U.S. patent application number 16/536287 was filed with the patent office on 2020-03-26 for silver-bismuth non-contact metallization pastes for silicon solar cells.
The applicant listed for this patent is Hitachi Chemical Co., Ltd.. Invention is credited to Stephen T. Connor, James Randy Groves, Brian E. Hardin, Craig H. Peters.
Application Number | 20200098938 16/536287 |
Document ID | / |
Family ID | 58095912 |
Filed Date | 2020-03-26 |
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United States Patent
Application |
20200098938 |
Kind Code |
A1 |
Hardin; Brian E. ; et
al. |
March 26, 2020 |
SILVER-BISMUTH NON-CONTACT METALLIZATION PASTES FOR SILICON SOLAR
CELLS
Abstract
Metallization pastes for use with semiconductor devices are
disclosed. The pastes contain silver particles, low-melting-point
base-metal particles, organic vehicle, and optional crystallizing
agents. Specific formulations have been developed that produce
stratified metal films that contain less silver than conventional
pastes and that have high peel strengths. Such pastes can be used
to make high contact resistance metallization layers on
silicon.
Inventors: |
Hardin; Brian E.; (San
Carlos, CA) ; Connor; Stephen T.; (San Francisco,
CA) ; Groves; James Randy; (Sunnyvale, CA) ;
Peters; Craig H.; (Belmont, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hitachi Chemical Co., Ltd. |
Tokyo |
|
JP |
|
|
Family ID: |
58095912 |
Appl. No.: |
16/536287 |
Filed: |
August 8, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15243830 |
Aug 22, 2016 |
10418497 |
|
|
16536287 |
|
|
|
|
62377369 |
Aug 19, 2016 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01B 1/22 20130101; Y02E
10/50 20130101; H01L 31/022425 20130101 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; H01B 1/22 20060101 H01B001/22 |
Goverment Interests
STATEMENT OF GOVERNMENT SUPPORT
[0002] This invention is made with Government support under
contract number IIP-1430721 awarded by the NSF. The Government may
have certain rights in this invention.
Claims
1. A solar cell comprising: a silicon substrate having a front
surface and a back surface; a plurality of fine grid lines on the
front surface of the silicon substrate; at least one front busbar
layer on the front surface of the silicon substrate, the front
busbar layer in electrical contact with at least a portion of the
plurality of fine grid lines; an aluminum layer on a portion of the
back surface of the silicon substrate; and at least one rear
tabbing layer on a portion of the back surface of the silicon
substrate; wherein the rear tabbing layer comprises a stratified
film that comprises at least two sublayers: a first sublayer over
the silicon substrate, the first sublayer comprising a
low-melting-point base-metal; and a second sublayer over the first
sublayer, the second sublayer comprising silver; wherein the second
sublayer has an exterior surface, and the exterior surface is
exposed to an outside environment; wherein the stratified film
contains a low temperature base-metal fraction greater than 20%.
wherein edges of the aluminum layer and edges of the rear tabbing
overlap to form overlap regions that comprise: a solid
aluminum-silicon eutectic layer on the silicon layer; a modified
rear tabbing layer over the solid aluminum-silicon eutectic layer;
and a portion of the aluminum layer over the modified rear tabbing
layer.
2. The solar cell of claim 1, wherein the stratified film further
comprises (MO.sub.x).sub.y(SiO.sub.2).sub.z, crystallites wherein
0.ltoreq.x.ltoreq.3, 1.ltoreq.y.ltoreq.10, and 0.ltoreq.z.ltoreq.1
and M is a material selected from the group consisting of bismuth,
tin, tellurium, antimony, lead, silicon, and alloys, composites,
and combinations thereof.
3. The solar cell of claim 1, wherein the first sublayer comprises
a material selected from the group consisting of bismuth, aluminum,
carbon, tin, tellurium, antimony, lead, silicon, and alloys,
oxides, composites, and combinations thereof.
4. The solar cell of claim 1, wherein the stratified film comprises
at least 0.5 wt % aluminum.
5. The solar cell of claim 1, wherein the stratified film comprises
at least 1 wt % tellurium.
6. The solar cell of claim 1, wherein the stratified film has a
thickness between 1 .mu.m and 15 .mu.m.
7. The solar cell of claim 1, wherein the first sublayer has a
thickness between 0.01 .mu.m and 5 .mu.m.
8. The solar cell of claim 1, wherein the second sublayer has a
thickness between 0.5 .mu.m and 10 .mu.m.
9. The solar cell of claim 1, wherein the second sublayer exterior
surface comprises at least 70 wt % silver.
10. The solar cell of claim 1, wherein the portion of the aluminum
layer in the overlap region has an exterior surface, and the
exterior surface is exposed to an outside environment.
11. The solar cell of claim 1, wherein the overlap region is
between 10 .mu.m and 500 .mu.m wide.
12. The solar cell of claim 1, wherein the rear tabbing layers have
a peel strength of more than 1 N/mm when soldered to tin-coated
copper tabbing ribbons.
13. The solar cell of claim 1, wherein a contact resistance between
the rear tabbing layer and the aluminum layer is between 0 and 5
m.OMEGA..
14. A solar cell comprising: a silicon substrate having a front
surface and a back surface; a plurality of fine grid lines on the
front surface of the silicon substrate; at least one front busbar
layer on the front surface of the silicon substrate, the front
busbar layer in electrical contact with at least a portion of the
plurality of fine grid lines; an aluminum layer on a portion of the
back surface of the silicon substrate; and at least one rear
tabbing layer on a portion of the back surface of the silicon
substrate; wherein at least one of the front busbar layers or the
rear tabbing layer comprises a stratified film that comprises at
least two sublayers: a first sublayer over the silicon substrate,
the first sublayer comprising a low-melting-point base-metal; and a
second sublayer over the first sublayer, the second sublayer
comprising silver; wherein the second sublayer has an exterior
surface, and the exterior surface is exposed to an outside
environment; wherein the stratified film contains a low temperature
base-metal fraction greater than 20%
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application 62/209,885, filed Aug. 26, 2015 and to U.S. Provisional
Patent Application 62/377,369, filed Aug. 19, 2016, both of which
are incorporated by reference herein.
BACKGROUND OF THE INVENTION
Field of the Invention
[0003] This invention relates to conductive metallization pastes,
and more specifically to metallization layers that have a high peel
strength after co-firing and subsequent soldering to a tabbing
ribbon. Such pastes may be especially well-suited for use in
silicon based solar cells.
[0004] In general, conductive metallization pastes contain silver
particles, glass frit, and an organic binder, all mixed with an
organic vehicle. The price of silver particles represents more than
80% of the materials cost of many conductive metallization pastes,
especially those used for front side and rear tabbing solar cell
applications. Reducing the amount of silver in the paste is highly
desirable and is done either by reducing the overall paste solids
content or by replacing silver with other particles to reduce the
silver fraction in the paste.
[0005] State-of-the-art commercial rear tabbing pastes contain
organic solvents and binder along with 40-55 wt % silver and 5-8 wt
% glass frit. When the total solids content of the paste is
reduced, the overall fired film thickness decreases. This strategy
has been successfully employed over the last five years in the rear
tabbing layer for PV (photovoltaic) cells, reducing the fired
silver film thickness from 7 .mu.m to 3 .mu.m. Such pastes result
in films where the Ag content is 83-90 wt % of the dried film.
Unfortunately, further reduction of the fired film thickness can
lead to increased film porosity, incomplete Ag coverage on the
silicon surface, and solder leaching, all of which reduce the
overall peel strength of the tabbed solder joint.
[0006] In the past, adding a large quantity of non-precious metal,
inorganic particles to Ag based metallization pastes has proven
challenging for several reasons. Adding large quantities of
particles that contain non-precious metal, inorganic materials such
as aluminum that alloy with silver at high temperatures can form
dense, strong films, but the resulting alloy may be unsolderable,
resulting in a soldered joint with an unacceptable low (e.g., less
than 1 N/mm) peel strength. Particles made from non-precious metal,
inorganic materials such as nickel, that are not miscible with
silver may produce mixed Ag/inorganic films that retain
solderability at high firing temperatures. However, silver does not
wet such materials during firing, which can cause de-sintering
around the inorganic particles and result in phase-separated films
that may be homogenous but may also be highly porous and
structurally weak. Furthermore, adding large quantities of
base-metal particles can negatively impact the contact resistance
between the tabbing layer and aluminum layer, causing an increase
in the overall series resistance of the solar cell.
[0007] There is a need to develop metallization pastes that contain
inorganic materials to reduce their silver content. It would be
especially useful if such pastes produced fired films with high
peel strengths and long-term joint reliability while maintaining
similar or better electrical performance than films fade from
conventional silver metallization pastes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The foregoing aspects and others will be readily appreciated
by the skilled artisan from the following description of
illustrative embodiments when read in conjunction with the
accompanying drawings.
[0009] FIG. 1 is a schematic cross-section drawing of a stratified
film 100 made from co-firing metallization pastes on a silicon
substrate, according to an embodiment of the invention.
[0010] FIG. 2 is a schematic drawing that shows the front (or
illuminated) side of a silicon solar cell, according to an
embodiment of the invention.
[0011] FIG. 3 is a schematic drawing that shows the rear side of a
silicon solar cell, according to an embodiment of the
invention.
[0012] FIG. 4 is a scanning electron microscope (SEM) image (10
k.times. magnification) of a stratified Ag:Bi film on a silicon
wafer, according to an embodiment of the invention.
[0013] FIG. 5 is an x-ray diffraction (XRD) pattern from a
stratified Ag:Bi film on a silicon substrate, according to an
embodiment of the invention.
[0014] FIG. 6A is a cross-section schematic illustration that shows
a portion of the rear side of a silicon solar cell before
co-firing, according to an embodiment of the invention.
[0015] FIG. 6B is a cross-section schematic illustration that shows
a portion of the rear side of the silicon solar cell shown in FIG.
6A after co-firing, according to an embodiment of the
invention.
[0016] FIG. 7 is a scanning electron microscope (SEM) image (3
k.times. magnification) of an overlap region on the silicon solar
cell wherein the rear tabbing layer does not contain aluminum-based
particles.
[0017] FIG. 8 is a scanning electron microscope (SEM) image (3
k.times. magnification) of an overlap region on the silicon solar
cell wherein the rear tabbing layer contains aluminum based
particles, according to an embodiment of the invention.
SUMMARY
[0018] In one embodiment of the invention, a solar cell is
disclosed. The solar cell has a silicon substrate, a plurality of
fine grid lines on the front surface of the silicon substrate, at
least one front busbar layer in electrical contact with at least a
portion of the plurality of fine grid lines, an aluminum layer on a
portion of the back surface of the silicon substrate, and at least
one rear tabbing layer on a portion of the back surface of the
silicon substrate. In one arrangement, the rear tabbing layer is a
stratified film that has at least two sublayers: a first
low-melting-point base-metal sublayer over the silicon substrate,
and a second silver-rich sublayer over the first sublayer. The
second sublayer may have an exterior surface that is exposed to the
outside environment. The stratified film has a low-temperature,
base-metal fraction greater than 20%. Some edges of the aluminum
layer may overlap with some edges of the rear tabbing to form
overlap regions. The overlap regions may contain a solid
aluminum-silicon eutectic layer in the underlying silicon layer, a
modified rear tabbing layer over the solid aluminum-silicon
eutectic layer, and an aluminum layer over the modified rear
tabbing layer. The stratified film may also contain
(MO.sub.x).sub.y(SiO.sub.2).sub.z, crystallites wherein
0.ltoreq.x.ltoreq.3, 1.ltoreq.y.ltoreq.10, and 0.ltoreq.z.ltoreq.1,
and M is any of bismuth, tin, tellurium, antimony, lead, silicon,
or alloys, composites, or combinations thereof.
[0019] The first sublayer may contain a material such as bismuth,
aluminum, carbon, tin, tellurium, antimony, lead, silicon, or
alloys, oxides, composites, or combinations thereof. The first
low-melting-point base-metal sublayer may contain bismuth and
oxygen.
[0020] The stratified film may contain at least 0.5 wt % aluminum.
The stratified film may have a low-temperature, base-metal fraction
greater than 30%, greater than 40%, or greater than 50%.
[0021] The aluminum layer may have a thickness between 20 .mu.m and
30 .mu.m.
[0022] The stratified film may have a thickness between 1 .mu.m and
15 .mu.m, between 1 .mu.m and 10 .mu.m, or between 1 .mu.m and 3
.mu.m. The first sublayer may have a thickness between 0.01 .mu.m
and 5 .mu.m. The first sublayer may have a thickness between 0.25
.mu.m and 3 .mu.m, or between 0.5 .mu.m and 2 .mu.m. The solar cell
of claim 1, wherein the second sublayer may have a thickness
between 0.5 .mu.m and 10 .mu.m, between 0.5 .mu.m and 5 .mu.m, or
between 1 .mu.m and 4 .mu.m.
[0023] The solar cell of claim 1, wherein the second sublayer
exterior surface may contain at least 90 wt %, at least 80 wt %, or
at least 70 wt % silver.
[0024] The aluminum layer in the overlap region may have an
exterior surface that is exposed to the outside environment. The
modified rear tabbing layer in the overlap region may have an
exterior surface that is exposed to the outside environment. The
overlap region may be between 10 .mu.m and 500 .mu.m or 100 .mu.m
and 300 .mu.m wide.
[0025] The modified rear tabbing layer may contain any of bismuth,
tin, tellurium, antimony, lead, silicon, oxides or alloys,
composites, or combinations thereof. The modified rear tabbing
layer may contain at least 0.5 wt % or at least 1 wt %
aluminum.
[0026] The stratified film may have a conductivity that is 2 to 50
times, 2 to 25 times, or 2 to 10 times less than the conductivity
of bulk silver. T
[0027] The rear tabbing layers may have a peel strength of more
than 1 N/mm when soldered to tin-coated copper tabbing ribbons.
[0028] The contact resistance between the rear tabbing layer and
the aluminum layer may be between 0 and 5 m.OMEGA., between 0.25
and 3 m.OMEGA., or between 0.3 and 1 m.OMEGA..
[0029] The rear surface of the silicon substrate may have a doped
silicon base layer, and the rear tabbing layers make electrical
contact to the silicon base layer with a contact resistance greater
than 100 m.OMEGA.-cm2.
[0030] The silicon substrate may contain be a monocrystalline or a
multi-crystalline silicon wafer and may have an n-type base or a
p-type base.
[0031] In another embodiment of the invention, a metallization
paste is disclosed. The metallization paste contains silver
particles low-melting-point, base-metal particles, optionally a
crystallizing agent and aluminum-based particles, which are all
mixed together in an organic vehicle. The low-temperature,
base-metal fraction in the metallization paste is greater than 20%,
greater than 30%, greater than 40%, or greater than 50%. The
metallization paste may contain a crystallizing agent. The
metallization paste may have a solids loading between 30 wt % and
70 wt %. The metallization paste may contain between 10 wt % and 45
wt % silver particles. The metallization paste may contain between
5 wt % and 35 wt % low-melting-point, base-metal particles
particles. The metallization paste may contain between 0.5 wt % and
3 wt % aluminum particles. The metallization paste may also contain
less than 2 wt % strong glass-forming frits.
[0032] The silver particles may be spherically shaped nanoparticles
with a D50 between 10 nm and 1 .mu.m, between 50 nm and 800 nm, or
between 200 nm and 500 nm. The silver particles may be spherically
shaped micron-sized particles with a D50 between 1 .mu.m and 10
.mu.m, between 1 .mu.m and 5 .mu.m, or between 1 .mu.m and 2 .mu.m.
The silver particles may be flake shaped particles with a width
between 1 and 10 .mu.m and a thickness between 100 nm and 500 nm.
The silver particles may be a mixture of nanoparticles and micron
sized particles. The silver particles may be a mix of 50 wt %
nanoparticles with a D50 of 500 nm, 30 wt % spherical micron sized
particles with a D50 of 2 .mu.m, and 20 wt % silver flakes with
diameter of 2 .mu.m. The silver particles may have either a
unimodal or a bimodal size distribution.
[0033] The low-melting-point, base-metal particles may contain any
of bismuth (Bi), tin (Sn), tellurium (Te), antimony (Sb), lead
(Pb), or alloys, composites, or other combinations thereof. The
low-melting-point, base-metal particles may contain bismuth. The
metallization low-melting-point, base-metal particles may be
crystalline. The low-melting-point, base-metal particles may have a
spherical shape with a D50 between 50 nm and 5 .mu.m, between 300
nm and 5 .mu.m, or between 300 nm and 2 .mu.m. The
low-melting-point, base-metal particles may have either a unimodal
or a bimodal size distribution.
[0034] The low-melting-point, base-metal particles may have bismuth
cores and each bismuth core is coated by a shell. The shell may
contain any of silver (Ag), nickel (Ni), nickel-boron (Ni:B) alloy,
tin (Sn), tellurium (Te), antimony (Sb), lead (Pb), molybdenum
(Mo), titanium (Ti), magnesium (Mg), boron (B), silicon oxides
(SiOx), composites, or other combinations thereof. The shell may
contain silver with a thickness is less than 200 nm. The shell may
contain a nickel-boron alloy with a boron content between 2-8 wt
%.
[0035] The aluminum-based particles may have a spherical shape with
a D50 between 50 nm and 15 .mu.m, between 300 nm and 10 .mu.m, or
between 300 nm and 4 .mu.m. The aluminum-based particles may have
either a unimodal or a bimodal size distribution. The metallization
paste may contain 35 wt % silver particles, 16 wt % low melting
point base-metal particles, 1 wt % aluminum-based particles, 48 wt
% organic vehicle, no crystallizing agent, and no glass frit.
[0036] The metallization paste may have a viscosity between 10,000
and 200,000 cP at 25.degree. C. and at a sheer rate of 4 sec-1.
[0037] The metallization paste may also contain a crystallizing
agent mixed together with the silver particles, the
low-melting-point base-metal particles, and the aluminum-based
particles in the organic vehicle. The metallization paste may
contain less than 1 wt % or less than 0.1 wt % crystallizing agent.
The crystallizing agent may contain any of tellurium, silicon ,
boron, zinc, or oxides or alloys thereof. The crystallizing agent
may contain tellurium and/or tellurium oxide. The crystallizing
agent may contain particles with an approximately spherical shape
and a D50 between 50 nm and 5 .mu.m, between 100 nm and 3 .mu.m, or
between 200 nm and 1 .mu.m. The crystallizing agent may contain
flake particles with diameters between 1 and 10 .mu.m and
thicknesses between 100 nm and 500 nm.
[0038] In another embodiment of the invention, an overlap region on
the back side of a silicon solar cell is disclosed. The overlap
region contains a solid aluminum-silicon eutectic region that
extends into the back side of the silicon solar cell, a modified
rear tabbing layer over the solid aluminum-silicon eutectic region,
and an aluminum layer over the modified rear tabbing layer. The
modified rear tabbing layer may contain any of bismuth, tin,
tellurium, antimony, lead, silicon, or alloys, composites, or
combinations thereof. The low-melting-point base-metal fraction
greater than 20%.
DETAILED DESCRIPTION
[0039] The preferred embodiments are illustrated in the context of
metallization layers for silicon-based solar cells. The skilled
artisan will readily appreciate, however, that the materials and
methods disclosed herein will have application in a number of other
contexts where making good electrical contact to semiconducting or
conducting materials is desirable, particularly where good adhesion
and low cost are important. These and other objects and advantages
of the present invention will become more fully apparent from the
following description taken in conjunction with the accompanying
drawings. All publications referred to herein are incorporated by
reference in their entirety for all purposes as if fully set forth
herein.
[0040] Metallization pastes are formulated to be applied onto
substrates, such as solar cells, using a process such as screen
printing. The pastes are dried and then co-fired in an oxidizing
ambient to compact the solid components and vaporize and oxidize
organic molecules in order to produce compact films. For the
purposes of this disclosure, the term "co-fired" is used to
describe heating to between 700.degree. C. and 850.degree. C. for
about 0.5 to 3 seconds in ambient air conditions using an IR belt
furnace or similar tool. The temperature profile of the substrate
is often calibrated using a DataPaq.RTM. system with a thermocouple
attached to a bare substrate.
[0041] Metrics for such films include: [0042] solderability [0043]
peel strength, [0044] bulk resistance, [0045] contact resistance
between the rear tabbing layer and the aluminum layer, and [0046]
contact resistance between the metallization layer and the
substrate.
[0047] Solderability is the ability to form a strong physical bond
between two metal surfaces by the flow of a molten metal solder
between them at temperatures below 400.degree. C. Soldering on a
solar cell may be performed after heating the cell in air to over
650.degree. C. for approximately one second. Soldering involves the
use of flux, which is a chemical agent that cleans or etches one or
both of the surfaces prior to reflow of the molten solder. This can
be difficult because many metal oxides are resistant to
commonly-used fluxes after oxidation above 650.degree. C.
[0048] For layers that contact a tabbing ribbon directly, it is
useful if the peel strength is greater than 1N/mm (Newton per
millimeter). The peel strength can be described as the force
required to peel a soldered ribbon, at a 180.degree. angle from the
soldering direction, divided by the width of the soldered ribbon.
It is common for the peel strength between rear tabbing layers and
the solar cell to be between 1.5 and 4 N/mm. Peel strength is a
metric of solder joint strength for solar cells and is an indicator
of module reliability.
[0049] Meier et al. describes how to use a four-point probe
electrical measurement to determine the resistivity of each
metallization layer on a completed solar cell (Reference: Meier et
al. "Determining components of series resistance from measurements
on a finished cell", IEEE (2006) pp1315). The bulk resistance of a
metallization layer is directly related to the bulk resistance of
the material from which it is made. As an example, the bulk
resistance of pure Ag is 1.6E-8 .OMEGA.-m. However, because of the
structural imperfections and impurities, Ag metallization layers
used on industrial solar cells can have a bulk resistance that is
1.5 times to 5 times higher than the bulk resistance of pure Ag. A
low bulk resistance is especially important for fine grid lines,
which must transport current over a relatively long (i.e., more
than 1 cm) length.
[0050] In a solar cell current will flow from an aluminum layer on
the back side and through a rear tabbing (or metallization) layer.
The contact resistance between the rear tabbing layer and the
aluminum layer in the solar cell can be measured by using the
transmission line measurement (TLM) (Reference: Meier et al. "Cu
Backside Busbar Tape: Eliminating Ag and Enabling Full Al coverage
in Crystalline Silicon Solar Cells and Modules", IEEE PVSC (2015)
pp. 1-6). The TLM is plotted as resistance versus distance between
electrodes. In the experimental set-up the rear tabbing layer is
printed directly onto a silicon wafer and then dried. An aluminum
layer is subsequently printed partially over the silver layer with
an overlap region of approximately 300 .mu.m surrounding all sides
of the tabbing layer. The wafer is subsequently dried and
eventually co-fired. The contact resistance between the rear
tabbing layer and the aluminum layer is half of the y-intercept
value of a linear fit of the resistance versus distance TLM plot.
The electrical resistance between busbars can be measured using a
Keithley 2410 Sourcemeter in a four-point probe setup that sourced
current between -0.5 A and +0.5 A and measured the voltage. In
various embodiments, the contact resistance between the rear
tabbing layer and the aluminum layer is between 0 and 5 m.OMEGA.,
0.25 and 3 m.OMEGA., 0.3 and 1 m.OMEGA., or any range subsumed
therein. The sheet resistance of the aluminum layer is determined
by the slope of the line times the length of the electrodes. The
contact resistance and sheet resistance can be used to numerically
determine the transfer length and subsequently the contact
resistivity.
[0051] The contact resistance between a metallization layer and an
underlying silicon wafer has an impact on the power conversion
efficiency of a solar cell. The contact resistance can be measured
on silicon substrates by using the TLM described above. For fine
grid lines on the front side of a silicon wafer, it is useful to
reduce the contact resistance to less than 30 m.OMEGA.-cm.sup.2 or
less than 10 m.OMEGA.-cm.sup.2 to maintain a high fill factor. For
the rear tabbing and front busbar layers the contact resistance can
be higher than for the fine grid lines and still not affect device
performance because most of the current is extracted from the wafer
through the fine grid lines on the front side and the aluminum
layer on the rear side. Furthermore, there can be a large contact
resistance between a silver metallization layer and a silicon
substrate because silver does not make ohmic contact to
non-degenerately doped, p-type silicon. Furthermore, such a contact
resistance tends to increase when a large quantity of base metal
particles are substituted for silver particles in the metallization
paste. In some embodiments, the contact resistance between the
stratified film and the silicon wafer is greater than 100
m.OMEGA.-cm.sup.2, or greater than 1 .OMEGA.-cm.sup.2, or greater
than 10 .OMEGA.-cm.sup.2.
Ag-LowT.sub.MBM Metallization Paste Components
[0052] This disclosure describes pastes made of silver and
low-melting-point, base metal particles (LowT.sub.MBMs)
metallization pastes (Ag-LowT.sub.MBM pastes). The Ag-LowT.sub.MBM
pastes contain silver particles, LowT.sub.MBM particles, aluminum
based particles, and an organic vehicle. Such pastes may also
contain crystallizing agent(s) and small amounts of strong
glass-forming frits. The term "solids loading" is often used in
connection with metallization pastes to mean the amount or
proportion of inorganic solids in a metallization paste. In an
exemplary embodiment, Ag-LowT.sub.MBM metallization paste has a
solids loading that includes silver particles, low-melting-point
base-metal particles, aluminum-based particles, no crystallizing
agent and no strong glass-forming frits. All metallization pastes
described herein also include an organic vehicle, although that may
not always be stated explicitly.
[0053] D50 is a common metric that is used to describe the median
diameter of particles. The D50 value is defined as the median value
at which half of the particle population has a diameter below and
half the particle population has a diameter above the value.
Measuring a particle diameter distribution is typically performed
with a laser particle size analyzer such as the Horiba LA-300.
Spherical particles are dispersed in a solvent in which they are
well separated and the scattering of transmitted light is directly
correlated to the size distribution from smallest to largest
dimensions. A common approach to express laser diffraction results
is to report the D50 values based on volume distributions. It
should be understood that the term "spherical shape" is used herein
to mean an approximately spherical or equiaxed shape. Particles do
not generally have perfect spherical shapes. It is also possible to
measure the D50 and particle size distribution of particles using a
scanning electron microscope and image processing software such as
ImageJ.
[0054] In one arrangement, the silver particles in the
metallization paste are spherically shaped, nanoparticles with a
D50 between 10 nm and 1 .mu.m, or between 50 nm and 800 nm, or
between 200 nm and 500 nm, or any range subsumed therein. In
another arrangement, the silver particles in the metallization
paste are spherically shaped, micron-sized particle with a D50
between 1 .mu.m and 10 .mu.m, or between 1 .mu.m and 5 .mu.m, or
between 1 .mu.m and 2 .mu.m, or any range subsumed therein. In
another arrangement, silver particles have flake, dendrite, or
filament shapes. In an exemplary embodiment, the silver particles
are flakes with a width between 1 and 10 .mu.m and a thickness
between 100 nm and 500 nm. A silver filament may have a diameter
between 200 nm and 1000 nm and a length greater than 1 .mu.m. In an
exemplary embodiment, the silver particles have a unimodal size
distribution. In another exemplary embodiment, the silver particles
have a bimodal particle size distribution. In general, silver
particles can include a mixture of spherical nanoparticles,
spherical micron-sized particles, flakes, dendrites and/or
filaments. In an exemplary embodiment, the silver particles contain
a mixture of nanoparticles and micron sized particles. In one
embodiment, the silver particles contain 50 wt % spherical
nanoparticles with a D50 of 500 nm, 30 wt % spherical micron sized
particles with a D50 of 2 .mu.m, and 20 wt % silver flakes with a
diameter of 2 .mu.m.
[0055] The term "low-melting-point, base-metal" (LowT.sub.MBM)
particle is used herein to describe any base-metal-containing or
base-metal alloy particle that has a melting point below
400.degree. C., below 350.degree. C., or below 300.degree. C. In an
exemplary embodiment the LowT.sub.MBMs have a solubility in silver
of less than 20 wt % or less than 10 wt % at 850.degree. C. In one
embodiment of the invention, LowT.sub.MBMs contain bismuth (Bi),
tin (Sn), tellurium (Te), antimony (Sb), lead (Pb), or alloys,
composites, or other combinations thereof. In one arrangement, the
LowT.sub.MBMs are primarily crystalline and metallic. In one
arrangement, the LowT.sub.MBM particles have a spherical shape with
a D50 between 50 nm and 5 .mu.m, between 300 nm and 5 .mu.m, or
between 300 nm and 2 .mu.m. In an exemplary embodiment, the
LowT.sub.MBM particles have a unimodal size distribution. In an
exemplary embodiment, the LowT.sub.MBM particles have a bimodal
particle size distribution. In another arrangement, the
LowT.sub.MBM particles have a flake, dendrite, or filament shape.
The flake may have a diameter between 1 .mu.m and 10 .mu.m and a
thickness between 100 nm and 500 nm. The filament may have a
diameter between 200 .mu.m and 1000 nm and a length greater than 1
.mu.m. In an exemplary embodiment the LowT.sub.MBM particles have a
D50 of 2 .mu.m.
[0056] In other embodiments, a low-melting-point, base-metal
particle has a core-shell morphology; a LowT.sub.MBM core particle
is coated with a thin shell. In some arrangements, the coat
completely encapsulates the core particle. In other arrangement,
the coat only partially encapsulates the core particle. In one
embodiment, a core-shell LowT.sub.MBM particle has a LowT.sub.MBM
core and a first shell coating the core. The first shell may
contain nickel, boron, silver, gold, platinum, copper, indium, tin,
zinc, lead, bismuth, antimony, or alloys or combinations thereof.
The first shell may have a uniform thickness or the thickness of
the shell may vary. In various embodiments, on average, the first
shell is less than 1000 nm thick, less than 500 nm thick, less than
200 nm thick, less than 50 nm thick or any range subsumed therein.
In an exemplary embodiment, the core-shell LowT.sub.MBM particle
has a metallic bismuth core and a first silver shell that is 200 nm
thick. In another embodiment, a core-shell LowT.sub.MBM particle
has a metallic bismuth core and a nickel-boron alloy (2-8 wt % B)
first shell.
[0057] Glass frits are glassy oxides of silicon or boron and one or
more additional elements such as barium, bismuth, lead, zinc,
tellurium, aluminum, strontium, sodium, lithium, or other trace
heavy metals. When used in conventional silver metallization
pastes, the composition of the frit is chosen to provide optimal
melting and flow properties for sintering of the silver metal
particles at temperatures between 400.degree. C. and 900.degree. C.
Commercially available glass frit (e.g., Ceradyne product #V2079)
and other additives can be used in front side metallization pastes
to penetrate through anti-reflective coatings, improve silver
sintering, and make ohmic contact to the silicon wafer.
[0058] The reactivity of a glass frit during co-firing determines,
at least partially, whether it will assist in crystallization or
impede crystallization. Strong glass-forming frits may contain, in
oxide and alloy forms, bismuth (Bi), lead (Pb), silicon (Si), and
sodium (Na). Such strong glass-forming frits can readily solubilize
metals into amorphous networks. Many glass frits commonly used in
PV metallization pastes are strong glass-forming frits. Strong
glass-forming frits can increase the oxidation of LowT.sub.MBM
making them much less effective in metallization paste. A film
formed from a metallization paste that contains strong
glass-forming frits, silver particles, and LowT.sub.MBM may have
poor solderability with solder-coated ribbons and may also have a
brittle internal structure due to the large volume of glassy
material when co-fired under conditions typically used for aluminum
back surface field (BSF) solar cells. In Ag-LowT.sub.MBM
metallization pastes, strong glass forming frits, such as those
containing, in oxide and alloy forms, mainly bismuth (Bi), lead
(Pb), silicon (Si), sodium (Na), and boron (B) are used sparingly
in order to prevent oxidation of LowT.sub.MBMs. In one arrangement,
such a metallization paste contains less than 2 wt % strong
glass-forming frits.
[0059] Weak glass-forming frits, which may contain, in oxide and
alloy forms, tellurium (Te), silicon (Si), boron (B), aluminum
(Al), or zinc (Zn). Such weak glass-forming frits solubilize metals
into amorphous networks only minimally. In some arrangements, films
formed from metallization pastes that contain such weak
glass-forming frit material show partial formation of crystalline
phases within a glassy phase. Such films have good high temperature
solderability and strong peel strengths.
[0060] The term "crystallizing agent" is used herein to describe a
material that reacts with LowT.sub.MBMs during co-firing to assist
in formation of crystals in a LowT.sub.MBM sublayer. Crystallizing
agents may also improve the adhesion and subsequent peel strength
of co-fired Ag:LowT.sub.MBMs layers. The crystallizing agent may be
in the form of particles. In one embodiment, crystallizing agents
are a specific subgroup of glass frits that contain tellurium (Te),
silicon (Si), boron (B), aluminum (Al), zinc (Zn), or alloys,
oxides, composites, or other combinations thereof. It is common to
add crystallizing agents to boron-oxide-containing or
silicon-oxide-containing glasses. In another embodiment, a
crystallizing agent contains metals or metallic alloys of tellurium
(Te), silicon (Si), boron (B), or zinc (Zn), composites, or other
combinations thereof. In an exemplary embodiment, a crystallizing
agent contains tellurium. In an exemplary embodiment, a
crystallizing agent contains tellurium oxide. In one arrangement,
the crystallizing agent is a particle with a spherical shape and a
D50 between 50 nm and 5 .mu.m, between 100 nm and 3 .mu.m, between
200 nm and 1 .mu.m, or any range subsumed therein. In another
arrangement, the crystallizing agent has a flake, dendrite, or
filament shape. The flake may have a diameter between 1 and 10
.mu.m and a thickness between 100 nm and 500 nm. In one exemplary
embodiment, crystallizing agent particles have a unimodal size
distribution. In another exemplary embodiment, crystallizing agent
particles have a bimodal particle size distribution.
[0061] In one embodiment of the invention, the organic vehicle is a
mixture of organic solvents and binders. Other additives may also
be included in the organic vehicle. The viscosity of metallization
pastes can be tuned by adjusting the amounts of binders and
solvents in the organic vehicle and by including thixotropic
agents. In one arrangement, the metallization paste has a viscosity
between 10,000 and 200,000 cP at 25.degree. C. and at a sheer rate
of 4 sec.sup.-1 as measured using a temperature controlled
Brookfield DV-II Pro viscometer. Common solvents include terpineol
and glycol ethers (diethylene glycol monobutyl ether, triethylene
glycol monobutyl ether, and texanol). Common organic binders
include ethyl cellulose, carboxymethyl cellulose, poly(vinyl
alcohol), poly(vinyl butyral), and poly(vinyl pyrrolidinone).
[0062] Aluminum-based particles may be added to a Ag:Bi
metallization paste in order to reduce contact resistance between
rear tabbing and aluminum layers. In an exemplary embodiment,
aluminum-based metal particles are made of metallic aluminum (Al).
In one embodiment aluminum-based particles are crystalline. In one
arrangement, aluminum-based particles have a spherical shape with a
D50 between 50 nm and 15 .mu.m, between 300 nm and 10 .mu.m, or
between 300 nm and 4 .mu.m. In an exemplary embodiment,
aluminum-based particles have a unimodal size distribution. In an
exemplary embodiment, aluminum-based particles have a bimodal
particle size distribution. In some arrangements, aluminum-based
particles have flake, dendrite, or filament shapes. The flake may
have a diameter between 1 .mu.m and 10 .mu.m and a thickness
between 100 nm and 500 nm. The filament may have a diameter between
200 .mu.m and 1000 nm and a length greater than 1 .mu.m.
[0063] In one embodiment of the invention, LowT.sub.MBM
metallization pastes and films also contain additional materials
that can getter oxygen or increase their shelf lives (e.g.,
Staybelite.TM.). In an exemplary embodiment, semiconducting
particles such as silicon are added to a bismuth-containing
metallization paste so that bismuth silicate crystals are formed
during co-firing in the LowT.sub.MBM layer formed from such a
paste, increasing the degree of crystallinity in the LowT.sub.MBM
layer.
[0064] In one embodiment of the invention, at least some silver
particle in a metallization paste are replaced by LowT.sub.MBMs.
Such pastes can be referred to as Ag:LowT.sub.MBM pastes.
Crystallizing agents can improve the adhesion and peel strength of
films made from co-fired Ag:LowT.sub.MBMs pastes.
Ag-LowT.sub.MBM Paste Formulations for Specific Metallization
Layers
[0065] The low-melting-point base-metal fraction is a useful metric
that can be used to describe the LowT.sub.MBM content in pastes and
films. The low-melting-point base-metal fraction is determined by
dividing the weight of the LowT.sub.MBM by the entire metal weight
in the metallization paste. As an example, if a paste contains 3.5
g of Ag and 2 g of bismuth, which is the LowT.sub.MBM, and 0.1 g of
aluminum based particles, then the low-melting-point base-metal
fraction in the paste is 35.7% and is calculated using the equation
below:
Low T m BP Metal Fraction = 2 g Bi 2 g Bi + 3.5 g Ag + 0.1 g Al *
100 % = 35.7 % ##EQU00001##
[0066] In exemplary embodiments, the metallization paste has a
low-melting-point base-metal fraction greater than 20%, greater
than 30%, greater than 40%, or greater than 50%.
[0067] Examples of formulations for rear tabbing and front busbar
pastes that contain silver particles, LowT.sub.MBMs, aluminum based
particles and crystallizing agents are shown in Table I. In one
embodiment of the invention, a Ag-LowT.sub.MBMs metallization paste
has a solids loading between 30 wt % and 70 wt %. In one
embodiment, the metallization paste has between 10 wt % and 45 wt %
silver particles. In one embodiment, the metallization paste has
between 5 wt % and 35 wt % LowT.sub.MBM particles. A small quantity
of aluminum-based particles can be included in the paste to reduce
contact resistance between a rear tabbing and an aluminum layer. In
another embodiment, the metallization paste has between 0.5 wt %
and 3 wt % aluminum based particles. Crystallizing agents are
typically added when it is desirable to increase the peel strength
of the rear tabbing layer or front busbar layer. In various
embodiments, a crystallizing agent accounts for less than 1 wt %,
less than 0.5 wt %, or less than 0.1 wt % of the paste. In one
embodiment, a metallization paste also contains less than 2 wt %,
less than 1 wt %, or 0 wt % strong glass-forming frits.
TABLE-US-00001 TABLE I Silver/LowT.sub.MBMs/Crystallization Agent
Metallization Paste Formulations (wt %) Crystal- Aluminum- Paste
Use Silver lizing based Organic (lines) particles LowT.sub.MBMs
Agent particles Vehicle Rear Tabbing 10-45 5-35 0.1-3 0 30-70 (I)
Rear Tabbing 10-45 5-35 .sup. 0-3 0.5-3 30-70 (II) Front Busbar
10-45 3-35 0.1-3 .sup. 0-3 30-70
[0068] Metallization paste formulation can be adjusted to achieve a
desired bulk resistance, contact resistance, film thickness, and/or
peel strength for a particular metallization layer. In an exemplary
embodiment, the metallization paste contains 35 wt % Ag particles,
16 wt % metallic bismuth particles (LowT.sub.MBMs), 1 wt %
aluminum-based particles, no glass frit, no crystallizing agent and
48 wt % organic vehicle. In another embodiment, the metallization
paste contains 35 wt % Ag particles, 16 wt % metallic bismuth
particles (LowT.sub.MBMs), 0.07 wt % TeO.sub.2 (crystallizing
agent), no glass frit, no aluminum-based particles and 48.93 wt %
organic vehicle.
Stratified Metallization Film Made From Ag-LowT.sub.MBM Pastes
[0069] Scanning electron microscopy (SEM) and energy dispersive
x-ray spectroscopy (EDX) (referred to collectively as SEM/EDX) as
used herein were performed using a Zeiss Gemini Ultra-55 analytical
field emission scanning electron microscope, equipped with a
Bruker) XFlash.RTM. 6|60 detector. Details about operating
conditions are described for each analysis. Cross-sectional SEM
images of the co-fired multilayer film stack were prepared by ion
milling. Samples are prepared by applying a thin epoxy layer to the
top of the co-fired multilayer stack and dried for at least 30
minutes. The sample was then transferred to a JEOL IB-03010CP ion
mill operating at 5 kV and 120 uA for 8 hours to remove 80 microns
from the sample edge. Milled samples were stored in a nitrogen box
prior to SEM/MX.
[0070] FIG. 1 is a schematic cross-section drawing of a stratified
film 100 that was formed during co-firing a Ag: LowT.sub.MBM paste
on a silicon substrate 110. It should be noted that the figure is
not drawn to scale. During co-firing, at least a portion of the
silver particles and LowT.sub.MBM particles in the Ag:LowT.sub.MBM
paste phase separate from each other. The LowT.sub.MBM particles
may melt and collect, forming a LowT.sub.MBM sublayer 120 adjacent
to the silicon substrate 110. The silver particles may sinter or
melt, forming a silver-rich sublayer 130 over the LowT.sub.MBM
sublayer 120. An outside surface 130S of the silver-rich sublayer
130 is exposed to the outside environment. In one embodiment, the
LowT.sub.MBM sublayer 120 contains bismuth, aluminum, carbon, tin,
tellurium, antimony, lead, silicon, or alloys, oxides, composites,
or combinations thereof. In an exemplary embodiment, the
LowT.sub.MBM sublayer 120 contains bismuth and oxygen (oxidized
bismuth). In one arrangement, elements such as oxygen, aluminum,
silicon, and/or carbon that are incorporated into the LowT.sub.MBM
sublayer 120 during co-firing.
[0071] The silver-rich sublayer 130 contains mostly silver. The
outermost surface 130S of the silver-rich sublayer is easily
soldered as it is also silver-rich. Plan view EDX was used to
determine the concentration of elements on the outermost surface
130S of the silver-rich sublayer 130. SEM/EDX was performed with
the equipment described above and at an accelerating voltage of 10
kV with a 7 mm sample working distance and 500 times magnification.
In various embodiments, at least 70 wt %, at least 80 wt %, or at
least 90 wt % of the outside surface 130S of the metallization
layer is silver. An interface 140 between the LowT.sub.MBM sublayer
120 and the silver-rich sublayer 130 may not be as sharply defined
as shown in FIG. 1. In some arrangements, there may be some
intermixing of the LowT.sub.MBM sublayer 120 and the silver-rich
sublayer 130 layers at the interface 140.
[0072] The thicknesses of the stratified film 100, the LowT.sub.MBM
sublayer 120 and the silver-rich sublayer 130 can be measured using
cross-sectional SEM/EDX of the stratified film. In some
embodiments, the total thickness of the stratified film 100 varies
between 1 .mu.m and 15 .mu.m, between 1 .mu.m and 10 .mu.m, between
1 .mu.m and 3 .mu.m, or any range subsumed therein. In some
embodiments, the silver-rich sublayer 130 has a thickness between
0.5 .mu.m and 10 .mu.m, between 0.5 .mu.m and 5 .mu.m, between 1
.mu.m and 4 .mu.m, or any range subsumed therein. In some
embodiments, the LowT.sub.MBM sublayer 120 has a thickness between
0.01 .mu.m and 5 .mu.m, between 0.25 .mu.m and 5 .mu.m, between 0.5
.mu.m and 2 .mu.m, or any range subsumed therein.
[0073] The low-melting-point base-metal fraction in the stratified
film 100 can be determined by Energy Dispersive X-Ray Spectroscopy
(EDX). EDX can be used to measure the ratio of Ag to Bi even if the
bismuth is in a crystalline or oxidized state. The EDX data was
collected for approximately three minutes using the equipment
described above at an accelerating voltage of 20 kV, and a working
distance of 7 mm over the entire image shown in FIG. 4. Elemental
quantification was performed on the spectra using Bruker Quantax
Esprit 2.0 software for automatic elemental identification,
background subtraction, and peak fitting. Only the metal peaks,
which were exclusively silver and bismuth, quantified from the EDX
spectrum to determine the low-melting-point base-metal fraction in
the film. In an exemplary embodiment, the stratified film has a low
temperature base-metal fraction greater than 20 wt %, or greater
than 30%, or greater than 40% or greater than 50%.
[0074] In the past, it was challenging to increase the
low-melting-point base-metal fraction in a silver-based
metallization paste beyond 16 wt % with materials such as bismuth
because of Bi oxidation during heating. In one embodiment, the
addition of the crystallizing agent and/or the aluminum-based
particles reduces oxidation, allowing an increase in the
LowT.sub.MBMs metal fraction beyond 20 wt %. In another embodiment,
the reduction of glass frits to less than 3 wt % can reduce
oxidation of the LowT.sub.MBM during co-firing, which improves
solderability.
[0075] A crystallizing agent may help the LowT.sub.MBM to form
crystalline compounds with silicon and oxygen during co-firing.
Such crystalline compounds can improve adhesion between the
LowT.sub.MBM sublayer 120 and the silicon surface 110. In one
embodiment of the invention, the stratified film 100 contains
(MO.sub.x).sub.y(SiO.sub.2).sub.z, crystallites, wherein
0.ltoreq.x.ltoreq.3, 1.ltoreq.y.ltoreq.10, and 0.ltoreq.z.ltoreq.1.
M may be any of bismuth (Bi), tin (Sn), tellurium (Te), antimony
(Sb), lead (Pb), and mixtures thereof. Such crystalline compounds
may be distributed throughout the film either homogeneously or
heterogeneously in a stratified manner and may account for 0.1-20
wt %, 0.5-10 wt %, 1-5 wt % (or any range subsumed therein) of the
co-fired film. In one embodiment, the stratified film 100 contains
less than 1 wt % crystallizing agent. The crystallizing agent may
contain tellurium (Te), silicon (Si), boron (B), zinc (Zn), or
oxides thereof. In an exemplary embodiment, the stratified layer
100 contains less than 1 wt % or less than 0.1 wt % tellurium. The
crystallites in the LowT.sub.MBM sublayer may increase the internal
strength of the sublayer 120, acting as a support structure.
Aluminum-based materials may also be included in Ag:LowT.sub.MBM
films to reduce contact resistance in overlap regions on the back
side of a solar cell, which be described in more detail below. In
an exemplary embodiment, the stratified film contains at least 0.5
wt % aluminum.
[0076] The contact resistance between the LowT.sub.MBM sublayer 120
and the silicon substrate 110 is greater than 100
m.OMEGA.-cm.sup.2, greater than 1 .OMEGA.-cm.sup.2, or greater than
10 .OMEGA.-cm.sup.2.
[0077] In some embodiments, there is a passivation layer (not
shown), such as a silicon nitride (Si.sub.3N.sub.4) layer between
the silicon substrate 110 and the stratified film 100. Such a
passivation layer lowers the surface recombination velocity at the
interface between the silicon substrate 110 and the stratified film
100. The stratified film 100 is in contact with the passivation
layer instead of directly contacting the surface of the silicon
substrate 110. Pastes that do not etch through the dielectric layer
is known as a "floating" metallization pastes. Floating
metallization pastes can be made by careful selection of
crystallizing agents and LowT.sub.MBMs. The degree to which the
paste etches through the Si.sub.3N.sub.4 can be measured by first
co-firing the paste on a Si.sub.3N.sub.4 coated silicon wafer
followed by etching the silver film in an appropriate strong acid
based etchant and measuring the area removed under the
metallization layer using an optical microscope. Silicon nitride
sections that are not etched remain blue while etched regions show
exposed silicon. When more than 90% of the metallized area contain
the Si.sub.3N.sub.4 after etching, a a paste can be considered to
be a floating metallization paste. For floating busbar pastes it is
advantageous to use non-fire through pastes so that penetration of
the passivation layer does not occur. In other exemplary
embodiments, the Ag:LowT.sub.MBM metallization paste is a floating
paste.
Solar Cell Fabrication Using Ag:LowT.sub.MBM Based Rear Tabbing
Pastes
[0078] FIG. 2 is a schematic drawing that shows the front (or
illuminated) side of a silicon solar cell 200. The silicon solar
cell 200 has a silicon wafer 210 with an anti-reflective coating
(not shown), and in some cases, a dielectric layer (not shown) on
the back side. In various embodiments, the silicon wafer is
monocrystalline, or multi-crystalline, and has an n-type, or a
p-type base. The solar cell has fine grid lines 220 and front side
bus bars lines 230. In one embodiment, Ag:LowT.sub.MBM
metallization pastes are used as drop-in replacements for
commercial Ag-based pastes to form the front side bus bars lines
230. First, a commercially-available front-side metallization paste
is screen printed and dried at 150.degree. C. to form the fine grid
lines 220. Then, the Ag:LowT.sub.MBM metallization paste is screen
printed and dried at 150.degree. C. to form the front busbar layers
230. In one embodiment, the front busbars do not penetrate through
the anti-reflective coating. The rear tabbing paste and rear
aluminum paste are subsequently printed and dried, and the entire
wafer is co-fired to over 750.degree. C. for approximately one
second in air. In one arrangement, screen-printed front busbar
layers 230 range in thickness from about 2 to 15 .mu.m after
co-firing. The front busbar layers 230 have a bulk conductivity
that is 2-50 times, or 2-25 times, or 2-10 times less than the bulk
conductivity of silver, which has a bulk resistivity of about
1.6E-8 .OMEGA.-m.
[0079] FIG. 3 is a schematic drawing that shows the rear (or back)
side of a silicon solar cell 300. Some regions of the rear side of
the solar cell are coated with an aluminum layer 330 and some
regions are coated with has rear side tabbing layers 340
distributed over a silicon wafer 310. In some embodiments, the
aluminum layer 330 extends over a portion of the rear tabbing
layers 340 creating overlap regions 350. In one embodiment,
Ag:LowT.sub.MBM metallization pastes are used as drop-in
replacements for commercial Ag-based metallization pastes to form
the rear tabbing layers 340. The Ag:LowT.sub.MBM rear tabbing paste
is screen printed onto portions of the silicon wafer 310 and dried
at 150.degree. C. to form the rear tabbing layer 340. An aluminum
paste is subsequently printed onto the remaining portions of
silicon substrate 310 that have not already been printed with the
Ag:LowT.sub.MBM rear tabbing paste to form the aluminum layer 330.
A portion of the aluminum layer 330 overlaps a portion of the rear
tabbing layers 340 in the overlap regions 350. The solar wafer is
subsequently dried at 250.degree. C. for 30 seconds to 10 minutes.
Prior to co-firing, the overlap region 350 contain a layer of dried
Ag:LowT.sub.MBM paste on the silicon surface and a dried aluminum
layer over the layer of dried Ag:LowT.sub.MBM paste. A
commercially-available front side metallization paste is screen
printed and dried at 150.degree. C. to form the fine grid lines 220
and busbar 230 layers (FIG. 2). Then the silicon solar cell is
co-fired at over 750.degree. C. for approximately one second, which
is a common temperature profile for aluminum BSF silicon solar
cells. During co-firing it is common for the regions of the
aluminum layer 330 that are in direct contact with the silicon
wafer to form both a back surface field and a solidified
aluminum-silicon eutectic layer. After co-firing the overlap region
350 contains a solid aluminum-silicon eutectic layer on the silicon
wafer 310, a modified rear tabbing layer on the solid
aluminum-silicon eutectic layer and an aluminum layer over the
modified rear tabbing layer. In one arrangement, screen-printed
rear tabbing layers range in thickness between about 1.5 and 7
.mu.m after co-firing. The rear tabbing layer has a bulk
conductivity that is 2-50 times, 2-25 tines, or 2-10 times less
conductive than bulk silver, which has a bulk resistivity of about
1.6E-8 .OMEGA.-m.
[0080] In another embodiment, the rear surface of the silicon
substrate has a dielectric layer, and the rear tabbing layers do
not penetrate through the dielectric layer. In another embodiment,
the rear surface of the silicon substrate has a doped silicon base
layer, and the rear tabbing layers make electrical contact to the
silicon base layer with a contact resistance greater than 100
m.OMEGA.-cm.sup.2.
[0081] In some embodiments silicon solar cells are connected to one
another by soldering tin-coated copper tabbing ribbons to the front
busbars and rear tabbing layers. Solder fluxes that are
commercially designated as either RMA (e.g., Kester.RTM. 186) or R
(e.g., Kester.RTM. 952) are deposited on the tabbing ribbon and
dried at 70.degree. C. A tinned copper ribbon, which is between 0.8
and 3 mm wide and 100-300 um thick, is then placed on the solar
cell and contacted to the front busbars and the rear tabbing layers
with a solder iron at a temperature between 200.degree. and
400.degree. C. The solder joints formed during this process have a
mean peel strength that is greater than 1 N/mm (e.g., a 2 mm
tabbing ribbon would require a peel force of greater than 2N to
dislodge the tabbing ribbon).
Silver Paste Containing LowT.sub.MBMs and Crystallizing Agent to
Improve Peel Strength
[0082] In the past, routine optimization of conventional glass frit
concentrations could not achieve optimal morphologies in
Ag-LowT.sub.MBM metallization pastes. Adjusting the proportion of
conventional frits that are used in solar metallization pastes
within the ranges that have been used in the past (i.e., 3-8 wt %)
produced only weak films even at the lowest frit loadings. The most
commonly used frits are bismuth-based or lead-based, which can be
strongly glass-forming and can cause amorphization of the
LowT.sub.MBM layer. Because the glassy phase formed from frit has
typically been considered to be a source of internal strength and
adhesion to the silicon substrate, it would have been
counterintuitive to attempt to crystallize the glassy phase of a
composite film formed under specific ratios of Ag, Bi, and weak
glass-forming frits, especially in the absence of the crystalline
phases.
[0083] Table II below shows the compositions of several pastes that
were screen printed as busbars and tabbing layers on a silicon
substrate, dried, and co-fired over a range of firing conditions
typically used for conventional Al BSF (aluminum back surface
field), multicrystalline solar cells. Each substrate with paste was
placed on a mat, heated to 65.degree. C., and soldered with a tip
temperature of 285.degree. C. using a copper ribbon with
Sn.sub.60Pb.sub.40 solder that was fluxed with Alpha.RTM. NR-205
and dried at 70.degree. C. The metal lines were subsequently peeled
at 180.degree. over a distance greater than 60 mm. Peel strengths
averaged over several co-firing conditions for each paste are also
shown in Table I. The data shows clearly the importance of
including a crystallizing agent and minimizing strong glass-forming
frits in the Ag:LowT.sub.MBMs system in order to maximize peel
strength.
TABLE-US-00002 TABLE II Silver/LowT.sub.MBMs/Crystallizing Agent
Metallization Paste Formulations (wt %) Average Silver LowT.sub.MBM
Crystallizing Strong Glass- Organic Peel Paste particles particles
Agent Forming Frit Vehicle Strength Paste A 35 20 0 0 45 2.4 N/mm
Paste B 35 20 1 0 44 4.3 N/mm Paste C 35 20 0 1 44 1.8 N/mm
[0084] Paste A is a control paste that contains 35 wt % Ag
particles, 20 wt % metallic Bi particles as the LowT.sub.MBM
particles, and no crystallizing agent or glass frit. Paste A has an
average peel strength of 2.4 N/mm. Peel strength for films made
from Paste A drop below 2 N/mm when co-fired at lower temperatures
typically used for passivated emitter rear contact (PERC) solar
cell architectures.
[0085] Paste B contains 35 wt % Ag particles, 20 wt % of metallic
Bi particles as the LowT.sub.MBMs, 1 wt % tellurium oxide frit,
which acts as the crystallizing agent, and no strong glass-forming
frits (i.e., no Bi.sub.2O.sub.3 or PbO containing frit), according
to an embodiment of the invention. Adding only 1 wt % crystallizing
agent increases the average peel strength of the film to 4.3 N/mm,
and these values are consistently high over a range of firing
temperatures greater than 100.degree. C. The viscosity of Paste B
is approximately 80,000 cP at 25.degree. C. and at a sheer rate of
4 sec.sup.-1.
[0086] Traditionally, glass frits used in Ag metallization pastes
have been made from Bi and Pb oxide derivatives, which are strong
glass-forming frits. These strong glass-forming frits do not make
strong films when large concentrations of LowT.sub.MBMs such as Bi
are also included in the paste. Traditional frits can aid in the
oxidation of Bi and result in films that have very poor
solderability. Paste C, shown in Table I, contains 35 wt % Ag
particles, 20 wt % of metallic Bi particles as the LowT.sub.MBMs
and 1 wt % of Bi.sub.2O.sub.3 containing frit. The average peel
strength for films made from Paste C is 1.8 N/mm, which is
significantly lower than the peel strength of Paste B which
contains the crystallizing agent.
[0087] FIG. 4 is a scanning electron microscope (SEM) image that
shows a polished cross section of a stratified metal film 400 on a
silicon substrate 410 made from Paste B. The image shows a
LowT.sub.MBM sublayer 420 adjacent to the silicon substrate 410.
The LowT.sub.MBM sublayer 420 is approximately 1 .mu.m-thick and
contains mainly bismuth, oxygen and silicon. The bright regions in
the LowT.sub.MBM sublayer 420 are crystallites 440 that have been
identified as bismuth silicates. There is a silver-rich sublayer
430 that is approximately 1-2 .mu.m-thick over the LowT.sub.MBM
sublayer 420. Many of the crystallites 440 extend from the silicon
substrate 410 into the silver-rich sublayer 430. In one embodiment,
the normalized weight ratio of silver to bismuth (Ag:Bi) in the
stratified metal film 400 is 1.8 to 1, resulting in a
low-melting-point base-metal fraction of 35.7 wt % (1/(1.8+1)). The
crystallizing agent in Paste B, which contains tellurium,
represents less than 1 wt % of the stratified film.
[0088] X-ray diffraction was performed using a Bruker ZXS D8
Discover GADDS x-ray diffractometer equipped with a V.ANG.NTEC-500
area detector and a cobalt x-ray source operated at 35 kV and 40
mA. Diffractograms were measured using the cobalt K.alpha.
wavelength in two 25.degree. frames which were combined for a total
window of 25-60.degree. in 2.THETA.. Each frame was measured for 30
minutes under x-ray irradiation. Background subtraction was
performed on diffraction patterns. FIG. 5 is an x-ray diffraction
(XRD) pattern of the stratified Ag:Bi film made from Paste B and
shown in FIG. 4. The small peaks in the pattern have been
identified as crystalline bismuth silicates. There are two families
of crystallites in the stratified Ag:Bi film made from Paste B and
shown in FIG. 4. They have been identified using XRD and EDX.
Crystallites formed by the interaction of the bismuth and silicon
in the presence of the crystallizing agent were found to be
(Bi.sub.2O.sub.3).sub.x(SiO.sub.2) (1.ltoreq.x.ltoreq.6); exemplary
crystallites contained the crystalline phases
(Bi.sub.2O.sub.3)(SiO.sub.2), (Bi.sub.2O.sub.3).sub.6(SiO.sub.2),
and (Bi.sub.2O.sub.3).
[0089] In one embodiment, the silver particles in the paste alloy
with the bismuth silicate material to form crystalline and glassy
compounds such as
Ag.sub.y(Bi.sub.2O.sub.3).sub.x(SiO.sub.2).sub.1-y-x
(0.1.ltoreq.x.ltoreq.0.9, 0.ltoreq.y.ltoreq.0.9), which can be seen
with EDX on metallization layers after they have been soldered and
peeled.
[0090] The bulk resistivity of the Ag/LowT.sub.MBM film resulting
from Paste B was determined to be 6E-6 ohm-cm, measured using the
four-point probe measurement. The bulk resistivity of
Ag/LowT.sub.MBM films is higher than the bulk resistivity of pure
Ag (1.6E-6 ohm-cm), which is to be expected as roughly one third of
the Ag/LowT.sub.MBM film contains a LowT.sub.MBMs layer that is
less conductive than Ag. However, the bulk resistivity of the
Ag/LowT.sub.MBM film is close to the values of rear tabbing layers
measured on commercially available PV cells. Thus, even though the
bulk resistivity of such novel films is less than for silver-only
films, the novel films have low enough resistivity to make them
useful for various solar cell metallization applications.
[0091] With reference to FIG. 1, although the LowT.sub.MBM layer
120 along the silicon substrate 100 does not greatly impede the
flow of current parallel to the surface of the silicon substrate,
the LowT.sub.MBM layer may decrease the flow of current from the
silicon substrate 110 to the Ag sublayer 130. Such a stratified
Ag:LowT.sub.MBM layer on the front side of a solar cell (i.e.,
n-type emitter layer/silicon nitride) has a contact resistance of
more than 10 .OMEGA.-cm.sup.2 when measured using TLM. It should be
noted that the contact resistance of commercially available front
side pastes are at least two orders of magnitude lower when
measured under the same conditions. Tellurium oxide based frits
have previously been used in fine grid line pastes to reduce
contact resistance. However, when LowT.sub.MBM represents more than
20 wt % versus the silver, the tellurium oxide based frit does not
greatly improve the contact resistance. The advantage of using
tellurium oxide based crystallizing agent in our system may not be
related to improving Ag crystallite formation at the Si interface.
Rather the tellurium oxide based crystallizing agent can, work to
crystallize the LowT.sub.MBMs layer between the Ag metallization
layer 130 and silicon substrate 110, further separating the Ag from
the silicon and may even impede Ag crystallite formation at the
silicon interface, which is necessary for low contact
resistance.
Silver Paste Containing Aluminum-Based Particles to Reduce Contact
Resistance Between the Metallization Layer and the Aluminum
Layer
[0092] One of the greatest challenges in increasing the amount of
LowT.sub.MBMs in metallization pastes is the possibility of high
contact resistance between a rear tabbing layer and an aluminum
layer. FIG. 6A is a cross-section schematic illustration that shows
a portion of the rear side of a silicon solar cell before
co-firing, according to an embodiment of the invention. FIG. 6A
includes a rear tabbing region 660, an overlap region 670, and an
aluminum layer region 680, as indicated by dashed lines. In the
rear tabbing region 660 and the overlap region 670 a silicon
substrate 610 is coated with a Ag:LowT.sub.MBM rear tabbing paste
layer 625. In the aluminum layer region 680, the silicon substrate
610 is coated with an aluminum layer 645. In one arrangement, the
structures in the overlap region 670 are created by printing the
rear tabbing paste layer 625 directly onto the silicon wafer 610,
drying the rear tabbing paste layer 625, and then printing the
aluminum layer 645 over the rear tabbing paste layer 625. In some
embodiments, the overlap region has a width between 10 .mu.m and
500 .mu.m, between 100 .mu.m and 300 .mu.m wide, or any range
subsumed therein. In another embodiment of the invention (not
shown), the structures in the overlap region 670 are created in
reverse order by printing the aluminum layer 645 directly onto the
silicon wafer 610, drying the aluminum layer 645, and then printing
the rear tabbing paste layer 625 over the aluminum layer 645.
[0093] FIG. 6B is a cross-section schematic illustration that shows
a portion of the rear side of the silicon solar cell shown in FIG.
6A after co-firing. FIG. 6B includes the same rear tabbing region
660, overlap region 670, and aluminum layer region 680 that are
shown in FIG. 6A. During co-firing, various reactions occur. In the
rear tabbing region 660 the exposed portion 627 of the
Ag:LowT.sub.MBM rear tabbing paste layer 625 has reacted to form a
stratified layer 600 that includes an Ag-rich sublayer and a
LowT.sub.MBM sublayer, as has been described above in reference to
FIG. 1.
[0094] In the aluminum layer region 680 the aluminum film 640
interacts with the silicon wafer 610 (at temperatures above
660.degree. C.) to form a solid aluminum-silicon eutectic layer
650. In one arrangement, there is a solid aluminum-silicon eutectic
layer 650 in both the aluminum layer region 680 and the overlap
region 670, as shown in FIG. 6B. In one arrangement, the solid
aluminum silicon layer 650 is continuous. In another arrangement,
the solid aluminum silicon layer 650 is not continuous. Aluminum is
a p-type dopant in silicon and, during firing, it can also form a
highly p-type doped region that is known as the back surface field
(not shown).
[0095] In the overlap region 670 aluminum from the aluminum layer
640 and silver from the covered portion 623 of the rear tabbing
paste layer 625 have interdiffused, forming a modified rear tabbing
layer 630. The solid aluminum-silicon eutectic layer 650 may or may
not form in the overlap region 670. For a rear tabbing paste layer
625 that contains more than 90 wt % silver, the Ag--Al
interdiffusion can result in the formation of a solid
aluminum-silicon eutectic layer 650 in the overlap region 670. Such
a solid aluminum-silicon eutectic layer in the overlap region 670
may play a key role in reducing contact resistance between the
modified tabbing layer 630 and the aluminum layer 640. For example,
pure Ag rear tabbing pastes with less than 10 wt % glass frit
typically form layers that have a contact resistance less than 3
m.OMEGA. between the rear tabbing and aluminum layers. For a rear
tabbing paste layer 625 in which a significant portion of silver
particles have been replaced by LowT.sub.MBMs, very little Ag--Al
interdiffusion may occur and there may be little or no formation of
a solid aluminum-silicon eutectic layer in the overlap region 670.
High loadings of LowT.sub.MBMs in rear tabbing pastes may form
layers that have extremely high contact resistance (i.e., more than
10 m.OMEGA.) between the rear tabbing and aluminum layers. During
co-firing most of the silver may interdiffuse into the aluminum
layer 640 leaving a modified rear tabbing layer 630 made of porous
LowT.sub.MBM material and little or no solid aluminum-silicon
eutectic layer 650. Silicon solar cells that have overlap regions
670 that contain porous modified tabbing layers 630 and no solid
aluminum-silicon eutectic may have high overall series
resistance.
[0096] Aluminum and silver may interdiffuse during co-firing. In
one arrangement, such interdiffusion result in a silver-aluminum
region (not shown) that can extend by as much as 100 .mu.m from the
aluminum layer 640 into the rear tabbing region 660.
[0097] In one embodiment of the invention, the backside of a solar
cell has an overlapping region that contains an aluminum layer and
a rear tabbing layer made from a Ag:LowT.sub.MBM paste that does
not contain aluminum-based particles. The low-melting-point
base-metal (bismuth) fraction in the paste is greater than 20 wt %.
A polished, cross-section sample of the overlapping region was
prepared for SEM imaging. An InLens mode scanning electron
micrograph of the sample is shown FIG. 7. The rear tabbing region
760, the overlap region 770, and the silicon substrate 710 are
indicated. The aluminum layer 740 is about 15 .mu.m thick and the
LowT.sub.MBM-containing rear tabbing layer 720 is about 2.5 .mu.m
thick. The overlap region 770 contains a relatively porous modified
rear tabbing layer 730 and does not have a solid Al--Si eutectic
layer. The contact resistance between the rear tabbing and aluminum
layer for this sample is greater than 10 m.OMEGA.. overlap
region.
[0098] Adding aluminum-based particles to the Ag:LowT.sub.MBM
stratified film can reduce the contact resistance between the
silver-based rear tabbing layer and the aluminum. In an exemplary
embodiment, the aluminum-based particles in the Ag:LowT.sub.MBM
film assist in the formation of the aluminum-silicon eutectic
formation during co-firing resulting in a solid Al--Si eutectic
layer.
[0099] In another embodiment of the invention, the backside of a
solar cell has an overlapping region that contains an aluminum
layer and a rear tabbing layer made from a Ag:LowT.sub.MBM paste
that contains aluminum-based particles. The low-melting-point
base-metal (bismuth) fraction in the paste is greater than 20 wt %.
A polished, cross-section sample of the overlapping region was
prepared for SEM imaging. An InLens mode scanning electron
micrograph of the overlap region 870 is shown FIG. 8. The rear
tabbing region 860, the overlap region 870, and the silicon
substrate 810 are indicated. The aluminum layer 840 is about 20
.mu.m thick and the LowT.sub.MBM-containing rear tabbing layer 820
is about 2 .mu.m thick. The overlap region 870 contains a modified
rear tabbing layer 830 that appears to be non-porous, as well as a
solid Al--Si eutectic layer 850. The contact resistance between the
rear tabbing and aluminum layer for this sample is less than 1.5
m.OMEGA., which is similar to the contact resistance for silver
rear tabbing pastes that contain no LowT.sub.MBM. In various
embodiments, the modified rear tabbing layer 830 contains at least
1 wt % Al, at least 2 wt %, or at least 4 wt % aluminum. In various
embodiments, the rear tabbing layer 820 contains at least 1 wt %
Al, at least 2 wt %, or at least 4 wt % aluminum.
Other PV Cell Architectures
[0100] Ag:LowT.sub.MBM based pastes can be used in other Si PV
architectures such as metal wrap through as well as passivated
emitter rear contact (PERC) solar cells. The Ag:LowT.sub.MBM based
paste, described herein, can be used as a drop-in replacement for
any metallization layers where a low contact resistance to silicon
is not required. For the PERC architecture the LowT.sub.MBMs and
crystallizing agents can be modified to initiate crystallization at
lower co-firing temperatures. In some instances, strong
forming-glass frits may be used to initiate crystallization at low
firing temperatures.
[0101] This invention has been described herein in considerable
detail to provide those skilled in the art with information
relevant to apply the novel principles and to construct and use
such specialized components as are required. However, it is to be
understood that the invention can be carried out by different
equipment, materials and devices, and that various modifications,
both as to the equipment and operating procedures, can be
accomplished without departing from the scope of the invention
itself.
* * * * *