Design,Synthesis And Electroluminescence Properties Of Novel Blue Thermally Activated Delayed Fluorescence Materials

Recently,organic light-emitting diodes(OLEDs)display technologies have been developed rapidly and applied in our daily life.Light-emitting materials are the fundamental part of OLEDs.The third generation of OLEDs based on thermally activated delayed fluorescence(TADF)materials has attracted massive attentions among researchers with the further development of light-emitting materials.Through reverse intersystem crossing(RISC)process,TADF materials can achieve 100% internal quantum efficiency.To date,the green and red TADF materials have achieved satisfactory results,however,the blue,especially the deep blue and pure blue TADF materials still lagged behind with many problems.Carbazole(Cz),owning to its weak electron-donating ability and good hole-transporting ability,has been regarded as an ideal donor for blue light-emitting materials.In this dissertation,a series of purely organic blue light-emitting materials were designed and synthesized based on the carbazole and its derivatives,and their physical properties and electroluminescence(EL)were investigated systematically.The main results are as following:(1)1,3,6,8-tetramethylcarbaole(TMCz)was selected as donor(D)and 1,3,4-oxadiazole(OXD)and 1,3,4-thiadiazole(THD)as acceptors(A)to construct two D-π-A-π-D type blue TADF emitters,namely,TMCz-OXD and TMCz-THD.The reference compound BCz Ox that contains OXD acceptor and Cz donor is only the normal fluorescence emitter,as described in previous literatures.In contrast,TADF was detected for present TMCz based analogues TMCz-OXD and TMCz-THD in their doped films.The corresponding device EL wavelengths were at 450 and 470 nm,and the external quantum efficiencies(EQEs)were 6.5% and 6.4%,respectively,which have surpassed the theoretical maximum efficiency(5%)of conventional fluorescent OLEDs.The above results have demonstrated that the blue emission carbazole based compounds can be transformed from the conventional fluorescence to TADF by altering donor from Cz to TMCz.(2)To drive the blue emission to pure blue or even deep blue region,9,9-dimethyl-9H-thioxanthene 10,10-dioxide(DMTD)was selected as acceptor along with Cz,3,6-diphenylcarbazole(Ph Cz)and TMCz as donors to construct D-A-D and D-A types blue emitters.For the present D-A-D type emitters,no delayed fluorescence was observed in Ph Cz-DMTD,the corresponding device exhibited deep blue emission at 429 nm with device efficiency close to 6.0%.However,TADF was successfully achieved in TMCz-DMTD.The corresponding TADF-OLED exhibited emission at 447 nm with CIE coordinates of(0.157,0.107)and EQE up to 8.7%.For the first time,the TMCz-based TADF-OLED realized EL wavelength below 450 nm and CIEy < 0.11.Compared with D-A-D type compounds,the emission of D-A type analogues has been hypsochromic shifted.In summary,it has been demonstrated that the rational modification on the Cz can be deemed as a feasible strategy to construct deep blue and pure blue light-emitting materials.