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

Project Goals
The aim of FULLSPECTRUM is the development of photovoltaic (PV) concepts capable of extracting
the most of every single photon available [1]. At this respect, each of the five activities envisaged in
this project to achieve this general goal confront its own challenges. The multijunction activity pursues
to develop solar cells that approach 40 % efficiency as much as possible. For that, it faces the challenge
of finding materials with a good compromise between lattice matching and bandgap energy. The
thermophotovoltaic activity bases part of its success in finding suitable emitters that can operate at
high temperatures and/or adapt their emission spectra to the gap of the cells. The other part relies in
the successful recycling of photons so that those that cannot be used effectively by the solar cells can
return to the emitter assisting in keeping it hot. The intermediate band solar cell approach defy the
challenge of proving its principles of operation to an extent in which these have not represent only
marginal effects in the performance of the cells. The molecular based concept activity devoted to
search of new molecules encounters the challenge of identifying molecules capable of undergoing
two photon processes, that is, molecules that can absorb two low energy photons to produced a
high energy excited state or, for example, dyes that can absorb one high energy photon and re-emit its
energy in the form of two photons of lower energy. An other aim is investigating the "flat-plate concentrator"
(FPC) concept, which is based on thin polymers sheets colored with special dyes 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 put on
the edges of the concentrator plate. Among all the above concepts, the multijunction approach appears
to be the most readily available for commercialization. For that, the manufacturing techniques
and pre-normative research activity is devoted specifically to speed up its path to market is developing
trackers, optics and manufacturing techniques that can integrate these cells in commercial concentrator
systems.
Short description of the project
The multijunction solar cell approach pursues
the better use of the solar spectrum by using a
stack of single gap solar cells to be incorporated
in a concentrator system in order to make it cost
effective (Fig. 1) . Within this approach, the project,
at its start, aimed to cells with an efficiency
of 35 %. This result has already been achieved
by FhG-ISE in the second year of the Project
and the Consortium aims now to achieve efficiencies
as close as possible to 40%.
Fig. 2. The principle of TPV conversion
FULLSPECTRUM, T. Meyer, Solaronix
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Fig. 1. Example of a structure of a monolithic triple-junction
solar cell made of Ga0.35In0.65P -,
Ga0.83In0.17As - and Ge-junctions interconnected
by internal tunnel diodes.
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
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