High Color Purity Red Cesium Lead Halide Perovskite Quantum Dot Light-Emitting Diodes

Cesium lead halide perovskite quantum dot light-emitting diode(LED)with a wide color gamut and narrow half peak width,has attracted wide attention in the field of high definition display.However,a single halogen perovskite,such as CsPbI3,whose emission spectrum is usually located after 680 nm,cannot meet the requirements of high color purity red(wavelength at 620-650 nm).Although the mixed halide component is easy to achieve pure red emission,it is easy to produce phase separation under an electric field,thus the spectral stability is poor.Using quantum confinement effect,pure red emission can be obtained by reducing the size of CsPbI3 quantum dots.However,the size reduction significantly increases surface defects and Auger recombination.Therefore,this paper develops a variety of surface ligand engineering to reduce surface defects and Auger recombination of quantum dots,improve the stability of quantum dots,and inhibit halogen ion migration,to achieve efficient and stable high color purity red LED.The main research results are as follows:This paper introduces zinc iodide into the synthesis process of quantum dots to form an iodide ion rich environment,to develop a synthesis technology of CsPbI3 strongly confined quantum dots(SQD).The researches show that this iodine-rich ion environment can induce the formation of iodine-rich surfaces to quantum dots,which hinders the adsorption of[PbI6]4-octahedrons on the surface of quantum dots,and ultimately inhibits the growth of quantum dots.When the molar ratio of zinc iodide to lead iodide is 3:1,the size of CsPbI3 quantum dots is reduced to 5 nm,and the photoluminescence(PL)spectrum is shifted to 642 nm.However,the fluorescence quantum yield(PLQY)of SQD is only 35%,because the reduction of quantum dot size brings a large number of surface defects,resulting in most excitons being trapped and quenched.This paper uses the SQD as the luminescent layer to produce LED,its electroluminescence(EL)peak is located at 655 nm,compared with the PL peak of SQD is significantly redshifted.LED has an external quantum efficiency(EQE)of 1.4%,a luminance of 675 cd/m2,and an operating half-life(T50)of only 1.3 min.The high surface defect state density of SQD and weak bonding of surface ligands are the main reasons for the poor performance of LED.To passivate surface defects of SQD and improve its stability,this paper develops a composite ligand engineering of 3-phenyl-l-propylamine(PPA)and tetrabutylammonium iodide(TBAI),which partially replaced oleylamine and oleic acid on the surface of quantum dots.The researches show that PPA ligands can improve the stability of SQD by forming strong bonding to lead ions,and inhibit the Stark effect induced by an electric field,stabilizing the EL spectrum of LED.The surface halogen vacancy defect of SQD is passivated by TBAI,and the PLQY of SQD is up to 85%and the radiation recombination rate is up to 5.6×107 s-1.The composite ligand engineering moves the valence band maximum of SQD from5.65eV to-4.65eV,thus decreasing the injection barrier of holes in the LED and reducing the interface charge accumulation.Based on SQD modified by composite ligand,the EL peak of LED is located at 636 nm pure red wavelength,EQE reaches 20.8%,and the luminance is 3775 cd/m2.When the bias voltage increases from 2.5 V to 6.3 V,the EL spectrum remains stable at the wavelength of pure red at 636 nm without a significant shift.However,the T50 of LED is still poor at only 7.3 min.This is because the large exciton binding energy of SQD causes serious Auger recombination,resulting in serious efficiency roll-off and poor operating stability of LED.To further improve the efficiency and operational stability of LED,this paper develops an inorganic ligand engineering using potassium iodide(KI)to partially replace the original long chain ligand on the surface of SQD to inhibit Auger recombination.The researches show that the surface defects of SQD accelerate the dissolution of KI in a non-polar solvent(n-octane),thus allowing KI to bond to the surface of the quantum dots without damaging them.KI inorganic ligand reduced the exciton binding energy of SQD from 305 meV to 51 meV,which was beneficial to reduce the Auger recombination of SQD.Moreover,compared to PPA ligands,KI does not cause surface lattice strain of quantum dots,thus better improving the stability of SQD.However,KI ligands greatly shorten the distance between SQDs,which induces a fluorescence resonance energy transfer(FRET)between SQDs,leading to broaden and redshift of the PL spectrum of SQD film.To this end,this paper fu rther introduces TBAI ligands with large steric hindrance,which effectively suppress the FRET and increase the PLQY of SQD to 97%.The LED based on this SQD has the following properties:EQE up to 22.3%,maximum brightness of 4165 cd/m2,EL peak of 639 nm,and Commission Internationale de I’Eclairage(CIE)coordinates are(0.706,0.294).In addition,the efficiency roll-off of LED is significantly reduced,when the brightness reaches 949 cd/m2,the EQE can still maintain 16%.The T50 has also been upgraded to 37 min from 7.3 min.In addition to surface defects and Auger recombination,the rapid ion migration of perovskite quantum dots under the action of an electric field is also one of the main reasons for the poor operating stability of LED.Therefore,this paper develops a ligand engineering to inhibit the halide ion migration of quantum dots to improve the operating stability of LED.Firstly,pure red mixed halide CsPbI3-xBrx quantum dots are prepared using hydrobromic acid as a bromine source.Then,1-dodecanethiol(1-DT)is used to replace the original long chain ligand on the surface of quantum dots.The results show that a strong bonding is formed between the sulfhydryl group of 1-DT and the lead dangling bond on the surface of the quantum dots,which blocks the migration channel of halide ions and increases the migration activation energy of halide ions from 0.10 eV to 0.14 eV,which significantly inhibit the migration of halide ions.The performances of pure red LED are as follows:EQE is 21.8%,EL peak is 637 nm,and CIE coordinates are(0.700,0.300).The EL spectrum of LED is still stable at a high bias voltage of 6.7 V,and the T50 is improved to 70 min.