Extremely Low-Cost, Scalable Oxide Semiconductors Employing Poly

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Extremely Low-Cost, Scalable Oxide Semiconductors Employing Poly(acrylic acid)-Decorated Carbon Nanotubes for Thin-Film Transistor Applications Gyu Ri Hong, Sun Sook Lee, Yejin Jo, Min Jun Choi, Yun Chan Kang, Beyong-Hwan Ryu, Kwun Bum Chung, Youngmin Choi, and Sunho Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08950 • Publication Date (Web): 20 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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ACS Applied Materials & Interfaces

Extremely Low-Cost, Scalable Oxide Semiconductors Employing

Poly(acrylic

acid)-Decorated

Carbon

Nanotubes for Thin-Film Transistor Applications

Gyu Ri Hong,a,b Sun Sook Lee,a Yejin Jo,a Min Jun Choi,c Yun Chan Kang,b Beyong-Hwan Ryu,a Kwun-Bum Chung,c Youngmin Choi,a,* Sunho Jeong a,*

a

Division of Advanced Materials, Korea Research Institute of Chemical Technology

(KRICT), 141 Kajeongro, Daejeon 305-600, Republic of Korea. b

Department of Materials Science and Engineering, Korea University, Seoul 136-713,

Republic of Korea c

Division of Physics and Semiconductor Science, Dongguk University, Seoul, 04620, Korea

Corresponding Authors; Y. Choi ([email protected]); S. Jeong ([email protected])

KEYWORDS: low-cost, scalable, oxide semiconductor, carbon nanotube, thin-film transistor

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ACS Paragon Plus Environment


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ABSTRACT In this study, we report, for the first time report, a simple bar-coating process of soluble metal oxide semiconductors, consuming the 0.1g of precursor solution in 4-inch sized devices with a cost of only 0.05 $. In order for resolving the issue of critical degradation in device performance observable in slow-evaporation based film formation processes, we incorporate the unprecedentedly-developed, poly(acrylic acid)-decorated multi-walled carbon nanotubes (MWNTs) in oxide semiconductors. It is demonstrated that a field-effect mobility is improved to the value of 7.34 cm2/Vs (improvement by a factor of 2) without any critical variation in threshold voltage and on/off current ratio.

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Thin-film transistors, composed of a gate electrode, source/drain electrode, gate dielectric, and semiconducting channel layer, are one of most crucial unit devices in modern electronic circuitries, and their performance is predominantly determined by the electrical properties of the semiconducting channel layer. To date, a variety of candidate materials has been intensively investigated for the semiconducting channel layer, and tremendous attention has specifically been devoted to oxide-based semiconductors, owing to their high mobility, optical transparency, environmental stability, and electrical stability.1,2 In particular, recent studies have suggested the possibility of realizing low-cost, high performance oxide-based electronic circuitries, in a conjunction with a solution-process based on a wet-chemical methodology, rather than a vacuum-deposition process. The major drawbacks to adopting solution-processed oxide semiconductors are the requirement of heat-treatment at high process temperatures, and their relatively low fieldeffect mobility.3 Low-temperature processability has been achieved by exploiting noble chemistries (including a combustion chemistry,4 a catalyst chemistry5-7 and a precursor chemistry8) and annealing techniques (including an ozone treatment9, deep-UV irradiation10 and a microwave irradiation11). The classic way of enhancing field-effect mobility is to incorporate elements that can generate a sufficient number of oxygen vacancies in a multicomponent oxide matrix.12 However, the addition of mobility-enhancing elements inevitably results in an undesirable negative shift of threshold voltage and a deterioration of the sub-threshold swing characteristic. As an alternative approach, high-quality carbon materials, such as graphene nanosheets and single-walled carbon nanotubes (SWNTs), have been suggested, and have been successfully demonstrated to have significantly improved electrical performance.13-16 However, both carbon materials are highly expensive, 3



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overshadowing their basic advantage of solution-processed oxide semiconductors, and a critical shift in threshold voltage has been reported to inevitably evolve with the improvement in field-effect mobility in the case of SWNT-incorporated oxide semiconductors. In addition, most of the previous studies have been based on the spin-coating method, which consumes and wastes an excessive amount of material, in order to uniformly deposit a few tens of nanometer-thick, ultrathin channel layer. Reducing this waste would address the present cost issue affecting the usage of such materials, especially in the case of large-area applications, and thereby remove an obstacle to the development of solution-processed oxide semiconductors. In a limited number of studies, ink-jet printing techniques have been suggested which utilize a drop-on-demand deposition process.17 However, the bar-coating process, which has been widely adopted in other large-area processes, has not been demonstrated for oxide semiconductors because it introduces a process-dependent degradation in device performance, unlike the cases of oxide gate dielectrics18 and organic semiconductors.19 The bar-coating method is one of large-area processes adoptable in practical applications, allowing for the extremely limited loss of material and in turn, lowcost/large-area processable fabrication of active devices. Thus, the low-cost oxide semiconducting channel layer applicable to large-area circuitries should be developed with a sophisticated design of inexpensive mobility-enhancing materials that are capable of improving the field-effect mobility (simultaneously, not shifting the threshold voltage and degrading the on/off current ratio), in a combination with a large-area process involved with a lower usage of materials. In this study, we demonstrate the fabrication of solution-processed oxide semiconductors, employing poly(acrylic acid) (PAA)-decorated multi-walled carbon 4


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nanotubes (MWNTs), in 4 inch-wafer scaled device, with a consumption of precursor solution as inexpensive as $0.05. The resulting oxide semiconductors exhibit a field effect mobility of 7.3 cm2/Vs, almost comparable to the value of those used in practical display applications. By incorporating well-engineered MWNTs, the field-effect mobility is improved by a factor of 2.0, with a slight variation of -1.2 V in threshold voltage. The basis for the improvement in device performance is elucidated with a comparative interpretation involving electrical performance and spectroscopy-based analytical results. In typical sol-gel derived chemistries, the metal-salt precursors are dissolved in a polar-solvent medium. 2-methoxyethanol (2-ME) is one of the solvents widely-used for this purpose, due to its appropriate polarity and intermediate boiling point. Figure 1a shows a schematic diagram of the surface-engineered MWNTs dispersed in the 2-methoxyethanol solvent medium. Carbon materials have been used as ballistic conduction pathways which allow charge carriers to effectively traverse an oxide semiconductor matrix, thereby improving the transport velocity of charge carriers in a channel layer.13-16 However, carbon nanotubes are typically hard to uniformly disperse in a polar medium, due to their surface hydrophobicity and a tendency to form highly-networked agglomerates. Well-known surfactants, such as Triton X-100, Tween 20, Tween 80, and sodium dodecyl sulfate, act as surface modifiers which allow for the dispersion of carbon nanotubes in aqueous media.20 As a conventional methodology, hydrophilic polar groups can be generated along the surface of carbon nanotubes by using a highly toxic, acid-based surface treatment.21 However, a nondestructive surface functionalization is more beneficial when it is necessary to ensure the critical role of carbon nanotubes as an efficient conduction pathway. Poly(acrylic acid) (PAA) is a kind of anionic polyelectrolyte with a carboxyl group in 5



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a repeating unit, which is deprotonated at neutral and basic pH in an aqueous medium, generating a negatively-charged, extended polymeric conformation. Under an acidic environment, a reversible protonation reaction occurs, and when the polymeric chains lose their characteristic surface charge, they tend to be coiled and form an agglomerated structure. This pH-dependent variation in morphology affects the dispersion of carbon nanotubes in an aqueous medium.22,23 When the PAA is negatively charged, PAA is well-dispersed in water by electrostatic repulsive force, and carbon nanotubes do not have a chemical/physical interaction with the deprotonated, charged unit in PAA. Once the pH drops below 4~5, the PAA starts to lose its electrostatic interaction and becomes coiled on the surface of the carbon nanotubes, due to the hydrophobic interaction between the hydrocarbons present in the main backbone of the PAA, and the pristine carbon nanotubes. As seen in Figure 1b, for an aqueous PAA-MWNT solution prepared at pH of 7, agglomerated MWNTs are observable on the surface, whereas the corresponding PAAMWNT solution formulated at a pH of 3 shows excellent dispersion stability. For pristine MWNTs, dispersion stability is critically poor, regardless of the pH value. Polyelectrolytes undergoing a pH-dependent protonation/deprotonation, in general, lose their ability to generate a surface charge in solvent media, except in an aqueous medium, and the pristine chemical structure (not deprotonated) is maintained with limited solubility in such solvent media, as shown in Figure 1c. In general, this limited solubility in a specific solvent medium does not allow facile surface-engineering approaches. However, in the case of PAA-MWNTs, the hybridized PAA-MWNT structure can be pre-formed in water by adjusting the pH, and its structure is rather well-maintained in a bad solvent, allowing for excellent dispersion stability in 2-methoxyethanol (Figure 1d). This controlled dispersion stability was also confirmed by 6


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monitoring the time-dependent formation of agglomerates, as shown in Figure S1. The PAAMWNT aqueous solution prepared at a pH of 7 includes significant agglomerates, which have accumulated at the bottom of the solution; this formation of agglomerates is represented by an increase in transmission. Agglomerate formation is drastically suppressed in the PAAMWNT aqueous solution prepared at a pH of 3, which shows little variation in the transmission of the overall solution, and this feature is well preserved when the PAAMWNTs are dispersed in 2-methoxylethanol. However, the PAA-free, MWNT aqueous solutions suffered from a critical variation in transmission, due to the gradual formation of aggregates over a prolonged time (Figure S2). Figure 2a shows the Fourier transform infrared spectroscopy (FT-IR) spectra for pristine MWNTs and PAA-MWNTs prepared in water with a pH of 3. As shown in Figure 2b, the PAA has distinctive absorption peaks at 2955 and 1572 cm-1 due to the methylene (CH2) stretching vibration of the alkyl chain of PAA, and the C=O stretching vibration of the – COOH group, respectively.24 The peaks positioned at 1417 and 1334 cm-1 are attributed to the asymmetric and symmetric stretching vibrations, respectively, of C-O in the –COOH groups.24 Distinct peaks for the PAA-MWNT hybrid are all clearly observable in the wavelength range of 1300-1600 cm-1. The presence of PAA was also confirmed by thermogravimetric analysis (TGA) results, as seen in Figure 2c. Compared with the TGA result for pristine MWNT, the PAA-decorated MWNT shows a more pronounced weight loss, as much as 14.55 wt%, prior to complete decomposition around 600 oC. Both samples were fully dried before TGA measurement by evaporating the adsorbed water molecules, which is indicated by the absence of weight loss around 100-200 oC; thus, the weight loss in the PAAMWNTs at the higher temperature is considered attributable to the PAAs anchored on the 7



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surface of the MWNTs. Note that the PAA decoration procedure was carried out with a repeated sonication process to prepare a homogeneous mixed solution, and a multiple centrifugation process was used for washing the excessive polymers. This implies that the interaction between the coiled PAA and MWNTs is strong enough to stably functionalize the surface of the MWNTs, enabling their long-term dispersion stability in solvents. For the PAA-MWNT hybrid obtained after solvent exchange from DI-water to 2-methoxyethanol, the weight loss observed in the TGA result diminished to some extent, with an amount of 7.66 wt%, indicative of the subtle loss of adsorbed PAA polymer; however, the PAA was still present in an amount sufficient to functionalize the surface of the MWNTs, taking into consideration the TGA result for pristine MWNTs, and this is a plausible origin for the superior dispersion stability in 2-methoxyethanol. To synthesize the oxide semiconductor precursor solutions, we chose a multicomponent composition comprised of In, Zn and O. In-Zn-O (IZO) is one of the representative oxide semiconductors, exhibiting a moderate field-effect mobility and a threshold voltage very close to 0 V, which could be obtained by adjusting the compositional ratio of In to Zn. In order to lower the annealing temperature to a level acceptable for current industrial display applications (at least, below 350 oC), we designed an activation of the solgel derived chemical reaction that could be assisted by the internal combustive exothermic heat produced by the combination of indium nitrate and zinc acetylacetonate.4 The molar ratio of both precursors is equivalent in a precursor solution. The synthesized precursor solutions were spin-coated or bar-coated on top of a heavily-doped Si wafer with a 100 nm-thick SiO2 gate dielectric, and were subsequently thermally annealed at 350 oC. The Al electrodes were evaporated through a shadow mask to define the source/drain electrodes. As shown in Figure 8


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S3, IZO oxide semiconductors were obtained with a field-effect mobility of 6.8 cm2/Vs, when the In and Zn salts were incorporated with an identical relative composition. The field-effect mobility performance of the device was improved significantly from 0.4 to 6.8 cm2/Vs, while the annealing temperature was increased from 250 to 350 oC (Figure S4). Figure 3 shows the device performance parameters, including field-effect mobility, threshold voltage, and an on/off current ratio for devices employing the spin-coated/barcoated MWNT-free IZO and bar-coated PAA-MWNT-added IZO channel layers. As for the X-ray photoelectron spectroscopy analysis, the atomic ratios of In to Zn were measured to be 1.02, 1.05 and 0.98 for spin-coated/MWNT-free, bar-coated/MWNT-free and barcoated/PAA-MWNT-added IZO semiconductors, respectively. For the CNT-added oxide semiconductor, the precursor solutions were spin-coated to form channel layers, but reproducible device data could not be obtained, due to the non-uniform spatial distribution of carbon nanotubes produced by the high-speed centrifugal force. As seen clearly in Figure 3b, when the channel layer was formed with a bar-coating process instead of a spin-coating process, the field-effect mobility was degraded significantly, down to 3.72 cm2/Vs. This is due to the fact that the rate of the sol-gel chemical reaction, which is influenced by solvent evaporation, is restricted in the case of bar-coated wet films, compared with the spin-coated films. In the ultra-thin wet spin-coated films produced by high-speed centrifugal rotation, volumetric contraction takes place instantaneously with vigorous solvent evaporation. In a previous study, the solvent evaporation triggered an immediate sol-gel reaction in a kinetically-controlled manner, and this was found to be one of the critical factors that determine the electrical properties of the soluble oxide semiconductors.25 The oxide frameworks including more intermediate hydroxide frameworks are produced with a 9



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suppressed formation of oxygen vacancy-involved oxide frameworks for the films derived with a limited solvent evaporation, degrading a device performance in oxide semiconductorbased thin-film transistors. Figure 4a shows the X-ray photoelectron spectroscopy (XPS) spectra for spin-coated/bar-coated MWNT-free IZO channel layers. The sub-peaks located at 528.6, 530.3, and 531.2 eV are attributable to a stoichiometric oxide lattice, an oxygendeficient oxide lattice, and a hydroxyl-involved oxide lattice, respectively.4,5 As seen in Figure 4b, for the bar-coated channel layer, the degree of oxide formation was predominantly suppressed, with a resulting reduction in the fraction of oxide lattice from 92.4 to 90.4%. The fraction of the oxygen-deficient lattice was correspondingly decreased, from 38.7 to 32.1%. This type of variation in chemical structure is one of the representative features of oxide semiconductor frameworks with limited device performance.26,27 The schematic of Figure 3a shows the efficient charge carrier transport through highly conductive carbon nanotubes inside channel layer. When the PAA-MWNTs were incorporated in the IZO oxide semiconductors, the field-effect mobility was improved from 3.72 to 7.34 cm2/Vs by increasing the concentration of PAA-MWNTs up to the level of 1x10-5 wt% (Figure 3b). In the precursor solution with a PAA-MWNT concentration of 1x10-5 wt%, the volumetric ratio of PAA-MWNTs to metal salt precursors was 1.1x10-4 (the density of the PAA-MWNTs is assumed to be 2.62 g/cm3 for carbon). The transfer characteristics of the TFTs employing the PAA-MWNT-added IZO channel layers are shown in Figure S5. As shown in Figure S6, the devices were almost free of variation in a threshold voltage under a forward/reverse bias sweep condition. The film thickness was ~20 nm for MWNT-free and PAA-MWNT-added IZO channel layers (Figure S7). Interestingly, even with the enhancement in field-effect mobility by a factor of 2.0, the threshold voltage merely changed 10


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from 8.4 to 7.21 V, maintaining an on/off current ratio of around 2x106 (Figure 3c and d). In the viewpoint of only field-effect mobility, a value approaching 8 cm2/Vs is almost comparable to that of the vacuum-deposited In-Ga-Zn-O semiconductors currently used in display industries, and the annealing temperature of 350 oC is also acceptable in terms of a thermal budget in practical display industries. It has been previously reported that the addition of pristine single-walled carbon nanotubes (SWNTs) in Zn-Mg-O semiconductors resulted in a significant negative shift, by as much as -40 V, in “turn-on” voltage14 and the incorporation of pristine SWNTs in In-Zn-O semiconductors is associated with a negative shift of over -35 V in “turn-on” voltage15. These pristine CNT-added IZO semiconductors have shown the significantly improved field-effect mobility even over 140 cm2/Vs (overcoming the theoretical limit of oxide semiconductors) with the incorporation of 1 wt% pristine SWNT; this is attributable to the formation of percolation-based current pathways inside the oxide semiconductor matrix, which inevitably evolve with the formation of conductive channel layers, causing the matrix to lose its characteristic semiconducting property. In the absence of any chemical/physical strategy for dispersing the carbon nanotubes in a precursor solution, an excessive amount of carbon nanotubes are normally incorporated in order to ensure that SWNTs are distributed throughout the oxide matrix. This trade-off behavior was managed to some extent with a relatively restricted negative shift, -18 V, in threshold voltage and a moderate field-effect mobility of 20.5 cm2/Vs by incorporating the less amount (0.005wt%) of destructively acid-treated SWNTs.16 In this case, the surfaces of the SWNTs were functionalized by generating hydrophilic polar groups with an acidtreatment, in order to uniformly disperse the carbon nanotubes, which then acted as charge transport pathways in the oxide matrix. However, the surface functional groups have a partial

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dipole moment, where electrons can be trapped under a repeated bias sweep condition, resulting in subsequent deterioration of the sub-threshold swing characteristic. Even though extraordinarily high field-effect mobilities were obtainable in previous CNT-added oxide semiconductors, this undesirable negative shift in threshold voltage, and degradation of the sub-threshold swing characteristic, can produce critical problems in practical electrical circuitries. The addition of another dopant material to resolve these issues has inevitably resulted in the degradation of field-effect mobility.14 It is conceivable that the improvement in field-effect mobility by a factor of 2.0 achieved in this study, without critical changes in other device parameters, can be attributed to the efficient incorporation of an extremely small amount (0.00001 wt%) of surfaceengineered carbon nanotubes, and the distribution of individual, perfectly-separated MWNTs without the formation of a continuous network of electrical conduction pathways. In addition, it should be noted that we have used multi-walled carbon nanotubes, instead of the singlewalled carbon nanotubes used commonly in previous studies. As is well known, the singlewalled carbon nanotube is much better qualified in terms of electrical properties, compared with multi-walled carbon nanotubes; however, the SWNT is 1000-times more expensive than the MWNT for lab-scale purchase. Thus, the desirable modulation of device performance achieved in this study by the incorporation of newly-surface engineered MWNTs also has a significant advantage in the additional aspect of cost-effectiveness. In order to examine the electronic structure near the conduction band of the MWNTfree/bar-coated IZO and the PAA-MWNT added/bar-coated IZO layers, near edge X-ray absorption spectroscopy (NEXAS) experiments were performed. Figure 5 shows the electronic structure of unoccupied states near the conduction band for both channel layers. 12


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Re-normalizations of the XAS spectra were carefully carried out by subtracting an X-ray beam background and scaling of the edge level from the raw data. The qualitative changes and a comparison of the conduction band features, such as the number and distribution of unoccupied states, could be analyzed by re-normalizations of the XAS spectra. The normalized oxygen K1 edge spectra of In2O3 and ZnO are directly related to the oxygen pprojected states of the conduction band, which consists of unoccupied hybridization orbitals for In 5sp + O 2p and Zn 4sp + O 2p.28 The conduction band of IZO semiconductors consists of two distinct hybridized orbital peaks; In 5sp, Zn 4s + O 2p (peak A) and In 5sp, Zn 4sp + O 2p (peak B). An interesting finding is that the intensity of peak A was increased by adding PAA-MWNTs, which means the crystal field splitting of the hybridized orbital structure related to In 5sp, Zn 4s + O 2p. These changes can induce the enhancement in charge transport, attributable to the characteristic electrical properties (a high mobility and a high carrier concentration) of carbon nanotubes, by increasing the unoccupied states near the conduction band.29,30 As shown in Figure 4 and Figure S8, for both films, there are no distinct variations in chemical structure, amorphous crystalline structure and surface morphology, as monitored by XPS analysis, X-ray diffraction results and atomic force microscopy observation, respectively. It is commonly observable that IZO semiconductors with an equivalent compositional ratio have an amorphous nature even with annealing at temperatures over 300

o

C.4 Therefore, it is highly conceivable that the significant

improvement in device performance observable for the PAA-MWNT added IZO layers is associated with the establishment of individually-located, efficient conduction pathways inside the semiconducting channel layer, and the appropriate modification of electronic structure in the IZO layer, due to the presence of carbon nanotubes.

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In order to demonstrate the extremely low-cost fabrication of an oxide channel layer using the bar-coating process and PAA-MWNT-added IZO precursor solution, we fabricated large-area thin-film transistors on a 4-inch heavily-doped Si wafer with a 100 nm-thick SiO2 gate dielectric (Figure 6a and Move S1). Using the 0.1 g precursor solution, a uniform channel layer was successfully formed by a simple, one-step bar-coating process. For comparison, the MWNT-free precursor solution was also bar-coated with an identical device architecture. As shown in Figure 6b and c, by incorporating the optimal amount of PAAMWNTs, the average field-effect mobility was improved by a factor of 2.75 even on 4-inch scaled devices, resulting in an average field-effect mobility of 4.4 cm2/Vs. The representative transfer characteristic for the 4-inch scaled TFTs employing the PAA-MWNT-added IZO channel layer is shown in Figure 6d. A threshold voltage of 7.34 V and an on/off current ratio of 2x105 was obtained, similar to the values for 4cm2-sized small devices. The cost of preparing the 0.1 g precursor solution was as cheap as $0.05, when calculated based on the price of chemicals purchased at lab-scale. Indium is one of the costly elements, and could be replaced with another inexpensive element, such as tin; but, even with after replacing indium precursors with tin precursors, the cost of preparing the precursor solutions is not lowered significantly (from $ 0.05 to $0.037), since the concentration of metal salt precursors needed to form the ultra-thin oxide channel layer is extremely low. The concentrations of In and Zn metal salt precursors were 1.45 and 1.27 wt%, respectively, for the 0.1 M precursor solutions synthesized in this study (in general, the molar concentration of metal salt precursors ranges from 0.05-0.3 M). Thus, in order for facilitating the cost-effective approach for oxide-based TFTs, one of most important factor is rather how to minimize the usage of precursor solutions in fabricating the large-area devices. The fabrication of 4-inch scaled devices with a cost of

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0.05 $ is much more inexpensive, compared with the vacuum-deposited counterparts, which has been achieved with the involvement of a previously unexploited bar-coating process for soluble oxide semiconductors and unprecedentedly developed PAA-MWNT-added oxide semiconductors that can compensate the device performance degradation in bar-coated oxide semiconducting channel layers. In summary, we have designed the poly(acrylic acid)-decorated multi-walled carbon nanotubes for improving the field-effect mobility of cost-effectively, large-area processed IZO semiconducting channel layer without a critical variation in other device performance parameters. The MWNTs well-dispersed in 2-methoxyethanol were obtainable by adjusting the pH of surrounding medium in preparing aqueous solutions, and the presence of PAA along the surface of MWNT was proven with FT-IR analysis and TGA results. The critical device performance degradation in bar-coated channel layers was effectively resolved by incorporating wet-chemically the PAA-MWNTs in IZO channel layers, evolving the fieldeffect mobility of 7.34 cm2/Vs (improved by a factor of 2) without any critical variation in threshold voltage and on/off current ratio. It is demonstrated that with this newly-formulated precursor solutions, the fabrication of 4-inch sized devices is achievable with consumption of 0.1g precursor solution as cheap as only 0.05 $. It is elucidated that the improvement of device performance is attributable to the generation of ballistic conduction pathways and the electronic structural modification beneficial to efficient charge transport, by a virtue of the presence of well-dispersed carbon nanotubes in oxide semiconductor matrix.

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a

poly(acrylic acid) (PAA)

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under neutral or basic environment negative surface charge by deprotonation under acidic environment nearly zero-surface charge

at pH 3

at pH 7 MWNT

PAA

b

dispersion in water PAA-MWNT pH 3

pristine MWNT

pH 7

pH 3

pH 7

as-prepared

After 1 hr

d dispersion in 2-ME

c

PAA-MWNT pH 3

pH 3

as-prepared

after 1 hr

Figure 1. (a) Schematic showing the pH-dependent dispersion mechanism of carbon nanotubes in an aqueous medium with poly(acrylic acid); (b) Photographs showing the dispersion stability of pristine MWNTs and PAA-MWNTs prepared in aqueous media (at pH: 3 and 7); (c) photographs showing the solubility limit of poly(acrylic acid) in various solvents such as DI-water, ethyl alcohol (EtOH), and 2-methodylethanol (2ME); (d) photographs showing the dispersion stability of the PAA-MWNT/2-methoxylethanol solution (the PAAMWNT was prepared from DI-water with a pH of 3).

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a 25

b 120

pristine MWNT PAA-MWNT

Transmittance(%)

Transmittance(%)

20 15 10 5 4000

3000

2000

PAA (in DI-water) DI-water

90 60 30 0 4000

1000

3000

2000

1000 -1

Wavenumber(cm )

-1

Wavenumber(cm )

c 100 Weight Loss (%)

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


80 60 40 pristine MWNT PAA-MWNT in Water PAA-MWNT in 2ME

20 0

0

200

400

600

800

o

Temperature ( C)

Figure 2. FT-IR spectra for (a) pristine MWNTs and PAA-MWNTs prepared from DI-water with a pH of 3 and (b) DI-water and poly(acrylic acid) dissolved in DI-water; (c) TGA results for pristine MWNTs, PAA-MWNTs obtained from DI-water based solution, and PAAMWNTs obtained from 2-methoxylethanol based solution. The PAA-MWNTs were prepared from DI-water with a pH of 3, and TGA analysis was carried out in air.

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channel “on”

a

b

source

Mobility (cm /Vs)

electrons

spin-coating (MWNT-free) bar-coating (MWNT-free) bar-coating (PAA-MWNT-added)

10 8

2

MWNT-free IZO

drain source MWNT-added IZO

MWNT

6 4 2 0

0

0

0.1 0.5

1

5 -5

Concentration of PAA-CNT (x10 wt%)

drain spin-coating (MWNT-free) bar-coating (MWNT-free) bar-coating (PAA-MWNT-added)

15

d

10

Ion/off

c Threshold Voltage (V)

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

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10

8

10

6

10

4

10

2

10

0

spin-coating (MWNT-free) bar-coating (MWNT-free) bar-coating (MWNT-added)

5

0

0

0

0.1 0.5

1

5 -5


0

0

0.1 0.5

1

5 -5


Figure 3. (a) Schematic showing the efficient charge transport through MWNT-added IZO channel layer. Variations in (b) field-effect mobility, (c) threshold voltage, and (d) on/off current ratio for TFTs prepared by spin-coated/MWNT-free and bar-coated/PAA-MWNTadded IZO channel layers, depending on the concentration of PAA-MWNTs. The sample size was 2cm x 2cm, and the channel dimension was 200 µm in length and 2000 µm in width.

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MWNT-free bar-coated

PAA-MWNT-added bar-coated

527

529

531

533

535

Binding Energy (eV)

b

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

MWNT-free spin-coated

a

527

529

531

533

535

527

Binding Energy (eV)

oxygen-deficient lattice

529

531

533

100

50

30 20

90

MWNT-free spin-coated MWNT-free bar-coated

Fraction (at%)

40

MWNT-free spin-coated MWNT-free bar-coated

535

Binding Energy (eV)

oxide lattice

Fraction (at%)

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


80



Figure 4. (a) XPS spectra and (b) semi-quantitative analytical results for the atomic contribution of each chemical component in the spin-coated/MWNT-free, bar-coated/MWNTfree, and bar-coated/PAA-MWNT-added IZO channel layers. The concentration of PAAMWNTs was 0.00001 wt% in the IZO semiconductor layer.

19


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Normalized Intensity (Arb. Units)


3.0

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Peak B In 5sp+O 2p Zn 4sp+O 2p

Peak A 2.5 In 5sp+O 2p Zn 4s+O 2p

2.0 1.5 1.0 0.5

MWNT-free PAA-MWNT-added

0.0 530

535

540

545

550

555

Photon Energy (eV) Figure 5. X-ray absorption spectroscopy (XAS) spectra of MWNT-free/bar-coated IZO and PAA-MWNT added/bar-coated IZO layers. Peak A and peak B indicate the orbital hybridization of In 5sp, Zn 4s + O 2p and In 5sp, Zn 4sp + O 2p, respectively. The concentration of PAA-MWNTs was 0.00001 wt% in the IZO semiconductor layer.

20


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Figure 6. (a) Photograph showing the extremely low-cost, large-area process, using barcoating and PAA-MWNT-added precursor solution. Graphs showing the distribution of fieldeffect mobility for TFTs fabricated by a bar-coating process on 4-inch sized substrates using (b) MWNT-free and (c) PAA-MWNT-added precursor solutions. The concentration of PAAMWNTs was 0.00001 wt% in the IZO semiconductor layer. (d) The representative transfer characteristics of TFTs fabricated by bar-coating process on 4-inch sized substrates using PAA-MWNT-added precursor solutions. The channel dimension was 200 µm in length and 2000 µm in width, and the drain voltage was 40 V in measuring the transfer characteristics.

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ASSOCIATED CONTENT Supporting Information. The turbiscan results for the PAA-MWNT and PAA-free MWNT solutions, transfer characteristics of TFTs employing IZO channel layers with different molar ratios of In/Zn and annealed at different temperatures, transfer characteristics of TFTs employing PAA-MWNT-IZO channel layers prepared from precursor solutions with different concentrations of PAA-MWNT, AFM images for bar-coated/MWNT-free and barcoated/PAA-MWNT added IZO layers, and the movie showing the large-area process of PAA-MWNT added IZO layers.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions S. Jeong and Y. Choi designed the experiment, supervised all phases of the project, and edited the manuscript. G. R. Hong and Y. Jo synthesized the precursor solutions. G. R. Hong fabricated the devices and measured the electrical performance of devices. S. S. Lee, B.-H. Ryu and Y. C. Kang analyzed the XPS spectra. M. J. Choi and K. B. Chung analyzed the XAS spectra. G. R. Hong and S. Jeong wrote the manuscript. All authors proofed the manuscript.

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ACKNOWLEDGMENT This research was supported by the Global Research Laboratory Program of the National Research Foundation (NRF) funded by Ministry of Science, Information and Communication Technologies and Future Planning (NRF-2015K1A1A2029679), and partially supported by the Nano·Material Technology Development Program through the National Research Foundation of Korea funded by the Ministry of Science, Information and Communication Technologies and Future Planning (NRF-2015M3A7B4050306).

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REFERENCES [1] Yu, X.; Marks, T. J.; Facchetti, A. Metal Oxides for Optoelectronic Applications. Nat.

Mater. 2016, 15, 383-396 [2] Huang, W.; Zeng, L.; Yu, X.; Guo, P.; Wang, B.; Ma, Q.; Chang, R. P. H.; Yu, J.; Bedzyk, M. J.; Marks, T. J.; Facchetti, A. Metal Oxide Transistors via Polyethylenimine Doping of the Channel Layer: Interplay of Doping, Microstructure, and Charge Transport. Adv.

Funct. Mater. 2016, DOI: 10.1002/adfm.201602069 [3] Jeong, S.; Moon, J. Low-Temperature, Solution-Processed Metal Oxide Thin Film Transistors. J. Mater. Chem. 2012, 22, 1243-1250 [4] Kang, Y. H.; Jeong, S.; Ko, J. M.; Lee, J.-Y.; Choi, Y.; Lee, C.; Cho, S. Y. TwoComponent Solution Processing of Oxide Semiconductors for Thin-Film Transistors via Self-Combustion Reaction. J. Mater. Chem. C 2014, 2, 4247-4256 [5] Jeong, S.; Lee, J.-Y.; Lee, S. S.; Oh, S.-W.; Lee, H. H.; Seo, Y.-H.; Ryu, B.-H.; Choi, Y. Chemically Improved High Performance Printed Indium Gallium Zinc Oxide Thin-Film Transistors. J. Mater. Chem. 2011, 21, 17066-17070 [6] Kim, S. J.; Jeon, H.-J.; Oh, S.-J.; Lee, S. S.; Choi, Y.; Park J.-S.; Jeong, S. Formamide Mediated, Air-Brush Printable, Indium-Free Soluble Zn−Sn−O Semiconductors for ThinFilm Transistor Applications. ACS Appl. Mater. Interfaces 2014, 6, 18429-18434 [7] Jeong, S.; Lee, J.-Y.; Lee, S. S.; Seo, Y.-H.; Kim, S.-Y.; Park, J.-U.; Ryu, B.-H.; Yang, W.; Moon, J.; Choi, Y. Metal Salt-Derived In–Ga–Zn–O Semiconductors Incorporating Formamide

as

a

Novel

Co-Solvent

for

Producing

Solution-Processed,

Electrohydrodynamic-Jet Printed, High Performance Oxide Transistors. J. Mater. Chem.

C 2013, 1, 4236-4243 [8] Banger, K. K.; Yamashita, Y.; Mori, K.; Peterson, R. L.; Leedham, T.; Rickard, J.; Sirringhaus, H. Low-Temperature, High-Performance Solution-Processed Metal Oxide Thin-Film Transistors Formed by a ‘Sol–Gel on Chip’ Process. Nat. Mater. 2011, 10, 4550 [9]

Han, S.-Y.; Herman, S. G.; Chang, C.-H. Low-Temperature, High-Performance, Solution-Processed Indium Oxide Thin-Film Transistors. J. Am. Chem. Soc. 2011, 133, 5166-5169

[10] Kim, Y.-H.; Heo, J.-S.; Kim, T.-H.; Park, S.; Yoon, M.-H.; Kim, J.; Oh, M. S.; Yi, G.-R.; 24


Page 25 of 28

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


Noh, Y.-Y.; Park, S. K. Flexible Metal-Oxide Devices Made by Room-Temperature Photochemical Activation of Sol–Gel Films. Nature 2012, 489, 128-132 [11] Teng, L.-F.; Liu, P.-T.; Lo, Y.-J.; Lee; Y.-J. Effects of Microwave Annealing on Electrical Enhancement of Amorphous Oxide Semiconductor Thin Film Transistor. Appl.

Phys. Lett. 2012, 101, 132901-1~132901-4 [12] Jeong, S.; Ha, Y.-G.; Moon, J.; Facchetti, A.; Marks, T. J. Role of Gallium Doping in Dramatically Lowering Amorphous-Oxide Processing Temperatures for Solution-Derived Indium Zinc Oxide Thin-Film Transistors. Adv. Mater. 2010, 22, 1346-1350 [13] Dai, M.-K.; Lian, J.-T.; Lin, T.-Y.; Chen, Y.-F. High-Performance Transparent and Flexible Inorganic Thin Film Transistors: a Facile Integration of Graphene Nanosheets and Amorphous InGaZnO. J. Mater. Chem. C 2013, 1, 5064-5071 [14] Liu, X.; Liu, W.; Xiao, X.; Wang, C.; Fan, Z.; Qu, Y.; Cai, B.; Guo, S.; Li, J.; Jiang, C.; Duan, X.; Liao, L. High Performance Amorphous ZnMgO/Carbon Nanotube Composite Thin-Film Transistors with a Tunable Threshold Voltage. Nanoscale 2013, 5, 2830-2834 [15] Liu, X.; Wang, C.; Cai, B.; Xiao, X.; Guo, S.; Fan, Z.; Li, J.; Duan, X.; Liao, L. Rational Design of Amorphous Indium Zinc Oxide/Carbon Nanotube Hybrid Film for Unique Performance Transistors. Nano Lett. 2012, 12, 3596-3601 [16] Son, G.-C.; Chee, S.-S.; Jun, J.-H.; Son, M.; Lee, S. S.; Choi, Y.; Jeong, S.; Ham, M.-H. Chemically Functionalized, Well-Dispersed Carbon Nanotubes in Lithium-Doped Zinc Oxide for Low-Cost, High-Performance Thin-Film Transistors. Small 2016, 12, 18591865 [17] Lee, D.-H.; Chang, Y.-J.; Herman, G. S.; Chang, C.-H. A General Route to Printable High-Mobility Transparent Amorphous Oxide Semiconductors. Adv. Mater. 2007, 19, 843-847 [18] Lee, W.-J.; Park, W.-T.; Park, S.; Sung, S.; Noh, Y.-Y.; Yoon, M.-H. Large-Scale Precise Printing of Ultrathin Sol–Gel Oxide Dielectrics for Directly Patterned Solution-Processed Metal Oxide Transistor Arrays. Adv. Mater. 2015, 27, 5043-5048 [19] Pierre, A.; Sadeghi, M.; Payne, M. M.; Facchetti, A.; Anthony, J. E.; Arias, A. C. AllPrinted Flexible Organic Transistors Enabled by Surface Tension-Guided Blade Coating.

Adv. Mater. 2014, 26, 5722-5727 [20] Rastogi, R.; Kaushal, R.; Tripathi, S. K.; Sharma, A. L.; Kaur, I.; Bharadwaj, L. M. 25



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

Page 26 of 28

Comparative Study of Carbon Nanotube Dispersion Using Surfactants. Science 2008, 328, 421-428 [21] Bak, S.-M.; Kim, K.-H.; Lee, C.-W.; Kim, K.-B. Mesoporous Nickel/Carbon Nanotube Hybrid Material Prepared by Electroless Deposition. J. Mater. Chem. 2011, 21, 19841990 [22] Aubin, K. S.; Poulin, P.; Saadaoui, H.; Maugey, M.; Zakri, C. Dispersion and FilmForming Properties of Poly(Acrylic Acid)-Stabilized Carbon Nanotubes. Langmuir 2009, 25(22), 13206-13211 [23] Grunlan, J. C.; Liu, L.; Kim, Y. S. Tunable Single-Walled Carbon Nanotube Microstructure in the Liquid and Solid States Using Poly(Acrylic Acid). Nano Lett. 2006, 6, 911-915 [24] Sarkar, S.; Hazra, C.; Chatti, M.; Sudarsan, V.; Mahalingam, V. Enhanced Quantum Efficiency for Dy3+ Emissions in Water Dispersible PbF2 Nanocrystals RSC Adv. 2012, 2, 8269-8272 [25] Kim, S. J.; Kim, A.; Jo, Y.; Yoon, J.-Y.; Lee, S. S.; Choi, Y.; Won, J.; Nahm, S.; Jang, K.-S.; Kim, Y. H.; Jeong, S. Polymeric Mold Soft-Patterned Metal Oxide Field-Effect Transistors: Critical Factors Determining Device Performance. J. Mater. Chem. C 2014, 2, 8486-8491 [26] Kim, W.-G.; Tak, Y.-J.; Ahn, B. D.; Jung T. S.; Chung, K.-B.; Kim, H. J. High-Pressure Gas Activation for Amorphous Indium-Gallium-Zinc-Oxide Thin-Film Transistors at 100 °C. Sci. Rep. 2016, 6, 23039(1-7) [27] Tak, Y. J.; Ahn, B. D.; Park, S. P.; Kim, S. J.; Song, A. R.; Chung, K.-B.; Kim, H. J. Activation of Sputter-Processed Indium–Gallium–Zinc Oxide Films by Simultaneous Ultraviolet and Thermal Treatments. Sci. Rep. 2016, 6, 21869(1-11) [28] Cho, D.-Y.; Kim, K.-H.; Kim, T.-W.; Noh, Y.-J.; Na, S.-I.; Chung, K.-B.; Kim, H.-K. Transparent and Flexible Amorphous InZnAlO Films Grown by Roll-to-Roll

Sputtering

for Acidic Buffer-Free Flexible Organic Solar Cells. Org. Electon. 2015, 24, 227-233 [29] Cho, D.-Y.; Na, S.-I.; Chung, K.-B.; Kim, H.-K. Embedment of Nano-Sized Ag Layer into Ag-Doped In2O3 Films for Use as Highly Transparent and Conductive Anode in Organic Solar Cells. Appl. Surf. Sci. 2015, 347, 88-95 [30] Lee, H-M.; Noh, Y.-J.; Na, S.-I.; Chung, K.-B.; Kim, H.-K.; PEDOT:PSS-Free Organic 26


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Solar Cells Fabricated on Buffer and Anode Integrated Ta-Doped In2O3 Films. Sol. Energ.

Mater. Sol. Cells 2014, 125, 145-154

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Table of Contents Graphic

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Extremely Low-Cost, Scalable Oxide Semiconductors Employing Poly

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