U.S. patent application number 11/726773 was filed with the patent office on 2008-09-25 for paste for back contact-type solar cell.
Invention is credited to Hideki Akimoto.
Application Number | 20080230119 11/726773 |
Document ID | / |
Family ID | 39773509 |
Filed Date | 2008-09-25 |
United States Patent
Application |
20080230119 |
Kind Code |
A1 |
Akimoto; Hideki |
September 25, 2008 |
Paste for back contact-type solar cell
Abstract
Disclosed is a paste for a back contact-type solar cell that
includes: (a) electrically conductive particles containing silver
particle and added particles selected from the group consisting of
(i) metal particles selected from the group consisting of Mo, Tc,
Ru, Rh, Pd, W, Re, Os, Ir and Pt particles, (ii) a metal alloy
containing the metal particles, and (iii) particles loaded with the
metal particles, (b) glass frit, and (c) a resin binder.
Inventors: |
Akimoto; Hideki;
(Tochigi-ken, JP) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1122B, 4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
39773509 |
Appl. No.: |
11/726773 |
Filed: |
March 22, 2007 |
Current U.S.
Class: |
136/255 ;
252/514; 252/515 |
Current CPC
Class: |
C03C 2214/16 20130101;
H01L 31/03529 20130101; C03C 8/18 20130101; H01B 1/22 20130101;
H01L 31/1804 20130101; C03C 8/04 20130101; C03C 2214/08 20130101;
Y02P 70/521 20151101; H01L 31/022433 20130101; Y02E 10/547
20130101; C03C 8/10 20130101; Y02P 70/50 20151101; H01L 31/022441
20130101; C03C 14/006 20130101 |
Class at
Publication: |
136/255 ;
252/514; 252/515 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; H01B 1/02 20060101 H01B001/02 |
Claims
1. A paste for a back contact-type solar cell, comprising: (a)
electrically conductive particles containing silver particles and
added particles selected from the group consisting of (i) metal
particles selected from the group consisting of Mo, Tc, Ru, Rh, Pd,
W, Re, Os, Ir and Pt particles (ii) a metal alloy containing the
metal particles and (iii) particles loaded with the metal
particles; (b) glass frit; and (c) a resin binder.
2. The paste according to claim 1, wherein the metal particles are
selected from the group consisting of Ru, Rh, Pd, W, and Pt
particles.
3. The paste according to claim 2, wherein the metal particles are
Ru, Pd or Pt particles.
4. The paste according to claim 1, wherein the content of the
silver particle is 40 to 90 wt %, and the content of the added
particle is 0.01 to 10 wt % based on the weight of the paste.
5. The paste according to claim 4, the content of a third
conductive particle, which is not included the concept of the
silver particles nor the added particle, is less than 2 wt % based
on the weight of the paste.
6. A method for producing a solar cell electrode, comprising the
steps of: (1) applying a paste comprising (a) electrically
conductive particles containing silver particle and added particle
selected from the group consisting of (i) metal particles selected
from the group consisting of Mo, Tc, Ru, Rh, Pd, W, Re, Os, Ir and
Pt particles, (ii) a metal alloy containing the metal particles,
and (iii) particles loaded with the metal particles, onto the
opposite side from the light receiving side of a back contact-type
solar cell substrate; and (2) firing the applied paste.
7. The method according to claim 6, wherein the paste is fired at
450 to 900.degree. C.
8. A solar cell electrode formed on the opposite side from the
sunlight receiving side, comprising as a conductive component:
silver particle; and added particle selected from the group
consisting of (i) metal particles selected from the group
consisting of Mo, Tc, Ru, Rh, Pd, W, Re, Os, Ir and Pt particles,
(ii) a metal alloy containing the metal particles, and (iii)
particles loaded with the metal particles.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a paste for a solar cell,
and more particularly, to an electrically conductive paste used to
form electrodes of a back contact-type solar cell.
[0003] 2. Technical Background
[0004] Silver paste is widely used for the electrode paste used in
solar cells, since electrode pastes for solar cells are required to
have low electrical resistance to facilitate improved efficiency.
In the case of contact between a metal and semiconductor, a
Schottky barrier is known to be formed that causes a considerable
increase in contact resistance. This type of contact is referred to
as Schottky contact. Since the electrical resistance of an
electrode is the sum of the conductor resistance and contact
resistance, in addition to lowering the conductor resistance of an
electrode paste, it is also necessary to reduce the contact
resistance with the conductor. The ideal contact of an electrode
material, free of a Schottky barrier, is referred to as Ohmic
contact; and contact resistance is known to decrease in this
contact state.
[0005] Methods for realizing Ohmic contact at the interface between
a metal and semiconductor consist of either reducing the Schottky
barrier height from the semiconductor to zero, or reducing the
thickness of the electrode material to narrow the width of the
Schottky barrier and approach the behavior of Ohmic resistance due
to a tunnel effect.
[0006] Examples of such methods include a technique in which the
thickness of a layer formed between a metal and semiconductor for
electrical continuity is decreased, a technique in which multilayer
thin films which have similar work function are inserted between
the metal and semiconductor, a technique in which a film interposed
between the metal and semiconductor is subjected to
high-temperature heat treatment to form an alloy, and a technique
in which contact resistance is decreased by high-temperature
annealing. In the case of solar cells, however, high-temperature
treatment results in the risk of lowering the power generation
efficiency of the cell. In addition, in the case of the application
of thin films technique, the production process is time-consuming
and complex, thereby preventing production from being carried out
at low cost.
[0007] Recently however, studies have been conducted on back
contact-type solar cells for the purpose of further enhancing the
power generation efficiency of solar cells. Back contact-type solar
cells refer to solar cells in which the electrodes are formed on
the opposite side from the sunlight receiving side (back side),
thereby making it possible to increase the light receiving surface
since the electrodes are not formed on the light receiving surface.
The structure of conventional crystalline solar cells in which the
electrodes are formed on the light receiving surface typically
consisted of the formation of an n layer on the light receiving
side of a p-type semiconductor. Consequently, electrically
conductive paste for electrodes used on the light receiving side
was also developed for use with an n-type semiconductor. An
aluminum paste is used to utilize BSF effects on the back side.
Since aluminum has an inferior adhesive strength as compared with
solder, silver paste is used at those portions requiring soldering.
This silver paste for the back side is used primarily for the
purpose of ensuring electrical connection with the aluminum paste
and physical connection with the substrate, and Ohmic resistance
with the p-type semiconductor is not required.
[0008] However, in the case of back contact-type solar cells, it is
necessary to develop electrodes having low contact resistance with
the p-type semiconductor layer and to develop an electrically
conductive paste for back contact applications.
[0009] Moreover, the method for forming electrodes employs a thin
film process using vapor deposition or sputtering, and a thick film
process using a paste. It is preferable to form the electrodes
using a thick film process in consideration of production costs. In
order to minimize the decrease in solar cell efficiency caused by
thermal damage, an electrode formation method is required that
allows electrodes to be formed at a low temperature and in a short
period of time. Low-temperature, short-duration production is also
important with respect to costs and production efficiency.
[0010] In the case of solar cells in which electrodes are formed on
both sides, the light receiving side paste usually contains as
basic components electrically conductive particles in the form of
Ag, binder, glass frit and a solvent (see, for example, Japanese
Patent Application Laid-open No. 2006-295197). In Japanese Patent
Application Laid-open No. 2006-295197, examples of electrically
conductive particles include metal particles selected from the
group consisting of Cu, Au, Ag, Pd, Pt, alloys of Cu, Au, Ag, Pd
and Pt, and mixtures thereof. On the other hand, a paste composed
primarily of aluminum is typically used for the paste for forming
sites on the back side that contact the semiconductor.
[0011] The present invention provides a paste able to be applied to
a back contact-type solar cell having low contact resistance
between formed electrodes and a semiconductor.
[0012] In addition, the present invention provided a method for
forming electrodes having low contact resistance with a
semiconductor on the back of a solar cell substrate using an
electrically conductive paste.
SUMMARY OF THE INVENTION
[0013] The present invention is a paste for a back contact-type
solar cell comprising: (a) electrically conductive particles
containing silver particles and added particle selected from the
group consisting of (i) metal particles selected from the group
consisting of Mo, Tc, Ru, Rh, Pd, W, Re, Os, Ir and Pt particles,
(ii) a metal alloy containing the metal particles, and (iii)
particles loaded with the metal particles, (b) glass frit, and (c)
a resin binder.
[0014] In addition, the present invention is a method for producing
a solar cell electrode, comprising the steps of: (1) applying a
paste comprising (a) electrically conductive particles containing
silver particle and added particle selected from the group
consisting of (i) metal particles selected from the group
consisting of Mo, Tc, Ru, Rh, Pd, W, Re, Os, Ir and Pt particles,
(ii) a metal alloy containing the metal particles, and (iii)
particles loaded with the metal particles, onto the opposite side
from the light receiving side of a back contact-type solar cell
substrate; and (2) firing the applied paste.
[0015] In addition, the present invention is a solar cell electrode
formed on the opposite side from the sunlight receiving side,
comprising as a conductive component: silver particles; and added
particles selected from the group consisting of (i) metal particles
selected from the group consisting of Mo, Tc, Ru, Rh, Pd, W, Re,
Os, Ir and Pt particles, (ii) a metal alloy containing the metal
particles, and (iii) particles loaded with the metal particles.
[0016] Electrodes formed using the paste of the present invention
have low contact resistance with the solar cell semiconductor.
[0017] In addition, the paste of the present invention is able to
form p-type electrodes having superior electrical characteristics
by applying onto a p-type semiconductor and firing. Moreover, the
electrodes can be formed by a thick film process, thereby enabling
the electrodes to be formed economically. The use of
low-temperature firing makes it possible to further enhance
efficiency and economy.
[0018] A solar cell provided with electrodes formed using the paste
of the present invention has low contact resistance between the
electrodes and semiconductor, and has superior power generation
characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1A is a cross-sectional schematic drawing of a portion
of a solar cell as claimed in the present invention, and FIG. 1B is
an overhead view showing an electrode pattern on a side opposite
from a light receiving side in a portion of a solar cell as claimed
in the present invention;
[0020] FIGS. 2A to 2E are drawings for explaining a production
process when producing a solar cell;
[0021] FIGS. 3A to 3E are drawings for explaining a production
process when producing a solar cell;
[0022] FIGS. 4A to 4D are drawings for explaining a production
process when producing a solar cell;
[0023] FIGS. 5A to 5C are drawings for explaining a production
process when producing a solar cell; and
[0024] FIGS. 6A and 6B are drawings showing the shape of a sample
for measuring the contact resistance of electrodes produced on an
Si substrate using the electrically conductive paste of the present
invention, while FIG. 6C is a drawing for explaining resistance
values measured between electrodes.
DETAILED DESCRIPTION OF THE INVENTION
[0025] A first embodiment of the present invention relates to a
paste for a back contact-type solar cell. The paste for a back
contact-type solar cell of the present invention comprises: (a)
electrically conductive particles containing silver particles and
added particle selected from the group consisting of (i) metal
particles selected from the group consisting of Mo, Tc, Ru, Rh, Pd,
W, Re, Os, Ir and Pt particles, (ii) a metal alloy containing the
metal particles, and (iii) particles loaded with the metal
particles, (b) glass frit, and (c) a resin binder.
[0026] In the art, because of high contact resistance with p-type
silicon, Ag paste is not used as an electrode for a back
contact-type solar cell. Addition of other components and,
particularly the addition of precious metals such as Pd or Pt is
avoided for conventional crystalline solar cell. Price increase of
the paste, and lowering electrical conductivity was expected by
adding precious metals. A preferable paste is obtained for use in a
back contact-type solar cell by lowering the contact resistance
through the addition of a predetermined metal to the paste.
[0027] In the present invention, no application of an aluminum
paste as the back contact-type solar cell is required.
Additionally, the paste(s) of the present invention provides low
contact resistance.
[0028] The following provides an explanation of each component of
the paste of the present invention.
1. Silver Particle
[0029] Silver (Ag) particles are used as an electrically conductive
metal. The silver particle may be in the shape of flakes, spheres
or they may be amorphous. Although there are no particular
limitations on the particle diameter of the silver particle from
the viewpoint of technical effects in the case of being used as an
ordinary electrically conductive paste, particle diameter has an
effect on the sintering characteristics of the silver (for example,
silver particle having a large particle diameter are sintered at a
slower rate than silver particle having a small particle diameter).
Thus, although the particle diameter (d.sub.50) is preferably
within the range of 0.1 to 10.0 .mu.m, the particle diameter of the
silver particle actually used is determined according to the firing
profile. Moreover, it is necessary that the silver particle having
a particle diameter suited for methods for applying an electrically
conductive paste (for example, screen printing). In the present
invention, two or more types of silver particle having different
particle diameters may be used as a mixture.
[0030] Normally, the silver preferably has a high purity (greater
than 99%). However, substances of lower purity can be used
depending on the electrical requirements of the electrode
pattern.
[0031] Although there are no particular limitations on the silver
content provided it is an amount that allows the object of the
present invention to be achieved, in the case of silver particle,
the silver content is preferably 40 to 90% by weight based on the
weight of the paste.
2. Added Particles
[0032] In the present invention, 3d or 4d transition metals
belonging to groups 6 to 11 of the periodic table are used in
addition to the silver particle. Namely, metal particles selected
from the group consisting of Mo (molybdenum), Tc (technetium), Ru
(ruthenium), Rh (rhodium), Pd (palladium), W (tungsten), Re
(rhenium), Os (osmium), Ir (iridium) and Pt (platinum) are added.
Ruthenium, palladium or platinum is preferable from the viewpoint
of lowering contact resistance.
[0033] In the present invention, alloy particles containing the
above-mentioned metals or particles loaded with the above-mentioned
metal particles can also be used preferably. Examples of alloys
containing the metal particles (also referred to as added particles
in the present description) include Ag/Pd alloy and Ni/Mo alloy.
Examples of particles loaded with the metal particles include
Pt-loaded graphite and Pd-loaded graphite.
[0034] The amount of the added particles is in the range of 0.01 to
10% by weight and any ranges contained therein, and preferably 0.1
to 5% by weight based on the weight of the paste. If the amount of
added particles is excessively low, effects are not obtained. In
addition, if the amount of added particles is excessively high,
conductor resistance increases, sinterability decreases and costs
increase.
[0035] As mentioned hereinbefore, the present invention is
characterized in that both the silver particle and the added
particle are used. The addition of a third conductive particle,
which is not included the concept of the silver particles nor the
added particle, is not excluded. However, the content of the third
conductive particles is preferably less than 2 wt % based on the
weight of the paste.
3. Glass Frit
[0036] The electrically conductive paste of the present invention
preferably contains an inorganic binder in the form of glass
frit.
[0037] Since the chemical composition of the glass frit is not
important in the present invention, any glass frit can be used
provided it is a glass frit used in electrically conductive pastes
for electronic materials. For example, lead borosilicate glass is
used preferably. Lead borosilicate glass is a superior material in
the present invention from the standpoint of both the range of the
softening point and glass adhesion. In addition, lead-free glass,
such as a bismuth silicate lead-free glass, can also be used.
[0038] Although there are no particular limitations on the content
of the inorganic binder in the form of the glass frit provided it
is an amount that allows the object of the present invention to be
achieved, it is 0.5 to 15.0% by weight and preferably 1.0 to 10.0%
by weight based on the weight of the paste. If the amount of the
inorganic binder is less than 0.5% by weight, adhesive strength may
become inadequate. If the amount of the inorganic binder exceeds
15.0% by weight, problems may be caused in the subsequent soldering
step due to floating glass and soon. In addition, the resistance
value as a conductor also increases.
4. Resin Binder
[0039] The electrically conductive paste of the present invention
contains a resin binder. In the present description, the concept of
a "resin binder" includes a mixture of a polymer and thinner. Thus,
an organic liquid (also referred to as a thinner) may be contained
in the resin binder. In the present invention, a resin binder
containing an organic liquid is preferable, and in the case of high
viscosity, an organic liquid can be added separately as a viscosity
adjuster as necessary.
[0040] In the present invention, any resin binder can be used. In
the present invention, examples of the resin binder include a pine
oil solution, ethylene glycol monobutyl ether monoacetate solution
or ethyl cellulose terpineol solution of a resin (such as
polymethacrylate) or ethyl cellulose. In the present invention, a
terpineol solution of ethyl cellulose (ethyl cellulose content: 5
to 50% by weight) is used preferably. Furthermore, in the present
invention, a solvent not containing a polymer, such as water or an
organic liquid, can be used as a viscosity adjuster. Examples of
organic liquids that can be used include alcohols, alcohol esters
(such as acetates or propionates), and terpenes (such as pine oil
or terpineol) The content of the resin binder is preferably 10 to
50% by weight based on the weight of the paste.
5. Additives
[0041] A thickener and/or stabilizer and/or other typical additives
may be or may not be added to the electrically conductive paste of
the present invention. Examples of other typical additives that can
be added include dispersants and viscosity adjusters. The amount of
additive is determined dependent upon the characteristics of the
ultimately required electrically conductive paste. The amount of
additive can be suitably determined by a person with ordinary skill
in the art. Furthermore, a plurality of types of additives may also
be added.
[0042] As is explained below, the electrically conductive paste of
the present invention has a viscosity within a predetermined range.
A viscosity adjuster can be added as necessary to impart a suitable
viscosity to the electrically conductive paste. Although the amount
of viscosity adjuster added changes dependent upon the viscosity of
the ultimate electrically conductive paste, it can be suitably
determined by a person with ordinary skill in the art.
[0043] The electrically conductive paste of the present invention
can be produced as desired by mixing each of the above-mentioned
components with a roll mixing mill or rotary mixer and the like.
Although the electrically conductive paste of the present invention
is preferably printed onto a desired site on the back side of a
solar cell by screen printing, in the case of being printed by this
type of printing, the paste preferably has a predetermined
viscosity range. The viscosity of the electrically conductive paste
of the present invention is preferably 50 to 350 PaS in the case of
using a #14 spindle with a Brookfield HBT viscometer and measuring
using a utility cup at 10 rpm and 25.degree. C.
[0044] As has been described above, the paste having electrical
conductivity of the present invention is used to form electrodes on
the opposite side from the light receiving side of a solar cell.
Namely, the paste of the present invention printed and dried on the
opposite side from the light receiving side of a solar cell.
[0045] Sintering after drying is carried out at temperature of
450.degree. C. to 900.degree. C. and preferably 450.degree. C. to
700.degree. C. Conventionally, the mixture of silver particles and
aluminum particle was occasionally used. The paste containing Al
particle requires a sintering at a high temperature to form an
alloy of Si and Al, which delivers a good contact resistance.
However, in case that the paste containing Al is applied for
back-contact electrode, the sintering at a high temperature may
infer a problem in terms of good P-N junctions. In other words, the
Al easily diffuses into the substrate and bring damage since the
P-N junction is very thin at the back side of solar cell. Sintering
at a low temperature offers the advantages of reducing damage to
P-N junctions, decreasing susceptibility to the occurrence of
destruction caused by thermal damage and lowering costs. In this
context, the content of Al is preferably less than 2 wt %, more
preferably less than 1 wt % based on the weight of the paste so
that lower sintering temperature can be adapted.
[0046] The following provides an explanation of a back contact-type
solar cell using the electrically conductive paste of the present
invention and an explanation of a production process of back
contact-type solar cell electrodes of the present invention using
the example of a solar cell having the structure shown in FIG. 1,
while also providing an explanation of an example of the
fabrication of a solar cell.
[0047] FIG. 1A is a cross-sectional drawing of a portion of a solar
cell as claimed in the present invention, while FIG. 1B is an
overhead view showing a portion an electrode pattern on the
opposite side from the light receiving side. A solar cell 100 is
composed of a light receiving section 102, a carrier generating
section 104 and an electrode section 106. The light receiving
section 102 has a textured structure, and the surface thereof is
covered with a anti-reflective film 108. The anti-reflective film
108 is a thin film composed of, for example, titanium dioxide
(TiO.sub.2) and silicon dioxide (SiO.sub.2). As a result of the
light receiving section 102 having a textured structure covered by
this anti-reflective film 108, more incident light enters the
carrier generating section 104, thereby making it possible to
increase the conversion efficiency of the solar cell 100.
[0048] The carrier generating section 104 is composed of a
semiconductor 110. When light from the light receiving section 102
(and particularly light having energy equal to or greater than the
band gap of the semiconductor 110) enters this semiconductor 110,
valence band electrons are excited to the conduction band, free
electrons are generated in the conduction band, and free holes are
generated in the valence band. These free electrons and free holes
are referred to as carriers. If these carriers reach the electrode
section 106 by diffusion prior to being recombined in the carrier
generating section 104, a current can be obtained from the
electrode section 106. Thus, in order to increase the conversion
efficiency of the solar cell 100, it is preferable to use a
semiconductor that impairs carrier recombination (namely, has a
long carrier life). For this reason, the semiconductor 110 used in
the carrier generating section 104 is preferably, for example,
crystalline silicon having high resistance.
[0049] The electrode section 106 is a section where current
generated in the carrier generating section 104 is obtained. This
electrode section 106 is formed on the opposite side from the side
of the light receiving section 102 of the semiconductor 110. The
electrode section 106 has an anode 112 and a cathode 114, and these
are alternately formed on the opposite side from the side of the
light receiving section 102 of the semiconductor 110. The anode and
the cathode are respectively formed in the form of V grooves 116
and 118 having triangular cross-sections. A p+ semiconductor layer
120 is formed in the V groove 116 of the anode, while an n+
semiconductor layer 122 is formed in the V groove 118 of the
cathode. The surface of the side opposite from the side of the
light receiving section 102 is covered with an oxide film 124. In
addition, electrodes 126 formed from the electrically conductive
paste of the present invention are embedded in the V grooves.
[0050] Next, an explanation is provided of the production process
of the back contact-type solar cell electrodes of the present
invention along with an explanation of the production process of a
back contact-type solar cell with reference to FIGS. 2 to 5.
[0051] The solar cell electrode production process of the present
invention is comprised of the following steps of: [0052] (1)
applying an electrically conductive paste containing (a)
electrically conductive particles containing silver particles and
added particle selected from the group consisting of (i) metal
particles selected from the group consisting of Mo, Tc, Ru, Rh, Pd,
W, Re, Os, Ir and Pt particles, (ii) a metal alloy containing the
metal particles, and (iii) particles loaded with the metal
particles, (b) glass frit, and (c) a resin binder, onto the
opposite side from the light receiving side of a back contact-type
solar cell substrate; and [0053] (2) firing the applied paste.
[0054] First, an explanation is provided of the production a back
contact-type solar cell substrate used to produce back contact-type
solar cell electrodes with reference to FIGS. 2 to 4.
[0055] A high-resistance silicon (100) substrate 202 (having a
thickness of, for example, 250 .mu.m) is prepared, and oxide films
204a and 204b are formed on both sides thereof (FIG. 2A). These
oxide films can be formed by, for example, thermal oxidation. Next,
the oxide film 204a on one side of the silicon substrate is removed
by photolithography or laser etching and so on to leave stripes of
a predetermined width (for example, width of 100 .mu.m and interval
of 300 .mu.m) (FIG. 2B).
[0056] Next, anisotropic etching is carried out with potassium
hydroxide (KOH) or tetramethyl ammonium hydroxide (TMAH) on the
side from which a portion of the oxide film has been removed, to
form V grooves 206 (at an interval of, for example, 300 .mu.m) in
the form of stripes having a triangular cross-section (FIG.
2C).
[0057] Next, the substrate in which the V grooves 206 have been
formed is placed in a diffusion furnace to diffuse the phosphorous.
As a result of these steps, an n+-type silicon layer 208 is formed
on the portions of the silicon where the V grooves 206 have been
formed as shown in FIG. 2D. In the diffusion furnace, by
interrupting the gas serving as the phosphorous material and
introducing only oxygen, the surfaces of the V grooves 206 can be
covered with an oxide film (FIG. 2E).
[0058] The oxide film is then removed from the substrate obtained
in this manner (FIG. 3A) at equal intervals by photolithography or
laser etching at the portions between the V grooves 206 of the
oxide film 204a (FIG. 3B). For example, in the case the oxide film
portion between the V grooves 206 has a width of 300 .mu.m, the
oxide film is removed so that the distance from the V grooves 206
on both sides of this oxide film portion is 100 .mu.m.
[0059] Next, anisotropic etching is carried out with potassium
hydroxide (KOH) or tetramethyl ammonium hydroxide (TMAH) and so on
at those locations where the oxide film has been removed to form V
grooves 302 (at a width of, for example, 100 .mu.m) in the form of
stripes having a triangular cross-section (FIG. 3C).
[0060] Next, the substrate in which the V grooves 302 have been
formed is placed in a diffusion furnace to diffuse the boron. As a
result, as shown in FIG. 3D, a p+-type silicon layer 304 is formed
on the silicon portions of the V grooves 302. In the diffusion
furnace, by interrupting the gas serving as the boron material and
introducing oxygen only, the surfaces of the V grooves 302 can be
covered with an oxide film (FIG. 3E).
[0061] After removing the oxide film on the other surface (the
surface on which the oxide film 204b is formed) of the silicon
substrate 202 in which two types of V grooves have been formed in
this manner (FIG. 4A), anisotropic etching is carried out with
potassium hydroxide (KOH) or tetramethyl ammonium hydroxide (TMAH)
and so on to form a textured structure 402 in the form of stripes
having a triangular cross-section (FIG. 4B). By then carrying out
dry oxidation in a diffusion furnace, an oxide film 404 is formed
on the other side of the substrate (FIG. 4C).
[0062] Subsequently, titanium dioxide (TiO.sub.2), for example, is
then deposited on the side of the oxide film 404 at normal
temperatures by sputtering and so on (titanium dioxide film: 406).
As a result, a light receiving side having an anti-reflective film
with a textured structure is formed on the other side of the
substrate.
[0063] Next, electrodes are formed using the electrically
conductive paste of the present invention (step (1) of the
production process of the present invention). In this step, the
electrically conductive paste 502 of the present invention is
embedded in the V grooves (FIG. 5B) of the substrate obtained using
the method described above (FIG. 5A). Embedding of the electrically
conductive paste can be carried out by a patterning method such as
screen printing, stencil printing or dispenser applying.
[0064] Next, the substrate filled with the electrically conductive
paste (FIG. 5A) is fired at a predetermined temperature (for
example, 450 to 900.degree. C.) (FIG. 5C) (step (2) of the
production process of the present invention). As a result,
electrodes 504 are formed.
[0065] In the present invention, in the case of an oxide film being
formed on the n+-type silicon layer 208 and the p+-type silicon
layer 304, by firing the electrically conductive paste to fire
through the oxide film during formation of the electrodes, the
electrode material is coupled directly to the semiconductor and
electrical contact is formed.
[0066] Back contact-type solar cell electrodes of the present
invention are produced according to the process shown in FIG.
5.
[0067] A solar cell (not shown) is then fabricated by covering the
side of the electrodes with a protective film, forming wiring by
applying with an electrically conductive ink and dicing.
EXAMPLES
[0068] Although the following provides an explanation of the
present invention through examples thereof, the present invention
is not limited to these examples.
(I) Preparation of Silver Pastes
[0069] Silver pastes 1 and 2 were produced to have the compositions
shown in Table 1 using the materials indicated below.
[0070] (i) Silver Particle
[0071] Flaked silver particle [d.sub.50 2.7 .mu.m (as determined
with a laser scattering-type particle size distribution measuring
apparatus)
(ii) Glass Frit
[0072] Leaded: Lead borosilicate glass frit
[0073] Components: SiO.sub.2/PbO/B.sub.2O.sub.3/ZnO
[0074] Softening point: 440.degree. C.
[0075] Lead-free: Lead-free bismuth glass frit
[0076] Components:
SiO.sub.2/Al.sub.2O.sub.3/B.sub.2O.sub.3/ZnO/Bi.sub.2O.sub.3/SnO.sub.2
[0077] Softening point: 390.degree. C.
(iii) Resin
[0078] Ethyl cellulose resin (Aqualon, Hercules)
(iv) Solvent
[0079] Terpineol
TABLE-US-00001 TABLE 1 (Parts by Silver Glass frit weight) particle
Leaded Lead-free Resin Solvent Total Paste 1 70.30 5.25 2.50 21.95
100.00 Paste 2 70.30 5.25 2.50 21.95 100.00
[0080] The silver particle, glass frit, resin and solvent were each
weighed, mixed and kneaded with a three-roll kneader to obtain
silver pastes.
(II) Preparation of Added Metal Pastes
[0081] Pastes A to I were prepared using the metal particles and
metal-loaded particles of each of the metals (to be referred to as
the added particles), resin and solvent shown in Table 2. The resin
and solvent were the same as those used to prepare the silver paste
described above.
TABLE-US-00002 TABLE 2 (Parts by Added weight) Type of added
particles particles Resin Solvent Total Paste A Ni 10.00 1.30 8.70
20 Paste B W 10.00 1.30 8.70 20 Paste C Pd 10.00 1.30 8.70 20 Paste
D Ag/Pd alloy 10.00 1.30 8.70 20 Paste E Pt 10.00 1.30 8.70 20
Paste F Au 10.00 1.30 8.70 20 Paste G Ru 10.00 1.30 8.70 20 Paste H
Rh 10.00 1.30 8.70 20 Paste I Pt-loaded graphite 5.00 1.95 13.05
20
[0082] The metal particles, resin and solvent were each weighed,
mixed and stirred for 2 minutes using a rotary mixer to obtain
added metal pastes.
(III) Preparation of Electrically Conductive Pastes
[0083] The silver paste prepared in (I) above and the pastes A to I
prepared in (II) above were mixed and stirred for 2 minutes with a
rotary mixer. Pastes A to I were added to the silver paste so that
the mixing ratio was 1% by weight of the added particles to the
weight of the silver in the silver paste (although two types of
electrically conductive pastes were prepared for the Pt-loaded
graphite having mixing ratios of 1% by weight and 2% by weight).
For example, a silver paste containing Pd metal was prepared by
adding 0.142 parts by weight of paste C to paste 1 (10.00 parts by
weight).
(IV) Evaluation Method
[0084] The resistance values (.OMEGA.) were measured for each of
the resulting electrically conductive pastes.
[0085] Samples were prepared as shown in FIGS. 6A and 6B, and
contact resistance was measured for both electrodes (indicated with
Ag in the drawings). Samples were prepared by cutting a
commercially available 4-inch single crystal silicon wafer
(Mitsubishi Materials Corp., crystal axis (1.0.0), P-type
conductivity) into squares measuring 20 mm on a side with a laser
scriber, and applying the electrically conductive paste onto the
wafer in a circular pattern at two locations followed by firing to
form electrodes.
[0086] Sample Shape:
[0087] Circular patterns having a diameter of 6 mm were formed at
an interval of 1 mm on an Si wafer cut to the shape of a square
measuring 20 mm on a side (see FIG. 6B).
[0088] Firing Conditions:
[0089] The wafers were fired under the following conditions using
an IR heating belt furnace.
[0090] Maximum set temperature: 600.degree. C.
[0091] Belt speed: 370 cpm
[0092] Furnace temperature profile: [0093] 400.degree. C. or
higher: 18 seconds [0094] 500.degree. C. or higher: 12 seconds
[0095] Resistance Value Measurement Conditions:
[0096] Current and voltage characteristics between the sample
electrodes formed under the conditions described above were
measured with the HSV-100 Cyclic Voltanometry apparatus available
from Hokuto Denko Corp. The average of inter-electrode resistance
over an applied voltage range of 0.2 to 0.4 V was used as the
resistance value.
[0097] As shown in FIG. 6C, since the resistance of the fabricated
electrodes was small enough to be able to be ignored in terms of
magnitude, the measured resistance values were taken to be the sum
of the resistance value of the Si wafer and the contact resistance
at the Si--Ag (electrode) interface. Since the resistance value of
the Si wafer is constant, the magnitude of the measured resistance
represents the magnitude of the contact resistance.
(V) Results
[0098] The results of measuring the resistance values of each of
the electrically conductive pastes are shown in Table 3 (using
paste 1 for the silver paste) and Table 4 (using paste 2 for the
silver paste).
TABLE-US-00003 TABLE 3 Resistance Values and Evaluation of
Electrically Conductive Pastes Using Paste 1 for the Silver Paste
Resistance Added Paste Value (.OMEGA.) Judgment* Comparative Not
added 87,000 Ineffective Example 1 Comparative Paste A Ni 88,500
Ineffective Example 2 Example 1 Paste B W 6,300 Effective Example 2
Paste C Pd 2,200 Very effective Example 3 Paste D Ag/Pd alloy 3,900
Very effective Example 4 Paste E Pt 400 Very effective Comparative
Paste F Au 82,000 Ineffective Example 3 Example 5 Paste G Ru 1,200
Very effective Example 6 Paste H Rh 8,400 Effective Example 7 Paste
I Pt-loaded 49,00- Ineffective graphite Example 8 Paste I Pt-loaded
24,000 Effective (2 wt %) graphite *Judgment: A resistance value
1/20 or less the resistance value in the case of not adding an
added paste (i.e., the Comparative Example resistance value) was
judged to be "very effective", while a value of 1/2 or less the
resistance value was judged to be "effective".
TABLE-US-00004 TABLE 4 Resistance Values and Evaluation of
Electrically Conductive Pastes Using Paste 2 for the Silver Paste
Resistance Added Paste Value (.OMEGA.) Judgment* Comparative Not
added 75,000 Ineffective Example 4 Comparative Paste A Ni 85,000
Ineffective Example 5 Example 9 Paste B W 3,600 Very effective
Example 10 Paste C Pd 128 Very effective Example 11 Paste D Ag/Pd
alloy 4,200 Effective Example 12 Paste E Pt 113 Very effective
Comparative Paste F Au 84,000 Ineffective Example 6 Example 13
Paste G Ru 1,100 Very effective Example 14 Paste H Rh 7,300
Effective Example 15 Paste I Pt-loaded 9,700 Effective graphite
Example 16 Paste I Pt-loaded 8,600 Effective (2 wt %) graphite
*Judgment: A resistance value 1/20 or less the resistance value in
the case of not adding an added paste was judged to be very
effective, while that 1/2 or less the resistance value was judged
to be effective.
[0099] On the basis of the above results, with the exception of
nickel and gold, the addition of tungsten, palladium,
silver/palladium alloy, platinum, ruthenium and rhodium to a silver
paste at 1% by weight was able to lower contact resistance.
Palladium, platinum and ruthenium in particular greatly contributed
to a decrease in contact resistance.
[0100] Even powders not consisting of a single component such as
the silver/palladium alloy and the platinum-loaded graphite clearly
contributed to a decrease in contact resistance. Furthermore, the
reason for the degree of the contribution to decreased contact
resistance being lower as compared with the case of using a single
metal is thought to be because, since the powder itself was added
at 1% by weight, the content of palladium or platinum in the silver
paste decreased correspondingly.
[0101] A silicon oxide film referred to as a natural oxide film
having a thickness of 10 to 15 Angstroms is known to normally be
present on untreated silicon wafers. The pastes of the examples are
believed to have fired through this silicon oxide film and make
direct contact with the semiconductor surface.
* * * * *