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Bandgap Tailoring for Improved Efficiency of p-p-n AlGaAs-GaAs-GaAs Solar Cells

IP.com Disclosure Number: IPCOM000108682D
Original Publication Date: 1992-Jun-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 122K

Publishing Venue

IBM

Related People

Mohammad, SN: AUTHOR

Abstract

Some design criteria are suggested for the improvement of conversion efficiency of p-n-p AlGaAs-GaAs-GaAs solar cells, which are attractive for space applications because of higher conversion efficiency (even at higher temperatures) and good resistance to space radiation damage. Currently AM1 conversion efficiency of these cells is about 21-22%. As conversion of light to electricity is perhaps the most viable option to obtaining electricity at higher altitudes, a 5-6% points improvement in their AM1 conversion efficiency, which is theoretically possible, is highly desirable.

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Bandgap Tailoring for Improved Efficiency of p-p-n AlGaAs-GaAs-GaAs Solar Cells

       Some design criteria are suggested for the improvement of
conversion efficiency of p-n-p AlGaAs-GaAs-GaAs solar cells, which
are attractive for space applications because of higher conversion
efficiency (even at higher temperatures) and good resistance to space
radiation damage.  Currently AM1 conversion efficiency of these cells
is about 21-22%. As conversion of light to electricity is perhaps the
most viable option to obtaining electricity at higher altitudes, a
5-6% points improvement in their AM1 conversion efficiency, which is
theoretically possible, is highly desirable.

      Quantum efficiency for photons to create electrical current in
the 2.5-3.2 eV region of the solar spectrum (see FIG. 1) is low.  As
ionized acceptors in the depletion regions BC and DC of the p-AlGaAs/
p-GaAs junction having interface at C (see FIG. 2) repel each other,
a large area of p-AlGaAs is depleted of free carriers.  These free
carriers are pushed away toward A.  As a result, both valence band
and conduction band edges are bent down at A, creating a large
potential Vp=Nv/Pop, (where Pop is the density of free carriers and
Nv is the effective density of states for holes in the valence band).
The Fermi level Ef in the p-A1GaAs region is pinned down, its pinning
position being almost mid-gap, ~ 1.0eV from a band edge.  These band
edges at position A thus become lower than those at B and C, causing
electrons in the conduction band of p-A1GaAs emitter to tend to move
toward the surface away from the depletion region BCD, and the holes
in the valence band to tend to move toward the depletion region BCD.
As the surface contains traps and dislocations, the photogenerated
electrons are trapped by the traps, when they reach the surface.
These are thus lost, contributing nothing to the photocurrent, and
causing a degradation in quantum efficiency of the solar cell.

      Recombination of electron-hole pairs in the bulk and in the
surface of various semiconductor regions is relatively high.  For
example, when electrons tend to flow through the interface at C, they
undergo recombination in the BCD depletion region.  Further
recombination occurs in the bulk of p-GaAs base, at the EFG depletion
region, and in the bulk of n-GaAs.  By the time these photoelectrons
reach the...