Environmental Engineering Reference
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provided by adding a highly doped N รพ -GaAs
layer. Light absorption in the rear N-layer is
enhanced by the BSF, which inhibits
recombination of minority holes with majority
electrons at the rear surface. These diagrams
make clear that light absorption primarily occurs
in the field-free regions of dimensions L p and L n
beyond the junction-forming depletion layer
([69], Figure 2.4, p. 25).
Figure 6.7 (a) Structure of epitaxially grown
GaAs solar cell as utilized in spacecraft [69]. (b)
Band diagram of epitaxially grown GaAs solar
cell, efficiency of 24.6%. (left) The sloping band
(built-in quasi-electric field) in the p-GaAs front
layer, where photoabsorption occurs, was
formed by Zn diffusion during LPE growth of the
wide-bandgap AlGaAs window. (Right)
Elaboration of cell with a back surface field
the sun is directly overhead. This spectrum is reduced from the top-of-the-atmo-
sphere spectrum, Air Mass 0 (AM 0), by about 28%, of which 18% is from absorption
and 10% from scattering. Scattering is well known to follow a l 4 law, which removes
blue light from the direct path but adds in some blue light scattered from the blue sky.
The resulting AM1.5 G spectrum (which also includes a diffuse light contribution) is
used to calculate the ideal conversion ef
ciency of a single-junction solar cell
operating at 300 K, as a function of the semiconductor bandgap energy. The results
for the AM 1.5 G spectrum (labeled, in Figure 6.8) peaks near 29% ef ciency at a
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