Imide-Functionalized Thiazole-Based Polymer Semiconductors

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Imide-Functionalized Thiazole-Based Polymer Semiconductors: Synthesis, Structure-Property Correlations, Charge Carrier Polarity, and Thin-film Transistor Performance Yongqiang Shi, Han Guo, Minchao Qin, Yuxi Wang, Jiuyang Zhao, Huiliang Sun, Hang Wang, Yulun Wang, Xin Zhou, Antonio Facchetti, Xinhui Lu, Ming Zhou, and Xugang Guo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03670 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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Chemistry of Materials

Imide-Functionalized Thiazole-Based Polymer Semiconductors: Synthesis, StructureProperty Correlations, Charge Carrier Polarity, and Thin-film Transistor Performance

Yongqiang Shi,†,‡,ǁ Han Guo,†,ǁ Minchao Qin,§ Yuxi Wang,† Jiuyang Zhao,† Huiliang Sun,† Hang Wang,† Yulun Wang,† Xin Zhou,† Antonio Facchetti,# Xinhui Lu,*,§ Ming Zhou,‡ Xugang Guo*,† †

Department of Materials Science and Engineering and The Shenzhen Key Laboratory for Printed

Organic Electronics, Southern University of Science and Technology (SUSTech), No. 1088, Xueyuan Road, Shenzhen, Guangdong, 518055, China ‡ School of Materials Science and Engineering, Southwest Petroleum University, Chengdu, Sichuan,

610500, China §

Department of Physics, The Chinese University of Hong Kong, New Territories, 999077, Hong

Kong # Department of Chemistry and the Materials Research Center, Northwestern University, Evanston,

IL 60208, USA

Abstract: Imide-functionalized arenes, exemplified by naphthalene diimides (NDIs), perylene diimides (PDIs), and bithiophene imides (BTIs), are the most promising building blocks for constructing high-performance n-type polymers. In order to reduce the steric hindrance associated with NDI and PDI-based polymers and to address the high-lying LUMO issue of BTI-based polymers, herein a highly electron-deficient imide-functionalized bithiazole, N-alkyl-5,5'bithiazole-4,4'-dicarboximide (BTzI), was successfully synthesized via an efficient C–H activation. Single crystal of BTzI model compound showed a planar backbone with close π-stacking distances (3.2–3.3 Å). The N,N'-bis(2-alkyl)-2,2'-bithiazolethienyl-4,4',10,10'-tetracarboxdiimide (DTzTI) was also used for constructing polymer semiconductors. Compared to DTzTI, BTzI is more 1 ACS Paragon Plus Environment

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electron-deficient, rendering it highly appealing for enabling n-type polymers. Based on BTzI and DTzTI, a series of polymers, including acceptor-acceptor homopolymers, and donor-acceptor and donor-acceptor-acceptor copolymers, were synthesized, which feature different contents of acceptor units in polymeric backbones. As imide content increases, the polymer FMO levels were gradually lowered, yielding a transition of charge carrier from ambipolarity to unipolar n-type in organic thin-film transistors (OTFTs). The acceptor-acceptor homopolymer PBTzI possesses the deepest LUMO/HOMO level of -3.94/-6.17 eV, enabling minimal off-current (Ioff) of 10–10–10–11 A in OTFTs. The highest electron mobility of 1.61 cm2 V−1 s−1 accompanied by small Ioff of 10–10– 10–11 A and high on-current/off-current ratio (Ion/Ioff) of 107–108 was achieved from OTFTs using PDTzTI homopolymer, showing the pronounced advantages of acceptor-acceptor homopolymer approach for developing unipolar n-type polymer semiconductors. The correlations between the FMO levels and the transistor performances underscore the significance of FMO tuning for enabling unipolar electron transport. The results demonstrate that imide-functionalized thiazoles are excellent units for constructing high-performance n-type polymers. Moreover, the synthetic routes to these highly electron-deficient imide-functionalized thiazoles and the polymer structureproperty correlations developed here are informative for materials invention in organic electronics.

Introduction π-conjugated organic and polymer semiconductors are currently studied with intensive efforts driven by their applications in various organic optoelectronic devices such as polymer solar cells (PSCs)1-3 and organic thin-film transistors (OTFTs).4-7 Compared to inorganic semiconductors, organic polymer semiconductors show distinctive advantages, including low-temperature solution processability, cost-effective and high throughput device manufacturing, light-weight, and mechanical flexibility and even stretchability.7-9 The advancement of organic electronics field is 2 ACS Paragon Plus Environment

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mainly facilitated by the development of new semiconducting materials,10-12 which rely on the design and synthesis of novel π-conjugated building blocks with well-tailored geometries and optimized optoelectronic properties.8, 11, 13-16 Their incorporation into π-conjugated polymers has yielded a great number of semiconductors with desirable physicochemical properties, improved self-assembly characteristics, and optimized film morphologies. Therefore, materials innovation combined with device engineering in the last decade has greatly improved semiconductor charge transport characteristics in OTFT devices and the corresponding integrated circuits.17-20 In the last two decades, a remarkable progress has been achieved in developing p-type (hole transporting) polymer semiconductors.21,

22

In comparison to a large variety of p-type semiconductors with

promising device performance, the development of n-type (electron transporting) organic semiconductors greatly lags behind, restrained by the limited number of available electrondeficient building blocks and challenges of synthesizing semiconductors with high content of acceptor moiety, especially acceptor-acceptor type polymers,23-25 thus leading to inferior materials performance not only in charge carrier mobility but also in device stability.8, 26, 27 However, highperformance n-type organic polymer semiconductors are essential for organic complementary logic circuits and p-n junction devices.28-30 Therefore, it is imperative to develop high-performance ntype polymer semiconductors. Imide functionalization is a highly effective and widely used approach to generate electrondeficient building blocks owing to the strong electron-withdrawing character and solubilizing capability of imide group.11,

31

Various imide-functionalized arenes have been successfully

developed and incorporated into polymer semiconductors,15, 32 exemplified by naphthalene diimide (NDI),33-37 perylene diimide (PDI),31, 38, 39 thieno[3,4-c]pyrrole-4,6-dione (TPD),40, 41 bithiophene imide (BTI) and its fused derivatives (BTIn)14,

23, 30, 42-44

(Figure 1). To achieve n-type

characteristics, the lowest unoccupied molecular orbitals (LUMOs) of polymer 3 ACS Paragon Plus Environment


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(a) electron-rich unit high-lying FMOs

TPD

BTI

TBI

(b) electron-neutral unit sizable steric hindrance

NDI

PDI

(c) electron-deficient unit reduced steric hindrance deep-lying FMOs

DTzTI

this work

BTzI

Figure 1. Chemical structures of representative imide-functionalized arene-based building blocks. (a) Thiophene-based electron-rich units with high-lying FMOs; (b) benzene-based electron-neutral units with sizable steric hindrance; (c) thiazole-based electron-deficient units with reduced steric hindrance and deep-lying FMOs (this work).

semiconductors must be sufficiently low-lying to allow efficient electron injection from source/drain electrodes, commonly gold, in OTFT devices, which can be fulfilled using diimidefunctionalized building blocks with electron-neutral cores, such as NDI and PDI (Figure 1b). In spite of such promise, their geometries of NDIs and PDIs result in sizable steric hindrance with neighboring arenes and therefore their polymer backbones typically show a high degree of distortion, especially for PDI polymers, which limits polymer chain packing and charge transport.11, 45, 46

Due to the electron-rich character of thiophene and functionalization with monoimide group,

TPD led to polymers with promising p-type performance in OTFTs and remarkable power conversion efficiencies (PCEs) as the donor materials in PSCs.47, 48 Marks and co-workers designed 4 ACS Paragon Plus Environment

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and synthesized a novel electron-deficient BTI (Figure 1a) featuring a highly planar backbone and close intermolecular stacking (3.43 Å). Moreover, the electron-withdrawing imide is bridged over bithiophene, which minimizes steric hindrance with neighboring arenes.42 Such geometry and structural features greatly promote the packing and film crystallinity of BTI-based polymers. However, similar to TPD polymers, BTI polymers also suffer from high-lying LUMOs, and the BTI polymers show tunable charge carrier polarity and the devices suffer from contact resistance and stability issue in OTFTs.23, 30, 44 To address the high-lying LUMO issue caused by the electron-rich thiophene in TPD and BTI-based polymers and tackle the steric hindrance challenge in NDI and PDI-based polymers, a feasible approach is to develop more electron deficient arenes with reduced steric hindrance.24, 4952

It is known that introducing electron-deficient thiazole into polymer backbones would result in

deeper-lying FMO energy levels, both LUMOs and highest occupied molecular orbitals (HOMOs), which can facilitate electron injection and suppress hole accumulation in OTFTs.53-55 Moreover, the replacement of thiophene with thiazole likely improves backbone planarity, owing to the less steric demanding sp2-hybridized N atom (versus C-H moiety in thiophene) and intramolecular noncovalent interactions promoted by N atom.50, 53, 56 Hence imide-functionalized thiazoles should be promising building blocks for constructing n-type polymer semiconductors. Marder and Ie54, 57-60 reported monocarbonyl and dicarbonyl-bridged dithiazole derivatives for n-type OTFTs. The electron-deficient thiazole core combined with the electron-withdrawing carbonyl bridge led to semiconductors with low-lying LUMOs for facile electron injection and optimal molecular packing for enhanced charge carrier transport. Thus, by using a carbonyl-bridged bithiazole derivatives as the active layer, the OTFT devices show a good electron mobility (e) of 0.06 cm2 V-1 s-1 in vacuum and a e of 0.014 cm2 V−1 s−1 under ambient condition.54 These results clearly indicate that a simple structural modification by replacing thiophene with electron-deficient thiazole is an effective 5 ACS Paragon Plus Environment


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strategy for promoting n-type performance in OTFTs and this strategy enables organic semiconductors with potential device air stability. Inspired by the excellent performance of both imide-functionalized organic semiconductors and thiazole derivatives in OTFTs, herein, two imide-functionalized electron-deficient units, bithiazole imide BTzI and DTzTI (Figure 1) were synthesized and incorporated into polymer semiconductors, including both alternating donor-acceptor (D-A) copolymers and novel acceptoracceptor (or all-acceptor, A-A) type homopolymers. Please note that DTzTI has been reported before and the all-acceptor type homopolymer PDTzTI showed highly promising unipolar n-type performance with a substantial e of 1.61 cm2 V−1 s−1, minimal offer-currents (Ioffs) of 10−10−10−11 A, and remarkable current on-current/off-current ratios (Ion/Ioffs) > 107 in OTFTs.61 We report here a facile synthetic route to BTzI as well as its all-acceptor type homopolymer PBTzI, which possesses remarkably deep-lying FMO levels and shows unipolar electron transport property in OTFTs. In addition, a series of D-A copolymers based on BTzI and thiazolothienyl imide dimer DTzTI were also synthesized and studied in parallel to systematically investigate their chemical

structure-materials

property-device

performance

correlations.

The

structural

modification from BTI to the new BTzI by incorporating thiazole instead of thiophene, results in a

greatly

different

chemistry

for

building

block

synthesis

and

the

following

halogenation/polymerization reactions. For BTzI, we devised an efficient 4-step synthetic route to its brominated monomer. The unique structural feature of BTzI, in which the electron-withdrawing imide group is installed on the center of bithiazole, minimizes the steric hindrance and enables the development of novel all-acceptor type homopolymers. Polymerization using the new dibrominated BTzI monomer appeared to be problematic under typical Pd-mediated Stille coupling condition. In order to address this issue, copper iodide (CuI)52 was utilized as the co-catalyst in the polymerization, which was proven to be effective in obtaining the target homopolymer PBTzI with 6 ACS Paragon Plus Environment

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a moderate molecular weight (5.8 kDa). For BTzI-based copolymers, two flanking thiophenes were first attached onto BTzI and then brominated to afford dibrominated monomer 10, which showed improved reactivity and afforded polymers with higher molecular weights (>10 kDa). However, the addition of the electron-rich thiophene flanks reduced the imide content in backbone, resulting in relatively shallow FMOs.44 The DTzTI, in which the reactive Br site is located on thiophene moiety, enables the ready synthesis of all-acceptor type homopolymer PDTzTI. In addition, when this imide dimer was incorporated into polymers, the semiconductors feature a donor-acceptoracceptor (D-A-A) type backbone with an increased content of acceptor component in the backbone, which should be beneficial to n-type characteristics. Through materials and device characterizations, the chemical structure-property-device performance correlations are established for these imide-functionalized thiazole-based polymers. In order to underscore the significance between the FMO levels and the transistor performances, a series of new polymers, including D-A, D-A-A, and A-A, were synthesized, showing widely tunable FMO levels. Compared to monoimide-based BTzI, the imide dimer DTzTI can further lower the polymer FMO levels due to their D-A-A type backbone, promoting n-type performance in OTFTs. As the imide content increases and the change of backbone motif from D-A to D-A-A and to A-A, the polymer FMO levels were gradually lowered, yielding a transition of charge carrier from ambipolarity to unipolar n-type in OTFTs. The results demonstrate that imide-functionalized thiazoles are promising building blocks for enabling high-performance polymers with unipolar ntype OTFT performance and offer great platforms for materials innovation with tunable charge carrier polarity in organic electronics. Results and Discussion Materials Synthesis

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The synthetic route to the bithiazole imide (BTzI, 7) and its dibrominated derivative (8) is shown in Scheme 1. It was found that the chemistry of thiazole greatly differs from that of thiophene, rendering the synthesis of BTzI far more challenging than that of BTI. Starting from commercial 2-bromothiazole as the initial reactant, the 2-position of thiazole was first protected by silylation to give 2-(triisopropylsilyl)thiazole (1) using n-butyl lithium (n-BuLi) followed by reaction with chlorotriisopropylsilane at room temperature since the 5-position of thiazole is more reactive than the 2-position.62 Then the 5 position of 1 was brominated to give 2-triisopropylsilyl-5bromothiazole (2) using n-BuLi, followed by quenching with Br2. Compound 2 was then subjected to the halogen dance reaction by lithium diisopropylamide (LDA) treatment at −78 °C, and the lithiated compound rearranges to afford the 4-bromo derivative. The reaction likely proceeds by deprotonation at the 4-position to provide a lithiated product. The 4-lithio intermediate is then converted to the 4-bromo-5-lithio compound via a 1,2-dance mechanism, in which the lithium is at the more acidic site. CuCl2-promoted oxidative coupling of the 5-lithio compound produces the corresponding dibromide compound, 4,4'-dibromo-2,2'-bis(triisopropylsilyl)-5,5'-dithiazole (3) in a good yield of 79%, which was then lithiated with n-BuLi. The generated dilithium salt was treated with dry CO2 to afford the lithium carboxylate. The lithium salt was acidified with HCl to afford a dicarboxylic acid 4, which can be used in the following reaction without further purification.


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Scheme 1. (a) Synthetic route to the dibrominated bithiazole imide (BTzI) and its derivative with flanking thiophene units; (b) improved synthetic route to dibrominated BTzI. Reagents and conditions: (i) n-BuLi, TIPSCl, −78 °C; (ii) n-BuLi, Br2; (iii) LDA, CuCl2, −78 °C; (iv) n-BuLi, CO2, −78 °C, then to r.t.; (v) HCl, reflux; (vi) SOCl2, 80 °C; (vii) 2-hexyldecan-1-amine, 140 °C; (viii) LiHMDS, BrCCl2CCl2Br, −78 °C; (ix) 2-trimethyltinchloride-3-dodecylthiophene, Pd(PPh3)4, DMF; (x) Br2, CHCl3/AcOH, r.t., 6 h. (xi) PdCl2(PhCN)2, AgF; (xii) NaOH, diluted HCl. Deprotection of TIPS group on the compound 4 was carried out in refluxing HCl to yield the novel key intermediate 4,4'-dicarboxylic acid-5,5'-bithiazole 5 in a quantitative yield. 5 can be readily converted to diacyl chloride 6 with SOCl2 at 80 C, which is condensed with 2-hexyldecyl amine at 140 C to afford the bithiazole imide 7 in a fair overall yield of 27%. During the optimization of imidization condition, it was found that using 0.7 equivalent alkylamine without any solvent can suppress the diamide byproduct formation and afford imide 7 as the major product. Subsequently, lithium hexamethyldisilazide (LiHMDS)63 was used to lithiate BTzI, which was then reacted with 1,2-dibromotetrachloroethane (BrCCl2CCl2Br)64 to afford the dibrominated compound 8 in an acceptable yield (50%). However, the overall synthetic route to monomer 8 is complicated and inefficient since it involved many steps. 9 ACS Paragon Plus Environment


In order to improve the synthetic efficiency, a much simplified new route to monomer 8 was developed, containing only 4 steps (Scheme 1b). Starting from the commercial methyl 2bromothiazole-4-carboxylate (11), the palladium-catalyzed65 homocoupling in the presence of AgF gave dimethyl 2,2'-dibromo-bis-5,5'-thiazole-4,4'-dicarboxylate (12) with a yield of 40%. Compound 12 was hydrolyzed to yield a dicarboxylic acid 13, which was then converted to dibrominated monomer 8 following similar procedures in Scheme 1a. To our best knowledge, this is the first example of C–H homocoupling of 2-bromothiazole-4-carboxylate, as C–H activation that takes place at the 5-position of thiazole is remarkably mild. Please note that in the route a, the high electron-deficiency of 7 rendered it quite challenging to brominate. In the new route b, the bromine atom was introduced at very beginning, which greatly simplified the synthesis and improved the yield, enabling easy access to monomer 8 in large quantity. Upon the successful synthesis of the dibrominated monomer 8, it was subjected to polymerizations with various comonomers under typical Stille coupling condition, which unfortunately only afforded oligomers with very low molecular weights (< 2 kDa) and poor yield due to the low reaction activity of thiazole based monomer, this may likely due to the chelatation of the Pd catalyst with the N atoms on thiazole moieties. Copper iodide (CuI) was then added as co-catalyst,52 which was found to be beneficial for polymerization here, giving homopolymer PBTzI with a decent number-averaged molecular weight (Mn) of 5.8 kDa. In addition, to address the low reactivity of brominate site on thiazole, the dibrominated imide (8) was reacted with 2trimethylstannyl-3-dodecylthiophene to afford a thiophene flanked bithiazole imide 9 in an acceptable yield of 30%, which can be readily brominated to give dibromo monomer 10. This monomer shows high reactivity in the following polymerizations and the polymers with good Mns (Table 1) can be readily obtained (vide infra).


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Scheme 2. Synthetic routes to copolymers PBTzI3T, PBTzI3T-2F, PDTzTIT and PDTzTIT-2F as well as homopolymers PDTzTI and PBTzI. Reagents and conditions: (i) Pd2(dba)3, P(o-tolyl)3, toluene, microwave, 140 C. (ii) Pd2(dba)3, P(o-tolyl)3, CuI as co-catalyst, toluene, microwave, 140 C. The synthesis of the homopolymer PBTzI was accomplished using bis(tributyltin) as the reactant and CuI as the co-catalyst (Scheme 2), and the resulting homopolymer features all-acceptor type backbone with the highest imide content. As for the imide-functionalized thiazole-based copolymers, they were synthesized under conventional Stille coupling condition using Pd2(dba)3 as the catalyst and P(o-tolyl)3 as the ligand. The 2,5-bis(trimethylstanny)thiophene and 3,411 ACS Paragon Plus Environment


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difluoro-2,5-bis(trimethylstanny)thiophene are chosen as the electron-rich comonomers due to its smallest thiophene number, which should lead to lower-lying FMOs, thus suppress p-type performance and promote n-type characteristics in OTFTs. The DTzTI contains two imide groups, which should increase the loading of key electron-withdrawing imide functionality in D-A-A type copolymer. This monomer has been used for the synthesis of A-A type homopolymer PDTzTI, showing remarkable unipolar n-type performance.61 This polymer was also included here for better comparison and understanding the structure-property correlations. Unlike monomer 8, the bromine-containing active site in DTzTI for polymerization is on the thiophene moiety, showing good reactivity in polymerization, which enabled the readily access of D-A-A type copolymers PDTzTIT and PDTzTIT-2F (Scheme 2). Table 1. Molecular weights, optical and electrochemical properties of polymers.

PBTzI3T

Mn (kDa)/PDIa 24/1.7

λmaxsol (nm)b 551

λmaxfilm (nm)c 555

λonsetfilm (nm) 653

PBTzI3T-2F

11/1.2

603

600

PDTzTIT

20/1.8

582

PDTzTIT-2F

14/1.2

PDTzTI PBTzI

Polymer

a

EHOMO (eV)e

ELUMO (eV)d

Egopt (eV)f

−5.16

−3.27

1.89

655

−5.22

−3.33

1.89

583

646

−5.59

−3.68

1.91

534

571

606

−5.75

−3.71

2.04

7.0/1.1

556

567

614

-5.78

-3.77

2.01

5.8/2.6

494

483

556

−6.17

−3.94

2.23

GPC versus polystyrene standard, trichlorobenzene as eluent at 150 C.

b

Absorption in

chloroform solution (10−5 M). c Absorption of as-cast film from 5 mg mL−1 chloroform solution. d ELUMO = −e(Eredonset + 4.80) eV, Eredonset determined using the Fc/Fc+ internal standard. e EHOMO = ELUMO − Egopt. f estimated from absorption onset of as-cast polymer film using the equation: Egopt = 1240/λonset (eV).

After polymerization, the polymer chains were end-capped with mono-functionalized thiophenes and the polymers were purified by successive Soxhlet extractions with methanol, 12 ACS Paragon Plus Environment

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acetone, hexane, dichloromethane (DCM), and chloroform in sequence. The final chloroform fractions were collected and re-precipitated into methanol to afford the copolymer products, and homopolymers were finally extracted with DCM. All polymers show good solubility in common organic solvents, and polymer molecular weights are measured by high-temperature (150 C) gel permeation chromatography (GPC) versus polystyrene standards. The polymer number-average molecular weights (Mns) and polydispersity index (PDI) data are summarized in Table 1. Single Crystal Structure of Model Compound In order to study the geometry of the new building block and gain insights into solid-state packing of the corresponding polymers, the thiophene flanked model compound BTzI-C6-2T with a linear n-hexyl chain on the imide group was synthesized (Scheme S1). The single crystal of BTzIC6-2T (CCDC: 1563479) was obtained by slow evaporation of its dichloromethane solution. Figure 2 shows the crystal structure of this model compound and important crystal parameters are summarized in Supporting Information along with synthetic and characterization details. The crystal structure analysis reveals that the seven-membered imide ring in BTzI-C6-2T is flat showing a small dihedral angle of 1.38 between the two thiazole planes (Figure 2b). This angle is smaller than that (4.32) between two thiophene planes in the previously reported BTI model compound.42 The flanking thiophenes and the central BTzI show small torsional angles (2–6, Figure 2b). The short distances (3.082 and 3.114 Å) between the S atom of thiophene and the N atom of thiazole indicate the presence of intramolecular noncovalent N…S interaction on both sides, since they are much shorter than the sum (3.35 Å) of the van der Waals radii of these two atoms.56 Such conformation locks are beneficial to backbone planarity and charge carrier delocalization in the corresponding polymers.66 Another benefit of introducing thiazole is that it leads to decreased steric hindrance with neighboring arenes by replacement of C–H moiety with the less steric demanding N atom. 13 ACS Paragon Plus Environment


(a)

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(c)

(b)

3.24Å 3.23Å 1.38°

Figure 2. (a) Chemical structure, (b) top view, and (c) side view of the BTzI-based model compound BTzI-C6-2T in single crystal.

Interestingly, the dithiazole imide cores in BTzI-C6-2T exhibit very short π-stacking distances of 3.23 or 3.24 Å depending on the stacking directions (Figure 2c). The tight π-stacking was likely attributed to increased intermolecular interaction with the replacement of thiophene with thiazole, evidenced by the gradually decreased π-stacking distance in single crystal compounds from BTI (3.43 Å) to DTzTI (3.35 Å)61 then to BTzI. The enhancement of intermolecular interaction might be beneficial for interchain charge transport in the π-stacking direction. The single crystal structures of BTzI-C6-2T and DTzTI-based model compound DTzTI-C861 indicate that both BTzI and DTzTI are promising building blocks for polymer semiconductors. Polymer Thermal Properties Thermal properties of these new polymer semiconductors were investigated using thermogravimetric analysis (TGA) in N2 with a heating ramp of 10 C min−1. A mass loss of 5% is defined as the threshold for thermal decomposition, and all polymers show good thermal stability with the decomposition onsets of 348, 323, 329, 328, 336 and 335 C for PBTzI3T, PBTzI3T-2F, PDTzTIT, PDTzTIT-2F, PDTzTI and PBTzI, respectively (Figure S25a). The data indicates that these polymers are sufficiently stable for thermal annealing and device optimization over a wide range. Differential scanning calorimetry (DSC) was used to characterize the polymer thermal transitions. On the basis of DSC thermograms (Figure S25b), all polymers show no distinctive 14 ACS Paragon Plus Environment

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exotherms or endotherms from 50 to 300 C, providing no evidence of mesophase transition in the investigated temperature range. Such thermal properties are similar to those of bithiophene imide homopolymers and bithiophene imide-thiophene copolymers.30 Polymer Optical Properties Figure 3 shows the UV-vis absorption spectra of polymers PBTzI3T, PBTzI3T-2F, PDTzTIT, PDTzTIT-2F, PDTzTI and PBTzI in diluted chloroform solutions (10−5 M) and as thin films, and their corresponding absorption data and bandgaps are summarized in Table 1. All the polymers are strongly aggregated even in diluted solutions, indicated by their structured absorption profiles and the small red-shifts of absorption peaks (maxs) when going from solution to film state. The strong aggregating properties of these polymers were further confirmed by temperature-dependent absorption measurement (Figure S26). When heated from room temperature to 100 C, majority of the polymer solutions were still aggregated except for PBTzI3T, as revealed by their structured absorption profiles and small blue-shifts of maxs.

(a)

(b)

Figure 3. Normalized UV-vis absorption spectra of the polymer semiconductors (a) in dilute chloroform solutions (10−5 M) and (b) at thin-film state. The film maxs of the BTzI-based copolymers and DTzTI-based polymers are located within a narrow range of ca. 560–600 nm, whereas the absorption of all-acceptor homopolymer PBTzI film 15 ACS Paragon Plus Environment


is greatly blue-shifted with a max around 480 nm. The blue-shift absorption and associated band gap widening is attributed to its greatly lowered HOMO level (EHOMO, vide infra) due to the absence of donor unit in PBTzI.23, 61 The optical bandgaps (Egopts) of PBTzI3T, PBTzI3T-2F, PDTzTIT, PDTzTIT-2F, PDTzTI and PBTzI are 1.89, 1.89, 1.91, 2.04, 2.01 and 2.23 eV, respectively, determined from the polymer film absorption onsets. Among all polymers, PDTzTIT-2F, PDTzTI and PBTzI show bandgaps > 2 eV, due to the reduced electron donating ability of difluorothiophene unit in the D-A-A type copolymer PDTzTIT-2F and the lack of intramolecular charge transfer character in the A-A type homopolymer PDTzTI and PBTzI. Such large Egopts reflect their very low-lying HOMOs of −5.75, −5.78, and −6.17 eV, respectively (Table 1). The Egopts and HOMOs of these BTzI and DTzTI-based polymers are quite different than those of prevailing high-performance n-type polymer semiconductors reported till today, such as NDI, isoindigo, and diketopyrrolopyrrole (DPP)-based polymers, which typically feature narrow Egopts with high-lying HOMOs.11, 19, 36, 67-69 Polymer Electrochemical Properties

(a)

(b)

LUMO

HOMO

Figure 4. (a) Cyclic voltammograms of polymer thin films measured in 0.1M tetrabutyl ammonium hexafluorophosphate acetonitrile solutions with the Fc/Fc+ redox couple as the internal standard at a scanning rate of 50 mV s−1. (b) The polymer FMO energy level diagram.


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The electrochemical properties of these polymers were characterized using cyclic voltammetry (CV) measurement, and the cyclic voltammograms are illustrated in Figure 4a and the results are summarized in Table 1. All these thiazole imide-based polymer semiconductors exhibit distinctive reduction peaks, indicative of their n-type characteristics. From top to bottom (Figure 4a), the reduction peaks become more pronounced accompanied by gradually weakened oxidation peaks, indicating that F addition on the thiophene and the increment of thiazole imide component resulted in improved n-type but suppressed p-type characteristics, which was in good agreement with the device performance (vide infra). As shown in Figure 4a, the onset reduction potentials are −1.53, −1.47, −1.12, −1.09, −1.03, and −0.86 eV, relative to the ferrocene/ferrocenium (Fc/Fc+) redox couple, which correspond to the LUMO levels (ELUMOs) of −3.27, −3.33, −3.68, −3.71, −3.77, and −3.94 eV for PBTzI3T, PBTzI3T-2F, PDTzTIT, PDTzTIT-2F, PDTzTI, and PBTzI, respectively. Among the series, the bithiazole imide homopolymer PBTzI showed the lowest-lying LUMO level (-3.94 eV), which is attributed to the combined effect of the most electron-deficient bithiazole backbone and the highest content of imide functionality. Due to the weak and irreversible oxidation peaks, the HOMO levels (EHOMOs) were calculated based on LUMO levels and the optical bandgaps using the equation: EHOMO = ELUMO − Egopt which were −5.16, −5.22, −5.59, −5.75, −5.78 and −6.17 eV for PBTzI3T, PBTzI3T-2F, PDTzTIT, PDTzTIT-2F, PDTzTI and PBTzI, respectively (Figure 4b). The actual EHOMOs should be even deeper than the currently reported values based on the equation: EHOMO = ELUMO – Efund (Efund: the fundamental gap),70 attributed to the fact that Efund is greater than Egopt due to the exicton binding energy, which further demonstrates the advantage of thiazole-imide based polymers for enabling unipolar n-type OTFTs. Such low-lying HOMOs should suppress hole injection. Compared to the HOMOs of the PBTzI3T and PBTzI3T-2F, the HOMOs of polymers PDTzTIT, PDTzTIT-2F, PDTzTI and PBTzI are further suppressed, originating from the higher 17 ACS Paragon Plus Environment


content of the electron-withdrawing imide group in these polymer semiconductors. On the basis of the LUMOs, the addition of F atoms slightly lowers the polymer LUMOs, and the lower loading of electron-rich thiophene unit results in deeper LUMOs for DTzTI-based polymers versus the BTzI-based copolymers. In comparison to the previous BTI-based polymer analogues,30 the introduction of thiazole results in far lower-lying ELUMO/EHOMOs, which will facilitate electron injection while suppress hole accumulation in OTFT devices. Particularly, the very low-lying ELUMO (−3.94 eV) of PBTzI should be beneficial for material and device stability in ambient environment.71 Theoretical Calculations In order to further understand the effect of thiazole incorporation on the polymer physicochemical properties and to establish chemical structure-materials property correlations, density functional theory (DFT)-based calculations were performed on oligomers at the B3LYP/631G(d) level using the Gaussian 09 program. To reduce the computation time, three repeating units were chosen as the simplified models and the 2-hexyldecyl side chains were replaced with methyl groups. The optimized trimer conformations are illustrated in Figure 5, which show that the torsional angles of typically < 5 between neighboring arenes. The calculated ELUMO/EHOMOs of PBTzI3T, PBTzI3T-2F, PDTzTIT, PDTzTIT-2F, PDTzTI and PBTzI are −2.93/−5.20, −3.02/−5.33, −3.41/−5.68, −3.48/−5.81, -3.54/-5.97 and −3.76/−6.41 eV, respectively. The calculation results indicate that as the loading of the electron-deficient imide increases, the polymer FMO levels were found to become lower-lying gradually. The EHOMO/ELUMO evolution is in excellent agreement with the experimentally determined CV results. F addition leads to suppressed FMOs and the DTzTI-based polymers show lower-lying FMOs than the BTzI-based copolymers. In addition, DFT calculations reveal that all polymers adopt highly planar backbone, which are in good agreement with the single crystal structures of the model compounds. Additionally, the DFT 18 ACS Paragon Plus Environment

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results reveal that the torsional angles between the difluorothiophene and the neighboring arene in the polymers PBTzI3T-2F and PDTzTIT-2F are smaller than those in PBTzI3T and PDTzTIT, which are likely attributed to intramolecular noncovalent S…F interactions.66 The S…F interactions enhance polymer

(a)

(b) 0.49° 3.05°

3.42° 0.06° 0.07°

4.37°

6.04°

0.52°

0.90°

11.95°

0.28°

0.11°

HOMO: -5.20 eV LUMO: -2.93 eV

0.26°


(d)

(c)

1.13° 1.25°

0.08°

0.65°

0.11°

0.14°

0.60° 0.45°

1.14°

1.09° HOMO: -5.81 eV LUMO: -3.48 eV


(e)

(f)

0.69°

0.35°

0.58°

0.18°

0.69°

0.32°

0.19°

0.60°

0.23°

0.20°



Figure 5. Chemical structures and optimized geometries for the trimers of the repeat units of (a) PBTzI3T, (b) PBTzI3T-2F, (c) PDTzTIT, (d) PDTzTIT-2F, (e) PDTzTI and (f) PBTzI. Calculations were performed at the DFT//B3LYP/6-31G(d) level; dihedral angles between neighboring arenes are indicated by red circles. The plots shown are the local minima, and alkyl substituents are truncated for calculation simplicity.



backbone planarity, thus assuming similar film morphologies, a higher charge carrier mobility is expected for PBTzI3T-2F and PDTzTIT-2F. It is interesting to note that the homopolymer PBTzI shows the smallest torsional angles (Figure 5f), which are likely due to the successive intramolecular noncovalent S…N interactions.72 Organic Thin-Film Transistor Performance The charge transport properties of these novel imide-functionalized thiazole-based polymers were evaluated by fabricating top-gate/bottom-contact (TG/BC) OTFTs. The source and drain electrodes (3 nm Cr and 30 nm Au) were pre-patterned on borosilicate glass prior to device fabrication using standard photolithography process. The semiconducting polymers were spincoated from their corresponding solutions and then annealed at various temperatures for 30 min. The dielectric layers were spin-coated onto the semiconductor using a diluted CYTOP (CTL809M:CT-SOLV180 = 2:1 volume ratio, Asahi Glass Co., Ltd.) or PMMA solution, then annealed at 100 °C for 30 min. The dielectric layer thickness is ~400 nm for CYTOP and ~500 nm for PMMA which results in a capacitance value of 4.4 and 6.2 nF cm−2, respectively. Finally, a 50 nm Al gate electrode was thermally evaporated on top to complete the device (channel length = 10, 20, or 50 m, channel width = 5000 m). To optimize the OTFT performance, we systematically investigated the effects of different processing solvents and post-deposition annealing temperatures for the polymer semiconductor layers. It was found that using chlorobenzene (CB) as the processing solvent consistently afforded better device performance. Furthermore, the OTFT performance highly depends on the solution casting temperature (Tsc) and semiconductor film annealing temperatures, with high Tsc and thermal annealing generally improving performance. In addition, bottom-gate devices with octadecyltrichlorosilane-treated SiO2 dielectric were also tested. Because of the film formation issue on the highly hydrophobic SiO2 surface at high Tsc, the bottomgate devices were fabricated at relatively low Tsc which led to unsatisfactory performance 20 ACS Paragon Plus Environment

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compared to the top-gate devices. The device performance parameters of the top-gate OTFTs fabricated under optimal conditions are summarized in Table 2 and the corresponding representative n-type I–V curves are plotted in Figure 6. All PBTzI3T, PBTzI3T-2F, PDTzTIT, PDTzTIT-2F, PDTzTI and PBTzI-based OTFT devices exhibited n-type dominating ambipolar or unipolar n-type transistor characteristics. PBTzI3T OTFTs annealed at 250 °C showed improved transport properties compared to the ascast films with an average electron mobility (e) of 0.015 cm2 V−1 s−1 and an average hole

10-4 10-5 |Is|(A)

Is (A)

-5x10-6

10-7

0.002

10-8 10-9

0 -3x10-4

20

40 Vd (V)

60

80

(f)

10

20 40 Vg (V)

60

80

10-5 |Is|(A)

10-9

10-7

10

-8x10-4

20

40 Vd (V)

60

80

0.01

10-8 0.005

10-3 10-4

-4x10-4

-6x10-4

10-6

80

|Is|(A)

10

0.02

-7

10-8

10

0

-2x10-5

0.01

20

40 Vd (V)

60

80

(n)

PBTzI

-11

10-3 10-4

40 Vd (V)

60

80

0

20 40 Vg (V)

60

80

-8x10

10-4 10-5

-6x10-4 -4x10-4

40 Vd (V)

60

80

0

20 40 Vg (V)

60

80

0

0.04

PDTzTI Off-center SC

0.03

10-6 0.02

10-7 10-8

0.01

10-9

0 0

0.006

-4x10-5

10-7 10-8

0.002

40 Vd (V)

60

80

10-11

(p)

10-3

PBTzI Off-center SC

-3x10-5

0.004

20

(o)

PBTzI

10-6

10-11 20

(l)

-2x10-4

0

10-9

0

10-11

10-3

10-4 10-5

-2x10-5 -1x10-5

0

20 40 Vg (V)

60

80

0

0.008

PBTzI Off-center SC

0.006

10-6 10-7

0.004

10-8 10-9

0.002

10-10

10-10 0

0.01

sqrt(|Is|)(A1/2)

|Is|(A)

20

PDTzTI Off-center SC

-4

sqrt(|Is|)(A1/2)

-1x10-5

10-8

10-10

10-5 Is (A)

-1x10-3

10-10

0

0

(k)

0.03

10-9

-2x10-4

0

0

0.04

PDTzTI

10-5

-4x10-4

(m)

60

|Is|(A)

10-4

0.02

10-7

sqrt(|Is|)(A1/2)

-8x10-4

20 40 Vg (V)

Is (A)

10-3

0

0.03

PDTzTIT-2F

10-9

|Is|(A)

(j)

PDTzTI

80

10 0

sqrt(|Is|)(A1/2)

Is (A)

-1x10-3

80

60

10-6

-2x10-4

Is (A)

(i)

60

20 40 Vg (V)

-10

10-11 40 Vd (V)

0

10-5

10 20

0.002

(h)

PDTzTIT-2F

-10

0

10

-6x10-4

0.01

0.004

-8

sqrt(|Is|)(A1/2)

Is (A)

(g)

0.006

10-7

-2x10-5

0

10-6

10-9

0

10

0

sqrt(|Is|)(A1/2)

-1x10-4

-4x10-5

0

0.02

PDTzTIT

0.008

-6

-11

0

0.01

PBTzI3T-2F

10-5

-6x10-5

10-10

-4

-2x10-4

10-4

-11

10-3

PDTzTIT

0.001

PBTzI3T-2F

10-10 10

0

(e)

10

0.003

-6

(d)10

-3

-5

sqrt(|Is|)(A1/2)

-1x10-5

(c) -8x10

0.004

PBTzI3T

|Is|(A)

10-3

|Is|(A)

PBTzI3T

Is (A)

(b)

-5

Is (A)

(a) -2x10

sqrt(|Is|)(A1/2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

20 40 Vg (V)

60

80

10-11

0

0

0

20

40 Vd (V)

60

80

0

20 40 Vg (V)

60

80

0

Figure 6. Top-gate/bottom-contact OTFT output and transfer (VD = 80 V) characteristics of (a, b) PBTzI3T, L = 10 µm; (c, d) PBTzI3T-2F, L = 10 µm; (e, f) PDTzTIT, L = 10 µm; (g, h) 21 ACS Paragon Plus Environment


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PDTzTIT-2F, L = 10 µm; (i, j) PDTzTI, L = 20 µm; (m, n) PBTzI, L = 10 µm, on-center spincasted devices and (k, l) PDTzTI, L = 50 µm; (o, p) PBTzI, L = 10 µm, off-center spin-casted (SC) device. W = 5 mm for all plots. The OTFTs were annealed at the optimal temperatures given in Table 2.

mobility (h) of 1.9 × 10−4 cm2 V−1 s−1. The appearance of p-type characteristics was related to its relative high-lying HOMO (−5.16 eV, Table 1). Compared to the previously reported p-type bithiophene imide-terthiophene polymer analogue (PBTI3T),30,

43, 44

the simple replacement of

electron-rich thiophene with electron-deficient thiazole led to terthiophene-based PBTzI3T with n-type dominating OTFT characteristics due to its lower-lying FMOs. In comparison to PBTzI3T, the F-containing polymer PBTzI3T-2F exhibited increased electron mobility under various annealing temperatures (Table S1). After annealed at 250 °C, PBTzI3T-2F OTFTs showed optimal performance with an average e of 0.047 cm2 V−1 s−1 and an average h of 2.0 × 10−4 cm2 V−1 s−1. Table 2. TG/BC OTFT performance parameters of polymers PBTzI3T, PBTzI3T-2F, PDTzTIT, PDTzTIT-2F, PDTzTI, and PBTzI fabricated under the optimal conditions. Tanneal (°C)

PBTzI3T

CYTOP

250 °C

0.027 (0.015)

PBTzI3T-2F

PMMA

250 °C

0.066 (0.047)

CYTOP

250 °C

CYTOP

PDTzTI PBTzI

PDTzTIT PDTzTIT-2F

a

μe

Dielectric

SC method

Polymer

On-center

μh

VT (V) b

Ion/Ioff

1.9 × 10-4

n: 49

n: 103-104

2.0 × 10-4

n: 46

n: 104-105

0.20 (0.17)

NA

n: 40

n: 105-106

200 °C

0.91 (0.68)

NA

n: 37

n: 106-107

CYTOP

200 °C

1.22 (0.87)

NA

n: 35

n: 107-108

CYTOP

250 °C

0.010 (0.0092)

NA

n: 32

n: 105-106

[cm2

V−1 s−1] a

[cm2

V−1 s−1] a

PDTzTI

Off-center

CYTOP

200 °C

1.61 (1.29)

NA

n: 24

n: 107-108

PBTzI

Off-center

CYTOP

250 °C

0.015 (0.012)

NA

n: 29

n: 105-106

Maximum mobilities with average values from at least 5 devices are shown in parentheses, and

the mobility values are extracted based on the average slope from 70 to 80 V in the I1/2 vs V plots. bAverage

threshold values are shown.


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For DTzTI-based copolymers, the p-type transport was further suppressed in the D-A-A type copolymer PDTzTIT and completely eliminated in the A-A type homopolymer PDTzT and PBTzI, which reflects the higher loading of the imide functionality and is in good agreement with the FMO evolvement. After annealing at 250 °C, the devices incorporating PDTzTIT active layer show an average e of 0.17 cm2 V−1 s−1, and the F-containing PDTzTIT-2F based OTFTs annealed at 200 °C exhibit unipolar n-type transport characteristics with greatly improved e of 0.68 cm2 V−1 s−1 and high Ion/Ioffs of 106–107 (Figure 6). For all-acceptor type homopolymer PBTzI-based OTFTs, annealing at 250 °C afforded a maximum e of 0.010 cm2 V−1 s−1 for the on-center spincasted device, which was improved to 0.015 cm2 V−1 s−1 by utilizing off-center spin-coating method under the same annealing temperature. This technique has been shown to enhance polymer chain alignment, thus to facilitate charge transport between source and drain electrodes, when the polymer chains are aligned in the charge transport direction.61, 73, 74 In addition, the output curves (Figure 6I) of PBTzI-based OTFTs did not have the undesirable ‘S’ shape at low source-drain voltage range, owing to its deep-lying LUMO that greatly reduces the electron injection barrier and the contact resistance in transistor devices. In comparison to the previous homopolymer PDTzTI with a maximum e of 1.61 cm2 V−1 s−1, the homopolymer PBTzI showed a substantially reduced mobility by more than 100 times, which is attributed to its great lower film crystallinity as revealed by x-ray diffraction (vide infra). Among all polymers, the all-acceptor type homopolymers PBTzI and PDTzTI-based OTFTs exhibited the low off-currents of 10−10–10−11 A, which differ from prevailing high mobility donoracceptor n-type polymers reported to date, typically showing higher Ioffs of 10−7–10−8 A with smaller Ion/Ioffs of 104–105.11, 19, 75 The small Ioffs of PDTzTI and PBTzI are mainly attributed to their very low-lying HOMOs of −5.78 and −6.17 eV, respectively. Most n-type polymers typically feature both acceptor and donor units in the backbone and the donor units lead to high-lying 23 ACS Paragon Plus Environment


HOMOs and facile hole injection. Therefore, the D-A polymers typically show high Ioffs and/or substantial hole mobilities in OTFTs. The A-A homopolymers PBTzI and PDTzTI contain only acceptor units in their backbones, leading to very low-lying HOMOs and suppressed hole injection and Ioffs. In general, going from PBTzI3T to PBTzI3T-2F and to PBTzI, the OTFTs show gradually reduced VTs, suppressed Ioffs (Table 2), and more ideal output/transfer characteristics (Figure 6), which are in good agreement with the evolvement of polymer FMO levels. A similar trend was observed for DTzTI-based polymer series from PDTzTIT to PDTzTIT-2F and to PDTzTI. In addition to FMO energy levels, chemical structure variation leads to polymer films with different morphologies, which also show great impact on charge carrier transport (vide infra). The device stability of these imide-functionalized thiazole-based polymers was studied by monitoring the temporal evolution of their OTFT performance in air with a relative humidity of ca. 50% (Figure S28). For the non-fluorinated polymers, the device stability increased in the order of PBTzI3T, PDTzTIT, PDTzTI and PBTzI, in good agreement with their gradually deepening ELUMOs. It was found that fluorination can improve the device stability as shown by PBTzI3T-2F and PDTzTIT-2F when compared to their non-fluorinated analogues, which has been observed in n-type isoindigo-based and benzodifurandione-based oligo(p-phenylene vinylene) (BDOPV)based polymers.76, 77 Benefited from its deepest ELUMO, PBTzI exhibited the best ambient OTFT stability among these imide-functionalized thiazole-based polymers. The air stability of PBTzI OTFTs appears to be comparable or even better than the benchmark n-type polymer semiconductor N2200,78 but is still inferior to the BDOPV-based n-type polymers whose ELUMOs are even deeper.77, 79, 80

Polymer Film Morphologies and Their Correlations to OTFT Performance Film morphologies and microstructures are critical for charge carrier transport in organic semiconductors.81 In order to investigate the origin of OTFT performance of these imide24 ACS Paragon Plus Environment

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functionalized thiazole-based polymers, atomic force microscopy (AFM) was first employed to study the polymer film morphology. The topography and phase images of the polymers under the conditions affording the best OTFT performance are shown in Figure 7. The root-mean-square (RMS) roughness of PBTzI3T, PBTzI3T-2F, PDTzTIT, PDTzTIT-2F, PDTzTI, and PBTzI films is 8.10, 0.88, 1.52, 0.80, 1.46, and 0.45 nm, respectively. Among them, the PBTzI3T film is the roughest one, which is likely related to its highest materials crystallinity as revealed by X-ray analysis (vide infra). Such rough surface also results in substantial grain boundaries and generates a great number of charge traps, reducing charge transport, which in combination with its highestpositioned LUMO yield its low electron mobility. On the contrary, the F addition on the thiophene moiety leads to polymer PBTzI3T-2F with greatly decreased RMS roughness, reduced grain boundaries, better domain connectivity, and hence increased charge carrier mobility. For the DTzTI-based PDTzTIT-2F and PDTzTI films, well ordered fibrillary structures are observed and the domain interconnectivity appears to be improved for PDTzTIT-2F and PDTzTI, which are beneficial for charge carrier transport. Compared to BTzI based-copolymers, the homopolymer PBTzI film exhibits a minimum roughness (0.45 nm), which corroborates its lowest crystallinity as revealed by X-ray study (vide infra). However, the homopolymer PDTzTI shows a much rougher surface with a RMS roughness of 1.46 nm with well-defined structural feature, which is in good with its greatly improved film crystallinity and substantially improved electron mobility in OTFT device. Therefore, both film crystallinity and grain boundary are important for OTFT mobilities.



Figure 7. Tapping-mode AFM height (top) and phase (bottom) images of (a, d) PBTzI3T; (b, e) PBTzI3T-2F; (c, f) PDTzTIT; (g, j) PDTzTIT-2F; (h, k) PDTzTI, and (i, l) PBTzI films deposited on silicon substrates from chlorobenzene solution annealed at 250, 250, 250, 200, 200, and 250 °C, respectively.

The molecular packing in the polymer films was characterized by grazing incidence wideangle X-ray scattering (GIWAXS).82,

83

The two-dimensional GIWAXS patterns and the

corresponding intensity profiles are presented in Figure 8. Table S2 summarizes the peak positions, lattice constants, and crystal correlation lengths for the (100) lamellar and (010) - stacking peaks. All these imide-functionalized thiazole-based polymer films share similar lamellar stacking 26 ACS Paragon Plus Environment

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distances of ~22 Å, due to the same 2-hexyldecyl chain on the imide group. On the basis of the (010) diffraction, all polymers show a compact -stacking distance of ~3.6 Å, which likely originates from the close -stacking distances (3.2–3.3 Å) of the model compounds and the highly planar backbone of these thiazole-based polymer semiconductors. Therefore, thiazole incorporation leads to the polymers with good materials packing and close inter-polymer chain stacking distance. On the basis of the diffraction patterns (Figure 8), these polymer chains PBTzI3T, PBTzI3T2F, PDTzTIT, PDTzTIT-2F, and PDTzTI exhibit a preferential “edge-on” oriented molecular packing, with the lamellar peak mainly located along the out-of-plane (qz) direction and the stacking peak concentrated along the in-plane (qr) direction. The “edge-on” orientation, known to be favorable for charge carrier transport in OTFT devices,84 is in agreement with the overall transport characteristics observed of our semiconductors. However, the homopolymer PBTzI shows a preferential “face-on” oriented molecular packing and poor crystallization property, which is in good agreement with its lowest mobility in the polymer series. Among all the polymer films, the film of the copolymer PBTzI3T exhibits up to the fifth order lamellar diffraction with relatively higher peak intensities and large crystal coherence lengths (CCLs) in both in-plane and out-ofplane directions (Table S2), indicative of their relatively high film crystallinity, which is in good agreement with its highest RMS roughness. It is known that when using an oligothiophene (nT) moiety as donor co-unit in alternating copolymers, as the oligothiophene length increases, the polymer film crystallinity is typically enhanced.44 However, oligothiophene extension also leads to undesired elevation of the LUMO energy level and thus increases the electron injection barrier, which is detrimental to n-type



(a)

(b)

Figure 8. (a) GIWAXS patterns of PBTzI3T, PBTzI3T-2F, PDTzTIT, PDTzTIT-2F, PDTzTI, and PBTzI films deposited on silicon substrates; (b) the corresponding in-plane (dash) and out-ofplane (solid) intensity profiles.

performance. Hence, in spite of its high crystallinity, the terthiophene-based copolymer PBTzI3T shows a moderate electron mobility of ~0.02 cm2 V-1 s-1 due to its high-lying LUMOs in combination with distinctive grain boundaries as shown by the AFM image. On the other hand, the 28 ACS Paragon Plus Environment

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poor film crystallinity of PBTzI is disadvantageous for electron transport, in agreement with the charge transport performance observed in OTFTs. Based on the diffraction pattern, the PDTzTI homopolymer shows a substantial crystallinity with the lamellar diffraction progressing to the fifth order, which combined with its low-lying LUMO and good domain connectivity yields the highest electron mobility in the series. Conclusions In summary, a new highly electron-deficient imide-functionalized bithiazole, N-alkyl-5,5'bithiazole-4,4'-dicarboximide (BTZI), was synthesized and incorporated into polymer semiconductors. Compared to thiophene, electron-deficient thiazole renders the synthesis of BTZI greatly different from the BTI. To our delight, a highly effective synthetic route was devised by taking advantage of the C–H activation, which greatly simplifies materials synthesis and improves the yield. Single crystal analysis reveals that BTzI features a planar backbone with a very short πstacking distance of ~3.3 Å. In addition, thiazole incorporation induces intramolecular noncovalent N…S interactions with reduced steric hindrance and promotes polymer backbone planarization. The new dibrominated BTzI shows low chemical reactivity under typical Stille-coupling based polycondensation. Interestingly, the addition of CuI co-catalyst allows successful synthesis of acceptor-acceptor (or all-acceptor) type homopolymer PBTzI, and the polymer exhibited very lowlying LUMO level of -3.94 eV. OTFTs based on PBTzI displayed unipolar n-type transport with an electron mobility of 0.015 cm2 V-1 s-1 and remarkably low Ioff of 10–10–10–11 A. Due to the greatly increased film crystallinity, homopolymer PDTzTI showed a substantially larger electron mobility of 1.61 cm2 V−1 s−1 accompanied by small Ioffs of 10–10–10–11 A and remarkable high Ion/Ioffs of 107–108, attributed to its highly suppressed FMO level enabled by the all-acceptor type backbone. The results demonstrate the superiority of all-acceptor approach for developing highperformance unipolar n-type polymers. 29 ACS Paragon Plus Environment


Incorporation of donor co-unit leads to BTzI-based donor-acceptor type copolymers with n-type predominating ambipolarity. Increasing the content of the electron-deficient imide gradually deepens the FMO levels of DTzTI-based donor-acceptor-acceptor type copolymers PDTzTIT and PDTzTIT-2F, which reduce hole injection and facilitate electron injection, enabling unipolar ntype transport with a maximum e of 0.91 cm2 V−1 s−1 in PDTzTIT-2F based OTFTs. The results reflect the effects of increasing acceptor content in polymer backbone on promoting n-type performance. The unique geometries and electronic properties of these imide-functionalized thiazoles enable the development of polymer semiconductors with distinct backbone motifs, including donor-acceptor, donor-acceptor-acceptor, and all-acceptor, as well as tunable charge carrier polarity. Thus, these imide-functionalized thiazoles are excellent building blocks for highperformance n-type polymers, and the structure-property correlations of these polymer semiconductors are informative for further development of n-type organic semiconductors.

Associated Content Supporting Information Supporting Information is available free of charge on the ACS Publications website. Experimental details, synthesis and characterization of model compounds, monomers, and polymers, single crystal structure data of model compounds (CCDC 1563479), TGA plots, DSC curves, details of OTFT fabrication and characterization, OTFT performance data, and GIWAXS data can be found in the online version at xxxxxxxxxxxxxxxxxxxxxxxxx.

Author Information Corresponding Authors *E-mail: [email protected] (X.L.) *E-mail: [email protected] (X.G) 30 ACS Paragon Plus Environment

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ORCID Antonio Facchetti: 0000-0002-8175-7958 Xugang Guo: 0000-0001-6193-637X Author Contributions ǁ

Y.S. and H.G. contributed equally.

Notes The authors declare no competing financial interest. Acknowledgements Y.S. and H.G. contributed equally. X.G. is grateful to NSFC (51573076), Shenzhen Basic Research Fund

(JCYJ20170817105905899),

and

Shenzhen

Peacock

Plan

Project

(KQTD20140630110339343). X.L. thanks the Research Grant Council of Hong Kong (General Research Fund No. 14314216) for financial support and the beam time and technical supports provided by 23A SWAXS beamline at NSRRC, Hsinchu. References: (1) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L., Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666-12731. (2) Li, H.; Kim, F. S.; Ren, G.; Hollenbeck, E. C.; Subramaniyan, S.; Jenekhe, S. A., Tetraazabenzodifluoranthene Diimides: Building Blocks for Solution-Processable n-Type Organic Semiconductors. Angew. Chem. Int. Ed. 2013, 52, 5513-5517. (3) Zhang, J.; Tan, H. S.; Guo, X.; Facchetti, A.; Yan, H., Material insights and challenges for non-fullerene organic solar cells based on small molecular acceptors. Nat. Energy 2018, 3, 720731. (4) Sirringhaus, H., 25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 2014, 26, 1319-1335. (5) Zhan, X.; Facchetti, A.; Barlow, S.; Marks, T. J.; Ratner, M. A.; Wasielewski, M. R.; Marder, S. R., Rylene and Related Diimides for Organic Electronics. Adv. Mater. 2011, 23, 268284. (6) Tsao, H. N.; Cho, D. M.; Park, I.; Hansen, M. R.; Mavrinskiy, A.; Yoon, D. Y.; Graf, R.; Pisula, W.; Spiess, H. W.; Müllen, K., Ultrahigh Mobility in Polymer Field-Effect Transistors by Design. J. Am. Chem. Soc. 2011, 133, 2605-2612. (7) Oh, J. Y.; Rondeau-Gagné, S.; Chiu, Y.-C.; Chortos, A.; Lissel, F.; Wang, G.-J. N.; Schroeder, B. C.; Kurosawa, T.; Lopez, J.; Katsumata, T.; Xu, J.; Zhu, C.; Gu, X.; Bae, W.-G.; 31 ACS Paragon Plus Environment


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Graphic Abstract


Imide-Functionalized Thiazole-Based Polymer Semiconductors

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