Research On Efficient And Stable Light-emitting Diodes Constructed By Surface Control Of CsPbI₃ Quantum Dot

All-inorganic perovskite CsPbX₃ quantum dots show potential application in photoelectronic devices such as solar cells,lasers and light-emitting diodes due to their high absorption coefficient,tunable bandgap and high photoluminescence quantum yield.The surface ligand plays an irreplaceable role in the synthesis of Cs Pb X₃ quantum dots.At present,oleic acid and oleylamine are commonly used ligands to prepare Cs Pb X₃ quantum dots.However,the stability of quantum dots is unsatisfied due to the dynamic combination between long-chain ligands and quantum dot surface.Meanwhile,long-chain ligands can form an insulating layer on the surface of quantum dots,which prevents the carriers injection and transmission at the device interface and further affects device performance.In addition,there is weak interaction between the traditional long-chain ligand and the surface of quantum dots.This makes surface ligands easy to fall off,resulting in agglomeration and increasing the density of surface defects.Therefore,it is crucial to achieve subtle control of quantum dots surface.Surface ligand engineering can enhance the luminescence performance and stability of Cs Pb X₃ quantum dots without changing their crystal structure,which has gradually become an important research direction of Cs Pb X₃quantum dots.In this paper,CsPbI₃ quantum dots are taken as the research object.We start our research from the structural design and chemical equilibrium regulation of the surface ligand.The following studies are carried out to solve the problems of poor luminescence performance and stability of CsPbI₃ quantum dots:(1)Octylamine ligand engineering improves the efficiency and stability of CsPbI₃ quantum dot LEDs.Using a trace of octylamine to regulate the dynamic balance between surface ligand adsorption and desorption,we develop a facile method to obtain stable CsPbI₃ quantum dot films that combine excellent optical.The octylamine additive maintains the integrity of quantum dots and inhibits the formation of defects,greatly enhances the phase-and the photoluminescence-stability,and thus the operational stability of the corresponding light-emitting devices(LEDs);enables in situ substitution of oleylamine,which reduces the spacing between adjacent quantum dots and thus leads to closely packed nanocrystal films with promoted charge carrier transport capability;and also increases the activation energy for ion migration,indicating that the number of ion vacancies-namely,the ion migration paths-has been decreased.As a result,the octylamine additive leads to a 4-and 6-fold increase in LED external quantum efficiency(EQE)and operational lifetime,respectively.LEDs based on octylamine-assisted Zn-doped CsPbI₃ quantum dot show a high EQE of 15.3%.(2)Polymer surface ligand-modified CsPbI₃ quantum dots for efficient and stable LEDs.According to the characteristics of hot-injection method,a polymer PMA with high density of passivating functional groups is designed.By adding PMA in the synthesis process,we successfully prepared highly efficient and stable?-CsPbI₃quantum dots for the fabrication of light-emitting diode devices.Further research indicates that PMA can regulate the crystallization kinetics of quantum dots by forming strong Pb-O bonds with Pb I₂ in the precursor.At the same time,the cross-linked PMA significantly can reduce the Pb _Cs inversion defects on the surface of?-CsPbI₃ QDs.Benefiting from the reduction of defect density and the improvement of crystal structure,the photoluminescence quantum yield of?-CsPbI₃ quantum dot films has significantly improved.The red quantum dot LEDs prepared based on this film has a peak external quantum efficiency of 17.8%at 690 nm,and an operating stability of 317 hours.(3)Synergistic control of polymers and small molecules to prepare efficient and stable CsPbI₃ quantum dots.On the basis of the previous work,we add organic small molecule ligands formamidine and o-toluidine into the precursor.Firstly,the dosage is optimized to further improve the luminescence performance,and the quantum dot with a high liquid-phase photoluminescence quantum yield of 95%are prepared.Through further analysis of the experimental results,we find that the performance improvement attributes to the passivation of surface defects by the adsorption of nitrogen-containing functional groups in formamidine and o-toluene biguanide to the surface of quantum dots through multiple hydrogen bond interactions.In addition,the hydrophobic functional groups in the ligands can prevent the water molecules from eroding and destroying the quantum dots.We put the prepared quantum dot films in water for 30 h under unencapsulated conditions and no phase transition is observed.Finally,the quantum dot light-emitting diode based on this polymer-organic small molecule synergistic modulation achieves a peak external quantum efficiency of 20.4%at 689 nm and a brightness of 1521 cd m^-2.