Design,Synthesis And Device Performance Of Efficient Phosphorescent Host Materials

In the past thirty years,great progress has been made in the field of organic light emitting diode(OLED).It has demonstrated practical applications in display and lighting device due to its superior inherent properties such as slim structure,wide viewing angle,fast response time.The rapid development of OLED devices is inseparable from the study of emitting layer materials.The currently commercialized guests are mainly phosphorescent materials.On the one hand,they can break through the theoretical 25%internal quantum efficiency(IQE)of traditional fluorescent OLED devices by using singlet and triplet excitons thus achieving 100%IQE;on the other hand,Phosphorescent materials can overcome problems such as insufficient color purity and serious efficiency roll-off of the thermally activated delayed fluorescence(TADF)devices.However,phosphorescent dopant still has many defects such as long lifetime,which may lead to concentration quenching,so we disperse it in the host matrix by using the host-guest structure.This strategy is necessary for suppressing the detrimental triplet-triplet annihilation process as well as ensuring the efficient energy transfer from host to dopant.Although green phosphorescent materials have been widely used,blue and red materials still have difficulties to be commercialized due to its low external quantum efficiency(EQE).For blue dopants,the large energy gaps,high triplet energy levels,would increase the difficulties in molecules design and synthesis.For red materials,the narrow band gaps,make it prone to proceed non-radiative transition which may result in lower device efficiency.In recent years,many stable host materials have been reported and applied in phosphorescence organic light emitting diode(PHOLED)to ensure efficient energy transfer for efficient EQE Therefore,the focus of this thesis is to design and synthesize highly efficient and stable host materials and apply them to red or blue PHOLED for improving the device performances.1.Three novel bipolar compounds(CNTPA-CZ,CNTPA-PXZ and CNTPA-PTZ)were designed and synthesized by modifying mCP,where three functional groups were attached to 1,3,5-positions of the central benzene ring,respectively.In these three compounds,carbazole/phenoxazine/phenothiazine(CZ/PXZ/PTZ)and benzonitrile were applied as electron-donating and electron-withdrawing groups,respectively,to achieve bipolar transport functionality which was confirmed by the hole-only and electron-only device results.Due to the efficient energy transfer from these compounds to the dopant Ir(MDQ)2(acac),the red phosphorescent OLEDs demonstrated high device performances with maximum EQE over 20%,the PHOLEDs hosted by CNTPA-PXZ and CNTPA-PTZ attained EQEs over 20%at a brightness of 500 cd/m2 Particularly,the red PHOLED hosted by CNTPA-PTZ achieved a maximum current efficiency(CE),power efficiency(PE),EQE of 42.5 cd/A,44.3 lm/w,23.4%with the EQE roll-off ratio of 3.4%from the peak value to that at a brightness of 500 cd/m2 Besides,it still had a high EQE of 21.4%at the brightness of 1000 cd/m22.Three novel thermally activated delayed fluorescence(TADF)materials(BPCN-Cz2Ph,BPCN-2Cz and BPCN-3Cz)were designed and synthesized Diphenyl-carbazole/bicarbazole/tercarbazole(Cz2Ph/2Cz/3 Cz)and 1,1 '-biphenyl-2-carbonitrile were applied as donors and acceptor,respectively,to achieve bipolar transport functionality.The compounds BPCN-Cz2Ph,BPCN-2Cz,BPCN-3Cz had the typical TADF characteristics with small singlet-triplet splitting(AEst)of 0.31,0.22,0.17 eV,respectively,ascribing to their twisted configurations Due to the efficient energy transfer from the high T1 energy levels over 2.7 eV of TADF host materials to the FIrpic dopant,the blue phosphorescent OLED demonstrated high device performances with low driving voltages and maximum EQE of 24%.Particularly,the blue PHOLED hosted by BPCN-2Cz achieved a maximum CE,PE of 50 cd/A,46.8 lm/w.Besides,the EQEs of BPCN-Cz2Ph and BPCN-2Cz hosted devices were 21.0 and 21.5%at a brightness of 1000 cd/m2 and the EQE roll-off ratios were 5 and 10%from the peak value to 1000 cd/m2,which were lower compared to many reported blue phosphorescence OLEDs.In summary,we designed and synthesized different D-A(Donor-Acceptor)type host materials,adjusted the molecular electroluminescence properties by changing different donor units.The PHOLED devices based on these materials all achieved good performances,which could as a guide in the development of efficient host materials.