Codoping of Cr³⁺ in Y₃Al₅O₁₂:Ce³⁺ phosphors for improving color rendering of white light-emitting diodes

Abstract

A series of Y₃Al₅O₁₂:Ce³⁺/Cr³⁺ phosphors were synthesized by a solid state reaction. Y₃Al₅O₁₂:Ce³⁺/Cr³⁺ phosphors show emission bands in yellow and red regions corresponding to Ce³⁺ and Cr³⁺ ions. It was found that the emission intensity of Ce³⁺ decreases with the increasing Cr³⁺ concentration because of the energy transfer from Ce³⁺ to Cr³⁺ in Y₃Al₅O₁₂:Ce³⁺/Cr³⁺ phosphors. The emission intensity of Cr³⁺ increases firstly and then decreases with the increasing Cr³⁺ concentration. The white light-emitting diode comprising of Y₃Al₅O₁₂:Ce³⁺/Cr³⁺ phosphor shows a lower CCT value of 4452 and a higher R_a value of 85.7 than those of values of 6720 and 68.2 for white light-emitting diode comprising of Y₃Al₅O₁₂:Ce³⁺ phosphor.

1 Introduction

Light is needed in fields of industry and daily life. It has been reported that electric lighting covers up to ~25% of the average energy budget, and lighting and displays occupy ~20% of the electricity budget in the world [1]. White light-emitting diodes (WLEDs) show promise in a variety of eco-friendly and energy saving lighting applications in the general lighting and display fields because of their advantages over traditional incandescent or fluorescent lightings, such as high luminous efficiency, long persistence, energy saving and therefore environmental benefits [2,3,4]. Currently, the most used fabrication of WLED is to combine a blue InGaN chip with a yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) phosphor. However, due to the absence of red emission in YAG:Ce phosphor excited by a blue InGaN chip, this type of WLEDs always show a low color rendering index (R_a < 80) and a high correlated color temperature (CCT > 6000 K) [5]. These drawbacks induce that it is inappropriate for applications in indoor lighting and full color displays. The general way to overcome theses drawbacks is to supplement red emission based on the excitation of blue InGaN chip.

Focusing on YAG:Ce phosphor, some ions are codoped to obtain red emission or lead to red shift of Ce³⁺ emission band. Lee et al. synthesized YAG:Ce³⁺/Eu³⁺ phosphors and they found that the YAG:Ce³⁺/Eu³⁺ phosphor presents a broad emission band in the green-yellow spectral region due to Ce³⁺, as well as a sharp emission peak around 610 nm in the red spectral region due to Eu³⁺ [6]. Chen et al. synthesized YAG:Ce³⁺/Gd³⁺ phosphors and they found that the emission peak of which can be regulated from approximately 543 nm to maximum 575 nm by tailoring the Gd³⁺ concentration until the collapse of crystal lattice [7]. Wang et al. synthesized YAG:Ce³⁺/Cr³⁺ phosphors via conventional solid state reaction, which exhibits enriched emission spectrum in the red region due to a nonradiative energy transfer from Ce³⁺ to Cr³⁺ [8]. The existence of energy transfer between Ce³⁺ and Cr³⁺ means that tunable luminescence can be obtained by changing concentration ratios of Ce³⁺ and Cr³⁺. This characteristic of YAG:Ce³⁺/Cr³⁺ phosphor leads to the obtaining of tunable R_a and CCT for the fabricated WLEDs.

In this work, we have synthesized a series of YAG:Ce³⁺/Cr³⁺ phosphors through a solid state reaction. The luminescence, mechanism of energy transfer from Ce³⁺ to Cr³⁺ and chromaticity diagrams of YAG:Ce³⁺/Cr³⁺ phosphors have been studied carefully. The performance of energy transfer leads to simultaneous observation of yellow emission from Ce³⁺ and deep red emission from Cr³⁺ upon blue light excitation. Moreover, performances of WLEDs encapsulated with YAG:Ce³⁺/Cr³⁺ phosphor have been studied.

2 Experiment

YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 0, 1, 2, 3, 4, 5 and 6) phosphors were synthesized by a solid state reaction. Y₂O₃ (99.99%), Al₂O₃ (99.99%), CeO₂ (99.99%) and Cr₂O₃ (99.99%) were used as raw materials. In the synthesis, raw materials with the stoichiometric proportions were weighted firstly. Then, the mixture was blended using a ball milling technique in ethyl alcohol media in a nylon bottle containing alumina balls for 10 h. Next, the slurry was dried at 80 °C for 12 h. Finally, the obtained powders were ground in an agate mortar and calcined at 1500 °C for 5 h in a reducing atmosphere of 5%H₂/95%N₂. After the system cooled to room temperature naturally, the product was ground again for further measurements.

The X-ray powder diffraction (XRD) measurements were carried out on a Rigaku-Dmax 2500 diffractometer using Cu Kα radiation (λ = 0.15405 nm). The excitation and emission spectra, as well as the decay of fluorescence of Ce³⁺ emission were measured by an Edinburgh Instrument FLS920 spectrophotometer equipped with a 150 W xenon lamp as the excitation source.

3 Results and discussion

Figure 1 shows the XRD patterns of YAG:4 mol%Ce³⁺ and YAG:4 mol%Ce³⁺/4 mol%Cr³⁺ phosphors. All of diffraction peaks are well according with JCPDs No. 33-0040, demonstrating that the obtained phosphors have cubic phase and the Ce³⁺ and Cr³⁺ ions have doped into the lattices of YAG host. In YAG, Y atoms occupy the eight-coordinate oxygen atom dodecahedral site and Al atoms have two positions. The site of Al₁ is surrounded by six oxygen atoms resulting in an octahedral structure and the site of Al₂ is coordinated by four oxygen atoms to form a tetrahedral structure. Due to the same valence and similar ion radii between Y³⁺ (1.019 Å, CN = 8) with Ce³⁺ (1.143 Å, CN = 8) and Al³⁺ (0.53 Å, CN = 4; 0.675 Å, CN = 6) with Cr³⁺ (0.755 Å, CN = 6), Ce³⁺ is expected to substitute Y³⁺ site and Cr³⁺ is likely to substitute Al₁ site in YAG host. As shown in Fig. 1, the diffraction peaks shift to smaller 2θ angels, which are induced by the larger radii of Ce³⁺ and Cr³⁺ than those of Y³⁺ and Al³⁺.

Fig. 1

XRD patterns of YAG:4 mol%Ce³⁺, YAG:4 mol%Ce³⁺/4 mol%Cr³⁺ phosphors and the standard data of JCPDs 33-0040

Figure 2 presents the excitation and emission spectra of YAG:4 mol%Ce³⁺ (A), YAG:2 mol%Cr³⁺ (B) and YAG:4 mol%Ce³⁺/4 mol%Cr³⁺ (C) phosphors. Ce³⁺ has only one electron in the 4f state. The ground state of Ce³⁺ is split into ²F_7/2 and ²F_5/2 with an energy difference of about 2200 cm⁻¹ [9]. The next higher state originates from the 5d state and 4f–5d transitions are parity and spin allowed. For the free-ion, the 5d electron of the excited 4f ¹5d ¹ configuration forms a ²D term, which is split into ²D_3/2 and ²D_5/2 states by spin–orbit coupling. Because that the radial wave function of the excited 5d electron extends spatially well beyond the closed 5s ²5p ⁶, its states are strongly perturbed by the crystal field. Therefore, there are three excitation bands locating at 232, 341 and 465 nm in the excitation spectrum. The excitation band peaking at 232 nm is ascribed to the electron transition from ground state 4f to 5d sub-band energy level [10]. The band at 232 nm is very weak because the upper 5d states of Ce³⁺ in YAG have energies within the conduction band of the host. Therefore, excitation in these levels results mainly in quenching [11]. The excitation bands centering at 341 and 465 nm respectively correspond to the transitions from the ground state 4f levels (²F_7/2, ²F_5/2) to the 5d ₁ and 5d ₂ levels [12]. The strongest excitation band peaking at 465 nm associates with the lowest-lying 5d state. The excitation band is deployed from the near-UV to the visible region. This means that the prepared phosphor is suitable for application in WLEDs excited by blue LED chip. Under the excitation at 465 nm, YAG:4 mol%Ce³⁺ shows a broad and asymmetric emission band ranging from 475 to 675 nm and peaking at 532 nm, which results from the 4f–5d transitions of Ce³⁺ ions. The emission band consists of doublet sub-emissions from 5d ₁ to ² F _2/7 and 5d ₁ to 2F _5/2, because the ground state of Ce³⁺ consists of ²F_7/2 and ²F_5/2 sublevels after considering the spin–orbit interaction [13].

Fig. 2

Excitation and emission spectra of YAG:4 mol%Ce³⁺ (a), YAG:2 mol%Cr³⁺ (b) and YAG:4 mol%Ce³⁺/4 mol%Cr³⁺ (c) phosphors

Figure 2b shows the excitation and emission spectra of YAG:2 mol%Cr³⁺ phosphor. In the excitation spectrum, an emission band peaking at 232 nm is a charge transfer band (CTB) relating to O²⁻Cr³⁺ [14]. The other three excitation bands peaking at 281, 445 and 611 nm can be ascribed to the spin-allowed electronic transitions of Cr³⁺ from the ⁴A₂ state to the ⁴T₁ (4 P), ⁴T₁ (4F) and ⁴T₂ (4F) states, respectively [15]. Under the excitation at 445 nm, YAG:2 mol%Cr³⁺ phosphor exhibits emission bands in the range of 600–750 nm, which are induced by the transitions form ²E state to ⁴A₂ state [16]. For phosphors with sensitizer and activator, the effective energy transfer occurs when the emission spectrum of sensitizer overlaps with the excitation spectrum of activator [17]. From Fig. 2a, b, one can clearly see that there is an overlap between the emission band of YAG:4 mol%Ce³⁺ with the excitation band of YAG:2 mol%Cr³⁺. Therefore, we can speculate that there is energy transfer from Ce³⁺ to Cr³⁺ in YAG:Ce³⁺/Cr³⁺. Figure 3c gives the excitation and emission spectra of YAG:4 mol%Ce³⁺/4 mol%Cr³⁺ phosphor. By monitoring the emission at 686 nm, both excitation band of Ce³⁺ and excitation bands of Cr³⁺ are observed in the excitation spectrum. Under the excitation at 465 nm, the emission spectrum of YAG:4 mol%Ce³⁺/4 mol%Cr³⁺ phosphor consists of a broad band in the yellow region and three emission bands in the red region, which respectively originate from Ce³⁺ and Cr³⁺. The luminescence properties confirm the occurrence of energy transfer from Ce³⁺ to Cr³⁺ in YAG:Ce³⁺/Cr³⁺.

Figure 3 exhibits the emission spectra of YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 0, 1, 2, 3, 4, 5 and 6) phosphors under the excitation at 465 nm. The emission intensities show clear concentration-dependence. The emission intensity of Ce³⁺ decreases gradually with the increases of the Cr³⁺ concentration. The emission intensity of Cr³⁺ increases firstly with the increases of Cr³⁺ concentration and reaches a maximum at x = 5. The emission intensity decreases if we further increase the Cr³⁺ concentration. The CIE chromaticity coordinates of YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 0, 1, 2, 3, 4, 5 and 6) phosphors are shown in Fig. 4 and Table 1. With the increases of Cr³⁺ concentration, the color of YAG:Ce³⁺/Cr³⁺ phosphors changes from yellow region to yellow–red region. The variation of the emission spectra reveals the energy transfer process from Ce³⁺ to Cr³⁺ in YAG:Ce³⁺/Cr³⁺ phosphors. And the decrease of Cr³⁺ emission intensity suggests the concentration quenching of Cr³⁺ for YAG:4 mol%Ce³⁺/6 mol%Cr³⁺ phosphor. The energy transfer efficiency (η_emission) from Ce³⁺ to Cr³⁺ can be calculated by the formula of ${{\upeta}}=1 - {\raise0.7ex\hbox{${{{I}_{d}}}$} \!\mathord{\left/ {\vphantom {{{{I}_{d}}} {{{I}_{{d}0}}}}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{${{{I}_{{d}0}}}$}},$ where I_d and I_d0 are the luminescence intensity of Ce³⁺in the presence and absence of the Cr³⁺ [18]. The values of η_emission for YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 1, 2, 3, 4, 5 and 6) phosphors are shown in Table 1.

Fig. 3

Emission spectra of YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 0, 1, 2, 3, 4, 5 and 6) phosphors under the excitation at 465 nm

Fig. 4

CIE chromaticity coordinates of YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 0, 1, 2, 3, 4, 5 and 6) phosphors

Table 1 CIE chromaticity coordinates and energy transfer efficiencies of YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 0, 1, 2, 3, 4, 5 and 6) phosphors

Fig. 5

Decay curves of YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 0, 1, 2, 3, 4, 5 and 6) phosphors

In order to further confirm the energy transfer from Ce³⁺ to Cr³⁺ in YAG:Ce³⁺/Cr³⁺ phosphors, the decay curves of Ce³⁺ emission in YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 0, 1, 2, 3, 4, 5 and 6) phosphors have been measured by monitoring the Ce³⁺ emission at 532 nm. As shown in Fig. 5, the decay curves are well according with the single exponential function. The lifetimes of Ce³⁺ emission in YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 0, 1, 2, 3, 4, 5 and 6) phosphors are 40.32, 35.17, 30.56, 27.23, 23.58, 19.66 and 16.72 ns, respectively. The lifetimes of Ce³⁺ emission decrease with the increasing Cr³⁺ concentrations, which is induced by the energy transfer from Ce³⁺ to Cr³⁺. The energy transfer efficiency (η_life) also can be calculated by the following formula of ${{\upeta}}=1 - {\raise0.7ex\hbox{${{\uptau}}$} \!\mathord{\left/ {\vphantom {{{\uptau}} {{{{\uptau}}_0}}}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{${{{{\uptau}}_0}}$}},$ where τ and τ₀ are the decay lifetimes of Ce³⁺ in the presence and absence of the Cr³⁺ [19]. The values of η_life for YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 1, 2, 3, 4, 5 and 6) phosphors are also shown in Table 1. The values of η_emission and η_life are similar, which gives an accurate energy transfer efficiencies from Ce³⁺ to Cr³⁺ in YAG:4 mol%Ce³⁺/xmol%Cr³⁺ (x = 1, 2, 3, 4, 5 and 6) phosphors.

On the basis of the luminescent properties of YAG:Ce³⁺/Cr³⁺ phosphors and the energy level diagrams of Ce³⁺ and Cr³⁺, the mechanisms of emission and energy transfer have been speculated. Under the excitation of 465 nm light, electrons are excited to the 5d state from the 4f ground state. Most of excited electrons transfer back to the ground state and emit light in yellow region. The energy transfer from Ce³⁺ to Cr³⁺ in YAG:Ce³⁺/Cr³⁺ phosphors occurs through two ways. As shown in Fig. 6, the ²E energy level of Ce³⁺ is little higher than the ⁴T_2g energy level of Cr³⁺, which leads to the energy transfer through the nonradiative process [20]. The second way is that Cr³⁺ is excited from ground state of ⁴A₂ to the excited state of ⁴T₂ by absorbing the energy of emission light from Ce³⁺ ions [21]. However, we can see from Fig. 2b that Cr³⁺ is difficult to absorb energy of 532 nm light. Therefore, the first way of energy transfer is dominating.

Fig. 6

Schematic energy level diagrams of Ce³⁺ and Cr³⁺ illustrating energy transfer process

To validate the availability of the synthesized phosphors for the InGaN based WLED, the WLEDs have been fabricate by combining the InGaN blue chip with the synthesized YAG:4 mol%Ce³⁺ and YAG:4 mol%Ce³⁺/4 mol%Cr³⁺ phosphors. Figure 7 presents the electroluminescence spectra of WLEDs comprising of YAG:4 mol%Ce³⁺ (A) and YAG:4 mol%Ce³⁺/4 mol%Cr³⁺ (B) phosphors under 20 mA current excitation. The spectrum of WLED based on InGaN blue chip and YAG:4 mol%Ce³⁺/4 mol%Cr³⁺ phosphor includes emission bands in blue, yellow and red regions. The blue emission band originates from the InGaN blue chip. The yellow emission band is induced by the YAG:Ce³⁺ under the excitation of InGaN blue chip. The red emission bands are ascribed to the Cr³⁺ ions in YAG:Ce³⁺/Cr³⁺ phosphor under the excitation of InGaN blue chip. The mixture of blue–yellow–red light results in the generation of white light. The CIE chromaticity coordinates of WLEDs without and with codoping Cr³⁺ ions are (0.312, 307) and (0.348, 0.289), respectively. The values of CCT decrease from 6720 to 4452 and the values of R_a increase from 68.2 to 85.7 by the addition of Cr³⁺ in YAG:Ce³⁺ phosphor.

Fig. 7

Electroluminescence spectra of WLEDs comprising of YAG:4 mol%Ce³⁺ (a) and YAG:4 mol%Ce³⁺/4 mol%Cr³⁺ (b) phosphors under 20 mA current excitation

4 Conclusion

We have synthesized a series of YAG:Ce³⁺/Cr³⁺ phosphors by a solid state reaction. The synthesized YAG:Ce³⁺/Cr³⁺ phosphors show emission bands in yellow and red regions corresponding to Ce³⁺ and Cr³⁺ ions. Tunable luminescence is obtained in YAG:Ce³⁺/Cr³⁺ phosphors by changing Cr³⁺ concentration. The energy transfer from the ²E energy level of Ce³⁺ to the ⁴T_2g energy level of Cr³⁺ through non-radiative transition is dominating in YAG:Ce³⁺/Cr³⁺ phosphor. The codoping Cr³⁺ in YAG:Ce³⁺ phosphor can decrease the CCT value and increase R_a value of WLED. The WLED comprising of YAG:Ce³⁺/Cr³⁺ phosphor shows a lower CCT value of 4452 and a higher R_a value of 85.7 than those of values of 6720 and 68.2 for WLED comprising of YAG:Ce³⁺ phosphor.