Near-Infrared Luminescence And Enenrgy Transfer Processes In LaOF:Ln³⁺,Yb³⁺?Ln=Nd,Pr?

At present,the commercial single-junction mono-and poly-crystalline Si solar cells dominate the photovoltaic market.However,the crystalline Si wafer cells are characterized by relatively low efficiencies,which is considerably below the theoretical limits based on Shockley-Queisser theory and below the efficiencies that have been obtained in the laboratory.The energy losses mainly originate from two reasons: first,a huge part of the energy of photons with energy larger than the band gap of Si is wasted through thermalization of charge carriers;second,the photons with energy lower than the bandgap of Si can not be absorbed by solar cells.Among the different approaches to improve the conversion efficiency of sunlight to electricity in solar cells,down-conversion or quantum cutting which means one high energy photon is converted into two low energy photons,is one of the interesting ways proposed to reduce the energy losses.At present,many down-conversion and quantum cutting luminescent materials have been reported,but the energy transfer processes of Nd³⁺/Yb³⁺ and Pr³⁺/Yb³⁺ are not unified and clearly understood.In this paper,we chose lanthanum oxyfluoride?LaOF?as the host lattice,and doped with Pr³⁺,Nd³⁺,Yb³⁺ and Bi³⁺,which were synthesized via a hydrothermal synthesis method.The phase purity of them was examined by the X-ray powder diffraction,and the concentration dependence of luminescence and decay properties of LaOF were investigated.The mechanisms of energy transfer processes between rare earth ions and the host were also analyzed.The primary results obtained are listed as follow:1.Nd³⁺ and Yb³⁺ singly and codoped LaOF were prepared by hydrothermal synthesis method.Cross-relaxation energy transfer from Nd³⁺(⁴D_3/2?⁴G_5/2)to Yb³⁺(²F_7/2?²F?5/2?)was confirmed not only based on the excitation spectra,but also by the decay property of Nd³⁺⁴D_3/2 state..However,the dominant process was multi-phonon relaxation from Nd³⁺ higher levels to ⁴F_3/2,and followed by energy transfer toYb³⁺²F_5/2.In addition,host energy transfer to Yb³⁺ and back energy transfer from Yb³⁺(²F_5/2?²F_7/2)to Nd³⁺(⁴I_9/2?⁴F_3/2)were also revealed.2.In order to improve the absorption coefficient of LaOF:Nd³⁺,Yb³⁺ in the energy range of near ultraviolet,Bi³⁺ ion was codoped and the luminescent properties and energy transfer processes were studied.From the excitation spectra and the emission spectra of LaOF:Bi³⁺,Nd³⁺,energy transfer from 1S0 state of Bi³⁺ to Nd³⁺ was revealed.The emission spectra of LaOF:Bi³⁺,Nd³⁺,Yb³⁺confirm the energy transfer from ¹S₀ state of Bi³⁺ to Yb³⁺ through Nd³⁺.The excitation spectra and emission spectra of Yb³⁺ indirectly confirm the energy transfer of Bi³⁺ to Yb³⁺,but the energy transfer processes are not clearlyunderstood.3.Pr³⁺ and Yb³⁺ singly and codoped LaOF were prepared by hydrothermal synthesis method.The excitation and emission spectra and decay times of LaOF:Pr³⁺,Yb³⁺ were measured and the energy transfer between Pr³⁺ and Yb³⁺were investigated.The energy transfer from Pr³⁺?3P0?1G4?to Yb³⁺(²F_7/2?²F_5/2)was confirmed not only based on the excitation spectra,but also by the decay times of Pr³⁺ 3P0 state.In addition,the excitation of LaOF:Pr³⁺ and LaOF:Pr³⁺,Yb³⁺ confirm the energy transfer from host and Pr³⁺?3P0 and 4f5d?to Yb³⁺.