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1. Recent Developments in LED Phosphors for Lighting
and Display Applications
Ravilisetty P. Rao and Daniel J. Devine
Specialty Phosphors Inc.
Cupertino, CA 95014, USA
Abstract
Phosphor-converted white-light LEDs are promising as the next
generation for lighting applications due to their solid-state nature,
energy savings, compact size, long life, and environmental
friendliness. In this presentation, recent developments in LED
phosphors for solid-state lighting (SSL) as well as displays (LCD
backlighting) will be presented.
Author Keywords
Phosphors; solid state lighting; lamps; UV; luminescence; color
temperature; rendering index; LED; displays; LCD; and
luminaires.
1. Introduction
Light emitting diodes (LEDs) are replacing conventional lighting
such as incandescent, halogen, fluorescent and compact fluorescent
lamps due to their long life, energy saving, compact size (small
form factor), higher efficiency, and environmental friendliness.
Due to higher cost and thermal management issues, LED lighting
is currently being used in limited applications such as museums,
art gallery, showcases, street lights, LCD backlighting, etc.
Significant efforts are being made to extend LED devices as the
next generation lighting source for general illumination. There are
two types of white LEDs, viz., phosphor converted LEDs (pc-
LEDs) and multi-LED packages. There are three different
approaches to generate pc-LED based white light: a) by mixing
red, green and blue LEDs, b) by using UV LED with red, green
and red phosphors, and c) by using blue LEDs with blue excitable
yellow phosphor, yellow and red, or green and red phosphors. The
fabrication of white light luminaires with blue LEDs and yellow
phosphors is reasonably easy and low cost.
When compared to near-UV LEDs using RGB phosphors, blue
LEDs with yellow phosphor have low efficiency; exhibit low CRI,
and low chromatic stability. For both blue or UV LEDs, the choice
of phosphor is a key component for functionality and commercial
success for lighting and display devices. A number of different
phosphor systems are currently being synthesized for LED based
lighting and display applications and are very well published.
However, very few are considered as practical phosphors, viz. rare
earth activated alkaline earth (AE) silicates, AE sulfides, AE thio-
gallates, rare-earth oxides (particularly garnets), AE silicon nitrides
and AE silicon-oxy-nitrides. Currently garnets, nitrides and
silicates are being used in SSL applications. Depending on the
nature of the desired lighting effect and end use, the combination
of yellow and red phosphors coated either on LED substrates or
used remotely varies. With the blending of two or more phosphors,
the efficiency goes down because of re-absorption between the
phosphors deepening on the Stokes shifts of the individual
phosphors. Remote phosphor technology is being encouraged
because of reliability, flexibility to tune to required CCT and CRI,
and beam uniformity.
Due to the high operation temperature of high current/brightness
LEDs, conformal as well as remote phosphor coatings using
silicone binders and encapsulations show degradation, cracking,
and discoloration over time. To avoid the usage of silicone or
similar binders, some phosphor/lighting companies are actively
pursuing development of luminescent ceramics and luminescent
plastic disks for conformal or remote applications.
This presentation will review commercially available blue and near
UV excitable phosphors along with their key optical and physical
properties such as: excitation, emission, conversion efficiency,
flux, absorption, reflection, scattering, FHHW, color temperature
(CCT), color coordinated (x,y), color rending index (CRI-Ra),
physical properties such as surface morphology, particle size
distribution, body color, surface charge, surface area, and
chemical, thermal and optical stability.
Chemical formulations, general characteristics, and stability of
various phosphors currently employed in pc-WLEDs are presented
in Table 1. Considering emission characteristics, white LEDs
appear to be promising alternatives to cold cathode fluorescent
lamps (CCFL) for backlight in LCDs.
2. Phosphor Requirements for Lighting and Display
Applications
For general lighting and display applications, phosphors need to
fulfill a number of required characteristics such as: spectral
response (excitation and emission), intensity and half width,
brightness and efficiency, decay (or persistence), color temperature
(CCT) and color coordinates (x,y) or color purity, color rendering
index (CRI-Ra), chemical, optical and thermal stability, long
lifetime (or slow aging or degradation), surface charge, reflectivity,
and morphology (size, shape, particle size distribution).
Most of these characteristics of individual phosphors will be
discussed in this presentation. Incorporation of specialized
phosphors into a particular device for lighting and display is very
critical. A number of current phosphor coatings or deposition
processes being employed by the industry are explained.
3. Blue Excitable Phosphors
Garnet structured Yttrium aluminum oxide activated with Cerium
(Ce3+
) also known as YAG yellow-emitting phosphors are widely used
for pc-WLEDs. The emission spectrum of a typical YAG:Ce phosphor
is shown in Fig. 1 (a). By partial replacement of Y in YAG:Ce
phosphor with Lutetium or Terbium, the emission spectrum can be
moved towards blue or red. Due to low red emission in YAG:Ce, the
CRI of WLEDs with blue InGaN chips is low. The CRI can be
improved slightly by co-activating YAG:Ce with Cr3+
, Sm3+
and Pr3+
.
To avoid intellectual property rights of others, efforts are being made
to replace YAG:Ce by improving traditional phosphors such as Eu2+
activated alkaline earth - AE (Ba,Ca,Sr) silicates (SiO4) (BOSE) and
Invited Paper 69.1 / R. P. Rao
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2. AE sulfides (AES:Eu) and AE thiogallates (AEGaS4:Ce) [1] and
developing new phosphors such as Eu2+
doped silicon-aluminum-oxy-
nitride (SiAlON) and Ce3+ activated lanthanum nitridosilicate
(La3Si6N11) phosphors. To achieve higher CRI (>80), there must be
sufficient red contribution from the phosphors. Red emitting BOSE
and AES are not stable particularly with high brightness LEDs due to
their thermal quenching and hygroscopic nature. Red emitting stable
divalent Europium (Eu2+
) activated AE2Si5N8 has been developed by
NIMS [2] and others [3]. The emission (color) from these phosphors
varies from orange to deep red, depending on the type and
concentration of alkaline earth metals (AE=Ca, Sr, and /or Ba). Due to
lower thermal quenching, emission in the red region and high quantum
efficiency, these nitridosilicate are widely used in the lighting industry
as a preferred red component. To enhance the color rendering
properties, two or more phosphors are applied as a blend or layers to
obtain the desired broad emission spectra [4].
4. Near Ultra Violet (NUV) Excitable Phosphors
Single phosphor with activator and co-activator can emit a broad
emission for example, Eu2+
- Mn2+
and Ce3+
- Mn2+
. There are few
single phosphors such as Ba3MgSi2O8:Eu2+
,Mn2+
which shows three
emission peaks at 442, 505 and 620 nm. But one has to work to
improve the performance of these phosphors for practical application.
There are few efficient phosphors in blue, green and red regions
suitable for near UV excitation with the exception of traditional lamp
phosphors. Some of them are tuned to be more suitable for SSL
applications. Eu2+
activated strontium chloro apatite exhibits emission
peaking at 447nm after excitation with NUV LED. Emission peak can
be shifted towards to longer wavelengths by partial replacement of Sr
by Ca. Another example is blue emitting BAM phosphors. Eu2+
and
Mn2+
activated barium magnesium aluminate is initially developed for
CCFL for sharper green peak. It belongs to BAM family with a partial
replacement of Mg2+
by Mn2+
. Green emission (515 nm) from this
phosphor corresponds to non-radiative energy transfer from Eu2+
to
Mn2+
. With the increase of Mn concentration, the energy transfer is
more effective, ie more green and less blue. Lower temperature
quenching with high Mn concentration is also favorable for high
brightness LEDs. Eu3+
activated rare earth oxides are known red
emitting phosphors for fluorescent as well as compact lamps. Other
than YOX, LnVO4, and Ln2O2S, there are only a few Mn2+
activated
phosphors described in the literature. By co-doping with Bi3+
some of
UV excitable phosphors can be tuned to NUV phosphors for pc-
WLEDs. Red emitting silicon nitride based phosphors absorb not only
NUV light but also blue and a part of green light. When blended with
blue and green phosphors, the efficiency drops due to re-absorption.
One has to give utmost care to re-absorption processes when blending
or layering different phosphors. Emission spectra from pc-WLED
(NUV-406 nm) with SCAP (blue), GBAM (green) and Ln2O2S:Eu
(red) phosphors are shown in Fig.2.
5. Phosphor in Ceramic and Resin Discs
Currently, silicone encapsulated phosphor is deposited on LED chips.
During the process of down conversion, a fair amount of visible light is
lost due to scattering. Stability of silicone at high operating
temperature is poor. Due to the difficulty in obtaining consistent color,
wide binning of LEDs is common. To minimize all these issues,
Philips, Osram and others are developing materials and processes to
embed phosphor particles in a ceramic medium [5,6]. Ceramic discs
with phosphor reduces the binning problem and the amount of organic
encapsulate and binders, allows tailoring of optical properties, and
maximizes packaging efficiency. On other hand, Mitsubishi, Intematix
and others are working on plastic/resin composites with selected
phosphors to be used in remote applications. The efficiency from these
discs increases by 25 to 30% [7] over conventional LED lighting
systems as the phosphor composite is precisely layered onto a
substrate, maximizing photon extraction and eliminating the diffuser.
By using a standard LED energy source, multiple CCT, CRI and Ra
requirements can be generated by simply replacing discs with various
phosphors and phosphor blends.
6. Conclusions
Yellow emitting YAG:Ce and LuAG:Ce are widely used for pc-
WLED systems for lower Ra. For higher Ra, red emitting silicon
nitride based phosphor is blended with yellow emitting phosphors.
Major phosphor manufacturers are claiming improved BOSE
phosphors with less thermal quenching. Narrow-band phosphors are
highly preferred in LCD back light applications due to color purity.
Known fluorescent lamp phosphors are being tuned for near UV
excitation. There are a number of phosphor formulations under
development. Some of them will lead to higher efficiency, extended
lifetime, and lower cost phosphors in near future for SSL applications.
Figure 1. Emission spectra from pc-WLED (blue) with YAG:Ce
phosphor [dotted] (Ra=72) and YAG:Ce and Sr2Si5N8:Eu phosphor
blend [solid] (Ra=82).
7. References
[1] D. Jia and X. J. Wang, Optical Materials 30, 375-379 (2007).
[2] Rong-Jun Xie , N. Hirosaki, Y. Q. Li and H. Yamamoto,
Nitride Phosphors and Solid State Lighting, CRC Press (2011).
[3] M. Zeuner, S. Pagano, and W. Schnick, Angew. Chem. Int. Ed.
50, 7754-7775 (2011)
[4] Y. Zhu and N. Naredran, Jpn. J. Appl. Phys 49, 100203-
1002107 (2010).
[5] H. Schmidt, et. al.,Tenth International Conference on Solid
State Lighting. Edited by I. Ferguson, M. Kane, H. Matthew,
N.Nadarajah and T. Taguchi, Proceedings of the SPIE, 7784,
77840W-77840W-11 (2010).
[6] M. Hannah, J. Kelso, M. Raukas, M. Stough, G. Wei, Y.
Zheng, N. Zink, K. Bergenek, R. Wirth, D. Eisert, and A.
Linkov, Abstract# 2663, 220th
ECS Meeting, October (2011).
[7] http://www.intematix.com /technology/chromalit-technology.
69.1 / R. P. Rao Invited Paper
934 • SID 2012 DIGEST

3. 0
10000
20000
30000
40000
50000
378 415 452 489 525 561 596 631 666
Wavelength, nm
Intensity,
AU
SUV LED
Blue
Green
Red
Figure 2. Emission spectra from pc-WLED (NUV-406 nm) with SCAP
(blue), GBAM (green) and Ln2O2S:Eu (red) phosphors
Table 1. Chemical formulations and characteristics of commercial phosphors for LED based solid-state lighting
Phosphor Chemical
Composition
Excitation
Nm
Emission
Max. nm
Emission
HW nm
x y IQE
%
Chem.
Stability
Thermal
Stability
YAG:Ce Y3Al5O12:Ce 440-470 555 120 0.450 0.532 95 Good Good
LuAG:Ce Lu3Al5O12:Ce 440-490 515 100 0.341 0.573 92 Good Good
BOSE (AE)2SiO4:Eu 200-490 525-620 80 -- -- 96 OK Bad
CALSIN CaAlSiN3:Eu 350-550 640 100 -- -- 94 V Good V Good
CaScxide (CaSC2O4:Ce 380-550 515 80 0.293 0.641 - V Good V Good
CALSION CaAlSi(ON)3:Eu 300-500 648 100 -- -- 94 V Good V Good
βSiAlON SiAlON:Eu 350-500 540 75 0.340 0.620 - V Good V Good
ThioGalat AEGaS4:Ce 400-530 534 50 0.278 0.684 - Bad Good
AESulfide AES:Eu 400-580 630 70 0.660 0.340 - Bad Good
LSN La3Si6N11:Ce 350-500 555 120 0.422 0.566 - Good Good
SCAP Sr10(PO4)6Cl2:Eu 270-420 447 60 0.148 0.101 98 Good Good
GBAM BaMgAl10O17:
Eu,Mn
300-420 515 78 0.221 0.620 79 Good Good
REX RE2O3:Eu 270-410 611 line 0.655 0.340 94 Good Good
REOS RE2O2S:Eu 350-420 627 line 0.625 0.352 93 Good Good
Invited Paper 69.1 / R. P. Rao
SID 2012 DIGEST • 935