U.S. patent application number 10/999926 was filed with the patent office on 2005-07-14 for method for forming deposition film and method for producing photovoltaic device.
This patent application is currently assigned to Canon Kabushiki Kaisha. Invention is credited to Hayashi, Ryo, Iwata, Masumitsu, Kondo, Takaharu, Miyamoto, Yusuke, Nishimoto, Tomonori, Sonoda, Yuichi, Toyama, Noboru, Yamada, Satoru.
Application Number | 20050150773 10/999926 |
Document ID | / |
Family ID | 34730159 |
Filed Date | 2005-07-14 |
United States Patent
Application |
20050150773 |
Kind Code |
A1 |
Yamada, Satoru ; et
al. |
July 14, 2005 |
Method for forming deposition film and method for producing
photovoltaic device
Abstract
A method for forming a deposition film from an aqueous solution
by electrochemical reaction includes the steps of: forming the
targeted deposition film under primary deposition conditions;
replacing at least part of members in contact with the solution or
removing deposit on surfaces of the members; and depositing a film
under secondary deposition conditions. These steps are performed in
that order. Then, the deposition film is formed again under the
primary deposition conditions. In the method, the resulting
deposition film exhibits desired characteristics even after
maintenance of the deposition apparatus.
Inventors: |
Yamada, Satoru; (Shiga-ken,
JP) ; Toyama, Noboru; (Shiga-ken, JP) ;
Hayashi, Ryo; (Shiga-ken, JP) ; Sonoda, Yuichi;
(Shiga-ken, JP) ; Nishimoto, Tomonori; (Shiga-ken,
JP) ; Iwata, Masumitsu; (Shiga-ken, JP) ;
Miyamoto, Yusuke; (Shiga-ken, JP) ; Kondo,
Takaharu; (Shiga-ken, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
Canon Kabushiki Kaisha
Tokyo
JP
|
Family ID: |
34730159 |
Appl. No.: |
10/999926 |
Filed: |
December 1, 2004 |
Current U.S.
Class: |
205/199 ;
205/223 |
Current CPC
Class: |
C25D 5/48 20130101; C25D
5/34 20130101; C23C 28/00 20130101 |
Class at
Publication: |
205/199 ;
205/223 |
International
Class: |
C25D 005/48; C23C
028/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 8, 2003 |
JP |
2003-408479 |
Claims
What is claimed is:
1. A method for forming a deposition film from an aqueous solution
by electrochemical reaction, comprising the steps of: forming the
deposition film under primary deposition conditions; performing one
of replacing at least part of members in contact with the solution
and removing deposit on surfaces of the members; and depositing a
film under secondary deposition conditions, wherein the steps are
performed in that order.
2. The method according to claim 1, further comprising the step of
forming again the deposition film under the primary deposition
conditions after the step of deposition under the secondary
conditions.
3. The method according to claim 1, wherein the secondary
deposition conditions allow a desired film to deposit on the
members in contact with the solution.
4. The method according to claim 1, wherein the deposition rate of
the secondary deposition conditions is higher than the deposition
rate of the primary deposition conditions.
5. The method according to claim 1, wherein the deposition under
the secondary deposition conditions is performed on a substrate
different from the substrate used under the primary deposition
conditions.
6. A method for producing a photovoltaic device, comprising the
steps of: forming a zinc oxide film by the method as set forth in
claim 1; and forming a silicon-based semiconductor layer after the
step of forming the zinc oxide film.
Description
[0001] This application claims priority from Japanese Patent
Application No. 2003-408479 filed Dec. 8, 2003, which is hereby
incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to methods for forming a
deposition film from an aqueous solution by electrochemical
reaction, and particularly to a method for depositing a zinc oxide
thin film. In addition, the invention relates to a method for
producing a photovoltaic device including the deposition film.
[0004] 2. Description of the Related Art
[0005] Many methods have been proposed for forming a deposition
film from a solution by electrochemical reaction. For example, U.S.
Pat. No. 5,804,466 (Japanese Patent Laid-Open No. 10-140373) has
disclosed a method for depositing a zinc oxide thin film by an
electrochemical reaction in a solution. This method can provide
zinc oxide thin films more inexpensively than vacuum processes,
such as vacuum deposition by resistance heating or electron
beaming, sputtering, ion plating, and CVD.
[0006] A solution used in such electrochemical reaction has been
disclosed in Japanese Patent No. 3273294. The solution contains
0.001 to 0.5 mol/L of zinc ion and 0.001 to 0.5 mol/L of nitrate
ion.
[0007] U.S. Pat. No. 6,576,112 (Japanese Patent Laid-Open No.
2002-167695) has disclosed a method to provide over a large area a
uniform zinc oxide film having a texture exhibiting a high optical
confinement effect, preventing the zinc oxide crystals from
abnormally growing in spherical or branched form and varying in
grain size. In this method, the aqueous solution used for
electrochemical deposition contains a polycarboxylic acid in which
a carboxyl group is bonded to each of carbon having sp2 hybrid
orbital, or an ester of the polycarboxylic acid, in addition to
nitrate ions and zinc ions.
[0008] These methods have provided zinc oxide film exhibiting
superior characteristics.
[0009] However, these methods do not produce a deposition film
having a satisfactory texture in some cases even though the
composition of the solution is supplemented by adding appropriate
constituents after repetitive deposition or long-time
deposition.
[0010] For example, repetition of long-time deposition of a zinc
oxide film on a strip substrate or other long substrate results in
difficulty controlling the shape and grain size of the zinc oxide
crystals constituting the resulting zinc oxide film. Abnormal shape
or grain size of the zinc oxide crystals changes optical
characteristics of the resulting zinc oxide film. If such a zinc
oxide film is used in photovoltaic devices, the resulting devices
cannot have uniform characteristics. Furthermore, such a zinc oxide
film may negatively affect electrical properties, such as
conversion efficiency and deterioration rate, and mechanical
characteristics associated with occurrence of film peeling or
cracks.
[0011] These problems are probably caused by: (1) unnecessary
deposit on apparatus members other than the substrate onto which
the film is to be deposited; or (2) precipitate produced in the
aqueous solution. Specifically, a deposit is formed on members in
contact with the solution of the deposition apparatus, such as the
wall of the solution bath, an electrode fastener, and a masking
plate for preventing deposition on the rear surface. In addition,
precipitation may occur in the solution. Furthermore, in the
solution, unnecessary impurities for the film deposition, which are
contained in supplementarily added zinc nitrite or additives, may
increase.
[0012] The zinc ions in the solution are consumed for film
formation. Accordingly, if the counter electrode for forming the
deposition film (zinc oxide film) is made of zinc, the zinc
electrode is dissolved in the solution to restore the zinc
concentration and the volume of the electrode is reduced.
Accordingly, it becomes necessary to replace the electrode.
[0013] It is considered that the solution for forming the
deposition film can be maintained stable if the amount of
unnecessary deposit on the solution bath wall, electrode fastener,
masking plate, or other members of the deposition apparatus and
precipitate in the solution are within a permissible range.
However, once the deposit or precipitate increases beyond the
permissible range, the solution probably becomes unstable and a
satisfactory texture cannot be provided to the resulting deposition
film.
[0014] Furthermore, as the thickness of the deposit on the members
of the deposition apparatus increases, the deposit becomes liable
to peel off. The peeled deposit and the precipitate in the solution
can be trapped on the substrate disadvantageously.
[0015] In order to solve these problems, appropriate maintenance is
performed after the number of times of deposition or the total
length of deposition reaches a predetermined level. For example,
the electrode, the electrode fastener, the masking plate, and the
like are replaced or cleaned, the solution is replaced, or the
precipitate is removed from the solution. In film deposition
immediately after such maintenance, however, zinc oxide crystals
constituting the zinc oxide film do no grow to a desired shape or
grain size in some cases even though the composition of the
solution is suitable for depositing a targeted zinc oxide film.
SUMMARY OF THE INVENTION
[0016] Accordingly, an object of the present invention is to
provide a method for forming a deposition film from an aqueous
solution by electrochemical reaction and in which a deposition film
exhibiting desired characteristics can be produced even after
maintenance of the deposition apparatus. Another object of the
present invention is to provide a method for producing photovoltaic
devices including the deposition film, the devices exhibiting
substantially the same characteristics both before and after
maintenance.
[0017] The inventors of the present invention have conducted
intensive research for a method for forming a deposition film
having satisfactory characteristics and obtained the following
findings.
[0018] The inventors have thought that the reason why a solution
having an optimum composition for forming a deposition film does
not result in a satisfactory texture immediately after maintenance
of the deposition apparatus is the variations of the surface
conditions of replaced members and the amount of precipitate in the
solution.
[0019] In the formation of a deposition film from a solution by
electrochemical reaction, the solution is prepared to have a
composition suitable for the targeted deposition film. The solution
is a mixture containing constituents necessary for the targeted
deposition film in water. In the method for forming deposition
films on substrates, deposit is produced on members in contact with
the solution of the deposition apparatus, and precipitate is also
produced in the solution. Since the deposit on its members and the
precipitate in the solution are at least partially removed by
maintenance of the apparatus, reactions occur to reproduce the
deposit and precipitate. These reactions probably proceed faster in
the early stage. The constituents in the solution are probably
consumed not only for the deposition film on the substrate, but
also for the deposit on the members and the precipitate in the
solution. Consequently, the resulting deposition film on the
substrate does not exhibit a satisfactory texture. The rates of the
reactions reproducing the deposit on the members and the
precipitate in the solution gradually decrease, and the changes in
the contents of the constituents in the solution are alleviated so
that the solution is substantially in equilibrium. The texture of
the deposition film becomes stable only after the solution has come
to this state.
[0020] However, since deposition over a long period of time makes
imbalance the contents of the constituents in the solution, it is
necessary to supplementarily add the constituents to the solution
in order to stabilize the texture.
[0021] The inventors have found that a method for forming a
deposition film which can stably and constantly provide a film
having a satisfactory texture even after maintenance. In the
method, the deposition film is formed under primary conditions,
maintenance is performed at a predetermined time interval,
subsequently a film is deposited under secondary conditions after
the maintenance, and then the deposition film is formed again under
the primary conditions.
[0022] Accordingly, the present invention is directed to a method
for forming a deposition film from an aqueous solution by
electrochemical reaction. The method comprises the steps of:
forming the deposition film under primary deposition conditions;
replacing at least part of members in contact with the solution or
removing deposit on surfaces of the members for maintenance; and
depositing a film under secondary deposition conditions. The steps
are performed in that order.
[0023] The method allows the solution to become stable, thereby
stably promoting the deposition under the primary conditions to
form a targeted deposition film.
[0024] Preferably, the method further comprises the step of forming
again the deposition film again under the primary deposition
conditions after the step of deposition under the secondary
conditions.
[0025] Thus, the deposition under the primary conditions can
proceed in a stable solution even after the maintenance, and the
resulting deposition film have the same satisfactory texture before
and after the maintenance.
[0026] Preferably, the secondary deposition conditions allow a
desired film to deposit on the members in contact with the
solution.
[0027] Thus, the deposition under the primary conditions can stably
proceed even after the maintenance.
[0028] Preferably, the deposition rate of the secondary deposition
conditions is higher than that of the primary deposition
conditions.
[0029] Thus, the primary deposition conditions can be stabilized in
a short time after the maintenance.
[0030] Preferably, the deposition under the secondary deposition
conditions is performed on a substrate different from the substrate
used in the deposition under the primary deposition conditions.
[0031] Thus, the primary deposition conditions can be restored by
use of an inexpensive substrate.
[0032] The present invention is also directed to a method for
producing a photovoltaic device, comprising the steps of: forming a
zinc oxide film by the foregoing method for forming a deposition
film; and forming a silicon-based semiconductor layer after the
step of forming the zinc oxide film.
[0033] Since this method stably produces photovoltaic devices from
the deposition film having a desired texture, the resulting devices
exhibit substantially the same characteristics before and after the
maintenance.
[0034] Further objects, features and advantages of the present
invention will become apparent from the following description of
the preferred embodiments (with reference to the attached
drawings).
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a schematic illustration of an apparatus for
depositing zinc oxide from a solution according to the present
invention.
[0036] FIG. 2 is a schematic sectional view of the layered
structure of a photovoltaic device according to the present
invention.
[0037] FIG. 3 is a schematic sectional view of a continuous
electrolytic deposition apparatus for depositing zinc oxide from a
solution according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] Preferred embodiment will now be described with reference to
the drawings.
[0039] Method of Electrochemical Deposition from Aqueous
Solution
[0040] FIG. 1 shows the structure of an apparatus for forming a
deposition film from an aqueous solution by electrochemical
reaction. The deposition apparatus includes a reactor 101
containing an aqueous solution 102, a conductive substrate 103, an
counter electrode 104, a power supply 105, a load resistor 106, a
solution outlet 107, a solution inlet 108, a solution intake pipe
109, a solution discharge pipe 110, a circulating pump 111, a sub
reactor 112, a heater 113, and a masking plate 114 for preventing
film deposition on the rear surface of the substrate 103.
[0041] Primary deposition conditions, which are intended for the
formation of a targeted deposition film, and secondary deposition
conditions include the composition and temperature of the solution
102, the structures and constituents of the conductive substrate
103 and counter electrode 104, the current applied between the
conductive substrate 103 and the counter electrode 104, and
deposition time.
[0042] To at least partially replace members in contact with the
solution of the deposition apparatus or remove deposit on the
surfaces of the members herein means to replace, for example, the
masking plate 114 or a fastener (not shown in the figure) fastening
the counter electrode 104 or to remove the films deposited on the
surfaces of those members.
[0043] The primary deposition conditions and the secondary
deposition conditions will be described below.
[0044] Secondary Deposition Conditions
[0045] The secondary deposition conditions are set for temporary
deposition for more stabilizing the state of the deposition
apparatus. For example, the secondary conditions are set for
forming a film on the members which have been replaced or cleaned
to remove deposit. This deposition on the members is intended to
produce deposit on the members to such an extent that the texture
of the resulting deposition film formed from an aqueous solution
for forming a targeted film does not significantly vary from the
texture of the targeted deposition film.
[0046] The secondary deposition conditions are preferably different
from the primary deposition conditions described below, but may be
the same.
[0047] The deposition rate of the secondary deposition conditions
is preferably higher than that of the primary deposition
conditions. Thus, the primary deposition conditions can be
stabilized in a short time after maintenance.
[0048] In order to set the deposition rate of the secondary
deposition conditions to be higher than that of the primary
deposition conditions, for example, the temperature of the solution
or the amount of current is increased, or the composition of the
solution or stir speed is varied. For varying the composition of
the solution, the ion concentrations or the additive content in the
solution for forming the targeted deposition film may be increased.
However, the change of the solution composition can involve large
change in concentration or other factors when the solution is
changed to that of the primary conditions.
[0049] The conductive substrate 103 used under the secondary
conditions may be different from that under the primary conditions.
While the substrate may be treated according to the application for
the primary deposition conditions, the substrate used under the
second deposition conditions does not require such treatment.
[0050] Primary Deposition Conditions
[0051] The primary deposition conditions are set as follows for
forming a deposition film having a desired texture.
[0052] Aqueous Solution 102:
[0053] The aqueous solution 102 contains ions for forming a
targeted deposition film. For a zinc oxide film, the solution 102
contains at least zinc nitrate and an additive, for example.
Deposition of a zinc oxide film will be described below.
[0054] Zinc nitrite dissociates into nitrate ions and zinc ions in
water. The concentrations of the nitrate ions and the zinc ions are
preferably in the range of 0.002 to 3.0 mol/L, more preferably 0.01
to 1.5 mol/L, and still more preferably 0.05 to 0.7 mol/L.
[0055] The additive may be a polycarboxylic acid in which a
carboxyl group is bonded to each of carbon having sp2 hybrid
orbital, or its ester. Preferably, the additive has a --C.dbd.C--
group whose carbons are each bonded to the carboxyl group or an
ester group, or an aromatic ring (benzene ring or heteroaromatic
ring) some of whose carbons are bonded to a carboxyl group.
Examples of such additives include phthalic acid, isophthalic acid,
maleic acid, naphthalic acid, and their esters. The concentration
of the polycarboxylic acid or its ester is preferably in the range
of 0.5 to 500 .mu.mol/L, more preferably 50 to 500 .mu.mol/L, and
still more preferably 150 to 500 .mu.mol/L. Such a concentration of
the polycarboxylic acid or its ester contributes to efficient
formation of a zinc oxide film having a texture exhibiting a high
optical confinement effect suitable as the transparent conductive
layer of a photovoltaic device described later.
[0056] The solution 102 may further contain saccharose or dextrin.
These additives properly control the electrolytic deposition to
prevent the zinc oxide film from abnormally growing, so that the
resulting film exhibits uniform surface and superior adhesion.
Thus, a zinc oxide film having a texture exhibiting a high optical
confinement effect can be produced in a high yield. If the solution
102 contains saccharose or dextrin, the saccharose content is
preferably in the range of 1 to 500 g/L and more preferably 3 to
100 g/L, and the dextrin content is preferably in the range of 0.01
to 10 g/L and more preferably 0.025 to 1 g/L.
[0057] The electrolytic deposition is performed while the
composition of the solution is restored by adding its constituents.
The solution is prepared so as to have a pH of at least 3 and an
electric conductivity of at least 10 mS/cm, and the solution
temperature is set at 60.degree. C. or more. Thus, efficient
deposition of a uniform zinc oxide film can be achieved without
abnormal growth.
[0058] Power Supply, Deposition Section:
[0059] The targeted deposition film is formed on the conductive
substrate 103. The conductive substrate 103 may be conductive
itself, or has a structure in which a conductive layer is formed
over an insulating substrate. The surface onto which the deposition
film is to be formed may be treated according to the
application.
[0060] The counter electrode 104 is intended for a reaction
together with the conductive substrate 103. The counter electrode
104 can be made of any conductive material. A stable material may
be suitable from the viewpoint of preventing constituents
unnecessary for the formation of the deposition film from being
supplied to the solution. The counter electrode 104 may also be
made of a material capable of supplying ions necessary for the
formation of the deposition film to the solution. For example, the
counter electrode 104 may be made of zinc for deposition of a zinc
oxide film.
[0061] The conductive substrate 103 and the counter electrode 104
are connected to the power supply 105 through the load resistor
106. The current applied between the conductive substrate 103 and
the counter electrode 104 is preferably in the range of 0.1 to 100
mA/cm.sup.2, more preferably 1 to 30 mA/cm.sup.2, and optimally 4
to 20 mA/cm.sup.2.
[0062] Solution Circulating System:
[0063] In order to stir the entire solution, a solution circulating
system is used which includes the solution inlet 108, the solution
outlet 107, the circulating pump 111, the solution intake pipe 109,
and the solution discharge pipe 110.
[0064] Photovoltaic Device
[0065] FIG. 2 is a schematic sectional view of the layered
structure of a photovoltaic device according to the present
invention. The device shown in the figure serves as a solar cell
and includes a substrate (support) 201, a metal layer (rear
reflector) 202, a hexagonal polycrystalline zinc oxide layer
(transparent conductive layer) 203, a semiconductor layer 204, a
transparent electrode layer 205, and a collector electrode 206.
When light enters through the substrate 201, the substrate is
transparent and the other layers 202 to 206 are disposed in inverse
order on the substrate 201.
[0066] The components of the photovoltaic device will be described
in detail below.
[0067] Substrate
[0068] The substrate 201 is made of metal, or resin, glass,
ceramic, or the like coated with a conductive material. The surface
of the substrate 201 may have a fine texture. The substrate 201 may
be transparent so that light enters through the substrate. A
substrate 201 made of a flexible material, such as stainless steel
or polyimide, can be so long as to be adoptable to continuous
deposition.
[0069] Metal Layer
[0070] The metal layer 202 serves as both an electrode and a
reflection layer for reflecting light reaching the substrate 201 to
reuse the light in the semiconductor layer 204. The metal layer 202
is formed of, for example, Al, Cu, Ag, Au, and the like by vapor
deposition, sputtering, electrolytic deposition, printing, or the
like. Also, by giving a texture to the surface of the metal layer
202, the optical path in the semiconductor layer 204 of reflected
light is lengthened to increase short-circuit current.
[0071] Transparent Conductive Layer
[0072] The transparent conductive layer 203 allows incident light
and reflected light to diffuse increasingly to lengthen the optical
path in the semiconductor layer 204. In addition, the transparent
conductive layer 203 prevents the constituent elements of the metal
layer 202 from diffusing or migrating into the semiconductor layer
204, and thus prevents the photovoltaic device from shunting.
Furthermore, by giving an appropriate resistance to the transparent
conductive layer 203, short circuits resulting from defects in the
semiconductor layer 204, such as pinholes, can be prevented.
Preferably, the surface of the transparent conductive layer 203, as
well as the metal layer 202, has a texture effective at exhibiting
an optical confinement effect.
[0073] The deposition method of the present invention is suitable
to form the transparent conductive layer 203. Preferably, a zinc
oxide film is formed on the metal layer 202 in advance by
sputtering, and then a further zinc oxide film is provided on the
previously formed zinc oxide film by the electrolytic deposition.
Such a process increases the adhesion between the metal layer 202
and the zinc oxide film or transparent conductive layer 203.
[0074] Semiconductor Layer
[0075] The semiconductor layer 204 is made of amorphous or
microcrystalline silicon, carbon, germanium, or their alloy. The
semiconductor layer 204 also contains hydrogen and/or halogen
atoms. The preferred content of hydrogen and/or halogen atoms is in
the range of 0.1 to 40 atomic percent. The semiconductor layer may
further contain oxygen, nitrogen, or other impurities. The
concentration of such impurities is preferably 5.times.10.sup.19
atoms/cm.sup.-3 or less. For a p-type semiconductor, a Group III
element is further added; for an n-type semiconductor, a Group V
element.
[0076] For a stack cell, it is preferable that the i-type
semiconductor layer of a pin junction close to the light incident
side has a wide band gap, and that the band gap is reduced as the
distance between the light incident side and the pin junction is
increased. Preferably, a minimum value of the band gap in the
i-type semiconductor layer is on the p-type semiconductor side of
the center of the thickness.
[0077] For a doped layer at the light incidence side, a crystalline
semiconductor having a low light absorption or a semiconductor
having a wide band gap is suitable.
[0078] For the formation of the semiconductor layer 204, microwave
(MW) plasma CVD or high-frequency (RF) CVD is suitably applied.
[0079] In this semiconductor deposition, a tip that "i-type layer
made of Graded SiGe having a Ge content of 20 to 70 atomic percent"
(Japanese Patent Laid-Open No. 6-21494) may be adopted.
[0080] Transparent Electrode Layer
[0081] The transparent electrode layer 205 can serve as an
antireflection layer by appropriately setting the thickness. The
transparent electrode layer 205 is formed of ITO, ZnO,
In.sub.2O.sub.3, or the like by a method, such as vapor deposition,
CVD, spraying, spin-on, or dipping. The transparent electrode layer
205 may further contain a material varying the conductivity.
[0082] Collector Electrode
[0083] The collector electrode 206 is intended to increase the
power collection efficiency. The collector electrode 206 may be
formed by depositing a metal in a power collection pattern by
sputtering through a mask, by printing a conductive paste or a
solder paste, or by bonding metal wires with a conductive
paste.
[0084] The photovoltaic device may be provided with protective
layers on its both surfaces, if necessary. A reinforcement, such as
a steel plate, may be used together with the protective layers.
EXAMPLES
[0085] Examples according to the present invention will now be
described, but the invention is not limited to these examples.
Example 1
[0086] A zinc oxide layer (transparent conductive layer) 203 as
shown in FIG. 2 was formed with a roll-to-roll type apparatus shown
in FIG. 3.
[0087] The roll-to-roll type apparatus included a delivery roller
301, a take-up roller 302, a rolled substrate 303, a transport
roller 304, a zinc oxide deposition bath 305 containing a solution
306 for depositing the zinc oxide film, an counter electrode 307, a
power supply 308, a rinse bath 309 containing water 310, a water
shower 311, a drier oven 312, an infrared heater 313, and a masking
plate 314 for preventing film deposition on the rear surface of the
substrate 303.
[0088] In the present example, silver was deposited to a 200 nm
thick metal layer (rear reflector) 202 in advance on a rolled
SUS4302D support (acting as the substrate 201 shown in FIG. 2) with
a DC magnetron sputtering apparatus for the roll-to-roll system. In
addition, a zinc oxide film was deposited to a thickness of 100 nm
on the metal layer 202 at a substrate temperature of 150.degree. C.
and a deposition rate of 5 nm/s with a similar DC magnetron
sputtering apparatus for the roll-to-roll system. The resulting
support was used as the rolled substrate 303.
[0089] Zinc oxide was further deposited to form the zinc oxide
layer or transparent conductive layer 203 on the substrate 303
under the below-described primary deposition conditions with the
roll-to-roll apparatus shown in FIG. 3.
[0090] The substrate 303 was transported to the zinc oxide
deposition bath 305 via the transport roller 304. The zinc oxide
deposition bath 305 had a sub deposition bath (not shown in the
figure). The solution 306 in the zinc oxide deposition bath 305 was
prepared by adding 60 g of zinc nitrate hexahydrate into 1 L of
water (to be 0.20 mol/L zinc nitrate) and further adding to the
solution potassium biphthalate (molecular weight: 204.22) at a
concentration of 0.05 mmol/L and 0.3 g of dextrin. The resulting
solution 306 was subjected to circulation to be stirred. The total
volume of the solution 306 was 1,000 L. The temperature of the bath
was maintained at 80.degree. C. and the pH was 4.0 to 5.0. The
electric conductivity was 65 mS/cm. The counter electrode 307 was a
zinc plate subjected to blasting and was used as the anode, and the
substrate 303 was used as the cathode. Thus, electrolytic
deposition was performed with the anode counter electrode 307 and
the cathode substrate 303 at a current density of 12 mA/cm.sup.2
(1.2 A/dcm.sup.2). The zinc oxide layer was deposited over a length
of 10,000 m. On the way of deposition, consumed zinc nitrate and
potassium biphthalate in the solution was supplementarily added.
The substrate 303 was transported at a speed of 2,000 mm/min, and
thus a zinc oxide thin-film of 1.8 .mu.m in thickness, serving as
the transparent conductive layer 203, was formed.
[0091] The substrate 303 having the zinc oxide layer 203 was
transported to the rinse bath 309 and the shower 311 to rinse out
the solution remaining on the surface of the substrate 303, and
drained with an air knife (not shown in the figure). Then, the
substrate 303 was passed through the drier oven 312 to be dried and
finally wound around the take-up roller 302.
[0092] Then, the masking plate 314, the counter electrode 307, and
the counter electrode fastener (not shown in the figure) were
replaced for maintenance.
[0093] After the maintenance, another deposition under the
secondary conditions was performed in the same manner as the
deposition under the primary conditions except that the simple
SUS4302D support was used as the substrate 303. Thus, zinc oxide
was deposited over a length of 100 m directly on the SUS4302D
support.
[0094] Then, the following deposition was performed under the same
conditions as before the maintenance, that is, the deposition under
the primary conditions was performed again. Thus, the zinc oxide
layer 203 was deposited over a length of 50 m.
[0095] Part of the substrate having the zinc oxide layer formed at
the point of the completion of the deposition under the primary
conditions after the maintenance was cut out for evaluation, and
the texture at the surface of the test piece was analyzed with a
scanning probe microscope Nanopics produced by Seiko Instruments
Inc. The results were shown in the table. In the table, the
inclination angle .theta., the surface roughness Ra, and the grain
size are those of the zinc oxide layer 203 formed at the point of
the completion of the deposition under the primary conditions after
the maintenance, and are represented by relative values to those
(each assumed to be 1) of the zinc oxide layer 203 at the point of
the completion of the deposition under the primary conditions
before the maintenance.
[0096] Then, a semiconductor layer 204 was formed on the zinc oxide
layer 203 by CVD. The semiconductor layer 204 included a 10 nm
thick n-type microcrystalline silicon sub layer acting as the
bottom cell, a 3,000 nm thick non-doped microcrystalline silicon
sub layer, and a 30 nm thick p-type microcrystalline silicon sub
layer. Subsequently, ITO was deposited to a thickness of 65 nm by
sputtering, thus forming a transparent electrode layer 205 serving
as an antireflective upper electrode. Finally, silver was deposited
in a grid by thermal vapor deposition to form an upper extraction
electrode serving as the collector electrode 206, thus completing a
photovoltaic device.
[0097] The short circuit current density and the conversion
efficiency of the photovoltaic device were measured under
artificial sunlight. The results were shown in the Table. In the
table, the short circuit current density and the conversion
efficiency are those of the photovoltaic device made from the
substrate having the zinc oxide layer formed at the point of the
completion of the deposition under the primary conditions after the
maintenance, and are represented by relative values to those (each
assumed to be 1) of the photovoltaic device made from the substrate
having the zinc oxide layer at the point of the completion of the
deposition under the primary conditions before the maintenance.
Example 2
[0098] Electrolytic deposition under the secondary conditions was
performed with the anode or counter electrode 307 and the cathode
or substrate 303 at a current density of 24 mA/cm.sup.2 (2.4
A/dcm.sup.2). Other conditions were the same as in Example 1.
Evaluation was also performed in the same manner as in Example 1.
The results were shown in the Table.
Example 3
[0099] In deposition under the secondary conditions, the substrate
was transported at a rate of 1,000 mm/min. Other conditions were
the same as in Example 1. Evaluation was also performed in the same
manner as in Example 1. The results were shown in the Table.
Example 4
[0100] In deposition under the secondary conditions, the solution
temperature was set at 90.degree. C. Other conditions were the same
as in Example 1. Evaluation was also performed in the same manner
as in Example 1. The results were shown in Table 1.
Example 5
[0101] For deposition under the secondary deposition, 120 g of zinc
nitrate hexahydrate was added into 1 L of water (to be 0.40 mol/L
zinc nitrate). Other conditions were the same as in Example 1.
Evaluation was also performed in the same manner as in Example 1.
The results were shown in the Table.
Comparative Example
[0102] The zinc oxide layer or transparent conductive layer 203 was
formed in the same manner as in Example 1, except that deposition
under the secondary conditions was not performed. Evaluation was
also performed in the same manner as in Example 1. The results were
shown in the Table.
1 TABLE 1 Surface Short circuit Inclination roughness Grain current
Conversion angle .theta. Ra size density efficiency Example 1 0.98
0.98 1.02 0.99 0.99 Example 2 0.95 0.95 1.05 0.97 0.97 Example 3
0.95 0.95 1.10 0.97 0.97 Example 4 0.95 0.96 1.00 0.96 0.96 Example
5 0.95 0.94 1.05 0.97 0.97 Comparative 0.70 0.70 1.30 0.60 0.60
example 1
[0103] The results of Example 1 shown in the Table suggest that a
zinc oxide layer having a stable texture can be provided through
the process in which deposition under the secondary conditions is
performed after maintenance on a substrate different from the
substrate used for the deposition under the primary conditions and
subsequently the deposition under the primary conditions was
performed again. In such a process, the short circuit current and
conversion efficiency of the photovoltaic device do not vary much
before and after maintenance.
[0104] The results of Example 2 suggest that a zinc oxide layer
having a stable texture can also be provided through the process in
which deposition under the secondary conditions was performed at a
higher current density than that of the primary deposition
conditions and subsequently the deposition under the primary
conditions was performed again. In such a process, the short
circuit current and conversion efficiency of the photovoltaic
device do not vary much before and after maintenance, and the
differences are permissible.
[0105] The results of Example 3 suggest that even though the
transport speed of the substrate of the secondary conditions is
varied, a zinc oxide layer having a stable texture can be provided.
In such a process, the short circuit current and conversion
efficiency of the photovoltaic device do not vary much before and
after maintenance, and the differences are permissible.
[0106] The results of Example 4 suggest that even though the
solution temperature of the secondary conditions is varied, a zinc
oxide layer having a stable texture can be provided. In such a
process, the short circuit current and conversion efficiency of the
photovoltaic device do not vary much before and after maintenance,
and the differences are permissible.
[0107] The results of Example 5 suggest that even though the
solution concentration of the secondary conditions is varied, a
zinc oxide layer having a stable texture can be provided. In such a
process, the short circuit current and conversion efficiency of the
photovoltaic device do not vary much before and after maintenance,
and the differences are permissible.
[0108] The results of the comparative example suggest that if
deposition under the secondary conditions is not performed, a
satisfactory texture cannot be provided to the zinc oxide layer.
The short circuit current and the conversion efficiency of the
photovoltaic device prepared from such a film are not
satisfactory.
[0109] While the present invention has been described with
reference to what are presently considered to be the preferred
embodiments, it is to be understood that the invention is not
limited to the disclosed embodiments. On the contrary, the
invention is intended to cover various modifications and equivalent
arrangements included within the spirit and scope of the appended
claims. The scope of the following claims is to be accorded the
broadest interpretation so as to encompass all such modifications
and equivalent structures and functions.
* * * * *