Impact of the Bonding Sites at the Inner or Outer Ï-Bridged Positions

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Impact of the Bonding Sites at the Inner or Outer #Bridged Positions for Non-fullerene Acceptors Shouli Ming, Cai'e Zhang, Pengcheng Jiang, Qinglin Jiang, Zaifei Ma, Jinsheng Song, and Zhishan Bo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02964 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Materials & Interfaces

Impact of the Bonding Sites at the Inner or Outer π-Bridged Positions for Non-fullerene Acceptors Shouli Ming,‡a Cai’e Zhang,‡a Pengcheng Jiang,a Qinglin Jiangd, Zaifei Mac, Jinsheng Songb,* and Zhishan Boa,* a

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of

Chemistry, Beijing Normal University, Beijing 100875, China. b Engineering

Research Center for Nanomaterials, Henan University, Kaifeng 475004,

China. c

Center for Advanced Low-dimension Materials, State Key Laboratory for

Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, P. R. China. d

College of Chemistry and Molecular Engineering, Qingdao University of Science &

Technology, Qingdao 266042, China. *Email: [email protected]; [email protected] ‡These

authors contributed equally to this work.

Keywords: non-fullerene acceptors, polymer solar cells, conjugated lateral chains, steric effect, and quantum calculation.

Abstract Two A-π-D-π-A type NFAs (IDT-ToFIC and IDT-TiFIC) with 5-hexylthienyl chain substituted at the inner and outer β-position of thiophene π-bridge have been

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designed, respectively. Impacts of varied positional modifications are systematically studied. Utilization of PBDB-T as the donor, polymer solar cells are constructed with these two molecules as the acceptors. Power conversion efficiencies (PCEs) of 11.09% and 9.46% are acquired for IDT-ToFIC and IDT-TiFIC based devices, respectively. Our studies have demonstrated that the use of thiophene spacers carrying one conjugated side chain at different positions can markedly enhance the photovoltaic properties in relative to the corresponding control molecule IDTT2F. Introduction Polymer solar cells (PSCs) possess typically lightweight, inexpensive, flexible and cost effective features, which guarantee it to be one alternative of great promise for solar energy utilization.1-7 Due to the drawbacks of fullerene derivatives, including fixed energy level, weak absorption, high cost etc, further development of its corresponding PSCs encounters the bottleneck. Significant progress of PSCs has been achieved after the emergence of non-fullerene acceptors (NFAs)8-13 and the up-to-date power conversion efficiencies (PCEs) reached above 14% by single junction PSC devices.14-17 As one typical category of NFAs, acceptor-donor-acceptor (A-D-A) type fused-ring electron acceptors (FREAs) have been intensively studied since 2015.18-29 A-D-A type molecules usually comprise a conjugated ladder core as the electron donor (D) in the center and two electron acceptor (A) units attached at both ends. Sometimes, the introduction of π-conjugated spacers between the D core and A terminals can tune the optoelectronic and film forming properties so as to further


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improve the photovoltaic performances of PSC devices.30-36 For example, Hou et. al. incorporated an alkoxythiophene as the π-spacer and the resulted acceptor IEICO exhibited a bathochromic absorption and stronger molecular interactions relative to the corresponding IEIC.30,31 Recently, we have inserted bis(alkoxy)-substituted benzenes or thiophenes as the π-spacers, the noncovalent S-O interaction could generate a planar molecular backbone. Such coplanar skeletons could enhance the electrostatic interaction and the intermolecular π-π stacking of terminal groups, resulting in an elevated electron transport ability.37,38 However, the most studied π-spacers do not carry or carry alkoxy and alkyl side chains, conjugated side chain is rarely used. Herein, two A-π-D-π-A type NFAs are designed and synthesized, which comprise π-spacers with one conjugated side chain. Indaceno[1,2-b:5,6-bʹ]dithiophene (IDT), 1,1-dicyanomethylene-5,6-difluoro-3-indanone (FIC) and 5-hexylthienyl substituted thiophene are adopted as the D core, A terminals and π-spacers, respectively. The chemical structures of these two acceptors (IDT-ToFIC and IDT-TiFIC) are shown in Figure 1. The only difference between these two acceptors is the substituted position of the side chain. The influence of the substituted position for the side chain is investigated in detail. The outer substituted IDT-ToFIC possesses a blue-shifting absorption spectrum and slightly higher LUMO value in comparison with the inner substituted IDT-TiFIC. When these acceptors are used to blend with PBDB-T (as shown in Figure 1) to fabricate PSCs, PCEs of 9.46% and 11.09% are acquired for devices based on IDT-TiFIC and IDT-ToFIC, respectively. Our studies have



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illustrated that incorporation of thiophene spacers accompanying with conjugated side chains into A-D-A type NFA can markedly enhance the device performances. Besides, the position of side chain attaching thiophene spacer also has impact on the photovoltaic properties. F

F

C6H13

C6H13

IDT-ToFIC R

O S

NC CN

S S

S

CN

S O

S

CN

O S

R outer position F

F

C6H13

C6H13

CN

F

F

S S

S

n

PBDB-T

S

IDT-TiFIC R

O

NC

C6H13

C6H13

O

S

S

S

S

CN

O

R inner position

CN

S

R= C6H13

C6H13

F

*

S

F

Figure 1. Molecular structures of the polymer donor and elelctron acceptors.

Results and Discussion Geometry Optimization by DFT Calculations


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Figure 2. (a) PES for the moieties (thiophene bridge unit and core) of IDT-ToFIC and IDT-TiFIC; (b) optimized conformations for the IDT-ToFIC and IDT-TiFIC: top, front and side views. Molecular configuration plays a great impact on the π-π stacking of the molecules in thin film, the blend morphology, the charge carrier mobility in the active layer, etc. Herein, the molecular configuration of the IDT-ToFIC and IDT-TiFIC are optimized via density functional theory (DFT) calculations at B3LYP/6-31G level. Considering the steric effect of lateral thienyl modification, the torsion angles of the bridged thiophene unit and the core (or the terminal unit) in both molecules is optimized. Firstly, conformational preference between the thiophene bridge unit and the core are investigated by relaxed potential energy scans (PES). The results are shown in Figure



2a with varied preferential configurations been observed. For simplicity, the target segment containing an FIC group, a thiophene bridge with an inner or outer lateral thienyl substituent and half of the IDT core is employed for this study. As shown in Figure 2a, the preferred configurations for both molecules are depicted and a super-planar conjugated backbone is observed for IDT-TiFIC with a perpendicular lateral thienyl chain (a dihedral angle of 85o). While, twisted terminal region is observed for the half moiety of IDT-ToFIC with torsion angles of 20o and 38o in relative to bridged thiophene for the FIC and lateral thienyl groups, respectively. Such observation is ascribed to the steric hindrance effects induced by the substituted thienyl unit. According to the acquired PES results, the optimal geometries of these IDT-TFIC acceptors are studied by the DFT calculation and the results are shown in Figure 2b, where the hexyl chains are substituted by methyl ones to facilitate the DFT prediction. In line with the PES results, the planar conformation is observed between the bridged thiophene and the core for IDT-ToFIC, but torsion angles of 19o and 43o are observed for the FIC and lateral thienyl groups in relative to bridged thiophene, respectively, due to the steric hindrance effect of the outer substituted thienyl groups. As expected, the optimized configuration for IDT-TiFIC possesses a super-planar structure for the whole skeleton as shown in Figure 2b. Such π-extended and planar ending skeleton may facilitate the favored terminal stacking and produce the bathochromic UV-vis absorption.


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Synthesis of the IDT-TFIC acceptors IDT-ToFIC and IDT-TiFIC were prepared according to the synthetic routes as depicted in Scheme 1. Beginning with 3-(5-hexylthien-2-yl)thiophene (compound 1), compounds 2 and 3 were obtained with different bromination methods. Via Stille coupling of compound 2 with IDT-diTin, the thiophene bridge with an outer lateral 5-hexylthienyl chain was attached to IDT core. Through the subsequent deprotonation with n-BuLi, the aldehyde precursor (IDT-To-CHO) was achieved by quenching with DMF in a yield of 63%. The inner thiophene bridge was prepared in four steps from compound 1 with satisfied yield. Compound 3 was obtained through the bromination reaction of Compound 1 in the presence of NBS and then the bromo substituent was converted to the TMS protecting group to furnish compound 4 after Br/Li exchange. The aldehyde group was incorporated using similar reaction conditions. Compound 6 was synthesized through the bromination of compound 5 with NBS in an 87% yield. Subsequently, via Stille coupling with IDT-diTin, the thiophene bridge with an inner lateral 5-hexylthienyl chain is attached to IDT core. Finally, through Knoevenagel condensation with FIC, the two aimed A-π-D-π-A typed acceptors (IDT-ToFIC and IDT-TiFIC) were afforded in 69% and 83% yields, respectively. Both IDT-ToFIC and IDT-TiFIC can be well dissolved in common organic solvents including dichloromethane, chloroform and chlorobenzene. The good solubility is favorable for the processing of organic solar cells.



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Scheme 1. Synthesis procedures for IDT-ToFIC and IDT-TiFIC. R

R

R CBr4 Br

S

2

40 oC

R

THF -78 oC

42%

S

CHCl3

1

R

n-BuLi

NBS

n-BuLi

Br

89%

n-BuLi

S

TMSCl TMS

S

3

87%

4

DMF

CHO

S

CHCl3

5

82%

C6H13

C6H13

C6H13

R S

S

THF -78 oC

R

R

2

OHC

63%

S

S

C6H13

S

CHO IDT-ToFIC CHCl3 50 oC

69%

R C6H13

C6H13

O

CN CN

IDT-To-CHO

Pd(PPh3)4 S

S

S

C6H13 IDT-To

C6H13

*

CHO

S

6

R DMF

n-BuLi

S

S

Br

87%

R= C6H13

R

NBS TMS

Br Toluene, Reflux 65%

C6H13

F

F

C6H13

FIC

C6H13

R R Sn

S S

Sn

+

Pd(PPh3)4 Br

S

6

CHO

OHC

S

S

Toluene, Reflux

84%

S

R

C6H13

CHO CHCl3 50 oC

IDT-TiFIC

83% C6H13

C6H13

S

C6H13

IDT-Ti-CHO

IDT-diTin

Photophysical and Electrochemical Properties As shown in Figure 3, the UV-vis absorption behaviors of IDT-ToFIC and IDT-TiFIC are depicted. The maximum absorption peaks (λmax), molar absorption coefficients and optical bandgaps (Eg,opt) of the two molecules are summarized in Table 1. Both molecules exhibit similar absorption curves in the chloroform solutions (Figure 3a) with the main absorption range from 550 nm to 820 nm, which are assigned to the intramolecular charge transfer. Remarkably, the substituent position of the lateral thienyl group at the thiophene bridge not only displays influence on the absorption maximum, but also impacts the molar absorption coefficient (ε). In chloroform solutions, the absorption maximum of IDT-ToFIC and IDT-TiFIC are 716 nm and 734 nm, respectively, and the non-planar IDT-ToFIC shows a higher ε of 1.68 × 105 M-1 cm-1, which is 1.3 times higher to its counterpart of IDT-TiFIC.


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Besides, the thin film absorption spectrum tends to be broader and red-shifted for each acceptor in relative to its solution. Red-shifts of 25 nm for IDT-ToFIC and 48 nm for IDT-TiFIC are observed in comparison with their corresponding solution absorption spectra, indicating these two acceptors could form J-aggregations in films. According to the formula: Eg,opt = 1240/λonset, where λonset is the film absorption onset, the Eg,opt of thin films are estimated to be 1.41 eV and 1.50 eV for IDT-TiFIC and IDT-ToFIC, respectively.

Figure 3. (a) UV-vis absorption properties of the NFAs in CHCl3 solutions; (b) solid state absorption specta; (c) CV results for the two NFAs in MeCN; (d) schematic diagram of energy levels .



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Table 1. Parameters obtained from optical and electrochemical measures of NFAs. Molecules

λmax/ nm

max,solution

/

λmax/ nm

(solution)

M-1 cm-1

(film)

716

1.68 × 105

741

734

105

782

IDT-ToFIC IDT-TiFIC a

aε

1.27 ×

solution molar absorption coefficient ;

bE g,opt/

b

eV

cE g,cv/

HOMO/ eV

LUMO/ eV

1.50

-5.55

-3.86

1.69

1.41

-5.57

-4.05

1.52

Eg,opt = 1240/λonset;

c

eV

Eg,cv = Eonset,ox -

Eonset,red. As shown in Figure 3c, the electrochemical properties of these two molecules are measured by cyclic voltammetry (CV). The HOMO levels of IDT-ToFIC and IDT-TiFIC are calculated to be -5.55 and -5.57 eV, respectively, on the basis of the formulas: EHOMO = -e(Eox - EFc/Fc+ + 4.80) eV; similarly, the LUMO levels are estimated to be -3.86 and -4.05 eV, respectively, according to ELUMO = -e(Ered - EFc/Fc+ + 4.80) eV. Since the Voc of the PSC devices could be influenced by the offset between the LUMO energy level of the acceptor and the HOMO energy level of the donor, the higher LUMO energy level for IDT-ToFIC is supposed to achieve an improved Voc.

Figure 4. HOMO and LUMO electron distributions for IDT-ToFIC and IDT-TiFIC. In order to facilitate the understanding of the electron distributions for IDT-ToFIC


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and IDT-TiFIC, the visualized frontier molecular orbitals (HOMO and LUMO) are drawn in Figure 4. Both molecules display rather resemble HOMO wave-functions along the conjugated backbones even with varied modification sites. While in the excited LUMO states, electrons tend to transfer to the terminal acceptor units, indicating the lateral thienyl substituent at either inner or outer position will not interferes with the main chain conjugation. However, varied electron distributions for the lateral thienyl groups are observed at the LUMOs and the perpendicular inner substituted thienyl unit cut off its conjugation to the backbone for IDT-TiFIC, while the contribution still could be detected for the outer substituted groups in IDT-ToFIC. Photovoltaic Performances Inverted PSCs device with structure of Ag/MoO3/active layer/ZnO/ITO was adopted to estimate the photovoltaic performance of IDT-ToFIC and IDT-TiFIC. The famous polymer PBDB-T was employed as the electron donating component in the light harvesting layer. The detailed PSCs preparation procedures are included in the Supporting Information. The external quantum efficiency (EQE) and current density-voltage (J-V) curves of the corresponding PSCs based on IDT-ToFIC and IDT-TiFIC are shown in Figure 5. The optimized weight ratios in between PBDB-T and acceptor are 1:1 for both IDT-TFIC molecules. The as-cast PSCs employing IDT-ToFIC as the acceptor displayed a PCE of 9.53% (Jsc = 16.76 mA cm−2, Voc = 0.88 V and FF = 65%). While the corresponding device for IDT-TiFIC presented a slightly decreased Jsc of 16.05 mA cm−2 and a lower FF of 59%, leading to a lowered PCE of 8.22%. After the processing solvent additive DIO was utilized for the light



harvesting layer, the PCE of IDT-ToFIC based PSCs was improved to 11.09%, due to the corresponding increased Jsc and FF. Via utilization of a higher DIO dosage of 2.5% (v/v), improved photovoltaic performance for IDT-TiFIC is also achieved with a PCE of 9.46%, which may benefit from the properly tuned morphology of the blend film (vide infra). Clearly, such Jsc enhancement for IDT-ToFIC might be ascribed to its higher molar absorption coefficients as mentioned above. Because of the steric effects from the lateral thienyl groups, twisted terminal region was favorable conformation for IDT-ToFIC. However, such steric hindrance would confine the terminal groups to a uniform configuration, which may beneficial to the terminal stacking and the charge transport. In addition, the non-planar conjugated backbones for IDT-ToFIC would take part in the morphology control and suppress the intramolecular aggregation when it blended with the donor polymer, which is helpful to form appropriate crystalline domains in blend film and beneficial to exciton diffusion, separation and improvement of Jsc and FF. Interestingly, the incorporation of lateral 5-hexylthienyl side chain either at the outer or the inner position can markedly improve the photovoltaic performance in relative to the control acceptor IDTT2F, which shows a PCE of 8.85%.39 More specifically, the elevation of the photovoltaic performance for the IDT-TFIC acceptors are attributed to their improved Voc and FF, which are directly correlated with the molecular skeleton change and the more balanced μe/μh. The EQE spectra of the PSCs utilizing these two molecules as acceptors are drawn in Figure 5b. The blend of PBDB-T:IDT-ToFIC exhibits a photocurrent response in the region of 300 nm to 860 nm; whereas, for


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IDT-TiFIC, the response edge at the long wavelength region is extended to 890 nm. Additionally, PBDB-T:IDT-ToFIC blend presents enhanced photocurrent responses in the region of 400-800 nm in relative to IDT-TiFIC no matter without or with DIO additive, which is in accordance with the diversity of their molar extinction coefficients. Meanwhile, the integrated Jsc data based on the EQE measurements are well agreed with those acquired in the J-V characterization as shown in Table 2 and the deviations are within 5%. Although these two acceptors possess different LUMO energy levels (vide supra), their devices exhibit almost identical Voc. In order to explain this phenomenon, Fourier transform photocurrent spectroscopy (FTPS) measurements are performed. Optimized devices based on the two NFAs show more or less similar FTPS spectra (Figure 5c), indicating that these two devices have identical charge transfer state energy (ECT). Since Voc is considered to be proportional to ECT according to the following empirical equation: ECT - qVoc ~ 0.5-0.6 eV,40 obtaining identical Voc for these two devices is reasonable. In addition, the external quantum efficiencies for electroluminescence (EQEEL) of devices based on IDT-ToFIC and IDT-TiFIC are also measured to be 1.9×10-3 % and 2.5×10-3 %, respectively. Consequently, the non-radiative voltage losses (∆V𝑛𝑟) are calculated to be 0.270 eV and 0.265 eV for IDT-ToFIC and IDT-TiFIC, respectively, according to the equation of ∆V𝑛𝑟 = 1

𝑘𝑇 𝑞

𝑙𝑛

, where k, T and q are the Boltzmann constant, temperature and elementary

𝐸𝑄𝐸𝐸𝐿

charge, respectively. Such low non-radiative voltage losses are impressive for PSCs.



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Figure 5. Curves of (a) J-V, (b) EQE and (c) FTPS to the devices based on IDT-ToFIC and IDT-TiFIC when blended with PBDB-T.

Table 2. Detailed photovoltaic data of the PSCs at the optimized conditions. DIO

Voc

(v %)

(V)

PBDB-T:IDT-ToFIC

--

0.88

16.76

16.47

65%

PBDB-T:IDT-ToFIC

0.3%

0.88

17.79

17.27

71%

PBDB-T:IDT-TiFIC

--

0.88

16.05

15.21

59%

PBDB-T:IDT-TiFIC

2.5%

0.86

16.97

16.34

65%

PBDB-T:IDTT2F39

--

0.81

18.51

17.66

59%

Active layer

aIntegrated

Jsc

Jcalc a

(mA/cm ) 2

(mA/cm ) 2

FF

PCE

μh

(%)

cm V s

9.53 (9.40)b 11.09 (10.84)b 8.22 (8.13)b 9.46 (9.37)b 8.85

μe

μe/μh

-1

cm V-1 s-1

1.94×10-4

2.45×10-4

1.26

2.83×10-4

3.88×10-4

1.37

9.71×10-5

1.41×10-4

1.45

1.34×10-4

2.10×10-4

1.57

1.03×10-5

3.10×10-6

0.30

2

-1

2

data from EQE curves. bAverage values obtained from 10 separated

devices. Hole and Electron Mobilities High charge mobility and balanced hole (μh)/electron (μe) ratio are the premise to achieve excellent performance PSCs. Herein, the space-charge limited current (SCLC) method is applied to evaluate the charge transporting characteristics for these two NFAs in the active layers. The results are as presented in Figure S1 and Table 2. The electron mobilities for IDT-ToFIC and IDT-TiFIC based active layers prepared at optimized conditions are 3.88×10-4 and 2.10×10-4 cm2 V-1 s-1, respectively. And, the hole mobilities for IDT-ToFIC and IDT-TiFIC based active layers are 2.83×10-4 and


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1.34×10-4 cm2 V-1 s-1, respectively. As seen, slightly better charge transport behavior is obtained for IDT-ToFIC with a relative balanced μe/μh value of 1.37. The results may account for the increased Jsc and FF of IDT-ToFIC based PSCs. Morphologies The morphology of blend film is close related to the exciton separation and transport as well, which would further affect the photovoltaic performance of PSCs. In order to explore the comprehensive morphology information of the active layer, 2D grazing-incidence wide-angle X-ray scattering (GIWAXS), atomic force microscope (AFM) and transmission electron microscope (TEM) are employed in this study. From the GIWAXS results as shown in Figure S2, we can observe broad diffraction peaks (010) both at in-plane (IP) and out-of-plane (OOP) directions in these NFAs pure films, from which the π-π stacking distances are estimated to be 4.13 Å and 4.10 Å for IDT-ToFIC and IDT-TiFIC, respectively. Besides, no preferred orientation can be detected. After blending these NFAs with PBDB-T, one clear (100) peak (q = 0.30 Å-1) in the IP direction and one (010) peak (q = 1.75 Å-1) in the OOP direction can be detected, demonstrating that the backbones of PBDB-T are inclined to generate face-on stacking style in contrast to the substrate. For the as-cast active layers of IDT-ToFIC and IDT-TiFIC, the root mean square (RMS) roughness values are calculated to be 1.28 nm and 1.56 nm for IDT-ToFIC and IDT-TiFIC, respectively. Enhanced RMS values are detected in the active layer accompany with the addition of DIO. As shown in the TEM images of all blend films in Figure 6, we can observe fiber-like structures all over the substrate, which is in favor of exciton dissociation



and charge transport. Thicker fibrils are obtained when DIO is utilized, agreeing well with the AFM results.

Figure 6. AFM images of IDT-ToFIC based blend film (a) without DIO and (b) with 0.3% DIO; IDT-ToFIC based blend film (c) without DIO and (d) with 2.5% DIO; and TEM images of IDT-ToFIC based blend film (e) without DIO and (f) with 0.3% DIO; IDT-TiFIC based blend film (g) without DIO and (h) with 2.5% DIO. Conclusion In conclusion, two NFAs IDT-ToFIC and IDT-TiFIC with lateral 5-hexylthienyl chain substituted at the inner and outer β-positions of the thiophene π-spacer are synthesized to study the structure-property relationships. Theoretical optimizations are adopted to predict molecular conformations of these two acceptors. The positions of lateral substituents (inner or outer) can influence molecular configurations, absorption properties, energy levels, molecular aggregation, etc. Devices based on IDT-ToFIC and IDT-TiFIC exhibit higher PCEs of 11.09% and 9.46%, respectively. Compared with the corresponding non-substituted acceptor, the incorporation of


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lateral 5-hexylthienyl side chain either at the outer or inner position can markedly improve the photovoltaic performance. These studies demonstrate that the insertion of π-spacer carrying one conjugated side chain between the D and A unit is a feasible method to enhance the photovoltaic properties of NFAs.

Supporting Information Synthetic procedures, NMR spectra, HRMS data, cyclic voltammetry procedure, details for the device optimization, charge transfer mobility, and GIWAXS results etc.

Acknowledgments This work was supported by the National Natural Science Foundation of China (U1704137, 21404031 and 21574013), Program for Changjiang Scholars and Innovative Research Team in University and Program sponsored by Henan Province (2016GGJS-021 and 19zx014).

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