Low-Band-Gap Polymer-Based Ambipolar Transistors and Inverters

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Low-Band Gap Polymer-Based Ambipolar Transistors and Inverters Fabricated Using a Flow-Coating Method Min Je Kim, Jae Hoon Park, Boseok Kang, Dongjin Kim, A-Ra Jung, Jee Hye Yang, Moon Sung Kang, Dong Yun Lee, Kilwon Cho, Hyunjung Kim, BongSoo Kim, and Jeong Ho Cho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01371 • Publication Date (Web): 14 Jun 2016 Downloaded from http://pubs.acs.org on June 23, 2016

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The Journal of Physical Chemistry

Low-Band Gap Polymer-Based Ambipolar Transistors and Inverters Fabricated Using a Flow-Coating Method Min Je Kim,1† Jae Hoon Park,1† Boseok Kang,2 Dongjin Kim,3 A-Ra Jung,4 Jeehye Yang,5 Moon Sung Kang,5 Dong Yun Lee,6 Kilwon Cho,2 Hyunjung Kim,3 BongSoo Kim,4,* and Jeong Ho Cho1,* 1

SKKU Advanced Institute of Nanotechnology, School of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 16419, Republic of Korea 2 Department of Chemical Engineering, Pohang University of Science and Technology, 77 Cheongam-ro, Nam-gu, Pohang 37673, Republic of Korea. 3 Department of Physics, Sogang University, 35 Baekbeom-ro, Mapo-gu, Seoul 04107, Republic of Korea 4 Department of Science Education, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea 5 Department of Chemical Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 06978, Republic of Korea 6 Department of Polymer Science and Engineering, Kyungpook National University, 80 Daehakro, Daegu, 41566, Korea †

M. J. Kim and J. H. Park equally contribute to this work.

*Corresponding authors: Professor BongSoo Kim: Telephone +82-2-3277-5954; Fax +82-2-3277-2369; Email: [email protected] Department of Science Education, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea Professor Jeong Ho Cho: Telephone +82-31-299-4165; Fax +82-31-299-4119; Email: [email protected] SKKU Advanced Institute of Nanotechnology, School of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 16419, Republic of Korea

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Abstract The performances of organic thin film transistors (OTFTs) produced by polymer solution casting 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

are tightly correlated with the morphology and chain-ordering of semiconducting polymer layers, which depends on the processing conditions applied. The slow evaporation of a high boiling point (bp) solvent permits sufficient time for the assembly of polymer chains during the process, resulting in improving the film crystallinity and inducing favorable polymer chain orientations for charge transport. The use of high bp solvents, however, often results in de-wetting of thin films formed on hydrophobic surfaces, such as the commonly used octadecyltrichlorosilane (ODTS)-treated SiO2 gate dielectric. De-wetting hampers the formation of uniform and highly crystalline semiconducting active channel layers. In this manuscript, we demonstrated the formation of highly crystalline dithienothienyl diketopyrrolopyrrole (TT-DPP)-based polymer films using a flow-coating method to enable the fabrication of ambipolar transistors and inverters. Importantly, unlike conventional spin-coating methods, the flow-coating method allowed us to use high bp solvents, even on a hydrophobic surface, and minimized the polymer solution waste. The crystalline orientations of the TT-DPP-based polymers were tuned depending on the solvent used (four different bp solvents were tested) and the employment of a thermal annealing step. The use of high bp solvents and thermal annealing of the polymer films significantly enhanced the crystalline microstructures in the flowcoated films, resulting in considerable carrier mobility increase in the OTFTs compared to the spin-coated films. Our simple, inexpensive, and scalable flow-coating method, for the first time employed in printing semiconducting polymers, presents a significant step toward optimizing the electrical performances of organic ambipolar transistors through organic semiconducting layer film crystallinity engineering.


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Introduction 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

Solution-processed organic thin film transistors (OTFTs) have been intensively studied for several decades for their applicability to flexible electronic applications, including flexible displays, e-paper, radiofrequency identification tags, and near-field communication-enabled tags.1-3 The performances of OTFTs have advanced over time, thanks to tremendous efforts applied toward developing new semiconducting materials. The carrier mobilities of both holes and electrons have recently exceeded >1 cm2V-1s-1, surpassing the carrier mobilities in amorphous silicon semiconductor-based devices.4-6 Most recently-developed highperformance OTFT materials consist of an electron-rich donor moiety (D),7-10 an electron-deficient moiety (A),11-16 and long alkyl side chains.17-23 These donor-acceptor (D-A) conjugated polymers display a highly crystallinity and tight packing in the film state.5, 24-25 The interactions among the polymer chains may be modulated through the selection of the processing solvent and the use of a thermal annealing process.17, 26-28 Solution-processed OTFT studies commonly use chloroform (CF) as the processing solvent and the spin-coating method as a very simple and popular method of generating the semiconducting polymer films. Bottom-gate OTFT devices based on octadecyltrichlorosilane (ODTS)-treated silicon dioxide (SiO2) dielectrics are one of the most basic test platforms, presumably because CF is a good solvent for most D-A polymers, and spin-coating of a polymer solution in CF is easy due to its compatibility with many substrates. On the contrary, the use of a high boiling point (bp) solvent as the processing solvent yields more crystalline cast films compared to those obtained from CF, and the carrier mobilities can be improved. Higher bp solvents of toluene (Tol), chlorobenzene (CB), and o-dichlorobenzene (DCB) than CF have better solvating powers for semiconducting polymers and provides longer drying times while forming polymer films because of their lower vapor pressures (the bps and vapor pressures of those solvents are summarized in Table 1). When dissolved in those high bp solvents, polymer chains can be fully disentangled, and they re-assembled with good organization during film drying.26, 29-31 Therefore, the use of those high bp solvents to improve DA polymer-based OTFT performances would be beneficial. However, it is difficult to form high quality polymer films by spin-coating such high bp solutions onto hydrophobic gate dielectric surfaces, such as ODTS-treated SiO2 gate dielectrics. This problem occurs because the surface energies of the high bp solvents are different from the ODTS-treated SiO2 surface, considering that the contact angles of CF, Tol, CB, and DCB on the ODTS-treated SiO2 surface are 25, 34, 44, and 53°, respectively.27 In an attempt to reduce the surface energy mismatch, we tested the spin-coating of polymer solutions prepared using binary solvents. Polymers dissolved in a CF:high bp solvent (vol. ratio = 16:1) successfully formed films on the ODTS-treated SiO2 gate dielectric, and the solution containing the high-bp solvent produced more crystalline D-A polymer films and higher carrier mobilities in the OTFT devices. Moreover, low bp solvents, such as CF, are not useful for large-area, high-throughput printing of D-A polymer solutions. A single highbp solvent is preferable. The spin-coating method tends to spread most of the polymer solution across the substrate during spinning, leading to a large amount of polymer solution waste. Therefore, appropriate 3 ACS Paragon Plus Environment


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solution-coating techniques must be developed to enable the use of high bp solvents for depositing large1 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

area uniform organic semiconducting thin films onto hydrophobic gate dielectric channel surfaces. Here, we describe the development of a flow-coating method of producing ambipolar transistors and inverters. This method was used to cast organic semiconducting films, enabling the facile application of high-bp processing solvents on hydrophobic surfaces and minimizing wasted polymer. Note that this technique may be used to fabricate conformal large-area polymer solution layers using flexible plastic blades and by imposing a shear force on the polymer solutions during coating.32 Poly(2,5-bis(2-decyltetradecyl)-3(5-(dithieno[3,2-b:2',3'-d]thiophen-2-yl)thieno[3,2-b]thiophen-2-yl)-6-(thieno[3,2-b]thiophen-2-yl)-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione) (PTTDPP-DT-DTT) solutions dissolved in four different solvents (CF, Tol, CB, or DCB) were flow-coated onto a hydrophobic ODTS-treated SiO2 dielectric surface. The combination of the high-bp solvent and thermal annealing dramatically enhanced the crystalline microstructures and long-range ordering in the films, thereby increasing the charge carrier mobilities of the OTFTs. This work reveals that a simple flow-coating method may be useful for applying organic semiconductor coatings onto a variety of hydrophobic surfaces to enhance the film crystallinity and optimize the electrical performances of the organic transistors.

Experimental section OTFTs based on PTTDPP-DT-DTT polymer were prepared and the electrical properties of OTFTs were measured as reported previously,27 except that the PTTDPP-DT-DTT film was deposited onto the octadecyltrichlorosilane-treated SiO2 surfaces by flow-coating a 0.2 wt% polymer solution in CF, Tol, CB, or DCB. The thicknesses of the flow-coated PTTDPP-DT-DTT films were ~48 nm. The detailed description of flow-coating method is provided below. UV-visible absorption spectroscopy was performed on a Perkin Elmer Lambda 9 UV-VIS spectrophotometer. Grazing incidence X-ray diffraction (GIXD) experiments on the flow-coated polymer films were done at the Pohang Light Source II in Republic of Korea with the 0.1110 nm X-ray source. The detectors were PI-SCX: 4300 charge-coupled device arrays. The flow-coated polymer film surface was probed by using Veeco Dimension D3100 atomic force microscopy (AFM) in tapping-mode.

Results and discussion Figure 1a shows a photographic image of our flow-coating setup. The angled (45°) polyethylene terephthalate (PET) blade was coupled to a vertical translation stage, and a linear translation stage was coupled to a piezo nanopositioner.33 The substrate temperature was heated mildly to 60°C by a hot plate positioned under the linear translation stage to accelerate solvent evaporation. The PTTDPP-DT-DTT solutions dissolved in four different solvents (CF, Tol, CB, or DCB) were introduced between the PET blade and the ODTS-treated SiO2/Si substrate. The solution was trapped by capillary forces, as shown in Figure 1b.32 As the linear translation stage was moved at a fixed velocity (0.2 mm/s), the semiconducting films 4 ACS Paragon Plus Environment

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formed uniformly onto the ODTS-treated SiO2/Si substrate. As mentioned above, spin-coating the PTTDPPDT-DTT solution in DCB onto the hydrophobic ODTS-treated substrate did not form a film on the surface due to both the inadequate wettability of the solution on the hydrophobic ODTS surface and the slow solvent evaporation rate (Figure 1c (bottom)). However, the polymer blade trapped the solvent under the blade, and mild heating of the substrate facilitated uniform film formation prior to de-wetting, as shown in Figure 1c (top). Note that only 0.02 mL of a 0.2 wt% polymer solution was required to achieve uniform and fully covered semiconducting films over the entire surface area (2 × 2 cm2), a much smaller amount than was needed in typical spin-coating methods (0.5 mL for CF). The molecular orientations and long-range order in the flow-coated PTTDPP-DT-DTT films prepared with four different solvents (CF, Tol, CB, or DCB) were studied using GIXD techniques. Thermal annealing can increase film crystallinity; therefore, the DCB-processed thin films were heated at 100 or 200 °C on a hot plate for 30 min, and the post-annealed films were examined. Figure 2 shows 2D GIXD images of the PTTDPP-DT-DTT films flow-coated using different solvents and post-annealed PTTDPP-DTDTT films. The (100) and (200) reflections in the out-of-plane (qz) direction and the (010) reflection in the in-plane direction (qxy) were observed in all samples, indicating an edge-on orientation among the PTTDPPDT-DTT polymers in the thin film state. The conjugated planes of the polymer chains were aligned parallel to the silicon wafer, whereas the 2-decyltetradecyl chains lay perpendicular to the silicon wafer.19-21 As the bp of the solvent increased, (100) and (200) reflections in the qz direction progressively intensified, and the higher-order peaks of (300) and (400) became visible. The (010) reflection along the qxy direction, in particular, was significantly enhanced in the DCB-used film, and post-annealing boosted the (010) reflections further.23, 34-35 Additional analyses were conducted using line-cut profiles along the qz or qy directions. Figure 3a shows the line-cut profiles in the out-of-plane (qz) direction at qy = 0.00 Å–1. All flow-coated PTTDPP-DTDTT films revealed intense (100) and (200) peaks at qxy = 0.29 and 0.58 Å–1, which corresponded to a d(h00) spacing of 21.7 Å. The (010) reflection was negligible along the out-of-plane direction, confirming the edgeon polymer chain orientation in the films. The use of high-bp solvents and a post-annealing process increased the intensities of the (100) and (200) peaks considerably compared to those of the films processed with low-bp solvents and also yielded pronounced high-order (h00) reflections, up to the 4th order increase in the crystalline portion of the thin films (Figure 3b). The full-width-at-half maximum (FWHM) values of the (100) reflections were similar for those obtained from all four solvents although they decreased with the annealing temperature. These results suggested that the higher intensities of the (100) reflections obtained using high-bp solvents resulted mainly from an increase of crystallite numbers and post-annealing increased the growth of the larger crystallites while generating longer-range organization of the polymer backbones. The correlation lengths determined according to the Scherrer equation are provided in Table S1 in the Supporting Information;36 the values varied from 20.6 nm to 29.9 nm, according to the use of the high-bp solvent and post-annealing step. The intensity profiles along the in-plane direction were obtained (Figure 5 ACS Paragon Plus Environment


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3c). All (010) reflections were observed at the same position, 1.76 Å–1, corresponding to a π-π interchain 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

stacking distance of 3.57 Å. The integrated areas and FWHM values of the in-plane (010) reflections followed a trend similar to that obtained from the (100) reflections along the out-of-plane direction (Figure 3d), further supporting the claim that the quantity of crystallites present in the as-cast films was related to the solvent evaporation rate, whereas the crystallite quality depended on the subsequent thermal annealing process. The GIXD results reflected that the flow-coated PTTDPP-DT-DTT thin films became quite crystalline through the use of a high-bp solvent, and subsequent thermal treatment induced an increased film crystallinity with an edge-on orientation of polymer chains that favored carrier transport between the source and the drain.20, 37-40 Note that the flow-coating did not form a noticeable degree of anisotropic crystallinity, which was revealed by probing a flow-coated polymer film with x-ray beams incident from two different directions (Figure S1). The UV-visible absorption characteristics of the flow-coated PTTDPP-DT-DTT films provided information about the interchain interactions among the PTTDPP-DT-DTT polymers. Figure 4a shows the UV-visible absorption spectra of the flow-coated PTTDPP-DT-DTT films obtained from CF, Tol, CB, and DCB. The spectrum of the as-coated film obtained from CF (black curve) included three main absorption peaks. The band I at 432 nm was attributed to electronic transitions from the low-lying sub-HOMO levels to the upper-lying unoccupied states, the band II at 701 nm was assigned to the HOMO-to-LUMO electronic transition, and the band III at 778 nm was attributed to interchain aggregation.27 As the solvent bp increased, the as-coated films exhibited a progressive red shift in the absorption peaks. The band III peaks were observed at 794, 798, and 798 nm for Tol, CB, and DCB, respectively. This red-shift suggests that the highbp solvents promoted the interchain interactions in the solid state.26, 31, 41 Thermal annealing further redshifted the absorption peaks, indicating a further enhancement in the interchain interactions, as indicated in Figure 4b,c. The higher bp solvent of DCB made a slightly larger change than CF upon thermal annealing. The UV-visible absorption spectra obtained in the films prepared using a high-bp solvent and subsequent thermal treatment were in good accord with the enhanced crystalline structures as evidenced by the GIXD results (Figure 2 and 3). The PTTDPP-DT-DTT film surfaces were imaged by using AFM and their morphologies are shown in Figure 5. The PTTDPP-DT-DTT films prepared from CF solution exhibited finely aggregated surfaces and thermal annealing increased surface roughness slightly (Figure 5a-c). The polymer films prepared from DCB solution displayed highly crystalline networks with a larger roughness that were originated from a high degree of interchain π-π interactions. The elongated fibrous features of the polymer aggregates were further enhanced in the post-annealed films (Figure 5d-f). Tol and CB solvents formed intermediate morphological structures (Figure 5g,h). The polymer film from Tol solution appeared similar to that from CF solution with slightly more grown aggregates. The polymer film from CB solvent looked closer to that from DCB solvent with narrower fibers. Thus, we concluded that the surface roughness increased gradually upon using the high-bp solvent and additional thermal annealing, accompanying the increase in crystallinity (see above). 6 ACS Paragon Plus Environment

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Table 1. Carrier Mobilities of the PTTDPP-DT-DTT OTFTs processed with four solvents and thermal treatment. Vapor Carrier 25 °C 100 °C 200 °C Boiling pressure at 20 °C Solvent typea (cm2V-1s-1) (cm2V-1s-1) (cm2V-1s-1) Point (oC) (mmHg) CF

61

Tol

111

CB

131

DCB a

180

160

22

8.8

1.2

h+

0.033 ± 0.003 0.042 ± 0.002 0.076 ± 0.005

e-

0.008 ± 0.001 0.016 ± 0.002 0.017 ± 0.002

h+

0.057 ± 0.005 0.084 ± 0.006 0.133 ± 0.004

e-

0.009 ± 0.002 0.014 ± 0.001 0.031 ± 0.003

h+

0.066 ± 0.007 0.095 ± 0.007 0.135 ± 0.008

e-

0.009 ± 0.002 0.019 ± 0.003 0.033 ± 0.005

h+

0.113 ± 0.013 0.114 ± 0.014 0.298 ± 0.017

e-

0.010 ± 0.001 0.045 ± 0.004 0.109 ± 0.007

hole: h+ and electron: e-

Bottom-gate and top-contact polymer thin-film transistors (PTFTs), where the PTTDPP-DT-DPP films were cast by the flow-coating method, were fabricated and their electrical properties were characterized. Figure 6a displays the transfer curves at drain voltages (VD = –60 or +60 V) for the PTTDPPDT-DTT PTFTs. The curve exhibited ambipolar characteristics in hole enhancement mode at VD = –60 V and electron enhancement at VD = +60 V mode. Note that the current level in the devices increased if highbp solvents were used during casting. The carrier mobilities of the ambipolar transistors were extracted in the respective saturation regimes based on the following equation:42 ID = CsµW(VG – Vth)2/2L, where Cs is the specific capacitance of 11 nF/cm2 for the 300 nm thick SiO2 dielectric layer, µ is the carrier mobility, W is the channel width, Vth is the threshold voltage, and L is the channel length. Figure 6b shows the carrier mobility of the PTTDPP-DT-DTT PTFTs depending on the solvent bp used during flow-coating. The PTFTs based on the PTTDPP-DT-DPP films flow-coated from a polymer solution in CF, the lowest bp solvent, exhibited a hole mobility of 0.033 ± 0.003 cm2V–1s–1 and an electron mobility of 0.008 ± 0.003 cm2V–1s–1. The use of a high-bp solvent increased the carrier mobility. For example, DCB, with the highest bp among the solvents tested, yielded enhanced carrier mobilities for hole (0.113 ± 0.013 cm2V–1s–1) and electron (0.010 ± 0.004 cm2V–1s–1) transport. The electrical performance of the PTFTs prepared using a PTTDPP-DT-DTT film flow-coated from DCB were dramatically enhanced by a post thermal annealing step, as shown in Figure 6c. The as-cast PTTDPP-DT-DTT films were annealed at 100 or 200 °C. As the annealing temperature increased, both carrier mobilities increased dramatically. For example, the hole mobility and electron mobility of the 200°C7 ACS Paragon Plus Environment


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annealed TTDPP-DT-DTT PTFTs devices were found to be 0.298 and 0.109 cm2V–1s–1, respectively. The 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

improved electrical properties as a result of the use of high-bp solvents and subsequent thermal treatment agreed well with the features of the better interchain packing and longer-range assembly of the PTTDPPDT-DTT polymers, which were confirmed by GIXD, UV-vis spectroscopy, and AFM results. Table 1 summarizes the carrier mobilities of the devices and Figure S2 shows the annealing temperature-dependent transfer characteristics for the PTTDPP-DT-DTT films that were flow-coated from CF, Tol, and CB solvents. It should be emphasized that the flow-coated films outperformed spin-coated films using CF or a mixed CF:high bp solvent,27 which is because of their improved crystallinity and edge-on orientations. It is also interesting to note that the surface roughness of the flow-coated film was relatively lower than that of the spin-coated film while the film crystallinity increased in the flow-coating process, which would be beneficial to a good contact between the polymer films and electrodes. The output curves (ID vs. VD) at various fixed VGs applied to the 200°C-annealed PTTDPP-DT-DTT PFETs (Figure 6d) exhibited rectifying curves at low gate biases and saturation at high gate biases (flow-coated from DCB). Finally, inverter devices were fabricated by linking two ambipolar PTFTs based on PTTDPP-DTDTT films that were prepared using a polymer solution of DCB and thermally treated at 200 °C. One of the transistors was linked to the supply voltage (VDD), the other was linked to the ground, and two transistors shared an input gate terminal (VIN) and an output terminal (VOUT). Figure 7 displays VOUT as a function of VIN at a constant VDD and a circuit diagram. The device exhibited a good inverter operation as sweeping the VIN. Signal inversion was observed in both the positive and negative VDD regions. Unlike typical complementary metal-oxide-semiconductor (CMOS) inverters, the ambipolar behavior of the constituent PTTDPP-DT-DTT PTFTs lead to no saturation of the output voltage to zero or VDD. The signal inverter gain, i.e. the absolute value of dVOUT/dVIN, of the inverter was obtained to be 4.6 at VDD = +70 V.

Conclusions In summary, we fabricated highly crystalline PTTDPP-DT-DTT semiconducting films using a flowcoating method, and their applications in ambipolar transistors and inverters were explored. The electrical performances of OTFTs improved with increasing the solvent bp because the high-bp solvents provided sufficient time for semiconductor polymer chains to self-organize. Thermal annealing further increased the OTFT performances. GIXD, UV-vis spectroscopy, and AFM measurements unveiled that the high-bp solvents promoted the crystallinity of flow-coated films, increased the degree of edge-on orientation, and improved the long-range crystalline ordering, resulting in higher carrier mobilities in the organic transistors. The flow-coating method not only allows us to use various processing solvents for the large-area printing but also provides a great potential in enhancing the electrical performances of solution-processable organic semiconductor-based transistors.


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Supporting Information 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

Table S1. FWHM values and the correlation lengths of the PTTDTT-DT-DTT thin films. Figure S1. X-ray diffraction patterns of the PTTDPP-DT-DTT film that was flow-coated from a DCB solution and subsequently annealed at 200 °C. Figure S2. The transfer curves for the PTTDPP-DT-DTT PTFTs processed from the polymer solutions of CF, Tol, and CB. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments This work was supported from the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (grant no. NRF-2015M1A2A2056218), Basic Science Research Program through NRF funded by the Ministry of Education

(grant

no.

NRF-2015R1D1A1A01058493,

NRF-2014R1A2A1A10052454,

NRF-

2013R1A1A2011897), and Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (grant no. NRF-2013M3A6A5073177).



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Figure 1. (a) Photographic and (b) schematic images of our flow-coating setup. The chemical structure of PTTDPP-DT-DTT is shown in (b). (c) Optical images of the PTTDPP-DT-DTT films fabricated using the flow-coating or spin-coating methods, from DCB as the processing solvent. The color of the spin-coated polymer film was nearly identical to that of the bare SiO2/Si substrate.



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Figure 2. 2D GIXD patterns obtained from the PTTDPP-DT-DTT films flow-coated from (a) CF, (b) Tol, (c) CB, or (d) DCB solvents, and from the PTTDPP-DT-DTT film flow-coated from DCB solvent and annealed at (e) 100°C or (f) 200°C.


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Figure 3. (a) Out-of-plane X-ray diffraction profiles, extracted along the qz direction at qxy = 0.00 Å–1. (b) Integrated peak area and FWHM of the out-of-plane (100) peak. (c) In-plane X-ray diffraction profiles, extracted along the qxy direction at qz = 0.03 Å–1. The reflection marked by an asterisk at 1.5 Å–1 revealed diffuse rings that may have arisen from disordered alkyl side chains.17-18 (d) Integrated peak area and FWHM of the in-plane (010) peak.



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Figure 4. (a) UV-visible absorption spectra of the PTTDPP-DT-DTT films flow-coated from various solvents: CF, Tol, CB, or DCB. (b) UV-visible absorption spectra of the PTTDPP-DT-DTT films flowcoated from CF solutions and annealed at two different temperatures, 100 or 200°C. (c) UV-visible absorption spectra of the PTTDPP-DT-DTT films flow-coated from DCB solutions and annealed at two different temperatures, 100 or 200°C.


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Figure 5. AFM images obtained from the PTTDPP-DT-DTT films flow-coated from (a, b, c) CF, (d, e, f) DCB, (g) Tol, and (h) CB solutions. (b, e) Images are for post-annealed at 100°C and (c, f) for post-annealed at 200°C.



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Figure 6. (a) Transfer characteristics at a fixed VD of –60 V or +60 V for the PTTDPP-DT-DTT films flowcoated from CF, Tol, CB, or DCB solutions. (b) Hole and electron mobilities of the PTTDPP-DT-DTT films extracted from (a). (c) Hole and electron mobilities of the PTTDPP-DT-DTT films coated from DCB as a function of the annealing temperature. (d) Output characteristics of the PTFTs based on a PTTDPP-DT-DTT film cast from a CF:DCB solution.


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Figure 7. Output voltage versus input voltage plots obtained from the inverters based on PTTDPP-DT-DTT films flow-coated from DCB and post-annealed at 200°C at a constant supply voltage.



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Low-Band-Gap Polymer-Based Ambipolar Transistors and Inverters

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