Electrical and Photoresponse Properties of Printed Thin-Film

(1-7) Printed thin film transistors (TFTs) are at the core of printable electronics because ..... and so on could also selectively sort sc-SWCNTs by t...
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Electrical and Photoresponse Properties of Printed Thin-Film Transistors Based on Poly(9,9-dioctylfluorene-co-bithiophene) Sorted Large-Diameter Semiconducting Carbon Nanotubes Long Qian,†,‡,∥ Wenya Xu,†,∥ XiaoFeng Fan,§ Chao Wang,† Jianhui Zhang,† Jianwen Zhao,*,† and Zheng Cui*,† †

Printable Electronics Research Centre, Suzhou Institute of Nanotech and nano-bionics, Chinese Academy of Sciences, No. 398 Ruoshui Road, SEID, Suzhou Industrial Park, Suzhou, Jiangsu Province 215123, PR China ‡ Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, Jiangsu Province 215123, PR China § College of Materials Science and Engineering, Jilin University, Changchun, Jilin Province, PR China ABSTRACT: In this work, a simple and effective method has been developed to selectively separate large-diameter semiconducting-SWCNTs (sc-SWCNTs) from commercial arc discharge SWCNTs using (9,9dioctylfluorene-co-bithiophene) (F8T2). The role of solvents in the separation and dispersion processes was studied. It has been proved that sc-SWCNTs mixed in arc discharge SWCNTs can be selectively sorted by F8T2 in toluene, xylene, or m-xylene with the aid of sonication and centrifugation, and the molecular dynamic simulation also agrees with the experiment results. The resulted sc-SWCNTs dispersion solution was used as a printable ink to print thin-film transistors (TFTs) via an aerosol jet printer. The printed TFTs showed good uniformity and electrical properties with on current up to 10−3 A, effective mobility up to 42.1 cm2 V−1 s−1 (±1.2 cm2 V−1 s−1), and on/off ratio of ∼107 after only four cycles of repeated printing. In addition, all of the printed devices exhibited rapid photocurrent response to light irradiation.

1. INTRODUCTION Printed electronics are drawing wide attention in recent years due to their potentials for applications in large-area and flexible electronic systems.1−7 Printed thin film transistors (TFTs) are at the core of printable electronics because they enable logic and data-processing function. However, printed TFTs have not yet become commercially viable due to poor performance and uniformity issues. The main challenge in printing highperformance TFTs is to obtain high-quality printable inks, particularly to formulate printable semiconducting inks that have high carrier mobility and chemical stability and are easy to process in solution form at room temperature.8,9 Single-walled carbon nanotubes (SWCNTs) have been widely regarded as one of the ideal materials for solar cell, Li ion batteries, sensors, and electronic devices due to its extraordinary physical, chemical, and mechanical properties.10−17 For making field-effect transistors, semiconducting SWCNTs (sc-SWCNTs) are the primary materials that are of high carrier mobility and excellent current carrying capacity and can be dispersed well in some organic solvents and aqueous solutions with the aid of sonication, therefore, are one of the most promising materials for preparing high-quality printable semiconducting inks. In particular, sc-SWCNTs with large diameters, which have small band gaps, exhibit better electrical properties than those of small diameter sc-SWCNTs. © 2013 American Chemical Society

To obtain high-purity sc-SWCNTs, various approaches have been developed to selectively remove or eliminate metallic species in commercial SWCNTs.18−50 However, most of these works are focused on separating small diameter sc-SWCNTs. In general, it is difficult to separate large diameter sc-SWCNTs from commercial SWCNTs. So far, only a few reports are about selectively separating large-diameter sc-SWCNTs from arcdischarge SWCNTs using density gradient ultracentrifugation (DGU), gel chromatography, and dielectrophoresis.19,48−50 However, these methods are time-consuming and not easy to scale up. As a consequence, commercial high-purity scSWCNTs are very expensive. For example, the price of commercial 99% large diameter sc-SWCNTs sorted by DGU is about 500 U.S. dollars per microgram. Polymer wrapping has become an attractive method to selectively separate sc-SWCNTs after Bao’s group reported in 2011 that regioregular poly(3-dodecylthiophene) (rr-P3DDT) could selectively wrap sc-SWCNTs and the sorted sc-SWCNTs exhibited excellent electrical properties.32 Since then, other groups also reported excellent electrical properties of TFTs made from the sc-SWCNTs of small diameter sorted by Received: June 4, 2013 Revised: August 7, 2013 Published: August 8, 2013 18243

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Interdigitated electrodes arrays with three fingers (finger dimensions: width 200 μm, length 20 μm, interfinger spacing 20 μm, that is, channel length and width are 20 and 600 μm, respectively) were patterned on SiO2/Si substrates by photolithography. The SWCNT TFTs were fabricated by both aerosol jet printing and drop-casting methods. For the printing process, sorted SWCNT solutions were printed onto the pretreated devices, followed by washing with m-xylene for three times. The printing procedure was repeated only four times, and the density of sorted sc-SWCNTs was high enough to form a percolation path and to reach the desired current level. The drop-casting procedure was similar to the printing procedure but needed to repeat 10 times. The prepared TFT devices were annealed at 200 °C for 30 min in oven; then, the electrical properties of SWCNT TFTs were measured after cooling to room temperature. The mobilities of SWCNT TFTs are estimated by μ = ((dId/ dVg) × (L/W) × (1/(CiVds))).8,9,50 Here Ci is the oxide capacitance per unit area, which is determined by ε0εrA/d, where ε0 is permittivity of free space (8.85 × 10−12), εr is relative permittivity (3.9), A is unit area, and d is the gate silicon oxide thickness (3 × 10−7). L and W represent the channel length and width, respectively.

polymers.33,34 However, these methods are usually timeconsuming because they need multiple ultracentrifugations for several hours to remove the excessive polymers or bundles of SWCNTs. Compared with small-diameter sc-SWCNTs, using large diameter sc-SWCNTs can effectively reduce Schottky barriers and significantly improve on-state currents and on/off ratio of SWCNT TFTs.51 Recently, there were reports that large-diameter SWCNTs could be selectively sorted by polymers.51−54 However, a few of them showed the electrical properties of TFTs made from the polymer-sorted largediameter SWCNTs except our previous work about selectively sorting sc-SWCNTs from arc-discharge SWCNTs by P3DDT.53 In the present work, (9,9-dioctylfluorene-co-bithiophene) (F8T2) has been investigated for sorting sc-SWCNTs of large diameters from commercial arc-discharge SWCNTs. The reason to choose F8T2 is that as a commonly used semiconducting material for making organic TFTs F8T2 has already been used to selectively separate sc-SWCNTs of small diameter from commercial SWCNTs together with sonication and centrifugation and it is air-stable. The sorted sc-SWCNT solutions were used directly for fabricating TFTs by printing and drop-casting methods. Printed TFTs with effective mobility as high as 42.1 cm2 V−1 s−1 (±1.2 cm2 V−1 s−1) and on/off ratio up to 107 have been achieved on SiO2/Si substrates with prepatterned interdigitated gold electrode arrays after printing only four cycles. Furthermore, all devices showed good photoresponse characteristics.

3. RESULTS AND DISCUSSION 3.1. Role of Solvents for Selectively Separating scSWCNTs from Arc Discharge SWCNTs. Although F8T2 has been used for selectively sorting sc-SWCNTs of small diameter, it has not proven that the polymer can work with largediameter sc-SWCNTs as well. The success or failure very much depends on choosing the right solvent. Different solvents have different dielectricity (dipole moments), which seriously influences the charge-transfer rate between polymeric molecules and SWCNTs. The successful separation requires the F8T2 polymer to be soluble in the solvent, whereas SWCNTs are not dispersible in the solvent. Solvents with different dipole moments were investigated, including hexane (0 D), p-xylene (0.07 D), m-xylene (0.32 D), toluene (0.36 D), o-xylene (0.54 D), chloroform (1.04 D), dichloromethane (1.60 D), THF (1.75 D), and xylene. SWCNTs and F8T2 were first dispersed in these solvents with the aid of probe-ultrasonication. The dispersed solution was then centrifuged at 21 000 g for 2 h. The resulting SWCNT supernatants were characterized by UV−vis-NIR absorption spectra. Figure 1a shows the dispersed solution before and after centrifugation. Figure 1b,c shows the adsorption spectra of pristine SWCNTs and sorted SWCNTs by F8T2 in various solvents. As shown in Figure 1b, the semiconducting peaks (S22) in the range of 900−1200 nm became very sharp, and the metallic peaks (M11) in the range of 600−800 nm were disappeared in m-xylene (0.32 D), toluene (0.36 D), and xylene, indicating that the metallic species have been selectively removed after centrifugation. However, no selectivity was observed in other solvents as the adsorption spectra shown in Figure 1c. The strong metallic peaks were still observed after centrifugation except in hexane (0 D). (No SWCNT peaks were observed after centrifugation.) SWCNTs−polymer composites showed very low solubility in p-xylene (0.07 D) with low dipole moment, and no selectivity was observed. Solubility of SWCNT−polymers increased with increasing the dipole moment of solvents, and SWCNT metallic peaks disappeared in m-xylene, xylene, and toluene with higher

2. EXPERIMENTAL SECTION Arc discharge (P2, diameter 1.2 to 1.6 nm) SWCNTs were purchased from Carbon Solution. Semiconducting SWCNT solutions of 99% purity were purchased from Nanointergris. (9,9-Dioctylfluorene-co-bithiophene) (F8T2) (Mw = 21 000 g mol−1) was purchased from Shenzhen (China) Derthon Optoelectronic Materials Science & Technology. All products were directly used without further purifications. A confocal Raman microscope (WITec CRM200) equipped with 785, 633, and 532 nm lasers was used for Raman measurements. Optical absorption measurements were performed in a Perkin-Elmer Lambda 750 UV−vis-Nir spectrometer. All electrical measurements were carried out in ambient using a Keithley semiconductor parameter analyzer (model 4200-SCS). A NSCRIPTOR DPN system (NanoInk, Skokie, IL) and Dimension 3100 AFM (Veeco, Santa Clara, CA) was used in AFM imaging. Sorted sc-SWCNT solutions were printed by an Optomec’s aerosol jet printing system. To obtain sorted sc-SWCNT solutions, we dispersed 2.5 mg of P2 in 20 mL solvents including toluene, xylene, m-xylene, hexane, p-xylene, o-xylene, chloroform, dichloromethane, and tetrahydrofuran (THF) with 10 mg F8T2 via probe-ultrasonication for 30 min (Sonics & Materials, model: VCX 130, 60W). Then, the resulting SWCNT solutions were centrifuged at 21 000 g for 2 h to remove metallic species and big bundles, and the supernatant was drawn out from the centrifuge tube and used to fabricate SWCNT TFTs without any other purification. To obtain the yield of sorted sc-SWCNTs, we filtered sorted sc-SWCNT suspension through a 0.22 μm PTFE membrane, followed by repeated washing with m-xylene and THF until the filtrate became colorless. The filtrate was then dried and the weight was measured after cooling to room temperature. The yield of sc-SWCNT is ∼5%. 18244

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of sc-SWCNTs, we measured Raman spectra of the pristine SWCNTs, sorted SWCNTs, and F8T2 samples by a confocal Raman microscope (WITec CRM200) with 785, 633, and 532 nm laser. As shown in Figure 2a, the strong metallic peaks at 163 cm−1 were observed in the 785 nm excited Raman spectra,49 However, they disappeared after reaction with F8T2 and centrifugation in both xylene and m-xylene. Three peaks, including one metallic peak at 200 cm−1 and two semiconducting peaks at 173 and 157 cm−1 (Figure 2b), were observed in pristine SWCNTs using 633 nm laser. However, metallic peak at ∼200 cm−1 and semiconducting peak at 173 cm−1 disappeared, and broad semiconducting peak at 165 cm−1 was observed in RBM region after being sorted by F8T2. According to the Kataura plot,55,56 semiconducting peaks in the range of 173 to 178 cm−1 and 194 cm−1 were observed under the 532 nm excitation. However, the peaks at 173, 178, and 194 cm−1 were disappeared, and two new semiconducting peaks at 176 and 157 cm−1 were observed after reaction with F8T2. Furthermore, tangential modes of sorted SWCNTs and pristine P2 using 633 nm laser also demonstrated that metallic species were selectively removed. As shown in Figure 2d, the G− peaks (from metallic SWCNTs) at 1550−1580 cm−1 became sharp; however, G+ peaks (from sc-SWCNTs) at 1590 cm−1 had no obvious changes after interaction with F8T2. As previously described, the semiconducting species with certain chirality can be preferentially sorted from arc-discharge SWCNTs by F8T2 with the aid of sonication and centrifugation. 3.3. Electrical Properties of Thin Film Transistors Based on Sorted sc-SWCNTs. The sc-SWCNTs solution was deposited on SiO2/Si substrates with prepatterned gold electrodes by aerosol jet-printing and drop-casting methods to make thin-film transistors (TFTs), and the electrical properties of TFTs were then measured using a Keithley 4200-SCS semiconductor parameter analyzer. Figure 3 shows the typical transfer curves and output curves of devices fabricated via drop-coating and aerosol jet-printing methods. The devices exhibited high on/off ratio and high charge mobility. The mobility could be further improved after annealing at 200 °C for 30 min in oven due to the removal of the solvents in the SWCNT thin films. Mobility and on/off

Figure 1. (a) Pictures of arc discharge SWCNTs-F8T2 composites dispersing in m-xylene (1) before and (2) after centrifugation. (b) Absorption spectra of pristine SWCNTs (curve 1) compared with SWCNTs-F8T2 composite in solvents of toluene (curve 2), xylene (curve 3), and m-xylene (curve 4) after centrifugation. (c) Absorption spectra of SWCNTs-F8T2 composite in solvents of hexane (curve 5), p-xylene (curve 6), o-xylene (curve 7), chloroform (curve 8), dichloromethane (curve 9), and THF (curve 10) after centrifugation. (d) Correlation between selectivity and dipole moment of solvents.

dipole moment. However, metallic peaks appeared again when the dipole moment of solvents was more than 0.54 D (curve 7−10 in Figure 1b). Xylene is composed of a mixture of three isomers: p-xylene, o-xylene, and m-xylene, so it is regarded that m-xylene plays a key role in selectively sorting sc-SWCNTs from arc discharge SWCNTs by F8T2 in xylene. Figure 1d summarizes the correlation between the separation selectivity and dipole moment of solvents. The selectivity can only be observed in toluene (0.36 D), m-xylene (0.32 D), and xylene. m-Xylene was therefore selected as the solvent in the following experiments, and the resulting SWCNT solutions was used for directly fabricating SWCNT TFTs after centrifugation by aerosol jet printing and drop casting. 3.2. Raman Spectra of sorted sc-SWCNTs from Arc Discharge SWCNTs. To further confirm the successful sorting

Figure 2. Raman spectra of arc discharge SWCNTs before and after being sorted by F8T2 using 785, 633, and 532 nm laser. 18245

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Figure 3. Typical transfer and output characteristics of thin-film transistors based on sorted sc-SWCNTs from arc-discharge SWCNTs before and after annealing at 200 °C for 30 min. (a,b) By drop casting and (c,d) by aerosol jet printing methods (Vds = 2 V).

ratio of printed devices were measured as high as 42.1 cm2 V−1 s−1 (±1.2 cm2 V−1 s−1) and 107, as shown in Figure 3c. AFM imaging was performed to characterize the morphology of deposited sc-SWCNTs. Figure 4 shows the typical AFM

unambiguously supported the conclusion that the sc-SWCNT species were well-sorted by F8T2 polymer wrapping and centrifugation. 3.4. Theoretical Calculation for Selectively Sorting scSWCNTs by F8T2 from Arc Discharge SWCNTs. The key to the successful sorting of sc-SWCNT with polymers is the formation of polymeric sc-SWCNTs composites, which are quasi-supermolecules and dissolvable in some solvents, whereas metallic SWCNTs, which cannot form the quasi-supermolecular structures, become sediments and can be filtered out after centrifugation. Previous experiments observed that a charge transfer took place at the metallic SWCNTs and polymer interface, whereas there was negligible charge transfer between sc-SWCNT and polymer.57,58 A hypothesis was then put forward that the charge transfer leads to an interface dipole that may favor a planar π-stacking between the polymer and SWCNT rather than a helical wrapping conformation, which may prevent the supramolecular structure formation needed for dispersing the metallic SWCNTs.32 To confirm the charge-transfer process, the molecular dynamics simulation with density functional theoretical (DFT) calculation was performed with the model shown in Figure 6a,b, where SWCNTs with chirality of (18, 0) as the metallic SWCNT and chirality of (17, 0) as the sc-SWCNT.59 We employ the method of projector-augmented wave potentials as implemented in the Vienna ab initio simulation package (VASP) code.60,61 The generalized gradient approximation (GGA) with the parametrization of Perdew−Burke− Ernzerhof (PBE) is used to express the exchange-correlation energy of interacting electrons.62 With the molecular dynamics simulation, we have found that there is the main contribution of the strong interaction between F8T2 and SWCNT is from the backbone of aromatic polymer.61 Therefore, both side chains (C8H17) of F8T2 with one unit are replaced by both CH3 in the model (as shown in Figure 6a) to calculate accurately the interaction between with the backbone and SWCNT by DFT. To study the difference of the interaction between F8T2 and sc-SWCNT and that between F8T2 and metallic SWCNT, the sc-SWCNT with chirality (17, 0) and metallic SWCNT with chirality (18, 0) are chosen to investigate. The supercell

Figure 4. Typical AFM images of sorted arc discharge SWCNTs in the channel by (a) drop casting and (b) aerosol jet printing methods. The insets are the heights of sorted SWCNTs.

image of sorted P2 SWCNTs deposited in the device channel. The lengths of SWCNTs range from 1 to 2 μm, and the diameters are in the range of 2 to 3 nm, indicating that the SWCNT thin films consist of individual SWCNTs and smaller SWCNT bundles. It is noted that there are some SWCNT bundles in SWCNT thin film deposited by drop-casting method. Figure 5 shows the histogram of the mobility and on−off ratios of the devices made with the sorted sc-SWCNTs. The statistics was obtained from the total number of 30 printed devices. As shown in Figure 5, all devices based on sorted scSWCNTs were of high effective mobility ranging from 3 to 42.1 cm2 V−1 s−1 and high on−off ratios ranging from 104 to 108. TFTs made by aerosol jet printing only needed to print four cycles, whereas TFTs made by drop casting had to drop-cast 10 times. The printed TFTs, however, exhibit better electrical properties than those of TFTs made by drop-casting method. It is possible that small SWCNT bundles in printable scSWCNTs inks were debundled to individual SWCNTs during the course of ultrasonic atomization in aerosol jet printer, resulting in a few SWCNT bundles in the printed thin film (Figure 4). In summary, such excellent electrical properties 18246

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Figure 5. Histogram of the mobility and on−off ratios of TFT devices based on sorted sc-SWCNTs after annealing at 200 °C for 30 min. (a,b) By drop casting and (c,d) by aerosol jet-printing methods.

which is calculated by the formula, 63 ΔCHA(r) = CHAF8T2‑SWCNT(r) − CHAF8T2(r) − CHASWCNT(r), where CHAF8T2‑SWCNT(r), CHAF8T2(r), and CHASWCNT(r) are the real-space charge distribution of the SWCNT with the adsorbed F8T2 and the independent SWCNT and SWCNT, respectively. A kinetic energy cutoff of 500 eV and k-points sampling with 0.05 Å−1 separation in the Brillouin zone are used. The structure optimizations were carried out with a force convergence criterion of 0.01 eV/Å. Figure 6c,d shows the calculated redistribution of charge when F8T2 is attached to the SWCNT, where blue denotes charge depletion and red denotes charge accumulation. It clearly shows more chargetransfer taking place for metallic SWCNT than for sc-SWCNT. There is also charge transfer in the region other than the adsorption interface with F8T2 for metallic SWCNT. The simulation has confirmed the selective charge-transfer process, which forms the basis for selective sorting of sc-SWCNTs. 3.5. Photoresponse Characteristics of Printed SWCNT TFTs. It is known that the dielectric constant of a material is related to its dipole polarizability. As the dielectric constant of metallic SWCNTs is much larger than that of sc-SWCNTs, the interface dipoles easily occur between F8T2 and metallic SWCNTs, which can prevent the formation of supramolecular structure of metallic SWCNT and F8T2. This explains why the dipole moment of solvents also has influence on the charge transfer between SWCNTs and F8T2. The present experiments have found that sc-SWCNTs can be selectively sorted by F8T2 only in toluene, xylene, or m-xylene. Furthermore, it was found that other poly(9,9-dialkyl-fluorene) derivates, such as poly[(9,9-dihexylfluorenyl-2,7-diyl)-co-(9,10-anthracene)](PFH-A), poly[9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-2,1-3thiadiazole)](PFO-BT), and so on could also selectively sort sc-SWCNTs by tuning types of solvents. F8T2 is a photosensitive polymer and can wrap tightly on the SWCNT surfaces; furthermore, it is reported that the energy can directly transfer from F8T2 to SWCNTs.59,64,65 The photoresponsive characteristics of SWCNT-F8T2 thin films were investigated. As shown in Figure 7a, the decreased

Figure 6. (a) Schematic representation of polymer F8T2 and 1-unit F8T2 with the simple side chains CH3 that replace the side chains C8H17, (b) the periodic supercell model of five-unit SWCNT with the modified F8T2, (c) the 3D charge difference isosurfaces of SWCNT (18, 0) with the modified F8T2, and (d) that of SWCNT (17, 0) with the modified F8T2. Notice that blue is for charge depletion and red is for charge accumulation.

method with periodic boundary condition is used, and the lattice constants a, b, and c of supercell are 25, 25, and 21.3 Å, respectively. The large lattice constants a = b = 25 Å are adopted to build a large vacuum separation in x−y plane to avoid the spurious coupling effect between SWCNTs of nearest-neighbor supercell. The lattice constant c = 21.3 Å is used due to the five-unit (17, 0) SWCNT or five-unit (18, 0) SWCNT in the supercell, as shown in Figure 6b. Therefore, the supercell model with 383 atoms is built to simulate the interaction between F8T2 and (17, 0) SWCNT, and the model with 403 atoms is built to simulate the interaction between F8T2 and (18, 0) SWCNT. The charge redistribution of F8T2 and (17, 0) SWCNT and that of F8T2 and (18, 0) SWCNT, 18247

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Figure 7. Photoresponse characteristic of printed TFTs based on (a) 99% sc-SWCNTs by DGU method and (b) sc-SWCNTs sorted by F8T2 at Vg = 2 V and Vds = 5 V.

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photocurrent was observed when semiconducting SWCNT film (99% purity) is irradiated with halogen lamp illumination (14.4 mW/cm2), which is attributed to the photodesorption of oxygen for the SWCNT network devices. In contrast with the decrease in photocurrent observed for the commercial 99% semiconducting SWCNT thin film, an enhancement in photoresponse was observed for the F8T2-SWCNT thin film. Figure 7b showed the photocurrents increased rapidly when light was turned on and decreased when light was turned off. The photocurrent of the F8T2-SWCNT thin film increased by ∼8% of magnitude under light illumination (14.4 mW/cm2) at 5 V bias, and a rise time (turn on) and fall time (turn off) were about 10 and 50 s, respectively. The relative photocurrent enhancement and the response speed of the F8T2-SWCNT thin films had no obvious changes after 18 cycles, indicating that F8T2-SWCNT thin films have good photosensitivity and stability.

ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of China (91123034, 61102046) and Basic Research Programme of Jiangsu Province (BK2011364).

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4. CONCLUSIONS In summary, a simple and scalable method has been developed to produce high-purity sc-SWCNTs of large diameters from arc discharge SWCNTs by F8T2. The role of organic solvents for sorting selectivity was investigated. Molecular dynamic simulation was performed, which revealed the charge-transfer process between F8T2 and SWCNTs. The absorption spectra, Raman spectra, and electrical properties of SWCNT TFT devices demonstrated that metallic species of P2 SWCNTs were effectively removed after interaction with F8T2 in toluene, xylene, and m-xylene with the aid of sonication and centrifugation. The printed TFTs exhibited excellent properties with mobility as high as 42.1 cm2 V−1 s−1 (±1.2 cm2 V−1 s−1) and on/off ratio up to 107. In addition, all TFTs based on SWCNT-F8T2 composites exhibit good photoresponsive characteristics with good stability and rapid response. The work is paving the way for making printable logic circuits, photoswitches, and solar cell based on SWCNT−polymer composites.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.Z.); [email protected]. cn (Z.C.). Author Contributions ∥

REFERENCES

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L. Qian and W. Y. Xu contributed equally to this work.

Notes

The authors declare no competing financial interest. 18248

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