A layer-stacking structure enabling K2TiF6:Mn4+ platelet crystals for inorganic high-power WLED packaging

Abstract

Conventional high power white LEDs (WLEDs) rely on the coating of micrometer-sized phosphor powder blends onto LED chip arrays using organic resins, which suffer from thermal accumulation and yellowing of the resin. Bulk phosphor crystals with a platelet morphology, geometrically compatible with chip surfaces, offer a solution to eliminate organic encapsulants. Red-emitting fluoride ceramics have been prepared, but no platelet crystals have been reported. Herein, the hexagonal K₂TiF₆:Mn⁴⁺ was specifically selected for its pronounced layered stacking of [TiF₆]²⁻ octahedral sheets perpendicular to the c-axis, favoring the growth of platelet crystals. Red-emitting platelet crystals up to 5 mm were successfully grown via a cooling-induced crystallization method, and the effects of precursor saturation, cooling protocols, and Mn⁴⁺ doping level on the crystal quality and photoluminescence were investigated. The optimized crystals exhibit internal and external quantum efficiencies of 90.6% and 70.2%, respectively, along with markedly enhanced moisture resistance compared to the powder counterpart. These K₂TiF₆:Mn⁴⁺ platelet crystals were then packaged onto high-power blue LED chips together with Y₃Al₅O₁₂:Ce³⁺-corundum ceramic plates in a pseudo all-inorganic manner that eliminates organic binders. The resulting WLEDs achieve a color-rendering index (R_a) of 85, an R₉ value of 42, a correlated color temperature of 3183 K, and a luminous efficacy of 79.3 lm W⁻¹. This work establishes a platelet crystal-growth strategy toward organic binder-free phosphor-converted WLEDs.