Yellow-Orange-Emitting CaAlSiN₃:Ce³⁺ Phosphor: Structure, Photoluminescence, and Application in White LEDs

Abstract

The crystal structure and photoluminescence properties of undoped and Ce3+-doped CaAlSiN3 as well as the application of white-light LEDs are reported. CaAlSiN3 and CaAlSiN3:Ce3+ have been synthesized, starting from Ca3N2, AlN, Si3N4, and CeN or CeO2 with and without Li3N, by a solid state reaction at 1700 °C for 4 h under high purity nitrogen atmosphere. Instead of an ideal CaAlSiN3, a more appropriate formula is proposed to be CaAl1−4δ/3Si1+δN3 (δ ≈ 0.3−0.4) with an Al/Si ratio of about 1:2 on the basis of the bond valence sum calculations, in which Al/Si is disorderly occupied on the 8b site within Cmc21 space group. Ce3+ can be incorporated into the host lattice of CaAlSiN3, and the estimated maximum solubility of Ce3+ is about x = 0.02 (e.g., 2.0 mol % with respect to Ca) of Ca1−2x Ce x Li x AlSiN3. CaAlSiN3:Ce3+ can be efficiently excited by blue light (450−480 nm) and yields yellow-orange emission with a broadband peaking in the range of 570−603 nm, originating from the 5d1 → 4f1 transition of Ce3+. With an increase of Ce concentration, the emission band of Ce3+ shifts to longer wavelengths due to the increased Stokes shift corresponding to structural relaxation and energy transfer of Ce3+. Upon excitation in blue light range (450−480 nm), the absorption and external quantum efficiency are about 70% and 56%, respectively, for both Ca1−2x CexLixAlSiN3 and Ca1−x CexAlSiN3−2x/3O3x/2 at x = 0.01. In addition, Ca1−2x Ce x Li x AlSiN3 and Ca1−x Ce x AlSiN3−2x/3O3x/2 show high thermal stability in air with the quenching temperature above 300 °C for x = 0.01. Using single CaAlSiN3:Ce3+ as the wavelength conversion phosphor combined with a blue InGaN LED-chip (450 nm), warm white-light LEDs can be generated, yielding the luminous efficacy of about 50 lm/W at color temperature 3722 K and the color rendering index (Ra) of 70, which demonstrates that CaAlSiN3:Ce3+ is a highly promising yellow-orange phosphor for use in white-light LEDs.

Photoluminescence Properties of Red-Emitting Mn2+-Activated CaAlSiN3Phosphor for White-LEDs

“…The value of the bandgap is close to the results published. 10,39 The intense reflection in the visible spectral range is in agreement with the observed gray-white daylight color of undoped CaAlSiN 3 Additionally, the absorption edge of CaAlSiN 3 : Mn 2+ shifts to longer wavelength, corresponding to the decrease in the optical bandgap with increasing Mn 2+ concentration. According to the band and electronic structure calculation results, the bottom of the conduction band (CB) of CaAlSiN 3 is mainly composed of Ca 3d orbitals, and Al/Si 3s/3p orbitals are hybridized at higher energy, while the top of valence band (VB) are dominated by the 2p states of N atoms.…”

“…It was found that Al/Si disorderly occupies the 8b site in the space group of Cmc2 1 with a ratio of about 1/2 with a more appropriate formula CaAl 1-4δ/3 Si 1+δ N 3 (δ = 0.3 -0.4) instead of 1/1 proposed for an ideal CaAlSiN 3 , probably due to the incomplete reaction resulting in a significantly lower Al solubility in CaAlSiN 3 . 10 Thus, a small amount of AlN can be detected for undoped and Mndoped CaAlSiN 3 samples.…”

“…40 CaAlSiN 3 : Eu 2+ and CaAlSiN 3 : Ce 3+ exhibit efficient red and yellow-orange emission respectively under blue irradiation. 10,39 In the present work, we focus on investigating the luminescence properties of Mn 2+ in CaAlSiN 3 and explore its potential possibilities to be used as a new kind of LED conversion phosphor, which is up to now only claimed but not proofed. 41 …”

“…3+ , Li + , 10 it is difficult to obtain undoped and Mn-doped CaAlSiN 3 samples as a single phase material according to the stoichiometric composition. It was found that Al/Si disorderly occupies the 8b site in the space group of Cmc2 1 with a ratio of about 1/2 with a more appropriate formula CaAl 1-4δ/3 Si 1+δ N 3 (δ = 0.3 -0.4) instead of 1/1 proposed for an ideal CaAlSiN 3 , probably due to the incomplete reaction resulting in a significantly lower Al solubility in CaAlSiN 3 .…”

Selecting Conversion Phosphors for White Light-Emitting Diodes

“…Nevertheless there are some indications of stability issues for certain (oxy)nitride compositions [40,134]. For instance, it appeared that (Ca,Sr)AlSiN 3 was more stable [82] against oxidation than M 2 Si 5 N 8 phosphors, the latter showing considerable degradation of the PL intensity when heated to 573K in air, with the stability depending on the stoichiometry [81].…”