U.S. patent application number 11/422570 was filed with the patent office on 2006-09-28 for thin-film photoelectric conversion device and a method of manufacturing the same.
This patent application is currently assigned to SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. Invention is credited to Yasuyuki Arai, Shunpei Yamazaki.
Application Number | 20060213550 11/422570 |
Document ID | / |
Family ID | 27469796 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060213550 |
Kind Code |
A1 |
Yamazaki; Shunpei ; et
al. |
September 28, 2006 |
THIN-FILM PHOTOELECTRIC CONVERSION DEVICE AND A METHOD OF
MANUFACTURING THE SAME
Abstract
A method of manufacturing a thin-film solar cell, comprising the
steps of: forming an amorphous silicon film on a substrate; placing
a metal element that accelerates the crystallization of silicon in
contact with the surface of the amorphous silicon film; subjecting
the amorphous silicon film to a heat treatment to obtain a
crystalline silicon film; depositing a silicon film to which
phosphorus has been added in contact with the crystalline silicon
film; and subjecting the crystalline silicon film and the silicon
film to which phosphorus has been added to a heat treatment to
getter the metal element from the crystalline film.
Inventors: |
Yamazaki; Shunpei;
(Atsugi-Shi, Kanagawa-Ken, JP) ; Arai; Yasuyuki;
(Atsugi-Shi, Kanagawa-Ken, JP) |
Correspondence
Address: |
FISH & RICHARDSON P.C.
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
SEMICONDUCTOR ENERGY LABORATORY
CO., LTD.
398 Hase
Atsugi-Shi, Kanagawa-ken
JP
|
Family ID: |
27469796 |
Appl. No.: |
11/422570 |
Filed: |
June 6, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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08907182 |
Aug 6, 1997 |
7075002 |
|
|
11422570 |
Jun 6, 2006 |
|
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|
08623336 |
Mar 27, 1996 |
5700333 |
|
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11422570 |
Jun 6, 2006 |
|
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Current U.S.
Class: |
136/252 |
Current CPC
Class: |
Y02E 10/547 20130101;
H01L 31/022441 20130101; H01L 31/1872 20130101; H01L 31/022425
20130101; H01L 31/02363 20130101; H01L 31/0682 20130101; H01L
31/186 20130101; Y02P 70/50 20151101; Y02P 70/521 20151101; H01L
31/1804 20130101 |
Class at
Publication: |
136/252 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 11, 1995 |
JP |
07110121 |
Mar 27, 1995 |
JP |
07129865 |
Mar 27, 1995 |
JP |
07129864 |
Claims
1. A thin-film device comprising: a semiconductor film formed over
a substrate; and an n-type semiconductor film formed over the
semiconductor film, wherein the n-type semiconductor film contains
a metal element.
2. A thin-film device according to claim 1, wherein the
semiconductor film comprises a crystalline silicon.
3. A thin-film device according to claim 1, further comprising
silicon oxide between the substrate and the semiconductor film.
4. A thin-film device according to claim 1, wherein the metal
element comprises at least one selected from the group consisting
of nickel, iron, cobalt and platinum.
5. A thin-film device according to claim 1, wherein the thin-film
device is a solar cell.
6. A thin-film device comprising: a semiconductor film formed over
a substrate, wherein the semiconductor film contains a metal
element at a concentration not higher than 5.times.10.sup.18
atoms/cm.sup.3; and an n-type semiconductor film formed over the
semiconductor film, wherein the n-type semiconductor film contains
the metal element.
7. A thin-film device according to claim 6, wherein the
semiconductor film comprises a crystalline silicon.
8. A thin-film device according to claim 6, further comprising
silicon oxide between the substrate and the semiconductor film.
9. A thin-film device according to claim 6, wherein the metal
element comprises at least one selected from the group consisting
of nickel, iron, cobalt and platinum.
10. A thin-film device according to claim 6, wherein the thin-film
device is a solar cell.
11. A thin-film device comprising: a semiconductor film formed over
a substrate; and an n-type semiconductor film formed over the
semiconductor film, wherein the n-type semiconductor film contains
phosphorus and a metal element.
12. A thin-film device according to claim 11, wherein the
semiconductor film comprises a crystalline silicon.
13. A thin-film device according to claim 11, further comprising
silicon oxide between the substrate and the semiconductor film.
14. A thin-film device according to claim 11, wherein the metal
element comprises at least one selected from the group consisting
of nickel, iron, cobalt and platinum.
15. A thin-film device according to claim 11, wherein the thin-film
device is a solar cell.
16. A thin-film device comprising: a semiconductor film formed over
a substrate; an n-type semiconductor film formed over the
semiconductor film, wherein the n-type semiconductor film contains
a metal element; and a transparent electrode formed over the n-type
semiconductor film.
17. A thin-film device according to claim 16, wherein the
semiconductor film comprises a crystalline silicon.
18. A thin-film device according to claim 16, further comprising
silicon oxide between the substrate and the semiconductor film.
19. A thin-film device according to claim 16, wherein the metal
element comprises at least one selected from the group consisting
of nickel, iron, cobalt and platinum.
20. A thin-film device according to claim 16, wherein the
transparent electrode comprises indium tin oxide.
21. A thin-film device according to claim 16, wherein the thin-film
device is a solar cell.
22. A thin-film device comprising: a semiconductor film formed over
a substrate, wherein the semiconductor film contains a metal
element at a concentration not higher than 5.times.10.sup.18
atoms/cm.sup.3; an n-type semiconductor film formed over the
semiconductor film, wherein the n-type semiconductor film contains
the metal element; and a transparent electrode formed over the
n-type semiconductor film.
23. A thin-film device according to claim 22, wherein the
semiconductor film comprises a crystalline silicon.
24. A thin-film device according to claim 22, further comprising
silicon oxide between the substrate and the semiconductor film.
25. A thin-film device according to claim 22, wherein the metal
element comprises at least one selected from the group consisting
of nickel, iron, cobalt and platinum.
26. A thin-film device according to claim 22, wherein the
transparent electrode comprises indium tin oxide.
27. A thin-film device according to claim 22, wherein the thin-film
device is a solar cell.
28. A thin-film device comprising: a semiconductor film formed over
a substrate; an n-type semiconductor film formed over the
semiconductor film, wherein the n-type semiconductor film contains
phosphorus and a metal element; and a transparent electrode formed
over the n-type semiconductor film.
29. A thin-film device according to claim 28, wherein the
semiconductor film comprises a crystalline silicon.
30. A thin-film device according to claim 28, further comprising
silicon oxide between the substrate and the semiconductor film.
31. A thin-film device according to claim 28, wherein the metal
element comprises at least one selected from the group consisting
of nickel, iron, cobalt and platinum.
32. A thin-film device according to claim 28, wherein the
transparent electrode comprises indium tin oxide.
33. A thin-film device according to claim 28, wherein the thin-film
device is a solar cell.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a thin-film photoelectric
conversion device, especially a solar cell which is formed on a
substrate, and more particularly to a thin-film solar cell having a
photoelectric conversion layer formed of a crystalline silicon
film.
[0003] 2. Description of the Related Art
[0004] A solar cell or a solar battery can be manufactured using a
variety of semiconductor materials or organic compound materials.
However, from an industrial viewpoint, silicon is mainly used for
the solar cell. The solar cells using silicon can be classified
into a bulk solar cell using a wafer of monocrystal silicon or
polycrystal silicon and a thin-film solar cell having a silicon
film formed on a substrate. Reduction of manufacturing costs is
required, and the thin-film solar cell is expected to have the
effect of reducing the costs because less raw materials are used
for the thin-film solar cell than for the bulk solar cell.
[0005] In the field of thin-film solar cells, an amorphous silicon
solar cell has been placed into practical use. However, since the
amorphous silicon solar cell is lower in conversion efficiency
compared with the monocrystal silicon or polycrystal silicon solar
cell and also suffers from problems such as deterioration due to
light exposure and so on, the use thereof is limited. For that
reason, as another means, a thin-film solar cell using a
crystalline silicon film has been also developed.
[0006] A melt recrystallization method and a solid-phase growth
method are used for obtaining a crystalline silicon film in the
thin-film solar cell. In both the methods an amorphous silicon
layer is formed on a substrate and recrystallized, thereby
obtaining a crystalline silicon film. In any event, the substrate
is required to withstand the crystallization temperature, whereby
usable materials are limited. In particular, in the melt
recrystallization method, the substrate has been limited to a
material that withstands 1,412.degree. C., which is the melting
point of silicon.
[0007] The solid-phase growth method is a method in which an
amorphous silicon film is formed on the substrate and crystallized
thereafter through a heat treatment. In such a solid-phase growth
method, in general, as the temperature becomes high, the processing
time may be shortened more. However, the amorphous silicon film is
hardly crystallized at a temperature of 500.degree. C. or lower.
For example, when the amorphous silicon film which has been grown
through a gas-phase growth method is heated at 600.degree. C. so as
to be crystallized, 10 hours are required. Also, when the heat
treatment is conducted at the temperature of 550.degree. C., 100
hours or longer is required for the heat treatment.
[0008] For the above reason, a high heat resistance has been
required for the substrate of the thin-film solar cell. Therefore,
glass, carbon, or ceramic was used for the substrate. However, from
the viewpoint of reducing the costs of the solar cell, those
substrates are not always proper, and it has been desired that the
solar cell be fabricated on a substrate which is most generally
used and inexpensive. However, for example, the #7059 glass
substrate made by Corning, which is generally used, has a strain
point of 593.degree. C., and the conventional crystallization
technique allows the substrate to be strained and largely deformed.
For that reason, such a substrate could not be used. Also, since a
substrate made of a material essentially different from silicon is
used, monocrystal cannot be obtained even through crystallization
is conducted on the amorphous silicon film through the above means,
and silicon having large crystal grains is hard to obtain.
Consequently, this causes a limit to an improvement in the
efficiency of the solar cell.
[0009] In order to solve the above problems, a method of
crystallizing an amorphous silicon film through a heat treatment is
disclosed in U.S. Pat. No. 5,403,772. According to the method
disclosed in this patent, in order to accelerate crystallization at
a low temperature, a small amount of a metal element is added to
the amorphous silicon film as a catalyst material. Further, it is
therein disclosed that a lowering of the heat treatment temperature
and a reduction of the treatment time are enabled. Also, it is
disclosed therein that a simple elemental metal substance, e.g.
nickel (Ni), iron (Fe), cobalt (Co), or platinum (Pt), or a
compound of any one of those metals and silicon, or the like is
suitable for the catalyst material.
[0010] However, since the catalyst materials used for accelerating
crystallization are naturally undesirable for crystalline silicon,
it has been desired that the concentration of the catalyst material
is as low as possible. The concentration of catalyst material
necessary for accelerating crystallization was
1.times.10.sup.17/cm.sup.3 to 1.times.10.sup.20/cm.sup.3. However,
even when the concentration is relatively low, since the above
catalyst materials are heavy metal elements, the material contained
in silicon forms a defect level, thereby lowering the
characteristics of a fabricated element.
[0011] The principle of operation of a solar cell containing a p-n
junction can be roughly described as follows. The solar cell
absorbs light and generates electron/hole charge pairs due to
absorbed light energy. The electrons move toward the n-layer side
of the junction, and the holes move toward the p-layer side due to
drift caused by the junction electric field and diffusion. However,
when the defect levels are high in silicon, the charges are trapped
by the defect levels while they are moving in the silicon, thereby
disappearing. In other words, the photoelectric conversion
characteristics are lowered. The period of time from when the
electrons/holes are generated until they disappear is called the
"life time". In the solar cell, it is desirable that the lifetime
is long. Hence, it has been necessary to reduce as much as possible
the heavy metal elements that generate the defect levels in
silicon.
SUMMARY OF THE INVENTION
[0012] The present invention has been made in view of the above
circumstances, and therefore an object of the present invention is
to provide a method of manufacturing a thin-film solar cell, which
retains the feature of crystallization due to the above catalyst
material and removes the catalyst material after the
crystallization has been completed.
[0013] Another object of the present invention is to provide a
solar cell which has an excellent photoelectric conversion
characteristic, using the above method.
[0014] In accordance with the primary feature of the present
invention, a method of manufacturing a photoelectric conversion
device includes a step of forming a gettering layer on a
crystallized semiconductor layer obtained by using a catalyst metal
such as nickel. The gettering layer may be either insulative or
semiconductive and contains phosphorus to absorb the catalyst metal
such as nickel from the semiconductor layer after it is
crystallized, thereby reducing the concentration of the catalyst
metal in the semiconductor layer. Specifically, the method includes
the steps of: [0015] disposing a metal containing layer in contact
with an upper or lower surface of a non-single crystalline silicon
semiconductor layer; [0016] crystallizing the non-single
crystalline silicon semiconductor layer by heating, wherein the
metal functions to promote the crystallization; [0017] forming a
gettering layer on or within said semiconductor layer after
crystallized, the gettering layer containing phosphorus; and [0018]
heating said semiconductor layer and the gettering layer in order
to getter the metal contained in the semiconductor layer.
[0019] As the metal element, it is possible to use one or more
elements chosen from Ni, Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, and
Au.
[0020] In accordance with a preferred embodiment of the invention,
the gettering layer may be a silicon layer to which phosphorus is
added during the deposition thereof onto the crystallized
semiconductor layer. In an alternative, the gettering layer may be
a phosphorus doped region formed within the crystallized
semiconductor layer, namely, a method of the present invention
includes a step of introducing phosphorus ions into a surface
region of the crystallized semiconductor layer by ion doping after
crystallizing the semiconductor layer by the use of the catalyst
metal. In a further alternative, the gettering layer may be a
phospho-silicate glass (PSG) layer deposited on the crystallized
semiconductor layer.
[0021] In accordance with another aspect of the invention, the
catalyst metal is provided by disposing the metal containing layer
in contact with an upper or lower surface of a non-single
crystalline semiconductor layer to be crystallized. In the case of
disposing the metal containing layer under the non-single
crystalline semiconductor layer, the metal containing layer may be
used also as a lower electrode of the photoelectric conversion
device.
[0022] In accordance with still another aspect of the invention, a
solar cell comprises a substrate, a first crystalline silicon film
having conductivity type formed on the substrate, and a second
crystalline silicon film having another conductivity type adjacent
to the first crystalline silicon film, wherein the first
crystalline silicon film contains a catalyst element for promoting
crystallization of silicon at a concentration not higher than
5.times.10.sup.18 atoms/cm.sup.3. The concentration value disclosed
in the present invention is determined by secondary ion mass
spectroscopy and corresponds to a maximum value of the measured
values.
[0023] In accordance with a further aspect of the invention, in the
above mentioned solar cell, the concentration of the catalyst
contained in the second crystalline silicon film is higher than the
concentration of the catalyst contained in the first crystalline
silicon film.
[0024] In accordance with a still further aspect of the invention,
the crystalline semiconductor film obtained by using the catalyst
metal such as nickel has a plurality of crystal grains in the form
of needles.
[0025] According to the present invention, the lifetime of carriers
in the crystalline silicon film is increased, and the excellent
characteristics of the thin-film solar cell are obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The above and other objects and features of the present
invention will be more apparent from the following description
taken in conjunction with the accompanying drawings.
[0027] FIGS. 1A to 1D are schematic diagrams showing a method of
manufacturing a thin-film solar cell in accordance with the present
invention;
[0028] FIGS. 2A to 2D are schematic diagrams showing a method of
manufacturing a thin-film solar cell in accordance with the present
invention;
[0029] FIG. 3 is a diagram showing an example of a cross-sectional
structure of a thin-film solar cell in accordance with the present
invention; and
[0030] FIG. 4 is a diagram showing an example of a cross-sectional
structure of a thin-film solar cell in accordance with the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] Now, a description will be given in more detail of
embodiments of the present invention with reference to the
accompanying drawings.
First Embodiment
[0032] The first embodiment shows a process of manufacturing a
thin-film solar cell through a method of forming an amorphous
silicon film in close contact with a metal element that accelerates
the crystallization of silicon, crystallizing said amorphous
silicon film through a heat treatment, and removing said metal
element remaining in the amorphous silicon film after the
crystallization.
[0033] This embodiment will be described with reference to FIGS. 1A
to 1D. In this embodiment, nickel is used as a metal element having
a catalyst action that accelerates the crystallization of silicon.
First, a silicon oxide film 102 having a thickness of 0.3 .mu.m is
formed on a glass substrate (for example, Corning 7059 glass
substrate) 101 as an underlying layer. The silicon oxide film 102
is formed through a plasma CVD technique using tetra ethoxy silane
(TEOS as a raw material), and also can be formed through a
sputtering technique as another method. Subsequently, an amorphous
silicon film 103 is formed using silane gas as a raw material
through a plasma CVD technique. The formation of the amorphous
silicon film 103 may be conducted using a low pressure thermal CVD
technique, a sputtering technique, or an evaporation method. The
above amorphous silicon film 103 may be a substantially-intrinsic
amorphous silicon film or may contain boron (B) at 0.001 to 0.1
atms %. Also, the thickness of the amorphous silicon film 103 is
set at 10 .mu.m. However, the thickness may be set at a required
value (FIG. 1A).
[0034] Subsequently, the substrate is immersed in an ammonium
hydroxide, hydrogen peroxide mixture and then held at 70.degree. C.
for 5 minutes, to thereby form an oxide film (not shown) on the
surface of the amorphous silicon film 103. The silicon oxide film
is formed in order to improve wettability in the next step process
of coating with a nickel acetate solution. The nickel acetate
solution is coated on the surface of the amorphous silicon film 103
by spin coating. The nickel functions as an element that
accelerates the crystallization of the amorphous silicon film
103.
[0035] Subsequently, the amorphous silicon film 103 is held at a
temperature of 450.degree. C. for one hour in a nitrogen
atmosphere, thereby eliminating hydrogen from the amorphous silicon
film 103. This is because dangling bonds are intentionally produced
in the amorphous silicon film, to thereby lower the threshold
energy in subsequent crystallizing. Then, the amorphous silicon
film 103 is subjected to a heat treatment at 550.degree. C. for 4
to 8 hours in the nitrogen atmosphere, to thereby crystallize the
amorphous silicon film 103. The temperature during crystallizing
can be set to 550.degree. C. because of the action of the nickel.
0.001 atms % to 5 atms % hydrogen is contained in crystallized
silicon film 104. During the above heat treatment, nickel
accelerates the crystallization of the silicon film while it is
moving in the silicon film.
[0036] In this way, the crystalline silicon film 104 is formed on
the glass substrate. Subsequently, a phospho-silicate glass (PSG)
105 is formed on the crystalline silicon film 104. The
phospho-silicate glass (PSG) 105 is formed, using a gas mixture
consisting of silane, phosphine, and oxygen, at a temperature of
450.degree. C. through an atmospheric CVD technique. The
concentration of phosphorus in the phospho-silicate glass is set to
1 to 30 wt %, preferably 7 wt %. The phospho-silicate glass (PSG)
105 is used to getter nickel remaining in the crystalline silicon
film. Even though the phospho-silicate glass 105 is formed at only
450.degree. C., its effect is obtained. More effectively, the
phospho-silicate glass 105 may be subjected to a heat treatment at
a temperature of 500 to 800.degree. C., preferably 550.degree. C.
for 1 to 4 hours in a nitrogen atmosphere. As another method, the
phospho-silicate glass 105 can be replaced by a silicon film to
which phosphorus of 0.1 to 10 wt % has been added with the same
effect (FIG. 1B).
[0037] Thereafter, the phospho-silicate glass 105 is etched using
an aqueous hydrogen fluoride solution so as to be removed from the
surface of the crystalline silicon film 104. As a result, the
surface of the crystalline silicon film 104 is exposed. On that
surface there is formed an n-type crystalline silicon film 106. The
n-type crystalline silicon film 106 may be formed through a plasma
CVD technique or through a low pressure thermal CVD technique. The
n-type crystalline silicon film 106 is desirably formed at a
thickness of 0.02 to 0.2 .mu.m, and in this embodiment, it is
formed at a thickness of 0.1 .mu.m (FIG. 1C).
[0038] Then, a transparent electrode 107 is formed through a
sputtering technique on the above n-type crystalline silicon film
106. The transparent electrode 107 is made of indium tin oxide
alloy (ITO) and has a thickness of 0.08 .mu.m. Finally, a process
of providing output electrodes 103 is conducted. In providing the
output electrodes 108, as shown in FIG. 1D, a negative side
electrode is disposed on the transparent electrode 107, and a
positive side electrode is disposed on the crystalline silicon film
104 by removing parts of the transparent electrode 107, the n-type
crystalline silicon film 106, and the crystalline silicon film 104.
The output electrodes 108 can be formed by sputtering or vacuum
evaporation, or using an aluminum or silver paste or the like.
Furthermore, after the provision of the output electrodes 108, the
product is subjected to a heat treatment at 150.degree. C. to
300.degree. C. for several minutes with the result that the
adhesion between the output electrodes 108 and the underlying layer
becomes high, thereby obtaining an excellent electric
characteristic. In this embodiment, the product is subjected to a
heat treatment at 200.degree. C. for 30 minutes in a nitrogen
atmosphere using an oven.
[0039] Through the above-mentioned processes, a thin-film solar
cell is completed.
Second Embodiment
[0040] In a second embodiment, there is described a thin-film solar
cell which is formed in a process where a metal element that
accelerates the crystallization of crystalline silicon is removed
after crystallization, through a method where phosphorus is
implanted into the surface of the crystalline silicon film via a
plasma doping method.
[0041] The second embodiment will be described with reference to
FIGS. 2A to 2D. Nickel is used in this embodiment as a metal
element functioning as a catalyst to accelerate the crystallization
to accellerate the crystallization of silicon. First, a silicon
oxide film 202 having a thickness of 0.3 .mu.m is formed on a glass
substrate (for example, Corning 7059 glass substrate) 201 as an
underlying layer. The silicon oxide film 202 is formed by plasma
CVD with tetra ethoxy silane (TEOS as a raw material), and also can
be formed through a sputtering technique as another method.
Subsequently, an amorphous silicon film 203 is formed with silane
gas as a raw material through a plasma CVD technique. The formation
of the amorphous silicon film 203 may be conducted using a low
pressure thermal CVD technique, a sputtering technique, or an
evaporation method. The above amorphous silicon film 203 may be a
substantially-intrinsic amorphous silicon film or an amorphous
silicon film to which boron (B) of 0.001 to 0.1 atms % has been
added. Also, the thickness of the amorphous silicon film 203 is set
at 20 .mu.m. However, the thickness may be set at any required
value (FIG. 2A).
[0042] Thereafter, the substrate is immersed in an ammonium
hydroxide, hydrogen peroxide mixture at 70.degree. C. for 5
minutes, to thereby form an oxide film (not shown) on the surface
of the amorphous silicon film 203. The silicon oxide film is formed
in order to improve wettability in the next step of coating with a
nickel acetate solution. The nickel acetate solution is coated on
the surface of the amorphous silicon film 203 by spin coating. The
nickel element functions as an element that accelerates the
crystallization of the amorphous silicon film 203.
[0043] Subsequently, the amorphous silicon film 203 is held at a
temperature of 450.degree. C. for one hour in a nitrogen
atmosphere, thereby eliminating hydrogen from the amorphous silicon
film 203. This is because dangling bonds are intentionally produced
in the amorphous silicon film, to thereby lower the threshold
energy in subsequent crystallizing. Then, the amorphous silicon
film 203 is subjected to a heat treatment at 550.degree. C. for 4
to 8 hours in a nitrogen atmosphere, to thereby crystallize the
amorphous silicon film 203. The temperature during crystallizing
can be set to 550.degree. C. because of the action of the nickel.
0.001 atms % to 5 atms % hydrogen is contained in a crystallized
silicon film 204. During the above heat treatment, nickel
accelerates the crystallization of the silicon film 204 while it is
moving in the silicon film.
[0044] In this way, the crystalline silicon film 204 can be formed
on the glass substrate. In this state, the implantation of
phosphorus (P) ions is conducted by a plasma doping method. The
dose amount may be set to 1.times.10.sup.14 to
1.times.10.sup.17/cm.sup.2, and in this embodiment, it is set to
1.times.10.sup.16/cm.sup.2. The accelerating voltage is set to 20
keV. Through this process, a layer containing phosphorus at a high
concentration is formed within a region of 0.1 to 0.2 .mu.m
depthwise from the surface of the crystalline silicon film 204.
Thereafter, a heat treatment is conducted on the crystalline
silicon film 204 in order to getter nickel remaining in the
crystalline silicon film 204. The crystalline silicon film 204 may
be subjected to a heat treatment at 500 to 800.degree. C.,
preferably 550.degree. C. for 1 to 4 hours in a nitrogen atmosphere
(FIG. 2B).
[0045] In the crystalline silicon film 204, since the region into
which phosphorus ions have been implanted has its crystallinity
destroyed, it becomes of a substantially amorphous structure
immediately after the ions have been implanted thereinto.
Thereafter, since that region is crystallized through said heat
treatment, it is usable as the n-type layer of the solar cell even
in this state. In this case, the concentration of nickel in the
i-type or p-type layer 204 is lower than in the phosphorus doped
n-type layer.
[0046] In accordance with a preferred embodiment of the invention,
the phosphorus implanted region is more desirably removed since
nickel that has functioned as a catalyst element is segregated in
this region. As the removing method, after a thin natural oxide
film on the surface has been etched using an aqueous hydrogen
fluoride aqueous solution, it is removed via dry etching using
sulfur hexafluoride and nitric trifluoride. With this process, the
surface of the crystalline silicon film 204 is exposed. An n-type
crystalline silicon film 205 is formed on that surface. The n-type
crystalline silicon film 205 may be formed by plasma CVD or low
pressure thermal CVD. The n-type crystalline silicon film 205 is
desirably formed at a thickness of 0.02 to 0.2 .mu.m, and in this
embodiment, it is formed at a thickness of 0.1 .mu.m (FIG. 2C).
[0047] Then, a transparent electrode 206 is formed via a sputtering
technique on the above n-type crystalline silicon film 205. The
transparent electrode 206 is made of indium tin oxide alloy (ITO)
and has a thickness of 0.08 .mu.m. Finally, a process of providing
output lead electrodes 207 is conducted. In providing the output
electrodes 207, as shown in FIG. 2D, a negative side electrode is
disposed on the transparent electrode 206, and a positive side
electrode is disposed on the crystalline silicon film 204 by
removing parts of the transparent electrode 206, the n-type
crystalline silicon film 205, and the crystalline silicon film 204.
The output electrodes 207 can be formed through a sputtering
technique or an evaporation method, or by using aluminum or silver
paste or the like. Furthermore, after the provision of the output
lead electrodes 207, the substrate is subjected to a heat treatment
at 150.degree. C. to 300.degree. C. for several minutes with the
result that the adhesion between the output electrodes 207 and the
underlying layer becomes high, thereby obtaining excellent electric
characteristics. In this embodiment, the substrate is subjected to
a heat treatment at 200.degree. C. for 30 minutes in a nitrogen
atmosphere using an oven.
[0048] Through the above-mentioned processes, a thin-film solar
cell is completed.
Third Embodiment
[0049] A third embodiment shows an example where in the process of
manufacturing the thin-film solar cell described with reference to
the first and second embodiments, the surface of the crystalline
silicon film is subjected to an anisotropic etching process so as
to make the surface of the solar cell irregular as shown in FIG. 3.
A technique by which that surface is made irregular so that
reflection from the surface of the solar cell is reduced is called
a "texture technique".
[0050] A silicon oxide film 302 having a thickness of 0.3 .mu.m is
formed on a glass substrate (for example, Corning 7059 glass
substrate) 301 as an underlying layer. The silicon oxide film 302
is formed by plasma CVD with tetra ethoxy silane (TEOS as a raw
material), and also can be formed by sputtering as another method.
Subsequently, an amorphous silicon film is formed by plasma CVD.
The formation of the amorphous silicon film may be conducted by low
pressure thermal CVD, sputtering, evaporation, or the like. The
above amorphous silicon film 303 may be a substantially-intrinsic
amorphous silicon film or an amorphous silicon film to which of
0.001 to 0.1 atms % boron (B) has been added. Also, the thickness
of the amorphous silicon film is set at 20 .mu.m. However, the
thickness may be set at any required value.
[0051] Subsequently, the substrate is immersed in an ammonium
hydroxide and hydrogen peroxide mixture and then held at 70.degree.
C. for 5 minutes, to thereby form an oxide film on the surface of
the amorphous silicon film. The silicon oxide film is formed in
order to improve wettability in the next step of coating nickel
acetate solution. The nickel acetate solution is coated on the
surface of the amorphous silicon film by spin coating. The nickel
functions as an element that accelerates the crystallization of the
amorphous silicon film.
[0052] Subsequently, the amorphous silicon film is held at a
temperature of 450.degree. C. for one hour in a nitrogen
atmosphere, thereby eliminating hydrogen from the amorphous silicon
film. This is because dangling bonds are intentionally produced in
the amorphous silicon film, to thereby lower the threshold energy
in subsequent crystallizing. Then, the amorphous silicon film is
subjected to a heat treatment at 550.degree. C. for 4 to 8 hours in
a nitrogen atmosphere, to thereby crystalline the amorphous silicon
film to obtain a crystalline silicon film 303. The temperature
during crystallizing can be set to 550.degree. C. because of the
action of nickel. 0.001 atms % to 5 atms % of hydrogen is contained
in the crystalline silicon film 303. During the above heat
treatment, nickel accelerates the crystallization of the silicon
film 303 while it is moving in the silicon film.
[0053] In this way, the crystalline silicon film 303 can be formed
on the glass substrate. Then, a gettering process is conducted on
the crystalline silicon film 304 in order to remove nickel
remaining in the crystalline silicon film 304. A method of
conducting the gettering process may include forming a
phospho-silicate glass (PSG) on the crystalline silicon film 303,
or implanting phosphorus ions into the surface of the crystalline
silicon film 303.
[0054] In the method comprising of forming the phospho-silicate
glass (PSG), the phospho-silicate glass film is formed via
atomspheric CVD, using a gas mixture consisting of silane,
phosphine, and oxygen, at a temperature of 450.degree. C. The
gettering process is then conducted by subjecting the crystalline
silicon film to a heat treatment at 550.degree. C. for 1 to 4 hours
in a nitrogen atmosphere. Thereafter, the phospho-silicate glass
film is desirably etched using an aqueous hydrogen fluoride aqueous
solution so as to be removed.
[0055] In the method comprising implanting phosphorus ions into the
surface of the crystalline silicon film, the implantation of ions
can be conducted through plasma doping. The dose amount may be set
to 1.times.10.sup.14 to 1.times.10.sup.17/cm.sup.2, and in this
embodiment, it is set to 1.times.10.sup.16/cm.sup.2. The
accelerating voltage is set to 20 keV. Through this process, a
layer containing phosphorus at a high concentration is formed
within a region of 0.1 to 0.2 .mu.m depthwise from the surface of
the crystalline silicon film. Thereafter, a heat treatment is
conducted on the crystalline silicon film in order to getter nickel
remaining in the crystalline silicon film. The heat treatment is
conducted at a temperature of 500 to 800.degree. C., preferably
550.degree. C. for 1 to 4 hours in a nitrogen atmosphere.
[0056] After the gettering process has been completed, a texture
process is conducted on the surface of the crystalline silicon
film. The texture process can be conducted using hydrazine or
sodium hydroxide aqueous solution. Hereinafter, a case of using
sodium hydroxide will be described.
[0057] The texture process is conducted by heating a 2% aqueous
solution of sodium hydroxide to 80.degree. C. Under this condition,
the etching rate of the crystalline silicon film thus obtained in
this embodiment is about 1 .mu.m/min. The etching is conducted for
five minutes, and thereafter the crystalline silicon film is
immersed in boiling water in order to immediately cease the
reaction and then the film is sufficiently cleaned by flowing
water. As a result of observing the surface of the crystalline
silicon film which has been subjected to the texture process
through an electron microscope, an unevenness of about 0.1 to 5
.mu.m is found on the surface although it is at random.
[0058] An n-type crystalline silicon film 304 is formed on that
surface. The n-type crystalline silicon film 304 may be formed
through a plasma CVD technique or through a low pressure thermal
CVD technique. The n-type crystalline silicon film 304 is desirably
formed at a thickness of 0.02 to 0.2 .mu.m, and in this embodiment,
it is formed at a thickness of 0.1 .mu.m.
[0059] Then, a transparent electrode 305 is formed by sputtering on
the above n-type crystalline silicon film 304. The transparent
electrode 305 is made of indium tin oxide alloy (ITO) and has a
thickness of 0.08 .mu.m. Finally, a process of providing output
electrodes 307 is conducted. In providing the output electrodes
307, as shown by the structure in FIG. 3D, a negative side
electrode is disposed on the transparent electrode 305, and a
positive side electrode is disposed on the crystalline silicon film
303 by removing parts of the transparent electrode 305, the n-type
crystalline silicon film 304, and the crystalline silicon film 303.
The output electrodes 306 can be formed by sputtering or vacuum
evaporation, or using an aluminum or silver paste or the like.
Furthermore, after the provision of the output lead electrodes 307,
the entire structure is subjected to a heat treatment at
150.degree. C. to 300.degree. C. for several minutes with the
result that the adhesion between the lead electrodes 207 and the
underlying layers becomes high, thereby obtaining excellent
electric characteristics. In this embodiment, the heat treatment
was conducted at 200.degree. C. for 30 minutes in a nitrogen
atmosphere using an oven.
[0060] Through the above-mentioned processes, a thin-film solar
cell having the texture structure on the surface is completed.
Fourth Embodiment
[0061] A fourth embodiment is a process of manufacturing a
thin-film solar cell, as shown in FIG. 4, in which a coating of a
metal element that accelerates the crystallization of silicon is
formed on a substrate, an amorphous silicon film is formed on the
coating of metal element, the amorphous silicon film is
crystallized through a heat treatment, and after crystallization,
the metal element diffused in the silicon film is removed
therefrom.
[0062] First, a coating of the metal element that accelerates the
crystallization of silicon is formed on a substrate. Nickel is used
as the metal element. A silicon oxide film having a thickness of
0.3 .mu.m is first formed on a glass substrate (for example,
Corning 7059 glass substrate) 401 as an underlying layer 402. The
silicon oxide film is formed through a plasma CVD technique with of
tetra ethoxy silane (TEOS) as a raw material, and also can be
formed through a sputtering technique as another method.
Subsequently, a nickel film 407 is formed on the substrate. The
nickel film 407 having 0.1 .mu.m is formed through an electron beam
evaporation method using a tablet made of pure nickel. Then, an
amorphous silicon film is formed through a plasma CVD technique.
The formation of the amorphous silicon film may be conducted
through low pressure thermal CVD, sputtering, evaporation, or the
like. The above amorphous silicon film may be a
substantially-intrinsic amorphous silicon film or an amorphous
silicon film to which 0.001 to 0.1 atms % boron (B) has been added.
Also, the thickness of the amorphous silicon film is set at 10
.mu.m. However, the thickness may be set at any required value.
[0063] Subsequently, the amorphous silicon film is held at a
temperature of 450.degree. C. for one hour in a nitrogen
atmosphere, thereby eliminating hydrogen from the amorphous silicon
film. This is because dangling bonds are intentionally produced in
the amorphous silicon film, to thereby lower the threshold energy
in subsequent crystallizing. Then, the amorphous silicon film is
subjected to a heat treatment at 550.degree. C. for 4 to 8 hours in
a nitrogen atmosphere, to thereby crystallize the amorphous silicon
film to obtain a crystalline silicon film 403. The temperature
during crystallizing can be set to 550.degree. C. because of the
action of the nickel. 0.001 atms % hydrogen to 5 atms % is
contained in a the crystalline silicon film 403. During the above
heat treatment, a small amount of nickel diffuses into the silicon
film from the nickel film disposed under the amorphous silicon
film, and accelerates the crystallization of the crystalline
silicon film 403 while it is moving in the silicon film.
[0064] In this way, the crystalline silicon film 403 is formed on
the glass substrate. Subsequently, a phospho-silicate glass (PSG)
is formed on the crystalline silicon film 403. The phospho-silicate
glass (PSG) is formed by atmospheric CVD gas, using a mixture
consisting of silane, phosphine, and oxygen, at a temperature of
450.degree. C. The concentration of phosphorus in the
phospho-silicate glass is set to 1 to 30 wt %, preferably 7 wt %.
The phospho-silicate glass is used to getter nickel remaining in
the crystalline silicon film. Even though the phospho-silicate
glass is formed at only 450.degree. C., its effect is obtained.
More effectively, the phospho-silicate glass may be subjected to a
heat treatment at a temperature of 500 to 800.degree. C.,
preferably 550.degree. C. for 1 to 4 hours in the nitrogen
atmosphere. As another method, the phospho-silicate glass can be
replaced with the same effect by a silicon film to which 0.1 to 10
wt % phosphorus has been added with the same effect.
[0065] Thereafter, the phospho-silicate class is etched using an
aqueous hydrogen fluoride solution so as to be removed from the
surface of the crystalline silicon film. As a result, the surface
of the crystalline silicon film 403 is exposed. An n-type
crystalline silicon film 404 is formed on that surface. The r-type
crystalline silicon film 404 may be formed by plasma CVD or low
pressure thermal CVD. The n-type crystalline silicon film 404 is
desirably formed at a thickness of 0.02 to 0.2 .mu.m, and in this
embodiment, it is formed at a thickness of 0.1 .mu.m.
[0066] Then, a transparent electrode 405 is formed by sputtering on
the above n-type crystalline silicon film 404. The transparent
electrode 405 is made of indium tin oxide alloy (ITO) and has a
thickness of 0.08 .mu.m. Finally, a process of providing output
electrodes 406 is conducted. In providing the output electrodes, as
shown in FIG. 4, a negative side electrode is disposed on the
transparent electrode 405, and a positive side electrode is
disposed on the crystalline silicon film 403 by removing parts of
the transparent electrode 405, the n-type crystalline silicon film
404 and the crystalline silicon film 403. The output electrodes 406
can be formed by sputtering or vacuum evaporation, or by using
aluminum or silver paste or the like. Furthermore, after the
provision of the output electrodes, the substrate is subjected to a
heat treatment at 150.degree. C. to 300.degree. C., for example at
200.degree. C. for 30 minutes in a nitrogen atmosphere, with the
result that the adhesion between the output electrodes and the
underlying layer becomes high, thereby obtaining excellent electric
characteristics.
[0067] Through the above-mentioned processes, a thin-film solar
cell is completed.
[0068] As was described above, in the method of manufacturing the
thin-film solar cell in accordance with the present invention, in a
process of crystallizing an amorphous silicon film by a heat
treatment, a catalyst material such as nickel is used, thereby
making it possible to obtain a crystalline silicon film at a heat
treatment temperature lower than in the conventional methods.
Furthermore, the method of the present invention enables the
concentration of the catalyst material remaining in the crystalline
silicon film obtained to be lowered. As a result, a thin-film solar
cell that uses an inexpensive glass substrate and is excellent in
photoelectric conversion characteristic can be obtained.
[0069] The foregoing description of a preferred embodiment of the
invention has been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and modifications and
variations are possible in light of the above teachings or may be
acquired from practice of the invention. The embodiment was chosen
and described in order to explain the principles of the invention
and its practical application to enable one skilled in the art to
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto, and their equivalents.
* * * * *