Design Of Doped Hole Injection Layers For Highperformance Organic Light-emitting Diodes (OLEDs)

Organic light-emitting diodes(OLEDs)have been widely studied in the past decades.Due to the advantages of low power consumption,ultra-thin structure,wide color gamut,self-luminous,flexibility etc.,OLED seems to be the most suitable technologies for display and lighting applications.However,in the commercialization process,there are still many issues that need to be dealt with.Currently,how to maintain low driving voltage,how to utilize both the triplet and singlet excitons to acquire high device efficiency,and how to improve the optical out-coupling have been studied in-depth.It is generally believed that the most efficient emissive layers for OLEDs are based on the host-guest system where charges are injected from the electrodes through buffer layers and transport layers into the host.In order to maintain low operating voltages in OLEDs,it is crucial to reduce energy barriers for charge carrier injection between the electrodes and the charge transport layers.Energy mismatch at the charge transport layer / electrode interfaces is the main reason which leads to poor charge injection and transport.For the charge carrier injection at the anode side,various hole-injection layers(HILs)have been introduced,which act as a injection buffer to reduce the injection barrier between the work function of the anode and the highest occupied molecular orbital(HOMO)of the HTLs.To further improve the electrical properties of the HILs,doping strategy is generally adopted.Therefore,this thesis mainly focuses on the research and design of new kinds of doped HILs for high-performance devices.Firstly,we introduced a low temperature combustion method to prepare solution-processed transition metal oxides(Ni Ox),instead of the conventional sol-gel method which needs high sintering temperatures of above 450?.The annealing temperature of as low as 200? in our combustion method is totally compatible with the commonly used ITO glass substrates.It is found that solution-processed Ni Ox thin layers incorporated with copper dopants resulted in a significantly improved electrical conductivity and hole-injection capability.UPS results indicate that both the Ni Ox and Cu:Ni Ox have a desired work functions(WF)that favours hole injection.With enhanced hole injection,the phosphorescent green device using 5mol % Cu:Ni Ox as the HIL has a maximum current efficiency and power efficiency of 85.3 cd A-1 and 77.3 lm W-1,respectively.The corresponding power efficiency is increased by 26.9% and 11.3% compared with the device without the HIL and with PEDOT:PSS as the HIL,respectively.Second,we adopted an effective p-type dopant,2,3,5,6-terafluoro-7,7,8,8-teracyanoquinodimethane(F4-TCNQ),combined with the good water solubility of TS-Cu Pc as a solution-processed composite HIL for blue phosphorescent devices.The crystallization of the TS-Cu Pc films is suppressed upon doping with F4-TCNQ,and hence its hole-injection ability can be improved.It is found that the F4-TCNQ doped TS-Cu Pc film facilitates hole injection and transport into the HTL(TCTA),which is attributed to the enhanced work function and hole mobility.The device using TS-Cu Pc:F4-TCNQ(30 vol.%)as the HIL exhibits the highest current efficiency and power efficiency of 45.6 cd A-1 and 46.4 lm W-1,respectively.In particular,the power efficiency is enhanced by 43% as compared to the device using conventional PEDOT:PSS as the HIL.