A reliable technique for evaluating a barrier film, which is a key component used to encapsulate flexible organic light-emitting diodes (OLEDs), is required to reliably appraise the lifetimes of such devices. The water vapor transmission rates (WVTR) is commonly used as an indication for a barrier film. In this paper, the variables affecting WVTR measurements were investigated because the results of such evaluations typically vary widely even at a level of 10−3 g m−2 day−1 at 40 °C and 90% relative humidity (RH). The reference films used for comparative measurements were prepared to eliminate the influence of the differences between individual barrier films. The measurement procedures were carefully investigated by using three WVTR measurement systems, which are based on different principles and different detectors. Consistency between the systems in terms of the WVTR was achieved at a level of 10−5 g m−2 day−1 at 40 °C and 90%RH. These results prove the reliability of not only our evaluation but also of these three systems, provided the measurements are performed correctly. The lag time was also analyzed to determine those factors that can affect the measurement time. It was found that the time required for a system to reach the adsorption-desorption equilibrium state can affect the measurement time.

Degradation resulting from the ingress of water vapor into devices is a particularly serious problem for those using flexible organic light emitting diodes (OLEDs) as vapor can lead to the growth of non-emissive spots (dark spots) and edge quenching.1 In this sense, the service life of a flexible OLED is determined by the time required for water to traverse its barrier seals. It is generally held that a water vapor transmission rate (WVTR) of less than 10−5 g m−2 day−1 is necessary to attain an OLED service life in excess of 10,000 hours.2 This estimate, however, has not yet been proven because it is difficult to quantify the degree of water vapor ingress into a device. It is also necessary to determine the correlation between the amount of water vapor ingress and the dark areas. In the first instance, we focused on the development of a reliable technique for evaluating a barrier film, which is a key component for the encapsulation of flexible OLEDs.

The WVTR is an indication of the transmission rate of water vapor though a film in a steady state. The evaluation of the WVTR of a barrier film incurs some problems. Although several systems have been proposed and are currently under development for measuring the WVTR,3–8 unfortunately it is not clear whether there is any correlation between the WVTR values obtained with the different systems, especially those based on different principles such as the equal pressure or differential pressure methods. Experimental comparisons of barrier films are reported in the literature.9,10 Differences in the WVTR of up to a factor of ten were observed depending on the system being used, even at a level of 10−3 g m−2 day−1 at 40°C and 90% relative humidity (RH).9 Moreover, the WVTR seems to differ when the measurement procedure differs, even when using the same system.11 Such variable factors are regarded as being more sensitive to lower WVTR values. On some occasions, a WVTR in excess of the system’s upper limit is observed due to damage caused by the handling of the barrier films. Especially, the area around the mounted film can become excessively stressed during the WVTR measurement. Unexpected peaks or sudden increases or decreases in the WVTR can occasionally be observed, although the cause is unknown. Another problem with the WVTR measurements is that it takes a very long time for the system to stabilize when dealing with lower WVTR values, especially in the case of multi-layered barrier films. For these reasons, comparative or repeatable measurements can be difficult.

The purpose of this study is to develop a reliable technique for evaluating barrier films. In other words, it is necessary to provide a standard evaluation procedure for a given barrier film to obtain the same WVTR value by different measurement systems. Standard films are also required to calibrate various measurement systems. In this paper, first we describe the fundamentals of the WVTR measurement systems, followed by the measurement procedures in the experimental section. A series of reference films were prepared in preparation for standard films development to enable repeated measurements, although they were unlike typical barrier films. The advantages of using the reference films are explained in detail. With the goal of developing a standard evaluation procedure for the WVTR measurements with different systems, the following factors were investigated. First, the amount of water vapor that is unrelated to the transmission through a film was identified by leak tests for all systems. Second, the reference films were inspected and their endurance and repeatability for WVTR measurements were confirmed. Third, the influence of the residual water affecting the WVTR was analyzed for different preheating conditions. A comparison of several WVTR measurement systems was performed using the common evaluation procedure developed in this study. The validity of the WVTR values obtained with the different systems as well as different sample sizes is discussed. Because the WVTR is measured in the steady state condition of the system, the factors that can affect the measurement time were also analyzed by comparing the lag times of each system.

A schematic of a typical WVTR measurement system is shown in Fig. 1. The systems commonly consist of a transmission unit and a detector. The transmission unit consists of a transmission cell with feed and detection chambers, with a film mounted between them. A sample film is cut into a larger piece than the inner area of the chamber and sealed using O-rings, gaskets, or grease. The sealing method has been steadily improved to reduce the leakage of water vapor into the detection chamber and is a notable feature of each system. The evaluation area shown in Fig. 1(b) is required to calculate the WVTR.

FIG. 1.

(a) Schematic of typical WVTR measurement system. The figure shows an example of a carrier gas system. The transmission unit is surrounded by dotted lines. (b) Schematic of transmission cell. Sample is mounted in the transmission cell with sealing materials. The evaluation area is also indicated. The systems are generally classified into two types, namely, the equal-pressure method and differential-pressure method.

FIG. 1.

(a) Schematic of typical WVTR measurement system. The figure shows an example of a carrier gas system. The transmission unit is surrounded by dotted lines. (b) Schematic of transmission cell. Sample is mounted in the transmission cell with sealing materials. The evaluation area is also indicated. The systems are generally classified into two types, namely, the equal-pressure method and differential-pressure method.

Close modal

The WVTR measurement systems are generally classified into one of two categories, namely, those using the equal-pressure method and those using the differential-pressure method. In a typical equal-pressure system, the water vapor is carried by an inert gas such as nitrogen or argon. The flow rate of the carrier gas is controlled to maintain the same total pressure in both the feed and detection chambers. In this method, the sample is not subjected to stress due to the pressure difference. On the other hand, the total pressure in each chamber is not equal in those systems based on the differential-pressure method. Both the feed and detection chambers are initially evacuated, and then water vapor is supplied to the feed chamber only. A pressure difference arises between the two chambers. With either method, the partial pressure difference of the water vapor between the feed and detection chambers should be identical under constant temperature and humidity conditions. This means that the force driving the water vapor into a sample is equal Therefore, the obtained WVTR should be identical regardless of which method is used, unless there are variable factors that can be attributed to the differences between the methods.

The process whereby the water vapor is introduced into the feed chamber differs between the systems. For example, the source of the water vapor is held at a different temperature to that in the transmission cell in order to introduce a given humidity into the feed chamber. The water vapor is transmitted into the sample, diffuses, and then effluents form in the sample over time. The effluent is analyzed by the likes of mass spectrometry, infrared spectroscopy, pressure sensors, coulometric sensors, and so on. Because the detected properties are of different dimensions depending on the detectors, the flux, J, having the same WVTR dimensions in units of g m−2 day−1 is calculated as follows. For example, a current caused by water molecule is obtained by mass spectrometry. It is converted to the molar fraction of water vapor, C [mol mol−1], using a calibration curve prepared in advance. The flux is calculated as

J = M w C F A ,
(1)

where Mw [g mol−1] is the molecular weight of the water, F [mol day−1] is the gas flow rate in the detection chamber, and A [m2] is the evaluation area of the film. The flux can also be derived from the pressure difference detected by a pressure sensor.

The WVTR measurement systems addressed by this study are summarized in Table I. Two equal-pressure systems (atmospheric pressure ionization mass spectrometry (API-MS) and cavity ring-down spectroscopy (CRDS)) and a differential pressure system (DP) were investigated. The API-MS system, which is based on the equal-pressure method, was developed by NIPPON API CO., Ltd. A feature of the API-MS detector is that it can ionize molecules under atmospheric pressure, unlike typical quadrupole mass spectrometers. This makes the detector more sensitive (the minimum detection limit of a molar fraction of water vapor is less than 10 pmol mol−1 (ppt)) and also makes the equal-pressure system easier to apply. The calibration curve was obtained by using standard nitrogen gas mixed with a molar fraction of 10 μmol mol−1 (ppm) water vapor. Unfortunately, the gas cannot be stably diluted to a molar fraction of less than 5 nmol mol−1 (ppb) in our calibration system. Therefore, when a lower level is to be measured, the molar fraction is extrapolated. A controlled concentration of water vapor in nitrogen is supplied to the feed chamber. Ultra-pure nitrogen is induced into the detection chamber, and water vapor transmitted through the film is carried to the detector. The effluent from the detection chamber is analyzed to determine the molar fraction of the water vapor. Two types of transmission cells are offered to accommodate different sizes of samples (60-mm and 100-mm diameter). The evaluation areas of these cells are 50 mm and 90 mm in diameter, respectively, when the samples are mounted. For clarity, they are described below as API-MS(50) and API-MS(90).

TABLE I.

Specifications of WVTR measurement systems compared in this study.

API-MS System CRDS System DP System
Abbreviation  API-MS (50)  API-MS (90)  CRDS-1 (50)  CRDS-2 (90)  DP(80) 
Transmission method  Equal-Pressure Method  Differential-Pressure Method 
Type of detector  Atmospheric pressure ionization mass spectrometry  Cavity ring-down spectroscopy  Diaphragm gauge 
Model of detector  FLEX-MS400 (NIPPON API Co.)  LaserTrace 2.5 (Tiger Optics LLC)  LaserTrace 3, (Tiger Optics LLC)  Baratron MKS121A (MKS Instruments, Inc.) 
Limit of detection  Below 10 pmol mol−1  500 pmol mol−1  250 pmol mol−1  0.2 Pa 
Measurement Temperature [°C]  Room temp. − 100  Room temp. − 60  Room temp. − 150 
Feed side relative humidity [%]  0 − 90  0 − 98 
WV supply method  Nitrogen with a given humidity is used  Pure water vapor 20 − 40 kPa corresponding RH is introduced 
Detection side preparation  Dry nitrogen with impurity level below 50 pmol mol−1 is used.  Evacuate below 100 Pa. 
Evaluation area in diameter [mm]  50 mm  90 mm  50 mm  90 mm  80 mm 
Sample diameter [mm]  60 mm  100 mm  60 mm  100 mm  100 mm 
Measurement range, WVTR [g m−2 day−1]a  10−2 − 10−5  10−3 − 10−6  100 − 10−4  10−1 − 10−5  102 − 10−5 
Commercial System  API-BARRIER (NIPPON API Co.)  Not available as a WVTR system  DELTAPERM - UH (Technolox Ltd.) 
Advantages 
  • Highly sensitive detector

  • No stress applied due to pressure difference

  • Method is described in ISO 15106-6

 
  • SI-traceable detector

  • No stress applied due to pressure difference

 
  • Sensitive pressure sensor

  • No carrier gas required

  • Method is described in ISO 15106-5

 
API-MS System CRDS System DP System
Abbreviation  API-MS (50)  API-MS (90)  CRDS-1 (50)  CRDS-2 (90)  DP(80) 
Transmission method  Equal-Pressure Method  Differential-Pressure Method 
Type of detector  Atmospheric pressure ionization mass spectrometry  Cavity ring-down spectroscopy  Diaphragm gauge 
Model of detector  FLEX-MS400 (NIPPON API Co.)  LaserTrace 2.5 (Tiger Optics LLC)  LaserTrace 3, (Tiger Optics LLC)  Baratron MKS121A (MKS Instruments, Inc.) 
Limit of detection  Below 10 pmol mol−1  500 pmol mol−1  250 pmol mol−1  0.2 Pa 
Measurement Temperature [°C]  Room temp. − 100  Room temp. − 60  Room temp. − 150 
Feed side relative humidity [%]  0 − 90  0 − 98 
WV supply method  Nitrogen with a given humidity is used  Pure water vapor 20 − 40 kPa corresponding RH is introduced 
Detection side preparation  Dry nitrogen with impurity level below 50 pmol mol−1 is used.  Evacuate below 100 Pa. 
Evaluation area in diameter [mm]  50 mm  90 mm  50 mm  90 mm  80 mm 
Sample diameter [mm]  60 mm  100 mm  60 mm  100 mm  100 mm 
Measurement range, WVTR [g m−2 day−1]a  10−2 − 10−5  10−3 − 10−6  100 − 10−4  10−1 − 10−5  102 − 10−5 
Commercial System  API-BARRIER (NIPPON API Co.)  Not available as a WVTR system  DELTAPERM - UH (Technolox Ltd.) 
Advantages 
  • Highly sensitive detector

  • No stress applied due to pressure difference

  • Method is described in ISO 15106-6

 
  • SI-traceable detector

  • No stress applied due to pressure difference

 
  • Sensitive pressure sensor

  • No carrier gas required

  • Method is described in ISO 15106-5

 
a

Rough estimates based on brochure specifications.

The CRDS system is not a commercially available product but was assembled from a CRDS detector12 and a transmission unit to configure a barrier evaluation system. These CRDS detectors were developed by Tiger Optics LLC. Two CRDS detectors (CRDS-1 and CRDS-2) are offered, differing in their lower detection limit, as shown in Table I. These detectors were calibrated within a range of 12 to 100 nmol mol−1 by the Chemicals Evaluation and Research Institute (CERI) of Japan in a manner conforming to the International System of Units (SI). The molar fraction is extrapolated as well as the API-MS system when they are less than 12 nmol mol−1. An example of a barrier evaluation system using a CRDS detector is introduced in Ref. 8. The transmission unit prepared in this study was, in principle, the same as that used by the API-MS system. The measurement involves almost the same procedure as that for the API-MS system. This proves the design of the API-MS detector provided there is consistency between the two systems. In this respect, a notable advantage of the CRDS detector is its SI-traceability. Two sizes of transmission cells are offered in addition to those of the API-MS system. CRDS-1 and CRDS-2 detectors are provided for 60-mm and 100-mm diameter samples, for which the evaluation areas are 50 mm and 90 mm in diameter, respectively. The descriptions below refer to these as CRDS-1(50) and CRDS-2(90).

The DP system is offered as a commercial system, named DELTAPERM, which was developed by Technolox Ltd.6 Unlike the two systems described above, it relies on the differential-pressure method, and no carrier gasses are required. Pressure sensors are used as the water vapor detectors. Both the feed and detection chambers are evacuated and closed, and then a controlled amount of water vapor is supplied to the feed chamber. The slope of the pressure curve as a function of time in the detection chamber is measured to determine the WVTR. It is not necessary to calibrate the absolute value for the pressure sensors because only pressure differences are used to calculate the WVTR. The DP system can mount 100-mm diameter samples with an evaluation area 80 mm in diameter. This is described as DP(80), below.

A sample barrier film was mounted in the transmission cell of each system. For every sample, the barrier-layer side was oriented toward the detection chamber to eliminate the influence of the transmission direction. There is theoretically no difference in WVTR, regardless of the transmission direction although, in fact, it could vary due to transmission around the edges of the sample. The systems were heated to reduce the amount of residual gas in the system, especially from within the samples. The WVTR can be affected by residual water in the samples, but to what degree is unknown. Comparisons of the preheating conditions were carried out and are discussed in section III C.

The value measured in the equilibrium state was recorded as the background flux, Jbg. The water vapor concentration or pressure in the effluent from the detection chamber starts to increase after a relative humidity of 90% is introduced into the feed chamber. This increase at the beginning of the measurement is evidence that the water vapor in the effluent originates from neither the outgas from the inner surface of the cell nor from the film itself. Nor does it result from a leakage of air. Rather, it is a result of transmission through the film. The measurements are continued until the flux stabilizes at a constant value, or a steady state. It is also difficult to judge the steady state because the detection value usually deviates to some extent with time. Because there is no standard judgment for the steady state in WVTR measurements, the following procedure was applied in this study. The average over one hour was calculated and checked to determine whether the serial differences are less than 3% for more than one fourth of the time that elapses. This judgment is temporary and may be modified later in accordance with the accumulated measurement data. The average during the steady state was recorded as the steady state flux, Jss.

The WVTR is determined as follows:

WV TR = J ss J bg .
(2)

Thus, the WVTR is defined as the incremental difference in the flux after the introduction of the water vapor.

The lag time was also analyzed. This was theoretically associated with the delay needed to reach the steady state. The derivation is explained, e.g. in the literature.11 According to the Fick’s second law, the fluence transmitted through a single-barrier layer film, Q is,

Q t = D t C 1 l l C 1 6 2 l C 1 π 2 n = 1 1 n n 2 exp D n 2 π 2 t l 2 ,
(3)

where D is the diffusivity of the vapor in a polymer film. C1 is the concentration of the vapor in the feed chamber while that in the detection chamber is constitutively zero, l is the thickness of the polymer film, and t is the time. As t becomes large (in the steady state), Eq. (3) can be reduced to the following equation,

Q t = D C 1 l t l 2 6 D .
(4)

The lag time, L is defined as an offset by a delay,

L = l 2 6 D .
(5)

An operative derivation is described in section III E.

A series of sample films, used as reference films, were prepared. A schematic of the reference films is shown in Fig. 2. They consist of aluminum foil, which is the actual barrier layer, attached to a polyethylene terephthalate (PET) base film with an adhesive layer (AL-PET®). The thickness of the aluminum foil is 30 μm, while that of the PET is 100 μm and that of adhesive is 3 μm. The samples are cut to diameters of either 60 mm or 100 mm. An artificial pinhole is created in the center of the A1 layer by etching. The water vapor can permeate into the PET and adhesive layers but is blocked by the aluminum foil. Water vapor can only pass through the artificial pinhole, such that the flux from the reference film is limited depending on the size of the pinhole. Because the size and position of the pinhole can be varied, the base film must have uniform water vapor barrier properties. In addition, it is preferable for the barrier properties to be stable over time and for them to be well documented. From these viewpoints, a PET film was selected as the base film for this study.

FIG. 2.

Schematic of reference films; (a) top view (b) cross-section AA’ across the center of the pinhole.

FIG. 2.

Schematic of reference films; (a) top view (b) cross-section AA’ across the center of the pinhole.

Close modal

The features of the films are as follows:

(1) The films are robust and thus can be reused. In the comparison described below, the same single WVTR film is mounted in the three different systems, so that differences between the individual systems can be avoided. Even among barrier films from the same batch, defect sizes and distributions in the barrier layers could be uneven which would lead to dispersion in the WVTR values. Moreover, deterioration of the barrier films could also be expected to affect the dispersion. Most comparisons in the literature use a different film for each evaluation because the films could be damaged by stresses arising during the measurement process. Therefore, the differences in the WVTR values output by each measurement system consist not only of the difference in the method but also the individual film dispersions, individual system differences, etc., which are difficult to separate from the experimental data.

(2) The evaluations are simple and quick to perform because of the simple structure of the systems. A low WVTR value could be achieved by restricting the pinhole size. As a result, this should lead to simple transmission behaviors such as a monotonic increase that is easy to describe, even when using an analytical function. Furthermore, the lag incurred when detecting the transferred water vapor after the introduction of water vapor to the feed would be short relative to common barrier films. This would allow for a series of rapid evaluations. Most barrier films with multilayered structures exhibit excellent barrier properties but complicated behaviors and long lag times during a WVTR evaluation.

(3) The WVTR can also be controlled by the evaluation area of the film. Because the flux of the effluent is determined only by the size of the pinhole, the larger evaluation area makes the WVTR lower. We prepared two sizes of transmission cells in order to confirm this advantage. This is advantageous for the reference films but not for typical barrier films. The total area of the pinholes increases linearly with an increase in the evaluation area of a barrier film that is assumed to have been fabricated uniformly over the whole area. In this sense, the WVTR, for which a flux per unit area is defined, remains constant even if the evaluation area increases.

Leak tests were performed for each system in order to determine the amount of water vapor escaping from the system, as well as impurities in the carrier gas, leakage from the cell mountings and pipe joints, and desorption from the surface of the system. A stainless sheet was mounted in the transmission cell so that the water vapor cannot pass from the feed to the detection chambers. The systems were heated to 60 °C and then ultra-pure nitrogen gas was introduced into both the feed and detection chambers of the API-MS and CRDS systems. Meanwhile, for the DP system, the chamber was evacuated at 70 °C. After at least 48 hours, the systems were held at 40 °C and a relative humidity of 90% was introduced into the feed chamber. The water vapor was monitored until it reached the equilibrium state. The judgment of the equilibrium state is as same as that for the steady state mentioned above. Typical fluxes are about 6 × 10−6 g m−2 day−1 for API-MS(90), about 2 × 10−5 g m−2 day−1 for CRDS-2(90), and about 4 × 10−5 g m−2 day−1 for DP (80). The values can be varied depending on the system conditions. Except for API-MS(90), the values could be dominated by the limits imposed by the detectors. The degree by which the amount of escaping water vapor has been reduced has not yet been identified for the measurement of a certain WVTR. To date, it has been confirmed that the value does not vary much every several months.

The WVTR measurement for the AL-PET® (a reference film without any artificial pinholes) was performed using the API-MS(90) system in advance. No effluent due to the transmission of the water vapor was observed for at least 50 hours, which confirmed that the WVTR was below a level of 10−6 g m−2 day−1 (the detection limit of the API-MS(90) system). It seems that the thickness of the Al layer (30 μm) is sufficiently impermeable for our evaluation range.

Different sizes of artificial pinhole were created with design range WVTR of 10−5 to 10−1 g m−2 day−1. The sizes of the pinholes were observed using an Olympus LEXT OLS4000 laser microscope. A representative image of a reference film, for which the target diameter of the pinhole is 0.1 mm, is shown in Fig. 3(a). The reflection intensity of the depth profile in the Al layer was obtained (Fig. 3(b)). The area was measured at the bottom of the pinhole as determined by the depth profile, and 1.3 × 10−8 m2 was obtained from Fig. 3(c). The opening equivalent circle diameter was also calculated from the area. This was 0.13 mm. The difference between the target and actual diameters arises from the difficulty of etching such a small pinhole. The results of all the measurements are listed in Table II. A careful visual inspection was also performed on the reference films in order to eliminate any unexpected effects on the WVTR measurements such as scratches, flaws, extra pinholes other than the artificial one, roughness in the mounting area, and so on.

FIG. 3.

(a) Artificial pinhole of 0.1-mm target equivalent circle diameter on 100-mm film observed by laser microscope. (b) Depth profile for reflection intensity across the pinhole detected by laser microscope. Threshold depth is determined analytically using the profile. (c) Opening area (colored white) determined from the depth profile.

FIG. 3.

(a) Artificial pinhole of 0.1-mm target equivalent circle diameter on 100-mm film observed by laser microscope. (b) Depth profile for reflection intensity across the pinhole detected by laser microscope. Threshold depth is determined analytically using the profile. (c) Opening area (colored white) determined from the depth profile.

Close modal
TABLE II.

Dimensions of pinhole created in barrier layer for a series of reference films.

Sample diameter (mm) Equivalent circle diameter of pinhole (mm) Opening area of pinhole (m2)
60  0.11  1.0 × 10−8 
  0.20  3.1 × 10−8 
  0.31  7.4 × 10−8 
  0.51  2.0 × 10−7 
  0.65  3.3 × 10−7 
  1.01  7.9 × 10−7 
  1.27  1.3 × 10−6 
  1.95  3.0 × 10−6 
  3.88  1.2 × 10−5 
  9.64  7.3 × 10−5 
100  0.10  7.6 × 10−9 
  0.13  1.3 × 10−8 
  0.14  1.6 × 10−8 
  0.18  2.4 × 10−8 
  0.23  4.3 × 10−8 
  0.41  1.3 × 10−7 
  1.15  1.0 × 10−6 
  1.98  3.1 × 10−6 
  4.94  1.9 × 10−5 
  9.64  7.3 × 10−5 
Sample diameter (mm) Equivalent circle diameter of pinhole (mm) Opening area of pinhole (m2)
60  0.11  1.0 × 10−8 
  0.20  3.1 × 10−8 
  0.31  7.4 × 10−8 
  0.51  2.0 × 10−7 
  0.65  3.3 × 10−7 
  1.01  7.9 × 10−7 
  1.27  1.3 × 10−6 
  1.95  3.0 × 10−6 
  3.88  1.2 × 10−5 
  9.64  7.3 × 10−5 
100  0.10  7.6 × 10−9 
  0.13  1.3 × 10−8 
  0.14  1.6 × 10−8 
  0.18  2.4 × 10−8 
  0.23  4.3 × 10−8 
  0.41  1.3 × 10−7 
  1.15  1.0 × 10−6 
  1.98  3.1 × 10−6 
  4.94  1.9 × 10−5 
  9.64  7.3 × 10−5 

As mentioned above, the barrier layer of a typical barrier film could be damaged by handling. This can sometimes lead to the failure of a measurement. An endurance test for a reference film was performed to confirm the reliability of a film when subjected to against repeated measurements. The 100-mm film with a 0.23-mm equivalent circle diameter pinhole was chosen as a representative sample for this test. Because the DP system, unlike the others uses grease to mount the sample, the test was performed using the API-MS(90) and CRDS-2(90) systems to prevent from contamination of systems. The transmission curves obtained over one year are shown in Fig. 4(a). The time dependency of WVTR, Jss, and Jbg is also plotted in Fig. 4(b). The WVTR values are found to be almost identical for the two systems, averaging 1.3 × 10−4 g m−2 day−1 with a standard deviation of 1.0 × 10−5 g m−2 day−1. The result indicates that this degree of deviation can be expected for the WVTR measurements, even when the same sample is being measured. This deviation could be caused by the difference in the background shown Fig. 4(b). However, this has not yet been clarified. In addition, a difference in the rising time of the flux can also be observed in Fig. 4(a). This could be due to the absorption of water vapor inside the measurement system, described in section III E. No correlation was observed between the WVTR and the measurement date. This proves not only the repeatability of the measurements but also the robustness of the films over an extended period.

FIG. 4.

(a) Transmission curves for reputation measurement for API-MS(90) and CRDS(90) systems using 100-mm reference film with a 0.23-mm pinhole. Fluxes, J were calculated using Eq.1. The water vapor was introduced at a 0 h. The backgrounds for all of the measurements were subtracted from the fluxes (JJbg). (b) Time dependency of WVTR, Jss, and Jbg of the reputation measurements for one year. No correlation between WVTR and measurement dates was found.

FIG. 4.

(a) Transmission curves for reputation measurement for API-MS(90) and CRDS(90) systems using 100-mm reference film with a 0.23-mm pinhole. Fluxes, J were calculated using Eq.1. The water vapor was introduced at a 0 h. The backgrounds for all of the measurements were subtracted from the fluxes (JJbg). (b) Time dependency of WVTR, Jss, and Jbg of the reputation measurements for one year. No correlation between WVTR and measurement dates was found.

Close modal

In the evaluation methods for the API-MS and CRDS systems, water in a sample is preferably eliminated by heating before taking the measurements. The preheating condition, however, is not regulated for other WVTR systems including the DP system. The measurements sometimes begin immediately or with conditioning for very short time after a sample is mounted in a system. In our previous study, two extreme conditions were compared and the difference in the WVTR was found due to preheating conditions using the DP system and the reference film.13 In the first case, the sample was placed in a vacuum with a pressure of less than 100 Pa for 2 hours at 40 °C before the measurement was taken. We can assume that there is residual water in the sample when water vapor is introduced in this case. The short drying time (2 hours) is applied so as not to exceed the detection limit of the system. In the second case, the sample was placed in a vacuum for 24 hours at 70 °C so that the residual water in the sample would evaporate sufficiently. This condition is comparable to the typical method for the API-MS and CRDS systems. The fluxes, J were measured for both preheating conditions for the 100-mm reference film (the evaluation area for the DP system is 80 mm in diameter) with a 0.23-mm pinhole. In this study, the same comparisons were performed on samples with other sizes of pinhole in order to investigate the dependency between the differences resulting from the preheating conditions and the WVTR. The steady-state flux, Jss under two different conditions is plotted as a function of the opening area of the pinhole in Fig. 5. They are almost identical when in excess of 1 × 10−3 g m−2 day−1, although a difference of about 30% can be seen when the values are lower. The difference in the WVTR derived using Eq. (2) would become much larger because Jbg for the first condition is larger than those for the second condition. Thus, it is possible to observe a trend whereby the WVTR value for the DP system, when measured with an insufficiently dry condition, is lower than that obtained with a sufficiently dry condition. The results also illustrated the importance of eliminating variable factors with lower WVTR measurements.

FIG. 5.

Comparison of fluxes in steady state, Jss at 40°C and 90%RH with different preheating conditions using DP(80) system. For condition 1 (Δ), the sample was dried for 2 h at 40°C before the measurement. Residual water was expected in this case. For condition 2 (•), the sample was dried out sufficiently (for 24 h at 70 °C). A difference in the fluxes can be seen when the values are lower than 1 × 10−3 g m−2 day−1.

FIG. 5.

Comparison of fluxes in steady state, Jss at 40°C and 90%RH with different preheating conditions using DP(80) system. For condition 1 (Δ), the sample was dried for 2 h at 40°C before the measurement. Residual water was expected in this case. For condition 2 (•), the sample was dried out sufficiently (for 24 h at 70 °C). A difference in the fluxes can be seen when the values are lower than 1 × 10−3 g m−2 day−1.

Close modal

The Jbg of the second condition was close to that of the leak test for the DP system. This confirms that the sample was dried out sufficiently for the second condition, although the water remained in the sample in the first condition. This residual water could become a variable factor for the WVTR measurements. A large amount of flux was observed in the first case and then the flux decreases with time. It is obvious that this is due to the effluence of the water remaining in the sample. The amount of water vapor transmitted into the sample cannot be determined from this result. The residual water appears to be removed relatively quickly in the case of the reference films, given the large artificial pinhole and the base material. On the other hand, the Jss for insufficiently dry conditions could increase if the water vapor continues to be desorbed from a sample over an extended duration. This could be expected, for example, for a barrier film with multiple layers or one with a base material that is prone to absorbing moisture. Another possibility regarding variations in Jss with insufficiently dry conditions is that the gradient of the water vapor concentration, that is, the force driving transmission, decreases owing to the residual water around the detection-side surface of the film. This could be the reason why the fluxes for the second condition are lower in Fig. 5. Regardless, it is possible for the WVTR value to vary when the sample is not sufficiently dry. As much residual water as possible was eliminated from each system in our evaluation.

Comparative measurements were performed with reference films and the three different systems mentioned above. The WVTR values derived from Eq. (2) for the reference films are plotted logarithmically as a function of the opening area of the pinhole in Fig. 6(a). The WVTR value seems to be controlled by the area of the opening and the evaluation area, as anticipated. The results also proved the applicability of our reference films to comparative measurements in a wide range by various systems. The WVTR values were provisionally normalized to an evaluation area of 90 mm in diameter, WVTR(90), using the following equation in order to compare different systems with different evaluation areas.

WV TR 90 = WV TR × A A ( 90 ) ,
(6)

where A(90) is the evaluation area for the 90-mm diameter. The WVTR(90) values are also plotted in Fig. 6(b). The values were found to be in good agreement for three systems with different evaluation areas up to a value of 10−5 g m−2 day−1. The value, which is about 40% lower than others, is relatively large deviation for the 100-mm film with a 0.10-mm pinhole, as obtained with a DP(80) system. It is not clear whether this deviation is as a result of the different principle on which the system is based, or whether an error is introduced as a result of approaching the detection limit. The uncertainty in terms of the WVTR value has not yet been clarified because there are only a small number of measurements available for each system, although a degree of deviation for a given sample could be expected from the results of the endurance test described in section III B. It will be necessary, therefore, to develop an evaluation method to determine an accurate value for a reference film. Such efforts are ongoing.

FIG. 6.

Comparative measurements using reference films for different measurement systems. The measurements were performed at 40°C 90%RH. (a) WVTR obtained by each system, (b) normalized WVTR for an evaluation area of 90 mm in diameter, WVTR(90). The dashed line of the slope of 1 indicates the WVTR is proportional to the opening area, the dotted line of the slope of 0.5 indicates that the WVTR is proportional to the opening diameter.

FIG. 6.

Comparative measurements using reference films for different measurement systems. The measurements were performed at 40°C 90%RH. (a) WVTR obtained by each system, (b) normalized WVTR for an evaluation area of 90 mm in diameter, WVTR(90). The dashed line of the slope of 1 indicates the WVTR is proportional to the opening area, the dotted line of the slope of 0.5 indicates that the WVTR is proportional to the opening diameter.

Close modal

The values lie on a straight line with a slope of 1 for films with a pinhole larger than 1 mm, which means the WVTR is proportional to the area of the opening. However, below 1 mm, the values deviate from the straight line and approach a slope of 0.5, implying that the WVTR is proportional to the opening diameter. This behavior can be attributed to the water vapor diffusion not being perpendicular to the film surface.14 It should be noted that the relationship between the WVTR and opening area of the pinholes is theoretically anticipated for the reference films, while the results of the practical experiments are in good agreement. In theory, the threshold between these two slopes is 0.2 mm (opening of 3.1 × 10−8 m2), which means that r = l, where r is the radius of the pinhole and l is the thickness of the PET layer. Nevertheless, the threshold as obtained from the results appears to be about 0.7 mm (opening of 3.8 × 10−7 m2). This shift in the threshold could be caused by the adhesive between the Al layer and PET layer. The value of the WVTR is expected to increase more as the size of the pinhole decreases due to the presence of the adhesive. Because the adhesive is thin (3 μm), water vapor diffusion perpendicular to the film surface is not greatly affected even if the diffusion coefficient of the water vapor in the adhesive is larger than that in the PET film. On the other hand, diffusion in plane direction to the adhesive layer could be attributed more to the WVTR with smaller pinholes like those mentioned above. However, further investigation is required to prove the influence of the existence of the adhesive.

The WVTR is obtained in the steady state, which means that the measurement has to be continued until the system becomes stable. The long measurement time is one of the problems of WVTR measurement. The lag time of each system has been analyzed to investigate the factors that affect the measurement time. Fig. 7(a) shows a representative time-dependent flux transmitted though the reference film of 0.23 mm by the API-MS(90) system. The flux, J can be integrated over time to give the fluence, Q, and the lag time, L = 5 h is obtained in Fig. 7(b). The lag times for all the measurements were also calculated and plotted as a function of the opening area in Fig. 8(a). The result shows that the lag time increases as the opening decreases for each system. This could be explained by the length of the diffusion path for the water vapor through the reference film becoming longer as the pinhole is made smaller.

FIG. 7.

(a) Representative time-dependent flux transmitted through a reference film of 0.23 mm by the API-MS(90) system. (b) The flux can be integrated over time to give the total mass, Q. The dashed line is a tangent line in the steady state. The lag time, L is defined as the intersection with the x-axis, and L = 5 h is obtained.

FIG. 7.

(a) Representative time-dependent flux transmitted through a reference film of 0.23 mm by the API-MS(90) system. (b) The flux can be integrated over time to give the total mass, Q. The dashed line is a tangent line in the steady state. The lag time, L is defined as the intersection with the x-axis, and L = 5 h is obtained.

Close modal
FIG. 8.

(a) Lag time of all measurements as a function of the opening area for both log scales. (b) Lag time of the API-MS system, extracted from the results shown in Fig. 8(a), for comparison with different sizes of transmission cells. The y-axis is plotted as a linear scale for clarity. The dashed curves merely provide an indication. The lag time increases with the size of the chambers.

FIG. 8.

(a) Lag time of all measurements as a function of the opening area for both log scales. (b) Lag time of the API-MS system, extracted from the results shown in Fig. 8(a), for comparison with different sizes of transmission cells. The y-axis is plotted as a linear scale for clarity. The dashed curves merely provide an indication. The lag time increases with the size of the chambers.

Close modal

Differences in the lag time can be observed between the systems and different evaluation areas in Fig. 8(a), for a given opening area. Because the lag time, L for a given sample should be identical under the same measurement conditions according to Eq. (5), the differences seem to stem from the delay required for each system to stabilize. When a system is dried out in advance, the transmitted water vapor would initially attach to the surfaces of the sample, chambers, and pipes, and then some of the vapor would detach from the surfaces and migrate to the detector. There is a delay in the WVTR entering the steady state because it takes time for the system to reach an adsorption-desorption equilibrium state. That is to say, the lag time obtained experimentally contains the lag incurred by the transmission of a film defined by Eq. (5) as well as the lag required for each system to stabilize. The time required to reach the equilibrium state varies with the conditions existing in the system, such as the pressure, WVTR, and so on. Moreover, the size of the chamber would affect the time because the amount of adsorbed water increases with the size of the chamber. Fig. 8(b) shows the lag time for the API-MS with different sizes of chambers, as extracted from Fig. 8(a) and converted to a linear scale to clarify the difference. The difference between the two chambers is only the internal surface area and there are no differences in the materials or their surface treatments. This, unfortunately, is not so clear due to scattering and the smaller number of points, but there appears to be a trend in that the lag time increases with the size of the chambers (API-MS(90) >API-MS(50)). A quantitative evaluation of the amount of adsorbed water will be performed in the near future to investigate the water vapor flux in the transient state.

The lag incurred with the DP system is obviously larger than that for incurred with the other two systems. For the API-MS and CRDS systems, the transmitted water vapor is carried into the detector by nitrogen gas. Meanwhile, the transmitted water vapor accumulates in the detection chamber of the DP system. The amount of adsorbed water vapor increases as their pressure increases. This could be the reason why it takes so long for this system to reach the adsorption-desorption equilibrium state.

A technique for evaluating the properties of a water vapor barrier, based on the use of a series of reference films, has been developed. The variable factors affecting the evaluation have been extracted and successfully eliminated. These are mainly individual differences in the barrier films and the residual water existing in a sample before measurements.

Differences in the WVTR due to preheating conditions were investigated using the DP system and the reference films. The influence of the residual water on the variations in the WVTR value increases as the value of WVTR decreases. The results imply the possibility of decreasing driving force for water vapor transmission by residual water.

Comparative measurements show that consistency between the systems in terms of the WVTR is achieved at a level of 10−5 g m−2 day−1 at 40 °C and 90%RH, provided the measurements have been undertaken correctly. It is notable that the relationship between the WVTR and opening area of a pinhole, which was theoretically anticipated for the reference films, is also in good agreement with the practical results.

The lag time was analyzed to compare the differences in the measurement time between the systems. The results showed that the time required to reach the adsorption-desorption equilibrium state for a system can affect the measurement time.

A number of comparative or repeated measurements proved the reliability of our evaluation technique. However, the WVTR values obtained in this study still feature a degree of uncertainty. The development of an evaluation method for determining an accurate value for a reference film is ongoing. Such films can be used as standard films in the calibration of various WVTR measurement systems. The improvement of the WVTR evaluation would encourage developments of high-barrier film fabrication for OLED encapsulation.

This development was partly carried out under “Development of Fundamental Evaluation Technology for Next-generation chemical Materials” commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan. The authors would like to thank Hisashi Abe, Minami Amano, Koji Hashiguchi of AIST for their numerous useful discussions especially on the CRDS systems. The authors are also grateful to Hideyuki Negishi of AIST for supporting the laser microscope observations and critically reading the manuscript.

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