U.S. patent application number 13/782176 was filed with the patent office on 2014-09-04 for photovoltaic devices and method of making.
This patent application is currently assigned to First Solar, Inc.. The applicant listed for this patent is First Solar, Inc.. Invention is credited to Jinbo Cao, William Hullinger Huber, Yong Liang.
Application Number | 20140246083 13/782176 |
Document ID | / |
Family ID | 51420317 |
Filed Date | 2014-09-04 |
United States Patent
Application |
20140246083 |
Kind Code |
A1 |
Liang; Yong ; et
al. |
September 4, 2014 |
PHOTOVOLTAIC DEVICES AND METHOD OF MAKING
Abstract
A photovoltaic device is presented. The photovoltaic device
includes a buffer layer disposed on a transparent conductive oxide
layer; a window layer disposed on the buffer layer; and an
interlayer interposed between the transparent conductive oxide
layer and the window layer. The interlayer includes a metal
species, wherein the metal species includes gadolinium, beryllium,
calcium, barium, strontium, scandium, yttrium, hafnium, cerium,
lutetium, lanthanum, or combinations thereof. A method of making a
photovoltaic device is also presented
Inventors: |
Liang; Yong; (Niskayuna,
NY) ; Cao; Jinbo; (Rexford, NY) ; Huber;
William Hullinger; (Niskayuna, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
First Solar, Inc. |
Tempe |
AZ |
US |
|
|
Assignee: |
First Solar, Inc.
Tempe
AZ
|
Family ID: |
51420317 |
Appl. No.: |
13/782176 |
Filed: |
March 1, 2013 |
Current U.S.
Class: |
136/256 ;
438/94 |
Current CPC
Class: |
Y02P 70/50 20151101;
H01L 31/022466 20130101; H01L 31/022475 20130101; H01L 31/0321
20130101; H01L 31/022483 20130101; Y02E 10/543 20130101; Y02P
70/521 20151101; H01L 31/073 20130101 |
Class at
Publication: |
136/256 ;
438/94 |
International
Class: |
H01L 31/032 20060101
H01L031/032; H01L 31/18 20060101 H01L031/18 |
Claims
1. A photovoltaic device, comprising: a buffer layer disposed on a
transparent conductive oxide layer; a window layer disposed on the
buffer layer; and an interlayer comprising a metal species
interposed between the transparent conductive oxide layer and the
window layer, wherein the metal species comprises gadolinium,
beryllium, calcium, barium, strontium, scandium, yttrium, hafnium,
cerium, lutetium, lanthanum, or combinations thereof.
2. The photovoltaic device of claim 1, wherein at least a portion
of the metal species is present in the interlayer in the form of an
elemental metal, a metal alloy, a metal compound, or combinations
thereof.
3. The photovoltaic device of claim 1, wherein the interlayer
further comprises tin, sulfur, oxygen, fluorine, zinc, cadmium, or
combinations thereof.
4. The photovoltaic device of claim 1, wherein the interlayer
comprises a metal compound comprising the metal species, tin, and
oxygen.
5. The photovoltaic device of claim 1, wherein the interlayer
comprises a metal compound comprising the metal species and
fluorine.
6. The photovoltaic device of claim 1, wherein the interlayer is
interposed between the transparent conductive oxide layer and the
buffer layer.
7. The photovoltaic device of claim 1, wherein the interlayer is
interposed between the buffer layer and the window layer.
8. The photovoltaic device of claim 1, wherein the transparent
conductive oxide layer comprises cadmium tin oxide, zinc tin oxide,
indium tin oxide, fluorine-doped tin oxide, indium-doped
cadmium-oxide, doped zinc oxide, or combinations thereof.
9. The photovoltaic device of claim 1, wherein the buffer layer
comprises tin dioxide, zinc oxide, indium oxide, zinc tin oxide, or
combinations thereof.
10. The photovoltaic device of claim 1, wherein the window layer
comprises cadmium sulfide, oxygenated cadmium sulfide, zinc
sulfide, cadmium zinc sulfide, cadmium selenide, indium selenide,
indium sulfide, or combinations thereof.
11. The photovoltaic device of claim 1, further comprising an
absorber layer disposed on the window layer.
12. The photovoltaic device of claim 11, wherein the absorber layer
comprises cadmium telluride, cadmium zinc telluride, cadmium sulfur
telluride, cadmium selenium telluride, cadmium manganese telluride,
cadmium magnesium telluride, copper indium sulfide, copper indium
gallium selenide, copper indium gallium sulfide, or combinations
thereof.
13. The photovoltaic device of claim 11, further comprising a
secondary interlayer interposed between the window layer and the
absorber layer, wherein the secondary interlayer comprises
magnesium, aluminum, zinc, nickel, gadolinium, or combinations
thereof.
14. The photovoltaic device of claim 1, wherein the interlayer has
a thickness in a range from about 0.2 nanometers to about 200
nanometers.
15. A photovoltaic device, comprising: a transparent conductive
oxide layer; a window layer; and an interlayer comprising a metal
species interposed between the transparent conductive oxide layer
and the window layer, wherein the metal species comprises
gadolinium, beryllium, scandium, yttrium, hafnium, cerium,
lutetium, lanthanum, or combinations thereof.
16. The photovoltaic device of claim 15, wherein at least a portion
of the metal species is present in the interlayer in the form of an
elemental metal, a metal alloy, a metal compound, or combinations
thereof.
17. The photovoltaic device of claim 15, wherein the interlayer
further comprises tin, sulfur, oxygen, fluorine, zinc, cadmium, or
combinations thereof.
18. The photovoltaic device of claim 15, wherein the interlayer
comprises a metal compound comprising the metal species, tin, and
oxygen.
19. The photovoltaic device of claim 15, wherein the interlayer
comprises a metal compound comprising the metal species and
fluorine.
20. The photovoltaic device of claim 15, wherein the interlayer is
disposed directly in contact with the transparent conductive oxide
layer.
21. The photovoltaic device of claim 15, wherein the window layer
comprises cadmium sulfide, oxygenated cadmium sulfide, zinc
sulfide, cadmium zinc sulfide, cadmium selenide, indium selenide,
indium sulfide, or combinations thereof.
22. A method of making a photovoltaic device, comprising: disposing
a buffer layer between a transparent conductive oxide layer and a
window layer; and disposing an interlayer comprising a metal
species between the transparent conductive oxide layer and the
window layer, wherein the metal species comprises gadolinium,
beryllium, scandium, yttrium, hafnium, cerium, lutetium, lanthanum,
calcium, barium, strontium, or combinations thereof.
23. The photovoltaic device of claim 22, wherein at least a portion
of the metal species is present in the interlayer in the form of an
elemental metal, a metal alloy, a metal compound, or combinations
thereof.
24. The method of claim 22, wherein the interlayer further
comprises tin, sulfur, oxygen, fluorine, zinc, cadmium, or
combinations thereof.
25. The method of claim 22, wherein the window layer comprises
cadmium sulfide, oxygenated cadmium sulfide, zinc sulfide, cadmium
zinc sulfide, cadmium selenide, indium selenide, indium sulfide, or
combinations thereof.
26. The method of claim 22, further comprising disposing an
absorber layer on the window layer.
27. The method of claim 26, wherein the absorber layer comprises
cadmium telluride, cadmium zinc telluride, cadmium sulfur
telluride, cadmium selenium telluride, cadmium manganese telluride,
cadmium magnesium telluride, copper indium sulfide, copper indium
gallium selenide, copper indium gallium sulfide, or combinations
thereof.
28. The method of claim 26, further comprising interposing a
secondary interlayer between the window layer and the absorber
layer, wherein the secondary interlayer comprises magnesium,
aluminum, zinc, nickel, gadolinium, or combinations thereof.
Description
BACKGROUND
[0001] The invention generally relates to photovoltaic devices.
More particularly, the invention relates to photovoltaic devices
that include an interlayer, and methods of making the photovoltaic
devices.
[0002] Thin film solar cells or photovoltaic (PV) devices typically
include a plurality of semiconductor layers disposed on a
transparent substrate, wherein one layer serves as a window layer
and a second layer serves as an absorber layer. The window layer
allows the penetration of solar radiation to the absorber layer,
where the optical energy is converted to usable electrical energy.
The window layer further functions to form a heterojunction (p-n
junction) in combination with an absorber layer. Cadmium
telluride/cadmium sulfide (CdTe/CdS) heterojunction-based
photovoltaic cells are one such example of thin film solar cells,
where CdS functions as the window layer.
[0003] However, thin film solar cells may have low conversion
efficiencies. Thus, one of the main focuses in the field of
photovoltaic devices is the improvement of conversion efficiency.
Absorption of light by the window layer may be one of the phenomena
limiting the conversion efficiency of a PV device. Thus, it is
desirable to keep the window layer as thin as possible to help
reduce optical losses by absorption. However, for most of the
thin-film PV devices, if the window layer is too thin, a loss in
performance can be observed due to low open circuit voltage
(V.sub.OC) and fill factor (FF). It is also desirable that the thin
window layer maintain its structural integrity during the
subsequent device fabrication steps, such that the interface
between the absorber layer and the window layer contains negligible
interface defect states.
[0004] Thus, there is a need for improved thin film photovoltaic
devices configurations, and methods of manufacturing these.
BRIEF DESCRIPTION OF THE INVENTION
[0005] Embodiments of the present invention are included to meet
these and other needs. One embodiment is a photovoltaic device. The
photovoltaic device includes a buffer layer disposed on a
transparent conductive oxide layer; a window layer disposed on the
buffer layer; and an interlayer interposed between the transparent
conductive oxide layer and the window layer. The interlayer
includes a metal species, wherein the metal species includes
gadolinium, beryllium, calcium, barium, strontium, scandium,
yttrium, hafnium, cerium, lutetium, lanthanum, or combinations
thereof.
[0006] One embodiment is a photovoltaic device. The photovoltaic
device includes a transparent conductive oxide layer; a window
layer; and an interlayer interposed between the transparent
conductive oxide layer and the window layer. The interlayer
includes a metal species, wherein the metal species includes
gadolinium, beryllium, scandium, yttrium, hafnium, cerium,
lutetium, lanthanum, or combinations thereof.
[0007] One embodiment is a method of making a photovoltaic device.
The method includes disposing a buffer layer between a transparent
conductive oxide layer and a window layer; and disposing an
interlayer between the transparent conductive oxide layer and the
window layer. The interlayer includes a metal species, wherein the
metal species includes gadolinium, beryllium, calcium, barium,
strontium, scandium, yttrium, hafnium, cerium, lutetium, lanthanum,
or combinations thereof.
DRAWINGS
[0008] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings, wherein:
[0009] FIG. 1 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0010] FIG. 2 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0011] FIG. 3 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0012] FIG. 4 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0013] FIG. 5 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0014] FIG. 6 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0015] FIG. 7 is a schematic of a semiconductor assembly, according
to some embodiments of the invention.
[0016] FIG. 8 is a schematic of a semiconductor assembly, according
to some embodiments of the invention.
[0017] FIG. 9 shows the performance parameters for photovoltaic
devices, according to some embodiments of the invention.
DETAILED DESCRIPTION
[0018] As discussed in detail below, some of the embodiments of the
invention include photovoltaic devices including an interlayer
disposed between a transparent conductive oxide layer and a window
layer. In some embodiments, the interlayer is disposed between a
buffer layer and a window layer. In some embodiments, the
interlayer is disposed between a transparent conductive oxide layer
and a buffer layer.
[0019] Approximating language, as used herein throughout the
specification and claims, may be applied to modify any quantitative
representation that could permissibly vary without resulting in a
change in the basic function to which it is related. Accordingly, a
value modified by a term or terms, such as "about", and
"substantially" is not to be limited to the precise value
specified. In some instances, the approximating language may
correspond to the precision of an instrument for measuring the
value. Here and throughout the specification and claims, range
limitations may be combined and/or interchanged, such ranges are
identified and include all the sub-ranges contained therein unless
context or language indicates otherwise.
[0020] In the following specification and the claims, the singular
forms "a", "an" and "the" include plural referents unless the
context clearly dictates otherwise. As used herein, the term "or"
is not meant to be exclusive and refers to at least one of the
referenced components (for example, a layer) being present and
includes instances in which a combination of the referenced
components may be present, unless the context clearly dictates
otherwise.
[0021] The terms "transparent region" and "transparent layer" as
used herein, refer to a region or a layer that allows an average
transmission of at least 70% of incident electromagnetic radiation
having a wavelength in a range from about 350 nm to about 850
nm.
[0022] As used herein, the term "layer" refers to a material
disposed on at least a portion of an underlying surface in a
continuous or discontinuous manner. Further, the term "layer" does
not necessarily mean a uniform thickness of the disposed material,
and the disposed material may have a uniform or a variable
thickness. As used herein, the term "disposed on" refers to layers
disposed directly in contact with each other or indirectly by
having intervening layers therebetween, unless otherwise
specifically indicated. The term "adjacent" as used herein means
that the two layers are disposed contiguously and are in direct
contact with each other.
[0023] In the present disclosure, when a layer is being described
as "on" another layer or substrate, it is to be understood that the
layers can either be directly contacting each other or have one (or
more) layer or feature between the layers. Further, the term "on"
describes the relative position of the layers to each other and
does not necessarily mean "on top of" since the relative position
above or below depends upon the orientation of the device to the
viewer. Moreover, the use of "top," "bottom," "above," "below," and
variations of these terms is made for convenience, and does not
require any particular orientation of the components unless
otherwise stated.
[0024] As discussed in detail below, some embodiments of the
invention are directed to a photovoltaic device including an
interlayer. A photovoltaic device 100, according to one embodiment
of the invention, is illustrated in FIGS. 1-2. As shown in FIGS.
1-2, the photovoltaic device 100 includes a transparent conductive
oxide layer 120, a buffer layer 130 disposed on the transparent
conductive oxide layer 120, and a window layer 140 disposed on the
buffer layer 130. As indicated in FIGS. 1-2, the photovoltaic
device 100 further includes an interlayer 150 interposed between
the transparent conductive oxide layer 120 and the window layer
140. In such instances, the interlayer 150 includes a metal
species, wherein the metal species include gadolinium, beryllium,
scandium, yttrium, hafnium, cerium, lutetium, lanthanum, calcium,
barium, strontium, or combinations thereof.
[0025] In some embodiments, the interlayer 150 is interposed
between the buffer layer 130 and the window layer 140, as indicated
in FIG. 1. In some other embodiments, the interlayer 150 is
interposed between the transparent conductive oxide layer 120 and
the buffer layer 130, as indicated in FIG. 2. Further, in such
instances, the interlayer 150 may be disposed directly in contact
with buffer layer 130 (as indicated in FIGS. 1 and 2), or,
alternatively may be disposed on an intervening layer (embodiment
not shown), which in turn is disposed on the buffer layer 130.
[0026] A photovoltaic device 200, according to another embodiment
of the invention, is illustrated in FIG. 4. As shown in FIG. 4, the
photovoltaic device 200 includes a transparent conductive oxide
layer 220, and a window layer 240 disposed on the transparent
conductive oxide layer 220. As indicated in FIG. 4, the
photovoltaic device 200 further includes an interlayer 250
interposed between the transparent conductive oxide layer 220 and
the window layer 240. In such instances, the interlayer 250
includes a metal species, wherein the metal species include
gadolinium, beryllium, scandium, yttrium, hafnium, cerium,
lutetium, lanthanum, or combinations thereof.
[0027] In such embodiments, the interlayer 250 may be disposed
directly in contact with the transparent conductive oxide layer
220, as indicated in FIG. 4. In such instances, the interlayer 250
may itself function as a buffer layer, and a separate buffer layer
may not be required in the photovoltaic device 200.
[0028] The term "metal species" as used herein refers to elemental
metal, metal ions, or combinations thereof. In some embodiments, at
least a portion of the metal species is present in the interlayer
150/250 in the form of an elemental metal, a metal alloy, a metal
compound, or combinations thereof. In some embodiments, the
interlayer 150/250 further includes tin, sulfur, oxygen, fluorine,
zinc, cadmium, or combinations thereof.
[0029] In some embodiments, at least a portion of the metal species
is present in the interlayer in the form of an elemental metal. In
some embodiments, the interlayer 150 includes elemental gadolinium,
elemental calcium, elemental barium, elemental strontium, elemental
beryllium, elemental scandium, elemental yttrium, elemental
hafnium, elemental cerium, elemental lutetium, elemental lanthanum,
or combinations thereof. In some embodiments, the interlayer 250
includes elemental gadolinium, elemental beryllium, elemental
scandium, elemental yttrium, elemental hafnium, elemental cerium,
elemental lutetium, elemental lanthanum, or combinations
thereof.
[0030] In some embodiments, at least a portion of the metal species
is present in the interlayer 150/250 in the form of a metal alloy.
In some embodiments, the interlayer 150/250 includes a metal alloy
of tin and at least one of the metal species, for example, an alloy
of tin and gadolinium. In certain embodiments, the interlayer
includes Gd.sub.xSm.sub.1-x, wherein x is a number greater than 0
and less than 1. In embodiments wherein the interlayer 150/250
includes two or more of the metal species, the interlayer 150/250
may include a metal alloy of two or more of the metal species, for
example, an alloy of gadolinium and strontium.
[0031] In some embodiments, at least a portion of the metal species
is present in the interlayer 150/250 in the form of a metal
compound. The term "metal compound", as used herein, refers to a
macroscopically homogeneous material (substance) consisting of
atoms or ions of two or more different elements in definite
proportions, and at definite lattice positions. For example,
gadolinium, tin and oxygen have defined lattice positions in the
crystal structure of a gadolinium tin oxide compound, in contrast,
for example, to tin-doped gadolinium oxide where tin may be a
dopant that is substitutionally inserted on gadolinium sites, and
not a part of the compound lattice
[0032] In some embodiments, the metal compound further includes
oxygen, sulfur, selenium, tellurium, or combinations thereof. In
some embodiments, the metal compound further includes zinc,
cadmium, or combinations thereof. In certain embodiments, the
interlayer includes a compound including the metal species, tin,
and oxygen. In certain embodiments, the interlayer includes a metal
compound including the metal species, zinc, tin, and oxygen. In
certain embodiments, the interlayer 150/250 includes a metal
compound including the metal species and fluorine.
[0033] In some embodiments, at least a portion of the metal species
is present in the interlayer 130 in the form of a binary metal
compound, a ternary metal compound, a quaternary metal compound, or
combinations thereof.
[0034] The term "binary metal compound" as used herein refers to a
compound including the metal species and one other different
element. In some embodiments, at least a portion of the metal
species is present in the interlayer 150/250 in the form of a
binary metal compound, such as, for example, a metal oxide, a metal
sulfide, a metal fluoride, a metal selenide, a metal telluride, or
mixtures thereof. Thus, by way of example, in certain embodiments,
the interlayer may include gadolinium oxide, gadolinium sulfide,
gadolinium fluoride, or mixtures thereof.
[0035] The term "ternary metal compound" as used herein refers to a
compound including the metal species and two other different
elements. Thus, by way of example, in certain embodiments, the
interlayer 150/250 includes gadolinium tin oxide. The term
"quaternary metal compound" as used herein refers to a compound
including the metal species and three other different elements.
Thus, by way of example, in certain embodiments, the interlayer
150/250 may include gadolinium zinc tin oxide.
[0036] In certain embodiments, the interlayer 150/250 includes a
metal tin oxide phase. Without being bound by any theory it is
believed that the formation of a compound including the metal
species, tin, and oxygen may preclude diffusion of tin from the
transparent parent conductive oxide layer 120/220, the buffer layer
130, or both to the junction-forming layers.
[0037] The interlayer 150/250 may be further characterized by the
concentration of the metal species in the interlayer 150/250. In
some embodiments, an atomic concentration of the metal species in
the interlayer 150/250 may be substantially constant across the
thickness of the interlayer 150/250. The term "substantially
constant" as used herein means that the concentration of the metal
species varies by less than about 5 percent across the thickness of
the interlayer 150/250. In some other embodiments, the metal
species may be compositionally graded across the thickness of the
interlayer 150/250.
[0038] In some embodiments, an average atomic concentration of the
metal species in the interlayer 150/250 is greater than about 10
percent. In some embodiments, an average atomic concentration of
the metal species in the interlayer 150/250 is greater than about
50 percent. In some embodiments, an average atomic concentration of
the metal species in the interlayer 150/250 is in a range from
about 10 percent to about 99 percent. The term "atomic
concentration" as used herein refers to the average number of atoms
per unit volume. As noted earlier, the interlayer 150/250 may
further include cadmium, sulfur, tin, oxygen, fluorine, or
combinations thereof.
[0039] The interlayer 150/250 may be further characterized by a
thickness. In some embodiments, the interlayer 150/250 has a
thickness in a range from about 0.2 nanometers to about 200
nanometers. In some embodiments, the interlayer 150/250 has a
thickness in a range from about 0.2 nanometers to about 100
nanometers. In some embodiments, the interlayer 150/250 has a
thickness in a range from about 1 nanometer to about 20 nanometers.
In some embodiments, it may be desirable to have a thin interlayer,
such that there are minimal optical losses in the interlayer
150/250 due to absorption.
[0040] As described earlier, the thickness of the window layer
140/240 is typically desired to be minimized in a photovoltaic
device to achieve high efficiency. With the presence of the
interlayer 150/250, the thickness of the window layer 140/240
(e.g., CdS layer) may be reduced to improve the performance of the
present device. Moreover, the present device may achieve a
reduction in cost of production because of the use of lower amounts
of CdS.
[0041] As noted, the interlayer 150/250 is a component of a
photovoltaic device 100/200. In some embodiments, the photovoltaic
device includes a "superstrate" configuration of layers. Referring
now to FIGS. 3 and 5, in such embodiments, the photovoltaic device
100/200 further includes a support 110/210, and the transparent
conductive oxide layer 120/220 (sometimes referred to in the art as
a front contact layer) is disposed on the support 110/220, as
indicated in FIGS. 3 and 5. As further illustrated in FIGS. 3 and
5, in such embodiments, the solar radiation 10 enters from the
support 110/210, and after passing through the transparent
conductive oxide layer 120/220, the buffer layer 130 (if present),
the interlayer 150/250, and the window layer 140/240, enters the
absorber layer 160/260, where the conversion of electromagnetic
energy of incident light (for instance, sunlight) to electron-hole
pairs (that is, to free electrical charge) occurs.
[0042] In some embodiments, the support 110/210 is transparent over
the range of wavelengths for which transmission through the support
110/210 is desired. In one embodiment, the support 110/210 may be
transparent to visible light having a wavelength in a range from
about 400 nm to about 1000 nm. In some embodiments, the support
110/210 includes a material capable of withstanding heat treatment
temperatures greater than about 600.degree. C., such as, for
example, silica or borosilicate glass. In some other embodiments,
the support 110/210 includes a material that has a softening
temperature lower than 600.degree. C., such as, for example,
soda-lime glass or a polyimide. In some embodiments certain other
layers may be disposed between the transparent conductive oxide
layer 120/220 and the support 110/210, such as, for example, an
anti-reflective layer or a barrier layer (not shown).
[0043] The term "transparent conductive oxide layer" as used herein
refers to a substantially transparent layer capable of functioning
as a front current collector. In some embodiments, the transparent
conductive oxide layer 120/220 includes a transparent conductive
oxide (TCO). Non-limiting examples of transparent conductive oxides
include cadmium tin oxide (Cd.sub.2SnO.sub.4 or CTO); indium tin
oxide (ITO); fluorine-doped tin oxide (SnO:F or FTO); indium-doped
cadmium-oxide; doped zinc oxide (ZnO), such as aluminum-doped
zinc-oxide (ZnO:Al or AZO), indium-zinc oxide (IZO), and zinc tin
oxide (ZnSnO.sub.x); or combinations thereof. Depending on the
specific TCO employed and on its sheet resistance, the thickness of
the transparent conductive oxide layer 120/220 may be in a range of
from about 50 nm to about 600 nm, in one embodiment.
[0044] The term "buffer layer" as used herein refers to a layer
interposed between the transparent conductive oxide layer 120 and
the window layer 140, wherein the layer 130 has a higher sheet
resistance than the sheet resistance of the transparent conductive
oxide layer 120. The buffer layer 130 is sometimes referred to in
the art as a "high-resistivity transparent conductive oxide layer"
or "HRT layer".
[0045] Non-limiting examples of suitable materials for the buffer
layer 130 include tin dioxide (SnO.sub.2), zinc tin oxide
(zinc-stannate (ZTO)), zinc-doped tin oxide (SnO.sub.2:Zn), zinc
oxide (ZnO), indium oxide (In.sub.2O.sub.3), or combinations
thereof. In some embodiments, the thickness of the buffer layer 130
is in a range from about 50 nm to about 200 nm.
[0046] The term "window layer" as used herein refers to a
semiconducting layer that is substantially transparent and forms a
heterojunction with an absorber layer 160/260 (indicated in FIGS. 3
and 5). Non-limiting exemplary materials for the window layer 140
include cadmium sulfide (CdS), indium III sulfide
(In.sub.2S.sub.3), zinc sulfide (ZnS), zinc telluride (ZnTe), zinc
selenide (ZnSe), cadmium selenide (CdSe), oxygenated cadmium
sulfide (CdS:O), copper oxide (Cu.sub.2O), zinc oxihydrate (ZnO:H),
or combinations thereof. In certain embodiments, the window layer
140/240 includes cadmium sulfide (CdS). In certain embodiments, the
window layer 140/240 includes oxygenated cadmium sulfide
(CdS:O).
[0047] The term "absorber layer" as used herein refers to a
semiconducting layer wherein the solar radiation is absorbed. In
one embodiment, the absorber layer 160/260 includes a p-type
semiconductor material. In one embodiment, the absorber layer
160/260 has an effective carrier density in a range from about
1.times.10.sup.13 per cubic centimeter to about 1.times.10.sup.16
per cubic centimeter. As used herein, the term "effective carrier
density" refers to the average concentration of holes and electrons
in a material.
[0048] In one embodiment, a photoactive material is used for
forming the absorber layer 160/260. Suitable photoactive materials
include cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe),
cadmium magnesium telluride (CdMgTe), cadmium manganese telluride
(CdMnTe), cadmium sulfur telluride (CdSTe), cadmium selenium
telluride (CdSeTe), zinc telluride (ZnTe), copper indium disulfide
(CIS), copper indium diselenide (CISe), copper indium gallium
sulfide (CIGS), copper indium gallium diselenide (CIGSe), copper
indium gallium sulfur selenium (CIGSSe), copper indium gallium
aluminum sulfur selenium (Cu(In,Ga,Al)(S,Se).sub.2), copper zinc
tin sulfide (CZTS), or combinations thereof. The above-mentioned
photoactive semiconductor materials may be used alone or in
combination. Further, these materials may be present in more than
one layer, each layer having different type of photoactive
material, or having combinations of the materials in separate
layers. In certain embodiments, the absorber layer 160/260 includes
cadmium telluride (CdTe). In certain embodiments, the absorber
layer 160/260 includes p-type cadmium telluride (CdTe).
[0049] In some embodiments, the window layer 140/240, the absorber
layer 160/260, or both the layers may contain oxygen. Without being
bound by any theory, it is believed that the introduction of oxygen
to the window layer 140/240 (e.g., the CdS layer) may result in
improved device performance. In some embodiments, the amount of
oxygen is less than about 20 atomic percent. In some instances, the
amount of oxygen is between about 1 atomic percent to about 10
atomic percent. In some instances, for example in the absorber
layer 160/260, the amount of oxygen is less than about 1 atomic
percent. Moreover, the oxygen concentration within the window layer
140/240, the absorber layer 160/260, or both the layers may be
substantially constant or compositionally graded across the
thickness of the respective layer.
[0050] In some embodiments, the window layer 140/240 and the
absorber layer 160/260 may be doped with a p-type dopant or an
n-type dopant to form a heterojunction. As used in this context, a
heterojunction is a semiconductor junction that is composed of
layers of dissimilar semiconductor material. These materials
usually have non-equal band gaps. As an example, a heterojunction
can be formed by contact between a layer or region of one
conductivity type with a layer or region of opposite conductivity,
e.g., a "p-n" junction.
[0051] In some embodiments, the window layer 140/240 includes an
n-type semiconductor material. In such instances, the absorber
layer 160/260 may be doped to be p-type and the window layer
140/240 and the absorber layer 160/260 may form an "n-p"
heterojunction. In some embodiments, the window layer 140/240 may
be doped to be n-type and the absorber layer 160/260 may be doped
such that it effectively forms an n-i-p configuration, using a
p+-semiconductor layer on the backside of the absorber layer
160/260.
[0052] In some embodiments, the photovoltaic device 100/200 may
further include an optional secondary interlayer 155 interposed
between the window layer 140/240 and the absorber layer 160/260, as
indicated in FIGS. 3 and 5. In such instances, without being bound
by any theory, it is believed that the first window layer 140/240
and the absorber layer 160/260 may form a heterojunction, such as,
a "p-n" junction or a "n-i-p" junction with the interlayer 155
positioned in between.
[0053] In some embodiments, the secondary interlayer 155 includes a
metal species including magnesium, aluminum, zinc, nickel,
gadolinium, or combinations thereof. The term "metal species" as
used in this context refers to elemental metal, metal ions, or
combinations thereof. In some embodiments, the secondary interlayer
155 may include a plurality of the metal species. In some
embodiments, at least a portion of the metal species is present in
the secondary interlayer 150 in the form of an elemental metal, a
metal alloy, a metal compound, or combinations thereof. In certain
embodiments, the secondary interlayer 155 includes magnesium,
gadolinium, or combinations thereof.
[0054] In some embodiments, the photovoltaic device 100/200 may
further include a p+-type semiconductor layer 170/270 disposed on
the absorber layer 160/260, as indicated in FIGS. 3 and 5. The term
"p+-type semiconductor layer" as used herein refers to a
semiconductor layer having an excess mobile p-type carrier or hole
density compared to the p-type charge carrier or hole density in
the absorber layer 160/260. In some embodiments, the p+-type
semiconductor layer has a p-type carrier density in a range greater
than about 1.times.10.sup.16 per cubic centimeter. The p+-type
semiconductor layer 170/270 may be used as an interface between the
absorber layer 160/260 and the back contact layer 180/280, in some
embodiments.
[0055] In one embodiment, the p+-type semiconductor layer 170/270
includes a heavily doped p-type material including amorphous Si:H,
amorphous SiC:H, crystalline Si, microcrystalline Si:H,
microcrystalline SiGe:H, amorphous SiGe:H, amorphous Ge,
microcrystalline Ge, GaAs, BaCuSF, BaCuSeF, BaCuTeF, LaCuOS,
LaCuOSe, LaCuOTe, LaSrCuOS, LaCuOSe.sub.0.6Te.sub.0.4, BiCuOSe,
BiCaCuOSe, PrCuOSe, NdCuOS, Sr.sub.2Cu.sub.2ZnO.sub.2S.sub.2,
Sr.sub.2CuGaO.sub.3S, (Zn,Co,Ni)O.sub.x, or combinations thereof.
In another embodiment, the p+-type semiconductor layer 170/270
includes a p+-doped material including zinc telluride, magnesium
telluride, manganese telluride, beryllium telluride, mercury
telluride, arsenic telluride, antimony telluride, copper telluride,
or combinations thereof. In some embodiments, the p+-doped material
further includes a dopant including copper, gold, nitrogen,
phosphorus, antimony, arsenic, silver, bismuth, sulfur, sodium, or
combinations thereof.
[0056] In some embodiments, the photovoltaic device 100/200 further
includes a back contact layer 180/280, as indicated in FIGS. 3 and
5. In some embodiments, the back contact layer 180/280 is disposed
directly on the absorber layer 160/260 (embodiment not shown). In
some other embodiments, the back contact layer 180/280 is disposed
on the p+-type semiconductor layer 170/270 disposed on the absorber
layer 160/260, as indicated in FIGS. 3 and 5.
[0057] In some embodiments, the back contact layer 180/280 includes
gold, platinum, molybdenum, tungsten, tantalum, titanium,
palladium, aluminum, chromium, nickel, silver, graphite, or
combinations thereof. The back contact layer 180/280 may include a
plurality of layers that function together as the back contact.
[0058] In some embodiments, another metal layer (not shown), for
example, aluminum, may be disposed on the back contact layer
180/280 to provide lateral conduction to the outside circuit. In
certain embodiments, a plurality of metal layers (not shown), for
example, aluminum and chromium, may be disposed on the back contact
layer 180/280 to provide lateral conduction to the outside circuit.
In certain embodiments, the back contact layer 180/280 may include
a layer of carbon, such as, graphite deposited on the absorber
layer 160/260, followed by one or more layers of metal, such as the
metals described above.
[0059] In alternative embodiments, as illustrated in FIG. 6, a
photovoltaic device 300 including a "substrate" configuration is
presented. The photovoltaic device 300 includes a back contact
layer 380 disposed on a support 390. Further, an absorber layer 360
is disposed on the back contact layer 380. A window layer 340 is
disposed on the absorber layer 360 and an interlayer 350 is
disposed on the window layer 340. A transparent conductive oxide
layer 320 is further disposed on the interlayer 350, as indicated
in FIG. 6. As illustrated in FIG. 6, in such embodiments, the solar
radiation 10 enters from the transparent conductive oxide layer 320
and after passing through the interlayer 350 and the window layer
340, enters the absorber layer 360, where the conversion of
electromagnetic energy of incident light (for instance, sunlight)
to electron-hole pairs (that is, to free electrical charge)
occurs.
[0060] In some embodiments, the composition of the layers
illustrated in FIG. 6, such as, the substrate 310, the transparent
conductive oxide layer 320, the window layer 340, the interlayer
350, the absorber layer 360, and the back contact layer 380 may
have the same composition as described above in FIG. 5 for the
superstrate configuration.
[0061] Some embodiments include a method of making a photovoltaic
device. In some embodiments, the method generally includes
disposing the interlayer 150/250 between the transparent conductive
oxide layer 120/220 and the window layer 220/240.
[0062] In some embodiments, the method further includes disposing a
buffer layer 130 between the transparent conductive oxide layer 120
and the window layer 140. In some embodiments, with continued
reference to FIG. 1, the method generally includes disposing the
interlayer 150 between the buffer layer 130 and the window layer
140. In some other embodiments, with continued reference to FIG. 2,
the method generally includes disposing the interlayer 150 between
the transparent conductive oxide layer 120 and the buffer layer
130.
[0063] As understood by a person skilled in the art, the sequence
of disposing the three layers or the whole device may depend on a
desirable configuration, for example, "substrate" or "superstrate"
configuration of the device.
[0064] In certain embodiments, a method for making a photovoltaic
device 100/200 in superstrate configuration is described. Referring
now to FIGS. 7 and 8, in some embodiments, the method includes
disposing a capping layer 152/252 on the buffer layer 130 (FIG. 7),
or directly on the transparent conductive oxide layer 220 (FIG. 8)
to form a semiconductor assembly 155/255.
[0065] The capping layer 152/252 includes the metal species. In
some embodiments, the metal species is present in the capping layer
152/252 in the form of an elemental metal, a binary metal compound,
a metal alloy, or combinations thereof. In certain embodiments, the
capping layer 152/252 includes a metal oxide, a metal fluoride, or
combinations thereof.
[0066] The capping layer 152/252 may be disposed using a suitable
deposition technique, such as, for example, sputtering, atomic
layer deposition, or combinations thereof. In certain embodiments,
the method includes disposing the capping layer 152/252 by atomic
layer deposition (ALD). In certain embodiments, the method includes
disposing the capping layer 152/252 by sputtering. Without being
bound by any theory, it is believed that deposition of the capping
layer 152/252 by ALD or sputtering may provide for a more conformal
layer in comparison to other deposition methods. A conformal layer
may provide for a more uniform contact of the subsequent interlayer
150/250 with the window layer 140/240. Further, deposition of the
capping layer by ALD/sputtering may provide for an interlayer
150/250 having lower number of pinholes when compared to layers
deposited using other deposition techniques.
[0067] The method further includes disposing a window layer 140/240
on the capping layer 152/252. Non-limiting examples of the
deposition methods for the window layer 140/240 include one or more
of close-space sublimation (CSS), vapor transport deposition (VTD),
sputtering (for example, direct current pulse sputtering (DCP),
electro-chemical deposition (ECD), and chemical bath deposition
(CBD).
[0068] The method further includes forming an interlayer 150/250.
The interlayer composition and configuration are as described
earlier. The step of forming the interlayer 150/250 may be effected
prior to, simultaneously with, or after the step of disposing the
window layer 140/240 on the capping layer 152/252.
[0069] In some embodiments, the interlayer 150/250 may be formed
prior to the step of disposing the window layer 140/240. In such
instances, the method may further include a step of thermally
processing the semiconductor assembly 155/255. The step of thermal
processing may include, for example, annealing of the semiconductor
assembly 155/255.
[0070] In some other embodiments, the interlayer 150/250 may be
formed simultaneously with the step of disposing the window layer
140/240. In some embodiments, the interlayer 150/250 may be formed
after the step of disposing the window layer 140/240, for example,
during the high-temperature absorber layer (e.g., CdTe) deposition
step, during the cadmium chloride treatment step, during the
p+-type layer formation step, during the back contact formation
step, or combinations thereof.
[0071] In some embodiments, the step of interlayer 150/250
formation may further include intermixing of at least a portion of
the metal species in the capping layer 152/252 with at least
portion of the transparent conductive oxide layer 120/220 material,
the buffer layer 130 material, or both. Without being bound by any
theory, it is believed that during the window layer-deposition step
or the post-deposition processing steps, recrystallization and
chemical changes may occur in the capping layer 152/252, and a
metal compound or a metal alloy may be formed in the resultant
interlayer 150/250.
[0072] In some instances, the method may further result in
formation of oxides of the metal species in the capping layer
152/252, and one or more of the metal species present in the
transparent conductive oxide layer 120/220 or the buffer layer 130,
during the interlayer 150/250 formation. In some instances, the
method may result in formation of a metal compound including the
metal species, tin, and oxygen during the interlayer 150/250
formation, for example, gadolinium tin oxide. In some instances,
the method may result in formation of a metal compound including
the metal species, zinc, tin, and oxygen during the interlayer
150/250 formation, for example, gadolinium zinc tin oxide.
[0073] As noted earlier, the photovoltaic device may further
include one or more additional layers, for example, a support
110/210, an absorber layer 160/260, a p+-type semiconductor layer
170/270, and a back contact layer 180/280, as depicted in FIGS. 3
and 5.
[0074] In some embodiments, the method further includes disposing
the transparent conductive oxide layer 120/220 on a support
110/210, as indicated in FIGS. 3 and 5. The transparent conductive
oxide layer 120/220 is disposed on the support 110/210 by any
suitable technique, such as sputtering, chemical vapor deposition,
spin coating, spray coating, or dip coating. Referring to FIG. 3,
in some embodiments, a buffer layer 130 may be deposited on the
transparent conductive oxide layer 120 using sputtering.
[0075] The method further includes disposing an absorber layer
160/260 on the window layer 140/240. In one embodiment, the
absorber layer 160/260 may be deposited using a suitable method,
such as, close-space sublimation (CSS), vapor transport deposition
(VTD), ion-assisted physical vapor deposition (IAPVD), radio
frequency or pulsed magnetron sputtering (RFS or PMS), plasma
enhanced chemical vapor deposition (PECVD), or electrochemical
deposition (ECD).
[0076] In some embodiments, a series of post-forming treatments may
be further applied to the exposed surface of the absorber layer
160/260. These treatments may tailor the functionality of the
absorber layer 160/260 and prepare its surface for subsequent
adhesion to the back contact layer(s) 180/280. For example, the
absorber layer 160/260 may be annealed at elevated temperatures for
a sufficient time to create a quality p-type layer. Further, the
absorber layer 160/260 may be treated with a passivating agent
(e.g., cadmium chloride) and a tellurium-enriching agent (for
example, iodine or an iodide) to form a tellurium-rich region in
the absorber layer 160/260. Additionally, copper may be added to
the absorber layer 160/260 in order to obtain a low-resistance
electrical contact between the absorber layer 160/260 and a back
contact layer(s) 180/280.
[0077] Referring again to FIGS. 3 and 5, a p+-type semiconducting
layer 170/270 may be further disposed on the absorber layer 160/260
by depositing a p+-type material using any suitable technique, for
example PECVD or sputtering. In an alternate embodiment, as
mentioned earlier, a p+-type semiconductor region may be formed in
the absorber layer 160/260 by chemically treating the absorber
layer 160/260 to increase the carrier density on the back-side
(side in contact with the metal layer and opposite to the window
layer) of the absorber layer 160/260 (for example, using iodine and
copper). In some embodiments, a back contact layer 180/280, for
example, a graphite layer may be deposited on the p+-type
semiconductor layer 170/270, or directly on the absorber layer
160/260 (embodiment not shown). A plurality of metal layers may be
further deposited on the back contact layer 180/280.
[0078] One or more of the window layer 140/240, the absorber layer
160/260, the back contact layer 180/280, or the p+type layer
170/270 (optional) may be further heated or subsequently treated
(for example, annealed) after deposition to manufacture the
photovoltaic device 100/200.
[0079] In some embodiments, other components (not shown) may be
included in the exemplary photovoltaic device 100/200, such as,
buss bars, external wiring, laser etches, etc. For example, when
the device 100/200 forms a photovoltaic cell of a photovoltaic
module, a plurality of photovoltaic cells may be connected in
series in order to achieve a desired voltage, such as through an
electrical wiring connection. Each end of the series connected
cells may be attached to a suitable conductor such as a wire or bus
bar, to direct the generated current to convenient locations for
connection to a device or other system using the generated current.
In some embodiments, a laser may be used to scribe the deposited
layers of the photovoltaic device 100/200 to divide the device into
a plurality of series connected cells.
EXAMPLES
Comparative Example 1
Method of Manufacturing a Cadmium Telluride Photovoltaic Device,
without an Interlayer
[0080] A cadmium telluride photovoltaic device was made by
depositing several layers on a cadmium tin oxide (CTO) transparent
conductive oxide (TCO)-coated substrate. The substrate was a 1.4
millimeters thick PVN++ glass, which was coated with a CTO
transparent conductive oxide layer and a thin high resistance
transparent zinc tin oxide (ZTO) buffer layer. The window layer
containing cadmium sulfide (CdS:O, 5 molar % oxygen in the CdS
layer) was then deposited on the ZTO layer by DC sputtering
followed by deposition of cadmium telluride (CdTe) layer at
550.degree. C., and back contact formation.
Example 1
Method of Manufacturing a Cadmium Telluride Photovoltaic Device
Including an Interlayer Between the Buffer Layer and the CdS
Layer
[0081] The method of making the photovoltaic device was similar to
the Comparative Example 1, except a capping layer of varying
thickness (3 nm and 6 nm) was deposited by sputtering on the ZTO
buffer layer to form an interlayer, prior to the deposition of the
CdS layer. The capping layer included elemental gadolinium,
elemental calcium, elemental strontium, hafnium oxide, or yttrium
oxide.
[0082] FIG. 9 illustrates the device efficiency values (normalized
with respect to Comparative Example 1) for devices with and without
an interlayer. As illustrated in FIG. 9, the efficiency values
showed improvement for the devices including Ca, Sr, or Gd-based
interlayer, when compared to the device without the interlayer
(Comparative Example 1). Further, the efficiency values for devices
including Y or Hf-based interlayers were comparable to the
efficiency value for the device without the interlayer (Comparative
Example 1).
[0083] The appended claims are intended to claim the invention as
broadly as it has been conceived and the examples herein presented
are illustrative of selected embodiments from a manifold of all
possible embodiments. Accordingly, it is the Applicants' intention
that the appended claims are not to be limited by the choice of
examples utilized to illustrate features of the present invention.
As used in the claims, the word "comprises" and its grammatical
variants logically also subtend and include phrases of varying and
differing extent such as for example, but not limited thereto,
"consisting essentially of" and "consisting of." Where necessary,
ranges have been supplied; those ranges are inclusive of all
sub-ranges there between. It is to be expected that variations in
these ranges will suggest themselves to a practitioner having
ordinary skill in the art and where not already dedicated to the
public, those variations should where possible be construed to be
covered by the appended claims. It is also anticipated that
advances in science and technology will make equivalents and
substitutions possible that are not now contemplated by reason of
the imprecision of language and these variations should also be
construed where possible to be covered by the appended claims.
* * * * *