Sheet Resistance Anisotropy in Transparent Conductive Films Containing Silver Nanowires

Transcription

1 p.1 Sheet Resistance Anisotropy in Transparent Conductive Films Containing Silver Nanowires Bill Devine, Aaron Clapp, Jason Payne 2016 AIMCAL Coating & Web Handling Conference Memphis, TN

2 p.2 Transparent Conductive Films (TCFs) Applications & Types Displays on mobile phones, tablets, notebooks, TVs Touch enabled wearable devices Electroluminescent displays Photo-voltaics OLED Lighting Ag Nanowires Carbon Nanotubes Graphene ITO Metal Mesh PEDOT:PSS Combinations of above Other Smart windows

3 p.3 Touch Panel Structure Glass / Film / Film (GFF) Protective Layer ( nm ) Ag Nanowires ( nm ) PET Substrate ( µm ) Hard Coat (1-5 µm ) Silver Nanowire TCF Structure Touch Panel Module (GFF) TCF Silver Nanowires Sheet resistance, R s (Ω/ ), key electrical parameter. Lower values required for larger screens. R s = f(l/d NW, ρ, ρ NW, AF NW, R w, R c ) AgNW L/D µm / nm R s decreased by increasing L/D, reducing contact resistance

4 p.4 Problem: Sheet Resistance Anisotropy Key Issues A = R TD /R MD R MD R TD Image : Polytype (modified by author) Sheet Resistance in transverse direction, R TD, usually higher than in machine direction, R MD. Anisotropy can exacerbate circuit sheet resistance differences between vertical and horizontal traces, leading to touch accuracy issues. Can reduce TCF film yield due to forced orientation of film. Goal: R TD / R MD < 1.3, ideally < 1.1

5 p.5 Approach Understand anisotropy in the film Identify Potential Root Causes of Anisotropy Nanowire Alignment Due to Flow/Shear Nanowire Morphology (L/D, bundling) Nanowire Concentration Develop Cause Maps for Each Root Cause Experiments to Develop Fundamental Understanding and Verify Cause Map Elements Cause Map Disciplined approach to a likely multi-variable solution

6 p.6 Sheet Resistance Anisotropy Measurement 6 x 7 TCF sample Laser etched channels 39 x 1 mm as shown Channel ends marked with silver paste Resistance measured with multimeter 15 x 1 strips cut from TCF Strip ends marked with silver paste Resistance measured with multimeter Use of handheld 4-point probe does not yield acceptable results

7 p.7 Anisotropy Variation Cross-web (TD) vs. Down-web (MD) 100 fpm slot die coating, 53 inch coated width Sheet resistance anisotropy measured every 200 ft downweb, 2-11 inches crossweb Lower anisotropy within 4-5 inches of coated edges Anisotropy is stable in down-web (MD) direction

8 p.8 TD, MD Variability R TD R MD Lower anisotropy at edges is due to higher R MD

9 p.9 Cause: AgNW Alignment Due to Shear Subjected AgNW dispersions to rotational shear flows in rheometer, transferred to microscope slide for characterization. Reversible process with wire alignment above a critical high shear (10 sec -1 ) and relaxation due to thermal effects at low shear. Viscosity opposes relaxation. Image : Zhou et. al. (2011) MD Wire alignment creates anisotropy in wire connectivity R s anisotropy Pre-shear TD Post-shear R MD <R TD Zhou et. al., Reversible Macroscopic Alignment of Ag Nanowires, Chemistry of Materials, 2011, 23,

10 p.10 Measurement of Wire Alignment MATLAB Image Analysis Eliminate small objects (non-wires) Identify wires Measure orientation of wires relative to MD (-90 ) and TD (-180, 0 ) Analyze distribution of wire orientations Initial work shows fair correlation between spread of wire alignment distribution (std dev) and sheet resistance anisotropy

11 p.11 Simplified Cause Map Coating Speed Dryer Configurations Flow After Coating Drying Conditions Coating Formulation (Viscosity, Wet Laydown, Surfactants) Dryer Temperatures Air Flows Slot Die Die Design Slot Height Potential Root Cause #1 Coating Flows Coating Method Alternative Methods Coating Conditions (speed, gaps) Mayer Rod Coating Formulation (Viscosity Wet Laydown) Spray Solution Velocity Delivery Line Diameter Gravure Flows Upstream of Coating Point (Solution Delivery System) Coating Formulation Coating Speed KEY Part of Current Study Residence Time Recirculation Delivery Line Length Potential Cause: Nanowire alignment due to flow / shear

12 p.12 Effect of Solution Delivery Residence time was increased only via solution line length, all other variables were kept constant Slight decrease in anisotropy with increased solution delivery line length

13 p.13 Shear Rate in Slot Die Process EXAMPLE S = 102 cm/s coating speed Q = 3.6 L/min volume flow rate G= 127 micron lip-to-web gap h= 127 micron slot height R= 0.95 cm delivery line radius W=1350 mm coating width DELIVERY LINE ϒ = 4Q/πR 3 = 90 sec -1 DIE SLOT ϒ = 6Q/h 2 W = 17,000 sec -1 COATING ϒ =S/G=8000 sec -1 Schweizer, P.M., Viscosity vs. Rheology, Converting Quarterly, Quarter , pp AgNW coating solution sees different shear rates throughout the coating process Need to separate out different parts of process to understand impact

14 p.14 Example: Effect of Slot Height γ = 6Q/h 2 W where, Q = volumetric flow rate h = slot height W = slot/coating width Slight decrease in anisotropy with increasing slot height

15 p.15 Example: Effect of Coating Speed γ = S/G where, S = coating speed G = lip-to-web gap Increase in Anisotropy with Coating Speed

16 p.16 Drying During Pilot Coating SLOT DIE Oven Nozzles HOT AIR DRYER 2 10 FT LONG EACH, INDEPENDENTLY CONTROLLED F oven temperatures 53 99% oven fan speeds (air velocity) Will drying variables of temperature and air velocity impact anisotropy?

17 p.17 Pilot Coating Zone 1 Air Flow Pilot Dryer Air Distributor (slot & baffle configuration) % Fan Speed vs. Air Velocity (TD Direction at Web) BAFFLE NOZZLE COATING WEB Baffles: 10 cm long, 1.5 cm from web Nozzles: 0.6 cm from baffle, 30 cm spacing Baffles reduce air velocity (10x) and direct impingement.

18 p.18 Effects of Drying Variables Oven temperature had little impact on sheet resistance anisotropy TD air velocity ( cm/s) did have a marked impact on anisotropy

19 p.19 Impact of Coating Method Mayer Rod Spray Coating Image analysis shows a distinct difference in wire alignment depending on the coating method. More investigation is required to optimize anisotropy

20 p.20 Effect of AgNW Dispersion Making Two different methods of post-synthesis purification & dispersion were used to modify silver nanowires prior to experimentation Significant effect of silver nanowire dispersion making process on sheet resistance anisotropy. More investigation is needed.

21 p.21 Conclusions and Recommendations for Future Work Conclusions Image analysis demonstrates correlation between nanowire alignment in MD direction and sheet resistance anisotropy. Anisotropy is minimized by dryer air flow optimization. Air flows immediately after coating point may be key. Spray and Mayer Rod coatings showed significantly less nanowire alignment in MD direction compared to slot die coatings. Silver nanowire dispersion processing can significantly influence anisotropy. Future Work Further investigation of alternative coating methods that may have lower tendency to orient wires. Examine effects of formulation parameters that may influence flow after coating through viscosity, wet thickness. Further work on understanding effects of dryer designs (countercurrent, concurrent, crossweb, for example). Study effects of fundamental nanowire characteristics (diameter, length, etc.) and AgNW dispersion processing.

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