The Influence Of Hole Transport Layer On The Performance Of Quantum Dot Light Emitting Devices

In recent years,OLED and QLED display technologies which have begun to access the market in various ways,would substitute that of LCD.Compared to LCD display technologies,OLED and QLED have many potential advantages and features,such as active emission,quicker response time,lower power consumption,and broad color gamut.In addition,OLED and QLED can be made into flexible displays to content people with need of displays with high-end and specific environment.In 2018,several major display companies such as Samsung,BOE,and Visionox carried out mass production on AMOLED.Compared with OLED,QLED has other advantages which mainly divide into two aspects.On the one hand,the major difference between QLED and OLED is that the materials of emitting layers are not the same.The emitting layers of QLED are based on inorganic quantum dots?QDs?,while the emitting layers of OLED use organic materials.Organic counterparts aren't as good as inorganic materials in physical and chemical stability.In theory,the stability of inorganic semiconductor QDs is higher than that of organic small molecule polymer.On the other hand,because of quantum dots confinement effect,QDs has narrow emission linewidth.Therefore,QDs process a better color purity.At present,the luminous efficiency of QDs has achieved commercial demand.However,the lifetime of the device is still short,especially blue QLEDs.We all know that understanding the working mechanism of devices is precondition of optimizing the devices.Hence,in this paper,we mainly discuss the working mechanism and optimize the device structure to improve device performance.Firstly,we investigated not only the role but also the influence of the hole transport layers in organic/inorganic hybrid QLED devices.We selected CuInS₂/ZnS?CIS?QDs as the emissive layer.And the architecture of devices is ITO/ZnO/CIS/HTL/MoO₃/Al.The four types of hole transport materials which possess different mobility and energy levels are used in the devices containing 4,4',4''-Tris?carbazol-9-yl?triphenylamine?TCTA?,4,4'-bis?9-carbazolyl?-2,2'-biphenyl?CBP?,di-[4-?N,Nditolylamino?-phenyl]cyclohexane?TAPC?,and N,N'-Bis?naphthalen-1-yl?-N,N'bis?phenyl?-benzidine?NPB?.We analyzed the carrier distribution and emitting mechanism of the devices by the device photoelectric characteristics,[for example,electroluminescence spectroscopy?EL?,capacitance-voltage?C-V?and transient EL?TREL?].We found that the character of hole transport layer determines devices emitting mechanism which is direct charge injection or energy transfer.Finally,we achieved high-performance QLED based on deep red CIS QDs.The peak current efficiency of the QLED is about2.0 cd/A,and the maximum luminance is about 3000 cd/m².However,the physicochemical stability of organic materials is lower than that of inorganic materials.Therefore,on the basis of the work on organic/inorganic hybrid QLED mentioned above,we also research all inorganic devices with NiO as hole transport layer and ZnO as electronic transport layer.As previously mentioned,the hole transport layer has a significant impact on device performance in the organic/inorganic hybrid QLED.Herein we adopted excellent Zn_xCd_1-xSe/ZnS QDs as emitting layers constructing all inorganic devices whose architecture is ITO/NiO/QDs/ZnO/Al.And we investigated that the mechanism of QDs caused by NiO hole transport layer in devices.The source of QDs quenching in device is mainly considered from two aspects:one is exciton dissociation of QDs caused by NiO;the other is QD charging result from NiO.Ultimately,we confirmed that charging effect of QDs is the primary reason for limiting the all-inorganic device performance of NiO as the hole transport layer.And this charging effect can be greatly suppressed by increasing thickness of the shell of the QDs.