Novel Magnetic Field Response And Physical Microscopic Mechanisms In Organic Doped Devices

Organic light-emitting diode?OLED?is a new type of flat panel display with wide viewing angle,high brightness,fast response,high efficiency and flexibility.In order to further improve the performance and practicability of the device,the researchers have made many improvements to the earlier OLED devices,which the doped light-emitting layers and new luminescent materials are commonly used.The luminescence of OLED is a very complicated process.There are various spin-pair-states?polarons and excitons,etc.?forming in the luminescent layer,and spin-pair-states inter-conversion process will have a great influence on the luminescence process.If the light-emitting layer is doped,the spin-pair-states inter-conversion processes will be more complicated,and the energy transfer process will also affect the luminescence.The researchers applied external magnetic field to the OLED device that does not contain magnetic materials,but its electroluminescence intensity and conductance change with the magnetic field.Studies have shown that these magneto-electroluminescence and magneto-conductance curves have a"fingerprint"response to the spin-pair-states inter-conversion process of the device.In the doped device,the spin-pair-states inter-conversion processes are so much.The question is also discussed in this paper that how the magnetic response curve of doped system reflects these spin-pair-states inter-conversion processes.In addition,the experiment also used thermally activated delayed fluorescence?TADF?material as a guest dopant to obtain higher quantum efficiency and enrich the research system.TADF material is the third generation of optoelectronic material following traditional fluorescent materials and phosphorescent materials.Because of its small difference between singlet and triplet states(?E_ST<100 meV),the triplet excitons?T₁?can be converted into singlet excitons?S₁?by the reverse intersystem crossing?RISC?process,achieving higher triplet exciton utilization.In this thesis,the microscopic mechanism of the doped OLED is studied by using the organic magnetic response.At the same time,we found some novel magnetic response curves in the doped OLED devices that have never been reported,which also reflects the complex and variable microscopic mechanism in the device.This work includes the four parts:?1?The first chapter describes the history of OLED,the structure of the device,and the carrier transport process in the device and spin-pair-states inter-conversion process in the light-emitting layer.In order to study the microscopic mechanism of OLED,the organic magnetic effect was applied and a novel magnetic effect curve was found.Therefore,the paper also introduces the theoretical basis of organic magnetic effects and some common magnetic effect curves.The energy transfer processes in organic doping system are also a problem that needs attention.In addition,this paper uses the new photoelectric material,TADF material,and its related knowledge is also introduced.?2?The second chapter introduces the preparation technology of OLED,including substrate cleaning,ozone treatment,polymer spin coating and molecular beam epitaxy.At the same time,the measurement equipment and technology of photoelectric performance and magnetic field effect of OLED are introduced.?3?The thermally activated delayed fluorescence?TADF?material 4CzTPN-Ph and the conventional fluorescence dopant DCJTB were used to co-dope into the host material CBP for fabrication of TADF-assisted fluorescent organic light-emitting diodes?OLED?.Some exceptional magnetic field effect?MFE?curves with abundant and tunable four components within a low magnetic field range??50 mT?were obtained,contrary sharply to the maximum of two components observed in typical OLED.These MFE components were easily tuned by the injection current,dopant concentration,working temperature,and dopant energy gap leading to a wide variety of MFE curve line shapes.The experimental results were caused by the spin-pair-state inter-conversions that were occurring in the device,including intersystem crossing?ISC?of CBP polaron pairs,ISC of 4CzTPN-Ph polaron pairs,reverse ISC?RISC?of 4CzTPN-Ph excitons,RISC of DCJTB polaron pairs,DCJTB triplet fusion,and DCJTB triplet-charge annihilation.Moreover,the exciton energy transfer processes among the host material and the guest dopants had pronounced impacts on the formation of these four components.This work gives a deeper understanding of the microscopic mechanisms of TADF-based co-doped system and the further development of organic magnetic field effects for the extensive field of OLED.?4?In the fourth chapter,the luminescence mechanism of NPB and TPBi blended doping devices is studied by using magneto-electroluminescence?MEL?curve.The structural device was reported by Jankus et al.in Adv.Mater.25₁455?2013?.Based on the experimental results of transient spectroscopy,it is believed that there may be a quenching?TF?process of triplet excitons inside the device.In this experiment,OLED with the same structure were fabricated,and the MEL curve was measured and analyzed.It was found that TF process didn't occur inside the device at room temperature,only at low temperature and large current the TF process will exist.A series of operations that are beneficial to the TF process are employed:switching the cathode of the lower work function,ozone O₃ treatment of the anode,increasing the injection current of the device by the order of magnitude,and still not observing TF.The magnetic conductance experimental results and theoretical simulations of the device show that the interaction between triplet excitons and holes may occur inside the system,ie TPI?triplet-polaron interaction?)process.This study has a good reference value for understanding the luminescence mechanism of NPB and TPBi blended devices.