Chalcogenophene-Sensitive Charge Carrier ... - ACS Publications

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Chalcogenophene-Sensitive Charge Carrier Transport Properties in A−D−A′′−D Type NBDO-Based Copolymers for Flexible Field-Effect Transistors Keli Shi,† Weifeng Zhang,*,† Yankai Zhou,†,‡ Congyuan Wei,† Jianyao Huang,† Qiang Wang,†,‡ Liping Wang,‡ and Gui Yu*,†,§

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†

CAS key Laboratory of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China § School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: The use of chalcogenophenes runs through the whole history of developing high performance π-conjugated materials toward organic electronics devices. In this work, we report three A−D−A′−D type (3E,7E)-3,7-bis(2-oxo-1Hpyrrolo[2,3-b]pyridin-3(2H)-ylidene)benzo[1,2-b:4,5-b′]difuran-2,6(3H,7H)-dione- (NBDO-) based π-conjugated copolymers containing different chalcogenophenes, i.e., 4,7di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TTT), 4,7-di(selenophen-2-yl)benzo[c][1,2,5]thiadiazole (STS), or 4,7di(selenophen-2-yl)benzo[c][1,2,5]selenadiazole (SSS). The effects of chalcogen atom on their optoelectronic properties were explored by a range of techniques, including thermal, optical, electrochemical, computational, molecular aggregation, and carrier transport properties. Interestingly, both PNBDOTTT and PNBDO-STS formed highly ordered, crystalline, and lamellar packing thin films with uniform intertwined fibrillar morphologies, whereas PNBDO-SSS only gave random molecular packing thin film with amorphous morphology, despite their similar chemical structures, optical and electrochemical properties. In consequence, PNBDO-TTT and PNBDO-STS-based flexible field-effect transistors on PET substrate exhibited high electron mobilities of 2.41 and 2.68 cm2 V−1 s−1, respectively, whereas PNBDO-SSS-based ones only showed a lowered electron mobility of 0.012 cm2 V−1 s−1.

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INTRODUCTION

meric semiconductors toward PFETs with good comprehensive performance are still indispensable and urgent. The use of chalcogenophenes runs through the whole history of developing high performance π-conjugated materials toward organic electronics devices. Poly(3-hexylthiophene) (P3HT) is a benchmark material of second generation πconjugated polymers. Meanwhile, poly(didodecylquarterthiophene) (PQT-12) and poly(2,5-bis(2thienyl)-3,6-dipentadecyl-thieno[3,2-b]thiophene) are also two outstanding representatives of second generation π-conjugated polymers.27,28 As for third generation D−A copolymers, actually, most of record high charge mobilities are also acquired from the chalcogenophene-based copolymers. In recent years, the use of chalcogenophenes are no longer limited to sulfur-containing π-conjugated systems, other chalcogenophenes containing oxygen, selenium, even tellurium atoms have also been used to develop high mobility polymer

In spite of the low crystallinity and structural uncertainty, πconjugated polymers have still been considered as one of the most promising semiconducting materials for next-generation electronics devices, including organic field-effect transistors (FETs)1−6 and organic photovoltaics,7−9 due to their superior solution processability and compatibility with flexible substrates. During the past few decades, striking achievements have been made in π-conjugated polymers-based FETs (PFETs) upon utilizing synergistic molecular design strategies such as planar conjugated backbone,10−12 long alkylated side chain,13−15 suitable frontier molecular orbital (FMO) energy levels,16−18 and ordered solid-state molecular packing,19,20 and device physics, for example, interface engineering.21 Nowadays, impressively high carrier mobilities (far exceeding 0.1−1.0 cm2 V−1 s−1 of amorphous silicon) have been achieved for pchannel,13,22 n-channel,14,23 and ambipolar PFETs based on well-designed donor−acceptor (D−A) copolymers;24−26 however, the air-stability and lifetime of these PFETs remains unsatisfactory, thereby continuous discovery of novel poly© XXXX American Chemical Society

Received: September 8, 2018 Revised: October 15, 2018

A

DOI: 10.1021/acs.macromol.8b01944 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules semiconductors (Table S1 in Supporting Information).29−33 For example, Kim and co-workers reported a diketopyrrolopyrrole (DPP)-based copolymer, poly[2,5-bis(7decylnonadecyl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione-(E)(1,2-bis(5-(thiophen-2-yl)selenophen-2-yl)ethene], which afforded higher hole mobilities of 12.04 cm2 V−1 s−1 than 10.54 cm2 V−1 s−1 obtained in its sulfur-containing analogue, poly[2,5-bis(7-decylnonadecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-(E)-1,2-di(2,2′-bithiophen-5-yl)ethene].13 Also, Takimiya et al. developed a series of naphthobischalcogenadiazole-based copolymers and found that naphtho[1,2c:5,6-c′]bis[1,2,5]oxadiazole and naphtho[1,2-c:5,6-c′]bis[1,2,5] -thiadiazole-based copolymers own similar optoelectronic properties compared to those of naphtho[1,2-c:5,6c′]bis[1,2,5]selenadiazole-based counterpart.32 Moreover, Ashraf et al. reported three DPP-based copolymers containing thiophene, selenophene, and tellurophene units.33 Among them, thiophene-containing copolymer gave better photovoltaic property, whereas both selenophene- and tellurophenecontaining derivatives afforded higher field-effect mobilities of 1.6 cm2 V−1 s−1. On the other hand, Seferos and co-workers also tried to unveil the effect of chalcogen atom using theoretical calculations and suggested that single atom substitution at key positions could be adopted in adjusting the bandgap of D−A copolymers.34 Both the experimental and theoretical results are important to research on studying the structure−property relationships of D−A copolymers. With the aim of developing high performance PFETs, material scientists recently developed series of π-conjugated polymers with more complex electronic structures than the binary system of conventional D−A copolymers. The most prominent example is ternary copolymers, including A−D− A′−D and A−D−A−D′ type copolymers.35 Because of their easily tunable bandgap gaps, especially, the A−D−A′−D type copolymers have great potential in the development of ambipolar and n-type polymer semiconductors.24 Therefore, exploring the effect of chalcogen atom on optoelectronic properties in A−D−A′−D type copolymers also become of great significance for the development of organic electronics. For all we know, however, the related studies are still rare to date. In this work, we developed a series of novel A−D−A′−D type (3E,7E)-3,7-bis(2-oxo-1H-pyrrolo[2,3-b]pyridin-3(2H)ylidene)benzo[1,2-b:4,5-b′]difuran-2,6(3H,7H)-dione (NBDO)-based copolymers PNBDO-TTT, PNBDO-STS, and PNBDO-SSS, having different chalcogenophenes, i.e., 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TTT), 4,7di(selenophen-2 -yl)benzo[c][1,2,5]-thiadiazole (STS), and 4,7-di(selenophen-2-yl)benzo[c][1,2,5] -selenadiazole (SSS), respectively. The NBDO unit herein was chosen as electronwithdrawing moiety because it owns low-lying FMO energy levels and planar π-extended conjugated backbone.24 The thermal, optical, electrochemical properties, and molecular aggregation of the three copolymers were explored by a range of techniques. Their charge mobilities were evaluated by fabricating flexible PFETs on flexible polyethylene terephthalate (PET) substrate, and the effect of chalcogen atom on their optoelectronic properties was specifically examined.

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water-sensitive reactions were performed under inert atmosphere. (3E,7E)-3,7-Bis(6-bromo-1-(4-octadecyldocosyl)-2-oxo-1H-pyrrolo[2,3-b]pyridin-3(2H)-ylidene)benzo[1,2-b:4,5-b’]difuran-2,6(3H,7H)-dione (8) was prepared according to the literature procedure.24 4,7-Bis(5-(trimethylstannyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (9) was purchased from Derthon Optoelectronic Materials Science Technology Co., Ltd. 4,7-Bis(5-(trimethylstannyl)selenophen-2-yl)benzo[c][1,2,5]thiadiazole (4). A 2.5 M solution of n-BuLi in hexane (2.0 mL, 5.0 mmol) was added dropwise to a solution of 3 (0.79 g, 2.0 mmol) in THF (50 mL) at −78 °C under argon. The reaction solution was stirred at −78 °C for 1 h and then slowly warmed to −40 °C and stirred for additional 30 min. After the suspension was cooled to −78 °C again, a 1.0 M solution of Me3SnCl in THF (6.0 mL, 6.0 mmol) was added in one portion. The reaction mixture was stirred overnight at room temperature, followed by being quenched with deionized water and being extracted with Et2O (60 mL × 3). After the removal of the organic solvent under reduced pressure, the resulting solid residue was crystallized from chloroform/methanol to give the desired monomer as brownish red needles (0.76 g, 53%). 1H NMR (300 MHz, CDCl3, δ): 8.25 (d, J = 3.0 Hz, 2H), 7.88 (s, 2H), 7.62 (d, J = 3.0 Hz, 2H), 0.43 (s, 18H). 13C NMR (75 MHz, CDCl3, δ): 152.65, 149.43, 149.08, 138.43, 129.40, 127.68, 125.37, −7.89. HRMS (MALDI−TOF): Calcd for C20H24N2SSe2Sn2: 721.8029. Found: 721.8032. 4,7-Bis(5-(trimethylstannyl)selenophen-2-yl)benzo[c][1,2,5]selenadiazole (7). The synthetic procedure is similar to that of monomer 6 using (0.66 g, 1.5 mmol), 2.5 M solution of n-BuLi in hexane (1.5 mL, 3.8 mmol), and 1.0 M solution of Me3SnCl in THF (4.0 mL, 4.0 mmol) to give the desired monomer as brownish red needles (0.53 g, 48%). 1H NMR (300 MHz, CDCl3, δ): 8.15 (d, J = 3.0 Hz, 2H), 7.82 (s, 2H), 7.61 (d, J = 3.0 Hz, 2H), 0.43 (s, 18H). 13C NMR (75 MHz, CDCl3, δ): 158.13, 149.60, 149.51, 138.04, 129.00, 128.91, 125.07, −7.91. HRMS (MALDI−TOF): Calcd for C20H24N2Se3Sn2: 767.7481. Found: 767.7479. General Synthetic Procedure for Stille Copolymerization. Pd2(dba)3 (4.0 mg), P(o-tol)3 (10.0 mg), and chlorobenzene (10.0 mL) were successively added to a Schlenk flask containing tin monomer, 8 (0.10 mmol), and monomer, 9 (or 4 and 7) (0.10 mmol). After being purged with argon for 30 min at −78 °C, the reaction mixture was heated to 115 °C and stirred for 12 h under argon. After being cooled to room temperature, the reaction system was poured into 200 mL of methanol containing 5 mL of HCl (aqueous 6 M), and stirred for 3 h. The resulting solid was filtered and purified via Soxhlet extraction in methanol, acetone, and hexane for 24 h, respectively, to remove the residual catalytic metal and lowmolecular-weight side-products. At last, the solid residue was extracted with o-dichlorobenzene and precipitated in methanol, filtered and dried in vacuo at room temperature to give the desired polymer material. PNBDO-TTT (170 mg, 92%). 1H NMR (400 MHz, d2-C2D2Cl4, δ): 8.43 (br), 7.27 (br), 6.83 (br), 3.72 (br), 1.36−1.28 (m). GPC: Mn = 46.1 kDa, Mw = 127.2 kDa, PDI = 2.76. Anal. Calcd for C118H174N6O6S3: C, 75.84; H, 9.38; N, 4.50. Found: C, 75.51; H, 9.25; N, 4.45. PNBDO-STS (176 mg, 90%). 1H NMR (400 MHz, d2-C2D2Cl4, δ): 8.49 (br), 7.36 (br), 6.77 (br), 3.73 (br), 1.63−0.87 (m). GPC: Mn = 63.8 kDa, Mw = 206.9 kDa, PDI = 3.24. Anal. Calcd for C118H174N6O6SSe2: C, 72.21; H, 8.94; N, 4.28. Found: C, 71.93; H, 8.83; N, 4.27. PNBDO-SSS (172 mg, 86%). 1H NMR (400 MHz, d2-C2D2Cl4, δ): 8.57 (br), 7.41 (br), 6.73 (br), 3.73 (br), 1.64−0.88 (m). GPC: Mn = 68.8 kDa, Mw = 196.1 kDa, PDI = 2.85. Anal. Calcd for C118H174N6O6Se3: C, 70.53; H, 8.73; N, 4.18. Found: C, 69.78; H, 8.52; N, 4.17. Device Fabrication and Characterization. PFETs with topgated bottom-contact (TGBC) configuration were fabricated on PET substrates. Gold source/drain electrodes were patterned on the PET substrates by vacuum evaporation technique, followed by modified by a 1-octanethiol (OT) solution in ethanol (OT: ethanol, 1:105, v/v).

EXPERIMENTAL SECTION

General Information. The catalyst and ligand, Pd2(dba)3 and P(o-tol)3, were purchased from Sigma-Aldrich, and other chemicals and solvents were used as received from Acros, Innochem, etc., without further purification unless otherwise stated. The air- and B


Article

Macromolecules Scheme 1. Synthetic Routes of the NBDO-Based Copolymers

Figure 1. DFT predicted frontier molecular orbital distribution and energy levels at B3LYP/6-31G(d) level for PNBDO-TTT (a), PNBDO-STS (b), and PNBDO-SSS (c) (in trimer). The semiconducting thin layer was spin-coated on the substrates from a polymer solution in o-dichlorobenzene (3 mg/mL) at a speed of 2000 rpm for 60 s in a glovebox filled with nitrogen, subsequently, thermal annealing on a hot plate at 140 °C for 40 min. A poly(methyl methacrylate) (PMMA, Mw = 1000 kDa) solution in n-butyl acetate (60 mg/mL) was used to fabricated dielectric layer on the surface of semiconducting thin layer by spin-coating method at 3000 rpm for 40s (PMMA thickness ∼ 960 nm; permittivity = 2.70). Then, the PMMA thin films were heated to 115 °C and baked for 60 min. Al (60 nm) gate electrode was deposited by thermal evaporation technique through a shadow mask. The channel width and channel length of all PFET devices are 4500 and 50 μm, respectively. Complementary-like inverters based on PNBDO-STS were fabricated on PET substrate with TGBC configuration and with a similar fabrication procedure of PFET devices aforementioned. The complementary-like inverters consisted of two ambipolar transistors,

in which a common drain as the output voltage (VOUT) and a common gate as the input voltage (VIN). Both PFET devices and complementary-like inverters were fabricated under nitrogen atmosphere. The electrical properties of each device were measured under ambient conditions on a Keithley 4200 semiconductor parameter analyzer. Hole and electron mobilities (μh and μe) were calculated from the saturation regime according to the following equation:

iW y IDS = jjj zzzCiμ(VG − VT)2 k 2L {

In the equation, W/L is the channel width/channel length, IDS denotes the saturation drain current, Ci is the capacitance per unit area of the gate dielectric layer, and VG and VT are the gate voltage and threshold voltage, respectively. C


Article

Macromolecules Table 1. Electrochemical Properties and Molecular Orbital Energy Levels of the NBDO-Based Copolymers polymer

a Eox onset (V)

a Ered onset (V)

b ECV HOMO (eV)

b ECV LUMO (eV)

c EDFT HOMO (eV)

c EDFT LUMO (eV)

PNBDO-TTT PNBDO-STS PNBDO-SSS

1.51 1.42 1.36

−0.52 −0.53 −0.51

−5.90 −5.81 −5.75

−3.87 −3.86 −3.88

−5.30 −5.28 −5.23

−3.68 −3.69 −3.68

a ox CV red Determined by CV under nitrogen atmosphere. bECV HOMO = −(Eonset + 4.80 − Eonset, Fc/Fc+) eV; ELUMO= −(Eonset + 4.80 − Eonset, Fc/Fc+) (Eonset, Fc/Fc+ = 0.41 eV). cCalculated using DFT at B3LYP/6-31G(d) level.

Figure 2. Normalized absorption spectra of the NBDO-based copolymers in chlorobenzene solution (a) and thin film spin-coated on quartz plates (b).

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RESULTS AND DISCUSSION Synthetic and Characterization. The synthetic routes of NBDO-based copolymers are outlined in Scheme 1. 4,7Di(selenophen-2-yl)benzo[c][1,2,5]thiadiazole (3) and 4,7di(selenophen-2-yl)benzo[c][1,2,5]selenadiazole (6) were individually synthesized by Stille coupling reactions of trimethyl(selenophen-2-yl)stannane (2) with 4,7-dibromobenzo[c][1,2,5]thiadiazole (1) or its selenium-containing analogue (5). The detailed synthetic procedures of 3 and 6 were collected in the Supporting Information. Then, the two intermediates reacted n-Buli and quenched by Me3SnCl giving their respective tin-containing monomers, 4 and 7. Modified Stille coupling polycondensation reactions of NBDO-based monomer (8) with 4,7-bis(5-(trimethylstannyl)thiophen-2yl)benzo[c][1,2,5]thiadiazole (9), or monomers 4 and 7 afforded crude polymer materials, PNBDO-TTT, PNBDOSTS, and PNBDO-SSS, respectively. After being purified by Soxhlet extraction and precipitation from methanol, the three desired copolymers were obtained, then characterized by high temperature 1H NMR (100 °C, CDCl2CDCl2) and elemental analysis. The results match well with their respective chemical structures. High temperature gel permeation chromatography (GPC) (150 °C, 1,2,4-trichlorobenzene) indicated that these copolymers have comparable molecular weight and distribution. The number-average molecular weight (Mn) and weightaverage molecular weight (Mw) of these copolymer locate at 46.1−68.8 and 127.2−206.9 kDa, respectively (Figure S1 and Table S2 in the Supporting Information). Although it is difficult to quantify the solubility, PNBDO-SSS shows apparently higher solubility than the other copolymers. Theoretical Calculations. The energy-minimized polymer backbone structures of the NBDO-based copolymers were computed using density functional theory (DFT) [B3LYP/631G(d) level] on a trimeric system, in which all the long alkyl chains were replaced by methyl groups for simplicity (Figure 1). Computational results reveal all these copolymers possesses similar and planar conjugated backbones (Figure S2 in the

Supporting Information). The highest occupied molecular orbitals (HOMOs) of these copolymers are well delocalized along the polymer backbone structures, wheareas the lowest unoccupied molecular orbitals (LUMOs) of all copolymers are almost localized on the NBDO unit in these A−D−A′−D πconjugated systems. The FMO energy levels of these copolymers were also therotically predicted using DFT as depicted in Figure 1. The HOMO energy levels (EDFT HOMO) of PNBDO-TTT, PNBDO-STS, and PNBDO-SSS were calculated to be −5.30, −5.28, and −5.23 eV, respectively, while their LUMO energy levels (EDFT LUMO) were estimated to be −3.68 to −3.69 eV (Table 1). These data imply that the introduction of selenium-containing π-conjugated systems only marginally raises the HOMOs, while the LUMOs are stabilized, which is in agreement with previous theoretical calculations.33 AccordDFT ingly, the predicted energy gaps (EDFT HOMO − ELUMO) of PNBDOTTT, PNBDO-STS, and PNBDO-SSS were computed to be 1.62, 1.59, and 1.55 eV, respectively (Table 1). Thermal Properties. The three NBDO-based copolymers were subjected to thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to investigate thermal properties. The decomposition temperatures (Tdec, 5% weight loss) of PNBDO-TTT, PNBDO-STS, and PNBDO-SSS are 398, 396, and 372 °C, respectively (Figure S3 in the Supporting Information). The high Tdec suggest that all these polymer materials have good thermal stabilities. As displayed in Figure S4, all the DSC curves (second heating−cooling cycle conducted at the same rate of 10 °C/min) show apparent endothermas corresponding glass transistion at temperature (Tg) of 120 °C for PNBDO-TTT, 165 °C for PNBDO-STS, and 171 °C for PNBDO-SSS (Table S2 of the Supporting Information). Meanwhile, the reversible exothermas were also observed upon cooling. In view of their similar molecular weight, we thought the increasing Tg stem from chalcogen atom effect that more selenium atoms in the polymeric conjugated backbones induce higher Tg. As we known, glass transistion could be considered as an indicative that the D


Article

Macromolecules

recorded from 30 to 90 °C at an interval of 10 °C (Figure S5 in the Supporting Information). In warm solutions, PNBDOTTT retains a mostly identical spectrum to that in the film, indicating that strong aggregation occurs even at 90 °C. For PNBDO-STS, there is partial aggregation in warm solutions due to a slight difference existing in their absorption spectra compared with that of thin film. However, the absorption spectra of PNBDO-SSS become structure-less upon heating from 30 to 90 °C, and they are much different to that of its thin film. This implies that PNBDO-SSS almost disaggregated at room temperature, revealing that the copolymer has weaker aggregation tendency than those of PNBDO-TTT and PNBDO-STS. The results could be used to interpret the increasing solubility of PNBDO-TTT, from PNBDO-STS to PNBDO-SSS,. For well eliciting the origin of these aggregation tendencies, we further computed in depth the molecular structure of TTT, STS, and SSS using DFT at B3LYP/631G(d) level. Though their electronic structures in the three molecules are slightly different (the N−S distance and N−C−C angle are 1.64 Å/125.57° for TTT, and the N−S distance and N−C−C angle are 1.64Å/125.54° for STS, and the N−Se distance and N−C−C angle are 1.79Å/123.58° for SSS), all the three molecules still keep almost planar conjugated backbone conformations (Figure S6 in Supporting Information). Thus, the conjugated backbone planarity of SSS unit is not responsible for the lower aggregation tendency of PNBDO-SSS than those of the other two copolymers. While several previous studies revealed that the Se-substitution in conjugated polymers might bring weakened intermolecular interaction and loose π−π stacking due to the larger van der Waals radius of Se atoms, therefore, we attribute the lowered aggregation tendency of PNBDO-SSS to its weaker intermolecular interactions induced by Se atoms.32,38 Electrochemical Properties. The electrochemical properties of these copolymers were explored by cyclic voltammetry (CV) (Figure 3a). The measuring process was performed by using the polymer film coated on platinum stick as the working electrode under argon atmosphere. The HOMO and LUMO CV energy levels (ECV HOMO and ELUMO) were estimated from their respective onset oxidation and reduction potentials. All copolymers have almost similar ECV LUMO of −3.86 to −3.88 eV due to the same electron-withdrawing unit, with gradually increasing ECV HOMO of −5.90 eV for PNBDO-TTT, −5.81 eV for PNBDO-STS, and −5.75 eV for PNBDO-SSS (Table 1). As displayed in the energy level diagrams in Figure 3b, the change

molecules can start to wiggle around. Polymer materials with low Tg might means that they could easily self-assemble when annealed at mild thermal annealing process. Nowadays, The annealing temperatures (up to 300 °C) of most PFETs devices required are apparently higher than the deformation temperatures of the commonly used flexible plastic substrates, for example PET (ca. 150 °C) and polyethylene naphthalate (PEN, ca. 175 °C).36 All in all, their low Tg of 120−171 °C endows the three NBDO-based copolymers with congenital advantage in fabricating flexible PFETs.37 Photophysical Properites. The normalized UV−vis−NIR absorption spectra of the three NBDO-based copolymers in chlorobenzene solution and thin film spin-coated on quartz plates are depicted in Figure 2. In solution, PNBDO-STS showed the absorption maxima (λabs max) at 847 nm, which bathochromically shifted by ca. 8 nm compared with PNBDOabs TTT (λabs of PNBDO-SSS max at 839 nm), whereas the λmax locates at 794 nm, having a large hypsochromical shift by 45 nm in comparison with that of PNBDO-TTT (Table 2). As Table 2. Optical Properties of the NBDO-Based Copolymers a λabs max (nm)

b λabs edge (nm)

polymer

solution

film

solution

film

c Eopt g (eV)


839 783, 847 794

763, 837 775, 855 792, 868

928 962 990

940 969 1009

1.32 1.28 1.23

a

Absorption maximum. bAbsorption edges. cCalculated from thinabs film absorption edges (Eopt g =1240/λedge).

expected, these copolymers possess broadened absorption spectra in thin films. The optical bandgap (Eopt g ) of PNBDOTTT was calculated to be 1.32 eV from the absorption edges opt (λabs of PNBDO-STS and edge, 940 nm). Similarly, the Eg PNBDO-SSS were estimated to be 1.28 and 1.23 eV based on their λabs edge of 969 and 1009 nm, respectively. Compared to PNBDO-TTT, PNBDO-STS and PNBDO-SSS have gradually red-shifted absorption spectra both in solution and thin film and own a narrowing of Eopt g , which is likely a consequence of the larger Se atoms in their single A−D−A′−D unit of the two copolymers. In order to cast light onto the aggregation tendency of these copolymers, their temperature-dependent UV−vis−NIR absorption spectra in dilute chlorobenzene solutions were also

Figure 3. CV traces (a) and energy level diagrams (b) of three NBDO-based copolymers. E


Article

Macromolecules Table 3. Device Performance of the TGBC PFETs Fabricated from Three NBDO-Based Copolymers p-channel

n-channel

polymer

μh,max (cm2 V−1s−1)

μh,avg (cm2 V−1s−1)

Vth (V)

μe,max (cm2 V−1s−1)

μe,avg (cm2 V−1s−1)

Vth (V)

d−d (Å)

π−π (Å)


0.13 0.27 0.0084

0.12 0.17 0.0049

−77 −78 −51

2.41 2.68 0.012

2.22 2.34 0.0054

74 73 22

27.67 27.03 −

3.55 3.56 −

The average values were calculated from more than 10 devices. bThe d−d and π−π stacking distances in respective annealed thin films.

a

respectively. However, PNBDO-SSS thin films only exhibited weak diffraction (100) Bragg peak and no diffraction (010) Bragg peaks were observed, suggesting that PNBDO-SSS adopts random molecular packing mode in solid state. Surface morphology of these polymer thin films were studied by tapping-mode atomic force microscopy (AFM). The AFM height images of as-spun and the annealed thin films deposited on PET substrates are presented in Figures S9d−f and 5d−f and in the Supporting Information, respectively. Both the annealed PNBDO-TTT and PNBDO-STS thin films display uniform intertwined fibrillar network with obviously crystallized zones, whereas the annealed PNBDO-SSS thin film shows an amorphous morphology. Because fibrillar polycrystalline networks are favorable for efficient charge transport,39 such observations can well interpret the different charge mobilities of the three copolymers. These copolymers have considerably different molecular packing and thin film morphology, and device performance despite their similar optical and electrochemical properties. This could be partly attributed to their different molecular selfassembly behaviors originated from the incorporation of selenium atoms. As mentioned above, the Tg of PNBDOTTT, PNBDO-STS, and PNBDO-SSS are 120, 163, and 171 °C, respectively, meaning that the molecular self-assembly become more difficult from PNBDO-TTT, PNBDO-STS, to PNBDO-SSS when their thin films were thermally annealed at 140 °C (the annealing temperature adopted in this study, lower than the deformation terperature of PET substrate). For PNBDO-STS, although its Tg of 163 °C is higher than the annealing teperature, effective molecular self-assembly could occur in its thin film when annealed for a certain time (40 min in this study); thus, ordered molecular packing is also formed in its thin film.3 The similar phenomenon was already observed in other high performance polymer semiconductors.24 For PNBDO-SSS, however, the difference of 30 °C between its Tg and the annealing temperature is too large for its polymer chains to conduct a valid molecular self-assembly. The diagram of PNBDO-TTT and PNBDO-STS self-assembling to orderly molecular packing in solid state upon thermal annealing at the temperatures close to or exceeding their respective Tg are shown in Figure 6. In the process, the scattered molecules marked as yellow, brown, green, or red wiggle to align with the ordered packing molecules marked as purple in thermal annealing process. Generally speaking, the poor charge transport property of PNBDO-SSS could be attributed to its lowered intermolecular interaction and molecular self-assembly ability induced by the introduction of selenophene atoms on polymeric conjugated backbone.

trends in FMO energy levels of these copolymers are essentially in agreement with those of computation results. CV In addition, the energy gaps (ECV HOMO − ELUMO) were also calculated to be 2.00, 1.92, and 1.85 eV for PNBDO-TTT, PNBDO-STS, and PNBDO-SSS, respectively. Charge Transport Properties. Flexible PFET devices with TGBC configuration were used to evaluate the carrier transport properties of the NBDO-based copolymers. The polymer active layer was fabricated on patterned Au/PET substrate by spin-coating method, then annealed at 140 °C for 40 min. All FET devices had a channel width (W) of 4500 μm and a channel length (L) of 50 μm. Details of the device fabrication are included in the Experimental Section. The three copolymers exhibited ambipolar carrier transport characteristics under ambient conditions. The highest hole/electron mobilities of PNBDO-TTT, PNBDO-STS, and PNBDO-SSS are 0.13/2.41, 0.27/2.68, and 0.0084/0.012 cm2 V−1 s−1, respectively (Table 3). Their typical transfer and output characteristics are displayed in Figures 4 and S7. It is notable that all these PFET devices have good air-stability. When storing in a desiccator under ambient conditions (RH = 40− 60%) for 3 weeks, the PNBDO-TTT- and PNBDO-STS-based devices still showed high hole/electron mobilities of 0.14/2.06 and 0.21/2.13 cm2 V−1 s−1, respectively (Table S3). In addition, we also fabricated PNBDO-STS-based flexible complementary-like inverter on PET substrate. The device configuration diagram and representative voltage transfer characteristic curves are shown in Figure S8 (Supporting Information). The PNBDO-STS-based flexible complementary-like inverter composed of two identical ambipolar PFET devices gave the gain of 64. Thin-Film Microstructure Studies. In the preceding section, we note that PNBDO-SSS showed much lower hole/ electron mobilities than those of PNBDO-TTT and PNBDOSTS. As the charge carrier mobility in OFETs are sensitive to π−π stacking distance and molecular packing mode;40 therefore, we further investigated thin film microstructure of these copolymer using 2D grazing incident X-ray diffraction (2D-GIXRD). For eliminating the strong diffraction peaks of PET substrate and acquiring clear diffraction patterns, we fabricated polymer thin films on SiO2/Si substrates in our GIXRD characterizations. The 2D-GIXRD diffraction patterns of the as-spun and annealed thin films are presented in Figures S9a−c and 5a−c, respectively. Compared to as-spun thin films, the annealed PNBDO-TTT and PNBDO-STS thin films exhibited enhanced diffraction (h00) Bragg peaks in out-ofplane (qz) orientation, especially PNBDO-STS even showed strong five out-of-plane peaks. These imply that that the two copolymers take highly ordered lamellar packing and predominantly edge-on oriented respective to the substrates in their annealed thin films. On the basis of their diffraction (010) and (100) Bragg peaks, the π−π and d−d stacking distances of the PNBDO-TTT and PNBDO-STS thin films were estimated to be 3.55/28.64 and 3.56/28.24 Å,

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CONCLUSION Three NBDO-based copolymers containing TTT, STS, or SSS units are developed. The investigation of thermal, optical, and electrochemical properties, and molecular packing were F


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Figure 4. TG/BC device configuration used in this study (a). Transfer characteristics of PFETs based on PNBDO-TTT (b), PNBDO-STS (c), and PNBDO-SSS (d) on flexible PET substrate (Annealed at 140 °C). VDS = −110/+110 V at hole-/electron-enhancement operation. G


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Figure 5. 2D-GIXRD patterns and taping mode AFM height images (2 μm × 2 μm) of the three polymer thin films after thermal annealing at 140 °C: (a, d) for PNBDO-TTT, (b, e) for PNBDO-STS, and (c, f) for PNBDO-SSS.

formed highly ordered, crystalline, lamellar packing thin films with uniform intertwined fibrillar morphologies, whereas PNBDO-SSS only gave random molecular packing thin films with amorphous morphologies, which match well with their charge carrier mobilities. The obviously different thin film microstructures were partly attributed to their different molecular self-assembly behaviors induced by the heavier selenium atoms. Our results demonstrate that that the type and number of chalcogen atoms exert remarkable influences on the optoelectronic properties of A−D−A′−D type copolymers.

Figure 6. Diagram of PNBDO-TTT and PNBDO-STS selfassembling to orderly molecular packing in solid state upon annealing at the temperatures close to or exceeding their respective Tg for a certain time. Polymer chains in thin film before (a) and after (b) annealing.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01944. TGA traces, DSC and CV curves, and temperaturedependent absorption profiles; predicted backbone conformation of benzochalcogenodiazole; output characteristics of PFETs; 2D-GIXRD patterns and AFM

systematically investigated. Flexible PFETs based on PNBDOTTT and PNBDO-STS exhibited ambipolar carrier transport characteristics with high electron mobilities up to 2.41 and 2.68 cm2 V−1 s−1, respectively, whereas PNBDO-SSS only afforded much lower charge mobility. Thin film microstructure studies revealed that both PNBDO-TTT and PNBDO-STS H


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(10) Jackson, N. E.; Kohlstedt, K. L.; Savoie, B. M.; Olvera de la Cruz, M.; Schatz, G. C.; Chen, L. X.; Ratner, M. A. Conformational Order in Aggregates of Conjugated Polymers. J. Am. Chem. Soc. 2015, 137, 6254−6262. (11) Huang, J. Y.; Mao, Z. P.; Chen, Z. H.; Gao, D.; Wei, C. Y.; Zhang, W. F.; Yu, G. Diazaisoindigo Based Polymers with HighPerformance Charge Transport Properties: From Computational Screening to Experimental Characterization. Chem. Mater. 2016, 28, 2209−2218. (12) Xin, H. S.; Ge, C. W.; Jiao, X. C.; Yang, X. D.; Rundel, K.; McNeill, C. R.; Gao, X. K. Incorporation of 2,6-Connected Azulene Units into the Backbone of Conjugated Polymers: Towards HighPerformance Organic Optoelectronic Materials. Angew. Chem., Int. Ed. 2018, 57, 1322−1326. (13) Kang, B.; Kim, R.; Lee, S. B.; Kwon, S.-K.; Kim, Y.-H.; Cho, K. Side-Chain-Induced Rigid Backbone Organization of Polymer Semiconductors through Semifluoroalkyl Side Chains. J. Am. Chem. Soc. 2016, 138, 3679−3686. (14) Kang, I.; Yun, H. J.; Chung, D. S.; Kwon, S. K.; Kim, Y. H. Record High Hole Mobility in Polymer Semiconductors via SideChain Engineering. J. Am. Chem. Soc. 2013, 135, 14896−14899. (15) Li, J.; Qiao, X. L.; Xiong, Y.; Li, H. X.; Zhu, D. B. Five-Ring Fused Tetracyanothienoquinoids as High-Performance and SolutionProcessable n-Channel Organic Semiconductors: Effect of the Branching Position of Alkyl Chains. Chem. Mater. 2014, 26, 5782− 5788. (16) Chang, J. J.; Ye, Q.; Huang, K.-W.; Zhang, J.; Chen, Z.-K.; Wu, J. S.; Chi, C. Y. Stepwise Cyanation of Naphthalene Diimide for nChannel Field-Effect Transistors. Org. Lett. 2012, 14, 2964−2967. (17) Wang, Y.; Hasegawa, T.; Matsumoto, H.; Mori, T.; Michinobu, T. High-Performance n-Channel Organic Transistors Using HighMolecular-Weight Electron-Deficient Copolymers and Amine-Tailed Self-Assembled Monolayers. Adv. Mater. 2018, 30, 1707164. (18) Chen, Z. H.; Zhang, W. F.; Huang, J. Y.; Gao, D.; Wei, C. Y.; Lin, Z. Z.; Wang, L. P.; Yu, G. Fluorinated Dithienylethene− Naphthalenediimide Copolymers for High-Mobility n-Channel FieldEffect Transistors. Macromolecules 2017, 50, 6098−6107. (19) Wang, M.; Ford, M. J.; Zhou, C.; Seifrid, M.; Nguyen, T.-Q.; Bazan, G. C. Linear Conjugated Polymer Backbones Improve Alignment in Nanogroove-Assisted Organic Field-Effect Transistors. J. Am. Chem. Soc. 2017, 139, 17624−17631. (20) Kim, H. G.; Kang, B.; Ko, H.; Lee, J.; Shin, J.; Cho, K. Synthetic Tailoring of Solid-State Order in Diketopyrrolopyrrole-Based Copolymers via Intramolecular Noncovalent Interactions. Chem. Mater. 2015, 27, 829−838. (21) Xu, H. X.; Zhou, Y. C.; Zhang, J.; Jin, J. Q.; Liu, G. F.; Li, Y. X.; Ganguly, R.; Huang, L.; Xu, W.; Zhu, D. B.; Huang, W.; Zhang, Q. C. Polymer-Assisted Single Crystal Engineering of Organic Semiconductors To Alter Electron Transport. ACS Appl. Mater. Interfaces 2018, 10, 11837−11842. (22) Kim, G.; Kang, S.-J.; Dutta, G. K.; Han, Y.-K.; Shin, T. J.; Noh, Y. Y.; Yang, D. A Thienoisoindigo-Naphthalene Polymer with Ultrahigh Mobility of 14.4 cm2/V·s That Substantially Exceeds Benchmark Values for Amorphous Silicon Semiconductors. J. Am. Chem. Soc. 2014, 136, 9477−9483. (23) Quinn, J. T. E.; Zhu, J. X.; Li, X.; Wang, J. L.; Li, Y. N. Recent Progress in the Development of n-Type Organic Semiconductors for Organic Field Effect Transistors. J. Mater. Chem. C 2017, 5, 8654− 8681. (24) Shi, K. L.; Zhang, W. F.; Gao, D.; Zhang, S. Y.; Lin, Z. Z.; Zou, Y.; Wang, L. P.; Yu, G. Well-Balanced Ambipolar Conjugated Polymers Featuring Mild Glass Transition Temperatures towards High-Performance Flexible Field-Effect Transistors. Adv. Mater. 2018, 30, 1705286. (25) Lee, J.; Han, A.-R.; Kim, J.; Kim, Y.; Oh, J. H.; Yang, C. Solution-Processable Ambipolar Diketopyrrolopyrrole−Selenophene Polymer with Unprecedentedly High Hole and Electron Mobilities. J. Am. Chem. Soc. 2012, 134, 20713−20721.

height images; device configuration diagram, transfer curves, and gain of complementary-like inverter; intermidates synthesis; and NMR spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(W.Z.) E-mail: [email protected]. *(G.Y.) E-mail: [email protected]. ORCID

Weifeng Zhang: 0000-0003-1336-771X Congyuan Wei: 0000-0003-3554-8001 Gui Yu: 0000-0001-8324-397X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21774134, 21474116, 51773016, and 21673258), the National Key Research and Development Program of China (2017YFA0204703 and 2016YFB0401100), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030100). The 2D-GIXRD analyses were performed at the BL14B1 Station of the Shanghai Synchrotron Radiation Facility (SSRF). The authors are very grateful to the assistance of scientists from the station during the experiments.

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