Incorporation of Heteroatoms in Conjugated Polymers Backbone

Article Cite This: Chem. Mater. 2018, 30, 5451−5459

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Incorporation of Heteroatoms in Conjugated Polymers Backbone toward Air-Stable, High-Performance n‑Channel Unencapsulated Polymer Transistors Feifei Wang,† Yanrong Dai,† Weiwei Wang,† Hongbo Lu,†,‡ Longzhen Qiu,*,‡ Yunsheng Ding,‡ and Guobing Zhang*,†,‡ †

Chem. Mater. 2018.30:5451-5459. Downloaded from pubs.acs.org by RMIT UNIV on 10/21/18. For personal use only.

National Engineering Lab of Special Display Technology, State Key Lab of Advanced Display Technology, Academy of Optoelectronic Technology, Hefei University of Technology, Hefei, 230009, China ‡ Key Laboratory of Advance Functional Materials and Devices, Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Anhui Province, Hefei 230009, China S Supporting Information *

ABSTRACT: Organic field-effect transistors (OFETs) without any encapsulation of polymer semiconductor layers that still exhibit unipolar n-type characteristics under air conditions are very rare. In this study, we use fluorinated bithiophene as a donor, and bis(2oxoindolin-3-ylidene)-benzodifuran-dione (BIBDF, P1) and azasubstituted BIBDF (P2) as acceptor units to develop air-stable and unipolar electron transport polymer semiconductors. Unencapsulated OFETs based on P1 and P2 were fabricated and directly evaluated under air conditions. The highest effective mobility 2 −1 −1 (μeff s was obtained for P2-based devices with e,max) of 0.23 cm V high Ion/Ioff ratio of >106 and low threshold voltage of 1.1 V. Moreover, P2 had high air stability and maintained unipolar 2 −1 −1 electron transport with μeff s and Ion/Ioff ratio of >106 during the 60 days of air storage. The work provides e,max of up 0.1 cm V an effective molecular design strategy to develop air-stable and high-performance n-channel unencapsulated polymer transistors that can be directly operated under air conditions.

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INTRODUCTION Polymer-based organic field-effect transistors (OFETs) have received much attention in the past three decades, because of the potential applications in integrated circuits for low-cost, lightweight, large-area, and flexible electronics.1−3 The OFET performances have been immensely improved; the mobility, in some cases, exceeded those of amorphous silicon transistors (mobility of ∼1.0 cm2 V−1 s−1) by optimizing the crystal structure and backbone orientation of polymer semiconductors.4−6 Moreover, the transport behavior of polymer semiconductors can also be conveniently varied by tuning the electronic structures such as the band-gap, lowest unoccupied molecular orbital (LUMO), and highest occupied molecular orbital (HOMO) energy levels.7−9 However, most polymer semiconductors containing different building units exhibit ptype transport and even ambipolar transport; the development of unipolar electron transport polymer transistors is obviously falling behind. In the past few years, the polymer transistors exhibiting unipolar electron behavior were evaluated either under vacuum or inner gas conditions.5,10,11 When the devices are exposed to air conditions, the polymer semiconductor layers should be encapsulated by a dielectric layer (PMMA or CYTOP), because of the electron vulnerability caused by water and oxygen.12,13 Consequently, OFETs without any encapsu© 2018 American Chemical Society

lation of polymer semiconductor layers that still exhibit unipolar n-type characteristics under air conditions are very rare.14−16 Moreover, the performances of this unencapsulated OFETs are very low, and the unipolar electron mobility did not surpass 0.1 cm2 V−1 s−1 when the devices were directly operated under air conditions.16 It was well-known that two main factors are important for the electron transport of n-channel OFETs under air conditions: (i) Chemical blocks of reaction between semiconductor layer and oxygen/water. Deep LUMO/HOMO energy levels are the key parameters for this problem. Low-lying LUMO energy levels are beneficial for electron injection, stable transport, and resisting charge carrier trapping under air conditions, while deep HOMO energy levels offer oxidative stability and hinder hole injection from the Au electrode to the semiconducting layers.17,18 (ii) Kinetic barriers for slowing oxidant intrusion. The fluorinated polymer and dense solid-state packing have the ability to create kinetic barriers to slow down the penetration Received: June 5, 2018 Revised: July 6, 2018 Published: July 7, 2018 5451

DOI: 10.1021/acs.chemmater.8b02359 Chem. Mater. 2018, 30, 5451−5459

Article

Chemistry of Materials

Figure 1. Molecular structures of the conjugated polymers (P1 and P2).

Scheme 1. Synthetic Route for the Polymers (P1 and P2)

of oxygen and water.19,20 For example, rylene dyes containing fluorinated side chain in the imide position exhibited excellent air stability for n-type OFETs, even though the perfluorinated substituents did not significantly lower the energy levels.21 Based on the energy level requirements mentioned above, three methods are generally available for the structural modification of polymer semiconductors to achieve lower LUMO/HOMO energy levels. First, the introduction of a strong electron-withdrawing unit into conjugated polymers not only decreases the LUMO/HOMO energy levels, but also increases the intermolecular and intramolecular interactions.18 For example, naphthalene diimides (NDI) is electron-deficient, because of the substitution of an aromatic fused ring with two sets of strongly electron-withdrawing imide groups.22 Facchetti et al. reported an NDI-based conjugated polymer with a deep LUMO energy level (approximately −4.0 eV) that exhibited electron mobilities as high as 0.85 cm2 V−1 s−1 under ambient conditions when using a top-gate/bottom-contact (TG/BC) configuration with encapsulation for the semiconductor layer.23 Second, the electron-deficient group such as a carbonyl group, which was incorporated into the side chain, can also decrease the LUMO/HOMO energy levels; the corresponding polymers exhibited unipolar electron or ambipolar transport characteristics.24,25 Third, electron-deficient heteroatoms and groups such as halogens, aza, and cyano were incorporated into the polymer backbone; this is another approach to effectively lower the molecular orbital energy levels and fine-tune the carrier transport behavior of conjugated polymers.26,27 For example, Chen synthesized a strongly electron-withdrawing benzo[c][1,2,5]oxadiazole (BOZ) by replacing the sulfur atom

in benzo[c][1,2,5]thiadiazole (BTZ) with an oxygen atom. The LUMO/HOMO energy levels (−3.98/−5.90 eV) of BOZ-based polymer can be further decreased, compared to the BTZ-based polymer (−3.81 eV/−5.84 eV). Therefore, the substitution increased the electron injection and blocked the hole injection; therefore, the BOZ-based polymer exhibited unipolar electron-transport behavior.28 In this study, two conjugated polymers (P1 and P2) were designed based on kinetic barriers and chemical block strategies using a fluorinated bithiophene as the donor unit and strongly electron-deficient bis(2-oxoindolin-3-ylidene)benzodifuran-dione (BIBDF, P1) and aza-substituted BIBDF (P2) as the acceptor units (Figure 1). On one hand, fluorination on the 2,2′-dithiophene not only deepens the LUMO/HOMO energy levels of the corresponding polymers, because of the electron-withdrawing nature of fluorine,2 but also increases the coplanarity of polymer backbone, because of noncovalent attractive interactions between F···S or F··· H.2,29,30 Most importantly, the hydrophobic property of fluorine favors the ability of the polymer to resist the penetration of water and oxygen.31,32 On the other hand, BIBDF is an excellent strong acceptor with two lactone groups in the central core and two lactam groups in the outer ring.33 The incorporation of heteroatoms (N) into BIBDF unit can significantly decrease the LUMO/HOMO energy levels of conjugated polymers,34,35 affording more-effective electron injection, transport, and chemical blocks of ambient oxidation. The results indicate that the strong electronegativity of fluorine and nitrogen atoms decreased the LUMO/HOMO energy levels and improved the coplanar backbone, thereby increasing 5452


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Table 1. Molecular Weights, Decomposition Temperatures, Absorption Properties, and Energy Levels of the Two Polymers λmax [nm] polymer

Mn/PDIa [kg/mol]

Tdb [°C]

solution

film

λonset [nm]

c Eopt g [eV]

EHOMOd [eV]

ELUMOd [eV]

Ecv g [eV]

P1 P2

28.3/1.8 34.2/1.7

395 400

830 868

825 859

980 980

1.27 1.27

5.79 6.04

4.0 4.10

1.79 1.94

a

Mn and PDI of the polymers were determined using polystyrene standards in trichlorobenzene. bTd represents the 5% weight loss temperature. c opt red + Eg = 1240/λonset (in film). dHOMO = −(4.75 + Eox onset) and LUMO = −(4.75 + Eonset); the redox Fc/Fc was located at 0.05 V, relative to Ag/ +

Ag .

Figure 2. Normalized absorption spectra of the polymers in solution and in thin films.

obtained due to poor solubility. The molecular weights of these polymers were characterized by gel permeation chromatography (GPC) at 100 °C using trichlorobenzene as the eluent. As shown in Table 1, the number-average molecular weights (Mn) of P1 and P2 were 28.3 and 34.2 kg/mol, respectively, and the corresponding polydispersity index (PDI) values were 1.8 and 1.7, respectively. The thermal properties of polymers were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere, as shown in Figure S1 in the Supporting Information. The TGA indicated excellent thermal stability for the two polymers with a 5% weight loss temperature (Td) of >395 °C. The DSC curves clearly showed endothermic transitions both at ∼26 °C during the heating and exothermic transitions at 22 °C during the cooling for both polymers. This can be attributed to side-chain phase transition.8 Optical and Electrochemical Properties. The ultraviolet-visible light-near infrared (UV-Vis-NIR) absorption spectra of the two polymers measured in diluted chlorobenzene solution and in solid thin films are shown in Figure 2, and the corresponding data are also summarized in Table 1. P1 and P2 had dual-band absorption both in the solution and thin films, typically observed in donor−acceptor polymers. The low-energy band ranged from 600 nm to 1000 nm; this can be assigned to intramolecular charge transfer (ICT) from the donor units to the acceptor cores. The high-energy band ranged from 310 nm to 500 nm; this can be attributed to the π−π* transition of polymer backbone.37,38 In the solution, the maximum absorption wavelengths of P1 and P2 were 830 and 868 nm, respectively. Both polymers showed slightly blue shifts (5−9 nm) for ICT in the thin films, compared with the solution. This is because of the formation of H-aggregates; similar blue-shift phenomena were also observed in isoindigo and its derivative-based donor−acceptor polymers.34,39 Compared with P1, the maximum absorption of P2 was significantly red-shifted to ∼38 and 34 nm for the solution and solid states, respectively, most probably because of the different electron-

the electron injection and blocking the hole injection. The unencapsulated bottom-gate/top-contact (BG/TC) OFETs based on polymers were directly evaluated under air conditions. P1 exhibited ambipolar behavior with a moderate mobility, whereas P2-based devices with the semiconductor layer directly exposed to air conditions still exhibited unipolar electron transport characteristic with the highest effective 2 −1 −1 mobility (μeff s and maintained high air e,max) of >0.23 cm V eff stability with unipolar μe,max up to 0.1 cm2 V−1 s−1 during the 60 days of air storage. To the best of our knowledge, this is the highest mobility for unencapsulated polymer transistors that were directly evaluated under air conditions. The results demonstrate that the design of a polymer based on fluorination for kinetic barriers and aza-substitution for chemical block is an effective strategy for the developments of air-stable and highperformance n-type unencapsulated polymer transistors.

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes of two polymers are shown in Scheme 1. 5,5′-Bis(trimethylstannyl)-3,3′-difluoro-2,2′-bithiophene, bis(6bromo-2-oxoindolin-3-ylidene)-benzodifuran-dione (2BrBIBDF), and bis(6-bromo-2-oxo-7-azaindolin-3-ylidene)-benzodifuran-dione (2Br-BABDF) were synthesized according to the reported procedures.11,33,36 The two polymers (P1 and P2) were synthesized through palladium-catalyzed Stille crosscoupling polymerization reaction. The detailed procedures are described in the Experimental Section (presented in the Supporting Information). The polymerization temperatures were controlled at about 110 °C, and a higher temperature resulted in insoluble polymers. Especially for P2, most of the resulting polymer (P2) was insoluble when the temperature was 130 °C. After Soxhlet extraction with methanol, dichloromethane, and chlorobenzene, two polymers were obtained as black solids by precipitating from methanol. The polymer components were examined by elemental analysis, and wellresolved 1H NMR spectra of the two polymers could not be 5453


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indicate that the fluorination on the dithiophene unit can cause a more coplanar backbone, because of the F···S noncovalent interactions.42,43 The calculated F···S distances on DFT are 2.93 and 2.92 Å for P1 and P2, respectively, which are much shorter than the sum of van der Waals radii for F···S (3.27 Å), indicating significant F···S interaction. Second, the dihedral angles between the donor and acceptor units are 20.4° and 0.01° for P1 and P2, respectively. P1 showed relatively large torsion angles, most probably because of the repulsion between the two hydrogen atoms in the thiophene unit of the donor and the benzene ring of the acceptor. This torsion can be effectively avoided by aza-substitution on the acceptor unit (Figure S3). Therefore, the fluorination on the donor and the aza-substitution on the acceptor both contributed to an almost-planar conformation for P2. The calculated LUMO/ HOMO energy levels are also shown in Figure S3. The LUMO energy levels of P1 and P2 are both concentrated on the acceptor units, whereas the HOMO energy levels are mainly located on the donor units. The fluorination on the donor unit (dithiophene) slightly deepens the LUMO/HOMO energy levels of P1, which is consistent with the trend of CV results. The LUMO/HOMO energy levels of P2 can be significantly decreased by aza-substitution, contributing to easier electron injection and more difficult hole injection from the Au electrode to the polymer semiconductor layer in OFET devices. Characteristics of Organic Field-Effect Transistors. Bottom-gate/top-contact (BG/TC) OFETs were fabricated following standard procedures (Supporting Information). All the devices without any encapsulation of semiconductor layer were directly evaluated under air conditions (relative humidity (RH) of 30%, 25 °C; see Figure S6a in the Supporting Information). The devices were annealed at different temperatures, and the corresponding performance data such as mobility (μ), current on/off ratio (Ion/Ioff), and threshold voltage (Vth) are listed in Table S1 in the Supporting Information. Note that OFET transfer characteristics are usually nonlinear; therefore, the claimed mobility (μclaimed) calculated by using eq 1 in the Experimental Section, only within a limited linear range or at the steepest part of the transfer curve, is likely an overestimation value. In this study, the reliability factor (r) was introduced and the mobility (called the effective mobility, μeff) was further calculated according to a recent commentary published by Choi et al.44 The detailed calculations are described in the Experimental Section. As shown in Figures S4 and S5 in the Supporting Information, P1 showed typical ambipolar transport characteristics in the V-shaped transfer curves in hole and electron enhancement mode operations when the devices were directly measured under air conditions. The nonannealing (N/A) devices exhibited the highest mobilities of 4.4 × 10−3 cm2 V−1 s−1 and 4.3 × 10−4 cm2 V−1 s−1 for electron and hole (see Figure S4), respectively. Both polymer-based OFETs exhibited improvements in carrier mobilities after the thermal annealing. This can be attributed to the enhanced crystallinity and decreased effect of residual solvent on carrier transport.45 For P1-based devices, optimized performances with an electron mobility of 0.047 cm2 V−1 s−1 and a hole mobility of 0.016 cm2 V−1 s−1 were obtained for the 210 °C-annealed devices (see Figure S5). These results indicated that fluorinated P1 has low performance when the devices without any encapsulation were directly measured under air conditions. This may be attributed to insufficiently low LUMO/HOMO energy levels; the

deficient characteristics of acceptor units (BIBDF and BABDF). The electron-deficient nature of BABDF unit with aza-substitution is much stronger than that of BIBDF unit due to the electronegative nature of nitrogen atoms, thus increasing the push−pull interactions within the P2 backbone, compared with the P1 backbone.15 Moreover, the significant red-shift might also be attributed to the improvement of coplanarity that arises from the incorporation of aza-atom.34 Interestingly, the absorption onsets of P1 and P2 in the solid state are almost the same and located at ∼980 nm, corresponding to the optical band gaps of 1.27 eV. The LUMO and HOMO energy levels of the polymers were investigated by cyclic voltammetry (CV) (see Figure 3a). The

Figure 3. (a) Cyclic voltammograms and (b) the electrochemical energy diagrams of the polymers.

data are summarized in Table 1, and the energy level diagram is illustrates in Figure 3b. For better comparison, PBIBDFDT13 was also measured under the same conditions as P1 and P2 (see Figure S2 in the Supporting Information). The LUMO/HOMO energy levels are −4.0 eV/−5.79 eV and −4.10 eV/−6.04 eV for P1 and P2, respectively. Compared with PBIBDF-DT without heteroatom substitution, the fluorination on dithiophene unit mainly decreased the HOMO energy level of the corresponding polymers (P1), whereas the substitution of the aza-atom on the acceptor unit can both deepen LUMO and HOMO energy levels. For fluorinated and aza-substituted P2, the deep-lying LUMO energy level (−4.10 eV) would effectively improve the electron injection, accumulation, and stable transport behaviors; the low HOMO energy level (−6.04 eV) would also form large hole injection barriers and, in turn, hinder hole injection from the electrode (such as Au) to the semiconductor layer. The electrochemical energy bandgaps of P1 and P2 were calculated from the difference of the LUMO and HOMO energy levels to be 1.79 and 1.94 eV, respectively, which are 0.5−0.7 eV higher than their optical bandgaps calculated from the absorption spectra of their thin films. These differences are probably caused by the large exciton binding energy.40,41 Density functional theory (DFT) calculations were used to investigated the effect of fluorination and aza-substitution on the conformations and energy levels of polymers by using Gaussian at the B3LYP/6-31G(d) level. To simplify the calculations, the large side chains were simplified into methyl groups, and the acceptor−donor−acceptor was used as the skeletal structure (see Figure S3 in the Supporting Information). First, the dihedral angles between the two thiophene units of P1 (0.42°) and P2 (0.01°) are much smaller than that of PBIBDF-DT (−10.5°). These results 5454


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Figure 4. (a)−(e) Transfer characteristics of fresh devices annealed at different temperatures. (f) Air stability of optimized device of P2 under air conditions, the mobilities were calculated by using the reliability factor (r), according to a recent commentary.44 All of the devices without any encapsulation were directly evaluated under air conditions.

using the reliability factor (r ≈ 30%). The μclaimed value calculated from the limited linear range by using eq 1 in the Experimental Section) were as high as 0.77 cm2 V−1 s−1 (see Table S1). The above-mentioned results confirm that the incorporation of heteroatoms significantly improves the stable transport of electrons. First, the aza-substitution on the acceptor unit significantly decreased the LOMO energy levels, which can ensure the thermodynamic stability of electron transport and resist the electron trap by oxygen and water under air conditions during the device operation. This was the chemical block between the oxygen/water and polymer. Moreover, the decreased LUMO levels match well with the work function of Au electrodes, resulting in efficient electron injection.47,48 Second, the hydrophobic nature of fluorinated materials might provide better air stability of devices by resisting the penetration of oxygen and water (see Figure S7 in the Supporting Information).31,32 Third, the close stacking of the polymer backbone caused by strong intermolecular interactions might also be another effective way to slow the diffusion of oxygen and water (see below). Therefore, these were the kinetic barriers between the oxygen/water and polymer semiconductor layer.19,49 Finally, note that the deep enough HOMO energy levels of P2 lowered by the incorporation of heteroatoms can form large hole injection barriers, thus effectively hindering hole injection from the Au electrode to the polymer semiconducting layers, even when the polymer semiconductor layers were directly exposed to air conditions.28 To investigate the air stability of the device, the optimized BG/TC devices of P2 without any encapsulation were stored under air conditions (RH = 30%, 25 °C) with their device performances being monitored for 60 days. As shown in Figure 4f, as well as Figure S7, P2 exhibited excellent unipolar electron transport stability. The unipolar electron mobility

electron carriers in P1 were easily trapped by the air oxidants, which might result in serious degradation and low performances.18 Furthermore, different aggregations in thin films might be the other reason for the low performances (see below). Similar results were also observed from fluorinated diketopyrrolopyrrole (DPP)-based polymers.46 P2-based devices exhibited much more excellent performances. Figure 4, as well as Figure S6 in the Supporting Information show the typical transfer and output curves of P2based fresh devices without any encapsulation. P2 exhibited unipolar electron transport characteristics with high electron mobilities, high Ion/Ioff ratio, and low Vth. The N/A device exhibited a maximum electron mobility of 0.026 cm2 V−1 s−1 and an average mobility of 0.024 cm2 V−1 s−1, with an Ion/Ioff ratio of 106 and Vth of 4.1 V (Figure 4a). With an increase in the annealing temperature, P2 showed greatly enhanced mobilities (more than 5−10 times higher, compared with N/ A devices (see Figures 4b−e). The optimized performances of 290 °C-annealing devices exhibited a mobility as high as 0.23 cm2 V−1 s−1 with a high Ion/Ioff ratio (>106) and a low Vth (1.1 V; see Figure 4e). It was reported that the unipolar electron transport behavior of BG devices without any encapsulation based on the milestone n-channel ladder polymer BBL, which displayed a unipolar electron mobility of 0.1 cm2 V−1 s−1.16 Facchetti et al. also synthesized a famous polymer P(NDI2ODT2); the BG/TC-based polymer transistors that evaluated air conditions directly also exhibited the highest unipolar electron mobility of 0.1 cm2 V−1 s−1.14 Therefore, to the best of our knowledge, the unipolar electron mobilities of P2 obtained in this study are the highest values to date in unencapsulated polymer transistors directly characterized under air conditions. All of the P2-based devices had very low Vth of ca. 1.0−6.5, which is very important for constructing low-power-consumption devices. Notably, the highest mobility (0.23 cm2 V−1 s−1) in this work was the effective mobility, which calculated by 5455


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Figure 5. Crystalline nature and molecular orientation of the two polymers films annealed at optimized temperatures: (a) 2D-GIXD patterns, (b) in-plane line cuts of GIXD, (c) out-of-plane line cuts, and (d) proposed polymer packing in the thin film of the two polymers.

decreased from 0.23 cm2 V−1 s−1 to 0.16 cm2 V−1 s−1 in 20 days, and was still as high as 0.10 cm2 V−1 s−1 with Ion/Ioff ratios of >106 after 60 days of air storage (see Figure S8 in the Supporting Information). The hole mobilities with very low Ion/Ioff ratios were observed to be 2−3 orders of magnitude lower than the electron mobility values after 10 days of exposure. It was reported that the mobility of famous polymer (PNDI2OD-T2)-based BG/TC devices degraded from 0.1 cm2 V−1 s−1 to 0.01 cm2 V−1 s−1 when exposed under air conditions for 6 weeks.14 Therefore, these results indicated that the fluorination and aza-substitution on polymer backbone does remarkably improve the device stability for the n-type OFETs that exhibited long-time unipolar electron transport under air conditions. Thin-Film Microstructural Characterization. To better understand the relationship between OFET performance and chemical structure, the annealing films of the two polymers (P1 and P2) were investigated by two-dimensional grazing incidence X-ray diffraction (2D-GIXD) and atomic force microscopy (AFM). Figure 5 shows the 2D-GIXD images and the corresponding diffractogram profiles of the annealed films fabricated under the same conditions as the OFET devices. P1 and P2 showed well-defined diffraction peaks corresponding to the (100)−(300) planes, and both also exhibited weak diffraction peaks (400) along the out-of-plane qz direction. These observations indicate that P1 and P2 have highly ordered lamellar structures, and the corresponding lamellar spacings were 32.37 and 29.90 Å for P1 and P2, respectively, calculated from the (100) peaks at qz = 0.194 and 0.210 Å−1. The clear (010) Bragg peaks that can be attributed to the π−π stacking were also observed for both polymers from the images and diffractogram profiles. P1 exhibited the (010) diffraction peaks located at both in-plane and out-of-plane orientations (Figures 5a and 5c), indicating that P1 had a dual packing, i.e., a mixture of face-on and edge-on packing, with respect to the substrate plane in thin films (Figure 5e). This may result in

negative effect on charge transport because the face-on orientation, relative to the substrate, might not be beneficial for parallel charge transport in OFETs, thus causing lower performances.47 The annealed film of P2 showed a strong (010) diffraction peak along the in-plane orientation, indicating that the P2 film favored edge-on orientation on the substrate (Figure 5f). The calculated π−π stacking distances of P1 and P2 were 3.49 and 3.41 Å. Compared with P1, P2 showed a smaller lamellar spacing and shorter π−π stacking distance. This is because P2 has more planar backbones, as shown in Figure S3, resulting in stronger intermolecular interactions. The close π−π stacking (3.41 Å) of P2 might also create kinetic barriers to prevent the diffusion of water and oxygen into the polymer semiconductor and was beneficial for electron transport under air conditions.19,49 The AFM images of P1 and P2 films deposited on OTStreated SiO2 substrates with and without annealing are shown in Figure 6. The N/A film of P1 showed fine, short, and clustered crystals with a low root-mean-square (RMS) roughness of 1.39 nm. In comparison, the annealed film of P1 exhibited relatively continuous nanofibrillar crystals over the area with an RMS of 1.45 nm. For P2 films, wide polymer fibers were clearly observed. The annealed film showed larger fibrillar domains, compared with the N/A film, indicating increased OFET performances of the annealed device. Moreover, the aza-substitution into polymer backbone (P2) provided much rougher surfaces and larger polycrystalline grains than P1. A larger polycrystalline grain might endow fewer grain boundaries and contribute to higher OFET performance. In short, by combining all the above-mentioned results, the high electron mobility of P2 can be attributed to aza-substitution (lower LUMO/HOMO energy levels) for chemical barriers, fluorination, and close π−π stacking for kinetic blocks, uniform edge-on orientation, and wellinterconnected fiberlike film morphology. 5456



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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G. Zhang). *E-mail: [email protected] (Z. Qiu). ORCID

Longzhen Qiu: 0000-0002-8356-6303 Guobing Zhang: 0000-0001-6053-2015 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 (NSFC, Grant Nos. 51573036 and 51703047), the Distinguished Youth Foundation of Anhui Province (No. 1808085J03), the Fundamental Research Funds for the Central Universities (Grant No. JZ2018HGPB0276), the Foundation of Anhui Provincial Education Department (No. kj2015jd15), and the China Postdoctoral Science Foundation (No. 2018M632524). The authors thank 9A beamlines (the Pohang Accelerator Laboratory in Korea) for providing the beam time. We also thank Ph.D candidate Li Xiang from Department of Chemistry, Pohang University of Science and Technology for helpful processing on GIXD.

Figure 6. AFM topography images (5 μm × 5 μm) of the polymer films spin-coated on OTS-modified SiO2/Si substrates with and without annealing.

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(1) Klauk, H. Organic Thin-Film Transistors. Chem. Soc. Rev. 2010, 39, 2643−2666. (2) Shi, L.; Guo, Y.; Hu, W.; Liu, Y. Design and Effective Synthesis Methods for High-Performance Polymer Semiconductors in Organic Field-Effect Transistors. Mater. Chem. Front. 2017, 1, 2423−2456. (3) Zaumseil, J.; Sirringhaus, H. Electron and Ambipolar Transport in Organic Field-Effect Transistors. Chem. Rev. 2007, 107, 1296− 1323. (4) Liu, Y.; Wang, F.; Chen, J.; Wang, X.; Lu, H.; Qiu, L.; Zhang, G. Improved Transistor Performance of Isoindigo-Based Conjugated Polymers by Chemically Blending Strongly Electron-Deficient Units with Low Content To Optimize Crystal Structure. Macromolecules 2018, 51, 370−378. (5) 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. (6) Fei, Z.; Han, Y.; Gann, E.; Hodsden, T.; Chesman, A. S. R.; McNeill, C. R.; Anthopoulos, T. D.; Heeney, M. Alkylated Selenophene-Based Ladder-Type Monomers via a Facile Route for High-Performance Thin-Film Transistor Applications. J. Am. Chem. Soc. 2017, 139, 8552−8561. (7) Yuan, Z.; Fu, B.; Thomas, S.; Zhang, S.; DeLuca, G.; Chang, R.; Lopez, L.; Fares, C.; Zhang, G.; Bredas, J. L.; Reichmanis, E. Unipolar Electron Transport Polymers: A Thiazole Based All-Electron Acceptor Approach. Chem. Mater. 2016, 28, 6045−6049. (8) Zhang, G.; Li, P.; Tang, L.; Ma, J.; Wang, X.; Lu, H.; Kang, B.; Cho, K.; Qiu, L. A Bis(2-oxoindolin-3-ylidene)-Benzodifuran-Dione Containing Copolymer for High-Mobility Ambipolar Transistors. Chem. Commun. 2014, 50, 3180−3183. (9) Facchetti, A. π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications. Chem. Mater. 2011, 23, 733−758. (10) Zhan, X.; Tan, Z.; Domercq, B.; An, Z.; Zhang, X.; Barlow, S.; Li, Y.; Zhu, D.; Kippelen, B.; Marder, S. R. A High-Mobility ElectronTransport Polymer with Broad Absorption and Its Use in Field-Effect Transistors and All-Polymer Solar Cells. J. Am. Chem. Soc. 2007, 129, 7246−7247. (11) Zhang, G.; Dai, Y.; Song, K.; Lee, H.; Ge, F.; Qiu, L.; Cho, K. Bis(2-oxo-7-azaindolin-3-ylidene)benzodifuran-Dione-Based Donor−

CONCLUSION Two conjugated polymers (P1 and P2) were designed by the incorporating heteroatoms with fluorinated bithiophene as the donor unit, and strongly electron-deficient BIBDF (P1) and aza-substituted BIBDF (P2) as the acceptor units. By incorporating heteroatoms, the LUMO/HOMO energy levels can be decreased, coplanar backbone can be improved, and π−π stacking can be reduced. All the devices without any encapsulation of the semiconductor layer were directly evaluated under air conditions. P1 exhibited ambipolar transport with low mobilities. P2 displayed unipolar electron transport characteristics under air conditions, because of kinetic barriers caused by the fluorination and close π−π stacking, and chemical blocks resulting from the deep LUMO/ HOMO energy levels that were lowered mainly by azasubstitution. The optimized effective mobility was 0.23 cm2 V−1 s−1 with a high Ion/Ioff ratio of >106 and a low Vth of 1.1 V. Moreover, P2-based devices had high air stability and maintained unipolar electron transport with effective mobility of up 0.1 cm2 V−1 s−1 and Ion/Ioff ratio of >106 during the 60 days of air storage. These results provide an effective molecular design strategy for developing air-stable and high-performance unipolar electron transport polymer transistors that can be directly operated under air conditions, especially for unencapsulated polymer transistors.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02359. Experimental section and other data (such as TGA, DSC, and OTFT performances) (PDF) 5457


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Incorporation of Heteroatoms in Conjugated Polymers Backbone

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