Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

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So far, experiments were performed using one cyanine dye with a mobile counter ion (CyC, Fig. 1)
and one dye with a bound counter ion (an internal salt). For the latter case, we concluded that van der
Waals forces are responsible for film break-up, and the theoretical predictions for thickness ranges
that result in stable, metastable or unstable films could be readily realized experimentally [5]. The
situation was different for CyC where all films were unstable, although a stable layered configuration
with PCBM wetting CyC would be expected from the surface tension values alone. From our analysis
we concluded that van der Waals forces and convective flows during spin coating do not play a major
role for structure formation. A characteristic of CyC is that the positively charged cyanine is associated
by the mobile negative perchlorate counter ion (Fig. 1). This can give rise to electrostatic forces in thin
films, and space charge can build up if the anions diffuse from the cyanine layer into the PCBM phase.
The thermodynamic model reveals that the transfer of ions lowers the free energy of the system. Surface
and interface fluctuations favour continuing ion transfer, which amplifies the film undulations and
effectively destabilizes the film.
Fig. 2: Atomic force microscopy images of PCBM/cyanine dye films coated on glass from solution. The
as-prepared film morphology consisted of circular PCBM domains with surprisingly uniform sizes
(right). The left panel displays the continuous matrix of the complementary cyanine dye material. The
individual components were made visible by treating the film with selective solvents.
We found that liquid-liquid dewetting results in a large variety of phase morphologies, with tuneable
dimensions well below 100 nm (Fig. 2). Qualitatively, the resulting type of morphology depends on the
ratio of the layer thicknesses, whereas the dominant wavelength of the domains is determined by the
absolute film thickness. These are material independent parameters and, therefore, the patterning
method is rather robust. In ongoing experiments, we study the quantitative aspects of the dewetting
process and adapt the concept to other cyanine dyes.
National / international collaborations
Collaboration with J. Groenewold (Uni Utrecht, NL) “Nanoscale structuring of heterojunction ionic organic
solar cells by liquid-liquid dewetting,” project submitted to “Polymol;” a FP6 financed PV ERA
NET project.
Collaboration with J.-E. Moser (EPFL), photophysics of cyanine dyes studied by fast spectroscopy.
Collaboration with U. Steiner (Cambridge), fundamental aspects of thin-film destabilization, external
forces as a means for film structuring.
Organic photovoltaic devices, F. Nüesch,Empa
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