Triply doped Y3Al5O12: Ce3+, Pr3+, Cr3+ phosphors are prepared by solid state reaction. The emission spectra are enriched in the red region with the luminescence of both Pr3+ and Cr3+ through Ce3+→Cr3+ and Ce3+→Pr3+→Cr3+ energy transfers. The properties of photoluminescence and fluorescence decay indicates larger macroscopic Ce3+→Cr3+ transfer rates in the triply doped phosphors in comparison to Ce3+ and Cr3+ doubly doped one, reflecting the effect of competition between Ce3+→Cr3+ and Ce3+→Pr3+ transfers. White LEDs fabricated using the triply doped phosphor coated on blue LED chips show a color rendering index of 81.4 higher than that either using Ce3+ and Cr3+ doubly doped or Ce3+ singly doped phosphor.
© 2010 OSA
Phosphor-converted white light–emitting diodes (pcWLEDs) are potential replacements for conventional light sources such as incandescent or fluorescent lamps . The general strategy for producing pcWLED is to combine blue LED with the yellow emitting Y3Al5O12: Ce3+ (YAG: Ce3+) phosphor at present . 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  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.  selected Cr3+ as an red emitting center to incorporate into YAG: Ce3+ phosphor to replace Al sites  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.
Powder phosphor samples were made using mixtures of high-purity Y2O3, CeO2, Al2O3, Cr2O3 and Pr6O11 in molar of (Y1-z-yCezPry)3 (Al1-xCrx)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 , 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 3HJ (J = 4, 5, 6) and the 3FJ (J = 2, 3, 4) manifolds, respectively . The group in the range of 450 – 600 nm originates from 3P0→3H4,5 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 O2- – 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+.
Figure 1(g) shows PL spectra of triply doped sample series A (solid): (Y0.985Ce0.01Pr0.005)3(Al1-xCrx)5O12 and Ce3+, Cr3+ doubly doped sample series B (dotted): (Y0.99Ce0.01)3(Al1-xCrx)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
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 (ICr/ICe) 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 (ICr/ICe) 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: (Y0.995Pr0.005)3(Al1-xCrx)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 Pr3+ 1D2→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 Pr3+ 1D2 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+.
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 ICr/ICe 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 ICr/ICe 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+.
To test the triply doped phosphors, (Y0.98Ce0.02)3Al5O12, (Y0.98Ce0.02)3(Al0.999Cr0.001)5O12 and (Y0.978Ce0.02Pr0.002)3(Al0.999Cr0.001)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.
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.
This work is financially supported by the National Nature Science Foundation of China (10834006, 10774141, 10904141, 10904140), the MOST of China (2006CB601104), the Scientific project of Jilin province (20090134, 20090524) and CAS Innovation Program.
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