High Efficiency/Low Roll-off White Organic Light Emitting Diodes Based On Carriers And Excitons Manipulation

White organic light emitting diodes?WOLEDs?are promising to become the mainstream technology for flat panel display and solid-state lighting in the future due to their unique advantages such as surface-emitting,low power consumption,soft color,light,thin and flexibility.In the past years,the performance of WOLEDs has been greatly improved.However,the severe efficiency roll-off and short lifetime are still pressing obstacles to industrial application of WOLEDs.The exciton-exciton and exciton-polaron annihilations at high voltage are the main reason for the efficiency roll-off of device,which are caused by the unbalanced distribution of carriers and aggregation of excitons in emitting layer because the carriers and excitons in the device can't be well managed.Based on above analysis,in this paper,we realized the effective management and utilization of carriers and excitons through designing novel device structure,and successfully developed a series of high efficiency and low efficiency roll-off WOLEDs.The specific research contents are as follows:1.By combining the emissions of blue fluorescence light and exciplex yellow light,a high efficiency/high color rendering index WOLED was prepared,in which Bepp₂ acts as blue fluorescent material and exciplex donor,m-MTDATA acts as exciplex acceptor,and m-MTDATA is doped in Bepp₂ to form exciplex.By adjusting the incorporation position,doping ratio,and thickness of exciplex doping layer,the carriers recombination zone and excitons distribution are well regulated in the device,and the ideal complementary color white light emission is successfully realized.The color rendering index for the optimized WOLED reaches 93.Simultaneously,this WOLED also achieves high device efficiency.For example,the maximum current efficiency,power efficiency and external quantum efficiency are up to 9.50 cd/A,11.08 lm/W,and4.43%,respectively.And at a luminance of 1000 cd/m²,the current efficiency,power efficiency,and external quantum efficiency still reach 5.10 cd/A,9.23lm/W,and 2.46%,respectively.2.In term of the management of carriers and excitons,we further proposed the sandwich host device structure,and successfully developed a series of monochrome and white OLEDs with ultra-high efficiency and extremly low efficiency roll-off.For example,the fabricated blue,green,yellow,red,and wihte OLEDs achieve the maximum external quantum efficiency reaching21.79%,21.71%,23.85%,23.99%,and 23.15%,respectively.And at a luminance of 5000 cd/m²,the external quantum efficiencies are still up to12.61%,20.34%,20.95%,20.07%,and 16.20%for green,yellow,red,and white OLEDs respectively.The achievement for high device performance is ascribed to a broaden and strictly limited carrier recombination zone by sandwich host device structure.3.To realize better color quality,a series of high efficiency/low roll-off tandem three-and four-color white OLEDs were demonstrated.Firstly,the high-efficiency/low roll-off two-color white OLEDs were fabricated by optimizing the charges generation unit.Then,combined with the advantages of above sandwich host structure and tandem device structure,a series of high efficiency/low roll-off tandem three-and four-color white OLEDs with two light emitting units are successfully realized.For example,the peak external quantum efficiency of two-color,three-color,and four-color tandem WOLEDs reach38.20%,35.40%,and 34.78%,respectively.At a luminance of 5000 cd/m²,the external quantum efficiencies of these devices were still 31.69%,28.55%,and24.56%,respectively.In addition,the optimized WOLED also achieves extremely high color quality and ultra-high color rendering index up to 91.