Structure Control And Electroluminescence Devices Of Blue CdSe Quantum Dots

Semiconductor quantum dots(QDs)have attracted great interests due to their unique size-dependent optical properties,solution processability,high photoluminescence quantum yield(PL QY),good photochemical stability,saturated pure color and so on.In particular,quantum dot light-emitting diode(QLED)has become one of the most active scientific research areas,showing great application prospects in the field of LEDs.In the past three decades,the performance of red and green QLED have satisfied the requirement of the industrialization in external quantum efficiency(EQE),luminance and operation lifetime.Nevertheless,the performance of blue QLED is much lower than those of red and green QLED,which has become a major bottleneck in QLED commercialization.Therefore,the development of high-performance blue QLED with high brightness,high efficiency and long operation lifetime is an important prerequisite to promote the process of QLED industry.At present,most of the high-performance QLED reported are widely organic-inorganic hybrid devices.However,the energy level matching is poor between blue QDs and the hole transport layer(HTL)due to the inherent deep valence band energy level of blue QDs,which leads to insufficient hole injection and the accumulation of excess electron in the device based on the QLED structure.These problems will become important factors to affect device efficiency,brightness and long operation lifetime.Therefore,how to further promote hole injection,improve carrier injection balance and suppress auger recombination are the key scientific issues to be solved to achieve high efficiency,high brightness and long operation lifetime of blue QLED.Meanwhile,previous reports on blue QLED are mostly focused on the ultraviolet-deep blue region(such as Cd S or ZnSe material system),but their photoluminescence properties are not very stable and the emission band(<470 nm)is harmful to the human eyes.All of the above problems will restrict the development of high-performance blue QLED.As is known to all,CdSe QDs have been become a research hotspots in the semiconductor field,whose optical tuning range spans the visible region of the spectrum.However,the studies is slow in the blue spectrum as CdSe core.Generally,it is easy to increase the density of surface defects for the small-sized of CdSe QDs,due to the large specific surface area.And it is easy to cause serious leakage of excitons and other problems when was coated the shell.These above problems will be critical to synthesis high-quality blue core-shell QDs as the small-sized CdSe seeds(?2 nm)under conventional reaction conditions.And the synthesis of high-quality blue core-shell structure QDs is also the difficulty in the current research field of blue QLED.Therefore,it still remains a challenge to develop new synthetic methods to make highly stability of blue CdSe-based QDs.In this paper,the high-temperature reverse thermal injection technology is adopted by using the small-sized CdSe seed,which are different from the traditional growth shell scheme.The energy level structure modulation of the core-shell QDs is realized through the shell material selection and alloying growth controlling,which can improve the energy level matching between the EML and the HTL.Furthermore,the interfacial properties and the corresponding physical mechanism of the device are studied in depth to realize blue high performance of QLED devices.Therefore,the research work of this paper is mainly divided into the following four parts:(1)Strategies for the synthesis of high-quality blue core-shell QDs by using CdSe seedAiming at the difficult of directly using CdSe seed to realize the blue range,the controllable synthesis of seed size,peak position and PL QY can be achieved by regulating CdSe seed,precursor types and concentration,nucleation temperature and reaction time and other factors.Combining the adjustment of reverse injection times,reverse temperature,and shell precursor types and components,we successfully explored a kind of good repeatability and suitable for large-scale production technology Subsequently,High quality blue QDs with the PL QY?90%,and luminescence covering the wavelength range from 450?490 nm were successfully obtained by using CdSe seed-mediated technology which can be adjust the appropriate shell materials.(2)Tailoring the shell composition and thickness and studying the performance of QLEDAfter the previous exploration,high temperature reverse thermal injection technology can effectively solve the problem of synthetising blue QDs as CdSe seed.On this basis,a series of different shell composition and thickness blue QDs were synthesized,and blue QLED was constructed by solution method based on these core-shell QDs.The external quantum efficiency(EQE)is up to?5%,and the brightness is about 32,705 cd/m~2 of the QLED device based on CdSe@ZnSe/2ZnSe/2ZnSe S/1ZnS core-shell QDs.Meanwhile,poly(n-vinyl carbazole)(PVK)was selected as HTL and Zn Mg O as the electron transport layer(ETL)by comparing different transport layer materials.The main reason is that the appropriate transport materials makes the carrier injection more balanced,which can suppress the probability of auger recombination,thereby the device efficiency is increased by about 2.6 times.(3)Interface engineering and architecture design for blue high-quality CdSe-based QLEDThe probability of fluorescence resonance energy transfer(FRET)is increased when the QDs with smaller particle size was spin-coated.However,the particle size was only about 8 nm in the previous chapter,which is difficult to suppress the effect of FRET.To overcome this problem,the?5 nm CdSe@ZnSe nuclei were grown into 8nm by�repeatedly reverse injection technology�.Followed the ZnS shell was coated to construct the QLED with the EQE only inproved by?1.2 times and the expected EQE has not been obtained.Subsequently,PVK has electronic defects after exploration.Here we demonstrate that PVK can trap electrons and hence resulting in low device efficiency from both theoretical and experiment respects for the first time,as well as their effect on the performance of blue QLED.An interlayer consisting of ZnSe/ZnS(I-QDs)is inserted between PVK and E-QD(emission layer,EML)to alleviate the electron trapping induced by PVK.The resulted device exhibits the maximum EQE of20.6%,increased by?35%compared with that of PVK-alone devices.This enhancement is mainly attributed to I-QDs interlayer that not only limits the electron trapping caused by PVK and reduces the probability of QDs charging,but also confines more charge carriers to QDs and increases the probability of exciton formation/recombination within EML.Additionally,a stepwise energy level alignment between PVK and E-QDs is formed,which enhances the hole injection and improves the charge-injection balance,thus solving the problem of low efficiency of blue QLED.(4)Improving the stability of blue light-emitting diodes based on large crystal core-shell engineeringIn this chapter,we carried out a targeted research on controlling the shell engineering based on the CdSe@ZnSe large nuclear through the experiment of precise design depthly.By adjusting the shell composition and thicknesses,it can improve the energy level matching between QDs and the HTL.Thus the balance of electron and hole injection was improved as promoting the hole injection,so as to suppress the auger recombination process.In this work,we have successfuly synthesized large-size(8 nm)large CdSe@ZnSe core through the developed�repeatedly reverse injection�method based on small-sized(?2 nm)CdSe seeds.Followed by the shell tailoring,the intermediate ZnSe layer and ultrathin ZnS(3ML)shells was coated.And the average size was?12 nm of CdSe@ZnSe/ZnSe/ZnS QDs with the near unity PL QYs?98%.Compared with the conventional Zn CdSe/ZnS QDs,the valance band was reduced by?0.36 e V,which can effectively improve the hole injection efficiency.The QLED operation lifetime is up to 3195 h based on CdSe@ZnSe/ZnSe/ZnS QDs.which is 1.78times than that of Zn CdSe/ZnS QDs.What's more,the resulted blue QLED with PVK HTL presents a record operation liftetime so far.