U.S. patent number 5,223,043 [Application Number 07/884,312] was granted by the patent office on 1993-06-29 for current-matched high-efficiency, multijunction monolithic solar cells.
This patent grant is currently assigned to The United States of America as represented by the United States. Invention is credited to Sarah R. Kurtz, Jerry M. Olson.
United States Patent |
5,223,043 |
Olson , et al. |
June 29, 1993 |
Current-matched high-efficiency, multijunction monolithic solar
cells
Abstract
The efficiency of a two-junction (cascade) tandem photovoltaic
device is improved by adjusting (decreasing) the top cell thickness
to achieve current matching. An example of the invention was
fabricated out of Ga.sub.0.52 In.sub.0.48 P and GaAs. Additional
lattice-matched systems to which the invention pertains include
Al.sub.x Ga.sub.1-x /GaAS (x= 0.3-0.4), GaAs/Ge and Ga.sub.y
In.sub.l-y P/Ga.sub.y+0.5 In.sub.0.5-y As (0<y<5).
Inventors: |
Olson; Jerry M. (Lakewood,
CO), Kurtz; Sarah R. (Golden, CO) |
Assignee: |
The United States of America as
represented by the United States (Washington, DC)
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Family
ID: |
27096531 |
Appl.
No.: |
07/884,312 |
Filed: |
May 11, 1992 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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653543 |
Feb 11, 1991 |
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Current U.S.
Class: |
136/249 |
Current CPC
Class: |
H01L
31/0687 (20130101); Y02E 10/544 (20130101) |
Current International
Class: |
H01L
31/06 (20060101); H01L 31/068 (20060101); H01L
031/078 () |
Field of
Search: |
;136/249TJ |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
J M. Olson et al, Conf. Record, 20th IEEE Photovoltaic Specialists
Conf. (1988) pp. 777-780. .
S. P. Tobin et al, Conf. Record, 20th IEEE Photovoltaic Specialists
Conf. (1988), pp. 405-410. .
"Monolithic AlGaAs/Ge Cascade Solar Cells" M. L. Timmons, J. A.
Hutchby, D. K. Wagner, and J. M. Tracy, Conference Record, 20th
IEEE Photovoltaic Specialists Conference (1988), pp. 602-606. .
"Quantum Yield Spectra and I-V Properties of a GaAs Solar Cell
Grown on a Ge Substrate", L. D. Partain, G. F. Virshup, and N. R.
Kaminar, Conference Record, 20th IEEE Photovoltaic Specialists
Conf. (1099), pp. 759-763. .
"High Altitude Current-Voltage Measurement of GaAs/Ge Solar Cells"
Russell E. Hart, Jr. and David J. Brinker, Keith A. Emery,
Conference Record, 20th IEEE Photovoltaic Specialists Conf. (1988),
pp. 764-765. .
"Lightweight GaAs/Ge Solar Cells", Rushikesh M. Patel, S. W.
Gersten, D. R. Perrachione, Y. C. M. Yeh, and D. K. Wagner, Applied
Solar Energy Corporation; and R. K. Morris Conference Record, 20th
IEEE Photovoltaic Specialists Conf. (1988), pp. 607-610..
|
Primary Examiner: Weisstuch; Aaron
Attorney, Agent or Firm: Richardson; Kenneth Albrecht; John
M. Moser; William R.
Government Interests
CONTRACTUAL ORIGIN OF THE INVENTION
The United States has rights in this invention under contract No.
DE-AC02 83CHI0093 between the U.S. Department of Energy and the
Solar Energy Research Institute, a division of Midwest Research
Institute.
Parent Case Text
This is a Continuation of application Ser. No. 07/653,543 filed
Feb. 11, 1991, now abandoned.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A high-efficiency multijunction photovoltaic solar cell,
consisting essentially of:
a top semiconductor cell fabricated from Ga.sub.x In.sub.l-x P
wherein x is (0<x<0.5) a light-sensitive n/p homojunction
therein for absorbing higher energy photons;
a bottom semiconducor cell fabricated from GaAs with a light
sensitive n/p homojunction therein for absorbing lower energy
photons; and wherein said top cell thickness is optimized by
thinning to from 0.5 to 1.7 micorsn and less than said bottom cell
thickness in order to provide current matching between said top
cell and said bottom cell in order to obtain improved conversion
efficiency for AMO and AM1.5;
a low-resistance attachment between the top cell and the bottom
cell, wherein said top cell is lattice matched to said bottom cell;
and
electrical contact means attached to opposite sides of said solar
cell to conduct current away from and into said solar cell.
2. The multijunction photovoltaic solar cell claim 1, wherein said
Ga.sub.x In.sub.l-x P has a band gap of about 1.9 eV.
3. The multijunction photovoltaic solar cell of claim 2 wherein
said bottom cell comprises a p.sup.+ GaAs substrate with a p-GaAs
absorber layer deposited on said substrate, and an n.sup.30 -GaAs
emitter layer deposited on said p-GaAs absorber layer.
4. The multijunction photovoltaic cell of claim 2, which, when
exposed to solar radiation, has an open circuit voltage (V.sub.oc)
equal to at least 2.3 V, a short circuit current (J.sub.sc) of at
least 13.6 mA/cm.sup.2 (ARC), a fill factor (FF) of at least 0.87,
and an efficiency of at least 27.3% Am1.
5. An improved method of converting solar radiation to electrical
energy, comprising the steps of:
directing the solar radiation into a first cell comprising Ga.sub.x
In.sub.l-x P, where X=0.51.+-.0.05, with a band gap of about 1.9
eV, and having an n/p homojunction therein for absorbing and
converting solar radiation of a wavelength of about 0.65 .mu.m or
less to electrical energy to product about 1.4V of open-circuit
voltage;
directing unabsorbed solar radiation into a second cell comprising
GaAs that is integrally grown and lattice connected with said
Ga.sub.x In.sub.l-x P and having an n/P homojunction therein for
absorbing and converting the remaining solar radiation of
wavelength of about 0.85 .mu.m or less to electrical energy to
produce about lV of open-circuit voltage, which GaAs second cell is
lattice-matched with said Ga.sub.x In.sub.l-x P first cell at a low
resistance tunnel heterojunction; and connecting said first and
second cells in series to an electrical load wherein said first
cell thickness is optimized by thinning from 0.5 to 1.7 microns and
less than the second cell thickness in order to provide current
matching between said first cell and said second cell to obtain
improved energy conversion efficiency for AMO and AM1.5.
6. A method of converting solar radiation to electrical energy,
comprising:
directing solar radiation into a Al.sub.y Ga.sub.l-y As top cell is
optimized by thinning from 0.5 to 1.7 microns and is less than the
thickness of the GaAs bottom cell; said top cell is latticed
matched at a p+/n+GaAs tunnel-junction interface with said bottom
cell and in which there is an n/p homojunction in the top cell and
another n/p homojunction in said bottom cell;
absorbing higher energy solar radiation of 0.65 .mu.m or greater
wavelengths in said top cell and converting it to electrical energy
in said top cell while allowing lower energy solar radiation to
pass through said top cell into said bottom cell;
absorbing lower energy radiation of 0.65 .mu.m or greater
wavelength and converting it in said bottom cell to electrical
energy;
connecting said device to an electric load; wherein said top
thickness is thinned to less than said bottom cell thickness in
order to provide current matching between said top cell and said
bottom cell to obtain improved current conversion efficiency;
and
converting the solar radiation to electrical energy of at least
V.sub.oc =2.9V, J.sub.sc =13.6 mA/cm.sup.2, and a FF=0.87 with an
efficiency of at least 27.3% AMl.
7. A high-efficiency multijunction photovoltaic solar cell,
consisting essentially of:
a top semiconductor cell fabricated from Al.sub.y Ga.sub.l-y Al,
wherein y=0.3-0.4, with a light-sensitive n/p homojunction therein
for absorbing higher energy photons;
a bottom semiconductor cell fabricated from GaAs with a light
sensitive n/p homojunction therein for absorbing lower energy
photons; and wherein said top cell thickness is optimized by
thinning from 0.5 to 1.7 microns and less than said bottom cell
thickness in order to provide current matching between said top
cell and said bottom cell in order to obtain improved conversion
efficiency for AMO and AM1.5;
a low-resistance attachment between the top cell and the bottom
cell wherein said top cell is lattice matched to said bottom cell;
and
electrical contact means attached to opposite sides of said solar
cell to conduct current away from and into said solar cell.
8. A high-efficiency multijunction photvoltaic solar cell,
consisting essentially of:
a top semiconductor cell fabricated from GaAs with a
light-sensitive n/p homojunction therein for absorbing higher
energy photons;
a bottom semiconductor cell fabricated from Ge with a light
sensitive n/p homojunction therein for absorbing lower energy
photons; and wherein said top cell thickness is optimized by
thinning from 0.5 to 1.7 microns and less than said bottom cell
thickness in order to provide current matching between said top
cell and said bottom cell in order to obtain improved conversion
efficiency for AMO and AM1.5;
a low-resistance attachment between the top cell and the bottom
cell wherein said top cell is lattice matched to said bottom cell;
and
electrical contact means attached to opposite sides of said solar
cell to conduct current away from and into said solar cell.
9. A high-efficiency multijunction photovoltaic solar cell,
consisting essentially of:
a top semiconductor cell fabricated from Ga.sub.x In.sub.l-x P,
wherein x is (0<x<0.5), with a light-sensitive n/p
homojunction therein for absorbing higher energy photons;
a bottom semiconductor cell fabricated from Ga.sub.x+0.5
In.sub.0.5-X As, wherein x is (0<x<0.5), with a light
sensitive n/p homojunction therein for absorbing lower energy
photons; and wherein said top cell thickness is optimized by
thinning from 0.5 to 1.7 microns and less than said bottom cell
thickness in order to provide current matching between said top
cell and said bottom cell in order to obtain improved conversion
efficiency for AMO and AM1.5;
a low-resistance attachment between the top cell and the bottom
cell wherein said top cell is lattice matched to said bottom cell;
and
electrical contact means attached to opposite sides of said solar
cell to conduct current away from and into said solar cell.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention generally relates to photovoltaic solar cells and,
more particularly, to high-efficiency multi-junction photovoltaic
solar cell devices in which the monolithic device uses GaInP as a
thin top cell in order to improve current matching between the top
and bottom cells. The current-matched device has a higher
efficiency than the current-mismatched device because of a higher
device current, and, in cases of low surface-recombination
velocities, a higher device voltage. Specific lattice-matched
material systems which also yield higher efficiencies under the
current invention include Al.sub.x Ga.sub.l-x As/GaAs (x=0.3-0.4),
GaAs/Ge and Ga.sub.y,In.sub.l-y P/Ga.sub.y+0.5-y As
(0<y<0.5).
2. Description of the Prior Art
Solar cells, also known as photovoltaic cells, are semi conductors
that convert electromagnetic energy, such as light or solar
radiation, directly to electricity. These semiconductors are
characterized by solid crystalline structures that have energy
bands gaps between their valence electron bands and their
conduction electron bands, so that free electrons cannot ordinarily
exist or remain in these band gaps. However, when light is absorbed
by the materials that characterize the photovoltaic cells,
electrons that occupy low-energy states are excited and jump the
band gap to unoccupied higher energy states. Thus, when electrons
in the valence band of a semiconductor absorb sufficient energy
from photons of solar radiation, they jump the band gap to the
higher energy conduction band.
Electrons excited to higher energy states leave behind them
unoccupied low-energy positions which are referred to as holes.
These holes may shift from atom to atom in the crystal lattice and
the holes act as charge carriers, in the valence bond, as do free
electrons in the conduction band, to contribute to the crystal's
conductivity. Most of the photons that are absorbed in the
semiconductor produce such electron-hole pairs. These electron-hole
pairs generate photocurrent and, in the presence of a built-in
field, the photovoltage of the solar cells.
Electron hole pairs produced by the light would eventually
recombine, and convert to heat or a photon the energy initially
used to jump the band gap, unless prevented from doing so. To
prevent this phenomenon, a local electric field is created in the
semiconductor by doping or interfacing dissimilar materials to
produce a space charge layer. The space charge layer separates the
holes and electrons for use as charge carriers. Once separated,
these collected hole and electron charge carriers produce a space
charge that results in a voltage across the junction, which is the
photovoltage. If these separated hole and charge carriers are
allowed to flow through an external load, they would constitute a
photocurrent.
It is well known that photon energies in excess of the threshold
energy gap or band gap between the valence and conduction bands are
dissipated as heat; thus they are wasted and do no useful work.
Specifically, there is a fixed quantum of potential energy
difference across the band gap in the semiconductor. For an
electron in the lower energy valence band to be excited to jump the
band gap to the higher energy conduction band, it must absorb a
sufficient quantum of energy from an absorbed photon with a value
at least equal to the potential energy difference across the band
gap.
A semiconductor is transparent to radiation with photon energy less
than the band gap. But if the electron absorbs more than the
threshold quantum of energy, e.g., from a larger energy photon, it
can jump the band gap. The excess of such absorbed energy over the
threshold quantum required for the electron to jump the band gap
results in an electron that is higher in energy than most of the
other electrons in the conduction band. Electrons that have energy
levels higher than the lower edge of the conduction band, i.e., the
top edge of the band gap, are referred to as "hot electrons". For
every electron excited out of its normal energy level, there is a
corresponding "hole". Thus, for each hot electron there can be a
corresponding hot hole; both are generally referred to as "hot
carriers".
These hot carriers lose their excess energy to the host lattice
very rapidly as heat. The process in which the hot carriers
dissipate their excess energy to the host lattice and equilibrate
with the lattice at ambient temperature is known as thermalization.
As a result, such thermalization of hot carriers reduces the
carriers in energy to the energy level at the edge of the
conduction band. Such thermalization normally occurs in about
10.sup.-12 seconds with the result that, the effective photovoltage
of a single band gap semiconductor is limited by the band gap.
In practice, the effect of the limitation is that the semiconductor
designer must sacrifice efficiencies in one area to achieve them in
another. For example, to capture as many photons from the spectrum
of solar radiation as possible, the semiconductor must be designed
with a small band gap so that even photons from lower energy
radiation can excite electrons to jump the band gap, but, in doing
so, there are at least two negative effects that must be
traded.
First, the small band gap results in a low photovoltage device, and
thus low power output occurs. Secondly, the photons from higher
energy radiation will produce many hot carriers with much excess
energy that will be lost as heat upon immediate thermalization of
these hot carriers to the edge of the conduction band. On the other
hand, if the semiconductor is designed with a larger band gap to
increase the photovoltage and reduce energy loss caused by
thermalization of hot carriers, then the photons from lower energy
radiation will not be absorbed. Therefore, in designing
conventional single junction solar cells, it is necessary to
balance these considerations and try to design a semiconductor with
an optimum band gap, realizing that in the balance, there has to be
a significant loss of energy from both large and small energy
photons. Materials, such as silicon with a band gap of 1.1 eV, are
relatively inexpensive and are considered to be good solar energy
conversion semiconductors for conventional single junction solar
cells; however, the band gap of GaAs is even better. Nevertheless,
a need exists for a device that can capture and use a larger range
of photon energies from the solar radiation spectrum, and yet not
sacrifice either photovoltage or excess energy loss to heat by
thermalization of hot carriers.
It was shown several years ago that two-junction photovoltaic cells
have the potential for achieving solar energy conversion
efficiencies than single junction cells..sup.1 The simplest
junction device is a monolithic, two-terminal, two-junction
structure, wherein the two junctions are stacked vertically. The
top junction is designed to absorb and convert the blue portion of
the solar spectrum and the bottom junction absorbs and converts the
red portion of the spectrum that is not absorbed by the top
junction. To achieve maximum energy conversion efficiency: 1) the
junctions must be fabricated from materials that are of high
electronic quality (usually achievable for systems which are
lattice matched), and 2) they must also be current matched, i.e.
generate equal currents when exposed in the tandem configuration to
the solar spectrum. The current matching is determined by the
relative band gap energies of the two materials.
Only a few material combinations are known that satisfy both these
criteria. Strictly speaking, only the AlxGaI.xAs/GaAs system allows
for both a lattice-matched and current-matched system for both
space (AMO) and terrestrial (AM 1.5) applications. However, the
high aluminum content makes it difficult to achieve material of
high electronic quality despite good lattice matching. Other
material combinations including Ga.sub.x In.sub.l-x P/Ga.sub.x+0.5
In.sub.0.5-x As and GaAs/Ge meet both criteria depending on the
spectrum under consideration, but are usually subject to some loss
in efficiency. For tandem solar cells which are not exactly current
matched the extra current that is generated either in the top or
the bottom cell is lost. Of the multiple publications which have
calculated the efficiencies of two-junction, III-V-like solar
cells, the only suggested method for recovering this lost current
is to use a 3.or 4-terminal device (using either independent or
parallel connection). Use of 3. and 4.terminal devices has been
considered to be substantially less convenient, yet a necessary
remedy to an otherwise unsolvable problem for material combinations
which lie away from the current-matched region.
In connection with amorphous silicon solar cells both U.S. Pat.
Nos. 4,272,641 and 4,271,328 teach that the current and therefore
the efficiency of a series-connected multi-junction solar cell
where all of the unit cells have the same optical band gap is
optimum when the thicknesses of unit cells closer to the incident
light surface are selected to be less than that of cells farther
from the incident light surface.
The problem that this invention addresses is taught by Hanak in
U.S. Pat. No. 4,272,641. He teaches that the conversion efficiency
of a single junction a-Si solar cell approaches a constant when the
intrinsic region thickness exceeds about 500 nm See column 4, lines
6-16). This is due to an inherent problem with the electronic
quality of a-Si. If a-Si could be made with better electronic
properties then cells thicker than 500 nm would yield higher
efficiencies. This problem is circumvented in a multi-junction cell
where all of the unit cells have a thickness less than or equal to
500 nm. Hanak specifically teaches that the top cell (Region 22,
FIGS. 1 & 2) is made to have a thickness between 40 and 500 nm
(see Col. 3, lines 9-11). The thickness of the bottom cell (Region
26, FIGS. 1 & 2) is then adjusted so that "the current produced
by said layer is about equal to the current produced by the first
active layer 2. . . " (see Col. 4, lines 1-5).
Hanak also teaches use of multiband gap, multi-junction devices as
an alternative method of matching currents in a tandem cell. The
reasoning is that a top cell with a high enough band gap will
obviate the need for current matching by the thinning of the top
cell. Hanak and Hamakawa et al. do not teach literally a combined
approach of thinning the top cell in a multiband gap, tandem solar
cell.
The line of reasoning outlined above does not apply to III-V
materials like GaAs, InP, AlGaAs, GaInPz, or InGaAs. They have
inherently excellent electronic properties, and the conversion
efficiency of devices made from these materials continue to
increase for thicknesses much larger than 500 nm. Furthermore, a
tandem device comprised of two GaAs unit cells (the GaAs analogue
of an a-Si tandem cell) would have an efficiency less than that of
a single junction GaAs cell.
We argue that the physical differences between a-Si and III-V
materials is a subtle cognitive barrier that precludes the obvious
foresighted extension of a-Si art to the III-V materials systems.
This is evident if one considers the scope and direction of III-V
tandem research and development since 1981. During this time period
there have appeared in the literature several analyses of the
efficiency of tandem solar cells as a function of top and bottom
cell band gaps (e.g. Fan et al. Proceedinqs of the 16th
Photovoltaic Specialists Conference, pp. 692-701, 1982; Nell and
Barnett, IEEE Trans. Electron Devices. Vol. 34, p. 257, 1987). All
of these calculations assume complete absorption by the respective
unit cell of light with energy greater than the band gap energy of
the unit cell, i.e. they only consider optically thick unit cells.
There are also reports in the literature of the efficiency of III-V
tandem devices including GaIn,P. 2/GaAs (Olson et al. 20th IEEE
PVSC p.777 1988), AlGaAs/GaAs (Virshup et al.20th IEEE PVSC p.441
1988) and GaAs/Ge (Timmons et al. 20th IEEE PVSC p.602 1988). All
Of these devices suffer a loss in efficiency because of poor
current matching. All try to compensate for this loss by increasing
the band gap of the top cell. In all cases this "cure" made the
overall efficiency worse for reasons that are unique to III-V
materials. In all three cases, the more effective cure would be to
simply reduce the thickness of the top cell to some optimum
thickness. That is exactly what Olson et al. have recognized and
done, and the other "equally-skilled-in-the-art" researchers have
not.
SUMMARY OF THE INVENTION
It is a general object of the invention to permit high-efficiency
two-junction monolithic solar cells to be made from a greater
number of material combinations, or to increase the efficiency of
two-junction monolithic solar cells. This is achieved by thinning
the top cell in order to equalize the currents generated in the top
and bottom cells.
It is a specific object of the invention to provide increased
efficiencies from two-junction monolithic solar cells made from top
cell/bottom cell material combinations of Ga.sub.x In.sub.lwx
P/GaAs, Al.sub.x Ga.sub.l-x As/GaAs (x=0.3-0.4), GaAs/Ge, and
Ga.sub.y In.sub.l-y P/Ga.sub.y+0.5 In.sub.0.5-y As (0<y<0.5)
by achieving current matching by thinning the top cell.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevation view schematic of the GaInP.sub.2 /GaAs
monolithic cascade cell exemplifying the present invention.
FIG. 2a is a graph showing the bulk electronic quality of
GaInP.sub.2 as a function of growth parameters where photocurrent
versus Tg and IV/III for GaInP.sub.2.
FIG. 2b is a graph showing the bulk electronic quality of
GaInP.sub.2 as a function of growth parameters where the
photocurrent versus band gap or lattice mismatch as measured by
x-ray diffraction.
FIG. 3 is a schematic of a tandem solar cell model used in the
invention.
FIG. 4a is an is efficiency plot for two-junction, series-connected
tandem structures under AM1.5 global, 1-sun illumination at 300K.
The top-cell thickness was adjusted for current matching.
FIG. 4b is an isoefficiency plot for two-junction, series-connected
tandem structures under AMl.5 global, 1-sun illumination at 300K.
The top-cell thickness was infinite.
FIG. 5a shows an isoefficiency plot for two-junction, series
connected tandem structures under AMl.5 global, 1-sun illumination
at 300K. The top-cell thickness was adjusted for current
matching.
FIG. 5b shows an isothickness plot showing the top-cell thicknesses
used for the calculation of FIG. 5a.
FIG. 5c shows an isoefficiency plot for two-junction,
series-connected tandem structures under AM1.5 global, 1-sun
illumination at 300K The top-cell thickness was infinite.
FIG. 6a shows efficiency and open-circuit voltage as a function of
top-cell band gap for a bottom cell with band gap of 1.42eV and AM
1.5 global irradiation. Curve A assumes an infinite thickness for
the top cell. Curve B varies the top-cell thickness to achieve
current matching but does not recalculate J.sub.o. Curve C repeats
Curve B, but includes a recalculation of J.sub.o, assuming no
surface recombination. In contrast to the other curves, curve D was
calculated for a four-terminal device, using the same models as for
Curves A-C and an infinite thickness for the top cell.
FIG. 6b shows efficiency curves A and C as shown in FIG. 6a, except
that AMO irradiance spectra were used.
FIG. 6c shows efficiency curves A and C as shown in FIG. 6a, except
that AM1.5 direct normal irradiance spectra were used.
FIG. 7 shows the effect of p-layer thickness and surface
recombination on the open-circuit voltage, wherein dimensionless
list variables are used for the p-layer thickness. The change in
open-circuit voltage is shown for various surface-recombination
ratios.
FIG. 8 shows the top-cell thicknesses as a function of top-cell
band gap to achieve the current-matched condition for a bottom cell
with band gap 1.424 eV, wherein parameters applicable to Ga.sub.x
In.sub.l.x P were used for the absorption calculation, and these
thicknesses were used for calculation for curves B and C in FIG.
6.
FIG. 9 in the lower portion of the Figure shows the top-cell (solid
lines) and bottom-cell (dashed lines) currents as a function of
top-cell thickness and band gap. The upper portion shows the
corresponding tandem-cell efficiencies. A band gap of 1.424 eV was
used for the bottom cell. Band gaps of A) 1.70 eV, B) 1.85 eV, C)
1.90 eV, D) 1.92 eV, E) 1.94 eV, and F) I.96 eV were used for the
top cells.
DETAILED DESCRIPTION OF THE INVENTION
In general, the invention pertains to a two-junction, tandem solar
cell in which the top cell has been thinned in order to match the
currents generated by the top and bottom cell. Ordinarily, the
thickness of a solar cell is chosen thick enough so that most of
the light with energy higher than the band gap of the material is
absorbed, yet thin enough that carriers generated toward the back
of the cell can still be collected, (i.e. the thickness should not
be much greater than the minority carrier diffusion length). In
this way the current generated by the cell is effectively a
maximum. For direct gap materials these criteria usually lead to a
thickness of about 3 .mu.m. According to the prior art, therefore,
a two-junction tandem solar cell is made by stacking a top and
bottom cell, both of which are about 3 .mu.m thick. In the present
invention, the bottom cell is made in the conventional manner, but
the top cell is made thinner so that some of the light with energy
above the band gap of the top cell passes through the top cell and
is absorbed by the bottom cell, thus increasing the current
generated by the bottom cell. In this way, the top- and bottom-cell
currents are equalized, optimizing the efficiency of the tandem
cell.
The invention pertains only to material systems that have top-cell
currents larger than bottom-cell currents. If the top-cell current
is smaller than the bottom-cell current, thinning the top-cell will
lead to a greater current mismatch. Although the bottom cell could
be thinned, in such a case, to achieve current matching, the
efficiency gain is negligible since the top-cell current will
remain fixed while the bottom-cell current is reduced. Thus, the
material systems which are affected by the current invention
include only those which have top-cell currents larger than the
bottom-cell currents. Specifically, these include Ga.sub.x
In.sub.l-x P/Ga.sub.x+0.5 In.sub.0.5-x As (0<x<0.5), Al.sub.y
Ga.sub.l-y As/GaAs (y=0.3-0.4) and GaAs/Ge.
The currents generated in the top and bottom cells are functions
not only of the band gaps of the two materials, but also of the
solar spectrum and the quantum efficiency. The solar spectrum
depends on the air mass (zenith angle of the sun) and various
atmospheric conditions including humidity, turbidity, and cloud
cover. Outside of the earth's atmosphere the solar spectrum is
markedly different than at the earth's surface. Also, the spectrum
will depend upon the geometry of collection: for cells operating
under concentration only the direct beam is collected, while for
flat-plate solar cells, off-normal radiation (which is rich in
short wavelength light) can also be collected. Although terrestrial
solar cells must operate under a range of conditions it is
customary to design the solar cell for optimal efficiency under a
set of conditions which will be representative of the application.
Typically, solar cells are designed for operation in space under
air mass 0 illumination, or for terrestrial operation under air
mass I.5 illumination.
Table 1 shows the estimated top cell thicknesses which will lead to
current matching for the noted band gap combinations and material
systems. We have selected two of the most commonly used spectra
rather than trying to make an exhaustive list. It is not possible
to specify exact thicknesses since these depend (as noted above)
not only on the spectrum, but on the quantum efficiency of the
device. The quantum efficiency is dependent on the quality of the
solar cell material, the quality (small absorption and good
passivation) of the window layer (front-surface field) and
back-surface passivation and/or reflection, and the reflectivity of
the entire device. For example, we estimate that a 30 nm layer of
Al.sub.0.5 In.sub.0.5 P (used as a window layer for the top cell)
and a 30 nm-thick GaAs tunnel junction each absorb approximately 3%
of the light destined for the respective underlying subcell. In
this case the losses do not affect the current matching, but in
another case it might.
TABLE 1. ______________________________________ Examples of Tandem
Cells With Optimized Top Cell Thickness Top Cell Thickness Top Cell
Thick- Bandgap combination for AM 1.5, Global ness for AMO Examples
(microns) (microns) ______________________________________
1.9/1.424 eV 1.0 0.7 Ga.sub.x In.sub.1-x P/GaAs or Al.sub.x
Ga.sub.1-x As/GaAs 1.85/1.424 eV 0.7 0.5 Ga.sub.x In.sub.1-x P/GaAs
or Al.sub.x Ga.sub.1-x As/GaAs 1.65/1.1 eV 0.8 0.7 Ga.sub.x
In.sub.1-x P/Ga.sub.y In.sub.1-y As 1.424/0.8 eV 1.0 1.6 GaAs/Ge
(direct (gap) 1.38/0.7 eV 0.7 1.7 InP/Ga.sub.0.5 In.sub.0.5 As
______________________________________
We describe in detail the fabrication of a Ga.sub.0.5 In.sub.0.5
P/GaAs two-junction solar cell, where the band gap combination is
1.85 and 1.424 eV. The fabrication of similar devices from the
other material combinations shown in Table 1 can be accomplished by
substituting the appropriate reagents and flows.
A schematic of the GaInP.sub.2 /GaAs monolithic cascade cell is
shown in FIG. 1. The structure was grown in a vertical, air-cooled
reactor at one atmosphere using organometallic chemical vapor
deposition (OMCVD), the detailed aspects of which are described
elsewhere..sup.2 The Group III source gases were trimethylindium,
trimethylgallium, and trimethylaluminum; the Group V source gases
were arsine and phosphine. The dopant sources were diethylzinc and
hydrogen selenide. The optoelectronic properties.sup.3 and
photovoltaic quality.sup.4 of the materials set forth above are
complex and coupled functions of the growth temperature, T.sub.g,
the V/III materials ratio, composition, dopant type and
concentration, and substrate quality and the effects of some of
these factors are demonstrated.
Generally, the cascade device is grown at T.sub.g =700.degree. C.,
The phosphides are grown with V/III =30 and a growth rate of
80-100nm/minute; the arsenides, with V/III =35 and a growth rate of
120-150 nm/minute, with the exception that the GaAs tunnel diode is
grown at a rate of 40 nm/minute.
The absorbers of both subcells are doped with Zn to a level of 1-4
x 10.sup.17 cm.sup.-3, and the emitters and window layers are doped
with Se at about 10.sup.18 cm.sup.-3. Both layers of the GaAs
tunnel diode are heavily doped at concentrations approaching
10.sup.19 cm.sup.-3. Tunnel diodes grown under conditions
simulating the fabrication of a full cascade device have a series
resistance of 10.sup.-3 -10.sup.-2 ohm cm.sup.2, and exhibit other
characteristics that are similar to those reported by Saletes et
al..sup.5 For example, they are relatively stable at 700.degree. C.
for at least 30-40 minutes.
The front and back contacts of these devices were electroplated
gold. A high dopant concentration is used in both the GaAs
substrate and the top GaAs contacting layer (not shown in the FIG.
1), and therefore, no thermal annealing of either contact is
required. The front contact is defined by photolithography and
obscures approximately 5% of the total cell area. The cell
perimeter is also defined by photolithography and a mesa etch that
uses a sequential combination of concentrated hydrochloric acid and
an ammonia:peroxide:water solution. The ammonia/peroxide solution
is also used to remove the GaAs contacting layer between the gold
grid fingers. The antireflection coating (ARC) is a double layer of
evaporated ZnS and MgF.sub.2, with thicknesses of 60 and 120 nm,
respectively.
Cell efficiency was measured using the multisource simulator method
of Glatfelter and Burdick..sup.6 The simulated solar spectrum was
adjusted using two reference cells. One reference cell was a
GaInP.sub.2 top cell and the second was a GaAs cell coated with the
GaAs tunnel junction and a layer of GaInPz to simulate the optical
transmission to the GaAs bottom cell in the actual tandem device.
The spectrum of the simulator was adjusted with filters until both
reference cells produced the correct ASTM E892 87 global,
short-circuit current at 1000 W/cm.sup.2. Using this spectrum, the
voltage characteristic of the cascade cell was measured.
The best efficiency measured for this device is 27.3% (1 sun, air
mass 1.5). The short circuit current density, J.sub.xc, open
circuit voltage, V.sub.oc, and fill factor, ff, were 13.6 mA
cm.sup.-2, 2.29V, and 0.87, respectively. The area of this device
was 0.25 cm.sup.2 and the band gap of the top cell was 1.85 eV.
This is the highest efficiency reported for a two-terminal,
tunnel-junction-interconnected tandem photovoltaic device, and
represents a significant improvement with respect to previously
reported work..sup.7 Part of this improvement is a direct result of
using a thinner top cell. Recently, Chung et al.,.sup.8 have
reported a 27.6% efficient monolithic AlGaAs/GaAs solar cell;
however, this device has a metal, as opposed to a tunnel-junction,
interconnect and includes a prismatic cover slip to eliminate the
photocurrent loss associated with grid shadowing. In the case of
the present invention, the use of the prismatic cover slip would
boost the efficiency from 27.3 to 28.7%.
Numerous factors affect the efficiency of a multi junction solar
cell, including, but not limited to, the electronic quality of the
top and bottom cell materials, the band gap and thickness of the
top cell, the design of the ARC, and the thickness and passivating
properties of the window layers.
Bulk electronic quality of GaInP.sub.2 as a function of growth
parameters is illustrated in FIGS. 2a and 2b. Plotted is the
relative white-light photocurrent generated at an
electrolyte/GaInP.sub.2 junction as a function of the GaInP.sub.2
growth temperature, the V/III ratio and lattice mismatch between
the GaInP and the GaAs. This photocurrent is a function mainly of
the minority carrier diffusion length in the GaInP.sub.2, and,
therefore, is a suitable quantity to use as a measure of
quality..sup.9 The highest relative currents shown in FIG. 2a for
GaInP.sub.2 are indicative of a minority carrier diffusion length
in the range of 3-6 .mu.m and a peak internal quantum efficiency of
about 90%. It is evident from FIG. 2a that the quality of
GaInP.sub.2 is relatively insensitive to Tg and V/III. This is
contrary to previous reports where the photoluminescence intensity
of GaInP.sub.2 grown by MOCVD was found to increase by a factor of
10.sup.3 when the growth temperature.sup.10 was decreased from
675.degree. C. to 650.degree. C. or the V/III ratio was increased
from 100 to 200.
The effect of lattice mismatch is shown in FIG. 2b. The lattice
mismatch (proportional to .phi., the Bragg peak separation) is
measured using the double-crystal rocking mode x-ray diffraction
technique. As expected, the photocurrent decreases rapidly for
tensional strains greater than some critical mismatch strain. For
material grown in compression, the photocurrent increases with
increasing mismatch. This increase is due to a concomitant decrease
in the band gap of the Ga.sub.x In.sub.l-x P. This result is
contrary to a previous report.sup.12 that suggested that precise
control of the composition of GaInP.sub.2 was required to achieve
device-quality material.
A model that couples the optical properties of the double layer ARC
with those of the underlying junctions materials was developed for
this study. The reflection coefficient, R, of an AIInP.sub.2
-coated GaInP.sub.2 epilayer is about 30% for wavelengths between
450 and 900 nm. A ZnS and MgF.sub.2 ARC with thicknesses of 60 and
120 nm, respectively, reduces R to less than 2% over the same range
of wavelengths. While these thicknesses are close to those that one
would calculate from simple quarter wavelength considerations, the
detailed effects of these antireflection layers is considerably
more complicated. For example, the optical modeling studies show
that the current matching between the top and bottom cells is a
strong function of the ZnS and MgF.sub.2 thicknesses.
By using a similar optical model, it was calculated that a 30 nm
layer of AlInP and 30 nm-thick GaAs tunnel junction each absorb
approximately 3% of the light destined for the respective
underlying subcell. It is apparent that most of this light is not
converted into external current, and that the tandem cell current
therefore suffers a net loss of 3%.
It has been shown that AlInPz passivates the surface of
GaAs,.sup.13 reducing the surface recombination velocity S, to less
than 100 cm/s. For the AlInPz/GaInPz interface, S has not been
measured; however, controlled experiments show that the blue
response and total short circuit current density of GaInP.sub.2
solar cells are enhanced by the use of an AlInP window layer.
For the GaInP.sub.2 /GaAs tandem cell, a significant potential loss
mechanism is associated with current matching between the top and
bottom cells. As stated previously, the top cell and bottom cell
currents, J.sub.T and J.sub.B, respectively, are determined
primarily by the band gaps of the top and bottom cell materials. It
was assumed in previous treatments of this problem that the
subcells were infinitely thick and that quantum efficiencies were
equal to 100%. With assumptions, for a bottom-cell band gap of 1.42
eV the optimum top cell band gap for an AM1.5 solar spectrum is
1.93 eV. Because the nominal or classical band gap of GaInpz is 1.9
eV, it is expected that for a thick, high quality GaInPz top cell
on a GaAs bottom cell, J.sub.T /J.sub.B >1. Worse yet, the band
gap of OMCVD-grown GaInP.sub.2 can be as low as 1.82 eV.
The solution to this problem is to reduce the thickness of the
GaInP.sub.2 top cell. The method used to calculate the top cell
thickness for the GaInP2/GaAs tandem cell is given in the next
section. These calculations suggest that top cell thicknesses in
the range of 500 to 100 nm, depending on the top cell band gap and
quantum efficiency yield a current-matched tandem structure at a
current level that changes only slightly with top cell band gap.
The device in FIG. 1 has top cell band gap of 1.85 eV and a top
cell of thickness of 0.7 .mu.m. This thickness is slightly larger
than the optimum value, so that this device may not be current
matched; however, it is clear that a GaInP.sub.2 /GaAs,
two-terminal tandem cell with a 1-sun, air mass 1.5 has a
conversion efficiency of 27.3%, the highest efficiency reported for
a tunnel-junction interconencted tandem device, thus factors
limiting the optimum efficiency of this device have been
identified.
METHOD
The theoretical efficiencies of two-junction, infinitely thick,
series-connected solar cells based on III-V-like (direct gap) solar
cells have been published by a number of researchers. However, the
theoretic efficiencies for similar devices with thinned top cells
have not been published. We present here a description of how using
optimal top-cell thicknesses increases the efficiencies primarily
because of an increase in short-circuit current, but also, in the
case of low back-surface recombination, by increasing the
open-circuit voltage.
The theoretical solar-cell efficiencies were calculated for air
mass (AM) 1.5 global, AM 1.5 direct normal, and, in some cases, AMO
spectra, using the irradiance standards published by Hulstrom, et
al., and trapezoidal integration. The total power densities are 964
and 768 W/m2, for the global and direct normal spectrums,
respectively.
In contrast to earlier referred to publications which assumed that
each cell was infinitely thick.sup.15 (i.e. absorbed all of the
light with energy greater than the material's band gap), the
short-circuit currents were calculated.sup.16 as a function of the
top-cell thickness as follows: ##EQU1## where J.sub.T and J.sub.B
are the photocurrents of the top and bottom cells, respectively, e
is the electronic charge, Io (.lambda.) is the incident intensity
as a function of wavelength (taken from the spectral tables of
Hulstrom, et al.),.sup.17 .alpha. is the absorption coefficient of
the top-cell material, and t is the thickness of the top cell. The
integration step width, was 1/30 of the wavelength step shown in
the tables..sup.18
The short-circuit current, J.sub.sc, of the cell was then taken as
the lesser of J.sub.T and J.sub.B. The thickness of the top cell
was adjusted to obtain J.sub.t =J.sub.B whenever possible (i.e.
whenever J.sub.T >J.sub.B for the infinite-thickness cells),
.+-.1 .mu.A/cm.sup.2. This arbitrary limit gives an accuracy of
about 0.1% for the thickness and 0.01% for J.sub.SC in most
cases.
The reverse saturation current density, J.sub.o, was calculated for
each cell as the sum of the currents for the n- and p-type layers
using the cell design in FIG. 1: ##EQU2## where N.sub.A and N.sub.D
are the acceptor and donor concentrations taken from FIG. 3 S.sub.h
and S.sub.e are the surface-recombination velocities in the n- and
p-type materials, x.sub.p and x.sub.n are the thicknesses of the p-
and n-type layers, respectively, D.sub.e and D.sub.h are the
diffusion constants for electrons and holes, respectively,
calculated from the Einstein relationship, n.sub.i is the intrinsic
carrier concentration, and .tau.e and .tau.h are the minority
carrier lifetimes.
Ideally, the reverse saturation currents would be calculated in
each case using the parameters for a known material of that band
gap. However, this is not practical, so literature values for GaAs
were used with scaling appropriate for band gap. In the cases where
an actual top-cell thickness is calculated, the absorption
coefficient and other parameters applicable to GaInP.sub.2 were
used.
The open-circuit voltage, V.sub.oc, was calculated.sup.19 from:
##EQU3## The power from the cell is given by the product of the
cell voltage V, and cell current J, and was maximized by satisfying
the condition ##EQU4## where the cell voltage was calculated from
the combined current-voltage curves: ##EQU5## J.sub.TO and J.sub.OB
are the reverse saturation currents for the top and the bottom
cells, respectively. This represents an improvement over the
previously-used method.sup.20 of calculating the maximum power
points of the two separate cells.
The isoefficiency contour plot is shown for the AM1.5 global
spectrum in FIG. 4a. For comparison, the corresponding plot
assuming an infinite thickness for the top cell is shown in FIG.
4b.
A comparison of FIGS. 4a and 4b or 5a, 5b, and 5c shows the effect
of varying the the thickness of the top cell to obtain
current-matched condition. The effect of thinning the top cell can
be understood better by studying curves A, B, and C of FIG. 6a,
which show the efficiency of a tandem structure with a bottom-cell
band gap of 1.42 eV as a function of top-cell band gap. Curve A
represents the efficiency when the top cell is required to have an
infinite thickness. Curve B was calculated by varying the thickness
of the top cell to obtain current-matched conditions, but without
recalculating J.sub.o. This gives an increase in J.sub.SC but no
change in V.sub.OC. Curve C shows the effect of recalculating
J.sub.O for the thinner layer. The smaller J.sub.O results in an
increase in V.sub.OC as shown in the upper half of the graph, and
therefore, an increase in efficiency. Comparison of curves A, B and
C shows that the primary advantage of using thinner top cells is a
gain in J.sub.SC, but that in cases of low surface recombination,
additional gain is made in the V.sub.OC.
The gain in V.sub.OC will be less than that shown in FIG. 6a when
the surface-recombination velocities are not zero. The combined
effects of surface-recombination velocity and thinning of the cell
are shown in FIG. 7. For simplicity, only the effect of
recombination at one surface is considered. In this case the
subscripts refer to recombination in the p-type material This is
done by setting S.sub.h L.sub.h /D.sub.h =1, where L.sub.h is the
minority carrier diffusion length of holes. This makes J.sub.o
independent of the n-layer thickness. The relative V.sub.oc is then
a function of x.sub.p /L.sub.e and S.sub.e L.sub.e /D.sub.3. which
are dimensionless expressions for the p-layer thickness and p-layer
surface-recombination velocity, respectively, and L.sub.e is the
diffusion length of minority carrier electrons. For S.sub.e L.sub.e
/D.sub.e =1 does not vary with the p-layer thickness. For lower
surface-recombination velocities, the V.sub.oc increases as the p
layer is thinned. However, it should be noted that the V.sub.oc is
expected to decrease as the cell is thinned when the
surface-recombination velocity is high. The same graph is equally
applicable to n-type material if all subscripts are replaced
appropriately.
The thickness which were used for the calculations in FIG. 6a are
noted at the top of FIG. 6a and are replotted for easy reference in
FIG. 7. The only practical limitation for implementing these
thinner cells is control of the layer thickness and uniformity.
This can be evaluated from FIG. 9, which shows the efficiency and
bottom-cell and top-cell currents as a function of top-cell
thickness and band gap, assuming a bottom cell with a band gap of
1.424 eV. If one assumes that the thickness can be controlled to
+/-10%, a band gap as low as 1.85 eV still has an ideal efficiency
of between 33% and 34%. However, a band gap as low as 1.70 eV is
not practical since a 10% variation of thickness would result in a
variation of several efficiency points. FIG. 9 is of particular
interest for the system GaAs/Ga.sub.0.5 In.sub.0.5 P since the band
gap of Ga.sub.0.5 In.sub.0.5 P varies anomalously with deposition
conditions from about 1.82 eV to about 1.90 eV.sup.21 . FIGS. 8 and
9 show an optimal top-cell thickness of about 0.75 .mu.m for a cell
GaAs/Ga.sub.0.5 In.sub.0.5 P with top-cell band gap of 1.85-1.86
eV. Thus, there was fabricated a current-matched, 27% efficient
tandem cell with a top-cell band gap of 1.85-1.86 eV using a
thickness of about 0.7 .mu.m. Given the assumptions for the model
(e.g. no surface recombination) which is an especially bad
assumption for the top cell) and experimental uncertainties (in the
absorption coefficient and thickness measurements) this is very
good agreement between theory and experiment.
Strictly speaking, the optimal top-cell thickness would be achieved
for the current-matched condition at the maximum-power point,
rather than under the short-circuit conditions as calculated.
However, this difference is quite small, for high band gap devices,
as can be seen from FIG. 9. This difference becomes significantly
greater for lower band gap cells that have small fill factors.
The efficiency of the series-connected tandem cells under different
irradiance spectra can be compared in FIGS. 6a, 6b, and 6c. At
lower air mass the spectrum has more short-wavelength light,
increasing the optimal top-cell band gap for a GaAs bottom cell.
However, the effect of thinning the top cell is roughly equivalent
in each case. The top-cell thicknesses used in FIG. 6a and plotted
in FIG. 8 would be almost identical for FIGS. 6b and 6c if
correction is made for the difference in optimal top-cell band
gap.
The figures showing the thicknesses required for current matching
were all obtained using the absorption coefficient of GaInP.sub.2
for the top cell, and these results can easily be used on other
systems by increasing or decreasing the thickness proportionally
when the absorption coefficient is smaller or larger,
respectively.
The foregoing calculated efficiencies of two-junction,
series-connected tandem solar cells with optimized top-cell
thicknesses and the thinning of the top-cell thickness in some
cases resulted in large increases in the calculated efficiencies.
Moreover, most of the increase came from a higher J.sub.sc since
current matching could be achieved. In cases of low
surface-recombination velocities, the V.sub.oc, also increased when
the top cell was thinned. It is clear that, a thin top cell in a
tandem structure is shown to be feasible even when the top-cell
band gap is decreased by 10 meV from the optimal band gap.
The description hereinabove is considered illustrative only of the
principles of the invention, and numerous modifications and changes
will readily occur to those skilled in the art. Accordingly, all
suitable modifications and equivalents may be resorted to falling
within the scope of the invention as defined by the claims that
follow.
* * * * *