Mediation Of Carrier Trap And Exciton Energy Transfer On Magnetic Field Effects In 4CzTPN-Ph Device

Organic Light Emitting Diodes?OLEDs?has become a hot topic in current research because of their great characteristics,such as light,thin,flexible,low price and simple preparation technology.It has gradually entered into the daily production and life of us and became an emerging technology in the field of lighting and display.Studies have shown that fluorescence-based OLEDs will produce 25%singlet excitons and 75%triplet excitons when excited by electricity.While the 75%triplet excitons cannot emit light due to spin confinement,only the 25%singlet excitons can radiate.Therefore,the internal quantum efficiency of such common fluorescent devices can only reach 25%at most.That is to say,the energy of 75%triplet excitons will be dissipated in some other form.Therefore,more and more scientists are trying to find a way to make full use of singlet excitons and triplet excitons.Studies have shown that phosphorescent OLEDs,who rely on noble metal elements such as iridium and platinum,could theoretically achieve internal quantum efficiency of 100%.Because the strong spin-orbit coupling of noble metal complexes such as iridium and platinum makes both single excitons and triplet excitons in devices fully utilized.However,these noble metal elements are very expensive and could cause environmental pollution,which prevent their further progress.Recently,the thermally activated delayed fluorescence?TADF?material proposed by Adachi's research group has effectively overcome the shortcomings of common fluorescence materials and phosphorescent materials.TADF materials could achieve the quantum efficiency of 100%in the device without noble metals.Due to the special molecular structure of TADF materials,the highest occupied molecular orbital?HOMO?and the lowest unoccupied molecular orbits?LUMO?of the electron cloud overlap is lesser,the electron and hole distance is far.And the spin exchange is weak,the singlet exciton and triplet exciton energy difference(?E_ST)is very small,so the triplet exciton can absorb environment heat through Reverse intersystem crossing?RISC?process convert into a singlet exciton,singlet exciton produce delayed fluorescence.In other words,TADF can realize the utilization of triplet excitons,and the quantum efficiency of TADF device can reach 100%in theory.However,the current researches on TADF-OLEDs are still in the initial exploration stage,and face many problems,such as how the luminescence and current of devices are regulated by the energy level trap?And how do exciton evolution and energy transfer in TADF devices regulate luminescence and current?Therefore,in this work,we had selected the TADF material 4CzTPN-Ph as the study object,and prepared a series of 4CzTPN-Ph doping devices.Then,we had measured their optoelectronic properties and MEL,MC curves.We had explained these curves,which are abnormal and influenced by the triplet energy of each function layer,by analyzing the structure of device,exciton evolution in device,and energy transfer process.The main content of this paper is as follows:In the first chapter,the organic light-emitting diodes?OLEDs?is briefly introduced at first,mainly including its development process,practical application value,structure of device,luminous principle and common microscopic mechanisms.Secondly,The TADF materials are introduced.Then the organic magnetic field effects and some relative magnetic field effect curves are introduced briefly.Finally,the theoretical significance and practical value of the work are described.In the second chapter,the preparation method of the device in this work is firstly introduced,and the vacuum evaporation deposition is emphatically introduced.Then,the measurement methods of the device's photoelectric are introduced,mainly including the I-B-V curves,electroluminescence spectrum and magnetic field effects of devices.In the third chapter,4CzTPN-Ph,which is one of the TADF materials,was selected as the dopant,and Alq3 was selected as the host material to prepare a series of dopant devices with varying doping concentrations.Compared with the reduced magnetic effect of common doped devices,these devices have obvious anomalous magnetic effect--stronger MEL and MC amplitude.Taking the experiment of 150?A at room temperature as an example,the MEL_300mT amplitude of the device,whose light emitting layer is Alq3:5%4CzTPN-Ph,reached about 10%,about 13 times that of the MEL_300mT00mT value?⁰.75%?of the reference device?light emitting layer is CBP:5%4CzTPN-Ph?,and the corresponding MC_300mT amplitude of the device reached about 6%,about 50times that of the reference device MC_300mT value?⁰.12%?.The analysis of the energy level structure of device shows that the Alq3:4CzTPN-Ph device has a special energy level distribution between the host and the guest,causing a weaker carrier energy level trap.When the external magnetic field inhibits the scattering of charge by the triplet exciton?TQI?,stronger MEL and MC amplitudes can be generated,and then cause abnormal magnetic effect,which is different from ordinary doped devices.In addition,because the concentration of triplet and carrier will be affected by doping concentration and temperature,when doping concentration and temperature changed,the ability of carrier trap to capture carriers also changes.So doping concentration and temperature can also effectively control this kind of abnormal magnetic field effects in these doped devices.In the fourth chapter,four 4CzTPN-Ph doped OLEDs were prepared,and materials with different triplet energies as host,hole transport layer and electron transport layer respectively.MEL and MC of these devices at room temperature and changing temperature were measured.Experimental observation showed that the MEL and MC curves of the devices could change when the hole transport layer,host or electron transport layer changed.When the hole transport layer,electron transport layer and doping host of these devices choose materials with different triplet energy levels respectively,their MEL and MC curves show different variation rules in the low magnetic field?|B|<20 mT?.And only when the hole transport layer,electron transport layer and the host are all with high triplet energy,the MEL and MC curves will show RISC process,which suppressed by magnetic field.When the energy of any of the three functional layers is low,the RISC process in the device will be weakened with varying degrees.This is because the hole transport layer,electron transport layer or host materials with different triplet energy have different binding capacity to the 4CzTPN-Ph triplet energy,which could result in different triplet energy transmission channels and energy loss channels of different degrees in each devices.Thus,different MEL and MC curves appear in the low magnetic field range of each device.