U.S. patent application number 12/736278 was filed with the patent office on 2011-05-26 for ultra-low reflectance broadband omni-directional anti-reflection coating.
This patent application is currently assigned to Rensselaer Polytechnic Institute. Invention is credited to Sameer Chhajed, Jong Kyu Kim, Mei-Ling Kuo, Shawn-Yu Lin, Frank W. Mont, David J. Poxson, E. Fred Schubert, Martin F. Schubert.
Application Number | 20110120554 12/736278 |
Document ID | / |
Family ID | 41114782 |
Filed Date | 2011-05-26 |
United States Patent
Application |
20110120554 |
Kind Code |
A1 |
Chhajed; Sameer ; et
al. |
May 26, 2011 |
ULTRA-LOW REFLECTANCE BROADBAND OMNI-DIRECTIONAL ANTI-REFLECTION
COATING
Abstract
An anti-reflection coating has an average total reflectance of
less than 10%, for example less than 5.9% such as from 4.9% to
5.9%, over a spectrum of wavelengths of 400-1100 nm and a range of
angles of incidence of 0-90 degrees with respect to a surface
normal of the anti-reflection coating. An anti-reflection coating
has a total reflectance of less than 10%, for example less than 6%
such as less than 4%, over an entire spectrum of wavelengths of
400-1600 nm and an entire range of angles of incidence of 0-70
degrees with respect to a surface normal of the anti-reflection
coating.
Inventors: |
Chhajed; Sameer; (Troy,
NY) ; Kim; Jong Kyu; (Watervliet, NY) ; Lin;
Shawn-Yu; (Niskayuna, NY) ; Kuo; Mei-Ling;
(Troy, NY) ; Mont; Frank W.; (Poughkeepsie,
NY) ; Poxson; David J.; (Troy, NY) ; Schubert;
E. Fred; (Troy, NY) ; Schubert; Martin F.;
(Troy, NY) |
Assignee: |
Rensselaer Polytechnic
Institute
|
Family ID: |
41114782 |
Appl. No.: |
12/736278 |
Filed: |
March 27, 2009 |
PCT Filed: |
March 27, 2009 |
PCT NO: |
PCT/US2009/038600 |
371 Date: |
December 17, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61039806 |
Mar 27, 2008 |
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61054289 |
May 19, 2008 |
|
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61197905 |
Oct 31, 2008 |
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Current U.S.
Class: |
136/259 ;
204/192.26; 257/E31.119; 359/586; 428/113; 428/216; 428/292.1;
428/311.11; 438/72; 977/902 |
Current CPC
Class: |
Y10T 428/24975 20150115;
Y10T 428/249962 20150401; Y10T 428/24124 20150115; H01L 31/0203
20130101; Y02E 10/50 20130101; Y10T 428/249924 20150401; C09D 5/006
20130101; G02B 1/115 20130101; C09D 1/00 20130101; H01L 31/02168
20130101 |
Class at
Publication: |
136/259 ;
428/292.1; 428/311.11; 428/113; 428/216; 204/192.26; 359/586;
438/72; 977/902; 257/E31.119 |
International
Class: |
H01L 31/0232 20060101
H01L031/0232; B32B 7/02 20060101 B32B007/02; B32B 18/00 20060101
B32B018/00; G02B 1/11 20060101 G02B001/11; H01L 31/18 20060101
H01L031/18 |
Goverment Interests
STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with U.S. Government support under
grant numbers DE-FG 02-06ER46347 awarded by the Department of
Energy--Basic Energy Services, FA 955006110431 awarded by AFOSR,
and NSF 0646439 awarded by the National Science Foundation. The
U.S. Government has certain rights in the invention.
Claims
1. An anti-reflection coating, wherein the anti-reflection coating
has an average total reflectance of less than 10% over a spectrum
of wavelengths of 400-1100 nm and a range of angles of incidence of
0-90 degrees with respect to a surface normal of the
anti-reflection coating.
2. The anti-reflection coating of claim 1, wherein the
anti-reflection coating has an average total reflectance of from
4.9 to 10% over a spectrum of wavelengths of 400-1100 nm and a
range of angles of incidence of 0-90 degrees with respect to a
surface normal of the anti-reflection coating.
3. The anti-reflection coating of claim 1, comprising at least one
porous layer and at least one non-porous layer.
4. The anti-reflection coating of claim 3, wherein the at least one
porous layer comprises nano-scale structures or porous silica.
5. The anti-reflection coating of claim 3, wherein the at least one
porous layers comprises one or more slanted nanorod layers.
6. The anti-reflection coating of claim 5, wherein: the one or more
slanted nanorod layers comprises at least a first slanted nanorod
layer and a second slanted nanorod layer located over the first
slanted nanorod layer; the first slanted nanorod layer has a first
porosity; and the second slanted nanorod layer has a second
porosity greater than the first porosity.
7. The anti-reflection coating of claim 6, wherein: the first
slanted nanorod layer comprises nanorods slanted in a first
direction; and the second slanted nanorod layer comprises nanorods
slanted in a second direction different from the first
direction.
8. The anti-reflection coating of claim 6, further comprising a
barrier layer located between the first slanted nanorod layer and
the second slanted nanorod layer.
9. The anti-reflection coating of claim 8, wherein the barrier
layer is transparent and chemically resistive.
10. The anti-reflection coating of claim 5, wherein: the one or
more slanted nanorod layers comprise nanorods made of at least one
of silicon oxide, titanium oxide, silicon oxynitride, aluminum
oxide, zinc oxide, transparent organic materials, or combinations
thereof; each of the one or more slanted nanorod layers has a
thickness of 20 nm to 500 nm; and the one or more slanted nanorod
layers having a porosity from 10% to 90%
11. The anti-reflection coating of claim 5, wherein: the one or
more slanted nanorod layers comprise nanorods made of at least one
of silicon oxide, titanium oxide, silicon oxynitride, aluminum
oxide, zinc oxide, transparent organic materials, or combinations
thereof; each of the one or more slanted nanorod layers has a
thickness of 20 nm to 100 nm; and the nanorods have a tilt angle of
0 to 60 degrees with respect to surface normal.
12. The anti-reflection coating of claim 3, wherein: the
anti-reflection coating comprises at least three layers having
index of refraction values that vary discretely from about 3.6 to
about 1; and an index of refraction profile of the anti-reflection
coating has an approximate Gaussian or Quintic distribution and is
smooth to a first derivative and a second derivative.
13. The anti-reflection coating of claim 3, wherein: the
anti-reflection coating comprises at least three layers having
index of refraction values that vary discretely from about 2.6 to
about 1.09; and an index of refraction profile of the
anti-reflection coating has an approximate Gaussian or Quintic
distribution and is smooth to a first derivative and a second
derivative.
14. The anti-reflection coating of claim 3, wherein: the
anti-reflection coating has a continuous index of refraction
profile varying from about 3.6 to about 1; and the index of
refraction profile has an approximate Gaussian or Quintic
distribution.
15. The anti-reflection coating of claim 3, wherein: the
anti-reflection coating has a continuous index of refraction
profile varying from about 2.6 to about 1.09; and the index of
refraction profile has an approximate Gaussian or Quintic
distribution.
16. The anti-reflection coating of claim 3, wherein: the at least
one non-porous layer has an index of refraction of greater than
1.5; and the at least one porous layer has an index of refraction
of 1.1 to 1.25 and a porosity of 90-95%.
17. The anti-reflection coating of claim 1, wherein: the
anti-reflection coating has a total thickness of 1-5 optical
wavelengths; the anti-reflection coating comprises at least three
layers; and each of the at least three layers has a thickness of
around 100 nm or less.
18. A solar cell, comprising: a first electrode located over a
substrate; at least one photovoltaic layer located over the first
electrode; a second electrode located over the at least one
photovoltaic layer; and the anti-refection coating according to
claim 1 located over the second electrode.
19. (canceled)
20. An anti-reflection coating, wherein the anti-reflection coating
has a total reflectance of less than 10% over an entire spectrum of
wavelengths of 400-1600 nm and an entire range of angles of
incidence of 0-70 degrees with respect to a surface normal of the
anti-reflection coating.
21. The anti-reflection coating of claim 20, wherein the
anti-reflection coating has a total reflectance of less than 6%
over a spectrum of wavelengths of 400-1600 nm and an entire range
of angles of incidence of 0-70 degrees with respect to a normal to
a surface normal of the anti-reflection coating.
22-39. (canceled)
40. An anti-reflection coating, comprising: one or more titanium
oxide layers; one or more intermixed layers located over the one or
more titanium oxide layers; and one or more slanted nanorod layers,
the one or more slanted nanorod layers being located over the one
or more intermixed layers.
41-54. (canceled)
55. An anti-reflection coating located over a substrate, the
anti-reflection coating comprising: one or more non-porous layers;
a first slanted nanorod layer having a first porosity located over
the non-porous layer; and a second slanted nanorod layer having a
second porosity located over the first slanted nanorod layer.
56. The anti-reflection coating of claim 55, wherein: the first
slanted nanorod layer comprising a first plurality of nanorods
slanted in a first direction; and the second slanted nanorod layer
comprising a second plurality of nanorods slanted in a second
direction different from the first direction.
57. The anti-reflection coating of claim 55, wherein: the first
nanorod layer and the second nanorod layer comprise independently
at least one of silicon oxide, titanium oxide, silicon oxynitride,
aluminum oxide, zinc oxide, transparent organic materials, or
combinations thereof; each of the first and second slanted nanorod
layers has a thickness of 20 nm to 500 nm; the second porosity is
greater than the first porosity.
58. The anti-reflection coating of claim 55, wherein: the
anti-reflection coating further comprises a third slanted nanorod
layer having a third porosity located over the second slanted
nanorod layer; and the third porosity is greater than the second
porosity.
59. The anti-reflection coating of claim 55, further comprising a
barrier layer located between the first slanted nanorod layer and
the second slanted nanorod layer.
60. The anti-reflection coating of claim 59, wherein the barrier
layer is transparent and chemically resistive.
61. The anti-reflection coating of claim 55, wherein: the
anti-reflection coating has an index of refraction profile varying
discretely from about 3.6 to about 1; and the index of refraction
profile of the anti-reflection coating has an approximate Gaussian
or Quintic distribution and is smooth to first derivative and
second derivative.
62-63. (canceled)
64. The anti-reflection coating of claim 55, wherein: the
anti-reflection coating has a continuous index of refraction
profile varying from about 2.6 to about 1.09; and the index of
refraction profile has an approximate Gaussian or Quintic
distribution.
65. (canceled)
66. An anti-reflection coating, comprising: a first slanted nanorod
layer having a first porosity; and a second slanted nanorod layer
having a second porosity located over the first slanted nanorod
layer; wherein the index of refraction profile of the
anti-reflection coating has an approximate Gaussian
distribution.
67-76. (canceled)
77. A method of making an anti-reflection coating, comprising:
depositing one or more titanium oxide layers; depositing one or
more co-sputtered layers of titanium oxide and silicon oxide over
the one or more titanium oxide layers; and depositing one or more
slanted nanorod layers over the one or more co-sputtered layers
titanium oxide and silicon oxide.
78. The method of claim 77, wherein the step of depositing one or
more slanted nanorod layers comprises: depositing a first slanted
nanorod layer having a first porosity; depositing a second slanted
nanorod layer having a second porosity located over the first
slanted nanorod layer; and the second porosity is greater than the
first porosity.
79. (canceled)
80. The method of claim 78, wherein: the first slanted nanorod
layer comprising nanorods slanted in a first direction; and the
second slanted nanorod layer comprising nanorods slanted in a
second direction different from the first direction.
81. The method of claim 78, further comprising depositing a barrier
layer over the first slanted nanorod layer, prior to the step of
depositing the second slanted nanorod layer.
82. The anti-reflection coating of claim 81, wherein the barrier
layer is transparent and chemically resistive.
83. The method of claim 77, wherein the step of depositing one or
more slanted nanorod layers comprises depositing slanted nanorods
with a predetermined tilt angle and porosity by oblique angle
deposition.
84. The method of claim 77, wherein: during the step of depositing
one or more slanted nanorod layers, a plurality of pores are formed
between the nanorods; and during the step of depositing the one or
more co-sputtered layers or the step of depositing the one or more
titanium oxide layers, substantially no pores are formed.
85. (canceled)
86. The method of claim 77, wherein an index of refraction profile
of the anti-reflection coating is graded and varies from about 3.6
to about 1.
87. (canceled)
88. The method of claim 77, wherein, during the step of depositing
the one or more co-sputtered layers at least one of sputtering
parameters varies in real time in such a way that the one or more
co-sputtered layers has a continuous composition profile, providing
a continuous index of refraction profile.
89. The method of claim 77, further comprising: depositing a first
electrode located over a substrate; depositing at least one
photovoltaic layer over the first electrode; depositing a second
electrode over the at least one photovoltaic layer, prior to the
step of depositing the one or more titanium oxide layers.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] This application claims benefit of priority of U.S.
Provisional Application Ser. No. 61/039,806 filed on Mar. 27, 2008,
U.S. Provisional Application Ser. No. 61/054,289 filed on May 19,
2008, and U.S. Provisional Application Ser. No. 61/197,905 filed on
Oct. 31, 2008, which are incorporated herein by reference in their
entirety.
FIELD OF THE INTENTION
[0003] The present invention relates to an anti-reflection coatings
and applications thereof.
SUMMARY OF THE INVENTION
[0004] One embodiment of this invention provides an anti-reflection
coating having an average total reflectance of less than 10%, for
example less than 5.9% such as from 4.9% to 5.9%, over a spectrum
of wavelengths of 400-1100 nm and a range of angles of incidence of
0-90 degrees with respect to a surface normal of the
anti-reflection coating.
[0005] Another embodiment of this invention provides an
anti-reflection coating having a total reflectance of less than
10%, for example less than 6% such as less than 4%, over an entire
spectrum of wavelengths of 400-1600 nm and an entire range of
angles of incidence of 0-70 degrees with respect to a surface
normal of the anti-reflection coating.
[0006] Another embodiment of this invention provides an
anti-reflection coating, comprising one or more titanium oxide
layers, one or more intermixed layers located over the one or more
titanium oxide layers, and one or more slanted nanorod layers, the
one or more slanted nanorod layers being located over the one or
more intermixed layers.
[0007] Another embodiment of this invention provides an
anti-reflection coating located over a substrate, the
anti-reflection coating comprising a non-porous layer, a first
slanted nanorod layer having a first porosity located over the
non-porous layer, and a second slanted nanorod layer having a
second porosity located over the first slanted nanorod layer.
[0008] Another embodiment of this invention provides
anti-reflection coating, comprising a first slanted nanorod layer
having a first porosity located over the non-porous layer and a
second slanted nanorod layer having a second porosity located over
the first slanted nanorod layer, where the index of refraction
profile of the anti-reflection coating has an approximate Gaussian
distribution.
[0009] Still another embodiment of this invention provides a method
of making an anti-reflection coating, comprising depositing one or
more titanium oxide layers, depositing one or more co-sputtered
layers of titanium oxide and silicon oxide over the one or more
titanium oxide layers, and depositing one or more slanted nanorod
layers over the one or more co-sputtered layers titanium oxide and
silicon oxide.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0010] FIG. 1 shows a Scanning Electron Microscopy image of rough
surface on a prior art solar cell.
[0011] FIG. 2 illustrates conventional single layer coating
(quarter-wavelength coating) on a prior art semiconductor solar
cell.
[0012] FIG. 3(a) shows total reflectance as a function of
wavelength at normal incidence with solar spectrum, and FIG. 3(b)
shows a scanning electron microscopy image of Quintic
graded-refractive-index 7-layer coating.
[0013] FIG. 4(a) shows an angular-dependent diffuse reflectance of
7-layer coating on silicon substrate at 633 nm; FIG. 4(b) shows
total reflectance as a function of angle. Compared to reflectance
between 7-layer coating and quarter-wavelength coating, the plot
shows 7-layer coating has a weak wavelength dependence and has low
reflectance from 0 to 70 degrees.
[0014] FIGS. 5(a)-(b) show angle-average total reflectance as a
function of wavelength at (a) TE polarization and (b) TM
polarization, respectively. Angle-average total reflectance is
defined by integration of total reflectance for 0 to 90 degrees at
each wavelength. Total reflectance values measured at incident
angle of 8 to 60 degrees are integrated. The 7-layer coating shows
a low reflectance for all angles and all wavelengths at both TE and
TM polarization.
[0015] FIG. 6 schematically illustrates the structure of
layer-by-layer coating with (1) thin film, (2) nanorods layer, and
(3) porous structure which has gradually changing index
profile.
[0016] FIG. 7 illustrates geometry of a gradient-index
anti-reflection coating.
[0017] FIG. 8 shows comparison of reflectance under Gaussian and
Gaussian-type profiles for a wavelength of .lamda.=1 .mu.m with d=1
.mu.m physical thickness. Inset is the Gaussian and Gaussian-type
profile.
[0018] FIGS. 9(a)-(b) show Gaussian and Quintic gradient-index
coating angular reflectance calculated for a wavelength of 1 .mu.m
with a physical thickness of (a) 1 .mu.m and (b) 3 .mu.m,
respectively.
[0019] FIG. 10(a) shows Gaussian and Quintic gradient-index coating
angular reflectance calculated with a physical thickness of 1
.mu.m. FIGS. 10(b)-(c) show the spectral reflectance calculated
with a physical thickness of 1 .mu.m for .theta..sub.0=70 degree
incident angle (b) and 80 degree incident angle (c),
respectively.
[0020] FIG. 11(a) shows index profile plotted for the Gaussian
(solid curve) and Quintic profile (dashed curve), respectively.
These two profiles are normalized to optical distance. FIG. 11(b)
shows refractive angle within the Gaussian and Quintic coating.
FIG. 11(c) shows reflectance within the Gaussian and Quintic
coating at incident angle of .theta..sub.0=75 degree.
[0021] FIG. 12(a) shows a five-layer discretized Gaussian profile.
FIG. 12(b) shows an angular reflectance with respect to different
numbers of discrete layers for a Gaussian profile.
[0022] FIG. 13(a) shows a schematic drawing of a setup used to
measure total reflectance. FIG. 13(b) shows total reflectance at
.theta.=8 degree as a function of wavelength, of (i) bare silicon,
(ii) .lamda./4 layer, and (iii) graded-index samples, respectively.
Light is TE-polarized. The dots refer to data points measured and
the curves refer to calculated results of a sample structure shown
in the inset. The sharp rise of reflectance at .lamda..about.1150
nm, where silicon's indirect band occurs, is due to an added
reflection from the bottom of the silicon substrate. FIG. 13(c)
shows a deducted total reflectance data (solid dots) from the top
anti-reflection coating surface only. This data demonstrates a low
total reflection (R=1-6%) over an ultra broad wavelength range,
.lamda.=400-1600 nm, at small incident angle, using the
graded-index anti-reflection coating.
[0023] FIG. 14 shows reflection coefficient of (left) silicon,
optimized (center) one-, and (right) three-layer anti-reflection
coatings for silicon image sensors versus wavelength and incident
angle.
[0024] FIG. 15 shows angle- and wavelength-averaged reflection
coefficient as a function of the number of layers for optimized
anti-reflection coatings for a silicon image sensor.
[0025] FIG. 16 shows reflection coefficient of (left) one-,
(center) two- and (right) four-layer anti-reflection coatings
optimized for silicon solar cells versus wavelength and incident
angle.
[0026] FIG. 17 shows angle- and wavelength-averaged reflection
coefficient as a function of the number of layers for optimized
anti-reflection coatings for silicon solar cells.
[0027] FIG. 18 shows reflectivity of (top to bottom) a bare
GaInP/GaAs/Ge triple junction solar cell, and triple-junction solar
cells with optimized one-, three-, and five-layer anti-reflection
coatings.
[0028] FIG. 19 shows angle- and wavelength-averaged reflection
coefficient as a function of the number of layers for optimized
anti-reflection coatings for GaInP/GaAs/Ge triple junction solar
cells.
[0029] FIG. 20 shows the solar spectrum and reflectance of Si
substrate as a function of wavelength with (a) no anti-reflection
coating, (b) .lamda./4 anti-reflection coating, and (c) 3-layer
anti-reflection coating.
[0030] FIGS. 21(a)-(c) show calculated wavelength and angle
resolved absolute reflectance of Si substrate with (a) no
anti-reflection coating, (b) .lamda./4 anti-reflection coating, and
(c) 3-layer graded-index anti-reflection coating.
[0031] FIGS. 22(a)-(b) show (a) Scanning Electron Micrograph, and
(b) Schematic of 3-layer graded-index anti-reflection coating. The
indicated refractive index values are measured at 550 nm.
[0032] FIGS. 23(a)-(c) show measured wavelength and angle resolved
absolute reflectance of Si substrate with (a) no anti-reflection
coating, (b) .lamda./4 anti-reflection coating, and (c) 3layer
graded-index anti-reflection coating.
[0033] FIGS. 24(a)-(c) show photograph of Si substrate with (a) no
anti-reflection coating, (b) .lamda./4 anti-reflection coating, and
(c) 3-layer graded-index anti-reflection coating.
DETAILED DESCRIPTION OF THE INVENTION
[0034] One embodiment of this invention provides an anti-reflection
coating having an average total reflectance of less than 10%, for
example less than 5.9% such as from 4.9% to 5.9%, over a spectrum
of wavelengths of 400-1100 nm and a range of angles of incidence of
0-90 degrees, such as 0-70 degrees, with respect to a surface
normal of the anti-reflection coating. Another embodiment of this
invention provides an anti-reflection coating having a total
reflectance of less than 10%, for example less than 6% such as less
than 4%, for example 1%-6% such as 1%-4%, over an entire spectrum
of wavelengths of 400-1600 nm, including 400-1100 nm such as
400-700 nm, and an entire range of angles of incidence of 0-70
degrees, including 0-60 degrees, with respect to a surface normal
of the anti-reflection coating.
[0035] In some embodiments, the anti-reflection coating comprises
at least one porous layer and at least one non-porous layer. In
some embodiments, the at least one non-porous layer has an index of
refraction of greater than 1.5, and the at least one porous layer
has an index of refraction of 1.1 to 1.25 and a porosity of 10%-95%
such as 90-95%. The at least one porous layer may comprise
nano-scale structures or porous silica. For example, in some
embodiments, the at least one porous layer comprises one or more
slanted nanorod layers.
[0036] The one or more slanted nanorod layers may comprise at least
a first slanted nanorod layer and a second slanted nanorod layer
located over the first slanted nanorod layer. In some embodiments,
the first slanted nanorod layer comprises nanorods slanted in a
first direction, and the second slanted nanorod layer comprises
nanorods slanted in a second direction different from the first
direction. In some embodiments, the first slanted nanorod layer has
a first porosity and a first tilt angle, defined as the angle of
slanted nanorods with respect to substrate surface normal, and the
second slanted nanorod layer has a second porosity different from
the second porosity and a second tilt angle different from the
first tilt angle. The anti-reflection coating may further comprise
a barrier layer located between the first slanted nanorod layer and
the second slanted nanorod layer. The barrier layer may be any
suitable material, which is transparent and chemically
resistive.
[0037] The nanorod layers may comprise nanorods made of any
materials. For example, the nanorods may be made of at least one of
silicon oxide, titanium oxide, silicon oxynitride, aluminum oxide,
zinc oxide, transparent organic materials, or combinations thereof.
Each of the one or more slanted nanorod layers may have a thickness
of 20 nm to 500 nm, for example 20 nm to 100 nm. The tilt angle of
the nanorods may be 0 to 70 degrees, for example 0 to 60 degrees.
The porosity of the nanorod layers may be any desired porosity, for
example 10% to 95% such as 10% to 90%.
[0038] In some embodiments, the anti-reflection coating may
comprise at least three layers having index of refraction values
that vary discretely from about 3.6 to about 1, for example from
about 2.6 to about 1.09. An index of refraction profile of the
anti-reflection coating may have an approximate Gaussian or Quintic
distribution and be smooth to a first derivative and a second
derivative. The anti-reflection coating has a total thickness of
1-5 optical wavelengths and each of the at least three layers has a
thickness of around 500 nm or less, for example 100 nm or less.
[0039] Alternatively, the anti-reflection coating has a continuous
index of refraction profile varying from about 3.6 to about 1, for
example from about 2.6 to about 1.09 and the index of refraction
profile has an approximate Gaussian or Quintic distribution.
[0040] In some embodiments, an anti-reflection coating may be
located over a substrate, and comprise a non-porous layer, a first
slanted nanorod layer having a first porosity located over the
non-porous layer, and a second slanted nanorod layer having a
second porosity located over the first slanted nanorod layer. The
second porosity is greater than the first porosity. In some
embodiments, the anti-reflection coating may further comprise a
third slanted nanorod layer having a third porosity greater than
the second porosity.
[0041] In some embodiments, an anti-reflection coating comprises
one or more titanium oxide layers, one or more intermixed layers
located over the one or more titanium oxide layers, and one or more
slanted nanorod layers, the one or more slanted nanorod layers
being located over the one or more intermixed layers.
[0042] In some embodiments, the one or more intermixed layers may
be non-porous and comprise at least a first intermixed layer having
a first composition and a second intermixed layer having a second
composition different from the first composition located over the
first intermixed layer. The one or more intermixed layers may
comprise at least one layer of co-sputtered titanium oxide and
silicon oxide.
[0043] The one or more slanted nanorod layers are porous and
comprise at least a first slanted nanorod layer and a second
slanted nanorod layer located over the first slanted nanorod layer.
In some embodiments, the first slanted nanorod layer comprises
nanorods slanted in a first direction and the second slanted
nanorod layer comprises nanorods slanted in a second direction
different from the first direction.
[0044] In some embodiments, the first titanium oxide layer has an
index of refraction of around 2.60, the second titanium oxide layer
has an index of refraction of around 2.52, the first intermixed
layer comprises a first composition of silicon oxide and titanium
oxide and has an index of refraction of around 2.24, the second
intermixed layer comprises a second composition of silicon oxide
and titanium oxide and has an index of refraction of around 1.86,
the first slanted nanorod layer has an index of refraction of
around 1.72, and the second slanted nanorod layer has an index of
refraction of around 1.09. In some other embodiments, the one or
more titanium oxide layers are non-porous and comprise a titanium
oxide layer, the one or more intermixed layers are non-porous and
comprise a co-sputtered layer of around 63% silicon oxide and
around 37% titanium oxide, the second intermixed layer comprises a
second composition of silicon oxide and titanium oxide and has a
porosity of 19%, where the first slanted nanorod layer has a
porosity of 71% and the second slanted nanorod layer has a porosity
of 90%.
[0045] Still another embodiment of this invention provides a method
of making an anti-reflection coating, comprising depositing one or
more titanium oxide layers, depositing one or more co-sputtered
layers of titanium oxide and silicon oxide over the one or more
titanium oxide layers, and depositing one or more slanted nanorod
layers over the one or more co-sputtered layers titanium oxide and
silicon oxide.
[0046] In some embodiments, the step of depositing one or more
slanted nanorod layers comprises depositing a first slanted nanorod
layer having a first porosity, depositing a second slanted nanorod
layer having a second porosity located over the first slanted
nanorod layer. The second porosity is greater than the first
porosity. In some embodiments, the first slanted nanorod layer
comprises nanorods slanted in a first direction, and the second
slanted nanorod layer comprises nanorods slanted in a second
direction different from the first direction. In some embodiments,
the step of depositing one or more slanted nanorod layers may
further comprise depositing a third slanted nanorod layer having a
third porosity greater than the second porosity.
[0047] The step of depositing one or more slanted nanorod layers
may comprises depositing slanted nanorods with a predetermined tilt
angle and porosity. The slanted nanorods may be deposited by any
suitable methods, for example, by oblique angle deposition. In some
embodiments, different tilt angles can be used for different
layers.
[0048] In some embodiments, the method further comprises depositing
a barrier layer over the first slanted nanorod layer, prior to the
step of depositing the second slanted nanorod layer. Preferably,
the barrier layer is transparent and chemically resistive.
[0049] During the step of depositing one or more slanted nanorod
layers, a plurality of pores are formed between the nanorods, while
during the step of depositing the one or more co-sputtered layers
or the step of depositing the one or more titanium oxide layers,
substantially no pores are formed.
[0050] The one or more slanted nanorod layers may comprise at least
one of silicon oxide, titanium oxide, silicon oxynitride, aluminum
oxide, zinc oxide, transparent organic materials, or combinations
thereof. Each of the one or more slanted nanorod layers has a
thickness of 20 nm to 500 nm, for example 20 nm to 100 nm, and the
nanorods have a tilt angle of 0 to 60 degrees.
[0051] In some embodiments, during the step of depositing the one
or more co-sputtered layers at least one of sputtering parameters
varies in real time in such a way that the one or more co-sputtered
layers has a continuous composition profile, providing a continuous
index of refraction profile. Examples for sputtering parameters
include but are not limited to flow of reactive gas, sputtering
speed, target composition, sputtering chamber pressure, bias added
to sample, temperature, etc.
[0052] The anti-reflection coating may be used for any suitable
applications, for example, an anti-reflection coating of solar
cells, light-emitting diodes, image sensors, photo detectors, or
any other optical components and devices where interfacial Fresnel
reflections are undesirable.
[0053] The anti-reflection coating(s) described above may be
applied to any type of solar cells. In one embodiment, the solar
cell comprises a first electrode located over a substrate, at least
one photovoltaic layer located over the first electrode, a second
electrode located over the at least one photovoltaic layer and the
anti-refection coating described above located over the second
electrode. The solar cell may further comprise a protective layer
located over the anti-reflection coating or between the
anti-reflection coating and the second electrode.
Embodiment I
[0054] By reducing reflective losses at interfaces, an
anti-reflection coating (AR coating) is an efficient application on
a solar cell. Since solar radiation provides a broadband spectrum
and solar cell devices can be made from several types of materials,
a high efficient anti-reflection coating is required to not only
have low-reflectance at all angles and all wavelengths but be
suitable for various types of solar devices, such as silicon,
silicon-film, gallium arsenide (GaAs), gallium antimonide (GaSb)
and others.
[0055] There is a market need for maximizing solar collection
efficiency from sun light and, thereby, increasing the net
solar-to-electricity conversion efficiency. Specifically, FIG. 1
shows a Scanning Electron Microscopy image of a rough surface on a
solar cell from a commercial solar cell. A conventional single
layer coating (quarter-wavelength coating) may be applied on
semiconductor solar cell to improve the collection efficiency, as
illustrated in FIG. 2. A single-layer quarter-wave anti-reflection
coating can reach low reflectance only at normal incidence and at
single wavelength. For a rough surface, it can confine incoming
light by reducing only certain amount reflectance.
[0056] In addition to single-layer quarter wave coating,
technologies such as periodic surface structure modification, and a
random surface structure may also be used to improve the collection
efficiency. However, most anti-reflection layer coating works for a
narrow band of wavelengths (.lamda.) and also over a small range of
angles (.theta.) near normal incidence. This constraint limits the
angle- and .lamda.-averaged reflectance to about 35% for a bare
silicon solar cell and 20% for a quarter-wave plate anti-reflection
coating. To accommodate the limited angle-of-acceptance, some solar
panels have to be built on a rotational tilt stage to track the sun
light to maximize its efficiency. Currently, it is believed that
there is no all-angle and all-wavelength (in solar spectrum)
anti-reflection coating in the market place.
[0057] In this embodiment, an anti-reflection coating on a silicon
substrate based on the Quintic profile design with seven
graded-refractive-index layers is demonstrated. The various index
of refraction profile can be obtained by varying structure and/or
materials of layers. As shown in FIG. 3(a), the total reflectance
of the graded-refractive-index coating shows the lowest total
reflectance over the entire wavelength range of solar spectrum at
normal incidence. A scanning electron microscopy image of a
non-limiting example is shown in FIG. 3(b), which has a
graded-refractive-index profile having an approximate Quintic
distribution. Other index-gradient profile such as Quintic profile,
Gaussian profile, or any other anti-reflection coating profile may
also be used. The detail of the theoretical design/simulation has
been described in Chen, M., et al. (Chen, M, et al., Applied Optics
2007, 46, 6533-6538).
[0058] FIG. 4(a) shows the measured reflectance plotted as a
function of .phi. for a series of light incident angle,
.phi.=-10.degree., -30.degree., -40.degree., -50.degree., and
-60.degree., respectively. Light is TE-polarized and at .lamda.=633
nm. The inset shows a schematic of the test setup. The data
exhibits a .delta.-function like sharp reflection peak at
|.phi.|=|.theta.| and a slight diffused component with
R.sub.diffu.ltoreq.10.sup.-4-10.sup.-7. The diffused component may
be fitted using a model calculation (dotted curves) and the
percentage of diffuse reflectance to the total reflectance is:
1.88%, 2.0%, 1.76%, 1.32%, and 0.765% at .theta.=-10.degree.,
-30.degree., -40.degree., -50.degree., and -60.degree.,
respectively. This data illustrates that an graded-index coating
can simultaneously accomplish a low total reflectance and a weak
diffuse reflectance for a wide range of .theta..
[0059] FIG. 4(b) shows a comparison of total reflectance vs .theta.
for the .lamda./4 layer and graded-index anti-reflection coating
samples at .lamda.=633 nm (top), 830 nm (middle), and 904 nm
(bottom), respectively. Both the measured (dots) and calculated
(curves) data are shown. The light is TE polarized. For
all-wavelengths, the total reflectance of the graded-index sample
displays a weak angle-dependence for .theta.=0.degree.-60.degree.
and also a weak wavelength-dependence. On the contrary, while the
reflectance of a .lamda./4 anti-reflection coating is low at
.lamda.=633 nm and for .theta.=0-30 degrees, it increases rapidly
at larger angles and at different wavelengths.
[0060] FIG. 5(a) shows measured and calculated
R.sub.angle-avg(.lamda.) as a function of wavelength. Angle-average
total reflectance is defined by integration of total reflectance
for 0 to 90 degrees at each wavelength. Total reflectance values
measured at incident angle of 8 to 60 degrees are integrated. The
7-layer coating shows a low reflectance for all-angles and all
wavelengths at both TE and TM polarization. The schematic of sample
and TE light polarization is shown in the inset. Comparing to the
.lamda./4 coatings (the dots and curve), the graded-index coating
(dots and curve) exhibits a much lower R.sub.angle-avg(.lamda.) for
the entire .lamda.-range and is also a nearly .lamda.-independent.
FIG. 5(b) shows R.sub.angle-avg(.lamda.) as a function of .lamda.
for TM-polarization. It exhibits a very similar functional
dependence as that for the TM polarization. The overall
R.sub.angle-avg(.lamda.) is lower for TM than that for TE
polarization due to the occurrence of Brewster angle for TM. For
example, any suitable variations of the above explained structure
having gradually changing index profile may be used. For example,
structures of layer-by-layer coating with (1) non-porous thin films
having different values of refractive index, with (2) a combination
of non-porous layers with nanorod layers, or with (3) porous
structures such as nano-engineered structures including
nano-particles and porous layers etc., as illustrated in plots
(1)-(3) in FIG. 6, respectively.
[0061] Alternatively, the index of refraction profile can be
continuous, by varying the index of refraction within each layer,
for example, by varying the deposition parameters in real-time to
continuously varying the composition or the porosity of the
layers.
[0062] As further described below, in this non-limiting embodiment,
an anti-reflection coating for eliminating Fresnel reflection at
the surface of a solar cell over the entire solar spectrum,
comprising a seven-layer graded-index coating with total
reflectance of 1-6 percent, can be used to increase
solar-to-electric efficiency by 20-42%. A graded-index coating
wherein the differential reflectance as each layer surface is
minimized by growing a discrete set of layered nanostructures (e.g.
TiO.sub.2 and/or SiO.sub.2 slanted rods) to approximate the
continuous index profile.
Embodiment II
[0063] Anti-reflection coatings are widely used to suppress
undesired interfacial Fresnel reflections in optical components and
devices. While the well-known single-layer quarter-wave film can,
in theory, lead to zero reflection at a single wavelength,
broadband anti-reflection coating is often needed for many
applications. In practice, coating materials with the required
refractive index for the quarter-wave antireflective film may not
be available. To address these issues, a multilayer stack of
homogeneous thin films was investigated extensively for over half a
century (A. Mussett and A. Thelen, Progress in Optics Vol. 8, E.
Wolf, ed. (North-Holland, 1970), pp. 203-237), resulting in the
development of a rich variety of multilayer thin film schemes (J.
A. Dobrowolski, Handbook of Optics, McGraw-Hill, 1995, pp.
42.19-42.34) and design methodologies (A. Thelen, Design of Optical
Interference Coatings, McGraw-Hill, 1989), which are hereby
incorporated by reference in their entireties.
[0064] An alternative broadband anti-reflection coating solution is
a layer of inhomogeneous film with gradient-index in which the
refractive index varies gradually and monotonically along its
thickness from the ambient (usually air) index to the substrate
index. Many specific gradient-index profiles including Quintic (W.
H. Southwell, Opt. Lett. 8, 584-586, 1983), Gaussian (E. Spiller et
al., Appl. Opt. 19, 3022-3026, 1980), Exponential (P. Yeh and S.
Sari, Appl. Opt. 22, 4142-4145, 1983), Exponential-Sine (P. G.
Verly, et al., Appl. Opt. 31, 3836-3846, 1992), and Klopfenstein
(E. B. Grann, et al., J. Opt. Soc. Am. A 12, 333-339, 1995) have
been investigated previously and are hereby incorporated by
reference in their entireties.
[0065] Compared to multilayer uniform films, gradient-index
anti-reflection coating can be less sensitive to the angle of
incidence, and is thus desirable for use in applications such as
solar cells (A. Gombert, et al., Sol. Energy 62, 177-188, 1998) and
light-emitting diodes (Y. Kanamori, et al., IEEE Photon. Technol.
Lett. 14, 1064-1066, 2002) that require effective anti-reflection
coating over a wide range of incident angles and where the
gradient-index can be implemented by techniques such as patterning
of subwavelength surface-relief "moth eye" structures (P. B.
Clapham and M. C. Hutley, Nature 244, 281-282, 1973), which are
hereby incorporated by reference in their entireties. Yet, while
numerous designs of multilayer anti-reflection coating for oblique
incident angles were previously reported, there is relatively
little literature on the design and performance of gradient-index
anti-reflection coating at grazing incident angles. Recently,
Poitras and his coworker (D. Poitras and J. A. Dobrowolski, Appl.
Opt. 43, 1286-1295, 2004) pointed out that at oblique angles, a
smooth variation of the spatially dependent refractive angle inside
a gradient-index anti-reflection coating is necessary to reduce
polarization splitting in reflectance of the film. They also noted
that performance is significantly degraded at large refractive
angles due to deformation of the index profile as seen by the
light. To partly remedy this, they showed that by applying a
spatial scaling to an index profile that effectively elongates the
portion close to the ambient side, the resultant modified profile
has improved performance at large incident angles over its original
counterpart. However, this comes at the expense of increased
physical thickness of the film and slight performance degradation
at near-normal incidence.
[0066] In this non-limiting embodiment, a new principle for the
design and selection of gradient-index anti-reflection coating
profiles has been demonstrated to be effective over a wide range of
incident angles as well as wavelengths, without the need for an
extension of the film's thickness. It is shown that large
variations in the optical path (which is characterized by the
refractive angle) of incident light inside a gradient-index film
directly lead to an increase in the overall reflection at oblique
incidence. This effect becomes more prominent at large angles.
Thus, it is the smoothness of variations in refractive angle rather
than that of the index profile itself that needs to be maximized
for wide-angle operation. As an example, the Gaussian profile
outperforms the Quintic profile at large incident angles.
[0067] A. Design of Optical Path
[0068] In some embodiments, the geometry of a gradient-index
anti-reflection coating shown in FIG. 7 may be used. The
inhomogeneous film can be placed on a substrate with an index
n.sub.max, and the ambient is assumed to have an index n.sub.min.
The ambient-film interface is at z=0 while the film-substrate
interface is at z=d, where z is the physical distance normal to the
film surface. Inside the film the refractive index n(z) varies
continuously from n.sub.min to n.sub.max. All media are assumed to
be non-dispersive and non-absorbing. Light is incident upon the
film surface from the ambience at an angle .theta..sub.0, and
inside the film the tangential propagation direction of light is
characterized by the spatially dependent refractive angle
.theta.(z).
[0069] To obtain a gradient-index profile, the optical thickness x
may be an important factor. In some embodiments, the incremental
optical thickness .DELTA.x can be related to the incremental
physical thickness .DELTA.z by .DELTA.z=.DELTA.x/n(x), so that
z = .intg. 0 x x ' n ( x ' ) . ( 1 ) ##EQU00001##
[0070] At oblique incidence, the optical characteristics of the
film are different for TE and TM incident polarizations. The
characteristic index for the two polarizations can be defined in
terms of the optical thickness as
n.sub.TE(x)=n(x)cos(.theta.(x)), (2a)
n.sub.TM(x)=n(x)/cos(.theta.(x)), (2b)
[0071] Where cos(.theta.(x)) is obtained by
cos ( .theta. ( x ) ) = [ 1 - n min n ( x ) sin ( .theta. 0 ) ] 1 /
2 . ( 3 ) ##EQU00002##
[0072] The refractive angle within the coating, .theta.(x), can be
obtained from Snell's law, that determines the optical path of
light propagating inside the coating. By the law of reflection at
the interface of two optical materials (Fresnel reflection), the
incremental reflectance dr.sub.TE(x) within the coating for the TE
polarization is then given by
dr TE ( x ) = [ n TE ( x ) - n TE ( x + dx ) n TE ( x ) + n TE ( x
+ dx ) ] 2 .apprxeq. [ dn TE ( x ) 2 n TE ( x ) ] 2 . ( 4 )
##EQU00003##
[0073] The incremental reflectance for TM polarization has exactly
the same form but with n.sub.TM(x) instead. The continuous index
function can be treated as an infinite series of discrete lamellar
layers of thickness of dx and calculate the reflectance as a
function of x directly by the Fresnel equation, Eq. (4). At a large
angle of incidence, cos(.theta.(x)) may approach zero, leading to a
great variation of the characteristic index in tiny increments of
.theta.(x). Therefore, the reflectance would be significantly
increased according to Eq. (4). Thus, without wishing to be bound
to a particular theory, the inventors believe that the variation of
refractive angle within the coating has great influence on the
performance of gradient-index anti-reflection coatings.
[0074] Smooth refractive index profile of the anti-reflection
coatings is favorable for an optimal reflectance. In some
embodiments, the refractive angle can be smoothly varied within the
coating to build an index profile. As a non-limiting example, the
variation of refractive angle of a well-known smooth sinusoidal
function at incident angle of .theta..sub.0=75.degree., as
described below, is used for building the index profile:
n ( x ) = n min sin .theta. 0 sin { .theta. 0 + ( .theta. 1 -
.theta. 0 ) [ 1 - cos ( .pi. x ) / 2 ] } , ( 5 ) ##EQU00004##
[0075] where .theta..sub.1 is the refractive angle in the
substrate. The index profile according to Eq. (5) are in fact very
close to a Gaussian profile as illustrated in FIG. 8, and thus are
referred to as "a Gaussian-type profile" in the following
description. Furthermore, its anti-reflective characteristics of a
layer is also similar to that the Gaussian profile as illustrated
in FIG. 8.
n ( x ) = n min + ( n max - n min ) exp [ - ( x - 1 0.4 ) 2 ] , ( 6
) ##EQU00005##
[0076] To illustrate the effect of smoothness of refractive angle
profile, the performance of the Gaussian according to Eq. (6) and
Quintic profile according to Eq. (7) are compared.
n(x)=n.sub.min+(n.sub.max-n.sub.min)(10x.sup.3-15x.sup.4+6x.sup.5).
(7)
[0077] These two index profiles are expressed in terms of their
optical distance, x. While the physical thickness is the actual
coating thickness, the optical thickness can be regarded as the
imaginary thickness seen by the electromagnetic wave. With optical
thickness, x=1.lamda..sub.0, where .lamda..sub.0 is the wavelength
in vacuum, Quintic exhibits physical thickness of
z.sub.Quintic=0.7.lamda..sub.0, while Gaussian has
z.sub.Gaussian=0.78.lamda..sub.0.
[0078] FIGS. 9(a)-(b) show the calculated reflectance of the two
profiles as a function of incident angle. Insets are the zoom-in
diagrams. The light is assumed to be incident from air
(n.sub.min=1.0) with wavelength of .lamda..sub.0=1 .mu.m and the
substrate is assumed to be aluminum nitride (n.sub.max=2.06). Two
physical thicknesses of 1 .mu.m (a) and 3 .mu.m (b) are used. As
shown in FIGS. 9(a) and (b), the Gaussian and Quintic profiles show
little difference at low incident angle. However, the significant
difference is revealed once the incident angle goes to above
60.degree.. Furthermore, the TE polarization is indiscernible to
the TM polarization under Gaussian while the polarizations are
split with Quintic. Without wishing to be bound to a particular
theory, the inventors believe that the reduced polarization
splitting under the Gaussian profile is an evidence that variation
of the refractive angle in Gaussian is smoother than that in the
Quintic.
[0079] A comparison of results shown in FIG. 9(a) and FIG. 9(b)
shows that a 3 .mu.m thick coating gives a better performance than
the 1 .mu.m coating. Without wishing to be bound to a particular
theory, the inventors believe that the better performance of the
structure having a thickness of 3 .mu.m may be attributed to a
smoother index variation. If an unpolarized light is illuminated
from .theta..sub.0=0 to .theta..sub.0=.pi. uniformly, the average
reflectance may be obtained by integrating the reflectance's
overall angle and dividing the integration by .pi.. For 1 .mu.m
thickness, the Gaussian and Quintic give an angle-averaged
reflectance of 5.93% and 7.53%, respectively. The result shows that
the Gaussian profile reduces reflectance by about 1.6% compared
with Quintic.
[0080] FIG. 10(a) shows the reflectance with a different wavelength
at coating thickness of 1 .mu.m. The unpolarized reflectance is
given by the average of TE and TM polarizations. Without wishing to
be bound to a particular theory, the inventors believe that that
the shorter wavelength performs better due to the relatively high
ratio (d/.lamda..sub.0) of coating thickness to wavelength. The
reflectance as a function of wavelength at two angles of incidence,
.theta..sub.0=70.degree. and .theta..sub.0=80.degree. is shown in
FIGS. 10(b) and (c), respectively. The Gaussian profile shows a
better performance over a wide range of wavelength. It was
previously shown that the Gaussian profile can have better
performance than the Quintic at normal incidence (E. B. Grann, et
al. J. Opt. Soc. Am. A 12, 333-339, 1995). It is further shown here
that the same holds true at oblique incidence, and we can consider
the refractive angle within the two profiles to gain some physical
understanding.
[0081] Turning to FIG. 11(a), results of Quintic and Gaussian
profiles are presented as a function of optical distance. FIG.
11(b) shows the refractive angle within the Gaussian as well as
Quintic profile as function of optical distance at
.theta.=75.degree., and FIG. 11(c) gives the incremental
reflectance as a function of optical distance for the two profiles.
The refractive angle .theta. in Quintic has a dramatic change at
the front region whereas that of the Gaussian is relatively
uniformly varied over the whole coating. Consequently, Quintic
gains its top reflectance at about x=0.18 as shown in FIG. 11(c)
while Gaussian has its top reflectance relatively close to the
central point. In a nonstrict sense, the integration of reflectance
over the whole coating indicates the total reflectance from the
inhomogeneous profile. The reflectance under Quintic covers more
area than that under Gaussian as demonstrated in FIG. 11(c),
showing that Gaussian may provides a better anti-reflection coating
performance than Quintic. At a given coating thickness, the
smoothness of the refractive angle may thus be used as a criterion
in the synthesis of optimal gradient-index profile for wide-angle
operation. Alternatively, given a pool of candidate profiles, the
best one for large incident angles may be selected by comparing
their respective refractive angle profiles.
[0082] B. Discretization
[0083] Further, the discretization of the continuous profiles and
how it affects the overall performance are tested. Any suitable
method for the discretization of the continuous profiles may be
used, for example, the methods described by W. H. Southwell (W. H.
Southwell, Appl. Opt. 24, 457-460, 1985), H. Sankur and his
coworker (H. Sankur and W. H. Southwell, Appl. Opt. 23, 2770-2773,
1984), and J. Q. Xi, et al (J. Q. Xi, et al, Opt. Lett. 31,
601-603, 2006), which are hereby incorporated by reference in their
entireties. A large range of arbitrary effective indexes may be
realized in a layer of nanorods grown by glancing angle deposition
(J.-Q. Xi, et al., Opt. Lett. 31, 601-603, 2006).
[0084] In this non-limiting embodiment, the refractive index and
thickness of each layer are obtained by the following procedure:
First, the refractive index is obtained by sampling the original
profile at the center of a layer and with equal layer spacing
(.DELTA.x); second, the thickness of each layer is obtained by
performing the integration of Eq. (1), with limits from -x/2 to
x/2, centering at the sample point.
[0085] FIG. 12(a) illustrates the geometry of an anti-reflection
coating consisting of five laminar layers to approximate Gaussian
profile. Note that the interference between both ends of each
laminar layer is the same. FIG. 12(b) shows the calculated results
with various numbers of layers. It can be seen that as few as five
discrete homogeneous layers are sufficient for approximating
Gaussian profile for anti-reflection coating between air and AlN
with a good performance. Without wishing to be bound to a
particular theory, the inventors believe that this result is a
consequence of the fact that the fine structure is not resolvable
under one-fifth of a wavelength with an index variation from 1 to
2.06. If increase the index variation is increased, more than five
layers may be necessary to approximate the continuous profile.
[0086] In summary, this non-limiting embodiment shows that, at
large incident angles, the magnitude of total reflection from a
gradient-index film is mainly influenced by the smoothness of the
optical path inside the film. The smoothness of the optical path
can thus be used as a design criterion for omni-directional
anti-reflection coating profile. This point of view can also be
used to explain the performance difference between gradient-index
profiles such as the Gaussian and Quintic. In addition, the
Gaussian profile can be sufficiently approximated by as few as five
discrete homogeneous layers for an anti-reflection coating between
air and AlN. While AlN was used as an exemplary system, other
substrate underlying materials may be used.
Embodiment III
[0087] To harness the full spectrum of solar energy, it favorable
to eliminate Fresnel reflection at the surface of a solar cell over
the entire solar spectrum. In this non-limiting embodiment, a
multi-layer nanostructure having a graded index profile is designed
(according to the theory described in Embodiment I above) and
fabricated to obtain a near perfect transmission of all-color of
sunlight. An ultra-low total reflectance of 1-6% has been achieved
over a spectrum of .lamda.=400 nm to 1600 nm, and over a range of
angles-of-incidence of .theta.=0-60 degrees. An angle- and
wavelength-averaged total reflectance as low as 3.79% is achieved
by using a seven layer graded-index coating. The corresponding
solar-to-electric efficiency over an uncoated silicon wafer is
consequently improved from 20.5% to 42.7% by going from a
single-layer quarter-wave coating to a seven layer graded-index
coating. This improvement of 22.2% makes such a multi-layer
anti-reflection coating ideal for any class of solar cell
application.
[0088] An anti-reflection coating is a type of coating applied to
the surface of a material to reduce light reflection and to
increase light transmission. The coating can improve solar
collection efficiency and, therefore, the overall
solar-to-electricity efficiency. As solar radiation is broadband,
the anti-reflection coating needs to be effective over the entire
solar spectrum from ultraviolet, visible to infrared wavelengths.
To ensure high collection efficiency over the entire course of a
day, the coating also has to be effective for all angles of light
incidence, .theta.. Hence, an ideal anti-reflection coating for
solar application should maintain a low reflectance for all colors
of sun light and all angles of incidence. For a smooth surface, the
law of reflection is dictated by the Fresnel equation. It predicts
a finite reflectance at normal incidence and a large reflectance at
large .theta., except for those near the Brewster angle. This
presents a fundamental constraint against the requirement of a low
reflectance at all angles.
[0089] A single layer of quarter-wave anti-reflection coating can
give zero reflection at a specific wavelength (.lamda.) by a
precise multiple interference of light within the layer. However,
such a precise interference occurs only for a small .lamda.-range
and a small .theta.-range. A double layer anti-reflection coating
has also been proposed to extend the range of low reflectance
regime from .lamda.=.about.450 nm to 700 nm. An alternative
approach to further increase the bandwidth is to create an
artificially modified surface structure. For example, a periodic
sub-.lamda. surface structure was shown to suppress Fresnel
reflection in the visible and near-infrared at .theta.=0 degree.
However, there is no angular-dependent study of the reflectance
except for .lamda.=633 nm. It has been reported that a random
silicon nanotip structure can give a total reflectance of less than
1% for .lamda.=0.2-2.5 .mu.m range. However, the claim of low
reflectance remains controversial for .lamda.>1.15 .mu.m. This
is because silicon is optically transparent for .lamda.>1.15 and
back reflection from the silicon substrate should give a greater
than 30% reflectance in this wavelengths range. In general, while
the process of random scattering can give a low total reflection
over a large bandwidth, it is not as effective at large .theta..
Thus, there is a need for a new anti-reflection coating scheme for
solar applications.
[0090] As explained in Embodiment I, the final reflectance of such
an anti-reflection coating depends on the smoothness of the index
profile. Without wishing to be bound to a particular theory, the
inventors believe that it is the differential reflectance at each
interface of the multi-layer structure that one must minimize to
obtain a low reflectance. As this minimization process does not
depend strongly on .lamda. or .theta., the resulting structure may
be an all-.theta. and all-.lamda. anti-reflection design in the
solar spectrum regime. The all-.theta. and all-.lamda. aspects of
the graded index nanostructure of this non-limiting embodiment make
it an ideal candidate for anti-reflection coating for solar energy
utilization. Theoretical modeling of several types of graded index
profiles, including the Quintic and Gaussian profiles, has
predicted a low reflectance for broad wavelength ranges. The
Quintic profile has the following functional form:
n(z)=n.sub.min=(n.sub.max-n.sub.min)(10z.sup.3-15z.sup.4+6z.sup.5).
In this embodiment, n.sub.min(air)=1, n.sub.max(silicon)=3.6 and z
refers to the vertical distance measured from the
air/anti-reflection coating interface. Experimentally, a discrete
set of layers having nanostructures was deposited to approximate
the continuous index-profile. A low reflectance under all-.lamda.
(400-1600 nm) and all-.theta. (0.degree.-60.degree.) is
accomplished simultaneously on a silicon substrate. This
performance is better than >98.5% transmission for
.lamda.=400-1000 nm and at .theta.=0 to 30 degrees which is
achieved on glass (n.about.1.45) substrate by Kennedy and his
coworkers (Kennedy, S. R. and Brett, M. J., Applied Optics 2003,
42, 4573-4579).
[0091] Such a multi-layer anti-reflection coating may be prepared
by any suitable deposition techniques, for example oblique angle
deposition may be used to deposit one or more layers of the stack.
Briefly, the oblique angle deposition can produce slanted nanorods
with a predesigned tilt angle and material porosity. A precise
control of the porosity can lead to a controllable refractive index
ranging from n=1.09-2.6, allowing the realization of almost any
graded-index profile for solar collection. The details of the
growth technique are described in Robbie et al. (Robbie, K et al.,
Nature 1996, 384, 616) and Zhao et al. (Zhao, Y. P. et al., Nano
Letter 2002, 2, 351-354), which are hereby incorporated by
reference in their entireties.
[0092] A scanning electron micrograph (SEM) image of the graded
index anti-reflection coating sample and the refractive-indexes of
each layer characterized by ellipsometry are shown in FIG. 3(b).
Particularly, the thicknesses of each layer, from bottom to top,
are 69 nm, 78 nm, 81 nm, 101 nm, 113 nm, 145 nm, and 156 nm,
respectively. The bottom two layers are made of TiO.sub.2. The
middle three layers are co-sputtered films using a combination of
SiO.sub.2 and TiO.sub.2 to tailor the refractive index. The top two
layers are made of slanted SiO.sub.2 nanorods with very low
refractive indexes of n=1.22 and 1.09, respectively. The slanted
SiO.sub.2 nanorods are randomly arranged, tilted along the same
direction, have a typical diameter of 20-30 nm and a rod-to-rod
spacing of 50-100 nm. This combination of the graded-index profile
and the extremely low index of the SiO.sub.2 slanted rods provides
a low reflectance and high transmittance over the entire solar
spectrum.
[0093] As explained previously in Embodiment I, FIG. 3(a) shows the
calculated reflectance spectra at normal incidence
(.theta.=0.degree.) for a bare silicon wafer, a .lamda./4 plate,
and a graded index anti-reflection coating. In this calculation, it
is assumed that the silicon substrate is infinitely thick and that
there is no reflection from the bottom surface. First of all, the
calculation shows that a bare silicon wafer has a reflectance of
R=30-49% in this wavelength regime (.lamda.=400-2000 nm). Second,
the reflectance of a single-layer .lamda./4 coating is zero at
.lamda.=550 nm, but increases rapidly to R=27% at .lamda.=2000 nm.
Thirdly, the graded index anti-reflection coating is expected to
have an overall reflectance of R=1%-6% throughout the solar
spectrum.
[0094] To characterize the anti-reflection property of the
graded-index coating, both total reflectance and diffuse
reflectance are measured. The total reflectance measurement is used
to account for all the reflected light. The diffuse reflectance
measurement is used to evaluate the effect of random scattering of
light, as the surface of the anti-reflection coating is not
perfectly smooth. The total reflectance measurements were performed
for a broad range of (1) wavelengths, (.lamda.=400-1600 nm) and (2)
incident angles, (.theta.=0-60.degree.) as well as for both (3) the
transverse electric (TE) and transverse magnetic (TM) polarizations
of light. The total reflectance is measured using a commercially
available integrating sphere, which integrates the reflected signal
over all 4.pi. of solid angles. A schematic of total reflectance
measurement setup is shown in FIG. 13(a). For the
wavelength-dependent study, several lasers were used that include:
.lamda.=454-514 nm from an Argon laser, .lamda.=633 nm from a
He--Ne laser, and .lamda.=780-1060 and .lamda.=1260-1600 nm from
laser diodes. To study the .theta.-dependence, the sample is
mounted at the center of the sphere and the incident angle is
varied by rotating the sample mount. The reflected light from the
sample is collected by a calibrated silicon photodetector in the
visible regime and an InGaAs detector in the near infrared
regime.
[0095] FIG. 13(b) shows the measured total reflectance spectra (the
solid dots) of the bare silicon, the .lamda./4, and the
graded-index samples, respectively. Light is incident at
.theta.=8.degree. and is TE polarized. The curves are from a
theoretical calculation that accounts for the finite thickness of
silicon substrate and its slightly rough back surface. In the
.lamda.=400-1100 nm range, all three samples exhibit similar
reflectance behavior to those predicted in FIG. 3(a). In brief, the
silicon has a high reflectance of R.about.31-49%, the .lamda./4 has
a minimum reflectance at .lamda.=550 nm and, finally, the graded
index sample has a low reflectance of R.about.5%. However, all
three samples exhibit an abrupt increase in their reflectance at
.lamda..about.1150 nm indicated by the vertical dashed lines. This
sharp rise of reflectance may be attributed to the onset of
silicon's optical transparency at .lamda..about.1150 nm, where
silicon's indirect band gap occurs. In these wavelengths, the
portion of light passing through the anti-reflection coating is
barely absorbed by silicon and would be reflected from the back
surface of the silicon substrate.
[0096] Assuming that the silicon substrate has no absorption in
this regime .lamda.>1150 nm and the backside silicon-air
interface is an optical flat surface with an effective index
(n.sub.eff) and an infinite thickness, so-called plane-wave based
Transfer-Matrix Method (Li, Z. Y. et al., Phys. Rev. E 2003, 67,
46607) is used for a model fit. Both the thickness and the
index-of-refraction of the multi-layer structure may be used as
input parameters. The model fit uses n.sub.eff as a single fitting
parameter. The model (the solid curves) fit the data well and yield
n.sub.eff=1.09, 1.35, 1.09 for the backside of the silicon,
.lamda./4 and graded-index anti-reflection coating samples,
respectively. Without wishing to be bound to a particular theory,
the inventors believe that the satisfactory fitting indicates that
the reflectance for .lamda.>1150 nm comes from two
contributions: one from the top surface and the other from the
bottom surface. Furthermore, once the n.sub.eff of the bottom
surface is known, it is possible to separate the contribution from
the back side surface and obtain reflectance due only to the
anti-reflection coating.
[0097] The deduced experimental reflectance data (solid dots) from
the top surface are plotted in FIG. 13(c), for silicon, .lamda./4
and graded index samples, respectively. The silicon reflectance
decreases monotonically as .lamda. is increased due to a slight
decrease in silicon's refractive index. The .lamda./4
anti-reflection coating has super low total reflectance of R<3%
for a small bandwidth, .lamda.=550.+-.50 nm. However, beyond this
.lamda.-range, reflectance increases rapidly and exhibits a strong
wavelength dependence. For the graded-index anti-reflection
coating, the total reflectance remains low, R=1-6%, for all visible
and near infrared wavelengths. It is noted that the theoretical
fits (the solid curves) in FIG. 13(c) are computed by assuming that
the substrate is infinite to eliminate the contribution from
backside reflection. The fitted result agrees well with the
experimental data for all-.lamda., except for .lamda.=1050 nm,
where silicon begins to become transparent. It is unexpected and
remarkable that the theoretical curve of the graded index sample
reproduces the weak reflectance oscillations between
.lamda.=450-1000 nm. This data is believed to represent the first
successful demonstration of an all-wavelength (.lamda.=400-1600 nm)
anti-reflection coating at small incident angle, having an ultralow
reflection (R=1-6%) for solar applications.
[0098] Next, the all-.theta. aspect of the seven-layer graded index
anti-reflection coating is demonstrated while the broadband nature
of the low-reflectance coating is obtained simultaneously. As shown
in the insert of FIG. 4(a), laser light is incident at an angle
.theta., and the reflected signal is detected at a different angle
.phi.. The detector used has a collecting solid angle of
.DELTA..OMEGA.=8.2.times.10.sup.-4 steradian. In FIG. 4(a), the
measured reflectance for TE polarization at .lamda.=633 nm is
plotted as a function of .phi. from -80.degree. to 80.degree. for
five different .theta.. First of all, the data exhibits a
.delta.-function-like sharp reflection peak at |.phi.|=|.theta.|
and a slight diffused component with
R.sub.diffu.ltoreq.10.sup.-4-10.sup.-7. In contrast to conventional
random surface structures, the nanorod structure used in this
embodiment is dominated by specular reflection. Without wishing to
be bound to a particular theory, the inventors believe that this is
because even though the SiO.sub.2 nanorods may be randomly
arranged, the surface is optically smooth to within .about.
1/10.lamda..
[0099] Second, the peak reflectance for TE polarization for .theta.
shows an initial decrease, reaches a minimum value of 2% at
.theta.=-40.degree., and finally increases to 9% at
.theta.=-60.degree.. This unusual angular dependence does not
follow the Fresnel equation and is a new feature of the
anti-reflection coating. Thirdly, the diffuse reflectance may be
modeled using an exponential scatter profile:
PSD = .sigma. rms 2 T 2 .pi. 2 1 1 + k 2 T 2 A 2 ##EQU00006##
(Ogilvy, J. A., Theory of Wave Scattering from Random Rough
Surface, 1990, 17-19). Here, .sigma..sub.rms is the root mean
square of the diffuser height and T is the correlation length of
the random surface. The model fits the diffused part of the
measured reflectance well and is shown in FIG. 4(a) as the dashed
curves. The percentage of diffuse reflectance as compared to the
total reflectance: 1.88%, 2.0%, 1.76%, 1.32%, and 0.765% at
.theta.=-10.degree., -30.degree., -40.degree., -50.degree.,
-60.degree., respectively, can be obtained from the fit results.
The data measured at .lamda.=633 nm show that the graded-index
anti-reflection coating can simultaneously accomplish a low total
reflectance and a weak diffuse reflectance under a wide range of
.theta..
[0100] This unique feature of low reflectance for all-.theta. at
.lamda.=633 nm applies equally well to other wavelengths. In FIG.
4(b), the measured (dots) and calculated (curves) total reflectance
of the graded-index sample vs .theta. for .lamda.=633 nm (top), 830
nm (middle), and 904 nm (bottom), respectively, are presented. TE
polarized light is used. For comparison purpose, total reflectance
of a .lamda./4 anti-reflection coating is also shown. At
.lamda.=633 nm, both the graded index and .lamda./4 sample exhibit
a similarly low reflectance at .theta.=8.degree. and a reasonable
reflectance of R.about.10% at .theta.=60.degree.. At .lamda.=830
nm, the total reflectance of the graded-index sample remains low
(R=2-5%) for all-.theta.. On the contrary, the total reflectance of
a .lamda./4 sample becomes quite high, i.e. R=11% at
.theta.=80.degree. and R=26% at .theta.=60.degree.. At an even
longer wavelength of .lamda.=904 nm, the graded-index sample still
maintains its low reflectance, while that for the .lamda./4
anti-reflection coating is as high as R=13-33%. Same measurements
were repeated for TM polarized light, and the data for the
graded-index sample shows a similar low reflectance at all-.theta.
and all-.lamda., demonstrating a superior all-.theta. and
all-.lamda. aspect of the graded-index anti-reflection coating.
[0101] Further, the performance of the graded-index anti-reflection
coating is compared with that of a conventional single-layer
.lamda./4 coating using the angle-averaged and .lamda.-averaged
total reflectance. To give an average value of the total
reflectance, the reflectivity function R(.lamda., .theta.) is
integrated over all measured angles from .theta.=8.degree. to
60.degree. and over all-.lamda. from .lamda.=400 nm to 2000 nm. The
angle-averaged total reflectance is defined using the following
formula:
R angle - avg ( .lamda. ) = .intg. 8 60 R ( .theta. , .lamda. )
.theta. .intg. 8 60 .theta. . ##EQU00007##
The range used for incident angles in this particular example is
limited by the testing system setup. As the laser beam spot is much
smaller than the sample size, there is no need to include a
cos(.theta.) term in the integration to account for the
angular-dependence of the incident light intensity.
[0102] Turning to FIG. 5(a), both the measured and calculated
R.sub.angle-avg(.lamda.) are presented as a function of .lamda. for
TE-polarization. Again, results of the TMM calculation simulate the
measured data well. Compared to the .lamda./4 coatings (the dots
and curve), the graded-index coating exhibits a much lower
R.sub.angle-avg(.lamda.) for the entire .lamda.-range and is also
nearly .lamda.-independent. As shown in FIG. 5(b),
R.sub.angle-avg(.lamda.) for TM-polarization exhibits a very
similar functional dependence as that for the TM polarization. For
the bare silicon, the overall R.sub.angle-avg(.lamda.) is lower for
TM than that for TE polarization, possibly due to the occurrence of
the Brewster angle for TM. Again, the R.sub.angle-avg(.lamda.)
value of the graded-index coating is significantly lower that of
the .lamda./4 coatings and is also nearly .lamda.-independent.
[0103] Finally, the average total reflectance is calculated by
integrating the fitted data, R.sub.angle-avg(.lamda.), from
.lamda.=400 nm to 2000 nm for both polarizations shown in FIGS.
5(a) and (b), respectively. The results are summarized in Table 1.
The average total reflectance over all angles, wavelengths of the
bare silicon, .lamda./4 coating and graded-index coating are 32.6%,
18.8% and 3.79%, respectively. By conservation of energy, the total
transmission efficiency of light and the total reflectance are
related by: T.sub.Total=1-R.sub.Total, Hence, the use of an
graded-index and .lamda./4 coating can achieve solar collection
efficiency (from .lamda.=400-2000 nm) of 81.2 and 96.21%,
respectively. It illustrates that optical-to-electrical power
conversion efficiency increases by 22.2% when switching from a
conventional single-layer .lamda./4 anti-reflection coating to a
seven layer graded index anti-reflection coating. The graded index
profile may also be tailored, when applied to solar cells based on
other materials, such as III-V multi junction and CdTe, according
to the refractive index of a specific material.
TABLE-US-00001 TABLE 1 .theta.- and .lamda.-Averaged Total
Reflectance and Efficiency of AR Coating for Silicon, Single-Layer
.lamda./4 Coating, and Graded-Index Coating Samples .eta. of AR
Improved .eta. R.sub.avg (%) Coating (%) over Silicon (%) Silicon
32.6 67.4 Single layer 18.8 81.2 20.5 Graded index 3.79 96.21
42.7
[0104] In summary, this embodiment demonstrates a multi-layer
nanostructure anti-reflection coating that can be engineered to
significantly reduce optical reflection over all wavelengths of sun
light and incident angles. This graded-index approach offers a
mechanism for minimizing Fresnel reflection fundamentally different
from either the traditional .lamda./4 anti-reflection coating or
the modified surface structures. The new design freedom afforded by
this graded index coating and the deposition technique of oblique
angle deposition technique allows the creation of this all-.theta.,
all-.lamda. anti-reflection coating. This non-limiting embodiment
may improve the solar-to-electrical conversion efficiency by 22.2%
compared to a conventional system using a conventional single-layer
.lamda./4.
Embodiment IV
[0105] Minimizing optical reflection at dielectric interfaces is a
fundamental challenge, and is vital for many applications in
optics. It is well known that normal-incidence reflection at a
specific wavelength can be minimized using a single layer coating
with quarter-wavelength optical thickness and refractive index n=
{square root over (n.sub.1n.sub.2)}, where n.sub.1 and n.sub.2 are
the refractive indices of the ambient and substrate, respectively.
However, a material with the required refractive index may not
exist, and additionally, omni-directional and broadband
anti-reflection characteristics are often required for applications
such as solar cells or image sensors.
[0106] Several methods exist that allow the tuning of refractive
index for optical thin films. Alternating layers of a high-index
and low-index material, each with thickness much less than the
wavelength, produces a film that can be treated as homogenous with
refractive index approximated by the volume ratio of the two
constituent materials (W. H. Southwell, Appl. Opt. 24, 457-460,
1985). By changing the relative thickness of each layer, the
effective refractive index of the film can be varied between that
of the two materials. Oblique angle deposition can also be used to
control the refractive index; in oblique angle deposition,
self-shadowing results in the formation of a nano-porous film of
high optical quality. The refractive index is related to the
porosity of the film, and can be varied by changing the deposition
angle. At deposition angles close to 90.degree., the porosity
becomes large and the index decreases to low values. The
nano-porous material is termed low-refractive-index (low-n)
material. Using SiO.sub.2, refractive indices as low as 1.05 have
been reported. A third method to create a film with specific
refractive index is co-sputtering, in which two materials such as
SiO.sub.2 and TiO.sub.2 are simultaneously deposited. The
refractive index can be tuned by varying the relative deposition
rates of the two materials.
[0107] The ability to tune the refractive index is important in
enabling broadband and omni-directional anti-reflection coatings.
Such coatings generally consist of multilayer stacks in which the
refractive index is graded between substrate value and that of air.
Using the appropriate refractive index is important in achieving
the best performance. In addition, the inclusion of layers with
refractive index close to that of air can greatly reduce
reflection. Well-known refractive index profiles for
anti-reflection coatings include the Quintic or modified-Quintic
profiles, which are continuous functions that vary between the
substrate refractive index and the index of the ambient
material.
[0108] However, these profiles sometimes do not give the optimum
profile when a finite number of layers is used. Additionally, these
profiles require high-refractive-index transparent materials--which
often do not exist--to be matched to high-refractive-index
substrates, such as silicon. Finally, material dispersion is not
considered although it may play a significant role, particularly
for broadband applications.
[0109] Optimization of multilayer anti-reflection coatings is
difficult because of the high cost of evaluating the performance
for a given structure. In addition, the parameter space generally
includes many local minima, which makes deterministic optimization
schemes that find the local minima unsuitable (H. Greiner, Appl.
Opt. 35, 5477-5483, 1996). To meet these challenges, genetic
algorithms have previously been applied in order to optimize a
variety of optical coatings, for example genetic algorithms
disclosed by H. Greiner (H. Greiner, Appl. Opt. 35, 5477-5483,
1996), S. Martin et al. (S. Martin, et al., Opt. Commun. 110,
503-506, 1994 and Appl. Opt. 34, 2247-2254, 1995), and J.-M. Yang
et al. (J. M. Yang et al., J. Light. Technol. 19, 559-570, 2001),
which are hereby incorporated by reference in their entireties.
Genetic algorithms mirror biological evolution in which the fitness
of a population is increased by the processes of selection,
crossover, and mutation. In this embodiment, genetic algorithm is
applied to optimize anti-reflection coatings for silicon image
sensors, silicon solar cells, and triple junction Ge/GaAs/GaInP
solar cells with air as the ambient medium. The calculations
consider coatings composed of co-sputtered and low-n materials and
take material dispersion into account.
[0110] A. Numerical Approach
[0111] Calculations begin with the generation of a population of
anti-reflection coatings with a fixed number of layers whose
thicknesses and compositions are randomly generated. A layer may be
composed of either nano-porous SiO.sub.2 or any combination of
SiO.sub.2/TiO.sub.2, corresponding to low-n and co-sputtered films,
respectively. The porosity of SiO.sub.2 may be up to 90%,
corresponding to a refractive index of 1.05, which has previously
been demonstrated. For each member of the population, the largest
thicknesses can be matched to compositions with the lowest
refractive index, and then sorted so that the high-index layers are
adjacent to the substrate. This increases the population near the
optimum anti-reflection coating--which may have monotonically
decreasing refractive index and increasing thickness when moving
away from the substrate--and reduces the computation time.
[0112] After the population has been formed, the fitness of each
member may be evaluated. The fitness may be determined by the
reflection coefficient averaged over the wavelength range and angle
range of interest, R.sub.ave, which is given by,
R ave = 1 .lamda. 2 - .lamda. 1 2 .pi. .intg. .lamda. 1 .lamda. 2
.intg. 0 .pi. / 2 R TE ( .lamda. , .theta. ) + R TM ( .lamda. ,
.theta. ) 2 .theta. .lamda. ( 1 ) ##EQU00008##
[0113] where R.sub.TE and R.sub.TM are the TE and TM reflection
coefficients. In practice, the fitness function may easily be
modified to give greater weight to certain angles of incidence or
to certain wavelengths to take into account the responsivity of a
particular solar cell, the solar spectrum, or the orientation of a
solar cell with respect to the sun, in order to maximize the power
produced by a solar cell, for example. The fittest member of the
population is the one with lowest average reflection coefficient.
The method for calculating the reflection coefficients of a
multilayer stack was described by Born and Wolf (M. Born and E.
Wolf, Principles of Optics, Pergamon, Oxford. 1980), which is
hereby incorporated by reference in its entirety. The population
may be sorted by fitness, and a percentage of the worst members are
then discarded. These are replaced by the offspring of two other
anti-reflection coatings, which are selected at random from the
remaining members of the population. Offspring anti-reflection
coatings are generated by a process of crossover and mutation. In
crossover, a set of layers for the new offspring is taken from one
parent, and the remainder is taken from the second parent. In
mutation, the composition and thickness of each layer is given a
random perturbation. Once the worst members of the population have
been replaced by new offspring, the fitness of each is evaluated,
and the process repeats until good convergence is achieved.
Finally, using a deterministic algorithm, the local minima near the
fittest member of the population is found.
[0114] B. Silicon Image Sensor
[0115] Silicon image sensors are widespread in digital cameras, and
generally capture light in the visible wavelength range. Low
reflection from the sensor surface is desirable to increase the
absorbed light and decrease the noise in the resultant image. The
reflection coefficient should also be low over a wide range of
incident angles; depending upon lens configuration, the angle of
incidence of light on the sensor surface can vary. Strong angular
dependence of reflection can produce undesirable vignetting.
Finally, the reflection coefficient must be consistently low across
the entire visible wavelength range of 400 to 700 nm.
[0116] Several layer compositions and thicknesses of these
optimized coatings are tested and listed in Table 2. Layer
thicknesses and compositions should be within several percent of
the specified values in order to achieve performance similar to the
given structure. FIG. 14 shows the reflection coefficient of (left)
silicon, optimized (center) one-, and (right) three-layer
anti-reflection coatings as a function of wavelength and incident
angle. The reflection for bare silicon is high throughout the range
of wavelengths and angles. The single-layer coating has a minimum
near .lamda.=540 nm at small angles of incidence, where the
reflection coefficient is below 0.5%, and reduced reflection
coefficient values throughout the range compared to bare
silicon.
[0117] Without wishing to be bound to a particular theory, the
inventors believe that the three-layer coating has three distinct
minima that combine to give reflection coefficients less than 2%
for the majority of wavelengths and incident angles. In some
embodiments, the number of local minima in reflection is equal to
the number of layers used in an optimized anti-reflection coatings
for the silicon image sensor, and about half of the layers are
composed of nano-porous low-n SiO.sub.2. Similar rules may be used
for optimizing anti-reflection coatings for other applications,
which will be shown below. This finding underscores the importance
of low-n materials in achieving high performance anti-reflection
coatings.
[0118] The reflection coefficient as a function of layer number for
optimized coatings is shown in FIG. 15. The reflection coefficient
initially decreases rapidly as more layers are added, and then
becomes almost constant. The angle- and wavelength-averaged
reflectivity of the three- and four-layer anti-reflection coatings
are similar at 4.9% and 4.4%, respectively; the top layers of the
three- and four-layer anti-reflection coatings each are composed of
90% porous SiO.sub.2 having the lowest allowed refractive index,
while the bottom layers of both coatings are pure or nearly-pure
TiO.sub.2 having the highest achievable refractive index. As
mentioned above, the three- and four-layer coatings have similar
reflection coefficients. Without wishing to be bound to a
particular theory, the inventors believe that this is a general
characteristic for anti-reflection coatings: once a sufficient
number of layers is used so that the optimum stack contains layers
with both the highest and lowest allowed refractive index,
increasing the layer number further has only a small effect on the
reflection coefficient. In Tables 2-4, "CS" refers to a
co-sputtered layer, and the percentage of a CS layer refers to a
weight percentage of the particular material composed of in the CS
layer. For example, "CS, 42% TiO.sub.2" refers to a co-sputtered
layer containing 42% TiO.sub.2 and 58% SiO.sub.2. "NP" refers to a
nano-porous low-n layer such as a nanorod layer, and the percentage
of a NP layer refers to one minus the porosity of the layer. For
example, "NP, 10% SiO.sub.2" refers to a nanoporous layer having a
porosity of 90%.
TABLE-US-00002 TABLE 2 Thickness t (in nm) and composition c of
individual layers for optimized silicon image sensor
anti-reflection coatings. 1-layer 2-layer 3-layer 4-layer t.sub.1
68.4 327.7 362.8 293.4 t.sub.2 -- 65.6 91.9 115.6 t.sub.3 -- --
42.7 75.4 t.sub.4 -- -- -- 41.7 c.sub.1 CS, 36% TiO.sub.2 NP, 14%,
SiO.sub.2 NP, 10% SiO.sub.2 NP, 10%, SiO.sub.2 c.sub.2 -- CS, 42%
TiO.sub.2 CS, 4% TiO.sub.2 NP, 35% SiO.sub.2 c.sub.3 -- -- CS, 98%
TiO.sub.2 CS, 15% TiO.sub.2 c.sub.4 -- -- -- CS, 100% TiO.sub.2
[0119] C. Silicon Solar Cell
[0120] The silicon solar cell is one of the most widespread
technologies for photovoltaics with a relevant spectral range of
400 to 1100 nm. One or two-layer anti-reflection coatings and
surface texturing are common methods used to reduce reflection from
the surface and increase efficiency. Using the genetic algorithm
approach disclosed above, anti-reflection coatings for silicon
solar cells with up to five layers with optimized performance can
be obtained. The reflection coefficient as a function of wavelength
and incident angle is shown in FIG. 16 for optimized one-, two-,
and four-layer anti-reflection coatings. As before, the number of
minima in reflection is equal to the number of layers in the
anti-reflection coating. The compositions of optimized coatings are
shown in Table 3. Again, nano-porous layers compose roughly half of
the layers in an anti-reflection coating with a given number of
layers.
[0121] Compared to the one- and two-layer coatings, the four-layer
coating yields substantially reduced reflection, particularly at
the largest incident angles and shortest wavelengths. Note that the
one- and two-layer coatings feature one co-sputtered layer and both
nano-porous low-n and co-sputtered layers, respectively, resulting
in enhanced performance compared to conventional one- and two-layer
coatings. The angle- and wavelength-averaged reflection
coefficients are plotted in FIG. 17 as a function of the number of
layers. As discussed above, when an optimized anti-reflection
coating includes layers with both the lowest allowed and highest
allowed refractive index, adding additional layers generally may
not provide significant benefit. In the case of the silicon solar
cell, which is identical to the image sensor with the exception
that the relevant wavelength range is broader, a larger number of
layers may be needed to reach the threshold. In the image sensor,
switching from three to four layers reduces average reflectivity by
10.5%, while for the solar cell, such a switch reduces the average
reflectivity by 29.4%. Adding a fifth layer to the solar cell
anti-reflection coating may further reduce the reflectivity by an
additional 5.6%.
TABLE-US-00003 TABLE 3 Thickness t (in nm) and composition c of
individual layers for optimized silicon solar cell anti-reflection
coatings. 1-layer 2-layer 3-layer 4-layer 5-layer t.sub.1 91.2
133.1 432.8 432.6 388.7 t.sub.2 -- 64.0 113.3 145.8 159.3 t.sub.3
-- -- 58.6 79.7 107.8 t.sub.4 -- -- -- 51.2 70.1 t.sub.5 -- -- --
-- 50.5 c.sub.1 CS, 36% TiO.sub.2 NP, 78% SiO.sub.2 NP, 11%
SiO.sub.2 NP, 10% SiO.sub.2 NP, 10% SiO.sub.2 c.sub.2 CS, 36%
TiO.sub.2 CS 70% TiO.sub.2 CS, 3% TiO.sub.2 NP, 54% SiO.sub.2 NP,
29% SiO.sub.2 c.sub.3 -- -- CS, 82% TiO.sub.2 CS, 28% TiO.sub.2 NP,
81% SiO.sub.2 c.sub.4 -- -- -- CS, 100% TiO.sub.2 CS, 37% TiO.sub.2
c.sub.5 -- -- -- -- CS, 100% TiO.sub.2
[0122] D. GaInP/GaAs/Ge Triple Junction Solar Cell
[0123] Multi junction solar cells have achieved the highest
efficiency of any photovoltaic technology available, for example
the technologies disclosed in N. H. Karam, et al. (N. H. Karam, et
al., Sol. Energy Mater. Sol. Cells 66, 453-466, 2001), D. J,
Friedman et al. (D. J, Friedman et al., Prog. Photovolt: Res. Appl.
9, 179-189, 2001), and Z. Q. Li, et al. (Z. Q. Li, et al., Proc.
SPIE 6339, 633909, 2006), which are hereby incorporated by
reference in their entireties. Because of the high cost associated
with fabrication, a primary intended use is in concentrator
systems, where lenses or reflectors are used to collect sunlight
over a large area and focus it on a small active area where the
solar cell is located. Generally, there is always some light
incident upon the solar cell at oblique angles because of the
nature of concentrator systems, making broadband and
omni-directional anti-reflection coatings especially important in
this application.
[0124] The structure used in calculations consists of a
GaInP/GaAs/Ge stack with thicknesses is described in Z. Q. Li, et
al. (Z. Q. Li, et al., Proc. SPIE 6339, 633909, 2006), which is
hereby incorporated by reference in its entirety. The bottom
germanium layer is assumed to be infinitely thick. The structure
used includes intermediate layers which act as tunnel junctions or
back surface field structures, however, the refractive indices of
materials used in some of these layers are not well reported.
Therefore, the triple junction solar cell is treated as a simple
three-layer stack, although, in principle, any number of layers
could be included in the calculation. The wavelength range
considered in for the anti-reflection coatings is 400 nm to 1500
nm. Table 4 shows the composition and thickness of each layer for
optimized anti-reflection coatings with up to six layers. FIG. 18
shows the reflectivity as a function of angle for the bare triple
junction solar cell, as well as the solar cell with optimized one-,
three-, and five-layer anti-reflection coatings.
TABLE-US-00004 TABLE 4 Thickness t (in nm) and composition c of
individual layers for optimized GaInP/GaAs/Ge triple junction solar
cell anti-reflection coatings. 1-layer 2-layer 3-layer 4-layer
5-layer 6-layer t.sub.1 162.7 294.9 544.4 550.9 525.0 489.8 t.sub.2
-- 132.0 137.5 168.1 195.8 206.5 t.sub.3 -- -- 78.3 94.0 108.9
127.7 t.sub.4 -- -- -- 63.0 74.1 90.1 t.sub.5 -- -- -- -- 53.2 66.5
t.sub.6 -- -- -- -- -- 52.2 c.sub.1 CS, 27% TiO.sub.2 NP, 36%
SiO.sub.2 NP, 11% SiO.sub.2 NP, 10% SiO.sub.2 NP, 10% SiO.sub.2 NP,
10% SiO.sub.2 c.sub.2 -- CS, 44% TiO.sub.2 CS, 0% TiO.sub.2 NP, 58%
SiO.sub.2 NP, 39% SiO.sub.2 NP, 28% SiO.sub.2 c3 -- -- CS, 61%
TiO.sub.2 CS, 25% TiO.sub.2 CS, 5% TiO.sub.2 NP, 69% SiO.sub.2
c.sub.4 -- -- -- CS, 82% TiO.sub.2 CS, 48% TiO.sub.2 CS, 17%
TiO.sub.2 c.sub.5 -- -- -- -- CS, 100% TiO.sub.2 CS, 57% TiO.sub.2
c.sub.6 -- -- -- -- -- CS, 100% TiO.sub.2
[0125] As shown in FIG. 18, the reflectivity at wavelengths longer
than 700 nm, and particularly longer than 900 nm shows pronounced
fringing. Without wishing to be bound to a particular theory, the
inventors believe that these longer wavelengths pass through the
top junction or both the top and middle junctions of the solar cell
without being absorbed; interference of light within these layers
produces the reflectivity fringes. When a single-layer
anti-reflection coating is added, reflectivity is initially reduced
at longer wavelengths. As more layers are added, reflectivity
across the entire range of wavelengths and incident angles is
reduced. FIG. 19 plots the reflectivity of the optimized
anti-reflection coatings as a function of total number of
layers.
[0126] In summary, this non-limiting embodiment describes a method
for optimizing anti-reflection coatings made of co-sputtered and
nano-porous low-refractive-index coatings. The method is based on a
genetic algorithm which is well suited for the task of optimizing
optical thin-film coatings, given the fact that the design space of
multi-layered optical coatings includes many local minima of the
fitness function, i.e., the average reflectivity. In this
non-limiting embodiment, the optimization method is applied to
silicon image sensors and solar cells, as well as a triple junction
GaInP/GaAs/Ge solar cell, but this optimization method may be
applied to any other suitable systems such as light-emitting diodes
or other optical components/devices that interfacial Fresnel
reflections are undesirable. As described above, in some
embodiments, nanoporous layers constitute roughly half of the total
number of layers in optimized anti-reflection coatings, which
underscores the importance of low-refractive-index materials for
high-performance anti-reflection coatings.
Embodiment V
[0127] Despite the early discovery of the photovoltaic (PV) effect
by Alexandre-Edmond Becquerel in 1839 and almost 125 years since
the first solar cell was built in 1883 by Charles Fritts, PV cells
have only seen limited commercial success to date. The primary
reason for this is the low efficiency and corresponding high cost
per kilowatt-hour of energy produced by PV cells. Even today, Si
solar cells have efficiency at best just over 20% and it has not
improved much over last decade. Reflection of incident light from
the surface of the solar cell is one of the major optical loss
mechanisms seriously affecting the solar cell efficiency. FIG. 20
shows broadband nature of solar irradiance spectrum. Nearly 90% of
commercial solar cells are made of crystalline Si. A polished Si
surface, due to its high refractive index value, reflects as much
as .about.37% light, when averaged over all angles of incidence
(0.degree. to 90.degree.) and over the range of wavelengths of the
solar spectrum that can be absorbed by Si (400 nm to 1100 nm).
[0128] For several years, the reduction of reflection from the
surface of the solar cell has been one of the primary focuses of
solar cell research. Conventionally, a single layer anti-reflection
coating with optical thickness equal to one quarter of the
wavelength of interest is used. Ideally such single layer .lamda./4
anti-reflection coating should have refractive index,
n.sub..lamda./4 as given by n.sub..lamda./4= {square root over
(n.sub.semiconductor.times.n.sub.air)}.
[0129] Often due to unavailability of materials desired, exact
value of the refractive index, the performance of such .lamda./4
anti-reflection coatings deviates from the optimum. For example,
Si.sub.3N.sub.4, which has refractive index value between that of
Si and air, is used for Si solar cells. However these single layer
anti-reflection coatings can fundamentally minimize reflection only
for one specific wavelength and for one specific angle of
incidence, typically for normal incidence. Thus, the conventional
Si.sub.3N.sub.4 single layer anti-reflection coating is inherently
unable to cover the broad range of wavelengths present in a solar
spectrum and the broad range of incident angles. These coatings
reduce the reflection to approximately .about.18%. In order to
further reduce reflection, surface texturing is often used, which
has shown to reduce reflection to .about.13%.
[0130] In this embodiment, the possibility of a near-perfect
anti-reflection coating by complete elimination of Fresnel
reflection is investigated. Lord Rayleigh, in 1880, mathematically
demonstrated that graded-refractive-index layers have broadband
anti-reflection properties. However, until recently, due to the
unavailability of optical materials with very low refractive
indices (n<1.4), such near-perfect graded-index anti-reflection
coatings could not been realized.
[0131] Recently, a novel class of low-n materials having refractive
index as low as 1.05 by using oblique angle deposition has been
fabricated, as disclosed in J. Q. Xi et al. (J.-Q. Xi, et al.,
Nature Photonics Vol. 1, 176-179, 2007), and E. F. Schubert et al.
(E. F. Schubert et al., Phys. Stat. Sol. (b) 244, No. 8, 3002-3008,
2007), which are hereby incorporated by reference in their
entireties. Consequently, it is now possible to tune the refractive
indices of an optical material to virtually any value between its
bulk value and that of air (.about.1).
[0132] In this embodiment, a systematic study of multilayer
anti-reflection coatings is performed. For multilayer
anti-reflection coatings, the refractive index of the layers is
gradually decreased from the semiconductor to air. Since the solar
spectrum, which is inherently broadband, is incident on the solar
cell over a wide range of angles during the course of the day, it
is important to use a figure of merit which gives a fair comparison
of the performance of various anti-reflection coatings. Therefore,
R.sub.avg defined as
R avg = 1 .DELTA..lamda. 1 .DELTA..theta. .intg. .lamda. min
.lamda. max .intg. .theta. min .theta. max R .theta. .lamda. ,
##EQU00009##
where .theta. is the zenith angle, is used as the figure of merit
in this embodiment.
[0133] A MATLAB program is used to obtain optimized parameters,
refractive index n and thickness t, for different anti-reflection
coatings and different solar-cell materials. All coatings are
optimized in the wavelength range of 400 nm to 1100 nm and incident
angle range of 0.degree. to 90.degree.. For validation of the
concept, a 3-layer anti-reflection coating optimized for polished
Si solar cells is fabricated and characterized, as described below.
For simplicity of design and fabrication, only the thickness t of
each layer in the 3-layer graded-index is varied during
optimization while keeping the refractive index n fixed for each.
FIG. 21 shows the surface plots of calculated reflectance for (a)
bare polished Si substrate with no anti-reflection coating, (b) Si
substrate with a conventional .lamda./4 Si.sub.3N.sub.4
anti-reflection coating, and (c) Si substrate with 3-layer
graded-index anti-reflection coating. The results show significant
advantages of the 3-layer graded-index anti-reflection coating in
terms of drastically reduced reflectance over a wide range of
wavelengths and angles of incidence.
[0134] Three samples are prepared using 1 cm.times.1 cm pieces of
polished crystalline Si cut by a diamond scriber. Sample (a) is
bare polished Si substrate with no anti-reflection coating. Sample
(b) is Si substrate with a conventional .lamda./4 Si.sub.3N.sub.4
anti-reflection coating. Sample (c) is Si substrate with 3-layer
graded-index anti-reflection coating. The .lamda./4 Si.sub.3N.sub.4
anti-reflection coating in sample (b) is deposited using a Plasma
Enhanced Chemical Vapor Deposition (PECVD) tool and is optimized
for lowest normal incidence reflection at 550 nm wavelength and has
refractive index, n=2.2 measured at 550 nm and thickness, t=62.5
nm. The 3-layers anti-reflection coating is deposited over a
polished crystalline Si substrate using RF sputtering for the first
two layers and oblique angle e-beam evaporation for the third
layer. As shown in FIG. 22(b), the anti-reflection coating is
composed of a first layer of TiO.sub.2 (n=2.66 at a wavelength of
550 nm), a second layer of SiO.sub.2 (n=1.47 at a wavelength of 550
nm), and a third layer of low-n SiO.sub.2 (n=1.07 at a wavelength
of 550 nm). The thickness of each layer is 45 nm, 120 nm, and 200
nm, respectively. The first layer of TiO.sub.2 is deposited by
reactive sputtering a 2 inch TiO.sub.2 target for 57 minutes, under
200 W of RF sputtering power, 5.0 sccm of Ar, 0.5 sccm of O.sub.2,
and an operating pressure of 2 mTorr. Substrate bias of 5 W and
substrate temperature of 500.degree. C. are used to achieve high
refractive index value of 2.66 at a wavelength of 550 nm. The
second layer of SiO.sub.2 is deposited by reactive sputtering a 2
inch SiO.sub.2 target for 60 minutes, under 200 W of RF sputtering
power, 5.0 sccm of Ar, 0.5 sccm of O.sub.2, and an operating
pressure of 2 mTorr. Substrate bias of 5 W is used without any
external heating. The third layer of porous SiO.sub.2 is deposited
using oblique angle e-beam evaporation technique. The desired low
refractive-index is achieved by mounting the sample in such a way
that the substrate normal is 85.degree. to the incoming flux.
Details of the oblique angle evaporation technique have been
described in Martin F. Schubert, et al. (Martin F. Schubert, et
al., App. Phys. Lett. 90, 141115, 2007), K. Robbie et al. (K.
Robbie et al., J. Vac. Set. Technol. A 15 (3), 1460-1465, 1997),
and K. Robbie, et al. (K. Robbie, et al., J. Vac. Sci. Technol. B
16 (3), 1115-1122, 1998), which are hereby incorporated by
reference in their entireties. The thickness and R.I. values are
measured using variable angle spectroscopic ellipsometry. Thickness
is confirmed by images of scanning electron microscopy (SEM) for
example the SEM image of the 3-layer anti-reflection coating as
shown in FIG. 22(a).
[0135] The absolute reflectance of samples (a), (b), and (c) is
measured using the VASE M44 variable angle spectroscopic
ellipsometry. For each sample, data are measured for 44 discreet
values of the wavelength between 400 nm and 750 nm, which were
predetermined by the instrument and for incident angle range
between 40.degree. and 80.degree. with 1.degree. increments. The
measurement is done for one specific polarization at a time. This
totals to 1804 data points for each polarization within the window
of desired range of wavelengths and incident angles for each
sample. A large number of data points are measured to ensure
accuracy in R.sub.avg. FIGS. 23(a)-(c) show the surface plots of
the measured absolute reflectance data of the samples. FIG. 23(c),
in contrast with FIG. 23(b), shows very-low reflectance over a wide
range of wavelengths and incident angles, clearly demonstrating the
broadband and omni-directional characteristics of the 3-layer
graded-index anti-reflection coating.
[0136] The measured absolute reflectance results are in excellent
agreement with the theoretically calculated values shown in FIG.
21. The R.sub.avg values of samples (a), (b), and (c) are 37.0%,
17.3%, and 5.9%, respectively. FIG. 24 shows the photograph of the
three samples for side-by-side comparison. The .lamda./4
anti-reflection coating shows blue color due to its significantly
high reflectance in the wavelength range below the zero-reflection
wavelength. The superiority of the 3-layer anti-reflection coating
is clearly observable.
[0137] In summary, this embodiment demonstrates an ultra-low
reflectance, broadband, omni-directional, graded-index
anti-reflection coating. The availability of the novel
nanostructured low-n materials deposited by oblique angle
deposition technique has allowed a design of near-perfect
anti-reflection coatings which can be used in wide variety of
applications. Measurements show dramatic reduction in reflection
over wide range of incident angles and broad range of wavelengths
in comparison with conventional .lamda./4 anti-reflection coatings.
The average reflectance, R.sub.avg, of 5.9% was measured for the
triple-layer graded-index anti-reflection coating as compared to
17.3% for the conventional Si.sub.3N.sub.4 .lamda./4
anti-reflection coating widely used for Si solar cells. These
values are in excellent agreement with the theoretical calculations
which predict R.sub.avg of 4.9% for the triple-layer graded-index
anti-reflection coating and 18.2% for the .lamda./4 anti-reflection
coating. This broadband and omni-directional character of the
anti-reflection coating of this embodiment is very well suited for
application in solar cells and other applications.
Embodiment VI
[0138] The antireflective coatings described herein may be used
with any suitable device in which antireflective coatings are used,
such as photodetectors and solar cells (i.e., photovoltaic cells).
A solar cell includes a first electrode, a second electrode and a
photovoltaic material located between the electrodes.
[0139] At least one of the electrodes which faces the Sun is
transparent to solar radiation. For example, the electrode is made
of a transparent material, such as a transparent conductive oxide
(e.g., indium tin oxide, zinc oxide, aluminum zinc oxide, etc.)
and/or is formed in a shape of a grid or mesh such that the solar
radiation may be incident on the photovoltaic material between the
grid or mesh lines. As used herein, transparent means transmitting
at least 50%, such as at least 75% of incident solar radiation and
should not be interpreted as being limited to 100% transmission of
solar radiation. The other electrode may comprise any suitable
conductor, such as a metal or metal alloy, including Al, Cu, Ti,
Au, Ag, steel, etc.
[0140] The photovoltaic material may comprise one or more layers of
organic and/or inorganic semiconductor photovoltaic material.
Organic photovoltaic materials include photovoltaic polymers.
Inorganic photovoltaic materials include Group IV semiconductors,
such as silicon, germanium, etc., and compound semiconductor
materials, such as binary, ternary or quaternary materials,
including CdTe, GaAs, InP, GaAlAs, etc. The photovoltaic material
may comprise a single junction (i.e., p-n or p-i-n junction) or
multi junction material. Alternatively, the photovoltaic material
may comprise a single layer of semiconductor material arranged in a
Schottky junction configuration (i.e., a junction between the
semiconductor material and an adjacent metal electrode).
[0141] The solar cell may be formed on a transparent substrate,
such as glass, quartz, plastic, polymer, etc., which faces the Sun.
In this case, the transparent electrode is located between the
transparent substrate and the photovoltaic material. The
anti-reflection coating may be located between the transparent
substrate and the transparent electrode. Alternatively, the
anti-reflection coating may be located between the photovoltaic
material and the transparent electrode, especially if the
transparent electrode has a grid or mesh shape. If desired, a
second anti-reflection coating may be added over the transparent
substrate instead of or in addition to the anti-reflection coating
located between the substrate and the rest of the solar cell.
[0142] Alternatively, the solar cell may be formed on a
non-transparent substrate, such as a metal, ceramic or
semiconductor substrate. In this case, the non-transparent
electrode is formed between the substrate and one side of the
photovoltaic material. The transparent electrode is formed over the
opposite side of the photovoltaic material. An optional transparent
encapsulating material, such as a polymer, glass or epoxy material,
is formed over the transparent electrode. The anti-reflection
coating may be located between the encapsulating material and the
transparent electrode. Alternatively, the anti-reflection coating
may be located between the photovoltaic material and the
transparent electrode, especially if the transparent electrode has
a grid or mesh shape. If desired, a second anti-reflection coating
may be added over the encapsulating material instead of or in
addition to the anti-reflection coating located between the
encapsulating material and the rest of the solar cell.
[0143] The solar cell may be a concentrator type cell (e.g., a
relatively small cell, such as a multi-junction cell used with a
concentrator device, such as a lens) or a large area panel type
cell.
[0144] Features from any embodiment may be used in any combination
with one or more features from the same or one or more different
embodiments. The forgoing description of the invention has been
presented for purpose of illustration and description. It is not
intended to be exhaustive or limit the invention to the precise
from disclosed, and modifications and variations are possible in
light of the above teachings or may be acquired from practice of
the invention. The description was chosen in order to explain the
principles of the invention. The description was chosen in order to
explain the principle of the invention and its practical
application. It is intended that the scope of the invention be
defined by the claims appended hereto, and their equivalents. All
references disclosed herein are incorporated by reference in their
entirety.
* * * * *