The UKDL KTN and the Printing and Graphics Science Group of the IOP organized a one-day seminar on measuring techniques for high barrier films.
Speakers from around Europe gave an overview of experimental techniques as well as simulation methods that would allow the accurate calculation of the permeability of different barriers developed. The result was a stimulating event that exposed the need for calibration of different techniques in order to get meaningful results from each one, as well as the fact that current techniques have the potential to accurately measure the low permeabilities that are required for flexible barriers for applications such as OLEDs and organic photovoltaics.
Dr Sharon Louch of the CPI for example, who has been doing a lot of work on preparation of barriers as well as measurements through the use of the Calcium test, reported the measurement of barriers that can achieve permeabilities as low as 10-5 g/m2.day. Dr Louch, who collaborates with DuPont Teijin Films in the development of flexible substrates, reported that the addition of an organic planarizing layer in combination with inorganic layers (e.g. alumina) is a very efficient way to lower the level of moisture permeation.
Dr Hazel Assender of Oxford University reported similar improvements when adding a layer of acrylate on alumina, which smoothes out the surface of the layer and lowers the number of pinholes on it.
The calcium test
Calcium is commonly employed in organic electronic devices and its degradation as it reacts with water inevitably leads to decline in performance or failure of such devices. In recent years this effect has been "turned on its head" by using calcium as a sensor to detect and measure moisture permeation (usually called Water Vapor Transmission rate: WVTR) and lag time through flexible barriers. The usual testing conditions are set at 60 ˚C and 90% humidity. Calcium that reacts with humidity that permeates through the barrier becomes transparent; hence, image analysis helps with the definition of the WVTR. A major advantage of the calcium test is its ability to also identify the exact failure mode, whether that's ingress through pinholes, defects in the barrier or diffusion across the whole area of the barrier film. All this is valuable information when looking to improve the performance of these types of barrier films.
Other barrier Measurement methods
The Mocon AQUATRAN seems to be the industry standard when it comes to measuring WVTR although one of its disadvantages is the long times each measurement takes, approximately a week. Mocon, which is based in Minneapolis, MN, has also developed the OXTRAN, which is used for oxygen permeation measurements (OTR).
Stephane Cros of the CEA in France presented at the seminar, describing the high sensitivity permeameter that has been developed there that measures both WVTR and OTR simultaneously, using mass spectrometer detection and isotope labeling. The apparatus is capable of measuring permeabilities down to 5.10-4 g/m2 a day and was patented in 2006.
Dr Jasper Michels of the Holst Centre in the Netherlands reported on the use of "Black Spot Analysis", which is a highly sensitive method of measuring permeation through barriers for OLEDs. Using layers of SiN, and intermediate organic planarization layers, the final film ends up having de-coupled pinholes, which lead to the extension of the lifetime of the OLED material, by giving moisture a more "complex" path in order to reach the sensitive active material. This method can also give a qualitative way to categorize the different types of black spots that appear on the OLED material: fast growing, slow growing and non-growing ones, each one of them describing a different type of defect on the barrier layer.
The correlation of the different values of permeability acquired from different tests and the need for calibration became apparent when different tests resulted in the acquisition of different values of permeability for the same materials. Calibration is therefore necessary in order to make sure that the numbers quoted are actually meaningful and determine specific properties rather than being apparatus.
For more attend Printed Electronics USA 2008.
