| Extensive research on organic light-emitting diodes(OLEDs)have been put forward due to their promising advantages in flat panel display and solid state lighting applications.Compared with the vacuum evaporation method,the OLEDs prepared by solution method exhibit the advantages of reducing the cost of preparation,simplifying the preparation process and improving the material usage,etc.They also show the high value of potential application in large-area luminescent and display devices.The purpose of this paper focuses on fabricating high performance OLEDs by solution method.Three types of composite hole injection layers have been designed and investigated.By optimizing their structures,they are introduced in OLEDs to improve the hole injection ability so as to achieve the low driving voltage and high performance of devices.The main contents of this work include the following aspects:1.A novel composite hole injection layer PTAA/AgNWs/PTAA was successfully fabricated.The surface morphology of the c-HIL film was tested and characterized by SEM and AFM.The energy level and carrier injection mechanism have been studied through the measurements of UPS and impedance spectroscopy.By adjusting the AgNWs’concentration and optimizing the film formation character,the c-HIL was applied to the flexible OLED device(FOLED)to improve its performance.Finally,the optimal device presents a highest luminance of 6051 cd/m~2 at 9 V and a maximum current efficiency of 1.76 cd/A at 55.73 mA/cm~2,which are increased by 45%and 75%compared to that of the control device without c-HIL.Meanwhile,the optimal device presents the low turn-on voltage of 2.7 V at 1 cd/m~2 and the driving voltage of 6.0 V at 1000 cd/m~2,which are decreased by 0.4 V and 1.9 V compared to that of the control device.In addition,through device performance tests and mechanism analysis,we found that an ohmic contact was formed between ITO and c-HIL interface,and the hole injection barrier in device was decreased,which resulted in the low driving voltage and high performance of FOLEDs.2.A new material HA-DHCA-AuNPs of gold nanoparticles(AuNPs)with conjugate 3,4-dihydroxyhydrocinnamic acid(DHCA)as template was synthesized.HA-DHCA-AuNPs was doped into conductive polymer PEDOT:PSS with a certain concentration,then spin-coated onto substrate to form the hole injection layer(HIL).The morphology of the HIL was measured and characterized by SEM and AFM.The surface conductivity of the HIL was tested by C-AFM.It was found that doping HA-DHCA-AuNPs in PEDOT:PSS could increase the conductivity of the HIL film.The energy levels and hole injection ability were tested and analyzed by Kelvin probe and impedance spectroscopy.The optimized HILs are respectively introduced into the solution-processed fluorescent OLED and phosphorescent OLED,which can improve the hole injection ability and reduce the hole injection barrier for the two kinds of devices.As a result,the performance of these devices can be effectively improved.3.A hole injection layer of polymer PTAA doped with molecule F4-TCNQ was prepared by solution method,then it was applied to the phosphorescent OLED to reduce the driving voltage and improve the efficiency.The optimal OLED with PTAA:F4-TCNQ presents a maximum current efficiency of 42 cd/A with a low turn-on voltage of 2.5 V and the driving voltage of 3.8 V at 1000 cd/m~2.The surface roughness of the HIL was tested by AFM,and the conductivity of the HIL were measured by C-AFM and impedance spectroscopy.It could be found that the conductivity of HIL could be increased by doping F4-TCNQ in PTAA.The UPS measurement was used to investigate the energy levels of the HIL.Moreover,the absorption and Raman spectra show that the charge transfer complex can be formed in PTAA:F4-TCNQ HIL,which will be beneficial for the improvement of hole injection ability for OLED. |
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