industry research
rhombus4
Figure 1: Right
Figure 2:
Far right
corresponding to an intensity of 1000 junctions made of semiconductors efficiency maximum is achieved for an
W/m
2
). Spectrolab has since raised with different bandgap energies infinite number of pn-junctions but
the bar even further to 41.6 percent. stacked on top of each other. In this one can also see from Fig. 2 that the
case short wavelength photons are benefit of adding more subcells
The conversion efficiency of a solar converted more efficiently by a first decreases with the number of
cell made of a single semiconductor high bandgap cell and longer junctions and at the same time the
material such as silicon is limited due wavelength radiation is transmitted to complexity of the solar cell device
to two reasons: long wavelength an underlying second pn-junction increases significantly.
photons are transmitted through the which can again be followed by
structure and carriers generated by further lower bandgap subcells in the In fact every subcell in a multi-
short wavelength photons quickly same way. Fig. 2 shows the junction solar cell typically exists of
thermalize to the band edge, loosing theoretical conversion efficiency for approximately 6-10 individual layers
part of their energy by the generation such a multi-junction stack calculated forming the emitter and base, as well
of lattice heat. for an ideal combination of bandgap as surrounding barrier layers and the
energies and at a concentration of tunnel diode for the series
These two fundamental losses limit 1000 suns (corresponding to 1000 interconnection of adjacent subcells.
the theoretical conversion efficiency times 1000 W/m
2
incident intensity). Today the highest efficiencies are
of a solar cell made of a single The maximum efficiency increases to reached for solar cells having three
semiconductor material to 40.8 % 55.9% for a dual-junction pn-junctions. An example of the
below left and But these losses can be reduced i.e. configuration to 63.8 % for an ideal complete layer structure of such a
right: Figure 3 through the use of several pn- triple-junction solar cell. The global device is shown in Fig. 3.
In the future it is well possible that
devices with four or more junctions
will show better performances.
However, this requires all the layers
to be optimized and to show
excellent material quality. In fact
material quality is a key requirement
for achieving high solar cell
conversion efficiencies, which
explains why III-V compound
semiconductors have been more
successful than multi-junction cells
made of polycrystalline or amorphous
materials.
[2]
Defects in the active region
of a solar cell lead to non-radiative
recombination of minority carriers and
22
www.compoundsemiconductor.net October 2009
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