Design,Synthesis And Optoelectronic Properties Of Thermally-Activated Delayed Fluorescence Materials

Being a new generation technology of displaying,organic light-emitting diodes(OLEDs)have been vastly commercialized in industrial field after years of research and development.As a key component of the OLED devices,the emitting materials have a significant impact on the efficiency of OLED deices,and have always been a key issue in the research field.Nowadays,thermally activated delayed fluorescence(TADF)materials have received extensive research attention as one of the alternatives to phosphorescent materials.Due to their small singlet-triplet energy gaps,TADF emitters can utilize the triplet excitons through the reverse intersystem crossing(RISC)process without using noble metals,as a result,the cost for mass production of OLED devices can be largely reduced.Solution-processed OLED devices are more suitable for large scale commercial production because of their convenience and low-cost preparation methods.However,the development of solution-processable TADF emitters lags behind the small molecule TADF emitters.Meanwhile,the development of the TADF emitters with orange-red emission lags behind their blue,green,and yellow counterparts.This dissertation focuses on these two main issues in the development of TADF emitters.By using different design strategies,solution processable TADF emitters and high efficiency orange-red TADF emitters were designed and synthesized.Furthermore,the optoelectronic properties of these materials were investigated.In addition,a newly developed TADF emitter was used in radiation detection to explore the advantages of TADF emitters in other fields.In chapter 1,the development history and current statues of OLED,the device structure of OLED,the working principle of OLED,and the factors that impact the device performance are introduced.Then the characteristics of the three generations of luminescent materials:fluorescence,phosphorescence,and TADF materials are introduced.The requirements of the molecular design and development of solution-processed TADF materials and orange-red TADF emitters are highlighted.Finally,the design ideas and research contents of these two types of TADF emitters in this thesis are described.In chapter 2,by introducing styrene groups with thermal crosslinking ability into the reported high-efficiency TADF emitter with green-yellow emission,two target molecules,DV-Cz DPPXZ and DVC6-Cz DPPXZ were designed and synthesized.Through thermodynamic test,both emitters thermal crosslinked under 200 ~oC.Photophysical tests proved that the introduction of the styrene group has no impact on the emission spectra of these two emitters.Further transient spectra showed that these two emitters own TADF properties after crosslinked.Film cleaning experiments using different solvents proved that both emitters have good solvent resistant properties after thermal crosslinked.Finally,the OLEDs based on DV-C6Cz DPPXZ achieved a lowest turn-on voltage at 5.5 V,a maximum current efficiency of 2.1 cd/A.In chapter 3,we used poly(aryl ether),which has a high triplet energy level as the mainchain to develop the blue TADF polymers.This novel mainchain were constructed with electron-donating carbazole derivate and electron-withdrawing diphenyl sulfone units,which makes this mainchain have better conductivity compared to total non-conjugated mainchains.By using side-chain engineering,blue TADF unit was successfully introduced into this polymeric system,blue TADF polymers,namely P5-P20 were developed.These polymers have favorable molecular weight and thermodynamic stability.Further transient spectra tests proved that these polymers have pronounced TADF properties inherited from the grafting TADF units.Finally,the OLEDs based on P5 achieved a maximum EQE of 14.5%,which is comparable with blue phosphorescence OLEDs.In chapter 4,by delicately molecular design,we directly use the polymer mainchain as the donor units for the TADF emitters.By using ortho linkage,the triazine was connected to the polymer mainchain as the acceptor unit of the TADF emitters.Two series of TADF polymers were successfully developed,namely OCz TRZ-5～OCz TRZ-20 and ODPATRZ-5～ODPATRZ-20.These polymers all have good thermal stability.The emission spectra of OCz TRZ-5～OCz TRZ-20 red-shifted to 520 nm and ODPATRZ-5～ODPATRZ-20 red-shifted to 550 nm.Further transient spectra tested proved that all these polymers have pronounced TADF properties.Finally,the OLED based on OCz TRZ-20 achieved a maximum EQE of 5.1%and the OLED based on ODPATRZ-5 achieved a maximum EQE of 5.4%.These device performances all surpassed the theoretical maximum EQE of traditional fluorescence OLEDs,which proved the TADF characteristics improved the efficiencies in these devices.In chapter 5,based on the result achieved in chapter 4,we use the poly-carbazole mainchain as the donor part of the TADF emitter.The triphenylboron derivate as the TADF acceptor unit was ortho linked to the mainchain.Target polymers OCz B-5～OCz-B-50 were successfully synthesized.These polymers all have good thermal stability according to thermodynamic tests.The emission spectra of these polymers peak around500 nm.Their TADF characteristics were proven by the transient spectra.Furthermore,when OCz B-50 doped into suitable host materials,the delayed component was enhanced and the PLQY was enhanced to 98.5%,which is among the best results of green TADF polymers.Finally,the doped OLED based on OCz B-50 achieved a maximum EQE of 8.6%.In chapter 6,fused polycycle design strategy was used to construct new donors used for orange-red TADF emitter based on a common TADF donor,acridine.By fusing benzothiophene and benzofuran to the acridine frame,new donor BTDMAc and BFDMAc were successfully developed.Through C-N coupling reactions,two new orange-red TADF emitters,namely BTDMAc-NAI and BFDMAc-NAI were synthesized based on these two new donors and 1,8-naphthalimide.According to the photophysical tests,the emission spectra of BTDMAc-NAI and BFDMAc-NAI were modulated by the fused benzothiophene and benzofuran.The TADF characteristics of BTDMAc-NAI and BFDMAc-NAI were proved by the transient spectra.Finally,the OLED device based on BFDMAc-NAI achieved a maximum EQE of over 20%and OLED device based on BTDMAc-NAI achieved a maximum EQE of 9.2%with its emission peaks at 641 nm.In chapter 7,based on the results of chapter 6,we further designed donor for TADF emitters using fused polycycle design strategy.9,9-diphenyl acridine,which is a commonly used donor unit for blue and green TADF emitters were chosen as the frame.A new donor,namely BTDPAc,were successfully developed by fusing electron-donating benzothiphene to the frame.BTDPAc-Ph NAI,an orange-red TADF emitter,was successfully synthesized.Through photophysical tests,BTDPAc-Ph NAI process pronounced TADF characteristics,and achieved near 60%PLQY in doped film with different concentration.These tests prove that BTDPAc,the new donor unit,not only processed stronger electron-donating ability than the frame,but also inherited the merit of being little affected by the concentration quenching from the DPAc frame.Finally,the OLED based on BTDPAc-Ph NAI achieved a maximum EQE near 20%,and retains at a high level at different concentrations.In chapter 8,we applied TADF materials into radiation detection.A new blue TADF emitter was designed and synthesized by elongating the alkyl chains of a reported high efficiency blue TADF emitter.By using this TADF emitter in the organic plastic scintillator,the resulting scintillators can utilize triplet excitons through RISC process,thus increasing the efficiency of the scintillator.This molecule was synthesized through a simple nucleophilic substitution reaction.The emission spectra of this emitter showed no difference compared to the model compound.The plastic scintillators based on this emitter showed high light yields,with the addition of wavelength shifter to reduce the self-absorption,the light yield was further improved to 6948 photons/Me V.In addition,pulse shape discrimination of different particles can be achieved when using the TADF emitters since TADF emitters exhibit second-order exponential fluorescence decays with a fast component from the singlet excitons and a slow component originated from the triplet excitons.The measuredα/γPSD figure of merit is 1.12 at the energy threshold of 100 ke Vee and n/γPSD figure of merit is 1.32 at the threshold of 1000 ke Vee.