Luminescence Enhancement Of Manganese Doped Perovskite Nanocrystals Based On Metal Ion Assistance

All inorganic cesium lead halide（CsPbX₃,X=Cl,Br,I）perovskite nanocrystals have attracted extensive attention because of their excellent photoelectric properties,such as narrow line width,tunable luminescence wavelength,high quantum yield,long carrier lifetime and diffusion length.In addition,halogen chemical components are easy to exchange at room temperature due to its excellent ion migration characteristics,so that the optical band gap of perovskite materials can be adjusted and cover the full visible range.This simple and low-cost room temperature preparation process is conducive to industrial production,and is expected to make practical application in photovoltaic cells,light-emitting diodes,lasers and other optoelectronic devices.It is found that doping B-site cations in lead halide based materials such as CsPbX₃can further improve the optical properties of perovskite nanocrystals and enhance the radiation luminescence,while the construction of impurity ions may produce hyperfine structure,degenerate splitting of energy levels and introduce new electronic states,which is of great help to the understanding and development of new optical properties of perovskite nanocrystals.In2016,Parobek et al.prepared Mn²⁺B-doped CsPbCl₃nanocrystals,without changing the intrinsic structure and band edge emission,and the 600 nm wide orange luminescence was successfully introduced to realize the dual-color emission in perovskite materials.Parobek et al.believe that this wide orange luminescence originated from exciton-to-Mn energy transfer,and obtained Mn doped perovskite nanocrystals with luminous efficiency up to60%.In 2018,Sun Hongtao et al.obtained Ni²⁺doped CsPbCl₃nanocrystals with near-unity quantum yield through metal ion doping strategy.They found that Ni²⁺ions doped into the host lattice can induce long-range lattice order,passivate defect states and enhance the radiative emission of perovskite nanocrystals.In addition,two-dimensional lead halide perovskite has also become an ideal candidate for optoelectronic materials in recent years.The low-dimensional sandwich structure shows strong quantum confinement in the octahedral coupling layer,and the excitons are effectively confined in the lattice.At the same time,the organic long-chain ligands covered on the surface make these two-dimensional materials have higher environmental stability.Mn²⁺ion doping has also been applied to modify the optical and magnetic properties of two-dimensional lead halide perovskite.Due to the strong quantum confinement,the energy transfer from exciton to Mn is more effective in two-dimensional perovskite.Therefore,Mn²⁺doped two-dimensional perovskite materials also show promising potential applications.The thesis mainly discusses the following four parts:（1）Preparation and luminescence properties of Mn²⁺:CsPbCl₃perovskite nanocrystals with CdCl₂post-treatment.The PL quantum yield of Mn²⁺ion doped CsPbCl₃perovskite nanocrystals was effectively improved to near-unity through the room temperature CdCl₂post-treatment strategy and the radiation luminescence of Mn was enhanced.Steady state and transient PL spectra show that the regulation of Cd²⁺ions to the host lattice will change the octahedral coordination field environment of Mn,resulting in the red shift of Mn²⁺emission and the increase of PL lifetime.This improvement may be that the Cdions induced lattice order effectively passivates the deep level defect state of perovskite nanocrystals,thus enhancing the energy transfer from exciton to Mn.In addition,the co-doping of Cd²⁺and Mn²⁺ions improves the stability of nanocrystals.Repeated purification will not change the phase structure and weaken the luminous efficiency of nanocrystals.（2）Optical properties of Mn²⁺:CsPbCl₃perovskite nanocrystals with tunable band gap and fabricated white light-emitting diodesMn²⁺:CsPbCl₃perovskite perovskite nanocrystals were post-treated with metal bromide,which effectively regulated the optical band gap energy,changed the exciton absorption and PL peak position,and achieved luminescence efficiency of 97%.Moreover,the radiation luminescence of Mn ⁴T₁-⁶A₁will not be quenched in the process of regulating the band gap.This enhanced stable luminescence is due to the introduction of new shallow energy levels by Cdions,which increases the energy transfer efficiency from exciton to Mn.A large amount of Cdion doping may lead to the generation of new electronic states and the formation of Pb-Cdalloy,so as to enhance the sp-d coupling between the host and Mn,improve radiation luminescence of Mn.（3）Temperature-dependent luminescence enhancement of Mn²⁺:CsPbX₃perovskite nanocrystalsWe studied the luminescence enhancement mechanism of Mn²⁺:CsPbX₃by steady-state and transient fluorescence spectra,and explained the abnormally increased PL intensity and increased exciton lifetime.The Mn²⁺emission is controlled by intermediate traps and the energy transfer rate from exciton to Mn²⁺.Cdion doping corrects the twist of Pb-X bond angle,passivates the deep level defect state at the band edge,which is conducive to the detrapping process,and makes re-formation of thermally activated electron hole pair.In addition,the treatment of Mn²⁺:CsPbCl₃nanocrystals with CdBr₂metal salt can adjust the optical band gap of perovskite.A large number of Cdions enter intrinsic lattice that will lead to a small amount of doping of Cdions into Cd-Pbmetal alloys.Experiments show that the deep valence band position caused by Cdion doping can effectively match the valence band with Mn ⁴T₁→⁶A₁energy level,and improve the exciton-to-Mn energy transfer efficiency.（4）Synthesis and optical properties of two-dimensional Mn doped PEA₂PbBr₄perovskite microcrystalsWe provide a simple strategy for preparing Mn²⁺:PEA₂PbBr₄two-dimensional perovskite at room temperature.The Mn²⁺:PEA₂PbBr₄perovskite microcrystals with high luminous efficiency were achieved by adjusting the doping level of Mn²⁺ions and volume of HBr.The prepared Mn₂₊doped perovskite microcrystals have excellent luminescence properties,the PL quantum efficiency is about 98%,and time-resolved radiative recombination of Mn shows a single exponential decay.The Mn²⁺ion doping content and two-dimensional layered structure were characterized by XRD and SEM.The variable temperature spectra show that the strong and stable Mn²⁺PL in the temperature range of80 K to 360 K.The PL enhancement is due to the surface passivation induced by HBr,and reduced nonradiative defect states.