Inverse bottleneck in Eu2+–Mn2+ energy transfer

ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1459–1463

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Inverse bottleneck in Eu2+–Mn2+ energy transfer U. Happek a,, A.A. Setlur b, J.J. Shiang b a b

Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, United States GE Global Research, 1 Research Circle, Niskayuna, NY 12309, United States

a r t i c l e in fo

abstract

Available online 17 April 2009

The energy transfer (ET) between Eu2+ and Mn2+ in Ca5(PO4)3Cl has been investigated. At low intensities under 405 nm excitation, time-resolved experiments provide microscopic parameters for the energy transfer between adjacent Eu2+ and Mn2+ ions. At high intensities, we observe a non-linear component in the energy transfer process due to ground state depletion of the Mn2+ ions, leading not to a reduction, but to an increase in the energy transfer rate between Eu2+ and Mn2+ ions and also energy transfer between excited Mn2+ ions. This results in a sub-linear response of both Eu2+ and Mn2+ luminescence as a function of intensity. Our observations are quantitatively described by a model using energy transfer to Mn2+ ions in both the ground and the excited state. & 2009 Elsevier B.V. All rights reserved.

Keywords: Energy transfer Luminescence LED

1. Introduction

2. Experimental procedure

Solid-state lighting using phosphor-converted LEDs (pcLEDs) has the potential to replace traditional incandescent and fluorescent lighting due to the promise of higher efficacies [1–3]. Fundamental understanding of the phosphor performance within LED packages will aid in the development of these lamps, especially towards understanding the effects of the high light flux (410 W/cm2) and temperatures (up to 150 1C) on the phosphor efficiency. Our initial studies [4] have focused on Eu2+–Mn2+-based phosphors, since this sensitizer–activator combination can be implemented in numerous hosts that can strongly absorb violet LED radiation through the allowed Eu2+ 4f7-4f65d1 transition and have efficient green to deep-red emission from forbidden Mn2+ d–d transitions. These phosphors also offer the potential advantage of separating the phosphor absorption and emission spectra, thereby reducing any potential losses due to reabsorption of phosphor emission. However, the relatively slow decay time of forbidden Mn2+ d–d emission band creates a relatively large Mn2+-excited state population at the excitation densities present in 5 mm LED packages [4]. The enhanced Mn2+excited state population creates additional paths for energy transfer (ET), leading to non-radiative losses under typical LED operation. In this report, we will describe and spectroscopically quantify these losses, also discussing the changes in the Eu2+Mn2+ energy transfer process as the Mn2+-excited state concentration increases.

Halophosphate samples were made using CaHPO4, Eu2O3, MnCO3, CaCO3, and CaCl2 with an excess of CaCl2 to account for the volatility of CaCl2. Samples were fired at 950–1100 1C in reducing atmospheres of N2/H2 and were crushed and washed after firing. The compositions in this report reflect the nominal Eu2+ and Mn2+ levels replacing Ca2+ in the starting mixtures. Phosphor emission measurements were made on powders pressed into an aluminum plaque using a SPEX Fluorolog 2 with corrections for Xe lamp intensity and instrument response. The Eu2+ yield as a function of the Mn2+ concentration in these halophosphates was estimated by taking the ratio of the Eu2+ emission to the Mn2+ emission (Fig. 1) and assumes that the overall quantum efficiency of the phosphor does not change at higher Mn2+ concentrations. Time-resolved measurements used 394 nm laser (PicoQuant) or a pulsed Xe lamp coupled into an Edinburgh F900 spectrofluorometer with a Peltier cooled R928-P Hamamatsu photomultiplier tube (PMT) detector. The FWHM of the laser pulse convoluted with the overall system response is 1 ns. All luminescence measurements were taken at room temperature unless specifically mentioned otherwise. In the LED experiments, a Ca5(PO4)Cl:Eu2+ (4%), Mn2+ (9%) halophosphate (HALO) was dispersed into a polymer, deposited onto 405 nm InGaN LEDs (7–8 mW optical power at 3.7 V/20 mA, 0.3 mm  0.3 mm active area), and packaged into 5 mm lamps. Initial evaluation of these pcLEDs used intensity measurements under pulsed operation (pulse frequency 5 Hz; variable pulse length of 0.5–110 ms) at room temperature. pcLEDs were placed into an integrating sphere (Labsphere) coupled into a spectrograph (Roper Scientific). The Mn2+ emission band is easily separated, while the Eu2+ intensity is estimated by integrating

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E-mail addresses: [email protected], [email protected] (U. Happek). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.12.029

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10000

0% Mn2+

λex=390 nm

1% Mn

2+

2% Mn2+

Intensity (a.u.)

4% Mn2+

Intensity (a.u.)

6% Mn2+ 9% Mn2+

Exchange

1000

Dipole-dipole

100 Dipole-quadrupole

0 450

500

550 600 Wavelength (nm)

650

700

1000 Time (ns)

1500

2000

750

Fig. 1. Emission spectra for (Ca0.999xEu0.001Mnx)5(PO4)3Cl phosphors (lex ¼ 390 nm).

from 430 to 510 nm to separate it from LED radiation bleeding through the phosphor coating. The pulse length was also significantly shorter than the pulse period to prevent excited state build-up over multiple pulses. In addition, the specific saturation processes for Eu2+ and Mn2+ in HALO-coated pcLEDs were determined by time-resolved luminescence measurements using a monochromator and photomultiplier tube detector (Hamamatsu).

3. Results and discussion

Fig. 2. Eu2+ decay profiles for (Ca0.909Eu0.001Mn0.09)5(PO4)3Cl (lex ¼ 394 nm) with least squares fits for Eu2+-Mn2+ (ground) energy transfer by dipole–dipole, dipole–quadrupole, and exchange mechanisms.

1.0 0.9 Relative Eu2+ efficiency

400

500

0.8 0.7 0.6 0.5

3.1. Energy transfer to the ground state of Mn2+ 0.4

Eu2+ energy transfer to the ground state of Mn2+ is the starting point for the key luminescent processes within these materials. Given the low oscillator strength of Mn2+ in HALO (4  107 [5]), it is likely that short-range energy transfer mechanisms, such as exchange or super-exchange, govern Eu2+-Mn2+ (ground) energy transfer in HALO. Given this, we have used the analysis method from Dornauf and Heber [6] to analyze the Eu2+ (donor) decay curves, since these methods incorporate the discrete crystal structure of the halophosphate host lattice, leading to a better accounting of short-range interactions. In this analysis of Eu2+Mn2+ (ground) energy transfer, the Eu2+ concentration is kept at 0.1%, replacing Ca2+, to minimize the effects of Eu2+–Eu2+ energy migration. Also, we have made some simplifying assumptions regarding the crystal structure of this halophosphate for this analysis, given that the crystal structure of Ca-chlorophosphate is relatively complex with five separate Ca2+ sites [7]. Instead, we use the typical hexagonal halophosphate structure with 90% Cl and 10% F in the halide sites [8]. In addition, we take the simplifying assumptions of Mn2+ occupying only the Ca(1) site as previously suggested [5,9] and that Eu2+ statistically occupies the Ca(1) and Ca(2) sites as has been determined through a Rietveld refinement of powder X-ray diffraction data for Eu2+-doped Sr-, Ca-chloroapatites [10]. The relevant sums are taken out to Eu2+–Mn2+ distances of 12 A˚. The Eu2+ decay profile (Fig. 2) for a 9% Mn2+ (replacing Ca2+) is fit using a least squares method for dipole–dipole (R6 dependence), dipole–quadrupole (R8 dependence), and exchange

0.3 0.00

0.02

0.04 Mn2+

0.06

0.08

0.10

concentration

Fig. 3. Experimental Eu2+ quantum efficiency versus Mn2+ concentration versus calculated values using Eu2+-Mn2+ (ground) energy transfer via exchange.

(exp[2R/L] dependence, where L is an effective Bohr radius for the ground and excited state of the donor and acceptor [11]) energy transfer mechanisms. In these fits, there is no angular dependence on the energy transfer rate. The least squares fit to the Eu2+ decay profile clearly favors Eu2+-Mn2+ (ground) energy transfer by exchange (Fig. 2) similar to previous studies for Sb3+–Mn2+ energy transfer in the halophosphates [9]. In this fit, the critical distance, defined as when the energy transfer rate is equivalent to the Eu2+ radiative rate (t580 ns), is determined to be 5.4 A˚. Taking into account the halophosphate crystal structure, this implies that energy transfer occurs only between Eu2+–Mn2+ neighbors that are o4 A˚ apart. The effective Bohr radius is calculated to be 0.67 A˚, a reasonable value given the spatial extension of the Eu2+ 4f65d1 excited state. Further support for the assignment of exchange for Eu2+-Mn2+ (ground) energy transfer is also given by the excellent fit for the relative Eu2+ efficiency versus Mn2+ concentration (Fig. 3).

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3.2. Energy transfer to the excited state of Mn2+ The primary rationale in this study for analyzing energy transfer to excited Mn2+ ions is the performance of HALO-coated pcLEDs at high duty cycles when pulsing these pcLEDs. When keeping a constant pulse period, the relative Eu2+ and Mn2+ efficiency is dramatically reduced at longer pulse lengths (Fig. 4): the Mn2+ efficiency is reduced by 40% and the Eu2+ efficiency reduced by 20% at the longest pulse lengths. Thermal effects are found to be negligible by analyzing the Mn2+ emission green shift and intensity versus temperature; the 150 1C efficiency of HALO is 490% of the room temperature value. Given the literature on the saturation of Mn2+ phosphors whose main cause is the slow Mn2+ emission (t11.3 ms in HALO) [12–17], it is reasonable to assign the Mn2+ quenching of HALO in these pcLEDs to saturation. However, standard models cannot explain the Eu2+ quenching due to the fast radiative decay time of Eu2+ (t580 ns

1.0

Relative efficiency

0.9

Eu2+

0.8 0.7 Mn2+

0.6

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for Ca5(PO4)3Cl:Eu2+), and experiments with Eu2+ phosphors in similar pcLEDs show virtually no quenching with variations in the LED excitation duty cycle. Time-resolved luminescence experiments with HALO pcLEDs show a reduction in the Eu2+ intensity occurs over a 20 ms timescale that is directly correlated to Mn2+-excited state population (Fig. 5). There are transitions from the Mn2+ emitting state resonant with the Eu2+ emission [18,19] that could have higher oscillator strengths than the parity and spin forbidden transitions from the ground state. Naı¨vely, one might think that an increase in the Mn2+-excited state population would create a bottleneck for Eu2+-Mn2+ energy transfer due to a reduction in the ground state population. However, these results show that excited Mn2+ ions are a strong sink for Eu2+ donors, enhancing the rate of energy transfer from Eu2+ without an increase in the Mn2+ intensity (‘‘inverse bottleneck’’). Apart from Eu2+-Mn2+ energy transfer, we also observe that the Mn2+ decay has additional non-radiative components at high duty cycles in these experiments (Fig. 6); a single exponential fit to the Mn2+ decay profile in HALO-coated pcLEDs has t ¼ 9.6 ms versus spectrometer measurements of 11.3 ms. The faster decay is indication that energy transfer to the Mn2+-excited state also occurs between excited Mn2+ ions [13,17,20]. We note that the ET quenching processes involving the Mn2+excited state are fundamentally similar to upconversion [21] (Fig. 8). Taking this similarity into account, we take the route of using rate equations to analyze both Eu2+ and Mn2+ emission quenching with the addition of ground state depletion [12,16,17] due to the long Mn2+ lifetime and relatively high violet LED fluxes    dE M GEu!Mn ¼ fsðE0 Þ  Gr;Eu þ 1  dt M0  M n E þ for 0otoT G M 0 Eu!Mn

0.5 0.4 0.3 1

10 Pulse length (ms)

100

Fig. 4. Integrated Eu2+ and Mn2+efficiencies in HALO-coated pcLEDs versus pulse length. The solid lines are calculated using the rate equations in the text [4].

(1)

   dE M GEu!Mn ¼  Gr;Eu þ 1  dt M0  M n E þ for TotoT 0 G M 0 Eu!Mn

(2)

1

Intensity (a.u.)

Eu2+

Intensity (a.u.)

Mn2+ low intensity Mn2+

Spectrometer

0.1

Calculated

pcLED

0.01

0 0

20

40

60

80 100 Time (ms)

120

140

160

Fig. 5. Eu2+ (lem470 nm) and Mn2+ luminescence (lem600 nm) in a HALO pcLED with a 100 ms pulse length. The Mn2+ decay in the pcLED is compared to the Mn2+ decay in a low-intensity spectrometer measurement [4].

10

20

30

40

50

Time (ms) Fig. 6. Mn2+ luminescence decay (lem600 nm) in a HALO-coated pcLED for a pulse length of 30 ms and a repetition rate of 2.5 Hz compared to the calculated decay profile using Eq. (3) and the decay profile measured in a spectrometer (lex ¼ 405 nm).

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dM ¼ dt

  M GEu!Mn E  MGr;Mn  kM 2 1 M0

for all t

(3)

where E is the excited Eu2+ density, f the excitation flux, s the absorption cross-section, E0 the total Eu2+ density, Gr,Eu the radiative rate of Eu2+ in HALO without Mn2+ (1.7  106 s1), M the excited Mn2+ density, M0 the total Mn2+ density, GEu-Mn the Eu2+-Mn2+(ground) ET rate (1.3  107 s1 using the relative Eu2+ QE), GEu-Mn* the Eu2+-Mn2+(excited) ET rate, T the pulse length, T0 the pulse period (5 Hz), Gr,Mn the radiative rate for Mn2+ (89 s1), and k rate constant for ET between excited Mn2+ ions [13,17,20]. There are two main assumptions that we have in using these rate equations. First, Mn2+ excited state absorption of LED radiation is neglected since parity-allowed transitions from the emitting Mn2+ level are at higher energy than the incident LED radiation [18,19]. Second, after energy transfer to excited Mn2+, we assume that Mn2+ directly returns to the lowest excited state, i.e. we exclude loss channels due to photoionization or level crossing based on the high efficiency of halophosphate phosphors under UV excitation [18,19]. The Eu2+ and Mn2+ efficiency versus pulse length are calculated with adaptive Runge–Kutta integration of Eqs. (1)–(3) [22]. The relative Eu2+ and Mn2+ efficiency is fit well using values of fs ¼ 30 s1, GEu-Mn* ¼ 7.0  107 s1, and k ¼ 2.2  1019 cm3s1 (Fig. 3). Approximately 7% of the Mn2+ ions are in the excited state under typical, steady state conditions. There is also quantitative agreement between model and experiment for the Mn2+ luminescence decay (Fig. 6) and build-up (Fig. 7). The somewhat larger deviations between model and experiment for the relative Eu2+ efficiency (o0.04 for all points) are likely due to the slight temperature increase at long pulses, changing ET rates due to band broadening and changes in oscillator strengths. The fit to the rate equations shows a 5  enhancement of the energy transfer rate for Eu2+-Mn2+ (excited) versus Eu2+-Mn2+ (ground). We believe that this enhancement is probably not due to a change in the mechanism for Eu2+-Mn2+ (excited) from exchange to a longer-range multipole–multipole mechanism. If Eu2+-Mn2+ (excited) energy transfer is still due to exchange, this would imply an 2.25  increase in the exchange integral [11] for Eu2+-Mn2+ (excited) energy transfer versus Eu2+-Mn2+ (ground)

No saturation

Intensity (a.u)

Experiment

X

X

ΓEu→Mn∗

k

Γr,Eu

hν

hν X

hν

ΓEu→Mn

Eu2+

Γr,Mn

Mn2+

Γr,Mn

Mn2+

Mn2+

Fig. 8. Schematic of Mn2+(excited)–Mn2+(excited) and Eu2+-Mn2+ (excited) energy transfer quenching processes. The transitions marked with ‘‘X’’ are nonradiative transitions [4].

energy transfer, assuming a similar spectral overlap for the relevant transitions. This relatively small change in the exchange integral could be due to changes in wavefunction overlap between the ground and excited states of Mn2+ with Eu2+. However, further understanding of the excited state transitions of Mn2+ in HALO will aid in definitively assigning the mechanism of Eu2+-Mn2+ (excited) energy transfer. It would also be useful to extend this to other materials such to further understand this quenching mechanism and its extent in other hosts.

4. Summary and conclusions We have described the energy transfer processes within Ca5(PO4)3Cl:Eu, Mn phosphors that could potentially be used in LED-based lighting systems. First, we quantitatively determine that the energy transfer mechanism between Eu2+ and Mn2+(ground) in these halophosphates is due to exchange as expected due to the low Mn2+ oscillator strength [5,9] as well as the prior work analyzing Sb3+–Mn2+ energy transfer [9]. Furthermore, we identify that the long radiative lifetime of Mn2+ can lead to additional energy transfer quenching paths in 5 mm pcLEDs using 405 nm LEDs. We demonstrate that these energy transfer paths are between Eu2+ and excited Mn2+ ions and between excited Mn2+ ions, leading to a reduction of both Eu2+ and Mn2+ luminescence. Taking these energy transfer processes into account and using simple rate equations, we are able to quantitatively understand the performance of Ca5(PO4)3Cl:Eu,Mn in LED packages.

Acknowledgements Calculated

0

5

10

15 Time (ms)

20

25

30

Fig. 7. Mn2+ luminescence build-up (lem600 nm) in a HALO pcLED using a pulse period of 2.5 Hz and pulse length of 30 ms versus a calculated build-up curve from the integrated solution to Eqs. (1)–(3) and in the absence of saturation [4].

We thank C.-L. Hsing for LED packaging; and A.M. Srivastava, D.D. Doxsee, E. Radkov, T.F. Soules, A. Meijerink, and H.-U. Gu¨del for helpful discussions. This work was supported by Lumination LLC and the US Department of Energy through Contract nos. DE-FC26-04NT41956 and DE-FC26-06NT42934. This report was prepared as an account of work partially sponsored by an agency of the United States Government. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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