Barrier Web 2009

Holger Nörenberg, Technolox Ltd. , Oxford, UK www.technolox.com

Measurement of Water Vapour Permeation: Current State of the Art and Future Challenges

“We invent and build permeation measurement equipment”

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• Problem: How to ensure a sufficiently long lifetime of flexible electronics to make money

• Key Aspect: Failure due to ingress of water vapour

• Solution: Barrier layer to prevent ingress of water vapour

• Good Barrier? Measurement of the permeation of WV

• Problem 1: Very small amount of water vapour to be measured • Problem 2: Short Measurement time, easy operation

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Diffusion D

=

Solubility S

&

Permeation P

Current State of the Art

Fickian Diffusion: P=D·S

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Applications of Barrier Layers

• Flexible OLED displays

• Lighting• Photovoltaics• E-paper• Thin-film devices

(batteries,…)

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Measurement of 10-4 … 10-6 g/m2/day: We are getting there

• Calcium Test• Tritium Test• MOCON Aquatran• Technolox Deltaperm

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Optical Calcium Test

• Observation of the corrosion of a thin calcium layer

• Position-resolved information

• Parallel processing

www.vitexsys.com

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Electrical Calcium Test

• Monitoring conductance and 1/f noise during Ca-degradation• Sensitivity <10-6 g/m2/day

• www.imre.a-star.edu.sg

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Tritium Test

• Sensitivity 10-6 g/m2/day• www.ga.com

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MOCON Aquatran

• Detection limit: 5x10-4 g/m2/day• www.mocon.com

H2O inCarrier gas (N2)

Coulometricsensor

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1: Evacuation of upstream side and downstream to remove the ambient air

sample

Watervapour

pressuresensor

upstream side

downstream side

PumpOFF

2: Admission of water vapour tothe upstream side.

sample

Watervapour

upstream side

downstream side

PumpON

valve

t

p

sample

Watervapour

pressuresensor

upstream side

downstream side

pumpOFF

PC

3: Measuring the total pressure p(t)means of a pressure sensor.

Total Pressure Method

• Sensitivity: <2x10-4 g/m2/day• suitable for gases and vapours• wide parameter range (100oC)

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Raw Data

0 100 200 300 400 5000

1

2OPP 60m

T=23oC

H2

He CO2

Ne O2

Ar WV Kr Xe N2

p [t

orr]

time [min]

• Information Contents: Rate of Permeation ~ Slope of p=f(t)

Barrier Web 2009

10-4g/m2/day and below

• Inorganic barrier on 100m PEN- substrate

• Information Contents: WVTR=f(T) “Activation Energy”• Can be (carefully) used for interpolation or extrapolation (“accelerated testing”

40oC

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Deltaperm Sensitivity

0 1000 2000 3000 4000

1.16

1.18

1.20

1.22

1.24

1.26

1.28

1.30Test SampleWVTR=5x10-5g/m2/dayT=40oC RH=0.9

p do

wnst

ream

[arb

. uni

ts]

time [min]

• @5x10-5 g/m2/day low noise level• Challenge: reduce or understand background signal

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Challenges• Sensitivity

• Sample conditioning - Measurement time• Distinguishing between good barriers caused by a long time-lag or

by low permeability

• Parameter range of measurement (temperature, RH): valuable tool

• Interpretation of “unwanted effects” (background signal, noise, edge permeation, outgasing, …)

• Easy operation

• Theoretical understanding of the permeation process

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Sensitivity: How small is 10-6g/m2/day?

• 10-6 x 0.1 x 3652 = 3.7x10-4g WV fits into a water droplet of Ø 0.9 mm

• Typical substrate: 100 m thick polyester (PET, PEN) S=0.005gWV/gPET

3x10-3

gWV/m2

(RH=50%)

10-6 g/m2/day

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A B C D

Sample Conditioning

Both sides of sampleunder ambient conditions

Upstream side exposed to water vapour (RH=0.9)

Downstream side of sampleunder vacuum

0 200 400 600 800 10000.00

0.01

0.02

0.03

0.04

WV

TR

Time [min]

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Single Barrier

• Sample can initially take up water

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Multilayers

0 200 400

0.008

0.010

0.012

0.014

0.016

0.018

WVT

R [r

el. u

nits

]

time [min]

• WVTR may fall first and increase later

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Time-lag• Information Contents: Rate of Permeation, Diffusion Coefficient, Solubility

• WV through PEN @23ºC

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Some Theory

dt

dp

)exp()1(2

6)(

2

22

122

111

L

tDn

n

LcLc

L

DtctQ

n

n

D = L2/6tlag

• Transient conditions:

• Stationary conditions:

P = const •

DP• S=

• This algorithm works for ideal gases and homogeneous materials• Multilayers: system of differential equations

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Outgasing of WV Uptake of WV

time

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PET or PEN

Outgasing: Water Vapour leaves the SampleUS

DS

3500 4000 4500 5000 5500 6000

0.0

0.5

1.0

1.5

2.0

2.5

water vapour removed by vacuum pump

outgasing

OutgasingTechnolox SampleT=40oC

p up

stre

am [r

el. u

nits

]

time [min]

• Water vapour leaves the sample (outgasing)• After some WV has accumulated outside the sample it is removed with a vacuum pump• More WV leaves the sample• WV removed again• And so on• Pressure increase due to outgasing is summed up and the total amount of WV coming from the sample is then calculated

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PET or PEN

Water Vapour enters the Sample

8000 9000 10000 11000

2

3

4

WV uptakeby sample

WV in

Uptake of Water VapourTechnolox Sample

T=40oC RH~0.9

p up

stre

am [r

el. u

nits

]

time [min]

• A dry sample is exposed to water vapour• Sample takes up WV• More WV is admitted• Sample takes up more WV• And so on• The pressure decrease of the WV is summed up and used to calculate the amount of WV that has gone into the sample

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Conclusions

• Various methods method suitable to measure WVTR in the 10-4 … 10-6 g/m2/day-range

• Wealth of information from permeation measurement• Measurement under stationary and transient conditions• Valuable tools: T, RH• Outgasing• Theoretical calculations should complement

experimental work

• A lot of challenges remain !!!

Thank you very much for your attention!