C L E A N E N E R G Y T R A N S I T I O N A N D S U S T A I N A B L E T E C H N O L O G I E S

S C I E N T I F I C H I G H L I G H T S

1 2 0 H I G H L I G H T S 2 0 2 3 I

X-ray spectroscopy probes samarium-doped borates for solar energy applications

Samarium-doped borate luminescent materials show promise for application in luminescent solar concentrators. High-energy-resolution fluorescence- detected X-ray absorption near-edge structure spectroscopy was used to investigate the oxidation state of the dopant ion in different borate structures.

A luminescent solar concentrator (LSC) is a device used as a large-area solar radiation collector that absorbs, converts and emits radiation and directs this radiation to photovoltaic cells located in the small side area of the device. Figure 98a illustrates that a basic LSC consists of a transparent waveguide with embedded luminescent material and a strategically placed photovoltaic cell on the edge. The large area of the waveguide collects a portion of the solar radiation, while the luminescent material absorbs the energy and downshifts it to longer wavelengths. Internal reflection directs the emitted photons towards smaller areas on the sides, where the photovoltaic cells are used to convert the concentrated light into electricity. This results in the concentration of solar energy with a resulting geometric gain since a large collection area is coupled to a limited photovoltaic cell area [1].

LSCs offer several key advantages when compared with conventional solar cells. Since with high geometrical gain only small photovoltaic cells are needed at the sides, high-efficiency and expensive per-unit-area cells can be used. Therefore, developing LSCs can provide an alternative route for cost-effective solar modules with higher efficiencies. This approach also allows energy to be transported optically, providing higher defect tolerances and easing the manufacturing processes requirements and associated costs [2]. Such a device can be developed to be thin and low-cost, which makes it ideal for large- area deployment. In addition to these advantages, LSCs are also excellent candidates for capturing oblique and diffuse radiation, where conventional solar panels do not perform well.

Using inorganic phosphor materials as the luminescent component in LSCs provides an interesting approach that can excel where other luminescent materials currently have limitations. These optically, chemically and thermally robust materials can feature low-cost synthesis, high light-conversion efficiency and spectral tuning. Inorganic phosphors are usually composed of a relatively small amount of dopant ions distributed inside a transparent microcrystalline matrix, also called the host. In most cases, the host is considered a medium for the dopant ions, and the dopant ions are regarded

Fig. 98: a) Edge-on schematics explaining the basic principle of a luminescent solar concentrator. The spheres represent

the luminescent material in the respective oxidation states. In the ideal scenario, the luminescent material absorbs solar

radiation; this energy is transferred, downshifted and emitted in the near-infrared region, and this radiation is guided

towards the edges of the waveguide, where an edge-mounted photovoltaic cell is used for energy-conversion.

b) HERFD-XANES spectrum of the samarium L3 absorption edge for divalent and trivalent samarium ions in strontium

hexaborate (SrB6O10), strontium tetraborate (SrB4O7) and strontium metaborate (SrB2O4) structures.