Highly efficient near-UV-excitable Ca2YHf2Al3O12:Ce3+,Tb3+ green-emitting garnet phosphors with potential application in high color rendering warm-white LEDs
Inorganic phosphors with broadband near-ultraviolet (near-UV) excitation and efficient visible emissions are highly important for fabricating high-performance near-UV-pumped white light-emitting diodes (LEDs). Herein, we reported on novel efficient near-UV-excitable Ce3+/Tb3+ ions co-activated Ca2YHf2Al3O12 (CYHA) green-emitting garnet phosphors, which were successfully prepared through a high-temperature solid-state reaction process. X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, elemental mapping, photoluminescence, CIE color coordinates, internal and external quantum efficiency, and temperature-dependent emission spectra were used to characterize the samples. Interestingly, due to the efficient energy transfer from Ce3+ to Tb3+ ions in CYHA, the CYHA:Ce3+,Tb3+ samples presented a wide excitation band in the 370-470 nm region peaking around 408 nm owing to the 4f-5d transition of Ce3+ ions, and under 408 nm excitation they showed intense characteristic sharp green emissions around 543 nm corresponding to the 5D4→7F0-6 transitions of Tb3+ ions. The energy transfer mechanism from Ce3+ to Tb3+ ions was ascribed to the quadruple-quadruple interaction, and the energy transfer efficiency reached as high as 93.2%. Notably, the composition-optimized CYHA:0.03Ce3+,0.6Tb3+ sample exhibited high internal quantum efficiency (IQE) of 78.5% and external quantum efficiency (EQE) of 56%, which were much higher than those of the Tb3+ singly-activated CYHA:0.6Tb3+ sample (IQE = 8%, EQE = 2.2%). Finally, a prototype white LED device was made by combining a 400 nm near-UV-emitting LED chip with the phosphor blend of the as-prepared CYHA:0.03Ce3+,0.6Tb3+ green phosphors, commercial BaMgAl10O17:Eu2+ blue phosphors and commercial CaAlSiN3:Eu2+ red phosphors, which emitted bright warm-white light with good CIE chromaticity coordinates of (0.391, 0.356), low correlated color temperature of 3528 K and high color rendering index of 92.3 under 120 mA driven current. This work opens up new opportunities for the development of efficient color converters toward near-UV-pumped warm-white LEDs with high color rendering index.