Substrate-Independent Surface Energy Tuning via Siloxane Treatment

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Substrate-Independent Surface Energy Tuning via Siloxane Treatment for Printed Electronics Stefan Schlisske, Martin Held, Tobias Rödlmeier, Silvia Menghi, Kathleen Fuchs, Marta Ruscello, Anthony John Morfa, Uli Lemmer, and Gerardo Hernandez-Sosa Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00304 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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Substrate-Independent Surface Energy Tuning via Siloxane Treatment for Printed Electronics Stefan Schlisske , Martin Held , Tobias Rödlmeier , Silvia Menghi , Kathleen Fuchs , Marta 1,2

1,2

1,2

1,2

1,2

Ruscello , Anthony J. Morfa , Uli Lemmer and Gerardo Hernandez-Sosa * 2

1,2

1,3

1,2

1

Light Technology Institute, Karlsruhe Institute of Technology, Engesserstrasse 13, 76131 Karlsruhe, Germany 2

InnovationLab GmbH, Speyerer Strasse 4, 69115 Heidelberg, Germany

3

Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

ACS Paragon Plus Environment

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Abstract:

Digital printing enables solution processing of functional materials and opens a new route to fabricate low-cost electronic devices. One crucial parameter that affects the wettability of inks for all printing techniques is the surface free energy (SFE) of the substrate. Siloxanes, with their huge variety of side chains and their ability to form self-assembled monolayers (SAMs), offer exhaustive control of the substrates SFE from hydrophilic to hydrophobic. Thus, siloxane treatment is a suitable approach to adjust the substrate conditions to the desired ink instead of optimizing the ink to an arbitrary substrate. In this work, the influence of different fluorinated and non-fluorinated siloxanes on the SFE of different substrates, such as polymers, glasses and metals are examined. By mixing several siloxanes we demonstrate a fine tuning of the surface energy. The polar and dispersive components of the SFE are determined by the Owens-WendtRabel-Kaelble (OWRK)-method. Furthermore, the impact of the siloxanes and therefore the SFE on the pinning of droplets and wet films is assessed via dynamic contact angle measurements. SFE-optimized substrates enable tailoring the resolution of inkjet printed silver structures. A nano-particulate silver ink was used for printing single drops, lines and source-drain electrodes for transistors. These were examined in terms of diameter, edge quality and functionality. We show that by adjusting the SFE of an arbitrary substrate, the printed resolution is substantially increased by minimizing the printed drop size by up to 70 %.

Keywords: inkjet printing; silanization; self-assembled monolayer; surface modification; surface energy control; contact angle measurement; wettability; organic field-effect transistor


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Introduction Microelectronic devices are generally fabricated via various and elaborate photolithographic or evaporation steps, which make the processes time and material intensive. As an alternative 1–3

processing route, especially for organic electronic devices, printing technologies have recently become a burgeoning research field aiming for cost efficient and large volume fabrication of electronics. A critical part of solution-based printing processes, is the control of the spreading 4–6

and flow of inks on the substrates after deposition. Particularly, controlling the wetting behavior 7–9

of the inks on a given substrate would be a desirable way to directly influence the feature size of printed structures.

1,10,11

A general rule is that small contact angles lead to bigger features sizes

compared to large contact angles. This defines for instance, in an inkjet printing process, the final drop size footprint and thus the actual printing resolution.

1,6,12

The wetting of liquids on substrates depends on the surface free energy (SFE) of the substrate and the surface tension (ST) of the liquid.

13,14

To achieve small feature sizes the SFE has to be

adjusted, which is usually done by plasma treatment treatment.

17–19

15,16

or by a self-assembled monolayer (SAM)

In the context of SAMs (organo-)siloxanes are widely used to form robust,

chemically bonded SAMs on a huge variety of substrates e.g. glass, Si-Wafer and plastic foils.

11,20

These (organo-)siloxanes are mostly used as coupling agents or nonstick-coatings for all kind of technical applications. Thus far, in the field of organic electronics, siloxane treatments have been widely used as adhesion layers for photolithography or molds and to locally modify the SFE to 20

produce dewetting structures for self-aligning structures.

7

In this work, we demonstrate the suitability of (organo-)siloxane layers as a method to decouple the printing process from the substrate. We deposited (organo-)siloxanes with the general chemical structure (C H O) -Si-R on a variety of substrates. The customizable sidechain n

3n-1

3


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(-R) has a direct influence on the surface conditions and offers the possibility of changing the SFE of the substrate significantly. This gives the possibility of adjusting the SFE of different 11

substrates to one defined value and, therefore, to match the SFE of different substrates to each other. Additionally, we show that mixing (organo-)siloxanes with highly divergent surface energies can be used to produce substrate surfaces with freely adjustable SFEs. The SAM formation is evaluated via static and dynamic contact angle (CA) measurements. This CA data is then used to determine the SFE of the substrates via the method of Owens-Wendt-Rabel-Kaelble (OWRK). Furthermore, the influence of the SFE on the printability of a commercial nanoparticle based silver (Ag) ink is investigated. For that purpose, single drops and lines are investigated in terms of diameter and line formation. Finally, inkjet printed electrode patterns are utilized to fabricate organic field-effect transistors (OFETs), with a focus on the printing quality, reduction of device footprint and the effect of the (organo-)siloxane layer on the electrical performance.

Experimental: The SAM deposition was primarily investigated on 25x25 mm borofloat33 glass slides 2

(Schott). All fabrication steps are performed at room temperature in ambient atmosphere under clean room conditions, unless mentioned otherwise. The glass substrates were cleaned for 10 min in an ultrasonic bath with first acetone and then isopropyl alcohol (IPA), and dried with nitrogen after each cleaning step. Prior to film preparation the glass substrates were exposed to oxygen plasma for 5 min (Tetra 30, Diener electronics GmbH + Co. KG). Siloxane SAM film formation was

examined

with

3-Aminopropyltriethoxysilane

(APTES)

(Acros

Organics

99%),

Triethoxyphenylsilane (TEPS) (Sigma-Aldrich >98 %), Pentafluorophenyltriethoxysilane (PFPTS) (Sigma-Aldrich 97 %), Trimethoxy-(3,3,3 trifluoropropyl)-silane (TTFPS) (AlfaAeser


4

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97

%),

Triethoxyoctylsilane

(TEOS)

(Sigma-Aldrich

98

%)

and

1H,1H,2H,2H-

Perfluorodecyltriethoxysilane (PFDTS) (AlfaAeser 97 %) (Fig. 1a). 250 µl of a 5 % siloxane vol.

solution in ethanol were spin cast on the glass substrates for 60 s at 1500 rpm with an initial acceleration of 500 rpm/s. The substrates were subsequently annealed on a hotplate for 10 min at 95 °C, rinsed with acetone and dried with nitrogen. These conditions resulted in maximum contact angles and SFE comparable to values from literature. For the binary siloxane mixtures, the respective siloxanes were mixed in different volume ratios at a constant total siloxane ratio of 0.1 %

vol.

The film deposition was performed as described above. The SAM formation was

subsequently examined on Soda-Lime-glass (VWR), n-doped silicon wafers (Active Business Company GmbH), copper-foil (SigmaAldrich), indium-tin oxide (ITO) covered glass slides (Kintec) and polyethylene terephthalate (PET) (Pütz GmbH+Co. Folien KG) foils. Samples of these substrates were prepared as described above. The SFEs were determined via CA measurements with a KRÜSS DSA 100 drop shape analyzing system. DI-Water, diiodomethane and ethylene glycol droplets with a nominal volume of 0.75 µl were placed on the substrate. The contours of the drops and their CAs were extracted with the help of the KRÜSS software. The SFE calculations, both the dispersive and polar parts of the SFE, were carried out following the OWRK-method.

18,19

In order to evaluate the pinning

behavior of the inks, dynamic contact angles were determined. Therefore a 0.75 µl drop of each test liquid was placed on the substrate and a cycle from 0.25 to 2.75 µl was performed. The advancing and receding CAs were extracted in a volume interval where the shape of the droplets remained constant. Atomic force microscopy (AFM) images were recorded with a DME DS 95 Dualscope AFM in ambient conditions in tapping mode. A highly doped Si-cantilever (NanoWorld Arrow NCR) with a resonance frequency of around 285 kHz and a tip radius of


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below 10 nm was used to determine the surface morphology of the glass as well as siloxane films. For evaluation of the influence of the SFE on the printed pattern size a Dimatix DMP 2831 was used with a 10 pl cartridge and a nanoparticle based Ag ink (Sigma-Aldrich Silverjet DGP 40LT 15C) with a surface tension of γ = 35.5 mN/m. At ideal jetting conditions, patterns 21

tot

consisting of single dots (droplets) and lines of several mm length with a drop spacing of 25 µm were printed. The maximum jetting frequency was set to 5 kHz, the print head temperature was set to 35 °C, and the substrate temperature was kept at 28 °C. These structures were examined with a light microscope and a Sensofar PLu neox optical 3D profiler. The resistivity of the printed silver electrodes was measured with a linear 4-point probe. Bottom-contact top-gate OFETs were fabricated utilizing the printed silver lines. Source-drain electrodes of 100 nm thickness were either ink-jet printed with Silverjet ink or thermally evaporated with silver through a shadow-mask, either on untreated glass or on PFPTS treated glass. The printed electrodes were annealed for 10 min at 115 °C. The channel geometry of the evaporated electrodes (channel length L = 10, 20, 50 µm, channel width W ≈ 1100 µm) is comparable to the printed electrodes (L = 40-80 µm, W ≈ 1100 µm). Errors in the W/L ratio created by local bulging of the printed electrodes are equal to the standard deviation of W and L, hence do not have a significant impact on the transistor parameters as discussed in Fig. S5. The electrode finger width was 40-50 µm for the evaporated electrodes (independent on the presence of PFPTS), 30-40 µm for printed electrode fingers on PFPTS and 60-90 µm on glass. A 60 nm Poly(3-hexylthiophene-2,5-diyl) (P3HT) (Sigma-Aldrich M 54,000 75,000) semiconducting n

layer was spin coated from an 8 g/l chlorobenzene solution and annealed at 100 °C for 5 min in inert atmosphere. 650 nm of poly(chloro-p-xylene) Parylene C (Special Coating Systems™)


6

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dielectric were deposited onto the substrates via chemical vapor deposition (CVD) from a PDS2010 coating system by SCS™. The gate electrode was either printed in ambient atmosphere or thermally evaporated in inert atmosphere, in correspondence to the source-drain electrodes. The transistors were characterized electrically with an Agilent 4156C Semiconductor Parameter Analyzer in ambient atmosphere. For the output characteristics the drain voltage V was swept d

from 0 V to -100 V at constant gate voltage V from 0 V to -80 V in steps of 10 V. The transfer g

characteristics were measured at a constant V of 60 V and V was swept from 20 V to -100 V in d

g

steps of 1 V. Saturation mobility and threshold voltage were directly extracted from the transfer characteristics. The contact resistance R and channel resistance R were determined by the C

ch

transmission line method from the output characteristics. The x-intercept of the total resistance R ∙ W versus channel length equals the transfer length. The reciprocal derivative of the total tot

resistance with respect to the channel length ∂L/∂(R ∙ W) served to extract the contacttot

resistance-corrected linear mobility and threshold voltage. Even though the resistivity of the printed silver electrodes (ρ = 1.96∙ 10 Ω∙ cm) exceeds the evaporated silver (ρ = 1.59∙ 10 -5

-6

Ω∙ cm) by an order of magnitude, their contribution to the overall resistance of a typical transistor at V = 20 V (L = 50 µm, W = 1100 µm, R ∙ W = 1∙ 10 Ω∙ cm, R ∙ W/L = 1.5∙ 10 Ω, 6

g

C

8

ch

electrode length 1 cm, electrode cross section area 40 µm ∙ 100 nm) is negligible as R

electrode

= 490

Ω, while R = 9∙ 106 Ω and R = 6.8∙ 106 Ω. C

ch

Results and discussion: The spincast siloxanes (Fig. 1a), deposited from ethanol solution were investigated by static contact angle measurements (Fig. 1b) using DI-water, ethylene glycol and diiodomethane as test fluids. The CA values, presented in Table 1, are comparable to reported literature, demonstrating


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that our procedure leads to a proper monolayer formation. Clearly, the contact angles of all test 11

liquids change significantly with different siloxane treatments. The bare glass and most SAMs exhibit CAs below 90°, which means that the test liquids wet the substrate. This behavior is related to a rather high surface energy of each substrate. The high contact angles of the e.g. PFDTS SAM indicate non-wetting behavior and thus a low surface energy. The chemical structure of the siloxane side chains strongly determine the contact angles. Polar side chains, 11

such as aminoalkyl groups tend to have small water CAs, while fluorinated side chains produce large water CAs that show dewetting behavior.

10,11

Table 1. Static contact angles (CA) of water, diiodomethane and ethylene glycol on different siloxane films on borofloat glass. CA Water [°]

CA Diiodomethane [°]

CA Ethylene glycol [°]

Glass

63.4 ± 2.1

46.3 ± 2.0

45.3 ± 1.6

APTES

60.3 ± 2.1

38.3 ± 2.2

41.6 ± 2.2

TEPS

63.1 ± 1.6

47.0 ± 0.5

44.9 ± 0.4

PFPTS

72.7 ± 1.4

61.1 ± 1.2

56.7 ± 1.4

TTFPS

87.2 ± 4.3

72.1 ± 0.5

71.5 ± 1.5

TEOS

84.3 ± 3.4

74.5 ± 2.7

74.1 ± 2.3

PFDTS

104.2 ± 4.0

93.3 ± 1.8

90.2 ± 4.3

The influence of the siloxane treatments on the wetting behavior becomes clearer by using the obtained contact angle data to calculate the SFE. Thus, Young’s equation (Eq. 1), connecting the contact angle with the SFE, was applied, with q as the measured contact angle of the deposited


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test liquid and γ the surface energy of the solid-gas (SG), liquid-solid (LS) and liquid-gas (LG) interface.

22

(1) As the interfacial energy of the liquid-solid interface is not directly accessible, models can be applied which connect Young’s equation with an expression for the interfacial energy of the solid-gas and liquid-gas, and more accessible parameters. The interactions between the two phases can be separated into different contributions like hydrogen-bonds or polar- and dispersive interactions. We applied the OWRK-method, which splits the surface free energy of a solid into a dispersive and a polar part.

23,24

With this method the interfacial energy can be expressed as a

composition of dispersive (D) and polar part (P) of the individual energies. (2) This equation 2 can be connected with the Young’s equation (1) and rearranged into a linear equation: (3)

In the above-mentioned equation (3), the dependent variable (

the independent variable (

), and

), contain only literature values of the test liquids and the

measured contact angle. Via a linear fit of at least three different test liquids the slope and the yintercept can be extracted, i.e. the dispersive and polar part of the SFE of the substrate (Table 2).


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Table 2. Surface energies (SFE) with polar and dispersive component, as calculated via the OWRK-method. SFE [mN/m] Polar [mN/m] Dispersive [mN/m] Glass

43.3 ±0.6

12.0 ± 0.2

31.3 ± 0.3

APTES 46.6 ± 0.7

12.4 ± 0.3

34.2 ± 0.4

TEPS

39.3 ± 0.2

5.9 ± 0.1

33.4 ± 0.1

PFPTS

34.6 ± 0.3

10.1 ± 0.1

24.5 ± 0.2

TTFPS

24.8 ± 0.2

5.3 ± 0.1

19.5 ± 0.1

TEOS

23.5 ± 0.7

6.4 ± 0.3

17.1 ± 0.4

PFDTS 13.4 ± 0.4

2.7 ± 0.2

10.6 ± 0.2

By means of the siloxane treatment, the total SFE of the glass can be tuned from 47 mN/m (APTES) to 13 mN/m (PFDTS). Comparing the SFE and the measured contact angles reveals a drastic increase to the water's contact angle for siloxanes with a nonpolar surface. Due to the different contributions of the polar and dispersive part of the SFE, the wetting behavior of liquids can be predicted. For a better overview and visualization of wetting, so called wetting envelopes 13

(WE) can be calculated. Therefore Eq. 3, obtained via the OWRK-method, is rearranged and γ

P LG

is plotted against γ for a given SFE of a siloxane treated glass at a given CA θ, in this case θ = D LG

0°, so for complete wetting. The obtained graph encloses an area which is then typically referred to as the wetting envelope or wetting range (Fig. 1c). Roughly speaking, liquids or ink formulations with surface tensions inside the area tend to wet the respective surface, whereas liquids and inks outside the area do not. The silver ink is located inside the WE of glass, APTES and TEPS, but outside of the WE of PFPTS, TTFPS, TEOS and PFDTS. Measuring the CA and modelling the wetting envelope serve as useful tools to predict the suitability of a SAM to tune


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the substrate surface energy before printing with a particular ink. Therefore, we added the surface tensions of some typical solvents used for printing applications in Fig. 1c. The reaction between the methoxy- or ethoxy-siloxane groups with OH-groups from the substrate offers the possibility of also transferring the silanization reaction to other substrates and to adjust the SFE of different substrates to one defined value. We applied the above-mentioned reaction mechanism on additional typical substrates used in printed organic electronics such as ITO, soda-lime glass, copper, silicon and PET. Due their different surface chemistries, the untreated substrates show different SFE. For each the substrates, the same siloxane treatment changes the SFE significantly. An overview of the leveling of the SFE of the substrates is displayed in Fig. 1d. The different SFE values for the same SAM on different substrates results mainly from different individual polar parts of the SFE (see Fig. S1 of the supporting information) attributed to different humidity in the cleanroom or on the substrate.

25,26

For example,

the varied results from the APTES treatment are due to a possible self-hydrolysis and the adsorption of the amine-group on the surface, instead of the ethoxy group, which results then in a different surface energy.

27


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Figure 1. a) Chemical structures of the used siloxanes. b) Representative photographs of the water contact angle of bare and SAM-borofloat glass. c) Wetting envelopes of the bare borofloat glass and siloxane treated substrates, along with fluid physical properties of the nano particulate silver ink and some typical solvents for inkjet applications (DMSO = dimethyl sulfoxide, EGME = ethylene glycol monoethyl ether). d) Leveling of the total SFE of different substrates.

Tuning printing features by SFE Printing Ag ink dots and lines on bare and SAM-treated substrates affirms the accuracy of the OWRK-model and validates the suitability of the SAMs to modify printing features. Single printed silver dots reveal a significant decrease in diameter from ~70 µm (APTES) to 22 µm


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(PFDTS) (Fig. 2.a-g). Three-dimensional images of the dots additionally show the increasing height of the printed dots with decreasing diameter due to the constant volume of the ejected droplets of the inkjet printer (Fig. 2.1-7). The increasing height with decreasing footprint of the droplets leads to steeper edges. As the drying conditions were kept constant for the different siloxane treatments, the influence of the coffee stain should be the same as well. However, as the edge gets steeper we approach the lateral resolution limit of the white-light interferometer therefore the dot edges might not be perfectly resolved. The reduction of the diameter of the printed silver dots correlates to the decreasing linewidth of the printed silver lines, where a linewidth of 132 ± 18 µm on the non-treated glass substrate is reduced to a minimum of 51.3 ± 20 µm by PFPTS treatment (Fig. 2.I-VII). The printed feature size, i.e. the diameter of the printed dots and lines, clearly decreases with decreasing SFE (Fig. 2.a-g; I-VII) and exhibits direct proportional dependence on the total SFE, which can be seen in Fig. 2h. This relation also holds true for any other substrate on which a direct proportional correlation between SFE and drop diameter can be seen (see supporting information Fig. S3). The error bars of the linewidth represent the edge quality or waviness of the patterns. The underlying effect in this relationship is wetting and dewetting behavior: As expected from the WE, Silverjet wets and forms a closed line on bare glass, APTES and TEPS films as the SFEs are 46.6 resp. 39.3 mN/m and the ST of the ink is 35.5 mN/m. Due to their low SFE (

Substrate-Independent Surface Energy Tuning via Siloxane Treatment

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