U.S. patent application number 14/584411 was filed with the patent office on 2015-07-23 for lead-tellurium inorganic reaction systems.
The applicant listed for this patent is Heraeus Precious Metals North America Conshohocken LLC. Invention is credited to Cuiwen Guo, Lei Wang, Li Yan.
Application Number | 20150206992 14/584411 |
Document ID | / |
Family ID | 52446193 |
Filed Date | 2015-07-23 |
United States Patent
Application |
20150206992 |
Kind Code |
A1 |
Guo; Cuiwen ; et
al. |
July 23, 2015 |
LEAD-TELLURIUM INORGANIC REACTION SYSTEMS
Abstract
An inorganic reaction system comprising a lead-tellurium-zinc
composition of Formula (I):
Pb.sub.a--Te.sub.b--Zn.sub.f-M.sub.d-O.sub.e, wherein 0<a, b, d,
or f.ltoreq.1, the sum of a, b, d and f is 1,
0.ltoreq.d.ltoreq.0.5, 0<f.ltoreq.0.2, a:b is between about
10:90 and about 90:10, (a+f+d):b is between about 10:90 and about
90:10, M is one or more elements, and e is a number sufficient to
balance the Pb, Te, Zn, and M components.
Inventors: |
Guo; Cuiwen; (Horsham,
PA) ; Yan; Li; (West Conshohocken, PA) ; Wang;
Lei; (Conshohocken, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Heraeus Precious Metals North America Conshohocken LLC |
West Conshohocken |
PA |
US |
|
|
Family ID: |
52446193 |
Appl. No.: |
14/584411 |
Filed: |
December 29, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61928744 |
Jan 17, 2014 |
|
|
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Current U.S.
Class: |
136/244 ;
106/425; 136/261; 252/514 |
Current CPC
Class: |
B05D 1/28 20130101; B05D
1/18 20130101; C01B 19/004 20130101; C01B 19/002 20130101; C09D
1/00 20130101; C09D 7/61 20180101; B05D 1/305 20130101; C03C 8/18
20130101; H01L 31/02168 20130101; H01L 31/022425 20130101; B05D
1/02 20130101; H01L 31/02008 20130101; H01L 31/1864 20130101; C09D
5/24 20130101; C03C 8/10 20130101; H01B 1/22 20130101; C03C 3/122
20130101; C09D 5/006 20130101; H01B 1/16 20130101; Y02E 10/50
20130101 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; C01B 19/00 20060101 C01B019/00; C09D 5/24 20060101
C09D005/24 |
Claims
1. An inorganic reaction system comprising a lead-tellurium-zinc
composition of Formula (I):
Pb.sub.a--Te.sub.b--Zn.sub.f-M.sub.d-O.sub.e, wherein 0<a, b, d,
or f.ltoreq.1, the sum of a, b, d and f is 1,
0.ltoreq.d.ltoreq.0.5, 0<f.ltoreq.0.2, a:b is between about
10:90 and about 90:10, (a+f+d):b is between about 10:90 and about
90:10, M is one or more elements, and e is a number sufficient to
balance the Pb, Te, Zn, and M components.
2. An inorganic reaction system comprising a
lead-tellurium-magnesium composition of Formula (II):
Pb.sub.a--Te.sub.b--(Mg.sub.w--Ca.sub.x--Sr.sub.y--Ba.sub.z)-M.sub.d-O.su-
b.e, wherein 0<a, b, or d.ltoreq.1, 0.ltoreq.w, x, y,
z.ltoreq.1, w+x+y+z=c, at least one of w, x, y and z is greater
than zero, the sum of a, b, c and d is 1, 0<c.ltoreq.0.2,
0.ltoreq.d.ltoreq.0.5, a:b is between about 10:90 and about 90:10,
(a+c+d):b is between about 10:90 and about 90:10, M is one or more
elements, and e is a number sufficient to balance the Pb, Te,
Mg--Ca--Sr--Ba and M components.
3. An inorganic reaction system comprising a
lead-tellurium-magnesium-zinc composition of Formula (III):
Pb.sub.a--Te.sub.b--(Mg.sub.w--Ca.sub.x--Sr.sub.y--Ba.sub.z)--Zn.sub.f-M.-
sub.d-O.sub.e, wherein 0<a, b, d, or f.ltoreq.1, 0.ltoreq.w, x,
y, z.ltoreq.1, w+x+y+z=c, at least one of w, x, y, and z is greater
than zero, the sum of a, b, c, d and f is 1, 0<c.ltoreq.0.2,
0<f.ltoreq.0.2, 0.ltoreq.d.ltoreq.0.5, a:b is between about
10:90 and about 90:10, (a+c+f+d):b is between about 10:90 and about
90:10, M is one or more elements, and e is a number sufficient to
balance the Pb, Te, Mg--Ca--Sr--Ba, Zn, and M components.
4. The inorganic reaction system according to claim 1, where d is
0.ltoreq.d.ltoreq.0.4.
5. The inorganic reaction system according to claim 1, wherein a:b
is between about 20:80 and about 80:20.
6. The inorganic reaction system according to claim 2, wherein
0<c.ltoreq.0.1.
7. The inorganic reaction system according to claim 1, wherein
0<f.ltoreq.0.1.
8. The inorganic reaction system according to claim 1, wherein the
inorganic reaction system is formed from at least about 5 wt %
lead-containing compound, preferably at least 8 wt %, and no more
than about 45 wt % lead-containing compound, preferably no more
than about 40 wt %, and most preferably no more than about 38 wt %,
based upon 100% total weight of the inorganic reaction system.
9. The inorganic reaction system according to claim 1, wherein the
inorganic reaction system is formed from at least about 20 wt %
tellurium-containing compound, preferably at least about 30 wt %,
and no more than about 70 wt % tellurium-containing composition,
and preferably no more than about 60 wt %, based upon 100% total
weight of the inorganic reaction system.
10. The inorganic reaction system according to claim 1, wherein the
inorganic reaction system is formed no more than about 15 wt %
zinc-containing compound, magnesium-containing compound, or both,
and preferably no more than about 10 wt %, based upon 100% total
weight of the inorganic reaction system.
11. The inorganic reaction system according to claim 1, wherein M
is selected from the group consisting of boron, aluminum, gallium,
silicon, germanium, tin, phosphorus, antimony, niobium, tantalum,
vanadium, titanium, molybdenum, tungsten, chromium, silver,
halides, chalcogenides, alkaline metals, alkaline earth metals, and
rare earth metals.
12. The inorganic reaction system according to claim 1, wherein the
inorganic reaction system further comprises bismuth.
13. An electroconductive paste comprising: metallic particles; at
least one of the inorganic reaction systems according to claim 1;
and an organic vehicle.
14. The electroconductive paste composition according to claim 13,
wherein the paste composition comprises at least about 50 wt %
metallic particles, preferably at least about 60 wt %, more
preferably at least about 70 wt %, and most preferably at least
about 80 wt %, and no more than about 95 wt % of metallic
particles, based upon 100% total weight of the paste.
15. The electroconductive paste composition according to claim 13,
wherein the metallic particles are selected from the group
consisting of silver, aluminum, gold, copper, nickel, and alloys or
mixtures thereof, preferably silver.
16. The electroconductive paste composition according to claim 13,
wherein the paste composition comprises at least about 0.1 wt % of
the IRS system, preferably at least about 0.5 wt %, and no more
than about 10 wt %, more preferably no more than about 5 wt %, and
most preferably no more than about 3 wt %, based upon 100% total
weight of the paste.
17. The electroconductive paste composition according to claim 13,
wherein the paste composition comprises at least about 0.01 wt %
organic vehicle, and no more than about 50 wt %, preferably no more
than about 30 wt %, and most preferably no more than about 20 wt %,
based upon 100% total weight of the paste.
18. The electroconductive paste composition according to claim 13,
wherein the organic vehicle comprises an organic solvent and one or
more of a binder, surfactant, and thixotropic agent, or any
combination thereof.
19. The electroconductive paste composition according to claim 18,
wherein the organic solvent is selected from the group consisting
of carbitol, terpineol, hexyl carbitol, texanol, butyl carbitol,
butyl carbitol acetate, dimethyladipate glycol ether, and any
combination thereof.
20. The electroconductive paste composition according claim 18,
wherein the binder is selected from the group consisting of ethyl
cellulose, phenolic resin, polyacrylic acid, polyvinyl butyral,
polyester resin, polycarbonate, polyethylene resin, polyurethane
resin, rosin derivative, and any combination thereof.
21. The electroconductive paste composition according to claim 18,
wherein the surfactant is selected from the group consisting of
polyethylene oxide, polyethylene glycol, benzotriazole,
poly(ethyleneglycol)acetic acid, lauric acid, oleic acid, capric
acid, myristic acid, linoleic acid, stearic acid, palmitic acid,
stearate salts, palmitate salts, and any combination thereof.
22. A solar cell produced by applying an electroconductive paste
according to claim 13 to a silicon wafer and firing the silicon
wafer.
23. The solar cell according to claim 22, wherein the
electroconductive paste is applied to an antireflective coating on
a surface of the silicon wafer.
24. A solar cell module comprising electrically interconnected
solar cells according to claim 22.
25. A method of producing a solar cell, comprising the steps of:
providing a silicon wafer having a front side and a backside;
applying an electroconductive paste according to claim 13 to the
silicon wafer; and firing the silicon wafer.
26. The method of producing a solar cell according to claim 25,
wherein the electroconductive paste is applied to the front side of
the silicon wafer.
Description
TECHNICAL FIELD
[0001] This invention relates to lead-tellurium-zinc (PTZ),
lead-tellurium-alkaline earth metal (PTM), and
lead-tellurium-alkaline earth metal-zinc (PTMZ) inorganic reaction
systems (IRS). In one aspect of the invention, an electroconductive
paste composition utilized in solar panel technology, especially
for forming front side electrical contacts, includes conductive
particles, an organic vehicle, and the PTZ, PTM, and/or PTMZ IRS of
the invention.
BACKGROUND
[0002] Solar cells are devices that convert the energy of light
into electricity using the photovoltaic effect. Solar power is an
attractive green energy source because it is sustainable and
produces only non-polluting by-products. Accordingly, a great deal
of research is currently being devoted to developing solar cells
with enhanced efficiency while continuously lowering material and
manufacturing costs. In operation, when light hits a solar cell, a
fraction of the incident light is reflected by the surface and the
remainder is transmitted into the solar cell. The photons of the
transmitted light are absorbed by the solar cell, which is usually
made of a semiconducting material such as silicon. The energy from
the absorbed photons excites electrons of the semiconducting
material from their atoms, generating electron-hole pairs. These
electron-hole pairs are then separated by p-n junctions and
collected by conductive electrodes applied on the solar cell
surface.
[0003] Solar cells typically have electroconductive pastes applied
to both their front and back surfaces. A front side paste, which
typically includes silver, is screen printed onto the front side of
the substrate to serve as a front electrode. A typical
electroconductive paste contains conductive metallic particles,
glass frit, and an organic vehicle. In some instances, the glass
frit etches through an antireflection coating, such as a silicon
nitride coating, on the surface of the silicon substrate upon
firing, helping to build electrical contact between the conductive
particles and the silicon substrate. On the other hand, it is
desirable that the glass frit is not so aggressive that it shunts
the p-n junction after firing. For example, glass frits which
include relatively high amounts of lead oxide and bismuth oxide may
damage the antireflection layer and degrade the p-n junction of the
substrate. As a result, the electrical performance of the solar
cell may be reduced. In addition, glass frits are known to have
wide melting temperature ranges, making their behavior strongly
dependent on their composition and processing parameters. As such,
the ability to predict glass processing parameters and behavior
under fast firing processes is difficult with known glass
frits.
[0004] Thus, an IRS which optimizes contact between the
electroconductive paste and the underlying substrate so as to
achieve improved solar cell efficiency, without being so aggressive
that it damages the antireflection layer and p-n junction, is
needed. Further, an IRS having more predictable processing
behaviors is also desirable.
SUMMARY
[0005] The invention provides a lead inorganic reaction system
(IRS) containing zinc and/or an alkaline earth metal which, when
used in an electroconductive paste, improves electrical contact
with the underlying substrate. Moreover, due to the improved
contact performance from the inclusion of zinc and/or an alkaline
earth metal in the IRS, the lead content can be reduced and more
predictable processing parameters can be achieved.
[0006] One aspect of the invention is an inorganic reaction system
comprising a lead-tellurium-zinc composition of Formula (I):
Pb.sub.a--Te.sub.b--Zn.sub.f-M.sub.d-O.sub.e, wherein 0<a, b, d,
or f.ltoreq.1, the sum of a, b, d and f is 1,
0.ltoreq.d.ltoreq.0.5, 0<f.ltoreq.0.2, a:b is between about
10:90 and about 90:10, (a+f+d):b is between about 10:90 and about
90:10, M is one or more elements, and e is a number sufficient to
balance the Pb, Te, Zn, and M components.
[0007] An inorganic reaction system comprising a
lead-tellurium-magnesium composition of Formula (II):
Pb.sub.a--Te.sub.b-(Mg.sub.w--Ca.sub.x--Sr.sub.y--Ba.sub.z)-M.sub.d-O.sub-
.e, wherein 0<a, b, or d.ltoreq.1, 0.ltoreq.w, x, y, z.ltoreq.1,
w+x+y+z=c, at least one of w, x, y and z is greater than zero, the
sum of a, b, c and d is 1, 0<c.ltoreq.0.2,
0.ltoreq.d.ltoreq.0.5, a:b is between about 10:90 and about 90:10,
(a+c+d):b is between about 10:90 and about 90:10, M is one or more
elements, and e is a number sufficient to balance the Pb, Te,
Mg--Ca--Sr--Ba and M components.
[0008] An inorganic reaction system comprising a
lead-tellurium-magnesium-zinc composition of Formula (III):
Pb.sub.a--Te.sub.b-(Mg.sub.w--Ca.sub.x--Sr.sub.y--Ba.sub.z)--Zn.sub.f-M.s-
ub.d-O.sub.e, wherein 0<a, b, d, or f.ltoreq.1, 0.ltoreq.w, x,
y, z.ltoreq.1, w+x+y+z=c, at least one of w, x, y, and z is greater
than zero, the sum of a, b, c, d and f is 1, 0<c.ltoreq.0.2,
0<f.ltoreq.0.2, 0.ltoreq.d.ltoreq.0.5, a:b is between about
10:90 and about 90:10, (a+c+f+d):b is between about 10:90 and about
90:10, M is one or more elements, and e is a number sufficient to
balance the Pb, Te, Mg--Ca--Sr--Ba, Zn, and M components. Another
aspect of the invention is a solar cell produced by applying the
electroconductive paste of the invention to a silicon wafer and
firing the silicon wafer.
[0009] The invention also provides a solar cell module comprising
electrically interconnected solar cells according to the
invention.
[0010] Another aspect of the invention is a method of producing a
solar cell, including the steps of providing a silicon wafer having
a front side and a backside, applying the electroconductive paste
of the invention to the silicon wafer, and firing the silicon
wafer.
DETAILED DESCRIPTION
[0011] The invention relates to PTZ, PTM, and PTMZ inorganic
reaction systems. While not limited to such an application, the IRS
compositions, or combinations thereof, may be used in an
electroconductive paste composition such as those used in silicon
solar cells. The electroconductive paste composition preferably
comprises conductive metallic particles, an organic vehicle, and at
least one of the PTZ, PTM, or PTMZ IRS compositions. The
electroconductive paste composition may comprise one or more
additional additives.
[0012] In other embodiments, the IRS may include a combination of
multiple PTZ, PTM and PTMZ glass compositions, PTZ, PTM and PTMZ
glass compositions with PTZ, PTM and PTMZ-containing compounds, or
PTZ, PTM and PTMZ-containing compounds (e.g., organometallic
compounds, salts) that form a PTZ, PTM and PTMZ IRS during physical
processing (e.g., mechanochemical processing, milling, grinding) or
chemical processing (e.g., firing, thermal decomposition, photo or
radiochemical decomposition). In other embodiments the elements
forming the PTZ, PTM and PTMZ may be present in a single component
or distributed among two or more components, which may be amorphous
or crystalline or partially crystalline.
[0013] When applied to silicon solar cells, such pastes may be used
to form an electrical contact layer or electrode, either on the
front side or backside of the silicon wafer.
[0014] In one preferred embodiment, the electroconductive paste is
used on the front side of a silicon wafer for a solar cell and
includes silver conductive particles, the IRS composition(s) of the
invention, and an organic vehicle.
Inorganic Reaction System
[0015] The invention relates to an IRS for use, for example, in an
electroconductive paste composition. The IRS serves multiple
functions when used in an electroconductive paste composition.
First, the IRS provides a delivery media for the conductive
particles, allowing them to migrate from the paste to the interface
of the semiconductor substrate. The IRS system also provides a
reaction media for the paste components to undergo physical and
chemical reactions at the interface when subjected to elevated
temperatures. Physical reactions include, but are not limited to,
melting, dissolving, diffusing, sintering, precipitating, and
crystallizing. Chemical reactions include, but are not limited to,
synthesis (forming new chemical bonds) and decomposition, reduction
and oxidation, and phase transitioning. Further, the IRS also acts
as an adhesion media that provides bonding between the conductive
particles and the semiconductor substrate, thereby improving
electrical contact performance during the lifetime of the solar
device. Although intended to achieve the same effects, existing
glass frit compositions can result in high contact resistance at
the interface of the electroconductive paste and the silicon wafer,
due to the insulative properties of the glass. The IRS of the
invention provides the desired delivery, reactivity, and adhesion
media, but also lowers contact resistance and improves overall cell
performance.
[0016] More specifically, the IRS provides improved Ohmic and
Schottky contact between the conductive particles and the
semiconductor substrate (e.g., silicon substrate) in the solar
cell. The IRS is a reactive media with respect to the silicon and
creates active areas on the silicon substrate that improve overall
contact, such as through direct contact or tunneling. The improved
contact properties provide better Ohmic contact and Schottky
contact, and therefore better overall solar cell performance.
Further, without being bound by any particular theory, the
inclusion of zinc and/or an alkaline earth metal in the IRS is
believed to improve the contact properties of the electroconductive
paste. Further, the combination of the IRS components, in certain
amounts, provides a paste with a widened range of glass transition
temperatures, softening temperatures, melting temperatures,
crystallization temperatures, and flowing temperatures, thus
broadening the processing window of the resulting paste. This
allows the resulting electroconductive paste to have improved
compatibility with a wide variety of substrates.
[0017] The IRS may include glass material(s), ceramic material(s),
any other compound(s) known in the art to form a reactive matrix at
an elevated temperature. In one embodiment, the IRS may include at
least one substantially amorphous glass frit. In another
embodiment, the IRS may incorporate crystalline phases or
compounds, or a mixture of amorphous, partially crystalline, and/or
crystalline materials. The IRS may also include other oxides or
compounds known in the art. For example, oxides of magnesium,
nickel, tellurium, tungsten, zinc, gadolinium, antimony, cerium,
zirconium, titanium, manganese, tin, ruthenium, cobalt, iron,
copper and chromium, or any combination of at least two thereof,
preferably zinc, antimony, manganese, nickel, tungsten, tellurium
and ruthenium, or a combination of at least two thereof, compounds
which can generate those metal oxides upon firing, or a mixture of
at least two of the aforementioned metals, a mixture of at least
two of the aforementioned oxides, a mixture of at least two of the
aforementioned compounds which can generate those metal oxides on
firing, or mixtures of two or more of any of the above mentioned,
may be used. Other glass matrix formers or glass modifiers, such as
germanium oxide, vanadium oxide, molybdenum oxides, niobium oxides,
indium oxides, other alkaline and alkaline earth metal (e.g., K,
Rb, Cs, Ca, Sr, and Ba) compounds, rare earth oxides (e.g.,
La.sub.2O.sub.3, cerium oxides), phosphorus oxides or metal
phosphates, and metal halides (e.g., lead fluorides and zinc
fluorides) may also be used as additives to adjust properties such
as the glass transition temperature of the IRS. In one embodiment,
the IRS may contain a combination of at least one glass and at
least one oxide or additive.
PTZ Inorganic Reaction System
[0018] According to one embodiment, the PTZ IRS may be expressed by
the following formula:
Pb.sub.a--Te.sub.b--Zn.sub.f-M.sub.d-O.sub.e (Formula I)
[0019] where 0<a, b, d, or f.ltoreq.1, the sum of a, b, d and f
is 1, 0.ltoreq.d.ltoreq.0.5, 0<f.ltoreq.0.2, M is one or more
metals which can act as glass formers, and the variable "e" charge
balances the Pb.sub.a--Te.sub.b--Zn.sub.f-M.sub.d components.
Preferably, 0.ltoreq.d.ltoreq.0.4. Preferably,
0<f.ltoreq.0.1.
[0020] M may be any metal including, but not limited to, alkaline
metals, alkaline earth metals, rare earth metals, boron, aluminum,
gallium, silicon, germanium, tin, phosphorus, antimony, niobium,
tantalum, vanadium, titanium, molybdenum, tungsten, chromium,
silver, lead and any combinations thereof. Preferably, M is
lithium, boron, silicon, or any combination thereof.
[0021] Preferably, the ratio of lead to tellurium, or a:b, is
between about 10:90 and about 90:10. More preferably, the a:b ratio
is between about 20:80 and about 80:20. The ratio of tellurium to
zinc, or b:f, is preferably between about 5:95 and about 95:5. More
preferably the b:f ratio is between about 1:1 and about 20:1.
Further, the ratio of lead and zinc to tellurium, or (a+f+d):b, is
preferably between about 10:90 to about 90:10, more preferably
between about 20:80 to about 40:60. The ratio of lead to zinc, or
a:f, is preferably between about 10:90 to about 90:10. More
preferably, the a:f ratio is between about 30:70 to about
70:30.
[0022] Formula I is preferably formulated as a one-glass
composition, whereby the starting components of the IRS (Pb, Te,
Zn, and additional metal) all chemically react to form one complex
compound or composite, instead of a glass having a physical mixture
of various oxides. Alternatively, the zinc component may be
included outside of Formula I as an additive to the IRS.
[0023] According to another embodiment, the PTZ IRS may be
expressed by the following formula:
Pb.sub.a--Te.sub.b--Zn.sub.f--Bi.sub.g-M.sub.d-O.sub.e (Formula
IA)
[0024] where a, b, d, e, and f, and all ratios thereof, as well as
M, are defined as they were in Formula I. Regarding the bismuth
component, the ratio a:g is between about 10:90 and about 90:10.
More preferably, the a:g ratio is between about 15:85 and about
85:15. The ratio of g:b is preferably between about 5:95 and about
95:5. More preferably, the g:b ratio is between about 10:90 and
about 80:20. The ratio f:g is preferably between about 10:90 and
about 90:10. More preferably, the f:g ratio is between about 15:85
and about 85:10.
[0025] Formula IA may be formulated as a one-glass composition
according to the same parameters set forth above for Formula I.
PTM Inorganic Reaction System
[0026] According to one embodiment, the PTM IRS may be expressed by
the following formula:
Pb.sub.a--Te.sub.b--(Mg.sub.w--Ca.sub.x--Sr.sub.y--Ba.sub.z)-M.sub.d-O.s-
ub.e (Formula II)
[0027] where 0<a, b, or d.ltoreq.1, 0.ltoreq.w, x, y,
z.ltoreq.1, at least one of w, x, y, and z is greater than zero,
w+x+y+z=c, 0<c.ltoreq.0.2, 0.ltoreq.d.ltoreq.0.5, the sum of a,
b, c and d is 1, and the variable "e" charge balances the
Pb.sub.a--Te.sub.b-(Mg.sub.w--Ca.sub.x--Sr.sub.y--Ba.sub.z).sub.c-M.sub.d
components. Preferably, "c", as set forth above, is less than or
equal to 0.1 and at least 0.005. The variable "d" is less than or
equal to 0.4.
[0028] While the use of magnesium is preferred, any of calcium,
strontium, or barium may be used instead of, or in addition to,
magnesium. Further, M may be any element or component including,
but not limited to, alkaline metals, alkaline earth metals, rare
earth metals, boron, aluminum, gallium, silicon, germanium, tin,
phosphorus, antimony, niobium, tantalum, vanadium, titanium,
molybdenum, tungsten, chromium, silver, halides, chalcogenides,
lead and any combinations thereof. Preferably, M is lithium, boron,
silicon, or any combination thereof.
[0029] In one embodiment, the ratio of lead to tellurium, or a:b,
is between about 10:90 and 90:10. More preferably, the a:b ratio is
between about 1:10 and about 10:1. The ratio of tellurium to the
Mg--Ca--Sr--Ba component, or b:c, is preferably between about 5:95
and about 95:5. More preferably, the b:c ratio is between about 1:1
and about 20:1. Further, the ratio of lead and the Mg--Ca--Sr--Ba
component to tellurium, or (a+c+d):b, is preferably between about
10:90 to about 90:10, and more preferably between about 20:80 to
about 40:60. The ratio of lead to the Mg--Ca--Sr--Ba component, or
a:c, is preferably between about 10:90 to about 90:10. More
preferably, the a:c ratio is between 30:70 to about 70:30.
[0030] Formula II is preferably formulated as a one-glass
composition, whereby the starting components of the IRS (Pb, Te,
Zn, Mg, or Ca, Sr, Ba, and additional metal) all chemically react
to form one complex compound or composite, instead of a glass
having a physical mixture of various oxides. Alternatively, the
Mg--Ca--Sr--Ba component may be included outside of Formula II as
an additive to the IRS.
[0031] According to another embodiment, the PTM IRS may be
expressed by the following formula:
Pb.sub.a--Te.sub.b--(Mg.sub.w--Ca.sub.x--Sr.sub.y--Ba.sub.z).sub.c--Bi.s-
ub.g-M.sub.d-O.sub.e (Formula IIA)
[0032] where a, b, c, d, and e, and all ratios thereof, as well as
M, are defined as they were in Formula II. Regarding the bismuth
component, the ratio a:g is between about 10:90 and about 90:10.
More preferably, the a:g ratio is between about 15:85 and about
85:15. The ratio of g:b is preferably between about 5:95 and about
95:5. More preferably, the g:b ratio is between about 10:90 and
about 80:20. The ratio f:g is preferably between about 10:90 and
about 90:10. More preferably, the f:g ratio is between about 15:85
and about 85:10.
[0033] Formula IIA may be formulated as a one-glass composition
according to the same parameters set forth above for Formula
II.
PTMZ Inorganic Reaction System
[0034] According to one embodiment, the PTMZ IRS may be expressed
by the following formula:
Pb.sub.a--Te.sub.b--(Mg.sub.w--Ca.sub.x--Sr.sub.y--Ba.sub.z)--Zn.sub.f-M-
.sub.d-O.sub.e (Formula III)
[0035] where 0<a, b, d, or f.ltoreq.1, 0.ltoreq.w, x, y,
z.ltoreq.1, w+x+y+z=c, at least one of w, x, y, and z is greater
than zero, 0<f.ltoreq.0.2, 0<d.ltoreq.0.5, the sum of a, b,
c, d, and f is 1, and the variable "e" charge balances the
Pb.sub.a--Te.sub.b-(Mg.sub.w--Ca.sub.x--Sr.sub.y--Ba.sub.z).sub.c--Zn.sub-
.f-M.sub.d components. Preferably, "c", as set forth above, is less
than or equal to about 0.2, and preferably less than or equal to
about 0.1. At the same time, "c" is at least 0.05. The variable "d`
is less than or equal to 0.4. The variable "f" is preferably less
than 0.1.
[0036] While the use of magnesium is preferred, any of calcium,
strontium, or barium may be used instead of, or in addition to,
magnesium. Further, M may be any element including, but not limited
to, alkaline metals, alkaline earth metals, rare earth metals,
boron, aluminum, gallium, silicon, germanium, tin, phosphorus,
antimony, niobium, tantalum, vanadium, titanium, molybdenum,
tungsten, chromium, silver, lead, halides, chalcogenides, and any
combinations thereof. Preferably, M is lithium, boron, silicon, or
any combination thereof.
[0037] In one embodiment, the ratio of lead to tellurium, or a:b,
is between about 10:90 and about 90:10. More preferably, the a:b
ratio is between about 15:85 and about 30:70. The ratio of
tellurium to the Mg--Ca--Sr--Ba component, or b:c, is preferably
between about 5:95 and about 95:5. More preferably, the b:c ratio
is between about 1:1 and about 20:1. The ratio of lead to the
Mg--Ca--Sr--Ba component, or a:c, is preferably between 10:90 and
90:10. More preferably, the a:c ratio is between 1:10 and 10:1. The
ratio of the Mg--Ca--Sr--Ba component to the zinc component, or
c:f, is between about 1:1 and about 20:1. The ratio of tellurium to
zinc, or b:f, is preferably between about 5:95 and about 95:5. More
preferably the b:f ratio is between about 1:1 and about 20:1.
Further, the ratio of lead and the Mg--Ca--Sr--Ba component and
zinc and other elements to tellurium, or (a+c+d+f):b, is preferably
between about 10:90 and about 90:10, more preferably between about
20:80 and about 40:60.
[0038] Formula III is preferably formulated as a one-glass
composition, whereby the starting components of the IRS (Pb, Te,
Zn, Mg, or Ca, Sr, Ba, Zn and additional metal) all chemically
react to form one complex compound or composite, instead of a glass
having a physical mixture of various oxides. Alternatively, the
Mg--Ca--Sr--Ba and/or zinc components may be included outside of
Formula III as an additive to the IRS.
[0039] According to yet another embodiment, the PTMZ IRS may be
expressed by the following formula:
Pb.sub.a--Te.sub.b--(Mg.sub.w--Ca.sub.x--Sr.sub.y--Ba.sub.z).sub.c--Bi.s-
ub.g--Zn.sub.f-M.sub.d-O.sub.e (Formula IIIA)
[0040] where a, b, c, d, e, and f, and all ratios thereof, as well
as M, are defined as they were in Formula III. Regarding the
bismuth component, the ratio a:g is between about 10:90 and about
90:10. More preferably, the a:g ratio is between about 15:85 to
about 85:15. The ratio of g:b is preferably between about 5:95 and
about 95:5. More preferably, the g:b ratio is between about 10:90
and about 80:20. The ratio f:g is preferably between about 10:90 to
about 90:10. More preferably, the f:g ratio is between about 15:85
to about 85:10.
[0041] Formula IIIA may be formulated as a one-glass composition
according to the same parameters set forth above for Formula
III.
[0042] In other embodiments, the IRS may include a combination of
multiple glass compositions, such as combinations of Formulas I, II
or III, glass compositions with PTZ, PTM, or PTMZ-containing
compounds, or compounds (e.g., organometallic compounds, salts)
that form a PTZ, PTM or PTMZ IRS during physical processing (e.g.,
mechanochemical processing, milling, grinding) or chemical
processing (e.g., firing, thermal decomposition, photo or
radiochemical decomposition).
[0043] The IRS may be formed of crystalline or partially
crystalline starting materials. The elements forming the IRS may be
present in a single component or distributed amount two or more
components. Preferably, the starting materials used to prepare the
IRS compositions are lead oxide (e.g., PbO), tellurium oxide (e.g.,
TeO.sub.2), and oxides of zinc and/or the alkaline earth metals,
such as zinc oxide (e.g, ZnO) and magnesium oxide (e.g., MgO).
However, any known lead, tellurium, zinc, and magnesium-containing
compositions which may be used to formulate a PTZ, PTM, or PTMZ
system according to Formulas I, II and III may be used.
[0044] According to one embodiment, the starting materials used to
prepare the IRS comprise at least about 5 wt % lead-containing
compound (e.g., PbO), and preferably at least about 8 wt %
lead-containing compound, based upon 100% total weight of the IRS.
At the same time, the composition comprises no more than about 45%
lead-containing compound, preferably no more than about 40 wt %,
and most preferably no more than about 38 wt %. Further, the
materials preferably include at least 20 wt % tellurium-containing
compound (e.g., TeO.sub.2), and preferably at least about 30 wt %,
based upon 100% total weight of the starting materials used to
prepare the IRS. At the same time, the materials preferably include
no more than about 70 wt % tellurium-containing compound, and
preferably no more than about 60 wt %. With respect to zinc and
alkaline earth metal (e.g., magnesium), the starting materials
preferably include no more than about 15 wt % of such compound, and
preferably no more than about 10 wt %, based upon 100% total weight
of the IRS. At the same time, the starting materials may include at
least about 0.1 wt % of such compound, and preferably at least
about 0.3 wt %, based upon 100% total weight of the IRS.
[0045] Other glass matrix formers may also be used to form the PBT
IRS, as designed by "M" in Formula I. Suitable compounds include,
but are not limited to, compounds of alkaline metals, alkaline
earth metals, rare earth metals, boron, aluminum, gallium, silicon,
germanium, tin, phosphorus, antimony, niobium, tantalum, vanadium,
titanium, molybdenum, tungsten, chromium, silver, halides,
chalcogenides, and any combinations thereof. Preferably, the
starting materials containing these metals are metal oxides, such
as, for example Li.sub.2O, Na.sub.2O, SiO.sub.2, Al.sub.2O.sub.3,
MoO.sub.3, MgO, Cr.sub.2O.sub.3, P.sub.2O.sub.5, B.sub.2O.sub.3,
and Ag.sub.2O. Metal halides, such as AgI or PbF.sub.2, may also be
used. In one preferred embodiment, the IRS includes lithium, boron,
silicon, or any combination thereof. In another embodiment, the IRS
includes bismuth, as set forth more fully herein. If present, the
starting materials of the IRS include at least about 0.1 wt % of
the above-referenced elements. At the same time, they include no
more than about 30 wt %, and preferably no more than about 20 wt %,
based upon 100% total weight of the IRS.
Forming IRS Composition
[0046] The IRS may be formed by any method known in the art,
including solid state synthesis, melting and quenching, or other
Chimie Douce (soft chemistry) processes. In a typical melting and
quenching process, the first step is to mix the appropriate amounts
of the starting materials (usually in powder form). This mixture is
then heated in air or in an oxygen-containing atmosphere to form a
melt. The melt is then quenched, and then it is ground, ball
milled, and screened, in order to provide a mixture with the
desired particle size. For example, components in powder form may
be mixed together in a V-comb blender. The mixture is then heated
(e.g., to around 800-1200.degree. C.) for about 30-40 minutes such
that the starting materials may react to form a one-glass system.
The IRS is then quenched, taking on a sand-like consistency. This
coarse powder is milled, such as in a ball mill or jet mill, until
a fine powder results. The IRS particles may be milled to an
average particle size (d.sub.50) of about 0.01-20 .mu.m, preferably
about 0.1-5 .mu.m. In one embodiment, the IRS particles may be
formed as nano sized particles having a d.sub.50 ranging from about
5 to about 100 nm.
[0047] Chimie Douce (soft chemistry) processes are carried out at
temperatures of about 20.degree. C. to about 500.degree. C. Chimie
Douce reactions are topotactic, meaning that structural elements of
the reactants are preserved in the product, but the composition
changes. Such processes include, but are not limited to, sol-gel
processes, precipitation, hydrothermal/solvothermal processes, and
pyrolysis.
[0048] Conventional solid state synthesis may also be used to
prepare the IRS system described herein. In this process, raw
starting materials are sealed in a fused quartz tube or tantalum or
platinum tube under vacuum, and then heated to about
700-1200.degree. C. The materials dwell at this elevated
temperature for about 12-48 hours and then are slowly cooled (about
0.1.degree. C./minute) to room temperature. In some cases, solid
state reactions may be carried out in an alumina crucible in
air.
[0049] Yet another process for preparing the IRS system is
co-precipitation. In this process, the metal elements are reduced
and co-precipitated with other metal oxides or hydroxides to form a
solution containing metal cations by adjusting the pH levels or by
incorporating reducing agents. The precipitates of these metals,
metal oxides or hydroxides are then dried and fired under vacuum at
about 400-800.degree. C. to form a fine powder.
Electroconductive Paste Composition
[0050] One aspect of the invention relates to an electroconductive
paste composition. A desired electroconductive paste is one which
is highly conductive, so as to optimize the resulting solar cell's
electrical performance. The electroconductive paste composition is
generally comprised of metallic particles, organic vehicle, and at
least one of the IRS compositions discussed herein. According to
one embodiment, the electroconductive paste comprises: (i) at least
about 50 wt % and no more than about 95 wt % metallic particles;
(ii) at least about 1 wt % and no more than about 10 wt % IRS; and
(iii) at least about 1 wt % and no more than about 25 wt % organic
vehicle, based upon 100% total weight of the paste.
[0051] The electroconductive paste of the invention includes at
least one of the IRS compositions of the invention, as set forth
herein. Preferably, the electroconductive paste includes at least
about 0.1 wt % of the IRS, and preferably at least about 0.5 wt %.
At the same time, the paste includes no more than about 10 wt % of
the IRS, preferably no more than about 5 wt %, and most preferably
no more than about 3 wt %, based upon 100% total weight of the
paste.
[0052] According to one embodiment of the invention, the IRS should
have a glass transition temperature range (T.sub.g) below the
desired firing temperature of the electroconductive paste.
Preferred IRS components have a T.sub.g range of at least about
250.degree. C., preferably at least 300.degree. C., and most
preferably at least 350.degree. C. At the same time, preferred IRS
materials have a T.sub.g range of no more than about 750.degree.
C., preferably no more than about 700.degree. C., and most
preferably no more than about 650.degree. C., when measured using
thermomechanical analysis. Specifically, the glass transition
temperature may be determined using a DSC apparatus, TA Instruments
SDT Q600 Simultaneous TGA/DSC (TA Instruments). For the
measurements and data evaluation, the measurement software TA
Universal Analysis 2000, V 4.5 A is applied. As pan for reference
and sample, Alumina sample cups (commercially available from TA
Instruments) with a diameter of 6.8 mm and a volume of about 90
.mu.l are used. An amount of about 20-50 mg of the sample is
weighted into the sample pan with an accuracy of 0.01 mg. The empty
reference pan and the sample pan are placed in the apparatus, the
oven is closed, and the measurement started. A heating rate of
10-50.degree. C./min is employed from a starting temperature of
25.degree. C. to an end temperature of 1000.degree. C. The balance
in the instrument is always purged with nitrogen (N.sub.2 5.0) and
the oven is purged with synthetic air (80% N.sub.2 and 20% O.sub.2
from Linde) with a flow rate of 50 ml/min. The first step in the
DSC signal is evaluated as glass transition using the software
described above and the determined onset value is taken as the
temperature for T.sub.g.
[0053] It is well known in the art that IRS solid particles can
exhibit a variety of shapes, sizes, and coating layers. For
example, a large number of shapes of IRS solid particles are known
to the person skilled in the art. Some examples include spherical,
angular, elongated (rod or needle like), and flat (sheet like,
flakes). IRS solid particles may also be present as a combination
of particles of different shapes (e.g., spheres and flakes). Glass
particles with a shape, or combination of shapes, which favor
advantageous adhesion of the produced electrode are preferred.
[0054] The median particle diameter d.sub.50 is a characteristic of
particles well known to the person skilled in the art. D.sub.50 is
the median diameter or the medium value of the particle size
distribution. It is the value of the particle diameter at 50% in
the cumulative distribution. Particle size distribution may be
measured via laser diffraction, dynamic light scattering, imaging,
electrophoretic light scattering, or any other method known in the
art. A Horiba LA-910 Laser Diffraction Particle Size Analyzer
connected to a computer with the LA-910 software program is used to
determine the particle size distribution of the glass frit. The
relative refractive index of the glass frit particle is chosen from
the LA-910 manual and entered into the software program. The test
chamber is filled with deionized water to the proper fill line on
the tank. The solution is then circulated by using the circulation
and agitation functions in the software program. After one minute,
the solution is drained. This is repeated an additional time to
ensure the chamber is clean of any residual material. The chamber
is then filled with deionized water for a third time and allowed to
circulate and agitate for one minute. Any background particles in
the solution are eliminated by using the blank function in the
software. Ultrasonic agitation is then started, and the glass frit
is slowly added to the solution in the test chamber until the
transmittance bars are in the proper zone in the software program.
Once the transmittance is at the correct level, the laser
diffraction analysis is run and the particle size distribution of
the glass is measured and given as d50.
[0055] In a preferred embodiment, the median particle diameter
d.sub.50 of the IRS particles is at least about 0.1 .mu.m, and
preferably no more than about 20 .mu.m, more preferably no more
than about 5 .mu.m, more preferably no more than about 2 .mu.m, and
most preferably no more than about 1 .mu.m.
[0056] The IRS particles may be present with a surface coating. Any
such coating known in the art and suitable in the context of the
invention can be employed on the IRS particles. Preferred coatings
are those coatings which promote improved adhesion characteristics
of the electroconductive paste. If such a coating is present, it is
preferred for that coating to be present in an amount of no more
than 10 wt %, preferably no more than about 8 wt %, more preferably
no more than about 5 wt %, more preferably no more than about 3 wt
%, and most preferably no more than about 1 wt %, in each case
based on the total weight of the IRS component.
[0057] Preferably, IRS particles have a specific surface area of at
least about 0.1 m.sup.2/g and no more than about 15 m.sup.2/g,
preferably at least about 1 m.sup.2/g and no more than about 10
m.sup.2/g. Methods of measuring specific surface area are known in
the art. As set forth herein, all surface area measurements were
performed using the BET (Brunauer-Emmett-Teller) method via a
Monosorb MS-22 analyzer (manufactured by Quantachrome Instruments
of Boynton Beach, Fla.) which operates according to the SMART
method. Samples are prepared for analysis in the built-in degas
station. Flowing gas sweeps away impurities, resulting in a clean
surface upon which adsorption may occur. The sample can be heated
to a user-selectable temperature with the supplied heating mantle.
Digital temperature control and display are mounted on the
instrument front panel. After degassing is complete, the sample
cell is transferred to the analysis station. Quick connect fittings
automatically seal the sample cell during transfer. With the push
of a single button, analysis commences. A dewar flask filled with
coolant is automatically raised, immersing the sample cell and
causing adsorption. The instrument detects when adsorption is
complete (2-3 minutes), automatically lowers the dewar flask, and
gently heats the sample cell back to room temperature using a
built-in hot-air blower. As a result, the desorbed gas signal is
displayed on a digital meter and the surface area is directly
presented on a front panel display. The entire measurement
(adsorption and desorption) cycle typically requires less than six
minutes. The technique uses a high sensitivity, thermal
conductivity detector to measure the change in concentration of an
adsorbate/inert carrier gas mixture as adsorption and desorption
proceed. When integrated by the on-board electronics and compared
to calibration, the detector provides the volume of gas adsorbed or
desorbed. A built-in microprocessor ensures linearity and
automatically computes the sample's BET surface area in m2/g.
Conductive Metallic Particles
[0058] The electroconductive paste also comprises conductive
metallic particles. The electroconductive paste may comprise at
least about 50 wt % metallic particles, preferably at least about
60 wt %, more preferably at least about 70 wt %, and most
preferably at least about 80 wt %, based upon 100% total weight of
the paste. At the same time, the paste preferably comprises no more
than about 95 wt % of metallic particles, based upon 100% total
weight of the paste.
[0059] All metallic particles known in the art, and which are
considered suitable in the context of the invention, may be
employed as the metallic particles in the electroconductive paste.
Preferred metallic particles are those which exhibit conductivity
and which yield electrodes having high efficiency and fill factor,
and low series and grid resistance. Preferred metallic particles
are elemental metals, alloys, metal derivatives, mixtures of at
least two metals, mixtures of at least two alloys or mixtures of at
least one metal with at least one alloy.
[0060] Preferred metals include at least one of silver, aluminum,
gold, copper, and nickel, and alloys or mixtures thereof. In a
preferred embodiment, the metallic particles comprise silver. In
another preferred embodiment, the metallic particles comprise
silver and aluminum. Suitable silver derivatives include, for
example, silver alloys and/or silver salts, such as silver halides
(e.g., silver chloride), silver nitrate, silver acetate, silver
trifluoroacetate, silver orthophosphate, and combinations thereof.
In one embodiment, the metallic particles comprise a metal or alloy
coated with one or more different metals or alloys, for example,
silver particles coated with aluminum.
[0061] Like the IRS particles, the metallic particles can exhibit a
variety of shapes, sizes, and coating layers. A large number of
shapes are known in the art. Some examples are spherical, angular,
elongated (rod or needle like) and flat (sheet like, flakes).
Metallic particles may also be present as a combination of
particles of different shapes (e.g., spheres and flakes). Metallic
particles with a shape, or combination of shapes, which favor
improved conductivity are preferred. One way to characterize such
shapes without considering the surface nature of the particles is
through the following parameters: length, width and thickness. In
the context of the invention, the length of a particle is given by
the length of the longest spatial displacement vector, both
endpoints of which are contained within the particle. The width of
a particle is given by the length of the longest spatial
displacement vector perpendicular to the length vector defined
above both endpoints of which are contained within the particle.
The thickness of a particle is given by the length of the longest
spatial displacement vector perpendicular to both the length vector
and the width vector, both defined above, both endpoints of which
are contained within the particle. In one embodiment, metallic
particles with shapes as uniform as possible are preferred (i.e.
shapes in which the ratios relating the length, the width and the
thickness are as close as possible to 1; preferably at least 0.7,
more preferably at least 0.8, and most preferably at least 0.9, and
preferably no more than about 1.5, preferably no more than about
1.3, and most preferably no more than about 1.2). Examples of
preferred shapes for the metallic particles in this embodiment are
spheres and cubes, or combinations thereof, or combinations of one
or more thereof with other shapes. In another embodiment, metallic
particles are preferred which have a shape of low uniformity,
preferably with at least one of the ratios relating the dimensions
of length, width and thickness being above about 1.5, more
preferably above about 3 and most preferably above about 5.
Preferred shapes according to this embodiment are flake shaped, rod
or needle shaped, or a combination of flake shaped, rod or needle
shaped with other shapes.
[0062] It is preferred that the median particle diameter d.sub.50,
as set forth herein, of the metallic particles is at least about
0.1 .mu.m, and preferably no more than about 10 .mu.m, preferably
no more than about 8 .mu.m, more preferably no more than about 7
.mu.m, and most preferably no more than about 5 .mu.m.
[0063] Further, preferable metallic particles have a specific
surface area of at least about 0.1 m.sup.2/g and no more than about
10 m.sup.2/g. According to a preferred embodiment, silver powders
having a specific surface area of at least about 0.2 m.sup.2/g,
preferably at least 0.5 m.sup.2/g, and at the same time no more
than about 5 m.sup.2/g are used. The specific surface area is
measured according to the parameters set forth herein.
[0064] Additional components which contribute to more favorable
contact properties and electrical conductivity are preferred. For
example, the metallic particles may be present with a surface
coating. Any such coating known in the art, and which is considered
to be suitable in the context of the invention, may be employed on
the metallic particles. Preferred coatings are those coatings which
promote the adhesion characteristics of the resulting
electroconductive paste. If such a coating is present, it is
preferred that the coating be no more than about 10 wt %,
preferably no more than about 8 wt %, and most preferably no more
than about 5 wt %, based on 100% total weight of the metallic
particles.
Organic Vehicle
[0065] The electroconductive paste of the invention also comprises
an organic vehicle. In one embodiment, the organic vehicle is
present in the electroconductive paste in an amount of at least
about 0.01 wt % and no more than about 50 wt %, preferably no more
than about 30 wt %, and most preferably no more than about 20 wt %,
based upon 100% total weight of the paste.
[0066] Preferred organic vehicles in the context of the invention
are solutions, emulsions or dispersions based on one or more
solvents, preferably organic solvent(s), which ensure that the
components of the electroconductive paste are present in a
dissolved, emulsified or dispersed form. Preferred organic vehicles
are those which provide optimal stability of the components of the
electroconductive paste and endow the paste with a viscosity
allowing effective printability.
[0067] In one embodiment, the organic vehicle comprises an organic
solvent and one or more of a binder (e.g., a polymer), a surfactant
and a thixotropic agent, or any combination thereof. For example,
in one embodiment, the organic vehicle comprises one or more
binders in an organic solvent.
[0068] Preferred binders in the context of the invention are those
which contribute to the formation of an electroconductive paste
with favorable stability, printability, viscosity and sintering
properties. All binders which are known in the art, and which are
considered to be suitable in the context of this invention, may be
employed as the binder in the organic vehicle. Preferred binders
(which often fall within the category termed "resins") are
polymeric binders, monomeric binders, and binders which are a
combination of polymers and monomers. Polymeric binders can also be
copolymers wherein at least two different monomeric units are
contained in a single molecule. Preferred polymeric binders are
those which carry functional groups in the polymer main chain,
those which carry functional groups off of the main chain and those
which carry functional groups both within the main chain and off of
the main chain. Preferred polymers carrying functional groups in
the main chain are for example polyesters, substituted polyesters,
polycarbonates, substituted polycarbonates, polymers which carry
cyclic groups in the main chain, poly-sugars, substituted
poly-sugars, polyurethanes, substituted polyurethanes, polyamides,
substituted polyamides, phenolic resins, substituted phenolic
resins, copolymers of the monomers of one or more of the preceding
polymers, optionally with other co-monomers, or a combination of at
least two thereof. According to one embodiment, the binder may be
polyvinyl butyral or polyethylene. Preferred polymers which carry
cyclic groups in the main chain are for example polyvinylbutylate
(PVB) and its derivatives and poly-terpineol and its derivatives or
mixtures thereof. Preferred poly-sugars are for example cellulose
and alkyl derivatives thereof, preferably methyl cellulose, ethyl
cellulose, hydroxyethyl cellulose, propyl cellulose, hydroxypropyl
cellulose, butyl cellulose and their derivatives and mixtures of at
least two thereof. Other preferred polymers are cellulose ester
resins, e.g., cellulose acetate propionate, cellulose acetate
buyrate, and any combinations thereof. Preferred polymers which
carry functional groups off of the main polymer chain are those
which carry amide groups, those which carry acid and/or ester
groups, often called acrylic resins, or polymers which carry a
combination of aforementioned functional groups, or a combination
thereof. Preferred polymers which carry amide off of the main chain
are for example polyvinyl pyrrolidone (PVP) and its derivatives.
Preferred polymers which carry acid and/or ester groups off of the
main chain are for example polyacrylic acid and its derivatives,
polymethacrylate (PMA) and its derivatives or
polymethylmethacrylate (PMMA) and its derivatives, or a mixture
thereof. Preferred monomeric binders are ethylene glycol based
monomers, terpineol resins or rosin derivatives, or a mixture
thereof. Preferred monomeric binders based on ethylene glycol are
those with ether groups, ester groups or those with an ether group
and an ester group, preferred ether groups being methyl, ethyl,
propyl, butyl, pentyl hexyl and higher alkyl ethers, the preferred
ester group being acetate and its alkyl derivatives, preferably
ethylene glycol monobutylether monoacetate or a mixture thereof.
Alkyl cellulose, preferably ethyl cellulose, its derivatives and
mixtures thereof with other binders from the preceding lists of
binders or otherwise are the most preferred binders in the context
of the invention. The binder may be present in an amount of at
least about 0.1 wt %, and preferably at least about 0.5 wt %, based
upon 100% total weight of the organic vehicle. At the same time,
the binder may be present in an amount of no more than about 10 wt
%, preferably no more than about 8 wt %, and more preferably no
more than about 7 wt %, based upon 100% total weight of the organic
vehicle.
[0069] Preferred solvents are components which are removed from the
paste to a significant extent during firing. Preferably, they are
present after firing with an absolute weight reduced by at least
about 80% compared to before firing, preferably reduced by at least
about 95% compared to before firing. Preferred solvents are those
which contribute to favorable viscosity, printability, stability
and sintering characteristics. All solvents which are known in the
art, and which are considered to be suitable in the context of this
invention, may be employed as the solvent in the organic vehicle.
Preferred solvents are those which exist as a liquid under standard
ambient temperature and pressure (SATP) (298.15 K, 25.degree. C.,
77.degree. F.), 100 kPa (14.504 psi, 0.986 atm), preferably those
with a boiling point above about 90.degree. C. and a melting point
above about -20.degree. C. Preferred solvents are polar or
non-polar, protic or aprotic, aromatic or non-aromatic. Preferred
solvents are mono-alcohols, di-alcohols, poly-alcohols,
mono-esters, di-esters, poly-esters, mono-ethers, di-ethers,
poly-ethers, solvents which comprise at least one or more of these
categories of functional group, optionally comprising other
categories of functional group, preferably cyclic groups, aromatic
groups, unsaturated bonds, alcohol groups with one or more O atoms
replaced by heteroatoms, ether groups with one or more O atoms
replaced by heteroatoms, esters groups with one or more O atoms
replaced by heteroatoms, and mixtures of two or more of the
aforementioned solvents. Preferred esters in this context are
di-alkyl esters of adipic acid, preferred alkyl constituents being
methyl, ethyl, propyl, butyl, pentyl, hexyl and higher alkyl groups
or combinations of two different such alkyl groups, preferably
dimethyladipate, and mixtures of two or more adipate esters.
Preferred ethers in this context are diethers, preferably dialkyl
ethers of ethylene glycol, preferred alkyl constituents being
methyl, ethyl, propyl, butyl, pentyl, hexyl and higher alkyl groups
or combinations of two different such alkyl groups, and mixtures of
two diethers. Preferred alcohols in this context are primary,
secondary and tertiary alcohols, preferably tertiary alcohols,
terpineol and its derivatives being preferred, or a mixture of two
or more alcohols. Preferred solvents which combine more than one
different functional groups are 2,2,4-trimethyl-1,3-pentanediol
monoisobutyrate, often called texanol, and its derivatives,
2-(2-ethoxyethoxyl)ethanol, often known as carbitol, its alkyl
derivatives, preferably methyl, ethyl, propyl, butyl, pentyl, and
hexyl carbitol, preferably hexyl carbitol or butyl carbitol, and
acetate derivatives thereof, preferably butyl carbitol acetate, or
mixtures of at least two of the aforementioned. The organic solvent
may be present in an amount of at least about 60 wt %, and more
preferably at least about 70 wt %, and most preferably at least
about 80 wt %, based upon 100% total weight of the organic vehicle.
At the same time, the organic solvent may be present in an amount
of no more than about 99 wt %, more preferably no more than about
95 wt %, based upon 100% total weight of the organic vehicle.
[0070] The organic vehicle may also comprise one or more
surfactants and/or additives. Preferred surfactants are those which
contribute to the formation of an electroconductive paste with
favorable stability, printability, viscosity and sintering
properties. All surfactants which are known in the art, and which
are considered to be suitable in the context of this invention, may
be employed as the surfactant in the organic vehicle. Preferred
surfactants are those based on linear chains, branched chains,
aromatic chains, fluorinated chains, siloxane chains, polyether
chains and combinations thereof. Preferred surfactants include, but
are not limited to, single chained, double chained or poly chained
polymers. Preferred surfactants may have non-ionic, anionic,
cationic, amphiphilic, or zwitterionic heads. Preferred surfactants
may be polymeric and monomeric or a mixture thereof. Preferred
surfactants may have pigment affinic groups, preferably
hydroxyfunctional carboxylic acid esters with pigment affinic
groups (e.g., DISPERBYK.RTM.-108, manufactured by BYK USA, Inc.),
acrylate copolymers with pigment affinic groups (e.g.,
DISPERBYK.RTM.-116, manufactured by BYK USA, Inc.), modified
polyethers with pigment affinic groups (e.g., TEGO.RTM. DISPERS
655, manufactured by Evonik Tego Chemie GmbH), other surfactants
with groups of high pigment affinity (e.g., TEGO.RTM. DISPERS 662
C, manufactured by Evonik Tego Chemie GmbH). Other preferred
polymers not in the above list include, but are not limited to,
polyethylene oxide, polyethylene glycol and its derivatives, and
alkyl carboxylic acids and their derivatives or salts, or mixtures
thereof. The preferred polyethylene glycol derivative is
poly(ethyleneglycol)acetic acid. Preferred alkyl carboxylic acids
are those with fully saturated and those with singly or poly
unsaturated alkyl chains or mixtures thereof. Preferred carboxylic
acids with saturated alkyl chains are those with alkyl chains
lengths in a range from about 8 to about 20 carbon atoms,
preferably C.sub.9H.sub.19COOH (capric acid), C.sub.11H.sub.23COOH
(Lauric acid), C.sub.13H.sub.27COOH (myristic acid)
C.sub.15H.sub.31COOH (palmitic acid), C.sub.17H.sub.35COOH (stearic
acid), or salts or mixtures thereof. Preferred carboxylic acids
with unsaturated alkyl chains are C.sub.18H.sub.34O.sub.2 (oleic
acid) and C.sub.18H.sub.32O.sub.2 (linoleic acid). The preferred
monomeric surfactant is benzotriazole and its derivatives. If
present, the surfactant may be at least about 0.01 wt %, based upon
100% total weight of the organic vehicle. At the same time, the
surfactant is preferably no more than about 10 wt %, preferably no
more than about 8 wt %, and more preferably no more than about 6 wt
%, based upon 100% total weight of the organic vehicle.
[0071] Preferred additives in the organic vehicle are those
materials which are distinct from the aforementioned components and
which contribute to favorable properties of the electroconductive
paste, such as advantageous viscosity, printability, stability and
sintering characteristics. Additives known in the art, and which
are considered to be suitable in the context of the invention, may
be used. Preferred additives include, but are not limited to,
thixotropic agents, viscosity regulators, stabilizing agents,
inorganic additives, thickeners, emulsifiers, dispersants and pH
regulators. Preferred thixotropic agents include, but are not
limited to, carboxylic acid derivatives, preferably fatty acid
derivatives or combinations thereof. Preferred fatty acid
derivatives include, but are not limited to, C.sub.9H.sub.19COOH
(capric acid), C.sub.11H.sub.23COOH (Lauric acid),
C.sub.13H.sub.27COOH (myristic acid) C.sub.15H.sub.31COOH (palmitic
acid), C.sub.17H.sub.35COOH (stearic acid) C.sub.18H.sub.34O.sub.2
(oleic acid), C.sub.18H.sub.32O.sub.2 (linoleic acid) and
combinations thereof. A preferred combination comprising fatty
acids in this context is castor oil.
Additives
[0072] According to another embodiment, the electroconductive paste
may include additives distinct from the conductive particles, IRS,
and organic vehicle. Preferred additives contribute to increased
performance of the electroconductive paste, of the electrodes
produced thereof, or of the resulting solar cell. All additives
known in the art, and which are considered suitable in the context
of the invention, may be employed as additives in the
electroconductive paste. Preferred additives include, but are not
limited to, thixotropic agents, viscosity regulators, emulsifiers,
stabilizing agents or pH regulators, inorganic additives,
thickeners and dispersants, or a combination of at least two
thereof. Inorganic additives are most preferred. Preferred
inorganic additives include, but are not limited to, alkaline and
alkaline earth metals, transition metals, such as nickel,
zirconium, titanium, manganese, tin, ruthenium, cobalt, iron,
copper and chromium tungsten, molybdenum, zinc; post-transition
metals such as boron, silicon, germanium, tellurium, gadolinium,
lead, bismuth, antimony, rare earth metals, such as lanthanum,
cerium, oxides, mixed metal oxides, complex compounds, or amorphous
or partially crystallized glasses formed from those oxides, or any
combination of at least two thereof, preferably zinc, antimony,
manganese, nickel, tungsten, tellurium and ruthenium, or a
combination of at least two thereof, oxides thereof, compounds
which can generate those metal oxides or glasses on firing, or a
mixture of at least two of the aforementioned metals, a mixture of
at least two of the aforementioned oxides, a mixture of at least
two of the aforementioned compounds which can generate those metal
oxides, mixed metal oxides, compounds or amorphous or partially
glasses on firing, or mixtures of two or more of any of the above
mentioned.
[0073] If present, the electroconductive paste composition may
include at least about 0.1 wt % additive, based upon 100% total
weight of the paste. At the same time, the paste preferably
includes no more than about 10 wt %, preferably no more than about
5 wt %, and more preferably no more than about 2 wt % additive(s),
based upon 100% total weight of the paste.
[0074] Forming the Electroconductive Paste Composition
[0075] To form the electroconductive paste composition, the IRS may
be combined with the conductive metallic particles and the organic
vehicle using any method known in the art for preparing a paste
composition. The method of preparation is not critical, as long as
it results in a homogenously dispersed paste. The components can be
mixed, such as with a mixer, then passed through a three roll mill,
for example, to make a dispersed uniform paste.
Solar Cells
[0076] In another aspect, the invention relates to a solar cell. In
one embodiment, the solar cell is formed from a semiconductor
substrate, for example a silicon wafer, and an electroconductive
paste composition according to any of the embodiments described
herein.
[0077] In another aspect, the invention relates to a solar cell
prepared by a process comprising applying an electroconductive
paste composition according to any of the embodiments described
herein to a semiconductor substrate and firing the semiconductor
substrate.
Silicon Wafer
[0078] Preferred wafers have regions, among other regions of the
solar cell, capable of absorbing light with high efficiency to
yield electron-hole pairs and separating holes and electrons across
a boundary with high efficiency, preferably across a p-n junction
boundary. Preferred wafers are those comprising a single body made
up of a front doped layer and a back doped layer.
[0079] Preferably, the wafer comprises appropriately doped
tetravalent elements, binary compounds, tertiary compounds or
alloys. Preferred tetravalent elements in this context are silicon,
Ge or Sn, preferably silicon. Preferred binary compounds are
combinations of two or more tetravalent elements, binary compounds
of a group III element with a group V element, binary com-pounds of
a group II element with a group VI element or binary compounds of a
group IV element with a group VI element. Preferred combinations of
tetravalent elements are combinations of two or more elements
selected from silicon, germanium, tin or carbon, preferably SiC.
The preferred binary compound of a group III element with a group V
element is GaAs. According to a preferred embodiment, the wafer is
silicon. The foregoing description, in which silicon is explicitly
mentioned, also applies to other wafer compositions described
herein.
[0080] The p-n junction boundary is located where the front doped
layer and back doped layer of the wafer meet. In an n-type solar
cell, the back doped layer is doped with an electron donating
n-type dopant and the front doped layer is doped with an electron
accepting or hole donating p-type dopant. In a p-type solar cell,
the back doped layer is doped with p-type dopant and the front
doped layer is doped with n-type dopant. According to a preferred
embodiment, a wafer with a p-n junction boundary is prepared by
first providing a doped silicon substrate and then applying a doped
layer of the opposite type to one face of that substrate.
[0081] Doped silicon substrates are well known in the art. The
doped silicon substrate can be prepared by any method known in the
art and considered suitable for the invention. Preferred sources of
silicon substrates are mono-crystalline silicon, multi-crystalline
silicon, amorphous silicon and upgraded metallurgical silicon, most
preferably mono-crystalline silicon or multi-crystalline silicon.
Doping to form the doped silicon substrate can be carried out
simultaneously by adding the dopant during the preparation of the
silicon substrate, or it can be carried out in a subsequent step.
Doping subsequent to the preparation of the silicon substrate can
be carried out by gas diffusion epitaxy, for example. Doped silicon
substrates are also readily commercially available. According to
one embodiment, the initial doping of the silicon substrate may be
carried out simultaneously to its formation by adding dopant to the
silicon mix. According to another embodiment, the application of
the front doped layer and the highly doped back layer, if present,
may be carried out by gas-phase epitaxy. The gas phase epitaxy is
preferably carried out at a temperature of at least about
500.degree. C., preferably at least about 600.degree. C., and most
preferably at least about 650.degree. C. At the same time, the gas
phase epitaxy is preferably carried out at a temperature of no more
than about 900.degree. C., more preferably no more than about
800.degree. C., and most preferably no more than about 750.degree.
C. The epitaxy is also preferably carried out at a pressure of at
least 2 kPa, preferably at least about 10 kPa, and most preferably
at least about 30 kPa. At the same time, the epitaxy is carried out
at a pressure of no more than about 100 kPa, preferably no more
than about 80 kPa, and most preferably no more than about 70
kPa.
[0082] It is known in the art that silicon substrates can exhibit a
number of shapes, surface textures and sizes. The shape of the
substrate may include cuboid, disc, wafer and irregular polyhedron,
to name a few. According to a preferred embodiment, the wafer is a
cuboid with two dimensions which are similar, preferably equal, and
a third dimension which is significantly smaller than the other two
dimensions. The third dimension may be at least 100 times smaller
than the first two dimensions.
[0083] Further, a variety of surface types are known in the art. In
one embodiment, silicon substrates with rough surfaces are
preferred. One way to assess the roughness of the substrate is to
evaluate the surface roughness parameter for a sub-surface of the
substrate, which is small in comparison to the total surface area
of the substrate, preferably less than about one hundredth of the
total surface area, and which is essentially planar. The value of
the surface roughness parameter is given by the ratio of the area
of the sub-surface to the area of a theoretical surface formed by
projecting that sub-surface onto the flat plane best fitted to the
sub-surface by minimizing mean square displacement. A higher value
of the surface roughness parameter indicates a rougher, more
irregular surface and a lower value of the surface roughness
parameter indicates a smoother, more even surface. The surface
roughness of the silicon substrate is preferably modified so as to
produce an optimum balance between numerous factors including, but
not limited to, light absorption and adhesion to the surface.
[0084] The two larger dimensions of the silicon substrate can be
varied to suit the application required of the resultant solar
cell. It is preferred for the thickness of the silicon wafer to be
at least about 0.01 mm. At the same time, the thickness is
preferably no more than about 0.5 mm, more preferably no more than
about 0.3 mm, and most preferably no more than about 0.2 mm.
According to one embodiment, the silicon wafer may have a minimum
thickness of 0.01 mm.
[0085] It is preferred that the front doped layer be thin in
comparison to the back doped layer. It is also preferred that the
front doped layer have a thickness of at least about 0.1 .mu.m, and
no more than about 10 .mu.m, preferably no more than about 5 .mu.m,
and most preferably no more than about 2 .mu.m.
[0086] A highly doped layer can be applied to the back face of the
silicon substrate between the back doped layer and any further
layers. Such a highly doped layer is of the same doping type as the
back doped layer and such a layer is commonly denoted with a +
(n+-type layers are applied to n-type back doped layers and p+-type
layers are applied to p-type back doped layers). This highly doped
back layer serves to assist metallization and improve
electroconductive properties. It is preferred for the highly doped
back layer, if present, to have a thickness of at least about 1
.mu.m, and no more than about 100 .mu.m, preferably no more than
about 50 .mu.m, and most preferably no more than about 15
.mu.m.
Dopants
[0087] Preferred dopants are those which, when added to the silicon
wafer, form a p-n junction boundary by introducing electrons or
holes into the band structure. It is preferred that the identity
and concentration of these dopants is specifically selected so as
to tune the band structure profile of the p-n junction and set the
light absorption and conductivity profiles as required. Preferred
p-type dopants are those which add holes to the silicon wafer band
structure. All dopants known in the art and which are considered
suitable in the context of the invention can be employed as p-type
dopants. Preferred p-type dopants include, but are not limited to,
trivalent elements, particularly those of group 13 of the periodic
table. Preferred group 13 elements of the periodic table include,
but are not limited to, boron, aluminum, gallium, indium, thallium,
or a combination of at least two thereof, wherein boron is
particularly preferred.
[0088] Preferred n-type dopants are those which add electrons to
the silicon wafer band structure. All dopants known in the art and
which are considered to be suitable in the context of the invention
can be employed as n-type dopants. Preferred n-type dopants
include, but are not limited to, elements of group 15 of the
periodic table. Preferred group 15 elements include, but are not
limited to, nitrogen, phosphorus, arsenic, antimony, bismuth, or a
combination of at least two thereof, wherein phosphorus is
particularly preferred.
[0089] As described above, the various doping levels of the p-n
junction can be varied so as to tune the desired properties of the
resulting solar cell.
[0090] According to certain embodiments, the semiconductor
substrate (i.e., silicon wafer) exhibits a sheet resistance above
about 60.OMEGA./.quadrature., such as above about
65.OMEGA./.quadrature., 70.OMEGA./.quadrature.,
90.OMEGA./.quadrature., 95.OMEGA./.quadrature., or
100.OMEGA./.quadrature..
Solar Cell Structure
[0091] One aspect of the invention is a solar cell obtainable from
the methods of the invention. Preferred solar cells are those which
have a high efficiency, in terms of proportion of total energy of
incident light converted into electrical energy output. Solar cells
which are lightweight and durable are also preferred. At a minimum,
a solar cell typically includes: (i) front electrodes, (ii) a front
doped layer, (iii) a p-n junction boundary, (iv) a back doped
layer, and (v) soldering pads. The solar cell may also include
additional layers for chemical/mechanical protection.
Antireflective Layer
[0092] An antireflective layer may be applied as the outer layer
before the electrode is applied to the front face of the solar
cell. Preferred antireflective layers are those which decrease the
proportion of incident light reflected by the front face and
increase the proportion of incident light crossing the front face
to be absorbed by the wafer. Antireflective layers which give rise
to a favorable absorption/reflection ratio, are susceptible to
etching by the electroconductive paste, are otherwise resistant to
the temperatures required for firing of the electroconductive
paste, and do not contribute to increased recombination of
electrons and holes in the vicinity of the electrode interface are
preferred. All antireflective layers known in the art and which are
considered to be suitable in the context of the invention can be
employed. Preferred antireflective layers include, but are not
limited to, SiN.sub.x, SiO.sub.2, Al.sub.2O.sub.3, TiO.sub.2 or
mixtures of at least two thereof and/or combinations of at least
two layers thereof. According to a preferred embodiment, the
antireflective layer is SiN.sub.x, in particular where a silicon
wafer is employed.
[0093] The thickness of antireflective layers is suited to the
wavelength of the appropriate light. According to a preferred
embodiment of the invention, the antireflective layers have a
thickness of at least about 20 nm, preferably at least about 40 nm,
and most preferably at least about 60 nm. At the same time, the
thickness is preferably no more than about 300 nm, preferably no
more than about 200 nm, and most preferably no more than about 90
nm.
Passivation Layers
[0094] One or more passivation layers may be applied to the front
and/or back side of the silicon wafer as an outer layer. The
passivation layer(s) may be applied before the front electrode is
formed, or before the antireflective layer is applied (if one is
present). Preferred passivation layers are those which reduce the
rate of electron/hole recombination in the vicinity of the
electrode interface. Any passivation layer which is known in the
art and which is considered to be suitable in the context of the
invention can be employed. Preferred passivation layers include,
but are not limited to, silicon nitride, silicon dioxide and
titanium dioxide. According to a preferred embodiment, silicon
nitride is used. It is preferred for the passivation layer to have
a thickness of at least 0.1 nm, preferably at least about 10 nm,
and most preferably at least about 30 nm. At the same time, the
passivation layer is preferably no more than about 2 .mu.m, more
preferably no more than about 1 .mu.m, and most preferably no more
than about 200 nm.
Additional Protective Layers
[0095] In addition to the layers described above which directly
contribute to the principle function of the solar cell, further
layers may be added for mechanical and chemical protection.
[0096] The cell can be encapsulated to provide chemical protection.
Encapsulations are well known in the art and any encapsulation
suitable for the invention can be employed. According to a
preferred embodiment, transparent polymers, often referred to as
transparent thermoplastic resins, are used as the encapsulation
material, if such an encapsulation is present. Preferred
transparent polymers include, but are not limited to, silicon
rubber and polyethylene vinyl acetate (PVA).
[0097] A transparent glass sheet may also be added to the front of
the solar cell to provide mechanical protection to the front face
of the cell. Transparent glass sheets are well known in the art and
any suitable transparent glass sheet suitable may be employed.
[0098] A back protecting material may be added to the back face of
the solar cell to provide mechanical protection. Back protecting
materials are well known in the art and any suitable back
protecting material may be employed. Preferred back protecting
materials are those having good mechanical properties and weather
resistance. The preferred back protection material is polyethylene
terephthalate with a layer of polyvinyl fluoride. It is preferred
for the back protecting material to be present underneath the
encapsulation layer (in the event that both a back protection layer
and encapsulation are present).
[0099] A frame material can be added to the outside of the solar
cell to give mechanical support. Frame materials are well known in
the art and any frame material considered suitable in the context
of the invention may be employed. The preferred frame material is
aluminum.
Method of Preparing Solar Cell
[0100] A solar cell may be prepared by applying an
electroconductive paste composition to an antireflective coating,
such as silicon nitride, silicon oxide, titanium oxide or aluminum
oxide, on the front side of a semiconductor substrate, such as a
silicon wafer, to form front side electrodes. The backside
electroconductive paste of the invention is then applied to the
backside of the solar cell to form soldering pads. The
electroconductive pastes may be applied in any manner known in the
art and considered suitable in the context of the invention.
Examples include, but are not limited to, impregnation, dipping,
pouring, dripping on, injection, spraying, knife coating, curtain
coating, brushing or printing or a combination of at least two
thereof. Preferred printing techniques are ink-jet printing, screen
printing, tampon printing, offset printing, relief printing or
stencil printing or a combination of at least two thereof. It is
preferred that the electroconductive paste is applied by printing,
preferably by screen printing. Specifically, the screens preferably
have finger line opening with a diameter of at least about 10
.mu.m, more preferably at least about 15 .mu.m, more preferably at
least about 20 .mu.m, and most preferably at least about 25 .mu.m.
At the same time, the finger line opening diameters is preferably
no more than about 100 .mu.m, more preferably no more than about 80
.mu.m, and most preferably no more than about 70 .mu.m.
[0101] An aluminum paste is then applied to the backside of the
substrate, overlapping the edges of the soldering pads formed from
the backside electroconductive paste, to form the BSF. The
substrate is then fired according to an appropriate profile
determined by the substrate and the composition of the
electroconductive paste.
[0102] Firing is necessary to sinter the printed electrodes and
soldering pads so as to form solid conductive bodies. Firing is
well known in the art and can be effected in any manner considered
suitable in the context of the invention. It is preferred that
firing be carried out above the T.sub.g of the IRS materials.
[0103] The maximum temperature set for firing is below about
900.degree. C., preferably below about 860.degree. C. Firing
temperatures as low as about 820.degree. C. have been employed for
obtaining solar cells. The firing temperature profile is typically
set so as to enable the burnout of organic binder materials from
the electroconductive paste composition, as well as any other
organic materials present. The firing step is typically carried out
in air or in an oxygen-containing atmosphere in a belt furnace. It
is preferred for firing to be carried out in a fast firing process
with a total firing time in the range from about 30 seconds (s) to
about 3 minutes, more preferably in the range from about 30 s to
about 2 minutes, and most preferably in the range from about 40 s
to about 1 minute. The time above 600.degree. C. is most preferably
in a range from about 3 to 7 s. The substrate may reach a peak
temperature in the range of about 700 to 900.degree. C. for a
period of about 1 to 5 s. The firing may also be conducted at high
transport rates, for example, about 100-500 cm/min, with resulting
hold-up times of about 0.05 to 5 minutes. Multiple temperature
zones, for example 3-12 zones, can be used to control the desired
thermal profile.
[0104] Firing of electroconductive pastes on the front and back
faces may be carried out simultaneously or sequentially.
Simultaneous firing is appropriate if the electroconductive pastes
applied to both faces have similar, preferably identical, optimum
firing conditions. Where appropriate, it is preferred for firing to
be carried out simultaneously. Where firing is carried out
sequentially, it is preferable for the back electroconductive paste
to be applied and fired first, followed by application and firing
of the electroconductive paste to the front face.
Measuring Performance of Electroconductive Paste
[0105] To measure the performance of a solar cell, a standard
electrical test is conducted. A sample solar cell having both front
side and backside pastes printed thereon is characterized using a
commercial IV-tester "cetisPV-CTL1" from Halm Elektronik GmbH. All
parts of the measurement equipment as well as the solar cell to be
tested are maintained at 25.degree. C. during electrical
measurement. This temperature is always measured simultaneously on
the cell surface during the actual measurement by a temperature
probe. The Xe Arc lamp simulates the sunlight with a known AM1.5
intensity of 1000 W/m.sup.2 on the cell surface. To bring the
simulator to this intensity, the lamp is flashed several times
within a short period of time until it reaches a stable level
monitored by the "PVCTControl 4.260.0" software of the IV-tester.
The Halm IV tester uses a multi-point contact method to measure
current (I) and voltage (V) to determine the cell's IV-curve. To do
so, the solar cell is placed between the multi-point contact probes
in such a way that the probe fingers are in contact with the bus
bars of the cell. The numbers of contact probe lines are adjusted
to the number of bus bars on the cell surface. All electrical
values are determined directly from this curve automatically by the
implemented software package. As a reference standard, a calibrated
solar cell from ISE Freiburg consisting of the same area
dimensions, same wafer material and processed using the same front
side layout is tested and the data compared to the certificated
values. At least five wafers processed in the very same way are
measured and the data interpreted by calculating the average of
each value. The software PVCTControl 4.260.0 provides values for
efficiency, fill factor, short circuit current, series resistance,
and open circuit voltage.
Solar Cell Module
[0106] Another aspect of the invention is a solar cell module
formed of the solar cells of the invention. A plurality of solar
cells may be arranged spatially and electrically interconnected to
form a collective arrangement called a module. Preferred modules
can have a number of arrangements, preferably a rectangular
arrangement known as a solar panel. A variety of ways to
electrically connect solar cells, as well as a variety of ways to
mechanically arrange and fix such cells to form collective
arrangements, are well known in the art. Any such methods known in
the art, and which are considered suitable in the context of the
invention, may be employed. Preferred methods are those which
result in a low mass to power output ratio, low volume to power
output ration, and high durability. Aluminum is the preferred
material for mechanical fixing of solar cells.
EXAMPLES
Example 1
[0107] A set of IRS compositions (G1 and G2) were prepared with the
starting materials set forth in Table 1 below. The control was
prepared using the same starting materials as G1 and G2 (lead,
tellurium, and M-oxides), except for the zinc or magnesium-based
compound. Samples were prepared in 100 g batches by mixing the
individual oxide constituents in the amounts designated in Table 1.
The oxide mixture was loaded into a 8.34 in.sup.3 volume Colorado
crucible. The crucible was then placed in an oven for 40 minutes at
600.degree. C. to preheat the oxide mixture. After preheating, the
crucible was moved into a refractory oven at 850.degree. C. for 15
minutes to melt the individual components into a glass mixture. The
molten glass was then removed from the oven and poured into a
bucket containing deionized water to quickly quench. This glass
material was further processed in a 1 L ceramic jar mill. The jar
mill was filled approximately halfway with 1/2''cylindrical alumina
media, or 2 mm diameters yttrium stabilized zirconia (YTZ) grinding
media, and deionized water. The glass was added to the jar mill and
rolled for 8 hours at 60-80 RPM. After milling, the glass was
filtered through a 325 mesh sieve and dried at 125.degree. C. for
12 hours. All amounts are based on 100% total weight of the
IRS.
TABLE-US-00001 TABLE 1 Exemplary PTZ and PTM IRS Compositions (G1
and G2) Control G1 G2 PbO 29.23% 27.88% 27.71% TeO.sub.2 50.19%
47.86% 47.57% ZnO -- 0.87% -- MgO -- -- 0.88% M-Oxides 20.58%
23.39% 23.85%
[0108] The IRS compositions were then mixed with silver particles
and organic vehicle to form exemplary electroconductive paste
compositions. To form each exemplary paste (P1 and P2) and the
control paste, about 2.3 wt % of each IRS composition, about 88.5
wt % silver particles, and about 9.2 wt % of organic vehicle, based
upon 100% total weight of the paste, were each combined.
[0109] Once the pastes were mixed to a uniform consistency, they
were screen printed onto the front side of a blank monocrystalline
silicon wafer using 250 mesh stainless steel, 5 .mu.m EOM, at about
a 30 .mu.m wire diameter. A commercially available backside paste
was used to form soldering pads, which extend across the full
length of the cell and are about 4 mm wide. Next, a commercially
available aluminum backside paste was printed all over the
remaining areas of the rear side of the cell to form an aluminum
BSF. The cell was then dried at an appropriate temperature. The
silicon substrate, with the printed front side and backside paste,
was then fired at a peak temperature of approximately
700-975.degree. C.
[0110] The electroconductive performance of the exemplary and
control pastes is set forth in Table 2 below. The efficiency (Eta,
%), short circuit current (Isc, m.OMEGA.), fill factor (FF, %),
open circuit voltage (Voc, V), and series resistance under three
standard lighting intensities (Rs3, .OMEGA.) were all calculated
according to the parameters set forth herein, and the values
provided in the table below have been normalized to 1 with respect
to the Control paste. Most notably, exemplary pastes P1 and P2 had
lower series resistance than the control paste and higher short
circuit current and fill factor.
TABLE-US-00002 TABLE 2 Electrical Performance of Exemplary Pastes
(P1 and P2) Control P1 P2 Eta (%) 1 0.9989 1.0021 Isc (m.OMEGA.) 1
1.0011 1.0009 Voc (V) 1 0.9967 0.9984 FF (%) 1 1.0012 1.0028 Rs3
(.OMEGA.) 1 0.9506 0.9331
Example 2
[0111] A set of PTZM, PTZ and PTM IRS compositions (G3-G5), as well
as another control IRS (Control 2) were prepared. The IRS
compositions were prepared with the starting materials set forth in
Table 3 below, according to the parameters set forth in Example 1.
Each exemplary IRS contained either MgO, ZnO, or both, as well as
Pb, Te, and M-Oxides. All amounts are expressed in 100% total
weight of the IRS.
TABLE-US-00003 TABLE 3 Exemplary PTM IRS Compositions (G3-G5)
Control 2 G3 G4 G5 PbO 28.26% 27.89% 28.04% 28.17% TeO.sub.2 48.53%
47.89% 48.15% 48.36% MgO -- 0.43% -- 0.42% ZnO -- 0.86% 0.85% --
M-Oxides 23.21% 22.93% 22.96% 23.06%
[0112] About 2 wt % of each IRS composition was then mixed with
about 89 wt % silver particles and about 9 wt % organic vehicle
according to the same parameters as set forth in Example 1 to form
Pastes P3-P5 and the Control 2 paste. The exemplary pastes were
then screen printed on a monocystalline silicon wafer according to
the parameters of Example 1. The electroconductive performance was
measured according to the parameters set forth herein and was
normalized to 1 with respect to the Control 2 paste. As shown in
Table 4, each of the exemplary pastes outperformed Control 2, with
specific improvements in efficiency and fill factor and reductions
in series resistance.
TABLE-US-00004 TABLE 4 Electrical Performance of Pastes P3-P5
Control 2 P3 P4 P5 Eta 1 1.0302 1.0233 1.0177 Isc 1 0.9990 0.9983
0.9985 Voc 1 1.0000 1.0000 1.0000 FF 1 1.0316 1.0256 1.0200 Rs3 1
0.6399 0.7052 0.7753
Example 3
[0113] A set of PTMZ and PTZ IRS compositions (G6-G9) was prepared
with the starting materials set forth in Table 5 below, according
to the parameters set forth in Example 1. All amounts are expressed
in 100% total weight of the IRS.
TABLE-US-00005 TABLE 5 Exemplary PTMZ and PTZ IRS Compositions
(G6-G9) G6 G7 G8 G9 PbO 28.06% 28.17% 28.03% 27.89% TeO.sub.2
48.18% 48.36% 48.12% 47.88% MO 22.05% 22.04% 21.94% 21.82% MgO
0.85% -- -- -- ZnO 0.86% 0.84% 0.84% 0.84% CaO -- 0.58% -- -- SrO
-- -- 1.07% -- BaO -- -- -- 1.57%
[0114] About 2 wt % of each PTMZ and PTZ IRS composition was mixed
with about 89 wt % silver particles and about 9 wt % organic
vehicle according to the same parameters as set forth in Example 1
to form Pastes P6-P9. A control paste having the Control 2 IRS of
Example 2 was also prepared with the same paste components. The
pastes were then screen printed on a monocrystalline silicon wafer
according to the parameters of Example 1. The electroconductive
performance was measured according to the parameters set forth
herein and normalized to 1 with respect to the Control 2 paste. As
can be seen in Table 6, pastes P6-P9 exhibited improved fill factor
and reduced series resistance.
TABLE-US-00006 TABLE 6 Electrical Performance of Pastes P6-P9
Control 2 P6 P7 P8 P9 Eta 1 0.9955 0.9947 0.9901 0.9981 Isc 1
0.9737 0.9748 0.9732 0.9750 Voc 1 0.9906 0.9938 0.9938 0.9938 FF 1
1.0315 1.0274 1.0248 1.0303 Rs3 1 0.6531 0.7107 0.7309 0.6741
Example 4
[0115] Another set of PBZ, PBM and PBMZ IRS compositions can be
prepared with the starting materials set forth in Table 7 below
according to the parameters set forth in Example 1. These
compositions further include Li.sub.2O and B.sub.2O.sub.3. All
amounts are based on 100% total weight of the IRS. The anticipated
electrical performance is set forth in Table 8 below.
TABLE-US-00007 TABLE 7 Prophetic PTZ, PTM and PTMZ IRS Compositions
(X1-X3) 1 X1 X2 X3 PbO 27.89% 28.04% 28.17% TeO.sub.2 47.89% 48.15%
48.36% MgO 0.43% -- 0.42% ZnO 0.86% 0.85% -- Li.sub.2O 5.80% 6.30%
6.70% B.sub.2O.sub.3 17.13% 16.66% 16.36%
TABLE-US-00008 TABLE 8 Electrical Performance of Prophetic Pastes
(X1-X3) Control X1 X2 X3 Eta 0 + + + Isc 0 + + + Voc 0 + + + FF 0 +
+ + Rs3 0 + + +
Example 5
[0116] Another set of PBT IRS compositions (X4-X6) can be prepared
with the starting materials set forth in Table 9 below according to
the parameters set forth in Example 1. These compositions all
contain only the PTZ, PTM and/or PTMZ system. All amounts are based
on 100% total weight of the IRS. The anticipated electrical
performance is set forth in Table 10.
TABLE-US-00009 TABLE 9 Prophetic PTZ, PTM and PTMZ IRS Compositions
(X4-X6) X4 X5 X6 PbO 36.19% 36.40% 36.61% TeO.sub.2 62.13% 62.49%
62.85% MgO 0.56% 0.55% ZnO 1.12% 1.10%
TABLE-US-00010 TABLE 10 Electrical Performance of Prophetic Pastes
(X4-X6) Control X4 X5 X6 Eta 0 + + + Isc 0 + + + Voc 0 + + + FF 0 +
+ + Rs3 0 + + +
[0117] These and other advantages of the invention will be apparent
to those skilled in the art from the foregoing specification.
Accordingly, it will be recognized by those skilled in the art that
changes or modifications may be made to the above described
embodiments without departing from the broad inventive concepts of
the invention. Specific dimensions of any particular embodiment are
described for illustration purposes only. It should therefore be
understood that this invention is not limited to the particular
embodiments described herein, but is intended to include all
changes and modifications that are within the scope and spirit of
the invention.
* * * * *