Polymer solar cells
Organic solar cells offer many new technological possibilities compared to traditional silicon-based solar cells. The greatest advantages here are probably the low manufacturing costs, the lower environmental impact and the mechanical flexibility. These properties open up a field of completely new applications.
Research is currently focused on improving the efficiency of solar cells, which is currently 8.3% (Konarka Inc.), and on extending their lifetime.
Our research is directed towards the morphology of the absorber layer, which is studied using various microscopic techniques. The investigated solar cells are fabricated by the group of U. Lemmer (Lichttechnisches Institut, KIT).
The absorber layer contains two organic semiconductor materials: as acceptor the fullerene derivative PCBM and as donor material P3HT (poly(3-hexylthiophene)). The degree of segregation of these two materials is crucial for the performance of the solar cell, since the excitons formed by the incident photons can only be separated at the interface between donor and acceptor due to their high binding energy. On the other hand, the two different polymer phases must be large enough to ensure charge transport to the electrodes. The figure on the left shows a simplified illustration of an organic solar cell with phase separation in the absorber layer.
Until now, there are few satisfactory methods to image the different phases in the photoactive layer. This is due to the fact that the average atomic numbers and densities of the two materials are very similar and both are weakly scattering materials. In addition, both materials are very sensitive to radiation.
To overcome these problems, we use several new electron microscopic techniques, such as low energy scanning transmission electron microscopy (low-keV STEM) or phase plate TEM to image the morphology of the absorber films. The two figures below show two HAADF ("high-angle annular-dark field") STEM images of an absorber layer consisting of P3HT:PCBM. The large-scale structures in the left image are thickness variations in the absorber layer. Only at higher magnification (right image) can the nanomorphology of the solar cell be seen in the form of crystalline P3HT needles.
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 dierent processing conditions analyzed
by low-energy scanning transmission electron microscopy.
Microscopy & Microanalysis 18, 1380-1388 (2012).
4] 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 buer 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)