Naphthalenediimide-Based Copolymers Incorporating Vinyl-Linkages ...

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Naphthalenediimide-Based Copolymers Incorporating VinylLinkages for High-Performance Ambipolar Field-Effect Transistors and Complementary-Like Inverters under Air Huajie Chen,† Yunlong Guo,† Zupan Mao,† Gui Yu,* Jianyao Huang, Yan Zhao, and Yunqi Liu* Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: We report the synthesis of two novel donor−acceptor copolymers poly{[N, N′-bis(alkyl)-1,4,5,8-naphthalene diimide-2,6-diyl-alt-5,5′-di(thiophen-2-yl)2,2′-(E)-2-(2-(thiophen-2-yl)vinyl)thiophene]} (PNVTs) based on naphthalenediimide (NDI) acceptor and (E)-2-(2-(thiophen-2-yl)vinyl)thiophene donor. The incorporations of vinyl linkages into polymer backbones maintain the energy levels of the lowest unoccupied molecular orbits at −3.90 eV, therefore facilitating the electron injection. Moreover, the energy levels of the highest occupied molecular orbits increase from −5.82 to −5.61 eV, successfully decreasing the hole injection barrier. Atomic force microscopy measurements indicate that PNVTs thin films exhibit larger polycrystalline grains compared with that of poly{[N, N′-bis(2-octyldodecyl)-1,4,5,8-naphthalene diimide-2,6diyl]-alt- 5,5′-(2,2′-bithiophene)} [P(NDI2OD-T2)], consistent with the stronger π−π stacking measured by grazing incidence X-ray scatting. To optimize devices performance, field-effect transistors (FETs) with three devices configurations have been investigated. The results indicate that the electron mobility of the vinyl-containing PNVTs exhibit about 3−5 times higher than that of P(NDI2OD-T2). Additionally, the vinyl-linkages in PNVTs remarkably enhance ambipolar transport of their top-gate FETs, obtaining high hole and electron mobilities of 0.30 and 1.57 cm2 V−1 s−1, respectively, which are among the highest values reported to date for the NDI-based polymers. Most importantly, ambipolar inverters have been realized in ambient, exhibiting a high gain of 155. These results provide important progresses in solution-processed ambipolar polymeric FETs and complementary-like inverters. KEYWORDS: naphthalenediimide, ambipolar transistors, complementary-like inverters, vinyl linkages, high performance

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performance ambipolar polymer semiconductors.14−18 Furthermore, most of high-performance ambipolar polymeric FETs are achieved under nitrogen. Very few of them can be operated under air.20,21 The development of ambiploar polymers for FETs which can be operated in ambient, however, are very important for practical applications in organic optoelectronic devices. Naphthalenediimide (NDI) as a famous electron-deficient core has been used in designing n-type polymer semiconductors because of their strong electron-deficient properties and good solubility supported by N-alkylation.22−24 For example, Facchetti et al reported a NDI-based copolymer P(NDI2OD-T2), exhibiting a high electron mobility of 0.20− 0.85 cm2 V−1 s−1 in ambient by employing a top-gate/bottomcontact (TGBC) geometry.24 However, because of large holeinjection energy barrier caused by deep-lying energy levels of the highest occupied molecular orbits (HOMO) (−5.82 eV, see Figure 1a), no obvious hole transport characteristics were detected in this FETs geometry. Recently, Noh et al reported a

INTRODUCTION Polymer field-effect transistors (FETs) have obtained tremendous attentions by worldwide academic and industrial researchers, due to their potential applications in low-cost flexible optoelectronic devices.1−3 Recently, significant progress has been made in the development of high-performance unipolar polymer semiconductors, exhibiting air-stable pchannel or n-channel mobilities exceeding commercially practical values (∼0.5−1.0 cm2 V−1 s−1).4−11 Additionally, by combining distinct p- and n-type semiconductors, some ambipolar complementary-like inverters have been successfully developed.12,13 However, this kind of inverter suffers from complex and time-consuming fabrication processes because of the requirements for lateral patterning of two distinct semiconductors.12,13 To minimize these complex processes, developing ambipolar polymers might be one of feasible approaches. However, there are limited ambipolar polymers with hole and electron moblities above 0.1 cm2 V−1 s−1,14−18 because of the existence of traps at the semiconductor/ dielectric interfaces and the scarcity of electron-deficient building blocks.19 So far only a few electron-deficient blocks, mainly focusing on diketopyrrolopyrrole (DPP) and benzobisthiadiazole cores, have been successfully developed for high© 2013 American Chemical Society

Received: April 8, 2013 Revised: August 19, 2013 Published: September 5, 2013 3589

dx.doi.org/10.1021/cm401130n | Chem. Mater. 2013, 25, 3589−3596

Chemistry of Materials

Article

Figure 1. Normalized absorption spectra: (a) P(NDI2OD-T2), (b) PNVT-8, and (c) PNVT-10.

and enhance charge transport.6 To our delight, the vinylcontaining PNVTs-based FETs exhibited a good ambiploar characteristic in ambient air (20−40% relative humidity) with hole and electron mobilities of 0.30 and 1.57 cm2 V−1 s−1, respectively. These mobilities are among the highest values observed to date for the NDI-based copolymers. Moreover, using single polymer PNVT-8 as active layer, high-performance ambipolar inverters have been realized in ambient, exhibiting a high gain of 155. Therefore, we provide an effective approach to enhance ambiploar charge transport for NDI-based copolymers by incorporating vinyl linkages into their backbones.

physical method through directional interface state modulation with a high permittivity poly(vinylidenefluoride-trifluoroethylene) dielectrics, successfully enhanced hole mobility of P(NDI2OD-T2) to ∼0.1 cm2 V−1 s−1, but decreased electron mobility to ∼0.1 cm2 V−1 s−1.12 Besides, by varying electrondonating blocks, Jenekhe et al demonstrated that strong donors could suitably increase the HOMO energy levels of NDI-based copolymers, therefore effectively enhancing their hole mobility up to 2.8 × 10−3 cm2 V−1 s−1, but resulting in a loss of electron mobility yet (∼ 0.038 cm2 V−1 s−1).25,26 To the best of our knowledge, there is no report on the NDI-based polymers that give the simultaneous enhancements of hole and electron mobilities, and it is a great challenge to further develop such ambipolar polymer semiconductors. Herein, strong electron-donating (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) units are first introduced into the NDIbased copolymer main-chains (PNVTs, Scheme 1) to further elevate their HOMO energy levels without changing their energy levels of the lowest unoccupied molecular orbits (LUMO), intending to improve their hole and electron mobilities simultaneously. Recently, our works have demonstrated that the incorporations of TVT units into polymer backbones can effectively promote intermolecular π−π stacking

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EXPERIMENTAL SECTION

General. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker ARX-400 (400 MHz) spectrometer. Electron impact mass spectra (EI-MS) were collected on a Bruker BIFLEX III mass spectrometer. Elemental analyses were carried out using a Carlo Erba model 1160 elemental analyzer. High temperature gel permeation chromatography (GPC) was carried out on a Polymer Laboratories PL 220 system at 150 °C using 1,2,4-trichlorobenzene as the eluent, and the concentrate of sample is 1 mg mL−1. UV−vis−NIR absorption spectra were measured on polymer diluted solutions in chloroform and thin films coated onto quartz glass using a Hitachi U-3010 spectrophotometer. Cyclic voltammetry (CV) experiments were carried out on an electrochemistry workstation (CHI660A, Chenhua Shanghai) using a three-electrode cell. The Platinum stick electrode coated with a layer of polymer thin-film was used as working electrode. Ag/AgCl (Ag in a 0.01 mol/L KCl) electrode was used as the reference electrode. Platinum wire was used as the counter electrode. An anhydrous and N2 saturated solution 0.1 M tetrabutylammonium hexylfluorophosphate in acetonitrile was employed as the electrolyte. Thermogravimetric analysis (TGA) was performed on a Perkin−Elmer TGA-7 with a heating/cooling rate of 10 °C min−1 under inert atmosphere. Differential scanning calorimetry analyses (DSC) were recorded on a METTLER TOLEDO Instrument DSC822 calorimeter with a heating/cooling rate of 10 °C min−1. Atomic force microscopy (AFM) measurements were carried out on a Nanoscope V instrument operated in a tapping mode. For grazing incidence X-ray scatting (GIXRD), polymer thin-film samples were illuminated at a constant incidence angle of 0.2° (λ = 2dsin θ = 1.54 Å). AFM and GIXRD thinfilm samples were identical to those used in FET performance analysis. The thickness of thin film was measured by XP-2 Profiler (Ambios Technology). General Procedures for FET Device Fabrication. Polymerbased FETs with a bottom-gate bottom-contact (BGBC) configuration were fabricated on a highly doped silicon wafer with 300 nm SiO2 insulator. Silicon was used as gate electrode. The source−drain gold electrodes (gold/titanium, 30 nm/5 nm) were prepared by photolithography. The substrates were then subjected to cleaning by using ultrasonication in acetone, deionized water, and ethanol. Next, octadecyltrichlorosilane (OTS) treatment was performed on the

Scheme 1. Chemical Structures and Synthetic Routes of the NDI-Based Copolymers

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Table 1. Molecular Weights, Optical Properties, and HOMO/LUMO Energy Levels of the NDI-Based Polymers λmax (nm) polymer

Mn/Mw (KDa)

solution

as-spun film

annealed film

HOMO (eV)

LUMO (eV)

P(NDI2OD-T2) PNVT-8 PNVT-10

18.8/43.4 24.3/64.1 23.7/58.4

370, 644 392, 692 390, 678

390, 700 403, 718 398, 723

394, 711 403, 719 400, 768

−5.82 −5.61 −5.62

−3.96 −3.93 −3.90

surface of SiO2 gate dielectrics in a vacuum to form an OTS selfassembled monolayer. Gold drain−source electrodes were then modified with/without pentafluorobenzenethiol (PFBT) according to the reported methods.27 In N2 glovebox, a layer of polymer semiconductor film (∼40 nm) was deposited on the OTS-treated substrates by spin-coating a polymer solution in o-dichlorobenzene (12 mg/mL) at a speed of 1200 rpm for 40 s. For annealing polymer films, the samples were further placed on a hot plate at 200 °C in N2 for 20 min before cooling down to room temperature. Three batches and 21 devices were measured for each configuration. BGBC FETs were determined in N2 by using a Keithley 4200 SCS semiconductor parameter analyzer. Different channel lengths (L) of the FET devices (L = 40 and 50 μm) and the same channel widths (W) of 1400 μm were used to optimize device performance. The field-effect mobility in saturation (μ) is calculated from equation

the Supporting Information), which were determined by high temperature gel permeation chromatography at 150 °C using 1,2,4-trichlorobenzene as the eluent. Because of the appropriately soluble side chains, the as-synthesized copolymers can be readily soluble in common organic solvents, such as chloroform, toluene, xylene, and dichlorobenzene at room temperature (>30 mg mL−1). Three polymers show high decomposition temperature (5% weight loss) over 430 °C, demonstrating that it is adequate for the fabrication processes of optoelectronic devices (see Figure S4a in the Supporting Information). During the heating scans of DSC measurements, three polymers exhibit the exothermic transition temperature at 251−310 °C, which is attributable to the melting of the polymer backbone (see Figure S4b in the Supporting Information). Photophysical and Electrochemical Properties. Three NDI-based copolymers exhibit a similar absorption behavior with typically dual band absorption: one absorption peak in Vis−NIR region (500−900 nm), corresponding to the strong charge transfer (CT) between donors (D) and acceptors (A);28 another one in UV−vis region (300−500 nm) arising from π−π* transitions (see Figure 1 and Table 1).29 In dilute chloroform solutions, the absorption spectra of PNVT-8 and PNVT-10 show absorption peaks at 692 and 678 nm, respectively, which were red-shifted about 0.13 and 0.10 eV than that of P(NDI2OD-T2) (λmax = 644 nm). This red-shifted phenomenon can be readily understood from molecular modeling studies by Density Functional Theory (DFT) calculations, using the Gaussian 03 program30 at the B3LYP 6-31G* level.31−33 As parameter approximation, N-alky chains were replaced by N-methyl groups. As shown in Figure S5 in the Supporting Information, the dihedral angles between NDI and adjacent thiophene units are θ1 = 39.8° and θ3 = 40.0°, respectively. The other ones between thiophene and adjacent vinyl or thiophene units are θ2 = 0.6° and θ4 = 0.7°, respectively. The calculation results demonstrate that the introductions of vinyl linkages into PNVTs do not obviously change the dihedral angles between NDI and adjacent thiophene units compared with those of P(NDI2OD-T2). Combining with stronger electron-donating ability for TVT units, PNVTs exhibited much stronger D−A effect than that of P(NDI2OD-T2), leading to more effective charge transport. Interestingly, the molecular orbital distributions, as shown in Figure S5, reveal that both the HOMOs and LUMOs of two NDI-based trimers (NDI-TVT and NDI-T2) are localized over their conjugated backbones, different from many other D−A polymers in which the HOMOs and LUMOs are mostly localized upon electron donors and acceptors, respectively. Therefore, the NDI-based polymers, P(NDI2OD-T2) and PNVTs may achieve an ambipolar feature by control of device architecture. Additionally, three polymers thin-films exhibit a red-shifted absorption peaks at 700 nm for P(NDI2OD-T2), 718 nm for PNVT-8, and 723 nm for PNVT-10. Annealing films lead to further red-shifting CT peaks and increasing absorption intensities of shoulders, suggesting that the orderly

IDS = (W /2L)Ciμ(VGS − Vth)2 where W/L is the channel width/length, Ci is the gate dielectric layer capacitance per unit area, and VGS and Vth are the gate voltage and threshold voltage, respectively. Top-gate/Bottom-contact (TGBC) FET devices were fabricated on bare glass substrates. First, the substrates were subjected to cleaning by using ultrasonication in acetone, deionized water, and ethanol. Next, gold was evaporated to form source/drain electrodes on the surface of glass substrates through a metal shadow mask (L/W = 80 μm/8800 μm). Then the substrates were washed with ethanol. In a N2 glovebox, the NDI-based polymer dissolved in o-DCB (8 mg mL−1) was spincoated onto the glass substrates, yielding a polymer film with a thickness of ∼30 nm. For annealed polymer films, the samples were further annealed at 200 °C for 5 min. Then polymethylmethacrylate (PMMA) (Mw = 100 KDa) solution in anhydrous n-butyl acetate (60 mg mL−1) was spin-coated onto the surface of the NDI-based polymer films. PMMA thickness is ∼850 nm with capacitance of 2.13 nF/cm2. The samples were then dried at 80 °C for 30 min in vacuum. The aluminum gate electrodes (thickness ∼100 nm) were then evaporated through a shadow mask onto the PMMA gate dielectric. Top-gate FETs were measured in ambient air with relative humidity of 20−40%. Three batches and 21 devices were measured. Ambipolar Complementary-Like Inverters Fabrication. The fabrication process of complementary-like inverters was similar with that of TGBC transistors. Ambipolar complementary-like inverters were fabricated on bare glass substrates by combining two identical ambipolar transistors, with a common gate as the input voltage (VIN) and a common drain as the output voltage (VOUT). The TGBC inverters were also measured in ambient.

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RESULTS AND DISCUSSION Synthesis and Characterization. Synthetic routes of three polymers are shown in Scheme 1. Synthetic details of monomers and polymers are provided in the Supporting Information. The Stille-coulping reactions between M1 (or M2) and M3 were employed to afford PNVT-8 and PNVT-10. To establish structure−property correlations within all assynthesized copolymers, P(NDI2OD-T2) was also synthesized according to the reported literature.24 Three polymers have high weight-average molecular weights [Mw = 64.1 KDa for PNVT-8, Mw = 58.4 KDa for PNVT-10, and Mw = 43.4 KDa for P(NDI2OD-T2), see Table 1], with the corresponding polydispersity indexes of 2.31−2.64 (see Figures S1−S3 in 3591



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PNVT-8 and PNVT-10 are −5.61 and −5.62 eV, respectively. Although PNVT-8 and PNVT-10 have lower ionization potentials than that of P(NDI2OD-T2), they still keep at ∼5.6 eV, and much higher than that of P3HT (4.76 eV).34 Compared with P(NDI2OD-T2), this properly increased HOMO values of PNVT-8 and PNVT-10 would effectively decrease hole injection energy barrier, therefore facilitating hole injection from Au electrodes to polymer semiconductor films, as shown in Figure 2b.25−27 Field-Effect Transistors and Complementary-Like Inverters. At first, BGBC FETs were fabricated on OTSmodified Si/SiO2 (300 nm) substrates to investigate field-effect performance of FET devices based on the NDI-based polymers. The device parameters were measured under nitrogen. Typical FETs curves and parameters are provided in panels b and c in Figure 3, Table 2, and Table S1 in the Supporting Information, respectively. P(NDI2OD-T2) exhibits a remarkable n-channel behavior, with the maximum electron mobility of 0.19 cm2 V−1 s−1 at a drain voltage (VDS) of 60 V (see Figure S6a, b in the Supporting Information). The vinyl-containing copolymers, PNVTs shows obviously ambipolar characteristics with higher charge transporting ability than P(NDI2OD-T2). This enhanced charge transport ability arises from strong backbone stacking, consistent with the results of AFM and GIXRD below. At VDS = ± 80 V, the average hole/electron mobilities for PNVT-8 are equal to 0.12 and 0.70 cm2 V−1 s−1, with the maxima of 0.15 and 0.78 cm2 V−1 s−1, respectively. Similar ambipolar results were also obtained for PNVT-10 (see Figure S7a, b in the Supporting Information), with the maximum

π−π stacking and the planarity of polymers were strengthened.6 This enhanced π−π stacking would be helpful for charge transport. As shown in Figure 2a, three NDI-based copolymers exhibit reversible and strong reduction processes, in good agreement

Figure 2. (a) CV of three polymers at the scan rates of 0.1 V s−1. (b) Experimental HOMO and LUMO energy levels for the compared systems.

with the strong electron-deficient nature of the NDI-based polymers. As we expected, the changes of alkyl-substituent length and the incorporations of vinyl groups into polymer backbones have a negligible influence on their LUMO energy levels, with the corresponding LUMO values of −3.96 eV for P(NDI2OD-T2), −3.93 eV for PNVT-8, and −3.90 eV for PNVT-10, respectively (see Table 1). However, the introduction of vinyl groups significantly increases HOMO energy levels exceeding 0.2 eV compared with that of P(NDI2OD-T2) (HOMO = −5.82 eV). Therefore, the HOMO energy levels of

Figure 3. Devices configurations: (a) BGBC FETs, (d) PFBT-BGBC FETs, and (g) TGBC FETs; Typical (b) transfer and (c) output curves of the PNVT-8 based FETs with the BGBC architecture (W = 1400 μm and L = 40 μm), exhibiting hole/electron mobilities of 0.12 and 0.73 cm2 V−1 s−1, respectively; typical (e) transfer and (f) output curves of the PNVT-8 based FETs with the PFBT-BGBC FETs architecture (W = 1400 μm and L = 50 μm), exhibiting hole/electron mobilities of 0.04 and 1.4 cm2 V−1 s−1, respectively; typical (h) transfer and (i) output curves of the PNVT-8 based FETs with the TGBC architecture (W = 8800 μm and L = 80 μm), affording hole/electron mobilities of 0.13 and 1.05 cm2 V−1 s−1, respectively. 3592



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Table 2. Performance Parameters of the FET Devices Based on Three NDI-Based Polymers with Different Devices Configurations n-channel polymer P(NDI2OD-T2)a PNVT-8a PNVT-10a P(NDI2OD-T2)b PNVT-8b PNVT-10b P(NDI2OD-T2)c PNVT-8c PNVT-10c

μe, avg (cm2 V−1 s−1) 0.18 0.70 0.50 0.23 1.10 0.95 0.30 1.05 1.10

(± 0.01) (± 0.05) (± 0.02) (± 0.03) (± 0.25) (±0.15) (±0.05) (±0.05) (±0.15)

p-channel

μe, max (cm2 V−1 s−1)

Ion/Ioff

0.19 0.78 0.55 0.27 1.4 1.13 0.40 1.13 1.57

>1 × 105 >1 × 103 >1 × 500 >1 × 105 >1500 > 400 >1 × 105 >1 × 104 >1 × 104

μh, avg (cm2 V−1 s−1) NA 0.12 0.10 NA 0.03 0.02 NA 0.15 0.18

(± 0.02) (± 0.01) (± 5 × 10−3) (± 4 × 10−3) (± 0.3) (± 0.3)

μh, max (cm2 V−1 s−1)

Ion/Ioff

NA 0.15 0.12 NA 0.04 0.04 NA 0.23 0.30

NA >50 >50 NA >550 >55 NA >1 × 104 >1 × 104

a

BGBC FETs without PFBT modification, measured in glovebox. bBGBC FETs with PFBT modification, measured in glovebox. cTGBC FETs were measured in ambient.

hole/electron mobility of 0.12 and 0.55 cm2 V−1 s−1, respectively. Obviously, the vinyl linkages in the NDI-based polymers not only successfully improve electron mobility, but also significantly enhance hole mobility. Furthermore, PFBTmodified Au electrodes led to a maximum electron mobility of 1.40 cm2 V−1 s−1 for PNVT-8, while maximum hole mobility decreased to 0.04 cm2 V−1 s−1 (see panels e and f in Figure 3, Table 2, and Table S1 in the Supporting Information). Previous works had demonstrated that Au electrodes modified by PFBT exhibited a higher work function (4.77 eV) than that of bare ones (5.1 eV) (see Figure 2b).35 Therefore, the dramatically enhanced electron mobility should be mainly ascribed to the decreased electron injection barrier, whereas the decreased hole mobility should be caused by the increased hole injection barrier.27,35 Considering that TGBC configurations can effectively enhance n-channel or ambipolar transport ability because of reducing the negative influences from H2O/O2.24,36 Therefore, TGBC FETs were also fabricated on bare glass substrates in glovebox, and then measured in air directly. Panels h and i in Figure 3 shows the typical I−V characteristics of PNVT-8 based TGBC FETs. And the corresponding hysteresis test is shown in Figure S8 in the Supporting Information. Compared with BGBC FETs, TGBC devices based on P(NDI2OD-T2) exhibit an enhanced electron mobility up to 0.40 cm2 V−1 s−1 (see Figure S6e, f in the Supporting Information), consistent with the reported literatures.24 To our delight, PNVTs-based FETs employing a PMMA TGBC configuration exhibit a significant increase in hole and electron mobilities, current on/off (Ion/Ioff) ratios, and air stability. The highest hole/electron mobilities reach 0.23/1.13 cm2 V−1 s−1 for PNVT-8 and 0.30/1.57 cm2 V−1 s−1 for PNVT-10, respectively. To study the air-stability of FET devices based on vinyllinking PNVT-8 and PNVT-10, we kept their TGBC devices in air and monitored the FET parameter for 720 h. After 720 h in laboratory air (20−40% humidity), the maximum hole/electron mobilities of devices based on PNVT-8 and PNVT-10 still kept at 0.10/0.70 cm2 V−1s−1 and 0.13/1.03 cm2 V−1 s−1, respectively (see Table S2 in the Supporting Information). Good air stability should be ascribed to the encapsulation effect of PMMA and aluminum gate-electrode, therefore reducing the negative influences from H2O/O2.6,24 However, for PNVT-8 based TGBC FETs, the p-channel Ion/Ioff ratios deteriorated with increasing cycle operation (see Figure S9 in the Supporting Information), which might be correlated to its irreversible oxidation process (see Figure 2a).

Given good ambipolar characteristic for PNVT-based topgate FETs, TGBC complementary-like voltage inverters based on PNVT-8 were fabricated by combining two identical ambipolar transistors (see Figure 4), with a common gate as

Figure 4. Transfer curves and gain of a complementary-like inverter comprised of two identical top-gate FETs based on PNVT-8 annealed at 200 °C. (Inset: inverter circuit configuration).

the input voltage (VIN) and a common drain as the output voltage (VOUT). All devices were directly measured in air with 20−40% air humidity. As shown in Figure 4, inverting functionality was clearly observed, whereas an asymmetry phenomenon in inverter transfer curves was displayed. This phenomenon might result from unbalanced mobilities and threshold voltages in p- and n-channel operation. Calculated from the steepness of inverter curve, a high gain of 155 is obtained, which is among the highest gain for complementarylike inverters based on one-component ambipolar polymers. After 60 days in laboratory air, PNVT-8 based TGBC-inverter kept a high gain of 45 (see Figure S10 in the Supporting Information), still higher than most of reported complementary-like inverters.16−18 Thin Film Morphology and Microstructure. To investigate the relationships between film morphologies and device performance, the surface morphologies of 200 °C annealed thin films on OTS-modified SiO2/Si and bare glass substrates were studied by AFM in a tapping mode. Figures 5a−f show the AFM images on OTS-modified SiO2/Si substrates. In contrast to P(NDI2OD-T2) films (see Figure 5a, b), PNVT-8 and PNVT-10 surface morphologies exhibit larger grains (see Figure 5c−f). Such largely interconnected grains have been suggested to facilitate charge-carrier transport in FETs based on polymer semiconductors, including DPPbased polymers6,7 and NDI-containing polymers.25,26 Figure 3593



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rough surface morphology with some small holes are observed (see Figures 5g-5h), probably caused by backbone melting. In contrast, PNVT-8 and PNVT-10 thin films deposited on glass substrates exhibit more smooth surface morphologies with interconnected polycrystalline grains (see Figure 5i−l), which is well-known to be critical for improving the performance of TGBC FETs.25,26 Because FETs performance can be correlated with the film crystalline reasonably,4−7 the microstructures of thin films annealed at 200 °C, deposited on OTS-modified SiO2/Si and bare glass substrates, were further studied by GIXRD. Typical out-of-plane and in-plane diffraction curves and data of peaks are shown in Figure 6 and Table 3, respectively. Out-of-plane and in-plane patterns show that three polymer films exhibit multiple (l00) diffraction peaks, which could be assigned to lamellar-stacking structures. For OTS-modified SiO2 and glass substrates, the observed lamellar-distances of PNVT-8 thinfilms are 22.92 and 22.57 Å, respectively; shorter than that of P(NDI2OD-T2) (d-spacing =23.85 Å, 2θ = 3.7°), indicating that the distance of polymer lamellae decreases due to the introduction of the vinyl linkage in PNVT-8 thin films. This relatively closed lamellar stacking will facilitate charge transport. Compared with PNVT-8, long alkyl substituents make PNVT10 film exhibit much larger interlayer distances (26.26 and 25.58 Å for two different substrates, respectively).

Figure 5. AFM topography and phase images (1 × 1 μm2) of the polymer thin films annealed at 200 °C: (a, b) P(NDI2OD-T2), (c, d) PNVT-8, and (e, f) PNVT-10 thin films morphology on OTSmodified SiO2/Si substrates; (g, h) P(NDI2OD-T2), (i, j) PNVT-8, and (k, l) PNVT-10 thin films morphology on bare glass substrates.

5g−l shows the AFM images on bare glass substrates. For P(NDI2OD-T2) thin films annealed at 200 °C, a relatively

Figure 6. (a, c) Out-of-plane and (b, d) in-plane GIXRD patterns of the polymer thin films annealed at 200 °C, fabricated on (a, b) OTS-modified SiO2/Si and (c, d) bare glass substrates. 3594



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Table 3. Peak Assignments for the Out-of-Plane XRD Patterns Obtained from Three NDI-Based Polymers Thin Films Depending on Annealing Treatment at 200 °C P(NDI2OD-T2)

PNVT-8

PNVT-10

substrates

peak

2θ (deg)

d-spacing (Å)

2θ (deg)

d-spacing (Å)

2θ (deg)

d-spacing (Å)

silicon

(100) (010) (100) (010)

3.70 22.80 3.70 22.80

23.85 3.90 23.85 3.90

3.85 23.40 3.91 23.35

22.92 3.80 22.57 3.81

3.36 23.10 3.45 23.50

26.26 3.85 25.58 3.78

glass

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Note that (l00) diffraction peaks can be observed in both diffraction patterns, indicating that three polymers take a mixture stacking structures with face-on and edge-on oriented molecules.37−39 Although (010) peaks could not be seen in the in-plane patterns (see Figure 6b, d), it was found in the out-ofplane patterns for both substrates clearly (see Figures 6a, c). It is concluded that three polymers take mainly face-on orientation respective to substrates, which are similar with many high-performance polymers.24,40−42 According to the outof-plane patterns on OTS-modified SiO2/Si substrates, PNVT8 and PNVT-10 thin films annealed at 200 °C exhibit (010) features at 23.4 and 23.1°, with the π−π stacking distances of 3.80 and 3.85 Å, respectively, which are shorter than that of P(NDI2OD-T2) (22.8 and 3.90 Å). The shorter π−π stacking distances indicate that the strongly intermolecular interactions exist due to the vinyl-extended polymer backbones as well as strong D−A interactions. Therefore, these results further explain why the vinyl-containing polymers PNVT-8 and PNVT-10 exhibit better charge transporting ability than P(NDI2OD-T2). For the bare glass substrates, PNVT-10 thin films exhibit (010) feature at 23.5°, with a close π−π stacking distance of 3.78 Å, shorter than those of other polymers thin films. Combining with the advantages of smooth surface morphologies and large polycrystalline grains, the closed π−π stacking structure ensure an effective charge transport for PNVT-10-based TGBC FETs.

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CONCLUSIONS

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ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G. Y.); [email protected] (Y. L.). Author Contributions †

Authors H. Chen, Y. Guo, and Z. Mao contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The GIXRD data was obtained at 1W1A, Beijing Synchrotron Radiation Facility. The authors gratefully acknowledge the assistance of scientists of Diffuse X-ray Scattering Station during the experiments. This work was supported by the National Natural Science Foundation of China (20825208, 61101051, 51233006, and 21021091) and the Major State Basic Research Development Program (2011CB808403, 2011CB932303).

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We had successfully synthesized two novel NDI-based copolymers for solution-processed ambipolar transistors and inverters. Compared with P(NDI2OD-T2), vinyl linkages extended polymers (PNVTs) conjugations, and in turn enhanced π−π orbital overlaps. Furthermore, CV measurements demonstrated that the vinyl linkages in NDI-based polymers suitably increased their HOMO energy levels while maintaining LUMO energy levels. In ambient, high hole/ electron mobilities of 0.3 and 1.57 cm2 V−1 s−1 had been demonstrated, which were among the highest values for NDIbased polymer semiconductors. For the first time, we demonstrated that both hole and electron mobilities of NDIbased polymer were enhanced simultaneously. Most importantly, complementary-like inverters based on PNVT-8 showed a sharp signal switching with a high gain of 155 in ambient. These results demonstrated that the copolymers constructed by the vinyl-linkages could be promising organic semiconductors for high-performance FETs and complementary-like inverters.

S Supporting Information *

Materials synthesis, TGA, DSC, theoretical calculations, and additional graphics. This material is available free of charge via the Internet at http://pubs.acs.org. 3595



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