The Molecular Design And Properties Of Thermally Activated Sensitizing Hosts For OLED Devices

Recently,the efficiency of phosphorescent and thermally activated delayed fluorescent?TADF?OLED devices have been greatly improved,the highest EQE of the TADF devices have exceeded 30%,but efficiency roll-off and short device lifetime have become main factors that restrict the application of phosphorescent and TADF-OLED.One way to improve roll-off and lifetime of phosphorescent and TADF-OLED devices is to use thermally activated sensitized hosts with TADF characteristics.However,there are few applications of thermally activated sensitized hosts for red emitters with small band gaps and blue emitters with high excited states.In this paper,the molecular design strategy of thermally activated sensitized host materials suitable for red and blue emitters is proposed,using these hosts,low efficiency roll-off,long-lifetime red and blue OLED devices are fabricated.The main research results are:1.Two thermally activated sensitized are fabricated using N-phenyl-indolcarbazole and triazine as electron donors and electron acceptors.The band gap and the energy of excited states of the molecules are reduced by introducing different electron-withdrawing effect groups on the triazine group,which make the hosts suitable for red emitters to prevent direct capture of carriers.The energy gap between singlet and triplet states(?E_ST)of the host are furture reduced,which increase the reverse intersystem crossing rate of the molecule and suppress the quenching of triplet excitons.Red and green phosphorescent and TADF-OLED devices were fabricated using these hosts,their roll-off and lifetime were significantly better than those devices with traditional CBP host.2.Two thermally activated sensitized are fabricated using N-phenyl-[2,3-a]-indolcarbazole and N-phenyl-[3,2-a]-indolcarbazole as electron donors,and triazine as electron acceptors.The excited states'energy level of these molecules are increased by introducing phenyl between the donor and accepter groups,and the?E_ST of these molecules are reduced by adjusting the position of donor and accepter groups on the phenyl ring.Sky-blue TADF-OLED devices were fabricated using these hosts,the hightest EQE of the devices is 16.6%,and there are almost no efficiency roll-off at the brightness of 1000 cd/m²,their roll-off and lifetime were significantly better than those devices with traditional mCBP host.3.An exciplex with a small radiative transition rate is used as thermally activated sensitized host,the singlet excitons with lower energy level can absorb heat and up-convert to a higher vibrational level,and then transfer energy to the excited states of blue emitters with higher energy levels,which can increase the lifetime of the blue OLED devices.The excited state energy levels of exciplex is increased by introducing pyrimidine in the electron acceptor material for the exciplex.The blue and sky-blue TADF-OLED devices were fabricated using this host,and the highest EQE of the devices are 13.5%and 16.9,respectively,their roll-off and lifetime were better than those of traditional mCBP host.