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Forschungsarbeit

Organic solar cells: Morphology correlated observation of charge transfer excitons at the polymer/fullerene interface

Von Ilka Kriegel (22.02.2010)

Solar cells are considered to be one of the most promising sources of clean energy for the future. So far costs of production have limited their widespread application. In contrast to the established systems, such as silicon modules, new thin film based technologies are attracting the interest of academic and industrial research groups around the world. The promise of thin film solar cells is the possibility to reach reasonable power conversion efficiencies, while maintaining low costs of production. Particularly appealing are organic solar cells, where the active material is a blend of two components forming an interpenetrating network at the nanoscale.

The two components consist of a conjugated polymer, which harvests light from the sun and a fullerene molecule, necessary to promote photoinduced charge transfer. This is the physical process by which an exciton in the conjugated polymer is converted into an electron and hole pair separated in space. Photoinduced charge transfer is, therefore, the fundamental step to generate current from the sun light. In this work we have shown that it is possible to monitor recombination after the initial photoinduced charge transfer, by near infrared photoluminescence (PL) spectroscopy [1].

Typical assembly of an organic polymer/fullerene solar cell.[Bildunterschrift / Subline]: Typical assembly of an organic polymer/fullerene solar cell (not to scale). The active layer consisting of the fullerene acceptor and polymer donor is magnified for clarity. The physical processes presented in the active layer are: (1) light absorption within the polymer an the genaration of an exciton, (2) exciton diffusion to the interface with the fullerene, (3) exciton dissociation and charge transfer from the polymer to the fullerene, (4) charge transport to the electrode.

Further, we have correlated the morphological features of the blends, probed by transmission electron microscopy, to the charge transfer exciton (CTE) emission intensity. Interestingly we have observed that the CTE intensity (proportional to recombination), is independent from the morphology, whereas it is controlled by the intrachain conformation of the conjugated polymer. The intrachain conformation of the polymer is known to depend on the chemical structure and the nature of the side group. In our work we have compared conjugated polymers with different degrees of rigidity and found that polythiophenes are particularly interesting for reducing CTE emission and thus loss channels in solar cells [2].

The work has an innovative character in the idea of combining morphological and optical studies on CTEs, these were indeed not observed before in such materials. The technological impact for applications is significant as recombination is a major loss mechanism in solar cells. The work represents a major step in understanding fundamental excitations in organic materials. In addition we demonstrate a facile, fast and non-destructive method to test organic solar cells and in particular the recombination losses.

References

[1] M. Hallermann, S. Haneder, E. Da Como, Appl. Phys. Lett. 93, 053307 (2008).

[2] M. Hallermann, I. Kriegel, E. Da Como, J. M. Berger, E. von Hauff, J. Feldmann Adv. Funct. Mater. (accepted) (2009).


Ilka Kriegel
Ilka Kriegel
* 1981, Erlangen

Stationen
  • since Oct. 2009
  • Member of the Doctorate Program NanoBioTechnology at the LMU München
  • since Aug. 2009
  • PhD at Photonics and Optoelectronics Group, Chair of Prof, Dr. Jochen Feldmann, LMU München
  • Oct. 2007 – April 2009
  • Elite graduate program "Advanced Materials Science" graduated with Master of Science of the TU München, LMU München and Universität Augsburg
  • Master thesis: Morphology correlated observation of charge transfer excitons in polymer/fullerene solar cells
  • Nov. 2004 – Aug. 2007
  • Bachelor program "Molecular Nano Science" graduated with Bachelor of Science of the Friedrich-Alexander-Universität Erlangen/Nürnberg
  • since Nov. 2004
  • Teacher of classical ballet

Veröffentlichungen
  • "Charge transfer excitons in polymer/fullerene blends: the role of morphology and chain conformation", M. Hallermann, I. Kriegel, E. Da Como, J. M. Berger, E. von Hauff, J. Feldmann, Adv. Funct. Mater. (accepted) (2009).