| Since Tang's report in 1987, the intense research activities are currently being untaken on organic light-emitting diodes (OLEDs) because of their potential applications in flat panel displays. At present, OLED display technology makes great progress towards commercialization. But it is endless for the researchers to pursue improved performance of OLEDs, for example saturated color purity, high luminescence efficiencies, long lifetime et al..Co-doping method is applied to fabricate efficient blue OLEDs. With the same structure of ITO/N, N'-bis(1- naphthyl)-,N,N'- diphenyl-1,1'-biphenyl-4,4' diamine (NPB) (80 nm)/light-emitting layer (30nm)/tris-(8-hydroxy-quinoline) aluminum (Alq3)(20nm)/LiF (1nm)/Al(120 nm), a set of devices were manufactured for comparison. For Device 1, 2, and 3, the light-emitting layers are 9,10-di(2-naphthyl) anthracene (ADN): 4,4'-(1,4-phenylenedi-2,1-ethenediyl)bis[N, N-bis(4-methylphenyl)-benzenamine](DPAVB) (1 wt%), ADN: 2,5,8,11-tetra-(t- butyl)-perylene (TBPE) (1wt%), and ADN: DPAVB(0.3 wt%): TBPE(0.7 wt%), respectively. It is found that the co-doped Device 3 is the best in maximum luminance, electroluminescence (EL) quantum efficiencies and color saturation. Further study on the effect of the co-dopants was through the relative photoluminescence(PL) quantum efficiencymeasurement. The result shows that the relative PL efficiency of Device 1, 2, and 3 are 15.6%, 19.3%, and 24% respectively using an integrating sphere system excited at 375nm. The co-doping method improves the EL efficiency intrinsically. Co-dopants of the heterogeneous light emitting molecules may decrease the possibility of self-quenching from the interaction of the homogenous molecules in the same total doping concentration. Furthermore, the decrease of the homogenous molecules interaction suppresses the light emission from the aggregations thus narrowing the emitting spectrum and results in saturated blue.High efficiency OLEDs employing an iridium complex with coumarin6 (C6) and acetylacetone (acac) ligands, Ir(C6)2(acac), are presented. The iridium complex possesses electron-transporting characteristics and shows high efficiency as light emitting dopant. Its PL quantum efficiency in dilute chloroform solution reaches 70%. By doping the iridium complex in Alq3 to form device of ITO/4,4',4"-tris(N-(2-Naphthyl)-N-phenyl-amino)triphenylamine (T-NATA)(40 nm)/NPB (40 nm)/Alq3: Ir(C6)2(acac)(1 wt%) (40 nm)/Alq3 (40nm)/LiF(1 nm)/Al(120nm), a green color (CIE coordinates x=0.23; y=0.63) and a maximum luminance of33000 cd/m2 and a maximum current efficiency of 12cd/A were obtained.Furthermore, co-doped Ir(C6)2(acac) (0.5 wt%) and C545T (0.5 wt%) in Alq3, a green color (CIE coordinates x=0.21; y=0.62) and a maximum luminance of 42000 cd/m2 and an maximum current efficiency of 20cd/A were achieved. More importantly, the devices show fiat efficiencies at high injection current density, which is attributed to the improved fluorescence from low doping concentration, which caused the suppression of phosphorescence and the charge balance of T-NATA.By the study on the co-dope tetraphenylnapthacene (Rubrene): Ir(C6)2(acac) in BAlq and Alq3, we found that the efficiencies of an OLED using Alq3 as host and Rubrene as yellow dopant were greatly increased by adding a small amount (0.3 wt%) of a phosphorescent Ir compound Ir(C6)2(acac) sensitizer. The device had a simple structure of ITO/T-NATA (40nm)/NPB(40 nm)/Alq3: Rubrene(0.7 wt%): Ir(C6)2(acac)(0.3 wt%)(40 nm)/Alq3(40 nm)/LiF (1nm)/Al(120 nm), and the current efficiencies keep a high value of 13.9 cd/A even the luminance rising to 37000 cd/m2. The CIE (Commission International d'Eclairage) coordinates x and y are 0.46, 0.44 of orange color. The high efficiency at the high luminance was proposed due to the phosphorescent Ir(C6)2(acac) can harvest the singlet and triplet excitons in emissive layer and then efficiently transfer to orange dopant Rubrene.Efficiencies of an OLED using Alq3 as host and 4-(dicyano methylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as red dopant were greatly increased by adding a small amount (0.3 wt%) of a phosphorescent (Ir(C6)2(acac)) sensitizer. The device had a simple structure of ITO/T-NATA (40 nm)/NPB (40 nm)/Alq3:DCJTB(0.7 wt%): Ir(C6)2(acac)(0.3 wt%)(40 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (120nm). In spite of the luminance changing from 0.1cd/m2 to 20000 cd/m2, the current efficiencies of this device keep almost flat around 13.8 cd/A. The Commission International d'Eclairage (CIE) coordinates x and y are 0.60, 0.37 of red color. The high efficiencies at the high luminance were proposed due to the phosphorescent Ir(C6)2(acac) can harvest the singlet and triplet excitons formed in Alq3 simultaneously and then efficiently transfer to red dopant DCJTB.An equipment for OLED fabrication was designed. Following aspects were considered: 1 it is advantageous for the protection of cleaned ITO substract to connect the plasma cavity with vacuum chamber; 2 Through in situ accurate measurement of film thickness, evaporation speed of various materials, e.g. the doping concentration can be controlled exactly; 3 the system can provide sixteen OLED devices with defferent fabricated conditions from every vacuum atmosphere.
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