Down-conversion Luminescence In Rare-earth Ions Doped LuBO₃Phossphor

With the development of science and technology, energy resource andenvironmental pollution have become a global problem. As a clean andpollution-free green sustainable energy, solar energy has attracted great attention.While photovoltaic solar cells, as an efficient photovoltaic device, will provide abroader platform for the effectively utilizing solar energy. However, thephotovoltaic efficiency of the silicon solar cells which dominate the presentphotovoltaic technologies is only up to a maximal29%(Shockley-Queisser limitefficiency) in theory and the actual conversion efficiency is only15%. Onemajor issue leading to low energy efficiency of the c-Si solar cells is themismatch between the incident solar spectrum and the spectral absorptionproperties of the c-Si semiconductor. The silicon semiconductor has a fixed bandgap (1.12eV), which can only absorb sunlight whose energy is greater than theenergy gap of c-Si semiconductor (λ <1100nm). Therefore, the extra energy iswasted as heat within the solar cell (thermalization loss), which will also reducethe efficiency of the silicon solar cells. To improve silicon solar cellsphotovoltaic efficiency, the spectral modification is being considered as efficientapproach. Among the methods of spectral modification, down-conversion, which can convert one photon with high energy into two photons with lowenergy is a promising approach to achieve the spectral modification because theluminescent efficiency of down-conversion can be up to200%. Therefore, inthis paper, The LuBO3: x%Ce3+, y%Tb3+, z%Yb3+phosphors were prepared bythe hydrothermal method and near infrared quantum cutting processes werestudied by scanning electron microscopy (SEM), X-ray diffraction, spectral andfluorescence decay curves in detailed. The findings is that under UV excitation,there are three kinds of energy transfer:Tb3+â†'Yb3+, Ce3+â†'Yb3+and Ce3+â†'Tb3+,The main contents in this paper are summarized as below:First of all, We have investigated the downconversion luminescence ofTb3+，Yb3+codoped LuBO3phosphor. Upon excitation of286nm UV light,efficient energy transfer is observed from Tb3+to Yb3+, which results in visibleemissions due to Tb3+:5D4â†'7FJ(J=6,5,4,3) transitions and near-infraredemission (900nm-1100nm) due to the Yb3+:2F5/2â†'2F7/2transition. Theexcitation peak located at286nm can be observed in all the samples bymonitoring the emissions of Tb3+and Yb3+ions. Energy transfer from Tb3+totwo neighbouring Yb3+ions occurs via cooperative energy transfer. And themeasured maximal near-infrared quantum efficiency is160.74%.LuBO3with different Ce3+, Yb3+doping concentration were synthesized byhydrothermal method. Evidence is provided by spectroscopic measurements toconfirm the occurrence of quantum cutting for the first time. Upon excitation ofCe3+5d level, near-infrared quantum cutting could occur from Ce3+to Yb3+by cooperation energy transfer. In this process, Ce3+absorded a high photon withenergy range in300–400nm and transfered the excited energy to two nearbyYb ions, leading to the emission of two low energy photons around970nm. Byanalyzing the Ce3+,and Yb3+energy level diagram, the energy transfer betweentwo ions is cooperation energy transfer, namely: Ce3+:5dâ†'Yb3+:2F5/2+Yb3+:2F5/2.Ce3+, Tb3+co-doped LuBO3phosphors were prepared by hydrothermalmethod. The emission spectrum of sample LuBO3:1%Ce3+upon370nm (Ce3+:4fâ†'5d) ultraviolet excitation overlaps with the excitation spectra of the sampleLuBO3:6%Tb3+monitored by543nm green light. As the proportion of Tb3+was increased, an energy shift from Ce3+to Tb3+was observed in the PL spectraof samples. We attributed the energy transfer process to be resonance energytransfer. From the emission of the LuBO3:1%Ce3+, x%Tb3+, the efficiency ofenergy transfer from Ce3+to Yb3+can be calculated. An efficiency of84.6%wasachieved in sample with6mol%Tb3+doping.On the basis of the above, we make an attempt to enhance thedownconversion luminescence of Ce3+, Yb3+codoped LuBO3phosphors byintroducing Tb3+ion. The photoluminescence properties of LuBO3: Ce3+, Tb3+,Yb3+phosphor are studied. Blue, green, and near-infrared emission wereobserved in PL spectra of Ce3+, Tb3+codoped LuBO3phosphors. Theobservation of Ce3+absorption bands in the excitation spectra of Yb3+confirmedthe presence of energy transfer from Ce3+to Yb3+ions.Two efficient energy-transfer processes of Ce3+â†'Tb3+direct resonant energy-transfer andsubsequent Tb3+â†'Yb3+cooperative energy-transfer were employed to realize theenhanced downconversion luminescence.