Improvement Of OLED Luminescence Performance By A New Composite Hole Injection Material/Electronic Transport Material

Organic light-emitting device?OLED?has progressed from laboratory to commercial application through decades of research and development.However,OLED still faces some issues which are needed to be resolved,such as improving the luminescent performances of OLEDs.To fulfill further achievements,it is an effective way to enhance the performances of OLEDs by improving the carrier injection and transport ability.In this paper,we improved the performances of three kinds of OLEDs from the aspects of hole injection and electron transport by means of material selection and structure optimization.The specific research works are listed as follows:1.A novel composite hole injection layer?c-HIL?MoO₃/Al/MoO₃ was fabricated and applied to OLED,which can effectively improve the hole injection ability and lower the driving voltage of the devices.We characterized the c-HIL and analyzed its mechanism through the tests and measurements of AFM,SEM,XPS,transmittance and so on.It is confirmed that the thin layer of Al deposited on MoO₃ would reduce Mo⁶⁺to a lower valence state,thus enhancing the hole injection ability of c-HIL.The theoretical calculation of space-charge-limited current and EIS measurements validate the improvement of the hole injection efficiency of OLEDs.Compared with the control device,the optimized OLED exhibits a 18.2%reduction in turn-on voltage,a 4.3-fold enhancement in brightness at 9 V and a 14.9%increase in current efficiency.2.The composite hole injection layer?c-HIL?PEDOT:PSS doped with MoO₃ was prepared by solution method,and the optimal c-HIL was obtained by adjusting the doping ratio.The surface morphology and optical properties of the c-HIL were characterized though the tests and measurements of AFM,SEM,transmittance and contact angle.The valence state,energy level and charge transfer characteristics of the c-HIL were analyzed by means of XPS,Kelvin probe and EIS.The results show that doping MoO₃ could optimize the film quality and increase the work function of c-HIL,which is beneficial to enhancing the hole injection ability of OLEDs.As a result,the turn-on voltage of the optimized OLED is reduced by about 0.3 V,and the current efficiency is increased from 3.2 cd/A to 4.1 cd/A,compared to those of the control device.3.Tin dioxide?SnO₂?thin film was prepared by spin-coated method and used as an composite electron transport layer?c-ETL?with PEI thin film in the inverted OLED.Through the optimization of device structure and introducing of doped emitting layer,the OLED exhibits an improved performance with a low turn-on voltage.SnO₂ films were characterized by XRD and AFM tests.The defects could be passivated by PEI modified layer,which could avoid the excitons'quenching in emitting layer.In addition,the analysis of steady PL and transient PL decay show that there are exciplex states in the doped emitting layer under electric field,which would enhance the efficiency of excitons'radiation.Compared with the control device,the turn-on voltage of optimal c-ETL OLED has been reduced by about 1V,the maximum brightness of optimal c-ETL device has been increased from 501 cd/m² to 8532 cd/m²,and the current efficiency of optimal c-ETL device has been also increased from 0.15 cd/A to 4.2 cd/A.