Soluble Metal Oxo Alkoxide Inks with Advanced Rheological

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Soluble Metal Oxo Alkoxide Inks with Advanced Rheological Properties for Inkjet-Printed Thin-Film Transistors Sebastian Meyer, Duy Vu Pham, Sonja Merkulov, Dennis Weber, Alexey Merkulov, Niels Benson, and Roland Schmechel ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12586 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on January 3, 2017

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ACS Applied Materials & Interfaces

Soluble Metal Oxo Alkoxide Inks with Advanced Rheological Properties for Inkjet-Printed Thin-Film Transistors Sebastian Meyer,* †,‡ Duy Vu Pham, † Sonja Merkulov, † Dennis Weber, † Alexey Merkulov, † Niels Benson, ‡ and Roland Schmechel‡ † Evonik Resource Efficiency GmbH, Electronic Solutions, 45772 Marl, Germany ‡ Institute of Technology for Nanostructures and CENIDE, University of Duisburg-Essen, 47057 Duisburg, Germany KEYWORDS: soluble metal oxo alkoxides, inkjet-printing, rheological stability, printability window, thin-film transistors

ABSTRACT: Semiconductor inks containing an indium-based oxo alkoxide precursor material were optimized regarding rheology requirements for a commercial 10 pL inkjet printhead. The rheological stability is evaluated by measuring the dynamic viscosity of the formulations for 12 hours with a constant shear rate stress under ambient conditions. It is believed that the observed superior stability of the inks is the result of effectively suppressing the hydrolysis and condensation reaction between the metal oxo alkoxide precursor complex and atmospheric water. This can be attributed to a strong precursor coordination and the resulting reduction in ligand exchange dynamics of the solvent tetrahydrofurfuryl alcohol which is used as the main solvent in the formulations. It is also shown that with a proper selection of co-solvents, having high polar Hansen solubility parameter values, the inks ACS Paragon Plus Environment


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drop formation properties and wettability can be fine-tuned by maintaining the inks rheological stability. Good drop jetting performance without satellite formation and high drop velocities of 8.25 m/s were found with the support of dimensionless numbers and printability windows. By printing single 10 pL ink dots onto short channel indium-tin-oxide electrodes, In2O3 calcination at 350 °C and a solutionprocessed back-channel protection, high average saturation mobility of approximately 10 cm2/Vs are demonstrated in a bottom-contact coplanar thin-film transistor device structure.

1. INTRODUCTION The advantages of metal-oxide semiconductors for thin-film transistor (TFT) backplanes are widely shown by leading display manufacturers.1 Due to their ability to provide high charge carrier mobility when deposited via vacuum processes, these materials are ideally suited for the TFTs driving the next generation of LCD and OLED flat panel display screens. Other exploited fabrication technologies are solution-based, for example the deposition by slot-die coating using functional precursor inks (soluble metal-oxides). Such an approach has many advantages in terms of equipment cost, flexibility of elemental film composition and large-area stoichiometric homogeneity.2-6 Nevertheless, regardless whether thin-films processed with a vacuum or a solution-based deposition, the resulting layers must be patterned by multiple and complex photoresist masking, developing and etching steps to avoid current superposition and cross-talk in the active matrix. Therefore, a non-lithographic, selective and resource efficient direct deposition of the functional semiconducting material is the goal of many research efforts today. One of the deposition technologies under investigation is inkjet-printing1,7-28 and the research for the implementation of metal-oxide inks is focused on binary (IZO, ZIO, ITO, IGO ZTO)14,17,19,20,22,23 and ternary formulations such as IZTO and IGZO15,16,19,21,24,25,27 which are generally demonstrating moderate charge carrier saturation mobility. It is difficult to evaluate the quality of an ink in terms of the maximum achievable charge carrier mobility, as process conditions, device layer structures, measurement conditions and underlying models for the extraction of electrical properties may vary significantly. ACS Paragon Plus Environment

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Moreover, it is important to mention that although the TFT-backplane application is frequently used as a motivation for this type of research, only a few devices are fabricated with single ink droplets (highest achievable inkjet-printing resolution) and characterized in short channel electrode geometries, typically used for display applications. This article suggest a general route for the testing and optimization of soluble metal oxo alkoxide formulations for inkjet-printed thin-film transistor applications, considering defined rheological, structural and electrical aspects. For the experimental evaluation a commercially available highperformance indium-based precursor material is used. We report the effects of the rheological ink stability under atmospheric conditions and provide a detailed printability study of the ink, by fine-tuning the organic solvent composition of the formulations and by analyzing the drop-formation using dimensionless We-Re as well as Ca-We plots. The inks were used to fabricate TFTs in a bottom-gate, bottom-contact coplanar device configuration with 10 µm indium-tin-oxide (ITO) short channel contacts (W/L = 1). Their characterization yielded high average saturation mobilites of approx. 10 cm²/Vs for maximum precursor conversion temperatures of 350 °C.

2. EXPERIMENTAL SECTION 2.1 Preparation of precursor ink formulations Inkjet formulations were fabricated by dissolving 12,5 g of the solid indium-based oxo alkoxide precursor material iXsenic® S (Evonik Resource Efficiency GmbH) under inert conditions and rigorous stirring for 24 hours at 40 °C in 250 mL of the organic solvents 1-methoxy-2-propanol (1M2P, Sigma Aldrich) or tetrahydrofurfuryl alcohol (THFA, Sigma Aldrich). After filtration with a 0.2 µm (PTFE) filter, these formulations were used as concentrates (indicated by *) and further diluted with other cosolvents like tri(propylene glycol) methyl ether (DowanolTM TPM, Dow), tri(propylene glycol) butyl ether (DowanolTM TPnB, Dow), tri(ethylene glycol) monoethyl ether (Ethoxytriglycol TGEE, Dow), acetonitrile (AC, Sigma Aldrich) and propylene carbonate (PC, Sigma Aldrich) to adjust the rheology and the wettability on the substrates. All solvents with their chemical formula, boiling point and Hansen ACS Paragon Plus Environment


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solubility parameter values are summarized in table 1. The final formulations (ink 1 - 8) with corresponding concentrate and co-solvent are shown in table 2. Table 1. Summary of organic solvents and their properties taken from HSPiP (Hansen Solubility Parameters in Practice) software dataset29 and solvent supplier product information,30 the abbreviation

Chemical formula

Organic solvent

Boiling point, °C

Hansen solubility (HSP), MPa0.5

parameter

D

P

H

T

1-methoxy-2-propanol – 1M2P29

C4H10O2

118.0

15.6

6.3

11.6

20.4

Tetrahydrofurfuryl alcohol – THFA29

C5H10O2

178.0

17.8

8.2

10.2

22.1

Tri(propylene glycol) butyl ether – C5H12O2 TPnB30

278.0

14.8

1.7

7.9

16.9

Tri(propylene glycol) methyl ether – C10H22O4 TPM30

242.2

15.1

2.5

8.7

17.6

Tri(ethylene glycol) monoethyl ether – C8H18O4 TGEE30

256.0

16.0

6.8

10.6

20.4

Propylene carbonate – PC29

C4H6O3

242.0

20.0

18.0

4.1

20.4

Acetonitrile – AC29

C2H3N

82.0

15.3

18.0

6.1

20.4

stands for the dispersion (D), polarity (P), hydrogen bonding (H) and total (T) amount

Table 2. Final formulations

with

concentrate- and co-solvent in a

Concentrate, 50 mg/ml

Co-solvent

Ink 1

1M2P*

TPnB

Ink 2

1M2P*

TPM

Ink 3

1M2P*

TGEE

Ink 4

THFA*

TPnB

ACS Paragon Plus Environment

composition

2:1 volume ratio

of

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Ink 5

THFA*

TPM

Ink 6

THFA*

TGEE

Ink 7

THFA*

PC

Ink 8

THFA*

AC

2.2 Thin-film transistor device fabrication TFT devices were fabricated on highly doped silicon (n++) substrates with thermally grown SiO2 (200 nm). Indium-tin-oxide (ITO) layers (30 nm) were deposited in a sputtering process and photolithographically patterned by wet chemical etching with oxalic acid at 45 °C for 3 minutes (1M, Sigma Aldrich). The TFTs have a quadratic channel area with a length (L) and width (W) of 10 µm. For direct ink deposition a Fujifilm Dimatix DMP-2831 drop-on-demand system is used with 10 pL drop volume cartridges (nozzle diameter 21.4 µm). During printing the head to substrate distance is fixed at 1500 µm. Neither substrate nor cartridge heating is applied. A drop watcher stroboscope LED delay camera system is used for investigating the drop formation process and optimization. Immediately before printing a substrate UV-ozone treatment ensures a good overall surface wetting and adjustment of the surface energy between channel contact and gate insulator interface. Single droplets are manually printed onto the TFT channel area. Right after inkjet-printing, the droplets are dried and cross-linked in an additional UV-ozone treatment and converted into the final In2O3 film at 350 °C on a hot plate for 1 hour.31 Thereafter, a Y2O3-based backchannel protection layer (iXsenic® P1301, Evonik Resource Efficiency GmbH) is spin-coated at 2000 rpm for 30 s on top with the same converting procedure.32

2.3 Characterization A Kruess Drop shape analyzer DSA 100 was used for the characterization of the inks surface tension. The ink density was determined with a 1 mL gas pycnometer at room temperature. Dynamic viscosity ACS Paragon Plus Environment


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and shear stress behavior were investigated by using a HAAKE RheoStress 600 in a double gap cylindrical container (DG41) with a 8 ml ink volume per measurement. For determination of the dynamic viscosity, the shear stress was measured by sweeping the shear rate between 10 - 1000 Hz. The viscosity versus shear rate data points were averaged over 10 s for each shear rate step. The dynamic viscosity change is characterized at a constant shear rate of 500 Hz for 12 hours under ambient conditions. Structural information about the printed semiconductor droplets was obtained by using a KLA Tencor Alpha-Step D-600 profilometer with a tip diameter of 2.5 µm. The scan parameters are 5 µm/s, 20 Hz with an applied stylus force of 2 mg. The root mean square roughness Sq of the final metal-oxide films was analyzed using a Nanosurf Mobile S atomic force microscope (AFM). The electrical characterization of TFTs is conducted with a semiconductor parameter analyzer (Agilent 4156C) under inert conditions in a glove box system with N2 (O2 < 1.0 ppm, H2O < 1.0 ppm) atmosphere.

3. RESULTS AND DISCUSSION 3.1 Inhibited sol-gel transition and rheological ink stability For reliable and error-free jet printing of functional materials in an industrial fabrication process, the drop formation properties of the ink, such as viscosity, density and surface tension need to be stable over time. Water as the most simple and critical reactant can initiate the sol-gel transition with hydrolysis (1) followed by condensation reactions (2 or/and 3) of metal-alkoxides according to the following reactions

M(OR)x + H2O M(OR)x-1OH + ROH

(1)

M(OH)(OR)x-1 + M(OR)x (OR)x-1M-O-M(OR)x-1 + HOR ACS Paragon Plus Environment

(2)

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2 M(OH)(OR)x-1 (OR)x-1M-O-M(OR)x-1 + H2O

(3)

As a consequence the formation of oligomers, polymers or even particle precipitation in the formulation can occur.33 In a rheological stable ink formulation these reactions need to be suppressed, as they may lead to nozzle clogging and misfires, resulting in layout errors and final device failure. Moreover, when there is a need for fine tuning the ink rheology or wettability by using a co-solvent, the understanding of ligand exchange reactions and their modification of the precursor complex on a molecular level is important, even if not always possible. The influence of these reactions on the change in formulation turbidity and rheology can be directly measured by optical microscopy and viscosimetry and the viscosity change from alkoxide solution to gel transition is indirectly linked to the degree of polymerization.34, 35 A row of pendent drop images of the indium-containing precursor complex in 1-methoxy-2-propanol (figure 1 a) and tetrahydrofurfuryl alcohol (figure 1 b) concentrate, diluted in a 2:1 volume ratio with the co-solvent TPnB (ink 1 and ink 4) are shown.

Figure 1. (a) Pendant drop images of 1M2P* concentrate diluted in a 2:1 volume ratio with TPnB (ink 1) as an example for unstable formulation and (b) a more stable formulation based on THFA* ACS Paragon Plus Environment


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concentrate (ink 4), (c) comparison of 1M2P* and THFA* precursor concentrate stability (d) influence of atmosphere and constant shear rate stress on unstable ink formulation (e) formulation with highest rheology stability based on THFA* concentrate and co-solvents with different viscosities and polar values The drops are illustrated for different time intervals in contact with the laboratory atmosphere (temperature 25 °C, relative humidity 40%). When using 1M2P as a starting point solvent for the concentrate, a visible gelation occurs within 60 seconds during the measurement. If stored under inert condition, the same formulation remains clear and is stable for months. The second formulation based on the THFA* concentrate, however, shows no visible change during the pendant drop measurement for a time frame of 10 minutes. To further characterize and compare the stability of the formulations, a constant viscosity shear rate stress test is used (described in the experimental section and schematically shown in the inset of figure 1 c). The 12-hour dynamic viscosity measurement at a constant shear rate of 500 Hz is plotted in figure 1 a, d and e for both concentrates and all inks except ink 1 which already showed a very low atmospheric stability. The higher stability of the THFA* concentrate is also found in the dynamic viscosity measurements plotted in figure 1 c. Here the viscosity is constant for 12 hours at approx. 7.3 mPas. The concentrate based on 1M2P* shows a continuous increase in viscosity from 4 mPas to 11 mPas within the first 6 hours. The instability of ink 1 should be therefore not only related to TPnB co-solvent incompatibility but could also be an effect of a poor solvent coordination or evaporation induced concentration due to the relative low boiling point of 1-methoxy-2-propanol (118.0 °C). All other formulations based on the 1M2P* concentrate (ink 2, ink 3) are also showing a lower stability and a dynamic viscosity increase of 3 - 4 mPas within the first two hours and four hours respectively (figure 1 d). However, when the measurement of ink 2 is taking place under nitrogen atmosphere, the viscosity is stable at 4.3 mPas. This measurement suggests that mostly atmospheric water is initiating the sol-gel reaction and viscosity increase and not the constant shear stress, which is applied during the measurement. The highest stability of the formulations based on the 1M2P* concentrate is found for the dilution with TGEE as a co-solvent (ink 3). This co-solvent stability effect ACS Paragon Plus Environment

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is also observed for the formulations based on tetrahydrofurfuryl alcohol concentrate (ink 4, 5 and 6). Furthermore, although ink 4 and ink 5 showing only a slight increase in viscosity after 12 hours, they have a strong turbidity thereafter. The dynamic viscosity of ink 6, based on the TGEE co-solvent, is stable at approximately 9 mPas for 12 hours under constant shear rate stress at 500 Hz. A high stability is also found for ink 7 at 6 mPas and ink 8 at 2.5 mPas, which are based on the THFA* concentrate and the aprotic co-solvents propylene carbonate and acetonitrile, respectively. These three formulations are the only ones, which do not show any turbidity after the 12-hour measurement. The overall higher stability of the precursor complex with the THFA solvent and the inhibited sol-gel transition can be confirmed in all experiments. A possible explanation for the higher stability can be attributed to the strong bonding of the THFA ligand to the oxo alkoxide complex and the resulting reduced ligand exchange kinetic with the water molecule. Furthermore, the experiments discussed in conjunction with figure 1 stresses that a proper co-solvent selection is necessary for the superior stability under atmospheric conditions. By comparing the polar Hansen solubility parameters of the co-solvents showing the highest stability in the final formulation (table 1), it is noticeable that they all have high values of 6.8 MPa0.5 (TGEE), 18.0 MPa0.5 (PC) and 18.0 MPa0.5 (AC). This may indicate that a high co-solvent polarity is required in order to keep the product of precursor complex and the coordinating solvent THFA stable in solution. In the following the influence of the single solvents THFA, PC, AC and TGEE on the precursor complex at different shear rates is investigated for ink 6, 7 and 8. Figure 2 a shows the dynamic viscosity η (left y-axis) and the shear stress τ (right y-axis) over shear rates γ from 10 - 1000 Hz for all single components (open circles) of the formulation and figure 2 b - e shows the molecular structure of the main solvents.



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Figure 2. (a) Measurement of shear stress τ and determined dynamic viscosity η vs. shear rate γ for the pure solvent THFA, TGEE, PC, AC (open circles) THFA*, ink 6, ink 7 and ink 8 (solid circles), molecular structure of the solvent (b) THFA, (c) PC, (d) AC and (e) TGEE The high boiling point solvent TGEE shows the highest viscosity of approx. 8 mPas at 1000 Hz. In the 2:1 dilution of the THFA* concentrate with TGEE (ink 6), η is measured at 7.66 mPas and is more close to the dynamic viscosity of the THFA* concentrate. A small non-Newtonian shear-thinning behavior is found for ink 6, which is demonstrated by the viscosity increase at lower shear rates. All other formulations and their solvents show Newtonian flow behavior, as indicated by the linear relationship between shear stress and shear rate. By comparing the pure THFA solvent with the concentrate, it can be seen, that the viscosity is increased by approximately 12% over the whole measured shear rate range. The viscosity increase can be directly related to the additional interaction between precursor-complex and THFA solvent molecules. By diluting the concentrate with the aprotic solvents PC (ink 7) and AC (ink 8) the viscosity is decreased down to 4.92 mPas and 2.13 mPas respectively. It should be pointed out, that although the fraction of the low-viscosity co-solvent AC is only 33% in the ink, the viscosity drop is approximately 71 %. The density of the formulations is found at approx. 1000 kg/m3 and the difference between the three inks is


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quite low (table 3). The surface tension is characterized using the pendant drop method and drop shape analysis (figure 3).

Figure 3. Pendant drop images for determining the surface tension of pure solvents and final ink formulations. (a) THFA* concentrate, (b) ink 6, (c) ink 7 and (d) ink 8 The highest surface tension is found to be 38.1 mN/m for ink 7 which is higher in comparison to the THFA* concentrate. The surface tension for ink 6 can be found at 35.1 mN/m and for ink 8 at 32 mN/m (table 3).

Table 3. Physical properties of ink formulations measured at 25 °C with a relative humidity of 40 %. The dynamic viscosity η is determined at a shear rate of 1000 Hz. Density and surface tension values σ of the pure solvents are taken from HSPiP (Hansen Solubility Parameters in Practice) software dataset29 and solvent supplier product information,30 ρ, kg/m3

η, mPas

σ, mN/m

TGEE30

1018

8.16

30.0

PC29

1200

3.16

41.1

AC29

780

0.51

29.3

THFA29

1060

6.53

37.0



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

THFA*

1078

7.34

35.0

Ink 6

1041

7.66

35.1

Ink 7

1100

4.92

38.1

Ink 8

1030

2.13

32.0

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3.2 Inkjet printability and drop formation properties The drop jetting mechanism is a complex process and determined by inertial, viscous and surface tension forces.18,36,37,38 The ratio of inertial to viscous forces for a specific fluid is defined by the Reynolds number (Eq. 4), whereas the ratio between inertial force and surface tension is given by the Weber number (Eq. 5). The capillary number (Eq. 6) describes the ratio between viscous forces and surface tension of the fluid. In these formulas ρ represents the formulation density, L the characteristic length (nozzle diameter 21.4 µm), ν the ejected drop velocity, η the dynamic viscosity and σ the interfacial surface tension. By introducing the Z number (Eq. 7), also called the inverse Ohnesorge number (Oh-1), the printability for different type of inks can be predicted.36

=

=

= (4) η

= (5) σ

=

f η = (6) σ

= ℎ"# =

√

=

% σ (7) η


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If the Z number is very low, the viscous forces are higher and a drop ejection is not always possible. Higher Z values are associated with large surface tension forces and the Plateau Rayleigh instability. This is leading to the formation of satellite droplets and the prevention of a stable single drop jetting behavior. Consequently, reliable inkjet printing is only possible within a defined parameter space. Different authors define this space in the range of 1 - 10,39 4 - 1440 or 1 - 60.37 The differences in the printability range can be related to variable ink system, print-head and waveform design. With equation 7 and the experimentally determined values for ink 6, ink 7 and ink 8 (table 3), the respective Z values can be calculated to be 3.9, 6.5 and 9.4. For the drop analysis we have used a customized waveform profile, which can be seen in figure 4 a. Even tough Z is independent on drop velocity, printability for different by a variation of the applied piezoelectric voltage pressure (figure 4 b) during the waveform optimization is observed. Thus the printability window can be specified more accurately. Under a certain threshold voltage, there is no sufficient energy to generate free droplets. For the optimized waveform (figure 4 a), this energy is found for threshold voltages of 18 V (ink 6), 15 V (ink 7) and 12 V (ink 8).



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Figure 4. (a) Optimized piezoelectric waveform profile which is used for the drop analysis, (b) We-Re and (c) Ca-We printability windows

With higher Z-values the threshold voltage is further increased for the possibility of drop generation. The lowest measurable velocity is found at approx. 2.4 m/s. This is in good agreement with the minimum velocity (equation 8) and voltage threshold (We > 4) for drop generation, as introduced by Duineveld et al.41 '()

4 σ ,.. = * + (8)


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Furthermore, the onset of splashing determined by the condition k < We0.5 ∙ Re0.25 is plotted in figure 4 b. The splashing parameter k can be found at 57.7 for smooth surfaces.42 With our experimental setup the onset of splashing could not be determined. In figure 5 the drop formation process for ink 6, 7 and 8 is given at different firing voltages using 1 kHz jetting frequency.

Figure 5. Time dependent drop formation process with satellite and head drop trajectory in respect to the nozzle distance for (a) ink 6 at 30 V, (b) ink 6 at 27 V, (c) ink 7 at 22 V and (d) ink 8 at 14 V firing voltages, the distance was determined out of the single images at the given time intervals from the printers drop-watcher setup

A drop velocity of 10 m/s is measured for ink 6 using a 30 V firing pulse. Satellite drop formation due to filament breakup as indicated in the scattering of head and satellite trajectory is observed (figure 5 a). By lowering the firing voltage to 27 V, a maximum velocity of approx. 8.25 m/s is determined with satellite to head catchup after 150 µs. Single droplets without satellites are only generated at lower firing voltages (22 V) for ink 7. Here a maximum velocity of 6.1 m/s can be reached. A reunion of satellite to the main drop is found at 14 V with 4.7 m/s for ink 8. The wider printability of ink 6 can be directly related to the higher viscosity and intermolecular forces, which are leading to higher filament stability. ACS Paragon Plus Environment


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It should be pointed out that for an adequate drop-placement accuracy the drop velocity should be in the range between 8 - 10 m/s (green area in figure 4 b without filament breakup and satellite formation). Figure 4 b and c are showing the Re–We plot and the Ca-We plot for the three inks at different firing voltages. The grey dots in figure 4 b indicate the range where printing is possible but satellite formation is dominant. The results are also compared with the printability window of Nallan et al. in figure 4 c. Because the Ca-We plot neglects the surface tension of the ink completely, all of the formulations can be found in the defined printability space. However, Nallan et al. predict a much lower minimum velocity. This could be related to a difference in printhead design and/or waveform-profile. As already pointed out, there is no relevance for printing with low drop velocity in production processes because the ink placement accuracy is negatively influenced. It is noticeable that ink 6 is already in the optimum Z range with the highest possible Weber numbers.

3.3 Ink wetting and single dot transistor device performance The three rheological stable ink formulations were printed with optimized parameters (maximum drop velocity without satellite formation) and characterized in a bottom-gate, bottom-contact TFT device structure as described in the experimental section and schematically shown in figure 6 a. The drain current (ID) level for the TFTs fabricated with ink 7 is found at approx. 1⋅10-10 A for the whole forward and backward gate voltage sweep. This current is equivalent to the noise level of the equipment (purple line in figure 6 b. For the TFTs fabricated with ink 8 only a small gate voltage response and a very poor electrical device performance with a maximum ID of approx. 2⋅10-7 A is measured (red line in figure 6 b). The TFTs fabricated with ink 6, demonstrate a good electrical performance. A typical transfer curve for this type of device is plotted in the linear regime with a constant VDS = 2 V (bright blue line in figure 6 b) and saturation regime with a constant VDS = 20 V (dark blue line in figure 6 b). All devices show ntype semiconductor behavior with an Ion/Ioff ratio of approximately 106. It should be noted that the offcurrent level is limited by the accuracy of the measurement equipment, which leads to an underestimation of the Ion/Ioff ratio. The threshold voltage is found at small negative gate voltage values, ACS Paragon Plus Environment

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which denotes a depletion mode device behavior with charge carriers already present at the channel interface without applied gate bias. The linear and saturation charge carrier mobility in response to the gate voltage is also plotted in figure 6 b. The maximum values for the shown device are found at 12.9 cm2/Vs and 9.0 cm2/Vs, respectively.

Figure 6. (a) Device structure with measurement parameters, (b) TFT transfer curves at VDS = 2 V (linear) and 20 V (saturation) with Y2O3 backchannel passivation for ink 6, and at VDS = 20 V for ink 7 and ink 8, (c) equation used for calculation of linear (VDS = 2 V) and saturation mobility (VDS = 20 V) of TFT fabricated with ink 6, (d) typical output curve for device fabricated with ink 6, (e) histogram for saturation mobility and (f) threshold voltage for 20 devices fabricated by direct drop deposition of ink 6 with optimized jetting parameters

The use of the equations (figure 6 c) for the given VDS and VGS values is justified by the linear and saturation drain current behavior shown in the output characteristic (figure 6 d). With the spin-coated Y2O3 backchannel passivation, as described in the experimental section, no hysteresis was observed for the electrical characterization. Stressing our devices for 100 cycles under inert conditions has led to no degradation of the device performance (supporting information figure S3).



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For optimized inkjet parameters the average saturation mobility is found at 9.9 ± 0.8 cm2/Vs and the average threshold voltage at -1.4 ± 1.0 V. The histogram of the statistics for 20 devices used to determine these values is plotted in figure 6 e and f. Fringe currents with might lead to an overestimation of the charge carrier mobility can be neglected for our experiment as substantiated with different transistor geometries as shown in S3. Up to our knowledge, such mobility values advanced the state of the art by approximately 30 % for single source indium-based precursors.

11-16, 19-22, 24-28

A

summary of our literature review in table form, backing this statement, is listed in the supporting information (supporting information S3). Microscope images and a corresponding profilometry measurements for all calcinated In2O3 dots printed on ITO source and drain electrodes are illustrated in figure 7 a and b, respectively.

Figure 7. (a) Microscope images of source and drain electrode with printed and converted metal-oxide dots for ink 6, ink 7 and ink 8, (b) corresponding profilometry data for the colored arrow in the microscope images indicating the scan direction during measurement, (c) microscope image of the atomic force microscope head, cantilever and the drop matrix, (d) 3D plots of the different surface topographies measured with the AFM


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The largest drop diameter of approx. 250 µm is found for ink 7, which is related to a strong spreading as the consequence of a high ink polarity and a high overall boiling point. The latter value leads to low evaporation rates and therefore longer spreading times. The drop diameter of ink 8 can be found at ≈ 150 µm with a very high coffee ring ratio (CRR = hout/hin) of 35. In the beginning of the film formation, the inks very low viscosity (2.13 mPas) leads to a strong spreading on the substrate. However, due to the high vapor pressure (low boiling point) of the co-solvent acetonitrile, there is a rapid increase in viscosity during the spreading process, which results in a pinned contact line and consequently in a pronounced ring film formation (hout). With ink 6 smaller drop diameters of approximately 70 µm are printed and a layer thickness of 4 - 6 nm with a low coffee-ring ratio of 1.4 is found. The root mean square roughness Sq of all final metal-oxide films are found very close to the SiO2 surface roughness at around 0.2 nm (figure 7 d). Because the generated drops are still wet after printing, a drying time of 78 s was estimated by video analysis with the print head fiducially camera (see figure S1 in the supporting information for the calculation of a drop drying time). It is believed, that the low coffee-ring ratio is a result of the longer drying time and a possible Marangoni effect induced recirculation flow44 (see movie S6 in the supporting information of the drop drying of ink 6 using the print head fiducial camera). The discussed structural drop on substrate analysis can be linked very nicely to the electrical TFT performance. The very low drain currents measured for TFTs fabricated with inks 7 and 8 can be explained by the very low overall thin-film thickness of < 2 nm in the channel area (figure 7 b). With such a low film thickness there should be no homogeneous metal-oxide channel formation between source and drain electrode, and consequently impeding charge carrier transport. The final drop thickness of ink 6 however, results in superior TFT characteristic as discussed with figure 6. Summarized in table 4 are all parameters for the individual inks which define their printability (Z, vmax), structural (d, CRR, Sq) thin-film and electronic properties (µsat, Vth). It is however not possible to correlate these parameters, as different solvent compositions might not influence the printability, while dramatically changing the inks rheological stability and the final structural/electronic properties of the thin film. ACS Paragon Plus Environment


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Table 4. Summary of the inks fluid properties, printability, final film quality and electrical parameters Ink 6

Ink 7

Ink 8

Inverse Ohnesorge number, Z

3.9

6.5

9.4

Maximum drop velocity, vmax, m/s

8.25

6.1

3.7

Drop diameter, d, µm

70

250

150

Coffee-ring ratio, CRR

1.4

2.6

35

Root-mean square roughness, Sq, nm

0.25

0.24

0.20

Saturation mobility, µsat, cm²/Vs

9.00

-

0.05

4. CONCLUSION The effect of solvent and co-solvent composition on the rheological stability of a commercial available indium-based oxo alkoxide precursor material (iXsenic® S) is studied. A superior stability of the ink formulations based on the concentrate with tetrahydrofurfuryl alcohol (THFA*) diluted in a 2:1 volumetric ratio with tri(ethylene glycol) monoethyl ether (ink 6), propylene carbonate (ink 7) and acetonitrile (ink 8) during a constant viscosity shear rate stress at 500 Hz for 12 hours is shown. We attribute the high ink stability to the stronger bonding of THFA ligands to the oxo alkoxide complex and the resulting reduced ligand exchange kinetic with atmosphere water molecules. Furthermore, it is shown, that for fine-tuning the inks drop-formation and wettability it is necessary to choose co-solvents with high polar Hansen solubility parameter (HSP) values to maintain the superior rheological stability of the THFA* precursor concentrate. The inverse Ohnesorge number (Z) for the respective inks is calculated by measuring surface tension, density and dynamic viscosity. Z values are found at 3.9 for ink 6, 6.5 for ink 7 and 9.4 for ink 8 respectively. All inks are lying within the printability range predicted by different authors but showing lower drop formation performance with increasing Z values. The widest printable window with satellite to head drop reunion and maximum drop velocities of 8.25 m/s is found for ink 6. With direct inkjet printing on UV-ozone treated SiO2 surfaces, this formulation showed the smallest drop diameter of 70 µm and the highest film thickness (4 - 6 nm) with a low coffee ACS Paragon Plus Environment

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ring ratio of 1.4. High average saturation mobility of 9.9 ± 0.8 cm2/Vs was found by printing on 30 nm thick ITO channel electrodes in a bottom-gate bottom-contact coplanar TFT device architecture. Almost no hysteresis and an onset voltage close to zero is observed, showing the great applicability of soluble metal-oxide inks in future display backplane fabrication processes and other printed electronic applications.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org Movie S1 showing the drop formation of ink 6 for 30 V (AVI) and Movie S2 for 27 V (AVI) Movie S3 showing the drop formation of ink 7 for 26 V (AVI) and Movie S4 for 22 V (AVI) Movie S5 showing the drop formation of ink 8 for 15 V (AVI) and Movie S6 for 14 V (AVI) All movies are recorded using the Dimatix DMP-2831 drop watcher stroboscope LED delay camera system. Movie S7 showing the single drop drying behavior of ink 6 (AVI) on the SiO2 surface. Figure S1 Images of the print-head positions and formula for the estimation of the single ink drop drying time. Figure S2 Typical transfer curve of a TFT devices fabricated with ink 6 before and after 100 electrical measurement cycles under inert conditions. Figure S3 (a) Microscope image of channel length and drop diameter with (b) the corresponding electrical TFT transfer curve measurement which yields a linear and saturation charge carrier mobility of 9,7 cm²/Vs and 10,6 cm²/Vs respectively. ACS Paragon Plus Environment


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Figure S4. Topography for different surfaces measured with the atomic force microscope, (a) SiO2 Substrate, (b) ink 6, (c) ink 7, (d) ink 8 after conversion AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. REFERENCES (1) Kamiya, T.; Hosono, H. Material Characteristics and Applications of Transparent Amorphous Oxide Semiconductors. NPG Asia Mater. 2010, 2 (1), 15-22. (2) Myny, K.; Smout, S.; Rockelé, M.; Bhoolokam, A.; Ke, T. H.; Steudel, S.; Cobb, B.; Gulati, A.; Rodriguez, F. G.; Obata, K.; Marinkovic, M.; Pham, D. V.; Hoppe, A.; Gelinck, G. H.; Genoe, J.; Dehaene, W.; Heremans, P. A Thin-Film Microprocessor with Inkjet PrintProgrammable Memory. Sci. Rep. 2014, 4, 7398.


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Table of content graphic (TOC)


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Soluble Metal Oxo Alkoxide Inks with Advanced Rheological

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