Permeation Measurements at 0.001 g/m2/day and below for Applications in Flexible Electronics Holger Nörenberg Technolox Ltd. www.technolox.com , 22 Ouseley Close, Oxford OX3 0JS, United Kingdom Abstract Electronic devices based on flexible plastic substrates require good protection against ingress of unwanted species such as moisture and oxygen to ensure a sufficient lifetime. Usually, a barrier layer is used to achieve rates of permeation for water vapour of 0.001g/m2/day and below. Such good barrier properties of thin films create challenges for measuring the rate of permeation. This is due to the long time it takes to reach stationary conditions and the small amount of gas or vapour to be measured. Good barrier properties can be achieved by a low rate of permeation, a very long time-lag or a combination of both. Introduction Electronics on flexible substrates for applications as displays or solar cells seem to have an irresistible attraction to the scientific and business communities. To profit from their real or perceived advantages over similar devices made on glass substrates, however, some technical challenges have to be overcome. One major challenge is to prevent ingress of unwanted species (water vapour, oxygen) which shorten the lifetime by undesired modification or even destruction of essential components in the device. Therefore, the electronic devices are protected by a barrier layer. It is necessary to characterize these barrier layers and measure the amount of gas or vapour they transmit. For application in photovoltaics and for flexible displays a permeation rate of water vapour (also called water vapour transmission rate WVTR) of 10-3… 10-6 g/m2/d is required.

Fig. 1: Principle of a permeation experiment In permeation experiments the test sample (film) is usually considered as “black box”, where one side is exposed to a gas or vapour and a response is measured on the opposite side by means of a suitable detector. From that response the am ount of gas or vapour transmitted by the sample is estimated. The permeation process itself is not that

simple as gas or vapour has to get dissolved in the sample and then diffuse through it before leaving it. Figure 1 shows the principle of a permeat ion experiment as described in standards for gas permeation [1]. The method can be used for studying the permeation of water vapour too [2]. Experiments The rate of permeation for different substrates and a variety of gases and water vapour was measured with a Deltaperm Permeation Tester made by Technolox.

Fig.2: Principle of the Deltaperm Permeation Tester Figure 2 shows the experimental set -up. A sample of A=50cm2 was exposed to gas at a pressure of 1 bar or water vapour at RH=100% on the upstream side. The total pressure was measured on the downstream side with a pressure gauge. Under stationary conditions (see next chapter) the pressure is a linear increasing function of time (see also fig. 4). The slope of this p(t) curve is a measure for the rate of permeation. Under transient

conditions (see next chapters) the p(t) curve may be non-linear. By measuring the time lag the coefficient of diffusion can be determined. Using the relationship P=D•S the solubility S can then be calculated from the permeability P and the coefficient of diffusion D. Permeation under Stationary Conditions In a permeation experiment a gas or vapour at a certain pressure p1 (or relative humidity in the case of water vapour) is supplied on the upstream side. The gas or vapour gets dissolved in the sample at a concentration c1, diffuses through the sample and leaves it. The downstream side is kept at a low pressure p2. Figure 3 shows a sample during a permeation experiment under stationary conditions [1]. Fick’s first law (with J the flux, D the coefficient of diffusion) states that the flux through the sample is proportional to the concentration gradient. There is a linear dependence of the concentration of the permeant species as function of the coordinate x.

Fig. 3: Permeation under stationary conditions Because of difficulties in assigning a concentration outside the sample and to assign a pressure inside the sample an appropriate continuous function is the chemical potential µ. The chemical potential gives the tendency for diffusion (diffusion proceeds from a higher potential to a lower potential). Figure 4 shows experimental results for water vapour permeation obtained with the Deltaperm permeation tester. After conditioning the samples sufficiently long, stat ionary conditions are assumed because the ∆p(t) curves shown in fig. 4 can be approximated with a straight line. The pressure on the downstream side increases linearly with time. The slope of the ∆p(t) curve, the sample size and

the downstream volume can be used to calculate the rate of permeation for the PEN substrate and the barrier layer as indicated in fig. 4. Because the PEN substrate (red curve) is much more permeable than a substrate with barrier layer (black curve) the red curve has a much steeper slope.

0 50 100 150 200 250 3000.0

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Permeation of Water Vapour Barrier PEN 50 µm∆p

[mba

r]

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Fig. 4: Permeation of water vapour through PEN and a barrier layer under stationary conditions downstream pressure as function of time Under stationary conditions the pressure increase is directly proportional to the rate of permeation. Next we want to discuss the transient behaviour of the ∆p(t) curves. Permeation under Transient Conditions The measurement of very low rates of permeation may be difficult because it may be time-consuming to wait for the permeation process to become stationary. Ideal gas transport through a film under transient conditions can be described by Fick’s 2nd law: For multilayers a system of differential equations has to be solved [3][4]. Figure 5 illustrates the change of the chemical potential under transient. Inside the sample the concentration c can be used instead. The sample is initially (t=0) free of permeant species. At t=0 the upstream side is exposed to a gas or vapour. After admitting gas to the upstream side, it takes time for the molecules to diffuse through the sample and to establish stationary conditions. State-of-the art barrier layers have water vapour transmission rates in the range 10 -3 … 10-6 g/m2/day. In many cases multilayer structures are used to achieve

2

2

xcD

tc

∂∂⋅=

∂∂

xc

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⋅−=

these excellent barrier properties by slowing down diffusion.

Fig. 5: Permeation under transient conditions The transient behaviour may cause a substantial time-lag before reaching steady-state conditions. It was shown, that in some structures an apparent low rate of permeation may be a transient effect of the order of the expected life-time of the device [2].

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PET 50µm

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Fig. 6: Time-lag measurements for Water Vapour through different materials Figure 6 shows time-lag measurements for water vapour through various materials. The steepest slope is observed for a 50µm thick PET sample (black curve). The slope for the 100µm thick PEN sample (red curve) is much lower because the sample is thicker and the constant of diffusion is lower for PEN compared to PET. For the sake of illustration the slope of the blue line in fig. 6 has been calculated for a single barrier layer (for example a thin oxide layer on a polyester substrate) with a rate of water vapour permeation of 0.001 g/m2/d. The better the barrier properties, the more difficult become experiments to determine the rate of permeation. The much reduced signal compared

to a polyester substrate is not the only problem. Depending on the structure of the barrier layer, the time-lag can be considerable (the time-lag for the 100µm PEN sample is several hours). Measurements under transient conditions can be used to estimate the constant of diffusion D from the time-lag according to [3]: D=L2/6tlag with L the thickness. Figure 7 shows three curves for 100 µm thick PET. The steepest pressure increase is observed for water vapour (red curve). Water vapour has a high solubility in many types of polyester which greatly contributes to the permeability P. The ∆p(t) curves in fig. 7 are determined by the diffusion D and solution S (P=D·S). The blue curve for CO2 in fig. 7 shows initially a slower take-off (larger time-lag) but will eventually have a steeper slope than the curve for oxygen (green curve). The reason for the first effect is the lower (by about one order of magnitude [5]) coefficient of diffusion D for CO2 compared to oxygen. The reason for the higher rate of permeation of CO2 later on is the much higher (by more than one order of magnitude [5]) solubility of CO2 in PET compared to oxygen.

0 500 1000 15000.0

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CO2

O2

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bar]

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Fig: 7. Timelag measurements on 100µm thick PET for Oxygen, CO2 and Water Vapour Permeation and Outgasing Some film materials such as polyesters have a high solubility for water vapour [5][6] . If such materials are used as substrates for barrier layers, their outgssing behavior has to be understood. Outgasing can be studied quantitatively by several methods. For example, chemical sensors and infrared-sensors have been used. Another method is, to measure the pressure change caused by outgasing. Figure 8 shows the total pressure as function of time for an outgasing experiment. The upstream side of the sample is not exposed to gas

or vapours. The pressure increase that is detected by the sensor on the downstream side has come out of the test sample. The test sample of 125 µm thick PEN was soaked in water prior to the experiment. To increase the sensitivity and to shorten the experiment, the outgased species are pumped away at regular intervals causing the steps visible in fig. 8. The pressure drop ∆pi during the pumping cycle is a measure for the amount of water vapour that has come out of the sample. Figure 9 shows the pressure drop ∆pi per pumping cycle. The amount of water vapour removed per pumping cycle decreases as the sample is depleted of water. The area under the curves shown in figure 9 is a measure for the total amount of water vapour originally present in the sample.

0 200 400 6000

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∆p2

∆p3

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sure

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Fig. 8: Outgassing measurement: pressure as function of time during outgasing with periodic pumping

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Fig. 9: Outgassing measurement: pressure change per outgasing cycle i with constant pumping time The experimental data shown in fig. 9 have been fitted with a sum of two exponential functions, which have been used to determine the value of the solubility for water vapour in PEN at RT as 0.005 gWV/gpolymer. This number seems small, but in the context of a substrate for a barrier layer it is a huge amount. The amount of water vapour dissolved in a

100 µm thick polyester substrate is more than will permeate through it at 10-4 g/m2/d in several years. Conclusions There are a number of points that should be considered when measuring very low rates of permeation. Improved barrier performance may be achieved by a low rate of permeation, by a long time-lag or both. Therefore, interpretation of experimental data obtained under transient conditions will be necessary as sophisticated barrier structures make it difficult or impossible to achieve stationary conditions within a reasonable time. This is particularly important for gases with a high solubility and vapours. With increasing barrier performance the transmission of gases or vapours becomes small so that other effects such as outgasing of the substrate materials needs to be considered. Progress in theoretical understanding of the transport of gases and vapours through barrier films is essential for data interpretation. References 1. Annual Book of ASTM Standards , vol. 15.09, p.

190, ASTM International, 2006. 2. H. Yasuda, “Special Problems and Methods in

the Study of Water Vapor Transport in Polymers” J. Macromol. Sci. – Phys., B3, 589, 1969.

3. G. L. Graff , R. E. Williford and P. E. Burrows, “Mechanisms of vapour permeation through multilayer barrier films: Lag time versus equilibrium permeation”, Journ. Appl. Phys., 96, 1840, 2004.

4. R. Ash, R. M. Barrer and D. G. Palmer, “Diffusion in multiple laminates”, Brit. J. Appl. Phys. 16, 1965, 873-884.

5. Permeability and Diffusion Data in: J. Brandrup and E. H. Immergut (eds.) Polymer Handbook, 3rd edition, p. VI/435, John Wiley & Sons, New York 1989.

6. J. E. Shelby, “Handbook of Gas Diffusion in Solids and Melts”, Materials Park 1996.