Research On Defect Control Of CsPbI₃ Quantum Dots And Its Optoelectronic Properties

As one of the most promising semiconductor materials,halide perovskite has achieved wide attention.Among these,perovskite quantum dots(QDs)with tunable wavelength in visible spectrum,wide color gamut,high color purity and solution processability have become hot materials in optoelectronic devices.However,its large specific surface area and weak ionic bonding make it prone to form internal and surface defects,which is not beneficial for the construction of high-efficiency lighting devices.Besides,due to the low forming energy of nonluminous phase(yellow phase)of CsPbI₃ QD,its luminous phase(black phase)will transform into nonluminous phase under inappropriate defect control,which causes great difficulty to the defect control of CsPbI₃ QD.Therefore,exploring the methods of CsPbI₃ QD defect control,decreasing defect and improving the efficiency of lighting device are of great significance to the further development of CsPbI₃ QD.Taking CsPbI₃ QD,synthesized through hot-injecction method,as the luminescent material,this paper researched on the methods of defect optimiazation from the angles of surface and internal defects.After study on different defect optimization methods(purification,surface ligand passivation and internal doping),CsPbI₃ QD with high luminescent efficiency and CsPbI₃ QD-based light-emitting diodes(QLEDs)with high external quantum efficiency(EQE)can be achieved.Main research contents are concluded as mentioned below:(1)Research on the surface defect control through purification of CsPbI₃ QD.Because inappropriate purification method could cause surface defect,this part will avoid the brought-in of surface defect by the means of adjusting main parts of purification(flocculation and dispersion).Through adjusting flocculation and disperse solvent with different dielectric constant and polarity,the defect introduction is minimized,which endows QLED with strong luminescence under low current injection.Ultimately,improved EQE(from 1.2%to 5.1%),current efficiency(CE,from 0.02 cd A^-1 to 0.135 cd A^-1)and highest luminance(678 cd m^-2)are achieved.(2)Research on the surface defect control through surface ligand passivation of CsPbI₃QD.Because the ionic bonding feature of CsPbI₃ QD,surface ligands and halide ions are prone to escape and leave surface defects.Hence,this part decreased surface defects through adding surface ligand to passivate surface defects by solution post-treatment method.Further,different ligands and dosage are employed to realize the balance between surface ligand passivation and electron injection.Based on these,EQE,CE and luminance of CsPbI₃ QLED are improved to7.8%,0.46 cd A^-1 and 852 cd m^-2 after surface ligand passivation.(3)Research on the internal defect control through Zn²⁺doping of CsPbI₃ QD.Due to the strain of CsPbI₃ QD,halide ions among the structure will escape to release the stain.Hence,internal defects are formed.This part restrained the form of internal defect through the doping of small radius Zn²⁺(74 pm).After Zn²⁺doping,the defect intensity is decreased from 4.6*10¹²cm^-3 to 2.3*10¹² cm^-3 measured by hole-only devices.In addition,the photoluminescence quantum yield(PLQY)is increased from 35%to 77%.And the EQE of QLED after Zn²⁺doping is increased from 7.5%to 14.6%,equating to a nearly 100%increase.The CE is also increased from 0.6 cd A^-1 to 0.9 cd A^-1.