U.S. patent application number 14/614601 was filed with the patent office on 2015-08-06 for monolithic multijunction power converter.
The applicant listed for this patent is SOLAR JUNCTION CORPORATION. Invention is credited to FERRAN SUAREZ ARIAS.
Application Number | 20150221803 14/614601 |
Document ID | / |
Family ID | 52472636 |
Filed Date | 2015-08-06 |
United States Patent
Application |
20150221803 |
Kind Code |
A1 |
SUAREZ ARIAS; FERRAN |
August 6, 2015 |
MONOLITHIC MULTIJUNCTION POWER CONVERTER
Abstract
Resonant cavity power converters for converting radiation in the
wavelength range from 1 micron to 1.55 micron are disclosed. The
resonant cavity power converters can be formed from one or more
lattice matched GaInNAsSb junctions and can include distributed
Bragg reflectors and/or mirrored surfaces for increasing the power
conversion efficiency.
Inventors: |
SUAREZ ARIAS; FERRAN; (SAN
JOSE, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SOLAR JUNCTION CORPORATION |
SAN JOSE |
CA |
US |
|
|
Family ID: |
52472636 |
Appl. No.: |
14/614601 |
Filed: |
February 5, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61936222 |
Feb 5, 2014 |
|
|
|
Current U.S.
Class: |
136/249 ;
136/255 |
Current CPC
Class: |
Y02P 70/50 20151101;
H01L 31/0547 20141201; H01L 31/054 20141201; Y02E 10/52 20130101;
H01L 31/022433 20130101; H01L 31/03048 20130101; H01L 31/02168
20130101; H01L 31/03046 20130101; Y02E 10/544 20130101; H01L
31/0725 20130101; H01L 31/0687 20130101 |
International
Class: |
H01L 31/0725 20060101
H01L031/0725; H01L 31/0216 20060101 H01L031/0216; H01L 31/0224
20060101 H01L031/0224; H01L 31/054 20060101 H01L031/054 |
Claims
1. A power converter, comprising: one or more GaInNAsSb junctions;
a first semiconductor layer overlying the one or more GaInNAsSb
junctions; and a second semiconductor layer underlying the one or
more GaInNAsSb junctions; wherein a thickness of the one or more
GaInNAsSb junctions, the first semiconductor layer and the second
semiconductor layer are selected to provide a resonant cavity at an
irradiated wavelength.
2. The power converter of claim 1, wherein each of the one or more
GaInNAsSb junctions, is lattice matched to GaAs; comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y, and z are 0.ltoreq.x.ltoreq.0.24, 0.01.ltoreq.y.ltoreq.0.07
and 0.001.ltoreq.z.ltoreq.0.20; and is characterized by a bandgap
corresponding to the energy of the irradiated wavelength.
3. The power converter of claim 1, wherein the wavelength is from
1.3 microns to 1.55 microns.
4. The power converter of claim 1, wherein the wavelength is 1.30
microns to 1.35 microns.
5. The power converter of claim 1, comprising: a first distributed
Bragg reflector overlying the first semiconductor layer; a second
distributed Bragg reflector underlying the second semiconductor
layer; or a first distributed Bragg reflector overlying the first
semiconductor layer and a second distributed Bragg reflector
underlying the second semiconductor layer.
6. The power converter of claim 1, comprising: a first distributed
Bragg reflector overlying the first semiconductor layer; a second
distributed Bragg reflector underlying the second semiconductor
layer; and a substrate underlying the second distributed Bragg
reflector.
7. The power converter of claim 1, comprising: a second distributed
Bragg reflector underlying the second semiconductor layer; and a
substrate underlying the second distributed Bragg reflector.
8. The power converter of claim 7, comprising an antireflection
coating overlying the first semiconductor layer.
9. The power converter of claim 1, comprising: a first distributed
Bragg reflector overlying the first semiconductor layer; and a back
mirror underlying the second semiconductor layer.
10. The power converter of claim 1, comprising: a second
distributed Bragg reflector underlying the second semiconductor
layer; and a back mirror underlying the second distributed Bragg
reflector.
11. The power converter of claim 1, comprising: a first distributed
Bragg reflector overlying the first semiconductor layer; a second
distributed Bragg reflector underlying the second semiconductor
layer; and a substrate overlying the first distributed Bragg
reflector.
12. The power converter of claim 1, comprising: a first distributed
Bragg reflector overlying the first semiconductor layer; a
substrate overlying the first distributed Bragg reflector; and a
back mirror underlying the second semiconductor layer.
13. The power converter of claim 1, comprising: a first lateral
conductive layer overlying the first semiconductor layer; and a
second lateral conductive layer overlying the second semiconductor
layer.
14. The power converter of claim 13, comprising: a first electrical
contact to the first lateral conductive layer overlying the first
semiconductor layer; and a second electrical contact to the second
lateral conductive layer overlying the second semiconductor
layer.
15. The power converter of claim 1, characterized by an efficiency
of at least 20% at an irradiated input power from 0.6 W to 6 W.
16. A power converter, comprising a plurality of the power
converters of claim 1 configured in a Pi structure.
17. A power converter, comprising a plurality of the power
converters of claim 1 interconnected in series.
Description
[0001] This application claims the benefit under 35 U.S.C. .sctn.
119(e) of U.S. Provisional Application No. 61/936,222, filed on
Feb. 5, 2014, which is incorporated by reference in its
entirety.
FIELD
[0002] The disclosure relates to the field of power conversion.
BACKGROUND
[0003] Power converters may be used in a number of applications to
charge electronic devices, such as cell phones, audio systems, home
theaters, or any other electronic devices, from a power source. It
is well known in the field that Ohmic losses are inversely related
to an increase in voltage and directly related to an increase in
current. It is advantageous, then, to increase the fill factor of
power converter devices by increasing the voltage of the
devices.
[0004] Prior art power converters in the field include
monolithically series-connected single layer converters made of
semiconductor wafers, such as GaAs. Such power converters may be
connected in series by wiring or sectored off by manufacturing the
converter on a semi-insulating substrate using insulating trenches
to provide electrical insulation between each sectored converter.
The energy source for such power converters is a monochromatic
light, such as a laser operating at a particular wavelength or
energy. In this particular application, the monochromatic light is
between 1 micron to 1.55 microns, in the infrared region of the
spectrum. Closer to 1 micron is less advantageous for home use due
to the potential dangers of the light source to the human eye, so
the focus of the embodiments disclosed herein is on light sources
between 1.3-1.55 microns, and in certain embodiments, around 1.3
microns. However, those skilled in the field may easily modify the
invention disclosed herein to convert light of a number of
wavelengths.
SUMMARY
[0005] The invention comprises a compact, monolithic multijunction
power converter, with two or more epitaxial layers of the same
material stacked on top of one another with tunnel junctions in
between each epitaxial layer. Because the epitaxial layers are
stacked on top of one another, each epitaxial layer is thinned to
collect a maximum amount of light and converts power in series to
increase the fill factor by increasing voltage of the overall
device and decreasing Ohmic losses (which increase with current
increase). Given the stacked epitaxial layers, light which is not
absorbed in one layer is absorbed in the next layer directly
beneath the first layer and so on. The power converter may reach an
overall efficiency of approximately 50%. There are minimal current
losses in these devices given that complex circuitry is avoided
using the vertical stacking of the epitaxial layers, compared to
the prior art, which requires interconnections between the
semiconductor light absorbing sectors.
[0006] In a first aspect, power converters are provided, comprising
one or more GaInNAsSb junctions; a first semiconductor layer
overlying the one or more GaInNAsSb junctions; and a second
semiconductor layer underlying the one or more GaInNAsSb junctions;
wherein a thickness of the one or more GaInNAsSb junctions, the
first semiconductor layer and the second semiconductor layer are
selected to provide a resonant cavity at an irradiated
wavelength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The drawings described herein are for illustration purposes
only. The drawings are not intended to limit the scope of the
present disclosure.
[0008] FIG. 1 shows an embodiment of a monolithic multijunction
power converter in which E.sub.1, E.sub.2, and E.sub.3 represent
semiconductor materials having the same bandgap.
[0009] FIGS. 2A and 2B show single junction and triple junction
resonant power converters, respectively, with dual distributed
Bragg reflectors (DBR), according to certain embodiments.
[0010] FIGS. 3A and 3B show single junction and triple junction
resonant power converters, respectively, with single DBRs,
according to certain embodiments.
[0011] FIGS. 4A and 4B show single junction and triple junction
resonant power converters, respectively, with a top DBR and a back
mirror, according to certain embodiments.
[0012] FIGS. 5A and 5B show single junction and triple junction
resonant power converters, respectively, with a back mirror,
according to certain embodiments.
[0013] FIGS. 6A and 6B show single junction and triple junction
resonant power converters, respectively, with two DBRs and a top
substrate, according to certain embodiments.
[0014] FIGS. 7A and 8B show single junction and triple junction
resonant power converters, respectively, with a substrate overlying
a top DBR and a back mirror, according to certain embodiments.
[0015] FIGS. 8A and 8B show single junction and triple junction
resonant power converters, respectively, with two DBRs and etched
back contacts to lateral conducting layers (LCL), according to
certain embodiments.
[0016] FIG. 9 shows a top view of a Pi structure having multiple
power converters interconnected in series, according to certain
embodiments.
[0017] FIGS. 10A and 10B show triple junction power converters
having a double pass configuration and characterized by a single
area (FIG. 10A) or four quadrant area (FIG. 10B), according to
certain embodiments.
[0018] FIGS. 11A and 11B show photographs of the top view of the
triple junction power converters shown schematically in FIGS. 10A
and 10B, respectively.
[0019] FIG. 12 shows the efficiency, power output, and voltage at
maximum power point (Mpp) as a function of laser input power for
single, double, and triple lattice-matched GaInNAsSb junction power
converters.
[0020] FIG. 13 shows the normalized density of current (J) as a
function of voltage for several laser input power levels for
single, double and triple lattice-matched GaInNAsSb junction power
converters.
[0021] Reference is now made in detail to embodiments of the
present disclosure. While certain embodiments of the present
disclosure are described, it will be understood that it is not
intended to limit the embodiments of the present disclosure to the
disclosed embodiments. To the contrary, reference to embodiments of
the present disclosure is intended to cover alternatives,
modifications, and equivalents as may be included within the spirit
and scope of the embodiments of the present disclosure as defined
by the appended claims.
DETAILED DESCRIPTION
[0022] In certain embodiments provided by the present disclosure,
two or more epitaxial layers of the same semiconductor material
grown on a substrate, such as GaInNAs, GaInNAsSb, GaAs, Ge, GaSb,
InP or other substrate known in the art, are stacked on top of one
another with tunnel junctions in between each epitaxial layer. FIG.
1 shows an embodiment of a monolithic multijunction power converter
in which E.sub.1, E.sub.2, and E.sub.3 represent semiconductor
materials having the same bandgap. Each epitaxial layer has the
same bandgap, which is roughly matched to the energy of the
monochromatic light source to minimize minority carrier and thermal
losses. In certain embodiments, the light source reaches the
uppermost epitaxial layer furthest from the substrate. In some
embodiments, the epitaxial layer material may be a dilute-nitride
material, such as GaInNAs or GaInNAsSb, or other dilute nitride
known in the art. In some embodiments, the monochromatic light
source is between 1 micron and up to 1.55 microns, and in certain
embodiments, the light source is approximately 1.3 microns. While
some current may be lost through light absorption by the tunnel
junction(s), light that is not collected in the first epitaxial
layer is collected in the second epitaxial layer, and so on. The
overall efficiency of such a device may reach at least 50% power
efficiency, such as from 50% to 60% or from 50% to 70%. In certain
embodiments, the power conversion efficiency of a single junction
power converter is at least 20% such as from 20% to 40%. In certain
embodiments, the power conversion efficiency of a single junction
power converter is at least 30% such as from 30% to 50%. In certain
embodiments, three junction devices provided by the present
disclosure exhibit a conversion efficiency from about 23% to about
25% over an input power from about 0.6 W to about 6 W when
irradiated with 1.32 micron radiation.
[0023] In certain embodiments, three or more epitaxial layers of
the same semiconductor material grown on a substrate such as
GaInNAs, GaInNAsSb, GaAs, Ge, GaSb, InP or other substrate known in
the art, are stacked on top of one another with tunnel junctions in
between each epitaxial layer. Increasing the number of junctions in
a power converter device can result in increased fill factor,
increased open circuit voltage (Voc) and decreased short circuit
current (Jsc). Each epitaxial layer has the same bandgap, which is
roughly matched to the energy of the monochromatic light source to
minimize minority carrier and thermal losses. In certain
embodiments, the light source reaches the bottom most epitaxial
layer closet to the substrate first. The substrate has a bandgap
that is higher than the bandgap of the epitaxial layers. Given that
the substrate has a higher bandgap than that of the epitaxial
layers, the light source passes through the substrate and the light
is absorbed by the epitaxial layers. An example of this employs
GaInNAs epitaxial layers (bandgap of 0.95 eV) and a GaAs substrate
(bandgap 1.42 eV). The light source in this example will not be
absorbed by the GaAs substrate and will be absorbed by the GaInNAs
active region. A heat sink can be coupled to the top of the
uppermost epitaxial layer, and can serve to cool the device and
prevent defects caused by overheating. In some embodiments, the
epitaxial layer material may be a dilute-nitride material, such as
GaInNAs or GaInNAsSb, or other dilute nitride known in the art. In
some embodiments, the monochromatic light source has a wavelength
between 1 micron and up to 1.55 microns, in certain embodiments,
from 1 micron to 1.4 micron, and in certain embodiments the light
source is approximately 1.3 microns. While some current may be lost
through light absorption by the tunnel junction(s), light that is
not collected in the first epitaxial layer can becollected in the
second epitaxial layer, and so on. The overall efficiency of such a
device may reach at least 50% power efficiency.
[0024] In certain embodiments, the light absorbing layer(s)
comprise GaInNAsSb. In certain of the embodiments, a GaInNAsSb
junction comprises Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z,
in which values for x, y, and z are 0.ltoreq.x.ltoreq.0.24,
0.01.ltoreq.y.ltoreq.0.07 and 0.001.ltoreq.z.ltoreq.0.20; in
certain embodiments, 0.02.ltoreq.x.ltoreq.0.24,
0.01.ltoreq.y.ltoreq.0.07 and 0.001.ltoreq.z.ltoreq.0.03; in
certain embodiments, 0.02.ltoreq.x.ltoreq.0.18,
0.01.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03; in
certain embodiments, 0.08.ltoreq.x.ltoreq.0.18,
0.025.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03; and in
certain embodiments, 0.06.ltoreq.x.ltoreq.0.20,
0.02.ltoreq.y.ltoreq.0.05 and 0.005.ltoreq.z>0.02.
[0025] In certain of the embodiments, a GaInNAsSb junction
comprises Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-ZSb.sub.z, in which
values for x, y, and z are 0.ltoreq.x.ltoreq.0.18,
0.001.ltoreq.y.ltoreq.0.05 and 0.001.ltoreq.z.ltoreq.0.15, and in
certain embodiments, 0.ltoreq.x.ltoreq.0.18,
0.001.ltoreq.y.ltoreq.0.05 and 0.001.ltoreq.z.ltoreq.0.03; in
certain embodiments, 0.02.ltoreq.x.ltoreq.0.18,
0.005.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03; in
certain embodiments, 0.04.ltoreq.x.ltoreq.0.18,
0.01.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03; in
certain embodiments, 0.06.ltoreq.x.ltoreq.0.18,
0.015.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03; and in
certain embodiments, 0.08.ltoreq.x.ltoreq.0.18,
0.025.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03.
[0026] In certain embodiments, a GaInNAsSb junction is
characterized by a bandgap of 0.92 eV and comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y, and z are: x is 0.175, y is 0.04, and
0.012.ltoreq.z.ltoreq.0.019.
[0027] In certain embodiments, a GaInNAsSb junction is
characterized by a bandgap of 0.90 eV and comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y, and z are: x is 0.18, y is 0.045, and
0.012.ltoreq.z.ltoreq.0.019.
[0028] In certain embodiments, a GaInNAsSb junction is comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y, and z are: 0.13.ltoreq.x.ltoreq.0.19,
0.03.ltoreq.y.ltoreq.0.048, and 0.007.ltoreq.z.ltoreq.0.02.
[0029] In certain embodiments, a GaInNAsSb junction comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y, and z are selected to have a band gap that matches or closely
matches the energy of the radiation used to deliver power to the
device. In certain embodiments, the GaInNAsSb junction is
substantially lattice matched to a GaAs substrate. It is to be
noted that the general understanding of "substantially lattice
matched" is that the in-plane lattice constants of the materials in
their fully relaxed states differ by less than 0.6% when the
materials are present in thicknesses greater than 100 nm. Further,
subcells that are substantially lattice matched to each other as
used herein means that all materials in the subcells that are
present in thicknesses greater than 100 nm have in-plane lattice
constants in their fully relaxed states that differ by less than
0.6%.
[0030] In certain embodiments, each of the epitaxial layers in the
power converter is lattice matched to a GaAs substrate.
[0031] In certain embodiments, the use of layering materials of
different refractive indices can produce distributed Bragg
reflectors (DBR) within the structure and is used to increase the
efficiency of the power converter. One such example uses a dilute
nitride material, which in certain embodiments is a GaInNAsSb
material, as the absorbing material in the epitaxial stack of the
structure. A cavity can be grown using a material such as
GaAs/AlGaAs as a DBR below the dilute nitride layer and above the
substrate, and another DBR grown above the dilute nitride layer,
that can be made of semiconductors or a number of oxides.
[0032] In certain embodiments, where the substrate has a higher
bandgap than the absorbing material, a back-side metal can be used
as structured mirror, allowing unabsorbed light to be reflected
from the back metal to be reabsorbed in the epitaxial layers above.
Examples of resonant cavity power converters using the double pass
configuration are shown in FIGS. 2A and 2B. FIG. 2A shows a single
junction resonant cavity with a top DBR and a bottom DBR. A single
GaInNAsSb junction is disposed between the two DBRs and separated
from the DBRs by semiconductor layers d1 and d2. Semiconductor
layers may be formed from a material that does not appreciably
absorb the incident radiation and that can be lattice matched to
GaAs and the absorbing layer, and in certain embodiments can be
GaAs. The thickness of d1, d2 and a GaInNAsSb junction can be
selected to provide a standing wave at the wavelength of the
incident radiation. FIG. 2B shows a similar configuration as shown
in FIG. 2A but includes multiple GaInNAsSb junctions with each of
the junctions separated by a tunnel junction. The thickness of the
GaInNAsSb junction can be from about 100 nm to about 1 micron. In
certain embodiments, the substrate is a semi-insulating or n-doped
GaAs substrate with a back-metal as the bottom-most layer of the
structure.
[0033] For use with 1 micron to 1.55 micron radiation, the mirror
layer can be, for example, gold or gold/nickel alloys.
[0034] In certain embodiments, the power converter structure uses
one DBR instead of two. Resonant power converters employing a
single DBR are shown in FIGS. 3A and 3B. FIG. 3A shows a single
GaInNAsSb junction disposed between two semiconductor layers d1 and
d2. These layers overly a bottom DBR, which overlies a substrate.
The upper surface of the device, such as the upper surface of layer
d1 facing the incident radiation may be coated with an
antireflection coating. The antireflection coating may be optimized
for the wavelength of the incident radiation to reduce scatteing.
FIG. 3B shows a single DBR resonant cavity configuration having
multiple GaInNAsSb junctions.
[0035] In certain embodiments, the power converter structure
includes one DBR and a back mirror below the substrate. Such device
configurations are shown in FIGS. 4A, 4B, 5A, and 5B. FIGS. 4A and
4B show power converters having a top DBR a resonant cavity
including a single GaInNAsSb junction between two semiconductor
layers d1 and d2, and a back mirror beneath semiconductor layer d2.
In certain embodiments, the back mirror can also serve as an
electrical contact. A multi junction power converter is shown in
FIG. 4B in which multiple GaInNAsSb junctions are disposed between
a top DBR and a back mirror.
[0036] In the power converters shown in FIGS. 5A and 5B both a DBR
and a back mirror are used at the bottom of the device. In this
configuration the thickness of the DBR can be reduced compared to a
configuration with a bottom DBR without the back mirror. As with
other devices, the upper surface of layer D1 may include an
antireflection coating. In certain embodiments, the substrate is
removed and a metal is used it its place as a back mirror. In such
structures, the light passes through the top DBR, then through the
epitaxial layers, then through the bottom DBR and finally hits the
back mirror. In these embodiments, the epitaxial layer comprises
GaInNAsSb as one or more absorbing layers.
[0037] In certain embodiments, the upper most layer of the
structure comprises an interface air-semiconductor above the
epitaxial layers, which may comprise of one or more layers of
GaInNAsSb. Below the epitaxial layer is a bottom DBR which overlays
a back mirror. In these embodiments, the light hits the upper most
layer of the interface air-semiconductor and moves to the epitaxial
layer, then the DBR and finally reflects back through the structure
after being reflected by the back mirror.
[0038] Resonant cavity configurations with two DBRs and a top
substrate layer are shown in FIGS. 6A and 6B. The top substrate
layer is substantially transparent to the incident radiation used
to generate the power. In certain embodiments, the substrate can be
GaAs such as n-type GaAs and can have a thickness from about 150
microns to about 250 microns, such as from 175 microns to 225
microns. The thickness of the substrate can be thinned, for
example, by grinding or etching to minimize absorption and in such
embodiments can be 50 microns or less. In certain embodiments, the
bottom DBR can be bonded to a heatsink. Bonding the DBR directly to
the heatsink can reduce the temperature of the power converter.
[0039] FIGS. 7A and 7B shown device configurations similar to those
shown in FIGS. 6A and 6B but with the bottom DBR replaced with a
back mirror.
[0040] In certain embodiments, the structure has intra-cavity
contacts to avoid resistivity from the DBR structures. The contact
is made in the cavity through lateral transport conducting layers
(LCL) bypassing the DBR structures. Power converters having
intra-cavity contacts are shown in FIGS. 8A and 8B. In these device
structures the epitaxial layers are etched down to either an LCL
overlying the bottom DBR or to an LCL overlying semiconductor layer
d1. The LCLs improve carrier mobility to the electrical contacts
(back contact and top contact) and can be formed, for example, from
doped GaAs such as n-type GaAs. LCLs and similar etch back
electrical contacts can be employed with other device structures
provided by the present disclosure.
[0041] In certain embodiments, the structure can be grown inverted.
In such cases, the substrate can be thinned down to a certain
thickness or removed after growth using a variety of lift off
techniques. The light passes through the substrate first before
passing through the epitaxy layers. In such structures, the bandgap
of the substrate is greater than the bandgap of the epitaxial
layers.
[0042] Multiple photovoltaic converters comprised of a number of
subcells connected in series can be constructed to increase the
output voltage. The subcells can be connected in parallel for
increasing output current. An example is a Pi structure as shown in
FIG. 9. Infrared absorbers are typically characterized by low
voltage; however, in certain application it is desirable to
increase the voltage of the power converter. This can be
accomplished by connecting multiple power converters in series. One
such configuration, of which a top-down view is shown in FIG. 9, is
referred to as a Pi structure in which multiple power converter
cells are disposed in concentric rings around a central axis, where
each cell is separated by an insulator and the multiple cells or
subsets of the multiple cells are connected in series. Such
structures can be fabricated using single junctions and provide a
high density of cells. The higher voltages provide improved DC-DC
converter efficiencies and lower Ohmic losses. Although later
currents can produce Ohmic losses this can be offset because the
increased number of sub-cells results in lower currents.
[0043] Other device structures are shown in FIGS. 10A and 10B. FIG.
10A shows single a triple junction double pass power converter.
FIG. 10B shows a four quadrant triple junction double pass power
converter. The dimensions of the devices are 300 microns by 300
microns. The four converters can be interconnected in series to
increase the voltage and/or decrease the current. The series
interconnection can also reduce the sensitivity to spatial
orientation of the incident radiation. Furthermore, for large area
power converters, separating the collection area into quadrants or
other sub-areas can reduce the Ohmic losses by bringing the
electrical contacts closer to the power generating surfaces.
Photographs of the single and four quadrant devices are shown in
FIGS. 11A and 11B.
[0044] The power converters shown in FIGS. 10A, 10B, 11A, and 11B
were fabricated using GaInNAsSb junctions. All epitaxial layers
were lattice matched to a GaAs substrate. A back mirror is disposed
at the bottom of the GaAs substrate. The resonant cavity of the
three junction structures was configured to support a standing wave
at about 1.3 microns, such as at 1.32 microns or at1.342 microns.
The bandgap of the GaInNAsSb junctions was about 0.92 eV for
devices configured for power conversion at 1.32 microns. Certain of
such devices exhibited a fill factor from about 65% to about 75%, a
Voc of from about 1.47 V to about 1.5 V and a Jsc from about 0.6 A
to about 1.4 A. The power conversion efficiency was from about 23%
to 25% at an input power from about 0.6 W to about 6 W.
[0045] In certain embodiments, the two or more epitaxial layers of
the same semiconductor material are of varying thicknesses. In
particular, the epitaxial layers can decrease in thickness the
further away from the light source. In certain embodiments, the
thicknesses of each of the epitaxial layers are the same. In
certain embodiments, the thicknesses of the epitaxial layers are
varied, either increasing nor decreasing depending on the light
source location.
[0046] In some embodiments, there is a window layer on top of the
upper most epitaxial layer.
[0047] In certain embodiments, the thickness, or height, of the
entire device may be between 1 micron and up to 10 microns. The
area of the power converter can be, for example, between 100
microns.times.100 microns, and up to 1 cm.times.1 cm, or more. For
example the total area is from 10.sup.-4 cm.sup.2 to 1 cm.sup.2.
The thickness of each epitaxial layer may be between a few hundred
nanometers up to a few microns.
[0048] FIG. 12 shows the efficiency, power output and voltage at
maximum power point (Mpp) as a function of laser input power for
single (open circle), double (square), and triple (plus) GaInNAsSb
junction power converters.
[0049] FIG. 13 shows the normalized current density (J) as a
function of voltage for several laser input power levels for single
(open circle), double (square), and triple (plus) GaInNAsSb
junction power converters.
[0050] Finally, it should be noted that there are alternative ways
of implementing the embodiments disclosed herein. Accordingly, the
present embodiments are to be considered as illustrative and not
restrictive. Furthermore, the claims are not to be limited to the
details given herein, and are entitled their full scope and
equivalents thereof.
* * * * *