Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

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Fig. 2. The principle of TPV conversion
In the thermophotovoltaic approach, the sun heats up, through a concentrator system, a material
called “emitter” leading it incandescent (Fig. 2). The radiation from this emitter drives an array of solar
cells producing electricity. The advantage of this approach is that, by an appropriate system of filters
and back reflectors, photons with energy above and below the solar cell bandgap can be directed
back to the emitter assisting in keeping it hot by recycling the energy of these photons that otherwise
would not be optimally converted by the solar cells. By the end of the project, it is expected that the
system, composed basically by the concentrator, emitter and solar cell array can be integrated and
evaluated.
The “intermediate band” approach pursues a better use of
the solar spectrum by using intermediate band materials
(Fig. 3). These materials are characterised by the existence
of an electronic energy band within what otherwise would
be a conventional semiconductor bandgap. According to
the principles of operation of this cell, the intermediate
band allows the absorption of low bandgap energy photons
and the subsequent production of enhanced photocurrent
without voltage degradation. The Project expects also to
identify as much intermediate band material candidates as
possible as well as to demonstrate experimentally the principles
of operation of the intermediate band solar cell by
using quantum dot solar cells as workbenches. Fig. 3. The principle of the IBC.
Diffuse light
Spectrally adapted solar cell(s)
Photonic layers.
Mirror for extremely
small band. Transmit
all the r est.
Fig. 4. Luminescent concentrator with
photonic crystal.
FULLSPECTRUM, T. Meyer, Solaronix
Mirror
As mentioned, under the “molecular based concepts”
heading, it is expected to find dyes and
molecules capable of undergoing two-photon
processes. Dyes -or quantum dots- suitable to be
incorporated into flat concentrators are also pursued.
Flat concentrators are essentially polymers
plates, that by incorporating these special dyes to
their structure, are capable of absorbing high energy
photons and re-emit them as low energy
photons that ideally match the gap of the solar
cells. This emitted light is trapped within the concentrator
usually by internal reflection and, if the
losses within the concentrator are small, can only
escape by being absorbed by the cells.
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