Asymmetric Conjugated Molecules Based on [1] Benzothieno [3, 2-b][1

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Asymmetric Conjugated Molecules Based on [1]Benzothieno[3,2b][1]benzothiophene for High-Mobility Organic ThinFilm Transistors: Influence of Alkyl Chain Length Keqiang He, Weili Li, Hongkun Tian, Jidong Zhang, Donghang Yan, Yanhou Geng, and Fosong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10675 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Applied Materials & Interfaces

Asymmetric Conjugated Molecules Based on [1]Benzothieno[3,2b][1]benzothiophene for High-Mobility Organic Thin-Film Transistors: Influence of Alkyl Chain Length Keqiang He,1,2 Weili Li,1 Hongkun Tian,*, 1 Jidong Zhang,1 Donghang Yan,1 Yanhou Geng,*, 1,3,4 and Fosong Wang1 1

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. 2 University of Chinese Academy of Sciences, Beijing 100049, P. R. China 3 School of Materials Science and Engineering and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072, P. R. China 4 Collaborative Innovation Center of Chemical Science and Engineering (Tianjin)Tianjin 300072, P. R. China E-mail: [email protected]; [email protected]

KEYWORDS: organic semiconductors, asymmetric, organic thin-film transistors, mobility, morphology

ABSTRACT: We herein report the synthesis and characterization of a series of [1]benzothieno[3,2-b][1]benzothiophene (BTBT)-based asymmetric conjugated molecules, i.e., 2-(5-alkylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-Tn, in which T and n represent thiophene and the number of carbons in the alkyl group, respectively). All the molecules with n ≥ 4 show mesomorphism and display smectic A, smectic B (n = 4) or smectic E (n > 4) phases and then crystalline phases in succession upon cooling from the isotropic state. Alkyl chain length has a noticeable influence on the microstructures of the vacuum-deposited

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films and therefore on the performance of the organic thin-film transistors (OTFTs). All molecules except for BTBT-T0 and BTBT-T2 showed OTFT mobilities above 5 cm2 V-1 s-1. BTBT-T6 and BTBT-T7 showed the greatest OTFT performance with reliable hole mobilities (µ) up to 10.5 cm2 V-1 s-1 since they formed highly ordered and homogeneous films with diminished grain boundaries.

INTRODUCTION Organic thin-film transistors (OTFTs) have attracted much attention in recent years because of their applications in low-cost and flexible electronics, such as electronic papers, radio frequency identification (RFID) tags and sensors.1-3 Because great effort has been devoted to the optimization of both the molecular design and fabrication processes, OTFT performance has improved tremendously in the past few years, and some materials, including small molecules and polymers, with mobilities above 8 cm2 V-1 s-1 have been reported.4-17 To achieve high OTFT mobility, organic semiconductors should not only have small reorganization energies and closely packed motifs for strong electronic interactions in multiple directions but also must be capable of forming homogeneous and defect-free films with diminished grain-boundaries.1, 18-20 [1]Benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives are characterized by low reorganization energies and large transfer integrals along different directions within the crystals.21-23 However, asymmetric conjugated molecules have recently attracted much attention because they have the advantage be being able to form large, uniform, smooth, crystalline films.4, 24-28 For example, vacuum deposited films of a monoalkylated BTBT derivative (mono-C13-BTBT) on substrates covered with ultrathin AlOx showed large, smooth grains in which the BTBT cores were densely packed.4 As reported by Hanna et al., mono-aryl


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(phenyl or thienyl)-substituted BTBT derivatives, i.e., 8-TP-BTBT (BTBT-T8 in the current paper)28 and Ph-BTBT-10,24 both exhibited smectic E (SmE) phases and can form uniform and molecularly flat polycrystalline films on bare SiO2/Si substrates through spin-coating in the temperature range of the SmE phase to afford OTFTs with superior device performance. However, systematic investigations into the relationship between molecular structures, thin-film microstructures and device performance in these types of materials are still highly desirable for offering guidelines for molecular design.22, 23 Therefore, in the current paper, we synthesized a series of asymmetric BTBT-based molecules, i.e., 2-(5-alkylthiophen-2-yl)[1]benzothieno[3,2b][1]benzothiophene (BTBT-Tn), in which T and n represent thiophene and the number of carbons in the alkyl group, respectively. The introduction of a thiophene ring between the BTBT core and alkyl chain could extend the conjugation, and this would both enhance intermolecular interactions and elevate the highest occupied molecular orbital (HOMO) energy level, which would reduce the hole-injection barrier between the electrodes and the semiconducting material. The influence of alkyl chain length on the crystal structures and thermal, electrochemical and charge carrier transport properties as well as thin-film microstructures was studied in detail. RESULTS AND DISCUSSION Synthesis. As shown in Scheme 1, BTBT-Tn molecules were synthesized via Stille crosscoupling reactions between 2-alkyl-5-tributylstannyl-thiophene and 2-iodo-BTBT29 in yields over 70%. All the compounds were purified by column chromatography on silica gel using petroleum ether as the eluent followed by recrystallization from toluene and then vacuum sublimation prior to characterization. Their chemical structures were confirmed by 1H NMR spectroscopy, elemental analysis and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (See Figures S24-32 in the Supporting Information (SI)).


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Scheme 1. Chemical structures and synthetic route of BTBT-Tns. Crystal Structure Analyses. Flake-like single-crystals of BTBT-T2, BTBT-T4 and BTBT-T7 for X-ray crystal structure analysis (CCDC: 1562813-1562815) were obtained from slow evaporation of solvent. All the crystallographic data are provided in Table S1. The thiophene groups form dihedral angles of ~ 10° with respect to the BTBT plane in these molecules, which is smaller than the angles (~22°) between phenyl rings and BTBT in Ph-BTBT-Cns (n = 5, 6, 8 and 10).22 These angles indicate that BTBT-Tns have better conjugation and stronger intermolecular interactions. All three molecules crystallize into triclinic (P-1) structures. As shown in Figure 1, they all have isomorphous bilayer-type crystal structures consisting of a headto-head alignment of asymmetric molecules in the unit cell, while the long axes of the molecules align nearly perpendicular to the layers. This type of packing structure is also found in PhBTBT-Cn (n ≥ 5), mono-Cn-BTBT (n ≥ 4) and BTTT-T-C12.22, 23, 27 However, in the current system, ethyl is long enough to force the molecules to form the aforementioned bilayer packing structures. BTBT-T moieties form a herringbone (HB) packing motif in the ab plane with a Tshaped HB angle of approximately 52° (determined from the dihedral angle between the leastsquares planes of the nearest neighbors including all non-hydrogen atoms in one molecule, as shown in Figure S1). The intermolecular short S···S and S···C contacts were found for all three molecules as outlined in Figure S1 and Table S2. With increasing n, the S···S contacts at the slipped parallel positions become shorter, but the S···C contacts do not exhibit a clear trend in variation. The intermolecular short S···S and S···C contacts between adjacent molecules for


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BTBT-T2, BTBT-T4 and BTBT-T7 are quite dense. In addition, from BTBT-T2 to BTBT-T4 and then to BTBT-T7, the centroid distances decrease at both the slipped parallel and T-shaped contacts, and the slipped distances between the conjugated cores along the short axes become shorter. These structural variations lead to a slight decrease in the volume of the HB packing of the conjugated cores (see Table S1) meaning the conjugated cores are packed more tightly, which can be attributed to the fastener effect of the long alkyl chain substituents.30-32 We further compared the difference in HB packing among BTBT-T7, Ph-BTBT-C1033 and mono-C9BTBT,23 and we found that BTBT-T7 has the shortest S···S contacts, centroid distances both at the slipped parallel and T-shaped contacts and slipped distance along the short axes (see Table S2). This indicates that it may have strong intermolecular electronic coupling. Powder X-ray diffractions show that other BTBT-Tns with n ≥ 2 also adopt bilayer packing structures in the solid state (Table S7).

Figure 1. Crystal packing structures of BTBT-Tns (n = 2, 4 and 7) projected along the molecular long axis (top) and the ab plane (bottom).


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Thermal and Electrochemical Properties. The thermal properties of the BTBT-Tns were investigated by thermogravimetric differential thermal analysis (TG-DTA) and differential scanning calorimetry (DSC) as shown in Figure S2. All the molecules showed 5% weight loss temperatures above 280 °C, which were attributed to sublimation. No phase transitions were found for BTBT-T0 because it sublimed before its phase transition occurred. For BTBT-T2, a single endothermic peak corresponding to the isotropic liquid-crystal transition was observed at 221 °C. Three endothermic peaks appeared for BTBT-Tn with n ≥ 4, suggesting that, before moving to the crystal phase, the isotropic liquid phase underwent two liquid crystal phase changes, which were identified using a polarized optical microscope (POM) and temperaturedependent X-ray diffraction (XRD) (Figure S3-S5). The change in phase transition temperatures as a function of the number of carbons in the alkyl chain is depicted in Figure 2. As the length of the alkyl chain increased, the transition temperatures decreased gradually, which can be attributed to the more dramatic thermal-induced motion of the alkyl chains.22 The hightemperature liquid crystalline phase, assigned to be SmA, was confirmed by the presence of the smooth, focal conic, fan-shaped textures in the POM images and a diffraction peak at ~3° (with d-spacing corresponding to the molecular length) in the XRD patterns.34 Upon further cooling, an SmB phase34 was observed for BTBT-T4 as indicated by the mosaic texture along with one peak at ~18° in the XRD pattern attributed to the (110) diffraction and two peaks at 4 and 8° from the ordered smectic layer (Figure S5a), suggesting the molecules have an intermolecular distance of 4.7 Å in the hexagonal lattice. Like Ph-BTBT-Cn (n ≥ 5),22, 24 BTBT-Tn (n ≥ 5) also exhibited an SmE phase34 as indicated by the fan-like texture with disclination lines and three peaks at ~18°, 21° and 26° in the XRD patterns, which can be attributed to the rectangular structure, together with a primary peak at a small angle of ~3° originating from the smectic layer.


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250

o

Temperature ( C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


crystal SmE SmB SmA

200

150

100

2 4 5 6 7 8 12 16 Number of carbons in the alkyl chains

Figure 2. Temperature ranges of different phases of BTBT-Tns. Pink, cyan, red and green columns represent temperature ranges of crystalline, SmE, SmB and SmA phases, respectively. The electrochemical properties of BTBT-Tn were evaluated by cyclic voltammetry in an anhydrous dichloromethane solution (BTBT-T7 is shown as an example in Figure S6). The HOMO energy levels were approximately -5.5 eV as calculated from the oxidation onset potentials versus Fc/Fc+ (Table S3), which revealed the weak influence of alkyl chain length on the frontier orbital energy levels. For comparison, 2,7-dihexyl-BTBT (di-C6-BTBT) was also measured, and its HOMO energy level was estimated to be -5.65 eV, which is similar to the value reported in the literature.35 As expected, the introduction of an additional thiophene unit can elevate the HOMO energy level. HOMO energy levels of BTBT-T7, Ph-BTBT-C10,33 diC8-BTBT36 and mono-C9-BTBT23 were also estimated by density functional theory (DFT) calculations using the conformations obtained from single-crystal structural analysis. Among the four molecules, BTBT-T7 showed the highest-lying HOMO energy level (calculated HOMO levels of BTBT-T7, Ph-BTBT-C10, di-C8-BTBT and mono-C9-BTBT are -5.21, -5.31, -5.33 and -5.40 eV, respectively.).


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Charge transport properties. OTFT devices with BTBT-Tns (n ≥ 0) were fabricated on octadecyltrimethoxysilane (OTMS)-modified SiO2 (200 nm)/Si substrates (SubOTMS) by vacuum deposition. The substrate temperature (TS) for active layer deposition was optimized for each molecule. OTFT performance was measured under ambient conditions, and the hole mobility (µ) was extracted from transfer curves in a saturation regime according to the widely used gradual approximation model.37 For the as-fabricated devices, although standard output characteristics can be observed, bending downward of square root of drain current (ID) against gate voltage (VG), i.e., a double slope, was observed as shown in Figure 3 and Figure S7. This non-ideal behavior is often observed in high mobility organic field-effect transistors.38-42 In this case, the mobility values extracted from the transfer curves in a saturation regime at low VG are incorrect or significantly overestimated as reported in the literature regarding the extraction of mobility values.41 However, to compare our values with those in the literature, we still calculated hole mobility at low VG (µ1) and at high VG (µ2), and those values are outlined in the SI (Table S4). Since the properties of the semiconductor/dielectric interface have an influence on this non-ideal electrical behavior,39, 43 we selected poly(α-methylstyrene) (PαMS) (Mw = 700 kDa, PDI = 1.08) to modify the substrates (SubPS) for fabricating OTFT devices with BTBT-Tns (n = 2, 4, 7, 16) because polymeric ultrathin films are an alternative to fully passivate SiO2 dielectric surfaces.44 As shown in Figure S8, the double slope behavior of ID against VG was still significant. This indicates that the interface effects may not be the main cause of the non-ideal electrical characteristics of the devices.


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Figure 3. Output (a, e, and i) and transfer (b, f, and j) curves, mobility as a function of gate bias (c, g, and k), and normalized resistances (d, h, and l) with and without annealing of BTBT-T2 (a, b, c, and d), BTBT-T7 (e, f, g, and h) and BTBT-T16 (i, j, k, and l)-based OTFT devices on SubOTMS. Inset values in the transfer curves plots are the mobilities of the devices with thermal annealing. 12 9

2

Mobility (cm / V⋅s)

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ID (mA)

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6 3 0

4 5 6 7 8 12 16 Number of carbons in the alkyl chains

Figure 4. Changes in device mobility as a function of the number of carbons in the alkyl chain on SubOTMS with annealing at 40 °C for 2 h under ambient conditions.


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Table 1. OTFT device performance of BTBT-Tns (n ≥ 2) deposited on SubOTMS at optimal substrate temperatures (TS) with annealing at 40 °C for 2 h.

a

Compounda

TS (°C)

µmax (cm2 V-1 s-1)

µave ± σ (cm2 V-1 s-1)

VT (V)

Ion/off

BTBT-T2

40

0.48

0.47 ± 0.08

0 – -5

106

BTBT-T4

80

6.7

6.2 ± 0.47

-9 – -13

107

BTBT-T5

80

8.2

7.4 ± 0.51

-7 – -10

107

BTBT-T6

80

10.5

8.7 ± 0.83

-3 – -13

107

BTBT-T7

80

9.9

8.6 ± 0.72

0 – -8

107

BTBT-T8

80

9.4

8.1 ± 0.83

-10 – -15

107

BTBT-T12

80

9.4

7.7 ± 0.88

-8 – -15

107

BTBT-T16

100

9.3

7.5 ± 1.1

-7 – -13

107

data were obtained from 15 devices. σ represents the standard derivation. Finally, we found that post-annealing is an effective way to improve transfer characteristics,45

much like with C10DNTT-based OTFTs. Therefore, OTFT devices with BTBT-Tns (n ≥ 2) fabricated on SubOTMS were annealed at 40 °C for two hours under ambient conditions, and the corresponding transfer curves and the dependence of mobility on VG are depicted in Figure 3 and Figure S7. As expected, the relationship between the square root of ID and VG was almost linear, and the values of VT shifted to ca. -10 V. Meanwhile, the mobility was almost independent of VG. These results were similar to what was observed by Takeya and Bittle.45, 46 It should be noted that the resultant mobility with annealing was comparable to that extracted from the high VG region without annealing as shown in Table 1 and Table S4. The mobilities of BTBT-Tns showed a weak dependence on alkyl chain length as shown in Figure 4. Devices based on BTBTT2 showed the lowest µave (0.47 cm2 V-1 s-1). With increasing n, µave increased to 6.2, 7.4, 8.7 and


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8.6 cm2 V-1 s-1 for BTBT-T4, BTBT-T5, BTBT-T6 and BTBT-T7, respectively. The highest mobilities of BTBT-T6 and BTBT-T7 were ca. 10 cm2 V-1 s-1. Further increasing the alkyl chain length resulted in a slight decrease in µave to 8.1, 7.7 and 7.5 cm2 V-1 s-1 for BTBT-T8, BTBTT12 and BTBT-T16, respectively. No odd-even effect was observed in the mobilities of the molecules. To understand above results, contact resistance (RC) with and without annealing was measured using a transmission line method (TLM).48 As depicted in Figure 3 and Figure S9, with annealing, RC in the low VG region decreased, and RC values in a wide VG range were approximately one order of magnitude smaller than RCHs. This indicated that contact effects have a negligible influence on mobility extractions as shown by Takeya and Bittle.45-47 Therefore, the aforementioned improvement should be attributed to the reduction in RC in the low VG region, and the mobilities extracted from the transfer curves of the annealing devices can demonstrate the meaningful values of charge transport. We also annealed the devices made by coating BTBTT7 onto SubPS under the same conditions. The transfer curves become more ideal and the contact effects become much weaker as shown in Figures S8, S12 and S13. A µave of 6.6 cm2 V-1 s-1 and a maximum value of 7.8 cm2 V-1 s-1 were obtained (Table S5). For comparison, we also fabricated OTFT devices based on mono-C13-BTBT4 with our device structure and using our fabrication conditions. As shown in Figure S14 and Table S6, mobility was significantly dependent on VG even after thermal annealing. The devices exhibited a peak mobility of 13-15 cm2 V-1 s-1, and the mobility values decreased to 7-10 cm2 V-1 s-1 at VG of ~ -60 V. In addition, OTFTs based on mono-C13-BTBT had much higher VT values (ca. -35 V versus ca. -10 V). These results indicate that BTBT-T6 and BTBT-T7 have charge carrier transport properties comparable to mono-C13BTBT, and confirm that introducing a thiophene ring is beneficial for charge injection due to the elevation of the HOMO energy level.


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Intensity (a.u.)


14

16 18 -1 qxy (nm )

20

500 0

14

16 -118 qxy (nm )

20

Figure 5. Out-of-plane XRD patterns (a-c) and in-plane line cuts from 2D-GIXD patterns (d-f) on SubOTMS of BTBT-Tns with n = 2 (a and d), 7 (b and e) and 16 (c and f) thin films obtained at the optimal substrate temperature. TS: 40 °C (BTBT-T2), 80 °C (BTBT-T7) and 100 °C (BTBTT16). Film microstructures and morphologies. Film microstructures and morphologies are critical for OTFT device performance and were therefore studied by using XRD and atomic force microscopy (AFM). We first studied thin films with ca. 30 nm of BTBT-Tns deposited on SubOTMS. All of them have well-ordered structures in which the molecules adopt an edge-on arrangement in the substrates as indicated by XRD measurements. As examples, the out-of-plane XRD patterns of thin films of BTBT-T2, BTBT-T7 and BTBT-T16 are shown in Figure 5, and those of the other compounds are displayed in Figure S15. For BTBT-T0 and BTBT-T2, the diffraction peaks up to the 4th and 3rd orders are clearly visible, and the interlayer distances (dspacings) calculated from the primary peaks are 15.4 and 17.6 Å (at 2θ = 5.8 and 5.0°), respectively, which correspond to their molecular lengths listed in Table S7. Unlike the bilayer structure in the bulk state, BTBT-T2 adopted monolayer packing in the thin film, which may be because short ethyl chains cannot provide enough interchain order in the film-forming process,


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which is faster than single-crystal growth. With increasing alkyl chain length, the film order and crystallinity were enhanced as indicated by the presence of high-order diffraction peaks (up to 8th order). The d-spacings of primary peaks of BTBT-Tn (n ≥ 4) are 40.0 (n = 4), 41.8 (n = 5), 44.2 (n = 6), 45.8 (n = 7), 48.8 (n = 8), 58.5 (n = 12) and 68.4 Å (n = 16), which are close to twice the lengths of the corresponding molecules (Table S7). This bilayer structure is consistent with that observed in single-crystal structural analysis for BTBT-T4 and BTBT-T7. Hanna et al. suggested that this bilayer structure, which consists of mono-alkylated π-conjugated molecules, could form a quasi-extended π-conjugated system and was beneficial to charge transport.24 The diffraction patterns of BTBT-Tn (n = 4-7) clearly exhibited a series of peaks assignable to (00l) and provided information regarding long-range film order. However, non-assignable peaks at 2θ = 3.10° (d-spacing of 28.5 Å, Figure S15g) for BTBT-T8, 2.72° (d-spacing of 32.4 Å, Figure S15h) for BTBT-T12 and 3.10° (d-spacing of 28.5 Å, Figure 5c) for BTBT-T16 were observed, implying the presence of different thin-film phases is possible. In-plane film XRD patterns were also recorded, and three peaks can be observed for all BTBTTn samples as shown in Figure S16. According to the single-crystal structures of BTBT-T4 and BTBT-T7, they can be assigned to (110), (020) and (120), which indicates that all these compounds show HB packing in the plane parallel to the substrate. With elongation of the alkyl chain, the in-plane peaks become stronger until n = 8 and then remain almost unchanged. Meanwhile, all these peaks shift to larger angles, which indicates that the size of the unit cells decrease as the alkyl chain is elongated. This phenomenon was also found in symmetric dialkylsubstituted BTBT derivatives36 and was attributed to the increased self-organization capability of the alkyl chains, which results in enhanced intermolecular interactions in the conjugated core layers.31, 32 However, for BTBT-Tns (n = 8, 12 and 16), unlike the different crystalline phases


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that were observed in the out-of-plane XRD pattern, only a set of diffraction peaks were observed. We speculated that this phenomenon is related to low instrument resolution. Therefore, we further studied the in-plane packing structures of all the thin films by two-dimensional grazing incidence XRD (2D-GIXD). As expected, two sets of peaks can be distinctly observed at qxy ≈ 13, 16 and 19 nm-1 for the above three compounds, which confirms the existence of different crystalline phases, and the other compounds with shorter alkyl chains only exhibit a set of diffraction peaks (Figure S17). The line cuts in the in-plane direction from the 2D-GIXD patterns of BTBT-T2, BTBT-T7 and BTBT-T16 are shown in Figure 5d-f as examples. As suggested in the literature,49 coexistence of different molecular stacking structures can cause a reduction in the charge transport ability of BTBT-Tns (n = 8, 12 and 16). AFM height images of all the BTBT-Tns films deposited on SubOTMS are shown in Figure S19 and S20. BTBT-T0 and BTBT-T2 formed discontinuous thin films comprising smaller domains (ca. 1 µm) with distinct grain boundaries (Figure S19a and S20a). In particular, many isolated grains are found in the films of BTBT-T0. This poor film morphology is consistent with their lower mobilities. BTBT-Tn derivatives with n ≥ 4 have similar film morphologies. The films are characterized by the large plate-like grains consisting of flat terraces and intimate coalescence. Meanwhile, the step heights correspond well to the lengths of one or two molecules. These highquality films should be beneficial for charge transport. From the aforementioned results, the length of the alkyl chain is vital to the formation of high-quality films, and the alkyl chains with n ≥ 4 can endow the molecules with self-organization capabilities.


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Figure 6. AFM images of thin films of various thicknesses of BTBT-T2 (a-d), BTBT-T7 (e-h) and BTBT-T16 (i-l) deposited on SubOTMS. TS: 40 °C (BTBT-T2), 80 °C (BTBT-T7) and 100 °C (BTBT-T-16). In the images, “sub” refers to the substrate, and the Arabic numerals represent the nominal thickness in term of the number of corresponding molecular monolayers. The accumulation and transport of charge carriers mainly occur in the first few layers near the semiconductor/insulator interface. Therefore, the film growth behavior of BTBT-Tns (n > 0) in the first few layers was further studied by AFM (Figure 6 and Figure S21). To conveniently describe the evolution of film morphology, the nominal film thickness (Θ) based on the number of monolayers (MLs) was used, which is estimated from the ratio of the target film thickness to the height of an ML of the standing molecules as determined by the out-of-plane XRD.50 During the early stage of deposition with Θ = 0.5 MLs, all BTBT-Tns (n > 0) formed independent


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islands with heights equal to 2 MLs. This may stem from the nature of the molecular structure, i.e., the rod-like asymmetric molecules tend to align themselves in a bilayer structure to reach the thermodynamically most stable state. As the thickness increased from ~0.5 to ~1.5 MLs, the bilayer coverage increased and the gaps (empty space) between the grains shrank, indicating that a percolation path in the films might start to form. This period is a key step in the formation of high-quality thin-films with larger domain sizes and fewer boundaries, and these properties affects their charge transport capabilities.51 For BTBT-T2, an increasing number of islands formed in the upper layers as Θ > 1.5 MLs, and the 4th, 6th and 8th layers were found at Θ ≈ 2.7, 3.8 and 4.8 MLs (Figure 6b-d), respectively. This growth behavior indicates that after an initial growth period with the bilayer structure, the density of the upper layer significantly increased, and the molecules deposited on the homo-surface tended to follow island growth with a monolayer aligned mode as determined from AFM analysis.50, 52 The stacking of monolayer terraces (~1.8 nm) makes the islands grow progressively more three-dimensional (3D), leading to ill-connected grains and inevitably limits the charge transport ability.52 Therefore, the poor film morphologies might be the main reason for the lower mobility of BTBT-T2. Based on the AFM images at different thicknesses, the film growth behavior of BTBT-Tn with moderate alkyl chain length (n = 4-8) is similar. The AFM images of BTBT-T7 are shown in Figure 6e-h as examples. Clearly, unlike the growth of BTBT-T2, BTBT-T7 exhibited a layer-by-layer growth mode. At Θ ≈ 1.5 MLs (Figure 6e), large grains with step heights (~ 4.6 nm) equal to twice the length of the BTBT-T7 molecule appeared. As the thickness increased, adjacent grains began to coalesce, and almost no nuclei of the 3rd layer emerged. At Θ ≈ 2.2 MLs (Figure 6f), the grains coalesced completely and a few regions of the 3rd layer can be found at the surface. At Θ = 3.5 MLs (Figure 6g), the 4th layer islands have coalesced and some 5th layer islands have appeared.


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When the thickness was further increased to 5.8 MLs (Figure 6h), the 4th layer islands have almost covered the whole substrate, and the 5th and 6th layer islands began to integrate. It is worth emphasizing that at Θ = 2.4 MLs, a continuous network with a degree of coverage beyond 90% had been formed on a 20 × 20 µm2 scale for BTBT-Tns (n = 4-7), and very few nuclei of the 3rd layer were present (Figure S22 and S23). This desirable growth behavior has never been reported previously for vacuum-deposited films. However, for BTBT-T8, the degree of film coverage was relatively low and many higher grains (bright spots) could be seen (Figure S23c). From the above information, it can be concluded that nuclei of the 3rd layer emerged just after the 1st bilayer coalesced completely, and then the 4th layer molecules were aligned on top of them in a head-to-head fashion, which lead to the formation of bilayer packing structures as suggested by the results from XRD analysis. This growth behavior remained throughout the film growth process, and it is also the reason why domains with monolayer step heights can be found in the AFM images of BTBT-Tn (n ≥ 4) as shown in Figure S19. BTBT-Tn derivatives with longer alkyl chains (n = 12 and 16) show slightly different growth behavior (Figure S21 and Figure 6i-l). More higher-layer nuclei emerged before the 1st bilayer domains completely coalesced. Taking BTBT-T16 as an example, the 4th layer began to emerge at Θ ≈ 1.4 MLs (Figure 6i). At Θ ≈ 2.3 MLs (Figure 6j), grains of bilayers had coalesced together with bigger 4th layer grains, which appeared at the gaps between the bilayer grains and hinder their integration. This situation can be clearly observed in the larger scale images (20 × 20 µm2) as shown in Figure S22d. When Θ was increased to 3.7 MLs (Figure 6l), these kinds of gaps were covered and are difficult to find; however, their existence must limit the charge transport ability of the films. In addition, the degree of coverage increased with the elongation of the alkyl chains up to n = 7 and then decreased.


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For comparison, the microstructures and morphologies of the films of ca. 30 nm of BTBT-Tn (n = 2, 4, 7 and 16) on SubPS were also studied. As shown in Figure S15 and S16, the main differences in the out-of-plane and in-plane diffraction patterns are the lower diffraction intensity and smaller qxy values on SubPS, which imply lower crystallinities and weaker intermolecular interactions in the plane parallel to the substrate surface. Although the surface morphologies of the ca. 30 nm films on both substrates was similar (as shown in Figure S19), the morphologies of the first two molecular layers were dependent on the substrates. The degree of coverage by the first two layers was lower on SubPS. For BTBT-T4 and BTBT-T7 on SubPS, unlike the large uniform area and continuous films on SubOTMS, higher layers were observed before the first two layer domains coalesced completely (Figure S22); however, the morphologies of the first two layers of BTBT-T2 and BTBT-T16 on these two substrates were slightly different. From above the discussion, we can conclude that the alkyl chain length of the molecules has a noticeable influence on their packing structures, film order and the number of grain boundaries, which all impact the charge transport capability of organic semiconductors. BTBT-T0 and BTBT-T2 formed low-quality films characterized by monolayer structures, distinct grain boundaries and relatively weak 2D packing order, and they therefore exhibited the lowest mobilities. For BTBT-Tn with n = 4 to 7, film order, especially in-plane order, was enhanced with increasing alkyl chain length. Meanwhile, the layer-by-layer growth mode of these molecules allowed the formation of homogeneous films with diminished grain boundaries, especially at the first few layers, resulting in higher mobilities. Further increasing the alkyl length, the competition between the enhanced intermolecular interactions and relatively poor film morphologies together with the existence of different crystalline phases lead to a slight decrease in mobility. OTFTs of BTBT-Tn (n = 2, 4, 7 and 16) on SubPS showed lower


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crystallinities, weaker intermolecular interactions and relatively poorer film growth behaviors, which caused a reduction in the mobilities. CONCLUSION In conclusion, a series of mono-alkylated asymmetric BTBT derivatives, i.e., BTBT-Tns, have been synthesized and studied. The length of their alkyl chains has a great influence on their phase transition temperatures, packing structures, film order and grain boundaries and thus the device performances. By optimizing the alkyl chain length, BTBT-T7 and BTBT-T6, which exhibited the tightest herringbone packing, highest film order and layer-by-layer growth, showed the best OTFT performances with reliable mobilities of ca. 10 cm2 V-1 s-1 after annealing. Our study indicated that BTBT-based asymmetric conjugated molecules substituted with proper alkyl chains are promising organic semiconductors for high performance OTFTs. EXPERIMENTAL SECTION Synthesis of materials. Stille cross-coupling reactions were performed in an argon atmosphere. All chemical reagents were purchased from Alfa Aesar, Acros, adamas or Aldrich and used as received. N,N-Dimethylformamide (DMF) was dried with CaH2 and distilled under reduced pressure. General procedure for the synthesis of BTBT-Tn. Compound 1 (1.00 g, 2.73 mmol), 2tributylstannylthiophene or 2-alkyl-5-tributylstannylthiophene (3.00 mmol) and Pd(PPh3)4 (35 mg, 0.03 mmol) and DMF (25 mL) were added into a Schlenk Tube. The mixture was stirred at 90 °C in dark for 12 h under an argon atmosphere. Then, the reaction was quenched by adding water. The precipitate was collected by filtration, washed with ethanol and dried in vacuum. The


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resulting crude product was purified by column chromatography on silica gel using petroleum ether as eluent, recrystallization from toluene and then vacuum sublimation. 2-(Thiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-T0). Yellow solid, 380 mg (72% yield). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.14 (s, 1H), 7.93 – 7.85 (m, 3H), 7.71 (d, J = 8.4 Hz, 1H), 7.49 – 7.39 (m, 3H), 7.33 (d, J = 4.8 Hz, 1H), 7.12 (d, J =4.8 Hz, 1H); Anal. Calcd for C18H10S3: C, 67.04; H, 3.13; S, 29.83; Found: C, 66.92; H, 3.34; S, 29.95; MS (MALDI-TOF, calcd: 322.0): found: 322.0. 2-(5-Ethylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-T2). Yellow solid, 480 mg (77% yield). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.08 (s, 1H), 7.95 – 7.78 (m, 3H), 7.66 (d, J = 8 Hz, 1H), 7.50 – 7.34 (m, 2H), 7.22 (d, J = 3.6 Hz, 1H), 6.80 (d, J = 3.6 Hz, 1H), 2.89 (q, J = 7.2 Hz, 2H), 1.37 (t, J = 7.2 Hz, 3H); Anal. Calcd for C20H14S3: C, 68.53; H, 4.03; S, 27.44; Found: C, 68.47; H, 4.10; S, 27.57; MS (MALDI-TOF, calcd: 350.0): found: 350.0. 2-(5-Butylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-T4). Yellow solid, 410 mg (63% yield). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.08 (s, 1H), 7.93 – 7.83 (m, 3H), 7.66 (d, J = 8 Hz, 1H), 7.48 – 7.38 (m, 2H), 7.22 (d, J = 3.6 Hz,1H), 6.79 (d, J = 3.6 Hz, 1H), 2.85 (t, J = 7.6 Hz, 2H), 1.75 – 1.68 (m, 2H), 1.49 – 1.40 (m, 2H), 0.97 (t, J = 7.2 Hz, 3H); Anal. Calcd for C22H18S3: C, 69.80; H, 4.79; S, 25.41; Found: C, 69.70; H, 4.77; S, 25.67; MS (MALDI-TOF, calcd: 378.1): found: 378.1. 2-(5-Pentylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-T5). Yellow solid, 530 mg (81% yield). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.07 (s, 1H), 7.93 – 7.83 (m, 3H), 7.66 (d, J = 8 Hz, 1H), 7.48 – 7.38 (m, 2H), 7.22 (d, J = 3.6 Hz, 1H), 6.78 (d, J = 3.6 Hz, 1H), 2.84 (t, J = 7.2 Hz, 2H), 1.73 (m, 2H), 1.39 (m, 4H), 0.92 (t, J = 7.2 Hz, 3H); Anal. Calcd for


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C23H20S3: C, 70.36; H, 5.13; S, 24.50; Found: C, 70.36; H, 5.25; S, 24.75; MS (MALDI-TOF, calcd: 392.1): found: 392.1. 2-(5-Hexylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-T6). Yellow solid, 510 mg (77% yield). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.07 (s, 1H), 7.93 – 7.83 (m, 3H), 7.66 (d, J = 8 Hz, 1H), 7.48 – 7.38 (m, 2H), 7.22 (d, J = 3.6 Hz, 1H), 6.78 (d, J = 3.6 Hz, 1H), 2.85 (t, J = 7.6 Hz, 2H), 1.76 – 1.69 (m, 2H), 1.46 – 1.29 (m, 6H), 0.91 (t, J = 6.8 Hz, 3H); Anal. Calcd for C24H22S3: C, 70.89; H, 5.45; S, 23.66; Found: C, 70.70; H, 5.40; S, 23.90; MS (MALDI-TOF, calcd: 406.1): found: 406.1. 2-(5-Heptylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-T7). Yellow solid, 500 mg (77% yield). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.08 (s, 1H), 7.93 – 7.83 (m, 3H), 7.66 (d, J = 8 Hz, 1H), 7.48 – 7.38 (m, 2H), 7.22 (d, J = 3.6 Hz, 1H), 6.78 (d, J = 3.6 Hz, 1H), 2.84 (t, J = 7.6 Hz, 2H), 1.76 – 1.69 (m, 2H), 1.46 – 1.29 (m, 8H), 0.90 (t, J = 6.6 Hz, 3H); Anal. Calcd for C25H24S3: C, 71.38; H, 5.75; S, 22.87; Found: C, 71.30; H, 5.77; S, 23.04; MS (MALDI-TOF, calcd: 420.1): found: 420.1. 2-(5-Octylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-T8). Yellow solid, 550 mg (77% yield). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.08 (s, 1H), 7.93 – 7.83 (m, 3H), 7.66 (d, J = 8 Hz, 1H), 7.48 – 7.38 (m, 2H), 7.22 (d, J = 3.6 Hz, 1H), 6.78 (d, J = 3.6 Hz, 1H), 2.84 (t, J = 7.4 Hz, 2H), 1.76 – 1.69 (m, 2H), 1.46 – 1.29 (m, 10H), 0.89 (t, J = 6.8 Hz, 3H); Anal. Calcd for C26H26S3: C, 71.84; H, 6.03; S, 22.13; Found: C, 71.82; H, 6.01; S, 22.41; MS (MALDI-TOF, calcd: 434.1): found: 434.1. 2-(5-Dodecylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene

(BTBT-T12).

Yellow

solid, 930 mg (72% yield). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.08 (s, 1H), 7.93 – 7.83 (m,


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3H), 7.66 (d, J = 8 Hz, 1H), 7.48 – 7.38 (m, 2H), 7.22 (d, J = 3.6 Hz, 1H), 6.78 (d, J = 3.6 Hz, 1H), 2.84 (t, J = 7.6 Hz, 2H), 1.76 – 1.68 (m, 2H), 1.43 – 1.27 (m, 18H), 0.88 (t, J = 7.4 Hz, 3H); Anal. Calcd for C30H34S3: C, 73.42; H, 6.98; S, 19.60; Found: C, 73.35; H, 6.66; S, 19.40; MS (MALDI-TOF, calcd: 490.2): found: 490.2. 2-(5-Hexadecylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-T16). Yellow solid, 580 mg (78% yield). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.08 (s, 1H), 7.93 – 7.83 (m, 3H), 7.66 (d, J = 8 Hz, 1H), 7.48 – 7.38 (m, 2H), 7.22 (d, J = 3.6 Hz, 1H), 6.78 (d, J = 3.6 Hz, 1H), 2.84 (t, J = 7.6 Hz, 2H), 1.76 – 1.68 (m, 2H), 1.43 – 1.27 (m, 26H), 0.88 (t, J = 6.8 Hz, 3H); Anal. Calcd for C34H42S3: C, 74.67; H, 7.74; S, 17.59; Found: C, 74.53; H, 7.63; S, 17.65; MS (MALDI-TOF, calcd: 546.2): found: 546.2. General Methods. 1H NMR spectra was recorded on Bruker AV 400-MHz spectrometer in CDCl3. Chemical shifts were reported as values (ppm) relative to internal tetramethylsilane. 13C NMR measurement was failed to perform because of the not sufficient solubility of these compounds for strong enough signal. Elemental analysis was carried out on a FlashEA1112 elemental analyzer. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were recorded in reflection mode on a Brucker/AutoflexIII mass spectrometer with dithranol (DIT) or 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene] malononitrile (DCTB) as the matrix. Thermogravimetric differential thermal analysis (TG-DTA) was carried out on a Perkin-Elmer TGA7 thermogravimetric analyzer at a heating rate of 10 °C/min at a nitrogen flow. Differential scanning calorimetry (DSC) was run on a Perkin-Elmer DSC7 at a heating/cooling rate of 10/-10 °C/min at a nitrogen flow. UV-vis absorption spectra were measured with Shimadzu UV3600 spectrometer. CV measurement was performed on a CHI660a electrochemical analyzer with a three electrode cell at a scan rate of 100 mV/s in


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dichloromethane solution of BTBT-Tn or 6-BTBT-6 (10-3 mol/L) with Bu4NPF6 (0.1 mol/L) used as electrolyte. A Pt disc electrode with a diameter of 2 mm, a Pt wire, and a saturated calomel electrode were used as the working, counter, and reference electrode, respectively. The HOMO energy levels (EHOMO) were estimated by the onset oxidation potential verses ferrocene ௢௡௦௘௧ using the following equation: EHOMO = -(4.80 + ‫ܧ‬௢௫ ) eV. Dielectric capacitance was measured

by Agilent E4980A. Polarizing optical microscopy (POM) observation was conducted on an Olympus BX51 polarizing optical microscopy equipped with an LTS 350 hot stage and a TMS 94 temperature programmer (Linkam). Temperature-dependent powder XRD was carried out on a Rigaku Smart Lab with CuKα source (λ = 1.54056 Å) under N2 atmosphere. Out-of-plane Xray diffraction (XRD) was measured on a Bruker D8 Discover thin-film diffractometer with CuKα radiation (λ = 1.54056 Å) operated at 40 kV and 40 mA in air. In-plane XRD was conducted on a Rigaku Smart Lab with CuKα source (λ = 1.54056 Å) in air. 2D-GIXD was measured at Shanghai Synchrotron Radiation Facility (SSRF) on beam line BL14B1 (λ = 1.24 Å) with a MarCCD area detector at incidence angle of 0.1°. Atomic force microscopy (AFM) measurements were carried out in tapping mode on a SPA400HV instrument with an SPI 3800 controller (Seiko Instruments). OTFT fabrication and characterization. Bottom gate and top contact devices were fabricated on heavily n-doped Si wafers covered with 200 nm SiO2 (Ci = 17.3 nF cm-2). The OTMS and PαMS modified substrates were done according to the literature method.44, 53 The polymer layers were ca. 10 nm thick with a capacitance of 16 nF cm-2. Semiconductor layers (ca. 30 nm thick) were vacuum deposited onto the modified substrates at a rate of 0.5 – 1 Å s-1 under a pressure of 10-4 Pa. Au source and drain electrodes (40 nm thick) were vacuum deposited through a shadow mask. The channel length was 200 µm, and the channel width was 1 mm. The as-fabricated


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devices were then annealed at 40 °C for two hours in ambient conditions. For the TLM, the channel lengths were 50, 80, 110, 140, 170 and 200 µm, respectively, and the channel width was 1 mm. The electrical measurements were performed with two Keithley 236 source/measure units in ambient conditions. Field-effect mobility was calculated in the saturation regime using the following equation, ID = (WCi/2L) µFET (VG – VT)2, where W and L are the channel width and length, Ci is the capacitance of the SiO2 insulator, and VG and VT are the gate and threshold voltages, respectively. ASSOCIATED CONTENT Supporting Information: The following files are available free of charge via the Internet at http://pubs.acs.org. Crystal structure, thermal and electrochemical characterization, the assignment of liquid crystal phases, OTFT performance, contact resistance extracted by TLM method, thin films XRD patterns, AFM images of BTBT-Tns, 1H NMR and MALDI-TOF mass spectra of BTBT-Tns were provided.

AUTHOR INFORMATION Corresponding Author Yanhou Geng, [email protected] Hongkun Tian, [email protected]


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ACKNOWLEDGMENT This work is supported by National Key R & D Program of “Strategic Advanced Electronic Materials” (No. 2016YFB0401100) of Chinese Ministry of Science and Technology, the National Natural Science Foundation of China (Nos. 51333006 and 51273192) and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB12010300). The authors also thank the Shanghai Synchrotron Radiation Facility (SSRF) for the help with 2D GIXD measurements. REFERENCES (1) Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect Transistors: the Path beyond Amorphous Silicon. Adv. Mater. 2014, 26, 1319-1335. (2) Di, C.-A.; Zhang, F.; Zhu, D. Multi-Functional Integration of Organic Field-Effect Transistors (OFETs): Advances and Perspectives. Adv. Mater. 2013, 25, 313-330. (3) Gelinck, G.; Heremans, P.; Nomoto, K.; Anthopoulos, T. D. Organic Transistors in Optical Displays and Microelectronic Applications. Adv. Mater. 2010, 22, 3778-3798. (4) Amin, A. Y.; Khassanov, A.; Reuter, K.; Meyer-Friedrichsen, T.; Halik, M. Low-Voltage Organic Field Effect Transistors with a 2-Tridecyl[1]benzothieno[3,2-b][1]benzothiophene Semiconductor Layer. J. Am. Chem. Soc. 2012, 134, 16548-16550. (5) Diao, Y.; Tee, B. C.-K.; Giri, G.; Xu, J.; Kim, D. H.; Becerril, H. A.; Stoltenberg, R. M.; Lee, T. H.; Xue, G.; Mannsfeld, S. C. B.; Bao, Z. Solution Coating of Large-Area Organic Semiconductor Thin Films with Aligned Single-Crystalline Domains. Nat. Mater. 2013, 12, 665-671. (6) He, P.; Tu, Z.; Zhao, G.; Zhen, Y.; Geng, H.; Yi, Y.; Wang, Z.; Zhang, H.; Xu, C.; Liu, J.; Lu, X.; Fu, X.; Zhao, Q.; Zhang, X.; Ji, D.; Jiang, L.; Dong, H.; Hu, W. Tuning the Crystal


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Asymmetric Conjugated Molecules Based on [1] Benzothieno [3, 2-b][1

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