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

Introduction / Project Goals
The concept of the luminescent solar concentrator (LSC) dates back to the 1970s [1]. The basic principle
of operation of a conventional LSC is illustrated in Fig. 1. The device consists of a transparent
plate (usually polymethylmethacrylate PMMA) containing luminescent centers, which absorb light entering
through the face of the plate. A fraction of the subsequently emitted photons is trapped by total
internal reflection and guided to the edges of the plate. As the edge area of the plate is much smaller
than the face area, the LSC operates as a concentrator of light.
Fig. 1 Principle of operation of a LSC. Only one luminescent center is shown. In order to function as
a concentrator, the face area needs to be larger than the edge area.
Conventional LSCs suffer from a series of problems, which lead to low efficiencies and have prevented
commercial application:
� Self-absorption: Organic dyes with high fluorescence quantum yield generally feature a considerable
overlap between absorption and emission spectra. An emitted photon with an energy corresponding
to this spectral overlap has a high probability of being re-absorbed on its path towards
the edge of the LSC plate. Successive re-absorption and re-emission steps lead to a larger path
length and thus to a higher susceptibility towards the various loss mechanisms, including radiationless
deactivation or emission into the escape cone.
� Escape cone losses: The angle of the escape cone depends on the refractive index of the material.
In the case of PMMA, an escape probability of 26 % is calculated for isotropic absorption and
emission.
� Stability: Problems with stability are mainly associated with the non-sufficient photostability of the
luminescent centers. To become commercially viable, a LSC should be stable enough to ensure
efficient operation over a period of more than 10 years.
We have developed a concept based on dye-zeolite composites that has the potential of solving the
above problems [2]. The goal of the project is the preparation of prototypes according to this new design
concept and the evaluation of the performance of the devices.
General Concept
The use of zeolite L as a host system offers possibilities to obtain high local concentrations of
supramolecularly organized monomeric dye molecules. Zeolite L is an aluminosilicate with onedimensional
channels running parallel to the c-direction of the crystals (Fig. 2). The morphology of the
crystals is typically cylindrical with channel entrances located at the base surfaces. A crystal with a
diameter of 500 nm contains approximately 66'000 parallel channels. Dye molecules can be included
by cation exchange or adsorption from the gas phase. As dye molecules cannot pass each other in
the channels, sequential introduction of different dyes results in defined dye domains. This allows for a
transport of electronic excitation energy along the channels by means of Förster resonance energy
transfer (FRET) [3]. The supramolecular organization of the dyes in the zeolite channels and the
option of extending this organization to the macroscopic scale by preparing layers of oriented crystals
is at the center of our concept for an advanced LSC. Zeolite-polymer hybrid materials can be prepared
with high transparency [4].
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Luminescent concentrators based on inorganic/organic nanomaterials, D. Brühwiler, University of Zurich
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