Page 22 - RSE - Results of the Apollon Project

t

Crystalline Si

and iii-v Mj high Effciency Solar Cells

(NAREC, ENEA, RSE, ENE)

The different approaches developed in the Photovoltaic Community to increase the solar cell effciency value can be
divided in two main categories: the approaches taken under the assumption outlined by Shockley and Queisser in 1961 10
and the approaches which extend in scope beyond the Shockley and Queisser limit. All the experiments carried out
in order to improve the combination of semiconductor materials and to implement new cell structures for enhancing
light capture and device voltage belong to the frst category. The second category includes the so called third generation
photovoltaics, which consider, for example, modifcation of the light spectrum or multiple carrier generation per photon.
The main concepts developed in the two categories are summarized in Table 3 (conventional single junction technologies
are not included). While the approaches which belong to frst category already brought to champion solar cell effciency
value, and the research related to these concepts/technologies can be further classifed as short term research, the concepts
developed in the second one have shown potential to eventually result in high effciency photovoltaics, therefore,
considerable scientifc efforts are still required from this second category before high effciency devices can emerge and
research related to these concepts/technologies can be further classifed as long term.
The solar cells investigated under APOLLON belong to the short term research category, and namely, two different
research paths have been taken for the development of Single junction (SJ) and Multijunction (MJ) devices: the frst has
been followed by the industry, in particular by ENE and NAREC, in order to develop customized solar cell devices for
point focus and mirror based spectrum splitting systems, while the second has been followed by the research institutes
of the Consortium in order to prove new concepts for MOCVD growth equipment, cell structures and materials. For both
research paths the objective was to increase the solar cell effciency value, while also improving the values of other solar
cell parameters like the concentrator factor, the yield and the cost. In other words, in order to get economical affordable
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solutions, solar cell development aimed to increase the value of the cell economic performance index (CEPI) . Referring
to Figure 12, it is possible to understand the importance of this objective to get competitive CPV.

FiguRE 12. Solar cell cost per Watt as a function of solar cell effciency and a) concentration factor or b) process yield
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values. The solar cells cost is taken to be constantly at 5.96 euro/cm . Solar irradiation = 850 W/m 2

10 William Shockley and Hans J. Queisser, Journ. of Appl. Phys. Vol. 32, n.3 March 1961, pp. 510-519.
11 CEPI= h*C *Y/C where: h = cell effciency C = concentrator factor Y = yield C = cell cost
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