Preparation,Luminescent Properties And Energy Transfer Mechanism Of Near-infrared Downconversion Luminescent Materials

Due to the depletion of fossil energy,the energy crisis has attracted much attention,so the development of new clean energy has become an urgent task.Solar energy is expected to be a substitute for fossil energy because of its inexhaustible features.However,the practical utilization efficiency of solar energy is not very high.The most popular commercial solar cell energy conversion efficiency is only about 18.5%.Therefore,enhancing the photoelectric conversion efficiency of solar cells has become the research focus.The main reason for the low conversion efficiency of solar cell is the spectral mismatch.Based on this,improving the efficiency of solar cells through spectral modification has received widespread attention.Quantum cutting,as an effective spectral modification method,can convert a high-energy photon into two or more near-infrared low-energy photons which can be efficiently absorbed by silicon solar cells and consequently enhance the conversion efficiency of solar cells.This thesis aims to study the possibility of improving the energy conversion efficiency of silicon-based solar cells through spectra modification without changing the structure of solar cells.In the first part,the transparent down-converting luminescent glass doped with Tm³⁺and Yb³⁺was prepared by melt-quenching method.This glass can absorb high-energy photons which located in the short-wave region of 250-370 nm and 465 nm,and then emitting 920-1130 nm near-infrared photons which match well with the silicon solar cells.Under the 465 nm laser excitation,the energy transfer mechanism can be expressed as Tm³⁺?¹G₄?+2Yb³⁺(²F_7/2)?Tm³⁺?³H₆?+2Yb³⁺(²F_5/2).The near-infrared down-conversion process were demonstrated.The maximum quantum efficiency of Tm³⁺to Yb³⁺was calculated to be 150.3%.In the second part,the down-conversion luminescence properties and energy transfer mechanism of Ho³⁺/Yb³⁺doped with Ca₃Ga₂Ge₃O₁₂ were discussed.Under 450nm excitation,Ho³⁺can transfer energy to Yb³⁺through cross-relaxation process CR1:Ho³⁺?⁵S₂,⁵F₄?+Yb³⁺(²F_7/2)?Ho³⁺?⁵I₆?+Yb³⁺(²F_5/2),and then ⁵I₆ and ²F_5/2/2 emitting two different wavelengths by radiative transition,respectively.There may also be a two-step energy transfer process in the system.The first step is CR2:Ho³⁺?⁵F₃?+Yb³⁺(²F_7/2)?Ho³⁺?⁵I₅?+Yb³⁺(²F_5/2);The second step CR3:Ho³⁺?⁵I₅?+Yb³⁺(²F_7/2)?Ho³⁺?⁵I₈?+Yb³⁺(²F_5/2).In addition,there is the energy transfer process of CR4:Ho³⁺?⁵F₅?+Yb³⁺(²F_7/2)?Ho³⁺?⁵I₇?+Yb³⁺(²F_5/2).At the same time,we also observed back energy transfer from Yb³⁺to Ho³⁺,BET1:Yb³⁺(²F_5/2)+Ho³⁺?⁵I₈??Yb³⁺(²F_7/2)+Ho³⁺?⁵I₆?and BET2 process:Yb³⁺(²F_5/2)+Ho³⁺?⁵I₆??Yb³⁺(²F_7/2)+Ho³⁺?⁵S₂?.In the third part,the near-infrared quantum cutting process of Tb³⁺as a sensitizer and Yb³⁺as an activator co-doped in SrMoO₄ was studied.The Tb³⁺ions are excited up to the ⁵D₄ state after the absorption of 489 nm photons,which is followed by spontaneous ⁵D₄?⁷F₅,⁷F₄,⁷F₃ emission and cooperative energy transfer of Tb³⁺?⁵D₄??2Yb³⁺(²F_5/2).The energy transfer process is expressed as:Tb³⁺?⁵D₄?+2Yb³⁺(²F_7/2)?Tb³⁺?⁷F₆?+2Yb³⁺(²F_5/2).The near-infrared?NIR?emission around 1003 nm from Yb³⁺were observed,and the downconversion was realized.Finally,the maximum quantum efficiency of Tm³⁺to Yb³⁺was calculated to be 182.2%.In the fourth part,the down-conversion luminescence and energy transfer of Tm³⁺,Yb³⁺co-doped SrMoO₄ were investigated.The strong visible emission around 473 nm from Tm³⁺?¹G₄?³H₆?and near-infrared?NIR?emission around 998 nm from Yb³⁺(²F_5/2?²F_7/2)of the SrMoO₄:Tm³⁺,Yb³⁺phosphors wer observed under 283nm excitation.When Tm³⁺and Yb³⁺doping concentration is 1%and 10%,the NIR emission intensity is strongest.The quenching concentration of Yb³⁺approaches 10mol%,which is higher than the level in other DC materials.The CET and phonon-assisted energy transfer are discussed as possible mechanisms for the NIR emission.In addition,according to the diffuse reflection spectra and CASTEP calculations,the direct character of band gap of SrMoO₄ was estimated.