Preparation And Studies On Luminescent Properties Of Cu,Zn,Cd-based Halide Perovskite

In recent years,metal halide materials have made great progress in photoelectric fields such as solar cells,light-emitting diodes（LEDs）and photodetectors owing to their changeable architecture,tunable electronic structure and excellent photovoltaic properties.Among them,metal halide single crystal materials have attracted much attention due to their excellent thermal stability,easy synthesis and other characteristics.However,their luminescence properties are a bit different compared to most studied and classical Pb-based nanocrystalline perovskite materials（Me PbX₃,CsPbX₃,where X=Cl,Br,I）.To improve the luminescence performance of metal halide single crystals and realize their practical applications,significant efforts have been made by researchers.On the one hand,dimensional regulation has been used to these single-crystal materials to obtain higher luminescence efficiency through quantum-limited effects.On the other hand,new luminescence centers can be introduced by ion doping strategy from changing the chemical components of metal halide single crystals,thus improving their luminescence performance.The above research methods have improved the luminescence efficiency of metal halide single crystals to a certain extent,but the effect is not significant for Cu（II）-based,Zn-based and three-dimensional（3D）Cd-based halides single crystal materials,and their luminescence mechanism still needs to be further explored.Herein,to systematically investigate the factors affecting the luminescence performance of metal halide single crystals,we investigate the effects of dimensional regulation and ion doping methods on the luminescence performance of the materials in terms of their intrinsic luminescence mechanisms,taking Cu（II）-based,Zn-based and3D Cd-based halides as the target of our study,which are generally low in luminescence efficiency at present.Based on this,a series of Cu（II）-based,Zn-based and 3D Cd-based halides with excellent luminescence properties are prepared and their luminescence mechanisms are investigated.It is shown that the dimensional regulation approach allows the materials to obtain strong luminescence through quantum-limited effects;the ion doping strategy can change the degree of crystal deformation and introduce new active luminescence centers,which is an extremely effective method for both low-dimensional and three-dimensional materials.In this paper,two methods,"dimensional regulation"and"ion doping",are used to improve the luminescence of materials,providing a new idea to obtain low-cost materials with outstanding luminescence and excellent stability.The specific research work is as follows:（1）Effect of Water on the Properties of Organic-inorganic Hybrid Copper-based Halide NanocrystalsCopper-based halides have drawn a lot of attention in the field of optoelectronics owing to their easy availability and low price of raw materials.So far,numerous works have been reported on Cu（I）-based halide materials with high luminous efficiency,however,monovalent copper is very easy to be oxidized,resulting the unsatisfactory air stability.Furthermore,the Cu（II）-based halides with valence stability are difficult to emit because of the redox reaction of Cu²⁺ions with halogen ions,which largely limits the practical application of such materials.Therefore,we chose the（R）-（-）-3-aminopiperidinium chloride as the organic cation to synthesize a green organic-inorganic hybrid copper halide C₅H₁₄N₂CuCl₄.For C₅H₁₄N₂CuCl₄ single crystal,the divalent copper forms a tetracoordination with four chlorine atoms,the inorganic parts of the material are separated and surrounded by organic cations to form a zero-dimensional structure.However,the single crystal structure does not show any luminescence properties.Therefore,the single crystal dissolution anti solvent method is used to reduce the dimension,and nanocrystals with blue emission（444 nm）are obtained with PLQY of 15.8%.To investigate their stability in water,deionized water is added to the synthesized nanocrystals.Unexpectedly,the emission intensity is greatly enhanced by the addition of water.Through the research,it was found that the XRD of the nanocrystals added with deionized water have no change significantly,but the emission position is red-shifted（452 nm）,and the fluorescence quantum yield increase three times（48.43%）.TEM results shows that the addition of water makes the nanocrystals become larger,and the water molecules are partially ionized into H₃O⁺and OH^-acid-base pairs,which reduces the defects of the nanocrystals and improves their luminescent efficiency.This work provides a new idea for the subsequent synthesis of similar organic-inorganic hybrid metal halide nanocrystals.（2）Effect of Native Defect in Organic-inorganic Hybrid Zinc-based Halide Luminescent MaterialsIn our previous work,a dimensional regulation approach was used to improve the luminescence properties of the Cu（II）-based halides,however,the synthesis steps of nanocrystal are lengthy and can not be easily prepared in large quantities.In contrast,the synthesis of single crystal materials is simple and more conducive to industrial production.Among the metal halide single crystal materials,zinc-based materials also have the advantage of being cheap and easy to obtain,but their luminescence properties and luminescence mechanism are still unclear and need to be further investigated.We know that all-inorganic zinc-based halides have very low or no luminescence efficiency,which is not conducive to the study of their own luminescence mechanism.Based on this,in the present work,we chose phenylethylamine as the organic cation and synthesize phenylethylamine zinc halides(C₈H₁₂N)₂ZnX₄（X=Cl,Br,I）,and photoluminescence quantum efficiency（PLQY）of 11.58%,5.74%,and 2.12%for blue-white emission are obtained,respectively.On this basis,the luminescence mechanism is further investigated,and a series of optical measurements show that the luminescence of the three materials mainly comes from their inherent defects.Electron paramagnetic resonance（EPR）test proves the existence of defects in the material.These intrinsic defects act as luminescence centers to promote the luminescence of zinc-based materials.However,compared with free exciton emission and self trapped exciton emission,intrinsic defects are easy to cause lattice damage,and the luminescence efficiency triggered is generally low.This work proves that the luminescence of zinc-based halides mainly comes from intrinsic defects in materials,and puts forward new ideas on the luminescence mechanism of this kind of materials.（3）Realizing Near-Unity Deep-Red Photoluminescence Efficiency in Sn²⁺-Doped Metal Halides Cs₂ZnX₄（X=Cl,Br）In the previous work,it was demonstrated that the main reason for the unsatisfactory luminescence performance of zinc-based halides is the presence of intrinsic defects in the material.If certain strategies can be adopted to suppress their defect luminescence,the purpose of enhancing their luminescence performance can be achieved.However,the organic cations in organic-inorganic zinc-based halide materials are prone to decomposition,which directly leads to poor environmental stability and is not conducive to practical applications.Therefore,Cs₂ZnX₄,an all-inorganic zinc-based halide with excellent stability,is used,as the target of our study and attempt to enhance the luminescence performance of zinc-based halide by ion doping strategy.Herein,we report a Sn²⁺-doped 0D metal halide Cs₂ZnCl₄（Cs₂ZnCl₄:Sn）with a broadband deep-red emission peaked at about 700 nm.By introducing Sn²⁺into Cs₂ZnCl₄,an unprecedented improvement of PLQY（～99.4%）and outstanding stability（the PLQY of Cs₂ZnCl₄:Sn only decreases 4%when it is exposed to the air with relative humidity of80%for 370 days）were realized.To the best of our knowledge,this is the best performance reported to date for any lead-free metal halides with deep-red emission.Detailed spectral characterizations and density functional theory（DFT）calculations revealed that the bright deep-red emission in Cs₂ZnCl₄:Sn originates from the self-trapped excitons（STEs）induced by the doped ion Sn²⁺.Especially,triplet state emission from the Sn-5s² orbital（³P₂→¹S₀）is observed at low temperatures owing to the breaking of the parity forbidden transition.This work provides an important guidance for the development of low-priced,high-efficiency and stable deep-red luminescent materials.（4）Realizing Efficient Emission in Three-Dimensional CsCdCl₃ Single Crystals by Introducing Separated Emitting CentersThe above three materials are all low-dimensional luminescent materials,and their luminescence performance has been greatly improved by the method of dimensional regulation and ion doping.For three-dimensional（3D）single crystal materials,which have good conductivity and play an important role in both photoluminescence and electroluminescence,their luminescence performance is still at a low level.In this context,we hope to take some measures to make a new breakthrough in the luminescence of three-dimensional materials.In this work,we have chosen CsCdCl₃,a3D single crystal material with a unique bonding pattern,which has been studied extensively to demonstrate its high luminescence potential.Unlike the high-temperature conditions of the Bridgman-Stockbarger method,which is the most reported method for synthesizing this material,the present work employs a simple solvothermal method to synthesize CsCdCl₃ single crystals with multiple self-traped exciton（STE）emission,which unsurprisingly exhibits a discouraging fluorescence quantum yield（PLQY）of about 4.8%.By embedding Mn²⁺ions into CsCdCl₃,the long Mn-Mn distance allows the resulting material to emit intense orange emission（PLQY up to 100%）from the d-d orbital transition（⁴T₁-⁶A₁）of Mn²⁺.Experimental and theoretical results demonstrate that the introduction of Mn²⁺ions successfully suppress the emission of non-Jahn-Teller type STEs in CsCdCl₃,reducing its band gap and effectively improving its luminescence efficiency.Besides,this is the highest PLQY among Mn-doped materials,and the doping amount of Mn is only 0.12%,which will be a major breakthrough in quantum efficiency for Mn-doped materials as well as 3D perovskite single crystals,which provides a favorable reference for the subsequent research of this material.