Methyl Thioether Functionalization of a Polymeric Donor for Efficient

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Methyl Thioether Functionalization of Polymeric Donor for Efficient Solar Cells Processed from Non-Halogenated Solvents Chun-Hui Zhang, Wei Wang, Wei Huang, Jiang Wang, Zhubin Hu, Zhichao Lin, Tingbin Yang, Fengyuan Lin, Yun Xing, Junxian Bai, Haitao Sun, and Yongye Liang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00926 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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Chemistry of Materials

Methyl Thioether Functionalization of Polymeric Donor for Efficient Solar Cells Processed from Non-Halogenated Solvents Chun-Hui Zhang,† Wei Wang,† Wei Huang,† Jiang Wang,† , ¶ Zhubin Hu,§ Zhichao Lin,† Tingbin Yang,*, †, ‡ Fengyuan Lin,† Yun Xing,† Junxian Bai,† Haitao Sun,*, §, # Yongye Liang*, † †

Shenzhen Key Laboratory of Printed Electronics, Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, P. R. China ¶ Analytical and Testing Center, Sichuan University of Science & Engineering, Zigong 643000, Sichuan Province, P. R. China § State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, P. R. China ‡ Materials Characterization and Preparation Center, Southern University of Science and Technology, Shenzhen 518055, P. R. China # Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi, 030006, P. R. China ABSTRACT: Polymer solar cells (PSCs) processed from non-halogenated solvents are favorable for large scale production. However, the photovoltaic performance of devices from non-halogenated solvents is generally inferior to the counterparts from halogenated solvents. Herein, we report the utilization of 5-alkyl-4-(methylthio)thiophene (MTT) as a conjugated side chain of the polymeric donor to achieve efficient PSCs processed from toluene. The derived polymer PMTT56 exhibits lower energy levels than the 5-alkyl-4-methoxythiophene (MOT) counterpart (PMOT39) and 5-(alkylthio)thiophene counterpart (PEHTT). PMTT56 is superior to PMOT39 with enhanced interchain interaction and hole mobility, and also shows better solubility than PEHTT in toluene. In toluene processed PSCs, the PMTT56 devices deliver PCE up to 12.6% with IT-2F as the acceptor. By introducing [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the second acceptor, a remarkable PCE of 13.2% is further achieved in the ternary devices, which is among the best for PSCs processed from toluene. Our study presents a fine-tuning approach to develop efficient organic photovoltaic materials processed from non-halogenated solvents.

1. INTRODUCTION Owing to solution processability, light weight, flexibility and semitransparency, polymer solar cells (PSCs) have been considered as a destructive photovoltaic technology. Very recently, the power conversion efficiencies (PCEs) of PSCs with fused-ring electron acceptors (FREAs)1–3 have been boosted to over 15% for single junction devices4–7 and over 17% for tandem devices,8,9 which are close to thin film inorganic counterparts. Bulk heterojunction (BHJ) structure formed by electron donor materials and electron acceptor materials is generally employed in PSCs.10,11 To achieve high efficiency, the structures of electron donor materials and electron acceptor materials require to be fine-tuned to optimize the absorption, energy levels, charge mobility and miscibility.11–13 Fluorination and chlorination have been demonstrated to be efficient structure fine-tuning approaches.7,14–16 Besides, introducing alkylsilyl or alkylthio side chains can also improve the photovoltaic performance.17,18 Processing solvent is also important for the performance of PSCs, and currently most efficient PSCs are processed from halogenated solvents. Such solvents are carcinogenic and cause environmental issues, therefore they are not favorable for practical fabrication.19 To this concern, facile structure fine-tuning approaches to afford efficient

PSCs processed from non-halogenated solvents are highly needed.19–22 5-alkyl-4-methoxythiophene (MOT) as a conjugated side chain has been developed as an efficient fine-tuning approach to construct polymeric donors. Compared to the generally used 5-alkylthiophene side chain, MOT can effectively lower the energy levels and enhance intermolecular interaction, affording enhanced open circuit voltage (Voc) and PCEs of PSCs.23–25 The resulting polymer PMOT16 delivered a PCE exceeding 10% in solar cells with IDIC as the acceptor.23 It should be noted that MOT also improves the polymer solubility, allowing efficient solar cells processed from non-halogenated solvents. 5(alkylthio)thiophene is also an efficient conjugated side chain to improve the photovoltaic properties.15,18 Due to the higher electron affinity and hyper-conjugation of S-atom compared to C-atom in 5-alkylthiophene, the resulting polymer devices can afford higher Voc and PCE.15,26,27 Herein, we develop a new conjugated side chain termed 5-alkyl-4-(methylthio)thiophene (MTT). A new polymer, PMTT56 (Scheme 1) is synthesized based on MTT modified benzo[1,2-b:4,5-b']dithiophene (BDT)28–30 and thieno[3,4c]pyrrole-4,6(5H)-dione (TPD)31–39 with octyl thieno[3,2b]thiophene (TT)40 as O ' * (tt-TPD)35,36. PMTT56 exhibits lower energy levels than the

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charge transfer (CT), PMTT56 possesses more pronounced localized excitation character that could result in higher extinction coefficient.

(a) 1.4 PMTT56

R.T. o 30 C o 40 C o 50 C o 60 C o 70 C o 80 C o 90 C o 100 C o 110 C

R.T.

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2.2. Binary polymer solar cells.

(b) 1.4 PMOT39

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R.T. o 40 C o 60 C o 80 C o 100 C o 110 C

0.4 110 o C

0.2 400

500

600

(a)

700

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5 0

-2

Absorbance

1.2

0.0

temperature-dependent absorption and reversible transition between aggregation and disaggregation state. Dilute toluene solutions exhibit similar absorption changes as those in oDCB (Figure S8). Notably, the shoulder peak of PMTT56 (at 606 nm) remains even being heated to 110 oC, while the shoulder peak in MOT-modified PMOT39 almost disappears. These observations suggest MTT modification induces stronger interchain interaction in PMTT56. Cyclic voltammetry (CV) was performed to determine the IP/EA levels of the polymers (Figure S9). With Fc/Fc+ couple as the external reference, IP/EA levels for PMTT56, PMOT39 and PEHTT are estimated to be -5.39/-3.40 eV, 5.34/-3.38 eV and -5.35/-3.39 eV, respectively. The energy level alignments of the polymeric donors, FREA IT-2F and fullerene acceptor PC71BM are presented in Figure 1b. MTT modified PMTT56 presents lower IP level than the MOT and EHTT modified counterparts, which is also consistent with the theoretical predictions (Table 1).

J (mA cm )

Absorbance

1.2

PMTT56/IT-2F PMOT39/IT-2F PEHTT/IT-2F

-5 -10 -15 -20

(c) 1.4 PEHTT 1.2

Absorbance

R.T.

1.0 0.8 0.6 110 o C

0.4

R.T. o 40 C o 60 C o 80 C o 100 C o 110 C

-0.2

0.0

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V (V) (b) 100 80

PMTT56:IT-2F PMOT39:IT-2F PEHTT:IT-2F

0.2 0.0

400

500

600

700

Wavelength (nm) Figure 3. Temperature-dependent UV-Vis absorption spectra of PMTT56, PMOT39 and PEHTT, measured in dilute o-DCB solutions.

The similarity of the absorption spectra in solution and film of these polymers suggests strong pre-aggregation.44 Temperature-dependent absorption spectra were measured in dilute o-DCB solutions to study the aggregating properties of the polymers (Figure 3 and Figure S8). Generally, the peak intensity gradually decreases and the absorption peak blueshifts upon heating and returns after cooling, suggesting

EQE (%)

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

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60 40 20 0 300

400

500

600

700

800

Wavelength (nm) Figure 4. (a) J-V curves and (b) EQE plots for PMTT56:IT-2F, PMOT39:IT-2F and PEHTT:IT-2F devices processed from toluene.


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Table 2. Photovoltaic data of PMOT39:IT-2F, PMTT56:IT-2F and PEHTT:IT-2F based photovoltaic devices, processed from toluene with 1% DPE as additive, tested under AM1.5G solar radiation (100 mW cm-2)

a b

Active layer

Voc a (V)

Jsc a (mA cm-2)

FF a (%)

PCE a (%)

Jcalc.b (mA cm-2)

PMTT56:IT-2F

0.945 (0.942 ± 0.008)

18.67 (18.50 ± 0.30)

71.4 (70.5 ± 1.0)

12.6 (12.4 ± 0.2)

18.06

PMOT39:IT-2F

0.897 (0.895 ± 0.012)

17.84 (17.60 ± 0.36)

65.8 (65.0 ± 0.9)

10.5 (10.4 ± 0.1)

17.05

PEHTT:IT-2F

0.890 (0.889 ± 0.010)

18.90 (18.60 ± 0.50)

64.4 (63.8 ± 1.2)

10.8 (10.5 ± 0.3)

18.17

Average data extracted from ten individual devices are listed in brackets. Calculated by integrating the EQE data.

Compared to halogenated solvents such as chloroform and chlorobenzene, toluene as a widely used solvent in industry, is less toxic and thus favorable for mass production of PSCs.16,19,45,46 Unfortunately, most photovoltaic materials deliver inferior performance when processed from toluene solutions possibly due to insufficient solubility.20 The superior solubility of PMTT56 in toluene may allow efficient toluene processed PSCs. Considering the intense complementary absorption over 650-800 nm, suitable lowlying EA and sufficient solubility in toluene, IT-2F is screened out as the partnered FREA.47 Herein, nonhalogenated solvent processed PSCs based on PMTT56:IT2F were systematically characterized in a conventional device structure: ITO/PEDOT:PSS/active layer/PDINO/Ag (Figure S10 and S11, Table S2-8), where 2,9-bis[3(dimethyloxidoamino)propyl]anthra[2,1,9-def:6,5,10-d'e'f'] diisoquinoline-1,3,8,10(2H,9H)-tetrone (PDINO)48 is used as the cathode interface layer. The preliminary results show that PMTT56:IT-2F could deliver PCEs of 11.2%, 10.5% and 10.3% when processed from o-xylene, tetrahydrofuran and anisole (Table S8), respectively, suggesting the superior performance of PMTT56 from non-halogenated solvents. In the optimized condition with 1% diphenyl ether (DPE) as the additive, the PMTT56:IT-2F devices processed from toluene deliver an average PCE of 12.4% (Table 2). The champion device presents a remarkable PCE of 12.6% with Jsc = 18.67 mA cm-2, Voc = 0.945 V and FF =

71.4%. The PCEs of PMTT56 devices are significantly higher than those of PMOT39 devices (with an average PCE of 10.4%) and PEHTT devices (with an average PCE of 10.5%), mainly due to the enhanced Voc and FF (Figure 4a and Table 2). Owing to the lower IP of PMTT56, its devices deliver higher Voc (0.945 V) than those of PMOT39 (0.897 V) and PEHTT (0.890 V). Further, the PMTT56 based device also exhibits higher FF (71.4%) compared to PMOT39 (65.8%) and PEHTT (64.4%). Higher Jsc in the PMTT56 based device than that of PMOT39 can be further confirmed by the external quantum efficiency (EQE) plots (Figure 4b), which show enhanced photoresponse over 400750 nm for PMTT56:IT-2F. 2.3. Ternary polymer solar cells. Introducing fullerene derivatives as the second acceptor has been proved to be an effective approach to improve the photovoltaic performance of FREA based PSCs.49–55 Here, PC71BM (25 wt% of IT-2F) was added to the PMTT56:IT-2F system, and the afforded ternary devices demonstrate further improved PCEs up to 13.2% (Figure5a, Table 3). Although the Voc is slightly reduced in the ternary device due to the relatively deeperlying EA of PC71BM, significant improvement of Jsc is observed in PMTT56:IT-2F:PC71BM (19.75 mA cm-2 vs 18.67 mA cm-2 for PMTT56:IT-2F). 50,52,54

Table 3. Photovoltaic data of PMTT56:IT-2F:PC71BM based devices

a b

Voc a (V)

Jsc a (mA cm-2)

FF a (%)

PCE a (%)

Jcalc.b (mA cm-2)

PMTT56:IT-2F

0.945 (0.942 ± 0.008)

18.67 (18.50 ± 0.30)

71.4 (70.5 ± 1.0)

12.6 (12.4 ± 0.2)

18.06

PMTT56:IT-2F:PC71BM

0.932 (0.930 ± 0.010)

19.75 (19.46 ± 0.46)

71.4 (70.8 ± 1.0)

13.2 (13.0 ± 0.2)

19.15

Average data extracted from ten individual devices are listed in brackets. Calculated by integrating the EQE data.



(a)

5

PMTT56:IT-2F PMTT56:IT-2F:PC71BM

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0.5

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700

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850

Wavelength (nm)

Figure 5. (a) J-V curves, (b) EQE plots, (c) UV-Vis absorption spectra for PMTT56:IT-2F and PMTT56:IT-2F:PC71BM devices processed from toluene, and (d) photoluminescence spectra for PMTT56, IT-2F, PMTT56:IT-2F and PMTT56:IT-2F:PC71BM.

To understand the effects of additional PC71BM, UVVis absorption and photoluminescence (PL) of PMTT56:IT2F and PMTT56:IT-2F:PC71BM were characterized. Introducing additional amount of PC71BM can contribute to the absorption enhancement in the short wavelength range (Figure 5c). Further, the PL characterizations (Figure 5d) indicate enhanced fluorescence quenching in the PC71BM added device, suggesting improved charge separation.52,54 The synergetic effects of better absorption and enhanced PL quenching may account for the improvement of short circuit current in the PC71BM containing devices. 2.4. Morphology and mobility. Atomic force microscope (AFM, Figure 6a-c) and transmission electron microscope (TEM, Figure 6d-f) were utilized to study the surface and bulk film morphology. As can be seen from the AFM topological images, PMTT56:IT-2F and PMOT39:IT-2F films show smooth surface with root mean square roughness (Rrms) of 1.63 nm and 1.24 nm, respectively, while large aggregates with size of around 100-150 nm appear in PEHTT:IT-2F blend (Figure 6c). The morphological characteristics can be further confirmed by TEM. The PMTT56 and PMOT39 blends exhibit uniform morphology with fine structures. In contrast, nanoparticles with size of over 100 nm can be clearly seen in the PEHTT:IT-2F blend (Figure 6f). Such large aggregates in PEHTT:IT-2F blend may be originated from the relatively lower solubility of PEHTT in toluene, and may account for the relatively low FF in PEHTT:IT-2F based devices.56 Comparing the TEM images for PMTT56:IT-2F and PMTT56:IT-2F:PC71BM (Figure 6d and 6g), similar morphology is presented, indicating negligible impact of additional PC71BM on morphology.

a) PMTT56:IT-2F

d) PMTT56:IT-2F

e) PMOT39:IT-2F b) PMOT39:IT-2F

f) PEHTT:IT-2F

c) PEHTT:IT-2F g) PMTT56:IT-2F:PC71BM

Figure 6. Topological AFM height images (a-c, 2.5 X0 × 2.5 X0B and TEM images (d-g) for PMTT56:IT-2F (a,d), PMOT39:IT-2F (b,e), PEHTT:IT-2F (c,f) and PMTT56:IT2F:PC71BM (g). The root mean square roughness (Rrms) for PMTT56:IT-2F (a), PMOT39:IT-2F (b) and PEHTT:IT-2F (c) is 1.63 nm, 1.24 nm and 6.53 nm, respectively.

Hole-only devices with a structure of “ITO/PEDOT:PSS/active layer/MoO3/Ag” and electron-only


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devices with a structure of “ITO/ZnO/active layer/PDINO/Ag” were fabricated to study the charge transport characteristics (Figure 7, Figure S12 and Table 4). In pristine polymer film, PMTT56 presents higher hole mobility of 8.0 × 10-4 cm2 V-1 s-1, compared with 6.3 × 10-4 cm2 V-1 s-1 and 4.4 × 10-4 cm2 V-1 s-1 for PMOT39 and PEHTT, respectively, which may result from the enhanced interchain interaction in PMTT56. In the blend film, the hole mobility values of these three polymers are on the similar level, and PMTT56:IT-2F exhibits slightly higher mobility of 6.3 × 10-4 cm2 V-1 s-1 compared to the other two blend films (Table 4). The improved charge mobility in PMTT56:IT-2F enables the carriers to be swept out to the external circuit efficiently prior to recombination and therefore account for the enhanced FF in the related PSCs. The charge mobility results agree well with the enhanced interchain interaction in MTT modified polymer.

(a) 120 -1

cm )

100

4.1. Materials. The BDT-MOT organotin monomer and Br2tt-TPD was synthesized according to literatures. Sn2-BDTEHTT was purchased from Derthon Co LTD (Shenzhen, China) and used as received without further purification. Synthetic procedures of BDT-MTT organotin monomer were illustrated in Figure S1. All solvents used for reactions were purified by the solvent purification system (Innovative Technology, Inc.) before using. 1H and 13C NMR spectra in CDCl3 were measured on a BRUKER AVANCE 400 system (400 MHz). Gel permeation chromatography (GPC) was performed at 170 °C on PL-GPC220 using polystyrene as the standard and 1,2,4-trichlorobenzene as the eluent.

J

1/2

(mA

0.5

80 60 40

2.0

2.5

3.0

Vapp-Vbi-Va (V) (b)

1/2

(mA

0.5

-1

cm )

160

counterpart PMOT39, PMTT56 exhibits higher absorption coefficient, lowered IP, enhanced charge mobility, and therefore delivers simultaneously improved Jsc, Voc and FF in IT-2F based PSCs. Compared to 5-(2ethylhexylthio)thiophene (EHTT) functionalized PEHTT, PMTT56 is advantageous with higher solubility in toluene and thus better morphology of blend films. Holding both the merits of better tailoring effects of S-atom and functionalization of 4-position, the MTT based polymer PMTT56 presents impressive PCE up to 12.6% in PSCs processed from toluene, with Voc = 0.94 V, Jsc = 18.67 mA cm-2, and FF = 71.4%. Further introducing PC71BM as the second acceptor increases the PCEs to up to 13.2%. This study highlights that PMTT56 is an efficient polymer donor to construct high performance PSCs processed from nonhalogenated solvents. It also suggests that MTT as a conjugated side chain is a promising fine-tuning approach for polymeric materials.

4. EXPERIMENTAL SECTION PMTT56:IT-2F PMOT39:IT-2F PEHTT:IT-2F

20 1.5

J

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


PMTT56:IT-2F PMOT39:IT-2F PEHTT:IT-2F

120 80 40 0 1.5

2.0

2.5

3.0

Vapp-Vbi-Va (V) Figure 7. J1/2-V curves for the hole-only devices (a) and the electron-only devices (b).

Table 4. Hole mobility and electron mobility of the polymer:IT-2F blend films measured by SCLC method Active layer

h

e

[cm2 V-1 s-1]

[cm2 V-1 s-1]

PMTT56:IT-2F

6.3 × 10-4

4.1 × 10-4

PMOT39:IT-2F

4.6 × 10-4

4.2 × 10-4

PEHTT:IT-2F

5.4 × 10-4

3.1 × 10-4

3. CONCLUSION In summary, 5-alkyl-4-(methylthio)thiophene (MTT) has been developed as a conjugated side chain to functionalize benzo[1,2-b:4,5-b']dithiophene (BDT) and PMTT56 is synthesized with such building unit. Compared to 5-(2ethylhexyl)-4-methoxythiophene (MOT) functionalized

4.2. Materials characterization. The thickness of the films was measured using a XP-200 Sylus Profilometer (KLA Tencor). Absorption spectra were measured by a UV-VisNIR spectrophotometer (UV3600, Shimadzu). The details for solubility measurement are as followed. First, an excess amount of the polymer was added into toluene to form a supersaturated solution. After sonication for 20 mins, the supersaturated solution was subjected to centrifugation. Subsequently, the upper saturated solution was carefully diluted proportionally down to ~0.01 mg mL-1. The exact concentration of the dilute solution was determined by UVVis absorption, with 0.01 mg mL-1 solution as the reference. The solubility of the polymer (concentration of the saturated solution) could be derived by considering the dilution ratio. Cyclic Voltammetry (CV) measurements were carried out using an electrochemical workstation (CHI600E) to determine the IP and EA values of the polymers. Polymer films were dip-coated from chloroform solutions on a glass carbon working electrode (2 mm in diameter). CV curves were measured under argon atmosphere in a CH3CN solution containing 0.1 M Bu4NPF6 with a Pt wire as the counter electrode and Ag/Ag+ as the reference electrode. The redox potential of ferrocene/ferrocenium (Fc/Fc+) under the same conditions is located at 0.73 V, which is assumed to have an absolute energy level of -4.8 eV to vacuum. AFM images of active layers were characterized with an Asylum MFP-3DStand Alone microscopy at tapping mode condition. TEM images were carried out on an FEI Helios Nanolab 600i. The TEM samples of the blend films were prepared on copper grids and dried in nitrogen filled glove-box. The PL were measured with a lifetime spectrometer (FL920, Edinburgh Instrument) equipped with an Oxford Instruments nitrogen cryostat (Optistat DN).



4.3. Details for DFT Simulations and Calculations. The ground-state (S0) geometries of the molecules were optimized using B3LYP/6-31G(d) method with the GD3GJ dispersion correction. The corresponding range-separation parameter AS in bohr_ ) for the polymers was optimally tuned based on the long-range corrected S)!+ functional with the 6-31G(d) basis set through the “GAPtuning” procedure employing the non-equilibrium polarizable continuum model (PCM) (with a dielectric constant of 1.3) to simulate the solid thin-film environment and the optimal S value is 0.065 bohr-1 for the polymers. We refer to the S)!+ functional with the optimally-tuned S value as S)!+*. The excited-state (S1) properties and vertical absorption wavelength of these polymers were obtained using time dependent A .B ) 1 S)!+*/631G(d) method. Natural transition orbital (NTO) distributions were analyzed using Multiwfn program and represented by VMD program to examine the nature of the excited-state (S1). All the DFT calculations were performed using Gaussian 16 code. More information about DFT calculations can be found in the SI.

4.4. Device fabrication. The device architecture was ITO/PEDOT:PSS/Active layer/PDINO/Ag. The ITO-coated glass substrates were sequentially cleaned with detergent/water, acetone and isopropyl alcohol in an ultrasonic bath for 20 min, and then dried in an oven at 80 oC. The ITO substrates were treated in UV-ozone for 15 min before use. The PEDOT:PSS solution was spin-coated at 2500 rpm for 30s on the ITO substrate and annealed at 150 oC for 15min in the air. The polymer:IT-2F in toluene with 1% diphenyl ether (DPE) as an additive was spin-coated on the PEDOT:PSS surface in the glove-box. The thickness of the active layer was about 130 nm. Subsequently, PDINO dissolved in ethyl alcohol was spin-coated on the active layer, with a thickness of about 10 nm. Finally, the Ag electrode with a thickness of about 100 nm was thermal evaporated onto the active layer under vacuum at a pressure of 2 × 10-6 mbar, with a device effective area of 0.045 cm2.

4.5. Characterization of PSCs. The J-V curves of the devices were measured under AM1.5G solar simulator illumination (100 mW cm-2) on a computer-controlled Keithley 2400 Source Measure Unit. The J-V characteristics of the devices in dark were measured on a computercontrolled Keithley 2635 Source Measure Unit. The EQE data were obtained by using the solar-cell-response measurement system (QE-R3011, Enlitech).

ASSOCIATED CONTENT Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website, including synthesis and structural characterization of the new polymers; detailed information for the DFT calculations; additional UV-Vis spectra and CV plots; additional photovoltaic device data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (T. B. Yang). * E-mail: [email protected] (H. T. Sun). * E-mail: [email protected] (Y. Y. Liang).

Author Contributions Dr. Chun-Hui Zhang, Wei Wang, Wei Huang, and Dr. Jiang Wang contributed equally to this work. All authors have given approval to the final version of the paper.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Dr. Tingbin Yang and Prof. Haitao Sun thank the National Natural Science Foundation of China (No. 51703092 and 21603074) for financial support. Dr. Chun-Hui Zhang thanks "Peacock Plan" project of Shenzhen (No.20181211686C) for financial support. Dr. Jiang Wang thanks the support from the Program of Education Department of Sichuan (No. 18ZB0429). Prof. Yongye Liang thanks the Shenzhen Key Lab funding (ZDSYS201505291525382) for financial support. The authors thank the Materials Characterization and Preparation Center in Southern University of Science and Technology (SUSTech) for the AFM and TEM measurements.


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