Screen Printing of Highly Loaded Silver Inks on. Plastic Substrates Using Silicon Stencils

Transcription

1 Supporting Information Screen Printing of Highly Loaded Silver Inks on Plastic Substrates Using Silicon Stencils Woo Jin Hyun, Sooman Lim, Bok Yeop Ahn, Jennifer A. Lewis, C. Daniel Frisbie*, and Lorraine F. Francis*, Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave. SE, Minneapolis, MN 55455, United States School of Engineering and Applied Science, Wyss Institute for Biologically Inspired Engineering at Harvard University, 52 Oxford St. Cambridge, MA 02138, United States *Corresponding Author S1

2 EXPERIMENTAL DETAILS Silver ink preparation Silver particles are produced by reducing aqueous silver nitrate solution, followed by concentration. In a typical procedure, 10 g of poly(acrylic acid) (PAA) (molecular weight = 5,000 g/mol, 50 wt% aqueous solution), 5 g PAA (molecular weight = 50,000 g/mol, 25 wt% aqueous solution), 200 g diethanolamine (DEA), and 250 g water are mixed by stirring for 2 h and room temperature. While stirring vigorously, a silver nitrate solution (100 g AgNO 3 in 100 ml H 2 O) is then added into this solution. When stirred for 22 h at room temperature, the solution gradually becomes dark black and generates silver nanoparticles (~5 nm in diameter). These nanoparticles are then ripened on a hot-plate at a solution temperature of 75 C for 2.5 h. The resulting silver particles are then collected by adding ethanol (1,300 ml), followed by centrifuging at 9000 rpm for 20 min. Because ethanol is a poor solvent for the PAA-coated silver particles, rapid coagulation occurs. After decanting the supernatant, the precipitate (solids loading ~ 90 wt%) is collected. Finally, the highly loaded silver ink is prepared for by homogenizing the precipitate after adding 7.5g of humectant solution (30 wt% ethylene glycol/70 wt% water). The ink solids loading (wt% as metallic silver) is determined by weighing the silver paste ink before and after annealing at 500 C for 1 h. After homogenization, a highly concentrated silver ink with solids loading of 77 wt% is obtained. Characterization of silver ink Transmission electron microscopy (TEM, JEOL 2010Lab6) and scanning electron microscopy (SEM, Ultra55, Zeiss) are used to measure the size and size distribution of the silver nanoparticles at various stages in the synthesis. The ink rheology is measured by both shear viscometry and an oscillatory technique using a controlled-stress rheometer (TA Instrument S2

3 LTD, 2000EX) equipped with cone and plate geometry at 25 C in the presence of a solvent trap to prevent evaporation. The apparent viscosity (η) is measured by varying shear rate ( s -1 ) in a logarithmically ascending series at room temperature. The elastic shear (G ) and viscous loss (G ) moduli are acquired in oscillatory mode of varying shear stress between 1 and 2000 Pa at a frequency of 1 Hz with increasing amplitude sweep. The electrical resistivity of the silver ink is measured as a function of annealing temperature and time by a four-point probe (Jandel, RM3000) using thin film samples. Silicon stencil fabrication A 4-inch silicon wafer with a thickness of 525 (±25) µm is immersed in a potassium hydroxide (KOH) bath (30 wt% KOH in deionized water) at 90 C for 105 min, rinsed with deionized water, and dried. A 90 (±5) µm thick silicon wafer is then obtained. After prebaking at 200 C for 5 min, as shown in Figure S1a, the thinned wafer is vapor-coated with hexamethyldisilazane for 3 min and spin-coated with photoresist (AZ9260) at 300 rpm for 10 sec and at 2500 rpm for 60 sec, sequentially. The photoresist is soft-baked at 110 C for 165 sec and exposed to UV light through the photomask with patterns for openings by using a mask aligner (MA6, Karl Suss). The wafer is put into a developer solution AZ400K diluted with deionized water (1:4 v/v) for 30 min, and rinsed with deionized water and dried. After etching the wafer by reactive ion etcher (SLR-770, Plasma-Therm) for 50 min to make openings, the photoresist is rinsed with acetone, ethanol, isopropanol, and deionized water. Spacer preparation The spacers are made employing poly(dimethylsiloxane) (PDMS) at a thickness of 2 mm by mixing PDMS pre-polymer with its curing agent (10:1 w/w, Sylgard 184, Dow Corning) and curing it in an oven at 70 C for 2 h. S3

4 Characterization of printed lines The printed silver is observed using an optical microscope (KH-7700, HIROX) and a scanning electron microscope (JSL-6500, JEOL). Surface profiles are obtained using a surface profiler (P- 7, KLA-Tencor). The electrical properties are characterized using a two-probe measurement technique with source measurement units (236 and 237, Keithley). Figure S1. Cross-sectional illustration of the screen printing with the silicon stencil and the silver ink. Before printing, the silicon stencil and the substrate are kept apart by the spacers. During printing, the squeegee pushes the ink and presses the stencil simultaneously, so that the stencil bends, providing contact between the stencil and the substrate. After printing, the stencil separates from the substrate, leaving the ink on the substrate. Figure S2. Photographs of (a) a silicon stencil showing photolithographically defined openings (in the black box) and (b) a polyimide film with printed silver patterns (in the black box). S4

5 Figure S3. (a) Schematic illustration of ink penetration into the gap of the silicon stencil and the substrate. (b) Length (α) of the penetrated silver ink for different w s. Figure S4. Optical microscopy (OM) images of (a) a stencil opening and (b) its corresponding printed silver pattern for a curved line. S5

6 Figure S5. Thickness of the printed silver lines after annealing at 250 C for 5 min. Figure S6. Relative resistance of screen-printed silver lines on flexible substrates for different bending radii of (a) 10, (b) 5, (c) 4, and (d) 2 mm, during 1000 bending cycles. ε is the bending strain. S6

7 Figure S7. Scanning electron microscopy (SEM) images of cracks on the printed silver lines prepared from the line openings with ws of (a) 10 and (b) 40 µm, after 1000 bending cycles at a bending radius of 2 mm. Figure S8. Relative resistance of the silver lines (for ws = 20 µm) annealed at 120 and 150 C for 30 min, after 1000 bending cycles for various bending radii and strains. S7