Investigation Of The Carrier Balance And Performance Improvement For Organic Light-emitting Devices

In recent years,organic light-emitting devices?OLEDs?have experienced great progress in solid state lighting and flat panel display.Choosing proper functional materials and designing reasonable structure to optimize the carrier balance are the most effective methods to enhance the current efficiency of OLEDs.In this paper,we explored how to balance the carriers in emitting layer of OLEDs fabricated by the vacuum deposition and solution process respectively.As a result,the performance of OLEDs was enhanced through different routes.Two aspects of the research have been listed as follows:1.On the aspect of vacuum evaporation OLEDs,pentacene with high carrier mobility was chosen as a hole buffer layer and the hole blocking mechanism is explored.The luminous performance of the device was improved because of the balanced carriers in the emitting layer by optimizing the thickness of pentacene layer and the position of pentacene layer in the hole transport layer?HTL?.When the pentacene film inserted in HTL is 25 nm away from the ITO and the thickness of pentacene layer is 5 nm,the device achieves the maximum current efficiency,which is about 162%higher than that of the control device.In addition,we further explored the characteristics of pentacene film.We performed the tests of scanning electron microscope,atomic force microscopy and X-ray diffraction to explore the effect of annealing on pentacene films.We found that the pentacene film develops into high crystallinity and the hole transport ability was promoted greatly at the annealing temperature of 120?.By incorporating with reasonable doping type electron transport layer,the current efficiency of optimal device has been improved effectively.Finally,we designed a novel composite hole buffer layer Al/Mo O₃,which is different from the traditional hole blocking layer.Based on the effective charge separation in composite hole buffer layer,we also improved the current efficiency of OLED significantly without compromise of the driving voltage.2.On the aspect of solution processed OLEDs,the solubility of Alq₃ in different solvents was studied.It is found that Alq₃ has good solubility property in DMF.By doping with PVK in the solution of Alq₃,the spin-coated thin film exhibited a low roughness and smooth surface morphology.Then the OLED based on Alq₃ as an emitting layer was successfully fabricated.In addition,in order to improve the performance of this type of device,we fabricated the blending film of PVK:Alq₃:TCTA:PBD by co-doping hole injecting material?TCTA?and electron injection material?PBD?into the above-mentioned PVK:Alq₃ system with an optimized ratio.The injection of carriers in the emitting layer was effectively balanced so as to confine the exciton recombination region,which is beneficial to the radiative of excitons.Finally the optimized OLED was achieved with the highest luminance of 7166.65 cd/m².