Fabrication Of Light-Emitting Diodes By Controlling The Ligand And Shell Thickness Of ZnCdSe/ZnS Core/Shell QUANTUM DOTS

In recent years,colloidal semiconductor quantum dots?QDs?have attracted much attention due to their high photoluminescence?PL?quantum yields?QYs?,good photostability,wide absorption spectra,narrow band-edge emissions,easy solution processsability,and so on.As the most promising luminescent materials,QDs have successfully extended their original fundamental research into many practical applications,including active-matrix light-emitting diodes,solar cells,and in vitro diagnostics,etc.Since the first report of QDs based light emitting diodes?QLEDs?in 1994,researchers have improved the devices performance through improving QYs of QDs,adjusting the devices architectures,optimizing carrier-transport materials,promoting and balancing the carriers injection efficiency,and so on.The properties of QDs?such as the QYs,surface capping ligand,and the grain size?always play crucial roles on the exceptional performance of the QLEDs.The organic ligands and QDs shell thickness are responsible for the solubility in solution and the rate of F?rster resonance energy transfer?FRET?,respectively.And they both are in charge of the conductivity between the QDs and adjacent charge transport layers when being transferred to a solid QDs film.Therefore,in this thesis,we mainly focus on the influence of the ligand and shell thickness on the performance of QLEDs:Surface ligand regulation:The QLEDs efficiency lies on the quantity of excitions recombined radiatively per ampere.To enhance the QLEDs efficiency,we should be devoted to the following points.?1?Improve the charge injection into QD sites.For QLEDs,it is embodied in decreasing the charge-injecting barriers and enhancing the charge transportation efficiency in QDs emitting layer;?2?maintain optimal charge balance.Unfortunately,the potential barriers of holes and electrons injecting into QDs and carrier mobility are different,which will induce charge injection imbalance at QD sites and inefficient trion emission.Consequently,it is critical for the QLEDs to maintain charge balance and increase the carriers injection.Based on the analysis above,the short-chain tridentate thiol?tris?mercaptomethyl?nonane,TMMN?capped QDs were chosen to fabricate QLEDs.The reasons are listed as follows.First,Dislodging ligands will induce the emergence of unbounded surface atoms and the surface-localized states,or the agglomeration of QDs,then lose the prominent characteristic of QDs.Due to the high affinity of thiolate groups?S^–?toward the surface of the QDs,the thiol ligand could adhere to the QDs surface firmly to stabilize the QDs and maintain their high optical property.Second,the short-chain ligand could enhance the charges injection efficiency.Finally and most importantly,thiol,which is an electron donor group,could raise the energy level and improve the injection efficiency of QDs.Compared with devices based on long-chain ligand?oleic acid,OA?capped QDs,the devices based on short-chain tridentate thiol?TMMN?capped QDs have lower turn-on voltage,higher luminance and higher efficiency with the highest external efficiency?EQE?of 16.5%.More importantly,the devices exhibit high color stability and high environmental stability.For the highest luminance devices?with emitting layer thickness of 25 nm?and the highest power efficiency devices?with emitting layer thickness of 38 nm?,the lifetimes are>480,000 h and>110,000 h,respectively.Shell thickness regulation:In the QLEDs,QDs are closely packed,so the prime determinant of QLEDs is the performance of QDs film instead of QDs solution.The key criterion for QLEDs is high PL QYs of the QDs film.As is known,FRET between the close-packed QDs will reduce the PL QY of QDs film.The rate of FRET,which follows the well-known R^-6 scaling?R is the donor-acceptor separation?,depends strongly on the distance between the adjacent QDs.Therefore,FRET process could be suppressed by increasing the shell thickness.From the perspective of suppressing FRET,the thicker the shell thickness is,the higher the efficiency of QLEDs is.However,in order to improve the QDs'stability,the materials of QDs shell often have wider band gap,so the thick shell will hinder carriers injecting into QDs.Therefore,it is important to study the influence of QDs'shell thickness on the QLEDs performance.According to the above analysis,we synthesized a series of nonblinking ZnCdSe-based core/shell QDs with different shell thickness by a“low temperature injection and high temperature growth”precisely controlled method.Due to the higher temperature shell growth procedure,the as-synthesized QDs can have higher chemical/photochemical stability than QDs prepared by low temperature shell growth method.Not only alloyed ZnCdSe core QDs with certain ratio of Cd and Zn have been pre-synthesized precisely,but also well-controlled ZnS shell with accurate different thickness?1-20 ZnS monolayers;monolayer is abbreviated to ML hereafter?.Due to the small lattice mismatch between ZnCdSe and ZnS,the trivial accumulation of interfacial strain between ZnS and CdSe is almost ignorable.The as-synthesized ZnCdSe/ZnS core/shell QDs have the uniform dimension and shape,high QYs?the highest QY of¹00%?,and PL emissions with narrow full width at half maximum.More importantly,for the QDs with?2 MLs of ZnS shell,the PL decay curves of QDs ensemble are single-exponential and the PL for single QD is nonblinking with the nonblinking threshold volume of¹37 nm³.After being transferred to solid films,the nonblinking QDs have been found to retain high optical characteristic and suppress FRET process effectively.By using ZnCdSe/ZnS core/shell QDs with 10 ML ZnS shell as emitter in QLEDs,the highest EQE is reached as high as 17%,which could compare favorably with the highest efficiency green QLEDs with traditional multilayered structures.