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Easily solution-processed, high-performance microribbon transistors based on a 2D condensed benzothiophene derivative†

Cite this: Chem. Commun., 2014, 50, 442 Received 5th October 2013, Accepted 30th October 2013

Xiaoxia Liu,‡ab Yingfeng Wang,‡a Jianhua Gao,*a Lang Jiang,*b Xiangye Qi,a Wanglong Hao,a Sufen Zou,a Haixia Zhang,a Hongxiang Li*c and Wenping Hu*b

DOI: 10.1039/c3cc47646d www.rsc.org/chemcomm

A 2D condensed benzothiophene derivative TBTDBT was synthesized. The thermal, optical and electrochemical properties were investigated. The single crystalline microribbons were grown by solution drop-casting and physical vapor transport methods respectively. The field effect transistors based on TBTDBT microribbons were fabricated and a mobility up to 2.62 cm2 V

1

s

1

and an on–off ratio greater than 105

could be achieved.

Recently, organic field-effect transistors (OFETs) have attracted considerable attention as an alternative to conventional silicon-based transistors because they can be potentially fabricated at low cost, in large areas and on flexible substrates.1 To realize these advantages in commercial application, the performance of OFET devices such as mobility and stability should be comparable with that of amorphous hydrogenated silicon. Therefore, although significant achievements have been made in the exploration of functional organic materials for OFETs and dozens of organic semiconductor materials exhibited high mobility over 1.0 cm2 V 1 s 1, to optimize the device fabrication techniques and develop new stable organic semiconductor materials with high charge carrier mobility are still intriguing challenges for OFETs.2 As we know, organic single crystals are regarded as important candidates for the fabrication of high performance devices because they are free of grain boundaries, possess few defects, and long-range order molecular packing.3 Different from inorganic semiconductors, the growth of organic crystals usually

a

Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 311121, P. R. China. E-mail: [email protected]; Fax: +86-571-28865135; Tel: +86-571-28865135 b Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: [email protected], [email protected]; Tel: +86-10-82615030 c Laboratory of Materials Science, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China. E-mail: [email protected]; Tel: +86-21-54925024 † Electronic supplementary information (ESI) available: Synthesis details and characterization data. See DOI: 10.1039/c3cc47646d ‡ Xiaoxia Liu and Yingfeng Wang contributed equally to this work.

442 | Chem. Commun., 2014, 50, 442--444

includes vapor-processed and solution-processed techniques. The vapor-processed techniques such as physical vapor transport are widely used to grow crystals of oligomers and low molecular weight materials. Compared with the vapor process, the solution process is lower cost and easier to carry out. It is usually performed by cooling the solution, evaporating the solvent or adding another poor solvent into the organic semiconductor solution. However, only a few organic materials reported high OFET performance based on solution processed single crystal devices due to their poor solubility which limits the application of this method.4 The design and synthesis of novel materials that possess high stability and mobility as well as easily growing crystals by the solution process are effective approaches for achievement of high performance OFETs. Currently, despite most effort being focused on the one dimensional (1D) linear p-conjugated molecule modulation,2 two dimensional (2D) p-conjugated molecules also attracted more and more interest because of their tendency to aggregate by p–p stacking which is favorable for intermolecular charge-transport.5 Important examples of 2D p-conjugated molecules include discotic molecules such as triphenylene6 and hexabenzocoronene (HBC) derivatives,7 star-shaped oligomers,8 phthalocyanines,9 rylene dyes,10 tetrathienoanthracenes,11 sulflower,12 etc. However, very few 2D p-conjugated molecules afford good single crystal OFET performance by the solution process. Herein, we report the synthesis and characterization of a novel butterfly 2D p-conjugated molecule 1,2,3,4,5,6,7,8-tetra(benzothieno)dibenzothiophene (TBTDBT, 4). The single crystalline microribbons of TBTDBT were grown using solution drop-casting and physical vapor transport methods, respectively, and their applications in OFET devices were also presented. TBTDBT was synthesized through three simple and controlled steps from commercially available benzo[b]thiophene as starting material (Scheme 1). Benzo[b]thiophene was lithiated and reacted with isopropoxyboronic acid pinacol ester to give 2 according to a known procedure described in the literature.13 An improved yield was obtained through using the dry THF as the solvent. Then 3 was prepared by Suzuki coupling reaction between tetrabromothiophene and quadruple 2. After oxidative cyclization with ferric chloride,11 a 2D fused

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Fig. 1 (a) TEM image of a single-crystal TBTDBT microribbon and (b) its corresponding electron diffraction pattern. (c) Micrograph of TBTDBT microribbons and the electrode formed by an organic wire mask. (d) XRD patterns of TBTDBT microribbons on an OTS modified SiO2. Scheme 1

Synthesis of TBTDBT.

ring compound TBTDBT was constructed via thienyl–thienyl carbon– carbon bond formation. The product was purified by extraction from chloroform to give pure TBTDBT as a yellow powder in 82% yield. The thermal and optical properties were measured by thermal gravimetric analysis (TGA) and UV-vis spectroscopy, respectively. A decomposition temperature of over 500 1C was determined (Fig. S1, ESI†), which indicates that TBTDBT exhibits very high thermal stability. The UV-vis spectra of TBTDBT in CH2Cl2 solution and a solid thin film are shown in Fig. S2 (ESI†). The solution of TBTDBT shows four absorption peaks at 399, 386, 334 and 242 nm. The maximum absorption peak of the vacuum-deposited thin film on the quartz substrate is red-shifted about 17 nm compared with that in solution, suggesting that strong intermolecular interactions occurred in the solid state. Performing UV-vis absorption measurement for the TBTDBT thin film with similar initial optical densities (OD), no decline of the intensity of absorption is observed after nearly one month under ambient conditions, further demonstrating the high stability of TBTDBT. The optical energy bandgap of TBTDBT estimated from the UV-vis absorption onset was 2.90 eV, indicating high photostability. The cyclic voltammetry of TBTDBT was done in CH2Cl2 by using ITO as a working electrode and Ag/ACl as reference (Fig. S3, ESI†). TBTDBT showed two irreversible oxidation waves (1.47 V, 1.70 V). The HOMO level of TBTDBT determined by using the onset position (1.19 V) of the oxidation peak according to the literature14 was 5.69 eV. The low-lying HOMO levels confirm the high oxidation stability of TBTDBT. To gain deeper insight into the electronic structures of the polycyclic aromatic TBTDBT, molecular-orbital (MO) calculations of the HOMO and LUMO levels were performed by using the density functional theory (DFT) method (B3LYP, 6-31G(d,p)) (Fig. S4, ESI†). The theoretical HOMO ( 5.45 eV) level is close to that estimated by cyclic voltammetry. However, the theoretical bandgap (3.55 eV) predicted from vertical transitions is significantly larger than the optical bandgap over 0.65 eV.

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Single-crystalline microribbons of TBTDBT were prepared by drop casting its solution of chlorobenzene onto the n-octadecyltrichlorosilane (OTS) modified SiO2 substrates. Similar microribbons were also obtained by physical vapor transport method under an argon atmosphere. Bright field transmission electron microscopic (TEM) observation of the TBTDBT microribbons (Fig. 1a) revealed that the microribbons exhibit regular shape. The corresponding electron diffraction pattern (Fig. 1b) shows sharp and well-defined reflection spots, this confirms the single crystalline structure of the microribbons. Furthermore, we used optical microscopy (OM) and X-ray diffraction (XRD) measurement to elucidate the structure of TBTDBT microribbons. The image of microribbons deposited on the OTS/SiO2/Si substrate and the electrode formed by an organic wire mask is shown in Fig. 1c, and the microribbons exhibited good long range regularity with a length of over 20 mm. The XRD profile revealed a single diffraction peak at 2y = 5.98 degree, which also indicates that the microribbons are high degree crystalline structures. A d spacing of 14.76 Å determined by XRD measurements is very close to the length of TBTDBT molecules, which suggests that the molecules are nearly oriented perpendicular to the substrate and the p–p stacking direction is parallel to the substrate. The field-effect transistors were fabricated using conventional techniques by using doped n-type Si as the gate electrode, Au as both source and drain electrodes and OTS modified SiO2 as the dielectric layer. The drain–source (D–S) contacts were fabricated by thermally evaporating a thin layer of Au onto the individual TBTDBT single crystalline microribbon which is supported by the OTS/SiO2/Si substrate and using the ‘‘organic ribbon mask technique’’15 at a vacuum pressure of 1.0  10 4 Pa. The OFET devices were measured in air, and the corresponding output and transfer characteristics are depicted in Fig. 2. The OFET devices based on the TBTDBT single crystalline microribbons which are grown by the solution method exhibited typical p-type semiconducting behaviors and the average hole mobility is as high as 1.14 cm2 V 1 s 1 for the fabricated ten

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Notes and references

Fig. 2

Transfer (top) and output (down) characteristics for OFETs of TBTDBT.

transistors. The highest mobility of up to 2.12 cm2 V 1 s 1 and an on–off ratio greater than 105 could also be achieved. The high OFET performance may result from the strong p–p stacking interactions between the 2D p-conjugated molecules. In comparison to the solution processed method, we also fabricated the OFET devices based on the TBTDBT single crystalline microribbons grown by physical vapor transport method. The devices exhibited the highest hole mobility of 2.62 cm2 V 1 s 1 and an on–off ratio greater than 105. These values are not much higher than those of solution drop-casting single crystal devices, which reflected that the TBTDBT was an excellent candidate for fabricating the single crystalline OFET devices by the solution processed method. In summary, a 2D condensed benzothiophene derivative TBTDBT was synthesized through three simple and controlled steps. This material possessed high stability which was confirmed by TGA, UV-vis spectroscopy and cyclic voltammetry. Moreover, the single crystalline microribbons were grown by solution drop-casting and physical vapor transport methods, respectively, and successfully applied to transistors. A mobility of up to 2.62 cm2 V 1 s 1 and an on–off ratio greater than 105 could be achieved. The strong p–p stacking interactions between the 2D molecules may contribute to the high performance. Further correlation between packing tendency and molecular dimension is underway. The present research was financially supported by the National Natural Science Foundation of China (51003022, 21272049), the Ministry of Science and Technology of China (2013CB933500) and HZNU (HSKQ0050).

444 | Chem. Commun., 2014, 50, 442--444

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