Bipolar Phosphorescent Hosts Based On Triphenylamine And 4,5-diazafluorene:Synthesis And Optoelectronic Properties

Electrophosphorescence has been widely studied to improve the efficiency of organic light-emitting diodes in the field of optoelectronics functional materials.The utilization of phosphorescent host materials plays a key role in promoting the commercialization of electrophosphorescent device.Host materials featuring a bipolar structure are able to banlance carriers transportation and extend the hole/electron recombination zone in the emitting layer,thereby improving the device efficiency,providing that the hosts possess high enough triplet energy,hihg thermal and chemical stability.In this thesis,an macrospirocyclic oligomer and three small molecules based on hole-transporting triphenylamine(TPA)and electron-transporting 4,5-diazafluorene(DAF)were synthesized by cleverly employing an electrophilic Friedel-Crafts reaction.The research is summarized as follows:1.A macrospirocyclic bipolar oligomer(TPA-DAF)3 was prepared using an electrophilic self-condensation Friedel-Crafts reaction.1H NMR and 13C NMR,mass spectroscopy were employed for the structural characterizations.Thermal gravity analysis,diffrential scanning calorimetry,UV-vis absorption,photoluminescence spectroscopy,and cyclic voltametry were utilized to investigate their thermal,photophysical and electrochemical properties.(TPA-DAF)3 showed excellent thermal stability,with a decomposition temperature of 485 0C and no glass transition detected.Blue phosphorescent device with the structure ITO/PEDOT:PSS(25 nm)/(TPA-DAF)3:15 wt%FIrpic(35 nm)/TPBI(35nm)/Ca(10 nm)/Ag,using(TPA-DAF)3 as the single host acheived a maximum current efficiency of 10.0 cd/A and a maximum luminance of 6368 cd/m2.At the luminance of 102,406,1086,2096,3262 cd/m2,the current efficiency is 9.6,9.6,9.4,8.7 and 7.4 cd/A,respectively,indicating the low efficiency roll-off of the device.When(TPA-DAF)3 was blended with TAPC(5:5 in weight)to serve as a mixed host,the maximum current efficiency and maximum luminance were increased to 13.5 cd/A and 23357 cd/m2,showing that TAPC can assist(TPA-DAF)3 to balance the the carrier transportation in the emitting layer.2.Three bipolar hosts TPA-DAF,TPA-(DAF)2,and TPA-(DAF)3 based on triphenylamine and 4,5-diazafluorene were synthezied through an electrophilic copolycondensation Friedel-Crafts reaction.The three hosts feature an electron/hole-transporting unit ratio of 1:1,2:1,3:1,respectively.1H NMR,13C NMR,and mass spectroscopy were employed for the structural characterizations.Thermal gravity analysis,diffrential scanning calorimetry,UV-vis absorption and photoluminescence spectroscopy were utilized to investigate their thermal and photophysical properties.The results showed that on increase of DAF unit in molecular structure,the thermal stability of the molecules was improved and the maxima of PL spectra blue shifted.Solution-processed blue phosphorescent devices based on the structure of ITO/PEDOT:PSS(25 nm)/15 wt%FIrpic:Host(30 nm)/TPBI(35 nm)/Ca(10 nm)/Ag(100 nm)were fabricated.The results showed that TPA-DAF is superior in balancing carriers transportation,and TPA-DAF based device ehxibited the best performance among the three,with a maximum current eficiency of 20.0 cd/A.Meanwhile,on increasing the luminance to 208,1706,2729,5586,11331 cd/m2,the current efficiency of the device remained as high as 17.4,20.5,20.6,19.5,and 16.5 cd/A,respectively,signifying a low efficiency roll-off.