Thin Film Slot Coating in a Rarefied Low Viscosity Gas

www.bradford.ac.uk Thin Film Slot Coating in a Rarefied Low Viscosity Gas H. Benkreira & J.B. Ikin* School of Engineering, Design & Technology University of Bradford-UK *Multicoat Ltd., 21 Turnfield Road, Cheadle, Cheshire, SK8 1JQ, England

Requirements for a Typical OPV (Organic Photo-Voltaic) layers typ. 100 nm to 500 nm thick coat in an oxygen-free atmosphere to ensure extended cell life Background figure courtesy Bjorkland,G.C. and Baer,T.M. Organic Thin-Film Solar Cell Research Conducted at Stanford University Photonics Spectra Nov. 2007, pp 70-76.

Typical Limitations using Slot-over-Roll Coating Lee,K-Y; Liu L-D & Liu,T-J [1992] Effect of Coating Gap H on Minimum Wet Thickness for Conventional Extrusion Slot Coating 50mPa.s Silicone Oil - 0.25 mm Slot Gap X denotes maximum coating speed 500 wet thickness (µm) 400 300 200 100 0 0 10 20 30 40 coating speed (cm/s) e.g. Q = 1 cc/s H = 1.0 mm H = 0.5 mm H = 0.3 mm H = 0.2 mm Q = 3 cc/s Q = 2 cc/s Q = 1 cc/s Q = 0.5 cc/s Q = 0.3 cc/s Q = 0.15 cc/s

Old Methods to reduce film thickness & increase speed in slot coating 1. Bead Vacuum: reduces minimum film thickness by more than 30% when it is sufficiently high. See Lee et al. Chem. Eng. Science, 47 (7) [1992]. Mechanism: coating bead is pulled back & downstream lip serves as a doctor blade & restricts net flow. 2. Tensioned-web slot coating: Dispense with backing roller and allow the gap to be reduced further without danger of clash between slot and steel roller. Films as thin as 0.5 microns can be produced but only at very low speeds, typically less than 10 m/min.

REPLACING AIR WITH HELIUM AND REDUCING THE PRESSURE SIGNIFICANTLY ENHANCES THE MAXIMUM COATING SPEED FOR DIP COATING entrainment speed (cm/s) Entrainment Speed vs Pressure - Clear Base Air 60 CO2 He Air 40 CO2 He 20 0 10 100 1000 pressure (mbar) tank containing silicone oil Why should helium be more effective than air or CO 2 when reducing the pressure?

1. The higher the viscosity of the gas, the greater the coupling force between the gas and the liquid, the more the gas will be dragged down by the web 2. Low viscosity gas exerts less coupling force on fluid and hence reduces size of vees and delays onset of gas entrainment relative viscosity 1.0 0.8 0.6 0.4 0.2 Relative bulk viscosity vs pressure for a typical gas (Starling & Woodhall [1961]) Viscosity measured between surfaces several mm apart fails to explain results 0.0 0.0 0.5 1.0 1.5 2.0 pressure (mbar) Gas Bulk viscosity at 20 C and 1 atmos. air 181 µp carbon dioxide 149 µp helium 199 µp almost constant down to typ. 0.6 mbar η = 2 3σ 2 mkt 3 π hence η ( P)

Mean Free Path vs Pressure Viscosity of a gas remains almost constant as pressure is reduced until mean free path approaches linear dimensions of measuring apparatus mfp (µm) 1000 100 10 1 0.1 Helium Air Carbon dioxide 0.01 1 10 100 1000 pressure (mbar) But Mues, Hens and Boiy [1989] found that air entrained at coating constrained to films typ. < 5µm thick So we need a measuring apparatus where solid boundaries are typ. 2µm apart

SOLUTION Andrews and Harris [1995] One plate set in oscillation in direction normal to other spring loaded plate measure relative motion and hence damping 1.0 relative viscosity 0.8 0.6 0.4 0.2 master curve for dry air, argon, N 2,A,CO 2,O 2 and H 2 0.0 0.1 1 10 100 d/λ Effective Viscosity vs Pressure d = gap λ = mean free path Plausible explanation for why helium performs well for dip coating So will it also benefit slot coating? viscosity (µp) 150 100 50 0 10 100 1000 pressure (mbar) Air Carbon Dioxide Helium

SLOT-OVER-ROLL MODE Challenging expts requiring developing miniature parts, controlled ramping-up and down web speeds+ tricky imaging optics/cameras/ SYNCRO-RECORDING optical tacho controls tilt angle controls gap gravity feed speed ramped up at constant acceleration to save base + oil Coating Fluid: Silicone oil, μ=50mpa.s Film thickness measured using a capacitive sensor supplied by Physik Instrumente GmbH

TENSIONED WEB SLOT COATING MODE Head withdrawal facility to avoid excess fluid draining down web at start-up Slot / bead visualised using INFINITY zoom + std. objective set up to view through the web Oil fed using micro-pump to achieve lower flow rates / film thicknesses problem with cavitation bubbles

PARAMETERS Slot exit = 0.425 mm x 4.5 mm Web width = 5 cm µ = 50mPa.s σ = 17.9 mn/m ρ = 985 kg/m 3 SLOT-OVER-ROLL TWSC MODE Web spans: 7.5 cm upstream 10.2 cm downstream Web deflection = 2mm Upstream land length = 0.2 mm Downstream land length = 1.5 mm Coating gap = 0.20 mm Upstream land length = 1.5 mm Downstream land length = 0.2 mm Web tension = 4.3 N

Four cameras used to display and record web speed, movement of coater from withdrawn to coat position, onset of entrainment/ribbing and flow rate web coating speed side view of slot coater micromanometer displays flow rate air/gas entrained at dwl

TYPICAL RECORDINGS - SLOT-OVER-ROLL coating speed at onset of air entrainment speed (cm/s) 20 15 10 5 Coating Speed Profile optical tacho O/P 0 0 2 4 6 8 10 12 14 16 18 20 time (s) Coating Thickness vs Time 400 350 wet thickness at onset of air entrainment thickness (µm) 300 250 200 150 100 50 0 0 2 4 6 8 10 12 14 16 18 20 time (s) capacitive sensor O/P

RESULTS - SLOT-OVER-ROLL MODE Results for air at atmos. pressure match those of Lee et al. [1992] quite well Minimum Wet Thickness for Slot-over-Roll Coating Mode minimum wet thickess (microns) 200 150 100 50 0 0 10 20 30 40 coating speed (cm/s) Lee et al [1992] air at 1 bar He at 25mbar Replacing air with He at low pressure reduces wet thickness by typically 17%

RESULTS - TWSC MODE Minimum Wet Thickness for TWSC Mode 20 limited by air entrainment minimum wet thickness (microns) 15 10 5 0 0 5 10 15 20 25 coating speed (cm/s) air 1 bar helium 45 mbar limited by ribbing Replacing air with He at low pressure reduces Hmin for given U by typically 50% or can increase Umax by >>2x for given H

CONCLUSIONS Replacing air at atmospheric pressure with helium at low pressure significantly reduces the low wet thickness limit or alternatively increases the coating speed for a given wet thickness for both slot-over-roll and TWSC configurations Gas should be selected through measurement of viscosity using apparatus replicating the very thin films actually entrained at coating Coating in an oxygen free environment is advantageous to prolonging endproduct life Further work is intended focused on exploring thinner coatings of the order of 0.5µm as required for the production of e.g. solar cells etc.

ACKNOWLEDGEMENTS We gratefully acknowledge the help of Physik Instrumente GmbH who kindly made available their capacitance probe system to measure wet film thickness without which these experiments would not have been possible