Study On WOLED Based On HLCT And Phosphorescent Molecules

Recently,organic light emitting diodes(OLEDs),have gained great attention and are developed rapidly,which have been widely used in the field of flat panel display.White organic light emitting devices(WOLED)with low energy consumption,healthy light color and environmental protection also have great potential in the application of solid-state lighting technology in the future.It is very promising to become the next generation of mainstream solid-state lighting technology.The performance of current WOLEDs,such as efficiency and brightness,have basically reached the practical level,but the power efficiency,color rendering index,color stability,efficiency roll-off and other indicators of solid-state lighting are still relatively low.The main reason is that the properties of blue light materials such as color purity,stability and preparation cost need to be further improved.In addition,the solution-processing technology(also known as wet process)has the characteristics of large area,flexibility,low cost,energy saving and green manufacturing,which is a revolutionary technology development trend of OLED industry and will lead the future direction of OLED production technology.Based on this,the principal line of this thesis is to obtain the high efficiency solution-processed blue materials.The host-type blue materials molecules with different emission mechanisms,including HLCT fluorescent molecules and bipolar phosphors,will be designed and synthesized.The WOLEDs based on fluorescence-phosphorescent hybrid strategy and the phosphor-phosphor type all-phosphor OLEDs are prepared by vacuum evaporation and solution processing technology,which have the advantages of simple structure,high efficiency,high quality and low efficiency roll-off.The research work of this thesis mainly includes the following three aspects(corresponding to three chapters):In Chapter II,a new D-A deep blue Hybridized Local and Charge-Transfer(HLCT)fluorescent molecule 2TPA-PPI is prepared by using phenylphenanthrazole acceptor unit and two diphenylamine donor units.The properties of the novel D-A type HLCT fluorescent molecule 2TPA-PPI are characterized.The high efficiency of the non-doped blue OLED is obtained,and the EQE of 8.14%can be maintained at the luminance of 1000 cd/m².The WOLED with max.EQE of 25.8%and max.luminance of 51280 cd/m² are achieved by doping 2TPA-PPI with two dopant phosphors with green and red light respectively.Because of the better solubility and film-forming properties of 2TPA-PPI in chlorobenzene,the solution processing non-doped blue OLED is prepared.The emission peak of the device is 460 nm,the max.EQE can reach 6.8%,and the max.PE can also reach 5.32 lm/W,which are the outstanding performance of the solution processing blue OLED.In chapter III,two new efficient HLCT blue molecules CPBA and CPTA are prepared by introducing the carbazole and triphenylamine groups respectively into2TPA-PPI.The theoretical calculation shows that the difference between S₁ and T₃ is only 0.04 e V,which has the obvious HLCT feather.In the films based on solution processing process,CPTA has more balanced carrier transporting characteristics than CPBA.The CIE coordinates of the non-doped deep blue OLED based on CPTA is(0.156,0.168),and the max.EQE can reach 7.89%.AFM test results shows that the spin coating film is very smooth with the roughness of 0.340 nm.The solution processing WOLEDs based on CPTA also show stable EL spectrum and high EQE of14.8%.In Chapter IV,employing 2,6-difluoro-4-methyl-2,3-bipyridine as chelating ligand,a novel solution processable phosphorescent host molecule mdfpyhx with rigid four membered ring skeleton is prepared by introducing alkyl carbon chain as solubilizing group into guanidine auxiliary ligand.Theoretical calculation shows that HOMO is mainly distributed on the chelating ligande,while LUMO is mainly distributed on the guanidine auxiliary ligand,which ensures the bipolar carrier transporting ability of mdfpyhx.The OLED using mdfpyhx as non-doped blue emitting layer fabricated by spin coating method shows the PE of 31.4 lm/W with low efficiency roll off.Furthermore,the max.EQE of OLED fabricated by spin coating method based on mdfpyhx doping with btph is 18.8%,which is also the quite high level for the solution-processed deep red OLEDs.