Device Physics Research Of Organic Light Emitting Diodes Based On Thermally Activated Delayed Fluorescence Materials

Since the advent of the first thermally activated delayed fluorescence(TADF)materials were successfully synthesized,TADF-based OLEDs have drawn enormous attention.TADF materials consist of singlet state(S1)and triplet state(T1)with a small energy difference,which enables excitons to up-convert from Ti to the Si via efficient reverse intersystem crossing(RISC)and hence gives fluorescence emission TADF materials have advantages of high internal quantum efficiency,ease of synthesis and low materials cost;Therefore,TADF materials are considered to be the third generation of luminescent materials which are able to further promote the OLED technology to a new era.Achieving high efficiency and long lifetime of white OLEDs are of very crucial to realize the commercialization of OLEDs for lighting as well as display applications,so how to prepare high-performance white OLEDs based on TADF materials has become a research hotspot.In the first research work of my thesis,I demonstrated the first TADF-based green-emitting tandem OLED with a maximum current efficiency(CE)and external quantum efficiency(EQE)of 117.5 cd/A and 32.5%,respectively.This is by far the highest EQE achieved to date for TADF devices.We also explored the first hybrid tandem white-emitting OLED using blue TADF and orange phosphorescent emitters.The device exhibited a maximum CE and EQE of 78.5 cd/A and 28.5%,respectively.Its CIE(x,y)at 1000 cd/m2 is(0.33,0.45).These results demonstrate that TADF materials combined with tandem structure have great potential for fabricating highly efficient and stable white OLEDs.Currently,TADF-based OLEDs suffer from severe efficiency roll-off particularly at high brightness.In the second part of my work,we tried to dope a small amount of blue phosphorescent material into the TADF devices.Strikingly,we successfully fabricated a TADF-based sky-blue OLED with a reduced efficiency roll-off by doping DMAC-DPS with 1%of FIrpic as an assistant phosphor.FIrpic helps reduce the average exciton lifetime of the devices.Our device had a maximum EQE of 19.1%,and the EQEs reached 16.5%and 11.2%even when the brightness of the device was increased to 1,000 cd/m2 and 5,000 cd/m2,respectively.The drop of EQE efficiency was very small(13.6%for a brightness at 1,000 cd/m2).This high EQE efficiency of 16.5%at a high brightness level of 1,000 cd/m2.Its efficiency roll-off is by for the lowest ever reported.