1. Introduction

Phosphor-converted white light–emitting diodes (pcWLEDs) are potential replacements for conventional light sources such as incandescent or fluorescent lamps [1]. The general strategy for producing pcWLED is to combine blue LED with the yellow emitting Y₃Al₅O₁₂: Ce³⁺ (YAG: Ce³⁺) phosphor at present [2]. However, YAG: Ce³⁺ has relatively weak emission in the red spectral region, leading to color rendering index (CRI) of pcWLEDs below 80. To meet the requirement of higher CRIs (> 80) for general illumination, co-doping red emitting ions as co-activators into YAG: Ce³⁺ was extensively studied [3–7]. Mueller-Mach and associates [3] added Pr³⁺ into YAG:Ce³⁺ to replace Y and consequently obtained a red emission line at 608 nm, originating from ¹D₂→³H₄ transition of Pr³⁺, through energy transfer from Ce³⁺ to Pr³⁺. However, increasing Pr³⁺ concentration over 0.015 for obtaining enough red components leads to a notable decrease of the red line due to self concentration quenching [4,5]. Recently, Wang et al. [7] selected Cr³⁺ as an red emitting center to incorporate into YAG: Ce³⁺ phosphor to replace Al sites [8] and observed a deep red emission line of Cr³⁺ at about 690 nm through Ce³⁺→Cr³⁺ energy transfer. While, no notable luminescence quenching of Cr³⁺ was observed for high Cr³⁺ concentration. Unsatisfactorily, both doubly doped YAG: Ce³⁺, Pr³⁺ and YAG: Ce³⁺, Cr³⁺ phosphors still need more red spectral component for high color rendering white LEDs because YAG: Ce³⁺, Pr³⁺ lacks for emission in deep red spectral region and YAG: Ce³⁺, Cr³⁺ lacks in light red region.

In this paper, we prepared triply doped YAG: Ce³⁺, Pr³⁺, Cr³⁺ phosphors by solid state reaction. The performance of energy transfer among these emitting centers leads to simultaneous observation of yellow emission from Ce³⁺, light red emission from Pr³⁺ and deep red emission from Cr³⁺ upon blue light excitation. The white LEDs fabricated using the triply doped phosphor shows higher CRI than that using singly or doubly doped YAG phosphors.

2. Experimental

Powder phosphor samples were made using mixtures of high-purity Y₂O₃, CeO₂, Al₂O₃, Cr₂O₃ and Pr₆O₁₁ in molar of (Y_1-z-yCe_zPr_y)₃ (Al_1-xCr_x)₅O₁₂ (x, y, z represent the concentration of Cr³⁺, Pr³⁺ and Ce³⁺, respectively), and fired under CO reducing condition at 1500°C for 3 h. The structure of the final products is characterized by powder X-ray diffraction (XRD). Photoluminescence (PL) and photoluminescence excitation (PLE) spectra are measured with a Hitachi Spextra-fluorometer (F-4500). The decay of the fluorescence from Ce³⁺ is measured by an FL920 fluorometer with a hydrogen flash lamp. In the measurements of fluorescence decay of Pr³⁺ and Cr³⁺, an optical parametric oscillator (OPO) is used as an excitation source. The signal is detected by a Tektronix digital oscilloscope (TDS 3052).

3. Results and discussion

Figure 1 shows the PL and PLE spectra of Ce³⁺, Pr³⁺, Cr³⁺ singly (a,b), doubly (d,e) and triply (f) doped YAG. The Ce³⁺ singly doped sample exhibits a well known yellow emission band peaking at around 530 nm due to the transition from the lowest-lying 5d state to the 4f ground state of Ce³⁺. The PLE spectrum of the yellow band consists of a band at 470 nm corresponding to the transition from the ground state to the lowest-lying 5d state, and two ultraviolet (UV) PLE bands corresponding to the upper 5d states, located at 340 nm and 230 nm, respectively [2], as shown in Fig. 1(a). The Pr³⁺ singly doped sample exhibits three groups of emissions of Pr³⁺ upon 288 nm excitation, as shown in Fig. 1(b). The group in the UV region consists of two strong bands located at 317 nm and 381 nm, which are originated to the transitions from the lowest lying 4f5d state to ³H_J (J = 4, 5, 6) and the ³F_J (J = 2, 3, 4) manifolds, respectively [9]. The group in the range of 450 – 600 nm originates from ³P₀→³H_4,5 transitions dominated by a intense blue emission line at 488 nm due to ³P₀→³H₄ transition. The other group in red originates from ¹D₂→³H₄ transitions dominated by a intense emission line at 608 nm with a weak satellite line at 640nm [10,11]. The PLE spectra of the three groups of emissions in Pr³⁺ singly doped sample are identical in the UV spectral range, showing two 4f5d PLE bands located at 288 nm and 238 nm, respectively. In Fig. 1(c), the principal features of the PLE spectra of Cr³⁺ singly doped sample are a O^2- – Cr ³⁺ related charge transfer band (CTB) peaking at 230nm and three broad bands peaking at 280nm, 443nm and 608nm, which belong to the spin-allowed electronic transitions from the ⁴A₂ to the ⁴T₁ (4 P), ⁴T₁ (4 F) and ⁴T₂ (4 F) states, respectively. The PL spectra are the typical emissions of Cr³⁺, which contains a ²E – ⁴A₂ zero-phonon line peaking at 685 nm with vibronic sidebands [12,13]. In Fig. 1(d), the PL spectrum of Ce³⁺ and Cr³⁺ doubly doped sample contains not only the yellow band of Ce³⁺ but also the red bands of Cr³⁺ when only Ce³⁺ is excited at 340 nm. Furthermore, the PLE spectrum monitoring Cr³⁺ 685nm emission includes two typical Ce³⁺ absorption bands at 340nm and 470nm, demonstrating occurrence of Ce³⁺→Cr³⁺ energy transfer. In the Fig. 1(e), the PL spectrum of Pr³⁺ and Cr³⁺ doubly doped sample exhibits Cr³⁺ typical emissions at 685nm as Pr³⁺ is only excited by 288 nm and the PLE spectrum monitoring the Cr³⁺ emission shows a clear Pr³⁺ absorption band at 288nm, indicating Pr³⁺→Cr³⁺ energy transfer. The PL spectrum of triply doped sample exhibits all typical emissions of Ce³⁺, Pr³⁺ and Cr³⁺ when only Ce³⁺ is excited at 340 nm, as shown in Fig. 1(f). This is the result of Ce³⁺→Cr³⁺, Ce³⁺→Pr³⁺ and Pr³⁺→Cr³⁺ energy transfers. The emission spectrum is considerably enriched in the red region with the luminescence of both Pr³⁺ and Cr³⁺.

 

Fig. 1 PL and PLE spectra of (Y_0.99Ce_0.01)₃Al₅O₁₂ (a), (Y_0.99Pr_0.005)₃Al₅O₁₂ (b), Y₃(Al_0.9925Cr_0.0075)₅O₁₂ (c), (Y_0.99Ce_0.01)₃(Al_0.9925Cr_0.0075)₅O₁₂ (d), (Y_0.99Pr_0.005)₃(Al_0.9925Cr_0.0075)₅O₁₂ (e) and (Y_0.985Ce_0.01Pr_0.005)₃(Al_0.9925Cr_0.0075)₅O₁₂ (f); PL spectra of (Y_0.99-yCe_0.01Pr_y)₃(Al_1-xCr_x)₅O₁₂, (x = 0, 0.0025, 0.005, 0.0075, 0.01, 0.0125, 0.015; y = 0 (dashed curves), 0.005 (solid curves)). The intensity of the yellow band in each spectrum is normalized(g); Dependence of W”of triply doped samples series A on W' of doubly doped samples series B(h).

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Figure 1(g) shows PL spectra of triply doped sample series A (solid): (Y_0.985Ce_0.01Pr_0.005)₃(Al_1-xCr_x)₅O₁₂ and Ce³⁺, Cr³⁺ doubly doped sample series B (dotted): (Y_0.99Ce_0.01)₃(Al_1-xCr_x)₅O₁₂ with variable Cr³⁺ concentration x in the range of 0 ~0.015 upon Ce³⁺ excitation at 340 nm. All spectra are normalized to the yellow band. The deep red emission of Cr³⁺ grows up with increasing x, reflecting the increase of the Ce³⁺→Cr³⁺ energy transfer efficiency since Cr³⁺ cannot be excited directly by 340 nm. When the upper 5d state of Ce³⁺ is excited at 340 nm, a rapid relaxation down to the lowest 5d state performs and subsequently the energy is transferred from Ce³⁺ to Cr³⁺.

One can find in Fig. 1(g) that the deep red emission of Cr³⁺ in the triply doped phosphor is always stronger than that in doubly doped one for the same x. To understand this behavior, we have measured the decay curves of the yellow fluorescence of Ce³⁺ and the deep red fluorescence of Cr³⁺ in both sample series A and B. The lifetimes of the yellow fluorescence (τ _Ce) and the red fluorescence (τ _Cr) are calculated by integrating the area under the corresponding decay curves with a normalized initial intensity, as listed in Table 1 . The lifetimes τ _Ce become shorter with increasing x, implying the enhanced Ce³⁺→Cr³⁺ energy transfer. The macroscopic Ce³⁺→Cr³⁺ energy transfer rate, W can be evaluated by

Table 1. Fluorescent lifetimes and transfer efficiencies in (Y_0.99Ce_0.01)₃(Al_1-xCr_x)₅O₁₂ and (Y_0.985Ce_0.01Pr_0.005)₃(Al_1-x Cr_x) ₅O₁₂

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In continuous excitation, the steady state rate equation concerning the population, n _Ce, of the lowest 5d state of Ce³⁺ and, n _Cr, of the ²E state of Cr³⁺ is written as Wn _Ce = n _Cr/τ _Cr. The intensity ratio (I_Cr/I_Ce) of Cr³⁺ to Ce³⁺ emission is proportional to Wτ _Cr, as expressed by

We found in Table 1 that the macroscopic Ce³⁺→Cr³⁺ energy transfer rate, W”, in the triply doped samples is always larger than W' in Ce³⁺ and Cr³⁺doubly doped one for the same x. This phenomenon can be well explained as described below. In the triply doped samples, the presence of Pr³⁺ shortens the intrinsic lifetime of the Ce³⁺ fluorescence from 57.8 ns to 46 ns due to Ce³⁺→Pr³⁺ energy transfer. As Cr³⁺ is added, there is a competition between Ce³⁺→Pr³⁺ energy transfer and Ce³⁺→Cr³⁺ energy transfer. The Ce³⁺ ions with small Ce³⁺→Cr³⁺ energy transfer rates are easily transferred to Pr³⁺, so that the Ce³⁺ ions in the excited state upon continuous excitation contain a larger number of fast Ce³⁺ ions in the presence of Pr³⁺ in comparison to the case of Pr³⁺ free. Therefore, a larger macroscopic Ce³⁺→Cr³⁺ energy transfer rate is expected in the triply doped samples. As we plot W” versus its corresponding W', they well satisfy a proportional relationship with a slope 1.36, as shown in Fig. 1(h). It means the value of W” is about 1.36 times that of W' for the same x. From Eq. (2), the intensity ratio (I_Cr/I_Ce) is proportional to Wτ _Cr. Taking into account the τ _Cr that exhibits a small difference between doubly and triply doped samples for the same x, it is evaluated that the intensity of Cr³⁺ emission in the triply doped samples [solid in Fig. 1(g)] should be averagely 1.32 times that in doubly doped samples [dotted in Fig. 1(g)] for the same x. We find in Fig. 1(g) that the ratio is around 1.59 larger than 1.32. While noticing the reduction of the red line at 608 nm of Pr³⁺ with increasing x, we attribute the extra ratio to the contribution made by Pr³⁺→Cr³⁺ energy transfer. In this case, the components of the deep red emission of Cr³⁺ in the triply doped samples are provided by not only Ce³⁺→Cr³⁺, but also Pr³⁺→Cr³⁺ energy transfer.

To study the effect of Pr³⁺→Cr³⁺ energy transfer, Pr³⁺, Cr³⁺ doubly doped sample series C: (Y_0.995Pr_0.005)₃(Al_1-xCr_x)₅O₁₂ with variable Cr³⁺ concentration x (x = 0 ~0.015) are synthesized. Figure 2(a) shows the PL spectra of sample series C as only Pr³⁺ is excited at 288 nm. All spectra are normalized by the intensity of Pr^{3+ 1}D₂→³H₄ red line. The PL spectra present enhancement of Cr³⁺ emission with increasing x due to Pr³⁺→Cr³⁺ energy transfer, which also results in shortening of the lifetimes of Pr^{3+ 1}D₂ as presented in the insert of Fig. 2(b). The decay curves (insert) of the ¹D₂ is measured by monitoring at 608 nm upon pulsed excitation at 288 nm. A descending dependence of Pr³⁺ emission intensities and its lifetimes on x are plotted in Fig. 2(b). They fit well for small x, but that the Pr³⁺ red emission intensity reduces faster than its lifetime for x higher than 0.005. This is considered to be the result of reabsorption of the red line by the spin-allowed ⁴A₂ – ⁴T₂ (4 F) transition of Cr³⁺.

 

Fig. 2 (a) PL spectra of (Y_0.995Pr_0.005)₃(Al_1-xCr_x)₅O₁₂, (x = 0, 0.0025, 0.005, 0.0075, 0.01, 0.0125, 0.015) under 288 nm excitation. The intensity of the pale red peak in each spectrum is normalized; (b) Pr³⁺ red fluorescence intensity and lifetime changed with increasing Cr³⁺ concentration x in (Y_0.985Pr_0.005Ce_0.01)₃(Al_1-xCr_x)₅O₁₂. Inset shows decay curves of the pale red fluorescence in (Y_0.985Pr_0.005Ce_0.01)₃(Al_1-xCr_x)₅O₁₂ for x = 0, 0.0025, 0.0075 and 0.015.

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According to the PL spectra in Fig. 2(a), the deep red component provided by Pr³⁺→Cr³⁺ energy transfer in the triply doped samples [see Fig. 1(g)] can be estimated from the corresponding intensity of the red line of Pr³⁺. Figure 3(a) shows x dependence of I_Cr/I_Ce with its two components (One is fed by Ce³⁺→Cr³⁺ energy transfer. Another one is fed by Pr³⁺→Cr³⁺ energy transfer) in the triply doped samples. The component fed by Ce³⁺→Cr³⁺ energy transfer is evaluated from I_Cr/I_Ce in Ce³⁺ and Cr³⁺ doubly doped samples multiplied by 1.32. Figure 3(a) demonstrates that the combination of the two components is in a good agreement with the total intensity of the deep red emission of Cr³⁺.

 

Fig. 3 (a) Dependence of the emission ratio (I_Ce/I_Cr) on Cr³⁺ concentration x in (Y_1-z-yCe_zPr_y)₃(Al_1-xCr_x)₅O₁₂; (b) EL spectra of the white LEDs using different phosphors coated on InGaN-based blue chips.

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To test the triply doped phosphors, (Y_0.98Ce_0.02)₃Al₅O₁₂, (Y_0.98Ce_0.02)₃(Al_0.999Cr_0.001)₅O₁₂ and (Y_0.978Ce_0.02Pr_0.002)₃(Al_0.999Cr_0.001)₅O₁₂ phosphors are selected to fabricate LEDs using the blue InGaN LED chips. The electroluminescence (EL) emission spectra of the white LEDs measured under a forward-bias current of 20 mA is shown in Fig. 3(b). The color coordinates and the color rendering indices (CRI) of the fabricated white LED are listed in Table 2 . The CRI of the white LED fabricated with the triply doped phosphor is 81.4 that is higher than 80.4 fabricated using the doubly doped phosphor and 78.3 using the singly doped phosphor. The luminous efficiency of the present white LEDs fabricated with singly, doubly and triply doped phosphors are 99.2 lm/W, 88.3 lm/W and 84.3 lm/W, respectively, yielding the luminous efficiency for the triply doped phosphor around 85% of the singly doped one. For the luminescence quantum yield of the triply doped phosphor that is not measured in this work could be higher than 85% of YAG:Ce³⁺ in view of vision function are insensitive to red light.

Table 2. Optical properties of white LED

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4. Conclusions

Y₃Al₅O₁₂: Ce³⁺, Pr³⁺, Cr³⁺ phosphors are prepared by solid state reaction. Three typical emission bands: yellow emission from Ce³⁺, light red emission from Pr³⁺ and deep red emission from Cr³⁺ are achieved upon blue light excitation on Ce³⁺. The study of photoluminescence and fluorescence decay indicates that there are Ce³⁺→Cr³⁺ and Ce³⁺→Pr³⁺→Cr³⁺ energy transfers. For the same Cr³⁺ concentration, the macroscopic Ce³⁺→Cr³⁺ energy transfer rate in the triply doped phosphor is larger than that in the doubly doped one, which is attributed to the competition between Ce³⁺→Pr³⁺ energy transfer and Ce³⁺→Cr³⁺ energy transfer. A white LED fabricated using a blue LED chip with the triply doped phosphor shows a color rendering index of 81.4 that is higher than that either using Ce³⁺, Cr³⁺ doubly doped or Ce³⁺ singly doped phosphors.