Luminescent Properties of RE-Doped Mullite (Al6Si2O13) Phosphors for Energy Conversion from UV to Visible Rays

Abstract

Introduction Leveraging of solar energy is a key issue in the world. The world’s energy consumption is increasing year by year due to rise in population and improvement of living quality. For the realization of sustainable development, utilizing alternative energy sources rather than fossil fuels is of critical importance. Using solar cells, such as silicon (Si), is the most popular way for harvesting solar energy. However, light absorbing materials have their intrinsic band gaps, and therefore, the energy range that cells can harvest is limited, which brings power conversion loss. To overcome the problem, combining a solar cell and a phosphor, which is a wavelength conversion material, is important. Here we report UV-excitable mullite (Al6Si2O13) phosphors doped with REs. Luminescence spectra can be tailored from 390 to 620 nm by changing doping rare-earth ions. Experimental Mullite samples were synthesized via a sol-gel process. Al(NO3)3•9H2O, tetraethylorthosilicate (TEOS) and a RE source (or sources) in a form of nitrate or oxide were dissolved in ethanol. Then the pH of the solution was increased to 10 by adding NH3 aq. The precipitate was filtered off by suction filtration and treated at 1400 °C for 6h. For Ce3+, Tb3+ and Eu2+ phosphors, the samples were heat-treated in 5% H2/Ar atmosphere for the reduction of the luminescent centres. For Eu3+ phosphor, the sample was treated in air. Results ＆ discussions Fig. 1 shows the emission spectra of the mullite phosphors doped with Ce3+, Eu2+, Ce3+-Tb3+ or Eu3+. Ce3+ and Eu2+ in mullite show relatively broad peaks around 380 and 410 nm, respectively. These peaks are typically assigned to the 5d-4f transitions in Ce3+ and Eu2+. The Ce3+-Tb3+ phosphor shows green emission with the sharp peaks attributed to the Tb3+ 4f-4f transitions. Absence of an emission peak of Ce3+ indicates the energy transfer from Ce3+ to Tb3+. Red luminescence with the sharp peaks in mullite are attributed to the 4f-4f transitions of Eu3+. Fig. 2 shows the thermal quenching properties of the mullite phosphors compared with commercially available YAG:Ce3+ and SrSiO4:Eu2+ phosphors [2]. The mullite phosphors maintain more than 50% of the initial intensities even at 200 °C. Conclusions We have successfully synthesized red, green and blue phosphors with mullite doped with Ce3+, Eu2+, Ce3+-Tb3+ and Eu3+. They all show good thermal quenching properties, which are comparable with the commercially available phosphors of YAG:Ce3+ and SrSiO4:Eu2+. Figure 1