Study Of The Carrier Transportation And Luminescence Characteristics Of The Direct Hole-injection Organic Light-emitting Device

Based on organic luminous Ir complexes, which are phosphorescent materials, novel direct hole-injection structure devices are designed and fabricated. The properities of the device that adopts this structure are investigated. The major Ir complex materials adopted in this paper are(t-bt)2Ir(acac) and Ir(ppy)3.Firstly, the yellow phosphorescent material(t-bt)2Ir(acac) is investigated, by using the device structure: ITO/(t-bt)2Ir(acac)/TCTA/TPBi:FIrpic/TPBi/Mg:Ag, the hole injection and transportation abilities of this Ir complex are verified. Based on this, direct hole-injection structure is designed as ITO/CBP:(t-bt)2Ir(acac)(50%)/CBP:(t-bt)2Ir(acac)(x%)/TPBi /Mg:Ag x = 5,10,15,20. Via adjusting the(t-bt)2Ir(acac) ratio of the emission layer, it is found that the performance of the direct hole-injection device increases with increasing doping ratio, and then decreases. Among the four devices that we made in this experiment, device with 10% doping ratio acquires the best performance, which indicates that dopant portion is extremely critical in phosphorescent OLEDs. Multiple parameters must be balanced when designs a light-emitting device, in order to fulfill the demand of illuminating and avoid heavy triplet-triplet annihilation at the same time. In order to possesss higher efficiency, dual emission layer structure is adopted in the device: ITO(150nm)/CBP:(t-bt)2Ir(acac)(50%)/CBP:(t-bt)2Ir(acac)(10%) /TPBi:(t-bt)2Ir(acac)(10 %, x nm) / TPBi(40-x nm) /Mg:Ag, x=10, 20, 30 and 40. With this structure, a device with maximum luminance of 39294 cd/m2, maximum current and power efficiencies of 38.22 cd/A and 20.85 lm/W is fabricated.As for another Ir complex Ir(ppy)3, hole injection ability of this material is investigated at first. Then, by manipulating doping ratio in the direct hole-injection layer, the influences on device performance that caused by this factor is investigated. The device structure is ITO/ CBP :Ir(ppy)3(x%)/ CBP :Ir(ppy)3(10%)/TPBi / Mg:Ag,x = 5%, 10%, 15% and 20%. Device efficiencies increases as the doping ratio increases, and the best performance is acquired at the ratio of 20%: maximum luminance of 35346 cd/m2, current efficiency of 36.23 cd/A and power efficiency of 18.85 lm/W. It indicates that in direct hole-injection device, the proportion of luminescence material in direct hole-injection layer should be way higher than that in ordinary emission layers, in order to maintain the function of carrier injection and transportation that this novel layer provides. Different host materials are also introduced into the experiment, TCTA and TPBi are selected to be the host material in the direct hole-injection layers respectively and the test result shows that there is only a negligible difference in the device performance. This phenomenon indicates that it’s the Ir(ppy)3 in the injection layer that affects the hole injection and transportation process and determine the characteristics of the direct hole-injection layer.