Synthesis of low band-gap 2D conjugated polymers and their application for organic field effect transistors and solar cells

Accepted Manuscript Synthesis of low band-gap 2D conjugated polymers and their application for organic field effect transistors and solar cells Fuchuan Liu, Hang Wang, Yangqian Zhang, Xin Wang, Shiming Zhang PII:

S1566-1199(18)30497-X

DOI:

10.1016/j.orgel.2018.09.032

Reference:

ORGELE 4898

To appear in:

Organic Electronics

Received Date: 18 May 2018 Revised Date:

25 July 2018

Accepted Date: 25 September 2018

Please cite this article as: Fuchuan Liu, Hang Wang, Yangqian Zhang, Xin Wang, Shiming Zhang, Synthesis of low band-gap 2D conjugated polymers and their application for organic field effect transistors and solar cells, Organic Electronics (2018), doi: 10.1016/j.orgel.2018.09.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

Synthesis of low band-gap 2D conjugated polymers and their application

2

for organic field effect transistors and solar cells

3

Fuchuan Liu1, Hang Wang1, Yangqian Zhang1, Xin Wang2, Shiming Zhang1*

4

1 Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

5

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech

6

University (Nanjing Tech), 30 South Puzhu Road, Nanjing 211816, China; [email protected]

7

(F.L.); [email protected] (H.W.); [email protected] (Y.Z.)

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2 Zhejiang Faguang Rubber Seal Parts Co., Ltd. Louao industrial park, Xinhe Town, Wenling,

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Zhejiang, China; [email protected]

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* Corresponding author. Key Laboratory of Flexible Electronics (KLOFE) & Institute of

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Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced

12

Materials (SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing

13

211816, China.

14

* Corresponding author. E-mail addresses: [email protected] (S. Zhang).

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Abstract

16

Two donor-acceptor (D-A) 2-dimensional (2D) conjugated polymers P1 and P2 based

17

on

18

4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b´]dithiophene

19

donor unit were prepared via Stille coupling and characterized by solution-possessed

20

organic field effect transistors (OFETs) and organic solar cells (OSCs), respectively.

21

By changing the electron-withdrawing component of the conjugated polymer

22

backbone from ID to TID, there have also been diversities in the optical absorption,

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isoindigo

(ID)

and

thienoisoindigo

1

(TID)

as

acceptor

unit

(BDT-T)

and as

ACCEPTED MANUSCRIPT thermal stability, molecular structure, electrochemical energy level, charge mobility

24

and photovoltaic properties of these two polymers. P1 and P2 exhibited intrinsic

25

p-type semiconductor characteristic with hole mobilities of 3.0×10-2 and 1.2×10-2 cm2

26

V-1 s-1, respectively. When blended with the [6,6]-phenyl-C61-butyric acid methyl

27

ester (PC61BM), the highest power conversion efficiency (PCE) of P1 and P2 were

28

2.40% and 1.28%, respectively. Our results suggest that ID and TID units are useful

29

building blocks for the further development of efficient organic optical-electrical

30

materials.

31

Keywords: isoindigo, thienoisoindigo, benzodithiophene, D-A conjugated polymers,

32

photoelectric properties

33

1. Introduction

34

In recent years, π-conjugated polymers based on the donor (D) and acceptor (A)

35

alternating structure are widely utilized for organic semiconductor devices, such as

36

organic light emitting diodes (OLEDs) [1-4], organic field effect transistors (OFETs)

37

[5-7] and organic solar cells (OSCs) [8-11] due to the low cost, light weight,

38

flexibility property and so on. Low band-gap D-A conjugated polymers play an

39

important role in organic optoelectronic materials among conjugated polymers for

40

organic photoelectric applications [12-14]. To date, a variety of D-A conjugated

41

polymers have been synthesized by traditional polymerization method, such as Stille

42

and Suzuki coupling polymerization [15, 16]. Recently, many novel D-A conjugated

43

polymers have obtained outstanding OFETs performance for fast charge mobility and

44

high power conversion efficiency (PCE) in bulk heterojunction (BHJ) OSCs devices

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ACCEPTED MANUSCRIPT [17-19]. One of the progresses is the incorporation of strong electron-deficient units

46

such as isoindigo (ID) and thienoisoindigo (TID) into polymer backbones because of

47

the lactam group for improving material properties [20-23]. With powerful

48

electron-withdrawing property and excellent absorption nature, ID has turned into an

49

significant acceptor unit for the synthesis of conjugated small molecules and polymers

50

since it had been first employed by Reynolds and coworkers as building block for

51

conjugated polymers, which exhibited a high PCE of 1.76% in OSCs [24]. However,

52

because of the steric repulsion between the hydrogens on the phenyl rings and the

53

oxygens of the lactams, ID molecular possesses the slightly twisted construction [25].

54

To improve the molecular planarity, the outer phenyl rings could be replaced with

55

other units. And also by changing the benzene ring into thiophene ring, TID can be

56

prepared through sample organic synthesis. The TID structure which served as an

57

acceptor unit in D-A conjugated polymers was first reported by McCulloch and

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coworkers in 2012, and the polymer showed an ambipolar property with high hole and

59

electron mobilities over 0.1 cm2 V-1 s-1 [26]. Up to now, TID moiety has been paid

60

more attentions as an excellent acceptor building block in low bandgap small

61

molecules and polymers [27-30]. The high planarity and charge delocalization due to

62

the S-O interactions and quinoidal structure of the backbone can lead to ordered

63

molecular stacking and efficient charge transport [31-33]. Because of these

64

advantages, the TID-based low bandgap polymer has realized a high hole mobility

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above 14 cm2 V-1 s-1, indicating the infinite potential of this type polymers for organic

66

optoelectrical applications [34].

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ACCEPTED MANUSCRIPT Most of the D-A conjugated polymers based on ID or TID acceptor unit show broad

68

absorption in UV-vis and near-infrared region, so they possess suitable absorption

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property with the solar radiation for harvesting more photons [31, 35]. However, the

70

distinction of donor units can also deeply affect the optical, thermal and

71

electrochemical properties of D-A conjugated polymers. The benzodithiophene (BDT)

72

molecule has a planar structure with ternary fused rings, which is beneficial for

73

electron delocalization within the molecular skeleton and enhances π-π interactions

74

between adjacent chains [36]. Lately, it has been found that the electrochemical

75

energy levels of BDT-based conjugated polymers can be fine controlled by different

76

substituents on the BDT units [37]. Compared with the polymers based on the

77

alkoxy-substituted BDT (BDT-O), the thienyl substituents on the BDT units (BDT-T)

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provide additional conjugation and more planar structure. The polymers based on

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BDT-T can display the improved optical absorption, lowered energy level and

80

enhanced charge motilities, which indicating BDT-T has been a promising building

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block for constructing high performance D-A conjugated polymers in organic

82

optoelectronic applications [38-41].

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Herein, two low band-gap D-A conjugated polymers (see the structure in Fig.1) with

84

alternating ID or TID acceptor unit and BDT-T donor compound have been designed

85

and synthesized for the application in organic electronic devices. Due to the structural

86

differentiation of the acceptor units, there are distinctions in the optical absorption,

87

thermal stability, molecular structure, electrochemical energy level, charge mobility

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and photovoltaic properties of these two polymers, indicating the change of acceptor

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4

ACCEPTED MANUSCRIPT units has a significant effect on the properties of conjugated polymer materials.

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OFETs performance showed these two polymers were intrinsic p-type semiconductors

91

with hole mobilities of 3.0×10-2 and 1.2×10-2 cm2 V-1 s-1, respectively. And OSCs

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fabricated based on P1 and PC61BM showed a higher efficiency of 2.40%, while

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OSCs from P2 with PC61BM obtained a modest PCE of 1.28%. It shows the structure

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of acceptor units has an effect on the photovoltaic property of the D-A conjugated

95

polymers, which can really supply a significant reference to obtain the highly efficient

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D-A conjugated polymer materials.

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R

R

R

S

N

O

N

S

S

S

S

O

N R

S R

S

S

n

S O

S

O

n

S

N

R

S R

P1

97 98

P2

Fig. 1. Molecular structures of conjugated polymers P1 and P2

2. Experimental section

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2.1. Materials and methods

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All reagents and starting materials were purchased from commercial sources and used

102

without

103

sodium/benzophenone immediately prior to use.

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2.2. General characterization methods

105

All nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AMX300

106

spectrometer and 1H NMR and

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further

purification.

Anhydrous

13

solvents

were

distilled

from

C NMR spectra were recorded in CDCl3 or 5

ACCEPTED MANUSCRIPT d6-DMSO at 303 K. Chemical shifts were reported in δ scale downfield from the peak

108

for tetramethylsilane (TMS). All chemical shifts were quoted in ppm, using the

109

residual solvent peak as a reference standard. High resolution mass spectra (HRMS)

110

were measured using a Solarix FF-ICR-MS Analyzer in the MALDI mode. Gel

111

permeation chromatography (GPC) was conducted by the PL-GPC 220 instrument

112

with dichlorobenzene (DCB) as eluent against polystyrene standards. UV-vis

113

absorption spectra were recorded on Shimadzu UV-1750 in high performance liquid

114

chromatography (HPLC) grade solvents or quartz plates. Cyclic voltammetry (CV)

115

was carried out on a CHI 660E electrochemical analyzer with a three-electrode cell in

116

a solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the

117

electrolyte and dissolved in anhydrous CH3CN at a scan rate of 0.1 V s−1. A platinum

118

carbon electrode, a platinum wire and an Ag/AgCl electrode were utilized as the

119

working electrode, the counter electrode and the reference electrode, respectively. The

120

potential was scanned from -2.0 to 2.0 V and was calibrated against the

121

ferrocene/ferrocenium (Fc/Fc+) internal reference. Thermogravimetric analysis (TGA)

122

was carried out on a TGA2 instrument at a heating rate of 10 °C min−1 under N2 flow,

123

and differential scanning calorimetry (DSC) was performed on a DSC 214 Polyma

124

instrument at a heating/cooling rate of 10 °C min−1 in N2 environment. Atomic force

125

microscopy (AFM) measurements are performed by using a Dimension Icon Scanning

126

Probe Microscope (Asylum Research, MFP-3D-Stand Alone) in tapping mode.

127

2.3. Synthesis of the monomers

128

The synthetic routes for monomers were shown in Scheme 1. and the detailed

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ACCEPTED MANUSCRIPT 129

synthesis steps were generally based on the previous literature [30, 42]. H H Br

O

N H

N +

O

Br

AcOH, HCl

N O

120

Br

Br

, reflux, 24 h

O

O H

1a H

RI PT

HE O

N

O

N

Br

K2CO3, DMF

Br +

100

Br

Br

Br

, 16 h

O

O

H

N

EH

S NH2

+

Cu, CuI, K3PO4 DMAE, 85

O

S

O

Et3N, DCM

+

, 48h

Cl

Cl

N H

Br HE

0

O

S

+ N

toluene

Lawesson reagent

EH

2b EH

O

65

O

, 6h

S

+

0

S

to r.t , 8h

N

O

EH

THF

NBS

O

N

HE

N

O

S

to rt, 12h

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O S

S O

N

EH

EH=

2c

130

EH

2d

Scheme 1. Synthetic routes to the monomers ID and TID.

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Br

2.4. Synthesis of the polymers

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The synthetic route for polymers was shown in Scheme 2. Polymerization steps for P1

134

and P2 were carried out through the traditional palladium-catalyzed Stille

135

cross-coupling reactions.

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7

ACCEPTED MANUSCRIPT EH HE

HE

S O

N

O

N

S

Pd(PPh3)4, toluene

Br

Br

110

, 72 h

S

EH

EH

EH

S HE

S

1b

S Sn

n

S N

O

N

O

Sn

P1

+

S EH

HE

N

O

N

HE

S

S

Br Pd(PPh3)4, toluene

Br

S

BDT-T

RI PT

EH

S

110 O

, 72 h

EH

O

S

S

S

O

N

S

n

S

N

EH

S

HE

EH=

2d

SC

136

P2

137

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2.4.1. Synthesis of P1

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Compound 1b (128.8 mg, 0.2 mmol) and BDT-T (180.9 mg, 0.2 mmol) were added

140

into a 25 mL Shrek tube with 10 mL of anhydrous toluene, after nitrogen bubble for

141

0.5 h, Pd(PPh3)4 (11.6 mg, 0.01 mmol) was added under the oxygen-free environment,

142

then the reaction mixture was heated to 110 ℃ and stirred for 72 h under the nitrogen

143

atmosphere, and then an excess amount of bromobenzene and trimethyl(thienyl)tin

144

were added to end-cap the trimethylstannyl and bromo groups for 8 h, respectively.

145

Stopped stirring and cooled down to room temperature, the reaction mixture was

146

dropped into 200 mL of methanol solution and stirred for 2 h, stopped and filtered.

147

The precipitate was collected and purified by the Soxhlet extraction with methanol

148

and acetone, followed by chloroform for 24 h, respectively. The chloroform fraction

149

was concentrated under reduced pressure and then poured into methanol solution. The

150

precipitate was collected and dried in vacuum to yield a black solid (164.4 mg,

151

77.5%). GPC (DCB at 120 ℃): Mn = 23.3 kg mol-1，Mw = 32.2 kg mol-1，PDI = 1.38,

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1

153

2H), 7.09 (d, 2H), 6.92 (d, 2H), 3.73 (d, 8H), 2.98 (m, 4H), 1.37 (m, 32H), 0.92 (t,

154

24H).

155

2.4.2. Synthesis of P2

156

Compound 2d (249.5 mg, 0.38 mmol) and BDT-T (343.7 mg, 0.38 mmol) were

157

added into a 25 mL Shrek tube with 10 mL of anhydrous toluene, after nitrogen

158

bubble for 0.5 h, Pd(PPh3)4 (21.9 mg, 0.019 mmol) was added under the oxygen-free

159

environment, then the reaction mixture was heated to 110 ℃ and stirred for 72 h under

160

the nitrogen atmosphere, and then an excess amount of bromobenzene and

161

trimethyl(thienyl)tin were added to end-cap the trimethylstannyl and bromo groups

162

for 8 h, respectively. Stopped stirring and cooled down to room temperature, then the

163

reaction mixture was dropped into 200 mL of methanol solution and stirred for 2 h,

164

stopped and filtered. The precipitate was collected and purified by the Soxhlet

165

extraction with methanol and acetone, followed by chloroform for 24 h, respectively.

166

The chloroform fraction was concentrated under reduced pressure and then poured

167

into methanol solution. The precipitate was collected and dried in vacuum to yield a

168

black solid (364.9 mg, 89.5%). GPC (DCB at 120 ℃): Mn = 18.0 kg mol-1, Mw = 35.4

169

kg mol-1, PDI = 1.96, 1H NMR (300MHz, CDCl3): δ (ppm) 7.49 (s, 2H), 7.22 (s, 2H),

170

7.06 (d, 2H), 6.92 (d, 2H), 3.69 (d, 8H), 2.98 (m, 4H), 1.42 (m, 32H), 0.98 (t, 24H).

171

Table 1

172

Molecular weight and thermal property of P1 and P2

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H NMR (300MHz, CDCl3): δ (ppm) 9.04 (s, 2H), 7.52 (d, 2H), 7.42 (d, 2H), 7.23 (s,

Polymer

Mna (kDa)

Mwa (kDa) 9

PDIa

Tdb (℃)

ACCEPTED MANUSCRIPT P1

23.3

32.2

1.38

340

P2

18.0

35.4

1.96

429

a

Determined by GPC at 120 ℃.

174

b

Measured by TGA.

175

2.5. Fabrication and Characterization of OFETs

176

Top-gate/bottom-contact (TG/BC) OFETs were used for device fabrication and

177

characterization. Source-drain electrodes (3 nm Cr and 30 nm Au) were patterned on

178

borosilicate glass by photolithography, with a channel length of 10, 20, 50, or 100 µm

179

and a channel width of 5 mm. The substrates were cleaned by sonication in acetone

180

and isopropanol for 10 min, respectively, followed by UV-ozone treatment for 1 h.

181

The polymer active layers were spin coated from hot chlorobenzene solutions (5 mg

182

mL-1, 100 °C), and then they were thermally annealed at 200 °C for 20 min. The

183

CYTOP dielectric layer was spin-coated (1500 rpm, 60 s) onto the semiconductor

184

from diluted solution (CTL-809M:CTSOLV180 = 2:1, volume ratio), then annealed at

185

100 °C on a hot plate for 20 min. The thickness of the CYTOP layer was about 380

186

nm and areal capacitance was of 4.54 nF cm-2 used for mobility calculation. Finally,

187

50 nm Al was evaporated on top as the gate electrode. The OFETs characterization

188

was carried out in a N2-filled glove box with Keithley S4200 semiconductor analyzer.

189

To calculate the linear mobility, the standard equation Isd = µ lin Ci (W/L) (Vg-VT) Vsd

190

was used, wherein Isd was the source drain current, µ lin was the linear mobility, Ci was

191

the dielectric capacitance per unit area, W was the channel width, L was the channel

192

length, Vg was the gate voltage, VT was the threshold voltage, and Vsd was the source

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10

ACCEPTED MANUSCRIPT drain voltage (Vsd = 10 V for linear regime). Similarly, Isd = µ sat Ci (W/2L) (Vg−VT)2

194

was used for the saturation field effect mobility extraction from Vsd = 80 V curve for

195

saturation regime, wherein µ sat was the saturation mobility.

196

2.6. Fabrication and Characterization of OSCs

197

BHJ OSCs were fabricated on ITO glass substrates with an inverted configuration of

198

ITO/ZnO/Polymer:PC61BM/MoO3/Ag. The ITO glass substrates were cleaned by

199

detergent first, and then sonification in deionized water, acetone and isopropyl alcohol,

200

respectively. After dried in a nitrogen stream, followed by an UV-ozone treatment.

201

Then an electron transport thin layer (40 nm) of ZnO, which new-prepared by the zinc

202

acetate with ethanol and ethanolamine mixed solution and stirred overnight was

203

spin-cast on the substrates at 4500 rpm and annealed at 200 ℃ for 30 min on a hot

204

plate [43, 44]. The active layer (100 nm) of Polymer:PC61BM was deposited by

205

spin-casting chlorobenzene solution at 2000 rpm for 30 s in nitrogen box.

206

Subsequently, the hole transport thin layer (8 nm) of MoO3 and silver cathode (80 nm)

207

were thermally evaporated onto the active layer through a shadow mask under a

208

pressure (<10-5 Pa). The effective device area was defined as 7 mm2. The current

209

density-voltage (J-V) characteristics were measured using a Keithley 2400 source

210

meter under a simulated AM 1.5G, 100 mW cm-2 conditions.

211

3. Results and discussion

212

3.1. Synthesis and characterization

213

The synthetic routes of the monomers (1b and 2d) and polymers (P1 and P2) were

214

described in Scheme 1 and Scheme 2. The synthesis of the monomers was generally

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ACCEPTED MANUSCRIPT based on the previous literature [30, 42]. Two polymers P1 and P2 were synthesized

216

by traditional palladium-catalyzed Stille coupling polymerization of BDT-T with 1b

217

or 2d. Both polymers showed good solubility in common organic solvents, such as

218

chloroform (CF), chlorobenzene (CB) and dichlorobenzene (DCB) due to the side

219

chains in the donor and acceptor repeat units. The number-average molecular weight

220

(Mn), weight-average molecular weight (Mw) and polydispersity index (PDI) were

221

measured by GPC using DCB as the eluant and polystyrenes as the internal standards,

222

and the results were listed in Table 1. The Mn of P1 and P2 were 23.4 kDa and 18.1

223

kDa, respectively, and the structures of two polymers were confirmed by 1H NMR

224

spectroscopy.

225

3.2. Thermal properties

226

Thermal stability was one of the important factors for the practical application of

227

organic optoelectronic materials. The thermal properties of P1 and P2 were measured

228

by TGA in N2 flow with a heating rate of 10 ℃ min-1. The decomposition temperature

229

(Td, corresponding to a 5% weight loss) located at 340 ℃ and 429 ℃ for P1 and P2,

230

respectively, indicating that these two polymers possessed favorable thermal stability

231

for the application of organic semiconductor devices, and the higher Td of P2 might

232

be owing to the better chain rigidity of the polymer backbone [41]. Phase transition

233

temperatures and enthalpies of P1 and P2 were investigated using DSC in N2

234

environment with a scanning rate of 10 ℃ min-1. From the DSC results of P1 and P2,

235

there were no obvious endothermic or exothermic phenomena occurred in the heating

236

and cooling process, indicating these two polymers did not appear phase transition

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ACCEPTED MANUSCRIPT 237

phenomenon with varying temperature [13]. The TGA and DSC curves of P1 and P2

238

were showed in Fig. 2 and the thermal performance data were summarized in Table 1.

100

(a)

P1 P2

0.6 (b)

P1 P2

0.4 90

70 60 50

0.0 -0.2 -0.4 -0.6 -0.8

40

-1.0 100

400

500

600

0

Temperature ( )

50

100

150

200

250

300

350

400

Temperature ( )

SC

240

300

Fig. 2. TGA (a) and DSC (b) curves of P1 and P2

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-1

Heat flow /Wg

Weight /%

0.2 80

3.3. Optical properties

242

The UV-Vis absorption spectra of P1 and P2 were conducted in dilute chloroform

243

solution and spin-coated thin film, respectively. The spectra were shown in Fig. 3 and

244

the relevant data were summarized in Table 2. Two broad absorption bands in the high

245

and low energy region were obtained for both solution and thin film of P1 and P2,

246

which the short-wavelength absorption bands between 300 and 500 nm were derived

247

from the π-π* transition of the donor units and the long-wavelength absorption bands

248

in the near-infrared region were attributed to the intra-molecular charge transfer (ICT)

249

from the donor to the acceptor units of the polymer backbone [45]. When compared

250

with P1, the absorption region and maximum absorption wavelength of P2 were

251

broader and larger obviously, which was due to the planar molecular structure and

252

strong electron-withdrawing ability of TID, resulting in the powerful intermolecular

253

and intramolecular interactions, fast charge transport and red-shifted absorption peak

254

[41]. On the other hand, from Fig. 3 (b), we could know the thin film absorption onset

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13

ACCEPTED MANUSCRIPT 255

(λonset) of P1 was 758 nm and of P2 was 1019 nm, respectively. Thus, the optical

256

band-gap of P2 was far less than P1 according to the equation of


257

[10, 46]. The polymers on spin-coated thin films showed broad absorption region and

258

displayed slightly red-shifted compared with their solution absorption spectrum,

259

indicating that some interchain interactions occurred in the solid state of the polymers,

260

which was benefited for the molecular arrangement and charge transport [47, 48]. The

261

optical band-gap of polymers was calculated from the absorption edge of thin-films

262

and summarized in Table 2. The results demonstrated that P2 displayed broader

263

absorption and narrower optical band-gap than P1.

SC

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(a)

0.4

0.2

400

600

800

1000

0.8

0.6

0.4

0.2

0.0 400

600

800

1000

Wavelength (nm)

EP

Wavelength (nm)

Normalized Absorption

0.6

P1 P2

1.0

TE D

Normalized Absorption

0.8

0.0

Fig. 3. UV-Vis absorption spectra of P1 and P2 in chloroform solution (a) and thin film (b)

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(b)

P1 P2

1.0

265

= 1240/λonset

RI PT



267

3.4. Electrochemical properties

268

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular

269

orbital (LUMO) energy levels of polymers were important parameters which affected

270

the performances of organic semiconductor devices. The electrochemical properties of

271

P1 and P2 were investigated by cyclic voltammetry (CV) while the polymers were

272

dropped as films on the working electrode, respectively. Electrochemical 14

ACCEPTED MANUSCRIPT measurement was conducted in anhydrous CH3CN with 0.1 M Bu4NPF6 as the

274

supporting electrolyte, a platinum carbon electrode, a platinum wire, and an Ag/AgCl

275

electrode used as the working electrode, the counter electrode and the reference

276

electrode, respectively, with a scan rate at 100 mV s-1. The CV curves were calibrated

277

by the ferrocene-ferrocenium (Fc/Fc+) redox couple (4.80 eV below the vacuum level).

278

The HOMO and LUMO energy levels of polymers were calculated from the formula

279



 EHOMO = - (4.80 +
  ) eV and ELUMO = - (4.80 +
  ) eV, where the onset

280



 oxidation potentials (
  ) and onset reduction potentials (
  ) were estimated

281

from the cyclic voltammogram and the corresponding
  were calculated from the

282



 equation
  = (
  -
  ) eV [49]. The result showed that these two polymers

283

possessed obviously reversible oxidation and reduction waves, suggesting that they

284

possessed stronger electron transport capacity and were intrinsic semiconductors [50,

285

51]. Moreover, from the CV results, P1 possessed relatively higher oxidation and

286

reduction potential that meant lower HOMO and LUMO energy levels and larger

287

energy band-gap, which were beneficial to prevent from the oxidation reactions under

288

ambient conditions. The lower HOMO and LUMO energy levels were in an ideal

289

range to ensure good air stability of the devices. The HOMO and LUMO energy

290

levels of P1 and P2 were -5.52/-3.92 eV and -5.28/-3.72 eV, respectively. When

291

compared with P1, P2 showed higher HOMO and LUMO energy levels, which were

292

ascribed to the planar quinoidal structure of TID increasing the charge delocalization

293

[52]. The CV curves were showed in Fig. 4, and the electrochemical data were

294

summarized in Table 2.

AC C

EP

TE D

M AN U

SC

RI PT

273

15

ACCEPTED MANUSCRIPT

RI PT

Current (norm.)

P1

0

M AN U

-2

SC

P2

2

Potential (V)

295 296

Fig. 4. Cyclic voltammetry curves of P1 and P2 Table 2

298

Photophysical and electrochemical properties of P1 and P2

TE D

297

P1

EHOMO

ELUMO

(nm)

(eV)a

(eV)b

(eV)c

631

758

1.64

-5.52

-3.92

809

1019

1.22

-5.28

-3.72

λonset, film

(nm)

EP

Polymer



λmax, film

AC C

P2






299

a

Determined by


300

b



Determined by CV. EHOMO = - (4.80 +
  ) eV

301

c

 Determined by CV. ELUMO = - (4.80 +
  ) eV

302

3.5. Computational analysis

303

To explore the molecular geometry and frontier orbital density distribution of these

304

two polymers, we utilized gas-phase density functional theory (DFT) to analyze

= 1240/λonset, film

16

ACCEPTED MANUSCRIPT truncated structures. All DFT calculations were performed with the Gaussian 09 and

306

the molecular geometries for these two polymers were optimized at the B3LYP level

307

of theory with the 6-31G (d, p) basis set. To simplify the calculation, all the alkyl

308

groups were replaced with methyl groups. Fig. 5 showed the calculated molecular

309

orbital geometry and energy levels on the model compound of the polymers. From the

310

molecular geometry graph, we could know the dihedral angle of P1 and P2 were 20.5°

311

and 12.6°, respectively. The dihedral angle of P1 was larger than P2, which was

312

attributed to the twisted structure of ID for the steric repulsion between the hydrogens

313

on the phenyl rings and the oxygens of the oxindoles [41, 53]. On the other hand,

314

because of the strong intramolecular S-O interactions and the quinoidal construction,

315

TID molecular possessed more planar structure [54, 55]. The theoretical HOMO and

316

LUMO energy levels of P1 and P2 were -4.99/-2.92 eV and -4.53/-2.94 eV,

317

respectively. The trend of this result was similar to the experimental data. The electron

318

density in the HOMO wave function of P2 was more delocalized along the whole

319

polymer backbone than P1, leading to the higher HOMO energy level of P2.

320

Moreover, the superior planarity of P2 was also beneficial for improving the HOMO

321

energy level and decreasing the band-gap [56], which might go against the high Voc

322

and stability of the photovoltaic devices [22, 57].

SC

M AN U

TE D

EP

AC C

323

RI PT

305

(a)

(b)

20.5°

12.6°

324 17

ACCEPTED MANUSCRIPT

RI PT

325

326 LUMO (-2.92 eV)

LUMO (-2.94 eV)

328 329

HOMO (-4.99 eV)

M AN U

SC

327

HOMO (-4.53 eV)

330

Fig. 5. Optimized molecular geometry and frontier orbital density distributions for P1 (a) and P2

331

(b) via DFT calculations

3.6. OFETs properties

333

The potential application of these two polymers for OFETs was investigated using a

334

top-gate/bottom-contact (TG/BC) device configuration. The polymer active layers

335

were spin coated from hot chlorobenzene solutions, and then they were thermally

336

annealed at 200 °C for 20 min. The typical output and transfer characteristics of the

337

devices are shown in Fig. 6 and the results are shown in Table 3. As shown in Table 3,

338

all the conjugated polymers exhibited p-type behavior with better charge mobility.

339

The highest hole mobility is 3.0×10-2 cm2 V−1 s−1 for P1, and 1.2×10-2 cm2 V−1 s−1 for

340

P2. The Ion/Ioff ratio and threshold voltage (VT) have been listed in Table 3. The hole

341

mobility values for both polymers were high and elegant, despite the effect of electron

342

donor unit and the difference of the dihedral angle between the acceptor unit and the

AC C

EP

TE D

332

18

ACCEPTED MANUSCRIPT 343

donor unit, which indicated these two polymers could be utilized in OSCs devices for

344

better performance [23, 30].

345

(a) -5

-3

0.007

-5

10

Vd = -80 V

-6

10

0.005

-7

10

0.004

Vd = -10 V

-9

10

-11

10

-12

0.0

10 0

-20

-40

-60

-80

0

Vd (V)

-40

-60

-5

-4

2.0x10

0.005

10

0V -10V -20V -30V -40V -50V -60V -70V -80V

Is (A)

0.0 -20

-40

-60

10

-8

10

Vd = -10 V

Vd = -80 V

-9

10

0.001 -10

10

-11

10

0

-80

Vd (V)

349

0.002

1/2

5.0x10

348

0.003

-7

-6

0

0.004

Vd = -80 V

-6

10

sqrt(Is) (A )

-5

1.0x10

-5

10

TE D

-5

1.5x10

0.000 -80

Vg (V)

(b)

Is (A)

-20

0.001

M AN U

346

SC

10

-5

347

0.002

-10

1.0x10

0.003

Vd = -80 V

1/2

-8

10

-5

2.0x10

0.006

sqrt(Is) (A )

-5

3.0x10

-4

10

Is (A)

-5

4.0x10

Is (A)

0.008

10 0V -10V -20V -30V -40V -50V -60V -70V -80V

RI PT

5.0x10

-20

-40

-60

0.000 -80

Vg (V)

EP

Fig. 6. Output and transfer curves of OFET devices: P1 (a), P2 (b) Table 3

351

OFET characteristics of P1 and P2

AC C

350

Polymer

P1

p-channel

µha,max

µha,avg

(cm2V−1 s−1)

(cm2V−1 s−1)

3.0×10-2

2.5×10-2

19

VTb (V)

Ion/Ioffc

-40

105

ACCEPTED MANUSCRIPT 1.2×10-2

P2

8.3×10-3

-28

104

a

TGBC configuration after thermal annealing at 200 °C.

353

b

Refers to the threshold voltage.

354

c

Refers to the on-to-off ratio.

355

3.7. Photovoltaic properties

356

To investigate and compare the photovoltaic properties of the polymers, BHJ OSCs

357

devices with a configuration of ITO/ZnO/Polymer:PC61BM/MoO3/Ag were fabricated

358

by the solution-processed method.

359

The external quantum efficiency (EQE) spectra of the P1 and P2 PSCs devices are

360

shown in Figure 7. Both P1 and P2 PSCs had a broad photon response from 400 to

361

800 nm, both the maximum EQE values of P1 and P2 reach 22%. P1 PSCs exhibits

362

higher EQE in the absorption spectrum than that of P2.

TE D

M AN U

SC

RI PT

352

25

EQE (%)

AC C

20

P1 P2

EP

30

15

10

5

0 400

500

600

700

Wavelength (nm) 363 364

Fig.7. EQE spectrum of P1 and P2 PSCs 20

800

ACCEPTED MANUSCRIPT 365

Atomic force microscopy (AFM) was utilized to observe the surface morphologies of

367

blend films. As shown in Figure 8a-b, the root mean square (RMS) surface roughness

368

increases from 3.3 nm for P1 blend film, to 4.5 nm for P2 blend film. Both P1 and P2

369

blends indicate phase separated morphologies with a large scale domains resulting in

370

losing charge carriers in the recombination of exciton.

M AN U

SC

RI PT

366

371

Fig.8. AFM images for a) P1 blend film and b) P2 blend film

373

Fig. 9 exhibited the J–V curves of the PSCs under illumination of AM 1.5G, 100 mW

374

cm-2. Table 4 summarized the detailed device performances. Finally, the best solar cell

375

obtained from P1:PC61BM showed a PCE of 2.40% with a Voc of 0.94 V, a Jsc of 5.94

376

mA cm-2 and a FF of 43.08% and P2 showed a PCE of 1.28% with a Voc of 0.66 V, a

377

Jsc of 5.25 mA cm-2 and a FF of 38.10% at the same condition. P1 and P2 showed

378

high and similar Jsc value, which might be originated from the broad absorption

379

characteristic, low band-gap and strong π–π stacking interaction. The Voc of P2 based

380

device was less than P1 due to the high-lying HOMO energy level, higher EQE and

381

better thin film morphology.[41]

AC C

EP

TE D

372

382 21

ACCEPTED MANUSCRIPT 383

Table 4

384

Photovoltaic properties of P1 and P2 Jsca (mA/cm2)

FFa (%)

PCEa (%)

P1

0.94

5.94

43.08

2.40

P2

0.66

5.25

38.10

Device configuration: ITO/ZnO/Polymer:PC61BM/MoO3/Ag.

1

) 2

-2 -3 -4

-7

TE D

Current Density (mA/cm

-1

-6

EP

0.0

387

AC C

386

1.28

M AN U

0

-5

RI PT

a

Voca (V)

SC

385

Polymer

P1 P2

0.5

1.0

Voltage (V)

Fig. 9. Photovoltaic properties of P1 and P2

388

4. Conclusion

389

In summary, two 2D conjugated polymers P1 and P2 based on ID and TID acceptor

390

units and BDT-T donor unit were designed and synthesized via Stille polymerization

391

and characterized by solution-possessed OFETs and OSCs. So far, ID and TID played

392

an important role in D-A conjugated polymers which served as excellent acceptor

393

units. Because of the structural differentiation of the acceptor units, there are 22

ACCEPTED MANUSCRIPT distinctions in the optical absorption, thermal stability, molecular structure,

395

electrochemical energy level, charge mobility and photovoltaic properties of these two

396

polymers, indicating the change of acceptor units has a significant effect on the

397

properties of D-A conjugated polymer materials. OFETs performance displays these

398

two polymers are intrinsic p-type semiconductors with the hole mobilities of 3.0×10-2

399

and 1.2×10-2 cm2 V-1 s-1, respectively. And OSCs fabricated based on P1 and PC61BM

400

showed a higher efficiency of 2.40%, while P2 shows a modest efficiency of 1.28%

401

due to the low Voc which arising from its high-lying HOMO energy level, higher EQE

402

and smooth surface. It shows the structure of acceptor units has an effect on the

403

photovoltaic property of the D-A conjugated polymer materials, including the ability

404

of optical absorption, energy level of electrochemical property, the planarity of

405

molecular construction and so on, which can really supply a significant reference to

406

obtain the highly efficient D-A conjugated polymer materials.

407

Acknowledgements

408

The authors acknowledge financial support from the National Key R&D Program of

409

‘‘Strategic Advanced Electronic Materials’’ (No. 2016YFB0401100) and the National

410

Natural Science Foundation of China (Grant No. 61574077).

411

Conflicts of Interest

412

The authors declare no conflict of interest.

413

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bis(dialkylthienyl)benzodithiophene

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Bandgap

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Conjugated

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Polymers.

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549

High-Performance Air-Processed and Air-Tested Fullerene-Free Organic Solar Cells.

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ACCEPTED MANUSCRIPT

Highlights

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2D conjugated polymers based on isoindigo (ID) and thienoisoindigo (TID) were synthesized. The ID and TID based polymers showed p type semiconductor characteristic with hole mobilities of 3.0×10-2 and 1.2×10-2 cm2 V-1 s-1, respectively. The ID based polymer exhibited better organic solar cell performance than the TID based polymer.

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