2D-Conjugated Benzodithiophene-Based Polymer Acceptor: Design

Sep 23, 2015 - All polymer photovoltaic cells offer unique potentials owing to the chemical and electronic tunability of both polymer donors and polym...
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2D-Conjugated Benzodithiophene-Based Polymer Acceptor: Design, Synthesis, Nanomorphology, and Photovoltaic Performance Long Ye,† Xuechen Jiao,‡ Hao Zhang,† Sunsun Li,† Huifeng Yao,† Harald Ade,*,‡ and Jianhui Hou*,† †

State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, United States S Supporting Information *

ABSTRACT: All polymer photovoltaic cells offer unique potentials owing to the chemical and electronic tunability of both polymer donors and polymer acceptors. Compared with the numerous π-conjugated polymer donors, choices of π-conjugated polymer acceptors are limited for photovoltaic applications. Although 2D-conjugated benzo[1,2-b:4,5-b′]dithiophene (BDT) units are widely used as building blocks in highly efficient donor polymers in recent years, polymer acceptors based on these units have not been reported yet. Herein, a novel 2D-conjugated polymer acceptor (PBDTNDI-T) based on naphthalene diimide (NDI) and alkylthiothiophene-substituted BDT was designed, synthesized, and in-depth characterized. The polymers’ photophysical, electrical, crystallinity, and morphological properties are addressed in homopolymer and blend films and well correlated with device performance. Under the weight ratio of 1.5:1 and 3 vol % of 1-chloronaphthalene, the PBDTNDI-T-based all polymer photovoltaic device exhibited a desirable PCE of nearly 3%, which is ascribed to the relatively high domain purity and small domain characteristic length observed by resonant soft X-ray scattering (R-SoXS) characterizations. These results demonstrated PBDTNDI-T is a novel polymer acceptor and also promising candidate material for efficient energy-related applications. N-alkylation.36−41 Therefore, high electron mobility, broad absorption spectrum, and suitable LUMO levels could be collectively realized in a variety of NDI-based polymer acceptors. In 2010, Chen and Shi et al. synthesized the first benzo[1,2-b:4,5-b′]dithiophene (BDT) and naphthalene diimide (NDI)-based copolymer, which exhibited promising absorption and electrochemical properties.42 However, the photovoltaic properties of PBDTNDI were not presented at that time. Xu et al. further reported two BDT-NDI copolymers with various alkyl chains, and PBDTNDI (see Scheme 1) exhibited a rather poor PCE of 0.02% in conventional PSC devices as polymer donor in 2013.43 Clearly, there is still much room to further improve the PCE of PBDTNDI-based devices. 2D-conjugated BDT units like alkylthiophene- or alkylthiothiophene-substituted BDTs constitute a promising class of high-performance π-conjugated polymer donors with excellent hole mobility and spectral coverage.44 For instance, Li’s group incorporated alkylthiothiophene in the BDT unit of the wellknown polymer PTB745 and synthesized a highly efficient 2Dconjugated BDT-based polymer donor, namely PBDTT-S-TT (Scheme 1), which produced high PCEs over 8.4% and a high

1. INTRODUCTION π-Conjugated polymers are receiving attention in fundamental and applied science due to their interesting optical, optoelectronic, charge storage, and transport properties.1−8 Compared with the rapid development of π-conjugated donor polymers, the development of π-conjugated polymer acceptors historically lags far behind that of their donor counterparts.9−11 Following the pioneer report12 of the naphthalenediimide− bithiophene π-conjugated polymer P(NDI2OD-T2), also known as Activink N2200, P(NDI2OD-T2) and its derivatives showed great potentials in energy-related devices and particularly as acceptor materials in all polymer solar cells (All-PSCs).13−35 Actually, most of the high performance AllPSCs with power conversion efficiencies surpassing 4% are limited by the use of P(NDI2OD-T2) as polymer acceptor.10,23−35 The development of new π-conjugated polymer acceptors with excellent electron mobility, broad absorption spectrum, and properly matched energy levels as well as controlled thermodynamic interactions such as miscibility with high performance π-conjugated polymer donors is still a major challenge in the field of All-PSCs. Of all the building blocks utilized for designing π-conjugated polymer acceptors, naphthalene diimide (NDI) is a vital building block due to its structural features, such as small π−π stacking distance, tunable solubility, and crystallinity via imide © XXXX American Chemical Society

Received: July 11, 2015 Revised: September 4, 2015

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Scheme 1. Molecular Structures of the π-Conjugated Polymer Donors (PTB7, PBDTT-S-TT, PBDTNDI) and the Target Polymer Acceptor (PBDTNDI-T)

Scheme 2. Synthesis of PBDTNDI-T by Pd(PPh3)2Cl2-Catalyzed Stille Coupling

Figure 1. Basic properties of PBDTNDI-T: (a) TGA curves; (b) absorption spectrum in solution and a thin film; (c) cyclic voltammogram of solid film; (d) dark J0.5−V curve of the electron-only device.

Voc of 0.84 V in the corresponding device.46 Our group further inserted linear alkylthiothiophene in PTB7 and prepared a

novel high-efficiency 2D-conjugated BDT-based polymer donor, named as PBDT-TS1.47 Compared with PTB7, the B

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was −0.82 eV, which corresponding to LUMO of −3.98 eV. With a deeper LUMO level relative to PBDTNDI, PBDTNDIT is expected to be beneficial for matching well with the stateof-the-art polymer donors in terms of energy levels. The electron transport properties of PBDTNDI-T thin films were characterized by using the SCLC method with the ITO/ Al/polymer/Al geometry. Compared with the electron mobility of P(NDI2OD-T2) reported by Facchetti and collaborators,31 the electron mobility of PBDTNDI-T is slightly lower with the value of 6 × 10−4 cm2 V−1 s−1(Figure 1d), which is the same order of magnitude as the electron mobility of (6,6)-phenylC61-butyric acid methyl ester (PCBM). In order to investigate polymer crystallinity and lamellar spacing, grazing incidence Xray diffraction (GIXRD) was performed to determine the molecular packing in both the out-of-plane (qz) and in-plane (qxy) directions.49 As observed from the 2D GIXRD patterns of neat PBDTNDI-T film and PBDTBDD-T film (Figures 2a and

improved PCE (∼10%) in PBDT-TS1-based photovoltaic devices are originated from the enhanced charge mobility and lowered energy levels.47,48 It can be concluded that the insertion of alkylthiothiophene is a feasible strategy to lower the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of BDT-based polymers. Although 2D-conjugated BDT units are widely used as building blocks in highly efficient donor polymers in recent years, polymer acceptors based on these units were seldom explored and also lack of in-depth device optimizations. When combining a 2D-conjugated BDT-based polymer donor with a 2D-conjugated BDT-based polymer acceptor in All-PSCs, the molecular interaction of the all polymer blend can be finely optimized for the similar features. On the basis of the above considerations, we inserted the alkylthiothiophene group on the BDT unit of polymer donor PBDTNDI and a 2D-conjugated BDT-based polymer acceptor (PBDTNDI-T) with alkylthio chain was developed for the first time, as shown in Scheme 1. The thermal, photophysical, morphological, and charge transport properties of the resulting polymer PBDTNDI-T and its application in All-PSC were well explored. The overall PCE of the All-PSC device based on PBDTNDI-T is nearly 3%. Therefore, these results implied that the side chains like alkylthiothiophene appended to the πconjugated polymer may substantially convert polymer donor to polymer acceptor without altering the backbone, which offers a novel method to design polymers with novel functions.

2. RESULTS AND DISCUSSION The synthesis routes of PBDTNDI-T are presented in Scheme 2. Starting with benzo[1,2-b:4,5-b′]dithiophene-4,8-dione, the 2D-conjugated BDT monomer (M1) with linear alkylthiothiophene side group can be synthesized through two steps with good yields. The synthesis details of the monomer M1 could be found in our recent report.44,47 M1 was copolymerized with a NDI monomer (M2) via Stille coupling reaction using Pd(PPh3)2Cl2 as catalyst. The novel polymer PBDTNDI-T can be easily dissolved into organic solvents like chloroform (CF), chlorobenzene (CB), and 1,2-dichlorobenzene (DCB) and also possesses good film-forming ability. For accurate determination, the number-average molecular weight (Mn) estimated by high temperature gel permeation chromatography (GPC) using 1,2,4-trichlorobenzene (TCB) as eluent is 80.22 kg/mol, with a polydispersity index (PDI) of 1.86. The thermal properties of PBDTNDI-T were investigated by thermogravimetric analysis (TGA). This polymer exhibits excellent thermal stability with decomposition temperature (5% weight loss) at 350 °C in a nitrogen atmosphere (see Figure 1a). Figure 1b shows the absorption spectra of PBDTNDI-T in dilute CF solution and solid film. Both of the absorption peaks in solution and thin film are ca. 650 nm, while PBDTNDI-T exhibit a red-shifted absorption in thin solid film compared to CF solution with a shift range of ∼50 nm. The film exhibits an absorption onset of 790 nm, which corresponds to a narrow optical band gap of 1.57 eV. Density functional theory (DFT) at the B3LYP/6-31G(d,p) level was used to gain insight into the π-electron distribution in the PBDTNDI-T, as illustrated in Figure S1. It can be speculated that the polymer PBDTNDI-T may have higher HOMO level than that of PBDTNDI. The reduction potential of PBDTNDIT was determined by cyclic voltammetry (CV) characterizations, and Fc+/Fc was used as the external standard, as shown in Figure 1c. The reduction potential of PBDTNDI-T

Figure 2. 2D GIXRD pattern of neat PBDTNDI-T film (a) and neat PBDTBDD-T film (b).

2b), a well-defined (001) peak associated with backbone repeat direction is located in-plane (IP). In the out-of-plane (OOP) direction, a pronounced π−π (010) diffraction peak is revealed at q = 1.5 Å−1 for PBDTNDI-T and q = 1.7 Å−1 for PBDTBDD-T. Lamellar (100) stacking exhibits very similar q value around 0.32 Å−1 for both neat films is also observed preferentially in the OOP direction. As a result, PBDTNDI-T and PBDTBDD-T tend to adopt a configuration and population distributions similar to a 2D powder, where the backbone is locked in plane. Determining the detailed orientation distribution is outside the scope of this paper. This is consistent with the high absorption coefficient of PBDTNDI-T. Because of narrow band gap, and desirable electron mobility as well as lower LUMO/HOMO level, PBDTNDI-T is promising acceptor material for efficient allpolymer photovoltaic devices. C

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PBDTNDI-T and PBDTBDD-T make it relative easy to compare the out-of-plane π−π diffraction (010) peak intensity, which is well-known to impact solar cell performance.1,56,57 As displayed in Figure 4, the blend processed with CN manifests stronger (010) diffraction intensity from PBDTNDI-T component and PBDTBDD-T component, while the (100) diffraction intensity is almost same for both blends (see Supporting Information for 2D GIXRD patterns, Figure S2). Hence, the overall molecular ordering of the PBDTBDDT:PBDTNDI-T blend film was relatively enhanced in the π−π direction after introducing CN as processing additive, consistent with the higher FF (59.6%) due to better charge transport.21,26−28 Additionally, the ratio of hole mobility/ electron mobility (Figure S3) suggested a relatively balanced charge transport occurring in the optimized devices. Furthermore, the real space morphologies of the PBDTBDD-T:PBDTNDI-T photoactive layers cast from CB and CB/CN were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Estimated from the AFM topography images (Figure 5a,c), the surface root-mean-square roughness (Rq) of PBDTBDDT:PBDTNDI-T blend film was decreased from 6.38 to 4.59 nm after incorporating CN as processing additive. Nanometer-scale domains are evident in both AFM phase images (Figure 5b,d) and TEM images (Figure S4). In contrast to the blend films cast from CB, much finer domain structure can be observed in CB/CN processed films. Considering the low contrast of polymer blends, resonant soft X-ray scattering (R-SoXS) was utilized as an emerging tool to probe the details of domain spacing (or characteristic length) and average domain purity in all polymer blend films for solar cell applications.19,27,28,31,56,58 Here, the near-edge X-ray absorption fine structure (NEXAFS) spectroscopy of neat polymer film is provided in Figure S5, and then the photon energy of 283.7 eV was selected to maximize the contrast of PBDTNDI-T and PBDTBDD-T. From R-SoXS profiles (Figure 6a,b), both blend films exhibited multiple-length-scale structural features and phase separation at three different length scales; namely, ∼200 nm (peak 1), ∼100 nm (peak 2), and ∼20 nm (peak 3) were observed. For a clear comparison, the domain spacing of like materials of each peak in both films are shown in Figure 6c and listed in Table 2. By incorporating CN as processing additive, the long periods correspond to all scattering peaks remains essentially unchanged. The slightly reduced domain spacing, i.e., from 110 to 80 nm for peak 2, could be linked with the minor enhancement of Jsc. Moreover, the integrated scattering intensity (ISI) of each peak as well as all scattering from both films are also listed in Table 2. According to our previous works, relative average domain purity is linearly linked with ISI.56,57 The ISI of each R-SoXS peak is substantially reduced in the absence of CN (Figure 6d), for instance, 0.07 for peak 1, 0.08 for peak 2, and 0.2 for peak 3 (ISI for each peak was normalized to the total ISI of CN processed sample), whereas the use of CN produces much higher relative average domain purity for all peaks. Higher domain purity is well correlated with higher FF due to reduced bimolecular recombination.21,56 Similar to some cases of high efficiency polymer/PCBM solar cells,59,60 the multilength scale nanomorphology is also formed in this all-polymer system. This finding may offer a new model for revealing the critical factors of organic photovoltaic morphology.

Besides the choice of polymer acceptors, recent investigations revealed that the polymer donor also has a critical role in All-PSC to yield optimal performance.25−27,31−35 Here, we selected a 2D-conjugated BDT-based polymer, namely poly{1(5-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-6-methylbenzo[1,2b:4,5-b′]dithiophen-2-yl)thiophen-2-yl)-5,7-bis(2-ethylhexyl)3-(5-methylthiophen-2-yl)benzo[1,2-c:4,5-c′]dithiophene-4,8dione} (PBDTBDD-T)50 as the polymer donor, because PBDTBDD-T performed well with multiple cases of acceptor materials including PCBM,51 Bis-PCBM,52 and small molecular non-fullerene molecule.53 Moreover, 2D-conjugated BDT is the mutual building block of both polymer donor and polymer acceptor, which might be anticipated for an enhanced overlap of π−π orbitals. PBDTBDD-T and PBDTNDI-T have the LUMO/HOMO levels of −3.40 eV/−5.23 eV and −3.98 eV/− 5.53 eV, respectively, which produce a well-matched energy level alignment with the appropriate energy offsets (>0.3 eV) for electron and hole transfer. The photovoltaic properties of PBDTNDI-T were evaluated in a conventional device structure of ITO/PEDOT:PSS/PBDTBDD-T:PBDTNDI-T/Mg/Al, as shown in Figure 1d. In this work, chlorobenzene (CB) was selected as the host solvent and different D/A weight ratios (2:1, 1.5:1, 1:1) of polymer donor to polymer acceptor were evaluated. The photovoltaic performance of All-PSCs based on various D/A weight ratios are enumerated in Table 1. When Table 1. Photovoltaic Properties of All-PSCs Processed with Different Donor:Acceptor Ratios

a

D/A ratio

Voc [V]

Jsc [mA/cm2]

FF [%]

2:1 1.5:1 1:1 1.5:1 3% CN

0.88 0.87 0.87 0.86

4.68 5.50 4.53 5.62

53.0 50.3 50.9 59.6

PCEa [%] 2.18 2.40 2.01 2.88

(±0.15) (±0.12) (±0.11) (±0.11)

The variations of eight devices are shown in the parentheses.

PBDTNDI-T weight content is 40%, the Jsc of device is much higher those at other ratios. The polymeric blend with D/A weight ratio of 1.5:1 yields a high PCE of 2.40% with a Voc of 0.87 V, Jsc of 5.50 mA/cm2, and FF of 0.503 under the illumination of AM 1.5G 100 mW/cm2. Recently, Neher et al. revealed that 1-chloronaphthalene (CN), a high boiling-point solvent additive, can suppress the aggregation of the polymer acceptor P(NDI2OD-T2) in solution, which results in a better device performance.54,55 Then, CN was also introduced in investigating solvent choices on blend characteristics, and 3% CN was found to fine-tune the morphology of photoactive layers. Under the optimized conditions, an average PCE up to 2.88% was achieved with a Voc of 0.86 V, a Jsc of 5.62 mA/cm2, and a FF of 59.6%. As shown in Figure 3b, the All-PSC devices exhibited a strong and broad photoresponse range from 500 to 700 nm with peak EQE over 40% (Figure 3c). The wavelength integration of the product of the EQE curves and the standard AM 1.5G solar spectrum yielded consistent Jsc within 4% error compared to the results from J−V tests. From the comparison of device performance, the PCE improvement of CB/CN processed device is mainly from FF enhancement (from 50.3% to 59.6%) and second from Jsc enhancement (from 5.50 to 5.62 mA/cm2). Because of the similar q values of (100) peak for PBDTNDI-T and PBDTBDD-T, it is not feasible to obtain pole figures of (100) and (010) peaks with high accuracy simultaneously. Nevertheless, the distinct q values of (010) peak for D

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Figure 3. (a) Schematic illustration of conventional All-PSC and molecular structure as well as energy levels of the involved materials: PBDTNDI-T and PBDTBDD-T. (b) J−V and (c) EQE curves of different conditions.

Figure 4. Out-of-plane 1D GIXRD profiles for PBDTNDIT:PBDTBDD-T blends with CN processed (red) and without CN processed (blue). Thickness correction is done for both blends.

3. CONCLUSIONS In conclusion, we inserted the alkylthiothiophene on the BDT unit of polymer donor PBDTNDI and a 2D-conjugated BDTbased polymer acceptor (PBDTNDI-T) was synthesized for the first time. Moderate diode electron mobility of 6 × 10−4 cm2 V−1 s−1, excellent crystalline feature, and broad absorption spectra covering 300−800 nm as well as suitable LUMO level of −3.98 eV are collectively achieved for PBDTNDI-T. Furthermore, a 2D-conjugated BDT-based polymer PBDTBDD-T was selected as polymer donor and PBDTNDI-T was applied as polymer acceptor to construct a conventional all-polymer photovoltaic cell, which afforded a respectable PCE of nearly 3% processed with CB/CN binary solvent. Compared to the device parameters of All-PSC devices processed without CN, the enhanced performance in CN processed devices is primarily due to higher average domain

Figure 5. AFM height and phase images of PBDTBDD-T:PBDTNDIT blend films processed with CN (a, b) and without CN (c, d), respectively.

purity and enhanced molecular ordering. Considering the numerous work done on 2D-conjugated polymer donors, further investigation is necessary to explore the potential of 2Dconjugated BDT unit in design and synthesis novel polymer acceptors. 2D-conjugated polymers like PBDTNDI-T is a novel and potential polymer semiconductor for application in high performance solar cells, transistors, and rechargeable Li batteries.

4. EXPERIMENTAL SECTION Synthesis of the Polymer PBDTNDI-T. All synthetic procedures were performed under an argon atmosphere. Monomer M1 was E

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Figure 6. R-SoXS results of PBDTBDD-T/PBDTNDI-T blend films processed without and with CN: (a) azimuthally averaged and Lorentz corrected profiles, with log-normal distributions overplotted as multiple peak fits; (b) q location of each fit peak; (c) corresponding characteristic length d (center-to-center spacing of like domains); (d) relative average domain purity normalized by the total ISI of CN processed sample. found to be around 80 nm. Finally, Mg (20 nm)/Al (80 nm) metal electrodes were thermally evaporated under vacuum of ca. 0.3 mPa, at a rate of ca. 0.1−0.3 nm/s, to complete the devices. The J−V curves were measured (2400 Source Meter, Keithley Instruments) under simulated AM 1.5 sunlight at 100 mW cm−2 irradiance generated by an Class AAA solar simulator, with the intensity calibrated with an NIM calibrated KG3 filtered Si reference cell. The mismatch factor (MMF) of all polymer photovoltaic cell was calculated to be unity. The AllPSCs were masked with a metal aperture to define the active area, typically 13.4 mm2. Eight devices were fabricated in parallel for each condition. EQE data were measured by the Quantum Efficiency Measurement System QE-R3011 (Enli Technology Co. Ltd., Taiwan). The SCLC mobility was extracted from dark J−V curves by fitting to the Mott−Gurney law. X-ray Scattering Experiments. GIXRD measurements were performed at beamline 7.3.3 at the Advanced Light Source, Lawrence Berkeley National Laboratory, CA.61 Samples were prepared using identical blend solutions as those used in devices on a PSS precoated Si substrate. The 10 keV X-ray beam was incident at a grazing angle of 0.12°, which maximized the scattering intensity from the samples. The scattered intensity was detected with a Pilatus detector. R-SoXS experiments were performed at beamline 11.0.1.2 at ALS Lawrence Berkeley National Laboratory.62 The measurement setup is under high vacuum (1 × 10−7 Torr) due to the high absorption of soft X-rays in air. Polymer blend films for the R-SoXS measurement were prepared the same way as that in All-PSC devices. Blend films were flowed and transferred onto Si3N4 (100 nm) substrate. Prior to measurement, near-edge X-ray absorption fine structure (NEXAFS) was recorded at beamline 5.3.2.2, and the energy of the soft X-rays was stepwise altered around the resonant scattering of carbon from 270 to 300 eV with a step size of 0.1 eV. The strongest contrast between PBDTNDI-T and PBDTBDD-T near the carbon K edge was found to be at 283.7 eV. The scattering signal was collected by a CCD camera with a size of 2048 pixels × 2048 pixels.

Table 2. Morphological Parameters (Surface Roughness, Domain Spacing, Relative Average Domain Purity) of AllPSCs Processed without and with CN All-PSCs

Rq [nm]

domain spacing [nm] peak 1/peak 2/peak 3

ISI peak 1/peak 2/peak 3

w/o CN w CN

6.38 4.59

225/110/15 220/80/15

0.07/0.08/0.2 0.4/0.23/0.37

synthesized according to literature procedures.46,47 PBDTBDD-T was synthesized in our laboratory according to our previous work.50 The other materials were commercially available and used as received. Monomer M2 was purchased from Solarmer Materials Inc., and the catalyst Pd(PPh3)2Cl2 was purchased from Frontiers Scientific Inc. The synthetic routes of PBDTNDI-T were as follows. 0.4 mmol of M1 and an equal quantity of M2 were mixed in 9 mL of toluene. After being purged with nitrogen for 5 min, Pd(PPh3)2Cl2 (31 mg) was added, and the mixture was then purged with argon for another 15 min. The reaction mixture was stirred and heated to reflux at 110 °C for 20 h. When the reaction mixture was cooled to room temperature, the polymer was precipitated by addition of 60 mL of methanol, filtered through a Buchner funnel, and then subjected to Soxhlet extraction with methanol, hexanes, and chloroform. The polymer was recovered as solid from the chloroform fraction by precipitation from methanol. The solid was dried under vacuum for 24 h. The yield and elemental analytical results of PBDTNDI-T are as follows. PBDTNDI-T. Yield: 50.6%. GPC (140 °C, TCB): Mn = 80.22 kg/mol, PDI = 1.86. Td (5% loss) = 350 °C. Elemental analysis calcd (%) C88H126N2O4S6: C 71.98, H 8.65, N 1.91; found: C 71.42, H 8.29, N 1.80. The 1H NMR of PBDTNDI-T is provided in Figure S6. Device Fabrication and Instruments. The fabrication details of All-PSC devices are as follows: ITO-coated glass substrates were cleaned in detergent, distilled water, acetone, and isopropanol under sonication sequentially for 30 min each. PEDOT:PSS solution was spin-casted onto the ITO substrates to give rise to ∼35 nm PEDOT:PSS layers, which were baked at 150 °C for 15 min before the deposition of the active layers. Blend solution of PBDTBDD/ PBDTNDI-T was prepared in chlorobenzene at a total concentration of 15 mg/mL, and the optimal D/A ratio is 1.5:1. All these solutions heated to 80 °C and stirred 6 h for complete dissolution. The thicknesses of the spin-cast active layers using such solutions were

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01537. F

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Figures S1−S6 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.H.). *E-mail: [email protected] (H.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS For financial support of this research, we thank the Chinese Academy of Sciences (XDB12030200), the National Basic Research Program 973 (2014CB643501), and the NSFC (51173189, 21325419, 91333204). X-ray characterization and analysis by X.J. and H.A. supported by ONR grant N000141410531. X-ray data were acquired at beamlines 11.0.1.2, 7.3.3, and 5.3.2.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231. L.Y. thanks Wenchao Zhao for kind assistance on mobility characterizations. J.H. thanks the CAS-Croucher Funding Scheme for Joint Laboratories for support.

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