High-resolution luminescence spectroscopy study of down-conversion routes in NaGdF4:Nd and NaGdF4:Tm using synchrotron radiation

Down-conversion in lanthanide doped luminescent materials is a promising route to significantly enhance the energy efficiency of silicon solar cells, plasma display panels, or mercury-free lighting tubes because it results in the emission of two photons for each absorbed higher energy photon. The Gd3+ /Eu3+ ion couple shows down-conversion of vacuum-ultraviolet light into visible light with an efficiency close to 190%. The low absorption strength of the G7/2 levels of Gd 3+ the starting point of the down-conversion process , however, prevents efficient excitation of the down-conversion process and therefore application. We have performed a high resolution luminescence spectroscopy study, using synchrotron radiation, in order to investigate the possibility to use the strong 4f→5d absorption transitions of Nd3+ and Tm3+ to sensitize the high energy G7/2 level of Gd3+ in the phosphors NaGdF4:2%Nd 3+ and NaGdF4:2%Tm 3+. Tm3+ appears to be an efficient sensitizer of the G7/2 state of Gd 3+. It was also found that sensitization is followed by two successive energy transfer processes exciting two Tm3+ ions in the H4 state which results in the emission of two infrared photons for one absorbed vacuum-ultraviolet photon. Nd3+ is not a good sensitizer of the G7/2 state in NaGdF4. Instead Nd3+ efficiently transfers its energy by cross relaxation to the lower energy DJ states of Gd 3+ but leaving the Nd3+ ion excited in the F3/2 state. Successive energy transfer from Gd 3+ back to Nd3+ excites a second Nd3+ ion in the F3/2 state. Also, in this case, two infrared photons can be emitted for one absorbed vacuumultraviolet photon.