Continuous Synthesis of Highly Stable Cs4PbBr6 Perovskite Microcrystals by a Microfluidic System and Their Application in White-Light-Emitting Diodes

Article abstract of DOI:10.1021/acs.inorgchem.8b01985

In this paper, we report a simple, rapid, and stable method for the continuous synthesis of highly stable Cs₄PbBr₆ perovskite microcrystals (MCs) using a microfluidic system. To demonstrate the potential application of Cs₄PbBr₆ MCs, the sample was fabricated with K₂SiF₆:Mn⁴⁺ phosphor onto InGaN blue chips as white-light-emitting diodes (LEDs). Our white-LED device achieved a high National Television Standards Committee value of 119% for backlight display, which indicated that the Cs₄PbBr₆ MC is a promising material for future applications.

Full text of DOI:10.1021/acs.inorgchem.8b01985

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Continuous Synthesis of Highly Stable Cs PbBr Perovskite
4
6
Microcrystals by a Microfluidic System and Their Application in
White-Light-Emitting Diodes
Zhen Bao, Hung-Chia Wang, Zhen-Feng Jiang, Ren-Jei Chung,* and Ru-Shi Liu*
†
†
‡
,‡
,†,§,⊥
†
§
Department of Chemistry and Advanced Research Center of Green Materials Science and Technology, National Taiwan
University, Taipei 106, Taiwan
⊥
‡
Department of Mechanical Engineering and Graduate, Institute of Manufacturing Technology, and Department of Chemical
Engineering and Biotechnology, National Taipei University of Technology, Taipei, 106, Taiwan
*
S Supporting Information
Recently, some reports demonstrated that reducing the
dimensionality of the perovskite structure improved the PLQY
ABSTRACT: In this paper, we report a simple, rapid, and
stable method for the continuous synthesis of highly
12−14
because the exciton binding energy increased.
Saidaminov
8
stable Cs PbBr perovskite microcrystals (MCs) using a
4
6
et al. synthesized zero-dimensional perovskite solids through
one-pot synthesis. The zero-dimensional Cs PbBr perovskite
microfluidic system. To demonstrate the potential
application of Cs PbBr MCs, the sample was fabricated
4
6
4
6
solids demonstrated long-term stability in ambient conditions
4
+
15
with K SiF :Mn phosphor onto InGaN blue chips as
2
6
(relative humidity of 57%). Chen et al. demonstrated a cooling
method to grow the centimeter-sized Cs PbBr crystals. Their
white-light-emitting diodes (LEDs). Our white-LED
device achieved a high National Television Standards
Committee value of 119% for backlight display, which
indicated that the Cs PbBr MC is a promising material
4
6
crystals can emit strong green light with absolute PLQY up to
7%, which is attributed to the embedded CsPbBr in the matrix
9
3
16
4
6
of the Cs PbBr crystals. However, Yang et al. demonstrated
4 6
for future applications.
that green photoluminescence (PL) of Cs PbBr materials may
4
6
be caused by the bromine-vacancy-related defect. Up to now, the
real mechanism for the green emission of Cs PbBr is still
underway. For practical applications, a microfluidic system is a
4
6
n recent years, perovskite nanocrystal has tremendous
potential for many applications, such as light-emitting diodes
I
considerable approach to mass production, which is another
1
−4
17
(
LEDs), solar cells, lasers, and photodetectors.
It has a
significant problem. Lignos et al. used a microfluidic reactor in
promising future in backlight display applications because of its
excellent optical performance of high photoluminescence
quantum yield (PLQY), narrow emission wavelength, and
the synthesis of CsPbX (X = Cl, Br, I) PQDs along with in situ
3
PLQY and absorption measurement. The reaction parameters
can be easily optimized through this microfluidic system. Ma et
18
5
al. built the one-dimensional−two-dimensional microreactors
by a microfluidic spinning technique to achieve continuous
CsPbBr3 PQDs/poly(methyl methacrylate) nanocomposite
production for various optoelectronic applications.
color-tunable property. Although it has outstanding optical
properties, many challenges, such as poor thermal stability,
photostability, water resistance, and anion exchange, still must
be overcome. To improve the stability of perovskite quantum
6
In this work, we successfully used the microfluidic system
dots (PQDs), Wang et al. first encapsulated CsPbX PQDs into
3
(
easy-Medchem; Figure S1) to continuous providely Cs PbBr
4 6
mesoporous (MP) silica matrix power, which enhanced the
thermal stability and photostability. However, the decrease of
the PLQY during the encapsulation of PQD nanocomposites
was quite difficult to overcome.
MC production. Different crystal growth temperatures and
washing methods were used to optimize the Cs PbBr MCs.
4
6
4+
Finally, the Cs PbBr MCs were fabricated with K SiF :Mn
4
6
2
6
red phosphor into white-LEDs to improve the color gamut.
The synthesis using a microfluidic system is illustrated in
To obtain high and stable PLQY performance, an alternative
is to reduce the structure dimensionality of the perovskite
material. Perovskite materials can be classified into four kinds of
dimensional crystal structures that depend on the connectivity
Scheme 1. The Cs PbBr MCs were synthesized through a
4
6
modified recrystallization procedure during heating. The PbBr2
and CsBr reagents were dissolved with oleic acid into dimethyl
sulfoxide (DMSO) as the precursor solution, and oleylamine
was dissolved in toluene as the precipitant solution. In the
reaction procedure, the polar solvent DMSO was a good solvent
property of the corner-sharing PbX64 octahedral.
−
7−9
The
general formula of a perovskite solid can be written as A BX2+n,
n
+
where A is a monovalent cation, such as Cs , B is divalent metal,
such as Pb , and X is a halogen anion, such as Cl , Br , and I .
The dimensionality of the perovskite material was calculated by
2
+
−
−
−
for dissolving PbBr and CsBr, while the nonpolar solvent
2
toluene thoroughly dissolved inorganic salts, which can promote
19
4
− n. Only a zero-dimensional Cs PbX perovskite structure
their reprecipitation. The precursor and precipitant solutions
4
6
4−
allows full isolation of each individual PbX6 octahedron. The
isolated structure can promote the recombination of excitons,
Received: July 15, 2018
10,11
which makes the PLQY more stable.
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01985
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Scheme 1. Schematic of the Synthesis of Cs PbBr Perovskite
4
6
MCs through a Microfluidic System
Figure 1. (a) XRD patterns of CsPbBr powder and Cs PbBr MCs
3
4
6
washed with different solvents. SEM images of Cs PbBr MCs (b)
4
6
unwashed, (c) washed with TBA, and (d) washed with DMSO. EDS
mapping images of (e) cesium, (f) lead, and (g) bromine that were
acquired at the same position as part d.
were loaded into two reagent bottles, and then they were
pumped into the microfluidic system. Before flowing into the
reactor, the solutions were mixed with a Micro-T mixer. During
mixing of the precursor and precipitant solutions, the Cs PbBr
is shown in Figure S6. Moreover, the X-ray photoelectron
spectroscopy (XPS) data (Figure S7) is consistent with the EDS
data, which indicates uniform elemental distribution in our
synthesized Cs PbBr MCs not only in the bulk but also on the
4
6
perovskite tended to nucleate with the coprecipitation of PbBr2
and CsBr. Then, the Cs PbBr perovskite crystal nuclear grew
4
6
into MCs under heating in the reactor. Different flow rates,
reagent ratios, and crystal growth temperatures were used to
optimize the Cs PbBr perovskite product (Tables S1 and S2).
4
6
surface.
The optical properties and thermal stability tests of Cs PbBr
6
4
6
4
The flow rates of both the precursor and precipitant solvents
were finally chosen as 1 mL/min, at which the reaction time was
MCs are shown in Figure 2. The PL intensity of unwashed
Cs PbBr MCs enhanced with an increase of the reaction
4
6
5
min and the highest product yield was obtained. Then, a series
temperature (Figure 2a). The Cs PbBr MCs synthesized at 150
4 6
of reagent ratios and reaction temperatures were used at this
°C emitted highest PL intensity, which was purified for further
flow rate. As the reaction temperature increased, the Cs PbBr
4
6
MCs can emit stronger under UV light (Figure S2).
To obtain pure Cs PbBr MCs, the Cs PbBr product was
4
6
4
6
washed with tert-butyl alcohol (TBA) or DMSO. TBA could
only dissolve the impurity in the product, while DMSO with
higher polarity could also partly dissolve the Cs PbBr
4
6
perovskite. The product could only be washed by DMSO at
most twice because all of the product was dissolved after washing
three times (Figure S3). As a consequence, the Cs PbBr MC
4
6
powder showed a clearer yellow-green color and emitted
stronger green light under UV light after each time of washing
(Figure S4). As a comparison, CsPbBr3 quantum dots were
synthesized via the hot-injection method and precipitated with a
hexane/TBA mixed solution (1:4 volume ratio). After washing
twice, all of the Cs PbBr MCs and CsPbBr powder samples
4
6
3
were dried at 90 °C for 12 h.
Figure 1a shows the structure characterization results and
powder X-ray diffraction (XRD) patterns of the CsPbBr and
3
Cs PbBr powders washed with different solvents. The CsPbBr
4
6
3
powder and both Cs PbBr MC samples exhibited a highly
4
6
crystalline structure and good agreement with the Crystallog-
raphy Open Database (COD) reference nos. 1533063 and
1
538416 respectively. This result proves that all of the samples
are a pure single phase. Scanning electron microscope (SEM)
images of the Cs PbBr MCs unwashed sample and samples
Figure 2. (a) PL spectra of unwashed Cs
PbBr MCs synthesized at
6
4
6
4
different temperatures. (b) PL excitation spectra of Cs PbBr MCs
washed with TBA or DMSO are shown in Figure 1b−d. The
4
6
monitored under an emission wavelength of 520 nm and emission
spectra excited by a wavelength of 460 nm. (c) PL spectra of the
CsPbBr powder and Cs PbBr MCs washed with different solvents.
surface morphology of Cs PbBr MCs washed with TBA did not
4
6
show an obvious difference compared with that of the unwashed
sample. However, because of the higher solubility of DMSO, the
surface morphology of the Cs PbBr MCs washed with DMSO
3
4
6
(
d) PL spectra of Cs PbBr MCs washed with DMSO after heating at
4 6
4
6
different temperatures. Inset: Normalized PL intensity of Cs PbBr
4
6
is much smoother. Parts e−g of Figure 1 show the energy-
dispersive spectroscopy (EDS) mapping images of cesium, lead,
and bromine that were acquired at the same position of Figure
MCs washed with DMSO and the CsPbBr powder after heating at
3
different temperatures. (e) PLQYs of DMSO-washed Cs PbBr MCs
4
6
synthesized at different temperatures. (f) PL decay times of Cs PbBr
4
6
1
d. The elemental analysis data of the yellow square in Figure 1d
MCs and the CsPbBr powder.
3
B
DOI: 10.1021/acs.inorgchem.8b01985
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
property tests. As shown in Figure 2b, the Cs PbBr MCs
4
6
washed with DMSO had board excitation spectra ranging from
40 to over 500 nm. While the emission spectra of the samples
3
were quite narrow with a peak position at 521 nm, the full width
at half-maximum (fwhm) was only 23 nm. As a comparison, the
CsPbBr powder and Cs PbBr MCs washed with TBA were
3
4
6
emitted at similar positions of 519 and 521 nm with broader
fwhm of 28 and 31 nm, respectively (Figure 2c). The Cs PbBr
4
6
MCs washed with DMSO were selected for the thermal stability
test. The Cs PbBr MCs had obviously higher thermal stability
4
6
compared with the CsPbBr powder (Figure 2d). After heat
3
treatment at 150 °C for 10 min, the Cs PbBr MCs still exhibited
4
6
9
2% PL intensity. On the contrary, the PL intensity of the
CsPbBr powder decreased to only 33% (Figure S8).
3
The quantum yields of the CsPbBr powder and Cs PbBr
3
4
6
MCs were measured by using an absolute PLQY spectrometer
c11347, Hamamatsu). The excitation wavelength was set at
60 nm to simulated the working status on the InGaN blue
(
4
Figure 3. Photographs of (a) LED devices fabricated with Cs PbBr
4
6
chips. The absolute quantum yield of the CsPbBr powder was
3
4+
MCs and K SiF :Mn phosphor and (b) their working status. (c)
2
6
quite low (PLQY < 0.1%), while the absolute quantum yields of
Cs PbBr MCs washed with TBA or DMSO can reach 15% and
Color coordinate of the white-LEDs. (d) EL spectra of the fabricated
white-LED. (e) Color gamut of the white-LEDs and the NTSC
standard.
4
6
2
4%, respectively. Moreover, the PLQYs of Cs PbBr MCs
4 6
synthesized at different temperatures increased with an increase
of the synthesis temperature, as shown in Figure 2e. The PL
decay times of Cs PbBr MCs and the CsPbBr powder are
4
6
3
shown in Figure 2f. The origin data were fitted by the following
formula:
In summary, we presented a simple, rapid, and stable method
to achieve the continuous production of high performance
Cs PbBr perovskite MCs.
fit = A + B e−
t/T1
+ B2e−t/T2 + B3e−t/T3
4
6
1
The fitting results of Cs PbBr MCs are T = 18.44 ns, T = 132.4
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01985.
4
6
1
2
■
ns, and T = 1303 ns and those of the CsPbBr powder are T =
S
3
3
1
7
.065 ns, T = 97.17 ns, and T = 561.0 ns. The Cs PbBr MCs
2 3 4 6
have obvious longer lifetimes than the CsPbBr powder. This
3
may lead to the higher PLQYs and stronger luminescence from
Cs PbBr MCs.
4
6
Experimental section, photographs of the microfluidic
system and different samples, lattice parameters, EDS
elemental analyst result, XPS, PL, and EL spectra, and the
color coordinate of LED, and structure of Cs PbBr
The Cs PbBr MCs were fabricated with CsPb(Br/I) red
4
6
3
PQDs onto InGaN blue chips to construct the white-LEDs. As
shown in the electroluminescence (EL) spectra (Figure S9), the
Cs PbBr MCs did not exhibit any ion exchange with CsPb(Br/
4
6
4
6
perovskite and its properties (PDF)
I)3 red PQDs. These LEDs had stable white-light color
coordinates (Figure S10). However, ion exchange occurred
when CsPbBr green PQDs were fabricated with CsPb(Br/I)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: rjchung@mail.ntut.edu.tw (R.J.C.).
*E-mail: rsliu@ntu.edu.tw (R.S.L.)
3
3
■
red PQDs (Figure S11), which caused the LED cannot emit
white light stably. This result illustrated that Cs PbBr MCs have
4
6
better structure stability than CsPbBr quantum dots. The EL
3
intensity of LEDs fabricated with Cs PbBr or CsPbBr depends
4
6
3
ORCID
on the operating time shown in Figure S12, which indicates that
the Cs PbBr MCs are much more stable than the CsPbBr one.
Ru-Shi Liu: 0000-0002-1291-9052
Author Contributions
The manuscript was written through the contributions of all
authors. All authors have given approval to the final version of
the manuscript.
4
6
3
To achieve better device performance and stability, the
4+
Cs PbBr MCs were fabricated with K SiF :Mn red phosphor.
4
6
2
6
The photographs of the fabricated LED devices and their
working statuses are shown in Figure 3a,b. These LED devices
mixed RGB color together to emit white light. The color
coordinate was optimized at (0.2879, 0.2434) in CIE 1931
Notes
The authors declare no competing financial interest.
(
Figure 3c), which is suitable for backlight display applications.
ACKNOWLEDGMENTS
As shown in Figure 3d, three peaks, corresponding to the InGaN
chip, Cs PbBr MCs, and K SiF :Mn red phosphor, were
■
4+
This work was financially supported by the Advanced Research
Center of Green Materials Science and Technology from The
Featured Area Research Center Program within the framework
of the Higher Education Sprout Project by the Ministry of
Education (107L9006) and the Ministry of Science and
Technology in Taiwan (Contracts MOST 107-2113-M-002-
008-MY3 and MOST 107-3017-F-002-001).
4
6
2
6
observed in the EL spectra of the white-LED. This white-LED
device exhibited a wide color gamut of 119% of National
Television Standards Committee (NTSC) standard (Figure 3e)
and a luminous efficiency of 13.91 lm/W. These results
demonstrated the success of fabricating white-LEDs by
Cs PbBr MCs.
4
6
C
DOI: 10.1021/acs.inorgchem.8b01985
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
(
19) Zhang, Y.; Saidaminov, M. I.; Dursun, I.; Yang, H.; Murali, B.;
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DOI: 10.1021/acs.inorgchem.8b01985
Inorg. Chem. XXXX, XXX, XXX−XXX