Low-Temperature Solution-processed NiO_x Intercalation To Construct High-Efficiency Quantum Dot Light Emitting Diodes

Due to their advantages of long life,low cost,wide color gamut and low energy consumption,QLED devices is expected to be widely applied in the field of the intelligent terminal,ultra-high definition display and etc.PEDOT:PSS has many advantages in high hole mobility,good film-forming properties and thermal stability,which is widely used in QLED devices as HIL.However,PEDOT:PSS adversely affects devices’stability due to its hygroscopic and acidic nature,which corrodes the ITO electrode and shortens device’s lifetime.Therefore,the introduction of TMOs with high transmittance and appropriate work function has attracted the attention of researchers,which can improve the stability of devices.Compared with the QLED devices based on PEDOT:PSS,the previous research results showed that the efficiency of the QLED devices based on TMOs is low due to the high injection potential barrier between the light-emitting layers,although the stability of the device constructed with TMOs has been effectively improved.In this dissertation,to address the stability of the anode and the high barrier for hole injection,p-type semiconductor NiOx with deep valence band energy level and wide band gap is selected as the intercalation to construct forward green QLED devices,which can passivate the anode and increase its work function.NiO_x films were prepared by two solution methods at low temperature.A series of adjustments were conducted for the NiO_x film to improve the hole transport ability,such as thickness,UV-O₃ treatment time,and post-annealing temperature.Then the hole injection efficiency can be increased,which will promote the balance of electron-hole injection.Meanwhile,the performance and stability of the device can be improved.The detailed work of this dissertation can be summarized into the following two parts:（1）Construction of QLED with NiO_x by solution combustion methodUsing Ni（NO₃）₂·6H₂O as oxidant and CH₃COCH₂COCH₃ as fuel,NiO_x films were prepared by solution combustion method.By optimizing the NiO_x film under different thickness,UV-O₃treatment time,annealing temperature and other experimental conditions,it shows that the NiO_x films with high transmittance can be formed on ITO substrate.The result of AFM shows that the roughness of NiO_x films under these optimal conditions is 3.55 nm.The prepared NiO_x film is used as intercalation to construct QLED devices.The results show that the device performance is the best when the precursor concentration of the NiO_x layer is 0.5 mol/L with UV-O₃ treatment time of 10 min and the annealing temperature of 130℃.The CE_max and EQE_max of QLED devices are 77.72 cd/A and 17.45%,respectively.When the initial brightness（L₀）is 100 cd/A,the T₅₀（that is,the time required to reduce the initial brightness to half the initial brightness at constant current）operation lifetime of the green QLED devices reaches to 8600 h,which is 1.5 times for the control device（5280 h）.（2）Construction of QLED with NiO_x by sol-gel methodThe NiO_x was synthesized by sol-gel method using Ni（CH₃COO）₂·4H₂O and NH₂CH₂CH₂OH as precursors and ethanol as solvent.Red,green and blue three kinds of QLED devices were constructed by the prepared NiO_x film as intercalation.The results show that the flat and compact NiO_x films can be formed on the surface of ITO electrodes at different annealing temperatures and other optimized conditions.KPFM results show that the work function of the ITO electrode increases from 4.70 eV to 4.88 eV after inserting NiO_x layer,which effectively reduces the hole injection barrier and improves the hole injection efficiency.Compared with the corresponding control devices based on PEDOT:PSS,when the precursor concentration of NiO_x layer is 1:1 with UV-O₃ treatment time of 15 min and the annealing temperature at 150℃,the performance of red,green and blue QLED devices are increased by 31.1%,42.4%and 26.8%,respectively.Finally,the EQE_max of red,green and blue QLED devices reach to 20.73%,23.04%and 12.59%,respectively.At the same time,the T₅₀ operation lifetime of green QLED devices is greatly increased to 28640 h（L₀=100cd/m²）.Finally,the efficiency and operation lifetime of QLED devices based NiO_x are improved at the same time.