Florian Baur, Beata Malysa and Thomas Jüstel
24th Symposium of the DAfP
Brunswick, Germany, June 7th, 2019
NIR Emitting Luminescent Materials for pcLEDs as Alternative to
Incandescent Lamps
2/21 DAfP-Symposium June 7th, 2019, Brunswick
Outline
1. Application Areas of NIR Emitter
2. Commercial NIR Sources
3. NIR Emitting Luminescent Materials
4. Summary
Near Infrared (IR-A) radiation: 780 – 1400 nm 12800 – 6700 cm-1
1. Near Infrared (NIR)
Solar spectrum and spectrum of a black radiator at 5525 K
• relatively low energy
• also called thermal radiation
• many technical application areas
3/21 DAfP-Symposium June 7th, 2019, Brunswick
Broad band NIR emitter are required for …..
NIR Spectroscopy Agricultural, food, and pharmaceutical industry, astronomy, blood sugar and ethanol determination, cancer diagnostics
Imaging Optical coherence tomography
NIR Illumination
Active night vision devices, iris recognition
1. Application Areas of NIR Emitter
4/21 DAfP-Symposium June 7th, 2019, Brunswick
2. Commercial NIR Sources
NIR Radiation sources Incandescent lamps Nernst radiator (ZrO2/Y2O3) Silicon carbide heating elements (Globar®) GaAs or (Al,Ga)As LEDs
700 750 800 850 900 9500,0
0,2
0,4
0,6
0,8
1,0
Inte
nsity
(a.u
.)
Wavelength (nm)
FWHM = 34 nm
Emission spectrum of a 3 W 850 nm LED
low efficiency high space requirements high energy consumption high temperature
Globar SLS203L and spectrum of a black body radiator at 1500 K
Only about 10% of the total spectrum are
located in the NIR range
5/21 DAfP-Symposium June 7th, 2019, Brunswick
Disadvantages of commercial NIR emitting LEDs − Relative narrow emission bands − Temperature sensitivity − Sensitivity towards humidity − Spectrum shifts with current, temperature, and lifetime
Dependence of the emission spectrum of an LED arrays comprising 3 W 850 nm LEDs on the current
Dependence of the emission spectrum of an NIR LED on the temperature
Reference: K.J. Reynolds et al., Br. J. Anaesth. 67 (1991) 638
760 780 800 820 840 860 880 9000,0
0,2
0,4
0,6
0,8
1,0
Nor
mal
ized
Inte
nsity
[a.u
.]
Wavelength [nm]
0.1 A 2.3 A
6/21 DAfP-Symposium June 7th, 2019, Brunswick
2. Commercial NIR Sources
3. Phosphor Converted LEDs
(In,Ga)N semiconductor
Little thermal quenching High wall plug efficiency Solely little spectral shift High lifetime (about 14000 hours)
Requirements on NIR emitting phosphors
High thermal quenching temperature (T1/2 > 450 K) & little spectral shift
Strong absorption in the blue or UV-A range
Broad band emission in the range from 750 to 1000 nm
Blue or UV-A LED and NIR phosphor
Decline of the luminous and radiation flux of a blue LED as function of the operation time Source: TU Darmstadt, Germany
7/21 DAfP-Symposium June 7th, 2019, Brunswick
Transition metal ions e.g. V2+, Cr3+, Mn4+, Co2+, Ni2+, Cu2+, mostly broad emission bands relatively strong absorption due to
spin-allowed 3d-3d transitions strong thermal quenching
Lanthanide ions e.g. Nd3+, Er3+ or Yb3+
show solely line emission only weak absorption due
to forbidden 4f-4f transitions high temperature stability
8/21 DAfP-Symposium June 7th, 2019, Brunswick
3. Potential Activators
3. Potential Activators
9/21 DAfP-Symposium June 7th, 2019, Brunswick
[Ar]3d3 Ions: V2+, Cr3+, Mn4+, Fe5+
Crystal field splitting in Al2O3: V2+ 15240 cm-1
Cr3+ 18145 cm-1 Mn4+ 21290 cm-1
Fe5+ > 22000 cm-1 Reference: J. Phys. Soc. Jpn. 81 (2012) 104709
→ Mn4+ shows solely line emission (2E-4A2) → Band emission (4T2-4A2) only for Cr3+ and V2+ possible → V2+ and Fe5+ stabilisation is difficult
SFH 4735 Osram Opto semiconductors The green body colour and the emission spectrum of the phosphor points to the presence of Cr3+
Emission spectrum of SFH 4735 LED
10/21 DAfP-Symposium June 7th, 2019, Brunswick
3. Commercial NIR pcLED Source
Osram presented the first phosphor converted LED (pcLED) for NIR applications at the end of 2016
Rather low NIR emission
Optical prperties are mainly determined by Crystal field splitting Dq 1300 – 1900 cm-1 Racah Parameter B and C B: 400 – 800 cm-1
C: 2500 – 3800 cm-1
Host lattice Dq /cm-1 B /cm-1 C /cm-1 Dq/B
Al2O3 1795 630 3218 2.85
SrAl12O19 1695 650 3225 2.60
GdAl3(BO3)4 1672 677 3143 2.47 05.3DqB8.1B9.7)E(E
C
22 +−
=
−
∆⋅
∆⋅−
∆
=8
DqE15
DqE10
DqE
DqB
2
250 300 350 400 450 500 550 600 650 7000,0
0,2
0,4
0,6
0,8
1,0 Al2O3
SrAl12O19
GdAl3(BO3)4
Inte
nsity
[a.u
.]
Wavelength [nm]
10)AT(E 2
42
4
Dq →=
11/21 DAfP-Symposium June 7th, 2019, Brunswick
Excitation spectra of Cr3+ phosphors
)R/1(~Dq 5
4A2 → 4T1 4A2 → 4T2
3. Optical Properties of Cr3+
Host Lattice Dq /cm-1 B /cm-1 E(2E) /cm-1 Dq/B
Al2O3 1795 630 14399 2,85
SrAl12O19 1707 636 14575 2,68
GdAl3(BO3)4 1672 677 14633 2,47
Sr8MgLa(PO4)7 1421 686 - 2,07 B Decreasing covalency of O-Cr bond
B Increasing covalency of O-Cr bond
500 550 600 650 700 750 800 850 900 950 10000,0
0,2
0,4
0,6
0,8
1,0
Inte
nsity
[a.u
.]
Wavelength [nm]
Al2O3
SrAl12O19
GdAl3(BO3)4
Sr8MgLa(PO4)7
2E→ 4A2
4T2→ 4A2
12/21 DAfP-Symposium June 7th, 2019, Brunswick
4T2→ 4A2
Emission spectra of selected Cr3+ phosphors Tanabe-Sugano Diagram d3
3. Optical Properties of Cr3+
13/21 DAfP-Symposium June 7th, 2019, Brunswick
200 300 400 500 600 700 800 900 10000,0
0,2
0,4
0,6
0,8
1,0
λex = 470 nm
Inte
nsity
[cou
nts]
Wavelength [nm]
CaSc2O4:Cr3+
SrSc2O4:Cr3+
Emission spectraExcitation spectra
λem = 800 nm
4 A2
4 T 2(4 F)
4 A2
4 T 1(4 P
)
CT band
4 A2
4 T 1(4 F)
4T2
4A2
Parameter CaSc2O4:Cr3+ SrSc2O4:Cr3+
Dq 1471 cm-1 1389 cm-1
Emission band max. 820 nm 860 nm
FWHM 164 nm (2404 cm-1) 168 nm (2213 cm-1)
T1/2 < 240 K < 240 K
50 100 150 200 250 300 350 400 450 500 5500.0
0.2
0.4
0.6
0.8
1.0
Rel
ativ
es E
mis
sion
sint
egra
lTemperatur /K
Emission integral of CaSc2O4:1%Cr3+ as function of temperature
Excitation and emission spectrum of CaSc2O4:Cr3+ and SrSc2O4:Cr3+
Ion Ionic radius
Sc3+ 88.5 pm
Cr3+ 75.5 pm
τ3K = 27 μs τRT = 8 μs
3. CaSc2O4:Cr3+ and SrSc2O4:Cr3+
14/21 DAfP-Symposium June 7th, 2019, Brunswick
3. Sr8MgLa(PO4)7:Cr3+
50 100 150 200 250 300 350 400 450 500 550
0.0
0.2
0.4
0.6
0.8
1.0
I = f(T) Fermi-Dirac Fit
Rel
ativ
es E
mis
sion
sint
egra
l
Temperatur /K
Emission integral of Sr8MgLa(PO4)7:Cr3+
as function of temperature
200 300 400 500 600 700 800 900 10000,0
0,2
0,4
0,6
0,8
1,0
λex = 490 nmλem = 850 nm
Excitation spectrum
Inte
nsity
[a.u
.]
Wavelength [nm]
Emission spectrum
4 A2
4 T 1(4 F)
4 A2
4 T 2(4 F)
CT band
4 A2
4 T 1(4 P
)
4T2 4A2
Ion Ionic radius
Mg2+ 86.0 pm
Cr3+ 75.5 pm
Parameter Sr8MgLa(PO4)7:Cr3+
Dq 1421 cm-1
Emission band max. 848 nm
FWHM 141 nm (1917 cm-1)
T1/2 300 K
Excitation and emission spectrum of Sr8MgLa(PO4)7:Cr3+
τ3K = 230 μs τRT = 120 μs
15/21 DAfP-Symposium June 7th, 2019, Brunswick
3. GdAl3(BO3)4:Cr3+
300 400 500 600 700 800 900 10000.0
0.2
0.4
0.6
0.8
1.0
4 T 2�4 A 2
4 A 2�4 T 2
4 A 2�4 T 1
λex = 420 nmλem = 720 nm
Excitation spectrum
Inte
nsity
[a.u
.]
Wavelength [nm]
Emission spectrum
8 H 7/2�6 P J
(Gd3+
)8 H 7/2�6 I J
(Gd3+
)
100 200 300 400 500 600 700 8000,0
0,2
0,4
0,6
0,8
1,0
I=f(T) Fermi-Dirac Fit
Rela
tive
emiss
ion
inte
gral
Temperature /K
Parameter GdAl3(BO3)4:Cr3+
Dq 1672 cm-1
Emission band max. 730 nm
FWHM 116 nm (2155 cm-1)
T1/2 650 K
Emission integral of GdAl3(BO3)4:Cr3+
as function of temperature Excitation and emission spectrum
of GdAl3(BO3)4:Cr3+
Ion Ionic radius
Al3+ 67.5 pm
Cr3+ 75.5 pm
τ77K = 1.2 ms τRT = 140 μs
16/21 DAfP-Symposium June 7th, 2019, Brunswick
3. X3Sc2Ga3O12:Cr3+
550 600 650 700 750 800 850 900 950 10000,0
0,2
0,4
0,6
0,8
1,0
Gd3Sc2Ga3O12:Cr3+(1%) La3Sc2Ga3O12:Cr3+(1%)
Lu3Sc2Ga3O12:Cr3+(1%) Y3Sc2Ga3O12:Cr3+(1%)
Inte
nsity
[a.u
.]Wavelength [nm]
4T2 4A2
250 300 350 400 450 500 550 600 650 700 7500,0
0,2
0,4
0,6
0,8
1,0
4A2 4T2(
4F)
Inte
nsity
[a.u
.]
Wavelength [nm]
4A2 4T1(
4F)
Lu3Sc2Ga3O12:Cr3+(1%) Y3Sc2Ga3O12:Cr3+(1%)
Gd3Sc2Ga3O12:Cr3+(1%) La3Sc2Ga3O12:Cr3+(1%)
4 A2
4T 1(4 P
)
Ionic radius X3+ /Å
Sc – O Distance /Å
X – O Distance /Å
LuSGG 1.12 1.993 2.490 YSGG 1.16 2.018 2.522 GSGG 1.19 2.041 2.550 LaSGG 1.30 2.086 2.607
Excitation spectra of XSGG:Cr3+ (X = Lu, Y, Gd, La)
Emission spectra of XSGG:Cr3+ (X = Lu, Y, Gd, La)
Dq [cm-1] Em. max FWHM Stokes Shift LuSGG 1626 722 nm 73 nm 2585 cm-1
YSGG 1587 740 nm 90 nm 2445 cm-1 GSGG 1563 754 nm 90 nm 2354 cm-1 LaSGG 1458 818 nm 145 nm 2392 cm-1
17/21 DAfP-Symposium June 7th, 2019, Brunswick
X = Lu, Y, Gd, La
Dq [cm-1] Em. max FWHM Stokes shift T1/2 LuSGG 1626 722 nm 73 nm 2585 cm-1 714 K YSGG 1587 740 nm 90 nm 2445 cm-1 660 K GSGG 1563 754 nm 90 nm 2354 cm-1 780 K LaSGG 1458 818 nm 145 nm 2392 cm-1 450 K
Emission intensity of XSGG:1%Cr3+ vs. temperature
0 100 200 300 400 500 600 700 800 9000,0
0,2
0,4
0,6
0,8
1,0
LuSGG:Cr3+
YSGG:Cr3+
GSGG:Cr3+
LaSGG:Cr3+
Nor
mal
ized
Inte
gral
Em
issi
on In
tens
ity [a
.u.]
Temperature [K]
Strong thermal quenching of LaSGG in comparison to XSGG due to Very little Dq and low energy
of the 4T2 state
Large Stoke‘sche Shift
(Huang-Rhys-Parameter: S = 6)
Large width of the 4T2 band
Decay time τ3K τRT
LuSGG 2.2 ms 314 μs
YSGG 1.2 ms 125 μs
GsGG 425 μs 102 μs
LaSGG 178 μs 104 μs
3. X3Sc2Ga3O12:Cr3+
18/21 DAfP-Symposium June 7th, 2019, Brunswick
X = Sc, Y, La, Gd, Lu Excitation and emission
spectra of Ca2XNbO6:Mn4+
Broad excitation bands
Emission band are around 700 nm
(deep red)
Narrow emission (multipel lines)
Potential application in horticulture
lighting
3. Ca2XNbO6:Mn4+
300 400 500 600 700 8000.0
0.5
1.0
λex = 308 nm
λem = 692 nm
Ca2ScNbO6:0.1% Mn4+
Ca2ScNbO6:0.25% Mn4+
Ca2ScNbO6:0.5% Mn4+
Ca2ScNbO6:1.0% Mn4+
Inte
nsity
(nor
mal
ized)
[a.u
.]
Wavelength [nm]
300 400 500 600 700 8000.0
0.5
1.0
λem = 328 nm
Ca2LaNbO6:0.1% Mn4+
Ca2LaNbO6:0.25% Mn4+
Ca2LaNbO6:0.5% Mn4+
Ca2LaNbO6:1.0% Mn4+
Inte
nsity
(nor
mal
ized)
[a.u
.]
Wavelength [nm]
λem = 697 nm
Verbindung B /cm-1 Dq /cm-1 Nb-O Abstand /pm
Em-Max
/nm Ca2ScNbO6:Mn4+ 1043 1938 193,4 692 Ca2LuNbO6:Mn4+ 1063 1938 200,1 683 Ca2YNbO6:Mn4+ 934 1946 197,6 682 Ca2GdNbO6:Mn4+ 972 1969 200,9 676 Ca2LaNbO6:Mn4+ 823 1931 200,7 697
19/21 DAfP-Symposium June 7th, 2019, Brunswick
X = Y, La, Gd Emission spectra and thermal quenching curve of Ca2GdNbO6:Mn4+
Very strong thermal quenching in
compounds according to Ca2XNbO6:Mn4+
Low energy of the 4T2 state and little
crystal field splitting causes a low
quenching temperature
3. Ca2XNbO6:Mn4+
0 100 200 300 400 500
Ca2GdNbO6:0.1% Mn4+
Boltzmann Fit
Inte
grat
ed e
mis
sion
inte
nsity
Temperature [K]
TQ1/2 = 300 K
Model BoltzmannEquation y = A2 + (A1-A2)/(1 + exp((x-
x0)/dx))
Reduced Chi-Sqr
1.47409E-4
Adj. R-Square 0.99913Value Standard Error
B A1 1 0B A2 0 0B x0 300.24299 1.49015B dx 50.22014 1.31297
600 625 650 675 700 725 750 775 80002468
1012141618202224
Inte
nsity
x10
-4 [c
ount
s]
Wavelength [nm]
77 K 100 K 150 K 200 K 250 K 300 K 350 K 400 K 450 K 500 K
Ca2GdNbO6:0.1% Mn4+
Compound T½ /K
Ca2YNbO6:Mn4+ 277
Ca2YSbO6:Mn4+ 296
Ca2GdNbO6:Mn4+ 300
Ca2LaNbO6:Mn4+ 299
Ca2LaNbO6:Mn4+ (heated in air) 284
20/21 DAfP-Symposium June 7th, 2019, Brunswick
Very little thermal quenching in inverse
garnets according to Ln2Mg3Ge3O12 :Mn4+
(Ln = Y, Gd, Lu) with deep red emission
Excitation and emission spectrum of Y2Mg3Ge3O12:Mn4+
3. Y2Mg3Ge3O12:Mn4+
250 300 350 400 450 500 550 600 650 700 750 8000.0
0.2
0.4
0.6
0.8
1.0 2A1g2T2g
4T1g4T2g
Rel
. in
tens
ity (
arb.
uni
ts)
Wavelength (nm)
Excitation (λEm= 659 nm) Emission (λEx= 288 nm)
2Eg2T1g
ZPLT1/2 > 800 K
Reference: T. Jansen, M. Brik, T. Jüstel, et al., ECS J. Solid State Sci. Technol., 7 (2018), R3086
ν6
ν4ν3
ν3
CT4T1
4T2
2EEner
gy
r0
4A2
nonradiativerelaxation
thermalpopulationEA
ν6ν4
100 200 300 400 500 600 700 8000,0
0,2
0,4
0,6
0,8
1,0
Rel
. em
issi
on in
tegr
als
(arb
. uni
ts)
Temperature (K)
High energy of the 4T1
state results in very high
quenching temperature
21/21 DAfP-Symposium June 7th, 2019, Brunswick
Commercial NIR sources (thermal radiators) show low effiency with respect to
the desired NIR emission
NIR LEDs exhibit a very high wall plug efficiency, but shortcomings are
lifetime, thermal quanching, and temperature stability
Phosphor converted light emitting diodes (pcLEDs) nicely combine the
efficiency and stability of blue emitting (In,Ga)N semiconductors with the
broad band NIR emission of phosphors
Particularly suitable are Cr3+ activated phosphors due to the suitable position
of the excitation bands and potential high quenching temperature
4. Summary
