| Quantum dots nanostructure,located at the junction of the macrocosm and the microcosm,is a bridge between the bulk and molecules.It could retain not only the excellent properties of bulk materials,but also the extra features endowed by its nano-size.Thus,QDs are widely used in various semi-conductor devices as potential semiconductors material.As a member of the quantum dot family,all-inorganic perovskite quantum dots own the excellent optoelectronic properties of perovskite structure,and their size is usually smaller than their exciton Bohr radius,which makes all-inorganic perovskite quantum dots have many excellent properties such as high quantum yield,tunable band gap,wide absorption,narrow emission,and high carrier mobility.At present,it has been involved in many fields such as display,laser,solar cell,biochemistry,etc.In the field of display,the mainstream trend of current research is still organic light-emitting diodes(OLEDs).While,in just a few years,perovskite quantum dot light-emitting technology has advanced by leaps and bounds,and the external quantum efficiency(EQE)has climbed from the initial 0.09%to more than 20%.The mechanism research is in the ascendant and has great potential.However,the rapid development is bound to magnify its shortcomings,and its stability problem has become a key factor limiting the large-scale fabrication and promotion of perovskite quantum dots.Due to the ionic nature of the perovskite structure itself and the smaller size means a larger specific surface area,many unstable factors were introduced into the perovskite quantum dots.In this work,we deeply analyze the surface issues of all-inorganic lead-halide perovskite quantum dots,and propose a new strategy to passivate deep red light CsPbI3 quantum dots with a combination of multidentate chelators.The various characterization methods were applied to analyze the mechanism of the multidentate chelators treatment on the surface of quantum dots.Then,it was fabricated into a deep red CsPbI3 quantum dot light-emitting diode as the emission layer,to analyze the electroluminescence performance.The experimental details and results are presented as follows:(1)The heavy-metal multidentate chelators,ethylenediaminetetraacetic acid(EDTA)and glutathione(GSH),which have been used in biomedicine act as the surface passivation ligands of CsPbI3 QDs.The ligand exchange process is performed on the CsPbI3 QDs by a post-processing method as a solid-liquid mixing reaction.Compared with the untreated samples,the photoluminescence quantum yield(PLQY)and fluorescence lifetime of the CsPbI3 quantum dots treated by multidentate chelators were significantly improved,especially the sample treated by both EDTA and GSH,which PLQY reached 94.3%.Through the fourier transform infrared spectroscopy(FTIR)test and the X-ray photoelectron spectroscopy(XPS),we deduce that both EDTA and GSH can chelate with the surface Pb2+,and passivate the surface Pb2+ dangling bonds,thereby reducing the density of defect states.At the same time,due to the strong binding of EDTA/GSH to the surface and the steric hindrance effect of the ligand end groups,the stability of CsPbI3 QDs in polar environments is greatly improved.The EDTA/GSH treated CsPbI3 QDs can keep strong fluorescence emission in ethanol solvent for more than 1 month,because the multidentate ligands protect CsPbI3 QDs with ionic properties from the damage by polar molecules.(2)Based on the above-mentioned EDTA/GSH passivation strategy,we prepare the deep red light-emitting diodes with the structure ITO/PEDOT:PSS/Poly-TPD/CsPbI3 NCs/TPBi/LiF/Al.Meanwhile,the hole transport layer is optimized for the problem of poor hole injection and unbalanced carrier transport.By doping PSS-Na into the HTL material PEDOT:PSS,the work function and film morphology of the hole transport layer are modified.The hole injection efficiency is enhanced,and the device is easier to achieve carrier balance injection.After that,the CsPbI3 quantum dots passivated by EDTA and GSH were used as the light-emitting layer,and the carrier injection efficiency was significantly improved after replacing the original long-chain ligands.Furthermore,with the synergistic effect of the double multidentate ligands,the carrier injection is more balanced,and the brightness and efficiency are jointly improved.Eventually,the EDTA+GSH treated deep red light CsPbI3 quantum dot light-emitting diode has a maximum brightness of 584.5 cd m-2,which is 2.6 times the value of the untreated sample,and the highest EQE is 8.15%,which is 4.4 times the value of the untreated sample.Therefore,this ligand exchange strategy provides a new idea for the application of all-inorganic perovskite quantum dots in the field of display luminescence. |
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