U.S. patent application number 13/175339 was filed with the patent office on 2012-01-05 for nanostructured solar cells.
Invention is credited to Dennis J. Flood.
Application Number | 20120000525 13/175339 |
Document ID | / |
Family ID | 39720289 |
Filed Date | 2012-01-05 |
United States Patent
Application |
20120000525 |
Kind Code |
A1 |
Flood; Dennis J. |
January 5, 2012 |
NANOSTRUCTURED SOLAR CELLS
Abstract
Improved photovoltaic devices and methods are disclosed. In one
embodiment, an exemplary photovoltaic device includes a
semiconductor layer and a light-responsive layer (which can be
made, for example, of a semiconductor material) which form a
junction, such as a p-n junction. The light-responsive layer can
include a plurality of carbon nanostructures, such as carbon
nanotubes, located therein. In many cases, the carbon
nanostructures can provide a conductive pathway within the
light-responsive layer. In other embodiments, exemplary
photovoltaic devices include semiconductor nanostructures, which
can take a variety of forms, in addition to the carbon
nanostructures. Further embodiments include a wide variety of other
configurations and features. Methods of fabricating photovoltaic
devices are also disclosed.
Inventors: |
Flood; Dennis J.; (Oberlin,
OH) |
Family ID: |
39720289 |
Appl. No.: |
13/175339 |
Filed: |
July 1, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12108500 |
Apr 23, 2008 |
7999176 |
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13175339 |
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60916727 |
May 8, 2007 |
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60944004 |
Jun 14, 2007 |
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60947139 |
Jun 29, 2007 |
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Current U.S.
Class: |
136/256 ;
136/252; 977/734; 977/742; 977/750; 977/752 |
Current CPC
Class: |
Y02E 10/541 20130101;
Y02E 10/544 20130101; H01L 31/03529 20130101; H01L 31/0749
20130101; H01L 31/035281 20130101; Y02P 70/521 20151101; Y02P 70/50
20151101; H01L 31/1836 20130101; H01L 31/0735 20130101; H01L
31/1828 20130101; H01L 31/1832 20130101; Y02E 10/543 20130101; H01L
31/073 20130101 |
Class at
Publication: |
136/256 ;
136/252; 977/734; 977/742; 977/750; 977/752 |
International
Class: |
H01L 31/02 20060101
H01L031/02; H01L 31/0224 20060101 H01L031/0224; H01L 31/0248
20060101 H01L031/0248 |
Claims
1. A photovoltaic device comprising: a first semiconductor layer;
and a second layer forming a junction with said first layer, said
junction including a depletion region, wherein the second layer
comprises: a semiconductor material; and a plurality of carbon
nanostructures substantially vertically oriented relative to an
electrically conductive layer, wherein the plurality of carbon
nanostructures contact the electrically conductive layer, and the
carbon nanostructures exhibit a vanishing band gap.
2. The photovoltaic device of claim 1, wherein the plurality of
carbon nanostructures are spaced apart at regular intervals.
3. The photovoltaic device of claim 1, wherein the plurality of
carbon nanostructures are grouped into bunches.
4. The photovoltaic device of claim 1, wherein the plurality of
carbon nanostructures are multi-wall carbon nanotubes.
5. The photovoltaic device of claim 1, wherein the plurality of
carbon nanostructures are single-wall carbon nanotubes.
6. The photovoltaic device of claim 1, further comprising a
plurality of semiconductor nanostructures disposed on portions of
an outer surface of at least one of said plurality of carbon
nanostructures.
7. The photovoltaic device of claim 6, wherein the portions of the
outer surface of the plurality of carbon nanostructures between the
plurality of semiconductor nanostructures are coated with an
insulating material.
8. The photovoltaic device of claim 1, wherein the semiconductor
nanostructures provide a carbon nanostructure core.
9. The photovoltaic device of claim 1, further comprising an
insulating layer separating the semiconductor material and the
electrical contact, wherein the plurality of carbon nanostructures
extend through the insulating layer.
10. The photovoltaic device of claim 1, wherein the semiconductor
material conformally coats at least one of the plurality of carbon
nanostructures.
11. The photovoltaic device of claim 1, wherein the junction is
substantially planar.
12. The photovoltaic device of claim 1, wherein the junction is
non-planar.
13. The photovoltaic device of claim 1, wherein the plurality of
carbon nanostructures have interstices therebetween, wherein the
interstices are sized such that electron-hole pairs generated in
the interstices are located a distance apart from any carbon
nanostructure that is less than about three diffusion lengths of
photo-generated minority carriers in the semiconductor material
included in the second layer.
14. The photovoltaic device of claim 1, wherein the semiconductor
material exhibits a crystalline structure.
15. A photovoltaic device comprising: a first semiconductor layer;
a second layer disposed adjacent said first layer so as to form a
junction therewith, said junction having a depletion region; a
plurality of carbon nanostructures distributed in said second
layer, wherein the carbon nanostructures are substantially
vertically oriented relative to an electrically conductive layer;
and a plurality of semiconductor nanostructures disposed on at
least some of said carbon nanostructures.
16. The photovoltaic device of claim 15, wherein the plurality of
carbon nanostructures are spaced apart at regular intervals.
17. The photovoltaic device of claim 15, wherein the plurality of
carbon nanostructures are grouped into bunches.
18. The photovoltaic device of claim 15, wherein the portions of
the outer surface of the plurality of carbon nanostructures between
the plurality of semiconductor nanostructures are coated with an
insulating material.
19. The photovoltaic device of claim 15, wherein the semiconductor
nanostructures provide a carbon nanostructure core.
20. The photovoltaic device of claim 15, further comprising an
insulating layer separating the semiconductor material and the
electrical contact, wherein the plurality of carbon nanostructures
extend through the insulating layer.
21. The photovoltaic device of claim 15, wherein the plurality of
carbon nanostructures have interstices therebetween, wherein the
interstices are sized such that electron-hole pairs generated in
the interstices are located a distance apart from any carbon
nanostructure that is less than about three diffusion lengths of
photo-generated minority carriers in the semiconductor material
included in the second layer.
22. The photovoltaic device of claim 15, wherein the semiconductor
material exhibits a crystalline structure.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/108,500 filed Apr. 23, 2008 and titled
"Nanostructured Solar Cells", which claims the benefit of U.S.
Provisional Application No. 60/916,727, titled "Nanostructured
Solar Cells" and filed May 8, 2007; U.S. Provisional Application
No. 60/944,004, titled "Nanostructured Solar Cells" and filed Jun.
14, 2007; and U.S. Provisional Application No. 60/947,139, titled
"Nanostructured Solar Cells" and filed Jun. 29, 2007.
FIELD OF THE INVENTION
[0002] This application generally relates to photovoltaics, and
more particularly to photovoltaic devices that convert solar energy
to electric energy.
BACKGROUND
[0003] Solar energy represents an attractive source of clean,
renewable energy. For many years, photovoltaic cells have been used
in attempts to efficiently tap into this energy source.
Photovoltaic cells convert light--be it light from the sun or
otherwise--into electrical energy. For example, one kind of a
photovoltaic cell contains layers of a semiconductor material such
as silicon, which are doped to form a p-n junction. Light absorbed
by the silicon creates charge carriers which can travel across the
p-n junction, producing a current. The power generated by the cell
can be tapped and utilized like other electrical power sources.
[0004] However, current photovoltaic devices have many drawbacks.
Despite constant efforts at improvement, their efficiency at
converting light to electrical energy remains low, and their
fabrication cost is high. Further, they are often produced in
bulky, rigid arrays, limiting their versatility.
[0005] There is a need for improved photovoltaics that offer
improved performance and that can be easily installed and used in
variety of applications both terrestrial and extra-terrestrial.
SUMMARY
Photovoltaic Devices and Methods
[0006] Photovoltaic devices and methods are disclosed. In one
exemplary embodiment, a photovoltaic device can include a first
semiconductor layer and a second light-responsive layer which forms
a junction with the first layer. The junction can include a
depletion region (such as a p-n junction). The second layer can
include a mesh of carbon nanostructures (e.g., carbon nanotubes,
such as single-wall or multi-wall tubes) at least partially
embedded therein to provide a conductive path out of the second
layer to an electrical contact.
[0007] In another embodiment, an exemplary photovoltaic device can
include a first semiconductor layer and a second light-responsive
layer. The first and second layers can form a junction, e.g., a
junction with a depletion region. The second layer can include a
mesh of carbon nanostructures at least partially embedded therein
to provide an ohmic contact out of the second layer to an
electrical contact.
[0008] In yet another embodiment, an exemplary photovoltaic device
can include a first semiconductor layer and a second
light-responsive layer. The first and second layers can form a
junction, which can include a depletion region. The second layer
can include a mesh of carbon nanostructures partially coated by the
second layer and partially extending beyond the second layer as a
mesh of uncoated carbon nanostructures, as well as an electrical
contact layer in ohmic contact with the mesh of uncoated carbon
nanostructures. The distance across which the uncoated carbon
nanostructures extend can be in a range of about 100 to 10,000
nm.
[0009] Any of the foregoing embodiments can have a wide variety of
other features. For example, the first and second layers can be
made of semiconductor materials and can both be light-responsive.
Possible semiconductor materials include Group II-VI semiconductor
materials, such as CdS, CdO, CdSe, ZnS, CdTe, and so on, as well as
Group I-III-VI, Group IV and Group III-V materials. Further, the
semiconductor material of the second layer can have an index of
refraction greater than a respective index of refraction of at
least a portion of the mesh of carbon nanostructures embedded
therein. The layers can have virtually any thickness, but in one
embodiment, the first or second layers can have a thickness in a
range of about 100 nm to 10 microns, and more preferably about 300
nm to 3000 nm. In some cases, the first and second layers can form
a planar junction, although the junction can have other profiles as
well. Further, the second layer can be spaced apart from the
electrical contact (e.g., by a gap), and the carbon nanostructures
can extend therebetween and can form an ohmic contact with the
electrical contact.
[0010] The mesh can be formed of intertwined carbon nanostructures
with interstitial spaces between them. The interstices can be sized
such that electron-hole pairs generated therein are located no
farther from any carbon nanostructure than about three, or in some
cases two, diffusion lengths of photo-generated minority carriers
in the semiconductor material included in the second layer. The
mesh can be a patterned arrangement, or be un-patterned. The mesh
can also be substantially randomly oriented along the width of the
layer (e.g., the second layer). Further, a portion of the second
layer, e.g., extending from the junction to a depth within the
second layer, can be substantially devoid of carbon nanostructures.
The depth can be less than about three diffusion lengths of
photo-generated minority carriers in the semiconductor material
included in the second layer.
[0011] Further Photovoltaic Devices and Methods
[0012] A wide variety of further embodiments are also disclosed.
For example, another embodiment of an exemplary photovoltaic device
includes a first semiconductor layer and a second light-responsive
layer forming a junction, e.g., a junction with a depletion region,
with the first layer. The second layer can comprise a semiconductor
material and have a plurality of carbon nanostructures distributed
in said second layer such that each of at least about 5% of said
nanostructures are at least partially coated by the semiconductor
material of the second layer. In other embodiments, at least about
10%, 25%, 50%, or 75% of the nanostructures can be at least
partially coated by the semiconductor material.
[0013] In another embodiment, an exemplary photovoltaic device
includes a first semiconductor layer and a second light-responsive
layer forming a junction (e.g., a junction with a depletion region)
with the first layer. The second layer can include a mesh of carbon
nanostructures and a semiconductor material can be interstitially
incorporated between the mesh of carbon nano structures.
[0014] In yet another embodiment, an exemplary photovoltaic device
includes a first semiconductor layer and a second light-responsive
layer forming a junction (e.g., a junction with a depletion region)
with the first layer. The second layer can include a plurality of
carbon nanostructures where each, or in some cases a majority, of
the carbon nanostructure in the plurality of carbon nanostructures
has at least a partial coating disposed thereon, and a
light-responsive material can fill in the interstices between
individual coated carbon nanostructures of the plurality of carbon
nanostructures. In some embodiments, the coating can be made of a
semiconductor material or an insulating material. In other
embodiments, at least one of the coating and the light-responsive
material (e.g., a semiconductor material) can have an index of
refraction greater than a respective index of refraction of at
least one of the plurality of carbon nanostructures. The coating
can be crystalline as well.
[0015] In yet another embodiment, an exemplary photovoltaic device
includes a first semiconductor layer and a second light-responsive
layer forming a junction (e.g., a junction with a depletion region)
with the first layer. The second layer can include a plurality of
carbon nanostructures (which in some cases can form a mesh) and a
semiconductor material that at least partially conformally coats at
least some individual carbon nanostructures in the plurality of
carbon nanostructures. Further, the at least partially conformally
coated individual carbon nanostructures can be located throughout
the second layer. In some embodiments, the semiconductor material
can circumferentially coat a plurality of individual carbon
nanostructures (e.g., carbon nanotubes) located in the second
layer.
[0016] In yet another embodiment, an exemplary photovoltaic device
includes a light-responsive layer comprising having a plurality of
carbon nanostructures at least partially embedded therein, where
individual carbon nanostructures in the plurality of embedded
nanostructures are at least partially coated with a semiconductor
material. In some embodiments, the coating and the light-responsive
layer can form a junction with a depletion region. The
light-responsive layer can include a light-responsive material
filling in the interstices between the at least partially coated
individual carbon nanostructures. Further, the plurality of carbon
nanostructures can be coupled to an electrical contact, and the
photovoltaic device can further include an insulating layer
disposed between the light-responsive layer and the electrical
contact.
[0017] In yet another embodiment, an exemplary photovoltaic device
includes a first semiconductor layer and a second light-responsive
layer forming a junction (e.g., a junction with a depletion region)
with the first layer. The second layer can include a mesh of carbon
nanostructures at least partially embedded therein, and at least
one of said plurality of carbon nanostructures can have a vanishing
band gap. In some embodiments, the vanishing band gap can be less
than about 0.1 eV, and in other embodiments, the vanishing band gap
can be less than about 0.01 eV.
[0018] In yet another embodiment, an exemplary photovoltaic device
includes a first semiconductor layer and coupled to a first
electrical contact and a second light-responsive layer forming a
junction with the first layer, the junction including a depletion
region. The second layer can include a plurality of carbon
nanostructures at least partially embedded therein to provide a
conductive path out of the second layer to a second electrical
contact. Further, the photovoltaic device can exhibit an efficiency
for conversion of incident solar energy to electrical energy equal
to or greater than about 4 percent, or in other embodiments equal
to or greater than about 8, 10, 12, 14, 16 or 18 percent.
[0019] Any of the foregoing embodiments can have a wide variety of
other features. For example, the first and second layers can be
made of semiconductor materials and can both be light-responsive.
Possible semiconductor materials include Group II-VI semiconductor
materials, such as CdS, CdO, CdSe, ZnS, CdTe, and so on, as well as
Group I-III-VI, Group IV and Group III-V materials. Further, the
semiconductor material of the second layer can have an index of
refraction greater than a respective index of refraction of at
least a portion of the mesh of carbon nanostructures embedded
therein. The layers can have virtually any thickness, but in one
embodiment, the first or second layers can have a thickness in a
range of about 100 nm to 10 microns, and more preferably about 300
nm to 3000 nm. In some cases, the first and second layers can form
a planar junction, although the junction can have other profiles as
well. The second layer can be spaced apart from the electrical
contact (e.g., by a gap), and the carbon nanostructures can extend
therebetween and can form an ohmic contact with the electrical
contact.
[0020] Further, in any of the foregoing embodiments, the carbon
nanostructures can form a mesh, or the carbon nanostructures can
also be aligned (e.g., upstanding, substantially vertically
aligned, substantially aligned on angle, and so on). In many cases
can be carbon nanotubes (e.g., including single-wall or multi-wall
tubes). The plurality of carbon nanostructures can have interstices
therebetween, which can be sized such that electron-hole pairs
generated in the interstices (e.g., in a semiconductor material
located therein) are located no further than about three, or in
some cases about one or about two, diffusion lengths from a carbon
nanostructure, the diffusion length representing the diffusion
length of photo-generated minority carriers in the semiconductor
material included in the second layer. Further, a portion of the
second layer, e.g., extending from the junction to a depth within
the second layer, can be substantially devoid of carbon
nanostructures. The depth can be less than about three diffusion
lengths of photo-generated minority carriers in the semiconductor
material included in the second layer.
[0021] Photovoltaic Devices And Flexible Substrates
[0022] In another embodiment, an exemplary photovoltaic device can
include a plurality of photovoltaic elements disposed on a flexible
substrate. At least one of the photovoltaic elements can include a
first semiconductor layer and a second light-responsive layer
forming a junction with the first layer, the junction including a
depletion region. The second layer can include a mesh of carbon
nanostructures at least partially embedded therein to provide a
conductive path out of the second layer to an electrical contact.
Further, a transparent conducting film can be disposed over the
first layer. A flexible radiation-transparent layer can be disposed
over the plurality of photovoltaic elements. In many embodiments,
the resulting photovoltaic device is sufficiently flexible that the
substrate (and, e.g., the photovoltaic device) can be rolled around
and unrolled from a 1 inch diameter cylinder without damage.
[0023] In another embodiment, an exemplary flexible photovoltaic
film can include a flexible upper radiation transparent layer, a
flexible lower substrate layer, and a plurality of photovoltaic
devices disposed between the two layers. At least one of the
photovoltaic layers can include a first semiconductor layer and a
second light-responsive layer forming a junction with the first
layer, the junction including a depletion region. The second layer
can include a mesh of carbon nanostructures at least partially
embedded therein to provide a conductive path out of the second
layer to an electrical contact.
[0024] Any of the foregoing embodiments can have a wide variety of
other features. For example, the first and second layers can be
made of semiconductor materials and can both be light-responsive.
Possible semiconductor materials include Group II-VI semiconductor
materials, such as CdS, CdO, CdSe, ZnS, CdTe, and so on, as well as
Group I-III-VI, Group IV and Group III-V materials. Further, the
semiconductor material of the second layer can have an index of
refraction greater than a respective index of refraction of at
least a portion of the mesh of carbon nanostructures embedded
therein. The layers can have virtually any thickness, but in one
embodiment, the first or second layers can have a thickness in a
range of about 100 nm to 10 microns, and more preferably about 300
nm to 3000 nm. In some cases, the first and second layers can form
a planar junction, although the junction can have other profiles as
well. The second layer can be spaced apart from the electrical
contact (e.g., by a gap), and the carbon nanostructures can extend
therebetween and can form an ohmic contact with the electrical
contact.
[0025] Further, in any of the foregoing embodiments, the carbon
nanostructures can form a mesh, or the carbon nanostructures can
also be aligned (e.g., upstanding, substantially vertically
aligned, substantially aligned on angle, and so on). In many cases
the carbon nanostructures can be carbon nanotubes (e.g., including
single-wall or multi-wall tubes). The plurality of carbon
nanostructures can have interstices therebetween, which can be
sized such that electron-hole pairs generated in the interstices
(e.g., in a semiconductor material located therein) are located no
further than about three, or in some cases about one or about two,
diffusion lengths from a carbon nanostructure, the diffusion length
representing the diffusion length of photo-generated minority
carriers in the semiconductor material included in the second
layer. Further, a portion of the second layer, e.g., extending from
the junction to a depth within the second layer, can be
substantially devoid of carbon nanostructures. The depth can be
less than about three diffusion lengths of photo-generated minority
carriers in the semiconductor material included in the second
layer.
[0026] Photovoltaic Devices and Methods of Fabricating Them
[0027] In one embodiment, an exemplary photovoltaic device can be
fabricated by the process of activating at least a surface portion
of a mesh of carbon nanostructures. In other embodiments, the
process can include activating a surface portion of a plurality of
carbon nanostructures (e.g., a mesh, an array of aligned
nanostructures, a carpet, as mentioned in previous embodiments and
so on). The process can further include catalyzing growth of a
first semiconductor material on the activated portions so as to at
least partially coat the mesh with the first semiconductor
material, the coated mesh forming at least part of a first
light-responsive semiconductor layer; and catalyzing growth of a
second semiconductor material on the coated carbon nanostructures
so as to form at least part of a second light-responsive
semiconductor layer, the first and second layers forming a junction
with a depletion region.
[0028] In another embodiment, an exemplary photovoltaic device can
be fabricated by the process of immersing a mesh of carbon
nanostructures in a chemical bath so as to catalyze growth of a
semiconductor coating on the plurality of carbon nanostructures,
the coated carbon nanostructures forming at least part of a first
light-responsive semiconductor layer; and immersing the coated mesh
of carbon nanostructures in a second chemical bath so as to
catalyze growth of a second semiconductor material on the mesh, the
second semiconductor material forming at least part of a second
light-responsive semiconductor layer. The first and second layers
can form a junction with a depletion region.
[0029] In yet another embodiments, an exemplary photovoltaic device
can be fabricated by the process of chemically functionalizing at
least a surface portion of a mesh of carbon nanostructures disposed
in a liquid; catalyzing growth of a first semiconductor material on
the functionalized surface portions so as to at least partially
coat the mesh of carbon nanostructures with the first semiconductor
material, the coated mesh forming at least part of a first
light-responsive semiconductor layer; and catalyzing growth of a
second semiconductor material on the coated mesh so as to form at
least part of a second light-responsive semiconductor layer. The
first and second layers can form a junction with a depletion
region.
[0030] Any of the foregoing embodiments can have a wide variety of
other features. For example, the process can further include
catalyzing growth of the first semiconductor material so as to form
a substantially planar surface for forming the junction. The
process also can include coupling the plurality of carbon
nanostructures to an electrical contact, coupling the second layer
to another electrical contact, and/or coupling at least one of the
first and second layers to a flexible substrate. The coating of the
mesh can include coating individual nanostructures within the mesh,
and/or incorporating the second semiconductor material in the
interstices between individual carbon nanostructures in the
mesh.
[0031] Further, the first and second layers can be made of
semiconductor materials and can both be light-responsive. Possible
semiconductor materials include Group II-VI semiconductor
materials, such as CdS, CdO, CdSe, ZnS, CdTe, and so on, as well as
Group I-III-VI, Group IV and Group III-V materials. Further, the
semiconductor material of the second layer can have an index of
refraction greater than a respective index of refraction of at
least a portion of the mesh of carbon nanostructures embedded
therein. The layers can have virtually any thickness, but in one
embodiment, the first or second layers can have a thickness in a
range of about 100 nm to 10 microns, and more preferably about 300
nm to 3000 nm. In some cases, the first and second layers can form
a planar junction, although the junction can have other profiles as
well. The second layer can be spaced apart from the electrical
contact (e.g., by a gap), and the carbon nanostructures can extend
therebetween and can form an ohmic contact with the electrical
contact.
[0032] Photovoltaic Devices and Semiconductor Nanostructures
[0033] In one embodiment, an exemplary photovoltaic device includes
a first semiconductor layer and a second light-responsive layer
disposed adjacent the first layer so as to form a junction
therewith, the junction having a depletion region. The photovoltaic
device further can include a plurality of carbon nanostructures
distributed in the second layer and a plurality of semiconductor
nanostructures disposed on at least some of the carbon
nanostructures. The semiconductor nanostructures can exhibit a
bandgap less than a bandgap of the second semiconductor layer.
[0034] In some embodiments, the difference between the band gap of
the semiconductor nanostructures and that of the second layer can
be in a range of about 0.1 eV to about 1 eV. Further, the first
layer can exhibit a band gap greater that a band gap of the second
layer.
[0035] The carbon nanostructures can be carbon nanotubes, such as
single-wall carbon nanotubes or multi-wall carbon nanotubes. The
carbon nanostructures can exhibit a vanishing band gap.
[0036] The first and second layers can be light-responsive and can
include semiconductor materials. For example, they can each be
formed of a Group IV, III-V, Group I-III-VI, or Group II-IV (e.g.,
CdSe) semiconductor material. The semiconductor nanostructures can
be formed of a Group II-VI semiconductor material, e.g., CdTe.
[0037] In some embodiments, the semiconductor nanostructures and
the second layer exhibit similar conductivity types. For example,
both of the semiconductor nanostructures and the second layer can
include an n-type dopant (e.g., to form an n-type CdSe), and the
first layer can include a p-type dopant (e.g., to form a p-type
CdTe). As another example, at least some of the semiconductor
nanostructures include an n+-type material and the second layer
includes an n-type material.
[0038] The photovoltaic device can further include a transparent
electrically conductive layer disposed on at least a portion of the
first layer. The transparent conductive layer can form an ohmic
contact with the first layer. The photovoltaic device can further
include another electrical contact layer disposed on at least a
portion of the second layer so as to form ohmic contact with at
least some of the carbon nanostructures and the second layer.
[0039] In another embodiment, an exemplary photovoltaic device can
include a first semiconductor layer and a second light-responsive
layer disposed adjacent said first layer to form a junction
therewith, the junction including a depletion region. The
photovoltaic device can also have a plurality of carbon
nanostructures (e.g., carbon nanotubes) distributed in at least one
of the layers, and a plurality of compound nanostructures disposed
on at least some of the carbon nanostructures. The compound
nanostructures can include a carbon bucky ball and a semiconductor
shell at least partially coating the bucky ball.
[0040] The first and second layers can be made of semiconductor
materials and can both be light-responsive. The carbon bucky balls
in the compound nanostructure can be formed of any of C.sub.60, a
C.sub.70, C.sub.84, C.sub.96, C.sub.108, and C.sub.120 molecule.
Further, the shell of the compound nanostructure can have a
thickness in a range of about 1 nm to about 100 nm. The shell can
be formed of a material having a lower bandgap than the band gap of
the semiconductor layer in which the carbon nanostructures are
distributed.
[0041] In yet another embodiment, an exemplary photovoltaic device
can include a first semiconductor layer and a second
light-responsive layer disposed adjacent the first layer to form a
junction therewith, the junction having a depletion region. The
photovoltaic device can further include a porous insulating layer
disposed adjacent the second layer and an electrically conductive
layer disposed adjacent the insulator layer, with the insulator
layer providing electrical insulation between the conductive layer
and the second layer. The photovoltaic device can further include a
mesh of carbon nanostructures distributed in the second layer such
that at least some of the carbon nanostructures extend through the
pores of the porous insulator layer to form an ohmic contact with
the electrically conductive layer. The photovoltaic device can also
have a plurality of composite nanostructures each having a core
comprising a carbon nanostructure and a shell comprising a
semiconductor distributed over the mesh of carbon
nanostructures.
[0042] The first and second layers can be made of semiconductor
materials and can both be light-responsive. The carbon
nanostructures of the mesh can be carbon nanotubes (e.g.,
single-wall nanotubes or multi-wall nanotubes), and the carbon
nanostructures of the composite nanostructures can be carbon bucky
balls. Further, the semiconductor shells of the composite
nanostructures can exhibit a conductivity type similar to that of
the second layer.
[0043] In yet another embodiment, an exemplary photovoltaic device
can include a light-responsive layer and a plurality of compound
nanostructures distributed in the layer. Each of the compound
nanostructures can include a carbon nanostructure, a plurality of
semiconductor nanostructures disposed on an outer surface of the
carbon nanostructure (e.g., so as to form junctions with the
light-responsive layer, the junction having depletion regions), and
an insulating material coating portions of the outer surface of the
carbon nanostructure located between the semiconductor
nanostructures.
[0044] The light-responsive layer can include a semiconductor
material. In some embodiments, the semiconductor material in the
light-responsive layer and that included in the semiconductor
nanostructures can have different conductivity types.
[0045] The photovoltaic devices can further include an electrical
contact layer adapted to form an ohmic contact with the carbon
nanostructures. Also, the photovoltaic devices can further include
an insulator coating providing electrical insulation between the
light-responsive layer and the electrical contact layer. The
insulator coating can have a plurality of pores distributed therein
through which said compound nanostructures form an ohmic contact
with the electrical contact layer.
[0046] The carbon nanostructures can include carbon nanotubes,
including any of single wall or multiwall carbon nanotubes. The
semiconductor nanostructures can include a core formed of a carbon
nanostructure, and a shell formed of a semiconductor material. The
shell can have any of a wide range of thicknesses. However, in one
embodiment, for example, the shell can have thickness in a range of
about 1 nm to about 100 nm, in a range of about 1 nm to about 50
nm, in a range of about 1 nm to about 30 nm, in range of about 1 nm
to about 20 nm, or in a range of about 1 nm to about 10 nm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] The photovoltaics discussed herein will be more fully
understood from the following detailed description taken in
conjunction with the accompanying drawings, in which:
[0048] FIG. 1 is a schematic view of an exemplary photovoltaic
device which includes a window layer and an absorption layer with a
mesh of carbon nanostructures embedded therein, the window layer
and the absorption layer forming a junction;
[0049] FIG. 2 is a detail view of the mesh of carbon nanostructures
shown in FIG. 1;
[0050] FIG. 3 is a schematic view of another exemplary photovoltaic
device which includes a window layer and an absorption layer with a
mesh of carbon nanostructures embedded therein, the window layer
and the absorption layer forming a distributed junction;
[0051] FIG. 4 is a schematic view of another exemplary photovoltaic
device which includes a window layer and an absorption layer with a
mesh of carbon nanostructures embedded therein, the mesh extending
across a gap to an electrical contact;
[0052] FIG. 5 is a schematic view of an exemplary photovoltaic
device which includes a window layer and a plurality of
substantially vertically oriented carbon nanostructures located in
an absorption layer, the window layer and the absorption layer
forming a junction;
[0053] FIG. 6 is a schematic view of another exemplary photovoltaic
device which includes a window layer and a plurality of
substantially vertically oriented carbon nanostructures located in
an absorption layer, the window layer and the absorption layer
forming a non-planar junction;
[0054] FIG. 7 is a schematic view of an exemplary photovoltaic
device which includes a plurality of coated carbon nanostructures
embedded in an absorption layer;
[0055] FIG. 8A is a schematic view of an exemplary photovoltaic
device which includes a window layer and an absorption layer with a
mesh of carbon nanostructures and a plurality of composite
nanostructures embedded therein;
[0056] FIG. 8B is a schematic view of an exemplary composite
nanostructure having a core made of a carbon nanostructure and a
shell made of a semiconductor material;
[0057] FIG. 9 is a schematic view of an exemplary photovoltaic
device which includes a window layer and a plurality of
substantially vertically oriented carbon nanostructures embedded in
an absorption layer, the carbon nanostructures having semiconductor
nanostructures disposed thereon;
[0058] FIG. 10 is a schematic view of an exemplary photovoltaic
device which includes an absorption layer with a plurality of
substantially vertically oriented carbon nanostructures embedded
therein, the carbon nanostructures having semiconductor
nanostructures disposed thereon and an insulating layer covering
portions of the carbon nanostructures;
[0059] FIG. 11 is a scanning electron microscope image of a mesh of
carbon nanotubes (on buckypaper) coated with CdSe which was
fabricated using a chemical bath deposition process;
[0060] FIG. 12 is a schematic view of a tandem solar cell that
incorporates the photovoltaic devices described herein;
[0061] FIG. 13A is a schematic view of an exemplary solar cell
module that incorporates the photovoltaic devices described herein;
and
[0062] FIG. 13B is a schematic view of an exemplary flexible solar
cell film that incorporates the photovoltaic devices described
herein.
DETAILED DESCRIPTION
[0063] Certain exemplary embodiments will now be described to
provide an overall understanding of the principles of the
structure, function, manufacture, and use of the devices and
methods disclosed herein. One or more examples of these embodiments
are illustrated in the accompanying drawings. The devices and
methods specifically described herein and illustrated in the
accompanying drawings are non-limiting exemplary embodiments, as
the scope of the present application is defined solely by the
claims. Throughout this application, the term "e.g." will be used
as an abbreviation of the non-limiting phrase "for example."
[0064] A variety of embodiments will be presented herein. It should
be understood that the features illustrated or described in
connection with one exemplary embodiment may be combined with the
features of other embodiments. Such modifications and variations
are intended to be included within the scope of the present
disclosure.
[0065] Generally, the devices and methods disclosed herein provide
improved photovoltaic cells for converting light, including in
particular sunlight, to electrical energy. It should be understood
that the terms "light" and "radiation" are used interchangeably
herein to refer to both visible and invisible radiation. These
devices and methods have a wide range of applications, including in
both terrestrial and extra-terrestrial settings, and can be
incorporated into panels, arrays, flexible films, sheets, or other
products.
[0066] FIG. 1 schematically illustrates one embodiment of a
photovoltaic device 100. In this case, the photovoltaic device 100
includes a substrate 101, a back electrical contact 102, an
absorption layer 104, a window layer 106, a top electrical contact
108, and an anti-reflective coating 110.
[0067] Typically, the absorption layer 104 is a light-responsive
layer--that is, it is capable of generating electron-hole pairs in
response to light of suitable wavelengths incident thereon. The
bulk of the absorption layer 104 can be formed from a semiconductor
material 112. The bulk of the window layer 106 can also be made of
a semiconductor material 114, and can also be light-responsive. In
many embodiments, the semiconductor materials 112 and 114 are of
differing conductivity types. As is known in the art, the
conductivity type of a material refers to the type of charge
carrier (e.g., electron or hole) that is predominantly responsible
for electrical conduction in the material. The junction between two
materials formed of different conductivity types can be
characterized by a depletion region that supports an electric
field, while the junction between two materials formed of similar
conductivity types does not produce a depletion region. By way of
example, the absorption layer 104 can be made of n-type CdSe while
the window layer 106 can be made of p-type CdS, thus forming a p-n
junction 130. As is well known in the art, a p-n junction can have
certain characteristics, including the aforementioned depletion
region, an electric field and built-in voltage, all of which follow
from the chosen semiconductor materials and doping profiles, among
other things. As one skilled in the art will understand, a variety
of semiconductors (e.g., Group II-VI, Group IV, or Group III-V) and
dopants can be used. In some embodiments, the junction 130 can be a
p-i-n junction that is formed by the junction of a p-type material,
an intrinsic layer (e.g., an undoped or lightly doped semiconductor
material), and an n-type material.
[0068] In FIG. 1, the absorption layer 104 has a plurality of
nanostructures embedded therein. In this embodiment, the
nanostructures are carbon nanotubes 122, although a wide variety of
carbonaceous or other nanostructures can be used. In many
embodiments, the term nanostructure is used herein to refer to a
material structure having a size in at least one dimension (e.g, a
diameter of a tube) that is less than about 1 micron, in other
embodiments less than about 500 nm, about 100 nm, about 20 nm or
about 1 nm. As shown, the plurality of carbon nanotubes 122 form a
mesh 124, which can be made up of many intertwined and/or
interconnected carbon nanotubes 122. For descriptive purposes, the
mesh 124 can be said to have an aspect ratio defined by a height
126 taken in a direction substantially normal to the substrate
surface 101' (a vertical direction 162 in FIG. 1) and a width taken
in a direction substantially parallel to the substrate surface 101'
(a horizontal direction 160 in FIG. 1). The carbon nanotubes 122
can be randomly distributed across their width 128. In some cases,
the carbon nanotubes 122 in the mesh can be substantially
horizontally oriented (as shown in FIG. 1) such that the nanotubes
predominantly extend farther in horizontal direction 160 than in
vertical direction 162, although this is not necessary. The
individual nanotubes 122 in the mesh 124 can be in contact (e.g.,
ohmic contact) with other nanotubes 122 along a portion of their
surfaces, creating a network of interconnected nanotubes 122. Ohmic
contact is known in the art; however, to the extent that any
additional explanation is necessary, ohmic contact refers to
contact that allows exchange of charge carriers therebetween (e.g.,
a flow of electrons) and is characterized by a substantially
voltage independent resistance. In many embodiments, the mesh 124
can provide a network of conductive pathways within the absorption
layer 104.
[0069] Although the mesh 124 is illustrated as an arrangement of
carbon nanotubes 122 without a discernible pattern, in other
embodiments an organized mesh, e.g., a patterned mesh, can be used.
In addition to whatever other characteristics (e.g., electrical
conductivity) it may provide, such a mesh can be used as a physical
template or screen for the fabrication of layers, semiconductor
nanostructures, or other features in the device 100, as will be
discussed in more detail below.
[0070] The density of the mesh 124 can vary widely. However, in
some embodiments, the density of the carbon nanotubes 122 of the
mesh 124 can be adjusted such that the interstitial spacing between
neighboring nanotubes produces desirable properties. For example,
as shown in FIG. 2, the spacing 200 between adjacent carbon
nanotubes 122 in the mesh 124 can be less than about three
diffusion lengths exhibited by photo-generated minority charge
carriers (e.g., upon photo-excitation) in the semiconductor
material 112 in which the mesh 124 is embedded, and more preferably
less than about two diffusion lengths. It should be understood that
while FIG. 2 is a two-dimensional illustration, in a
three-dimensional device the interstitial spacing 200 can represent
the distance across three-dimensional voids separating carbon
nanotubes 122 in the mesh 124.
[0071] As shown in FIG. 1, the carbon nanotubes 122 in the mesh 124
can be coated with the semiconductor material 112. In many cases,
the semiconductor material 112 conformally coats individual
nanotubes 122 in the mesh 124. A conformal coating can take the
shape of the contours and/or underlying three-dimensional profile
of the carbon nanotubes 122, and in some cases can cover surfaces
of the carbon nanotubes 122 in many or substantially all
directions, e.g., in the nature of a circumferential coating. In
some embodiments, the coating can at least partially cover at least
about 5 percent of the nanotubes 122 (or other nanostructures) in
the material; in other embodiments, between 5 and 100 percent of
individual nanotubes are at least partially coated. More narrow
ranges are possible. For example, the coating can at least
partially cover 5 and 10 percent of carbon nanotubes, between 5 and
20 percent, between 5 and 30 percent, about 40 percent, about 50
percent, about 75 percent, and so on.
[0072] The semiconductor material 112 can also fill in the spaces
between carbon nanotubes 122, such as the spaces 200 and/or
three-dimensional voids discussed above in connection with FIG. 2.
In FIG. 1, the semiconductor material 112 coating on the carbon
nanotubes 122 has been built up such that it covers the mesh 124.
In a three dimensional device, the junction 130 can be
substantially planar. However, in other embodiments, the
semiconductor material 112 can coat the nanotubes 122 without being
built up, and a window layer can be disposed on top of the coated
nanotubes 122 to create a non-planar junction, as will be discussed
in more detail below in connection with FIG. 3.
[0073] The semiconductor material 112 coating the carbon nanotubes
122 can be crystalline, including both single-crystal and/or
polycrystalline coatings, and e.g., hexagonal phase crystalline
CdSe. In some cases, a region of crystalline material (e.g., single
crystal) can surround the carbon nanotubes and/or each individual
carbon nanotube. In some cases, other regions (e.g., regions beyond
this surrounding crystalline region) can be polycrystalline or
amorphous. However, in other embodiments, a substantial portion of
the bulk of the absorption layer 104 can be crystalline (e.g., 80%
or more). Crystalline regions can have advantageous electrical
properties, e.g., they can promote high-efficiency current
generation and collection in the absorption layer 104. In many
embodiments, the foregoing semiconductor materials and coatings can
be fabricated using CBD procedures, which will be described in more
detail below.
[0074] The carbon nanotubes 122 can be coupled to the back
electrical contact 102, e.g., electrostatically or via an adhesive
material, such that the carbon nanotubes 102 form a contact (e.g.,
an ohmic contact) to the back electrical contact 102. The back
electrical contact 102 (and/or substrate 101) can have a roughened
or textured top surface so as to improve the anchoring of the
carbon nanotubes 122 thereto. By way of example, the top surface
can include micron or sub-micron sized undulations or can be a
porous surface, e.g., with micron or sub-micron sized pores. Such a
textured or roughened surface can be created using a variety of
techniques, as will be described in more detail below.
[0075] In many cases, the back electrical contact 102 is formed
from an electrically conductive material. It can be rigid or
flexible, transparent or opaque. For example, the electrical
contact 102 can be a film of electrically conducting material
(e.g., a metal such as aluminum or copper) disposed atop a
substrate 101, which itself can be rigid (e.g., a glass substrate)
or flexible (e.g., plastic). The back electrical contact 102 can
also take the form of trace connections atop (e.g., patterned and
deposited metals) or through the substrate 101 (e.g., drilled
through the substrate 101). Further, in some embodiments, the back
electrical contact 102 and the substrate 101 can be combined, as
they need not be separate components or materials. The use of
flexible electrical contacts 102 and/or substrates 101 can be
advantageous for producing flexible photovoltaic films. The
flexibility of such films, and of such electrical contacts 102
and/or substrates 101, can be such that the film can be rolled and
unrolled, e.g., for transport, storage, and installation. In some
embodiments, the flexibility of the back electrical contact 102
and/or substrate 101 can be such that will allow them to be rolled
around and unrolled from a 1 inch diameter cylinder repeatedly
without damage.
[0076] Returning to FIG. 1, a top electrical contact 108 can be
disposed over the window layer 106. The top electrical contact 108
can be a layer formed from a transparent conductive polymer (TCP),
metal oxide, or polyimide, or can have any of a wide variety of
other configurations, including an arrangement of fine metal lines
as is known in the art. In many embodiments, the top contact can be
radiation-transparent (for example, transparent to solar radiation,
which can include radiation having a wavelength in a range of about
200 nm to 2.5 microns). The top electrical contact 108 also can be
configured as previously described with respect to the back
electrical contact 102. An anti-reflective coating 110, such as a
silicon oxynitride thin film, can be disposed over the window layer
106 and the top electrical contact 108. The photovoltaic device 100
can be coated with or encapsulated in a protective material (e.g.,
to provide physical and/or environmental protection), such as a
transparent polymer or PTFE. The anti-reflective coating 110 and
protective material are typically radiation-transparent.
[0077] In use, and without being limited by theory, the device 100
can be exposed to solar radiation that passes through the top
electrical contact 108 without any substantial absorption to reach
the window layer 106. Some of the photons passing through the
window layer 106 can be absorbed by the semiconductor material 114
of that layer to generate electron-hole pairs. Other incident
photons pass through that layer 106 to be absorbed by the
semiconductor material 112 of the absorption layer 104 so as to
generate electron-hole pairs therein. The generation of
electron-hole pairs occurs, e.g., by promoting an electron in the
valence band of the material to its conduction band. As previously
mentioned, and for explanatory purposes only, the absorption layer
104 can be made of n-type semiconductor (e.g., CdSe), while the
window layer can be made of p-type semiconductor (e.g., doped CdS).
In such an embodiment, the electric field in the junction 130
causes the separation of such electron-hole pairs in the vicinity
thereof. Electrons can travel across the junction 130 to the n-type
semiconductor and the holes can travel across the junction 130 to
the p-type semiconductor. Additionally, electron-hole pairs are
photo-generated outside the vicinity of the depletion region in
both the absorption layer 104 and window layer 106. Such electrons
and holes can move (e.g., diffuse, as dictated by factors such as
carrier concentration and thermal effects in the semiconductor)
within the absorption layer 104 and window layer 106, as the case
may be. This movement can be quantitatively described by their
diffusion lengths (e.g., average distance traveled by a charge
carrier before recombination).
[0078] Whether generated inside or outside the vicinity of the
junction 130, photo-generated electrons can migrate (e.g., via
diffusion) through the n-type absorption layer 104 to the carbon
nanotubes 122 of the mesh 124 (e.g., before recombining). The
carbon nanotubes 122 can provide a conductive pathway out of the
absorption layer 104 to the back electrical contact 102, which can
reduce the chances that an electron will recombine with a hole
before it can exit the absorption layer 104. (In this embodiment,
electrons can also pass directly from the absorption layer 104 to
the back electrical contact 102.) The removal of electrons can also
reduce the recombination rate for holes in the absorption layer 104
by reducing the number of electrons with which they can recombine,
which enhances the probability of reaching the depletion region of
the junction 130 and the p-type window layer 106. Photo-generated
holes can migrate through the p-type window layer 106 to the top
electrical contact 108. The electrons in the back electrical
contact 102 can travel through an external load 150 to the p-type
window layer 106 to recombine with the holes that have migrated
through the window layer 106. This flow of electrons represents a
current which, in conjunction with the built-in potential of the
junction 130, represents electrical power.
[0079] As one skilled in the art will understand, in other
embodiments the absorption layer 104 can be made of p-type
material, while the window layer can be made of n-type material,
and the operation of such a device will change accordingly. In such
an embodiment, photo-generated electrons in the window layer 106
travel to the top electrical contact 108, through an external load
150, and through the back electrical contact 102 and the carbon
nanotubes 122 to recombine with holes in the p-type absorption
layer 104.
[0080] Without being limited by theory, in many cases, the mesh 124
of carbon nanotubes 122 (or other carbon nanostructures) in the
photovoltaic device 100 can provide a conductive pathway and
thereby reduce recombination of generated electron-hole pairs in
the absorption layer 104, for example, by allowing free electrons a
low-resistance path out of the absorption layer 104 to the back
electrical contact 104. Further, as noted above, in many
embodiments, the density of the carbon nanotubes 122 forming the
mesh 124 is sufficient to ensure that a charge carrier (e.g., an
electron) generated in the interstitial space between the carbon
nanotubes 122 is likely to reach a carbon nanotube 122 before
recombining (e.g., with a hole) or undergoing other absorption
and/or scattering event that would prevent them from flowing out of
the absorption layer 104 to the external load 150. Use of carbon
nanotubes 122 can enable a relatively thick absorption layer (which
can be advantageous, e.g., for absorbing a greater proportion of
incident radiation) to have the same or a lower electron-hole
recombination rate as a relatively thin absorption layer (which can
be advantageous to enhance efficiency of the device). In short, the
device 100 can have the advantages of a thick absorbing layer with
the lower recombination rate of a thin absorbing layer.
[0081] A wide variety of materials can be used in the photovoltaic
device 100. As previously mentioned, the absorption layer 104 and
window layer 106 can be formed from semiconductor materials 112,
114, including both single-element or compound semiconductors.
Typically, the absorption layer 104 and window layer 106 can have
differing conductivity types so that the junction therebetween can
form a depletion region. Solely by way of non-limiting example,
some potential Group II-VI semiconductors which can be used for the
layers 104, 106 include CdSe, CdS, CdO, ZnS, CdTe, ZnO, ZnSe, and
ZnTe. Group IV and III-IV semiconductors, or other semiconducting
materials, also can be used. In addition, in some embodiments,
alternative materials can be substituted for the Group II material
in a nominally Group II-VI compound semiconductor, e.g., a Group
I-III-VI semiconductor. For example, Copper Indium di-Selenide
(CuInSe.sub.2) can be used, with the Copper Indium compound
substituting for Cadmium (Group II) in its pairing with Selenium
(Group VI) to create a Group I-III-VI semiconductor. The absorption
and windows layers 104, 106 can include different semiconductor
materials (e.g., n-type CdSe for the semiconductor material 112 in
the absorption layer 104 and p-type CdS for the semiconductor
material 114 in the window layer 106, or vice versa). In other
embodiments, the absorption and windows layers 104, 106 can include
the same semiconductor material, doped to be of differing
conductivities (e.g., n-type CdSe for the semiconductor material
112 in the absorption layer 104 and p-type CdSe for the
semiconductor material 114 in the window layer 106, or vice versa).
As one skilled in the art will understand, the chosen semiconductor
materials and dopants can vary. Typically, some Group II-VI
materials are intrinsically n-type (that is, without doping), such
as CdSe and CdS. Others typically are intrinsically p-type (that
is, without doping), such as CdTe and ZnTe. Such materials need not
be doped; however, in many embodiments dopants such as Cu can be
used as a p-type dopant (e.g., to change CdSe to a p-type material
via appropriate dosages).
[0082] The thickness of the absorption layer 104 and window layer
106 can vary widely and be designed to achieve suitable absorption
of incident radiation. However, in one embodiment, the absorption
layer 104 can have a thickness in a range of about 100 nm to 10
microns, and more preferably each can have a thickness in a range
of about 300 nm to 3000 nm. The window layer 106 can have a
thickness in a range of about 10 nm to 10,000 nm, and preferably 50
to 2000 nm, and more preferably about 50 nm to 500 nm. Referring to
FIG. 1, the mesh 124 of carbon nanotubes 122 embedded in the
absorption layer 104 can extend virtually to any height 126 within
the layer. A region 132 of the absorption layer, preferably
adjacent the junction 130, can be substantially devoid of
nanostructures. Although the region 132 devoid of nanostructures
can have virtually any thickness, it is preferably thicker than the
depletion region of the junction 130 within the absorption layer
104. Further, while it is not necessary, the region 132 preferably
can have a thickness that is less than about three, or in some
cases about two, diffusion lengths of photo-generated minority
carriers in the semiconductor material included in the second
layer. For example, in one embodiment the mesh 124 of carbon
nanostructures 122 can extend to a height 162 of about 500 nm
within the absorption layer 104, and a region 132 devoid of
nanostructures can be about 500 nm thick, which can result in an
overall absorption layer 104 thickness of about 1000 nm. However,
all such dimensions are merely by way of illustration and can vary
widely.
[0083] It should be understood that while FIG. 1 is shown with an
absorption layer 104 and window 106, multiple layers can be
included. For example, additional absorption layers and/or window
layers can be disposed above the window layer 106 (or in other
embodiments, below the absorption layer 104) to form a
multiple-junction device. Such additional layers can include
nanostructures such as carbon nanotubes 122 embedded therein, but
need not do so.
[0084] As previously mentioned, the nanostructures shown in FIG. 1
can be a mesh 124 of carbon nanotubes 122, which can be multi-wall
carbon nanotubes or single-wall carbon nanotubes. A wide variety of
other nanostructures (both carbon and non-carbon) also can be used,
including cylindrical, spherical, elongate, ovoid, oblate, and
other shapes, as well as carbon nanostructures formed from C.sub.60
molecules, C.sub.72 molecules, C.sub.84 molecules, C.sub.96
molecules, O.sub.108 molecules, or C.sub.120 molecules. In many
embodiments, carbon nanostructures are formed primarily of carbon
atoms (e.g., carbon can constitute 90% or more of a nanostructure's
composition). However, they can include other constituents, for
example, a plurality of catalyzing iron atoms from carbon
nanostructure fabrication.
[0085] The fabrication of carbon nanostructures, including single
and multi-wall carbon nanotubes, is known in the art. By way of
example, carbon nanotubes can be fabricated using a variety of
techniques, including chemical vapor deposition, laser-ablation,
and are discharge. Methods of fabricating carbon nanotube are
disclosed in more detail in U.S. Pat. Nos. 7,125,534 (Smalley et
al., "Catalytic growth of single- and double-wall carbon nanotubes
from metal particles"), 7,150,864 (Smalley et al., "Ropes comprised
of single-walled and double-walled carbon nanotubes") and
7,354,563, (Smalley et al., "Method for purification of as-produced
fullerene nanotubes"), which are hereby incorporated by reference
in their entirety. Further, suitable carbon nanostructures can be
obtained from commercial suppliers, such as Nanocyl of Sambreville,
Belgium (US office in Rockland, Mass., USA), Bayer Materials
Science AG of Leverkusen, Germany, and Showa Denko K. K. of
Japan.
[0086] In many cases, the nanostructures are more conductive than
the surrounding material in the absorption layer 104. The
nanostructures can advantageously have a band gap less than that of
the material forming the bulk absorption layer 104 and preferably a
vanishing band gap. For example, the nanostructures can have a band
gap of 0.1 eV or lower, and preferably 0.01 eV or lower. The band
gaps of some single-wall carbon nanotubes have been measured to be
about 0.6 eV, and the band gaps (Eg) of some multi-wall carbon
nanotubes can be calculated using Eg=0.6/d where d is the outer
diameter of the multi-wall carbon nanotube in nanometers, as is
known in the art.
[0087] In some embodiments, it can be advantageous for the carbon
nanotubes 122 or other nanostructures to be formed of a material
having an index of refraction less than that of the material
forming the bulk of the absorption layer 104. Light traveling
through the absorption layer 104 can be internally reflected within
the semiconductor material 112 at the interfaces between the carbon
nanotubes 122 and the semiconductor material 112, increasing the
opportunity for photons to be absorbed by the layer 104 and to
generate electron-hole pairs. For example, the index of refraction
for some carbon nanotubes has been measured to be about 2, while
the index of refraction of CdSe has been measured to be about
2.6.
[0088] In one embodiment of a method for fabricating the
above-described photovoltaic device 100, carbon nanotubes can be
deposited onto an electrical contact 102 and/or substrate 101. As
previously mentioned, the back electrical contact 102 and/or
substrate 101 can be roughened or textured, which can be
accomplished, for example, via mechanical abrasion or chemical
etching. The carbon nanotubes 102 can be deposited via any of a
variety of suitable techniques, such as a Langmuir-Blodgett
process, spin coating, inkjet printing, or spraying. The density of
the carbon nanotubes 122 can be controlled by adjusting the amount
deposited in an area. In other embodiments, buckypaper or other
commercially available nanotube sheets or films (e.g., pre-formed)
can be used. Such sheets are available, for example, from Nanocomp
Technologies, of Concord, N.H., USA. To create an absorption layer
104, the semiconductor material 112, such as CdSe, can be grown on
the carbon nanotubes 122 using a chemical bath deposition (CBD)
technique. Typically, a CBD reaction involves preparing aqueous or
non-aqueous solutions containing appropriate precursor compounds
(for example, Cadmium precursor solution and Selenium precursor
solution) and appropriate ligands. Aliquots of these solutions can
be combined in a CBD container, and the object (e.g., carbon
nanostructure, wafer, or otherwise) onto which the film will be
deposited can be immersed in the resulting chemical bath. The
object remains immersed for the time required to form a film of the
desired thickness. After removal, the objects are rinsed to remove
excess reactants and dried for use. It should be understood that
the foregoing is a general description and by way of illustration
only. CBD processes are described in more detail in U.S. Pat. No.
7,253,014 (Barron et al., "Fabrication Of Light Emitting Film
Coated Fullerenes And Their Application For In-Vivo Light
Emission"), and in U.S. Patent Publication No. 2005/0089684 (Barron
et al., "Coated Fullerenes, Composites And Dielectrics Made
Therefrom"), both of which are hereby incorporated by reference in
their entirety.
[0089] The thickness of the semiconductor coating on the carbon
nanotubes 122 can be grown so as to fill in the interstices between
carbon nanotubes 122 in the mesh 124, and can be built up above the
mesh 124, e.g., to cover the carbon nanotubes 122 and to form a
uniform surface for a planar junction 130, as shown in FIG. 1.
[0090] The CBD process can provide a crystalline semiconductor
coating (including a CdSe crystalline coating having a hexagonal
phase) on the carbon nanotubes 122. For example, the crystalline
coating can be a single crystal and/or have crystalline regions
formed therein. In some cases, the carbon nanotubes 122 can promote
the growth of crystalline regions in the semiconductor material
112, for example by nucleating growth of the semiconductor material
112 on the carbon nanotube surfaces. As previously mentioned, the
crystalline regions can have advantageous electrical properties,
e.g., they can promote high-efficient current generation and
collection in the absorption layer 104. In some embodiments, the
deposited semiconductor material 112 can be annealed to facilitate
the production of a coating of crystalline material. By way of
example, such annealing can be performed at an elevated temperature
(e.g., in a range of about 300-1000 degrees Celsius) and for a
suitable duration (e.g., a few second to a few hours).
[0091] In some embodiments, the window layer 106 can be deposited
on the absorption layer 104 via CBD (e.g., via another bath
deposition again using CBD techniques). The top electrical contact
108 can be fused into place with another CBD bath or deposited
directly with CBD.
[0092] The foregoing is by way of example only, and a range of
variations are possible and are intended to be within the scope of
this disclosure. For example, the carbon nanotubes 122 can be
coated with the semiconductor material 112 (to act, for example, as
a seed layer) before being deposited onto the back electrical
contact 102/substrate 101. In such an embodiment, another CBD bath
can be performed after the carbon nanotubes 122 are deposited to
increase the thickness of the coating of the semiconductor material
112. Further, in other embodiments, other processes for depositing
the semiconductor materials and/or other layers of the photovoltaic
device can be used, including chemical vapor deposition, molecular
beam epitaxy, atomic layer deposition, and electrochemical
deposition.
[0093] A variety of other embodiments of photovoltaic devices and
methods are presented below; however it should be understood that
any of them can employ any of the features already described in
connection with FIGS. 1-2 (including, for example, the materials,
fabrication processes, dimensions, and so on), as they are intended
to build on the foregoing discussion.
[0094] FIG. 3 illustrates another embodiment of a photovoltaic
device 300. In this embodiment, the photovoltaic device 300 has an
absorption layer 304 that includes a light-responsive material
(here, semiconductor material 312) conformally coating a mesh 324
of carbon nanotubes 322. A window layer 306, which can be made of
semiconductor material 314 and as previously mentioned can also be
light-responsive, is disposed on the absorption layer 304 to form a
junctions 330 at the interfaces of the two layers. Although here
the window layer 306 has been built up to a uniform surface, it
need not be so (for example, alternatively the front electrical
contact 308 can be a transparent conductor (e.g., a doped metal
oxide) deposited on the window layer 306 and built up to a uniform
level). The mesh 324 shown in FIG. 3 extends out of the absorption
layer 304 and through an insulating layer 336 to an electrical
contact 302, which is disposed on a substrate 301. The insulating
layer 336, which can be made for example of silicon dioxide, can be
provided between the back electrical contact 302 and the absorption
layer 304 to prevent a short circuit path around the junction 330.
In addition, a top electrical contact 308 and an anti-reflective
coating 310 can be disposed on top of the window layer 306. These
and other aspects of the photovoltaic device 300 can be as
described in connection with the photovoltaic devices of previous
Figures.
[0095] The photovoltaic device 300 shown in FIG. 3 can be
fabricated using the techniques described above in connection with
FIG. 1, with some adjustments. For example, instead of building up
the absorption layer 104 by filling in the interstitial spaces
between the coated carbon nanotubes to form a planar junction 130,
the CBD process can be used (for example, by controlling bath
concentrations and immersion times used with the CBD bath) to
create a conformal coating on the carbon nanotubes 322 that does
not necessarily fill up the interstitial spaces. In some cases, the
thickness of the semiconductor coating can be in a range of about
10 nm to 1000 nm. The window layer 306 can be deposited onto the
absorption layer 304 via CBD.
[0096] FIG. 4 illustrates another embodiment of a photovoltaic
device 400 that includes a substrate 401, a back electrical contact
402, an absorption layer 404, a window layer 406, a top contact
408, and an anti-reflective coating 410. In this embodiment, the
absorption layer 404 and the back electrical contact 402 are
separated by a gap 450. As shown, the gap 450 is open, although in
some embodiments it can be filled with a material (e.g., an
insulating material). The mesh 424 of carbon nanotubes 422 can
extend out of the absorption layer 404 to the back electrical
contact 402 to form a contact (e.g., an ohmic contact) therewith.
The portion of the mesh 424 that spans the gap 450 can include
uncoated carbon nanotubes 422. In some embodiments, the size of the
gap 450 (e.g., the size across the gap from the absorption layer
404 to the back electrical contact 402) can be in a range of about
100 nm to 10,000 nm; or more preferably in a range of about 500 nm
to 1,000 nm. Other aspects of the photovoltaic device 400 can be as
described in connection with the photovoltaic devices of previous
Figures. In some embodiments, the photovoltaic device 400 can
include two meshes of carbon nanostructures, one of which is at
least partially disposed in the absorption layer 404 and the other
of which is attached to the back electrical contact 402, the two
meshes being electrically connected.
[0097] The photovoltaic device 400 shown in FIG. 4 can be
fabricated using the techniques described above in connection with
FIG. 1, with some adjustments. For example, the CBD process can be
used to partially coat the carbon nanotubes 422 with a
semiconductor material 412. These partially coated nanotubes can be
deposited on the back electrical contact 402 to create a mesh 424,
leaving the portion of the mesh 424 closest to the back electrical
contact 402 substantially uncoated. The absorption layer 404 can be
formed by filling in the interstitial spaces of the mesh 424. In
some embodiments, the mesh 424 of carbon nanotubes 422 can be
coated with a semiconductor material 412 (e.g., via CBD or other
process), and then etched on the bottom surface to remove some of
the deposited material. The partially coated nanotubes 122 can then
be disposed on the back electrical contact 402.
[0098] Although the foregoing embodiments have illustrated a mesh
of carbon nanotubes, a variety of other configurations are
possible. FIG. 5 shows an exemplary photovoltaic device 500 with an
absorption layer 504 that includes a "carpet" of aligned
nanostructures, e.g., substantially vertically oriented
nanostructures, which in this embodiment are carbon nanotubes 522.
As used herein, substantially vertically oriented means that the
nanotubes are nearly, but not exactly, normal to the substrate 501.
In many embodiments, the nanotubes can be substantially vertically
oriented such that the angle between the nanostructure (e.g., along
its height 526) and a vector 562 that is normal to the substrate
surface 501' is less than about 45 degrees. In some cases, the
orientation of the embedded carbon nanotubes 522 can be such that
the majority of nanotubes extend farther in a direction
substantially normal to the surface 501' of the substrate 501 (a
vertical direction 562 in FIG. 5) that is greater than they extend
in a direction substantially parallel to the surface 501' of the
substrate 501 (a horizontal direction 560 in FIG. 5), although this
is not necessary. In some cases, the carbon nanotubes 522 are
upstanding (e.g., the carbon nanotubes can have a sidewall 522a and
an end cap 522b, and they can be substantially supported on the
substrate by the end cap 522b).
[0099] The carpet of carbon nanotubes 522 can be arranged in a wide
variety of ways. In FIG. 5, the carbon nanotubes 522 are spaced at
substantially regular intervals from one another. However, in other
embodiments, the spacing need not be regular. Further, the carbon
nanotubes 522 can be placed in bunches, such bunches being spaced
apart from one another. As previously mentioned, it can be
advantageous to arrange the spacing 580 between adjacent carbon
nanotubes 522 to be less than about three diffusion lengths
exhibited by photo-generated minority charge carriers (e.g., upon
photo-excitation) in the semiconductor material 512 incorporated
therebetween, and more preferably less than about two diffusion
lengths. It should be understood that while FIG. 5 is a
two-dimensional illustration, in a three-dimensional device the
interstitial spacing 580 can represent the distance across
three-dimensional voids separating carbon nanotubes 522.
[0100] The carbon nanotubes 522 can have virtually any size, but
preferably they do not extend into the junction 530. For example,
the carbon nanotubes 522 can have a height of about 200 nm to 5000
nm and can be embedded in an absorption layer 504 that is about 3
diffusion lengths thicker than the nanotubes are tall (for example,
the nanotubes can be about 500 nm) in a region about 1000 nm thick,
leaving a region 532 devoid of about 500 nm devoid of
nanostructures. However, all such dimensions are merely by way of
illustration and can vary widely.
[0101] As shown, a semiconductor material 512 which forms the bulk
of the absorption layer 504 conformally coats the individual carbon
nanotubes 522, filling in the spaces in between them and embedding
them in the absorption layer 504. The material 512 can have
crystalline regions 570 therein, which can be as previously
described. A window layer 506 can be disposed over the absorption
layer 504 and can be made of a semiconductor material 514, with the
interface of the absorption layer 504 and window layer 506 creating
a junction 530 with a depletion region.
[0102] The photovoltaic device 500 can also include a substrate
501, a back electrical contact 502, a top electrical contact 508,
and an anti-reflective coating 510. These and other aspects of the
photovoltaic device 500 can be as described in connection with the
photovoltaic devices of previous Figures.
[0103] Operation of the photovoltaic device 500 with upstanding
carbon nanotubes 522 can be similar to that of the photovoltaic
device 100 in many respects. Photons incident on the photovoltaic
device 500 can generate electron-hole pairs in the absorption layer
504 and/or in the window layer 506. As previously mentioned, and
for explanatory purposes only, the absorption layer 504 can be made
of an n-type semiconductor (e.g., CdSe), while the window layer can
be made of a p-type semiconductor (e.g., doped CdS). In such an
embodiment, the electric field in the junction 530 causes the
separation of such electron-hole pairs in the vicinity thereof.
Such electrons can travel across the junction 530 to the n-type
semiconductor and the holes can travel across the junction 530 to
the p-type semiconductor. Additionally, electron-hole pairs are
photo-generated outside the vicinity of the depletion region in
both the absorption layer 504 and window layer 506. Such electrons
and holes can move (e.g., diffuse) within the absorption layer 504
and window layer 506.
[0104] Whether generated inside or outside the vicinity of the
junction 530, photo-generated electrons can migrate through the
n-type absorption layer 504 to the carbon nanotubes 522, which can
provide a conductive pathway out of the absorption layer 504 to the
back electrical contact 502, although in this embodiment electrons
can also pass directly from the absorption layer 504 to the back
electrical contact 502. Photo-generated holes can migrate through
the p-type window layer 506 to the top electrical contact 508. The
electrons in the back electrical contact 502 can travel through an
external load 550 to the p-type window layer 506 to recombine with
the holes that have migrated through the window layer 506.
[0105] The photovoltaic device 500 can be fabricated using the
techniques described above in connection with the photovoltaic
device 100, with some adjustments. For example, the CBD process can
be used to coat aligned (e.g., vertically aligned) carbon nanotubes
722 rather than the mesh 124 described in conjunction with
photovoltaic device 100. The fabrication of aligned carbon
nanotubes is known in the art through a variety of techniques. For
example, aligned nanotubes can be grown chemical vapor deposition
(CVD), such as plasma-enhanced hot filament chemical vapor
deposition using acetylene as a carbon source and ammonia as a
dilution and catalytic agent, as described in Huang et al., "Growth
of highly oriented carbon nanotubes by plasma-enhanced hot filament
chemical vapor deposition," Applied Physics Letters, Vol. 73 No.
26,3845 (1998), and Ren et al., "Synthesis of Large Arrays of
Well-Aligned Carbon Nanotubes on Glass," Science 282, 1105 (1998),
which are hereby incorporated by reference. Aligned nanotubes have
been grown using CVD techniques on patterned silicon substrates
using Fe/Mo nanoparticles as catalysts and CO and H.sub.2 as feed
gases. Such techniques are described in Huang et al., "Growth
Mechanism of Oriented Long Single Walled Carbon Nanotubes Using
`Fast-Heating` Chemical Vapor Deposition Process," Nano Letters,
Vol. 4, No. 6, 1025-1028 (2004), which is hereby incorporated by
reference. Other alignment techniques include the use of magnetic
fields, mechanical shear, and gel extrusion, as discussed in
Fischer et al., "Magnetically aligned single wall carbon nanotubes
films: Preferred orientation and anisotropic properties," Journal
of Applied Physics, Vol. 93 No. 4, 2157 (2003), which is hereby
incorporated by reference. Inkjet printing can be used in some
circumstances. Further, arrays of carbon nanotubes can be
commercially obtained from suppliers, as previously mentioned. More
details on the formation and alignment of carbon nanotubes can be
obtained with reference to U.S. Patent Publication Nos.
2005/0260120 (Smalley et al., "Method For Forming An Array Of
Single-Wall Carbon Nanotubes In An Electric Field And Compositions
Thereof") and 2005/0249656 (Smalley et al, "Method For Forming A
Patterned Array Of Single-Wall Carbon Nanotubes").
[0106] In FIG. 5, the semiconductor material 512 has been built up
to cover the tops of the carbon nanotubes 522 and create a
substantially planar junction 522. However, these semiconductor
material 512 (relative to that shown in FIG. 5) need not be so
thick. FIG. 6 illustrates such an alternate embodiment. FIG. 6
shows a photovoltaic device 600 which has semiconductor material
612 conformally coating individual carbon nanotubes 622 in a carpet
of substantially vertically oriented carbon nanotubes 622. A window
layer 606, which is made of semiconductor material 614, is disposed
on the absorption layer 604 to form a non-planar junction 630. The
carbon nanotubes extend to an electrical contact 602, which is
disposed on substrate 601. In addition, a top electrical contact
608 and an anti-reflective coating 610 can be disposed on top of
the window layer 606. These and other aspects of the photovoltaic
device 600 can be as described in connection with the photovoltaic
devices of previous Figures.
[0107] FIG. 7 illustrates another exemplary photovoltaic device 700
which includes a light-responsive absorption layer 704 made of a
semiconductor material 712 in which are embedded nanostructures (in
this case carbon nanotubes 722) that are conformally coated with a
film made of a semiconductor material. The absorption layer 704 and
the coating 706 can be doped to have differing conductivity types
(e.g., n-type and p-type) so as to form a junction 730 at the
interface of the coating of each carbon nanotubes 722 and the
semiconductor material 712 with a depletion region. In this manner,
the carbon nanotubes 722 extend through a porous insulating layer
736, such as a silica layer, which separates the absorption layer
704 from the back contact 702 and substrate 701. A top electrical
contact 708 can be provided above the absorption layer 704, and an
anti-reflective-layer 710 can be provided above the top electrical
contact 708. These and other aspects of the photovoltaic device 700
can be as described in connection with the photovoltaic devices of
previous Figures.
[0108] In use, photons incident on the photovoltaic device 700 can
generate electron-hole pairs in the semiconductor material 712 of
the absorption layer 704 and in the semiconductor material of the
coating 706. For explanatory purposes only, the absorption layer
704 can be made of an n-type semiconductor, while the coating 706
can be made of a p-type semiconductor. In such an embodiment, the
electric field in the junction 730 causes the migration of holes
through the p-type coating 706 to carbon nanotubes 722, which in
turn provide an electrically conductive path to the back electrical
contact 702. The electric field also causes the migration of
electrons through the n-type absorption layer 704 to the top
electrical contact 708. The electrons travel through an external
load 750, through the back electrical contact 702 and the carbon
nanotubes 722, and recombine with the holes in the p-type coating
706.
[0109] The photovoltaic device 700 can be fabricated using the
techniques described above in connection with the photovoltaic
device 100, with some adjustments. For example, the coating 706
(e.g., semiconductor 714, such as p-type CdTe) can be grown on each
individual carbon nanotube 722 (e.g., the carbon nanotubes can have
a sidewall 722a and an end cap 722b, and both sidewall 722a and
endcap 722b can be coated), followed by a deposition of a seed
layer of semiconductor material 712 (e.g., n-type CdSe). The coated
carbon nanotubes 722 can then be deposited on the back electrical
contact 702, e.g., in a manner previously described. The absorption
layer 704 can be grown over the carbon nanotubes 722 (e.g.,
building up the thickness of semiconductor material 712) and the
top electrical contact 708 can be fused thereon.
[0110] Other nanostructures can be incorporated into the exemplary
photovoltaic devices disclosed herein, including in particular
semiconductor nanostructures. Such nanostructures may exhibit
quantum effects, e.g., acting as quantum dots, or may be
substantially free of quantum confinement effects.
[0111] FIG. 8A schematically depicts an exemplary photovoltaic
device 800 accordingly to another embodiment of the invention that
includes absorption layer 804 and window layer 806, which can be
semiconductor light-responsive layers, as previously described. As
shown, the absorption layer 804 and window layer 806 are in contact
with one another along a planar junction 830. By way of
illustration, in this embodiment, the absorption layer 804 can be
made of a semiconductor material 812 doped to be n-type while the
window layer 806 can be made of a semiconductor material 814 doped
to be p-type. In other embodiments, the n- and p-type doping of the
layers can be reversed.
[0112] A plurality of carbon nanostructures 822 (e.g., in this
case, carbon nanotubes, which can be single-wall or multi-wall
tubes) forming a mesh 824 are distributed within the absorption
layer 804. As previously described, however, a portion of the
absorption layer 804 in the vicinity of the junction 830 can remain
substantially free of such carbon nanostructures 822 to ensure that
the carbon nanostructures 822 do not provide a conductive path
across the junction 830.
[0113] In this embodiment, the absorption and window layers 804,
806 are sandwiched between two electrically conductive layers
(shown as back electrical contact 802 and top electrical contact
808) such that the absorption layer 804 forms a contact (e.g.,
ohmic contact) with the back electrical contact 802 and the window
layer 806 forms a contact (e.g., ohmic contact) with the top
electrical contact 808. The mesh 824 of carbon nanostructures is
also in contact (e.g., ohmic contact) with the back electrical
contact 802 to provide an electrically conductive path out of the
absorption layer 804 thereto. The layer 810 can be formed of an
antireflective and transparent material to allow the passage of
photons incident thereon to the absorption and window layers 804,
806.
[0114] With continued reference to FIG. 8A, a plurality of
composite nanostructures 870 are disposed over (and in some cases,
in between) the mesh 824 of carbon nanostructures 822 so as to be
in electrical contact therewith. In this embodiment, the composite
nanostructures 870 include a core formed primarily of carbon and a
shell formed of a semiconductor material. The semiconductor shell
of the composite nanostructures 870 can have the same conductivity
type as that of the material 812 forming the bulk of the absorption
layer 804 (e.g., both can include n-type doping, such as
n-type/n-type or n+-type/n-type) to prevent the formation of a
depletion region at the interface of the semiconductor shells and
the material 812.
[0115] By way of illustration and with reference to FIG. 8B, a
composite nanostructure 870 can include a core 870b formed of a
carbon nanostructure and a semiconductor shell 870a. By way of
example, the composite nanostructures 870 can be in the form of
substantially spherical particles having a carbon core with a
diameter, e.g., in a range of about 0.7 nm to about 100 nm, and a
semiconductor shell 870a. In many embodiments, the thickness of the
semiconductor shell 870a is selected such that the optical
properties of the shell 870a (e.g., its band gap) are not dominated
by quantum confinement effects. By way of example, the
semiconductor shell 870a can have a thickness in a range of about 1
nm to about 100 nm, or in a range of about 1 nm to about 50 nm, or
in a range of about 1 nm to about 30 nm, or in a range of about 1
nm to about 20 nm, or in a range of about 1 nm to about 10 nm. In
other embodiments, the semiconductor shells 870a of the
nanocomposite structures 870 can provide quantum confinement (e.g.,
they can function as quantum dots). The semiconductor material of
the shell 870a can advantageously be crystalline.
[0116] By way of example, in this embodiment, the core 870b can be
formed of C.sub.60 molecules, C.sub.72 molecules, C.sub.84
molecules, C.sub.96 molecules, C.sub.108 molecules, or C.sub.120
molecules, which are herein referred to as buckyballs, while in
other embodiments the core 870b can be formed of a carbon nanotube
(e.g., a single-wall or multi-wall nanotube). The semiconductor
shell 870a can in turn be formed of any suitable semiconductor
material, such as Group II-VI semiconductor materials. Although in
this embodiment the semiconductor shell 870a is shown as completely
coating the core 870b, in other embodiments the shell 870a can
partially coat the core 870b such that a portion of the core
remains exposed. In some cases, the uncoated portions of such
partially coated cores can be in electrical contact with the carbon
nanostructures 822 of the mesh 824 to facilitate the transfer of
charge carriers generated in the semiconductor shell 870b (e.g., in
response to absorption of a photon) to the mesh 824.
[0117] In many embodiments, the semiconductor shells can be formed
of a semiconductor material having a smaller band gap than that of
the material forming the bulk of the absorption layer 804 so as to
enhance the absorption of photons passing through that layer. It
should also be understood that the composite nanostructures 870 can
be replaced by nanostructures without a composite structure (e.g.,
a nanostructure formed of a semiconductor material without a carbon
core), which can also be formed of a semiconductor material having
a smaller band gap than that of the material forming the bulk of
the absorption layer 804.
[0118] In use, the conductive layer 808 can be exposed to solar
radiation that passes through that layer 808 without any
substantial absorption to reach the window layer 806. Some of the
photons passing through the window layer 806 can be absorbed by the
material 814 of that layer to generate electron-hole pairs. Other
incident photons pass through that layer 806 to be absorbed by the
material 812 of the absorption layer 804 so as to generate
electron-hole pairs therein. In some embodiments, the band gap of
the material 814 forming the layer 806 is greater than the band gap
of the material 812 forming the bulk of the absorption layer 804.
Further, as noted above, the band gap of the semiconductor material
forming the shells 870b of the composite nanostructures 870 can be
less than that of the material 812 forming the bulk of the
absorption layer 804. Such "cascading" of the band gaps can
advantageously enhance the absorption of photons by the
photovoltaic cell 800 as the layer 804 can absorb, in addition to
high energy photons, some of the lower energy photons to which
layer 806 is transparent due to its larger band gap. Likewise, the
semiconductor shells 870b can absorb some of the photons having
energies less than that corresponding to the band gap of the
material 812 forming the bulk of the layer 804, although the shells
870b may also absorb high energy photons that were not absorbed
(e.g., by the materials 812, 814 in the layers 804, 806) before
reaching the composite nanostructures 870.
[0119] The carbon nanostructures 822 can also produce
light-trapping effects (e.g., by providing an index of refraction
less than that of the material 812 in the bulk of the layer 804 so
as to cause internal reflection therein), as described
previously.
[0120] The absorbed photons can generate electron-hole pairs. As
previously described in connection with other embodiments, in the
context of an n-type material 812 and p-type material 814,
electrons can migrate to the back electrical contact 802 via the
carbon nanostructures 822 (or directly). Holes can migrate to the
front electrical contact 808. Electrons can pass through the load
850 and arrive at the front electrical contact 808 and the window
layer 806 to recombine with the holes that have migrated within
that layer.
[0121] The photovoltaic device 800 shown in FIG. 8A can be
fabricated using the techniques described above in connection with
FIG. 1, with some adjustments. In some embodiments, the composite
nanostructures 870 can be fabricated by initially forming the
carbon nanostructure cores 870b via any of a number of fabrication
techniques known in the art, as previously mentioned. In some
embodiments, the carbon cores 870b can then be coated with a
semiconductor shell 870ba by utilizing CBD techniques, such as
those previously discussed. The thickness of the semiconductor
shell 870a can be adjusted by allowing the chemical bath deposition
to proceed for a selected time period. Alternatively, other
techniques, such as atomic layer deposition, molecular beam
epitaxy, or chemical vapor deposition, can be utilized to coat
carbon cores with a semiconductor shell.
[0122] The use of a carbon core 870b as a "scaffolding" for the
formation of the semiconductor shell 870a can provide certain
advantages. For example, it can allow for the generation of a
plurality of nanoparticles whose sizes are narrowly distributed.
More specifically, it can facilitate the formation of a
semiconductor shell with a desired nanosized thickness, and
consequently a desired band gap. For example, the use of bucky
balls as the carbon cores allows the formation of a plurality of
substantially spherical nanoparticles with a carbon core and
semiconductor shell having diameters within a narrow distribution
about an average diameter. The ability to generate the composite
nanostructures 870 with substantially uniform sizes allows the
nanoparticles to have substantially similar band gaps, e.g., in the
above embodiment of FIG. 8A the band gap value less than that of
the bulk of the absorption layer 804.
[0123] As noted above, a variety of semiconductor materials can be
utilized to form the semiconductor shell 870a, including any of the
materials 812, 814 used in the absorption layers 804 and window
layer 806, as detailed above in connection with FIG. 1. By way of
example, in one embodiment the bulk of the absorption layer 804 is
formed of CdS (e.g., with n-type or p-type doping), the window
layer 806 is formed of ZnO, and the semiconductor shells 870a of
the composite nanostructures 870 are formed, e.g., of CdSe or CdTe,
having the same conductivity type as that of the bulk of the
absorption layer 804.
[0124] Although FIG. 8A illustrates the incorporation of
nanostructures 870 into a photovoltaic device which has a mesh 824
of carbon nanostructures 822, in other embodiments, semiconductor
nanostructures can be similarly incorporated into photovoltaic
devices with upstanding carbon nanostructures, such as the
photovoltaic device shown in FIG. 5.
[0125] FIG. 9 illustrates another exemplary photovoltaic device
900, which has an absorption layer 904 formed from a semiconductor
material 912 (e.g., an n-type semiconductor, such as n-type CdSe)
and a window layer 906 formed from a semiconductor material 914
(e.g., a p-type semiconductor, such as p-type doped CdS). As shown,
the absorption layer 904 and window layer 906 form a junction 930
(e.g., a p-n junction).
[0126] A plurality of carbon nanostructures (in this case, carbon
nanotubes 922) are embedded in the absorption layer 904.
Semiconductor nanostructures 970 can be disposed on portions of the
outer surfaces of the carbon nanotubes 922. The semiconductor
nanostructures 970 can be made (e.g., entirely or predominantly) of
a semiconductor material of similar conductivity type to the
semiconductor material 912. By way of illustration, the
semiconductor material 912 can be an n-type material and the
semiconductor nanostructures 970 can comprise an n- or n+-type
material (or alternatively, p-type and p+-type, respectively), or
the semiconductor material 912 and the semiconductor nanostructures
970 both can be n-type (or p-type) semiconductor materials. Again,
the use of different bandgap materials in the semiconductor
nanostructures 970 can be advantageous for tuning the absorption
capabilities of the absorption layer 904.
[0127] The photovoltaic device 900 can further include a substrate
901, a back electrical contact 902, a top electrical contact 908,
and an anti-reflective coating 910. These and other aspects of the
photovoltaic device 900 can be as described in connection with the
photovoltaic devices of previous Figures.
[0128] The photovoltaic device 900 shown in FIG. 9 can be
fabricated using the techniques described above in connection with
photovoltaic device 500 shown in FIG. 5, with some adjustments. For
example, the CBD process can be used to grow semiconductor
nanostructures 970 onto carbon nanotubes 922 before they are
covered with semiconductor material 912. The immersion time of the
carbon nanotubes can be limited such that semiconductor
nanostructures 970 of desired sizes form (typically growing from
nucleation or displacement sites on the carbon nanotube
sidewall).
[0129] Although FIG. 9 illustrates the incorporation of
semiconductor nanostructures 970 into a photovoltaic device with
predominantly upstanding carbon nanostructures, in other
embodiments, such semiconductor nanostructures 970 can be similarly
incorporated into photovoltaic devices having a mesh of carbon
nanostructures, such as the photovoltaic device shown in FIG.
1.
[0130] FIG. 10 illustrates another exemplary photovoltaic device
1000. In this embodiment, the photovoltaic device 1000 has an
absorption layer 1004 formed from a semiconductor material 1012. A
plurality of carbon nanostructures (in this case, carbon nanotubes
1022) can be embedded in the absorption layer 1004. Semiconductor
nanostructures 1070 can be disposed on portions of the outer
surfaces of the carbon nanotubes 922. In this embodiment, the
semiconductor nanostructures 1070 are made (e.g., entirely or
predominantly) of a semiconductor material doped to be of a
different conductivity type than the semiconductor material 1012.
By way of illustration, the semiconductor material 1012 can be an
n-type material and the semiconductor nanostructures 1070 can
comprise an p-type material. The interfaces of the semiconductor
material 1014 and the semiconductor nanostructures form a plurality
of junctions 1030 (e.g., each being a p-n junction), so that the
photovoltaic device 1000 essentially includes a distributed
heterojunction.
[0131] In this embodiment, portions of the carbon nanotubes 1022
that are between the semiconductor nanostructures 1070 are coated
with an insulating material 1038 (e.g., silica). Insulating layer
1036 can be disposed between the absorption layer 1004 and a back
electrical contact 1002, with the carbon nanotubes 1022 extending
through the insulating layer 1036 to make contact (e.g., ohmic
contact) with the back electrical contact 1002.
[0132] The photovoltaic device 1000 can further include a substrate
1001, a top electrical contact 1008, and an anti-reflective coating
1010. These and other aspects of the photovoltaic device 1000 can
be as described in connection with the photovoltaic devices of
previous Figures.
[0133] As previously mentioned, and for explanatory purposes only,
the absorption layer 1004 can be made of n-type CdSe, while the
semiconductor nanostructures 1070 layer can be made of p-type CdTe.
In use, photons incident on the photovoltaic device 1000 can
generate electron-hole pairs in the semiconductor material 1012 in
the absorption layer 1004. They also can produce multi-exciton
generation (MEG) in the semiconductor nanostructures 1070. Charge
separation occurs across the heterojunction formed between the
n-type CdSe absorption layer 1004 and the p-type semiconductor
nanostructures 1070 such that holes from the absorption layer 1004
are swept into the semiconductor nanostructures 1070 and electrons
resulting from MEG in the semiconductor nanostructures 1070 are
swept into the n-type CdSe absorption layer 1004. The
photo-generated electrons can pass through an external load 1050
and return via the back electrical contact 1002 and the carbon
nanotubes 1022 to combine with holes created in the semiconductor
nanostructures 1070.
[0134] The photovoltaic device 1000 shown in FIG. 10 can be
fabricated using the techniques described above in connection with
photovoltaic device 900 shown in FIG. 9, with some adjustments. For
example, in one embodiment, the carbon nanotubes 1022 can have
semiconductor nanostructures 1070 deposited on them in a CBD bath
(e.g., as described in connection with FIG. 9). The semiconductor
nanostructures 1070 can be chemically capped, as is known in the
art, to prevent deposition of the insulating layer 1038 (e.g.,
silica) on them. The insulating layer 1038 can be deposited on the
exposed surface portions of the carbon nanotubes 122. The chemical
cap can be removed from the semiconductor nanostructures 1070 and
the semiconductor material 112 can be grown on the semiconductor
nanostructure/carbon nanotube composite structure 1070/1022 (e.g.,
as a seed coating). The composite structure 1070/1022 can be
deposited on the back electrical contact 1002 by suitable
techniques (e.g., inkjet printing or spin coating, or other
techniques as previously described) and the semiconductor material
1012 can be grown in to fill the volume between the composite
structures 1070/1022 to create absorption layer 1004. In other
embodiments, the step of depositing the seed coating can be omitted
and instead the composite structures 1070/1022 can be deposited and
then the semiconductor material 1012 grown to form the absorption
layer 1004. The top electrical contact 1008 and other elements can
be assembled as previously described.
[0135] Although FIG. 10 illustrates the incorporation of
semiconductor nanostructures 1070 into a photovoltaic device with
upstanding carbon nanostructures, in other embodiments, such
semiconductor nanostructures 1070 can be similarly incorporated
into photovoltaic devices having a mesh of carbon nanostructures,
such as the photovoltaic device shown in FIG. 1.
[0136] The photovoltaic devices disclosed herein can efficiently
convert light to electrical energy. Without being limited by
theory, the nanostructures incorporated into many of the
embodiments described above can provide several
efficiency-enhancing benefits. In some cases, the nanostructures
can provide a conductive pathway out of an absorption layer and
thereby reduce recombination of generated electron-hole pairs
therein, e.g., by ensuring that a higher proportion of charge
carriers are able to escape the absorption layer before
recombination. The nanostructures also can provide light trapping
structures (e.g., by providing a material with an index of
refraction less than that of the material that can make up the bulk
of the absorption layer) and thereby can increase the opportunity
for light to be absorbed in the absorption layer. Further, the
nanostructures can act as nucleation sites for the growth of
crystalline materials in the absorption layer. Such crystalline
material can provide advantageous electrical properties in the
layers. As a result, the photovoltaic devices described herein can
convert light to electrical energy with improved efficiencies over
existing technologies, e.g., for a given absorption layer
thickness.
[0137] In some embodiments, the addition of nanostructures to seed
the growth of crystalline layers around the nanostructures and
promote polycrystalline growth in the bulk of the absorber layer
will enhance minority carrier lifetimes and also facilitate
electron removal from (or insertion into) the absorber layer to
yield a total efficiency for the nanostructured solar cell equal or
greater than about 4%, or about 8%, preferably 10% and more
preferably 15%, e.g., in a range from about 14% to about 17% (e.g.,
in a range of about 14.5% to about 16.5%). Moreover, the addition
of composite nanostructures having a lower band gap than that of
the material forming the bulk of the absorption layer can enhance
the absorption of photons. Such enhancement of the photon
absorption can reasonably be expected to further improve the device
efficiency (e.g., by about 2 to 4 percentage points), to yield an
efficiency for the device in the range of about 16% to about
20%.
[0138] It should be understood that the foregoing theoretical
discussion is illustrative only and that such features and/or
efficiencies are not necessary to a photovoltaic device constructed
in accordance with the teachings herein.
Example
[0139] The following procedures, which are illustrative in nature
and provided only as non-limiting examples, describe CBD procedures
that have been used for depositing semiconductor materials on
carbon nanotubes. These procedures can be used for the creation of
absorption and window layers and for forming photovoltaic devices
such as those described herein. The procedures described in
aforementioned U.S. Pat. No. 7,253,014 (Barron et al., "Fabrication
Of Light Emitting Film Coated Fullerenes And Their Application For
In-Vivo Light Emission"), and in U.S. Patent Publication No.
2005/0089684 (Barron et al., "Coated Fullerenes, Composites And
Dielectrics Made Therefrom"), can also be used. [0140] I. Exemplary
Procedure for preparation of Chemical Bath Deposition (CBD)
solution for deposition of Cadmium Selenide (CdSe) onto carbon
nanotube (CNT) substrates
[0141] A. Preparation of Stock Solutions (10 mL Total Volumes).
[0142] 1. 0.1 M Cadmium Sulfate solution: dissolve 0.209 g of
Cadmium Sulfate (CdSO.sub.4) with de-ionized water (d.i. HO) to a
final volume of 10 mL. [0143] 2. 0.8 M Sodium Citrate solution:
dissolve 2.32 g of Sodium Citrate Dihydrate
(Na.sub.3C.sub.6H.sub.5O.sub.7.2 H.sub.2O) with d.i. H.sub.2O to a
final volume of 10 mL. [0144] 3. 1.5 M Ammonia solution: dilute 1
mL of concentrated Ammonium Hydroxide (NHOH) solution (conc=15 M
NH.sub.3) with d.i. H.sub.2O to a final volume of 10 mL. [0145] 4.
0.01 M Sodium Sulfite: dissolve 0.013 g of Sodium Sulfite
(Na.sub.2SO.sub.3) with d.i. H.sub.2O to a final volume of 10 mL.
NOTE: This solution was prepared immediately before use. [0146] 5.
0.1 M DMSU solution (stabilized): dissolve 0.151 g of
1,1-Dimethyl-2-Selenourea (C.sub.3H.sub.8N.sub.2Se, DMSU) with 0.01
M Sodium Sulfite solution to a final volume of 10 mL. [0147] NOTE:
This solution was prepared immediately before use. [0148] 6. 0.005
M Mercury (II) Chloride solution: dissolve 0.014 g of Mercury (II)
Chloride (HgCl.sub.2) with d.i. H.sub.2O to a final volume of 10
mL.
[0149] B. Preparation of CBD Solution (10 mL Total Volume, Final
pH=9.6-9.7). [0150] 1. Place 1.7 mL of d.i. H.sub.2O in a vial.
[0151] 2. Add 3.0 mL of the 0.1 M Cadmium Sulfate solution to the
vial. [0152] 3. Add 1.5 mL of the 0.8 M Sodium Citrate solution to
the vial. [0153] 4. Add 1.2 mL of the 1.5 M Ammonia solution to the
vial. [0154] 5. Add 2.6 mL of the 0.1 M DMSU solution (stabilized)
to the vial.
[0155] C. Preparation of the Substrate. [0156] 1. CNT substrates
can be immersed into enough d.i. H.sub.2O to completely cover their
surfaces at a rate that is slow enough to gently displace any
trapped air with water. [0157] 2. The substrates are left soaking
in water for 10 min or until they are needed for Step D1. [0158] 3.
Upon removing the substrates from the water, excess water is
allowed to drip from the surface; however, they are not allowed to
dry, but are instead dipped into the CBD solution wet (Step
D1).
[0159] D. Coating of the Substrate. [0160] 1. Immediately after
completing Step C3, immerse substrate into the resultant CBD
solution within the vial. Leave at room temperature for 12 h. Note:
Substrate was oriented vertically or with the side of interest
tilted face down to minimize the unwanted deposition of bulk
precipitate due to gravity. [0161] 2. After 12 h. have passed,
remove the substrate from the CBD solution and rinse it with
copious amounts (.about.50 mL) of d.i. H.sub.2O to remove any
reagents and adsorbed precipitate. [0162] 3. Samples are allowed to
dry in air at room temperature before characterization.
[0163] E. Doping of the Coated Substrates. [0164] 1. Doping of
coated-CNT substrates with Hg to enhance the n-type conductivity of
the CdSe coating may be accomplished by immersing the substrates
after Step D2 into a 0.005 M HgCl.sub.2 solution for 15 min at room
temperature. [0165] 2. After 15 minutes have passed, remove the
substrate from the solution and rinse it with copious amounts
(.about.50 mL) of d.i. H.sub.2O to remove any reagents. [0166] 3.
Samples are allowed to dry in air at room temperature before
characterization.
[0167] F. Thermal Annealing of the Coated Substrates. [0168] 1.
Thermal annealing of either doped coated-CNT substrates from Step
E3 or undoped coated-CNT substrates from Step D3, for improving the
photoconductivity of the CdSe coatings, can be affected by
placement in an oven at 300.degree. C. under a normal atmosphere of
air for 1 h. [0169] 2. After 1 h. has passed, the samples are
removed from the oven and allowed to cool to room temperature
before further modification or characterization. [0170] II.
Exemplary procedure for preparation of Chemical Bath Deposition
(CBD) solution for deposition of in-situ Cu-doped Cadmium Sulfide
(CdS) onto CdSe-coated carbon nanotube (CNT) substrates.
[0171] A. Preparation of stock solutions (10 mL total volumes).
[0172] 1. 1.0 M Cadmium Sulfate solution: dissolve 2.09 g of
Cadmium Sulfate (CdSO.sub.4) with de-ionized water (d.i. H.sub.2O)
to a final volume of 10 mL. [0173] 2. 15 M Ammonia solution: use
concentrated Ammonium Hydroxide (NH.sub.4OH) solution (conc=15 M
NH.sub.3) as purchased. [0174] 3. 1.0 M Thiourea solution: dissolve
0.761 g of Thiourea (CH.sub.4N.sub.2S, TU) with de-ionized water
(d.i. H.sub.2O) to a final volume of 10 mL. [0175] 4. 3.75 M
Triethanolamine solution: dissolve 5.59 g Triethanolamine
(C.sub.6H.sub.15NO.sub.3, TEA) with d.i. H.sub.2O to a final volume
of 10 mL. [0176] 5. 0.1 M Cupric Chloride solution: dissolve 0.170
g of Cupric Chloride Dihydrate (CuCl.sub.2.2 H.sub.2O) with
de-ionized water (d.i. H.sub.2O) to a final volume of 10 mL.
[0177] B. Preparation of CBD Solution (10 mL Total Volume). [0178]
1. Place 7.91 mL of d.i. H.sub.2O in a vial. [0179] 2. Add 0.500 mL
of the 1.0 M Cadmium Sulfate solution to the vial. [0180] 3. Add
0.467 mL of the 3.75 M Triethanolamine solution to the vial. [0181]
4. Add 0.500 mL of the 15 M Ammonia solution to the vial. [0182] 5.
Add 0.120 mL of the 0.1 M Cupric Chloride solution to the vial.
[0183] 6. Add 0.500 mL of the 1.0 M Thiourea solution to the
vial.
[0184] C. Preparation of the Substrate. [0185] 1. Coated-CNT
substrates are immersed into enough d.i. H.sub.2O to completely
cover their surfaces at a rate that is slow enough to gently
displace any trapped air with water. [0186] 2. The substrates are
left soaking in water for a minimum of 10 min or until they are
needed for Step D1. [0187] 3. Upon removing the substrates from the
water, excess water is allowed to drip from the surface; however,
they are not allowed to dry, but are instead dipped into the CBD
solution wet (Step D1).
[0188] D. Coating of the Substrate. [0189] 1. Immediately after
completing Step C3, immerse substrate into the resultant CBD
solution within the vial. Note: Substrate was oriented vertically
or with the side of interest tilted face down to minimize the
unwanted deposition of bulk precipitate due to gravity. [0190] 2.
Place vial in a heating bath @ 80.degree. C. for 2 h. [0191] 3.
After 2 h. have passed, remove the substrate from the CBD solution
and rinse it with copious amounts (-50 mL) of d.i. H.sub.2O to
remove any reagents and adsorbed precipitate. [0192] 4. Samples are
allowed to dry in air at room temperature before characterization.
[0193] III. Exemplary procedure for preparation of Chemical Bath
Deposition (CBD) solution for deposition of un-doped Cadmium
Sulfide (CdS) onto CdSe-coated carbon nanotube (CNT) substrates and
ex situ Cu doping.
[0194] A. Preparation of stock solutions (10 mL total volumes).
[0195] 1. 1.0 M Cadmium Sulfate solution: dissolve 2.09 g of
Cadmium Sulfate (CdSO.sub.4) with de-ionized water (d.i. H.sub.2O)
to a final volume of 10 mL. [0196] 2. 15 M Ammonia solution: use
concentrated Ammonium Hydroxide (NH.sub.4OH) solution (conc=15 M
NH.sub.3) as purchased. [0197] 3. 1.0 M Thiourea solution: dissolve
0.761 g of Thiourea (CH.sub.4N.sub.2S, TU) with de-ionized water
(d.i. H.sub.2O) to a final volume of 10 mL. [0198] 4. 3.75 M
Triethanolamine solution: dissolve 5.59 g Triethanolamine
(C.sub.6H.sub.15NO.sub.3, TEA) with d.i. H.sub.2O to a final volume
of 10 mL. [0199] 5. 0.1 M Cupric Chloride solution: dissolve 0.170
g of Cupric Chloride Dihydrate (CuCl.sub.2.H.sub.2O) with
de-ionized water (d.i. H.sub.2O) to a final volume of 10 mL. [0200]
6. 0.005 M Cupric Chloride solution: dilute 0.5 mL of 0.1 M Cupric
Chloride solution with de-ionized water (d.i. H.sub.2O) to a final
volume of 10 mL.
[0201] B. Preparation of CBD Solution (10 mL Total Volume). [0202]
1. Place 8.03 mL of d.i. H.sub.2O in a vial. [0203] 2. Add 0.500 mL
of the 1.0 M Cadmium Sulfate solution to the vial. [0204] 3. Add
0.467 mL of the 3.75 M Triethanolamine solution to the vial. [0205]
4. Add 0.500 mL of the 15 M Ammonia solution to the vial. [0206] 5.
Add 0.500 mL of the 1.0 M Thiourea solution to the vial.
[0207] C. Preparation of the Substrate. [0208] 1. Coated-CNT
substrates are immersed into enough d.i. H.sub.2O to completely
cover their surfaces at a rate that is slow enough to gently
displace any trapped air with water. [0209] 2. The substrates are
left soaking in water for a minimum of 10 min or until they are
needed for Step D1. [0210] 3. Upon removing the substrates from the
water, excess water is allowed to drip from the surface; however,
they are not allowed to dry, but are instead dipped into the CBD
solution wet (Step D1).
[0211] D. Coating of the Substrate. [0212] 1. Immediately after
completing Step C3, substrates are immersed into the resultant CBD
solution within the vial. Note: Substrates are oriented vertically
or with the side of interest tilted face down to minimize the
unwanted deposition of bulk precipitate due to gravity. [0213] 2.
Place vial in a heating bath @ 80.degree. C. for 2 h. [0214] 3.
After 2 h. have passed, remove the substrate from the CBD solution
and rinse it with copious amounts (.about.50 mL) of d.i. H.sub.2O
to remove any reagents and adsorbed precipitate. [0215] 4. Samples
are allowed to dry in air at room temperature before
characterization.
[0216] E. Ex Situ Doping of the Coating. [0217] 1. Immerse
CdS-coated substrate from Step D4 into the 0.005 M Cupric Chloride
solution for 30 s. The film color will change from bright orange to
dark brown as Cu doping occurs. [0218] 2. After 30 s have passed,
remove sample from solution and rinse it with copious amounts
(.about.50 mL) of d.i. H.sub.2O to remove any reagents. [0219] IV.
Exemplary Procedure for preparation of Chemical Bath Deposition
(CBD) solution for deposition of in situ Cu-doped Zinc Sulfide
(ZnS) onto CdSe-coated carbon nanotube (CNT) substrates.
[0220] A. Preparation of Stock Solutions (10 mL Total Volumes).
[0221] 1. 1.0 M Zinc Sulfate solution: dissolve 2.88 g of Zinc
Sulfate Heptahydrate (ZnSO.sub.4.7 H.sub.2O) with de-ionized water
(d.i. H.sub.2O) to a final volume of 10 mL. [0222] 2. 0.8 M Sodium
Citrate solution: dissolve 2.32 g of Sodium Citrate Dihydrate
(Na.sub.3C.sub.6H.sub.5O.sub.7.2 H.sub.2O) with d.i. H.sub.2O to a
final volume of 10 mL. [0223] 3. 15 M Ammonia solution:
concentrated Ammonium Hydroxide (NH.sub.4OH) solution (conc=15 M
NH.sub.3) as purchased. [0224] 4. 3.75 M Triethanolamine solution:
dissolve 5.59 g Triethanolamine (C.sub.6H.sub.15NO.sub.3, TEA) with
d.i. H.sub.2O to a final volume of 10 mL. [0225] 5. 0.1 M Cupric
Chloride solution: dissolve 0.170 g of Cupric Chloride Dihydrate
(CuCl.sub.2.2 H.sub.2O) with de-ionized water (d.i. H.sub.2O) to a
final volume of 10 mL. [0226] 6. 1.0 M Thiourea solution: dissolve
0.761 of Thiourea (CH.sub.4N.sub.2S, TU) with de-ionized water
(d.i. H.sub.2O) to a final volume of 10 mL.
[0227] B. Preparation of CBD Solution (10 mL Total Volume, Final
pH=10.0-10.1). [0228] 1. Place 7.16 mL of d.i. H.sub.2O in a vial.
[0229] 2. Add 0.150 mL of the 1.0 M Zinc Sulfate solution to the
vial. [0230] 3. Add 0.560 mL of the 0.8 M Sodium Citrate solution
to the vial. [0231] 4. Add 0.200 mL of the 15 M Ammonia solution to
the vial. [0232] 5. Add 0.400 mL of the 3.75 M Triethanolamine
solution to the vial. [0233] 6. Add 0.036 mL of the 0.1 M Cupric
Chloride solution to the vial. [0234] 7. Add 1.50 mL of the 1.0 M
Thiourea solution to the vial.
[0235] C. Preparation of the Substrate. [0236] 1. Coated-CNT
substrates are immersed into enough d.i. H.sub.2O to completely
cover their surfaces at a rate that is slow enough to gently
displace any trapped air with water. [0237] 2. The substrates are
left soaking in water for a minimum of 10 M in or until they are
needed for Step D1. [0238] 3. Upon removing the substrates from the
water, excess water is allowed to drip from the surface; however,
they are not allowed to dry, but are instead dipped into the CBD
solution wet (Step D1).
[0239] D. Coating of the Substrate. [0240] 1. Immediately after
completing Step C3, immerse substrate into the resultant CBD
solution within the vial. Note: Substrates are oriented vertically
or with the side of interest tilted face down to minimize the
unwanted deposition of bulk precipitate due to gravity. [0241] 2.
Place vial in a heating bath @ 80.degree. C. for 4 h. [0242] 3.
After 4 h. have passed, remove the substrate from the CBD solution
and rinse it with copious amounts (.about.50 mL) of d.i. H.sub.2O
to remove any reagents and adsorbed precipitate. [0243] 4. Samples
are allowed to dry in air at room temperature before
characterization. [0244] V. Exemplary Procedure for preparation of
Chemical Bath Deposition (CBD) solution for deposition of un-doped
Zinc Sulfide (ZnS) onto CdSe-coated carbon nanotube (CNT)
substrates and ex situ Cu doping.
[0245] A. Preparation of Stock Solutions (10 mL Total Volumes).
[0246] 1. 1.0 M Zinc Sulfate solution: dissolve 2.88 g of Zinc
Sulfate Heptahydrate (ZnSO.sub.4.7 H.sub.2O) with de-ionized water
(d.i. H.sub.2O) to a final volume of 10 mL. [0247] 2. 0.8 M Sodium
Citrate solution: dissolve 2.32 g of Sodium Citrate Dihydrate
(Na.sub.3C.sub.6H.sub.5O.sub.7.2 H.sub.2O) with d.i. H.sub.2O to a
final volume of 10 mL. [0248] 3. 15 M Ammonia solution: use
concentrated Ammonium Hydroxide (NH.sub.4OH) solution (conc=15 M
NH.sub.3) as purchased. [0249] 4. 3.75 M Triethanolamine solution:
dissolve 5.59 g Triethanolamine (C.sub.6H.sub.15NO.sub.3, TEA) with
d.i. H.sub.2O to a final volume of 10 mL. [0250] 5. 0.1 M Cupric
Chloride solution: dissolve 0.170 g of Cupric Chloride Dihydrate
(CuCl.sub.2.2 H.sub.2O) with de-ionized water (d.i. H.sub.2O) to a
final volume of 10 mL. [0251] 6. 0.005 M Cupric Chloride solution:
dilute 0.5 mL of 0.1 M Cupric Chloride solution with de-ionized
water (d.i. H.sub.2O) to a final volume of 10 mL. [0252] 7. 1.0 M
Thiourea solution: dissolve 0.761 g of Thiourea (CH.sub.4N.sub.2S,
TU) with de-ionized water (d.i. H.sub.2O) to a final volume of 10
mL.
[0253] B. Preparation of CBD Solution (10 mL Total Volume, Final
pH=10.0-10.1). [0254] 1. Place 7.20 mL of d.i. H.sub.2O in a vial.
[0255] 2. Add 0.150 mL of the 1.0 M Zinc Sulfate solution to the
vial. [0256] 3. Add 0.560 mL of the 0.8 M Sodium Citrate solution
to the vial. [0257] 4. Add 0.200 mL of the 15 M Ammonia solution to
the vial. [0258] 5. Add 00400 mL of the 3.75 M Triethanolamine
solution to the vial. [0259] 6. Add 1.50 mL of the 1.0 M Thiourea
solution to the vial.
[0260] C. Preparation of the Substrate. [0261] 1. Coated-CNT
substrates are immersed into enough d.i. H.sub.2O to completely
cover their surfaces at a rate that is slow enough to gently
displace any trapped air with water. [0262] 2. The substrates are
left soaking in water for a minimum of 10 min or until they are
needed for Step D1. [0263] 3. Upon removing the substrates from the
water, excess water is allowed to drip from the surface; however,
they are not allowed to dry, but are instead dipped into the CBD
solution wet (Step D1).
[0264] D. Coating of the Substrate. [0265] 1. Immediately after
completing Step C3, immerse substrate into the resultant CBD
solution within the vial. Note: Substrate should be oriented
vertically or with the side of interest tilted face down to
minimize the unwanted deposition of bulk precipitate due to
gravity. [0266] 2. Place vial in a heating bath @ 80.degree. C. for
4 h. [0267] 3. After 4 h. have passed, remove the substrate from
the CBD solution and rinse it with copious amounts (.about.50 mL)
of d.i. H.sub.2O to remove any reagents and adsorbed precipitate.
[0268] 4. Samples are allowed to dry in air at room temperature
before characterization.
[0269] E. Ex Situ Doping of the Coating. [0270] 1. Immerse
CdS-coated substrate from Step D4 into the 0.005 M Cupric Chloride
solution for 30 s. The film color will change from bright orange to
dark brown as Cu doping occurs. [0271] 2. After 30 s have passed,
remove sample from solution and rinse it with copious amounts
(.about.50 mL) of d.i. H.sub.2O to remove any reagents.
VI. Reagent Specifications
TABLE-US-00001 [0272] Cadmium Sulfate (CdSO.sub.4) FW: 208.46 g/mol
Purity: 99% Grade: ACS Reagent Vendor: Sigma Aldrich Product No.:
383082-100G Sodium Citrate Dihydrate
(Na.sub.3C.sub.6H.sub.5O7.cndot.2H.sub.2O) FW: 294.10 g/mol Purity:
Meets USP Spec Vendor: Sigma Aldrich Product No.: S1804-1KG Sodium
Sulfite (Na.sub.2SO.sub.3) FW: 126.04 g/mol Grade: Certified ACS
Vendor: Fisher Scientific Product No.: S430-500 Thiourea
(CH.sub.4N.sub.2S, TU) FW: 76.12 g/mol Purity: 99.0% Grade: ACS
Reagent Vendor: Sigma Aldrich Product No.: T8656-500G Cupric
Chloride Dihydrate (CuCl.sub.2.cndot.2 H.sub.2O) FW: 170.48 g/mol
Purity: 99.0% Grade: ACS Reagent Vendor: Sigma Aldrich Product No.:
307483-100G 1,1-Dimethyl-2-Selenourea (C.sub.3H.sub.8N.sub.2Se,
DMSU), Note: Stored in Argon glove box. FW: 151.07 g/mol Purity:
97% Vendor: Sigma Aldrich Product No.: 278882-1G Ammonium Hydroxide
(NH.sub.4OH) solution, concentrated FW: 35.05 g/mol Grade:
Certified ACS Vendor: Fisher Scientific Product No.: A669-212
Ammonium Chloride (NH.sub.4Cl) FW: 53.49 g/mol Grade: Certified ACS
Vendor: Fisher Scientific Product No.: A661-500 Zinc Sulfate
Heptahydrate (ZnSO.sub.4.cndot.7 H.sub.2O) FW: 287.56 g/mol Purity:
99.0% Grade: ACS Reagent Vendor: Sigma Aldrich Product No.:
221376-500G Triethanolamine (C.sub.6H.sub.15NO.sub.3, TEA) FW:
149.19 g/mol Purity: 98% Vendor: Sigma Aldrich Product No.:
T1377-1L Mercury (II) Chloride (HgCl.sub.2) FW: 271.50 g/mol
Purity: 99.5% Grade: ACS Reagent Vendor: Sigma Aldrich Product No.:
215465-100G
[0273] By way of illustration, FIG. 11 shows an exemplary scanning
electron microscope image of a mesh of carbon nanotubes (on
buckypaper) coated with CdSe. The CdSe coating was created using
the CBD procedure described above for coating CNT substrates with
CdSe (Procedure I). As can be seen from the image, the CdSe has
been built up to cover the mesh of carbon nanotubes.
[0274] The photovoltaic devices and methods described herein can be
incorporated into films, modules, arrays, and other products. FIG.
12 is a schematic view of an exemplary tandem solar cell (also
known as a multi-junction cell) which includes a plurality of
photovoltaic cells. In this embodiment, a first cell 1202 is
stacked on a second cell 1204 such that some portion of solar
radiation is absorbed by the first cell 1202, and radiation that is
not absorbed passes through the first cell 1202 and can be absorbed
by the second cell 1204. More specifically, the first and second
cells 1202, 1204 can be designed (e.g., by selecting a materials
having suitable band gaps, by selecting the thickness of the
absorbing layers) to absorb photons different wavelengths. Such
cascading of cells, of which there can be several, can
advantageously enhance the overall absorption of the tandem cell
1200. In many embodiments, the first cell 1202 can capture
high-energy photos while passing lower-energy photons to be
absorbed by subsequent cells. The output of the cells 1202, 1204
can be combined, e.g., at junction box 1206. In other embodiments,
the tandem solar cells can be monolithically integrated, as is
known in the art.
[0275] Any of the cells 1202, 1204 can be constructed in accordance
with the teachings of this disclosure, e.g., they can represent any
of the photovoltaic devices previously described. Further, a cell
that does not utilize the teachings of this application (e.g., a
solar cell constructed previously known, or later developed) can be
combined with one that does, e.g., as a new or a retrofitted
product.
[0276] FIG. 13A is a schematic view of an exemplary solar cell
module 1300 that incorporates the photovoltaic cells described
herein. In this example, a module substrate 1301, which can be
rigid or flexible, supports an array of several photovoltaic cells
1302 (e.g., which can be photovoltaic devices 100, or others). The
photovoltaic cells 1302 can be electrically coupled (e.g., in
series, parallel, etc.) in virtually any arrangement to achieve
desired voltage and current characteristics for the module 1300,
although in other embodiments the cells 1302 can be electrically
isolated. (In FIG. 13A, the cells 1302 are coupled into groups of
three by lines 1303, although is merely illustrative.) The module
1300 be electrically coupled to a transformer 1304, which can
convert the electrical output of the module (produced by the cells
1302) as desired for transmission, use, or otherwise.
[0277] FIG. 13B is a schematic view of an exemplary flexible film
1306 that incorporates the photovoltaic devices and principles
described herein. In this example, the flexible film 1306 is made
of a substantially planar sheet in which photovoltaic cells (here,
photovoltaic device 100) are incorporated. In many embodiments, the
photovoltaic cells can be disposed between two flexible substrates,
at least one of which is transparent to solar radiation. A
plurality of separate electrically connected cells, each one
constructed, e.g., in the manner of photovoltaic cell 100, can be
disposed on the film (e.g., on a flexible substrate) and can be
electrically connected to one another. In other embodiments, the
film 1306 can represent a single cell, e.g., the photovoltaic
device 100.
[0278] U.S. Patent Application Publication No. 2006/0145194 (Barron
et al., "Method For Creating a Functional Interface Between A
Nanoparticle Nanotube or Nanowire, And A Biological Molecule Or
System") is hereby incorporated by reference in its entirety.
[0279] One skilled in the art will appreciate further features and
advantages based on the above-described embodiments. Accordingly,
the claims are not to be limited by what has been particularly
shown and described. All publications and references cited herein
are expressly incorporated herein by reference in their
entirety.
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