Boosting the Brightness of Short-Wave Infrared Emission in YPO₄ :Yb³⁺/Er³⁺ Phosphors: Optimal Photoluminescence Quantum Yield versus Particle Size

The use of luminescent tracers in plastic recycling presents a novel application opportunity for classical phosphor materials, such as co-doped YPO₄. In this study, we report the optimization of the photoluminescence quantum yield (PLQY) of YPO₄:Yb³⁺/Er³⁺ phosphors via a flux-assisted solid-state synthesis approach. Upon excitation of Yb³⁺ ions at 940 or 980 nm, efficient energy transfer to Er³⁺ ions enables strong emission at 1540 nm, with a maximum PLQY of 78% achieved under optimized synthesis conditions. This performance was obtained by annealing the phosphor at 1100 °C for 12 h in the presence of LiCl flux. Notably, a reduced synthesis temperature of 1000 °C and a much shorter annealing time of 3 h still yielded a high PLQY (72%) when the flux was present. To demonstrate practical applicability, the phosphors were integrated into two model systems: (1) dispersion of 300 ppm phosphor in transparent silicone (emulating a bulk polymer), and (2) surface printing on polyethylene foil with a loading of 10 µg cm⁻² (emulating a label). In both cases, the measured brightness was significantly lower than that of a commercial Y₂O₂S:Yb³⁺/Er³⁺ phosphor, despite its much lower PLQY of only 7%. This discrepancy was attributed to the non-optimal particle size distribution of the YPO₄ phosphor, which induced non-optimal scattering, absorption, and emission losses in both demonstrator matrices. After optimizing particle size via dry milling, the luminescence performance of the YPO₄-based phosphor surpassed that of the commercial reference in both configurations, confirming its suitability for use in luminescent tagging of plastics.