High-Performance Organic Field-Effect Transistors Fabricated Based

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High-Performance Organic Field-Effect Transistors Fabricated Based on a Novel Ternary #-Conjugated Copolymer Wenkai Zhong, Sheng Sun, Lei Ying, Feng Liu, Linfeng Lan, Fei Huang, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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ACS Applied Materials & Interfaces

High-Performance

Organic

Field-Effect

Transistors

Fabricated Based on a Novel Ternary π-Conjugated Copolymer

Wenkai Zhong,†,║ Sheng Sun,†,║ Lei Ying,*,† Feng Liu,§ Linfeng Lan,† Fei Huang,† and Yong Cao†

†

Institute of Polymer Optoelectronic Materials and Devices, and State Key

Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China

§

Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai

200240, P. R. China

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ABSTRACT: In this study, we developed a ternary conjugated polymer IFBT-TT consisting

of

centro-symmetric

thieno[3,2-b]thiophene

as

the

indaceno[1,2-b:5,6-b']dithiophene

electron-donating

units

and

an

and

asymmetric

5-fluorobenzo[c][1,2,5]thiadiazole as the electron-accepting unit. The target copolymer was synthesized using an acceptor-donor-acceptor (A-D-A) type of macromonomer, which gave the target copolymer a precisely defined D1-A-D2-A architecture. Theoretical simulation revealed that the IFBT-TT features C-H···N and F···S non-bonding interactions, leading to a highly rigid and planar molecular backbone. Although the spin-cast IFBT-TT films exhibited an amorphous morphology lacking in ordered structures, the fabricated field effect transistors presented remarkable p-type transport properties with high mobility of up to 5.0 cm2 V-1 s-1 and excellent ambient stability. These observations highlight that the integration of a three-component D1-A-D2-A type backbone framework is an effective molecular design strategy for high-mobility conjugated polymers.

KEYWORDS: ternary conjugated polymers, organic field-effect transistors, high mobility, regioregular polymers, donor-acceptor type polymers


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INTRODUCTION Solution-processable organic semiconducting materials with high charge-carrier mobility have attracted tremendous interest in the fabrication of cost-effective and flexible organic field-effect transistors for use in displays, radio frequency identification tags, and so forth.1,2,3 Recently, remarkable progress has been made in achieving high carrier mobilities, 4 , 5 , 6 , 7 , 8 , 9 primarily due to improved rational molecular designs and the synthesis of donor-acceptor type of conjugated copolymers, which typically have alternating configurations of donor and acceptor unit across the main chain of polymers. These types of D–A copolymers typically have a strong aggregation preference due to their strong interactions in intermolecular fashion, and strong electronic coupling property, which facilitate efficient charge carrier transportation.10,11,12,13,14, It is well established that the interchain interactions can be affected by modifying the side groups, which can induce changes in the polymer assembly and backbone orientation and thus improve the charge carrier transportation.15 For copolymers consisting of asymmetric donor or acceptor units, however, it is highly worthwhile to construct regioregular polymers that have strictly confined monomeric units along the polymer backbone, because the molecular geometries, frontier orbital energy levels, electronic structures, and overall optoelectronic properties might be disturbed due to the uncertain molecular structures16,17,18. Much attention has been paid to the control over the regioregularity of D–A-type conjugated polymers, which focused on asymmetric electron-accepting units such as 3 ACS Paragon Plus Environment


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[1,2,5]thiadiazolo[3,4-c]pyridine17, 19 , 20 , 3-fluorothieno[3,4-b]thiophene-2-carboxylic acid 2-ethylhexyl ester18,21, and 5-fluorobenzo[c][1,2,5] thiadiazole (FBT)22,23. The regioselective reaction of the asymmetric dibromo-FBT affords the A–D–A type of macromonomer with a precisely defined orientation of fluorine atoms24, which can translate into the target copolymer in the subsequent copolymerization procedure. This strategy allows for the rational selection of an additional electron-donating unit, which offers the appreciable solubility and improved optoelectronic properties associated with enhanced intermolecular interactions. The fluorine atom in an FBT unit has strong electronegativity associated with strong interchain stacking through the non-covalent F···S or F···H interaction with the adjacent fragments25,26, which is favorable for the formation of a planar architecture in the polymer main chain and thus facilitate charge-carrier transport. Based on the aforementioned merits of FBT-based conjugated polymers, in this study we designed and synthesized a ternary conjugated polymer with a well-defined backbone, which consists of the FBT as the electron-withdrawing moiety and two centro-symmetric thieno[3,2-b]thiophene (TT) and indaceno[1,2-b:5,6-b']dithiophene (IDT) moieties as the electron-donating moieties. The constructed copolymer, IFBT-TT, bears the backbone, with the FBT unit evenly distributed across the two electron-donating units in a D1-A-D2-A fashion, where the fluorine atom in the FBT unit, which is also adjacent to the TT unit, can potentially form the non-covalent F···S interaction to facilitate the co-planarity of the polymer backbone. Because the electron delocalization in such fused IDT and TT units is less favorable than in a single 4 ACS Paragon Plus Environment

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thiophene ring, the resulting copolymer IFBT-TT should have a lower highest-occupied molecular orbital level, which is beneficial for ambient stability.27,28,29 The combination of these merits ultimately leads to high hole mobility of about 5.0 cm2 V-1 s-1 of the resulting copolymer IFBT-TT as measured by organic field effect transistors (OFETs).

RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis of the IFBT-TT polymer is illustrated in Scheme 1. To avoid the structural uncertainties arising from the discrepancy

in

the

reaction

activity

of

the

asymmetric

compound

4,7-dibromo-5-fluorobenzo[c][1,2,5]thiadiazole (FBT-Br2), a symmetric A-D-A type intermediate

compound,

2,7-bis(4-bromo-5-fluoro-7-benzo[c][1,2,5]thiadiazole)-4,9-dihydro-4,4,9,9-tetrahexa decyl-s-indaceno[1,2-b:5,6-b']dithiophene (2), was synthesized as a macromonomer. The

synthesis

was

carried

out

using

a

bisstanyl

compound

2,7-bis(trimethyltin)-4,9-dihydro-4,4,9,9-tetrahexadecyl-s-indaceno[1,2-b:5,6-b']dithi ophene (1) and the FBT-Br2 as the starting materials in a palladium-catalyst coupling reaction. As the bromo atom in the meta-position of the fluorine atom in the FBT-Br2 compound is more reactive than the bromine atom in the ortho-position, 30 the fluorine atoms of the resulting monomer 2 were distal to the central IDT unit. The proposed molecular structure of monomer 2 was confirmed by the 1H-1H nuclear overhauser effect (NOE) and

19

F nuclear magnetic resonance (NMR) spectroscopy 5

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(Figure S1 and Figure S2, Supporting Information, SI). The target copolymer IFBT-TT was synthesized by microwave-assisted Stille polymerization

using

the

dibromo-monomer

2

and

bisstannyl

2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene as the monomers. The copolymer was purified by Soxhlet extraction by successively using the eluent of methanol, acetone, hexane, dichloromethane, and chloroform. The copolymer was precipitated in methanol from chlorobenzene (CB) solution. The molecular structure of the target copolymer IFBT-TT was confirmed by 1H NMR spectroscopy (Figure S11 in the SI). The peak located at 8.7 ppm can be assigned to the proton in the TT unit, as the electron-withdrawing fluorine atom in the adjacent FBT unit led to the down-shift of the proton, which is consistent with the NMR spectrum of the model compound 2,5-bis(5-fluoro-7-(4-(2-octyldodecyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thi eno[3,2-b]thiophene (S2) (see Scheme S1, Figures S12-S13 in the SI). Similar downshielded proton signals have been observed in other molecular frameworks consisting of fluorinated conjugated species.18, 31 The number-average molecular weight (Mn) of the IFBT-TT polymer was estimated as 138 kDa (dispersity, Đ = 1.98), as evaluated by high temperature gel permeation chromatography measurement using 1,2,4-trichlorobenzene as the eluent, and the linear polystyrene as the standard. Thermogravimetric analysis revealed a decomposition temperature of 424 oC (Td, weight loss of 5%) (Figure S3 in the SI). Differential scanning calorimetry (DSC) analysis revealed no discernible glass transition temperature ranged from 30 to 320 oC under nitrogen (Figure S4 in the SI). 6 ACS Paragon Plus Environment

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Scheme 1. Synthesis of Monomer 2 and the Target Polymer IFBT-TT Using Compound 1 as the Starting Material

Optical and Electrochemical Properties. Figure 1 shows the UV-vis absorption spectra of the IFBT-TT. The absorption spectra in CB solution show the absorption profiles with dual characteristics, which have the short wavelength band from 350 to 450 nm that is attributable to localized π-π* transition, and the long wavelength band peaking at 658 nm can be ascribed to the intramolecular charge-transfer effect. The temperature-dependent absorption indicates a gradual increase in the absorption intensity of the 0-0 peak as the temperature increases, with a slight blue-shift from 683 to 649 nm. The absorption profile in thin film shows an absorption peak at 683 nm, and the absorption profile is highly similar to that in solution except for a slight bathochromic shift. It is also worth noting that the absorption profiles barely changed upon thermal annealing at 100 and 200 oC. These observations are consistent with the strong aggregation of the highly planar polymer backbone.32 The optical band gap (Egopt) calculated from the onset of absorption profile was 1.7 eV. 7 ACS Paragon Plus Environment


Cyclic voltammetry (CV) measurement was utilized to probe the electrochemical properties of the copolymer, with the corresponding characteristics shown in Figure 1c. The CV curve presents an onset potential of oxidation (Eox) of 1.09 V, and an onset potential of reduction (Ered) of 1.15 V, relative to ferrocene/ferrocenium (Fc/Fc+). Under the same condition, the onset of Fc/Fc+ was measured as 0.44 V. The highest occupied molecular orbital energy level, calculated by the equation EHOMO = –e(4.80 – 0.44 + 1.09) eV, was –5.45 eV, and the lowest unoccupied molecular orbital energy level, calculated by the equation ELUMO = –e(4.80 – 0.44 – 1.15) eV, was –3.21 eV. It is also worth pointing out that the relatively deep EHOMO was lower than ~ –5.30 eV, which is favorable for ambient stability.33,34 The detailed parameters of optical and electrochemical properties are listed in Table 1. 1

(b) 0-0

Room Temperature

0.4

0.2 135 oC

0

400

500

600

700

800

Normalized Absorbance

0-1

(c)

As cast 1

0.8

0.6

1.2

IFBT-TT

0-0 o

TA @ 100 C

Ferrocene

o

TA @ 200 C

0.8

0-1

Current

(a)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.4 0.2 0

400

Wavelength (nm)

500

600

700

800

Wavelength (nm)

-2

-1.5

-1

-0.5

0

0.5

1

1.5

Potential (V)

Figure 1. UV-vis absorption of IFBT-TT in CB (about 1 × 10–5 g mL-1) at various temperatures (a), and as thin films (b); CV characteristics (c) of IFBT-TT.

Theoretical Calculations. To understand the backbone geometry and the distribution of molecular orbitals, we carried out the density functional theory calculation at the B3LYP/6-31G* level.

The simulation was performed by modeling

the dimer structure of the repeating unit using the Gaussian 09 package35. To simplify 8 ACS Paragon Plus Environment

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the

calculation,

the

alkyl

chains

in

the

modeling

dimer

compound

(IDT-FBT-TT-FBT)2 were truncated as methyl groups. Figure 2a shows that the dimer structure exhibits a linear molecular backbone with a curvature angle of 168o. The front and side views (Figure 2b-c) illustrate the almost-planar configuration of the dimer, where the dihedral angle between the IDT or TT rings and FBT unit is close to 0o. The coplanar configuration correlates with the potential non-bonding interactions, such as C-H···N and F···S, between the FBT units and the adjacent IDT and TT units, leading to a “conformation lock” along the conjugated backbone. 36,37 It is also noteworthy that the HOMO is delocalized along the molecular backbone; in contrast, the LUMO is mostly confined on the FBT units (Figure S5 in the SI). The electrostatic potential map depicted in Figure 2d illustrates the continuous electron distribution along the molecular backbone. To gain insight into the conformational preference of the molecular backbone, the calculation of potential energy surfaces was carried out by twisting the FBT unit relative to the IDT and TT motif. Figure 2a shows the relative energy of all conformers as the function of the angle of the interring bond. The potential energy surface profile indicates that both the IDT-FBT and the TT-FBT reached the minimum values at 0o/360o and 180o, which can be correlated to the completely planar conformation in the fashion of anti- and syn-, respectively. The energy differences between these two minimum energy conformations were estimated to be 0.92 and 0.72 kcal mol-1 for IDT-FBT and TT-FBT, respectively, due to the higher binding energies of C-H···N (2.20 kcal mol-1) and F···S (0.44 kcal mol-1) interactions 9 ACS Paragon Plus Environment


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compared with C-H···F (0.94 kcal mol-1) and N···S (0.46 kcal mol-1) interactions38. The slight difference in the two conformations at the minimum energy level implies that a planar backbone can be achieved via statistical anti- and syn- conformations.39 Figure 2f shows the conformer distribution inferred from the Boltzmann distribution at room temperature (298 K). The conformer distribution exhibits two global maxima at 0o/360o and 180o, indicating two different coplanar conformers (Figure 2f, insert) with minimum energy. These observations demonstrate that the polymer backbone is coplanar and rather rigid, which can promote inter-chain packing that is favorable for charge-carrier transport.

Figure

2.

Molecular

structure

and

conformational

lock

of

the

dimer

(IDT-FBT-TT-FBT)2 (a), the curvature angles are calculated from connecting line of C1-C2-C3 (marked as “•”); front view (b) and side view (c) of dimer; ESP map (in unit of Hartree) of dimer (d); potential energy surface steming from the rotation of FBT units relative to IDT and TT unit (e); relative densities of probability that populates the energy distribution (at 25 oC), and conformers with nonbonding 10 ACS Paragon Plus Environment

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interactions corresponding to dihedral angle of 0o/360o and 180o (f).

Organic Field-Effect Transistors. Organic field-effect transistors with the configuration of top-gate, bottom-contact consisting of IFBT-TT were fabricated using a glass/Au/polymer/CYTOP/Au architecture (Figure S6 in the SI). Au source/drain electrodes (20 nm) with a channel width (W) of 500 µm and channel length (L) of 70 µm were used to measure the field-effect mobilities of the copolymer. The copolymer was spin-coated with a chlorobenzene solution at a concentration of ~5 mg mL-1. The resulting polymer semiconducting layers were annealed at 100 and 200 oC before depositing the dielectric layer. The gate dielectric layer was spin-casted atop the polymer semiconductor layer using a commercially available fluoropolymer (CYTOPTM) (the chemical structure is shown in Figure S6) with a thickness of about 400 nm. Finally, an Au gate electrode (40 nm) was deposited by thermal evaporation. Figure 3 displays the representative transfer characteristics upon sweeping gate bias (VG) of devices from –100 to 0 V under a source-drain voltage (VD) of –100 V, and the output curves taken at various VG and the detailed OFETs parameters are listed in Table 1. The pristine films exhibited a high hole mobility of 3.6 cm2 V-1 s-1 with on/off current ratios (Ion/Ioff) in the order of 106. The initial measurement of device based on IFBT-TT film after thermal annealing at 100oC provided an increased hole mobility of 4.6 cm2 V-1 s-1, with an average hole mobility (µaverage) of 3.03 cm2 V-1 s-1. The highest hole mobility of 5.01 cm2 V-1 s-1 with Ion/Ioff > 106 was achieved upon thermal annealing at 200 oC. However, there were unanticipated 11 ACS Paragon Plus Environment


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variations in the transfer characteristics of the gate bias from about –20 V to about – 40 V for devices based on thermal annealed IFBT-TT thin film, while a relatively smooth transfer curve was recorded for the device based on the as-cast film. This observation might be associated with the overlap of n-type and p-type behavior, as the ambipolar behavior of devices based on conjugated polymers consisting of indaceno[1,2-b:5,6-b']dithiophene unit is easily observed.40 Ensuring the stable operation of the device is a critical issue for potential commercialization. 41 Thus, we investigated the bias-stability of OFETs with IFBT-TT as the semiconducting layer under both negative and positive bias stress, as the low-lying HOMO energy level of IFBT-TT can provide good ambient stability for the resulting devices. The measurement was carried out by applying a negative bias (VG = −60 V, VD = −60 V) and positive bias (VG = 60 V, VD = 60 V) as the electrical stress for 60 min, and the transfer curves were recorded every 15 min (Figures S7a and b in the SI). The device showed a slight shift in the threshold voltage (Vth) and good hysteresis reversion of the transfer curves after 60 min (Figures S7c and d in the SI), while the calculated hole mobility remained at about 4.0 cm2 V-1 s-1, indicating the remarkable bias-stability of the OFETs fabricated by the resulting copolymer.


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V = -100 V D

0.01

As cast

G

10

-9

0.006

D

G

-8

0.008

o

TA@200 C

-6

10

0.004

1/2

D

10

0.008

o

TA@100 C

0.006

-7

10

10-8

0.004

-9

10

-10

10

-10

10

0.002

0.002

-9

10

0

-20

-40

-60

-80

0 -100

10

-12

10-11

10

0

-20

-40

-60

-80

0 -100

-40

(d)

(f)

VG =

0 to -100 V @ -20 V step

As cast

0 -100

-80

-100

VG = 0 to -100 V @ -20 V step

-5

-8x10

o

TA@100 C

-6x10

-80

-4

-1x10

0 to -100 V @ -20 V step -5

-60 G

G

VG = -4

-20

V (V)

-8x10-5

-1x10

0

V (V)

G

(b)

0.002

-10

-11

10

V (V) -1.2x10-4

1/2

-7

0.01

VD = -100 V

-5

10

D

1/2

-6

10

10

(e)

D

10

-4

0.01

V = -100 V

1/2

1/2

0.004

-8

10

-5

D

1/2

0.006

-7

10

-4

10

(II I) (A )

D

10

10

(II I) (A )

0.008 -6

II I, II I (A)

-5

10

II I, II I (A)

(c)

G

-4

10

D

0.012

(a)

(II I) (A )

o

TA@200 C

-5

-8x10

-5

I (A)

-5

-4x10

D

-5

-6x10

D

D

I (A)

-6x10

I (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


II I, II I (A)

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-5

-4x10 -5

-4x10

-2x10-5

-5

-2x10

-5

-2x10

0 0

0 0

-20

-40

-60

-80

-100

V (V)

0 -20

-40

-60

-80

-100

0

V (V)

D

-20

-40

-60

V (V)

D

D

Figure 3. Transfer (a, c, e) and output (b, d, f) characteristics for OFETs based on IFBT-TT.

Table 1. OFETs Properties of Pristine and Thermally Annealed IFBT-TT Films

a

Annealing Temperature

µaverage

µmax

Ion/Ioff

(oC)

(cm2 V-1 s-1)

(cm2 V-1 s-1)

None

2.34a

3.60

1×106

−38

100

3.03b

4.60

1×107

−60

200

3.54b

5.01

2×106

−62

Vth (V)

Obtained from the average values of 7 devices; The CYTOP layer was without

annealed. b Obtained form average 10 devices for each thermal annealing temperature.

Microstructure of Thin Films. The molecular stacking of IFBT-TT polymer at various thermal annealing temperatures were investigated by grazing incident X-ray diffraction (GIXD), and surface morphologies of such films are investigated by 13 ACS Paragon Plus Environment


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atomic force microscopy (AFM). The GIXD patterns illustrated in Figures 4a-c clearly show a strong ring with a q value of about ~1.52 Å-1 in the qz direction for all pure polymer films, corresponding to the distance of π-π stacking of about 0.41 nm. No detectable signal correlated with the alkyl-alkyl stacking in the qz direction, indicating the lack of obvious ordered packing in such pure polymer films. Moreover, the azimuthal spreading of the (010) reflections in the qz direction demonstrated that the thermal treatment of the pure polymer films did not lead to obviously ordered structures, while the molecular stacking slightly improved, as indicated by the (010) signal after thermal treatment (see the line-cut characteristics in Figure S8 in the SI). The lack of obviously ordered structures in such pure polymer films might be associated with the different orientations of alkyl side chains connected in the two carbon bridges of the IDT unit, which are not favorable for crystallization42. The atomic force microscopy characterizations in Figures 4d-f indicate that the film morphology did not exhibit distinct variation upon thermal treatment at various temperatures, with very flat film topography and root-mean-square (RMS) roughness values in the range of 0.29-0.39 nm. These observations imply that high charge-carrier mobility can be achieved with such amorphous film. Much higher field effect mobility might be achieved by utilizing more advanced film deposition technologies43; for instance, by using nano-structured substrates to create well-ordered polymer chains44, or using lattice strain to tune the charge transport in organic semiconductors that are sheared in solution45.


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Figure 4. Patterns of GIXD measurements (a-c), and surface morphologies measured by AFM (2 µm × 2 µm) (d-f) of the pristine and thermally annealed IFBT-TT films.

CONCLUSION In conclusion, a novel D1-A-D2-A type regioregular conjugated copolymer was designed and synthesized. The highly rigid and planar molecular backbone led to a remarkable p-type charge transport capability with mobility up to 5.0 cm2 V-1 s-1. The resulting polymer exhibited excellent bias stability due to the relatively deep highest occupied molecular orbital energy level. It is of particular interest that the polymer showed high mobility even though the resulting thin films were amorphous. Integrating this three-component D1-A-D2-A type backbone framework with precisely defined fluorine atom orientations offers an effective molecular design strategy for the production of high-mobility conjugated polymers with excellent ambient stability.



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ASSOCIATED CONTENT Supporting Information Additional ﬁgures as mentioned in the text, including synthesis of monomer and polymer, TGA and DSC curves, NMR spectra of monomer and polymer, and device fabrication and characterizations. This information is available free of charge via http://pubs.acs.org/.

Corresponding Authors *E-mail: [email protected] (L. Y.) Author Contributions ║

W. K. Zhong and S. Sun contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the National Nature Science Foundation of China (No. 21520102006 and 51673069). Portions of this research were carried out at beamline 7.3.3 at the Advanced Light Source, Lawrence Berkeley National Laboratory, which was supported by the DOE, Office of Science, and Office of Basic Energy Sciences.

REFERENCES 16 ACS Paragon Plus Environment

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480, 504-508.


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Graphic Abstract


High-Performance Organic Field-Effect Transistors Fabricated Based

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