Design Synthesis And Performance Of Thermally Activated Delayed Fluorescence Conjugated Polymers

Organic light-emitting diodes(OLEDs)technology has been widely regarded as a new-generation efficient display and lighting technology due to the inimitable advantages of self-illumination,low-energy consumption,fast response,and potential application on ultrathin and flexible display.In the OLEDs devices featuring multilayer structures,the reasonable design and synthesis of emitting layer materials are the crucial factors to gain high-performance luminescent device.With respect to the luminous mechanism,the emitting layer materials employing thermally activated delayed fluorescence(TADF)mechanism can break the limitation of 25%internal quantum efficiency(IQE)for traditional fluorescence materials and achieve 100%IQE value theoretically through reverse intersystem crossing(RISC)process to harvest both of singlet and triplet excitons,resulting in high efficiency OLEDs device.In addition,this method can also avoid the utilization of heavy metal atoms required in phosphorescent materials,and effectively reduce the cost.From the perspective of device fabrication,TADF conjugated polymers served as emitting layer can not only be processed by relatively simple wet process,but they can also provide the channel for the rapid migration and recombination of carriers due to the delocalized conjugate skeletons,thus achieving higher device efficiency and lower efficiency roll off.Therefore,the research focus of this paper is to design and synthesize high-efficiency conjugate polymeric materials with TADF characteristics and to study their electroluminescence properties.In this paper,the small molecules with verified TADF characteristics were modified and then copolymerized with the host units.Thus,a series of TADF conjugated polymeric emitters with good thermal stability and electrochemical stability were obtained.By altering guest and host units and regulating the corresponding proportion,the photophysical properties of TADF polymers including excited state characteristics and luminous color could be tuned effectively.Furthermore,we carefully studied the effects of halogenation on the electrochemical properties,photophysical processes and molecular aggregation structure of TADF molecules and the related conjugated polymers.Finally,the electroluminescent devices were assembled by solution processing fabrication,and the effects of molecular structure,halogen effect and device structure on device performance were further characterized here.The main contents of each chapter are shown as following:1.The Chapter 1 systematically summarizes the development history and current situation of emitting layer materials for OLEDs,depicting the development process from the first generation of fluorescent materials to the second generation of phosphorescent materials,and then to the third generation of TADF materials,and the major breakthroughs proposed in molecule designing and device fabricating to improve device efficiency are also described here.After that,the designing principles and developing process of TADF materials are reviewed in detail,and the structural characteristics of efficient TADF materials and the corresponding photophysical process analysis are highlighted.From the perspective of assembling device,the developing processes of TADF based OLEDs are summarized as wells,and the advantages of different types of devices are also under detailed comparison here.Finally,combining with the research content of this paper,the macromolecular materials integrated with TADF characteristics,especially polymeric materials,are mainly introduced in this part.We also analyze and compare the advantages/disadvantages and current developments of different types of polymeric materials toward the molecular structure and properties of polymers.Combining with the above analysis,the design ideas of this paper are proposed at the end of this chapter,and the main contents are also briefly expounded.2.In the chapter 2,series of TADF conjugated polymers were designed and synthesized by using phenothiazine-dibenzothiophene-S,S-dioxide(DBTO2-PTZ)as TADF unit and carbazole derivatives with high ET value and excellent charge transferring characteristics as host units,respectively.We found that changing proportion of carbazole derivatives can not only manipulate the distribution of molecular orbital and energy splitting value of lowest singlet and triplet energy level(?EST),but it can also improve the composition of excited states.With increasing of the proportion of carbazole derivatives,the photoluminescence quantum yield(PLQY)of solid blended film increases gradually.Finally,the solution processing device using TADF polymers as emitting layer can achieve the maximum external quantum efficiency(EQEmax)of 15.7%,providing a designing idea for studying the efficient TADF conjugated polymers.3.In the chapter 3,TADF warm white conjugated polymers with double emission was designed,named PTDP-5 and PTDP-10,in which the DBTO2-PTZ as TADF guest unit can emit orange-yellow light,while triphenylamine derivatives(TPAs)with strong blue fluorescence emission and high ET value serve as host unit.With the increased component of TPAs,the luminescence color can be regulated from yellow emission for PTDP-10 to the warm white emission of PTDP-5.Furthermore,the exciton quenching can be further inhibited with the increase of the host units,and consequently the device efficiency can be boosted.Finally,the solution-processed doped device based on PTDP-5 can achieve EQEmax of 7.1%with typical warm white light emission of Commission Internationale de l'Eclairage(CIE)coordinate of(0.42,0.33).4.In chapter 4,the typical TADF molecule of 9,9-dimethyl acridine-dibenzophenone(DMAC-BP)was modified with fluorine,chlorine,bromine and iodine halogen atom,respectively,to study the effect of halogen effect on the luminescence performance of TADF molecule.We found that the introduction of different halogen atoms can regulate the dipole moment of TADF molecules,intramolecular charge transfer and other characteristics,so as to effectively improve the radiation decay process.In addition,halogen atoms can also regulate the transient decay process and the efficiency of RISC process,which is of great significance for reducing the accumulation of triplet excitons,inhibiting exciton quenching,and achieving efficient luminescence.Further molecular simulation and transient absorption spectroscopy(TAS)were performed to show that the interaction between the lowest excited singlet(S1)and triplet(T1)is dominated by the hyperfine coupling(HFC).With the increased atomic number of halogen atoms,the intensity of HFC interaction is increased gradually,resulting in the enhancement of exciton conversion efficiency and highly efficient luminescence.5.In the chapter 5,the fluorinated and chlorinated TADF conjugated polymers were synthesized by the Yamamoto polymerization by using DMAC-BP derivates as TADF units and carbazole derivatives as host units,respectively.Halogenation can effectively regulate the transition dipole moment and photophysical properties of the excited states of TADF conjugated polymers.It can also enhance the SOC between the singlet and triplet excited states through the nature conversion of dominant component for triplet excited state,coming from the triplet charge transfer state(3CT)to the triplet locally excited state(3LE),and thus improve the efficiency of RISC process.In addition,with the increase of the molecular dipole moment,the proportion of horizontal dipole orientation((?)//)of TADF doped polymeric films or pure films can be improved.Therefore,the light out-coupling efficiency(?out)and luminescence properties of the device can also be effectively enhanced.Finally,the optimized solution processed OLEDs devices can achieve high device performance,in which the EQEmax value of the chlorinated TADF polymeric emitter can reach 22.0%,while EQEmax value of the fluorinated TADF polymer can reach 20.0%.It is worth mentioning that due to the superior RISC process,the efficiency roll-off of fluorinated TADF polymer can be effectively restricted,and so it can still maintain EQE values of 18.2%at the brightness of 1000 cd/m2,which is the highest value of solution processing devices based on TADF at this brightness.6.In the chapter 6,blue light conjugated polymeric materials with verified TADF characteristics were designed and synthesized by two strategies here.Firstly,TADF small molecule of 2-(9,9-dimethylacridan)-9,9-dimethyl-thioxanthene-S,S-dioxide(DMOTX-DMAC)with deep blue emission were designed and synthesized,and they were adopted as the guest unit to incorporate with the host units of derivatives of carbazole and triphenylamine,respectively,and then the conjugated polymers named PCzDD-50 and PTDD-50 were obtained.These two polymers exhibit emission peaks located around 480 nm along with determinated TADF characteristics.Moreover,PCzDD-50 has more 3LE components than PTDD-50,which is conducive to enhancing the SOC between S1 and T1,realizing the more effective utilization of triplet excitons.Therefore,the solution processed OLEDs device based on PCzDD-50 has 7.3%EQEmax value with the CIE coordinates of(0.23,0.36).Secondly,high-efficiency blue TADF molecule of 10H-spiro(acridine-9,9'-fluorene)-triphenyltriazine with spiral ring structure was used as the guest unit to construct random copolymerization with carbazole derivatives as host units.Then the blue TADF conjugated polymer named PCzSpiro-10 featuring distinct TADF characteristics was obtained using spiral ring to isolate the conjugated backbone and TADF moiety.Through a series of simulation calculations and photophysical analysis,the participation and TADF characteristics of the triplet excitons are determined.More importantly,the introduction of spiral ring structure can interrupt the conjugate effect between TADF unit and main chain,thus realizing blue light emission.