The Synthesis And Property Investigation Of Thermally Activated Delayed Fluorescence Materials

In resent years,thermally activated delayed fluorescence(TADF)materials ha ve become a research focus in the field of organic light-emitting diodes(OLED).TADF materials can achieve a theoretical internal quantum efficiency of as high as 100% with pure-organic molecular structures.Compared with organic fluorescent and phosphorescent materials,TADF materials demonstrate compresensive advantages including high efficiency,low cost,and environmental friendliness,etc,and they are regarded as the third generation of O LED materials.The major focus of this thesis is the design,synthesis and photoelectric property study of TADF materials.In C hapter 2,we conceived and synthesized three OLED materials based on nitrogencontaining five-membered aromatic rings.We introduce 1,2,3-triazole-based acceptors in the research of TADF materials for the first time,and optimized the synthetic conditions of 4,5-bis(4-bromophenyl)-2-phenyl-2H-1,2,3-triazole,enhancing the yield of the precursor,and obtained two 1,2,3-triazole-based materials TAZDCz and TAZDPXZ,between which TAZDPXZ is proved to be a TADF material.Devices with TAZDPXZ as the emitter reached a maximal external quantum efficiency(EQE)of 9.48%,which is by far the best performance of small molecule O LED emitters with triazole-based acceptors.We also linked an oxadiazole core with two dimethylacridines,and acquired a blue-emitting TADF material as expected.EQE of the divice with this material as the emitter reaches 21.6%.In Chapter 3,we linked carbazole with two carbazoles or two diphenylamines,obtaining two branch-shaped donor units respectively,and connected the donor units with benzophenone,acquiring two D-A typed TADF materials,CCDC and CCDD.Such branchshaped donors where carbazoles or diphenylamines are connected can realize ideal HOMO/LUMO separation with moderately twisted molecular configurations.In this way,a low ? is promised without sacrificing oscillator strength.A large donor unit also enhances the delocalization of HOMO,boosting photophysical properties.Devices with these two TADF materials as emitters display outstanding performance.A maximal EQE of 15.9% is achieved in blue-emitting devices with CCDC as the emitter,which is the highest value among all benzophenone-based blue emitters.Besides,green-emitting devices where CCDD is utilized as the emitter are well-performed concerning their overall properties.Not only do their maximal EQE exceed 22%,but also an on-set voltage as low as 2.6 V is achieved with a 30% doping concentration.Moreover,efficiency roll-off of the CCDD-based devices is wellcontrolled.An EQ E of nearly 20% is maintained at the luminance of 1000 cd m-2.The outstanding performance of CCDC and CCDD can be reflected from their short delayed lifetimes,which are 12.6 μs and 10.2 μs on doped films of CCDC and CCDD,respectively,which indicate fast reversed intersystem crossing(RISC)procedures.Also,we found that CCDC and CCDD demonstrate converse phenomena in THF/H2 O mixed solvent despite their structural similarity.The photoluminance of CCDC drops drastically as the water proportion goes up,which is caused by aggregation caused quenching(ACQ),while photoluminance of CCDD increases with rising water ratio,displaying traits of aggregation caused quenching induced emission(AIE).However,whether in PLQY tests or in devices,CCDD shows better performance in doped films compared with non-doped ones.Thus,we infer that the dampened luminance of CCDD in good solvents may attribute to the vibration and rotation of phenyl rings on the diphenylamine donors.