Page 93 - RSE - Results of the Apollon Project

Results of the APolloN PRoject ANd coNceNtRAtiNg PhotovoltAic PeRsPective

a. the solar cell un-yielded cost is constant, regardless of the effciency value;
b. the module, the tracker and the fxed costs are constant, regardless of the concentration factor.

Like in the car sector, where year after year it is possible to fnd cars with improved performances (for example,
featuring engines which consume less fuel) for the same cost, we can assume that we can reach higher solar cell
effciency values at the same solar cell cost. Indeed, in this publication DJ-III-V based solar cells were reported
as having the same cost of TJ-III-V based solar cells (see chapter 10: Environmental and Cost Impact Analysis).
The same hypothesis can hold for the CPV module: we can assume that the CPV module technology can improve
in terms of developing modules operating at higher concentration factor with comparable acceptance angle,
keeping the same costs.
In particular, in the cost analysis, the module area has been kept as constant, regardless of the concentration
factor, by assuming to install optics with comparable aperture area but adopting solar cells with different
areas, getting smaller as the concentration factor increases. By maintaining the area of the module constant,
the tracker area is constant as well, regardless of the concentration factor, therefore, considering that we need
similar tracking accuracy, regardless of the concentration factor (given that we assume to get similar module
acceptance angles) we can also keep the tracker cost constant, regardless of the concentration factor. Referring to
equation (1) and Figure 13, we could straightforwardly point out that when the concentration factor and process
yield values are low (low value of CEPI), the frst term of equation (1) plays a relevant role in the total system
cost, therefore, highly effcient systems can happen to cost more than systems having a lower effciency value.
Therefore, it is important to recognize that choosing project targets focusing only on the solar cells effciency
value means risking giving a false idea on the competitiveness of the CPV technology.
On the other hand, by keeping the process yield high enough (>80%), the effciency value over 40 % and by
properly decreasing the solar cell, module, tracker and fxed costs, it is possible to achieve competitive CPV
technology at concentration factor ≥1,000 suns. A further system cost decrease can be obtained up to 1,500 suns,
over this values, further concentration values increment do not produce any substantial system cost decrease.
In future research program on CPV, it is clear that the development efforts should be addressed both to increase
the solar cells effciency values with target approaching 45% and the concentration factor with target approaching
1,500 X in order to decrease module and tracker cost and increase the competitiveness of the CPV technology.

6. In order to increase the solar cell effciency approaching the theoretical limit several routes can be followed. So
far, several material engineering approaches have been pursued in order to increase the MJ solar cell effciency
(see Table 3); however, concerning the short term research, the challenge to bring practical performance close to
theoretical limits, reducing at the same time the process cost, will surely rely on the proposition of new approaches
to material deposition in order to properly increase the number of junctions in the solar cell devices.
In the framework of APOLLON, RSE has demonstrated that with a proper growth chamber design and adopting
proper growth procedures, it is possible to grow group IV and group III-V elements in the same MOCVD equipment;
this technological option will allow seeking for the combination of semiconductors to be used to increase the
MJ cell effciency, for example, by joining III-V and Ge alloys for developing InGaP/InGaAs/SiGeSn (0.9-1 eV) /
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Ge three or four junction solar cells, with a target effciency value >44% . The use of SiGeSn ternary layers is
particularly interesting since they represent the frst group-IV alloys with a tuneable electronic structure, at a
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fxed lattice constant, effectively decoupling band gap and strain . SiGeSn alloys can be grown with 1eV energy
gap lattice matched on Ge, possibly allowing the replacement of dilute nitrides, in the creation of high effciency
InGaP/GaAs/SiGeSn/Ge multi-junction solar cells. So far such devices have been proposed, but not yet turned
into reality, considering the different growth equipment needed, in particular using UHV-CVD for growing the Ge
alloy and the MOCVD for growing the III-V elements.
The possibility to grow group IV and group III-V elements with the same MOCVD equipment, without, therefore,
adding a substantial cost increase in the growth process which would be required if separate growth chambers
for group IV and groups III-V elements were utilized, puts Europe in an outstanding position in the road map
towards the realization of high effciency 4 junction solar cells.

52 D.J.Friedman, Sarah Kurz abd J.F Geisz, Proceeding of IEEE 2002 and Benjamin R. Conley at al. Proceeding of IEEE 2011.
53 Kouvetakis at al. IEEE Photonic Journal 2 (6) 2010 pp 924-941.

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