Single-Junction Binary-Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency

Abstract

A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71BM (PCE = 5.22%).

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Single-Junction Binary-Blend Nonfullerene Polymer Solar
Cells with 12.1% Efficiency
Fuwen Zhao, Shuixing Dai, Yang Wu, Qianqian Zhang, Jiayu Wang, Li Jiang,
Qidan Ling, Zhixiang Wei, Wei Ma, Wei You, Chunru Wang,* and Xiaowei Zhan*
DOI: 10.1002/adma.201700144
As a promising technology for clean and
renewable energy conversion, organic
solar cells (OSCs) have attracted consider-
able attention in recent years since they
have a number of attractive features, such
as low cost, light weight, flexibility, and
semi-transparency.[1–6] Fullerene deriva-
tives (e.g., PC61BM and PC71BM) are
the classical electron acceptors in OSCs
during the last two decades and they
exhibit high electron affinity and isotropic
charge transport with good mobility, due
to unique spherical geometry.[7,8] The
power conversion efficiency (PCE) over
11% has been achieved for fullerene-based
OSCs;[9,10] however, its weak absorption in
the visible region and limited tunability in
energy levels restrict further development
of the OSCs.
Nonfullerene acceptors, as the alter-
native to fullerene acceptors, attracted
increasing attention in recent years,
because they present good light harvesting
capability and facile energy level modula-
tion, which are beneficial to achieving high
short-circuit current density (JSC) and high
A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and
synthesized by introducing fluorine (F) atoms onto the end-capping group
1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of
F would improve intramolecular interaction, enhance the push–pull effect
between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor
unit IC due to electron-withdrawing effect of F, and finally adjust energy levels
and reduce bandgap, which is beneficial to light harvesting and enhancing
short-circuit current density (JSC). On the other hand, incorporation of F
would improve intermolecular interactions through CF···S, CF···H, and
CF···
π
noncovalent interactions and enhance electron mobility, which is
beneficial to enhancing JSC and fill factor. Indeed, the results show that fluori-
nated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and
higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, non-
fullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron
acceptor and a wide-bandgap polymer donor FTAZ based on benzodithio-
phene and benzotriazole exhibit power conversion efficiency (PCE) as high as
12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The
PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junc-
tion binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show
much better efficiency and better stability than the control devices based on
FTAZ:PC71BM (PCE = 5.22%).
F. Zhao, S. Dai, J. Wang, Prof. X. Zhan
Department of Materials Science and Engineering
College of Engineering
Key Laboratory of Polymer Chemistry and Physics
of Ministry of Education
Peking University
Beijing 100871, China
E-mail: xwzhan@pku.edu.cn
F. Zhao, Dr. L. Jiang, Prof. C. Wang
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, China
E-mail: crwang@iccas.ac.cn
S. Dai, Prof. Q. Ling
Fujian Key Laboratory of Polymer Materials
College of Materials Science and Engineering
Fujian Normal University
Fuzhou 350007, China
Y. Wu, Prof. W. Ma
State Key Laboratory for Mechanical Behavior
of Materials
Xi’an Jiaotong University
Xi’an 710049, China
Q. Zhang, Prof. W. You
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599-3290, USA
Prof. Z. Wei
National Center for Nanoscience and Technology
Beijing 100190, China
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open-circuit voltage (VOC), respectively.[11] Some high-perfor-
mance nonfullerene acceptors have been developed and afforded
PCEs as high as 9%, such as perylene diimide and naphthalene
diimide small molecules and related polymers.[12–33]
Recently, our group reported a series of fused-ring electron
acceptors (FREAs) based on fused-ring electron donor units,
such as indacenodithiophene (IDT) and indacenodithieno[3,2-
b]thiophene (IDTT), flanked with two compact strong elec-
tron acceptor units such as 1,1-dicyanomethylene-3-indanone
(IC).[34–40] These FREAs such as ITIC (IDTT-IC based electron
acceptor),[34] ITIC-Th (IDTT-IC based electron acceptor),[38]
and IDIC (IDT-IC based electron acceptor)[37,39] present strong
absorption at long wavelength (i.e., small bandgaps) and appro-
priate energy levels. Moreover, the side chain on the fused-
ring donor unit can inhibit the excessive aggregation of the
molecules, while the compact acceptor units at the ends retain
the strong intermolecular interaction. After blending with
some high-performance donors with complementary absorp-
tion spectra, the PCEs of FREA-based solar cells achieved over
11%.[41–48] For example, ITIC-Th[38] exhibits a higher electron
mobility than its counterpart ITIC with phenyl side chains[34]
because the easy polarization of sulfur atom and sulfur–sulfur
interaction can enhance the intermolecular interaction. As a
result, the PCE of the ITIC-Th-based OSCs was improved to
9.6% when blending with a wide-bandgap polymer PDBT-T1
based on dithienobenzodithiophene and benzodithiophen-
edione.[38] Later, ITIC-Th was widely used by other groups. For
example, Sun and co-workers used ITIC-Th to fabricate ternary
blend OSCs, which exhibited a PCE of 10.27%.[49] In another
example, Yan and co-workers used ITIC-Th to blend with other
polymer donors and got a PCE of 10.88%.[47]
Here, we design and synthesize a new fluorinated ITIC-Th,
ITIC-Th1 (Figure 1a), by introducing fluorine (F) atoms onto
the end-capping group IC. On the one hand, incorporation
of F would improve intramolecular interaction, enhance the
push–pull effect between the donor unit IDTT and the acceptor
unit IC due to the electron-withdrawing effect of F, and finally
adjust energy levels and reduce the bandgap, which is benefi-
cial to light harvesting and enhancing JSC. On the other hand,
incorporation of F would improve intermolecular interactions
through CF···S, CF···H, and CF···
π
noncovalent inter-
actions and enhance electron mobility, which is beneficial to
enhancing JSC and fill factor (FF). Indeed, our results show that
fluorinated ITIC-Th1 exhibits redshifted absorption, a smaller
optical bandgap and a higher electron mobility than those of
nonfluorinated ITIC-Th. Furthermore, nonfullerene OSCs
based on fluorinated ITIC-Th1 electron acceptor and a wide-
bandgap polymer donor PBnDT-FTAZ (herein abbreviated as
FTAZ)[50] (Figure 1a) exhibit PCEs as high as 12.1%, signifi-
cantly higher than that of the device based on FTAZ and non-
fluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest
value in fullerene and nonfullerene-based single-junction
binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-
Th1 show much better efficiency and stability than the control
devices based on FTAZ:PC71BM.
Compound ITIC-Th1 was synthesized through facile Kno-
evenagel condensation reaction between aldehyde 1 and fluori-
nated IC (2)[51] (Scheme S1, Supporting Information). ITIC-
Th1 was fully characterized by spectroscopic methods and
elemental analysis (see Supporting Information). ITIC-Th1
exhibits good solubility in common organic solvents (e.g., chlo-
roform, chlorobenzene (CB), and o-dichlorobenzene (o-DCB))
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Figure 1. a) Chemical structures of FTAZ, ITIC-Th, and ITIC-Th1. b) UV–vis absorption spectra of FTAZ, ITIC-Th, and ITIC-Th1 in thin film. c) Estimated
energy levels of FTAZ, ITIC-Th, and ITIC-Th1 from electrochemical cyclic voltammetry.

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and good thermal stability (5% weight loss
at 271 °C in thermogravimetric analysis,
Figure S1, Supporting Information). ITIC-
Th1 in dichloromethane solution (10−6 m)
exhibits strong absorption in the region of
500–750 nm with a maximum extinction
coefficient of 1.8 × 105 m−1 cm−1 at 677 nm
(Figure S2, Supporting Information), which
is slightly higher than that of ITIC-Th in
dichloromethane solution (1.5 × 105 m−1 cm−1
at 668 nm).[38] From solution to thin film,
ITIC-Th1 presents a broader absorption range and the max-
imum absorption peak shifts from 677 to 728 nm, indicating
strong intermolecular interactions among ITIC-Th1 molecules
in the solid state (Figure S2, Supporting Information). The
absorption of ITIC-Th1 film exhibits an obvious redshift com-
pared with that of ITIC-Th film (Figure 1b), due to enhanced
push–pull effect after fluorination. The optical bandgap of
ITIC-Th1 estimated from the absorption onset is 1.55, 0.05 eV
smaller than that of ITIC-Th, which would benefit harvesting
low-energy photons and enhancing JSC. The HOMO (highest
occupied molecular orbital) and LUMO (lowest unoccupied
molecular orbital) energy levels of ITIC-Th1 film are estimated
to be −5.74 and −4.01 eV (Figure 1c), according to the onset
oxidation and reduction potentials from electrochemical cyclic
voltammetry (Figure S3, Supporting Information), respectively.
The HOMO and LUMO of ITIC-Th1 are relatively lower than
those of ITIC-Th (HOMO = −5.66 eV; LUMO = −3.93 eV),[38]
due to the electron-withdrawing effect of fluorine.
Our previously reported wide-bandgap polymer donor
FTAZ exhibits strong absorption at 400–620 nm with a molar
extinction coefficient of 9.8 × 104 m−1 cm−1,[50] which is com-
plementary to the absorption spectra of ITIC-Th and ITIC-Th1
(Figure 1b). Indeed, FTAZ:ITIC-Th (1:1.5, w/w) and FTAZ:ITIC-
Th1 (1:1.5, w/w) blended films exhibit broad and strong
absorption in 300–800 nm; FTAZ:ITIC-Th1 blend exhibits
redshifted and stronger absorption relative to FTAZ:ITIC-Th
blend (Figure S4, Supporting Information). The energy levels
of FTAZ (HOMO = −5.38 eV; LUMO = −3.17 eV) match with
those of ITIC-Th and ITIC-Th1 very well (Figure 1c). FTAZ
exhibits a high hole mobility of 1.2 × 10−3 cm2 V−1 s−1.[52]
Photoluminescence intensities of pure donor FTAZ and pure
acceptor ITIC-Th or ITIC-Th1 are significantly quenched when
blending FTAZ with ITIC-Th or ITIC-Th1, suggesting efficient
exciton dissociation and charge transfer between FTAZ and
ITIC-Th or ITIC-Th1 (Figure S5, Supporting Information).
Thus, we used FTAZ as the donor and ITIC-Th or ITIC-Th1 as
the acceptor to fabricate bulk heterojunction (BHJ) OSCs with
an inverted device structure of indium tin oxide (ITO)/ZnO/
FTAZ:acceptor/MoOx/Ag. We optimized the device fabrication
conditions, such as processing solvent, donor/acceptor weight
ratio, additive content, and film thickness (Tables S1–S3, Sup-
porting Information). The devices processed with chloroform
exhibit higher performance than those processed with CB and
o-DCB and therefore we choose chloroform as processing sol-
vent (Table S1, Supporting Information). The optimized FTAZ/
acceptor weight ratio is 1:1.5, and the optimized content for
the processing additive, 1,8-diiodooctane, is 0.25% (v/v) when
using chloroform as processing solvent (Table S2, Supporting
Information). Changing thickness of the active layer from 80
to 150 nm leads to insignificant variation of PCE from 10% to
12%, while further increasing the thickness leads to significant
decrease in PCE; the optimized thickness is 120 nm (Table S3,
Supporting Information). The optimized performance para-
meters are summarized in Table 1 and the corresponding
J–V curves are shown in Figure 2a. The ITIC-Th:FTAZ-based
device affords the best PCE of 8.88% with a VOC of 0.915 V,
a JSC of 15.84 mA cm−2, and an FF of 61.26%, while the
ITIC-Th1:FTAZ-based device achieves the best PCE of 12.1%
with a VOC of 0.849 V, a JSC of 19.33 mA cm−2, and an FF of
73.73%. Compared with ITIC-Th, the ITIC-Th1-based devices
exhibit a slightly lower VOC but much higher JSC and FF; the
lower VOC is mainly due to the deeper LUMO of ITIC-Th1,
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Table 1. Best photovoltaic performance of OSCs based on ITIC-Th, ITIC-Th1, and PC71BM via
the same fabrication process. The average values and standard deviations of 20 devices are
shown in parentheses.
Acceptor Voc
[V]
Jsc
[mA cm−2]
FF
[%]
PCE
[%]
ITIC-Th 0.915 (0.914 ± 0.003) 15.84 (15.67 ± 0.23) 61.26 (61.14 ± 0.86) 8.88 (8.67 ± 0.15)
ITIC-Th1 0.849 (0.847 ± 0.002) 19.33 (19.22 ± 0.18) 73.73 (72.56 ± 0.29) 12.1 (11.9 ± 0.1)
PC71BM 0.786 (0.785 ± 0.008) 10.20 (10.19 ± 0.23) 65.09 (62.09 ± 1.69) 5.22 (4.97 ± 0.14)
Figure 2. a) Current density versus voltage characteristics. b) EQE curves of the devices based on FTAZ:ITIC-Th, FTAZ:ITIC-Th1, and FTAZ:PC71BM
under the same conditions.

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while the higher JSC is partially due to redshifted and stronger
absorption of ITIC-Th1. Relative to the PC71BM-based control
devices (5.22%), ITIC-Th and ITIC-Th1-based OSCs exhibit sig-
nificantly improved PCEs, mainly due to the complementary
absorption.
The external quantum efficiency (EQE) spectra of non-
fullerene OSCs exhibit much broader photoresponse than
that of the device based on PC71BM (Figure 2b), ascribable to
strong absorption of the nonfullerene acceptors (i.e., ITIC-Th
and ITIC-Th1) at long wavelengths. The EQE of the ITIC-Th1-
based cell shows even wider and stronger photoresponse than
that of the ITIC-Th-based cell, due to the stronger and fur-
ther redshifted absorption of ITIC-Th1. In particular, the EQE
response of FTAZ:ITIC-Th1-based solar cell is stronger than
that of FTAZ:ITIC-Th counterpart in the region of 350–500 nm
since FTAZ:ITIC-Th1 blended film shows stronger absorption
than FTAZ:ITIC-Th counterpart in this region (Figure S4, Sup-
porting Information). The integrated photocurrents from EQE
spectra are 10.13, 15.00, and 18.60 mA cm−2 for PC71BM, ITIC-
Th, and ITIC-Th1-based cells, respectively, which are similar
to the JSC values obtained from J–V measurements under the
1 sun condition.
To study the initial stability of the unencapsulated devices,
stress (such as heat, light, and air) was employed. For thermal
stability test, the devices were continuously heated at 100 °C
(Figure S6, Supporting Information). In the first 5 min, the
PCEs of both ITIC-Th and ITIC-Th1 cells decay relatively fast
to 85% of their original values. Both devices retain 80% of their
original PCEs after heating for 600 min, while the PC71BM-
based devices only retain 60% of their original PCEs. Therefore,
the nonfullerene OSCs exhibit better thermal stability than the
fullerene devices. For FTAZ:ITIC-Th and FTAZ:ITIC-Th1-based
devices, JSC and FF mainly contribute to the decay dynamics.
For FTAZ:PC71BM-based devices, VOC, JSC, and FF all contribute
to the decay dynamics. The morphology change of the active
layer induced by heating may be the main reason for the decay.
The stability tests of the devices based on nonfullerene accep-
tors under continuous illumination and in air are shown in
Figures S7 and S8 (Supporting Information), respectively. After
continuous illumination for 120 min, the PCE of FTAZ:ITIC-Th
and FTAZ:ITIC-Th1-based devices retains 73% and 72% of their
original value, respectively. Stored in air for 21 d, the PCE of
FTAZ:ITIC-Th and FTAZ:ITIC-Th1-based devices retains 89%
and 90% of their original value, respectively. Both devices exhibit
good light stability and air stability without obvious difference.
Additionally, we studied the charge generation, charge trans-
port, charge extraction, and charge recombination in ITIC-Th
and ITIC-Th1-based devices. To investigate the charge gen-
eration, dissociation, and extraction properties, we measured
the photocurrent density (Jph) versus the effective voltage (Veff)
(Figure 3a). It is assumed that all the photogenerated exci-
tons are dissociated into free charge carriers and collected by
electrodes at a high Veff (that is, Veff = 2 V), so the saturation
photo current density (Jsat) is only limited by the total amount
of absorbed incident photons.[53] Jsat of the ITIC-Th1-based cell
is 20.15 mA cm−2, higher than that of the ITIC-Th-based cell
(16.37 mA cm−2), partially due to better light harvesting and
exciton generation. Jsc/Jph for ITIC-Th and ITIC-Th1-based cells
are >95%, indicating excellent charge extraction in both devices.
To investigate carrier recombination in the active layers of ITIC-
Th and ITIC-Th1-based cells, Jsc was measured as a function
of incident light intensity (Plight) and the data were fitted to the
power law: Jsc ∝ Plight
α
(Figure 3b).[54] The exponent
α
for ITIC-
Th and ITIC-Th1-based cells is 0.991 and 0.993, respectively,
indicating very weak bimolecular recombination at the short
circuit condition in the active layers of both devices. We also
studied monomolecular recombination via treating Voc as a func-
tion of Plight. The data were fitted to the linear law: Voc ∝ InPlight
(Figure 3c).[55] The slope for ITIC-Th and ITIC-Th1-based cell is
1.83 kT/q and 1.36 kT/q, respectively; the smaller slope for ITIC-
Th1 cell indicates lower monomolecular recombination.
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Figure 3. a) Photocurrent density versus effective voltage curves;
b) dependence of Jsc on light intensity; c) dependence of Voc on light
intensity for OSCs based on FTAZ:ITIC-Th and FTAZ:ITIC-Th1.

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Space charge limited current method was conducted
to obtain electron and hole mobility in the blended films
(Figure S9, Supporting Information). The electron mobility
of ITIC-Th and ITIC-Th1 in blended films are 4.5 × 10−3 and
7.6 × 10−3 cm2 V−1 s−1, respectively, while the hole mobility
of ITIC-Th and ITIC-Th1 blended films are 2.4 × 10−2 and
2.7 × 10−2 cm2 V−1 s−1, respectively.
μ
h/
μ
e of ITIC-Th and
ITIC-Th1 blends are 5.3 and 3.5, respectively. The higher
hole/electron mobility and more balanced charge transport in
FTAZ:ITIC-Th1 blended film are responsible for its higher Jsc
and higher FF. The above exciton/charge dynamics data imply
that both ITIC-Th and ITIC-Th1-based OSCs exhibit good
charge extraction and very weak bimolecular recombination.
However, the ITIC-Th1-based cells exhibit better exciton gener-
ation and dissociation, less monomolecular recombination, and
faster and more balanced hole/electron transport than the ITIC-
Th counterpart, thus contributing to higher Jsc and high FF.
The morphology of the active layer based on ITIC-Th1 and
ITIC-Th was studied by atomic force microscopy (AFM), trans-
mission electron microscopy (TEM), grazing-incidence wide-
angle X-ray scattering (GIWAXS), and resonant soft X-ray scat-
tering (RSoXS). In the AFM images (Figure S10, Supporting
Information), the root-mean-square roughness of FTAZ:ITIC-
Th and FTAZ:ITIC-Th1 films is 4.32 and 7.69 nm, respectively.
In the phase images, both films exhibit uniform nanoscale
aggregation domains. In the TEM images (Figure S11, Sup-
porting Information), the FTAZ:ITIC-Th1 blended film presents
finer and clearer aggregation domains than the FTAZ:ITIC-Th
blended film.
GIWAXS is used to investigate the molecular packing of
FTAZ:ITIC-Th and FTAZ:ITIC-Th1. The
π
–
π
stacking peaks
of ITIC-Th and ITIC-Th1 are located at ≈1.8 Å. As shown in
Figure 4, it is obvious that almost all the peaks are stronger/
sharper for FTAZ:ITIC-Th1, indicating the enhanced molec-
ular packing in FTAZ:ITIC-Th1 films. The coherence length
of ITIC-Th and ITIC-Th1
π
–
π
stacking is calculated (via the
Scherrer equation)[56] to be 3.7 and 4.0 nm, respectively. The
improved molecular packing is known to benefit the charge
transport. Thus, FTAZ:ITIC-Th1 shows higher mobility, which
leads to higher FF and thus higher PCE.
Resonant soft X-ray scattering (R-SoXS) is also employed to
obtain the phase separation information of FTAZ:ITIC-Th and
FTAZ:ITIC-Th1 blend films (Figure 5).[57] The photon energy of
284.8 eV is selected to obtain enhanced material contrast. The
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Figure 4. a) 2D GIWAXS patterns and b) scattering profiles of in-plane and out-of-plane for FTAZ:ITIC-Th and FTAZ:ITIC-Th1 blend films.
Figure 5. Normalized R-SoXS profiles in log scale for FTAZ:ITIC-Th and
ITIC-Th1 blend films.

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mode of the distribution smode of the scattering corresponds
to the characteristic mode length scale,
ξ
. It is noted that the
mode domain size is the half of
ξ
. The mode domain size of
FTAZ:ITIC-Th and FTAZ:ITIC-Th1 is calculated to be 29 and
15 nm, respectively. These results are consistent with the TEM
results. Due to the limited exciton diffusion length (10–20 nm),
smaller domains are favorable for the charge separation. There-
fore, FTAZ: ITIC-Th1 shows higher JSC.
In summary, we designed and synthesized a fluorinated
FREA, ITIC-Th1. Compared with the nonfluorinated counter-
part ITIC-Th, ITIC-Th1 presents a slightly deeper LUMO, red-
shifted absorption, and a smaller bandgap. FTAZ:ITIC-Th1 BHJ
films exhibit stronger molecular packing and longer coherence
length, leading to higher electron and hole yet balanced mobili-
ties. Moreover, FTAZ:ITIC-Th1 films present smaller domain
sizes to afford more D/A interfaces for exciton dissociation
and reduce monomolecular recombination. As a result, non-
fullerene OSCs based on FTAZ:ITIC-Th1 exhibit PCEs as high
as 12.1%, significantly higher than those of BHJ devices based
on FTAZ:nonfluorinated ITIC-Th (8.88%) and FTAZ:PC71BM
(5.22%). The PCE of 12.1% is among the highest values in
fullerene and nonfullerene-based single-junction OSCs. More-
over, the nonfullerene OSCs show better thermal stability than
the PC71BM-based control devices. Our results demonstrate
that the fluorinated acceptor ITIC-Th1 is very promising for
high-performance OSCs.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
X.Z. thanks the 973 Program (Grant No. 2013CB834702) and the
National Natural Science Foundation of China (Grant No. 91433114).
Q.Z. and W.Y. were supported by the Office of Naval Research (Grant
No. N000141410221) and National Science Foundation (Grant No.
DMR-1507249). W.M. thanks for the support from Ministry of Science
and Technology (Grant No. 2016YFA0200700) and NSFC (Grant Nos.
21504066 and 21534003). X-ray data were acquired at beamlines 7.3.3
and 11.0.1.2 at the Advanced Light Source, which was supported by
the Director, Office of Science, Office of Basic Energy Sciences, of the
U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
The authors thank Chenhui Zhu at beamline 7.3.3 and Cheng Wang at
beamline 11.0.1.2 for assistance with data acquisition.
Received: January 8, 2017
Revised: February 8, 2017
Published online:
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