Spectral Properties and Energy Transfer of a Potential Solar Energy Converter

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Article

Spectral Properties and Energy Transfer of a Potential Solar Energy ConverterLei Zhou, Weijie Zhou, Fengjuan Pan, Rui Shi, Lin Huang, Hongbin Liang,

Peter A. Tanner, Xueyan Du, Yan Huang, Ye Tao, and Lirong Zheng

Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00763 • Publication Date (Web): 29 Mar 2016

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Spectral Properties and Energy Transfer of a Potential Solar Energy

Converter

Lei Zhoua, Weijie Zhoua, Fengjuan Pana, Rui Shia, Lin Huanga, Hongbin

Liang,*a Peter A. Tanner* b, Xueyan Duc, Yan Huangc, Ye Taoc, Lirong

Zhengc

aMOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic

Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University,

Guangzhou 510275, P.R. China.

E-mail: [email protected];

bThe Hong Kong Institute of Education (Education University of Hong Kong, designate), 10 Lo Ping

Road, Tai Po, Hong Kong S.A.R., P.R. China

E-mail: [email protected];

cBeijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Sciences, Beijing 100039, P.R. China

ABSTRACT: The energy transfer between Ce3+ and Eu2+ has been investigated in the host

Ca3Sc2Si3O12 (CSS), prepared by a modified sol-gel method. Excitation and emission measurements

from the near infrared to the vacuum ultraviolet spectral regions have been performed upon CSS,

Ce3+-doped CSS, Eu2+-doped CSS and Ce3+, Eu2+-co-doped CSS, at various concentrations, including

experiments at temperatures range of 15-460 K. The energy transfer efficiency from Ce3+ to Eu2+ can

approach 90% and the Ce3+ donor decay curves for different Eu2+ acceptor concentrations in the

co-doped system were fitted by the Inokuti-Hirayama method, indicating that it is energy transfer

induced by electric dipole interaction. The use of the Ce3+, Eu2+ couple in the CSS host as a wideband

harvester with an emission profile tailored to the response of the silicon solar cell in solar energy

conversion suffers from two main drawbacks relating to valence instability and emission quenching of

Eu2+. Possible solutions are suggested.

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1. INTRODUCTION

The World Energy Outlook 2015 released in November 20151 calls for the

understanding of the implications of the shifting energy landscape for economic and

environmental goals and for energy security. Oil price collapse, geopolitical

instability, trends in CO2 emissions and energy inefficiency all provoke us to seek

renewable and stable energy resources. Recent developments in flexible copper

indium gallium selenide,2 transparent solar cells,3 upconversion multicolor tuning4

and lanthanide ion-doped quantum dots5 provide optimism in this respect. Our goal

has been to harness as much of the solar flux as possible and herein we report a

wideband harvester with an emission profile tailored to the response of the silicon

solar cell.

The cubic silicate garnet host Ca3Sc2Si3O12 (CSS) is endowed with unique and

versatile spectroscopic properties. Its structure comprises Ca2+ in dodecahedral (8-)

and Sc3+ in octahedral 6-coordination to oxygen.6 Upon doping with Eu3+, the bands

of the transition 5D0 →7F4 located between 800-900 nm are unusually strong,

7

attributed to the selection rules for the distorted EuO8 coordination.8,9 The Tb3+-doped

CSS exhibits strong green luminescence with a lifetime of 3 ms and a rather slow

population of 5D4 from5D3.

7 The afterglow of the scintillator CSS:Pr 3+ can be

removed by co-doping with Mg2+ and hence the performance is improved.10 A

phosphor with color rendering index Ra > 90 for white light-emitting diode (wLED)

use was synthesized by co-doping CSS with Ce3+, Mn2+ and a charge-compensating

lanthanide ion.11 Perhaps the most striking property of the host is that it can extend the

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luminescence of divalent europium into the near infrared spectral region12 and this is

exploited in the present work.

The synthesis of the CSS host material may be problematic due to the presence

of various secondary phases when using solid state methods.6,8 Wu et al. have

proposed that the substitution of Sc3+ by Al3+ (below 40%) in CSS:Ce3+ in solid state

synthesis at 1450 oC can inhibit the formation of the impurity phases Sc2O3 and CeO2,

improve crystallinity, and enhance the photoluminescence intensity.13 Recently, a

freeze-drying method and increased heat treatment temperature have been utilized to

reduce the presence of secondary phases.14 The X-ray diffraction pattern of a sample

prepared by sol-gel combustion synthesis and subsequently fired at 1100 oC does not

exhibit other phases.15,16 Several candidates are available for the doping of this host

with divalent lanthanide ions, of which Eu2+ and Yb2+ are the most promising.17

The luminescence and excitation spectra of some Ce3+-activated silicate garnets

have previously been reported at room temperature.18,19 The Ce3+ electronic energies

were interpreted on the basis of two factors: the centroid shift from the free ion and

the 5d crystal field splitting. There have been numerous studies regarding of the

energy transfer between Ce3+ and Eu2+ in various solid-state hosts. To our knowledge,

the aim of these studies has been the development of near-ultraviolet-pumped white

light emitting diodes (wLEDs). For example, the Ca1.65Sr 0.35SiO4:Ce3+, Li+, Eu2+

phosphor has been recommended as a candidate for color-tunable blue-green

components of wLEDs, with emission between 465-550nm;20 Ba2ZnS3:Ce3+, Eu2+ has

application as a blue-converting phosphor for wLEDs, with emission maximum

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intensity at ~650 nm.21 The Ca4(PO4)2O:Ce3+, Eu2+ wLED phosphor has Eu2+

emission maximum at 650 nm.22

The format of this work is as follows. The structure and luminescence of the

host material, luminescence of Ce3+ or Eu2+-doped systems, and the energy transfer

from Ce3+ to Eu2+ in the CSS host material are presented and discussed in Section 2.

A comment is included in Section 3 concerning the applicability of Ce3+-Eu2+

co-doped CSS in the field of solar energy conversion. Some conclusions from the

experimental results and data analyses are made in the final Section 4.

2. RESULTS AND DISCUSSION

Ca2.54

Ce0.2

Eu0.06

Na0.2

Sc2Si

3O

12

Ca2.97

Eu0.03

Sc2Si

3O

12

Ca2.94

Ce0.03

Na0.03

Sc2Si

3O

12

Ca3Sc

2Si

3O

12

20 30 40 50 60 70 80

R e l a t i v e i n t e n s i t y ( a r b . u n i t s )

2θθθθ (degree)

ICDD PDF 2 card # 72-1969 Ca3Sc

2Si

3O

12

Figure 1. Representative XRD patterns of samples at room temperature.

2.1. Structure refinement. The X-ray diffraction (XRD) patterns of samples

Ca3-2 xCe x Na xSc2Si3O12 ( x = 0.001-0.2), Ca3- xEu xSc2Si3O12 ( x = 0-0.09),

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Ca2.8-2 xCe0.1Eu x Na0.1Sc2Si3O12 ( x = 0-0.09), Ca2.94-2 xCe xEu0.06 Na xSc2Si3O12 ( x = 0-0.2)

were measured at room temperature. The Na+ ions from Na2CO3 serve as charge

compensators for Ce3+. Figure 1 shows the X-ray diffraction patterns of some

representative samples. All of the samples were verified to comprise a single phase

and they are consistent with the standard file of Ca3Sc2Si3O12 (ICDD no.72-1969),

without other impurities such as Sc2Si2O7, Sc2O3, SiO2 and Ca2SiO4 etc. The

compound Ca3Sc2Si3O12 (CSS) crystallizes in the cubic system with the Ia-3d space

group (No. 230).13 In the structure, each Ca2+ is surrounded by 8 O2- ions to form a

distorted dodecahedron (D2 point symmetry) with four long Ca-O distances of

2.5660(14) Å and four short Ca-O distances of 2.4324(11) Å. Each Sc3+ is

coordinated with six equidistant oxygens at the distance of 2.1062(15) Å to form an

octahedron.13 The coordination environments are depicted in Figure S1. The crystal

data of CSS was used as the initial model for structural analysis and the rare earth ions

were assumed to be dispersed randomly in this host lattice. The refinement patterns of

the three representative samples (CSS host; CSS:0.03Ce; CSS:0.03Eu) were

processed using the software TOPAS23 and the results for the CSS host at room

temperature are shown in Figure S2, with the values of Rwp, R p, RB being in the range

of 2.1% - 4.4%, indicating a good fitting quality. Table S2 also presents the

crystallographic data and refined structure parameters for the three systems. The ionic

radii of eight-fold coordinated Ce3+ and Eu2+ are 114.3 and 125.0 pm,24 respectively,

so that it is suggested that Ce3+/Eu2+ occupy the Ca2+(VIII) (112.0 pm24) site because

of similar ionic sizes. In fact, Shimomura et al.6 have shown from X-ray absorption

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(λex = 206 nm; λem = 385 nm) at room temperature. (c) The excitation spectra (λem =

385 nm) at different temperatures.

2.2 The luminescence of Ca3Sc2Si3O12 host. The vacuum ultraviolet (VUV)

spectroscopy of pure Ca3Sc2Si3O12 was investigated as a prerequisite for the study of

the luminescence properties of doped samples. The emission spectrum upon

synchrotron radiation excitation at 206 nm and the VUV excitation spectrum by

monitoring at the wavelength of 385 nm emission at 15 K are shown in Figure 2(a). A

broad excitation band was observed with a maximum at 206 nm (6.02 eV) and is

associated with near-defect excitons. At higher energy, a broad shoulder band (184 nm,

6.74 eV) is present and is assigned to self-trapped exciton (STE) absorption. Upon

206 nm excitation, the emission exhibits a broad band with maximum at 385 nm. The

spectrum is similar to that reported by Ivanovskikh et al.26 who pointed out that the

broad emission band in the time-integrated excitation spectrum is attributed to

excitons localized near defects or to direct electron–hole recombination. These

authors also reported several other weak bands which are not present in Figure 2(a).

Figure 2(b) shows the monoexponential decay curve of the emission intensity I ,

fitted to Eq (1).

ln( I ) = ln A - kt (1)

where the slope k is the reciprocal of the intrinsic lifetime (τ0) and A is a constant.

The lifetime is fitted to be 5.5 µs. The decay is little slower than that previously

reported for CSS at 10 K (2 s27) and also in comparison with the decay time of

self-trapped exciton luminescence in pure LaCl3 (3.5 µs at room temperature28).

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Figure 2(c) shows the normalized excitation spectra of Ca3Sc2Si3O12 at different

temperatures between 15 K and 296 K. The shape and position of the band remain

unchanged relative to the situation at 15 K and are insensitive to temperature

variation.

100 200 300 400 500 600 700

λλλλem = 550 nm (UV)

λλλλem = 550 nm (VUV)

A

R e l a t i v e i n t e n s i t y

( a r b . u n i t s )

Wavelength (nm)

H

E

C

G

B

Ca2.8

Na0.1

Ce0.1

Sc2Si

3O

12 RT

F

D

λλλλex

= 440 nm

Figure 3．VUV-UV excitation spectra (λem = 550 nm) of Ca2.8Ce0.1 Na0.1Sc2Si3O12 and

the UV emission spectrum of Ca2.8Ce0.1 Na0.1Sc2Si3O12. The features in the excitation

spectrum at wavelengths longer than 350 nm are beyond the collection range of the

experimental setup at BSRF so that the excitation spectrum in the 280-530 nm range

was recorded with a spectrometer in our laboratory and normalized on the band D.

2.3. Spectra of Ca2.8Ce0.1Na0.1Sc2Si3O12 at room temperature (RT). The VUV-UV

excitation spectra of Ca2.8Ce0.1 Na0.1Sc2Si3O12 at room temperature are shown in

Figure 3. The far-left curve is the excitation spectrum using synchrotron radiation in

the 125-350 nm range when monitoring 550 nm emission of Ce3+. Six excitation

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bands (A-F) are distinguished in the 125-530 nm range as labeled in Figure 3. The

bands A (182 nm, 6.81 eV) and B (215 nm, 5.77 eV) are analogous to the exciton

absorptions observed in the VUV-UV excitation spectrum of Ca3Sc2Si3O12 in Figure 2,

but with different relative intensities and a red-shift of the latter by several nm. With

an excess of 8% for exciton binding energy,29 the band gap of CSS:Ce3+ is estimated

to be 7.35 eV at room temperature.

The three lower energy bands C (239 nm, 5.18 eV), D (308 nm, 4.02 eV) and F

(440 nm, 2.82 eV) are attributed to 4f-5d transitions of Ce3+, in line with previous

studies.12,30 In CSS-Ce the Ce3+ ion occupies the D2 symmetry Ca2+ site, so that the 5d

level is initially split into a lower doublet (Eg) and a higher triplet (T2g) by the cubic

component of the crystal field and then further split by the distortion from cubic

symmetry. A first-principles study27 has calculated the energies of the 4f 1 and 5d1

levels of Ce3+ in Ca3Sc2Si3O12 with different types of charge compensation

mechanisms. Assignments for the 5d(1)-5d(5) energy levels of Ce3+ in garnet host

lattices (mostly expressed as vibronic band maxima) are collected in Table 1 and in

the present case the assignments for 5d(1)-5d(3) appear secure. There is some

disagreement in the literature concerning the assignment of the higher 5d levels in

YAG:Ce3+. From the first principles calculation, the crystal field splitting (CFS) of

Ca3Sc2Si3O12:Ce3+

, Na+

is calculated to be about 27773 cm-1

(3.44 eV).

Table 1. Calculated (calc.) and observed (obs.) energies (in eV) for selected garnets

doped with Ce3+. Values refer to band maxima except for the calculation of Ref. 32 for

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YAG:Ce3+ which refers to zero phonon line energies. In YAG:Ce3+, the 5d(1) and 5d(2)

zero phonon lines are at 0.15 eV and 0.11 eV to low energy of the excitation band

maxima, respectively.32 Measurement temperatures are included, where appropriate.

Ref. System 5d(1) 5d(2) 5d(3) 5d(4) 5d(5)

This, 15 K, obs. Ca2.8Ce0.1 Na0.1Sc2Si3O12 2.82 4.03 5.19

12, 77 K, obs. Ca2.97Ce0.03Sc2Si3O12 2.79 4.03 5.23

27, calc. Ca3Sc2Si3O12:Ce3+, Na+ 2.87 3.94 5.59 6.10 6.31

31, obs./calc. Y3Al5O12:Ce3+ 2.71 3.65 5.50 ~5.50 6.06

32, calc. Y3Al5O12:Ce3+ 2.53 3.57 5.39 6.09 7.51

A discussion of the calculation of the centroid shift is included in the Supporting

Information and the calculated value is 1.42 eV (11455 cm-1). Hence, as also

remarked by Berezovskaya et al.,12 the observed relatively long-wavelength position

of the lowest 5d state for Ce3+ in CSS is attributed to the low 5d centroid shift energy

from the Ce3+ free ion level and the large crystal field splitting of the 5d1

configuration in this host (Figure S5).

In the emission spectrum of Ca2.8Ce0.1 Na0.1ScSi3O12 excited by 440 nm radiation

(Figure 3), a broad band with peak maximum at 505 nm and a shoulder at 550 nm are

observed, corresponding to the electronic transition from the relaxed lowest 5d state

to the 4f 1 J -multiplets 2F5/2 and2F7/2 of Ce

3+, respectively. Although there is a change

in total intensity of the emission bands, the shape and position are unchanged for

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excitation wavelengths in the range of 410-480 nm (Figure 4) so that is considered

that Ce3+ occupies only one crystal site: that of Ca2+, in CSS.

400 500 600 700

300

400

500

0

0.2

0.5

0.7

1.01300 400 500

0

10

20

30

40

Ca2.8Ce0.1Na0.1Sc2Si3O12 RT

300 400 500

0

50

100

150

I n t e n s i t y ( a r b . u n i t s )

5d(1)5d(2)

λλλλem

= 390 nm

λλλλem

= 550 nm

Wavelength (nm)Emission wavelength (nm)

E x c i t a t i o n w a v e l e n g t h ( n m )

5d(2)

5d(1)

Figure 4. The excitation spectra map of Ca2.8Ce0.1 Na0.1ScSi3O12 by monitoring

different emission wavelengths. The right-hand figures show the representative

excitation spectra by monitoring 390 and 550 nm emissions, respectively.

2.4. Spectral impurity bands in Ca2.8Ce0.1Na0.1ScSi3O12. The weak Band E (Figure

3: 330-360 nm) has not yet been assigned. This band is evident in the excitation

spectra of Liu et al.16 for CSS:Ce3+ samples prepared by the gel-combustion method

in air and by solid-state reaction method, but not in the spectrum of CSS:Ce3+

prepared by the gel-combustion method in carbon. The band was attributed by Ding et

al.27 to the second 4f → 5d transition of Ce3+, with Sc3+ substituting Si4+ in its local

environment. Our XANES shows that cerium is present in the 3+ oxidation state in

our samples (Figure S6). In order to further investigate whether the broad absorption

band E corresponds to defect or impurity absorption present beyond the limit of XRD

detection, the continuous excitation spectrum was recorded by monitoring the

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emission from 340 nm to 700 nm, as shown in Figure 4. The remarkable intense

absorption between 400 - 500 nm corresponds to the vibronic band of the 4f - 5d(1)

transition of Ce3+. The transition to 5d(2) shows weak absorption intensity (as marked

in Figure 4), whilst three additional weaker bands at 296, 330 and 360 nm appear

when monitoring emission at ~390 nm (Figure 4: top right-hand panel), which is at a

similar wavelength to that of the host emission (Figure 2). These excitation bands are

more prominent when monitoring the emission at 390 nm than at 550 nm, as shown

Figure 4, right-hand panels, Figure S7 and Figure S8. Furthermore, excitation at 330

nm or 360 nm gives an emission band ~390 nm in addition to the Ce3+ emission at

longer wavelength (Figure S9). The former emission band is shifted a few nm to

higher energy, at 385 nm, when exciting at 237 nm and 206 nm at 15 K (Figure S10).

The decay curves of the Ce3+ and defect site emissions are compared in Figure S11.

The life time of this defect site emission is 20±2 ns which is shorter than the 67 ns

decay of Ce3+. The defect sites may arise from aliovalent substitutions near Ce3+ ions

since their population leads not only to self-trapped excitonic emission but also to

Ce3+ emission. Since the relevant spectral features of these defect sites are at high

energy, their presence does not interfere with our study on Ce3+- Eu2+ energy transfer

and application.

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300 400 500 600 700

Ca3-2 x

Ce x Na

x Sc

2Si

3O

12 RT

N o r m a l i z e d i n t e n s i t y ( a r b . u n i t s )

Wavelength (nm)

x = 0.001

x = 0.03

x = 0.10

x = 0.15

x = 0.20

λλλλex

= 440 nm

λλλλem

= 550 nm

a

100 200 300 400 500

b

Time (ns)

λλλλex

= 440 nm, λλλλem

= 550 nm

Figure 5. (a) The normalized excitation spectra (λem = 550 nm) and emission spectra

(λex = 440 nm) for Ca3-2 xCe x Na xSc2Si3O12 ( x = 0.001-0.2) at room temperature. (b)

The corresponding decay curves of these samples.

2.5. Concentration and temperature dependence of Ce3+

emission in

Ca3-2 x Ce x Na x Sc2Si3O12. The effect of Ce3+ concentration upon the room

temperature emission spectrum (a) and lifetime (b) of CSS:Ce3+, Na+ is illustrated

in Figure 5. The increase in Ce3+ concentration markedly broadens features in the

excitation spectrum but exhibits a smaller effect upon the emission bands.

Notably, the defect site bands located at 330-360 nm are absent in the samples x =

0.001 and 0.03. It is remarkable that the Ce3+

lifetime remains monoexponential

and within the range of 67.0±0.7 for x = 0.001-0.1 (as also reported for CSS:Ce3+

by Shimomura et al.6), and only decreases to 62.6±0.1 for x = 0.2. This feature is

taken to indicate the absence of migration between Ce3+ ions and consequent

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photoluminescence of CSS:Ce3+ is far superior to that of the commercial phosphor

(Y,Gd)3Al5O12:Ce3+.

Figure S12 depicts the corresponding changes in the excitation spectrum of

Ca2.8Ce0.1 Na0.1Sc2Si3O12 when the temperature increases from 320 K to 480 K. The

5d(1) band broadens at low energies due to transitions from thermally-occupied

crystal field levels of 2F5/2. The defect site energy levels at 320-380 nm act as traps

and they are emptied with increasing temperature.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

VB

HS Ln2+

G

43

1

Lu

V R B E ( e V )

number of electrons n in Ln3+

4f n

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb

2

5

Ln3+

E

Ln3+

G

LS

CB

Figure 7. VRBE scheme for lanthanide Ln3+ 4f n and 4f n-15d states (blue color), and

Ln2+ 4f n states (red color) in Ca3Sc2Si3O12. The excited energy levels of lanthanide

ions are taken from Refs. 33,34. Refer to the text for explanation.

2.6. Vacuum referred binding energy scheme (VRBE) scheme. The vacuum

referred binding energy (VRBE) scheme for Ln3+ 4f n and 4f n-15d states and Ln2+ 4f n

states in Ca3Sc2Si3O12 is displayed in Figure 7 by using published methods.33,34

Various approximations are involved, including the use of the same band gap for all

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ions, the use of band maxima instead of zero phonon line energies, and the separations

of high and low-spin states. The required data for the construction in the CSS host are

(i) the energy of the lowest 4f - 5d transition of Ce3+ (arrow 1: 2.84 eV); (ii) the host

band gap energy (arrow 2: 7.35 eV); (iii) the energy of the charge transfer band of

Eu3+ (arrow 3, see Figure S13: 4.86 eV); (iv) the Coulomb correlation energy, i.e.,

energy difference between the ground states of Eu2+ and that of Eu3+ (not shown, 6.93

eV); and (v) the centroid shift (Figure S5: 1.4 eV). The exchange splitting energy

between the low spin and high spin 4f - 5d transitions of Tb3+ in CSS is taken from

Velázquez et al.14 (arrows 4 and 5: 0.79 eV). The ground state of Eu2+ is found to be

above the Fermi energy level in Figure 7, indicating that the Eu2+ is unstable in this

host with respect to oxidation. This fact is supported by the calculated sum of the

bond valences (Eq. (1) in Ref. 35) coordinating Eu2+ in the CSS host (equal to the

oxidation number of Eu) and found to be 3.1 in the present case from the Ca-O bond

distances in CSS.

300 400 500 600 700 800 900 1000 1100 1200

λλλλex

= 405 nm, λλλλem

= 873 nm

Ca2.94

Eu0.06

Sc2Si

3O

12 RT

λλλλem

= 873 nm

Wavelength (nm)

R e l a

t i v e i n t e n s i t y ( a r b . u n i t s )

a

λλλλex

= 360 nm

λλλλex

= 520 nm

150 300 450 600 750 900 1050

Time (ns)

ττττ ~ 95.1 ns

b

Figure 8. (a) The excitation and emission spectra of Ca2.94Eu0.06Sc2Si3O12 at room

temperature. (b) The decay curve of Eu2+ emission in Ca2.94Eu0.06Sc2Si3O12 excited by

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a laser light source at room temperature.

2.7. Luminescence of Ca3- x Eu x Sc2Si3O12. The excitation and emission spectra of

Ca2.94Eu0.06Sc2Si3O12 at room temperature are shown in Figure 8(a). In this figure and

subsequently, the Eu2+ concentrations, x, are taken as the nominal amounts added in

the syntheses. A broad absorption band with maximum at 510 nm occupies the range

of 450-720 nm, with a stronger higher-energy band at 353 nm having a shoulder at

412 nm. These features correspond to the 4f 7→ 4f 65d transitions of Eu2+ ions. The

lower-energy band is derived from excitation to the 5d Eg orbital and 4f 6 core states

which are at lower energies than in Eu3+ since the divalent ion is larger. The

complexity of the energy levels in addition to the progressions in totally-symmetric

vibrational modes are responsible for the broad band. The higher-energy band

corresponds in addition to excitation to the 5d T2g orbital and 4f 6 core states. Upon

excitation at 360 nm or 520 nm (Figure 8a), a relatively intense band in the range

from 700 nm to 1100 nm is observed, with peak maximum at 873 nm. Low

temperature spectra taken at 13 K by varying the emission and excitation wavelengths

show similar spectral shape and position and are more clearly resolved than the room

temperature spectra (Figure S14). The emission band corresponds to the transition

from the lowest 4f 6

5d state to the 4f 7

ground state (8

S7/2) of Eu2+

.36

Note the absence

of Eu3+ emission. Dorenbos37 has given an empirical equation relating the energies of

the lowest 5d bands of Ce3+ and Eu2+. Using the energy of the Ce3+ emission band

(2.46 eV), the peak maximum for Eu2+ emission is estimated to be at 590 nm,

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700 800 900 1000 1100 1200

Ca3- x

Eu x Sc

2Si

3O

12 RT

R e l a t i v e i n t e n s i t y ( a r b . u n i t s )

Wavelength (nm)

873 nmλλλλ

ex = 520 nm

x = 0

x = 0.001

x = 0.01 x = 0.03

x = 0.06

x = 0.09

0.00 0.04 0.08

I n t e n s i t y

Concentration x

assuming the same Stokes shift, and this is far from the observed peak wavelength.

The decay curve of Eu2+ emission in Ca2.94Eu0.06Sc2Si3O12 was excited by 405 nm

radiation (Figure 8(b)) and can be well fitted by the monoexponential Eq. (1) with the

lifetime of 95.1 ns.

Figure 9. Emission spectra of the samples Ca3- xEu xSc2Si3O12 ( x = 0 – 0.09) at room

temperature. The inset shows the intensity-concentration dependence of emission.

Figure 9 displays the emission spectra for Ca3- xEu xSc2Si3O12, where x varies

from 0 to 0.09. The change of Eu2+ dopant ion concentration has negligible effect on

the location of the emission peak. The inset shows the luminescence intensity of

Ca3- xEu xSc2Si3O12 as a function of Eu2+

concentration x between 0 and 0.09 under 520

nm excitation at room temperature. With increasing Eu2+ concentration, the emission

intensity increases gradually and reaches a maximum when the Eu2+ concentration is x

= 0.06, after which quenching occurs. Figure S15 shows the corresponding excitation

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19

spectra which also exhibit the maximum intensity at x = 0.06. Therefore, in

Ca3- xEu xSc2Si3O12 the optimal doping concentration of Eu2+ is x = 0.06. In addition,

the decay curves for different doping contents x in Ca3- xEu xSc2Si3O12 (Figure S16) do

not show a significant lifetime change.

The temperature-dependence of the Eu2+ emission in Ca2.94Eu0.06Sc2Si3O12 is

rather unusual (Figure S17(a)). The emission intensity increases from 13 K up to 50 K

and then decreases. The initial increase is attributed to the thermal population of Eu 2+

excited states.38 A further increase in temperature produces photoionization. The Eu2+

ground state has been located in Figure 7 at 4.02 eV (from the CT transition of Eu3+).

The 4f – 5d zero phonon line energy of roughly 1.8 eV (Figure 8a) therefore places

the lowest Eu2+ 5d level at ~2.22 eV, i.e. ~0.66 eV (~5300 cm-1) below the conduction

band. The activation energy fit to T > 150 K in Figure S17(b) gives a linear plot with

the value of 0.05 eV (400 cm-1). Part of the discrepancy in value with 0.66 eV arises

from the uncertainty of location of the Eu2+ ground state (from the Eu3+ charge

transfer band maximum) and the band gap (from the excitation spectra, Figures 2(a), 3,

rather than from absorption measurements) and another part from the omission of

other parameters in the activation energy fit (see, for example, Ref. 38). However, it

can be concluded that the temperature stability of Eu2+ emission is poor.

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0.00 0.05 0.10 0.15 0.200.3

0.4

0.5

0.6

0.7

0.8

0.9

Ca2.94-2x

Cex Eu

0.06Na

x Sc

2Si

3O

12

R e

l a t i v e i n t e n s i t y

Ce3+

content (x)

(b)

350 400 450 500 550 600 650 700 750

0.2

0.4

0.6

0.8

1.0

1.2

Ca2.94-2 x

Ce x Eu

0.06Na

x Sc

2Si

3O

12

Wavelength (nm)

x = 0

x = 0.001

x = 0.03

x = 0.10

x = 0.20

λem

= 870 nm

N o r m a l i z e d i n t e n s i t y

RT

(a)

number of Ce3+ luminescence centers. By contrast, the emission intensity of Eu2+

increases up to x = 0.03 and then decreases. The variation of the integrated emission

areas for Ce3+ and Eu2+ is shown in the inner panel of the figure.

Figure 11. (a) The normalized excitation spectra (λem = 873 nm) of

Ca2.94-2 xCe xEu0.06 Na xSc2Si3O12 at room temperature. (b) Excitation intensity at 370 nm

relative to that at 520 nm.

The excitation spectra, normalized at 520 nm, of Eu2+ emission (λem = 873 nm)

from Ca2.94-2 xCe xEu0.06 Na xSc2Si3O12 at room temperature (Figure 11(a)) show a

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The energy transfer rates were estimated from the decay curves of the Ce3+:5d1→

4f 1 emission at 550 nm in Ca2.8- xCe0.1 Na0.1Eu xSc2Si3O12 ( x = 0-0.09) (Figure 12) at

room temperature. With ascending Eu2+ concentration, the lifetimes decrease

markedly and deviate from the monoexponential behavior of the sample x = 0.

Therefore, the curves were fitted by the double-exponential equation:

+

(2)

Where τ1 and τ2 are the fast and slow components of the luminescent lifetime,

respectively. A1 and A2 are the corresponding fitting parameters. The average lifetime

can be further evaluated by the following equation:

τ (τ + τ)( + ) (3)

The fitting results are presented in Table 2, with goodness of fit, for all the

samples. The Ce3+ lifetime is drastically reduced from 66.8 ns and follows a

monoexponential decay with Eu dopant ion concentration as shown in the top panel of

Figure 13. The linear relationship of k ET with Eu2+ dopant ion concentration x as

shown in the bottom panel of this figure indicates a direct transfer from Ce3+ to Eu2+,

with the transfer efficiency rising as high as 88.9% (Table 2, final column).

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Table 2. Lifetimes, fitting parameters, energy transfer rates and efficiency for

Ca2.8-2 xCe0.1 Na0.1Eu xSc2Si3O12 from measurements of Ce3+ emission at 295 K.

Eu2+

τ1 τ2 A1 A2 R2

adj τak ET (µs)

-1 η (%)

0 66.8 ± 0 66.8 ± 0 0.4874 0.4874 0.9984 66.8 0.0 0.0

0.001 18.0 ± 0.6 55.4 ± 0.5 0.2965 0.6886 0.9987 50.8 4.7 24.0

0.01 14.8 ± 0.3 48.1 ± 0.5 0.3855 0.5820 0.9984 42.4 8.6 36.5

0.03 6.9 ± 0.08 27.4 ± 0.2 0.5336 0.4332 0.9985 22.5 29.4 66.3

0.06 4.3 ± 0.04 17.1 ± 0.2 0.5995 0.2903 0.9985 12.8 63.4 80.9

0.09 2.6 ± 0.02 10.6±0.1 0.7053 0.2602 0.9990 7.4 120.4 88.9

ak ET = 1/τ - 1/τ0, where τ is the average lifetime at concentration x, and τ0 is at x =

0.001. η = 100(1 - τ/τ0).

0.00 0.02 0.04 0.06 0.08 0.100

20

40

60

E n e r g

y t r a n s f e r r a t e

k E T

( µ µµ µ s ) -

1

L i f e t i m e , τ ττ τ

( n s )

y = (52.9±6.7)exp[- x /(0.024±0.009)]+(7.2±6.2)

Ca2.8- x

Ce0.1

Eu x Na

0.1Sc

2Si

3O

12

RT

R 2

adj = 0.9395

0.00 0.02 0.04 0.06 0.08 0.10

0

40

80

120

R 2

adj = 0.9700

Ca2.8- x Ce0.1Eu x Na0.1Sc2Si3O12

Doping level, x

RTy ̀ = ( 1269±100) x ̀ - (2.6±4.6)

Figure 13. The Ce3+ emission lifetimes and the energy transfer rate as a function of

Eu2+ dopant ion concentration in Ca2.8-xCe0.1Eux Na0.1Sc2Si3O12.

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Table 3. The fitting parameters and the C A/C 0 ratios obtained from the I-H model.

Ca2.8- xCe0.1 Na0.1Eu xSc2Si3O12

C A(Eu2+

) Radj2 C A/C 0

0 0.9984 -

0.001 0.9839 0.34

0.01 0.9973 0.55

0.03 0.9944 1.31

0.06 0.8466 2.10

0.09 0.9806 2.94

Neglecting the value for x = 0.001 in Table 3, the average value of C 0, the critical

transfer concentration, is x = 0.025±0.006. Taking the intrinsic lifetime τ0 as 66.8 ns,

with the corresponding deactivation rate is 15 (µs)-1, then from the equation in the

lower Figure 13, the value of C 0 is calculated to be slightly lower at x = 0.014.

There are 8 x Eu2+ acceptors in the unit cell volume of 1837. 8 Å3, so that with

the use of Eq. 25 in Ref. 39:

300 4

3

RC

π

= (5)

the value of R0 is determined to be 13 Å. This critical transfer distance, R0, represents

the separation of an isolated Ce3+-Eu2+ pair when the energy transfer rate, k ET = 1/τ0,

i.e., the same rate as the spontaneous deactivation of Ce3+. The spectral overlap

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27

calculation in the Supporting Information gives the value of R0 as 25 Å.

Figure 14. Spectral conversion design for solar cell applications, involving the AM

1.5 solar spectrum, spectral response of c-Si, and the PLE and PL spectra of

Ca2.88Ce0.03Eu0.06 Na0.03Sc2Si3O12.

3. APPLICATION TO SOLAR CELLS

Figure 14 illustrates a schematic diagram for spectral conversion design in the solar

cell application of the phosphor CSS:Ce,Eu. The AM 1.5 solar spectrum commences

from the wavelength of 300 nm and shows the maximum absorption around 500 nm.

When sunlight falls, the photons are firstly absorbed by the phosphor layer and

converted to longer wavelengths more suitable for the response of the solar cell. The

presence of Ce3+ ions makes up for the smaller absorption of Eu2+ in the range of

400-450 nm, so that the excitation spectrum of the Ce3+

, Eu2+

co-doped sample

Ca2.88Ce0.03Eu0.06 Na0.03Sc2Si3O12 shows a continuous strong absorption from the UV

through the visible spectral range and matches very well with the incident solar flux,

implying a strong absorption of sunlight. Because the corresponding broad NIR

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emission is located in the region of highest spectral response of the c-Si solar cell, the

CSS: Ce, Eu phosphor could be a solar energy converter material for application

together with the c-Si photovoltaic solar cell. The energy transfer efficiency from Ce3+

to Eu2+ is nearly 90%. The two main drawbacks have been identified herein: the poor

valence stability of Eu2+ in the CSS host and the luminescence quenching with

temperature increase.

4. CONCLUSIONS

This study has comprised two facets: the spectral properties and energy transfer of

Ce3+ and Eu2+ in the garnet host and the suitability for solar energy harvesting. Indeed,

the CSS garnet host exhibits many unique characteristics. The energy levels of Ce 3+

are subject to a strong crystal field so that the 4f 1 -5d1 lowest energy zero phonon line

is located in the blue spectral region. The Ce3+ ion occupies the Ca2+ site and charge

compensation by Na+ has been included. The monoexponential emission decay does

not exhibit noticeable quenching for Ce3+ concentration below x = 0.2 in

Ca3-2 xCe x Na xSc2Si3O12, and the intensity only decreases by 20% at 460 K. By contrast,

the emission of Eu2+-doped CSS is more sensitive to concentration and temperature

quenching. However, the emission band is located in the near infrared spectral region

and broad, intense 4f 7

→ 4f 6

5d absorption bands span the visible and ultraviolet

regions. The spectral overlap between Ce3+ emission and Eu2+ absorption promotes

resonant energy transfer and the Ce3+ decay kinetics have been fitted by the

Inokuti-Hirayama formalism for electric dipole – electric dipole energy transfer, in

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agreement with the mechanism deduced from selection rules.

The broad, electric dipole allowed absorption spectrum of CSS:Ce3+, Eu2+

enables ~60% of the sunlight spectrum (from spectral overlap) to be utilized for solar

energy conversion. The emission is more intense than that of Nd3+ or Yb3+ systems

and it effectively targets the response of the c-Si solar cell. Although CSS:Ce3+ has

excellent emission intensity and thermal performance, the drawbacks of valence

instability and temperature quenching of emission when co-doping with Eu2+ have

been found. Some suggestions are included for research to solve these problems

which are related to host structure. First, the valence stability in a cubic garnet can be

tuned by variation of the central cation size and the introduction of larger cation sites

(e.g., Sr 2+, Ba2+) may be beneficial.41 The thermal instability due to photoionization

may be improved through the introduction of cations to improve structural rigidity

(e.g., Mg2+, Al3+, Ge3+); to increase the host bandgap;42 and to utilize core-shell

protected nanocrystals.43 A good host candidate could be Lu2CaMg2Si3O12, which

exhibits longer wavelength Ce3+ emission.44,45 It is also important to modify the form

of the solar cell device into a transparent glass or ceramic, or a phosphor in glass.46

We will pursue the above modifications.

ACKNOWLEDGMENTS

This work is financially supported by the National Natural Science Foundation of

China (21171176, U1232108, and U1432249).

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SUPPORTING INFORMATION

Experimental details; Centroid shift of Ce3+ in CSS; Energy transfer formulae; Figures

and Tables.

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Spectral Properties and Energy Transfer of a Potential Solar Energy Converter