Well-Balanced Carrier Mobilities in Ambipolar Transistors Based on

Article pubs.acs.org/JPCC

Well-Balanced Carrier Mobilities in Ambipolar Transistors Based on Solution-Processable Low Band Gap Small Molecules Min Je Kim,†,⊥ Minwoo Jung,‡,⊥ Woonggi Kang,† Gukil An,§ Hyunjung Kim,§ Hae Jung Son,‡ BongSoo Kim,*,∥ and Jeong Ho Cho*,† †

SKKU Advanced Institute of Nanotechnology (SAINT), School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea ‡ Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea § Department of Physics, Sogang University, Seoul 121-742, Republic of Korea ∥ Department of Science Education, Ewha Womans University, Seoul 120-750, Republic of Korea S Supporting Information *

ABSTRACT: We synthesized a solution-processable low band gap small molecule, Si1TDPP-EE-COC6, for use as a semiconducting channel material in organic thin film transistors (OTFTs). The Si1TDPP-EECOC6 is composed of electron-rich thiophene−dithienosilole−thiophene (Si1T) units and electron-deficient diketopyrrolopyrrole (DPP) and carbonyl units. SiTDPP-EE-COC6-based OTFTs with Au source/drain electrodes were fabricated, and their electrical properties were systematically investigated with increasing thermal annealing temperature. The hole and electron mobilities of as-spun Si1TDPP-EE-COC6 were 3.3 × 10−4 and 1.7 × 10−4 cm2 V−1 s−1, respectively. The carrier mobilities increased significantly upon thermal annealing at 150 °C, yielding a hole mobility of 0.003 cm2 V−1 s−1 and an electron mobility of 0.002 cm2 V−1 s−1. The performance enhancement upon thermal annealing was strongly associated with the formation of a layered edge-on structure and a reduction in the π−π intermolecular spacing. Importantly, the use of atomically thin single-layer graphene (SLG) source/drain electrodes that were grown by the chemical vapor deposition (CVD) method further increased the carrier mobilities. The 150 °C annealed Si1TDPP-EE-COC6-based OTFTs with SLG source/drain electrodes exhibited a hole mobility of 0.011 cm2 V−1 s−1 and an electron mobility of 0.015 cm2 V−1 s−1. The improved electrical performances of the SLG OTFTs were attributed to the stepless flat surface of the SLG electrodes and the better interfacial contact between the Si1TDPP-EE-COC6 molecules and the SLG electrodes compared to the Au electrodes. This work suggests that careful chemical design is essential to enhance balanced ambipolar transistor performance based on small conjugated molecules, and the SLG is a good electrode material to promote the carrier mobilities.

1. INTRODUCTION A large number of donor (D)−acceptor (A) type conjugated polymers structured with alternating electron-rich donor and electron-deficient acceptor blocks in the main backbone have been synthesized for use as the semiconducting channel materials in organic thin film transistors (OTFTs).1,2 A variety of electron-rich donors, such as thiophenes3 and selenophenes,4−6 and electron-deficient acceptor blocks, such as isoindigos,7−10 diketopyrrolopyrroles (DPP),5,11−13 benzothiadiazoles,9,14 and naphthalenedicarboximides,15,16 have been reported. In particular, DPP acceptor blocks have received considerable attention because they form strong π−π interactions among their planar backbones and they offer high carrier mobilities exceeding 1 cm2 V−1 s−1.17,18 Meanwhile, solution-processable low band gap D−A type small molecules have emerged because they can offer several advantages as alternatives to donor−acceptor-type polymers: their syntheses © XXXX American Chemical Society

tend to be straightforward, purification tends to be easy, they are readily functionalized, they provide consistent chemical and physical properties among different batches, and their molecular weight distributions are unity. Although these advantages have been informed, only a few studies have examined small molecules.19,20 New classes of DPP-based small molecules with high carrier mobilities warrant exploration.20−22 Outstanding improvements in OTFT performances have been achieved by (i) modifying the thin film formation methods, e.g., spin-coating, drop-casting, or blade-coating, (ii) introducing postprocesses (such as thermal annealing or solvent annealing), and (iii) developing device architectures, i.e., top-gate structures with polymeric gate dielectrics.20,23,24 Received: March 9, 2015 Revised: July 2, 2015

A

DOI: 10.1021/acs.jpcc.5b02308 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Synthetic Route of the Si1TDPP-EE-COC6

on structure and the reduced π−π intermolecular distance. Note that compared to our previous report on the Si1TDPPEE-C6 molecule19 the introduction of ketone groups into Si1TDPP-EE-C6 not only increased carrier mobilities significantly but also improved carrier mobility imbalance. Moreover, the OTFT performance was further improved using atomically thin CVD-grown SLG as the source/drain electrodes, which led to even higher and more balanced carrier mobilities, with an average hole mobility of 0.011 cm2 V−1 s−1 and an average electron mobility of 0.015 cm2 V−1 s−1. The use of CVD-grown SLG electrodes provided a facile method for improving the ambipolar OTFT performance based on small molecule semiconductors.

Regarding source/drain contact electrodes that are also an important component to determine a contact resistance and a majority carrier type, only metallic electrodes such as Al or Au have been utilized as the source−drain electrodes in DPP-based OTFTs.25 Recently, graphene, which consists of an atomicscale honeycomb lattice composed of carbon atoms, has been utilized as a good candidate electrode material for use in OTFTs owing to its transparency, mechanical flexibility, and environmental stability.26−28 High-quality large-area graphene with a preferential thickness can be easily synthesized using chemical vapor deposition (CVD) methods.28 A few research groups have used CVD-grown single-layer graphene (SLG) as source−drain electrodes in OTFTs prepared based on pentacene, triethylsilylethynyl-anthradithiophene, and poly(3hexylthiophene).27,29−31 Here, we report the synthesis and electrical properties of a strongly electron-accepting silole-based small molecule, Si1TDPP-EE-COC6. This molecule contains electron-donating thiophene−dithienosilole−thiophene (Si1T) units and electron-withdrawing diketopyrrolopyrrole (DPP) units with an extra electron-withdrawing ketone group on the side alkyl chains. Scheme 1 displays the synthetic route and molecular structure of this molecule. As-spun Si1TDPP-EE-COC6 devices with Au source/drain electrodes exhibited ambipolar transport characteristics with hole and electron mobilities of 3.3 × 10−4 and 1.7 × 10−4 cm2 V−1 s−1, respectively. Thermal annealing yielded a 10-fold increase in the carrier mobility to hole and electron mobilities of 0.003 and 0.002 cm2 V−1 s−1, respectively, due to the extensive formation of a layered edge-

2. EXPERIMENTAL SECTION 2.1. Materials. Chemicals of 2-bromothiophene, heptanoyl chloride, N-bromosuccinimde (NBS), AlCl3, hexamethylditin, bis(triphenylphosphine)palladium(II) dichoride (PdCl2(PPh3)2), 1.6 M n-butyllithium (BuLi) solution in hexanes, N,N,N′,N′-tetramethylethylenediamine (TMEDA), 1.0 M trimethyltin chloride solution in tetrahydrofuran (THF), and tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) were bought from Sigma-Aldrich and Tokyo Chemical Industry C., Ltd. 3-(5-Bromothiophen-2-yl)-2,5bis(2-ethylhexyl)-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione was acquired from Lumtec. Common organic solvents were purchased from Daejung and J. T. Baker. THF and toluene were dried over sodium and benzophenone prior to use. All other chemicals and solvents (HCl, NaOH, B


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The Journal of Physical Chemistry C

4.0 Hz), 7.47 (d, 1H, J = 4.4 Hz), 7.65 (d, 1H, J = 4.0 Hz), 7.68 (d, 1H, J = 4.8 Hz), 7.70 (d, 1H, J = 4.8 Hz), 8.88 (d, 1H, J = 4.0 Hz), 8.89 (d, 1H, J = 4.8 Hz). 3-(5-Bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)-6-(5′-heptanoyl-[2,2′-bithiophen]-5-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (5). To a 100 mL round-bottomed (RB) flask with a magnetic bar, compound 4 (350 mg, 0.487 mmol) and 10 mL of DCM were added. The reaction mixture was cooled to 0 °C using an iced water bath. While keeping the reaction temperature at 0 °C, NBS (86.6 mg, 0.487 mmol) was added to the RB flask, and the reaction mixture was stirred for 2 h. Then, 100 mL of H2O was added to the mixture, and desired product was extracted with 100 mL of CHCl3 three times. The organic layer was combined and dried over MgSO4, and the organic solvent was removed under reduced pressure. The crude product was purified by silica gel column chromatography using DCM:hexane (1:1, v/v) as an eluent to provide a purple solid (Yield: 310 mg, 80%). 1H NMR (CD2Cl2, δ ppm) 0.86−0.90 (m, 15H), 1.26−1.34 (m, 22H), 1.69−1.74 (m, 2H), 1.83−1.86 (m, 2H), 2.87−2.91 (m, 2H), 3.97−4.07 (m, 4H), 7.29 (d, 1H, J = 3.6 Hz), 7.37 (d, 1H, J = 4.0 Hz), 7.50 (d, 1H, J = 4.4 Hz), 7.68 (d, 1H, J = 4.4 Hz), 8.66 (d, 1H, J = 4.4 Hz), 8.92 (d, 1H, J = 3.6 Hz). 6,6′-(5,5′-(4,4-Bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(thiophene-5,2-diyl))bis(2,5-bis(2ethylhexyl)-3-(5′-heptanoyl-[2,2′-bithiophen]-5-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (Si1TDPP-EE-COC6). To a flame-dried 25 mL reaction tube, compound 3 (131 mg, 0.177 mmol), compound 5 (310 mg, 0.388 mmol), 8 mL of dried toluene, and 2 mL of anhydrous DMF were added. The reaction solution was deoxygenated by three FPT cycles and kept under argon atmosphere. PdCl2(PPh3)2 (6.2 mg, 0.009 mmol) was then added to the reaction solution. The reaction mixture was stirred at 90 °C for 8 h. After the reaction finished, the reaction mixture was cooled to room temperature and poured into 250 mL of methanol. Black colored precipitates were filtered and purified by silica gel column chromatography using CHCl3:hexane (2:1, volume ratio) as an eluent to give a pure black solid (Yield: 225 mg, 69%). 1H NMR (CD2Cl2, δ ppm) 0.90−0.91 (m, 42H), 1.13 (m, 4H), 1.28−1.36 (m, 60H), 1.70−1.72 (m, 4H), 1.90 (m, 4H), 2.84−2.89 (m, 4H), 4.03 (m, 8H), 7.03 (d, 2H, J = 4.0 Hz), 7.22 (d, 2H, J = 3.6 Hz), 7.25 (d, 2H, J = 4.0 Hz), 7.34 (s, 2H), 7.57 (d, 2H, J = 4.0 Hz), 8.80 (d, 2H, J = 4.0 Hz), 8.96 (d, 2H, J = 3.6 Hz). 13C NMR (CD2Cl2, δ ppm) 192.88, 161.26, 160.92, 149.18, 145.37, 143.68, 143.57, 143.36, 140.60, 140.02, 138.55, 138.10, 137.74, 136.11, 132.29, 131.08, 127.42, 126.07, 124.88, 124.31, 108.80, 107.77, 46.05, 39.12, 35.92, 35.80, 35.81, 31.68, 30.36, 29.06, 29.04, 28.95, 28.90, 28.61, 28.59, 24.67, 23.71, 23.13, 22.55, 17.73, 14.26, 14.11, 14.07, 10.78, 10.59. Anal. Calcd For C106H142N4O6S8Si: C, 68.71; H, 7.73; N, 3.02; O, 5.18; S, 13.84; Si, 1.52. Found: C, 68.14; H, 7.72; N, 2.85; S, 13.14. MS (MALDI-TOF) [M + H]+, 1853.9; Found, 1854.1 (1H NMR, 13 C NMR, and MALDI-TOF spectra are provided in Figures S1−S3 in the Supporting Information). 2.3. Characterization. 1H NMR and 13C NMR spectra were collected on a Bruker Advance 400 spectrometer (400 MHz). Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was performed on a Bruker Autoflex instrument. Differential scanning calorimetry (DSC) measurements were run on a PerkinElmer Pyris 1 DSC instrument with a heating rate of 10 °C/min under nitrogen. UV−visible absorption spectroscopy was conducted using a

dichloromethane (DCM), N,N-dimethylformamide (DMF), CH3OH, and CHCl3) were used as received without further purification. Compounds 1 and 3 were synthesized according to previously reported procedures.19,32 2.2. Synthesis. 1-(5-Bromothiophen-2-yl)heptan-1-one (1). To a 100 mL flame-dried reaction flask with a magnetic bar, 2-bromothiophene (2.0 g, 12.267 mmol) and heptanoyl chloride (2.3 g, 15.334 mmol) were dissolved in anhydrous benzene (18.4 mL), and aluminum chloride (AlCl3) was added in small portions with stirring over 10 min. The resulting brown solution was then refluxed for 2 h and cooled to room temperature. The greenish brown reaction mixture was then quenched with 2 M HCl (18.4 mL). The organic layer was separated, washed with 2 M HCl, 2 M NaOH, and deionized H2O. The organic layer was dried over MgSO4, and the organic solvent was removed under reduced pressure. The crude product was purified by silica gel column chromatography using DCM:hexane (1:1, volume ratio) as an eluent to yield a colorless solid compound 1 (Yield: 2.5 g, 81%). 1H NMR (CD2Cl2, δ ppm) 0.85−0.91 (m, 15H), 1.26−1.39 (m, 22H), 1.69−1.76 (m, 2H), 1.84−1.88 (m, 2H), 2.87−2.90 (m, 8H), 7.10 (d, 1H, J = 4.0 Hz), 7.44 (d, 1H, J = 4.0 Hz). 1-(5-(Trimethylstannyl)thiophen-2-yl)heptan-1-one (2). To a 50 mL flame-dried reaction tube with a magnetic bar compound 1 (460 mg, 1.671 mmol) and hexamethylditin (1.1 g, 3.343 mmol) were added. An amount of 20 mL of dried toluene was added to the reaction tube. The reaction solution was deoxygenized by three freeze−pump−thaw (FPT) cycles and kept under argon atmosphere. Pd(PPh3)4 (193.1 mg, 0.167 mmol) was then added to the reaction solution. The reaction mixture was stirred at 100 °C for 24 h. After reaction completion, 100 mL of H2O was added to the reaction mixture, which was extracted with 100 mL of ether three times. The extracted organic layers were combined and dried over MgSO4. After the removal of organic solvent, the crude mixture was precipitated using CHCl3 and CH3OH and filtered, and the resulting filtrate was concentrated under vacuum to yield a brown solid (the product was used for the next reaction without further purification) (Yield: 350 mg, 58%). 1H NMR (CDCl3, δ ppm) 0.40 (m, 9H), 0.87−0.90 (m, 3H), 1.26−1.37 (m, 6H), 1.72−1.80 (m, 2H), 2.87−2.91 (m, 2H), 7.28 (d, 1H, J = 4.0 Hz), 7.61 (d, 1H, J = 4.0 Hz). 2,5-Bis(2-ethylhexyl)-3-(5′-heptanoyl-[2,2′-bithiophen]-5yl)-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (4). To a flame-dried 100 mL reaction tube with a magnetic bar, 3-(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)-6-(thiophen-2yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (300 mg, 0.497 mmol), compound 2 (359 mg, 0.994 mmol), 8 mL of dried toluene, and 2 mL of anhydrous DMF were added. The reaction solution was deoxygenized by three FPT cycles and kept under argon atmosphere. Bis(triphenylphosphine)palladium(II) dichoride (PdCl2(PPh3)2) (17.4 mg, 0.025 mmol) was then added to the reaction solution. The reaction mixture was stirred at 90 °C for 6 h. After the reaction finished, the reaction mixture was cooled to room temperature and dropped into 200 mL of methanol while stirring, yielding red colored precipitates. The precipitates were filtered and purified by silica gel column chromatography using DCM:hexane (1:1, volume ratio) as an eluent to provide a reddish purple solid (Yield: 290 mg, 81%). 1H NMR (CD2Cl2, δ ppm) 0.85−0.92 (m, 15H), 1.26−1.39 (m, 22H), 1.69−1.76 (m, 2H), 1.84−1.88 (m, 2H), 2.86−2.90 (m, 2H), 4.00−4.03 (m, 4H), 7.28 (dd, 1H, J = 4.0, 4.8 Hz), 7.30 (d, 1H, J = 4.0 Hz), 7.34 (d, 1H, J = C


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Table 1. Carrier Mobilities of Si1TDPP-EE-COC6 OTFTs with Au and SLG Source/Drain Electrodes at Different Annealing Temperaturesa electrodes Au SLG a

carrier type

25 °C

100 °C

hole electron hole electron

3.3 (±1.0) × 10−4 1.7 (±0.5) × 10−4 4.9 (±0.9) × 10−4 0.0017 (±0.0004)

0.0010 (±0.0004) 6.2 (±2.2) × 10−4 0.004 (±0.001) 0.010 (±0.003)

150 °C 0.003 0.002 0.011 0.015

(±0.001) (±0.001) (±0.001) (±0.002)

Unit: cm2 V−1 s−1.

Table 2. Threshold Voltages and On/Off Current Ratios of Si1TDPP-EE-COC6 OTFTs with Au and SLG Source/Drain Electrodes at Different Annealing Temperatures electrodes Au

SLG

carrier type Vth (hole) Vth (electron) Ion/Ioff (hole) Ion/Ioff (electron) Vth (hole) Vth (electron) Ion/Ioff (hole) Ion/Ioff (electron)

25 °C 38.0 (±5.7) V 18.9 (±8.4) V 0.8 (±0.2) × 0.10 (±0.04) 17.1 (±3.8) V 21.1 (±5.9) V 0.3 (±0.1) × 0.8 (±0.3) ×

100 °C

104 × 104

104 104

26.5 27.5 1.7 0.4 18.2 29.2 2.0 4.0

(±7.6) (±4.2) (±0.4) (±0.2) (±4.8) (±9.1) (±0.7) (±1.4)

V V × × V V × ×

150 °C

104 104

104 104

31.9 27.1 2.6 0.8 21.3 17.7 0.9 4.8

(±4.5) (±7.1) (±0.6) (±0.3) (±3.9) (±5.3) (±0.4) (±1.7)

V V × × V V × ×

104 104

104 104

Figure 1. (a) Energy levels with a band gap (ΔE) (left) and molecular orbitals of HOMO+1, HOMO, LUMO, and LUMO+1 of Si1TDPP-EECOC1 (right). (b) Molecular geometry of the energy-minimized Si1TDPP-EE-COC1 molecule in face view and in side view. (c) UV−visible absorption spectrum of the Si1TDPP-EE-COC1 (red), predicted using TD-DFT (B3LYP) calculations of the electronic excitation energies and oscillator strengths (blue).

electrode potential was calibrated against the ferrocene oxidation (Fc/Fc+) potential (assumed to be 4.8 eV below the vacuum level). The electronic structures were acquired from ultraviolet photoelectron spectroscopy (UPS) spectra acquired on a PHI 5000 Versaprobe spectrometer (Ulvac-PHI) with a He I (hv = 21.2 eV) light source. Photoemitted electrons were detected at normal emission with a pass energy of 2.95 eV at a base pressure of 7 × 10−8 Pa. The temperature-dependent crystalline microstructure of the Si1TDPP-EE-COC6 films was

PerkinElmer Lamb 9 UV−vis spectrophotometer. Threeelectrode-based cyclic voltammetry was run using a CH instrument electrochemical analyzer at a voltage scan rate of 50 mV/s. Degassed acetonitrile solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) that was purged with high purity nitrogen for more than 20 min was used as the electrolyte solution. The working electrode was a Si1TDPP-EE-C6-coated Pt wire; the counter electrode was a Pt wire; and the reference electrode was Ag/Ag+. The reference D


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the 6-31+G(d,p) basis set. The surface plots of the molecular HOMO and LUMO levels displayed well-delocalized orbitals over the entire π-conjugated system, including the ketone groups (Figure 1a). The resulting molecular geometries indicated that the molecule is quite planar with dihedral angles of zero (Figure 1b). The good π-orbital overlapping and planar geometry suggested that the ketone groups increased the quinoidal character in the conjugation system and facilitated effective intermolecular coupling of the molecular π-orbitals via close intermolecular packing. The calculated orbital energy levels of Si1TDPP-EE-COC1 were compared with those of Si1TDPP-EE-C1, a simplified Si1TDPP-EE-C6 molecule, indicating that the HOMO/LUMO levels and the molecular HOMO−LUMO band gap could be lowered slightly by incorporating ketone groups, as intended. A predicted UV− visible absorption spectrum was obtained using TD-DFT calculations (Figure 1c). The shorter wavelength absorption band originated from the collective electronic excitations from (HOMO−2 and HOMO−1) to LUMO, (HOMO−1 and HOMO) to LUMO+1, and HOMO to LUMO+2. The longer wavelength absorption band was attributed to an excitation from HOMO to LUMO. The optical properties of Si1TDPP-EE-COC6 were investigated using UV−visible absorption spectroscopy. The UV− visible absorption spectra of Si1TDPP-EE-COC6 in solution and in thin film are shown in Figure 2. In solution, Si1TDPP-

investigated by grazing incidence X-ray diffraction (GIXD) measurements at the 9A beamline of the Pohang Light Source II, Korea. Tapping mode atomic force microscopy (AFM) images were taken on a XE-100 (Park systems). The OTFT performances were measured using Keithley 2400 and 236 source measure units under a pressure of 10−5 Torr in the dark. 2.4. Fabrication of OTFT Devices. OTFT devices were fabricated using a substrate composed of heavily doped Si supporting a 300 nm thickness of a silicon dioxide (SiO2) layer. The Si wafer was cleaned with piranha solution at 100 °C for 30 min and was washed with a copious amount of deionized water. The SiO2 layer was surface-modified with octadecyltrichlorosilane (ODTS) that was purchased from Gelest, Inc. The ODTS-treated surface was hydrophobic with a contact angle of around 110°. 50 nm thick Au source/drain electrodes were formed by thermal deposition at high vacuum through a shadow mask onto the ODTS-treated substrate. The channel length was 50 μm, and the channel width was 800 μm. Finally, 50 nm thick Si1TDPP-EE-COC6 film was deposited onto the ODTS-treated SiO2 surface by spin-coating a chloroform solution containing 0.5 wt % molecules. The resulting films were placed in a vacuum chamber for 24 h and then annealed at 25, 100, or 150 °C for 30 min. The SLG source/drain electrode was fabricated as follows: SLG layers were first grown onto Cu foil (t ∼ 25 μm, Alfa Aesar) by chemical vapor deposition (CVD).28 The patterned SLG source/drain electrodes were fabricated by conventional photolithography and reactive ion etching. A poly(methyl methacrylate) (PMMA) supporting layer (46 mg/mL in chlorobenzene) was formed by spincoating (4200 rpm, 30 s) onto the patterned SLG graphene/Cu substrates. The Cu catalyst was then electrochemically etched with ammonium persulfate. Then, the graphene patterns on the PMMA support were transferred onto the ODTS-treated Si/ SiO2 substrate, and the PMMA layer was cleaned with hot acetone. For a given condition, more than 10 transistors were fabricated. Their electrical properties were measured, and then all the electrical parameters were extracted. For each electrical parameter of carrier mobility (μ), threshold voltage (Vth), and on−off current ratio (Ion/Ioff), the averages and standard deviations of collected data were summarized in Table 1 and Table 2.

Figure 2. UV−visible absorption spectra of Si1TDPP-EE-COC6 in the solution state, in the as-cast film state, and in the thermally annealed film state.

3. RESULTS AND DISCUSSION The low band gap small molecule semiconductor, Si1TDPPEE-COC6, was synthesized, as shown in Scheme 1. We previously reported the electrical properties of Si1TDPP-EEC6.19 Si1TDPP-EE-C6 molecule-based OTFTs showed unbalanced ambipolar transport characteristics with hole and electron mobilities of 3.7 × 10−3 and 5.1 × 10−4 cm2 V−1 s−1, respectively. To improve the electron mobility, electronwithdrawing ketone groups into the side alkyl chains of the Si1TDPP-EE-C6 molecule were inserted. This chemical modification constituted an attempt to lower the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of Si1TDPPEE-C6. To identify the electronic effect of ketone groups in the newly designed Si1TDPP-EE-COC6, we carried out quantum mechanical calculations using density function theory (DFT) and time-dependent DFT (TD-DFT) methods (Figure 1). To save calculation time, a Si1TDPP-EE-COC1 molecule where the C6H13 (C6) side chains were replaced by CH3 (C1) groups was geometrically optimized to an energy minimum. The Gaussian 09 program was used at the DFT B3LYP level with

EE-COC6 showed two absorption bands with peaks at 391 and 678 nm. Compared to the UV−visible absorption spectrum predicted by TD-DFT, Si1TDPP-EE-COC6 displayed blueshifted features,33,34 but the overall shapes agreed well. The UV−visible absorption spectrum of the as-cast film was redshifted by 20 nm at the peaks and displayed a shoulder in the long wavelength region. The UV−visible absorption spectra of the thermally annealed films were further red-shifted, and the shoulder peak became more pronounced. Both the large red shift and the clear shoulder were signatures of the increased intermolecular interactions among the Si1TDPP-EE-COC6 molecules.35,36 Compared to the UV−visible absorption spectrum of Si1T-DPP-EE-C6, the spectrum of Si1TDPP-EECOC6 was slightly red-shifted in both the solution and film states (Figure S4, Supporting Information), as predicted by the DFT calculations. The onsets of the absorption spectra were 765 and 862 nm in the solution and film states, respectively, which corresponded to 1.62 and 1.44 eV. The electrochemical E


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Figure 3. (a) GIXD images of the as-spun and 150 °C annealed Si1TDPP-EE-COC6 films. (b) Out-of-plane X-ray diffraction line profiles extracted along the qz direction at qy = 0.00 Å−1. The inset shows the radial profile at the (200) peak. (c) In-plane X-ray diffraction line profiles extracted along the qy direction at qz = 0.03 Å−1.

the (010) peak along the qy direction were much more pronounced. Moreover, much less diffuse scattering around the (h00) diffraction peaks was observed. More detailed information was obtained by extracting the 1D profiles along the qz and qy directions from the GIXD patterns of the Si1TDPP-EE-COC6 films. Figure 3b shows the out-ofplane profiles extracted along the qz direction at qy = 0.00 Å−1. The as-spun Si1TDPP-EE-COC6 film exhibited strong diffraction peaks at qz = 0.44 and 0.86 Å−1, corresponding to an (h00) reflection with a d(h00) spacing of 14.5 Å. Annealing the Si1TDPP-EE-COC6 film at 150 °C resulted in the increased (h00) diffraction peak intensity, and the (200) diffraction peak grew in distinctly. The lamellar spacing in the out-of-plane direction increased from 14.5 to 15.6 Å, presumably because the 2-ethylhexyl chains were stretched out. Figure 3c shows the in-plane profiles extracted along the qy direction at qz = 0.03 Å−1. Upon thermal annealing, the intensity of the (010) peak increased, and the estimated π−π stacking distance decreased from 3.67 to 3.51 Å, consistent with the results obtained from the out-of-plane profile. Thermal annealing enforced a layered edge-on orientation of the Si1TDPP-EE-COC6 molecules along the (h00) axis, such that the π−π stacking among the molecules was oriented parallel to the substrate. The thermally induced highly oriented close π−π stacking among molecules improved carrier transport along the OTFT channel parallel to the gate substrate.27,29 The angular distribution of the crystalline plane orientation with respect to the substrate could be assessed from the full width at half-maximum (fwhm) of the (h00) peak profile along the azimuthal angle (χ). The inset of Figure 3b shows the intensity along the azimuthal angle through the (200) peak obtained from the as-spun Si1TDPP-EE-COC6 films annealed at 150 °C. The fwhm of the (200) peak was reduced dramatically from 12.6 to 9.4° upon thermal annealing. Therefore, the thermal annealing-induced carrier mobility enhancement could be understood as the development of both a higher degree of crystalline ordering and better alignment of both (h00) and (0k0) planes with respect to the substrate surface.

properties were measured by cyclic voltammetry. The Si1TDPP-EE-COC6 film displayed an onset of oxidation curve (Eonset,ox) at 0.35 V (Figure S5, Supporting Information), which corresponded to a HOMO of −5.15 eV, based on the equation: HOMO = −(4.8 + Eonset,ox). Considering the HOMO and the optical band gap, the LUMO was located at −3.52 eV. The HOMO/LUMO levels of the Si1TDPP-EE-COC6 were slightly lower than those of the Si1TDPP-EE-C6 molecules, as expected. Note that for Au (work function 5.2 eV) source/drain electrodes, an injection of holes into the Si1TDPP-EE-COC6 channel would be more favored than an injection of electrons. These results suggested that the use of the SLG electrodes (work function 4.5 eV) would be better than Au electrodes to attain the balanced hole and electron injection.27,28 (The UPS spectra of the Au and SLG are shown in Figure S6, Supporting Information). The thermal properties of the molecules were analyzed by differential scanning calorimetry (DSC). The DSC curve displayed a melting temperature (Tm) at 199.5 °C and a crystallization temperature (Tc) of 161.8 °C, as shown in Figure S7 (Supporting Information). The Tm and Tc are higher than those of the Si1TDPP-EE-C6 (Tm = 189 °C and Tc = 158 °C) presumably because of the increased dipolar interaction via the insertion of ketone groups, which increased the optimal thermal annealing temperature from 110 to 150 °C in the film preparation and device fabrication (see below). The thermal annealing effects of the Si1TDPP-EE-COC6 thin films were understood by analyzing GIXD patterns that were attained from the synchrotron X-ray beam. Figure 3a displays the GIXD patterns of the as-spun and 150 °C annealed Si1TDPP-EE-COC6 films. The as-spun film showed intense (h00) diffraction peaks along with second- and third-order peaks, a weak (010) diffraction intensity along the out-of-plane (qz) direction, and a weak (010) diffraction peak along the inplane (qy) direction. These results indicated that the backbones of the molecules were parallel to the substrate.3,37,38 The molecular chains in the as-spun film readily formed a layered edge-on structure on the substrate during solvent evaporation due to the strong intermolecular interactions and effective π−π stacking among the planar molecules.5,9,13,39 After thermal annealing, the (h00) diffraction peaks along the qz direction and F


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in the as-spun and thermally annealed films. The average hole and electron mobilities were extracted from ID−VG plots measured in the respective saturation regimes according to the relationship2

The surface profiles of the as-spun and 150 °C annealed Si1TDPP-EE-COC6 films were revealed using AFM. Figures 4a

ID = CiμW (VG − Vth)2 /2L

where Ci is the specific capacitance of the gate dielectric (11 nF/cm2); μ is the carrier mobility; L and W are the channel length and width, respectively; and Vth is the threshold voltage. The as-spun Si1TDPP-EE-COC6 OTFT exhibited a hole mobility of 3.3 × 10−4 cm2 V−1 s−1 and an electron mobility of 1.7 × 10−4 cm2 V−1 s−1. The fact that the hole mobility exceeds the electron mobility is mainly related with a more favorable hole injection from the Au to the Si1TDPP-EE-COC6 active channel.1,15,17 Thermal annealing of the film at 100 or 150 °C significantly improved the carrier mobilities. The 150 °C annealed Si1TDPP-EE-COC6 film yielded hole and electron mobilities of 0.003 and 0.002 cm2 V−1 s−1, respectively. On/off current ratios also increased gradually upon thermal annealing. Table 1, Table 2, and Figure 5b summarize the changes in the carrier mobilities, threshold voltages, and on/off current ratios upon thermal annealing. These carrier mobilities exceeded the previously reported values obtained from small molecule-based ambipolar OTFTs.19,40 It should be noted that as intended the introduction of ketone groups into the side chains of the Si1TDPP-EE-C6 molecules effectively improved the electron mobility by slightly lowering the molecular LUMO level. The enhanced carrier mobilities were attributed to the enhanced layered edge-on structures and the film morphologies of the Si1TDPP-EE-COC6 film, as discussed in the GIXD and AFM results. Figure 5c shows the ID−VD plot obtained from Au electrode-based OTFTs prepared using 150 °C annealed Si1TDPP-EE-COC6, with hole or electron accumulation. The device exhibited both a superlinear increase and saturation behaviors in ID. The electrical performances of the Si1TDPP-EE-COC6 OTFTs were further improved by applying SLG source− drain electrodes. Note that this is the first example of ambipolar OTFTs prepared with SLG electrodes and small molecule semiconductors. Figure 6a schematically illustrates the fabrication of the patterned SLG source/drain electrodes. The fabrication procedure of patterned SLG electrodes is described in the Experimental Section. The Si1TDPP-EE-COC6 solution was spin-coated to form OTFTs and then thermally annealed at 25, 100, or 150 °C. Figure 6b shows the transfer characteristics

Figure 4. Topographic [(a) and (b)] and phase images [(c) and (d)] of as-spun and 150 °C annealed Si1TDPP-EE-COC6 films.

and 4b show the topographic and phase images of both the molecular films. The as-spun Si1TDPP-EE-COC6 films contained fine granular morphologies, whereas the 150 °C annealed films induced much larger crystalline nanostructures. The development of the crystalline nanostructures was accompanied by an increase of the surface roughness values from 2.21 to 4.18 nm. Overall, the GIXD and AFM results clearly showed that thermal annealing improved the film crystallinity and morphology, which affected the performances of OTFTs prepared based on Si1TDPP-EE-COC6. The electrical properties of the Si1TDPP-EE-COC6 films were investigated after fabricating bottom-contact bottom-gate OTFT devices. Source/drain electrodes were formed with Au. Figure 5a shows the drain current (ID)−gate voltage (VG) plots at a drain voltage (VD) of −80 (hole-enhancement mode) or +80 V (electron-enhancement mode), in OTFTs prepared based on Si1TDPP-EE-COC6 annealed at 25 (as-spun), 100, and 150 °C. Ambipolar charge transport behavior was observed

Figure 5. (a) Transfer characteristics at a VD of −80 V or +80 V for Au source/drain electrode-based OTFTs prepared with a Si1TDPP-EE-COC6 film active material annealed at three temperatures: 25 (black), 100 (red), or 150 °C (blue). (b) Carrier mobilities of the Au-based Si1TDPP-EECOC6 OTFTs with increasing annealing temperature. (c) The output characteristics of the Au source/drain electrode-based OTFTs prepared with 150 °C annealed Si1TDPP-EE-COC6 films. G


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Figure 6. (a) Schematic diagram showing the procedure used to fabricate SLG source−drain electrodes. (b) Transfer characteristics at a VD of −80 V or +80 V, obtained from OTFTs prepared with SLG source−drain electrodes based on Si1TDPP-EE-COC6 films annealed at three temperatures: 25, 100, or 150 °C. (c) Carrier mobilities of the SLG electrode-based Si1TDPP-EE-COC6 OTFTs with increasing the annealing temperature. (d) Output characteristics of OTFTs prepared with SLG electrodes, using 150 °C annealed Si1TDPP-EE-COC6 films.

both the hole and electron transport in the SLG-based OTFTs reflected that the better interfacial contact contributed more in charge transport than the lowered work function did. Third, it is interesting that the balanced transport was observed in both the Au- and SLG-based OTFTs upon 150 °C annealing. The thermal annealing must have changed the molecular ordering and alignment on/near the electrode favorably for both carriers, as we observed the increased molecular crystallinity and better molecular alignment in the channel region upon thermal annealing. In conclusion, the enhanced and balanced transport on using the SLG electrodes is a result of the interplay between interfacial contact, interfacial molecular ordering and alignment, and energy level alignment, although the details on the molecular crystallinity and orientation at the interface between molecules and electrodes are still under investigation.

of SLG OTFTs prepared based on Si1TDPP-EE-COC6 annealed at 25 (black), 100 (red), or 150 °C (blue). The current levels at a fixed gate voltage were higher than those obtained from the Au source/drain electrode-based OTFTs. The devices prepared based on as-spun Si1TDPP-EE-COC6 yielded a hole mobility of 4.9 × 10−4 cm2 V−1 s−1 and an electron mobility of 1.7 × 10−3 cm2 V−1 s−1. The carrier mobilities were further improved by thermal annealing. Figure 6c summarizes the carrier mobilities of the SLG OTFTs after thermal annealing. 150 °C annealed SLG OTFTs yielded an average hole mobility of 0.011 cm2 V−1 s−1 and an average electron mobility of 0.015 cm2 V−1 s−1. Figure 6d shows the ID−VD plot obtained from the SLG OTFTs prepared based on the 150 °C annealed Si1TDPP-EE-COC6, indicating wellbalanced hole and electron conduction. Here we made several important observations. First, the major transporting carrier type switched from holes in the Aubased OTFT to electrons in the SLG-based OTFT for 25 °C annealed Si1TDPP-EE-COC6. The higher electron mobility in the SLG-based OTFT, as compared to the hole mobility, can be ascribed to the lower electron injection barrier between Si1TDPP-EE-COC6 and SLG when the energetic alignment between molecular HOMO/LUMO and the employed electrode Fermi levels was considered. Second, both the hole and electron mobilities of the SLG-based OTFTs exceeded the corresponding values of the Au-based OTFTs, suggesting that the improved electrical performance originated from the better contact at the interface of Si1TDPP-EE-COC6/SLG electrodes. We can expect that the better interfacial contact can be made by the extremely low thickness (3−4 Å) of the SLG electrode and also by the structural similarities between the SLG electrode and the π-conjugated molecules, compared to the Au electrode.27 Moreover, the significant improvement in

4. CONCLUSIONS We demonstrated the synthesis of a low band gap dithienosilole-based small molecule, Si1TDPP-EE-COC6, and its application to OTFTs. The as-spun Si1TDPP-EE-COC6 films exhibited ambipolar transport behavior with relatively low hole and electron mobilities on the order of ∼10−4 cm2 V−1 s−1. Thermal annealing significantly improved the carrier mobilities by providing a hole mobility of 0.003 cm2 V−1 s−1 and an electron mobility of 0.002 cm2 V−1 s−1 upon thermal annealing the Si1TDPP-EE-COC6 OTFTs at 150 °C. The improved OTFT performance was strongly correlated with the enhanced layered edge-on structure and the reduced π−π intermolecular spacing via thermal treatment. The use of the SLG electrode further increased the carrier mobilities to provide a hole mobility of 0.011 cm2 V−1 s−1 and an electron mobility of 0.015 cm2 V−1 s−1 in SLG-based OTFTs prepared with 150 °C annealed Si1TDPP-EE-COC6 films. This work demonstrates H


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that the insertion of electron-withdrawing ketone groups between π-conjugated backbones and the alkyl side chains effectively improved the electron mobility, and flat SLG provides an excellent electrode material for boosting ambipolar transistor performance.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR and 13C NMR spectra, MALDI-TOF spectrum, CV voltammogram, and DSC thermogram of Si1TDPP-EE-COC6. Comparison of the UV−visible absorption spectra of Si1TDPPEE-C6 and Si1TDPP-EE-COC6. UPS spectra of Au and SLG. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02308.

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AUTHOR INFORMATION

Corresponding Authors

*Prof. BongSoo Kim. Tel.: +82 2 3277 5954. Fax: +82 2 3277 2684. E-mail: [email protected]. *Prof. Jeong Ho Cho: Tel.: +82 31 299 4165. Fax: +82 31 299 4119. E-mail: [email protected]. Author Contributions ⊥

W. Kang and M. Jung equally contributed to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by New and Renewable Energy Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Goverment Ministry of Trade, Industry & Energy (MTIE) (20113030010030 and 20133030000130), and by Korea Institute of Science and Technology (KIST) Internal Project, and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013R1A1A2011897 and 2009-0083540).

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REFERENCES

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Well-Balanced Carrier Mobilities in Ambipolar Transistors Based on

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