Page 65 - Alternative Energy Systems in Building Design

42 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

Germanium substrate Triple-junction cells consisting of indium-gallium-
phosphide, gallium-arsenide or indium-gallium-arsenide, and germanium are fabricated
on germanium wafers. Much like GaAs, owing to the large bandgap difference
between GaAs (1.42 eV) and Ge (0.66 eV), the current match becomes very poor, and
as a result, current throughput suffers limited output efficiency. At present, efficiencies
for InGaP, GaAs, and Ge cells are within the 25–32 percent range. In a recent labora-
tory test of cells, using additional junctions between the GaAs and Ge junction pro-
duced efficiencies that exceeded 40 percent.

Indium-phosphide substrate Indium-phosphide is also used as a substrate to
fabricate cells with bandgaps between 1.35 and 0.74 eV. Indium-phosphide has a
bandgap of 1.35 eV. Indium-gallium-arsenide (In 0.53 Ga 0.47 As), lattice matched to indium-
phosphide, has a bandgap of 0.74 eV. An alloy composed of four elements, indium,
gallium-arsenide, and phosphide, has resulted in the fabrication of optically matched
lattices that perform with greater efficiency.
Recently, multijunction cell efficiencies have been improved through the use of con-
centrator lenses. This has resulted in significant improvements in solar energy conver-
sion and price reductions that have made the technology competitive with silicon flat-
panel arrays.

Polymer Solar Cells

Polymer solar cells, also referred to as plastic cells, are a relatively new technology that
coverts solar energy to electricity through the use of polymer materials. This class of
solar cells, unlike conventional semiconductors, is based on PV system technologies
(described earlier). Neither silicon nor any alloy material is used in their fabrication.
At present, polymer solar cells are being researched by a number of universities,
national laboratories, and several companies around the world. Compared with silicon-
based devices, polymer solar cells are lightweight, biodegradable, and inexpensive to
fabricate. The use of polymer substances renders the cells flexible, facilitating greater
design possibilities and diverse applications.
Because fullerene, a plastic-based material, is inexpensive and readily available, these
solar cells are extremely easy to mass produce at a cost of approximately one-third that
of traditional silicon solar cell technology. Some potential uses of polymer solar cells
include applications in a wide variety of commercial products, including small televi-
sions, cell phones, and toys.

BIONANOGENERATORS

Bionanogenerators are biologic cells that function like a fuel cell on a nanoscale,
molecular level. Bionanocells are essentially electrochemical devices that function like
galvanic cells. They use blood glucose, drawn from living cells, as a reactant or fuel.
This is similar to how the body generates energy from food. The bionanogeneration