U.S. patent application number 13/572387 was filed with the patent office on 2013-03-14 for multilayer thin-film back contact system for flexible photoboltaic devices on polymer substrates.
This patent application is currently assigned to ASCENT SOLAR TECHNOLOGIES, INC.. The applicant listed for this patent is Joseph H. Armstrong, Hobart Stevens, Lawrence M. Woods. Invention is credited to Joseph H. Armstrong, Hobart Stevens, Lawrence M. Woods.
Application Number | 20130061927 13/572387 |
Document ID | / |
Family ID | 46875950 |
Filed Date | 2013-03-14 |
United States Patent
Application |
20130061927 |
Kind Code |
A1 |
Woods; Lawrence M. ; et
al. |
March 14, 2013 |
Multilayer Thin-Film Back Contact System For Flexible Photoboltaic
Devices On Polymer Substrates
Abstract
A polymer substrate and back contact structure for a
photovoltaic element, and a photovoltaic element include a CIGS
photovoltaic structure, a polymer substrate having a device side at
which the photovoltaic element can be located and a back side
opposite the device side. A layer of dielectric is formed at the
back side of the polymer substrate. A metal structure is formed at
the device side of the polymer substrate.
Inventors: |
Woods; Lawrence M.;
(Littleton, CO) ; Stevens; Hobart; (Golden,
CO) ; Armstrong; Joseph H.; (Littleton, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Woods; Lawrence M.
Stevens; Hobart
Armstrong; Joseph H. |
Littleton
Golden
Littleton |
CO
CO
CO |
US
US
US |
|
|
Assignee: |
ASCENT SOLAR TECHNOLOGIES,
INC.
Denver
CO
|
Family ID: |
46875950 |
Appl. No.: |
13/572387 |
Filed: |
August 10, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61522209 |
Aug 10, 2011 |
|
|
|
Current U.S.
Class: |
136/262 ;
136/252 |
Current CPC
Class: |
H01L 31/022425 20130101;
H01L 31/0264 20130101; H01L 31/03923 20130101; H01L 31/03926
20130101; Y02E 10/541 20130101; H01L 21/02485 20130101 |
Class at
Publication: |
136/262 ;
136/252 |
International
Class: |
H01L 31/0264 20060101
H01L031/0264; H01L 31/02 20060101 H01L031/02 |
Claims
1. A polymer substrate and back contact structure for a
photovoltaic element, comprising: a polymer substrate having a
device side at which the photovoltaic element can be located and a
back side opposite the device side; a layer of dielectric formed at
the back side of the polymer substrate; and a metal structure
formed at the device side of the polymer substrate.
2. The structure of claim 1, wherein the photovoltaic element
comprises a CIGS structure.
3. The structure of claim 1, wherein the dielectric comprises at
least one of SiO.sub.2, Al.sub.2O.sub.3, and silicone resin.
4. The structure of claim 3, further comprising a thin adhesion
layer disposed between the layer of dielectric and the back side of
the polymer substrate.
5. The structure of claim 4, wherein the adhesion layer comprises
at least one of molybdenum, aluminum, chromium, titanium, titanium
nitride (TiN), a metal oxide, and a metal nitride.
6. The structure of claim 1, wherein the metal structure comprises
a first metal layer, the first metal layer comprising at least one
of aluminum, brass, bronze and copper.
7. The structure of claim 6, wherein the metal structure further
comprises a layer of molybdenum formed over the first metal
layer.
8. The structure of claim 6, wherein the metal structure further
comprises a thin adhesion layer disposed between the first metal
layer and the device side of the polymer substrate.
9. The structure of claim 8, wherein the thin adhesion layer
comprises at least one of molybdenum, aluminum, chromium, titanium,
titanium nitride (TiN), a metal oxide, and a metal nitride.
10. The structure of claim 1, wherein the metal structure further
comprises a thin adhesion layer in contact with the device side of
the polymer layer.
11. The structure of claim 10, wherein the thin adhesion layer
comprises at least one of molybdenum, aluminum, chromium, titanium,
titanium nitride (TiN), a metal oxide, and a metal nitride.
12. A photovoltaic element, comprising: a CIGS photovoltaic
structure; a polymer substrate having a device side at which the
CIGS photovoltaic structure can be located and a back side opposite
the device side; a layer of dielectric formed at the back side of
the polymer substrate; and a metal structure formed at the device
side of the polymer substrate between the polymer substrate and the
CIGS photovoltaic structure.
13. The photovoltaic element of claim 12, wherein the dielectric
comprises at least one of SiO.sub.2, Al.sub.2O.sub.3, and silicone
resin.
14. The photovoltaic element of claim 12, wherein the dielectric
comprises an oxynitride formed from one of aluminum and
silicon.
15. The photovoltaic element of claim 12, further comprising a thin
adhesion layer disposed between the layer of dielectric and the
back side of the polymer substrate.
16. The photovoltaic element of claim 15, wherein the adhesion
layer comprises at least one of molybdenum, aluminum, chromium,
titanium, titanium nitride (TiN), a metal oxide, and a metal
nitride.
17. The photovoltaic element of claim 12, wherein the metal
structure comprises a first metal layer, the first metal layer
comprising at least one of aluminum, brass, bronze and copper.
18. The photovoltaic element of claim 17, wherein the metal
structure further comprises a layer of molybdenum formed over the
first metal layer.
19. The photovoltaic element of claim 17, wherein the metal
structure further comprises a thin adhesion layer disposed between
the first metal layer and the device side of the polymer
substrate.
20. The photovoltaic element of claim 19, wherein the thin adhesion
layer comprises at least one of molybdenum, aluminum, chromium,
titanium, titanium nitride (TiN), a metal oxide, and a metal
nitride.
21. The photovoltaic element of claim 12, wherein the metal
structure further comprises a thin adhesion layer in contact with
the device side of the polymer layer.
22. The photovoltaic element of claim 21, wherein the thin adhesion
layer comprises at least one of molybdenum, aluminum, chromium,
titanium nitride (TiN), a metal oxide, and a metal nitride.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This U.S. Nonprovisional Patent Application claims the
benefit of U.S. Provisional Patent Application No. 61/522,209,
filed in the U.S. Patent and Trademark Office on Aug. 10, 2011, the
entire contents of which are incorporated herein by reference.
BACKGROUND
[0002] 1. Technical Field
[0003] This disclosure relates to photovoltaic modules and methods
of manufacturing photovoltaic modules and, more particularly, to
photovoltaic modules and methods of manufacturing photovoltaic
modules in which mechanical distortion in the modules is
substantially reduced or eliminated.
[0004] 2. Discussion of the Related Art
[0005] One type of flexible photovoltaic (PV) module is formed as a
thin-film device on a polymeric substrate. An example of such
devices is the Copper-Indium-Gallium-Selenide (CIGS) device. CIGS
devices present many challenges in terms of the thin-film
deposition processes, device patterning, and final
assembly/packaging. Polymer substrates are of great significance
since high-temperature variations of the material are adequate to
accommodate CIGS processing while the material maintains its
dielectric properties, which enables monolithic integration without
any additional insulating films.
[0006] A fundamental challenge in flexible CIGS devices is in the
deposition of a metallic back contact onto the polymer prior to the
deposition of the CIGS p-type absorber layer. This back contact
makes ohmic contact to the CIGS and allows for current to flow
through the device and be collected through interconnects to the
leads attached to the electrical load. Thus, this back contact,
which is usually a metal, must maintain high electrical
conductivity, both before and after device processing. It must also
survive the deposition environment for the subsequent thin film
deposition steps.
SUMMARY
[0007] According to a first aspect, a polymer substrate and back
contact structure for a photovoltaic element is provided. The
structure includes a polymer substrate having a device side at
which the photovoltaic element can be located and a back side
opposite the device side. A layer of dielectric is formed at the
back side of the polymer substrate. A metal structure is formed at
the device side of the polymer substrate.
[0008] According to another aspect, a photovoltaic element is
provided. The photovoltaic element includes a CIGS photovoltaic
structure and a polymer substrate having a device side at which the
CIGS photovoltaic structure can be located and a back side opposite
the device side. A layer of dielectric is formed at the back side
of the polymer substrate. A metal structure is formed at the device
side of the polymer substrate between the polymer substrate and the
CIGS photovoltaic structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The foregoing and other features and advantages will be
apparent from the more particular description of preferred aspects,
as illustrated in the accompanying drawings, in which like
reference characters refer to the same parts throughout the
different views. The drawings are not necessarily to scale. In the
drawings, the thickness of layers and regions may be exaggerated
for clarity.
[0010] FIG. 1 includes a graph of intrinsic stress in Mo as a
function of Ar pressure during a vacuum-based sputtering Mo
deposition process.
[0011] FIG. 2 includes a schematic cross-sectional view of a back
contact for a flexible monolithically integrated CIGS photovoltaic
device on a polymer utilizing a metallic multilayer as a top
contact and an oxide as a back-side coating, according to some
exemplary embodiments.
[0012] FIG. 3 includes an image of a
dielectric-polymer-metal-Mo-CIGS stack structure, according to some
exemplary embodiments.
DETAILED DESCRIPTION
[0013] For CIGS devices, molybdenum (Mo) has been a common choice
of material for a back contact, regardless of the substrate. While
Mo can be deposited in a straightforward manner using DC sputtering
or other thin film deposition methods, the wide range of stress
states possible with sputtering can particularly complicate
deposition onto flexible substrates, particularly those that do not
exhibit significant stiffness, such as polymers. Unlike rigid
substrates where the film stresses can readily be borne by the
substrate, film stresses can have a significant impact upon the
life, surface topology, and physical properties of flexible
substrates, particularly substrates made from polymers. This class
of substrates, while exhibiting excellent dielectric properties
that allow monolithic integration, also typically exhibits high and
inconsistent thermal expansion coefficient compared to the metals
and semiconductors of the CIGS layer stack. Thus, there exist
extrinsic stresses that combine with intrinsic stresses that can
warp, wrinkle, distort and otherwise diminish the integrity of
these flexible substrates. In addition, the electrical and
mechanical properties of a back contact also affect the device
performance and adhesion.
[0014] FIG. 1 contains a graph of intrinsic stress state of
sputtered Mo as a function of Argon pressure during a vacuum-based
sputtering Mo deposition process. A careful balance of intrinsic
and extrinsic stresses in the back contact deposition step is thus
desirable to provide a viable flexible photovoltaic device. The
method of deposition, deposition pressure, rates, web process
gasses, web speed, and number of passes are all variables that are
balanced to provide the best back contact for the device.
[0015] According to the present disclosure, a multilayer approach
using two or more different metals in the back contact is used to
replace the prior Mo film deposited onto both sides of a
high-temperature polymeric substrate. According to the disclosure,
the polymeric substrate can be, for example, polyimide,
polybenzobisoxazole (PBO), insulated metal foils, or other such
material for flexible, monolithically integrated CIGS modules using
a high-temperature CIGS deposition process, such as multi-source
evaporation. Unlike prior processes which use Mo films on both
sides of the polymer in order to balance the stresses of this
process, along with subsequent CIGS, CdS and TCO depositions,
according to some exemplary embodiments, a stress-balanced back
contact is formed using a dielectric film on the back side of the
polymer substrate, a primary high-conductivity but low-modulus and
low-cost metallic film layer, for example, aluminum (Al), applied
to the front side of the polymer, followed by a thin cap of Mo over
the Al film layer. The Mo may be disposed onto the Al with or
without added oxygen.
[0016] FIG. 2 contains a schematic cross-sectional view of a back
contact for a flexible monolithically integrated CIGS photovoltaic
device on a polymer utilizing a metallic multilayer as a top
contact and an oxide as a back-side coating, according to some
exemplary embodiments. Referring to FIG. 2, the polymer substrate
14 may be prepared to receive the disposed materials by plasma
cleaning, annealing, or other processes best suited for a given
combination of substrate and photovoltaic (PV) device. The plasma
treatment involves one or more gases. The amounts and percentage of
each gas may vary to optimize the treatment for a particular
material being deposited. The power density of the plasma and the
duration of treatment may also be varied to optimize the treatment.
Annealing or heating the substrate before, during, or after plasma
treatment may further optimize the treatment. The device 10
according to some exemplary embodiments includes the dielectric
film 12, which can be, for example, an oxide such as SiO.sub.2,
Al.sub.2O.sub.3, a nitride, an oxynitride such as an oxynitride of
Al or Si, and which, in this particular exemplary embodiment, is
Al.sub.2O.sub.3, formed at the back side of the polymer substrate
14. Other dielectric coating possibilities include high-temperature
silicone, silicone resins, and other polyimides that may not have
the structural properties to function as a stand-alone substrate,
but that have high-temperature and high-emissivity properties and
that are capable of adding compressive stress to the polymer
substrate. An optional adhesion layer 13 may be formed on the back
side of the polymer substrate 14 before the dielectric film 12 is
formed. The adhesion layer 13 can include at least one of
molybdenum, aluminum, chromium, titanium, titanium nitride (TiN), a
metal oxide, and a metal nitride. The optional adhesion layer 13
can be made very thin, i.e., thin enough to have very low
conductivity and having little to no impact on the back side
emissivity. The optional adhesion layer 13 may oxidize some during
subsequent oxide deposition of the dielectric film 12, forming, for
example, Mo oxide, Cr oxide, Ti oxide, etc. The polymer substrate
14 can be, for example, polyimide, polybenzobisoxazole (PBO),
insulated metal foil, or other such material. Another optional
adhesion layer 15 can be formed over the polymer substrate 14 to
aid in adhesion of the subsequent metallic film layer 16. The
adhesion layer 15 can include at least one of molybdenum, aluminum,
chromium, titanium, titanium nitride (TiN), a metal oxide, and a
metal nitride. The metallic film 16 is formed on the front side of
the polymer substrate 14 or formed on the front side of the
adhesion layer 15 if it is present. The metallic film 16 can be a
high-conductivity but low-modulus and low-cost metallic film made
of, for example, aluminum, copper, brass, bronze, or other such
material. The thin cap layer 18 of Mo is formed over the metallic
film 16. The Mo cap layer 18 may be formed with or without added
oxygen. The CIGS layer 20 is formed over the Mo cap layer 18, which
enables the proper chemical, mechanical and electrical interface to
the CIGS layer 20. A buffer layer 22, formed of, for example, CdS,
may be formed over the CIGS layer 20, and a transparent conductive
oxide (TCO) layer 24 may be formed over the buffer layer 22.
[0017] FIG. 3 contains an image of the
dielectric-polymer-metal-Mo-CIGS stack structure of the inventive
concept, with various (four) thicknesses of the Al.sub.2O.sub.3
back side dielectric layer 12. The four exemplary thicknesses of
the dielectric layer 12 are 0.0 nm (no back side dielectric layer
or coating), 210 nm, 350 nm and 640 nm. As illustrated in FIG. 3,
according to the inventive concept, stress balancing is achieved.
The combination of back side dielectric film 12, the top-side
metallic contacts 16 that serve as the electrical back contact, and
subsequent depositions, all balance their respective stresses to
achieve a flat material that is better suited for mass production
processes.
[0018] Referring to FIG. 3, the stack of
dielectric-polymer-metal-Mo-CIGS according to the inventive concept
has very little compressive stress compared to similar Mo-only back
contact films. This is due to the presence of the metal film 16.
With the addition of the dielectric film 12 on the back side, the
substrate begins to flatten and at a thickness of, for example, 640
nm, all stresses are balanced. According to some exemplary
embodiments, depositing a film that can maintain sufficient
electrical conductivity while surviving a high-temperature CIGS
deposition process in which it is subjected to high temperatures
(exceeding 400.degree. C.) in a selenium (Se)-rich environment is a
major advancement in the scale-up of flexible monolithically
integrated CIGS devices.
[0019] Mo presents a challenge in that, not only can the material
exhibit dramatically different inherent stresses due to variations
in process parameters, but mismatches in the coefficient of thermal
expansion (CTE) between Mo and the underlying substrate coupled
with high-temperature processing, the stiffness of the substrate,
and ultimately, the mechanical properties of the subsequent films,
can all lead to large stresses in the resultant multilayer
construction. Mo can be deposited in various intrinsic stress
states ranging from tensile to compressive in nature, as shown in
FIG. 1. With as-deposited Mo films, a transition between tensile
and compressive intrinsic stresses in Mo occurs approximately at 6
mTorr with the compressive stress state exhibiting a maxima at
approximately 1.2 Pa. However, regardless of the as-deposited
stress state of Mo on the polymer, a compressive stress state is
the result of Mo on polymer after a high-temperature exposure,
e.g., CIGS deposition temperature. These stresses can lead to
cracking of the thin films, or even the substrate, particularly if
extrinsic stresses are added in the form of bending or otherwise
flexing the coated substrate. Stress balancing of the highly
compressive Mo back contact, in consideration of subsequent
deposition steps, is achieved by depositing a compressive film to
the substrate backside. In order to achieve a flat material, the
stress state is balanced, and as the top surface has multiple
metal, semiconductor, and oxide layers, a corresponding Mo layer
applied to the bottom side of the substrate is required to balance
the multiple layers on the top side, although in most cases the
type of Mo film used on the back side (for stress balancing) is
deposited differently and to a different thickness than the Mo film
on the front (for back side electrical conductor). Wrinkle
reduction is one of the primary reasons that batch processing of
panels through the patterning cell is performed to prevent damage
to the closely-moving ink head printing operations. However
balancing the front and back stresses is much more difficult when
the stress levels are high.
[0020] Table 1 illustrates the challenge in depositing a metal,
particularly Mo, onto a high-temperature polymeric substrate. Both
Mo and Al have a much higher modulus by an order of magnitude than
the polymer, while the thermal expansion may be a closer match
between Al and the polymer than Mo. More importantly, the yield
stress of the Al is much lower than Mo, and the stress at 5%
elongation of the polymer is closer to Al than Mo. Finally, the
ultimate stress of the Mo is nearly twice that of the polymer.
TABLE-US-00001 TABLE 1 MECHANICAL PROPERTIES OF ALUMINUM AND
MOLYBDENUM COMPARED TO A TYPICAL HIGH-TEMPERATURE POLYMERIC
SUBSTRATE Thermal Linear Young's Conductivity Expansion Melting
Yield Ultimate Specific Modulus Poisson's (at 0.degree. C.)
Coefficient Point Stress Stress Metal Gravity GPa Ratio W/(m
.degree. K) .times.10.sup.-6 .degree. C. K MPa MPa Al 2.7 68.95
0.33 237 25 933 30-140 60-140 Mo 10.2 275.80 0.32 138 5 2893
585-690 690-827 Thermal Linear Stress at Tensile Conductivity
Expansion Max 5% Tensile Specific Modulus Elongation (at 0.degree.
C.) Coefficient Temp Elongation Strength Substrate Gravity GPa (%)
W/(m .degree. K) .times.10.sup.-6/.degree. C. K MPa MPa UpilexR 1.5
6.9-9.1 42-50 0.29 12-20 ~723 210-260 360-520
[0021] In accordance with some exemplary embodiments, the overall
stress state in the polymer is reduced, and, as a result, a more
planar, wrinkle-free substrate is provided. Because Mo is used for
a proper interface to CIGS, but is a major reason for the high
stresses in the substrate, according to the inventive concept, its
use has been minimized to the minimum required to mask the work
function of the underlying primary metallic film, as shown in Table
2. In some exemplary embodiments, the primary metallic film of
choice is aluminum (Al), although formulations using copper (Cu)
and other highly electrically conductive materials, for example,
brass or bronze, can be used. The CIGS device relies on the proper
work function of its metallic back contact to function properly.
While it is possible to use metallic foils (without insulting
layers) with subsequent Mo deposition to mask the work function of
the metal foil substrate, the inherent stiffness of the
non-polymeric substrates enables the ability to apply greater Mo
film thicknesses without the Mo stress overwhelming the substrate.
With the polymeric process according to embodiments of the
inventive concept, and their lower mechanical properties, the
desirable masking effect by the Mo of the work function of the
underlying primary thin film back contact material (Al, Mo, etc.)
is carefully balanced with the high stresses in Mo that can
increase with greater Mo thickness. Furthermore, the use of
metallic foils without insulating layers precludes the
straightforward ability to integrate monolithically the
photovoltaic device, and as such, limit device construction to
discrete individual cells.
TABLE-US-00002 TABLE 2 ELECTRICAL PROPERTIES OF ALUMINUM AND
MOLYBDENUM (AT 20.degree. C.) COMPARED TO A TYPICAL POLYMERIC
SUBSTRATE Material .rho. [.OMEGA. m] .sigma. [S/m] Work Function
(eV) Aluminum 2.82 .times. 10.sup.-8 3.5 .times. 10.sup.7 4.08
Molybdenum 5.34 .times. 10.sup.-8 1.8 .times. 10.sup.7 4.60 Upilex
PI ~10.sup.+17 ~10.sup.-17
[0022] The AlMo stack of some exemplary embodiments provides
several advantages over conventional single or multi-layer Mo back
contacts. [0023] 1) The film can be made with the bulk of the
stress state dictated by the Al film 16, which is far thicker than
the Mo cap 18. Thus, the overall stress state in the front side
metallization is reduced. [0024] 2) The AlMo stack achieves a far
greater electrical in-plane conductivity than the baseline Mo film,
exceeding an order of magnitude improvement as is shown in Table 2.
This results in the ability to carry greater current than prior
devices, and enables greater cell pitch (width) for monolithically
integrated modules. Larger cells equates to fewer interconnects,
which reduces the interconnect-related losses. Measurements with
samples indicate an order of magnitude reduction in sheet
resistance, dropping from baseline 2 .OMEGA./square to 0.2
.OMEGA./square. This improvement allows for cell width (pitch) to
increase to almost double that demonstrated in baseline conditions,
thereby reducing the interconnects by a factor of two as well.
[0025] 3) While Mo has adequate electrical conductivity for some
applications, it constrains the performance of CIGS that possesses
high current density (>30 mA/cm.sup.2). By using only a thin Mo
cap 18, and relying on the conductivity of Al to provide the bulk
of the electrical conductivity, the stacked material of the
embodiments provides very little sheet resistance. Table 2 also
compares the electrical properties of Cu, Al and Mo. Mo has
approximately half the electrical conductivity of Al and less than
1/3 the electrical conductivity of Cu. However, as the work
function of Al is significantly lower than that of Mo, and that Al
would diffuse readily into CIGS, a cap of Mo is retained to shield
the low Al work function from CIGS. Likewise, Cu would diffuse into
the CIGS during deposition when using Cu, brass or bronze as metal
layer 16. Thus, by using a Mo cap, the best electrical properties
are retained while providing the proper work function interface to
ensure a successful photoelectric effect. [0026] 4) As an added
benefit to the electrical conductor construction, the thin Mo cap
18 presents a much lower electrical resistance pathway through the
P2 laser scribe, e.g., via scribe, into the higher conductivity Al.
Thus, while the baseline P2 interconnect resistance under the
process of record (POR) is nominally between 500-1000 m.OMEGA.-cm,
the P2 resistance for this new interconnect drops to 2 m.OMEGA.-cm.
This alone will account for approximately 5% boost in power output
for a given module by reducing module losses.
[0027] Because Mo that is sufficiently thick to provide adequate
electrical conductivity on polymer contributes adversely to the
stress state in the photovoltaic stack, minimizing the Mo content
of the device back contact allows for another material, other than
Mo, to serve as a back-side film, according to exemplary
embodiments. According to the present disclosure, by eliminating
dependence upon Mo on the back-side film, and by minimizing it in
the back contact, significant advantages over the prior art are
realized. [0028] a) Mo serves as an interface to the CIGS, and
thus, masks the Al work function in order to allow the device to
work optimally. Other metallic elements or alloys can be utilized
as desired for new substrates as they become available. [0029] b)
The overall reduced stress state in the back contact film provides
options for the back side film. In one case, an inexpensive alumina
(Al.sub.2O.sub.3) film that is a good insulator and provides some
level of moisture protection for the polymer can be employed.
However, other oxide films can be employed to enhance bonding
strength to packaging, and oxynitrides can be substituted for
better moisture protection as well. [0030] c) By virtue of reducing
the stress state in the films on either side of the polymer
substrate, the resultant stress experienced by the polymer is also
reduced. Particularly for high-temperature polymers used in
roll-to-roll deposition, the reduced stress state will result in
reduced wrinkling and waviness of the web, particularly after
high-temperature excursions such as those experienced in CIGS
deposition. [0031] d) As new flexible, non-conductive substrates
are developed, such as Poly(p-phenylene-2,6-benzobisoxazole) (PBO)
and are introduced into the flexible CIGS market, experience in
reducing the stress imparted by the back contact can result in a
construction that may eliminate the need for the back-side film
altogether. [0032] e) As achieving the desired Mo stress state is
important, deposition rate is limited with standard Mo films, often
requiring multiple thinner passes to achieve the desired electrical
and stress properties. State-of-the-art films using the process of
record (POR) is 390 nm on the front side and 620 nm on the back
side of the substrate, for a total of just over a micron (1,010
nm). The nominal Mo thickness with the new construction according
to the exemplary embodiments is approximately 100-200 nm, or an
80-90% reduction in the amount of Mo in the device. Using the back
contact of the exemplary embodiments significantly reduces the need
to deposit in multiple layers, and furthermore, as the film is
significantly thinner, at least a 5.times. throughput increase from
the back contact chambers should result, as Al is much easier to
deposit at high web rates. [0033] f) Mo is a relatively expensive
film in the CIGS device, and is approximately 35 times the cost of
Al. As noted above, Mo reduction and substitution of common
elements (Al, Al.sub.2O.sub.3) reduces the cost of the back contact
dramatically. Even in replacing the back-side Mo with
Al.sub.2O.sub.3 should have a noticeable effect. [0034] As noted
above, the Al--Mo back contact has demonstrated dramatically lower
sheet resistance and P2 interconnect resistance. Combined, these
effects will account for a percentage point of efficiency when
module design is optimized to take full advantage of the effects.
Even with the same module design, module power should increase by
5% due to the reduced P2 resistance.
[0035] According to the exemplary embodiments, elimination of a
metal back side film and replacing it with a dielectric layer
provides thermal management in the device, in addition to stress
management, as described herein in detail. Heating of substrates in
a vacuum includes conductive heating (direct contact to a
substrate) and/or radiative heating (energy radiating from one
source to another). Radiative heating is the most common means of
transferring thermal energy to the substrate, but the degree to
which energy is conveyed is dependent upon the substrate's
absorptivity (ability to absorb energy) and emissivity (ability to
radiate heat into the environment). Metals typically have lower
emissivity than, for example, oxide films; thus, metal surfaces do
not give up their heat as easily as oxides. Thus a polymer coated
with metal on both sides can trap the heat within the sandwiched
polymer substrate. In a vacuum, a surface coated with a
high-emittance coating, such as an oxide or nitride, can provide
radiative cooling to that surface and the substrate. A cooler back
side coating and substrate helps to keep the substrate from
degrading and embrittling during high device-side temperatures, and
thus enables higher device-side temperatures that can lead to
higher quality solar absorber layers.
[0036] Exemplary embodiments have been described herein. For
example, exemplary embodiments have been described in terms of
specific exemplary polymeric substrates and particular exemplary
coatings or layers. It will be understood that the exemplary
embodiments relate to stress balancing to provide a back contact in
an improved photovoltaic device. Therefore, the present disclosure
is applicable to other substrate materials and other back side
coatings or layers. In fact, the disclosure may be applicable to
structures with alternate substrates and the elimination of back
side coating altogether.
Combinations of Features
[0037] Various features of the present disclosure have been
described above in detail. The disclosure covers any and all
combinations of any number of the features described herein, unless
the description specifically excludes a combination of features.
The following examples illustrate some of the combinations of
features contemplated and disclosed herein in accordance with this
disclosure.
[0038] In any of the embodiments described in detail and/or claimed
herein, the photovoltaic element can comprise a CIGS structure.
[0039] In any of the embodiments described in detail and/or claimed
herein, the dielectric can comprise at least one of SiO.sub.2,
Al.sub.2O.sub.3, and silicone resin.
[0040] In any of the embodiments described in detail and/or claimed
herein, a thin adhesion layer can be disposed between the layer of
dielectric and the back side of the polymer substrate.
[0041] In any of the embodiments described in detail and/or claimed
herein, the adhesion layer can comprise at least one of Mo, Cr, and
Ti.
[0042] In any of the embodiments described in detail and/or claimed
herein, the metal structure can comprises a first metal layer, the
first metal layer comprising at least one of aluminum, brass,
bronze and copper.
[0043] In any of the embodiments described in detail and/or claimed
herein, the metal structure can further comprise a layer of
molybdenum formed over the first metal layer.
[0044] In any of the embodiments described in detail and/or claimed
herein, the metal structure can further comprise a thin adhesion
layer disposed between the first metal layer and the device side of
the polymer substrate.
[0045] In any of the embodiments described in detail and/or claimed
herein, the thin adhesion layer can comprise at least one of
molybdenum, aluminum, titanium and chromium.
[0046] In any of the embodiments described in detail and/or claimed
herein, the metal structure can further comprise a thin adhesion
layer in contact with the device side of the polymer layer.
[0047] In any of the embodiments described in detail and/or claimed
herein, the thin adhesion layer can comprise at least one of
molybdenum, aluminum, chromium, titanium nitride (TiN), a metal
oxide, and a metal nitride.
[0048] While the present disclosure makes reference to exemplary
embodiments, it will be understood by those of ordinary skill in
the art that various changes in form and details may be made
therein without departing from the spirit and scope of the present
disclosure.
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