Photoluminescence And Energy Transfer Of Trivalent Cerium And Terbium Ions In Borate Glasses

Photovoltaic energy conversion,based on the sustainable supply of sunlight and the photoelectric properties of materials,has been studied extensively for several decades.However,the production of solar energy is still expensive because of the low power conversion efficiency of solar cells.Among factors limiting the efficiency of conventional solar cells,a major reason is the battery device can only use the part of sun radiation,most of the ultraviolet and infrared radiation energy is wasted.It was reported that the classical efficiency of silicon-based solar cells could be improved from 30.9%to 38.6%via spectral modification.Down conversion?DC?is a promising approach to enhance the performance of solar cells,which shifts the shorter wavelengths to longer ones as down-shifters.In DC materials,rare earth ions such as Tb³⁺ is often used as activators because of high luminescence quantum efficiency in visible band by preventing the non-radiative dacays.On the other hand,Ce³⁺ ions present a broad UV absorption with a large corss-section due to 4f-5d transitions are allowed by Laporte parity selection rules,and a broad UV-blue emission band,turning them into good candidates for down-shifting process as a sensitizer of Tb³⁺.Among various oxide glasses,the alkaline-earth borate glass system has good chemical stability,uniform optical properties,low melting point and perfect dispersibility of rare earth ions.Furthermore,the RE ions usually have larger absorption and emission cross-sections in borate glasses,so it would be a promising candidate using for DC approach.Based on the above considerations,we synthesized the single doped Ce³⁺,the single doped Tb³⁺ and Ce³⁺/Tb³⁺ co-doped alkaline earth borate glass?RMB?with high temperature solid state melting method.The emission spectra,excitation spectra and energy transfer processes of Ce³⁺ and Tb³⁺ are analyzed by fluorescence spectroscopy.The relationship between the fluorescence emission intensity and doping concentration of Ce³⁺ and Tb³⁺ in the glass is found.The main conclusions are as follows:1.For the Tb³⁺ doped RBM glass with different concentrations under the excitation at 365 nm,four emission bands peaked at 488,543,584 and 622 nm are attributed to the transitions ⁵D₄?⁷F_J?J=6,5,4,3?,and one of the strongest emission green fluorescent is located at 543nm.Under the sane conditions,with the increase of Tb³⁺ cocentration,the green fluorescence increases gradually.It is easy to obtain effective green fluorescence under the excitation of300?380nm UV light range.2.Under the excitation of UV light,the emission spectrum of Ce³⁺ doped RBM glass is not sharp peak,but it is broadband.The Ce³⁺ doped RBM glass in the wavelength range of 280?380nm is easy to be excited,and it will convert high-energy ultraviolet light into visible blue light through the 5d?4f transition emission.Moreover,in a certain concentration range,with the increase of Ce³⁺ concentration,the blue fluorescence intensity of Ce³⁺ doped RBM glass increases.3.Since wide-band emission of Ce³⁺ ions and presence of many excitation lines of Tb³⁺ions in the wavelength range of 340?385nm,the overlapping between Ce³⁺ emission and Tb³⁺excitation occurs,leading to efficient energy transfer process from Ce³⁺ to Tb³⁺ through 5d?4f transition.After Ce³⁺ absorbs high energy ultraviolet light,it transfers the energy of ultraviolet photons to Tb³⁺,which makes Tb³⁺ get a strong emission.4.In the Ce³⁺/Tb³⁺ Co-doped RBM glass,with the intervention of Ce³⁺,the green emissio of Tb³⁺ is obviously enhanced,which shows that Ce³⁺ is a good sensitizer for Tb³⁺.Through the study of 0.02Ce³⁺-xTb³⁺ and 0.05Ce³⁺-xTb³⁺?x=0.5,1,2?co-doped RBM glass under UV excitation,when the concentration of Tb³⁺ is constant,with the increase of Ce³⁺concentration,the fluorescence emission intensity of Tb³⁺ increases.And when the concentration of Ce³⁺ is constant,the emission intensity of Tb³⁺ increases with the increase of Tb³⁺ concentration in a certain range of Tb³⁺ concentration.When the concentration of Tb³⁺ is constant,the emission of 0.05Ce³⁺ doped sample is stronger than that of 0.02Ce³⁺ doped sample.The emission spectra of 0.02Ce³⁺-2.0Tb³⁺ doped and 0.05Ce³⁺-2.0Tb³⁺ doped RBM glass are the strongest by comparing the emission spectra under 308nm excitation.By estimating the energy transfer efficiency of Ce³⁺ and Tb³⁺,when the concentration of Ce³⁺ is certain,with the increase of Tb³⁺ concentration,the energy transfer efficiency increases.These results shows that Ce³⁺/Tb³⁺ co-doped RMB glasses would have promising potential as an efficient UV ? visible radiation conversion layer for use on improving the efficiency of solar cells.