Study On Exciton Dynamics Of High-performance Organic Light-emitting Diodes Based On Anthracene Derivatives

Since Ching W.Tang first prepared organic light-emitting diodes（OLEDs）in 1987,OLEDs have been widely studied,and the development of OLEDs have been greatly promoted.After 30 years of development,OLEDs have been successfully commercialized and well used in display fields such as smartphones,TVs and wearable electronic devices due to its advantages such as surface light,high efficiency,flexible screens and thinness.Compared with red and green OLEDs,the efficiency and lifetime of blue OLEDs are still the biggest problems.Blue OLEDs usually have low efficiency,short lifetime,and serious efficiency roll-off phenomenon.Therefore,how to design efficient and stable blue light emitting materials has become an important research topic of OLEDs.Among the many blue materials,anthracene derivatives,due to their broadband energy levels,the first excited state singlet level（S₁）close to the the high energy excited state triplet level（T_n）,and the much lower first excited state triplet level（T₁）,can obtain better blue emission as well as high exciton utilization rate and low efficiency roll-off through triplet-triplet annihilation process（TTA）up-conversion and hot exciton processes.However,the device efficiency of this kind of material still needs to be improved,and the exciton loss mechanism still needs to be further studied.In this thesis,the evolution behavior of excitons of OLEDs prepared based on anthracene derivatives is studied in depth by exciton dynamics.The main research results are summarized as follows:1.The exciton dynamics of the sensitized TTA process was analyzed in detail,and the nonlinear characteristics of brightness and current density in the sensitized TTA-OLEDs was obtained,with a slope from 2 to 1.Using the anthracene derivative PIAn CN as emitting molecule and electron acceptor,the formed exciplex between m-MTDATA/PIAn CN to sensitize PIAn CN’s triplet state exciton is used to lead to TTA up-conversion luminescence,and then the blue OLEDs with ultra-low turn-on voltage is obtained.The blue emission with a peak value of 460 nm（2.23 e V）can be obtained at a bias voltage less than 2 V.2.The efficient blue OLEDs were prepared by using PIAn CN,3Cz An Bzt,CZPA and ADN anthracene derivatives as pure emitting layer.The exciton dynamics of non-sensitized TTA process and the parameters affecting the device efficiency were analyzed.It was found that the triplet exciton lifetime is the key to affect the efficiency of TTA devices.The quenching of the triplet excitons is inhibited by doping strategy,and the lifetime of the triplet excitons is increased from 53.62μs to 236μs,which makes the efficiency of the doped device increased by 26%compared with that of the non-doped device.3.The exciton dynamics of the anthracene derivative blue materials MADN,PAC,2M-ph-p Cz An Bzt and 2M-p Cz An Bzt were compared.It was found that MADN devices exhibit a pure TTA mechanism,while TTA mechanism and hot exciton mechanism exist simultaneously in PAC,2M-ph-p Cz An Bzt and 2M-p Cz An Bzts devices.The proportion of the excited state energy transferred from high energy T_n to singlet S₁ and low energy triplet T₁was measured by theoretical model and transient electroluminescence measurements.On this basis,the exciton loss from S₁ to T_n is reduced by doping strategy,and the blue OLEDs with efficiency up to 9.23%was fabricated.4.The blue material PL-CN with dianthracene was designed and its luminescence mechanism was explored.The results show that PL-CN has simultaneously TTA and hot exciton processes,which can realize the efficient utilization of excitons.The intramolecular TTA process of PL-CN was studied,and comparing with p Cz An Bzt with monoanthracene,it was found that the introduction of dianthracene allows the TTA process to occur within the intramolecular.The PL-CN doped deep blue OLEDs was prepared and its EQE reached9.33%,the luminescence peak was 452 nm,and the CIE coordinate was（0.150,0.051）at 6 V bias-voltage.