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Monday, 30 July, 2012

Charge separation made easy

Organic Solar Cells

Organic solar cells are thin, flexible and cheap, but not terribly efficient. Materials that capture the red and infrared components of sunlight could help to change that. LMU researchers have now revealed some unexpected facets of such compounds.

The present generation of organic solar cells can convert sunlight into electricity with an overall efficiency of about 9%. To make the technology commercially viable this figure must be further increased. Novel materials called low-bandgap polymers have the potential to do just that. As their name suggests, these compounds are characterized by particularly narrow bandgaps. The size of the bandgap is a measure of the energy that must be imparted to an electron to raise it to an excited state.

Red light suffices

It actually takes very little energy to excite the electrons in low-bandgap polymers. Red light will do the job. But it turns out that the favorable size of the bandgap alone cannot account for the measured improvement in efficiency. Some other factor must be involved.

A team of researchers led by LMU’s Professor Jochen Feldmann has now identified the missing ingredient. On the basis of systematic studies on various low-bandgap polymers with the aid of ultrafast spectroscopy, the team gained new and surprising insights into their mode of function, and were able to show that their chemical structure accommodates significantly more weakly bound pairs of charge carriers than do conventional polymers.

Weakly bound – rapidly separated

When electrons are stimulated by photons they are raised to a higher energy level, leaving a “hole” behind. The process of charge separation, the physical separation in space of negatively charged electrons from positively charged holes, is a prerequisite for the generation of electric current. In conventional polymers only mixing with fullerenes with large energy offsets can help in separating charges, this results in a substantial drop in the photo-voltage of solar cells. This loss could in future be considerably reduced by the use of low band-gap polymers, because their molecular structure can accommodate a larger fraction of weakly bound charge carrier pairs, known as polaron pairs.

“In conventional polymers, the fraction of polaron pairs that can be generated is less than 10%, but in many of the low-bandgap polymers we tested, we measured values of up to 25%,” explains Raphael Tautz, first author on the study. “In the new work we have shown for the first time that the distinctive structure of polymer chains consisting of alternately repeating subunits has a significant influence on the type of charge-carrier pairs produced. This feature could open up new opportunities for the improvement of cells made with these materials.”

The study was performed under the auspices of the Nanosystems Initiative Munich, a Cluster of Excellence at LMU, and was based on fruitful collaborations with colleagues at the Universities of Wuppertal, Mons (Belgium) and Freiburg. (Nature Communications, 2012) (NIM)

Contact:

Dr. Enrico Da Como
Department of Physics, University of Bath
Claverton Down, Bath BA2 7AY, United Kingdom
Email: edc25(at)bath.ac.uk
Phone: +44-1225-384368
Fax: +44-1225-386110

Professor Dr. Jochen Feldmann
Chair of Photonics and Optoelectronics
Department of Physics
Ludwig-Maximilians-Universität (LMU) München
Email: feldmann(at)lmu.de
Phone: +49-89-2180-3356 or -3357 (Secretary’s Office)
Fax: +49-89-2180-3441
www.phog.physik.lmu.de

Publication:
Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics. R. Tautz, E. da Como, T. Limmer, J. Feldmann, H.-J. Egelhaaf, E. von Hauff, V. Lemaur, D. Beljonne, S. Yilmaz, I. Dumsch, S. Allard and U. Scherf. Nature Communications 4: XYZ, 10. July 2012

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