Two dimeric trimeric phenylenvinylene derivative 2, 5, 2', 5'-tetrakis (4'-fluorostyryl) biphenyl (P-F-TSB) and 2, 5, 2', 5'-tetra (p-trifluoromethylstyryl)-biphenyl (TFM-TSB) have been synthesized as new electroluminescent (EL) materials. P-F-TSB exhibits good color purity, high luminance of blue light-emission in organic light-emitting devices. Maximum brightness and luminous efficiency are of 1828 cd/m2 and of 1.92 cd/A, respectively. CIE coordinates are x=0.20 and y=0.22. Interestingly, we can fabricate single layer white light-emitting device using TFM-TSB as emitting layer. The broad electroluminescence emission band may attribute to long-wavelength excimer and electromer emission in addition to the blue component from singlet excited state of individual TFM-TSB molecule. Furthermore, white-light emission can also be obtained with a typical three-layer structure of ITO/ NPB (50 nm)/ TFM-TSB (50 nm)/ Alq3 (30 nm)/LiF/Al device. The maximum brightness of this device is 809 cd/m2 at 217 mA/cm2 and 13V, and the maximum luminous efficiency is 1.49 cd/A at 11 mA/ cm2 and 8V.
We studied electron injection form Al cathode into the tris(8-hydroxyquinoline)aluminum (Alq3). When a thin CsCl layer is inserted between Alq3 and Al, a substantial enhancement in electron injection is observed. The results show that the device with the cathode containing the ultrathin CsCl layer has a higher brightness and electroluminescent efficiency than the device without this layer. Further, organic light-emitting devices (OLEDs) based on tris-(8-hydroxyquinoline)aluminum using a trilayer of CsCl/LiF/Al as cathodes have been fabricated. The results show that the device with the cathode containing 0.5 nm CsCl layer and 1.0 nm LiF layer has a higher brightness and electroluminescent efficiency than that of the device with LiF/Al or CsCl/Al cathodes. The maximum EL efficiency of the CsCl/LiF/Al cathode device is 3.41 cd/A, which is higher than the 2.74 cd/A of the LiF/Al device and 2.49 cd/A of the CsCl/Al device.
Blue and white organic light-emitting devices using a novel dimeric trimeric phenylenvinylene (TPV) derivative , 2,5,2',5'-tetrakis(4'-biphenylenevinyl)-biphenyl (TBVB) containing a biphenyl center are fabricated. Structures of these devices are simple, where tris (8-hydroxyquinoline) aluminum (Alq) and N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,4'-diamine (NPB) are used as electron-transporting and hole-transporting layers, respectively. In blue device, TBVB is used as light-emitting layer. The peak of electroluminescent (EL) spectra of the device with TBVB is at 468 nm, and its maximum efficiency is greatly higher than that of the reported oligomer poly(phenylenvinylene) (PPV) devices. By inserting an ultra thin layer of rubrene between TBVB and Alq layers, a fairy pure white OLED with CIE coordinates of (0.33, 0.34) is realized. Its maximum luminances and efficiencies are 4025 cd/m2 and 3.2 cd/A, respectively.
Efficient white organic light-emitting devices are demonstrated by inserting a thin layer of tris (8-hydroxyquinoline) aluminum (Alq) doped with 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl) (DCJTB) into N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,4'-diamine (NPB) layer. Alq without doping is used as an electron-transporting layer and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine or BCP) as an exciton-blocking layer. NPB layers are separated by the doped Alq layer, the layer that sandwiched between BCP and doped Alq layers acts as a blue-emitting layer, and the other as a hole-transporting layer. The doped Alq layer acts as a red and green-emitting as well as chromaticity-tuning layer, whose thickness and position as well as the concentration of DCJTB in Alq permit the tuning of the device spectrum to achieve a balanced white emission with Commission Internationale De L'Eclairage coordinates of (0.33, 0.33), which are largely insensitive to the applied voltages, especially at high brightness (>1000cd/m2). The device have maximum luminance and efficiency of 6745 cd/m2 and 2.56 cd/A, respectively.
A bilayer is used as an electrode for organic light-emitting devices. The bilayer consists of a Sn layer adjacent to Alq layer and an Al outerlayer. The effects of a controlled Sn buffer layer on the behavior of ITO/CuPc/NPB/Alq/Sn/Al light-emitting devices are described. It is found that both electron injection efficiency and electroluminescence are significantly enhanced when the thickness of Sn layer is suitable. The enhancement is believed to be due to increased electron density of Sn layer near the Sn/Al interface .
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