A study on white organic light-emitting diodes co-doped with red fluorescent and blue phosphorescent dopants.

White organic light-emitting diodes (WOLEDs) have drawn increasing attention due to their potential use in various applications such as solid-state lighting and backlight of liquid crystal displays and full-color OLEDs of red, green, and blue pixel. N,N'-dicabazolyl-3,5-benzene (mCP), the host material, was co-doped with Iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2']-picolinate (FIrpic), which functions not only as phosphorescent sensitizer but also blue emitter, and (2Z,2'Z)-3,3'-[4,4"-bis (dimethylamino)-1,1':4',1"-terphenyl-2',5'-diyl]bis (2-phenylacrylonitrile) (ABCV-P), which is a red fluorescent material. The fabricated device structures were as follows: (device A) Indium tin oxide (ITO)/N,N'-bis-(1-naphyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB)/(mCP)/mCP:ABCV-P (1%)/4,7-diphenyl-1,10-phenanthroline (Bphen)/lithium quinolate (Liq)/aluminum (Al), (device B) ITO/NPB/mCP/mCP:FIrpic (8%)/Bphen/Liq/Al and (device C) ITO/NPB/mCP/mCP:FIrpic:ABCV-P (8%, 1%)/Bphen/Liq/Al, respectively. Phosphorescent FIrpic harvesting both singlet and triplet excitions not only emitted blue light but also transferred energy to fluorescent ABCV-P. The maximum luminance efficiency, external quantum efficiency, and luminance of white light device were measured to be 5.95 cd/A, 2.45% and 2500 cd/m2, respectively. The white device gave practically white light with the Commision Internationale de l'Eclairage (CIE(xy)) coordinate of (0.44, 0.49) which was close to warm white color (CIE(xy) = 0.45, 0.45).

A non-doped organic light emitting diode with pure red emission using a new host emitter.

A red fluorescent material (2E,2'E)-3,3'-[4,4"-bis(dimethylamino)-1,1': 4',1 "-terphenyl-2',5'-diyl]bis[2-(2-thienyl)acrylonitrile] (ABCV-Th) was synthesized for use in organic light emitting diodes (OLEDs) as the host emissive material. It has been reported some green and blue host emissive materials used in OLEDs revealed high device performance but, owing to concentration quenching, comparable red light emitting materials are still rare in OLEDs application. Non-doped organic light emitting diodes, with the structure of ITO/NPB/ABCV-Th (30 nm and 50 nm)/BCP/Alq3/Liq/Al were fabricated using ABCV-Th as the host emitter. The peak wavelength and full width at half maximum (FWHM) of electroluminescence (EL) were 629.5 nm and 68.5 nm, respectively. The maximum brightness and turn on voltage of the device were measured to be 1330 cd/m2 at 14.6 V and 3.4 V, respectively. The device exhibited authentic red emission (Commission Internationale De L'Eclairage (CIE(xy)) = 0.65, 0.34) which is almost close to the standard red (CIE(xy) = 0.67, 0.33) demanded by the national television system committee (NTSC).

Dependence of surface morphology on molecular structure and its influence on the properties of OLEDs.

Most organic light-emitting diodes (OLEDs) have a multilayer structure composed of organic layers such as a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL) and an electron injection layer (EIL) sandwiched between two electrodes. The organic layers are thin solid films with a thickness from a few nano meters to a few tenths nano meter, respectively. Surface morphology of an organic thin solid film in OLEDs depends on the molecular structure of the organic material and has an affect on device performance. To analyze the effect of surface morphology of an organic thin solid film on fluorescence and electroluminescence (EL) properties, thin solid films of 4-(dicyanomethylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran (DCM2) and new red fluorophores, (2E,2'E)-3,3'-[4,4''-bis(dimethylamino)-1,1':4',1''-terphenyl-2',5'-diyl]bis[2-(2-thienyl)acrylonitrile] (ABCV-Th) and (2Z,2'Z)-3,3'-[4,4''-bis(dimethylamino)-1,1':4',1''-terphenyl-2',5'-diyl]bis(2-phenylacrylonitrile) (ABCV-P) were investigated by atomic force microscopy (AFM). The samples for EL and AFM measurement were fabricated by the high-vacuum thermal deposition (8 x 10(-7) Torr) of organic materials onto the surface of indium tin oxide (ITO)-coated glass substrate, in which the layer structures of samples for AFM measurement and those for EL measurement were ITO/NPB (40 nm)/red emitters (80 nm) and ITO/NPB (40 nm)/red emitters (80 nm)/BCP (30 nm)/Liq (2 nm)/Al (100 nm), respectively. The analysis based on AFM measurements well supported that the photoluminescence properties and the device performance were very much dependent upon surface morphology of an organic thin layer.