Construction And Its Electroluminescence Properties Of QLED Based On Ni_1-xO Synthesized By A Low-temperature Solution Method

Quantum dots light-emitting diodes show many advantages compared with organic light-emitting diodes?OLEDs?,such as high emitting intensity,high fluorescence efficiency,tunable emitting spectrum,pure color,long-life of luminescence and so on.In addition,the discrete energy band of quantum dots is benifcal to the combination of the excitons.In view of the great advantages mentioned above,QLED will be the promising candidate for the next generation of flat panel display and solid-state lighting device.However,the imbalance of the carrier transportation is the most important factor to limit the efficiency of QLED.For the effective mass of hole is larger than that of electron,leading to the lower mobility of hole,in addition,the injection barrier of hole is larger than that of electron as well,causing the imbalance of the carrier transportation.In order to balance the carrier transportation and to decrease the barrier of hole injection,PEDOT:PSS,as an organic hole injection layer,has been widely applied in QLED.However,PEDOT:PSS based QLEDs obtain relative short-life and bad stability,due to the hygrobic property,acidic corrosion,bad thermal stability of PEDOT:PSS.Further,the work function of PEDOT:PSS?5.2 eV?is far lower than that of the valence band of QDs?6.5 eV?,leading to the increase hole injection barrier and non-radiation transfer,such as over charging of QDs,exciton quenching and Auger recombination.Transition metal oxides?TMOs?,such as Ni_1-x-x O,MoO₃,WO₃,CuO and V₂O₅ have caused great concerns dur to their high work functions,good carrier transportation capability as well as excellent stabiltity.TMOs were initially applied in organic photovoltaics?OPVs?and organic light emitting diodes?QLEDs?due to its ability to decrease the hole-injection barrier at anode/organic interfaces.Among various kinds of TMOs mentioned above,particularly,only Ni_1-xO is an intrinsic p-type wide bandgap transfer semiconductor metal oxide with a high ionizaition,while the others are n-type materials and can't serve as hole selective and electron blodking materials.The p-type conductivity of Ni_1-xO originates from positive charge composition at the thermodynamically favored Ni²⁺vacancies to form oxygen interstitial defects,making Ni_1-xO thin films an ideal alternative as hole injection layer?HIL?and hole transport layer?HTL?in QLEDs.However,most preparation methods need expensive sputtering equipment and high vacuum condition,not meeting the modern mass production demanding in a large scale.The sputtered Ni_1-xO thin film has some disadvanges,such as large particle size,bad crystallinity,high surface roughness and low conductivity.In addition,in all the previous sol-gel synthesis of Ni_1-x-x O thin films,the precursors have to be subject to high temperature annealing more than 420oC to induce decomposition and crystallization,which will not be proper to produce in soft substrate,leading to the inferior quality of Ni_1-xO film with many cracks in the surface due to the different coefficient of thermal expansion between the substrate and the precursors,lead ing to the bad performance of ITO,increase the leakage current?the leakage current is current that transports the QLEDs device but does not form the excitons?and decrease the efficiency of QLEDs.Furthermore,the performance of indium tin oxide?ITO?substrate will be damaged as well due to the thermal mismatch and crystal mismatch under high annealing temperature.The pure intrinsic NiO crystal is a kind of Mott insulator with a resistivity of 10¹³?m at room temperature due to the quantum entanglement effect between the electrons,which is a great neck bottle in QLED application because of its high resistivity and low carrier mobility.One hand,high resisrtivity of Ni_1-xO film can lead to a higher average electric field across the QLED device and more Joule heating,which will seriously deteriorate QLED decice performance,on the other hand,low holes carrier mobility of Ni_1-xO film will make carrier transportation more imbalance,inducing excess electrons into the QDs layer,leading to an increased electrons leakage current and a recombination zone close to the anode.Furthermore,lattice defects including nickel vacancy and interstitial oxygen will play a carrier trap role to limit holes transportation.As a result,the threshold voltage is relative high in previous reports?more than 6 V?,since most of the voltages is applied in Ni_1-xO layer.PMMA and Al₂O₃ were apllied in QLEDs as electron blocking layer?EBL?to balance carrier transportation due to their excellent insulator properties.The mechanism of EBL is the tunneling current for the wave effect of electron to block a part of electrons.However,the blocked electrons will charge the PMMA or Al₂O₃ layer,further,the Joule heat will produce due to the large resistivity of PMMA and Al₂O₃ layer,leading to the decrease of QLED lifetime and efficiency.According to the exsited problems mentioned above,three works are concerned in the dissertation:1 In the work of chapter 2,according to the drawbacks of Ni_1-x-x O production including high vacuum,high-temperature annealing over 420oC,we apply a simple solvothermal method to fabricate Ni_1-xO nanocrystal,followed by a spinning-coatingmethod and low-tempeature annealing process at the temperature of 60oC to obtain Ni_1-xO nanocrystals film.Furthermore,the band gap and valence band edge can be adjusted by changing the solvothermal reaction time.The best results are obtained under 24 h solvothermal reaction time using butyl alcohol and nickel acetylacetonate as precursors at 200oC.The transmittance of Ni_1-xO nanocrystals film is 88%,the band gap is 3.72 eV,the size is 3.5 nm.High bright orange-red QLED with peak luminance up to²5580 cd m^-2,and current efficiency of 5.38 cdA^-1 are achieved successfully based on Ni_1-xO HIL.In particular,the lifetime of the Ni_1-xO-based QLED device is11491 h,which was improved by more than 5-fold as compared to 1839 h for the device based on PEDOT:PSS.In addition,the Ni_1-xO nanocrytals were surface treated by UV-ozone and annealed in air to increase the amount of Ni vacancies in Ni_1-xO crytal lattice to enhance its conductivity.The corresponding QLED performance is enhanced as well.The UV-ozone treatment for 10 min obtains the best result.2 In the work of chapter 3,on the basis of the work in chapter 2,in order to overcome the drawbacks of the low hole mobility and conductivity of Ni_1-x-x O film,the Cu,Fe-doped Ni_1-x-x Onanocrystals are obtained based on the pure Ni_1-xO nanocrystals.The copper and iron are transfer metal elements,with variable chemical valence,the changing of the chemical valence of an element is similar to the transfer of hole,leading to the increase hole mobility of Ni_1-xO nanocrystals.Further,the Fermi level will transfer to the top of valence band after Cu or Fe doping,decreasing the holes injection barrier.In addition,Cu and Fe-doping can decrease the surface deficiency of Ni_1-xO nanocrystals,so as to decrease the quenching of QDs.The amount of 5%Cu doping obtains the best result.The luminance,current efficiency and EQE is 2109 cd m^-2,0.85 cd A^-1 and 0.423%,respectively,in all-inorganic QLED device.In the compound QLED device,the luminance,current efficiency and EQE is 29563 cd m^-2,6.22 cd A^-1 and 4.07%,respectively.Similarly,proper amount of Fe-doping cann improve the QLED performance as well,which is mainly ascribed to the enhanced carrier concentration,the decreased injection barrier,film resistivity and surface defect.3 In the work of chapter 4,considering that instrinsic Ni_1-xO is a kind of p-type semiconductor,on the basis of the previous work in chapter 2 and chapter 3,we applied Cu-doped Ni_1-xO nanocrystals film as HIL and HTL,the intrinsic Ni_1-xO nanocrystals films were applied as electron blocking layer?EBL?in organic-inorganic and all-inorganic QLEDs to balance the carrier transportation through the optimation of the thickness of Ni_1-xO nanocrystals thin film,decrease the leakage current and enhance its luminance.The electron transportation is recrified and limited by the pn junction internal electric field and the grain boundary barrier existed between p-type Ni_1-xO and n-type ZnO ETL.The results show that the proper thickness of Ni_1-x-x O film EBL can decrease the leakage current,the QDs charging and fluorescence quenching,increase the probability of radiative recombination.The threshold voltage is not changed.However,the electron tunneling probability decreased when the thickness of Ni_1-x-x O is too thick,leading to the increased threshold voltage and bad device performance.When the concentration of Ni_1-xO NCs is 0.1mg ml^-1,the spinning speed is 3000 rpm,the QLED obtains the best performance.The novelties of the dissertation are listed as below,1 The Ni_1-xO nanocrystals hole injection layer in QLED device can be applied by a low-temperature annealing process through a simple solvothermal method.The QLED performance can be optimized by adjusting the solvothermal reaction time.The Ni_1-xO nanocrystals as hole injection layers were surface post-reated by UV-ozone and annealed in air to improve its conductivity and to enhance the QLED2 A certain amount of Cu and Fe ions are doped into the lattice of Ni_1-xO nanocrystal,with the increase of the carrier concentration,the carrier mobility is increased properlydue to the conversion of Cu^2+,Cu⁺and Fe³⁺,Fe²⁺,which overcome the decrease of carrier mobility after doping owing to the increased carrier diffusion.Further,the hybrid orbit can be formed between the Cu3d¹00 and the top of Ni_1-xO valence band,which can attenuate the locality effect of hole carriers by O2p6 energy level,so as to enhance the carrier mobility of Ni_1-xO.In addition,Cu,Fe doping can increase the conductivity of Ni_1-xO,decrease the hole injection barrier,surface defect and fluorescence quenching to balance the carrier transportation,so as to improve the performance of QLED device.3 The p-type Ni_1-xO NCs is firstly applied as EBL in QLED to rectify and prevent electron transportation to balance carrier transportation due to its high conduction band position,the pn junction internal electric field and the grain boundary barrier existed between Ni_1-xO EBL and ZnO ETL,which overcome the single electron tunneling mode to prevent electrons using PMMA or Al₂O₃ insulator layer.