U.S. patent application number 16/099081 was filed with the patent office on 2019-05-09 for high absorption photovoltaic material and methods of making the same.
This patent application is currently assigned to RENSSELAER POLYTECHNIC INSTITUTE. The applicant listed for this patent is RENSSELAER POLYTECHNIC INSTITUTE. Invention is credited to Sergey Leonidovich EYDERMAN, Mei-Li HSIEH, Sajeev Oommen JOHN, Ping KUANG, Shawn Yu LIN, Anthony POST.
Application Number | 20190140115 16/099081 |
Document ID | / |
Family ID | 60203627 |
Filed Date | 2019-05-09 |
United States Patent
Application |
20190140115 |
Kind Code |
A1 |
KUANG; Ping ; et
al. |
May 9, 2019 |
HIGH ABSORPTION PHOTOVOLTAIC MATERIAL AND METHODS OF MAKING THE
SAME
Abstract
A high absorption photovoltaic material and method of making the
material for use in a solar cell are disclosed. The photovoltaic
material includes a surface modified with a layer of repeating
photonic crystal structures. The photonic crystal structures are
approximately inverse conically shaped and have a curved sidewall
that has an approximately Gaussian shape. The photonic crystal
structures generally have a high vertical depth and sidewall angle.
The structures also have a gradient refractive index profile and
exhibit the parallel-to-interface refraction light trapping effect.
An anti-reflective coating is disposed over the photonic crystal
structure layer. The photovoltaic material exhibits near unity
light absorption over a broad range of visible and near infrared
wavelengths and incidence angles, even at reduced thicknesses. The
photovoltaic structures are formed via a combined photolithography
and reactive-ion etching method at low power with a gas mixture
having a high ratio of an etchant component to a passivation
component.
Inventors: |
KUANG; Ping; (Latham,
NY) ; LIN; Shawn Yu; (Troy, NY) ; POST;
Anthony; (Troy, NY) ; JOHN; Sajeev Oommen;
(Mississauga, CA) ; EYDERMAN; Sergey Leonidovich;
(Toronto, CA) ; HSIEH; Mei-Li; (New Taipei City,
TW) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
RENSSELAER POLYTECHNIC INSTITUTE |
Troy |
NY |
US |
|
|
Assignee: |
RENSSELAER POLYTECHNIC
INSTITUTE
Troy
NY
|
Family ID: |
60203627 |
Appl. No.: |
16/099081 |
Filed: |
May 8, 2017 |
PCT Filed: |
May 8, 2017 |
PCT NO: |
PCT/US17/31556 |
371 Date: |
November 5, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62332531 |
May 6, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/02168 20130101;
Y02E 10/52 20130101; H01L 31/18 20130101; H01L 31/02363
20130101 |
International
Class: |
H01L 31/0236 20060101
H01L031/0236; H01L 31/0216 20060101 H01L031/0216; H01L 31/18
20060101 H01L031/18 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with government support under grant
no. DE-FG02-06ER46347 awarded by the Department of Energy. The
government has certain rights in the invention.
Claims
1. A high absorption photovoltaic material comprising: a
photovoltaic material including a photovoltaic material surface and
an opposite second surface; and a photonic crystal structure layer
at said photovoltaic material surface, said photonic crystal layer
including photonic crystal structures having an approximately
inverse conical shape including an approximately Gaussian-shaped
side wall having a gradient refractive index profile; wherein said
photonic crystal structures have a substantially simple cubic
symmetry.
2. The material according to claim 1, wherein said gradient
refractive index profile is substantially continuous.
3. The material according to claim 1, wherein said photonic crystal
structures have a width (a) and a depth (d), wherein a value of
d/(a/2) for said photonic crystal structures is greater than 2.
4. The material according to claim 3, wherein said photonic crystal
structures have a width of about 1.2 .mu.m and a depth of about 1.4
.mu.m.
5. The material according to claim 1, wherein said photonic crystal
structures have a sidewall angle greater than about 55 degrees.
6. The material according to claim 1, further comprising an
anti-reflective coating on said photonic crystal structure
layer.
7. The material according to claim 6, wherein said anti-reflective
coating is SiO.sub.2, ZnO, AlO.sub.3, Si.sub.3N.sub.4, or a
combination thereof.
8. The material according to claim 6, wherein said anti-reflective
coating has a thickness less than about 0.1 .mu.m.
9. The material according to claim 1, wherein said photovoltaic
material includes a thin layer photovoltaic material having a
thickness less than about 10 .mu.m.
10. The material according to claim 1, wherein said photovoltaic
material is composed of inorganic materials such as crystalline
silicon, multi-crystalline silicon, amorphous silicon, GaAs, CIGS,
CdTe, perovskite, organic photovoltaic materials, or a combination
thereof.
11. A method of making a high absorption photovoltaic material
comprising: providing a photovoltaic material including a
photovoltaic material surface and an opposite second surface;
applying a photoresist layer to said photovoltaic material surface,
said photoresist layer having an array of holes extending through
said photoresist layer; and dry etching said photovoltaic material
surface having said photoresist layer using a gas mixture including
an etchant component and a passivation component at a predetermined
wattage for limiting etching damage and surface roughness; wherein
said gas mixture has a high ratio of said etchant component to said
passivation component.
12. The method according to claim 11, further comprising removing
said photoresist layer from said photovoltaic material surface
after dry etching; and depositing an anti-reflective coating on
said photovoltaic material surface.
13. The method according to claim 12, wherein said anti-reflective
coating is SiO.sub.2, ZnO, AlO.sub.3, Si.sub.3N.sub.4, or a
combination thereof.
14. The method according to claim 11, wherein said etchant
component and said passivation component each include a fluorine or
a chlorine.
15. The method according to claim 14, wherein said etchant
component and said passivation component are SF.sub.6 and CHF.sub.3
or Cl.sub.2 and BCl.sub.3.
16. The method according to claim 11, wherein said dry etching is a
reactive-ion etching process.
17. The method according to claim 11, wherein said predetermined
wattage is less than 100 watts.
18. The method according to claim 11, wherein said high ratio of
said etchant component to said passivation component is at least
about 3 to 1 etchant component to passivation component.
19. The method according to claim 11, wherein dry etching said
photovoltaic material surface further comprising dry etching to a
depth of about 1 .mu.m to about 1.5 .mu.m.
20. A high absorption photovoltaic material comprising: a
photovoltaic material including a photovoltaic material surface and
an opposite second surface, said photovoltaic material having a
thickness; and a photonic crystal structure layer at said
photovoltaic material surface, said photonic crystal layer
including photonic crystal structures having an approximately
inverse conical shape including an approximately Gaussian-shaped
side wall having a gradient refractive index profile; wherein said
photovoltaic material maintains an average absorption percentage
from .lamda.=400-1000 nm of up to about 98.5% when said thickness
is about 500 .mu.m, and said photovoltaic material maintains an
average absorption percentage from .lamda.=400-1000 nm of up to
about 94.7% when said thickness is about 10 .mu.m.
Description
CROSS REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a national stage filing of International
Patent Application No. PCT/US2017/031556, filed May 8, 2017, which
claims the benefit of U.S. Provisional Application No. 62/332,531,
filed May 6, 2016, which is incorporated by reference as if
disclosed herein in its entirety.
BACKGROUND
[0003] The development and utilization of solar power and solar
cells have steadily increased in recent years in an effort to
create a sustainable, renewable, and clean energy resource. As a
result, the market for photovoltaic materials for use in these
solar cells is one of the fastest growing markets globally and in
the United States.
[0004] Traditional silicon-based photovoltaic materials are
multi-layer constructions that struggle with high dollar-per-Watt
cost due to the amount of material required to produce them. To
save on cost, the thickness of these materials is reduced, often
from the order of hundreds of micrometers to tens of micrometers.
However, these thinner photovoltaic materials suffer large
decreases in efficiency due to insufficient light absorption. This
disadvantageous decrease in light absorption is particularly
evident in the longer, near-infrared wavelength range of the solar
spectrum.
SUMMARY
[0005] Some embodiments of the disclosed subject matter are
directed to a photovoltaic material having a surface modified with
a layer of repeating photonic crystal structures. The photonic
crystal structures are approximately inverse conically shaped and
have simple cubic geometry. The photonic crystal structures have a
curved sidewall that has an approximately Gaussian shape and a
gradient refractive index profile. In some embodiments, an
anti-reflective coating is disposed over the photonic crystal
structure layer. The photonic crystal structures exhibit a light
trapping effect known as parallel-to-interface refraction. This
effect, combined with the gradient refractive index profile and the
anti-reflective coating combine to produce a photovoltaic material
with near-unity absorption of light at visible wavelengths and
vastly improved absorption at longer, near-infrared wavelengths,
even on thinner photovoltaic material wafers.
[0006] In some embodiments, the photonic crystal structures are
made using a combination photolithography and reactive-ion etching
process. A 2-D photoresist hole array is deposited on the surface
of the photovoltaic material. The photovoltaic material surface
left exposed is then subjected to low power reactive-ion etching
with a gas mixture having a high concentration of an etchant
component to a passivation component. The result is a more
isotropic etch while the sidewalls remain relatively smooth to
prevent undesired light reflection.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The drawings show embodiments of the disclosed subject
matter for the purpose of illustrating the invention. However, it
should be understood that the present application is not limited to
the precise arrangements and instrumentalities shown in the
drawings, wherein:
[0008] FIG. 1 is a front isometric scanning electron microscope
image of a high absorption photovoltaic material according to some
embodiments of the present disclosure;
[0009] FIG. 1B is an enlarged front isometric scanning electron
microscope image of a photonic crystal structure on the high
absorption photovoltaic material according to some embodiments of
the present disclosure;
[0010] FIG. 1C is a schematic representation of a refractive index
profile of the high absorption photovoltaic material according to
some embodiments of the present disclosure;
[0011] FIG. 2 is a series of charts of light absorption data
exhibiting the improved absorbance properties of the high
absorption photovoltaic material according to some embodiments of
the present disclosure; and
[0012] FIG. 3 is a chart of a method of making a high absorption
photovoltaic material according to some embodiments of the present
disclosure.
DETAILED DESCRIPTION
[0013] Referring now to FIG. 1A, aspects of the disclosed subject
matter include a high absorption photovoltaic material 100 having a
photovoltaic material surface 102 and an opposite second surface
104. In some embodiments, photovoltaic material 100 is composed of
inorganic materials such as crystalline silicon, multi-crystalline
silicon, amorphous silicon, GaAs, CIGS, CdTe, perovskite, etc.,
organic photovoltaic materials, other suitable photovoltaic
materials, or a combination thereof. In some embodiments,
photovoltaic material 100 has a thickness 106 less than about 100
.mu.m, e.g., about 50 .mu.m, 20 .mu.m, 10 .mu.m, or less. In some
embodiments, photovoltaic material 100 is a thin layer photovoltaic
material, e.g., a silicon-on-insulator wafer or other thin film
photovoltaic material, such as fabricated by chemical vapor
deposition or physical vapor deposition technique processes.
[0014] Photovoltaic material 100 as described above includes a
photonic crystal structure layer 108 that defines photonic material
surface 102. In some embodiments, photovoltaic material 100
includes an anti-reflective coating 110 on photonic crystal
structure layer 108. In some embodiments, anti-reflective coating
110 is SiO.sub.2, ZnO, AlO.sub.3, Si.sub.3N.sub.4, or a combination
thereof. In some embodiments, anti-reflective coating 110 is only a
single layer. In some embodiments, anti-reflective coating 110 has
a thickness less than about 1 .mu.m. In some embodiments,
anti-reflective coating 110 has a thickness of about 0.03 .mu.m to
about 0.12 .mu.m. In some embodiments, anti-reflective coating 110
has a thickness less than about 0.1 .mu.m.
[0015] Photonic crystal structure layer 108 includes photonic
crystal structures 112 having an approximately inverse conical
shape. As used herein, the terms "approximately" and "about" are
used to indicate that insubstantial changes to the limitation are
also envisioned. For example, the term "approximately inverse
conical shape" is used to convey that the shape of photonic crystal
structures 112 resembles a generally conical shape. However,
insubstantial deviations from the inverse conical shape, which have
little to no impact on the absorptive properties and performance of
the present disclosure discussed herein, are also envisioned. In
some embodiments, photonic crystal structures 112 have a
substantially simple cubic symmetry. In some embodiments, photonic
crystal structures 112 repeat across substantially all of
photovoltaic material surface 102. In some embodiments, photonic
crystal structures 112 are present on only a portion of
photovoltaic material surface 102. In some embodiments, photonic
crystal structures 112 have a width 114 and a depth 116 where the
relative sizes of the width and depth are defined according to a
value of Equation 1:
(d)/(a/2), (1)
where (a) is width 114 and (d) is depth 116. In some embodiments,
the value of (d)/(a/2) for photonic crystal structures 112 is
greater than 2. In the example portrayed in FIG. 1, (d)/(a/2) is
about 2.3.
[0016] Photonic crystal structures 112 thus typically, but not
always, have a relatively large vertical depth, for example
compared to a typical KOH-etched inverted pyramid structure profile
having a (d)/(a/2) value of only 1.3, allowing for better light
catching by the photonic crystal structures. In some embodiments,
photonic crystal structures 112 have a width 114 of about 1.2 m. In
some embodiments, photonic crystal structures 112 have a depth 116
of about 1 .mu.m to about 1.5 .mu.m. In some embodiments, photonic
crystal structures 112 have a depth 116 of about 1.4 .mu.m.
[0017] Photonic crystal structures 112 include a sidewall 118. In
some embodiments, photonic crystal structures 112 include multiple
sidewalls 118. Because of the increased vertical depth of photonic
crystal structures 112, sidewall 118 is typically relatively steep,
which allows for better light catching. In some embodiments,
photonic crystal structures 112 have a sidewall angle 120 greater
than about 55 degrees. In some embodiments, photonic crystal
structures 112 have a sidewall angle 120 greater than about 70
degrees.
[0018] Referring now to FIG. 1B, sidewall 118 is approximately
Gaussian-shaped. The expression for the Gaussian shape is shown in
Equation 2 below. As used herein, the term "approximately
Gaussian-shaped" is used to convey that sidewall 118 do not need to
rigorously adhere to the definition portrayed in Equation 2.
Rather, sidewall 118 generally follows Equation 2 and if they do
deviate, they do so insubstantially enough so as not to remove the
advantageous refractive properties of sidewall 118. Equation 2 is
defined as:
( x ) = n min + ( n max - n min ) e - ( x - 1 b ) 2 ( 2 )
##EQU00001##
where n.sub.min and n.sub.max are the minimum and maximum
refractive index, respectively, x is the optical distance, and
b=0.52.+-.0.2, which represents the shape width of the profile.
[0019] The approximately Gaussian-shaped sidewall 118 is curved
towards a central axis A. Sidewall 118 also increases in a
thickness 122 as depth 116 of photonic crystal structures 112
increases. In some embodiments, thickness 122 of sidewall 118
increases continuously and smoothly.
[0020] Photovoltaic material 100 having photonic crystal structures
112 and specifically the shape and size of sidewall 118 provide
light absorptive (anti-reflection) properties to photonic crystal
structure layer 108 and thus to photovoltaic material surface 102
and photovoltaic material 100. Referring now to FIG. 1C, sidewall
118 has a gradient refractive index profile, where the refractive
index increases with depth 116, which is known to be advantageously
anti-reflective. In some embodiments, a gradient refractive index
profile for sidewall 118 is substantially continuous. Photovoltaic
material 100 having photonic crystal structures 112 and
specifically the shape and size of sidewall 118 also exhibit
parallel-to-interface refraction, or another mechanism that
exhibits nearly parallel-to-interface light bending phenomena, the
effect of which is a positive or negative refraction of light
inside the photonic crystal structure. As a result, light
interacting with photonic crystal structures 112 may be bent nearly
perpendicularly. The optical path length of the light thus
increases and vortex-like concentration of light at "hot spots"
within photovoltaic material 100 occurs.
[0021] Referring now to FIG. 2, the practical results of this light
bending and trapping by photonic crystal structures 112 and
photovoltaic material 100 are greatly increased absorption of
light, particularly at high wavelengths such in the near-infrared
spectrum, as well as over a broader range of incidence angles.
Compared to a similar thickness planar photovoltaic material, i.e.,
without photonic crystal structures 112, photovoltaic material 100
enhances absorption by as much as about 2.3, about 4.5, and about
13 times at .lamda.=800, 900, and 1000 respectively. Overall,
photovoltaic material 100 maintains an average absorption
percentage from .lamda.=400-1000 nm of up to about 98.5%, with no
noticeable decrease in absorption percentage at higher incidence
angles. Additionally, photonic crystal structures 112 have shown to
limit the dependence of light absorbance on thickness of material.
Thin 10 .mu.m planar photovoltaic material was shown to have
inferior absorbance compared to thicker 500 .mu.m planar
photovoltaic material. As shown in FIG. 2, photovoltaic material
100, i.e., with photonic crystal structures 112, having a thickness
106 of 10 .mu.m showed vastly improved performance over a wide
range of wavelengths compared to the 500 .mu.m planar photovoltaic
material, but also showed comparable performance to photovoltaic
material 100 having a thickness 106 of 500 .mu.m. For example,
photovoltaic material 100 having a thickness 106 of 500 .mu.m
maintained an average absorption percentage from .lamda.=400-1000
nm of up to about 98.5%; while photovoltaic material 100 having a
thickness 106 of 10 .mu.m still maintained an average absorption
percentage from .lamda.=400-1000 nm of up to about 94.7%.
[0022] Referring now to FIG. 3, some embodiments of the disclosed
subject matter include a method 300 of making a high absorption
photovoltaic material. At 302, a photovoltaic material is provided
including a photovoltaic material surface and an opposite second
surface. At 304, a photoresist layer is applied to the photovoltaic
material surface. In some embodiments, photoresist layer is applied
304 using a suitable photolithography process. At 306, at least one
hole is provided in the photoresist layer. In some embodiments, the
at least one hole is provided 306 after the photoresist is applied
304. In some embodiments, the at least one hole is provided 306 as
the photoresist is being applied 304. In some embodiments, the at
least one hole is an array of holes. In some embodiments, the array
of holes is a uniform array. In some embodiments, the array of
holes has simple cubic geometry. In some embodiments, the holes are
spaced apart between about 1 .mu.m and about 1.5 .mu.m.
[0023] At 308, the photovoltaic material surface having the
photoresist layer is dry etched. In some embodiments, dry etching
308 is a reactive-ion etching process. The holes discussed above
extend through the photoresist layer to expose the photovoltaic
material surface beneath and define where photonic crystal
structures 112 are etched. In some embodiments, the photovoltaic
material surface is dry etched at a predetermined wattage. In some
embodiments, the predetermined wattage is relatively low for
limiting etching damage and surface roughness at sidewall 118. In
some embodiments, the predetermined wattage is about 100 watts. In
some embodiments, the predetermined wattage is below 100 watts. In
some embodiments, predetermined wattage is below about 50
watts.
[0024] In some embodiments, the photovoltaic material surface is
dry etched using a gas mixture including an etchant component and a
passivation component. The gas mixture has a high ratio of the
etchant component to the passivation component. In some
embodiments, the gas mixture ratio is greater than about 2 to 1
etchant component to passivation component. In some embodiments,
the gas mixture ratio is at least about 3 to 1 etchant component to
passivation component. In some embodiments, the gas mixture ratio
is greater than about 3 to 1 etchant component to passivation
component.
[0025] In some embodiments, the etchant component and the
passivation component each include a halogen atom. In some
embodiments, the etchant component and the passivation component
each include a fluorine or a chlorine. In some embodiments, the
etchant component and the passivation component are SF.sub.6 and
CHF.sub.3 or Cl.sub.2 and BCl.sub.3.
[0026] As discussed above, the arrangement of the photonic crystal
structures etched into the photovoltaic surface follows the
arrangement of the holes in the photoresist layer. Because of the
high ratio of etchant component to passivation, dry etching 308 is
more isotropic, etching in horizontal as well as vertical
directions. However, the undesired roughness one might expect from
the high etchant component ratio is mitigated by the presence of
the passivation component. The passivation component creates a
passivation layer at the surface during etching, which slows the
etching down and also limits isotropic undercutting.
[0027] In some embodiments, at 310, the photoresist layer is
removed from the photovoltaic material surface after dry etching.
In some embodiments, at 312, an anti-reflective coating is
deposited on the photovoltaic material surface. In some
embodiments, an oxidation process is used to deposit the
anti-reflective coating. In some embodiments, an annealing process
is used with the oxidation process to deposit the anti-reflective
coating. In some embodiments, a chemical vapor deposition process
is used to deposit the anti-reflective coating, e.g.,
plasma-enhanced chemical vapor deposition. In some embodiments, an
atomic layer deposition process is used to deposit the
anti-reflective coating.
[0028] The photovoltaic materials of the present disclosure include
a photonic crystal structure layer that increases absorption versus
a planar photovoltaic material, particularly at higher wavelengths.
Because of this photonic crystal structure layer, more light is
therefore available within the photovoltaic material when
incorporated into a solar cell, and the solar cell can operate at a
higher efficiency, i.e., produce more energy per unit time. The
photovoltaic materials can also be produced using significantly
less material, on the order of 50 times less, without sacrificing
performance. The material cost for each solar cell thus decreases,
allowing for the production of more solar cells that in turn
results in the production of more energy.
[0029] Although the disclosed subject matter has been described and
illustrated with respect to embodiments thereof, it should be
understood by those skilled in the art that features of the
disclosed embodiments can be combined, rearranged, etc., to produce
additional embodiments within the scope of the invention, and that
various other changes, omissions, and additions may be made therein
and thereto, without parting from the spirit and scope of the
present invention.
* * * * *