A British partnership working within a European collaboration is attempting to take advantage of these technologies by demonstrating flexible organic photovoltaic (OPV) panels that can be bent thousands of times with no reduction in performance.
Flexible solar cells are lightweight due to the lack of heavy glass sheets and metal frames, significantly reducing transportation and deployment costs. Benefits from this energy portability include delivery of electrical power during disaster relief, road work sites, open air festivals and military applications, amongst many others. Additionally, organic PVs work very well in harvesting energy indoors with higher conversion efficiency than conventional silicon photovoltaics. Adding flexibility also opens up interesting possibilities to use OPVs to power the internet-of-things (IoT), a concept where multiple sensors inside buildings will be in constant communication.
An EU project called TREASORES, a 3-year EU funded initiative to develop the flexible electrode and barrier layers for roll-to-roll manufacturing of bendable solar cells and lighting, had the aim of scaling up these exciting photovoltaic technologies, as well as dramatically reducing the cost of production. The project, led by the Swiss organisation EMPA, pooled the know-how of nine companies and six research institutions across five nations to do this, creating seven patents and a dozen peer-reviewed publications laying down the science behind the technology.
A major roadblock to scaling up these technologies is whether the device performance, electrical properties, or module lifetime are affected by repeated bending of the devices; a key criteria when the cornerstone of the product is its flexibility. Yet there are currently no industry standards available for testing the mechanical stability of flexible solar cells, limiting customer trust and therefore whether they can be brought to market.
The manufacturers of the flexible solar cells, Eight19 Ltd, as part of the TREASORES initiative, partnered with the UK’s national measurement institute, the National Physical Laboratory (NPL) to develop such a test. NPL is a world leading centre dedicated to developing and applying the most accurate measurement standards, science and technology to build trust in industry. This trust is delivered through standards that are critical to commercial research and development.
Eight19 manufactures the flexible organic photovoltaic modules by printing the active layers on a plastic barrier material, and sandwiching the final result between transparent sheets. Eight19 and NPL partnered with flexible device manufacturers and conductive plastic substrate producers to develop a scalable, repeatable and consistent test method to measure the effect of repeated bending on these flexible devices and components, such as transparent electrodes.
Whilst testing how a solar cell performs when bent may appear simple, performing it reproducibly and in a manner which allows analysis of the failure mechanism is not. NPL developed a test apparatus that can measure electrical characteristics in-situ whilst a device is being bent, and can test the device reproducibly from flat conditions to a very tight bend with a radius as low as 2mm. Such a test is important as it allows early identification of failure and the investigation of changes in behaviour when the panels are curved. When Eight19’s flexible modules were tested with this system the devices didn’t exhibit any reduction in performance after 1,000 bends.
Another potential issue is that the bending of the module may fracture or damage the encapsulating adhesive that protects the device from oxygen and water in the air, affecting the module lifetime in the long term. However a blind accelerated aging test of as-produced modules and modules after being subjected to cyclic bending showed no difference in lifetime. These results are extremely encouraging for the commercialisation of flexible organic photovoltaic technologies. They are a clear indication of the reliability of the modules when under mechanical stress that is likely to be experienced during production, handling and installation, ensuring they will live up to customer expectations of robustness.
In the TREASORES project the NPL bending system has been essential for the development of the low-cost, flexible transparent electrode materials which reached the technology demonstrator stage during the project. The reproducible test is also a key milestone on the pathway to international standards for these flexible materials through the International Electrotechnical Commission.
Industrial development of organic photovoltaic devices has been on the rise thanks to recent demonstrations of long lifetimes and high efficiency panels. Eight19 and NPL have helped the technology take a step closer to commercialisation by demonstrating that such OPV panels are very mechanically robust. With independent and reliable verification of the technology in place, Eight19 will now develop the scalable process capability that will help make the OPV-based devices a commercial reality in off-grid and energy harvesting markets. Commercialised products will therefore soon be on sale, helping to drastically increase the use of solar panels as a clean source of energy.
The research leading to these results received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement 314068.
ABOUT THE AUTHOR
Dr George Dibb is a Higher Research Scientist within the Materials Division at NPL.
FURTHER INFORMATION
NPL, http://www.npl.co.uk/