Development of high-performance NIR-to-SWIR down-shifting Li$_3$Ba$_2$Gd$_3$(MoO$_4$)$_8$ :Yb$^{3+}$ /Tm$^{3+}$ and Yb$^{3+}$ /Ho$^{3+}$ for phosphor conversion LEDs

Abstract

High-performance short-wave infrared (SWIR) phosphors are of increasing interest for phosphor-converted light-emitting diodes (pc-LEDs) used in sensing, imaging, and analytical applications. In this work, Li$_3$Ba$_2$Gd$_3$(MoO$_4$)$_8$ (LBGMO) phosphors co-doped with Yb$^{3+}$/Tm$^{3+}$ and Yb$^{3+}$ /Ho$^{3+}$ were developed as efficient near-infrared (NIR)-to-SWIR down-shifting (DS) materials. Phase purity and crystal structure were confirmed by X-ray diffraction, demonstrating successful incorporation of rare-earth ions into the LBGMO host lattice. Under 940 nm excitation, efficient sensitization by Yb$^{3+}$ ions enables strong SWIR emission from both activatorions. The Yb$^{3+}$/ Tm$^{3+}$ system exhibits broadband emission centered at approximately 1800 nm, originating from the Tm$^{3+}$: $^3$F$_4$ → $^3$H$_ 6$ transition, whereas the Yb$^{3+}$/Ho$^{3+}$ system shows dominant emission near 2050 nm associated with the Ho$^{3+}$ : $^5$I$_7$ → $^5$I$_8$ transition. High photoluminescence quantum yields (PLQY) of 63% and 69% were achieved for the Yb$^{3+}$/Tm$^{3+}$ and Yb$^{3+}$/Ho$^{3+}$ phosphors, respectively, accompanied by efficient energy transfer from Yb$^{3+}$ to Tm$^{3+}$ (95%) and Ho$^{3+}$ (96%). The moderate cut-off phonon energy of 930 cm$^{-1}$ supports efficient population of the lower-energy SWIR-emitting states, while the rate of non-radiative relaxation remains sufficiently low to prevent effective competition with radiative transitions. Phosphor-converted LED devices using 940 nm excitation and fabricated with the developed phosphor conversion layers exhibited maximum wall-plug efficiencies of 1.6% for Yb$^{3+}$ /Tm$^{3+}$ and 3.6% for Yb$^{3+}$ /Ho$^{3+}$ . Demonstration of SWIR imaging through visibly opaque materials and spectral overlap with water absorption bands confirms the potential application of these materials.