Publication date: 15th December 2025
Rare-earth (RE) ion-doped fluoride nanocrystals, particularly those based on the NaYF4 matrix doped with the Yb3+-Er3+ pair, are highly promising photon recyclers for enhancing solar cell efficiency through up-conversion (UC) luminescence. These materials absorb near-infrared (NIR) photons and emit visible light, mitigating non-absorption losses in solar cells. However, subtle variations in composition and atomic concentration (such as the inclusion of Gd3+ in the matrix) as well as synthesis conditions (such as temperature) have a significant impact on the luminescence output.
We have synthesized a series of NaY80-xGdxF4:Yb18,Er2 nanocrystals using the hydrothermal method, systematically varying the Y/Gd ratio content (x = 0%-80%) and the synthesis temperature. Prior structural (XRD, SEM, TEM/EDX) and optical (PL, CL) analysis have revealed a strong structural modification (cubic-to-hexagonal transition) and changes in particle morphology and surface-to-volume ratio, which correlate with the observed shifts in the red/green UC emission ratio. Specifically, the ratio of red (650 nm) to green (540 nm) luminescence bands decreases with increasing Gd content/decreasing particle size. This suggests a crucial dependence of the Yb3+-Er3+ energy transfer (ET) mechanism on the local coordination environment and atomic disposition near the surface.
To precisely elucidate this correlation, we have performed a comprehensive study using Synchrotron X-ray Absorption Spectroscopy (XAS). Specifically, EXAFS measurements at the Y-K edge (17.0 keV) and Gd-L3 edge (7.2 keV) to determine the local structure and relative bonding distances as a function of Gd doping, as well as XANES analysis at the Er-L3 edge (8.3 keV) to try to clarify the electronic properties and oxidation state of this activator ion. By correlating these high-resolution data with the luminescence output, we aim to identify the optimal composition and growth parameters for maximizing up-conversion efficiency, thereby paving the way for the functional integration of this material into next-generation photovoltaic devices.
UC3M project-2024/00734/001 "Ayudas para la Actividad Investigadora de los Jóvenes Doctores, Programa Propio de Investigación de la UC3M”.
These experiments were performed at BL22 – CLAESS beamline at ALBA Synchrotron with the collaboration of ALBA staff.
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