Li(Sc_0.70Ga_0.24Cr_0.06)Ge₂O₆ in Polylactic Acid for Robust Near-Infrared Phosphor-Converted Light-Emitting Diodes

Abstract

Near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) are promising for various applications, including bioimaging, sensing, and security monitoring. Although there is significant attention to NIR phosphor design, systematic investigations for integrating these phosphors into pc-LED devices are still lacking. Herein, we explore the structure–property relationship of Cr³⁺ ion-activator-based Li(Sc_0.70Ga_0.24Cr_0.06)Ge₂O₆ phosphor and its polymer composites for broadband NIR pc-LED applications. Under blue excitation (470 nm), the phosphor emits broad NIR radiation with a peak at 890 nm and a full width at half maximum of 164 nm (256 meV). Temperature-dependent (6.3–350 K) photoluminescence (PL) spectroscopy provides mechanistic insights into the excitation and emission processes originating from the Cr³⁺ d-d transitions. For pc-LED fabrication, Li(Sc_0.70Ga_0.24Cr_0.06)Ge₂O₆ is embedded into three different polymer matrices, namely, poly(methyl methacrylate), polylactic acid (PLA), and polycarbonate. The optimized pc-LED device is found with PLA, a biodegradable polymer. PLA-based NIR pc-LEDs exhibit an output power of ∼190 mW at 800 mA with a wall-plug efficiency of 15.6% at 15 mA. Importantly, the Li(Sc_0.70Ga_0.24Cr_0.06)Ge₂O₆–PLA pc-LED shows long-term stability under a high driving current and during humidity stress tests (95% RH, 353 K, 250 mA current). Finally, an NIR pc-LED panel containing 88 pc-LEDs was fabricated, demonstrating noninvasive NIR imaging. Our Li(Sc_0.70Ga_0.24Cr_0.06)Ge₂O₆–PLA composite, combining properties of both phosphor and matrix, shows potential for robust and environmentally friendly NIR pc-LEDs.