Subtle Side-Chain Engineering of Random Terpolymers for High

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Subtle Side-Chain Engineering of Random Terpolymers for High-Performance Organic Solar Cells Lijun Huo, Xiaonan Xue, Tao Liu, Wentao Xiong, Feng Qi, Bingbing Fan, Dongjun Xie, Feng Liu, Chuluo Yang, and Yanming Sun Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00510 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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Chemistry of Materials

Subtle Side-Chain Engineering of Random Terpolymers for High-Performance Organic Solar Cells Lijun Huo,† Xiaonan Xue,† Tao Liu,† Wentao Xiong,† Feng Qi,† Bingbing Fan,† Dongjun Xie,§ Feng Liu,*,‡ Chuluo Yang,§ and Yanming Sun*,†

†

Heeger Beijing Research and Development Center, School of Chemistry and Environment, Beihang University, Beijing 100191, China

‡

Department of Physics and Astronomy, Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiaotong University, Shanghai 200240, China

§

Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, China *Email: [email protected]; [email protected]

Abstract: Terpolymers comprising three different components in backbone have emerged as a new design strategy for conjugated polymers. However, compared with the backbone modification, less efforts have been devoted to the alkyl side chain engineering, especially the subtle side chain modifications of random polymers. In this contribution, we designed and synthesized a series of random terpolymers, in which the subtle side chain regioregularity is used to finely tune the optical, electronic and morphological properties. PB55 was found to outperform the other copolymers, with better mixing with fullerene and higher photovoltaic performance. Moreover, the utilization of these random terpolymers in non-fullerene solar cells has been investigated. A well-known non-fullerene acceptor, ITCPTC, has been used as the acceptor. Among the terpolymers, PB55 yielded the best photovoltaic performance with an impressive PCE of ~12.1%, representing the highest value reported in the literature so far for non-fullerene OSCs based on random terpolymers. This work demonstrates the importance of subtle side chain engineering of random terpolymers for high-performance organic solar cells. 1

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Introduction Rapid progress has been made in organic solar cells (OSCs) in the past few years to develop lightweight, cost-effective and flexible device panels. The state-of-the-art OSCs have reached power conversion efficiencies (PCEs) over 10%.1-16 To design high-performance photovoltaic polymers, the chemical structure has to be synchronized with processing parameter to yield a suitable morphology.17-19 In this sense, material physical parameters, which guide solubility, crystallinity and material interactions, should be taken into account.20-25 There are urgent needs to relate the morphological control to polymer structure, which adds a new handle that can be tailored to fine-tune the nanostructure. The donor/acceptor approach has been widely used to construct donor polymers, in which an electron-rich donor (D) and an electron-deficient acceptor (A) are alternately incorporated in the polymer backbone to tune backbone electronic structure that determines absorption and aggregation.26-30 Other factors, such as aliphatic side chains, or decorative atoms, are also used to induce solubility and lamellar packing.31-36 A general selection rule is to use alkyl chains to provide necessary solubility of conjugated polymer while maintaining high chromophore density.37,38 There are studies of using different alkyl side chains to tune the lamellar packing crystallinity and chain orientation.3,15,39-44 We have shown previously that the optical, electrical, molecular packing, crystallinity and photovoltaic properties of copolymers can be tuned by simultaneously controlling the alkyl side-chain length on the donor and acceptor units by precise chemical synthesis.15 It is seen that terpolymers comprising three different

2


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components in backbone have emerged as a new design strategy for conjugated polymers.45-55 The multiple components in backbone alters the electrical and morphological properties of the resulting polymer. The new design is successful with exciting results of using random terpolymers in solar cell devices to obtain a PCE up to 8%.45,47,55 However, we noticed that compared with the backbone modification, less efforts have been devoted to the alkyl side chain engineering of random polymers, especially the subtle side chain modifications. In this contribution, we designed and synthesized several random terpolymers, in which ethylhexyl and butyloctyl have been used as the alkyl side chain. The influence of the subtle side chain difference on optical and electronic properties of terpolymers has been systematically studied. We found that higher photovoltaic performance of PB55 is obtained than the other copolymers. The champion devices based on PB55:PC71BM and PB55:ITCPTC56 blends showed high PCEs of 10.1% and 12.1%, respectively. To the best of our knowledge, the 12.1% efficiency is the highest values reported in the literature for non-fullerene OSCs based on random terpolymers. The results suggested that the subtle side chain engineering is crucially important for developing high-performance random terpolymers.

Experimental Section Solar cell fabrication and characterization: OSCs were fabricated with a conventional architecture of ITO/PEDOT:PSS/active layer/ZrAcac/Al. After cleaning according to the procedure reported previously 9, the indium tin oxide (ITO)-coated 3



glass

substrates

were

ready

for

use.

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

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A

40

nm

(PEDOT:PSS)

thick (Heraeus

Clevios P VP A 4083) layer was firstly spin cast on top of the ITO substrates and then annealed on a hotplate at 150 oC for 10 min under the ambient condition. The polymers were mixed with PC70BM at various blending ratio in CHCl3 and concentration is fixed at 6 mg/mL. Then they were spin cast atop the PEDOT:PSS to form the active layer. The optimal thickness of the active layers was 130 nm measured using an Ambios Technology XP-2 surface profilometer. For the fabrication of non-fullerene OSCs, the concentration of PB01-PB10 is also fixed at 6 mg/mL in CHCl3. The active layers were annealed at 100 oC for 5 min in a glove box. A thin layer of ZrAcac was spin cast atop the active layer and functions as the cathode interfacial layer. Finally, a 100 nm-thick Al electrode were successively deposited on top of the active layers. The active area of devices is 4.50 mm2. During measurements, an aperture with the area of 3.14 mm2 was used. Current density-voltage (J-V) characteristics were measured using a Keithley 2400 Source Measure Unit. The currents were measured under 100 mW/cm2 simulated 1.5 Global (AM 1.5 G) solar simulator (Enli Technology Co., Ltd, SS-F5-3A). The light intensity was calibrated by a standard Si solar cell (SRC-2020, Enli Technology Co., Ltd). IPCE spectra were measured by using a QEX10 Solar Cell IPCE measurement system (PV measurements, Inc.).

Results and Discussion The new random terpolymers, PB37-PB73, were synthesized by Stille coupling 4


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polymerization

with

three

monomers

of

(4,8-bis(5-((2-ethylhexyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl) bis(trimethylstannane)

(BDT-EH),

(4,8-bis(5-((2-butyloctyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)b is(trimethylstannane)

(BDT-BO),

and

1,3-bis(5-bromothiophen-2-yl)-5,7-bis(2-ethylhexyl)-4H,8H-benzo[1,2-c:4,5-c']dithio phene-4,8-dione (T1) (Figure 1a). The reference copolymers, PB01 and PB10 were also synthesized for comparison. The chemical structures of PB01-PB10 were confirmed by the NMR data (Figure S1). Both the reference parent copolymers and the terpolymers show good solubility in chloroform and dichlorobenzene. The averaged molecular weight (Mn) and polydispersity index (PDI) of PB01-PB10 are 24.9, 26.6, 23.6, 20.9 22.4 kDa and 1.6, 2.3, 2.0, 2.7, and 2.0, respectively, measured by gel permeation chromatography (GPC) in chloroform at room temperature. (b)

S

Normalized absorption

(a)

S

S S

S O S Sn

Sn

+

Br

S

O S

S S

+

Br

S

Sn

Sn S S

S S

S

PB01 PB37 PB55 PB73 PB10

1.0 0.8 0.6 0.4 0.2 0.0

400

500 600 Wavelength (nm)

700

(c) -3.5 S

S

S

S

Stille polymerization

S

S O

O

S

O

S

S

O

S

S S

S

x

S

S

S

S

1-x

S

S

PB01 PB37 PB55 PB73 PB10

x=0 x = 0.3 x = 0.5 x = 0.7 x=1

S

Energy levels (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3.54

-3.56

-3.57

-3.59

-3.60

PB37

PB55

PB73

PB10

-5.44

-5.44

-5.46

-4.0 -4.5

PB01

-5.0 -5.5

-5.40

-6.0

5


-5.42


Figure 1. (a) Synthetic routes of the terpolymers. (b) Absorption spectra of PB01-PB10 films. (c) Energy levels of PB01-PB10.

The absorption spectra of PB-PB10 are shown in Figure 1. In a dilute chloroform solution, all the polymers exhibit similar absorption profiles and two distinctive absorption bands located at 300-400 nm and 500-700 nm, corresponding to the π-π* transition and intramolecular charge transfer (ICT) from the donor to the acceptor units, respectively. Meanwhile, PB01-PB10 show pronounced shoulder peaks at 613-618 nm, indicating their strong aggregated state even in the dilute solution (Figure S2). In the solid state, the two strong vibronic peaks are still observed for all the polymers. The peak located at long wavelength is commonly referred to as the 0-0 transistion and other peak is 0-1 transistion.58,59 It has been shown that the types of aggregate of the polymers can be distinguished according to the ratio of the two vibronic peak intensities.57,58 From Figure 1b, it can be seen that the ratio of 0-0/0-1 vibronic peak for PB01 film is smaller than unity (H-aggregate), but the ratio values increased steadily from PB01 to PB10 films and for PB73 fim, the ratio is larger than unity (J-aggregate). The results sugested that the aggregation types of PB01-PB10 films are gradually changed from H- to J-aggregate, due to the difference of steric hindrance caused by the ethylhexyl and butyloctyl. Optical bandgap (Egopt) values of these polymers can be calculated from the onset of optical absorption edge of films and they are ranging from 1.83 to 1.85 eV. Cyclic voltammetry (CV) was performed to determine the HOMO/LUMO levels. As displayed in Figure S3, both p- and n-doping processes are 6


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reversible for all polymers. The corresponding HOMO/LUMO levels of PB01-PB10, are -5.40 eV/-3.54 eV, -5.42 eV/-3.56 eV, -5.44 eV/-3.57 eV, -5.44 eV/-3.59 eV and -5.46 eV/-3.60 eV, respectively (Figure 1c). (b)

(a)

0

0 PB01:PC71BM PB55:PC71BM

-5

PB01:ITCPTC PB37:ITCPTC PB55:ITCPTC PB73:ITCPTC PB01:ITCPTC

) 2

J (mA/cm

J (mA/cm

2 )

PB37:PC71BM PB73:PC71BM PB01:PC71BM

-10

-5 -10 -15

-15

0.0

0.2

(c)

0.4 0.6 Voltage (V)

0.8

1.0

0.0 80

60

60

40

PB01:PC71BM PB37:PC71BM PB55:PC71BM

20

0.2

(d)

80

IPCE (%)

IPCE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.6 Voltage (V)

40

PB01:PC71BM

0 300

400 500 600 Wavelength (nm)

0 300

700

1.0

PB01:ITCPTC PB37:ITCPTC PB55:ITCPTC PB73:ITCPTC PB01:ITCPTC

20

PB73:PC71BM

0.8

400

500 600 700 Wavelength (nm)

800

Figure 2. J–V characteristics and the corresponding IPCE spectra of OSCs based on terpolymer:PC71BM (a, c) and terpolymer:ITCPTC (b, d) under the illumination of AM1.5G (100 mW/cm2). Table 1. Summary of device parameters of of OSCs based on terpolymer:PC71BM and terpolymer:ITCPTC under the illumination of AM1.5G (100 mW/cm2). Active layer PB01:PC71BM PB37:PC71BM PB55:PC71BM PB73:PC71BM PB10:PC71BM PB01:ITCPT PB37:ITCPT PB55:ITCPT PB73:ITCPT PB10:ITCPT a)

Voc [V] 0.92±0.01 0.91±0.01 0.94±0.01 0.92±0.01 0.92±0.01 0.91±0.1 0.91±0.1 0.93±0.1 0.91±0.1 0.91±0.1

Jsc [mA/cm2] 12.6±0.1 13.5±0.1 13.7±0.2 13.2±0.1 13.3±0.2 15.3±0.3 15.9±0.6 17.0±0.1 16.5±0.4 15.6±0.3

FF [%] 76.0±0.4 73.1±0.4 76.5±1.2 72.0±0.5 68.6±0.8 72.3±0.6 72.6±0.8 74.8±0.7 71.6±0.7 68.9±0.8

The values are average PCEs from 20 devices. 7


PCEa [%] 8.8±0.1 8.9±0.1 9.9±0.2 8.7±0.1 8.4±0.1 10.1±0.3 10.5±0.4 11.8±0.2 10.8±0.2 9.8±0.3

PCEmax [%] 8.9 9.0 10.1 8.8 8.6 10.3 11.0 12.1 11.0 10.1


To assess the photovoltaic performance of PB01-PB10, organic solar cells have been fabricated using PC71BM as the acceptor. The best weight ratio of polymer and PC71BM and the 1, 8-diiodooctane (DIO) content were optimized to be 1:1 and 0.5%, respectively. Typical current density–voltage (J–V) curves of OSCs are shown in Figure 2, and the corresponding device parameters were summarized in Table 1. We firstly evaluate the initial performace of PB01-PB10 (Figure S4 and Table S1). Without any optimization, the PB55-based device showed a high PCE of ~7.5% with a Voc of 0.99 V, a short-circuit current (Jsc) of 11.9 mA/cm2, and a fill factor (FF) of 63.1%. In comparison, PB10 and PB01 yielded much lower photovoltaic performance than PB55 (PCEs in the range of 4-5%). With DIO, the champion PB55:PC71BM device showed a Jsc of 13.78 mA/cm2, a Voc of 0.94 V, and an impressive high FF of 78.0%, producing a high PCE of ~10.1%, which is significantly higher than devices based on the other polymers. The high and balanced hole and electron mobilities help to explain the high Jsc and FF achieved in PB55:PC71BM devices (Figure S6 and Table S2). OSCs with a large device area (19.6 mm2) were also fabricated for comparison (Figure S5). The low sheet resistance of ITO leads to high series resistance in large-area devices and then cause a decrease in FF. As a result, the device showed a PCE of ~9.4% with a Voc of 0.91 V, a Jsc of 13.8 mA/cm2, and a FF of 74.5%, which is slightly lower than that (10.1%) of devices with a small device area (4.5 mm2), mainly due to the reduced FF. The corresponding incident photon conversion efficiency (IPCE) spectra are shown in Figure 2. The IPCE curves exhibited a broad photoresponse in the range 8


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of 300-700 nm for all the devices. Especially for PB55:PC71BM device, IPCE exceeds 70% from 380 to 660 nm, with a peak of 77% at 550 nm. Therefore, PB55-based device yielded the highest photocurrent. The Jsc calculated from the IPCE spectra is 13.41 mA/cm2, which is consistent with the value (13.78 mA/cm2) measured from the J-V curves. The utilization of random terpolymers in non-fullerene solar cells has not attracted so much attention. Therefore, we further fabricated non-fullerene solar cells using the random terpolymers as the donor and a well-known non-fullerene acceptor, ITCPTC, as the acceptor (see Scheme S1 for its chemical structure). We noticed that non-fullerene OSCs based on PB01-PB10 showed similar trend to that in polymer:fullerene OSCs. Among the five polymers, PB55 yielded the best photovoltaic performance with an impressive PCE of ~12.1%, with a Jsc of 17.05 mA/cm2, a Voc of 0.93 V, a FF of 75.8%. To the best of our knowledge, the efficiency of ~12.1% represents the highest value reported in the literature so far for non-fullerene OSCs based on random terpolymers. neat film

qz [A-1]

105

105

PB01

PB10

102

PB55

100

1

qxy [A-1]

Q vector

101

100

[A-1]

DIO PB01

104

Intensity [au]

Intensity [au]

Intensity [au]

qz [A-1]

PB10

101

qxy [A-1]

105

PB01

103

103

102

qxy [A-1 ]

as cast

104

104

qz [A-1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


103

PB10 102

PB55

1

Q vector [A-1]

101

100

PB55

1

Q vector [A-1]

Figure 3. (a) 2D GIWAXS patterns of PB01, PB10, PB55 neat and blend films. (b-d) The in-plane (dotted line) and out-of-plane (solid line) line-cut profiles of PB01, PB10, PB55 neat and blend films. 9



Grazing incident X-ray diffraction (GIXD) was performed to look into the crystalline differences between these polymers in neat film and BHJ blends.59 2D GIXD patterns and the corresponding in-plane and out-of-plane line-cuts were shown in Figure 3. The crystal coherence lengths (CCL) of the (010) π–π stacking in the out-of-plane direction and (100) lamellar stacking in the in-plane direction can be obtained using the Scherrer equation, which were summarized in Table S3. All these polymers in neat film showed a face-on orientation, with pronounced π-π stacking shown in out-of-plane direction. PB01 exhibited the highest crystallinity among the three polymers, as evidenced by the strong in-plane (100) and out-of-plane (010) scattering intensity. PB10 showed a low crystalline order in both in-plane (100) and out-of-plane (010) directions. PB55 showed intermediate values compared with the parent polymers. Thus copolymerization that hybrids two materials together in a random manner can adjust crystalline properties in detail. More interestingly, the random side chain distribution leads to backbone packing differences, as seen from the π-π stacking distances, in which PB55 showed a medium value. The side chain engineering does have negative effect in the self-assembly in (100) directions, in which PB55 showed the lowest (100) CCL value. Thus terpolymers with side chain engineering can be a quite interesting methodology in fine-tuning the crystal packing and lamellar order in two crystal dimensions, which is of particular importance in regulating morphology. These results agreed well with solid state UV-vis absorption and the aggregate types of polymers as mentioned before. The SCLC hole-mobilities were measured and the values are 2.1×10-3, 1.8×10-3, and 1.9×10-3 cm2/Vs for 10


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PB01, PB10, and PB55 (Figure S7 and Table S2), respectively, which are consistent with the GIXD measurements. GIXD of BHJ blends are shown in Figure 3. In as cast thin films, all three polymers showed reduced crystalline order and the π-π stacking became weak due to the presence of PC71BM. PB01, PB10 and PB55 showed a (100) CCL of 9.3, 8.2 and 6.9 nm respectively, which is similar to that in pure films. When DIO additive is used, the ordering of donor polymers showed drastic improvement. The (100) CCL was increased to 14.5, 10.5 and 10.7 nm for PB01, PB10 and PB55 blends, respectively. The presence of DIO during film drying could help to order the disordered side chains to improve the lamellae stacking quality. The π-π stacking became sharp and located in out-of-plane direction. Thus DIO could help to induce chain self-assembly of conjugated polymers in 3-D. In comparison with PB01, PB10 blends, PB55 films with DIO showed a more preferred face-on orientation, as evidenced by the relatively weaker (100) lamellar stacking in the out-of-plane and more pronounced peak in in-plane, which can facilitate the charge transport. The surface and bulk morphology of BHJ thin films were investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM) and resonant soft x-ray scattering (RSoXS).60 As illustrated in Figure 4g, as cast BHJ thin films for all three polymers had large scaled phase separations, which are unfavorable to charge separation and and thus leads to inferior device performances. Among them, PB55 blends showed the lowest phase separation size (71 nm), due to improved mixing with PC71BM that suppresses large scaled phase separation, which 11



was confirmed by the results of higher PCEs achieved for the unoptimized PB55-based OSCs. When the DIO additive is used in thin film preparation, the

material interaction changes and much smoother films are obtained. As seen for PB01 blends, an obvious scattering hump is seen at ~0.014 A-1, giving a distance of 45 nm. PB10 blends give multi-length scaled scattering with one hump at ~0.0082 A-1 (corresponding to a distance of 76 nm) and one hump at ~0.021 A-1 (corresponding to a distance of 30 nm). PB55 blends give a scattering hump at ~0.02 A-1 (corresponding to a distance of 31 nm) with the lowest scattering intensity, an indication of reduced extent of phase separation induced by mixing. The reduced length scale of phase separation gives rise to enhanced device short-circuit current, which agrees well with device characterizations. From TEM and AFM micrographs, PB01 blends show quite obvious fibril network morphology. For PB10 and PB55 blends, the phase image is not as clear as that of PB01 blends (Figure S8), yet the films are quite smooth and polymer fibril content is still visible but smaller, agreed well with GIXD investigations. Improved crystallinity and better phase separation boost the solar cell performances, and the fine-tuned crystal quality and mixing via random copolymerization lead to best device performances. More balanced charge transport in PB55 blends leads to enhanced device fill factor, making the random copolymer best in the series (Table ).

12


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(b)

(a)

RMS = 2.06 nm

(d)

(c)

PB01 as cast PB01 DIO PB10 as cast PB10 DIO PB55 as cast PB55 DIO

RMS = 1.08 nm

(f)

(e)

RMS = 1.10 nm

Figure 4. TEM and AFM heigh (2 × 2 µm) images of (a, b) PB01:PC71BM blend films with 0.5% DIO additive, (c, d) PB10:PC71BM blend films 0.5% DIO additive, and (e, f) PB55:PC71BM blend films 0.5% DIO additive. (g) The RSoXS profiles of PB01:PC71BM, PB10:PC71BM, and PB55:PC71BM as cast films and films with 0.5% DIO. Conclusion In summary, random terpolymers PB01-PB10 were designed and synthesized via a terpolymerization approach, in which the subtle side chain regioregularity is controlled to improve the optical, electronic and morphological properties. PB55 was found to outperform the other copolymers, with better mixing with PC71BM and higher photovoltaic performance. As a result, PCEs of ~10.1% in PB55:PC71BM based solar cells. More exciting results were seen in non-fullerene solar cells, with a ~12.1% PCE in and PB55:ITCPTC blends, respectively, which represent the highest 13



values so far reported in the literature for any types of terpolymers. The results showed that the random terpolymerization is a feasible and promising synthetic strategy in developing high-performance polymer materials. This work also demonstrated the importance of subtle side chain engineering in regulating the morphology in organic solar cells, which can help to improve the mixing with electron acceptor materials.

ASSOCIATED CONTENT Supporting Information Experimental details, synthesis and characterizations, chemical structure of ITCPTC, 1H NMR spectra, absorption spectra,electrochemical cyclic voltammetry measurements of PB01-PB10, device data, AFM images, SCLC measurements, device performance, coherence length data and the d spacings of thin films. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author [email protected]; [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 51473009, 21674007, 21572171), the International Science & Technology Cooperation Program of China (No. 2014DFA52820). Y.S. gratefully acknowledges Prof. Yuanping Yi (ICCAS) for fruitful discussions. Portions of this research were carried out at beamline 7.3.3 and 11.0.1.2 at the Advanced Light Source, Molecular Foundry, and National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, which was supported by the DOE, Office of Science, and Office of Basic Energy Sciences. 14


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