Dicyanodistyrylbenzene-Based Copolymers for Ambipolar Organic

Oct 10, 2018 - Kranthiraja, Long, Sree, Cho, Cho, Zaheer, Lee, Noh, and Jin. 2018 51 (15), pp 5530–5536. Abstract: Improving π-conjugated polymer e...
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Dicyanodistyrylbenzene-Based Copolymers for Ambipolar Organic Field-Effect Transistors with Well-Balanced Hole and Electron Mobilities Hwa Sook Ryu,† Min Je Kim,‡,§ Moon Sung Kang,∥ Jeong Ho Cho,*,‡,§ and Han Young Woo*,† †

Department of Chemistry, Korea University, Seoul 136-713, Republic of Korea SKKU Advanced Institute of Nanotechnology (SAINT) and §Department of Nano Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea ∥ Department of Chemical Engineering, Soongsil University, Seoul 156-743, Korea

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‡

S Supporting Information *

ABSTRACT: We report three types of dicyanodistyrylbenzene (DCS)-based copolymers (PBDT-DCS, PT-DCS, and PNDI-DCS), which present highly balanced ambipolar charge transport characteristics in organic field-effect transistors (OFETs). The introduction of the DCS moiety in a polymer backbone not only lowers the lowest unoccupied molecular orbital (LUMO) level but also increases the crystalline ordering via interchain dipole−dipole interactions. As a result, the LUMO levels for PBDT-DCS, PT-DCS, and PNDI-DCS were decreased to −3.76, −4.00, and −3.99 eV, respectively, which is beneficial for efficient electron injection from Au electrode for improving ambipolar charge transport. The determined hole/ electron mobilities of the OFETs were 0.064/0.014, 0.492/0.181, and 0.420/0.447 cm2/(V s) for PBDT-DCS, PT-DCS, and PNDI-DCS, respectively, after thermal annealing at 250 °C. By incorporating the electron-deficient naphthalene diimide (NDI) unit in the copolymers, the n-channel transport was enhanced, with decreasing frontier molecular orbitals with enhanced electron injection and impeded hole injection from the Au electrode. Therefore, PNDI-DCS provided completely symmetric output curves in the positive and negative drain voltage regions with almost equivalent hole and electron mobilities. Benefiting from the balanced ambipolar feature of the PNDI-DCS OFETs, a complementary inverter was successfully fabricated.

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close to, −4.0 eV to achieve facile hole and electron injection.3−5 Most homopolymer-type semiconducting organic materials (such as polythiophene and polyfluorene) have an energy band gap greater than ∼2 eV, which creates a high barrier (∼1 eV) to inject electrons or holes from the metal electrode. Therefore, it is necessary to synthesize an organic material with a low band gap for ambipolar OFETs. Copolymerization of electron-rich donor (D) and electrondeficient acceptor (A) units, to form D−A-type narrow band gap polymers with appropriately positioned FMO level orbitals, results in single component ambipolar structures with small injection barriers for both hole and electrons.6−9 However, in these types of OFET devices, most of the D−Atype ambipolar structures show superior hole mobility compared with electron mobility, which is mainly caused by electron traps and the difficulty of injecting electrons from the Au electrode to the LUMO level. The resulting drain current levels at the same gate voltages with opposite polarities are not equivalent. Only a few reports have been published regarding the well-balanced ambipolar OFETs. Yang et al. reported

INTRODUCTION π-Conjugated semiconducting organic materials have been intensively studied for applications in flexible and solutionprocessed electronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaic cells (OPVs). Their frontier molecular orbital (FMO) energy levels and electronic structures can be facilely tuned by modification of the molecular design. The OFET technology has great potential in realizing low cost, flexible logic circuits, and displays. In particular, ambipolar OFETs are very useful for the construction of various organic electronic devices, such as organic light-emitting transistors and complementary logic circuits, by controlling the electron/hole balance in the transistor channel by the gate voltage.1,2 Balanced charge (electron and hole) transport is particularly important for these applications. Although ambipolar charge transport has been previously demonstrated using blends or bilayers of p- and ntype organic semiconducting materials, the achievement of ambipolarity based on a single molecule is an ideal strategy because of easy device fabrication. Organic ambipolar semiconductors must meet the highest occupied molecular orbital (HOMO) energy level below ca. −5.0 eV and lowest unoccupied molecular orbital (LUMO) level below, or at least © XXXX American Chemical Society

Received: August 7, 2018 Revised: September 30, 2018

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DOI: 10.1021/acs.macromol.8b01700 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Routes to DCS-Based Polymersa

Reagents and conditions: (i) THF, n-BuLi, DMF, −73 °C; (ii) t-BuOK, t-BuOH, 50 °C; (iii) THF, n-BuLi, (CH3)3SnCl, −78 °C; (iv) Pd2(Dba)3, P(o-Tol)3, hexabutylditin, toluene, 90 °C; and (v) Pd2(dba)3, P(o-Tol)3, toluene.

a

suitable substituent to induce both ambipolar characteristics as well as high crystallinity.19−21 In this study, three different types of DCS substituted semiconducting polymers were synthesized, and their ambipolar charge transport characteristics were investigated by fabricating OFET devices. The cyano substituents effectively lower the LUMO level because of their superior electron-withdrawing capability. The band gap of each synthesized polymer was less than 1.8 eV, and the determined LUMO levels were −3.76, −4.00, and −3.99 eV for PBDTDCS, PT-DCS, and PNDI-DCS, respectively. This property is advantageous for electron injection from the Au electrode and yields ambipolar charge transport due to their narrow band gap. All of the resulting polymers were thermally stable, and the charge mobility increased significantly with thermal treatment. The hole/electron mobilities after thermal annealing at 250 °C were 0.064/0.014, 0.492/0.181, and 0.420/0.447 cm2/(V s) for PBDT-DCS, PT-DCS, and PNDI-DCS, respectively. In particular, PNDI-DCS had almost equivalent hole and electron mobilities with symmetric output curves in the positive and negative drain voltage regions. Finally, the complementary inverter was successfully fabricated by connecting two PNDI-DCS OFETs in series.

PDTDPP-alt-BTZ containing two types of electron-deficient moieties of diketopyrrolopyrrole (DPP) and benzothiadiazole (BTZ),10 with symmetrical transfer/output characteristics and well-balanced hole and electron mobilities of 0.1 and 0.09 cm2/(V s), respectively. The inverter using p- and n-channel OFETs was also fabricated using the ambipolar polymer PDTDPP-alt-BTZ, which showed a relatively high gain value of 35 in a single-component-based OFET inverter. The electron-rich and -deficient substituents, such as alkoxy, fluorine, and cyano, can be introduced to induce intermolecular dipole−dipole interactions and enhanced intermolecular ordering. Among these, the cyano groups are not only strong electron-withdrawing groups but also induce high crystalline ordering.11−16 The incorporation of highly electronwithdrawing cyano groups on the backbone induces a permanent dipole in the molecule, which increases the intermolecular Coulombic interactions between neighboring molecules. As a result, the backbone becomes planar in a solid film state, and the cofacial π−π stacking interaction between adjacent molecules becomes stronger. In several cases, J-type aggregates have been reported for CN-containing molecules by stacking in a sideway fashion via the repulsion between the neighboring bulky cyano groups. Furthermore, the aggregation-induced planarization extends the effective conjugation length. Park et al. also reported the n-type acceptor (2E,2′E)3,3′-(2,5-bis(hexyloxy)-1,4-phenylene)bis(2-(5-(4-(N-(2-ethylhexyl)-1,8-naphthalimide)yl)-thiophen-2-yl)acrylonitrile) (NIDCS-HO), demonstrating a power conversion efficiency of 7.64% with PPDT2FBT as a donor polymer.17 In the molecular structure of NIDCS-HO, β-dicyanodistyrylbenzene (DCS) containing two CN substituents between phenylene and thiophene moieties effectively lowers the LUMO level and induces high crystallinity in the solid state. A β-DCS-based ptype polymer donor (PBDCS) was also reported, with a deep HOMO (−5.59 eV), low band gap (1.75 eV), and nanocrystalline structure.18 Therefore, the cyano groups can be a

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RESULTS AND DISCUSSION The synthetic routes and molecular structures of three DCSbased polymers are depicted in Scheme 1. Four monomers (M1−M4) were synthesized by modifying the previously reported procedures.18,22−24 The Stille coupling polymerizations of the DCS monomer (M1) and stannylated monomers (M2, M3, and M4) were performed in toluene using Pd2(dba)3 and P(o-tol)3 as a catalyst system in a microwave reactor to produce three copolymers (PBDT-DCS, PT-DCS, and PNDI-DCS) in 50−80% yields. The chemical structures of the monomers and polymers were confirmed by 1 H NMR spectroscopy (Supporting Information). The B

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Figure 1. Normalized UV−vis absorption spectra in (a) chlorobenzene and (b) thin films. (c) Cyclic voltammograms of DCS-based copolymers.

Table 1. Summary of the Molecular Weights as Well as the Thermal, Optical, and Electrochemical Properties PBDT-DCS PT-DCS PNDI-DCS

Mn

PDI

Tda [°C]

λabs/sol [nm]

λabs/film [nm]

Eoptb [eV]

HOMOc [eV]

LUMOd [eV]

41.3 12.9 4.74

2.55 1.35 1.33

352.3 391.5 382.8

572.5 603.5 (660)e 549.5

594.5 608.5 (660.5)e 574.5 (643)e

1.77 1.72 1.78

−5.53 −5.72 −5.77

−3.76 −4.00 −3.99

Decomposition temperature with 5% weight loss by TGA under nitrogen. bOptical band gap was determined from absorption onset in film. HOMO level was estimated from the oxidaton onset potential of CV measurement. dLUMO level was estimated from the HOMO level and optical band gap in film. eAbsorption shoulder peak.

a c

the solution spectra, the PT-DCS and PNDI-DCS films showed noticeably enhanced shoulder peaks at 660.5 and 643 nm, respectively, suggesting tighter interchain packing in the films. In particular, PT-DCS showed a clear shoulder peak at 660 nm even in solution, suggesting a significant preaggregation in solution. The temperature-dependent UV−vis spectra (Figure S2) were also measured in chlorobenzene with changing temperatures from 30 to 80 °C. With increasing temperature, the spectra of PT-DCS were gradually blueshifted with decreased shoulder peaks, suggesting gradual disaggregation. The other two polymers also showed a similar trend, indicating a strong interchain interaction of the DCScontaining polymers via intermolecular dipole−dipole Coulombic interactions. This pronounced intermolecular organization may be beneficial for forming crystalline morphology in films and improving charge carrier transport in OFET devices. Based on the absorption onset of the films, the determined optical band gaps were 1.77, 1.72, and 1.78 eV for PBDT-DCS, PT-DCS, and PNDI-DCS, respectively.26 The electrochemical properties were investigated by cyclic voltammetry (CV) using Ag/AgCl as a reference electrode, a platinum wire as a counter electrode, and a platinum electrode coated with a thin polymer film as a working electrode. The measured onset oxidation potentials of PBDT-DCS, PT-DCS, and PNDI-DCS were 0.66, 0.85, and 0.91 V, respectively, relative to the internal standard (ferrocene/ferrocenium couple, Fc/Fc+). Based on the equation EHOMO = −(4.8 + + Eoxonset − E1/2Fc/Fc ), the HOMO energy levels of the polymers were determined to be −5.53, −5.72, and −5.77 eV for PBDTDCS, PT-DCS, and PNDI-DCS, respectively. The estimated LUMO energy levels of PBDT-DCS, PT-DCS, and PNDIDCS were −3.76, −4.00, and −3.99 eV, respectively, based on their HOMO level and corresponding optical band gap. The incorporation of electron-deficient NDI moiety significantly decreased both the HOMO and LUMO energy levels, to induce an enhanced n-type character for PNDI-DCS. The three semiconducting polymers showed HOMO and LUMO levels close to −5.6 eV and −4.0 eV, respectively, which can lead to the efficient injection of electrons and holes from the

molecular weights of the polymers were determined by hightemperature gel-permeation chromatography (GPC) at 80 °C using 1,2-dichlorobenzene as the eluent, relative to polystyrene as a standard. The number-average molecular weights (Mn) and polydispersity indices (PDI) for PBDT-DCS, PT-DCS, and PNDI-DCS were determined to be 41 kDa and 2.55, 13 kDa and 1.35, and 5 kDa and 1.33, respectively. The low molecular weight of PNDI-DCS is probably due to the significantly decreased reactivity of the Stille coupling between two electron-deficient monomers.20,25 All of the synthesized polymers showed good solubility in common organic solvents, i.e., chlorobenzene, dichlorobenzene, chloroform, and toluene. As shown in Figure S1, the thermal properties were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The TGA data showed that the thermal decomposition temperatures (Td, 5% weight loss) of PBDTDCS, PT-DCS, and PNDI-DCS were 352.3, 391.5, and 382.8 °C, respectively, showing good thermal stabilities. For the DSC data (with a heating rate of 5 °C/min), no obvious thermal transitions were observed for all three polymers in the temperature range 30−350 °C under a nitrogen atmosphere. The UV−vis absorption spectra were measured in dilute chlorobenzene and in film (Figure 1). In the chlorobenzene solution, PBDT-DCS, PT-DCS, and PNDI-DCS reached the maximum absorptions at 572.5, 603.5, and 549.5 nm, respectively. PBDT-DCS and PT-DCS had red-shifted absorptions relative to that of PNDI-DCS because of the stronger intramolecular charge transfer (ICT) interaction from the electron-sufficient benzodithiophene or thiophene to the electron-deficient DCS unit. In the case of PNDI-DCS, the ICT interaction is relatively weaker due to the electrondeficient nature of both the NDI and DCS moieties. In addition, the relatively lower Mn of PNDI-DCS may also contribute to the blue-shift in the absorption of PNDI-DCS in solution by influencing its solubility and the resulting morphology in solution. The films showed broader and redshifted absorption compared to the spectra in the solution, which is attributed to the enhanced backbone planarity and strong intermolecular π−π stacking in a solid state. Contrary to C

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Figure 2. DFT calculated frontier molecular orbital structures and energy-minimized conformations of three polymers based on a dimeric unit.

Figure 3. (a) Transfer characteristics of the OFETs based on DCS-based semiconducting polymers by varying the annealing temperatures. (b) Comparison of the hole and electron mobilities of as-cast films. (c) Variation of the carrier mobilities as a function of the annealing temperature. (d) Output characteristics of the OFETs based on 250 °C-annealed PNDI-DCS film.

Au electrode, to realize ambipolar properties in OFET devices. The optical, thermal, and electrochemical properties are summarized in Table 1. Density functional theory (DFT) calculations based on B3LYP/6-31G* were performed to gain a theoretical insight into the electronic structures and energy minimum geometries of DCS-based polymers (Figure 2). To simplify the calculation, the alkyl side chains were replaced with methyl groups, and the calculation was performed based on a dimeric unit. The calculated HOMO/LUMO levels of PBDT-DCS, PT-DCS, and PNDI-DCS were −5.04/−3.02, −5.03/−3.00, and −5.39/ −3.54 eV, respectively, showing good agreement with the experimental measurements. According to the DFT calculation, PBDT-DCS and PT-DCS have a planar molecular backbone, but PNDI-DCS has a tilted backbone with a torsional angle of ∼55° between the DCS and NDI moieties. However, this calculation was performed in the gas phase, and the real chain conformation in a solid state may differ. In a solid film, the intermolecular Coulombic and π−π stacking interactions between adjacent molecules become stronger and aggregation-induced planarization occurs. The presence of the ambipolarity of the DCS-based polymers is partially explained by the delocalized HOMO and LUMO on the entire conjugated backbone (see the HOMO and LUMOs for PBDT-DCS and PT-DCS in Figure 2). Normally D−A-type

conjugated copolymers show hindered electron transport because of the localized character of the LUMO on the electron-deficient regions. PNDI-DCS is also expected to have delocalized HOMO and LUMO in a solid film state, which will be discussed in the following section. Three DCS-based copolymer semiconductors were applied as the channel layers of the OFETs (Figure 3). The OFETs with bottom-contact and top-gate geometries were fabricated onto a Si wafer. First, the Cr/Au (3/17 nm) source and drain electrodes were patterned though thermal evaporation. The channel length and width were 100 and 800 μm, respectively. The chloroform (CF) or chlorobenzene (CB) solution containing 5 mg/mL polymers were spin-coated on the substrate with prepatterned source-drain electrodes and then dried in a vacuum chamber to remove residual solvent. The thickness of three polymer films was in the range of 38−43 nm. The polymer films were thermally annealed at 100, 150, 200, and 250 °C in a vacuum chamber. The poly(methyl methacrylate) (PMMA) gate dielectric layer with a thickness of ca. 495 nm was deposited onto the semiconductor film via spin-coating 70 mg/mL n-butyl acetate polymer solution (1000 rpm for 30 s). Finally, the 50 nm thick Al gate electrode was patterned onto the channel region. The OFET measurements were done under vacuum condition (∼10−3 Torr). D

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Macromolecules Table 2. Carrier Mobilities of the DCS-Based OFETs (Units: cm2 V−1 s−1) carrier PBDT-DCS PT-DCS PNDI-DCS

hole electron hole electron hole electron

25 °C 0.003 0.001 0.085 0.032 0.040 0.042

(±0.001) (±0.0005) (±0.008) (±0.003) (±0.004) (±0.01)

100 °C 0.004 0.004 0.140 0.069 0.082 0.096

150 °C

(±0.002) (±0.002) (±0.03) (±0.019) (±0.022) (±0.03)

0.008 0.005 0.212 0.110 0.118 0.134

(±0.003) (±0.002) (±0.031) (±0.031) (±0.031) (±0.033)

200 °C 0.025 0.012 0.342 0.118 0.144 0.153

250 °C

(±0.003) (±0.006) (±0.034) (±0.031) (±0.034) (±0.035)

0.064 0.014 0.492 0.181 0.420 0.447

(±0.006) (±0.004) (±0.049) (±0.038) (±0.042) (±0.044)

Figure 4. 2D-GIXD images of the (a) PBDT-DCS, (b) PT-DCS, and (c) PNDI-DCS films with varying annealing temperature.

°C for CF) allowed sufficient time for a well-ordered assembly of polymer chains during the drying process, improving the crystalline microstructure of the semiconductor film.31−34 The as-spun PT-DCS OFETs exhibited ∼1 order higher carrier mobilities of 0.085 cm2/(V s) for the holes and 0.032 cm2/(V s) for the electrons compared with those of the PBDT-DCS OFETs (Figure 3b). These improved carrier mobilities must be related to the enhanced crystalline film morphology of the PT-DCS film, which showed a good agreement with most pronounced shoulder peak in the UV−vis spectrum of PTDCS. The detailed intermolecular packing and film morphologies will be discussed in the following section. Note that the NDI unit in the backbone yielded OFETs with very wellbalanced mobilities of 0.04 cm2/(V s) for both the holes and electrons. The balanced carrier mobilities are attributed to the enhanced n-type character by incorporating the electronwithdrawing NDI moiety in the DCS-based polymeric backbone. Although both the PBDT-DCS and PT-DCS polymers showed ambipolar OFET characteristics, they both

The black curve in Figure 3a shows the transfer characteristics of the OFETs based on the as-spun DCS-based semiconductor films prepared from CB solutions in the holeenhancement (VD = −60 V) and electron-enhancement (VD = +60 V) modes. All three curves exhibited clear V-shaped ambipolar charge transport characteristics. There was negligible hysteresis between the forward and reverse sweeps, which shows strong contrast with the SiO2 back-gate device case, indicating the lower charge trapping density at the DCS polymers−PMMA interface.27,28 The carrier mobilities (μ) of the OFETs were calculated in the saturation regimes according to the equation ID = CSμW(VG − VTH)2/2L,6,29,30 where ID is drain current, CS is the specific capacitance of PMMA, W and L are the channel width and length, respectively, and VG and VTH are gate and threshold voltages, respectively. The hole and electron mobilities of the as-spun PBDT-DCS were 0.003 and 0.001 cm2/(V s), respectively. Note that these values were superior to those prepared with CF (Figure S3). The slow evaporation of CB with a high boiling point of 131 °C (vs 61 E

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Figure 5. GIXD line-cut profiles in the (a) OOP and (b) IP directions.

Two-dimensional grazing incidence X-ray diffraction (2DGIXD) measurements were performed with changing annealing temperatures to investigate the interchain packing and orientations in semiconductor films (Figures 4 and 5). The pristine PBDT-DCS film showed face-on dominant bimodal orientation, with (100) lamellar scatterings in both the out-ofplane (OOP) and in-plane (IP) directions together with an OOP (010) π−π stacking peak (qz ∼ 1.49 Å−1and d-spacing 4.2 Å). Upon thermal annealing at 150 and 250 °C, the π−π stacking diffraction became pronounced with a reduced dspacing from 4.2 to 4.0 Å, indicating a tighter interchain packing with thermal treatments. The higher order lamellar diffraction peaks up to (300) in the OOP and IP directions appeared with thermal annealing, suggesting the coexistence of face- and edge-on crystallites. The as-cast PT-DCS film exhibited OOP lamellar diffractions up to (400) and a (010) π−π stacking diffraction (qxy ∼ 1.60 Å−1 and d-spacing 3.93 Å) in the IP direction, indicating predominant edge-on orientation, which can lead to more efficient carrier transport in the parallel direction in the OFET devices,35 compared with PBDT-DCS film. Based on the OOP (200) lamellar diffraction peak, the estimated d-spacing was 21.4 Å for the as-cast PTDCS film. The lamellar d-spacing slightly decreased to 20.9 Å with thermal annealing at 250 °C. We also calculated the crystal coherence length (CCL) based on the full width at halfmaximum (fwhm) values of the OOP (200) peak to compare the relative crystallinity.36,37 The CCL value increased from 65.0 to 85.7 and 92.7 Å before and after thermal annealing at 150 and 250 °C, showing a clear increase in the crystallinity of thermally annealed PT-DCS. This shows a good agreement with the enhanced carrier mobilities by thermal annealing,4,38 which can be attributed to the enhanced intermolecular packing or ordering. In the case of the PNDI-DCS, the as-cast film exhibited Bragg spots39,40 and is expected to have mixed edge- and face-on bimodal orientation. The Bragg spots became clear upon 150 °C annealing, and after 250 °C annealing, the multiple (h00) peaks in the OOP direction become sharp, and the other Bragg scattering peaks almost disappeared. This indicates an enhanced edge-on orientation

still had p-dominant transport properties because of the electron-rich BDT and thiophene moieties. By replacing these moieties with the electron-deficient NDI, the n-type character was enhanced with decreasing p-type character, yielding wellbalanced ambipolar charge transport in PNDI-DCS. Furthermore, the lowered HOMO level of NDI-DCS may have impeded the hole injection from the Au electrode, which is balanced with the electron injection from Au to the LUMO level of PNDI-DCS. The values of VTH and the on−off current ratio (ION/IOFF) in both sides of the hole and electron transports were also well-balanced for the PNDI-DCS OFETs. The electrical properties including μ, VTH, and ION/IOFF of all of the OFETs are summarized in Table 2 and Table S1. The DCS-based semiconductor films were thermally annealed by varying the temperature up to 250 °C. Figure 3c shows the variation of the carrier mobilities as a function of annealing temperature. The thermal annealing dramatically improved both the hole and electron mobilities of the OFETs. For example, the hole and electron mobilities of the 250 °Cannealed PBDT-DCS OFETs were 0.064 and 0.014 cm2/(V s), respectively, showing noticeable improvement by 10−20 times compared to the as-spun film. The other two polymers also showed a similar enhancement of the carrier mobilities by thermal annealing. The measured carrier mobilities of the 250 °C-annealed films were 0.492 cm2/(V s) (hole) and 0.181 cm2/(V s) (electron) for PT-DCS and 0.420 cm2/(V s) (hole) and 0.447 cm2/(V s) (electron) for PNDI-DCS. This remarkable mobility enhancement is strongly related to the annealing-induced development of the edge-on orientation (discussed later in detail). The ION/IOFF ratio of the OFETs with the three 250 °C-annealed copolymers was approximately 105−106. Figure 3d shows the output characteristics of the OFETs with 250 °C-annealed PNDI-DCS, exhibiting wellbalanced ambipolar charge transport characteristics. Note that the PNDI-DCS OFETs exhibited nearly identical (and symmetrical) output curves in both sides of hole and electron transports compared with other PBDT-DCS and PT-DCS cases (Figure S4). F

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Figure 6. Pole figures extracted from the (100) lamellar diffraction of (a) PBDT-DCS, (b) PT-DCS, and (c) PNDI-DCS. (d) Az/Axy value as a function of thermal annealing temperatures.

polymer crystallites. The larger Az/Axy values of PT-DCS and PNDI-DCS agree well with the much higher carrier mobility of PT-DCS and PNDI-DCS compared with PDBT-DCS. This result demonstrates that changes in the crystalline orientation greatly influence the charge mobility and also suggests good agreement with the OFET charge transport characteristics with thermal annealing. The surface morphology of the DCS-based polymer films with different annealing temperatures was monitored by atomic force microscopy (Figure S5). The measured surface root-mean-square (RMS) roughness values were 1.1, 1.4, and 2.2 nm for the as-spun PBDT-DCS, PTDCS, and PNDI-DCS films, respectively. These values increased slightly (1.6 nm for PBDT-DCS and 2.6 nm for PNDI-DCS) or substantially (4.2 nm for PT-DCS) after thermal annealing at 250 °C, which is related to the enhanced interchain packing and enlarged crystallites in the annealed films. Benefiting from the balanced ambipolar feature of the PNDI-DCS OFETs, the complementary inverter, a key building block of digital logic circuits, was successfully fabricated as shown in Figure 7. Two identical PNDI-DCS OFETs were connected in series. One OFET was connected to ground, while the other OFET was connected to the supply electrode. Both transistors shared the same input (VIN) and output (VOUT) terminals. The corresponding circuit diagram of the complementary inverter is shown in the inset. Figure 7 displays the voltage transfer characteristics of the complementary inverter at different supply voltages (VDD). The tunable conduction between the holes and electrons of the ambipolar OFETs yielded operation in the first (positive VDD and VIN) and third (negative VDD and VIN) quadrants. As the absolute values of VDD increased from 10 to 60 V, a signal

with highly ordered molecular packing, which is consistent with the ∼10 fold mobility enhancement with thermal annealing at 250 °C. The 2D-GIXD data are consistent with the UV−vis absorption and device characteristics, both with and without thermal annealing. The PNDI-DCS was calculated to have a larger torsional angle (∼55°) than the other two polymers in the gas phase, but the polymer is expected to have a well-ordered structure even in neat film before thermal annealing based on the OFET and 2D-GIXD data. We postulate that the high carrier mobilities of PNDI-DCS can be attributed to the facile charge transport owing to the strong intermolecular cofacial packing by planarized NDI rings in a solid state and the extended π-conjugation via the NDI moiety, which can further reorganize into a well-ordered and tightly packed structure with thermal treatment. To further investigate the interchain orientation changes by thermal annealing, pole figure analyses were also performed by choosing the (100) lamellar diffraction peaks of three polymer films (Figure 6). The pole figures were then integrated according to each polar angle region: the integrated peak areas in the range of q = 0−45° and 135−180° (Axy) correspond to the face-on orientation, and q = 55−125° (Az) corresponds to the edge-on orientation fraction (see the inset of Figure 6a).41,42 An increase in the ratio Az/Axy implies an enhanced edge-on orientation alignment. The thermal annealing at higher temperature in the PT-DCS and PNDI-DCS films exhibited increased Az/Axy values. In particular, PT-DCS has the highest Az/Axy value of 1.64 with thermal annealing at 250 °C among the three polymers. In the case of the PNDI-DCS film with 250 °C annealing, the Az/Axy value was 1.53 times larger than that of the as-cast film, indicating a substantially increased edge-on orientation portion in the PNDI-DCS G

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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/acs.macromol.8b01700.

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Synthesis of monomers and polymers, TGA/DSC thermograms, additional OFET device characteristics, AFM morphology, and NMR spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Moon Sung Kang: 0000-0003-0491-5032 Jeong Ho Cho: 0000-0002-1030-9920 Han Young Woo: 0000-0001-5650-7482 Notes

The authors declare no competing financial interest.

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Figure 7. Voltage transfer characteristics of the complementary inverter based on the PNDI-DCS OFETs. The inset shows the circuit diagram of the inverter.

ACKNOWLEDGMENTS H. S. Ryu and M. J. Kim contributed equally to this work. This work was supported by the National Research Foundation (NRF) of Korea (2016M1A2A2940911, 2015M1A2A2057506, and 2015R1D1A1A09056905) and the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (NRF2013M3A6A5073177), Korea.

inversion was clearly observed depending on VIN: VOUT remained comparable to the VDD values obtained at low | VIN|, while VOUT approached 0 V at high |VIN|. The signal gain, an absolute value of dVOUT/dVIN of inverter, was determined to be ∼3 at VDD = +60 V. Note that almost the same inversion properties were obtained in the first and third quadrants, which may be attributed to the symmetric conduction between hole and electron.

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REFERENCES

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CONCLUSION

Three types of DCS-based copolymers were synthesized, and their ambipolar charge transport characteristics were studied by fabrication of bottom-contact and top-gate OFET devices. Their charge transport properties with thermal treatments were in good agreement with the optical, electrochemical, and morphological analyses. The DCS building block effectively lowered the LUMO level because of the strong electronwithdrawing capability of the cyano substituents, yielding LUMO levels of −3.76, −4.00, and −3.99 eV for PBDT-DCS, PT-DCS, and PNDI-DCS, respectively. In addition, the wellordered semicrystalline morphology was obtained via the favorable intermolecular Coulombic interactions originating from a permanent dipole by the incorporation of cyano substituents. As a result, the measured hole/electron OFET mobilities for PBDT-DCS, PT-DCS, and PNDI-DCS were 0.064/0.014, 0.492/0.181, and 0.420/0.447 cm2/(V s), respectively, after thermal annealing at 250 °C. Interestingly, PT-DCS and PNDI-DCS showed enhanced edge-on orientation with thermal annealing, as revealed by a pole figure analysis with significantly increased Az/Axy values with the thermal treatments. The well-balanced hole/electron transports were achieved in the PNDI-DCS OFETs by incorporating the electron-deficient NDI unit in the copolymers. H

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