U.S. patent application number 15/165702 was filed with the patent office on 2016-09-22 for back contact for a photovoltaic module.
The applicant listed for this patent is First Solar, Inc.. Invention is credited to Sreenivas Jayaraman.
Application Number | 20160276521 15/165702 |
Document ID | / |
Family ID | 45465941 |
Filed Date | 2016-09-22 |
United States Patent
Application |
20160276521 |
Kind Code |
A1 |
Jayaraman; Sreenivas |
September 22, 2016 |
BACK CONTACT FOR A PHOTOVOLTAIC MODULE
Abstract
The present invention relates to photovoltaic modules and
methods of manufacturing photovoltaic modules.
Inventors: |
Jayaraman; Sreenivas;
(Holland, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
First Solar, Inc. |
Perrysburg |
OH |
US |
|
|
Family ID: |
45465941 |
Appl. No.: |
15/165702 |
Filed: |
May 26, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13183209 |
Jul 14, 2011 |
|
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15165702 |
|
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61364664 |
Jul 15, 2010 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/073 20130101;
H01L 31/022425 20130101; Y02P 70/50 20151101; H01L 31/0749
20130101; Y02P 70/521 20151101; Y02E 10/541 20130101; H01L
31/022466 20130101; H01L 31/1884 20130101; Y02E 10/543 20130101;
H01L 31/0322 20130101; H01L 31/18 20130101; H01L 31/022441
20130101; H01L 31/1864 20130101 |
International
Class: |
H01L 31/18 20060101
H01L031/18; H01L 31/0224 20060101 H01L031/0224; H01L 31/0749
20060101 H01L031/0749; H01L 31/032 20060101 H01L031/032 |
Claims
1. A method for manufacturing a back contact for a photovoltaic
module, the method comprising: forming a semiconductor window layer
over a substrate; forming a semiconductor absorber layer over the
window layer, the semiconductor absorber layer comprising one of
CdTe and CIGS; removing oxides from the absorber layer surface;
forming a first metal layer in direct physical contact with the
oxide removed surface of the absorber layer, the first metal layer
comprising a material selected from the group consisting of
molybdenum, tungsten, nickel, cobalt, titanium, molybdenum nitride,
titanium nitride, tungsten nitride, mercury tellurium, or any
combination thereof; heat treating the first metal layer; and
forming a second metal layer adjacent to the first metal layer.
2. The method of claim 1, wherein the first metal layer comprises a
material selected from the group consisting of molybdenum,
molybdenum nitride, or any combination thereof.
3. The method of claim 1, wherein the step of heat treating the
first metal layer comprises a temperature of about 100.degree. C.
to about 400.degree. C. and a duration of about 30 seconds to about
30 minutes.
4. The method of claim 1, wherein the step of heat treating the
first metal layer comprises a temperature of about 200.degree. C.
to about 300.degree. C. and a duration of about 1 minute to about
20 minutes.
5. The method of claim 1, wherein the first metal layer has a
thickness of about 5 angstroms to about 300 angstroms.
6. The method of claim 1, wherein the first metal layer has a
thickness of about 50 angstroms to about 250 angstroms.
7. The method of claim 1, wherein the second metal layer comprises
a material selected from the group consisting of aluminum,
chromium, iron, vanadium, manganese, zinc, ruthenium, silver, gold,
copper, platinum, tungsten, nickel, cobalt, titanium, molybdenum
nitride, titanium disilicide, titanium silicide, titanium nitride,
tungsten nitride, and mercury tellurium, or any combination
thereof.
8. The method of claim 1, wherein the step of forming a back
contact layer further comprises heat treating the second metal
layer at a temperature of about 50.degree. C. to about 400.degree.
C. for a duration of about 1 minute to about 30 minutes.
9. The method of claim 1, wherein the step of forming a back
contact layer further comprises heat treating the second metal
layer at a temperature of about 50.degree. C. to about 150.degree.
C. for a duration of about 5 minutes to about 20 minutes.
10. The method of claim 1, wherein the second metal layer has a
thickness of about 500 angstroms to about 10000 angstroms.
11. The method of claim 1, wherein the second metal layer has a
thickness of about 1000 angstroms to about 5000 angstroms.
12. The method of claim 1, further comprising: cleaning a surface
of the formed first metal layer before forming the second metal
layer
Description
CLAIM OF PRIORITY
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/183,209, filed Jul. 14, 2011, which claims
priority under 35 U.S.C. .sctn.119(e) to U.S. Provisional Patent
Application Ser. No. 61/364,664 filed on Jul. 15, 2010, which are
hereby incorporated by reference in their entireties.
TECHNICAL FIELD
[0002] The present invention relates to photovoltaic modules and
methods of manufacturing photovoltaic modules.
BACKGROUND
[0003] A photovoltaic device may include a semiconductor material
deposited over a substrate. The semiconductor may contain a first
layer serving as a window layer and a second layer serving as an
absorber layer. The semiconductor window layer may allow solar
radiation to reach the absorber layer, and the absorber layer,
which may contain cadmium telluride, may convert the solar
radiation to electricity. The photovoltaic device may also include
a back contact to facilitate connectivity. However, the back
contact contributes electrical resistance to the photovoltaic
device which reduces the device's overall efficiency.
DESCRIPTION OF DRAWINGS
[0004] FIG. 1 is a side view of a photovoltaic device.
[0005] FIG. 2 is a side view of a transparent conductive oxide
stack.
[0006] FIG. 3 is a side view of a back contact.
[0007] FIG. 4 is a method of manufacturing a photovoltaic
device.
[0008] FIG. 5 is a method of manufacturing a back contact layer for
a photovoltaic device.
[0009] FIG. 6 is a method of manufacturing a back contact layer for
a photovoltaic device.
[0010] FIG. 7 is a method of manufacturing a back contact layer for
a photovoltaic device.
[0011] FIG. 8 is a method of manufacturing a back contact layer for
a photovoltaic device.
[0012] FIG. 9 is a method of manufacturing a back contact layer for
a photovoltaic device.
DETAILED DESCRIPTION
[0013] A photovoltaic device may include an optically transparent
substrate, a transparent conductive oxide layer adjacent to the
substrate, and a semiconductor material adjacent to the transparent
conductive oxide layer. In addition, one or more metal layers may
be deposited on a back surface of the semiconductor material to
form a back contact. With the transparent conductive oxide layer
acting as a front contact, the front and back contacts may serve as
electrodes for transporting photo-generated current away from the
photovoltaic device.
[0014] The layers of semiconductor material may include a bi-layer,
which may include an n-type semiconductor window layer, and a
p-type semiconductor absorber layer. The n-type window layer and
the p-type absorber layer may be positioned in contact with one
another to form a p-n junction. As a result of diffusion across the
junction, negative acceptor ions are formed on the p-type side and
positive donor ions are formed on the n-type side. The presence of
the ions creates a built-in electric field across the junction.
When a photon is absorbed within the p-n junction, an electron hole
pair is formed. The electrons are then swept towards the n-type
layer and holes are swept towards the p-type layer. Electrons can
then flow back to the p-type side via an external current path. The
resulting electron flow provides current, which combined with the
resulting voltage from the electric field, creates power. The
result is a conversion of photon energy into electrical power.
[0015] The transparent conductive oxide layer may be deposited
between the substrate and the semiconductor bi-layer to serve as a
front contact. The transparent conductive oxide layer may include,
for example, cadmium stannate, since it exhibits high optical
transmission and low electrical resistance. The transparent
conductive oxide may be part of a three-layer stack. For instance,
the transparent conductive oxide stack may include a barrier layer,
a transparent conductive oxide layer, and a buffer layer. The
buffer layer may be included between the transparent conductive
oxide layer and the semiconductor window layer to decrease the
likelihood of irregularities occurring during the formation of the
semiconductor window layer. Also, the barrier layer can be
incorporated between the substrate and the transparent conductive
oxide layer to lessen diffusion of sodium or other contaminants
from the substrate to the semiconductor layers, which could result
in poor performance and degradation of the photovoltaic devices.
The barrier layer may include, for example, silicon dioxide.
[0016] The back contact layer may transport electrical charge away
from the device and may include one or more metal layers deposited
adjacent to the semiconductor absorber layer. In particular, the
back contact may include a first metal layer deposited adjacent to
the semiconductor absorber layer. The first metal layer may be
deposited using any suitable process or combination of processes.
For instance, the first metal layer may be deposited by physical
vapor deposition techniques such as magnetron sputtering, thermal
evaporation or laser ablation. Alternate methods of depositing a
first layer may include chemical vapor deposition, wet methods such
as electrochemical or electroless deposition, or even mechanical
roll coating. After the first metal layer is deposited, it may be
heat treated to alter its physical and electrical properties.
During the deposition process, nitrogen may be introduced into the
back contact metal to improve the overall efficiency of the
photovoltaic device. The second metal layer may be deposited by
physical vapor deposition, electrochemical or electroless
deposition, chemical vapor deposition, mechanical roll coating, or
a combination thereof.
[0017] A variety of materials are available for the first and
second metal layers, including molybdenum, aluminum, chromium,
iron, nickel, titanium, vanadium, manganese, cobalt, zinc,
ruthenium, tungsten, silver, gold, copper, mercury tellurium,
titanium disilicide, titanium silicide, molybdenum nitride,
titanium nitride, tungsten nitride and platinum. Molybdenum nitride
functions particularly well as a back contact metal due to its
relative stability at processing temperatures and low contact
resistance. Similarly, silver, gold, and copper function well as
back contact metals since they are low-resistance electrical
conductors.
[0018] The heat treating process may include annealing or any other
suitable heat treating process. Post-deposition annealing of the
first metal layer may relieve stress as well as induce desirable
reactions between the metal and the semiconductor layer.
Post-deposition annealing may also transform the metal layer to
form a desirable metallurgical phase. For instance, annealing may
reduce the contact resistance of the first metal layer. Contact
resistance is defined as a contribution to the total resistance of
a device resulting from electrical leads and connections. By
reducing the contact resistance, the overall efficiency of the
photovoltaic device may be increased.
[0019] When heat treating the back contact, high temperatures may
be desirable to cause inter-diffusion between the semiconductor
absorber layer and the first metal layer. High temperatures may
also be desirable to transform the first metal layer to a desired
phase. However, to prevent dopant redistribution, the thermal
budget should be carefully controlled. Thermal budget is defined as
the cumulative thermal energy imparted to the photovoltaic panel by
all thermal processing steps during manufacturing. If high
temperatures are required during manufacturing, a moderate thermal
budget may be achieved by limiting the duration of the process.
Similarly, if a process requires significant time to complete, the
temperature must be reduced to avoid an excessive thermal
budget.
[0020] While a high temperature may be desirable when forming the
first metal layer of the back contact, the temperature must be
controlled to avoid reducing the integrity of the photovoltaic
device. In particular, the layers of metal used to create the back
contact may have coefficients of thermal expansion that differ from
those of the semiconductor, TCO, and substrate layers. Adding heat
to layers having differing coefficients of thermal expansion may
induce strain that can result in cracking or even gross
delamination of the layers. Accordingly, excessive heat treatment
temperatures and durations should be avoided.
[0021] To produce a reliable back contact, the semiconductor
surface should be extremely clean prior to forming the back contact
layer adjacent to the semiconductor surface. Under certain
conditions, unwanted oxides may form on the semiconductor surface.
Before a first metal layer can be deposited on the semiconductor,
the oxides must be removed. Surface cleaning may be performed by
sputter-etching, chemical etching, reactive gas etching, ion
milling, or any other suitable process.
[0022] In one aspect, a photovoltaic module may include a
substrate, a transparent conductive oxide layer adjacent to the
substrate, a semiconductor layer adjacent to the transparent
conductive oxide layer, and a back contact layer adjacent to the
semiconductor layer. The back contact layer may include a first
metal layer formed adjacent to the substrate layer and a second
metal layer foamed adjacent to the first metal layer. In
particular, the first metal layer may include a material selected
from the group consisting of molybdenum, tungsten, nickel, cobalt,
titanium, molybdenum nitride, titanium nitride, tungsten nitride
and mercury tellurium. The first metal layer may be heat treated at
a temperature of about 100 C to about 400 C for a duration of about
30 seconds to about 30 minutes. Preferably, the first metal layer
may be heat treated at a temperature of about 200 C to about 300 C
for a duration of about 1 minute to about 20 minutes. The first
metal layer may have a thickness of about 5 angstroms to about 300
angstroms. Preferably, the first metal layer may have a thickness
of about 50 angstroms to about 250 angstroms. The second metal
layer may include a material selected from the group consisting of
silver, gold, copper, and aluminum and may be heat treated at a
temperature of about 50 C to about 400 C for a duration of about 1
minute to about 30 minutes. Preferably, the second metal layer may
be heat treated at a temperature of about 50 C to about 150 C for a
duration of about 5 minutes to about 20 minutes. The second metal
layer may have a thickness of about 500 angstroms to about 10000
angstroms. Preferably, the second metal layer may have a thickness
of about 1000 angstroms to about 5000 angstroms.
[0023] In another aspect, a method of manufacturing a photovoltaic
device may include providing a substrate, forming a transparent
conductive oxide layer adjacent to the substrate, forming a
semiconductor layer adjacent to the transparent conductive oxide
layer, and forming a back contact layer adjacent to the
semiconductor layer. The step of forming a back contact layer may
include forming a first metal layer adjacent to the substrate
layer, heat treating the first metal layer, and forming a second
metal layer adjacent to the first metal layer. The first metal
layer may include a material selected from the group consisting of
molybdenum, tungsten, nickel, cobalt, titanium, molybdenum nitride,
titanium nitride, tungsten nitride, and mercury tellurium. The step
of heat treating the first metal layer may include a temperature of
about 100 C to about 400 C for a duration of about 30 seconds to
about 30 minutes. Preferably, the heat treating process may occur
at a temperature of about 200 C to about 300 C for a duration of
about 1 minute to about 20 minutes. The first metal layer may have
a thickness of about 5 angstroms to about 300 angstroms.
Preferably, the first metal layer may have a thickness of about 50
angstroms to about 250 angstroms. The step of forming a back
contact may include applying a second metal layer. The second metal
layer may include a material selected from the group consisting of
silver, gold, copper, and aluminum. The second metal layer may have
a thickness of about 500 angstroms to about 10000 angstroms.
Preferably, the second metal layer may have a thickness of about
1000 angstroms to about 5000 angstroms. The second metal layer may
or may not require the application of a thermal treatment.
[0024] In another aspect, a photovoltaic module can include a
plurality of photovoltaic cells adjacent to a substrate and a back
cover adjacent to the plurality of photovoltaic cells. Each one of
the plurality of photovoltaic cells can include a transparent
conductive oxide layer adjacent to the substrate, a semiconductor
layer adjacent to the transparent conductive oxide layer, and a
back contact layer adjacent to the semiconductor layer. The back
contact layer can include a first metal layer formed adjacent to
the semiconductor layer and a second metal layer formed adjacent to
the first metal layer.
[0025] The first metal layer can include a material including
molybdenum, tungsten, nickel, cobalt, titanium, molybdenum nitride,
titanium nitride, tungsten nitride, or mercury tellurium. The first
metal layer can have a thickness of about 5 angstroms to about 300
angstroms. The second metal layer can include a material selected
from the group consisting of silver, gold, copper, and aluminum The
second metal layer can have a thickness of about 500 angstroms to
about 10000 angstroms.
[0026] In another aspect, a method for generating electricity can
include illuminating a photovoltaic cell with a beam of light to
generate a photocurrent and collecting the generated photocurrent.
The photovoltaic cell can include a substrate, a transparent
conductive oxide layer adjacent to the substrate, a semiconductor
layer adjacent to the transparent conductive oxide layer, and a
back contact layer adjacent to the semiconductor layer. The back
contact layer can include a first metal layer formed adjacent to
the semiconductor layer a second metal layer formed adjacent to the
first metal layer.
[0027] As shown in FIG. 1, a photovoltaic device 100 may include a
substrate 105, a transparent conductive oxide stack 110, a
semiconductor bi-layer including a semiconductor window layer 115
and a semiconductor absorber layer 120, a back contact layer 125,
and a back support 130. The substrate 105 may include an optically
transparent material, such as soda-lime glass. However, since the
primary function of the substrate 105 is to protect the device from
physical damage caused by moisture or debris while permitting
penetration of solar radiation, any suitable transparent material
may be used. Similar to the substrate 105, the back support 130 may
serve to protect and enclose the photovoltaic device 100. The back
support 130 may be any suitable material, such as soda-lime
glass.
[0028] As shown in FIG. 2, a transparent conductive oxide stack 110
may include a barrier layer 205, a transparent conductive oxide
layer 210, and a buffer layer 215. The barrier layer 205 may be
formed adjacent to the substrate 105. One or more barrier layers
205 may include any suitable material, including, for example, a
silicon oxide, aluminum-doped silicon oxide, boron-doped silicon
oxide, phosphorous-doped silicon oxide, silicon nitride,
aluminum-doped silicon nitride, boron-doped silicon nitride,
phosphorous-doped silicon nitride, silicon oxide-nitride, titanium
oxide, niobium oxide, tantalum oxide, aluminum oxide, zirconium
oxide, tin oxide, or combinations thereof. The transparent
conductive oxide layer 210 may be formed adjacent to the barrier
layer 205 and may include any suitable material. For instance, the
transparent conductive oxide layer 210 may include cadmium
stannate. Alternately, the transparent conductive oxide layer 210
may include any suitable material or materials. For instance, the
transparent conductive oxide layer 210 may include a layer of
cadmium and tin and may be any suitable thickness. The transparent
conductive oxide layer 210 may have a thickness ranging from, for
example, 100 to 1000 nm. The transparent conductive oxide stack 110
may also include a buffer layer 215 which may be formed adjacent to
the transparent conductive oxide layer 210. The presence of the
buffer layer 215 during manufacturing may decrease the likelihood
of irregularities occurring during the formation of the
semiconductor window layer 115. The transparent conductive oxide
stack 110 may be manufactured using a variety of deposition
techniques, including, for example, low pressure chemical vapor
deposition, atmospheric pressure chemical vapor deposition,
plasma-enhanced chemical vapor deposition, thermal chemical vapor
deposition, DC or AC sputtering, spin-on deposition, or
spray-pyrolysis. Each deposition layer can be of any suitable
thickness, for example, in the range of about 10 to about 5000
angstroms.
[0029] The semiconductor window layer 115 may be formed adjacent to
the transparent conductive oxide stack 110. The semiconductor
absorber layer 120 may formed adjacent to the semiconductor window
layer 115. Together, the semiconductor window layer 115 and the
semiconductor absorber layer 120 form a semiconductor bi-layer. The
semiconductor bi-layer may include cadmium telluride (CdTe).
Alternately, the semiconductor bi-layer may include any suitable
compound, such as copper indium gallium selenide (CIGS). The window
layer 115 may be an n-type semiconductor window layer, and the
absorber layer 120 may be a p-type semiconductor absorber layer.
The n-type window layer 115 and the p-type absorber layer 120 may
be positioned in contact with one another to create an electric
field.
[0030] The back contact layer 125 may be formed adjacent to the
semiconductor absorber layer 120. The back contact layer 125 may
cover a portion or an entire surface of the semiconductor absorber
layer 120. As shown in FIG. 3, the back contact layer 125 may
include a first metal layer 305 and a second metal layer 310. The
first metal layer may be adjacent to the semiconductor absorber
layer 120, and the second metal layer 310 may be adjacent to the
first metal layer 305. The first metal layer 305 may include a
material selected from the group consisting of molybdenum,
aluminum, chromium, iron, nickel, titanium, vanadium, manganese,
cobalt, zinc, ruthenium, tungsten, silver, gold, copper, mercury
tellurium, titanium disilicide, titanium silicide, molybdenum
nitride, titanium nitride, tungsten nitride, and platinum or a
combination thereof. Alternately, any suitable material may be
used. The first metal layer 305 may have a thickness ranging from 5
angstroms to 300 angstroms. Preferably, the first metal layer may
have a thickness ranging from 50 to 250 angstroms.
[0031] The first metal layer 305 may be heat treated to alter, for
instance, its electrical and mechanical properties. The heat
treating of the first metal layer 305 may include annealing and may
result in the formation of an ohmic contact between, for example,
the cadmium telluride semiconductor and the first metal layer 305.
The annealing may occur in the presence of a gas such as, for
example, nitrogen gas selected to control the atmosphere of the
annealing process. The annealing may be aided by providing an
oxygen-depleting or oxygen-reducing environment. The first metal
layer 305 may be annealed under any suitable pressure, for example,
under reduced pressure at about 0.01 Pa (10-4 Torr). Also, the
first metal layer 305 may be annealed at any suitable temperature
or temperature range. For example, the first metal layer 305 may be
annealed at about 100 to about 400 C. Preferably, the first metal
layer 305 may be annealed at about 200 to about 300 C. In addition,
the first metal layer 305 may be annealed for any suitable
duration. For example, the first metal layer 305 may be annealed
for a duration ranging from about 30 seconds to about 30 minutes.
Preferably, the first metal layer 305 may be annealed for a
duration ranging from about 1 minute to about 20 minutes. By
annealing the first metal layer 305 at a high temperature,
inter-diffusion may occur between the semiconductor absorber layer
120 and the first metal layer 305. The first metal layer 305 may be
transformed into a metallurgical phase that affords low contact
resistance to the semiconductor absorber layer 120. The selection
of temperature and duration values must account for the physical
limitations and properties of the various layers of the device 100.
For instance, the thermal budget must be large enough to cause a
reduction in contact resistance of the first metal layer 305, but
the thermal budget must also be small enough to avoid degrading the
other layers (e.g. 110,115, 120).
[0032] Once the first metal layer 305 is deposited adjacent the
semiconductor absorber layer 120, a second metal layer 310 may be
deposited on the first metal layer 305. The second metal layer 310
may include a material selected from the group consisting of
molybdenum, aluminum, chromium, iron, nickel, titanium, vanadium,
manganese, cobalt, zinc, ruthenium, tungsten, silver, gold, copper,
mercury tellurium, titanium disilicide, titanium silicide,
molybdenum nitride, titanium nitride, tungsten nitride, and
platinum or a combination thereof. Alternately, any suitable
material may be used. Similar to the first metal layer, the second
metal layer 310 may be heat treated to improve its electrical and
mechanical properties. In particular, the second metal layer 310
may be heat treated to improve adhesion to the first metal layer
305 and to reduce its electrical resistance. For instance, an
annealing process may refine the microstructure of the second metal
layer to improve its current carrying ability. The choice of
temperature and duration values for the heat treatment process may
be limited by the existing layers (e.g. 110, 115, 120, and 305).
For instance, the temperature and duration values must be low
enough to avoid degrading the existing layers or causing
detrimental interactions amongst the layers (e.g. 110, 115, 120,
and 305). Alternately, the heat treatment process may be omitted if
the second metal layer 310 exhibits desirable physical and
electrical properties.
[0033] As shown in FIG. 4, a method of manufacturing a photovoltaic
device 100 may include providing a substrate 405, forming a
transparent conductive oxide layer adjacent to the substrate 410,
forming a semiconductor layer adjacent to the transparent
conductive oxide 415, and forming a back contact layer 420 adjacent
to the semiconductor layer. Forming a back contact layer 420 may be
accomplished through methods described in FIGS. 5-10. In
particular, as shown in FIG. 5, forming a back contact layer may
include forming a first metal layer adjacent to the first
semiconductor layer 505, heat treating the first metal layer 510,
and forming a second metal layer adjacent to the first metal layer
515. As shown in FIG. 6, the method of FIG. 5 may include an
additional step of heat treating the second metal layer 620. This
step can be achieved by using an established technique like
microwave heating to selectively heat a given layer. Alternately,
as shown in FIG. 7, the method of FIG. 5 may include an additional
step of heat treating both the first and second metal layers
720.
[0034] Once the first metal layer 305 is deposited adjacent to the
semiconductor absorber layer 120, a passivating layer (not shown)
may be deposited adjacent the first layer 305. The passivating
layer may cap the first metal layer 305 and protect against
oxidation and grain boundary grooving prior to, during, and after
the heat treating process. The passivating layer may be any
suitable material. The passivating layer may be incorporated into
the back contact layer, or it may be stripped via dry etching, wet
chemical, or any other suitable process prior to forming the second
metal layer 310. For example, as shown in FIG. 8, the method may
include forming a first metal layer adjacent to the semiconductor
layer 805, forming a passivating layer adjacent to the first metal
layer 810, heat treating the first metal layer 815, removing the
passivating layer 820, and forming a second metal layer adjacent to
the first metal layer 825. The second metal layer may then be heat
treated in a subsequent step. In addition, the step of removing the
passivating layer may omitted as shown in the method of FIG. 9. In
this way, the passivating layer may become a permanent component of
the back contact layer.
[0035] Although the steps in the aforementioned methods are shown
in particular orders in FIGS. 5-9, this is not limiting. For
instance, the forming steps and heat treating steps may occur prior
to forming the back contact layer adjacent to the semiconductor
layer. Although only two layers are shown, the back contact layer
125 may include two or more layers. For instance, the back contact
layer 125 may include two, three, four, five, or six layers. The
additional layers may be formed in sequential steps similar to the
two-part back contact described herein. The additional layers may
be heat treated as described herein to improve their electrical and
mechanical properties. In addition, subsequent layers may be formed
and heat treated separately from the photovoltaic device to avoid
high thermal budgets from adversely affecting the semiconductor
bi-layer.
[0036] Details of one or more embodiments are set forth in the
accompanying drawings and description. Other features, objects, and
advantages will be apparent from the description, drawings, and
claims. Although a number of embodiments of the invention have been
described, it will be understood that various modifications may be
made without departing from the spirit and scope of the invention.
It should also be understood that the appended drawings are not
necessarily to scale, presenting a somewhat simplified
representation of various features illustrative of the basic
principles of the invention.
* * * * *