Far-red-emitting Cr3+-doped CaAl12O19 phosphors with excellent color purity and good quantum efficiency for plant growth LEDs

Abstract

Single-phase far-red-emitting CaAl12O19:x%Cr3+ (x = 0.1 – 3.0 mol%) phosphors were successfully synthesized by a sol-gel method. Besides a small absorption peak at 263 nm, the phosphors have two broad excitation bands in the UV-blue (350 – 475 nm) and green-red (500 – 600 nm) regions, centered at 414 and 573 nm, respectively. They give a broad emission band in the 650 – 800 nm range and several narrow emission peaks at 665, 688, 695, 700, and 709 nm under violet (414 nm) and orange (573 nm) excitations. The optimum annealing temperature and doping concentration of Cr3+ ions are 1500 °C and 0.3%, respectively. Further, the optimized CAO: 0.3%Cr3+ phosphor exhibits an excellent color purity of 100%, activation energy of 0.4 eV, a long lifetime of 4.2 ms, and good quantum efficiency of 46.2%. A prototype of plant growth LED with the CIE coordinate of (x = 0.3050; y = 0.1264) was fabricated by coating CaAl12O19:0.3%Cr3+ phosphor on a violet (410 nm) LED chip. The obtained EL spectrum of plant growth LED is well-matched with the far red phytochrome (PFr) absorption spectra, demonstrating that CaAl12O19:Cr3+ is a potential far-red-emitting phosphor for violet-pumped plant growth LEDs.