Environmental Engineering Reference
In-Depth Information
Figure 8.10 Photovoltaic efficiencies rise above 0.51 at one sun in theoretical investigation of the
AlP:Cr system [107]. Adapted from Ref. [107], Figure 8.3.
structure diagram is split into two parts, the left for spinup and the right for
spindown. The splitting of the narrow peak from the Cr ion indicates ferromagnetic
order. The Cr plays the role of a deep level as was mentioned above in connection
with doping of semiconductors. The Cr level is indicated as half-
lled, so this band
structure is similar to that needed for the intermediate band solar cell.
The authors point out that a single absorbermaterial, rather than amultijunction
system, helps avoid optical and tunnel junction losses, and generally might be
simpler to construct than a competing tandem system. On the other hand, the
particular compound has not yet been made, but its properties have been surveyed
theoretically as a start. The parent semiconductor is AlP, in the zinc-blende
structure with an indirect bandgap of 2.43 eV. The effect of random substitution
of Cr on the Al sites was estimated theoretically. The material was found to be a
dilute magnetic semiconductor of ferromagnetic type, with a critical temperature
estimated at 160 K.
The ef
ciency of the resulting solar cell was found to exceed 50% in a suitable
range of parameters, as shown in Figure 8.10. This is a very encouraging result for
the proposed AlP:Cr cell, since the largest possible single bandgap cell efficiency is
only 31%.
The optimumpredicted behavior for photovoltaic conversionwas 52%efficiency at
one sun, at bandgap 2.43 eV, and
E
2
/
E
G
about 0.38. This remarkably high estimate is
