Synthesis And Optoelectronic Properties Of Solution-processed Carbazole-based Thermally Activated Delayed Fluorescent Dendrimers

In recent years,more and more companies,like Samsung,LG,etc.,employ organic light-emitting diodes(OLEDs)in flat-panel display and lighting fields,attributed to some unique characteristics of organic semiconductor,such as accessibility on large area processing,relatively low fabrication cost and excellent optical and electrical properties.With profound study of OLED materials,abundant original high-efficient fluorescent and phosphorescent materials are reported successively,especially thermally activated delayed fluorescent(TADF)materials,due to not only molecules without heavy metal,but also fulfillment of 100%internal quantum efficiency through reverse intersystem-crossing(RISC)process.Hence,TADF materials are regarded as the third-generation emitting materials in OLEDs.Until now,most of OLED featuring TADF are made via vacuum evaporation which has precise operation skill and is not easy for large area processing.On the contrary,solution-processed OLED perfectly compensates the disadvantages of vacuum evaporation.However,compared with researches on TADF emitters via vacuum evaporation,the researches on solution-processed TADF emitters are lagged behind.Therefore,it is important to develop solution-processed TADF emitters,especially non-doped ones,owing to prevention of phase separation in spin-coating.Consequently,this thesis aims to the synthesis and the properties of the non-doped solution-processed TADF materials.In chapter 1,we simply introduce the development and the basic principle of OLEDs,and illustrate some major kinds of emitting mechanisms in OLED.Among the mechanisms,we emphasize the TADF and state its development.Finally,we exhibit our major design thought and content.In chapter 2,two novel TADF carbazole dendrimers,named CDE1 and CDE2,are synthesized based on DMACBP,a reported TADF emitter.Two dendrimers exhibit excellent thermostability,solution processability and obvious TADF feature.Additionally,the non-doped solution-processed OLEDs based on two dendrimers exhibit outstanding performance.The device,CDE1 as emitting layer(EML)and TmPyPB as electron transporting layer(ETL),has the maximum external efficiency(EQE)of 13.8%,which was one of the highest EQE in non-doped solution-processed OLEDs at the time of publication.Furthermore,the reason of the high efficiency is two parallel emissive channels in this system:TADF emission and interfacial exciplex emission.In chapter 3,two TADF dendrimers,named 2CzSO and 3CzSO,are designed and synthesized with green emission based on a reported TADF emitter(lCzSO).Compared with lCzSO,two novel dendrimers exhibit larger twist angle between electron donor and acceptor units,smaller ΔEST,shorter τDF,and larger kRISC.Moreover,the optimized double-layered non-doped solution-processed OLED with 2CzSO reaches a maximum EQE of 10.7%.In chapter 4,we regard a TADF emitter(AcDBTO)with small ΔEST as foundation and synthesize two novel TADF dendrimers,named CzAcDBTO and 2CzAcDBTO.Both of them present obvious TADF feature.Additionally,a series of double-layered non-doped solution-processed OLEDs are fabricated,and the OLEDs exhibit great performance.Among those OLEDs,the device based on 2CzAcDBTO has the best performance:a maximum current efficiency(ηc,max)of 13.9 cd A-1 and a maximum power efficiency(ηP,max)of 5.5 Im W-1.