Page 63 - Alternative Energy Systems in Building Design

40 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

Multijunction PV Cells

Multijunction solar cells were first developed and deployed for satellite power applica-
tions, where the high cost was offset by the net savings offered by the higher efficiency.
Multijunction PV cells are a special class of solar cells that are fabricated with multiple
levels of stacked thin-film semiconductor PN junctions. Their production technique is
referred to as molecular-beam epitaxy or the metal-organic vapor-phase process. Each
type of semiconductor is designed with a characteristic valance bandgap that allows for
the absorption of a certain bandwidth or spectrum of solar electromagnetic radiation.
In a single-layered or bandgap solar cell, valance-band efficiency is limited owing
to the inability of the PN junction to absorb a broad range of electromagnetic rays or
photons in the solar spectrum. Photons below the bandgap in the blue spectrum either
pass through the cell or, owing to molecular agitation, are converted within the mate-
rial into heat. Energy in photons above the bandgap in the red spectrum are also lost
because only the energy necessary to generate the electron-hole pair is used. The
remaining energy is converted into heat. Multijunction solar cells, which have multi-
ple layers and therefore junctions with several bandgaps, allow different portions of
the solar spectrum to be converted by each junction at greater efficiency. Figure 3.9
shows the epitaxial layers of a four-junction solar cell.

MULTIJUNCTION SOLAR CELL CONSTRUCTION
Multijunction PV cells use many layers of film deposition, or epitaxy. By using differing
alloys within the eighth column of the periodic table, the semiconductors, the bandgap of
each layer is tuned to absorb a specific band of solar electromagnetic radiation. The
efficiency of multijunction solar cells is achieved by the precise alignment of respective
superimposed bandgaps.
To achieve maximum output efficiency, all epitaxial layers that are in series are opti-
cally aligned from top to bottom such that the first junction receives the entire spectrum.
Photons above the bandgap of the first junction are absorbed in the first layer (red-spec-
trum photons). Green and yellow photons below, which pass through the first layer, are
absorbed by the second bandgap, and finally, the third bandgap absorbs the high-ener-
gy blue-spectrum photons.
Most commercial cells use a tandem P-N electrical connection, which allows series
cumulative or composite current output through positive and negative terminals. An
inherent design constraint of tandem-cell configuration is that in series connection, the
current through each junction becomes limited by the ohmic resistance of the material,
and since the point current of each junction is not the same, the efficiency of the cell
is reduced.

MULTIJUNCTION SOLAR CELL MATERIALS

In general, most multijunction cells are categorized by the substrate used for cell manu-
facture. Depending on the bandgap characteristics, multijunction solar cell substrates
are constructed from various epitaxial layers that make use of different combinations