EXCURSION TO FRAUNHOFER IAP

At the Fraunhofer Institute for Applied Polymer Sciences (IAP) in Potsdam-Golm, research is carried out on the development of organic solar cells, a field in which the Fraunhofer Institute for Applied Polymer Sciences (IAP) is located. Dr. Christine Boeffel, head of the department for OLED (Organic Light Emitting Diode) and OPV (Organic Photovoltaic) showed us the institute, the production and process technology and the current state of research during one day. 

In a short presentation Dr. Boeffel explained the structure of OPV cells. These are thin-film solar cells, with a thickness of a few 100 nanometers to a maximum of 1 micrometer. The process for manufacturing an OPV cell does not require high temperatures. The materials used are soluble so that they can be printed using an inkjet process. The IAP has such a printer in its laboratory, which is located in a clean room and has special nozzles that can apply the viscous liquid to almost any thin-layer surface.

The layers of a classical OPV are structured as follows: 

Translucent top layer
Cathode: here mostly silver net structure
Active layer: semiconducting organic material (polymer)
Anode: ITO (indium tin oxide)
Substrate

A solar panel is built up in small cells which are then connected in series. This gives the solar panel its known stripy appearance. The energy production of a single cell results in a voltage of about 0.8 V and an electric current of about 11 mA per 1 cm². When connected in series, this results in about 8 V and 10 mA for a 120 cm² panel, since the gaps between the cells must be included in the total size.
Since the cells are connected in series, the total current is based on the weakest cell. Optimally, all individual cells have the same total area. The design of the individual cells, as long as they are all the same size, can be freely chosen, as the example of the snowflake-like solar cell used in the German Pavilion 2015 in Milan shows.
The service life of an OPV cell varies depending on the substrate and covering material used. A cell in glass is more durable, but the great advantage of the OPV is that it can also be used on flexible substrates such as PET. However, this reduces the life span to about 1 year. 
The manufacturing costs for OPVs are still comparatively high, given the small quantities produced by the Fraunhofer Institute. PET costs only 1 €/qm², the layers to be used 10-100 € and the transparent barrier film about 100-150 €. These costs could be significantly lower with higher production volumes. 

Almost any surface can be used to apply OPV, be it an ICE train, wind turbines, noise barriers or even a truck tarpaulin. The individual modules can also be designed interchangeably, so that in the event of a defective module, only the individual module needs to be replaced, without having to replace the entire system. In addition, research is currently being conducted on a safety mechanism that can compensate for defective individual cells.

The recycling of an OPV is only possible to a limited extent. The carrier layer (e.g. PET) is easily recyclable, but the individual organic or silver layers are not, or only with great difficulty. However, these layers are extremely thin.

Currently, research in the OPV area of the Fraunhofer is unfortunately no longer very active, as the production of silicon solar cells is extremely cheap and research into alternatives is not very attractive from an economic point of view. 

Thanks to Christine Boeffel for the insights and guided tour

iap.fraunhofer.de

text: Nikolaus Hößle, Leopold Seiler, Anniek Timmermann, Lu Meiying