Study On The Application Of BN-Ullazine Derivatives In Organic Light-Emitting Device

With the rapid development of science and technology,organic light-emtting devices(OLEDs)show great potential applications in the field of displays and lightings.The synthesis of high performance organic semiconductor materials,the key device fabrication process and packaging technology are particularly important in this field.At present,organic semiconductor materials used in OLEDs normally exhit the emission wavelengths in the visible light range.While there are few types of organic materials with ultraviolet and deep-blue emissive wavelength ranging from 300 nm to 420 nm,and their performances need to be improved.In this thesis,novel BN-Ullazine derivatives with ultraviolet and deep-blue emission bands were synthesized based on main Ullazine skeleton by replacing C=C bond with isoelectronic and isostructural B-N bond and introducing different functional groups.Accordingly,these derivatives were used as emitters to fabricate OLEDs by further studying their photophysical properties.A novel composite hole transport layer was introduced to match the BN-Ullazine derivatives,which effectively improved the device performances.In addition,the highly efficient phosphorescent OLEDs were demonstrated using BN-Ullazine derivatives as the host materials.The main contents of this work are listed as follows:1.Six compounds were obtained by introducing different functional groups to the BN-Ullazine skeleton.The photophysical properties were investigated by absorption spectra,photoluminescence spectra(PL),time-resolved photoluminescence spectra(TRPL)and cyclic voltammetry(C-V)measurements.Through analysis,it is found that these compounds have a wide optical band-gaps.Except compound 1b,other compounds all exhibited ultraviolet and deep-blue emissions.At the same time,the introduction of benzyl group increases the thermal stability,optical band-gaps and fluorescence lifetime of compounds 2a-2c.Based on the density function theory(DFT)calculations and Lippert-Mataga model analysis of the solvent effect,it is found that the dihedral angle in the molecular structure could be changed by introducing benzyl group,which leads to HLCT state formed by equivalent hybridization between locally excited state(LE)and charge transfer state(CT).Moreover,the morphology and carrier mobilities of compouds 2a-2c thin films were analyzed by AFM and space charge limited current(SCLC)theory,respectively.For exploring their electroluminescent(EL)characteristics,the ultraviolet organic light-emitting devices were prepared using BN-Ullazine derivatives as the emitting layers.Among them,device 2c exhibited the best EL performances with a turn-on voltage of 7 V,a maximum irradiance of 0.129m W/cm² and a maximum external quantum efficiency(EQE)of 0.85%,respectively.2.BN-Ullazine derivatives with wide band-gaps would result in higher carrier injection barriers in the device.Therefore,we introduced a composite hole transport layer(HTL)using p-type and wide band-gap polymer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine](PTAA)doped with deep energy levels electron acceptor F4-TCNQ to optimize the device structure.Through the analyis of absorption spectra,PL spectra,XRDs,IR spectra and electron paramagnetic resonance(EPR)measurements,two doping mechanisms of integer charge transfer(ICT)and charge transfer complex(CTC)were observed to coexsist in the PTAA:F4-TCNQ system,which are related to the HJ-aggregation of PTAA.With the increase of F4-TCNQ doping ratio,the proportion of CTC increased gradually.While ICT is affected by the polymer aggregation and showed a trend of increase first and then decrease.The F4-TCNQ doping ratio could be optimized to 15%when ICT reaches saturation.The UV-OLED prepared with optimal composite HTL shows more superior performances compared to the control device.The turn-on voltage was decreased by 3 V,the irradiance was increased by 51.2%and the EQE was also increased by 25.0%.3.Considering the wide band-gaps and high triplet energy levels of BN-Ullazine derivatives,compounds 2a-2c were used as the host of phosphorescent material Ir(MDQ)₂acac.The mixed emitting layers of BN-Ullazine derivatives codoped with Ir(MDQ)₂acac and PTAA were solution-processed and characterized by SEM and AFM tests.The triplet energy levels of BN-Ullazine derivatives are close to the LUMO of the electron transport layer(ETL),which would result in the triplet-triplet annihilation(TTA).Although this process results in a lower device turn-on voltage being smaller than the optical bandgap energy,it is not conducive to improve the radiative exciton recombination.To avoid the TTA process,we inserted a thin BCP layer with different thicknesses between EML and ETL as a hole blocking layer.The results show that the3 nm BCP layer can balanced carrier injection and effectively improve the radiative exciton recombination.A highly efficient red Ph OLED was obtained with a maximum luminance of 10539 cd/cm²,a maximum current efficiency of 29.8 cd/A,and a maximum power efficiency of 15.6 lm/W,respectively.