High-quality ZnCdS Quantum Dots For Blue Light Emitting Diodes With High Efficiency By Shell Regulation

Quantum dots?QDs?have received great attention in the field of light-emitting devices?LEDs?because of their advantages of high photoluminescence?PL?quantum yield?QY?,narrow band-edge emissions,tunable emission spectra,easy synthesis and so on.Quantum dot light-emitting diodes?QDs?have occupied an important position in the field of light-emitting devices because of their advantages over organic light-emitting diodes in terms of a wider gamut,higher thermal and color stability,and longer lifetime.QLED with high efficiency requires high-quality quantum dots.At present,the quantum dots commonly used in the blue QLED luminescent layer are mainly based on a core/shell structure with cores such as CdS,CdZnS,CdZnSeS and ZnSe.Due to the quantum confinement effect,the smaller the size of nanocrystalline,the larger the width of its energy gap and the shorter its emission wavelength.Traditional binary quantum dots can only adjust their luminescence range by adjusting their size.However,when the quantum dots are less than 2 nm,their stability is extremely poor,and it is not conducive to the epitaxial growth of the shell layer to passivate the surface.Alloy quantum dots faciliate tunning the size of and components of quantum dots to control their wavelengths,and it is easier to obtain high-quality blue quantum dots.The poor device performance of deep blue QLEDs is mainly attributable to three points.First,the quantum dots for blue QLEDs are commonly core-shell structures,the carrier injection barrier produced by the inherent structure of deep blue QDs creates a bottleneck.Second,the HOMO level of blue QDs is lower than that of green and red QDs,rendering a larger potential energy barrier between the hole transport layer?HTL?and QDs,which hinders the hole injection from the neighboring HTL into the QD layer,leading to a poor EQE.Third,the photometric quantity of luminance,which is determined by the relative sensitivity of human vision to different wavelengths,shows a much lower luminous efficacy for blue light versus green and red counterparts.Since the third one is an inherent factor,the strategies for improving the performance of deep blue QLEDs are mainly focused on the first two points.We will improve the structure of blue quantum dots by regulating shell materials,so as to make up for the shortcomings of ZnCdS/ZnS core-shell quantum dots and prepare high-quality blue quantum dots.This is of great significance not only for the synthesis of quantum dots,but also for improving the performance of blue QLED.Based on the above problems,The main content of this paper is as follows:?1?ZnCdS/ZnS blue quantum dots with fluorescent quantum yield of more than 90%were synthesized by component control.High quality deep blue ZnCdS/Cd_xZn_1-xS/ZnS quantum dots were prepared by using the same synthesis method.The effect of x value of Cd content in alloy shell on the quantum yield and the characterization of optical properties and morphology of quantum dots were analyzed.When x=0.225,the quantum yield can approach100%,which indicates that the lattice mismatch of quantum dots is the smallest and the wave function of electrons can be effectively localized in the stone.ZnCdS/Cd_xZn_1-xS/ZnS quantum dots can maintain their stability during epitaxial growth at higher temperatures,and have good monodispersity,higher photoluminescence quantum yield and higher color purity?FWHM<18 nm?.?2?The QLED devices constructed by ZnCdS/ZnS and ZnCdS/CdxZn_1-xS/ZnS quantum dots are compared,the influence of shell thickness on their properties is studied,and the EL spectra,UPS and single carrier devices based on the best experimental conditions are characterized.The experimental results show that ZnCdS/Cd_xZn_1-xS/ZnS quantum dots have good chemical/photochemical stability and more efficient carrier transport performance compared with ZnCdS/ZnS core/shell QDs.The presence of the Cdx Zn1-xS shell has a profound impact on device performances such as the external quantum efficiency and current efficiency.The corresponding light-emitting diodes exhibited a peak current efficiency of 3.4cd A^-1,a high EQE exceeding 18%and low efficiency roll-off.Such excellent results of ZnCdS/Cd_xZn_1-xS/ZnS-based QLEDs are likely attributable to the QD's high PLQY and very thin ZnS outermost shell which did not sacrifice the charge injection efficiency in QLEDs.