Wide-bandgap Aryloimidazole Derivatives and Their Application in Organic Light-Emitting Diodes

Author:Li Zhi Qiang

Supervisor:wang yue


Degree Year:2019





The blue light-emitting materials have a wide range of applications in organic light-emitting diodes(OLEDs).Benifiting from their wide bandgaps,they can be used not only directly as a luminescent materials,but also as hosts to generate light of other color through energy transfer.However,although the existing blue phosphorescent materials and TADF materials have high device efficiencies,the lifetime of these devices can hardly meet the application requirements;while the traditional fluorescent materials are less efficient.Therefore,developing new high stability,high performance,multifunctional wide-bandgap materials is critical for OLEDs,because this could further improve the performance as well as reduce the cost for OLED products.The aryloimidazole derivatives,which have a wide band gap and tunable photophysical and electrical properties,are a promising class of OLED materials,but still suffer from deficiencies.Firstly,although theoretically some of the aryloimidazole compounds can achieve 100%exciton utilization when used as a luminescent material,the efficiency of their blue devices are still very low.Secondly,when an aryloimidazole material is used as host materials or non-doped luminescent layer,how to obtain high mobility while maintain its high triplet energy level plays an important role in improving device performance.In addition,previous studies have mainly focused on the relationship between the structure and properties of single molecules,but little research has been done on the influence of supramolecular structures,such as intermolecular interactions and stacking methods,on light-emitting materials yet.In this thesis we focus on aryloimidazole derivatives,and explores the influence of molecular structure and molecular aggregation structure on properties of these wide band gap materials.The main contents are as follows:In Chapter 2,we obtained three isomers with wide band gap by adjusting the substitution sites of N-Phenylcarbazole on Phenanthroimidazole.It is found that the formation of multi-channel transport mode,originating from aggregation of donor and acceptor into different regions and intermolecularπ…πinteractions,greatly increases the carrier mobility of PI33PPCz.In contrast,PI22PPCz has a highly distorted molecular structure due to the meta-linkage strategy,forming only intramolecularπ...πinteractions and CH...πinteractions between molecules,which may make PI22PPCz molecules behave like independent carrier traps.Therefore,the carrier mobility of PI22PPCz is significantly lower than the other two materials.Meanwhile,the localized triplet spin density distribution characteristics of these materials also effectively reduce the device efficiency roll-off when they were used as host.Finally,the green phosphorescence OLED(PhOLED)using PI33PPCz as host has a maximum brightness of 131300 cd/m2,maximum external quantum efficiency(EQEmax)of 25.84%,and the EQE is still larger than 20%when the brightness reaches 5000 cd/m2.The red PhOLED based on PI33PPCz as host also has a high brightness(34540 cd/m2)and a very low efficiency roll-off,and its EQEmax is as high as 21.12%,which is nearly 50%higher than previously reported.In Chapter 3,we utilized triphenylamine as donor and fluoro-phenanthrene as acceptor,so that the luminescence of phenamimidazole compound is red-shifted from near-ultraviolet region as shown in Chapter 2 to the deep blue region.The substitution of fluorine atoms makes phenandylimidazole compound form intermolecular hydrogen bonds during aggregation,and their packing is more compact,so that these materials have good carrier transport properties,wherein their hole transport abilities are substantially the same as NPB.At the same time,the introduction of fluorine effectively adjusts the excited state properties of these molecules,so that they can achieve high-efficiency hybridized local-charge transfer state(HLCT)while maintaining a wide band gap.Non-doped OLED devices based on these fluorinated materials have high exciton utilization and are much better than their analogues in terms of luminescent color purity and device efficiency.Among them,the multilayer device prepared by 2FPPIDPA has an exciton utilization ratio of up to 98%,color coordinate of(0.156,0.046),and EQE of 6.49%;the EQE of the double-layer device of employing2FPPIDPA as both the hole transport layer and the light-emitting layer is still as high as6.73%.The EQE of the multilayer device based on 2FPPITPA is up to 8.47%and its color coordinates is(0.152,0.083),which is one of the best"hot exciton"materials with samilar color purity.In Chapter 4,we developed a nonsymmetrically linking strategy,and successfully used this strategy to construct one functional group-based donor-acceptor structure in a molecule.Such nonsymmetrically linked molecules are characterized by high triplet energy levels,narrow half-widths,high fluorescence quantum yields and high exciton utilization.Moreover,the nonsymmetrically linked bis-aryloimidazole molecules possess enhanced carrier mobility and carrier balance due to accumulation of donor and acceptor into different partitions and formation of intermolecularπ...πinteractions.This nonsymmetrically linking strategy make these bis-aryloimidazole molecules achieve nearly optimal levels of all properties,so that both their blue nondoped devices and doped phosphorescent devices achieve good performances.Among them,the color coordinate of the nondoped deep blue light device based on PIBI is(0.159,0.044),and the EQEmax is 5.02%.The green PhOLED based on vDNI as host achieves a power efficiency of more than 100 lm/W and EQEmax up to 24.04%,and the EQEmax of its red PhOLED is also as high as 18.88%.In Chapter 5,based on the results of Chapter 4,we designed and synthesized the nonsymmetric linking molecules PImPI and PIpPI which composed of two phenanthroimidazole group.These two unconventional type donor-acceptor molecules have deep blue emissions with high fluorescence quantum yields and balanced carrier mobility.Thanks to the horizontal orientation of the molecular transition dipoles in the film and their ability to utilize triplet excitons,the EQEmax of non-doped device based on PIpPI is as high as 8.84%,and the color coordinates is(0.15,0.07).The EQEmax of PImPI-based OLED also reaches 6.83%,and the color coordinate is(0.16,0.05).Moreover,these deep blue devices have high brightness and low efficiency roll-offs.Due to the good bipolar transport properties,the doped PhOLEDs prepared with these two materials also have low turn-on voltages(2.7 V),high brightness and low efficiency roll-offs.Their green PhOLEDs both have EQEs of more than 22%,and their red PhOLEDs both have EQEs of more than 16%.These two materials with very simple structure can be used as high-performance deep blue materials and host materials at the same time,which show great application potentials.