Title: Transparent conductive film formation process, photovoltaic device production process, transparent conductive film, and photovoltaic device
Abstract: In a process for forming on a substrate a transparent conductive film having crystallizability, the process comprises a first step of forming a film at a first film formation rate and a second step of forming a film at a second film formation rate, and the relationship between film formation rates in the respective steps satisfies:
which provides a process for producing a transparent conductive film by a deposition process advantageous for cost reduction, which can form in a short time a transparent conductive film having an uneven surface profile with a high light-confining effect, and can bring about an improvement in photovoltaic performance and enjoy a high mass productivity when applied to the formation of multi-layer structure of photovoltaic devices.
Patent Number: 6,930,025 Issued on 08/16/2005 to Nakayama, et al.
| Inventors:
|
Nakayama; Akiya (Nara, JP);
Echizen; Hiroshi (Nara, JP);
Takai; Yasuyoshi (Nara, JP);
Okada; Naoto (Nara, JP);
Kiso; Shigeo (Nara, JP)
|
| Assignee:
|
Canon Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
059168 |
| Filed:
|
January 31, 2002 |
Foreign Application Priority Data
| Feb 01, 2001[JP] | 2001-025062 |
| Jul 31, 2001[JP] | 2001-230963 |
| Current U.S. Class: |
438/478; 438/485; 438/486 |
| Intern'l Class: |
H01L 021/20; H01L 021//36 |
| Field of Search: |
438/478,485,486
136/243,256
|
References Cited [Referenced By]
U.S. Patent Documents
| 5453165 | Sep., 1995 | Bachmann.
| |
| 5510151 | Apr., 1996 | Matsuyama et al.
| |
| 5549763 | Aug., 1996 | Sano et al.
| |
| 5620530 | Apr., 1997 | Nakayama.
| |
| 5620924 | Apr., 1997 | Takizawa et al.
| |
| 5981867 | Nov., 1999 | Toyama et al.
| |
| 5998730 | Dec., 1999 | Shiozaki et al.
| |
| 6043427 | Mar., 2000 | Nishimoto.
| |
| 6140570 | Oct., 2000 | Kariya.
| |
| 6172296 | Jan., 2001 | Iwasaki et al.
| |
| 6737123 | May., 2004 | Kondo et al.
| |
| 6855621 | Feb., 2005 | Kondo et al.
| |
| 6858308 | Feb., 2005 | Kondo et al.
| |
| 2001/0051388 | Dec., 2001 | Shiozaki et al.
| |
| 2003/0127127 | Jul., 2003 | Inamasu et al.
| |
| 2003/0143822 | Jul., 2003 | Kondo et al.
| |
| Foreign Patent Documents |
| 63-456 | Jan., 1988 | JP.
| |
| 5-343715 | Dec., 1993 | JP.
| |
| 6-116722 | Apr., 1994 | JP.
| |
| 8-32094 | Feb., 1996 | JP.
| |
| 10-310862 | Nov., 1998 | JP.
| |
| 11-220154 | Aug., 1999 | JP.
| |
Primary Examiner: Coleman; W. David
Assistant Examiner: Nguyen; Khiem D.
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper and Scinto
Claims
1. A process for forming on a substrate a transparent conductive film having
crystallizability, the process comprising:
a first step of forming a film at a first film formation rate;
a second step of forming a film at a second film formation rate; and
a third step of forming a film at a third film formation rate, with the third
step being carried out after the second step,
wherein the relationship between film formation rates in the respective steps
satisfies:
2. The transparent conductive film formation process according to claim 1, wherein,
in the first step, nuclei are formed on the substrate, and, in the second step,
the film is so formed as to cover the substrate surface and the surfaces of the
nuclei over their whole area.
3. The transparent conductive film formation process according to claim 1, wherein,
in the first step, the film is formed in a thickness of from 1 nm to 100 nm.
4. The transparent conductive film formation process according to claim 1, wherein,
in the second step, the film is formed in a thickness of 5 μm or less.
5. The transparent conductive film formation process according to claim 1, wherein,
in the second step, the film is formed in a thickness of 1 μm or less.
6. The transparent conductive film formation process according to claim 1, wherein,
in the first step, the first step is carried out such that the average distance
between apexes of hills themselves of the film formed in the first step and the
average distance between the apexes of hills of the film formed in the first step
and the substrate surface are in a ratio of from 1:3 to 4:1.
7. The transparent conductive film formation process according to claim 1, wherein
the transparent conductive film is formed by a roll-to-roll method in which a continuous
substrate is put across rollers and transported therebetween.
8. The transparent conductive film formation process according to claim 1, wherein,
in the third step, the film is formed in a thickness of 5 μm or less.
9. The transparent conductive film formation process according to claim 1, wherein
the transparent conductive film is formed by sputtering.
10. The transparent conductive film formation process according to claim 9, wherein,
in the second step, a target having been subjected to pre-sputtering is used.
11. A process for producing a photovoltaic device, the process comprising the
steps of:
forming a transparent conductive film by the process according to claim 1; and
forming a semiconductor layer.
12. A transparent conductive film formed by the process according to claim 1.
13. A photovoltaic device comprising a transparent conductive film formed by
the process according to claim 1.
14. The transparent conductive film formation process according to claim 1, wherein
the transparent conductive film comprises zinc oxide, tin oxide, indium oxide,
titanium oxide, or a composite of any of these oxides.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for forming a transparent conductive film
superposed on a substrate by means of a sputtering system or the like, a process
for producing a photovoltaic device, a transparent conductive film formed by that
process, and a photovoltaic device making use of the transparent conductive film.
2. Related Background Art
Photovoltaic devices comprised of hydrogenated amorphous silicon, hydrogenated
amorphous silicon germanium, hydrogenated amorphous silicon carbide, microcrystalline
silicon, polycrystalline silicon or compound semiconductor are provided with reflecting
films on the back sides of devices in order to improve collection efficiency at
long wavelengths. It is desirable for such reflecting films to exhibit reflection
performance effective at wavelengths which are near to energy band ends of semiconductor
materials and at which their absorption is small, i.e., wavelengths of from 800
nm to 1,200 nm. Those which can well satisfy this condition are metals such as
gold, silver, copper and aluminum.
An uneven layer which is optically transparent within a stated range of reflection
is also provided in some cases in order to confine in a semiconductor active layer
the light having entered a photovoltaic device. Such an uneven layer is commonly
provided between the reflecting film formed of any of the above metals and the
semiconductor active layer so that the reflected light can effectively be utilized
to improve short-circuit current density Jsc. As methods by which such a transparent
conductive film used as a light-confining layer is made to have an uneven surface
profile, the following techniques are conventionally known.
Japanese Patent Application Laid-Open No. 63-456 discloses that, in a method
of forming a film of a polycrystalline metal oxide by vacuum deposition, a film
with uneven surface structure can be formed at the surface of a transparent conductive
film where a metal oxide material is vacuum-deposited at a rate of 15 to 30 angstroms/second
with heating.
Japanese Patent Application Laid-Open No. 8-32094 discloses that a highly
reliable solar cell can be obtained where a transparent conductive film consists
of at least two layers and film formation temperature and/or film formation rate
is/are controlled so that one layer of them is formed as a crystalline transparent
conductive film and one layer as an amorphous transparent conductive film.
Japanese Patent Application Laid-Open No. 10-310862 discloses that a reflecting
film and a transparent conductive film which have the desired textural structure,
have a high reflectance, are inexpensive and have a high reliability can stably
be obtained where a film-forming chamber is once heated and then cooled, thereafter
a metallic reflecting film is formed on a substrate, and, after active oxygen is
brought into contact therewith, a transparent conductive film is formed thereon.
Japanese Patent Application Laid-Open No. 11-220154 discloses that, after
a metallic reflecting film is formed on a substrate, a first transparent conductive
film is formed in an atmosphere containing 50% or more of oxygen and a second transparent
conductive film is formed in an atmosphere of argon and at a temperature higher
than that at the time of the formation of the first transparent conductive film,
so that in the first transparent conductive film the c-axes of crystal grains stand
parallel to the substrate and in the second transparent conductive film the c-axes
of crystal grains stand vertical to the substrate, thus a transparent conductive
film the surface unevenness of which is free of any ridges can be formed and a
solar cell improved in photovoltaic characteristics and having a high reliability
can be obtained.
Japanese Patent Application Laid-Open No. 6-116722 discloses that a metal
layer and a transparent conductive film are continuously formed by sputtering on
a continuous substrate while it is moved (a roll-to-roll method).
Thus, deposited films having a certain good quality can be obtained by regulating
the rate of film formation and forming the transparent conductive film in multiple layers.
However, in order to mass-produce good transparent conductive films by spurring
at a low cost, there are problems as stated below which should be settled.
In order that the transparent conductive film used as a light-confining layer
is grown to have an uneven surface profile which is effective for the characteristics
of photovoltaic devices, the film has had to be formed by deposition in an extremely
large film thickness and/or at an extremely low rate of film formation. Such a
method of forming thin films has problems of a high material cost and a long film
formation time, resulting in a very high production cost for the photovoltaic devices.
Also, it is difficult to provide any good uneven surface profile only by simply
making the film formation rate higher, undesirably bringing about an unstable sputtering
discharge and a low film quality.
SUMMARY OF THE INVENTION
Accordingly, a chief object of the present invention is to provide a
process for producing a transparent conductive film by a deposition process advantageous
for cost reduction, which can form in a short time a transparent conductive film
having an uneven surface profile with a high light-confining effect, and can bring
about an improvement in photovoltaic performance and enjoy a high mass productivity
when applied to the formation of multi-layer structure of photovoltaic devices.
In order to achieve both film characteristics and stability at a high level when
films are formed by sputtering at a high rate, the present inventors made extensive
studies especially taking note of the relationship between conditions of film formation
rate, deposited-film surface profile and device construction. As the result, they
have discovered that conditions of film formation rate and film thickness at the
initial stage of sputtering on the substrate are effective factors for improving
film characteristics and film structure of transparent conductive films and for
improving device performance, and also greatly effective factors for the production
of transparent conductive films in a shorter time, and have accomplished the invention
having the following construction.
As an embodiment, the present invention provides a process for forming a transparent
conductive film on a substrate having at least a surface layer formed of a metal
layer, the process comprising:
a first step of forming a film at a first film formation rate; and
a second step of forming a film at a second film formation rate;
the relationship between film formation rates in the respective steps satisfying:
In the present invention, the term "film" is the concept which implies "nucleus".
As more preferred features, the transparent conductive film formation process
of the present invention may embrace:
that, in the first step, nuclei are formed on the substrate, and, in the second
step, a transparent conductive film is so formed as to cover the substrate surface
and the surfaces of the nuclei over their whole area;
that, in the first step, the film is formed in a thickness of from 1 nm to
100 nm;
that, in the second step, the film is formed in a thickness of 5 μm or
less, and preferably 1 μm or less;
that the process has, after the second step, a third step of forming a transparent
conductive film at a third film formation rate, and the relationship between film
formation rates in the respective steps satisfies:
that, in the third step, the film is formed in a thickness of 5 μm or less;
that the transparent conductive film is crystal;
that the transparent conductive film is formed by sputtering; and
that, in the second step, a target having been subjected to pre-sputtering
is used.
As another feature in the transparent conductive film formation process of the
present invention, the first transparent conductive film (the film formed in the
first step) may preferably be so formed that the average distance between the apexes
of hills themselves of the first transparent conductive film and the average distance
between the apexes of hills of the first transparent conductive film and the substrate
surface are in a ratio of from 1:3 to 4:1.
As a still another feature, the transparent conductive film may preferably be
formed by a roll-to-roll method in which a continuous substrate is put across rollers
and transported therebetween.
The present invention also provides a process for producing a photovoltaic device,
comprising the steps of forming a transparent conductive film by the transparent
conductive film formation process of the present invention, and forming a semiconductor layer.
The present invention still also provides a transparent conductive film formed
by the transparent conductive film formation process of the present invention.
The present invention further provides a photovoltaic device having a transparent
conductive film formed by the transparent conductive film formation process of
the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are diagrammatic views showing cross-sectional structure of
examples of the photovoltaic device according to the present invention.
FIGS. 2A and 2B are schematic views showing examples of a production system
for forming the photovoltaic device of the present invention.
FIG. 3 is a diagrammatic view showing cross-sectional structure of a photovoltaic
device made in Comparative Examples.
FIGS. 4A and 4B are schematic views showing an example of the production system
of a roll-to-roll method, used for forming the photovoltaic device of the present
invention on a continuous substrate.
FIG. 5. is a graph showing the relationship between film formation rate
ratio and photoelectric conversion efficiency in an experiment made in the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the transparent conductive film formation process according
to preferred embodiments of the present invention, the ratio of film formation
rates between the steps of forming transparent conductive films is controlled so
that a surface profile most favorable as a back reflecting film of a photovoltaic
device can be materialized and also the film formation rate of the whole transparent
conductive film can be made higher, thus a transparent conductive film having an
uneven surface profile with a high light-confining effect can be formed in a short
time. Also, using this transparent conductive film, a photovoltaic device having
a good photoelectric conversion efficiency and a high reliability can be produced
at a low cost.
The present invention having the above construction and the conventional techniques
described previously differ greatly from each other in conceptional and specific construction.
More specifically, the disclosure in Japanese Patent Application Laid-Open No.
63-456 concerns a technique taking note of the film formation rate, but there is
no disclosure about the changing of conditions of film formation rate and the formation
of films in a short time.
The disclosure in Japanese Patent Application Laid-Open No. 8-32094 takes note
of a feature that the transparent conductive film is a multiple layer, but concerns
combination of crystal and amorphous, and the transparent conductive film has different
construction. Also, there is no disclosure at all about the changing intentionally
of conditions of film formation rate during the formation of the crystal transparent
conductive film. Still also, there is no disclosure about the changing of conditions
of film formation rate to control the film formation rate and layer structure.
The disclosure in Japanese Patent Application Laid-Open No. 10-310862 takes note
of an improvement in reflectance which is achievable by forming a transparent conductive
film after the exposure of a metallic reflecting film to active oxygen, and does
not suggest at all any change in surface profile which is attributable to the changing
of conditions of film formation rate for the transparent conductive film.
The disclosure in Japanese Patent Application Laid-Open No. 11-220154 takes note
of forming a transparent conductive film in double layer and changing film formation
gas and film formation temperature, but is different from the present invention
in which conditions of film formation rate are changed to attain the surface profile
with a proper surface unevenness.
Embodiments of the present invention are exemplified below. The present
invention is by no means limited to these examples. In the following embodiments,
transparent conductive films formed in the respective steps are all so formed as
to have crystallizability.
FIG. 1A shows cross-sectional structure of an example of the photovoltaic device
according to an embodiment of the present invention, which comprises a substrate
101 and superposed thereon a reflecting film
102, a first transparent
conductive film
103a, a second transparent conductive film
103b,
a semiconductor layer
105, a transparent electrode
106 and a collector
electrode
107 in order. The reflecting film
102, the first transparent
conductive film
103a and the second transparent conductive film
103b
constitute a back reflecting film
104. In the present specification,
the wording "on a substrate" embraces also expression "on a film (such as a reflecting
film or a metal layer) formed on a substrate".
FIG. 2A shows an example of a production system for forming on a substrate the
photovoltaic device shown in FIG.
1A. Films are formed using this system
in the following procedure.
(1) Substrates
205 are set on a substrate holder
204, and the inside
of a film-forming chamber
201 is evacuated by means of a vacuum pump (not
shown). Each substrate may be made of a conductive material or an electrically
insulating material, or may also be an electrically insulating substrate the surface
of which has been subjected to conductive treatment. It may also be a semiconductor
crystal bulk. It may still also be a light-transmitting substrate such as glass.
It may preferably be a substrate free of deformation or strain and having the desired
strength, which may preferably include thin sheets of metals such as Fe, Ni, Cr
and Al, alloys of any of these, and stainless steel, composites of any of these,
and films of heat-resistant synthetic resins such as polyester and polyethylene.
(2) After the inside of the film-forming chamber
201 has been evacuated
to a stated pressure, a material gas feed system (not shown) feeds argon gas thereinto
through a gas feed pipe
206, and the internal pressure is regulated to a
stated pressure by adjusting the valve travel of an evacuation valve (not shown).
The substrate holder
204 is rotated by driving a rotating shaft
203.
(3) A plurality of infrared lamps of a heater
202 are lighted to heat
the substrates to a temperature for forming a reflecting film.
(4) A DC power source
210 is switched on to cause argon plasma to take
place on a target
207 for the reflecting film, and then a shutter
215
is opened to form the reflecting film
102. After the reflecting film has
been deposited in a stated film thickness, the shutter
215 is closed, and
the DC power source is switched off. As materials for the reflecting film, materials
having a high reflectance are preferred, and may preferably be gold, silver, copper,
aluminum and alloys of any of these.
(5) Next, the heater
202 is set so as to provide temperature for forming
a transparent conductive film. After the substrates have been heated to a stated
temperature, a DC power source
211 is switched on to cause argon plasma
to take place on a target
208 for a first transparent conductive film, and
then a shutter
216 is opened to deposit the first transparent conductive
film. After the first transparent conductive film has been formed by a stated film
thickness at a stated film formation rate, the shutter
216 is closed, and
the DC power source
211 is switched off.
(6) Next, a DC power source
212 is switched on to cause argon plasma to
take place on a target
209 for a second transparent conductive film, and
then a shutter
217 is opened to deposit the second transparent conductive
film. After the second transparent conductive film has been formed by a stated
film thickness at a stated film formation rate, the shutter
217 is closed,
and the DC power source
212 is switched off.
As materials for the transparent conductive films, materials having a high transmittance
are preferred, and may preferably be oxides of zinc, tin, indium and titanium and
composites of any of these. Also, it is effective in the present invention to optionally
introduce oxygen together with the argon as a sputtering material gas of the materials
for transparent conductive films, and water vapor.
Here, surface unevenness with hills of hundreds of nanometers in size can be
produced at the surface of the back reflecting film
104, and the effect
of scattering light can be utilized. In FIG. 1A, this surface unevenness is diagrammatically
exaggeratively illustrated.
In this way, the reflecting film and the transparent conductive films can continuously
be prepared on the substrate.
Using another deposition system, the semiconductor layer
105 and the
transparent electrode
106 are further prepared and the collector electrode
107 is formed thereon. A protective resin may further be provided.
The transparent conductive films formed on the reflecting film prepared according
to such a procedure have a surface profile having no ridges and a small density
of hills, thus the reflected light is effectively absorbed in the semiconductor
layer and the photoelectric conversion efficiency can be improved.
The present inventors have discovered that the film formation rate for the first
transparent conductive film after the preparation of the reflecting film in the
step (5) correlates with the surface profile and the photoelectric conversion efficiency
of the photovoltaic device. Stated specifically, the first transparent conductive
film
103a to be formed in the step (5) is formed at a low film formation
rate in a short time, and the second transparent conductive film
103a
is formed at a high film formation rate, controlling the ratio of these film
formation rates within a specific range. This enables formation of a back reflecting
film
104 more improved in photoelectric conversion efficiency, also having
a superior adhesion even when deposited on a flexible substrate, being tough against
any film peeling, being dense, having superior weatherability and electrical properties
and having an optimum uneven surface profile.
The relationship between optimum film formation rate ratio and surface unevenness
for the first transparent conductive film
103a and the second transparent
conductive film
103b is described below.
According to the steps described above, back reflecting films
104
were prepared in the same manner except that the transparent conductive films were
prepared setting the film formation rate of the first transparent conductive film
103a at 1 nm/sec, setting at 1:1 the ratio of the average distance
between the apexes of hills themselves of the film to the average distance between
the apexes of hills of the film and the substrate surface, setting the film thickness
of the film to be 5 nm, and changing the film formation rate of the second transparent
conductive film
103b, and the semiconductor layer, transparent electrode
and collector electrode were formed to produce photovoltaic devices. To examine
their initial characteristics, photoelectric conversion efficiency was measured
with a solar simulator (AM 1.5, 100 mW/cm
2, surface temperature of 25° C.).
The results of measurement are shown in FIG.
5. The photoelectric conversion
efficiency (relative value) is plotted as ordinate and the ratio of film formation
rates (second transparent conductive film/first transparent conductive film) as
abscissa. As can be seen from FIG. 5, the photoelectric conversion efficiency lowers
when the ratio of film formation rates (film formation rate of second transparent
conductive film
103b/film formation rate of first transparent conductive
film
103a) is outside the desired range.
Next, back reflecting films
104 were prepared in the same manner as
the above except that the transparent conductive films were formed in different
film thickness to change their surface profile within the range of from 1:5 to
5:1 in respect of the ratio of the average distance between the apexes of hills
themselves of the film to the average distance between the apexes of hills of the
film and the substrate surface, and fixed to 10 the ratio of film formation rates
(film formation rate of second transparent conductive film
103b/film
formation rate of first transparent conductive film
103a), and the
semiconductor layer, transparent electrode and collector electrode were formed
to produce photovoltaic devices. To examine their initial characteristics, photoelectric
conversion efficiency was measured with a solar simulator (AM 1.5, 100 mW/cm
2,
surface temperature of 25° C.).
The results of measurement are shown in Table 1. As can be seen from Table 1,
the photoelectric conversion efficiency lowers when the ratio of the average distance
between the apexes of hills themselves of the film to the average distance between
the apexes of hills of the film and the substrate surface is outside the desired range.
| TABLE 1 |
| |
| Surface Profile of First Transparent Conductive Film |
| And Photoelectric Conversion Efficiency |
| |
| |
| a:b = |
1:5 |
1:4 |
1:3 |
1:2 |
1:1 |
2:1 |
3:1 |
4:1 |
5:1 |
| Photoelectric |
C |
C |
B |
A |
AA |
AA |
A |
B |
C |
| conversion |
| efficiency: |
| |
What letter symbols indicate (numerical value: relative value):
a: Average distance between the apexes of hills themselves of the film formed
on the substrate.
b: Average distance between the apexes of hills of the film and the substrate surface.
AA: 1.00.
A: 0.95 to less than 1.
B: 0.90 to less than 0.95.
C: 0.80 to less than 0.90.
The cause of this lowering of photoelectric conversion efficiency is considered
to be due to the fact that, the density of nuclei increases when the first transparent
conductive film is formed at a higher rate and/or in a film thickness larger than
100 nm, so that adjoining hills tend to become connected with each other to form
ridges. Also, the effect of the present invention is not obtainable when the first
transparent conductive film is formed in a film thickness smaller than 1 nm. More
specifically, a thin film formed thereon tends to have a larger thickness at the
ridge areas than other ridge-free areas. Hence, the open-circuit voltage may lower
correspondingly to such ridge areas, resulting in a lowering of photoelectric conversion
efficiency as the whole device.
Little ridges are formed in the present invention, and hence the whole device
can have a high open-circuit voltage. Also, in the present invention, valley lines
are held in a proportion smaller than conventional ones, and hence the open-circuit
voltage of the photovoltaic device can be improved.
In general, in valley areas, hill areas which surround the valley areas may be
an obstacle to the particles to be sputtered, and hence the film may be formed
there with difficulty. As the result, the film formed may have a small film thickness
at the valley areas, and hence a leak current may occur at such microscopic areas,
resulting in a lowering of open-circuit voltage of the whole device. In the present
invention, however, it is considered that there are fewer valley areas and hence
the open-circuit voltage may less lower.
From the above results of studies, the optimum ratio of the average distance
between the apexes of hills themselves of the first transparent conductive film
103a to the average distance between the apexes of hills of the film
and the substrate surface in the present embodiment may preferably be in the range
of from 1:3 to 4:1, more preferably from 1:2 to 3:1, and most preferably from 1:1
to 2:1. The film thickness may preferably be in the range of from 1 to 100 nm,
and more preferably from 2 to 80 nm, and most preferably 3 to 50 nm.
In the present embodiment, the controlling of the film formation rate and film
thickness in the steps (5) and (6) to the above specific ranges is very important
for the following reasons.
As a result of studies made by the present inventors, it has been discovered
that
the film formation rate and film thickness have a great influence on the surface
unevenness formed at the film surface.
For example, when the film is formed at a high rate (e.g., 10 nm/sec), target's
particles having been sputtered are in a large quantity and hence the particles
are deposited on the substrate in a very short time, so that the hills are formed
at a small distance between their apexes and also the hills are low.
The subsequent growth of the film proceeds chiefly at the hills, and hence the
growth is limited to the small distance between the apexes. As the result, with
an increase in film thickness with progress of the film formation, the surface
unevenness at the surface of the film deposited becomes more gentle and it becomes
difficult for the surface unevenness to be optimum when the film is used as, e.g.,
the back reflecting film of photovoltaic devices.
On the other hand, when the film is formed at a low rate (e.g., 1 nm/sec), target's
particles having been sputtered are in a small quantity. Hence, the particles deposited
on the substrate are fewer, and the hills are formed at a large distance between
their apexes and also high. However, even in this case where the film is formed
at a low rate, too, the hills are formed at a little small distance between their
apexes because it takes a long time to form the film, resulting in a too large
film thickness, and, with the growth of hills, additional hills become formed between
adjoining hills. As the result, the surface unevenness tends to be a little gentle.
This can be known from the fact that, when the first transparent conductive film
described above has a film thickness larger than 100 nm, the ratio of the average
distance between the apexes of hills themselves of the film to the average distance
between the apexes of hills of the film and the substrate surface comes outside
the desired range to cause a lowering of photoelectric conversion efficiency. Thus,
it can be said that it is necessary to control the film thickness together with
the film formation rate.
Even when the film is formed at a high rate, there is also a tendency that the
surface unevenness becomes gentle with progress of film deposition where a sufficient
surface unevenness has been formed on a film having already been deposited (i.e.,
an underlying layer). However, the surface unevenness can be maintained to a certain
extent, and the time necessary for forming the transparent conductive film can
greatly be shortened.
Hence, when, e.g., the surface unevenness necessary for the back reflecting
film of photovoltaic devices is formed, a film may be deposited in a first step
at a low film formation rate and in a specific film thickness to previously form
a surface unevenness which is large to a certain extent, and then, in a second
step a film may further be formed at a high film formation rate on the film deposited
in the first step. Thus, the surface unevenness necessary for the back reflecting
film of photovoltaic devices can be formed in a short time.
From the above results of studies, the optimum ratio of film formation rates
for the first transparent conductive film
103a and second transparent
conductive film
103b in the present embodiment may preferably be
in the range of from 2 to 100, more preferably from 5 to 50, and most preferably
from 7 to 20.
The first transparent conductive film in the first step may preferably be formed
in a thickness ranging from 1 to 100 nm, more preferably from 2 to 80 nm, and most
preferably from 3 to 50 nm. The second transparent conductive film in the second
step may be formed in a thickness of 5 μm or less, preferably 1 μm
or less, and more preferably 500 nm or less.
As described above, it is very important in the present invention to control
the
film formation rate and the film thickness simultaneously within optimum ranges.
FIG. 1B shows cross-sectional structure of another example of the photovoltaic
device according to another embodiment of the present invention, which comprises
a substrate
101 and superposed thereon a reflecting film
102, a transparent
conductive film
103 (consisting of a first-step portion (first transparent
conductive film)
103a, a second-step portion (second transparent
conductive film)
103b and a third-step portion (third transparent
conductive film)
103c), a semiconductor layer
105, a transparent
electrode
106 and a collector electrode
107 in order. The reflecting
film
102 and the first transparent conductive film
103 (films
103a,
103b and
103c) constitute a back reflecting film
104.
FIG. 2B shows an example of a production system for forming on a substrate the
photovoltaic device shown in FIG.
1B. An example of a process for producing
a photovoltaic device according to the present invention by means of this system
is described below.
(1) Substrates
205 are set on a substrate holder
204, and the inside
of a film-forming chamber
201 is evacuated by means of a vacuum pump (not
shown). Each substrate may be made of a conductive material or an electrically
insulating material, or may also be an electrically insulating substrate the surface
of which has been subjected to conductive treatment. It may also be a semiconductor
crystal bulk. It may still also be a light-transmitting substrate such as glass.
It may preferably be a substrate free of deformation or strain and having the desired
strength, which may preferably include thin sheets of metals such as Fe, Ni, Cr
and Al, alloys of any of these, and stainless steel, composites of any of these,
and films of heat-resistant synthetic resins such as polyester and polyethylene.
(2) After the inside of the film-forming chamber
201 has been evacuated
to a stated pressure, a material gas feed system (not shown) feeds argon gas thereinto
through a gas feed pipe
206, and the internal pressure is regulated to a
stated pressure by adjusting the valve travel of an evacuation valve (not shown).
(3) The substrate holder
204 is rotated by driving a rotating shaft
203.
(4) A plurality of infrared lamps of a heater
202 are lighted to heat
the substrates to a temperature for forming a reflecting film
102.
(5) A DC power source
211 is switched on to cause argon plasma to take
place on a target
207 for the reflecting film, and then a shutter
215
is opened to form the reflecting film
102. After the reflecting film has
been deposited in a stated film thickness, the shutter
215 is closed, and
the DC power source is switched off. As materials for the reflecting film, materials
having a high reflectance are preferred, and may preferably be gold, silver, copper,
aluminum and alloys of any of these.
(6) Next, the heater
202 is set so as to provide temperature for forming
a transparent conductive film. After the substrates have been heated to a stated
temperature, a DC power source
212 is switched on to cause argon plasma
to take place on a target
208 used for a first step in forming the transparent
conductive film, and then a shutter
216 is opened to form the first-step
portion (nuclei; herein regarded as the first transparent conductive film)
103a
of the transparent conductive film
103. After the nuclei have been formed
by a stated size at a stated film formation rate, the shutter
216 is closed,
and the DC power source
212 is switched off.
(7) Next, a DC power source
213 is switched on to cause argon plasma to
take place on a target
209 used for a second step in forming the transparent
conductive film, and then a shutter
217 is opened to deposit the second-step
portion
103b of the transparent conductive film
103. After
the second-step portion has been formed by a stated film thickness at a stated
film formation rate, the shutter
217 is closed, and the DC power source
213 is switched off.
(8) Next, a DC power source
214 is switched on to cause argon plasma to
take place on a target
210 used for a third step in forming the transparent
conductive film, and then a shutter
218 is opened to deposit the third-step
portion
103c of the transparent conductive film
103. After
the third-step portion has been formed by a stated film thickness at a stated film
formation rate, the shutter
218 is closed, and the DC power source
214
is switched off.
As materials for the transparent conductive film, materials having a high transmittance
are preferred, and may preferably be oxides of zinc, tin, indium and titanium and
composites of any of these. Also, it is effective in the present invention to optionally
introduce oxygen together with the argon as a sputtering material gas of the materials
for transparent conductive film, and water vapor.
Here, surface unevenness with hills of hundreds of nanometers in size can be
produced at the surface of the back reflecting film
104, and the effect
of scattering light can be utilized. In FIG. 1B, this surface unevenness is diagrammatically
exaggeratively illustrated.
In this way, the reflecting film
102 and the transparent conductive film
103 (layers
103a to
103c) can continuously be
prepared on the substrate. Using another deposition system, the semiconductor layer
105 and the transparent electrode
106 are further prepared and the
collector electrode
107 is formed thereon. A protective resin may further
be provided.
The transparent conductive film
103 (layers
103ato
103c)
prepared according to such a procedure has an uneven surface profile, thus the
reflected light is effectively absorbed in the semiconductor layer
105 and
the photoelectric conversion efficiency can be improved.
In such a method of forming the transparent conductive film
103 (layers
103a to
103c), it is characterized in the present embodiment,
too, to control conditions of its film formation rate.
Stated specifically, the first-step portion
103a of the transparent
conductive film, formed in a first step, the step (6), is formed at a low film
formation rate in a short time. Thus, the nuclei, which finally serving as bases
of hills of the transparent conductive film, can be formed in the desired density.
Next, in a second step, the step (7), the second-step portion
103b
is formed at a high film formation rate. In this step, the nuclei formed in
the first step are made to grow rapidly, without damaging the density of nuclei.
At the stage where the matter thus formed covers the whole surfaces of the nuclei
formed in the first step, i.e., the matter formed is transformed from nuclei to
a film, the process proceeds to a third step, the step (8).
In the third step, the third-step portion
103c is formed at a film
formation rate which is higher than that in the first step and lower than that
in the second step. The film formation at a rate higher than that in the first
step enables the film formation time for the whole film to be shortened while keeping
any new nuclei from being formed at the valleys of the film. Also, the film formation
at a rate lower than that in the second step enables formation of the film in a
denser film quality.
Here, the first step and the second step in the present embodiment are the
step of forming nuclei and the step of glowing the nuclei, respectively, and the
matter formed can be said to be a film with difficulty. Accordingly, in a strict
meaning, it is not suitable to refer to "film formation rate" in the first step
and second step. Hence, the film formation rate referred to in the present embodiment
is defined to be the rate of film formation when all the underlying surfaces are
compositionally the same as the film (i.e., the thickness of a film become deposited
per unit time). In other words, it refers to the rate of film formation when the
deposits on the substrate are transformed from island-like nuclei to a film and
the deposits grow uniformly into a film over the whole surface. This is the film
formation rate in the step corresponding to the third step in the present embodiment.
Thus, the first to third steps can be compared by film formation rate commonly
used. The factors that determine the film formation rate may commonly include the
voltage and electric current applied to targets, the composition and concentration
of sputtering gases, the temperature and pressure in the film-forming chamber,
and the composition and surface properties of targets. In the present invention,
these may appropriately be controlled so that the desired film formation rate may
consequently be achieved.
Thus, controlling the ratio of film formation rates between the respective
steps within a specific range enables formation of a back reflecting film
104
more improved in photoelectric conversion efficiency, also having a superior adhesion
even when deposited on a flexible substrate, being tough against any film peeling,
being dense, having superior weatherability and electrical properties and having
an optimum uneven surface profile.
The relationship between optimum film formation rate ratio and surface unevenness
in the respective steps in the present embodiment is described below.
As a result of studies made by the present inventors, it has been discovered
that
the film formation rate and film thickness in the steps of forming the transparent
conductive film greatly influence the unevenness formed finally at the film surface.
For example, when the film is formed at a high rate (e.g., 10 nm/sec or more)
from the first step, i.e., the step of forming nuclei, target's particles having
been sputtered are in a large quantity and hence the particles are deposited on
the substrate in a very short time and also in a high density of nuclei. Hence,
the nuclei are formed at a small distance between them, and the nuclei come small
in height because the target's particles contribute to the growth of nuclei in
a dispersed state. Thereafter, the part of nuclei comes to hills as the film is
formed on, and hence the small distance between the nuclei is maintained as it
is, as the distance between hills of the film. As the result, with an increase
in film thickness with progress of the film formation, the unevenness at the surface
of the film deposited becomes more gentle and it becomes difficult for the film
to have the optimum uneven surface profile when the film is used as, e.g., the
back reflecting film of photovoltaic devices. However, even when the film is formed
at a high rate, the uneven surface profile can be maintained to a certain extent,
although there is a tendency that the surface unevenness becomes gentle with progress
of film deposition where a sufficient surface unevenness has been formed on a film
having already been deposited as in the second and third steps in the present embodiment.
When the film is formed at a high rate, the time necessary for forming the transparent
conductive film can also greatly be shortened. In view of the denseness of the
film, however, it is better for the film formation rate to be low. Especially in
respect of the transparent conductive film coming into contact with the semiconductor
layer, like the film of the third step, it is better for its surface to have a
denser structure. If the surface is not dense, cracks or the like may occur to
cause a lowering of open-circuit voltage of the whole device due to a leak current
caused by such cracks. In view of adhesion, too, it is better for the transparent
conductive film surface to have dense structure.
In addition, where the film formation rate is set high by controlling various
factors which determine the above film formation rate, any sputtering discharge
may not rise or, even if it rises, a phenomenon of short circuit may occur in the
middle of film formation to make the discharge intermit, or make the discharge
stop as it is. This phenomenon of short circuit refers to a phenomenon in which
a voltage flows abnormally during discharge to make the discharge unstable or make
the discharge break off temporarily. Once the phenomenon of short circuit has occurred,
it may cause a decrease in film formation rate on the contrary or may have bad
influences such as a lowering of film quality due to non-uniform film thickness
distribution or film defects. Hence, this matters a great deal especially when
films are continuously formed. The occurrence of the phenomenon of short circuit
may further make the discharge not rise as it is, without any self reversion. This
matters a great deal also in respect of productivity.
Accordingly, in the step having a great influence on the formation of
surface unevenness of the film as in the second step of the present embodiment,
it is better for the film formation rate to be set as high as possible in order
to maintain the density of the nuclei formed in the first step and transform them
into a film rapidly and in a short time. In the step which does not so much influence
the formation of surface unevenness of the film and in which the film is deposited
until it has the desired film thickness, as in the third step, it is better to
set optimum conditions for film formation in view of the balance between film quality,
film formation time and discharge stability.
On the other hand, when the film is formed at a low rate (e.g., 1 nm/sec), target's
particles having been sputtered are in a small quantity and hence the particles
deposited on the substrate are fewer and also in a low density of nuclei. Hence,
the nuclei are formed at a large distance between them, and the growth of nuclei
proceeds concentratedly on the nuclei formed, so that the nuclei come high. However,
even in this case where the film is formed at a low rate, too, the nuclei are formed
at a small distance between them because, as the film formation time becomes longer,
additional nuclei become formed between adjoining nuclei with the growth of nuclei.
As the result, the surface unevenness of the film comes gentle. This tendency is
remarkable when the nuclei formed in the step of forming nuclei (the first step)
has a thickness larger than 100 nm. Also, when the film is formed at a low rate,
it inevitably takes a time to deposit the transparent conductive film until it
has the desired film thickness, leaving a problem on productivity.
In order to form the film under stable conditions of film formation rate immediately
after the start of discharging, it is preferable to use a target having been subjected
to pre-sputtering. When a virgin target is used, the target changes in surface
properties between the one immediately after the start of discharging and immediately
before the completion of discharging, and the conditions of film formation rate
change on with time. On the other hand, the target having been subjected to pre-sputtering
has originally a surface standing rough appropriately, and hence the conditions
of film formation rate immediately after the start of discharging and immediately
before the completion of discharging can be made alike. In the first step, having
a low film formation rate, and in the third step, having not so much influence
on the uneven surface profile of the film even if the film formation rate deflects
more or less, the stability of this film formation rate does not especially matter
so much. However, this stability is greatly effective in the second step, which
is a step important for the formation of surface unevenness of the film.
Accordingly, when, e.g., the surface unevenness necessary for the back
reflecting film of photovoltaic devices is formed, the following film formation
method is most preferred in which the conditions of film formation rate are changed
for each step.
First, in the first step, nuclei with a specific size are formed under conditions
of a low film formation rate. Next, in the second step, the nuclei are made to
grow rapidly under conditions of a high film formation rate using a target having
been subjected to pre-sputtering, and made to transform swiftly from the state
of nuclei to the state of a film. Finally, in the third step, the film is formed
under conditions of a film formation rate a little lower than that in the second
step, thus a denser film can be formed. The time taken to form the film can also
be shortened as a whole, and the surface unevenness and film quality necessary
for the back reflecting film of photovoltaic devices can be obtained.
As a result of studies made by the present inventors, the optimum ratio of film
formation rates between the second step and the first step and between the third
step and the first step may preferably be in the range of from 2 to 100, more preferably
from 5 to 50, and most preferably from 7 to 20. Also, the third step may be under
conditions of a film formation rate lower than the second step, where the optimum
ratio of film formation rates between the second step and the third step may preferably
be in the range of from 1 to 10, more preferably from 1 to 3, and most preferably
from 1.1 to 2.
The film (nuclei) in the first step may preferably be formed in a thickness ranging
from 1 to 100 nm, more preferably from 2 to 80 nm, and most preferably from 3 to
50 nm. If it is in a thickness smaller than 1 nm, any function as the nuclei can
not be achieved and any effect of the present invention can not be obtained. If
it is larger than 100 nm, the nuclei may be in so high a density as to make it
difficult for the transparent conductive film to have the desired uneven surface profile.
The film in the second step may be formed in a thickness of 5 μm or less,
preferably 1 μm or less, and more preferably 500 nm or less. In this thickness,
the transformation from the state of nuclei to the state of a film is substantially
completed, and the uneven surface profile intended in the second step can be provided.
The thickness of the film formed in the third step greatly influences the characteristics
of the final products photovoltaic devices, and may preferably be 5 μm or
less taking account of strain resistance, impact resistance and adhesion.
Incidentally, a batch method is shown here as an example. Without limitation
to this method, the present invention may also be applied to a sheet-by-sheet method
or a method in which a continuous substrate is transported, such as a roll-to-roll method.
The present invention is described below in greater detail by giving Examples
and with reference to the accompanying drawings. The present invention is by no
means limited to these Examples.
EXAMPLE 1
In this Example, the photovoltaic device having the construction shown in the
FIG. 1A cross-sectional diagrammatic view was produced by means of the system shown
in FIG. 2A.
To state specifically, thin substrates 205 made of stainless steel were
set on the substrate holder 204, and the inside of the film-forming chamber
201 was evacuated by means of a vacuum pump (not shown). After the inside
of the film-forming chamber 201 was evacuated to a stated pressure, the
material gas feed system (not shown) fed 30 sccm of argon gas thereto through the
gas feed pipe 206, and the internal pressure was regulated to 0.3 Pa by
adjusting the valve travel of an evacuation valve (not shown). Then the substrate
holder 204 was rotated by driving the rotating shaft 203. The heater
202 was so set that the substrates were heated to 100° C., and the
infrared lamps were lighted to heat the substrates. After the substrates came to
the stated temperature, the DC power source 210 was switched on to cause
argon plasma to take place on an aluminum target 207 for the reflecting
film, and then the shutter 215 was opened to form the reflecting film 102.
The reflecting film 102 was deposited in a thickness of about 150 nm. Then
the shutter 215 was closed, and the DC power source was switched off.
Next, the heater 202 was so set that the substrates were heated to 200°
C. After the substrates came to the stated temperature, the DC power source 211
was switched on to cause argon plasma to ta
