Mesoporous Silica Particles Integrated with All-Inorganic CsPbBr3Perovskite Quantum-Dot Nanocomposites (MP-PQDs) with High Stability and Wide Color Gamut Used for Backlight Display

Article abstract of DOI:10.1002/anie.201603698

All-inorganic CsPbX₃(X=I, Br, Cl) perovskite quantum dots (PQDs) have been investigated because of their optical properties, such as tunable wavelength, narrow band, and high quantum efficiency. These features have been used in light emitting diode (LED) devices. LED on-chip fabrication uses mixed green and red quantum dots with silicone gel. However, the ion-exchange effect widens the narrow emission spectrum. Quantum dots cannot be mixed because of anion exchange. We address this issue with a mesoporous PQD nanocomposite that can prevent ion exchange and increase stability. We mixed green quantum-dot-containing mesoporous silica nanocomposites with red PQDs, which can prevent the anion-exchange effect and increase thermal and photo stability. We applied the new PQD-based LEDs for backlight displays. We also used PQDs in an on-chip LED device. Our white LED device for backlight display passed through a color filter with an NTSC value of 113 % and Rec. 2020 of 85 %.

Full text of DOI:10.1002/anie.201603698

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201603698
German Edition: DOI: 10.1002/ange.201603698
Perovskite Quantum Dot LEDs
Mesoporous Silica Particle Integrated with All-Inorganic CsPbBr3
Perovskite Quantum-Dot Nanocomposite (MP-PQDs) with High
Stability and Wide Color Gamut Used for Backlight Display
Hung-Chia Wang, Shin-Ying Lin, An-Cih Tang, Bheeshma Pratap Singh, Hung-Chun Tong,
Ching-Yi Chen, Yu-Chun Lee, Tzong-Liang Tsai, and Ru-Shi Liu*
Abstract: All-inorganic CsPbX3 (X = I, Br, Cl) perovskite
quantum dots (PQDs) have been investigated because of their
optical properties, such as tunable wavelength, narrow band,
and high quantum efficiency. These features have been used in
light emitting diode (LED) devices. LED on-chip fabrication
uses mixed green and red quantum dots with silicone gel.
However, the ion-exchange effect widens the narrow emission
spectrum. Quantum dots cannot be mixed because of anion
exchange. We address this issue with a mesoporous PQD
nanocomposite that can prevent ion exchange and increase
stability. We mixed green quantum-dot-containing mesoporous
silica nanocomposites with red PQDs, which can prevent the
anion-exchange effect and increase thermal and photo stability.
We applied the new PQD-based LEDs for backlight displays.
We also used PQDs in an on-chip LED device. Our white
LED device for backlight display passed through a color filter
with an NTSC value of 113% and Rec. 2020 of 85%.
phosphor exhibits low efficiency and entails high costs for this
RG phosphor with a blue chip.[6] RG QDs have been
commonly used in LED[5b] because their emission wave-
lengths can be simply tuned to increase luminescence and its
CRI. QDs are optimum backlight materials because of their
narrow emission wavelength and superior color purity. QD-
based white LEDs exhibit advantage over conventional
lightening owing to its design flexibility and optimization of
the color performance and NTSC value through color filters.[7]
The NTSC for commercial LCDs and wide-color-gamut TVs
are approximately 72% and 96%, respectively. The NTSC for
CdSe QD-based white LED devices reaches 104% at CIE
coordinates (0.24, 0.21).[1] During LED fabrication, packaging
approaches should also be considered. On-film type[8] is the
most common packaging approach; in this approach, QDs are
in a thin-film form and placed over an entire display area. The
demand on materials in this approach is much higher than
that of other approaches. In this study, fabricated on-chip type
QDs can decrease the demand on materials and provide
a convenient packaging technique. Two types of perovskite-
based semiconductors have been used: hybrid perovskite
semiconductors and all-inorganic perovskite QDs. An exam-
ple of a hybrid perovskite semiconductor is MAPbX3 (MA =
CH3NH3, X = Cl, Br, I) that can be potentially used for solar
cells.[9] An organic ligand (MA) can be replaced with a cesium
cation to develop a new type of perovskite semiconductor,
namely, all-inorganic perovskite QDs.[10] For example, all-
inorganic perovskite QD CsPbX3 (X = Cl, Br, I) can be used
more efficiently than Cd-based QD (CdSe)[11] and Cd-free
QD (InP,[12] CuInS2[13]) systems for backlight displays. Cd-
based QDs for backlight display have also been extensively
explored because of their high quantum efficiency and narrow
emission wavelength. However, Cd-based (CdSe, CdTe) QDs
have been synthesized under harsh reaction conditions and
long reaction time; these conditions limit their commercial
production. Cd-based QDs are also restricted in many
countries because of the development of green chemistry.
Cd-free InP QDs should be improved in terms of their
quantum efficiency, and their full width to half-maximum
should be decreased to enhance their color purity. In 2015,
Protesescu et al.[10] reported a facile method to synthesize all-
inorganic PQDs at a reaction temperature of approximately
1808C and a reaction time of 5 s. Synthesizing PQDs with high
quantum efficiency (CsPbBr3 up to 90%) and narrow
emission wavelength (12–42 nm) is easier than fabricating
traditional QDs. In all-inorganic PQDs with a tunable wave-
length, their halide ratio and core growth temperature can be
Q
uantum dots (QDs) can be used in many applications,
such as light emitting diodes (LEDs),[1] organic LEDs
OLEDs,[2] solar cells,[3] and bioimaging.[4] QDs are an
excellent candidate for backlight displays because of their
high luminescence, narrow emission wavelength, and tunable
color. QD-based white LEDs have been widely explored and
have been further subdivided into two categories: one
category is based on lighting[5] and the other category is
used for backlight. These two categories are entirely different.
For lighting, the most important factors are color rendering
index (CRI) and luminescence at 1931 CIE coordinates (0.33,
0.33). The most general commercially available approach is
the YAG phosphor mixed with silicone gel and InGaN blue
placed on LED. However, its CRI is low. To increase CRI, we
can mix red (R) phosphor with green (G) phosphor, but green
[*] H. C. Wang, S. Y. Lin, A. C. Tang, Dr. B. P. Singh, Prof. Dr. R. S. Liu
Department of Chemistry
National Taiwan University
Taipei, 106 (Taiwan)
E-mail: rsliu@ntu.edu.tw
Prof. Dr. R. S. Liu
Department of Mechanical Engineering and Graduate Institute of
Manufacturing Technology, National Taipei University of Technology
Taipei 106 (Taiwan)
H. C. Tong, Dr. C. Y. Chen, Dr. Y. C. Lee, Dr. T. L. Tsai
Lextar Electronic Corporation
Hsinchu, 300 (Taiwan)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201603698.
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easily tuned to obtain different emission wavelengths. Fur-
thermore, recently researchers used a fast anion-exchange
method[14] to tune different emission wavelengths by adding
Grignard reagents (MeMgX) or oleylammonium halides
(OAmX) to CsPbX3. Our research focuses on the synthesis
and fabrication of PQDs and their use for backlight displays.
The use of all-inorganic PQDs for LEDs is challenging. In
a previous study, a silica coating on QDs was used to improve
stability;[15] the most common methods are Stçber[16] and
reverse microemulsion[17] methods. In silica coating, surface
ligand exchange initially occurs and phase changes from oil to
water. Ligand exchange is difficult to facilitate because of the
low stability of PQDs. Other polymer encapsulating methods
also encounter issues on solvent polarity.[18] Generally the
strong anion-exchange effect was observed when green and
red all-inorganic QDs were mixed with silicone resin.
We propose an efficient and simple method to prevent the
anion-exchange effect. We mixed green CsPbBr3 PQDs with
purchased mesoporous silica[19] whose pore size is approx-
imately 12–15 nm. The synthesis is shown in Scheme 1. We
can use non-polar solvents, such as hexane and toluene, to
prevent the solvent effect of polymers and to synthesize
mesoporous silica green PQD nanocomposite (MP-G-PQDs;
Scheme 1) with mesoporous silica.
Figure 1. Photoluminescence and UV/Vis spectra of a) blue
CsPb(Br0.5Cl0.5)3, b) green CsPbBr3, and c) red CsPb(Br0.4I0.6)3 perovskite
QDs. Colloidal RGB PQDs dispersed in d) hexane under sunlight,
e) hexane under UV-light (365 nm) and f) color gamut spectra of RGB
PQDs.
CsPbX3 PQDs was measured by using an absolute photo-
luminescence quantum yield spectrometer (c11347, HAMA-
MATSU). An excitation wavelength of 460 nm was set to
calculate the quantum yield in an InGaN blue chip. Fig-
ure S2a illustrates the absolute quantum yield and full-width
at half wavelength of CsPbX3 PQDs with different emission
wavelengths (457–698 nm). The absolute quantum yields of
green CsPbBr3 and red CsPb(I0.6Br0.4)3 were 55% and 70%,
respectively. The full width of half wavelength of CsPbX3
PQDs were approximately 13–35 nm.
As shown in Figure 2, the red shift of emission wavelength
decreases with the increasing average particle size of the
RGB PQDs. This phenomenon indicated that the emission
wavelength of PQDs was dominated by the halide ratio and
quantum size effect. CsPbX3 was cubic, which is a highly
stable structure.
Scheme 1. The synthesis process of mesoporous silica green PQD
nanocomposite (MP-PQDs).
CsPbX3 PQDs crystallize to form an orthorhombic or
cubic phase because of the synthesis temperature. In our
All-inorganic CsPbX3 (X = Cl, Br, I) QDs were synthe-
sized by using previous methods. We tuned different Br/I and
Br/Cl ratios to obtain various emission wavelengths. In the
application of white LED (photoluminescence) and QD-
LED (electroluminescence), red–green–blue (RGB) PQDs
are the most important applications. Figure 1 shows the
emission and UV/Vis spectra of RGB PQDs. We synthesized
blue (460 nm) CsPb(Br0.5Cl0.5)3, green (515 nm) CsPbBr3, and
red (625 nm) CsPb(Br0.4I0.6)3 QDs with a narrow emission
bandwidth of 15 nm (blue), 20 nm (green), and 30 nm (red).
For backlight applications, the CIE coordinates of blue (0.17,
0.01), green (0.07, 0.75), and red (0.66, 0.34) are shown in
Figure 1 f. The wide color gamut spectra of RGB PQDs are
attributed to the narrow emission wavelength.
The PL and UV/Vis spectra of different ratios of CsPb-
(Br1ÀxIx)3 are shown in Figure S1 in the Supporting Informa-
tion. The emission spectra can be tuned from 515 nm to
690 nm by adjusting the different halide ratios. UV/Vis
spectra revealed the first absorption maximum peak shifted
because of different halide ion ratios. The quantum yield of
Figure 2. TEM spectra of a) blue CsPb(Br0.5Cl0.5)3, b) green CsPbBr3,
c) red CsPb(Br0.4I0.6)3 perovskite QDs, and d) Particle size of RGB
perovskite QDs and pore size of mesoporous silica powder.
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synthesis, the reaction temperature was approximately 170–
1908C; therefore, the phase of our CsPbX3 QDs is cubic.
Figure 3a reveals CsPbX3 with various Br/I ratios; the phase
of CsPbX3 was cubic in contrast to the standard phase of
CsPbI3 (ICSD-181288), CsPbBr3 (ICSD-29073), and CsPbCl3
(ICSD-29072). The XRD result with various Br/Cl ratios is
shown in Figure S3.
Figure 4. a) Blue, b) green, and c) red MP-PQDs under sunlight illumi-
nation. d) Blue, e) green, and f) red MP-PQDs under UV light
(365 nm) excitation.
Figure 3. a) X-ray diffraction patterns of PQDs with various Br/I ratios,
b) crystal structure of cubic CsPbBr3, and c) high-resolution TEM
images of CsPbBr3 PQDs.
For structural characterization, high-resolution-TEM
results are shown in Figure 3c. Green CsPbBr3 PQDs were
single-crystalline cubic phase with d-spacing of 5.8 ꢀ. Ele-
mental analysis was performed by using an energy dispersive
X-ray analyzer (EDS) to confirm the elemental composition
ratios of green CsPbBr3 QDs (Figure S4). The element ratio
of Cs, Pb, and Br was approximately 1:1:3, which fitted the
experimental ratio.
Figure 5. Thermal stability test of MP-CsPbBr3 and CsPbBr3.
CsPbBr3 QDs with 10 nm particle size and cubic phase
structure were integrated with mesoporous silica nanoparti-
cles. A RGB MP-PQD nanocomposite is illustrated in
Figure 4. The mesoporous silica material was purchased.
The TEM and SEM images of mesoporous silica nano-
particles are shown in Figure S5. The size of this mesoporous
silica nanoparticle was 200–500 nm and its pore size was
approximately 12–15 nm. The PL spectrum of MP-CsPbBr3
and CsPbBr3 is shown in Figure S6. The emission wavelength
of MP-CsPbBr3 was shifted approximately 4 nm from 515 nm
to 519 nm. The stability of PQDs can be enhanced by this
novel PQD nanocomposite. If we aim to use PQDs in an on-
chip system, we should improve the stability of PQDs.
We used a thermal controller system to test the thermal
stability (Figure 5). The experimental temperature ranged
from 258C to 1008C. The relative intensity of CsPbBr3 PQDs
decreased when the temperature increased. The thermal
cycling results were shown in Figure S7. MP-CsPbBr3 exhib-
ited higher thermal stability and thermal recycling than
CsPbBr3. When the temperature was decreased to room
temperature, the intensity of MP-CsPbBr3 was nearly the
same as that before the temperature treatment. The relative
intensity of green CsPbBr3 PQDs was decreased to 60% after
heat treatment was administered.
A photostability test of CsPbBr3 PQDs was implemented
under continuous UV-light (365 nm, 6 W) irradiation. The
test period was from 30 min to 96 h. In Figure 6, CsPbBr3
PQDs were dispersed in hexane and exposed to UV light
irradiation for 96 h. After 96 h, the relative intensity of
CsPbBr3 was decreased to 40%. As seen for previously
designed core/shell structures of QDs, a passivation shell[20]
can prevent photooxidation during UV light irradiation.
However, PQDs only have a core structure, which is easily
exposed to oxygen and thus causes surface defects. The
relative intensity decreased quickly and because of surface
defects, and PQDs decomposed. We also observed some
CsPbBr3 residues after the PQDs were exposed to UV light
irradiation. The mesoporous silica shell can act as a protective
shell for the core of CsPbBr3 PQDs. After 96 h, MP-CsPbBr3
powder is approximately equal to 80% under UV light
irradiation. MP-CsPbBr3 nanocomposite not only exhibits
better thermal and photostability but also resolves the anion-
exchange phenomena of PQDs.
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Figure 6. Photostability test of MP-CsPbBr3 and CsPbBr3.
Figure 7. Spectra of green, red, and mixed (green+red) PQDs was
under the excitation of a 450 nm blue chip.
In LED packaging, green CsPbBr3 was initially mixed
with red CsPb(Br0.4I0.6)3 PQDs in silicone resin and then
dropped in the blue chip. The spectra of green CsPbBr3 PQDs
and red CsPb(Br0.4I0.6)3 PQDs under the excitation of the blue
chip (450 nm) are shown in Figure 7. The emission wave-
lengths of green and red QDs were 515 and 625 nm,
respectively. The mixture spectra are also illustrated in
Figure 7. Red and green strongly shifted to yellow when the
narrow emission spectra widened. Considering the strong ion-
exchange effect, we cannot use this narrow emission wave-
length and high quantum yield of PQDs for LEDs.
Green MP-PQD nanocomposite was mixed with red
perovskite QDs in silicone resin to solve this problem. The
obtained spectra are shown in Figure 8. No spectrum shift of
this device was observed. The color coordinates of red PQDs
and MP-green PQDs were (0.66, 0.34) and (0.19, 0.69),
respectively.
Figure 8. Spectra of MP-CsPbBr3 mixed with red PQDs under blue
chip (450 nm) excitation. The inset spectrum reveals the color gamut
of white light LED.
A wide color gamut was also determined as shown in
Figure 9. The color coordinates of the PQD-white LED were
optimized at (0.24,
0.28) in CIE 1931 and
luminous
efficiency
30 lm/w. We first suc-
cessfully used PQDs
packaged on-chip in
a
backlight display.
The advantages of
PQDs included a nar-
rower emission wave-
length and
quantum
a
higher
efficiency
than those of tradi-
tional QDs. For back-
light display, the NTSC
value of the device
should also be calcu-
lated. The emission
wavelengths of RGB
before the PQDs were
filtered were 625, 519,
and 450 nm. PQD-
white LED passed
through the color filter
Figure 9. a) Spectra of PQD-based white LED before color filter, b) color gamut of PQD-LED, phosphor LED, and
The RGB CIE coordi- Cd-QDs white LED c) The table features data of the three types of display.
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nates were (0.69, 0.30), (0.19, 0.73), and (0.14, 0.04) after the
PQDs were passed through the color filter. The color gamut
overlap of the NTSC space was approximately 113%, which
was higher than a previously designed regular phosphor LED
(NTSC 86%) and Cd-QD LED (NTSC 104%). This result is
attributed to the narrow emission wavelength of the green
and red PQDs. The new definition of color space is Rec 2020.
The RGB CIE coordinates were (0.70, 0.29), (0.17, 0.79), and
(0.13, 0.04) because of the wider color gamut of the display.
The overlap area of the Rec 2020 of the PQD-white LED was
approximately 85%.
In summary, the anion-exchange phenomenon of all-
inorganic PQDs was successfully resolved. In previous
studies, the anion-exchange effect is a simple mechanism
used to tune wavelengths. In LED packaging, this effect
should be prevented. We created a mesoporous PQD nano-
composite, which can prevent the anion exchange and
increase stability. We mixed green MP-CsPbBr3 PQD nano-
composite with red PQDs in silicon resin and subjected to
excitation by using a blue InGaN chip. Our white LED device
for backlight display was passed through a color filter. The
NTSC value is 113% and the Rec 2020 value is 85%.
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Experimental Section
Typical synthesis process of colloidal CsPbBr3 perovskite QDs
followed the previous procedure. The detailed synthesis process was
shown in Supporting Information. Synthesized mesoporous silica
CsPbBr3 perovskite QDs nanocomposite (MP-PQDs): 10 mgmLÀ1
green CsPbBr3 was mixed with 100 mg mesoporous silica powder in
hexane solution and stirred for 1 h. After that, the precipitate was
collected by centrifugation at 4000 rpm and solvent was removed for
30 min at 408C. We can obtain MP-PQDs nanocomposite powder.
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Acknowledgements
We thank the Ministry of Science and Technology of Taiwan
(Contract No. MOST 104-2113-M-002-012-MY3) for finan-
cially supporting this research. This work is also supported by
the Lextar Electronics Corporation. We also express our
gratitude to Ms. C. Y. Chien of the Precious Instrument
Center (National Taiwan University) for her assistance in our
TEM experiments.
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Keywords: all-inorganic perovskite quantum dots · backlight ·
light emitting diode · white light
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Received: April 16, 2016
Published online: && &&, &&&&
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Perovskite Quantum Dot LEDs
Points of light: Green CsPbBr3 perovskite
quantum dots (PQDs), embedded in
mesoporous silica (MP), are mixed with
red CsPb(Br0.4I0.6)3 quantum dots in a sil-
icone resin and placed on an InGaN blue
chip. The green and red QDs were excited
by 450 nm blue light. The resulting PQD-
white light emitting diode (LED) exhibits
a wide color gamut because of its narrow
emission wavelength.
H. C. Wang, S. Y. Lin, A. C. Tang,
B. P. Singh, H. C. Tong, C. Y. Chen,
Y. C. Lee, T. L. Tsai,
R. S. Liu*
&&&& — &&&&
Mesoporous Silica Particle Integrated
with All-Inorganic CsPbBr3 Perovskite
Quantum-Dot Nanocomposite (MP-
PQDs) with High Stability and Wide Color
Gamut Used for Backlight Display
6
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