Study On White Tandem OLEDs With Alq₃: Mg/MoO₃ As Charges Generation Layer

Organic light-emitting devices (OLEDs) have been attracting much attention due to their potential applications in flat-panel displays. Since Tang demonstrated the high brightness OLEDs at low operating voltage for the first time in 1987, materials and devices for OLEDs have developed rapidly, and now, OLEDs are on the way to industrialization.Nowadays, white organic light-emitting devices (WOLEDs) are considered as full-color displays, lighting and flat panel displays. A lot of revolution devices fabricated on soft substrate for display and lighting were invented. OLED will change the people's life, especially white OLED. Organic light-emitting diodes (OLEDs) have been attracting more and more attention Since C.W. Tang reported organic light-emitting devices (OLEDs) with low operating voltage for the first time in 1987. As a new flat panel display technology, OLEDs have many merits such as: light weight, thin thickness, low cost, broad visual angle, fast response speed, active emitting, low energy consume, high brightness and efficiency, broad operating temperature, more choice of materials, availability for full color display and flexible display, etc. Owing to the research on new materials, optimization of device structure and improvement of fabrication processes, OLEDs have developed rapidly. Because of their potential use in backlight, full color applications, as well as in lighting purposes, white organic light-emitting devices (WOLEDs) have attracted intense attention. In order to generate the desired white light, WOLEDs with various configurations have been proposed, such as using multiple emission layers in which each layer emits a different color light to generate white-light emission, using single emission layer with multiple dyes, using excimer or exciplex emission, using microcavity structure and so on. Among these approaches, WOLEDs employing phosphorescent materials are most effective because phosphorescent materials can harvest both singlet and triplet excitons which leads to the potential for achieving 100% internal emission efficiency. But the instability and the low efficiency of the blue phosphorescent dyes, as well as their demand for wide band-gap host materials, will hamper their application in field of display or lighting. In general, the emission spectra of organic materials are broader than that of inorganic materials, thus two complementary colors can produce white light emission. So the combined use of blue fluorescent and orange phosphorescent dyes may solve these problems and obtain efficient and stable WOLEDs. Since the current efficiency and luminance can scale linearly with the number of emitting units, stacked OLEDs consisting of vertically stacked multiple emitting units in a device in series via charge generating layer (CGL) attract particular interest in recent years. We have demonstrated tandem organic light-emitting diodes employing Mg: tri(8-hydroxyquinoline) aluminum/MoO3 as charges generation layer. We analyzed the devices'performance by adjusting the thicknesses of Alq3:Mg and MoO3 layer respectively. The structures we used are ITO/m-MTDATA(30 nm)/NPB(20 nm)/CBP:8%(F-BT)2Ir(acac)(30 nm)/Bphen(20 nm)/Alq3 (10 nm)/LiF(0.8 nm)/Al and ITO/m-MTDATA(30 nm)/NPB(20 nm)/CBP:8%(F-BT)2Ir(acac)(30 nm)/Bphen(20 nm)/Alq3 (10 nm)/Alq3:Mg(X nm)/MoO3(Y nm)/m-MTDATA(30 nm)/NPB (20 nm)/ CBP:8%(F-BT)2Ir(acac)(30 nm)/Bphen(20 nm)/Alq3(10 nm)/LiF(0.8 nm)/Al, where X=10, 20, 30, 40nm and Y=1, 3, 5, 10, 15, 25nm. When current density was 10 mA/cm2, the current efficiency of tandem OLED was 34.0 cd/A. Furthermore, we find that MoO3, without Alq3:Mg, as charges generation layer have charges generating ability. But when we added Alq3:Mg, the charges generation layer, Alq3:Mg/MoO3, have a obvious charges generating ability.We demonstrate white organic light-emitting devices based on (F-BT)2Ir(acac) and 4,4,8-bis(2,28-diphenylvinyl)-1,18-biphenyl (DPVBi) used as orange phosphorescent dye and blue fluorescent material, respectively. The structures we used are ITO/NPB(20 nm)/DPVBi(15 nm)/CBP(2 nm)/CBP:8%(F-BT)2Ir(acac)(X nm)/Bphen (20 nm)/Alq3 (20 nm)/Alq3:Mg (30 nm)/MoO3 (3 nm)/NPB (40 nm)/DPVBi (15nm)/CBP (2 nm)/ CBP:8%(F-BT)2Ir(acac)(X nm)/Bphen (20 nm)/Alq3 (20 nm)/LiF (0.8 nm),where X is 7,10,12,15nm. By introducing a thin undoped CBP layer between the blue fluorescent layer and the orange phosphorescent emission layer, the intensity of orange phosphorescent dye were enhanced by adjusting the thickness of CBP:8%(F-Bt)2Ir(acac). When the voltage was 25V, the current efficiency of device with 12nm CBP:8%(F-Bt)2Ir(acac) was 16.9 cd/A.The research work of this paper is not only a research for the mechanism of tandem OLED's charge generation layer, but also basis for fabricating white tandem OLED. I hope that we can use the conclusion we got in the research to fabricate white tandem OLED with higher performances in the future.