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Significant Improvement of Unipolar n-Type Transistor Performances by Manipulating the Coplanar Backbone Conformation of Electron-Deficient Polymers via Hydrogen-Bonding Yang Wang, Tsukasa Hasegawa, Hidetoshi Matsumoto, and Tsuyoshi Michinobu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12499 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019
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Journal of the American Chemical Society
Significant Improvement of Unipolar n-Type Transistor Performances by Manipulating the Coplanar Backbone Conformation of Electron-Deficient Polymers via Hydrogen-Bonding Yang Wang*, Tsukasa Hasegawa, Hidetoshi Matsumoto, and Tsuyoshi Michinobu* Department of Materials Science and Engineering, Tokyo Institute of Technology, 2‐12‐1 Ookayama, Meguro‐ku, Tokyo 152‐8552, Japan KEYWORDS: unipolar n‐type organic transistors, semiconducting polymers, electron transport, conformation anal‐ ysis, noncovalent interactions, hydrogen bonding ABSTRACT: The development of high‐performance unipolar n‐type semiconducting polymers still remains a significant challenge. Only a few examples exhibit a unipolar electron mobility over 5 cm2 V–1 s–1. In this study, a series of new poly(benzothiadiazole‐naphthalenediimide) derivatives with the high unipolar electron mobility (μe) up to 7.16 cm2 V–1 s–1 in thin film transistors are reported. The dramatically increased μe is achieved by finely optimizing the coplanar backbone conformation through the introduction of vinylene bridges which can form intramolecular hydrogen bonds with the neighboring fluorine and oxygen atoms. The hydrogen bonding functionalities are fused to the backbone to ensure a much more planar conformation of the conjugated π‐system, as demonstrated by the density functional theory (DFT)‐ based calculations. The theoretical prediction is in good agreement with the experimental results. As the coplanarity is promoted by the hydrogen bonding, the thin‐film crystallinity and molecular packing strength are also improved, which is evidenced by the synchrotron two‐dimensional grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) and atomic force microscopy (AFM) measurements. Notably, the GIWAXS measurements reveal an extremely short π‐π stacking dis‐ tance of 3.40 Å. Overall, this study marks a significant advance in the unipolar n‐type semiconducting polymers and offers a general approach for further increasing the electron mobility of semiconducting polymers in organic electronics.
INTRODUCTION Since the first solution‐processed polymer light‐ emitting‐diode based on a poly(phenylenevinylene) (PPV) derivative in 1990, semiconducting polymers have attract‐ ed more and more attention due to their vast application possibilities in flexible, light‐weight, and large‐area or‐ ganic electronics.1,2 Among the organic electronic devices, polymer thin‐film transistors (PTFTs) are of special inter‐ est as the basic elements of circuits for their potential use in flexible displays and skin electronics.3 According to the polarity of the dominant charge carriers in the transistor channel, semiconducting polymers can be typically classi‐ fied as p‐type (hole dominant), n‐type (electron domi‐ nant), and ambipolar (both carriers). Significant efforts have been devoted to the development of p‐type semi‐ conducting polymers with the state‐of‐the‐art hole mobil‐ ities over 10 cm2 V–1 s–1, which are superior to those of amorphous silicon‐based thin film transistors.4 Compared to the well‐developed p‐type semiconduct‐ ing polymers, however, the emergence and development of complementary n‐type (electron‐deficient) semicon‐ ducting polymers have been slow with the highest unipo‐ lar electron mobility of 6.50 cm2 V–1 s–1.5 This is mainly because the development of high‐performance n‐type semiconducting polymers is still highly challenging due to
the following reasons: (1) Synthetic difficulty in develop‐ ing n‐type semiconducting polymers with deep lowest unoccupied molecular orbital (LUMO) energy levels, which is due to a lack of strong electron‐deficient mono‐ mers and suitable polymer design strategies; and (2) Lim‐ ited stability of the devices under ambient conditions. Since most electron‐deficient polymers are vulnerable to water and oxygen under actual device operating condi‐ tions, long‐term stable n‐type PTFTs using a top con‐ tact/bottom gate device structure without any encapsula‐ tion are rarely reported. Despite these issues, n‐type sem‐ iconducting polymers are highly desired as electron‐ transporting materials for realizing potential p‐n‐ junction‐based flexible and wearable optoelectronic de‐ vices, such as complementary metal‐oxide‐semiconductor (CMOS)‐like logic circuits, all‐polymer solar cells (all‐ PSCs), organic photodetectors (OPDs), and organic ther‐ moelectrics (OTEs).6,7 Therefore, how to effectively devel‐ op high‐mobility n‐type (electron‐deficient) semicon‐ ducting polymers with a long‐term stability remains a fundamental issue in this research field.8 From the viewpoint of the energy levels, an effective approach to achieve promising n‐type semiconducting polymers is to deepen their LUMO energy levels to near or below −4.0 eV.9 Therefore, the following two effective molecular design strategies could be adopted: (1) to intro‐
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Figure 1. (a) Examples of high‐performance n‐type (ambipolar) semiconducting polymers using the recently developed strategy of “dual‐acceptor”; (b) Manipulation of the backbone planarity via hydrogen bonding in this study, as demonstrat‐ ed by the optimized geometry of BBTV, BBTV‐F, NDI‐T, and NDI‐V‐T units using the DFT B3LYP/6‐31G(d) level of theory; (c) Chemical structures of the new n‐type copolymers developed in this study.
duce electron‐withdrawing groups (EWGs, such as fluo‐ rine, sp2‐nitrogen, and cyano groups) into the D‐A poly‐ mer backbones; and (2) to introduce two or more accep‐ tor building blocks in one polymer repeat unit.10 While the former approach may readily achieve high electron mobilities by modifying the backbone structures of previ‐ ously reported p‐type D‐A polymers with high hole mo‐ bilities, the latter will benefit the straightforward synthe‐ sis by using cross‐coupling reactions, as represented by the “acceptor dimerization” strategy (Figure 1a left).11 During the course of our study and manuscript prepa‐ ration, several polymers using the “dual‐acceptor” strate‐ gy (such as “D‐A‐A” or D‐A1‐D‐A2 backbone, Figure 1a and Figure S1) were reported.11‐13 For example, bis(pyridal[2,1,3]thiadiazole) (BPT), which is a dimeric form of the pyridalthiadiazole (PT) unit (Figure 1a)11, pro‐ duced a semiconducting polymer (PBPTV) with a high electron affinity and extended π‐conjugation. As a conse‐ quence, PTFTs based on PBPTV exhibited excellent elec‐ tron‐dominant ambipolar transistor performances with the hole/electron mobilities of 6.87/8.94 cm2 V−1 s−1. As another example, a D‐A1‐D‐A2 type electron‐deficient copolymer composed of naphthalenediimide (NDI) and the benzobisthiadiazole derivative (SN), namely pSNT, demonstrated a high EA and remarkable unipolar elec‐ tron mobility of 5.35 cm2 V−1 s−1 (Figure 1a).13 On the other hand, the backbone coplanarity of poly‐ mers also plays a critical role in their opto‐electrical prop‐ erties and film morphologies, which exert a profound im‐ pact on the performance of organic electronics. Generally speaking, n‐type polymers more easily suffer from the
problem of “nonplanar backbones”. This is because the electron‐withdrawing substituents (e.g., C=O and C≡N groups), which can enable n‐type charge transport prop‐ erties, are usually bulky and may induce a large steric hindrance on their neighboring subunits.14 This issue be‐ came even more severe in the cases of the recently devel‐ oped A‐A type homopolymers or “dual‐acceptor” type polymers.11,13 For instance, density functional theory (DFT) calculations suggested that the backbones of the above “dual‐acceptor” type polymers, PBPTV and pSNT, had the relatively large dihedral angles (θ) of 25° and 40°, respec‐ tively (Figure 1a and Figure S1).11,13 Consequently, the twisted polymer backbones would have potentially short‐ ened the coherent conjugation length, hampered the in‐ trachain delocalization of the HOMO/LUMO orbitals, and inhibited interchain carrier transport as a result of a poor molecular packing and low thin‐film crystallinity.14 One strategy to solve the aforementioned “nonplanar backbone” issue of the state‐of‐the‐art n‐type polymers is to design new acceptor units based on five‐membered‐ ring (e.g., thiophene)‐fused structures. For example, Guo et al. reported a series of n‐type polymers based on the bithiophene imide (BTI) or thiazolothienyl imide (TzTI) units, which showed the μe values up to 3.71 and 1.61 cm2 V−1 s−1, respectively (e.g., polymers PBTI and PDTzTI, see Figure 1a).15,16 These high electron mobilities benefited from the minimized steric hindrance between the adja‐ cent BTI or TzTI units since the electron‐withdrawing imide group is located on the center of the bithiophene or thiazolothiophene moieties, which provides enough free space for their neighboring co‐monomers (Figure 1a).
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Scheme 1. Synthesis of the monomers and copolymers.
However, to obtain these fused ring structures, a compli‐ cated molecular design and tedious synthesis are often needed. Another approach to enhance the coplanarity of the polymer backbones is to simply introduce vinylene spacers. For example, Yang et al. reported that a vinylene‐ bridged bis(benzothiadiazole) (BBTV) derivative shows a planar backbone and promising electron transport prop‐ erties when incorporated into the PDPP‐BBTV copoly‐ mers (Figure 1a).17 Nonetheless, the vinylene spacers were, in most cases, introduced into the donor segments (Fig‐ ure S2a), and vinylene‐incorporated acceptor segments were rarely reported.18 For example, the key acceptor unit of 1,2‐bis(7‐bromobenzo[c][1,2,5]thiadiazol‐4‐yl)ethene was not readily obtained by the one‐pot Stille coupling even when a large excess of 4,7‐ dibromobenzo[c][1,2,5]thiadiazole was employed (
