U.S. patent application number 12/414422 was filed with the patent office on 2010-09-30 for semitransparent flexible thin film solar cells and modules.
This patent application is currently assigned to SoloPower, Inc.. Invention is credited to Bulent M. BASOL.
Application Number | 20100248415 12/414422 |
Document ID | / |
Family ID | 42710931 |
Filed Date | 2010-09-30 |
United States Patent
Application |
20100248415 |
Kind Code |
A1 |
BASOL; Bulent M. |
September 30, 2010 |
SEMITRANSPARENT FLEXIBLE THIN FILM SOLAR CELLS AND MODULES
Abstract
A method of manufacturing partially light transparent thin film
solar cells generally includes forming a solar cell structure stack
and forming multiple openings through the solar cell structure
stack. The solar cell structure stack includes a flexible foil
substrate, a contact layer formed over the flexible foil substrate,
a Group IBIIIAVIA absorber layer formed over the contact layer and
a transparent conductive layer formed over the Group IBIIIAVIA
absorber layer. A terminal structure including at least one busbar
and a plurality of conductive finger patterns is deposited onto a
top surface of the transparent conductive layer forming a
semi-transparent solar cell.
Inventors: |
BASOL; Bulent M.; (Manhattan
Beach, CA) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
SoloPower, Inc.
San Jose
CA
|
Family ID: |
42710931 |
Appl. No.: |
12/414422 |
Filed: |
March 30, 2009 |
Current U.S.
Class: |
438/73 ;
257/E21.002 |
Current CPC
Class: |
Y02P 70/521 20151101;
H01L 31/03928 20130101; H01L 31/0468 20141201; Y02P 70/50 20151101;
Y02E 10/541 20130101; H01L 31/0749 20130101 |
Class at
Publication: |
438/73 ;
257/E21.002 |
International
Class: |
H01L 21/02 20060101
H01L021/02 |
Claims
1. A method of manufacturing a semitransparent thin film solar cell
structure stack, comprising: forming a solar cell structure stack
wherein the solar cell structure stack includes a flexible foil
substrate, a contact layer formed over the flexible foil substrate,
a Group IBIIIAVIA absorber layer formed over the contact layer and
a transparent conductive layer formed over the Group IBIIIAVIA
absorber layer; and mechanically forming a plurality of openings
through the solar cell structure stack to create a hole pattern in
the solar cell structure stack, each of the openings in the hole
pattern extending through the transparent conductive layer, the
Group IBIIIAIVA absorber layer, the contact layer and the flexible
foil substrate, thereby making the semitransparent solar cell
structure stack.
2. The method of claim 1 further comprising forming a terminal
including a busbar and fingers on a top surface of the transparent
conductive layer, thereby forming a semitransparent thin film solar
cell.
3. The method of claim 2, wherein the top surface of the
transparent conductive layer includes a passive region on which the
terminal is formed and an active region on which the plurality of
openings are located.
4. The method of claim 3, wherein a transparency range of the
semitransparent thin film solar cell is 10-50%.
5. The method according to claim 2 further comprising the step of
stringing together a plurality of semitransparent thin film solar
cells, thereby creating a semitransparent thin film solar cell
module.
6. The method according to claim 5 wherein the step of stringing
further includes a plurality of non-transparent thin film solar
cells.
7. The method according to claim 5 wherein the semitransparent thin
film solar cells and the non-transparent thin film solar cells are
arranged in a pattern to provide an overall transparency
pattern.
8. The method of claim 1 further comprising forming a terminal
including a busbar and fingers on a top surface of the transparent
conductive layer, thereby forming a thin film solar cell prior to
the step of mechanically forming the plurality of openings and
thereby resulting in a semitransparent solar cell.
9. The method of claim 8, wherein the top surface of the
transparent conductive layer includes a passive region on which the
terminal is formed and an active region on which the plurality of
openings are located.
10. The method of claim 9, wherein a transparency range of the
semitransparent thin film solar cell is 10-50%.
11. The method according to claim 8 further comprising the step of
stringing together a plurality of semitransparent thin film solar
cells, thereby creating a semitransparent thin film solar cell
module.
12. The method according to claim 11 wherein the step of stringing
further includes a plurality of non-transparent thin film solar
cells.
13. The method according to claim 11 wherein the semitransparent
thin film solar cells and the non-transparent thin film solar cells
are arranged in a pattern to provide an overall transparency
pattern.
14. The method of claim 2, wherein a diameter of the plurality of
openings is in the range of 0.1-2 cm.
15. The method of claim 1, wherein the step of mechanically forming
is performed using at least one of die cutting, punching and
drilling.
16. The method of claim 1 wherein the step of mechanically forming
mechanically forms all of the plurality of openings for the hole
pattern at the same time.
17. The method of claim 1 wherein the hole pattern creates a
substantially uniform overall transparency across the solar cell
structure stack.
18. The method of claim 1 wherein the hole pattern creates a
non-uniform overall transparency across the solar cell structure
stack.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to fabrication of
semitransparent flexible thin film solar cells and modules.
BACKGROUND
[0002] Solar cells are photovoltaic devices that convert sunlight
directly into electrical power. The most common solar cell material
is silicon, which is in the form of single or polycrystalline
wafers. However, the cost of electricity generated using
silicon-based solar cells is higher than the cost of electricity
generated by the more traditional methods. Therefore, since early
1970's there has been an effort to reduce cost of solar cells for
terrestrial use. One way of reducing the cost of solar cells is to
develop low-cost thin film growth techniques that can deposit
solar-cell-quality absorber materials on large area substrates and
to fabricate these devices using high-throughput, low-cost
methods.
[0003] Group IBIIIAVIA compound semiconductors comprising some of
the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group
VIA (O, S, Se, Te, Po) materials or elements of the periodic table
are excellent absorber materials for thin film solar cell
structures. Especially, compounds of Cu, In, Ga, Se and S which are
generally referred to as CIGS(S), or Cu(In,Ga)(S,Se).sub.2 or
CuIn.sub.1-xGa.sub.x(S.sub.ySe.sub.1-y).sub.k, where
0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1 and k is approximately 2,
have already been employed in solar cell structures that yielded
conversion efficiencies approaching 20%. Absorbers containing Group
IIIA element Al and/or Group VIA element Te also showed promise.
Therefore, in summary, compounds containing: i) Cu from Group IB,
ii) at least one of In, Ga, and Al from Group IIIA, and iii) at
least one of S, Se, and Te from Group VIA, are of great interest
for solar cell applications.
[0004] The structure of a conventional Group IBIIIAVIA compound
photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te).sub.2 thin film
solar cell is shown in FIG. 1. The device 10 is fabricated on a
substrate 11, such as a sheet of glass, a sheet of metal, an
insulating foil or web, or a conductive foil or web. The absorber
film 12, which comprises a material in the family of
Cu(In,Ga,Al)(S,Se,Te).sub.2, is grown over a conductive layer 13 or
contact layer, which is previously deposited on the substrate 11
and which acts as the electrical contact to the device. The
substrate 11 and the contact layer 13 form a base 20. Various
conductive layers comprising Mo, Ta, W, Ti, and stainless steel
etc. have been used in the solar cell structure of FIG. 1. If the
substrate itself is a properly selected conductive material, it is
possible not to use a contact layer 13, since the substrate 11 may
then be used as the ohmic contact to the device. After the absorber
film 12 is grown, a transparent layer 14 such as a CdS, ZnO or
CdS/ZnO stack is formed on the absorber film. Radiation 15 enters
the device through the transparent layer 14. Metallic grids (not
shown) may also be deposited over the transparent layer 14 to
reduce the effective series resistance of the device. The preferred
electrical type of the absorber film 12 is p-type, and the
preferred electrical type of the transparent layer 14 is n-type.
However, an n-type absorber and a p-type window layer can also be
utilized. The preferred device structure of FIG. 1 is called a
"substrate-type" structure. A "superstrate-type" structure can also
be constructed by depositing a transparent conductive layer on a
transparent superstrate such as glass or transparent polymeric
foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te).sub.2 absorber
film, and finally forming an ohmic contact to the device by a
conductive layer. In this superstrate structure light enters the
device from the transparent superstrate side. A variety of
materials, deposited by a variety of methods, can be used to
provide the various layers of the device shown in FIG. 1. It should
be noted that although the chemical formula is often written as
Cu(In,Ga)(S,Se).sub.2, a more accurate formula for the compound is
Cu(In,Ga)(S,Se).sub.k, where k is typically close to 2 but may not
be exactly 2. For simplicity we will continue to use the value of k
as 2. It should be further noted that the notation "Cu(X,Y)" in the
chemical formula means all chemical compositions of X and Y from
(X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga)
means all compositions from CuIn to CuGa. Similarly,
Cu(In,Ga)(S,Se).sub.2 or CIGS(S) means the whole family of
compounds with the Ga/(Ga+In) molar ratio varying from 0 to 1, and
the Se/(Se+S) molar ratio varying from 0 to 1.
[0005] Cu(In,Ga)(S,Se).sub.2 type compound thin films may be
deposited over the selected substrate by various methods such as
co-evaporation, sputtering/co-sputtering, ink deposition,
electrodeposition, etc., One technique for growing
Cu(In,Ga)(S,Se).sub.2 type compound thin films for solar cell
applications is a two-stage process where metallic components of
the Cu(In,Ga)(S,Se).sub.2 material are first deposited onto a
substrate, and then reacted with S and/or Se in a high temperature
annealing process. For example, for CIS or CuInSe.sub.2 growth,
thin layers of Cu and In are first deposited on a substrate and
then this stacked precursor layer is reacted with Se at elevated
temperature. If the reaction atmosphere also contains sulfur, then
a CIS(S) or CuIn(S,Se).sub.2 layer can be grown. Addition of Ga in
the precursor layer, i.e. use of a Cu/In/Ga stacked film precursor,
allows the growth of a CIGS(S) or Cu(In,Ga)(S,Se).sub.2 absorber.
The precursor layers may be deposited by various methods such as
evaporation, sputtering, ink deposition, electrodeposition,
etc.
[0006] Two-stage process approach may also employ stacked layers
comprising Group VIA materials. For example, a CIGS or
Cu(In,Ga)Se.sub.2 film may be obtained by depositing In--Ga--Se and
Cu--Se layers in an In--Ga--Se/Cu--Se stack and reacting them in
presence of Se. Similarly, stacks comprising Group VIA materials
and metallic components may also be used. Stacks comprising Group
VIA materials include, but are not limited to In--Ga--Se/Cu stack,
Cu/In/Ga/Se stack, Cu/Se/In/Ga/Se stack, etc. The stacks may be
deposited over the substrate using the various methods listed
above.
[0007] Building Integrated Photovoltaics (BIPV) used in buildings
often needs semitransparent solar cells and modules that can be
utilized on building facades. These devices, while generating
electricity, also let some predetermined amount of the light
impinging on the solar module into the building. The
semitransparent PV modules may have a transparency in the 10-70%
range.
[0008] One way of achieving semi-transparency in PV modules is to
use a transparent front protective sheet and back protective sheet
in the module structure and leave large gaps between the solar
cells in the module. The larger the gaps are between the solar
cells, the more sun light passes from the front side of the module
to the back side. However, this method is not attractive because as
the space between cells increases the interconnection wiring or the
interconnection ribbons that electrically connect each solar cell
with its neighboring cell becomes more and more visible.
[0009] Modules made of amorphous silicon can also be made partially
transparent by employing thin absorber layers and transparent
contacts. In such amorphous silicon applications, the transparency
of the solar cell itself can be controlled by the thickness and
type of the amorphous silicon. This method cannot be used for CIGS
and CdTe type solar cells because these solar cells employ metallic
contact layers that are opaque.
[0010] One other method used to achieve semi-transparency for
crystalline silicon solar cells involves mechanical texturization
of the front and rear side of the silicon wafer before the cell is
fabricated. In this method, perpendicular grooves made on the front
and rear sides of the silicon wafer create holes at their crossing
points if the depth of each groove is larger than half of the
thickness of the Si wafer. The size of the holes can be controlled
by controlling the depth of the grooves, deeper grooves creating
larger holes. The holes in silicon solar cells result in a partial
optical transparency of the device. A hole size obtained with such
methods is typically in the range of 100-200 .mu.m diameter. These
devices are extremely fragile because the grooves penetrate into
the silicon along substantially the whole length of the solar cell
and reduce the mechanical strength of the substrate, which
typically have a total thickness in the 200-400 .mu.m range.
Therefore, in order to optimize the mechanical strength of the
device, the hole size is limited so that the transparency of the
device is typically in the range of 15-25% range. The grooves
opened in the Si substrate also make solar cell processing
difficult. Thin film solar cells have absorber layer thicknesses in
the 1-10 .mu.m range. Therefore, the grooving method cannot be used
in such devices. Grooves with depths of 0.5-8 microns would yield
holes with diameters in the range of only a few microns.
Furthermore, fragile nature of thin films would not allow defect
free grooving with the required precision.
[0011] Therefore there is a need for robust, semi-transparent thin
film flexible solar cells that can be handled without concern for
breakage.
SUMMARY
[0012] The present invention provides a method to form partially
light transparent solar cells with continuous flexible substrates.
Accordingly a method of manufacturing partially light transparent
or semitransparent thin film solar cells generally includes forming
a solar cell structure stack and forming multiple openings through
the solar cell structure stack for a predetermined amount of light
to go through. In one embodiment the solar cell structure stack
includes a flexible foil substrate, a contact layer formed over the
flexible foil substrate, a Group IBIIIAVIA compound absorber layer
formed over the contact layer and a transparent conductive layer
formed over the Group IBIIIAVIA absorber layer.
[0013] Each of the openings extends through the transparent
conductive layer, the Group IBIIIAIVA absorber layer, the contact
layer and the flexible foil substrate; thereby making the solar
cell structure stack partially light transparent. A terminal
structure including at least one busbar and a plurality of
conductive finger patterns is deposited onto a top surface of the
transparent conductive layer forming a semi-transparent solar cell.
Alternatively, the step of forming the one or more openings can be
performed after the terminal, including the at least one busbar and
the plurality of conductive finger patterns, is deposited onto the
top surface of the transparent conductive layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a cross-sectional view of a solar cell employing a
Group IBIIIAVIA absorber layer;
[0015] FIG. 2A-2B are schematic perspective and cross sectional
views of a semitransparent solar cell of the present invention;
[0016] FIGS. 3A-3C are schematic views of various exemplary
semi-transparent thin film solar cells with holes; and
[0017] FIG. 4 is a schematic view of a module employing
semi-transparent thin film solar cells with holes.
DETAILED DESCRIPTION
[0018] The present inventions provide a partially light transparent
solar energy conversion device such as a solar cell or solar cell
module, and a variety of processes to manufacture the partially
light transparent solar energy device with adjustable light
transparency. The solar cells of the present inventions are
fabricated on flexible foil substrates, such as flexible polymeric
or metallic substrates. In one embodiment, the manufacturing
process includes forming a solar cell structure stack by depositing
a contact layer on a flexible foil substrate, forming an absorber
layer on the contact layer, and forming a transparent conductive
layer on the absorber layer. After forming the solar cell structure
stack, through-holes or apertures, which permit light to pass
through the stack, are formed in the solar cell in a determined
pattern or arrangement. Each hole extends from a top opening in a
front surface of the solar cell structure stack to a bottom opening
in a bottom surface of the substrate. The top openings of the holes
are distributed within an active region of the front surface.
Further, a passive region of the front surface is designated for a
finger pattern of the solar cell. After the holes formed, the
finger pattern is deposited on the passive region of the front
surface of the solar cell structure stack. In another embodiment,
the holes are formed within the active region after depositing the
finger patterns on the passive region of the front surface of the
solar cell structure stack. Light transparency of the solar cell is
provided by holes formed through the thickness of the device after
the solar cell is fabricated. The ratio of the area of the active
region occupied by the holes to the total area of the solar cell
gives the transparency rate of the solar cell.
[0019] FIG. 2A shows in perspective view a solar cell 100 including
a flexible foil substrate 102, a contact layer 104 formed over the
flexible foil substrate, an absorber layer 106 such as a Group
IBIIIAVIA absorber layer formed over the contact layer 104 and a
transparent conductive layer 108 formed over the Group IBIIIAVIA
absorber layer. The flexible foil substrate 102, the contact layer
104, the absorber layer 106 and the transparent conductive layer
108 form a solar cell structure stack 100A. A terminal structure
110 or current collecting electrode including a busbar 112 and
fingers 114 is deposited onto a top surface 116 of the transparent
conductive layer, which is also the light receiving surface of the
solar cell 100. Holes 120 or openings are formed through the solar
cell structure stack 100A using a mechanical mechanism, such as by
die cutting, punching, drilling, or the like. In a preferred
embodiment, the mechanical mechanism will create all of the holes
that are desired for the entire solar cell/solar cell structure
stack at the same time, in a predetermined pattern, by having a
plurality of punches or the like corresponding to each hole that is
made. FIG. 2B is a cross sectional view of FIG. 2A taken along the
line 2B-2B. As shown in FIG. 2B, the holes 120 are formed and
extend through the substrate 102, the contact layer 104, the
absorber layer 106 and the transparent conductive layer 108. Holes
120 make the solar cell 100 semi-transparent to light. Holes 120
may be formed after depositing the terminal structure 110 over the
surface of the transparent conductive layer 108, i.e. after
completion of the solar cells, or can be formed prior to depositing
the terminal structure 110 over the surface of the transparent
conductive layer 108.
[0020] The terminal structure 110 is formed over a passive region
122 of the top surface 116 whereas the holes 120 are formed in an
active region 124 of the surface 116. It should be noted that the
passive region 122 shown in FIG. 2A refers to the region right
under the terminal structure 110. Each hole 120 formed within the
active area of a solar cell with a total area of "A" reduces the
area of the available active region 124 while increasing the
transparency of the solar cell 100. Therefore, the ratio between
the area occupied by the total number of holes (e.g., B) to the
total area of the solar cell gives the transparency of the solar
cell structure stack or the solar cell. For example, by adjusting
the B/A ratio a transparency range of the solar cell structure
stack may be adjusted in a range of 10-50% or higher.
[0021] FIGS. 3A, 3B and 3C are top views (illuminated surface
views) of exemplary devices. In FIG. 3A, a solar cell 20 has holes
23 and a terminal structure 24 or current collecting electrode. The
terminal structure 24 is formed of fingers 24A and busbars 25. The
holes 23 are formed in areas avoiding damage to the terminal
structure 24. As is shown in FIG. 3A, the holes 23 in the solar
cell may have different shapes and sizes, as well as patterns.
Therefore, many different appearances may be achieved using
different hole geometries and patterns and terminal structure
designs. The appearance can be one in which a substantially uniform
transparency results across the entire solar cell 20, and can also
be one in which a predetermined illumination gradient, which
corresponds to an overall transparency pattern that results from
the hole geometries and patterns and terminal design. Different
ways can be used to achieve the same results in terms of
appearance. FIG. 3B shows a solar cell with wavy finger pattern 26
and large holes 23. FIG. 3C shows another device 22 with a mesh 28
as the current collecting grid and smaller holes 23 in between the
mesh conductor. In both the FIG. 3B and the FIG. 3C embodiments,
the resulting appearance is one in which there is substantially
uniform transparency, as the holes are located in a substantially
uniform pattern across the entire solar cell. It is noted that
although holes 23 do not exist in four different grid locations
formed by the terminal structure design mesh 28, the absence of
these few holes will not result in an appreciable difference in the
appearance of the transparency.
[0022] If the hole pattern of a semitransparent solar cell is such
that the hole size and distribution over the solar cell is
substantially uniform, such cells may be used in module structures
that yield an overall transparency that is also substantially
uniform. It is however, also possible to mix solar cells with
substantially uniform hole patterns and substantially non-uniform
hole patterns or even totally opaque (hole-free) solar cells in a
module structure to create the desired visual effect. For example,
the logo of a company may be written on a module or an array of
modules by designing holes that form the logo on otherwise
hole-free solar cells or modules. As can be seen from the
discussion above, solar cells with different degrees of
transparency and different visual effects can be easily fabricated
using hole designs that fit the need for the BIPV application.
Solar cell modules having a perceived uniform transparency can be
made by stringing together solar cells that each have a
substantially uniform transparency. It is also possible to create
an overall transparency pattern by stringing together solar cells
that have perceived uniform transparency and solar cells that have
either no transparency or a perceived non-uniform
transparency.[
[0023] As discussed previously, in prior art silicon devices with
holes, the silicon substrate is first grooved forming the holes.
The solar cell is then fabricated on the Si substrate with the
holes. Thin film solar cell devices of the present invention are
fabricated in other ways, described hereinafter, due to the
differences between the types of devices. It should be noted that
grooving methods are not practical for thin film solar cell
structures, especially for flexible thin film solar cell
structures.
[0024] In one embodiment, a complete solar cell is fabricated on
the foil substrate, the solar cell comprising a base, an absorber
layer, a transparent layer through which light enters the device,
and a grid pattern to collect the generated current. Holes are then
punched through the solar cell, avoiding damage to the grid
pattern. Alternately, holes may be first opened in the solar cell
structure stack without a current collecting grid in a
predetermined fashion and the grid pattern may then be deposited on
the cell on a predetermined location in a way that avoids
deposition of the grid onto areas with holes.
[0025] For thin film solar cells employing a CIGS(S) type absorber
layer, holes providing certain degree of transparency to the device
may be formed during the various process steps used for the
fabrication of the device. For example, the typical process flow
for CIGS solar cell fabrication comprises the steps of; i)
providing a flexible foil substrate, ii) depositing a contact layer
on a surface of the substrate, iii) forming a CIGS(S) absorber
layer on the contact layer, iv) depositing a transparent conductive
layer (such as a CdS/TCO stack) on the absorber layer, and v)
depositing a grid pattern on the transparent layer. Holes may be
formed in the structure after any of the steps (i), (ii), (iii),
(iv), and (v) listed above and the rest of the process steps may be
completed after the formation of the holes. Holes may also be
formed in a roll to roll fashion in technologies that process CIGS
solar cells in a roll-to-roll fashion.
[0026] Thin film flexible solar cells such as CIGS(S) based solar
cells with hole patterns may be packaged in rigid or flexible
module structures to yield semi-transparent rigid or flexible
modules if a transparent back protective sheet such as a
transparent polymeric foil, preferably with a moisture barrier
coating is employed in the structure. One such exemplary module is
shown in FIG. 4, the module 30 comprising twenty interconnected
solar cells 31. The interconnections and the grid patterns of the
solar cells are not shown in this figure for simplification. The
solar cells 31 comprise an active region 32 that is shown as shaded
regions, and holes 33 that give the module 30 transparency, along
with the gaps 34 between the solar cells 31. By adjusting the sizes
of the gaps 34 and the holes 33, the transparency of the module 30
may be adjusted. A preferred range of the size of the holes 33 is
0.1-2 cm. A more preferred range is 0.2-1 cm.
[0027] Although the present invention is described with respect to
certain preferred embodiments, modifications thereto will be
apparent to those skilled in the art.
* * * * *