Study On The Application Of Imidazole And Triphenylamine Derivatives In Organic Light-Emitting Diode

Exploiting efficient deep-blue emitters are of great importance to promote the practical application of organic light-emitting diodes(OLEDs)in full color displays and solid-state lightings.In this work,the new wide-bandgap imidazole and triphenylamine small molecules have been synthesized,and used as the emitting layer and the hole transport layer in deep-blue OLEDs,respectively.Furthermore,the device performances have been improved by optimizing the device structure.Besides,we designed a novel charge generation layer(CGL)by which the high efficiency tandem deep-blue OLED has been successfully fabricated.The key contents include the following aspects:1.A series of wide-bandgap bis-aryl substituted imidazole derivatives a-e(PIM)were synthesized.They exhibit good thermal stability,high fluorescence quantum yield and high carrier mobilities.Combined with the solution method and the vacuum evaporation method,we fabricated a series of deep-blue OLEDs based on these compounds.The device structure could be further optimized by incorporating an efficient n-doped electron injection layer(TPBi:Liq).The films morphology of TPBi:Liq were tested and characterized by SEM and AFM,and the energy level structure and electrical characteristics were investigated by using ultraviolet photoelectron spectroscopy(UPS)and impedance spectroscopy.The results show that the optimized device performance is better.The turn-on voltage of the optimized device is reduced to 3.6 V,and its maximum brightness is increased to 10836 cd/m~2.It also exhibits a maximum current efficiency of 5.25 cd/A.It has a low efficiency roll-off(19%).2.We synthesized a series of bis-triphenylamine derivatives(TPA)3a-3e with a twisted naphthalene bridge connection,which exhibited high thermal stability and hole mobilities.These new materials are used as the hole transport layers in imidazole-based deep-blue OLEDs.The surface morphology and electrical conductivity of hole transport layers were characterized by SEM,AFM and C-AFM tests.The effects of five triphenylamine derivatives on device performances were investigated by means of single carrier devices,impedance spectroscopy and Kelvin probes.The results reveal that the optimal device with TPA-3e as the hole transport layer achieved the high performance.Compared to the control device with NPB as the hole transport layer,the maximum brightness and maximum current efficiency of optimal device were increased by 45%and 50%,respectively,and its turn-on voltage was reduced by 0.5 V.3.We fabricated high efficiency tandem deep-blue OLED with TPBi:Liq/HAT-CN/TPA-3e as a novel charge generation layer(CGL).The energy level structure,carrier dissociation ability and impedance property of CGL were characterized by UPS,single CGL device and impedance spectroscopy.Compared the CGL using NPB as the hole transport layer,the novel CGL presents more efficient carrier dissociation and transport abilities.The results indicated that the CGL with TPA-3e can obviously improve the current efficiency of tandem device.The maximum current efficiency of optimal device is 8.56 cd/A,which was increased by63%and 35%compared to the single emitting-unit device and the tandem device with CGL containing NPB,respectively.