U.S. patent application number 11/643271 was filed with the patent office on 2007-08-09 for tandem photovoltaic cells.
This patent application is currently assigned to KONARKA TECHNOLOGIES, INC.. Invention is credited to Christoph Brabec, Russell Gaudiana, Christoph Waldauf.
Application Number | 20070181179 11/643271 |
Document ID | / |
Family ID | 38332768 |
Filed Date | 2007-08-09 |
United States Patent
Application |
20070181179 |
Kind Code |
A1 |
Brabec; Christoph ; et
al. |
August 9, 2007 |
Tandem photovoltaic cells
Abstract
Tandem photovoltaic cells having a recombination layer, as well
as related systems, methods, and components, are disclosed.
Inventors: |
Brabec; Christoph; (Linz,
AT) ; Gaudiana; Russell; (Merrimack, NH) ;
Waldauf; Christoph; (Innsbruck, AT) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
KONARKA TECHNOLOGIES, INC.
|
Family ID: |
38332768 |
Appl. No.: |
11/643271 |
Filed: |
December 21, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60752608 |
Dec 21, 2005 |
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60790606 |
Apr 11, 2006 |
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60792485 |
Apr 17, 2006 |
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60792635 |
Apr 17, 2006 |
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60793442 |
Apr 20, 2006 |
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60795103 |
Apr 26, 2006 |
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60797881 |
May 5, 2006 |
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60798258 |
May 5, 2006 |
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Current U.S.
Class: |
136/263 |
Current CPC
Class: |
H01L 51/0037 20130101;
H01L 51/4226 20130101; H01L 2251/308 20130101; H01L 51/0039
20130101; H01L 51/0043 20130101; Y02E 10/549 20130101; H01L 31/0725
20130101; H01L 51/441 20130101; H01L 27/302 20130101; H01L 51/4253
20130101; H01L 51/0036 20130101; B82Y 10/00 20130101; H01L 51/0047
20130101 |
Class at
Publication: |
136/263 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Claims
1. A system, comprising: first and second electrodes; a
recombination layer between the first and second electrodes, the
recombination layer comprising a semiconductor material; a first
photoactive layer between the first electrode and the recombination
layer; and a second photoactive layer between the second electrode
and the recombination layer; wherein the system is configured as a
photovoltaic system.
2. The system of claim 1, wherein the semiconductor material
comprises a p-type semiconductor material and an n-type
semiconductor material.
3. The system of claim 2, wherein the p-type semiconductor material
comprises a polymer selected from the group consisting of
polythiophenes, polyanilines, polyvinylcarbazoles, polyphenylenes,
polyphenylvinylenes, polysilanes, polythienylenevinylenes,
polyisothianaphthanenes, polycyclopentadithiophenes,
polysilacyclopentadithiophenes, polycyclopentadithiazoles,
polythiazolothiazoles, polythiazoles, polybenzothiadiazoles,
poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s,
polythiadiazoloquinoxalines, polybenzoisothiazoles,
polybenzothiazoles, polythienothiophenes, poly(thienothiophene
oxide)s, polydithienothiophenes, poly(dithienothiophene oxide)s,
polytetrahydroisoindoles, and copolymers thereof.
4. The system of claim 3, wherein the p-type semiconductor material
comprises poly(3,4-ethylene dioxythiophene).
5. The system of claim 2, wherein the p-type semiconductor material
comprises a metal oxide.
6. The system of claim 5, wherein the metal oxide comprises an
oxide selected from the group consisting of copper oxides,
strontium copper oxides, and strontium titanium oxides.
7. The system of claim 5, wherein the p-type semiconductor material
comprises a p-doped metal oxide.
8. The system of claim 7, wherein the p-doped metal oxide comprises
p-doped zinc oxides or p-doped titanium oxides.
9. The system of claim 2, wherein the n-type semiconductor material
comprises a metal oxide.
10. The system of claim 9, wherein the metal oxide comprises an
oxide selected from the group consisting of titanium oxides, zinc
oxides, tungsten oxides, molybdenum oxides, and combinations
thereof.
11. The system of claim 9, wherein the n-type semiconductor
material comprises a material selected from the group consisting of
fullerenes, inorganic nanoparticles, oxadiazoles, discotic liquid
crystals, carbon nanorods, inorganic nanorods, polymers containing
CN groups, polymers containing CF.sub.3 groups, and combinations
thereof.
12. The system of claim 2, wherein the p-type and n-type
semiconductor materials are blended into one layer.
13. The system of claim 2, wherein the recombination layer
comprises two layers, one layer comprising the p-type semiconductor
material and the other layer comprising the n-type semiconductor
material.
14. The system of claim 1, wherein the first or second photoactive
layer comprises an electron donor material and an electron acceptor
material.
15. The system of claim 14, wherein the electron donor material
comprises a polymer selected from the group consisting of
polythiophenes, polyanilines, polyvinylcarbazoles, polyphenylenes,
polyphenylvinylenes, polysilanes, polythienylenevinylenes,
polyisothianaphthanenes, polycyclopentadithiophenes,
polysilacyclopentadithiophenes, polycyclopentadithiazoles,
polythiazolothiazoles, polythiazoles, polybenzothiadiazoles,
poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s,
polythiadiazoloquinoxaline, polybenzoisothiazole,
polybenzothiazole, polythienothiophene, poly(thienothiophene
oxide), polydithienothiophene, poly(dithienothiophene oxide)s,
polytetrahydroisoindoles, and copolymers thereof.
16. The system of claim 15, wherein the electron donor material
comprises a polymer selected from the group consisting of
polythiophenes, polycyclopentadithiophenes, and copolymers
thereof.
17. The system of claim 16, wherein the electron donor material
comprises poly(3-hexylthiophene) or
poly(cyclopentadithiophene-co-benzothiadiazole).
18. The system of claim 14, wherein the electron acceptor material
comprises a material selected from the group consisting of
fullerenes, inorganic nanoparticles, oxadiazoles, discotic liquid
crystals, carbon nanorods, inorganic nanorods, polymers containing
CN groups, polymers containing CF.sub.3 groups, and combinations
thereof.
19. The system of claim 18. wherein the electron acceptor material
comprises a substituted fullerene.
20. The system of claim 19, wherein the substituted fullerene
comprises PCBM.
21. The system of claim 1, wherein the first photoactive layer has
a first band gap and the second photoactive layer has a second band
gap different from the first band gap.
22. The system of claim 1, further comprising a hole carrier layer
between the first photoactive layer and the first electrode.
23. The system of claim 22, wherein the hole carrier layer
comprises a polymer selected from the group consisting of
polythiophenes, polyanilines, polyvinylcarbazoles, polyphenylenes,
polyphenylvinylenes, polysilanes, polythienylenevinylenes,
polyisothianaphthanenes, and copolymers thereof.
24. The system of claim 1, further comprising a hole blocking layer
between the second photoactive layer and the second electrode.
25. The system of claim 24, wherein the hole blocking layer
comprises a material selected from the group consisting of LiF,
metal oxides, and combinations thereof.
26. The system of claim 1, wherein the system comprises a tandem
photovoltaic cell.
27. A system, comprising: first and second electrodes; first and
second photoactive layers between the first and second electrodes,
the first photoactive layer comprising a first semiconductor
material and the second photoactive layer comprising a second
semiconductor material; and a third layer between the first and
second photoactive layers, the third layer comprising a third
semiconductor material different from the first or second
semiconductor material; wherein the system is configured as a
photovoltaic system.
28. A system, comprising: first and second electrodes; first and
second photoactive layers between the first and second electrodes;
a third layer comprising an n-type semiconductor material, the
first photoactive layer being between the first electrode and the
third layer and the third layer being between the first and second
photoactive layers; and a fourth layer comprising an p-type
semiconductor material, the second photoactive layer being between
the second electrode and the fourth layer and the four layer being
between the second photoactive layer and the third layer; wherein
the system is configured as a photovoltaic system.
29. The system of claim 28, further comprising a hole carrier layer
between the first electrode and the first photoactive layer.
30. The system of claim 29, further comprising a hole blocking
layer between the second electrode and the second photoactive
layer.
31. A system, comprising: first and second electrodes, at least one
of the first and second electrodes comprising a mesh electrode; a
recombination layer between the first and second electrodes, the
recombination layer comprising a semiconductor material; a first
photoactive layer between the first electrode and the recombination
layer; and a second photoactive layer between the second electrode
and the recombination layer; wherein the system is configured as a
photovoltaic system.
32. A method, comprising: preparing a photovoltaic system having a
recombination layer by a roll-to-roll process.
33. The method of claim 32, wherein the method further comprising
disposing the recombination layer onto a photoactive layer.
34. The method of claim 33, wherein the disposing comprises
disposing a first layer containing a first semiconductor material
onto the photoactive layer and disposing a second layer containing
a second semiconductor material onto the first layer, the second
semiconductor being different from the first semiconductor.
35. The method of claim 34, wherein one of the first and second
semiconductor materials is an n-type semiconductor material and the
other of the first and second semiconductor materials is a p-type
semiconductor material.
36. The method of claim 33, wherein the recombination layer is
disposed on the photoactive layer using at least one process
selected from the group consisting of solution coating, ink jet
printing, spin coating, dip coating, knife coating, bar coating,
spray coating, roller coating, slot coating, gravure coating, and
screen printing.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Pursuant to 35 U.S.C. .sctn. 119(e), this application claims
priority to U.S. Provisional Application Ser. No. 60/752,608, filed
Dec. 21, 2005, U.S. Provisional Application Ser. No. 60/790,606,
filed Apr. 11, 2006, U.S. Provisional Application Ser. No.
60/792,485, filed Apr. 17, 2006, U.S. Provisional Application Ser.
No. 60/792,635, filed Apr. 17, 2006, U.S. Provisional Application
Ser. No. 60/793,442, filed Apr. 20, 2006, U.S. Provisional
Application Ser. No. 60/795,103, filed Apr. 26, 2006, U.S.
Provisional Application Ser. No. 60/797,881, filed May 5, 2006, and
U.S. Provisional Application Ser. No. 60/798,258, filed May 5,
2006, the contents of which are hereby incorporated by
reference.
TECHNICAL FIELD
[0002] The invention relates to tandem photovoltaic cells having a
recombination layer, as well as related systems, methods, and
components.
BACKGROUND
[0003] Photovoltaic cells are commonly used to transfer energy in
the form of light into energy in the form of electricity. A typical
photovoltaic cell includes a photoactive material disposed between
two electrodes. Generally, light passes through one or both of the
electrodes to interact with the photoactive material to generate
electricity. As a result, the ability of one or both of the
electrodes to transmit light (e.g., light at one or more
wavelengths absorbed by a photoactive material) can limit the
overall efficiency of a photovoltaic cell. In many photovoltaic
cells, a film of semiconductive material (e.g., indium tin oxide)
is used to form the electrode(s) through which light passes
because, although the semiconductive material may have a lower
electrical conductivity than electrically conductive materials, the
semiconductive material can transmit more light than many
electrically conductive materials.
[0004] There is an increasing interest in the development of
photovoltaic technology due primarily to a desire to reduce
consumption of and dependency on fossil fuel-based energy sources.
Photovoltaic technology is also viewed by many as being an
environmentally friendly energy technology. However, for
photovoltaic technology to be a commercially feasible energy
technology, the material and manufacturing costs of a photovoltaic
system (a system that uses one or more photovoltaic cells to
convert light to electrical energy) should be recoverable over some
reasonable time frame. But, in some instances the costs (e.g., due
to materials and/or manufacture) associated with practically
designed photovoltaic systems have restricted their availability
and use.
SUMMARY
[0005] The invention relates to tandem photovoltaic cells having a
recombination layer, as well as related systems, methods, and
components.
[0006] In one aspect, this invention features a system that
includes first and second electrodes, a recombination layer between
the first and second electrodes, a first photoactive layer between
the first electrode and the recombination layer, and a second
photoactive layer between the second electrode and the
recombination layer. The recombination layer includes a
semiconductor material. The system is configured as a photovoltaic
system.
[0007] In another aspect, this invention features a system that
include first and second electrodes, first and second photoactive
layers between the first and second electrodes, and a third layer
between the first and second photoactive layers. The first
photoactive layer includes a first semiconductor material and the
second photoactive layer includes a second semiconductor material.
The third layer includes a third semiconductor material different
from the first or second semiconductor material. The system is
configured as a photovoltaic system.
[0008] In another aspect, this invention features a system that
includes first and second electrodes, first and second photoactive
layers between the first and second electrodes, a third layer
including an n-type semiconductor material, and a fourth layer
include an p-type semiconductor material. The first photoactive
layer is between the first electrode and the third layer, which is
between the first and second photoactive layers. The second
photoactive layer is between the second electrode and the fourth
layer, which is between the second photoactive layer and the third
layer. The system is configured as a photovoltaic system.
[0009] In another aspect, this invention features a system that
includes first and second electrodes, a recombination layer between
the first and second electrodes, a first photoactive layer between
the first electrode and the recombination layer, and a second
photoactive layer between the second electrode and the
recombination layer. At least one of the first and second
electrodes includes a mesh electrode. The recombination layer
includes a semiconductor material. The system is configured as a
photovoltaic system.
[0010] In still another aspect, this invention features a method
that includes preparing a photovoltaic system having a
recombination layer by a roll-to-roll process. Embodiments can
include one or more of the following features. In some embodiments,
the semiconductor material in the recombination layer includes a
p-type semiconductor material and an n-type semiconductor
material.
[0011] In some embodiments, the p-type semiconductor material
includes a polymer selected from the group consisting of
polythiophenes (e.g., poly(3,4-ethylene dioxythiophene) (PEDOT)),
polyanilines, polyvinylcarbazoles, polyphenylenes,
polyphenylvinylenes, polysilanes, polythienylenevinylenes,
polyisothianaphthanenes, polycyclopentadithiophenes,
polysilacyclopentadithiophenes, polycyclopentadithiazoles,
polythiazolothiazoles, polythiazoles, polybenzothiadiazoles,
poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s,
polythiadiazoloquinoxalines, polybenzoisothiazoles,
polybenzothiazoles, polythienothiophenes, poly(thienothiophene
oxide)s, polydithienothiophenes, poly(dithienothiophene oxide)s,
polytetrahydroisoindoles, and copolymers thereof.
[0012] In some embodiments, the p-type semiconductor material
includes a metal oxide. For example, the metal oxide can include an
oxide selected from the group consisting of copper oxides,
strontium copper oxides, and strontium titanium oxides. In certain
embodiments, the p-type semiconductor material includes a p-doped
metal oxide (e.g., p-doped zinc oxides or p-doped titanium
oxides).
[0013] In some embodiments, the n-type semiconductor material
includes a metal oxide. For example, the metal oxide can include an
oxide selected from the group consisting of titanium oxides, zinc
oxides, tungsten oxides, molybdenum oxides, and combinations
thereof. In other embodiments, the n-type semiconductor material
includes a material selected from the group consisting of
fullerenes, inorganic nanoparticles, oxadiazoles, discotic liquid
crystals, carbon nanorods, inorganic nanorods, polymers containing
CN groups, polymers containing CF.sub.3 groups, and combinations
thereof.
[0014] In some embodiments, the p-type and n-type semiconductor
materials are blended into one layer.
[0015] In some embodiments, the recombination layer includes two
layers, one layer including the p-type semiconductor material and
the other layer including the n-type semiconductor material.
[0016] In some embodiments, the first or second photoactive layer
includes an electron donor material and an electron acceptor
material.
[0017] In some embodiments, the electron donor material includes a
polymer selected from the group consisting of polythiophenes,
polyanilines, polyvinylcarbazoles, polyphenylenes,
polyphenylvinylenes, polysilanes, polythienylenevinylenes,
polyisothianaphthanenes, polycyclopentadithiophenes,
polysilacyclopentadithiophenes, polycyclopentadithiazoles,
polythiazolothiazoles, polythiazoles, polybenzothiadiazoles,
poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s,
polythiadiazoloquinoxaline, polybenzoisothiazole,
polybenzothiazole, polythienothiophene, poly(thienothiophene
oxide), polydithienothiophene, poly(dithienothiophene oxide)s,
polytetrahydroisoindoles, and copolymers thereof. For example, the
electron donor material can include a polymer selected from the
group consisting of polythiophenes (e.g., poly(3-hexylthiophene)
(P3HT)), polycyclopentadithiophenes (e.g.,
poly(cyclopentadithiophene-co-benzothiadiazole)), and copolymers
thereof.
[0018] In some embodiments, the electron acceptor material includes
a material selected from the group consisting of fullerenes,
inorganic nanoparticles, oxadiazoles, discotic liquid crystals,
carbon nanorods, inorganic nanorods, polymers containing CN groups,
polymers containing CF.sub.3 groups, and combinations thereof. For
example, the electron acceptor material can include a substituted
fullerene (e.g., C61-phenyl-butyric acid methyl ester (PCBM)).
[0019] In some embodiments, the first photoactive layer has a first
band gap and the second photoactive layer has a second band gap
different from the first band gap.
[0020] In some embodiments, the system further includes a hole
carrier layer between the first photoactive layer and the first
electrode. The hole carrier layer can include a polymer selected
from the group consisting of polythiophenes, polyanilines,
polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes,
polysilanes, polythienylenevinylenes, polyisothianaphthanenes, and
copolymers thereof.
[0021] In some embodiments, the system further includes a hole
blocking layer between the second photoactive layer and the second
electrode. The hole blocking layer can include a material selected
from the group consisting of LiF, metal oxides, and combinations
thereof.
[0022] In some embodiments, the system includes a tandem
photovoltaic cell.
[0023] In some embodiments, the method further includes disposing
the recombination layer onto a photoactive layer. The disposing can
include disposing a first layer containing a first semiconductor
material onto the photoactive layer and disposing a second layer
containing a second semiconductor material different from the first
semiconductor onto the first layer. In some embodiments, one of the
first and second semiconductor materials is an n-type semiconductor
material and the other of the first and second semiconductor
materials is a p-type semiconductor material.
[0024] In some embodiments, the recombination layer is disposed on
the photoactive layer using at least one process selected from the
group consisting of solution coating, ink jet printing, spin
coating, dip coating, knife coating, bar coating, spray coating,
roller coating, slot coating, gravure coating, and screen
printing.
[0025] Embodiments can provide one or more of the following
advantages.
[0026] In some embodiments, the recombination layer can be prepared
by using a solution process that can be readily used in a
continuous roll-to-roll process. Such a process can significantly
reduce the cost of preparing a photovoltaic cell.
[0027] In some embodiments, the photoactive layer can include a low
band gap electron donor material, such as a polymer having an
absorption wavelength at the red and near IR regions (e.g., 650-800
nm) of the electromagnetic spectrum, which is not accessible by
most other conventional polymers. When such a polymer is
incorporated into a photovoltaic cell together with a conventional
polymer, it enables the cell to absorb the light in this region of
the spectrum, thereby increasing the current and efficiency of the
cell.
[0028] In some embodiments, the first and second photoactive layers
have different band gaps. Thus, light not absorbed by one
photoactive layer can be absorbed by another photoactive layer,
thereby increasing the efficiency of the photovoltaic cell.
[0029] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features and advantages of the invention will be apparent
from the description and drawings, and from the claims.
DESCRIPTION OF DRAWINGS
[0030] FIG. 1 is a cross-sectional view of an embodiment of a
tandem photovoltaic cell.
[0031] FIG. 2 is an elevational view of an embodiment of a mesh
electrode.
[0032] FIG. 3 is a cross-sectional view of the mesh electrode of
FIG. 2.
[0033] FIG. 4 is a cross-sectional view of another embodiment of a
tandem photovoltaic cell.
[0034] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0035] FIG. 1 shows a tandem photovoltaic cell 100 having a cathode
110, a hole carrier layer 120, a photoactive layer 130, a
recombination layer 140, a photoactive layer 150, a hole blocking
layer 160, an anode 170, and an external load 180 connected to
photovoltaic cell 100 via cathode 110 and anode 170.
[0036] In general, a recombination layer refers to a layer in a
tandem cell where the electrons generated from a first cell
recombine with the holes generated from a second cell.
Recombination layer 140 typically includes a p-type semiconductor
material and an n-type semiconductor material. In general, n-type
semiconductor materials selectively transport electrons and p-type
semiconductor materials selectively transport holes. As a result,
electrons generated from the first cell recombine with holes
generated from the second cell at the interface of the n-type and
p-type semiconductor materials.
[0037] In some embodiments, the p-type semiconductor material
includes a polymer and/or a metal oxide. Examples p-type
semiconductor polymers include polythiophenes (e.g.,
poly(3,4-ethylene dioxythiophene) (PEDOT)), polyanilines,
polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes,
polysilanes, polythienylenevinylenes, polyisothianaphthanenes,
polycyclopentadithiophenes, polysilacyclopentadithiophenes,
polycyclopentadithiazoles, polythiazolothiazoles, polythiazoles,
polybenzothiadiazoles, poly(thiophene oxide)s,
poly(cyclopentadithiophene oxide)s, polythiadiazoloquinoxaline,
polybenzoisothiazole, polybenzothiazole, polythienothiophene,
poly(thienothiophene oxide), polydithienothiophene,
poly(dithienothiophene oxide)s, polytetrahydroisoindoles, and
copolymers thereof. The metal oxide can be an intrinsic p-type
semiconductor (e.g., copper oxides, strontium copper oxides, or
strontium titanium oxides) or a metal oxide that forms a p-type
semiconductor after doping with a dopant (e.g., p-doped zinc oxides
or p-doped titanium oxides). Examples of dopants includes salts or
acids of fluoride, chloride, bromide, and iodide. In some
embodiments, the metal oxide can be used in the form of
nanoparticles.
[0038] In some embodiments, the n-type semiconductor material
includes a metal oxide, such as titanium oxides, zinc oxides,
tungsten oxides, molybdenum oxides, and combinations thereof. The
metal oxide can be used in the form of nanoparticles. In other
embodiments, the n-type semiconductor material includes a material
selected from the group consisting of fullerenes, inorganic
nanoparticles, oxadiazoles, discotic liquid crystals, carbon
nanorods, inorganic nanorods, polymers containing CN groups,
polymers containing CF.sub.3 groups, and combinations thereof.
[0039] In some embodiments, the p-type and n-type semiconductor
materials are blended into one layer. In certain embodiments, the
recombination layer includes two layers, one layer including the
p-type semiconductor material and the other layer including the
n-type semiconductor material.
[0040] In some embodiments, recombination layer 140 includes at
least about 30 wt % (e.g., at least about 40 wt % or at least about
50 wt %) and/or at most about 70 wt % (e.g., at most about 60 wt %
or at most about 50 wt %) of the p-type semiconductor material. In
some embodiments, recombination layer 140 includes at least about
30 wt % (e.g., at least about 40 wt % or at least about 50 wt %)
and/or at most about 70 wt % (e.g., at most about 60 wt % or at
most about 50 wt %) of the n-type semiconductor material.
[0041] Recombination layer 140 generally has a sufficient thickness
so that the layers underneath are protected from any solvent
applied onto recombination layer 140. In some embodiments,
recombination layer 140 can have a thickness at least about 10 nm
(e.g., at least about 20 nm, at least about 50 nm, or at least
about 100 nm) and/or at most about 500 nm (e.g., at most about 200
nm, at most about 150 nm, or at most about 100 nm).
[0042] In general, recombination layer 140 is substantially
transparent. For example, at the thickness used in a tandem
photovoltaic cell 100, recombination layer 140 can transmit at
least about 70% (e.g., at least about 75%, at least about 80%, at
least about 85%, or at least about 90%) of incident light at a
wavelength or a range of wavelengths (e.g., from about 350 nm to
about 1,000 nm) used during operation of the photovoltaic cell.
[0043] Recombination layer 140 generally has a sufficiently low
resistivity. In some embodiments, recombination layer 140 has a
resistivity of at most about 1.times.10.sup.5 ohm/square, (e.g., at
most about 2.times.10.sup.5 ohm/square, at most about
5.times.10.sup.5 ohm/square, or at most about 1.times.10.sup.6
ohm/square).
[0044] Without wishing to be bound by theory, it is believed that
recombination layer 140 can be considered as a common electrode
between two sub-cells (one including cathode 110, hole carrier
layer 120, and photoactive layer 130, and the other include
photoactive layer 150, hole blocking layer 160, and anode 170) in
photovoltaic cells 100.
[0045] Cathode 110 is generally formed of an electrically
conductive material. Examples of electrically conductive materials
include electrically conductive metals, electrically conductive
alloys, and electrically conductive polymers. Exemplary
electrically conductive metals include gold, silver, copper,
aluminum, nickel, palladium, platinum and titanium. Exemplary
electrically conductive alloys include stainless steel (e.g., 332
stainless steel, 316 stainless steel), alloys of gold, alloys of
silver, alloys of copper, alloys of aluminum, alloys of nickel,
alloys of palladium, alloys of platinum and alloys of titanium.
Exemplary electrically conducting polymers include polythiophenes
(e.g., PEDOT), polyanilines (e.g., doped polyanilines),
polypyrroles (e.g., doped polypyrroles). In some embodiments,
combinations of electrically conductive materials are used.
[0046] In some embodiments, cathode 110 can include a mesh
electrode. Examples of mesh electrodes are described in commonly
owned co-pending U.S. Patent Application Publication Nos.
20040187911 and 20060090791, the contents of which are hereby
incorporated by reference.
[0047] FIGS. 2 and 3 shows a mesh cathode 110 that includes solid
regions 112 and open regions 114. In general, regions 112 are
formed of electrically conducting material so that mesh cathode 110
can allow light to pass therethrough via regions 114 and conduct
electrons via regions 112.
[0048] The area of mesh cathode 110 occupied by open regions 114
(the open area of mesh cathode 110) can be selected as desired.
Generally, the open area of mesh cathode 110 is at least about 10%
(e.g., at least about 20%, at least about 30%, at least about 40%,
at least about 50%, at least about 60%, at least about 70%, at
least about 80%) and/or at most about 99% (e.g., at most about 95%,
at most about 90%, at most about 85%) of the total area of mesh
cathode 110.
[0049] Mesh cathode 110 can be prepared in various ways. In some
embodiments, mesh cathode 110 is a woven mesh formed by weaving
wires of material that form solid regions 112. The wires can be
woven using, for example, a plain weave, a Dutch, weave, a twill
weave, a Dutch twill weave, or combinations thereof. In certain
embodiments, mesh cathode 110 is formed of a welded wire mesh. In
some embodiments, mesh cathode 110 is an expanded mesh formed. An
expanded metal mesh can be prepared, for example, by removing
regions 114 (e.g., via laser removal, via chemical etching, via
puncturing) from a sheet of material (e.g., an electrically
conductive material, such as a metal), followed by stretching the
sheet (e.g., stretching the sheet in two dimensions). In certain
embodiments, mesh cathode 110 is a metal sheet formed by removing
regions 114 (e.g., via laser removal, via chemical etching, via
puncturing) without subsequently stretching the sheet.
[0050] In certain embodiments, solid regions 112 are formed
entirely of an electrically conductive material (e.g., regions 112
are formed of a substantially homogeneous material that is
electrically conductive), such as those described above. In some
embodiments, solid regions 112 can have a resistivity less than
about 3 ohm per square.
[0051] In some embodiments, solid regions 112 are formed of a first
material that is coated with a second material different from the
first material (e.g., using metallization, using vapor deposition).
In general, the first material can be formed of any desired
material (e.g., an electrically insulative material, an
electrically conductive material, or a semiconductive material),
and the second material is an electrically conductive material.
Examples of electrically insulative material from which the first
material can be formed include textiles, optical fiber materials,
polymeric materials (e.g., a nylon) and natural materials (e.g.,
flax, cotton, wool, silk). Examples of electrically conductive
materials from which the first material can be formed include the
electrically conductive materials disclosed above. Examples of
semiconductive materials from which the first material can be
formed include indium tin oxide, fluorinated tin oxide, tin oxide
and zinc oxide. In some embodiments, the first material is in the
form of a fiber, and the second material is an electrically
conductive material that is coated on the first material. In
certain embodiments, the first material is in the form of a mesh
(see discussion above) that, after being formed into a mesh, is
coated with the second material. As an example, the first material
can be an expanded metal mesh, and the second material can be PEDOT
that is coated on the expanded metal mesh.
[0052] Generally, the maximum thickness of mesh cathode 110 should
be less than the total thickness of hole carrier layer 120.
Typically, the maximum thickness of mesh cathode 110 is at least
0.1 micron (e.g., at least about 0.2 micron, at least about 0.3
micron, at least about 0.4 micron, at least about 0.5 micron, at
least about 0.6 micron, at least about 0.7 micron, at least about
0.8 micron, at least about 0.9 micron, at least about one micron)
and/or at most about 10 microns (e.g., at most about nine microns,
at most about eight microns, at most about seven microns, at most
about six microns, at most about five microns, at most about four
microns, at most about three microns, at most about two
microns).
[0053] While shown in FIG. 2 as having a rectangular shape, open
regions 114 can generally have any desired shape (e.g., square,
circle, semicircle, triangle, diamond, ellipse, trapezoid,
irregular shape). In some embodiments, different open regions 114
in mesh cathode 110 can have different shapes.
[0054] Although shown in FIG. 3 as having square cross-sectional
shape, solid regions 112 can generally have any desired shape
(e.g., rectangle, circle, semicircle, triangle, diamond, ellipse,
trapezoid, irregular shape). In some embodiments, different solid
regions 112 in mesh cathode 110 can have different shapes. In
embodiments where solid regions 112 have a circular cross-section,
the cross-section can have a diameter in the range of about 5
microns to about 200 microns. In embodiments where solid regions
112 have a trapezoid cross-section, the cross-section can have a
height in the range of about 0.1 micron to about 5 microns and a
width in the range of about 5 microns to about 200 microns.
[0055] In some embodiments, mesh cathode 110 is flexible (e.g.,
sufficiently flexible to be incorporated in photovoltaic cell 100
using a continuous, roll-to-roll manufacturing process). In certain
embodiments, mesh cathode 110 is semi-rigid or inflexible. In some
embodiments, different regions of mesh cathode 110 can be flexible,
semi-rigid or inflexible (e.g., one or more regions flexible and
one or more different regions semi-rigid, one or more regions
flexible and one or more different regions inflexible).
[0056] In general, mesh electrode 110 can be disposed on a
substrate. In some embodiments, mesh electrode 110 can be partially
embedded in the substrate.
[0057] Hole carrier layer 120 is generally formed of a material
that, at the thickness used in photovoltaic cell 100, transports
holes to cathode 110 and substantially blocks the transport of
electrons to cathode 110. Examples of materials from which layer
120 can be formed include polythiophenes (e.g., PEDOT),
polyanilines, polyvinylcarbazoles, polyphenylenes,
polyphenylvinylenes, polysilanes, polythienylenevinylenes,
polyisothianaphthanenes, and copolymers thereof. In some
embodiments, hole carrier layer 120 can include combinations of
hole carrier materials.
[0058] In general, the thickness of hole carrier layer 120 (i.e.,
the distance between the surface of hole carrier layer 120 in
contact with first photoactive layer 130 and the surface of cathode
110 in contact with hole carrier layer 120) can be varied as
desired. Typically, the thickness of hole carrier layer 120 is at
least 0.01 micron (e.g., at least about 0.05 micron, at least about
0.1 micron, at least about 0.2 micron, at least about 0.3 micron,
or at least about 0.5 micron) and/or at most about five microns
(e.g., at most about three microns, at most about two microns, or
at most about one micron). In some embodiments, the thickness of
hole carrier layer 120 is from about 0.01 micron to about 0.5
micron.
[0059] Each of photoactive layers 130 and 150 generally contains an
electron acceptor material and an electron donor material.
[0060] Examples of electron acceptor materials include fullerenes,
oxadiazoles, carbon nanorods, discotic liquid crystals, inorganic
nanoparticles (e.g., nanoparticles formed of zinc oxide, tungsten
oxide, indium phosphide, cadmium selenide and/or lead sulphide),
inorganic nanorods (e.g., nanorods formed of zinc oxide, tungsten
oxide, indium phosphide, cadmium selenide and/or lead sulphide), or
polymers containing moieties capable of accepting electrons or
forming stable anions (e.g., polymers containing CN groups,
polymers containing CF.sub.3 groups). In some embodiments, the
electron acceptor material is a substituted fullerene (e.g., PCBM).
In some embodiments, a combination of electron acceptor materials
can be used in photoactive layer 130 or 150.
[0061] Examples of electron donor materials include conjugated
polymers, such as polythiophenes, polyanilines,
polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes,
polysilanes, polythienylenevinylenes, polyisothianaphthanenes,
polycyclopentadithiophenes, polysilacyclopentadithiophenes,
polycyclopentadithiazoles, polythiazolothiazoles, polythiazoles,
polybenzothiadiazoles, poly(thiophene oxide)s,
poly(cyclopentadithiophene oxide)s, polythiadiazoloquinoxalines,
polybenzoisothiazoles, polybenzothiazoles, polythienothiophenes,
poly(thienothiophene oxide)s, polydithienothiophenes,
poly(dithienothiophene oxide)s, polytetrahydroisoindoles, and
copolymers thereof. In some embodiments, the electron donor
material can be polythiophenes (e.g., poly(3-hexylthiophene)),
polycyclopentadithiophenes, and copolymers thereof. In certain
embodiments, a combination of electron donor materials can be used
in photoactive layer 130 or 150.
[0062] In some embodiments, the electron donor materials or the
electron acceptor materials can include a polymer having a first
comonomer repeat unit and a second comonomer repeat unit different
from the first comonomer repeat unit. The first comonomer repeat
unit can include a cyclopentadithiophene moiety, a
silacyclopentadithiophene moiety, a cyclopentadithiazole moiety, a
thiazolothiazole moiety, a thiazole moiety, a benzothiadiazole
moiety, a thiophene oxide moiety, a cyclopentadithiophene oxide
moiety, a polythiadiazoloquinoxaline moiety, a benzoisothiazole
moiety, a benzothiazole moiety, a thienothiophene moiety, a
thienothiophene oxide moiety, a dithienothiophene moiety, a
dithienothiophene oxide moiety, or a tetrahydroisoindoles
moiety.
[0063] In some embodiments, the first comonomer repeat unit
includes a cyclopentadithiophene moiety. In some embodiments, the
cyclopentadithiophene moiety is substituted with at least one
substituent selected from the group consisting of C.sub.1-C.sub.20
alkyl, C.sub.1-C.sub.20 alkoxy, C.sub.3-C.sub.20 cycloalkyl,
C.sub.1-C.sub.20 heterocycloalkyl, aryl, heteroaryl, halo, CN, OR,
C(O)R, C(O)OR, and SO.sub.2R; R being H, C.sub.1-C.sub.20 alkyl,
C.sub.1-C.sub.20 alkoxy, aryl, heteroaryl, C.sub.3-C.sub.20
cycloalkyl, or C.sub.1-C.sub.20 heterocycloalkyl. For example, the
cyclopentadithiophene moiety can be substituted with hexyl,
2-ethylhexyl, or 3,7-dimethyloctyl. In certain embodiments, the
cyclopentadithiophene moiety is substituted at 4-position. In some
embodiments, the first comonomer repeat unit can include a
cyclopentadithiophene moiety of formula (1): ##STR1## In formula
(1), each of H, C.sub.1-C.sub.20 alkyl, C.sub.1-C.sub.20 alkoxy,
C.sub.3-C.sub.20 cycloalkyl, C.sub.1-C.sub.20 heterocycloalkyl,
aryl, heteroaryl, halo, CN, OR, C(O)R, C(O)OR, or SO.sub.2R; R
being H, C.sub.1-C.sub.20 alkyl, C.sub.1-C.sub.20 alkoxy, aryl,
heteroaryl, C.sub.3-C.sub.20 cycloalkyl, or C.sub.1-C.sub.20
heterocycloalkyl. For example, each of R.sub.1 and R.sub.2,
independently, can be hexyl, 2-ethylhexyl, or
3,7-dimethyloctyl.
[0064] An alkyl can be saturated or unsaturated and branch or
straight chained. A C.sub.1-C.sub.20 alkyl contains 1 to 20 carbon
atoms (e.g., one, two, three, four, five, six, seven, eight, nine,
10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms).
Examples of alkyl moieties include --CH.sub.3, --CH.sub.2--,
--CH.sub.2.dbd.CH.sub.2--, --CH.sub.2--CH.dbd.CH.sub.2, and
branched --C.sub.3H.sub.7. An alkoxy can be branch or straight
chained and saturated or unsaturated. An C.sub.1-C.sub.20 alkoxy
contains an oxygen radical and 1 to 20 carbon atoms (e.g., one, two
, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14,
15, 16, 17, 18, 19, and 20 carbon atoms). Examples of alkoxy
moieties include --OCH.sub.3 and --OCH.dbd.CH--CH.sub.3. A
cycloalkyl can be either saturated or unsaturated. A
C.sub.3-C.sub.20 cycloalkyl contains 3 to 20 carbon atoms (e.g.,
three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15,
16, 17, 18, 19, and 20 carbon atoms). Examples of cycloalkyl
moieities include cyclohexyl and cyclohexen-3-yl. A
heterocycloalkyl can also be either saturated or unsaturated. A
C.sub.3-C.sub.20 heterocycloalkyl contains at least one ring
heteroatom (e.g., O, N, and S) and 3 to 20 carbon atoms (e.g.,
three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15,
16, 17, 18, 19, and 20 carbon atoms). Examples of heterocycloalkyl
moieties include 4-tetrahydropyranyl and 4-pyranyl. An aryl can
contain one or more aromatic rings. Examples of aryl moieties
include phenyl, phenylene, naphthyl, naphthylene, pyrenyl, anthryl,
and phenanthryl. A heteroaryl can contain one or more aromatic
rings, at least one of which contains at least one ring heteroatom
(e.g., O, N, and S). Examples of heteroaryl moieties include furyl,
furylene, fluorenyl, pyrrolyl, thienyl, oxazolyl, imidazolyl,
thiazolyl, pyridyl, pyrimidinyl, quinazolinyl, quinolyl,
isoquinolyl, and indolyl.
[0065] Alkyl, alkoxy, cycloalkyl, heterocycloalkyl, aryl, and
heteroaryl mentioned herein include both substituted and
unsubstituted moieties, unless specified otherwise. Examples of
substituents on cycloalkyl, heterocycloalkyl, aryl, and heteroaryl
include C.sub.1-C.sub.20 alkyl, C.sub.3-C.sub.20 cycloalkyl,
C.sub.1-C.sub.20 alkoxy, aryl, aryloky, heteroaryl, heteroaryloxy,
amino, C.sub.1-C.sub.10 alkylamino, C.sub.1-C.sub.20 dialkylamino,
arylamino, diarylamino, hydroxyl, halogen, thio, C.sub.1-C.sub.10
alkylthio, arylthio, C.sub.1-C.sub.10 alkylsulfonyl, arylsulfonyl,
cyano, nitro, acyl, acyloxy, carboxyl, and carboxylic ester.
Examples of substituents on alkyl include all of the above-recited
substituents except C.sub.1-C.sub.20 alkyl. Cycloalkyl,
heterocycloalkyl, aryl, and heteroaryl also include fused
groups.
[0066] The second comonomer repeat unit can include a
benzothiadiazole moiety, a thiadiazoloquinoxaline moiety, a
cyclopentadithiophene oxide moiety, a benzoisothiazole moiety, a
benzothiazole moiety, a thiophene oxide moiety, a thienothiophene
moiety, a thienothiophene oxide moiety, a dithienothiophene moiety,
a dithienothiophene oxide moiety, a tetrahydroisoindole moiety, a
fluorene moiety, a silole moiety, a cyclopentadithiophene moiety, a
fluorenone moiety, a thiazole moiety, a selenophene moiety, a
thiazolothiazole moiety, a cyclopentadithiazole moiety, a
naphthothiadiazole moiety, a thienopyrazine moiety, a
silacyclopentadithiophene moiety, an oxazole moiety, an imidazole
moiety, a pyrimidine moiety, a benzoxazole moiety, or a
benzimidazole moiety. In some embodiments, the second comonomer
repeat unit is a 3,4-benzo-1,2,5-thiadiazole moiety.
[0067] In some embodiments, the second comonomer repeat unit can
include a benzothiadiazole moiety of formula (2), a
thiadiazoloquinoxaline moiety of formula (3), a
cyclopentadithiophene dioxide moiety of formula (4), a
cyclopentadithiophene monoxide moiety of formula (5), a
benzoisothiazole moiety of formula (6), a benzothiazole moiety of
formula (7), a thiophene dioxide moiety of formula (8), a
cyclopentadithiophene dioxide moiety of formula (9), a
cyclopentadithiophene tetraoxide moiety of formula (10), a
thienothiophene moiety of formula (11), a thienothiophene
tetraoxide moiety of formula (12), a dithienothiophene moiety of
formula (13), a dithienothiophene dioxide moiety of formula (14), a
dithienothiophene tetraoxide moiety of formula (15), a
tetrahydroisoindole moiety of formula (16), a thienothiophene
dioxide moiety of formula (17), a dithienothiophene dioxide moiety
of formula (18), a fluorene moiety of formula (19), a silole moiety
of formula (20), a cyclopentadithiophene moiety of formula (21), a
fluorenone moiety of formula (22), a thiazole moiety of formula
(23), a selenophene moiety of formula (24), a thiazolothiazole
moiety of formula (25), a cyclopentadithiazole moiety of formula
(26), a naphthothiadiazole moiety of formula (27), a thienopyrazine
moiety of formula (28), a silacyclopentadithiophene moiety of
formula (29), an oxazole moiety of formula (30), an imidazole
moiety of formula (31), a pyrimidine moiety of formula (32), a
benzoxazole moiety of formula (33), or a benzimidazole moiety of
formula (34): ##STR2## ##STR3## ##STR4## ##STR5## ##STR6## ##STR7##
##STR8## ##STR9## In the above formulas, each of X and Y,
independently, is CH.sub.2, O, or S; each of R.sub.5 and R.sub.6,
independently, is H, C.sub.1-C.sub.20 alkyl, C.sub.1-C.sub.20
alkoxy, C.sub.3-C.sub.20 cycloalkyl, C-C.sub.20 heterocycloalkyl,
aryl, heteroaryl, halo, CN, OR, C(O)R, C(O)OR, or SO.sub.2R, in
which R is H, C.sub.1-C.sub.20 alkyl, C.sub.1-C.sub.20 alkoxy,
aryl, heteroaryl, C.sub.3-C.sub.20 cycloalkyl, or C.sub.1-C.sub.20
heterocycloalkyl; and each of R.sub.7 and R.sub.8, independently,
is H, C.sub.1-C.sub.20 alkyl, C.sub.1-C.sub.20 alkoxy, aryl,
heteroaryl, C.sub.3-C.sub.20 cycloalkyl, or C.sub.3-C.sub.20
heterocycloalkyl. In some embodiments, the second comonomer repeat
unit includes a benzothiadiazole moiety of formula (2), in which
each of R.sub.5 and R.sub.6 is H.
[0068] The second comonomer repeat unit can include at least three
thiophene moieties. In some embodiments, at least one of the
thiophene moieties is substituted with at least one substituent
selected from the group consisting of C.sub.1-C.sub.20 alkyl,
C.sub.1-C.sub.20 alkoxy, aryl, heteroaryl, C.sub.3-C.sub.20
cycloalkyl, and C.sub.3-C.sub.20 heterocycloalkyl. In certain
embodiments, the second comonomer repeat unit includes five
thiophene moieties.
[0069] The polymer can further include a third comonomer repeat
unit that contains a thiophene moiety or a fluorene moiety. In some
embodiments, the thiophene or fluorene moiety is substituted with
at least one substituent selected from the group consisting of
C.sub.1-C.sub.20 alkyl, C.sub.1-C.sub.20 alkoxy, aryl, heteroaryl,
C.sub.3-C.sub.20 cycloalkyl, and C.sub.3-C.sub.20
heterocycloalkyl.
[0070] In some embodiments, the polymer can be formed by any
combination of the first, second, and third comonomer repeat units.
In certain embodiments, the polymer can be a homopolymer containing
any of the first, second, and third comonomer repeat units. In some
embodiments, the polymer can be ##STR10## in which n can be an
integer greater than 1.
[0071] The monomers for preparing the polymers mentioned herein may
contain a non-aromatic double bond and one or more asymmetric
centers. Thus, they can occur as racemates and racemic mixtures,
single enantiomers, individual diastereomers, diastereomeric
mixtures, and cis- or trans-isomeric forms. All such isomeric forms
are contemplated.
[0072] The polymers described above can be prepared by methods
known in the art, such as those described in commonly owned
co-pending U.S. application Ser. No 11/601,374, the contents of
which are hereby incorporated by reference. For example, a
copolymer can be prepared by a cross-coupling reaction between one
or more comonomers containing two alkylstannyl groups and one or
more comonomers containing two halo groups in the presence of a
transition metal catalyst. As another example, a copolymer can be
prepared by a cross-coupling reaction between one or more
comonomers containing two borate groups and one or more comonomers
containing two halo groups in the presence of a transition metal
catalyst. The comonomers can be prepared by the methods described
herein or by the methods know in the art, such as those described
in U.S. patent application Ser. No. 11/486,536, Coppo et al.,
Macromolecules 2003, 36, 2705-2711 and Kurt et al., J. Heterocycl.
Chem. 1970, 6, 629, the contents of which are hereby incorporated
by reference.
[0073] Without wishing to be bound by theory, it is believed that
an advantage of the polymers described above is that their
absorption wavelengths shift toward the red and near IR regions
(e.g., 650-800 nm) of the electromagnetic spectrum, which is not
accessible by most other conventional polymers. When such a polymer
is incorporated into a photovoltaic cell together with a
conventional polymer, it enables the cell to absorb the light in
this region of the spectrum, thereby increasing the current and
efficiency of the cell.
[0074] In some embodiments, photoactive layer 130 has a first band
gap and photoactive layer 150 has a second band gap different from
the first band gap. In such embodiments, light not absorbed by one
photoactive layer can be absorbed by another photoactive layer,
thereby increasing the efficiency of photovoltaic cell 100.
[0075] Generally, photoactive layer 130 or 150 is sufficiently
thick to be relatively efficient at absorbing photons impinging
thereon to form corresponding electrons and holes, and sufficiently
thin to be relatively efficient at transporting the holes and
electrons. In certain embodiments, photoactive layer 130 or 150 is
at least 0.05 micron (e.g., at least about 0.1 micron, at least
about 0.2 micron, at least about 0.3 micron) thick and/or at most
about one micron (e.g., at most about 0.5 micron, at most about 0.4
micron) thick. In some embodiments, photoactive layer 130 or 150 is
from about 0.1 micron to about 0.2 micron thick.
[0076] In general, photoactive layer 130 or 150 can be formed by
using a suitable process, such as solution coating, ink jet
printing, spin coating, dip coating, knife coating, bar coating,
spray coating, roller coating, slot coating, gravure coating, or
screen printing.
[0077] Hole blocking layer 160 is generally formed of a material
that, at the thickness used in photovoltaic cell 100, transports
electrons to anode 170 and substantially blocks the transport of
holes to anode 170. Examples of materials from which hole blocking
10 layer 160 can be formed include LiF and metal oxides (e.g., zinc
oxide, titanium oxide).
[0078] Typically, hole blocking layer 160 is at least 0.02 micron
(e.g., at least about 0.03 micron, at least about 0.04 micron, at
least about 0.05 micron) thick and/or at most about 0.5 micron
(e.g., at most about 0.4 micron, at most about 0.3 micron, at most
about 0.2 micron, at most about 0.1 micron) thick.
[0079] Anode 170 is generally formed of an electrically conductive
material, such as one or more of the electrically conductive
materials noted above. In some embodiments, anode 170 is formed of
a combination of electrically conductive materials. In certain
embodiments, anode 170 can be formed of a mesh electrode.
[0080] Without wishing to be bound by theory, it is believed that
tandem photovoltaic cell 100 achieves the highest efficiency when
photoactive layers 130 and 150 generate substantially the same
amount of current.
[0081] FIG. 4 shows a tandem photovoltaic cell 400 having a cathode
410, a hole carrier layer 420, a photoactive layer 430, a
recombination layer 440, a photoactive layer 450, a hole blocking
layer 460, an anode 470, and an external load 480 connected to
photovoltaic cell 400 via cathode 410 and anode 470. Recombination
layer 440 includes a layer 442 that contains an an-type
semiconductor material and a layer 444 that contains a p-type
semiconductor material. In some embodiments, recombination layer
440 can include a layer of mixed n-type and p-type semiconductor
material at the interface of layer 442 and layer 444.
[0082] In some embodiments, a two-layer recombination layer can be
prepared by applying a layer of an n-type semiconductor material
and a layer of a p-type semiconductor material separately. For
example, when titanium oxide nanoparticles are used as an n-type
semiconductor material, a layer of titanium oxide nanoparticles can
be formed by (1) dispersing a precursor (e.g., a titanium salt) in
a solvent (e.g., an anhydrous alcohol) to form a dispersion, (2)
coating the dispersion on a photoactive layer, (3) hydrolyzing the
dispersion to form a titanium oxide layer, and (4) drying the
titanium oxide layer. As another example, when a polymer (e.g.,
PEDOT) is used a p-type semiconductor, a polymer layer can be
formed by first dissolving the polymer in a solvent (e.g., an
anhydrous alcohol) to form a solution and then coating the solution
on a photoactive layer. In some embodiments, a one-layer
recombination layer can be prepared by applying a blend of an
n-type semiconductor material and a p-type semiconductor material
on photoactive layer. For example, an n-type semiconductor and a
p-type semiconductor can be first dispersed and/or dissolved in a
solvent together to form a dispersion or solution and then coated
the dispersion or solution on a photoactive layer to form a
recombination layer. The coating process mentioned above can be
achieved by using at least one process selected from the group
consisting of solution coating, ink jet printing, spin coating, dip
coating, knife coating, bar coating, spray coating, roller coating,
slot coating, gravure coating, and screen printing.
[0083] Without wishing to bound by theory, it is believed that the
solution process described above can be readily used in a
continuous manufacturing process, such as a roll-to-roll process,
thereby significantly reducing the cost of preparing a photovoltaic
cell. Examples of roll-to-roll processes have been described in,
for example, U.S. Application Publication No. 2005-0263179.
[0084] The following examples are illustrative and not intended to
be limiting.
EXAMPLE 1
[0085] A tandem photovoltaic cell having the structure of
ITO/TiO.sub.2/P3HT:PCBM/ PEDOT/TiO2/P3HT:PCBM/PEDOT/Ag was prepared
as follows. A substrate with ITO (having a resistivity of 13
ohm/square) was cleaned sequentially with acetone and isopropanol
for 10 minutes in an ultrasonic bath at room temperature.
Tetra-n-butyl-titanate (TYZOR; E. I. du Pont de Nemours and
Company, Wilmington, Del.) diluted 1:199 in anhydrous isopropanol
was applied onto the ITO via doctor-blading (40 mm/s; 600 .mu.m
slot at 40.degree. C.) and hydrolyzed by distilled water. The
coating thus obtained was dried for 10 minutes to give a titanium
oxide layer having a thickness of 10.+-.5 nm. A solution of
poly-(3-hexylthiophen) (P3HT): C61-phenyl-butyric acid methyl ester
(PCBM) in ortho-xylene (1.5 mg:1.2 mg:100 .mu.l) was then applied
onto the titanium oxide layer via doctor-blading (7.5 mm/s; 600
.mu.m slot at 65.degree. C.) to give a P3HT:PCBM layer having a
thickness of 100.+-.10 nm. A solution of PEDOT in isopropanol (1
ml:5 ml) was subsequently coated on the P3HT:PCBM layer via
doctor-blading (2.times.5 mm/s; 150 .mu.m slot at 85.degree. C.) to
give in a PEDOT layer of 30.+-.10 nm. After the device thus
obtained was baked for 10 minutes at 140.degree. C. in nitrogen
atmosphere, tetra-n-butyl-titanate diluted 1:199 in anhydrous
isopropanol was applied onto the PEDOT layer via doctor-blading (40
mm/s; 600 .mu.m slot at 40.degree. C.). The coating was hydrolyzed
and dried for 10 minutes to give a second titanium oxide layer of
10.+-.5 nm. The PEDOT layer and the second titanium oxide layer
obtained above constituted as the recombination layer in the final
tandem photovoltaic cell. A solution of P3HT:PCBM in ortho-xylene
(1.5 mg:1.2 mg:100 .mu.l) was then applied onto the second titanium
oxide layer via doctor-blading (65 mm/s; 600 .mu.m slot at
65.degree. C.) to give a second P3HT:PCBM layer having a thickness
of 300.+-.30 nm. Subsequently, a solution of PEDOT in isopropanol
(1 ml:5 ml) was applied onto the second P3HT:PCBM layer via
doctor-blading (2.times.5 mm/s; 150 .mu.m slot at 85.degree. C.) to
give a second PEDOT layer having a thickness of 30.+-.10 nm. After
the device thus obtained was baked for 20 minutes at 140.degree. C.
in nitrogen atmosphere, a 100 nm layer of silver was applied onto
the second PEDOT layer via thermal evaporation (0.05-0.5 nm/s at
3.times.10.sup.-6 mbar) to give a tandem photovoltaic cell.
[0086] A single photovoltaic cell having the structure of
ITO/TiO.sub.2/P3HT:PCBM/ PEDOT/Ag was also prepared. The titanium
oxide layer, the P3HT:PCBM layer, the PEDOT layer, and the silver
layer were prepared using the same methods described in the
preceding paragraph.
[0087] The tandem photovoltaic cell and single cell were tested for
their properties. The open circuit voltage of both cells were
measured at zero current using a Source Measurement Unit (SMU)
Keithley 2400 when the device was illuminated by a solar simulator
(Oriel) at 1 kW/m.sup.2 Air Mass 1.5 global. The results show that
the open circuit voltage of the tandem photovoltaic cell was 1.025
V, twice as much as that of a single photovoltaic cell having the
structure of ITO/TiO.sub.2/P3HT:PCBM/PEDOT/Ag.
[0088] Other embodiments are in the claims.
* * * * *