U.S. patent application number 16/032531 was filed with the patent office on 2018-11-08 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 Kristian William Andreini, Holly Ann Blaydes, Jongwoo Choi, Adam Fraser Halverson, Eugene Thomas Hinners, William Hullinger Huber, Yong Liang, Joseph John Shiang.
Application Number | 20180323334 16/032531 |
Document ID | / |
Family ID | 52004414 |
Filed Date | 2018-11-08 |
United States Patent
Application |
20180323334 |
Kind Code |
A1 |
Blaydes; Holly Ann ; et
al. |
November 8, 2018 |
Photovoltaic Devices and Method of Making
Abstract
Embodiments of a photovoltaic device are provided herein. The
photovoltaic device can include a layer stack and an absorber layer
disposed on the layer stack. The absorber layer can include a first
region and a second region. Each of the first region of the
absorber layer and the second region of the absorber layer can
include a compound comprising cadmium, selenium, and tellurium. An
atomic concentration of selenium can vary across the absorber
layer. The first region of the absorber layer can have a thickness
between 100 nanometers to 3000 nanometers. The second region of the
absorber layer can have a thickness between 100 nanometers to 3000
nanometers. A ratio of an average atomic concentration of selenium
in the first region of the absorber layer to an average atomic
concentration of selenium in the second region of the absorber
layer can be greater than 10.
Inventors: |
Blaydes; Holly Ann;
(Perrysburg, OH) ; Andreini; Kristian William;
(Perrysburg, OH) ; Huber; William Hullinger;
(Perrysburg, OH) ; Hinners; Eugene Thomas;
(Perrysburg, OH) ; Shiang; Joseph John;
(Perrysburg, OH) ; Liang; Yong; (Perrysburg,
OH) ; Choi; Jongwoo; (Perrysburg, OH) ;
Halverson; Adam Fraser; (Perrysburg, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
First Solar, Inc. |
Tempe |
AZ |
US |
|
|
Assignee: |
First Solar, Inc.
Tempe
AZ
|
Family ID: |
52004414 |
Appl. No.: |
16/032531 |
Filed: |
July 11, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13912782 |
Jun 7, 2013 |
10062800 |
|
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16032531 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/1828 20130101;
H01L 31/1832 20130101; H01L 31/02966 20130101; H01L 31/065
20130101; Y02P 70/50 20151101; H01L 31/073 20130101; Y02E 10/543
20130101; H01L 31/022425 20130101; Y02P 70/521 20151101 |
International
Class: |
H01L 31/18 20060101
H01L031/18; H01L 31/073 20060101 H01L031/073; H01L 31/065 20060101
H01L031/065; H01L 31/0224 20060101 H01L031/0224; H01L 31/0296
20060101 H01L031/0296 |
Claims
1. A photovoltaic device, comprising: a layer stack; and an
absorber layer disposed on the layer stack, wherein: the absorber
layer comprises a compound comprising cadmium, selenium, and
tellurium, the compound comprising cadmium, selenium, and tellurium
has a first region and a second region, an atomic concentration of
selenium varies across the absorber layer; the first region of the
compound comprising cadmium, selenium, and tellurium has a
thickness between 100 nanometers to 3000 nanometers, the second
region of the compound comprising cadmium, selenium, and tellurium
has a thickness between 100 nanometers to 3000 nanometers, and a
ratio of an average atomic concentration of selenium in the first
region of the compound comprising cadmium, selenium, and tellurium
to an average atomic concentration of selenium in the second region
of the compound comprising cadmium, selenium, and tellurium is
greater than 10.
2. The photovoltaic device of claim 1, wherein a peak of a
Se/(Se+Te) ratio of the absorber layer is less than 0.40.
3. The photovoltaic device of claim 1, wherein an average atomic
concentration of selenium in the absorber layer is in a range from
0.001 atomic percent to 40 atomic percent of the absorber
layer.
4. The photovoltaic device of claim 1, wherein the atomic
concentration of selenium in the absorber layer has a profile
comprising an exponential profile, a top-hat profile, a step-change
profile, a saw-tooth profile, a square-wave profile, a power law
profile, or combinations thereof.
5. The photovoltaic device of claim 1, wherein the absorber layer
comprises sulfur, oxygen, copper, chlorine, lead, zinc, mercury, or
combinations thereof.
6. The photovoltaic device of claim 1, wherein at least a portion
of the compound comprising cadmium, selenium, and tellurium is a
ternary compound or a quaternary compound.
7. The photovoltaic device of claim 1, wherein the compound
comprising cadmium, selenium, and tellurium further comprises
mercury.
8. The photovoltaic device of claim 7, an amount of mercury varies
across a thickness of the absorber layer.
9. The photovoltaic device of claim 1, wherein the compound
comprising cadmium, selenium, and tellurium further comprises
zinc.
10. The photovoltaic device of claim 9, an amount of zinc varies
across a thickness of the absorber layer.
11. A photovoltaic device, comprising: a layer stack comprising a
transparent conductive oxide; and an absorber layer disposed on the
layer stack, wherein: the absorber layer comprises a compound
comprising cadmium, selenium, and tellurium, the compound
comprising cadmium, selenium, and tellurium comprising a first
region disposed proximate to the layer stack relative to a second
region, the first region of the compound comprising cadmium,
selenium, and tellurium is disposed at a front interface of the
absorber layer, the second region of the compound comprising
cadmium, selenium, and tellurium is disposed at a back interface of
the absorber layer, the first region of the compound comprising
cadmium, selenium, and tellurium has a thickness between 200
nanometers to 1500 nanometers, the second region of the compound
comprising cadmium, selenium, and tellurium has a thickness between
200 nanometers to 1500 nanometers, and a ratio of an average atomic
concentration of selenium in the first region of the compound
comprising cadmium, selenium, and tellurium to an average atomic
concentration of selenium in the second region of the compound
comprising cadmium, selenium, and tellurium is greater than 2.
12. The photovoltaic device of claim 11, wherein the compound
comprising cadmium, selenium, and tellurium further comprises
mercury.
13. The photovoltaic device of claim 12, an amount of mercury
varies across a thickness of the absorber layer.
14. The photovoltaic device of claim 11, wherein the compound
comprising cadmium, selenium, and tellurium further comprises
zinc.
15. The photovoltaic device of claim 14, an amount of zinc varies
across a thickness of the absorber layer.
16. The photovoltaic device of claim 11, wherein an average atomic
concentration of selenium in the absorber layer is in a range from
0.01 atomic percent to 25 atomic percent of the absorber layer.
17. The photovoltaic device of claim 16, wherein a peak of a
Se/(Se+Te) ratio of the absorber layer is located at the front
interface of the absorber layer.
18. The photovoltaic device of claim 17, wherein the peak of the
Se/(Se+Te) ratio of the absorber layer is less than 0.40.
19. The photovoltaic device of claim 18, wherein the photovoltaic
device is substantially free of a cadmium sulfide layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 13/912,782, filed on Jun. 7, 2013, now U.S.
Pat. No. ______.
BACKGROUND
[0002] The invention generally relates to photovoltaic devices.
More particularly, the invention relates to photovoltaic devices
including selenium, and methods of making the photovoltaic
devices.
[0003] 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.
[0004] 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. Further, a
lattice mismatch between the window layer and absorber layer (e.g.,
CdS/CdTe) layer may lead to high defect density at the interface,
which may further lead to shorter interface carrier lifetime. 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 (Voc)
and fill factor (FF).
[0005] Thus, there is a need for improved thin film photovoltaic
devices configurations, and methods of manufacturing these.
BRIEF DESCRIPTION OF THE INVENTION
[0006] Embodiments of the present invention are included to meet
these and other needs. One embodiment is a photovoltaic device. The
photovoltaic device includes a layer stack; and an absorber layer
is disposed on the layer stack. The absorber layer includes
selenium, and an atomic concentration of selenium varies
non-linearly across a thickness of the absorber layer.
[0007] One embodiment is a photovoltaic device. The photovoltaic
device includes a layer stack including a transparent conductive
oxide layer disposed on a support, a buffer layer disposed on the
transparent conductive oxide layer, and a window layer disposed on
the buffer layer. The layer stack further includes an absorber
layer disposed on the layer stack, wherein the absorber layer
includes selenium, and an atomic concentration of selenium varies
non-linearly across a thickness of the absorber layer.
[0008] One embodiment is a method of making a photovoltaic device.
The method includes providing an absorber layer on a layer stack,
wherein the absorber layer includes selenium, and wherein an atomic
concentration of selenium varies non-linearly across a thickness of
the absorber layer.
DRAWINGS
[0009] 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:
[0010] FIG. 1 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0011] FIG. 2 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0012] FIG. 3 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0013] FIG. 4 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0014] FIG. 5 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0015] FIG. 6 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0016] FIG. 7 is a schematic of a photovoltaic device, according to
some embodiments of the invention.
[0017] FIG. 8 is a schematic of a method of making a photovoltaic
device, according to some embodiments of the invention.
[0018] FIG. 9 shows the Se concentration as a function of depth, in
accordance with one embodiment of the invention.
[0019] FIG. 10 shows the log-log plot of Se concentration as a
function of depth, in accordance with one embodiment of the
invention.
[0020] FIG. 11 shows the Se concentration as a function of depth,
in accordance with some embodiments of the invention.
DETAILED DESCRIPTION
[0021] As discussed in detail below, some of the embodiments of the
invention include photovoltaic devices including selenium.
[0022] 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.
[0023] 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.
[0024] 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 1000
nm.
[0025] 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. Furthermore, the term "a layer" as used herein refers to
a single layer or a plurality of sub-layers, unless the context
clearly dictates otherwise.
[0026] 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.
[0027] 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.
[0028] As discussed in detail below, some embodiments of the
invention are directed to a photovoltaic device including selenium.
A photovoltaic device 100, according to some embodiments of the
invention, is illustrated in FIGS. 1-5. As shown in FIGS. 1-5, the
photovoltaic device 100 includes a layer stack 110 and an absorber
layer 120 disposed on the layer stack 110. The absorber layer 120
includes selenium, and an atomic concentration of selenium varies
non-linearly across a thickness of the absorber layer 120.
[0029] The term "atomic concentration" as used in this context
herein refers to the average number of selenium atoms per unit
volume of the absorber layer. The terms "atomic concentration" and
"concentration" are used herein interchangeably throughout the
text. The term "varies non-linearly across the thickness" as used
herein means that the rate-of-change in concentration itself varies
across the thickness of the absorber layer 120.
[0030] As used herein the term "linear gradient" refers to the
first derivative of a given property, which when measured respect
to a dimensional parameter, such as the distance from the front
contact is both continuous and constant. For example, a step-wise
distribution with a fixed concentration of selenium (Se) at the
front contact, which then abruptly transitions to a different
concentration after some distance away from the front contact, is
non-linear due to the fact that the first derivative is
non-continuous at the point where the concentration of Se
transitions from one value to another. An exponentially varying
distribution is another example of a non-linear distribution since
the value of the first derivative continuously changes as a
function of distance. The linearity of a given distribution may be
readily assessed by plotting the logarithm of the measured property
versus the logarithm of the dimensional parameter. A linear
gradient implies that the data when plotted this manner can be fit
to a line with a unity slope. A super-linear distribution will have
a slope greater than unity and a sub-linear distribution will have
a slope less than 1.
[0031] Measurement of a first derivative of a material property in
a real material implies averaging of the material property over a
defined dimension and length scale, since the atomic nature of real
materials may lead to local discontinuities of the first
derivative. The non-linear distributions of interest according to
some embodiments of the invention are in the axis that goes from
the front contact to the back contact, which will be referred to as
the z-axis or z-dimension. Thus, to measure the non-linearity of
the distribution of a property along the z-axis, it may be useful
to average the measured properties over the orthogonal axes, x, y
in order to minimize the effect of grain-boundaries and other local
inhomogeneities on the measurement.
[0032] A lower limit for the averaging window is set by the polaron
radius of the material which scales the typical "size" of a charge
carrier within a real material:
r p = h 4 .pi. m .omega. ##EQU00001##
where h is Planck's constant, m is the effective mass of the charge
carrier, and .omega. is the highest angular frequency of a typical
vibration of the lattice, which is typically an optical phonon. In
cadmium telluride (CdTe), the effective mass of the electron is
about 0.1 me, where me is the mass of an electron in free space and
the phonon angular frequency is about 2.1.times.10.sup.13. Thus,
the calculated polaron radius is about 5 nm and a calculated
polaron diameter is about 10 nm. Since proto-typical Gaussian or
exponential wave functions have significant amplitude about 2-3
times their nominal characteristic size, then an estimate of the
`size` of charge carrier in CdTe based material is about 30 nm. A
typical charge carrier in a CdTe type material will sample a 30 nm
diameter sphere at any given time, and its behavior will to a large
extent be determined by the average physical properties within this
sphere. Thus, to determine the degree of non-linearity relevant to
the performance of the photovoltaic cells in accordance with some
embodiments of the invention, it may not be necessary to resolve
non-linearities in the distribution of a given property or
composition below a length scale of about 30 nm. An upper limit on
the averaging required is set by the need to sample a sufficient
number of points, i.e. 3, along the z axis so that the linearity of
the distribution may be determined.
[0033] In some embodiments, there is a step-change in the
concentration of selenium across the thickness of the absorber
layer. In such instances, the selenium concentration may remain
substantially constant for some portion of the thickness. The term
"substantially constant" as used in this context means that the
change in concentration is less than 5 percent across that portion
of the thickness.
[0034] In some embodiments, the concentration of selenium varies
continuously across the thickness of the absorber layer 120.
Further, in such instances, the variation in the selenium
concentration may be monotonic or non-monotonic. In certain
embodiments, the concentration of selenium varies non-monotonically
across a thickness of the absorber layer. In some instances, the
rate-of-change in concentration may itself vary through the
thickness, for example, increasing in some regions of the
thickness, and decreasing in other regions of the thickness. A
suitable selenium profile may include any higher order non-linear
profile. Non-limiting examples of suitable selenium profiles
include an exponential profile, a top-hat profile, a step-change
profile, a square-wave profile, a power law profile (with exponent
greater than 1 or less than 1), or combinations thereof. FIG. 11
illustrates a few examples of representative non-linear selenium
profiles in the absorber layer 120. As will be appreciated by one
of ordinary skill in the art, the profile of the selenium
concentration may further vary after the processing steps, and the
final device may include a diffused version of the profiles
discussed here.
[0035] In some embodiments, the selenium concentration decreases
across the thickness of the absorber layer 120, in a direction away
from the layer stack 110. In some embodiments, the selenium
concentration monotonically decreases across the thickness of the
absorber layer 120, in a direction away from the layer stack 110.
In some embodiments, the selenium concentration continuously
decreases across a certain portion of the absorber layer 120
thickness, and is further substantially constant in some other
portion of the absorber layer 120 thickness.
[0036] In certain embodiments, the absorber layer 120 includes a
varying concentration of selenium such that there is lower
concentration of selenium near the front interface (interface
closer to the front contact) relative to the back interface
(interface closer to the back contact). In certain embodiments, the
absorber layer 120 includes a varying concentration of selenium
such that there is higher concentration of selenium near the front
interface (interface closer to the front contact) relative to the
back interface (interface closer to the back contact).
[0037] In certain embodiments, the band gap in the absorber layer
120 may vary across a thickness of the absorber layer 120. In some
embodiments, the concentration of selenium may vary across the
thickness of the absorber layer 120 such that the band gap near the
front interface is lower than the band gap near the back
interface.
[0038] Without being bound by any theory, it is believed that a
higher concentration of selenium near the front interface relative
to the back interface may further allow for a higher fraction of
incident radiation to be absorbed in the absorber layer 120.
Moreover, selenium may improve the passivation of grain boundaries
and interfaces, which can be seen through higher bulk lifetime and
reduced surface recombination. Further, the lower band gap material
near the front interface may enhance efficiency through photon
confinement.
[0039] In some embodiments, the photovoltaic device 100 is
substantially free of a cadmium sulfide layer. The term
"substantially free of a cadmium sulfide layer" as used herein
means that a percentage coverage of the cadmium sulfide layer (if
present) on the underlying layer (for example, the interlayer or
the buffer layer) is less than 20 percent. In some embodiments, the
percentage coverage is in a range from about 0 percent to about 10
percent. In some embodiments, the percentage coverage is in a range
from about 0 percent to about 5 percent. In certain embodiments,
the photovoltaic device is completely free of the cadmium sulfide
layer.
[0040] In certain embodiments, the absorber layer 120 may include a
heterojunction. As used herein, a heterojunction is a semiconductor
junction that is composed of layers/regions 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 having an excess electron concentration
with a layer or region having an excess of hole concentration e.g.,
a "p-n" junction.
[0041] As will be appreciated by one of ordinary skill in the art,
by varying the concentration of selenium in the absorber layer 120,
a particular region of the absorber layer 120 may be rendered
n-type and another region of the absorber layer 120 may be rendered
p-type. In certain embodiments, the absorber layer 120 includes a
"p-n" junction. The "p-n" junction may be formed between a
plurality of regions of the absorber layer 120 having different
band gaps. Without being bound by any theory, it is believed that
the variation in selenium concentration may allow for a p-n
junction within the absorber layer 120 or formation of a junction
between the absorber layer and the underlying TCO layer.
[0042] In some embodiments, the photovoltaic device may further
include a window layer (including a material such as CdS). In some
embodiments, the absorber layer 120 may form a p-n junction with
the underlying buffer layer or the window layer. As described
earlier, the thickness of the window layer (including a material
such as CdS) is typically desired to be minimized in a photovoltaic
device to achieve high efficiency. With the presence of the varying
concentration of selenium in the absorber layer, the thickness of
the window layer (e.g., CdS layer) may be reduced or the window
layer may be eliminated, 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 or
elimination of CdS.
[0043] In some embodiments, as indicated in FIG. 2, the absorber
layer 120 includes a first region 122 and a second region 124. As
illustrated in FIG. 2, the first region 122 is disposed proximate
to the layer stack 110 relative to the second region 124. In some
embodiments, an average atomic concentration of selenium in the
first region 122 is greater than an average atomic concentration of
selenium in the second region 124.
[0044] In some embodiments, the selenium concentration in the first
region 122, the second region 124, or both the regions may further
vary across the thickness of the respective regions. In some
embodiments, the selenium concentration in the first region 122,
the second region 124, or both the regions may continuously change
across the thickness of the respective regions. As noted earlier,
in some instances, the rate-of-rate-of-change in concentration may
itself vary through the first region 122, the second region 124, or
both the regions, for example, increasing in some portions, and
decreasing in other portions.
[0045] In some embodiments, the selenium concentration in the first
region 122, the second region 124, or both the regions may be
substantially constant across the thickness of the respective
regions. In some other embodiments, the selenium concentration may
be substantially constant in at least a portion of the first region
122, the second region 124, or both the regions. The term
"substantially constant" as used in this context means that the
change in concentration is less than 5 percent across that portion
or region.
[0046] The absorber layer 120 may be further characterized by the
concentration of selenium present in the first region 122 relative
to the second region 124. In some embodiments, a ratio of the
average atomic concentration of selenium in the first region 122 to
the average atomic concentration of selenium in the second region
124 is greater than about 2. In some embodiments, a ratio of the
average atomic concentration of selenium in the first region 122 to
the average atomic concentration of selenium in the second region
124 is greater than about 5. In some embodiments, a ratio of the
average atomic concentration of selenium in the first region 122 to
the average atomic concentration of selenium in the second region
124 is greater than about 10.
[0047] The first region 122 and the second region 124 may be
further characterized by their thickness. In some embodiments, the
first region 122 has a thickness in a range from about 1 nanometer
to about 5000 nanometers. In some embodiments, the first region 122
has a thickness in a range from about 100 nanometers to about 3000
nanometers. In some embodiments, the first region 122 has a
thickness in a range from about 200 nanometers to about 1500
nanometers. In some embodiments, the second region 124 has a
thickness in a range from about 1 nanometer to about 5000
nanometers. In some embodiments, the second region 124 has a
thickness in a range from about 100 nanometers to about 3000
nanometers. In some embodiments, the second region 124 has a
thickness in a range from about 200 nanometers to about 1500
nanometers.
[0048] Referring again to FIG. 2, in some embodiments, the first
region 122 has a band gap that is lower than a band gap of the
second region 124. In such instances, the concentration of selenium
in the first region 122 relative to the second region 124 may be in
a range such that the band gap of the first region 122 is lower
than the band gap of the second region 124.
[0049] The absorber layer 120 also includes a plurality of grains
separated by grain boundaries. In some embodiments, an atomic
concentration of selenium in the grain boundaries is higher than
the atomic concentration of selenium in the grains.
[0050] Selenium may be present in the absorber layer 120, in its
elemental form, as a dopant, as a compound, or combinations
thereof. In certain embodiments, at least a portion of selenium is
present in the absorber layer in the form of a compound. The term
"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, cadmium, tellurium, and selenium have
defined lattice positions in the crystal structure of a cadmium
selenide telluride compound, in contrast, for example, to
selenium-doped cadmium telluride, where selenium may be a dopant
that is substitutionally inserted on cadmium sites, and not a part
of the compound lattice
[0051] In some embodiments, at least a portion of selenium is
present in the absorber layer 120 in the form of a ternary
compound, a quaternary compound, or combinations thereof. In some
embodiments, the absorber layer 120 may further include cadmium and
tellurium. In certain embodiments, at least a portion of selenium
is present in the absorber layer in the form of a compound having a
formula CdSe.sub.xTe.sub.1-x, wherein x is a number greater than 0
and less than 1. In some embodiments, x is in a range from about
0.01 to about 0.99, and the value of "x" varies across the
thickness of the absorber layer 120.
[0052] In some embodiments, the absorber layer 120 may further
include sulfur. In such instances, at least a portion of the
selenium is present in the absorber layer 120 in the form of a
quaternary compound including cadmium, tellurium, sulfur, and
selenium. Further, as noted earlier, in such instances, the
concentration of selenium may vary across a thickness of the
absorber layer 120.
[0053] The absorber layer 120 may be further characterized by the
amount of selenium present. In some embodiments, an average atomic
concentration of selenium in the absorber layer 120 is in a range
from about 0.001 atomic percent to about 40 atomic percent of the
absorber layer 120. In some embodiments, an average atomic
concentration of selenium in the absorber layer 120 is in a range
from about 0.01 atomic percent to about 25 atomic percent of the
absorber layer 120. In some embodiments, an average atomic
concentration of selenium in the absorber layer 120 is in a range
from about 0.1 atomic percent to about 20 atomic percent of the
absorber layer 120.
[0054] As noted, the absorber layer 120 is a component of a
photovoltaic device 100. In some embodiments, the photovoltaic
device 100 includes a "superstrate" configuration of layers.
Referring now to FIGS. 3-6, in such embodiments, the layer stack
110 further includes a support 111, and a transparent conductive
oxide layer 112 (sometimes referred to in the art as a front
contact layer) is disposed on the support 111. As further
illustrated in FIGS. 3-6, in such embodiments, the solar radiation
10 enters from the support 111, and after passing through the
transparent conductive oxide layer 112, the buffer layer 113, and
optional intervening layers (for example, interlayer 114 and window
layer 115) enters the absorber layer 120. The conversion of
electromagnetic energy of incident light (for instance, sunlight)
to electron-hole pairs (that is, to free electrical charge) occurs
primarily in the absorber layer 120.
[0055] In some embodiments, the support 111 is transparent over the
range of wavelengths for which transmission through the support 111
is desired. In one embodiment, the support 111 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 111 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 111
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 112 and the support
111, such as, for example, an anti-reflective layer or a barrier
layer (not shown).
[0056] 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 112 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 (ZnSnOx); or combinations thereof. Depending on the specific
TCO employed and on its sheet resistance, the thickness of the
transparent conductive oxide layer 112 may be in a range of from
about 50 nm to about 600 nm, in one embodiment.
[0057] The term "buffer layer" as used herein refers to a layer
interposed between the transparent conductive oxide layer 112 and
the absorber layer 120, wherein the layer 113 has a higher sheet
resistance than the sheet resistance of the transparent conductive
oxide layer 112. The buffer layer 113 is sometimes referred to in
the art as a "high-resistivity transparent conductive oxide layer"
or "HRT layer".
[0058] Non-limiting examples of suitable materials for the buffer
layer 113 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 113
is in a range from about 50 nm to about 200 nm.
[0059] In some embodiments, as indicated in FIGS. 3-6, the layer
stack 110 may further include an interlayer 114 disposed between
the buffer layer 113 and the absorber layer 120. The interlayer may
include a metal species. Non limiting examples of metal species
include magnesium, gadolinium, aluminum, beryllium, calcium,
barium, strontium, scandium, yttrium, hafnium, cerium, lutetium,
lanthanum, 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 interlayer 114 may
include a plurality of the metal species. In some embodiments, at
least a portion of the metal species is present in the interlayer
114 in the form of an elemental metal, a metal alloy, a metal
compound, or combinations thereof. In certain embodiments, the
interlayer 114 includes magnesium, gadolinium, or combinations
thereof.
[0060] In some embodiments, the interlayer 114 includes (i) a
compound including magnesium and a metal species, wherein the metal
species includes tin, indium, titanium, or combinations thereof; or
(ii) a metal alloy including magnesium; or (iii) magnesium
fluoride; or combinations thereof. In certain embodiments, the
interlayer includes a compound including magnesium, tin, and
oxygen. In certain embodiments, the interlayer includes a compound
including magnesium, zinc, tin, and oxygen.
[0061] As indicated in FIGS. 3 and 4, in certain embodiments, the
absorber layer 120 is disposed directly in contact with the layer
stack 110. However, as further noted earlier, in some embodiments,
the photovoltaic device 100 may include a discontinuous cadmium
sulfide layer interposed between the layer stack 110 and the
absorber layer 120 (embodiment not shown). In such instances, the
coverage of the CdS layer on the underlying layer (for example,
interlayer 114 and the buffer layer 113) is less than about 20
percent. Further, at least a portion of the absorber layer 120 may
contact the layer stack 110 through the discontinuous portions of
the cadmium sulfide layer.
[0062] Referring now to FIGS. 5 and 6, in some embodiments, the
layer stack 110 may further include a window layer 115 disposed
between the interlayer 114 and the absorber layer 120. The term
"window layer" as used herein refers to a semiconducting layer that
is substantially transparent and forms a heterojunction with an
absorber layer 120. Non-limiting exemplary materials for the window
layer 115 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
115 includes cadmium sulfide (CdS). In certain embodiments, the
window layer 115 includes oxygenated cadmium sulfide (CdS:O).
[0063] In some embodiments, the absorber layer 120 may function as
an absorber layer in the photovoltaic device 100. The term
"absorber layer" as used herein refers to a semiconducting layer
wherein the solar radiation is absorbed, with a resultant
generation of electron-hole pairs. In one embodiment, the absorber
layer 120 includes a p-type semiconductor material.
[0064] In one embodiment, a photoactive material is used for
forming the absorber layer 120. Suitable photoactive materials
include cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe),
cadmium magnesium telluride (CdMgTe), cadmium manganese telluride
(CdMnTe), cadmium telluride sulfide (CdTeS), zinc telluride (ZnTe),
lead telluride (PbTe), mercury cadmium telluride (HgCdTe), lead
sulfide (PbS), 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.
[0065] As will be appreciated by one of ordinary skill in the art,
the absorber layer 120 as described herein further includes
selenium. Accordingly, the absorber layer 120 may further include a
combination of one or more of the aforementioned photoactive
materials and selenium, such as, for example, cadmium selenide
telluride, cadmium zinc selenide telluride, zinc selenide
telluride, and the like. In certain embodiments, cadmium telluride
is used for forming the absorber layer 120. In certain embodiments,
the absorber layer 120 includes cadmium, tellurium, and
selenium.
[0066] In some embodiments, the absorber layer 120 may further
include sulfur, oxygen, copper, chlorine, lead, zinc, mercury, or
combinations thereof. In certain embodiments, the absorber layer
120 may include one or more of the aforementioned materials, such
that the amount of the material varies across a thickness of the
absorber layer 120. In some embodiments, one or more of the
aforementioned materials may be present in the absorber layer as a
dopant. In certain embodiments, the absorber layer 120 further
includes a copper dopant.
[0067] In some embodiments, the absorber layer 120, the window
layer 115, 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 115 (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 120, the amount of oxygen is less than about 1 atomic
percent. Moreover, the oxygen concentration within the absorber
layer 120 may be substantially constant or compositionally graded
across the thickness of the respective layer.
[0068] In some embodiments, the photovoltaic device 100 may further
include a p+-type semiconductor layer 130 disposed on the absorber
layer 120, as indicated in FIGS. 3-5. 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
120. 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 130 may be used as an interface between the absorber layer
120 and the back contact layer 140, in some embodiments.
[0069] In one embodiment, the p+-type semiconductor layer 130
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 130 includes
a p+-doped material including zinc telluride, magnesium telluride,
manganese telluride, beryllium telluride, mercury telluride,
arsenic telluride, antimony telluride, copper telluride, elemental
tellurium 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.
[0070] In some embodiments, the photovoltaic device 100 further
includes a back contact layer 140, as indicated in FIGS. 3-5. In
some embodiments, the back contact layer 140 is disposed directly
on the absorber layer 120 (embodiment not shown). In some other
embodiments, the back contact layer 140 is disposed on the p+-type
semiconductor layer 130 disposed on the absorber layer 120, as
indicated in FIGS. 3-5.
[0071] In some embodiments, the back contact layer 140 includes
gold, platinum, molybdenum, tungsten, tantalum, titanium,
palladium, aluminum, chromium, nickel, silver, graphite, or
combinations thereof. The back contact layer 140 may include a
plurality of layers that function together as the back contact.
[0072] In some embodiments, another metal layer (not shown), for
example, aluminum, may be disposed on the back contact layer 140 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
140 to provide lateral conduction to the outside circuit. In
certain embodiments, the back contact layer 140 may include a layer
of carbon, such as, graphite deposited on the absorber layer 120,
followed by one or more layers of metal, such as the metals
described above.
[0073] Referring again to FIG. 6, as indicated, the absorber layer
120 further includes a first region 122 and a second region 124. As
further illustrated in FIG. 6, the first region 122 is disposed
proximate to the layer stack 110 relative to the second region 124.
In some embodiments, the first region 122 is disposed directly in
contact with the window layer 115. In some embodiments, the first
region 122 is disposed directly in contact with the buffer layer
113 (embodiment not shown). Further, as discussed earlier, an
average atomic concentration of selenium in the first region 122 is
greater than an average atomic concentration of selenium in the
second region 124. In other embodiments, an average atomic
concentration of selenium in the first region 122 is lower than an
average atomic concentration of selenium in the second region
124.
[0074] In alternative embodiments, as illustrated in FIG. 7, a
photovoltaic device 200 including a "substrate" configuration is
presented. The photovoltaic device 200 includes a layer stack 210
and an absorber layer 220 disposed on the layer stack. The layer
stack 210 includes a transparent conductive oxide layer 212
disposed on the absorber layer, as indicated in FIG. 7. The
absorber layer 220 is further disposed on a back contact layer 230,
which is disposed on a substrate 240. As illustrated in FIG. 7, in
such embodiments, the solar radiation 10 enters from the
transparent conductive oxide layer 212 and enters the absorber
layer 220, where the conversion of electromagnetic energy of
incident light (for instance, sunlight) to electron-hole pairs
(that is, to free electrical charge) occurs.
[0075] In some embodiments, the composition of the layers
illustrated in FIG. 7, such as, the substrate 240, the transparent
conductive oxide layer 212, the absorber layer 220, and the back
contact layer 230 may have the same composition as described above
in FIG. 5 for the superstrate configuration.
[0076] A method of making a photovoltaic device is also presented.
In some embodiments, the method generally includes providing an
absorber layer on a layer stack, wherein the absorber layer
includes selenium, and wherein an atomic concentration of selenium
varies non-linearly across a thickness of the absorber layer. With
continued reference to FIGS. 1-6, in some embodiments the method
includes providing an absorber layer 120 on a layer stack 110.
[0077] In some embodiments, as indicated in FIG. 2, the step of
providing an absorber layer 120 includes forming a first region 122
and a second region 124 in the absorber layer 120, the first region
122 disposed proximate to the layer stack 110 relative to the
second region 124. As noted earlier, in some embodiments, an
average atomic concentration of selenium in the first region 122 is
greater than an average atomic concentration of selenium in the
second region 124.
[0078] The absorber layer 120 may be provided on the layer stack
110 using any suitable technique. In some embodiments, the step of
providing an absorber layer 120 includes contacting a semiconductor
material with a selenium source. The terms "contacting" or
"contacted" as used herein means that at least a portion of the
semiconductor material is exposed to, such as, in direct physical
contact with a suitable selenium source in a gas, liquid, or solid
phase. In some embodiments, a surface of the absorber layer may be
contacted with the suitable selenium source, for example using a
surface treatment technique. In some other embodiments, the
semiconductor material may be contacting with a suitable selenium
source, for example, using an immersion treatment.
[0079] In some embodiments, the semiconductor material includes
cadmium. Non-limiting examples of a suitable semiconductor material
include cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe),
cadmium magnesium telluride (CdMgTe), cadmium manganese telluride
(CdMnTe), cadmium sulfur telluride (CdSTe), zinc telluride (ZnTe),
lead telluride (PbTe), lead sulfide (PbS), mercury cadmium
telluride (HgCdTe), or combinations thereof. In certain
embodiments, the semiconductor material includes cadmium and
tellurium.
[0080] The term "selenium source" as used herein refers to any
material including selenium. Non-limiting examples of a suitable
selenium source include elemental selenium, cadmium selenide,
oxides of cadmium selenide, such as, for example, cadmium selenite
(CdSeO.sub.3), hydrogen selenide, organo-metallic selenium, or
combinations thereof.
[0081] The portion of the semiconductor material contacted with the
selenium source may depend, in part, on the physical form of the
selenium source during the contacting step. In some embodiments,
the selenium source is in the form of a solid (for example, a
layer), a solution, a suspension, a paste, vapor, or combinations
thereof. Thus, by way of example, in some embodiments, for example,
the selenium source may be in the form of a solution, and the
method may include soaking at least a portion of the semiconductor
material in the solution.
[0082] In some embodiments, the selenium source may be in the form
a vapor, and the method may include depositing the selenium source
using a suitable vapor deposition technique. In some embodiments,
for example, the absorber layer 120 may be heat treated in the
presence of a selenium source (for example, selenium vapor) to
introduce selenium into at least a portion of the absorber layer
120.
[0083] In some embodiments, for example, the selenium source may be
in the form of a layer, and the method may include depositing a
selenium source layer on the semiconductor material, or,
alternatively, depositing the semiconductor material on a layer of
the selenium source. In some such embodiments, the method may
further include subjecting the semiconductor material to one or
more post-processing steps to introduce the selenium into the
semiconductor material.
[0084] Referring now to FIG. 8, in some embodiments, the step of
providing an absorber layer includes (a) disposing a selenium
source layer 125 on the layer stack 110; (b) disposing an absorber
layer 120 on the selenium source layer 125; and (c) introducing
selenium into at least a portion of the absorber layer 120. It
should be noted, that the steps (b) and (c) may be performed
sequentially or simultaneously.
[0085] In some embodiments, the selenium source layer 125 may be
disposed on the layer stack 110 using any suitable deposition
technique, such as, for example, sputtering, sublimation,
evaporation, or combinations thereof. The deposition technique may
depend, in part, on one or more of the selenium source material,
the selenium source layer 125 thickness, and the layer stack 110
composition. In certain embodiments, the selenium source layer 125
may include elemental selenium and the selenium source layer 125
may be formed by evaporation. In certain embodiments, the selenium
source layer 125 may include cadmium selenide, and the selenium
source layer 125 may be formed by sputtering, evaporation, or
sublimation.
[0086] The selenium source layer may include a single selenium
source layer or a plurality of selenium source layers. The selenium
source may be the same or different in the plurality of source
layers. In some embodiments, the selenium source layer includes a
plurality of selenium source layers, such as, for example, a stack
of elemental selenium layer and a cadmium selenide layer, or vice
versa.
[0087] The selenium source layer 125 may have a thickness in a
range from about 1 nanometer to about 1000 nanometers. In some
embodiments, the selenium source layer 125 has a thickness in a
range from about 10 nanometers to about 500 nanometers. In some
embodiments, the selenium source layer 125 has a thickness in a
range from about 15 nanometers to about 250 nanometers.
[0088] As noted, the method further includes disposing an absorber
layer 120 on the selenium source layer 125. In some embodiments,
the absorber layer 120 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), chemical
vapor deposition (CVD), plasma enhanced chemical vapor deposition
(PECVD), or electrochemical deposition (ECD).
[0089] The method further includes introducing selenium into at
least a portion of the absorber layer 120. In some embodiments, the
method includes introducing selenium into at least a portion of the
absorber layer 120 such that a concentration of selenium varies
non-linearly across the thickness of the absorber layer 120.
[0090] In some embodiments, at least a portion of selenium is
introduced in the absorber layer 120 simultaneously with the step
of disposing the absorber layer 120. In some embodiments, at least
a portion of selenium may be introduced after the step of disposing
the absorber layer 120, for example, during the cadmium chloride
treatment step, during the p+-type layer formation step, during the
back contact formation step, or combinations thereof.
[0091] In some embodiments, the step of providing an absorber layer
120 includes co-depositing a selenium source material and a
semiconductor material. Suitable non-limiting examples of
co-deposition include co-sputtering, co-sublimation, or
combinations thereof. Non-limiting examples of a suitable selenium
source material in such instance includes elemental selenium,
cadmium selenide, hydrogen selenide, cadmium telluride selenide, or
combinations thereof. Thus, by way of example, in some embodiments,
an absorber layer 120 may be provided by depositing the
semiconductor material in the presence of selenium source (for
example, selenium containing vapor or hydrogen selenide vapor).
[0092] In some embodiments, the absorber layer 120 may be provided
by sputtering from a single target (for example, cadmium selenide
telluride target) or a plurality of targets (for example, cadmium
telluride and cadmium selenide targets). As will be appreciated by
one of ordinary skill in the art, the concentration of selenium in
the absorber layer 120 may be varied by controlling one or both of
target(s) composition and sputtering conditions.
[0093] As noted earlier, the photovoltaic device 100 and the layer
stack 110 may further include one or more additional layers, for
example, a support 111, a transparent conductive oxide layer 112, a
buffer layer 113, an interlayer 114, a p+-type semiconductor layer
130, and a back contact layer 140, as depicted in FIGS. 3-5.
[0094] 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.
[0095] In certain embodiments, a method for making a photovoltaic
100 in superstrate configuration is described. Referring now to
FIGS. 3-6, in some embodiments, the method further includes
disposing the transparent conductive oxide layer 112 on a support
111. The transparent conductive oxide layer 112 is disposed on the
support 111 by any suitable technique, such as sputtering, chemical
vapor deposition, spin coating, spray coating, or dip coating.
Referring again to FIGS. 3-6, in some embodiments, a buffer layer
113 may be deposited on the transparent conductive oxide layer 112
using sputtering. The method may further including disposing an
interlayer 114 on the buffer layer 113 to form a layer stack 110,
as indicated in FIG. 4.
[0096] The method may further include disposing a window layer 115
on the interlayer 114 to form a layer stack 110, as indicated in
FIGS. 5 and 6. Non-limiting examples of the deposition methods for
the window layer 115 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).
[0097] The method further includes providing an absorber layer 120
on the layer stack 110, as described in detail earlier. In some
embodiments, a series of post-forming treatments may be further
applied to the exposed surface of the absorber layer 120. These
treatments may tailor the functionality of the absorber layer 120
and prepare its surface for subsequent adhesion to the back contact
layer(s) 140. For example, the absorber layer 120 may be annealed
at elevated temperatures for a sufficient time to create a quality
p-type layer. Further, the absorber layer 120 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 120.
Additionally, copper may be added to absorber layer 120 in order to
obtain a low-resistance electrical contact between the absorber
layer 120 and a back contact layer(s) 140.
[0098] Referring again to FIGS. 3-6, a p+-type semiconducting layer
130 may be further disposed on the absorber layer 120 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
120 by chemically treating the absorber layer 120 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 120
(for example, using iodine and copper). In some embodiments, a back
contact layer 140, for example, a graphite layer may be deposited
on the p+-type semiconductor layer 130, or directly on the absorber
layer 120 (embodiment not shown). A plurality of metal layers may
be further deposited on the back contact layer 140.
[0099] One or more of the absorber layer 120, the back contact
layer 140, or the p+-type layer 130 (optional) may be further
heated or subsequently treated (for example, annealed) after
deposition to manufacture the photovoltaic device 100.
[0100] In some embodiments, other components (not shown) may be
included in the exemplary photovoltaic device 100, such as, buss
bars, external wiring, laser etches, etc. For example, when the
device 100 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 to divide the device into a plurality
of series connected cells.
EXAMPLES
Example 1: Method of Fabricating a Photovoltaic Device with a
Non-Linear Gradient Profile
[0101] 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. A
magnesium-containing capping layer was then deposited on the ZTO
buffer layer to form an interlayer. The window layer (approximately
40 nanometers thick) containing cadmium sulfide (CdS:O, with
approximately 5 molar % oxygen in the CdS layer) was then deposited
on the interlayer by DC sputtering and then annealed at an elevated
temperature. An approximately 500 nm thick Cd(Te,Se) film was then
deposited by close space sublimation from a source material with a
Se/(Se+Te) ratio of approximately 40%. The pressure was fixed at
approximately 15 Torr with a small amount of oxygen in the He
background gas. After deposition, the stack was treated with
CdCl.sub.2 and then baked at temperature greater than 400.degree.
C. Following the bake excess CdCl.sub.2 was removed. Approximately
3.5 microns CdTe film was then deposited by close space sublimation
in the presence of about 1 Torr of O.sub.2. After the second
deposition, a second CdCl.sub.2 treatment and subsequent bake
followed by removal of excess CdCl.sub.2 was performed before
forming a back contact.
[0102] The Se deposition profile in the device was measured using
dynamic secondary ion mass spectroscopy (DSIMS) performed. Prior to
the measurement, the samples were polished to reduce the effects of
surface roughness. The results for Se ion concentration (in
atoms/cm.sup.3) are shown in FIG. 9. The peak of the Se
concentration is near the location window and buffer layers. The
depth axis is the distance in microns from the polished edge of the
sample. Since the polishing procedure removes some amount of CdTe,
the total thickness of the alloy layer is less than the thickness
of the Cd(Se)Te alloy layer of the original solar cell.
[0103] To assess the non-linear nature of the distribution of the
Se within the absorber layer the data was filtered to remove points
after the peak of the Se distribution in the in data. The data was
then plotted on a log-log plot. The data is shown in FIG. 10. Two
functions were fitted on the log-log plot: one a linear fit which a
slope of 1.27, which is indicative of super-linear distribution.
Since the overall fit quality was poor, the log-log data was also
fit to an exponentially rising function, which gave a significantly
better fit indicating that the measured Se distribution is highly
non-linear.
Examples 2-4 Simulation Tests for Different Non-Linear Se
Concentration Profiles in a CdTe Layer
[0104] To illustrate some of the non-linear profiles, simulations
were carried out using the one-dimensional solar cell simulation
program SCAPS v.3.2.01 (M. Burgelman, P. Nollet and S. Degrave,
"Modelling polycrystalline semiconductor solar cells", Thin Solid
Films 361-362 (2000), pp. 527-532) The program numerically solves
the Poisson and continuity equations for electrons and holes in a
single dimension to determine the band-diagram of the device and
its response to illumination, voltage bias, and temperature.
Performance calculations were made using simulated W sweeps in the
simulation under illumination by the AM1.5G spectrum at 100
mW/cm.sup.2 of intensity and 300K, also known as Standard Test
Conditions (STC). The model parameters for CdTe and device design
were set according to the parameters given by Gloeckler et. al. for
CdTe solar cells. (M. Gloeckler, A. Fahrenbruch and J. Sites,
"Numerical modeling of CIGS and CdTe solar cells: setting the
baseline", Proc. 3rd World Conference on Photovoltaic Energy
Conversion (Osaka, Japan, may 2003), pp. 491-494, WCPEC-3, Osaka
(2003)), except that the CdTe absorber layer thickness was
increased to 4.5 microns and the nature of the deep trap in the
CdTe absorber layers was changed from `donor` to neutral. The CdSe
parameters were set to have the same values as the CdTe parameters,
except that the deep trap density in the CdSe is a factor of ten
lower and the band gap is 1.7 eV. A model for the variation in the
properties of the alloy material CdTe.sub.1-xSe.sub.x as a function
of x, the faction Se substitution, was constructed. The model
assumes that the Eg of the CdTe is equal to 1.5 eV, the gap of the
CdSe is equal to 1.7 and a bowing parameter, b=0.8. The band gap of
the alloy is given by:
E.sub.g,alloy=xE.sub.g,CdSe+(1-x)E.sub.g,CdTe+bx(1-x).
The other material properties, such as carrier mobilities and
dielectric constant values were assumed to be independent of alloy
composition and the deep donor concentration varied linearly
between the CdTe and CdSe values as function of x.
[0105] In Example 2, simulation was conducted using the measured
DSIMS Se profile as input. The measured DSIMS profile was fit to a
bi-exponential decay profile and the parameters from the fit used
to calculate a Se concentration profile throughout the 4.5 micron
thickness of the absorber layer.
[0106] In Example 3, an exponential Se concentration profile was
assumed, rising from about x=0.006 in the back to 0.2 in the front.
The total amount of Se in the device was about 4.4 times that of
the device described in Example 1.
[0107] In Example 4, a top-hat Se concentration profile was
assumed. In the particular top-hat profile, x=0 from the back of
the device until about 0.4 microns from the front interface,
whereupon it rises. From this point, x=0.4 until the front of the
absorber layer is reached. The total amount of Se in the device was
about 3 times that of the device described in Example 1.
Comparative Example 1 Simulation Test for a Conventional CdS/CdTe
Photovoltaic Device
[0108] For this simulation, the device had no Se and used the
inputs as specified by Gloeckler except for the modifications noted
previously. The calculated performance metrics of this model cell
(efficiency, Voc, Jsc, and fill factor (FF)) were used as the
reference levels for the other examples and their respective
performance metrics were normalized to this baseline case.
Comparative Example 2 Simulation Test for a Linear Se Concentration
Profile in CdTe Layer
[0109] For this simulation, a linear Se concentration profile was
used assuming the same total amount of Se as determined via the
DSIMS profile. In this calculation, a linear gradient in Se
concentration was input into the device model. The value of x was
set to 0 at the back contact and to about 0.025 in front.
Comparative Example 3 Simulation Test for a Constant Se
Concentration Profile in CdTe Layer
[0110] For this simulation, a constant Se concentration profile
with x=0.4 was assumed throughout the absorbing layer of the
device. The total amount of Se in the device was about 32 times
that of the device described in Example 1.
Comparative Example 4 Simulation Test for a Linear Se Concentration
Profile in CdTe Layer
[0111] For this simulation, the Se concentration profile was
assumed to be a linear ramp starting from x=0 at the back contact
and rising to x=0.4 at the front of the device. The total amount of
Se in the device is about 16 times that of the device described in
Example 1.
[0112] The performance metrics of Examples 2-4 and Comparative
Examples 2-4 relative to the baseline cell of Comparative Example 1
are reported in Table 1. FIG. 11 shows the Se concentration profile
as a function of CdTe thickness for Comparative Examples 2-4 and
Examples 2-4.
[0113] As illustrated in Table 1, the device performance parameters
showed improvement for the devices with a non-linear graded CdTeSe
layer (Examples 2-4) when compared to the device without a CdTeSe
layer (Comparative Example 1). For the same amount of Se, the
device performance parameters further showed improvement for the
devices with a non-linear graded CdTeSe layer (Example 2) when
compared to the device with a linear gradient of Se in CdTeSe layer
(Comparative Example 2). Both the `exponential` and `top-hat`
non-linear Se concentration profiles (Examples 3-4) demonstrated
superior efficiency to cells that had either constant or linearly
graded Se concentration profiles (Comparative Examples 3 and 4),
despite have a much lower total amount of Se present in the
layer.
[0114] It should be noted that while the profiles in the example
set are primarily confined the front, some degree of shifting of
the Se profile may lead to improvement in overall device
performance, particularly if the doping profiles or the energy
levels of the front and back contacts are adjusted. In such cases
it is possible that optimal position of the peak of the Se is not
at the front interface next to the buffer layer.
TABLE-US-00001 TABLE 1 Simulation results for performance
parameters for different Se concentration profiles in CdTe Effi-
Relative ciency Amount Peak Example (%) Voc Jsc FF of Se Peak
Location Comparative 1.000 1.000 1.000 1.000 0.0 0.00 none Example
1 Comparative 1.004 0.991 1.014 0.998 1.0 0.03 front Example 2
Comparative 1.060 0.893 1.186 1.002 32.0 0.40 none Example 3
Comparative 1.041 0.924 1.169 0.963 16.0 0.40 front Example 4
Example 2 1.033 0.999 1.028 1.006 1.0 0.25 front Example 3 1.102
0.998 1.100 1.004 4.4 0.20 front Example 4 1.102 0.998 1.100 1.004
3.0 0.40 front 0.4 microns
[0115] 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.
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