Preparation And Electroluminescent Properties Of Blue Thermally Activated Delayed Fluorescent Materials Based On Carbazole Derivatives

Organic light-emitting materials are the cornerstone of OLED?organic light-emitting diodes?technology.At present,the shortage of high-efficiency blue materials has become a bottleneck restricting the development of OLED industry.Blue phosphorescence materials can not be widely used in industry due to factors such as device stability and material cost.Blue TADF?thermally activated delayed fluorescence?materials have become one of the potential substitutes for blue phosphorescence materials because of their high device efficiency and wider sources of materials.After nearly ten years'development,blue TADF materials still face the problems of single material system,unclear relationship between structure and properties,and serious device efficiency roll-off.In this thesis,we intend to design and synthesize high-efficiency blue TADF materials and explore its application in OLED devices from the following aspects:?1?expanding the electron-donating groups available for blue TADF molecules;?2?exploring the relationship between molecular configuration and the performance of TADF materials;?3?studying the mechanism of efficiency roll-off of TADF devices.The details are as follows:?1?By designing and synthesizing two isomeric TADF materials with different stereo configurations,the effects of stereo configurations on the properties of TADF are investigated.Both compounds possess small energy gap between singlet and triplet(?E_ST)about 0.11-0.13 eV and relatively high quantum yield?0.71-0.78?.IndCzpTr-1 with large spatial steric hindrance can reduce exciton quenching,and eventually achieves a maximum external quantum efficiency(EQE_max)of 22.1%in sky-blue doped device and10.2%green non-doped device,which indicates that IndCzpTr-1 is not only an excellent emitter but also a good host material.The planar configuration of IndCzpTr-2 shows a high portion of horizontal molecular orientation,which increases the optical coupling efficiency and endows the green TADF doped device an EQE_max of 30.0%.?2?A new 3,4-position substituted fluorene-carbazole hybrid donor group is firstly designed and synthesized.Four TADF materials are prepared by changing the bridging mode with 4,6-diphenyltriazine.The effects of different bridging modes and bridging groups on the properties of TADF are studied.These compounds all show good thermal stability because of the rigid structure.Among them,the adjacent-connected molecule SFI34oTz has the smallest?E_ST and PLQY because of its large twist angle.Photophysical measurements show that SFI34oTz produces both locally excited?LE?state and charge transfer?CT?state emissions in solution and thin film,and the transition from ¹LE to ¹CT configuration could occur in high polarity host or high energy excitation.The doped device of SFI34oTz achieves a EQE_max of 22.4%with green emission.The para-connected molecule SFI34pTz possesses relatively small?E_ST and high PLQY simultaneously,and the doped device reaches a EQE_max of 25.3%with sky-blue emission.?3?Two new donor groups are formed by the substitution of spirodifluorene at 1,2and 2,3 positions of carbazole.The effects of different substitution positions of carbazole on the performance of TADF are compared with that of 3,4-position fluorene-carbazole in the previous chapter.The photophysical properties show that the order of electron-donating ability is 3,4-position substitution>2,3-position substitution>1,2-position substitution,and the strength of TADF is SFI34pTz>SFI23pTz>SFI12pTz.The doped device based on SFI12pTz achieves deep blue emission with EQE_max of 8.44%and the color coordinate CIE_x,y,y of?0.15,0.11?.The doped device based on SFI23pTz exhibits pure blue emission with a maximum EQE of 17.15%with CIE_x,y,y of?0.15,0.18?.?4?The energy levels of excited states and TADF properties are regulated by changing substituents and substitution positions on fluorene-carbazole hybrid.The rigid molecular structures endow them with high PLQY.Although temperature-dependent time-resolved spectroscopic measurements show that these compounds exhibit a relatively small contribution of TADF under photo excited,the vibration coupling between the lowest triplet ³LE and ³CT improves the reverse system transition from ³LE to ¹CT.The EQE_max of the doped device basing on InCz34DPhTz reaches 25.9%with the CIE_x,y of?0.15,0.24?.The mechanism of the device roll-off efficiency is mainly due to the TTA quenching effect caused by the long triplet exciton lifetime.?5?Three deep blue TADF materials are designed and synthesized by connecting SFI23,SFI34 and 3CzPh groups with different steric hindrance to asymmetric pyrimidine.The relationship between the properties and structure of the three molecules in the device as guest and host materials was investigated.The doped device based on SFI34pPM achieves a EQE_max of 8.22%with CIE_x,y of?0.15,0.09?,and the EQE_max of non-doped device is 4.59%with CIE_x,y of?0.15,0.10?.Besides,these compounds working as the host of yellow TADF molecule achieve higher device efficiencies and lower efficiency roll-off than that of traditional host material CBP.