Controlled Preparation And Optoelectronic Properties Of Multinary Copper-based Chalcogenide Semiconductor Nanocrystals

Quantum dots light-emitting diode(QLED)based on colloidal semiconductor nanocrystals(NCs)have been developed rapidly in the past few decades,and their performance indicators have approached organic electroluminescent diodes.However,most of the colloidal NCs currently contain heavy metal elements,which does not meet the requirements of environmental protection and commercialization.Therefore,the development of cadmium-free semiconductor NCs has very important scientific significance.Among the many cadmium-free semiconductor NC,multinary copper-based chalcogenide semiconductor NCs have become one of the important candidates for the next generation of quantum dot lighting and display due to their advantages of not containing heavy metal elements,rich source of raw materials and simple preparation.At present,the performance of red QLED based on multinary copper-based chalcogenide semiconductor NCs constructed by the all-solution processed needs to be further improved.Therefore,regulating the composition of multinary copper-based chalcogenide semiconductor NCs,elucidating the formation process and luminescence mechanism,and optimizing the luminescence performance of them,is one of the effective methods to construct high performance of red QLED based on multinary copper-based chalcogenide semiconductor NC.The Cu-In-Se bulk material has a band gap of 1.05 e V,which is a potential candidate for red copper-based chalcogenide materials.However,according to literature reports,the current luminous intensity of ternary Cu-In-Se NCs is low.Researchers usually coat an inorganic shell layer on its surface or introduce zinc to construct a multnary alloy material to optimize its luminescence performance.This thesis aims to construct high-performance multinary copper-based chalcogenide semiconductor NCs and QLED,starting from the controllable preparation of Cu-In-Zn-based semiconductor NCs,focusing on its formation process,optical performance optimization,luminescence mechanism and the construction of QLED employed the NCs as light-emitting layer.The main research methods and innovations are as follows:1.Cu-In-Zn-Se-S NCs were prepared by a one-step reaction method with n-dodecanethiol(DDT)as the sulfur source and selenium powder as the selenium source.By adjusting the amount of copper precursor,the luminescence range was adjusted from560 to 670 nm.Its luminescence performance was optimized and the highest photoluminescence quantum yield reached 48%via adjusting the amount of selenium and DDT.After that,the structure of the five-component Cu-In-Zn-Se-S NCs was analyzed,and it was proved that a large amount of zinc ions were enriched on the surface to form a"shell-like layer".On this basis,five-component Cu-In-Zn-Se-S NCs were prepared by hot-injection method,and compared the optical properties with the one-pot method were.It is believed that copper ion doping and zinc ion diffusion are the both contribute to its formation processes.2.The ternary Cu-In-S NCs as an original template to quaternary Cu-In-Se-S NCs and finally five-component Cu-In-Zn-Se-S NCs by introducing the Se and Zn,and their formation process and optical properties were studied.The results show that the formation of Cu-In-S NCs is a combination of sulfur atoms released by the cracking of DDT at high temperature(220 ~oC)and Cu atoms and In atoms.After adding Se powder,the quaternary Cu-In-Se-S NCs can be nucleated at 120 ~oC in the one-pot method,and the probability of recombination from the conduction band to the acceptor level increases.By adding different dosages of Zn precursors,the nucleation temperature,emission peak position,and full width at half maximum(FWHM)of the obtained NCs were compared,indicating that Zn ions have an inhibitory effect on the formation of NCs,and the more dosage of zinc precursor,the more obvious the inhibitory effect,while the average lifetime of the gradually increases.In addition,the results of thermogravimetric experiments show that the release rate of Zn ions in the system is slower than that of the other three ions.Therefore,the growth of Cu-In-Se-S and the diffusion of Zn ions proceed simultaneously,and finally the five-component Cu-In-Zn-Se-S NCs are formed.During the formation of five-element Cu-in-Zn-Se-S NCs,Cu-In-Se-S growth and zinc ion diffusion occur simultaneously,and is mainly controlled by cationic diffusion reaction3.The Cu-In-Zn-S and Cu-In-Zn-Se-S NCs were synthesized by the one-pot method.The elemental analysis results showed that the introduction of Se replaced about 83%of the S,while the content of the remaining atoms and the chemical environment were constant.The bandgap of these two NCs were calculated using first-principles calculations and it is clear that the introduction of Se mainly reduces the position of the bottom of the conduction band,while the position of the top of the valence band remains unchanged.By fitting the luminescence intensity,emission peak position and FWHM of the two NCs with temperature changes from 78 K to 300 K,it shows that the surface defect state carriers activation energy of Cu-In-Zn-Se-S NCs is less than that of Cu-In-Zn-S NCs,and the Huang-Rhys factor in the five-component NCs is greater than that of the Cu-In-Zn-S,which means that the phonon-electron coupling effect in the five-component NCs is stronger.In addition,the broadening of the FWHM of the two NCs is mainly due to the coupling between acoustic phonons and excitons.Finally,the luminescence mechanism of mutinary Cu-based NCs was verified by calculating the temperature-dependent transient emission spectra and defect state density.4.A red-emitting QLED based on Cu-In-Zn-Se-S NCs was constructed by solution-processed.The performance of the device was improved through compositional engineering.First,the effect of the amount of Se on the performance of device was studied.The results show that when the amount of Se powder is 0.8 mmol,the maximum EQE reached 1.5%.On this basis,the maximum EQE of the red QLED was improved by optimizing the copper content in the nanocrystalline,and the maximum value reached4.2%.In summary,this thesis systematically studied the preparation of five-component Cu-In-Zn-Se-S NCs,made it clear that DDT provides sulfur and optimized its preparation conditions.On this basis,the formation process and luminescence mechanism were systematically studied,and it was proved that zinc ions diffuse into Cu-In-Se-S to form five-component Cu-In-Zn-Se-S NCs.Subsequently,through variable-temperature photoluminescence spectra and first-principles calculations,the electron-phonon coupling effect in the multinary copper-based chalcogenide NCs was clarified,and its luminescence mechanism was verified.Finally,the red NCs were applied to QLED,and the device performance was optimized through component engineering,so that the maximum EQE of the device reached 4.2%,reaching a relatively high level in the same period.This thesis clarifies some key scientific issues of the preparation and formation process of Cu-In-Zn-Se-S NCs and the luminous mechanism.It provides a reference for the further optimization of the optical performance of the multinary copper-based NCs and the improvement of the performance of QLED.