Study On Near-infrared Luminescence And Enenrgy Transfer Mechanism Of K₂YF₅ Activated By Rare Earth Ions

In order to alleviate energy crisis,reduce pollution and cut the emission of greenhouse gasses,solar energy with renewable,green and pollution-free has attracted wide attention and research.At present,the commercial Mono-or poly-crystalline silicon-based solar cells play a leading role in the photovoltaic market.However,the low photoelectric conversion efficiency is a serious impediment to the commercial production and market application of solar cells.The main reason is that the spectral mismatch between absorption of solar cells and solar spectrum.Photons with energy lower than the band gap of solar cells cannot be utilized at all,and photons with much higher energy than the band gap will lose a huge part of energy through the thermalization of lattice vibrations.One of the effective methods to improve the photoelectric conversion efficiency of silicon-based solar cells is to modify the solar spectrum.The way of spectral modulation by down-conversion luminescent materials is that converting a high-energy ultraviolet or visible solar photon into one or more lower energy near-infrared photons that match the band gap of silicon-based solar cells well,which greatly reduces the energy loss due to non-radiation relaxation and then improves the photoelectric conversion efficiency.Therefore,down-conversion luminescent materials have always been a hot topic of research in various fields.At present,there are quite a few studies on down-conversion and quantum tailoring luminescent materials.The energy transfer between rare earth ions is a common physical phenomenon in the luminescence processes.Meanwhile,the transfer efficiency between rare earth ions directly affects the luminescence efficiency of the prepared luminescent materials.The understandings of the energy transfer mechanisms between rare earth ions in luminescent materials such as Ce³⁺-Yb³⁺,Nd³⁺-Yb³⁺or Ho³⁺-Yb³⁺have not been consistent and clear enough in reported studies.Therefore,it is of great significance to study the energy transfer mechanism between rare earth ions doped in different kinds of hosts.In this thesis,the K₂YF₅ is selected as the host lattice that doped with different rare earth ions,which was prepared by hydrothermal synthesis method.The X-ray powder diffraction（XRD）was used to comfirm the crystal structure.Moreover,the luminescence spectrascopic properties and decay dynamics were investigated.The specific research contents are listed as follows:1.A series of Nd³⁺and Yb³⁺singly and co-doped K₂YF₅ samples were synthesized by hydrothermal synthesis method.By analyzing the excitation and emission spectra and decay curves of Nd³⁺⁴D_3/2 and ²P_3/2levels,the energy transfer mechanism between Nd³⁺and Yb³⁺was studied.The results indicate that Nd³⁺⁴D_3/2 or ²P_3/2 level cannot occur CRET between neighboring Nd³⁺ions,and on the other hand,the electrons will fast non-radiatively relax from Nd³⁺⁴D_3/2 and ²P_3/2 level to ⁴F_3/2 level,resulting dominant energy transfer process through cross-relaxation from Nd³⁺(⁴F_3/2→⁴I_9/2)to Yb³⁺(²F_7/2→²F_5/2).In addition,the potential cross-relaxation energy transfer process between Nd³⁺⁴G_5/2 and Yb³⁺²F_7/2level is also revealed.2.K₂YF₅:1.0%Ho³⁺,x%Yb³⁺（x=0,2.0,5.0,8.0,15.0 and 20.0）samples were prepared by the hydrothermal synthesis method.The energy transfer mechanisms between Ho³⁺-Ho³⁺and Ho³⁺-Yb³⁺were investigated.There have two routes of energy transfer were proposed that one is from Ho³⁺（⁵G₅→⁵F₅）to Yb³⁺(²F_7/2→²F_5/2)and then from Ho³⁺（⁵F₅→⁵I₇）to Yb³⁺(²F_7/2→²F_5/2),another is from Ho³⁺（⁵F₄/⁵S₂→⁵I₆）to Yb³⁺(²F_7/2→²F_5/2).We prove the energy transfer process from Ho³⁺（⁵F₄/⁵S₂→⁵I₆）to Yb³⁺(²F_7/2→²F_5/2)not only depending on the luminescence spectra but also the decay curve.Meanwhile,the back energy transfer process from Yb³⁺²F_7/2 level to Ho³⁺⁵I₆ level was first confirmed by decay curves of the Ho³⁺:⁵I₆→⁵I₈（1198 nm）under 356 nm and 940 nm excitation,respectively.3.The Ce³⁺ions are attractive owing to the large absorption cross section in the ultraviolet and visible region.By analyzing the characteristics of excitation and emission spectra and fluorescence decay curves that varying with concentration and temperature,respectively,the luminescent properties and energy transfer mechanisms of the samples were obtained.The energy transfer occurs from Ce³⁺to Yb³⁺was single step energy transfer via Ce⁴⁺-Yb²⁺change transfer state（CTS）.The values of the average decay lifetime,energy transfer efficiency and activation barrier were also calculated.