U.S. patent application number 13/764667 was filed with the patent office on 2013-08-15 for photovoltaic cells having electrical contacts formed from metal nanoparticles and methods for production thereof.
This patent application is currently assigned to LOCKHEED MARTIN CORPORATION. The applicant listed for this patent is LOCKHEED MARTIN CORPORATION. Invention is credited to Andrew Fried, Sidney Hu, Alfred A. ZINN.
Application Number | 20130206225 13/764667 |
Document ID | / |
Family ID | 48944615 |
Filed Date | 2013-08-15 |
United States Patent
Application |
20130206225 |
Kind Code |
A1 |
ZINN; Alfred A. ; et
al. |
August 15, 2013 |
PHOTOVOLTAIC CELLS HAVING ELECTRICAL CONTACTS FORMED FROM METAL
NANOPARTICLES AND METHODS FOR PRODUCTION THEREOF
Abstract
Photovoltaic cells having copper contacts can be made by using
copper nanoparticles during their fabrication. Such photovoltaic
cells can include a copper-based current collector located on a
semiconductor substrate having an n-doped region and a p-doped
region. The semiconductor substrate is configured for receipt of
electromagnetic radiation and generation of an electrical current
therefrom. The copper-based current collector includes an
electrically conductive diffusion barrier disposed on the
semiconductor substrate and a copper contact disposed on the
electrically conductive diffusion barrier. The copper contact is
formed from copper nanoparticles that have been at least partially
fused together. The electrically conductive diffusion barrier
limits the passage of copper therethrough.
Inventors: |
ZINN; Alfred A.; (Palo Alto,
CA) ; Fried; Andrew; (Saint Paul, MN) ; Hu;
Sidney; (Sunnyvale, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LOCKHEED MARTIN CORPORATION; |
|
|
US |
|
|
Assignee: |
LOCKHEED MARTIN CORPORATION
Bethesda
MD
|
Family ID: |
48944615 |
Appl. No.: |
13/764667 |
Filed: |
February 11, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61597680 |
Feb 10, 2012 |
|
|
|
Current U.S.
Class: |
136/256 ; 438/98;
977/773; 977/890 |
Current CPC
Class: |
Y02E 10/50 20130101;
H01L 31/02167 20130101; B82Y 30/00 20130101; H01L 31/1804 20130101;
H01L 31/18 20130101; B82Y 40/00 20130101; H01L 21/288 20130101;
H01B 1/22 20130101; H01L 31/022425 20130101; H01L 31/1864
20130101 |
Class at
Publication: |
136/256 ; 438/98;
977/773; 977/890 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; H01L 31/18 20060101 H01L031/18 |
Claims
1. A photovoltaic cell comprising: a copper-based current collector
located on a semiconductor substrate, the copper-based current
collector comprising: an electrically conductive diffusion barrier
disposed on the semiconductor substrate, the electrically
conductive diffusion barrier limiting the passage of copper
therethrough; and a copper contact disposed on the electrically
conductive diffusion barrier, the copper contact being formed from
copper nanoparticles that have been at least partially fused
together; wherein the semiconductor substrate comprises an n-doped
region and a p-doped region, and the semiconductor substrate is
configured for receipt of electromagnetic radiation and generation
of an electrical current therefrom.
2. The photovoltaic cell of claim 1, wherein the copper-based
current collector is located on a face of the photovoltaic cell
that receives the electromagnetic radiation.
3. The photovoltaic cell of claim 1, wherein the copper-based
current collector is located on a face of the photovoltaic cell
opposite a face of the photovoltaic that receives the
electromagnetic radiation.
4. The photovoltaic cell of claim 1, wherein the copper-based
current collector is in contact with the n-doped region of the
semiconductor substrate.
5. The photovoltaic cell of claim 4, further comprising: an
electrical connection in contact with the p-doped region of the
semiconductor substrate.
6. The photovoltaic cell of claim 1, wherein the semiconductor
substrate comprises a silicon substrate.
7. The photovoltaic cell of claim 6, wherein the electrically
conductive diffusion barrier is selected from the group consisting
of TiN, TaN, WN, TiW, W and any combination thereof.
8. The photovoltaic cell of claim 7, wherein the electrically
conductive diffusion barrier is formed from nanoparticles that have
been at least partially fused together.
9. The photovoltaic cell of claim 1 wherein the copper contact
further comprises a corrosion-resistant substance.
10. The photovoltaic cell of claim 9, wherein a corrosion-resistant
coating is disposed on the copper contact, the corrosion-resistant
coating being selected from the group consisting of a Sri coating,
an Ag coating, a SnAgCu coating, an Al coating, a Si coating, a
polymer coating, and any combination thereof.
11. The photovoltaic cell of claim 1, wherein the copper
nanoparticles are mixed with micron-scale copper particles while
being formed into the copper contact.
12. The photovoltaic cell of claim 1, wherein the copper contact
further comprises a conductive additive selected from the group
consisting of carbon black, pyrene, phenanthrene, carbon nanotubes,
graphene, and any combination thereof.
13. The photovoltaic cell of claim 12, wherein the conductive
additive is mixed with the copper nanoparticles while being formed
into the copper contact.
14. The photovoltaic cell of claim 12, wherein the conductive
additive is disposed on the copper contact.
15. The photovoltaic cell of claim 1, further comprising:
antireflective coating disposed on a face of the semiconductor
substrate that receives the electromagnetic radiation.
16. A method for forming a photovoltaic cell, the method
comprising: applying an electrically conductive diffusion barrier
onto a semiconductor substrate comprising an n-doped region and a
p-doped region, the electrically conductive diffusion barrier
limiting the passage of copper therethrough; applying copper
nanoparticles onto the electrically conductive diffusion barrier;
and heating the copper nanoparticles to a temperature sufficient to
at least partially fuse the copper nanoparticles together, thereby
forming a copper contact on the electrically conductive diffusion
barrier.
17. The method of claim 16, wherein the electrically conductive
diffusion barrier and the copper contact are disposed on the
n-doped region of the semiconductor substrate.
18. The method of claim 16, wherein the copper nanoparticles are
applied to the electrically conductive diffusion barrier as a
dispensible nanoparticle paste formulation comprising an organic
matrix in which the copper nanoparticles are dispersed.
19. The method of claim 18, wherein at least a portion of the
copper nanoparticles are about 20 nm in size or smaller.
20. The method of claim 18, wherein the dispensible nanoparticle
paste formulation further comprises micron-scale copper particles,
a conductive additive, a corrosion-resistant substance, or any
combination thereof.
21. The method of claim 16, wherein the semiconductor substrate
comprises a silicon substrate.
22. The method of claim 21, wherein the electrically conductive
diffusion barrier is selected from the group consisting of TiN,
TaN, WN, TiW, W and any combination thereof.
23. The method of claim 22, wherein the electrically conductive
diffusion barrier is applied to the semiconductor substrate by
plating, physical vapor deposition, or chemical vapor
deposition.
24. The method of claim 22, further comprising: applying a
plurality of nanoparticles to the semiconductor substrate; and at
least partially fusing the nanoparticles together to form the
electrically conductive diffusion barrier.
25. The method of claim 16, further comprising: adhering the
semiconductor substrate to a surface while at least partially
fusing the copper nanoparticles together.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn.119 from U.S. Provisional Patent Application
61/597,680, filed Feb. 10, 2012, which is incorporated herein by
reference in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
FIELD OF THE INVENTION
[0003] The present invention generally relates to photovoltaic
cells, and, more specifically, to photovoltaic cells formed from
nanoparticles.
BACKGROUND
[0004] With growing consumer demands for energy, especially clean
energy derived from renewable resources, photovoltaic devices
(i.e., solar cells) have garnered increased interest and are
becoming much more widely deployed in spite of their high cost.
Silicon-based photovoltaic cells currently dominate the global
photovoltaics market and are projected to continue to do so for the
foreseeable future, with an estimated revenue growth of 10-25%
taking place over the next 10 years. As a result of the expense of
most photovoltaic cells, the photovoltaics industry is under
tremendous pressure to cut costs, which can be a primary driver for
a product's success in the marketplace. In many commercial
applications, cost can outrank even a photovoltaic cell's
performance and photon conversion efficiency.
[0005] Materials constitute a large portion of the total cost of
silicon-based photovoltaic cells, and these costs are continuing to
rise. The expense of high purity silicon is the leading cost driver
for silicon-based photovoltaic cells. Alternative semiconductor
materials are available, but they are also very expensive. The
silver paste used to form electrical connections in conventional
silicon-based photovoltaic cells ranks a close second behind
silicon in terms being a cost driver. Silver prices have steadily
increased and become quite volatile over the past several years,
and there are limited opportunities to increase global silver
production capacity. Despite these issues, there are currently no
low cost alternative materials that can suitably replace silver in
silicon-based photovoltaic cells, at least without necessitating
significant changes to their manufacturing process.
[0006] During the fabrication of many conventional silicon-based
photovoltaic cells, silver paste is applied to the top surface of
the cell and converted into current collectors by a high
temperature processing step (>800.degree. C.). The high
temperature processing step facilitates a glass frit etch of a SiN
antireflective coating on the substrate, such that the silver can
make electrical contact with the semiconductor. However, the
micron-scale silver particles within the silver paste do not melt
or become fused together with one another during the high
temperature processing step. Instead, electrical conduction is
established through grain-to-grain contact of the silver particles,
thereby decreasing the obtainable electrical conductivity. The high
temperature processing step also places significant restrictions on
the types of materials that can be used prior to that operation.
Thermally stable substrates capable of withstanding the processing
temperatures of silver paste can also significantly add to the cost
of current photovoltaic cells.
[0007] One of the most desirable features of silver in regard to
the manufacturing of photovoltaic cells is its high electrical
conductivity. Although other metals can display similar electrical
properties, silver paste is still more readily processed than are
any potential replacement bulk metals, thereby compensating for its
high cost. Of potential metallic replacements for silver, copper
presents particular advantages due to its similar electrical
conductivity, much lower cost, and relatively low price volatility.
By utilizing copper in place of silver in photovoltaic cells,
material costs could be reduced by as much as 10%. However, copper
presents particular challenges as a direct replacement for silver
that have not allowed this change to be made.
[0008] Although photovoltaic cells having copper-based current
collectors have the potential to fulfill an unmet need in the art,
satisfactory means for fabricating such photovoltaic cells have yet
to be developed. The present invention satisfies the foregoing need
and provides related advantages as well.
SUMMARY
[0009] In some embodiments, the present disclosure describes
photovoltaic cells including a copper-based current collector
located on a semiconductor substrate having an n-doped region and a
p-doped region. The semiconductor substrate is configured for
receipt of electromagnetic radiation and generation of an
electrical current therefrom. The copper-based current collector
includes an electrically conductive diffusion barrier disposed on
the semiconductor substrate and a copper contact disposed on the
electrically conductive diffusion barrier. The copper contact is
formed from copper nanoparticles that have been at least partially
fused together. The electrically conductive diffusion barrier
limits the passage of copper therethrough.
[0010] In some embodiments, the present disclosure describes
methods for forming photovoltaic cells. The methods include
applying an electrically conductive diffusion barrier onto a
semiconductor substrate having an n-doped region and a p-doped
region, applying copper nanoparticles onto the electrically
conductive diffusion barrier, and heating the copper nanoparticles
to a temperature sufficient to at least partially fuse the copper
nanoparticles together, thereby forming a copper contact on the
electrically conductive diffusion barrier. The electrically
conductive diffusion barrier limits the passage of copper
therethrough.
[0011] The foregoing has outlined rather broadly the features of
the present disclosure in order that the detailed description that
follows can be better understood. Additional features and
advantages of the disclosure will be described hereinafter, which
form the subject of the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] For a more complete understanding of the present disclosure,
and the advantages thereof, reference is now made to the following
descriptions to be taken in conjunction with the accompanying
drawings describing specific embodiments of the disclosure,
wherein:
[0013] FIGS. 1 and 2 show presumed structures of copper
nanoparticles suitable for use in forming the photovoltaic cells
described herein;
[0014] FIGS. 3 and 4 show schematics of illustrative photovoltaic
cells having a front-side copper-based current collector disposed
thereon;
[0015] FIG. 5 shows a schematic of an illustrative photovoltaic
cell having a back-side copper-based current collector disposed
thereon;
[0016] FIGS. 6A and 6B show precision secondary ion mass
spectrometry for wafer 4, pre- and post-anneal, respectively;
[0017] FIGS. 7A and 713 show precision secondary ion mass
spectrometry for wafer 3, pre- and post-anneal, respectively;
[0018] FIGS. 8A and 8B show precision secondary ion mass
spectrometry for wafer 5, pre- and post-anneal, respectively;
and
[0019] FIGS. 9A and 9B show precision secondary ion mass
spectrometry for wafer 2, pre- and post-anneal, respectively.
DETAILED DESCRIPTION
[0020] The present disclosure is directed, in part, to photovoltaic
cells having current collectors formed from metal nanoparticles,
particularly copper nanoparticles. The present disclosure is also
directed, in part, to methods for making photovoltaic cells from
metal nanoparticles, particularly copper nanoparticles.
[0021] Nanoparticles can exhibit a number of physical and chemical
properties that differ significantly from those of the
corresponding bulk substance. One property of nanoparticles that
can be of particular importance is nanoparticle fusion that occurs
at or above the nanoparticles' fusion temperature. As used herein,
the term "fusion temperature" will refer to the temperature at
which a nanoparticle liquefies, thereby giving the appearance of
melting. As used herein, the terms "fused" and "fusion" will refer
to the coalescence or partial coalescence of nanoparticles with one
another. The drop in apparent melting point can be a particularly
significant feature for metal nanoparticles. Upon decreasing in
size, particularly below about 20 nm in equivalent spherical
diameter, the temperature at which metal nanoparticles can be
liquefied drops dramatically from that of the corresponding bulk
metal. For example, copper nanoparticles having a size of about 20
nm or less can have fusion temperatures of about 220.degree. C. or
below, in comparison to bulk copper's melting point of 1083.degree.
C.
[0022] The low fusion temperature of copper nanoparticles places
them in a temperature regime in which they can be processed at
significantly lower temperatures than the silver pastes currently
used in the manufacturing of photovoltaic cells. The ability to
process copper nanoparticles at significantly reduced temperatures
relative to silver represents a distinct process advantage compared
to current approaches for manufacturing photovoltaic cells.
Moreover, since copper nanoparticles can become at least partially
fused together with one another to form a bulk lattice of
polycrystalline copper at or above the fusion temperature, higher
electrical conductivities can be realized than with only
grain-to-grain contact being present, as in silver pastes. As an
additional advantage, the reduced processing temperatures of copper
nanoparticles offer the opportunity to utilize lower quality and
lower cost substrates that are not particularly thermally resistant
during manufacturing processes.
[0023] Despite the potential advantages that copper nanoparticles
can provide in the fabrication of photovoltaic cells, copper does
not represent a simple "drop-in" replacement for silver in their
manufacture, as copper presents significant challenges that are not
encountered with silver. Foremost, copper atoms diffuse into
silicon along grain boundaries much faster than do silver atoms. If
excessive metal diffusion into the semiconductor occurs,
particularly into the n-doped region of a p-type semiconductor
(i.e., into the p-n junction), performance of the photovoltaic cell
can become diminished due to the occurrence of trapped states.
Although silver also somewhat presents a diffusion problem in this
regard, it can be readily addressed through the in situ formation
of a diffusion barrier when glass frit is used to etch through a
SiN antireflective coating on the surface of silicon-based
semiconductors during manufacturing of photovoltaic cells, where
the glass frit serves as the diffusion barrier. Due to copper's
much higher effective diffusion rate, such diffusion barriers can
be ineffective for this metal, since excessive copper can pass into
the semiconductor while etching is taking place. Furthermore, over
the decade or more working lifetime of photovoltaic cells, such
diffusion barriers can be insufficient to limit the slow diffusion
of copper into the semiconductor. Implementation of effective,
long-lived diffusion barriers for copper in photovoltaic cells is
described hereinbelow.
[0024] Another issue that can be encountered with copper
nanoparticles is that of developing a suitable formulation that can
be deposited with precision on a semiconductor substrate in order
to maintain high cell efficiencies. As-produced copper
nanoparticles are often prone to clumping and are difficult to use
directly, particularly in precision applications such as screen and
ink-jet printing techniques, for example. In the manufacture of
conventional photovoltaic cells, screen and ink-jet printing
techniques are commonly used due to their high throughput
capabilities, instead of the more precise hut lower throughput and
more costly lithographic techniques, despite the tatter's ability
to produce photovoltaic cells having higher efficiencies. For
example, photovoltaic cells that are fabricated by lithography
techniques can have a 1-1.5%-fold increase in cell efficiency
relative to photovoltaic cells that are fabricated by printing
techniques. Nanoparticle paste formulations that can be utilized in
precision deposition techniques and their implementation in the
fabrication of photovoltaic cells are also described herein. Such
nanoparticle paste formulations can achieve printed line widths as
low as about 5-10 microns, in contrast to printed line widths of
about 100-150 microns in conventional photovoltaic cells that are
fabricated from silver paste. The printed line widths attainable
with copper nanoparticle paste formulations can be comparable with
those attainable by lithography techniques. Thus, using copper
nanoparticles in a dispensable nanoparticle paste formulation can
allow cell efficiencies to be improved without the throughput
issues associated with lithography techniques. In this regard,
copper nanoparticle paste formulations can permit high aspect ratio
copper-based current collectors to be fabricated, thereby
minimizing shadowing effects on the photovoltaic cells.
[0025] Copper nanoparticles also can provide additional advantages
in the fabrication of photovoltaic cells that are often not
attainable in conventional photovoltaic cells manufactured from
silver pastes. First, by using copper nanoparticles, photovoltaic
cells can be readily fabricated in which there are no electrical
contacts on a face of the photovoltaic cells that receives
electromagnetic radiation. That is, the copper-based current
collector can be located on a face of the photovoltaic cell
opposite a face of the photovoltaic cell that receives
electromagnetic radiation. Disposition of the copper-based current
collector in this manner can increase the effective surface area of
the photovoltaic cell that is available for receipt of
electromagnetic radiation, thereby increasing cell efficiency. Of
course, in other embodiments, the copper-based current collector
can also be located on a face of the photovoltaic cell that
receives electromagnetic radiation. Each of these configurations is
described in more detail herein.
[0026] Even more advantageously, when the photovoltaic cell
contains a copper-based current collector located on a face of the
photovoltaic cell opposite the face of the photovoltaic cell that
receives or is configured to receive electromagnetic radiation, the
copper nanoparticles can perform multiple rotes in addition to
forming the copper contact of the copper-based current collector.
First, as the copper nanoparticles become at least partially fused
together with one another, they can simultaneously adhere the
semiconductor substrate to another surface, thereby sandwiching the
copper contact between the semiconductor substrate and the other
surface and leaving the photovoltaic cell configured to receive
electromagnetic radiation on its open face. In the fabrication of
conventional photovoltaic cells, a separate bonding operation is
usually necessary. Second, the copper contact cart provide art
excellent heat transfer pathway to the other surface. Use of fused
copper nanoparticles for heat management and transfer is described
in commonly owned U.S. patent application Ser. No. 13/079,757,
filed Apr. 4, 2011, which is incorporated herein by reference in
its entirety. If the other surface is thermally stable, it can
serve as a heat sink to dissipate heat generated in the
photovoltaic cell during its operation. The ability to form a heat
transfer pathway at the low processing temperatures of copper
nanoparticles represents a distinct advantage over the high
temperature processes used in the fabrication of conventional
photovoltaic cells. A final advantage of locating the copper
contact in the above manner is that it introduces roughness to the
"back-side" of the photovoltaic cell and reduces reflectance of
electromagnetic radiation therefrom, again increasing the cell
efficiency.
[0027] As used herein, the terms "partially fused," "partial
fusion," and other derivatives and grammatical equivalents thereof
will refer to the partial coalescence of nanoparticles with one
another. For example, whereas totally fused metal nanoparticles
retain essentially none of the structural morphology of the
original unfused metal nanoparticles (i.e., they resemble bulk
polycrystalline or multi-crystalline metal, or a lattice thereof),
partially fused metal nanoparticles retain at least some of the
structural morphology of the original unfused metal nanoparticles.
The properties of partially fused metal nanoparticles can be
intermediate between those of the corresponding hulk metal and the
original unfused metal nanoparticles.
[0028] As used herein, the term "nanoparticles" will refer to
particles that are about 100 nm or less in size, without particular
reference to the shape of the particles. As used herein, the term
"copper nanoparticles" will refer to copper particles that are
about 100 nm or less in size, without particular reference to the
shape of the copper particles.
[0029] As used herein, the term "organic matrix" will refer to a
continuous fluid phase containing one or more organic
compounds.
[0030] As used herein, the term "micron-scale metal particles" will
refer to metal particles that are about 100 nm or greater in size
in at least one dimension. As used herein, the term "micron-scale
copper particles" will refer to copper particles that are about 100
nm or greater in size in at least one dimension.
[0031] In various embodiments, photovoltaic cells described herein
can include a copper-based current collector located on a
semiconductor substrate having an n-doped region and a p-doped
region. The semiconductor substrate is configured for receipt of
electromagnetic radiation and generation of an electrical current
therefrom. The copper-based current collector includes an
electrically conductive diffusion barrier disposed on the
semiconductor substrate and a copper contact disposed on the
electrically conductive diffusion barrier. The copper contact is
formed from copper nanoparticles that have been at least partially
fused together. The electrically conductive diffusion barrier
limits the passage of copper therethrough.
[0032] In various embodiments, methods for forming photovoltaic
cells from copper nanoparticles can include applying an
electrically conductive diffusion barrier onto a semiconductor
substrate having an n-doped region and a p-doped region, applying
copper nanoparticles onto the electrically conductive diffusion
barrier, and heating the copper nanoparticles to a temperature
sufficient to at least partially fuse the copper nanoparticles
together, thereby forming a copper contact on the electrically
conductive diffusion barrier. The electrically conductive diffusion
barrier limits the passage of copper therethrough.
[0033] Although some embodiments of photovoltaic cells are
described herein with reference to copper nanoparticles, it is to
be recognized that other types of metal nanoparticles can be used
as well. Suitable alternative metals can include, but are not
limited to, aluminum or nickel, for example. In addition, in some
embodiments, mixtures of different types of metal nanoparticles can
be used when fabricating the photovoltaic cells described herein.
For example, in some embodiments, a mixture of copper nanoparticles
and nickel nanoparticles can be used to fabricate the photovoltaic
cells. Mixtures of metal nanoparticles can create alloys once
nanoparticle fusion takes place, and the alloys can display
increased corrosion resistance compared to copper alone. Suitable
alloys having corrosion resistance can be envisioned by one having
ordinary skill in the art. Likewise, admixture of copper
nanoparticles with micron-scale particles of other metals can also
produce metal alloys having increased corrosion resistance.
[0034] In some embodiments, at least a portion of the copper
nanoparticles used in forming the photovoltaic cells can be about
20 nm or smaller in size. As discussed above, copper nanoparticles
in this size range can have fusion temperatures that are
significantly lower than that of bulk copper and readily undergo
fusion with one another as a result. For example, copper
nanoparticles that are about 20 nm or smaller in size can have
fusion temperatures of about 220.degree. C. or below (e.g., a
fusion temperature in the range of about (150.degree. C. to about
220.degree. C.), or about 200.degree. C. or below. In some
embodiments, at least a portion of the copper nanoparticles can be
about 10 nm or less in size, or about 5 nm or less in size. In some
embodiments, at least a portion of the copper nanoparticles can
range between about 1 nm in size to about 20 nm in size, or between
about 1 nm size and about 10 nm in size, or between about 1 nm in
size to about 5 nm in size, or between about 3 nm in size to about
7 nm in size, or between about 5 nm in size to about 20 nm in size.
In some embodiments, larger copper nanoparticles can be combined
with copper nanoparticles that are about 20 nm in size or smaller.
For example, in some embodiments, copper nanoparticles ranging from
about 1 nm in size to about 10 nm in size can be combined with
copper nanoparticles that range from about 25 nm in size to about
50 nm in size, or from about 25 nm in size to about 100 nm in size.
As further discussed below, micron-scale copper particles or other
types of particles can also be combined with the copper
nanoparticles in some embodiments. Although larger copper
nanoparticles and micron-scale copper particles may not be
liquefiable at low temperatures, they can still become fused with
one another upon liquefying copper nanoparticles at or above their
fusion temperature, as generally discussed above.
[0035] In some embodiments, the copper nanoparticles can have a
surfactant coating thereon, where the surfactant coating contains
one or more surfactants. The surfactant coating can be formed on
the copper nanoparticles during their synthesis. Formation of a
surfactant coating on the copper nanoparticles during their
synthesis can desirably limit the ability of the copper
nanoparticles to fuse to one another, limit agglomeration of the
copper nanoparticles, and promote the formation of a population of
copper nanoparticles having a narrow size distribution. Further
details regarding the synthesis of copper nanoparticles and
suitable surfactants are discussed in more detail below.
[0036] Any suitable technique can be employed for forming the
copper nanoparticles used in the embodiments described herein.
Particularly facile metal nanoparticle fabrication techniques are
described in commonly owned U.S. Pat. Nos. 7,736,414, 8,105,414,
and 8,192,866 and commonly owned U.S. patent application Ser. Nos.
13/656,590, filed Oct. 19, 2012; 13/228,411, filed Sep. 8, 2011;
13/040,207, filed Mar. 3, 2011; and 12/813,463, filed Jun. 10,
2010, each of which is incorporated herein by reference in its
entirety. As described therein, metal nanoparticles can be
fabricated in a narrow size range by reduction of a metal salt in a
solvent in the presence of a suitable surfactant system, which can
include one or more different surfactants. Further description of
suitable surfactant systems follows below. Without being bound by
any theory or mechanism, it is believed that the surfactant system
can mediate the nucleation and growth of the metal nanoparticles,
limit surface oxidation of the metal nanoparticles, and/or inhibit
metal nanoparticles from extensively aggregating with one another
prior to being at least partially fused together. Suitable organic
solvents for solubilizing metal salts and forming metal
nanoparticles can include, for example, formamide,
N,N-dimethylformamide, dimethyl sulfoxide, dimethylpropylene urea,
hexamethylphosphoramide, tetrahydrofuran, and glyme, diglyme,
triglyme, and tetraglyme. Reducing agents suitable for reducing
metal salts and promoting the formation of metal nanoparticles can
include, for example, an alkali metal in the presence of a suitable
catalyst (e.g., lithium naphthalide, sodium naphthalide, or
potassium naphthalide) or borohydride reducing agents (e.g., sodium
borohydride, borohydride, potassium borohydride, or
tetraalkylammonium borohydrides).
[0037] FIGS. 1 and 2 show presumed structures of copper
nanoparticles suitable for use in forming the photovoltaic cells
described herein. As shown in FIG. 1, copper nanoparticle 10
includes copper core 12 and surfactant layer 14 overcoating copper
core 12. Surfactant layer 14 can contain any combination of
surfactants, as described in more detail below. Copper nanoparticle
20 shown in FIG. 2 is similar to that depicted in FIG. 11, but
copper core 12 is grown about nucleus 211, which can be a metal
that is the same as or different than that of the copper of copper
core 12. Because nucleus 211 is buried deep within copper core 12
in copper nanoparticle 20, it is not believed to significantly
affect the overall nanoparticle properties. In some embodiments,
the copper nanoparticles can have an amorphous morphology.
[0038] In various embodiments, the surfactant system used to
prepare the copper nanoparticles can include one or more
surfactants. The differing properties of various surfactants can be
used to tailor the properties of the copper nanoparticles. Factors
that can be taken into account when selecting a surfactant or
combination of surfactants for use in synthesizing copper
nanoparticles can include, for example, ease of surfactant
dissipation from the copper nanoparticles during nanoparticle
fusion, nucleation and growth rates of the copper nanoparticles,
and the like.
[0039] In some embodiments, an amine surfactant or combination of
amine surfactants, particularly aliphatic amines, can be used
during the synthesis of copper nanoparticles. In some embodiments,
two amine surfactants can be used in combination with one another.
In other embodiments, three amine surfactants can be used in
combination with one another. In more specific embodiments, a
primary amine, a secondary amine, and a diamine chelating agent can
be used in combination with one another. In still more specific
embodiments, the three amine surfactants can include a long chain
primary amine, a secondary amine, and a diamine having at least one
tertiary alkyl group nitrogen substituent. Further disclosure
regarding suitable amine surfactants follows hereinafter.
[0040] In some embodiments, the surfactant system can include a
primary alkylamine. In some embodiments, the primary alkylamine can
be a C.sub.2-C.sub.18 alkylamine. In some embodiments, the primary
alkylamine can be a C.sub.7-C.sub.10 alkylamine. In other
embodiments, a C.sub.5-C.sub.6 primary alkylamine can also be used.
Without being bound by any theory or mechanism, the exact size of
the primary alkylamine can be balanced between being long enough to
provide an effective inverse micelle structure versus having ready
volatility and/or ease of handling. For example, primary
alkylamines with more than 18 carbons can also be suitable for use
in the present embodiments, but they can be more difficult to
handle because of their waxy character. C.sub.7-C.sub.10 primary
alkylamines, in particular, can represent a good balance of desired
properties for ease of use.
[0041] In some embodiments, the C.sub.2-C.sub.18 primary alkylamine
can be n-hexylamine, n-heptylamine, n-octylamine, n-nonylamine, or
n-decylamine, for example. While these are all straight chain
primary alkylamines, branched chain primary alkylamines can also be
used in other embodiments. For example, branched chain primary
alkylamines such as, for example, 7-methyloctylamine,
2-methyloctylamine, or 7-methylnonylamine can be used. In some
embodiments, such branched chain primary alkylamines can be
sterically hindered where they are attached to the amine nitrogen
atom. Non-limiting examples of such sterically hindered primary
alkylamines can include, for example, t-octylamine,
2-methylpentan-2-amine, 2-methylhexan-2-amine,
2-methylheptan-2-amine, 3-ethyloctan-3-amine,
3-ethylheptan-3-amine, 3-ethylhexan-3-amine, and the like.
Additional branching can also be present. Without being bound by
any theory or mechanism, it is believed that primary alkylamines
can serve as ligands in the copper coordination sphere but be
readily dissociable therefrom during copper nanoparticle
fusion.
[0042] In some embodiments, the surfactant system can include a
secondary amine. Secondary amines suitable for forming copper
nanoparticles can include normal, branched, or cyclic
C.sub.4-C.sub.12 alkyl groups bound to the amine nitrogen atom. In
some embodiments, the branching can occur on a carbon atom bound to
the amine nitrogen atom, thereby producing significant steric
encumbrance at the nitrogen atom. Suitable secondary amines can
include, without limitation, dihexylamine, diisobutylamine,
di-t-butylamine, dineopentylamine, di-t-pentylamine,
dicyclopentylamine, dicyclohexylamine, and the like. Secondary
amines outside the C.sub.4-C.sub.12 range can also be used, but
such secondary amines can have undesirable physical properties such
as low boiling points or waxy consistencies that can complicate
their handling.
[0043] In some embodiments, the surfactant system can include a
chelating agent, particularly a diamine chelating agent. In some
embodiments, one or both of the nitrogen atoms of the diamine
chelating agent can be substituted with one or two alkyl groups.
When two alkyl groups are present on the same nitrogen atom, they
can be the same or different. Further, when both nitrogen atoms are
substituted, the same or different alkyl groups can be present. In
some embodiments, the alkyl groups can be C.sub.1-C.sub.6 alkyl
groups. In other embodiments, the alkyl groups can be
C.sub.1-C.sub.4 alkyl groups or C.sub.3-C.sub.6 alkyl groups. In
some embodiments, C.sub.3 or higher alkyl groups can be straight or
have branched chains. In some embodiments, C.sub.3 or higher alkyl
groups can be cyclic. Without being bound by theory or mechanism,
it is believed that diamine chelating agents can facilitate copper
nanoparticle formation by promoting nanoparticle nucleation.
[0044] In some embodiments, suitable diamine chelating agents can
include N,N'-dialkylethylenediamines, particularly C.sub.1-C.sub.4
N,N'-dialkylethylenediamines. The corresponding methylenediamine,
propylenediamine, butylenediamine, pentylenediamine or
hexylenediamine derivatives can also be used. The alkyl groups can
be the same or different. C.sub.1-C.sub.4 alkyl groups that can be
present include, for example, methyl, ethyl, propyl, and butyl
groups, or branched alkyl groups such as isopropyl, isobutyl,
s-butyl and t-butyl groups. Illustrative
N,N'-dialkylethylenediamines that can be suitable for use in
forming metal nanoparticles include, for example,
N,N'-di-t-butylethylenediamine, N,N'-diisopropylethylenediamine,
and the like.
[0045] In some embodiments, suitable diamine chelating agents can
include N,N,N',N'-tetraalkylethylenediamines, particularly
C.sub.1-C.sub.4 N,N,N',N'-tetraalkylethylenediamines. The
corresponding methylenediamine, propylenediamine, butylenediamine,
pentylenediamine or hexylenediamine derivatives can also be used.
The alkyl groups can again be the same or different and include
those mentioned above. Illustrative
N,N,N',N'-tetraalkylethylenediamines that can be suitable for use
in forming metal nanoparticles include, for example,
N,N,N',N'-tetramethylethylenediamine,
N,N,N',N'-tetraethylethylenediamine, and the like.
[0046] Surfactants other than aliphatic amines can also be present
in the surfactant system. In this regard, suitable surfactants can
include, for example, pyridines, aromatic amines thiols, or any
combination thereof. These surfactants can be used in combination
with an aliphatic amine, including those described above, or they
can be used in a surfactant system in which an aliphatic amine is
not present. Further disclosure regarding suitable pyridines,
aromatic amines, phosphines, and thiols follows below.
[0047] Suitable aromatic amines can have a formula of
ArNR.sup.1R.sup.2, where Ar is a substituted or unsubstituted aryl
group and R.sup.1 and R.sup.2 are the same or different. R.sup.1
and R.sup.2 can be independently selected from H or an alkyl or
aryl group containing from 1 to about 16 carbon atoms. Illustrative
aromatic amines that can be suitable for use in forming metal
nanoparticles include, for example, aniline, toluidine, anisidine,
N,N-dimethylaniline, N,N-diethylaniline, and the like. Other
aromatic amines that can be used in conjunction with forming copper
nanoparticles can be envisioned by one having ordinary skill in the
art.
[0048] Suitable pyridines can include both pyridine and its
derivatives. Illustrative pyridines that can be suitable for use in
forming copper nanoparticles include, for example, pyridine,
2-methylpyridine, 2,6-dimethylpyridine, pyridazine, and the like.
Chelating pyridines such as bipyridyl chelating agents can also be
used. Other pyridines that can be used in conjunction with forming
copper nanoparticles can be envisioned by one having ordinary skill
in the art.
[0049] Suitable phosphines can have a formula of PR.sub.3, where R
is an alkyl or aryl group containing from 1 to about 16 carbon
atoms. The alkyl or aryl groups attached to the phosphorus center
can be the same or different. Illustrative phosphines that can be
used in forming copper nanoparticles include, for example,
trimethylphosphine, triethylphosphine, tributylphosphine,
tri-t-butylphosphine, trioctylphosphine, triphenylphosphine, and
the like. Phosphine oxides can also be used in a like manner. In
some embodiments, surfactants that contain two or more phosphine
groups configured for forming a chelate ring can also be used.
Illustrative chelating phosphines can include 1,2-bisphosphines,
1,3-bisphosphines, and bis-phosphines such as BINAP, for example.
Other phosphines that can be used in conjunction with forming
copper nanoparticles can be envisioned by one having ordinary skill
in the art.
[0050] Suitable thiols can have a formula of RSH, where R is an
alkyl or aryl group having from about 4 to about 16 carbon atoms.
Illustrative thiols that can be used for forming copper
nanoparticles include, for example, butanethiol,
2-methyl-2-propanethiol, hexanethiol, octanethiol, benzenethiol,
and the like. In some embodiments, surfactants that contain two or
more thiol groups configured for forming a chelate ring can also be
used. Illustrative chelating thiols can include, for example,
1,2-dithiols (e.g., 1,2-ethanethiol) and 1,3-dithiols (e.g.,
1,3-propanethiol). Other thiols that can be used in conjunction
with forming copper nanoparticles can be envisioned by one having
ordinary skill in the art.
[0051] In general, the disposition of the n-doped region and the
p-doped region relative to one another within the semiconductor
substrate is not believed to be particularly limited. In some
embodiments, the n-doped region and the p-doped region can be
vertically disposed relative to one another. In other embodiments,
the n-doped region and the p-doped region can be horizontally
disposed relative to one another. In some embodiments, the n-doped
region, the p-doped region, or both can be located on a face of the
photovoltaic cell that receives or is configured to receive
electromagnetic radiation. In other embodiments, the n-doped
region, the p-doped region, or both can be located on a face of the
photovoltaic cell that is opposite a ace of the photovoltaic cell
that receives or is configured to receive electromagnetic
radiation. In some embodiments, the copper-based current collector
can be in contact with the n-doped region of the semiconductor
substrate. In further embodiments, the photovoltaic cells described
herein can also include an electrical connection in contact with
the p-doped region of the semiconductor substrate.
[0052] In some embodiments, when the n- and p-doped regions of the
semiconductor substrate are vertically disposed relative to one
another, the copper-based current collector can form a front-side
electrical contact and the electrical connection in contact with
the p-doped region can form a back-side electrical contact. As used
herein, the term "front-side electrical contact" will refer to an
electrical contact on the same face of the photovoltaic cell upon
which electromagnetic radiation is incident, and the term
"back-side electrical contact" will refer to an electrical contact
on the opposite face of the photovoltaic cell relative to the face
upon which electromagnetic radiation is incident. In some
embodiments, a back-side electrical contact can cover a significant
portion or all of the back face of the semiconductor substrate,
since there is no need to collect incident electromagnetic
radiation thereon. In contrast, a front-side electrical contact is
desirably made to have a footprint that is as small as possible,
with small line widths, in order to maximize the amount of incident
electromagnetic radiation received by the semiconductor substrate,
white still maintaining enough surface area to achieve a desired
degree of electrical conduction. That is, front-side electrical
contacts desirably have a high aspect ratio that maximizes surface
area of the electrical contact white limiting the area in contact
with the semiconductor substrate.
[0053] In embodiments where the n- and p-doped regions of the
semiconductor substrate are horizontally disposed relative to one
another, the electrical contacts can be located on both the
front-side and the back-side of the semiconductor substrate, or
they can all be located on the back-side of the semiconductor
substrate. Each configuration is described in more detail
below.
[0054] In some embodiments, the semiconductor substrate can be a
silicon substrate, which can include any of single crystal silicon,
polycrystalline amorphous silicon, or any combination thereof. For
example, in some embodiments, the semiconductor substrate can be a
silicon substrate having a region of n-doped silicon and a region
of p-doped silicon. Suitable dopants and techniques for n- and
p-doping of silicon will be familiar to one having ordinary skill
in the art. In other embodiments, suitable semiconductor substrates
can include, for example, CdTe, CuInGaSe (CIGS), GaAs, quantum
solar dots (e.g., CdS, CdSe, Sb.sub.2S.sub.3, PbS, and the like),
Gratzel cell type solar cells, dye sensitized type solar cells
(DSSC or DSC), and organophotovoltaic materials. Suitable organic
photovoltaic materials will be familiar to one having ordinary
skill in the art, and can include polymers such as
poly(phenylenevinylene), poly(3-hexylthiophene), and like
substances. Is to be noted that the electrically conductive
diffusion barrier may optionally be omitted with some of these
semiconductor substrates due to a reduced diffusion rate of copper
atoms therein (e.g., organophotovoltaic materials)
[0055] Unlike the in situ-formed diffusion barriers for silver in
conventional photovoltaic cells, the diffusion barriers for copper
in the photovoltaic cells described herein are desirably
electrically conductive by themselves and facilitate the adhesion
of metal nanoparticles to the semiconductor substrate. Moreover,
suitable diffusion barriers are intended to shield the
semiconductor substrate from copper for an extended period of time
(e.g., at least about 10-20 years). For deposition and fusion of
metal nanoparticles on a silicon substrate, suitable electrically
conductive diffusion barriers for copper can include thin films of
refractory substances such as, for example, electrically conductive
metal nitrides, electrically conductive metal carbides,
electrically conductive metal borides, or any combination thereof
in some or other embodiments, suitable electrically conductive
diffusion barriers for copper can include metal tungstides, metals,
or any combination thereof. In some embodiments, the electrically
conductive diffusion barrier can be a metal nitride. In more
particular embodiments, the electrically conductive diffusion
barrier can include substances such as, for example, TiN, TaN, WN,
TiW, W, or any combination thereof.
[0056] In general, any suitable technique can be used to deposit
the electrically conductive diffusion barrier. In some embodiments,
chemical vapor deposition or physical vapor deposition can be used.
Suitable chemical vapor deposition techniques can include, for
example, thermochemical vapor deposition of a metal-organic
precursor or plasma enhanced chemical vapor deposition. Suitable
physical vapor deposition techniques can include atomic layer
deposition and sputtering (e.g., magnetron sputtering), for
example. Use of such vapor deposition techniques for forming thin
films of refractory substances will be familiar to one having
ordinary skill in the art. In still other embodiments, plating,
evaporation, spin coating or lithographic deposition techniques can
also be used to deposit the electrically conductive diffusion
barrier.
[0057] In some embodiments, the electrically conductive diffusion
barrier can be deposited on the semiconductor substrate while in
nanoparticle form, with nanoparticle fusion taking place
thereafter. More specifically, in some embodiments, methods
described herein can further include applying a plurality of
nanoparticles to a semiconductor substrate, and at least partially
fusing the nanoparticles together to form the electrically
conductive diffusion barrier. Fusion of the nanoparticles forming
the diffusion barrier can take place in a manner similar to that
described above for fusing the copper nanoparticles together.
Suitable techniques for forming refractory nanoparticles are
described in commonly owned U.S. patent application Ser. No.
12/191,975, filed Aug. 14, 2008, which is incorporated herein by
reference in its entirety. More particularly, in some embodiments,
the diffusion barrier of the photovoltaic cells described herein
can be formed from nanoparticles of TiN, TaN, WN, TiW, W, or any
combination thereof. After formation of the electrically conductive
diffusion barrier from nanoparticles, copper nanoparticles can then
be deposited thereon, and construction of the photovoltaic cell can
be completed as generally described herein. In some embodiments,
micron-size particles can also be present in combination with the
nanoparticles being used to form the diffusion barrier.
[0058] In some embodiments, nanoparticles being used to form the
diffusion barrier can optionally be combined with an aggressive or
non-aggressive glass frit. For example, glass frit combined with
the nanoparticles can be used to etch through a SiN antireflective
coating on the semiconductor substrate, in much the same manner as
a glass frit etch is conducted when annealing photovoltaic cells
formed from silver pastes, as discussed above. However, when
forming a diffusion barrier from nanoparticles containing glass
frit, copper nanoparticles are subsequently deposited as described
above, in order to make a copper contact configured for removal of
current from the photovoltaic cells.
[0059] In some embodiments, the photovoltaic cells described herein
can further include an antireflective coating disposed on a face of
the semiconductor substrate that receives or is configured to
receive electromagnetic radiation. In some embodiments, the
antireflective coating can be disposed on the same face of the
semiconductor substrate as the copper-based current collector.
Specifically, in some embodiments, the antireflective coating can
be disposed on the front-side of the semiconductor substrate in
order to increase the amount of electromagnetic radiation that
optically interacts with the semiconductor substrate. In some
embodiments, the antireflective coating can be a SiN coating,
particularly when the semiconductor substrate is a silicon
substrate. Other types of suitable antireflective coatings will be
familiar to one having ordinary skill in the art and can be
substituted for a SiN antireflective coating in other embodiments
of the present disclosure.
[0060] In addition to copper nanoparticles, other substances can be
present when forming the copper contact or present in or on the
copper contact after it is formed. In some embodiments, the copper
contact can further include a corrosion-resistant substance. In
some embodiments, the corrosion-resistant substance can be metal
nanoparticles that are admixed with the copper nanoparticles and
confer corrosion-resistance thereto. For example, in some
embodiments discussed above, nickel (e.g., nickel nanoparticles)
can be included with the copper nanoparticles to improve corrosion
resistance. Other suitable substances for improving corrosion
resistance can include substances having elements such as, for
example, Zn, Si, Al, P, B, Ni, Sn, Ag, or SnAgCu, any of which can
be in nanoparticle form. When mixed with copper nanoparticles, the
corrosion-resistant substance can be present in an amount ranging
between about 0.1 to about 5% by weight of the copper
nanoparticles. In some or other embodiments, the copper contact can
be coated with a corrosion-resistant substance. In addition to the
corrosion-resistant substances noted above, a polymer coating can
also be applied as a final coating to the photovoltaic cells to
confer corrosion resistance thereto.
[0061] In some or other embodiments, the copper nanoparticles can
be mixed with micron-scale copper particles or other micron-scale
metal particles while being formed into the copper contact. In
various embodiments, an amount of the micron-scale copper particles
can range between about 0.1% to about 15% relative to the weight of
the copper nanoparticles. In various embodiments, the micron-scale
copper particles can range in size from about 500 nm to about 100
microns in at least one dimension.
[0062] In some or other embodiments, the copper contact can further
include a conductive additive associated therewith. In some
embodiments, the conductive additive can be mixed with the copper
nanoparticles while they are being formed into the copper contact.
In some or other embodiments, the conductive additive can be
disposed on the copper contact after its formation. In some
embodiments, the conductive additive can include substances such as
carbon black, pyrene, phenanthrene, carbon nanowires, carbon
nanotubes, graphene, any combination thereof, or any derivative
thereof.
[0063] In some embodiments, copper nanoparticles can be applied to
semiconductor substrates described herein when the copper
nanoparticles are dispersed in a dispensable nanoparticles paste
formulation. Any of the additional components described above can
also be present in the nanoparticle paste formulations including,
for example, micron-scale copper particles or other micron-scale
metal particles, conductive additives, corrosion-resistant
substances, or any combination thereof. In addition to the
foregoing additives, various thickening and rheology control agents
can also be present in order to impart a desired viscosity to the
nanoparticle paste formulations. In general, suitable nanoparticle
paste formulations can contain the copper nanoparticles dispersed
in an organic matrix and be dispensable through micron-size
apertures, thereby enabling deposition of the copper nanoparticles
by techniques such as screen printing, stencil printing, or ink-jet
printing, for example. Other suitable deposition techniques can
include, for example, spray coating and spin coating of the
dispensable nanoparticle paste formulations.
[0064] In some embodiments, the nanoparticle paste formulations can
be formulated as described in commonly owned U.S. Pat. No. ______
entitled "Nanoparticle Paste Formulations and Methods for
Production and Use Thereof," filed concurrently herewith and
incorporated herein by reference in its entirety. Desirably, by
including a high level of solids and carefully tailoring the
organic matrix, the nanoparticle paste formulations can be readily
dispensed and promote copper nanoparticle fusion white minimizing
the occurrence of significant crack and void formation. As-produced
copper nanoparticles, in contrast, can crack and form voids during
nanoparticle fusion due to volume contraction resulting from
surfactant and/or solvent loss. The nanoparticle paste formulations
are described in brief hereinafter.
[0065] some embodiments, the nanoparticle paste formulations can
contain at least about 30% copper nanoparticles by weight,
particularly about 30% to about 90% copper nanoparticles by weight
of the nanoparticle paste formulation, or about 50% to about 90%
copper nanoparticles by weight of the nanoparticle paste
formulation, or about 70% to about 90% copper nanoparticles by
weight of the nanoparticle paste formulation. Moreover, in some
embodiments, in addition to copper nanoparticles, micron-scale
copper particles can be present in the nanoparticle paste
formulations in an amount ranging between about 0.01% to about 15%
by weight of the nanoparticle paste formulation. In some
embodiments, the micron-size copper particles can range between
about 100 nm and about 5 microns in size, or between about 500 nm
and about 5 microns in size, or between about 500 nm and about 100
microns in size in at least one dimension. Although such
micron-scale copper particles need not necessarily be present, they
have been found to desirably promote the fusion of copper
nanoparticles to one another to form a consolidated mass.
[0066] In some embodiments, the nanoparticle paste formulations can
contain an organic matrix that includes one or more organic
solvents. Without being bound by any theory or mechanism, it is
believed that the organic solvents can promote solubilization of
the surfactants forming the surfactant coating, such that they are
no longer associated with the copper nanoparticles and thereby
promote nanoparticle fusion. The one or more organic solvents can
include any combination of hydrocarbons, alcohols, amines, and
organic acids. Remaining unbound by any theory or mechanism, it is
believed that hydrocarbon and alcohol solvents can passively
solubilize surfactant molecules liberated from the copper
nanoparticles by Brownian motion, whereas amine and organic acid
solvents can actively chemically interact with the surfactant
molecules such that they are no longer available for recombination
with the copper nanoparticles.
[0067] In some embodiments, the nanoparticle paste formulations can
include the combination of one or more hydrocarbons, one or more
alcohols, one or more amines, and one or more acids. In some
embodiments, between about 2 and about 10 members of each class of
organic solvents can be present. The number of members of each
class of organic solvent can be the same or different. In some
embodiments, the members of each class can have boiling points that
are separated from one another by a desired degree, so as to reduce
the suddenness of solvent loss while fusing the nanoparticles
together with one another and decreasing the rate of volume
contraction during nanoparticle fusion. More specifically, in some
embodiments, the various members of each class can have boiling
points that are separated from one another by about 20.degree. C.
to about 50.degree. C. Furthermore, in some embodiments, the
various organic solvents in the organic matrix can have boiling
points ranging between about 50.degree. C. and about 200.degree. C.
or between about 50.degree. C. and about 250.degree. C.
[0068] Illustrative but non-limiting examples of alcohol and
hydrocarbon solvents that can be present in the nanoparticle paste
formulations include, for example, light aromatic petroleum
distillate (CAS 64742-95-6), hydrotreated fight petroleum
distillates (CAS 64742-47-8), tripropyleneglycol methyl ether,
ligroin (CAS 68551-17-7, a mixture of C.sub.10-C.sub.13 alkanes),
diisopropyleneglycol monomethyl ether, diethyleneglycol diethyl
ether, 2-propanol, 2-butanol, t-butanol, 1-hexanol,
2-(2-butoxyethoxy)ethanol, terpineol, monohydric alcohols, diols,
triols, glycol ethers (e.g., diethylene glycol and triethylene
glycol), alkanolamines (e.g., ethanolamine, triethanolamine, and
the like), or any combination thereof. In some embodiments,
polyketone solvents can be used in a like manner.
[0069] Illustrative but non-limiting examples of amine solvents
that can be present in the nanoparticle paste formulations include,
for example, tallowamine (CAS 61790-33-8), alkyl (C.sub.8-C.sub.18)
unsaturated amines (CAS 68037-94-5), di(hydrogenated tallow)amine
(CAS 61789-79-5), dialkyl (C.sub.8-C.sub.20) amines (GAS
68526-63-6), alkyl (C.sub.10-C.sub.16)dimethyl amine (CAS
67700-98-5), alkyl (C.sub.14-C.sub.18) dimethyl amine (CAS
68037-93-4), &hydrogenated tallowmethyl amine (CAS 61788-63-4),
and trialkyl (C.sub.6-C.sub.12) amines (CAS 68038-01-7).
[0070] Illustrative but non-limiting examples of organic acid
solvents that can be present in the nanoparticle paste formulations
include, for example, octanoic acid, nonanoic acid, decanoic acid,
caprylic acid, pelargonic acid, undecylic acid, lauric acid,
tridecylic acid, myristic acid, pentadecanoic acid, palmitic acid,
margaric acid, stearic acid, nonadecylic acid, .alpha.-linolenic
acid, stearidonic acid, oleic acid, and linoleic acid.
[0071] In accordance with some embodiments described above, FIGS. 3
and 4 show schematics of illustrative photovoltaic cells having a
front-side copper-based current collector disposed thereon. For
convenience, photovoltaic cells 30 and 31 depicted in FIGS. 3 and
4, respectively, will be described with reference to a silicon
semiconductor substrate. However, as described above, any suitable
semiconductor can be used. As depicted in FIGS. 3 and 4, silicon
semiconductor substrate 32 is typically p-doped and contains
n-doped region 34 (i.e., n-doped silicon) and p-doped region 36
(i.e., p-doped silicon) therein. The silicon semiconductor
substrate configurations depicted in FIGS. 3 and 4 can be
fabricated starting from p-doped silicon and patterning n-doped
region 34 though a suitable doping technique, such as
ion-implantation of an n-dopant. In FIG. 3, n-doped region 34 and
p-doped region 36 are vertically disposed relative to one another,
and in FIG. 4, they are disposed side-by-side. Either configuration
can be used in the embodiments described herein. Incident
electromagnetic radiation 35 denotes the face of the photovoltaic
cells upon which the electromagnetic radiation is incident.
[0072] With continued reference to FIGS. 3 and 4, antireflective
coating 38 is disposed on the front-side of silicon semiconductor
substrate 32, upon which electromagnetic radiation 35 is incident.
Portions of antireflective coating 38 are patterned and/or removed
to provide electrical exposure to silicon semiconductor substrate
32. Thereafter, electrically conductive diffusion barrier 40 is
directly disposed on n-doped region 34, and copper contact 42 is
formed thereon. As described above, copper contact 42 can be formed
from at least partial fusion of copper nanoparticles. In FIG. 3,
electrical connection 44 is formed as aback-side electrical
connection in electrical communication with p-doped region 36. As
depicted, electrical connection 44 spans the entirety of p-doped
region 36, but it need not necessarily do so. Contrastingly, in
FIG. 4, electrical connection 44 is formed as a front-side
electrical connection in electrical communication with p-doped
region 36. In the embodiment depicted in FIG. 4, electrical
connection 44 can be formed from copper nanoparticles that have
been at least partially fused together on diffusion barrier 40.
[0073] As described above, the copper-based current collectors can
also be configured as back-side electrical contacts. FIG. 5 shows a
schematic of an illustrative photovoltaic cell having a back-side
copper-based current collector disposed thereon. The side-by-side
disposition of n-doped region 34 and p-doped region 36 in
photovoltaic cell 50 is similar to that depicted in FIG. 4. In the
case of a back-side electrical contact, a significant fraction of
incident electromagnetic radiation 35 is absorbed near the top
surface of silicon semiconductor substrate 32 before reaching the
p-n junction. Accordingly, the overall thickness of silicon
semiconductor substrate 32 is kept as small as possible in order to
create a large fraction of charge carriers within the diffusion
length of the p-n junction. As in FIG. 4, electrical connection 44
in electrical communication with p-doped region 36 can also be
fabricated from copper nanoparticles. Optionally, a compliant
material (not depicted) can be used to till the voids between
copper contact 42 and electrical connection 44. The compliant
material can desirably reduce the amount of CTE mismatch that can
be present when adhering photovoltaic cell 50 to another
surface.
[0074] It bears mention that although the foregoing FIGURES have
depicted only a single copper contact 42 or electrical connection
44 in electrical communication with n-doped region 34 and p-doped
region 36, respectively, multiple copper contacts 42 and electrical
connections 44 can be present, particularly if copper contact 42
and electrical connection 44 are on the same face of the
photovoltaic cell. If multiple copper contacts 42 or electrical
connections 44 are present, they can be connected in parallel to
like elements via a busbar. The busbar can consolidate the flow of
electrical current from the photovoltaic cell. Such techniques for
connecting multiple electrical elements in parallel to one another
will be familiar to one having ordinary skill in the art.
[0075] Various techniques can be used to form the copper-based
current collector on the semiconductor substrate. Some of these
illustrative techniques will be described in more detail below.
However, it should be recognized that these techniques are merely
illustrative in nature, and other protocols for forming
copper-based current collectors on the photovoltaic cells described
herein can be envisioned by one having ordinary skill in the
art.
[0076] In some embodiments, the electrically conductive diffusion
barrier can be deposited on the semiconductor substrate by plating,
CVD, PVD or the like. Thereafter, a photoresist material can be
applied, and the photoresist material can then be etched to remove
undesired material. Deposition of copper nanoparticles can then
follow. In other embodiments, a lift-off technique can be
performed, where the photoresist is deposited first, followed by
patterning, etching, diffusion barrier deposition, and copper
nanoparticle deposition. The photoresist and material deposited
thereon can be removed to re-expose the surface of the
semiconductor substrate. Suitable deposition and patterning
techniques will be familiar to one having ordinary skill in the
art. Thereafter, an antireflective coating can be applied to the
face of the semiconductor substrate that receives electromagnetic
radiation, which can also convey additional oxidation protection to
the current collectors. Similar techniques can be used to apply a
copper-based current collector to the face of the semiconductor
substrate that is opposite the face that receives electromagnetic
radiation.
[0077] As a non-limiting example, the following describes an
illustrative lift-off technique in which high aspect ratio copper
contacts with minimized shadows effects can be fabricated. First, a
thick layer (10-30 microns) of photoresist SU8) can be deposited
and patterned, followed by a thin layer of the electrically
conductive diffusion barrier and then copper nanoparticles into the
patterned areas. Here, the copper nanoparticles can be applied as
if the photoresist is a stencil, with deposition taking place into
the recessed areas. In such embodiments, the line widths of the
copper contacts can be as small as between about 5-10 microns or as
large as between about 100-300 microns. The line widths can be
about 30 microns or greater in some embodiments, or between about
30-100 microns in other embodiments. Such line widths can greatly
reduce shadowing effects, thereby promoting an overall rise in
efficiency of the photovoltaic cells by increasing the degree of
illumination thereon. In some embodiments, the copper nanoparticles
can be spray coated or spin coated to fill the recesses in the
photoresist. In other embodiments, the copper nanoparticles can be
direct printed into the recesses using screen printing,
micro-dispensing, inkjet printing, or like deposition
techniques.
[0078] In still other embodiments, a combination approach can be
applied wherein the electrically conductive diffusion barrier is
applied by a photolithography technique, and the copper
nanoparticles are patterned using screen printing,
micro-dispensing, ink-jet printing, or like techniques. In cases
where photolithography techniques are used, improvements in
throughput can be achieved during the photolithography step by
creating a duplicated mask and exposing many semiconductor
substrates in succession or at one time. Either inline or batch
photolithography techniques can be applied in order to reduce
costs.
[0079] To facilitate a better understanding of the embodiments
described herein, the following experimental examples are provided.
The examples are provided for purposes of illustration only and
should be considered non-limiting.
EXAMPLES
Example 1
Testing of Diffusion Barriers for Copper on a Silicon Substrate
[0080] A 100 mm diameter CZ <100> silicon wafer was coated
with a 50 nm thick layer of diffusion barrier as set forth in Table
1. A 50 nm layer of copper was then deposited on top of the
diffusion barrier. Bulk copper was deposited by sputtering
techniques. The sample indicated as nanoCu was deposited by a
stencil deposition technique described below.
[0081] The nanoCu sample was deposited using Kapton tape as a rough
stencil. Thereafter, a copper nanoparticle paste containing 3.5%
micron-scale copper flakes and 15% organic matrix was applied to
the rough stencil. The copper nanoparticles were then dried and
fused for 20 minutes at 50.degree. C., 20 minutes at 90.degree. C.,
and 4 minutes at 210.degree. C.
[0082] The samples were aged for 2 weeks at 353.degree. C. in a
vacuum oven in an accelerated aging test, which is equivalent to 25
years of use at 50.degree. C. As shown in Table 1, the diffusion
barriers in wafers 2 and 5 were visually determined to best
mitigate the diffusion of copper into the silicon.
TABLE-US-00001 TABLE 1 Wafer Layer 1 Layer 2 Layer 3 Post Aging
Visual Inspection 1 Cu 50 nm -- -- Baseline, barely a tinge of
copper color apparent, mostly silver color 2 Ti 20 nm TiN 30 nm Cu
50 nm Still copper colored, little change from before 3 Ti 50 nm Cu
50 nm -- Burnt copper color, has a deep blue edge 4 Ni 50 nm Cu 50
nm -- Copper color mostly gone, slightly more apparent than in no
barrier 5 Ta 50 nm Cu 50 nm -- Still copper colored, little change
from before 6 Ti 20 nm TiN 30 nm nanoCu Not performed (stencil
deposition)
[0083] Secondary precision ion mass spectrometry was performed to
determine the degree of copper incursion into the silicon wafers.
FIGS. 6A and 6B show precision secondary ion mass spectrometry for
wafer 4, pre- and post-anneal, respectively. FIGS. 7A and 7B show
precision secondary ion mass spectrometry for wafer 3, pre- and
post-anneal, respectively. FIGS. 8A and 8B show precision secondary
ion mass spectrometry for wafer 5, pre- and post-anneal,
respectively. FIGS. 9A and 9B show precision secondary ion mass
spectrometry for wafer 2, pre- and post-anneal, respectively. As
shown by the secondary ion mass spectrometry results, the diffusion
barriers in wafers 2 and 5 best retained the copper ion profile and
produced less knock on. These results are consistent with the
visual analyses presented in Table 1.
[0084] Although the invention has been described with reference to
the disclosed embodiments, those skilled in the art will readily
appreciate that these are only illustrative of the invention. It
should be understood that various modifications can be made without
departing from the spirit of the invention. The invention can be
modified to incorporate any number of variations, alterations,
substitutions or equivalent arrangements not heretofore described,
but which are commensurate with the spirit and scope of the
invention. Additionally, while various embodiments of the invention
have been described, it is to be understood that aspects of the
invention may include only some of the described embodiments.
Accordingly, the invention is not to be seen as limited by the
foregoing description.
* * * * *