High Efficiency And Color-stable White Organic Light-emitting Diodes

Organic light-emitting diodes(OLEDs) are quite promising for next generation of flat panel displays instead of liquid crystal displays(LCDs) for the merits of low power consumption, broad visual angle, fast response speed, low-cost, manufacturing feasibility, and flexible display. White OLEDs (WOLEDs) have potential of producing highly efficient saturated white light, with advantages of low-driving voltage, large area available, and flexible display, hence presenting tremendous potential applications in solid-state lighting(SSL) and backlights in liquid-crystal displays.In this thesis, we report several kinds of high efficiency and color stable WOLEDs based on different host or co-host including two-spectrum and three-spectrum type by selecting the appropriate dopant and host, combining with energy levels, the charge injection balance, the carrier injection efficiency, device physics, harvesting the exciton energy, etc. The physical mechanism of devices is discussed. In addition, through the design and optimization of device structure, highly efficiency organic blue device is also preparation.Firstly, highly efficiency and color-stable two-spectrum WOLED combining an orange phosphor Ir(BT)2acac and a blue fluorescent dye DSA-Ph are fabricated. The device structure is ITO/m-MTDATA/NPB/TCTA:6 wt.%Ir(BT)2acac/CBP:3wt.% DSA-Ph/Bhpen/LiF/Al, in which CBP doped with DSA-Ph as blue emitting layer and TCTA doped Ir(BT)2acac as orange emitting layer, respectively. Different interlayer was introduced between the two emitting layer. The current efficiency of the device with TCTA interlayer is 7.0 cd/A at 1000 cd/m2,6.9 cd/A at 5000 cd/m2. respectively. It shows efficiency roll-off of 1.4%. The chromaticity coordinates varies from (0.343.0.369) to (0.347,0.366) with increasing luminance from 1000 cd/m2 to 5000 cd/m2, corresponding to a shift of color coordinates ofΔCIExy(x, y)<±(0.004,0.003), It closes to the point of equal energy (0.33.0.33). TCTA can prevent electron transfer to orange layer, tune the distribution of carrier in emitting layers and enhance blue intensity. When the interlayer is Bphen, the device exhibits current efficiency roll-off of 3.9% with increasing luminance from 1000 to 5000 cd/m2, which can contribute to Bphen preventing hole transfer to blue layer. The color coordinates of the divice closes to the warm white standard illuminant A(0.45,0.41).By using DSA-Ph and Ir(BT)2acac doped in co-host CBP, high efficiency and color-stable WOLED with a structure of ITO/m-MTDATA(40 nm)/NPB(10 nm)/CBP:3 wt.%DSA-Ph(x nm)/CBP:6 wt.%Ir(BT)2acac(20 nm)/Bphen(40-x nm)/LiF(0.6 nm)/Al has been demonstrated in this thesis. The maximum current efficiency and power efficiency is 13.9 cd/A and 10.9 lm/W, respectively. When the luminance is 1000 cd/m2, the current efficiency and power efficiency are 11.5 cd/A and 10.1 lm/W, respectively. The CIE coordinates of the device change from (0.441,0.452) at 1000 cd/m2 to (0.397,0.440) at 10000 cd/m2, which indicate a stabilized white color with a shift of color coordinates ofΔCIEx,y<±(0.013,0.014). These results indicate that the dominant excitation mechanism is in self-recombination in the dopant as carrier trap at low current density, while in high current density, the excitation mechanism is included not only in self-recombination in the dopant, but also the Forster energy transfer from host to dopant. In addition, the carrier injection balance also contributed to the color stability of the device.Phosphorescence is inherently a slow and inefficient process, but triplet states constitute the majority of electrogenerated excited states (～75%), so the successful utilization of the triplet manifold to produce light should increase the overall luminance of OLED. In this paper, we demonstrate a high efficiency WOLED employing phosphorescent dye Firpic to replace DSA-Ph. The structure of WOLED based on Firpic is ITO/NPB(40 nm)/mCP(3 nm)/mCP:10%Firpic(5 nm)/BCP(3 nm)/Bphen:6% Ir(BT)2acac(15 nm)/Bphen(30 nm)/LiF(0.6 nm)/Al. A maximum current efficiency is 6.3cd/A at 1000cd/m2. CIE coordinates ranges within ACIEx,y<±(0.024,0.011) when the luminance changes from 1000 to 10000 cd/m2. In order to improve the color rendering index (CRI) of the WOLEDs, the green emitting layer of mCP doped with Ir(ppy)3 was intrudeced into the device. After optimization, a maximum current efficiency of 15.9 cd/A and maximum power efficiency of 9.4 lm/W are obtained at 1000 cd/m2. The CIE coordinates of the device are shifted from (0.321,0.431) at 7 V to (0.323,0.435) at 15 when the luminance changes from 2335 to 5523cd/m2. It ranges within ACIEx>y<±(0.002,0.004)Although the blue OLEDs based phosphorescent dye exhibits high efficient, its poor lifetime and color stability problems have never been resolved. In this thesis, we also fabricated high-efficiency blue fluorescence OLEDs with DPVBi inserted in the doping emmision layer(EML). The OLEDs with a configuration of ITO/2T-NATA/NPB /DPVBi:DSA-Ph(inserted with DPVBi thin layer)/Alq3/LiF/Al were fabricated, using 2T-NATA as hole injection layer, NPB as hole transport layer, DPVBi:DSA-Ph as emission layer and Alq3 as electron transport layer, respectively. The introduction of DPVBi thin layer inserted in EML leads to an increase in device efficiency as a result of an improvement of the balanced carrier injection, which result in an efficient radiative recombination in the emission zone. In addition, DPVBi's ability of hole blocking can also be another reason for the improvement in the luminous gain. Hence, high radiative recombination is expected in DPVBi:DSA-Ph emission layer. This highly efficient recombination results in high brightness and enhanced efficiency in our OLEDs. By optimizing the location and the number of layers of DPVBi thin layer, a maximum current efficiency of 6.77 cd/A achieved at a current density 6.84 mA/cm2, which is nearly 67.6%more than of non-inserted device. At luminance of 1000 cd/m2, the current efficiency of the optimizing device is 6.49 cd/A at 6.7 V with a CIE (0.179, 0.317).