Studies On Organic Light Emitting Devices

Organic light-emitting diodes (OLEDs) have been attracting considerable attention as an important flat panel display technology due to their ease of production, light weight, low cost, broad visual angle, fast response time, active emitting, high brightness, high efficiency, availability for full color display and compatibility to flexible substrates, etc. The research works in this field gained rapid development since C. W. Tang demonstrated the high brightness OLEDs at low operating oltage for the first time in 1987. Owing to the research on new materials, optimization of device structure and improvement of fabrication processes, enormous progress has been made in the development and improvement of OLEDs for display applications. Intense research in both academia and industry has yielded OLEDs with remarkable color fidelity, device efficiencies and operational stability.To improve the efficiency of OLEDs primarily, we researched about phosphorescence OLEDs, phosphorescence and fluorescence OLEDs and OLEDs of cathode or anode modified by ZnO, respectively. It offers reliable experiment gist for improving the capability of OLEDs again.Firstly, we used CDBP instead of CBP as the host for blue dopant FIrpic, the triplet energy level of CBP (2.6 eV) is lower than that of FIrpic(2.7 eV), can't transfer energy sufficiently between CBP triplet states and FIrpic triplet states; And the triplet energy level of CDBP(3.0 eV) was much higher than those of CBP and FIrpic, there was a very sufficient energy transfer from CDBP triplet states to FIrpic triplet states, the luminance and efficiency of the devices improved too. We had made the devices by FIrpic in CDBP and Ir(ppy)3 and Ir(piq)2(acac) in CBP. We decide best the concentration of FIrpic in CDBP by the structure ITO/NPB (40 nm) /CDBP:FIrpic(30nm,x%)/TPBI (50 nm)/LiF(0.8 nm)/Al is 10%, and the best concentration of CBP: Ir(ppy)3: Ir(piq)2(acac) is 1:5%:3% by the structure ITO/NPB(40 nm)/CBP: Ir(ppy)3: Ir(piq)2(acac) (20 nm,x%,y%)/TPBI (50 nm)/LiF(0.8 nm)/Al, and we fabricated the structure ITO/NPB(40 nm)/ CDBP:Firpic /CBP: Ir(ppy)3: Ir(piq)2(acac) /TPBI (50 nm)/LiF(0.8 nm)/Al by changing the thickness of the layer CDBP:Firpic and the layer CBP: Ir(ppy)3: Ir(piq)2(acac), it all gave bad blue light. So we fabricated the structure A by using CBP between the blue layer (CDBP:Firpic) and CBP: Ir(ppy)3: Ir(piq)2(acac) layer. The devices is good white one, the maximum luminance and power efficiency of the WOLED are 37100 cd/m2 at 17 v and 6.28 cd/A at 4 v, 7.37 lm/w at 5 v, respectively. The CIE of the devices change (0.41, 0.42) at 1000cd/m2 from to (0.37, 0.39) at 25800 cd/m2, giving pure white light, the efficiency is as great as the pure phosphorescent devices.Secondly, we fabricated the WOLED by using blue phosphorescent and yellow fluorescent matter, to research about the influence in lightness and efficiency of the devices different lighting layer position. Firstly, the structure ITO/ m-MTDATA(30 nm)/NPB(20 nm)/CBP: FIrpic (10%,30 nm)/rubrene(x nm)/ Bphen(40-x nm)/LiF(0.8nm)/Al was fabricated, after choice, the device gave white light as x=0.05 nm. The maximum luminance and power efficiency of the WOLED A are 4520 cd/m2 and 4.60 cd/A, respectively. The current efficiency is 2.60 cd/A at 1000 cd/m2. The CIE of the devices change (0.34,0.37)at 777 cd/m2 from to (0.29, 0.29)at 4520 cd/m2, giving pure white light. For improving efficiency, we made another structure ITO/m-MTDATA(30 nm)/NPB(20 nm)/rubrene(x nm)/CBP: FIrpic (10%, 30 nm) /Bphen(40-x nm)/LiF(0.8 nm)/Al. When x=0.1 nm, its'maximum luminance and power efficiency of the WOLED B are 8265 cd/m2 and 8.25 cd/A, respectively. When bright change from 897 cd/m2 to 8265 cd/m2, the CIE of the devices change from(0.36,0.43)to(0.31,0.37). It gave pure white light near white equal point.In devices A, from energy level graph, the exciton was formed at interface of NPB/CBP: FIrpic, because electrons injecting was difficultly than caves', when rubrene near cathode, some electrons was captured, the amount of electrons get to CBP: FIrpic decreased comparatively, the exciton was lower formed at the interface of NPB/CBP: FIrpic, FIrpic gave poor light, the efficieny of the devices was lower comparatively; In devices B, for rubrene was very thinner, the exciton also was think be formed at interface of NPB/CBP: FIrpic, but caves was easy be injecting than electrons, caves was injected more, caves was captured by rubrene beside forming enough excitons, then catch some electrons, complex and gave the yellow light, so the lightness and the efficiency of the devices were high, comparatively.Finally, we researched about anode and cathode modification by the numbers with ZnO for the first time. One for cathode modification directly by ZnO, one for cathode modification by it after LiF'modified, and the third for anode modification by it.The devices with modification directly by different thickness ZnO, the structure is ITO/NPB(50 nm)/Alq3(50 nm)/ZnO(x nm)/Al, the luminance and efficiency increased with the thickness of ZnO increased, the brightness and efficiency is great as 1.5nm. The maximum luminance and efficiency of the OLED are 7822 cd/m2 at 16 v and 2.64 cd/A at 10 v, 1.02 lm/w at 7 v, respectively. The current efficiency is 1.81 cd/A at 1000 cd/m2.Becase the electron tunneling, the injecting of it is propitious with proper ZnO, it is favorable for injecting balance of cavity and electron, improving the compound of them, so the brightness and efficiency of the devices increase; thick too much of ZnO, tunneling cut down, balance break, luminance and efficiency cut down too.The efficiency of the devices with ZnO is grater than those without one. The brightness and efficiency is increasing as the thickness of ZnO increasing, it shows that, ZnO is propitious to injecting of electron. It due to the LUMO of ZnO is lower, and it can connect with metal cathode, electron can inject roll one's hoop.But the efficiency of devices will cut down with the over thickness of ZnO due to the bad electron transmit ability.The cause for the increasing with efficiency, one for improve injecting of electron, one for connect well with metal cathode, all to increasing the efficiency.Changing the thickness of ZnO, after the modification by LiF, the brightness and current density of the devices increase as the thickness increase, shows as contrast table 4.2.4. And the efficiency is most as 1nm. We fabricated another structure ITO/NPB(50 nm)/Alq3(50 nm)/ZnO(x nm)/LiF(y nm)/Al, the thickness of ZnO/LiF were x/y=1 nm/0.5 nm; x/y=1.5 nm/0 nm; x/y=0 nm/1.5 nm; x/y=0.5 nm/1 nm, separately. We summing-up after experiment, the Luminance and efficiency of devices modified by ZnO and LiF is higher than that of devices modified by ZnO only. The maximum luminance and power efficiency of the OLED are 20209.2 cd/m2 at 16 v and 3.67cd/A at 10 v, 1.21 lm/w at 9 v, respectively. The current efficiency is 3.52 cd/A at 11 v at 1000 cd/m2.Finally, we research about the devices using ZnO at anode, and find it is effective too, and the maximum luminance and current efficiency of the OLED are 33130 cd/m2 and 4.42 cd/A, respectively. The current efficiency is 4.23 cd/A at 1000 cd/m2.