Barrier layer assemblies
Traditional silicon-based photovoltaics needs some physical protection from the weather when mounted on the outside of a building. This is most commonly achieved by assembling the active layers between two sheets of rigid material, usually glass or plastic. These act as a physical barrier, preventing damage to the electrically-active assemblies. However, some of the newer PV technologies have additional needs to keep them stable; a chemical barrier shielding them from the external environment.
The reason being that some of the materials used in thin-film electronics are very sensitive to environmental exposure. The principal chemical entities causing the damage are ubiquitous – water and oxygen. Sheets of rigid glass provide a very effective chemical barrier to water and oxygen. But if market demands cause a move away from rigid assemblies, a barrier layer is required. The purpose of these barrier layers is to prevent the passage of these small molecules from the surroundings into the electronics assemblies.
Using the new thin-film technologies, photovoltaics can now be produced on flexible substrates. This allows new design freedoms and it is anticipated that this will become an increasingly important market sector. However, as these assemblies now have to move away from the protection afforded by rigid sheets of glass, additional chemical barrier shielding must be provided. This can be provided by a multilayer barrier film glued or deposited onto the product to be protected.
Multiple products needed for multilayer barrier
Multilayer barrier requires several types of products that also dictate the standardization regime. There are prefabricated barrier films that are sold with a self-adhesive layer to protect sensitive products. There are also barrier adhesives that allow assemblies to be constructed without providing a conduit for water and oxygen into the device.
There are two major electrotechnical product groups that have a need for barrier layers and it is useful to compare and contrast them from this perspective. The first is Organic Light Emitting Diode (OLED) for both display and lighting, the second is photovoltaics. OLED materials used for display in items such as smartphones and televisions and in emerging lighting modules exhibit a high sensitivity to atmospheric oxygen and water. However, they are in general used in a comparatively benign environment and in general are expected to have product lifetimes measured in small numbers of years.
Photovoltaics is a rather different proposition in terms of barrier performance expectations. The emerging thin-film technologies are rather less sensitive to water and oxygen than the materials used in OLED devices. However, these products need to survive in an outdoor environment for a service life of at least 20 years. As a result the overall performance requirements for barrier layer assemblies may actually be more similar than at first sight.
As a consequence the testing of barrier layer performance is a critical part of the assessment of the potential lifetime of OLED display and lighting and photovoltaics. And this illustrates the need for standardization in this area.
Standardization of barrier layer performance
Barrier layers are not used exclusively for electrotechnology. Indeed the main use of barrier layers is in plastic films incorporated into packaging to exclude moisture and oxygen from food products. Consequently the International Standards for the testing of these products have evolved not within the IEC but within ISO TC 61: Plastics. ISO TC 61/SC 11: Products, has two working groups (WGs) covering barrier layer technology. The first, WG 3: Plastics films and sheeting, is defining test methods for barrier layer assemblies. The second, WG 5: Polymeric adhesives, is starting a similar task for barrier layer adhesives.
However, the interest in this work is not confined to ISO. For example, the Organic Electronics Association (OE-A) is an active supporter of International Standards for flexible electronics and has established a WG entitled Encapsulation. Their goal is to further facilitate the International Standards development process by conducting round robin testing of adhesive and barrier layer materials. They have also organized workshops in this area.
There is also understandable interest in this work through the IEC TC structure. For example IEC TC 110: Electronic display devices, and IEC TC 82: Solar photovoltaic energy systems, have interest as barrier layer technology is important in these applications. In addition, WG 6 of IEC TC 47: Semiconductor Devices, has commenced work on a test method for barrier layer performance for flexible and stretchable semiconductor devices.
Barrier layer performance is also of importance in the field of printed electronics. As a result IEC TC 119: Printed Electronics, has resolved to start a liaison relationship with ISO TC 61/SC 11 to contribute to the work on barrier layer performance.
As photovoltaics moves into the area of flexible substrates, the use of printing techniques to manufacture electronics assemblies becomes an attractive prospect. This is because these techniques allow industry to fabricate devices and structures over a wide area. And printing processes are also amenable to roll-to-roll processing, as described in e-tech August 2016.
TC 119 is working on Standards for the terminology, materials, processes and equipment that will facilitate the industrialization of printed electronics. Printing could be a significant benefit to photovoltaics as it moves into new thin-film technology. For example, printing technology has the potential of significantly reducing the cost of CIGS (copper indium gallium selenide) photovoltaics over the alternative sputtering process. It can increase materials utilisation whilst facilitating high speed roll-to-roll deposition. As an example companies are aiming to develop a production-scale CIGS ink to accelerate the adoption of this technology.
Printing will also allow photovoltaics to be incorporated into other electronics systems into the future. The use of photovoltaics for energy harvesting at low light levels is currently attracting significant attention. The ability to replace the thick and rigid coin cell batteries with a combination of printed photovoltaics and energy storage could facilitate a number of applications.
Wearable devices, as described in e-tech January 2016 is one area where this could become important and where standardization is needed.
IEC SMB Strategy Group (SG) 10: Wearable Smart Devices, has resulted in an SMB decision to create a new TC in this area, and working with other Technical Committees, IEC TC 119 will be contributing to this. As an example of this contribution IEC TC 119 has published IEC TR 62899-250:2016, a technical report on the material technologies required in printed electronics for wearable smart devices. Further work is now being initiated on standards documents covering materials for flexible wearable smart devices, such as stretchable substrates and inks.
PV technology potential move into wearables
As PV technology moves into flexible form factors, new technologies such as barrier layers and printed electronics become a priority. International Standards are developed to support this from a number of directions. There is the potential for these new photovoltaics to move into wearable electronics too and structures within the IEC are growing to accommodate this.