Copolymers of Bis-Diketopyrrolopyrrole and Benzothiadiazole

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Copolymers of Bis-Diketopyrrolopyrrole and Benzothiadiazole Derivatives for High-Performance Ambipolar Field-Effect Transistors on Flexible Substrates Jinyang Chen,#,†,‡ Yingying Jiang,#,†,‡ Jie Yang,#,†,§ Yunlong Sun,†,‡ Longxian Shi,†,‡ Yang Ran,†,‡ Qingsong Zhang,†,‡ Yuanping Yi,† Shuai Wang,§ Yunlong Guo,*,† and Yunqi Liu*,† †

Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China S Supporting Information *

ABSTRACT: We develop an “acceptor dimerization” strategy by a bis-diketopyrrolopyrrole (2DPP) for an ambipolar organic semiconductor. Copolymers of 2DPP and benzothiadiazole (BTz) derivatives, P2DPP-BTz and P2DPP-2FBTz, are designed and synthesized. Both of the polymers exhibit narrow optical bandgaps of ca. 1.30 eV. The strong electronwithdrawing property of 2DPP results in low-lying lowest unoccupied molecular orbital (LUMO) energy levels of the polymers, improving the electron mobilities. 2D grazing incident X-ray diffraction and atomic force microscopy indicate that the P2DPP-BTz exhibits a small π−π stacking distance of 3.59 Å and a smooth interface, thus promoting high mobility. To take full advantage of the flexibility of organic semiconductors, flexible field-effect transistors (FETs) were fabricated on poly(ethylene terephthalate) (PET) substrates. The FETs based on P2DPP-BTz show high performance with hole and electron mobilities of 1.73 and 2.58 cm2 V−1 s−1, respectively. Our results demonstrate that the 2DPP acceptor is a promising building block for high-mobility ambipolar polymers. KEYWORDS: bis-diketopyrrolopyrrole, “acceptor dimerization” strategy, field-effect transistor, ambipolar polymer, flexible substrate

1. INTRODUCTION

Therefore, it is a critical issue to develop high-mobility ambipolar polymers. In the past several years, donor−acceptor (D−A) polymers with DPP as a benchmark acceptor in OFETs have attracted lots of attention due to the planar backbone, good πconjugation, and strong intermolecular interactions.7 Because of the moderate electron-withdrawing property of the DPP core, polymers containing DPP and electron-rich donors such as thiophene or benzene derivatives7 generally hold high-lying LUMO energy levels, thus leading to μh over 1 order of magnitude higher than μe. To improve the μe of DPP-based polymers, a promising strategy is to lower the LUMO energy levels for easier electron injection by enhancing the electronwithdrawing property of the DPP core or to replace the donors with strong electron-deficient units.7 Our group recently

Field-effect transistors (FETs) based on inorganic semiconductors are generally fabricated on rigid substrates and processed with vacuum evaporation technology. In contrast, organic field-effect transistors (OFETs) offer the advantages of mechanical flexibility, solution processability, and tunable optoelectronic properties endowed by flexible organic semiconducting layers.1−4 In recent years, various high-performance organic semiconductors have been developed based on alternating conjugated polymers containing electron-rich (donor) and electron-deficient (acceptor) units.5,6 In general, electron-deficient units such as diketopyrrolopyrrole (DPP),7 isoindigo (IID),8,9 naphthalene diimide (NDI),10,11 and thienopyrroledione (TPD)12 are the best choices for highmobility polymers. Unipolar polymers with hole mobilities (μh) over 10 cm2 V−1 s−1 or electron mobilities (μe) over 5 cm2 V−1 s−1 have been achieved.13,14 By contrast, the advances in highperformance ambipolar polymers fall behind those of p-type or n-type counterparts.15−20 Ambipolar polymers are of paramount importance for construction of easy-fabrication and lowcost organic logic circuits or light-emitting transistors.21−25 © XXXX American Chemical Society

Special Issue: Materials and Interfaces for Next Generation Thin Film Transistors Received: October 30, 2017 Accepted: January 16, 2018

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DOI: 10.1021/acsami.7b16516 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Molecular structure of (a) typical A−D polymers and (b) A−A−D polymers by the “acceptor dimerization” strategy. A represents an acceptor, while D represents a donor. Polymers based on (c) DPP dimer, (d) TPD dimer, and (e) BTI dimer.

introduced the “acceptor dimerization” strategy (shown in Figure 1) and successfully developed a novel bis-DPP (2DPP) acceptor, which was more electron-deficient than the monoDPP core.18 As expected, P2DPP-BT exhibited balanced ambipolar transport with a μh/μe of 2.99/2.60 cm2 V−1 s−1. In contrast, PDPP-BT exhibited a comparable μh of 1.33 cm2 V−1 s−1 but a much inferior μe of 0.0146 cm2 V−1 s−1. The obvious increase of μe from PDPP-BT to P2DPP-BT was ascribed to the significantly lowered LUMO energy levels (from −3.40 to −3.50 eV).18 Similarly, Li et al. reported bi-TPDbased polymers, which even showed higher μe than μh.26 Recently, a bithiophene imide (BTI) dimer, s-BTI2, which exhibits a strong electron-withdrawing property, was developed by Guo et al., and the resulting polymer showed a μe of 0.82 cm2 V−1 s−1 (Figure 1).27 All the results demonstrate that the “acceptor dimerization” strategy is a promising method to obtain stronger electron-withdrawing acceptors for ambipolar or even unipolar n-type polymers. Due to facile chemical modifications of the DPP core, there have been near hundreds of reported DPP-based polymers, most of which exhibit high mobilities.5,7 In consideration that 2DPP-based polymers exhibited a higher μh and μe than those of DPP-based ones, numerous new high-performance polymers are expected to be obtained by replacing DPP with 2DPP.18 As an electron-deficient building block, benzothiadiazole (BTz) and its derivatives have been demonstrated to be efficient for high-performance ambipolar or n-type polymers.14,28,29 Therefore, we designed copolymers of 2DPP and BTz derivatives for ambipolar OFETs. Copolymers of 2DPP and BTz derivatives such as 4,7-di(thiophen-2-yl)-benzo[c][1,2,5]thiadiazole (BTz2T) and 5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (2FBTz-2T), P2DPP-BTz and P2DPP-2FBTz, were prepared. Both of the polymers exhibited narrow

bandgaps and low-lying LUMO energy levels due to the good coplanarity and the strong electron-withdrawing property of 2DPP core. OFETs are essential elements of flexible electronic devices for wearable electronics.30−32 Therefore, we selected plastic substrates to fabricate flexible OFETs instead of rigid substrates such as silica or glass. Annealed at a mild temperature of 160 °C on poly(ethylene terephthalate) (PET), the 2DPP-based OFETs exhibited ambipolar transport characteristics. Especially, P2DPP-BTz exhibited a small π−π stacking distance of 3.59 Å and a smooth interface, thus demonstrating high performance with μh and μe up to 1.73 and 2.58 cm2 V−1 s−1, respectively. Our results demonstrate that the 2DPP acceptor designed by the “acceptor dimerization” strategy is a promising building block for high-performance ambipolar polymers.

2. EXPERIMENTAL SECTION General Procedures for Polymer Synthesis and Characterization. All starting reagents were purchased from Aldrich, Acros, or Alfar Aesar and used directly without further purification. 2DPP-2Br,18 4,7-bis(5′-trimethylstannyl-2′-thienyl)-2,1,3-benzothiadiazole,33 and 5,6-difluoro-4,7-bis(5′-trimethylstannyl-2′-thienyl)-2,1,3-benzothiadiazole33 were synthesized according to the literature. Synthesis of P2DPP-BTz. 2DPP-2Br (100.0 mg, 0.0476 mmol), 4,7-bis(5′-trimethylstannyl-2′-thienyl)-2,1,3-benzothiadiazole (BTz2T-2SnMe3) (29.8 mg, 0.0476 mmol), Pd2(dba)3 (1.3 mg), P(otol)3 (3.5 mg), and chlorobenzene (3 mL) were added to a Schlenk tube. The tube was charged with argon through a freeze−pump−thaw cycle three times. The mixture was stirred for 10 h at 120 °C, cooled down to room temperature, and poured into methanol (100 mL) containing hydrochloric acid (5 mL) and stirred for 3 h. The precipitated product was filtered and purified via Soxhlet extraction with methanol (10 h), acetone (10 h), hexane (10 h) and was finally collected with chloroform. The chloroform fraction was concentrated by evaporation and precipitated into methanol (100 mL) and filtered B


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ACS Applied Materials & Interfaces Scheme 1. Synthetic Route to 2DPP-Based Monomer and Polymers

off to afford the target polymer (100 mg, 93.7%). GPC: Mn = 78.1 kDa, Mw = 191.3 kDa, PDI = 2.45. Anal. Calcd for C138H210N6O4S7: C 73.94, H 9.44, N 3.75; found: C 73.90, H 9.43, N 3.59. Synthesis of P2DPP-2FBTz. 2DPP-2Br (100.0 mg, 0.0476 mmol), 5,6-difluoro-4,7-bis(5′-trimethylstannyl-2′-thienyl)-2,1,3-benzothiadiazole (2FBTz-2T-2SnMe 3) (31.5 mg, 0.0476 mmol), Pd2(dba)3 (1.3 mg), P(o-tol)3 (3.5 mg), and chlorobenzene (3 mL) were added to a Schlenk tube. The tube was charged with argon through a freeze−pump−thaw cycle three times. The mixture was stirred for 4 h at 120 °C, cooled down to room temperature, and poured into methanol (100 mL) containing hydrochloric acid (5 mL) and stirred for 3 h. The precipitated product was filtered and purified via Soxhlet extraction with methanol (10 h), acetone (10 h), and hexane (10 h) and was finally collected with chloroform. The chloroform fraction was concentrated by evaporation and precipitated into methanol (100 mL) and filtered off to afford the target polymer (54 mg, 49.8%). GPC: Mn = 85.3 kDa, Mw = 210.7 kDa, PDI = 2.47. Anal. Calcd for C138H208F2N6O4S7: C 72.77, H 9.21, N 3.69; found: C 71.56, H 9.18, N 3.64. OFET Device Fabrication and Characterization. Polymeric FETs (PFETs) with a top-gate bottom-contact (TGBC) configuration were fabricated on PET substrates. The substrate surfaces were cleaned by ultrasonication with deionized water, ethanol, and acetone. Then, the substrates were annealed at 120 °C for 20 min. The source and drain gold electrodes were prepared by thermal evaporation with a shadow mask (width/length = 4400 μm/50 μm). The polymer solutions (5 mg/mL in 1,2-dichlorobenzene) were spin-coated onto PET sheets at a speed of 2000 rpm for 1 min in a nitrogen glovebox. Then the film samples were annealed at 160 °C for 10 min. Poly(methyl methacrylate) (PMMA, Mw = 120 kDa) solution (60 mg/ mL in anhydrous n-butyl acetate) was spin-coated onto the surface of the polymeric films (PMMA thickness ≈ 900 nm, dielectric constant = 3.6). The samples were annealed at 90 °C for 30 min. Finally, the aluminum gate electrodes (thickness ≈ 50 nm) were evaporated on PMMA through a shadow mask. All the FET devices were determined under ambient conditions using a Keithley 4200 SCS semiconductor parameter analyzer. The field-effect mobility in the saturation region (μsat) was calculated according to the equation IDS = (W/2L) Ciμsat (VGS − Vth)2, where IDS is the drain current, W is the channel width, L is the channel length, Ci

is the capacitance per unit area of the gate dielectric layer, VGS is the gate voltage, and Vth is the threshold voltage.

3. RESULTS AND DISCUSSION Synthesis and Characterization. Scheme 1 shows the synthetic route to P2DPP-BTz and P2DPP-2FBTz. Except for the 2DPP acceptor, BTz-2T and 2FBTz-2T were selected as the other acceptors, which were demonstrated as good building blocks for ambipolar polymers.14 The monomer 2DPP-2Br (1) was synthesized according to our previous work (Scheme 1).18 The polymers were synthesized by conventional Stille coupling polymerizations between BTz-2T, 2FBTz-2T, and 2DPP-2Br. The synthetic details of the polymers are provided in Experimental Section. It is worth noting that the polymerization time of P2DPP-2FBTz should be much shorter than P2DPP-BTz due to the worse solubility of P2DPP-2FBTz in chlorinated solvents. The purification of the crude polymers was performed by Soxhlet extraction with methanol, acetone, and hexane. Finally, the target polymer products were extracted with chloroform. We conducted solubility tests (Figures S1 and S2) of the polymers in different solvents. P2DPP-BTz shows good solubility (up to 20 mg/mL) in chlorinated solvents such as chloroform, chlorobenzene, and dichlorobenzene. In contrast, P2DPP-2FBTz shows moderate solubility (up to 10 mg/mL) in the same solvents. The results indicate that fluorination of polymers decreases the solubility in chlorinated solvents. Unfortunately, both of the polymers are nearly insoluble in nonchlorinated solvents (xylene). Number-average molecular weights (Mn) of the polymers were determined by high-temperature gel permeation chromatography (GPC) at 150 °C using 1,2,4-tricholorobenzene as the eluent. P2DPPBTz and P2DPP-2FBTz showed high molecular weights with Mn of 78.1 and 85.3 kDa, respectively. We also measured their thermal properties by thermogravimetric analysis (TGA). TGA curves indicated that both of the polymers exhibited good thermal stability with decomposition temperatures over 410 °C (Figure S3). C


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ACS Applied Materials & Interfaces Table 1. Summary of Molecular Weights and Optical and Electrochemical Properties of the Polymers Mn

λmax

Egopta

HOMOb

LUMOc

HOMOd

LUMOd

polymer

[kDa]/PDI

[nm]

[eV]

[eV]

[eV]

[eV]

[eV]

P2DPP-BTz P2DPP-2FBTz

78.1/2.45 85.3/2.47

847e/853f 845/852

1.29 1.30

−5.48 −5.63

−4.19 −4.33

−4.69 −4.74

−3.14 −3.17

a

Determined from the onset of thin-film absorption. bDetermined from the onset of oxidation and reduction potentials of cyclic voltammetry. Calculated by ELUMO = EHOMO + Egopt. dCalculated from DFT calculations. eAbsorption maximum in chloroform solution. fAbsorption maximum in film. c

Figure 2. UV−vis absorption spectra of (a) P2DPP-BTz and (b) P2DPP-2FBTz in chloroform solution and in thin film.

and 1.30 eV, respectively. These values are among the smallest bandgaps for the reported D−A polymers,7 indicating effective π-conjugation and good coplanarity (Figure S2) of 2DPP-based polymers. The electrochemical properties of the polymers were investigated by cyclic voltammetry (CV) (Figure S5). The CV behaviors of the polymers in film were measured in an acetonitrile solution containing n-Bu4NPF6 as the supporting electrolyte. Both of the polymers showed oxidation and reduction peaks during the positive and negative scans, suggesting that they could act as ambipolar semiconductors. In particular, the reduction peak of P2DPP-2FBTz was more obvious than that of P2DPP-BTz, indicating easier electron transport. After calibration with use of ferrocene as a reference, the HOMO and LUMO energy levels of the polymers were estimated from the onset of the oxidation and reduction peaks. The HOMO energy levels were calculated by using the equation E = −(Eonset + 4.40) eV and the LUMO energy levels by ELUMO = EHOMO + Egopt. As shown in Table 1, the HOMO and LUMO energy levels were −5.48 and −4.19 eV for P2DPP-BTz and −5.63 and −4.33 eV for P2DPP-2FBTz. In agreement with DFT computational results, both the HOMO and LUMO energy levels were lowered after introducing the electron-withdrawing fluorine atoms. A similar trend can be observed in reported BTz and 2FBTz-based polymers.14,37 Thin-Film Crystalline Natures. To explore the crystalline natures of the polymers, we conducted 2D grazing incident Xray diffraction (2D-GIXRD). 2D-GIXRD and corresponding inplane or out-of-plane patterns of the polymeric films annealed at 160 °C are shown in Figures 3 and S6, respectively. Along the in-plane and out-of-plane directions, both of the polymeric films displayed three diffraction peaks, which were attributed to the (h00) lamellar diffractions (Figure S6). P2DPP-BTz and P2DPP-2FBTz showed lamellar d-spacing of 24.05 and 23.94 Å corresponding to out-of-plane (100) diffractions. For both of the polymers, the (010) diffraction peaks corresponding to the π−π stacking were mainly observed along the out-of-plane direction, suggesting that the π−π stacking crystallites predominantly adopted a face-on orientation. Such face-on

Density functional theory (DFT) calculations were carried out to investigate the planarity and frontier molecular orbital (FMO) energy levels of the polymers using Gaussian 09 at the B3LYP/6-31G(d) level.41 For simplicity, DFT calculations were performed on methyl-substituted dimers of P2DPP-BTz and P2DPP-2FBTz. The optimized geometries of the polymers were nearly coplanar, as shown in (Figure S4).18 Good planarity can extend the effective conjugation and promote π−π stacking, which is helpful for improving intramolecular and intermolecular charge transport.34−36 Table 1 shows the calculated highest occupied molecular orbital (HOMO) and LUMO energy levels of P2DPP-BTz and P2DPP-2FBTz. The simulated HOMO and LUMO energy levels of P2DPP-BTz were −4.69 and −3.14 eV, respectively. Due to the electronwithdrawing property of fluorine atoms, P2DPP-2FBTz showed lower HOMO and LUMO energy levels. The photophysical properties of the polymers were examined by UV−vis absorption spectra. As shown in Figure 2, the polymers exhibited typically dual absorption bands in chloroform solution and in thin film, which were similar to the reported P2DPPs.18 The high-energy absorption bands from 300 to 500 nm were attributed to the π−π* transitions, whereas the low-energy absorption peaks from 600 to 1000 nm mainly originated from the intense intramolecular charge transfer (ICT). From analysis of the UV data, P2DPP-BTz and P2DPP2FBTz were found to exhibit similar spectra both in chloroform and in film. In chloroform, P2DPP-BTz and P2DPP-2FBTz exhibited the maximum absorptions (λmax) at 847 and 845 nm, respectively (Table 1). The λmax of P2DPP-BTz in film (853 nm) showed a red shift compared to that in solution (847 nm). This phenomenon was also observed for P2DPP-2FBTz, suggesting the planarization of the polymer backbones in the solid state. Note that obvious bathochromic shifts of λmax for P2DPP-BTz and P2DPP-2FBTz in comparison with the reported DPP-based polymers in solution as well as in film could be observed, which was ascribed to the extended effective π-conjugation and enhanced HOMO/LUMO coupling.18 The optical bandgaps of P2DPP-BTz and P2DPP-2FBTz estimated from the onset of their thin-film absorption spectra were 1.29 D


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PMMA was used as a dielectric layer due to its encapsulation effect.14,19 The semiconducting layers were spin-coated in a nitrogen glovebox, and all of the OFET devices were measured under ambient conditions. Figure S7 shows the corresponding mobilities of the polymers at different annealing temperatures. Both polymers showed the highest mobilities at 160 °C. Figure 4 shows the typical output and transfer curves of the polymers by sweeping the gate voltage in both the forward and reverse directions at the optimal annealing temperature (160 °C). The transfer characteristics of the two polymers showed small hysteresis, indicating that there were only a few traps for electrons and holes.21,39 Weak dependences of the μh and μe on VGS were observed for OFET devices based on the polymers, suggesting identical mobilities beyond the threshold voltage. As shown in Figure 4, both of the polymers showed ambipolar transport behaviors with typical V-shaped transfer curves. The carrier mobilities, threshold voltages, and on/off current ratios extracted from the transfer characteristics in the saturation regimes are summarized in Table 2. P2DPP-BTz exhibited the highest μh of 1.73 cm2 V−1 s−1 with an average μh of 1.61 cm2 V−1 s−1 and the highest μe of 2.58 cm2 V−1 s−1 with an average μe of 2.37 cm2 V−1 s−1. To the best of our knowledge, this polymer is one of the rarely reported ambipolar polymers with both μh and μe over 1.5 cm2 V−1 s−1.19 In contrast, P2DPP-2FBTz showed inferior ambipolar performance with μh and μe up to 0.36 and 0.68 cm2 V−1 s−1, respectively. High on/off current ratios of 103−105 were found in both p- and n-channel operations for the polymers, which was important for the application of ambipolar OFETs in complementary logic circuits.40 Unfortunately, P2DPP-BTz showed a slightly high threshold voltage of 81 V due to the poor electron injection caused by the LUMO energy level (−3.44 eV). To demonstrate the mechanical robustness of our FETs for flexible electronics, we conducted bending tests (Figure S8). Flexible P2DPP-BTz-based FETs on PET substrates were bent repeatedly at a bending radius of 7.5 mm. No obvious degradation in the transfer characteristics was

Figure 3. 2D-GIXRD patterns of (a) P2DPP-BTz and (b) P2DPP2FBTz films annealed at 160 °C.

π−π stacking patterns could be found in the reported P2DPPs and other high-performance ambipolar or n-type polymers.15,18,38 According to the (010) diffraction peaks, P2DPP-BTz and P2DPP-2FBTz showed π−π stacking distances of 3.59 and 3.57 Å, respectively. The short π−π stacking distances of the two polymers are among the smallest values for DPP-based polymers,7 promising efficient intermolecular charge transport for carriers. Overall, no obvious differences of 2D-GIXRD results were observed for P2DPPBTz and P2DPP-2FBTz. OFET Performance. OFET devices with a top-gate bottomcontact (TGBC) architecture were fabricated to investigate the charge transport properties of P2DPP-BTz and P2DPP-2FBTz. The device configuration of TGBC devices is shown in Figure 4a. We fabricated the devices on PET substrates by spin-coating polymer solutions. Until now, most of OFET devices based on reported polymers have been fabricated on rigid substrates such as silica or glass. Here, we chose PET as the substrate to develop flexible transistors, which could be applied to flexible organic electronics in the future. To prevent PET deformation, the annealing temperatures of active layers were controlled below 160 °C. Gold source/drain electrode and aluminum gate electrodes were thermally evaporated via a shadow mask.

Figure 4. (a) Schematic device configuration of a flexible OFET fabricated on a PET substrate. (b,c) Typical output and (e,f) transfer characteristics of TGBC OFETs based on (b,e) P2DPP-BTz and (c,f) P2DPP-2FBTz. (d) Maximum hole and electron mobilities of the polymers extracted from the transfer curves. E


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ACS Applied Materials & Interfaces Table 2. OFET Characteristics of the Polymers μha

polymer 2

[cm V P2DPP-BTz P2DPP-2FBTz a

−1

Vth −1

s ]

1.73 (1.61) 0.36 (0.33)

Vth

[cm V−1 s−1]

[V]

2.58 (2.37) 0.68 (0.61)

81 (±2) 68 (±2)

2

[V] −7 (±3) −14 (±2)

μea

Ion/Ioff 104−105 102−103

Ion/Ioff

n/p ratio

104−105 104−105

1.49 1.89

Maximum mobilities extracted from the transfer curves in the saturation regimes. The average values are listed in parentheses.

Root-mean-square (RMS) analyses of height images were used to evaluate surface roughnesses of the polymeric films. Notably, the interface of P2DPP-BTz film (RMS = 0.21 nm) was much more smooth compared to that of P2DPP-2FBTz. In top-gated OFET devices, more smooth interfaces reveal less traps for intermolecular charge transport and thus resulting in higher mobility, consistent with other reported high-mobility ambipolar polymers.21,34 Further experiments to illustrate the origin of the mobility changes, including the effect of fluorination on the morphology of the polymers, are ongoing. P2DPP-BTz and P2DPP-2FBTz showed better OFET performance compared to that of other BTz-based or 2FBTzbased polymers,26,33 which demonstrated that 2DPP is a promising building block for high-mobility polymers.

observed after 2000 bending cycles (Figure S8b,c). One of the devices showed an initial μh and μe of 1.52 and 2.22 cm2 V−1 s−1, respectively. After 2000 bending cycles, the μh and μe decreased to 1.45 (4.6% loss) and 1.61 cm2 V−1 s−1 (27.5% loss), respectively. The results demonstrated that our FETs showed favorable flexibility. Compared to the reported 2DPP-based polymers, P2DPPBTz and P2DPP-2FBTz exhibited similar ambipolar transport properties.18 P2DPP-2FBTz showed lower HOMO and LUMO energy levels compared to P2DPP-BTz due to the electron-withdrawing property of fluorine atoms. Therefore, the hole injection became more difficult, but electron injection turned easier. Accordingly, the n/p ratios increased from 1.49 to 1.89 (Table 2). In general, fluorination of the polymeric backbone can promote better molecular crystallization endowed by intermolecular F···H or F···S noncovalent interactions, thus enhancing the mobility.17,19 But that is not always the case. Interestingly, it seems that 2FBTz-based polymers generally show inferior mobilities compared to those of BTz-based polymers.14,26,33,37 Because P2DPP-BTz and P2DPP-2FBTz showed similar GIXRD results, we tried to elucidate the origin of the difference in mobilities between the polymers by atomic force microscopy (AFM). Because of the nature of the OFET architecture (TGBC), the morphology of the top of the polymeric film, which is in direct contact with the dielectric, is important in the carrier transport of a FET device.33 Figure 5 shows the AFM height and phase images of annealed polymeric films, which were prepared under the same conditions as those of their device fabrication. Both annealed P2DPP-BTz and P2DPP-2FBTz films showed good continuity.

4. CONCLUSION We synthesized a 2DPP acceptor by the “acceptor dimerization” strategy. Two 2DPP-based polymers, P2DPP-BTz and P2DPP-2FBTz, were developed. Both of the polymers exhibited narrow bandgaps and low-lying LUMO energy levels due to the good coplanarity and strong electron-withdrawing property of the 2DPP acceptor. To take full advantage of the flexibility of organic semiconductors, we chose PET substrates to fabricate flexible OFETs. Both of the polymers exhibited ambipolar transport characteristics. In particular, P2DPP-BTz exhibited small π−π stacking distance and smooth interface. OFETs based on P2DPP-BTz exhibited high performance with μh and μe of 1.73 and 2.58 cm2 V−1 s−1, respectively, which are among the highest values for ambipolar polymers. Our results demonstrate that 2DPP acceptor designed by the “acceptor dimerization” strategy is a promising building block for highperformance ambipolar polymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16516. Experimental details and additional figures and tables (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.G.) *E-mail: [email protected] (Y.L.) ORCID

Yuanping Yi: 0000-0002-0052-9364 Yunlong Guo: 0000-0003-1602-769X Yunqi Liu: 0000-0001-5521-2316 Author Contributions #

J.C., Y.J., and J.Y. contributed equally to this work.

Figure 5. AFM (a) height and (b) phase images (3 × 3 μm) of P2DPP-BTz (left) and P2DPP-2FBTz (right) annealed films at 160 °C.

Notes

The authors declare no competing financial interest. F


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ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of “Strategic Advanced Electronic Materials” (No. 2016YFB0401100), the National Natural Science Foundation of China (Grant Numbers: 21673247, 51233006, and 21633012), the Major State Basic Research Development Program (2013CB733700), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB12030100 and XDPB06) and Key Research Program of Frontier Sciences, CAS, Grant NO. QYZDY-SSW-SLH029. 2D-GIXRD results were obtained at 1W1A Station of Beijing Synchrotron Radiation Facility. The authors gratefully acknowledge the assistance of scientists of the Diffuse X-ray Scattering Station during the experiments.

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Copolymers of Bis-Diketopyrrolopyrrole and Benzothiadiazole

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