U.S. patent application number 12/369045 was filed with the patent office on 2010-08-12 for method and apparatus for the solution deposition of high quality oxide material.
This patent application is currently assigned to United Solar Ovonic LLC. Invention is credited to Arindam Banerjee, Vincent Cannella, Bud Dotter, II, Subhendu Guha, Chaolan Hu, Shengzhong Liu, Jeffrey Yang, Kais Younan, Yanhua Zhou.
Application Number | 20100200408 12/369045 |
Document ID | / |
Family ID | 42539495 |
Filed Date | 2010-08-12 |
United States Patent
Application |
20100200408 |
Kind Code |
A1 |
Liu; Shengzhong ; et
al. |
August 12, 2010 |
METHOD AND APPARATUS FOR THE SOLUTION DEPOSITION OF HIGH QUALITY
OXIDE MATERIAL
Abstract
A metal and oxygen material such as a transparent electrically
conductive oxide material is electro deposited onto a substrate in
a solution deposition process. Process parameters are controlled so
as to result in the deposition of a high quality layer of material
which is suitable for use in a back reflector structure of a high
efficiency photovoltaic device. The deposition may be carried out
in conjunction with a masking member which operates to restrict the
deposition of the metal and oxygen material to specific portions of
the substrate. In particular instances the deposition may be
implemented in a continuous, roll-to-roll process. Further
disclosed are semiconductor devices and components of semiconductor
devices made by the present process, as well as apparatus for
carrying out the process.
Inventors: |
Liu; Shengzhong; (Rochester
Hills, MI) ; Hu; Chaolan; (Rochester Hills, MI)
; Zhou; Yanhua; (Rochester Hills, MI) ; Younan;
Kais; (Rochester Hills, MI) ; Dotter, II; Bud;
(Shelby Township, MI) ; Cannella; Vincent;
(Beverly Hills, MI) ; Banerjee; Arindam;
(Bloomfield Hills, MI) ; Yang; Jeffrey; (Troy,
MI) ; Guha; Subhendu; (Bloomfield Hills, MI) |
Correspondence
Address: |
GIFFORD, KRASS, SPRINKLE,ANDERSON & CITKOWSKI, P.C
PO BOX 7021
TROY
MI
48007-7021
US
|
Assignee: |
United Solar Ovonic LLC
Auburn Hills
MI
|
Family ID: |
42539495 |
Appl. No.: |
12/369045 |
Filed: |
February 11, 2009 |
Current U.S.
Class: |
205/50 ;
204/230.2; 205/91 |
Current CPC
Class: |
C25D 21/12 20130101;
C25D 5/36 20130101; C25D 5/10 20130101; C25D 5/20 20130101; C25D
21/10 20130101; C25D 9/08 20130101; C25D 17/12 20130101; C25D 5/18
20130101 |
Class at
Publication: |
205/50 ; 205/91;
204/230.2 |
International
Class: |
C25D 5/00 20060101
C25D005/00; C25D 7/00 20060101 C25D007/00; C25B 15/00 20060101
C25B015/00 |
Goverment Interests
STATEMENT OF GOVERNMENT INTEREST
[0001] This invention was made, at least in part, under U.S.
Government, Department of Energy, Contract No. DE-FC36-07G017053.
The Government may have rights in this invention.
Claims
1. In a method for the electroplating of a layer of a metal and
oxygen material onto a substrate wherein said substrate is disposed
in an electrolyte in a spaced apart relationship with an electrode,
and wherein a power supply is operative, when energized, to
establish a flow of electrical current through said electrode, said
electrolyte, and said substrate so as to deposit a layer of said
metal and oxygen material on said substrate, characterized in that
said deposition process includes at least two steps from the group
consisting of: inputting ultrasonic energy into said electrolyte
during at least a portion of the time while said layer of metal and
oxygen material is being deposited onto said substrate;
periodically interrupting the flow of electrical current between
said electrode, said electrolyte, and said substrate, while said
layer of said metal and oxygen material is being deposited;
maintaining said substrate in a partiphobic orientation while said
layer of metal and oxygen material is being deposited thereupon;
bubbling a gas through said electrolyte; and energizing said power
supply at a first level during the time that a first portion of
said metal and oxygen material is being deposited on said substrate
so that said first portion is deposited at a first deposition rate
and thereafter energizing said power supply at a second level
during the time that a second portion of said layer is being
deposited atop the first portion wherein said second level of power
is selected so that the deposition rate of said second portion is
less than the deposition rate of said first portion.
2. The method of claim 1, wherein the improvement comprises
implementing said process so as to employ at least three of said
steps.
3. The method of claim 1, including at least one further step
selected from the group consisting of: monitoring the composition
of the electrolyte bath; monitoring the level of a dopant in the
deposited metal and oxygen material; utilizing a dimensionally
stable electrode; utilizing an electrode configured as a hollow
basket having particles of said metal contained therein; and
utilizing a filter-shielded electrode.
4. The method of claim 1, wherein said method is operative to
deposit a layer of a zinc and oxygen material onto the
substrate.
5. An apparatus for carrying out the method of claim 1.
6. A layer of a zinc and oxygen material made by the method of
claim 4.
Description
FIELD OF THE INVENTION
[0002] This invention relates generally to the electro deposition
of transparent, electrically conductive oxide materials and in
particular to the deposition of transparent, electrically
conductive metal oxide materials.
BACKGROUND OF THE INVENTION
[0003] A number of electronic devices incorporate one or more
layers of transparent, electrically conductive material. Such
devices include, but are not limited to, semiconductor devices such
as electronic memory, photovoltaic devices, photo sensors, other
photo responsive devices, display devices and the like. These
layers are typically fabricated from transparent, electrically
conductive metal oxide (TCO) materials; and, zinc oxide based
materials comprise one particular TCO material. Transparent,
electrically conductive zinc oxide materials are often not
stoichiometrically pure, but typically incorporate species such as
suboxides, hydroxides, ionic species, dopants and the like which
can function to enhance electrical conductivity of the electronic
device. Therefore, within the context of this disclosure, it is to
be understood that "metal and oxygen materials" are meant to
include materials based thereon and may also include suboxides,
hydroxides, and other species. For example, materials based on zinc
and oxygen (sometimes referred to as "zinc oxide" or "zinc oxide
material") may also include suboxides of zinc, hydroxides of zinc
such as Zn(OH).sub.2, Zn.sup.2+ ions (typically in the form of zinc
salts) and other such species. Likewise other metal and oxygen
materials, such as tin and indium based material, may include
oxides, suboxides, hydroxides and ionic species. It is also to be
understood that in the context of this disclosure, the metal and
oxygen materials may also include dopants or modifiers such as
boron, which can function to tailor the electrical conductivity of
the deposited oxide material (e.g. ZnO) layer and/or control the
physical morphology of the deposited layer.
[0004] Zinc oxide materials are one metal and oxygen material which
has significant utility as components of the back reflector
structure of high efficiency photovoltaic devices and the present
invention will be explained with reference to such materials;
however, it is to be understood that the principles of this
invention are applicable to the deposition of other metal and
oxygen materials. The back reflector is an important component of
such devices. It is disposed at the back surface of the
photovoltaic device, typically as a portion of the support
substrate, and functions to reflect and redirect unabsorbed photons
which have passed through the overlying, photovoltaically active
semiconductor layers back through those layers for reabsorption. A
typical back reflector structure includes a highly reflective metal
layer such as a layer of silver or aluminum having a microtextured
layer of transparent, electrically conductive zinc oxide material
disposed thereatop. The textured nature of the zinc oxide material
serves to scatter the reflected photons of incident light that were
not absorbed on the initial pass through the superposed solar cell
material thereby allowing for their subsequent absorption in their
secondary pass through said solar cell.
[0005] In order to maximize the efficiency of the photovoltaic
device, the electronic, optical and physical properties of the zinc
oxide material must be carefully controlled. The zinc oxide
material must have good electrical conductivity, since photo
current generated by the overlying semiconductor layers must pass
through the zinc oxide material for collection in the subjacent
substrate electrode. Hence, the electrical resistivity of the oxide
material represents a parasitic loss in the photovoltaic device.
Likewise, the material must have good optical transparency, since
reflected photons may pass through the layer numerous times
(depending upon the absorption characteristics of the semiconductor
material of the photovoltaic device and the scattering
characteristics of the zinc oxide and back reflector layers), and
any optical absorption will also represent a loss in device
efficiency. Finally, the microtexture of the layer needs to be
controlled so as to optimize the scattering of the reflected
photons so as to maximize the opportunity of those photons to be
absorbed by the overlying semiconductor layers. Therefore, the
controllable deposition of high quality zinc oxide materials is
important to the preparation of high efficiency photovoltaic
devices.
[0006] The prior art has generally utilized vacuum deposition
processes, such as sputtering, for the deposition of zinc oxide
materials. However, such processes are inherently equipment
intensive and relatively slow deposition rates coupled with high
capital expenditure costs and high operational expenses adversely
impact the cost of producing photovoltaic devices. In addition,
such deposition processes are inherently slow and represent a
bottleneck in the photovoltaic device deposition process.
Therefore, if high volume deposition processes are to be attempted,
the back reflector fabrication stations must be extremely large and
expensive.
[0007] Because of the problems associated with the vacuum
deposition of such materials, the prior art has attempted to
deposit zinc oxide materials by high speed, low cost electro
deposition processes wherein zinc oxide materials are electroplated
onto substrates in an aqueous bath. Some such processes are
disclosed, for example, in U.S. Pat. Nos. 6,133,061; 6,224,736;
6,238,808; and 6,379,521. Despite various attempts, the prior art
has not, heretofore, been able to reliably and repeatedly electro
deposit zinc oxide materials having electrical, optical and
physical properties which maximize their utility in back reflector
structures of high efficiency photovoltaic devices. Furthermore,
prior art processes have encountered problems of compatibility when
such materials were deposited on particular substrates.
[0008] As will be explained in detail hereinbelow, the present
invention provides a method and apparatus whereby high quality zinc
oxide and other transparent conductive oxide materials may be
electro deposited onto a variety of substrates of the type utilized
in high efficiency photovoltaic devices. Furthermore, the present
invention provides a method and apparatus whereby the deposition of
the zinc oxide and other transparent conductive oxide materials may
be limited to preselected portions of the substrate. Finally, the
present invention provides a method and apparatus which is
compatible with the high speed, roll-to-roll fabrication of large
area, high efficiency, photovoltaic devices. These and other
advantages of the present invention will be apparent from the
drawings, description, and discussion which follow.
BRIEF DESCRIPTION OF THE INVENTION
[0009] The present invention is directed to a method for electro
depositing a layer of a metal and oxygen material, such as a zinc
and oxygen material, onto a substrate. In a first aspect of the
present invention, the metal and oxygen material is electroplated
onto a substrate in a process wherein a first portion of the
thickness of the layer is deposited on the substrate at a first
deposition rate, and thereafter a second portion of the thickness
of the layer is deposited atop the first portion of the thickness
at a second deposition rate which differs from the first deposition
rate. In a specific instance, the second deposition rate is slower
than the first deposition rate.
[0010] In another aspect of the present invention, a metal oxygen
material is electro deposited onto a substrate in a process wherein
at least a portion of the substrate is covered with a masking
member which prevents the deposition of the metal and oxygen
material onto those portions of the substrate to which it is
affixed. The masking member may, in some instances, be magnetically
affixable to the substrate. In specific instances, the electro
deposition process is carried out on an elongated web of substrate
material which is continuously advanced through a deposition system
which includes a deposition station wherein the metal and oxygen
material is deposited on the substrate. In this embodiment of the
invention, a belt-like body of masking material is brought into
contact with a back surface of the substrate member while it is in
the deposition station and while the metal and oxygen material is
being deposited onto the front surface of the web of substrate
material. In some specific instances, the deposition system may
include a biasing member such as a platen or series of rollers
which urge the belt of masking material into contact with the
substrate.
[0011] In yet another aspect of the present invention, the
substrate member is maintained in a partiphobic orientation while
the metal and oxygen material is being deposited thereonto so as to
at least partially inhibit the incorporation of particulate
material into the depositing layer of metal and oxygen
material.
[0012] In another aspect of the present invention, a layer of metal
and oxygen material is electroplated onto a substrate which is
disposed in an electrolyte in a spaced apart relationship with an
electrode. In this process, a power supply is operative, when
energized, to establish a flow of electrical current through the
electrode, the electrolyte and the substrate so as to deposit a
layer of metal and oxygen material on the substrate. In this
process, at least two of the following steps are implemented:
inputting ultrasonic energy into the electrolyte during at least a
portion of the time while the layer of metal and oxygen material is
being deposited onto the substrate; periodically interrupting the
flow of electrical current between the electrode, the electrolyte
and the substrate while the layer of metal and oxygen material is
being deposited; maintaining the substrate in a partiphobic
orientation while the layer of metal and oxygen material is being
deposited thereupon; bubbling a gas through the electrolyte; and
energizing the power supply at a first level while a first portion
of the metal and oxygen material is being deposited on the
substrate so that the first portion is deposited at a first
deposition rate, and thereafter energizing the power supply at a
second level during the time that a second portion of the layer is
being deposited atop the first portion so that the second portion
is deposited at a second deposition rate. In a specific instance,
the second deposition rate is less than the first deposition rate.
In some particular instances, at least three of the foregoing steps
are implemented. In further embodiments of this aspect of the
invention, at least one more step from the following group is
implemented: monitoring the composition of the electrolyte bath;
monitoring the level of a dopant in the deposited metal and oxygen
material; utilizing a dimensionally stable electrode; utilizing an
electrode configured as a hollow basket having particles of the
metal contained therein and utilizing a filter shielded
electrode.
[0013] The present invention may be implemented in a continuous
process, and in specific instances may be utilized to fabricate
back reflector structures for high efficiency photovoltaic
devices.
[0014] The present invention also includes substrates having metal
and oxygen materials deposited thereupon in accord with the
foregoing. The substrates of the present invention may be used as
back reflector structures for photovoltaic devices. In specific
instances, the present invention is directed to substrates which
include a layer of a highly reflective metal such as aluminum or
silver disposed thereupon and having a highly adherent metal and
oxygen layer, such as a zinc and oxygen layer, electro deposited
thereupon wherein these substrates are characterized in that they
do not include any vacuum deposited seed layer of a metal and
oxygen material thereupon so that all of the metal and oxygen
material deposited upon the reflective metal is deposited from a
solution in an electro deposition process in accord with the
present invention.
[0015] The present invention is also directed to apparatus for
carrying out the aforedescribed methods and for manufacturing the
aforedescribed articles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a cross-sectional view of a photovoltaic device
showing a back reflector structure which includes a zinc oxide
material deposited in accord with the present invention;
[0017] FIG. 2 is a cross-sectional view of a schematic
electroplating apparatus which may be utilized to carry out the
method of the present invention;
[0018] FIG. 3 is a flowchart depicting one embodiment of the
present invention;
[0019] FIG. 4 is a schematic depiction of an apparatus for
implementing the method of the present invention in a continuous
process;
[0020] FIG. 5 is an enlarged view of a portion of a deposition
station of the apparatus of FIG. 4 better illustrating the masking
system; and
[0021] FIG. 6 is a depiction of a deposition station generally
similar to that of FIG. 5 but including a biasing platen.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The present invention will be described with reference to
the deposition of metal oxides such as zinc oxide materials in
connection with the fabrication of back reflector structures for
high efficiency photovoltaic devices. However, it is to be
understood that the principles of the present invention may be
readily extended to any application wherein high quality metal
oxide materials are electro deposited in a high speed, high volume
process. As noted above, such applications may include the
fabrication of display devices, sensor devices, light emitting
devices, and the like.
[0023] Referring now to FIG. 1, there is shown a cross-sectional
view of a generalized high efficiency photovoltaic device 10. The
device incorporates a substrate 12 which functions to support the
remainder of the device and operates to provide a bottom, current
collecting, electrode for the device. In the illustration, the
substrate 12 is comprised of two separate layers. The first layer
14 is a body of stainless steel. Disposed thereatop is a relatively
thin layer of a highly reflective metal 16, such as aluminum or
silver. In other embodiments of photovoltaic device, the substrate
may be comprised of a body of electrically insulating material such
as a polymer, glass, ceramic or the like, provided that a layer of
electrically conductive material is disposed thereupon.
[0024] Disposed atop the substrate 12 is a layer of transparent,
electrically conductive metal oxide material, in an exemplary
embodiment a zinc oxide material, 18. As noted above, this layer is
primarily comprised of ZnO, but may further include other zinc
based species as well as dopants and the like. The material
comprising the zinc oxide layer 18 is at least partially
crystalline and as such the surface of this layer may have a
texture corresponding to the crystalline features of the material.
In general, it is preferable that the crystalline features have a
size range of approximately 200-1000 nanometers so as to maximize
the scattering of visible light therefrom. The layer 18 has good
electrical conductivity and good optical transparency.
[0025] Disposed atop the zinc oxide layer 18 is a body of
photovoltaic semiconductor material 20. The active semiconductor
layers of this body 20 operate to absorb incident photons and
create carrier pairs which are collected by the electrodes of the
device. As is known in the art, this body 20 may be comprised of a
number of layers of semiconductor materials disposed in various
configurations. In one particular embodiment, the semiconductor
body 20 is comprised of hydrogenated silicon alloy materials, and
as such may comprise one or more stacked triads, each triad
comprised of a layer of substantially intrinsic semiconductor
material interposed between p-doped and n-doped semiconductor
layers.
[0026] Disposed atop the photovoltaic body 20 is a top electrode
layer 22, which in the instance of this particular configuration of
device, is fabricated from an optically transparent, electrically
conductive material such as ZnO or another TCO material. As is
known in the art, current collecting structures such as bus bars,
grids and the like may be disposed upon the top electrode 22.
[0027] In the operation of the photovoltaic device, photons pass
into the device through the top electrode layer 22 and are absorbed
by the photovoltaic body 20 wherein they generate electron-hole
pairs. The inherent, built-in electric field of the photovoltaic
body 20 separates the photogenerated holes and electrons of these
carrier pairs and they are collected by the respective top
electrode 22 and substrate 12. Photons which are not absorbed by
the photovoltaic body 20 pass through the zinc oxide layer 18 and
are reflected by the reflective layer 16. The textured nature of
the zinc oxide layer 18 scatters the reflected photons so that
their angulated path back through the photovoltaic body 20 is
increased as compared to non-scattered photons. And in some
embodiments, the reflective layer 16 will also include a textured
configuration to also aid in scattering the reflected photons.
[0028] Referring now to FIG. 2, there is shown a generalized system
30 as may be employed for the deposition of zinc oxide materials in
accord with the present invention. The system 30 includes a tank 32
which is configured and operable to retain a volume of electrolyte
material 34 therein. The apparatus further includes an electrode
station having a deposition electrode 36 supported therein. As
shown in FIG. 2, the electrode 36 is configured as a plate,
comprised primarily of a metallic material such as zinc metal. It
is to be understood that the apparatus of FIG. 2 is generalized,
and in some instances the electrode may be configured as a mesh,
and/or as a nonplanar body. In one embodiment, the electrode is a
hollow, basket-like, perforated body comprised of a material which
is inert to the deposition process, such as Ti, Pt, Pd, Au, or the
like. Zinc particles in the form of shot or the like are disposed
in the hollow body. In another embodiment, a filter is positioned
about the electrode to shield the electrode and prevent particulate
matter from reaching the surface of the substrate upon which the
deposition is taking place. In one embodiment the filter is in the
form of a porous, polyethylene filter bag, disposed so as to
surround the electrode. In another embodiment, the electrode is an
inert, dimensionally stable electrode fabricated from an inert
material such as titanium. As is known in the art, in
electroplating processes of this type, all of the metal ions which
form the deposited metal and oxygen layers are provided from the
electrolyte. As is further to be understood, the electrode station
may also include fixturing members such as clamps, brackets and the
like for supporting the electrode body. Also, as will be further
discussed hereinbelow, in some instances the electrode station may
include a plurality of discrete electrodes.
[0029] The system of FIG. 2 supports a substrate 38 in the body of
electrolyte material 34. As described above, the substrate 38 may
comprise a single layered structure or a multilayered
structure.
[0030] The electrode 36 and the substrate 38 are both in electrical
communication with a power supply station which includes power
supply 40 which in turn is controlled by a controller 42. The power
supply 40 is a DC power supply, and the electrode 36 is in
communication with the positive terminal of the power supply 40 and
the substrate is in electrical communication with the negative
terminal of the power supply 40. The illustrated embodiment of FIG.
2 includes a single power supply 40; however, it is to be
understood that in other embodiments, the power supply station may
include a number of power supplies operative to energize a
plurality of discrete electrodes and/or to provide different levels
of power.
[0031] As is further illustrated, the system 30 includes a heater
44 disposed in the tank 32. The heater 44 is operative to maintain
the electrolyte 34 at a preselected temperature, and in that
regard, the heater 44 has a controller 46 associated therewith. As
illustrated herein, the heater 44 is an electrical resistance
heater; although, other types of heater as is known in the art may
be likewise utilized.
[0032] The system 30 also preferably includes a gas bubbler 48
disposed in the tank. The bubbler 48 has a gas supply 50 associated
therewith and is operable, when activated, to bubble a gas, such as
air or nitrogen, through the electrolyte 34, so as to keep the
electrolyte stirred.
[0033] The system further includes an ultrasonic transducer 52
disposed in the tank. The transducer is energized by a controller
54 and is operative, when energized, to introduce ultrasonic energy
into the electrolyte material 34. While not wishing to be bound by
speculation, the inventors hereof presume that the ultrasonic
energy may act to maintain the cleanliness of the surface of the
deposition substrate and/or the cleanliness of the depositing layer
by removing unwanted species therefrom.
[0034] The systems of the present invention may further include a
monitoring station for measuring the composition of the electrolyte
during the deposition process, so as to determine the concentration
of metal ions, dopants and other species. Such monitoring is
preferably done in situ and in real time, and assures the
uniformity and consistency of the deposited materials. Monitoring
may be by techniques including potentiometric techniques, chemical
techniques such as EDTA titration, spectroscopic techniques and the
like. Monitoring can be utilized in combination with reagent supply
systems operating in a feedback mode. Thus, for example, if the
metal concentration of the electrolyte is too low, additional metal
can be added. Or, if the pH is too high, acid can be automatically
added. Likewise, the system can control and adjust dopant reagent
levels based upon measured levels in the electrolyte and/or the
deposited layer.
[0035] In FIG. 2, the substrate material 38 is shown as having a
body of masking material 56 affixed to one surface thereof. The
masking material operates to shield portions of the substrate so
that in the process, zinc oxide material is unable to be deposited
onto those shielded portions of the substrate. This feature is
optional in the practice of the present invention; however, in a
number of processes and device configurations it has been found
beneficial to so restrict the deposit of the zinc oxide material.
The masking material may be variously configured and adhered to the
substrate and as such may comprise a polymeric resist coating.
However, in one specific embodiment of the present invention, the
masking material 56 comprises a sheet of material which is
magnetically affixable to at least a portion of one surface of the
substrate. In this regard, the masking material 56 may comprise a
sheet of magnetized metal, or it may comprise a body of polymeric
material having magnetized particles dispersed therein. In specific
instances, the masking material is electrically insulating, so as
to preclude deposition thereonto.
[0036] In a typical process for the deposition of zinc oxide
material in accord with the present invention, the electrolyte
material 34 comprises an approximately 0.03 molar solution of
Zn(NO.sub.3).sub.2. In some embodiments, the electrolyte will also
include relatively small amounts of adhesion promoting material
such as ethylenediaminetetraacetic acid (EDTA). Other chelating
materials and/or adhesion promoters such as fumaric acid, malic
acid, various other compounds having multiple functional groups, as
well as compounds such as sucrose may likewise be included.
Typically, the concentration of these materials is in the range of
1-200 ppm. The electrolyte material may also include one or more
dopant or modifying species which operate to enhance the electrical
conductivity of the deposited zinc oxide material. One specific
doping species utilized in the present invention comprises boron,
and it may be present in the electrolyte in the form of boric acid
at a concentration in the range of 0.1%-1.0% by weight. The
electrolyte is generally maintained at a temperature in the range
of 50-100.degree. C. during the deposition process, and in a
typical instance, the electrolyte is maintained at a temperature of
approximately 80.degree. C.
[0037] In those instances where tin and oxygen based materials are
being deposited, the electrolyte will include one or more tin salts
such as tin chloride, tin acetate, tin sulfate, and the like. The
deposition of indium based materials will employ an electrolyte
which includes indium salts such as indium chloride, indium
nitrate, indium sulfate and the like.
[0038] The power supply is activated so as to establish an
electrical potential of approximately 0.5 to 20 volts between the
electrode 36 and the substrate 38. This potential will cause the
deposition of zinc oxide material onto the substrate, and the rate
of deposition will be proportional to the power density at the
substrate. Therefore, the control of deposition power will allow
for the control of the deposition rate. In a typical deposition,
power density at the substrate will be in the range of 0.5-20
mA/cm.sup.2.
[0039] In order to enhance the uniformity of the deposited zinc
oxide, the electrolyte bath 34 is at least periodically stirred,
and this may be done by use of a recirculation pump (not shown)
and/or by bubbling a gas through the electrolyte from the bubbler
48. It has been found, in this process, that air or nitrogen may be
employed for this purpose; however, other gases which are inert or
do not otherwise degrade the deposition process may likewise be
employed.
[0040] In accord with another aspect of the present invention, it
has been found that the quality of the deposited zinc oxide
material is improved if ultrasonic energy is at least periodically
introduced into the electrolyte bath. In one embodiment, the
ultrasonic transducer 52 is energized at a power level of
approximately 500 watts, for example. The configuration of the
ultrasonic energy system employed will depend on the configuration
of the electrical device and other aspects of the electro
deposition system.
[0041] In another aspect of the present invention, it has been
found advantageous to operate the power supply 40 in a pulsed mode
wherein the DC current applied to the electrode 36 and substrate 38
is periodically interrupted. In a typical process, the current is
pulsed at a rate of 1 to 10 Hz. While not wishing to be bound by
speculation, Applicant presumes that operation in the pulsed mode
allows for equilibration of deposition conditions at the surface of
the substrate and thereby promotes the deposition of materials
having optimum compositions and morphology.
[0042] In accord with yet a further aspect of the present
invention, the inventors hereof have found that very high quality
deposits of zinc oxide material may be prepared in a
multi-deposition rate process. In this embodiment of the present
invention, the substrate is initially coated with a first layer of
zinc oxide material in a relatively high rate deposition process.
High rate deposition may be achieved by controlling the power
supply so as to energize the electrode 36 and substrate 38 with a
relatively high level of power. This produces a relatively fast
deposition of a relatively thick portion of the body of zinc oxide
material. Thereafter, the power supply energizes the electrode 36
and substrate 38 at a lower level of power so as to deposit zinc
oxide material upon the previously deposited layer, at a lower
rate. It is believed that this lower rate material manifests a very
good crystalline structure which optimizes the performance of the
zinc oxide layer. Use of the dual rate process thus achieves the
benefits of high average deposition rate while producing a body of
zinc oxide material having superior electrical, optical and
physical properties. In further refinements of this process, the
body may be deposited at three or more deposition rates Also, it is
to be noted that the change in deposition rate need not be abrupt,
and within the context of this aspect of the invention, the
deposition rate may be varied on a continuous basis, by varying
current density, so that the material transitions from high rate to
low rate, or from a low rate to a high rate, in a non-stepwise, or
only partially stepwise manner.
[0043] In yet another aspect of the present invention, it has been
found that superior quality materials are prepared when the
substrate 38 is maintained in an orientation which will allow
gravity to inhibit the accumulation of particulate matter
thereupon. As such, the substrate 38 may be oriented vertically as
is shown in FIG. 2. However, other orientations which will inhibit
particle accumulations may be employed. For example, the substrate
may be disposed in a horizontal orientation with the deposition
surface facing downward. In other instances, the substrate may be
disposed in an angled relationship with a vertical axis, provided
that the deposition surface is downwardly inclined so as to inhibit
particulate accumulation. Within the context of this disclosure,
all of such orientations of the substrate, wherein gravity acts (at
least in part) to inhibit particle accumulation on the deposition
surface, are referred to as "partiphobic".
[0044] The zinc oxygen materials produced by the present invention
have very good physical, optical and electronic properties which
make them ideally suited for use in back reflector structures of
photovoltaic devices. It is believed that this combination of
properties is resultant from the independent and/or synergistic
effect of at least two and perhaps more of the aforedescribed
features of the present invention, namely the use of pulsed power,
deposition of the material in an at least dual layered structure at
differing power levels, ultrasonic cleaning of the depositing layer
during the deposition process, and use of a partiphobic substrate
orientation which precludes particulate inclusions. Other factors
which can contribute to the quality of the materials produced by
the present process include the use of in situ monitoring of
electrolyte bath composition; in situ monitoring of dopant
composition and profiles; and the use of electrode structures such
as the hollow basket, dimensionally stable electrode and/or filter
shielded electrode previously discussed. Typical layer thicknesses
in back reflector structures are on the order of 0.1 to 3 microns,
and the high speed nature of the deposition process of the present
invention greatly enhances the economics and physical
implementation of the fabrication process as compared to methods
wherein the layer is entirely deposited by vacuum processes.
[0045] While the present invention provides for the high speed
electrochemical deposition of zinc oxide materials, it is to be
understood that in some instances, the invention may be implemented
in connection with an overall fabrication process wherein some
portions of the zinc oxide material may be deposited in a vacuum
process such as sputtering. For example, commonly employed
substrates for photovoltaic devices comprise stainless steel having
a reflective coating of silver or aluminum deposited thereupon. The
reflective layer is fairly thin and is often deposited by
sputtering or some other vacuum process. In some instances, it has
been found advantageous to vacuum deposit a relatively thin "seed"
layer of zinc oxide material atop the reflective layer. This
deposition is typically carried out by sputtering, and the total
layer thickness is on the order of 5-100 nanometers; consequently,
deposition time is relatively fast. It has been found that in some
instances, the use of the vacuum deposited seed layer facilitates
the deposition and adhesion of the electrochemically deposited zinc
oxide material. It should be noted that the use of a seed layer is
optional, and in accord with the present invention, the inventors
herein have been able to electro deposit a high quality TCO
material having very good adhesion properties and device
operational parameters atop various reflective substrates without a
seed layer, thereby reducing manufacturing costs considerably.
Elimination of the seed layer is particularly important in those
instances where the reflective layer is deposited by
electroplating, since this allows for a total atmospheric pressure
process. It has been found that the inclusion of adhesion promoters
such as EDTA in the electrolyte enhances the adhesion of the
electro deposited layer to the reflective metal, and thereby
eliminates the need for a seed layer. In an experimental series it
was found that the adhesion of zinc oxide layers directly electro
deposited onto silver layers from an EDTA containing bath was at
least as good as that of comparable layers of zinc oxide electro
deposited onto a silver layer having a vacuum coated seed layer of
zinc oxide thereupon. If the adhesion promoter is eliminated from
the bath, adhesion of the zinc oxide layer is poor in the absence
of the seed layer. In this experimental series, adhesion was
measured by the tape lift-off method.
[0046] In one specific implementation of the present invention, the
substrate is approximately 5 mils thick layer of stainless steel.
In those instances where a reflective layer is to be sputtered
thereatop, an approximately 100 nanometer thick adhesion layer of
titanium is vacuum deposited upon the stainless steel.
Subsequently, a reflective layer of silver or aluminum, having a
thickness in the range of 100-500 nanometers is deposited upon the
substrate. Thereafter, a seed layer of zinc oxygen material having
a thickness of approximately 40 nanometers is deposited atop the
reflective layer. The thus prepared substrate is coated with a
layer of zinc oxide material in the process of the present
invention. The thickness of this layer is generally in the range of
0.1-3 microns depending upon specific applications.
[0047] Referring now to FIG. 3, there is shown a generalized
flowchart depicting one embodiment of the present invention. As is
shown in FIG. 3, the process employs a substrate which, as
mentioned above, may optionally include a seed layer thereupon. In
a first portion of the deposition process, the zinc oxide material
is deposited onto the substrate at a relatively high deposition
rate, which in some instances is approximately 10 nm/sec. This
initial deposition is carried out at a temperature in the range of
50-100.degree. C., and typically at a temperature of 80.degree. C.
The electrolyte in the deposition tank is agitated by activating
the gas bubbler system; however, agitation may optionally be
carried out by pumps, stirrers or the like. After a portion of the
layer (typically 30-80; and in specific instances 50-70% of its
thickness) has been deposited, ultrasonic energy is input to the
deposition tank. Deposition conditions are maintained at a high
rate, and agitation of the bath is also continued. The ultrasonic
energy serves to remove undesirable solution particulates from the
depositing layer. Any pitting left by the removal of the loosely
adherent materials is filled in by the depositing zinc oxide
material. In this second stage of the process, the remainder of the
thickness of the final zinc oxide layer is deposited.
[0048] In the third stage of the deposition process, a further
portion of the layer of zinc oxide material is deposited at a
relatively low deposition rate. In particular instances, this rate
is in the range of approximately 1-5 nm/sec. The deposition bath is
maintained at approximately the same temperature it was in the
first two stages, and agitation of the electrolyte is maintained
through the use of the bubbler or other means.
[0049] Other modes of deposition may be employed. In one instance,
the initial deposition may be at a low rate, followed by high rate
deposition; and optionally followed by a second low rate
deposition. In general, it is believed that low rate deposition
promotes the formation of a layer having larger crystals which
operate to promote optimum light scattering. Also, the low rate
material can provide good adhesion to subjacent layers. In
addition, low rate material can provide a template for subsequently
deposited high rate material so that the crystalline structure of
the high rate material resembles that of the low rate material to
some degree.
[0050] Once the total thickness of the layer of zinc oxide material
is deposited, the substrate is then rinsed with water and dried.
Drying is typically carried out utilizing atmospheric air either in
an oven or through the use of a blower. Drying is generally carried
out at elevated temperatures, typically in the range of
25-200.degree. C. for times of approximately 2 minutes. The drying
step serves to remove water, but also allows for the at least
partial conversion of zinc hydroxide species into zinc oxide
species. The drying also can function to anneal the material,
thereby further increasing its adherence to the substrate. In one
particular instance, the drying/annealing is at 120.degree. C. for
two minutes; in another, it is at 150.degree. C. for 1 minute.
Following the drying/annealing, the process is complete, and the
substrate may be subsequently processed into photovoltaic
devices.
[0051] The process of the present invention may be readily
implemented in a continuous, roll-to-roll process for the
preparation of photovoltaic substrate material, and one such
implementation is shown in FIG. 4. Depicted therein is a
roll-to-roll deposition apparatus 60 for the coating of an
elongated substrate web with a zinc/oxygen material. The system 60
of FIG. 4 includes a payoff station 62 which supports and feeds out
a web of substrate material 38 from a supply roll 64. As is known
in the art, the payoff station may include turning rollers,
steering rollers, a tensioning mechanism and the like.
[0052] The system 60 includes three deposition stations 66, 68 and
70, although it is to be understood that in other implementations,
greater or lesser numbers of deposition stations may be employed.
In this particular implementation, the stations 66, 68 and 70 are
configured to carry out the three stages of the deposition as
described with reference to FIG. 3. As such, the first station 66
carries out a relatively high speed deposition wherein the
electrolyte material is agitated by the bubbler 48. In the second
station 68, high speed deposition is carried out utilizing bubbler
agitation as well as ultrasonic energy input from the ultrasonic
transducer 52. The third deposition station 70 is used for the low
rate deposition. It also includes a bubbler 48 for maintaining
agitation of the electrolyte.
[0053] Each of the stations includes a heater 44, and it is notable
that in this embodiment, each deposition station 66, 68 and 70
includes two deposition electrodes. In this regard, the first
station includes electrodes 36a, 36b, the second station includes
electrodes 36c, 36d and the third includes electrodes 36e, 36f. Use
of dual electrodes speeds up the deposition process. As described
with reference to FIG. 2, the electrodes 36 are all in
communication with an appropriate power supply and energized at
power levels sufficient to provide a desired deposition rate.
[0054] As discussed above, it is frequently desirable to include a
body of masking material which operates to prevent deposition of
the zinc/oxygen material onto particular portions of the substrate.
In the illustrated embodiment, each deposition station includes a
masking system which as illustrated is comprised of two portions
72a, 72b.
[0055] Referring now to FIG. 5, there is shown an enlarged view of
a portion of the first deposition station of FIG. 4, better
illustrating the masking system. As depicted, a portion of the
substrate web 38 is advanced past a first deposition electrode 36a,
about a turning roller 76, and past a second deposition electrode
36b. The first masking system 72a is disposed so as to contact the
back surface of the substrate 38 with a body of masking material
78, when it is in the region of the first electrode 36a. The
masking material 78 is flexible, electrically insulating and
magnetic, and as such may comprise a polymer having a magnetic
substance embedded therein. The masking material 78 is configured
as a continuous web, and it is supported by a first 80 and a second
82 roller. In the operation of the system, the web 38 advances
through the deposition station and is contacted by the magnetic
material 78 which adheres thereto. The web of magnetic material 78
travels along with the substrate, past the electrode 36a. The
magnetic nature of the masking material maintains it in contact
with the substrate. After the substrate 38 leaves the region of the
first electrode, the second roller 82 pulls the masking material 78
away from the substrate 38. The second masking system 72b is
disposed in association with the second electrode 36b and operates
in a similar manner to the first masking system 72a.
[0056] The substrate masking system may be configured to include
rollers, platens, and the like which can assist in biasing the
masking member against the substrate. These biasing systems may be
used in combination with a magnetically affixable masking member;
although, in some instances, the biasing force may be sufficient to
assure good contact between the substrate and the biasing member so
that magnetic attraction need not be employed. Referring now to
FIG. 6, there is shown one embodiment of biasing system as
configured to be utilized in a deposition station of the type
generally shown in FIG. 5; and in that regard, similar elements
will be identified by similar reference numerals. The deposition
station of FIG. 6 includes a first and a second deposition
electrode 36a, 36b disposed and operative to electro deposit a
layer of zinc oxide material onto a web of substrate material 38
passing through the deposition station. The deposition station of
FIG. 6 further includes a first masking system 72a and a second
masking system 72b which, as previously described, include a
flexible, electrically insulating body of masking material 78
supported by a first 80 and a second 82 roller. The system of FIG.
6 further includes a curved biasing platen 84 which is disposed so
as to contact the belt of masking material 78 and urge that
material against a portion of the substrate 38. A second such
platen 86 is associated with the second masking system 72b. Biasing
may be accomplished by otherwise configured members. For example,
the biasing platens 84, 86 may be replaced by one or more rollers.
Since the biasing platens urge the masking material into contact
with the substrate, the masking material need not be magnetic,
although a magnetic body may be utilized.
[0057] Returning now to FIG. 4, it will be seen that the system 60
further includes a rinsing station 84 disposed downstream of the
deposition stations 66, 68 and 70. The rinsing station 84 comprises
a tank configured so that the coated substrate passes therethrough
wherein it is rinsed with water. The rinsing station 84 may further
include agitators, stirrers or the like for enhancing the rinse
action. It may also include a flow-through system for continuously
replacing the rinse water. In some instances, the rinse station may
comprise two or more discrete rinse tanks.
[0058] Downstream of rinsing station 84 is a drying station 86
wherein the coated web is dried as described above. The drying
station may comprise an oven, a drying tunnel, or the like and may
include radiant heaters, hot air blowers or the like. Following the
drying, the substrate material is then wound onto a take-up reel 88
in a take-up station 90.
[0059] The coating of the zinc and oxygen material may also be
implemented into a single, continuous process in which a reflective
layer is electroplated onto a stainless steel web and thereafter
coated with the zinc and oxygen material. In this regard the
apparatus may include a first deposition station wherein the
substrate is electroplated with a reflective layer of silver or
aluminum. For example, silver may be electroplated onto the
stainless steel from an electrolyte bath comprising: 37.5 g/l
dimethylhydantoin, 12.0 g/l silver nitrate, 0.38 g/l thiamine
hydrochloride, 7.5 g/l potassium chloride and 7 g/l potassium
hydroxide. Plating takes place at a temperature of 60-90.degree.
C., using a silver electrode at a current density of about 3
mA/cm.sup.2 and deposits a highly reflective silver layer at a rate
of about 2 nm/sec. Aluminum may likewise be electroplated by
processes known in the art.
[0060] The system of FIG. 4 produces an elongated web of substrate
material which may subsequently be employed in a continuous process
for the fabrication of photovoltaic devices. In that regard, the
roll of material may be transferred to a photovoltaic deposition
apparatus. In yet other instances, the substrate coating system may
be placed in line with, or incorporated into, a photovoltaic
deposition apparatus. For example, a roll to roll process may be
implemented in which a reflective layer of silver or aluminum is
first electroplated onto the substrate, and thereafter a layer of a
zinc and oxygen material is electroplated onto the substrate in the
same apparatus. The coated substrate may then be conveyed to a
series of semiconductor deposition chambers associated with the
same apparatus, or it may be subsequently conveyed to separately
disposed semiconductor deposition chambers.
[0061] The present invention provides a method and apparatus for
the rapid, efficient deposition of high quality layers of metal
oxide material, such as zinc oxide material. The invention has been
described with regard to particular apparatus and particular
operating conditions as specifically adapted for the preparation of
substrates for high efficiency photovoltaic devices. However, it is
to be understood that the principles of the present invention may
be extended to other methods and apparatus and to processes for the
preparation of devices and materials other than those for use in
photovoltaic applications. As such, numerous modifications and
variations of the invention will be apparent to those of skill in
the art in view of the teaching presented herein. It is to be
understood that the foregoing drawings, discussion and description
are illustrative of specific embodiments of the invention, but are
not meant to be limitations upon the practice thereof. It is the
following claims, including all equivalents, which define the scope
of the invention.
* * * * *