An up-conversion luminophore with high quantum yield and brightness based on BaF2_{2}:Yb3+^{3+},Er3+^{3+} single crystals

Up-conversion (UC) of near-infrared radiation to visible light has received much attention because of its use in the conversion of solar radiation, luminescence thermometry, biosensing, and anti-counterfeiting applications. However, the main issue hindering the successful utilization of UC is the relatively low quantum efficiency of the process. In order to design new UC systems with high quantum yield (ϕ$_{UC}$) values, we synthesized two series of co-doped BaF$_{2}$ single crystals with nominal concentrations of Yb$^{3+}$ (2–15 mol%)/Er$^{3+}$ (2 mol%) as well as Yb$^{3+}$ (3 mol%)/Er$^{3+}$ (2–15 mol%). The highest ϕ$_{UC}$ value of 10.0% was demonstrated for the BaF$_{2}$:Er$^{3+}$ (2 mol%) and Yb$^{3+}$ (3 mol%) sample under 490 W cm$^{-2}$ of 976 nm excitation. To study the natural limit of UC efficiency, quantum yield values upon direct excitation (ϕ$_{DS}$) of the $^{4}$S$_{3/2}$ (ϕ$_{DS}$ ≤ 26%) levels were measured. Comparison of experimental values of quantum yields to the ones obtained using Judd–Ofelt theory reveals strong quenching of the $^{4}$S$_{3/2}$ state for all investigated compositions. In addition, we observed an unusually strong contribution of the Er$^{3+}$:4I$_{9/2}$ excited state to both UC and down-shifting luminescent processes. This contribution becomes possible due to the very low maximum phonon energy of BaF$_{2}$ crystals (240 cm$^{-1}$)

Simultaneous measurement of the mission quantum yield and local temperature: The illustrative example of SrF2:Yb3+/Er3+ Single Crystale

The emission quantum yield is one of the key figures of merit to evaluate the photoluminescence performance of luminescent materials. The emission quantum yield of upconverting materials is still not widely reported due to technical difficulties and intricate dependence on the excitation power density that is mirrored in a temperature increase. This work describes the simultaneous determination of the emission quantum yield (for both downshifting and upconverting processes) and of the temperature by using the output of a commercial integrating sphere. The temperature is calculated by primary luminescence thermometry through the Boltzmann equation, analyzing the intensity ratio between the 2H11/2, 4S3/2→4I15/2 transitions. The procedure is illustrated using of SrF2: Yb3+/Er3+ single crystals with distinct Yb3+ compositions and the effect of the Yb3+ content on the emission quantum yield and the temperature increase of the sample.publishe

Spectral and Cathodoluminescence Decay Characteristics of the Ba_1−xCe_xF_2+x (x = 0.3–0.4) Solid Solution Synthesized by Precipitation from Aqueous Solutions and Fusion

Single-phase samples of the Ba1−xCexF2+x solid solution (x = 0.3–0.4) were synthesized by directional crystallization in the form of single crystals and by co-precipitation from aqueous nitrate solutions using potassium fluoride as a fluorinating agent in the form of nanopowders. The cathodoluminescence of the pressed powder samples was studied in comparison with the BaF2: Ce single crystals in 250–460 nm (2.7–5 eV) spectral range upon excitation by an electron accelerator. The cathodoluminescence spectra of the samples revealed a wide band in the range of 3.0–4.0 eV, which consists of two typical components of Ce3+ with decay time 23 ns in the case of single crystals and three decay times 27 ns, 140–170 ns, and ~600 ns in the case of pressed powders. The decay time of the short-wavelength component (27 ns) in the case of pressed powders is close to the lifetime of the excited state of the Ce3+ ion. The developed X-ray phosphors can be applied for embedding in diamonds for diamond–nanoparticle composite preparation

Harvesting Sub-bandgap Photons via Upconversion for Perovskite Solar Cells

Lanthanide-based upconversion (UC) allows harvesting sub-bandgap near-infrared photons in photovoltaics. In this work, we investigate UC in perovskite solar cells by implementing UC single crystal BaF2:Yb3+, Er3+ at the rear of the solar cell. Upon illumination with high-intensity sub-bandgap photons at 980 nm, the BaF2:Yb3+, Er3+ crystal emits upconverted photons in the spectral range between 520 and 700 nm. When tested under terrestrial sunlight representing one sun above the perovskite’s bandgap and sub-bandgap illumination at 980 nm, upconverted photons contribute a 0.38 mA/cm2 enhancement in the short-circuit current density at lower intensity. The current enhancement scales non-linearly with the incident intensity of sub-bandgap illumination, and at higher intensity, 2.09 mA/cm2 enhancement in current was observed. Hence, our study shows that using a fluoride single crystal like BaF2:Yb3+, Er3+ for UC is a suitable method to extend the response of perovskite solar cells to near-infrared illumination at 980 nm with a subsequent enhancement in current for very high incident intensity