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

Introduction and objectives
Excitonic heterojunction solar cells based on semiconducting organic small molecules and polymers
are promising devices for inexpensive, large-scale solar energy conversion. Organic solar cells can be
fabricated by simple coating processes from solution, thereby preserving the low-cost potential that
these materials offer. Organic components with matched optical and electronic properties are obviously
important for high light-to-current conversion. In addition, the nanoscale arrangement of the materials
in the thin-film geometry needs to be carefully controlled. Indeed, charge generation within the
~10 nm diffusion length of the primary photoexcitation and loss-free charge transport to the electrodes
via percolating paths requires phase-separated domains on multiple length scales. Detailed studies on
the organization of phase-separating organic blends, however, are still very limited, and the driving
forces are poorly understood.
This project specifically relates to the controlled arrangement of ionic cyanine dyes in thin films of twocomponent
mixtures. Cyanine dyes have several attractive properties for use in organic photovoltaic
devices [1-4]: Cyanine dyes have good film-forming properties, very high extinction coefficients and
the light absorption can simply be tuned over the visible to the near-infrared region using structural
analogues. As generally true for small molecules, cyanines are more easily to purify than their polymeric
counterparts. We recently reported a planar bilayer solar cell with a promising efficiency of 1.2%
when using a cyanine dye as electron donor and buckminsterfullerene C60 as acceptor [2]. Increased
performance was achieved via light-induced cyanine doping by oxygen and water. From this we conclude
that a cyanine dye solar cell with an optimized morphology can potentially reach much higher
efficiencies and effectively compete with other materials.
Work performed and results obtained
We found that thin films coated from a cyanine dye / PCBM (a soluble C60 derivative) mixture show
sub-micrometer phase-separated morphologies (Fig. 1) [5,6]. We identified that the mechanism leading
to these morphologies occurs by liquid-liquid dewetting. Liquid-liquid dewetting proceeds via a
transient bilayer that forms during coating driven by differences in surface tension, and this layer is
destabilized by long range interfacial interactions. The destabilization forces do not only act at the
liquid-air interface, but also at the liquid-liquid interface, leading to significant interfacial roughening.
The dispersive force driving the instability is counterbalanced by the inherent energy cost of having a
large interface, leading to thickness fluctuations with a characteristic spinodal wavelength.
Fig. 1. Left: Diagram showing how dewetting patterns evolve from destabilizing double layers. Right:
The developing morphology depends on the processing parameters [5,6] and can consist, for example,
of droplets of one component embedded in the film-forming second component, or of laterally
demixed structures.
Organic photovoltaic devices, F. Nüesch,Empa
132/290
2/4