Luminescence of Tb₃Al₅O₁₂ phosphors co-doped with Ce³⁺/Gd³⁺ for white light-emitting diodes

Introduction

Currently, the most popular fabrication model of white light-emitting diodes (WLEDs) is to combine blue chips with yellow Y₃Al₅O₁₂:Ce³⁺ phosphors, which has the disadvantages of low colour-rendering index (CRI) and high correlated colour temperature (CCT) [1,2]. At the same time, this type of WLEDs has the advantages of long lifetime, eco-friendliness, high luminous efficiency and low energy consumption, which helps to mitigate two serious issues in the world, namely ecological crisis and energy dilemma. As a result, various attempts have been made to address the shortcomings in this type of WLED. To date, two common methods are red-light compensation and the red-shift of Ce³⁺ emission band in Y₃Al₅O₁₂. Red-light compensation is generally achieved by adding a red-emission phosphor, such as Eu²⁺-doped materials [3], materials doped with trivalent lanthanide ions (e.g., Eu³⁺ and Sm³⁺) [4-8], Mn⁴⁺-doped materials [9-12], or Ce³⁺/Cr³⁺-co-doped Y₃Al₅O₁₂ [13,14]. The red-shift of the Ce³⁺ emission band in Y₃Al1₅O₁₂ is achieved, in general, through ion substitution, such as Ca²⁺–Mg²⁺–Si⁴⁺ [15], Si⁴⁺–N³⁻ [16,17], Mg²⁺–Si⁴⁺/Ge⁴⁺ [18-20], or Gd³⁺ [21,22].

Tb₃Al₅O₁₂ has a garnet structure similar to Y₃Al₅O₁₂. A series of doped Tb₃Al₅O₁₂ phosphors have been synthesized, such as Tb₃Al₅O₁₂:Ce³⁺ [23-25], Tb₃Al₅O₁₂:Ce³⁺/Eu³⁺ [26], Tb₃Al₅O₁₂:Eu³⁺ [27], and Tb₃Al₅O₁₂:Ce³⁺/Ga³⁺ [28]. The results show that Tb₃Al₅O₁₂ is also a good host for various ions and the luminescent properties could be tuned by co-doping different ions into the Tb₃Al₅O₁₂ host. The Tb₃Al₅O₁₂:Ce³⁺ phosphor also shows a yellow emission band. But the emission wavelength is longer than that of the Y₃Al₅O₁₂:Ce³⁺ phosphor because Tb³⁺ ions produce a stronger crystal field effect [23-25]. The longer emission wavelength of Tb₃Al₅O₁₂:Ce³⁺ is more suitable for WLEDs used as indoor illumination than that of Y₃Al₅O₁₂:Ce³⁺. It is known that the sensitivity of human eyes to red light decreases strongly as soon as the wavelength is longer than 611 nm [9]. We aimed to shift the emission wavelength of Tb₃Al₅O₁₂:Ce³⁺ to a longer wavelength that is, however, still shorter than 611 nm. In this work, we report the synthesis and luminescence of a series of Ce³⁺/Gd³⁺-co-doped Tb₃Al₅O₁₂ phosphors. The effect of co-doping Gd³⁺ on the luminescence of Tb₃Al₅O₁₂:Ce³⁺ was investigated. It is found that the co-doped Gd³⁺ leads to a red-shift of the Tb₃Al₅O₁₂:Ce³⁺ emission.

Results and Discussion

The phase of the synthesized Tb_2.96-xCe_0.04Gd_xAl₅O₁₂ phosphors was confirmed by using XRD analysis. As shown in Figure 1, the diffraction peaks of Tb_2.96−xCe_0.04Gd_xAl₅O₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25) phosphors are well in accordance with the JCPDs card no. 17-1735, meaning that Ce³⁺/Gd³⁺ ions have been doped into the Tb₃Al₅O₁₂ host entirely and formed a solid solution. Moreover, the diffraction peaks shift to lower 2θ angles with increasing x values, which is induced by the substitution of Tb³⁺ with Ce³⁺/Gd³⁺. The ionic radii of Tb³⁺, Ce³⁺ and Gd³⁺ are 1.040 Å (CN = 8), 1.143 Å (CN = 8) and 1.053 Å (CN = 8), respectively. Due to the same valence and similar ionic radii of Tb³⁺, Ce³⁺, and Gd³⁺, Tb³⁺ ions are replaced by Ce³⁺ and Gd³⁺ ions in Ce³⁺/Gd³⁺ co-doped Tb₃Al₅O₁₂ phosphors. The larger ionic radii of Ce³⁺ and Gd³⁺ lead to the increase of the cell volume, which induces the shifts to lower 2θ angles of the diffraction peaks.

The excitation spectra of Tb_2.96−xCe_0.04Gd_xAl₅O₁₂ phosphors, which show the excitation bands of Tb³⁺ and Ce³⁺ ions, is given in Figure 2. The excitation bands in the range of 250–300 nm with two excitation peaks at 275 and 286 nm correspond to the 4f⁸→4f⁷5d¹ inter-configurational transitions of Tb³⁺ [28]. The weak excitation band with a peak at 375 nm is induced by the ⁷F₆→⁵D₃ transition of Tb³⁺ [29]. Moreover, the 4f⁸→4f⁷5d¹ transition of Tb³⁺ overlaps with the 4f→5d₂ transition of Ce³⁺, which results in the excitation band with a peak at 331 nm [22,26]. The strongest excitation band with a peak at 457 nm corresponds to the 4f→5d₁ transition of Ce³⁺ [22]. It can be seen from Figure 2 that the excitation band corresponding to the 4f→5d₁ transition of Ce³⁺ shifts to shorter wavelengths gradually with the increase of x, which is induced by the splitting of the Ce³⁺ 5d state. The increasing Gd³⁺ concentration leads to an intensified crystal field, which results in a stronger of splitting of the Ce³⁺ 5d state. As a result, the 4f→5d₁ transition of Ce³⁺ shifts to shorter wavelengths, but the 4f→5d₂ transition of Ce³⁺ shifts to longer wavelength. Herein, the shift of the 4f→5d₂ transition of Ce³⁺ to longer wavelengths cannot be seen clearly because of its overlaps with the 4f⁸→4f⁷5d¹ transition of Tb³⁺.

Under excitation at 460 nm, Tb_2.96−xCe_0.04Gd_xAl₅O₁₂ phosphors show the characteristic emission band of Ce³⁺, as shown in Figure 3. One feature is the red-shift of the Ce³⁺ emission with increasing Gd³⁺ concentration and the other is the decrease of emission intensity with increasing Gd³⁺ concentration. The emission band of the Tb_2.96Ce_0.04Al₅O₁₂ phosphor peaks at about 554 nm. For the Tb_2.71Ce_0.04Gd_0.25Al₅O₁₂ phosphor, the peak wavelength of the emission band shifts to 610 nm. It is well known that the emission of Ce³⁺ depends on the crystal field splitting. The crystal field splitting can be calculated through the following equation:

where 10Dq is the crystal field splitting parameter, Z is the anion charge, e is the electron charge, r is the radial distance of the d orbital from the nucleus, and R is the bond length [30,31]. Gd³⁺ has a larger ionic radius than of Tb³⁺. As a result, the Ce³⁺–O²⁻ bond length in Tb₃Al₅O₁₂:Ce³⁺/Gd³⁺ decreases when Tb³⁺ ions are replaced by Gd³⁺ ions. The decrease of the Ce³⁺–O²⁻ bond length results in an increase of the crystal field splitting, which in turn leads to the red-shift of the Ce³⁺ emission. This result is in accordance with the excitation spectra. Moreover, the substitution of Tb³⁺ with Gd³⁺ causes lattice deformation and generates numerous structural and chemical defects, which results in a decrease of the Ce³⁺ emission intensity [32]. The decay characteristics of the synthesized Tb_2.96−xCe_0.04Gd_xAl₅O₁₂ phosphors were also investigated. Figure 4 gives the decay curves of Tb_2.96−xCe_0.04Gd_xAl₅O₁₂ phosphors. The decay curves of the Ce³⁺ emission fit well with the second-order exponential formula

where I is the emission intensity, A₁ and A₂ are constants, t is the time, and τ₁ and τ₂ are the rapid and the slow lifetime, respectively. The average lifetime (τ*) can be calculated through

The calculated τ* values for Tb_2.96−xCe_0.04Gd_xAl₅O₁₂ with x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25 are 35.23, 31.46, 28.52, 26.37, 23.58 and 19.45 ns, respectively.

The thermal stability of a phosphor is crucial for its applications in WLEDs. Thus, the emission spectra of a typical phosphor (Tb_2.81Ce_0.04Gd_0.15Al₅O₁₂) at different temperatures were measured and the results are shown in Figure 5. The emission intensity decreases continuously with the increasing temperature in the range of 300–540 K. As the temperature increases from 300 to 390 K, the emission intensity decreases by about 49%. Photon interaction plays an important role in thermal quenching, in which emission centres are thermally activated and the energy is released through a nonradiative transition [31]. It is known that the probability of nonradiative transitions increases with increasing temperature. As a result, the emission intensity decreases with increasing temperature because of the higher number of nonradiative transitions.

WLEDs were fabricated by combining a blue LED chip (460 nm) with Y_2.96Ce_0.04Al₅O₁₂, Tb_2.96Ce_0.04Al₅O₁₂ and Tb_2.81Ce_0.04Gd_0.15Al₅O₁₂ phosphors. The electroluminescence spectra under an operating current of 20 mA for the fabricated WLEDs are given in Figure 6. All of spectra consist of the blue excitation band of the LED chip and the emission band of the phosphor. The emission bands of the phosphors shift from 532 nm for Y_2.96Ce_0.04Al₅O₁₂ (Figure 6A) through 545 nm for Tb_2.96Ce_0.04Al₅O₁₂ (Figure 6B) to 589 nm for Tb_2.81Ce_0.04Gd_0.15Al₅O₁₂ (Figure 6C). The CIE chromaticity coordinates for the light from these three WLEDs are (0.325, 0.349), (0.368, 0.351), and (0.376, 0.338), respectively. The CCT values of the light from these three WLEDs are 5828, 4158, and 3767, respectively. These results suggest that Tb_2.81Ce_0.04Gd_0.15Al₅O₁₂ is a suitable phosphor for applications in WLEDs with low CCT for indoor lighting.

Generally, the colour stability of a LED device can be examined through measuring the colour deviation under different driving currents [33]. Figure 7 shows the electroluminescence spectra of a WLED containing the Tb_2.81Ce_0.04Gd_0.15Al₅O₁₂ phosphor under forward-bias currents of 5, 10, 20, 30, 40, and 50 mA. It can be seen that the intensity of the WLED increases with increasing current. Moreover, the shape and the peak of the bands corresponding to the LED chip and phosphor are consistent under different driving currents. This suggests the outstanding colour stability of the fabricated WLEDs.

Conclusion

We synthesized a series of Tb_2.96−xCe_0.04Gd_xAl₅O₁₂ phosphors through solid-state reactions. The doping with Ce³⁺/Gd³⁺ ions does not lead to a phase change of the Tb₃Al₅O₁₂ host but induces a slight increase of cell volume. Under excitation at 460 nm, the Tb_2.96Ce_0.04Al₅O₁₂ phosphor shows the characteristic emission band of Ce³⁺ with a peak wavelength of about 554 nm. The co-doped Gd³⁺ ions lead to a red-shift of the Ce³⁺ emission band and the red-shifts become larger with increasing Gd³⁺ concentration. Due to the larger ionic radius of Gd³⁺ compared with Tb³⁺, the substitution of Tb³⁺ with Gd³⁺ decreases the bond distance between Ce³⁺ and O²⁻, which leads to an increase of crystal field splitting. The increasing crystal field splitting induces the red-shift of the Ce³⁺ emission. The longer peak-wavelength of the Ce³⁺ emission for Gd³⁺ co-doped phosphors leads to a warmer light in WLEDs. The fabricated WLEDs by integrating a blue LED chip and the Ce³⁺/Gd³⁺ co-doped Tb₃Al₅O₁₂ phosphors show outstanding colour stability when they are driven under different currents.

Experimental

A series of Tb_2.96−xCe_0.04Gd_xAl₅O₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25) phosphors were synthesized through solid-state reactions in a reduction atmosphere (5% H₂/95% N₂). Al₂O₃ (99.9%), Tb₄O₇ (99.9%), CeO₂ (99.99%) and Gd₂O₃ (99.95%) were used as starting materials. For the purpose of decreasing the reaction temperature, 4 wt % H₃BO₃ (99.5%) was added as flux. In a typical synthesis, we firstly weighted the raw materials according to stoichiometric ratios. Then, the raw materials were mixed in an agate mortar by grinding for 30 min and the mixture was calcined at 1350 °C for 5 h in an alumina crucible. Finally, the product was collected and reground after the temperature decreased to room temperature.

The X-ray diffraction (XRD) measurements were performed on a Rigaku D/max-RA X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å) with the experimental parameters of 40 kV, 30 mA and 2°/min. The measurements of excitation, emission, temperature-dependence of emission and decay curves were carried out in an Edinburgh Instrument FLS920 spectrophotometer equipped with a 450 W xenon lamp as the excitation source. The measurements were spectrally corrected. The samples were heated to a certain temperature and kept at this temperature for 5 min by using a temperature controller. The rate of temperature increase was less than 1 °C/ min and the temperature deviation was less than 0.1 °C.