Environmental Engineering Reference
In-Depth Information
6.1.2
GaAs Epitaxially Grown Solar Cells
GaAs, as shown in Figure 6.3, has a suitable bandgap and direct absorption structure
for solar cells. GaAlAs layers can be grown epitaxially by liquid and vapor-phase
methods. Figure 6.7a shows an excellent cell using GaAlAs layers.
Features of this single-junction multilayer AlGaAs/GaAs solar cell include a thin
p
-AlGaAs window, grown by low-temperature liquid-phase epitaxy, LPE, and a
prismatic cover made of silicone (above the antire
ection coating, ARC), which
minimizes the optical losses caused by contact grid shadowing and re
ection from
the semiconductor surface. An ef
ciency of 24.6%was recorded with such cells, in a
100 times concentrated air mass zero, AM 0, spectrum. More details of the band
structure in the cell are shown in Figure 6.7b.
6.1.3
Single-Junction Limiting Conversion Efficiency
Some crude estimates may be useful. For example, the open-circuit voltage has an
upper limit
E
g
/
e
, where
E
g
is the semiconductor bandgap. The analysis [71] assumes a
single junction and a single uniform bandgap energy. It is assumed that photons
whose energy is less than the bandgap energy
E
g
do not contribute to the photo-
current. It is also assumed that the excess energy
hc
/l
E
g
is lost to heat in the
semiconductor. In practical terms, the junction structure should be thick enough that
all of the light of energy
hc
/l
0 is absorbed.
Adisadvantage of the single-junction cell at bandgap
E
g
is that all photons of energy
less than
E
g
are lost, are not absorbed but pass through the cell. The energy output of
the cell is the same for
N
photons of 2
E
g
or 3
E
g
as for
N
photons of energy
E
g
. For these
reasons, the ef
ciency of the single-junction cell is inherently limited and depends on
the spectrum of photon energies that are incident. Roughly, the bandgap for best
ef
ciency, within the single-junction assumption, should be close to the peak of the
incident spectrum.
Recalling Eq. 5.1 and Figure 5.2 the light spectrum from the sun is approx-
imately that of a black body of
E
g
>
temperature 5973 K, which peaks at
10
6
/5973) nm
¼
¼
l
m
(2.9
486 nm, which corresponds to an energy 1240/
¼
486
2.55 eV. (The spectrum at ground level (Air Mass 1) is substantially
altered, by strong absorptions from several minor constituents of the atmo-
sphere, including ozone, water vapor, and compounds of nitrogen and carbon. It
is also weakened and redshifted by scattering in the atmosphere.) Adopting the
hypothetical black body spectrum, for simplicity, if the bandgap of silicon is
1.12 eV, then the many photons in the range below 1.12 eV are completely lost,
and an increasing fraction of the energy of the more energetic photons is also
lost. The most probable photon at energy 2.55 eV will contribute not more than
will a 1.12 eV photon, the difference energy 1.43 eV contributing only to heat.
The specialists in photovoltaic conversion have adopted an effective spectrum Air
Mass 1.5 G, correcting for an average daytime light path length 1.5 larger than when
