Orange-Red Thermally Activated Delayed Fluorescent Materials And Devices Based On B,N-containing Polycyclic Aromatic Hydrocarbons

Pure organic thermally activated delayed fluorescent(TADF)materials have become the most promising and attractive emitters of organic light-emitting diodes(OLEDs).At present,orange-red TADF materials are inferior to blue and green materials in terms of external quantum efficiency(EQE)and device operating stability.The focus of this thesis is to modify the design of orange-red TADF molecule to improve the device efficiency and alleviate the efficiency roll-off.Herein,we propose and practice two innovative molecular design strategies,which open up a new way to design orange-red TADF molecules with high-efficiency and low roll-off.1)Design strategy of regulating intramolecular LR/SR-CT excitation dominanceBoron,nitrogen-containing polycyclic aromatic hydrocarbons(B,N-PAHs)are widely used in the construction of blue and green TADF emitters,due to the short-rang charge transfer(SR-CT)characteristics and rapid radiative transition,but there are few reports in the red region.In this work,we introduce peripheral donors with different intensities on the B,N skeleton,and achieve a wide range of red-shifting and full-color emission by regulating the long-rang/short-range charge transfer(LR/SR-CT)excitation.Noteworthy,BN-PZ-based devices exhibit orange-red emission at 612 nm,with a maximum EQE of 25.0%and low efficiency roll-off,and EQE maintains 17.2%at the brightness of 1000 cd m-2,which is in the first class of high-performance orange-red materials.2)Design strategy of A-D-A configuration with hybrid LR/SR-CT excitationIn order to further improve the device efficiency and reduce the efficiency roll-off,two rod-like A-D-A type orange-red TADF emitters were constructed by bridging two B,N-PAHs receptors with dihydrophenozine donor.Both materials exhibit hybrid LR/SRCT excitation and possess rapid singlet radiative transition and triplet up-conversion(kr>>kRISC>106 s-1)and high horizontal dipole orientation factor(Φ//>90%).The maximum EQE of 2BNtCz-PZ based orange-red device is up to 31.0%with slight roll-off,and the EQE remains 22.2%at the brightness of 1000 cd m-2,which is the best performance of TADF-OLEDs with emission peak over 600 nm.In addition,the maximum EQE of single emission layer white OLEDs reach 30.6%.This work reveals the great potential of the "rod-like A-D-A configuration with hybrid LR/SR-CT excitation" design strategy for the construction of high efficiency,low roll-off red TADF emitters.