Organic solar cells offer – compared to the traditional silicon-based solar cells – many novel technological possibilities. The most prominent advantages are probably the low manufacturing costs and the mechanical flexibility. These properties open a completely new field of application. The focus of the research lies at present in the enhancement of the efficiency of the solar cell whose maximum is at present at approximately 5% and the improvement of long-term stability.
Our research focuses on the morphology of the absorber film which is investigated by different electron microscopic techniques. The examined solar cells are manufactured in the group of U. Lemmer (Light Technology Institute, University of Karlsruhe).
The absorber-films contain two organic semiconducting materials: As acceptor-doped PCBM (a fullerene derivative) and as donor-doped materials P3HT (Poly(3-Hexylthiophen)). The grade of segregation of these two materials is crucial for the performance of the photovoltaic device, because the excitons that are generated by the incident photons, can only be separated at the interface between electron- and hole-conducting regions. On the other hand the phases have to be large enough to afford the charge transport to the electrodes. On the left a simplified image of an organic solar cell with the separated phases of the absorber film is shown.
So far there are few satisfactory methods to image the different phases, because the average atomic numbers and the densities of the two materials are very similar. Besides, the carbon-based composites are highly sensitive towards high-energy electrons. Therefore we are using different innovative electron microscopical techniques, like low voltage scanning transmission electron microscopy (STEM), phase-plate TEM or electron holography to image the carbon based absorber film. The following image shows a STEM-image taken in HAADF-Mode of an absorber-film consisting of a 1:0.9 ratio of P3HT:PCBM.
Selected conference poster presentations:
Organic Solar Cells:
 M. Pfaff, E. Müller, D. Gerthsen, M.F.G. Klein, A. Colsmann, U. Lemmer,
Low-energy electron transmission measurements of thin polymer films in a scanning electron microscope,
in W. Grogger, F. Hofer, P. Pölt (Eds.), Proc. MC2009 Microscopy Conference, Graz, Austria (2009),
Vol. 1: Materials Science, p. 215, Verlag der TU Graz 2009
 M. F. G. Klein, M. Pfaff , E. Müller, J. Czolk, M. Reinhard, S. Valouch, U. Lemmer, A. Colsmann and D. Gerthsen.
Poly(3-hexylselenophene) solar cells: Correlating the optoelectronic device performance and nanomorphology imaged by low-energy scanning Transmission electron microscopy.
Journal of Polymer Science Part B: Polymer Physics 50, 198-206 (2012)
 M. Pfaff , M. F. G. Klein, E. Müller, P. Müller, A. Colsmann, U. Lemmer and D. Gerthsen.
Nanomorphology of P3HT:PCBM-based absorber layers of organic solar cells after di erent processing conditions analyzed
by low-energy scanning transmission electron microscopy.
Microscopy & Microanalysis 18, 1380-1388 (2012)
 M. F. G. Klein, F. M. Pasker, S. Kowarik, D. Landerer, M. Pfaff , M. Isen, D. Gerthsen, U. Lemmer, S. Höger and A. Colsmann.
Carbazole-phenylbenzotriazole copolymers as absorber material in organic solar cells.
Macromolecules 46, 3870-3878 (2013).
 M. Pfaff , P. Müller, P. Bockstaller, E. Müller, J. Subbiah, W. W. H. Wong, M. F. G. Klein, W. Pisula, A. Kiersnowski,
S. R. Puniredd, W. Pisula, A. Colsmann,D. Gerthsen and D. J. Jones.
Bulk heterojunction nanomorphology of fluorenyl hexa-peri-hexabenzocoronene-fullerene blend lms.
ACS Applied Materials and Interfaces 5, 11554-11562 (2013)
S. Höfle, H. Do, E. Mankel, M. Pfaff , Z. Zhang, D. Bahro, W. Jaegermann, D. Gerthsen, C. Feldmann, U. Lemmer and A. Colsmann.
Molybdenum oxide anode bu er layers for solution processed, blue phosphorescent small molecule organic light emitting diodes.
Organic Electronics 14, 1820-1824 (2013)
S. Höfle, M. Pfaff , H. Do, C. Bernhard, D. Gerthsen, U. Lemmer and A. Colsmann.
Suppressing molecular aggregation in solution processed small molecule organic light emitting diodes.
Organic Electronics 15, 337–341 (2014)