U.S. patent application number 09/824041 was filed with the patent office on 2001-08-09 for method for forming a zinc oxide layer and method for producing a photovoltaic device.
Invention is credited to Arao, Kozo, Miyamoto, Yusuke, Sonoda, Yuichi, Tamura, Hideo, Toyama, Noboru.
Application Number | 20010012569 09/824041 |
Document ID | / |
Family ID | 26362776 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012569 |
Kind Code |
A1 |
Arao, Kozo ; et al. |
August 9, 2001 |
METHOD FOR FORMING A ZINC OXIDE LAYER AND METHOD FOR PRODUCING A
PHOTOVOLTAIC DEVICE
Abstract
Provided are a substrate with a zinc oxide layer, in which at
least a zinc oxide layer is provided on a support substrate,
wherein the zinc oxide layer comprises a zinc oxide layer having
the c axis perpendicular to the support substrate and a zinc oxide
layer having the c axis slantindicular to the support substrate in
the order from the side of the support substrate; and a
photovoltaic device in which a semiconductor layer is formed on the
substrate with the zinc oxide layer. Thus provided is the
inexpensive photovoltaic device with excellent reflective
performance and optical confinement effect and with high
photoelectric conversion efficiency.
Inventors: |
Arao, Kozo; (Nara-shi,
JP) ; Tamura, Hideo; (Nara-shi, JP) ; Toyama,
Noboru; (Osaka, JP) ; Sonoda, Yuichi;
(Nara-shi, JP) ; Miyamoto, Yusuke; (Kyotanabe-shi,
JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
26362776 |
Appl. No.: |
09/824041 |
Filed: |
April 3, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09824041 |
Apr 3, 2001 |
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09233328 |
Jan 20, 1999 |
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6238808 |
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Current U.S.
Class: |
428/632 ;
257/E31.126; 257/E31.13; 428/336; 428/472; 428/623 |
Current CPC
Class: |
Y10S 428/935 20130101;
H01L 31/056 20141201; H01L 31/022483 20130101; Y10T 428/1259
20150115; Y10T 428/12764 20150115; Y10T 428/12618 20150115; Y10T
428/1275 20150115; Y10T 428/12611 20150115; Y10T 428/12743
20150115; Y10T 428/12757 20150115; Y10T 428/12639 20150115; H01L
31/02366 20130101; Y10T 428/12549 20150115; Y10T 428/12979
20150115; Y10T 428/12986 20150115; Y10T 428/265 20150115; Y02P
70/50 20151101; H01L 31/1884 20130101; H01L 31/206 20130101; Y02E
10/52 20130101; C25D 9/08 20130101 |
Class at
Publication: |
428/632 ;
428/472; 428/336; 428/623 |
International
Class: |
B21D 039/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 23, 1998 |
JP |
10-025180 |
Jan 23, 1998 |
JP |
10-025181 |
Claims
What is claimed is:
1. A substrate with a zinc oxide layer, in which at least the zinc
oxide layer is provided on a support substrate, wherein the zinc
oxide layer comprises a zinc oxide layer having the c axis
perpendicular to the support substrate and a zinc oxide layer
having the c axis slantindicular to the support substrate in the
order from the side of the support substrate.
2. The substrate with the zinc oxide layer according to claim 1,
wherein the support substrate is an electrically conductive
substrate.
3. The substrate with the zinc oxide layer according to claim 1,
wherein a metal layer is interposed between the support substrate
and the zinc oxide layer.
4. The substrate with the zinc oxide layer according to claim 3,
wherein the metal layer is a metal aluminum layer.
5. The substrate with the zinc oxide layer according to claim 4,
wherein an aluminum oxide layer is interposed between the metal
aluminum layer and the zinc oxide layer.
6. The substrate with the zinc oxide layer according to claim 5,
wherein the aluminum oxide layer is formed by oxidizing the metal
aluminum layer by an oxygen plasma process.
7. The substrate with the zinc oxide layer according to claim 1,
wherein an average of inclination angles of crystal grains in a
surface of the zinc oxide layer having the c axis slantindicular to
the support substrate on an opposite side to the support substrate
is not less than 15.degree..
8. The substrate with the zinc oxide layer according to claim 1,
wherein surface roughness Ra of a surface of the zinc oxide layer
having the c axis slantindicular to the support substrate on an
opposite side to the support substrate is not more than 80 nm.
9. The substrate with the zinc oxide layer according to claim 1,
wherein a total reflectance of light incident from the side of the
zinc oxide layer is not less than 60% at 800 nm.
10. The substrate with the zinc oxide layer according to claim 1,
wherein a scattering reflectance of light incident from the side of
the zinc oxide layer is not less than 20% at 800 nm.
11. The substrate with the zinc oxide layer according to claim 1,
wherein a thickness of the zinc oxide layer having the c axis
slantindicular to the support substrate is not less than 5000
.ANG..
12. The substrate with the zinc oxide layer according to claim 1,
wherein the support substrate is an SUS sheet.
13. The substrate with the zinc oxide layer according to claim 12,
wherein the SUS sheet has a 2D-surface.
14. The substrate with the zinc oxide layer according to claim 12,
wherein the SUS sheet is of a long rolled form.
15. The substrate with the zinc oxide layer according to claim 4,
wherein a thickness of the metal layer is 1000 .ANG. to 2500
.ANG..
16. The substrate with the zinc oxide layer according to claim 1,
wherein a thickness of the zinc oxide layer having the c axis
perpendicular to the support substrate is 1500 .ANG. to 2500
.ANG..
17. The substrate with the zinc oxide layer according to claim 1,
wherein electric resistance in a direction normal to a surface of
the support substrate is not more than 20 .OMEGA./cm.sup.2.
18. A method for forming a zinc oxide layer, comprising: forming on
a substrate a zinc oxide layer having the c axis perpendicular to
the substrate by a sputtering method; and forming a zinc oxide
layer having the c axis slantindicular to the substrate on the zinc
oxide layer having the c axis perpendicular to the substrate, by an
electrodeposition method.
19. The forming method of the zinc oxide layer according to claim
18, wherein the substrate is an electrically conductive
substrate.
20. The forming method of the zinc oxide layer according to claim
18, wherein a temperature of the substrate is set at 380.degree. C.
or less in the step of forming the zinc oxide layer having the c
axis perpendicular to the substrate.
21. The forming method of the zinc oxide layer according to claim
18, wherein a metal aluminum layer is formed on the substrate by
sputtering and thereafter the zinc oxide layer having the c axis
perpendicular to the substrate is formed on the metal aluminum
layer.
22. The forming method of the zinc oxide layer according to claim
21, wherein a temperature of the substrate is set at 100.degree. C.
or less in the step of forming the metal aluminum layer.
23. The forming method of the zinc oxide layer according to claim
21, wherein after the step of forming the metal aluminum layer, an
aluminum oxide layer is formed by oxidizing a surface of the metal
aluminum layer by an oxygen plasma.
24. The forming method of the zinc oxide layer according to claim
18, wherein the substrate is an SUS substrate rolled in a long roll
form.
25. The forming method of the zinc oxide layer according to claim
18, wherein the zinc oxide layer having the c axis perpendicular to
the substrate is formed in a thickness of 1500 .ANG. to 2500
.ANG..
26. The forming method of the zinc oxide layer according to claim
18, wherein a temperature of the substrate in the step of forming
the zinc oxide layer having the c axis perpendicular to the
substrate is so set that an average grain size of crystal grains
observed in a surface of the zinc oxide layer having the c axis
slantindicular to the substrate on an opposite side to the
substrate is not more than 2 .mu.m and that a scattering
reflectance of light incident from the side of the zinc oxide layer
having the c axis slantindicular to the substrate is not less than
20% at 800 nm.
27. The forming method of the zinc oxide layer according to claim
18, wherein a temperature of the substrate in the step of forming
the zinc oxide layer having the c axis perpendicular to the
substrate is so set that surface roughness Ra of a surface of the
zinc oxide layer having the c axis slantindicular to the substrate
on an opposite side to the substrate is not more than 80 nm and
that an average of inclination angles of crystal grains of the
surface is not less than 15.degree..
28. The forming method of the zinc oxide layer according to claim
18, wherein the zinc oxide layer having the c axis slantindicular
to the substrate is formed in a thickness of not less than 5000
.ANG..
29. The forming method of the zinc oxide layer according to claim
18, wherein the zinc oxide layer having the c axis slantindicular
to the substrate is formed by electrodeposition using a zinc
nitrate solution having a concentration of not less than 0.15
mol/l.
30. The forming method of the zinc oxide layer according to claim
23, wherein power of the oxygen plasma in the oxygen plasma process
is so set that a total reflectance of light incident from the side
of the zinc oxide layer having the c axis slantindicular to the
substrate is not less than 60% and that electric resistance in a
direction normal to a surface of the substrate is not more than 20
.OMEGA./cm.sup.2.
31. A photovoltaic device comprising a substrate with a zinc oxide
layer in which at least a zinc oxide layer is provided on a support
substrate, and a semiconductor layer, wherein the zinc oxide layer
comprises a zinc oxide layer having the c axis perpendicular to the
support substrate and a zinc oxide layer having the c axis
slantindicular to the support substrate in the order from the side
of the support substrate.
32. The photovoltaic device according to claim 31, wherein the
support substrate is an electrically conductive substrate.
33. The photovoltaic device according to claim 31, wherein a metal
layer is interposed between the support substrate and the zinc
oxide layer.
34. The photovoltaic device according to claim 33, wherein the
metal layer is a metal aluminum layer.
35. The photovoltaic device according to claim 34, wherein an
aluminum oxide layer is interposed between the metal aluminum layer
and the zinc oxide layer.
36. The photovoltaic device according to claim 35, wherein the
aluminum oxide layer is formed by oxidizing the metal aluminum
layer by an oxygen plasma process.
37. The photovoltaic device according to claim 31, wherein an
average of inclination angles of crystal grains in a surface of the
zinc oxide layer having the c axis slantindicular to the support
substrate on an opposite side to the support substrate is not less
than 15.degree..
38. The photovoltaic device according to claim 31, wherein surface
roughness Ra of a surface of the zinc oxide layer having the c axis
slantindicular to the support substrate on an opposite side to the
support substrate is not more than 80 nm.
39. The photovoltaic device according to claim 31, wherein a total
reflectance of light incident from the side of the zinc oxide layer
in the substrate with the zinc oxide layer is not less than 60% at
800 nm.
40. The photovoltaic device according to claim 31, wherein a
scattering reflectance of light incident from the side of the zinc
oxide layer in the substrate with the zinc oxide layer is not less
than 20% at 800 nm.
41. The photovoltaic device according to claim 31, wherein a
thickness of the zinc oxide layer having the c axis slantindicular
to the support substrate is not less than 5000 .ANG..
42. The photovoltaic device according to claim 31, wherein the
support substrate is an SUS sheet.
43. The photovoltaic device according to claim 42, wherein the SUS
sheet has a 2D-surface.
44. The photovoltaic device according to claim 42, wherein the SUS
sheet is of a long rolled form.
45. The photovoltaic device according to claim 34, wherein a
thickness of the metal layer is 1000 .ANG. to 2500 .ANG..
46. The photovoltaic device according to claim 31, wherein a
thickness of the zinc oxide layer having the c axis perpendicular
to the support substrate is 1500 .ANG. to 2500 .ANG..
47. The photovoltaic device according to claim 31, wherein electric
resistance in a direction normal to a surface of the support
substrate of the substrate with the zinc oxide layer is not more
than 20 .OMEGA./cm.sup.2.
48. A method for producing a photovoltaic device, comprising:
producing a substrate with a zinc oxide layer by forming on a
support substrate a zinc oxide layer having the c axis
perpendicular to the support substrate by a sputtering method and
forming a zinc oxide layer having the c axis slantindicular to the
support substrate on the zinc oxide layer having the c axis
perpendicular to the support substrate, by an electrodeposition
method; and forming a semiconductor layer on the substrate with the
zinc oxide layer.
49. The producing method of the photovoltaic device according to
claim 48, wherein the support substrate is an electrically
conductive substrate.
50. The producing method of the photovoltaic device according to
claim 48, wherein a temperature of the support substrate is set at
380.degree. C. or less in the step of forming the zinc oxide layer
having the c axis perpendicular to the support substrate.
51. The producing method of the photovoltaic device according to
claim 48, wherein a metal aluminum layer is formed on the support
substrate by sputtering and thereafter the zinc oxide layer having
the c axis perpendicular to the support substrate is formed on the
metal aluminum layer.
52. The producing method of the photovoltaic device according to
claim 51, wherein a temperature of the support substrate is set at
100.degree. C. or less in the step of forming the metal aluminum
layer.
53. The producing method of the photovoltaic device according to
claim 51, wherein after the step of forming the metal aluminum
layer, an aluminum oxide layer is formed by oxidizing a surface of
the metal aluminum layer by an oxygen plasma.
54. The producing method of the photovoltaic device according to
claim 48, wherein the support substrate is an SUS substrate rolled
in a long roll form.
55. The producing method of the photovoltaic device according to
claim 48, wherein the zinc oxide layer having the c axis
perpendicular to the support substrate is formed in a thickness of
1500 .ANG. to 2500 .ANG..
56. The producing method of the photovoltaic device according to
claim 48, wherein a temperature of the support substrate in the
step of forming the zinc oxide layer having the c axis
perpendicular to the support substrate is so set that an average
grain size of crystal grains observed in a surface of the zinc
oxide layer having the c axis slantindicular to the support
substrate on an opposite side to the support substrate is not more
than 2 .mu.m and that a scattering reflectance of light incident
from the side of the zinc oxide layer having the c axis
slantindicular to the support substrate in the substrate with the
zinc oxide layer is not less than 20% at 800 nm.
57. The producing method of the photovoltaic device according to
claim 48, wherein a temperature of the support substrate in the
step of forming the zinc oxide layer having the c axis
perpendicular to the support substrate is so set that surface
roughness Ra of a surface of the zinc oxide layer having the c axis
slantindicular to the support substrate on an opposite side to the
support substrate is not more than 80 nm and that an average of
inclination angles of crystal grains of the surface is not less
than 15.degree..
58. The producing method of the photovoltaic device according to
claim 48, wherein the zinc oxide layer having the c axis
slantindicular to the support substrate is formed in a thickness of
not less than 5000 .ANG..
59. The producing method of the photovoltaic device according to
claim 48, wherein the zinc oxide layer having the c axis
slantindicular to the support substrate is formed by
electrodeposition using a zinc nitrate solution having a
concentration of not less than 0.15 mol/l.
60. The producing method of the photovoltaic device according to
claim 53, wherein power of the oxygen plasma in the oxygen plasma
process is so set that a total reflectance of light incident from
the side of the zinc oxide layer having the c axis slantindicular
to the support substrate is not less than 60% and that electric
resistance in a direction normal to a surface of the support
substrate is not more than 20 .OMEGA./cm.sup.2.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a substrate with a zinc
oxide layer (hereinafter often referred to a ZnO-layered substrate)
used, for example, as a component of a photovoltaic device such as
a solar cell or the like, a method for forming the zinc oxide
layer, a photovoltaic device, and a method for producing the
photovoltaic device and, more particularly, to a substrate provided
with a zinc oxide layer as a part of a reflecting layer for
improving the long-wavelength sensitivity of the solar cell and a
method for forming
[0003] 2. Related Background Art
[0004] The applicant of the present application already suggested a
combination of a metal layer with a transparent conductive layer
made by sputtering, which was used as a reflecting layer of the
photovoltaic device (solar cell). In this suggestion, sputter
conditions of the metal layer for suppressing decrease of
reflectance of the metal layer are detailed in order to obtain the
reflecting layer with good reflection characteristics. The
sputtering method, however, requires time and labor for production
of a target even if inexpensive materials are used. Thus, the cost
of the target is not low and the efficiency of utilization thereof
is as low as approximately 20%. The material cost is, therefore,
very high. Moreover, a sputter apparatus is a vacuum device and the
apparatus is thus expensive. As a result, the depreciation cost is
also high. These are hindrance to providing inexpensive solar cells
to solve environmental issues.
[0005] The applicant of the present application also suggested a
method for depositing two kinds of zinc oxide layers on a long
rolled substrate (hereinafter referred to as "long substrate") by
electrolytic deposition (electrodeposition) being excellent in the
optical confinement effect and industrially very inexpensive. This
method at the present time, however, does not allow the zinc oxide
layers to be deposited directly on an aluminum layer which is
inexpensive and which has high reflectance. Grounds thereof are
that the surface of aluminum is modified into boehmite by a hot
acid solution, thereby extremely degrade the reflection and that
the boehmite-modified surface is of crumplelike structure whereby
growth of zinc oxide is also of the crumple shape.
[0006] An object of the present invention is, therefore, to
inexpensively provide a ZnO-layered substrate excellent in
reflection and the optical confinement effect and useful as a
substrate with a reflecting layer for solar cell, solving the above
problems.
SUMMARY OF THE INVENTION
[0007] The present invention provides a ZnO-layered substrate in
which at least a zinc oxide layer is provided on a support
substrate, wherein the zinc oxide layer comprises a zinc oxide
layer having the c axis perpendicular to the support substrate and
a zinc oxide layer having the c axis slantindicular to the support
substrate in the order from the side of the support substrate.
[0008] The present invention also provides a method for forming a
zinc oxide layer, comprising a step of forming on a substrate a
zinc oxide layer having the c axis perpendicular to the substrate
by a sputtering method, and a step of forming a zinc oxide layer
having the c axis slantindicular to the substrate on the zinc oxide
layer having the c axis perpendicular to the substate, by an
electrodeposition method.
[0009] Further, the present invention provides a photovoltaic
device comprising a ZnO-layered substrate in which at least a zinc
oxide layer is provided on a support substrate, and a semiconductor
layer, wherein the zinc oxide layer comprises a zinc oxide layer
having the c axis perpendicular to the support substrate and a zinc
oxide layer having the c axis slantindicular to the support
substrate in the order from the side of the support substrate.
[0010] In addition, the present invention provides a method for
producing a photovoltaic device, comprising a step of producing a
ZnO-layered substrate by forming on a support substrate a zinc
oxide layer having the c axis perpendicular to the support
substrate by a sputtering method and forming a zinc oxide layer
having the c axis slantindicular to the support substrate on the
zinc oxide layer having the c axis perpendicular to the support
substrate, by an electrodeposition method, and a step of forming a
semiconductor layer on the ZnO-layered substrate.
[0011] In the present invention, the substrate (support substrate)
is preferably an electrically conductive substrate. The support
substrate is preferably an SUS sheet and the SUS sheet preferably
has a 2D-surface. It is also preferable to use an SUS substrate of
a long roll form.
[0012] In the ZnO-layered substrate and the photovoltaic device of
the present invention, a metal layer is preferably interposed
between the support substrate and the zinc oxide layer. The
thickness of the metal layer is preferably 1000 .ANG. to 2500
.ANG.. The metal layer is preferably a metal aluminum layer. A
temperature of the substrate in forming the metal aluminum layer by
sputtering is preferably set at 100.degree. C. or less. It is
preferable to interpose an aluminum oxide layer between the metal
aluminum layer and the zinc oxide layer. The aluminum oxide layer
is preferably one formed by oxidizing the metal aluminum layer by
an oxygen plasma process. On that occasion, the power of the oxygen
plasma in the oxygen plasma process is preferably so set that a
total reflectance of light incident from the side of the zinc oxide
layer having the c axis slantindicular to the substrate is not less
than 60% and that electric resistance in a direction normal to the
surface of the substrate is not more than 20 .OMEGA./cm.sup.2.
[0013] Further, an average of inclination angles of crystal grains
of a surface of the zinc oxide layer having the c axis
slantindicular to the support substrate on an opposite side to the
support substrate is preferably not less than 15.degree.; surface
roughness Ra of the opposite surface of the layer to the support
substrate is preferably not more than 80 nm; the thickness of the
layer is preferably not less than 5000 .ANG.. The zinc oxide layer
having the c axis slantindicular to the substrate is preferably
formed by electrodeposition using a zinc nitrate solution having a
concentration of not less than 0.15 mol/l.
[0014] The thickness of the zinc oxide layer having the c axis
perpendicular to the substrate is preferably 1500 .ANG. to 2500
.ANG.. The temperature of the substrate in forming the layer by
sputtering is preferably set at 380.degree. C. or less.
[0015] The temperature of the substrate in forming the zinc oxide
layer having the c axis perpendicular to the substrate is
preferably so set that an average grain size of crystal grains
observed in the surface of the zinc oxide layer having the c axis
slantindicular to the substrate on the opposite side to the
substrate is not more than 2 .mu.m and that a scattering
reflectance of light incident from the side of the zinc oxide layer
having the c axis slantindicular to the substrate is not less than
20% at 800 nm. The temperature of the substrate in forming the zinc
oxide layer having the c axis perpendicular to the substrate is
preferably so set that surface roughness Ra of the surface of the
zinc oxide layer having the c axis slantindicular to the substrate
on the opposite side to the substrate is not more than 80 nm and
that an average of inclination angles of crystal grains in the
surface is not less than 15.degree..
[0016] Further, a total reflectance of light incident from the side
of the zinc oxide layer into the ZnO-layered substrate of the
present invention is preferably not less than 60% at 800 nm and a
scattering reflectance is preferably not less than 20% at 800
nm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1A and FIG. 1B are schematic sectional views of
respective ZnO-layered substrates according to the present
invention;
[0018] FIG. 2 is a schematic diagram to show the structure of an
electrodeposition system which can be applied to the forming method
of the present invention;
[0019] FIG. 3 is a schematic diagram to show the structure of a
sputtering system which can be applied to the forming method of the
present invention;
[0020] FIG. 4 is an SEM image of a surface of a zinc oxide layer
having the c axis perpendicular to the substrate, made by
sputtering;
[0021] FIG. 5 is an SEM image of a surface of a zinc oxide layer
having the c axis slantindicular to the substrate, made by
electrodeposition;
[0022] FIG. 6A and FIG. 6B are SEM images of electrodeposited ZnO
films in the surface of the reflecting layer where the ZnO films
were formed at the substrate temperature of 400.degree. C. in the
sputtering step;
[0023] FIG. 7A and FIG. 7B are SEM images of electrodeposited ZnO
films in the surface of the reflecting layer where the ZnO films
were formed at the substrate temperature of 350.degree. C. in the
sputtering step;
[0024] FIG. 8A and FIG. 8B are SEM images of electrodeposited ZnO
films in the surface of the reflecting layer where the ZnO films
were formed at the substrate temperature of 300.degree. C. in the
sputtering step;
[0025] FIG. 9A and FIG. 9B are SEM images of electrodeposited ZnO
films in the surface of the reflecting layer where the ZnO films
were formed at the substrate temperature of 250.degree. C. in the
sputtering step;
[0026] FIG. 10A and FIG. 10B are SEM images of electrodeposited ZnO
films in the surface of the reflecting layer where the ZnO films
were formed without heating the substrate in the sputtering
step;
[0027] FIG. 11 is a diagram to show the dependence of average grain
size of crystal grains in the surface of electrodeposited ZnO film,
on substrate temperature during deposition of sputtered ZnO
film;
[0028] FIG. 12 is a diagram to show the dependence of reflectance
of reflecting layer, on substrate temperature during deposition of
sputtered ZnO film;
[0029] FIG. 13 is a diagram to show the dependence of average of
inclination angles of crystal grains in the surface of
electrodeposited ZnO film, on substrate temperature during
deposition of sputtered ZnO film;
[0030] FIG. 14 is a diagram to show the dependence of surface
roughness Ra of electrodeposited ZnO film, on substrate temperature
during deposition of sputtered ZnO film; and
[0031] FIG. 15 is a diagram to show the dependence of series
resistance and reflectance of photovoltaic device of the present
invention, on power of oxygen plasma.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] The present invention is directed to effectively forming
surface texture and depressions without decreasing the reflectance
of the ZnO-layered substrate suitable for the reflecting layer for
solar cell, thereby increasing collected photocurrent. At the same
time as it, the present invention is also directed to forming the
reflecting layer industrially at low cost and on a stable basis. In
the present invention, the reflecting layer is formed by stacking
appropriate films, respectively using a vacuum device and a wet
device. Specifically, an aluminum film and a zinc oxide layer
having c axis perpendicular to the substrate are made by sputtering
capable of forming a dense film with excellent adhesion on any
substrate, and a zinc oxide layer having the c axis slantindicular
to the substrate is made by electrodeposition by which the surface
texture can be controlled readily and by which the film can be made
at low production cost.
[0033] In order to accomplish the object of the present invention,
it is conceivable to employ a process for forming the thin aluminum
film and zinc oxide layer by the sputtering method and for forming
the thick zinc oxide layer thereon by the electrodeposition method.
Conceptually, it is possible to determine cost distribution in
proportion to the thickness. For example, if the thickness of the
sputtered film is 10 or less % of the total thickness, the cost
impact due to the sputtering is 10 or less % of the total;
therefore, the actual cost can be not more than two to three times
the cost where the film is formed by only electrodeposition. This
cost impact varies depending upon the instrument cost of the
sputter device and electrodeposition device, the material cost of
the respective materials, fuel and light cost, labor cost, the
operating rate and yield of each device, and so on. Basically, it
is more preferable in terms of the cost to make the sputtered film
as thin as possible.
[0034] It was, however, clarified by the inventors from studies on
stacking of the sputtered film and electrodeposited film that
different results were achieved depending upon conditions of the
two films. This will be described based on the results of
experiments conducted by the inventors.
[0035] FIG. 4 shows a typical example of a surface SEM (scanning
electron microscope) image of a ZnO film formed by sputtering. The
temperature of the substrate during sputtering of ZnO was
400.degree. C. for forming texture in the surface and the film of
ZnO was deposited in the thickness of about 2 .mu.m. Since the
sputtered film is dense and reflects the composition of a target as
it is, it has an advantage of capability of readily being adapted
to change in doping and composition. In most cases, the structure
thereof is of hexagonal polycrystals having the c axis
perpendicular to the substrate.
[0036] The ZnO film illustrated in FIG. 4 has also c axis
perpendicular to the substrate and grain sizes of its crystal
grains are not more than several hundred .ANG., which are much
smaller than the size of the texture shown in FIG. 4. This is clear
from the ion image of cross section of film by FIB (focused ion
bombardment). Namely, the projections and depressions observed as
texture in FIG. 4 are of the secondary structure. This secondary
structure of projections and depressions is formed both where the
ZnO film is deposited on an uneven metal layer and where the ZnO
film is deposited on a flat metal layer. Further, it was verified
that, in the case where the metal layer+ZnO layer (this stack
structure will also be referred to as a reflecting layer) were used
to make a solar cell, this structure had the effect of increasing
the short-circuit current density J.sub.SC of the solar cell
because of optical confinement. This effect of increasing J.sub.SC
is also observed with the ZnO film of FIG. 4, and the effect
becomes gradually clearer as the thickness is increased to about 4
.mu.m. This is probably because the size of the projections and
depressions becomes close to the wavelength region of reflection
utilization light of the solar cell. Therefore, J.sub.SC will be
decreased if the temperature for forming ZnO during sputtering or
the thickness is decreased to the contrary.
[0037] As described above, the solar cell having the very good
effect of increasing J.sub.SC can be obtained when the ZnO film is
formed in the thickness over 4 .mu.m by sputtering. In this case
there are, however, significant industrial issues, including (1)
the use amount of material is large, thereby decreasing the
throughput of the deposition apparatus and in turn increase the
depreciation cost, thus posing the problem of high cost and (2) a
thick and dense film is fragile when deposited on a flexible
substrate, thus posing a problem in application thereof, for
example, where the solar cell is formed on such a flexible
substrate as an SUS plate.
[0038] FIG. 5 shows an example of a surface SEM image of a ZnO film
which was deposited by electrodeposition on a flat surface in which
silver was deposited on an SUS430 substrate having a flat surface
as treated by BA (bright annealing). The ZnO film was
electrodeposited in the thickness of about 1.2 .mu.m at 85.degree.
C. and at the current density of 3.6 mA/cm.sup.2 with a zinc
nitrate solution of 0.2 mol/l mixed with dextrin of 0.07 g/l.
[0039] Crystal grains are observed in the above ZnO film made by
electrodeposition and, according to analysis with XRD (X-Ray
diffraction system), the crystals are exactly hexagonal ZnO but
have the inclined c axis. The c axis direction is considered to be
normal to the facet of the crystal grains. The contrast of the SEM
image is enhanced and, therefore, the actual projections and
depressions are not so large as those judged from FIG. 5 and are
approximately 60 to 80% of the thickness even in portions appearing
black. It is thus easy to form the solar cell with little shunt by
forming necessary members on this layer. The solar cell obtained
exhibits the exceedingly desired result as to J.sub.SC.
[0040] In the polycrystal grain structure seen in FIG. 5, the
direction of the c axis can be changed by the concentration of the
electrodeposition solution in the electrodeposition step and the
grain sizes of crystal grains can be changed to some extent by
either of the concentration and temperature of the
electrodeposition solution, the current density of
electrodeposition, and texture of the base substrate. Therefore, an
optimum layer can be selected for the reflecting layer of solar
cell by combination of these electrodeposition conditions.
[0041] Since the above ZnO film by the electrodeposition method
basically has the great advantage of capability of being formed
industrially at low cost and demonstrates the sufficient optical
confinement effect even in the form of a relatively thin film, the
solar cells can be provided as practical products to be applied in
the wide range.
[0042] ZnO itself is transparent to the sunlight that the solar
cell utilizes, and thus the metal layer as a reflecting member is
necessary for achieving the optical confinement effect. For
utilizing the solar cells as products to be applied in the wide
range, it is better to form a resin-laminated module, but it is
known that this solar cell module allows moisture to permeate into
the film, thereby causing electrochemical migration or corrosion.
Therefore, the above metal layer needs to be selected from two
aspects of the optical reflection property and the electrochemical
stability (to prevent the electrochemical migration and
corrosion).
[0043] Materials satisfying the above selection conditions include
aluminum and some alloys thereof. However, the alloys are
industrially disadvantageous in high volume production, because of
great dependence of the optical reflection property and the
electrochemical stability on the composition. Therefore, the metal
layer that can be used is limited to aluminum, including one doped
with a small amount of dopant, in terms of the production cost.
[0044] Any method for forming a good electrodeposited ZnO film on
an aluminum layer has never been established yet heretofore,
however. It can be thus conceivable to employ a combination of the
ZnO film by sputtering with the ZnO film by electrodeposition, but
practical stacking of these films is not easy.
[0045] The inventors investigated films of a small area. If the
thickness of the ZnO film by sputtering was smaller than 500 .ANG.
dissolution thereof sometimes proceeded due to the
electrodeposition bath, to expose the base metal aluminum. This
often resulted in unsuccessful progress of deposition of the ZnO
film in the subsequent electrodeposition step. If the thickness of
the ZnO film by sputtering was smaller than 1500 .ANG. film
formation onto aluminum sometimes failed to catch up with change of
morphology (shape), thereby resulting in exposing the aluminum
layer. This could result in unsuccessful deposition of the ZnO film
by the subsequent electrodeposition method. For stably supplying
the reflecting layer, the thickness of the ZnO film by the
sputtering method is desirably not less than 1500 .ANG.
accordingly.
[0046] As the thickness of the above sputtered ZnO film approaches
1 .mu.m, the cost merit vanishes rare. It was further clarified
that, for example, where the thickness of the sputtered ZnO film
was set to 2000 .ANG., the uneven shape of the surface of the ZnO
film deposited by electrodeposition varied depending upon the
thickness of the base aluminum layer and the film-forming substrate
temperature in the sputtering step.
[0047] The inventors conducted further studies using an apparatus
adapted to a long substrate advantageous in production. Using the
long substrate of SUS430 having a 2D-treated surface, a metal
aluminum film was deposited in the thickness of 1000 .ANG. or 2000
.ANG. in the sputtering apparatus illustrated in FIG. 3, which will
be described hereinafter, and the ZnO film 2000 .ANG. thick was
deposited with changes of the substrate temperature in the range of
70.degree. C. to 400.degree. C. by the sputtering method. Then the
ZnO layer was further electrodeposited in the thickness of 1.2
.mu.m on each sputtered ZnO film by the electrodeposition apparatus
of FIG. 2 described hereinafter, thereby forming the reflecting
layer. Surface SEM images of the respective reflecting layers are
shown in FIGS. 5, 6A, 6B, 7A, 7B, 8A, 8B, 9A, 9B, 10A and 10B. A
surface oxidation process by an oxygen plasma was carried out
immediately after the formation of the metal aluminum layer by
sputtering. This is a process for preventing reduction at the
interface of the sputtered ZnO film to the metal aluminum layer so
as to prevent degradation of characteristics, rather than to
achieve enhancement of the characteristics. They were produced
under the same conditions except for the thicknesses of the metal
aluminum layer and the substrate temperatures for deposition of the
sputtered ZnO film.
[0048] In each of FIG. 6A, FIG. 7A, FIG. 8A, FIG. 9A, and FIG. 10A
the thickness of the metal aluminum layer is 1000 .ANG. and in each
of FIG. 6B, FIG. 7B, FIG. 8B, FIG. 9B, and FIG. 10B the thickness
of the metal aluminum layer is 2000 .ANG.. FIGS. 6A and 6B show the
samples made at the substrate temperature of 400.degree. C., FIGS.
7A and 7B the samples at the substrate temperature of 350.degree.
C., FIGS. 8A and 8B the samples at the substrate temperature of
300.degree. C., FIGS. 9A and 9B the samples at the substrate
temperature of 250.degree. C., and FIGS. 10A and 10B the samples at
the effective substrate temperature of 70.degree. C. without
heating. As apparent from FIGS. 6A, 6B to FIGS. 10A, 10B, there are
great differences in the morphology of the films even though the
ZnO films were electrodeposited in the thickness of 1.2 .mu.m by
the same electrodeposition apparatus. This is the fact first found
by the inventors. The films have respective physical quantities and
industrial quantities different from each other.
[0049] FIG. 11 shows the dependence of the average grain size of
crystal grains in the outermost surface of the reflecting layers
shown in FIGS. 6A, 6B to FIGS. 10A, 10B, on the substrate
temperature during the deposition of the sputtered ZnO film. The
average grain size is an average of longer diameters of the
sputtered ZnO film. The total reflectance is a factor including
direct reflectance and scattering reflectance measured by an
integrating sphere and the scattering reflectance is a factor
obtained by subtracting the direct reflectance from the total
reflectance. Greater values are more preferable for the both
reflectances. It is, however, noted that the illustration of FIG.
12 represents evaluation of values at 800 nm from spectra of the
total reflectance and scattering reflectance. The evaluation of
values at 800 nm is based on an average of peaks and bottoms due to
interference in the reflection spectrum around 800 nm. Accordingly,
they are evaluated as being smaller than actual values of
reflectance. Values of peaks and bottoms show almost same values in
the case of stacked specular films as long as their thicknesses and
indices of refraction are equal in the same wavelength region. In
the cases of the reflecting films shown in FIGS. 6A, 6B to FIGS.
10A, 10B, their average thicknesses and refractive indices can be
assumed to be almost equal, and thus it can be considered that they
almost reflect the slate of scattering of light.
[0050] It is seen from FIG. 12 that the total reflectance is
constant, regardless of the morphology of the surface, (which
indicates that there is no abnormal absorption of light due to
formation of the textured surface) and the scattering reflectance
is gradually increasing as the substrate temperature during
deposition of the sputtered ZnO film increases over a specific
value. This is similar to the dependence of average grain size of
crystal grains shown in FIG. 11, from which it is clear that the
increase of projections and depressions contributes to scattering
reflection. It is, however, necessary to pay attention to the fact
that the scattering reflection of light also appears great, for
example, where a mirror is inclined, i.e., the scattering
reflection is not always effective in increasing J.sub.SC of solar
cell.
[0051] FIG. 13 shows the dependence of the average of inclination
angles of crystal grains in the surfaces of the reflecting layers
shown in FIGS. 6A, 6B to FIGS. 10A, 10B, on the substrate
temperature during deposition of the sputtered ZnO film. The
average of inclination angles of crystal grains was obtained by
scanning the surface of each of the reflecting layers of FIGS. 6A,
6B to FIGS. 10A, 10B by AFM (atomic force microscope). The average
increases as inclined portions increase. Namely, it is basically
mentioned that the average increases as the projections and
depressions become larger. Even with development of projections and
depressions, it becomes, however, more difficult to make an
accurate judgment of inclined portions where the peak and bottom
portions include flat portions or where the scanning direction of
probe traverses an inclined portion. Since the AFM itself is not
always suitable for scanning of a wide area, reliability thereof is
not so high for large projections and depressions. In practice,
FIG. 13 shows that a clear rise appears against the substrate
temperature during deposition of the sputtered ZnO film in either
case of the thickness of the metal aluminum layer of 1000 .ANG. and
2000 .ANG.. According to the SEM images, the average of inclination
angles at 400.degree. C. seems to support the increase of flat
portions of peaks and bottoms where the thickness of the metal
aluminum layer is 1000 .ANG.; whereas it seems in the case of the
thickness of 2000 .ANG. that an accurate slope was not obtained
because of exceeding growth of crystal grains. In either case the
inclination angle is smaller than that at 350.degree. C.
[0052] FIG. 14 shows the dependence of the surface roughness Ra of
the reflecting layers shown in FIGS. 6A, 6B to FIGS. 10A, 10B, on
the substrate temperature during deposition of the sputtered ZnO
film. Ra indicates the average roughness, which is an average of
vertical deflections of the probe of the AFM. Ra almost linearly
increases where the metal aluminum layer is 2000 .ANG.; whereas Ra
starts decreasing from midway where the metal aluminum layer is
1000 .ANG.. It is judged that this result reproduces actual degrees
of peaks and bottoms in the projections and depressions relatively
well. The reason why the average roughness starts decreasing from
midway where the metal aluminum layer is 1000 .ANG. is considered
to be that the grain sizes of crystal grains became too large to
leave a room for vertical growth of electrodeposited ZnO. It is
assumed that the same phenomenon possibly occurs at lower
temperatures (though it is not practical) even if the metal
aluminum layer is 2000 .ANG..
[0053] Solar cells were actually made in the same structure using
the above reflecting layers and tendency thereof was investigated.
The solar cells produced were of the triple structure comprised of
a stack of three pin structures using amorphous silicon and
amorphous silicon germanium for three types of active layers, in
which the light absorbing region ranged from 300 nm to 1100 nm and
in which, particularly in the region of not less than 700 nm, the
light having passed once was reflected by the reflecting layer to
be reused. In order to evaluate absorption of light contributing to
collected current, total current by Q-measurement was evaluated
instead of measuring J.sub.SC itself. J.sub.SC can be maximized by
adjusting thicknesses of the triple active layers so as to
eliminate a current mismatch. As a consequence, there were
differences in the total current; 23.5 mA/cm.sup.2 with the solar
cells using the reflecting layers of FIG. 7A, FIG. 8A, and FIG. 8B;
23.1 mA/cm.sup.2 with the solar cell using the reflecting layer of
FIG. 7B; approximately 21 to 22 mA/cm.sup.2 with the solar cells
using the other reflecting layers. Particularly, in the case of the
solar cell using the reflecting layer of FIG. 6B, it showed the
reflection property and inclination angle not too bad, but it was
inferior in the Q total current value, 21.8 mA/cm.sup.2.
[0054] From the results of these studies, to increase the
scattering reflectance and the inclination angle is effective in
order to maximize J.sub.SC, i.e., in order to achieve effective
optical confinement, but excessive increase thereof will cause the
inverse effect; it is, therefore, understood that values of those
should be increased within the scope not exceeding the values
determined by the average grain size of crystal grains and Ra easy
to judge. On the other hand, the longer diameters of crystal grains
and Ra are not appropriate for determination of small values,
because errors become large. It is thus concluded that the lower
limit of the substrate temperature during deposition of the
sputtered ZnO film is preferably set based on the scattering
reflectance and the inclination angle and the upper limit thereof
is preferably set based on the average grain size of crystal grains
and Ra.
[0055] In the above studies, after the metal aluminum layer was
formed by sputtering as described above, the surface thereof was
subjected to an oxidation process with an oxygen plasma. Omission
of this process will result in decreasing the reflectance, thereby
making the surface look black in visual observation. Such a layer
is rather unpreferable as a reflecting layer. However, to effect
this process too strong is not preferred, either.
[0056] The forming steps and each of the members of the reflecting
layer according to the present invention will be described
below.
[0057] First described referring to the schematic diagram of FIG. 2
is an electrodeposition apparatus used in the step of forming the
zinc oxide layer having the c axis slantindicular to the substrate
in the present invention. The electrodeposition apparatus has an
electrodeposition tank, washing facilities, and a drying device
placed between a substrate feeding mechanism and a substrate
winding mechanism, as illustrated in FIG. 2.
[0058] The long substrate 2001 is conveyed in the packing style of
the coil shape rolled around a bobbin to this apparatus. In this
apparatus, this coil is set on substrate feed roller 2002 and the
substrate is conveyed toward substrate winding roller 2062 while a
slip sheet interleaved for protection of surface is unwound by slip
winding roller 2003. Specifically, the substrate 2001 moves via
tension detecting roller 2005 and power supply roller 2006 into the
electrodeposition tank 2009. Inside the electrodeposition tank 2009
the substrate is positioned by support rollers 2013 and 2014 and
electrodeposition is effected thereon. Then the substrate is fed
into the washing tank 2030 to be washed by water. Positioning
inside the washing tank 2030 is effected by support rollers 2031
and 2066. Further, the substrate is dried in hot air drier 2051 and
then passes support roller 2057 and meander correcting roller 2059
to be corrected for horizontal deviation. The substrate is wound up
around the substrate winding roller 2062 while a new slip sheet is
interleaved from slip feed roller 2060 for protection of the
surface of deposited film. It is then sent to the next step as
occasion may demand.
[0059] The tension detecting roller 2005 operates to detect dynamic
winding tension of the long substrate 2001 and feed the detection
result back to an unrepresented brake means such as a powder clutch
or the like linked to the shaft of substrate feed roller 2002,
thereby keeping the tension constant. The apparatus is designed so
that this arrangement can keep the conveying path of the long
substrate 2001 under a predetermined value of tension between the
support rollers. Particularly, in the case of the apparatus of FIG.
2, which is constructed in such structure that no roller touches
the film-forming surface, weak tension will result in posing a
problem that the long substrate 2001 is dismounted from the support
rollers or a problem that the long substrate 2001 becomes suspended
down at the entrance or the exit of the electrodeposition tank 2009
or the washing tank 2030, thereby scrubing and damaging the
film-forming surface. The structure of the apparatus adapted to
avoid the contact of the film-forming surface has the advantage of
preventing the film-forming surface from being damaged or
contaminated and is preferable, particularly, in the application
where the micron size texture has to be formed on a thin film, for
example, in the case of the reflecting layer of the solar cell.
[0060] The power supply roller 2006 is provided for applying a
cathode-side potential to the long substrate 2001, is placed as
near to the electrodeposition bath 2016 as possible, and is
connected to the negative electrode of power supply 2008.
[0061] The electrodeposition tank 2009 is arranged to retain the
electrodeposition bath 2016 and to define the path of the long
substrate 2001. Anodes 2017 are set with respect to the path of
long substrate and a positive potential is applied from the power
supply 2008 via power supply bar 2015 to the anodes 2017. This
allows the electrochemical, electrolytic deposition process with
the long substrate 2001 being negative and the anodes 2017 being
positive in the electrodeposition bath 2016. When the
electrodeposition bath 2016 is kept at high temperature, a
considerable amount of water vapor evolves therefrom. Therefore,
the water vapor is exhausted through vapor exhaust ducts 2010 to
2012. In order to agitate the electrodeposition bath 2016, air is
introduced through agitating air inlet pipe 2019 to effect bubbling
of air from air outlet pipe 2018 inside the electrodeposition tank
2009.
[0062] An electrodeposition circulation tank 2025 is provided for
replenishing the high-temperature electrodeposition bath solution
to the electrodeposition tank 2009. A heater 2024 is set in this
tank 2025 to heat the electrodeposition bath solution and this bath
solution is supplied from bath circulation pump 2023 through
electrodeposition bath supply pipe 2020 to the electrodeposition
tank 2009. The electrodeposition bath solution overflowing out of
the electrodeposition tank 2009 and a part of the bath solution to
be positively returned are returned through an unrepresented return
path to the electrodeposition circulation tank 2025 to be heated
again. When the discharge of the pump is constant, the bath supply
amount from the electrodeposition circulation tank 2025 into the
electrodeposition tank 2009 can be controlled by valves 2021 and
2022, as illustrated in FIG. 2. Specifically, for increasing the
supply amount, the valve 2021 is opened more and the valve 2022 is
closed more. The inverse operation is carried out in order to
decrease the supply amount. The retained level of the
electrodeposition bath 2016 is controlled by adjusting this supply
amount and the return amount not illustrated.
[0063] The electrodeposition circulation tank 2025 is provided with
a filter circulation system comprised of a circulation pump 2027
and a filter so as to be able to remove particles in the
electrodeposition circulation tank 2025. If the supply and return
amounts are sufficiently large between the electrodeposition
circulation tank 2025 and the electrodeposition tank 2009, the
sufficient particle removing effect can be achieved by the
structure in which only the electrodeposition circulation tank 2025
is provided with the filter as described above.
[0064] In the apparatus illustrated in FIG. 2, the
electrodeposition circulation tank 2025 is also provided with a
vapor exhaust duct 2026 so as to exhaust water vapor. Particularly,
the heater 2024 is set as a heating source in the electrodeposition
circulation tank 2025, so that water vapor evolves considerably
therein; therefore, the above structure is very effective in cases
where unintentional discharge or dew condensation of water vapor
evolving is not preferred.
[0065] An electrodeposition preliminary tank 2029 is provided in
order to prevent the heated bath solution from flowing into the
existing waste liquid system at once so as to damage the disposal
system and is arranged to temporarily retain the electrodeposition
bath 2016 from the electrodeposition tank 2009 and to eventually
evacuate the electrodeposition tank 2009 in order to enhance the
efficiency of work.
[0066] The long substrate 2001 after electrodeposition in the
electrodeposition tank 2009 is then conveyed into the washing tank
2030 to be washed with water. Inside the washing tank 2030 the long
substrate 2001 is positioned by the support rollers 2031 and 2066
and passes through first washing tank 2032, second washing tank
2033, and third washing tank 2034 in this order. Each washing tank
is provided with a washing circulation tank 2047 to 2049 and a
water circulation pump 2044 to 2046 and a water supply amount to
the washing tank 2030 is determined by two valves, i.e., valves
2038 and 2041, or 2039 and 2042, or 2040 and 2043. The cleaning
water is supplied through the supply pipe 2035 to 2037 to each
washing tank 2032 to 2034. The method for controlling the water
supply amount by the two valves is the same as the bath supply
amount control method in the electrodeposition tank 2009. Just as
in the electrodeposition tank 2009, it can also be contemplated
that overflowing water is collected or part of unrepresented return
water to be positively returned is fed back to each washing
circulation tank 2050.
[0067] In the three-stage washing system as illustrated in FIG. 2,
normally, the purity of cleaning water becomes higher and higher
from the upstream washing tank on the upstream of conveyance of
substrate, i.e., from the first washing tank 2032, toward the
downstream washing tank, i.e., toward the third washing tank 2034.
This means that cleanliness of the long substrate 2001 becomes
higher as the long substrate 2001 is conveyed to the end of the
process. This realizes the structure, as illustrated in FIG. 2, in
which the cleaning water is first replenished to the third washing
circulation tank 2049, then the cleaning water overflowing from the
third washing circulation tank 2049 is replenished to the second
washing circulation tank 2048, and the cleaning water overflowing
from the second washing circulation tank 2048 is further
replenished to the first washing circulation tank 2047, whereby the
use amount of water can be decreased drastically.
[0068] The long substrate 2001 after completion of washing is
dehydrated by air knife 2065 mounted in a portion of the washing
tank 2030 and is then conveyed to the hot air drier 2051. In this
drier the substrate is dried by convective air of temperature
enough to evaporate water. The convective air for the drying is
supplied by guiding hot air generated by hot air generator 2055
through filter 2054 to decontaminate the hot air and ejecting the
hot air from hot air inlet pipe 2052. Overflowing air is again
collected by hot air collecting pipe 2053 to be mixed with the
external air from external air inlet pipe 2056 and the mixed air is
sent to the hot air generator 2055. The conveyance path of the long
substrate 2001 in the hot air drier 2051 is positioned by the
support roller 2066 and support roller 2057.
[0069] The meander correcting roller 2059 is adapted to correct
deviation of the long substrate 2001 in the widthwise direction and
roll the substrate around the substrate winding roller 2062. A
deviation amount is detected by an unrepresented sensor and the
widthwise position of the meander correcting roller 2059 is
controlled by rotating the roller 2059 about the fulcrum of an
unrepresented arm. Normally, deviation amounts detected by the
sensor and actuation amounts of the meander correcting roller 2059
both are very small, within 1 mm. On the occasion of winding of the
long substrate 2001, new slip paper is also supplied from the slip
feeding roller 2060 for protection of the surface.
[0070] Stopper 2007 and stopper 2058 are actuated simultaneously to
keep the long substrate 2001 method of the present invention is
more advantageous, as compared with the sol-gel method, the coating
method using organic substance, the spray pyrolysis method, and so
on.
[0071] (5) The above advantages can also be enjoyed in formation of
oxide. The waste liquid can be disposed of readily, an impact on
the environment is also little, and the cost for prevention of
contamination of the environment is also low.
[0072] Next described referring to the schematic diagram of FIG. 3
is a sputter apparatus used in the steps of forming the metal
aluminum layer, the thin layer of aluminum oxide, and the zinc
oxide layer having the c axis perpendicular to the substrate in the
present invention. The sputter apparatus, as illustrated in FIG. 3,
has chambers respectively for sputtering of aluminum, for an oxygen
plasma process, and for sputtering of zinc oxide between a
substrate feed mechanism enclosed in a vacuum chamber and a
substrate winding mechanism enclosed in a vacuum chamber, the
chambers being coupled to each other through a gate. The sputter
apparatus will be described in detail.
[0073] The long substrate 3003 after cleaned is set in the form of
the coil rolled around the bobbin, onto substrate feed roller 3002.
While a slip sheet interleaved for protection of surface is unwound
by slip winding roller 3004, the long substrate 3003 is conveyed
toward substrate winding roller 3028. In more detail, the long
substrate 3003 is positioned by feed control roller 3005,
thereafter the long substrate 3003 is conveyed successively via gas
gate 3007, Al sputter chamber 3009, gas gate 3013, oxygen plasma
chamber 3015, gas gate 3019, ZnO sputter chamber 3021, and gas gate
3025 to winding chamber 3027, the widthwise winding position onto
the substrate winding roller 3028 is accurately controlled by
meander correcting roller 3030, and the substrate is wound up onto
the substrate winding roller 3028 with interleaving a slip sheet
from slip feed roller 3029.
[0074] The gas gates 3007, 3013, 3019, 3025 between the chambers
have a substrate conveyance path in the form of a slit as thin as
0.1 mm to 10 mm, and non-reactive gas such as argon or the like is
allowed to flow thereto at several hundred sccm, whereby cross
contamination is prevented between a certain chamber and a chamber
connected thereto. The gas gate for prevention of cross
contamination is extremely effective, particularly, where the
difference in pressure between chambers is 10 or more times or
where different gases are used between chambers.
[0075] In cases where the process temperature is low in the
subsequent chamber, the gas at the gas gate also functions to
enable temperature control in the subsequent chamber with cooling
the long substrate 3003 even if the conveying speed of the long
substrate 3003 is relatively high. Normally, increase of
temperature can be achieved by placing a heater of several kW near
the substrate, but it is hard to achieve cooling in short time.
Since hydrogen gas has the great cooling effect, hydrogen gas is
effectively used where it is allowed to be mixed into the
chambers.
[0076] An independent exhaust device not illustrated is connected
to each of the substrate feed chamber 3001, Al sputter chamber
3009, oxygen plasma chamber 3015, ZnO sputter chamber 3021, and
substrate winding chamber 3027 to effect evacuation as exhaust
3006, 3010, 3016, 3022, or 3031 to keep each of the chambers under
corresponding low pressure. The gas at the gas gate between the
chambers is evacuated through the exhausts of the adjacent
chambers. Operating pressures of these chambers are not more than
{fraction (1/100)} atm and sometimes reach 1.times.10.sup.-7
Pa.
[0077] In order to realize these low pressures (which can be said
as a vacuum), the exhaust devices are selected from rotary pumps,
mechanical booster pumps, diffusion pumps, cryopumps,
turbo-molecular pumps, sublimation pumps, and so on, with
consideration to the vacuum level, the exhaust speed, and the
instrument cost. An exhaust device may be a combination of these
pumps. Each of the chambers and gas gates is made of SUS, Al,
glass, or the like in order to maintain these vacuums and they are
welded if necessary. Connection between the elements is achieved by
use of a valve, fittings, gaskets, and O-rings.
[0078] There is an Al target 3012 placed inside the Al sputter
chamber. Al is not only pure Al but may also be one containing a
small amount of a metal such as Mg, Si, Fe, Mn, Ni, Co, Cr, Zn, P,
or Cu. Al is normally placed on a copper backing plate with keeping
electrical conduction and thermal conduction well and this backing
plate is cooled on the bottom surface to radiate heat generated. A
negative DC potential from an external power supply is applied to
the backing plate, whereby a plasma is generated as supported by
magnetism from a magnet placed behind the backing plate. In order
to keep the plasma in good order, Ar gas is introduced up to the
pressure of several mTorr to several hundred mTorr. The strength of
the magnet is 100 G to several kG.
[0079] A cathode 3018 is placed inside the oxygen plasma chamber
3015. The cathode 3018 may be of any metal, but it is preferably
the same material as the target used in the sputtering in the
preceding step, particularly, from the aspect of preventing cross
contamination. The cathode 3018 is also placed on a copper backing
plate with keeping electric conduction and thermal conduction well,
similar to the Al target in the Al sputtering, and this backing
plate is cooled on the bottom surface to radiate heat generated. A
negative DC potential from an external power supply is also applied
to the backing plate in the similar fashion to above and the plasma
is generated as supported by magnetism from a magnet placed behind
the backing plate. In order to keep the plasma in good order, Ar
gas is introduced up to the pressure of several mTorr to several
hundred mTorr. In addition to the Ar gas, oxygen gas is also
introduced at the rate of 0.01% to 100% into the oxygen plasma
chamber 3015. Namely, the Ar gas can be null where the plasma is
generated stably and the power is very small even if the flow rate
of oxygen is very low. The process power in this oxygen plasma
chamber 3015 can be determined based on the reflection
characteristics, and series resistance in the form of the solar
cell, as detailed hereinafter, but it is normally not more than 1
kW. On the other hand, too high power is not preferable, because
the substance of the cathode 3018 is sputtered to form a film. This
limit can be obtained experimentally.
[0080] A ZnO target 3024 is placed inside the ZnO sputter chamber
3021. Mechanical arrangement and components are similar to those
around the Al target. Since ZnO is an oxide having a larger
electric resistance than the Al target, the DC potential applied
thereto is higher. There are thus restrictions such as requirements
of exacting design against abnormal discharge such as arcing or the
like. Specifically, it is necessary to space the target an optimum
distance apart from the earth shield so that the Ar gas may form
the plasma dark space at predetermined pressure. The distance is
about 4 mm in the apparatus of FIG. 3. In order to optimize the
magnetron effect, the magnetic flux density by the magnet is
selected in combination with the Ar pressure and the DC potential
applied. Since the magnetic flux density to accomplish the
magnetron condition is normally a hundred G to several hundred G,
the strength of the magnet placed at some distance is set to
several hundred to a thousand and several hundred G.
[0081] When sputtering of the DC magnetron method is selected, the
electric resistance of the target has to be set low to a certain
extent. This requires that the target is produced by the hot press
method and it is thus pointed out everywhere that the cost of the
target is increased thereby. It is also possible to implement RF
sputtering using a target with a high electric resistance made by
the cold press method. In this case, it is better to use the magnet
to stabilize the discharge, but the magnet does not always have to
be used. This can considerably decrease the target cost. However,
the DC magnetron method is easier in designing of the target in the
discharge portion and can perfectly eliminate influence due to
discharge leak, particularly, in the case of the long continuous
process as illustrated in FIG. 3.
[0082] The sputter deposition of ZnO is usually carried out in Ar
gas. Oxygen may be added in some cases, but the amount thereof is
small. The power of discharge has to be sufficiently high and it is
over 1 kW in the apparatus of FIG. 3.
[0083] The sputter apparatus as illustrated in FIG. 3 and the
deposited films by the apparatus have the following advantages.
[0084] (1) The apparatus is clean because of the vapor phase
process and generates no toxic substance, because the gases used
are Ar, oxygen, and hydrogen. Because of the vapor phase process,
influence of degassing mainly of water is little on the subsequent
process. The apparatus can be adapted to the temperature range from
room temperature to several hundred .degree. C. and can thus be
used for deposition of many kinds of films.
[0085] (2) The films obtained are generally dense and excellent in
adhesion. A compound in a composition deviated from a
stoichiometric ratio can also be deposited without much trouble. A
fine or small amount of doping can also be made by using a target
containing a dopant or by supplying the dopant from gas.
[0086] (3) It is known that the reflectance at 800 nm is increased
by adding about 1% metal (Cu, Mg, or Ni), boron, or oxygen to Al.
In this Al case, however, because absorption at the W point in the
Brillouin zone is decreased by destroying crystallinity,
reflectances at the other wavelengths than 800 nm are somewhat
decreased at the same time.
[0087] (4) The doping effect is great with materials whose
resistance is easy to vary depending upon the deposition
conditions, such as ZnO or the like. If the target contains such
I-element as Li or the like, it compensates for oxygen defects in
the ZnO film deposited to decrease the carrier density and increase
the resistance. Conversely, if the target contains such III-element
as Al or the like, the carrier density is increased, thereby
decreasing the resistance.
[0088] The morphology of ZnO by this sputtering is polycrystals of
the columnar shape having the c axis perpendicular to the
substrate, though the reason is not clear yet. Diameters of the
columns are generally smaller than 1000 .ANG. and projections and
depressions grow in the size larger than the diameters but in the
very small level difference in the surface of film. If the
thickness of the film is small, these projections and depressions
are insufficient as those used for optical confinement of the solar
cell, aimed by the present invention.
[0089] FIG. 1A is a schematic sectional view of an example of the
ZnO-layered substrate of the present invention. In the drawing,
reference numeral 1001 designates a support substrate (which will
also be referred to hereinafter simply as "substrate"), 1002 a
metal aluminum layer, 1003 a zinc oxide layer having the c axis
perpendicular to the substrate, and 1004 a zinc oxide layer having
the c axis slantindicular to the substrate. FIG. 1B is a schematic
sectional view of another example of the ZnO-layered substrate of
the present invention. In the drawing, numeral 1005 designates an
aluminum oxide layer.
[0090] (Substrate)
[0091] In the present invention the substrate 1001 is preferably
the long substrate suitable for the electrodeposition apparatus and
vacuum apparatus described above and can be any material that
allows electrical conduction to the film-forming surface and that
is resistant to the electrodeposition bath 2016. Specifically, it
is selected from metals such as SUS, Al, Cu, Fe, Cr, and so on.
Among these materials, SUS is relatively inexpensive and excellent
in anticorrosion in consideration to carrying out the
device-forming process in the subsequent step and is also excellent
as a long substrate. The surface of the substrate 1001 may be
either flat or rough. Further, another electroconductive material
may also be deposited on these substrates and an appropriate
substrate is selected depending upon the purpose of
electrodeposition.
[0092] (Metal aluminum layer)
[0093] The aluminum for formation of the metal aluminum layer 1002
suitable for the metal layer of the present invention can be
selected from pure Al (the purity 5 N), Al containing Si, Cu, Mg,
Zn, Ni, Cr or the like, and alloys of Al. The material suitable for
formation by the sputtering method is Al metal in the same
composition as a desired film. When the substrate temperature is
not more than 100.degree. C. during sputtering, it is easy to
obtain a flat and unblackened metal aluminum layer, which is
preferable.
[0094] The pure Al has absorption of light near 800 nm. This
overlaps with the effective light utilization area of the solar
cells, particularly, those using a-Si (amorphous Si), a-SiGe
(amorphous SiGe), microcrystalline Si, or crystalline Si, thereby
decreasing utilization of reflected light. This is somewhat
inconvenient. The reflectance at 800 nm may be improved by adding
about 1% metal such as Si, Cu, Mg, Zn, Ni, or Cr, oxygen, nitrogen,
boron, or the like. In that case, the reflectances at the other
wavelengths than 800 nm are also decreased, and, therefore, the
additive amount suitable for the device has to be selected. Optical
characteristics can be considered to be the same as pure Al if the
additive amount is not more than 0.1%.
[0095] The morphology of the Al film deposited varies, depending
upon the composition of the sputter target. Particularly, in
systems with the dopant of Si, growth of microcrystals of
approximately 1000 .ANG. is observed. This is because grain
boundaries become easier to create. In the present invention
projections and depressions are positively made in the succeeding
layer, and thus there is no need for formation of crystal grains
herein in particular, though there is no specific effect
thereby.
[0096] In the present invention the thickness of the metal aluminum
layer 1002 is preferably not less than 500 .ANG. nor more than 1
.mu.m. The thickness is preferably not less than 1000 .ANG.,
particularly, in order to assure the optical reflectance and it is
desirably not more than 5000 .ANG. in order to assure mechanical
adhesion. Further, the thickness is more preferably not more than
2500 .ANG. in order to keep the reflection performance of light.
Where the surface is oxidized by the oxygen plasma, the total
thickness of the above layer and the oxidized portion (aluminum
oxide layer) is preferably determined in the above thickness
ranges.
[0097] It is desirable in the present invention to form the thin
film 1005 of aluminum oxide by oxidizing the surface of the metal
aluminum layer 1002 in the oxygen plasma. This aluminum oxide layer
1005 is obtained by exposing the surface of the metal aluminum
layer deposited by sputtering to the plasma in the oxygen plasma
and has a very small thickness. This thin film of aluminum oxide
1005 contains less oxygen or has the smaller thickness than
aluminum oxide made of the starting material of Al.sub.2O.sub.3 in
a vacuum process, aluminum oxide made in a surface of aluminum in a
solution by anodization or anodic oxidation, and even a film of
aluminum oxide made when exposed to severe conditions in the oxygen
plasma. From the principle of production thereof, there are no
possibilities that other substances than aluminum oxide (which
means that a film contains oxygen and aluminum) are made, and with
this aluminum oxide, it is almost impossible to determine the
thickness and refractive index from interference by optical
measurement or to analyze the composition by XMA (X-ray
microanalyzer).
[0098] It should be, however, considered that the thin film of
aluminum oxide is made for sure from the increase of electric
resistance, which is observed with increase of oxygen mixed into
Al, and from the relaxation of the reflectance decrease with
deposition of zinc oxide in the succeeding process. The extent of
the formation of the aluminum oxide layer is determined from values
of reflectance and electric resistance at the time of completion of
film deposition in the succeeding process.
[0099] Namely, the electric resistance increases with increase in
the oxygen amount and thickness of aluminum oxide by increasing the
mixture amount of oxygen. This is probably because characteristics
of Al.sub.2O.sub.3 being an insulator appear principal because of
the increase of the stoichiometric oxygen amount. On the other
hand, if the oxygen amount of the thin film of aluminum oxide is
too small, cross movement of oxygen between the aluminum layer and
the zinc oxide layer occurs to separate oxygen from the zinc oxide
layer, thereby forming metal zinc. This decreases the reflectance.
An example of this case will be explained with FIG. 15 in Example 6
described hereinafter. In this example, it can be said that the
zone between two dashed lines is the characteristic range of
aluminum oxide suitable for the present invention.
[0100] (Zinc oxide layer having the c axis perpendicular to
substrate)
[0101] In the present invention the zinc oxide layer is comprised
of the zinc oxide layer 1003 having the c axis perpendicular to the
substrate 1001 and the zinc oxide layer 1004 having the c axis
slantindicular to the substrate 1001. C axis is the principal axis
of a crystal, and is in a zinc oxide crystal grain constituting the
layer and can be identified by X-ray diffraction. The c axis
slantindicular to the substrate means that c axis is inclined to
perpendicular (or normal) of the substrate. Typical surface SEM
(scanning electron microscope) images of the respective layers are
shown in FIG. 4 and FIG. 5.
[0102] The zinc oxide layer 1003 having the c axis perpendicular to
the substrate 1001 is made by sputtering and can be identified from
the fact that most diffraction thereof by XRD (X-Ray diffraction
apparatus) is the <0002> peak of ZnO. According to
observation by FIB (focused ion bombardment), the columnar
structure of columns growing as slightly spreading to the end can
be observed and the diameters of the columns are approximately 1000
.ANG.. In the SEM image shown in FIG. 4, the surface projections
and depressions of the secondary structure are observed. They are
assembly of several columnar crystals and only inclination smaller
than 10.degree. is observed by AFM (atomic force microscope) where
the thickness of zinc oxide is about 5000 .ANG..
[0103] The thickness of the zinc oxide layer 1003 having the c axis
perpendicular to the substrate 1001 is 500 .ANG. to 1 .mu.m and is
preferably not less than 1500 .ANG., because there is the
possibility that it is dissolved in the subsequent
electrodeposition process and Al directly exposed will be the cause
of abnormal growth. The thickness should be determined as thin as
possible in terms of preventing occurrence of cracks or
delamination during electrodeposition and it is preferably not more
than 2500 .ANG..
[0104] (Zinc oxide layer having the c axis slantindicular to
substrate)
[0105] The zinc oxide layer 1004 having the inclined c axis is made
simply by electrodeposition where the concentration of zinc nitrate
is higher than 0.1 mol/l and it can be identified based on the fact
that the <0002> peak of ZnO by XRD is smaller than that of
powder sample (the data of JCPDS card). As shown in the SEM image
of FIG. 5, the polycrystal structure is extremely characteristic
and is comprised of crystal grains approximately equal to the
wavelengths of light that is intended to undergo the optical
confinement effect purposed by the present invention. The crystal
grains also grow similarly while the thickness is still small. When
the thickness is over 1 .mu.m, the inclination by AFM becomes far
greater than 20.degree. and sometimes reaches 40.degree.. When the
thickness of the zinc oxide layer 1004 having the inclined c axis
is over 5000 .ANG., the sufficient effect of projections and
depressions can be exhibited. The thickness is preferably not less
than 1 .mu.m.
[0106] When the zinc oxide layer having the c axis slantindicular
to the substrate is deposited, for example, on a flat surface of
copper or the like, it very often suffers delamination during
bending of the substrate without any difficulties. When the zinc
oxide layer having the c axis slantindicular to the substrate is
deposited on the zinc oxide layer having the c axis perpendicular
to the substrate, delamination is rarely observed by visual
observation even with 180.degree. bending of substrate and even in
the structure in which the zinc oxide layer having the c axis
perpendicular to the substrate is as thin as 1000 .ANG. and the
zinc oxide having the inclined c axis is as thick as 2 .mu.m,
however. Further, the zinc oxide having the inclined c axis seems
easier to break from the SEM image, but no structural change is
observed even after kept under the hydrostatic pressure of 300
kg/cm.sup.2.
[0107] The total reflectance (the rate of intensity of direct
light+purely scattered light to incident light) of the reflecting
layer of the present invention is measured by a reflection
spectrometer using the integrating sphere. Calibration is conducted
with a smooth surface sample of silver. Therefore, where a film
surface is a very good scattering surface, there is the possibility
that the total reflectance thereof becomes a little numerically
small, but it is defined always using the same diffuse plate (the
one attached to the spectrometer). The direct component can be
measured in measurement without the diffuse plate and the
scattering component, i.e., the scattering reflectance can be
evaluated by subtracting the direct component from the total
reflectance.
[0108] The total reflectance indicates the degree of absorption of
light inside the reflecting layer formed and values thereof closer
to 100% are better. When the scattering reflectance of a certain
reflecting layer is close to the total reflection, the reflecting
layer can be used as an excellent reflecting layer for the solar
cell. Since the structure is the stack of the metal reflecting
layer and the dielectric layer, the reflection spectrum
demonstrates strong and weak patterns due to interference. The
total reflectance and the scattering reflectance of reflection
herein are defined each as an average of maxima (maximum) and
minima (minimum) around the border of 800 nm (when the dielectric
layer is thick, there may be maxima and minima.). The total
reflectance will never be 100% as long as interference occurs. This
value is not reflected so vivid in numerical values than when
observed. As a practical value, the total reflectance of 50%
appears a dark surface, by which collection of light of the solar
cell is poor and J.sub.SC (short-circuit current density) is
extremely low. On the other hand, the total reflectances over 60%
show good characteristics and the total reflectances exceeding 70%
cannot be achieved as long as Al is used.
[0109] The reason why 800 nm is used as an index of reflectance in
the present invention is that Al has absorption thereat to show a
minimum of reflection. Another reason is that 800 nm is also the
wavelength that truly reflects the optical confinement effect at
the same time.
[0110] The average grain size of crystal grains of the zinc oxide
layer having the c axis slantindicular to the substrate, which
forms the outermost surface of the reflecting layer of the present
invention, is read from the SEM image (the longer diameter is read
where an ellipse is observed). The grain sizes of the zinc oxide
having the inclined c axis by the electrodeposition method become
greater as the projections and depressions of the substrate become
larger, as the concentration of the solution becomes higher, or as
the temperature becomes lower. The grain sizes can also be
controlled greatly by an additive such as phthalic acid.
[0111] Experiments were conducted with changes of grain sizes as
described, and it was clarified from measurement of light
collection efficiency (Q curve measurement) that preferable results
were obtained where the grain sizes were 0.4 to 1.2 .mu.m.
Specifically, it was found that without the zinc oxide layer having
the c axis slantindicular to the substrate, the current values by
the measurement of light collection efficiency (which is reduced to
current from carriers collected and which is the total current of
bottom, middle, and top cells in the solar cell of the triple
structure) were approximately 20 mA/cm.sup.2, whereas with the zinc
oxide layer having the c axis slantindicular to the substrate and
having the grain sizes in the range of 0.4 to 1.2 .mu.m, an
improvement of about 10% was able to be achieved. Therefore, the
average grain size is preferably 0.4 to 1.2 .mu.m in the present
invention as well.
[0112] The inclination angles of the crystal grains in the surface
of the zinc oxide layer having the c axis slantindicular to the
substrate, which forms the outermost surface of the reflecting
layer of the present invention, are measured as an average angle of
motions of the probe tip, using the AFM. Such a mode is normally
set in the AFM.
[0113] It is known that zinc oxide having grains having the c axis
perpendicular to the substrate grows in the electrodeposition
method, for example, where the concentration of the solution is
0.005 mol/l. When it is formed on zinc oxide made by sputtering and
having the surface slightly etched with acetic acid of 0.2%, the
deposited film is the polycrystal film of zinc oxide having the c
axis perpendicular to the substrate and having the grain size of
0.8 .mu.m. When comparison was made in this manner, it was found
that the collected current became larger, generally, with
increasing inclination of the c axis, and thus with increasing
inclination angle by the AFM.
[0114] Very good collected current is expected when the inclination
angle by AFM is not less than 15.degree.; but it is very difficult
to achieve it by the sputtering method. On the other hand, use of
the electrodeposition method makes it possible by selecting the
substrate, the concentration of solution, and the temperature. For
that, a good result is normally achieved when the temperature is 75
to 95.degree. C. and the concentration of solution is 0.1 mol/l.
The electric resistance of the reflecting layer of the present
invention is defined as a resistance in the direction normal to the
substrate per cm.sup.2 of the area of film. The electric resistance
in the direction parallel to the film does not have to be taken
into substantial consideration. The best evaluation method is one
for forming a solar cell element by making a cell having a known
resistance on the reflecting layer of the present invention and
obtaining the resistance of the reflecting layer by subtracting the
resistance of the above cell from the vertical resistance obtained
by I-V measurement of the solar cell element. It may also be
measured simply by forming electrodes of a sandwich type.
[0115] The electric resistance of the reflecting layer of the
present invention is the sum of resistances of the respective bulks
of the substrate 1001, the metal aluminum layer 1002, the zinc
oxide layer having the c axis perpendicular to the substrate 1003,
and the zinc oxide layer having the c axis slantindicular to the
substrate 1004, (also including the aluminum oxide layer 1005 in
the case of the surface of the metal aluminum layer being processed
by the oxygen plasma process) and resistances of the interfaces.
The inventors investigated this resistance by forming solar cells
and checking applicable values thereof from I-V of the solar cells
and it was not more than 20 .OMEGA. per cm.sup.2 of the area of
film. When the resistance was over this value, anomaly of I-V
occurred and environmental stability of film was considerably
degraded.
EXAMPLE 1
[0116] The ZnO-layered substrate was formed using the long SUS430
substrate having the width of 350 mm and the thickness of 0.15 mm
with the 2D-treated surface and using the systems illustrated in
FIG. 2 and FIG. 3.
[0117] The deposition conditions by the sputter apparatus of FIG. 3
were as follows; the conveyance speed of the long substrate 800
mm/min, the Al sputter power 3.2 kW, the flow rate of Ar gas during
the Al sputtering 50 sccm, the substrate temperature during the Al
sputtering 70.degree. C., the power during the oxygen plasma
process 0.1 kW, the flow rate of oxygen gas during the oxygen
plasma process 10 sccm, the substrate temperature of the oxygen
plasma process 60.degree. C., the ZnO sputter power 3.2 kW, the
flow rate of Ar gas during the ZnO sputtering 50 sccm, the
substrate temperature during the ZnO sputtering 350.degree. C., and
the gas flow rates of all the gas gates 50 sccm of Ar. The
thickness of Al was set to 1000 .ANG. and the thickness of ZnO to
2000 .ANG..
[0118] The deposition conditions by the electrodeposition apparatus
of FIG. 2 were as follows; the conveyance speed of the long
substrate 500 mm/min, the concentration of the electrodeposition
bath 0.2 mol/l, the temperature of the electrodeposition bath
85.degree. C., and the current density of electrodeposition 1.2
mA/cm.sup.2. Under these conditions, the zinc oxide layer having
the c axis slantindicular to the substrate, comprised of
polycrystals having the grain size of 1.0 .mu.m, was formed in the
thickness of 1.1 .mu.m. The total reflectance at 800 nm was 67% and
the scattering reflectance was 55%. Only interference colors looked
faint in visual observation, so that the surface was a good
scattering surface. The inclination angles were measured by the AFM
and the average thereof was 21.degree.. Ra was 65 nm.
[0119] Produced on this ZnO-layered substrate were triple cells of
the pin structure having the active layers of a-Si in the top,
a-SiGe in the middle, and a-SiGe in the bottom. The upper electrode
of ITO was formed thereon and measurement of the solar cell was
conducted under the solar simulator of AM 1.5. The series
resistance per cm.sup.2 was 34 .OMEGA. and the series resistance of
only the triple layers was 28 .OMEGA.. Therefore, the thickwise
resistance of the reflecting layer of the present example was 6
.OMEGA.. The photocurrent collected was 23.4 mA/cm.sup.2, so that a
great improvement was demonstrated, as compared with 20.5
mA/cm.sup.2 of the reflecting layer made by only sputtering.
EXAMPLE 2
[0120] The layers were deposited according to the same procedures
as in Example 1 except that the SUS substrate with the 2D-surface
in Example 1 was replaced by a BA-treated substrate with a highly
flat surface. It was also noted, however, that the substrate
temperature during the ZnO sputtering was 380.degree. C., instead
of 350.degree. C., because growth of projections and depressions
was slow at 350.degree. C.
[0121] The same triple cells as in Example 1 were produced on the
ZnO-layered substrate thus obtained, the upper electrode of ITO was
formed thereon, and the measurement of the solar cell was conducted
under the solar simulator of AM 1.5. As a result, the series
resistance per cm.sup.2 was 30 .OMEGA. and the series resistance of
only the triple layers was 28 .OMEGA.; therefore, the thickwise
resistance of the reflecting layer of the present example was 2
.OMEGA.. The photocurrent collected was 23.1 mA/cm.sup.2, so that a
great improvement was demonstrated, as compared with 20.5
mA/cm.sup.2 of the reflecting layer made by only sputtering, as in
Example 1.
EXAMPLE 3
[0122] The current density of the electrodeposition was changed
from 1.2 mA/cm.sup.2 in Example 1 to 5 mA/cm.sup.2. The degree of
formation of the projections and depressions was lowered by the
degree of increase of the current. The dependence of the scattering
reflectance due to the projections and depressions on the substrate
temperature during deposition of the sputtered ZnO film started
rising at 300.degree. C. and it was also found that the temperature
of 400.degree. C. was too high from the judgment of the average
grain size of crystal grains. Therefore, the temperature of
350.degree. C. was selected as an optimum point.
[0123] The reflecting layer was made under the above conditions and
the solar cell was formed in the same manner as in Example 1. Then
the measurement was carried out. As a result, the series resistance
per cm.sup.2 was 35 .OMEGA. and, by subtracting the series
resistance 28 .OMEGA. of only the triple layers therefrom, the
thickwise resistance of the reflecting layer of the present example
was 7 .OMEGA.. The photocurrent collected was 22.9 mA/cm.sup.2, so
that a great improvement was observed, as compared with 20.5
mA/cm.sup.2 of the reflecting layer made by only sputtering, as in
Example 1.
EXAMPLE 4
[0124] The ZnO-layered substrate was formed using the long SUS430
substrate having the width of 350 mm and the thickness of 0.15 mm
with the 2D-treated surface and using the systems illustrated in
FIG. 2 and FIG. 3.
[0125] The deposition conditions by the sputter apparatus of FIG. 3
were as follows; the conveyance speed of the long substrate 800
mm/min, the Al sputter power 3.2 kW, the flow rate of Ar gas during
the Al sputtering 50 sccm, the substrate temperature during the Al
sputtering 70.degree. C., the power during the oxygen plasma
process 0.1 kW, the flow rate of oxygen gas during the oxygen
plasma process 10 sccm, the substrate temperature of the oxygen
plasma process 60.degree. C., the ZnO sputter power 3.2 kW, the
flow rate of Ar gas during the ZnO sputtering 50 sccm, the
substrate temperature during the ZnO sputtering 400.degree. C., and
the gas flow rates of all the gas gates 50 sccm of Ar. The
thickness of Al was set to 2000 .ANG. and the thickness of ZnO to
2000 .ANG..
[0126] The deposition conditions by the electrodeposition apparatus
of FIG. 2 were as follows; the conveyance speed of the long
substrate 500 mm/min, the concentration of the electrodeposition
bath 0.2 mol/l, the temperature of the electrodeposition bath
85.degree. C., and the current density of electrodeposition 1.2
mA/cm.sup.2. Under these conditions, the zinc oxide layer having
the c axis slantindicular to the substrate, comprised of
polycrystals having the grain size of 1.0 .mu.m, was formed in the
thickness of 1.1 .mu.m. The total reflectance at 800 nm was 67% and
the scattering reflectance was 55%. Only interference colors looked
faint in visual observation, so that the surface was a good
scattering surface. The inclination angles were measured by the AFM
and the average thereof was 24.degree..
[0127] Produced on this ZnO-layered substrate were triple cells of
the pin structure having the active layers of a-Si in the top,
a-SiGe in the middle, and a-SiGe in the bottom. The upper electrode
of ITO was formed thereon and the measurement of the solar cell was
conducted under the solar simulator of AM 1.5. The series
resistance per cm.sup.2 was 34 .OMEGA. and the series resistance of
only the triple layers was 28 .OMEGA.. Therefore, the thickwise
resistance of the reflecting layer of the present example was 6
.OMEGA.. The photocurrent collected was 23 mA/cm.sup.2, so that a
great improvement was demonstrated, as compared with 20.5
mA/cm.sup.2 of the reflecting layer made by only sputtering.
EXAMPLE 5
[0128] The SUS substrate with the 2D-surface in Example 4 was
replaced by a substrate with a BA (bright annealing)-treated
surface which was almost a mirror surface. On this substrate, the
metal aluminum layer, the thin layer of aluminum oxide, and the
zinc oxide layer having the c axis perpendicular to the substrate
were made respectively in the same manner as in Example 1, using
the sputter apparatus of FIG. 3. Surfaces of the films made by the
sputter apparatus were almost mirror surfaces, in which a face of
an observer looked reflected when visually observed. The zinc oxide
layer having the c axis slantindicular to the substrate was made
thereon using the electrodeposition apparatus of FIG. 2 in the same
manner as in Example 1 except that the current density of
electrodeposition was 2 mA/cm.sup.2 to obtain the thickness of 1.8
.mu.m.
[0129] The total reflectance at 800 nm of the reflecting layer
obtained was 64%, the scattering reflectance was 47%, and the
average of inclination angles was 19.degree.; therefore, the
obtained surface was a good scattering surface. The same triple
cells as in Example 1 were produced on this reflecting layer and
evaluation was carried out. The resistance per cm.sup.2 in the
direction normal to the reflecting layer was 1.3 .OMEGA. and the
photocurrent collected was 22.7 mA/cm.sup.2. Therefore, the good
result was obtained with the BA-substrate.
EXAMPLE 6
[0130] The reflecting layers were made in the same manner as in
Example 1 except that the aluminum oxide layer thereof was made
with changes in the power in the oxygen plasma process at intervals
of 0.1 kW from 0 to 0.5 kW. The total reflectances of the
reflecting layers, and series resistances with the same triple
cells as in Example 1 were checked. The results are shown in FIG.
15. It was clear from FIG. 15 that the thin film of aluminum oxide
to be formed by the oxygen plasma process was made well by the
power in the range of a little under 0.1 kW to 0.2 kW.
[0131] On the other hand, there appeared no great change in the
solar cell characteristics with changes in the thickness of the
metal aluminum layer and in the thickness of the zinc oxide layer
having the crystal grains having the c axis perpendicular to the
substrate. As the thickness of the zinc oxide layer having the
crystal grains of the inclined c axis was increased, the collected
photocurrent of the solar cell was gradually increased with
increasing thickness.
EXAMPLE 7
[0132] The ZnO-layered substrate was made using the long SUS430
substrate having the width of 355 mm and the thickness of 0.125 mm
with the 2D-surface and using the systems illustrated in FIG. 2 and
FIG. 3.
[0133] The deposition conditions by the sputter apparatus of FIG. 3
were as follows; the conveyance speed of the long substrate 800
mm/min, the power of the Al sputtering 3.2 kW, the flow rate of Ar
gas during the Al sputtering 50 sccm, the substrate temperature
during the Al sputtering 70.degree. C., the power of the ZnO
sputtering 2.6 kW, the flow rate of Ar gas during the ZnO
sputtering 50 sccm, the substrate temperature during the ZnO
sputtering 250.degree. C., and the flow rates of the gas gates all
50 sccm of Ar. The thickness of Al was set to 1000 .ANG. and the
thickness of ZnO to 1200 .ANG.. In the present example the oxygen
plasma chamber was used only for simple passage of the
substrate.
[0134] The deposition conditions by the electrodeposition apparatus
of FIG. 2 were as follows; the conveyance speed of the long
substrate 1000 mm/min, the concentration of the electrodeposition
bath 0.08 mol/l, the temperature of the electrodeposition bath
85.degree. C., and the current density of electrodeposition 1.8
mA/cm.sup.2. Under these conditions, the zinc oxide layer having
the c axis slantindicular to the substrate, comprised of
polycrystals having the grain size of 0.9 mm, was made in the
thickness of 0.5 mm. The total reflectance at 800 nm was 60% and
the scattering reflectance was 20%. Interference colors were
observed in visual observation and looked a little faint. The
inclination angles were measured by the AFM and the average of
inclined surfaces was 15.degree.. Ra was 20 nm.
[0135] This reflecting layer was applied as a lower layer of a
solar cell. Specifically, a single cell of the pin structure having
the active layer of a-Si was made on this ZnO-layered substrate.
The upper electrode of ITO was formed thereon and the measurement
of the solar cell was carried out under the solar simulator of AM
1.5. The series resistance per cm.sup.2 was 8.2 .OMEGA. and the
series resistance of the single pin structure was 7.9 .OMEGA.. This
means that the thickwise resistance of the reflecting layer of the
present example was 0.3 .OMEGA.. The collected photocurrent was
15.3 mA/cm.sup.2, so that a great improvement was shown, as
compared with 15.0 mA/cm.sup.2 of the reflecting layer made by only
sputtering.
[0136] As detailed above, the ZnO-layered substrate of the present
invention is excellent in the reflection performance and the
optical confinement effect as a reflecting layer of a photovoltaic
device and the application thereof to the solar cell will allow the
photovoltaic devices with high efficiency to be formed stably and
at low cost as a result.
[0137] Provision of the thin film of aluminum oxide can increase
the reflectance.
[0138] When the average of inclination angles of crystal grains in
the surface of the zinc oxide layer having the c axis
slantindicular to the substrate is not less than 15.degree., the
optical confinement effect is achieved in the long-wavelength
region and the application thereof to the photovoltaic device
allows increase of J.sub.SC.
[0139] When the surface roughness Ra of the zinc oxide delamination
during the electrodeposition, and it can enhance the reliability
and environmental stability of the photovoltaic device with
increased efficiency.
[0140] When the long SUS roll having the 2D-surface is used as a
substrate, stable film formation can be carried out with little
elution from the substrate and with little hindrance of
impurities.
[0141] When the electric resistance in the direction normal to the
substrate is not more than 20 .OMEGA. per cm.sup.2, the application
of the reflecting layer to the photovoltaic device allows the
characteristics thereof to be optimized readily.
[0142] The method for forming the zinc oxide layer of the present
invention can form the zinc oxide layer suitable for the reflecting
layer on the stable basis and at low cost and, in turn, can provide
the photovoltaic device with high efficiency on the stable basis
and at low cost.
[0143] When the thickness of the zinc oxide layer by the
electrodeposition method is not less than 5000 .ANG., the
sufficient scattering effect and optical confinement effect can be
achieved.
[0144] When the zinc oxide layer by the electrodeposition method is
deposited by electrodeposition from the solution of zinc nitrate
having the concentration of not less than 0.15 mol/l, the
projections and depressions with the excellent scattering effect
can be effectively formed in the surface.
[0145] When the SUS sheet rolled in the long roll shape is used as
a substrate, the layer by the sputtering method and the layer by
the electrodeposition can be continuously formed in good order and
on the stable basis, thereby improving the durability and
throughput of product.
[0146] When the substrate temperature is controlled during the
formation of the zinc oxide layer by the sputtering method, the
point to optimize optical confinement can be determined readily and
it can provide the optimum reflecting layer for formation of the
solar cell with optimum J.sub.SC.
[0147] When oxidation is effected by use of the oxygen plasma, the
thin film of aluminum oxide can be made in the same apparatus as
the metal aluminum layer and the zinc oxide layer by sputtering;
therefore, the increase of cost can be minimized and the
reflectance of the reflecting layer obtained can be increased by
formation of the thin film of aluminum oxide.
[0148] When the average grain size of crystal grains forming the
zinc oxide layer having the c axis slantindicular to the substrate
is 0.4 to 1.2 .mu.m, the optical confinement effect can be expected
when used as a reflecting layer for many photovoltaic devices.
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