which was 28.7% higher than those without ultrathin red EMLs, and the maximum external quantum efficiency was 11.81% at 500 cd/m².     

High efficiency in blue organic light-emitting diodes using an anthracene-based emitting material

A highly efficient deep blue emitting material based on anthracene core structure, 9,10-bis-[4-(2-(4-naphthalene-1-yl-phenyl)-vinyl)-phenyl]anthracene (NSA), was synthesized and the device performances of blue organic light-emitting diodes (OLEDs) with NSA as an emitting material were investigated. High efficiency value of 7.75 Candela (cd)/A was obtained in NSA blue devices compared with 3.6 cd/A of 9,10-bis(4-(2,2-diphenylvinyl)phenyl anthracene devices. The introduction of a phenylanthracene core and a rigid naphthylphenyl side group gave high thermal stability due to non-coplanar structure and limited intermolecular interactions, resulting in high efficiency in blue OLEDs.     

Triplet to singlet transition induced low efficiency roll-off in green phosphorescent organic light-emitting diodes

Phosphorescent organic light-emitting diodes (PHOLEDs) with an emitting layer of 4,4′-N,N′-dicarbazole-biphenyl codoped with phosphor fac-tri(phenylpyridine)iridium(III) [Ir(ppy)₃] and fluorophore N,N’-dimethy-quinacridone (DMQA) are investigated. Predominant emission from DMQA due to the efficient energy transfer from Ir(ppy)₃ to DMQA is observed. Such an energy transfer results in the transition of Ir(ppy)₃ triplet to DMQA singlet, which reduces the Ir(ppy)₃ exciton lifetime and hence suppresses the triplet-triplet annihilation and triplet-polaron annihilation of Ir(ppy)₃ excitons, leading to dramatical reduction of the efficiency roll-off of the PHOLEDs. This transition of triplet to singlet strategy provides a method to improve the efficiency roll-off of the PHOLEDs.     

A high triplet energy phosphine oxide derivative as a host and exciton blocking material for blue phosphorescent organic light-emitting diodes

We have designed and synthesized a blue phosphorescent host material based on a phosphine oxide moiety. 2-(diphenylphosphine oxide)-9,9′-spirobifluorene (SPPO1) was compared with N,N′-dicarbazolyl-3,5-benzene (mCP) as a blue host material in blue phosphorescent organic light-emitting diodes (PHOLEDs). The SPPO1 was effective as a host for blue PHOLEDs and the SPPO1 based blue PHOLEDs showed much higher quantum efficiency than common mCP based blue PHOLEDs. A high quantum efficiency of 16.3% and a current efficiency of 31.4 cd/A were obtained in the blue PHOLED with iridium(III) bis(4,6-(di-fluorophenyl)-pyridinato-N,C2′) picolinate (FIrpic) as a blue phosphorescent dopant. In addition, SPPO1 was also effective as an exciton blocking material for the blue PHOLED.     

Enhancement of the light extraction efficiency in organic light emitting diodes utilizing a porous alumina film

The light extraction efficiencies of organic light emitting diodes (OLEDs) utilizing various kinds of porous alumina films with different pore diameters were investigated. The OLEDs with the porous alumina film deposited on the glass surface were fabricated to improve their light extraction efficiency. The porous alumina film was fabricated by using a two step anodizing electrochemical procedure. The current densities as functions of the applied voltage do not significantly change, regardless of the existence and the magnitude of the pore diameter in the porous alumina film. The luminance efficiency of the OLEDs increased with increasing pore diameter. The luminance efficiency of the OLEDs utilizing the porous alumina film with a pore diameter of 70 nm was enhanced approximately 9% in comparison with that of the OLEDs without the porous alumina film. These results indicate that highly efficient OLEDs can be fabricated using a porous alumina film with an optimum pore diameter.     

Highly efficient white phosphorescent organic light emitting diodes using a mixed host structure in deep blue emitting layer

Highly efficient phosphorescent white organic light-emitting diodes (PHWOLEDs) were developed using a deep blue phosphorescent emitter doped into a mixed host of high triplet energy host materials. The deep blue emitting layer was combined with a red:green emitting layer to fabricate PHWOLEDs. A high quantum efficiency of 19.5% with a color coordinate of (0.29,0.38) and 19.8% with a color coordinate of (0.39,0.46) were achieved in the PHWOLEDs using the mixed host emitting layer doped with a deep blue phosphorescent dopant. In addition, a low optimum doping concentration below 5% in red, green and blue dopants was realized in the PHWOLEDs.     

Pyridine substituted spirofluorene derivative as an electron transport material for high efficiency in blue organic light-emitting diodes

The quantum efficiency of blue fluorescent organic light-emitting diodes was enhanced by 20% using a pyridine substituted spirofluorene-benzofluorene derivative as an electron transport material. 2′,7′-Di(pyridin-3-yl)spiro[benzofluorene-7,9′-fluorene] (SPBP) was synthesized and it was used as the electron transport material to block the hole leakage from the emitting layer. The improvement of the quantum efficiency and power efficiency of the blue fluorescent organic light-emitting diodes using the SPBP was investigated.     

Red organic light emitting devices with reduced efficiency roll-off behavior by using hybrid fluorescent/phosphorescent emission structure

Organic light emitting device (OLED) with a fluorescence-interlayer-phosphorescence emissive structure (FIP EML) is proposed to solve efficiency roll-off issue effectively. By doping fluorescent emitter of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) and phosphorescent emitter of tris(1-phenylisoquinolinolato-C2,N)iridium(III) (Ir(piq)₃) into the different regions of emission zone to form FIP EML in red OLED, an improvement of more than 20% in luminance efficiency roll-off compared with that of typical phosphorescent OLED with single EML in 10-500 mA/cm² range has been obtained. Detailed mechanisms have been studied. Such improvement should be attributed to the distinct roles of the two emitters, where DCJTB mainly used to influence the carrier transport leading to an improved balance of charge carriers while Ir(piq)₃ functions as the radiative decay sites for most generated excitons. Meanwhile, with the help of the formation of FIP EML, the redistribution of excitons in recombination zone, the suppression of non-radiative exciton quenching processes and the elimination of energy transfer loss also contribute to the enhancement of efficiency roll-off. The method proposed here may provide a route to develop efficient OLED for high luminance applications.     

High efficiency phosphorescent white organic light-emitting diodes using a spirofluorene based phosphine oxide host material

High efficiency phosphorescent white organic light-emitting diodes (PHWOLEDs) were developed by using a spirofluorene based phosphine oxide (SPPO1) as a host material in blue emitting layer. A stack structure of red:green/blue with an interlayer was used and the device performances of PHWOLEDs were investigated according to host composition in red:green emitting layer and the interlayer thickness. The use of SPPO1 as a host in the blue emitting layer resulted in a high quantum efficiency of 13.5% and a current efficiency of 27.6 cd/A with a color coordinate of (0.37, 0.43).     

Effect of blue doping concentration on the light emission of two-color phosphorescent white organic light emitting diodes

The effect of the doping concentration of the blue dopant on the light emission of two color phosphorescent white organic light-emitting diodes (PHWOLEDs) doped with a blue dopant and a red dopant was investigated. The red emission was intensified at high blue doping concentration due to direct energy transfer from the blue dopant to the red dopant by charge trapping. The changes in the electroluminescence spectra could be well correlated with the light emission mechanism of the PHWOLEDs.     

High efficiency green phosphorescent organic light emitting device with (TCTA/TCTA0.5TPBi0.5/TPBi): Ir(ppy)3 emission layer

A new high efficiency green light emitting phosphorescent device with an emission layer consisting of {4,4',4'-tris(N-carbazolyl)-triphenylamine[TCTA]/TCTA_0.5TPBi_0.5/1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene[TPBi]}:tris(2-phenylpyridine)iridium(III)[Ir(ppy)₃] was fabricated and its electroluminescence characteristics were evaluated in comparison with those of devices with emission layers made of (TCTA_0.5TPBi_0.5):Ir(ppy)₃ and (TCTA/ TPBi):Ir(ppy)₃.The device with the emission layer consisting of (TCTA/TCTA_0.5TPBi_0.5/TPBi):Ir(ppy)₃ showed a luminance of 11,000 cd/m² at an applied voltage of 8 V and maximum current efficiency of 63 cd/A under a luminance of 500 cd/m². The peak wavelength in the electroluminescent spectral and color coordinate on the Commission Internationale de I'Eclairage(CIE) chart were 513 nm and (0.31, 0.62) in this device, respectively. Under a luminance of 10000 cd/m², the current efficiency of this device was 55 cd/A, which is 1.4 and 1.1 times better than those of the devices with the emission layers made of (TCTA_0.5TPBi_0.5):Ir(ppy)₃ and (TCTA/TPBi):Ir(ppy)₃, respectively.     

Efficient triplet exciton confinement of white organic light-emitting diodes using a heavily doped phosphorescent blue emitter

We demonstrated efficient white electrophosphorescence with a heavily doped phosphorescent blue emitter and a triplet exciton blocking layer (TEBL) inserted between the hole transporting layer (HTL) and the emitting layer (EML). We fabricated white organic light-emitting diodes (WOLEDs) (devices A, B, C, and D) using a phosphorescent red emitter; bis(2-phenylquinolinato)-acetylacetonate iridium III (Ir(pq)₂acac) doped in the host material; N,N′-dicarbazolyl-3,5-benzene (mCP) as the red EML and the phosphorescent blue emitter; bis(3,5-Difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III (FIrpic) doped in the host material; p-bis(triphenylsilyly)benzene (UGH2) as the blue EML. The properties of device B, which demonstrate a maximum luminous efficiency and external quantum efficiency of 26.83 cd/A and 14.0%, respectively, were found to be superior to the other WOLED devices. It also showed white emission with CIE_x,y coordinates of (x = 0.35, y = 0.35) at 8 V. Device D, which has a layer of P-type 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) material between the HTL and TEBL, was compared with device A to determine the 430 nm emission peak.     

4,4′,4″-Tris(N-carbazolyl)-triphenylamine interlayer in highly efficient phosphorescent organic light emitting diodes based on tris[4-methyl-2-2(4′-trimethylsilylphenyl)pyridine]iridium complex

We developed highly efficient yellowish-green emitting phosphorescent organic light emitting diodes based on tris[4-methyl-2-2(4′-trimethylsilylphenyl)pyridine] [Ir(msippy)₃]. The device showed electroluminescence emission from the dopant emitter at 521 nm with CIE color coordinates of (0.32, 0.62). We investigated the roll of 4,4′,4″-Tris(carbazol-9-yl)triphenylamine (TcTa) interlayer between hole transporting and emissive layers on the higher device performances using different hole transporting materials. TcTa interlayer in the device using Di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane as hole transporting layer confines the excitons within the emissive layer and improves the charge balance, resulting in higher device external quantum efficiency of 25.6% and current efficiency of 84.4 cd/A with improved efficiency roll-off.     

Enhancement of light extraction efficiency of ultraviolet light emitting diodes by patterning of SiO2 nanosphere arrays

Here we introduce a simple and robust method to improve the light extraction efficiency of ultraviolet light emitting diodes (UV LEDs). Although many previous efforts have focused on etching the GaN surfaces, we employed a simple solution process to texture the GaN surface. Arrays of SiO₂ nanosphere monolayers were spun cast onto a polymer layer, consisting of benzocyclobutene (BCB) resins; subsequently, the bottom half of the SiO₂ nanospheres sunk into the BCB layer. The resulting array formed in a hexagonal-like pattern of ‘nano-lenses’ and the photoluminescence measurement exhibited that these patterns enhanced the light extracting efficiency of UV LEDs by 23%.     

Synthesis and electroluminescent properties of blue emitting materials based on arylamine-substituted diphenylvinylbiphenyl derivatives for organic light-emitting diodes

This paper reports the synthesis and electroluminescent properties of a series of blue emitting materials with arylamine and diphenylvinylbiphenyl groups for applications to efficient blue organic light-emitting diodes (OLEDs). All devices exhibited blue electroluminescence with electroluminescent properties that were quite sensitive to the structural features of the dopants in the emitting layers. In particular, the device using dopant 4 exhibited sky-blue emission with a maximum luminance, luminance efficiency, power efficiency, external quantum efficiency and CIE coordinates of 39,000 cd/m², 12.3 cd/A, 7.45 lm/W, 7.71% at 20 mA/cm² and (x = 0.17, y = 0.31) at 8 V, respectively. In addition, a blue OLED using dopant 2 with CIE coordinates (x = 0.16, y = 0.18) at 8 V exhibited a luminous efficiency, power efficiency and external quantum efficiency of 4.39 cd/A, 2.46 lm/W and 2.97% at 20 mA/cm², respectively.     

An aromatic imine group enhances the electro-luminescence efficiency and color purity of blue emitter for organic light emitting diodes

DPBP-EPY has the same structure with DPBP-EIF, one of blue-light-emitting materials, except their cores, in which the former has two imine groups, but the latter has only carbon-containing groups. The electro-optical properties modulated by the adoption of the different core structures were systematically examined. It was confirmed that the maximum values in the UV-visible and PL spectra of DPBP-EPY were about 19-46 nm red-shifted from those of DPBP-EIF due to the electron-withdrawing effect of the imine groups whether in solution or in solid. In addition, in case of DPBP-EPY where imine group is substituted, LUMO level of DPBP-EPY decreased while HOME level did not show any significant change. Furthermore, the results of the non-doped OLED device built with these two materials for an emitting layer indicated that DPBP-EPY needed about 2 V lower operation-voltage, and produced higher quantum yield than DPBP-EIF. In particular, it was shown that DPBP-EPY emitted purer and deeper blue-light with CIE coordinate (0.157,0.131) than DPBP-EIF with CIE coordinate is (0.179, 0.191).