Investigation On Photoluminescence Mechanism Of Zn(Cu)-In-Se(S) Multiple Quantum Dots

Semiconductor quantum dots?QDs?,as versatile fluorescent nanomaterials,have broad potential applications in light-emitting diodes?LEDs?,solar cells,and bioimaging.Eco-friendly I-III-VI QDs possessed several interesting properties including tunable band gap,large Stokes shift,and high photoluminescence?PL?quantum yield,which were developed to luminescent materials for high color rendering index QD-LEDs.Band gap and defect level modulation can be achieved by doping transition-metal elements into QD lattice to meet different optoelectronic device requirements.The surface-bound ligands with high chemical activity acting has an important impact on the chanrge transport between nanocrystals?NCs?.Study on the photophysical mechanism of electronic transition of NCs,which related to band structure and surface modification,provide theoretical basis and scientific basis for achieving highly efficient and stable photoelectric materials.This paper focuses on the key issues of Zn?Cu?-In-Se?S?multiple QDs such as electronic motion and luminescence efficiency to investigate the effect of crystal structure and energy band structure on the luminescent properties of Cu-doped Zn-In-Se QDs through cation ion substitution at sites within the QD lattice.Furthermore,the electronic structure of Cu-doped Zn-In-Se QDs was examined by altering the surface-bound ligands.Finally,the spectral shift mechanism of Zn-Cu-In-S QDs was discussed through ZnS shell deposition,and the Zn-Cu-In-S/ZnS QDs/PVK multilayer films were fabricated.The influence of charge trapping densities on the electroluminescence?EL?spectra modulation was discussed based on the device band structure and J-V curves.The main research contents and results are summarized as follows:1.The tunable PL emission of Cu-doped Zn-In-Se QDs was achieved with In³⁺ions being substituted by Zn²⁺ions charge compensation.As the Zn/In ratio increases,a systematic blue-shift in the dominant broad emission occurs from 648 nm to 552 nm,with a fixed weak peak at 465 nm.X-ray diffraction?XRD?,Raman spectroscopy and high resolution transmission electron microscopy?HRTEM?analyses suggest that the formation of multiple phases for the samples including Zn In₂Se₄,ZnSe,and In₂Se₃.Such spectral shifts are not due to the quantum size effect,but this might be ascribed to the widened band gap as a result of the up-shift in the conduction band edge position with increasing Zn/In ratio from–4.17 eV to–3.17 eV.PL results indicate that the 465 nm emission is ascribed to zinc interstitial?Zn_i?defects level,and the large blue-shift in the broad band is attributed to the Zn_i and indium-zinc antisite(In _Zn)donor levels moving up as the average conduction edge up-shifts.2.Considering the effect of surface-bound ligands on the electronic transitions in a QD nanoparticle,the PL properties of Cu-doped Zn-In-Se QDs were investigated through the comparison of oleylamine?OLA?and 6-mercaptohexanol?MCH?molecules.PL spectral shapes and widths have no significant changes after ligand exchange,suggesting that their emissions originate from Zn_i and In _Zn donor levels rather than ligand molecules.PL quantum yield?QY?is reduced from 49%to 38%,indicating that the increased MCH ligand density and the stronger ligand electron-donating ability resulted in an increase in the nonradiative transition probability.The temperature-dependent PL spectra indicate the lower PL quenching transition temperature for MCH-capped QDs due to an increased electron-acoustic phonon coupling of MCH molecules.3.High-efficiency Zn-Cu-In-S/ZnS QDs were prepared by thermal injection method.A single broad red emission was achieved for Zn-Cu-In-S QDs,in which dodecanethiol was used as sulfur source.After ZnS is deposited,the emission band of the Zn-Cu-In-S QDs blue shift from 635 nm to 586 nm,with the PL QY increasing from 45%to 76%.This is attributed to the increase in the radiative recombination from conduction band to V_Cu level with a decrease in the recombination through surface states?V_s?.The average lifetimes were prolonged from 204 to 306 ns after ZnS shell deposition,suggesting that nonradiative recombination probability through surface states were significantly reduced.4.The organic/inorganic hybrid LEDs with different thicknesses of Zn-Cu-In-S/ZnS QD layer were fabricated.The EL spectra showed an obvious red shift from 560 nm to 590 nm as the QD layer thickness increased from 10 nm to 27 nm.Distinguished by the decomposed emission peaks using Gaussian fitting and energy level structure of the device,the EL spectra were constituted by the emission overlap of PVK and Zn-Cu-In-S/Zn S QDs.The charge trapping density was enhanced from 6.5×10¹88 cm^-33 to 9.0×10¹88 cm^-33 with the increase of QD layer thickness.The origin of the red shift is associated with the increased trap density of Zn-Cu-In-S/ZnS QDs that induces the injected electrons to be trapped in the deep donor level.The current conduction mechanism based on the J-V curves was discussed.The current transport was transited from the trapped-charge-limited current model to the space-charge-limited current model from low voltage?2.6 V<V<9 V?to high voltage region?V>9 V?due to the effective filling of injected electrons in the trap states of QDs.