Regioregular Bis-Pyridal[2,1,3]thiadiazole-Based Semiconducting

Nov 21, 2017 - Our work demonstrates that the BPT unit is a promising building block for designing high-performance electron-transporting semiconducto...
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Regioregular Bis-Pyridal[2,1,3]thiadiazole-Based Semiconducting Polymer for High-Performance Ambipolar Transistors Chunguang Zhu, Zhiyuan Zhao, Huajie Chen, Liping Zheng, Xiaolin Li, Jinyang Chen, Yanming Sun, Feng Liu, Yunlong Guo, and Yunqi Liu J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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Regioregular Bis-Pyridal[2,1,3]thiadiazole-Based Semiconducting Polymer for High-Performance Ambipolar Transistors Chunguang Zhu†,‡, Zhiyuan Zhao§,‡, Huajie Chen†,*, Liping Zheng†, Xiaolin Li†, Jinyang Chen§, Yanming Sunǁ, Feng Liu┴, Yunlong Guo§,*, Yunqi Liu§,* †

Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, P. R. China § Beijing National Laboratory for Molecular Sciences, Chinese Academy of Sciences, Beijing 100190, P. R. China ǁ Heeger Beijing Research and Development Center, School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China ┴ Department of Physics and Astronomy, and Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiao Tong University, Shanghai 200240, P. R. China

Supporting Information Placeholder ABSTRACT: We report a regioregular bis-pyridal[2,1,3]thiadiazole (BPT) acceptor strategy to construct the first ambipolar pyridal[2,1,3]thiadiazole-based semiconducting polymer (PBPTV). The use of BPT unit enables PBPTV to achieve high electron affinity, low LUMO level, and extended πconjugation. All these factors provide PBPTV with encouraging hole and electron mobilities up to 6.87 and 8.49 cm2 V−1 s−1, respectively. Our work demonstrates that the BPT unit is a promising building block for designing high-performance electron-transporting semiconductors in organic electronics.

Ambipolar polymeric semiconductors, which can transport hole and electron simultaneously, exhibit great potential to enable costeffective fabrication of ambipolar field-effect transistors (FETs), complementary circuits, and light-emitted transistors[1]. Currently, the most effective synthetic strategy for high-mobility ambipolar polymers is proved to construct donor (D)-acceptor (A) architecture, in which the energy levels of the polymers are mainly controlled by the acceptor segment[1–3]. As a result, this approach has aroused extensive interest to design and synthesize novel acceptor moieties for high-mobility ambipolar polymers[1–3]. However, only few of them can achieve both hole and electron mobilities of above 3.0 cm2 V–1 s–1 simultaneously (see Table S1 and Figure S1)[4–10]. On the other hand, the development of novel acceptor moieties generally suffers from complex molecule design and great synthetic challenge. Therefore, one may ask whether ultrahigh ambipolar mobility can be achieved in the polymers that developed from a facile acceptor via an effective design strategy. Due to highly electron-withdrawing nature and good coplanarity, pyridal[2,1,3]thiadiazole (PT) was widely utilized as the acceptor unit to construct D−A type copolymers for organic solar cells and p-type polymeric FETs[11]. Especially, its asymmetric backbone cased by pyridal N atom has also attracted extensive research on the development of the regioregular PT-based copolymers[11b,11c]. For example, Bazan et al. reported the first regioregular PT-based copolymer (PCDTPT), whose hole mobility (0.6 cm2 V−1 s−1) exhibits 120 times higher than that of the random

counterpart, as a result of higher degree of structural order within the resulting films[11b]. On the basis of an one-dimensional nanotemplate technique, the macroscopic alignment of the PCDTPT thin films achieves sharply improved hole mobility of > 23 cm2 V−1 s−1, as calculated in the low VG regime (VG < 20 V)[11e,11g]. Despite of great advances in p-type PT-based polymers, so far, no ambipolar transport performance was observed in any of these polymer materials.

Figure 1. Optimized HOMO/LUMO levels of PT and BPT acceptors; and molecular structures of BPT acceptor and its copolymer PBPTV. Herein, we reported an effective regioregular dual-acceptor strategy for designing the first ambipolar PT-containing copolymer, PBPTV (Figure 1), where the regioregular bispyridal[2,1,3]thiadiazole (BPT) unit is used as the acceptors and the alkyl-substituted (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) unit is used as the donors. Unlike previously reported method[11], our synthetic strategy enables two PT acceptors to link together, thus developing a novel electron-withdrawing unit (BPT). The calculated LUMO value of BPT unit is –3.42 eV (Figure 1), which is lower than that of PT (–3.13 eV) and even can be comparable to the unsubstituted naphthalene diimide (NDI, –3.40 eV)[12]. The result thus reveals that our developed BPT unit has a highly electron-withdrawing feature, which would enable low LUMO level and high electron affinity for the resulting BPT-

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based polymers. Additionally, the extended backbone conjugation of BPT acceptors and TVT donors may also contribute to the overlaps of intermolecular electron orbitals, thereby facilitating interchain carriers transport[13]. More importantly, our synthetic method can ensure structural regioregularity of PBPTV, in which the pyridal nitrogens will point in the same direction along the main chain. We expect that this well-defined backbone can enhance interchain organization and thereby improve carriers mobilities[11b]. Using PBPTV as a active layer, top-gate FETs exhibited air-stable ambipolar behaviours under ambient conditions, achieving ultrahigh hole and electron mobilities up to 6.87 and 8.49 cm2 V−1 s−1, respectively. The observed values can be comparable to most of the highest hole or electron mobilities measured from the well-known polymers bearing isoindigo (IDG)[4,5], diketopyrrolopyrrole (DPP)[8–10,14], and NDI moities[15] (see Table S1 and Figure S1).

a suitable energy-level structure (EHOMO = –5.61 eV; ELUMO = – 3.66 eV, Figure 2a) for both hole and electron injections from Au electrodes to semiconductor films. Note that PBPTV exhibits strong redox peaks; moreover, the intensity of oxidative process is very similar with reductive one, indicating that PBPTV will exhibit typical ambipolar transport. The optimized model structure, as shown in Figure 2b, reveals that PBPTV possesses an extended π-conjugation with relatively small torsion dihedral angles of < 25.3 ° throughout the main chain, which is much smaller than that (ca. 40°) of well-known P(NDI2OD-T2)[17]. In general, an enlarged conjugation would facilitate strong intramolecular charge transfer and molecular organization in the polymer chains[13]. Furthermore, the molecular orbital distributions indicate that both the HOMO and LUMO are highly delocalized over the whole polymer chain (Figure 2b), thus giving partial explanation for the observed ambipolar characteristic of the PBPTV films[4–10].

Scheme 1. Synthetic route to the regioregular PBPTV. Synthetic route to PBPTV is provided in Scheme 1. On the basis of the different reactivity of the two C-Br sites in 4,7dibromo[1,2,5]thiadiazolo[3,4-c]pyridine (PT-2Br)[11b], Stille coupling between PT-2Br and 3-(2-decyltetradecyl)-5tributylstannylthiophene was controlled to occur at the C-Br site near to the pyridyl N atom, thereby affording the key intermediate TPT-Br in high yields (> 90%). Then another Stille coupling of PT-Br and hexabutyldistannane gave BPT in 55% yields, which was then converted to the target dibrominated monomer (BPT2Br) through N-bromobutanimide-promoted electrophilic reaction. The proposed regioregular structure of BPT-2Br was confirmed by 1H–1H nuclear overhauser effect spectroscopy, in which no obvious cross-correlation peaks can be detected between PT (δ = 9.42 ppm) and thiophene (δ = 8.44 ppm) proton resonances (Figure S2). Finally, Stille-coupling polymerization of BPT-2Br and (E)-1,2-bis(tributylstannyl)ethane afforded the target copolymer (PBPTV) in moderate yields (80%). Solubility tests confirmed that PBPTV was readily soluble in common organic solvents at room temperature, such as chlorinated solvent, THF, and xylene (Figure S3). Satisfactory number-average molecular weight of 22.7 kDa and narrow polydispersity index of 2.23 were achieved for PBPTV (Figure S4). Thermogravimetric analysis of PBPTV samples revealed high decomposition temperature over 420 °C (Figure S5). Differential scanning calorimetry did not provide any evidences on phase transition in range of –35 °C to 300 °C (Figure S6). In dilute chlorobenzene (CB) solution at 30 °C, PBPTV shows a strong intramolecular charge transfer (ICT) band (500–900 nm) with the absorption peak (λmax) at 756 nm (Figure S7a). Moreover, a distinct absorption shoulder at ca. 696 nm is observed, which suggests aggregation of the polymer chains[16]. Heating CB solution from 30 to 90 °C reveals a breakup of aggregated chains because a significant hypsochromic shift in λmax from 756 to 688 nm can be observed in Figure S7b[16]. In comparison to the solution at 30 °C, PBPTV thin film achieves ca. 44 nm red shift with the λmax of ca. 800 nm. These absorption features indicate that PBPTV has a strong tendency to crystalline and organization in solid-state film, which would facilitate high-performance carrier transport[13]. Cyclic voltammetry (CV) measurements indicate that PBPTV has

Figure 2. (a) CV curve of the PBPTV film. (b) Optimized molecular geometries and HOMO/LUMO orbitals. Our primary interest was to determine whether the assynthesized PBPTV would achieve high-performance ambipolar transport. To this end, top-gate bottom-contact (TGBC, Figure 3a) FET devices were fabricated on octadecyltrichlorosilane (OTS)treated SiO2 (300 nm)/Si substrate. Representative transfer and output curves of the TGBC transistors are shown in Figures 3c and 3d, respectively. As expected, unprecedented ambipolar behaviours were observed under ambient conditions. Moreover, dual sweep test of the transfer curves showed a small hysteresis (Figure S8). For as-cast thin films, the average hole and electron mobilities, extracted from the saturation regimes of transfer curves at VDS = ±100 V, are equal to 4.47 and 5.25 cm2 V–1 s–1 (Figure S9), respectively. The result indicates that PBPTV thin films could establish efficient carrier transport channels at low annealing temperature. Further investigation of thermal annealing effects reveals that 180 °C annealed thin films achieve the best-performing performance, affording the average hole and electron mobilities of up to 6.64 and 8.13 cm2 V–1 s–1 (Figures 3b and S10), respectively. For the hero device (Figures 3c and 3d), the maximum hole and electron mobilities of 6.87 and 8.49 cm2 V–1 s–1 were demonstrated. To our knowledge, PBPTV represents the first ambipolar polymer semiconductor observed to date that achieved both hole and electron mobilities of above 6.8 cm2 V–1 s–1 simultaneously (Figure S1)[4–10]. It should be mentioned that these ultrahigh ambipolar mobilities were achieved only with a weak dependence of gate voltage, as determined from both positive and negative gate sweeping measurements (Figure S11). To investigate the effect of air humidity on device stability, the hole and electron mobilities of TGBC devices were tested under different relative humidity conditions (RH, 20–98%, Figure S12). We found that our devices showed excellent ambient-stability even under high RH > 90%. After being stored in 98% RH for 24 h, the average hole and electron mobilities of devices still kept at a high level of above 6.6 and 8.0 cm2 V–1 s–1, respectively. Under 20~40% RH, the devices exhibited an unprecedented air-stability, giving a negligible decay in hole and electron mobilities in more than 100 days (Figure S13). The results indicate that, although the LUMO value does not match with the reported requirement of thermodynamically stable electron transport[18], TGBC devices still achieved excellent ambient stability. As a contrast, we pre-

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Journal of the American Chemical Society pared bottom-gate bottom-contact FETs (Figure S14) to elaborate the encapsulation effect of PMMA dielectric layer. The results reveal that thick (≈1350 nm) PMMA layer can establish an effective barrier to H2O and O2, thereby stabilizing transport, especially for electrons[4,15c]. Furthermore, little changes in the transfer curves were observed when the TGBC FET devices were operated for 1000 times (Figure S15).

Figure 3. (a) TGBC device structure. (b) Mobility statistics calculated from 30 devices. (c) Transfer and (d) output curves of the hero FET device, achieving ultrahigh hole and electron mobilities of 6.87 and 8.49 cm2 V–1 s–1, respectively. Surface topography images of PBPTV thin films were investigated by atomic force microscopy (AFM). As shown in Figures 4a and 4b, both film samples (as-cast thin film and 180 °C annealed one) exhibit smooth topography images with very small surface roughness of 0.65 and 1.12 nm, respectively. In general, for topgate FET devices smooth film morphology would facilitate good interface contact, which is critical for achieving high-mobility carrier transport[4,15c]. Thermal annealing leads to larger polycrystalline grains and forming well-connected and ordered sheet-like structures (Figure 4b). These observations show evidence that compared with as-cast thin films, the annealed one obtain higher order bulk organization, thus achieving improved carrier mobilities. The structure order of PBPTV thin films was proved by 2D grazing-incidence wide-angle X-ray scattering (GIWAXS). As shown in Figures 4c and 4e, the primary (100) lamellae stacking peak is located at 0.26 A–1, giving a layer-to-layer distance of 2.41 nm. The crystal coherence length was estimated to be 13.8 nm (calculated by Scherrer’s equation, see SI), corresponding to the crystalline domain with 5−6 stacks. Such a structure feature will make the polymer chain extended in in-plane direction. From the difference in out-of-plane and in-plane scattering intensities (Figures 4c and 4d), one can conclude that polymer chains arrange mainly in an edge-on orientation geometry. However, in in-plane direction, a strong diffusive scattering halo at ~1.49 A–1 was observed, which is larger than liquid type scattering halo from amorphous conjugated polymers. Such a packing mode indicates that in thin film, PBPTV chains adopt a geometry that conjugated plans might form tilted or twisted close contact, which is similar to the reported herringbone-like polymorph[19]. We thus believe that the extended polymer chain and this unique inter-chain contact would facilitate intra-/inter-chains carrier transport. After annealing, the strong scattering halo at ~1.50 A–1 still dominated (Figure 4d). However, annealing treatment led to better lamellae stacking since the out-of-plane (100) coherence length increased to 20.9 nm, which represents 8-9 stacks in high degree ordering (Figure 4f). Consequently, the enhanced lamellae ordering leads to improved carrier transport.

Figure 4. AFM height images and GIWAXS data of the PBPTV thin films deposited on OTS-treated SiO2/Si substrates. (a, c, e) as cast thin films. (b, d, f) 180 °C annealed thin films. In conclusion, a regioregular dual-acceptor strategy has been successfully utilized to develop the first PT-containing ambipolar polymer semiconductor PBPTV. We have demonstrated that the use of strongly electron-withdrawing BPT unit improves electron affinity of PBPTV, and endows it with suitable HOMO/LUMO levels. Investigation of FET performance demonstrates that PBPTV achieves ultrahigh hole and electron mobilities of 6.87 and 8.49 cm2 V–1 s–1, respectively. These findings indicate that our regioregular dual-acceptor strategy is an effective method, which provides a versatile design platform for developing broad electron-transporting polymers in organic electronics. Further investigations on the effect of structural regioregularity, alkyl chain, and molecular weight on carriers transport of BPT-based derivatives are underway.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed experimental procedures, device fabrication, and characterization data, including Table S1 and Figures S1–S22.

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected]; [email protected]

Author Contributions ‡

These authors contributed equally.

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Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

J. Am. Chem. Soc. 2012, 134, 18303. [18] Jones, B.; Facchetti, A.; Wasielewski, M.; Marks, T. J. Am. Chem. Soc. 2007, 129, 15259. [19] Fei, Z.; Chen, L.; Han, Y.; Gann, E.; Chesman, A.; McNeill, C.; Anthopoulos, T.; Heeney, M. Pietrangelo, A. J. Am. Chem. Soc. 2017, 139, 8094.

This work was supported by the National Natural Science Foundation of China (51403177 and 51233006), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030100 and XDPB06), the Research Foundation of Education Bureau of Hunan Province (17B253), and the Science and Technology Planning Project of Hunan Province (2017RS3048). Portions of this research were carried out at beamline 7.3.3 at the Advanced Light Source, Lawrence Berkeley National Laboratory, which was supported by the DOE, Office of Science, and Office of Basic Energy Sciences.

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