All Inkjet-Printed Metal-Oxide Thin-Film Transistor Array with Good

Publication Date (Web): February 23, 2017 .... Inkjet printing of oxide thin film transistor arrays with small spacing with polymer-doped metal nitrat...
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All Inkjet-Printed Metal-Oxide Thin-Film Transistor Array with Good Stability and Uniformity Using Surface-Energy Patterns Yuzhi Li, Linfeng Lan,* Sheng Sun, Zhenguo Lin, Peixiong Gao, Wei Song, Erlong Song, Peng Zhang, and Junbiao Peng* State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: An array of inkjet-printed metal-oxide thin-film transistors (TFTs) is demonstrated for the first time with the assistance of surface-energy patterns prepared by printing pure solvent to etch the ultrathin hydrophobic layer. The surfaceenergy patterns not only restrained the spreading of inks but also provided a facile way to regulate the morphology of metal oxide films without optimizing ink formulation. The fully printed InGaO TFT devices in the array exhibited excellent electron transport characteristics with a maximum mobility of 11.7 cm2 V−1 s−1, negligible hysteresis, good uniformity, and good stability under bias stress. The new route lights a general way toward fully inkjet-printed metal-oxide TFT arrays. KEYWORDS: inkjet printing, metal oxide, thin-film transistors, surface-energy patterns, Cytop

1. INTRODUCTION Over the past decade, intensive efforts have been devoted to the development of organic and inorganic thin-film transistors (TFTs) for its potential applications in flat panel displays (FPDs), radio frequency identification tags, sensors, etc.1,2 The advantages of TFTs based on metal oxide, including high mobility, high electrical stability, and visible light transparency, make it one of the most competitive candidates for applications in FPDs.3 Recently, most attention has been paid on the solution-processed metal-oxide TFTs for the advantage of low cost compared to that of traditional physical vapor deposition ones.4,5 Inkjet printing, as one of the state-of-the-art solution processing techniques, which achieves directly patterned films without photolithography, is attracted in TFT fabrication.6−9 Although the first metal-oxide TFT based on inkjet-printed semiconducting layer was reported in 2007 by Lee et al.,10 the fully inkjet-printed metal-oxide TFT has not been reported until recently (by Jang et al.).11 However, they demonstrated only a single TFT device, indicating there still exists serious difficulties from single device to device array. To realize fully printed uniform and stable metal-oxide TFT arrays, several issues should be addressed appropriately: (i) preparing printable ink; (ii) controlling the spreading of inks on different areas of substrates; (iii) suppressing coffee-ring formation. In previous literature on TFTs fabricated by spin coating, the solvents, such as 2-methoxyethanol or water, were generally used as precursor solvent to prepare oxide films.12−16 Generally, the relatively low viscosity of the solvents is not in favor of formation of long-time stable ink droplets in piezoelectric printers. Thus, adding solvents with relatively © 2017 American Chemical Society

high viscosity, such as ethylene glycol, is required, and it will change ink viscosity as well as surface tension, which is one of the parameters that determines the ink spreading on substrate. In addition, the ink formulation also has direct influence on the morphology of printed films.17 Therefore, it is inevitable to devote a lot of effort to optimize ink formulation to achieve printable ink, which is a serious issue for the application of inkjet printing in fabricating electronic devices. To fabricate TFT arrays by printing, the spreading of inks should be well-controlled, especially for the layers such as dielectric layer and source/drain electrodes deposited across two or more predeposited patterned films. Because of the existence of gradient and/or step in the predeposited patterned films, the printed ink tends to flow toward the lower areas, leading to accumulation of solute on these areas. In addition, the difference of surface energy leads to different spreading radius of ink on different area, and the ink will spontaneously flow to the areas with higher surface energy. To solve the problem, separators are widely employed to limit the spreading of ink in printing organic light-emitting diodes (OLEDs).18−21 However, most of the separators are produced through the traditional photolithography, which will increase the fabrication cost. Coffee ring is a phenomenon that is frequently observed in printed films, and intensive efforts have been devoted to eliminate the effect.22−24 The existence of coffee ring in the Received: January 10, 2017 Accepted: February 14, 2017 Published: February 23, 2017 8194

DOI: 10.1021/acsami.7b00435 ACS Appl. Mater. Interfaces 2017, 9, 8194−8200

Research Article

ACS Applied Materials & Interfaces preprinted films will cause nonuniformity of the following printed films, which lower the reliability of TFTs, so elimination of the coffee ring is necessary.25 Restraining the outward capillary flow and/or enhancing the inward Marangoni flow are two strategies to suppress the coffee-ring effect, which generally rely on the adjustment of ink formulation for a certain substrate. Lim et al.26 obtained the desired morphology of printed films by adjusting the ink formulation to regulate the capillary flow and Marangoni flow in droplets during the drying process. Kim et al.27 improved the morphology of printed films by adding a portion of ethylene glycol to the ink, which boosted inward Marangoni flow to eliminate the coffee ring. However, there may be different surface topologies (or even breakpoints) for printed lines across different areas due to the difference in surface energy.28 Jang et al.11 reported the fabrication of printed oxide films by adjusting solvent formulation and heating the substrate to suppress the coffee ring. However, the heated substrate will heat the nozzle, resulting in the instability of droplets. In addition, the increase in substrate temperature will increase the partial pressure of the solvent surrounding the droplet, and thus affects the adjacent droplets more seriously.29 As a result, it is difficult to achieve printed TFT array. In this work, we developed a new route to achieve fully inkjet-printed metal-oxide TFT array. Solvent etching method and oxygen plasma treatment were utilized to prepare surfaceenergy patterns. The surface patterns not only restrained the spreading of inks effectively but also provided a facile way to modulate the surface topology of deposited films, which relied only on the adjustment of the drop spacing or solute concentration of ink. Inspiringly, the multilayers of TFT, including gate, insulator, semiconducting layer, and source/ drain electrodes, were all printed based on the same route with the assistance of surface-energy patterns, and the TFT array was successfully fabricated with good field-effect characteristics. To the best of our knowledge, it is the first fully inkjet-printed metal-oxide TFT array.

ink was printed into the surface-energy patterns with drop spacing of 20 μm. After the ITO precursor was dried for 5 min, the film was softbaked at 80 °C for 5 min and hard-baked at 450 °C for 1 h in sequence under ambient conditions. The fabrication process of the following layers, including ZrOx dielectric, ITO source/drain, and InGaO semiconductor, was similar to that of the ITO gate layer. The process parameters are listed in Table S1 of the Supporting Information. Finally, the TFT array was passivated by Cytop. The channel length and width of fabricated TFTs were ca. 40 μm and ca. 60 μm, respectively. 2.3. Characterization. Surface profiles and 3D morphology images were characterized by Veeco NT 9300. Polarizing microscope images were obtained from Nikon Eclipse E600 POL. Transmission electron microscopy (TEM, FEI Titan Themis 200) equipped with an energy dispersive X-ray spectrometer (EDS) was used to obtain the structure and chemical information on the printed oxide TFT. The surface tension and viscosity of inks were obtained using a contactangle analyzer (Biolin Scientific, Theta Lite 101) and a viscometer (Brookfield, DV-I+), respectively. The electrical characteristics of TFTs were measured using a semiconductor parameter analyzer (Keithley 4200-SCS) in ambient atmosphere. The mobility (μ) and subthreshold slope (SS) of TFTs were calculated using IDS =

WμC i (VGS − Vth)2 2L

⎛ d(log I ) 10 DS ⎟ SS = ⎜⎜ ⎟ dVGS ⎝ ⎠

(1)

−1 ⎞−1

(2)

where Ci is the areal capacitance of the dielectric and Vth is the threshold voltage obtained by fitting the saturation region of IDS1/2 vs VGS plots and extrapolating the fitted line to IDS = 0. W and L are the channel width and length, respectively.

3. RESULTS AND DISCUSSION Figure 1 shows the basic process flow for printing oxide films, and each layer of TFT was printed by the identical method.

2. EXPERIMENTAL SECTION 2.1. Preparation of Solution and Inks. Solute (CTL-107MK) and solvent (CT-SOLV180) with a volume ratio of 1:10 were mixed and stirred at room temperature for 12 h to prepare Cytop solution. The indium tin oxide (ITO) precursor ink was prepared by dissolving In(NO3)3·xH2O (0.27 M) and SnCl2·xH2O (0.03 M) into a mixture of 2-methoxyethanol and ethylene glycol with a volume ratio of 1:1. The ITO ink was stirred vigorously at 50 °C for 12 h. The InGaO precursor ink was prepared by dissolving In(NO3)3·xH2O (0.19 M) and Ga(NO3)3·xH2O (0.01 M) into a mixture of 2-methoxyethanol and ethylene glycol with a volume ratio of 1:1. The ZrOx precursor ink was prepared by adding 0.6284 g of ZrOCl2·8H2O into a blended solvent containing 3 mL of 2-methoxyethanol, 3 mL of ethylene glycol, and 0.5 mL (35 wt %) of H2O2. Both of the InGaO and ZrOx inks were stirred at room temperature for 12 h. The solution and inks were all filtered through a 0.22 um filter before use. 2.2. TFTs Fabrication. A Dimatix (DMP-2800) printer with a 10 pL cartridge was used to print pure solvent and inks for desired patterns. The orifice diameter of the inkjet nozzle is 21 μm. During the printing, the substrate temperature of the printer was set at 28 °C. Bottom-gate, bottom-contact structure was employed here to realize fully printed oxide TFTs. For the ITO gate fabrication, the Cytop solution was first spin-coated on glass at 3000 rpm for 40 s to prepare ultrathin Cytop layer. Then the Cytop layer was etched to obtain surface-energy patterns by printing pure Cytop solvent with drop spacing of 45 μm. The patterned Cytop layer was treated with oxygen plasma for 3 min to thoroughly remove residue in the surface-energy patterns, and then it was annealed at 120 °C for 10 min. Next, the ITO

Figure 1. (I) Ultrathin Cytop layer deposited on substrate by spin coating. (II) Printing pure solvent to etch the Cytop layer. (III) Printing oxide precursor into surface-energy pattern. (IV) Formation of oxide film after annealing.

First, an ultrathin hydrophobic polymer layer was spin-coated on the substrate, and then the layer was etched selectively by inkjet-printing pure solvent to form surface-energy patterns. Then the oxygen plasma treatment was performed to remove the residue in the inner part of patterns. After annealing, oxide precursor ink was printed into the inner part of surface-energy patterns. Then oxide film formed after hard annealing. Cytop was chosen as the hydrophobic polymer layer for its low surface energy property, even after treatment by oxygen plasma.30 As shown in the inset of Figure 2, there existed obvious coffee stripes after the solvent etching, which was induced by the coffee-ring effect. When the solvent droplets landed on the 8195

DOI: 10.1021/acsami.7b00435 ACS Appl. Mater. Interfaces 2017, 9, 8194−8200

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ACS Applied Materials & Interfaces

Figure 2. Plot of width of linear surface-energy pattern versus drop spacing. The inset is the polarizing microscope image of surface-energy patterns.

Figure 3. 3D morphology images of printed (a) ITO, (b) ZrOx, and (c) InGaO films with various drop spacing (D). The corresponding surface profiles were shown in Figure S1.

substrate, the underlying layer was dissolved, and the dissolved solute migrated to the three-phase contact line along with the outward capillary flow during the evaporation of the solvent, resulting in the coffee stripes.31 The width of lines obtained by inkjet printing can be estimated by the following equation:32

phase diagram” (Figure 4a), the surface topology of printed films is determined by capillary flow and Marangoni flow which

2πd 2

w= 3p

(

θ sin 2 θ

−

cos θ sin θ

)

(3)

where d is the diameter of droplet in air, θ is the contact angle of the droplet on the substrate, and p is the drop spacing. By fixing the parameters of d and θ, the line width (w) decreases with the increase of drop spacing, which is consistent with the plot of Figure 2. This phenomenon provides an easy way to control the width of printed metal oxide films. To prepare printable oxide precursor inks, 2-methoxyethanol was chosen to dissolve the metal salts, and ethylene glycol was added to improve the viscosity of inks for stable drop formation during inkjet printing. The viscosity and surface tension for the prepared ITO, ZrOx, and InGaO inks are summarized in Table S2. Z number was used to evaluate the drop stability of piezoelectric inkjet printing in previous literature, and the value for the printable inks should be in the range 1−14.33,34 Z numbers of the inks were calculated using the following equation:

Z=

Figure 4. (a) “Surface-topology phase diagram” of printed films for the contact line with pinning. (b) Schematic diagram of the cross-sectional printed lines.

aργ η

(4)

where a is the inner diameter of the nozzle, ρ is the density, γ is the surface tension, and η is the viscosity of the ink. The calculated Z numbers for the ITO, ZrOx, and InGaO inks were 3.6, 4.7, and 4.0, respectively, all of which were within the printable range. To investigate the surface topology of the printed oxide films with the change of drop spacing, we printed the ITO, ZrOx, and InGaO precursor inks into linear surface-energy patterns with width of ca. 75 μm, ca.112 μm, and ca. 70 μm, respectively. Interestingly, the evolution of surface topology for oxide films with the alteration of drop spacing exhibited trending identical to that shown in Figure 3. With the decrease of drop spacing, the oxide films changed from concave to a convex surface topology. It has been reported that the sol−gel transition of metal salt precursor inks occurs at a late stage of solvent evaporation, which is analogous to the drying process of polymer inks.34 In this regard, the existence of fluid flow during the drying process should be considered to comprehend the evolution of surface topology. As shown in “surface-topology

cause the redistribution of solute in the droplets. The outward capillary flow, which is derived from the higher evaporation flux at the edge than that in the center, causes outward migration of solute, and thus the concave surface topology tends to form for the deposited films.35 The outward Marangoni flow intensifies stacking of solute on the periphery, while the inward one carries the solute to the center to replenish the loss caused by the outward migration, which may decrease the coffee-ring effect or avoid the formation of coffee ring. In our experiment, 2methoxyethanol (boiling point, 119 °C; surface tension, 29.7 mN/m) and ethylene glycol (boiling point, 197 °C; surface tension, 46.49 mN/m) were chosen as solvents for inks. Because the volatility of 2-methoxyethanol is higher than that of the ethylene glycol, the proportion of 2-methoxyethanol in the periphery reduced faster than that in the center during the drying process. As a result, the surface tension of the fluid near the periphery was higher than that near the center, which led to the formation of outward Marangoni flow.36 It meant that the evolution of surface topology was ascribed to the change of 8196

DOI: 10.1021/acsami.7b00435 ACS Appl. Mater. Interfaces 2017, 9, 8194−8200

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the glass substrate with drop spacing of 40 μm showed concave surface topology (Figure 3c). Figure 6a shows the cross-sectional high-resolution transmission electron microscopy (HRTEM) image in the channel

fluid-flow velocity, which may be caused by the alteration of evaporation flux and/or the change of the viscosity of fluid. The ratio of evaporation flux near the periphery (J(R − r)) to that of the center (J(R)) has the form35 J(R − r ) ⎛ R − r ⎞−λ ⎟ ∼⎜ ⎝ R ⎠ J (R )

λ=

π − 2θ 2π − 2θ

(5)

(6)

where R is the spreading radius, r is the distance from the point near the periphery to the center of the droplet, and θ is the contact angle. The J(R − r)/J(R) increases with the decrease of contact angle, which indicates that the intensity of outward capillary flow increases with the decrease of contact angle. As shown in Figure 4b, the contact angle of initial printed lines increased with the reduction of drop spacing because the spreading of droplets was limited by surface-energy pattern. The printed line (Line 1) with the higher initial contact angle (θ1) had the lower intensity of outward migration of solute during the initial drying process. With the evaporation of some solvent, θ1 would reduce to θ3 of initial Line 3, and thus J(R − r)/J(R) would be similar for Line 1 and Line 3. As a result, without the consideration of the influence of viscosity, the final deposited films would exhibit similar surface topology. However, this deduction was contradictory with the results. It indicated that the viscosity of fluid played an important role in determining the final films’ surface topology. When θ1 of Line1 reduced to θ3 of initial Line 3, the solute concentration in Line 1 was higher than that in Line 3. The higher solute concentration meant the higher the viscosity, as shown in Figure S2, which in turn reduced fluid-flow velocity and left a higher proportion of solute in the center of the droplet. Thus, the coffee ring was suppressed for the films printed with the smaller drop spacing. To further confirm this mechanism, the experiment of printing different concentration inks with the same drop spacing was performed. As shown in Figure S3, increase in solute concentration is in favor of formation of convex surface topology. This discovery provides a universal and facile way to print oxide films with desired surface topology. On the basis of the aforementioned methods, a TFT array was successfully fabricated on glass substrate as shown in Figure 5a. It could be seen that the devices in the array were uniform, indicating the interaction of adjacent droplets was negligible. The 3D morphology image (Figure 5b) shows that all of the visible layers are free of coffee ring. The convex surface topology of InGaO film was ascribed to the convex morphology of the underneath ZrOx layer because InGaO layer printed onto

Figure 6. (a) Cross-sectional TEM image of the channel region of TFT (bar = 50 nm). Cross-sectional TEM images and corresponding FFT patterns of (b) InGaO layer, (c) ZrOx layer, and (d) ITO layer, respectively (bar = 5 nm).

region of the printed oxide TFT, and Figure 6b−d display the close-up HRTEM images and the corresponding fast Fourier transform (FFT) patterns for the functional layers. And energydispersive X-ray spectrometer (EDS) point scan profiles (Figure S4) were obtained to collect the chemical information from the films. Thicknesses of the InGaO, ZrOx, and ITO films were measured to be ca. 11 nm, ca. 42 nm, and ca. 45 nm, respectively. The close-up HRTEM images and FFT patterns clearly revealed that both of the InGaO and ITO films were dominated by nanocrystalline structure, while the ZrOx film exhibited amorphous structure. The lattice spacing of crystal in InGaO layer was measured to be ca. 0.29 nm, which matches well with the (222) crystal plane of In2O3.37 The amorphous phase of the ZrOx film was in favor of reducing the leakage current compared to the crystalline ones, in which the crystal boundaries provided pathways for the leakage current. Capacitors with a structure of ITO/ZrOx/ITO were fabricated to investigate the insulating properties of the printed ZrOx films. Figure 7 shows the areal capacitance as a function of frequency, while the inset shows leakage current density versus electric field. The capacitance density at 1 kHz was ca. 303 nF/ cm2, and the calculated dielectric constant was ca. 15.4, similar to the value of spin-coated ZrOx reported elsewhere.38 The

Figure 5. (a) Polarizing microscope image of fully printed oxide TFT array with a scale bar of 200 μm. The inset is a close-up image of printed TFT. (b) 3D morphology image of printed TFT.

Figure 7. Areal capacitance and leakage current density (inset) of the printed ZrOx dielectric. 8197

DOI: 10.1021/acsami.7b00435 ACS Appl. Mater. Interfaces 2017, 9, 8194−8200

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Figure 8. (a) Typical output and (b) transfer characteristics of the printed TFT. (c) Variation of the transfer curves of the printed TFT under positive gate bias stress for 1000 s.

smaller dielectric constant of printed ZrOx compared to ZrO2 ceramic (>30) is ascribed to the amorphous phase of ZrOx, and the existence of organic residues and unconverted hydroxides. It was reported that the dielectric constant of metal oxide films containing organic residues and unconverted hydroxides is lower than that of pure oxide, and the crystallinity also has an influence on the dielectric constant of metal oxides.39 The capacitance decreased as the frequency increased, which was mainly attributed to the hydroxyl groups and other impurities in ZrOx insulator. The orientation polarization for hydroxyl groups and other impurities generally requires a long time (in the range of 10−9 to 10−5 s approximately), so the areal capacitance decreases with increasing frequency.40 The printed ZrOx exhibited a low leakage current density of 5.1 × 10−7 A/ cm2 at 1 MV, and the breakdown electric field was about 2.05 MV/cm. Figure 8a,b shows the output and transfer curve of the printed InGaO TFT, respectively. The TFT exhibited typical nchannel characteristics with a saturation mobility of 7.5 cm2 V−1 s−1, a threshold voltage of 0.04 V, a subthreshold slope of 0.14 V/dec, an Ion/Ioff of larger than 107, and negligible hysteresis, which indicated low density of mobile impurity ions near the channel region and defect states in the channel/dielectric interface.38,41 Furthermore, the total trap density (Nt), including the traps existing in the semiconductor bulk and channel/dielectric interface, was evaluated using the following formula,42 SS =

q2Nt ⎞ kBT ln 10 ⎛ ⎜1 + ⎟ q Ci ⎠ ⎝

Figure 9. Histograms of the (a) mobility and (b) threshold voltage for printed TFTs.

11.7 cm2 V−1 s−1. The results indicated that the route to fabricate TFT arrays was promising for applications in FPDs.

4. CONCLUSIONS In summary, the fully inkjet-printed oxide TFT array was demonstrated based on our developed route. Successful integration of each layer of TFT was achieved by the introduction of surface-energy patterns produced by inkjet printing and plasma treatment. The printed oxide TFTs exhibited excellent electrical properties with an average mobility of 7.4 cm2 V−1 s−1, a maximum mobility of 11.7 cm2 V−1 s−1, and good stability under gate bias stress. All of these results indicate that the route is a feasible way to realize fully printed metal-oxide TFT arrays.

(7)

where q is the electron charge, kB is the Boltzmann’s constant, and SS is the subthreshold swing of the transfer curve. The calculated Nt was ca. 2.59 × 1012 eV−1 cm−2. Although the fully printed oxide TFTs have been reported elsewhere, the electrical stability of the devices under bias stressing have not been presented.11,43 To investigate the electrical stability of the printed oxide TFT, positive gate bias stress with a gate voltage of 3 V for 1000 s was performed, as shown in Figure 8c. It could be seen that there was only small Vth shift (0.08 V) after stressing for 1000 s. The excellent electrical stability was ascribed to the low trap states in semiconducting bulk and good interface coupling between the insulator and channel, which is consistent with the value of Nt. To evaluate the device uniformity, the mobility and threshold voltage extracted from 50 devices were collected, as shown in Figure 9. The statistical data shows that the devices exhibited an average mobility of 7.4 cm2 V−1 s−1 with a maximum mobility of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00435. (1) Surface profiles of oxide films for different drop spacings (Figure S1); (2) plot of viscosity versus precursor concentration for ZrOx ink (Figure S2); (3) 3D morphology images and surface profiles of ZrOx films (Figure S3); (4) STEM images and EDS point scan profiles of printed TFT (Figure S4); (5) process parameters of TFT fabrication (Table S1); (6) surface tension and viscosity of prepared inks (Table S2) (PDF) 8198

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Linfeng Lan: 0000-0002-6477-2830 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Key Research and Development Program of Strategic Advanced Electronic Materials under Grant 2016YFB0401105, in part by the National Natural Science Foundation of China under Grant 51673068, Grant 61204087, and Grant 51173049, in part by the Pearl River S&T Nova Program of Guangzhou under Grant 2014J2200053, and in part by the Guangdong Province Science and Technology Plan under Grant 2014B010105008, Grant 2014B090916002, Grant 2015B090914003, and Grant 2016B090906002.

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DOI: 10.1021/acsami.7b00435 ACS Appl. Mater. Interfaces 2017, 9, 8194−8200