U.S. patent application number 10/723456 was filed with the patent office on 2004-08-05 for apparatus and method for optimizing the efficiency of a bypass diode in solar cells.
Invention is credited to Clevenger, Marvin Brad, Sharps, Paul R., Stan, Mark A..
Application Number | 20040149331 10/723456 |
Document ID | / |
Family ID | 25546513 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040149331 |
Kind Code |
A1 |
Sharps, Paul R. ; et
al. |
August 5, 2004 |
Apparatus and method for optimizing the efficiency of a bypass
diode in solar cells
Abstract
Apparatus and Method for Optimizing the Efficiency of a Bypass
Diode in Solar Cells. In a preferred embodiment, a layer of TiAu is
placed in an etch in a solar cell with a contact at a doped layer
of GaAs. Electric current is conducted through a diode and away
from the main cell by passing through the contact point at the GaAs
and traversing a lateral conduction layer. These means of
activating, or "turning on" the diode, and passing the current
through the circuit results in greater efficiencies than in prior
art devices. The diode is created during the manufacture of the
other layers of the cell and does not require additional
manufacturing.
Inventors: |
Sharps, Paul R.;
(Albuquerque, NM) ; Clevenger, Marvin Brad;
(Albuquerque, NM) ; Stan, Mark A.; (Albuquerque,
NM) |
Correspondence
Address: |
Daniel McGlynn / Emcore Corporation
145 Belmont Drive
Somerset
NJ
08873
US
|
Family ID: |
25546513 |
Appl. No.: |
10/723456 |
Filed: |
November 26, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10723456 |
Nov 26, 2003 |
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09999598 |
Oct 24, 2001 |
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6680432 |
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Current U.S.
Class: |
136/255 ;
136/249; 257/E27.123; 257/E31.022 |
Current CPC
Class: |
H01L 31/0735 20130101;
H01L 31/1852 20130101; H01L 27/1421 20130101; Y02E 10/544 20130101;
Y02P 70/50 20151101; H01L 31/0725 20130101; H01L 31/0687 20130101;
H01L 31/184 20130101; H01L 31/03046 20130101; Y02E 10/547 20130101;
Y10S 136/293 20130101 |
Class at
Publication: |
136/255 ;
136/249 |
International
Class: |
H01L 031/00 |
Claims
What is claimed is:
1. A solar cell comprising: a multijunction solar cell structure
including at least one or more epitaxially grown layers on a
substrate, and having at least a first, second, and third subcell;
the first photovoltaic subcell having a first photoactive junction
monolithically grown above the substrate; the second photovoltaic
subcell having a second photoactive junction monolithically grown
above the first subcell; the third photovoltaic subcell having a
third photoactive junction created through diffusion of epitaxially
grown material into the substrate; and an epitaxially grown diode
that is integral to at least a portion of the first subcell.
2. The multijunction solar cell as defined in claim 1, wherein such
epitaxially grown diode is electrically connected across at least
said first and second cells to protect such first and second cells
against reverse biasing.
3. The multijunction solar cell as defined in claim 1, the
epitaxially grown diode having a Schottky contact.
4. The multijunction solar cell as defined in claim 1, further
comprising a Ge substrate.
5. The multijunction solar cell as defined in claim 1, further
comprising a second subcell fabricated at least in part of
InGaP.
6. The multijunction solar cell as defined in claim 1, further
comprising a first subcell fabricated at least in part of GaAs.
7. The multijunction solar cell as defined in claim 1, further
comprising a first subcell fabricated at least in part of
InGaAs.
8. The multijunction solar cell as defined in claim 1, further
comprising at least a third subcell fabricated at least in part of
Ge.
9. The multijunction solar cell described in claim 1, wherein said
bypass diode is space qualified to operate in an AM 0
environment.
10. The multijunction solar cell described in claim 1, wherein said
diode is about 0.5 to 3.0 microns thick.
11. A solar cell comprising: a multijunction solar cell structure
including at least one or more epitaxially grown layers on a
substrate and having at least a first, second, and third subcell;
the first photovoltaic subcell having a first photoactive junction
monolithically grown above the substrate; the second photovoltaic
subcell having a second photoactive junction monolithically grown
above the first subcell; the third photovoltaic subcell having a
third photoactive junction created by diffusion of epitaxially
grown material into the substrate; and an epitaxially grown diode
having a Schottky contact that is integral to at least a portion of
the first subcell.
12. The multijunction solar cell as defined in claim 11, wherein
such epitaxially grown diode is electrically connected across at
least said at first and second cells to protect such first and
second cells against reverse biasing.
13. The multijunction solar cell as defined in claim 11, further
comprising a Ge substrate.
14. The multijunction solar cell as defined in claim 11, further
comprising a second subcell fabricated at least in part of
InGaP.
15. The multijunction solar cell as defined in claim 11, further
comprising a first subcell fabricated at least in part of GaAs.
16. The multijunction solar cell as defined in claim 11, further
comprising a first subcell fabricated at least in part of
InGaAs.
17. The multijunction solar cell as defined in claim 11, further
comprising at least a third subcell fabricated at least in part of
Ge.
18. The multijunction solar cell described in claim 11, wherein
said bypass diode is space qualified to operate in an AM 0
environment.
19. The multijunction solar cell described in claim 11, wherein
said diode is about 0.5 to 3.0 microns thick.
20. A solar cell comprising: a multijunction solar cell structure
including at least one or more epitaxially grown layers on a
substrate and having at least a first, second, and third subcell;
the first photovoltaic subcell having a first photoactive junction
monolithically grown above the substrate; the second photovoltaic
subcell having a second photoactive junction monolithically grown
above the first subcell; the third photovoltaic cell having a third
photoactive junction created by diffusion into the substrate; and
an epitaxially grown diode having a lateral conduction layer.
21. The multijunction solar cell as defined in claim 20, wherein
the lateral conduction layer is comprised of highly doped
n.sup.+-GaAs.
22. The multijunction solar cell as defined in claim 20, wherein
the lateral conduction layer is comprised of InGaAs.
23. The multijunction solar cell as defined in claim 20, further
comprising a Ge substrate.
24. The multijunction solar cell as defined in claim 20, further
comprising a second subcell fabricated at least in part of
InGaP.
25. The multijunction solar cell as defined in claim 20, further
comprising a first subcell fabricated at least in part of GaAs.
26. The multijunction solar cell as defined in claim 20, further
comprising a first subcell fabricated at least in part of
InGaAs.
27. The multijunction solar cell as defined in claim 20, further
comprising at least a third subcell fabricated at least in part of
Ge.
28. The multijunction solar cell defined in claim 20, wherein said
bypass diode is space qualified to operate in an AM 0
environment.
29. The multijunction solar cell defined in claim 20, wherein said
diode is about 0.5 to 3.0 microns thick.
30. The multijunction solar cell defined in claim 20, wherein said
GaAs lateral conduction layer is about 0.1 to 2.0 microns
thick.
31. A monolithic diode integral to a multijunction solar cell
comprising: a Schottky contact and a structure comprised of at
least a GaAs buffer layer, an InGaP contact layer, and a lateral
conduction layer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to solar cells and methods for
their fabrication, and more particularly to optimizing the
efficiency of a bypass diode within solar cells.
DESCRIPTION OF THE RELATED ART
[0002] Photovoltaic cells, also called solar cells, are one of the
most important new energy sources that have become available in the
past several years. Considerable effort has gone into solar cell
development. As a result, solar cells are currently being used in a
number of commercial and consumer-oriented applications. While
significant progress has been made in this area, the requirement
for solar cells to meet the needs of more sophisticated
applications has not kept pace with demand. Applications such as
satellites used in mobile and telephone communications have
dramatically increased the demand for solar cells with improved
power and energy conversion characteristics.
[0003] In satellite and other space related applications, the size,
mass, and cost of a satellite power system is dependent on the
power and energy conversion efficiency of the solar cells used.
Putting it another way, the size of the payload and the
availability of on-board services are proportional to the amount of
power provided. Thus, as the payloads become more sophisticated,
solar cells, which act as the power conversion devices for the
on-board power systems, become increasingly more important.
[0004] Solar cells are often used in arrays, an assembly of solar
cells connected together in a series. The shape and structure of an
array, as well as the number of cells it contains, are determined
in part by the desired output voltage and current.
[0005] When all of the solar cells in an array are receiving
sunlight or are illuminated, each cell will be forward biased.
However, if any of the cells are not illuminated, because of
shadowing or damage, those cells may become reversed biased in
order to carry the current generated by the illuminated cells. This
reverse biasing can degrade the cells and can ultimately render the
cells inoperable. In order to prevent reverse biasing, a diode
structure is often implemented.
[0006] The purpose of the bypass diode is to draw the current away
from the shadowed or damaged cell. The current flows through the
bypass diode and it becomes forward biased when the shadowed cell
becomes reverse biased. Rather than forcing current through the
shadowed cell, the diode draws the current away from the shadowed
cell and maintains the connection to the next cell.
[0007] Different types of bypass diodes have been utilized in prior
art. In some configurations the bypass diode is connected to the
exterior of a solar cell array. This is a difficult device to
manufacture, and charges the array assembler with a task perhaps
better performed by the cell manufacturer. Another conventional
method to provide bypass diode protection to a solar cell array has
been to connect a bypass diode between adjacent cells, with the
anode of the bypass diode connected to one cell and the cathode of
the diode connected to an adjoining cell. However, this technique
is complicated to manufacture and requires a very difficult and
inefficient assembly method. Another technique for providing a
bypass diode for each cell involves a recess formed onto the back
of each cell and a bypassed diode being placed into this recess.
This technique has not yet proven efficient to manufacture because
of the delicacy of the cells and because the technique requires the
connection of the adjoining cells to be formed by the assembler of
the array as opposed to the cell manufacturer.
[0008] Given the foregoing, there is a necessity for an integral
bypass diode which can be manufactured by the cell manufacturer as
an integral part of the cell itself, taking the responsibility for
the bypass away from the array assembler. Rather than working with
an array assembler to develop the most efficient means of including
a bypass diode as part of the array, the device demonstrated by
this invention allows the cell manufacturers to simply give the
array assemblers an all-in-one cell that can be tightly packed and
arrayed without the necessity of adding an additional bypass diode
device to the array.
[0009] U.S. Pat. No 6,278,054 (the "'054 Patent") describes an
integral bypass diode that attempts to address these drawbacks.
However, the '054 Patent requires additional epitaxial layers to be
grown on top of the multijunction cell structure, and the bypass
diode described in the '054 Patent is for a homojunction made out
of GaAs. Most of this additional epitaxial layer has to be etched
off the front of the cell, leaving only a small area to be used as
the bypass diode.
[0010] European Patent no. 1 056 137 A1, Application Serial No.
00109681.7 discloses a Schottky diode for a two junction or single
junction device, in contrast to the device described herein.
SUMMARY OF THE INVENTION
[0011] A preferred embodiment of the present invention provides a
method for manufacturing a bypass diode in a multijunction solar
cell with at least three junctions.
[0012] In accordance with the purpose of the invention, as embodied
and broadly described herein, the invention relates to an
alternative and more effective way of optimizing the efficiency of
and manufacturing a bypass diode so as to create a monolithic
integral bypass diode that can be manufactured with a high rate of
efficiency and when operable will require less voltage than prior
art devices.
[0013] Advantages of the invention will be set forth, in part, in
the description that follows and, in part, will be understood by
those skilled in the art from the description herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate an embodiment
of the invention and, together with the description, serve to
explain the principles of the invention.
[0015] FIG. 1 illustrates one embodiment of the present invention,
a multijunction solar cell, after the completion of all processing
steps, illustrating the composition of such embodiment;
[0016] FIG. 2 illustrates the two paths current in the cell
illustrated in FIG. 1 can take, given a particular set of
circumstances;
[0017] FIG. 3 illustrates one embodiment of the present invention,
a multijunction solar cell, prior to any processing steps;
[0018] FIG. 4 illustrates a first processing step used to construct
one embodiment of the present invention; and
[0019] FIG. 5 illustrates the second and third processing steps
used to construct one embodiment of the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0020] The following describes the present invention more fully
with reference to the accompanying drawings, in which a preferred
embodiment of the invention is shown. The present invention may,
however, be embodied in many different forms and should not be
construed as being limited to the embodiment set forth herein; this
embodiment is provided so that this disclosure will be thorough and
complete and will fully convey the invention to those skilled in
the art.
[0021] The present invention relates to a multijunction solar cell
with at least one integral monolithic bypass diode. The layers
comprising the solar cell are particularly chosen for their
combination of efficiency and manufacturability. As discussed
below, one embodiment consists of a multijunction structure with at
least three junctions, with a unique modified buffer structure.
[0022] The process of manufacturing the solar cell with an integral
monolithic bypass diode is comprised of five distinct steps, which
are described below.
[0023] FIG. 1 is an illustration of an embodiment of the invention,
a monolithic solar cell with an integral bypass diode. FIG. 2 is a
series of schematic drawings of the two possible current paths
through the cell.
[0024] FIG. 1 shows a multijunction solar cell 100 with a cell of
Indium Gallium Phosphorus (InGaP) 101 and a cell of Gallium
Arsenide (GaAs) 102 over a GaAs buffer 103 on top of a Germanium
(Ge) substrate 104. When the solar cell 100 is illuminated, both a
voltage and a current are generated. FIG. 2A represents the solar
cell as seen in FIG. 4, without the metalization 107 and lateral
conduction layer 113 described below. If the solar cell is
illuminated, there will be no barrier to the current following the
cell path 201 through the layers of the solar cell: the Ge junction
104, the GaAs junction 102, and the InGaP junction 101.
[0025] However, when the solar cell 100 is not receiving sunlight,
whether because of shading by a movement of the satellite, or as a
result of damage to the cell, then resistance exists along the cell
path 201. As solar cells exist in an array, current from
illuminated cells must pass through shaded cells. If there were no
diode, the current would force its way through the cell path 201,
reversing the bias of such cells and degrading, if not destroying
them.
[0026] If the cell contains a diode, however, the current can be
offered an alternate, parallel path 202, and the shaded cells will
be preserved. The problem with this concept has been the difficulty
in creating a diode that is relatively easy to manufacture and
which uses a very low level of voltage to turn on and operate. The
invention described herein solves these problems.
[0027] If a cell is shaded or otherwise not receiving sunlight, in
order for the current to choose the diode path 202, the turn on
voltage for the diode path 202 must be less than the breakdown
voltage along the cell path 201. The breakdown voltage along the
cell path will typically be at least five volts, if not more. The
Schottky contact 207 requires a relatively small amount of voltage
to "turn on"--600 milivolts. However, to pass through the Ge
junction 104, the bias of the Ge junction 104 must be reversed,
requiring a large voltage. Reversing the bias of the Ge junction
104 requires approximately 9.4 volts, so nearly ten volts are
needed for the current to follow the diode path 202 in FIG. 2A. Ten
volts used to reverse the bias of the Ge junction is ten volts less
than otherwise would be available for other applications. The
device illustrated by FIG. 4 is therefore a functioning bypass
diode, but an inefficient one from a power utilization
perspective.
[0028] To address this inefficiency, in the metalization process in
which the Titanium Gold (TiAu) contacts 109, 110 are added to the
solar cell, an additional layer of metal 107 is added as well. In
the embodiment shown in FIG. 1, the metal is TiAu, although
practitioners in the art will be well aware that other metals can
also be used.
[0029] The effect of the metal 107 is to "short" the Ge junction
104 to the base of the Ge cell 104. Because of the short, a minimal
voltage is required to pass current between the layer 113 and the
Ge substrate. No longer is a high voltage required to force the
current through the Ge junction 104. The current flows easily
through the "short path" 107. FIG. 2B provides a schematic
representation. If the solar cell is shaded, no longer is the cell
forced into reverse bias to pass the current of the array string.
There is a much less resistive path, requiring a much lower voltage
drop, for the current to pass through the bypass diode 202. With
the addition of the metalization 107, the Ge cell 104 is shorted.
As a result, rather than a reverse biased diode with a 9.4 turn-on
voltage, the current instead encounters an ohmic resistance path
represented by the resistor 204.
[0030] The layer is doped to about 7 to 8 times 10.sup.7 cm.sup.3
to do two things. First, it reduces the contact resistance of the
metal layer 107 and second, it provides a low resistance path for
the lateral conduction layer. Without the lateral conduction layer,
the resistance at the resistor 204 is approximately 20 ohms. 20
ohms represents a significant drain on the current of the solar
cell. To reduce this resistance, a lateral conduction layer 113 is
added to the solar cell. FIG. 2C represents the current paths in
the solar cell as depicted in FIG. 1. When the solar cell is
shaded, the current will flow to the resistor 204. Because of the
presence of the lateral conduction layer 113, the resistance at the
resistor can be as low as 0.4 ohms.
[0031] The manufacturing process for the solar cell 100 comprises
five steps. FIG. 3 shows a multijunction solar cell 100 and the
component parts: the multijunction structure 301 and the buffer
structure 302. In the illustrated embodiment, a top cell comprised
of an n-on-p InGaP.sub.2 101 is grown over a cell of n-on-p GaAs
102. A third diffused Ge junction 104 is formed due to diffusion of
As during the growth of buffer layers 103.
[0032] A buffer exists between the upper junctions in the solar
cell and the Ge substrate 104, because the upper junctions are
fabricated of III-V material, and the entire cell is grown on a Ge
substrate 104. Ge is a group IV element, so it has different
lattice parameters than group III-V elements. Lattice matching is
generally accepted among those skilled in the art as a way to
increase the efficiency of a solar cell, and it follows that
lattice mismatching decreases a cell's overall efficiency. To
achieve lattice matching, the buffer layer is inserted in the
manufacture process; normally it is a thick layer of GaAs grown
over the Ge substrate. An InGaP layer lattice matches with a GaAs
layer much better than with a Ge layer.
[0033] The buffer structure 302 is comprised of the following: an
InGaP Schottky contact 303 as the top layer of the buffer structure
302. This will later form the Schottky diode. The buffer structure
302 is also comprised of an additional etch stop 304. The etch stop
304 enables the device to be more easily manufactured. Upon "wet
etching" the etch stop creates barriers during processing which
facilitate formation of the bypass diode. The lateral conduction
layer 113 exists in this buffer layer, to more efficiently guide
the current out of the diode, as discussed above.
[0034] As shown on FIG. 4, the first step in the manufacturing
process is to make a "wet etch" that comes down into the cell 100
and terminates at the InGaP layer 403.
[0035] A TiAu contact 110 is formed upon the InGaP layer 403. A
TiAu contact 109 is formed at the top of the cell to make an ohmic
contact with the n.sup.+-GaAs layer 112. The TiAu contact 110 on
the InGaP layer 403 makes a Schottky contact, which is non-ohmic.
In other words, instead of looking like a resistor, such contact
403 and the TiAu contact 110 forms a diode.
[0036] The second step in the manufacturing process is demonstrated
in FIG. 5. FIG. 5 shows a "mesa etch" 501 down to the level of the
Ge cell 104. The primary purpose of this step is to create a true
diode 106, electrically isolating the junctions 105 within the
solar cell from the diode 106. When the entire solar cell is
manufactured and the metal contacts are bound and the cell is
packaged, the cell 105 and the diode 106 will be parallel, yet
electrically separate.
[0037] The third step is a "shunt etch" 502, which provides a
"shelf" on which the metal 107 in the next step will be laid. To
make etches in the middle of manufacturing without etch stops among
the layers, one would have to use a "minute etch" which would be
extremely difficult to use with any degree of precision in this
instance. The etch stop 304 allows the solar cell to be
manufactured more efficiently.
[0038] The fourth step is the metalization process. The TiAu
contacts 109, 110 are added, and the metal layer 107 is added. In
this embodiment the metal comprising the layer is TiAu. Where the
TiAu contact 110 meets the InGaP layer 403, a Schottky contact is
created.
[0039] At the TiAu contact 109 on top of the cell, the TiAu makes
an ohmic contact to n-type GaAs 112. That is an ordinary cell
conduction for this type of cell to persons skilled in the art.
With the TiAu contact 110 at the InGaP layer 403, a Schottky
contact is created. However, because the object is to "short out"
the Ge cell 104, the contact was made to the highly doped n+-GaAs
cell 113. Layer 113 is also a lateral conduction layer. Upon making
the contact to the GaAs layer 113, a resistor is created. The
resistance at the resistor 204 was approximately 20 ohms, as
illustrated in FIG. 2A. 20 ohms of power dissipation can make the
cell too inefficient from a power utilization perspective.
[0040] The etch stop at the GaAs buffer contact 304 alleviates this
problem. The GaAs buffer contact 113 is n+ doped at the same level
as the GaAs buffer contact 112 at the top of the cell. This creates
a cell with the same quality of contact between the TiAu contact
109 and the GaAs contact layer 112 at the top of the cell as the
metal contact 107 with the GaAs layer 113.
[0041] Modifying the thicknesses of the various layers in the diode
106 is another way to decrease resistance in the diode 106.
[0042] The lateral conduction layer 113 also alleviates the
resistance through the diode 106, from 20 ohms to as low as 0.4
ohms. The current path passes through the diode 106, and the
thickness of the diode would ordinarily cause some resistance, but
the lateral conduction layer 113 helps the current move to the
metal more efficiently. In this embodiment, the lateral conduction
layer is made of highly doped n.sup.+-GaAs. The shunt layer 107 can
also be made to partially or completely surround the contact 110,
further lowering the series resistance.
[0043] The lateral conduction layer 113 and the metalization 107
are the two most important means to lessen the amount of voltage
needed to "turn on" the diode and bypass the shaded cell. By
reducing the series resistance, the amount of localized I.sup.2R
heating is also reduced. The process is also unique becaues the
amount of processing steps are reduced (compared to the '054
Patent), as the bypass diode layers are grown internally to the
buffer layers of the cell, rather than as additional layers that
have to be grown on top of the cell (as in the '054 Patent). The
current device provides for a low bypass diode turn on, as well as
a low series resistance bypass diode. Completion of the bypass
diode circuit requires a soldered or welded interconnect made
between contacts 109 and 110. This can be done as part of the usual
interconnect weld.
[0044] The fifth step in the manufacturing process is to apply the
anti-reflective coating and include etches where external contacts
will be attached.
[0045] As can be seen from the foregoing, the process by which the
diode is manufactured is integral to the manufacture of the cell,
and does not require additional manufacturing steps or additional
layers to be grown on the cell.
[0046] While we have described our preferred embodiments of the
present invention, it is understood that those skilled in the art,
both now and in the future, may make various improvements and
enhancements which fall within the scope of the claims which
follow. These claims should be construed to maintain the proper
protection for the invention first disclosed.
* * * * *