U.S. patent application number 14/371669 was filed with the patent office on 2015-01-08 for glass frit, and conductive paste composition and solar cell comprising the same.
The applicant listed for this patent is HANWHA CHEMICAL CORPORATION. Invention is credited to Mi-Kyoung Kim, Choong-Hoon Paik, You-Jin Sim, Won Il Son.
Application Number | 20150007880 14/371669 |
Document ID | / |
Family ID | 48781696 |
Filed Date | 2015-01-08 |
United States Patent
Application |
20150007880 |
Kind Code |
A1 |
Son; Won Il ; et
al. |
January 8, 2015 |
GLASS FRIT, AND CONDUCTIVE PASTE COMPOSITION AND SOLAR CELL
COMPRISING THE SAME
Abstract
The present invention relates to a glass frit, a conductive
paste composition comprising the glass frit, and a solar cell
fabricated using the conductive paste composition. The glass frit
of the present invention comprises SiO2, PbO, and at least one
selected from the group consisting of Al2O3, ZrO2, ZnO, and Li2O.
Further, the conductive paste composition of the present invention
comprises a silver (Ag) powder, a lithium titanium oxide, a glass
frit, a binder, and a solvent. The conductive paste composition of
the present invention can be used to provide a solar cell having
low contact resistance to enhance photoelectric efficiency.
Inventors: |
Son; Won Il; (Seoul, KR)
; Sim; You-Jin; (Daejeon, KR) ; Paik;
Choong-Hoon; (Daejeon, KR) ; Kim; Mi-Kyoung;
(Seoul, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HANWHA CHEMICAL CORPORATION |
Seoul |
|
KR |
|
|
Family ID: |
48781696 |
Appl. No.: |
14/371669 |
Filed: |
January 11, 2013 |
PCT Filed: |
January 11, 2013 |
PCT NO: |
PCT/KR2013/000246 |
371 Date: |
July 10, 2014 |
Current U.S.
Class: |
136/256 ;
136/252; 252/514; 501/22; 501/23 |
Current CPC
Class: |
C04B 35/462 20130101;
C03C 8/18 20130101; Y02E 10/547 20130101; C01G 23/005 20130101;
C03C 8/20 20130101; C04B 2235/5445 20130101; H05K 1/092 20130101;
C01P 2004/64 20130101; Y02E 10/50 20130101; C03C 8/16 20130101;
H01B 1/22 20130101; C04B 2235/3201 20130101; C01G 33/006 20130101;
C03C 8/12 20130101; C03C 8/10 20130101; C04B 2235/3251 20130101;
C01P 2004/62 20130101; C01P 2004/03 20130101; C04B 2235/5454
20130101; H01L 31/022425 20130101; C01P 2002/54 20130101; C04B
2235/3203 20130101; C03C 3/074 20130101; C03C 3/07 20130101; H01B
1/02 20130101 |
Class at
Publication: |
136/256 ;
136/252; 501/22; 501/23; 252/514 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; C03C 8/12 20060101 C03C008/12; H01B 1/22 20060101
H01B001/22; C03C 8/10 20060101 C03C008/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 13, 2012 |
KR |
10-2012-0004387 |
Claims
1. A glass frit comprising SiO.sub.2, PbO, and at least one
selected from the group consisting of Al.sub.2O.sub.3, ZrO.sub.2,
ZnO, and Li.sub.2O.
2. The glass frit as claimed in claim 1, wherein the glass frit
comprises 5 to 30 wt % of SiO.sub.2, 50 to 90 wt % of PbO, 0.1 to
10 wt % of Al.sub.2O.sub.3, and 0.1 to 10 wt % of ZrO.sub.2.
3. The glass frit as claimed in claim 1, wherein the glass frit
comprises 5 to 30 wt % of SiO.sub.2, 50 to 90 wt % of PbO, 0.1 to
10 wt % of Al.sub.2O.sub.3, 0.1 to 10 wt % of ZrO.sub.2, 0.1 to 10
wt % of ZnO, and 0.1 to 10 wt % of Li.sub.2O.
4. The glass frit as claimed in claim 1, wherein the glass frit has
an average particle diameter of 0.5 to 10 .mu.m.
5. A conductive paste composition comprising a silver (Ag) powder,
a lithium titanium oxide, the glass frit as claimed in claim 1, a
binder, and a solvent.
6. The conductive paste composition as claimed in claim 5, wherein
the conductive paste composition comprises, with respect to 100
parts by weight of the paste composition, 60 to 95 parts by weight
of the silver powder, 0.1 to 10 parts by weight of the glass frit,
0.1 to 5 parts by weight of the lithium titanium oxide, 1 to 20
parts by weight of the binder, and 1 to 20 parts by weight of the
solvent.
7. The conductive paste composition as claimed in claim 5, wherein
the lithium titanium oxide is a compound represented by the
following formula 1: Li.sub.4Ti.sub.5-xM.sub.xO.sub.12 [Formula 1]
wherein x denotes the molar ratio of each component, satisfying
0.ltoreq.x.ltoreq.0.25; and M is a metal selected from the group
consisting of Nb, Zn, Mn, Mg, Fe, Ni, Ca, Bi, Al, Zr, V, Cu, Na, K,
and P.
8. The conductive paste composition as claimed in claim 5, wherein
the lithium titanium oxide has an average particle diameter of 10
to 500 nm.
9. The conductive paste composition as claimed in claim 5, wherein
the conductive paste composition further comprises at least one
metal oxide selected from the group consisting of ZnO, ZrO.sub.2,
and a mixture thereof.
10. The conductive paste composition as claimed in claim 6, wherein
the conductive paste composition further comprises 0.1 to 10 parts
by weight of at least one metal oxide selected from the group
consisting of ZnO, ZrO.sub.2, and a mixture thereof.
11. The conductive paste composition as claimed in claim 5, wherein
the silver powder is a spherical or flake powder having an average
particle diameter of 0.5 to 4 .mu.m.
12. The conductive paste composition as claimed in claim 5, wherein
the binder includes at least one photosensitive resin containing a
carboxyl group selected from the group consisting of a
photosensitive resin containing a carboxyl group as a copolymer of
an unsaturated carboxylic acid and a compound having an unsaturated
double bond, a photosensitive resin containing a carboxyl group as
a copolymer of an unsaturated carboxylic acid and a compound having
an unsaturated double bond, wherein the copolymer has an
ethylene-based unsaturated group added as a pendant group, and a
photosensitive resin containing a carboxyl group as obtained by
reacting a copolymer of an acid anhydride having an unsaturated
double bond and a compound having an unsaturated double bond with a
compound having a hydroxide group and an unsaturated double
bond.
13. The conductive paste composition as claimed in claim 5, wherein
the solvent includes at least one selected from .alpha.-terpinol,
butyl carbitol acetate, texanol, butyl carbitol, and di-propylene
glycol monomethyl ether.
14. The conductive paste composition as claimed in claim 6, wherein
the conductive paste composition further comprises 0.1 to 10 parts
by weight of at least one additive selected from the group
consisting of a dispersing agent, a thickening agent, a thixotropic
agent, and a leveling agent.
15. A solar cell comprising: a substrate having a first
conductivity; an emitter layer having a second conductivity and
being formed on the substrate; an anti-reflection layer formed on
the emitter layer; a front surface electrode being connected to the
emitter layer as passing through the anti-reflection layer and
prepared using the conductive paste composition as claimed in claim
5; and a back surface electrode formed on the back surface of the
substrate.
16. The solar cell as claimed in claim 15, wherein the substrate is
doped with P type impurities, the emitter layer being doped with N
type impurities.
17. The solar cell as claimed in claim 15, wherein the emitter
layer has a sheet resistance of 60 to 120 .OMEGA./sq.
18. The solar cell as claimed in claim 15, wherein the
anti-reflection layer includes any one monolayer selected from the
group consisting of silicon nitride layer, hydrogen-containing
silicon nitride layer, silicon oxide layer, silicon oxynitride
layer, MgF.sub.2 layer, ZnS layer, TiO.sub.2 layer, and CeO.sub.2
layer; or multiple layers comprising a combination of at least two
of the monolayers.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a glass frit, and a
conductive paste composition and a solar cell comprising the same
and, more particularly to a glass frit, and a conductive paste
composition and a solar cell comprising the same that exhibit low
contact resistance and high efficiency.
[0002] This application claims priority to and the benefit of
Korean Patent Application No. 10-2012-0004387 filed on Jan. 13,
2012, the contents of which are incorporated herein by
reference.
BACKGROUND
[0003] Interest in alternative energy sources is rising along with
the forecasts on the steady depletion of energy sources, such as
petroleum or charcoal. Among the alternative energy sources, the
solar cells are catching on as a next-generation cell technology
using a semiconductor device for converting solar energy directly
into electricity. The solar cells are largely classified into
silicon solar cells, compound semiconductor solar cells, and tandem
solar cells. Silicon solar cells are by far the predominant
technology.
[0004] For high efficiency of the silicon solar cells, there have
been developed a variety of technologies, such as shallow emitters,
selective emitters, or the like. The term "shallow emitter" as used
herein refers to an emitter layer with high sheet resistance of 60
to 120 .OMEGA./sq. Such a shallow emitter features some advantages,
such as having low recombination rate and using short-wavelength
sun light.
[0005] The emitter layer formed on the substrate in the crystalline
silicon solar cell has a pn junction using the substrate as a base.
The high Rs cell has high photoelectric conversion efficiency,
since it has a relatively high sheet resistance of 60 to 120
.OMEGA./sq with respect to the conventional crystalline solar cells
of which the emitter layer has a sheet resistance of 40 to 50
.OMEGA./sq. In other words, the high-efficiency solar cells are
designed to have enhanced efficiency by reducing the potential
portion of the dead layer (i.e., the region where the electrons
produced by the solar cell are inhibited from forming the
electrical current due to the surplus semiconductor impurity
concentration) on the surface layer among the emitter layers formed
on the front surface of the solar cell substrate.
[0006] In such a high-efficiency solar cell, the emitter layer may
have a thickness of 100 nm to 500 nm and a semiconductor impurity
concentration of 1.times.10.sup.16 to 1.times.10.sup.21
atom/cm.sup.3. In the fabrication method for the conventional solar
cells, the emitter is formed to have such a small thickness as of
an ultrathin type and doped with a low concentration of
semiconductor impurities.
[0007] In contrast to the regular solar cell having an emitter
layer with a thickness of at least 600 nm, the high-efficiency
solar cell has an ultrathin emitter layer 100 nm to 500 nm in
thickness and thus potentially encounters a problem that its
electrode during formation gets in contact with the base substrate
as passing through the ultrathin emitter layer, ending up with
occurrence of short-circuit. In other words, to put the
high-efficiency solar cells having an ultrathin emitter layer into
commercial uses, there is a need for a process not only to
facilitate a contact of the thin emitter layer but to make the
electrode out of contact with the silicon substrate which acts as a
base, thereby preventing occurrence of short-circuit.
[0008] The silver (Ag) paste has been used to form a front surface
electrode of the ultrathin emitter. The silver paste contains a
silver powder, an organic binder, glass frits, and so forth.
However, the existence of glass frits in the silver paste results
in poor Ohmic contact and, worse, occurrence of short-circuit.
Particularly, a high-temperature process at about 800.degree. C.
for a short period is necessary in forming a contact region of the
front surface electrode. A failure to precisely control the
high-temperature process potentially ends up with high serial
resistance or low shunt resistance.
[0009] Conventionally, the silver paste has been prepared simply by
mixing silver powder with glass frits and/or optionally metal
oxides as an inorganic additive, but with limitation in the
inhibitory effect on the resistance between the formed electrode
and the substrate. Thus, there is still a difficulty in achieving
high efficiency when applying the silver paste to the
electrode.
[0010] For example, a paste composition comprising zinc oxide as an
inorganic additive to form a front surface electrode is disclosed
in US Patent Application No. 20080223446. The cited patent
describes a conductive paste containing a glass frit and 7- to 100
nm-diameter ZnO as an inorganic additive, where the glass frit
comprises 21 to 29 wt % of SiO.sub.2, 0.1 to 8 wt % of
Al.sub.2O.sub.3, 50 to 62 wt % of PbO, 7 to 10 wt % of
B.sub.2O.sub.3, 0 to 4 wt % of ZnO, 0 to 0.1 wt % of Li.sub.2O, and
2 to 7 wt % of TiO.sub.2. However, the front surface electrode
using the paste composition disclosed in the cited document has
some problems that it has unsatisfactory "fired-through" state with
the SiN film mainly used as an anti-reflection layer for solar
cells and exhibits high serial resistance, thus making it difficult
to provide high-efficiency solar cells.
SUMMARY OF THE INVENTION
[0011] To solve the problems with the prior art, it is an object of
the present invention to provide a glass frit having a novel
composition.
[0012] It is another object of the present invention to provide a
conductive paste composition comprising the glass frit and having a
low contact resistance.
[0013] It is still another object of the present invention to
provide a solar cell fabricated using the conductive paste
composition.
[0014] To achieve the objects, the present invention provides a
glass frit comprising SiO.sub.2, PbO, and at least one selected
from the group consisting of Al.sub.2O.sub.3, ZrO.sub.2, ZnO, and
Li.sub.2O.
[0015] The present invention also provides a conductive paste
composition comprising a silver (Ag) powder, a lithium titanium
oxide, a glass frit, a binder, and a solvent.
[0016] The present invention also provides a solar cell comprising:
a substrate having a first conductivity; an emitter layer having a
second conductivity and being formed on the substrate; an
anti-reflection layer formed on the emitter layer; a front surface
electrode being connected to the emitter layer as passing through
the anti-reflection layer and prepared using the conductive paste
composition; and a back surface electrode formed on the back
surface of the substrate.
[0017] The present invention can provide a solar cell having low
contact resistance to enhance photovoltaic efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a cross-sectional view showing the structure of a
solar cell according to one embodiment of the present
invention.
[0019] FIG. 2 is an SEM image with a magnification of 50K showing
the lithium titanium oxide particles according to the Preparation
Example 2-1 of the present invention.
DESCRIPTION OF REFERENCE NUMERALS
[0020] 1: substrate [0021] 2: emitter layer [0022] 3:
anti-reflection layer [0023] 4: front surface electrode [0024] 5:
back surface electrode
DETAILED DESCRIPTION
[0025] In accordance with one embodiment of the present invention,
there is provided a glass frit comprising SiO.sub.2, PbO, and at
least one selected from the group consisting of Al.sub.2O.sub.3,
ZrO.sub.2, ZnO, and Li.sub.2O.
[0026] In accordance with another embodiment of the present
invention, there is provided a conductive paste composition
comprising a silver (Ag) powder, a lithium titanium oxide, the
glass frit, a binder, and a solvent.
[0027] In accordance with still another embodiment of the present
invention, there is provided a solar cell comprising: a substrate
having a first conductivity; an emitter layer having a second
conductivity and being formed on the substrate; an anti-reflection
layer formed on the emitter layer; a front surface electrode being
connected to the emitter layer as passing through the
anti-reflection layer and prepared using the conductive paste
composition; and a back surface electrode formed on the back
surface of the substrate.
[0028] It will be understood that, although the terms "first",
"second", etc. may be used herein to describe various elements,
they are only used to distinguish one element from another.
[0029] It will also be understood that when a layer or an element
is referred to as being "on" or "upon" another layer or element, it
can be directly on the other layer or element, or intervening
layers or elements may be present therebetween.
[0030] While example embodiments of the present invention are
susceptible to various modifications and alternative forms,
specific embodiments thereof will herein be described in detail. It
should be understood, however, that there is no intent to limit
example embodiments of the invention to the particular forms
disclosed, but conversely, example embodiments of the invention are
to cover all modifications, equivalents, and alternatives falling
within the spirit and scope of the invention.
[0031] Hereinafter, a detailed description will be given as to a
glass frit, and a conductive paste composition and a solar cell
comprising the same according to the present invention with
reference to the accompanying drawings.
Glass Frit
[0032] The glass frit of the present invention comprises at least
one selected from the group consisting of Al.sub.2O.sub.3,
ZrO.sub.2, ZnO, and Li.sub.2O, in combination with SiO.sub.2 and
PbO.
[0033] The glass frit is used to effectively form an electrode in a
fired pattern deprived of pin holes. The glass frit of the present
invention includes, as well as SiO.sub.2 and PbO, at least one
metal oxide selected from the group consisting of Al.sub.2O.sub.3,
ZrO.sub.2, ZnO, and Li.sub.2O.
[0034] More specifically, in accordance with one embodiment of the
present invention, the glass frit may comprise 5 to 30 wt % of
SiO.sub.2, 50 to 90 wt % of PbO, 0.1 to 10 wt % of Al.sub.2O.sub.3,
and 0.1 to 10 wt % of ZrO.sub.2.
[0035] In accordance with another embodiment of the present
invention, the glass frit may comprise 5 to 30 wt % of SiO.sub.2,
50 to 90 wt % of PbO, 0.1 to 10 wt % of Al.sub.2O.sub.3, 0.1 to 10
wt % of ZrO.sub.2, 0.1 to 10 wt % of ZnO, and 0.1 to 10 wt % of
Li.sub.2O.
[0036] In the conductive paste composition of the present
invention, the components of the glass frit, such as
Al.sub.2O.sub.3, ZrO.sub.2, ZnO, and Li.sub.2O, are not only to
form a stable glass phase in the interface reaction but to maintain
low viscosity. The low viscosity of the glass component in the
interface reaction enhances the possibility that PbO gets in
contact with the anti-reflection layer, allowing etching to occur
in the larger area. The etching in a relatively large area leads to
an increase in the region for the front surface electrode formed by
recrystallization of silver, which lowers the contact resistance
between the substrate and the front surface electrode to enhance
the performance of the resultant solar cell and furthermore improve
the contact strength between the substrate and the front surface
electrode.
[0037] In accordance with one embodiment of the present invention,
the glass frit may have an average particle diameter (D50) in the
range of about 0.5 to about 10 .mu.m, preferably about 0.8 to about
5 .mu.m. The average particle size of the glass frit within the
above-defined range leads to formation of the electrode in a fired
pattern deprived of pin holes with effectiveness.
[0038] Beside the aforementioned components, another components
typically used in the glass frit may be further added. For example,
the glass frit may further comprise, if not specifically limited
to, Na.sub.2O.
[0039] The components of the glass frit of the present invention
are melted together. The step of melting the components of the
glass frit according to one embodiment of the present invention is
conducted to disconnect the bonding between the molecules in the
individual components to lose the properties peculiar to the metal
oxide, so that the melted components are homogeneously mixed
together to provide the vitric properties through the subsequent
cooling step. In the melting step, the melting temperature may be
selected without specific limitation as the temperature at which
all the individual components are sufficiently melted. For example,
the melting temperature may be, if not specifically limited to,
about 900 to about 1,500.degree. C. Further, the melting time may
be determined, without specific limitation, as the time period
during which all the components are sufficiently melted at the
above-defined melting temperature, and selected appropriately
depending on the types of the components and the melting
temperature. For example, the melting time may be, if not
specifically limited to, about 10 minutes to about one hour.
[0040] The melted mixture is then cooled down to acquire a glass
frit in a solid state. The cooling step hardens the individual
melted components to form a solid glass frit. The cooling rate may
be controlled appropriately depending on the types of the
components of the glass frit. Generally, the fast cooling for the
melted mixture is preferable. The specified cooling conditions may
be determined with reference to the phase diagram based on the
individual components. An extremely low cooling rate may cause
crystallization during the cooling step, consequently with the
failure to form the glass phase. For example, the melted mixture
may be cooled down, if not specifically limited to, at about 25 to
about 50.degree. C. for about 1 to about 25 minutes under the
atmospheric pressure. As a means for acquiring such a high cooling
rate, the typical methods known in the related art may be used,
such as, if not specifically limited to, conducting the sheet
extrusion of the melted mixture to increase the surface area, or
immersion in water.
[0041] Subsequently, the solid glass frit is ground into powder.
The solid glass frit is too bulky to be blended into a metal paste
and thus preferably subjected to comminution into powder. The
average particle diameter of the powder after comminution may be,
if not specifically limited to, about 1 to about 10 .mu.m. With the
particle diameter in the above-defined range, the glass frit can be
relatively uniformly dispersed in the metal paste to cause the
interface reaction with considerable efficiency. The comminution
method for the solid glass frit obtained by the cooling step into
powder may include, without limitation, any typical comminution
method known in the related art. For efficiency, the comminution
process may be carried out in two stages. In this case, the first
and second comminution stages may involve a repetition of the same
process; otherwise, the first comminution is crushing, and the
second comminution is fine-grinding. The term "crushing" as used
herein refers to comminution of the solid glass frit to such a
particle size adequate to the subsequent fine-grinding method to
facilitate the fine-grinding process, rather than limiting the
particle size to a given average particle diameter. The term
"fine-grinding" as used herein refers to comminution of the crushed
glass frit into glass frit powder having a desired average particle
diameter. In addition, the individual comminution stages, when
necessary, may selectively adopt either dry comminution or wet
comminution. As typically known in the related art, the wet
comminution may involve adding, if not specifically limited to,
water, ethanol, or the like.
Conductive Paste Composition
[0042] The conductive paste composition of the present invention
comprises a silver (Ag) powder, a lithium titanium oxide, a glass
frit, a binder, and a solvent.
[0043] Hereinafter, a description in further detail will be given
as to the individual ingredients of the conductive paste
composition.
[0044] In the conductive paste composition of the present
invention, silver (Ag) powder is used as a conductive powder to
provide electrical characteristics. In the present invention, the
silver powder comprises, as well as pure silver powder, silver
oxide, silver alloys, silver compounds, or other substances that
can be precipitated from the silver powder by firing, which may be
used alone or as a mixture of at least two.
[0045] The silver powder may be a spherical or flake powder
commercially available. Alternatively, the flake powder may be used
to prepare a silver powder by a known method.
[0046] The particle diameter of the silver powder can be controlled
in an appropriate range in consideration of the desired sintering
rate or its effect on the formation of an electrode. In accordance
with one embodiment of the present invention, the silver powder may
have an average particle diameter (d50) in the range of about 0.5
to about 4 .mu.m.
[0047] Further, the purity of the silver powder is not specifically
limited as long as it meets the typical requirements for
electrodes, and may be at least 90%, preferably at least 95%.
[0048] The silver powder may be contained in the conductive paste
composition of the present invention in an amount of about 60 to
about 95 parts by weight, preferably about 70 to about 85 parts by
weight, with respect to 100 parts by weight of the conductive paste
composition. The content of the silver powder less than 60 parts by
weight leads to phase separation or reduced viscosity, causing the
problem in printability, while the content of the silver powder
greater than 95 parts by weight increases the viscosity,
consequently with difficulty in printing and a rise of the cost.
Further, the content of the silver powder in the solid powder other
than the organic solvent in the paste composition may be 92 to 99
wt % with respect to the total weight of the solid powder.
[0049] In accordance with one embodiment of the present invention,
the silver powder may be used in a way that its surface is coated
with a surfactant. The specific examples of the surfactant
applicable to the coating may include, but are not limited to,
stearic acid, palmitic acid, lauric acid, oleaic acid, caprilic
acid, myristic acid, linoleic acid, and their salts or
mixtures.
[0050] The conductive paste composition of the present invention
comprises a lithium titanium oxide (LTO).
[0051] The lithium titanium oxide may be represented by the
following formula 1:
Li.sub.4Ti.sub.5-xM.sub.xO.sub.12 [Formula 1]
[0052] In the formula, x denotes the molar ratio of each component,
satisfying 0.ltoreq.x.ltoreq.0.25; and M is a metal selected from
the group consisting of Nb, Zn, Mn, Mg, Fe, Ni, Ca, Bi, Al, Zr, V,
Cu, Na, K, and P.
[0053] The lithium titanium oxide may be a compound doped with a
metal or not. For example, when 0<x.ltoreq.0.25 in the formula
1, the lithium titanium oxide is a compound obtained by doping with
a metal M.
[0054] In accordance with one embodiment of the present invention,
the lithium titanium oxide may have an average particle diameter of
about 10 to about 500 nm, preferably about 3 to about 200 nm.
[0055] The lithium titanium oxide may be obtained by a synthesis
method, which is not specifically limited and may include,
according to its phase, a gas phase method, a liquid phase method,
or a solid phase method.
[0056] Among the synthesis methods, the gas phase method involves
vaporization of a metal or a metal precursor and then reaction of
the gaseous metal or metal precursor with oxide or the like. The
gas phase method may be classified into flame combustion pyrolysis,
laser vaporization, plasma vaporization, or spray pyrolysis.
[0057] The solid phase method includes firing synthesis and
mechanochemical synthesis. The firing synthesis is a traditional
synthesis method for inorganic particles, which involves long-term
pyrolysis and oxidization of a metal precursor in a
high-temperature furnace to prepare a metal oxide,
recrystallization of the metal oxide, and then comminution of the
metal oxide crystals into microparticles. The mechanochemical
synthesis is activating the surface of a metal precursor using the
mechanical strength with high-speed and high-energy technique, such
as by ball milling, to cause a desired reaction.
[0058] The liquid phase method includes hydrothermal synthesis and
sol-gel synthesis. The hydrothermal synthesis which is most
predominantly used as the liquid phase method can prepare the
lithium titanium oxide by thermal synthesis using water as a
reaction medium or a reactant.
[0059] Further, the lithium titanium oxide can be prepared in the
form of nano-size ultrafine particles without needing a firing
process by continuous reactions of water and an aqueous solution of
at least two metal salts under the sub-critical or super-critical
conditions.
[0060] The conductive paste composition of the present invention
comprises a glass frit.
[0061] The glass frit is used to effectively form an electrode in a
fired pattern deprived of pin holes.
[0062] In accordance with one embodiment of the present invention,
the glass frit is contained in an amount of about 0.1 to about 10
parts by weight with respect to 100 parts by weight of the paste
composition. The content in the above-defined range enables it to
maintain low viscosity during the interface reaction, lowers the
contact resistance between the substrate and the front surface
electrode, and secures considerably high contact strength.
[0063] The glass frit comprises at least one selected from the
group consisting of Al.sub.2O.sub.3, ZrO.sub.2, ZnO, and Li.sub.2O,
in combination with SiO.sub.2 and PbO.
[0064] In the conductive paste composition of the present
invention, Al.sub.2O.sub.3, ZrO.sub.2, ZnO, and Li.sub.2O included
in the glass frit not only form the stable glass phase in the
interface reaction but maintain low viscosity. The glass component
with low viscosity in the interface reaction increases the
possibility of the contact between PbO and the anti-reflection
layer, allowing etching to occur in more regions. The more the
etching occurs, the wider the region of the front surface electrode
formed by recrystallization of silver becomes, leading to a reduced
contact resistance between the substrate and the front surface
electrode, to enhance the performance of the solar cell and improve
the contact strength between the substrate and the front surface
electrode.
[0065] A further detailed description on the glass frit can be
given as above.
[0066] The conductive paste composition of the present invention
may further comprise a metal oxide as an inorganic additive. The
metal oxide can be added to enhance the adhesiveness of the paste
composition with the crystalline wafer. The metal oxide added as
the inorganic additive can be selected from zinc oxide (ZnO),
zirconium oxide (ZrO.sub.2), or a mixture of them.
[0067] The metal oxide may be included in an amount of about 0.1 to
about 10 parts by weight, preferably about 1 to about 5 parts by
weight, with respect to 100 parts by weight of the composition. In
addition, the average particle diameter (d50) may be in the range
of about 500 to about 1,000 nm.
[0068] The conductive paste composition of the present invention
comprises a binder.
[0069] The binder, which functions as a binding material for the
individual ingredients prior to a firing of the electrode pattern,
can be preferably prepared by suspension polymerization.
[0070] The binder may include a resin containing a carboxyl group,
such as, for example, a photosensitive resin containing a carboxyl
group with or without an ethylene-based unsaturated double bond.
More specifically, the binder may include, but are not specifically
limited to, a photosensitive resin containing a carboxyl group that
is a copolymer of an unsaturated carboxylic acid and a compound
having an unsaturated double bond; a photosensitive resin
containing a carboxyl group that is a copolymer of an unsaturated
carboxylic acid and a compound having an unsaturated double bond in
which an ethylene-based unsaturated group is added as a pendant
group; or a photosensitive resin containing a carboxyl group that
is obtained by reacting a copolymer of an acid anhydride having an
unsaturated double bond and a compound having an unsaturated double
bond with a compound having a hydroxide group and an unsaturated
double bond.
[0071] The binder may be contained in an amount of about 1 to about
20 parts by weight with respect to 100 parts by weight of the paste
composition. The content of the binder less than 1 part by weight
possibly leads to the non-uniform distribution of the binder in the
electrode pattern, making it difficult to achieve patterning by
selective exposure and development, whereas the content of the
binder greater than 20 parts by weight causes pattern collapse
during a firing of the electrode and increases the resistance of
the electrode due to carbon ash after the firing.
[0072] The conductive paste composition of the present invention
comprises a solvent.
[0073] The solvent can be selected as to dissolve the binder and
become well miscible with other additives. The examples of the
solvent may include, but are not specifically limited to,
.alpha.-terpinol, butyl carbitol acetate, texanol, butyl carbitol,
di-propylene glycol monomethyl ether, etc.
[0074] The solvent may be contained in an amount of about 1 to
about 20 parts by weight with respect to 100 parts by weight of the
paste composition. The content of the solvent less than 1 part by
weight leads to a non-uniform coating of the paste, whereas the
content of the solvent greater than 20 parts by weight results in
insufficient conductivity of the electrode pattern and
deterioration of the adhesion of the electrode with the
substrate.
[0075] The conductive paste composition of the present invention
may further comprise additives, such as a dispersing agent, a
thickening agent, a thixotropic agent, a leveling agent, etc. in
addition to the aforementioned constitutional ingredients. Under
necessity, such additives may be contained in an amount of about 1
to about 20 parts by weight with respect to 100 parts by weight of
the paste composition.
[0076] The examples of the dispersing agent may include, but are
not specifically limited to, DISPERBYK.RTM.-180, 110, 996, 997,
etc. as produced by BYK.
[0077] The examples of the thickening agent may include, but are
not specifically limited to, BYK.RTM.-410, 411, 420, etc. as
produced by BYK.
[0078] The examples of the thixotropic agent may include, but are
not specifically limited to, ANTI-TERRA.RTM.-203, 204, 205, etc. as
produced by BYK.
[0079] The examples of the leveling agent may include, but are not
specifically limited to, BYK.RTM.-3932 P, BYK.RTM.-378,
BYK.RTM.-306, BYK.RTM.-3440, etc. as produced by BYK.
Solar Cell
[0080] The solar cell according to one embodiment of the present
invention comprises: a substrate having a first conductivity; an
emitter layer having a second conductivity and being formed on the
substrate; an anti-reflection layer formed on the emitter layer; a
front surface electrode being connected to the emitter layer as
passing through the anti-reflection layer and prepared using the
conductive paste composition; and a back surface electrode formed
on the back surface of the substrate.
[0081] FIG. 1 is a cross-section view showing the structure of the
solar cell according to one embodiment of the present
invention.
[0082] Referring to FIG. 1, the solar cell of the present invention
comprises: a substrate having a first conductivity; an emitter
layer having a second conductivity and being formed on the
substrate 1; an anti-reflection layer 3 formed on the emitter layer
2; a front surface electrode 4 being connected to the emitter layer
2 as passing through the anti-reflection layer 3 and prepared using
the conductive paste composition; and a back surface electrode 4
formed on the back surface of the substrate 1.
[0083] In accordance with one embodiment of the present invention,
the substrate 1 is a silicon semiconductor substrate having a first
conductivity that may be doped with P type impurities, which are
trivalent elements, such as boron (B), gallium (Ga), or indium
(In). The silicon may be crystalline silicon, such as
monocrystalline silicon or polycrystalline silicon, or amorphous
silicon.
[0084] The emitter layer 2 has a second conductivity as opposite to
the substrate 1 and may be doped with N type impurities, which are
elements in Group 5, such as phosphorus (P), arsenic (As), antimony
(Sb).
[0085] As the substrate 1 and the emitter layer 2 are doped with
opposite conductivity type impurities to each other, a P-N junction
is formed in the interface between them.
[0086] In accordance with one embodiment of the present invention,
the emitter layer 2 can have such a high sheet resistance that
secures high photoelectric conversion efficiency. For example, the
solar cell of the present invention may comprise the emitter layer
2 having a high sheet resistance of about 60 to about 120
.OMEGA./sq.
[0087] In accordance with one embodiment of the present invention,
the thickness of the emitter layer 2 may be in the range of about
100 to about 500 nm.
[0088] The anti-reflection layer 3 passivates the defects on the
surface or in the bulk of the emitter layer 2 and reduces the
reflectivity of an incident sun light that strikes the front
surface of the substrate 1. The passivation of the defects on the
emitter layer 2 leads to elimination of sites for recombination of
minority carriers to increase the open-circuit voltage Voc of the
solar cell. Further, the quantity of light arriving at the region
of the P-N junction increases with a decrease in the reflectivity
of the sun light, to increase the short-circuit current Isc of the
solar cell. In this manner, the anti-reflection layer 3 enhances
the photoelectric conversion efficiency as much as an increase in
the open-circuit voltage and the short-circuit current of the solar
cell.
[0089] The anti-reflection layer 3 may have a monolayer structure
comprising any one monolayer selected from the group consisting of,
if not specifically limited to, silicon nitride layer,
hydrogen-containing silicon nitride layer, silicon oxide layer,
silicon oxynitride layer, MgF.sub.2 layer, ZnS layer, TiO.sub.2
layer, and CeO.sub.2 layer; or a multilayer structure comprising a
combination of at least two of the monolayers. The anti-reflection
layer 3 may be formed by vacuum deposition, chemical vapor
deposition, spin coating, screen printing, or spray coating.
Further, the thickness of the anti-reflection layer 3 may be, if
not specifically limited to, about 30 to about 100 nm.
[0090] The front surface electrode 4 may be formed using the
conductive paste composition of the present invention.
[0091] The conductive paste composition comprises a silver (Ag)
powder, a lithium titanium oxide, a glass frit, a binder, and a
solvent. The detailed description of those ingredients is as
specified in the descriptions of the glass frit and the conductive
paste composition.
[0092] In accordance with one embodiment of the present invention,
the conductive paste composition is subjected to screen printing
and then heat treatment to form the front surface electrode 4. More
specifically, the conductive paste composition is applied to
printing with a screen printer and then dried out in an infrared
firing furnace at about 250 to about 350.degree. C. for about 0.5
to about 5 minutes. Subsequently, it is fired in the infrared
firing furnace at about 600 to about 900.degree. C. for about 1 to
about 5 minutes to form the front surface electrode 4. As the
silver contained in the paste liquefies at high temperature and
then recrystallizes into solid through the firing process, a
connection to the emitter layer 2 is formed by punch-through
phenomenon that the silver goes through the anti-reflection layer 3
via the glass frit and the lithium titanium oxide.
[0093] The back surface electrode 5 is formed on the back surface
of the substrate 1 and contains aluminum. For example, aluminum
paste is printed on the back surface of the substrate 1 and then
subjected to heat treatment to form the back surface electrode
5.
[0094] The aluminum contained in the back surface electrode 5
diffuses through the back surface of the substrate 1 to form a back
surface field layer in the interface between the back surface
electrode 5 and the substrate 1. The formation of the back surface
field layer prevents migration of carriers towards the back surface
of the substrate 1 and recombination. As the recombination of
carriers is inhibited, the open-circuit voltage increases to
enhance the efficiency of the solar cell.
[0095] The solar cell of the present invention can have an enhanced
efficiency by including the front surface electrode prepared using
the conductive paste composition having an optimized composition
enough to provide low contact resistance.
[0096] Hereinafter, the present invention will be described in
further detail by way of the following examples, which are given
for illustrations only and not intended to limit the scope of the
present invention.
EXAMPLES
Preparation of Glass Frit
Preparation Example 1-1
[0097] 17.0 wt % of SiO.sub.2, 8.7 wt % of Al.sub.2O.sub.3, 66.0 wt
% of PbO, 6.0 wt % of ZnO, 1.7 wt % of Li.sub.2O, and 0.6 wt % of
ZrO.sub.2 were mixed using a ball mill and dried out at 80.degree.
C. The mixture was melted at 1,000.degree. C. and then quenched at
the room temperature. The dried mixture was crushed by disc milling
and fine-ground using a planetary mill to prepare a glass frit
(hereinafter, denoted by "G/F-1") having an average particle
diameter of 5 .mu.m.
Preparation Example 1-2
[0098] The procedures were performed in the same manner as
described in Preparation Example 1-1, excepting that 9.3 wt % of
SiO.sub.2, 2.4 wt % of Al.sub.2O.sub.3, 84.5 wt % of PbO, 6.0 wt %
of ZnO, and 3.8 wt % of ZrO.sub.2 were mixed to prepare a glass
frit (hereinafter, denoted by "G/F-2").
[0099] The compositions of the glass frits according to the
Preparation Examples 1-1 and 1-2 are presented in the following
Table 1.
TABLE-US-00001 TABLE 1 Glass frits SiO.sub.2 Al.sub.2O.sub.3 PbO
ZnO Li.sub.2O ZrO.sub.2 Preparation Example 1-1 17.0 8.7 66.0 6.0
1.7 0.6 (G/F-1) Preparation Example 1-2 9.3 2.4 84.5 -- -- 3.8
(G/F-2)
Preparation of Lithium Titanium Oxide
Preparation Example 2-1
[0100] The hydrothermal method was used to prepare a lithium
titanium oxide (hereinafter, denoted by
"Li.sub.4Ti.sub.5O.sub.12-150") having an average particle diameter
of 150 nm.
[0101] More specifically, of Li.sub.2CO.sub.3 and TiO(OH).sub.2
were used as starting materials. Li.sub.2CO.sub.3 and TiO(OH).sub.2
at a weight ratio of 1.05:1 (Li.sub.2CO.sub.3: TiO(OH).sub.2) were
added to 100 g of deionized (DI) water and then mixed in a rotary
mill at 100 rpm for 2 hours to prepare a slurry.
[0102] The slurry was collected and dried out with a spray drier at
100.degree. C. The dried powder thus obtained was transferred to a
high temp furnace for calcination at 750.degree. C. for 5 hours.
Then, the powder was blended and ground in a rotary mill for 24
hours. The resultant powder was placed in a high temp firing
furnace for sintering at 1,050.degree. C. for 5 hours to prepare
crystalline lithium titanium oxide.
[0103] The lithium titanium oxide particles obtained in the
Preparation Example 2-1 were measured in regard to the particle
diameter using a scanning microscope with a magnification of 50K.
The results are shown in FIG. 2. Referring to FIG. 2, it can be
seen that lithium titanium oxide particles having an average
particle diameter of 150 nm were formed.
Preparation Example 2-2
[0104] The procedures were performed in the same manner as
described in Preparation Example 2-1, excepting that the powder was
ground in a rotary mill at 500 rpm for 4 hours to prepare a lithium
titanium oxide (hereinafter, denoted by
"Li.sub.4Ti.sub.5O.sub.12-100") having an average particle diameter
of 100 nm.
Preparation Example 2-3
[0105] The procedures were performed in the same manner as
described in Preparation Example 2-1, excepting that the powder was
ground in a rotary mill at 1,000 rpm for 48 hours to prepare a
lithium titanium oxide (hereinafter, denoted by
"Li.sub.4Ti.sub.5O.sub.12-30") having an average particle diameter
of 30 nm.
Preparation Example 2-4
[0106] The procedures were performed in the same manner as
described in Preparation Example 2-1, excepting that 0.1 g of
Nb(OH).sub.5 was added to prepare a Nb-doped lithium titanium oxide
(hereinafter, denoted by
"Li.sub.4Ti.sub.4.75Nb.sub.0.25O.sub.12-150") having an average
particle diameter of 150 nm.
Preparation Example 2-5
[0107] The procedures were performed in the same manner as
described in Preparation Example 2-1, excepting that 1.0 g of NaOH
was added to prepare a Na-doped lithium titanium oxide
(hereinafter, denoted by
"Li.sub.4Ti.sub.4.75Na.sub.0.25O.sub.12-150") having an average
particle diameter of 150 nm.
Preparation of Conductive Paste Composition
Example 1
[0108] For a silver (Ag) powder, there were used 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 32.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m.
[0109] 1.0 wt % of the Li.sub.4Ti.sub.5O.sub.12-150 as obtained in
the Preparation Example 2-1 was used as a lithium titanium
oxide.
[0110] 5.0 wt % of G/F-2 as obtained in the Preparation Example 1-2
was used as a glass frit.
[0111] Further, 2.0 wt % of ethyl cellulose (Std 10 produced by
DOW) as a binder, 4.0 wt % of butyl carbitol acetate (BCA) as a
solvent, and 1.0 wt % of a thixotropic agent (Anti-terra 204
produced by BYK) as an additive were blended to prepare a
conductive paste composition.
Example 2
[0112] The procedure were performed in the same manner as described
in Example 1 to prepare a conductive paste composition, excepting
that there were used, as the silver (Ag) powder, 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 31.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m; and 2.0 wt % of
Li.sub.4Ti.sub.5O.sub.12-150 as the lithium titanium oxide.
Example 3
[0113] The procedure were performed in the same manner as described
in Example 1 to prepare a conductive paste composition, excepting
that there were used, as the silver (Ag) powder, 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 30.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m; and 3.0 wt % of
Li.sub.4Ti.sub.5O.sub.12-150 as the lithium titanium oxide.
Example 4
[0114] The procedure were performed in the same manner as described
in Example 1 to prepare a conductive paste composition, excepting
that there were used, as the silver (Ag) powder, 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 29.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m; and 4.0 wt % of
Li.sub.4Ti.sub.5O.sub.12-150 as the lithium titanium oxide.
Example 5
[0115] The procedure were performed in the same manner as described
in Example 1 to prepare a conductive paste composition, excepting
that there were used, as the silver (Ag) powder, 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 28.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m; and 5.0 wt % of
Li.sub.4Ti.sub.5O.sub.12-150 as the lithium titanium oxide.
Comparative Example 1
[0116] For a silver (Ag) powder, there were used 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 33.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m.
[0117] 5.0 wt % of G/F-2 as obtained in the Preparation Example 1-2
was used as a glass frit.
[0118] Further, 2.0 wt % of ethyl cellulose (Std 10 produced by
DOW) as a binder, 4.0 wt % of butyl carbitol acetate (BCA) as a
solvent, and 1.0 wt % of a thixotropic agent (Anti-terra 204
produced by BYK) as an additive were blended to prepare a
conductive paste composition.
[0119] The constitutional ingredients and the compositions of the
paste compositions according to the Examples 1 to 5 and the
Comparative Example 1 are presented in the following Table 2.
TABLE-US-00002 TABLE 2 Comparative Example (unit: wt %) Example 1
Ingredient Type 1 2 3 4 5 (unit: wt %) Silver Particles with 55.0
55.0 55.0 55.0 55.0 55.0 powder average particle diameter of 2.0
.mu.m Particles with 32.0 31.0 30.0 29.0 28.0 33.0 average particle
diameter of 0.8 .mu.m Lithium Preparation 1.0 2.0 3.0 4.0 5.0 0
titanium Example 2-1 oxide (Li.sub.4Ti.sub.5O.sub.12-150) Glass
frit Preparation 5.0 5.0 5.0 5.0 5.0 5.0 Example 1-2 (G/F-2) Binder
Ethyl cellulose 2.0 2.0 2.0 2.0 2.0 2.0 Solvent BCA 4.0 4.0 4.0 4.0
4.0 4.0 Organic Anti-terra 204 1.0 1.0 1.0 1.0 1.0 1.0 additive
Total 100 100 100 100 100 100
Example 6
[0120] For a silver (Ag) powder, there were used 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 32.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m.
[0121] 1.0 wt % of Li.sub.4Ti.sub.5O.sub.12-150 as obtained in the
Preparation Example 2-1 was used as a lithium titanium oxide.
[0122] 5.0 wt % of the G/F-1 as obtained in the Preparation Example
1-1 was used as a glass frit.
[0123] Further, 2.0 wt % of ethyl cellulose (Std 10 produced by
DOW) as a binder, 4.0 wt % of butyl carbitol acetate (BCA) as a
solvent, and 1.0 wt % of a thixotropic agent (Anti-terra 204
produced by BYK) as an additive were blended to prepare a
conductive paste composition.
Example 7
[0124] The procedure were performed in the same manner as described
in Example 6 to prepare a conductive paste composition, excepting
that there were used, as the silver (Ag) powder, 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 31.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m; and 2.0 wt % of
Li.sub.4Ti.sub.5O.sub.12-150 as the lithium titanium oxide.
Example 8
[0125] The procedure were performed in the same manner as described
in Example 6 to prepare a conductive paste composition, excepting
that there were used, as the silver (Ag) powder, 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 30.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m; and 3.0 wt % of
Li.sub.4Ti.sub.5O.sub.12-150 as the lithium titanium oxide.
Example 9
[0126] The procedure were performed in the same manner as described
in Example 6 to prepare a conductive paste composition, excepting
that there were used, as the silver (Ag) powder, 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 29.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m; and 4.0 wt % of
Li.sub.4Ti.sub.5O.sub.12-150 as the lithium titanium oxide.
Example 10
[0127] The procedure were performed in the same manner as described
in Example 6 to prepare a conductive paste composition, excepting
that there were used, as the silver (Ag) powder, 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 28.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m; and 5.0 wt % of
Li.sub.4Ti.sub.5O.sub.12-150 as the lithium titanium oxide.
[0128] The constitutional ingredients and the compositions of the
paste compositions according to the Examples 6 to 10 are presented
in the following Table 3.
TABLE-US-00003 TABLE 3 Example (unit: wt %) Ingredient Type 6 7 8 9
10 Silver Particles with average 55.0 55.0 55.0 55.0 55.0 powder
particle diameter of 2.0 .mu.m Particles with average 32.0 31.0
30.0 29.0 28.0 particle diameter of 0.8 .mu.m Lithium Preparation
1.0 2.0 3.0 4.0 5.0 titanium Example 2-1 oxide
(Li.sub.4Ti.sub.5O.sub.12-150) Glass frit Preparation 5.0 5.0 5.0
5.0 5.0 Example 1-1 (G/F-1) Binder Ethyl cellulose 2.0 2.0 2.0 2.0
2.0 Solvent BCA 4.0 4.0 4.0 4.0 4.0 Organic Anti-terra 204 1.0 1.0
1.0 1.0 1.0 additive Total 100 100 100 100 100
Example 11
[0129] For a silver (Ag) powder, there were used 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 30.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m.
[0130] 1.0 wt % of Li.sub.4Ti.sub.5O.sub.12-150 as obtained in the
Preparation Example 2-1 and 1.0 wt % of Li.sub.4Ti.sub.5O.sub.12-30
as obtained in the Preparation Example 2-3 were used as lithium
titanium oxides.
[0131] 5.0 wt % of the G/F-1 as obtained in the Preparation Example
1-1 was used as a glass frit.
[0132] Further, 2.0 wt % of ethyl cellulose (Std 10 produced by
DOW) as a binder, 4.0 wt % of butyl carbitol acetate (BCA) as a
solvent, and 1.0 wt % of a thixotropic agent (Anti-terra 204
produced by BYK) as an additive were blended to prepare a
conductive paste composition.
Example 12
[0133] The procedure were performed in the same manner as described
in Example 11 to prepare a conductive paste composition, excepting
that there were used, as the silver (Ag) powder, 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 31.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m; and 2.0 wt % of
Li.sub.4Ti.sub.5O.sub.12-100 of the Preparation Example 2-2 as the
lithium titanium oxide.
Example 13
[0134] The procedure were performed in the same manner as described
in Example 11 to prepare a conductive paste composition, excepting
that there were used, as the silver (Ag) powder, 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 31.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m; and 2.0 wt % of
Li.sub.4Ti.sub.5O.sub.12-30 of the Preparation Example 2-3 as the
lithium titanium oxide.
[0135] The constitutional ingredients and the compositions of the
paste compositions according to the Examples 11, 12, and 13 are
presented in the following Table 4.
TABLE-US-00004 TABLE 4 Example (unit: wt %) Ingredient Type 11 12
13 Silver powder Particles with average 55.0 55.0 55.0 particle
diameter of 2.0 .mu.m Particles with average 30.0 31.0 31.0
particle diameter of 0.8 .mu.m Lithium titanium Preparation Example
2-1 2.0 -- -- oxide (Li.sub.4Ti.sub.5O.sub.12-150) Preparation
Example 2-2 -- 2.0 -- (Li.sub.4Ti.sub.5O.sub.12-100) Preparation
Example 2-3 1.0 -- 2.0 (Li.sub.4Ti.sub.5O.sub.12-30) Glass frit
Preparation Example 1-1 5.0 5.0 5.0 (G/F-1) Binder Ethyl cellulose
2.0 2.0 2.0 Solvent BCA 4.0 4.0 4.0 Organic additive Anti-terra 204
1.0 1.0 1.0 Total 100.0 100.0 100.0
Example 14
[0136] For a silver (Ag) powder, there were used 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 28.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m.
[0137] 2.0 wt % of Li.sub.4Ti.sub.5O.sub.12-150 as obtained in the
Preparation Example 2-1 and 1.0 wt % of Li.sub.4Ti.sub.5O.sub.12-30
as obtained in the Preparation Example 2-3 were used as lithium
titanium oxides.
[0138] 5.0 wt % of the G/F-1 as obtained in the Preparation Example
1-1 was used as a glass frit.
[0139] 2.0 wt % of ZnO was used as an inorganic additive.
[0140] Further, 2.0 wt % of ethyl cellulose (Std 10 produced by
DOW) as a binder, 4.0 wt % of butyl carbitol acetate (BCA) as a
solvent, and 1.0 wt % of a thixotropic agent (Anti-terra 204
produced by BYK) as an additive were blended to prepare a
conductive paste composition.
Example 15
[0141] The procedure were performed in the same manner as described
in Example 14 to prepare a conductive paste composition, excepting
that 2.0 wt % of ZrO.sub.2 was used as the inorganic additive.
Example 16
[0142] The procedure were performed in the same manner as described
in Example 14 to prepare a conductive paste composition, excepting
that 1.0 wt % of ZnO and 1.0 wt % of ZrO.sub.2 were used as the
inorganic additives.
[0143] The constitutional ingredients and the compositions of the
paste compositions according to the Examples 14, 15, and 16 are
presented in the following Table 5.
TABLE-US-00005 TABLE 5 Example (unit: wt %) Ingredient Type 14 15
16 Silver powder Particles with average 55.0 55.0 55.0 particle
diameter of 2.0 .mu.m Particles with average 28.0 28.0 28.0
particle diameter of 0.8 .mu.m Lithium titanium Preparation Example
2-1 2.0 2.0 2.0 oxide (Li.sub.4Ti.sub.5O.sub.12-150) Preparation
Example 2-2 -- -- -- (Li.sub.4Ti.sub.5O.sub.12-100) Preparation
Example 2-3 1.0 1.0 1.0 (Li.sub.4Ti.sub.5O.sub.12-30) Inorganic
additive ZnO 2.0 -- 1.0 ZnO.sub.2 -- 2.0 1.0 Glass frit Preparation
Example 1-1 5.0 5.0 5.0 (G/F-1) Binder Ethyl cellulose 2.0 2.0 2.0
Solvent BCA 4.0 4.0 4.0 Organic additive Anti-terra 204 1.0 1.0 1.0
Total 100.0 100.0 100.0
Example 17
[0144] For a silver (Ag) powder, there were used 55.0 wt % of
silver particles (4-8F produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 2.0 .mu.m and 29.0 wt % of
silver particles (2-1C produced by DOWA MINING CO., LTD.) having an
average particle diameter (d50) of 0.8 .mu.m.
[0145] 2.0 wt % of Li.sub.4Ti.sub.4.75Nb.sub.0.25O.sub.12-150 as
obtained in the Preparation Example 2-4 was used as a lithium
titanium oxide.
[0146] For a glass frit, 5.0 wt % of G/F-1 as obtained in the
Preparation Example 1-1 was used.
[0147] 1.0 wt % of ZnO and 1.0 wt % of ZrO.sub.2 were used as
inorganic additives.
[0148] Further, 2.0 wt % of ethyl cellulose (Std 10 produced by
DOW) as a binder, 4.0 wt % of butyl carbitol acetate (BCA) as a
solvent, and 1.0 wt % of a thixotropic agent (Anti-terra 204
produced by BYK) as an additive were blended to prepare a
conductive paste composition.
Example 18
[0149] The procedure were performed in the same manner as described
in Example 17 to prepare a conductive paste composition, excepting
that 2.0 wt % of Li.sub.4Ti.sub.4.75Na.sub.0.25O.sub.12-150 as
obtained in the Preparation Example 2-5 was used as the lithium
titanium oxide.
[0150] The constitutional ingredients and the compositions of the
paste compositions according to the Examples 17 and 18 are
presented in the following Table 6.
TABLE-US-00006 TABLE 6 Example (unit: wt %) Ingredient Type 17 18
Silver powder Particles with average particle 55.0 55.0 diameter of
2.0 .mu.m Particles with average particle 29.0 29.0 diameter of 0.8
.mu.m Lithium titanium Preparation Example 2-4 2.0 -- oxide
(Li.sub.4Ti.sub.4.75Nb.sub.0.25O.sub.12-150) Preparation Example
2-5 -- 2.0 (Li.sub.4Ti.sub.4.75Na.sub.0.25O.sub.12-150) Inorganic
additive ZnO 1.0 1.0 ZnO.sub.2 1.0 1.0 Glass frit Preparation
Example 1-1 (G/F-1) 5.0 5.0 Binder Ethyl cellulose 2.0 2.0 Solvent
BCA 4.0 4.0 Organic additive Anti-terra 204 1.0 1.0 Total 100.0
100.0
Fabrication of Solar Cell
Example 19
[0151] A 156 mm polycrystalline silicon wafer was doped with
phosphorus (P) through a diffusion process using POCL.sub.3 at
900.degree. C. in a tube furnace to form an emitter layer having a
sheet resistance of 100 .OMEGA./sq.
[0152] On the emitter layer was deposited a silicon nitride layer
by PECVD to form an 80 nm-thick anti-reflection layer.
[0153] An aluminum paste (ALSOLAR.RTM. produced by Toyo Aluminum K.
K) was used to screen-print the back surface of the wafer.
Subsequently, the aluminum paste was dried out in a belt firing
furnace at 300.degree. C. for 60 seconds and then sintered in the
belt firing furnace at 900.degree. C. for 60 seconds to form a back
surface electrode. The thickness of the back surface electrode
formed after the sintering process was about 30 .mu.m.
[0154] The conductive paste composition obtained in the Example 1
was used to screen-print the surface of the anti-reflection layer
with a screen printer and then dried out in an infrared firing
furnace at 180.degree. C. for one minute. Subsequently, a firing
process was conducted with the infrared firing furnace at a
temperature of 600 to 900.degree. C. for 1 to 5 minutes to form a
front surface electrode.
Examples 20 to 36
[0155] The procedures were performed in the same manner as
described in Example 19 to fabricate a solar cell, excepting that
the conductive paste compositions of the Examples 2 to 18 were
used, respectively, to form a front surface electrode.
Comparative Example 2
[0156] The procedures were performed in the same manner as
described in Example 19 to fabricate a solar cell, excepting that
the conductive paste composition of the Comparative Example 1 was
used to form a front surface electrode.
Evaluation of Electrical Performance
[0157] The solar cells fabricated in the Examples 19 to 36 and the
Comparative Example 2 were measured in regard to the electrical
performances with Model NCT-M-180A manufactured by NPC Inc. (Dumont
N.J., U.S.A.) under the AM 1.5 solar condition according to the
ASTM G-173-03.
[0158] The results are presented in the following Table 7, where
Jsc means the short-circuit current density measured at zero output
voltage; Voc means the open-circuit voltage measured at zero output
voltage; FF (%) means the fill factor; and Eta (%) means the
efficiency.
TABLE-US-00007 TABLE 7 Eta (%) Jsc (mA/cm.sup.2) Voc (V) FF (%)
Example 19 13.80 34.58 0.61 65.61 Example 20 14.10 34.48 0.61 66.57
Example 21 15.30 34.49 0.61 72.40 Example 22 13.92 34.58 0.61 65.61
Example 23 14.09 34.91 0.61 65.92 Example 24 16.53 34.43 0.61 77.34
Example 25 16.45 35.25 0.61 76.11 Example 26 16.69 35.04 0.62 76.99
Example 27 16.20 34.17 0.62 76.77 Example 28 16.18 34.17 0.62 76.65
Example 29 16.70 35.05 0.62 76.80 Example 30 16.64 35.18 0.62 76.39
Example 31 16.54 35.03 0.62 76.08 Example 32 16.70 34.98 0.62 77.40
Example 33 16.70 34.94 0.62 77.50 Example 34 16.80 34.92 0.62 78.91
Example 35 17.10 35.40 0.62 77.80 Example 36 17.20 35.52 0.62 77.90
Comparative 8.72 34.38 0.61 43.96 Example 2
[0159] As can be seen from Table 7, the solar cells of the Examples
19 to 36 using the conductive paste compositions of the present
invention had considerable enhancement in efficiency and fill
factor in comparison with the solar cell of the Comparative Example
2.
* * * * *