Research On Improving Luminescence Stability Of CsPbX₃ Nanomaterials By Doping And Surface Optimization

All-inorganic perovskite CsPbX3(X=Cl,Br,I)nanomaterials have become a hot research topic in recent years because of their excellent optoelectronic properties and wide applications in devices.Although many breakthroughs have been made in the research of CsPbX3 in both materials and devices,some difficulties and challenges are still faced.Among them,stability is one of the reasons that limit its application.Firstly,the thermal stability of CsPbX3 perovskite is poor,and it is difficult to cope with the heat generated during device operation,so lattice transformation,material decomposition or agglomeration occurs,which eventually leads to a significant shortening of the device lifetime as well.Secondly,CsPbX3 perovskite with ionic properties can easily exchange anions and cations with the outside world,and if the components are changed,the forbidden band width will change and thus affect the material and device performance,which is not conducive to practical applications.Again,perovskite materials with mixed anions are prone to phase separation after preparation into optoelectronic devices,which in turn makes the optoelectronic properties of the devices change and cannot be used anymore.Finally,the improvement of photostability is also crucial.Solar cells or detectors of perovskite need to avoid problems such as phase change,decomposition or agglomeration under light exposure.For CsPbX3 quantum dot light-emitting diodes(QLEDs),it is important to prepare a perovskite layer with stable photovoltaic performance and few defects,taking into account carrier injection efficiency and injection balance.This is optimized from the material to ultimately improve the device performance.In this Ph.D.paper,the thermal stability,spectral stability and optical stability of CsPbX3 nanocrystals(NCs)were improved using doping,core-shell structure and surface engineering.QLEDs were prepared from the optimized CsPbBr3 NCs.The main work and innovation of the thesis are as follows:(1)The thermal stability of CsPbI3 was significantly improved by doping YCl3 in CsPbI3,and rare earth ion doping regulated the luminescence peak position of CsPbI3.Theoretical calculation shows that YCl3 doping improves the formation energy of perovskite(from-21.482 eV to-24.529 eV).After 30 minutes of reaction at high temperature,the photoluminescence quantum yield(PLQY)of doped CsPbI3 nanosheets is 96%under 365 nm ultraviolet irradiation,while the undoped CsPbI3 NCs become yellow phase within 5 minutes after reaction at high temperature.The doping of YCl3 affects the surface energy and makes the growth of CsPbI3 anisotropic,resulting in a new morphology of cross-shaped nanosheets.Experimental and theoretical calculations demonstrate that the doping of YCl3 broadens the forbidden band width of perovskite and blueshifts the luminescence of the nanosheets under UV illumination.In order to demonstrate that metal ion doping can regulate the forbidden band width of CsPbI3,the effect of various metal ions on the forbidden band width was investigated.Among them,the doping of InCl3 shifted the luminescence wavelength of all-inorganic perovskite with pure iodine from near-infrared(690 nm)to red emission(620 nm).In addition,Clpassivated the surface defects and effectively suppressed the nonradiative recombination,so that the PLQY of CsPbI3 was improved after metal chloride salt doping.(2)CsPbX3@Cs4PbX6 core-shell structure was prepared,and the encapsulation of single perovskite NCs was successfully achieved to improve the stability of the luminescence spectrum of CsPbX3 perovskite.The study in the previous section demonstrated that improving the stability of the luminescence spectrum of perovskite requires the suppression of the effects produced by the exchange of anions or cations on the perovskite components.Therefore,in this section,the core-shell structure was prepared by coating Cs4PbX6 on the surface of CsPbX3(X=Cl,Br)to inhibit anion and cation exchange.To demonstrate the protective ability of the shell layer,pure CsPbCl3 and CsPbBr3 NCs were mixed,and it was found by photoluminescence testing that the anion exchange between the two was rapidly completed and an alloy was formed.However,the blue and green luminescence peaks from CsPbCl3@Cs4PbCl6 and CsPbBr3@Cs4PbBr6,respectively,could still be observed simultaneously under the excitation of UV light after CsPbCl3@Cs4PbCl6 and CsPbBr3@Cs4PbBr6 being mixed for 3 hours,which confirmed that the shell layer Cs4PbX6 could effectively inhibit the anion exchange,which ensures the component and spectral stability of perovskite.In order to investigate whether Cs4PbBr6 can prevent the exchange of divalent cations and Pb2+ in CsPbBr3@Cs4PbBr6 and the conditions to achieve the exchange,different cation salts ZnBr2,MnBr2 and EuBr2 were put into the prepared pure Cs4PbBr6 nanoparticle dispersions,and the physicochemical properties of Cs4PbBr6 at room temperature did not change.The necessary conditions for the lattice transition of Cs4PbBr6 were further investigated,and the inhibition of cation exchange and the protection of spectral stability by the shell layer under certain conditions were demonstrated.This lattice transition process can be used as a new doping method,and the luminescence peak of Mn2+ was obtained for the first time in pure bromine inorganic perovskite.(3)The photostability of CsPbBr3 and the performance of QLEDs were improved by using the synergistic optimization of ZnBr2 doping and ligand exchange from inside and outside.Based on the previous research work,the effectiveness of metal halide salt doping to improve the stability and optimized performance of CsPbBr3 was proved,and the study in the previous chapter found that the ZnBr2-doped CsPbBr3 has the advantages of fewer defects and higher PLQY.Therefore,in this chapter,CsPbBr3 with optimized doping is prepared into QLEDs.At first,Zn-CsPbBr3 Quantum dots(QDs)were prepared by doping ZnBr2 into lead precursor,and then DT-Zn-CsPbBr3 QDs were prepared by replacing the long-chain ligands on QDs surface with Didodecyldimethylammonium bromide(DDAB)and Tetrabutylammonium bromide(TOAB).For the material,because Zn2+increases the order of perovskite lattice from the inside and improves the stability,the Br-introduced by doping and ligand exchange reduces the anion defects inside and on the surface and further improves the stability.Therefore,the stability of DT-Zn-CsPbBr3 QDs is significantly improved under long-term ultraviolet irradiation and high-energy electron beam bombardment during TEM test.For QLED prepared from DT-Zn-CsPbBr3 QDs,because doping has improved the quality of perovskite,ligand exchange is also helpful to prepare high-performance thin films and improve the conductivity of perovskite layer.The final DT-Zn-CsPbBr3 QLED with synergistic optimization inside and outside of doping and ligand exchange achieves electroluminescence with the wave peak located at 516 nm under DC drive.The maximum brightness and EQE are 3518.9 cd/m2 and 5.07%,which are 3.6 times and 2.1 times of CsPbBr3 QLED,respectively.