Solar cells out of a printer?

However, the technology has its challenges. To manufacture the components on an industrial scale, semiconducting or insulating layers must be printed onto a carrier film in a predefined order. A further challenge is the contacting between flexible, conducting layers. Previously, electronic contacts made of crystalline indium tin oxide were frequently used. However, the oxide is more brittle than the polymer layers over them, which limits the flexibility of the cells. In addition, indium is a rare element that exists only in very limited quantities.

Earlier this year, researchers from the Lawrence Berkeley National Laboratory in California for the first time succeeded in observing the cross-linking of polymer molecules in the active layer of an organic solar cell during the printing process. In collaboration with his colleagues in California, Professor Peter Müller-Buschbaum, Chair of Functional Materials at TU München, took advantage of this technology to improve the characteristics of the polymer electronic elements.

The researchers used X-ray radiation generated in the Berkley synchrotron for their investigations. The X-rays are directed to the freshly printed synthetic layer and scattered. The arrangement and orientation of the molecules during the curing process of the printed films can be determined from changes in the scattering pattern.

In Berkeley, the physicist from the TUM investigated the ‘blocking layer’ that sorts and selectively transports the charge carriers in the organic electronic components. The TUM research team is now, together with its US colleagues, publishing the results in the trade journal Advanced Materials.

“In our work, we showed for the first time ever that even small changes in the physico-chemical process conditions have a significant influence on the build-up and properties of the layer,” says Claudia M. Palumbiny, Research Associate, TUM. “Adding solvents with a high boiling point, for example, improves segregation in synthetics components. This improves the crystallisation in conducting molecules. The distance between the molecules shrinks and the conductivity increases.

In this manner, stability and conductivity can be improved to such an extent that the material can be deployed not only as a blocking layer, but even as a transparent, electrical contact. This can be used to replace the brittle indium tin oxide layers. To make all of this possible one day, TUM researchers want to continue investigating and optimising the electrode material further and make their know-how available to industry.

The research was supported by the GreenTech Initiative Interface Science for Photovoltaics of the EuroTech Universities together with the International Graduate School of Science and Engineering at TUM and by the Cluster of Excellence Nanosystems Initiative Munich (NIM). Further support came from the Elite Network of Bavaria’s International Doctorate Program NanoBioTechnology, the Center for NanoScience and from Polymer-Based Materials for Harvesting Solar Energy, an Energy Frontier Research Center funded by the U.S. Department of Energy’s Office of Basic Energy Sciences. Portions of the research were carried out at the Advanced Light Source, which receives support by the Office of Basic Energy Sciences.