Solution Processed Hole Injection Layers For High Efficiency White OLEDs

Organic Light-Emitting Devices(OLEDs)have been widely used in solid-state lighting and flat panel displays currently due to their high brightness,high efficiency,light weight,flexibility and easy preparation of large areas.They show great potential for social and commercial applications.At present,vacuum evaporation is one of the major manufacture processes for preparing high-efficiency OLEDs,but it still exists disadvantages such as complex processes,high manufacturing cost,and difficulty in large-area industrial production.Therefore,the solution processing method is a more ideal manufacturing technology to overcome these limitations easily for OLED fabrication.More importantly,hole-injection materials with excellent electrical conductivity,high optical transparency and suitable work function play an important role in improving the balance of charge injection and transport to reduce the turn-on voltage.Based on the construction of different hole injection layers,this paper discusses the preparation technology of efficient white OLEDs by solution processing method.At the same time,the physical and electrochemical properties of different hole injection materials are systematically explored.The main research contents are summarized as follows:1.Using ammonium molybdate tetrahydrate(AMT)as precursor material to prepare hole injection film after low-temperature thermal annealing and white OLEDs are fabricated to explore the effect of different ultraviolet-ozone(UV-O₃)treatment time on the surface of AMT film.The device performance is optimized with the AMT concentration of 2.5 mg/ml and the UV-O₃ treatment time of 8 minutes and the hole injection capability is significantly improved.Compared with the device without UV-O₃ treatment,the current efficiency of treated white OLED increases from 8.9cd/A to 20.3 cd/A,achieving an enhancement of 128%.2.By doping a certain proportion of AMT in PEDOT:PSS as a mixed hole injection layer,high-efficiency white phosphorescent OLEDs are prepared,in which the hole injection layer is optimized at different doping ratios and different temperatures.Test results show the layer has a smooth film surface and is suitable for direct device fabrication.Also,the work function of the film is increased from 5.12 e V to 5.20 e V,and the hole injection capability is significantly enhanced.When 0.5 wt.%AMT is doped in PEDOT:PSS,and the hole injection layer annealing temperature is 120?,the performance of the device is the best.At this time,the power efficiency has reached 23.4 lm/W which is 40%higher than the PEODT:PSS only device(16.9 lm/W).3.Cuprous thiocyanide(Cu SCN)and nickel oxide(Ni O_x)instead of the most commonly used but more expensive PEDOT:PSS are chosen as the hole-injection material(HIM)to prepare efficient white OLED.However,the luminescence quenching caused by the interface defects of Cu SCN and Ni O_x surfaces limits the performance of the device.Therefore,we inserted an ultra-thin graphene oxide(GO)layer as a passivation layer between the hole-transport layer and the emission layer.The ultra-thin GO passivation layer can not only improve the work function of HIL,but also promote hole injection.Compared with devices without GO layer,the efficiency of Cu SCN/GO devices has increased from 18.1 cd/A(6.6 lm/W)to 30.3 cd/A(19.8 lm/W).Similarly,when Ni O_x is used as the hole injection layer,the power efficiency of the device is increased from 10.1 lm/W to 20.0 lm/W.