U.S. patent application number 10/988208 was filed with the patent office on 2006-05-18 for method of making solar cell contacts.
This patent application is currently assigned to Ferro Corporation. Invention is credited to Chandrashekhar S. Khadilkar, Steve S. Kim, Tung Pham, Aziz S. Shaikh, Srinivasan Sridharan.
Application Number | 20060102228 10/988208 |
Document ID | / |
Family ID | 36384917 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060102228 |
Kind Code |
A1 |
Sridharan; Srinivasan ; et
al. |
May 18, 2006 |
Method of making solar cell contacts
Abstract
Formulations and methods of making solar cells are disclosed. In
general, the invention presents a solar cell contact made from a
mixture wherein the mixture comprises a solids portion and an
organics portion, wherein the solids portion comprises from about
85 to about 99 wt % of silver, and from about 1 to about 15 wt % of
a glass component wherein the glass component comprises from about
15 to about 75 mol % PbO, and from about 5 to about 50 mol %
SiO.sub.2, and preferably with no B.sub.2O.sub.3.
Inventors: |
Sridharan; Srinivasan;
(Strongsville, OH) ; Pham; Tung; (Vista, CA)
; Khadilkar; Chandrashekhar S.; (Broadview Heights,
OH) ; Shaikh; Aziz S.; (San Diego, CA) ; Kim;
Steve S.; (Goleta, CA) |
Correspondence
Address: |
RANKIN, HILL, PORTER & CLARK, LLP
925 EUCLID AVENUE, SUITE 700
CLEVELAND
OH
44115-1405
US
|
Assignee: |
Ferro Corporation
Cleveland
OH
|
Family ID: |
36384917 |
Appl. No.: |
10/988208 |
Filed: |
November 12, 2004 |
Current U.S.
Class: |
136/256 |
Current CPC
Class: |
C03C 3/07 20130101; Y02E
10/50 20130101; C03C 8/18 20130101; H01L 31/022425 20130101; H01B
1/22 20130101; C03C 3/072 20130101 |
Class at
Publication: |
136/256 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Claims
1. A solar cell contact made from a mixture wherein the mixture
comprises: a. a solids portion and b. an organics portion, c.
wherein the solids portion comprises i. from about 85 to about 99
wt % of a silver component, and ii. from about 1 to about 15 wt %
of a glass component iii. wherein the glass component comprises a.
from about 15 to about 75 mol % PbO, b. from about 5 to about 50
mol % SiO.sub.2, and c. no B.sub.2O.sub.3.
2. The solar cell contact of claim 1 wherein the weight ratio of
the solids portion to the organics portion is from about 20:1 to
about 1:20.
3. The solar cell contact of claim 1 wherein the glass component
further comprises about 1 to about 30 mol % Bi.sub.2O.sub.3.
4. The solar cell contact of claim 1 wherein the glass component
further comprises about 0.1 to about 15 mol % Al.sub.2O.sub.3.
5. The solar cell contact of claim 4 wherein the glass component
further comprises about 0.1 to about 10 mol % Ta.sub.2O.sub.5.
6. The solar cell contact of claim 5 wherein the glass component
further comprises about 0.1 to about 10 mol % ZrO.sub.2.
7. The solar cell contact of claim 6 wherein the glass component
further comprises about 0.1 to about 8 mol % P.sub.2O.sub.5.
8. The solar cell contact of claim 1 wherein the glass component
further comprises about 0.1 to about 15 mol %
HfO.sub.2+In.sub.2O.sub.3+Ga.sub.2O.sub.3.
9. The solar cell contact of claim 1 wherein the glass component
further comprises about 0.1 to about 10 mol %
Y.sub.2O.sub.3+Yb.sub.2O.sub.3.
10. The solar cell contact of claim 4 wherein the glass component
further comprises about 0.1 to about 15 mol % HfO.sub.2.
11. The solar cell contact of claim 4 wherein the glass component
further comprises about 0.1 to about 10 mol % ZrO.sub.2.
12. The solar cell contact of claim 11 wherein the glass component
further comprises about 0.1 to about 8 mol % P.sub.2O.sub.5.
13. The solar cell contact of claim 4 wherein the glass component
further comprises about 0.1 to about 3 mol % B.sub.2O.sub.3.
14. The solar cell contact of claim 13 wherein the glass component
further comprises about 0.1 to about 10 mol % Sb.sub.2O.sub.5.
15. The solar cell contact of claim 14 wherein the glass component
further comprises about 0.1 to about 10 mol % ZrO.sub.2.
16. The solar cell contact of claim 4 wherein the glass component
comprises: a. about 26 to about 34 mol % PbO, b. about 27 to about
33 mol % SiO.sub.2, c. about 5 to about 11 mol % A1.sub.2O.sub.3,
d. about 0.1 to about 2 mol % Ta.sub.2O.sub.5, e. and further
comprises about 27 to about 33 mol % ZnO.
17. The solar cell contact of claim 4 wherein the glass component
further comprises about 0.1 to about 3 mol % MoO.sub.3.
18. The solar cell contact of claim 1 wherein the solids portion
further comprises a crystalline additive selected from the group
consisting of Bi.sub.2O.sub.3, Sb.sub.2O.sub.3, In.sub.2O.sub.3,
Ga.sub.2O.sub.3, SnO, ZnO, Pb.sub.3O.sub.4, PbO, SiO.sub.2,
ZrO.sub.2, Al.sub.2O.sub.3, B.sub.2O.sub.3, Ta.sub.2O.sub.5,
4PbO.SiO.sub.2, 3PbO.SiO.sub.2, 2PbO.SiO.sub.2, 3PbO.2SiO.sub.2,
PbO.SiO.sub.2, ZnO.SiO.sub.2, and ZrO.sub.2.SiO.sub.2, and reaction
products thereof and combinations thereof.
19. The solar cell contact of claim 1 wherein the solids portion
comprises about 60 to about 95 wt % of flaked silver or powdered
silver, and about 0.1 to about 20 wt % of colloidal silver.
20. The solar cell contact of claim 1 wherein the silver component
comprises silver selected from the group consisting of present as
flakes, powder, or colloidal particles of silver, wherein the
solids portion further comprises phosphorus, at least a portion of
which is present as a coating on at least a portion of the silver
flakes, powder or colloidal particles.
21. The solar cell contact of claim 1 wherein the silver component
contains a compound selected from the group consisting of an oxide
of silver or a salt of silver, or combinations thereof.
22. The solar cell contact of claim 1 wherein the solids portion
further comprises about 0.5 to about 25 wt % of a first metal
selected from the group consisting of Pb, Bi, Zn, In, Ga, and Sb
and alloys thereof with at least one second metal.
23. The solar cell contact of claim 22 wherein the at least one
second metal is silver.
24. The solar cell contact of claim 22 wherein the first metal is
zinc.
25. The solar cell contact of claim 24 wherein the at least one
second metal is silver.
26. The solar cell contact of claim 1 wherein the glass component
comprises a first glass composition and a second glass composition,
wherein a. the first glass composition comprises: i. about 26 to
about 34 mol % PbO, ii. about 27 to about 33 mol % SiO.sub.2, iii.
about 20 to about 33 mol % ZnO, and iv. about 5 to about 11 mol %
Al.sub.2O.sub.3, b. the second glass composition comprises: i.
about 58 to about 70 mol % PbO and ii. about 5 to about 50 mol %
SiO.sub.2, c. wherein the weight ratio between the first and second
glass compositions is from about 1:20 to about 20:1.
27. The solar cell contact of claim 26 wherein the weight ratio
between the first and second glass compositions is from about 1:3
to about 3:1.
28. A process for making a solar cell contact comprising: a.
applying a silver-containing paste on an antireflective silicon
wafer and b. firing the paste to form a coating, c. wherein the
paste comprises a solids portion and an organics portion, the
solids portion comprising: i. about 85 to about 99 wt % silver, and
ii. about 1 to about 15 wt % of a glass component; d. wherein the
glass component comprises i. about 15% to 75 mol % PbO, ii. about
5% to about 50 mol % SiO.sub.2, and iii. less than about 3 mol %
B.sub.2O.sub.3.
29. The process of claim 28 wherein the paste is fired at a furnace
set temperature of about 650.degree. C. to about 1000.degree. C.
for about 1 second to about 5 minutes.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a paste composition and a method
of making contacts for solar cells as well as other related
components used in fabricating photovoltaic cells.
BACKGROUND
[0002] Solar cells are generally made of semiconductor materials,
such as silicon (Si), which convert sunlight into useful electrical
energy. Solar cells are, in general, made of thin wafers of Si in
which the required PN junction is formed by diffusing phosphorus
(P) from a suitable phosphorus source into a P-type Si wafer. The
side of silicon wafer on which sunlight is incident is in general
coated with an anti-reflective coating (ARC) to prevent reflective
loss of incoming sunlight, and thus to increase the efficiency of
the solar cell. A two dimensional electrode grid pattern known as a
front contact makes a connection to the N-side of silicon, and a
coating of aluminum (Al) on the other side (back contact) makes
connection to the P-side of the silicon. These contacts are the
electrical outlets from the PN junction to the outside load.
SUMMARY OF THE INVENTION
[0003] The present invention provides glass compositions for use in
front contact paste materials that provide low series resistance
(Rs) and high shunt resistance (R.sub.sh) to give high performance
solar cells, as measured by efficiency (.eta.) and fill factor
(FF). Generally, the present invention includes a solar cell
contact made from a mixture of ingredients, wherein the mixture
comprises a solids portion and an organics portion. The solids
portion comprises from about 85 to about 99 wt % of a metal
component preferably silver, and from about 1 to about 15 wt % of a
glass component. The glass component comprises from about 15 to
about 75 mol % PbO, and from about 5 to about 50 mol % SiO.sub.2.
The metal component comprises silver flakes, silver power,
colloidal silver, and/or phosphorus-coated silver powder. Methods
for making solar cells using the above ingredients and amounts are
also envisioned.
[0004] The compositions and methods of the present invention
overcome the drawbacks of the prior art by facilitating optimized
interaction, bonding, and contact formation between front contact
components, typically Ag and Si, through the glass medium. A
conductive paste containing glass and silver is printed on a
silicon substrate, and fired to fuse the glass and sinter the metal
therein. Upon firing, Ag/Si conductive islands are formed providing
conductive bridges between bulk paste and silicon wafer. Leaded
glasses allow low firing temperatures owing to their excellent flow
characteristics relatively at low temperatures.
[0005] The foregoing and other features of the invention are
hereinafter more fully described and particularly pointed out in
the claims, the following description setting forth in detail
certain illustrative embodiments of the invention, these being
indicative, however, of but a few of the various ways in which the
principles of the present invention may be employed.
DETAILED DESCRIPTION
[0006] The foregoing and other features of the invention are
hereinafter more fully described. Silver- and glass-containing
thick film pastes are used to make front contacts for silicon-based
solar cells to collect current generated by exposure to light. The
cell electrical performance as measured by cell efficiency (.eta.)
and fill factor (FF) is strongly affected by the microstructure and
the electrical properties of the silver/silicon interface. The
electrical properties of the solar cell are also characterized by
R.sub.S and R.sub.Sh. The composition and microstructure of the
front contact interface largely determine R.sub.S. While the paste
is generally applied by screen-printing, methods such as extrusion,
pad printing, and hot melt printing may be used. Solar cells with
screen-printed front contacts are fired to relatively low
temperatures (550.degree. C. to 850.degree. C. wafer temperature;
firing furnace set temperatures of 650.degree. C. to 1000.degree.
C.) to form a low resistance contact between the N-side of a
phosphorus doped silicon wafer and a silver based paste. The front
contact paste, before firing, contains a silver-containing compound
in one or more forms (powder, flake, colloid) and a glass
component, and/or other additives. The glass component contains at
least PbO and SiO.sub.2.
[0007] The sequence and rates of reactions occurring as a function
of temperature are factors in forming the low resistance contact
between the silver paste and silicon wafer. The interface structure
consists of multiple phases: substrate silicon, Ag/Si islands, Ag
precipitates within the insulating glass layer, and bulk silver.
The glass forms a nearly continuous layer between the silicon
interface and the bulk silver.
[0008] Solar cells include a front contact made from pastes or inks
consisting of a mixture of ingredients. These mixtures, prior to
firing, comprise a solids portion and an organics portion. The
solids portion comprises a conductive metal and a glass component.
Up to about 30 wt % of other (i.e., inorganic) additives,
preferably up to about 25 wt % and more preferably up to about 10
wt %, may be included as needed. The solids portion for the paste
composition according to the present invention comprises: (a) a
metal component comprising silver, an alloy of silver, or an oxide
of silver, or a silver compound (i.e., silver component), from
about 85 to about 99 wt %, preferably from about 88 to about 95 wt
% of the solids portion; and, (b) a glass component, from about 1
to about 15 wt %, preferably about 2 to about 9 wt %, and more
preferably from about 3 to about 8 wt % of the solids portion. The
organics portion of the pastes herein comprises (a) at least about
80 wt % organic solvent; (b) up to about 15 wt % of a thermoplastic
resin; (c) up to about 4 wt % of a thixotropic agent; and (d) up to
about 2 wt % of a wetting agent. The use of more than one solvent,
resin, thixotrope, and wetting agent is also envisioned. Although a
variety of weight ratios of the solids portion to the organics
portion are envisioned, one embodiment includes a weight ratio of
the solids portion to the organics portion from about 20:1 to about
1:20. In preferred embodiments the weight ratio is from about 15:1
to about 1:15, and most preferably the ratio is about 10:1 to about
1:10. Each of the major ingredient types (glass, metal, organics)
is detailed hereinbelow.
[0009] Paste Glasses. The glass component comprises, prior to
firing, one or more glass compositions. Each glass composition
comprises oxide frits including, at a minimum, PbO and SiO.sub.2.
Zinc oxide (ZnO) may replace a portion of the PbO in the glass
component herein. In particular, in various embodiments of the
present invention, a glass composition comprises the ingredients of
Table 1. When at least two glass compositions are present, the
selection of their makeup and proportions has an effect on the
quality of the solar cell contact. The use of a (first) glass
composition containing a high proportion of ZnO (e.g., up to about
35 mol %) provides minimum penetration into silicon. Such a glass
composition is exemplified by embodiments V and VII in Table 3, and
composition A in Table 4. On the other hand the use of a (second)
zinc-free glass composition with high proportion of PbO (e.g., up
to about 75 mol %) provides more penetration into silicon. Such a
glass composition is exemplified by embodiments VI, VIII, IX, X,
and XI in Table 3, and compositions B, C, D, and E in Table 4.
Regardless of the number of glass compositions used, the total
content of PbO and SiO.sub.2 in the glass component overall will
fall within the range of about 15 to about 75 mol % PbO, and from
about 5 to about 50 mol % SiO.sub.2. Varying proportions of the
first and second glass compositions can be used in forming a solar
cell contact to control, the extent of penetration into silicon,
and hence the resultant solar cell properties. For example, within
the glass component, the first and second glass compositions may be
present in a weight ratio of about 1:20 to about 20:1, and
preferably about 1:3 to about 3:1. The glass component preferably
contains no cadmium or oxides of cadmium. Further, a portion of PbO
can be replaced by Bi.sub.2O.sub.3 to provide a glass composition
used in making a solar cell within the scope of the present
invention. For example, about 1 to about 30 mol % of
Bi.sub.2O.sub.3 can be used.
[0010] Other embodiments may further comprise Al.sub.2O.sub.3,
Ta.sub.2O.sub.5, Sb.sub.2O.sub.5, ZrO.sub.2, HfO.sub.2,
In.sub.2O.sub.3, Ga.sub.2O.sub.3, Y.sub.2O.sub.3, Yb.sub.2O.sub.3
and combinations thereof. An entry such as Y.sub.2O.sub.3 means
that Y.sub.2O.sub.3 or Yb.sub.2O.sub.3 or a combination of the two
is present in the specified amount. The embodiments set forth in
Table 1, may in addition include the following oxide frit
ingredients as shown in Table 2. TABLE-US-00001 TABLE 1 Oxide frit
ingredients in mole percent of the glass component. Glass
Composition III (more Ingredient I (broad) II (preferred)
preferred) PbO 15-75 25-66 30-64 SiO.sub.2 5-50 15-40 20-35 ZnO
0-50 5-35 20-33 PbO + ZnO 15-80 -- --
[0011] TABLE-US-00002 TABLE 2 Additional oxide frit ingredients in
embodiments of Table 1 in mole percent of the glass component.
Glass Composition III (more Ingredient I (broad) II (preferred)
preferred) Al.sub.2O.sub.3 0-15 1-11 2-10 Ta.sub.2O.sub.5 0.1-10
0.1-3 0.2-2 Sb.sub.2O.sub.5 0.1-10 0.1-3 0.2-2 ZrO.sub.2 0.1-10
0.5-5 1-2 P.sub.2O.sub.5 0.1-8 1-5 2-4 MoO.sub.3 0.1-3 -- --
HfO.sub.2 + In.sub.2O.sub.3 + Ga.sub.2O.sub.3 0.1-15 1-10 3-8
Y.sub.2O.sub.3 + Yb.sub.2O.sub.3 0.1-10 1-8 3-8
[0012] A given embodiment need not contain all frit ingredients as
noted in Table 2, but various combinations are possible. Other
specific embodiments may contain various amounts of the
aforementioned ingredients in mole percent as shown in Table 3.
TABLE-US-00003 TABLE 3 Further embodiments of glass compositions in
the glass component in mole percent of the glass component. Glass
Composition Ingredient IV V VI VII VIII IX X XI PbO 58-64 25-40
58-64 26-34 58-66 58-66 58-70 58-66 SiO.sub.2 25-31 20-31 22-32
27-33 20-31 20-31 20-31 20-32 ZnO 0-10 5-34 27-33 Al.sub.2O.sub.3
2-11 4-10 1-10 5-11 1-9 1-9 1-11 1-9 Ta.sub.2O.sub.5 0-2 0.1-2
0.1-2 P.sub.2O.sub.5 0.1-4 HfO.sub.2 + In.sub.2O.sub.3 +
Ga.sub.2O.sub.3 0.1-8 ZrO.sub.2 0.1-5 0.1-2 0.1-4 B.sub.2O.sub.3
0-3 Sb.sub.2O.sub.5 0.1-3
[0013] Silver Component. The source of the silver in the silver
component can be one or more fine powders of silver metal, or
alloys of silver. A portion of the silver can be added as silver
oxide (Ag.sub.2O) or as silver salts such as AgCl, AgNO.sub.3 or
AgOOCCH.sub.3 (silver acetate). Additionally, the silver may be
coated with various materials such as phosphorus. Alternately, the
silver oxide can be dissolved in the glass during the glass
melting/manufacturing process. The silver particles used in the
paste may be spherical, flaked, or provided in a colloidal
suspension, and combinations of the foregoing may be used. Any of
the aforementioned silver sources may be used to contribute silver
to the silver component of the solar cell contacts herein. For
example the solids portion of the paste may comprise about 80 to
about 99 wt % spherical silver particles or about 75 to about 90 wt
% silver particles and about 1 to about 10 wt % silver flakes.
Another alternative composition of the solids portion comprises
about 75 to about 90 wt % silver flakes and about 1 to about 10 wt
% of colloidal silver. In general, the solids portion may comprise
about 60 to about 95 wt % of silver powder or silver flakes and
about 0.1 to about 20 wt % of colloidal silver. Suitable commercial
examples of silver particles are spherical silver powder Ag3000-1,
silver flakes SF-23, and colloidal silver suspension RDAGCOLB, all
commercially available from Ferro Corporation, Cleveland, Ohio.
[0014] Inorganic/Other Additives. Phosphorus can be added to the
paste in a variety of ways to reduce the resistance of the front
contacts. For example, certain glasses can be modified with
P.sub.2O.sub.5 in the form of a powdered or fritted oxide, or
phosphorus can be added to the paste by way of phosphate esters and
other organo-phosphorus compounds. More simply, phosphorus can be
added as a coating to silver particles prior to making a paste. In
such case, prior to pasting, the silver particles are mixed with
liquid phosphorus and a solvent. For example, a blend of from about
85 to about 95 wt % silver particles, from about 5 to about 15 wt %
solvent and from about 0.5 to about 10 wt % liquid phosphorus is
mixed and the solvent evaporated. Phosphorus coated silver
particles help ensure intimate mixing of phosphorus and silver in
the inventive pastes.
[0015] Other additives such as fine silicon or carbon powder, or
both, can be added to the paste to control the silver reduction and
precipitation reaction. The silver precipitation at the interface
or in the bulk glass, can also be controlled by adjusting the
firing atmosphere (e.g., firing in flowing N.sub.2 or
N.sub.2/H.sub.2/H.sub.2O mixtures). Fine low melting metal
additives (i.e., elemental metallic additives as distinct from
metal oxides) such as Pb, Bi, In, Ga, Sn, and Zn and alloys of each
with at least one other metal can be added to provide a contact at
a lower temperature, or to widen the firing window. Zinc is the
preferred metal additive, and silver is the preferred metal with
which the metal additive is alloyed. A zinc-silver alloy is most
preferred.
[0016] A mixture of (a) glasses or a mixture of (b) glasses and
crystalline additives or a mixture of (c) one or more crystalline
additives can be used to formulate a glass component in the desired
compositional range. The goal is to reduce the contact resistance
and improve the solar cell electrical performance. For example,
second-phase crystalline materials such as Bi.sub.2O.sub.3,
Sb.sub.2O.sub.3, Sb.sub.2O.sub.5, In.sub.2O.sub.3, Ga.sub.2O.sub.3,
SnO, ZnO, Pb.sub.3O.sub.4, PbO, SiO.sub.2, ZrO.sub.2,
Al.sub.2O.sub.3, B.sub.2O.sub.3, and Ta.sub.2O.sub.5 may be added
to the glass component to adjust contact properties. Combinations
and reaction products of the aforementioned oxides can also be
suitable to design a glass component with desired characteristics.
For example, low melting lead silicates, either crystalline or
glassy, formed by the reaction of PbO and SiO.sub.2 such as
4PbO.SiO.sub.2, 3PbO.SiO.sub.2, 2PbO.SiO.sub.2, 3PbO.2SiO.sub.2,
and PbO.SiO.sub.2, either singly or in mixtures can be used to
formulate a glass component. A second phase of lead silicates may
optionally be used. Other reaction products of the aforementioned
oxides such as ZnO.SiO.sub.2 and ZrO.sub.2.SiO.sub.2 may also be
used. However, the total amounts of the above oxides must fall
within the ranges specified for various embodiments disclosed
elsewhere herein.
[0017] The inventors herein have found that boron content (as
B.sub.2O.sub.3) has an effect on contact formation. The presence of
high amounts (>10 mol %) of B.sub.2O.sub.3 can cause poor
contact formation, especially contacts with high R.sub.S.
Accordingly, in a preferred embodiment, the glass component
contains no more than about 3 mol % of B.sub.2O.sub.3, preferably
no more than about 1 mol % B.sub.2O.sub.3. Most preferably, the
glass component contains no B.sub.2O.sub.3.
[0018] The inventors herein have also found that certain glasses
containing oxides of hafnium (HfO.sub.2), indium (In.sub.2O.sub.3),
and/or gallium (Ga.sub.2O.sub.3) increase both the size and
quantity of the conductive Ag/Si islands. Hence, up to 15 mol % of
HfO.sub.2 and/or In.sub.2O.sub.3 and/or Ga.sub.2O.sub.3 may be
included in the glass component.
[0019] Oxides of tantalum and molybdenum reduce glass viscosity and
surface tension of the glass during firing, facilitating better
wetting of the wafer by the molten glass. Accordingly, up to about
10 mol % Ta.sub.2O.sub.5, and up to about 3 mol % MoO.sub.3 can be
included in the glass component.
[0020] Kinetics of silver dissolution and precipitation from the
glass compositions can be significantly altered by the presence of
alkali metal oxides. In that regard, -the compositions of the
present invention may further comprise oxides of alkali metals, for
example Na.sub.2O, K.sub.2O, and Li.sub.2O and combinations
thereof. In particular, the glass components of certain embodiments
herein may contain from about 0.1 to about 15 mol %
Na.sub.2O+K.sub.2O+Li.sub.2O, or more preferably from about 0.1 to
about 5 mol % of those alkali metal oxides.
[0021] The glass in the front contact paste or ink plays many key
roles in forming an efficient front contact silver-silicon
interface. The front contact paste glass corrodes the
antireflective coating, typically made of silicon nitride
(SiN.sub.x) or titanium dioxide (TiO.sub.2) to form fired through
contacts to underlying Si. The glass also takes part in a
self-limiting interaction with Si to oxidize and dissolve a portion
of Si into the glass as SiO.sub.2. Because the local concentration
of SiO.sub.2 increases the viscosity of the glass, this increase
will eventually limit further dissolution of Si as SiO.sub.2,
giving rise to a self-limiting interaction of the glass with Si to
preserve the PN junction. The glass also dissolves Ag metal into
the glass, transports Ag ions to the silicon interface, and
precipitates Ag from the glass to form beneficial Ag/Si islands at
the interface. Finally, the glass serves to enhance densification
of the silver paste to reduce bulk silver resistivity and enhances
bonding (adhesion) between the silicon wafer and the fired (silver)
paste.
[0022] Organic Vehicle. The vehicle or carrier for most conductive
compositions is typically a solution of a resin dissolved in a
solvent and, frequently, a solvent solution containing both resin
and a thixotropic agent. The solvent usually boils from about
130.degree. C. to about 350.degree. C. The most frequently used
resin for this purpose is ethyl cellulose. However, resins such as
ethyl hydroxy ethyl cellulose, wood rosin, mixtures of ethyl
cellulose and phenolic resins, polymethacrylates of lower alcohols
and the monobutyl ether of ethylene glycol monoacetate can also be
used. The most widely used solvents for thick film applications are
terpenes such as alpha- or beta-terpineol or higher boiling
alcohols such as Dowanol.RTM. (diethylene glycol monoethyl ether),
or mixtures thereof with other solvents such as butyl Carbitol.RTM.
(diethylene glycol monobutyl ether); dibutyl Carbitol.RTM.
(diethylene glycol dibutyl ether), butyl Carbitol.RTM. acetate
(diethylene glycol monobutyl ether acetate), hexylene glycol,
Texanol.RTM. (2,2,4-trimethyl-1,3-pentanediol monoisobutyrate), as
well as other alcohol esters, kerosene, and dibutyl phthalate. The
vehicle can contain organometallic compounds, for example those
based on nickel, phosphorus or silver, to modify the contact.
Various combinations of these and other solvents can be formulated
to obtain the desired viscosity and volatility requirements for
each application. Other dispersants, surfactants and rheology
modifiers, which are commonly used in thick film paste
formulations, may be included. Products useful in the organic
carrier may be obtained commercially under any of the following
trademarks: Texanol.RTM. (Eastman Chemical Company, Kingsport,
Tenn.); Dowanol.RTM. and Carbitol.RTM. (Dow Chemical Co., Midland,
Mich.); Triton.RTM. (Union Carbide Division of Dow Chemical Co.,
Midland, Mich.), Thixatrol.RTM. (Elementis Company, Hightstown
N.J.), and Diffusol.RTM. (Transene Co. Inc., Danvers, Mass.).
N-DIFFUSOL.RTM. is a stabilized liquid preparation containing an
n-type diffusant with a diffusion coefficient similar to that of
elemental phosphorus.
[0023] Among commonly used organic thixotropic agents is
hydrogenated castor oil and derivatives thereof. A thixotrope is
not always necessary because the solvent/resin properties coupled
with the shear thinning inherent in any suspension may alone be
suitable in this regard. Furthermore, wetting agents may be
employed such as fatty acid esters, e.g.,
N-tallow-1,3-diaminopropane di-oleate; N-tallow trimethylene
diamine diacetate; N-coco trimethylene diamine, beta diamines;
N-oleyl trimethylene diamine; N-tallow trimethylene diamine; and
N-tallow trimethylene diamine dioleate, and combinations
thereof.
[0024] It should be kept in mind that the foregoing compositional
ranges are preferred and it is not the intention to be limited to
these ranges where one of ordinary skill in the art would recognize
that these ranges may vary depending upon specific applications,
specific components and conditions for processing and forming the
end products. The paste according to the present invention may be
conveniently prepared on a three-roll mill. The amount and type of
carrier utilized are determined mainly by the final desired
formulation viscosity, fineness of grind of the paste, and the
desired wet print thickness. In preparing compositions according to
the present invention, the particulate inorganic solids are mixed
with the carrier and dispersed with suitable equipment, such as a
three-roll mill, to form a suspension, resulting in a composition
for which the viscosity will be in the range of about 100 to about
500 kcps, preferably about 300 to about 400 kcps, at a shear rate
of 9.6 sec.sup.-1 as determined on a Brookfield viscometer HBT,
spindle 14, measured at 25.degree. C.
[0025] Printing and Firing of the Paste. The aforementioned paste
compositions may be used in a process to make a solar cell contact
or other solar cell components. The inventive method of making
solar cell contacts comprises (1) applying a silver-containing
paste to the silicon substrate, (2) drying the paste, and (3)
firing the paste to sinter the metal and make contact to silicon.
The printed pattern of the paste is fired at a suitable
temperature, such as about 650-950.degree. C. furnace set
temperature, or about 550-850.degree. C. wafer temperature.
Preferably, the furnace set temperature is about 750-930.degree.
C., and the paste is fired in air. During the firing the
antireflective SiN.sub.X layer is believed to be oxidized and
corroded by the glass and Ag/Si islands are formed on reaction with
the Si substrate, which are epitaxially bonded to silicon. Firing
conditions are chosen to produce a sufficient density of Ag/Si
islands on the silicon wafer at the silicon/paste interface,
leading to a low resistivity, high efficiency, high-fill factor
front contact and solar cell.
[0026] A typical ARC is made of a silicon compound such as silicon
nitride, generically SiNx, such as Si.sub.3N.sub.4. This layer acts
as an insulator, which tends to increase the contact resistance.
Corrosion of this ARC layer by the glass component is hence a
necessary step in front contact formation. The inventors herein
have discovered that reducing the resistance between the silicon
wafer and the paste is facilitated by the formation of epitaxial
silver/silicon conductive islands at the interface. That is, the
silver islands on silicon assume the same crystalline structure as
is found in the silicon substrate. When such an epitaxial
silver/silicon interface does not result, the resistance at that
interface becomes unacceptably high. Until now, the processing
conditions to achieve a low resistance epitaxial silver/silicon
interface have been very narrow and difficult to achieve. The
pastes and processes herein now make it possible to produce an
epitaxial silver/silicon interface leading to a contact having low
resistance under broad processing conditions--a minimum firing
temperature as low as about 650.degree. C., but which can be fired
up to about 850.degree. C. (wafer temperature). The pastes herein
can be fired in air.
[0027] The formation of a low resistance front contact on a silicon
solar cell is technically challenging. Both the interactions among
paste constituents (silver metal, glass, additives, organics), and
the interactions between paste constituents and silicon substrate
are complex. However the interaction between paste constituents and
silicon substrate must be controlled. The rapid furnace processing
makes all the reactions highly dependent on kinetics. Further, the
reactions of interest must take place within a very narrow region
(<0.5 micron) of silicon in order preserve the P-N junction.
[0028] The properties discussed herein are believed to depend on a
variety of variables, including the glass composition, amount of
glass in the paste, silver morphology, and firing conditions.
Several physical and chemical phenomena within the glass component
must take place in order to form a front contact having low series
resistance (R.sub.S). Optimization of front contact properties
requires fine tuning of firing temperature and conditions because
small changes in the temperature--temperature variations with in
the PV cell, between cells, furnace-to-furnace, cell lot to cell
lot--can have a large effect on the performance of a cell: It is
believed that resistance is decreased and conductivity is increased
by increasing the number and quality of contacts between silver and
silicon (that is, epitaxial silver-silicon islands) provided the
interlayer glass thickness is minimized. If the paste is fired at
too low a temperature, a high series resistance results because
silver and silicon fail to react sufficiently at the Ag/Si
interface. Conversely, if the paste is fired at too high a
temperature, the PN junction in the silicon wafer is affected by
excessive silver diffusion into silicon (and away from the
interface), thereby reducing cell performance due to reduced
R.sub.sh. A high R.sub.sh is needed for good cell performance.
[0029] Method of Front Contact Production. A solar cell contact
according to the present invention may be produced by applying any
conductive paste disclosed herein to a substrate, for example by
screen-printing, to a desired wet thickness, e.g., from about 40 to
about 80 microns. Automatic screen-printing techniques can be
employed using a 200-325 mesh screen. The printed pattern is then
dried at 200.degree. C. or less, preferably at about 120.degree. C.
for about 5-15 minutes before firing. The dry printed pattern can
be fired for as little as 1 second up to about 5 minutes at peak
temperature, in a belt conveyor furnace in air. During firing, the
glass is fused and the metal is sintered.
[0030] Nitrogen (N.sub.2) or another inert atmosphere may be used
if desired. The firing is generally according to a temperature
profile that will allow burnout of the organic matter at about
300.degree. C. to about 550.degree. C., a period of peak furnace
set temperature of about 650.degree. C. to about 1000.degree. C.,
lasting as little as about 1 second, although longer firing times
as high as 1, 3, or 5 minutes are possible when firing at lower
temperatures. For example a three-zone firing profile may be used,
with a belt speed of about 1 to about 4 meters (40-160 inches) per
minute, preferably 3 meters/minute (about 120 inches/minute). In a
preferred example, zone 1 is about 7 inches (18 cm) long, zone 2 is
about 16 inches (40 cm) long, and zone 3 is about 7 inches (18 cm)
long. The temperature in each successive zone is typically higher
than the previous, for example, 700-790.degree. C. in zone
1,800-850.degree. C. in zone 2, and 800-970.degree. C. in zone 3.
Naturally, firing arrangements having more than 3 zones are
envisioned by the present invention, including 4, 5, 6, or 7, zones
or more, each with zone lengths of about 5 to about 20 inches and
firing temperatures of 650 to 1000.degree. C.
[0031] Experimental Examples: Polycrystalline silicon wafers, 12.5
cm.times.12.5 cm, thickness 250-300 .mu.m, were coated with a
silicon nitride antireflective coating. The sheet resistivity of
these wafers was about 1 .OMEGA.-cm.
[0032] Commercially available back surface field aluminum paste
(Ferro CN53-038) and backside silver paste (Ferro CN33-451) were
used for the back contact. The front contact pattern was printed
using a 280 mesh screen with 100 .mu.m openings for finger lines
and with about a 2.8 mm spacing between the lines. Glass
compositions used in the exemplary pastes were prepared by known
glass-making techniques, and are presented in Table 4, the
properties of those glass compositions are in Table 5, and the
paste compositions are in Table 6. Samples were dried at about 100
to about 150.degree. C. for about 3 to about 15 minutes after
printing the front contacts. The printed wafers were co-fired using
a 3-zone infrared (IR) belt furnace with a belt speed of about 3
meters (120'') per minute, with temperature settings of 780.degree.
C., 810.degree. C., and 930.degree. C. for the three zones. The
zones were 7'', 16'', and 7'' long, respectively. The fired finger
width for most samples was about 120 to about 170 .mu.m, and the
fired thickness was about 10 to 15 .mu.m.
[0033] Electrical performance of the solar cells was measured with
a solar tester, Model 91193-1000, Oriel Instrument Co., Stratford,
Conn., under AM 1.5 sun conditions, in accordance with ASTM
G-173-03. The results of this electrical testing are presented in
Table 7. TABLE-US-00004 TABLE 4 Exemplary Glass Compositions Glass
mole % A B C D E PbO 31.3 61.6 61.5 58.9 61.9 ZnO 30.0 SiO.sub.2
29.8 30.3 27.2 28.7 30.1 Al.sub.2O.sub.3 8.0 3.3 5.6 7.7
B.sub.2O.sub.3 2.4 Ta.sub.2O.sub.5 0.9 ZrO.sub.2 1.6 2.0
P.sub.2O.sub.5 3.3 Sb.sub.2O.sub.5 1.4 Ga.sub.2O.sub.3 8.0
HfO.sub.2 4.8
[0034] Properties of the glass compositions A-E are set forth in
the following Table 5. Tg stands for glass transition temperature;
TCE is thermal coefficient of expansion over the range of
25-300.degree. C. TABLE-US-00005 TABLE 5 Glass Properties Glass
Composition Glass properties A B C D E Tg, .degree. C. 498 404 390
-- 404 TCE, (25-300.degree. C.) .times. 10.sup.-7/.degree. C. 68
105 98 105 97 Density, gm/cc 5.8 6.7 6.5 6.8 6.3
[0035] Paste formulations in Table 6 were made using organic
vehicles V131 and V132, commercially available from Ferro
Corporation, Cleveland, Ohio. All amounts in Table 6 are in weight
percent of the paste, including the solids portion and the organics
portion. TABLE-US-00006 TABLE 6 Paste Formulations Paste
Ingredients in wt % 1 2 3 4 5 6 Glass component A B C D E B Glass
component in paste 4.5 4.5 4.5 4.5 4.5 4.5 Silver flake, SF-23 73.0
Silver powder, Ag3000-1 68 68 68 68 68 Colloidal silver, RDAGCOLB
12.4 12.4 12.4 12.4 12.4 5.0 Vehicle V131 0.6 6.5 0.6 0.6 0.6 17.5
Vehicle V132 14.5 8.6 14.5 14.5 14.5
[0036] The front contact pastes in Table 6 were fired according to
the firing profile disclosed herein. The electrical properties of
the resultant solar cells are set forth in Table 7. TABLE-US-00007
TABLE 7 Properties of Solar cells made with front contact pastes of
Table 6. Paste 1 2 3 4 5 6 Glass A B C D E B Isc, A 4.73 4.93 4.73
4.83 4.72 4.89 Voc, mV 597 605 601 602 598 600 Efficiency, % 13.9
14.7 14.1 14.3 11.0 14.1 Fill Factor, % 76.5 77.2 76.1 76.6 61.1
75.2 Rs, m.OMEGA. 8.5 8.1 8.8 8.8 21.7 8.8 Rsh, .OMEGA. 31.7 29 20
38 22.3 8.0
[0037] Isc means short circuit current, measured at zero output
voltage; Voc means open circuit voltage measured at zero output
current; R.sub.S and R.sub.sh were previously defined.
[0038] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
illustrative example shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general invention concept as defined by the
appended claims and their equivalents.
* * * * *