Studies On Novel Structure And High Performance Of Organic Light Emitting Devices

Organic light-emitting diodes (OLEDs) have been attracting more andmore attention as an important flat panel display technology for the merits oflight weight, low cost, broad visual angle, fast response speed, active emitting,high brightness, high efficiency, availability for full color display, etc. Theresearch works in this field gained rapid development especially after 1987when C.W.Tang for the first time reported the high brightness OLED at lowoperating voltage. In recent ten years or so, OLED has become a project on thecutting-edge of scientific research that relates to many intercrossed branches ofscience and advanced technology. The design of EL structure for used inOLEDs is critical to device's performance. Great strides have been madetowards the development and improvement of small molecular OLEDs fordisplay applications. Intense research in both academia and industry over thelast 4–5 years has yielded OLEDs with remarkable full color, deviceefficiencies and operational stability. In this thesis, we will report novelstructures in whtie,blue and red colors organic EL research, and a techniquefor the OLED display whtie balance.Recently, White Organic light emitting devices (WOLEDs) with highbrightness properties is expected, not only for a display device, but also for theback light of a liquid crystal display. The WOLEDs show excellent propertiesof low driving voltage, high efficiency, compact and bright emission. A varityof methods have been proposed to achieve the WOLEDs. In this thesis, wereport on a voltage independently and high luminous efficiency white lightemitting device based on an incomplete energy transfer, in which emissionlayer composes of a layer of blue-emitting material DPVBi, uniformly dopedwith orange emitting rubrene. The thickness of the emitting layer and rubreneconcentration in the DPVBi layer are varied to obtain the desired emissioncolor. The structure of the WOLED is ITO/NPB(40 nm)/ DPVBi :0.8%rubrene(20 nm)/Alq3(50 nm)/Lif (0.8 nm) /Al The device turned on at 4V.Its luminance was 16230 cd/m2at 17V and maximum efficiency was 5.7cd/A at6V. CIE coordinates range from (0.39, 0.41)to (0.30, 0.27) when the forwardbias changes from 5 to 18V. In order to improve the performance of theWOLED, the DPVBi is replaced by blue material BePP2, and use m-MTDATAas hole injection layer. The structure of WOLED based on BePP2 isITO/m-MTDATA (30nm) / NPB (10 nm)/ BePP2 : 0.2% rubrene(30nm)/Alq3(40 nm)/LiF (0.8 nm) /Al. A maximum brightness of approximately29010cd/m2 at 15V and a maximum power efficiency of 6.3cd/A at 7V,respectively. CIE coordinates range from(0.35,0.36)to(0.32,0.34)whenthe forward bias changes from 100cd/m2 to 1000cd/m2.Usually, phosphorescence is inherently a slow and inefficient process, buttriplet states constitute the majority of electrogenerated excited states (~75%),so the successful utilization of the triplet manifold to produce light shouldincrease the overall luminance of organic light emitting devices (OLEDs).Since a Princeton University group has demonstrated a way to break throughthe efficiency limitation by using phosphorescence materials which harvestedsinglet and triplet excitons, the efficiency for OLEDs makes a great advance. Inthis paper, we demonstrate a high efficiency red OLEDs employing4,4'-N,N'-dicarbazole-biphenyl (CBP) as the host and exciton donor, arhenium complex (Butbpy) Re (CO) 3Cl (Butbpy = 4, 4'-bi (tert-butyl)-2,2'-bipyridine) as phosphorescent sensitizer and the red fluorescent dye4-(dicyanomethylene)-2-t-butyl-6-(1, 1, 7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran(DCJTB) as the acceptor. Red electroluminescence achieves from excitonemission in ultra thin DCJTB layer, via a two-step excitation energy transferfrom host to phosphor sensitizer and then from phosphor sensitizer to DCJTBmolecules. The structure of red OLED is NPB (50 nm)/CBP: 9 % (Butbpy) Re(CO) 3Cl (30 nm)/ (d nm) DCJTB /BCP (10 nm)/ Alq3 (40 nm)/LiF (0.8 nm)/Al.When d=0.4 nm, the CIE coordinate is (0.61, 0.37), the maximum efficiency of3.38 cd/A at 6V and maximum brightness of 5000 cd/m2 at 17V are achieved.Our work illuminates that rhenium complexes adapt to phosphor-sensitizer forimproving fluorescence, and provides a feasible route for fabricationfluorescent acceptor with ultrathin layer instead of double-doped structure.OLED color screen can be used to show TV programs. At present there isno source of images and standard used specially for OLED color screen;Basedon the theory of primary color transformation, true-color display technique forOLED display screen with RGB three primary colors is put forward in thisthesis. In order to get the brightness equation, the RGB signals must beprocessed with color switching method. In our work, the color switchingmethod is described in detail.We deduce the formulas of calculation for OLED color screen, providingthe theoretical basis so as to control the brightness of OLED precisely, andbringing out a practical example:Blue devcie:ITO/NPB (50 nm)/BCP (10 nm)/Alq3 (50 nm)/LiF (0.5 nm)/AlGreen device:ITO/NPB (50 nm) /Alq3 (60 nm)/LiF (0.5 nm)/AlRed device:ITO/NPB (50 nm)/ Alq3: 2% DCJTB (30 nm)/Alq3 (50 nm)/LiF(0.5 nm)/AlIn the situation of white balacne, the RGB tri-stimulus values brightness ratiois 0.22 ∶ 0.68 ∶ 0.10.Amorphous molecular, distyrylbenzene derivative 2, 5, 2', 5', 2'',5''-hexastyryl-[1, 1';4', 1'']terphenyl (HSTP), is sandwiched between NPBand Alq3 as blue light emitting material in typical multilayer organic lightemitting devices, where NPB and Alq3 are 1,4-bis(1-naphylphenylamino)工团biphenyl and tris(8-hydroxyquinoline)aluminum, respectively. Formation ofexciplex at the interface of NPB and HSTP layer is verified by study onphotoluminescence and electroluminescence (EL) spectra. The performance ofEL can be greatly improved by optimizations of devices, a pure blue devicewith CIE coordination (0.16, 0.13) and maximum brightness of 15830 cd/m2and current efficiency of 4.88 cd/A is obtained.