Synthesis, crystal structure, and characterizations of Eu3+-activated Ca2LnHf2(AlO4)3 (ln = Lu, Y and Gd) red-emitting phosphors for phosphors-converted white LEDs

Abstract

High-brightness red-emitting Ca2LnHf2Al3O12:Eu3+ (Ln = Lu, Y and Gd) garnet phosphors with high photoluminescence efficiency are demonstrated for application in white light-emitting diodes (LEDs), and they have been successfully prepared using a high-temperature solid-state method. X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, elemental mapping, photoluminescence excitation and photoluminescence spectra, decay lifetimes, photoluminescence quantum yield and temperature-dependent emission spectra have been used to characterize these samples. Under near-ultraviolet excitation into the 7F0→5L6 transition of Eu3+ at 394 nm, the Ca2LnHf2Al3O12:Eu3+ (Ln = Lu, Y and Gd) samples produce the characteristic red emissions of Eu3+ ions with peaks around 591, 611, 656, and 709 nm, corresponding to the 5D0→7F1, 5D0→7F2, 5D0→7F3, and 5D0→7F4 transitions. These samples exhibit significant concentration-dependent luminescence behaviors, and the highest emission intensity is achieved at 50 mol% Eu3+ doping concentration owing to the concentration quenching effect caused by the dipole-dipole interaction between neighboring Eu3+ activators. The CIE color coordinates of these optimal Ca2LnHf2Al3O12:50%Eu3+ (Ln = Lu, Y and Gd) phosphors are (0.6339, 0.3654), (0.6341, 0.3653), and (0.6343, 0.3651), along with respective color purity of 92.4%, 92.6%, and 92.6%. The photoluminescence quantum yields of Ca2LnHf2Al3O12:50%Eu3+ are determined to be 39.2%, 56.6%, and 53.3% for Ln = Lu, Y, Gd, respectively. A white LED device has been fabricated through utilizing the Ca2YHf2Al3O12:50%Eu3+ phosphor as red-emitting color converter, which generates bright warm-white light with high color rendering index (80.8), low correlated color temperature (3920 K), CIE chromaticity coordinates (0.3827, 0.3742), and high luminous efficacy (11.56 lmW−1) under 40 mA driving current.