Page 64 - Alternative Energy Systems in Building Design

MULTIJUNCTION PV CELLS 41

CONTACT
CAP
n+GaAS
nAIGaInP WINDOW
nGaInP EMITTER

pGaInP BASE

TUNNEL JUNCTION
nGaInP EMMITER

nGaAs BASE
p+GaAs TRANSFERRER LAYER
n+InP 1/2 TUNNEL JUNCTION
nInGaAsP EMMITER

pInGaAsP BASE

TUNNEL JUNCTION
nInGaAs EMMITER

pInGaAs BASE

p+InP TRANSFERRER LAYER
N+Si

pSi SUPPORT SUBSTRATE

CONTACT
Figure 3.9 Multijunction solar cell epitaxial layers.

of semiconductors, metals, and rare earth metals alloys such as germanium, gallium-
arsenide, and indium-phosphide.

Gallium arsenide substrate Twin-junction cells with indium-gallium-phosphide
(InGsP) and gallium-arsenide (GaAs) are constructed on GaAs wafers. Alloys of
In Ga P through In 0.53 Ga 0.47 P are used as high-bandgap alloys. This alloy range
0.5
0.5
provides bandgaps with voltages ranging from 1.92 to 1.87 eV. GaAs, on the other hand,
allows for the fabrication of junctions with a lower bandgap of 1.42 eV. Since the solar
spectrum has a considerable quantity of photons in its lower regions than does the
GaAs bandgap, a significant portion of the energy is lost as heat, limiting the efficiency
of the GaAs substrate cells.