Improving Electron Transport in a Double-Cable Conjugated Polymer

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Improving Electron Transport in a Double-Cable Conjugated Polymer via Parallel Perylenetriimide Design Fan Yang,†,‡,⊥ Junyu Li,§,⊥ Cheng Li,‡ and Weiwei Li*,†,‡,∥ †

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P. R. China Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China § DSM DMSC R&D Solutions, P.O. Box 18, 6160 MD Geleen, The Netherlands ∥ Institute of Applied Chemistry, Jiangxi Academy of Sciences, Nanchang 330096, P. R. China Downloaded via IDAHO STATE UNIV on May 8, 2019 at 01:34:26 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Double-cable conjugated polymer can be applied to singlecomponent organic solar cells (SCOSCs), which have great potential to improve the stability and to simplify the fabrication procedure compared to twocomponent organic solar cells. However, SCOSCs always show low powerconversion efficiencies (PCEs), which is mainly due to the difficulty to tune the nanophase separation in double-cable polymers for charge transport. Herein, we are able to find a way to improve the electron transport in double-cable polymers. The idea starts by introducing a parallel and large benzo[ghi]perylenetriimide into the side chains, different from the conventional perylene bisimide (PBI) side units in double-cable polymers. The new electron-deficient side units were found to lower the crystallinity of conjugated backbone and enhance the contact region between different acceptors. This could help in electron transport in the new double-cable polymers, as confirmed by spacecharge limited current measurement. Therefore, the new double-cable polymer provided a high PCE of 4.34% in SCOSCs compared to 1.92% based on the polymer with PBI side units. Our results demonstrate that by rationally designing electrondeficient side units, the electron transport in double-cable polymers can be optimized toward efficient SCOSCs.

1. INTRODUCTION

advantages, the SCOSCs are promising candidates for industry application in the future. Conjugated materials for SCOSCs mainly contain diblock conjugated polymers, double-cable conjugated polymers, and molecular dyads. Among them, diblock conjugated polymers, including rod−rod and rod−coil structures, have been widely studied due to their tendency to form donor/acceptor lamellar structures, which are recognized as the ideal morphology for charge separation and transport in OSCs.21−30 The PCEs of SCOSCs based on diblock conjugated polymers have been close to 4%.30 Molecular dyads containing electron donor and acceptor have also been explored for SCOSCs with PCEs around 3%.31−38 Double-cable conjugated polymers with conjugated backbone as donor and aromatic side units as acceptor have also received much attention in the last twenty years,39−51 in which the PCEs of double-cable polymer-based SCOSCs could reach more than 5%.51 Therefore, the PCEs of SCOSCs are still far behind those of two-component OSCs, which also weaken the researchers’ enthusiasm into this field. We are particularly interested with double-cable conjugated polymers, since they can be easily prepared via Suzuki, Stille, or direct-arylation polymerization.46−50 Therefore, huge amount

Bulk-heterojunction organic solar cells (OSCs) that contain electron donor and electron acceptor in the photoactive layers have attracted much attention in the last two decades.1−5 In recent years, with rapid development of nonfullerene electron acceptors, the power-conversion efficiencies (PCEs) have reached 16% in single-junction OSCs.6−8 These promising achievements open the door for the application of OSCs in flexible and large-area devices.9 PCEs, stability, and cost are the three key parameters for outdoor application of organic photovoltaic modules. Two-component OSCs surfer from instability issues under heat and light during long-term operation.10 Both electron donor and acceptor tend to form self-aggregated nanostructures due to their miscibility, so that the optimized microphase separation will be destroyed, resulting in poor charge generation efficiency and low PCEs in OSCs. An efficient route to improve the stability is to covalently link electron donor and acceptor into one conjugated material so as to suppress the self-aggregation tendency.11 This is also called “single-component organic solar cells” (SCOSCs), since only one conjugated material is applied into the photoactive layer.12−20 Besides excellent stability, SCOSCs can also simplify the fabrication procedure concerning one conjugated material compared to twocomponent OSCs with donor and acceptor. With these © XXXX American Chemical Society

Received: March 11, 2019 Revised: April 19, 2019

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DOI: 10.1021/acs.macromol.9b00495 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. Chemical structures of the double-cable polymers (a) PTPDPBI and (b) PTPDBPTI, and the proposed mechanism for electron transport. (c) The chemical structures of the donor polymer PTPDBDT and the acceptors PBI and PBTI-C8.

Scheme 1. Synthetic Routes for the Monomers and the Polymera

(i) K2CO3, DMF, 80 °C. (ii) Stille polymerization using Pd2(dba)3/P(o-tolyl)3, toluene, 115 °C.

a

to form ordered structures, as shown in Figure 1a. In this case, electrons have difficulty transporting via PBI units, resulting in severe charge recombination in SCOSCs. Therefore, it is necessary to find a method to construct an electron-transport channel to facilitate the high and balanced charge transport in SCOSCs. In this work, we have successfully developed a new method to solve electron-transport issues in these double-cable conjugated polymers toward efficient SCOSCs. The idea starts from selecting a new electron-deficient side unit benzo[ghi]perylene triimide (BPTI, Figure 1c).53−56 BPTI has large aromatic skeleton with improved crystallinity compared to PBI unit, which will help to form ordered structures during competition with conjugated backbone. In addition, we speculate that this parallel structure will create more chance for the contact between different BPTI units, so that electron transport could be allowed (Figure 1b). With these ideas, we synthesized a new double-cable polymer PTPDBPTI as compared to the polymer PTPDPBI with PBI side units (Figure 1). PTPDBPTI exhibited a more than doubled PCE of 4.34% in SCOSCs than the PCE of 1.92% in PTPDPBI-based

of electron-rich and electron-deficient units that have been widely used in the so-called “donor−acceptor” conjugated polymers52 can be potentially introduced into the conjugated backbone and side units, so that their absorption, energy levels, crystallinity, and charge carrier mobility can be systematically adjusted. With this method, we have designed a series of double-cable polymers, in which perylene bisimide (PBI) side units were attached to the conjugated backbones via benzodithiophene,47 thienopyrroledione (TPD),49 isoindigo,50 and diketopyrrolopyrrole units.46,48 Among these polymers, we found that the length of alkyl chains as linkers46 and the shape of conjugated backbone47 played important role in the nanophase separation, and hence cause the distinct photovoltaic performance in SCOSCs. It is very difficult to tune the nanophase separation in double-cable polymers, which is also the key factor to limit the PCEs. In general, both polymer backbone and PBI side units show strong crystalline properties due to their π−π interactions, but their crystalline behavior will be hampered when they are covalently linked. A possible mechanism is that polymer backbones with large conjugated systems have the priority to form crystalline structures, so that PBI side units fail B


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Figure 2. (a) Optical absorption spectra of PBI and BPTI-C8 in chloroform solution. Optical absorption spectra of BPTI-C8, PTPDPBI, PTPDBPTI, and PTPDBDT in (b) chloroform solution and (c) thin films. (d) PL spectra of PTPDBDT, PTPDPBI, and PTPDBPTI in thin films. All films were excited by light at a wavelength of 540 nm.

Figure 3. (a) J−V characteristics of optimized solar cells of PTPDPBT and PTPDBPTI. (b) EQE spectra of the corresponding devices.

shown in Figure 2a. BPTI-C8 shows a blue-shifted absorption compared to PBI, and the molar extinction coefficients of PBI and BPTI-C8 are 8.8 × 104 and 6.4 × 104 cm−1 M−1, respectively. Three bands of BPTI-C8 at λ = 410, 436, and 466 nm are assigned to the S0 to S1 transition energy, which is along the long-axis direction, and the bands in the range of 350−400 nm are assigned to the S0 to S2 transition energy, which is along the short-axis direction.53 It is worth mentioning that PBTI can show a much higher triplet excited state (∼1.68 eV) than that of PBI (∼1.2 eV), which might be helpful in reducing the charge recombination from chargetransfer state to triplet excited state.53 We then studied the optical properties of the double-cable polymers PTPDPBI and PTPDBPTI in solution and solid states, as shown in Figure 2b,c. Both polymers show similar absorption spectra in solution and thin films with an optical band gap of 1.90 eV, indicating the preaggregation in the solution. PTPDBPTI covers the whole spectra from 300 to 700 nm, including the absorption from BPTI and TPD-polymer backbone. Interestingly, the strong absorption intensity in 300−400 nm was observed for PTPDBPTI, which could be

cells. We will also provide systematical studies about the charge transport and nanophase separation in the two polymers.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization. The synthetic procedures of the monomers and polymer are shown in Scheme 1. The compounds 149 and 254 were synthesized according to the literatures. The double-cable polymer PTPDBPTI was prepared via Stille polymerization by using the catalyst Pd2(dba)3/P(o-tolyl)3 in toluene and purified via extraction by acetone, n-hexane, dichloromethane, and chlorobenzene (CB) in turns. PTPDBPTI show a numberaverage molecular weight (Mn) of 14.1 kDa and dispersity (ĐM) of 1.58, which was determined by high-temperature gelpermeation chromography (GPC) measurement (Figure S1). The thermal stability was tested by thermogravimetric analysis (TGA). The decomposition temperatures (5% weight loss) of PTPDPBI and PTPDBPTI are all above 400 °C, indicating the excellent thermal stability of the polymers (Figure S2). 2.2. Optical and Electrochemical Properties. We first compared the optical properties of the two electron acceptors PBI47 and BPTI-C853 (Figure 1c) in chloroform solutions, as C


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Macromolecules Table 1. Characteristics of Optimized SCOSCs Based on PTPDPBI and PTPDBPTI PTPDPBI PTPDBPTI

Jsc (mA cm−2)

Voc (V)

FF

PCEa (%)

5.76 (5.44 ± 0.14) 8.31 (8.26 ± 0.31)

1.01 (1.01 ± 0.005) 1.05 (1.05 ± 0.007)

0.33 (0.32 ± 0.01) 0.50 (0.50 ± 0.02)

1.92 (1.77 ± 0.08) 4.34 (4.29 ± 0.04)

a

Statistics in parentheses from 6 independent cells.

Figure 4. (a) Photocurrent density Jph vs effective voltage Veff based on solar cells. (b) Light intensity dependence of Voc in solar cells.

Figure 5. Mobilities of the optimized solar cells from SCLC measurements: (a) hole mobilities and (b) electron mobilities.

mA cm−2, Voc = 1.01 V, and FF = 0.33). The higher Voc is attributed to a slightly higher LUMO level of the BPTI end group as reported in the literature.53 The enhanced Jsc was also reflected by their external quantum efficiencies (EQEs) (Figure 3b). PTPDBPTI-based cells show EQEs up to 0.63, while EQEs of PTPDPBI-based cells were below 0.40. In addition, we also observe high EQE in the range of 400−500 nm, which could be contributed to the light absorption of the BPTI side units. This result confirms the photocurrent contribution both from PTPDBDT backbone and BPTI side units. 2.4. Study of Charge Transport in These SCOSCs. It is clearly shown that PTPDBPTI could provide significantly improved photocurrent and FF compared to PTPDPBI in SCOSCs, indicating their distinct charge-transport properties. We first studied the charge dissociation probability P(E, T) to probe the charge recombination situations. The photocurrent density (Jph, defined by JL − JD, where JL and JD are the photocurrent densities under illumination and dark conditions, respectively) versus effective voltage (Veff, defined by V0 − V; V0 is the voltage where Jph =0 and V is the applied voltage bias) plots for the SCOSCs are shown in Figure 4a. When the efficient voltage is 2 V, the Jph of PTPDPBI does not reach a saturation value, but the Jph of PTPDBPTI is at the saturation value, according to the Jph and Jsc, the P(E, T) can be calculated as 92.8%. This result means the introduction of BPTI side chain can efficiently decrease the charge recombination in PTPDBPTI-based SCOSCs. We also measured the light

due to the contribution from both conjugated backbone and PBTI side units (Figure 2b). The frontier energy level of PTPDBPTI was determined by cyclic voltammetry (CV) measurement, as shown in Figure S3. PTPDBPTI has an occupied molecular orbital (HOMO) level at −5.60 eV and the lowest unoccupied molecular orbital (LUMO) level at −3.59 eV, which are similar to those of PTPDPBI with HOMO and LUMO levels of −5.61 and −3.62 eV, respectively.49 Photoluminescence (PL) property of the double-cable polymers was also studied, in which both PTPDPBI and PTPDBPTI present a completely quenched PL compared to their donor polymer PTPDBDT (Figure 1c) with a strong PL emission (Figure 2d). The quenched PL ensures that exciton can be effectively dissociated into free charges. 2.3. Photovoltaic Properties. The photovoltaic properties of PTPDBPTI were studied with an inverted device configuration of ITO/ZnO/active layer/MoOx/Ag. The photoactive layer containing PTPDBPTI was spin-coated from the CB solution without additive and thermal annealing. The PTPDPBI-based SCOSCs also achieved optimized photovoltaic performance by using the same fabrication condition.49 As shown in Figure 3 and Table 1, the PTPDBPTI-based SCOSCs exhibit a short-circuit current density (Jsc) of 8.31 mA cm−2, an open-circuit voltage (Voc) of 1.05 V, and a fill factor (FF) of 0.50. All these parameters are higher than those of the PTPDPBI-based cells (with Jsc = 5.76 D


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Macromolecules dependence of Voc. The plot of Voc versus the natural plot of light intensity will have a slope equal to kT/q. As shown in Figure 4b, the two devices show similar slope of 1.21 kT/q for PTPDPBI and 1.28 kT/q for PTPDBPTI. The result demonstrates that the bimolecular recombination is the dominant mechanism in these devices. We then perform space charge limited current (SCLC) measurement to study the charge-carrier mobilities in SCOSCs, with the structure of ITO/PEDOT:PSS/polymer/ Au for hole mobilities and ITO/ZnO/polymer/Ca/Al for electron mobilities, as shown in Figure 5 and Table 2.57

is hampered so as to generate a better nanophase separation between donor and acceptor. It is interesting to mention that the domain size in the PTPDBPTI thin film is consistent with that in two-component donor/acceptor systems, in which 10− 20 nm was recognized as the optimized size for OSCs, since the diffusion length of excitons was also in this range.58 The GIWAXS patterns show a clear difference is the weak (010) diffraction peak in the out-of-plane direction for PTPDBPTI thin film, indicating that π−π stacking of conjugated backbone or BPTI side units is significantly suppressed (Figure 7). It is not easy to point out the origin

Table 2. Hole and Electron Mobilities of Optimized SCOSCs Based on PTPDPBI and PTPDBPTI from SCLC measurements active layer

μh (cm2 V−1 s−1)

μe (cm2 V−1 s−1)

μh/μe

PTPDPBI PTPDBPTI

1.44 × 10−4 1.47 × 10−4

2.55 × 10−7 7.11 × 10−6

565 21

PTPDPBI and PTPDBPTI-based devices have similar hole mobilities of 1.44 × 10−4 and 1.47 × 10−4 cm2 V−1 s−1, respectively, indicating that large BPTI side chains have negligible effect on the hole-transport channels. Interestingly, the electron mobility of PTPDBPTI-based device is nearly 30 times that of the PTPDPBI-based device. This observation indicates that BPTI side units might build a continuous channel for electron transport, and thus reduce the charge recombination for the improved Jsc and FF in SCOSCs. 2.5. Crystalline Properties and Morphology. We further studied the crystalline properties of the two polymers by using atomic force microscopy (AFM) and grazingincidence wide-angle X-ray scattering (GIWAXS) measurement. PTPDPBI thin film showed large domains (>50 nm) on the surface with a roughness of 1.22 nm, while PTPDBPTI thin film performed small domains (10−20 nm) with a roughness of 0.58 nm (Figure 6). This indicates that crystallinity of the polymer backbone or aromatic side units

Figure 7. GIWAXS images of the polymers (a) PTPDPBI and (b) PTPDBPTI. (c, d) In-plane and out-of-plane plots of the corresponding GIWAXS images.

of the (010) diffraction peak in PTPDPBI, whether it is from conjugated backbone or PBI side units. However, in our recent study, we find that the polymer PTPDBDT has strong aggregation tendency in thin films, which can be suppressed via a cross-linking strategy.59 From this aspect, we speculate that the large BPTI could hamper the aggregation/crystallization of the PTPDBDT backbone, and therefore provide a better hole and electron transport in PTPDBPTI-based SCOSCs. It is worth mentioning that although the electron mobility of PTPDBPTI is much improved compared to that of PTPDPBI, PTPDBPTI still performed unbalanced hole and electron mobilities (with μh/μe = 21 in Table 2). This indicates that in PTPDBPTI, the electron mobility is required to be further improved to reduce the charge recombination. From the AFM images and GIWAXS pattern, it seems that the conjugated backbones and BPTI side units in PTPDBPTI form mix phase, which may explain the high charge recombination and low FF in solar cells. The mix phase with poor nanophase separation could be further confirmed by grazing-incidence small-angle Xray scattering (GISAXS) measurement, in which both PTPDPBI and PTPDBPTI showed no diffraction peaks (Figure S4). This is different from our previous observation in double-cable polymers with linear conjugated backbone in which diffraction peaks could be observed in the GISAXS measurement.47 Currently, it is hard to claim a clear strategy to improve the performance of this system. We envision that the crystallinity of the conjugated backbone and aromatic side units might be the key issue to control the nanophase separation in double-cable polymers. In TPD-based doublecable polymers, the crystallinity of conjugated backbones could

Figure 6. AFM (a, b) height and (c, d) phase images (500 × 500 nm2) of the photoactive layers. The root mean square roughness for PTPDPBI and PTPDBPTI thin films is 1.22 and 0.58 nm, respectively. E


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in a vacuum oven to yield PTPDBPTI (62.5 mg, 96.2%) as a dark solid. GPC (o-DCB, 140 °C): Mn = 14.1 kDa, Mw = 22.2 kDa, and ĐM = 1.58.

be further improved, such as by introducing extra thiophenes/ selephenes/thieno[3,2-b]thiophenes into the backbone, which is also in progress in our laboratory.

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3. CONCLUSIONS In this work, we have applied a parallel perylene triimide side chain to design a double-cable conjugated polymer. The new polymer showed similar absorption onset and energy levels, but provided significantly improved photovoltaic performance in SCOSCs. A PCE of 4.34% in PTPDBPTI-based SCOSCs was achieved, which is much higher than that of PTPDPBI with perylene dimide as side units (1.92%). To the best of our knowledge, 4.34% is also among the highest PCEs in SCOSCs. The improvement could be attributed to the less-crystalline backbone/side units in PTPDBPTI and the large BPTI side units. These facilitate to build a good electron-transport channel to obtain a balanced hole and electron transport. Therefore, our results demonstrate that the electron transport is a key parameter to improve the PCEs of SCOSCs, which could be realized by introducing new electron-deficient side units.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00495. Materials and measurements, GPC, TGA, CV, and GISAXS of the polymers, and NMR and HR-MALDI (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Cheng Li: 0000-0001-9377-9049 Weiwei Li: 0000-0002-7329-4236 Author Contributions ⊥

4. EXPERIMENTAL SECTION

F.Y. and J.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

4.1. Measurements. The detailed information about the measurement, characterization, and device fabrication are present in the Supporting Information. 4.2. Synthesis. Compound 2 was synthesized according to the literature procedure.54 1H NMR (CDCl3, 400 MHz): δ (ppm) 10.37 (s, 2H), 9.36−9.39 (d, 2H), 9.19 (s, 2H), 8.03 (s, 1H), 5.30−5.31 (d, 2H), 2.34−2.37 (d, 4H), 1.97−1.98 (d, 4H), 1.30−1.33 (m, 24H), 0.81−0.86 (t, 12H). 13C NMR (125 MHz, CDCl3) δ (ppm) 167.6, 133.4, 128.3, 128.0, 127.8, 125.1, 124.2, 123.5, 55.4, 32.5, 31.8, 26.8, 22.6, 14.1. (HR-MALDI-TOF): m/z: 791.3945. (Calcd for C50H53N3O6: 791.3934). BPTI-C8 was synthesized according to the literature procedure.53 1 H NMR (CDCl3, 400 MHz): δ (ppm) 10.00−10.20 (d, 2H), 8.99− 9.20 (m, 4H), 5.30 (s, 2H), 3.97 (s, 2H), 2.37 (s, 4H), 1.98−2.00 (m, 6H), 1.29−1.36 (m, 34H), 0.84−0.86 (t, 15H). 13C NMR (100 MHz, CDCl3) δ (ppm) 167.7, 132.2, 127.1, 126.8, 126.4, 123.8, 123.3, 122.3, 55.5, 38.9, 32.6, 32.1, 32.0, 29.4, 28.8, 27.2, 27.0, 22.8, 14.3. (HR-MALDI-TOF): m/z: 903.5190. (Calcd for C58H69N3O6: 903.5186). 4.2.1. Monomer M1. Compounds 1 (142 mg, 0.25 mmol), 2 (221.5 mg, 0.28 mmol), and anhydrous potassium carbonate (71 mg, 0.51 mmol) were dissolved in N,N-dimethylformamide (20 mL). The reaction mixture was stirred at 80 °C for 12 h, and the solvent was removed by vacuum evaporation. The resulting solid was purified by silica gel chromatography (dichloromethane as eluent) to afford M1 as an orange solid (147 mg, 49.9%). 1H NMR (CDCl3, 400 MHz): δ (ppm) 10.36 (s, 2H), 9.30−9.33 (d, 2H), 9.14 (s, 2H), 5.32 (s, 2H), 3.96−4.00 (t, 2H), 3.53−3.56 (t, 2H), 2.36 (s, 4H), 1.86−1.99 (m, 6H), 1.30−1.38 (m, 42H), 0.85−0.87 (t, 12H). 13C NMR (100 MHz, CDCl3) δ (ppm) 168.1, 160.4, 134.9, 132.8, 127.5, 127.3, 127.2, 124.4, 123.8, 123.0, 112.9, 55.4, 38.9, 32.6, 32.0, 29.6, 29.5, 29.4, 29.2, 28.9, 26.9, 22.8, 14.2. (HR-MALDI-TOF): m/z: 1268.3725. (Calcd for C68H76Br2N4O8S: 1268.3730). 4.2.2. PTPDBPTI. To a degassed solution of monomer M1 (52.27 mg, 41.2 μmol), M2 (37.25 mg, 41.2 μmol) in toluene (2 mL), Pd2(dba)3 (1.12 mg, 1.2 μmol), and tri(o-tolyl)phosphine (3.01 mg, 9.9 μmol) were added. The mixture was stirred at 115 °C for 48 h in nitrogen atmosphere and then precipitated in methanol and filtered through a Soxhlet thimble. The polymer was extracted with acetone, hexane, and dichloromethane. Then, the polymer was dissolved in chlorobenzene and filtered, the solvent was evaporated, and the polymer was precipitated in acetone. The polymer fraction in chlorobenzene was used for studies in this work. The polymer was collected by filtering over a 0.45 μm PTFE membrane filter and dried

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ACKNOWLEDGMENTS This study is jointly supported by MOST (2018YFA0208504, 2017YFA0204702) and NSFC (51773207, 21574138, 51603209, 91633301) of China. This work was further supported by the Strategic Priority Research Program (XDB12030200) of the Chinese Academy of Sciences, Fundamental Research Funds for the Central Universities (XK1802-2), and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, CAS.

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