Studies On White Organic Light Emitting Devices Based On Blue Fluorescence And Orange Phosphorescence

Organic light-emitting diodes (OLEDs) have been attracting more and more attention Since C.W. Tang reported organic light-emitting devices 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.We fabricated highly efficient orange organic light-emitting devices based an Iridium complex bis (2-(2-fluorphenyl)-1,3- benzothiozolato-N,C2′) iridium (acetylacetonate) (F-BT)2Ir(acac) by adjusting the concentration of phosphorescent materials. The structure of organge OLED is m-MTDATA (30 nm)/NPB (20 nm)/CBP: Xwt% (F-BT)2Ir(acac) (30 nm)/BCP (10 nm)/Alq3 (40 nm)/LiF (0.8 nm)/Al. The performance is best when the doped concentration is 12 wt%, The maximum luminance and current efficiency is 52340 cd/m2 and 30.4 cd/A. According to the energy-level diagram, the highly efficient efficiency of the orange organic light-emitting devices may be attributed to the carrier trapping of the phosphorescent materials.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 of the devices: m-MTDATA(30 nm)/NPB (20 nm)/DPVBi (15 nm)/CBP (Y nm)/CBP: 8 wt% (F-BT)2Ir(acac) (8 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (0.8 nm)/Al. The thickness is 0, 2, 5, 8 nm for device A, B, C, D, respectively. By introducing a thin undoped CBP layer between the blue fluorescent layer and the orange phosphorescent emission layer, additional light emission from (F-BT)2Ir(acac) is obtained which is attributed to the eliminated Dexter energy transfer between the two emitters. More efficient WOLEDs are demonstrated by adjusting the thickness of CBP: (F-BT)2Ir(acac). The structures of WOLEDs are as follows: ITO/m-MTDATA (30 nm)/NPB (20 nm)/DPVBi (15nm)/CBP (2 nm)/CBP: 12wt% (F-BT)2Ir(acac) (Y nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (0.8 nm)/Al. The device with 10-nm-thick doping CBP: (F-BT)2Ir(acac) layer emits highly efficient pure white light with CIE coordinates (0.33, 0.34) which is close to the equi-energy white point (0.33, 0.33). The white device has the maximum current efficiency of 13.4 cd/A and the maximum luminance of 40962 cd/m2 obtained at 13 V. Moreover, the CRI of the device is 79.Because one singlet versus three triplets are produced by electrical excitation, so the blue fluorescent emitting layer can only use the singlet exitons, which causes the efficiency of the WOLEDs based on blue fluorescent and other phosphorescent materials lower than that of the WOLEDs based on all- phosphorescent materials. But highly efficient WOLEDs can be demonstrated if we introduce a different device concept that uses a fluorescent emitting dopant to harness all electrically generated high energy singlet excitons for blue emission, and orange phosphorescent dopant to harvest the remainder of lower-energy triplet excitons orange emission. The WOLEDs have high efficiency which can be compared to similarly ideal all-phosphor devices.In order to fix the thickness of the blue fluorescent emitting layer at the interface of the emitting layer, we fabricated blue organic light-emitting device. The structure are as follows: m-MTDATA (30 nm)/NPB (15 nm)/Ir(ppz)3 (5 nm)/CBP:5wt%BCzVBi (X nm)/CBP (Y nm)/CBP:5wt%BCzVBi (X nm)/Bphen (30 nm)/LiF (0.8 nm)/Al. 2X+Y=30. X=2, Y=26 for divice A; X=5, Y=20 for divice B; X=10, Y=10 for divice C; X=15, Y=0 for divice D; We find all the singlets can almost used when X is 5. Device B has a maximum current efficiency of 3.0 cd/A and the luminance is 6693 cd/m2 at 13V. We fix the the thickness of the blue fluorescent emitting layer at 5 nm and adjust the thickness of orange phosphorescent emitting layer to obtain white light emission. The configurations of WOLEDs are:ITO/m-MTDATA (30 nm)/NPB (20 nm)/CBP: 5wt%BCzVBi (5 nm)/CBP (X nm)/CBP:8wt%(F-BT)2Ir(acac) (Y nm)/CBP (X nm)/CBP: 5wt%BCzVBi (5 nm)/Bphen (20 nm)/Alq3 (20 nm)/LiF (0.8 nm)/Al. For device E, X=0, Y=5;Device F, X=3, Y=5;Device G, X=3, Y=10;Device H, X=3, Y=15。3 nm undoped host layer larger than the F?rster radius is placed between the blue fluorescent and the phosphorescent to prevent direct energy transfer from the blue dopant to orange phosphor. The optimized white device has a maximum current efficiency of 21.2 cd/A and maximum luminance of 40870 cd/m2. The power efficiency of the device at 100 cd/m2, 1000 cd/m2 is 14.4 lm/W and 10.1lm/W. Moreover, the white device has good color stability at high luminance (>1000 cd/m2).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 stacked white organic light-emitting diodes employing Mg: tri(8-hydroxyquinoline) aluminum/MoO3 as charge generation layer. To illuminate the function of Alq3:20wt%Mg/MoO3, we fabricated two devices. The configurations of the control and two-unit stacked OLED are as follows:Control device: ITO/m-MTDATA (30 nm)/NPB (20 nm)/CBP: 8wt% (F-BT)2Ir(acac) (30 nm)/Bphen (10 nm)/ Alq3 (30 nm)/LiF (0.8 nm)/Al. Stacked device: ITO/m-MTDATA (30 nm)/NPB (20 nm)/CBP: 8wt% (F-BT)2Ir(acac) (30 nm)/Bphen (10 nm)/Alq3 (30 nm)/Alq3:20wt%Mg (30 nm)/MoO3 (3 nm)/m-MTDATA (30 nm)/NPB (30 nm)/CBP: 8wt% (F-BT)2Ir(acac) (30 nm)/Bphen (10 nm)/Alq3 (30 nm)/LiF (0.8 nm)/Al. The current efficiency of the two-unit stacked device is two times as high as that of the control device. For example, at 1.8 mA/cm2, the efficiency of the stacked device is 48.0 cd/A while that of the control device is 22.3 cd/A, which proves Alq3:20wt%Mg/MoO3 to be an effective CGL for stacked OLEDs.We have demonstrated two kinds of stacked white organic light-emitting diodes employing Alq3:20wt%Mg/MoO3 as charge generation layer. The configurations of stacked WOLEDs are:ITO/ m-MTDATA (30 nm)/NPB (20 nm)/DPVBi (30 nm)/Bphen (20 nm)/Alq3 (20 nm)/Alq3:20wt%Mg (30 nm)/MoO3 (Z nm)/m-MTDATA (30 nm)/NPB (30 nm)/CBP: 8wt% (F-BT)2Ir(acac) (30 nm)/Bphen (20 nm)/Alq3 (20 nm)/LiF (0.8 nm)/Al. Z=0.5 1.5 4 8 is for devices E, F, G, H. The stacked WOLED with individual blue fluorescent and orange phosphorescent emissive units has much better color stability and higher efficiency than that with double white emissive units, which is attributed to the avoidance of the movement of excitons recombination zone and elimination of the Dexter energy transfer between the blue and orange emission layers occurring in the latter device. Device H has a maximum current efficiency of 36.3 cd/A. Our stacked white device can obtain high efficiency at high luminance. For example, current efficiencies at the luminance of 1000 cd/m2 and 10000 cd/m2 are 35.9 cd/A, 27.8 cd/A, respectively.