Research On High Performance Blue Phosphorescent Organic Light-emitting Devices And Their Working Mechanisms

In recent years,the organic light emitting devices?OLEDs?have attracted wide attention in academia and business for its applications in flat panel display and solid-state lighting.Introducing phosphorescent emission materials that can utilize both singlet and triplet excitons in OLED is one of the main ways to improve the luminescence efficiency of devices.At present,red and green phosphorescent OLEDs?PhOLEDs?have made great progress in luminous properties,while blue PhOLEDs need to be further studied in efficiency,stability,efficiency roll-off and so on.The preparation of blue PhOLEDs with excellent performances and high stability is a crucial step in the commercialization of OLED full-color display and white lighting.Many researchers continue to conduct in-depth research in many aspects include the development of more efficient and low-coat organic materials,the improvement of the structure of devices,the optimization of device preparation methods,hoping to produce high-performance blue PhOLEDs that can be widely used in display and lighting terminals to meet the requirements of market.The research of this paper is mainly focused on the light-emitting layer?EML?of blue PhOLEDs,including the design of novel EML structure of blue PhOLEDs which could broaden exciton recombination zone,the introduction of excellent new fluorescent host materials in EML,and the change of device preparation methods to improve the performances of blue PhOLEDs.And the carrier dynamics process in EML of different blue PhOLEDs was analyzed and discussed in detail to explore the main reasons for the improved performances.The main contents of the study include:?1?A novel blue PhOLEDs structure that can balance the transport and distribution of electrons and holes in the EML and broaden the exciton recombination zone is proposed to solve the problem that the blue PhOLEDs based on low triplet energy level?T₁?host material 4,4'-N,N'-dicarbazol-biphenyl?CBP?usually have low device efficiency.The efficiency of blue PhOLEDs based on this structure is greatly improved compared with that of device with conventional structure.CBP is a common host material for the preparation of high performance PhOLEDs,but it is rarely used in the preparation of blue PhOLEDs,which is mainly attributed to the fact that the low T₁ of CBP can lead to energy inversion between it and blue phosphorescent emission materials.In order to improve the device efficiency of CBP-based blue PhOLEDs,we inserted pure CBP as carrier adjustment layer?CAL?into the EML which based CBP as host and commonly used blue phosphorescent emission material bis[?4,6-difluoro-phenyl?-pyridinato-N,C^2']?picolinate?iridium?III??FIrpic?as guest,and the resulting blue device can achieve a maximum external quantum efficiency(EQE_max)of 16%,which is 53.8%higher than that of blue PhOLEDs with a conventional EML structure?FIrpic doped CBP?.We draw the conclusion by simulating the transport and distribution of carrier in the EML:after the introduction of CAL in EML,more balanced distribution of electrons and holes in the EML of blue PhOLEDs can be realized,and the exciton recombination zone is wider and the utilization of excitons is improved,which greatly enhances the efficiency of the device based on CBP host.?2?In order to solve the problem that the efficiency roll-off of blue PhOLEDs based on host-guest doping system falls seriously at high luminance,a new host-guest system which can effectively reduce the triplet-polaron quenching?TPQ?process at high brightness is proposed.Firstly,we designed a new fluorescent host material5-?5-9H-carbazol-9-yl?pyridin-2-yl)-8-?9H-carbazol-9-yl?-5H-pyrido[3,2-b]indole?p2PCB2CZ?and studied its thermal chemical,optoelectronic and carrier transport properties in detail.The results show that p2PCB2CZ has good thermal stability,high T1,balanced electron and hole transport characteristics and other excellent properties that should be possessed as the host material.What's more,p2PCB2CZ has a higher HOMO energy level compared with FIrpic,which is main reason why we chose it as the host material of blue PhOLEDs for analysis.Compared with blue PhOLEDs based on charge-capture-type EML?FIrpic doped26DCZppy?,the novel EMLstructure based on p2PCB2CZ as host has great advantages in reducing the efficiency roll-off of blue PhOLEDs at high luminance.We compared and analyzed the energy levels of the host and guest materials,different from 26DCZppy?-6.05 eV?with lower HOMO energy level than FIrpic?-5.9 eV?,p2PCB2CZ has a higher HOMO of-5.57 eV than that of FIrpic,which eliminates the hole trap formed on the guest in blue device.At high luminance,fewer hole traps on the guest can effectively reduce the TPQ process,thus decreasing the the efficiency roll-off of blue PhOLEDs.In order to verify this conclusion,we further investigated the carrier transport and charge capture in EML by analyzing the transient electroluminescence?Transient EL?of the devices,and simulated the triplet-triplet annihilation?TTA?and the TPQ process of different doping systems.?3?In view of the problems of vacuum thermal evaporation technology?VTET?such as the complicated device preparation process,low material utilization rate and unsuitability for large area device preparation,we prepared blue PhOLEDs by solution process?s-PhOLED?with advantages of simple preparation process and high material utilization rate.We have prepared blue s-PhOLED using the host material p2PCB2CZ with excellent bipolar transport performance,and compared with the devices based on the common polymer host material PVK and unipolar small-molecule host material TcTa in terms of EL efficiency and device stability.The results show that the small-molecule hosts have greater potential in preparing high stability s-OLEDs.For the serious efficiency roll-off phenomenon of blue PhOLEDs,we further used the mixed host EML structure?MH-EML?instead of the single host EML structure?SH-EML?o balance the transport and distribution of holes and electrons in EML and better limit the excitons to reduce the accumulation of excitons and carriers in EML.The HOMO of TcTa can provide an energy level gradient that can effectively improve the hole injection,and TcTa is a very excellent material for the hole transport type host.Therefore,we chose TcTa as the hole transport part of the MH-EML in this work to study different MH-EML structures and concluded that the introduction of unipolar electron transport materials such as TPBI and TmPyPB in the MH-EML with 4,4',4''-tris?carbazole-9-yl?-triphenylamine?TcTa?as hole transport part is not conducive to the formation of balanced carrier system.It is mainly because the low HOMO energy level of electron transport materials hinders the effective injection and transport of holes.The introduction of bipolar material2,6-bis?3-?carbazol-9-yl?phenyl?pyridine?26DCZppy?can effectively solve this problem,and the blue PhOLEDs based on TcTa:26DCZppy hybrid host demonstrate higher efficiency,lower turn-on voltage(V_on)and lower efficiency roll-off.