U.S. patent application number 13/565882 was filed with the patent office on 2014-02-06 for thick-film paste containing lead-vanadium-based oxide and its use in the manufacture of semiconductor devices.
This patent application is currently assigned to E I DU PONT DE NEMOURS AND COMPANY. The applicant listed for this patent is KENNETH WARREN HANG, Esther Kim, Brian J. Laughlin, Kurt Richard Mikeska, Ahmet Cengiz Palanduz. Invention is credited to KENNETH WARREN HANG, Esther Kim, Brian J. Laughlin, Kurt Richard Mikeska, Ahmet Cengiz Palanduz.
Application Number | 20140038346 13/565882 |
Document ID | / |
Family ID | 47189794 |
Filed Date | 2014-02-06 |
United States Patent
Application |
20140038346 |
Kind Code |
A1 |
HANG; KENNETH WARREN ; et
al. |
February 6, 2014 |
THICK-FILM PASTE CONTAINING LEAD-VANADIUM-BASED OXIDE AND ITS USE
IN THE MANUFACTURE OF SEMICONDUCTOR DEVICES
Abstract
The present invention provides a thick-film paste for printing
the front side of a solar cell device having one or more insulating
layers and a method for doing so. The thick-film paste comprises a
source of an electrically conductive metal and a
lead-vanadium-based oxide dispersed in an organic medium. The
invention also provides a semiconductor device comprising an
electrode formed from the thick-film paste.
Inventors: |
HANG; KENNETH WARREN; (Cary,
NC) ; Kim; Esther; (Cary, NC) ; Laughlin;
Brian J.; (Apex, NC) ; Mikeska; Kurt Richard;
(Hockessin, DE) ; Palanduz; Ahmet Cengiz; (Durham,
NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HANG; KENNETH WARREN
Kim; Esther
Laughlin; Brian J.
Mikeska; Kurt Richard
Palanduz; Ahmet Cengiz |
Cary
Cary
Apex
Hockessin
Durham |
NC
NC
NC
DE
NC |
US
US
US
US
US |
|
|
Assignee: |
E I DU PONT DE NEMOURS AND
COMPANY
Wilmington
DE
|
Family ID: |
47189794 |
Appl. No.: |
13/565882 |
Filed: |
August 3, 2012 |
Current U.S.
Class: |
438/98 ; 252/512;
252/514; 501/41; 501/46; 501/49 |
Current CPC
Class: |
C03C 8/18 20130101; H01L
31/022425 20130101; C03C 8/08 20130101; H01L 31/022408 20130101;
H01B 1/22 20130101; C03C 8/12 20130101; C03C 8/10 20130101; H01B
1/16 20130101; Y02E 10/50 20130101 |
Class at
Publication: |
438/98 ; 252/512;
252/514; 501/41; 501/46; 501/49 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224 |
Claims
1. A thick-film paste composition comprising: a) 80-99.5 wt % of a
source of electrically conductive metal; b) 0.5 to 20 wt % of a
lead-vanadium-based oxide; and c) an organic medium; wherein the
source of electrically conductive metal and the lead-vanadium-based
oxide are dispersed in the organic medium and wherein the above wt
% are based on the total weight of the source of electrically
conductive metal and the lead-vanadium-based oxide, the
lead-vanadium-based oxide comprising 52-80 wt % PbO, 10-45 wt %
V.sub.2O.sub.5 and one or more additional oxides with a liquidus
temperature of 900.degree. C. or less, said one or more additional
oxides comprising at least one oxide selected from the group
consisting of 4-18 wt % Bi.sub.2O.sub.3, 0.5-8 wt % P.sub.2O.sub.5,
1-3 wt % B.sub.2O.sub.3, and 0.5-6 wt % TeO.sub.2, wherein said
oxide wt % are based on the total weight of the lead-vanadium-based
oxide.
2. (canceled)
3. The thick-film paste composition of claim 1, the
lead-vanadium-based oxide further comprising one or more of 0.1-2
wt % Li.sub.2O, 0.1-4 wt % TiO.sub.2, 0.1-5 wt % Fe.sub.2O.sub.3
and 0.1-5 wt % Cr.sub.2O.sub.3, wherein said oxide wt % is based on
the total weight of the lead-vanadium-based oxide.
4. The thick-film paste composition of claim 1, the
lead-vanadium-based oxide comprising 55-63 wt % PbO, 18-30 wt %
V.sub.2O.sub.5 and 5-11 wt % of the additional oxide
Bi.sub.2O.sub.3, wherein said wt % are based on the total weight of
the lead-vanadium-based oxide.
5. The thick-film paste composition of claim 4, the
lead-vanadium-based oxide further comprising one or more additional
oxides selected from the group consisting of 0.8-7 wt %
P.sub.2O.sub.5, 1.5-1.9 wt % B.sub.2O.sub.3, and 1-6 wt %
TeO.sub.2, wherein said wt % are based on the total weight of the
lead-vanadium-based oxide.
6. The thick-film paste composition of claim 5, the
lead-vanadium-based oxide further comprising one or more of 0.2-1.1
wt % Li.sub.2O, 0.5-2 wt % TiO.sub.2, 0.1-5 wt % Fe.sub.2O.sub.3
and 0.1-5 wt % Cr.sub.2O.sub.3, wherein said wt % is based on the
total weight of the lead-vanadium-based oxide.
7. The thick-film paste composition of claim 1, wherein the
electrically conductive metal is selected from the group consisting
of Ag, Cu, Au, Pd, Pt, Sn, Al and Ni.
8. A lead-vanadium-based oxide comprising 52-80 wt % PbO, 10-45 wt
% V.sub.2O.sub.5 and one or more additional oxides with a liquidus
temperature of 900.degree. C. or less, said one or more additional
oxides comprising at least one oxide selected from the group
consisting of 4-18 wt % Bi.sub.2O.sub.3, 0.5-8 wt % P.sub.2O.sub.5,
1-3 wt % B.sub.2O.sub.3, and 0.5-6 wt % TeO.sub.2, wherein said
oxide wt % are based on the total weight of the lead-vanadium-based
oxide.
9. (canceled)
10. The lead-vanadium-based oxide of claim 8, the
lead-vanadium-based oxide further comprising one or more of 0.1-2
wt % Li.sub.2O, 0.1-4 wt % TiO.sub.2, 0.1-5 wt % Fe.sub.2O.sub.3
and 0.1-5 wt % Cr.sub.2O.sub.3, wherein said wt % is based on the
total weight of the lead-vanadium-based oxide.
11. The lead-vanadium-based oxide of claim 8, the
lead-vanadium-based oxide comprising 55-63 wt % PbO, 18-30 wt %
V.sub.2O.sub.5 and 5-11 wt % of the additional oxide
Bi.sub.2O.sub.3, wherein said wt % are based on the total weight of
the lead-vanadium-based oxide.
12. The lead-vanadium-based oxide of claim 11, the
lead-vanadium-based oxide further comprising one or more additional
oxides selected from the group consisting of 0.8-7 wt %
P.sub.2O.sub.5, 1.5-1.9 wt % B.sub.2O.sub.3, and 1-6 wt %
TeO.sub.2, wherein said wt % are based on the total weight of the
lead-vanadium-based oxide.
13. The lead-vanadium-based oxide of claim 12, the
lead-vanadium-based oxide further comprising one or more of 0.2-1.1
wt % Li.sub.2O, 0.5-2 wt % TiO.sub.2, 0.1-5 wt % Fe.sub.2O.sub.3
and 0.1-5 wt % Cr.sub.2O.sub.3, wherein said wt % is based on the
total weight of the lead-vanadium-based oxide.
14. A process comprising: (a) providing an article comprising one
or more insulating films disposed onto at least one surface of a
semiconductor substrate; (b)(e) applying a thick-film paste
composition onto the one or more insulating films to form a layered
structure, the thick-film paste composition comprising: i) 80-99.5
wt % of a source of electrically conductive metal; ii) 0.5 to 20 wt
% of a lead-vanadium-based oxide; and iii) an organic medium,
wherein the source of electrically conductive metal and the
lead-vanadium-based oxide are dispersed in the organic medium and
wherein the above wt % are based on the total weight of the source
of electrically conductive metal and the lead-vanadium-based oxide,
the lead-vanadium-based oxide comprising 52-80 wt % PbO, 10-45 wt %
V.sub.2O.sub.5 and one or more additional oxides with a liquidus
temperature of 900.degree. C. or less, said one or more additional
oxides comprising at least one oxide selected from the group
consisting of 4-18 wt % Bi.sub.2O.sub.3, 0.5-8 wt % P.sub.2O.sub.5,
1-3 wt % B.sub.2O.sub.3, and 0.5-6 wt % TeO.sub.2, wherein said
oxide wt % are based on the total weight of the lead-vanadium-based
oxide; and (c) firing the semiconductor substrate, the one or more
insulating films, and the thick-film paste wherein the organic
medium of the thick film paste is volatilized, thereby forming an
electrode in contact with the one or more insulating layers and in
electrical contact with the semiconductor substrate.
15. (canceled)
16. The process of claim 14, the lead-vanadium-based oxide further
comprising one or more of 0.1-2 wt % Li.sub.2O0.0.1-4 wt %
TiO.sub.2O, 1-5 wt % Fe.sub.2O.sub.3 and 0.1-5 wt %
Cr.sub.2O.sub.3, wherein said wt % is based on the total weight of
the lead-vanadium-based oxide.
17. The process of claim 14, the lead-vanadium-based oxide
comprising 55-63 wt % PbO, 18-30 wt % V.sub.2O.sub.5 and 5-11 wt %
of the additional oxide Bi.sub.2O.sub.3, wherein said wt % are
based on the total weight of the lead-vanadium-based oxide.
18. The process of claim 17, the lead-vanadium-based oxide further
comprising one or more additional oxides selected from the group
consisting of 0.8-7 wt % P.sub.2O.sub.5, 1.5-1.9 wt %
B.sub.2O.sub.3, and 1-6 wt % TeO.sub.2, wherein said wt % are based
on the total weight of the lead-vanadium-based oxide.
19. The process of claim 18, the lead-vanadium-based oxide further
comprising one or more of 0.2-1.1 wt % Li.sub.2O, 0.5-2 wt %
TiO.sub.2, 0.1-5 wt % Fe.sub.2O.sub.3 and 0.1-5 wt %
Cr.sub.2O.sub.3, wherein said wt % is based on the total weight of
the lead-vanadium-based oxide.
20. A semiconductor device comprising an electrode formed from a
thick-film paste composition comprising: i) 80-99.5 wt % of a
source of electrically conductive metal; ii) 0.5 to 20 wt % of a
lead-vanadium-based oxide; and iii) an organic medium, wherein the
source of electrically conductive metal and the lead-vanadium-based
oxide are dispersed in the organic medium and wherein the above wt
% are based on the total weight of the source of electrically
conductive metal and the lead-vanadium-based oxide, the
lead-vanadium-based oxide comprising 52-80 wt % PbO, 10-45 wt %
V.sub.2O.sub.5 and one or more additional oxides with a liquidus
temperature of 900.degree. C. or less, said one or more additional
oxides comprising at least one oxide selected from the group
consisting of 4-18 wt % Bi.sub.2O.sub.3, 0.5-8 wt % P.sub.2O.sub.5,
1-3 wt % B.sub.2O.sub.3, and 0.5-6 wt % TeO.sub.2, wherein said
oxide wt % are based on the total weight of the lead-vanadium-based
oxide and wherein said thick film paste composition has been fired
to remove the organic medium and form said electrode.
21. (canceled)
22. The semiconductor device of claim 20, the lead-vanadium-based
oxide further comprising one or more of 0.1-2 wt % Li.sub.2O, 0.1-4
wt % TiO.sub.2, 0.1-5 wt % Fe.sub.2O.sub.3 and 0.1-5 wt %
Cr.sub.2O.sub.3, wherein said wt % is based on the total weight of
the lead-vanadium-based oxide.
23. The semiconductor device of claim 20, the lead-vanadium-based
oxide comprising 55-63 wt % PbO, 18-30 wt % V.sub.2O.sub.5 and 5-11
wt % of additional oxide Bi.sub.2O.sub.3, wherein the said wt % are
based on the total weight of the lead-vanadium-based oxide.
24. The semiconductor device of claim 23, the lead-vanadium-based
oxide further comprising one or more additional oxides selected
from the group consisting of 0.8-7 wt % P.sub.2O.sub.5, 1.5-1.9 wt
% B.sub.2O.sub.3, and 1-6 wt % wherein said wt % are based on the
total weight of the lead-vanadium-based oxide.
25. The semiconductor device of claim 24, the lead-vanadium-based
oxide further comprising one or more of TeO.sub.2, 0.2-1.1 wt %
Li.sub.2O, 0.5-2 wt % TiO.sub.2, 0.1-5 wt % Fe.sub.2O.sub.3 and
0.1-5 wt % Cr.sub.2O.sub.3, wherein said wt % is based on the total
weight of the lead-vanadium-based oxide.
Description
FIELD OF THE INVENTION
[0001] The present invention provides a thick-film paste for
printing the front side of a solar cell device having one or more
insulating layers. The thick-film paste comprises a source of an
electrically conductive metal, a lead-vanadium-based oxide and an
organic medium.
TECHNICAL BACKGROUND
[0002] A conventional solar cell structure with a p-type base has a
negative electrode that is typically on the front side (sun side)
of the cell and a positive electrode on the back-side. Radiation of
an appropriate wavelength falling on a p-n junction of a
semiconductor body serves as a source of external energy to
generate hole-electron pairs in that body. As a result of the
potential difference which exists at a p-n junction, holes and
electrons move across the junction in opposite directions and
thereby give rise to flow of an electric current that is capable of
delivering power to an external circuit. Most solar cells are in
the form of a silicon wafer that has been metalized, i.e., provided
with metal contacts that are electrically conductive.
[0003] Conductive inks are typically used to form the conductive
grids or metal contacts. Conductive inks typically comprise a glass
frit, a conductive species (e.g., silver particles), and an organic
medium. To form the metal contacts, conductive inks are printed
onto a substrate as grid lines or other patterns and then fired,
during which electrical contact is made between the grid lines and
the semiconductor substrate.
[0004] However, crystalline silicon solar cells are typically
coated with an anti-reflective coating such as silicon nitride,
titanium oxide, or silicon oxide to promote light adsorption, which
increases the cells' efficiency. Such anti-reflective coatings also
act as an insulator which impairs the flow of electrons from the
substrate to the metal contacts. To overcome this problem, the
conductive ink should penetrate the anti-reflective coating during
firing to form metal contacts having electrical contact with the
semiconductor substrate. Formation of a strong bond between the
metal contact and the substrate and solderability are also
desirable.
[0005] The ability to penetrate the anti-reflective coating and
form a strong bond with the substrate upon firing is highly
dependent on the composition of the conductive ink and firing
conditions. Efficiency, a key measure of solar cell performance, is
also influenced by the quality of the electrical contact made
between the fired conductive ink and the substrate.
[0006] Alternatively, a reverse solar cell structure with an n-type
silicon base is also known. This cell has a front p-type silicon
surface (front p-type emitter) with a positive electrode on the
front-side and a negative electrode to contact the back-side of the
cell. Solar cells with n-type silicon bases (n-type silicon solar
cells) can in theory produce higher efficiency gains compared to
solar cells with p-type silicon bases owing to the reduced
recombination velocity of electrons in the n-doped silicon.
[0007] To provide an economical process for manufacturing solar
cells with good efficiency, there is a need for thick-film paste
compositions that can be fired at low temperatures to penetrate an
anti-reflective coating and provide good electrical contact with
the semiconductor substrate.
SUMMARY OF THE INVENTION
[0008] This invention provides a thick-film paste composition
comprising: [0009] a) 80-99.5 wt % of a source of electrically
conductive metal; [0010] b) 0.5 to 20 wt % of a lead-vanadium-based
oxide; and [0011] c) an organic medium; wherein the source of
electrically conductive metal and the lead-vanadium-based oxide are
dispersed in the organic medium and wherein the above wt % are
based on the total weight of the source of electrically conductive
metal and the lead-vanadium-based oxide, the lead-vanadium-based
oxide comprising 52-80 wt % PbO, 10-45 wt % V.sub.2O.sub.5 and one
or more additional oxides with a liquidus temperature of
900.degree. C. or less, the one or more additional oxides selected
from the group consisting of B.sub.2O.sub.3, P.sub.2O.sub.5,
Bi.sub.2O.sub.3, As.sub.2O.sub.3, Sb.sub.2O.sub.3, MoO.sub.3,
TeO.sub.2, and SeO.sub.2, wherein the oxide wt % are based on the
total weight of the lead-vanadium-based oxide.
[0012] This invention also provides a lead-vanadium-based oxide
comprising 52-80 wt % PbO, 10-45 wt % V.sub.2O.sub.5 and one or
more additional oxides with a liquidus temperature of 900.degree.
C. or less, the one or more additional oxides selected from the
group consisting of B.sub.2O.sub.3, P.sub.2O.sub.5,
Bi.sub.2O.sub.3, As.sub.2O.sub.3, Sb.sub.2O.sub.3, MoO.sub.3,
TeO.sub.2, and SeO.sub.2, wherein the oxide wt % are based on the
total weight of the lead-vanadium-based oxide.
[0013] This invention further provides a process comprising: [0014]
(a) providing an article comprising one or more insulating films
disposed onto at least one surface of a semiconductor substrate;
[0015] (b) applying a thick-film paste composition onto the one or
more insulating films to form a layered structure, the thick-film
paste composition comprising: [0016] i) 80-99.5 wt % of a source of
electrically conductive metal; [0017] ii) 0.5 to 20 wt % of a
lead-vanadium-based oxide; and [0018] iii) an organic medium,
[0019] wherein the source of electrically conductive metal and the
lead-vanadium-based oxide are dispersed in the organic medium and
wherein the above wt % are based on the total weight of the source
of electrically conductive metal and the lead-vanadium-based oxide,
the lead-vanadium-based oxide comprising 52-80 wt % PbO, 10-45 wt %
V.sub.2O.sub.5 and one or more additional oxides with a liquidus
temperature of 900.degree. C. or less, the one or more additional
oxides selected from the group consisting of B.sub.2O.sub.3,
P.sub.2O.sub.5, Bi.sub.2O.sub.3, As.sub.2O.sub.3, Sb.sub.2O.sub.3,
MoO.sub.3, TeO.sub.2, and SeO.sub.2, the oxide wt % are based on
the total weight of the lead-vanadium-based oxide; and [0020] (c)
firing the semiconductor substrate, the one or more insulating
films, and the thick-film paste wherein the organic medium of the
thick film paste is volatilized, thereby forming an electrode in
contact with the one or more insulating layers and in electrical
contact with the semiconductor substrate.
[0021] This invention, in addition, provides a semiconductor device
comprising an electrode formed from a thick-film paste composition
comprising: [0022] i) 80-99.5 wt % of a source of electrically
conductive metal; [0023] ii) 0.5 to 20 wt % of a
lead-vanadium-based oxide; and [0024] iii) an organic medium,
wherein the source of electrically conductive metal and the
lead-vanadium-based oxide are dispersed in the organic medium and
wherein the above wt % are based on the total weight of the source
of electrically conductive metal and the lead-vanadium-based oxide,
the lead-vanadium-based oxide comprising 52-80 wt % PbO, 10-45 wt %
V.sub.2O.sub.5 and one or more additional oxides with a liquidus
temperature of 900.degree. C. or less, the one or more additional
oxides selected from the group consisting of B.sub.2O.sub.3,
P.sub.2O.sub.5, Bi.sub.2O.sub.3, As.sub.2O.sub.3, Sb.sub.2O.sub.3,
MoO.sub.3, TeO.sub.2, and SeO.sub.2, wherein the oxide wt % are
based on the total weight of the lead-vanadium-based oxide and
wherein the thick film paste composition has been fired to remove
the organic medium and form the electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a process flow diagram illustrating the
fabrication of a semiconductor device. Reference numerals shown in
FIG. 1 are explained below.
[0026] 10: p-type silicon substrate [0027] 20: n-type diffusion
layer [0028] 30: insulating film [0029] 40: p+ layer (back surface
field, BSF) [0030] 60: aluminum paste disposed on back side [0031]
61: aluminum back electrode (obtained by firing back-side aluminum
paste) [0032] 70: silver or silver/aluminum paste disposed on back
side [0033] 71: silver or silver/aluminum back electrode (obtained
by firing back-side silver paste) [0034] 500: thick-film paste
disposed on front side [0035] 501: front electrode (formed by
firing the thick-film paste)
DETAILED DESCRIPTION OF THE INVENTION
[0036] As used herein, "thick film composition" refers to a
composition which, upon firing on a substrate, has a thickness of 1
to 100 microns. The thick film compositions contain a conductive
material, a lead-vanadium-based oxide composition, and an organic
medium. The thick film composition may include additional
components. As used herein, the additional components are termed
"additives".
[0037] The composition described herein include one or more
electrically functional materials and one or more glass frits
dispersed in an organic medium. This composition is a thick-film
paste composition. The composition may also include one or more
additive(s). Exemplary additives include metals, metal oxides or
any compounds that can generate these metal oxides during
firing.
[0038] In an embodiment, the electrically functional powders may be
conductive powders. In an embodiment, the composition is used in a
semiconductor device. In an aspect of this embodiment, the
semiconductor device may be a solar cell or a photodiode. In a
further aspect of this embodiment, the semiconductor device may be
one of a broad range of semiconductor devices.
Electrically Conductive Metal
[0039] The thick film composition includes a functional component
that imparts appropriate electrically functional properties to the
composition. The electrically functional component is an
electrically conductive metal.
[0040] The source of the electrically conductive metal can be in a
flake form, a spherical form, a granular form, a crystalline form,
a powder, or other irregular forms and mixtures thereof. The
electrically conductive metal can be provided in a colloidal
suspension.
[0041] In one embodiment, the source of the electrically conductive
metal is from about 80 to about 99.5 wt % of the solid components
of the thick-film paste composition. Solids are defined herein as
the total composition of the thick-film paste composition minus the
organic medium. These wt % are then based on the total weight of
the source of electrically conductive metal and the
lead-vanadium-based oxide. In a further embodiment, the source of
the electrically conductive metal is from about 90 to about 95 wt %
of the solid components of the thick-film paste composition. The
solid components are defined herein as the electrically conductive
metal and the lead-vanadium-based oxide.
[0042] The electrically conductive metal is selected from the group
consisting of Ag, Cu, Au, Pd, Pt, Sn, Al, Ni and mixtures thereof.
In an embodiment, the conductive particles may include silver (Ag).
In a further embodiment, the conductive particles may include
silver (Ag) and aluminum (Al). In a further embodiment, the
conductive particles may, for example, include one or more of the
following: Cu, Au, Ag, Pd, Pt, Al, Ni, Ag--Pd, Pt--Au. In an
embodiment, the conductive particles may include one or more of the
following: (1) Al, Cu, Au, Ag, Pd and Pt; (2) an alloy of Al, Cu,
Au, Ag, Pd and Pt; and (3) mixtures thereof.
[0043] When the metal is silver, it can be in the form of silver
metal, alloys of silver, or mixtures thereof. The silver can also
be in the form of silver oxide (Ag.sub.2O), silver salts such as
AgCl, AgNO.sub.3, AgOOCCH.sub.3 (silver acetate), AgOOCF.sub.3
(silver trifluoroacetate), silver orthophosphate
(Ag.sub.3PO.sub.4), or mixtures thereof. Other forms of silver
compatible with the other thick-film paste components can also be
used.
[0044] In an embodiment, the functional phase of the composition
may be coated or uncoated silver particles which are electrically
conductive. In an embodiment in which the silver particles are
coated, they are at least partially coated with a surfactant. In an
embodiment, the surfactant may include one or more of the following
non-limiting surfactants: stearic acid, palmitic acid, a salt of
stearate, a salt of palmitate, lauric acid, palmitic acid, oleic
acid, stearic acid, capric acid, myristic acid and linoleic acid,
and mixtures thereof. The counter ion may be, but is not limited
to, hydrogen, ammonium, sodium, potassium and mixtures thereof.
[0045] The particle size of the silver is not subject to any
particular limitation. In an embodiment, the average particle size
may be less than 10 microns, and, in a further embodiment, no more
than 5 microns. In an aspect, the average particle size may be 0.1
to 5 microns, for example. As used herein, "particle size" is
intended to mean "average particle size"; "average particle size"
means the 50% volume distribution size. The 50% volume distribution
size can be denoted as D.sub.50. Volume distribution size may be
determined by a number of methods understood by one of skill in the
art, including but not limited to LASER diffraction and dispersion
method using a Microtrac particle size analyzer.
[0046] In an embodiment, the silver may be 60 to 90 wt % of the
paste composition. In a further embodiment, the silver may be 70 to
85 wt % of the paste composition. In a further embodiment, the
silver may be 75 to 85 wt % of the paste composition. In a further
embodiment, the silver may be 78 to 82 wt % of the paste
composition.
[0047] In an embodiment, the silver may be 90 to 99 wt % of the
solids in the composition. The solids are defined herein as the
total thick-film composition minus the organic medium. The solids
weight percent is therefore based on the total weight of the source
of electrically conductive metal and the lead-vanadium-based oxide.
In a further embodiment, the silver may be 92 to 97 wt % of the
solids in the composition. In a further embodiment, the silver may
be 93 to 95 wt % of the solids in the composition.
[0048] In an embodiment, the solids portion of the thick-film paste
composition includes about 80 to about 90 wt % spherical silver
particles. In one embodiment, the solids portion of the thick-film
paste composition includes about 80 to about 90 wt % silver
particles and about 1 to about 9.5 wt % silver flakes.
[0049] In one embodiment, the thick-film paste composition includes
coated silver particles that are electrically conductive. Suitable
coatings include phosphorous and surfactants. Suitable surfactants
include polyethyleneoxide, polyethyleneglycol, benzotriazole,
poly(ethyleneglycol)acetic acid, lauric acid, oleic acid, capric
acid, myristic acid, linolic acid, stearic acid, palmitic acid,
stearate salts, palmitate salts, and mixtures thereof. The salt
counter-ions can be ammonium, sodium, potassium, and mixtures
thereof.
Lead-Vanadium-Based Oxide Compositions
[0050] The invention relates to lead-vanadium-based oxide
(Pb--V--O) compositions with one or more additional oxides with a
liquidus temperature of 900.degree. C. or less. In an embodiment,
the lead-vanadium-based oxide with an additional oxide or
combination of oxides comprises lead oxide, vanadium oxide, and an
additional oxide or combination of oxides having a liquidus
temperature of 900.degree. C. or less. The liquidus temperature of
an oxide or combination of oxides can be found on phase diagrams
such as those published by the American Ceramic Society (ACerS-NIST
Phase Equilibria Diagrams, CD-ROM Database, Version 3.2.0,
Westerville Ohio, 2009, www.ceramics.org/phase) or can be measured
according to the ASTM method C829-81 (ASTM Standard C829-1,
reapproved 2010, "Standard Practices for Measurement of Liquidus
Temperature of Glass by the Gradient Furnace Method," ASTM
International, West Conshohocken, Pa., 2010, DOI:
0.15201C0829-81R10, www.astm.org.). One or more additional oxides
with a liquidus temperature of 900.degree. C. or less are selected
from the following non-limiting list: B.sub.2O.sub.3,
P.sub.2O.sub.5, Bi.sub.2O.sub.3, As.sub.2O.sub.3, Sb.sub.2O.sub.3,
MoO.sub.3, TeO.sub.2, or SeO.sub.2. As used herein, "additional
oxides" refers to the oxides with a liquidus temperature of
900.degree. C. or less. In one embodiment, the lead-vanadium-based
oxide further comprises one or more of Li.sub.2O, TiO.sub.2,
Fe.sub.2O.sub.3 and Cr.sub.2O.sub.3.
[0051] In an embodiment, the lead of the Pb--V--O may be from lead
oxides, lead salts, metallic lead, or organometallic lead. In a
further embodiment, lead oxide can be PbO, PbO.sub.2,
Pb.sub.3O.sub.4, Pb.sub.2O.sub.3, Pb.sub.12O.sub.19, or the like.
In a still further embodiment, the lead oxide can be coated for
example by silica or SiO.sub.2. In a further embodiment, the lead
salts can be lead halides such as PbCl.sub.2 or PbBr.sub.2 or lead
fluoride such as PbF.sub.2. In a further embodiment, the metallic
lead may be Pb or alloys such as Pb--V or Pb--Sn. In a further
embodiment, organometallic lead may be lead resinate such as lead
2-ethylhexanoate (Pb(C.sub.2H.sub.15O.sub.2).sub.2), lead octoate
(Pb[CH.sub.3(CH.sub.2).sub.3CH(C.sub.2H.sub.5)COO].sub.2), or lead
oxalate (PbC.sub.2O.sub.4).
[0052] In an embodiment, the vanadium of the Pb--V--O may be from
vanadium oxides, vanadium salts, metallic vanadium, or
organometallic vanadium. In a further embodiment, vanadium oxide
can be VO, VO.sub.2, V.sub.2O.sub.5, V.sub.2O.sub.3, or the like.
In a still further embodiment, the vanadium oxide can be coated for
example by silica or SiO.sub.2. In a further embodiment, the
vanadium salts can be vanadium halides such as VCl.sub.4,
VBr.sub.5, VBr.sub.4, VCl.sub.5 or vanadium fluoride such as
VF.sub.4 or VF.sub.5. In a further embodiment, the metallic
vanadium may be V or alloys such as Pb--V or V--Fe. In a further
embodiment, organometallic vanadium may be vanadium resinate,
vanadocene dichloride (C.sub.10H.sub.10Cl.sub.2V), or vanadium
carbonyl (V(CO).sub.6). In an embodiment, the vanadium may be an
inorganic vanadium compound. Exemplary inorganic compounds include,
but are not limited to carbonates, nitrates, phosphates,
hydroxides, peroxides, halogen and mixtures thereof, with the
proviso that such materials possess a liquidus temperature of
900.degree. C. or less and yield a miscible liquid upon heating to
temperatures of 900.degree. C. or less.
[0053] In an embodiment, these compositions may be glass
compositions. In a further embodiment, these compositions may be
crystalline, partially crystalline, amorphous, partially amorphous,
or combinations thereof. In an embodiment, the Pb--V--O composition
may include more than one glass composition. In an embodiment, the
Pb--V--O composition may include a glass composition and an
additional composition, such as a crystalline composition. Herein,
all these compositions are referred to as glasses or glass
frits.
[0054] The following paragraph provides mole % ranges consistent
with the weight % ranges presented in other areas of this text.
[0055] Typically, the mixture of PbO and V.sub.2O.sub.5 in Pb--V--O
includes 38 to 76 mole %, 40 to 65 mole %, or 45 to 60 mole % of
lead oxide and 10 to 53 mole %, 10 to 47-mole %, 15 to 45 mole %,
20-35 mole %, or 25-30 mole % of vanadium oxide, based on the
Pb--V--O composition powders. In an embodiment the mole ratio of
lead oxide to vanadium oxide of the lead-vanadium-oxide is between
42/58 and 88/12, 45/55 and 70/30, or 50/50 and 60/40. In one
embodiment, the mixture of PbO and V.sub.2O.sub.5 powders includes
45 to 63 mol % of lead oxide and 18 to 38 mol % of vanadium oxide,
based on the combined powders.
[0056] The lead-vanadium-oxide (Pb--V--O) may be prepared by mixing
PbO, V.sub.2O.sub.5, the one or more of the additional oxides (or
other materials that decompose into the desired oxides when heated)
and any other oxides present using techniques understood by one of
ordinary skill in the art. Such preparation techniques may involve
heating the mixture in air or an oxygen-containing atmosphere to
form a melt, quenching the melt, and grinding, milling, and/or
screening the quenched material to provide a powder with the
desired particle size. Melting the mixture of lead and vanadium
oxides is typically conducted to a peak temperature of 800 to
1200.degree. C. The molten mixture can be quenched, for example, on
a stainless steel platen or between counter-rotating stainless
steel rollers to form a platelet. The resulting platelet can be
milled to form a powder. Typically, the milled powder has a
D.sub.50 of 0.1 to 3.0 microns. One skilled in the art of producing
glass frit may employ alternative synthesis techniques such as but
not limited to water quenching, sol-gel, spray pyrolysis, or others
appropriate for making powder forms of glass.
[0057] In one embodiment, the starting mixture used to make the
Pb--V--O comprises (based on the weight of the total starting
mixture): 52 to 80 wt %, 10 to 45 wt % V.sub.2O.sub.5 and the one
or more additional oxides selected from the group consisting of
B.sub.2O.sub.3, P.sub.2O.sub.5, Bi.sub.2O.sub.3, As.sub.2O.sub.3,
Sb.sub.2O.sub.3, MoO.sub.3, TeO.sub.2, and SeO.sub.2, wherein said
oxide wt % are based on the total weight of the lead-vanadium-based
oxide. In one embodiment, the lead-vanadium-based oxide further
comprises one or more of Li.sub.2O, TiO.sub.2, Fe.sub.2O.sub.3 and
Cr.sub.2O.sub.3.
[0058] In one embodiment, the one or more additional oxides are
selected from the group consisting of 4-18 wt % Bi.sub.2O.sub.3,
0.5-8 wt % P.sub.2O.sub.5, 1-3 wt % B.sub.2O.sub.3, and 0.5-6 wt %
TeO.sub.2, wherein said oxide wt % are based on the total weight of
the lead-vanadium-based oxide. In one such embodiment, the
lead-vanadium-based oxide further comprises one or more of 0.1-2 wt
% Li.sub.2O, 0.1-4 wt % TiO.sub.2, 0.1-5 wt % Fe.sub.2O.sub.3 and
0.1-5 wt % Cr.sub.2O.sub.3 wherein said oxide wt % is based on the
total weight of the lead-vanadium-based oxide.
[0059] In another embodiment, the starting mixture used to make the
Pb--V--O comprises (based on the weight of the total starting
mixture): 55 to 63 wt % PbO, 18 to 30 wt % V.sub.2O.sub.5 and 5 to
11 wt % of the additional oxide Bi.sub.2O.sub.3. In one embodiment,
one or more additional oxides are selected from the group
consisting of 0.8-7 wt % P.sub.2O.sub.5, 1.5-1.9 wt %
B.sub.2O.sub.3, and 1-6 wt % TeO.sub.2, wherein said oxide wt % are
based on the total weight of the lead-vanadium-based oxide. In one
such embodiment, the lead-vanadium-based oxide further comprises
one or more of 0.2-1 wt % Li.sub.2O, 0.5-2 wt % TiO.sub.2, 0.1-5 wt
% Fe.sub.2O.sub.3 and 0.1-5 wt % Cr.sub.2O.sub.3, wherein said
oxide wt % is based on the total weight of the lead-vanadium-based
oxide.
[0060] In the various embodiments, the starting mixture used to
make the Pb--V--O includes one or more additional oxides with a
liquidus temperature of 900.degree. C. or less. These are selected
from the following non-limiting list: B.sub.2O.sub.3,
P.sub.2O.sub.5, Bi.sub.2O.sub.3, As.sub.2O.sub.3, Sb.sub.2O.sub.3,
MoO.sub.3, TeO.sub.2, or SeO.sub.2. Thus, the P--V--O may further
include one or more of Li.sub.2O, TiO.sub.2, Fe.sub.2O.sub.3 and
Cr.sub.2O.sub.3, such that [0061] the Bi.sub.2O.sub.3 may be 0.5 to
25 wt %, 4 to 18 wt %, or 5 to 11 wt %; [0062] the P.sub.2O.sub.5
may be 0.1 to 18 wt %, 0.5 to 8 wt %, or 0.8 to 7 wt %; [0063] the
B.sub.2O.sub.3 may be 0.25 to 15 wt %, 1 to 3 wt %, or 1.5 to 1.9
wt %; [0064] the TeO.sub.2 may be 0.1 to 15 wt %, 0.5 to 6 wt %, or
1 to 6 wt %; [0065] the As.sub.2O.sub.3 may be 0.1 to 15 wt %, or
0.1 to 5 wt %; [0066] the Sb.sub.2O.sub.3 may be 0.1 to 15 wt %, or
0.1 to 10 wt %; [0067] the MoO.sub.3 may be 0.1 to 15 wt %, or 0.1
to 10 wt %; [0068] the SeO.sub.2 may be 0.1 to 15 wt %, or 0.1 to
10 wt %; [0069] the Li.sub.2O may be 0.1 to 3 wt %, 0.1 to 2 wt %,
or 0.2 to 1.1 wt %; [0070] the TiO.sub.2 may be 0.1 to 10 wt %, 0.1
to 4 wt %, or 0.5 to 2 wt %; [0071] the Fe.sub.2O.sub.3 may be 0.1
to 10 wt %, Or 0.1 to 5 wt %; [0072] the Cr.sub.2O.sub.3 may be 0.1
to 10 wt %, Or 0.1 to 5 wt %;.
[0073] In a further embodiment, in addition to the above oxides,
the starting mixture used to make the Pb--V--O may include one or
more of ZrO.sub.2, P.sub.2O.sub.5, Na.sub.2O, WO.sub.3,
Nb.sub.2O.sub.5, SnO, SiO.sub.2, ZnO, PbF.sub.2, or BiF.sub.3. In
aspects of this embodiment (based on the weight of the total
starting mixture):
[0074] the ZrO.sub.2 may be 0 to 10 wt %, 0 to 8 wt %, or 2 to 5 wt
%;
[0075] the Na.sub.2O may be 0 to 3 wt %, 0 to 2 wt %, or 0 to 1 wt
%;
[0076] the WO.sub.3 may be 0 to 10 wt %, 0 to 6 wt %, or 0 to 4 wt
%;
[0077] the Nb.sub.2O.sub.5 may be 0 to 5 wt %, or 0 to 2 wt %;
[0078] the SnO may be 0 to 15 wt %, 0 to 12 wt %, or 8 to 12 wt
%;
[0079] the SiO.sub.2 may be 0 to 5 wt %, or 0 to 3 wt %;
[0080] the ZnO may be 0 to 5 wt %, or 0 to 4 wt %;
[0081] the PbF.sub.2 may be 0 to 30 wt %, 0 to 20 wt %, or 0 to 10
wt %; or
[0082] the BiF.sub.3 may be 0 to 30 wt %, 0 to 20 wt %, or 0 to 10
wt %.
[0083] In an embodiment, the Pb--V--O may be a homogenous powder.
In a further embodiment, the Pb--V--O may be a combination of more
than one powder, wherein each powder may separately be homogenous.
The composition of the overall combination of the multiple powders
is within the ranges described above. For example, the Pb--V--O may
include a combination of two or more different powders; separately,
these powders may have different compositions, and may or may not
be within the ranges described above; however, the combination of
these powders is within the ranges described above.
[0084] Glass compositions, also termed glass frits, are described
herein as including percentages of certain components.
Specifically, the percentages are the percentages of the components
used in the starting material that was subsequently processed as
described herein to form a glass composition. Such nomenclature is
conventional to one of skill in the art. In other words, the
composition contains certain components, and the percentages of
those components are expressed as a percentage of the corresponding
oxide form. As indicated above the Pb and V may be supplied by
various sources such as oxides, halides, carbonates, nitrates,
phosphates, hydroxides, peroxides, halogen compounds and mixtures
thereof. Similarly with the components of the one or more
additional oxides with a liquidus temperature of 900.degree. C. or
less or other components used in making the lead-vanadium-based
oxide. Any such source of an additional oxide having a liquidus
temperature of 900.degree. C. or less that will provide the oxide
at that temperature is suitable. Herein, the composition of the
lead-vanadium-based oxide is given in terms of the equivalent
oxides no matter the source of the various components. As
recognized by one of ordinary skill in the art in glass chemistry,
a certain portion of volatile species may be released during the
process of making the glass. An example of a volatile species is
oxygen.
[0085] In an embodiment, some or all of the Li.sub.2O and/or
Na.sub.2O may be replaced with K.sub.2O, Cs.sub.2O, or Rb.sub.2O,
resulting in a glass composition with properties similar to the
compositions listed above. In this embodiment, the total alkali
metal oxide content may be 0.1 to 5 wt %, 0.1 to 3 wt %, or 0.1 to
2 wt %.
[0086] In an embodiment, the Pb--V--O composition(s) herein may
include one or more of compound constituents, such as AgVO.sub.3,
Ag.sub.2V.sub.2Te.sub.2O.sub.10, Pb.sub.2V.sub.2O.sub.7,
Ag.sub.2V.sub.4O.sub.11, Ag.sub.3VO.sub.4, BiVO.sub.4,
BiPb.sub.3V.sub.3O.sub.12, Pb.sub.3V.sub.2O.sub.8 although such
compounds are not limited to this set of examples. Other exemplary
compounds include such compounds of constituents as defined herein
and having a liquidus temperature of less than 900 Celsius.
[0087] In a further embodiment, the Pb--V--O composition(s) herein
may include one or more of another set of components:
Al.sub.2O.sub.3, GeO.sub.2, Ga.sub.2O.sub.3, In.sub.2O.sub.3, NiO,
CoO, CaO, MgO, SrO, MnO, BaO, SeO.sub.2, Y.sub.2O.sub.3,
As.sub.2O.sub.3, La.sub.2O.sub.3, Nd.sub.2O.sub.3, Ta.sub.2O.sub.5,
HfO.sub.2, CdO, Mn.sub.2O.sub.3, CuO, La.sub.2O.sub.3,
Pr.sub.2O.sub.3, Gd.sub.2O.sub.3, Sm.sub.2O.sub.3, Dy.sub.2O.sub.3,
Eu.sub.2O.sub.3, Ho.sub.2O.sub.3, Yb.sub.2O.sub.3, Lu.sub.2O.sub.3,
CeO.sub.2, SnO.sub.2, Ag.sub.2O, K.sub.2O, Rb.sub.2O, Cs.sub.2O,
and metal halides (e.g., NaCl, KBr, NaI, LiF).
[0088] Therefore as used herein, the term "Pb--V--O" may also
include metal oxides that contain one or more elements selected
from the group consisting of Al, Ge, Ga, In, Ni, Co, Ca, Mg, Sr,
Mn, Ba, Se, Y, La, Nd, Ta, Fe, Hf, Cr, Cd, Mn, Cu, La, Pr, Gd, Sm,
Dy, Eu, Ho, Yb, Lu, Ce, Ag, K, Rb, and Cs.
[0089] Table 1 lists some examples of powder mixtures containing
PbO, V2O5, and other compounds that can be used to make
lead-vanadium-boron-oxides. This list is meant to be illustrative,
not limiting. In Table 1, the amounts of the compounds are shown as
weight percent, based on the weight of the total glass
composition.
[0090] If starting with a fired glass, one of ordinary skill in the
art may calculate the percentages of starting components described
herein using methods known to one of skill in the art including,
but not limited to: Inductively Coupled Plasma-Emission
Spectroscopy (ICPES), Inductively Coupled Plasma-Atomic Emission
Spectroscopy (ICP-AES), and the like. In addition, the following
exemplary techniques may be used: X-Ray Fluorescence spectroscopy
(XRF); Nuclear Magnetic Resonance spectroscopy (NMR); Electron
Paramagnetic Resonance spectroscopy (EPR); Mossbauer spectroscopy;
electron microprobe Energy Dispersive Spectroscopy (EDS); electron
microprobe Wavelength Dispersive Spectroscopy (WDS);
Cathodo-Luminescence (CL).
[0091] One of ordinary skill in the art would recognize that the
choice of raw materials could unintentionally include impurities
that may be incorporated into the glass during processing. For
example, the impurities may be present in the range of hundreds to
thousands ppm. The presence of the impurities would not alter the
properties of the glass, the thick-film composition, or the fired
device. For example, a solar cell containing the thick-film
composition may have the efficiency described herein, even if the
thick-film composition includes impurities.
Lead-Vanadium-Boron Oxide Compositions
[0092] An aspect of the invention relates to lead-vanadium-boron
oxide (Pb--V--B--O) compositions. In an embodiment, the
lead-vanadium-boron oxide may contain an additional oxide or
combination of oxides, where the additional oxide or combination of
oxides have a liquidus temperature of 900.degree. C. or less. One
or more additional oxides, or combination of additional oxides,
with a liquidus temperature of 900.degree. C. or less may be
selected from the following non-limiting list: P.sub.2O.sub.5,
Bi.sub.2O.sub.3, As.sub.2O.sub.3, Sb.sub.2O.sub.3, MoO.sub.3,
TeO.sub.2, or SeO.sub.2.
[0093] In an embodiment, the starting mixture used to make the
Pb--V--B--O may include (based on the weight of the total starting
mixture): PbO that may be 52 to 80 wt % or 55 to 63 wt %;
V.sub.2O.sub.5 that may be 10 to 45 wt %, or 18 to 30 wt %; and
B.sub.2O.sub.3 that may be 0.25 to 10 wt %, 1 to 3 wt % or 1.5 to
1.9 wt %.
[0094] In a further embodiment, in addition to the above PbO,
V.sub.2O.sub.5B.sub.2O.sub.3 and the other oxides already disclosed
above, the starting mixture used to make the Pb--V--B--O may
include one or more of ZrO.sub.2, P.sub.2O.sub.5, Na.sub.2O,
WO.sub.3, Nb.sub.2O, SnO, SiO.sub.2, ZnO, PbF.sub.2, or BiF.sub.3.
In aspects of this embodiment (based on the weight of the total
starting mixture):
[0095] the ZrO.sub.2 may be 0 to 10 wt %, 0 to 8 wt %, or 0 to 5 wt
%;
[0096] the P.sub.2O.sub.5 may be 0 to 15 wt %, 0 to 10 wt %, or 1.5
to 7 wt %;
[0097] the Na.sub.2O may be 0 to 3 wt %, 0 to 2 wt %, or 0 to 1 wt
%;
[0098] the WO.sub.3 may be 0 to 5 wt %, 0 to 4 wt %, or 3 to 4 wt
%;
[0099] the Nb.sub.2O.sub.5 may be 0 to 5 wt %, or 0 to 2 wt %;
[0100] the SnO may be 0 to 15 wt %, or 0 to 12 wt %;
[0101] the SiO.sub.2 may be 0 to 5 wt %, or 0 to 3 wt %;
[0102] the ZnO may be 0 to 5 wt %, or 0 to 4 wt %;
[0103] the PbF.sub.2 may be 0 to 30 wt %, 0 to 20 wt %, or 0 to 10
wt %; or
[0104] the BiF.sub.3 may be 0 to 30 wt %, 0 to 20 wt %, or 0 to 10
wt %.
Lead-Vanadium-Phosphorus Oxide Compositions
[0105] An aspect of the invention relates to
lead-vanadium-phosphorus-oxide (Pb--V--P--O) compositions. In an
embodiment, the lead-vanadium-phosphorus oxide may have an
additional oxide or combination of oxides, where the additional
oxide or combination of oxides have a liquidus temperature of
900.degree. C. or less. One or more additional oxides, or
combination of additional oxides, with a liquidus temperature of
900.degree. C. or less may be selected from the following
non-limiting list: B.sub.2O.sub.3, Bi.sub.2O.sub.3,
As.sub.2O.sub.3, Sb.sub.2O.sub.3, MoO.sub.3, TeO.sub.2, or
SeO.sub.2.
[0106] In an embodiment, the starting mixture used to make the
Pb--V--B--O may include (based on the weight of the total starting
mixture): PbO that may be 52 to 80 wt % or 55 to 63 wt %;
V.sub.2O.sub.5 that may be 10 to 45 wt %, or 18 to 30 wt %; and
P.sub.2O.sub.5 that may be 0.25 to 15 wt %, 0.5 to 8 wt %, or 0.8
to 7 wt %.
[0107] In a further embodiment, in addition to the above PbO,
V.sub.2O.sub.5, P.sub.2O.sub.5 and the other oxides already
disclosed above, the starting mixture used to make the Pb--V--P--O
may include one or more of ZrO.sub.2, Na.sub.2O, TeO.sub.2,
WO.sub.3, Nb.sub.2O.sub.5, SnO, SiO.sub.2, ZnO, PbF.sub.2, or
BiF.sub.3. In aspects of this embodiment (based on the weight of
the total starting mixture):
[0108] the ZrO.sub.2 may be 0 to 10 wt %, 0 to 8 wt %, or 0 to 5 wt
%;
[0109] the Na.sub.2O may be 0 to 3 wt %, 0 to 2 wt %, or 0 to 1 wt
%;
[0110] the WO.sub.3 may be 0 to 5 wt %, 0 to 4 wt %, or 3 to 4 wt
%;
[0111] the MoO.sub.3 may be 0 to 10 wt %, 0 to 6 wt %, or 0 to 1 wt
%;
[0112] the Nb.sub.2O.sub.5 may be 0 to 5 wt %, or 0 to 2 wt %;
[0113] the SnO may be 0 to 15 wt %, or 0 to 12 wt %;
[0114] the SiO.sub.2 may be 0 to 5 wt %, or 0 to 3 wt %;
[0115] the ZnO may be 0 to 5 wt %, or 0 to 4 wt %;
[0116] the PbF.sub.2 may be 0 to 30 wt %, 0 to 20 wt %, or 0 to 10
wt %; or
[0117] the BiF.sub.3 may be 0 to 30 wt %, 0 to 20 wt %, or 0 to 10
wt %.
Lead-Vanadium-Boron-Phosphorus Oxide Compositions
[0118] An aspect of the invention relates to
lead-vanadium-boron-phosphorus oxide (Pb--V--B--P--O) compositions.
In an embodiment, the lead-vanadium-boron-phosphorus oxide may be
with an additional oxide or combination of oxides, where the
additional oxide or combination of oxides having a liquidus
temperature of 900.degree. C. or less. One or more additional
oxides, or combination of additional oxides, with a liquidus
temperature of 900.degree. C. or less may be selected from the
following non-limiting list: Bi.sub.2O.sub.3, As.sub.2O.sub.3,
Sb.sub.2O.sub.3, MoO.sub.3, TeO.sub.2, or SeO.sub.2.
[0119] In an embodiment, the starting mixture used to make the
Pb--V--B--P--O may include (based on the weight of the total
starting mixture): PbO that may be 52 to 80 wt % or 55 to 63 wt %;
V.sub.2O.sub.5 that may be 10 to 45 wt %, or 18 to 30 wt %;
B.sub.2O.sub.3 that may be 0.25 to 10 wt %, 1 to 3 wt % or 1.5 to
1.9 wt % and P.sub.2O.sub.5 that may be 0.25 to 15 wt %, 0.5 to 8
wt %, or 0.8 to 7 wt %.
[0120] In a further embodiment, in addition to the above PbO,
V.sub.2O.sub.5, B.sub.2O.sub.3, P.sub.2O.sub.5 and the other oxides
already disclosed above, the starting mixture used to make the
Pb--V--B--P--O may include one or more of ZrO.sub.2, Na.sub.2O,
Li.sub.2O, WO.sub.3, Nb.sub.2O.sub.5, SnO, SiO.sub.2, ZnO,
PbF.sub.2, or BiF.sub.3. In aspects of this embodiment (based on
the weight of the total starting mixture):
[0121] the ZrO.sub.2 may be 0 to 5 wt %, 0 to 4 wt %, or 0 to 2 wt
%;
[0122] the Na.sub.2O may be 0 to 3 wt %, 0 to 2 wt %, or 0 to 1 wt
%;
[0123] the WO.sub.3 may be 0 to 5 wt %, 0 to 4 wt %, or 3 to 4 wt
%;
[0124] the MoO.sub.3 may be 0 to 10 wt %, 0 to 6 wt %, or 0 to 1 wt
%;
[0125] the Nb.sub.2O.sub.5 may be 0 to 5 wt %, or 0 to 2 wt %;
[0126] the SnO may be 0 to 15 wt %, or 0 to 12 wt %;
[0127] the SiO.sub.2 may be 0 to 5 wt %, or 0 to 3 wt %;
[0128] the ZnO may be 0 to 5 wt %, or 0 to 4 wt %;
[0129] the PbF.sub.2 may be 0 to 30 wt %, 0 to 20 wt %, or 0 to 10
wt %; or
[0130] the BiF.sub.3 may be 0 to 30 wt %, 0 to 20 wt %, or 0 to 10
wt %.
Lead-Vanadium-Boron-Phosphorus-Bismuth-Oxide Compositions
[0131] An aspect of the invention relates to
lead-vanadium-boron-phosphorus-bismuth oxide (Pb--V--B--P--Bi--O)
compositions. In an embodiment, the
lead-vanadium-boron-phosphorus-bismuth oxide may be with an
additional oxide or combination of oxides, where the additional
oxide or combination of oxides having a liquidus temperature of
900.degree. C. or less. One or more additional oxides, or
combination of additional oxides, with a liquidus temperature of
900.degree. C. or less may be selected from the following
non-limiting list: As.sub.2O.sub.3, Sb.sub.2O.sub.3, MOO.sub.3,
TeO.sub.2, or SeO.sub.2.
[0132] In an embodiment, the starting mixture used to make the
Pb--V--B--P--Bi--O may include (based on the weight of the total
starting mixture): PbO that may be 52 to 80 wt % or 55 to 63 wt %;
V.sub.2O.sub.5 that may be 10 to 45 wt %, or 18 to 30 wt %;
B.sub.2O.sub.3 that may be 0.25 to 10 wt %, 1 to 3 wt % or 1.5 to
1.9 wt %, P.sub.2O.sub.5 that may be 0.25 to 15 wt %, 0.5 to 8 wt
%, or 0.8 to 7 wt % and Bi.sub.2O.sub.3 that may be 1 to 25 wt %, 4
to 18 wt %, or 5 to 11 wt %.
[0133] In a further embodiment, in addition to the above PbO,
V.sub.2O.sub.5, B.sub.2O.sub.3, P.sub.2O.sub.5, Bi.sub.2O.sub.3 and
the other oxides already disclosed above, the starting mixture used
to make the Pb--V--B--P--Bi--O may include one or more of
ZrO.sub.2, Na.sub.2O, WO.sub.3, Nb.sub.2O.sub.5, SnO, SiO.sub.2,
ZnO, PbF.sub.2, or BiF.sub.3. In aspects of this embodiment (based
on the weight of the total starting mixture):
[0134] the ZrO.sub.2 may be 0 to 5 wt %, 0 to 4 wt %, or 0 to 2 wt
%;
[0135] the Na.sub.2O may be 0 to 3 wt %, 0 to 2 wt %, or 0 to 1 wt
%;
[0136] the WO.sub.3 may be 0 to 5 wt %, 0 to 4 wt %, or 3 to 4 wt
%;
[0137] the Nb.sub.2O.sub.5 may be 0 to 5 wt %, or 0 to 2 wt %;
[0138] the SnO may be 0 to 15 wt %, or 0 to 12 wt %;
[0139] the SiO.sub.2 may be 0 to 5 wt %, or 0 to 3 wt %;
[0140] the ZnO may be 0 to 5 wt %, or 0 to 4 wt %;
[0141] the PbF.sub.2 may be 0 to 30 wt %, 0 to 20 wt %, or 0 to 10
wt %; or
[0142] the BiF.sub.3 may be 0 to 30 wt %, 0 to 20 wt %, or 0 to 10
wt %.
Lead-Vanadium-Tellurium Oxide Compositions
[0143] An aspect of the invention relates to
lead-vanadium-tellurium oxide (Pb--V--Te--O) compositions. In an
embodiment, the lead-vanadium-tellurium-oxide may be with an
additional oxide or combination of oxides, where the additional
oxide or combination of oxides having a liquidus temperature of
900.degree. C. or less. One or more additional oxides, or
combination of additional oxides, with a liquidus temperature of
900.degree. C. or less may be selected from the following
non-limiting list: As.sub.2O.sub.3, Sb.sub.2O.sub.3, MoO.sub.3,
B.sub.2O.sub.3, P.sub.2O.sub.5, Bi.sub.2O.sub.3. Or SeO.sub.2.
[0144] In an embodiment, the starting mixture used to make the
Pb--V--B--O may include (based on the weight of the total starting
mixture): PbO that may be 52 to 80 wt % or 55 to 63 wt %;
V.sub.2O.sub.5 that may be 10 to 45 wt %, or 18 to 30 wt %; and
TeO.sub.2 that may be 0.1 to 15 wt %, 0.5 to 6 wt %, or 1 to 6 wt
%.
[0145] In a further embodiment, in addition to the above PbO,
V.sub.2O.sub.5, and TeO.sub.2 and the other oxides already
disclosed above, the starting mixture used to make the Pb--V--Te--O
may include one or more of ZrO.sub.2, Na.sub.2O, WO.sub.3,
Nb.sub.2O.sub.5, SnO, SiO.sub.2, or ZnO. In aspects of this
embodiment (based on the weight of the total starting mixture):
[0146] the ZrO.sub.2 may be 0 to 5 wt %, 0 to 4 wt %, or 0 to 2 wt
%;
[0147] the Na.sub.2O may be 0 to 3 wt %, 0 to 2 wt %, or 0 to 1 wt
%;
[0148] the WO.sub.3 may be 0 to 5 wt %, 0 to 4 wt %, or 3 to 4 wt
%;
[0149] the Nb.sub.2O.sub.5 may be 0 to 5 wt %, or 0 to 2 wt %;
[0150] the SnO may be 0 to 15 wt %, or 0 to 10 wt %;
[0151] the SiO.sub.2 may be 0 to 5 wt %, or 0 to 3 wt %; or
[0152] the ZnO may be 0 to 5 wt %, or 0 to 4 wt %.
Various Lead-Vanadium-Based Oxide Compositions
[0153] In an embodiment, the Pb--V--B--P--Bi--O composition which
may include Li.sub.2O and/or TiO.sub.2 may be a homogenous powder.
In a further embodiment, the Pb--V--B--P--Bi--O may be a
combination of more than one powder, wherein each powder may
separately be a homogenous population. The composition of the
overall combination of the multiple powders may be within the
ranges described above. For example, the Pb--V--B--P--Bi--O may
include a combination of two or more different powders; separately,
these powders may have different compositions, and may or may not
be within the ranges described above; however, the combination of
these powders may be within the ranges described above.
[0154] In an embodiment, some or all of the Li.sub.2O and/or
Na.sub.2O may be replaced with K.sub.2O, Cs.sub.2O, or Rb.sub.2O,
resulting in a glass composition with properties similar to the
compositions listed above. In this embodiment, the total alkali
metal oxide content may be 0 to 2 wt %, 0.1 to 2 wt %, or 0.2 to
1.1 wt %.
[0155] In a further embodiment, the Pb--V--B--P--Bi--O
composition(s) herein may include one or more of a third set of
components: GeO.sub.2, Ga.sub.2O.sub.3, In.sub.2O.sub.3, NiO, CoO,
ZnO, CaO, MgO, SrO, MnO, BaO, SeO.sub.2, MOO.sub.3, WO.sub.3,
Y.sub.2O.sub.3, As.sub.2O.sub.3, La.sub.2O.sub.3, Nd.sub.2O.sub.3,
Bi.sub.2O.sub.3, Ta.sub.2O.sub.5, V.sub.2O.sub.5, FeO, HfO.sub.2,
Cr.sub.2O.sub.3, CdO, Sb.sub.2O.sub.3, PbF.sub.2, ZrO.sub.2,
Mn.sub.2O.sub.5, P.sub.2O.sub.5, CuO, La.sub.2O.sub.3,
Pr.sub.2O.sub.3, Nd.sub.2O.sub.3, Gd.sub.2O.sub.3, Sm.sub.2O.sub.3,
Dy.sub.2O.sub.3, Eu.sub.2O.sub.3, Ho.sub.2O.sub.3, Yb.sub.2O.sub.3,
Lu.sub.2O.sub.3, CeO.sub.2, BiF.sub.3, SnO, SiO.sub.2, Ag.sub.2O,
Nb.sub.2O.sub.5, TiO.sub.2 and metal halides (e.g., NaCl, KBr, NaI,
LiF).
[0156] Therefore as used herein, the term "Pb--V--B--P--Bi--O" may
also include metal oxides that contain one or more elements
selected from the group consisting of Si, Sn, Li, Ti, Ag, Na, K,
Rb, Cs, Ge, Ga, In, Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, W, Y, As, La,
Nd, Co, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd,
Sb, Bi, F, Zr, Mn, P, Cu, Ce, and Nb.
[0157] In an embodiment, the Pb--V--B--P--Bi--Te--O composition
which may include Li.sub.2O and/or TiO.sub.2 may be a homogenous
powder. In a further embodiment, the Pb--V--B--P--Bi--Te--O may be
a combination of more than one powder, wherein each powder may
separately be a homogenous population. The composition of the
overall combination of the multiple powders may be within the
ranges described above. For example, the Pb--V--B--P--Bi--Te--O may
include a combination of two or more different powders; separately,
these powders may have different compositions, and may or may not
be within the ranges described above; however, the combination of
these powders may be within the ranges described above.
[0158] In an embodiment, some or all of the Li.sub.2O and/or
Na.sub.2O may be replaced with K.sub.2O, Cs.sub.2O, or Rb.sub.2O,
resulting in a glass composition with properties similar to the
compositions listed above. In this embodiment, the total alkali
metal oxide content may be 0 to 2 wt %, 0.1 to 2 wt %, or 0.2 to
1.1 wt %.
[0159] In a further embodiment, the Pb--V--B--P--Bi--Te--O
composition(s) herein may include one or more of a third set of
components: GeO.sub.2, Ga.sub.2O.sub.3, In.sub.2O.sub.3, NiO, CoO,
ZnO, CaO, MgO, SrO, MnO, BaO, SeO.sub.2, MoO.sub.3, WO.sub.3,
Y.sub.2O.sub.3, As.sub.2O.sub.3, La.sub.2O.sub.3, Nd.sub.2O.sub.3,
Bi.sub.2O.sub.3, Ta.sub.2O.sub.5. V.sub.2O.sub.5, FeO, HfO.sub.2,
Cr.sub.2O.sub.3, CdO, Sb.sub.2O.sub.3, PbF.sub.2, ZrO.sub.2,
Mn.sub.2O.sub.3, P.sub.2O.sub.5, CuO, La.sub.2O.sub.3,
Pr.sub.2O.sub.3, Nd.sub.2O.sub.3, Gd.sub.2O.sub.3, Sm.sub.2O.sub.3,
Dy.sub.2O.sub.3, Eu.sub.2O, Ho.sub.2O.sub.3, Yb.sub.2O.sub.3,
Lu.sub.2O.sub.3, CeO.sub.2, BiF.sub.3, SnO, SiO.sub.2, Ag.sub.2O,
Nb.sub.2O, TiO.sub.2 and metal halides (e.g., NaCl, KBr, NaI,
LiF).
[0160] Therefore as used herein, the term "Pb--V--B--P--Bi--Te--O"
may also include metal oxides that contain one or more elements
selected from the group consisting of Si, Sn, Li, Ti, Ag, Na, K,
Rb, Cs, Ge, Ga, In, Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, W, Y, As, La,
Nd, Co, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd,
Sb, Bi, F, Zr, Mn, P, Cu, Ce, and Nb.
[0161] In an embodiment, a composition Pb--V--B--P--Bi--Ti--Li--O
and a composition Pb--V--B--P--Bi--Ti--Li--Te--O maybe blended as
powders and may have different compositions, and may or may not be
within the ranges described above; however, the combination of
these powders may be within the ranges described above. In a
further embodiment, a composition such as
Pb--V--B--P--Bi--Ti--Li--O may be blended with constituent Te
inorganic compound, Te organic compound, Te metal, Te resinate,
multiple constituent Te compounds such Pb--Li--Te--O, Li--B--Te--O.
The compositions may or may not be within the ranges described
above; however, the combination of these powders may be within the
ranges described above. In an aspect of this embodiment, the
powders may be melted together to form a uniform composition. In a
further aspect of this embodiment, the powders may be added
separately to a thick-film composition.
[0162] In an embodiment, a composition free of particular
constituents such as a composition Pb--V--B--Ti--Li--O and a
composition Bi--V--P--Ti--Li--O having different constituents in
each when blended may be within the ranges described above.
Organic Medium
[0163] The inorganic components of the thick-film paste composition
are mixed with an organic medium to form viscous pastes having
suitable consistency and rheology for printing. A wide variety of
inert viscous materials can be used as the organic medium. The
organic medium can be one in which the inorganic components are
dispersible with an adequate degree of stability during
manufacturing, shipping, and storage of the pastes, as well as on
the printing screen during the screen-printing process.
[0164] Suitable organic media have rheological properties that
provide stable dispersion of solids, appropriate viscosity and
thixotropy for screen printing, appropriate wettability of the
substrate and the paste solids, a good drying rate, and good firing
properties. The organic medium can contain thickeners, stabilizers,
surfactants, and/or other common additives. The organic medium can
be a solution of polymer(s) in solvent(s). Suitable polymers
include ethyl cellulose, ethylhydroxyethyl cellulose, wood rosin,
cellulose esters, mixtures of ethyl cellulose and phenolic resins,
polymethacrylates of lower alcohols, and the monobutyl ether of
ethylene glycol monoacetate. Suitable solvents include terpenes
such as alpha- or beta-terpineol or mixtures thereof with other
solvents such as kerosene, dibutylphthalate, butyl carbitol, butyl
carbitol acetate, hexylene glycol and alcohols with boiling points
above 150.degree. C., and alcohol esters. Other suitable organic
medium components include: bis(2-(2-butoxyethoxy)ethyl adipate,
dibasic esters such as DBE, DBE-2, DBE-3, DBE-4, DBE-5, DBE-6,
DBE-9, and DBE 1B, octyl epoxy tallate, isotetradecanol, and
pentaerythritol ester of hydrogenated rosin. The organic medium can
also include volatile liquids to promote rapid hardening after
application of the thick-film paste composition on a substrate.
[0165] The optimal amount of organic medium in the thick-film paste
composition is dependent on the method of applying the paste and
the specific organic medium used. Typically, the thick-film paste
composition contains 70 to 95 wt % of inorganic components and 5 to
30 wt % of organic medium.
[0166] If the organic medium includes a polymer, the polymer may
include 8 to 15 wt % of the organic composition.
Preparation of the Thick-Film Paste Composition and its Use
[0167] In one embodiment, the thick-film paste composition can be
prepared by mixing the conductive metal powder, the Pb--V--O
powder, and the organic medium in any order. In an embodiment, the
thick-film paste composition may also include two powders of
Pb--V--Te--O or Pb--V--O and Pb--Te--O. In some embodiments, the
inorganic materials are mixed first, and they are then added to the
organic medium. The viscosity can be adjusted, if needed, by the
addition of one or more solvents. Mixing methods that provide high
shear may be useful. In an embodiment, the thick-film paste may
include lead-vanadium-based oxide in an amount of 0.5 to 15 wt %,
0.5 to 7 wt %, or 1 to 3% by weight based on the total weight of
the solids.
[0168] One aspect of the present invention is a process comprising:
[0169] (a) providing an article comprising one or more insulating
films disposed onto at least one surface of a semiconductor
substrate; [0170] (b) applying a thick-film paste composition onto
the one or more insulating films to form a layered structure, the
thick-film paste composition comprising: [0171] i) 80-99.5 wt % of
a source of electrically conductive metal; [0172] ii) 0.5 to 20 wt
% of a lead-vanadium-based oxide; and [0173] iii) an organic
medium, [0174] wherein the source of electrically conductive metal
and the lead-vanadium-based oxide are dispersed in the organic
medium and wherein the above wt % are based on the total weight of
the source of electrically conductive metal and the
lead-vanadium-based oxide, the lead-vanadium-based oxide comprising
52-80 wt % PbO, 10-45 wt % V.sub.2O.sub.5 and one or more
additional oxides with a liquidus temperature of 900.degree. C. or
less, said one or more additional oxides selected from the group
consisting of B.sub.2O.sub.3, P.sub.2O.sub.5, Bi.sub.2O.sub.3,
As.sub.2O.sub.3, Sb.sub.2O.sub.3, MoO.sub.3, TeO.sub.2, and
SeO.sub.2, said oxide wt % are based on the total weight of the
lead-vanadium-based oxide; and [0175] (c) firing the semiconductor
substrate, the one or more insulating films, and the thick-film
paste wherein the organic medium of the thick film paste is
volatilized, thereby forming an electrode in contact with the one
or more insulating layers and in electrical contact with the
semiconductor substrate.
[0176] In one embodiment, a semiconductor device is manufactured
from an article comprising a junction-bearing semiconductor
substrate and a silicon nitride insulating film formed on a main
surface thereof. The process includes the steps of applying (for
example, coating or screen-printing) onto the insulating film, in a
predetermined shape and thickness and at a predetermined position,
a thick-film paste composition having the ability to penetrate the
insulating layer, then firing so that thick-film paste composition
reacts with the insulating film and penetrates the insulating film,
thereby effecting electrical contact with the silicon
substrate.
[0177] One embodiment of this process is illustrated in FIG. 1.
[0178] FIG. 1(A) shows a mono-crystal silicon or multi-crystalline
silicon p-type substrate 10.
[0179] In FIG. 1(B), an n-type diffusion layer 20 of the reverse
polarity is formed to create a p-n junction. The n-type diffusion
layer 20 can be formed by ion implantation or thermal diffusion of
phosphorus (P) using phosphorus oxychloride (POCl.sub.3) as the
phosphorus source. In the absence of any particular modifications,
the n-type diffusion layer 20 is formed over the entire surface of
the silicon p-type substrate. The depth of the diffusion layer can
be varied by controlling the diffusion temperature and time, and is
generally formed in a thickness range of about 0.3 to 0.75 microns.
The n-type diffusion layer may have a sheet resistivity of several
tens of ohms per square up to about 120 ohms per square.
[0180] After protecting one surface of the n-type diffusion layer
20 with a resist or the like, as shown in FIG. 1(C), the n-type
diffusion layer 20 is removed from most surfaces by etching so that
it remains only on one main surface. The resist is then removed
using an organic solvent or the like.
[0181] Next, in FIG. 1(D), an insulating layer 30 which also
functions as an anti-reflection coating is formed on the n-type
diffusion layer 20. The insulating layer is commonly silicon
nitride, but can also be a SiN.sub.x:H film (i.e., the insulating
film includes hydrogen for passivation during subsequent firing
processing), a titanium oxide film, a silicon oxide film, or a
silicon oxide/titanium oxide film. A thickness of about 700 to 900
.ANG. of a silicon nitride film is suitable for a refractive index
of about 1.9 to 2.0. Deposition of the insulating layer 30 can be
by sputtering, chemical vapor deposition, or other methods.
[0182] Next, electrodes are formed. As shown in FIG. 1(E), a
thick-film paste composition of this invention is screen-printed on
the insulating film 30, and then dried. In addition, aluminum paste
60 and back-side silver paste 70 are screen-printed onto the back
side of the substrate, and successively dried. Firing is carried
out at a temperature of 750 to 850.degree. C. for a period of from
several seconds to several tens of minutes.
[0183] Consequently, as shown in FIG. 1(F), during firing, aluminum
diffuses from the aluminum paste into the silicon substrate on the
back side, thereby forming a p+ layer 40, containing a high
concentration of aluminum dopant. This layer is generally called
the back surface field (BSF) layer, and helps to improve the energy
conversion efficiency of the solar cell. Firing converts the dried
aluminum paste 60 to an aluminum back electrode 61. The back-side
silver paste 70 is fired at the same time, becoming a silver or
silver/aluminum back electrode 71. During firing, the boundary
between the back-side aluminum and the back-side silver assumes the
state of an alloy, thereby achieving electrical connection. Most
areas of the back electrode are occupied by the aluminum electrode,
owing in part to the need to form a p+ layer 40. At the same time,
because soldering to an aluminum electrode is impossible, the
silver or silver/aluminum back electrode is formed on limited areas
of the back side as an electrode for interconnecting solar cells by
means of copper ribbon or the like.
[0184] On the front side, the thick-film paste composition 500 of
the present invention sinters and penetrates through the insulating
film 30 during firing, and thereby achieves electrical contact with
the n-type diffusion layer 20. This type of process is generally
called "fire through." This fired-through state, i.e., the extent
to which the paste melts and passes through the insulating film 30,
depends on the quality and thickness of the insulating film 30, the
composition of the paste, and on the firing conditions. When fired,
the paste 500 becomes the electrode 501, as shown in FIG. 1(F).
[0185] In one embodiment, the insulating film is selected from
titanium oxide, aluminum oxide, silicon nitride, SiN.sub.x:H,
silicon oxide, and silicon oxide/titanium oxide films. The silicon
nitride film can be formed by sputtering, plasma-enhanced chemical
vapor deposition (PECVD), or a thermal CVD process. In one
embodiment, the silicon oxide film is formed by thermal oxidation,
sputtering, or thermal CVD or plasma CVD. The titanium oxide film
can be formed by coating a titanium-containing organic liquid
material onto the semiconductor substrate and firing, or by a
thermal CVD.
[0186] In one embodiment of this process, the semiconductor
substrate can be single-crystal or multi-crystalline silicon.
[0187] Suitable insulating films include one or more components
selected from: aluminum oxide, titanium oxide, silicon nitride,
SiN.sub.x:H, silicon oxide, and silicon oxide/titanium oxide. In
one embodiment of the invention, the insulating film is an
anti-reflection coating (ARC). The insulating film can be applied
to a semiconductor substrate, or it can be naturally forming, such
as in the case of silicon oxide.
[0188] In one embodiment, the insulating film includes a layer of
silicon nitride. The silicon nitride can be deposited by CVD
(chemical vapor deposition), PECVD (plasma-enhanced chemical vapor
deposition), sputtering, or other methods.
[0189] In one embodiment, the silicon nitride of the insulating
layer is treated to remove at least a portion of the silicon
nitride. The treatment can be a chemical treatment. The removal of
at least a portion of the silicon nitride may result in an improved
electrical contact between the conductor of the thick-film paste
composition and the semiconductor substrate. This may result in
improved efficiency of the semiconductor device.
[0190] In one embodiment, the silicon nitride of the insulating
film is part of an anti-reflective coating.
[0191] The thick-film paste composition can be printed on the
insulating film in a pattern, e.g., bus bars with connecting lines.
The printing can be by screen-printing, plating, extrusion, inkjet,
shaped or multiple printing, or ribbons.
[0192] In this electrode-forming process, the thick-film paste
composition is heated to remove the organic medium and sinter the
metal powder. The heating can be carried out in air or an
oxygen-containing atmosphere. This step is commonly referred to as
"firing." The firing temperature profile is typically set so as to
enable the burnout of organic binder materials from dried
thick-film paste composition, as well as any other organic
materials present. In one embodiment, the firing temperature is 750
to 950.degree. C. The firing can be conducted in a belt furnace
using high transport rates, for example, 100-500 cm/min, with
resulting hold-up times of 0.05 to 5 minutes. Multiple temperature
zones, for example 3 to 11 zones, can be used to control the
desired thermal profile.
[0193] Upon firing, the electrically conductive metal and Pb--V--O
mixture penetrate the insulating film. The penetration of the
insulating film results in an electrical contact between the
electrode and the semiconductor substrate. After firing, an
interlayer may be formed between the semiconductor substrate and
the electrode, wherein the interlayer includes one or more of
vanadium, vanadium compounds, lead, lead compounds, and silicon
compounds, where the silicon may originate from the silicon
substrate and/or the insulating layer(s). After firing, the
electrode includes sintered metal that contacts the underlying
semiconductor substrate and may also contact one or more insulating
layers. [0194] Another aspect of the present invention is a
semiconductor device comprising an electrode formed from a
thick-film paste composition comprising: [0195] i) 80-99.5 wt % of
a source of electrically conductive metal; [0196] ii) 0.5 to 20 wt
% of a lead-vanadium-based oxide; and [0197] iii) an organic
medium, [0198] wherein the source of electrically conductive metal
and the lead-vanadium-based oxide are dispersed in the organic
medium and wherein the above wt % are based on the total weight of
the source of electrically conductive metal and the
lead-vanadium-based oxide, the lead-vanadium-based oxide comprising
52-80 wt % PbO, 10-45 wt % V.sub.2O.sub.5 and one or more
additional oxides with a liquidus temperature of 900.degree. C. or
less, said one or more additional oxides selected from the group
consisting of B.sub.2O.sub.3, P.sub.2O.sub.5, Bi.sub.2O.sub.3,
As.sub.2O.sub.3, Sb.sub.2O.sub.3, MoO.sub.3, TeO.sub.2, and
SeO.sub.2, wherein said oxide wt % are based on the total weight of
the lead-vanadium-based oxide and wherein said thick film paste
composition has been fired to remove the organic medium and form
said electrode.
[0199] In one embodiment, the electrode is a front-side electrode
on a silicon solar cell. In one embodiment, the device further
includes a back electrode.
Lightly Doped Emitter (LDE) Wafers
[0200] Si solar cells are made by adding controlled impurities
(called dopants) to purified Si. Different dopants impart positive
(p-type) and negative (n-type) semiconducting properties to the Si.
The boundary (junction) between the p-type and n-type Si has an
associated (built in) voltage that provides power to electrical
charge carriers in the solar cell. Dopant concentration must be
controlled to achieve optimal cell performance. High dopant
concentration imparts low electrical resistivity within the Si and
at the Si surface (to metal contacts) decreasing resistance losses.
It also introduces crystalline defects or electrical perturbations
in the Si lattice that increase recombination losses.
[0201] The most common Si solar cell design consists of a 200
micron thick p-type Si wafer coated with a 0.4 micron layer n-type
Si. The p-type wafer is the base. The n-type layer is the emitter.
It is made by either diffusion or ion implantation of phosphorous
(P) dopant into the Si wafer. Emitters made with low dopant
concentration at the wafer surface are called lightly (or lowly)
doped emitters.
[0202] The lightly doped emitter (LDE) improves solar cell
performance by decreasing electron-hole recombination at the front
surface. The concentration of P dopant at the front surface
([P.sub.surface]) above .about.1.times.10.sup.20 atoms/cm.sup.3 in
Si leads to various types of recombination. Recombined charge
carriers are bound to the Si lattice and unable to be collected as
electrical energy. The solar cell energy loss results from a
decrease of both Voc (open circuit voltage) and Isc (short circuit
current).
[0203] Typical highly doped Si emitters (HDE) have total
[P.sub.surface] ranging from 9 to 15.times.10.sup.20 atoms/cm.sup.3
and active [P.sub.surface] ranging from 3 to 4.times.10.sup.20
atoms/cm.sup.3. Lightly doped emitters have total [P.sub.surface]
ranging from 0.9 to 2.9.times.10.sup.20 atoms/cm.sup.3 and active
[P.sub.surface] ranging from 0.6 to 2.0.times.10.sup.20
atoms/cm.sup.3. P dopant in excess of the active concentration
(Inactive P) leads to Shockley-Read-Hall (SRH) recombination energy
loss. Active P dopant above 1.times.10.sup.20 atoms/cm.sup.3 leads
to Auger recombination energy loss.
[0204] Total dopant concentration is typically measured using the
SIMS (secondary ion mass spectrometry) depth profiling method.
[Diffusion in Silicon, S. W. Jones, IC Knowledge LLC 2008 pages
56-62, see page 61]. Active dopant concentration is often measured
using SRP (spreading resistance probing) [Diffusion in Silicon, op.
cit., page 61] or ECV (electrochemical capacitance voltage
[Diffusion in Silicon, op. cit., page 57] methods.
[0205] Metal contacts to lightly doped emitters have larger energy
barriers to charge carrier tunneling than contacts to highly doped
emitters. The larger barriers decrease tunneling current and
increase contact resistivity. The high contact resistance to LDE is
decreased by increasing contact area with improved interfacial film
nanostructure.
EXAMPLES
[0206] Illustrative preparations and evaluations of thick-film
paste compositions are described below.
Lead-Vanadium-Based Oxide Preparation of Glasses 1-88 of Table
1
[0207] Lead-vanadium-based oxide (Pb--V--O) compositions of the
invention were prepared by mixing and blending Pb.sub.3O.sub.4 and
V.sub.2O.sub.5, and various oxides as shown in Table 1, TeO.sub.2,
Nb.sub.2O.sub.5, Sb.sub.2O.sub.3, Fe.sub.2O.sub.3, WO.sub.3,
MoO.sub.3, P.sub.2O.sub.5, PbF.sub.2, SiO.sub.2, BiF.sub.3,
SnO.sub.2, Li.sub.2O, Bi.sub.2O.sub.3, ZnO, Na.sub.2O, TiO.sub.2,
ZrO.sub.2, or B.sub.2O.sub.3. The blended powder batch materials
were loaded into a platinum alloy crucible and then inserted into a
furnace at 900-1000.degree. C. using an air- or O.sub.2-containing
atmosphere. The duration of the heat treatment was 20 minutes
following the attainment of a full solution of the constituents.
The resulting low viscosity liquid resulting from the fusion of the
constituents was then quenched by metal roller. The quenched glass
was then milled, and screened to provide a powder with a D.sub.50
of 0.1 to 3.0 microns. The compositions in Table 1 are displayed as
weight percent, based on the weight of the total glass
composition.
TABLE-US-00001 TABLE 1 Glass frit compositions in weight percent
Pb-V-O com- posi- tion PbO ZrO.sub.2 B.sub.2O.sub.3 Na.sub.2O
Li.sub.2O Bi.sub.2O.sub.3 P.sub.2O.sub.5 WO.sub.3 Fe.sub.2O.sub.3
TiO.sub.2 V.sub.2O.sub.5 Sb.sub.2O.sub.3 TeO.sub.2 1 77.21 2.29
8.01 2.25 10.24 2 70.11 1.34 4.68 3.29 20.58 3 69.23 1.54 1.45
27.79 4 68.23 0.71 3.13 0.54 27.39 5 68.03 0.83 0.71 3.12 27.31 6
67.58 3.53 1.74 27.15 7 64.93 2.17 1.92 1.77 1.99 27.21 8 64.66
3.18 0.34 2.16 1.22 28.43 9 64.29 1.85 0.57 2.69 3.03 27.58 10
63.93 2.28 1.47 3.48 1.31 27.54 11 63.85 5.14 1.86 29.14 12 63.79
1.79 4.68 1.76 27.99 13 63.76 1.65 5.77 3.42 25.39 14 62.62 3.19
3.79 1.83 28.58 15 62.37 1.95 0.83 6.80 1.91 26.14 16 62.30 2.13
7.44 28.13 17 61.92 1.61 0.42 6.61 1.51 0.57 27.36 18 61.77 1.83
6.41 1.80 28.19 19 61.71 1.82 4.77 1.79 29.90 20 60.81 2.53 1.09
12.69 1.74 21.14 21 60.74 3.67 1.82 6.35 27.43 22 60.41 1.88 1.04
5.41 3.29 1.24 26.73 23 60.03 1.80 5.23 1.77 25.80 5.37 24 59.97
1.93 0.55 6.26 1.38 1.77 28.14 25 59.89 1.25 1.07 12.50 1.78 1.72
21.80 26 59.72 1.74 0.43 8.34 1.52 1.14 27.10 27 59.31 1.56 0.41
6.41 1.71 0.82 21.50 8.28 28 59.23 1.77 0.54 8.47 1.86 1.68 26.45
29 57.96 1.68 0.46 0.55 8.64 2.11 0.59 28.00 30 57.72 1.80 0.22
6.89 4.72 1.77 26.88 31 57.63 2.40 12.03 1.65 26.30 32 57.46 1.78
0.65 10.20 1.55 28.35 33 56.57 1.80 0.66 10.31 1.57 1.18 27.91 34
56.22 2.35 0.66 10.24 1.60 1.20 27.74 35 56.18 1.80 0.72 11.21 3.15
26.93 36 55.59 1.54 0.46 0.55 8.58 1.57 0.72 27.46 3.53 37 55.48
1.15 0.41 0.49 7.68 0.47 16.22 18.11 38 55.33 1.76 0.65 10.08 1.54
3.34 27.30 39 55.28 1.50 0.45 0.43 10.07 1.53 27.28 3.45 40 55.24
1.70 0.42 14.67 1.49 26.48 41 55.00 1.60 0.33 7.35 3.15 1.18 25.77
5.62 42 54.42 1.75 0.43 13.37 1.53 1.32 27.19 43 53.18 1.71 0.31
16.33 2.98 25.49 44 56.45 1.79 0.44 6.83 3.64 1.17 26.00 4.68 45
56.46 1.76 0.54 8.42 3.59 1.44 22.02 5.77 46 56.89 1.77 0.22 6.79
5.17 1.16 24.51 3.49 47 60.01 1.78 6.75 1.75 29.71 48 55.42 1.64
7.65 1.62 33.67 49 56.09 6.42 37.48 50 67.65 4.74 27.61 51 65.13
4.89 26.59 52 69.32 2.38 28.30 53 66.45 1.67 4.76 27.12 54 69.02
0.83 27.70 55 64.63 4.20 4.80 26.38 56 66.25 4.30 2.41 27.04 57
61.08 1.56 0.42 9.78 0.99 4.47 1.68 19.99 58 55.68 1.65 0.30 7.24
3.54 1.33 26.05 4.22 59 56.36 1.70 0.27 7.12 3.93 1.48 26.32 2.81
60 56.70 1.73 0.26 7.06 4.13 1.55 26.46 2.11 61 59.24 0.90 0.86
28.45 10.56 62 54.21 1.62 44.17 63 69.76 1.81 28.43 64 59.56 7.76
3.78 27.32 65 59.23 0.53 8.19 4.89 27.16 66 60.02 1.02 2.29 36.68
67 62.00 2.00 8.00 28.00 68 66.86 11.34 21.80 69 56.66 23.09 20.26
70 57.20 15.54 27.27 71 64.29 2.06 0.22 5.90 27.54 72 66.01 1.72
2.81 29.46 73 61.08 1.62 0.43 6.69 2.04 1.15 26.99 74 52.11 26.28
13.83 75 52.79 29.89 3.31 14.02 76 54.58 1.22 1.95 42.25 77 54.24
1.17 1.96 36.93 78 52.42 1.13 8.87 1.89 35.69 79 41.72 47.42 80
57.16 1.78 0.44 10.23 2.60 4.09 1.75 21.96 81 57.31 1.53 0.22 8.55
4.69 5.27 1.76 20.66 82 57.94 1.81 0.22 6.91 4.74 1.78 24.69 83
56.49 1.64 7.58 5.13 1.16 26.30 1.15 84 61.05 0.97 0.42 9.78 3.48
6.70 1.12 16.48 85 56.25 0.97 0.42 14.64 3.47 6.69 1.12 16.45 86
60.86 0.97 0.42 9.75 3.47 4.46 1.11 18.97 87 60.67 0.97 0.42 9.72
3.45 2.22 1.11 21.44 88 57.02 1.78 0.44 10.20 2.59 2.33 1.75 23.90
Pb-V-O com- posi- tion SiO.sub.2 ZnO Nb.sub.2O.sub.5 SnO MoO.sub.3
PbF.sub.2 Cr.sub.2O.sub.3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
40 41 42 43 44 45 46 47 48 49 50 51 3.39 52 53 54 2.45 55 56 57 58
59 60 61 62 63 64 1.59 65 66 67 68 69 70 71 72 73 74 7.78 75 76 77
5.70 78 79 10.86 80 81 82 1.92 83 0.55 84 85 86 87 88
Paste Preparation for Table 3a-3c
[0208] Pastes of the invention were prepared using the following
procedure. The appropriate amount of solvent, medium, and
surfactant from Table 2 were weighed and put into a plastic jar.
Then a stirring bar was placed in a jar and the mixture was stirred
for 1 hr until all ingredients were well blended.
TABLE-US-00002 TABLE 2 Component Wt. %
2,2,4-trimethyl-1,3-pentanediol monoisobutyrate 5.57 Ethyl
Cellulose (50-52% ethoxyl) 0.14 Ethyl Cellulose (48-50% ethoxyl)
0.04 N-tallow-1,3-diaminopropane dioleate 1.00 Hydrogenated castor
oil 0.50 Pentaerythritol tetraester of perhydroabietic acid 1.25
Dimethyl adipate 3.15 Dimethyl glutarate 0.35
[0209] 1.2-3 wt. % glass frit of a glass from Table 1 was added to
the organic mixture of Table 2 and 87.35-89.35 wt. % Ag powder was
added incrementally to ensure good wetting. After all ingredients
were well mixed by hand or table mixer, the paste was put into
Thinky mixer for 1-3 min at 2000 rpm. The paste was then repeatedly
passed through a 3-roll mill at progressively increasing pressures
from 0 to 250 psi. The gap of the rolls was set to 2 mils. The
degree of dispersion was measured by fineness of grind (FOG). A
typical FOG value for a paste was less than 10 microns for the
fourth longest, continuous scratch and less than 5 microns for the
point at which 50% of the paste was scratched.
[0210] The paste viscosity was measured using a Brookfield
viscometer (Brookfield, Inc., Middleboro, Mass.) with a #14 spindle
and a #6 cup. The viscosity of the paste was measured after 12-24
hrs at room temperature. Viscosity was measured after 3 minutes at
10 RPM in a viscometer and the paste viscosity was adjusted to
between 270-340 Pas by adding solvent and medium and then mixing
for 1-2 minute at 2000 RPM. This step was repeated until the
desired viscosity was achieved.
Solar Cell Preparation of the Examples in Table 3a-3c
[0211] Solar cells for testing the performance of the thick-film
pastes of the invention shown in Table 3a were made from 200 micron
multi-crystalline silicon wafers with a 65-68.OMEGA./.quadrature.
phosphorous-doped p-type emitter layer. Those multi-crystalline
wafers were obtained from Gintech Energy Corporation, Taiwan or
DeutscheCell, Germany. The solar cells used were textured by
isotropic acid etching and had an anti-reflection coating (ARC) of
SiN.sub.X:H. Wafers of 6 inch square were cut down to 3 inch square
using a laser scribing tool from OpTek System, UK. Efficiency and
fill factor were measured for each sample.
[0212] Solar cells for testing the performance of the thick-film
pastes of the invention shown in Table 3b were made from 200 micron
mono-crystalline silicon wafers with a 70.OMEGA./.quadrature. or
80.OMEGA./.quadrature. phosphorous-doped p-type emitter layer.
Those mono-crystalline wafers were obtained from Gintech Energy
Corporation, Taiwan. The solar cells used were textured by
pyramidal acid etching and had an anti-reflection coating (ARC) of
SiN.sub.X:H. Wafers of 6 inch pseudo-square were cut down to 2.5
inch square.
[0213] Solar cells for testing the performance of the thick-film
pastes of the invention shown in Table 3c were made from 180 micron
mono-crystalline silicon wafers with a 70-75.OMEGA./.quadrature.
phosphorous-doped p-type lightly doped emitter (LDE) layer. Those
mono-crystalline wafers were obtained from DuPont Innovalight,
Sunnyvale, Calif. The solar cells used were textured by pyramidal
acid etching and had an anti-reflection coating (ARC) of
SiN.sub.X:H. Wafers of 6 inch square were cut down to 2.5 inch
square.
[0214] For the Examples in Tables 3a-3c, each sample was made by
screen-printing using an AMI Presco MSP-885 printer set with a
squeegee speed of 100-150 mm/sec. The screen used for 3 inch square
had a pattern of 31 finger lines with a 70 or 80 .mu.m opening. The
screen used for 2.5 inch square had a pattern of 26 finger lines
with an 80 .mu.m opening or 29 finger lines with a 70 .mu.m
opening. All patterns had a single bus bar with a 1.5 mm opening on
a 15 .mu.m or a 20 .mu.m emulsion in a screen with 325 mesh and 23
.mu.m wires. An experimental Al paste was printed on the
non-illuminated (back) side of the device.
[0215] After each side was printed, the device with the printed
patterns was dried for 10-15 minutes in a drying oven with a
150.degree. C. peak temperature. The substrates were then fired
front up with a CF7214 Despatch 6-zone IR furnace using a 560
cm/min belt speed and the first five zones were set to
500-550-610-700-800 or 550-600-660-750-850 and the sixth zone was
set to the temperatures indicated in Table 3a-3c. The actual
temperature of the part was measured during processing. The
estimated peak temperature of each part was 770-830C and each part
was above 650.degree. C. for a total time of 4-6 seconds. The fully
processed samples were then tested for PV performance using a
calibrated Berger I-V tester.
Test Procedure: Efficiency for Table 3
[0216] The solar cells built according to the method described
above were tested for conversion efficiency. A solar cell was
placed in a commercial I-V tester for measuring efficiencies
(BERGER Lichttechnik GmbH & Co. KG, a manual cell testing
system comprised with PCC1011, PSL SCD, PT100 and work station with
manual probing unit,). The Xe arc lamp in the I-V tester simulated
the sunlight with a known intensity, AM 1.5, and irradiated the
front surface of the cell. The tester used a multi-point contact
method to measure current (I) and voltage (V) to determine the
cell's I-V curve. Both fill factor (FF), efficiency (Eff) and
series resistance (Rs) (data not shown for Rs) were calculated from
the L-V curve. Ideality factor was determined using the Suns-VOC
technique (Suns-VOC data not shown). Efficiency and fill factor
were measured for each sample and the results shown in Table 3a-3c.
For each paste, the mean values of the efficiency and fill factor
for 6 to 12 samples are shown.
TABLE-US-00003 TABLE 3a Eff % and FF of paste using select glass
frits of Table 1 on Multi-crystalline solar cells. Mean Mean
Efficiency (%) FF (%) Paste Glass Frit 915 930 930* 945 945* 915
930 930* 945 945* 139A 50 2% 14.69 14.31 68.33 65.03 143A 52 2%
15.69 16.23 16.63 16.38 73.23 73.15 74.94 75.62 139B 53 2% 14.78
15.95 67.51 73.00 143D 79 2% 10.97 9.59 9.02 11.29 63.71 47.38
41.95 52.33 139E 62 2% 11.87 12.71 53.24 57.05 147F 17 2% 16.97
16.8 17.24 76.38 76.55 77.88 143F 17 2% 16.16 16.46 16.58 16.48
74.17 75.84 76.81 77.29 143B 63 2% 15.69 16.23 16.63 16.37 73.75
73.02 74.82 75.81 143G 64 2% 10.72 14.70 16.31 16.67 51.01 73.05
74.78 76.44 139C 18 2% 16.74 16.78 77.17 77.18 139D 67 2% 16.25
16.75 74.01 76.73 139F 68 2% 12.33 14.77 57.74 67.93 143C 15 2%
13.93 15.46 16.52 16.94 70.67 69.12 75.74 77.34 147A 12 2% 16.27
16.42 17.18 73.68 74.22 77.75 147B 30 2% 16.467 16.67 16.89 75.8
76.42 77.12 147D 33 2% 16.69 17.35 17.02 76.97 77.68 77.53 147E 22
2% 16.62 17.05 17.06 76.26 77.37 77.77 153A 23 2% 11.35 12.24 13.03
12.83 51.98 62.54 60.34 62.52 153B 38 2% 13.73 16.43 14.93 16.18
63.3 75.76 69.28 78.98 153C 51 2% 7.87 8.92 7.89 9.77 37.82 41.61
37.38 45.85 153D 54 2% 12.42 15.67 13.39 13.25 57.42 72.10 61.94
62.53 153E 57 2% 16.71 16.54 16.68 16.85 76.21 76.23 76.03
77.04
TABLE-US-00004 TABLE 3b Eff % and FF of paste using select glass
frits of Table 1 on Mono-crystalline solar cells. Mean Mean
Efficiency (%) FF (%) Paste Glass Frit 915 930 945 945* 915 930 945
945* 147B 30 2% 18.56 18.61 78.81 79.12 147D 33 2% 18.57 18.6 79.15
79.38 147D1 33 1.60% 18.66 18.91 77.51 78.4 147E 22 18.3 18.51
77.91 79.14 147F 17 2% 18.31 18.56 78.44 79.29 147F1 17 1.60% 18.57
18.66 77.2 78.36 150BC1 41 2.00% 17.49 18.04 18.3 18.69 72.95 75.41
76.25 77.06 150BC2 58 2.00% 18.26 18.54 18.48 18.59 75.4 76.34 76.6
76.45 150D1 33 1.20% 18.16 18.81 75.39 77.8 150E.1 22 1.20% 17.26
18.84 71.32 78.08 150F1 17 1.20% 18.01 18.85 74.85 78.29
TABLE-US-00005 TABLE 3c Eff % and FF of paste using select glass
frits of Table 1 on Mono-crystalline lightly doped emitter (LDE)
solar cells. Mean Mean Efficiency (%) FF (%) Paste Glass Frit 930
945 945* 960 930 945 945* 960 147B 30 2% 18.3 18.48 77.13 77.69
147D 33 2% 17.68 18.54 74.72 78.36 147E 22 2% 18.26 18.56 76.92
78.3 147F 17 2% 17.83 18.25 75.13 77.15 147D1 33 1.6% 17.96 18.62
73.17 76.49 147F1 17 1.6% 17.08 18.5 69.8 75.93 149B 35 2% 10.32
13.57 15.88 44.81 55.22 65.08 149C 43 2% 16.45 17.42 18.39 66.22
69.91 74.24 149D 28 2% 12.49 15.13 17.71 51.22 61.31 72.5 139E 39
2% 13.73 11.37 10.83 81.77 64.44 45.22 149F 29 2% 10.26 13.39 14.49
44.52 60.19 61.98 150BC1 41 2% 17.37 18.57 71.54 76.25 150D1 33
1.2% 16.47 17.84 67.7 73.36 150E.1 22 1.2% 15 15.85 61.43 65.7
150F1 17 1.2% 13.84 13.73 56.98 58.46 152 30 2% 18 18.24 18.41 73.5
74.59 75.05 152A 58 2% 18.44 18.55 18.59 18.49 75.62 76.01 76.13
74.18 152B 59 2% 18.5 18.61 18.66 18.08 75.6 76.3 76.4 74.18 152E
60 2% 17.5 18.27 18.45 18.51 71.46 74.76 75.97 75.96
Test Procedure: Adhesion Test for Table 4
[0217] An adhesion test was carried out using semi-automated
soldering equipment, Semtek SCB-160. The sample was loaded over a
hot stage where its temperature is pre-set for 180.degree. C. An
array of heated pins which press loaded ribbon over the busbar were
pre-set for their temperature at 180.degree. C. The solder ribbon
consisted of a 0.127 mm thick and 2 mm wide copper core layer which
was coated with 16-20 .mu.m solder layer, 62Sn/36Pb/2Ag solder
alloy. Solder ribbon was supplied from the spool and dipped into
Kester 952S flux before it was attached onto the busbar in the hot
plate. When solder ribbon was bonded onto the busbar and cooled to
room temperature, the solder ribbon was pulled in a 180.degree.
direction in a MOGRL pull tester. Adhesion peel strength in Newton
(N) was recorded when the solder ribbon was pulled off the
underlying busbar of each sample with a speed of 120 mm/min. The
results are shown in Table 4.
TABLE-US-00006 TABLE 4 Busbar adhesion pull test result of paste
using select glass frits of Table 1 Mean Paste Glass Frit Wafer
Size Adhesion (N) 139A 50 2% Multi 3'' .times. 3'' 0.15 139B 53 2%
Multi 3'' .times. 3'' 0.16 139C 18 2% Multi 3'' .times. 3'' 1.70
139D 67 2% Multi 3'' .times. 3'' 1.17 139E 62 2% Multi 3'' .times.
3'' 0.14 139F 68 2% Multi 3'' .times. 3'' 0.14 143A 52 2% Multi 3''
.times. 3'' 0.44 143B 63 2% Multi 3'' .times. 3'' 0.47 143C 15 2%
Multi 3'' .times. 3'' 0.84 143F 17 2% Multi 3'' .times. 3'' 1.94
143G 64 2% Multi 3'' .times. 3'' 0.91 147A 12 2% Multi 3'' .times.
3'' 0.48 147B 30 2% Multi 3'' .times. 3'' 2.43 147D 33 2% Multi 3''
.times. 3'' 2.49 147E 22 2% Multi 3'' .times. 3'' 2.62 147F 17 2%
Multi 3'' .times. 3'' 1.94 147D1 33 1.6%.sup. Mono 2.5'' .times.
2.5'' 3.39 147F1 17 1.6%.sup. Mono 2.5'' .times. 2.5'' 1.36 150BC1
41 2% Mono 2.5'' .times. 2.5'' 1.84 150BC2 58 2% Mono 2.5'' .times.
2.5'' 2.19 147B 30 2% Mono-LDE 2.5'' .times. 2.5'' 2.54 147D 33 2%
Mono-LDE 2.5'' .times. 2.5'' 2.51 147E 22 2% Mono-LDE 2.5'' .times.
2.5'' 2.86 147F 17 2% Mono-LDE 2.5'' .times. 2.5'' 2.53 152 30 2%
Mono-LDE 2.5'' .times. 2.5'' 1.99 152A 58 2% Mono-LDE 2.5'' .times.
2.5'' 2.19 152B 59 2% Mono-LDE 2.5'' .times. 2.5'' 2.15 152E 60 2%
Mono-LDE 2.5'' .times. 2.5'' 2.34
Comparative Experiments
Comparative Experiment A, B and C
[0218] Comparative Experiments A, B and C demonstrate the decrease
in photovoltaic (PV) cell performance as the frit PbO concentration
is decreased. The compositions of the glass used in Comparative
Experiments A, B and C are shown in Table 5.
TABLE-US-00007 TABLE 5 Comparative Experiment Glasses - Wt %
Pb--V--O composition # PbO B.sub.2O.sub.3 P.sub.2O.sub.5 TiO.sub.2
V.sub.2O.sub.5 A 46.02 1.37 9.49 1.34 41.77 B 36.34 1.08 11.39 1.06
50.12 C 5.44 0.16 17.45 0.16 76.79
Paste Preparation
[0219] Pastes were prepared for the Comparative Experiment glasses
A, B and C and for glasses 47 and 48 of Table 1 using the following
procedure: The appropriate amount of solvent, medium, and
surfactant from Table 6 were weighed, poured into a plastic jar and
mixed with a stir bar for about 1 hour until all ingredients were
well blended.
TABLE-US-00008 TABLE 6 Component Wt. %
2,2,4-trimethyl-1,3-pentanediol monoisobutyrate 5.57 Ethyl
Cellulose (50-52% ethoxyl) 0.14 Ethyl Cellulose (48-50% ethoxyl)
0.04 N-tallow-1,3-diaminopropane dioleate 1.00 Hydrogenated castor
oil 0.50 Pentaerythritol tetraester of perhydroabietic acid 1.25
Dimethyl adipate 3.15 Dimethyl glutarate 0.35
[0220] 2 wt. % glass frit was added to the organic mixture in Table
6 and 87.35-88.35 wt. % Ag powder was added incrementally to ensure
good wetting. After all ingredients were well mixed by hand or
table mixer, the paste was mixed twice in a Thinky mixer
(Thinky.RTM. USA, Inc.) for 25 seconds at 2000 rpm. The paste was
then consecutively passed through a roll mill three times for each
value of pressures 0 psi, 100 psi, and 150 psi. The gap of the
rolls was set to 2 mils. The degree of dispersion was measured by
fineness of grind (FOG). A typical FOG value of 3/2 was
measured.
[0221] The paste viscosity was measured using a Brookfield
viscometer (Brookfield, Inc., Middleboro, Mass.) with a #14 spindle
and a #6 cup. The viscosity of the paste was measured after the
paste stayed for 12-24 hrs at room temperature. Viscosity was
measured after 3 minutes at 10 RPM in a viscometer and the paste
viscosity was adjusted to between 270-340 Pas by adding solvent and
medium from Table 6 and then mixing for 1-2 minute at 2000 RPM.
This step was repeated until the desired viscosity was
achieved.
Solar Cell Preparation
[0222] The pastes for the Comparative Experiments A, B and C and
those of glasses 47 and 48 of Table 1, were applied to
1''.times.1'' multi-crystalline or mono-crystalline silicon solar
cells with a phosphorous-doped emitter on a p-type base creating a
62-68.OMEGA./.quadrature. emitter. The solar cells used were
textured by isotropic acid etching and had an anti-reflection
coating (ARC) of SiN.sub.X:H. Each sample was made by
screen-printing using an AMI Presco MSP-885 printer set with a
squeegee speed of 100-150 mm/sec. The screen used had a pattern of
11 finger lines with an 80 .mu.m opening and 1 bus bar with a 0.89
mm opening in a screen with 290 mesh and 20 .mu.m wires. An
experimental Al paste was printed on the non-illuminated (back)
side of the device.
[0223] The device with the printed patterns was dried for 10-15
minutes in a drying oven with a 150.degree. C. peak temperature
after each side is printed. The substrates were then fired front up
with a CF7214 Despatch 6-zone IR furnace using a 560 cm/min belt
speed and the first five zones were set to 500-550-610-700-800 and
the sixth zone was set to the temperatures indicated in table 7.
The actual temperature of the part was measured during processing.
The estimated peak temperature of each part was 770-830.degree. C.
and each part was above 650.degree. C. for a total time of 4-5
seconds.
Test Procedure
[0224] Efficiency and fill factor were measured for each of the
solar cells. The method of testing is provided below.
[0225] Solar cells built according to the method described above
were placed in a commercial I-V tester for measuring efficiencies
(ST1000). The Xe arc lamp in the I-V tester simulated the sunlight
with a known intensity and irradiated the front surface of the
cell. The tester used a multi-point contact method to measure
current (I) and voltage (V) to determine the cell's I-V curve. Both
fill factor (FF), efficiency (Eff) and series resistance (Rs) were
calculated from the I-V curve. For each paste, the mean values of
the efficiency, the fill factor and the series resistance for 5 to
10 samples are shown in Table 7.
The performance of the frits of Comparative Examples A, B and C can
be compared to those of glass 47 and 48 which provide Examples of
the invention. The results for glasses 47 and 48 show considerably
higher efficiencies and form factors and lower series resistance
than the frits of the Comparative Experiments. Glasses 47 and 48
have PbO content of 56.89 wt % and 60.01 wt %, respectively, while
those of Comparative Experiments A, B and C were 46.02 wt %, 36.34
wt % and 5.44 wt % respectively, demonstrating the decrease in
photovoltaic (PV) cell performance as the frit PbO concentration is
decreased.
TABLE-US-00009 TABLE 7 Eff % and FF of paste using glass frits of
Tables 1 and 5 Pb--V--O Compo- Frit Eff FF Rs siton # w % 945C 945C
945C Note 47 2 14.276 70.8533 0.289827 1'' Gintech Poly Rsheet = 68
Ohms/square 48 2 13.8542 67.8417 0.394525 1'' Gintech Poly Rsheet =
68 Ohms/square A 2 11.3007 56.8933 0.70924 1'' Gintech Poly Rsheet
= 68 Ohms/square B 2 10.3833 51.9667 0.846713 1'' Gintech Poly
Rsheet = 68 Ohms/square C 2 6.34133 34.4467 1.72539 1'' Gintech
Poly Rsheet = 68 Ohms/square
Comparative Experiments D and E
[0226] Comparative Experiments D and E further demonstrate that
frit PbO concentrations in the frit of the paste must be 52 wt % or
higher to achieve fire through the one or more insulating films of
silicon nitride on a solar cell.
[0227] A lead-phosphorus-oxide sample for use as Comparative
Experiment D was made according to the melting and quenching
procedure described for Table 1 using 14.63 wt % P.sub.2O.sub.5 and
85.37 wt % V.sub.2O. The quenched flake was dry milled in a ball
mill to a D.sub.50=1.2 microns. Attempts to mill this composition
in water or isopropyl alcohol resulted in a gel that could not be
reduced to powder.
[0228] The lead-phosphorus-oxide powder was mixed 1 to 1 by weight
with a Si.sub.3N.sub.4 powder (Sigma Aldrich
Si.sub.3N.sub.4#334103, 99.9+% purity, submicron). The mixture was
placed in an alumina crucible and heated in a simultaneous
differential thermal analyzer (DTA), thermogravometric analyzer
(TGA) (TA Instruments Model 2960, Wilmington Del.). The weight
change was recorded by the TGA during heating at 10.degree. C./min.
This experiment assesses the lead-phosphorus-oxide reactivity with
silicon nitride and is used as a means to exemplify the ability of
the lead-phosphorus to "fire-through" the silicon nitride ARC of a
solar cell according the equation below.
##STR00001##
[0229] The oxide powder assists the oxidation of the silicon
nitride at temperatures practical to solar cell firing
(<800.degree. C.). The conversion of 1 to 1 by weight mixture of
V--P--O and Si.sub.3N.sub.4 to V--P--O+SiO.sub.2 theoretically
results in a weight gain of 14% by weight if all the
Si.sub.3N.sub.4 is oxidized. Only when there is at least a 10%
weight gain at less than 800.degree. C. is there sufficient
reactivity with nitride to form a "fire through" contact and
realize practical efficiency, fill factor and series
resistance.
[0230] The results of the measurements are shown in Table 8. The
V--P--O:Si3N4 Comparative Experiment D weight gain onset was
recorded as 535.degree. C. The gain was only 9% weight by
1050.degree. C. Similar measurements were made with a glass
comprising 35.38 wt % PbO, 55.17 wt % V.sub.2O.sub.5 and 9.45 wt %
P.sub.2O.sub.5. (Comparative Experiment E) and glasses 49 and 3
from Table 1. Comparative Experiment E weight gain onset was
recorded as 535.degree. C. 10% wt gain was not achieved until
815.degree. C. In contrast powders of glasses 49 and 3 from Table 1
started to gain weight at <475.degree. C. and gained 10% weight
by 734.degree. C. and 775.degree. C., respectively. Pastes made
with the glasses of Comparative Experiment E and glasses 49 and 3
following essentially the description relating to Table 3 were
screen printed onto silicon substrates containing an antireflection
layer essentially as described in connection with Table 3. The
paste of Comparative Experiment E showed no contact with the
silicon substrate, i.e., no fire through. The paste of glass 49
showed fire through and a cell efficiency of 12.49%, a fill factor
of 59.1% and a series resistance of 70.0 mOhms. The paste of glass
3 showed fire through and a cell efficiency of 16.52%, a fill
factor of 76.5% and a series resistance of 16.1 mOhms. These
Comparative Experiments indicate that the lead-vanadium based oxide
must comprise greater than 52 wt % PbO as part of the composition
to achieve contact through one or more a insulating films
comprising silicon nitride to a solar cell emitter.
TABLE-US-00010 TABLE 8 Comparison of Composition Influence on
Performance Potential wt max T at T at wt wt PbO P2O5 V2O5 B2O3
gain on- wt off- 10% 14% gain % gain at glass # wt % wt % wt % wt %
start set rise set gain gain at 990.degree. C. max T Compara- 14.63
85.37 536 658 798 903 n/o n/o 8.6 9.1% tive @ Exper. D 1088.degree.
C. Compara- 35.38 9.45 55.17 425 596 709 781 815 n/o 11.1 11.8%
tive @ Exper. E 1088.degree. C. Table1 56.09 6.42 37.48 373 570 659
736 734 1073 12.7 12.6% Compo- @ sition #49 990.degree. C. Table1
69.23 1.45 27.79 1.54 474 686 726 768 775 n/o 12.6 12.6% Compo- @
sition #3 990.degree. C.
Comparative Experiments F and G
Preparation of Lead-Vanadium Oxide Glasses
[0231] Two mixtures of V.sub.2O.sub.5 powder (99+% purity) and PbO
powder were tumbled in a suitable container for 15 to 30 minutes to
mix the starting powders. The first--Comparative Experiment
F--contained 70.98 wt % PbO and 29.02 wt % V.sub.2O.sub.5. The
second--Comparative Experiment G--contained 60.1 wt % PbO and 39.9
wt % V.sub.2O.sub.5. Each starting powder mixture was placed in a
platinum crucible and heated in air at a heating rate of 10.degree.
C./min to 900.degree. C. and then held at 900.degree. C. for one
hour to melt the mixture. The melt was quenched from 900.degree. C.
by removing the platinum crucible from the furnace and pouring the
melt onto a stainless steel platen. The resulting material was
ground in a mortar and pestle to less than 100 mesh. The ground
material was then ball-milled in a polyethylene container with
zirconia balls and isopropyl alcohol until the D.sub.50 was 0.5-0.7
microns. The ball-milled material was then separated from the
milling balls, dried, and run through a 230 mesh screen to provide
the flux powders used in the thick-film paste preparations.
Thick-Film Paste Preparation of Comparative Experiments F and G
[0232] The organic components of the thick-film paste and the
relative amounts are given in Table 9.
TABLE-US-00011 TABLE 9 Organic components of the thick-film paste
Component wt % 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate 3.70
Ethyl Cellulose (50-52% ethoxyl) 0.18 Ethyl Cellulose (48-50%
ethoxyl) 0.05 N-tallow-1,3-diaminopropane dioleate 1.29
Hydrogenated castor oil 0.65 Pentaerythritol tetraester of
perhydroabietic acid 1.61 Dimethyl adipate 4.06 Dimethyl glutarate
0.45
[0233] The organic components (.about.4.65 g) were put into a
plastic jar. Then a stirring bar was placed in a jar and mixture
was stirred for 1 hr until all ingredients were well blended. The
inorganic components (Pb--V--O powder and silver conductive powder)
were tumble-mixed in a jar for 15 min. The total weight of the
inorganic components was 44.0 g, of which 41.0-43.0 g was silver
powder and 1.0-3.0 g was the Pb--V--O powder. One third of
inorganic components were then added to the Thinky jar containing
the organic components and mixed for 1 minute at 2000 RPM. This was
repeated until all of the inorganic components were added and
mixed. The paste was then roll-milled at a 2 mil gap for 3 passes
at 0 psi, 3 passes at 100 psi and 1 pass at 150 psi. The degree of
dispersion was measured by fineness of grind (FOG). The FOG value
was typically equal to or less than 10/5 for thick-film pastes. The
viscosity of the paste was measured after 24 hrs at room
temperature. Viscosity was measured after 3 minutes at 10 RPM in a
viscometer within 200 and 500. The paste viscosity was adjusted to
approximately 300 Pas by adding solvent and then mixing for 1
minute at 2000 RPM. This step was repeated until the desired
viscosity was achieved. Different loadings of the two glasses were
used in the pastes -2 wt %, 4 wt % and 6 wt %.
[0234] Table 10 shows the results of photovoltaic cell performance.
All showed low efficiencies. These results indicate that the
lead-vanadium based oxide must comprise a third component, i.e.,
one or more an additional oxides with a liquidus temperature of
900.degree. C. or less as part of the composition to achieve
contact through one or more a insulating films comprising silicon
nitride to a solar cell emitter.
TABLE-US-00012 TABLE 10 Comparative Experiments - Eff %, FF, and Rs
paste data for PbO-V2O5 only compositions Compar- ative Experi- wt
% wt % loading Eff % FF R series (.OMEGA.) ment PbO V2O5 wt % 870
885 900 915 930 870 885 900 915 930 870 885 900 915 F 70.98 29.02 2
0.70 1.79 2.48 1.71 4.55 35.8 35.8 27.9 33.8 31.5 40.10 29.200
4.521 20.700 F 70.98 29.02 4 0.17 0.31 0.84 0.68 1.60 47.3 34.2
30.0 29.0 28.8 72.200 37.190 14.670 16.360 F 70.98 29.02 6 0.36
0.31 0.64 1.16 1.68 36.5 36.1 31.3 30.6 29.4 45.900 43.640 20.920
9.560 G 60.1 39.9 2 0.97 1.29 3.80 3.54 6.32 28.6 29.1 29.7 31.6
37.2 11.380 11.350 2.660 8.290 G 60.1 39.9 4 0.06 0.16 0.39 0.69
1.39 44.9 85.8 91.4 28.4 27.9 71.500 61.300 11.310 16.860 G 60.1
39.9 6 0.15 0.19 0.31 0.70 0.78 47.5 38.3 29.7 28.6 22.300 75.100
42.200 17.930
* * * * *
References