Copper Oxide(CuO_x) As Hole Injection Layer For Quantum Dot Light-emitting Diodes

Quantum dot-based light emitting diodes have extensively been investigated over the past twodecades in order to utilize high color purity,high photoluminescence quantum yield and photophysical stability of quantum dots.In a typical QD-LEDs with a standard structure,PEDOT:PSS is the most widely used as a hole injection layer,because of its high transparency,high work-unction and high conductivity.However,the PEDOT:PSS layer tends to etch the ITO due to its acidic and hygroscopic nature,which is the main factor for the long-term stability and the performance of the devices.In order to solve this problem,one can simply introduce other interfacial materials to improve the performance of the devices.Transition metal-oxides?TMOs?such as tungsten oxide?WO₃?,molybdenum oxide?MoO₃?,nickel oxide?NiO?,copper oxide?CuO_x?,and vanadium oxide?V₂O₅?have been proposed as promising alternatives to PEDOT:PSS,because of their good stability and excellent carrier mobility.However,these TMOs are usually achieved by high-cost thermal evaporation under vacuum.Among these TMOs,copper oxide as a typical p-type semiconductor,has certain advantages in the aspect of improving hole injection.And it has made successful application in the field of solar cells.A variety of methods for the preparation of CuO_x have been developed,including vacuum deposition and chemical methods.In particular,sol-gel processed was chosen to synthesize Cu O_x due to its low cost and simplicity.In this work,we demonstrated a simple solution-processed ultrathin CuO_x film as hole injectionlayer in the QD-LEDs.The different structures based on CuO_x device were fabricated to investigate its hole injection ability.Meanwhile,The CuO_x/PEDOT:PSS layer is designed to balance carrier concentration and reduce corrosion of ITO.Herein,the work in this thesis can be summarized as follows:?1?We prepared the Cu O_x with sol-gel process.In the process of preparation,we used cupric acetylacetone as the precursor material and chlorobenzene as solvent.From the AFM images,they were observed that the roughness of the CuO_x was relatively small with different concentrationsof precursor solution.That was to say,the surface of the ITO electrode could effectively smoothed with CuO_x.At the same time,the film had very high transmittance.What's more,it was mixed with Cu⁺and Cu²⁺of the CuO_x film,which could ensure CuO_x to be a p-type semiconductor.?2?We fabricated QD-LEDs with Cu O_x as a hole injection layer.And the structure of the device was ITO/CuO_x/TFB/QDs/ZnO/Al.By optimizing the concentration of the precursors,the annealed temperatures,and the time of UV-O₃,We got the different chemical composition,the proper proportion and valence band location.Finally,the performance of the CuO_x-based device was the maximum luminance of27860 cd/m²,the maximum current efficiency of 11.34 cd/A,the maximum power efficiency of 5.74 lm/W.As a result,the performance of the device was improved.In the meantime,we found that the CuO_x could affect the energy level of TFB,so that it reduced the hole injection barrier.In other words,it was more convenient for hole injection,making the device performance improvement.?3?We introduced CuO_x/PEDOT:PSS as an effective hole injection layer for QD-LEDs.By optimizing some factors of the device,we got the best performance of the device.And we investigated the mechanism of this improvement.On one hand,using the structure of Cu O_x/PEDOT:PSS had a superior morphological uniformity,which could fill the defects to make the surface more smooth.On the other hand,because the valance band of CuO_x was between the ITO and PEDOT:PSS,it could make the energy level more match.After optimizing,the optimizing device presented a higher luminance of 60200 cd/m²,which increased about 24%compared with the 45810 cd/m² of the standand device.The current efficiency and power efficiency respectively of the optimizing device increased to 33.03 cd/A and 24.70 lm/W.And the increasing rate was 32%and 39%respectively.At the same time,the EQE had obvious improvement.The largest EQE reached to 7.80%.The lifetime of these two different structure devices were also tested.As could be seen in the result,a better device stability was achieved for CuO_x/PEDOT:PSS-based device when compared with PEDOT:PSS-based one.