U.S. patent application number 12/562086 was filed with the patent office on 2010-04-01 for method and structure for thin film tandem photovoltaic cell.
This patent application is currently assigned to STION CORPORATION. Invention is credited to Howard W.H. Lee.
Application Number | 20100078059 12/562086 |
Document ID | / |
Family ID | 42056085 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100078059 |
Kind Code |
A1 |
Lee; Howard W.H. |
April 1, 2010 |
METHOD AND STRUCTURE FOR THIN FILM TANDEM PHOTOVOLTAIC CELL
Abstract
A tandem photovoltaic cell. The tandem photovoltaic cell
includes a bifacial top cell and a bottom cell. The top bifacial
cell includes a top first transparent conductive oxide material. A
top window material underlies the top first transparent conductive
oxide material. A first interface region is disposed between the
top window material and the top first transparent conductive oxide
material. The first interface region is substantially free from one
or more entities from the top first transparent conductive oxide
material diffused into the top window material. A top absorber
material comprising a copper species, an indium species, and a
sulfur species underlies the top window material. A top second
transparent conductive oxide material underlies the top absorber
material. A second interface region is disposed between the top
second transparent conductive oxide material and the top absorber
material. The bottom cell includes a bottom first transparent
conductive oxide material. A bottom window material underlies the
first bottom transparent conductive oxide material. A bottom
absorber material underlies the bottom window material. A bottom
electrode material underlies the bottom absorber material. The
tandem photovoltaic cell further includes a coupling material free
from a parasitic junction between the top cell and the bottom
cell.
Inventors: |
Lee; Howard W.H.; (Sratoga,
CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
STION CORPORATION
San Jose
CA
|
Family ID: |
42056085 |
Appl. No.: |
12/562086 |
Filed: |
September 17, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61101641 |
Sep 30, 2008 |
|
|
|
Current U.S.
Class: |
136/244 ;
257/E31.127; 438/69 |
Current CPC
Class: |
Y02E 10/548 20130101;
H01L 31/076 20130101; H01L 31/0296 20130101; H01L 31/18 20130101;
H01L 31/1828 20130101; H01L 31/0725 20130101; Y02E 10/544 20130101;
Y02E 10/541 20130101; H01L 31/0322 20130101; H01L 31/0687 20130101;
Y02E 10/543 20130101 |
Class at
Publication: |
136/244 ; 438/69;
257/E31.127 |
International
Class: |
H01L 31/042 20060101
H01L031/042; H01L 31/0232 20060101 H01L031/0232 |
Claims
1. A tandem photovoltaic cell comprising: a bifacial top cell
comprising: a top first transparent conductive oxide material; a
top window material underlying the top first transparent conductive
oxide material; a first interface region between the top window
material and the top first transparent conductive oxide material,
the first interface region being substantially free from one or
more entities from the top first transparent conductive oxide
material being diffused into the top window material; a top
absorber material underlying the top window material, the top
absorber material comprising a copper species, an indium species,
and a sulfur species; a top second transparent conductive oxide
material underlying the top absorber material; a second interface
region between the top second transparent conductive oxide material
and the top absorber material, the second interface region being
substantially free from one or more entities from the top first
transparent conductive oxide material being diffused into the top
absorber material; and a bottom cell comprising: a bottom first
transparent conductive oxide material; a bottom window material
underlying the first bottom transparent conductive oxide material;
a bottom absorber material underlying the bottom window material;
and a bottom electrode material underlying the bottom absorber
material; a coupling material between the top cell and the bottom
cell, the coupling material being free from a parasitic junction
between the top cell and the bottom cell.
2. The photovoltaic cell of claim 1 wherein the first transparent
conductive oxide material is selected from a group consisting of:
indium tin oxide (ITO), aluminum doped zinc oxide (ZnO:Al), or
Fluorine doped tin oxide (SnO.sub.2:F).
3. The photovoltaic cell of claim 1 wherein the top window material
is selected from: cadmium sulfide (CdS), zinc sulfide (ZnS), zinc
selenium (ZnSe), zinc oxide (ZnO), or zinc magnesium oxide
(ZnMgO).
4. The photovoltaic cell of claim 1 wherein the top absorber
material is selected from: copper indium disulfide (CIS), copper
indium aluminum disulfide, copper indium gallium disulfide (CIGS),
or (Ag,Cu)(In,Ga)S2.
5. The photovoltaic cell of claim 1 wherein the top second
transparent conductive oxide is selected from: cadmium sulfide
(CdS), zinc sulfide (ZnS), zinc selenium (ZnSe), zinc oxide (ZnO),
or zinc magnesium oxide (ZnMgO)
6. The photovoltaic cell of claim 1 wherein the bottom first
transparent conductive oxide is selected from: cadmium sulfide
(CdS), zinc sulfide (ZnS), zinc selenium (ZnSe), zinc oxide (ZnO),
or zinc magnesium oxide (ZnMgO)
7. The photovoltaic cell of claim 1 wherein the bottom window
material is selected from: cadmium sulfide (CdS), zinc sulfide
(ZnS), zinc selenium (ZnSe), zinc oxide (ZnO), or zinc magnesium
oxide (ZnMgO).
8. The photovoltaic cell of claim 1 wherein the bottom electrode
material is selected from a transparent conductive oxide material
or a metal material.
9. The photovoltaic cell of claim 1 wherein the bottom absorber
material is selected from: a copper indium disulfide thin film
material, copper indium aluminum disulfide thin film material, or a
copper indium gallium disulfide material.
10. The photovoltaic cell of claim 1 wherein the bottom absorber
material is selected from: Cu.sub.2SnS.sub.3; Cu(In,Ga)Se.sub.2;
CuInSe.sub.2; or FeSi.sub.2.
11. A tandem photovoltaic cell comprising: a bifacial top cell
comprising: a top first TCO material; a top window material
underlying the top first TCO material; a top absorber material
underlying the top window material, the top absorber material
comprising a copper species, an indium species, and a sulfur
species; a top second TCO material underlying the top absorber
material; and a bottom cell comprising: a bottom first TCO
material; a bottom window material underlying the first bottom TCO
material; a bottom absorber material underlying the bottom window
material; and a bottom electrode material underlying the bottom
absorber material; a coupling material between the top cell and the
bottom cell, the coupling material being free from a parasitic
junction between the top cell and the bottom cell.
12. The photovoltaic cell of claim 11 wherein the first transparent
conductive oxide material is selected from a group consisting of:
indium tin oxide (ITO), aluminum doped zinc oxide (ZnO:Al), or
Fluorine doped tin oxide (SnO.sub.2:F).
13. The photovoltaic cell of claim 11 wherein the top window
material is selected from: cadmium sulfide (CdS), zinc sulfide
(ZnS), zinc selenium (ZnSe), zinc oxide (ZnO), or zinc magnesium
oxide (ZnMgO).
14. The photovoltaic cell of claim 11 wherein the top absorber
material is selected from: copper indium disulfide (CIS), copper
indium aluminum disulfide, copper indium gallium disulfide (CIGS),
or (Ag,Cu)(In,Ga)S2.
15. The photovoltaic cell of claim 11 wherein the top second
transparent conductive oxide is selected from: cadmium sulfide
(CdS), zinc sulfide (ZnS), zinc selenium (ZnSe), zinc oxide (ZnO),
or zinc magnesium oxide (ZnMgO)
16. The photovoltaic cell of claim 11 wherein the bottom first
transparent conductive oxide is selected from: cadmium sulfide
(CdS), zinc sulfide (ZnS), zinc selenium (ZnSe), zinc oxide (ZnO),
or zinc magnesium oxide (ZnMgO)
17. The photovoltaic cell of claim 11 wherein the bottom window
material is selected from: cadmium sulfide (CdS), zinc sulfide
(ZnS), zinc selenium (ZnSe), zinc oxide (ZnO), or zinc magnesium
oxide (ZnMgO).
18. The photovoltaic cell of claim 11 wherein the bottom absorber
material is selected from: a copper indium disulfide thin film
material, copper indium aluminum disulfide thin film material, or a
copper indium gallium disulfide material.
19. The photovoltaic cell of claim 11 wherein the bottom absorber
material is selected from: Cu.sub.2SnS.sub.3; Cu(In,Ga)Se.sub.2;
CuInSe.sub.2; or FeSi.sub.2.
20. A method for fabricating a tandem photovoltaic cell,
comprising: forming a bifacial top cell, comprising: providing a
top first transparent conductive oxide material; forming a top
window material underlying the top first transparent conductive
oxide material; forming a first interface region between the top
window material and the top first transparent conductive oxide
material, the first interface region being substantially free from
one or more entities from the top first transparent conductive
oxide material being diffused into the top window material; forming
a top absorber material underlying the top window material, the top
absorber material comprising a copper species, an indium species,
and a sulfur species; forming a top second transparent conductive
oxide material underlying the top absorber material; forming a
second interface region between the top second transparent
conductive oxide material and the top absorber material, the second
interface region being substantially free from one or more entities
from the top first transparent conductive oxide material being
diffused into the top absorber material; and forming a bottom cell,
comprising: forming a bottom first transparent conductive oxide
material; forming a bottom window material underlying the first
bottom transparent conductive oxide material; forming a bottom
absorber material underlying the bottom window material; and
forming a bottom electrode material underlying the bottom absorber
material; providing a coupling material disposed between the
bifacial top cell and the bottom cell, the coupling material being
free from a parasitic junction between the bifacial top cell and
the bottom cell.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application No. 61/101,641, filed Sep. 30, 2008, commonly assigned,
and hereby incorporated by reference in its entirety herein for all
purpose. This application is related to Provisional Application No.
61/101,642 (Attorney Docket Number 026335-005800US) filed Sep. 30,
2008, commonly assigned, and hereby incorporated by reference
herein for all purpose. This application is also related to PCT
Application No.: PCT/US09/46161 (Attorney Docket Number
026335-002510PC) filed Jun. 3, 2009, commonly assigned, and hereby
incorporated by reference herein for all purpose.
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED
RESEARCH AND DEVELOPMENT
[0002] NOT APPLICABLE
REFERENCE TO A "SEQUENCE LISTING," A TABLE, OR A COMPUTER PROGRAM
LISTING APPENDIX SUBMITTED ON A COMPACT DISK
[0003] NOT APPLICABLE
BACKGROUND OF THE INVENTION
[0004] The present invention relates generally to photovoltaic
materials and manufacturing method. More particularly, the present
invention provides a method and structure for a thin film tandem
photovoltaic cells. Merely by way of example, the present method
and structure include absorber materials comprising copper indium
disulfide species.
[0005] From the beginning of time, mankind has been challenged to
find way of harnessing energy. Energy comes in the forms such as
petrochemical, hydroelectric, nuclear, wind, biomass, solar, and
more primitive forms such as wood and coal. Over the past century,
modern civilization has relied upon petrochemical energy as an
important energy source. Petrochemical energy includes gas and oil.
Gas includes lighter forms such as butane and propane, commonly
used to heat homes and serve as fuel for cooking Gas also includes
gasoline, diesel, and jet fuel, commonly used for transportation
purposes. Heavier forms of petrochemicals can also be used to heat
homes in some places. Unfortunately, the supply of petrochemical
fuel is limited and essentially fixed based upon the amount
available on the planet Earth. Additionally, as more people use
petroleum products in growing amounts, it is rapidly becoming a
scarce resource, which will eventually become depleted over
time.
[0006] More recently, environmentally clean and renewable sources
of energy have been desired. An example of a clean source of energy
is hydroelectric power. Hydroelectric power is derived from
electric generators driven by the flow of water produced by dams
such as the Hoover Dam in Nevada. The electric power generated is
used to power a large portion of the city of Los Angeles in
California. Clean and renewable sources of energy also include
wind, waves, biomass, and the like. That is, windmills convert wind
energy into more useful forms of energy such as electricity. Still
other types of clean energy include solar energy. Specific details
of solar energy can be found throughout the present background and
more particularly below.
[0007] Solar energy technology generally converts electromagnetic
radiation from the sun to other useful forms of energy. These other
forms of energy include thermal energy and electrical power. For
electrical power applications, solar cells are often used. Although
solar energy is environmentally clean and has been successful to a
point, many limitations remain to be resolved before it becomes
widely used throughout the world. As an example, one type of solar
cell uses crystalline materials, which are derived from
semiconductor material ingots. These crystalline materials can be
used to fabricate optoelectronic devices that include photovoltaic
and photodiode devices that convert electromagnetic radiation into
electrical power. However, crystalline materials are often costly
and difficult to make on a large scale. Additionally, devices made
from such crystalline materials often have low energy conversion
efficiencies. Other types of solar cells use "thin film" technology
to form a thin film of photosensitive material to be used to
convert electromagnetic radiation into electrical power. Similar
limitations exist with the use of thin film technology in making
solar cells. That is, efficiencies are often poor. Additionally,
film reliability is often poor and cannot be used for extensive
periods of time in conventional environmental applications. Often,
thin films are difficult to mechanically integrate with each other.
These and other limitations of these conventional technologies can
be found throughout the present specification and more particularly
below.
[0008] From the above, it is seen that improved techniques for
manufacturing photovoltaic materials and resulting devices are
desired.
BRIEF SUMMARY OF THE INVENTION
[0009] According to embodiments of the present invention, a method
and a structure for forming a photovoltaic cell is provided. More
particularly, the present invention provides a method and structure
for forming thin film tandem photovoltaic cell. Merely by way of
example, embodiments according to the present invention have been
implemented using thin film semiconductor material. But it would be
recognized that embodiments according to the present invention can
have a much broader range of applicability.
[0010] In a specific embodiment, a tandem photovoltaic cell is
provided. The tandem photovoltaic cell includes a top cell. The top
cell is a bifacial cell in a specific embodiment. The top cell
includes a top first transparent conductive oxide material. A top
window material underlies the top first transparent conductive
oxide material. In a specific embodiment, the top cell includes a
first interface region disposed between the top window material and
the top first transparent conductive oxide material. The first
interface region is substantially free from one or more entities
from the top first transparent conductive oxide material being
diffused into the top window material. The top cell also includes a
top absorber material underlying the top window material. The top
absorber material comprise a copper species, an indium species, and
a sulfur species in a specific embodiment. The top cell includes a
top second transparent conductive oxide material underlying the top
absorber material and a second interface region disposed between
the top second transparent conductive oxide material and the top
absorber material. The second interface region is substantially
free from one or more entities from the top first transparent
conductive oxide material being diffused into the top absorber
material.
[0011] The tandem photovoltaic includes a bottom cell. The bottom
cell includes a bottom first transparent conductive oxide material.
A bottom window material underlies the first bottom transparent
conductive oxide material. In a specific embodiment, a bottom
absorber material is provided underlying the bottom window material
and a bottom electrode material is provided underlying the bottom
absorber material. In a specific embodiment, a coupling material is
disposed between the top cell and the bottom cell. The coupling
material is free from a parasitic junction between the top cell and
the bottom cell in a preferred embodiment.
[0012] In an alternative embodiment, an alternative tandem
photovoltaic cell is provided. The alternative tandem photovoltaic
cell includes a top cell. The top cell is a bifacial cell in a
specific embodiment. The top cell includes a top first transparent
conductive oxide material. A top window material underlies the top
first transparent conductive oxide material. The top cell also
includes a top absorber material underlying the top window
material. The top absorber material comprise a copper species, an
indium species, and a sulfur species in a specific embodiment. The
top cell includes a top second transparent conductive oxide
material underlying the top absorber material.
[0013] The alternative tandem photovoltaic includes a bottom cell.
The bottom cell includes a bottom first transparent conductive
oxide material. A bottom window material underlies the first bottom
transparent conductive oxide material. In a specific embodiment, a
bottom absorber material is provided underlying the bottom window
material and a bottom electrode material is provided underlying the
bottom absorber material. In a specific embodiment, a coupling
material is disposed between the top cell and the bottom cell. The
coupling material is free from a parasitic junction between the top
cell and the bottom cell in a preferred embodiment.
[0014] In a specific embodiment, a method of forming a tandem
photovoltaic cell is provided. The method includes forming a
bifacial top cell. The bifacial top cell is formed by providing a
top first transparent conductive oxide material. A top window
material is formed underlying the top first transparent conductive
oxide material. In a specific embodiment, the method forms a first
interface region between the top window material and the top first
transparent conductive oxide material. In a specific embodiment,
the first interface region is substantially free diffusion of from
one or more entities from the top first transparent conductive
oxide material into the top window material. The method forms a top
absorber material underlying the top window material. The top
absorber material includes a copper species, an indium species, and
a sulfur species in a specific embodiment. A top second transparent
conductive oxide material is formed underlying the top absorber
material. The method includes forming a second interface region
between the top second transparent conductive oxide material and
the top absorber material for the bifacial top cell. The second
interface region is substantially free from one or more entities
from the top first transparent conductive oxide material diffused
into the top absorber material in a preferred embodiment. The
method includes forming a bottom cell. The bottom cell is formed by
providing a bottom first transparent conductive oxide material and
forming a bottom window material underlying the first bottom
transparent conductive oxide material. A bottom absorber material
is formed underlying the bottom window material. A bottom electrode
material is formed underlying the bottom absorber material to form
the bottom cell. In a specific embodiment, the method provides a
coupling material disposed between the top bifacial cell and the
bottom cell. Preferably, the coupling material provides for a
junction free from parasitic potential between the top bifacial
cell and the bottom cell.
[0015] Many benefits can be achieved by ways of the embodiments
according to the present invention. For example, the thin film
tandem photovoltaic cell can be fabricated using techniques without
substantial modification to the conventional equipment.
Additionally the present thin film tandem photovoltaic cell has an
improved conversion efficiency compared to a conventional
photovoltaic cell and provides a cost effective way to convert
sunlight into electric energy. Depending on the embodiment, one or
more of these benefits may be achieved. These and other benefits
will be described in more detailed throughout the present
specification and particularly below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a simplified diagram of a tandem photovoltaic cell
according to an embodiment of the present invention.
[0017] FIGS. 2 through 9 are schematic diagrams illustrating a
method and structure for forming a thin film photovoltaic device
according to an embodiment of the present invention.
[0018] FIGS. 10 through 17 are schematic diagrams illustrating a
method and structure for forming a thin film photovoltaic device
according to an embodiment of the present invention.
[0019] FIG. 18 is a simplified diagram illustrating a structure for
a thin film tandem photovoltaic cell according to an embodiment of
the present invention.
[0020] FIG. 19 is a simplified diagram illustrating an alternative
structure for a thin film tandem photovoltaic cell according to an
embodiment of the present invention.
[0021] FIG. 20 is a simplified diagram illustrating a test result
for a thin film tandem photovoltaic cell according to an embodiment
of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] According to embodiments of the present, a method and a
structure for forming a photovoltaic cell are provided. More
particularly, embodiments according to the present invention
provide a method and structure for forming a thin film tandem
photovoltaic cell. Merely by way of example, embodiments according
to the present invention have been implemented using thin film
semiconductor material. But it would be recognized that embodiments
according to the present invention can have a much broader range of
applicability.
[0023] FIG. 1 is a simplified diagram of a tandem photovoltaic cell
according to an embodiment of the present invention. The diagram is
merely an illustration and should not unduly limit the scope of the
claims herein. One of ordinary skill in the art would recognize
other variations, modifications, and alternatives. As an example,
the tandem photovoltaic cell can also be described in U.S.
Provisional No. 61/092,732, Attorney Docket number
026335-003400US), commonly assigned, and hereby incorporated by
reference herein. As shown, a four terminal tandem photovoltaic
cell device 100 is provided. The four terminal tandem photovoltaic
cell includes a lower cell 103 and an upper cell 101 operably
coupled to the lower cell. The terms "lower" and "upper" are not
intended to be limiting but should be construed by plain meaning by
one of ordinary skill in the art. In general, the upper cell is
closer to a source of electromagnetic radiation than the lower
cell, which receives the electromagnetic radiation after traversing
through the upper cell. Of course, there can be other variations,
modifications, and alternatives.
[0024] In a specific embodiment, the lower cell includes a lower
glass substrate material 119, e.g., a transparent glass material.
The lower cell also includes a lower electrode layer made of a
reflective material overlying the lower glass substrate material.
The lower cell includes a lower absorber layer overlying the lower
electrode layer. As shown, the absorber and electrode layer are
illustrated by reference numeral 117. In a specific embodiment, the
absorber layer is made of a semiconductor material having a band
gap energy Eg in a range of about 1.2 eV to about 2.2 eV and
preferably in a range of about 1.6 eV to about 1.9 eV, but can be
others. In a specific embodiment, the lower cell includes a lower
window layer overlying the lower absorber layer and a lower
transparent conductive oxide layer 115 overlying the lower window
layer.
[0025] In a specific embodiment, the upper cell includes a p+ type
transparent conductor layer 109 overlying the lower transparent
conductive oxide layer. In a preferred embodiment, the p+ type
transparent conductor layer is characterized by a sheet resistance
of less than or equal to about 10 Ohms/square centimeters and an
optical transmission of 90 percent and greater. In a specific
embodiment, the upper cell has an upper p type absorber layer
overlying the p+ type transparent conductor layer. In a preferred
embodiment, the p type conductor layer made of a semiconductor
material has a band gap energy Eg in a range of about 1.2 eV to
about 2.2 eV and preferably in a range of about 1.6 eV to about 1.9
eV, but can be others. The upper cell also has an upper n type
window layer overlying the upper p type absorber layer. Referring
again to FIG. 1, the window and absorber layer for the upper cell
are illustrated by reference numeral 107. The upper cell also has
an upper transparent conductive oxide layer 105 overlying the upper
n type window layer and an upper glass material (not shown)
overlying the upper transparent conductive oxide layer. Of course,
there can be other variations, modifications, and alternatives.
[0026] In a specific embodiment, the tandem photovoltaic cell
includes four terminals. The four terminals are defined by
reference numerals 111, 113, 121, and 123. Alternatively, the
tandem photovoltaic cell can also include three terminals, which
share a common electrode preferably proximate to an interface
region between the upper cell and the lower cell. In other
embodiments, the multi junction cell can also include two
terminals, among others, depending upon the application. Examples
of other cell configurations are provided in U.S. Provisional
Patent Application No. 61/092,383, Attorney Docket No:
026335-001600US, commonly assigned and hereby incorporated by
reference herein. Of course, there can be other variations,
modifications, and alternatives. Further details of the four
terminal cell can be found throughout the present specification and
more particularly below.
[0027] FIG. 2-17 are a schematic diagrams illustrating a method for
forming a top cell for a thin film tandem photovoltaic device
according to an embodiment of the present invention. These diagrams
are merely examples, which should not unduly limit the claims
herein. One skilled in the art would recognize other variations,
modifications, and alternatives. As shown in FIG. 2, a substrate
110 is provided. In an embodiment, the substrate includes a surface
region 112 and is held in a process stage within a process chamber
(not shown). In another embodiment, the substrate is an optically
transparent solid material. For example, the substrate can be a
glass, quartz, fused silica, or a plastic, or metal, or foil, or
semiconductor, or other composite materials. Depending upon the
embodiment, the substrate can be a single material, multiple
materials, which are layered, composites, or stacked, including
combinations of these, and the like. Of course, there can be other
variations, modifications, and alternatives.
[0028] As shown in FIG. 3, the method includes forming a first
electrode layer 120 overlying the surface region of the substrate.
The first electrode layer can be formed using a suitable metal
material such as molybdenum, or tungsten, but can be others. These
other metal materials may include copper, chromium, aluminum,
nickel, platinum, or others. Such metal material can be deposited
using techniques such as sputtering, evaporation (e.g., electron
beam), electro plating, combination of these and the like in a
specific embodiment. A thickness of the first electrode layer can
range from about 100 nm to 2 micron, but can be others. First
electrode layer 120 is preferably characterized by a resistivity of
about 10 Ohm/cm2 and less according to a specific embodiment. In a
preferred embodiment, the electrode layer is provided by
molybdenum. In a specific embodiment, the first electrode layer may
be provided using a transparent conductive oxide (TCO) material
such as In.sub.2O.sub.3:Sn (ITO), ZnO:Al (AZO), SnO2:F (TFO), but
can be others. Of course, there can be other variations,
modifications and alternatives.
[0029] Referring to FIG. 4, the method for forming the thin film
photovoltaic cell includes forming a copper layer 130 overlying the
electrode layer formed. The copper layer can be formed using a
sputtering process such as a DC magnetron sputtering process in a
specific embodiment. The DC magnetron sputtering process may be
provided at a deposition pressure of about 6.2 mTorr, controlled by
using an inert gas such as argon. Such pressure can be achieved
using a gas flow rate of about 32 sccm. The sputtering process can
be perform at about room temperature without heating the substrate.
Of course, minor heating of the substrate may be resulted due to
the plasma generated during the deposition process. According to
certain embodiments, a DC power in a range from 100 Watts to 150
Watts, and preferably about 115 Watts may be used, depending on the
embodiment. A deposition time for a Cu layer of 330 nm thickness
can be about 6 minutes or more. Of course, the deposition condition
can be varied and modified according to a specific embodiment.
[0030] Depending on the embodiment, the method forms a barrier
layer 125 overlying the electrode layer to form an interface region
between the electrode layer and the copper layer. In a specific
embodiment, the interface region is maintained substantially free
from a metal disulfide layer having a semiconductor characteristic
that is different from the copper indium disulfide material during
later processing steps. Depending upon the embodiment, the barrier
layer has suitable conductive characteristics and can be reflective
to allow electromagnetic radiation to reflect back or can also be
transparent or the like. In a specific embodiment, the barrier
layer is selected from platinum, titanium, chromium, or silver. Of
course, there can be other variations, modifications, and
alternatives.
[0031] As shown in FIG. 5, the method includes providing an indium
(In) layer 140 overlying the copper layer. In particular, the
indium layer 140 is formed overlying the copper layer 130. The
indium layer is deposited over the copper layer using a sputtering
process. In one example, the indium layer is deposited using a DC
magnetron sputtering process is under a similar process condition
for depositing the Cu layer. The deposition time for the indium
layer may be shorter than that for Cu layer. For example, 2 minutes
and 45 seconds may be sufficient for depositing an In layer of a
thickness of about 410 nm according to a specific embodiment. Other
suitable deposition methods such as electroplating or others may
also be used depending on the embodiment.
[0032] In a specific embodiment, the copper layer and the indium
layer form a multilayer structure for the thin film photovoltaic
cell. In a specific embodiment, the copper layer and the indium
layer are provided in a certain stoichiometry that forms a copper
rich material having a copper to indium atomic ratio ranging from
about 1.2:1 to about 2.0:1. In an alternative embodiment, the
copper to indium atomic ratio ranges from about 1.35:1 to about
1.60:1. In another embodiment, the copper to indium atomic ratio is
selected to be 1.55:1. In a preferred embodiment, the copper to
indium atomic ratio provides a copper rich film for the
photovoltaic cell. In another specific embodiment, the indium layer
is deposited overlying the electrode layer prior to the deposition
of the copper layer. Of course there can be other variations,
modifications, and alternatives.
[0033] As shown in FIG. 5, the multilayered structure 150
comprising at least an indium layer and a copper layer is subjected
to a thermal treatment process 200 in an sulfur species 210 bearing
environment. The thermal treatment process uses a rapid thermal
process while the multilayer structure is subjected to the sulfur
bearing species. In a specific embodiment, the rapid thermal
process uses a temperature ramp rate ranging from about 10 Degrees
Celsius/second to about 50 Degrees Celsius/second to a final
temperature ranging from about 400 Degrees Celsius to about 600
Degrees Celsius. In a specific embodiment, the thermal treatment
process further maintains at the final temperature for a dwell time
ranging from about 1 minute to about 10 minutes, but can be others.
The thermal treatment process also includes a temperature ramp down
in an inert ambient or other suitable environment. The inert
ambient can be provided using gases such as nitrogen, argon,
helium, and others, which stops reaction to alloy the metal
material with the sulfur species. Further details of the
temperature ramp process is described throughout the present
specification and more particularly below.
[0034] In a specific embodiment, the sulfur bearing species can be
applied using a suitable technique. In an example, the sulfur
bearing species are in a fluid phase. As an example, the sulfur can
be provided in a solution, which has dissolved Na.sub.2S, CS.sub.2,
(NR.sub.4).sub.2S, thiosulfate, and others. Such fluid based sulfur
species can be applied overlying one or more surfaces of the
multilayered copper/indium structure according to a specific
embodiment. In another example, the sulfur bearing species 210 is
provided by hydrogen sulfide gas or other like gas. In other
embodiments, the sulfur can be provided in a solid phase, for
example elemental sulfur. In a specific embodiment, elemental
sulfur can be heated and allowed to vaporize into a gas phase,
e.g., S.sub.n. and allowed to react with the indium/copper layers.
Other sulfur bearing species may also be used depending on the
embodiment. Taking hydrogen sulfide gas as the sulfur bearing
species as an example. The hydrogen sulfide gas can be provided
using one or more entry valves with flow rate control into a
process chamber. Any of these application techniques and other
combinations of techniques can also be used. The process chamber
may be equipped with one or more pumps to control process pressure.
Depending on the embodiment, a layer of sulfur material may be
provided overlying the multilayer structure comprising the copper
layer and the indium layer. The layer of sulfur material can be
provided as a patterned layer in a specific embodiment. In other
embodiment, sulfur material may be provided in a slurry, a powder,
a solid, a paste, a gas, any combination of these, or other
suitable form. Of course, there can be other variations,
modifications, and alternatives.
[0035] Referring again to FIG. 6, the thermal treatment process
cause a reaction between copper indium material within the
multilayered structure and the sulfur bearing species 210, thereby
forming a layer of copper indium disulfide thin film material 220.
In one example, the copper indium disulfide material is formed by
incorporating sulfur ions and/or atoms evaporated or decomposed
from the sulfur bearing species into the multilayered structure
with indium atoms and copper atoms mutually diffused therein. In a
specific embodiment, the thermal treatment process results in a
formation of a cap layer overlying the copper indium disulfide
material. The cap layer comprises a thickness of substantially
copper sulfide material 221 substantially free of indium atoms. The
copper sulfide material 221 includes a surface region 225. In a
specific embodiment, the formation of the copper sulfide cap layer
is under a Cu-rich conditions for the Cu--In bearing multilayered
structure 150. Depending on the embodiment, the thickness of the
copper sulfide material is in an order of about five to ten
nanometers and greater depending on the multilayered structure. In
a specific embodiment, thermal treatment process allows the first
electrode layer using a TCO material to maintain at a sheet
resistance of less than or equal to about 10 Ohms per square
centimeters and an optical transmission of 90 percent and greater
after the copper indium disulfide thin film material is formed. Of
course, there can be other variations, modifications, and
alternatives.
[0036] As shown in FIG. 7, copper sulfide material 221 is subjected
to a dip process 300. The copper sulfide material overlies copper
indium disulfide thin film 220. The dip process is performed by
exposing the surface region of the copper sulfide material to about
a solution comprising a 10% by weight of potassium cyanide 310
according to a specific embodiment. Potassium cyanide solution
provides an etching process to selectively remove copper sulfide
material 221 from the copper indium disulfide material surface
exposing a surface region 228 of underlying copper indium disulfide
material according to a specific embodiment. In a preferred
embodiment, the etch process has a selectivity of about 1:100 or
more between copper sulfide and copper indium disulfide. Other
etching species can be used depending on the embodiment. In a
specific embodiment, the etching species can be hydrogen peroxide.
In other embodiments, other etching techniques including
electro-chemical etching, plasma etching, sputter-etching, or any
combination of these may be used. In a specific embodiment, the
copper sulfide material can be mechanically removed, chemically
removed, electrically removed, or any combination of these, and
others. In a specific embodiment, the absorber layer made of copper
indium disulfide can have a thickness ranging from about one micron
to about 10 microns, but can be others. Of course, there can be
other variations, modifications, and alternatives.
[0037] In a specific embodiment, the copper indium disulfide film
has a p type impurity characteristics. In certain embodiments, the
copper indium disulfide material is further subjected to a doping
process to adjust a p-type impurity density therein to optimize an
I-V characteristic of the high efficiency thin film photovoltaic
devices. For example, the copper indium disulfide material may be
doped using an aluminum species. In another example, the copper
indium disulfide material can be intermixed with a copper indium
aluminum disulfide material to form the absorber layer. Of course,
there can be other variations, modifications, and alternatives.
[0038] Referring again to FIG. 8, the method includes forming a
window layer 310 overlying the copper indium disulfide material,
which has a p-type impurity characteristics. The window layer can
be selected from a group of materials consisting of a cadmium
sulfide (CdS), a zinc sulfide (ZnS), zinc selenium (ZnSe), zinc
oxide (ZnO), zinc magnesium oxide (ZnMgO), or others. These
material may be doped with a suitable impurities to provide for a
n+ type impurity characteristic. The window layer and the absorber
layer forms an interface region for a PN junction associated with a
photovoltaic cell. The window layer is heavily doped to form a
n+-type semiconductor layer. In one example, indium species are
used as the doping material for a CdS window layer to cause
formation of the n+-type characteristic associated with the window
layer. In certain embodiments, ZnO may be used as the window layer.
ZnO can be doped with an aluminum species to provide for the n+
impurity characteristics. Depending on the material used, the
window layer can range from about 200 nanometers to about 500
nanometers. Of course, there can be other variations,
modifications, and alternative.
[0039] As shown in FIG. 9, a conductive layer 330 is form overlying
a portion of a surface region of the window layer. Conductor layer
330 provides a top electrode layer for the thin film photovoltaic
cell. In one embodiment, conductive layer 330 is a transparent
conductive oxide (TCO). For example, the transparent conductive
oxide can be selected from a group consisting of In.sub.2O.sub.3:Sn
(ITO), ZnO:Al (AZO), SnO2:F (TFO), but can be others. In a specific
embodiment, the TCO layer is patterned to maximize the efficiency
of the thin film photovoltaic devices. In certain embodiments, the
TCO layer can also function as a window layer, which eliminates the
need of a separate window layer. Of course there can be other
variations, modifications, and alternatives.
[0040] FIG. 10 through 17 are simplified diagrams illustrating a
method to form a photovoltaic cell in a superstrate configuration
for the thin film tandem photovoltaic cell according to an
alternative embodiment of the present invention. These diagrams are
merely examples and should not unduly limit the scope of the claims
herein. One skilled in the art would recognize other variations,
modifications, and alternatives. As shown in FIG. 10, a substrate
1010 is provided. In an embodiment, the substrate includes a
surface region 1012 and is held in a process stage within a process
chamber (not shown). In a specific embodiment, the transparent
substrate is an optically transparent solid material. For example,
the optically transparent solid material can be glass, quartz,
fused silica, or a polymer material. Other material such as metal,
or foil, or semiconductor, or other composite materials may also be
used in other embodiments. Depending upon the embodiment, the
substrate can be a single material, multiple materials, which are
layered, composites, or stacked, including combinations of these,
and the like. Of course there can be other variations,
modifications, and alternatives.
[0041] As shown in FIG. 11, the method includes forming a first
electrode layer 1102 including a electrode surface region overlying
the surface region of the substrate. The first electrode layer is
preferably made of a transparent conductive oxide, commonly called
TCO. For example, the transparent conductive oxide can be selected
from a group consisting of In.sub.2O.sub.3:Sn (ITO), ZnO:Al (AZO),
SnO2:F (TFO), but can be others. In a specific embodiment, the TCO
layer is patterned to maximize the efficiency of the thin film
photovoltaic devices. A thickness of the electrode layer can range
from about 100 nm to 2 micron, but can be others. Electrode layer
120 is preferably characterized by a resistivity of less than about
10 Ohm/cm.sup.2 according to a specific embodiment. Of course there
can be other variations, modifications, and alternatives.
[0042] In a specific embodiment, the method includes forming a
window layer 1202 overlying the first electrode layer as shown in
FIG. 12. The window layer can be selected from a group of materials
consisting of a cadmium sulfide (CdS), a zinc sulfide (ZnS), zinc
selenium (ZnSe), zinc oxide (ZnO), zinc magnesium oxide (ZnMgO), or
others. These material may be doped with a suitable impurities to
provide for a n+ type impurity characteristic. In one example,
indium species are used as the doping material for a CdS window
layer to cause formation of the n+-type characteristic associated
with the window layer. In certain embodiments, ZnO may be used as
the window layer. ZnO can be doped with an aluminum species to
provide for the n+ impurity characteristics. Depending on the
material used, the window layer can range from about 200 nanometers
to about 500 nanometers. Of course, there can be other variations,
modifications, and alternative.
[0043] Referring to FIG. 13, the method includes providing a copper
layer 1302 overlying the window layer. The copper layer can be
formed using a sputtering process such as a DC magnetron sputtering
process in a specific embodiment. The DC magnetron sputtering
process may be provided at a deposition pressure of about 6.2
mTorr, controlled by using an inert gas such as argon. Such
pressure can be achieved using a gas flow rate of about 32 sccm.
The sputtering process can be perform at about room temperature
without heating the substrate. Of course, minor heating of the
substrate may be resulted due to the plasma generated during the
deposition process. According to certain embodiments, a DC power in
a range from 100 Watts to 150 Watts, and preferably about 115 Watts
may be used, depending on the embodiment. As merely an example, a
deposition time for a Cu layer of 330 nm thickness can be about 6
minutes or more. Of course, the deposition condition can be varied
and modified according to a specific embodiment.
[0044] As shown in FIG. 14, the method includes providing an indium
(In) layer 1402 overlying the copper layer. The indium layer is
deposited over the copper layer using a sputtering process in a
specific embodiment. In one example, the indium layer is deposited
using a DC magnetron sputtering process is under a similar process
condition for depositing the Cu layer. The deposition time for the
indium layer may be shorter than that for Cu layer. For example, 2
minutes and 45 seconds may be sufficient for depositing an In layer
of a thickness of about 410 nm according to a specific embodiment.
Other suitable deposition methods such as electroplating or others
may also be used depending on the embodiment.
[0045] In a specific embodiment, the copper layer and the indium
layer form a multilayer structure 1404 for the thin film
photovoltaic cell. In a specific embodiment, the copper layer and
the indium layer are provided in a certain stoichiometry that forms
a copper rich material. In a specific embodiment, the copper rich
material can have a copper to indium atomic ratio ranging from
about 1.2:1 to about 2.0:1. In an alternative embodiment, the
copper to indium atomic ratio ranges from about 1.35:1 to about
1.60:1. In another embodiment, the copper to indium atomic ratio is
selected to be 1.55:1. In a preferred embodiment, the copper to
indium atomic ratio provides a copper rich film for the
photovoltaic cell. In another specific embodiment, the indium layer
is deposited overlying the electrode layer prior to the deposition
of the copper layer. Of course there can be other variations,
modifications, and alternatives.
[0046] As shown in FIG. 15, the multilayered structure comprising
at least an indium layer and a copper layer is subjected to a
thermal treatment process 1502 in an sulfur species 1504 bearing
environment. The thermal treatment process uses a rapid thermal
process while the multilayer structure is subjected to the sulfur
bearing species. In a specific embodiment, the rapid thermal
process uses a temperature ramp rate ranging from about 10 Degrees
Celsius/second to about 50 Degrees Celsius/second to a final
temperature ranging from about 400 Degrees Celsius to about 600
Degrees Celsius. In a specific embodiment, the thermal treatment
process further maintains at the final temperature for a dwell time
ranging from about 1 minute to about 10 minutes, but can be others.
The thermal treatment process also include a temperature ramp down
in an inert ambient or other suitable environment that can stop the
reaction of formation of the alloy material in a specific
embodiment. The inert ambient can be provided using gases such as
nitrogen, argon, helium, and others. Further details of the
temperature ramp process is described throughout the present
specification and more particularly below.
[0047] In a specific embodiment, the sulfur bearing species can be
applied using a suitable technique. In an example, the sulfur
bearing species are in a fluid phase. As an example, the sulfur can
be provided in a solution, which has dissolved Na.sub.2S, CS.sub.2,
(NR.sub.4).sub.2S, thiosulfate, and others. Such fluid based sulfur
species can be applied overlying one or more surfaces of the
multilayered copper/indium structure according to a specific
embodiment. In another example, the sulfur bearing species 210 is
provided by hydrogen sulfide gas or other like gas. In other
embodiments, the sulfur can be provided in a solid phase, for
example elemental sulfur. In a specific embodiment, elemental
sulfur can be heated and allowed to vaporize into a gas phase,
e.g., S.sub.n. and allowed to react with the indium/copper layers.
Other sulfur bearing species may also be used depending on the
embodiment. Taking hydrogen sulfide gas as the sulfur bearing
species as an example. The hydrogen sulfide gas can be provided
using one or more entry valves with flow rate control into a
process chamber. Any of these application techniques and other
combinations of techniques can also be used. The process chamber
may be equipped with one or more pumps to control process pressure.
Depending on the embodiment, a layer of sulfur material may be
provided overlying the multilayer structure comprising the copper
layer and the indium layer. The layer of sulfur material can be
provided as a patterned layer in a specific embodiment. In other
embodiment, sulfur material may be provided in a slurry, a powder,
a solid, a paste, a gas, any combination of these, or other
suitable form. Of course, there can be other variations,
modifications, and alternatives.
[0048] In a specific embodiment, the thermal treatment process
maintains the absorber layer substantially free from species that
may diffuse from the window layer and/or the transparent conductive
oxide layer. The method also eliminates using a thick window layer
to protect the transparent conductive oxide layer from diffusion of
species from the absorber layer. The method provides a photovoltaic
cell that can have a conversion efficiency greater than about 8
percent or greater than 10 percent, and others. Of course, there
can be other variations, modifications, and alternatives.
[0049] Referring again to FIG. 15, the thermal treatment process
causes a reaction between copper and indium materials within the
multilayered structure and the sulfur bearing species, thereby
forming a layer of copper indium disulfide thin film material 1506.
In one example, the copper indium disulfide thin film material is
formed by incorporating sulfur ions and/or atoms evaporated or
decomposed from the sulfur bearing species into the multilayered
structure with indium atoms and copper atoms mutually diffused
therein. In a specific embodiment, the thermal treatment process
results in a formation of a cap layer overlying the copper indium
disulfide material. The cap layer comprises a thickness of
substantially copper sulfide material 1508 substantially free of
indium atoms. The copper sulfide material includes a surface region
1510. In a specific embodiment, the formation of the copper sulfide
cap layer is under a Cu-rich conditions for the Cu--In bearing
multilayered structure. Depending on the embodiment, the thickness
of the copper sulfide material is in an order of about five to ten
nanometers and greater depending on the multilayered structure. In
a specific embodiment, the thermal treatment process allows the
first electrode layer to maintain at a sheet resistance of less
than or equal to about 10 Ohms per square centimeters and an
optical transmission of 90 percent and greater after the copper
indium disulfide thin film material is formed. Of course, there can
be other variations, modifications, and alternatives.
[0050] As shown in FIG. 16, the copper sulfide material is
subjected to a dip process 1602. The dip process is performed by
exposing the surface region of the copper sulfide material to a
solution 1604 comprising potassium cyanide as an etching species at
a concentration of about a 10% by weight according to a specific
embodiment. Potassium cyanide solution provides an etching process
to selectively remove copper sulfide material from the copper
indium disulfide material surface exposing a surface region 1606 of
underlying copper indium disulfide material according to a specific
embodiment. In a preferred embodiment, the etching process has a
selectivity of about 1:100 or more between copper sulfide and
copper indium disulfide. Other etching species can be used
depending on the embodiment. In a specific embodiment, the etching
species can be hydrogen peroxide. In other embodiments, other
etching techniques including electro-chemical etching, plasma
etching, sputter-etching, or any combination of these may be used.
In a specific embodiment, the copper sulfide material can be
mechanically removed, chemically removed, electrically removed, or
any combination of these, and others In a specific embodiment, the
absorber layer made of copper indium disulfide can have a thickness
ranging from about one micron to about 10 microns, but can be
others. Of course, there can be other variations, modifications,
and alternatives.
[0051] In a specific embodiment, the copper indium disulfide film
has a p type impurity characteristics and provide for an absorber
layer for the thin film photovoltaic cell. In certain embodiments,
the copper indium disulfide material is further subjected to a
doping process to adjust a p-type impurity density therein to
optimize an I-V characteristic of the high efficiency thin film
photovoltaic devices. For example, the copper indium disulfide
material may be doped using an aluminum species. In another
example, the copper indium disulfide material can be intermixed
with a copper indium aluminum disulfide material to form the
absorber layer. The window layer and the absorber layer forms an
interface region for a PN-junction associated with a photovoltaic
cell. Of course, there can be other variations, modifications, and
alternatives
[0052] As shown in FIG. 17, the method forms a second electrode
layer 1702 overlying the absorber layer. The second electrode layer
can be a transparent conductive oxide (TCO) in a specific
embodiment. For example, the transparent conductive oxide can be
selected from a group consisting of In.sub.2O.sub.3:Sn (ITO),
ZnO:Al (AZO), SnO2:F (TFO), but can be others. In certain
embodiments, the second electrode layer may be provided using a
metal material such as tungsten, gold, silver, copper or others. In
other embodiments, the second electrode layer can be reflective to
reflect electromagnetic radiation back to the photovoltaic cell and
improves the conversion efficiency of the photovoltaic cell. Of
course there can be other variations, modifications, and
alternatives.
[0053] In a specific embodiment, the method includes coupling the
top cell and the bottom cell to form the thin film tandem cell as
illustrated in FIG. 1. In a specific embodiment, the top cell and
the bottom cell may be coupled using a suitable optical transparent
material such as ethyl vinyl acetate but can be others depending on
the application. Of course, there can be other variations,
modifications, and alternatives. In a specific embodiment, other
substrate configurations are described below.
[0054] FIGS. 18 and 19 are simplified diagrams illustrating
structures of a tandem photovoltaic cell according to embodiments
of the present invention. As shown in FIG. 18, a structure for a
tandem photovoltaic cell is provided includes a top cell 1802 and a
bottom cell 1804. The top cell can be a bifacial cell in a specific
embodiment. The top cell includes a top first transparent
conductive oxide material 1806. The top first transparent
conductive oxide (TCO) material can include materials such as
indium tin oxide (ITO), aluminum doped zinc oxide (ZnO:Al),
Fluorine doped tin oxide (SnO.sub.2:F), or others. The top cell
includes a top window material 1802 underlying the first
transparent conductive oxide material. In a specific embodiment,
the top window material uses an n type semiconductor thin film
material such as cadmium sulfide (CdS), zinc sulfide (ZnS), zinc
selenium (ZnSe), zinc oxide (ZnO), or zinc magnesium oxide (ZnMgO),
but can be others. The n type semiconductor material is preferably
heavily doped to have a n+ type impurity characteristic. Of course
there can be other variations, modifications, and alternatives.
[0055] Referring again to FIG. 18, the top cell includes a first
interface region 1810 disposed between the top first transparent
conductive oxide and the top window material. The first interface
region is maintained substantially free from one or more entities
from a diffusion of the top first transparent conductive oxide
material into the top window material. Depending upon the
embodiment, the barrier layer has suitable conductive
characteristics and can be optically transparent. Of course, there
can be other variations, modifications, and alternatives.
[0056] The top cell includes a top absorber material 1812
underlying the top window layer. The top absorber material has a p
type impurity characteristic in a preferred embodiment. In a
specific embodiment, the top absorber material comprises at least a
top absorber material underlying the top window material, the top
absorber material comprising a copper species, an indium species,
and a sulfur species in a specific embodiment. In certain
embodiment, the top absorber material can include a copper indium
disulfide thin film material, copper indium aluminum disulfide thin
film material, a copper indium gallium disulfide material, or a
(Ag,Cu)(In,Ga)S.sub.2 material, but can also be others, depending
on the application. Of course, there can be other variations,
modifications, and alternatives.
[0057] As shown in FIG. 18, the top cell includes a top second
transparent conductive oxide material 1814 underlying the top
absorber material. The second top second transparent conductive
oxide material can include materials such as indium tin oxide
(ITO), aluminum doped zinc oxide (ZnO:Al), Fluorine doped tin oxide
(SnO.sub.2:F), or others, depending on the embodiment.
[0058] In a specific embodiment, the top cell includes a second
interface region 1816 dispose between the top second transparent
conductive oxide and the top absorber material. The second
interface region is maintained substantially free from one or more
entities from a diffusion of the top second TCO material into the
top absorber material. Depending upon the embodiment, the barrier
layer has suitable conductive characteristics and can be optically
transparent. Of course, there can be other variations,
modifications, and alternatives.
[0059] Referring to again to FIG. 18. Bottom cell 1804 includes a
bottom first transparent conductive oxide material 1818. The bottom
first transparent conductive oxide (TCO) material can include
materials such as indium tin oxide (ITO), aluminum doped zinc oxide
(ZnO:Al), Fluorine doped tin oxide (SnO.sub.2:F), or others. The
bottom cell includes a bottom window material 1820 underlying the
bottom first transparent conductive oxide material. In a specific
embodiment, the bottom window material uses an n type semiconductor
thin film material such as cadmium sulfide (CdS), zinc sulfide
(ZnS), zinc selenium (ZnSe), zinc oxide (ZnO), or zinc magnesium
oxide (ZnMgO), but can be others. The n type semiconductor material
is preferably heavily doped to have a n+ type impurity
characteristic. Of course there can be other variations,
modifications, and alternatives.
[0060] The bottom cell includes a bottom absorber material 1822
underlying the window layer. The bottom absorber material has a p
type impurity characteristic in a preferred embodiment. In a
specific embodiment, the bottom absorber material comprises at
least a copper species, an indium species, and a sulfur species in
a specific embodiment. In certain embodiment, the bottom absorber
material can include a copper indium disulfide thin film material,
copper indium aluminum disulfide thin film material, or a copper
indium gallium disulfide material, but can also be others,
depending on the application. In other embodiments, the bottom
absorber material can be Cu.sub.2SnS.sub.3; Cu(In,Ga)Se.sub.2;
CuInSe.sub.2; or FeSi.sub.2. Of course, there can be other
variations, modifications, and alternatives.
[0061] As shown in FIG. 18, the bottom cell includes a bottom
electrode material 1824 underlying the bottom absorber material.
The bottom electrode material can include a transparent conductive
oxide material such as indium tin oxide (ITO), aluminum doped zinc
oxide (ZnO:Al), Fluorine doped tin oxide (SnO.sub.2:F), and the
like. The bottom electrode material may also include a metal
material such as copper, nickel, gold, tungsten and others,
depending on the embodiment. In a specific embodiment the bottom
electrode material is provided using a molybdenum material. Of
course, there can be other variations, modifications, and
alternatives.
[0062] In a specific embodiment, the tandem thin film photovoltaic
cell includes a coupling material 1826 provided between the top
cell and the bottom cell. The coupling material is preferably an
optically transparent material and free from a parasitic junction
between the top cell and the bottom cell in a specific embodiment.
In a specific embodiment, the optically transparent material can
include material such as ethyl vinyl acetate and the like. Of
course, there can be other variations, modifications, and
alternatives.
[0063] Depending on the embodiment, the first interface region and
the second interface region for the top cell can be optional. That
is, the top cell is configured to have the top window material to
underlie the top first transparent conductive oxide and the top
second transparent conductive oxide to underlie the top absorber
material as shown in FIG. 19. Of course there can be other
variations, modifications, and alternatives.
[0064] FIG. 20 is an exemplary solar cell I-V characteristics plot
measured from a copper indium disulfide based thin film
photovoltaic cell according to an embodiment of the present
invention. The diagram is merely an example, which should not
unduly limit the claims herein. One skilled in the art would
recognize other variations, modifications, and alternatives. As
shown in FIG. 20, a current density of a high efficiency copper
indium disulfide thin film photovoltaic cell made according to an
embodiment of the present invention is plotted against a bias
voltage. Further details of the thin film photovoltaic cell and the
experimental results are described in PCT Application No.:
PCT/US09/46161 (Attorney Docket Number 026335-002510PC) filed Jun.
3, 2009, commonly assigned, and hereby incorporate by reference.
The curve intersects the y-axis with a short circuit current value
at about 0.0235 A/cm.sup.2 and intersects a zero current line with
a bias at about 0.69 V. The corresponding photovoltaic cell has an
absorber layer made from copper indium disulfide thin film
according to an embodiment of the present invention. In particular,
the absorber layer is about 1.5 .mu.m in thickness and an atomic
ratio of Cu:In at about 1.5:1. Based on standard formula, a cell
conversion efficiency .eta. can be estimated:
.eta. = J S C V O C F F P in ( AM 1.5 ) ##EQU00001##
where J.sub.SC is the short circuit current density of the cell,
V.sub.OC is the open circuit bias voltage applied, FF is the
so-called fill factor defined as the ratio of the maximum power
point divided by the open circuit voltage (Voc) and the short
circuit current (J.sub.SC). The input light irradiance (P.sub.in,
in W/m.sup.2) under standard test conditions [i.e., STC that
specifies a temperature of 25.degree. C. and an irradiance of 1000
W/m2 with an air mass 1.5 (AM1.5) spectrum.] and the surface area
of the solar cell (in m.sup.2). Thus, a 10.4% efficiency can be
accurately estimated for this particular cell made from a method
according to embodiments of the present invention. In a specific
embodiment, the bandgap is about 1.45 eV to 1.5 eV. Of course,
there can be other variations, modifications, and alternatives.
[0065] Although the above has been illustrated according to
specific embodiments, there can be other modifications,
alternatives, and variations. For example, the method can be used
to fabricate a photovoltaic cell that has an absorber layer that
forms using a high temperature process. Although the above has been
described in terms of a specific absorber material, other absorber
materials such as Cu(InAl)S.sub.2 Cu(InGa)S.sub.2, Cu.sub.2SnS,
Cu.sub.2ZnSnS.sub.4, SnS, any combinations of these, and others can
be used. It is understood that the examples and embodiments
described herein are for illustrative purposes only and that
various modifications or changes in light thereof will be suggested
to persons skilled in the art and are to be included within the
spirit and purview of this application and scope of the appended
claims.
* * * * *