Multisubstituted Azaisoindigo-Based Polymers for High-Mobility

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Organic Electronic Devices

Multisubstituted Azaisoindigo-Based Polymers for HighMobility Ambipolar Thin-Film Transistors and Inverters Zhihui Chen, Xuyang Wei, Jianyao Huang, Yankai Zhou, Weifeng Zhang, Yuchai Pan, and Gui Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11608 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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ACS Applied Materials & Interfaces

Multisubstituted Azaisoindigo-Based Polymers for High-Mobility Ambipolar Thin-Film Transistors and Inverters Zhihui Chen,†,‡,# Xuyang Wei,†,‡,# Jianyao Huang,† Yankai Zhou,†,‡ Weifeng Zhang,*,† Yuchai Pan,†,‡ and Gui Yu*,†,‡ †

Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for

Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡

School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P.

R. China

ABSTRACT: Ambipolar semiconducting materials offer great potentials in complementary-like organic logic circuits. Accessing such logic circuits demands balanced hole and electron mobilities. However, the lack of ambipolar high-mobility polymer semiconductors with balanced charge carrier transporting properties precludes the rapid development of organic logic circuits. In this context, structural modification of semiconductor materials to enhance the electron/hole transport is of great urgency. Herein, a multi-functionalization strategy is used to achieve this goal. Combined electron-withdrawing moieties involving fluorine and pyridinic nitrogen atoms can not only reduce the frontier molecular orbital energies but also planarize the polymer

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backbone, demonstrating synergetic effects on the control over carrier injection process at the metal–semiconductor interface and microstructure-sensitive charge transport in the channel. A balanced ambipolar behavior with electron/hole mobilities of 3.88/3.44 cm2 V−1 s−1 was observed, and complementary-like inverters with high gains of greater than 200 were achieved. Microstructure and thin-film morphology was characterized to further reveal the relationship between device performances and macroscopic observables. This multi-functionalization strategy bodes well for developing new ambipolar semiconducting materials.

KEYWORDS: azaisoindigo-based polymers, ambipolar semiconductor, organic field-effect transistors, complementary-like organic inverters, balanced charge carrier transport

INTRODUCTION High-mobility polymer semiconductors are emerging electronic materials for use in flexible display backplanes and digital logic that enable mainstream technologies.1-3 Developing new semiconducting polymers has attracted increasing attention. The ability to control electronic structures by combination of different conjugation moieties leads to a plethora of such materials.4-16 However, one of the major obstacles to fully realize their potentials is the uneven development in which performances of n-type materials cannot rival those of p-type counterparts. The need for balanced hole- and electron-transport is becoming an imperative to construct complementary-like

logic

circuit.

Therefore,

designing

high-performance

polymer

semiconductors with balanced hole and electron mobilities is a cutting-edge research challenge in this area.


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The primary difficulty in obtaining ambipolar semiconductors lies in the mismatched energy levels between frontier molecular orbitals and electrode work functions. Except for naphthalenediimide based materials, most of polymer semiconductors do not possess sufficiently deep-lying lowest unoccupied molecular orbital (LUMO) energy levels. To enhance the electron injection process, an ambipolar semiconducting material requires a high electron affinity originating from the introduction of multiple electron-withdrawing groups to a core structure of interest. These groups include halogen,17 carbonyl,18-19 trifluoromethyl,20 and cyano moieties21. Among them, heteroatom substitution has enjoyed widespread use as an effective backbone functionalization toward tuning the frontier orbital energies and enhancing intramolecular short contacts. For example, the multifluorination strategy has proven powerful to suppress the interring rotation, thus forming preferably planar backbone motif and facilitating long-range order.22 This method leads to variation of charge transport polarity toward n-channel, delivering ambipolar or n-type semiconductors. Introducing pyridinic nitrogen atoms into the backbone exemplifies another important method for simultaneously eliminating unfavorable steric effects and reducing the electron injection barrier.23 These strategies involve the use of hydrogen or chalcogen bonds, including C–H∙∙∙N, C–H∙∙∙O, S∙∙∙N, and S∙∙∙O noncovalent interactions, which are crucial to reduce the torsional freedom.24-28 These conformational controls further exert great effects on the structural evolvement of polymeric semiconductors. With such pioneering works in mind, we sought to develop ambipolar materials using a combined fluorine- and nitrogen-substitution strategy. This strategy strikes a balance between molecular complexity and synthetic availability, providing new opportunities to explore the chemical space of these materials by structural and conformational engineering, both of which accord with modern design rationales for semiconducting polymers. Two polymers consist of


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azaisoindigo (AIID) and fluorinated thiophene derivative subunits were then designed and synthesized. The presence of abundant C–H∙∙∙N, S∙∙∙N, S∙∙∙F, and H∙∙∙F contacts has a great propensity for planarized conjugated backbone, suppressed single-bond rotation, and enhanced π-stacking interactions. In addition, the strengthened electron affinity further facilitates the electron injection, resulting in ambipolar transport behaviors with balanced electron and hole mobilities both exceeding 3.4 cm2 V−1 s−1. We explored the microstructural information and molecular effects to correlate with the enhanced charge carrier mobilities. Complementary-like organic inverters with high gains of greater than 200 were fabricated, indicating the potential use in organic logic circuits. RESULTS AND DISCUSSION Design, Synthesis, and Structural Characterization. Our previous work showed that the azaisoindigo-based polymers had promising applications in ambipolar field-effect transistors.23, 29

The embedded pyridinic nitrogen atoms at 7,7’-positions not only raise the electron affinity

but also exert significant conformational control over the backbone. These azaisoindigo-based polymers showed predominant hole transport behaviors when bithiophene or dithienylethene functioned as the electron-donating moiety. To further lower the LUMO energy levels, we attempt to introduce electron-withdrawing fluorine atoms into the donor moiety at proper positions. This strategy provides more intramolecular conformational locking sites to planarize the single-bonded subunits.27 The hydrophobicity of fluorinated structures serves to circumvent the penetration of water into the polymer film (in particular, water is an oft-encountered species for electron trapping), thus aiding the construction of ambipolar transistor materials.17 Upon the design of such polymers, we notice that a synthetic compromise should be made among the reactivity of monomers, the rigidity of backbones, and the solubility of the polymers. Empirically,


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multiple electron-deficient functional groups likely lead to a high electron affinity at the expense of reduced molecular weight and mechanical robustness.30 This dilemma was due mainly to the propagating electron-deficient chain retarding the cross-coupling reactivity,31 and in part to the reduced π-densities on the backbone accelerating the aggregation that precipitate out from the catalytic cycle.32-33 On the basis of these considerations, we introduced two fluorine atoms to immobilize the interring single bond linkage and tune the electronic structures of the backbone for efficient charge carrier injection. In addition, a branched side chain was appended to ensure the solubility and optimize the aggregation of polymers. Scheme 1. Synthesis of Azaisoindigo-Based Polymers

We prepared the polymers using the conventional Stille coupling reaction. We used a 4decyltetradecyl side chain because of its availability and interdigitation tendency. 34 As shown in Scheme 1, dibromoazaisoindigo was copolymerized with the organostannane derivative of fluorinated bithiophene (FBT) and dithienylethene (FDTE) under Pd-mediated conditions. Although the introduction of electron-withdrawing fluorine atoms slightly hampers the transmetalation step because of the reduced reactivity of the electrophilic cleavage of the C–Sn bond,31 the resulting polymers PAIID-FBT and PAIID-FDTE were afforded with good numberaverage molecular weights of 39.8 and 34.9 kDa with polydispersity indices (PDI) of 3.39 and


5


3.08, respectively. Compared with those of the nonfluorinated counterparts,23,

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29

the reduced

molecular weights are also attributable to the enhanced backbone rigidity. We note that both polymers formed free-standing films during the post-polymerization treatment, indicating their applicability in solution-processed techniques. Thermogravimetric analyses show that the 5% weight loss temperatures are 392 and 370 °C for PAIID-FBT and PAIID-FDTE, respectively (Figure S1), demonstrating their good stability during the device optimization process (vide infra). We performed density functional theory (DFT) calculations to illustrate the

Figure 1. DFT-optimized structures of azaisoindigo-based polymer trimers and schematic illustration of intramolecular interactions.


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conformational control and the electronic structures.35-37 Figure 1 shows the optimized structures calculated at the B3LYP/6-31G(d) level of theory and Figure S2 shows the distribution of frontier molecular orbitals. The frontier molecular orbitals were evenly distributed across the whole backbone, indicative of good conjugation. The single-bond rotation is highly suppressed because the optimized structure for PAIID-FBT shows planar conformations with dihedral angles of small than 1.3°. By contrast, the FDTE-containing polymers show more planar conformations with optimized dihedral angles being 0°along the whole backbone. These results are in good accordance with the design purpose that both nitrogen- and fluorine-substitutions can stabilize the frontier orbitals and minimize the preference of twisted conformations. The electrostatic potential surfaces were plotted to illustrate the effect of heteroatom substituents. We also plotted electrostatic potential maps for nonfluorinated counterparts for comparison (Figure S3). The region along the extension of C–H or C–S bonds show positive electrostatic potentials that can interact with the negative lone pairs of nitrogen or fluorine atoms, thus forming typical σ-hole interactions. The introduced fluorine atoms concurrently lead to reduced electrostatic potential indicating the enhanced π–π interactions between backbones. UV–visible–near infrared absorption spectra were recorded in chloroform, as shown in Figure 2. The optical properties are summarized in Table 1. The dual-band absorption profiles are typical features of donor–acceptor polymers. The high-energy absorption band is attributed to π– π* transitions, and the low-energy absorption band has intramolecular charge transfer character due to the D–A structural motif. The latter absorption band is also overlapped to some extent by the vibronic coupling originating from the aggregation. The absorption maxima in the solutions are 738 nm for PAIID-FBT and 744 nm for PAIID-FDTE. These peaks negligibly shift from solution to film because of the intrinsic rigidity of the backbone. The estimated optical gaps,


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which are calculated from the onset of the absorption profiles, are 1.56 and 1.53 eV for PAIIDFBT and PAIID-FDTE, respectively. The narrower optical gap and red-shifted absorption maximum for PAIID-FDTE are due to the inserted vinylene linkage extending the conjugation length. To further compare the frontier molecular orbital energy levels, cyclic voltammetry analyses were conducted (Figure S4). The highest occupied molecular orbital (HOMO) and LUMO energy levels for PAIID-FBT and PAIID-FDTE were estimated to be −5.76/−3.64 eV and −5.70/−3.64 eV, respectively, using the equation E = – (Eonset + 4.80 – EFc/Fc+) eV (Fc = ferrocene). The corresponding electrochemical gaps are 2.12 and 2.06 eV, respectively; the

Figure 2. UV–visible–near infrared absorption spectra of AIID-based polymers in dilute chloroform solution (ca. 10−5 mol L−1) and in thin films deposited on quartz. Table 1. Optical and Electrochemical Properties

polymer

λmax (nm) soln.

film

PAIID-FBT

738

738

PAIID-FDTE

744

745

EHOMO (eV)

ELUMO (eV)

Egcv (eV)

1.56

−5.76

−3.64

2.12

1.53

−5.70

−3.64

2.06

Egopt

(eV)


8

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tendency is in good agreement with that of the optical gaps. Compared with unfluorinated materials, the introduced fluorine atoms function as a contributor to stabilized HOMO energy levels, whereas the effect to lower LUMO energy levels is less significant.23, 29 In addition, the presence of both oxidation and reduction waves promises the applications in electronics devices. Field-effect Transistors and Inverters. To explore the potential in electronics applications, we fabricated thin-film transistors with a top-gate/bottom-contact (TGBC) configuration. It is noteworthy that we adopted the low-cost glass substrates rather than expensive silicon wafers, although the high-performance transistors were mostly fabricated on the latter. A Corning glass substrate with pre-patterned Au source-drain electrodes, a spin-coated semiconducting thin film, a poly(methyl methacrylate) (PMMA) dielectric layer, and an aluminum top gate comprise the device structure. This device configuration that has an encapsulation effect can alleviate oxygen and water doping.38 Devices based on both polymers exhibit typical ambipolar charge carrier transport behaviors. The fabrication conditions were first evaluated and the optimal annealing temperatures were observed at 180 °C (Figure S5). The representative transfer and output characteristics are shown in Figure 3. Hole mobilities were evaluated from the negative gate bias in the range of 0 to −110 V whereas electron mobilities were extracted from the positive gate bias. The electrical properties are collected in Table 2. The PAIID-FDTE based thin-film transistors show a relatively balanced transport behavior with the highest electron and hole mobilities of 3.88 and 3.44 cm2 V−1 s−1, respectively, whereas PAIID-FBT based transistors exhibit predominant n-channel transport behavior with the highest electron and hole mobilities of 2.23 and 0.31 cm2 V−1 s−1, respectively. The average electron/hole mobilities calculated from 10 devices are 1.84/0.258 and 3.27/2.94 cm2 V−1 s−1 for PAIID-FBT and PAIID-FDTE based


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Figure 3. Charge transport properties of thin-film transistors based on azaisoindigo polymers: transfer and output curves for (a,b,e,f) PAIID-FBT and (c,d,g,h) PAIID-FDTE based devices. The red lines in transfer curves are fitted to calculate the mobilities in saturation regimes. devices, respectively. The relatively low hole mobilities for PAIID-FBT is likely due to the its unsuitable deep-lying HOMO energy level. We observed that the mobilities of PAIID-FDTE based polymers are among the highest values for ambipolar transistors fabricated on glass substrates to date.39-40 We performed stability tests of the devices for 30 d in ambient. The electron and hole mobilities remain 75 and 82% of the maximum values (2.91 and 2.81 cm2 V−1 s−1, respectively) for PAIID-FDTE based devices after up to 30 days of exposure to air (Figure S6). The relatively good air stability is likely due to the electronic structure engineering by fluorination that lowers the frontier molecular orbital energies and the encapsulation effect of the top-gate PMMA layer. On the basis of the balanced ambipolar behavior of PAIID-FDTE based devices, we fabricated complementary-like inverters to further evaluate its electronics


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Figure 4. Complementary like inverters based on PAIID-FDTE thin films. (a) Circuit diagram of the inverter; (b) three-dimensional device architecture of a complementary-like inverter on a glass substrate; (c,d) voltage transfer characteristics and gains of the PAIID-FDTE based inverters at negative and positive supply voltages. Table 2. Electrical Properties of Polymer Based Thin-Film Transistors

 (cm2 V−1 s−1)

Ion/Ioff

VTH (V)

n-channel

1.84 ±0.39

104–105

63 ±4

p-channel

0.258 ±0.053

103–104

−69 ±5

n-channel

3.27 ±0.61

103–104

77 ±4

p-channel

2.94 ±0.50

103–104

−87 ±3

polymer

PAIID-FBT

PAIID-FDTE

applications. Notably, identical channel length and width of 50 and 4500 μm for both transistors were used to investigate the performance of inverters. Typical voltage transfer characteristics of ambipolar inverters are shown in the first and third quadrants with supply (Vdd) and input (Vin)


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voltages applied in the channel. As shown in Figure 4, high gains of 212 and 218 are obtained at near 1/2 Vdd, demonstrating the great potential in complex logic circuits. Surface Morphology and Thin-film Microstructure. To gain insight into the film morphology, we analyzed the tapping-mode atomic force microscopy images. As shown in Figure 5, all of the polymer thin films exhibit fibrillar grains with smooth surfaces. The calculated root mean square (RMS) surface roughness values are 1.10 and 0.88 nm for as-spun PAIID-FBT and PAIID-FDTE films, respectively, and slightly increase to 1.24 and 1.07 nm after thermal treatment. The reduced the intergranular disconnections account for the enhancement in charge transport behaviors. To reveal the intrinsic microstructural information of the polymer thin films, especially the crystallinity and ordering, we performed two-dimensional grazingincidence wide-angle X-ray scattering (GIWAXS). The diffraction patterns are shown in Figure 6 and the parameters are collected in Table 3. All of the d–d and π–π distances are calculated from the cross-sectional out-of-plane and in-plane profiles (Figures S7 and S8). Both polymer thin films show diffractions up to the fourth order with typical edge-on packing modes that are favorable to interchain charge hopping within the channel. The pole figures corroborate the improved edge-on packing for both polymer thin films after thermal treatment, because the ratios of edge-on to face-on crystallites increase for both polymers (Figure S9). The π–π distances are 3.56 and 3.54 Å for PAIID-FBT and PAIID-FDTE thin films, respectively. Notably, these distances are shorter than those of nonfluorinated counterparts, demonstrating the reduced electron densities within the backbone greatly facilitating intermolecular dispersion interactions.29 Thermal treatment greatly improves the crystallinity of the films, and concurrently enhances the π–stacks. After annealing, the


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Figure 5. AFM images of polymer thin films: (a,b) as-spun and annealed PAIID-FBT, and (c,d) as-spun and annealed PAIID-FDTE thin films (image size: 3 × 3 μm2).

coherence lengths for (100) diffraction peaks, which are estimated from the Scherrer analysis, increase from 19.5 nm to 27.8 nm for PAIID-FBT thin film, and from 15.3 to 29.8 nm for PAIID-FDTE thin film.41 The π–π interactions are also slightly enhanced with the distances reducing to 3.53 and 3.51 Å for PAIID-FBT and PAIID-FDTE, respectively. These shortened distances promote the interchain charge hopping rate, indicative of the improved charge transport behaviors. Paracrystalline disorder parameter (g) can also reveal the enhanced crystallinity of thin films.42-45 The estimated g values for as-spun PAIID-FBT and PAIID-FDTE thin films are 7.48% and 6.44%, respectively, whereas these values reduce to 5.63% and 5.52% for annealed thin films. The improved crystallinity indicates the reduced traps within crystalline grains, facilitating the charge transport in the channel.


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Table 3. Microstructural Information for the PAIID-FBT and PAIID-FDTE Thin Films

polymer

PAIID-FBT

PAIID-FDTE

d–d (nm)

π–π (nm)

CL (nm)

g (%)

As spun

2.368

0.356

19.5

7.48

Annealed

2.387

0.353

27.8

5.63

As spun

2.325

0.354

15.3

6.44

Annealed

2.314

0.351

29.8

5.52

Figure 6. 2D-GIWAXS patterns of (a) as-spun PAIID-FBT, (b) annealed PAIID-FBT, (c) asspun PAIID-FDTE, (d) annealed PAIID-FDTE thin films (annealing temperature: 180 °C).

EXPERIMENTAL SECTION Synthesis. To a 50 mL Schlenk tube were added ditin monomer (0.20 mmol), 1,1’-bis(4decyltetradecyl)-6,6’-dibromo-7,7’-diazaisoindigo

(0.20

mmol),


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tris(dibenzylideneacetone)dipalladium (8.0 mg), and tri(o-tolyl)phosphine (24 mg). The reaction mixture was dissolved in 6 mL of chlorobenzene under an argon atmosphere and then stirred at 120 °C for 6 h. After the mixture was allowed to cool to room temperature, a solution of conc. HCl (5 mL) in methanol (100 mL) was added. The crude product was collected by filtration and washed with methanol and acetone. The polymer was further purified by Soxhlet extraction with methanol, acetone, and hexane to remove the low-molecular-weight fraction. The residue was extracted with chloroform to afford the high-molecular-weight fraction as a dark-purple freestanding film. PAIID-FBT: 220.0 mg, yield: 96.9%. 1H NMR (300 MHz, C2D2Cl4, 373K, δ, ppm): 9.20 (m, 2H), 7.80–6.50 (br, 4H), 3.80 (br, 4H), 2.0–0.7 (m, 94H). GPC: Mn = 39.8 kDa, PDI = 3.39. Elemental analysis (%) Calcd. for (C70H104F2N4O2S2)n: C, 74.03; H, 9.23; N, 4.93; Found: C, 73.85; H,9.32; N, 4.87. PAIID-FDTE: 226.2 mg, yield: 97.3%. 1H NMR (300 MHz, C2D2Cl4, 373K, δ, ppm): 9.10 (m, 2H), 7.70–6.60 (br, 6H), 3.80 (br, 4H), 2.0–0.8 (m, 94H). GPC: Mn = 34.9 kDa, PDI = 3.08. Elemental analysis (%) Calcd. for (C72H106F2N4O2S2)n: C, 74.44; H, 9.20; N, 4.82; Found: C, 73.79; H,9.02; N, 4.76. Field-effect Transistor Fabrication. Top-gate/bottom-contact configuration was adopted to fabricate the organic field-effect transistors. The glass substrates were rinsed by deionized water, ethanol, and acetone, and further dried under a nitrogen flow. The Au (30 nm) drain/source electrodes were vacuum-evaporated onto the clean glass slide through shadow mask with the channel length/width (L/W) of 30/1400 μm. A 4 mg/mL polymer solution in o-dichlorobenzene and a 60 mg/mL PMMA (Mn = 996 kDa) solution in n-butyl acetate were then sequentially spincoated and annealed at 180 and 80 °C in the glove box, respectively (annealing time: polymer, 5


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min; PMMA 30 min). The thickness and relative permittivity of the PMMA layer were measured to be 650 nm and 3.5. An 80 nm-thick Al gate electrode were eventually deposited through vacuum evaporation. The corresponding electrical performance were measured by Keithley 4200 under ambient condition. The extraction of saturation mobility followed the equation: IDS = (W/2L)Ciµ(VGS − Vth)2, where VGS is the gate voltage, VDS is the drain/source voltage, Vth is the threshold voltage, IDS is the drain/source current, Ci is the capacity per unit, and μ is the mobility. Fabrication of Complementary Inverters. The procedures for fabricating inverters were similar to those of TGBC transistors. Due to the balanced carrier ambipolarity, the inverter consisted of two transistors with the identical channel width and length of 4500 and 50 μm. The inverter gain is defined as dVout/dVin.

CONCLUSION In conclusion, we demonstrate a multi-functionalization strategy with a combined introduction of electron-withdrawing fluorine and pyridinic nitrogen to achieve ambipolar semiconducting polymers. This strategy realizes the structural and conformational engineering of the polymer backbone, affording highly ordered polymer chain and corresponding high-performance ambipolar charge transport behaviors. The fully planar conformation in PAIID-FDTE is demonstrated as the major factor for enhanced mobilities. We achieved the electron and hole mobilities of up to 3.88 and 3.44 cm2 V−1 s−1, respectively, which are among the highest values for devices fabricated on glass substrates. Complementary-like inverters with gains of greater than 200 further highlight the potential applications in logic circuits. This multi-substitution approach can extend the scope of present high-mobility materials and inspire easy access to highly ordered, conformer-free semiconducting polymers.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website Experimental details, analysis of the new compounds, and additional figures (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (W. Z.) *E-mail: [email protected] (G. Y.) ORCID Jianyao Huang: 0000-0003-4177-6393 Weifeng Zhang: 0000-0003-1336-771X Gui Yu: 0000-0001-8324-397X Author Contributions # Z. C. and X. W. contributed equally. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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The authors acknowledge the financial support from the National Key Research and Development Program of China (2017YFA0204703 and 2016YFB0401100), the National Natural Science Foundation of China (Grants 21673258, 21774134, and 21474116), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030100). The GIWAXS results were tested at BL14B1 Station of Shanghai Synchrotron Radiation Facility (SSRF). The authors are grateful for the assistance during the test. The authors thank Dr. Deyang Ji and Prof. Jidong Zhang for helpful discussions. REFERENCES (1)

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