Importance of Interface Diffusion and Climate in Defect Dominated

Jul 15, 2016 - often only a single number is provided as water vapor transmission rate. However, this value is highly dependent on the aging climate. ...
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Importance of Interface Diffusion and Climate in Defect Dominated Moisture Ultrabarrier Applications Frederik Nehm,*,† Felix Dollinger,† John Fahlteich,‡ Hannes Klumbies,† Karl Leo,† and Lars Müller-Meskamp† †

Institut für Angewandte Photophysik, Technische Universität Dresden, 01062 Dresden, Germany Fraunhofer Institut für Organische Elektronik, Elektronenstrahl- und Plasmatechnik (FEP), Winterbergstraße 28, 01277 Dresden, Germany

‡

ABSTRACT: OLEDs and organic photovoltaic (OPV) devices require encapsulation from water vapor using a permeation barrier system. As a benchmark for barrier quality, often only a single number is provided as water vapor transmission rate. However, this value is highly dependent on the aging climate. So far, little scientific effort has been undertaken to characterize ultrahigh moisture barriers at different temperatures and relative humidities. We present Catest studies on sputtered Zinc−Tin-Oxide and atomic layer deposited AlOx barriers in extensively varied climates. Relative humidities are changed at constant temperatures, and temperatures are changed at constant absolute humidity. We find Henry’s law to apply for sorption and discover a fundamental change of the diffusion regime with time related to the interface between the test and the barrier thin-film. KEYWORDS: water vapor barrier, encapsulation, diffusion, permeation, climate, activation energy

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and relative humidities from 0% to 90% are applied to flexible Zn2SnO4 (ZTO) barrier films as well as atomic layer deposited (ALD) AlOx thin-film encapsulation. WVTRs are found to be highly dependent on the climate, ranging from 2 × 10−5 g/m2/ d to 2 × 10−2 g/m2/d. Furthermore, we find the WVTR to be directly proportional to the absolute water content in the outside air for a constant temperature, highlighting Henry’s law for sorption in the evaluated humidity range. In addition, we clearly see temperature activation of the permeation for both barriers. This temperature activation changes with progressing test time. We show that this phenomenon is not related to the individual materials but to diffusion along the interface between the barrier and the test cell. This highlights how encapsulation of organic devices is not only dependent on the used barrier material but also barrier integration into the device concept.

INTRODUCTION Recent rapid progress in the field of organic electronics1,2 has created the need for thin-film gas ultrabarriers to allow for longer device lifetime. In most cases, water vapor can be identified as the driving factor in degradation.3−6 Without sufficient protection from moisture, flexible organic solar cells and light emitting diodes cannot reach full industrial production and successfully enter the market. While packaging for conventional products, like food or pharmaceuticals, only requires water vapor transmission rates (WVTRs) in the (10− 10−2) g/m2/d7,8 regime, organic devices demand 10−4 g/m2/d and below.9,10 However, it is often neglected that WVTRs are highly dependent on climate conditions. So far, few studies have investigated how the applied aging climate influences permeation mechanisms through defect-driven high quality barriers with regard to sorption and diffusion. Moreover, WVTR testing mostly focuses on new barrier materials and processing techniques while neglecting the influence of interfaces in moisture permeation. In this work, we study the permeation behavior of water in different encapsulation systems by extensive climate variations and changes in test architecture. Central point is not the characterization of different barriers but the influence of the complete permeation system. We use an electrical calcium corrosion test (Ca-test) setup with thin-film architecture, which is made to closely resemble an encapsulated organic device. This way, permeation in WVTR tests and actual aging devices become as similar as possible. Temperatures from 20 to 68 °C © 2016 American Chemical Society

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EXPERIMENTAL SECTION

The following moisture barrier systems are investigated with an electrical Ca-test at different temperatures and relative humidities: a) Flexible moisture barriers consisting of 144 nm Zinc−Tin-Oxide (ZTO) on 75 μm Melinex PET (DupontTeijin Films). b) 20 nm ALD AlOx thin-film encapsulation on glass substrates with predeposited Catests. All sample sizes are (2.5 × 2.5) cm2. ZTO barrier films are deposited by reactive dual-magnetron sputtering in roll-to-roll vacuum Received: April 17, 2016 Accepted: July 15, 2016 Published: July 15, 2016 19807

DOI: 10.1021/acsami.6b04561 ACS Appl. Mater. Interfaces 2016, 8, 19807−19812

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ACS Applied Materials & Interfaces chambers (labFlex 200, FHR Anlagenbau GmbH, Germany). Each target has an area of (349.6 × 121) mm2 and is alternately switched as cathode or anode with a frequency of 50 kHz. Targets consist of 52 wt % Zn and 48 wt % Sn (atomic ratio of 2:1), oxidation is done via oxygen-inlet. Sputtering power is 4 kW, and oxygen flow is set by means of a closed-loop control using the optical emission of excited Zn atoms in the plasma as control variable. The PET substrate is cooled to room-temperature from the backside. For detailed information and thin-film characterization, see refs 11 and 12. ALD barriers are deposited directly on electrical Ca-tests on glass substrates. This is done in a Sentech SI ALD LL plasma-enhanced deposition system (Sentech Instruments GmbH, Berlin, Germany) at 100 °C reactor temperature. 220 cycles of electronic grade trimethylaluminum (TMA) and oxygen plasma precursors are used with 99.9999% purity nitrogen as purge gas. One cycle consists of 80 ms TMA pulse, 5 s N2 purge, 5 s oxygen plasma, and 2 s N2 purge. To inhibit corrosion of the ALD barrier, a 125 μm Melinex PET film is glued onto it after deposition13 using UV-curing XNR 5592 (Nagase Chemtex Corporation, Japan). ALD alumina thin-films exhibit root-mean-square roughness values of 0.18 nm on average in atomic force microscope images made with an AIST-NT Combiscope (Novato, California, USA) in tapping mode and TAP-Al-G tips by BudgetSensors with a resonance frequency of approximately 320 kHz. All polymer films (including ZTO barrier films) are heated out in nitrogen atmosphere with residual oxygen and water concentrations below 0.1 ppm at 80 °C for 72 h. Furthermore, all investigated barriers are deposited in the same run for each material type to enhance comparability. Electrical Ca-tests are utilized to measure WVTR values. Here, a metallic calcium sensor on one side of the barrier reacts with ingressing water and turns into calcium hydroxide. The sensor’s electric conductance gradually decreases and its derivative is linearly proportional to the WVTR.14 To keep the test layout close to an actual OPV or OLED device, electrical Ca-test structures are deposited as thin-films in the same deposition chambers used for organic device manufacturing. For this, thermal evaporation is done in a custom multisource vacuum deposition system (K.J. Lesker, UK) at a base pressure of 10−8 mbar using shadow masks. Two Ca-tests are employed: One for flexible ZTO barrier films and one for thin-film encapsulation with ALD barriers. For flexible barriers, the following layers are deposited onto the ZTO side of the barrier film: (1): 20 nm of C60 fullerene as decoupling layer between the barrier and the Ca sensor stripe. This layer mitigates stress, which builds up on the inorganic barrier layer due to expansion of the calcium upon its reaction to calcium hydroxide.15 (2): 100 nm calcium sensor. (3): 100 nm Al four-point-probe electrode fingers to avoid series resistances.16 On the test backside, the active area of the Ca-test is encapsulated with a cavity glass to reduce the background water ingress. Cavity glasses are glued on under nitrogen atmosphere using XNR-5592 UV-curing resin (Nagase Chemtex Corporation, Japan) applied to the rim of the glass. In the course of the investigation, Ca-tests are also conducted on ZTO barriers in f lipped geometry. Here, the layers of the Ca-test are evaporated onto the PET side of the barrier film. For the measurement of ALD thin-film encapsulation, Ca-tests are produced on 1.1 mm thick Borofloat 33 glass substrates (Schott, Germany). Here, a different layer stack is used: 100 nm of C60 for mechanical decoupling, 25 nm Cu to prevent bottleneck corrosion at the electrodes, 60 nm Ca sensor, 100 nm Al four-point probe electrode fingers, another 150 nm of C60 for mechanical decoupling, and 20 nm of ALD barrier. As mentioned above, a PET film is glued onto the sample after barrier deposition to prevent barrier corrosion. Further information on this stack design can be found in refs 17 and 18. Due to the thin-film nature of the tests and since only single barrier layers are used in this investigation, lagtimes in all Ca-tests are very short. For Ca-test aging, samples are screwed onto custom-made circuit boards using aluminum adapters with rubberized pads. These press the samples against gold spring contacts in the circuit boards for electrical contact. Holes through the adapters allow the ingress of humid air, which is provided by saturated salt solutions. This way, humidity is only applied to the active test area and not to the sample edges. Circuit boards and samples are kept in plastic boxes, partially filled with silica

gel, to provide low-humidity background air and keep edge diffusion minimal. The plastic boxes are kept in insulation boxes with heating/ cooling elements in a feedback loop to keep the temperature constant. Custom measurement software measures each sample every 5 min using a Keithley 2400 source measuring unit and custom-made sample switches. For further information on the setup, see refs 17 and 19. Theoretical Background. The permeation of water from a volume of humid air through a moisture barrier can be described in a simple approach using the coefficients of sorption (S) and diffusion (D). On a homogeneous material, water molecules are first adsorbed, resulting in a concentration c1 directly below the surface. The relation between c1 and the outside air is determined by the material properties as well as its surface structure (e.g., capillaries) as described in various sorption models. A water flux J then establishes in the direction of a concentration gradient dc/dx as proposed by Fick’s law: J = − D × dc /dx

(1)

Once water molecules reach the other side, they are desorbed from the surface and eventually participate in a reaction such as the degradation of an organic device or calcium corrosion. If a fast reaction is assumed at this point, i.e. assuming a concentration of 0 at the device, we can approximate the concentration gradient through the barrier with thickness h as

dc /dx = c1/h = S × p1 /h

(2)

using the sorption coefficient and the water vapor partial pressure p1. A WVTR can thus be written as WVTR = − J = D × S × p1 /h

(3)

Both sorption and diffusion are activated by temperature T and follow an Arrhenius relation

D , S ∼ exp(− EA /(RT ))

(4)

with the ideal gas constant R and activation energy EA. For a barrier film consisting of two or more layers with different materials, contributions from the single materials as well as from interfaces must be considered for a more complex approach. Considering the defectdriven nature of moisture barriers for organic devices, it is easy to see that diffusion splits in at least two contributions: Through the polymer substrate to a barrier defect and from the defect to a reaction site. It should be noted at this point that diffusion can be driven by structural properties as opposed to material properties. This happens when the material can no longer be considered bulk-like but exhibits smaller structures.20−22

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RESULTS AND DISCUSSION Figure 1 shows the WVTR of ZTO barriers vs absolute humidity of the test climate at three different temperatures. For each temperature, we see a linearly increasing WVTR with increasing water vapor partial pressure within the investigated humidity range. We can exclude a saturation of the water concentration inside the test materials, because no WVTR saturation is visible in the data. The diffusion coefficient should thus be unaffected by a change in water concentration c1 − we mainly see the contribution of sorption: The linear WVTRhumidity relation indicates the validity of Henry’s law of sorption: c1 = S × p1 , with S = const

(5)

We also observe a temperature activation of the permeation process − a rise in curve slope and offset with higher temperature. As mentioned above, this can be caused by various mechanisms. In the following, we try to find the exact cause of the activation. Figure 2 displays normalized single Ca-test data curves for all three investigated temperatures. Each curve is exemplary for the 19808

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results in the commonly known circular degradation patterns around the pinhole defects. However, it also means that diffusion can take place through different materials and structures over time. Early on, it is mostly dominated by bulk diffusion through the polymer substrate. As diffusion paths below the barrier lengthen, the diffusion along this interface starts to dominate the overall permeation. This is depicted schematically in Figure 3. Since each test represents an average over all its defects with different size and geometry, the regime change in Figure 2 appears spread out over time.

Figure 1. WVTR vs absolute humidity for ZTO barrier films at 3 different temperatures. Linear dependency at a given temperature shows the applicability of Henry’s law of sorption. A thermal activation of the WVTR can be observed. The inlet shows a detail of the same data sets at low humidities. Each data point is averaged over 3−4 samples.

Figure 3. Schematic cross-section of a Ca-test (not to scale). At test beginning, degradation takes place in direct defect vicinity. Later, Ca near the defect is already corroded, and water needs to diffuse further below the barrier. This mechanism equals the predominant degradation process of organic devices. The different structural dimensions near the defects and far below the interface can strongly influence permeation.

According to the two regimes, also two WVTRs exist for each Ca-test. In Figure 1, the late regime is chosen, because it represents steady-state diffusion more adequately. Using the linear fits (WVTR vs humidity) from both late and early regimes, temperature dependencies of the WVTR can be extracted at constant absolute humidity. Here, 20 mbar are used. In Figure 4, the WVTR is displayed in an Arrhenius plot (against 1/T). For each regime, the slope of the respective

Figure 2. Normalized, single Ca-test curves for 3 different temperatures. A change of the diffusion can be observed after some time. At 20 °C, the slope steepens over time. The opposite can be seen for 60 °C. At 38 °C, no change is visible. This suggests a change of the permeation mechanics and thermal activation with time.

complete data set at the corresponding temperature. Although taken at six different relative humidities for each temperature, measurements always follow the same qualitative curves: For 60 °C, first a steep slope (high WVTR) is measured, which flattens (lower WVTR) over time. For 20 °C, it appears to be the opposite − a slope steepening over time. At 38 °C, no change is seen over time. These changes typically appear after many hours to several days into the measurement. This rules out temperature adjustment of the measurement setup, which happens in under an hour. It should be mentioned at this point that the time scale in Figure 2 is not absolute, because the tests obviously age faster or slower depending on the aging climate. On the observed time scale, only one significant process occurs in the tests: Ca corrosion. As for all high quality moisture barriers, water ingress through the ZTO films is driven by defects. This means that water molecules ingressing at the beginning of the test always degrade Ca in the direct vicinity of the defect. At later stages, Ca in defect vicinity is already corroded, so water molecules must diffuse below the barrier along the interface to the Ca-test to find metallic Ca. This

Figure 4. Arrhenius plot of the WVTR through ZTO barriers for both permeation regimes. Data points at 20 mbar constant absolute humidity are extracted from linear fits of WVTR over relative humidity at constant temperature (see Figure 1). A clear drop in activation energy can be seen during the test. Each data point is averaged over 3−4 samples. 19809

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ACS Applied Materials & Interfaces linear fit characterizes thermal activation of the permeation process. We measure a drop in activation energy over time. This implies a change in permeation mechanics. Activation energies should not be mistaken to be characteristic for the barrier material. The values more likely result from the method of measurement and exact interface composition. In order to exclude a contribution of the substrate to the thermal activation of water permeation, we measure the climate dependent WVTRs for pure PET. This is shown in Figure 5.

contributions from the ZTO layer itself and the PET-ZTO interface, but the interface from ZTO to the Ca-test is omitted. Just as for pure PET substrates, no significant temperature activation can be observed for the WVTR vs absolute humidity from 20 to 68 °C. To clarify, PET and ZTO barriers have certain activation energies. However, both of them do not significantly contribute to the effect observed in Figure 1. More information on the materials’ thermal activation was published by Fahlteich et al.12 This means that we can further exclude the following for temperature activation: Diffusion inside the ZTO, sorption at the ZTO, interface effects between PET and ZTO, any kind of structural rearrangement of the ZTO. We conclude that the diffusion along the interface between ZTO and Ca-test (or a device) dominates the permeation. This effect is especially well visible if we compare WVTRs for the ZTO barrier films in normal and flipped-barrier architecture. Figure 7 indicates 1

Figure 5. WVTR vs absolute humidity for pure PET films at 4 different temperatures. Linear dependency shows the applicability of Henry’s law of sorption. No thermal activation is visible. Each data point is averaged over 3−4 samples.

Here, WVTRs are plotted against absolute humidity from 20 to 68 °C. We observe a linear dependency of WVTR and absolute humidity almost independent of temperature. We can thus neglect a significant thermal activation caused by either diffusion in or sorption on the PET. Next, the contribution of the ZTO layer is investigated. Unfortunately, there is no possibility of measuring a free-standing ZTO thin-film. Instead, we measure the climate-dependent WVTR for ZTO barriers in flipped-barrier architecture, see Figure 6. This describes the ZTO thin-film facing the outside air instead of the dry Ca-test side. Such an experiment still allows the measurement of all

Figure 7. WVTR vs relative humidity for ZTO barrier films in normal and flipped barrier architecture at 20 °C. Even though the same barriers are measured, completely different WVTRs are observed. The flipped architecture samples exhibit WVTRs that are 1 order of magnitude higher. Each data point is averaged over 3−4 samples.

order of magnitude higher WVTRs for the flipped-barrier type. This shows the strong influence of the described interface diffusion process on the aging performance and potential device lifetime. The interface between ZTO thin-film and Ca-test appears to effectively slow the complete permeation. The observed difference does not originate from increased edge diffusion, because the test setup applies humidity only to the test area. Sample edges are subjected to dried air. It is worth noting that Ca-test data for both test architectures do not show barrier failures in the form of sudden, rapid current decreases. This would indicate barrier destruction, which can happen to barriers that are directly exposed to high relative humidity climates.13 Additionally, the linear fit for the flipped-barrier samples in Figure 6 shows the validity of Henry’s law for these samples as well. No qualitative change in sorption mechanisms can be observed, highlighting the comparability between the two different Ca-test architectures. For a more precise investigation of thermal activation, we measure WVTRs at different temperatures but equal absolute humidity. Within this approach the number of data points is increased from 3 (Figure 4) to 6 (temperatures from 28.9 to 72.3 °C). This is done in the following, using an ALD AlOx thin-film barrier.

Figure 6. WVTR vs absolute humidity for flipped ZTO barrier films at 4 different temperatures. Linear dependency shows the applicability of Henry’s law of sorption. No thermal activation is visible. Each data point is averaged over 3−4 samples. 19810

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ACS Applied Materials & Interfaces First, basic permeation mechanics of the two systems are compared: WVTRs are taken for six different relative humidities at a constant temperature of 38 °C. Figure 8

Figure 9. Arrhenius plot showing WVTR (naturally logarithmic scaling) of ALD AlOx barrier thin-films against reciprocal temperature for both test regimes. Linear fits are applied for activation energy acquisition and depicted as dashed lines. For the early regime (red circles) an activation energy of 40 kJ/mol is measured. For the late regime (black squares), the activation energy drops to 27 kJ/mol. Each data point is averaged over 3 samples.

Figure 8. WVTR vs absolute humidity for ALD AlOx thin-films at 38 °C. Linear dependency shows the applicability of Henry’s law of sorption. Each data point is averaged over 3 samples.

normally expect very different values for completely different barriers, if the values were related to the materials. Instead, similar interface composition yields similar activation behavior. This investigation clearly shows an influence of the exact barrier interface on the permeation. However, further research is necessary for advice on interface construction. A systematic study of the organic material and its thickness below the interface or observations on a single-defect system would be highly valuable but are beyond the scope of this investigation. Since the Ca-test used here is very similar to an actual organic device, these results apply not only to other WVTR measurements but also to aging of organic solar cells and OLEDs as a whole. When performing aging experiments, the resulting interface, the applied climate conditions, and possible interrelations should be kept in mind.

displays the WVTR of 20 nm AlOx barriers vs humidity of the aging climate. First of all, we notice the high barrier quality which manifests in WVTRs in the 10−5 g/m2/d regime. This highlights the relevance of this data for the application on organic devices. Next, we see no qualitative difference to ZTO barrier films: WVTRs and the humidity are found to be linearly related within the uncertainties of the measurement. This shows the good comparability of gathered data. The equal validity of Henry’s law for ZTO barrier films in normal and flipped geometry, as well as for ALD AlOx thin-films, strongly suggests a general applicability of this sorption mechanism for all defect-driven high moisture barrier systems. However, this cannot generally be assumed and must be ascertained individually for every system. In order to quantify activation energies for the permeation system, another measurement series is performed on equivalent barriers. Here, six different temperatures (from 28.9 to 72.3 °C) are applied. Relative humidities are chosen so that the absolute humidity stays constant at approximately 35 mbar. Therefore, the amount of water provided is equal in every test, and differences in WVTR only originate from thermal activation. As Melinex PET is also used on top of this barrier system against barrier corrosion, we can exclude an influence of adsorption from Figure 5. The WVTR data from this investigation are depicted in an Arrhenius plot in Figure 9. Data sets for both regimes follow an Arrhenius behavior as indicated by the linear fits. Activation energies of this permeation system are 40 kJ/ mol for the early regime and 27 kJ/mol for the late regime − a deviation by a factor of 1.5. The activation energy decreases with time, as already seen for ZTO from Figure 4. Again, this showcases the importance of the interface and the structural dimensions. Not only the WVTR changes when water diffuses far below the barrier interface but also the complete diffusion mechanism. It is crucial to understand at this point that all activation energies ascertained within this investigation are not necessarily related to the barrier materials. Instead, the geometry and composition of the tests likely dominates these values. This point is emphasized by the fact that activation energies for ZTO and AlOx are very similar. One would

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CONCLUSION We show climate-dependent water permeation studies on sputtered Zn−Sn-oxide (ZTO) single layer barriers on flexible PET substrates as well as ALD AlOx thin-films. With an electrical Ca-test setup that closely resembles organic devices, these barriers are each measured in a variety of climates. ZTO barrier films exhibit WVTRs from 5.5 × 10−5 g/m2/d to 2 × 10−2 g/m2/d at temperatures from 20 to 60 °C and from 0% to 90% RH. The WVTRs are directly linearly related to the humidity at a given temperature, which shows the applicability of Henry’s law for sorption. For temperature dependence of permeation, classic Arrhenius activation is observed with changing activation energy over progressing test time. By measuring similar climate variations for the pure substrate and flipped ZTO barrier films, the cause for this activation is determined: Water diffusion at the interface between Ca-test or device and the barrier. Here, a regime change occurs when the diffusion path elongates below the barrier thin-film with progressing test time. To verify the observations on a second system, ALD barriers are tested as thin-film encapsulation in two different climate variations: Humidity is varied at a constant temperature and the temperature is varied at constant absolute humidity. The first investigation confirms Henry’s law 19811

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(10) Klumbies, H.; Karl, M.; Hermenau, M.; Rösch, R.; Seeland, M.; Hoppe, H.; Müller-Meskamp, L.; Leo, K. Water Ingress into and Climate Dependent Lifetime of Organic Photovoltaic Cells Investigated by Calcium Corrosion Tests. Sol. Energy Mater. Sol. Cells 2014, 120, 685−690. (11) Fahlteich, J.; Schönberger, W.; Fahland, M.; Schiller, N. Characterization of Reactively Sputtered Permeation Barrier Materials on Polymer Substrates. Surf. Coat. Technol. 2011, 205, S141−S144. (12) Fahlteich, J.; Fahland, M.; Schönberger, W.; Schiller, N. Permeation Barrier Properties of Thin Oxide Films on Flexible Polymer Substrates. Thin Solid Films 2009, 517, 3075−3080. (13) Nehm, F.; Klumbies, H.; Richter, C.; Singh, A.; Schroeder, U.; Mikolajick, T.; Mönch, T.; Hoßbach, C.; Albert, M.; Bartha, J. W.; Leo, K.; Müller-Meskamp, L. Breakdown and Protection of ALD Moisture Barrier Thin-films. ACS Appl. Mater. Interfaces 2015, 7, 22121−22127. (14) Paetzold, R.; Winnacker, A.; Henseler, D.; Cesari, V.; Heuser, K. Permeation Rate Measurements by Electrical Analysis of Calcium Corrosion. Rev. Sci. Instrum. 2003, 74, 5147. (15) Klumbies, H.; Müller-Meskamp, L.; Nehm, F.; Leo, K. Influence of Calcium Corrosion on the Performance of an Adjacent Permeation Barrier. Rev. Sci. Instrum. 2014, 85, 016104. (16) Schubert, S.; Klumbies, H.; Müller-Meskamp, L.; Leo, K. Electrical Calcium Test for Moisture Barrier Evaluation for Organic Devices. Rev. Sci. Instrum. 2011, 82, 094101. (17) Nehm, F.; Dollinger, F.; Klumbies, H.; Leo, K.; MüllerMeskamp, L. Device-like Electrical Calcium Corrosion Test for WVTR Measurements of Ultra-barriers. In 58th Annual Technical Conference, Proceedings of the Society of Vacuum Coaters, Santa Clara, USA, 2015. (18) Nehm, F.; Müller-Meskamp, L.; Klumbies, H.; Leo, K. Note: Inhibiting Bottleneck Corrosion in Electrical Calcium Tests for UltraBarrier Measurements. Rev. Sci. Instrum. 2015, 86, 126110. (19) Klumbies, H.; Müller-Meskamp, L.; Schubert, S.; Moench, T.; Hermenau, M.; Leo, K. Diffusion Barriers for Organic Devices and their Evaluation with Calcium Corrosion Tests. In 56th Annual Technical Conference, Proceedings of the Society of Vacuum Coaters, Providence, USA, 2013. (20) Knudsen, M. Die Gesetze der Molekularströmung und der inneren Reibungsströmung der Gase durch Röhren. Ann. Phys. (Berlin, Ger.) 1909, 333, 1521−3889. (21) Roth, A. Vacuum technology, 3rd ed.; Elsevier Science: Amsterdam, The Netherlands, 2012; pp 62−119. (22) Casanova, F.; Chiang, C. E.; Li, C.-P.; Roshchin, I. V.; Ruminski, A. M.; Sailor, M. J.; Schuller, I. K. Gas Adsorption and Capillary Condensation in Nanoporous Alumina Films. Nanotechnology 2008, 19, 315709.

for sorption; the second also shows a drop in activation energy over time. Activation energy values are likely not related to the barrier material but to the exact interface structure and composition. This strongly highlights the importance of interfaces in permeation barrier applications. Successful encapsulation of organic devices will require consideration of the resulting interfaces as well as the climates during application.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Bundesministerium für Bildung und Forschung (BMBF) within the Innoprofile Transfer project 03IPT602A is gratefully acknowledged. The authors thank Tobias Günther and Andreas Wendel for sample preparation as well as Sven Kunze for maintenance of the measurement setup.

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DOI: 10.1021/acsami.6b04561 ACS Appl. Mater. Interfaces 2016, 8, 19807−19812