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PRINCIPAL PUBLICATION AND AUTHORS

Making Eu2+- and Sm2+-Doped Borates Fit for Solar Energy Applications, L.J.B. Erasmus (a,b), P.F. Smet (b), R.E. Kroon (a), D. Poelman (b), J.J. Terblans (a), J.J. Joos (b), D. Van der Heggen (b), H.C. Swart (a), ACS Photon. 10, 609-622 (2023); https:/doi.org/10.1021/acsphotonics.2c01571 (a) Department of Physics, University of the Free State, Bloemfontein (South Africa) (b) Department of Solid State Sciences, Ghent University, Ghent (Belgium)

REFERENCES

[1] S. Castelletto & A. Boretti, Nano Energy 109, 108269 (2023). [2] C. Li et al., Sci. Rep. 5, 1-9 (2015). [3] E.M.J. Weber et al., PHOSPHOR Physics and Chemistry of Photochromic Glasses, 2nd ed. CRC Press (1998).

as the centres for luminescence [3]. An appropriate choice of dopant ions and the composition of the host can minimise self-absorption, which is a prerequisite for realising large-area LSCs beyond the typical sizes in laboratory studies. Depending on the host material, divalent samarium (Sm2+) ions can address the optical requirements for this specific application. The challenge, however, is to reduce the amount of trivalent samarium (Sm3+) ions in the materials, which cause a decrease in the photoluminescence efficiency.

This work aimed to identify a suitable host for these ions and, by empirical methods, optimise the phosphor material for application in solar technology. One of the main functions of the host material is to stabilise the dopant ions. Strontium borate materials are viable due to their unique ability to stabilise some lanthanide dopants in a divalent oxidation state. Strontium borates are also of interest due to their wide bandgap (>7 eV), ensuring little optical and electronic interference with Sm2+ ions. Therefore, over a wide energy range, the excitation and emission properties are driven by the dopant ions only.

High-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy was used at beamline ID26 to obtain quantitative information on the oxidation state of the Sm ions within the different structures in order to explore the effect of different strontium borate host materials to stabilise the Sm ions in the divalent oxidation state. As seen in Figure 98b, there was a substantial increase in the fraction of Sm2+ ions in the strontium hexaborate (SrB6O10) host material when compared to strontium tetraborate (SrB4O7) and strontium metaborate (SrB2O4). Therefore, the SrB6O10 structure is the resulting optimised material.

Further experiments using a combination of techniques showed that this material has a broad excitation region ranging from 220 to 600 nm and exhibited strong and narrow emission lines in the region from 650 to 850 nm. Also, the significant Stokes shift reduced the probability of reabsorption. An internal luminescence quantum efficiency of 79% (λex = 508.5 nm) was measured. All these characteristics make this phosphor material a promising candidate for solar conversion and LSC applications.