U.S. patent application number 13/100619 was filed with the patent office on 2011-09-29 for thick-film pastes and solar cells made therefrom.
This patent application is currently assigned to E. I. DU PONT DE NEMOURS AND COMPANY. Invention is credited to KURT RICHARD MIKESKA, RAJ G. RAJENDRAN, DAVID HERBERT ROACH, SEIGI SUH.
Application Number | 20110232747 13/100619 |
Document ID | / |
Family ID | 44583755 |
Filed Date | 2011-09-29 |
United States Patent
Application |
20110232747 |
Kind Code |
A1 |
MIKESKA; KURT RICHARD ; et
al. |
September 29, 2011 |
THICK-FILM PASTES AND SOLAR CELLS MADE THEREFROM
Abstract
This invention relates to thick-film pastes and processes for
using such pastes to make solar cell contacts and other circuit
devices. In particular, the thick-film pastes comprise a
lead-tellurium-oxide frit component, an organic vehicle, and a
conductive metal component comprising a silver component and a
nickel component.
Inventors: |
MIKESKA; KURT RICHARD;
(Hockessin, DE) ; ROACH; DAVID HERBERT;
(Hockessin, DE) ; RAJENDRAN; RAJ G.; (Hockessin,
DE) ; SUH; SEIGI; (Cary, NC) |
Assignee: |
E. I. DU PONT DE NEMOURS AND
COMPANY
Wilmington
DE
|
Family ID: |
44583755 |
Appl. No.: |
13/100619 |
Filed: |
May 4, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61331006 |
May 4, 2010 |
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61440117 |
Feb 7, 2011 |
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61445508 |
Feb 22, 2011 |
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61467003 |
Mar 24, 2011 |
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Current U.S.
Class: |
136/256 ;
252/513; 257/741; 257/E21.159; 257/E23.01; 438/660 |
Current CPC
Class: |
H01L 2924/0002 20130101;
B22F 7/04 20130101; H01B 1/16 20130101; B22F 2007/047 20130101;
Y02E 10/50 20130101; H01L 31/1884 20130101; Y02E 10/52 20130101;
H01L 31/022425 20130101; B22F 1/007 20130101; C03C 8/12 20130101;
C04B 35/01 20130101; B22F 7/08 20130101; H01L 31/0264 20130101;
B22F 1/0059 20130101; H01B 1/22 20130101; C03C 8/10 20130101; H01L
2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
136/256 ;
252/513; 438/660; 257/741; 257/E21.159; 257/E23.01 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; H01L 31/0216 20060101 H01L031/0216; H01B 1/22
20060101 H01B001/22; H01L 21/283 20060101 H01L021/283; H01L 23/48
20060101 H01L023/48 |
Claims
1. A thick-film paste comprising: (a) a conductive metal portion
comprising a silver component and a nickel component; (b) a
lead-tellurium-oxide frit component; and (c) an organic
vehicle.
2. The thick-film paste of claim 1, wherein the nickel component is
a nickel alloy.
3. The thick-film paste of claim 1, wherein the nickel component
comprises nickel powder, nickel flake or mixtures thereof.
4. The thick-film paste of claim 1, wherein the silver component
comprises silver powder, silver flake or mixtures thereof.
5. The thick-film paste of claim 1, wherein the conductive metal
portion comprises: (a) from about 10-99.9 wt % of a silver
component; and (b) from about 0.1-90 wt % of a nickel
component.
6. The thick-film paste of claim 1, wherein the conductive metal
portion comprises: (a) about 50-99.9 wt % silver; and (b) about
0.1-50 wt % nickel.
7. The thick-film paste of claim 1, wherein the conductive metal
portion comprises: (a) about 70-99.9 wt % silver and (b) about
0.1-30 wt % nickel.
8. The thick-film paste of claim 1, wherein the conductive metal
portion comprises: (a) 10-80 wt % silver; (b) about 5-85 wt %
nickel; and (c) about 0.1-10 wt % of a metal selected from the
group consisting of aluminum, chromium and combinations
thereof.
9. The thick-film paste of claim 1, wherein the
lead-tellurium-oxide frit component comprises 1-10 wt % of the
solids portion of the thick-film paste.
10. The thick-film paste of claim 1, wherein the
lead-tellurium-oxide further comprises lithium.
11. The thick-film paste of claim 10, further comprising
titanium.
12. The thick-film paste of claim 1, wherein the
lead-tellurium-oxide frit component comprises particles having a
particle size less than or equal to about 2 microns.
13. A process comprising: (a) providing an article comprising an
insulating film disposed onto a surface of a semiconductor
substrate; (b) applying a thick-film paste composition onto at
least a portion of the insulating film to form a layered structure,
wherein the thick-film paste composition comprises: (i) a
conductive metal portion comprising a silver component and a nickel
component; (ii) a lead-tellurium-oxide frit component; and (iii) an
organic vehicle; and (c) firing the layered structure to form an
electrode that is in contact with the insulating layer and is in
electrical contact with the semiconductor substrate.
14. The process of claim 13, wherein the thick-film paste
composition is applied pattern-wise onto the insulating film.
15. (canceled)
16. The process of claim 13, wherein the lead-tellurium-oxide
further comprises lithium.
17. An article comprising: (a) a semiconductor substrate; (b) an
insulating layers on the semiconductor substrate; and (c) an
electrode that is in contact with the insulating layer and is in
electrical contact with the semiconductor substrate, wherein the
electrode comprises silver, nickel, lead, and tellurium.
18. The article of claim 17, wherein the article is a semiconductor
device.
19. The article of claim 18, wherein the semiconductor device is a
solar cell.
20. The solar cell of claim 19, wherein the electrode further
comprises lithium.
21. The solar cell of claim 19, wherein the electrode further
comprises an element selected from the group consisting of cobalt,
iron, silicon, molybdenum, niobium, tantalum, manganese, vanadium,
antimony, boron, and combinations thereof.
Description
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 61/331,006 filed May 4, 2010, U.S.
Provisional Patent Application No. 61/440,117 filed Feb. 7, 2011,
U.S. Provisional Patent Application No. 61/445,508 filed Feb. 22,
2011, and U.S. Provisional Patent Application No. 61/467,003 filed
Mar. 24, 2011, all of which are herein incorporated by
reference.
FIELD OF THE INVENTION
[0002] This invention relates to thick-film pastes and processes
for using such pastes to make solar cell contacts and other circuit
devices.
BACKGROUND
[0003] Solar cells are typically made of a semiconductor material,
e.g., silicon, which converts sunlight into useful electrical
energy. Such solar cells comprise thin wafers of silicon, in which
a PN junction is formed by diffusing phosphorus (P) from a suitable
phosphorus source into a p-type silicon wafer. The side of the
silicon wafer on which sunlight falls is often coated with an
anti-reflective coating (ARC) to prevent reflective loss of
incoming sunlight, thus increasing the efficiency of the solar
cell. A two-dimensional electrode grid pattern, known as a "front
contact," makes a connection to the n-side of the silicon, and a
coating of aluminum (Al) on the opposite side (back contact) makes
connection to the p-side of the silicon. These contacts are the
electrical outlets from the PN junction to the outside load.
[0004] The front contacts of silicon solar cells are generally
formed by screen-printing a thick-film paste. Typically, the paste
contains fine silver particles (75-80 wt %), glass (1-5 wt %), and
an organic medium (15-20 wt %). After screen-printing, the wafer
and paste are fired in air, typically at furnace set temperatures
of about 650-1000.degree. C. for a few seconds to form a dense
solid of highly conductive silver traces. During this step, glass
reacts with the anti-reflective coating, etches the silicon
surface, and facilitates the formation of intimate silicon-silver
contact. The organic components are also burned away in this
step.
[0005] Although silver is a highly electrically conductive metal,
it is also expensive and is periodically in short supply. This has
motivated attempts to substitute less-expensive metals for at least
a portion of the silver in thick-film pastes. Thick-film paste
compositions have been disclosed in which 1-90 wt % of the silver
has been replaced by nickel or a nickel alloy.
[0006] Nevertheless, it is desirable to develop thick-film paste
compositions that provide improved performance properties (e.g.,
efficiency, fill factor, and adhesion) when used in PV devices and
other applications.
SUMMARY
[0007] One aspect of this invention is a thick-film paste
comprising:
a) a conductive metal portion comprising a silver component and a
nickel component; b) a frit component comprising
lead-tellurium-oxide; and c) an organic vehicle.
[0008] Another aspect of this invention is a process for making a
solar cell contact comprising:
a) applying a thick-film paste to a silicon wafer, wherein the
thick-film paste comprises: [0009] i) a conductive metal portion
comprising a silver component and a nickel component; [0010] ii) a
frit component comprising lead-tellurium-oxide; and [0011] iii) an
organic vehicle b) firing the silicon wafer at a time and
temperature sufficient to sinter the conductive metal portion.
[0012] Another aspect of this invention is a solar cell comprising
a front contact, wherein the front contact is formed by firing a
thick-film paste of this invention.
[0013] In addition to solar cells, the pastes of the invention can
be used to produce a variety of circuit devices.
BRIEF DESCRIPTION OF THE FIGURES
[0014] FIGS. 1A-1F illustrate selected steps in fabricating a
semiconductor device. Reference numerals shown in FIGS. 1A-1F are
explained below.
[0015] 10: p-type silicon substrate [0016] 20: n-type diffusion
layer [0017] 30: insulating film [0018] 40: p+ layer (back surface
field, BSF) [0019] 60: aluminum paste disposed on back-side [0020]
61: aluminum back electrode (obtained by firing back-side aluminum
paste) [0021] 70: silver or silver/aluminum paste disposed on
back-side [0022] 71: silver or silver/aluminum back electrode
(obtained by firing back-side silver paste) [0023] 500: thick-film
paste disposed on front-side [0024] 501: front electrode (formed by
firing the thick-film paste)
DETAILED DESCRIPTION
[0025] The features of the invention are hereinafter more fully
described and particularly pointed out in the claims. The following
description sets forth in detail certain illustrative embodiments
of the invention, these being indicative of but a few of the
various ways in which the invention may be employed.
[0026] The thick-film pastes described herein can be used to make
front contacts for silicon-based solar cells to collect current
generated by exposure to light. The pastes can also be used to make
back contacts to conduct electrons to an outside load. The pastes
can also be used to create tabs used on solar cells.
[0027] Conductive metal portion. The conductive metal portion of
the thick-film paste comprises a silver component and a nickel
component, typically from about 10-99.9 wt % silver and 0.1-90 wt %
nickel; or about 50-99.9 wt % silver and 0.1-50 wt % nickel; or
about 70-99.9 wt % silver and 0.1-30 wt % nickel; or about 80-99.9
wt % silver and 0.1-20 wt % nickel, wherein the silver (or nickel)
weight percent is calculated on the basis of the silver (or nickel)
content of the conductive metal portion.
[0028] Typically, the conductive metal portion comprises 50-95 wt %
of the thick-film paste, and is calculated on the basis of the
silver components (e.g., silver metal particles or silver salts)
and nickel components (e.g., nickel metal or nickel alloy).
[0029] Silver Component. The silver 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. Particles of silver or silver alloy can be coated with other
materials, e.g., phosphorus. Alternatively, silver can be coated
onto glass, or silver oxide can be dissolved in the glass during
the glass melting/manufacturing process.
[0030] In one embodiment, the solids portion of the thick-film
paste composition comprises about 80 to about 90 wt % silver
particles and about 1 to about 9.5 wt % silver flakes.
[0031] In one embodiment, the thick-film paste composition
comprises 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
or mixtures thereof.
[0032] The particle size of the silver is not subject to any
particular limitation. In one embodiment, an average particle size
is less than 10 microns; in another embodiment, the average
particle size is less than 5 microns; in another embodiment, the
average particle size is less than 3 microns; in another
embodiment, the average particle size is less than 1 micron; in
another embodiment, a mixture of particle sizes is used.
[0033] Nickel Component. The nickel component is selected from the
group consisting of nickel metal and nickel alloys. Nickel metal is
typically in the form of powders or flakes. In some embodiments,
the nickel particles have an average particle size of between 0.2
and 10.0 microns, e.g., about 0.5 microns, 1.0 micron, 2.5 microns,
5.0 microns, 6.6 microns, or 10 microns. In some embodiments, the
nickel particle size is approximately the same as the silver
particle size.
[0034] The nickel can be provided in the form of essentially pure
nickel powder, nickel flake, colloidal nickel, or as nickel alloyed
with one or more other metals including any or all of aluminum,
chromium, silicon, iron, molybdenum, antimony, vanadium and
niobium/tantalum. As is known in the art, the proportion of niobium
and tantalum are given together owing to their tendency to
intimately alloy/mix and the difficulty in purifying one from the
other.
[0035] In some embodiments, the thick-film paste comprises a nickel
alloy comprising about 70-99 wt % nickel and about 1-30 wt %
aluminum, or about 80-99 wt % nickel and 1-20 wt % aluminum, or
about 80-96 wt % nickel and about 4-20 wt % aluminum.
[0036] Instead of, or in addition to, a Ni--Al alloy, a Ni--Cr
alloy may be present in the pastes, such as Ni--Cr alloy comprising
about 48-81 wt % nickel and about 19-52 wt % chromium. In another
embodiment of the invention, the paste comprises a Ni--Cr alloy,
the Ni--Cr alloy comprising about 1-60 wt % chromium.
[0037] In another embodiment, the conductive metal portion
comprises from about 10-99.9 wt % silver, and (b) from about 0.1-90
wt % of a nickel alloy selected from the group consisting of a
Ni--Al alloy, a Ni--Cr alloy, a Ni--Al--Cr alloy, and combinations
thereof.
[0038] When a Ni--Al alloy is used, it may comprise about 1-30 wt %
aluminum and about 70-99 wt % nickel. In yet another embodiment,
the conductive metal portion comprises (a) about 37.5-75 wt %
silver and (b) about 25-62.5 wt % of a nickel alloy. In still
another embodiment, the conductive metal portion comprises (a)
about 13.8-87.5 wt % silver and (b) about 12.5-86.2 wt % of a
nickel alloy selected from the group consisting of Ni--Al, Ni--Cr,
Ni--Al--Cr, and combinations thereof.
[0039] The nickel alloy can further comprise an element selected
from the group consisting of cobalt, iron, silicon, molybdenum,
niobium, tantalum, manganese, vanadium, antimony, boron, and
combinations thereof. For example, some embodiments can include at
least one of the following: about 1-30 wt %, or about 5-25 wt %, or
about 10-20 wt % chromium; about 0.1-10 wt %, or about 0.3-8 wt %,
or about 1-5 wt % iron; about 0.1-5 wt %, or about 1-4 wt %, or
about 1.5-3 wt % silicon; about 1-10 wt %, or about 2-8 wt %, or
about 3-7 wt % molybdenum; about 0.1-5 wt %, or about 0.25-4 wt %
manganese; about 0.1-10 wt %, or about 0.3-8 wt %, or about 1-5 wt
% niobium+tantalum; about 0.5-8 wt %, or about 1-7 wt %, or about
2-6 wt % vanadium; and 0.5-9 wt %, or about 1-8 wt %, or about 2-6
wt % antimony.
[0040] The metals and alloys can be provided in the form of
powders, flakes, or colloids. The particles of the metals silver,
nickel, aluminum, (Al alone being only in a back contact) and
alloys containing combinations of silver, aluminum, nickel and
nickel alloys, have an average size of less than about 10 microns,
or less than about 5 microns, or less than about 1 micron. In other
embodiments, the metal or alloy particles have average particle
sizes of less than about 750 nm, less than about 500 nm or less
than about 250 nm.
[0041] Frit Component. The frit component comprises a
lead-tellurium oxide and optionally other metal compounds, metal
oxides or glasses. Typically, the frit comprises 1-10 wt % of the
thick-film paste, based on the weight of the solids.
[0042] The lead-tellurium-oxide (Pb--Te--O) can be prepared by
mixing TeO.sub.2 and PbO powders, heating the powder mixture in air
or an oxygen-containing atmosphere to form a melt, quenching the
melt, grinding and ball-milling the quenched material, and
screening the milled material to provide a powder with the desired
particle size. Firing the mixture of lead and tellurium 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 thick platelet. The resulting platelet can be milled to form
a powder. Typically, the milled powder has a D.sub.50 of 0.1-3.0
microns.
[0043] Typically, the mixture of PbO and TeO.sub.2 powders
comprises 5-95 mol % of lead oxide and 5-95 mol % of tellurium
oxide, based on the combined powders. In one embodiment, the
mixture of PbO and TeO.sub.2 powders comprises 30-85 mol % of lead
oxide and 15-70 mol % of tellurium oxide, based on the combined
powders.
[0044] In some embodiments, the mixture of PbO and TeO.sub.2
powders further comprises one or more other metal compounds.
Suitable other metal compounds include PbF.sub.2, SiO.sub.2,
B.sub.2O.sub.3, Li.sub.2O, Na.sub.2O, K.sub.2O, Rb.sub.2O,
Cs.sub.2O, MgO, CaO, SrO, BaO, TiO.sub.2, V.sub.2O.sub.5, ZrO,
MoO.sub.3, Mn.sub.2O.sub.3, Ag.sub.2O, ZnO, Ga.sub.2O.sub.3,
GeO.sub.2, In.sub.2O.sub.3, SnO.sub.2, Sb.sub.2O.sub.3,
Bi.sub.2O.sub.3, P.sub.2O.sub.5, CuO, SeO.sub.2, and CeO.sub.2.
[0045] Table 1 lists some examples of powder mixtures containing
PbO, TeO.sub.2 and other optional metal compounds that can be used
to make lead-tellurium oxides. This list is meant to be
illustrative, not limiting.
TABLE-US-00001 TABLE 1 Illustrative examples of powder mixtures
that can be used to make suitable lead-tellurium oxides. Powder Wt
% Wt % Wt % Wt % Wt % Wt % Wt % Wt % Wt % mixture PbO TeO.sub.2
PbF.sub.2 SiO.sub.2 B.sub.2O.sub.3 P.sub.2O.sub.5 SnO.sub.2
Ag.sub.2O Li.sub.2O A 32.95 67.05 B 38.23 51.26 10.50 C 67.72 32.28
D 72.20 27.80 E 80.75 19.25 F 59.69 9.30 16.19 14.82 G 75.86 9.26
14.88 H 48.06 51.55 0.39 I 48.16 51.65 0.19 J 47.44 50.88 1.68 K
47.85 51.33 0.82 L 41.76 44.80 0.32 0.80 12.32 M 46.71 50.10 3.19 N
46.41 49.78 3.80 O 45.11 48.39 6.50 P 44.53 47.76 7.71 Q 48.05
51.54 0.41 R 47.85 51.33 0.82 S 47.26 50.70 2.04 T 45.82 49.19 U
48.04 51.53 V 39.53 28.26 W 48.04 51.53 0.42
[0046] As used herein, the term "Pb--Te--O" refer to compositions
that comprise lead-tellurium oxides and may further comprise metal
oxides or carbonates that contain one or more elements selected
from the group consisting of Si, Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba,
Ti, V, Zr, Mo, Mn, Zn, B, P, Sn, Ga, Ge, In, Sb, Bi, Ce, Cu, and
Ag.
[0047] In some embodiments, the lead-tellurium-oxide frit component
comprises a mixture of a lead-tellurium-oxide and a tellurium-free
oxide, e.g., a
lead-lithium-boron-silion-aluminum-zirconium-sodium-oxide.
[0048] Organic Vehicle. The organic vehicle is typically a solution
of a resin dissolved in a solvent, optionally further comprising a
thixotropic agent. The solvent typically has a boiling point
between about 130 and 350.degree. C. In some embodiments, the resin
is ethyl cellulose, ethyl hydroxyethyl cellulose, wood rosin, a
mixture of ethyl cellulose and phenolic resins, a polymethacrylate,
the monobutyl ether of ethylene glycol monoacetate, or mixtures
thereof.
[0049] Suitable solvents include terpenes (e.g., alpha- or
beta-terpineol; Hercules Inc., Wilmington, Del.); Dowanol.RTM.
(diethylene glycol monoethyl ether; Dow Chemical Co., Midland,
Mich.); butyl Carbitol.RTM. (diethylene glycol monobutyl ether; Dow
Chemical Co.); dibutyl Carbitol.RTM. (diethylene glycol dibutyl
ether; Dow Chemical Co.); butyl Carbitol.RTM. acetate (diethylene
glycol monobutyl ether acetate; Dow Chemical Co.); hexylene glycol;
Texanol.RTM. (2,2,4-trimethyl-1,3-pentanediol monoisobutyrate;
Eastman Chemical Company, Kingsport, Tenn.); alcohol esters;
kerosene; dibutyl phthalate and mixtures thereof. 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.
[0050] Various combinations of these and other solvents can be
formulated to obtain the desired viscosity and volatility
requirements for each application.
[0051] Suitable organic thixotropic agents include hydrogenated
castor oil and derivatives thereof. A thixotrope is not always
necessary because the solvent/resin properties, coupled with the
shear thinning inherent in any suspension, may be sufficient.
Suitable wetting agents include fatty acid esters and diamines,
e.g., N-tallow-1,3-diaminopropane di-oleate, N-tallow trimethylene
diamine diacetate, N-coco trimethylene diamine, beta-diamines,
N-oleyl trimethylene diamine, N-tallow trimethylene diamine,
N-tallow trimethylene diamine dioleate, and combinations
thereof.
[0052] The organic vehicle can further comprise dispersants,
surfactants and/or rheology modifiers, which are commonly used in
thick-film paste formulations.
[0053] 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.
[0054] If the organic medium comprises a polymer, the polymer may
constitute 8 to 15 wt % of the organic medium.
[0055] Inorganic/Other Additives. Phosphorus can be added to the
paste in a variety of ways to reduce the resistance of the front
contacts. For example, certain glasses can be modified with
P.sub.2O.sub.5 in the form of a powdered or fritted oxide, or
phosphorus can be added to the paste by way of phosphate esters and
other organo-phosphorus compounds. Phosphorus can also be added as
a coating to silver and/or nickel and or Ag/Ni alloy particles
prior to making a paste. In such case, the silver and/or nickel and
or Ag/Ni alloy particles are mixed with liquid phosphorus and a
solvent. For example, a blend of about 85-95 wt % silver and/or
nickel and/or Ag/Ni particles, about 5-15 wt % solvent and about
0.5-10 wt % liquid phosphorus are mixed and the solvent evaporated.
Phosphorus-coated silver and/or nickel and/or Ag/Ni alloy particles
help ensure intimate mixing of phosphorus and silver and/or nickel
and/or Ag/Ni alloy in the pastes.
[0056] Other additives such as fine silicon and/or carbon powder
can be added to the paste to control the silver reduction and
precipitation reaction. The silver precipitation at the interface
or in the bulk glass can also be controlled by adjusting the firing
atmosphere (e.g., by firing in flowing N.sub.2 or
N.sub.2/H.sub.2/H.sub.2O mixtures). However, no special atmosphere
is required. Fine low-melting metal additives (i.e., elemental
metallic additives as distinct from metal oxides) such as Pb, Bi,
In, Ga, Sn, Ni, and Zn or alloys of each with at least one other
metal can be added to provide a contact at a lower firing
temperature, or to widen the firing window. Typically such metal
additives are present at less than about 1 wt % of the conductive
metal portion of the pastes. Organometallic compounds providing
aluminum, barium, bismuth, magnesium, zinc, strontium and potassium
can be used, such as, for example, the acetates, acrylates,
formates, neodeconates, methoxides, ethoxides, methoxyethoxides,
and stearates of the named metals. Potassium silicate is also a
suitable source of potassium.
[0057] Embodiments. In one embodiment, the thick-film paste
comprises an organic vehicle, a lead-tellurium-oxide frit component
and a conductive metal portion, wherein the conductive metal
portion comprises: (a) from about 10-99 wt % silver and (b) from
about 1-90 wt % of a nickel alloy selected from the group
consisting of a Ni--Al alloy, a Ni--Cr alloy, a Ni--Al--Cr alloy,
and combinations thereof.
[0058] Another embodiment of the invention is a thick-film paste
comprising: (a) a lead-tellurium-oxide frit component comprising
frit particles having a particle size no greater than about 2
microns, and (b) a conductive metal portion comprising: (i) from
about from about 10-99 wt % silver and (ii) from about 0.05-90 wt %
of a nickel alloy selected from the group consisting of a Ni--Al
alloy, a Ni--Cr alloy, a Ni--Al--Cr alloy, and combinations
thereof.
[0059] Another embodiment of the invention is a thick-film paste
comprising: (a) a lead-tellurium-oxide frit component; and (b) a
metal portion comprising about 5-85 wt % nickel; about 10-80 wt %
silver, and about 0.1-10 wt % of a metal selected from the group
consisting of aluminum, chromium, and combinations thereof.
[0060] Another embodiment of the invention is a thick-film paste
comprising: a lead-tellurium-oxide frit component and a conductive
metal portion, said conductive metal portion comprising: from about
10-99 wt % silver and from about 1-90 wt % of a nickel alloy
selected from the group consisting of a Ni--Al alloy, a Ni--Cr
alloy, a Ni--Al--Cr alloy, and combinations thereof.
[0061] Another embodiment of the invention is a thick-film paste
comprising a vehicle, a lead-tellurium-oxide frit component and a
conductive metal portion, where the conductive metal portion
comprises: (a) from about 10-99 wt % silver and (b) from about 1-90
wt % of a nickel alloy selected from the group consisting of a
Ni--Al alloy, a Ni--Cr alloy, a Ni--Al--Cr alloy, and combinations
thereof.
[0062] Another embodiment of the invention is a thick-film paste
comprising a metal portion comprising about 5-85 wt % nickel; about
10-80 wt % silver and about 0.1-10 wt % of a metal selected from
the group consisting of aluminum, chromium, and combinations
thereof.
[0063] Another embodiment of the invention is a thick-film paste
comprising (a) a lead-tellurium-oxide frit component comprising
frit particles having a particle size no greater than about 2
microns, and (b) a conductive metal portion comprising (i) from
about from about 10-99 wt % silver and (ii) from about 0.05-90 wt %
of a nickel alloy selected from the group consisting of a Ni--Al
alloy, a Ni--Cr alloy, a Ni--Al--Cr alloy, and combinations
thereof.
[0064] If a nickel alloy is used, the thick-film paste may comprise
as little as about 10% by weight silver. Use of the nickel in the
paste provides a distinct advantage, since the cost of nickel is
typically less than half of the cost of silver.
[0065] Solar cells. Another embodiment of the invention is a solar
cell comprising an electrical contact, wherein the contact
comprises a metal portion comprising, prior to firing: about 5-85
wt % nickel; about 10-80 wt % silver, and about 0.1-10 wt % of a
metal selected from the group consisting of aluminum, chromium, and
combinations thereof.
[0066] Another embodiment of the invention is a solar cell
comprising a front electrical contact, said front electrical
contact formed by firing a thick-film paste composition comprising
a lead-tellurium-oxide frit component and a conductive metal
portion, said conductive metal portion comprising silver and at
least about 8 wt % nickel.
[0067] Another embodiment of the invention relates to a process for
making a solar cell contact, the process comprising (a) applying a
thick-film paste to a silicon wafer, wherein the thick-film paste
comprises (i) a lead-tellurium-oxide frit component and (ii) a
conductive metal portion, said conductive metal portion comprising
(1) from about 10-99.1 wt % silver and (2) from about 0.1-90 wt %
of a nickel alloy selected from the group consisting of a Ni--Al
alloy, a Ni--Cr alloy, a Ni--Al--Cr alloy, and combinations
thereof, and (b) firing the silicon wafer at a time and temperature
sufficient to sinter the metal portion.
[0068] Another embodiment of the invention is a process for making
a solar cell contact, comprising (a) applying a thick-film paste to
a silicon wafer, wherein the thick-film paste comprises (i) a
lead-tellurium-oxide frit component and (ii) a conductive metal
portion, said conductive metal portion comprising (1) silver and
(2) at least about 0.1 wt % nickel, and (b) firing the silicon
wafer at a time and temperature sufficient to sinter the metal
portion.
[0069] Another embodiment of the invention is a solar cell
comprising a front electrical contact, where the front electrical
contact is formed by firing a thick-film paste composition
comprising a lead-tellurium-oxide frit component and a conductive
metal portion. The conductive metal portion comprises Ag and at
least about 1 wt % nickel.
[0070] Another embodiment of the invention is a solar cell
comprising an electrical contact, wherein the electrical contact
comprises a metal portion comprising, prior to firing: about 5-85
wt % nickel; about 10-80 wt % silver, and about 0.1-10 wt % of a
metal selected from the group consisting of aluminum, chromium, and
combinations thereof.
[0071] Another embodiment is a solar cell front electrical contact
comprising silver, a nickel alloy, and aluminum.
[0072] Another embodiment of the invention is a process for making
a solar cell electrical contact, the process comprising (a)
applying a thick-film paste to a silicon wafer, wherein the
thick-film paste comprises (i) a lead-tellurium-oxide frit
component and (ii) a conductive metal portion, said conductive
metal portion comprising (1) from about 10-99.9 wt % silver and (2)
from about 0.1-90 wt % of a nickel alloy selected from the group
consisting of a Ni--Al alloy, a Ni--Cr alloy, a Ni--Al--Cr alloy,
and combinations thereof, and (b) firing the silicon wafer at a
time and temperature sufficient to sinter the metal portion.
[0073] Another embodiment of the invention is a process for making
a solar cell contact, comprising (a) applying a thick-film paste to
a silicon wafer, wherein the paste comprises (i) a
lead-tellurium-oxide frit component and (ii) a conductive metal
portion, said conductive metal portion comprising (1) silver and
(2) at least about 1 wt % nickel, and (b) firing the silicon wafer
at a time and temperature sufficient to sinter the metal
portion.
[0074] Preparation of the Thick-Film Paste Composition. A Paste
According to the invention can be prepared either by mixing the
individually prepared silver and nickel paste in various
proportions or by mixing silver and nickel powder in required
proportions prior to making the paste.
[0075] In one embodiment, the thick-film paste composition can be
prepared by mixing the conductive metal powder, the
lead-tellurium-oxide powder, and the organic medium in any order.
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 solvents. Mixing methods
that provide high shear may be useful.
[0076] Another aspect of the present invention is a process
comprising:
a) providing an article comprising one or more insulating films
disposed onto at least one surface of a semiconductor substrate;
(b) applying a thick-film paste composition onto at least a portion
of the one or more insulating films to form a layered structure,
wherein the thick-film paste composition comprises: [0077] i) 90 to
99.9% by weight based on solids of a source of an electrically
conductive metal; [0078] ii) 0.1 to 10% by weight based on solids
of a lead-tellurium-oxide, wherein the mole ratio of lead to
tellurium of the lead-tellurium-oxide is between 5/95 and 95/5; and
[0079] iii) an organic medium; and (c) firing the semiconductor
substrate, one or more insulating films, and thick-film paste,
wherein the organic medium of the thick-film paste is volatilized,
forming an electrode in contact with the one or more insulating
layers and in electrical contact with the semiconductor
substrate.
[0080] 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 (e.g.,
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.
[0081] One embodiment of this process is illustrated in FIG. 1.
[0082] FIG. 1(A) shows a single-crystal silicon or
multi-crystalline silicon p-type substrate 10.
[0083] 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 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 0.5 microns. The n-type
diffusion layer may have a sheet resistivity of several tens of
ohms per square.
[0084] 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,
e.g., using an organic solvent.
[0085] Next, in FIG. 1(D), an insulating layer 30 which also
functions as an antireflection 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 comprises hydrogen for passivation during subsequent firing
processing), a titanium oxide film, or a silicon 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.
[0086] 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.
[0087] 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 backside as an electrode for interconnecting solar cells by
means of copper ribbon or the like.
[0088] 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).
[0089] 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, a 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 CFD or plasma CFD. 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.
[0090] In this process, the semiconductor substrate can be a
single-crystal or multi-crystalline silicon electrode.
[0091] 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.
[0092] In one embodiment, the insulating film comprises 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.
[0093] 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.
[0094] In one embodiment, the silicon nitride of the insulating
film is part of an anti-reflective coating.
[0095] 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.
[0096] 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 3-11 zones, can be used to control the desired
thermal profile.
[0097] Upon firing, the electrically conductive metal and
lead-tellurium-oxide frit component 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
comprises one or more of tellurium, tellurium 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 comprises sintered metal that contacts the
underlying semiconductor substrate and may also contact one or more
insulating layers.
[0098] Another aspect of the present invention is an article formed
by a process comprising:
(a) providing an article comprising one or more insulating films
disposed onto at least one surface of a semiconductor substrate;
(b) applying a thick-film paste composition onto at least a portion
of the one or more insulating films to form a layered structure,
wherein the thick-film paste composition comprises: [0099] i) 90 to
99% by weight based on solids of a source of an electrically
conductive metal; [0100] ii) 1 to 10% by weight based on solids of
a lead-tellurium-oxide frit component, wherein the mole ratio of
lead to tellurium of the lead-tellurium-oxide is between 5/95 and
95/5; and [0101] iii) an organic medium, and (c) firing the
semiconductor substrate, one or more insulating films, and
thick-film paste wherein the organic medium of the thick-film paste
is volatilized, forming an electrode in contact with the one or
more insulating layers and in electrical contact with the
semiconductor substrate.
[0102] Such articles may be useful in the manufacture of
photovoltaic devices. In one embodiment, the article is a
semiconductor device comprising an electrode formed from the
thick-film paste composition. In one embodiment, the electrode is a
front-side electrode on a silicon solar cell. In one embodiment,
the article further comprises a back electrode. It will be
appreciated that although the examples herein primarily concern a
conductive composition for use in forming a conductor paste for use
in the formation of solar cell contacts, the present invention also
contemplates the use of the principles disclosed herein to form
resistor pastes, semiconductor pastes, inks, or tapes. Furthermore,
such compositions may be considered as materials for use in forming
thick films. Thus, the compositions disclosed herein can be used to
form conductive, resistive or semiconducting paths or patterns on
substrates. Such conductive compositions can assume various forms,
including an ink, a paste, or a tape. Deposition of the composition
on a substrate can be by screen printing, plating, extrusion,
inkjet, contact printing, stencil printing, shaped or multiple
printing, or ribbons.
[0103] Substrates other than silicon may be employed in connection
with the pastes of the present invention. The use of the
compositions disclosed herein in a variety of electronic components
and devices is also envisioned.
[0104] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
illustrative examples shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general invention concept as defined by the
appended claims and their equivalents.
EXAMPLES
[0105] Thick-film pastes were prepared by mixing the individually
prepared silver and nickel pastes in various proportions. The
silver and nickel pastes were made as detailed below.
[0106] Frit Preparation
[0107] Mixtures of TeO.sub.2 powder (99+% purity), PbO powder (ACS
reagent grade, 99+% purity) and Li.sub.2CO.sub.3 in the % cation
ratio Te:Pb:Li of 57:38:5 were tumbled in a polyethylene container
for 30 min to mix the starting powders. The 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 h 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
100 mesh screen to provide the PbO--TeO.sub.2--Li.sub.2O powder
(PTOL) used in the thick-film paste preparations.
[0108] Thick-Film Paste Preparation
[0109] Silver Paste: A 50 g batch of silver paste was made by
placing silver powder (44.72 g) in a glass jar, to which was added
1.04 g of PbO--TeO.sub.2--LiO.sub.2 powder (PTOL), prepared as
above. The powders were then tumble-mixed for about 15 min. An
organic medium containing solvents and binders was prepared by
mixing the various components in respective amounts as listed in
Table 2 in a plastic jar using a planetary centrifugal mixer,
THINKY.RTM. ARE-310 (THINKY USA, Inc., Laguna Hills, Calif.) for 1
min at 2000 rpm. To this organic medium, approximately one third of
the silver and PTOL powder mix was added and mixed using
THINKY.RTM. ARE-310 for 1 min at 2000 rpm. This step was repeated
with the second and third portion of silver and PTOL powder mix,
making sure the inorganic powder mix was thoroughly dispersed in
the medium. The dispersed mixture was then blended with triple roll
mill (Charles Ross & Son Company, Floor Model, 4''.times.8'')
at a 1 mil gap for three passes at zero psi and three passes at 100
psi to obtain a thick paste. The viscosity of the blended paste was
adjusted with 0.14 g of Texanol (Eastman Chemical Company, TN) to
obtain a printable paste.
[0110] The solid content of final paste was measured in duplicate
by weighing small quantities (1-2 g) into an alumina boat and
firing in a muffle furnace at 450.degree. C. for 30 min to remove
organics, and reweighing the alumina boat and contents. The average
solid content of the paste was determined to be 90.2%.
[0111] The paste viscosity was measured using a Brookfield HADV-I
Prime viscometer (Brookfield Engineering Laboratories, Inc.,
Middleboro, Mass.) with the thermostatted small-sample adapter at
about 10 rpm and was found to be 278 Pas.
TABLE-US-00002 TABLE 2 Composition of the organic medium Component
Weight (g) 50-52% ethoxyl ethyl cellulose resin, viscosity =
150-250 cps, 0.6776 dissolved in Texanol 48-50% ethoxyl ethyl
cellulose resin, viscosity = 18-24 cps, 0.2612 dissolved in Texanol
Amine oleate surfactant 0.5200 Foralyn (hydrogenated rosin ester),
50 wt % dissolved in 1.3002 Texanol
(2,2,4-trimethyl-1,3-pentanediol monoisobutyrate) Hydrogenated
Castor oil derivative 0.2610 Dibasic ester-3 1.8206
[0112] Nickel paste: A 50 g batch of nickel paste was made
following the procedure described above for the preparation of the
silver paste, except that nickel powder was used in place of silver
powder. The solid content of final paste was measured to be 90.8%
and its viscosity was measured to be 170 Pas.
[0113] Silver/Nickel Pastes: Paste containing silver and nickel in
approximately a 3:1 ratio (i.e., approximately 75 wt % silver and
25 wt % nickel) was prepared by mixing silver paste and nickel
paste in a 3:1 weight ratio in a THINKY.RTM. ARE-310 planetary
mixture for 1 min at 2000 rpm. Similarly, a paste containing
approximately 50 wt % silver and 50 wt % nickel was prepared by
mixing silver paste and nickel paste in a 1:1 weight ratio. For
each paste mixture, the mixing was repeated for three more times to
obtain a thoroughly mixed blended paste. Pastes formulations
prepared using PTOL frit are labeled as "Paste A"
Solar Cell Fabrication
[0114] Solar cells for testing the performance of the thick-film
pastes were made from 200 micron-thick multi-crystalline silicon
wafers (Deutsche Solar AG) with a 65 ohm/sq phosphorus-doped
emitter layer which had an acid-etched textured surface and 70-80
nm thick PECVD SiN.sub.x antireflective coating. The wafers were
cut into 28 mm.times.28 mm wafers using a diamond wafering saw.
[0115] Wafers were screen-printed full ground-plane with commercial
aluminum paste, PV381 (E. I. du Pont de Nemours and Company,
Wilmington, Del.) for back side contact using a screen on an
8''.times.10'' frame (Sefar Inc., Depew, N.Y.) with a square
opening of 26.99 mm.times.26.99 mm and a screen printer, MSP 885
(Affiliated Manufacturers Inc., North Branch, N.J.). This left a
nominal 0.5 mm border of bare Si (i.e., without aluminum paste)
around the edges. The wet weight of aluminum paste was targeted to
be around 60 mg, which produced an aluminum loading after firing of
about 5.9 mg Al/cm.sup.2. After printing, the aluminum paste was
dried in a mechanical convection oven with vented exhaust for 30
min at 150.degree. C., resulting in a dried film thickness of 25-30
microns.
[0116] The silver paste, or nickel blend silver pastes, were
screen-printed using a screen on 8''.times.10'' frames (Sefar Inc.)
and a screen printer, MSP 485 (Affiliated Manufacturers Inc.) on
the silicon nitride layer of the front surface of silicon wafers
and dried at 150.degree. C. for 30 min in a convection oven to give
25-30 microns thick grid lines and a bus bar. The screen-printed
silver paste (or silver-nickel paste) had a pattern of eleven
fingers/grid lines of 100-125 microns width, connected to a bus bar
of 1.25 mm width located near one edge of the cell. The screen for
printing the silver paste used 325-mesh wires of 23 microns
diameter at 30.degree. angle and 32 microns thick emulsion.
[0117] The dried cells were fired in a 4-zone furnace (BTU
International, North Billerica, Mass.; Model PV309) at a belt speed
of 221 cm/min, with the following furnace setpoint temperatures:
zone 1 at 610.degree. C., zone 2 at 610.degree. C., zone 3 at
585.degree. C., and the final zone 4 set at peak temperature,
T.sub.max, in the range of 860.degree. C. to 960.degree. C. The
wafers took about 5.2 sec to pass through zone 4. In Table 3, only
the peak firing temperature of zone 4 is reported, which is
approximately 100-125.degree. C. greater than the actual wafer
temperature. After the firing process, the wafer is a functional
photovoltaic cell. For each paste composition, a number of
duplicate photovoltaic cells were fabricated. These photovoltaic
cells were divided into 4-5 groups, with 4-5 cells in each group,
all fired at the same temperature. Each cell group gave optimum
median cell efficiency at a firing temperature, which might be
different for the different paste compositions.
Solar Cell Electrical Measurements
[0118] A commercial Current-Voltage (JV) tester ST-1000
(Telecom-STV Ltd., Moscow, Russia) was used to make efficiency
measurements of the polycrystalline silicon photovoltaic cells. Two
electrical connections, one for voltage and one for current, were
made on the top and the bottom of each of the photovoltaic cells.
Transient photo-excitation was used to avoid heating the silicon
photovoltaic cells and to obtain JV curves under standard
temperature conditions (25.degree. C.). A flash lamp with a
spectral output similar to the solar spectrum illuminated the
photovoltaic cells from a vertical distance of 1 m. The lamp power
was held constant for 14 milliseconds. The intensity at the sample
surface, as calibrated against external solar cells was 1000
W/m.sup.2 (or 1 Sun) during this time period. During the 14
milliseconds, the JV tester varied an artificial electrical load on
the sample from short circuit to open circuit. The JV tester
recorded the light-induced current through, and the voltage across,
the photovoltaic cells while the load changed over the stated range
of loads. A power versus voltage curve was obtained from this data
by taking the product of the current times the voltage at each
voltage level. The maximum of the power versus voltage curve was
taken as the characteristic output power of the solar cell for
calculating solar cell efficiency. This maximum power was divided
by the area of the sample to obtain the maximum power density at 1
Sun intensity. This was then divided by 1000 W/m.sup.2 of the input
intensity to obtain the efficiency which is then multiplied by 100
to present the result in percent efficiency. Other parameters of
interest were also obtained from this same current-voltage curve.
One such parameter is fill factor (FF) which is obtained by taking
the ratio of the maximum power from the solar cell to the product
of open circuit voltage and short circuit current. For reasonably
efficient cells, an estimate of the series resistance
(R.sub.series) was obtained from the reciprocal of the local slope
of the current voltage curve near the short circuit point. The FF
is defined as the ratio of the maximum power from the solar cell to
the product of V.sub.oc and I.sub.sc.
[0119] Median values for optimum cell efficiency, fill factor and
series resistance, for solar cells prepared using the thick-film
pastes of Examples 1-3 are summarized in Table 3.
TABLE-US-00003 TABLE 3 Electrical Performance of Examples 1 and 2
and Comparative Example A Peak Median Median Wt Wt Firing Median
Fill Series % % Temp. Efficiency Factor Resistance Ex. Frit Ni Ag
(.degree. C.) (%) (%) (ohm cm.sup.2) A PTOL 0 100 925 15.71 78.7
1.33 1 PTOL 25 75 940 15.61 79.4 1.32 2 PTOL 50 50 930 15.16 77.5
1.51
[0120] These results demonstrate that the Ag/Ni blend thick-film
pastes can provide solar cells with good performance
characteristics.
Paste Adhesion Measurements
[0121] Paste adhesion to the silicon wafer is a critical
performance requirement for stability and long-term durability of
solar cell devices. As described below, adhesion can be assessed by
attaching the fired paste to a solder ribbon, and then pulling the
soldered tab and measuring the force required at break. Fired
pastes exceeding 2.5 N force at break are generally considered to
meet the industry requirement.
[0122] Test samples for the adhesion measurements were printed and
fired the same way as detailed in `solar cell fabrication` section,
except that the samples were printed with three bus bars instead of
grid lines and a bus bar using a screen on 8''.times.10'' frame
(Sefar Inc.) with 325-mesh wires of 27 microns diameter at
30.degree. angle and 27 microns thick emulsion.
[0123] The print bar of the fired test sample was 2 mm.times.20 mm,
with 4 mm spacing between the bars. The back side of the fired test
sample was glued onto an alumina substrate using a 2-part epoxy,
Hardman.RTM. (Royal adhesives and sealants, CA), and cured for at
least for 15 min. A 2 mm wide, 3'' long, tinned-copper ribbon
(Sn/Cu/Ag in 62/36/2 ratio; Ulbrich Inc, CT) was cut and flattened,
and then a thin layer of no-clean flux, 959T (Kester Inc, IL) was
applied onto a 1'' long portion and dried for 15 min. The dried
flux-coated portion was then placed on top of the fired bus bar.
Heat at 320.degree. C. was applied with the solder rod for
approximately 5 sec, resulting in the attachment of the ribbon to
the fired paste. This step was repeated for the other two bus bars
of the sample.
[0124] The adhesion test was performed by pulling the soldered tab
at a 90.degree. angle and measuring the force required at break
with Instron.RTM. Model 5569 (Instron Inc, MA). The average force
required to pull each tab was recorded. Four samples for each paste
were tested, for a total of 12 tab pulls per paste composition. The
average data from 4 samples are presented in Table 4.
[0125] The thick-film paste used for Comparative Example B is
similar to that of Example 1, but it does not contain PTOL.
TABLE-US-00004 TABLE 4 Results from the Adhesion Tests of Example 1
and Comparative Examples A, B and C Paste Formulation Average pull
(Frit; Ni/Ag) data (N) Example 3 4.89 (PTOL; 25/75) Comparative
Example A 2.01 (PTOL; 0/100) Comparative Example B 3.59 (Commercial
frit; 0/100) Comparative Example C 1.87 (Commercial lead silicate
frit, no PTOL; 25/75)
[0126] These results demonstrate that a paste of this invention
containing Ag, Ni and PTOL has a significant improvement in
adhesion to silicon wafers, relative to Comparative Examples A-C,
which lack either Ni or PTOL.
Examples 4-15 and Comparative Examples D-H
[0127] Paste preparations were accomplished with the following
procedure: The appropriate amount of solvent, medium and surfactant
was weighed then mixed in a Thinky.RTM. mixer for 30-60 sec, then
glass frits and metal additives were added and mixed for 1-2 min.
When well-mixed, the paste was passed through a 3-roll mill a few
times, at 0-250 psi. The gap of the rolls were typically adjusted
to 1 mil. The degree of dispersion was measured by fineness of
grind (FOG). A typical FOG value is generally equal to or less than
20/15 for conductors.
[0128] Compositions of the nickel, nickel alloy powders,
lead-tellurium oxides, and organic media used in Examples 4-21 are
described in Tables 5-7. Table 8 shows the metal (Ag+Ni or Ni
alloy) and lead-tellurium-oxide (glass frit) compositions in wt %
of the total composition.
TABLE-US-00005 TABLE 5 Nickel and nickel alloy powders Ni Ni
Ni--B--Cr--Fe Ni--Cr Type 1 Type II alloy alloy Particle 6 1 3.2 --
size in .mu.m (D.sub.50)
TABLE-US-00006 TABLE 6 Lead-tellurium-oxide compositions Glass
Glass Glass Glass I II III IV PbO 48.25 48.04 33.77 81.46 Li.sub.2O
0.42 2.39 0.2 TiO.sub.2 2.13 TeO.sub.2 51.75 51.54 61.71
B.sub.2O.sub.3 1.86 SiO.sub.2 15.76 Al.sub.2O.sub.3 0.2 ZrO.sub.2
0.42 Na.sub.2O 0.1 Sum 100 100 100 100
TABLE-US-00007 TABLE 7 Composition of the organic media for
Examples 4-15 and Comparative Examples D-H (in wt % of the total
weight of the thick-film paste) Examples 8% EC 11% EC F110 TST Duo
DBE3 Tex D, F, G, 0.74 0.74 2.6 0.5 1.04 2.6 3.78 H, 4, 5, 8-15 E,
6, 7 0.74 0.74 2.6 0.5 1.04 2.6 3.43 8% EC = 8 wt % of 48-50%
ethoxyl ethyl cellulose, dissolved in Texanol 11% EC = 11 wt % of
50-52% ethoxyl ethyl cellulose, dissolved in Texanol F110 = Foralyn
110 (hydrogenated rosin ester), 50 wt % dissolved in Texanol TST =
Thixatrol ST, an hydrogenated castor oil derivative Duo = Duomeen,
an amine oleate surfactant DBE3 = Dibasic ester-3 solvent (DBE-3)
Tex = Texanol, a solvent
TABLE-US-00008 TABLE 8 Metal (Ag + Ni or Ni alloy) and
lead-tellurium- oxide (frit) compositions in wt % of total
composition Ni Ni Glass Glass Glass Glass Ex. Ag Type I Type II
Ni--B--Cr--Fe Ni--Cr I II III IV D 88 2 4 77 11 2 5 66 22 2 E 87.65
2 0.7 6 76.69 10.96 2 0.7 7 65.74 21.91 2 0.7 F 88 1 1 8 77 11 1 1
9 66 22 1 1 G 88 2 10 77 11 2 11 66 22 2 12 77 11 2 13 66 22 2 H 88
1 1 14 77 11 1 1 15 66 22 1 1 Ni type I = 6.5 micron powder Ni type
II = 1 micron powder
Test Procedure
Efficiency
[0129] The solar cells built according to the method described
above were placed in a Berger IV tester to measure the
efficiencies. The light bulb in the IV tester simulated the
sunlight with a known intensity and irradiated the front surface of
the cell. The bus bars printed in the front of the cell were
connected to the multiple probes of the IV tester and the
electrical signals were transmitted through the probes to the
computer for calculating efficiencies.
Test Procedure
Adhesion
[0130] After firing, a solder ribbon (copper coated with
62Sn/36Pb/2Ag) was soldered to the bus bars printed on the front of
the cell. The soldering was typically carried out at 200.degree. C.
for 1-2 sec. The flux used was Kester.RTM. 959. The soldered area
was approximately 1.8 mm.times.145 mm. The adhesion strength was
obtained by pulling the ribbon at an angle of 180.degree. to the
surface of the cell at a speed of 120 mm/min.
TABLE-US-00009 TABLE 9 Performance results for Examples 4-8, 10-14
and Comparative Examples D-H Solderability Adhesion Example Eff(%)
FF (%) (N) D 17.76 76 90 2.5 E 17.74 75.8 75 2.5 F nm nm 75 2.4 G
17.71 75.5 nm 2.1 H 17.29 74.7 90 2.4 4 17.78 75.6 90 3.9 5 17.48
75.1 nm 4.7 6 17.77 76.1 75 5 7 17.77 75.9 75 2.9 8 nm nm 75 3 10
13.8 59.7 nm 3.1 11 10 42 nm 0.5 12 10.97 54 nm 3.5 13 9.55 44.1 nm
nm 14 16.46 71 75 2 nm = not measured
[0131] In similar experiments using Ni powders of different sizes
and 6 inch wafers, the following results were obtained, as shown in
Tables 10 and 11. In these examples, the frit component comprises
Glass II and Glass IV (see Table 8).
TABLE-US-00010 TABLE 10 Mean Adhesion Data (N) for thick-film
pastes using 0.4-6.6 micron-sized nickel powders Ni Ni/(Ag + Ni)
.times. 100% (microns) 0 0.5 5 12.5 20 30 50 0.4 4.42 4.93 nm 0.57
0.19 nm nm 1 4.42 4.55 nm 0.97 0.26 nm nm 2.5 4.42 4.07 4.42 2.06
0.97 nm <0.10 5 4.42 3.29 4.32 3.8 2.47 1.35 0.2 6.6 4.42 nm nm
4.54 3.44 nm nm Nm = Not measured
TABLE-US-00011 TABLE 11 Median Eff (%) at best temperature
(925-985.degree. C.) for thick-film pastes using 0.4-6.6
micron-sized nickel powders Ni Ni/(Ag + Ni) .times. 100% (microns)
0 0.5 5 12.5 20 30 50 0.4 16.50 16.66 nm 9.46 8.39 nm nm (985)
(985) (925) (925) 1 16.50 16.60 nm 15.60 14.31 nm nm (985) (985)
(925) (925) 2.5 16.50 16.68 16.63 16.51 15.74 nm 11.99 (985) (985)
(985) (945) (925) (925) 5 16.50 16.63 16.64 16.87 16.21 15.98 14.53
(985) (985) (965) (945) (945) (925) (925) 6.6 16.50 nm nm 16.22
16.44 nm nm (985) (985) (945) Nm = not measured
* * * * *