Construction And Performance Of QLEDs With Alkali Metal Doped MoO₃ As Hole Injection Layer

QLEDs is a new kind of electroluminescent device based on colloidal QDs as luminescent material.It has the advantages of wide spectral coverage,high color purity and low energy consumption,which is expected to be the next generation of mainstream technology for lighting and display.For typical QLEDs,as hole injection material,PEDOT:PSS reduces devices’stability due to its hygroscopic and acidic nature,which corrodes the ITO electrode and damages the adjacent functional layers.Therefore,the development of new hole injection materials meets the needs of national industrial development.Transition metal oxides（V₂O₅,NiO,MoO₃,WO₃,CuO）prepared by solution method have the advantages of high transmittance and high air stability,which provides the possibility for them to be used as hole injection layer.Among these transition metal oxides,molybdenum oxide（MoO₃）,not only has the common advantages of transition metal oxides,but also has the advantages of high work function and non-toxicity.So it can be widely used in QLEDs as hole injection material.The previous results show that the efficiency of the QLEDs is low due to its poor charge transport ability and high hole injection barrier with the light-emitting layer.Therefore,on the basis of improved stability for QLEDs based on transition metal oxides,the improvement of device efficiency is limited.In order to reduce the hole injection barrier and enhance the hole injection ability,the MoO₃ prepared by sol-gel method was selected as the material basis in this dissertation.The alkali metal ions（Li⁺,Na⁺,K⁺）with low ionic radius,low activation energy and high ionic mobility were doped to MoO₃,which will move down the conduction band of MoO₃,reduce the hole injection barrier and increase the mobility of MoO₃film.Specifically,the forward QLEDs were constructed with the alkali doped MoO₃ as the hole injection layer and the green Zn Cd Se S/Zn S QDs as the emitting materials.So it will improve the hole injection efficiency,the lifetime and efficiency of the device can be increased at the same time.The main work of this dissertation is summarized into the following two parts:（1）K-MoO₃/PEDOT:PSS double hole injection layer to construct QLED devicesThe K⁺doped MoO₃（K-MoO₃）films were successfully prepared by sol-gel method,and the green QLEDs was constructed with K-MoO₃ as hole injection layer.The results of UPS and KPFM show that after K⁺doping,the conduction band of K-MoO₃ film moves down and the work function increases.The C-AFM results show that the conductivity in micro-region of MoO₃ film can be improved by K⁺doping,which can improve the hole injection.The corresponding single-hole devices also verify this increasing.The effects of the K-MoO₃ doping concentration,spin coating speed,annealing temperature and UV-O₃treatment time on the performance of the device were optimized.The optimization results show that when the volume ratio of K⁺and MoO₃ precursor solution is 1:10 with UV-O₃ treatment of 10 minutes and annealing temperature at 150℃,the performance of QLEDs is the best.The L_max and EQE_max is achieved to287700 cd/m² and is 23.04%,respectively.With the initial attenuation brightness is 100 cd/m²,the T₅₀operation lifetime of this optimized device reaches 64452 h.Compared with control device,the EQE_max and T₅₀ lifetime of the doped devices are increased by 48.8%and 11 times,respectively.（2）Li（Na）-MoO₃/PEDOT:PSS double hole injection layer to construct QLED devicesWith smaller ion radius and lower activation energy,Li⁺or Na⁺were also doped MoO₃,which were used as hole injection layer to construct red,green and blue QLEDs.UPS results show that the conduction band bottom of Li-MoO₃ and Na-MoO₃ also moves down,and their work function increases to 5.45 eV and5.38 eV,respectively.C-AFM results show that the conductivity of MoO₃ films can also be increased after doping of Li⁺,Na⁺.The effects of doping concentration and rotating speed of Li-MoO₃ and Na-MoO₃ on device performance are preliminarily optimized.The T₅₀operation lifetimes of Li-MoO₃ and Na-MoO₃based devices are 85885 h and 78693 h at the initial brightness is 100 cd/m²,respectively.Compared with the devices based on K-MoO₃ as hole injection layer,the T₅₀ lifetime of devices is further improved.When the emitting material is extended to red and blue QDs,the EQE_max of the QLEDs is increased to 24.49%and 12.28%,respectively.Alkali doped MoO₃ has certain universality when it was used as a hole injection layer to construct high-efficiency QLEDs.