U.S. patent application number 13/947205 was filed with the patent office on 2015-01-22 for segmented thin film solar cells.
This patent application is currently assigned to International Business Machines Corporation. The applicant listed for this patent is International Business Machines Corporation. Invention is credited to Hans-Juergen Eickelmann, Ruediger Kellmann, Markus Schmidt.
Application Number | 20150020863 13/947205 |
Document ID | / |
Family ID | 52342581 |
Filed Date | 2015-01-22 |
United States Patent
Application |
20150020863 |
Kind Code |
A1 |
Eickelmann; Hans-Juergen ;
et al. |
January 22, 2015 |
SEGMENTED THIN FILM SOLAR CELLS
Abstract
Use of chemical mechanical polishing (CMP) and/or pure
mechanical polishing to separate sub-cells in a thin film solar
cell. In one embodiment the CMP is only used to separate the
active, thin film layer into sub-cells, with scribing still being
used to achieve sub-cell separation in conductive layers above and
below the active, thin film layer. Also, the active layer may be
placed over a series of protrusions so that the CMP removes the
active layer that is over the protrusion, while leaving intact the
flat, planar portions of the active layer. In this way, the removed
active layer, from over the protrusions then becomes the division
between sub-cells in the active layer.
Inventors: |
Eickelmann; Hans-Juergen;
(Nieder-Hilbersheim, DE) ; Kellmann; Ruediger;
(Mainz, DE) ; Schmidt; Markus; (Seibersbach,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation |
Armonk |
NY |
US |
|
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
52342581 |
Appl. No.: |
13/947205 |
Filed: |
July 22, 2013 |
Current U.S.
Class: |
136/244 ; 438/57;
438/98 |
Current CPC
Class: |
H01L 31/0392 20130101;
H01L 31/03923 20130101; H01L 31/0508 20130101; H01L 31/03925
20130101; Y02P 70/50 20151101; Y02E 10/541 20130101; H01L 31/0463
20141201; Y02P 70/521 20151101 |
Class at
Publication: |
136/244 ; 438/57;
438/98 |
International
Class: |
H01L 21/306 20060101
H01L021/306; H01L 31/05 20060101 H01L031/05; H01L 31/18 20060101
H01L031/18 |
Claims
1-13. (canceled)
14. An electromagnetic radiation cell comprising: a lower
conductive layer; a substrate; and an active layer; wherein: the
substrate includes an upper surface having a set of protrusions
including at least one protrusion with each protrusion including
two upstanding lateral surfaces; and the lower conductive layer is
located between the substrate and the active layer so that: (i) the
lower conductive layer extends over the upstanding surfaces of the
protrusions, and (ii) the active layer is segmented into a
plurality of mutually isolated sub-cells by the lower conductive
layer and the upstanding portions of the protrusions underlying the
lower conductive layer.
15. The cell of claim 14 wherein the active layer is
photovoltaic.
16. The cell of claim 14 further comprising: an upper conductive
layer located on top of at least a substantial portion of the
active layer.
17. the cell of claim 16, wherein: the lower conductive layer is
divided into sub-cells corresponding to the sub-cells of the active
layer; and the upper conductive layer is divided into sub-cells
corresponding to the sub-cells of the active layer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to the field of thin
film cells that transduce electromagnetic radiation into electrical
energy (herein generically called e/m radiation cells), more
particularly to thin film solar cells.
BACKGROUND OF THE INVENTION
[0002] Two known types of solar cells are: (i) thin film solar
cells; and (ii) crystalline solar cells. This document deals with
thin film solar cells, and does not deal with crystalline solar
cells. Thin film solar cells are segmented into sub-cells in order
to optimize cell performance in terms of voltage and current. The
segments are reconnected again, preferably in a monolithic way
(where the electrical connections are created in situ) without
further added interconnection wiring. The segmentation is currently
done by: (i) mechanical scribing; or (ii) laser evaporation (also
called laser scribing). The conventional process for thin film
solar cell segmentation includes the following scribe operations:
(i) deposit and scribe into sub-cell formations the conducting back
contact layer (also called the "lower conducting layer"); (ii)
deposit and scribe into sub-cell formations the absorber (that is,
the active layer); (iii) deposit and scribe into sub-cell
formations the transparent conducting oxide (also called the "upper
conducting layer"); and (iv) edge scribe.
[0003] Thin film solar cells are expected to deliver substantial
cost savings compared to first generation bulk crystalline solar
cells. They are manufactured by large area deposition of thin
films. For practical purposes, the module voltage has to be
increased by a series connection of several sub-cells. In order to
take advantage of monolithic integration, the scribing processes,
mentioned in the previous paragraph, are used to mechanically
separate, while maintaining electrical connections between,
respective sub-cell areas from each other.
SUMMARY
[0004] According to an aspect of the present invention, there is a
process for at least partially making an electromagnetic radiation
cell. The process includes the following actions (not necessarily
in the following order): (i) providing a first intermediate state
electromagnetic radiation cell sub-assembly that includes an active
layer of a thin film type; and (ii) performing polishing on the
first intermediate state electromagnetic radiation cell
sub-assembly to remove at least a portion of the active layer to
yield a second intermediate state electromagnetic radiation cell
sub-assembly.
[0005] According to a further aspect of the present invention,
there is a process for at least partially making a photovoltaic
cell. The process includes the following actions (not necessarily
in the following order): (i) providing a first intermediate state
photovoltaic cell sub-assembly; (ii) performing polishing on the
first intermediate state photovoltaic cell sub-assembly to yield a
second intermediate state photovoltaic cell sub-assembly; and (iii)
depositing an upper conductive layer on the second intermediate
state photovoltaic cell sub-assembly to yield a third intermediate
state photovoltaic cell sub-assembly. The first intermediate state
thin film electromagnetic cell sub-assembly includes a substrate
layer, a lower conductive layer on top of the substrate layer and a
photovoltaic layer of a thin film type on top of the lower
conductive layer. The performing of the polishing removes a portion
of the photovoltaic layer so that the photovoltaic layer is divided
into a plurality of mutually isolated sub-cells.
[0006] According to a further aspect of the present invention, an
electromagnetic radiation cell includes: a substrate; and an active
layer. The substrate includes an upper surface having a set of
protrusions including at least one protrusion. The active layer is
of a thin film type. The active layer is divided into a plurality
of mutually isolated sub-cells. The divisions between the sub-cells
are located over the protrusions in the substrate.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0007] FIG. 1 is a flowchart view of a first manufacturing process
according to the present disclosure;
[0008] FIG. 2 is a transverse cross-sectional view (cross hatching
omitted for clarity of illustration reasons) of a solar cell at an
intermediate stage of manufacture according to the first
manufacturing process; and
[0009] FIG. 3 is a transverse cross-sectional view (cross hatching
omitted for clarity of illustration reasons) of a solar cell after
completion of the first manufacturing process.
DETAILED DESCRIPTION
[0010] The present invention recognizes that the scribing processes
conventionally used to make the sub-cell structures of thin film
e/m radiation cells are not ideal, and can potentially reduce
system performance through scribing-induced defects. The present
disclosure further recognizes that: (i) conventional processes for
making thin film solar cells are potentially detrimental to
performance because they usually induce defect in the active
semiconducting layers of the thin film solar cell; and (ii) the
mechanical scribe step of the conventional process limits the
spacing distance of scribes, resulting in a large dead area in
between adjacent sub-cells of the thin film solar cell. This
Detailed Description section is divided into the following
sub-sections: (i) Operation of Embodiment(s) of the Present
Invention; (ii) Further Comments and/or Embodiments; and (iii)
Definitions.
I. Operation of Embodiment(s) of the Present Invention
[0011] A process embodiment 50 of the present disclosure will now
be discussed with reference to FIGS. 1-3. It is noted that the
cross-sections of FIGS. 2 and 3 are not necessarily to scale.
[0012] As shown in FIG. 1, processing begins at step S55 where a
substrate (for example, glass substrate, polymeric substrate, steel
with barrier layer) is provided as starting stock for process 50.
The starting stock substrate (not shown in the Figures) has
protruding areas on its top surface area. For example, the
protruding surface areas may be in the shape of stripes
corresponding to the location of planned divisions between the
sub-cells of the final product. For glass substrates, the
protrusions may be made by: (i) chemical etching; (ii) hot
embossing; or (iii) a combination of (i) and (ii). For polymeric
substrates, protrusions may be made by: (i) hot embossing; (ii)
steel embossing; or (iii) a combination of (i) and (ii).
[0013] Processing proceeds to step S60 where a deposition device
deposits a lower conducting layer on the top surface of the
substrate, including over the protruding areas mentioned above in
connection with step S55. The lower conducting layer may be any
material suitable for a lower conducting layer in a thin film e/m
radiation cell, now conventional or to be developed in the future.
In some embodiments, it may be preferable to use a material that is
resistant to CMP (or at least some types of CMP) so that CMP
material removal of the active layer (to be discussed below in
connection with step S85) does not impact the geometry of the lower
conducting layer. The conducting layer may be made of any metal
capable of forming a good ohmic contact to the active layer (for
example, Ag, Al or Mo).
[0014] Processing proceeds to step S70 where a scribing device (not
shown in the Figures) scribes the lower conducting layer in order
to separate the lower conducting layer into sub-cell structures.
This step may be accomplished by any appropriate type of scribing
now known or to be developed in the future. Alternatively, other
material removal processes could be used to break the lower
conducting layer into multiple sub-cell structures.
[0015] Processing proceeds to step S80 where a deposition device
(not shown in the Figures) deposits an absorber layer (also
sometimes referred to as an active layer).
[0016] FIG. 2 shows solar cell 100 in its intermediate sub-assembly
state 100a, which occurs just after step S80 of process 50 has been
completed. Intermediate state solar cell 100a includes: substrate
102 (including protruding area 150); first sub-cell portion 104a of
lower conducting layer 104; second sub-cell portion 104b of
conducting layer 104; active layer 106; and first scribed gap 152
in the lower conducting layer. Protruding area 150 was discussed
above in connection with step S55. First scribed gap 152 was formed
by the scribing discussed, above, in connection with step S70. Note
that protruding area 150 causes a discontinuity in the lower
surface of active layer 106. More specifically, the lower surface
of active layer is generally flat and planar, except the portion of
the active layer that is over protrusion which is a discontinuity
in the shape of a recess in the lower surface of the active
layer.
[0017] Processing proceeds to step S85 where a CMP device (not
shown in the Figures) performs CMP on the top surface of
intermediate state solar cell 100a. This CMP removes part, but not
all, of active layer 106 in a top downwards manner. More
specifically, the CMP removes a portion of the active layer from
the initial top surface down to the level of its lower surface
where the discontinuity caused by protrusion 150 is. Because the
CMP removes material all the way down to the lower surface in the
vicinity of the discontinuity in the lower surface, this means that
the active surface is completely gone over the topmost portion of
protrusion 150. On the other hand, in areas where the lower surface
of the active layer is flat and planar, there is still plenty of
active layer remaining over these portions, despite the material
removal accomplished by the CMP. These remaining portions of active
layer become sub-cells, but the completely removed portions of
active layer are now breaks between sub-cells that electrically
isolate the sub-cells from each other. It is the discontinuity in
the lower surface of the active layer that causes these "breaks" in
the active layer to occur.
[0018] Any type of CMP, now known, or to be developed in the
future, may be used. In at least some embodiments of the present
disclosure, the CMP of step S85 does not adversely affect the
radiation-to-electricity transduction functionality of the
remaining portion of active layer 106, which is advantageous from
the perspective of solar cell performance. Alternatively, the
material removal of step S85 can be accomplished by pure mechanical
polishing, as an alternative to CMP. The CMP of step S85 has to be
selective by not removing the bottom conductive layer, while
reliably removing the active layer material. The chemical agent for
the CMP of step S85 contains a mixture of acidic oxidizing and
stabilizing agents.
[0019] Processing proceeds to step S87 where a deposition device
(not shown in the Figures) deposits a buffer layer on top of the
remaining portion of the active layer.
[0020] Processing proceeds to step S90 where a deposition device
(not shown in the Figures) deposits a transparent conducting layer
(also called an upper conducting layer) on top of the remaining
portion of the buffer layer deposited at step S87. The upper
conducting layer (see definition of "layer," below, in the
definitions sub-section) includes a buffer sub-layer and a
transparent front contact layer.
[0021] The upper conducting layer, or its constituent sub-layers,
may be any materials suitable for a lower conducting layer in a
thin film e/m radiation cell, now conventional or to be developed
in the future. In order to be suitable for use as an upper
conducting layer, the material and/or thickness of the upper
conducting layer must be chosen so that the upper conducting layer
is at least somewhat transmissive with respect to the wavelengths
of e/m radiation that the active layer is designed to transduce
into electricity. In some embodiments, it may be preferable to use
a material for the upper conducting layer that can be easily
removed by CMP (or at least some types of CMP) so that a CMP
process can be used to divide the upper conducting layer into
sub-cell structures. However, it should be understood that process
50, shown in FIG. 3, does not use CMP to divide the upper
conducting layer into sub-cell structures. In this embodiment, the
upper conducting layer is made of a doped transparent conducting
oxide such as indium tin oxide (ITO), or aluminum doped zinc oxide
(ZnO:Al).
[0022] Processing proceeds to step S95 where a scribing device (not
shown in the Figures) scribes the upper conducting layer in order
to separate the upper conducting layer into sub-cell structures.
This step may be accomplished by any appropriate type of scribing
now known or to be developed in the future. Alternatively, other
material removal processes could be used to break the lower
conducting layer into multiple sub-cell structures.
[0023] FIG. 3 shows solar cell 100 in its final state 100b, which
occurs when process 50 has been completed. Final state solar cell
100b includes: substrate 102 (including protruding area 150); first
sub-cell portion 104a of lower conducting layer 104; second
sub-cell portion 104b of conducting layer 104; first scribed gap
152; first sub-cell portion 106a of active layer 106; second
sub-cell portion 106b of active layer 106; second scribed gap 154;
first sub-cell portion 107a of buffer sub-layer 107; second
sub-cell portion 107b of buffer sub-layer 107; first sub-cell
portion 108a of transparent contact sub-layer 108; second sub-cell
portion 108b of transparent contact sub-layer 108. Taken together,
sub-layers 107 and 108 make up the upper conducting layer.
[0024] As shown by comparing FIGS. 2 and 3, it can be seen that
because of the CMP of step S85, the height of layer 106 was reduced
down to the height of protruding surface area 150. However,
protruding surface area 150 and the portion of layer 104 extending
over the protruding surface area is not substantially affected by
the CMP of step S85.
[0025] As shown in FIG. 3, the scribing of step S95 causes scribing
gap 154 in upper conducting layer 107, 108, which divides
sub-layers 107 and 108 into sub-cell structures 107a, 107b, 108a
and 108b. In this embodiment, gap 154 exposes a part of layer 104b
to separate the sub-cells. In this embodiment, the upper conducting
layer of the first sub-cell 108a contacts the lower conducting
layer of the second sub-cell 104b to provide the monolithic series
connection of the sub-cells to achieve a higher module voltage.
Alternatively, in other embodiments, the sub-cells could be
completely electrically isolated from each other.
II. Further Comments and/or Embodiment(s)
[0026] According to some embodiments of the present disclosure, a
process for making e/m radiation cells (for example, solar cells)
includes one or more of the following: (i) use of a substrate with
protruded areas; and/or (ii) replacing at least a portion of the
scribing operations (for example, the P2 scribe of the absorber) by
chemical mechanical polishing (CMP) operation(s). A method for
segmenting thin film solar cells will now be discussed. Thin film
solar cells usually require segmentation into sub-cells in order to
optimize cell performance in terms of voltage and current. The
segments are electrically reconnected again, usually in a
monolithic way without further added interconnection wiring. A
method embodiment of the present disclosure includes the following
operations: (i) provide a substrate; (ii) scribe the conducting
back contact layer (also referred to as the P1 scribe); (iii)
perform absorber-related CMP; (iv) scribe the transparent
conducting oxide (also referred to as the P3 scribe); and (v) edge
scribe (also referred to as the P4 scribe). Some embodiments of
this process: (i) reduce or eliminate defects in the active
semiconducting layers; and (ii) reduce or eliminate the dead area
in between sub-cells. Some embodiments of the present disclosure
target CIGS (copper indium gallium selenide) solar cells, but other
embodiments are more broadly directed to: (i) any thin film solar
cells that has sub-cells; and/or (ii) any thin film e/m radiation
cell that has sub-cells.
[0027] As mentioned above, some embodiments of this disclosure
apply chemical mechanical polishing methods in order to separate
sub-cells. In these embodiments, layers are: (i) deposited onto
dimples (that is protrusions made by making dimples in the
substrate); or, alternatively or additionally, (ii) a flat
substrate is modified by small humps (or protrusions). In these
embodiments, an e/m-voltaic (such as a photovoltaic layer) active
layer (also called the thin film) is deposited in and/or on both:
(i) the protrusions, and (ii) the flat portion of the substrate.
Active layer material above the protrusions is removed by chemical
mechanical polishing. This removal of the protruded areas leaves
separate segments of thin film material as deposited photovoltaic
material such that the thin film is divided into sub-cell
structures. Some embodiments of the present disclosure provide a
new method, employing chemical mechanical polishing to separate
sub-cells reducing the defects generated by scribing. The present
disclosure recognizes that there is a need to improve the current
scribing process for thin film solar cells, especially for CIGS
(copper indium gallium selenide) active layer, CZTS (copper zinc
tin sulfide) active layer, or CdTe (cadmium telluride) active layer
solar cells.
[0028] Thin film solar cells are photovoltaic devices manufactured
by deposition of a thin film of the absorber material onto large
substrates, for example glass plates the size of window panes.
After deposition of the absorber and the formation of a suitable
photovoltaic junction, the film would appear as a single diode with
a voltage related to the internal bandgap of the semiconducting
material and the electrical properties of the network, much like a
silicon wafer with added contact grid but with larger dimensions.
In this case the voltage would be a fraction of a volt. If the
module is able to deliver a power in the range of 60 to 100 watts,
then relatively large currents result. In many applications, a
higher voltage and lower current is beneficial, because this
reduces ohmic losses in series resistance. The transparent
conducting oxide (TCO) which is used as a front window contact
material is far from being ideal as a conductor. For this reason,
the large absorber is cut into segments and the segments are
interconnected again. This interconnection is done via soldered
stripes for silicon wafers but needs to be done in a different way
for thin film solar cells. Fortunately, a monolithic approach is
possible, segmenting the different films and interconnecting them
again via deposition of thin films. In that case, no wiring or
soldering is required for thin film solar cells. Moreover, no
contact grid deposition is required.
[0029] A process embodiment of the present disclosure is as
follows: (i) provide substrate with protruded surface areas, for
example in stripes; (ii) deposit backside contacting layer; (iii)
scribe backside contacting layer; (iv) deposit absorber; (v)
chemical mechanical polishing of the surface and recess to level of
protrusion, exposing the backside conducting layer; (vi) deposit
frontside transparent conducting layer; (vii) scribe front side
transparent conducting layer; and (viii) module fabrication. The
method is quite general and not limited to a particular material
system and can be applied to any thin film semiconducting thin film
for singulation and series connection. The CMP scribing method can
also be applied for roll-to-roll manufacturing of thin film solar
cells and also on polymeric/metal sheet substrates.
[0030] Some embodiments of the present invention may have one, or
more, of the following features, characteristics and/or advantages:
(i) increases the module efficiency relative to panels that
exclusively use scribing to separate sub-cells; (ii) prevent damage
to the absorber layer; and/or (iii) separate the absorber without
damaging the material by inducing defects or changing its phase
state through heating, etc.
[0031] In some embodiments of the present disclosure, in order to
apply CMP to thin film solar cells for separation into sub-cells,
substrate topography is modified. This modification can be achieved
by etching or deposition methods. In one embodiment, the active
layer(s) are deposited into dimples in the substrate. In another
embodiment, a flat substrate is modified by deposition of small
humps, creating a similar shaped surface topography. In either type
of embodiment, the active layer(s) cover substantially the entire
substrate, including its flat, planar areas and also its dimples
(that is, a protrusion made by dimpling)/humps (collectively called
protrusions). The protruded areas are then removed by chemical
mechanical polishing. This leaves separate flat, planar, mutually
isolated sub-sell segments of the active layer(s).
III. Definitions
[0032] Present invention: should not be taken as an absolute
indication that the subject matter described by the term "present
invention" is covered by either the claims as they are filed, or by
the claims that may eventually issue after patent prosecution;
while the term "present invention" is used to help the reader to
get a general feel for which disclosures herein that are believed
as maybe being new, this understanding, as indicated by use of the
term "present invention," is tentative and provisional and subject
to change over the course of patent prosecution as relevant
information is developed and as the claims are potentially
amended.
[0033] Embodiment: see definition of "present invention"
above--similar cautions apply to the term "embodiment."
[0034] and/or: non-exclusive or; for example, A and/or B means
that: (i) A is true and B is false; or (ii) A is false and B is
true; or (iii) A and B are both true.
[0035] Electrically Connected: means either directly electrically
connected, or indirectly electrically connected, such that
intervening elements are present; in an indirect electrical
connection, the intervening elements may include inductors and/or
transformers.
[0036] Mechanically connected: Includes both direct mechanical
connections, and indirect mechanical connections made through
intermediate components; includes rigid mechanical connections as
well as mechanical connection that allows for relative motion
between the mechanically connected components; includes, but is not
limited, to welded connections, solder connections, connections by
fasteners (for example, nails, bolts, screws, nuts, hook-and-loop
fasteners, knots, rivets, quick-release connections, latches and/or
magnetic connections), force fit connections, friction fit
connections, connections secured by engagement caused by
gravitational forces, pivoting or rotatable connections, and/or
slidable mechanical connections.
[0037] Over: if thing A is "over" thing B, then this should not be
taken to necessarily imply that thing A and thing B are in
contact.
[0038] Layer: a single layer or a set of consecutive layers that
acts as a single layer; for example, an active layer may be a
single layer, or a set of discrete and consecutive layers that
transduce electromagnetic radiation into electricity; as another
example, a conductive layer may include multiple discrete layers,
but still herein be referred to as a singular "layer."
[0039] Polishing: CMP and/or pure mechanical polishing.
* * * * *