U.S. patent application number 12/174626 was filed with the patent office on 2009-01-22 for hybrid multi-junction photovoltaic cells and associated methods.
Invention is credited to Prem Nath, Rosine M. Ribelin, Lawrence M. Woods.
Application Number | 20090020149 12/174626 |
Document ID | / |
Family ID | 39798104 |
Filed Date | 2009-01-22 |
United States Patent
Application |
20090020149 |
Kind Code |
A1 |
Woods; Lawrence M. ; et
al. |
January 22, 2009 |
Hybrid Multi-Junction Photovoltaic Cells And Associated Methods
Abstract
A multi-junction photovoltaic cell includes a substrate and a
back contact layer formed on the substrate. A low bandgap Group
IB-IIIB-VIB.sub.2 material solar absorber layer is formed on the
back contact layer. A heterojunction partner layer is formed on the
low bandgap solar absorber layer, to help form the bottom cell
junction, and the heterojunction partner layer includes at least
one layer of a high resistivity material having a resistivity of at
least 100 ohms-centimeter. The high resistivity material has the
formula (Zn and/or Mg)(S, Se, O, and/or OH). A conductive
interconnect layer is formed above the heterojunction partner
layer, and at least one additional single-junction photovoltaic
cell is formed on the conductive interconnect layer, as a top cell.
The top cell may have an amorphous Silicon or p-type Cadmium
Selenide solar absorber layer. Cadmium Selenide may be converted
from n-type to p-type with a chloride doping process.
Inventors: |
Woods; Lawrence M.;
(Littleton, CO) ; Ribelin; Rosine M.; (Lakewood,
CO) ; Nath; Prem; (Denver, CO) |
Correspondence
Address: |
LATHROP & GAGE LC
4845 PEARL EAST CIRCLE, SUITE 300
BOULDER
CO
80301
US
|
Family ID: |
39798104 |
Appl. No.: |
12/174626 |
Filed: |
July 16, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60950087 |
Jul 16, 2007 |
|
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60956107 |
Aug 15, 2007 |
|
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61031652 |
Feb 26, 2008 |
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Current U.S.
Class: |
136/244 ;
136/256; 427/76; 427/97.6 |
Current CPC
Class: |
Y02E 10/543 20130101;
H01L 31/0392 20130101; H01L 31/0336 20130101; H01L 31/073 20130101;
H01L 21/0256 20130101; H01L 31/1828 20130101; H01L 31/046 20141201;
H01L 31/18 20130101; Y02P 70/50 20151101; Y02P 70/521 20151101;
H01L 31/1884 20130101; H01L 31/0725 20130101; H01L 21/02579
20130101; H01L 21/385 20130101 |
Class at
Publication: |
136/244 ;
136/256; 427/97.6; 427/76 |
International
Class: |
H01L 31/042 20060101
H01L031/042; H01L 31/00 20060101 H01L031/00; B05D 5/12 20060101
B05D005/12 |
Goverment Interests
U.S. GOVERNMENT RIGHTS
[0002] Certain claims with this invention were made with Government
support under Advanced Technical Program Grant No. 70NANB8H4070
awarded by the United States Department of Commerce (NIST), and a
Small-Business Innovative Research program contract No.
FA9453-03-C-0216 awarded by the United States Department of
Defense. The Government has certain rights in this invention.
Claims
1. A photovoltaic cell, comprising: a transparent conductor layer;
a first heterojunction partner layer; a p-type Cadmium Selenide
layer in contact with the first heterojunction partner layer; and a
first electrical contact layer.
2. The photovoltaic cell of claim 1, wherein the first
heterojunction partner layer comprises a material selected from the
group consisting of Zinc Selenide, Cadmium Sulfide, Cadmium Zinc
Selenide, Cadmium Selenide, Zinc Sulfide, Cadmium Oxide, Zinc
Oxide, Zinc Magnesium Oxide, Tin Oxide, and Cadmium Zinc
Sulfide.
3. The photovoltaic cell of claim 1, further comprising a buffer
layer, between the transparent conductor layer and the first
heterojunction partner layer.
4. The photovoltaic cell of claim 3, wherein the buffer layer is
formed of a material selected from the group consisting of undoped
Zinc Oxide, Zinc Magnesium Oxide, and Tin Oxide.
5. The photovoltaic cell of claim 1, wherein the first electrical
contact layer is a first back contact layer.
6. The photovoltaic cell of claim 1, wherein the first electrical
contact layer is a bilayer.
7. The photovoltaic cell of claim 6 wherein the first electrical
contact layer contains a layer of doped ZnTe and a layer of a
material selected from the group consisting of a metal, and a
transparent conducting oxide.
8. The photovoltaic cell of claim 1, wherein the first electrical
contact layer comprises a second transparent conductor layer, and
further comprising: a second heterojunction partner layer, a
IB-IIIB-VIB.sub.2 semiconductor layer, and a second back contact
layer.
9. The photovoltaic cell of claim 8 wherein the IB-IIIB-VIB.sub.2
semiconductor layer comprises a layer of Copper-Indium-Diselenide
(CIS).
10. The photovoltaic cell of claim 8, wherein the first
heterojunction partner layer comprises a material selected from the
group consisting of Zinc Selenide, Cadmium Sulfide, Cadmium Zinc
Selenide, Cadmium Selenide, Zinc Sulfide, Cadmium Oxide, Zinc
Oxide, Zinc Magnesium Oxide, Tin Oxide, and Cadmium Zinc
Sulfide.
11. The photovoltaic cell of claim 8, further comprising a buffer
layer, between the transparent conductor layer and the first
heterojunction partner layer.
12. The photovoltaic cell of claim 11, wherein the buffer layer is
formed of a material selected from the group consisting of undoped
Zinc Oxide, Zinc Magnesium Oxide, and Tin Oxide.
13. The photovoltaic cell of claim 8 wherein the IB-IIIB-VIB.sub.2
semiconductor layer is selected from the group of materials having
the approximate formula Cu(In, Ga, or Al) (S, or Se).sub.2.
14. The photovoltaic cell of claim 8, wherein the first electrical
contact layer comprises a bilayer.
15. The photovoltaic cell of claim 14, wherein the bilayer
comprises a layer of doped ZnTe and a layer of a material selected
from the group consisting of a metal and a conductive oxide.
16. The photovoltaic cell of claim 1, wherein the p-type Cadmium
Selenide layer is made by a process comprising: depositing a layer
of Cadmium Selenide; coating the layer of Cadmium Selenide with a
solution comprising at least one of chloride selected from the
group consisting of chlorides of Group IA elements, chlorides of
group IB elements, and chlorides of Group IIIB elements; and
heat-treating the layer of Cadmium Selenide while at least
partially preventing evaporation of Selenium from the layer of
Cadmium Selenide.
17. The photovoltaic cell of claim 16, wherein Selenium is at least
partially prevented from evaporating from the layer of Cadmium
Selenide during the step of heat-treating by executing the step of
heat-treating in a Selenium enriched atmosphere.
18. The photovoltaic cell of claim 16, wherein Selenium is at least
partially prevented from evaporating from the layer of Cadmium
Selenide during the step of heat-treating by physically impeding
the flow of Selenium vapor from the layer of Cadmium Selenide.
19. The photovoltaic cell of claim 16, the solution further
comprising Cadmium Chloride.
20. A hybrid multi-junction photovoltaic cell, comprising: a
polymer substrate for providing mechanical support for the
photovoltaic cell; a back contact layer formed on the substrate; a
bottom solar absorber layer formed on the back contact layer, the
bottom solar absorber layer including a material selected from the
group consisting of a low bandgap Group IB-IIIB-VIB.sub.2 material
having bulk p-type character, and an alloy of a low bandgap Group
IB-IIIB-VIB.sub.2 material having bulk p-type character; a
heterojunction partner layer formed on the bottom solar absorber
layer; a layer of p-type semiconductor formed above the
heterojunction partner layer; a layer of intrinsic semiconductor
formed on the layer of p-type semiconductor; and a layer of n-type
semiconductor formed on the layer of intrinsic semiconductor.
21. The photovoltaic cell of claim 20, the polymer substrate being
formed of polyimide.
22. The photovoltaic cell of claim 20, the bottom solar absorber
layer comprising a material selected from the group consisting of
Copper Indium DiSelenide, Copper Indium DiTelluride, and an alloy
formed of Copper Indium DiSelenide and at least one of Gallium,
Aluminum, Tellurium and Sulfur.
23. The photovoltaic cell of claim 20, the heterojunction partner
layer comprising an n-type material.
24. The photovoltaic cell of claim 20, the heterojunction partner
layer comprising at least one layer of a high resistivity material
having a resistivity of at least 100 ohms-centimeter, the high
resistivity material being a material having the formula (Zn and/or
Mg)(S, Se, O, and/or OH).
25. The photovoltaic cell of claim 24, wherein the at least one
layer of high resistivity material of the heterojunction partner
layer has a resistivity of at least 1,000 ohms-centimeter.
26. The photovoltaic cell of claim 24, the heterojunction partner
layer being formed using a chemical vapor deposition process.
27. The photovoltaic cell of claim 20, further comprising an
interconnect layer disposed between the heterojunction partner
layer and the layer of p-type semiconductor.
28. The photovoltaic cell of claim 27, the interconnect layer
comprising a material selected from the group consisting of doped
Zinc Oxide, undoped Zinc Oxide, Indium Tin Oxide, doped Tin Oxide,
undoped Tin Oxide, n-type amorphous Silicon, n-type amorphous
Silicon Germanium, hydrogenated amorphous Silicon Carbide, and
n-type microcrystalline Silicon.
29. The photovoltaic cell of claim 20, the back contact layer
comprising Molybdenum.
30. The photovoltaic cell of claim 20, the layer of p-type
semiconductor and the layer of n-type semiconductor each being
formed from a material selected from the group consisting of
hydrogenated amorphous Silicon, hydrogenated amorphous Silicon
Germanium, hydrogenated amorphous Silicon Carbide, nanocrystalline
Silicon, and microcrystalline Silicon.
31. The photovoltaic cell of claim 20, the layer of intrinsic
semiconductor being formed of hydrogenated amorphous Silicon
Germanium.
32. The photovoltaic cell of claim 20, further comprising at least
one additional single junction photovoltaic cell formed above the
layer of n-type semiconductor.
33. A module of a plurality of hybrid multi-junction photovoltaic
cells, comprising: a substrate for providing mechanical support for
the photovoltaic cells; a back contact layer formed on the
substrate; a bottom solar absorber layer formed on the back contact
layer, the bottom solar absorber layer including a material
selected from the group consisting of a low bandgap Group
IB-IIIB-VIB.sub.2 material having bulk p-type character, and an
alloy of a low bandgap Group IB-IIIB-VIB.sub.2 material having bulk
p-type character; an n-type heterojunction partner layer formed on
the bottom solar absorber layer; a contact layer of p-type
semiconductor formed above the heterojunction partner layer; a
primary solar absorber layer formed on the contact layer of p-type
semiconductor, the primary solar absorber layer formed of a
material selected from the group consisting of an intrinsic
semiconductor and a p-type semiconductor; a layer of n-type
semiconductor formed on the primary solar absorber layer; a top
contact layer formed on the layer of n-type semiconductor; at least
one first isolating scribe extending at least from a top surface of
the layer of n-type semiconductor to a top surface of the
substrate, the first isolating scribe being filled with an
insulating material; at least one connecting scribe extending at
least from the top surface of the layer of n-type semiconductor to
a top surface of the back contact layer, the connecting scribe
being filed with a conductive material; and at least one second
isolating scribe extending at least from a top surface of the top
contact layer to a top surface of the bottom solar absorber
layer.
34. The module of claim 33, the heterojunction partner layer being
formed of at least one layer of a high resistivity material having
a resistivity of at least 100 ohms-centimeter, the high resistivity
material being a material having the formula (Zn and/or Mg)(S, Se,
O, and/or OH).
35. The module of claim 34, wherein the heterojunction partner
layer is formed using a chemical vapor deposition process.
36. The module of claim 34, wherein the at least one layer of high
resistivity material of the heterojunction partner layer has a
resistivity of at least 1,000 ohms-centimeter.
37. The module of claim 33, further comprising a conductive
interconnect layer disposed between the heterojunction partner
layer and the contact layer of p-type semiconductor, the conductive
interconnect layer comprising a material selected from the group
consisting of doped Zinc Oxide, Indium Tin Oxide, doped Tin Oxide,
n-type amorphous Silicon, n-type amorphous Silicon Germanium,
n-type hydrogenated amorphous Silicon Carbide, and n-type
microcrystalline Silicon.
38. The module of claim 33, the contact layer of p-type
semiconductor and the layer of n-type semiconductor each being
formed from a material selected from the group consisting of
hydrogenated amorphous Silicon, hydrogenated amorphous Silicon
Germanium, hydrogenated amorphous Silicon Carbide, nanocrystalline
Silicon, and microcrystalline Silicon, and the primary solar
absorber layer being formed of hydrogenated amorphous Silicon
Germanium.
39. The module of claim 33, the contact layer of p-type
semiconductor being doped ZnTe, and the primary solar absorber
layer being p-type CdSe.
40. The module of claim 33, further comprising at least one
additional single junction photovoltaic cell disposed between the
heterojunction partner layer and the contact layer of p-type
semiconductor.
41. A multi-junction photovoltaic cell, comprising: a substrate for
providing mechanical support for the photovoltaic cell; a back
contact layer formed on the substrate; a solar absorber layer
formed on the back contact layer, the solar absorber layer being
formed of a low bandgap Group IB-IIIB-VIB.sub.2 material having
bulk p-type character; a heterojunction partner layer formed on the
solar absorber layer, the heterojunction partner layer comprising
at least one layer of a high resistivity material having a
resistivity of at least 100 ohms-centimeter, the high resistivity
material being a material having the formula (Zn and/or Mg)(S, Se,
O, and/or OH); a conductive interconnect layer formed above the
heterojunction partner layer; and at least one additional
single-junction photovoltaic cell formed on the conductive
interconnect layer.
42. The photovoltaic cell of claim 41, wherein the at least one
layer of high resistivity material of the heterojunction partner
layer has a resistivity of at least 1,000 ohms-centimeter.
43. The photovoltaic cell of claim 41, the heterojunction partner
layer being formed using a chemical vapor deposition process.
44. The photovoltaic cell of claim 41, the substrate being formed
from a material selected from the group consisting of a polymer, a
reinforced polymer, polyimide, a metal foil, an insulated metal
foil, and glass.
45. The photovoltaic cell of claim 41, the low bandgap Group
IB-IIIB-VIB.sub.2 material being selected from the group consisting
of Copper Indium DiSelenide, Copper Indium DiTelluride, an alloy
formed of Copper Indium DiSelenide and at least one of Gallium,
Aluminum, Tellurium and Sulfur, and an alloy formed of Copper
Indium DiTelluride and at least one of Gallium, Aluminum, Selenium
and Sulfur.
46. The photovoltaic cell of claim 41, further comprising a buffer
layer disposed between the heterojunction partner layer and the
conductive interconnect layer.
47. The photovoltaic cell of claim 41, the additional
single-junction photovoltaic cell comprising a solar absorber layer
formed of a material selected from the group consisting of a Cu(In,
Ga, Al)Se.sub.2 compound, a Cu(In, Ga, Al)S.sub.2 compound,
hydrogenated amorphous Silicon, hydrogenated amorphous Silicon
Germanium alloy, a (Cd, Zn, Mg, Mn)Te compound, and Cadmium
Selenide.
48. The photovoltaic cell of claim 41, the additional
single-junction photovoltaic cell comprising a solar absorber layer
formed of p-type Cadmium Selenide.
49. A method of making a p-type Cadmium Selenide semiconductor
material, comprising: depositing a layer of Cadmium Selenide;
coating the layer of Cadmium Selenide with a solution comprising a
solvent and at least one of chloride selected from the group
consisting of chlorides of Group IA elements, chlorides of group IB
elements, and chlorides of Group IIIB elements; and heating the
layer of Cadmium Selenide in an environment having an ambient
temperature of between 300 and 500 degrees Celsius for a time
between three and thirty minutes while at least partially
preventing the evaporation of Selenium from the layer of Cadmium
Selenide.
50. The method of claim 49, wherein Selenium is at least partially
prevented from evaporating from the layer of Cadmium Selenide
during the step of heating by executing the step of heating in a
Selenium enriched atmosphere.
51. The method of claim 49, wherein Selenium is at least partially
prevented from evaporating from the layer of Cadmium Selenide
during the step of heating by physically impeding the evaporation
of Selenium from the layer of Cadmium Selenide.
52. The method of claim 49, wherein the solution comprises Copper
Chloride and a solvent.
53. The method of claim 49, wherein the solution comprises Gallium
Chloride and a solvent.
54. The method of claim 49, wherein the solution comprises Copper
Chloride, Gallium Chloride, and a solvent.
55. The method of claim 49, wherein the solution further comprises
Cadmium Chloride.
56. A method of making a photovoltaic device, comprising:
depositing a contact layer; depositing a layer of Cadmium Selenide;
coating the layer of Cadmium Selenide with a solution comprising a
solvent and at least one of chloride selected from the group
consisting of chlorides of Group IA elements, chlorides of group IB
elements, and chlorides of Group IIIB elements; heating the layer
of Cadmium Selenide in an environment having an ambient temperature
of between 300 and 500 degrees Celsius for a time between three and
thirty minutes while at least partially preventing the evaporation
of Selenium from the layer of Cadmium Selenide; depositing a
heterojunction partner layer; and depositing a transparent
conductor layer.
57. The method of claim 56, wherein Selenium is at least partially
prevented from evaporating from the layer of Cadmium Selenide
during the step of heating by executing the step of heating in a
Selenium enriched atmosphere.
58. The method of claim 56, wherein Selenium is at least partially
prevented from evaporating from the layer of Cadmium Selenide
during the step of heating by physically impeding the evaporation
of Selenium from the layer of Cadmium Selenide.
59. The method of claim 56, wherein the solution comprises Copper
Chloride and a solvent.
60. The method of claim 56, wherein the solution comprises Gallium
Chloride and a solvent.
61. The method of claim 56, wherein the solution comprises Copper
Chloride, Gallium Chloride, and a solvent.
62. The method of claim 56, wherein the solution further comprises
Cadmium Chloride.
63. The method of claim 56, wherein the heterojunction partner
layer is a material selected from the group consisting of Zinc
Selenide, Cadmium Sulfide, Cadmium Zinc Selenide, Cadmium Selenide,
Zinc Sulfide, Cadmium Oxide, Zinc Oxide, Zinc Magnesium Oxide, Tin
Oxide, and Cadmium Zinc Sulfide.
64. The method of claim 56, further comprising depositing an
undoped buffer layer between the heterojunction partner layer and
the transparent conductor layer.
65. The method of claim 56, wherein the contact layer is a
bilayer.
66. The method of claim 65, wherein the contact layer comprises a
layer of doped ZnTe and a layer of a material selected from the
group consisting of a metal, and a transparent conducting
oxide.
67. A method of making a photovoltaic device, comprising:
depositing a transparent conductor layer; depositing a
heterojunction partner layer; depositing a layer of Cadmium
Selenide; coating the layer of Cadmium Selenide with a solution
comprising a solvent and at least one of chloride selected from the
group consisting of chlorides of Group IA elements, chlorides of
group IB elements, and chlorides of Group IIIB elements; heating
the layer of Cadmium Selenide in an environment having an ambient
temperature of between 300 and 500 degrees Celsius for a time
between three and thirty minutes while at least partially
preventing the evaporation of Selenium from the layer of Cadmium
Selenide; and depositing a contact layer.
68. The method of claim 67, wherein Selenium is at least partially
prevented from evaporating from the layer of Cadmium Selenide
during the step of heating by executing the step of heating in a
Selenium enriched atmosphere.
69. The method of claim 67, wherein Selenium is at least partially
prevented from evaporating from the layer of Cadmium Selenide
during the step of heating by physically impeding the evaporation
of Selenium from the layer of Cadmium Selenide.
70. The method of claim 67, wherein the solution comprises Copper
Chloride and a solvent.
71. The method of claim 67, wherein the solution comprises Gallium
Chloride and a solvent.
72. The method of claim 67, wherein the solution comprises Copper
Chloride, Gallium Chloride, and a solvent.
73. The method of claim 67, wherein the solution further comprises
Chloride.
74. The method of claim 67, wherein the heterojunction partner
layer is a layer of a material selected from the group consisting
of Zinc Selenide, Cadmium Sulfide, Cadmium Zinc Selenide, Cadmium
Selenide, Zinc Sulfide, Cadmium Oxide, Zinc Oxide, Zinc Magnesium
Oxide, Tin Oxide, and Cadmium Zinc Sulfide.
75. The method of claim 67, further comprising depositing an
undoped buffer layer between the transparent conductor layer and
the heterojunction partner layer.
76. The method of claim 67, wherein the contact layer is a
bilayer.
77. The method of claim 76, wherein the contact layer comprises a
layer of doped ZnTe and a layer of a material selected from the
group consisting of a metal layer, and a transparent conducting
oxide layer.
78. A process for forming a hybrid multi-junction photovoltaic
cell, comprising: forming a first single-junction photovoltaic cell
on a substrate, including the steps of: forming a first back
contact layer on the substrate, forming a first solar absorber
layer on the back contact layer, the first solar absorber layer
being formed of a low bandgap Group IB-IIIB-VIB.sub.2 material
having bulk p-type character, forming a first heterojunction
partner layer on the first solar absorber layer, the first
heterojunction partner layer comprising at least one layer of a
high resistivity material having a resistivity of at least 100
ohms-centimeter, the high resistivity material being a material
having the formula (Zn and/or Mg)(S, Se, O, and/or OH); forming a
conductive interconnect layer above the first heterojunction
partner layer of the first single-junction photovoltaic cell; and
forming at least one additional single-junction photovoltaic cell
above the conductive interconnect layer.
79. The process of claim 78, wherein the at least one layer of high
resistivity material of the heterojunction partner layer has a
resistivity of at least 1,000 ohms-centimeter.
80. The process of claim 78, the low bandgap Group
IB-IIIB-VIB.sub.2 material being selected from the group consisting
of Copper Indium DiSelenide, Copper Indium DiTelluride, an alloy
formed of Copper Indium DiSelenide and at least one of Gallium,
Aluminum, Tellurium and Sulfur.
81. The process of claim 78, the step of forming the first
heterojunction partner layer including one of performing a chemical
vapor deposition process, a wet chemical bath deposition process,
and a low energy sputtering process.
82. The process of claim 81, the first heterojunction partner layer
being formed using a chemical vapor deposition process.
83. The process of claim 78, the step of forming the first solar
absorber layer including performing at least of one a selenization
process, a sulfurization process, a tellurization process, a
thermal evaporation process, an electron beam evaporation process,
a sputtering process, an electrodeposition process, a molecular
beam epitaxy process, and a chemical vapor deposition process.
84. The process of claim 78, the step of forming at least one
additional single-junction photovoltaic cell comprising: forming a
second back contact layer on the conductive interconnect layer, and
forming a second solar absorber layer on the second back contact
layer, the second solar absorber layer having a higher bandgap
energy than the first solar absorber layer.
85. The process of claim 84, the second solar absorber layer being
formed of a material selected from the group consisting of a Cu(In,
Ga, Al)Se.sub.2 compound, a Cu(In, Ga, Al)S.sub.2 compound,
hydrogenated amorphous Silicon, hydrogenated amorphous Silicon
Germanium alloy, a (Cd, Zn, Mg, Mn)Te compound, and Cadmium
Selenide.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of priority of U.S.
Provisional Patent Application Ser. No. 60/950,087 filed 16 Jul.
2007, U.S. Provisional Patent Application Ser. No. 60/956,107 filed
15 Aug. 2007, and U.S. Provisional Patent Application Ser. No.
61/031,652 filed 26 Feb. 2008. All of the aforementioned
applications are hereby incorporated herein by reference.
BACKGROUND
[0003] Photovoltaic cells convert electromagnetic energy (e.g.,
sunlight) into an electric current; accordingly, they are commonly
used to provide electric power in a diverse range of applications.
For example, photovoltaic cells may be incorporated in and provide
electric power for devices as diverse as hand-held calculators and
space vehicles.
[0004] Photovoltaic cells having a variety of characteristics have
been developed. One class of photovoltaic cells that is currently
the subject of significant research is the thin-film class.
Thin-film photovoltaic cells include a plurality of layers of thin
films formed on a substrate.
[0005] As mentioned above, photovoltaic cells (e.g., thin-film
photovoltaic cells) may be operated from sunlight. Sunlight
generally includes a plurality of colors of light, including light
in the infrared and near ultraviolet bands, where each color of
light has a different energy. For example, blue light has greater
energy per photon than red light.
[0006] It is known that absorption of light in a photovoltaic cell
typically requires that each photon have energy greater than the
bandgap energy of the absorbing material, such that carriers can be
excited into the free or nearly free carrier bands (conduction and
valence) in the absorbing material. These carriers then move to
appropriate sides of the junction, providing an output voltage and
current. Absorbed photon energy in excess of this bandgap energy is
usually wasted in that additional carriers are not always
generated. Semiconductor materials that have electrons as the
dominant free carriers are defined as n-type materials, and
materials that have holes, or the absence of an electron, as the
dominant free carriers are defined as p-type materials.
[0007] The maximum output voltage of the cell is related to the
bandgap of the materials used to form the junction. For example,
higher bandgap materials can provide higher output voltages. As a
result, photovoltaic cells optimized to absorb long wavelength, or
low energy, photons will also absorb short-wavelength photons, but
will waste much energy by producing a lower output voltage than
attainable with cells optimized to absorb the short wavelength,
high energy, photons.
[0008] A photovoltaic cell optimized to absorb short wavelength,
e.g. blue, light will fail to efficiently absorb long wavelength,
or red and infrared, light because the bandgap of the material is
greater than the available energy in each photon, and therefore,
photons are not absorbed and carriers are not promoted into the
conduction.
SUMMARY
[0009] In an embodiment, a photovoltaic cell includes a transparent
conductor layer, a first heterojunction partner layer, a p-type
Cadmium Selenide layer in contact with the first heterojunction
partner layer, and a first electrical contact layer.
[0010] In an embodiment, a hybrid multi-junction photovoltaic cell
includes a substrate for providing mechanical support for the
photovoltaic cell. The substrate is a polymer substrate. A back
contact layer is formed on the substrate, and a bottom solar
absorber layer is formed on the back contact layer. The bottom
solar absorber layer includes one of the following materials: a low
bandgap Group IB-IIIB-VIB.sub.2 material having bulk p-type
character, and an alloy of a low bandgap Group IB-IIIB-VIB.sub.2
material having bulk p-type character. A heterojunction partner
layer is formed on the bottom solar absorber layer, and a layer of
p-type semiconductor is formed above the heterojunction partner
layer. A layer of intrinsic semiconductor is formed on the layer of
p-type semiconductor, and a layer of n-type semiconductor is formed
on the layer of intrinsic semiconductor.
[0011] In an embodiment, a module of a plurality of hybrid
multi-junction photovoltaic cells includes a substrate for
providing mechanical support for the photovoltaic cells and a back
contact layer formed on the substrate. A bottom solar absorber
layer is formed on the back contact layer, and the bottom solar
absorber layer includes one of the following materials: a low
bandgap Group IB-IIIB-VIB.sub.2 material having bulk p-type
character, and an alloy of a low bandgap Group IB-IIIB-VIB.sub.2
material having bulk p-type character. An n-type heterojunction
partner layer is formed on the bottom solar absorber layer, and a
contact layer of p-type semiconductor is formed above the
heterojunction partner layer. A primary solar absorber layer is
formed on the contact layer of p-type semiconductor, and the
primary solar absorber layer is formed of an intrinsic
semiconductor or a p-type semiconductor. A layer of n-type
semiconductor is formed on the primary solar absorber layer, and a
top contact layer is formed on the layer of n-type semiconductor.
At least one first isolating scribe extends at least from a top
surface of the layer of n-type semiconductor to a top surface of
the substrate, and the first isolating scribe is filled with an
insulating material. At least one connecting scribe extends at
least from the top surface of the layer of n-type semiconductor to
a top surface of the back contact layer, and the connecting scribe
is filled with a conductive material. At least one second isolating
scribe extends at least from a top surface of the top contact layer
to a top surface of the bottom solar absorber layer.
[0012] In an embodiment, a multi-junction photovoltaic cell
includes a substrate for providing mechanical support for the
photovoltaic cell and a back contact layer formed on the substrate.
A solar absorber layer is formed on the back contact layer, and the
solar absorber layer is formed of a low bandgap Group
IB-IIIB-VIB.sub.2 material having bulk p-type character. A
heterojunction partner layer is formed on the solar absorber layer,
and the heterojunction partner layer includes at least one layer of
a high resistivity material having a resistivity of at least 100
ohms-centimeter. The high resistivity material has the formula (Zn
and/or Mg)(S, Se, O, and/or OH). A conductive interconnect layer is
formed above the heterojunction partner layer, and at least one
additional single-junction photovoltaic cell is formed on the
conductive interconnect layer.
[0013] In an embodiment, a method of making a p-type Cadmium
Selenide semiconductor material includes depositing a layer of
Cadmium Selenide and coating the layer of Cadmium Selenide with a
solution. The solution includes a solvent and at least one of the
following chlorides: chlorides of Group IA elements, chlorides of
group IB elements, and chlorides of Group IIIB elements. The coated
layer of Cadmium Selenide is heated in an environment having an
ambient temperature of between 300 and 500 degrees Celsius for a
time between three and thirty minutes while at least partially
preventing the evaporation of Selenium from the coated layer of
Cadmium Selenide.
[0014] In an embodiment, a method of making a photovoltaic device
includes depositing a contact layer, depositing a layer of Cadmium
Selenide, and coating the layer of Cadmium Selenide with a
solution. The solution includes a solvent and at least one of the
following chlorides: chlorides of Group IA elements, chlorides of
group IB elements, and chlorides of Group IIIB elements. The coated
layer of Cadmium Selenide is heated in an environment having an
ambient temperature of between 300 and 500 degrees Celsius for a
time between three and thirty minutes while at least partially
preventing the evaporation of Selenium from the coated layer of
Cadmium Selenide. A heterojunction partner layer is deposited, and
a transparent conductor layer is deposited.
[0015] In an embodiment, a method of making a photovoltaic device
includes depositing a transparent conductor layer, depositing a
heterojunction partner layer, and depositing a layer of Cadmium
Selenide. The layer of Cadmium Selenide is coated with a solution
including a solvent and at least one of the following chlorides:
chlorides of Group IA elements, chlorides of group IB elements, and
chlorides of Group IIIB elements. The coated layer of Cadmium
Selenide is heated in an environment having an ambient temperature
of between 300 and 500 degrees Celsius for a time between three and
thirty minutes while at least partially preventing the evaporation
of Selenium from the coated layer of Cadmium Selenide. A contact
layer is deposited.
[0016] In an embodiment, a process for forming a hybrid
multi-junction photovoltaic cell includes forming a first
single-junction photovoltaic cell on a substrate. The step of
forming the first single-junction photovolatic cell includes: (1)
forming a first back contact layer on the substrate, (2) forming a
first solar absorber layer on the back contact layer, the first
solar absorber layer being formed of a low bandgap Group
IB-IIIB-VIB.sub.2 material having bulk p-type character, and (3)
forming a first heterojunction partner layer on the first solar
absorber layer, the first heterojunction partner layer including at
least one layer of a high resistivity material having a resistivity
of at least 100 ohms-centimeter, the high resistivity material
being a material having the formula (Zn and/or Mg)(S, Se, O, and/or
OH). A conductive interconnect layer is formed above the first
heterojunction partner layer of the first single-junction
photovoltaic cell. At least one additional single-junction
photovoltaic cell is formed above the conductive interconnect
layer.
BRIEF DESCRIPTION OF DRAWINGS
[0017] FIG. 1 is a top perspective view of one module of
photovoltaic cells, in accordance with one embodiment.
[0018] FIG. 2 is a cross sectional view of one photovoltaic cell,
in accordance with one embodiment.
[0019] FIG. 3 is a cross sectional view of one two-junction
photovoltaic cell, in accordance with one embodiment.
[0020] FIG. 4 is a cross sectional view of one three-junction
photovoltaic cell, in accordance with one embodiment.
[0021] FIG. 5 is a graph of idealized spectral response versus
wavelength of electromagnetic energy of an embodiment of the
photovoltaic cell of FIG. 3.
[0022] FIG. 6 is a cross sectional view of one multi-junction
photovoltaic cell, in accordance with one embodiment.
[0023] FIG. 7 is a cross sectional view of one multi-junction
photovoltaic cell, in accordance with one embodiment.
[0024] FIG. 8 is a cross sectional view of one multi-junction
photovoltaic cell, in accordance with one embodiment.
[0025] FIG. 9 is a cross sectional view of a plurality of
multi-junction photovoltaic cells monolithically integrated onto a
common substrate, in accordance with one embodiment.
[0026] FIG. 10 shows one method of converting n-type Cadmium
Selenide to p-type Cadmium Selenide, in accordance with one
embodiment.
[0027] FIG. 11 is a cross sectional view of one two-junction
photovoltaic cell, in accordance with one embodiment.
[0028] FIG. 12 is a cross sectional view of one two-junction
photovoltaic cell, in accordance with one embodiment.
[0029] FIGS. 13 and 14 show one method of fabricating the
photovoltaic cell of FIG. 11, in accordance with one
embodiment.
[0030] FIGS. 15 and 16 show one method of fabricating the
photovoltaic cell of FIG. 12, in accordance with one
embodiment.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0031] It is noted that, for purposes of illustrative clarity,
certain elements in the drawings may not be drawn to scale.
Specific instances of an item may be referred to by use of a
numeral in parentheses (e.g., photovoltaic cell 108(1)) while
numerals without parentheses refer to any such item (e.g.,
photovoltaic cells 108).
[0032] FIG. 1 is a top perspective view of one module 100 of
photovoltaic cells 108. Although module 100 is illustrated as
having six photovoltaic cells 108, module 100 may have any quantity
of photovoltaic cells. Furthermore, although photovoltaic cells 108
are illustrated in FIG. 1 as having a rectangular shape,
photovoltaic cells 108 can have other shapes (e.g., a square or
circle). Photovoltaic cells 108 are operable to convert
electromagnetic energy 102 (e.g., sunlight) into an electric
current. If module 100 includes several photovoltaic cells 108, the
cells may be electrically connected in series to increase the
voltage of module 100, in parallel to increase the maximum current
capability of module 100, or in combinations of parallel and series
to increase both the voltage and maximum current capability of
module 100. The electric current generated by photovoltaic cells
108 is accessible via electrical terminals 104 and 106.
[0033] Photovoltaic cells 108 are thin-film photovoltaic cells;
that is, photovoltaic cells 108 are formed of a plurality of
thin-film layers on a substrate. In embodiments of module 100,
several photovoltaic cells 108 are formed on a single common
substrate; in such cases, the several photovoltaic cells 108 are
considered to be "monolithically integrated" on the substrate.
[0034] FIG. 2 is a cross sectional view of one photovoltaic cell
200. Photovoltaic cell 200 is an embodiment of photovoltaic cell
108 of FIG. 1. Photovoltaic cell 200 as illustrated in FIG. 2 is a
single-junction photovoltaic cell; however, photovoltaic cell 200
may be combined with one or more additional single-junction
photovoltaic cells to form a multi-junction photovoltaic cell, as
discussed below. Furthermore, several photovoltaic cells 200 may be
monolithically integrated onto substrate 202. Photovoltaic cell 200
absorbs electromagnetic energy 102 incident on its top surface
214.
[0035] Substrate 202 forms a base and provides mechanical support
for photovoltaic cell 200. Substrate 202 affects characteristics of
photovoltaic cell 200; accordingly, substrate 200 is chosen in
accordance with the required characteristics of photovoltaic cell
200. For example, if photovoltaic cell 200 is to be flexible,
substrate 202 must be flexible. As another example, if photovoltaic
cell 200 is not to be electrically shorted to adjacent cells on the
same substrate, substrate 202 should have an insulating surface.
Further, if photovoltaic cell 200 is to be able to withstand high
processing temperatures, substrate 202 must be able to withstand
such high processing temperatures. Examples of materials that may
be used to form substrate 202 include one of a polymer material
(e.g., polyimide), a reinforced polymer material (e.g. carbon-fiber
or glass-fiber reinforced polymer), metal foil, insulated metal
foil (e.g., polymer coated metal foil), and glass.
[0036] Back contact layer 204 is formed on substrate 202. Back
contact layer 204 provides electrical interface to photovoltaic
cell 200. Back contact layer 204 is formed of a material that is
compatible with (e.g., forms a good contact with) solar absorber
layer 206; in an embodiment, back contact layer 204 includes
molybdenum.
[0037] Solar absorber layer 206 is formed on back contact layer
204; solar absorber layer 206 is formed of a material having bulk
p-type character. The term bulk p-type character means that at
least a majority of the material is p-type. However, portions
(e.g., a surface or a near surface, or doped portions) of a
material having a bulk p-type character may not be p-type. For
example, a surface of a material having a bulk p-type character may
be n-type.
[0038] Solar absorber layer 206 has a photovoltaic effect such that
solar absorber layer 206 is operable to at least partially absorb
electromagnetic energy 102 having a certain range of wavelengths
and create corresponding electron-hole pairs. In an embodiment,
solar absorber layer 206 is formed of a Group IB-IIIB-VIB.sub.2
material having bulk p-type character. The term Group
IB-IIIB-VIB.sub.2 material refers to a compound having a
photovoltaic effect that is formed of at least one element from
each of Groups IB, IIIB, and VIB of the periodic table, wherein
there are typically two atoms of the Group VIB element for every
one atom of the Group IB and IIIB elements. In the context of this
disclosure, Group IB elements include Copper, Silver, and Gold;
Group IIIB elements include Boron, Aluminum, Gallium, Indium, and
Thallium; and Group VIB elements include Oxygen, Sulfur, Selenium,
and Tellurium. Examples of materials that may be used to form solar
absorber layer 206 therefore include Copper Indium DiSelenide,
Copper Indium DiTelluride, an alloy formed of Copper Indium
DiSelenide and at least one of Gallium, Aluminum, and Sulfur (e.g.,
Copper Indium Gallium DiSelenide), and an alloy formed of Copper
Indium DiTelluride and at least one of Gallium, Aluminum, and
Sulfur. In an embodiment, solar absorber layer 206 is formed of a
low bandgap p-type Group IB-IIIB-VIB.sub.2 material. A low bandgap
material for purposes of this disclosure and the corresponding
claims is a material that is tuned to absorb electromagnetic energy
in the red and/or infrared portions of the electromagnetic
spectrum. An embodiment of photovoltaic cell 200 having solar
absorber layer 206 formed of a low bandgap material may be useful
if photovoltaic cell 200 is combined with another single-junction
photovoltaic cell tuned to absorb electromagnetic energy of a
different portion of the electromagnetic spectrum (e.g.,
ultraviolet or blue light). In an embodiment, solar absorber layer
206 is formed using at least one of a sequential selenization
process, a sulfurization process, a tellurization process, a
thermal evaporation process, an electron beam evaporation process,
a sputtering process, an electrodeposition process, a molecular
beam epitaxy process, and a chemical vapor deposition process.
[0039] As another example, solar absorber layer 206 may be formed
of Cadmium Selenide having a bulk p-type character ("p-type Cadmium
Selenide"). Such p-type Cadmium Selenide may be obtained by
converting n-type Cadmium Selenide to p-type as discussed below
with respect to FIG. 10. For purposes of this disclosure and the
corresponding claims, Cadmium Selenide is a material that is tuned
to absorb electromagnetic energy in the red/yellow to blue and
ultraviolet portions of the electromagnetic spectrum. An embodiment
of photovoltaic cell 200 having solar absorber layer 206 formed of
a Cadmium Selenide material may be useful if photovoltaic cell 200
is combined with another single-junction photovoltaic cell tuned to
absorb electromagnetic energy of a different portion of the
electromagnetic spectrum (e.g., red and infrared light).
[0040] Window layer or heterojunction partner layer 208 is formed
on solar absorber layer 206. Heterojunction partner layer 208, for
example, forms a heterojunction with the material of solar absorber
layer 206 having bulk p-type character. Heterojunction partner
layer 208 is, for example, formed of a high resistivity material
having the formula (Zn and/or Mg)(S, Se, O, and/or OH) where the
material may be formed of any combination of Zn and/or Mg, and one
or more of S, Se, O, or OH. In an embodiment, heterojunction
partner layer 208 is formed of Zinc Oxide. In another embodiment,
heterojunction partner layer 208 is formed of a Zinc Magnesium
Oxide alloy. Forming heterojunction partner layer 208 of such
materials may advantageously help photovoltaic cell 200 to
withstand higher processing temperatures than some other
heterojunction partner layer materials, as discussed below. In the
context of this disclosure and corresponding claims, the term high
resistivity means a resistivity of at least 100 ohms-centimeter. In
embodiments, heterojunction partner layer 208 has a resistivity of
at least 1,000 ohms-centimeter. Forming heterojunction partner
layer 208 of a high resistivity Zn compound (e.g., Zinc Oxide, Zinc
Magnesium Oxide) may advantageously improve the high-temperature
durability of photovoltaic cell 200 over a photovoltaic cell with a
Zn compound having a lower resistivity. In an embodiment,
heterojunction partner layer 208 is preferably formed using a
chemical vapor deposition process (e.g., metal-organic chemical
vapor deposition) such that a top surface of solar absorber layer
206 is not damaged by the deposition process.
[0041] As another example, heterojunction partner layer 208 may be
formed of Cadmium Sulfide, such as when solar absorber layer 206 is
formed of p-type Cadmium Selenide.
[0042] Photovoltaic cell 200 optionally includes buffer layer 210
formed on heterojunction partner layer 208; an exemplary material
that may be used to form buffer layer 210 is undoped Zinc Oxide.
Conductive top contact layer or interconnect layer 212 is formed on
buffer layer 210, if present, or is formed on heterojunction
partner layer 208 if buffer layer 210 is not present. Layer 212 is
referred to as a top contact layer if photovoltaic cell 200 is not
combined with an additional photovoltaic cell. Layer 212 is
referred to as an interconnect layer if photovoltaic cell 200 is
combined with one or more additional single-junction photovoltaic
cells 200 to form a multi-junction photovoltaic cell; in such case
interconnect layer 212 provides electrical connection between
photovoltaic cell 200 and an additional single-junction
photovoltaic cell formed on top surface 214 of photovoltaic cell
200. Top contact/interconnect layer 212 is electrically conductive
and is at least partially transparent to electromagnetic energy 102
in order to allow electromagnetic energy 102 to reach solar
absorber layer 206. Exemplary materials that may be used to form
top contact/interconnect layer 212 include doped Zinc Oxide,
undoped Zinc Oxide, Indium Tin Oxide, doped Tin Oxide, undoped Tin
Oxide, n-type amorphous Silicon, n-type amorphous Silicon
Germanium, hydrogenated amorphous Silicon carbide, and n-type
microcrystalline amorphous Silicon.
[0043] One embodiment of photovoltaic cell 200 has the following
configuration: substrate 202 formed of glass, back contact layer
204 formed of molybdenum, solar absorber layer 206 formed of Copper
Indium DiSelenide, heterojunction partner layer 208 formed of Zinc
Oxide, buffer layer 210 formed of Zinc Oxide, and top
contact/interconnect layer 212 formed of Indium Tin Oxide.
[0044] As stated above, in embodiments, heterojunction partner
layer 208 is formed of a high resistivity material having the
formula (Zn and/or Mg)(S, Se, O, and/or OH). This heterojunction
partner layer construction may result in photovoltaic cell 200
having a greater ability to withstand heat and/or vacuum stresses
that some other thin-film photovoltaic cells. For example, in a
thin-film photovoltaic cell including a solar absorber layer formed
Copper Indium DiSelenide or Copper Indium DiTelluride and a
heterojunction partner layer (window layer) formed of Cadmium
Sulfide, heat and/or vacuum stresses may result in excessive
cross-diffusion of constituent elements of the solar absorber layer
and the heterojunction partner layer across their junction. This
cross-diffusion causes degradation of the junction and overall
performance of the photovoltaic cell; accordingly, such
photovoltaic cell has a relatively low ability to withstand heat
and vacuum stresses. However, if heterojunction partner layer 208
is formed of a high resistivity material having the formula (Zn
and/or Mg)(S, Se, O, and/or OH), junction degradation may be
reduced or eliminated.
[0045] Forming heterojunction partner layer 208 of a high
resistivity material having the formula (Zn and/or Mg)(S, Se, O,
and/or OH) also may allow photovoltaic cell 200 to have a greater
ability to operate from electromagnetic energy including little or
no blue colored light compared to some other thin-film photovoltaic
cells. For example, a thin-film photovoltaic cell including a solar
absorber layer formed from Copper Indium DiSelenide or Copper
Indium DiTelluride and a heterojunction partner layer formed of
Cadmium Sulfide generally performs better when the electromagnetic
energy incident upon the window layer contains at least some blue
colored light. This requirement is due to defects in the Cadmium
Sulfide. However, this problem can be avoided by forming
heterojunction partner layer 208 of a high resistivity material
having the formula (Zn and/or Mg)(S, Se, O, and/or OH).
[0046] Forming an additional photovoltaic cell on a first
photovoltaic cell commonly exposes the first photovoltaic cell to
significant heat and/or vacuum stresses. Additionally, one or more
additional photovoltaic cells formed on the first photovoltaic cell
often blocks transmission of blue electromagnetic energy to the
first photovoltaic cell. Accordingly, photovoltaic cell 200 may be
an attractive photovoltaic cell for use as a bottom photovoltaic
cell in a multi-junction photovoltaic cell due to photovoltaic cell
200's relatively low sensitivity to heat and vacuum stresses and
ability to reliably operate from electromagnetic energy devoid of
blue light.
[0047] A multi-junction photovoltaic cell includes two or more
single-junction photovoltaic cells optically aligned with each
other, and each of the single-junction photovoltaic cells is tuned
to respond to (i.e., absorb) electromagnetic energy having a
different range of wavelengths. For example, FIG. 3 is a cross
sectional view of one two-junction photovoltaic cell 300, and FIG.
4 is a cross sectional view of one three-junction photovoltaic cell
400. Two or more of the single-junction photovoltaic cells in a
multi-junction photovoltaic cell may have different solar absorber
layer materials; in such case, the multi-junction photovoltaic cell
may be referred to as a hybrid photovoltaic cell.
[0048] A two junction or tandem photovoltaic cell 300 includes a
second single-junction photovoltaic cell 304 formed on a first
single-junction photovoltaic cell 302. Each of first and second
photovoltaic cells 302 and 304 are designed to absorb portions of
electromagnetic energy 102 having different ranges of wavelengths.
FIG. 5, which is a graph 500 of idealized spectral response versus
wavelength, illustrates one possible design of two-junction
photovoltaic cell 300. A vertical axis 502 represents the magnitude
of the spectral response of a photovoltaic cell; the spectral
response essentially represents the photovoltaic cell's conversion
of electromagnetic energy into an electric current. A horizontal
axis 504 represents wavelength of electromagnetic energy. A curve
506 represents the spectral response of second photovoltaic cell
304 as a function of wavelength of electromagnetic energy, and a
curve 508 represents the spectral response of first photovoltaic
cell 302 as a function of wavelength of electromagnetic energy. As
can be observed from FIG. 5, in an embodiment of two-junction
photovoltaic cell 300 designed according to FIG. 5, second
photovoltaic cell 304 responds to electromagnetic energy having a
shorter wavelength than first photovoltaic cell 302. For example,
second photovoltaic cell 304 may respond to (that is convert to an
electric current), light generally having a blue color while first
photovoltaic cell 302 responds to light generally having a red
and/or infrared color. Thus, two-junction photovoltaic cell 300 is
operable to generate an electric current from electromagnetic
energy having two ranges of wavelengths as illustrated by curves
506 and 508.
[0049] In photovoltaic cell 300, electromagnetic energy 102
impinges a top surface 308 of photovoltaic cell 300; a portion of
electromagnetic energy 102 having a certain range of wavelengths is
absorbed by second photovoltaic cell 304 and is converted to an
electric current. A portion of electromagnetic energy 102 that is
not absorbed by second photovoltaic cell 304 passes to first
photovoltaic cell 302 as indicated by arrows 306.
[0050] Three-junction photovoltaic cell 400 includes a third
single-junction photovoltaic cell 406 formed on a second
single-junction photovoltaic cell 404 which is in turn formed on a
first single-junction photovoltaic cell 402. Each of first, second,
and third single junction photovoltaic cells 402, 404, and 406 is
designed to absorb portions of electromagnetic energy 102 having
different ranges of wavelengths. Electromagnetic energy 102
impinges a top surface 412 of photovoltaic cell 400; a portion of
electromagnetic energy 102 having a certain range of wavelengths is
absorbed by third photovoltaic cell 406 and converted to an
electric current. A portion of electromagnetic energy 102 that is
not absorbed by third photovoltaic cell 406 passes to second
photovoltaic cell 404 as indicated by arrows 410. Second
photovoltaic cell 404 absorbs a portion of a certain range of
wavelengths of electromagnetic energy 410, and the remainder of
electromagnetic energy 410 passes to first photovoltaic cell 402 as
indicated by arrows 408. A portion of electromagnetic energy 408
having a certain range of wavelengths is absorbed by first
photovoltaic cell 402.
[0051] A multi-junction photovoltaic cell may have advantages over
a single-junction photovoltaic cell. First, as discussed above with
respect to FIGS. 3-5, a multi-junction photovoltaic cell has a
plurality of single-junction photovoltaic cells each of which are
tuned to respond to electromagnetic energy having a certain range
of wavelengths. Accordingly, there may be a smaller difference
between the energy level of at least some of the electromagnetic
energy spectrum that creates `free` electron-hole pairs in a solar
absorber layer and the bandgap energy of the solar absorber layer
in a multi-junction photovoltaic cell than in a single-junction
photovoltaic cell. Therefore, a multi-junction photovoltaic cell
may have a smaller energy loss due to thermalization of free
carriers with excess energy above a solar absorber layer's bandgap
energy level than a single-junction photovoltaic cell. As a result,
a multi-junction photovoltaic cell may operate more efficiently
than a single-junction photovoltaic cell.
[0052] Second, a multi-junction photovoltaic cell generally has a
higher output voltage than a single-junction photovoltaic cell.
This may be advantageous because the higher output voltage reduces
the output current required to power a given load. Reducing the
output current magnitude reduces resistive losses which directly
increases the efficiency of the solar cell and module. Also, a
multi-junction photovoltaic cell generally has a lower temperature
coefficient than a single-junction photovoltaic cell, where the
temperature coefficient quantifies additional power loss in the
photovoltaic cell due to an increase in the photovoltaic cell's
operating temperature. A lower temperature coefficient is
beneficial because it reduces the power loss of the photovoltaic
cell at elevated operating temperatures, which commonly occur in
practical photovoltaic cell applications. Furthermore, increasing
the output voltage of a photovoltaic cell may be advantageous in
that it reduces the quantity of photovoltaic cells that must be
electrically connected in series in order to provide a required
output voltage. Reducing the quantity of series connected
photovoltaic cells may in turn reduce monolithic integration cost
and area of a module lost due to electrical interconnection of
photovoltaic cells.
[0053] FIG. 6 is a cross sectional view of one multi-junction
photovoltaic cell 600. Photovoltaic cell 600, which is an
embodiment of photovoltaic cell 108, includes a second
single-junction photovoltaic cell 602 formed on photovoltaic cell
200. Although photovoltaic cell 600 is illustrated in FIG. 6 as
being a two-junction photovoltaic cell, photovoltaic cell 600 may
have more than two junctions. For example, photovoltaic cell 600
could be a three-junction photovoltaic cell if a third
single-junction photovoltaic cell were formed on a top surface 606
of second photovoltaic cell 602. Multiple photovoltaic cells 600
may be monolithically integrated onto substrate 202; and one or
more substrates 202 with cells may be combined into a module or
solar panel.
[0054] As discussed above with respect to FIG. 2, photovoltaic cell
200 may optionally include a buffer layer (not shown in FIG. 6)
disposed between heterojunction partner layer 208 and interconnect
layer 212. In addition to providing an interface between
photovoltaic cell 200 and second photovoltaic cell 602 (FIG. 6), an
embodiment of interconnect layer 212 may reflect some
electromagnetic energy that passes through second photovoltaic cell
602 back into second photovoltaic cell 602 for absorption by it. As
discussed above, embodiments of photovoltaic cell 200 may withstand
higher temperatures and/or vacuum stresses better than some other
thin-film photovoltaic cells; accordingly, it is desirable that
photovoltaic cell 200 can withstand deposition of at least one
additional photovoltaic cell (e.g., second photovoltaic cell 602)
on top surface 608 of interconnect layer 212. Additionally, as
discussed above, certain embodiments of bottom photovoltaic cell
200 do not need to absorb blue colored light in order to operate
well. Accordingly, photovoltaic cell 200 may operate well even if
second photovoltaic cell 602 (and any additional single-junction
photovoltaic cells) absorbs some or all of the blue light of
electromagnetic energy 102.
[0055] Second photovoltaic cell 602 is tuned to be complimentary to
photovoltaic cell 200. For example, second photovoltaic cell 602 is
optimized to absorb a certain range of wavelengths of
electromagnetic energy that first photovoltaic cell 200 is not
optimized to absorb. Examples of second photovoltaic cells 602
include a solar absorber layer formed of a material including one
of a Cu(In, Ga, Al)Se.sub.2 compound, a Cu(In, Ga, Al)S.sub.2
compound, amorphous Silicon (e.g., hydrogenated amorphous Silicon),
a hydrogenated amorphous Silicon Germanium alloy, a (Cd, Zn, Mg,
Mn)Te compound, and p-type Cadmium Selenide. By a Cu (In, Ga, Al)
Se.sub.2 compound, a compound is meant that comprises Copper,
Selenium, and at least one of Indium, Gallium, or Aluminum. By a
Cu(In, Ga, Al)S.sub.2 compound, a compound is meant that comprises
Copper, Sulfur, and at least one of Indium, Gallium, or Aluminum.
By a (Cd, Zn, Mg, Mn)Te compound, a compound is meant that
comprises Tellurium and at least one of Cadmium, Zinc, Magnesium,
or Manganese.
[0056] FIG. 7 is a cross sectional view of one multi-junction
photovoltaic cell 700, which is an embodiment of photovoltaic cell
600. Photovoltaic cell 702 is an embodiment of photovoltaic cell
602. A plurality of photovoltaic cells 700 may be monolithically
integrated onto substrate 202.
[0057] Second photovoltaic cell 702 optionally includes a second
back contact layer 706 formed on interconnect layer 212. Second
back contract layer 706 is at least partially transparent to
electromagnetic energy having a certain range of wavelengths
intended to be absorbed by solar absorber layer 206 of photovoltaic
cell 200. If second back contact layer 706 is not included in
photovoltaic cell 700, interconnect layer 212 serves as a back
contact for photovoltaic cell 702.
[0058] Second solar absorber layer 708 is formed on second back
contact layer 706 (or an interconnect layer 212 if second back
contact layer 706 is not present). In an embodiment, solar absorber
layer 708 is formed of a material including one of a Cu(In, Ga,
Al)Se.sub.2 compound, a Cu(In, Ga, Al)S.sub.2 compound, amorphous
Silicon (e.g., hydrogenated Amorphous Silicon), a hydrogenated
amorphous Silicon Germanium alloy, Cadmium Telluride, a (Cd, Zn,
Mg, Mn)Te compound, and p-type Cadmium Selenide. Second solar
absorber layer 708 has a higher bandgap energy than solar absorber
layer 206 such that second solar absorber layer 708 absorbs
electromagnetic energy having shorter wavelengths than that
absorbed by solar absorber layer 206.
[0059] Second heterojunction partner layer 710 is formed on second
solar absorber layer 708. In an embodiment, second heterojunction
partner layer 710 is formed of Cadmium Sulfide and second solar
absorber layer 708 is formed of p-type Cadmium Selenide.
[0060] Top contact layer 712 is formed on second heterojunction
partner layer 710. A buffer layer (not shown in FIG. 7) is
optionally disposed between second heterojunction partner layer 710
and top contact layer 712. Top contact layer 712 and second
heterojunction partner layer 710 are each at least partially
transparent to wavelengths of electromagnetic energy 102 intended
to be absorbed by second solar absorber layer 708 and solar
absorber layer 206.
[0061] FIG. 8 is a cross sectional view of one multi-junction
photovoltaic cell 800, which is an embodiment of photovoltaic cell
108. Photovoltaic cell 800 is illustrated in FIG. 8 as including a
second single-junction photovoltaic cell 826 formed on a first
single-junction photovoltaic cell 824. Accordingly, photovoltaic
cell 800 is illustrated in FIG. 8 as being a two-junction or tandem
photovoltaic cell. However, photovoltaic cell 800 may include more
than two stacked single-junction photovoltaic cells and thereby
have more than two junctions. For example, photovoltaic cell 800
could be a three-junction photovoltaic cell if a third
single-junction photovoltaic cell were formed on a top surface 822
of photovoltaic cell 800. Photovoltaic cell 800 may be considered
to be a hybrid photovoltaic cell because at least two of its single
junction photovoltaic cells (e.g., photovoltaic cells 824 and 826)
have different materials in their solar absorber layers.
[0062] First single-junction photovoltaic cell 824 includes a
substrate 802, a back contact layer 804, a solar absorber layer
806, a heterojunction partner layer 808, an optional buffer layer
810, and an optional interconnect layer 812. Several photovoltaic
cells 800 may be monolithically integrated onto substrate 802.
Substrate 802 forms a base and provides mechanical support for
photovoltaic cell 800. Examples of materials that may be used to
form substrate 802 include polymers (e.g., polyimide), reinforced
polymers, insulated metal foil (e.g., polymer coated metal foil),
and glass. If substrate 802 is formed of a polymer or insulated
metal foil, photovoltaic cell 800 may advantageously be flexible
and lightweight compared to photovoltaic cells having a glass
substrate. Additionally, forming substrate 802 of a polymer or
insulated metal foil may advantageously allow photovoltaic cell 800
to support monolithic integration in contrast to photovoltaic cells
having an uninsulated metal substrate. It should be noted, however,
that substrate 802 could be formed of metal foil if photovoltaic
cell 800 will not be used in monolithic integration
applications.
[0063] Back contact layer 804 is formed on substrate 802. Back
contact layer 804 provides an electrical interface to photovoltaic
cell 800. Back contact layer 804 is formed of a material that is
compatible with (e.g., forms a good contact with) solar absorber
layer 806; in an embodiment, back contact layer 804 includes
molybdenum.
[0064] Solar absorber layer 806 is formed on back contact layer
804; solar absorber layer 806 is formed of a material having a bulk
p-type character and a photovoltaic effect such that solar absorber
layer 806 is operable to at least partially absorb electromagnetic
energy 102 and create corresponding electron-hole pairs. In
particular, solar absorber layer 806 is formed of a Group
IB-IIIB-VIB.sub.2 material having bulk p-type character or its
alloys. The Group IB-IIIB-VIB.sub.2 material or its alloys may be a
low bandgap material such that solar absorber layer 806 responds to
relatively long wavelengths of electromagnetic energy (e.g., red
and/or infrared light). Examples of materials that may be used to
form solar absorber layer 806 include Copper Indium DiSelenide,
Copper Indium DiTelluride, an alloy formed of Copper Indium
DiSelenide and at least one of Gallium, Aluminum, and Sulfur, and
an alloy formed of Copper Indium DiTelluride and at least one of
Gallium, Aluminum, and Sulfur.
[0065] Heterojunction partner layer 808 is formed on solar absorber
layer 806. Heterojunction partner layer 808 may form a
heterojunction with the material of solar absorber layer 806 having
bulk p-type character. In an embodiment, Heterojunction partner
layer 808 is formed of n-type material (e.g., Cadmium Sulfide). In
another embodiment, heterojunction partner layer 808 is similar to
heterojunction partner layer 208 of photovoltaic cell 200 of FIG. 2
in that heterojunction partner layer 808 is formed of a high
resistivity material having the formula (Zn and/or Mg)(S, Se, O,
and/or OH).
[0066] Photovoltaic cell 800 optionally includes buffer layer 810;
an exemplary material that may be used to form buffer layer 810 is
undoped or high-resistivity ZnO. Conductive interconnect layer 812
is optionally formed on buffer layer 810, if present, or on
heterojunction partner layer 808 if buffer layer 810 is not
present. Conductive interconnect layer 812 is required if
heterojunction partner layer 808 is formed of a high resistivity
material; however, if heterojunction partner layer 808 is formed of
a conductive n-type material, interconnect layer 812 is not
included in an embodiment of photovoltaic cell 800. Interconnect
layer 812 functions as an electrical and an optical interface
between first photovoltaic cell 824 and second photovoltaic cell
826. If interconnect layer 812 is not used, second photovoltaic
cell 826 is formed directly on first photovoltaic cell 824.
Interconnect layer 812 may include a doped Zinc Oxide, undoped Zinc
Oxide, Indium Tin Oxide, a doped Tin Oxide, undoped Tin Oxide,
n-type amorphous Silicon, n-type amorphous Silicon Germanium,
hydrogenated amorphous Silicon Carbide, and n-type microcrystalline
Silicon. Interconnect layer 812 is at least partially transparent
to electromagnetic energy intended to be absorbed by first
photovoltaic cell 824. In an embodiment of photovoltaic cell 800,
interconnect layer 812 serves to reflect some electromagnetic
energy that passes through second photovoltaic cell 826 back into
second photovoltaic cell 826 for absorption by it.
[0067] Second photovoltaic cell 826 includes layers 814, 816, and
818 of semiconductor material (e.g., amorphous Silicon) and an
optional top contact layer 820. In particular, a layer 814 of
p-type semiconductor (e.g., p-type amorphous Silicon) is formed on
interconnect layer 812 (if present), or on a top surface (e.g.,
surfaces 828 or 830) of first photovoltaic cell 824 if interconnect
layer 812 is not present. A layer 816 of intrinsic semiconductor
(e.g., intrinsic amorphous Silicon) is formed on layer 814 of
p-type semiconductor, and a layer 818 of n-type semiconductor
(e.g., n-type amorphous Silicon) is formed on the layer 816 of
intrinsic semiconductor. In an embodiment, layers 814, 816, and 818
are formed of hydrogenated amorphous Silicon. In another
embodiment, layers 814 and 818 are formed of hydrogenated amorphous
Silicon, and layer 816 is formed of hydrogenated amorphous Silicon
Germanium. Layers 814 and 818 may also be formed of
microcrystalline Silicon in an embodiment. In another embodiment,
layers 814 and 818 are each formed from one of hydrogenated
amorphous Silicon Germanium, hydrogenated amorphous Silicon
Carbide, and nanocrystalline Silicon. In embodiments of
photovoltaic cell 800, layer 816 has a graded composition or is
formed of several discrete sublayers. Layer 816, for example, is
formed of a plurality of different materials, where each material
has a different Silicon germanium alloy composition. As another
example, layer 816 has a variable level of small crystalline
domains of specified sizes to achieve specified bandgaps. As yet
another example, layer 816 may be a graded bandgap layer of
intrinsic amorphous Silicon germanium. Such designs of layers of
intrinsic amorphous Silicon are discussed in the art (for example:
X. Liao et al, Proc. 31.sup.st IEEE PVSC, 2005, and V.
Suntharalingam et al, Proc. 1.sup.st WCPEC, 1994) and are used, for
example, to reduce efficiency losses at interfaces to layer
816.
[0068] Top contact layer 820 is optionally formed on layer 818 of
n-type semiconductor. If one or more additional photovoltaic cells
are formed on multi-junction photovoltaic cell 800, top contact
layer 820 need not be included. Top contact layer 820 is at least
partially transparent to electromagnetic energy 102 in order to
allow electromagnetic energy 102 to reach first and second
photovoltaic cells 824 and 826. Examples of materials that may be
used to form top contact layer 820 include one of doped Zinc Oxide,
Indium Tin Oxide, and doped Tin Oxide. A top surface 822 of top
contact layer 820 may be roughened to increase light trapping of
second photovoltaic cell 826, thereby increasing its current
generation and efficiency.
[0069] Embodiments of photovolatic cell 800 advantageously have a
relatively high efficiency despite being formed on a polyimide
substrate. A polyimide substrate generally cannot support high
processing temperatures required to fabricate a high efficiency
photovoltaic cell. However, even if embodiments of photovoltaic
cell 800 are fabricated at low temperatures, the multijunction
structure of photovoltaic cell 800 may enable the embodiments to
nevertheless obtain high efficiencies.
[0070] One embodiment of photovoltaic cell 800 has the following
configuation: substrate 802 formed of glass, back contact layer 804
formed of molybdenum, solar absorber layer 806 formed of Copper
Indium DiSelenide, heterojunction partner layer 808 formed of Zinc
Oxide, buffer layer 810 formed of Zinc Oxide, interconnect layer
812 formed of doped Zinc Oxide, layer 814 formed of p-type
amorphous Silicon, layer 816 formed of intrinsic amorphous Silicon,
and layer 818 formed of n-type amorphous Silicon.
[0071] As has been previously discussed, a plurality of the
photovoltaic cells of the present disclosure may be monolithically
integrated onto a common substrate. For example, several
photovoltaic cells 200, 300, 400, 600, 700, or 800 may be
monolithically integrated onto a common substrate. FIG. 9 is a
cross sectional view of a plurality 900 of multi-junction
photovoltaic cells 938 monolithically integrated onto a common
substrate; such plurality of photovoltaic cells is an embodiment of
module 100. FIG. 9 illustrates plurality 900 including three
multi-junction photovoltaic cells 938(1), 938(2), and 938(3) (only
parts of photovoltaic cells 938(1) and 938(3) are illustrated in
FIG. 9); however, plurality 900 may include any number of
photovoltaic cells 938 greater than 1. Each photovoltaic cell 938
includes N single-junction photovoltaic cells, where N is an
integer that is greater than or equal to one. It should be noted
that several photovoltaic cells 200, 300, 400, 600, 700, or 800 may
be monolithically integrated onto a common substrate in manners
different from that illustrated in FIG. 9.
[0072] Bottom single junction photovoltaic cells 904 are formed on
substrate 902. Photovoltaic cells 904 include bottom contacts 910
(e.g., back contact layers formed of molybdenum) formed on
substrate 902. Junctions 912 are formed on bottom contacts 910;
junctions 912 each include, for example, a solar absorber layer and
a heterojunction partner layer. Top contacts 914 are optionally
formed on junctions 912; top contacts 914 includes at least one of
a conductive top contact layer (e.g., doped Zinc Oxide, undoped
Zinc Oxide, Indium Tin Oxide, doped Tin Oxide, undoped Tin Oxide,
n-type amorphous Silicon, n-type amorphous Silicon Germanium,
hydrogenated amorphous Silicon Carbide, and n-type microcrystalline
amorphous Silicon) and a buffer layer.
[0073] One or more intermediate layers of single-junction
photovoltaic cells (represented by reference character 906) are
sequentially formed on bottom single-junction photovoltaic cells
904. The individual layers of single-junction photovoltaic cells
906 are not shown in FIG. 9 in order to promote illustrative
clarity. It should be noted that if plurality 900 includes only two
single-junction photovoltaic cell layers (904 and 908),
intermediate single-junction photovoltaic cell layers 906 will not
be included in plurality 900.
[0074] Top single junction photovoltaic cells 908 are formed on a
top surface 936 of single junction photovoltaic cell layers 906 (or
directly on bottom single-junction photovoltaic cells 904 if single
junction photovoltaic cell layers 906 are not present). Similar to
photovoltaic cells 904, photovoltaic cells 908 include bottom
contacts 916 formed on top surface 936, junctions 918 formed on
bottom contacts 916, and top contacts 920 formed on junctions 918.
Junctions 918 may include, for example, N-1-P or P-I-N
structures.
[0075] Scribes at least partially delineate each multi-junction
photovoltaic cell 938. For example, FIG. 9 illustrates photovoltaic
cells 938(1), 938(2), and 938(3) being at least partially defined
by scribes. First isolation scribes 922 each extend from a top
surface 928 of photovoltaic cells 908, or from a top surface 940 of
junctions 918 (if top contacts 920 are sufficiently conductive), to
a top surface 930 of substrate 902, or to a top surface of an
insulator layer (not shown in FIG. 9) disposed on top surface 930.
First isolation scribes 922 are filled with an insulating material.
Connecting scribes 924 each extend from top surface 928 of
photovoltaic cells 908, or from top surface 940 of junctions 918,
to a top surface 932 of bottom contacts 910. Connecting scribes 924
are filled with a conductive material. If top contacts 920 are
scribed, then the conductive material of each connecting scribe 924
extends over an adjacent first isolating scribe 922; accordingly,
each connecting scribe 924 electrically connects a section of top
surface 928 of one photovoltaic cell 938 to a bottom contact of an
adjacent photovoltaic cell 938. Second isolating scribes 926 each
extend from top surface 928 of photovoltaic cells 938 to a top
surface 934 of a respective junction of a bottom single junction
photovoltaic cell 904, provided the solar absorber of the bottom
single junction photovoltaic cell is sufficiently resistive,
otherwise second isolating scribe 926 may extend from top surface
928 to top surface 932 of bottom contacts 910. Second isolating
scribes 926 are illustrated as being filled with air in FIG. 9,
however, in other embodiments, isolating scribes 926 may be filled
with another relatively non-conductive material.
P-Type Cadmium Selenide Absorber Layers
[0076] As deposited, Cadmium Selenide tends to be n-type. In many
of the structures described herein, a p-type Cadmium Selenide layer
is desired.
[0077] FIG. 10 shows one process 1000 of converting n-type Cadmium
Selenide to p-type Cadmium Selenide. Process 1000 may enable use
CdSe in structures and processes resembling those previously
demonstrated in lower (1.5 electron volt) bandgap CdTe TFPV cells.
Process 1000 also allows fabrication of a CdSe cell having a
correct polarity for monolithic connection to an underlying p-type
Cu(In, Ga, Al) (S, Se).sub.2 bottom cell. By a Cu(In, Ga, Al)(S,
Se).sub.2, a material is meant that comprises Copper, at least one
of Sulfur or Selenium, and at least one of Indium, Gallium, or
Aluminum. For example, as discussed above, some embodiments of
solar absorber layers 206 and 708 are formed of p-type CdSe, and
process 1000 may be used to form such embodiments. As another
example, process 1000 may be used to form the top CdSe solar
absorber layers 1115 and 1216 of photovoltaic cells 1100 (FIG. 11)
and 1200 (FIG. 12), respectively. Furthermore, process 1000 may be
used to provide p-type CdSe for use in applications other than
photovoltaic cell solar absorber layers.
[0078] Process 1000 begins with step 1002 where a layer of Cadmium
Selenide is deposited. An example of step 1002 is sputtering a
layer of Cadmium Selenide on a photovoltaic cell's back contact
layer. Optional step 1004 is performed concurrently with step 1002.
In step 1004, a dopant is deposited concurrently with the layer of
Cadmium Selenide to form a doped Cadmium Selenide layer. Some
commonly known potential p-type dopants that can be used for group
IIB-VIB materials include, for example, at least one of a Group IA
element, a Group IB element, and a Group VB element. However the
effectiveness of these dopants in highly compensated group IIB-VIB
materials such as CdSe, is tenuous at best, especially if
subsequent high-temperature processing of the CdSe is desired to
improve the properties of the CdSe and subsequent device
efficiency. In an embodiment, Group IIIB and a Group VIIB elements
are used in step 1004. An example of step 1004 is depositing a
Group IIIB elements, preferably lower cost elements such as Ga and
Al, along with the layer of Cadmium Selenide.
[0079] In step 1006, the layer of Cadmium Selenide (which may
include a dopant deposited in optional step 1004) is coated with a
solution including a solvent (e.g., methanol) and at least one
chloride selected from the group consisting of chlorides of Group
IA elements, chlorides of Group IB elements, and chlorides of Group
IIIB elements. Examples of such solution include a preferable
GaCl.sub.2 solution, a CuCl.sub.2 solution, a mixed CuCl.sub.2 and
GaCl.sub.2 solution, a mixed CuCl.sub.2 and CdCl.sub.2 solution, a
mixed GaCl.sub.2 and CdCl.sub.2 solution, and a mixed CuCl.sub.2,
GaCl.sub.2, CdCl.sub.2 solution. An example of step 1006 is spin
coating the layer of Cadmium Selenide with a solution of Gallium
Chloride and methanol at a concentration of 0.1 to 0.00001
(preferably 0.001) Molar and allowing the coated layer to dry. As
another example, the layer of Cadmium Selenide may be submersed in
the solution and heated to 50 C-60 C for a period of 10 minutes to
20 minutes, then allowed to dry.
[0080] In step 1008, the coated layer of Cadmium Selenide is heated
in an environment having an ambient temperature of between 300 and
500 degrees Celsius (preferably 400 degrees Celsius) for a time
between three and thirty minutes in a manner which minimizes loss
(evaporation) of Selenium from the layer of Cadmium Selenide. For
example, the Cadmium Selenide may be heated in a Selenium
containing environment or a Selenium enriched atmosphere. As
another example, the two layers of Cadmium Selenide may be disposed
such that their surfaces are touching or nearly touching to
physically impede evaporation of Selenium from the layers'
surfaces. As yet another example, a non-porous cap may be placed on
the coated layer of Cadmium Selenide during heating to physically
impede the evaporation of Selenium from the Cadmium Selenide.
[0081] It should be noted that execution of optional step 1004 may
advantageously allow additional freedom in selection of chlorides
and chloride concentrations used in step 1006. For example, if step
1004 is executed, step 1006 may be executed with the use of solely
Cadmium Chloride as the chloride. Use of Cadmium Chloride may be
advantageous because it may help to prevent erosion of Cadmium
Selenide during the chloride treatment.
[0082] FIGS. 11 and 12 are cross sectional views of tandem
photovoltaic cells 1100 and 1200, respectively. Each of
photovoltaic cells 1100 and 1200 are an embodiment of photovoltaic
cell 108 of FIG. 1 and include a top solar absorber layer optimized
for absorbing short wavelength "blue" photons, defined as those
photons having wavelengths shorter than red light. The top solar
absorber layers of photovoltaic cells 1100 and 1200 are formed of
p-type Cadmium Selenide having a wide bandgap of 1.72
electron-volts. Embodiments of photovoltaic cells 1100 and 1200
advantageously have potential to reach high efficiencies, possibly
above twenty percent, and high specific power (potentially greater
than 2,000 watts per kilogram) when built on lightweight flexible
substrates.
[0083] The CdSe junction is fabricated on top of a high-efficiency,
but lower-bandgap, thin film photovoltaic junction ("TFPV") bottom
junction having a photon absorber layer of a IB-IIIB-VIB.sub.2
material such as a Copper-Indium-DiSelenide and related alloys with
Gallium, Aluminum or Sulfur having the approximate formula Cu(In,
Ga, or Al) (S, or Se).sub.2. As another example, the bottom
junction may be formed of Copper Indium Gallium Selenide.
[0084] In photovoltaic cells 1100 and 1200, the CdSe top cell
absorbs light toward the blue end of the spectrum, transmitting
unabsorbed light for absorption by the underlying Cu(In, Ga, Al)
(S, Se).sub.2 cell. The CdSe layers of photovoltaic cells 1100 and
1200 are, for example, fabricated using an embodiment of process
1000, FIG. 10.
Structure of Photovoltaic Cell 1100:
[0085] With reference to FIG. 11, incident light 102 approaches the
illuminated side of photovoltaic cell 1100. Photovoltaic cell 1100
then has layers as follows.
[0086] Antireflective Coatings layer 1103 forms the top of
photovoltaic cell 1100. This layer, which may include several
sublayers, serves to impedance-match incoming light 102 to
underlying layers of photovoltaic cell 1100 to enhance efficiency
by avoiding loss of incident photons to reflection. Layer 1103
admits light to photovoltaic cell 1100. Additional antireflective
layers may also be used between other layers of photovoltaic cell
1100 having different indexes of refraction, such as between
encapsulation/protection layer 1105 and transparent conductor layer
1107.
[0087] Encapsulation/Protection layer 1105 is disposed below layer
1103. Layer 1105 is a layer of a transparent material intended to
provide mechanical and/or chemical protection to photovoltaic cell
1100. Layer 1105 may include one or more polymers such as
polyimides or silicones, or may include a sputter-deposited metal
oxide layer or a transparent glass such as Silicon Dioxide.
[0088] Metal collection grid 1109 is disposed on transparent
conductor layer 1107 as shown in FIG. 11. This layer is optional.
This layer or layers aid the lateral conduction to the cell
terminals and is patterned so as to cover only a small percentage
of the surface of photovoltaic cell 1100 while providing low
resistance interconnection to transparent conductor layer 1107.
Collection grid 1109 collects electrical power from the transparent
conductor layer 1107 and conducts it to terminals of photovoltaic
cell 1100. The collection grid 1109 and bottom collection or back
contact layers 1125 may be patterned, and may contact each other at
selected locations as known in the photovoltaic art, to
electrically connect cells serially on the same substrate to obtain
higher output voltages than those available from a single pair of
junctions. Typical metal collection grids are multilayers, with the
first layer being an adhesion and/or diffusion barrier layer, such
as nickel or titanium, and another layer for conduction, such as
aluminum or silver. Additional conducive layers may be used on top
of the conduction layer, such as a layer of nickel.
[0089] A transparent conductor layer 1107 is disposed below
encapsulation/protection layer 1105. Conductor layer 1107 is, for
example, a single layer of doped Tin Oxide, doped Zinc Oxide, or an
Indium Tin Oxide compound. Conductor layer 1107 is thin,
transparent, and conductive. It serves to carry current from the
top cell to the metal collection grid 1109. Optionally, a
transparent buffer layer 1111 may be included. For example,
conductor layer 1107 may be formed of relatively conductive
material (e.g., Indium Tin Oxide) and buffer layer 1111 may be used
and formed of a less conductive material, such as undoped Zinc
Oxide.
[0090] An n-type heterojunction partner layer or window layer 1113
is disposed below conductor layer 1107 (or buffer layer 1111 if
present). Heterojunction partner layer 1113 is, for example,
Cadmium Sulfide (CdS). As another example, heterojunction partner
layer 1113 may be formed of one of Indium Selenide, Zinc Selenide,
Cadmium Zinc Selenide, Cadmium Selenide, Zinc Sulfide, Cadmium
Oxide, Zinc Oxide, Zinc Magnesium Oxide, Tin Oxide, and Cadmium
Zinc Sulfide. Heterojunction partner layer 1113 layer helps form
the PN junction with the underlying CdSe layer 1115 which directs
carriers generated by absorbed photons. Alternatively,
heterojunction partner layer 1113 may be omitted if conductor layer
1107 is used in combination with a buffer layer 1111, such as with
buffer layer 1111 formed of undoped Zinc Oxide, undoped Zinc
Magnesium Oxide, or undoped Tin Oxide.
[0091] A Cadmium Selenide (CdSe) layer 1115 is disposed below
heterojunction partner layer 1113. CdSe layer 1115 has been
deposited as p-type, or converted to p-type, by post-treatment
diffusion of a dopant, or by use of a process such as process 1000
of FIG. 10. P-type CdSe layer 1115 absorbs photons toward the blue
end of the solar spectrum.
[0092] A transparent interconnect layer 1150 is disposed below CdSe
layer 1115. Interconnect layer 1150 may be a bilayer and therefore
include two layers--back contact interface layer 1117 and conductor
layer 1119--as shown in FIG. 11. Alternately, interconnect layer
1150 may be formed of a single layer. For example, interconnect
layer 1150 may be formed of a single layer of one of doped Tin
Oxide, doped Zinc Oxide, or Indium Tin Oxide. As another example,
interconnect layer 1150 may include interface layer 1117 (e.g.,
formed of doped Zinc Telluride), and conductor layer 1119 formed of
a conductive oxide. Interconnect layer 1150 serves as a back
contact for the top photovoltaic junction.
[0093] An n-type second heterojunction partner layer 1121 is
disposed below interconnect layer 1150. Second heterojunction
partner layer 1121 is, for example, one of Indium Selenide, Zinc
Sulfide, Zinc Selenide, Cadmium Sulfide, Cadmium Zinc Sulfide, Zinc
Oxide, or Zinc Magnesium Oxide. Second heterojunction partner layer
1121 helps to form a junction with layer 1123.
[0094] A p-type solar absorber layer 1123 is disposed below second
heterojunction interconnect layer 1121. Solar absorber layer 1123
is formed of a group IB-IIIB-VIB.sub.2 semiconductor such as Copper
Indium DiSelenide, to form a low bandgap solar absorber, or a
related alloy Cu(In, Ga, Al) (S, Se).sub.2 to optimize the bandgap
and tandem device performance. In a particular embodiment, layer
1123 is CuInSe.sub.2.
[0095] A metal back contact and current collection layer 1125 is
disposed below solar absorber layer 1123. Layer 1125 is, for
example, formed of Molybdenum metal.
[0096] An optional layer 1127 of insulation may be disposed between
back contact layer 1125 and substrate 1129. Insulating layer 1127
is used, for example, when substrate 1129 is a conductive metal
foil. Insulating layer 1127 is formed, for example, of an undoped
oxide or a polymeric material such as a silicone or polyimide.
[0097] A substrate layer 1129 is disposed below back contact layer
1125 (or insulating layer 1127 if present). Substrate layer 1129
is, for example, metal foil, insulated metal foil, glass, or a
heat-stable polymer such as polyimide or reinforced silicone.
[0098] One embodiment of photovoltaic cell 1100 has the following
configuration: substrate 1129 formed of polyimide, no insulating
layer 1127, back contact layer 1125 formed of Molybdenum, solar
absorber layer 1123 formed of Copper Indium DiSelenide, second
heterojunction partner layer 1121 formed of Zinc Oxide, transparent
conductor layer 1119 formed of Indium Tin Oxide, back contact
interface layer 1117 formed of doped Zinc Telluride, solar absorber
layer 1115 formed of p-type Cadmium Selenide, first heterojunction
partner layer 1113 formed of Cadmium Sulfide, buffer layer 1111
formed of undoped Tin Oxide, transparent conductor layer 1107
formed of doped Tin Oxide, metallic grid 1109 formed of silver on a
nickel adhesion layer, layer 1105 formed of Silicon Oxide, and
antireflective coatings 1103 formed of Magnesium Flouride.
Structure of Photovoltaic Cell 1200:
[0099] In photovoltaic cell 1200 (FIG. 12), a transparent substrate
1206 is used. Photovoltaic cell 1200 is an alternate embodiment of
photovoltaic cell 1100 (FIG. 11). In photovoltaic cell 1200,
incident light 102 enters through an antireflection coating layer
1204 into the substrate 1206. Light then passes through gaps in the
optional collection grid metal 1210 and through transparent
conductor layer 1208. Transparent conductor layer 1208 may have a
buffer layer 1212 as heretofore described with 1107 and 1111 of
photovoltaic cell 1100 (FIG. 11).
[0100] Light passes from the transparent conductor layer 1208 (and
buffer layer 1212 if present) and through a first heterojunction
partner layer 1214 into the p-type CdSe absorber layer 1216 of the
top junction. In layer 1216, photons toward the blue end of the
solar spectrum are absorbed, but photons toward the red end of the
solar spectrum mostly pass through a back contact interface layer
1218 and a second transparent conductor layer 1220 into the bottom
junction. (It should be noted that photovoltaic cell 1200 need not
include both back contact interface layer 1218 and conductor layer
1220. If both back contact interface layer 1218 and conductor layer
1220 are present, they may be collectively referred to as a
bilayer.) The bottom junction has a heterojunction partner layer
1222 of Cadmium Sulfide, Zinc Sulfide, Cadmium Zinc Sulfide, Zinc
Oxide, or Zinc Magnesium Oxide in contact with a low bandgap p-type
group IB-IIIB-VIB.sub.2 semiconductor layer 1224 such as the Cu(In,
Ga, Al) (S, Se).sub.2 material previously discussed. The
transmitted light is absorbed in this p-type layer 1224.
[0101] A metallic back contact layer 1226, preferably of molybdenum
metal, serves as an electrical back contact to photovoltaic cell
1200. Finally, a protective passivation coating or backing material
1228, which need not be transparent, protects photovoltaic cell
1200. The various layers are patterned as needed using a series of
scribes or, alternatively photoetching, to expose the metallic grid
layer 1210 and back contact layer 1226 for connection of leads to
the device, to isolate particular areas of the device as individual
cells, and to electrically connect individual cells in series to
provide higher voltage output than attainable from a single
tandem-junction pair.
[0102] One embodiment of photovoltaic cell 1200 has the following
configuration: back contact layer 1226 formed of Molybdenum, solar
absorber layer 1224 formed of Copper Indium DiSelenide, second
heterojunction partner layer 1222 formed of Zinc Oxide, transparent
conductor layer 1220 formed of Indium Tin Oxide, transparent back
contact layer 1218 formed of p-type Zinc Telluride, solar absorber
layer 1216 formed of Cadmium Selenide, first heterojunction partner
layer 1214 formed of undoped Tin Oxide, buffer layer 1212 formed of
undoped Tin Oxide, transparent conductor layer 1208 formed of doped
Tin Oxide, no grid 1210, substrate 1206 formed of glass, and
antireflective coatings 1204 formed of Magnesium Flouride.
Fabrication of Photovoltaic Cells 1100:
[0103] Method 1300 of FIGS. 13 and 14 may be used to fabricate
photovoltaic cell 1100 of FIG. 11. However, it should be noted that
photovoltaic cell 1100 might be fabricated using a different
method.
[0104] Method 1300 may make use conventional sputtering or of
roll-to-roll sputter processing, as known in the art and described
in U.S. Pat. No. 6,783,640, for deposition of most layers. A
disclosure of a conventional sputtering process for thin-film
photovoltaic devices using similar materials is contained in U.S.
Pat. No. 5,393,675 to Compaan, which is hereby incorporated by
reference. For this discussion, a flexible metal strip is assumed
for substrate 1129, although, as previously stated, other
substrates are usable.
[0105] First, the metal substrate 1129 is coated 1302 with
insulator layer 1127. For the following deposition steps, the
substrate is loaded into a suitable sputtering machine with a
target chosen to give the desired composition of each layer, and
deposition performed as known in the art.
[0106] While RF sputter deposition is preferred for most layers, as
it can be used with a wide variety of materials, alternative
thin-film deposition methods may be used for some layers.
Alternative thin-film deposition methods that may be suitable for
some layers include evaporation, printing, pulsed laser deposition,
close-space sublimation, electro-chemical deposition, chemical
vapor deposition, wet chemical processing and molecular beam
epitaxy.
[0107] Metal back contact layer 1125, preferably Molybdenum, is
deposited 1304, following which the p-type Cu(In, Ga or Al)
(S.sub.2 or Se.sub.2) low bandgap solar absorber layer 1123 is
deposited preferably using a co-evaporation process 1306. Solar
absorber layer 1123 is then topped with a high-resistivity
Zinc-Oxide partner layer 1121, deposited 1308 preferably using
chemical vapor deposition.
[0108] Next is sputter deposited 1310, 1312 conductor layer 1119
and back contact interface layer 1117, respectively. Conductor
layer 1119 is, for example, Indium Tin Oxide. Back contact
interface layer 1117 is preferably Cu doped ZnTe or is optionally
nitrogen doped per the method described by Compaan et al, contained
in U.S. Pat. No. 6,852,614, which is hereby incorporated herein by
reference.
[0109] Next, a layer 1115 of Cadmium Selenide (CdSe) is deposited
1316, preferably by sputtering, but alternatively this layer can be
deposited via any of the known deposition techniques used for II-VI
materials. Some common thin-film deposition techniques of II-VI
materials include but are not limited to: RF sputtering,
evaporation, printing, pulsed laser deposition, close-space
sublimation, electro-chemical deposition, wet chemical processing,
chemical vapor deposition, or molecular beam epitaxy.
[0110] The CdSe is then made p-type by a high-temperature chloride
process that includes coating the CdSe with a solution including at
least one chloride selected from the following: chlorides of Group
IA elements, chlorides of Group IB elements, and chlorides of group
IIIB elements. Examples of chlorides that may be used include, but
are not limited to, Copper Chloride (CuCl.sub.2), and/or Gallium
Chloride (GaCI.sub.2) with optional Cadmium Chloride (CdCl.sub.2).
The chloride coated CdSe is then heated, as discussed below, while
at least partially preventing the evaporation of Selenium from the
chloride coated CdSe.
[0111] For example, the CdSe may be made p-type as follows.
Following the deposition 1316 of the doped or undoped CdSe layer, a
wet GaCl.sub.2 solution (with optional CuCl.sub.2 and CdCl.sub.2)
is applied 1318 uniformly to the surface of the CdSe film. In one
embodiment, the GaCl.sub.2 is mixed in methanol (CH.sub.3OH) at a
concentration of 0.1 to 0.00001 (preferably 0.001) M. The solution
is deposited on the CdSe surface via spin coating to uniformly
distribute the solution across the CdSe surface. The volume of
solution is preferably 0.04 ml per cm.sup.2 of CdSe surface.
Alternatively, the solution may be drop coat and knife "doctor"
bladed across the CdSe surface. Alternatively, the solution may be
sprayed or misted uniformly onto the CdSe surface. Alternatively,
the chlorides may be deposited using a dry technique, such as
chemical vapor deposition. Alternatively, the chlorides may be
deposited from submersion in a heated chloride bath.
[0112] After applying 1318 the solution, the solution is allowed to
dry on the CdSe surface, leaving the dried chloride on the surface.
The CdSe and dried chloride are then placed into a chamber and
heated 1320 in an environment having an ambient temperature in the
range of 300 to 500.degree. C., preferably 390 C for a time ranging
from a three minutes to 30 minutes, preferably 20 minutes, with a
strong selenium-containing background gas supplied immediately
adjacent to the CdSe surface. The Selenium background gas helps
prevent the evaporation of Selenium from the coated layer of CdSe.
Alternately, instead of using a Selenium background, the
evaporation of Selenium from the coated layer of CdSe may be
physically impeded, such as by used of a non-porous cap on the CdSe
layer, or by placing two or more layers of CdSe together or nearly
touching. When a non-porous cap layer is used on the CdSe layer to
impede evaporation of the selenium, this cap layer may be removed
after the heating step, and need not remain as a permanent part of
the photovoltaic cell.
[0113] In one embodiment, the selenium ambient may be provided via
evaporation with the Se source in close proximity (within 12
inches) and directed at the CdSe surface. Alternatively, the Se is
provided by passing the CdSe over a heated Se source plate for
close-space sublimation of the Se to the CdSe surface in a
background gas of air, Ar, O.sub.2, or N.sub.2. The concentrated Se
ambient above the surface of the CdSe suppresses the loss of
Selenium vapor from the surface during the high-temperature
treatment. The high-temperature chloride treatment modifies the
CdSe electronic and optical properties, making it suitable as a
p-type solar absorber layer. Residual chloride is removed by
washing 1321 with methanol.
[0114] Next, the heterojunction partner layer 1113 is deposited
1322, preferably CdS by wet chemical processing on the now-p-type
CdSe layer 1115. In an alternative embodiment, the high-temperature
treatment with CdCl.sub.2, and/or CuCl.sub.2 and/or GaCl.sub.2
described previously, is performed after the deposition of this
layer. In yet another alternate embodiment a second
high-temperature treatment is performed using CdCl.sub.2 only after
the deposition of the CdS layer. In yet another alternative
embodiment, no n-type heterojunction partner layer is used to form
the junction--the heterojunction partner layer is replaced by an
undoped oxide layer, buffer layer 1111 such as Tin Oxide, Zinc
Oxide, or Zinc Magnesium Oxide, and the transparent conductor layer
1107.
[0115] Following the heterojunction partner layer deposition, the
optional undoped oxide buffer layer 1111, such as Tin Oxide is
deposited 1324 in some embodiments by sputtering.
[0116] Next, the photovoltaic cell 1100 is patterned. This may be
performed by scribing as known in the art. Scribing may be
performed by laser, mechanical scribing, roll print or photo etch
with dry or wet etching, or other methods of selective removal of
material from the photovoltaic device surface. A deep isolation
scribing 1326 cuts a trench by selectively removing all layers
above insulation layer 1127 in an isolation trench, and backfills
the resulting trench with insulation 1131, such as a non-conductive
ink or oxide. A second contact scribing 1328 cuts a trench by
selectively removing all layers above metal layer 1125, with the
resulting trench backfilled with a conductive material 1133, such a
conductive ink or oxide, which may be the next deposited conductor
layer 1107. The contact scribe thereby connects metal layer 1125 in
particular predetermined areas to transparent conductor layer 1107
to connect individual cells in series to obtain high voltage
output.
[0117] Next, Indium Tin Oxide transparent conductor layer 1107 is
deposited 1330, preferably by sputtering.
[0118] An interconnect scribing 1332 is performed to remove at
least the transparent conductor layer 1107 in a trench 1135 between
individual cells. This trench 1135 may be left empty or backfilled
with an insulator.
[0119] The metal collection grid 1109 is next deposited 1334, if
used, and patterned 1336 so it covers only a specific portion or
pattern on the surface. This layer, when finished, occupies only a
small percentage of the cell so as to admit light to the absorbing
layers beneath it.
[0120] Next, encapsulation/protection layer 1105 and antireflective
coating 1103 layer are deposited 1338. Then, openings are etched
1340 in the encapsulation/protection and antireflective coating
layers so that leads may be attached 1342 and the device is cut
into finished devices.
Fabrication of Photovoltaic Cells 1200:
[0121] FIGS. 15-16 illustrate a method 1500 fabricating
photovoltaic cell 1200 of FIG. 12. Method 1500 is an embodiment of
method 1300 of FIGS. 13-14. It should be noted that photovoltaic
cell 1200 may be fabricated using methods other than method
1500.
[0122] Method 1500 begins with deposition 1502 and patterning of
the optional collection grid 1210 on a transparent substrate 1206.
Following patterning of this layer, a transparent conductor layer
1208 is deposited 1504, preferably Indium Tin Oxide but
alternatively transparent conductors such as doped Tin Oxide
(SnO.sub.2), and doped Zinc Oxide (ZnO) may be used. Conductor
layer 1208 is deposited by sputter deposition and is thin,
transparent, and conductive. It serves to carry current from the
top cell to the metal collection grid 1210. Optionally, buffer
layer 1212 may be sputtered 1506. Buffer layer 1212 is, for
example, undoped Zinc Oxide or undoped Tin Oxide.
[0123] Next, a heterojunction partner layer 1214, preferably of
CdS, is deposited 1508, preferably by wet chemical processing
although other deposition methods known in the art such as
sputtering may be used. Following this, a layer of CdSe 1216 is
deposited, preferably by sputtering 1510.
[0124] The CdSe is then made p-type by a high-temperature chloride
process that includes coating CdSe layer 1216 with a solution
including at least one chloride selected from the following
chlorides: chlorides of Group IA elements, chlorides of Group IB
elements, and chlorides of Group IIIB elements. Examples of
solutions that may be used to coat CdSe layer 1216 include, but are
not limited to, Copper Chloride (CuCl.sub.2), and/or Gallium
Chloride (GaCl.sub.2) with optional Cadmium Chloride (CdCl.sub.2).
The coated CdSe layer 1216 is subsequently heated while at least
partially preventing the evaporation of Selenium from the coated
CdSe layer 1216. For example, the coated CdSe layer may be heated
in a Selenium environment.
[0125] The following is one example of how CdSe layer 1216 may be
made p-type. Following the deposition 1510 of the doped or undoped
CdSe layer, a wet GaCl.sub.2 solution (with optional CuCl.sub.2 and
CdCl.sub.2) is applied 1512 uniformly to the surface of the CdSe
film. In one embodiment, the GaCl.sub.2 is mixed in methanol
(CH.sub.3OH) at a concentration of 0.1 to 0.00001 (preferably
0.001) Molar. The solution is deposited on the CdSe surface via
spin coating to uniformly distribute the solution across the CdSe
surface. The volume of solution is preferably 0.04 ml per cm.sup.2
of CdSe surface. Alternatively the solution may be drop coat and
knife "doctor" bladed across the CdSe surface. Alternatively, the
solution may be sprayed or misted uniformly onto the CdSe surface.
Alternatively, the chloride is applied by chemical vapor deposition
(CVD). Alternatively, the chlorides may be deposited from
submersion in a heated chloride bath.
[0126] After applying 1512 the solution, the solution is allowed to
dry on the CdSe surface, leaving the chloride on the surface. The
device, including the CdSe film coated with dried chloride, is then
placed into a chamber and heated 1514 in an environment having an
ambient temperature ranging from 300 to 500.degree. C., preferably
390 C for a time ranging from a three minutes to 30 minutes,
preferably 20 minutes, with a strong Selenium containing background
gas environment supplied immediately adjacent to the CdSe surface
to prevent evaporation of Selenium from the CdSe surface. As an
alternative to providing a CdSe environment, evaporation of
Selenium from the CdSe surface may be physically impeded, such as
by placing two of more layers of CdSe such that their surfaces
touch, or nearly touch, or by placing a non-porous cap on the CdSe
layer. In one embodiment, the Selenium ambient may be provided via
evaporation with the Se source in close proximity (within 12
inches) and directed at the CdSe surface. Alternatively, the Se is
provided by passing the CdSe over a heated Se source plate for
close-space sublimation of the Se to the CdSe surface in a
background gas of air, Ar, O.sub.2, or N.sub.2. The concentrated Se
ambient above the surface of the CdSe suppresses evaporative loss
of Se from the surface during the high-temperature treatment. The
high-temperature chloride treatment modifies the CdSe electronic
and optical properties, making it suitable as a p-type solar
absorber layer. Residual chloride is removed by washing 1516 with
methanol.
[0127] Next is sputter deposited 1518 a transparent conductor
interface layer 1218. This is ZnTe with optional Cu doping or with
optional nitrogen doping per the method described by Compaan et al,
contained in U.S. Pat. No. 6,852,614. Next, a conductive back
contact layer 1220, preferably a doped Tin Oxide or an Indium Tin
oxide layer, is deposited 1520.
[0128] Next, the red-optimized photovoltaic junction is formed. To
do this, processing continues with deposition 1522 of a
high-resistivity Zinc-Oxide heterojunction partner layer 1222 is
deposited by sputtering, or chemical vapor deposition. Then, the
p-type Cu(In, Ga or Al) (S.sub.2 or Se.sub.2) low bandgap solar
absorber layer 1224 is deposited, preferably using co-evaporation
1524.
[0129] Next, some patterning steps are performed to separate the
device into several separate photovoltaic cells that can be strung
together in series to obtain higher voltage output than available
with a single hybrid stack. A first isolation scribeline 1526 cuts
through all layers (excluding substrate 1206 and antireflective
coatings 1204) and is backfilled with insulator, such as a
non-conductive ink or oxide. A second contact scribeline 1528 cuts
through all layers above (excluding substrate 1206 and
antireflective coatings 1204), but stops at transparent conductor
layer 1208 so as to permit contact to be made to, either the
transparent conductor layer 1208 or the collection grid layer
1210.
[0130] Back contact metal layer 1226 is then deposited 1530. A
third isolation scribeline 1532 is performed that cuts through back
contact 1226 but stops above transparent conductor layer 1208 or
collection grid layer 1210. The cell is then finished by depositing
1534 protection layers 1228, with openings in this layer for lead
attachment; and depositing 1536 an antireflective coating 1204 on
the opposite, illuminated, side of the substrate 1206.
[0131] For purposes of this document, the term blue includes
wavelengths that are shorter than those of red light, including
some red light but predominately yellow through the blue end of the
spectrum, including ultraviolet light, but may also contain some
absorption of the entire visible spectrum. Similarly, the term red
includes wavelengths that are predominately near the red end of the
spectrum including infrared. While the top, blue-optimized, cell of
photovoltaic cells 1100 and 1200 is largely transparent to red and
infrared light, it may absorb small amounts of red and infrared
light because of defects and impurities. The bottom, red-optimized,
cell may also absorb some short wavelength light, including blue
light, that escapes absorption in the top cell.
[0132] Similarly, when a layer is described as transparent, it need
transmit a majority of light incident at appropriate angles; it may
absorb, scatter, and reflect small amounts of light.
[0133] Changes may be made in the above methods and systems without
departing from the scope hereof. It should thus be noted that the
matter contained in the above description or shown in the
accompanying drawings should be interpreted as illustrative and not
in a limiting sense. The following claims are intended to cover
generic and specific features described herein, as well as all
statements of the scope of the present method and system, which, as
a matter of language, might be said to fall therebetween.
* * * * *