| In 1987,Kodak Company reported the invention of an organic electroluminescent device with a sandwich structure.Over the past three decades,organic light emitting diodes(OLEDs)related research areas have made great progress.Organic light-emitting device(OLED)has lots of advantages such aswide angle of view,fast response,self-emiting,low power consumption,high contrast andcapability of resisting bending.Currently,due to the development of OLED technology,the LCD and LED market have been greatly took over by OLED products.Conventional fluorescent materials can only utilize singlet exciton states,which can onlytheoreticallyreach 25%internal quantum efficiency.However,phosphorescent materials can utilize both singlet and triplet exciton states,so they can approach 100%internal quantum efficiency theoretically.Although thermally activated delayed fluorescent materials(TADF)can also reach 100%internal quantum efficiency,yet these materials are far from practical commercial application.So far the matrix and emitter materials for green and red phosphorescent OLEDs have been successfully developed,and are in commercial use today.Nevertheless,limit progress has been made in the development of blue phosphors and the corresponding hosts,despite the importance of blue phosphorescent OLEDs.This will include an in-depth study of highly efficient bipolar blue phosphorescent host materials.In the first part of the work,four novel bipolar hosts,namely 2CzPm,2TCzPm,2BFCzPm and 2BFCzTrz were designed and synthesized with diphenylpyrimidine and diphenyltriazine as electron-transporting units and carbazole derivatives as hole-transporting motifs for the application in blue phosphorescent organic light-emitting diodes(PHOLEDs).These electron-accepting and-donating functional groups were attached to the central phenylene bridge in an ortho-substituted fashion,which led to high triplet energies(2.97-3.00 eV)and wide bandgap(3.43-3.55 eV).The effect of modulation of electron-accepting and donating groups on the photophysical properties,frontier orbital energy levels,charge carrier transport properties and device performance of these four hosts has been investigated.2BFCzPm and 2BFCzTrz featured with large conjugation system exhibited high thermal stability as compared to 2CzPm and 2TCzPm.The FIrpic based blue PHOLEDs hosted by 2BFCzPm exhibited excellent electroluminescence performance with a peak current efficiency of 38.2 cd/A and a maximum external quantum efficiency of 19.0%,which could be ascribed to the enhanced thermal stability,high triplet energy and good bipolar charge transport properties of the host material.In the second part of the work,inspired by the results of the first part of the work,the electron transport groups(pyrimidine and triazine)was replaced by pyridine group.Meantime one of ortho substituents is positioned para to pyridine.Firstly,the electron-withdrawing property of pyridine is weaker than that of pyrimidine and triazine,which can enhance the lowest unoccupied molecular orbital(LUMO)energy level of the material.Secondly,the electron transport group is in the para-position of the hole transport group,which is beneficial to improve the carrier transport properties.Then,we designed and synthesized a series of new bipolar blue phosphorescent host materials(2CzPy,2TCzPy,2BFCzPy,2PxzPy and 2PBCzPy).The FIrpic based blue PHOLEDs hosted by 2CzPy exhibited excellent electroluminescence performance with a maximum external quantum efficiency of 17.8%,a peak power efficiency of 34.0 lm/W and a peak current efficiency of 40.9 cd/A.The follow-up workis now under investigation.In summary,a series of highly efficient bipolar blue phosphorescent host materials were designed and synthesized by adjusting the electron-donating or withdrawing capibility of the functional groups and changing the relative positions of thesehole transport groups to the electron transport group.These work may pave a new way for the design of efficient bipolar blue phosphorescent host materials. |
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