Preparation And Investigation Of White Organic Light Emitting Diodes Based On Fluorescent Materials

White Organic light-emitting diodes(WOLEDs)offer a wide variety of application prospects in the full-color display and solid-state lighting fields attributed to their unique merits of surface light source,low heat dissipation and flexibility compared with inorganic LED.Although phosphorescent OLEDs can effectively utilize all the excitons of triplets and singlets state to achieve high electroluminescence(EL)efficiency,the poor stability of phosphorescent materials and the spin-coupling effect of heavy-atoms inevitably reduce device lifetime and increase the complexity and cost of manufacturing.In order to prepare energy-saving and green light sources,we designed and optimized device structures based on conventional fluorescent materials and thermally activated delayed fluorescence(TADF)materials to enable the production of high quailty all-fluorescent WOLEDs by effectively improving luminous efficiency while reducing costs.In this paper,we mainly investigated the EL mechanisms,the optimization of fabrication technologies and the design of device structure.(1)The luminous efficiency and stability of blue-emitting materials have been the bottleneck for commercial development.Thus,bis(2-(2-hydroxyphenyl)pyridine)beryllium(Bepp2)and 9,10-bis[4-(6-methylbenzothiazol-2-yl)phenyl]anthracene(DBz A)were selected as guest materials and the matched host materials were chosen to optimize the EL performance of blue fluorescent OLEDs due to their energy levels and emission properties,respectively.Finally,we chose DBz A as the blue emitter and 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidn-4-yl-vinyl)-4H pyran(DCJTB)and 10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-[1]benzopyrano[6,7,8-ij]quinolizin-11-one(C545T)as the red and green emitter,respectively.And a series of WOLEDs based on traditional fluorescent materials with excellent EL performance have been prepared.The single light-emitting layer(EML)WOLED achieves a color temperature range from 2700 to 12104 K and excellence spectral stability of double-EMLs and triple-EMLs WOLEDs achieve a high color rendering index(CRI)of 85 and 88,respectively.(2)Thermally activated delayed fluorescence(TADF)sensitization of conventional fluorescence is a promising strategy to improve the EL performance of OLED.Here,the EL performance of blue OLEDs based on TADF materials10,10’-(4,4’-Sulfonylbis(4,1-phenylene))bis(9,9-dimethyl-9,10-dihydroacridine)(DMAC-DPS)and 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-spiro[acridine-9,9’-fluorene](Spiro Ac-TRZ)were optimized,respectively.All-fluorescent WOLEDs based on N-(4-tert-butylphenyl)-1,8-naphthyridine-9,9-dimethyl-9,10-dihydroacridine(NAI-DMAC)and DCJTB emitter were prepared by selecting Spiro Ac-TRZ as the sensitizer.Ultimately,the hyperfluorescent WOLEDs exhibited lower efficiency roll-off and longer lifetime up to 10 hours.(3)The combination of TADF materials and conventional fluorescent materials exhibited excellent EL performance,but the CRI of dual-band WOLEDs still needs to be improved.Therefore,we discuss a novel design of tandem three-color WOLED constructed with a blue-green unit based on TADF emitter2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile(4Cz IPN)and conventional fluorescent emitter DBz A connected with an orange unit based on TADF emitter N-(4-Tert-butylphenyl)-1,8-naphthalimide-9,9-diphenyl-9,10-dihydroacridine(NAI-DPAC).To further improve the special rendering index R9 to enhance the ability to restore the saturated red,the deep-red TADF emitter 7,10-Bis(4-(diphenylamino)phenyl)-2,3-dicyanopyrazino-phenanthrene(TPA-DCPP)unit were inserted to prepare a three-unit all-fluorescent tandem WOLEDs.Since tandem devices are slightly affected by the direct energy transfer from the EML enables the spectrum of white devices more stable,which can effectively retain the characteristics of each light-emitting unit and improve EL efficiency and lifetime.The optimal three-unit tandem WOLED obtains maximum brightness,current efficiency and power efficiency of 15411 cd/m~2,101.2 cd/A and 46.7 lm/W,respectively,with a maximum CRI of up to 95.