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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 24 — Nov. 22, 2010
  • pp: 25177–25182
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Enriching red emission of Y3Al5O12: Ce3+ by codoping Pr3+ and Cr3+ for improving color rendering of white LEDs

Lei Wang, Xia Zhang, Zhendong Hao, Yongshi Luo, Xiao-jun Wang, and Jiahua Zhang  »View Author Affiliations


Optics Express, Vol. 18, Issue 24, pp. 25177-25182 (2010)
http://dx.doi.org/10.1364/OE.18.025177


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Abstract

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

1. Introduction

Phosphor-converted white light–emitting diodes (pcWLEDs) are potential replacements for conventional light sources such as incandescent or fluorescent lamps [1

1. J. K. Kim and E. F. Schubert, “Transcending the replacement paradigm of solid-state lighting,” Opt. Express 16(26), 21835–21842 (2008). [CrossRef] [PubMed]

]. The general strategy for producing pcWLED is to combine blue LED with the yellow emitting Y3Al5O12: Ce3+ (YAG: Ce3+) phosphor at present [2

2. J. R. Oh, S. H. Cho, Y. H. Lee, and Y. R. Do, “Enhanced forward efficiency of Y3Al5O12:Ce3+ phosphor from white light-emitting diodes using blue-pass yellow-reflection filter,” Opt. Express 17(9), 7450–7457 (2009). [CrossRef] [PubMed]

]. 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

3. R. Mueller-Mach, G. O. Mueller, M. R. Krames, and T. Trottier, “High-Power Phosphor-Converted Light-Emitting Diodes Based on III-Nitrides,” IEEE J. Sel. Top. Quantum Electron. 8(2), 339–345 (2002). [CrossRef]

7

7. W. Wang, J. Tang, S. T. V. Hsu, J. Wang, and B. P. Sullivan, “Energy transfer and enriched emission spectrum in Cr and Ce co-doped Y3Al5O12 yellow phosphors,” Chem. Phys. Lett. 457(1-3), 103–105 (2008). [CrossRef]

]. Mueller-Mach and associates [3

3. R. Mueller-Mach, G. O. Mueller, M. R. Krames, and T. Trottier, “High-Power Phosphor-Converted Light-Emitting Diodes Based on III-Nitrides,” IEEE J. Sel. Top. Quantum Electron. 8(2), 339–345 (2002). [CrossRef]

] added Pr3+ into YAG:Ce3+ to replace Y and consequently obtained a red emission line at 608 nm, originating from 1D23H4 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

4. H. S. Jang, W. B. Im, D. C. Lee, D. Y. Jeon, and S. S. Kim, “Enhancement of red spectral emission intensity of Y 3 Al 5 O 12:Ce3+ phosphor via Pr co-doping and Tb substitution for the application to white LEDs,” J. Lumin. 126(2), 371–377 (2007). [CrossRef]

,5

5. H. H. Yang and Y. S. Kim, “Energy transfer-based spectral properties of Tb-, Pr-, or Sm-codoped YAG:Ce nanocrystalline phosphors,” J. Lumin. 128(10), 1570–1576 (2008). [CrossRef]

]. Recently, Wang et al. [7

7. W. Wang, J. Tang, S. T. V. Hsu, J. Wang, and B. P. Sullivan, “Energy transfer and enriched emission spectrum in Cr and Ce co-doped Y3Al5O12 yellow phosphors,” Chem. Phys. Lett. 457(1-3), 103–105 (2008). [CrossRef]

] selected Cr3+ as an red emitting center to incorporate into YAG: Ce3+ phosphor to replace Al sites [8

8. K. M. Kinsman, J. McKittrick, E. Sluzky, and K. Hesse, “Phase Development and Luminescence in Chromium-Doped Yttrium Aluminum Garnet (YAG:Cr) Phosphors,” J. Am. Ceram. Soc. 77(11), 2866–2872 (1994). [CrossRef]

] 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 (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(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

Table 1. Fluorescent lifetimes and transfer efficiencies in (Y0.99Ce0.01)3(Al1-xCrx)5O12 and (Y0.985Ce0.01Pr0.005)3(Al1-x Crx) 5O12

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. 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

W=1/τCe1/τCe,0.
(1)

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 Cr, as expressed by
ICr/ICe= (γCr/γCe)WτCr,
(2)
where the radiative transiton rate γ’s are considered to be independent on Cr3+ concentration.

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 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)
Fig. 2 (a) PL spectra of (Y0.995Pr0.005)3(Al1-xCrx)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 (Y0.985Pr0.005Ce0.01)3(Al1-xCrx)5O12. Inset shows decay curves of the pale red fluorescence in (Y0.985Pr0.005Ce0.01)3(Al1-xCrx)5O12 for x = 0, 0.0025, 0.0075 and 0.015.
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+ 1D23H4 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 4A24T2 (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)
Fig. 3 (a) Dependence of the emission ratio (ICe/ICr) on Cr3+ concentration x in (Y1-z-yCezPry)3(Al1-xCrx)5O12; (b) EL spectra of the white LEDs using different phosphors coated on InGaN-based blue chips.
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

Table 2. Optical properties of white LED

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. 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.

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.

Acknowledgements

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.

References and links

1.

J. K. Kim and E. F. Schubert, “Transcending the replacement paradigm of solid-state lighting,” Opt. Express 16(26), 21835–21842 (2008). [CrossRef] [PubMed]

2.

J. R. Oh, S. H. Cho, Y. H. Lee, and Y. R. Do, “Enhanced forward efficiency of Y3Al5O12:Ce3+ phosphor from white light-emitting diodes using blue-pass yellow-reflection filter,” Opt. Express 17(9), 7450–7457 (2009). [CrossRef] [PubMed]

3.

R. Mueller-Mach, G. O. Mueller, M. R. Krames, and T. Trottier, “High-Power Phosphor-Converted Light-Emitting Diodes Based on III-Nitrides,” IEEE J. Sel. Top. Quantum Electron. 8(2), 339–345 (2002). [CrossRef]

4.

H. S. Jang, W. B. Im, D. C. Lee, D. Y. Jeon, and S. S. Kim, “Enhancement of red spectral emission intensity of Y 3 Al 5 O 12:Ce3+ phosphor via Pr co-doping and Tb substitution for the application to white LEDs,” J. Lumin. 126(2), 371–377 (2007). [CrossRef]

5.

H. H. Yang and Y. S. Kim, “Energy transfer-based spectral properties of Tb-, Pr-, or Sm-codoped YAG:Ce nanocrystalline phosphors,” J. Lumin. 128(10), 1570–1576 (2008). [CrossRef]

6.

Y. Pan, M. Wu, and Q. Su, “Tailored photoluminescence of YAG:Ce phosphor through various methods,” Phys. Chem. Solids 65(5), 845–850 (2004). [CrossRef]

7.

W. Wang, J. Tang, S. T. V. Hsu, J. Wang, and B. P. Sullivan, “Energy transfer and enriched emission spectrum in Cr and Ce co-doped Y3Al5O12 yellow phosphors,” Chem. Phys. Lett. 457(1-3), 103–105 (2008). [CrossRef]

8.

K. M. Kinsman, J. McKittrick, E. Sluzky, and K. Hesse, “Phase Development and Luminescence in Chromium-Doped Yttrium Aluminum Garnet (YAG:Cr) Phosphors,” J. Am. Ceram. Soc. 77(11), 2866–2872 (1994). [CrossRef]

9.

G. G. Özen, O. Forte, and B. Di Bartolo, “Down-conversion and upconversion dynamics in Pr-doped Y3Al5O12 crystals,” J. Appl. Phys. 97(1), 013510 (2005). [CrossRef]

10.

G. G. Özen, O. Forte, and B. Di Bartolo, “Upconversion dynamics in Pr-doped YAlO3 and Y3Al5O12 laser crystals,” Opt. Mater. 27(11), 1664–1671 (2005). [CrossRef]

11.

M. Malinowski, P. Szczepanski, W. Woliñski, R. Wolski, and Z. Frukacz, “Inhomogeneity study of Pr3+-doped yttrium aluminium garnet using time-resolved spectroscopy,” J. Phys. Condens. Matter 5(35), 6469–6482 (1993). [CrossRef]

12.

Y. R. Shen and K. L. Bray, “Effect of pressure and temperature on the lifetime of Cr3+ in yttrium aluminum garnet,” Phys. Rev. B 56(17), 10882–10891 (1997). [CrossRef]

13.

W. W. Jia, H. Liu, S. Jaffe, W. M. Yen, and B. Denker, “Spectroscopy of Cr3+ and Cr+4 ions in forsterite,” Phys. Rev. B 43(7), 5234–5242 (1991). [CrossRef]

OCIS Codes
(160.5690) Materials : Rare-earth-doped materials
(230.3670) Optical devices : Light-emitting diodes
(250.5230) Optoelectronics : Photoluminescence
(260.2160) Physical optics : Energy transfer

ToC Category:
Optical Devices

History
Original Manuscript: September 8, 2010
Revised Manuscript: October 24, 2010
Manuscript Accepted: October 28, 2010
Published: November 17, 2010

Citation
Lei Wang, Xia Zhang, Zhendong Hao, Yongshi Luo, Xiao-jun Wang, and Jiahua Zhang, "Enriching red emission of Y3Al5O12: Ce3+ by codoping Pr3+ and Cr3+ for improving color rendering of white LEDs," Opt. Express 18, 25177-25182 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-24-25177


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References

  1. J. K. Kim and E. F. Schubert, “Transcending the replacement paradigm of solid-state lighting,” Opt. Express 16(26), 21835–21842 (2008). [CrossRef] [PubMed]
  2. J. R. Oh, S. H. Cho, Y. H. Lee, and Y. R. Do, “Enhanced forward efficiency of Y3Al5O12:Ce3+ phosphor from white light-emitting diodes using blue-pass yellow-reflection filter,” Opt. Express 17(9), 7450–7457 (2009). [CrossRef] [PubMed]
  3. R. Mueller-Mach, G. O. Mueller, M. R. Krames, and T. Trottier, “High-Power Phosphor-Converted Light-Emitting Diodes Based on III-Nitrides,” IEEE J. Sel. Top. Quantum Electron. 8(2), 339–345 (2002). [CrossRef]
  4. H. S. Jang, W. B. Im, D. C. Lee, D. Y. Jeon, and S. S. Kim, “Enhancement of red spectral emission intensity of Y 3 Al 5 O 12:Ce3+ phosphor via Pr co-doping and Tb substitution for the application to white LEDs,” J. Lumin. 126(2), 371–377 (2007). [CrossRef]
  5. H. H. Yang and Y. S. Kim, “Energy transfer-based spectral properties of Tb-, Pr-, or Sm-codoped YAG:Ce nanocrystalline phosphors,” J. Lumin. 128(10), 1570–1576 (2008). [CrossRef]
  6. Y. Pan, M. Wu, and Q. Su, “Tailored photoluminescence of YAG:Ce phosphor through various methods,” Phys. Chem. Solids 65(5), 845–850 (2004). [CrossRef]
  7. W. Wang, J. Tang, S. T. V. Hsu, J. Wang, and B. P. Sullivan, “Energy transfer and enriched emission spectrum in Cr and Ce co-doped Y3Al5O12 yellow phosphors,” Chem. Phys. Lett. 457(1-3), 103–105 (2008). [CrossRef]
  8. K. M. Kinsman, J. McKittrick, E. Sluzky, and K. Hesse, “Phase Development and Luminescence in Chromium-Doped Yttrium Aluminum Garnet (YAG:Cr) Phosphors,” J. Am. Ceram. Soc. 77(11), 2866–2872 (1994). [CrossRef]
  9. G. G. Özen, O. Forte, and B. Di Bartolo, “Down-conversion and upconversion dynamics in Pr-doped Y3Al5O12 crystals,” J. Appl. Phys. 97(1), 013510 (2005). [CrossRef]
  10. G. G. Özen, O. Forte, and B. Di Bartolo, “Upconversion dynamics in Pr-doped YAlO3 and Y3Al5O12 laser crystals,” Opt. Mater. 27(11), 1664–1671 (2005). [CrossRef]
  11. M. Malinowski, P. Szczepanski, W. Woliñski, R. Wolski, and Z. Frukacz, “Inhomogeneity study of Pr3+-doped yttrium aluminium garnet using time-resolved spectroscopy,” J. Phys. Condens. Matter 5(35), 6469–6482 (1993). [CrossRef]
  12. Y. R. Shen and K. L. Bray, “Effect of pressure and temperature on the lifetime of Cr3+ in yttrium aluminum garnet,” Phys. Rev. B 56(17), 10882–10891 (1997). [CrossRef]
  13. W. W. Jia, H. Liu, S. Jaffe, W. M. Yen, and B. Denker, “Spectroscopy of Cr3+ and Cr+4 ions in forsterite,” Phys. Rev. B 43(7), 5234–5242 (1991). [CrossRef]

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