Title of Invention

A FLEXIBLE METAL OXIDE COATED STEEL PRODUCT

Abstract A coated steel product comprises a metallic strip material which has a coating comprising an electrically insulating layer doped with sodium. The thermal expansion coefficient of said metallic strip material is less than 12.10-6 K-1 in the temperature range 0-600oC. Said product may be coated with an electrically conducting layer of molybdenum. The coated steel is useful as a substrate for flexible Cu (In, Ga) Se2 (CIGS) solar cells.
Full Text The present invention relates generally to a coated
metallic substrate material suitable for manufacturing of
flexible solar cells and a method of manufacturing of a
metal oxide coated metal strip product in a roll-to-roll
process. This is achieved by coating a metallic strip with
an electrically insulating inner layer in accordance with
claim 1 and also optionally with an electrically conducting
surface layer.
Background of the Invention
The most common substrate material used today by
manufacturers of thin film Cu(In,Ga)Se2 (abbreviated CIGS)
solar cells is sodalime glass. Two examples of solar cells
with glass substrates are DE-A-100 24 882 and US-A-5 994
163. A positive effect by the use of sodalime glass is an
increased efficiency of the solar cell, due to the
diffusion of an alkali metal (primarily sodium) from the
glass into the CIGS layer. This fact is known from, e.g.,
the Thesis by Karin Granath (19 99): The Influence of Na on
the. Growth of Cu(Tn,Ga)Se2 Layers for Thin Film Solar
Cells, Acta Universitatis Upsaliensis, Comprehensive
Summaries of Uppsala Dissertations from the Faculty of
Science and Technology 491, Uppsala ISBN 91-554-4591-8,
hereby incorporated into the present disclosure by this
reference. However, the batch-like production of CIGS on
glass substrates is expensive and, therefore, it is an
advantage to use roll-to-roll production of solar cells,
which lowers the production cost. Moreover, there are
several technical advantages with flexible solar cells
produced by a roll-to-roll process, for instance, the
flexible solar cells can be folded or rolled into compact:


packages and they may be used for making light weight solar
cells, which is desirable for portable, spatial and
mi litary applications.
Several materials have been tested as substrate
materials for flexible CIGS solar cells, including polymers
such as polyimide and metals such as molybdenum, aluminium
and titanium foils, bearing in mind that they all have to
fulfil certain criteria. Thus, the substrate material
should be thermally resistant in order to withstand further
process steps in the production of thin film flexible CIGS
solar cells, and this may include heat treatments at
temperatures up to 600°C under corrosive atmosphere. The
flexible metallic substrate should be insulated from the
electrical back contact if CIGS modules with integrated
series connections are to be produced. Therefore, it is
essential that the thermal expansion coefficient (TEC) of
the substrate material should be as close as possible to
the TEC of the electrical insulating metal oxide layer(s)
tc avoid thermal cracking or spallation of the insuJ ating
metal oxide layer.
Common conventional substrate materials for the
production of CIGS solar cells are:
Using sodalime glass substrates in batch-like:
processes;
Depositing a molybdenum back, contact material directly
onto the metal strip that constitutes the substrate,-
Depositing insulating silicon oxide (SiOx or SiO2)
layers onto metal strips in batch type deposition
processes.
One example of a known solar cells are disclosed in
Thin Solid Films 403-404 (2002) 384-309 by K. Herz ot al . :
"Dielectric barriers for flexible CIGS solar modules",
hereby incorporated into the present disclosure by this
reference. According to this article, excellent electrical
insulation for the preparation of CIGS solar modules was


obtained on metal substrates by using SiOx and/or Al2O3
barrier layers. However, due to the lack of sodium, the
'oltage produced by the solar cell was inferior.
A further example of known solar cells making use of
inless steel substrates are disclosed in Solar Energy
Materials & Solar Cells 75 (2003) 65-71 by Takuya Satoh et
"Cu(In,Ga)Se2 solar cells on stainless steel
substrates covered with insulating layers", hereby
incorporated into the present disclosure by this reference,
However, according to this article, the CIGS solar cells on
the stainless steel decreased open-circuit voltage compared
with that on the soda-lime glass substrates.
Moreover, in WO 03/007386 (hereby incorporated into
the present disclosure by this reference) a thin-film solar
is described. It comprises a flexible metallic
substrate having a first surface and a second surface. A
back metal contact layer is deposited on the first surface
of the flexible metallic substrate. A semiconductor
absorber layer is deposited on the back metal contact. A
photoactive film deposited on the semiconductor absorber
layer forms a heterojunction structure and a grid contact
deposited on the heterojunction structure. The flexible
metal substrate can be constructed of either aluminum or
stainless steel. Furthermore, a method of constructing a
solar cell is disclosed. This method comprises providing an
aluminum substrate, depositing a semiconductor absorber
layer on the aluminum substrate, and insulating the
aluminum substrate from the semiconductor absorber layer to
Inhibit reaction between the aluminum substrate and the
semiconductor absorber layer.
Although this known solar cell works satisfactorily,
it does not attain the open-voltage level of a solar cell
with a soda-lime glass substrate because of the lack of
sodiurn doping.


Thus, all these conventional methods have their
disadvantages. All processes based on batch-type
will always increase the cost and it is
herefore essential that the production will be on a roll-
process to decrease the cost.
Hence, when using sodalime glass, it is impossible to
nrodace flexible CIGS, and the batch-type process is
pensive. Further, the deposition of Mo back contact
onto the flexible metal strip substrate will limit
production of CIGS modules with integrated series
connections. Furthermore, the SiOx or SiO2 insulating
ayers have a too low TEC, which may lead to the formation
and pinholes during the following process steps.
by not adding an alkali metal in the Si02 layer,
(primarily soddum) has to be added in a later production
step if higher efficiency CIGS is to be produced. The
addition of one or more process steps in a production line
always associated with extra costs.
It is therefore a primary object of the present
iivencion to provide a flexible and light metallic
bstrate for solar cell production with a thermal
expansion coefficient as similar as possible, to the
insulating metal oxide layer(s).
Yet another object of the present invention is to
flexible substrate for solar cells that is
expensive and which may be produced in a continuous roll-
process .
Still another object of the present invention is to
make possible the production of flexible solar cells with
-increased efficiency as to the voltage attained.
These and other objects have been attained in a
-xrprising manner by creating a coated steel product with
he features according to the characterizing clause of
Further preferred embodiments are defined in the
dependent claims.


Brief Description of the Invention
Thus, the above objects and further advantages are
achieved by applying a thin continuous, uniform,
electrically insulating layer of a metal oxide, such as
aluminum oxide, on the top of a metal strip serving as
substrate. To the insulating metal oxide a small amount of
an alkali metal is added to increase the efficiency of the
solar cell. The metal oxide layer should be smooth and
dense in order to avoid any pinholes, which may otherwise
function as pathways for electrical conduction when the
material is further processed. If so desired, and in order
to ensure safe electrical insulation from the metal strip
substrate, multi-layers (ML) of metal oxides can be
deposited. The advantage of an ML structure is that it will
exclude any pinholes or conducting pathways through the
insulating oxide layer. Furthermore, by depositing a
continuous uniform dense metal oxide layer on top of the
metallic substrate, it is easier to control the insulating
properties as well as the thickness of the metal oxide
layer, compared to for instance anodized oxide layers on
metallic strips. Moreover, the metal oxide layer will also
have an enhanced adhesion to the substrate, in comparison
with thermally grown oxide layers. The added alkali metal
(primarily sodium) will diffuse into the CIGS layer during
the further process steps in the CIGS production. Further,
if so required, on top of said metal oxide layer, there may
then be deposited a molybdenum layer, this for obtaining a
back electrical contact for the production of a thin film
flexible solar cell.
When several layers of metal oxide(s) are deposited,
these layers may be of the same metal oxide or of different
metal oxides.


BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows a schematic cross-section of a first
embodiment of the present invention.
Figure 2 shows a schematic cross-section of a second
embodiment of the present invention.
Figure 3 shows schematic cross-sections of two further
embodiments of the present invention.
Figure 4 shows schematically a production line for the
manufacturing of a coated metal strip material according to
the invention.
Detailed Description of the Invention
The Metal Strip to be coated
One of the key issues of the underlying metallic strip
is that it should have a low thermal expansion coefficient
(TEC) in order to avoid spallation or cracking of the
deposited metal oxide layers. Therefore, it is desirable
thai: the TEC of the metallic strip be lower than 12-lCTe K"1
in the temperature range of 0 to 600°C. This will include
materials such as ferritic chromium steels, titanium and
some nickel alloys. It is also preferred that the material
in the metal strip be sufficiently corrosion-resistant to
withstand the environment in which the solar cell.will
work. The physical shape of the metal is in a strip or toil
whose thickness should be in the range of 5 to 200 urn,
preferably 10 to 100 urn. Another important parameter is the
surface roughness of the metal strip, which should be as
as possible; a Ra value of less than 0,2 µm is
preferably less than 0,1 µm.
The Insulating Oxide Layer
The electrically insulating oxide layers should adhere
well to the metallic strip, in order to ensure highest

possible flexibility of the solar cell. This is achieved by
careful pre-treatment of the metal strip prior to the
coating, first by cleaning it in a proper way to remove oil
residues, etc., which may affect the efficiency of the
coating process, and the adhesion and quality of the
coating. Thereafter, the metal strip is treated by means of
an in-line ion assisted etching process. Moreover, the
oxide layer should also be a good electrical insulator in
order to avoid any electrical connection between the
meaallic strip and the molybdenum back contact. This can be
achieved by depositing dense and smooth oxide layers to
vring about better insulating properties, it being repeated
b.ai. multi-layered structures may also be deposited. The
number of individual oxide layers in a multi-layered
structure can be 10 or less. As mentioned above, a multi-
layered oxide structure will terminate any pinholes or
electrical pathways through the overall metal oxide layer
and ensure good electrical insulation of the metallic
This fact is illustrated in Figure 3, in which the
aanholes are terminated by the adjacent oxide layers. When
there are more than one insulating metal oxide layer, then
thickness of each individual oxide layer may be between
10 ma and up to 2 pi, preferably between 0,1 and 1,5 urn.
The total thickness of the overall metal oxide layer, both
the case of a single mono layer and multi layers (2 co
layers), may be up to 20 un, preferably 1 to 5 um.
The chemical composition of the oxide layer could be
my dielectric oxide such as A1203, Ti02, Hf02, Ta205 and
or mixtures of these oxides, preferably Ti02 and/or
Al203, most preferably Al203, although other oxide layers
are feasible, both stoichiometric and non-stoichometric
Furthermore, when the metal oxide coating consists of
plurality of layers (multi-layer) , then each individual
may be of the same metal oxide, or of different metal

oxides. An individual layer may also consist of a mixture
of metal oxides.
Moreover, according to the present invention, the
oxide layer is doped with an amount of an alkali metal,
suitably lithium, sodium or potassium, preferably sodium.
The alkali metal concentration in the deposited oxide layer
should be between 0,01 and 10% (by weight), preferably 0,1
and 6%, and most preferably 0,2-4%, in order to improve the
efficiency of the CIGS solar cell by Na diffusion through
olie back contact layer in a way similar to the one observed
for CIGS deposited on sodalime glass. It is indeed
surprising for the skilled man, that the alkali metal in
the alkali metal doped oxide layer manages to penetrate
through the back contact layer and in a decisive manner
influence the performance of the CIGS layer.
When sodium is used, the Na source can be any sodium-
containing compound and the Na compound is preferably mixed
with the oxide source material prior to the deposition, or
the Na can be independently added to the oxide coating in a
separate process step. The concentration of Na in the oxide
source should be the ones mentioned above. The following Na
compounds are useful as Na sources for the oxide layer: Na,
Ma20, NaOH, NaF, NaCl, Nal., Na2S, Na2Se, NaN03 and Na?.C03,
to list a few.
According to one embodiment of the present invention,
when a plurality of metal oxide layers are deposited on the
substrate, only the most distal layer, or possibly the two
most distal layers, is/are doped with an alkali metal. The
reason is of course that mainly this layer or these layers
contribute to the diffusion of alkali metal into and past
the molybdenum layer and into the CIGS layer in a solar
cell.


Description of the Back Contact Layer
Depending upon further processing steps, and on the
specific conditions dictated by the individual client, a
layer consisting substantially of molybdenum is applied
on top of the oxide layer. This top layer should be dense
and adhere well to the underlying, previously deposited
oxide layer, while simultaneously allowing the penetration
of the alkali metal(s). The thickness of the molybdenum top
layer should be 0,01-5,0 urn, preferably 0,1-2,0 urn, most
preferably around 0,5 um.
Descrip_tion of Coating Method
Advantageously, the coating method is integrated in a
rcll-to-roll strip production line. In this roll-to-roll
production line, the first production step is an ion-
assisted etching of the metallic strip surface, in order to
achieve good adhesion of the adjacent insulating oxide
layer. The insulating oxide layer is deposited by means of
electron beam evaporation (EB) in a roll-to-roll process.
This process is well known for the skilled man and is,
e.g., comprehensively described in the book Electron Beam
"ochnology by Siegfried Schiller, Ullrich Heisig and
Siegfried Panzer, Verlag Technik GmbH Berlin 1995, ISBN 3-
341-01153-6, hereby incorporated into the present
disclosure by this reference.
The insulating oxide layer may be either a single or
mono layer, or a plurality of layers, so called multi
layers. While the mono layer usually works satisfactorily,
the multi-layer embodiment gives more safety as to cracks
and pinholes. The formation of multi-layers can be achieved
by integrating several EB deposition chambers in-line (see
Tigure 4), or by running the strip several times through
die same EB deposition chamber. If a stoichiometric oxide
is desired, then the deposition of oxides should be made
under reduced pressure with a partial pressure of oxygen as


reactive gas in the chamber. In such a production line, the
last chamber should be the EB chamber for the deposition of
the molybdenum for the back contact layer. The deposition
of Mo should be done under reduced atmosphere at a maximum
pressure of 1-10-2 mbar.
Preferred Embodiment of the Invention
Firstly, the substrate materials are produced by
ordinary metallurgical steelmaking to a chemical
composition as described above. They are then hot rolled
down to an intermediate size, and thereafter cold-rolled in
several steps with a number of recrystallization steps
between said rolling steps, until a final thickness of
about 0,042 mm and a width of a maximum of 1000 mm. The
surface of the substrate material is then cleaned in a
proper way to remove all oil residuals from the rolling.
In Figure 1 a typical cross section of a flexible
metallic substrate for the production of thin film CIGS
colar cell is illustrated. The substrate material is a
flexible metal strip (1), which can consist of stainless
steel, or any other metal or alloy which has a TEC lower
than 12 x 10-6 K-1, in the temperature range 0-600 °C. The
nurface roughness of the metallic strip should be kept as
low as possible. The thickness of the metallic strip should
be in the range of 5 - 200 urn, preferably 10-100 urn to
ensure good flexibility.
On top of the surface of the metallic strip substrate
:'l) , a single layered alkali metal (in. this case sodium)
doped aluminum oxide (4) may be deposited in a roll-to-roll
KB process, directly on top of the flexible metal strip as
illustrated in Figure 2. On top of the electrically
insulated single layered alkali metal doped aluminum oxide,
also a molybdenum layer can be deposited by means of
electron beam deposition in a roll-to-roll process.


As an altrnative to the single or mono layer (4)
according to Figure 2, an electrically insulating aluminum
oxide multi-layer structure (2) may be deposited, also by
EB deposition in a roll-to-roll process. The aluminum oxide
multi-layer structure should be well adherent to the metal
strip as well as dense and smooth.
The deposited aluminum oxide is doped with a small
amount of alkali metal, preferably sodium. To create a back
contact for the CIGS solar cell, a molybdenum layer (3) may
be deposited on top of the electrically insulated metallic
strip. The molybdenum layer should be dense and well
adherent to the metal oxide coating to avoid cracking or
spallation. Furthermore, the molybdenum layer should have a
i-.kickness between 0,1-5 \xm, preferably 0,4-2 Jim.
Another variation to the two above-mentioned examples
is that no molybdenum back contact layer is deposited on
top of the electrically insulating aluminum oxide multi-
layer structure (2) or the electrically insulating aluminum
oxide single layer (4) deposited by EB deposition in the
roll-to-roll process. This is illustrated in Figure 3. In
the figure the benefit of a multi-layer metal oxide
structure is illustrated by the termination of any pinholes
(5) and/or electrical pathways (5) through the metal oxide
multi-layers.
The roll-to-roll, electron beam evaporation process is
illustrated in Figure 4. The first part of such a
production line is the uncoiler (6) within a vacuum chamber
(7), then the in-line ion assisted etching chamber (8),
followed by a series of EB evaporation chambers (9), the
number of EB evaporation chambers needed can vary from 1 up
::o 10 chambers, this to achieve the wanted multi-layered
metal oxide structure. All the metal oxide EB evaporation
chambers (9) are equipped with EB guns (10) and water cold
copper crucibles (11) for the evaporation. The following

chamber is a separate chamber (12) for the EB evaporation
of molybdenum top layer, this chamber is also equipped with
an EB gun (13) and a crucible (14) for the molybdenum melt.
The need for a separate EB evaporation chamber for the
molybdenum can be excluded if only metal oxide coated
strips are to be produced. After this chamber comes the
exit vacuum chamber (15) and the recoiler (16) for the
coated strip material, the recoiler being located within
vacuum chamber (15). The vacuum chambers 7 and 15 may also
be replaced by an entrance vacuum lock system and an exit
vacuum lock system, respectively. In the latter case, the
uncoiler 6 and the coiler 16 are placed in the open air.


WE CLAIM :
1. A coated steel product comprising a metallic strip material, characterized in that said
strip has a coating comprising an electrically insulating layer doped with an alkali
metal or a mixture of alkali metals, the thermal expansion coefficient of said metallic
strip material being less than 12.10-6 K-1 in the temperature range 0-600°C, the
electrically insulating layer comprises at least one oxide layer and the oxide layer
consists essentially of any of the following dielectric oxides : Al2O3, TiO2, HfO2 /Ta2O5
and Nb2O5 or mixtures of these oxides, preferably AI2O3 and/or TiO2
2. Coated steel product as claimed in claim 1, wherein the metallic strip material has a
thickness of 5 to 200 µm, preferably 10 to 100 µm.
3. Coated steel product as claimed in claim 1 or 2, wherein the electrically insulating
layer has a multi-layer constitution of 2 to 10 layers, to ensure efficient electrical
insulation.
4. Coated steel product as claimed in claim 3, wherein each individual oxide layer has a
thickness of between 0.01 and 2 µm, preferably between 0.1 to 1.5 µm.
5. Coated steel product as claimed in claim 1 or 4, wherein only the layer, or the two
layers, most distal from the metallic strip substrate is/are doped with alkali metal(s).
6. Coated steel product as claimed in any of the preceding claims, wherein the total
thickness of the oxide coating may be up to 20 µm preferably 1 to 5 µm.
7. Coated steel product as claimed in any of the preceding claims, wherein the
electrically insulating layer is coated by a conducting layer, preferably mainly made
of molybdenum.
8. Coated steel product as claimed in claim 7, wherein the molybdenum layer has a
thickness of between 0.01 and 5 µm preferably 0.1 and 2 µm.

9. Coated steel product as claimed in any of the preceding claims, wherein the alkali
metal is either Li, Na or K, or mixtures thereof, preferably Na.
10. Coated steel product as claimed in claim 3 or 4, wherein the individual layers in the
multi-layer structure are either made of the same metal oxide or of different metal
oxides and that each individual layer is made of one metal oxide or of a mixture of
two or more metal oxides.
11. Coated steel product as claimed in any of the previous claims, wherein it is suitable
as a substrate material for the production of flexible Cu (In, Ga) Se2 (CIGS) solar
cells.
12. Method for producing a coated steel product as claimed in any of claims 1-11,
wherein the electrically insulating layer(s) and the electrically conducting layer(s) are
all deposited in a roll-to-roll electronic beam evaporation process.
13. A flexible Cu(ln, Ga) Se2 (CIGS) solar cell wherein it comprises a coated steel
product as claimed in any of claims 1-11.


A coated steel product comprises a metallic strip material which has a coating comprising
an electrically insulating layer doped with sodium. The thermal expansion coefficient of
said metallic strip material is less than 12.10-6 K-1 in the temperature range 0-600oC. Said
product may be coated with an electrically conducting layer of molybdenum. The coated
steel is useful as a substrate for flexible Cu (In, Ga) Se2 (CIGS) solar cells.

Documents:

00065-kolnp-2006-abstract.pdf

00065-kolnp-2006-claims.pdf

00065-kolnp-2006-description complete.pdf

00065-kolnp-2006-drawings.pdf

00065-kolnp-2006-form 1.pdf

00065-kolnp-2006-form 2.pdf

00065-kolnp-2006-form 3.pdf

00065-kolnp-2006-form 5.pdf

00065-kolnp-2006-international publication.pdf

00065-kolnp-2006-international search authority.pdf

65-KOLNP-2006-(02-04-2012)-PETITION UNDER RULE 138.pdf

65-KOLNP-2006-(30-04-2012)-FORM-27.pdf

65-KOLNP-2006-CORRESPONDENCE-1.2.pdf

65-kolnp-2006-correspondence.pdf

65-kolnp-2006-examination report.pdf

65-KOLNP-2006-FORM 1 1.1.pdf

65-kolnp-2006-form 18.pdf

65-kolnp-2006-form 3.pdf

65-kolnp-2006-form 5.pdf

65-kolnp-2006-gpa.pdf

65-kolnp-2006-granted-abstract.pdf

65-kolnp-2006-granted-claims.pdf

65-kolnp-2006-granted-description (complete).pdf

65-kolnp-2006-granted-drawings.pdf

65-kolnp-2006-granted-form 1.pdf

65-kolnp-2006-granted-form 2.pdf

65-kolnp-2006-granted-specification.pdf

65-kolnp-2006-reply to examination report.pdf

abstract-00065-kolnp-2006.jpg


Patent Number 249651
Indian Patent Application Number 65/KOLNP/2006
PG Journal Number 44/2011
Publication Date 04-Nov-2011
Grant Date 01-Nov-2011
Date of Filing 06-Jan-2006
Name of Patentee SANDVIK INTELLECTUAL PROPERTY AB
Applicant Address S-811 81 SANDWIKEN, SWEDEN
Inventors:
# Inventor's Name Inventor's Address
1 SCHUISKY, MIKAEL MOSSVÄGEN 75C, S-811 34 SANDVIKEN, SWEDEN
2 CEDERGREN, MAGNUS FURUVÄGEN 6, S-811 36 SANDVIKEN, SWEDEN
PCT International Classification Number H01L 31/02
PCT International Application Number PCT/SE2004/001173
PCT International Filing date 2004-08-09
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 0302206-8 2003-08-12 Sweden