The Hybrid-p-doped Organic Light-emitting Diodes

Due to the remarkable developments during the past decades organic light emitting diodes（OLEDs）can be used in high quality flat-panel displays and solid-state lighting.But regardless of the flat-panel displays or solid-state lighting need to be more efficient,stable,and cheap.They are still problems hindering the development of OLEDs.This thesis mainly do the research of the following three aspects:First,the MoO₃ doped 4,4'-[N-（1-naphtyl）-N-phenyl-amino]biphenyl（NPB:MoO₃ in2:1 mass ratio）and MoO₃ doped 4,4'-bis（N-carbazolyl）-2,2'-biphenyl（CBP:MoO₃ in 2:1mass ratio）as p-doped hole transport layers have been used in inverted organic light emitting diodes（IOLEDs）.Compared to the NPB/20 nm NPB:MoO₃ structure,the NPB/10 nm CBP:MoO₃/10 nm NPB:MoO₃ structure showed increased device performance,mostly because the hole transport barrier from CBP:MoO₃ to NPB was smaller than that from NPB:MoO₃ to NPB;it also presented improved device performance than the NPB/20nm CBP:MoO₃ structure,ascribed to the higher conductivity of NPB:MoO₃ than that of CBP:MoO₃.The influence of electrical conductivity and transport barrier to the device was discussed.Secondly,in order to get better doping efficiencies,the IOLEDs have been fabricated using the hybrid-p-doped hole transport layer consisting of MoO₃-doped NPB（NPB:MoO₃）and 2,3,5,6-Tetrafluoro-7,7,8,8,-tetracyano-quinodimethane doped NPB（NPB:F4-TCNQ）.Compared with the IOLED using the 20 nm NPB:MoO₃/Al,the one using the 10 nm NPB:F4-TCNQ/10 nm NPB:MoO₃/Al showed increased performance.Finally,because the material like CPB is suitable to serve as a hole-transporting host to phosphorescent dyes.Hence,the study based on p-doping to enhance hole current into CBP is of much value.F₄-TCNQ doped NPB was used to work together with NPB:MoO₃ and CBP:MoO₃,due to its much higher conductivity.The resultant combination containing three hybrid-p-doped HTLs showed improved device performance than those employing two p-doped HTLs.The underlying mechanism was discussed as well.