The Study On Electrically Doped Organic Light-emitting Diodes

Organic light emitting diode（OLED）has many merits,such as high efficiency,good color purity,wide viewing angle,fast response speed,light weight and thinness,flexibility and so on.There is a broad application prospect in the field of flat-panel displays and lighting.It has attracted high attention from the industry and academia and become a hot topic of the frontier for science and technology.To further reduce the driving voltage of OLEDs and improve the market competitiveness of their products,this thesis develops the structure of electrically doped organic light-emitting diode（p-i-n OLED）and also one n-doped composite material,in attempt to solve the self-heating effect of OLED and push forward the development of OLED display and lighting technology.The research work done in the thesis includes the following aspects:（1）Phosphorescent OLED needs to deal with the problem of low quantum efficiency under high current density.In this thesis,the p-i-n structure is used to reduce the power loss of OLED;the double p-doped layers CBP:MoO₃/NPB:MoO₃ and ternary luminescent layer NPB:CBP:Ir（piq）₂（acac）are designed to enhance hole injection and reduce triplet polaron annihilation.It is found that compared with the NPB:MoO₃,CBP:MoO₃/NPB:MoO₃combination promotes the hole current injected into emissive layer,is mainly due to the hole injection from indium tin oxide（ITO）to CBP:MoO₃ more efficient than that from ITO to NPB:MoO₃.Compared with NPB:Ir（piq）₂（acac）binary emissive layer,NPB:CBP:Ir（piq）₂（acac）ternary emissive layer increases the current efficiency of the device,because the bipolar CBP is able to transport electrons,reducing the number of electrons conducted by Ir（piq）₂（acac）and thereby triplet-polaron annihilation.（2）In order to improve electron injection from cathode to organic layer,an n-type doped composite material BCP:Li F is prepared by means of co-sublimation.It is found that BCP:Li F can form ohmic contact with Li F/Al and has good electron injection capability.Compared with conventional lithium fluoride/aluminum cathode,the BCP:Li F/lithium fluoride/aluminum cathode weakens the electron current injected into Alq3.When the thickness of BCP:Li F is 10 nm,the carriers balance is improved and the current efficiency of devices is improved.The BCP:Li F/Li F/Al composite cathode shows better performance than the traditional Li F/Al cathode.（3）We design a three-p-doped-layer structure of CBP:MoO₃/NPB:MoO₃/CBP:MoO₃in order to improve the hole current of device,where the CBP:MoO3 lying at the left side of NPB:MoO₃ contacts ITO anode.The thickness of this CBP:MoO₃ was adjusted to optimize the performance of the combined three p-doped layers.It is found that compared with single-p-doped-layer structure NPB:MoO₃ and double-p-doped-layer structure NPB:MoO₃/CBP:MoO₃,the three-p-doped-layer structure reduces the injection loss at anode and the barrier loss at organic interface.Especially at high current density,the efficiency based on three p-doped layers is better than those based on single p-doped layer and double p-doped layers.The hole current generated by ITO/CBP:MoO₃ x nm/NPB:MoO₃ 10 nm/CBP:MoO₃ 5 nm（x=5,10,15,20）is compared,indicating that when x=10,the device performance is the best.