An oligomer semiconductor with an asymmetric cyclohexylhexyl end group for solution-processed organic field-effect transistors

Journal Pre-proof An oligomer semiconductor with an asymmetric cyclohexylhexyl end group for solution-processed organic field-effect transistors Yeong Hean Jeong, Tae Kyu An, Min Cho Lee, Min-Jung Lee, Sun Young Jung, Yong Jin Jeong, Yun-Hi Kim PII:

S0254-0584(19)31213-1

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122398

Reference:

MAC 122398

To appear in:

Materials Chemistry and Physics

Received Date: 3 September 2019 Revised Date:

16 October 2019

Accepted Date: 2 November 2019

Please cite this article as: Y.H. Jeong, T.K. An, M.C. Lee, M.-J. Lee, S.Y. Jung, Y.J. Jeong, Y.-H. Kim, An oligomer semiconductor with an asymmetric cyclohexylhexyl end group for solution-processed organic field-effect transistors, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/ j.matchemphys.2019.122398. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

An oligomer semiconductor with an asymmetric cyclohexylhexyl end group for solution-processed organic field-effect transistors Yeong Hean Jeong1,a, Tae Kyu An 2,a, Min Cho Lee 2, Min-Jung Lee1, Sun Young Jung 3, Yong Jin Jeong 3,*, and Yun-Hi Kim 1,* 1

Department of Chemistry and Research Institute for Green Energy Convergence Technology (RIGET), Gyeongsang National University, Jinju 52828, Republic of Korea

2

Department of Polymer Science & Engineering and Department of IT Convergence, Korea National University of Transportation, Chungju, 27469, Republic of Korea

3

Department of Materials Science & Engineering, Korea National University of Transportation, Chungju 27469, Republic of Korea

* Corresponding authors: [email protected] (Y.J.J.); [email protected] (Y.-H.K.) a

Yeong Hean Jeong1 and Tae Kyu An contributed equally to this work.

Abstract: We report on the synthesis of an oligomer semiconductor, 2-(6-((6cyclohexylhexyl)oxy)naphthalen-2-yl)anthracene (C-HNA), and research its potential application as a thin-film active layer for organic field-effect transistors (OFETs). The measured field-effect mobility of the spin-coated C-HNA films for OFETs was 2.3 × 10−2 cm2/(V·s). The performance of the resulting OFETs was attributed to the packing tendency of the asymmetric oligomer, where the asymmetric cyclohexylated alkyl chains were vertically aligned to the substrates, which contributed to favorable charge transfer.

Keywords: organic semiconductor; asymmetric cyclohexylhexyl end group; crystal; fused acene; organic field-effect transistor (OFET)

Highlights - An asymmetric oligomer semiconductor is synthesized for organic electronics. - The crystalline semiconductor film is successfully formed from the synthesized oligomer. - The OFETs with the oligomer exhibited fairly good performances.

Graphical Abstract

1. Introduction Research in organic semiconductors has progressed to the point where these materials can potentially be substituted for amorphous Si in the fabrication of sensors, radiofrequency identification (RFID) tags, thermoelectrics, electronic paper, memory chips, and driving circuits for flexible displays.[1-6] The attractive aspect of these organic semiconductors, which are still under active development, is that their electrical and optical properties can be easily tuned, enabling molecular and structural optimization.[7, 8] The strategic design of a backbone for oligomers and polymers provides a basic platform for the easy synthesis of organic semiconductors with high physical and chemical stabilities and introduces the feasibility of fabricating organic semiconductors with well-tuned electronic properties.[9, 10] In parallel, control of the side chains and end groups enables improvements in the crystallinities of oligomers and polymers and in their solubilities in organic solvents, which in turn enables the use of low-cost solution processing methods such as spin-coating, dip-coating, and other printing techniques.[9, 11, 12] In many classes of these organic semiconductors, the molecular ordering and crystallinity strongly influence the hopping behavior of charge carriers between each molecule, which is a key factor in improving the electrical performance of devices such as organic field-effect transistors (OFETs), organic photovoltaics (OPVs), and organic diodes.[13, 14] Therefore, intensive efforts have been devoted to synthesizing novel organic semiconductors and controlling the molecular ordering and crystallinity in their thin films.[9, 11] As an example, the use of linearly fused acene molecules as oligomer backbones improved the intermolecular π–π overlap in their thin films, which provided efficient pathways for charge transport.[15-17] When the aromatic ring (benzene) structures are connected multiple times, π-state orbitals are more delocalized, and the resulting molecules have long conjugation length. Therefore, it is easy to obtain large π-overlap area in the

crystallin thin films of the molecules having multiple linked benzene rings. End groups in the oligomers are also considered important, and some asymmetric end groups have been found to determine the aggregation mode of molecules. For example, Kim et al. found that the quaterthiophene derivatives with asymmetrically substituted cyclohexyl end-groups produced H-type aggregation, whereas the materials with symmetrically substituted cyclohexyl formed J-type aggregation.[18] The alignment of oligomers composed of cyclohexylated alkyl end groups also tends to depend on symmetry.[19] In this regard, molecular symmetry and backbone structure should be carefully considered in the strategic design of oligomers that will induce a crystal orientation favorable for charge transport. In this study, we report on the feasibility of using a newly designed asymmetric alkoxy naphthyl anthracene derivative containing a cyclohexylated alkyl end group, 2-(6-((6cyclohexylhexyl)oxy)naphthalen-2-yl)anthracene (C-HNA) (Fig. 1a), as an active layer of OFETs. Two and three benzene rings were linearly connected by single bond to prepare linearly fused acenes, which improved solubility while keeping long conjugation lengths and large π-overlap area. We investigated the molecular orientation of a fused acene-based oligomer semiconductor by introducing asymmetric bulky end groups. Through polarized optical microscopy (POM), atomic force microscopy (AFM), two-dimensional grazing incidence X-ray diffraction (2D-GIXD) analysis, and ultraviolet–visible–near-infrared (UV– vis–NIR) absorption spectroscopy, we confirmed that the C-HNA molecules prefer vertical ordering with their cyclohexylhexyl group perpendicular to the substrate. We also found that the crystalline C-HNA thin films resulted in favorable hole transport, which in turn improved the electrical properties of OFETs within a saturation mobility of 2.3 × 10−2 cm2/(V·s).

(a)

O

(b)

Figure 1. (a) Molecular structure of C-HNA. (b) Scheme of the C-HNA synthesis route.

2. Materials and Methods 2.1. Synthesis of 6-cyclohexylhexyl 4-methylbenzenesulfonate (1) 2-Bromoanthracene, tetrakis(triphenylphosphine)palladium(0), and solvents were purchased from TCI. Fifty milliliters (0.24 mol) of 6-cyclohexylhexan-1-ol and 45.92 mL (0.72 mol) of pyridine were added to a 500 mL three-neck flask. p-Toluenesulfonyl chloride (40.03 g, 0.21 mol) was added to the mixture at 0 °C. After the mixture was stirred overnight, the reaction was terminated by addition of 2 N HCl and the products were extracted with ethyl acetate. Water was removed by addition of anhydrous MgSO4, and the solvent was subsequently removed. The crude product was recrystallized from ethanol. Yield: 53 g (65%); 1H NMR

(300 MHz, chloroform-d) δ 7.78 (d, 2H, J = 8.3 Hz), 7.34 (d, 2H, J = 8.3 Hz), 4.01 (t, 2H, J = 6.5 Hz), 2.44 (s, 3H), 1.71-1.55 (m, 7H), 1.32-1.07 (m, 12H), 0.90-0.78 (m, 2H).

2.2. Synthesis of 2-bromo-6-((6-cyclohexylhexyl)oxy)naphthalene (2) To a 250 mL two-neck round-bottom flask, 10 g (44.82 mmol) of 6-bromo-2-naphthanol, 300 mL of dimethylformamide, 18.58 g (134.48 mmol) of K2CO3, and 22.76 g (67.23 mmol) of 1 were added. After the mixture was stirred for 14 h at 100 °C, the reaction was terminated by addition of 2 N HCl. Methylene chloride was used for extraction, and water was subsequently removed by addition of anhydrous MgSO4. The solvent was removed using a rotary evaporator, and the product was separated by column chromatography using n-hexane as the eluent. Yield: 11 g (63%); 1H NMR (300 MHz, chloroform-d) δ 8.48 (s, 1H), 8.03 (d, 1H), 7.95 (d, 1H), 7.40 (d, 1H), 7.33 (s, 1H), 7.21 (d, 1H), 4.11 (t, 2H), 1.82-1.73 (m, 7H), 1.311.08 (m, 10H), 0.90-0.85 (m, 2H).

2.3. Synthesis of (6-((6-cyclohexylhexyl)oxy)naphthalen-2-yl)boronic acid (3) Ten grams (25.16 mmol) of 2 and 150 mL of anhydrous tetrahydrofuran (THF) were added to a 250 mL three-neck round-bottom flask. After the flask was cooled to less than −78°C, 30.82 mmol of 2.5 M n-butyl lithium (n-BuLi) was slowly added over a period of 1 h. After the flask was again cooled to less than −78°C, 4.50 g (30.82 mmol) of triethyl borate was slowly added to the mixture. The mixture was stirred for 24 h at room temperature, and the reaction was terminated by addition of 2 N HCl. The organic layer was extracted using ethyl acetate, and the reaction mixture was filtered through MgSO4. After the solvent was removed, the product was separated by hot filteration using hexane. Yield: 5.2 g (57%); 1H NMR (300 MHz, chloroform-d) 8.25 (s, 1H), 7.79 (d, 2H), 7.71 (d, 1H), 7.24 (s, 1H), 7.11 (d, 1H), 4.214.19 (t, 2H), 1.93-1.83 (m, 2H), 1.84-1.70 (m, 6H), 1.46-1.40 (m, 4H), 1.35-1.12 (m, 7H),

0.97-0.91 (m, 2H).

2.4. Synthesis of 2-(6-((6-cyclohexylhexyl)oxy)naphthalen-2-yl)anthracene (4) To a solution of 3 g (8.47 mmol) of 3 and 6.75 g (8.72 mmol) of 2-bromoanthracene, 40 mL of toluene, 12 mL of 2 M K2CO3, 15 mL of THF solvent, and 0.20 g (5 mol%) of tetrakis(triphenylphosphine)palladium(0) were added at 85°C. The solution was stirred for 48 h under a stream of N2. After the temperature was lowered to room temperature, the reaction was terminated by addition of 2 N HCl. After the solvent was removed, the product was obtained by column chromatography using dichloromethane. The synthetic route for C-HNA is shown in Fig. 1b. Yield: 2.11 g (51%) 1H NMR (300 MHz, chloroform-d) δ 8.57 (s, 1H), 8.52 (s, 1H) 8.38 (d, 1H, J = 0.6Hz), 8.22-8.17 (dd, 2H, J = 11.9Hz, 5.1Hz), 8.13-8.06 (m, 2H), 8.08-7.81 (m, 4H), 7.56-7.48 (m, 2H), 7.27-7.21 (m, 2H), 4.18-4.13 (t, 2H, J = 6.6Hz), 1.91-1.83 (m, 2H), 1.80-1.64 (m, 6H), 1.45-1.38 (m, 4H), 1.33-1.14 (m, 7H), 1.01-0.88 (m, 2H);

13

C NMR (500 MHz, chloroform-d) δ 157.5, 140.0, 138.7, 137.6, 134.1, 132.1, 131.8,

130.8, 129.1, 128.7, 128.1, 128.0, 127.3, 126.4, 125.9, 125.8, 125.3, 124.6, 119.5, 107.7, 106.3, 68.2, 37.7, 37.5, 33.5, 29.8, 29.3, 26.8, 26.5, 26.1; HRMS (ESI) calcd for C36H38O: 486.2923, found : 486.2924.

2.5. Film and device fabrication To fabricate bottom-gate–top-contact OFETs with C-HNA thin films, heavily n-doped Si substrates with 300-nm-thick SiO2 (SiO2/Si) were used as gate (Si) and dielectric (SiO2) layers. The surface of the SiO2 layer was modified with an octadecyltrichlorosilane (ODTS) self-assembled monolayer by immersing the SiO2/Si substrate in the ODTS/toluene solution for 90 min (processed at room temperature). A C-HNA solution was prepared at a concentration of 0.7 wt% in solvent and stirred at 40°C for 30 min. The C-HNA thin film was

spin-coated onto the ODTS-modified SiO2/Si substrates at a spin-coating speed of 2000 rpm for 60 s. On top of the semiconductor layers (40 nm), source and drain electrodes were deposited via thermal evaporation of Au with a patterned shadow mask. The channel width (W) and length (L) were 1000 µm and 100 µm, respectively.

2.6. Characterization 1

H NMR spectra were recorded using an Avance 300 MHz and 500 MHz spectrometer. Mass

spectra were recorded using a Jeol JMC-700 mass spectrometer and a GCmate2 gas chromatograph. Thermogravimetric analysis (TGA) was performed under a N2 atmosphere with a TA Instruments 2050 TGA thermogravimetric analyzer. Samples were heated at 10 °C/min. Differential scanning calorimetry (DSC) was conducted under N2 on a TA Instruments 2100 differential scanning calorimeter. Each sample was heated at 20 °C/min from 0 °C to 250 °C. The surface morphologies of the prepared C-HNA thin films were investigated by POM (Carl Zeiss Axioplan) and AFM (Bruker Nanoscope). 2D-GIXD patterns of the C-HNA thin film were obtained using the synchrotron X-ray beam source at the 3C beamline of the Pohang Accelerator Laboratory (PAL). UV–vis–NIR absorption spectra of C-HNA solutions and thin films were obtained using a Cary UV–vis spectrophotometer (Varian Co.). The electrical characteristics of the OFETs with C-HNA thin films were measured with a Keithley 4200-SCS (Keithley Instruments, USA). Cyclic voltammetry (CV) measurement was implemented on an EG and G Parc model 273 Å potentiostat/galvanostat system with a three-electrode cell in a solution of Bu4NCIO4 (0.1 M) in chloroform solvent. The saturation field-effect mobilities (µsat) were calculated in the saturation regime from the slope of the plots of the square root of the source–drain current (IDS) vs. the gate voltage (VG) in conjunction with the equation IDS = (WCi/2L)µsat(VG − Vth)2, where Ci is the capacitance per unit area of the dielectric (~10 nF/cm2) and Vth is the

threshold voltage.

3. Results

Figure 2. (a) POM image of the C-HNA thin film. (b) AFM image of the C-HNA thin film. (c) 2D-GIXD patterns of the C-HNA thin film. All intensities in the 2D-GIXD pattern were normalized with the probed material volume and X-ray exposure time. The cross-sectional intensity profile along the (d) out-of-plane and (e) in-plane directions, all of which were extracted from (c).

A new asymmetric

alkoxy naphthyl

anthracene derivative containing a

cyclohexylated alkyl end group, C-HNA, was synthesized by nucleophilic substitution and Suzuki coupling reaction. The structure of C-HNA was confirmed through various spectroscopic

analyses,

including

1

H-NMR,

13

C-NMR,

and

high-resolution

mass

spectrometry (HR-MS) (Fig. S1). The thermal stability of the C-HNA was characterized by TGA and DSC (Fig. S2). The 5% decomposition temperature (Td-5%) of C-HNA was 345 °C,

and its glass-transition temperature (Tg), melting temperature (Tm), and crystallization temperature (Tc) were approximately 197 °C, 205 °C, and 201 °C, respectively. The results of these analyses suggest that the asymmetric acenes with cyclohexylhexyl groups exhibit highly crystalline structures with good thermal stability. Table 1 summarized physical properties of the synthesized C-HNA. The highest occupied molecular orbital (HOMO) level of C-HNA was estimated from CV plot (Fig. S3), whose value was -5.48 eV. The lowest unoccupied molecular orbital (LUMO) level was -2.61 eV, calculated by using the relation LUMO = HOMO + band gap. Since C-HNA molecules were found to be highly soluble in chloroform solvent, C-HNA solution was well coated by spin-coating method after filtering with 0.2 µm PTFE filter. Fig. 2a shows a POM image of the spin-coated C-HNA film. Bright colors and color contrast are distinctly observed in the POM image, which suggests that the C-HNA molecules self-assembled to form millimeter-sized crystals when spin coated. In addition, the AFM image in Fig. 2b reveals that the morphology of the C-HNA film includes ring-type boundary lines that suggested crystalline structures from continuous layer-by-layer growth

Table 1. Physical properties of synthesized C-HNA.

Semiconductors

UV-edge [nm]

Band gap (optical) [eV]

HOMO (electrochemical) [eV]

LUMO [eV]

C-HNA

432

2.87

-5.48

-2.61

To further investigate the crystalline structures and molecular orientation of the CHNA film, its 2D-GXID pattern was collected. As shown in Fig. 2c, the diffraction patterns of the C-HNA film were observed along both the in-plane and out-of-plane directions but not

in the diagonal direction. The out-of-plane profiles of the C-HNA film, which were extracted along the out-of-plane direction from Fig. 2c, show XRD peaks corresponding to diffraction of the [l00] planes and some peaks corresponding to other molecular ordering (Fig. 2d).[20] The d-spacing of the C-HNA crystals was 30.0 Å, as calculated from the (100) peak (q = 0.21 Å−1) and using Bragg’s law (2d sinθ = nλ) and (q = 2π/d).[21] From literature studies on the length of alkoxy naphthyl anthracene and cyclohexylated alkyl end group, we speculated that this d-spacing value approximately matches the c-axis length of the C-HNA unit cell.[15, 19] These [l00] diffraction peaks and the corresponding d-spacings suggest that the C-HNA molecules preferred to vertically align with their cyclohexylhexyl groups perpendicular to the substrate.[18, 19] In addition, the packing trend of asymmetric oligomers with cyclohexylated alkyl chains in their side was found to avoid proximity between the bulky side groups and to minimize steric hindrance generated by them.[22] The other peaks in the extracted out-ofplane profiles indicate the existence of additional polymorphic molecular packing structures, which might arise from the bulky cyclohexylhexyl groups not being sufficiently selfassembled or regularly arranged to minimize steric hindrance between the bulky groups during a short spin-coating time. Figure 2d shows the in-plane profile of the C-HNA film, which was extracted along the in-plane direction from Fig. 2c. This profile includes the (010) diffraction at q = 1.62 Å−1, corresponding to a π–π stacking distance of 3.88 Å−1. Because charge carriers in organic semiconductors move along π-overlapping areas in the in-plane direction, this π–π stacking distance could enhance the charge-carrier mobility in OFETs prepared using a C-HNA active layer.[23]

Figure 3. (a) UV–vis–NIR absorption spectra of the C-HNA solution (black) and thin film (gray), respectively. (b) Schematics of an OFET fabricated with the C-HNA thin film developed in the present study. (c) Transfer characteristics of the OFETs with a C-HNA thin film.

In general, intermolecular interactions in a solid-state material can be characterized by their UV–vis–NIR absorption spectra, which are widely used for studying intermolecular interactions and molecular aggregation of thin films.[24] Therefore, UV–vis–NIR absorption spectra were collected to obtain information about molecular structures of the C-HNA semiconductor. Figure 3a shows UV–vis–NIR absorption spectra obtained from the samples including (1) a C-HNA solution and (2) a spin-coated C-HNA film. In the spectrum of the solution, absorption peaks were located at 351, 370, and 392 nm, corresponding to the characteristic absorption peaks of the anthracene unit. The absorption peaks in the spectrum of the C-HNA film prepared by spin coating a C-HNA solution onto a glass substrate were red-shifted to 370, 393, and 417 nm, respectively. The spectral shift from solution to film occurred because of the molecular packing geometry resulting from intermolecular ordering

within the anthracene derivatives.[22, 25] That is, spin coating of the C-HNA solution induced self-assembly of C-HNA molecules such that intermolecular interactions between the anthracene units were enhanced. Several researchers have reported that oligomer semiconductors consisting of asymmetrically substituted side groups, such as cyclohexylated alkyl chains, favor parallel stacking to induce H-aggregation of the molecules.[18, 19] Because the absorption peaks in UV–vis–NIR absorption spectra of H-aggregated films are blue-shifted compared with those in the spectra of the solution sample as a consequence of the cancellation of the transition dipole moments of the lower-lying state, we concluded that C-HNA molecules did not undergo H-aggregation despite being vertically aligned to the substrate.[24] This result implies that the asymmetric oligomers with cyclohexylated alkyl chains in their side groups do not always undergo H-aggregation; rather, the type of molecular aggregation is also governed by the intermolecular interaction between central groups as well as that between the central group and the side group.

Table 2. Electrical characteristics of the OFETs prepared with C-HNA semiconductor thin films.

Semiconductors

Avg. µsat [cm2/(V·s)]

Threshold Voltage (Vth) [V]

On/Off Ratio

Subthreshold Swing [V/dec]

C-HNA

2.3 × 10−2

−0.24

3.0 × 104

4.28

As the schematics show in Fig. 3b, the spin-coated C-HNA semiconductor thin films were used as active layers for OFETs. The transfer characteristics in Fig. 3c reveal that the OFETs with C-HNA thin films exhibited typical p-type operation, with a µsat of 2.3 × 10−2 cm2/(V·s) and an on/off ratio greater than 104. Table 1 summarizes the OFET performance

parameters. The OFET performances and µsat values of the C-HNA OFETs were similar to those of other cyclohexyl-terminated oligomer semiconductors.[18, 19] Although C-HNA molecules did not self-assemble with H-aggregation, the vertical orientation of the molecules was sufficient to enhance the traverse charge transfer across the active layer in the OFETs. Collectively, the bulky cyclohexylated alkyl chains of the asymmetric oligomer C-HNA induced molecular ordering in the vertical direction because of their steric hindrance, which led to enhanced OFET performance.

4. Conclusions In this research, a semiconducting C-HNA oligomer consisting of asymmetric cyclohexylated alkyl and linearly fused acene groups was applied to OFETs. The spin-coated C-HNA induced crystalline growth with vertical alignment of the cyclohexylhexyl groups perpendicular to the substrates. These crystalline C-HNA films exhibited favorable charge transfer characteristics when used as an active layer in OFETs, enhancing the field-effect mobility to 2.3 × 10−2 cm2/(V·s). We believe that this work provides another option for designing oligomer-type semiconductor molecules by exploiting the good packing behavior of asymmetric oligomers, which can lead to satisfactory performance of OFETs in which they are incorporated.

Acknowledgments This research was supported by the National Research Foundation of Korea (NRF) funded by MSIP (2018R1A2A1A05078734). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry

of Education (2018R1A6A1A03023788). This study was supported by the Basic Science Research Capacity Enhancement Project (National Research Facilities and Equipment Center) through the Korea Basic Science Institute funded by the Ministry of Education (Grant No. 2019R1A6C1010047), Republic of Korea.

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Declarations of interest: none