Pushing the Limits: Down-Converting Er³⁺-Doped BaF₂ Single Crystals with Photoluminescence Quantum Yield Surpassing 100%

Down-conversion (DC) is a phenomenon that can enable the observation of photoluminescent quantum yield (PLQY) values exceeding 100%. A comprehensive study of the DC properties of BaF2-based single crystals with different Er3+ doping levels (1-25 mol.%) is presented. The samples exhibit a PLQY of 110% in the 1550-1650 nm under 405 nm excitation. This remarkable PLQY is attributed to a cross-relaxation process within the energy levels of Er3+. Furthermore, when considering all emitted photons in the 1000-1650 nm range, the PLQY reaches 153% for a sample doped with 5 mol.% of Er3+. By integrating the emission of this single crystal with a Ge diode, serving as an example of a photovoltaic device sensitive to short-wave infrared light, a significant enhancement in short-circuit current is demonstrated. In a broader context, the presented material holds promise for improving the spectral response of low-bandgap photovoltaic devices.