Performance Of PhOLEDs Based On Monodisperse Oligomer Hosts And CP-OLEDs Based On Chiral Luminescent Materials

Organic light emitting diodes(OLEDs)are characterized by high brightness,high efficiency,low driving voltage,and flexibility,et al.,which enable their promising applications in solid-state lighting,panel displays,and flexible displays.In order to enhance the efficiency of OLEDs,new emission mechanisms including phosphorescence,thermally acitvated delayed fluorescence,and aggregation induced emission,et al.have been introduced to the design of luminescent materials,and thus boosting the development of new generations of OLEDs.Harvesting the triplet exciton for light emission(phosphorescence)is one of the effective methods to enhance the efficiency of OLEDs.In phosphorescent OLEDs(PhOLEDs),the emitting layers are comprising of host and guest materials,where the phosphorescent emitters acting as dopants(guest)are finely dispersed in the organic matrix(host).This thesis focuses on the design of a series of phosphorescenct host materials based on novel monodisperse oligomers with desirable properties,such as high triplet energy levels,excellent film-forming ability,good morphological stablility,balanced charge transport ability,ease of solution-processability,and excellent compatibility with the dopants.The applicability of the oligomers as host materials for blue,green,red,and white phosphorescent OLEDs will be discussed.In addition,the circularly polarized OLEDs(CP-OLEDs)based on chiral small molecules and polymers will also be studied.The main contents and results are summarized as follows:(1)Electroluminescent properties of an oligomer host material based triphenylamine-pyridineA bipolar oligomer host material TPA-(PyF)3 based on triphenylamine-fluorene-pyridine was designed and synthesized,in which the triphenylamine and pyridine moieties,bridged by the sp3-hybridized C-9's of the fluorene to feature a non-conjugated mode,serve as hole-transporting and electron-transporting units,respectively.TPA-(PyF)3 exhibits a high triplet energy level(2.83 eV),excellent thermal stability,excellent film-forming ability and stability.Solution-processed blue phosphorescent OLEDs based on the following structure were fabricated:ITO/PEDOT:PSS(25 nm)/TPA-(PyF)3:15 wt%FIrpic(35 nm)/TPBI(35 nm)/Ca(10 nm)/Ag(100 nm).The device achieved a turn-on voltage(Von),maximum current efficiency(CEmax)and brightness(Lmax)of 5.6 V,18.5 cd/A,and 13838 cd/m2,respectively,with a low efficiency roll-off lower than 10%at the practical brightness of 100-1000 cd/m2.What's more,green thermally activated delayed fluorescence(TADF)OLEDs based on the structure of ITO/PEDOT:PSS(25 nm)/TPA-(PyF)3:20 wt%4CzIPN(35 nm)/TPBI(35 nm)/Ca(10 nm)/Ag(100 nm)showed a Von,CEmax,and Lmax of 5.3 V,12.7 cd/A and 7705 cd/m2.Based on the same device structure,blue PhOLEDs and green TADF OLEDs using the commercial host of 26DCzPPy showed relatively lower CEmax of 12.7 and 10.2 cd/A,respectively.These results demonstrate that TPA-(PyF)3 can serve as a universal solution-processable host for both phosphorescent and TADF OLEDs.(2)Electrophosphorescent propeties of oligomer hosts based on triphenylamine-diazafluorene and carbazole-fluoreneTwo monodisperse macrospirocyclic oligomers,(T-D)3 and(Cz-F)4,based on triphenylamine/4,5-diazafluorene and carbazole/fluorene were designed and synthesized.Triphenylamine/4,5-diazafluorene and carbazole/fluorene in(T-D)3 and(Cz-F)4 are connected through sp3-hybridized carbon atoms to limit the conjugation of the groups,imparting them with high triplet energy levels(2.88 and 2.82 eV,respectively),excellent thermal stability,and desirable solution processability.Blue phosphorescent OLEDs based on(T-D)3 and(Cz-F)4 hosts were fabricated with the basic structure of ITO/PEDOT:PSS(25 nm)/emitting layer(35 nm)/TPBI(35 nm)/Ca(10 nm)/Ag(100 nm).PhOLEDs based on(T-D)3 exhibited a Von,CEmax,and Lmax of 3.0 V,17.7 cd/A,and 28368 cd/m2,respectively,and those values for the(Cz-F)4 based device were 3.2 V,18.8 cd/A,and 11243 cd/m2.These results indicate that(T-D)3 and(Cz-F)4 are highly efficient solution-processable host materials for blue PhOLEDs.(3)Electroluminescent properties of an oligomer based on triphenylamine-diphenylphosphine oxideA monodisperse macrospirocyclic oligomer(TPA-PO)3 based on triphenylamine and diphenylphosphine oxide was utilized as the host material for fabricating PhOLEDs,and their electroluminescent properties were investigated.In(TPA-PO)3,the triphenylamine units are connected by the sp3-hybridized carbon atom of fluorene to form a cyclic molecular skeleton,and the diphenylphosphine oxide units are appended to the triphenylamine units,affording(TPA-PO)3 a highly steric structure with a hollow in the center and excellent doping compatibility with dopants.PhOLEDs based on the structure of ITO/PEDOT:PSS(25 nm)/emitting layer(35 nm)/TPBI(35 nm)/Ca(10 nm)/Ag(100 nm)were fabricated.Blue PhOLEDs based on FIrpic achieved a CEmax and Lmax of 21.7 cd/A and 22475 cd/m2,respectively.The values are 21.9 cd/A and 67759 cd/m2 for green device based on Ir(ppy)2acac,6.4 cd/A and 8046 cd/m2 for red device based on Ir(piq)2acac.Following these results,highly efficient four-component white PhOLEDs with single emitting layer based on FIrpic,Ir(ppy)2acac,Ir(bt)2acac,Ir(piq)2acac emitters were fabricated,and the CEmax were 18.9 cd/A.The CE can be maintained at 17.3 cd/A at the brightness of 1000 cd/m2,signifying a low efficiency roll-off.In addition,the device featured a warm white emission with CCT lower than 3700 K and an excellent CRI of 91.These results demonstrate that(TPA-PO)3 is a highly efficient solution-processable host for phosphorescent OLEDs.(4)Circularly polarized electroluminescence of organic chiral small molecules and polymersTwo types of circularly polarized luminescence materials,including the small-molecule chiral Zn(II)complexes R/S-ZnL based on 1,2-diaminocyclohexane ligand and the AIE-active chiral conjugated polymers R/S-P1 based on the BINOL and arylethene groups were utilized as emitters for the fabrication of circularly polarized organic light emitting diodes(CP-OLEDs)to investigate the consistency of their fluorescence spectra and CPEL signals.The results showed that CP-OLEDs based on R/S-ZnL exhibited orange emission with a main peak at 576 nm originating from the excimer.The R/S-ZnL complexes showed mirror-image CPEL and gem signals,and the Von,CEmax,Lmax,and gem are 5.6 V,0.181-0.191 cd/A,860-1000 cd/m2,and-0.05/+0.05,respectively,for their CP-OLEDs.However,the gem values at the emission maxima were small.The CP-OLEDs using R/S-P1 as the emitters also showed mirror-image CPEL and gem signals,and the devices showed the intrinsic emission of polymers.Among them,the device based on S-P1 showed a Von,CEmax,and Lmax of 4.7 V,0.557 cd/A,and 340 cd/m2,respectively.The maximum gem achieved was +0.035,and the gem value was+0.021 at the maximum emission wavelength(516 nm).The results demonstrate that AIE-active chiral conjugated polymers show high consistency in their fluorescence spectra,CPEL signals,and gem,which promises their potential applications in high-performance CP-OLEDs.