Chlorinated 2,1,3-Benzothiadiazole-Based ... - ACS Publications

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Chlorinated 2,1,3-Benzothiadiazole-Based Polymers for Organic Field-Effect Transistors So-Huei Kang,† Grace Dansoa Tabi,‡ Junghoon Lee,† Gyoungsik Kim,† Yong-Young Noh,*,‡ and Changduk Yang*,† †

Department of Energy Engineering, School of Energy and Chemical Engineering, Perovtronics Research Center, Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 44919, Republic of Korea ‡ Department of Energy and Materials Engineering, Dongguk University, 30 Pildong-ro 1-gil, Jung-gu, Seoul 04620, Republic of Korea S Supporting Information *

ABSTRACT: The vital role of introducing chlorine (Cl) atoms onto conjugated polymers, which affects their semiconducting properties, is not yet well understood. A series of donor−acceptor polymers based on dichlorinated-2,1,3-benzothiadiazole (2ClBT) and four different donor moieties with various conjugation lengths (thiophene (T), thieno[3,2-b]thiophene (TT), 2,2′bithiophene (DT), and (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT)) were synthesized and used in organic field-effect transistors (OFETs). The structure−property relationship associated with the 2ClBT-based polymers was thoroughly investigated via a range of techniques, and it was found that a change in the conjugation length of the main backbone could alter energy levels, morphology, and optoelectronic properties, which had a significant effect on the charge transport property. P2ClBT-TVT exhibited superior qualities relative to the other samples with respect to the degree of uniform film-forming ability and molecular organization and charge carrier transport, which resulted in the best hole mobility of 0.147 cm2 V−1 s−1. Furthermore, we also emphasize that for all the polymers no substantial changes were observed in the OFET transfer-curve slopes during 200 testing cycles, indicating excellent operational stability. This study demonstrates that the design of semiconducting polymers possessing Cl atoms was effective at improving operating stability in the OFETs manufactured from them.

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INTRODUCTION

(ICT) interactions from the donor to the acceptor moieties.22−27 Introducing strong electron-withdrawing fluorine (F) atoms into D−A polymer backbones is capable of not only modulating the electronic properties but also strengthening noncovalent interactions of polymers without deleterious steric effects.24,27 As a result, fluorinated BTs have recently emerged as stronger electron accepting moieties relative to BT and have been incorporated into many D−A polymers for use in the field of organic electronics.23,25,26,28 Such intensive research allows

Conjugated polymeric semiconductors are core materials for constructing printed organic field-effect transistors (OFETs), which have potential applications for low-cost radio-frequency identification (RFID) tags, flexible and large area displays, sensors, etc.1−10 It is well recognized that the most successful approach to access high mobility polymer semiconductors in OFETs is to utilize the alternating donor−acceptor (D−A) architecture that ensures better π-stacking structures between large overlaps between cofacial polymer backbones.11−21 Among the various D−A alternating polymers, 2,1,3benzothiadiazole (BT) has been widely used and has been proven to be one of the most promising acceptor units for topperformance D−A polymers owing to its strong electron affinity, which can facilitate intramolecular charge transfer © XXXX American Chemical Society

Received: May 2, 2017 Revised: June 3, 2017

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DOI: 10.1021/acs.macromol.7b00900 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Molecular structure of the 2ClBT-based polymers and (b) the visualized dipole moments of BT derivatives according to the type and number of halogens.

Scheme 1. Synthesis Routes of the 2ClBT-Based Polymers

ethylhexyl)dithieno[3,2-b:2′,3′-d]silole (DTS) moieties to generate a new planar chromophore of 2ClBT flanked by the DTS units (DTS-2ClBT-DTS); the DTS units are able to provide an additional driving force for enhancing intermolecular interactions through their strong electron-donating ability and highly coplanar nature. To investigate the dichlorinated BT impact on the properties of semiconducting polymers, we report the synthesis and characterization of four D−A type polymers incorporating DTS-2ClBT-DTS and different donor units: thiophene (T), thieno[3,2-b]thiophene (TT), 2,2′bithiophene (DT), and (E)-2-(2-(thiophen-2-yl)-vinyl)thiophene (TVT) (see the polymer structures in Figure 1). Furthermore, differences in the optical and electrochemical properties and the molecular structures of the resultant polymers (P2ClBT-T, P2ClBT-TT, P2ClBT-DT, and

us to clearly understand the intriguing impact of the F atom and the working principles. However, limited attention has been given to the chlorine (Cl) atom, which is the second-highest electronegative element in the halogen group, despite its favorable properties for semiconducting materials, e.g., its higher capability to hold electron density and easier accessibility with lower cost compared to the F atom. Besides, Bao and co-workers29 reported that the stability of OFETs can be improved by the lowering of energy levels induced by substitution with Cl atoms. Nonetheless, so far, there has not been one single report that elucidates the detailed molecular properties and device performance of chlorinated BT-containing D−A polymers.29−32 In this study, we chose the dichlorinated BT acceptor (2ClBT) as a core unit and implanted it into two 4,4′-bis(2B


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including chloroform, chlorobenzene, and dichlorobenzene but showed limited solubility in tetrahydrofuran. The molecular weights of the polymers were determined with high-temperature gel permeation chromatography (HT-GPC) at 120 °C with 1,2,4-trichlorobenzene as an eluent. The number-average molecular weights (Mns) of P2ClBT-T, P2ClBT-TT, P2ClBTDT, and P2ClBT-TVT were determined to be 27.4, 24.5, 44.2, and 37.0 kDa, respectively, with polydispersity indices (PDI) of 1.99, 2.51, 2.62, and 2.67, respectively, by HT-GPC. Optical and Electrochemical Properties. The optical properties of the polymers in chloroform solution and as thin films were studied using UV−vis near-infrared (UV−vis NIR) spectroscopy (Figure 2). All the polymers showed typical D−A

P2ClBT-TVT) are thoroughly discussed. Besides, we show the OFET characteristics and ordering structures in the thin films and establish their overall structure−property correlations. Among them, the OFET made from P2ClBT-TVT was shown to have the best hole mobility of up to 0.147 cm2 V−1 s−1, which is due to the extended conjugation length of TVT over the other donor units. Moreover, we are also aware of the high operational stability in OFETs based on the 2ClBT unit. To our knowledge, this study is the first to use 2ClBT in polymer backbones and showed it to be a high-potential acceptor unit for stable semiconducting materials.

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RESULTS AND DISCUSSION Synthesis and Characterization. In our attempt to understand the nature of the 2ClBT unit, we first compared the calculated dipole moments of the BT, mono/difluorinated BT, and mono/dichlorinated BT units. We observed the changes in direction and magnitude of the dipole moments as a function of the type and number of halogen atoms (Figure 1b and Table S1). The chemical structures and synthesis routes of the chlorinated BT-based polymers are shown in Scheme 1. First, 5,6-dichloro-2,1,3-benzothiadiazole (2, dichlorinated BT (2ClBT)) was synthesized by the ring-closure reaction of 4,5dichlorobenzene-1,2-diamine (1) with thionyl chloride and triethylamine and followed by bromination with N-bromosuccinimide (NBS) under acidic conditions (H2SO4), generating 4,7-dibromo-5,6-dichloro-2,1,3-benzothiadiazole (3) in a 75.3% yield. After this, a microwave-assisted Stille coupling reaction between 3 and monostannylated DTS (4) moieties afforded the key intermediate 4,7-bis(4,4-bis(2-ethylhexyl)-4Hsilolo[3,2-b:4,5-b′]dithiophen-2-yl)-5,6-dichloro-2,1,3-benzothiadiazole (DTS-2ClBT-DTS, 5) at a 54.2% yield. It is worth pointing out that the reactivity of the coupling reaction was completely lost when conventional heating was applied; one plausible reason for this may be that the larger size of the Cl atom increases steric demand, causing reduced reactivity of the two bromide functionalities in 3. The purity and structure of 5 were clearly confirmed through both 1H and 13C NMR spectroscopy, matrix assisted laser desorption/ionization timeof-flight (MALDI-TOF) mass spectrometry, and elemental analysis (EA). Finally, adding 2 equiv of NBS into the THF solution of 5 yielded the monomer 4,7-bis(6-bromo-4,4-bis(2ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophen-2-yl)-5,6-dichloro-2,1,3-benzothiadiazole (6) at a 92.7% yield. The various electron-donating stannylated comonomers (2,5-bis(trimethylstannyl)thiophene (T), 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (TT), 5,5′-bis(trimethylstannyl)-2,2′-bithiophene (DT), and (E)-1,2-bis(5-trimethylstannyl)thiophen-2-yl)ethene (TVT)) were polymerized with 6 in a Stille-type polycondensation to yield a series of 2ClBTbased polymers. After work-up, sequential fractionation with methanol, acetone, hexane, and chloroform yielded the polymers P2ClBT-T, P2ClBT-TT, P2ClBT-DT, and P2ClBTTVT. The typical yield of the polymerizations was in the range of 65%−83%. P2ClBT-T and P2ClBT-TT were obtained as dark blue powders, while P2ClBT-DT and P2ClBT-TVT were both dark purple in color (see Figure S1 for the visual images of the polymers). The molecular structure of the polymers was verified by 1H NMR and EA data. The synthesis details and complete characterization of all the intermediates and polymers can be found in the Experimental Section of the Supporting Information. All the resulting polymers were readily soluble in common organic solvents

Figure 2. Absorption profiles of the polymers (a) in dilute chloroform solution and (b) as thin films on a quartz plate.

polymer absorption behaviors with two absorption bands in the 400−500 and 500−800 nm regions, corresponding to localized π−π* transition and ICT bands, respectively. Similar optical features such as absorption maxima (λmax), and onsets were seen in the solution spectra of all the polymers with slightly different intensities between the π−π* and ICT bands. As for the absorption spectra of many conjugated polymers,33−36 both red-shift and band-broadening occurred when transitioning from the solution to the film state, especially for P2ClBT-DT and P2ClBT-TVT, reflecting a relatively stronger aggregation of their backbone chains in the films. This is attributed to the extended π-conjugation lengths of the DT and TVT units over other donor chromophores. Note also that vibrational absorption possibly caused by additional polymer packing arises as a shoulder peak at approximately 730−770 nm in the P2ClBT-TVT film, indicating better π−π stacking between the polymer backbones which can potentially enhance the charge transporting property. The order of the bandgap-lowering abilities of these four co-units is TVT ≅ DT > T > TT, as summarized in Table 1. C


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Macromolecules Table 1. Photophysical and Electrochemical Properties of the Polymers polymer

λmaxsol [nm]

λmaxfilm [nm]

λonset [nm]

Egopt a [eV]

EHOMOb [eV]

ELUMOb [eV]

EgCV [eV]

P2ClBT-T P2ClBT-TT P2ClBT-DT P2ClBT-TVT

580 558 573 574

620 596 661 632

772 752 805 803

1.61 1.65 1.54 1.54

−5.37 −5.34 −5.32 −5.29

−3.42 −3.38 −3.50 −3.45

1.95 1.96 1.82 1.84

Calculated from the absorption band edge of the polymer film, Egopt = 1240/λedgefilm. bThin films in n-Bu4NPF6/CH3CN versus ferrocene/ ferrocenium at 100 mV s−1. HOMO and LUMO were estimated from the onset oxidation and reduction potentials, respectively, assuming the absolute energy level of ferrocene/ferrocenium to be 4.8 eV below vacuum. a

P2ClBT-TT, P2ClBT-DT, and P2ClBT-TVT with internal ferrocene/ferrocenium (Fc/Fc+) as a reference, the oxidation onset potential values were determined to be 0.57, 0.54, 0.52, and 0.49 V, respectively, and the reduction onset potentials were determined to be −1.38, −1.42, −1.30, and −1.35 V, respectively. For an alternating D−A polymer, although one might anticipate that the lowest unoccupied molecular orbital (LUMO) level would be selectively affected by changing the donor units within the same platform, varying them in the 2ClBT-based polymers had a simultaneous effect on both the highest unoccupied molecular orbital (HOMO) and LUMO levels. The HOMO and LUMO levels were calculated based on HOMO (eV) = −(E(ox)onset + 4.8) and LUMO (eV) = −(E(red)onset + 4.8), respectively. Accordingly, the resulting HOMO/LUMO energy levels of polymers P2ClBT-T, P2ClBT-TT, P2ClBT-DT, and P2ClBT-TVT were −5.37/− 3.42, − 5.34/−3.38, −5.32/−3.50, and −5.29/−3.45 eV, respectively, with corresponding electrochemical bandgaps of 1.95, 1.96, 1.82, and 1.84 eV, respectively, which follows the same trend as the optical bandgaps above (the detailed electrochemical properties are listed in Table 1). We can see that despite a nonlinear change in their LUMO values, the HOMO-lying levels of the polymers were slightly raised in the

The electrochemical behavior of the polymers was evaluated by cyclic voltammetry (CV). All polymers showed reversible oxidation and reduction behavior with stronger oxidative peaks than reductive ones (Figure 3). For polymers P2ClBT-T,

Figure 3. Cyclic voltammograms of the polymers in n-Bu4NPF6/ CH3CN solution (scan rate: 100 mV s−1).

Figure 4. Optimized geometries of the dimeric repeating units of 2ClBT-based polymers by DFT calculation and the charge density isosurfaces for the HOMO and LUMO levels. D


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DT, where the dipole of the latter was slightly inclined toward the z-axis. Electrical Characterization and Performance of the OFETs. We evaluated the charge carrier transport properties of the 2ClBT-based polymer OFETs using staggered top-gate/ bottom-contact geometry fabricated on a glass substrate under ambient atmospheric conditions (the detailed fabrication procedure is described in the Experimental Section). To investigate the optimum temperature for the devices, the spincast semiconductor layers (from 5 mg mL−1 in chlorobenzene at 2000 rpm for 60 s) were annealed at various temperatures (150 and 200 °C) for 30 min and cooled. 500 nm thick poly(methyl methacrylate) (PMMA) dielectric (Ci = 6.20 nF cm−2) was subsequently deposited followed by a 50 nm Al gate electrode via thermal evaporation.6,28,39−41 All of the transfer and output plots of the devices showed typical p-type transistor behavior. A summary of the OFETs’ electrical parameters, including maximum hole mobility (μmax), average hole mobility (μavg), threshold voltage (Vth), subthreshold slope (SS), and on/off current ratio (Ion/off), is presented in Table 2. The hole mobilities of the polymers listed in Table 2 with on/off ratios were calculated from the transfer characteristics of the devices in the saturation regime. The hole mobilities increased as the annealing temperature increased up to 200 °C, except for P2ClBT-T, which exhibited the best performance when annealed at 150 °C, as observed from its 2FBT analogue polymer.28 Figure 6 displays the representative transfer and output curves (see Figure S11 for the other data from testing in this study). In addition, Figures 7a and 7b show the hole mobilities for the 2ClBT-based polymers according to annealing temperature and the trend of their maximum hole mobilities associated with Vth values, respectively. It is clear that the mobilities of both the P2ClBT-DT and P2ClBT-TVT polymers were higher than those of either the P2ClBT-T or P2ClBT-TT polymers. Among the tested devices, the OFET made of P2ClBT-TVT revealed the highest hole mobility of 1.47 × 10−1 cm2 V−1 s−1 after annealing at 200 °C. It is interesting to note that the observed mobilities of the four polymers were spread over 3 orders of magnitude despite the small difference in their HOMO energy levels, suggesting that other factors such as directionality, amplitude of the dipole moments, and morphology could also have influenced the charge transport properties. The morphology study is detailed in the next subsection. Furthermore, we also carried out a bias stress test on the 2ClBT-based polymer OFETs under a continuous gate voltage bias (VG = VD = −100 V) for 200 cycles.42 As shown in Figure 8, for all of the samples, there was almost no hysteresis of the OFET transfer-curve slopes during the 200 cycles where in addition to a negligible difference of the on-current levels, only a small decrease was observed in the off-current, which led to an increase in the on−off current ratio indicating high performance with less power leakage. Especially, P2ClBTTVT showed a very low threshold voltage shift of less than ±1 V during the cycling test. Apart from high mobility, stable operation is also a critical issue for commercialization of OFETs. Such impressive operational stability means that introducing 2ClBT units into a polymer backbone is a promising approach toward improving OFETs’ stability. Thin-Film Microstructural Analyses. The thin films were characterized by atomic force microscopy (AFM) and twodimensional grazing incidence X-ray diffraction (2D-GIXD) to understand the relationships between molecular structure, film

order P2ClBT-TVT > P2ClBT-DT > P2ClBT-TT > P2ClBTT, which is essentially consistent with the increasing trend of the conjugation lengths in the repeating backbones. Note that compared to P2ClBT-T, its difluorinated BT (2FBT) analogue previously reported has a similar absorption feature with slightly higher-lying HOMO/LUMO levels (−5.24 eV/−3.37 eV).28 To obtain additional insights into the electronic features, distributions of the HOMO and LUMO levels, and geometryoptimized structures of the polymers depending on the molecular structures of the donor partners (Figure 4), each modeling dimeric repeating unit was subjected to density functional theory (DFT) computation at the B3LYP/6-31G level. The HOMO densities were well delocalized along the whole backbones, while the LUMO densities were mostly localized on the electron-accepting 2ClBT cores. Moreover, all the models showed a high coplanarity (ϕ = 0°−3°) within the donating cores (DTS-T-DTS), while the large torsional angles (ϕ = 32°−34°) between the internal thiophene planes (of DTS) and 2ClBT planes were observed, which is probably correlated to the large size of the Cl atom. This observation agrees with the previous finding that chlorinated polymers have a larger dihedral angle than that of corresponding polymers containing smaller-sized F atoms, resulting in a relatively wider bandgap in the chlorinated polymers.29,30,37,38 For example, the 2FBT analogue of P2ClBT-T showed somewhat smaller torsional angle of ∼15°.28 We also extracted the dipole moments and directions of the dimeric models and collected the results in Figure S10 and Table S2. Figure 5 schematically illustrates the directionality

Figure 5. Dipole moments of the dimeric repeating units of the polymers shown in 3-dimensional space calculated by DFT and a comparison of the magnitude of the dipole vectors along the z-axis (the xy-plane represents the planes of the DTS-Ar-DTS core of the dimeric repeating units).

and amplitude of the dipole moments. The net dipole moments are in the order of P2ClBT-TT (0.65 D) < P2ClBT-TVT (6.51 D) < P2ClBT-T (6.98 D) < P2ClBT-DT (9.22 D). The directions of the dipoles were almost perpendicular to the molecular plane of the DTS-Ar-DTS block in both P2ClBT-TT and P2ClBT-TVT, while they were observed to be in a parallel direction to the molecular plane in P2ClBT-T and P2ClBTE


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Macromolecules Table 2. Electrical Characteristics of the 2ClBT-Based Polymer OFET Devices insulator: PMMA polymera P2ClBT-T

P2ClBT-TT

P2ClBT-DT

P2ClBT-TVT

T [°C] as-cast 150 200 as-cast 150 200 as-cast 150 200 as-cast 150 200

μh,maxb [cm2 V−1 s−1] 3.00 5.30 3.70 4.00 8.10 7.14 1.61 5.51 8.91 2.95 7.01 1.47

× × × × × × × × × × × ×

−4

10 10−4 10−4 10−4 10−4 10−3 10−2 10−2 10−2 10−2 10−2 10−1

μh,avgc [cm2 V−1 s−1] (2.00 (4.40 (3.20 (3.00 (7.60 (6.45 (1.35 (4.19 (7.06 (2.25 (5.61 (1.03

± ± ± ± ± ± ± ± ± ± ± ±

0.006) 0.006) 0.046) 0.030) 0.040) 0.134) 0.190) 1.253) 1.697) 0.852) 1.220) 0.250)

× × × × × × × × × × × ×

−4

10 10−4 10−4 10−4 10−4 10−3 10−2 10−2 10−2 10−2 10−2 10−1

SS [V dec−1]

Vth [V] −27.09 −36.61 −42.90 −25.84 −42.99 −52.01 −57.12 −54.97 −55.04 −50.19 −53.21 −55.03

± ± ± ± ± ± ± ± ± ± ± ±

0.76 1.12 2.31 1.36 0.54 1.89 0.88 0.60 1.41 1.23 1.65 1.67

d

13.77 16.58 17.86 16.13 17.05 19.14 19.96 16.51 17.06 22.03 17.40 16.83

± ± ± ± ± ± ± ± ± ± ± ±

0.39 0.52 0.65 1.69 0.53 0.41 0.86 1.59 1.39 1.93 1.12 0.47

Ion/off (8.07 (3.51 (1.60 (8.26 (4.62 (6.70 (1.80 (1.65 (1.89 (1.06 (1.15 (1.31

± ± ± ± ± ± ± ± ± ± ± ±

0.65) 2.18) 0.39) 1.09) 1.25) 0.26) 0.13) 0.21) 0.12) 0.30) 0.08) 0.07)

× × × × × × × × × × × ×

102 102 102 102 102 102 103 103 103 103 103 103

The OFET performances were tested in a nitrogen atmosphere. bThe maximum of the FET devices (L = 10 μm and W = 1000 μm) obtained from at least 7 devices. cThe average mobility of the FET devices (L = 10 μm and W = 1000 μm) obtained from at least 7 devices. dThe standard deviation.

a

Figure 6. Transfer (a, b) and output (c, d) characteristics of the optimized 2ClBT-based polymer OFET devices at 200 °C with PMMA as the gate dielectric. Figure 8. Transfer characteristics for drain-source voltage of bias stress for 200 cycles. During bias stress, a constant gate-source voltage of −100 V and a constant drain-source voltage of −100 V were applied to all 2ClBT-based polymer OFET devices.

morphology/crystallinity, and device performance. All the films were prepared under the same conditions as the device fabrication. As shown in Figure 9 and Figure S12, the morphologies of all of the films underwent some change, but it was difficult to observe a certain uniform trend with increasing annealing temperature. However, upon thermal annealing, it is clear that P2ClBT-DT and P2ClBT-TVT acquired relatively smaller root-mean-square (RMS) roughness values than those obtained from P2ClBT-T and P2ClBT-TT. Therefore, we speculate that the relatively high performance of P2ClBT-DT and P2ClBT-TVT might be the result of their smooth and uniform thin-film morphology.

Figure 7. (a) Mobility change according to annealing temperature for 2ClBT-based polymer OFETs and (b) mobility change and threshold voltage trend for the 2ClBT-based polymer OFETs under optimum conditions.

F


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Figure 9. AFM height images of the spin-coated 2ClBT-based polymer films: P2ClBT-T (a, e, i), P2ClBT-TT (b, f, j), P2ClBT-DT (c, g, k), and P2ClBT-TVT (d, h, l) at three different annealing temperatures: (a−d) pristine films, (e−h) annealed films at 150 °C, and (i−l) annealed films at 200 °C.

Figure 10. 2D-GIXD images of 2ClBT-based polymer films: P2ClBT-T (a, e, i), P2ClBT-TT (b, f, j), P2ClBT-DT (c, g, k), and P2ClBT-TVT (d, h, l) at three different annealing temperatures: (a−d) pristine films, (e−h) annealed films at 150 °C, and (i−l) annealed films at 200 °C.

the substrate surface.42 These patterns are well-correlated with the observed enhancement in the mobilities of the polymers upon thermal annealing. In particular, compared to the P2ClBT-T and P2ClBT-TT samples, the annealed films of both P2ClBT-DT and P2ClBT-TVT exhibited more intense and sharper (100) diffraction peaks with smaller π−π stacking distances (∼3.60 Å), indicating their improved microstructural ordering with closer π−π intermolecular interactions between the main backbone chains. Actually, this observation is a critical feature for enhancing charge carrier transport. On the basis of a combination of the AFM and 2D-GIXD data, one can conclude

The 2D-GIXD data show that the four polymers adopted distinct orientations in the thin films (Figure 10) (corresponding peak assignments and 1D-line cuts along the out-of-plane (qz) and in-plane (qxy) directions are given in Figure S13 and Table S3). All as-cast 2ClBT polymer films exhibited welldefined lamellar (100) Bragg peaks in both the out-of-plane and the in-plane direction. Thermal annealing of the polymer films resulted in longer-range ordered (h00) diffraction peaks in the out-of-plane direction and a strong (010) π−π stacking peak in the in-plane direction, signifying the formation of highly ordered lamellar structures with edge-on orientation relative to G


Macromolecules that the relative degree of crystallinity and film roughness uniformity of the polymers was able to influence the variations in mobility efficacy.

CONCLUSION In summary, we developed and analyzed four novel 2ClBTbased polymers (P2ClBT-T, P2ClBT-TT, P2ClBT-DT, and P2ClBT-TVT), in which a dichlorinated BT-accepting unit (2ClBT) was introduced into the DTS-based donating backbones in order to investigate the effect of chlorination on the nature of the conjugated polymers. It appears that manipulating counter comonomers (T, TT, DT, and TVT) of varying conjugation length within the backbones of the polymers had a solid effect on their optical properties, electronic structure (HOMO/LUMO energy levels), dipole moment, morphology, charge transport characteristics, and in turn OFET performance. For example, compared to the others, P2ClBT-DT and P2ClBT-TVT had relatively stronger aggregation, smaller bandgaps, finer morphology, and moreordered molecular packing in the films. Ultimately, the best hole mobility of 1.47 × 10−1 cm2 V−1 s−1 was achieved by P2ClBT-TVT with an enhanced conjugation system. More importantly, all OFETs based on 2ClBT units, especially P2ClBT-TVT, showed an excellent operating stability, as evidenced by a negligible change in OFET transfer-curve slopes during 200 cycles. Our study underscores the significance of understanding the structure−property relationship associated with chlorinated conjugated polymers and demonstrates their promising potential in developing high operational stable semiconductors via utilizing chlorination.

ACKNOWLEDGMENTS

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REFERENCES

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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.7b00900. Detailed synthesis procedures and fabrication method, additional figures (DFT calculations for BT units, NMR spectra of new materials, DFT calculations for the dipole moments of the dimeric repeating units, transfer and output characteristics of 2ClBT-based polymers under other annealing temperatures, AFM phase images, and 2D-GIXD data and profiles and summary of crystallographic parameters according to annealing temperatures) (PDF)

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This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2015R1A2A1A10053397 and 2014K1A3A1A19066591) and the Center for Advanced SoftElectronics (2013M3A6A5073183) funded by the Ministry of Science, ICT & Future Planning. The 2D-GIXD experiment at the PLS-II 6D UNIST-PAL beamline was supported in part by MEST, POSTECH, and UNIST-UCRF.

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yong-Young Noh: 0000-0001-7222-2401 Changduk Yang: 0000-0001-7452-4681 Present Address

G.K.: Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea. Author Contributions

S.-H.K. and G.D.T. contributed equally. Notes

The authors declare no competing financial interest. H


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