U.S. patent application number 09/813296 was filed with the patent office on 2001-10-18 for photovoltaic element and method for manufacture thereof.
This patent application is currently assigned to Sanyo. Invention is credited to Ienaga, Teruhiko, Kadonaga, Yasuo, Nakai, Takuo, Taniguchi, Hiroyuki.
Application Number | 20010029978 09/813296 |
Document ID | / |
Family ID | 13352238 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010029978 |
Kind Code |
A1 |
Nakai, Takuo ; et
al. |
October 18, 2001 |
Photovoltaic element and method for manufacture thereof
Abstract
A photovoltaic element which directly converts an optical energy
such as solar light into an electric energy. After many uneven
sections are formed on the surface of an n-type crystalline silicon
substrate (1), the surface of the substrate (1) is isotropically
etched. Then the bottoms (b) of the recessed sections are rounded
and a p-type amorphous silicon layer (3) is formed on the surface
of the substrate (1) through an intrinsic amorphous silicon layer
(2). The shape of the surface of the substrate (1) after isotropic
etching is such that the bottoms of the recessed sections are
slightly rounded and therefore the amorphous silicon layer can be
deposited in a uniform thickness.
Inventors: |
Nakai, Takuo; (Osaka,
JP) ; Taniguchi, Hiroyuki; (Osaka, JP) ;
Ienaga, Teruhiko; (Osaka, JP) ; Kadonaga, Yasuo;
(Osaka, JP) |
Correspondence
Address: |
ARENT FOX KINTNER PLOTKIN & KAHN, PLLC
Suite 600
1050 Connecticut Avenue, N.W.
Washington
DC
20036-5339
US
|
Assignee: |
Sanyo
|
Family ID: |
13352238 |
Appl. No.: |
09/813296 |
Filed: |
March 21, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09813296 |
Mar 21, 2001 |
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09355311 |
Jul 29, 1999 |
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6207890 |
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09355311 |
Jul 29, 1999 |
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PCT/JP98/01204 |
Mar 19, 1998 |
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Current U.S.
Class: |
136/258 ;
136/261; 438/96; 438/97 |
Current CPC
Class: |
H01L 31/035281 20130101;
H01L 31/02363 20130101; Y02E 10/50 20130101; H01L 31/0747 20130101;
H01L 31/03529 20130101 |
Class at
Publication: |
136/258 ;
136/261; 438/96; 438/97 |
International
Class: |
H01L 021/00; H01L
031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 21, 1997 |
JP |
67690/1997 |
Claims
1. A photovoltaic element in which an amorphous or micro
crystalline silicon layer is formed on a crystalline silicon
substrate having many uneven sections, wherein bottoms of said
uneven sections on the substrate are rounded.
2. The photovoltaic element according to claim 1, wherein said
bottom has a curved surface of a larger curvature than that of the
top of the protruded section.
3. The photovoltaic element according to claim 2, wherein said
bottom is a curved surface of which radius is larger than 0.005
.mu.m.
4. The photovoltaic element according to claim 2, wherein said
bottom is a curved surface of which radius is in the range 0.01-20
.mu.m.
5. A photovoltaic element in which an amorphous or micro
crystalline silicon layer of different conductivity type is formed
on a front surface of a crystalline silicon substrate of one
conductivity type having many uneven sections, wherein bottoms of
said uneven sections on the substrate are rounded.
6. The photovoltaic element according to claim 5, wherein a
substantially intrinsic amorphous or micro crystalline silicon
layer is interposed between said crystalline silicon substrate of
one conductivity type and said amorphous or micro crystalline
silicon layer of different conductivity type.
7. The photovoltaic element according to claim 5 or 6, wherein said
bottom has a curved surface of a larger curvature than that of the
top of the protruded section.
8. The photovoltaic element according to claim 7, wherein said
bottom is a curved surface of which radius is larger than 0.005
.mu.m.
9. The photovoltaic element according to claim 7, wherein said
bottom is a curved surface of which radius is in the range 0.01-20
.mu.m.
10. The photovoltaic element according to claims 5-9, wherein a
high doping layer of one conductivity type is formed on the back
surface of said crystalline silicon substrate of one conductivity
type.
11. The photovoltaic element according to claim 10, wherein a high
doping layer of one conductivity type containing an amorphous or
micro crystalline silicon is formed on the back surface of said
crystalline silicon substrate.
12. The photovoltaic element according to claim 11, wherein a
substantially intrinsic amorphous or micro crystalline silicon
layer is interposed between said crystalline silicon substrate and
said high doping layer of one conductivity type.
13. A photovoltaic element in which an amorphous or micro
crystalline silicon layer of different conductivity type is formed
on a crystalline silicon substrate of one conductivity type having
many uneven sections on both front and back surface of the
substrate, wherein bottoms of said uneven sections on the front
surface of the substrate are rounded.
14. The photovoltaic element according to claim 13, wherein a
substantially intrinsic amorphous or micro crystalline silicon
layer is interposed between said crystalline silicon substrate of
one conductivity type and said amorphous or micro crystalline
silicon layer of different conductivity type.
15. The photovoltaic element according to claim 13 or 14, wherein
said bottom has a curved surface of a larger curvature than that of
the top of the protruded section.
16. The photovoltaic element according to claim 15, wherein said
bottom is a curved surface of which radius is larger than 0.005
.mu.m.
17. The photovoltaic element according to claim 15, wherein said
bottom is a curved surface of which radius is in the rage 0.01-20
.mu.m.
18. The photovoltaic element according to claims 13-17, wherein a
transparent electrode is formed on said amorphous or micro
crystalline silicon layer of different conductivity type, a
comb-like collecting electrode is formed on the transparent
electrode, a transparent electrode is also formed on the back
surface of the substrate, and a comb-like collecting electrode is
formed thereon.
19. The photovoltaic element according to claim 18, wherein a high
doping layer of one conductivity type is formed on the back surface
of said crystalline silicon substrate of one conductivity type.
20. The photovoltaic element according to claim 18, wherein a high
doping layer of one conductivity type containing an amorphous or
micro crystalline silicon is formed on the back surface of said
crystalline silicon substrate.
21. The photovoltaic element according to claim 20, wherein a
substantially intrinsic amorphous or micro crystalline silicon
layer is interposed between said crystalline silicon substrate and
said high doping layer of one conductivity type.
22. A method for manufacturing a photovoltaic element, wherein many
uneven sections are formed on a crystalline silicon substrate, a
surface of said substrate is isotropically etched, bottoms of the
uneven sections on the front surface of said substrate are rounded,
a substantially intrinsic amorphous or micro crystalline silicon
layer is formed on the front surface of the substrate, and thereon
an amorphous or micro crystalline silicon layer is formed.
23. The method for manufacturing a photovoltaic element according
to claim 22, wherein wet etching using a mixed solution of hydrogen
fluoride and nitric acid is used as isotropic etching.
24. The method for manufacturing a photovoltaic element according
to claim 22, wherein many uneven sections on the crystalline
silicon substrate are formed in a first process of cleaning a
surface of the crystalline silicon substrate and a second process
of processing the crystalline silicon substrate surface in an
alkaline solution to form uneven sections on the front surface.
25. The method for manufacturing a photovoltaic element according
to claim 24, wherein an alkaline solution of the same kind as that
in the second process is used in the first process.
26. The method for manufacturing a photovoltaic element according
to claim 24, wherein an alkaline solution containing an interface
active agent is used in the second process.
27. The method for manufacturing a photovoltaic element according
to claim 24, wherein an alkaline solution used in said second
process is in the range 0.2-8 wt. % of NaOH aqueous solution.
28. The method for manufacturing a photovoltaic element according
to claim 24, wherein an alkaline solution used in said second
process is in a range 3-6 wt. % aqueous solution of KOH.
29. The method for manufacturing a photovoltaic element according
to claim 24, wherein said second process is performed in a state
where said crystalline silicon substrate is oscillated.
30. The method for manufacturing a photovoltaic element according
to claim 26, wherein a surface tension of said interface active
agent is less than 47 dyn/cm.
Description
FIELD OF THE INVENTION
[0001] This invention is related to a photovoltaic element for
directly converting an optical energy such as solar light into an
electric energy and a method for manufacturing the same.
BACKGROUND OF THE INVENTION
[0002] A heterojunction type photovoltaic element, in which an
amorphous silicon layer or a micro crystalline silicon layer are
deposited on a single crystalline silicon substrate, is well-known.
The heterojunction can have its distinguish function when an
impurity is doped on an amorphous silicon layer or a micro
crystalline silicon layer.
[0003] In the amorphous silicon layer or mircocrystalline silicon
layer which is doped, however, defects caused by doping increase
and the characteristic of the heterojunction interface is degraded.
The degradation of the interface characteristic results in a lower
conversion efficiency because of a recombination of carriers in the
case where these silicon layers are used for a photovoltaic
element.
[0004] To overcome this problem, Japanese Patent Laid-Open
No.70183/1991 (IPC:H01L 31104) has proposed a photovoltaic element
in which the heterojunction interface characteristic is improved by
interposing a substantially intrinsic amorphous silicon layer
between a single crystalline silicon substrate and an amorphous
silicon layer for the purpose of decreasing defects at the
interface.
[0005] In a conventional photovoltaic element, many uneven sections
of line- or lattice-shape etc. are formed on a surface of a
substrate by such processes as etching which uses resist, or
anisotropic etching which employs alkaline solutions such as
potassium hydroxide (KOH) or sodium hydroxide (NaOH) solutions or
mechanical groove in order to improve short circuit current brought
by the optical confinement effect.
[0006] FIG. 11 illustrates a structure of a photovoltaic element
having the optical confinement which improves the heterojunction
interface characteristic (hereinafter it is referred as an HIT
structure). As shown in FIG. 11, an intrinsic amorphous silicon
layer 2 is formed on an n-type crystalline silicon substrate 1 of
which front surface has many uneven sections. A p-type amorphous
silicon layer 3 is formed on the intrinsic amorphous silicon layer
2. A front electrode 4 is formed on the whole region of the p-type
amorphous silicon layer 3 and a comb-like collecting electrode 5 is
formed on the front electrode 4. A back electrode 6 is formed on
the back surface of the substrate 1.
[0007] Although the comb-like collecting electrode 5 appears to be
formed on the top of the pyramid-shape protruded section in FIG.
11, the actual width of the comb-like collecting electrode 5 is no
less than 100 .mu.m. To help an understanding about the notion of
the comb-like collecting electrode 5, the figure describes the
electrode appears to be formed only on the top of the pyramid-shape
protruded section. The actual comb-like collecting electrode 5 has
a width equivalent to ten to twenty protruded sections of
pyramid-shape.
[0008] In the above described conventional structure of the front
surface of the substrate 1, a problem may occur when the intrinsic
amorphous silicon layer 2 is formed on the substrate 1 by a plasma
CVD method. When an amorphous semiconductor layer such as amorphous
silicon is formed by a plasma CVD method, the thickness of
amorphous semiconductor layer may not be uniform in the top a, the
bottom b of the uneven section on the front surface, and the plain
surface between a and b. As the thickness of the amorphous
semiconductor film on the top a is thick and thin on the bottom b,
particularly the amorphous semiconductor film may not be
sufficiently deposited at the bottom b. In FIG. 11, the intrinsic
amorphous silicon layer 2 and the p-type amorphous silicon layer 3
become thin at the bottom b, and it causes a lower open circuit
voltage and short circuit between the electrode and the substrate,
resulting in extremely degraded output characteristic of a
photovoltaic element.
[0009] This invention has an objective to provide a photovoltaic
element which solves the conventional problem as described above
and improve an output characteristic and yields, and a method for
manufacturing the same.
DESCLOSURE OF THE INVENTION
[0010] An amorphous or micro crystalline silicon layer on a
crystalline silicon substrate having many uneven sections is formed
on a photovoltaic element of the present invention, and bottoms of
the uneven sections on the substrate are rounded.
[0011] When the bottom of the uneven section is rounded, the
thickness of the amorphous or micro crystalline silicon layer which
is formed thereon can be uniform.
[0012] An amorphous or micro crystalline silicon layer of different
conductivity type on a front surface of a crystalline silicon
substrate of one conductivity type having many uneven sections is
formed on a photovoltaic element of the present invention, and
bottoms of the uneven sections on the substrate are rounded.
[0013] A substantially intrinsic amorphous or micro crystalline
silicon layer is preferably interposed between the front surface of
the crystalline silicon substrate of one conductivity type and the
amorphous or micro crystalline silicon layer of different
conductivity type.
[0014] When the bottoms of the uneven sections are rounded, the
thickness of the amorphous or micro crystalline silicon layer of
different conductivity type which is formed thereon can be uniform.
In particular, a open circuit voltage and fill factor of a
photovoltaic element having an HIT structure which improves a
characteristic of the heterojunction interface by interposing the
substantially intrinsic amorphous or micro crystalline silicon
layer. The substantially intrinsic amorphous or micro crystalline
silicon layer reduces defects at the heterojunction interface with
a crystalline silicon substrate and improves the characteristic of
the heterojunction interface. Thus, the layer does not affect the
improvement of the heterojunction interface even when dopant is
diffused on the intrinsic amorphous or micro crystalline silicon
layer in the subsequent processes.
[0015] The bottom is preferably formed so as to have a curved
surface of a larger curvature than that of the top of the protruded
section.
[0016] Furthermore, the bottom is preferably a curved surface of
which radius is larger than 0.005 .mu.m, more preferably in the
range 0.01-20 .mu.m.
[0017] A high doping layer of one conductivity type can be formed
on the back surface of the crystalline silicon substrate of one
conductivity type. By providing the high doping layer of one
conductivity type, a BSF-type photovoltaic element can be
obtained.
[0018] A high doping layer of one conductivity type containing an
amorphous or micro crystalline silicon can be formed on the back
surface of the crystalline silicon substrate. A substantially
intrinsic amorphous or micro crystalline silicon layer is
preferably interposed between the crystalline silicon substrate and
the high doping layer of one conductivity type containing an
amorphous or micro crystalline silicon layer.
[0019] By using this structure, a BSF type photovoltaic element can
be obtained in a low temperature process. The substantially
intrinsic amorphous or micro crystalline silicon layer can reduce
defects at the heterojunction interface with a crystalline silicon
substrate and improve the characteristic of heterojunction
interface.
[0020] An amorphous or micro crystalline silicon layer of different
conductivity type on a crystalline silicon substrate of one
conductivity type having many uneven sections on both front and
back surface of the substrate is formed on a photovoltaic element
of the present invention, and bottoms of the uneven sections are
rounded.
[0021] A substantially intrinsic amorphous or micro crystalline
silicon layer is preferably interposed between the crystalline
silicon substrate of one conductivity type and the amorphous or
micro crystalline silicon layer of different conductivity type.
[0022] The bottom is preferably formed so as to have a curved
surface of a larger curvature than that of the protruded
section.
[0023] Furthermore, the bottom is preferably a curved surface of
which radius is larger than 0.005 .mu.m, more preferably in the
rage 0.01 to 20 .mu.m.
[0024] The bottom is preferably formed so as to have a curved
surface of a larger curvature than that of the top of protruded
section.
[0025] A transparent electrode is formed on the amorphous or micro
crystalline silicon layer of different conductivity type, and a
comb-like collecting electrode is formed on the transparent
electrode. A transparent electrode is also formed on the back
surface of the substrate, and a comb-like collecting electrode is
formed on the transparent electrode.
[0026] By using this structure, a warp of the substrate is
prevented even when the substrate is made thinner.
[0027] In a manufacturing method of a photovoltaic elements
according to the present invention, many uneven sections are formed
on a crystalline silicon substrate, a surface of the substrate is
isotropically etched, and bottoms of the uneven sections on the
front surface of the substrate are rounded. A substantially
intrinsic amorphous or micro crystalline silicon layer is formed on
the front surface of the substrate, and thereon an amorphous or
micro crystalline silicon layer is formed.
[0028] A substantially intrinsic amorphous or micro crystalline
silicon layer is formed by depositing an intrinsic amorphous or
micro crystalline silicon layer by plasma resolution using raw
material gas such as silane without mixing dopant gas when forming
a layer. In the subsequent processes, dopant may be diffused in the
substantially intrinsic or micro crystalline silicon layer.
However, the substantially intrinsic amorphous or micro crystalline
silicon layer is formed so as to avoid defects at the
heterojunction interface with a single crystalline silicon
substrate, and the interface characteristic is improved when the
layer is formed without containing dopant gas. Thus, the interface
characteristic is not affected even if dopant is diffused after
forming a layer.
[0029] Wet etching using a mixed solution of hydrogen fluoride and
nitric acid is preferably used as isotropic etching.
[0030] Many uneven sections on the crystalline silicon substrate
can be formed in a first process of cleaning a surface of the
crystalline silicon substrate and a second process of processing
the crystalline silicon substrate surface in an alkaline solution
to form uneven sections on the surface.
[0031] In the first process, an alkaline solution of the same kind
as that in the second process is preferably used.
[0032] An alkaline solution containing an interface active agent is
preferably used in the second process. In the above composition,
the appearance of the surface after isotropically etched does not
affect the strength of the short circuit current of a photovoltaic
element since the bottoms of the uneven sections are slightly
rounded. And the variety of the thickness of the amorphous or micro
crystalline layer is prevented, eliminating the lowered open
circuit voltage and short circuit between the substrate and the
electrode. Therefore, it is possible to achieve an improved output
characteristic of a photovoltaic element and high yields.
THE BELIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a cross sectional view illustrating a photovoltaic
element according to an embodiment of the present invention;
[0034] FIG. 2 is a schematic view illustrating isotropic etching
performed in the present invention;
[0035] FIG. 3 illustrates a relation between the bottom shape of
the uneven section on the substrate and the conversion
efficiency;
[0036] FIG. 4 is a cross sectional view illustrating a photovoltaic
element according to a second embodiment of the present
invention;
[0037] FIG. 5 is a cross sectional view illustrating a substrate
which is etched;
[0038] FIG. 6 is a cross sectional view illustrating a photovoltaic
element according to a third embodiment of the present
invention;
[0039] FIG. 7 is a cross sectional view illustrating a photovoltaic
element according to a forth embodiment of the present
invention;
[0040] FIG. 8 is a cross sectional view illustrating a photovoltaic
element according to a fifth embodiment of the present
invention;
[0041] FIG. 9 is a characteristic diagram describing a relation
between the density of NaOH used in the second process for forming
an uneven section on a substrate in the present invention and the
photoelectric conversion characteristic of a photovoltaic
element;
[0042] FIG. 10 is a characteristic diagram describing a relation
between the surface tension of an interface active agent used in
the second process for forming an uneven section on a substrate in
the present invention and the photoelectric conversion
characteristic of a photovoltaic element;
[0043] FIG. 11 is a cross sectional view illustrating a
conventional photovoltaic element of an HIT structure having the
optical confinement.
THE PREFERRED EMBODIMENTS OF THE INVENTION
[0044] Detailed explanation on this invention will be made by
referring to the drawings. FIG. 1 is a cross sectional view
illustrating a single crystalline silicon photovoltaic element
manufactured by a method according to the present invention. As
like above described FIG. 11, a comb-like collecting electrode 5 is
illustrated as that it is formed only on the top of a pyramid shape
protruded section to help an understanding about the notion of a
comb-like collecting electrode 5. Other cross sectional views
according to the other embodiments illustrate the comb-like
collecting electrode in the same manner as in FIG. 1.
[0045] In a photovoltaic element of the present invention, an
intrinsic amorphous silicon layer 2 is formed on an n-type single
crystalline. silicon substrate 1 having many uneven sections on its
front surface, and a p-type amorphous silicon layer 3 is formed on
the intrinsic amorphous silicon layer 2. A front electrode 4
comprising ITO is formed on the whole region of the p-type
amorphous silicon layer 3 and a comb-like collecting electrode 5
comprising silver (Ag) is formed on the front electrode 4. A back
electrode 6 comprising aluminum (Al) is formed on the back surface
of the single crystalline silicon substrate 1.
[0046] The present invention is featured by that a bottom b of the
uneven section on the front surface of the single crystalline
silicon substrate 1 is rounded. As described above, an amorphous
silicon layer deposited on the front surface of the silicon
substrate 1 in the conventional photovoltaic element is thick at
the top a of pyramid shape protruded section of the uneven section
and is thin at the bottom b, causing variety in thickness of the
layer. On the other hand, the present invention can achieve a
uniform thickness of amorphous silicon layers 2 and 3 formed on the
substrate 1 by rounding the bottom b.
[0047] In the present invention, an uneven section is formed on the
substrate 1 and a bottom rounded before the amorphous silicon layer
2 is deposited on the single crystalline silicon substrate 1. That
is, before depositing the amorphous silicon layers 2 and 3 by a
plasma CVD method, an uneven section of a pyramid shape is formed
on the front surface of the substrate 1 and its bottoms are
rounded. Explanation on this process will be made as follows.
[0048] First, a method for forming an uneven section, which is most
suitable for optical confinement, on the substrate 1 will be
explained.
[0049] As a first step, a front surface of a substrate is cleaned
for eliminating attachments on the surface and modifying
deformations of a surface of a crystalline semiconductor substrate.
A second step is anisotropic etching for forming an uneven
structure. The deformation means a deformation in a crystalline
structure in a region of as long as tens .mu.m in depth from the
front surface of the substrate generated when slicing a substrate
from a single crystalline silicon ingot.
[0050] It is preferred to etch to a depth of tense m from the front
surface of the substrate since the first step is performed in order
to eliminate attachments on the surface and deformations which
extend as deep as tens .mu.m from the front surface of the
substrate.
[0051] It is also preferred to use an etching solution of high
density to fasten the etching rate of a crystalline semiconductor
since process time is required to shorten in a case of mass
production.
[0052] Another reason for using an etching solution of high density
is that it is necessary to increase the times of using an etching
solution since the etching solution contaminated by attached
subjects to the surface of the substrate may affect the etching
characteristics in the present invention.
[0053] The objective of the second step is to form an uneven
structure in a photovoltaic element. The most suitable uneven
structure for a photovoltaic element is one having a length of
10-50 .mu.m between tops of the uneven sections and the protruded
section's vertical angle of less than 90.degree. and this structure
should be formed accurately. Therefore, it is difficult to obtain
the most suitable and accurate uneven structure and reproduce it
when anisotropic etching rate is too fast. Thus, the density of an
etching solution is required to be relatively low.
[0054] As an etching solution used in the second process is
required to isotropically etch a crystalline semiconductor
substrate, alkaline solutions such as NaOH and KOH etc. are used.
In use of these chemicals, it is necessary to pay attentions to
maintenance of the chemicals and the durability of the equipment
against etching solutions. The cost and time necessary for the
maintenance of chemicals and the measures to be taken for the
durability of the equipment increase as the number of chemicals
used increases.
[0055] In conjunction with this, it is preferred to use the same
alkaline solution in the first and second processes;
[0056] In the second process, a silicon crumb or a reactive product
generated during anisotropic etching is reattached to the
substrate, making the front surface of the substrate rough. To
prevent this, an interface active agent or isopropyl alcohol (IPA)
should be mixed in the alkaline solution.
[0057] To be concrete, many bubbles are generated on the front
surface of the substrate in the process of etching the crystalline
semiconductor substrate by the alkaline solution. It is expected
that IPA or an interface active agent minimize the size of bubbles
and facilitate the bubbles to detach from the front surface of the
substrate, thereby preventing silicon crumbs or reactive products
in the bubbles from reattaching to the substrate.
[0058] The effect of facilitating detachment of bubbles by the
interface active agent is enhanced when the surface tension is
under the predetermined value.
[0059] By such ways as oscillating the substrate up and down,
supersonically oscillating the substrate, supersonically
oscillating the etching solution, or bubbling the etching solution
by inert gases such as nitrogen (N.sub.2) or argon (Ar), the
substrate is oscillated directly or indirectly, thereby the effect
of bubble detachment by IPA or an interface active agent is further
facilitated.
[0060] The detail embodiment of the first and second processes will
be explained.
[0061] An n-type crystalline silicon substrate 1 which is sliced
along (100) surface and of which resistivity is 0.1-10 .OMEGA.cm
and of which thickness is 200-400 .mu.m is prepared. As a first
process, the substrate 1 is dipped into 5 wt. % aqueous solution of
NaOH about 85.degree. C. for ten minutes to remove deformation on
the front surface of the substrate 1 generated during the process.
By this process, the deformation as deep as about 10 .mu.m from the
front surface of the substrate is removed, and a single crystalline
silicon substrate of good crystallization can be obtained. The
present invention uses an NaOH aqueous solution as an etching
solution in the first process. This etching solution of higher
density fastens etching rate and etches excessively, to form
relatively moderate uneven sections on the surface of the
substrate.
[0062] In the second process, the front surface of the substrate 1
is anisotropically etched by using a mixed aqueous solution of 2
wt. % of sodium hydrogen (NaOH), which is lower density than that
of the first process, and isopropyl alcohol (IPA). In the second
process, uneven sections of pyramid shape are formed on the surface
of the substrate 1. A single crystalline silicon is anisotropically
etched by an alkaline aqueous solution. The etching rate for (111)
surface is extremely slower in comparison with that of the other
crystal orientation. Accordingly, the single crystalline substrate
1 sliced in (100) surface is etched by a mixed aqueous solution
containing 2% of NaOH and IPA to anisotropically etch the silicon
along the (111) surface, and form 1-10 .mu.m deep uniform protruded
sections of pyramid shape on the front surface of the substrate 1.
Therefore, recessed sections which are V-shape in cross section and
are formed by four faces deposited on (111) surface are formed on
the whole surface.
[0063] In the above mentioned second process, hydrogen is generated
when alkali contained in the etching solution and the silicon
substrate react each other. And bubbles of hydrogen attach to the
front surface of the substrate. Attached bubbles make it impossible
for the region attached with the bubbles to form uniform uneven
sections since the region can not react with alkali. Thus, an IPA
aqueous solution is mixed in an alkali aqueous solution in order to
prevent bubbles from attaching to the surface.
[0064] Since IPA is volatile, it is necessary to control strictly
the density of IPA and supply it in the second process.
[0065] An interface active agent can substitute for the above
mentioned IPA in order to prevent hydrogen generated in the second
process from attaching to the front surface of the substrate. For
example, in this second process, an etching solution in which an
interface active agent is added in the ratio of about 1 wt. % into
about 1.5 wt. % aqueous solution of NaOH is used. The above
mentioned etching to form uneven sections as in the case of IPA can
be performed by dipping the substrate into the above etching
solution of about 85.degree. C. for 30 minutes. The interface
active agent is not volatile, therefore, it is not necessary to
supply the interface active agent or control its density, resulting
in easier process.
[0066] In this embodiment, Sintrex by Nippon Oil and Fat Co. Ltd.
is used although other interface active agent can also be used.
[0067] By the first and second processes of this embodiment, uneven
sections of pyramid shape which is 5 .mu.m in width and 5 .mu.m in
depth (height) and is most suitable to achieve optical confinement
can be formed on the front surface of the substrate.
[0068] When manufacturing 100 units of substrates having uneven
sections by the above first and second processes, it has not been
necessary to replace an etching solution through the first and
second processes.
[0069] Next, bottoms b are rounded after uneven sections are formed
on the substrate 1. After cleaning the substrate 1 having the
uneven sections with isonized water, the substrate is dipped into a
hydrogen fluoride aqueous solution mixing hydrogen fluoride (HF)
and water (H.sub.2O) in the ratio 1:100 to remove the oxide film on
the front surface of the substrate, and is cleaned with water.
[0070] Then, the substrate 1 is dipped into an aqueous solution
mixing ammonia (NH.sub.4OH), hydrogen peroxide (H.sub.2O.sub.2),
and water (H.sub.2O) in the ratio 1:1:5 to oxide the silicon
surface by catching particles and organic materials on the front
surface of the substrate, and the substrate is cleaned with water.
Further, the substrate 1 is dipped into a hydrogen fluoride aqueous
solution mixing hydrogen fluoride and water in the ratio 1:100 to
remove the oxide layer on the front surface of the substrate, and
is cleaned with water, thus removing particles and organic
materials on the front surface of the substrate.
[0071] Next, the substrate 1 is dipped into an aqueous solution
mixed 35% hydrochloric acid (HCl), 30% hydrogen peroxide
(H.sub.2O.sub.2), and water (H.sub.2O) in the ratio 1:1:6 to oxide
the front surface of the substrate by catching heavy metals such as
aluminum, iron, and magnesium and alkaline component such as sodium
which have negative effects on solar cell characteristics. Then the
substrate is dipped into a hydrogen fluoride solution mixing
hydrogen fluoride and water in the ratio 1:100 to remove the oxide
layer on the front surface of the substrate, and is cleaned by
water, thus removing the heavy metals on the front surface of the
substrate.
[0072] Further, the substrate 1 is dipped into an aqueous solution
mixing hydrogen fluoride (HF) and nitric acid (HNO.sub.3) in the
ratio 1:20 for about 30 seconds to eliminate the front surface of
the substrate about 2 .mu.m in depth by isotropically etching. The
bottom of the uneven section on the front surface of the substrate
is rounded by the isotropic etching. After the isotropic etching,
the substrate is cleaned with water and dipped into a hydrogen
fluoride aqueous solution mixing hydrogen fluoride and water in the
ratio 1:100 to eliminate the oxide layer on the front surface of
the substrate, and is cleaned with water.
[0073] After processing the surface of the substrate 1, the surface
of the substrate 1 is processed by H.sub.2 plasma. Further an
intrinsic amorphous silicon layer 2 of 50 to 200 .ANG. in thick is
deposited by a plasma CVD method using silane (SiH.sub.4), and a
p-type amorphous silicon layer 3 of 50-200 .ANG. in thick is
deposited on the intrinsic amorphous silicon layer 2 to form a p-n
junction by using silane (SiH.sub.4) and diborane (B.sub.2H.sub.6)
as dopant gas.
[0074] Further, a front electrode 4 containing ITO of 1000 .ANG. in
thick is formed by sputtering and a comb-like collecting electrode
5 containing silver of 10 .mu.m in thick is formed by sputtering. A
back electrode 6 containing aluminum of 2 .mu.m in thick is formed
on the back surface of the substrate 1 by vacuum deposition. By
these processes, a photovoltaic element of the present invention
can be obtained.
[0075] In the meantime, although the intrinsic amorphous silicon
layer 2 is interposed in the above photovoltaic element to improve
hetrojunction interface characteristics between the single
crystalline silicon substrate 1 and the p-type amorphous silicon
layer 3, p-type dopant may be diffused in the intrinsic amorphous
silicon layer 2 during formation of the p-type amorphous silicon
layer 3 and heat treatment thereafter. Since the intrinsic
amorphous silicon layer 2 is formed to reduce defects at the
heterojunction interface, the junction interface characteristic is
not affected by diffusion of p-type dopant in the subsequent
processes. As shown in FIG. 2, the above isotropic etching is
performed for predetermined etching time to etch the front surface
1a of the substrate 1 as deep as r. When the front surface of the
substrate becomes 1b, a curved surface, of which radius is r and of
which center is the bottom of the pre-etched substrate, is
formed.
[0076] The intrinsic amorphous silicon layer 2 in the photovoltaic
element of an HIT structure should be more than 50 .ANG. (0.005
.mu.m) in thick. Accordingly, in order to form amorphous silicon
layer of uniform and satisfying thickness, a curved surface of 50
.ANG. in thick is necessary at the bottom. When a curved surface is
less than 50 .ANG. in radius, it may cause various thickness of the
amorphous silicon layers 2 and 3. Therefore, the radius of the
curved surface at the bottom b should be more than 50 .ANG..
[0077] By isotropic etching, the surface of the substrate 1 can be
etched to make its front surface plain rather than pyramid shape.
FIG. 3 shows the results of conversion efficiency of photovoltaic
elements different in isotropic etching times and radius of
curvature at bottoms b from one another. From FIG. 3, it is found
that the best radius of curvature of round shape at bottom is in
the range of 0.01-20 .mu.m.
[0078] A photovoltaic element formed by the method of the present
invention and a photovoltaic element which is formed by the same
method except that only uneven section of pyramid shape is formed
on a substrate 1 and isotropic etching is not performed and are
prepared. The photovoltaic characteristics of them are measured
under the solar simulator of which brightness is AM 1.5, 100
mW/m.sup.2. The results are in Table 1.
1 TABLE 1 Voc (V) Isc (A) F.F. .eta. (%) Present invention 0.65
3.49 0.75 17.0 Conventional example 0.62 3.52 0.72 15.7
[0079] As shown in Table 1, the photovoltaic element of the present
invention improves an open circuit voltage (Voc) 5%, a fill factor
(F.F.) 4%, and a conversion efficiency (.eta.) 8% as compared with
the conventional photovoltaic element. This is due to a round shape
of bottoms of pyramid structure in the photovoltaic element of the
present invention by isotropic etching. Thus, the variety in the
thickness of the amorphous silicon layer laminated on the silicon
substrate is reduced and unequal electric field strength short
circuit in a front electrode and silicon substrate do not
occur.
[0080] The same effect can be obtained when the front surface of
the substrate 1 is uneven structure of line- or lattice-shape.
[0081] In the above embodiment, a mixed solution of hydrogen
fluoride and nitric acid is used for isotropic etching. Other
isotropic etching such as wet etching using a mixed solution of
HF/HNO.sub.3/CH.sub.3COOH or dry etching using CF.sub.3/O.sub.2 gas
are also applicable.
[0082] Although the comb-like collecting electrode 5 is formed by
sputtering and the back electrode is by vacuum deposition in the
above embodiment, the comb-like collecting electrode 5 can be
formed by screen printing using Ag paste and the back electrode 6
can be formed by screen printing using Al paste.
[0083] Since calcination temperature for Ag paste is about
200.degree. C., the comb-like collecting electrode 5 can be formed
by screen-printing Ag paste after forming the amorphous silicon
layers 2 and 3 and the front electrode 4 on the substrate and
calcinating them. Al paste, of which calcination temperature is
about 700.degree. C., can not be calcinated after forming the
amorphous silicon 2 and 3 in conjunction with heat treatment. Thus,
when using Al paste as the back electrode 6, Al paste is printed
and calcinated on the whole back surface of the substrate 1 before
forming an amorphous silicon layer to form the back electrode 6.
Then, the amorphous silicon layers 2 and 3 and the front electrode
4 and the comb like collecting electrode 5 can be formed.
[0084] FIG. 4 is a cross sectional view illustrating the second
embodiment of the present invention. In this embodiment, a BSF
(Back Surface Field) type photovoltaic element in which an internal
electric field is introduced on the back surface of the substrate 1
to prevent an effect from the recombination of carriers near the
back surface of the substrate 1. An n-type high doping layer 7 is
formed on the back surface of the n-type substrate 1 as in FIG.
4.
[0085] The photovoltaic element in FIG. 4 can be formed by the
following processes for example. An n-type crystalline silicon
substrate 1 which is sliced along (100) surface and of which
resistivity is 0.1-10 .OMEGA.cm and of which thickness is 200-400
.mu.m is prepared. As a first process, the substrate 1 is dipped
into a 5 wt. % aqueous solution of sodium hydroxide (NaOH) about
85.degree. C. for ten minutes to remove deformation on the surface
of the substrate 1. As a second process, the substrate 1 is dipped
into a mixed solution containing 1.5 wt. % aqueous solution of
NaOH, lower density than that of the first process, which is added
1 wt. % of an interface active agent. In the second process, uneven
sections of pyramid shape are formed on the front surface of the
substrate 1 by anisotropic etching.
[0086] Further, phosphorus (P) is diffused for 15-20 minutes under
about 550.degree. C. by using POCl.sub.3 gas to form an n-type
layer about 0.5 .mu.m in depth in peripheral of the substrate.
[0087] After cleaning with water, the substrate 1 is dipped into an
hydrogen fluoride aqueous solution mixing hydrogen fluoride (HF)
and water (H.sub.2O) in the ratio 1:100 to remove the oxide layer
on the front surface of the substrate and is cleaned with
water.
[0088] Then, the substrate 1 is dipped into an aqueous solution
mixing ammonia (NH.sub.4OH), hydrogen peroxide (H.sub.2O.sub.2),
and water (H.sub.2O) in the 1:1:5 ratio to oxide the silicon
surface by catching particles and organic materials on the front
surface of the substrate, and the substrate is cleaned with water.
Further, the substrate is dipped into a hydrogen fluoride solution
mixing hydrogen fluoride and water in the 1:100 ratio to remove the
oxide layer on the surface of the substrate, and is cleaned with
water in order to remove particles and organic materials on the
front surface of the substrate.
[0089] Next, the substrate 1 is dipped into an aqueous solution
mixing 35% hydrochloric acid (HCl) and 30% hydrogen peroxide
(H.sub.2O.sub.2) and water (H.sub.2O) in the 1:1:6 ratio to oxide
the front surface of the substrate by catching heavy metals such as
aluminum, iron, and magnesium and alkaline component such as sodium
which are attached to the front surface of the substrate and have
negative effects on solar cell characteristics. Then the substrate
1 is dipped into a hydrogen fluoride solution mixing hydrogen
fluoride and water in the 1:100 ratio to remove the oxide layer on
the front surface of the substrate, and is cleaned with water, thus
removing the heavy metals on the front surface of the
substrate.
[0090] Further, after coating the back surface of the substrate 1
with resist etc., the substrate 1 is dipped into an aqueous
solution mixing hydrogen fluoride (HF) and nitric acid (HNO.sub.3)
in the 1:20 ratio for about 30 seconds to remove the front surface
of the substrate about 2 .mu.m in depth by isotropic etching. The
bottom b of the uneven section on the front surface of the
substrate is rounded and an n-layer on the front and side surfaces
of the substrate is removed by the isotropic etching, obtaining an
n-type high doping layer 7 only on the back surface of the
substrate 1.
[0091] After isotropically etched, the substrate is cleaned with
water and dipped in a hydrogen fluoride aqueous solution mixing
hydrogen fluoride and water in the 1:100 ratio to remove the oxide
layer on the surface of the substrate. Then the substrate is
cleaned with water.
[0092] After the process of the surface of the substrate 1, an
intrinsic amorphous silicon layer 2 and a p-type amorphous silicon
layer 3 are deposited by a plasma CVD method as described in the
above embodiment to form a p-n junction. A front electrode 4
containing ITO of 1000 .ANG. in thick is formed by sputtering and a
comb-like collecting electrode 5 containing silver is formed by
screen printing using Ag paste thereon. Further, a back electrode 6
containing aluminum of 2 .mu.m in thick is formed by vacuum
deposition on an n-type high doping layer 7 disposed on the back
surface of the substrate 1. By these processes, a BSF-type
photovoltaic element can be obtained.
[0093] When forming a back electrode 6 by screen printing using Al
paste, as described in the above description, a back electrode 6 is
formed on the back surface of the substrate 1 before an amorphous
silicon layer and so on are formed.
[0094] The above embodiments is described as that the uneven
sections are formed only on the front surface of the substrate 1.
Some measures such as mask against the back surface of the
substrate 1 should be taken so as not to be formed uneven sections
by etching, otherwise as shown in FIG. 5, uneven sections having
rounded bottoms are formed on both front and back surfaces of the
substrate 1 by etching.
[0095] As shown in FIG. 6 of the third embodiment, an intrinsic
amorphous silicon layer 2 and a p-type amorphous silicon layer 3
are formed on the front surface of the substrate 1 having uneven
sections by a plasma CVD method. A front electrode 4 containing ITO
of 1000 .ANG. in thick is formed by sputtering on the p-type
amorphous silicon layer 3. A comb-like collecting electrode 5
containing silver is formed by screen printing using Ag paste on
the front electrode 4. A back electrode 6 of about 20-25 .mu.m in
thick is formed on the back surface of the substrate 1 by using Al
paste.
[0096] It is convenient to form a back electrode 6 on the whole
back surface of the substrate 1 by screen printing using Al paste
from the viewpoint of mass production. In this method, a back
electrode 6 containing Al of about 20 .mu.m in thick is formed by
pasting Al paste on the whole back surface of the substrate 1 by
screen printing and calcinating the paste by approximately
700.degree. C. heat treatment.
[0097] Recently, in the meanwhile, the substrate has become thinner
and thinner in order to reduce the cost for materials. In
conjunction with this, the substrate 1 warps due to the difference
of coefficients of thermal expansion between Al and silicon in the
heat treatment for calcinating paste. Because of the warps the
substrate 1 may break in the subsequent processes, resulting in
decrease of yields.
[0098] FIG. 7 of the forth embodiment illustrates a photovoltaic
element in which a substrate 1 does not warp up while its thickness
is smaller. A transparent electrode 8 containing ITO as like as on
the front surface of the substrate 1 is formed on the back surface
of the substrate 1. A comb-like collecting electrode 9 is formed on
the transparent electrode 8 by screen printing using Ag paste. On
the front surface of the substrate 1, the comb-like collecting
electrode 5 is also formed on the transparent electrode 4
containing ITO.
[0099] The comb-like collecting electrodes 5 and 9 having the same
shape are formed on both front and back surface of the substrate 1
respectively through the transparent electrodes 4 and 8. Thus, the
concentration of stress in a certain direction can be eliminated
and warps of the substrate 1 can be prevented.
[0100] FIG. 8 of the fifth embodiment illustrates a BSF-type
photovoltaic element manufactured in a low-temperature process
without using thermal diffusion. An intrinsic amorphous silicon
layer 10 of 50-100 .ANG. in thick is deposited on the back surface
of the substrate 1 by a plasma CVD method of low-temperature
process using silane (SiH.sub.4) where the temperature of the
substrate is about 170.degree. C. A high doping n-type amorphous
silicon layer 11 of 50-500 .ANG. in thick is deposited on the
intrinsic amorphous silicon layer 10 by using silane (SiH.sub.4)
and phospine (PH.sub.3)as a dopant gas. A transparent electrode on
a back surface 12 containing ITO of 1000 .ANG. in thick is formed
by sputtering. A comb-like collecting electrode 9 is formed on the
transparent electrode on a back surface 12 by using Ag paste. By
these processes, a substantially intrinsic amorphous silicon is
interposed between the crystalline silicon substrate and the
amorphous silicon layer on the back surface in order to reduce
defects at their interface and improve the characteristic of the
heterojunction interface. As a result, a BSF structure can be
obtained by a low-temperature process without using a thermal
diffusion method.
[0101] In this embodiment, n-type dopant may be diffused on the
intrinsic amorphous silicon layer 10 in the subsequent processes.
The intrinsic amorphous silicon layer 10 is formed for improving
the characteristics of the heterojunction interface as described in
the above embodiment. Thus, the diffusion of dopant thereafter does
not affect the interface characteristics.
[0102] Explanation will be made on an embodiment where the density
of an alkaline solution included in the etching solutions used in
the second process is varied.
[0103] In this embodiment, after the first process, a substrate
having uneven sections is manufactured by varying the density of
NaOH used in the second process within the range of 0.1-10 wt. %.
By using these substrates, a photovoltaic element is formed through
the same processes as those in the first embodiment to compare
their characteristics of photovoltaic conversion.
[0104] FIG. 9 is a diagram showing the relation between the NaOH
density used in the second process and the characteristic of
photovoltaic conversion of the photovoltaic element. FIG. 9 shows
relative values when a value of the photovoltaic element having the
most effective conversion efficiency is one. It is found that high
characteristics of photovoltaic conversion can be obtained when the
NaOH density is 0.1-8 wt. %, in particular 1.5-3 wt. %.
[0105] When KOH is used instead of NaOH, high photovoltaic
conversion characteristics can be obtained at the KOH density of
3-6 wt. %.
[0106] Further, explanations will be made on an embodiment where
the surface tension of interface active agents used in the second
process of this invention is varied.
[0107] FIG. 10 is a characteristic diagram showing a relation
between the surface tension of an interface active agent and
photovoltaic conversion characteristic of a photovoltaic element.
FIG. 10 shows relative values when a value of the photovoltaic
element having the most effective conversion efficiency is one. It
is found that a solar cell having high characteristics of
photovoltaic conversion can be obtained when the surface tension of
the interface active agent is less than 47 dyn/cm, in particular
less than 40 dyn/cm.
[0108] The reason for the improvement of photovoltaic conversion
characteristics by reducing the surface tension of the interface
active agent is considered as follows.
[0109] In the second process, hydrogen is generated when alkali
contained in the etching solution and silicon substrate react each
other. The bubbles of hydrogen attach to the surface of the
substrate. Adhesiveness of the etching solution to the substrate
becomes grater when the surface tension of the interface active
agent is smaller, making the bubbles which do not become larger
detach from the surface of the substrate.
[0110] On the contrary, when the surface tension of the interface
active agent is great, adhesiveness of the etching solution to the
substrate becomes great, preventing the bubbles from detaching
before the bubbles become larger. When the bubbles attach to the
surface of the substrate until they become large, the sections
attached by the bubbles can not react to alkali, thus uneven
sections can not be obtained uniformly.
[0111] When the surface tension of the interface active agent is
less than 47 dyn/cm, the detachment of bubbles form the substrate
can be facilitated and the uniform texture can be formed on the
whole surface of the substrate, achieving high photovoltaic
conversion characteristics.
[0112] In this embodiment, controls of the surface tension are made
by varying the density of an interface active agent. Table 2 shows
the relative relation between the density of the interface active
agent and the surface tention. It is found that the surface tension
becomes less than 47 dyn/cm when the density of the interface
active agent is less than 1 wt. %. In this examination, the surface
tension can not be less than 20 dyn/cm.
2TABLE 2 Density of interface 0.1 0.5 2 5 20 40 active agent (%)
Surface tension 56 50 46 40 30 20 (dyn/cm)
[0113] As described above, in this invention, since the first
process for cleaning the surface of the substrate is performed
before the second process, deformations created on the surface of
the substrate which is sliced from an ingot can certainly be
eliminated, and at the same time attachment on the surface of the
substrate can also be removed. Thus, a substrate having uniform and
most suitable uneven sections can be manufactured repeatedly. And a
photovoltaic element of high photovoltaic conversion efficiency can
be accurately reproduced by using a substrate manufactured by this
invention.
[0114] Although the above description does not mention, by such
ways as oscillating a substrate down and up, supersonically
oscillating a substrate, supersonically oscillating an etching
solution, and bubbling an etching solution by inert gas such as
N.sub.2 and Ar, a substrate can be oscillated directly or
indirectly, thus facilitating the detachment of bubbles by the
interface active agent.
[0115] Although the above embodiments are about a single
crystalline silicon substrate, the present invention is applicable
to a polycrystalline silicon substrate. The above mentioned first
and second processes are applicable to a crystalline semiconductor
substrate in general such as single- and poly-crystalline Germanium
based substrate.
[0116] In this embodiment, an intrinsic amorphous silicon layer is
interposed between a single crystalline silicon substrate and a p-
or n-type amorphous silicon layer. The present invention is also
applicable when a p- or n-type amorphous silicon layer is formed
directly on a single crystalline silicon substrate. In such a case,
uniform thickness of the amorphous silicon layer formed on the
substrate can be obtained when bottoms of uneven sections on the
substrate are rounded.
[0117] As described, in this invention, bottoms of many uneven
sections on the surface of the substrate are etched by isotropic
etching and are made round, to reduce variety of the thickness of
amorphous silicon layer deposited thereon. Thus, an open circuit
voltage and a fill factor of the photovoltaic element are improved
and its output can increase.
* * * * *