ThiopheneâVinyleneâThiophene-Based DonorâAcceptor Copolymers

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Thiophene-Vinylene-Thiophene-Based Donor-Acceptor Copolymers with Acetylene-Inserted Branched Alkyl Side Chains to Achieve High Field-Effect Mobilities De-Yang Chiou, Yen-Chen Su, Kai-En Hung, Jhih-Yang Hsu, Tze-Gang Hsu, Tung-Yu Wu, and Yen-Ju Cheng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02801 • Publication Date (Web): 21 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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Chemistry of Materials

Thiophene-Vinylene-Thiophene-Based Donor-Acceptor

Copolymers

with

Acetylene-Inserted Branched Alkyl Side Chains to Achieve High Field-Effect Mobilities De-Yang Chiou,a Yen-Chen Su, a Kai-En Hung, a Jhih-Yang Hsu, a Tze-Gang Hsu, a Tung-Yu Wua and Yen-Ju Chengab* aDepartment

of Applied Chemistry, National Chiao Tung University, 1001 University Road,

Hsinchu, 30010, Taiwan bCenter

for Emergent Functional Matter Science, National Chiao Tung University, 1001

University Road, Hsinchu, 30010, Taiwan Email: [email protected]

ABSTRACT: 2-Alkyl(1)alkyl(2)-type aliphatic side chains with a branching point position at the C2-position (such as 2-ethylhexyl or 2-octayldodecyl) have been widely implanted into numerous donor-acceptor conjugated copolymers for solution processable transistors or organic solar cells. However, the tertiary branching site located at the second carbon inevitably imposes steric

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hindrance that twists the main-chain coplanarity and attenuates interchain interactions. In this research, we developed a new 2-dimensonal thiophene-vinylene-thiophene (TVT) derivative where a carbon-carbon triple bond is inserted between the thiophene unit and the 2-octyldodecyl group.

This

acetylene-incorporated

TVT

(aTVT)

5,10-di(thiophen-2-yl)naphtho[1,2-c:5,6-c']bis([1,2,5]thiadiazole)

was

copolymerized (DTNT)

with and

5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTFBT) to form the polymers PaTVT-NT and PaTVT-FBT, respectively. PTVT-FBT without the triple bond was also prepared for comparison. The insertion of a linear triple bond moves the tertiary carbon away from the main chain to reduce the steric hindrance, thereby improving the main-chain coplanarity and facilitating the interchain interactions. The acetylene-incorporated copolymers show better thermal stability, red-shifted absorption spectra, stronger intermolecular aggregation, lower-lying electron affinity and much higher solid-state crystallinity. Due to the linear and coplanar polymeric backbone supported by theoretical calculation, PaTVT-NT exhibits high crystallinity and adopts strong stacking with an edge-on orientation in the thin film evidenced by 2D-GIXRD, 5

leading to a high p-type OFET mobility up to 1.27 cm2 V-1 s-1 with an on–off ratio of 9.22 × 10 . This value represents the highest value among the NT-based polymers. PaTVT-FBT also achieved a high mobility of 0.78 cm2 V-1 s-1 which greatly outperforms the corresponding


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non-acetylene

PTVT-FBT

counterpart.

Most

importantly,

the

preparation

of

2-alkyl(1)alkyl(2)-acetylenyl side chain is synthetically feasible, which can be easily applied to create new conjugated polymers for high-performance solution-processable optoelectronics.

INTRODUCTION

Donor-acceptor (D-A) alternating copolymers have been widely used in organic photovoltaics (OPVs)1-7 and organic field effect transistors (OFETs)7-16. Numerous researches have been conducted to develop new donor monomers for making high-mobility D-A copolymers.17-24 Reducing the conformational distortion of polymeric backbone in solid state to promote strong intermolecular π-π interactions is a guideline to develop high-performance conjugated polymers.25-30 The incorporation of trans-1,2-vinylene (CH=CH) linkage in the polymeric conjugated backbone not only sterically enhances the coplanarity but also electronically prolongs the conjugation. Poly(phenylene vinylene) (PPV) is the most representative polymer which has been extensively studied for polymer light-emitting diode applications.31-32 Alternatively, (E)-2-[2-(thiophen-2-yl)vinyl]thiophene unit, where a central vinylene is linked with two thiophenes (also denoted as TVT), emerges as a superior electron-rich donor to make D-A copolymers particularly for OPVs and OFETs applications.33-39 To impart



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sufficient solubility of the polymers for solution-processability, it is necessary to copolymerize an unsubstituted TVT unit with an electron-accepting unit such as diketopyrrolopyrrole (DPP)40, perylene diimide (PDI)41 and naphthalene diimide (NDI)42 which contain nitrogen atoms to install aliphatic solubilizing groups. These unsubstituted-TVT-based copolymers have delivered superior

OFET

mobilities.

Recently,

several

compact

electron

acceptors

such

as

difluorobenzothiadiazole (FBT) and naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT) have been utilized to make various D-A copolymers which have shown high OFET mobilities and OPV efficiencies.43-55 It is of great interest to further combine the TVT donor with FBT or NT acceptors for developing new promising TVT-based polymers. However, it is synthetically unable to introduce aliphatic side chains to these thiadiazole-based acceptors. As a result, development of a new TVT derivative which is substituted with aliphatic side chains is thus required for versatile applications. Introduction of a branched alkyl group at the 3-position of two thiophene units in a TVT unit has been employed (Scheme 1). The tertiary chiral center is commonly located at the second carbon of a linear alkyl chain (i.e. 2-octyldodecyl in Scheme 1). However, the branching site in close proximity to the main conjugated backbone inevitably increases the steric repulsion to cause the backbone distortion and attenuate intermolecular interactions. Consequently, these TVT polymers with the C2-brainching side chains generally did


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not exhibit satisfactory OFET mobilities.56-59 Side-chain engineering thus plays a key role to circumvent this problem. It was documented that the branching point of the side-chain plays an important role in determining the polymer packing properties.60-62 By shifting the branching chiral center from the second carbon to the outer carbon or introduced tertiary silyl groups at the end of an alkyl chain, the main-chain coplanarity can be retained without sacrificing sufficient solubility, leading to the enhanced crystallinity and thus the improved OFET mobilities.63-67 However, preparation of aliphatic side chains with different branching position is synthetically tedious.

Scheme 1. Chemical Structure of Unsubstituted TVT, Alkylated TVT and Alkynylated TVT (aTVT). C8H17 C10H21

C8H17 S S

2.59Å

4.94Å

S S

TVT

C10H21

C10H21

S S

C8H17

alkylated TVT

C10H21

C8H17

alkynylated TVT (aTVT)

In this research, we simply insert a carbon-carbon triple bond between the thiophene and the branched 2-octyldodecyl side chain to form a new alkynylated TVT (aTVT). With the linear and



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compact acetylene moiety as a conjugated spacer, the distance between the branching site and the thiophene unit is approximately increased from 2.59 Å to 4.94 Å (see Scheme 1). Moreover, incorporation of the acetylene moiety in the lateral direction not only prolongs the intrachain conjugation but also facilitates the intermolecular π-π interaction. Consequently, the 2-dimensional conjugated TVT structure could be advantageous for light absorption and charge transport. The newly designed alkynylated TVT unit was polymerized with non-alkylated acceptor units DTNT (5,10-di(thiophen-2-yl)naphtho[1,2-c:5,6-c']bis([1,2,5]thiadiazole)) and DTFBT (5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole) to form PaTVT-NT and PaTVT-FBT, respectively. To study the triple-bond effect, 2-octyldodecyl TVT without the additional triple bond was also copolymerized with DTFBT to form PTVT-FBT for comparison (Scheme 2). Their photophysical, electrochemical, theoretical, semiconducting properties, and device characterization were carefully investigated and discussed. Incorporation of the triple bonds in the TVT unit reduces the side-chain steric hindrance and improves the main-chain coplanarity, leading to the enhanced crystallinity. As a result, alkynylated PaTVT-FBT showed the much better hole mobility of 0.78 cm2 V-1 s-1 than the corresponding non-acetylene PTVT-FBT counterpart. PaTVT-NT with the more linear polymeric backbone further enhances the

intermolecular

π–π

interactions

and

polymeric


packing.

The

thermal-annealed

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PaTVT-NT-based device exhibited an excellent hole mobility up to 1.27 cm2 V-1 s-1. To the best of our knowledge, this value represents the highest hole mobility among the NT-based polymers reported in the literature. Meanwhile, the PaTVT-NT:PC71BM-based solar cell showed a good power

conversion

efficiency

of

7.75%,

which

also

greatly

outperforms

the

PTVT-FBT:PC71BM-based device. RESULTS AND DISCUSSION Synthesis. The synthesis of the stannylated TVT monomer is illustrated in Scheme 2. McMurry coupling of compound 1 with TiCl4/Zn furnished dimerized compound 2. The Sonogashira alkynylation of compound 2 with 9-(prop-2-yn-1-yl)nonadecane afforded the compound 3.68 The lithiation of compound 3 to react with trimethyltin chloride obtained the distannylated TVT monomer 4. The alkylated monomer 5 was also synthesized according to the literature.56 The monomer 4 was polymerized with Br-DTNT and Br-DTFBT monomers via Stille coupling to yield PaTVT-NT, and PaTVT-FBT, while PTVT-FBT was obtained by reacting monomer 5 with Br-DTFBT respectively (Scheme 1). The molecular weights of PaTVT-NT (Mn = 34.7 kDa; PDI = 2.1), PaTVT-FBT (Mn = 26.9 kDa; PDI = 2.0) and PTVT-FBT (Mn = 30.0 kDa; PDI = 2.0) were determined by gel permeation chromatography.



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Scheme 2. The Synthesis of TVT Monomers and Their Corresponding Polymers. C8H17 Zn, TiCl4 pyridine

Br O S

Br S

THF 78%

1

S

C10H21

C8H17

PdCl2(PPh3)2, CuI PPh3

S

2

C8H17

S

C10H21 4 C8H17

C8H17

4

C10H21 N SN S

S

S

S N N S Br-DTNT

n

N N S C10H21

C8H17

C8H17 PaTVT-NT

C10H21 N SN

S N N

SnMe3 +

C10H21 4

Me3Sn

SnMe3

S

C10H21

Br

C10H21

S

C8H17

S

S

C8H17

S Me3Sn

Br

Me3Sn

C8H17

S N N SnMe3

S

C8H17

C10H21

S Me3Sn

1) n-BuLi 2) SnMe3-Cl THF 85%

C10H21 3

C10H21

C8H17

S

DIPA/THF 93%

Br

C10H21

Br

S

C8H17

C10H21

S S

Br

Pd2dba3, P(o-tolyl)3

C10H21

C8H17

S

S S

SnMe3 Br

C10H21 5

C8H17

S

S

F

n

F

chlorobenzene

F F Br-DTFBT

N N

S

S

S

C10H21 S

Br

F F Br-DTFBT

C8H17 PaTVT-FBT

S

N SN S

S F

C10H21

C8H17

F

n

PTVT-FBT

Table 1. Thermal and Electrochemical Properties of PaTVT-NT, PaTVT-FBT and PTVT-FBT.


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PDI

Td (°C)

Ionization potential (eV)

Electron affinity (eV)

Egele (eV)

34.7

2.1

430

5.58

3.89

1.69

PaTVT-FBT

26.9

2.0

330

5.61

3.85

1.76

PTVT-FBT

30.0

2.0

318

5.62

3.68

1.94

Copolymer

Mn (kDa)

PaTVT-NT

Thermal properties. From the thermogravimetric analysis (TGA), the PaTVT-NT, PaTVT-FBT and PTVT-FBT exhibited sufficiently high decomposition temperatures (Td) of 432, 372 and 318 °C, respectively (Figure S1). It should be noted that introduction of the triple bonds in PaTVT-FBT turns out to improve its thermal stability significantly. From the differential scanning calorimetry (DSC) measurements (Figure 1), both PaTVT-NT and PaTVT-FBT exhibited a melting point (Tm) at 304 and 265 °C during heating and a crystallization point (Tc) at 286 and 241 °C during cooling. However, PTVT-FBT shows more amorphous nature without observing a melting point. The result indicates that the alkynylated PaTVT-NT and PaTVT-FBT have much higher crystallinity than the non-acetylene PTVT-FBT.



Figure 1. Differential scanning calorimetry of PaTVT-NT, PaTVT-FBT and PTVT-FBT with scanning rate of 10 °C/min.

Electrochemical properties. Cyclic voltammetry (CV) was conducted to examine the electrochemical properties and estimate the ionization potential/electron affinity of the polymers (Table 1 and Figure 2). The ionization potentials were estimated to be 5.58 eV for PaTVT-NT, 5.61 eV for PaTVT-FBT and 5.62 eV for PTVT-FBT. Besides, the electron affinities are approximately located at 3.89 eV for PaTVT-NT, 3.85 eV for PaTVT-FBT and 3.68 eV for PTVT-FBT. Therefore, the electrochemical bandgap is 1.69 eV for PaTVT-NT, 1.76 eV for PaTVT-FBT and 1.94 eV for PTVT-FBT. The attachment of alkynyl group does not affect the ionization potential much but downshifts the electron affinity, resulting in a smaller bandgap.69 This phenomenon is consistent with trends reported in previous studies of diethynylated pentacenes and pentacene derivatives.70-71 Notice that the electron affinity of the polymers are


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higher than the that of the PC71BM (3.98 eV) for favorable electron transfer.72-74

Figure 2. (a) Cyclic voltammogram of PaTVT-NT, PaTVT-FBT and PTVT-FBT measured in acetonitrile with a scanning rate of 80 mV/s; (b) energy diagram of PaTVT-NT, PaTVT-FBT, PTVT-FBT and PC71BM.

Optical properties. The UV–vis absorption spectra of the polymers are shown in Figure 3. Two distinct bands were observed for the polymers. Compared to PaTVT-FBT with the absorption maximum (λmax) at 714 nm in ortho-dichlorobenzene (o-DCB) (Table 2 and Figure 3), PaTVT-NT exhibited broader absorption bands and a bathochromic shift of λmax at 762 nm, suggesting that NT unit is a stronger electron-withdrawing acceptor than FBT. On the other hand, the PTVT-FBT showed a blue-shifted λmax at 702 nm in o-DCB solution, implying that the 2-dimensional acetylene-incorporated TVT in PaTVT-FBT results in a smaller optical bandgap. The optical bandgap is 1.49 eV for PaTVT-NT, 1.60 eV for PaTVT-FBT and 1.63 eV for



Page 12 of 46

PTVT-FBT. The trend of Egopt is qualitatively consistent with their Egele values. Temperature-dependent absorption spectra measured from 30 °C to 160 °C were used to study the intermolecular interactions (Figure 4). Both the acetylene-incorporated PaTVT-FBT and PaTVT-NT show a distinct and strong vibronic shoulder at ca.720 nm at low temperatures due to the strong intermolecular aggregation. When the temperature of the solution increases, the absorbance of these polymers is gradually blue-shifted with dramatic disappearance of the vibronic

structure

as

a

result

of

the

temperature-induced

disaggregation.

This

temperature-induced aggregation-disaggregation is commonly observed in many highly crystalline polymers.75 However, the PTVT-FBT without the acetylene-incorporated side chain does not exhibit obvious vibronic peak and the temperature-induced aggregation-disaggregation phenomenon is also less apparent. PaTVT-NT, PaTVT-FBT and PTVT-FBT exhibited λmax at 720, 715 and 710 nm, respectively, from the thin film absorption spectra (Figure 3). The optical band gaps (Egopt) were estimated to be 1.49 eV for PaTVT-NT, 1.60 eV for PaTVT-FBT and 1.63 eV for PTVT-FBT.


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Figure 3. Normalized absorption spectra of (a) PaTVT-NT, (b) PaTVT-FBT and (c) PTVT-FBT in o-DCB solution and thin film.



Page 14 of 46

`

Figure 4. Temperature-dependent absorption spectra of (a) PaTVT-NT, (b) PaTVT-FBT and (c)


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PTVT-FBT in o-DCB from 30 °C to 160 °C.

Table 2. Optical Properties of PaTVT-NT, PaTVT-FBT and PTVT-FBT. Copolymer

λmax (nm)

λonset (nm)

Egopt (eV)

o-DCB

Film

PaTVT-NT

762

720

832

1.49

PaTVT-FBT

714

715

777

1.60

PTVT-FBT

702

710

762

1.63

Theoretical calculations. The optimized geometry of alkyl TVT and alkynyl aTVT was computed at the B3LYP/6-311G (d, p) to study the side-chain effect. In Figure 5, the distance between the branched center of side chain and the β-carbon of the thiophene subunit is estimated to be 2.59 Å for TVT and 4.94 Å for aTVT. The insertion of a linear triple bond releases the steric crowdedness near the main chain.



Figure 5. The optimized geometry of (a) TVT and (b) aTVT monomer.

To gain perception into main-chain conformation of the TVT-based polymers, optimized geometry of simplified trimer model 3aTVT-NT, 3aTVT-FBT and 3TVT-FBT were calculated at the wB97XD/6-311G (d, p) level of theory. All the 2-octyldodecyl groups are replaced by isobutyl groups. The side view and top view of the optimized backbone configurations were plotted with a-, b- and c-axis pointing to the polymeric backbone direction, lateral direction and vertical direction, respectively. The side view of the optimized trimer structures is shown in Figure 6. Viewing from the b-axis (side view), the angle between the linear lines contacting the center of two adjacent acceptor units (NT or FBT) is used to approximately quantify the degree of coplanarity. 3aTVT-NT and 3aTVT-FBT have good coplanar polymer backbones with the


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angle of 165o and 166o, whereas 3TVT-FBT has a larger twisted angle of 154o. Similarly, seeing from the direction perpendicular to the plane of polymeric backbone (c-axis), the angle between the lines contacting the center of two adjacent acceptor units is also used to examine the linearity of the polymer backbone (Figure 7).76 3aTVT-NT shows an angle of 178o, while both of the 3aTVT-FBT and 3TVT-FBT exhibit the angle of 165 o, indicating that the NT-based polymer has more linear polymer backbone than the FBT-based polymers. The difference might originate in the different symmetry, C2v and C2h for FBT and DTNT, respectively. The two thiophene units tend to adapt anti form in DTNT, however, the thiophene subunits tends to be in syn form in DTFBT.54 The more coplanar and linear polymer backbone of PaTVT-NT leads to the better aligned packing in the solid state, which might achieve better OFET mobility and OPV efficiency.



Figure 6. Side view of the optimized geometry of (a) 3aTVT-NT, (b) 3aTVT-FBT and (c) 3TVT-FBT.


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Figure 7. Top view of the optimized geometry of (a) 3aTVT-NT, (b) 3aTVT-FBT and (c) 3TVT-FBT.

Transistor characterization. The bottom-gate/top-contact devices with 40 nm gold source/drain electrodes were fabricated to evaluate the OFET mobilities of the TVT-based polymers. A self-assembled monolayer (SAM) of octadecyltrichlorosilane (ODTS) was formed on the surface of SiO2 as a gate dielectric. The polymer films deposited by spin coating were thermally annealed at different temperatures for 10 min. The typical output and transfer plots of the devices are shown Figure 8. The hole mobilities were estimated from the transfer characteristics in the saturation regime (Figure 8). The PaTVT-NT and PaTVT-FBT devices without thermal annealing exhibited a mobility of 0.19 cm2 V−1 s−1 and 0.14 cm2 V−1 s−1, respectively. After thermal annealing at 200 oC for 10 min, the mobility of PaTVT-NT improved dramatically to 1.27 cm2 V−1 s−1, respectively. In a similar manner, the PaTVT-FBT film annealed at 200 and 220 oC exhibited much improved mobilities of 0.35 and 0.78 cm2 V−1 s−1, respectively, which are attributed to the enhanced crystallinity with ordered molecular packing induced by the thermal annealing. Notably, the optimized mobility of 1.27 cm2 V−1 s−1 is the highest value among the NT-based alternating D-A copolymers. Nevertheless, PTVT-FBT-based -3

device showed a much lower mobility of 2.28 × 10 cm2 V−1 s−1. After thermal annealing at 200



oC

Page 20 of 46

-2

-2

and 220 oC, the mobility was only slightly improved to 9.71 × 10 and 8.91 × 10 cm2 V−1 s−1

respectively. Compared to PTVT-FBT, the acetylene-incorporated PaTVT-FBT exhibited a much higher mobility. Insertion of the acetylene unit in the lateral direction is indeed successful to improve the charge transport. The saturation mobility dependent on gate voltage and plot of the optimized devices is shown in Figure S2. The hysteresis test of the devices is shown in Figure S3.


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Figure 8. Typical output curves (a, c, e) and transfer plots (b, d, f) of the PaTVT-NT (200 oC), PaTVT-FBT (220 oC) and PTVT-FBT (200 oC) devices, respectively.

To study the air-stability of the new materials for OFET application, the mobility of OFET PaTVT-NT-based and PaTVT-FBT-based devices without encapsulation were monitored over 12 days under ambient condition. The devices were fabricated and directly exposed to air, light and moisture. As shown in Figure S4, the mobilities of PaTVT-NT and PaTVT-FBT devices are gradually decreased probably due to the oxidation in air. However, it should be also stressed that the device showed fairly stable air-stability after PMMA encapsulation or under nitrogen atmosphere.

Table 3. P-type OFET Characteristics of The Polymers Obtained from 10 Devices


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Copolymer

Annealing T (oC)

Vth (V)

Ion/off

Mobility (cm2 V-1 s-1)a

PaTVT-NT

non

−12.2

2.81 × 10

PaTVT-NT

200 oC

−18.5

9.22 × 10

PaTVT-NT

220 oC

−9.36

1.17 × 10

PaTVT-FBT

non

−7.80

1.14 × 10

PaTVT-FBT

200 oC

−8.55

2.55 × 10

PaTVT-FBT

220 oC

−2.68

2.55 × 10

PTVT-FBT

non

−7.40

1.46 × 10

PTVT-FBT

200 oC

−2.56

4.06 × 10

PTVT-FBT

220 oC

−6.46

3.13 × 10

5

0.19 (0.13 ± 0.03)

5

1.27 (1.09 ± 0.11)

6

1.16 (1.01 ± 0.09)

6

0.14 (0.11 ± 0.02)

6

0.35 (0.26 ± 0.06)

6

0.78 (0.66 ± 0.10)

3

6

7

-3

2.28 (2.15 ± 0.06) × 10

-2

9.71 (9.23 ± 0.47) × 10

-2

8.91 (8.21 ± 0.31) × 10

aThe

average values and standard deviations of carrier mobilities from 10 devices (W = 1 mm and L = 100 μm) are indicated in parentheses

Photovoltaic

characteristics.

ITO/ZnO/polymer:PC71BM/MoO3/Ag

Inverted were

bulk

fabricated.

heterojunction The

current

devices

density–voltage

characteristics and the corresponding external quantum efficiency (EQE) spectra are depicted in Figure 9. The PaTVT-NT:PC71BM (1:2 wt%) device with 5 vol% diphenyl ether (DPE) as an additive showed a PCE of 7.75% with a Voc of 0.66 V, a Jsc of −16.09 mA cm−2, and a fill factor



Page 24 of 46

(FF) of 73.04% (Table 4). For comparison, the PaTVT-FBT:PC71BM-based device using the identical fabrication conditions showed a PCE of 5.61%, with a Jsc of −15.24 mA cm−2, Voc of 0.62 V, and FF of 68.41%. In sharp contrast, the PTVT-FBT:PC71BM-based device only delieverd a PCE of 0.27%, with a Jsc of −1.03 mA cm−2, Voc of 0.74 V, and FF of 34.99%. The low efficiency might be associated with the poorly ordered orientation of the polymer.

Figure 9. The J-V curves (a) and EQE plots (b) of PaTVT-NT, PaTVT-FBT and PTVT-FBT-based highest performance devices.

In addition, the hole mobilities of polymer:PC71BM films were investigated by the space charge-limited current (SCLC) method (Table S1).The PTVT-FBT film does not exhibit SCLC mobility due to the poor film-forming properties. The PaTVT-NT:PC71BM film show higher -5

-5

hole mobility (5.78 × 10 cm2 V-1 s-1) than the PaTVT-FBT:PC71BM film (3.86 × 10 cm2 V-1 s-1), which results in the highest Jsc and efficiency.


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Table 4. Photovoltaic Parameters of ITO/ZnO/Polymer:PC71BM=1:2(wt%)/MoO3/Ag Obtained from 15 Devicesab

Copolymer

Voc (V)

Jsc (mAcm-1)

PaTVT-NT

0.66 (0.66 ± 0.00)

−16.09 (−15.86 ± 0.45)

7.75 (7.44 ± 0.19)

73.04 (71.06 ± 1.24)

PaTVT-FBT

0.62 (0.62 ± 0.00)

−13.26 (−13.24 ± 0.11)

5.61 (5.46 ± 0.09)

68.41 (66.37 ± 1.38)

PTVT-FBT

0.74 (0.78 ± 0.04)

−1.03 (−0.68 ± 0.24)

0.27 (0.17 ± 0.07)

34.99 (31.81 ± 2.55)

PCE

(%)

FF

(%)

a5wt%

DPE is added and spin rate is 450 rpm. bThe average values and standard deviations from 15 devices are indicated in parentheses

X-ray diffraction measurements. 2D grazing-incidence wide-angle X-ray diffraction (2D-GIWAXRD) was used to study the thin film morphologies of the neat polymers. (Figure 10). PaTVT-NT and PaTVT-FBT thin films thermally annealed at 240 oC for 10 min exhibited strong high-ordered lamellar stacking peaks in the out-of-plane direction and π-stacking peaks at qxy = 1.74 Å−1 and 1.73 Å−1 respectively in the out-of-plane direction. These results indicate that PaTVT-NT and PaTVT-FBT adopt edge-on orientations relative to the substrate with the periodic π-stacking distance of ca. 3.61 Å and 3.63 Å, respectively, (Figure 10 and 11), which is beneficial for charge transport along the horizontal transport channel in OFETs. In sharp contrast, PTVT-FBT did not show distinct diffraction peaks, indicating that PTVT-FBT is not able to



Page 26 of 46

form ordered molecular packing presumably due to the more twisted backbone and weaker intermolecular interactions. These results clearly indicate that the insertion of acetylene moieties indeed enhances the molecular packing. As a consequence, thermal-annealed PaTVT-NT and PaTVT-FBT devices exhibited high mobilities of 1.27 cm2 V−1 s−1 and 0.78 cm2 V−1 s−1, -2

respectively. However, PTVT-FBT-based device showed a much lower mobility of 9.71 × 10 cm2 V−1 s−1.

2D-GIWAXRD was used to study the morphologies of the polymer:PC71BM blends (Figure S5). All the thin films show a distinct PC71BM stacking arc at ca. 1.36 Å-1.49 PaTVT-NT:PC71BM and PaTVT-FBT:PC71BM films show a polymer lamella peak at qz = 0.27 Å-1 and 0.31 Å-1, respectively. (Figure S6), whereas the PTVT-FBT:PC71BM film does not exhibit the similar lamella peak. This result implies that PaTVT-NT and PaTVT-FBT polymers have better crystallinity, resulting in the much higher efficiencies of the OPV devices.


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Figure 10. The 2D-GIXRD patterns of (a) PaTVT-NT, (b) PaTVT-FBT and (c) PTVT-FBT neat films before thermal annealing; (c) PaTVT-NT, (d) PaTVT-FBT and (e) PTVT-FBT neat films after thermal annealing at 240 oC for 10 min on the device substrates.



Figure 11. 1-D GIWAXS profiles of the polymers neat films: out-of-plane (a) and in-plane (b) before thermal annealing; out-of-plane (c) and in plane (d) after thermal annealing at 240 oC for 10 min.

Morphology images. The surface morphologies of the PaTVT-NT, PaTVT-FBT and


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PTVT-FBT thin films thermally annealed at 240 °C were investigated by atomic force microscopy (AFM). In Figure 12, PaTVT-NT and PaTVT-FBT exhibited a smoother surface with a RMS roughness of 0.82 nm and 2.33 nm, respectively, while PTVT-FBT shows a more inhomogenous film with a high roughness of 5.16 nm. This result suggests that the crystalline PaTVT-NT and PaTVT-FBT polymers are aligned homogenously in the thin films.

Figure 12. Atomic force microscopy height (a, b and c) and phase (d, e and f) images for PaTVT-NT, PaTVT-FBT and PTVT-FBT neat films thermally annealed at 240 oC for 10 min, respectively.

CONCLUSION We have designed and synthesized a 2-dimensonal thiophene-vinylene-thiophene (TVT)



derivative where a carbon-carbon triple bond is inserted between the thiophene unit and the 2-octyldodecyl aliphatic side chain. This alkynyl-substituted TVT unit was copolymerized with non-substituted DTNT and DTBT acceptors to form PaTVT-NT and PaTVT-FBT copolymers with sufficient solution-processability. The non-acetylene counterpart PTVT-FBT was also prepared to investigate the acetylene effect. The presence of the linear, compact and conjugated carbon-carbon triple bond moves the branched carbon away (ca. 2.35 Å elongation) from the main chain to dramatically reduce the steric hindrance, improving the main-chain coplanarity and facilitating the interchain interactions. The acetylene-containing copolymers show better thermal stability, red-shifted absorption spectra, stronger intermolecular aggregation, lower-lying electron affinity energy levels and much higher solid-state crystallinity. PaTVT-NT and PaTVT-FBT with linear and coplanar polymeric backbone adopts strong edge-on stacking in the thin film after thermal annealing. As a result, PaTVT-NT showed a high p-type OFET mobility up to 1.27 cm2 5

V-1 s-1 with an on–off ratio of 9.22 × 10 , which represents the highest value among the NT-based polymers. PaTVT-FBT also showed an impressive hole mobility of 0.78 cm2 V−1 s−1 with an 6

on–off ratio of 2.55 × 10 . However, PTVT-FBT can not form well-ordered solid-state structure, thus giving a much lower p-type mobility. It should be emphasized that compared to other side-chain (branching point) engineering, preparation of 2-alkyl(1)alkyl(2)-acetylenyl side chain is


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synthetically more feasible, which can be easily applied to create new high-performance conjugated polymers for solution-processable optoelectronics.

Supporting information: Synthetic procedures, device fabrication, thermogravimetric analysis, theoretical calculations, higher-temperature OFET devices and 13C and 1H NMR spectra Acknowledgements This work is supported by Ministry of Science and Technology, Taiwan (grant No. MOST107-3017-F009-003) and Ministry of Education, Taiwan (SPROUT Project-Center for Emergent Functional Matter Science of National Chiao Tung University). We thank the National Center of High-Performance Computing (NCHC) in Taiwan for computer time and facilities. We also thank the National Synchrotron Radiation Research Center (NSRRC), and Dr. U-Ser Jeng and Dr. Chun-Jen Su at BL23A1 station for the help with the GIXS experiments. References (1) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868−5923. (2) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer Based Organic Solar Cells. Chem. Rev. 2007, 107, 1324−1338.



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Table of content

Insertion of a carbon-carbon triple between the branched alkyl side chain and the conjugated TVT unit significantly reduces steric hindrance and strengthens intermolecular interactions. The copolymers with the acetylene-incorporated side chains exhibited enhanced crystallinity and a high OFET mobility up to 1.27 cm2 V-1 s-1, which is significantly higher than that of the corresponding non-acetylene polymer.


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ThiopheneâVinyleneâThiophene-Based DonorâAcceptor Copolymers

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