Device Engineering Of Phosphorescent And Thermally Activated Delayed Fluorescent Based White Organic Light-Emitting Diodes

Organic light-emitting diodes(OLEDs)have many attractive characteristics for next-generation display and solid state lighting applications,such as low power consumption,wide color gamut,free of blue hazard,thin,light weight and flexibility.Although small-sized and TV-sized OLED displays and a small number of OLED lighting panels are commercially available,it is still very crucial for us to pursue higher efficiency,longer lifetime and lower cost of OLED panels.To acquire high-efficiency white OLEDs,one of the most common ways is to utilize all phosphorescent emitters.The phosphorescent materials are able to realize 100%internal quantum efficiency(IQE)and achieve very high efficiency since both the singlet and triplet excitons can be fully utilized.Another way to obtain an IQE of 100%is to adopt thermally activated delayed fluorescence(TADF)materials which are basically fluorescent emitters.Till now,the efficiency,lifetime and manufacturing cost of OLEDs are the most challenging obstacles for OLED display and lighting.In my thesis,device engineering of both the phosphorescent and TADF based OLEDs is investigated and discussed.In the first part of this thesis,a novel device structure with doped ultrathin emissive layer for two-color-based phosphorescent white OLEDs are designed.By doping the phosphorescent red emitter into an ultrathin phosphorescent blue emitter and optimizing the doping concentrations and thicknesses of the emitting layer,we fabricated high-efficiency white OLEDs with a current efficiency and power efficiency of 33 cd/A and 37 lm/W respectively,without using light out-coupling techniques.The CIE(x,y)of(0.40,0.40)was obtained,which is very stable at high brightness up to 10,000 cd/m2.The device covers the required brightness for practical applications.In addition,the turn-on voltage of our device was below 2.8 V,which is lower than that in theconventional host-guest based devices.We also propose a mechanism for the enhanced performance of OLEDs with doped ultrathin emissive layer.In the second part,two-color fluorescence white OLEDs based on a traditional fluorescent blue emitter and a fluorescent orange emitter with TADF material as a co-host are studied and discussed.By modifying the thickness and doping concentration of the two-color based white OLED device,we obtained a quite stable white emission with a CIE(x,y)of(0.38,0.38).The current efficiency of the all-fluorescent white OLED is 55 cd/A while the EQE reaches 21.7%.Using TADF materials as hosts of fluorescent emitters would be a new approach and promising to achieve high-efficiency white OLEDs.Finally,aiming at reducing the efficiency roll-off for phosphorescent OLEDs,a novel aminoborane-basedcompound named BCzBMes acting as the bipolar host for phosphorescent OLEDs was synthesized.Photophysical and DFT analysis reveal that BCzBMes not only possesses high triplet energy that can satisfy the requirement of the blue phosphorescent materials,but also exhibits bipolar behavior which can indeed reduce the efficiency roll-off.To examine the material property,a two-color based white phosphorescent OLED was fabricated with the novel bipolar host material,which exhibits a high current efficiency of 52.0 cd/A,and a low efficiency roll-off of 19.2%(50.2 cd/A to 42.0 cd/A)when the luminance reaches 10000 cd/m2.