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

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Fig. 2. (A) Framework of zeolite L viewed along the c-axis,
(B) side view of a main channel,
(C) projection of a main channel with dimensions.
The drawbacks of conventional LSCs are addressed as follows:
� Self-absorption: The zeolite crystals are filled with light-absorbing dyes (donor molecules) and
emitting dyes (acceptor molecules). The concentration of donor molecules is high to ensure
efficient light absorption. Multiple types of dye can be employed to cover the visible spectrum and
establish an energy transfer cascade, leading to a transport of electronic excitation energy along
the channels to acceptor molecules located at the channel entrances. The acceptor molecules
subsequently emit light in a wavelength range where the donor molecules do not absorb. As the
concentration of acceptor molecules is kept low, self-absorption of the acceptor luminescence
should become negligible.
� Escape cone losses: Disc-shaped zeolite L crystals are used to prepare oriented layers (Fig. 3).
For a given dye molecule in zeolite L, the possible orientations of its electronic transition dipole
moment can be described by a double cone distribution (Fig. 4) [3]. The opening angle of this
cone largely depends on the size of the guest molecule. Orientation of the electronic transition
dipole moment of the acceptor molecules along the channel axis, i.e., perpendicular to the face of
the LSC, is expected to minimize the escape cone losses.
� Stability: The diffusion of potential reactants (H2O, O2, fragments due to incomplete
polymerization of the organic matrix, etc.) through the narrow dye-filled zeolite channels is
hindered and can be quantitatively blocked by appropriate sealing of the channel entrances.
Furthermore, large amplitude motions of the included dye molecules are unlikely due to the
strong confinement. We therefore expect enhanced dye stability after inclusion into the zeolite
host crystals.
Fig. 3 Scanning electron microscopy image of oriented zeolite L crystals. The channels run
perpendicular to the surface of the substrate.
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Luminescent concentrators based on inorganic/organic nanomaterials, D. Brühwiler, University of Zurich