Synthesis And Electroluminescent Property Of Bipolar Host Materials And Thermally Activated Delayed Fluorescence Materials

Organic light-emitting diodes（OLEDs）have been widely used in flat-panel displays,including smart phones and large-screen displays,and have achieved very large economic benefits.Phosphorescent materials and thermally activated delayed fluorescence（TADF）materials can simultaneously use singlet and triplet excitons to emit light,and the theoretical internal quantum efficiency can reach 100%,so have become research hotspots.Both phosphorescent OLEDs（Ph OLEDs）and TADF-OLEDs usually adopt host-guest doping technique to promote carrier transfer balance,reduce exciton quenching and improve device efficiency.Therefore,the host materials and the light emitters play a decisive role to determine the overall performance of OLEDs.In addition,compared with doped devices,the non-doped OLEDs have the advantages of simple manufacturing process,low cost,and good repeatability.Accordingly,this thesis focuses on the development of new bipolar host materials and TADF materials and preparation of high-performance phosphorescent and TADF OLEDs,especially those non-doped TADF-OLEDs.The specific work is as follows:（1）A series of bipolar host materials o-Cz DPSCN,m-Cz DPSCN and m CPDPSCN were designed and synthesized by using cyanophenyl as the electron transport group,diphenyl sulfide as the bridging unit and carbazole as the hole transport group.The triplet energy and charge transporting mobilities were tuned by varying the ortho-and meta-linking style of the functional groups and the molar ratio of n-type cyanophenyl unit to p-type carbazole group.The sky blue and green Ph OLEDs with m-Cz DPSCN as the host material and FIrpic and Ir（ppy）₃ as the dopants exhibited the maximum external quantum efficiencies(EQE_max)of26.7%(maximum current efficiency of 48.6 cd A^-1)and 28.3%(93.3 cd A^-1),respectively.（2）Bipolar host materials,BTPOSCz and BTPODCz,were designed and synthesized by adjusting the n/p ratio of diphenylphosphine oxide and carbazole groups and with benzothiophene（BT）as the bridging group.Compared to BTPOSCz,BTPODCz have better charge balance ability,and its blue and green Ph OLEDs realized the EQE_max of 24.7%(46.4cd A^-1)and 28.1%(95.5 cd A^-1).In addition,the BTPODCz hosted green TADF-OLEDs achieved a EQE_max of 22.1%(70.1 cd A^-1)and low efficiency roll-off.It is verified that in the diphenylphosphine oxide-benzothiophene-carbazole type bipolar host materials,the n/p molar ratio of 1:2 led to the better charge balance and superior emission efficiency.（3）Benzophenone was decorated by cyano group to enhance acceptor（CN-BP）strength,and a series of TADF emitters CN-BP-PXZ,CN-BP-DMAC and CN-BP-BCz were designed using PXZ,DMAC and 3,9-BCz as electron donors.Through tuning the intramolecular charge transfer extent by varying the donors,the light-emitting color of these emitters were adjusted from sky blue via green and orange red.Their doped TADF-OLEDs exhibited efficiencies of 4.5%(9.1 cd A^-1),13.1%(40.6 cd A^-1)and 20.9%(50.4 cd A^-1).（4）“Self-host TADF materials”concept was raised up to develop TADF emitters for non-doped OLEDs application.9-phenyl-9H-3,9’-bicarbazole（PBCz）was used as the host moiety to design novel TADF emitters PBCz-BP-PXZ and PBCz-BP-DMAC with benzoyl acceptor and PXZ and DMAC donors.Intramolecular energy transfer from PBCz to the emitting center BP-PXZ or BP-DMAC were proved through photophysical study,along with the balanced positive and negative charge transportations and high PLQYs in neat films,confirming the self-host nature of these two emitters.The non-doped devices of both materials exhibited excellent efficiencies of 23.2%(72.2 cd A^-1)and 23.0%(57.8 cd A^-1),quite close to 26.7%(80.1 cd A^-1)and 27.5%(69.9 cd A^-1)of their doped OLEDs.In addition,PBCz-BP-DMAC can sensitize green iridium phosphor to produce a EQE_max of 23.6%(81.2cd A^-1).Furthermore,PBCz-BP-DMAC was capable of acting as blue host emitter to fabricate all-TADF white OLED that exhibited a EQE_max of 20.6%(49.9 cd A^-1).As far as we know,PBCz-BP-DMAC is the first reported multi-functional self-host TADF emitter.（5）A new acceptor Py DCN was designed by attaching two benzonitrile on pyridine ring to enlarge LUMO conjugation and reduce acceptor strength in comparison with triazine.Then two novel TADF materials,Py DCNPXZ and Py DCNDMAC,were designed and synthesized,whose emission were both blue shifted relative to their triazine analogues.Py DCNPXZ showed the EQE_max of 26.9%(81.9 cd A^-1)and 18.2%(51.6 cd A^-1)in green doped and non-doped OLED,and Py DCNDMAC exhibited the high efficiencies of 25.6%(45.6 cd A^-1)and6.8%(16.8 cd A^-1)in its sky-blue doped and non-doped devices.In addition,Py DCNDMAC functioned as sky-blue host emitter in single-emitting-layer all-TADF WOLED and showed a EQE_max of 18.5%(44.8 cd A^-1).