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Thiazole-Flanked Diketopyrrolopyrrole Polymeric Semiconductors for Ambipolar Field-Effect Transistors with Balanced Carrier Mobilities Zhihui Chen, Dong Gao, Jianyao Huang, Zupan Mao, Wei-Feng Zhang, and Gui Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08543 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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ACS Applied Materials & Interfaces

Thiazole-Flanked Diketopyrrolopyrrole Polymeric Semiconductors for Ambipolar Field-Effect Transistors with Balanced Carrier Mobilities Zhihui Chen,#,†,‡ Dong Gao,#,†,‡ Jianyao Huang,† Zupan Mao,† Weifeng Zhang,† and Gui Yu*,†,‡ †

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy

of Sciences, Beijing 100190, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

KEYWORDS: thiazole, diketopyrrolopyrrole-based polymer, field-effect transistor, balanced mobilities, ambipolar semiconductor

ABSTRACT: In this paper we report three thiazole-flanked diketopyrrolopyrrole-based donor– acceptor alternating copolymers as new ambipolar semiconductors and their field-effect transistor devices with balanced hole and electron mobilities. Nitrile groups are introduced into polymer backbone and the substituent effect on electronic structures is studied. Different side chains are also involved to tune the interdigitation of the polymers. To probe the structural effects that contribute to the device performances, we provide insight into the thin film microstructures and morphologies. Top-gate bottom-contact transistors fabricated under ambient

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conditions exhibit the impressive balanced hole and electron mobilities as high as 1.46 and 1.14 cm2 V−1 s−1, respectively, which are among the highest values reported for ambipolar thiazoleflanked diketopyrrolopyrrole-based polymers. Additionally, this class of ambipolar polymers also shows promise for complementary-like inverters with a high gain value of 163.

Introduction

π-Conjugated polymers have attracted significant attention because of their potential applications in organic electronics. These materials exhibit excellent solution processability and intriguing properties to afford low-cost, large-area, and flexible circuits.1–4 Recently, device performance of organic field-effect transistors (OFETs) has received significant development and their mobilities are comparable to those of benchmark amorphous silicon-based materials (ca. 1 cm2 V−1 s−1).5–16 To meet the criteria for applications in complementary logic circuits, both pand n-channel transistors with matching mobilities are of particular importance.17–20 The obstacle in complementary-like inverters distinctly combining p- and n-type semiconductor components can be simply overcome by ambipolar materials, because these materials are readily processed without the additional and complex lateral patterning of the individual transistor.21 Holetransport polymeric semiconductors have been made dramatic progress in performance over the last decade, whereas electron-transport or ambipolar counterparts remain insufficiently developed. For instance, among the polymeric semiconductors reported up to date, hole mobility as high as a few dozen cm2 V−1 s−1 has been obtained,22–24 whereas ambipolar polymeric semiconductors with balanced hole and electron mobilities are rare. The difficulty in obtaining the polymeric semiconductors with high electron mobilities is the chemical modification for finetuning the molecular frontier orbitals to afford adequately deep-lying lowest unoccupied molecular orbitals (LUMOs) and to reduce electron-trapping, in which electron-deficient


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moieties or substituents are involved.25 In contrast to the electron-rich building blocks, electrondeficient ones, e.g. thiazole, halogen, and nitrile groups, generally encounter synthetic difficulties associated with sensitivities to basic conditions. The attainment of balanced ambipolar carrier mobilities also requires a compromise between intrinsic molecular electronic structures and device fabrication techniques. Despite that suitable energy levels for effective charge injection are generally required, there is always a fundamental problem in predicting hole and electron mobilities.26 Therefore, developing new ambipolar polymers is still a challenge in organic electronics. Preparing polymers with donor–acceptor (D–A) structures has been one of the most successful strategies to modify the frontier orbital energy levels.3,27 This class of polymers not only provides noncovalent intramolecular interactions planarizing backbones and ensuring excellent π-conjugation, but also engenders preferable cross-axis dipoles to stack closely and in turn enhancing intermolecular charge hopping.7,17 Backbone modification and side chain engineering of D–A polymers can lead to significant enhancement of carrier mobilities.28–30 Combining with a bislactam as the acceptor and a chalcogenophene derivative as the donor, one can readily develop a semiconducting polymer. In this paper, we report the synthesis and characterization of thiazole-flanked diketopyrrolopyrrole (DPP) based polymers. Thiazole-flanked DPP has proven to be good electron-accepting moiety for the application in organic photovoltaics.31–32 Unlike the thiophene-flanked DPP with conformations containing C–H···O interactions, thiazole-flanked DPP exhibits a predominant S···O conformational locking occurring along the backbone.33 The difference in preferential conformations leads to distinct backbone linearity and molecular orientation, providing an insight into the influences of heteroatoms on intrinsic charge transport properties. The introduction of 2,3-bis(thiophen-2-yl)acrylonitrile (CNTVT) into the polymer


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backbones was regarded as an effective method to improve n-channel charge transport, attaining a high electron-mobility of 7.0 cm2 V−1 s−1.26,34–35 In spite of its asymmetrical molecular structure, the nitrile group substituted at the vinylene linkage does not break the coplanarity, thereby maintaining the long-range ordering of the polymer backbone. We accordingly prepared three alternating copolymers with thiazole-flanked DPP acceptor and CNTVT donor. These polymers exhibited good ambipolar charge transport behaviors with balanced hole and electron mobilities as high as 1.46 and 1.14 cm2 V−1 s−1, respectively. The complementary-like logic circuits with a high gain value of 163 were fabricated. Side chain engineering was used to optimize the molecular stacking and in turn to enhance the device performances. Experimental Section Instruments and Measurement. The starting materials, alkylated thiazole-DPPs and (E)-2,3bis(5-(trimethylstannyl)thiophen-2-yl)acrylonitrile, were synthesized according to previously reported procedures.31,34 1H NMR spectra were measured on a Bruker DMX 300 spectrometer. Chemical shifts were reported as δ values (ppm) relative to internal tetramethylsilane (TMS). UV-vis-NIR absorption spectra were recorded on a J–570 spectrophotometer. Cyclic voltammetric measurements were performed on a CHI660c electrochemical workstation at a scanning rate of 100 mV s−1 with a glassy carbon disc as the working electrode, a Pt wire as the counter electrode, an Ag/AgCl electrode as the reference electrode, and 0.1 M tetrabutylammonium hexafluorophosphate dissolved in CH3CN as the supporting electrolyte. Gel permeation chromatography (GPC) on a PL-220 system was used to determine molecular weights. 1,2,4-Trichlorobenzene was used as the eluent and the column temperature was 150 °C. Detailed Procedures for Device Fabrication. Bottom-gate bottom-contact (BGBC) OFETs were fabricated on a highly doped n++-silicon wafer with a 300-nm SiO2 dielectric layer. Silicon


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was used as the bottom-gate electrode. The source−drain gold electrodes were prepared by photolithography. The substrate surfaces were cleaned by ultrasonication in acetone, deionized water, and ethanol. Octadecyltrichlorosilane (OTS) self-assemble monolayer was formed on the gate insulator after treatment in vacuum. Polymer thin film was then spin-coated onto the OTStreated substrates from a hot solution in o-dichlorobenzene (concentration: 5 mg/mL). The spincoating speed was 2000 rpm. Further annealing treatment was performed on a hot plate at optimized temperatures for 5 min in nitrogen before being allowed to cool to room temperature. OFET devices with top-gate bottom–contact (TGBC) architecture were fabricated on OTStreated SiO2 substrates with gold source–drain electrodes. The procedures of cleaning and spincoating were identical to the above mentioned method. Polymer films were annealed on a hot plate at 40 °C for 5 min in nitrogen. Polymethylmethacrylate (PMMA) (Mw = 1000 kDa) was dissolved in anhydrous n-butyl acetate (Aldrich, concentration: 80 mg/mL) and stirred for 3 h. Then the solution was spin-coated onto the semiconductor surface at a speed of 2000 rpm (thickness of PMMA layer: ~ 1 μm). The prepared samples were then dried at 80 °C for 30 min before evaporation of an aluminum gate electrode. The thickness of Al gate was about 100 nm. OFET parameters were measured on a Keithley 4200 SCS semiconductor parameter analyzer. BGBC FETs were measured in nitrogen and TGBC FETs were measured under ambient conditions. The relative humidity was 30–40% during testing. The mobilities were extracted from saturated regions using the equation, IDS = (W/2L)Ciμ(VGS − Vth)2, where L is channel length (50 μm), W is the channel width (1400 μm), IDS is source–drain saturation current, Ci is the capacitance per unit area of the insulator, μ is the mobility, VGS is the gate voltage, and Vth is the threshold voltage.


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The complementary-like inverters were fabricated similarly to TGBC transistors. The complementary-like inverters consisting of two ambipolar transistors were fabricated on Corning glass substrates, with a common input gate terminal (VIN) and a common output terminal (VOUT). The inverters were also tested under ambient conditions. Scheme 1. Synthetic Approach towards Copolymers

Reagents and conditions: i) NaH, RI, DMF, 110 °C; ii) NBS, CHCl3, reflux; iii) Pd2(dba)3, P(otol)3, toluene, 110 °C.

Synthesis of the polymer PTDCNTVT. Compounds 1 (Scheme 1), 2a, and 3a were synthesized

according

to

reported

procedures.36

3,6-Bis(thiazol-2-yl)-2,5-bis(2-

decyltetradecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2a). 1H NMR (300 MHz, CDCl3, δ): 8.05 (d, J = 3 Hz, 2H), 7.69 (d, J = 3 Hz, 2H), 4.36 (d, J = 9 Hz, 4H), 1.87 (s, 2H), 1.1 (br, 80H), 0.88 (t, J = 9 Hz, 12H). 13C NMR (75 MHz, CDCl3, δ): 160.79, 154.80, 143.75, 137.58, 123.37, 110.12, 46.39, 37.45, 31.49, 31.00, 29.60, 29.26, 29.24, 29.21, 29.18, 28.92, 25.94, 22.25, 13.67. HR-MS (MALDI-TOF): calcd for C60H102N4O2S2, 974.74442; found, 974.74446. 3,6-Bis(thiazol-2-yl)-2,5-bis(4-decyltetradecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2b). To 20 mL of anhydrous DMF in a 2-neck round bottom flask were added compound 1 (1.0 g, 3.3


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mmol) and NaH (60% mineral oil, 0.33 g, 8.25 mmol) under an argon atmosphere. Then the mixture was refluxed for 1 h. After cooling down the reactant to room temperature, 4decyltetradecyl iodide (4.60 g，9.9 mmol) was added and subsequently was heated at 110 °C for 12 h. The mixture was then cooled to room temperature and extracted with chloroform and washed three times with water. The organic solution was dried over Na2SO4 and concentrated, and the crude product was purified by silica gel chromatography with the eluent of hexane and tetrahydrofuran (THF) (5:1) to yield the title compound as a purple solid (1.38 g, 43%). 1H NMR (300 MHz, CDCl3, δ): 8.07 (d, J = 3 Hz, 2H), 7.71 (d, J = 3 Hz, 2H), 4.36 (t, J = 7.5 Hz, 4H), 1.68 (m, 4H), 1.24 (br, 78H), 0.88 (t, J = 9 Hz, 12H).

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C NMR (75 MHz, CDCl3, δ): 160.89,

155.35, 144.46, 137.59, 123.91, 110.53, 42.91, 37.33, 33.61, 33.35, 31.94, 30.18, 29.74, 29.68, 29.38, 26.68, 24.03, 22.70, 14.09. HR-MS (MALDI-TOF): calcd for C60H102N4O2S2, 974.74442; found, 974.74444. 3,6-Bis(thiazol-2-yl)-2,5-bis(5-decylpentadecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2c). This compound was synthesized according to the method of 2b (1.84 g, 55%). 1H NMR (300 MHz, CDCl3, δ): 8.08 (d, J = 3 Hz, 2H), 7.71 (d, J = 3 Hz, 2H), 4.40 (t, J = 7.5 Hz, 4H), 1.69 (dd, J = 10.5 Hz, 9Hz, 4H), 1.25 (br, 82H), 0.88 (t, J = 9 Hz, 12H). 13C NMR (75 MHz, CDCl3, δ): 160.86, 155.35, 144.43, 137.60, 123.91, 110.52, 63.61, 43.22, 37.22, 37.02, 33.57, 31.94, 30.43, 30.13, 29.72, 29.67, 29.36, 26.66, 22.77, 14.12. HR-MS (MALDI-TOF): calcd for C62H106N4O2S2, 1002.77572; found, 1002.77433. 3,6-Bis(5-bromo-thiazol-2-yl)-2,5-bis(2-decyltetradecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)dione (3a). 1H NMR (300 MHz, CDCl3, δ): 7.91 (s, 2H), 4.27 (d, J = 6Hz, 4H), 1.86 (s, 2H), 1.22 (br, 80H), 0.87 (t, J = 7.5 Hz, 12H).

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C NMR (75 MHz, CDCl3, δ): 161.03, 156.33, 145.73,

137.00, 115.83, 110.73, 47.02, 37.94, 31.94, 31.43, 30.05, 29.72, 29.70, 29.64, 29.38, 26.39,


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22.71, 14.13. HR-MS (MALDI-TOF): calcd for C60H100Br2N4O2S2, 1130.56545; found, 1130.56680. 3,6-Bis(5-bromo-thiazol-2-yl)-2,5-bis(4-decyltetradecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)dione (3b). Compoud 2b (1.0 g, 1.03 mmol) was dissolved in 50 mL of chloroform under argon atmosphere and fit with a condenser in the dark. N-Bromosuccinimide (NBS) (0.45 g, 2.52 mmol) was added and the reactant was refluxed for 24 h. Thereafter, additional 0.45 g of NBS was added and the reaction was continued for another 24 h. The reactant system was then quenched with saturated sodium sulfite and extracted with chloroform. The organic extracts was dried over Na2SO4 and concentrated, and the crude product was purified by column chromatography using hexane and THF (6:1) as the eluent to yield the target compound 3b as a purple solid (0.52 g, 43%). 1H NMR (300 MHz, CDCl3, δ): 7.93 (s, 2H), 4.32 (t, J = 7.5Hz, 4H), 1.66 (m, 4H), 1.25 (br, 78H), 0.88 (t, J = 7.5Hz, 12H).

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C NMR (75 MHz, CDCl3, δ): 160.66, 156.40, 145.85,

136.65, 115.88, 110.80, 43.34, 37.02, 33.57, 31.94, 30.40, 30.12, 29.74, 29.68, 29.37, 26.61, 22.70, 14.12. HR-MS (MALDI-TOF): calcd for [M+H] C60H101Br2N4O2S2, 1131.57327; found, 1131.57217. 3,6-Bis(5-bromo-thiazol-2-yl)-2,5-bis(5-decylpentadecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)dione (3c). This compound was synthesized according to the method of 3b (0.54 g, 47%). 1H NMR (300 MHz, CDCl3, δ):. 7.93 (s, 2H), 4.32 (m, 4H), 1.84–1.48 (m, 4H), 1.25 (br, 82H), 0.88 (t, J = 6.6 Hz, 12H).

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C NMR (75 MHz, CDCl3, δ): 160.68, 156.37, 145.81, 136.63, 115.87,

110.80, 43.02, 37.32, 33.61, 33.34, 31.94, 30.18, 29.74, 29.68, 29.38, 26.69, 24.00, 22.70, 14.12. HR-MS (MALDI-TOF): calcd for C62H104Br2N4O2S2, 1158.59675; found, 1158.59710. General Procedures for Polymerization Reaction. Thiazole-flanked DPP (0.2 mmol), (E)2,3-bis(5-(trimethylstannyl)thiophen-2-yl)acrylonitrile

(0.2

mmol),


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tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3, 9 mg], and tri(o-tolyl)phosphine [P(o-tol)3, 12.3 mg] were dissolved in toluene (20 mL). The obtained solution was purged with argon through three freeze-pump-thaw degassing circles. Then the reaction mixture was heated at 100 °C with stirring for 24 h under argon atmosphere. The resulting mixture was poured into methanol and was stirred for 2 h. The crude polymer was filtered and the low-molecular-weight fractions were removed by Soxhlet extraction with methanol, acetone, hexane, and chloroform. The residue was finally extracted with chlorobenzene to give the final product. PTDCNTVT-1. GPC: Mn = 53.7 kDa, PDI = 3.61. 1H NMR (300 MHz, C2D2Cl4, δ): 7.92 (Br, 8H), 6.91 (Br, 1H), 4.24 (Br, 4H), 1.90–0.80 (m, 94H). PTDCNTVT-2. GPC: Mn = 22.4 kDa, PDI = 2.48. 1H NMR (300 MHz, C2D2Cl4, δ): 7.95 (Br, 8H), 6.98 (Br, 1H), 4.37 (Br, 4H), 1.68–0.81(m, 94H). PTDCNTVT-3. GPC: Mn = 35.2 kDa, PDI = 2.81. 1H NMR (300 MHz, C2D2Cl4, δ): 7.84 (Br, 8H), 6.95 (Br, 1H), 4.27 (Br, 4H), 1.40–0.80 (m, 98H).

Results and Discussion

Synthesis. The thiazole-flanked diketopyrrolopyrrole monomers were synthesized according to previous methods.36 Alkyl iodides with different branched sites were employed in the step of N-alkylation of the diketopyrrolopyrrole moieties and the bromination with NBS afforded the brominated monomers. The Stille coupling reaction was used to synthesize all three polymers. Tris(dibenzylideneacetone)dipalladium(0) and tri(o-tolyl)phosphine were employed as the catalyst and ligand, respectively. The obtained polymers were fully characterized by NMR and high-temperature GPC against the polystyrene standards. The number average molecular weights are determined as 53.7, 22.4, and 35.2 kDa for PTDCNTVT-1, PTDCNTVT-2, and


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PTDCNTVT-3, respectively. The molecular weights are comparable to most of n-type semiconducting polymers and are solution-processable.37–38

Figure 1. Dihedral torsional energies of (a,b) CNTVT and (c) TVT subunits. Geometries were calculated at the B3LYP/6-31G(d) level, and the single point energies were calculated at the MP2/cc-pVTZ level. Theoretical Calculations. Intermolecular packing and self-assembly of polymers have proven to be affected by conformations. To access the conformational preferences of the polymers, we performed geometry optimizations using density functional theory (DFT) at the B3LYP/6-31G(d) level.39 In the calculations, methyl groups were used as the alkyl side chains of all polymers for simplicity. First, the conformation of 2,3-bis(thiophen-2-yl)acrylonitrile moiety was optimized and dihedral potential energy scans (PESs) were carried out to confirm the energetically favorable conformation. Relaxed PESs were performed at 10°intervals and single-point Møller– Plesset perturbation theory (MP2) calculations at the cc-pVTZ level were further employed to determine the torsional energies. We also calculated the torsional potential of unsubstituted di(2thienyl)ethene (TVT) at the same level of theory for comparison. As shown in Figure 1, the nitrile substituent does not cause distortion of the molecular structure. This is mainly due to the absence of steric hindrance of the S-trans conformer. When the dihedral angle Φ1 is set to 0°, the total energy is destabilized by the CH–CH van der Waals repulsions in the eclipsed S-cis


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conformer, as well as the weak repulsion between the lone pair electrons of the sulfur atom and π-electrons of the nitrile group. The energy difference between the two rotamers of TVT is 4.69 kJ/mol with a torsional barrier of 22.24 kJ/mol. For CNTVT, the energy difference increases to 6.10 kJ/mol with a local energy-minimum at a dihedral angle of about −20°, indicating both the conformational fluctuations and the destabilizing effect of the nitrile substituent on the eclipsed rotamer. The transition-state conformer, however, is slightly stabilized by the π-orbital coupling, which is the primary reason for the decreased torsional barrier. The other inter-ring dihedral potential was also investigated. The torsional barrier is 26.07 kJ/mol and the energy difference between two planar rotamers (Φ2 = –180° and Φ2 = 0°) is 1.32 kJ/mol. The nitrile group is conjugated to this rotational thiophene ring, which provides the stabilization to planar conformations. In light of the above results, we consequently summarize the substituent effect of this nitrile group: 1) the less bulky nitrile group substituted at the vinylene linkage allows maintenance of the all-trans planar conformation, which is demonstrated by single crystal characterization of the similar 2,3-di(thiophen-2-yl)fumaronitrile derivative,40 although the dihedral torsional barriers and energy differences are slightly altered; 2) the nitrile group is conjugated to one thiophene ring and is cross-conjugated to the other, revealing populations of inequivalent bond lengths (Figure S1); 3) a noticeable molecular dipole moment (calculated value: 3.79 Debye) can induce a shift in electrostatic potential and subsequently affect intermolecular packing; 4) the molecular asymmetry also indicates asymmetric charge distributions, which can affect the regioselectivity of reaction sites. The transmetalation step in Stille reaction is promoted by electron-donating organotin compounds.41 The more negative charge the bonding carbon atom has, the more reactive the reaction site is. The Mulliken charges were calculated to be –0.211 and –0.173 for


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carbon atoms at 5-positions of the 2- and 3-thiophenyl rings, respectively. Therefore, the reaction site at the thiophene cross-conjugated to the nitrile group is deduced to be more reactive, obtaining a possible symmetrical intermediate triad during polymerization, although the selectivity is not sufficient to form a regioregular alternating copolymer. This result indicates the presence of local pseudosymmetry in spite of the overall global backbone regioirregularity.

Figure 2. Frontier molecular orbitals, energy levels, and inter-ring dihedral angles of regioregular and regioirregular trimers of PTDCNTVTs. We further study the electronic structures of PTDCNTVT trimers. To illustrate the discrepancy in frontier molecular orbitals of regioregular (RR) and regioirregular (RI) polymers, we changed the substitution position of nitrile group in the central unit. For each input geometry guess, an all trans conformation with respect to neighboring moieties was used and the inter-ring dihedral angle for thiophene-thiazole subunit was initially set to 0.5°according to previously reported torsional potential studies. Figure 2 shows the optimized structures. Both trimers exhibit good planarity, with frontier orbitals being delocalized over the backbone. The calculated dihedral


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angles between each conjugated ring are close to 0° (< 1°). The regioregular trimer exhibits larger dihedral angles because the less linear backbone and larger dipole moment increase interring torsions. The frontier orbital coefficients are slightly different. The HOMO and LUMO of RR-trimer are delocalized on the peripheral rings, whereas those of RI-trimer are more localized on the central rings. The more extended π-orbitals of RR-trimer are due to the cumulative resonance effects of nitrile groups, which pull electrons in the same direction without disrupting effective conjugation. On the contrary, the cancellation of electron-pulling in the opposite directions in RI-trimer not only slightly suppresses the π-extension, but also results in a smaller dipole moment and a more linear polymer main chain. The calculated HOMO energy levels of the two trimers are identical because of the absence of nitrile’s contribution. Given that the electron-acceptor resembles the LUMO of the hybridized π-orbital, the more electron-deficient nature of the central thiazole-DPP acceptor in RI-trimer leads to a lower LUMO energy level. Optical and Electrochemical Properties. The UV-vis-NIR absorption spectra of PTDCNTVT-n in solution and thin film are shown in Figure 3. The corresponding results are summarized in Table 1. The broad absorption band in the range of 500–900 nm is a typical feature of D–A polymers, which is mainly attributed to intramolecular charge transfer character. The solution absorption maxima are all 770 nm for PTDCNTVT-1, PTDCNTVT-2, and PTDCNTVT-3. The introduction of different side chains slightly changes the relative intensities of the two main absorption peaks. The polymer thin films exhibit the wider absorption bands than those in solution. Side chains exert great effects on solid-state aggregation, leading to changes in absorption edges. The optical energy gaps were calculated to be 1.42, 1.39, and 1.41 eV for PTDCNTVT-1, PTDCNTVT-2, and PTDCNTVT-3, respectively. Compared with


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previously reported materials with different side chains,42–45 we attribute the great solution-tosolid absorption changes to intermolecular heteroatom-heteroatom interactions in these polymers.

Figure 3. UV-vis-NIR absorption spectra of PTDCNTVT-n in solution (a) and thin films (b). Cyclic voltammograms (CVs) were measured to estimate frontier orbital energy levels of the polymeric semiconductors. In Figure S2, all polymer thin films show reduction waves in the electrochemical window of the electrolytic solution. The LUMO energy levels were evaluated from the onset of reduction potential to be −3.66, –3.72, and –3.72 eV for PTDCNTVT-1, PTDCNTVT-2, and PTDCNTVT-3, respectively. The HOMO energy levels are in the range of – 5.73 to −5.81 eV, which demonstrates the strong electron-deficient nature of PTDCNTVT-n polymers stabilizing the frontier molecular orbitals. The relatively low LUMO energy levels of these polymers are expected to be favorable for electron injection because of the decreased energetic mismatch between the work function of Au electrode and the LUMO energy levels. Table 1. Optical and Electrochemical Properties of PTDCNTVT-n polymers

Polymer

λmax (nm)

Egopt (eV)

ELUMO (eV)

EHOMO (eV)

soln.

film

PTDCNTVT-1

770

702

1.42

–3.66

–5.73

PTDCNTVT-2

770

776

1.39

–3.72

–5.80

PTDCNTVT-3

770

702

1.41

–3.72

–5.81


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Electrical Properties. The electrical properties of the polymers were initially studied in BGBC transistor devices under nitrogen atmosphere. The mobilities at different annealing temperatures were optimized (Supporting Information, Figure S3). The highest hole and electron mobilities of PTDCNTVT-1 are 0.153 and 0.0453 cm2 V−1 s−1, respectively.

Figure 4. Transfer and output curves of the TGBC FETs based on polymers: (a,b) PTDCNTVT1, (c,d) PTDCNTVT-2, and (e,f) PTDCNTVT-3. Through side chain engineering technique, corresponding parameters of PTDCNTVT-2 increase to 0.325 and 0.0924 cm2 V−1 s−1. PTDCNTVT-3 exhibits hole and electron mobilities of 0.0737


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and 0.0395 cm2 V−1 s−1, respectively. The changes in mobilities are rationalized by thin film microstructures in the following discussion. TGBC transistor devices generally exhibit good electrical properties under ambient conditions. A PMMA solution in butyl acetate (80 mg/mL, with a thickness of about 1000 nm) was spincoated on semiconductors as the dielectric layer. OFET characteristics of the polymers are collected in Table 2. All polymers exhibit ambipolar transport characteristics, as shown in Figure 4. The leakage current was on the order of 10 nA, allowing the accurate estimation of carrier mobilities from the transfer curves (Figure S4). Especially, ambipolar transport behaviors of PTDCNTVT-2 are achieved with balanced hole and electron mobilities of 1.46 and 1.14 cm2 V−1 s−1, respectively, which are among the highest values of thiazole-flanked DPP-based polymers.31– 32,46

The relatively low hole and electron mobilities were obtained for PTDCNTVT-1 (0.772 and

0.494 cm2 V−1 s−1). Meanwhile, the polymer PTDCNTVT-3 shows the charge carrier transporting characteristics with hole and electron mobilities of 0.489 and 0.202 cm2 V−1 s−1, respectively. This trend is in accordance with that measured in the BGBC configurations.

Figure 5. Complementary inverter based on PTDCNTVT-2. As mentioned above, OFET based on PTDCNTVT-2 shows balanced hole and electron mobilities. Therefore, PTDCNTVT-2 was used as the active layer to prepare the complementary inverter as shown in Figures 5, S5 and S6. We measured the performance of the inverters under ambient conditions based on the TGBC architecture. A typical voltage transfer curve of


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ambipolar inverters would show nonsaturation behavior. As illustrated in Figure S6, the inset shows the zoomed-in view of the transfer curve at the high supply voltage (VDD). Indeed, the curve does display the nonsaturation characteristics. The repeated measurements would greatly increase the value of threshold voltage. The voltage transfer characteristics curve at VDD = 120 V was the last measured one. As a result, the curve only exhibits the nonsaturation behavior in the high-voltage region. A high gain value of 163 was achieved, providing potential in ambipolar complementary inverters. Table 2. OFET Characteristics of the PTDCNTVT-n Polymers

Polymer

TGBC devicesa μh (cm2 V−1 s−1)

Ion/Ioff

Vth (V)

μe (cm2 V−1 s−1) p-channel n-channel p-channel n-channel

PTDCNTVT-1

0.67 ±0.10

0.40 ±0.097

102 – 103 102 – 103 −40 ±15

15 ±5

PTDCNTVT-2

1.28 ±0.18

0.97 ±0.17

102 – 103 102 – 103 −40 ±15

18 ±4

PTDCNTVT-3

0.43 ±0.059

0.18 ±0.022

103 – 104 103 – 104 −40 ±15

40 ±5

a

Channel width = 1400 μm, channel length = 50 μm. The average values were extracted from 20 devices. Thin Film Microstructures and Surface Morphologies. To understand the structural features of the polymer thin films, we performed grazing incidence X-ray diffraction (GIXRD) tests at various annealing temperatures. The GIXRD patterns of the PTDCNTVT-n thin films are shown in Figures 6 and S7-9, and the corresponding data are collected in Table S1. All thin films exhibit lamellar structures, although the GIXRD patterns vary greatly. GIXRD patterns generally display two models of packing, edge-on and face-on model, where molecules pack in the directions perpendicular and parallel to the substrates, respectively. PTDCNTVT-1 exhibits bimodal orientation distribution of edge-on and face-on domains with apparent (010) peaks observed in both out-of-plane and in-plane directions. Up to the third order diffraction peak was


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observable along the qz axis, indicating the long-range ordered lamellar structure. The annealing procedure increases the fraction of edge-on texture, which is the thermodynamically stable orientation.47 Moving the branching site away from the backbone increased the edge-on orientational preferences. Both PTDCNTVT-2 and PTDCNTVT-3 exhibit edge-on textures revealed by the presence of the (010) signal in the in-plane direction. The difference in mobilities of PTDCNTVT-2 and PTDCNTVT-3 is attributed to the odd–even effects of alkyl chains on crystalline lattice. Our results also suggest that the edge-on orientation has positive effect on charge transport properties of thiazole-DPP polymer-based devices. In addition to the orientational preferences, noticeable differences in lamellar stacking distances were observed. The values of interchain d-spacing for PTDCNTVT-1, PTDCNTVT-2, and PTDCNTVT-3 calculated from the out-of-plane (h00) peaks are 21.68, 23.03 and 24.04 Å, respectively. These results are attributed to the increasing side chain lengths and change in interdigitation.48–49 Annealing procedure is revealed to further increase the d-spacing distances. Upon annealing treatment, d-spacing distance increases to 21.86 Å for PTDCNTVT-1, as well as 24.48 Å for PTDCNTVT-3. For comparison, PTDCNTVT-2 exhibits negligible changes in dspacing distance, indicating the similar crystalline tendency and self-assembly of the as-spun and annealed thin films of PTDCNTVT-2. On the basis of the peak positions of (010), π-π stacking distances for PTDCNTVT-1, PTDCNTVT-2, and PTDCNTVT-3 are estimated to be 3.58, 3.53, and 3.53 Å, respectively. The close π-π stacking distance in PTDCNTVT-2 thin film promotes the π-stacking interactions and results in a densely ordered structure preventing water and oxygen penetration, consistent with the highest ambipolar carrier mobilities.2 It is obvious that the different branching position has a dramatic effect on the side chain interdigitation.2,23,50–52


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Figure 6. 2D-GIXRD patterns of as-spun (left) and annealed (right) polymer thin films: (a) PTDCNTVT-1, (b) PTDCNTVT-2, and (c) PTDCNTVT-3. To elucidate the effect of the film morphology on device performance, atomic force microscopy (AFM) was performed. As shown in Figures 7 and S10-12, all polymer thin films display homogeneous microcrystalline features without distinct variations in nanoscale morphology.53 The surface morphology of all semiconductors became coarser upon heating, with the observable formation of larger grains. Given that PTDCNTVT-2 provides large interconnected domains sizes after annealing treatment, the mobility is higher than those of the other two polymers. For comparison, PTDCNTVT-3 formed comparatively poor film surface morphology with an obvious increase of surface roughness. Larger grains were unfavorably interconnected with numerous and enhanced grain boundaries which hinder the charge transport. This result strongly suggests that surface morphologies correlate well with electrical properties.


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Figure 7. AFM topography images (3 × 3 μm2) of the polymer thin films: (a) as-spun PTDCNTVT-1, (b) PTDCNTVT-1 annealed at 140 °C, (c) as-spun PTDCNTVT-2, (d) PTDCNTVT-2 annealed at 140 °C, (e) as-spun PTDCNTVT-3, and (f) PTDCNTVT-3 annealed at 140 °C. Conclusion

We synthesized three thiazole-flanked DPP based polymers and fabricated OFETs based on them. The introduction of nitrile groups in polymer backbone leads to low-lying LUMO energy levels, despite the regioirregularity of the polymers. Theoretical calculations enable the evaluation of the substituent effects of nitrile groups, which are amenable to experimental observations. Ambipolar device behaviors with balanced hole and electron mobilities of 1.46 and 1.14 cm2 V−1 s−1 and inverters with a gain value of 163 have been achieved in terms of side-chain engineering and device optimization. Thin film microstructures provide significant insights into


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the differences of device performances. The PTDCNTVT-2 thin film, which adopts predominantly edge-on orientations, gives the highest mobilities among the three polymers. The present results bode well for the development of ambipolar semiconducting polymers by introducing strong electron-deficient groups.

ASSOCIATED CONTENT Supporting Information. Experimental details, analysis of the new compounds, and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected]. Author Contributions # Z. C. and D. G. contributed equally. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21474116, 21673258 and 51233006), the National Key Research and Development Program of China (2016YFB0401100), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030100). The GIXRD results were tested at BL14B1 Station of Shanghai Synchrotron Radiation Facility (SSRF), 23A1 Station of


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Thiazole-Flanked Diketopyrrolopyrrole Polymeric ... - ACS Publications

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