U.S. patent application number 13/642526 was filed with the patent office on 2013-03-21 for composition for forming p-type diffusion layer, method of forming p-type diffusion layer, and method of producing photovoltaic cell.
The applicant listed for this patent is Shuichiro Adachi, Mitsunori Iwamuro, Keiko Kizawa, Yoichi Machii, Takeshi Nojiri, Kaoru Okaniwa, Tetsuya Sato, Masato Yoshida. Invention is credited to Shuichiro Adachi, Mitsunori Iwamuro, Keiko Kizawa, Yoichi Machii, Takeshi Nojiri, Kaoru Okaniwa, Tetsuya Sato, Masato Yoshida.
Application Number | 20130071968 13/642526 |
Document ID | / |
Family ID | 44834292 |
Filed Date | 2013-03-21 |
United States Patent
Application |
20130071968 |
Kind Code |
A1 |
Machii; Yoichi ; et
al. |
March 21, 2013 |
COMPOSITION FOR FORMING P-TYPE DIFFUSION LAYER, METHOD OF FORMING
P-TYPE DIFFUSION LAYER, AND METHOD OF PRODUCING PHOTOVOLTAIC
CELL
Abstract
The composition for forming a composition for forming a p-type
diffusion layer, the composition containing a glass powder and a
dispersion medium, in which the glass powder includes an acceptor
element and a total amount of a life time killer element in the
glass powder is 1000 ppm or less. A p-type diffusion layer and a
photovoltaic cell having a p-type diffusion layer are prepared by
applying the composition for forming a p-type diffusion layer,
followed by a thermal diffusion treatment.
Inventors: |
Machii; Yoichi;
(Tsukuba-shi, JP) ; Yoshida; Masato; (Tsukuba-shi,
JP) ; Nojiri; Takeshi; (Tsukuba-shi, JP) ;
Okaniwa; Kaoru; (Tsukuba-shi, JP) ; Iwamuro;
Mitsunori; (Tsukuba-shi, JP) ; Adachi; Shuichiro;
(Tsukuba-shi, JP) ; Sato; Tetsuya; (Tsukuba-shi,
JP) ; Kizawa; Keiko; (Tsukuba-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Machii; Yoichi
Yoshida; Masato
Nojiri; Takeshi
Okaniwa; Kaoru
Iwamuro; Mitsunori
Adachi; Shuichiro
Sato; Tetsuya
Kizawa; Keiko |
Tsukuba-shi
Tsukuba-shi
Tsukuba-shi
Tsukuba-shi
Tsukuba-shi
Tsukuba-shi
Tsukuba-shi
Tsukuba-shi |
|
JP
JP
JP
JP
JP
JP
JP
JP |
|
|
Family ID: |
44834292 |
Appl. No.: |
13/642526 |
Filed: |
April 22, 2011 |
PCT Filed: |
April 22, 2011 |
PCT NO: |
PCT/JP2011/059970 |
371 Date: |
December 5, 2012 |
Current U.S.
Class: |
438/98 ; 252/500;
252/513; 257/E31.124 |
Current CPC
Class: |
H01L 31/032 20130101;
C03C 8/18 20130101; H01L 21/2225 20130101; H01L 21/2255 20130101;
C03C 8/16 20130101; Y02E 10/547 20130101; H01L 31/1804 20130101;
Y02P 70/521 20151101; H01L 31/022425 20130101; H01L 31/1864
20130101; Y02P 70/50 20151101 |
Class at
Publication: |
438/98 ; 252/500;
252/513; 257/E31.124 |
International
Class: |
H01B 1/08 20060101
H01B001/08; H01B 1/02 20060101 H01B001/02; H01L 31/0224 20060101
H01L031/0224 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 23, 2010 |
JP |
2010-100223 |
Claims
1. A composition for forming a p-type diffusion layer, the
composition comprising a glass powder and a dispersion medium,
wherein the glass powder includes an acceptor element and a total
amount of a life time killer element in the glass powder is 1000
ppm or less.
2. The composition for forming a p-type diffusion layer according
to claim 1, wherein the acceptor element is at least one selected
from boron (B), aluminum (Al) or gallium (Ga).
3. The composition for forming a p-type diffusion layer according
to claim 1, wherein the glass powder comprises: at least one
acceptor element-containing material selected from B.sub.2O.sub.3,
Al.sub.2O.sub.3 or Ga.sub.2O.sub.3, and at least one glass
component material selected from SiO.sub.2, K.sub.2O, Na.sub.2O,
Li.sub.2O, BaO, SrO, CaO, MgO, BeO, ZnO, PbO, CdO, Tl.sub.2O, SnO,
ZrO.sub.2 or MoO.sub.3.
4. The composition for forming a p-type diffusion layer according
to claim 1, wherein the life time killer element is at least one
selected from iron (Fe), copper (Cu), nickel (Ni), manganese (Mn),
chromium (Cr), tungsten (W) or gold (Au).
5. A method of forming a p-type diffusion layer, the method
comprising: applying the composition for forming a p-type diffusion
layer of claim 1; and conducting a thermal diffusion treatment.
6. A method of producing a photovoltaic cell, the method
comprising: applying, on a semiconductor substrate, the composition
for forming a p-type diffusion layer of claim 1; conducting a
thermal diffusion treatment to form an p-type diffusion layer; and
forming an electrode on the p-type diffusion layer.
Description
TECHNICAL FIELD
[0001] The present invention relates to a composition for forming a
p-type diffusion layer of a photovoltaic cell, a method of forming
a p-type diffusion layer, and a method of producing a photovoltaic
cell. More specifically, the present invention relates to a
technique for forming a p-type diffusion layer, which enables
reduction in internal stress of a silicon serving as a
semiconductor substrate, whereby damage to a crystal grain boundary
can be suppressed and increase in crystal defects and warpage can
be suppressed.
BACKGROUND ART
[0002] A related art procedure of a silicon photovoltaic cell is
described hereinbelow.
[0003] First, in order to realize high efficiency by promoting
optical confinement effects, a p-type silicon substrate having a
texture structure formed thereon is prepared, and subsequently
subjected to a treatment at a temperature of from 800 to
900.degree. C. for several tens of minutes under a mixed gas
atmosphere of phosphorus oxychloride (POCl.sub.3), nitrogen and
oxygen, thereby uniformly forming an n-type diffusion layer.
According to this related art method, since diffusion of phosphorus
is carried out using a mixed gas, the n-type diffusion layer is
formed not only on the surface, but also on the side face and the
rear surface. For these reasons, there has been a need for a side
etching process to remove the n-type diffusion layer of the side
face. Further, the n-type diffusion layer of the rear surface needs
to be converted into a p.sup.+-type diffusion layer, and
correspondingly an aluminum paste is applied to the n-type
diffusion layer of the rear surface and then sintered to achieve
conversion of the n-type diffusion layer into the p.sup.+-type
diffusion layer and also formation of ohmic contact at the same
time.
[0004] However, aluminum paste has low conductivity, and therefore,
it is generally necessary to form a thick aluminum layer of about
10 to 20 .mu.m after sintering on the entire rear surface in order
to reduce the sheet resistance. Further, the coefficient of thermal
expansion of aluminum is considerably different from the
coefficient of thermal expansion of silicon, and therefore, such a
difference results in generation of large internal stress in the
silicon substrate during the sintering and cooling processes, which
contributes to damage to a crystal grain boundary, increase in the
crystal defects, and the warpage.
[0005] In order to solve this problem, there has been a method to
reduce the thickness of the rear surface electrode by decreasing
the amount of a paste composition to be coated. However, when the
coating amount of the paste composition is decreased, the amount of
aluminum diffused from a surface of a p-type silicon substrate into
an internal portion is insufficient. As a result, a desirable BSF
(Back Surface Field) effect (an effect in which collection
efficiency of generated carriers is increased due to the presence
of a p.sup.+-type layer) is not achieved, resulting in the problem
of a decrease in properties of a photovoltaic cell.
[0006] For these reasons, for example, in Japanese Patent
Application Laid-Open (JP-A) No. 2003-223813, there has been
proposed a paste composition including an aluminum powder, an
organic vehicle, and an inorganic compound powder whose coefficient
of the thermal expansion is lower than that of aluminum, and whose
at least one of melting temperature, softening temperature and
decomposition temperature is lower than the melting temperature of
aluminum.
SUMMARY OF INVENTION
Technical Problem
[0007] Even when the paste composition described in JP-A No.
2003-223813 is used, however, it is found that a warpage cannot be
sufficiently suppressed in some cases.
[0008] The present invention has been made in view of the above
problems exhibited by the background art, and it is an object of
the present invention to provide: a composition for forming a
p-type diffusion layer in a manufacturing process of a photovoltaic
cell using a silicon substrate, which is capable of forming a
p-type diffusion layer, suppressing generation of internal stress
and warpage of a silicon substrate, and which does not
significantly shorten a life time of a carrier in the resultant
substrate including the p-type diffusion layer; a method of forming
a p-type diffusion layer; and a method of producing a photovoltaic
cell.
Means for Solving Problems
[0009] The above-stated problems are addressed by the following
means.
[0010] <1 > A composition for forming a p-type diffusion
layer, the composition containing a glass powder and a dispersion
medium, in which the glass powder includes an acceptor element and
a total amount of a life time killer element in the glass powder is
1000 ppm or less.
[0011] <2> The composition for forming a p-type diffusion
layer according to <1>, in which the acceptor element is at
least one selected from boron (B), aluminum (Al) or gallium
(Ga).
[0012] <3> The composition for forming a p-type diffusion
layer according to <1> or <2>, in which the acceptor
element-containing glass powder contains: [0013] at least one
acceptor element-containing material selected from B.sub.2O.sub.3,
Al.sub.2O.sub.3 or Ga.sub.2O.sub.3; and [0014] at least one glass
component material selected from SiO.sub.2, K.sub.2O, Na.sub.2O,
Li.sub.2O, BaO, SrO, CaO, MgO, BeO, ZnO, PbO, CdO, Tl.sub.2O, SnO,
ZrO.sub.2 or MoO.sub.3.
[0015] <4> The composition for forming a p-type diffusion
layer according to any one of <1> to <3>, in which the
life time killer element is at least one selected from iron (Fe),
copper (Cu), nickel (Ni), manganese (Mn), chromium (Cr), tungsten
(W) or gold (Au).
[0016] <5> A method of forming a p-type diffusion layer, the
method including: [0017] applying the composition for forming a
p-type diffusion layer of any one of <1> to <4>; and
[0018] conducting a thermal diffusion treatment.
[0019] <6> A method of producing a photovoltaic cell, the
method including: [0020] applying, on a semiconductor substrate,
the composition for forming a p-type diffusion layer of any one of
<1> to <4>; [0021] conducting a thermal diffusion
treatment to form a p-type diffusion layer; and [0022] forming an
electrode on the p-type diffusion layer.
Advantageous Effects of Invention
[0023] The present invention enables the formation of a p-type
diffusion layer in the manufacturing process of a photovoltaic cell
using a silicon substrate, suppressing generation of internal
stress and warpage of a silicon substrate, and which does not
significantly shorten a life time of a carrier in the substrate
including the resultant p-type diffusion layer.
DESCRIPTION OF EMBODIMENTS
[0024] First, a composition for forming a p-type diffusion layer in
accordance with the present invention will be described, and then a
method of forming a p-type diffusion layer and a method of
producing a photovoltaic cell, using the composition for forming a
p-type diffusion layer, will be described.
[0025] In the present specification, the term "process" denotes not
only independent processes but also processes that cannot be
clearly distinguished from other processes as long as a purpose is
accomplished by the process.
[0026] Furthermore, in the present specification, "from . . . to .
. . " denotes a range including each of the minimum value and the
maximum value of the values described in this expression.
[0027] Further, in the case in which the plurality of the materials
corresponding to each component are present in the composition, the
amount of each component in the composition means a total amount of
plural materials present in the composition unless otherwise
specified.
[0028] The composition for forming a p-type diffusion layer in
accordance with the present invention includes at least a glass
powder and a dispersion medium, in which the glass powder includes
at least an acceptor element and a total amount of a life time
killer element of the glass powder is 1000 ppm or less
(hereinafter, often referred to simply as "glass powder") and a
dispersion medium, and may further contain other additives as
necessary, taking into consideration coatability or the like.
[0029] As used herein, the term "composition for forming a p-type
diffusion layer" refers to a material which contains an acceptor
element and is capable of forming a p-type diffusion layer through
thermal diffusion of the acceptor element after application of the
material to a silicon substrate. The use of the composition for
forming a p-type diffusion layer ensures that a process of forming
a p.sup.+-type diffusion layer and a process of forming ohmic
contact are separated, whereby the options for the electrode
material for forming ohmic contact are expanded, and the options
for the structure of the electrode are also expanded. For example,
when a low resistance material like Ag is applied to an electrode,
an electrode having a thin film thickness and low resistance can be
achieved. Further, there is no need to form an electrode on the
whole surface, and therefore, the electrode may be partially formed
such as a comb-shaped electrode. As mentioned above, due to forming
a thin or partial electrode such a comb-shaped electrode, it is
possible to form a p-type diffusion layer, while suppressing an
internal stress in a silicon substrate and warpage of the
substrate.
[0030] Accordingly, when the composition for forming a p-type
diffusion layer in accordance with the present invention is
employed, internal stress in a silicon substrate and warpage of the
substrate, which occur in the conventionally widely used method,
namely a method in which an aluminum paste is applied to the n-type
diffusion layer and then sintered to convert the n-type diffusion
layer into the p.sup.+-type diffusion layer and also to form ohmic
contact at the same time, are suppressed.
[0031] Furthermore, since the acceptor element included in the
glass powder is hardly vaporized during sintering, formation of the
p-type diffusion layer in areas other than a desired area due to
vaporization of the acceptor element is suppressed. It is assumed
that the reason for this is that the acceptor component combines
with an element in a glass powder, or is absorbed into the glass,
as a result of which the acceptor component is hardly
volatilized.
[0032] In addition, in the composition for forming a p-type
diffusion layer according to the present invention, since a total
amount of the life time killer element in the glass powder of the
composition is 1000 ppm or less, a life time of a carrier in the
substrate having the p-type diffusion layer does not significantly
shorten. Details of the life time killer element are described
later.
[0033] The acceptor element-containing glass powder in accordance
with the present invention will be described in more detail.
[0034] As used herein, the term "acceptor element" refers to an
element which is capable of forming a p-type diffusion layer by
doping thereof on a silicon substrate. As the acceptor element,
elements of Group XIII of the periodic table can be used. Examples
of the acceptor element include B (boron), aluminum (Al) and
gallium (Ga).
[0035] Examples of the acceptor element-containing material which
is used for introducing the acceptor element into the glass powder
include B.sub.2O.sub.3, Al.sub.2O.sub.3 and Ga.sub.2O.sub.3. At
least one selected from B.sub.2O.sub.3, Al.sub.2O.sub.3 and
Ga.sub.2O.sub.3 is preferably used.
[0036] Further, the melting temperature, softening point,
glass-transition point, chemical durability, and the like of the
acceptor element-containing glass powder may be controlled by
adjusting the component ratio, if necessary. Further, a
below-mentioned glass component material(s) may be preferably
contained.
[0037] Examples of the glass component material include SiO.sub.2,
K.sub.2O, Na.sub.2O, Li.sub.2O, BaO, SrO, CaO, MgO, BeO, ZnO, PbO,
CdO, Tl.sub.2O, V.sub.2O.sub.5, SnO, ZrO.sub.2, MoO.sub.3,
La.sub.2O.sub.3, Nb.sub.2O.sub.5, Ta.sub.2O.sub.5, Y.sub.2O.sub.3,
TiO.sub.2, GeO.sub.2, TeO.sub.2, and Lu.sub.2O.sub.3. At least one
selected from SiO.sub.2, K.sub.2O, Na.sub.2O, Li.sub.2O, BaO, SrO,
CaO, MgO, BeO, ZnO, PbO, CdO, Tl.sub.2O, SnO, ZrO.sub.2 and
MoO.sub.3 is preferably used.
[0038] Specific examples of the acceptor element-containing glass
powder include those including both the acceptor element-containing
material and the glass component material such as, for example,
B.sub.2O.sub.3 based glass which includes B.sub.2O.sub.3 as the
acceptor element such as B.sub.2O.sub.3-SiO.sub.2 (the acceptor
element-containing material and the glass component material are
listed in this order, and are listed in the same order below) based
glass, B.sub.2O.sub.3-ZnO based glass, B.sub.2O.sub.3-PbO based
glass or single B.sub.2O.sub.3 based glass; Al.sub.2O.sub.3 based
glass which includes Al.sub.2O.sub.3 as the acceptor element such
as Al.sub.2O.sub.3-SiO.sub.2 based glass; and Ga.sub.2O.sub.3 based
glass which includes Ga.sub.2O.sub.3 as the acceptor element such
as Ga.sub.2O.sub.3-SiO.sub.2 based glass.
[0039] The acceptor element-containing glass powder may include two
or more acceptor element-containing materials such as
Al.sub.2O.sub.3-B.sub.2O.sub.3, Ga.sub.2O.sub.3-B.sub.2O.sub.3 or
the like.
[0040] Although composite glasses containing one or two components
are illustrated in the above, composite glass containing three or
more components, such as B.sub.2O.sub.3-SiO.sub.2-Na.sub.2O or
B.sub.2O.sub.3-SiO.sub.2-CeO.sub.2, may also be possible.
[0041] A total amount of an element which makes a life time of a
carrier shortened (life time killer element) is 1000 ppm or less,
preferably 500 ppm or less, more preferably 100 ppm or less, and
still more preferably 50 ppm or less, in a glass power.
[0042] A life time killer element includes Fe, Cu, Ni, Mn, W and
Au. The amount of the element may be analyzed by ICP mass
spectrometer, ICP optical emission spectrometer or atomic
absorption spectrometer. A life time of a carrier may be measured
by microwave reflectance photoconductivity decay (.mu.-PCD)
method.
[0043] The content of the glass component material in the glass
powder is preferably appropriately set taking into consideration
the melting temperature, the softening point, the glass-transition
point, and chemical durability. Generally, the content of the glass
component material in the glass powder is preferably from 0.1% by
mass to 95% by mass, and more preferably from 0.5% by mass to 90%
by mass.
[0044] The softening point of the glass powder is preferably in the
range of from 200.degree. C. to 1000.degree. C., and more
preferably from 300.degree. C. to 900.degree. C., from the
viewpoint of diffusivity during the diffusion treatment, and
dripping. The softening point of the glass powder may be measured
by a known differential thermal analysis (DTA) and using an
endothermic peak thereof.
[0045] The shape of the glass powder includes a substantially
substantially spherical shape, a flat shape, a block shape, a plate
shape, a scale-like shape, and the like. From the viewpoint of
coating property and uniform dispersion property of a composition
for forming n-type diffusion layer including a glass powder, it is
preferably a substantially spherical shape, a flat shape, or a
plate shape.
[0046] The particle diameter of the glass powder is preferably 50
.mu.um or less. When a glass powder having a particle diameter of
50 .mu.m or less is used, a smooth coated film may be easily
obtained. Further, the particle diameter of the glass powder is
more preferably 10 .mu.m or less. The lower limit of the particle
diameter is not particularly limited, and preferably 0.01 .mu.m or
more.
[0047] The particle diameter of the glass powder means the average
particle diameter, and may be measured by laser diffraction
particle size analyzer or the like.
[0048] The acceptor element-containing glass powder is prepared
according to the following procedure.
[0049] First, raw materials are weighed and filled in a crucible.
The crucible may be made of platinum, platinum-rhodium, iridium,
alumina, quartz, carbon, or the like, which is appropriately
selected taking into consideration the melting temperature,
atmosphere, reactivity with melted materials, and the like.
[0050] Next, the raw materials are heated to a temperature
corresponding to the glass composition in an electric furnace,
thereby preparing a melted liquid. At this time, stirring is
preferably applied such that the melted liquid becomes
homogenous.
[0051] Subsequently, the melted liquid is allowed to flow on a
zirconia plate, carbon plete or the like to result in vitrification
of the solution.
[0052] Finally, the glass is pulverized into a powder. The
pulverization can be carried out by using a known method such as
jet mill, bead mill or ball mill.
[0053] The content of the acceptor element-containing glass powder
in the composition for forming a p-type diffusion layer is
determined taking into consideration coatability, diffusivity of
acceptor elements, and the like. Generally, the content of the
glass powder in the composition for forming a p-type diffusion
layer is preferably from 0.1% by mass to 95% by mass, and more
preferably from 1% by mass to 90% by mass.
[0054] Hereinafter, a dispersion medium will be described.
[0055] The dispersion medium is a medium which disperses the glass
powder in the composition. Specifically, a binder, a solvent or the
like is employed as the dispersion medium.
[0056] For example, the binder may be appropriately selected from a
dimethylaminoethyl (meth)acrylate polymer, polyvinyl alcohol,
polyacrylamides, polyvinyl amides, polyvinyl pyrrolidone,
poly(meth)acrylic acids, polyethylene oxides, polysulfonic acid,
acrylamide alkyl sulfonic acid, cellulose ethers, cellulose
derivatives, carboxymethylcellulose, hydroxyethylcellulose,
ethylcellulose, gelatin, starch and starch derivatives, sodium
alginates, xanthane, guar and guar derivatives, scleroglucan and
scleroglucan derivatives, tragacanth and tragacanth derivatives or
dextrin and dextrin derivatives, acrylic acid resins, acrylic acid
ester resins, butadiene resins, styrene resins, copolymers thereof,
silicon dioxide, and the like. These compounds may be used
individually or in a combination of two or more thereof.
[0057] The molecular weight of the binder is not particularly
limited and is preferably appropriately adjusted taking into
consideration a desired viscosity of the composition.
[0058] Examples of the solvent include ketone solvents such as
acetone, methylethylketone, methyl-n-propylketone,
methyl-iso-propylketone, methyl-n-butylketone,
methyl-iso-butylketone, methyl-n-pentylketone,
methyl-n-hexylketone, diethylketone, dipropylketone,
di-iso-butylketone, trimethylnonanone, cyclohexanone,
cyclopentanone, methylcyclohexanone, 2,4-pentanedione and
acetonylacetone; ether solvents such as diethyl ether, methyl ethyl
ether, methyl-n-propyl ether, di-iso-propyl ether, tetrahydrofuran,
methyl tetrahydrofuran, dioxane, dimethyl dioxane, ethylene glycol
dimethyl ether, ethylene glycol diethyl ether, ethylene glycol
di-n-propyl ether, ethylene glycol dibutyl ether, diethylene glycol
dimethyl ether, diethylene glycol diethyl ether, diethylene glycol
methyl ethyl ether, diethylene glycol methyl n-propyl ether,
diethylene glycol methyl n-butyl ether, diethylene glycol
di-n-propyl ether, diethylene glycol di-n-butyl ether, diethylene
glycol methyl n-hexyl ether, triethylene glycol dimethyl ether,
triethylene glycol diethyl ether, triethylene glycol methyl ethyl
ether, triethylene glycol methyl n-butyl ether, triethylene glycol
di-n-butyl ether, triethylene glycol methyl n-hexyl ether,
tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl
ether, tetradiethylene glycol methyl ethyl ether, tetraethylene
glycol methyl n-butyl ether, diethylene glycol di-n-butyl ether,
tetraethylene glycol methyl n-hexyl ether, tetraethylene glycol
di-n-butyl ether, propylene glycol dimethyl ether, propylene glycol
diethyl ether, propylene glycol di-n-propyl ether, propylene glycol
dibutyl ether, dipropylene glycol dimethyl ether, dipropylene
glycol diethyl ether, dipropylene glycol methyl ethyl ether,
dipropylene glycol methyl n-butyl ether, dipropylene glycol
di-n-propyl ether, dipropylene glycol di-n-butyl ether, dipropylene
glycol methyl n-hexyl ether, tripropylene glycol dimethyl ether,
tripropylene glycol diethyl ether, tripropylene glycol methyl ethyl
ether, tripropylene glycol methyl n-butyl ether, tripropylene
glycol di-n-butyl ether, tripropylene glycol methyl n-hexyl ether,
tetrapropylene glycol dimethyl ether, tetrapropylene glycol diethyl
ether, tetradipropylene glycol methyl ethyl ether, tetrapropylene
glycol methyl n-butyl ether, dipropylene glycol di-n-butyl ether,
tetrapropylene glycol methyl n-hexyl ether, and tetrapropylene
glycol di-n-butyl ether; ester solvents such as methyl acetate,
ethyl acetate, n-propyl acetate, i-propyl acetate, n-butyl acetate,
i-butyl acetate, sec-butyl acetate, n-pentyl acetate, sec-pentyl
acetate, 3-methoxybutyl acetate, methyl pentyl acetate, 2-ethyl
butyl acetate, 2-ethyl hexyl acetate, 2-(2-butoxyethoxy) ethyl
acetate, benzyl acetate, cyclohexyl acetate, methyl cyclohexyl
acetate, nonyl acetate, methyl acetoacetate, ethyl acetoacetate,
diethylene glycol monomethyl ether acetate, diethylene glycol
monoethyl ether acetate, diethylene glycol mono-n-butyl ether
acetate, dipropylene glycol monomethyl ether acetate, dipropylene
glycol monoethyl ether acetate, glycol diacetate, methoxy triglycol
acetate, ethyl propionate, n-butyl propionate, i-amyl propionate,
diethyl oxalate, di-n-butyl oxalate, methyl lactate, ethyl lactate,
n-butyl lactate, n-amyl lactate, ethylene glycol methyl ether
propionate, ethylene glycol ethyl ether propionate, ethylene glycol
methyl ether acetate, ethylene glycol ethyl ether acetate,
diethylene glycol methyl ether acetate, diethylene glycol ethyl
ether acetate, diethylene glycol-n-butyl ether acetate, propylene
glycol methyl ether acetate, propylene glycol ethyl ether acetate,
propylene glycol propyl ether acetate, dipropylene glycol methyl
ether acetate, dipropylene glycol ethyl ether acetate,
.gamma.-butyrolactone, and .gamma.-valerolactone; aprotic polar
solvents such as acetonitrile, N-methyl pyrrolidinone, N-ethyl
pyrrolidinone, N-propyl pyrrolidinone, N-butyl pyrrolidinone,
N-hexyl pyrrolidinone, N-cyclohexyl pyrrolidinone, N,N-dimethyl
formamide, N,N-dimethyl acetamide, and dimethyl sulfoxide; alcohol
solvents such as methanol, ethanol, n-propanol, i-propanol,
n-butanol, i-butanol, sec-butanol, t-butanol, n-pentanol,
i-pentanol, 2-methylbutanol, sec-pentanol, t-pentanol, 3-methoxy
butanol, n-hexanol, 2-methylpentanol, sec-hexanol, 2-ethylbutanol,
sec-heptanol, n-octanol, 2-ethylhexanol, sec-octanol, n-nonyl
alcohol, n-decanol, sec-undecyl alcohol, trimethylnonyl alcohol,
sec-tetradecyl alcohol, sec-heptadecyl alcohol, phenol,
cyclohexanol, methylcyclohexanol, benzyl alcohol, ethylene glycol,
1,2-propylene glycol, 1,3-butylene glycol, diethylene glycol,
dipropylene glycol, triethylene glycol, and tripropylene glycol;
glycol monoether solvents such as ethylene glycol methyl ether,
ethylene glycol ethyl ether, ethylene glycol monophenyl ether,
diethylene glycol monomethyl ether, diethylene glycol monoethyl
ether, diethylene glycol mono-n-butyl ether, diethylene glycol
mono-n-hexyl ether, ethoxy triglycol, tetraethylene glycol
mono-n-butyl ether, propylene glycol monomethyl ether, dipropylene
glycol monomethyl ether, dipropylene glycol monoethyl ether, and
tripropylene glycol monomethyl ether; terpene solvents such as
.alpha.-terpinene, .alpha.-terpinenol, myrcene, allo-ocimene,
limonene, dipentene, .alpha.-pinene, (.beta.-pinene, terpinenol,
carvone, ocimene and phellandrene; water, and the like. These
materials may be used individually or in a combination of two or
more thereof.
[0059] From the viewpoint of the coating property of the
composition for forming a p-type diffusion layer at a substrate,
.alpha.-terpinenol, diethylene glycol mono-n-butyl ether or
2-(2-butoxyethoxy) ethyl acetate is preferable.
[0060] The content of the dispersion medium in the composition for
forming a p-type diffusion layer is determined taking into
consideration coatability and acceptor concentration.
[0061] The viscosity of the composition for forming a p-type
diffusion layer is preferably from 10 mPas to 1,000,000 mPas, and
more preferably from 50 mPas to 500,000 mPas, from the viewpoint of
coatability.
[0062] In the case in which the total amount of the life time
killer element in the glass powder is 1000 ppm or less, the total
amount of the life time killer element in the composition for
forming a p-type diffusion layer is approximately 1100 ppm or less.
Therefore, the total amount of the life time killer element in the
composition for forming a p-type diffusion layer is preferably 500
ppm or less, and more preferably 100 ppm or less.
[0063] Hereinafter, the method of producing a p-type diffusion
layer and a photovoltaic cell in accordance with the present
invention will be described.
[0064] First, an alkaline solution is applied to a silicon
substrate which is a p-type semiconductor substrate, thereby
removing the damaged layer, and a texture structure is obtained by
etching.
[0065] Specifically, the damaged layer of the silicon surface,
which is caused at the time of being sliced from an ingot, is
removed by using 20% by mass of caustic soda. Then, a texture
structure is formed by etching with a mixture of 1% by mass of
caustic soda and 10% by mass of isopropyl alcohol. The photovoltaic
cell achieves high efficiency through the formation of a texture
structure on a light-receiving side (front surface) to promote
optical confinement effects.
[0066] Next, an n-type diffusion layer is uniformly formed by
subjected to a treatment at a temperature of from 800 to
900.degree. C. for several tens of minutes under a mixed gas
atmosphere of phosphorus oxychloride (POCl.sub.3), nitrogen and
oxygen. At this time, according to the method using phosphorus
oxychloride atmosphere, the n-type diffusion layer is formed not
only on the surface, but also on the side face and the rear
surface. For these reasons, there has been a need for a side
etching process to remove the n-type diffusion layer of the side
face.
[0067] Further, the composition for forming a p-type diffusion
layer is applied on the n-type diffusion layer formed on the rear
surface, i.e., non-light receiving surface. In the present
invention, although there is no limit to the application method,
for example, a printing method, a spinning method, brush
application, a spray method, a doctor blade method, a roll coater
method, an inkjet method or the like may be used.
[0068] The coating amount of the composition for forming a p-type
diffusion layer is not particularly limited, and for example, may
be from 0.01 g/m.sup.2 to 100 g/m.sup.2, and preferably from 0.1
g/m.sup.2 to 10 g/m.sup.2 as an amount of the glass powder.
[0069] Further, depending on the composition of the composition for
forming a p-type diffusion layer, a drying process for
volatilization of the solvent contained in the composition may be
required after the application thereof, if necessary. In this case,
the drying is carried out at a temperature of from 80 to
300.degree. C., for from 1 minute to 10 minutes when using a hot
plate, or for from 10 minutes to 30 minutes when using a dryer or
the like. Since these drying conditions are dependent on the
solvent composition of the composition for forming a p-type
diffusion layer, the present invention is not particularly limited
to the above-stated conditions.
[0070] The semiconductor substrate, to which the composition for
forming a p-type diffusion layer was applied, is subjected to a
heat treatment at a temperature of from 600 to 1200.degree. C. This
heat treatment results in diffusion of an acceptor element into the
semiconductor substrate, thereby forming an p.sup.+-type diffusion
layer. The heat treatment may be carried out using a known
continuous furnace, batch furnace, or the like. When performing the
thermal diffusion treatment, the furnace atmosphere may be
appropriately adjusted with air, oxygen, nitrogen, or the like.
[0071] The treatment time of the thermal diffusion may be
appropriately selected depending on the content of an acceptor
element contained in the composition for forming a p-type diffusion
layer. For example, the treatment time of the thermal diffusion may
be in the range of from 1 minute to 60 minutes, and preferably from
5 minutes to 30 minutes.
[0072] As a glass layer is formed on the surface of the
p.sup.+-type diffusion layer, the glass layer is removed by
etching. The etching may be carried out by using a known method,
including a method of dipping a subject in an acid such as
hydrofluoric acid, a method of dipping a subject in an alkali such
as caustic soda, or the like.
[0073] In the conventional production method, an aluminum paste is
applied to the rear surface and then sintered, thereby converting
the n-type diffusion layer into the p.sup.+-type diffusion layer
and also forming an ohmic contact at the same time. However, since
a aluminum paste has low conductivity, in order to reduce a sheet
resistance, it is generally necessary to form a thick aluminum
layer of about 10 to 20 .mu.m after sintering on the entire rear
surface. Furthermore, the coefficient of thermal expansion of
aluminum is considerably different from the coefficient of thermal
expansion of silicon, and therefore, such a difference results in
generation of large internal stress in the silicon substrate during
the sintering and cooling processes, which contributes to warpage
of the silicon substrate.
[0074] The internal stress leads to the problem of damage to a
crystal grain boundary resulting in an increase in power loss. The
warpage makes a photovoltaic cell prone to damage during conveying
of the cell in a module process or during connecting to a copper
line which is referred to as a tub line. Recently, owing to
improvement in slicing techniques, the thickness of the silicon
substrate continues to be mode thinner, whereby the cell is more
readily cracked.
[0075] On the other hand, according to the production method of the
present invention, an n-type diffusion layer is converted into a
p.sup.+-type diffusion layer with a composition for forming a
p-type diffusion layer, and then an electrode is made on the
p.sup.+-layer as another process. Accordingly, the material used
for an electrode of the rear surface is not limited to aluminum.
For example, Ag (silver), Cu (copper) or the like may also be used,
so the thickness of the electrode of the rear surface may be
further reduced as compared to the related art, and in addition,
there is no need to form an electrode on the whole rear surface. As
a result, it is possible to inhibit the generation of internal
stress in a silicon substrate and warpage in sintering and cooling
processes.
[0076] An antireflective film is formed over the n-type diffusion
layer. The antireflective film is formed by using a known
technique. For example, when the antireflective film is a silicon
nitride film, the antireflective film is formed by a plasma CVD
method using a mixed gas of SiH.sub.4 and NH.sub.3 as a raw
material. In this case, hydrogen diffuses into crystals, and an
orbit which does not contribute to bonding of silicon atoms, that
is, a dangling bond binds to hydrogen, which inactivates a defect
(hydrogen passivation).
[0077] More specifically, the antireflective film is formed under
the conditions of a mixed gas NH.sub.3/SiH.sub.4 flow ratio of from
0.05 to 1.0, a reaction chamber pressure of from 0.1 to 2 Ton, a
film-forming temperature of from 300 to 550.degree. C., and a
plasma discharge frequency of 100 kHz or higher.
[0078] A metal paste for a front surface electrode is printed and
applied on the antireflective film of the front surface
(light-receiving side) by a screen printing method, followed by
drying to form a front surface electrode. The metal paste for a
front surface electrode contains metal particles and glass
particles as essential components, and optionally a resin binder,
other additives, and the like.
[0079] Then, a rear surface electrode is also formed on
p.sup.+-type diffusion layer. As described hereinbefore, the
constitutive material and forming method of the rear surface
electrode are not particularly limited in the present invention.
For example, the rear surface electrode may also be formed by
applying the rear surface electrode paste containing a metal such
as aluminum, silver or copper, followed by drying. In this case,
the rear surface may also be partially provided with a silver paste
for forming a silver electrode, for connection between cells in the
module process.
[0080] Electrodes are sintered to complete a photovoltaic cell.
When the sintering is carried out at a temperature of from
600.degree. C. to 900.degree. C. for from several seconds to
several minutes, the front surface side undergoes melting of the
antireflective film which is an insulating film, due to the glass
particles contained in the electrode-forming metal paste, and the
silicon surface is also partially melted, by which metal particles
(for example, silver particles) in the paste form a contact with
the silicon substrate, followed by solidification. In this manner,
electrical conduction is made between the formed surface electrode
and the silicon substrate. This type of process is called
fire-through.
[0081] Hereinafter, the shape of the surface electrode is
described. The surface electrode is made up of a bus bar electrode
and a finger electrode intersecting the bus bar electrode.
[0082] The surface electrode may be formed, for example, by the
above-stated screen printing of a metal paste, or plating of
electrode materials, deposition of electrode materials by electron
beam heating under high vacuum, or the like. The surface electrode
made up of the bus bar electrode and the finger electrode is well
known since it is typically used as an electrode of the
light-receiving surface side, and a known method for the formation
of the bus bar electrode and the finger electrode of the
light-receiving surface side may be applied.
[0083] In the above methods for producing a p-type diffusion layer
and a photovoltaic cell, in order to form an n-type diffusion layer
on a silicon serving as a p-type semiconductor substrate, a mixed
gas of phosphorus oxychloride (POCl.sub.3), nitrogen and oxygen is
used. However, a composition for forming an n-type diffusion layer
may be used to form the n-type diffusion layer. The composition for
forming an n-type diffusion layer contains an element of Group XV
of the periodic table such as phosphorous (P), antimony (Sb) or the
like as a donor element.
[0084] In the method using a composition for forming an n-type
diffusion layer in order to form the n-type diffusion layer, first,
the composition for forming an n-type diffusion layer is applied on
a front surface of the p-type semiconductor substrate which is a
light receiving surface, the composition for forming an p-type
diffusion layer in accordance with the present invention is applied
on a rear surface, and then a thermal treatment is carried out at
from 600 to 1200.degree. C. This thermal treatment results in
diffusion of the donor element into the front surface of the p-type
semiconductor substrate to form an n-type diffusion layer, and in
diffusion of an acceptor element into the rear surface of the
p-type semiconductor substrate to form a p.sup.+-type diffusion
layer. Aside from these processes, a photovoltaic cell is produced
according to the same processes as in the method described
above.
[0085] The disclosures of Japanese Patent Application
No.2010-100223 is incorporated by reference herein in their
entireties.
[0086] All the literature, patent applications, and technical
standards cited herein are also herein incorporated to the same
extent as provided for specifically and severally with respect to
an individual literature, patent application, and technical
standard to the effect that the same should be so incorporated by
reference.
EXAMPLES
[0087] Hereinafter, Examples in accordance with the present
invention will be described in more detail, but the present
invention is not limited thereto. Unless specifically indicated,
the chemicals all used reagents. Unless specifically indicated, "%"
refers to "% by mass".
[0088] The term "a life time of a carrier" denotes a relative value
of a life time of a carrier in an n-type silicon substrate having a
p-type diffusion layer formed in each of the Examples or
Comparative Examples, relative to a life time of a carrier in an
n-type silicon substrate having a p-type diffusion layer formed by
applying a B.sub.2O.sub.3-containing solution and then conducting a
thermal diffusion treatment. A life time of a carrier of 70% or
more is deemed acceptable from a practical perspective.
Example 1
[0089] 20 g of SiO.sub.2-B.sub.2O.sub.3 based glass powder whose
particle shape is substantially spherical and average particle
diameter is 3.3 .mu.m (SiO.sub.2: 50 mol %, B.sub.2O.sub.3: 50 mol
%, life time killer element: 980 ppm), 3 g of ethylcellulose and 77
g of 2-(2-butoxyethoxy) ethyl acetate were mixed with an automatic
mortar kneading machine and made into a paste to prepare a
composition for forming a p-type diffusion layer.
[0090] Regarding a life time killer elements in the glass powder,
the content and type of the elements were analyzed with an
Inductively Coupled Plasma-Optical Emission Spectrometer and an
Inductively Coupled Plasma-Mass Spectrometer. The same applies for
the below Examples. In the glass powder, Fe, Cu and Ni were
contained as a life time killer element.
[0091] The particle shape of the glass powder was judged by
observation with a scanning electron microscope (trade name:
TM-1000, manufactured by Hitachi High-Technologies Corporation).
The average diameter of the glass powder was calculated with a
laser diffraction particle size analyzer (measurement wave length:
632 nm, trade name: LS 13 320, manufactured by Beckman Coulter,
Inc.).
[0092] Next, the prepared paste was applied to an n-type silicon
substrate surface by screen printing, and dried on a hot plate at
150.degree. C. for 5 minutes. Subsequently, a thermal diffusion
treatment was carried out in an electric furnace at 1000.degree. C.
for 10 minutes. Then, in order to remove the glass layer, the
substrate was dipped in hydrofluoric acid for 5 minutes, followed
by washing with running water and drying.
[0093] The surface at the side where the composition for forming a
p-type diffusion layer was applied exhibited sheet resistance of 80
.OMEGA./.quadrature. and the formation of a p-type diffusion layer
through diffusion of B (boron). On the other hand, the rear surface
exhibited an unmeasurable sheet resistance of 1,000,000
.OMEGA./.quadrature. or higher and it was judged that no p-type
diffusion layer was substantially formed. A life time of carrier
was 74%. The warpage of the substrate did not occur.
Example 2
[0094] A p-type diffusion layer was formed in the same manner as in
Example 1, except that the glass powder was changed to
SiO.sub.2-B.sub.2O.sub.3 based glass powder (SiO.sub.2: 50 mol %,
B.sub.2O.sub.3: 50 mol %, life time killer element: 530 ppm,
particle shape: substantially spherical, average particle diameter:
3.8 .mu.m). In the glass powder, Fe, Cu and Ni were contained as a
life time killer element.
[0095] The surface at the side where the composition for forming a
p-type diffusion layer was applied exhibited sheet resistance of 82
.OMEGA./.quadrature. and the formation of a p-type diffusion layer
through diffusion of B (boron). On the other hand, the rear surface
exhibited an unmeasurable sheet resistance of 1,000,000
.OMEGA./.quadrature. or higher and it was judged that no p-type
diffusion layer was substantially formed. A life time of carrier
was 82%. The warpage of the substrate did not occur.
Example 3
[0096] A p-type diffusion layer was formed in the same manner as in
Example 1, except that the glass powder was changed to
SiO.sub.2-B.sub.2O.sub.3 based glass powder (SiO.sub.2: 50 mol %,
B.sub.2O.sub.3: 50 mol %, life time killer element: 470 ppm,
particle shape: substantially spherical, average particle diameter:
3.3 .mu.m). In the glass powder, Fe, Cu and Ni were contained as a
life time killer element.
[0097] The surface at the side where the composition for forming a
p-type diffusion layer was applied exhibited sheet resistance of 78
.OMEGA./.quadrature. and the formation of a p-type diffusion layer
through diffusion of B (boron). On the other hand, the rear surface
exhibited an unmeasurable sheet resistance of 1,000,000
.OMEGA./.quadrature. or higher and it was judged that no p-type
diffusion layer was substantially formed. A life time of carrier
was 87%. The warpage of the substrate did not occur.
Example 4
[0098] A p-type diffusion layer was formed in the same manner as in
Example 1, except that the glass powder was changed to
SiO.sub.2-B.sub.2O.sub.3 based glass powder (SiO.sub.2: 50 mol %,
B.sub.2O.sub.3: 50 mol %, life time killer element: 120 ppm,
particle shape: substantially spherical, average particle diameter:
3.7 .mu.m). In the glass powder, Fe, Cu and Ni were contained as a
life time killer element.
[0099] The surface at the side where the composition for forming a
p-type diffusion layer was applied exhibited sheet resistance of 81
.OMEGA./.quadrature. and the formation of a p-type diffusion layer
through diffusion of B (boron). On the other hand, the rear surface
exhibited an unmeasurable sheet resistance of 1,000,000
.OMEGA./.quadrature. or higher and it was judged that no p-type
diffusion layer was substantially formed. A life time of carrier
was 91%. The warpage of the substrate did not occur.
Example 5
[0100] A p-type diffusion layer was formed in the same manner as in
Example 1, except that the glass powder was changed to
SiO.sub.2-B.sub.2O.sub.3 based glass powder (SiO.sub.2: 50 mol %,
B.sub.2O.sub.3: 50 mol %, life time killer element: 85 ppm,
particle shape: substantially spherical, average particle diameter:
3.5 .mu.m). In the glass powder, Fe, Cu and Ni were contained as a
life time killer element.
[0101] The surface at the side where the composition for forming a
p-type diffusion layer was applied exhibited sheet resistance of 80
.OMEGA./.quadrature. and the formation of a p-type diffusion layer
through diffusion of B (boron). On the other hand, the rear surface
exhibited an unmeasurable sheet resistance of 1,000,000
.OMEGA./.quadrature. or higher and it was judged that no p-type
diffusion layer was substantially formed. A life time of carrier
was 95%. The warpage of the substrate did not occur.
Example 6
[0102] A p-type diffusion layer was formed in the same manner as in
Example 1, except that the glass powder was changed to
SiO.sub.2-B.sub.2O.sub.3 based glass powder (SiO.sub.2: 50 mol %,
B.sub.2O.sub.3: 50 mol %, life time killer element: 20 ppm,
particle shape: substantially spherical, average particle diameter:
3.2 .mu.m). In the glass powder, Fe, Cu and Ni were contained as a
life time killer element.
[0103] The surface at the side where the composition for forming a
p-type diffusion layer was applied exhibited sheet resistance of 85
.OMEGA./.quadrature. and the formation of a p-type diffusion layer
through diffusion of B (boron). On the other hand, the rear surface
exhibited an unmeasurable sheet resistance of 1,000,000
.OMEGA./.quadrature. or higher and it was judged that no p-type
diffusion layer was substantially formed. A life time of carrier
was 98%. The warpage of the substrate did not occur.
Example 7
[0104] A p-type diffusion layer was formed in the same manner as in
Example 1, except that the glass powder was changed to
SiO.sub.2-B.sub.2O.sub.3 based glass powder (SiO.sub.2: 50 mol %,
B.sub.2O.sub.3: 50 mol %, life time killer element: 8 ppm, particle
shape: substantially spherical, average particle diameter: 3.5
.mu.m). In the glass powder, Fe, Cu and Ni were contained as a life
time killer element.
[0105] The surface at the side where the composition for forming a
p-type diffusion layer was applied exhibited sheet resistance of 81
.OMEGA./.quadrature. and the formation of a p-type diffusion layer
through diffusion of B (boron). On the other hand, the rear surface
exhibited an unmeasurable sheet resistance of 1,000,000
.OMEGA./.quadrature. or higher and it was judged that no p-type
diffusion layer was substantially formed. A life time of carrier
was 100%. The warpage of the substrate did not occur.
Comparative Example 1
[0106] 20 g of SiO.sub.2-B.sub.2O.sub.3 based glass powder whose
particle shape is substantially spherical and average particle
diameter is 3.3 .mu.m (SiO.sub.2: 50 mol %, B.sub.2O.sub.3: 50 mol
%, life time killer element: 1180 ppm), 3 g of ethylcellulose and
77 g of 2-(2-butoxyethoxy) ethyl acetate were mixed and made into a
paste to prepare a composition for forming a p-type diffusion
layer. In the glass powder, Fe, Cu and Ni were contained as a life
time killer element.
[0107] Next, the prepared paste (composition for forming a p-type
diffusion layer) was applied to a p-type silicon substrate surface
by screen printing, and dried on a hot plate at 150.degree. C. for
5 minutes. Subsequently, a thermal diffusion treatment was carried
out in an electric furnace at 1000.degree. C. for 10 minutes. Then,
in order to remove the glass layer, the substrate was dipped in
hydrofluoric acid for 5 minutes, followed by washing with running
water and drying.
[0108] The surface at the side where the composition for forming a
p-type diffusion layer was applied exhibited sheet resistance of 80
.OMEGA./.quadrature. and the formation of a p-type diffusion layer
through diffusion of B (boron). On the other hand, the rear surface
exhibited an unmeasurable sheet resistance of 1,000,000
.OMEGA./.quadrature. or higher and it was judged that no p-type
diffusion layer was substantially formed. A life time of carrier
was 68%, which was lower.
* * * * *