Regulation Of Magnetic Effects Of Three-Body Doped OLEDs By Energy Transfer And Carrier Traps

In the study of organic light emitting diodes(OLEDs),reverse intersystem crossing(RISC)is a very important physical mechanism and a research hotspot.This is because it can transform a large number of triplet excitons that should be wasted into singlet excitons under the auxiliary action of ambient thermal energy,which makes a great contribution to the high efficiency luminescence of OLEDs.So what are the conditions under which OLEDs can effectively perform RISC processes? The research up to now mainly includes the following two situations:(1)the emitting layer of OLEDs was prepared by using thermally activated delayed fluorescence material.The difference of energy level of this material is very small,which provides favorable conditions for the transformation from triplet exciton to singlet exciton.(2)exciplex-OLEDs.In this device,there is a large energy barrier between the electron transport layer material and the hole transport layer material,leading to a large number of electrons and holes are accumulated at the interface or in the body to form the exciplex with basically equal energy levels in the single and triplet energy level,thus RISC process occurs.In addition,in order to reduce the self-quenching effect of the fluorescence dopant in the research process,it is usually doped with some host materials such as CBP(two-body doping),and the concentration of the dopant is very low.Moreover,on the basis of the two-body doping mentioned above,a guest was added as an auxiliary dopant,such as CBP:Rubrene:DCJTB(three-body doping),to further optimize the luminous performance of OLEDs.With the difference in the number of materials in OLEDs emitting layer,it exists spin pairs such as polarons and excitons of different materials in the luminescence process.Their complex interactions will have a non-negligible influence on the luminescence of OLEDs,which involves the microscopic mechanisms(energy transfer and carrier traps)existing in devices.Organic magnetic effect is a good way to reflect the complex and variable interactions between spin pairs and the regulation of microscopic mechanisms.And considering the practical application of OLEDs,in this thesis we choose exciplex(TCTA:PO-T2T)and fluorescence dopant(DCJTB or 4CzTPN-Ph)to prepare three-body doping device.Through analyzing the organic magnetic effect curves and using Lorentzian and non-Lorentzian functions to fit and quantify,we have gained a deeper understanding of the microscopic mechanisms and spin interactions in OLEDs.The main contents of this dissertation are listed as follows:The first introductory chapter introduces some basic information of OLEDs,including concept,development and device structure.Since we study the electroluminescence of OLEDs,the luminescence mechanisms and exciton formation of OLEDs under electric injection are also introduced.Secondly,considering the characteristics of the materials themselves,we also briefly introduced the fluorescent materials used in this study.Then,the research methods of organic magnetic effect are introduced,as well as the basic and widely used mechanism models(i.e.hyperfine interaction model,triplet-triplet exciton quenching model and exciton-charge interaction model).Finally,our research work is described,which is to analyze the complex and changeable magnetic response curves of the interaction among the spin states of three-body doped OLEDs.The second chapter is the introduction of the experiment,we focused on experimental equipments and the measurement of the basic performance(optical-electrical-magnetic characteristics),as well as the processing of relevant datas.In the third chapter,magnetoelectroluminescence(MEL)curves and microscopic mechanisms of 2,3,5,6-tetrakis(3,6-diphenylcar-bazol-9-yl)-1,4-dicyanobenzene(4CzTPN-Ph)and 2,4,6-Tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine:4,4’,4’’-Tri(9-carbazoyl)tripheny-lamine(PO-T2T:TCTA)co-doped OLEDs were introduced detailed.In order to facilitate the analysis,we also measured and analyzed MEL of 4CzTPN-Ph-doped OLEDs which are doped PO-T2 T and TCTA respectively.It was found that current dependent MEL curves of the PO-T2T:4CzTPN-Ph OLED showed abnormal current dependence(that is,it increased with the increase of current)at room temperature,while that of TCTA:4CzTPN-Ph OLED showed normal current dependence(decreased with the increase of current).However,the concentration dependent MEL curves of TCTA:PO-T2T:4CzTPN-Ph OLED showed the above two situations,that is,the normal current dependence was shown when the doping concentration of 4CzTPN-Ph was low,and the abnormal current dependence was shown when the concentration was high.Therefore,we believe that these three OLEDs have similarities and differences.By analyzing their basic characteristics,we conclude that the energy transfer between the host and the guest exists in the three OLEDs simultaneously,and whether the carrier traps exists or not will play a decisive role in the MEL curve of OLEDs.In the fourth chapter,we also focus on the study of exciplex and fluorescent guest doped OLEDs.In this chapter,the traditional red fluorescent doping material 4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran(DCJTB)is selected as the guest,and use the same research method as in chapter 3 to study the MEL curves and microscopic mechanism of OLEDs.The difference is that we found that there is an obvious RISC process in these OLEDs.But DCJTB is a material with large energy gap,and there is no RISC process.By analyzing the MEL curves of OLEDs at different temperatures and currents,we found that this RISC process was enhanced at high current and low temperature,which is quite different from the RISC process in the general thermal activation delay fluorescence materials.From this we inferred that the RISC is from among DCJTB polaron pairs.In addition,the electron-hole spacing distance changed as the concentration of DCJTB increased,leading to the transition from RISC to ISC.Furthermore,we obtained an external quantum efficiency of up to 10.17%,which indicates that the RISC of polaron pairs has a non-negligible effect on the luminescence of OLEDs.