Influence of Branched Alkyl Ester-Labeled Side Chains on Specific

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 40681−40691

Influence of Branched Alkyl Ester-Labeled Side Chains on Specific Chain Arrangement and Charge-Transport Properties of Diketopyrrolopyrrole-Based Conjugated Polymers Hyung Jong Kim,†,§ Mingyuan Pei,‡,§ Joong Se Ko,‡ Min Hee Ma,† Gi Eun Park,† Jimin Baek,‡ Hoichang Yang,*,‡ Min Ju Cho,*,† and Dong Hoon Choi*,† †

Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 02841, South Korea Department of Applied Organic Materials Engineering, Inha University, Incheon 22212, South Korea

‡

ACS Appl. Mater. Interfaces 2018.10:40681-40691. Downloaded from pubs.acs.org by YORK UNIV on 12/03/18. For personal use only.

S Supporting Information *

ABSTRACT: A series of diketopyrrolopyrrole (DPP)-based copolymers, with DPP and bithiophene (BT) as the electron-acceptor and donor backbone units, respectively, are synthesized with branched alkyl side chains that are either directly coupled to the N-positions of DPP or separated by an alkyl ester group. The ester moieties in the side chains induce specific cohesive molecular interactions between these side chains, as compared to the alkyl-only side chains with weak van der Waals interactions. Structure analysis of the DPPBT-based copolymers demonstrated that the introduction of a proper alkyl ester spacer to the branched alkyl chains can shorten the π−π stacking distance between the DPPBT backbones down to 3.61 Å and promote the development of twodimensionally extended domains. DPPBT-based copolymers, including different branched alkyl ester-labeled side chains, are spun-cast on polymer-treated SiO2 dielectrics from dilute chloroform solutions for organic thin-film transistors. A DPPBT-based copolymer with properly engineered side chains (i.e., 2-decyltetradecyl esterlabeled side chains) shows the highest hole mobility of 2.30 cm2 V−1 s−1 and an on/off current ratio of above 106. KEYWORDS: diketopyrrolopyrrole, side chain engineering, cohesive side chain interaction, alkyl ester, conjugated polymer, organic thin-film transistor

1. INTRODUCTION Solution-processable semiconducting polymers have been studied extensively for various optoelectronic applications, owing to their great potential to achieve low-cost fabrication, large area deposition, and flexibility of devices.1−13 Among organic semiconductors, π-conjugated polymers have recently demonstrated highly promising electrical properties in organic thin-film transistors (OTFTs), specifically, a charge-carrier mobility (μ) that is much greater than the 0.5−1.0 cm2 V−1 s−1 found in amorphous silicon-based thin-film transistors (TFTs).14−19 It is known that semiconducting polymers composed of repeating units with alternating electron donor (D) and acceptor (A) can facilitate better charge-carrier transport due to intra- and interchain interactions.17−20 In this case, diketopyrrolopyrrole (DPP) derivatives are popular as A units, due to their planar fused-ring structures and strong intermolecular interactions.21 There are two general design strategies of these D−A polymers, specifically, DPP-based copolymers reported elsewhere. The first strategy is to design and synthesize novel D backbone moieties, including various conjugated groups or linkages, that precisely modulate the optoelectrical properties of the resulting copolymers.22−26 Another strategy is to © 2018 American Chemical Society

introduce a precisely engineered linear alkyl side chain at the N-positions of the DPP units located in the conjugated copolymers. In comparison to linear alkyl side chains, branched alkyl substituents (e.g., 2-hexyldecyl, 2-octyldodecyl (OD), and 2-decyltetradecyl (DD)) are highly effective in improving the solubility and crystalline chain-packing behavior without significantly altering the intra- and intermolecular ordering of side chain-engineered conjugated copolymers.27−29 Recently, many studies have focused on improving the carrier mobility in OTFTs by optimizing the size and bifurcation points of bulky branched alkyl chains on the conjugated backbone segments.30−32 For example, Kim et al. controlled the bifurcation points of long alkyl side chains on DPP moieties in D−A conjugated copolymers. They reported that π−π overlapping and localized ordering of the DPP-based copolymers were significantly improved with increasing distance of the bifurcation points of the alkyl side units from the polymer backbones, yielding μ values of up to 5.2 cm2 V−1 s−1 in spun-cast polymer film-based OTFTs.33−36 Wang et al. Received: August 4, 2018 Accepted: November 1, 2018 Published: November 1, 2018 40681

DOI: 10.1021/acsami.8b13292 ACS Appl. Mater. Interfaces 2018, 10, 40681−40691

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthetic Schemes for Four DPPBT-Based Copolymers

van der Waal forces.44−46 We synthesized a series of DPPBTbased copolymers bearing alkyl ester-labeled side chains, which were easily designed via a single-step esterification of two bromoalkanoyl chlorides (i.e., Br−(CH2)3CO−Cl and Br− (CH2)5CO−Cl) and branched alkyl (i.e., OD and DD) alcohols (Scheme 1). Charge-carrier-transport properties of the resulting copolymers, P(ODE3-DPPBT), P(ODE5DPPBT), P(DDE3-DPPBT), and P(DDE5-DPPBT), were characterized and compared to those of P(OD-DPPBT) and P(DD-DPPBT) without ester groups in the branched side chains. We found that the ester moieties induced relatively strong interactions between these side chains in comparison to the alkyl-only system with weak van der Waals interactions. The DPPBT-based copolymers were deposited on a polymergrafted SiO2 dielectric surface from dilute chloroform (CF) solutions. It was shown that the introduction of an additional alkyl ester spacer to branched alkyl chains can shorten the π−π overlapping distance between the well-ordered DPPBT-based polymer chains down to 3.61 Å and produce ordered side chains to form two-dimensional (2D) π-conjugated domains, originating from the Cα−H···O cohesive interactions. The optimal DPPBT-based copolymer film shows the highest μ value of 2.30 cm2 V−1 s−1 in an OTFT, with a high on/off current ratio (Ion/Ioff > 106).

reported DPP-based copolymers with asymmetric alkyl substituents: one branched and another linear chains at each DPP unit.37 The presence of linear alkyl chains in the DPP units could improve the π−π overlapping between the copolymer chains, yielding μ values of up to 9.4 cm2 V−1 s−1 in OTFTs. In addition, a few studies have focused on alkyl side chains with specific functional ends that act as electronegative atoms or groups.38−40 Lee et al. reported that DPP-based copolymers with siloxane-bridged alkyl side chains could facilitate better π−π overlap, producing high hole and electron mobilities in OTFTs.39 Recently, Yao et al. have demonstrated that DPP-based copolymers with urea-bridged alkyl side chains could exhibit good electrical properties.40 Although these kinds of DPP-based copolymers showed high performance in OTFTs, it is difficult to prepare these alkyl groups because of their complicated synthesis routes. Additionally, there are limited studies related to the structures of DPP-based copolymers including alkyl side chains with polar functional units, which can induce other interactions besides van der Waals interactions. Hydrogen bonds are strong molecular interactions and are a key feature of various ordered structures, such as proteins.41,42 Hydrogen bonds are formed when a proton can be shared by two electronegative atoms, such as nitrogen or oxygen. In addition, various proteins exhibit a close contact between a hydrogen bonded to an α-carbon (Cα−H) and X (where Cα is a highly polarized methyl and X is an electronegative atom).43 Cα−H···O-type hydrogen bonds are observed mainly in wellknown protein structures. Here, we modified alkyl side chains to include alkyl ester moieties, which can induce Cα at the nearest methyl group. Novel D−A conjugated copolymers including DPP-bithiophene (DPPBT) derivatives were synthesized with esterbridged alkyl substituents tethered to the N-positions of the DPP moieties. The electronegative O atom in the ester moiety and the H atom bonded to the Cα located at the nearest ester moiety can induce intermolecular attraction, resulting in a better conjugated structure in the D−A backbones compared to the hydrocarbon-only side chain systems, which have weak

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. Materials and all other solvents were used without further purification. The DPPBT-based conjugated copolymers were polymerized via multistep procedures, as reported in the literature (Scheme 1).25 2.1.1. General Synthetic Procedure for the DPPBT-Based Copolymers Bearing Alkyl Ester Side Chains. Tris(dibenzylideneacetone)dipalladium(0) (2.0 μM) and tri(o-tolyl)phosphine (4.0 μM) were added to a toluene solution (12 mL) of compound Cx (C1−C4) (0.1 mM) and 2,5-bis(trimethylstannyl)thiophene (0.1 mM) under a nitrogen atmosphere. The mixture was stirred at 100 °C for 24 h. After cooling the reaction mixture to room temperature, it was added to vigorously stirred methanol (200 mL). The precipitated solid was filtered and washed with methanol. The crude product was purified using Soxhlet extraction with acetone, hexane, and CF successively. The CF fraction was concentrated under 40682

DOI: 10.1021/acsami.8b13292 ACS Appl. Mater. Interfaces 2018, 10, 40681−40691

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Figure 1. Chemical structures of the four DPPBT-based copolymers.

Table 1. Physical and Electrochemical Properties of the DPPBT-Based Copolymers PDI

absorption (nm)

polymer

Mn (kDa)

PDI

solution

P(OD-DPPBT) P(DD-DPPBT) P(ODE3-DPPBT) P(ODE5-DPPBT) P(DDE3-DPPBT) P(DDE5-DPPBT)

77.4 162.6 196.0 192.8 173.2 177.7

1.76 2.40 2.69 3.43 2.70 2.36

778 780 797 789 791 786

energy level (eV)

film 719, 717, 748, 739, 739, 734,

782 776 792 798 785 781

λcut‑offa (nm)

Egopt,b (eV)

Eoxonset,c (V)

HOMOc

LUMOd

958 906 907 953 905 918

1.29 1.37 1.37 1.30 1.37 1.35

0.86 0.84 0.64 0.60 0.66 0.59

−5.31 −5.29 −5.09 −5.05 −5.11 −5.04

−4.02 −3.92 −3.72 −3.75 −3.74 −3.69

Film. b1240/λcut‑off. cObtained from CV sample film on Pt electrode. dLowest unoccupied molecular orbital (LUMO) = highest occupied molecular orbital (HOMO) + Egopt. Eoxonset of ferrocene = 0.35 eV. a

films were deposited on the dielectric surfaces from dilute polymer solutions by the spin-coating method, where each polymer (3 mg) was dissolved in 1 mL of CF. Some copolymer films were annealed at different temperatures (TAs) of 150 and 250 °C each for 30 min.

reduced pressure. The copolymer was then precipitated in methanol and completely dried under vacuum for 24 h to obtain the DPPBTbased copolymers. 2.2. Characterization. 1H nuclear magnetic resonance (NMR) spectra of the intermediate products were obtained with a Bruker 500 MHz spectrometer after dissolving them in deuterated CF (CDCl3, Cambridge Isotope Laboratories, Inc.). Elemental analysis to confirm the C, H, N, and S contents was conducted using an elemental analyzer (Thermo Scientific Flash 2000, Thermo Fisher Scientific). The number-average molecular weights (Mn), weight-average molecular weights (Mw), and polydispersity indices (PDIs, Mw/Mn) of the copolymers synthesized in this study were measured by gel permeation chromatography (GPC; Agilent 1200 Series GPC) with polystyrene (PS) standards in o-dichlorobenzene at 80 °C. The absorption spectra of DPP-based copolymer solutions and thin films were recorded using an ultraviolet−visible absorption spectrometer (Agilent 8453, wavelength (λ) = 190−1100 nm). The redox potentials of the DPPBT-based copolymer films were monitored by cyclic voltammetry (CV, potentiostat Model EQ161, eDAQ, scan rate = 50 mV s−1).30 In addition, the thermal properties of DPPBT-based copolymers were measured by differential scanning calorimetry (DSC, Q20, TA Instruments, heating rate = 10 °C min−1). Atomic force microscopy (AFM, MultiMode 8, Bruker) and synchrotron-based grazing incidence X-ray diffraction (GIXD, 6D and 9A beamlines, Pohang Accelerator Laboratory, Korea) were performed on the DPPBT-based copolymer films on polymer-treated SiO2 surfaces.47 We measured the current−voltage (I−V) characteristics of DPPBT-based copolymer OTFTs using an I−V analyzer (Keithley, 4200 SCS). The μ and threshold voltage (Vth) parameters were calculated by drain current−gate voltage (ID−VG) transfer curves and the following equation: ID = (W/2L)Ciμ(VG − Vth)2, where Ci is the capacitance per unit area (10.8 nF cm−2).47 2.3. Sample Preparation. The PS-grafted SiO2 dielectrics and TFT devices used in this study were fabricated in accordance with the methods described in our previous study.47 DPPBT-based copolymer

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of DPPBT-Based Copolymers. Most studies related to side chain engineering for semiconducting polymers have been limited to characterizing the material and electrical properties of π-conjugated polymer chains including alkyl side chains. Proper side chains substituted to the conjugated backbone moieties can enhance the solubility, flexibility, and specific chain arrangement of the resulting copolymers when solidified from solution.30−36 However, relatively few studies have attempted to functionalize alkyl side chains.39,40 Here, well-known DPP and BT units were selected as A and D units for organizing the π-conjugated polymer backbones, respectively. We investigated the influence of branched alkyl ester-labeled side chains on the chain arrangement and chargecarrier-transport properties of D−A conjugated copolymers. Four different types of branched alkyl ester-labeled side chains (referred to as ODE3, ODE5, DDE3, and DDE5) were introduced to each N-position of the DPP moieties, and the resulting DPPBT-based copolymers were named as follows: P(ODE3-DPPBT), P(DDE3-DPPBT), P(ODE5-DPPBT), and P(DDE5-DPPBT), respectively (Figure 1). To introduce different side chains at the N-positions on the DPP units, we first synthesized the ester-bifurcated alkyl derivatives (R′-Br) by the one-step esterification of bromoalkanoyl chlorides and branched alkyl alcohols according to a reported procedure.48 Intermediates B1−B4 were synthesized 40683

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broader and separated absorption peaks, originating from the 0−1 and 0−0 transitions (Table 1).34 The λmax values in the absorption spectra for E3- and E5-spacer-linked side chain systems for both solutions and films were slightly red-shifted compared to those of P(OD-DPPBT) and P(DD-DPPBT), owing to the minimized geometrical hindrance of the branched alkyl side chains caused by the insertion of these spacers.31,32,34,35 The spun-cast films of P(OD-DPPBT) and P(DD-DPPBT) showed relatively higher onset oxidation potential (Eox) values of +0.86 and +0.84 V, respectively, indicating highest occupied molecular orbital energy level (EHOMO) values of −5.31 and −5.29 eV, respectively (Figure 4). These latter values were calculated from the following equation: EHOMO = −e(Eox + 4.45 V) (Table 1). In contrast, the EHOMO values of the E3and E5-spacer polymer systems were shifted to −5.04 and −5.11 eV, respectively. The HOMO levels became higher as the bifurcation points of the OD and DD chains moved away from the DPPBT backbones, suggesting that the alkyl esterlabeled side chains can induce better conjugated structures for the D−A conjugated backbones. 3.3. Specific Chain Arrangement of DPPBT-Based Copolymers in Spun-Cast Films: Effects of Alkyl Ester Spacers. Figures 5 and 6 show typical AFM topographies of the DPPBT-based copolymer films spun-cast on the polymertreated SiO2 surfaces before and after annealing at 150 and 250 °C for 30 min, respectively. PDPPBT-based copolymer films showed distinct morphologies depending on which side chains were on the D−A conjugated backbones. The as-spun P(ODDPPBT) and P(DD-DPPBT) films contained irregularly percolated nanodomains (10−30 nm in size) (Figures 5a and 6a) because the fast solvent evaporation (80%). In addition, P(OD-DPPBT) and P(DD-DPPBT) were synthesized as reference polymers bearing only the branched alkyl side chains.28,49 The four DPPBT-based copolymers, including the alkyl ester-bifurcated side chains, exhibited better solubility in solvents (e.g., dichloromethane, CF, and chlorobenzene), in comparison to P(OD-DPPBT) and P(DD-DPPBT), which only had branched alkyl side chains. Mn and PDI values of all of the DPPBT-based copolymers determined by the GPC analysis are shown in Table 1. Among the DPPBT-based copolymers, only the P(ODDPPBT) and P(DD-DPPBT) copolymers showed endotherms that could be attributed to semicrystalline melting transition (Figure 2). The peak melting temperature (Tm) of the P(OD-

Figure 2. DSC heating curves of DPPBT-based copolymers (endothermic direction is down).

DPPBT) powder was observed at 303.4 °C during DSC heating (up to 350 °C). P(DD-DPPBT) had a relatively lower Tm of 289.0 °C. However, other D−A copolymers with alkyl ester linkers in the side chain did not show any clear endotherms during heating. The annealing temperature (TA) for the spun-cast DPPBT-based copolymer films was chosen to be below 260 °C based on the onset Tm of P(DD-DPPBT) in the DSC heating curve. 3.2. Optical and Electrochemical Properties of DPPBT-Based Polymers. Absorption spectra and CV profiles of the copolymers were measured and are displayed in Figures 3 and 4a, respectively. The corresponding parameters are also listed in Table 1. Absorption spectra of the DPPBT-based copolymer solutions and films showed dual characteristic bands in the range of λ = 300−1000 nm; low- and highintensity bands were evident at λ = 400−500 and 600−1000 nm, respectively. These bands correspond to π−π* transition and intramolecular charge-transfer transition, respectively.50,51 Unlike the P(OD-DPPBT) and P(DD-DPPBT) solutions, which presented absorption maxima at λ (λmax) = 778 and 780 nm, respectively, the corresponding spun-cast films showed 40684

DOI: 10.1021/acsami.8b13292 ACS Appl. Mater. Interfaces 2018, 10, 40681−40691

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Figure 3. Absorption spectra of the DPPBT-based copolymers in (a, c) solutions and (b, d) films.

Figure 4. (a) CV curves and (b) energy-level band diagrams for the DPPBT-based copolymers.

In conjugated polymer thin films, the localized chain orientation used to achieve an efficient charge-carrier transport is crucial to extend the π-conjugated polymer textures with fewer defect sites. Two-dimensional (2D) GIXD patterns and one-dimensional (1D) X-ray profiles of these π-conjugated

semiconducting copolymer films showed typical X-ray reflections (Figures 7, 8, S7, and S8). The structural parameters are summarized in Table 2. Two-dimensional GIXD patterns of the DPPBT-based copolymer films supported that the chain conformation and 40685

DOI: 10.1021/acsami.8b13292 ACS Appl. Mater. Interfaces 2018, 10, 40681−40691

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ACS Applied Materials & Interfaces

Figure 5. AFM topographies of (a−c) P(OD-DPPBT), (d−f) P(ODE3-DPPBT), and (g−i) P(ODE5-DPPBT) cast films before and after annealing at different TA for 30 min: (a, d, g) as-spun, (b, e, h) 150 °C, and (c, f, i) 250 °C.

Figure 6. AFM topographies of (a−c) P(DD-DPPBT), (d−f) P(DDE3-DPPBT), and (g−i) P(DDE5-DPPBT) cast films before and after annealing at different TA for 30 min: (a, d, g) as-spun, (b, e, h) 150 °C, and (c, f, i) 250 °C.

20.2−20.6 Å. The d(010) value (referred to as the π−π overlapping distance, d(010)) varied from 3.80 to 3.86 Å depending on the TA (see Table 2). The larger π−π overlapping distance of P(OD-DPPBT) was mainly related to the bulkiness of the alkyl side chains closely linked to the D−A backbone, making it difficult to form coplanar and extended chain structures. In contrast, introduction of the alkyl ester spacers between the OD and DPP backbone moieties significantly improved the edge-on chain conformation of the D−A conjugated chains in both as-spun and annealed films on polymer-treated SiO2 surfaces. This was comparable to the engineered side chain D−A copolymer systems with similar alkyl spacer lengths reported elsewhere.34 In addition, OD segments located farther from the π-conjugated D−A backbones produced better coplanar chain conformations, which

orientation of the D−A copolymers were considerably affected by the different substituents tethered to the N-positions of the DPP derivatives. First, most of polymer chains in the as-spun P(OD-DPPBT) film were oriented with an “edge-on” chain conformation, but some chains formed a “face-on”-oriented structure, where the π-stacking direction between the conjugated backbones was normal to the dielectric surface (marked as (010)face‑on in Figures 7a and S7a). In addition, 2D GIXD patterns of the annealed P(OD-DPPBT) films displayed strong reflection intensities due to increased crystallinity; the edge-on chain conformation was more dominant than the faceon one, although the as-spun film contained a small amount of the latter oriented chains. In all of the P(OD-DPPBT) films, the layer spacing values of (100) crystal plane (d(100) = 2π/ΔQ(h00)) were found to be 40686

DOI: 10.1021/acsami.8b13292 ACS Appl. Mater. Interfaces 2018, 10, 40681−40691

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resulted in closer d(010) values of 3.61−3.64 Å.34 After annealing the P(ODE3-DPPBT) and P(ODE5-DPPBT) films, the d(100) values of the crystal planes of the backbone chains that were vertically separated by side chains increased to 27.5−28.6 and 29.5−30.3 Å, respectively. These values were greater than those of P(OD-DPPBT). Interestingly, these values were even higher than those (approximately 26.4 Å) of common D−A copolymers including branched alkyl side chains with linear alkyl spacers, which have a number of similar carbons corresponding to the E3 and E5 segments.34 The results were attributed to the more cohesive interactions between the Cα−H and ester segments in comparison to the van der Waals force experienced by methylene groups.42 The strong interactions between the Cα−H and OC− segments can lead to extended, aligned conformations of the ester-linked side chains, resulting in a closer π−π overlap and an extended structure of D−A π-conjugated backbones. Unlike the P(OD-DPPBT) series, the P(DD-DPPBT) films displayed less intense, oriented X-ray reflections of the (h00) and (010) crystal planes in the 2D GIXD patterns. Specifically, the face-on-oriented chains were increased in the as-spun film and the one annealed at 150 °C (Figures 8 and S8). For the P(DD-DPPBT) films, the d(100) and d(010) values were approximately 23.5−23.8 and 3.84−3.85 Å, respectively. Similar to the P(OD-DPPBT) film, the introduction of the E3 spacers between DD and DPP moieties was found to improve the edge-on chain conformation and provide a closer π−π overlap distance of the D−A conjugated polymer backbones in both the as-spun and annealed films; the d(010) values were determined to be 3.62−3.63 Å. Also, the values of d(100) were 29.4−30.5 and 31.4−32.3 Å for the P(DDE3DPPBT) and P(DDE5-DPPBT) annealed films, respectively. These values were greater than the ∼23.8 Å obtained for P(DD-DPPBT). Similarly, the values were even higher than those of the branched alkyl side chain systems separated by the linear alkyl spacers with a number of similar carbons, corresponding to the E3 and E5 segments. The high charge-transport capability of semiconducting polymer films has been mainly associated with the efficiency of the charge-carrier transport along conjugated (1) chain backbones, (2) domains, and (3) grains.47 Sirringhaus et al. also reported that an indacenodithiophene-based D−A copolymer formed an amorphous-like texture film, but its highly planar chain conformation could yield unexpectedly high μ values in OTFTs.19 3.4. Charge-Carrier Transport Properties of DPPBTBased Copolymers. In our study, solution-cast DPPBTbased copolymer films could be controlled to develop various ordered structures depending on the side chains. P(OD-

Figure 7. Two-dimensional GIXD patterns of (a−c) P(OD-DPPBT), (d−f) P(ODE3-DPPBT), and (g−i) P(ODE5-DPPBT) cast films before and after annealing at different TA for 30 min: (a, d, g) as-spun, (b, e, h) 150 °C, and (c, f, i) 250 °C.

Figure 8. Two-dimensional GIXD patterns of (a−c) P(DD-DPPBT), (d−f) P(DDE3-DPPBT), and (g−i) P(DDE5-DPPBT) cast films before and after annealing at different TA for 30 min: (a, d, g) as-spun, (b, e, h) 150 °C, and (c, f, i) 250 °C.

Table 2. Structural Information for the DPPBT-Based Copolymer Films TA (°C) P(OD-DPPBT) 150 250 P(ODE3-DPPBT) 150 250 P(ODE5-DPPBT) 150 250

d(100) (Å)

d(010) (Å)

20.2 20.5 20.6 28.3 28.6 27.5 29.5 30.3 30.3

3.81 3.80 3.86 3.63 3.64 3.63 3.62 3.61 3.62

TA (°C) P(DD-DPPBT) 150 250 P(DDE3-DPPBT) 150 250 P(DDE5-DPPBT) 150 250 40687

d(100) (Å)

d(010) (Å)

23.5 23.8 23.5 29.4 30.5 30.1 32.3 31.9 31.4

3.84 3.84 3.85 3.63 3.62 3.62 3.62 3.62 3.62

DOI: 10.1021/acsami.8b13292 ACS Appl. Mater. Interfaces 2018, 10, 40681−40691

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Figure 9. ID−VG transfer curves for (a) P(OD-DPPBT)-, (b) P(ODE3-DPPBT)-, (c) P(ODE5-DPPBT)-, (d) P(DD-DPPBT)-, (e) P(DDE3DPPBT)-, and (f) P(DDE5-DPPBT)-based OTFTs (at drain voltage = −40 V).

DPPBT) and P(DD-DPPBT) without alkyl ester spacers formed conjugated domains with a larger π−π overlap distance of 3.80−3.86 Å, while the corresponding alkyl ester-linked systems formed self-assembled structures with much smaller d(010) values of 3.61−3.64 Å. I−V characteristics of top-contact S/D electrode OTFTs containing DPPBT-based films (channel length, L = 100 μm and channel width, W = 1500 μm) were measured. Figure 9 shows typical ID−VG transfer curves for these OTFTs; some devices had a VG-sweep hysteresis. The μ values for DPPBT-based OTFTs are summarized in Figure 10, and other device parameters are shown in Table 3. The P(OD-DPPBT) film-based OTFTs yielded μ values ranging from 0.28 to 0.62 cm2 V−1 s−1. The

Table 3. Electrical Performance of DPPBT-Based OTFTs polymer

TA (°C)

P(OD-DPPBT) 150 250 P(ODE3-DPPBT) 150 250 P(ODE5-DPPBT) 150 250 P(DD-DPPBT) 150 250 P(DDE3-DPPBT) 150 250 P(ODE5-DPPBT) 150 250

μ (cm2 V−1 s−1)

Vth (V)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

−1.50 −6.20 2.20 5.10 8.70 7.90 0.50 0.10 0.10 −0.40 −3.0 −4.4 1.80 −0.80 1.80 2.60 −4.20 1.70

0.28 0.57 0.62 0.35 0.60 0.84 0.50 0.55 1.56 0.80 1.42 1.52 1.10 2.30 1.55 1.15 1.36 2.00

0.03 0.04 0.05 0.05 0.05 0.06 0.05 0.03 0.12 0.05 0.10 0.13 0.05 0.21 0.23 0.15 0.17 0.30

Ion/Ioff >106 ∼2 × ∼2 × >105 ∼2 × ∼4 × ∼2 × >107 >107 ∼2 × ∼104 ∼104 ∼105 ∼105 ∼105 ∼104 >105 ∼2 ×

105 105 105 105 106

104

105

P(ODE3-DPPBT)-based OTFTs showed higher μ values of 0.35−0.84 cm2 V−1 s−1, which is due to an enhancement in πconjugated ordering, specifically, edge-on chain conformation of the copolymer along the two electrodes. In addition, the P(ODE5-DPPBT) with the longest alkyl ester (E5) spacer showed significantly enhanced μ values in OTFTs; specifically, the 250 °C-annealed film exhibited the highest μ value of 1.56 cm2 V−1 s−1 (Table 3). As shown in the AFM and 2D GIXD results, the P(ODE5-DPPBT) films that included the long-

Figure 10. Variations in μ of OTFTs made from the DPPBT-based copolymer films before and after annealing. 40688

DOI: 10.1021/acsami.8b13292 ACS Appl. Mater. Interfaces 2018, 10, 40681−40691

Research Article

ACS Applied Materials & Interfaces Author Contributions

range-ordered edge-on chain conformation showed considerable enhancement in charge-carrier transport. However, the P(DD-DPPBT) system had μ values of around 0.80 cm2 V−1 s−1 (for as-spun film) in the OTFT, which was much higher than that (0.28 cm2 V−1 s−1) of the asspun P(OD-DPPBT)-based devices, and the 250 °C-annealed film-based OTFTs could yield μ values of up to 1.52 cm2 V−1 s−1. The optimal P(DDE3-DPPBT) film annealed at 150 °C showed excellent device performance, presenting a μ value of approximately 2.30 cm2 V−1 s−1, a Vth of −1.8 V, and an Ion/Ioff of about 105. In contrast, the P(DDE5-DPPBT) films showed a relatively degraded electrical performance in comparison to the P(DDE3-DPPBT) system. Optimized P(DDE5-DPPBT) OTFTs showed μ values of up to 2.0 ± 0.30 cm2 V−1 s−1, specifically, for the 250 °C-annealed film. For OTFTs, efficient charge-carrier transport is expected along the π−π overlapped textures of organic semiconductors between the two electrodes. In this case, the charge-carrier mobility is significantly enhanced by the small π−π overlapping distance and planar chain conformation. π-Conjugated domains with significantly smaller π−π stacking distances for the E3- and E5-spacer-linked side chain systems created an efficient conducting path for carrier hopping along semiconducting polymer chains due to the ester group-induced attraction between the side chains. The branched alkyl DD chains that were tethered by the optimal alkyl spacer to the D− A backbones yielded better intra- and intermolecular π−π overlap, producing high-performance OTFTs.

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H.J.K. and M.P. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF2017R1A2B4009313 and NRF2012R1A2A1A01008797) and the Key Research Institute Program (NRF20100020209).

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ABBREVIATIONS DPP, diketopyrrolopyrrole BT, bithiophene D−A, donor−acceptor OTFT, organic thin-film transistor μ, charge-carrier mobility OD, 2-octyldodecyl DD, 2-decyltetradecyl Cα, α-carbon CF, chloroform CV, cyclovoltammetry DSC, differential scanning calorimetry AFM, atomic force microscopy GIXD, grazing incidence X-ray diffraction VD, drain voltage Vth, threshold voltage Mn, number-average molecular weight Mw, weight-average molecular weight Tm, melting temperature

4. CONCLUSIONS Six different diketopyrrolopyrrole (DPP)-based copolymers, including DPP and bithiophene (BT) as electron-acceptor and donor backbone units, respectively, were synthesized to contain branched alkyl side chains, which were either directly coupled to the N-positions of DPP or separated by an alkyl ester group. It was found that the ester moieties form pseudohydrogen bonds with the nearest side chains in comparison to the alkyl-only system with weak van der Waals forces. DPPBTbased copolymers were spun-cast on polymer-treated dielectrics from dilute chloroform solutions for use as channel materials for OTFTs. The optimized P(DDE3-DPPBT) film showed the highest hole mobility of 2.30 cm2 V−1 s−1 with an on/off current ratio greater than 106.

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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.8b13292.

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REFERENCES

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Synthetic procedure, elemental analysis, 1H NMR spectra, and 1D out-of-plane and in-plane X-ray profiles of DPPBT-based D−A copolymer films (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Y.). *E-mail: [email protected] (M.J.C.). *E-mail: [email protected] (D.H.C.). ORCID

Hoichang Yang: 0000-0003-0585-8527 Dong Hoon Choi: 0000-0002-3165-0597 40689

DOI: 10.1021/acsami.8b13292 ACS Appl. Mater. Interfaces 2018, 10, 40681−40691

Research Article

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DOI: 10.1021/acsami.8b13292 ACS Appl. Mater. Interfaces 2018, 10, 40681−40691