'Longlife' organic solar cells embedded in glass

10 Jul 2014

Fraunhofer IAP works with Corning glass to extend life of carbon-based PV system.

While organic solar modules have some performance advantages over silicon solar cells, one critical problem is their shorter operating life. Researchers at the Fraunhofer Institute for Applied Polymer Research, Potsdam, Germany, are working on a solution based on flexible glass as a carrier substrate to better protect the components.

Organic photovoltaics, embedded in film, are a promising alternative to silicon-based cells, say the researchers. The OPV materials can also be processed at atmospheric pressure, cutting costs and complexity of fabrication. However, the main advantage is such modules can be manufactured using printing technology, which is faster and more efficient that processes needed to fabricate inorganic components.

The Fraunhofer IAP team is working with a new carrier material and embedding the solar modules inside a thin layer of glass. Danny Krautz, Project Manager for Photonics at Berlin Partner for Business & Technology, commented, “Glass is not only the ideal encapsulating material, it also tolerates process temperatures of up to 400 degrees.”

A specialized glass from Corning is used in the research work. Due to its special physical properties, layers can be made that are only 100µm thick. The glass is not only fracture-resistant, it is so flexible that it can be gently bowed even in its solid form.

Production on rolls

Using this material, the team has already created the first working OPVs by processing stacks sheet-by-sheet.
The team’s goal is to be able to fabricate these modules on rolls as well. The carrier substrate will be wound on a roll in this case, in a similar way to printing newspapers. Photoactive layers and electrodes are printed in several steps; relatively large surfaces can be manufactured effectively in series by this fabrication technique.

The team from IAP has already begun a first test of how the flexible glass could be processed in this way. Dr. Armin Wedel, who leads the division of functional polymer system at the Fraunhofer IAP and the group of functional material and devices, said, “We were immediately successful on our first run in producing homogenous layers on smaller substrate dimensions.”

”The technology still needs to be modified at many points for the process to meet the demands of industrial applications – and the Potsdam team is already working on these. Long-lived, robust, high-performance OPVs can be fabricated with this technology for use in a wide range of applications – from tiny solar cells in mobile phones to large-scale photovoltaic modules.”

Interview

optics.org interviewed Krautz and Wedel about the development and how the Fraunhofer IAP’s organic approach could offer advantages over silicon-based PV solar cells.

What are the possible cost-benefits of OPV compared with silicon cells?

Conventional silicon solar cells usually require a frame system on which the wafers can be mounted. A lamination system will also protect the solar modules mechanically. But the production capacity a conventional solar system is limited because the size of a solar cell is limited by the diameter of the Si ingot and is therefore based on an individual sheet system.

Our approach is based on thin-film PV and a continuous production process so our production capacity is quite flexible and depends on the printing speed,which is somehow between 1-10m/min. The installation of such a production line is therefore quite simple compare to a Si-fab and the payback/ energy payback time should be significant lower.

How does solar conversion efficiency compare?

The absorber material we are using now has the potential to reach 9%, which is still around 50% lower than that of conventional solar cells. However, the types of solar technology can be used for indoor applications, does not depend on direct sun radiation and is even available in different colors. The cost per watt-peak can be reduced below €1, depending on improvement in the materials and cutting the costs of a roll-to-roll production line.

Is the resistance of your organic cells to oxygen more important than technical performance?

Both aspects are important for successful market penetration. What we observed when discussing projects with partners was that the lifetime of the OPV module seems to be more important than the efficiency. More work is required to enhance the power conversion of such organic devices but this will depend on research activities of the industrial key players for this type of absorber materials.

What are the implications for useful operational lifetime of the organic cell?

First developments are focused in particular on power supplies, based on OPV for easy applications such as school satchels or for recharging laptop or smart-phone batteries. But the key application will be building integration of OPV system in the near future due to the possible aesthetic and design variation.

OPV will be successful only when these systems can be integrated into buildings and used in power supplies for higher power consumption applications. For that, the lifetime and the weight are important factors specially if we are considering integration of OPV modules in architecture. The flexibility of the substrate is mostly important for lowering the cost of the manufacturing process. We know that a lifetime of at least 10 years must be achieved for a successful marketing story.

Is your flexible glass a better approach than a printed, flexible film?

Using flexible glass could significantly extend OPV device lifetime due to increased barrier functionality. Even ultra-thin glass provides an excellent encapsulation with superior WVTR coefficient below 10^-6 g/m²/day compared to 10^-2 to 10^-5 g/m²/day for barrier foil and is therefore an excellent barrier towards external degradation factors of OPV devices.

About the Author

Matthew Peach is a contributing editor to optics.org.