Synthesis And Properties Of Imidazole-based Host Materials

Organic light-emitting diode（OLED）has attracted much attention due to its low power consumption and simple manufacturing process,showing extraordinary potential in display and lighting applications,and has been widely used in smartphones.However,commercial Ph OLEDs still have some problems such as low efficiency and high efficiency roll-off,which have aroused great interest in researchers.The choice of host materials palys an important role in high-efficient OLED devices.To achieve high-efficient and low roll-off devices,the host materials need to have not only a match of the HOMO/LUMO energy levels with emitters and hole/electron transport materials,but also a high enough triplet energy level,Because imidazole groups have the characteristics of high triplet energy level（E_T）,high thermal stability,and good photoluminescence efficiency,deeper HOMOs can also be used as hole blocking layers Therefore,so this thesis designed and synthesized a series of group phosphorescence host materials based on imidazoles group,and studied their applications in green phosphorescent devices,The specific research contents are as follows:1.Based on the pyridine imidazole group,two kinds of bipolar phosphorescent materials were designed and synthesized with and without a benzene ring between benzocarbazole and dibenzofuran.Both two compounds have high thermal stability and morphological stability through photophysical study,and the energy level of the triplet state Cz DFDp is greater than Cz PDFDp.It is possible that the local conjugation of Cz PDFDp molecule is enhanced after the addition of the benzene ring,and the degeneracy of the molecular orbital is stronger,leading to the red-shift of Cz PDFDp low-temperature fluorescence,so Cz PDFDp has reduced three-wire energy level.Therefore,GD-Ir-based doped devices using Cz DFDp and Cz PDFD as host matrix exhibit a low driving voltage(V_on,1 cd m^-2)of only 2.6 V,the maximum current efficiency of 44.9 and 47.2 cd A^-1,and the maximum power efficiency of 50.4 and 57 lm W^-1.Compared to the device using the conventional host CBP(3.6 V,14.4 cd A^-1,5.8 Im W^-1),these two host materials-based devices show 1 V lower in the driving voltage,the current efficiency is increased by more than 3 times,and the power efficiency is increased by more than 8 times,indicating that these two novel bipolar host materials have good transmission properties and can effectively balance carriers,and are excellent green host materials.2.In order to further study the photophysical properties and electron transport properties of imidazoles,a benzene ring was added to the imidazopyridine group to form an imidazoisoquinoline group,two phosphorescent main molecules were synthesized with or without a benzene ring in the benzocarbazole group.The two compounds have high thermal stability,morphological stability and high triplet energy level through the photophysical study,which is beneficial to the device evaporation film,which is of great significance for the stable operation of the device.In gaussian simulation,HOMO and LUMO of DPDCz molecules are not completely separated,and the LUMO orbital electron cloud is mainly distributed on the groups of dibenzocarbazole and phenylimidaz-oline-isoquinoline,indicating that the electron-giving ability of dibenzocarbazole is weakened after the addition of benzene ring.Among them,DPBCz molecule,through the guest Gd-IR doping to make green phosphors and single-carrier devices,the maximum brightness reached 35686 cd m^-2,EQE_max,CE,_max,and PE_maxreached 13.13%,47.31 cd A^-1and 25.15 lm W^-1,respectively.The single-carrier device proves that the larger turn-on voltage is due to the fact that the hole transport capacity of DPBCz material is greater than the electron transport capacity,and it cannot effectively balance holes and electrons,so that light emission occurs at the interface between the electron transport layer and the light-emitting layer.