U.S. patent application number 12/946580 was filed with the patent office on 2011-07-14 for high transmittance optical windows and method of constructing the same.
This patent application is currently assigned to MAGNOLIA SOLAR, INC.. Invention is credited to Sameer Chhajed, Jaehee Cho, Frank W. Mont, David J. Poxson, E. Fred Schubert, Ashok K. Sood, Roger E. Welser.
Application Number | 20110168261 12/946580 |
Document ID | / |
Family ID | 44257581 |
Filed Date | 2011-07-14 |
United States Patent
Application |
20110168261 |
Kind Code |
A1 |
Welser; Roger E. ; et
al. |
July 14, 2011 |
HIGH TRANSMITTANCE OPTICAL WINDOWS AND METHOD OF CONSTRUCTING THE
SAME
Abstract
Designs for ultra-high, broadband transmittance through windows
over a wide range of incident angles are disclosed. The
improvements in transmittance result from coating the windows with
a new class of materials consisting of porous nanorods. A high
transmittance optical window comprises a transparent substrate
coated on one or both sides with a multiple layer coating. Each
multiple layer coating includes optical films with a refractive
index intermediate between the refractive index of the transparent
substrate and air. The optical coatings are applied using an
oblique-angle deposition material synthesis technique. The coating
can be performed by depositing porous SiO.sub.2 layers using
oblique angle deposition. The high transmittance window coated with
the multiple layer coating exhibits reduced reflectance and
improved transmittance, as compared to an uncoated transparent
substrate.
Inventors: |
Welser; Roger E.;
(Providence, RI) ; Sood; Ashok K.; (Brookline,
MA) ; Poxson; David J.; (Troy, NY) ; Chhajed;
Sameer; (Pohang, KR) ; Mont; Frank W.; (Troy,
NY) ; Cho; Jaehee; (Troy, NY) ; Schubert; E.
Fred; (Troy, NY) |
Assignee: |
MAGNOLIA SOLAR, INC.
Woburn
MA
RENNSELAER POLYTECHNIC INSTITUTE
Troy
NY
|
Family ID: |
44257581 |
Appl. No.: |
12/946580 |
Filed: |
November 15, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61293469 |
Jan 8, 2010 |
|
|
|
Current U.S.
Class: |
136/259 ;
257/E31.127; 359/586; 438/69 |
Current CPC
Class: |
H01L 31/02165 20130101;
H01L 31/0725 20130101; H01L 31/184 20130101; H01L 31/1844 20130101;
H01L 31/022475 20130101; H01L 31/0445 20141201; H01L 31/1884
20130101; H01L 31/02168 20130101; H01L 31/03046 20130101; H01L
31/048 20130101; H01L 31/022425 20130101; H01L 31/0735 20130101;
Y02E 10/544 20130101; H01L 31/035236 20130101; H01L 31/035263
20130101; Y02E 10/52 20130101; H01L 31/0203 20130101; G02B 1/115
20130101; H01L 31/056 20141201; H01L 31/065 20130101 |
Class at
Publication: |
136/259 ;
359/586; 438/69; 257/E31.127 |
International
Class: |
H01L 31/0232 20060101
H01L031/0232; G02B 1/10 20060101 G02B001/10; H01L 31/18 20060101
H01L031/18 |
Goverment Interests
GOVERNMENT SUPPORT
[0002] This invention was supported in part by Small Business
Innovative Research (SBIR) contract # W31P4Q-08-C-0300 from the
Defense Advanced Research Projects Agency (DARPA) to Magnolia
Optical Technologies, Inc., 52 B Cummings Park, Suite 314, Woburn,
Mass. 01801. The government may have certain rights in this
invention.
Claims
1. A high transmittance optical window comprising: a transparent
substrate coated on both sides with a multiple layer coating; and
the multiple layer coating comprising a plurality of optical films,
and the multiple layer coating defining a refractive index
intermediate between the refractive index of the transparent
substrate and air.
2. The high transmittance window of claim 1 wherein the transparent
substrate comprises at least one of glass, quartz, and sapphire
materials.
3. The high transmittance window of claim 1 wherein the multiple
layer coating comprises at least one of SiO.sub.2, TiO.sub.2,
Si.sub.3N.sub.4, BaF.sub.2, CdTe, and diamond like carbon
materials.
4. The high transmittance window of claim 1 wherein the multiple
layer coating comprises a transparent conductive oxide including at
least one of indium tin oxide and zinc oxide.
5. The high transmittance window of claim 1 wherein the multiple
layer coating is deposited by oblique-angle deposition.
6. The high transmittance window of claim 1 wherein the multiple
layer coating comprises at least two layers having a similar
chemical composition but a different porosity and thus a different
refractive index.
7. The high transmittance window of claim 6 wherein the transparent
substrate comprises sapphire and the index of refraction for each
of the plurality of optical films is varied from 1.5 to 1.1 over
two steps, with the plurality of deposited layers defining
approximately 230 nm of dense SiO.sub.2 (n.about.1.46) and
approximately 300 nm of porous SiO.sub.2 (n.about.1.18).
8. The high transmittance window of claim 1 wherein the multiple
layer coating contains one of (i) at least one layer of the AR
coating comprises a single dense material, (ii) at least one layer
of the AR coating comprises a solid solution of two different dense
materials, and (iii) at least one layer of the AR coating comprises
a porous material.
9. The high transmittance window of claim 6 further comprising a
pore closing coating.
10. A photovoltaic device comprising: a glass window coated on a
top, sun-facing surface with a multiple layer coating comprising a
plurality of optical films, and the multiple layer coating defining
a refractive index intermediate between the refractive index of the
glass window (n.about.1.5) and air (n.about.1); and an underlying
semiconductor solar cell device.
11. The photovoltaic device of claim 10 wherein the glass window
forms a cover glass that is attached to the underlying
semiconductor device with transparent epoxy.
12. The photovoltaic device of claim 10 wherein the glass window
forms a transparent superstrate upon which a semiconductor thin
film solar cell structure is deposited.
13. The photovoltaic device of claim 10 wherein the multiple layer
coating comprises at least two layers having a similar chemical
composition but a different porosity and thus a different
refractive index.
14. The photovoltaic device of claim 13 wherein the index of
refraction in the topmost coating is varied from 1.5 to 1.1 over
three steps, with the plurality of optical films defining
approximately 192 nm of porous SiO.sub.2 (n.about.1.36),
approximately 179 nm of porous SiO.sub.2(n.about.1.19), and
approximately 260 nm of porous SiO.sub.2(n.about.1.10).
15. The photovoltaic device of claim 13 further comprising a pore
closing coating covering the topmost layer in the antireflection
coating.
16. A method of manufacturing a thin film solar cell comprising:
providing a transparent substrate having a front surface and a back
surface; and coating the transparent substrate on at least one side
with a multiple layer optical coating comprising a plurality of
optical films, and the multiple layer optical coating defining a
refractive index intermediate between the refractive index of the
transparent substrate and the refractive index of air.
17. The method of claim 16 wherein the step of coating the
transparent substrate comprises the deposition of porous SiO.sub.2
layers using oblique-angle deposition.
18. The method of claim 16 wherein the step of coating the
transparent substrate comprises the deposition of porous TiO.sub.2
layer using oblique-angle deposition.
19. The method of claim 16 wherein the step of coating the
transparent substrate comprises the depositing of a porous layers
consisting of SiO.sub.2, TiO.sub.2, Si.sub.3N.sub.4, BaF.sub.2,
CdTe, and diamond like carbon materials using oblique-angle
deposition.
20. The method of claim 16 wherein the multiple layer optical
coating is applied on the front surface after forming a thin film
solar cell device on the back surface of the transparent substrate.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of co-pending U.S.
Provisional Patent Application Ser. No. 61/293,469, filed on Jan.
8, 2010 entitled EFFICIENT SOLAR CELL EMPLOYING MULTIPLE ENERGY-GAP
LAYERS AND LIGHT-SCATTERING STRUCTURES AND METHODS FOR CONSTRUCTING
THE SAME, which is expressly incorporated herein by reference.
FIELD OF THE INVENTION
[0003] This invention relates to transparent optical windows for
detectors, sensors, and other optical devices; and to
semiconductor-based photovoltaic energy converters, also known as
"solar cells," and to the design and fabrication of the same.
BACKGROUND OF THE INVENTION
[0004] Transparent windows are employed in a wide range of military
and commercial applications, including optical lenses and
photovoltaic cover glass. Glass, sapphire, and quartz are
well-known materials used to form high transmittance optical
windows for a wide range of applications. Because these materials
have very low absorption coefficients over a wide range of photon
energies, optical transmittance through glass, sapphire, and quartz
windows is typically limited by reflection losses. Fresnel
reflection losses in optical windows arise from the difference in
index of refraction between air (n.about.1) and the window material
(n.about.1.4-1.8). Although Fresnel reflection losses are typically
relatively low at normal incidence, they can become quite
substantial for off-angle light incidence. For example, Fresnel
reflection from uncoated glass generally varies from over 4% at
normal incidence to as much as 40% at an incident angle of
75.degree..
[0005] Reducing optical reflection from surfaces is highly
desirable to many applications in optics. Reducing reflection is
commonly achieved through coating or texturing the surface of
interest. Numerous applications involving dielectric or
semiconducting materials use the light that is transmitted through
the material's surface. Examples of such an application are optical
lenses, windows, photovoltaic devices, and photodetectors. Glass
(amorphous SiO.sub.2) is an example of a dielectric material widely
used in a variety of optical applications (e.g. lenses, windows)
and as a cover or encapsulation for semiconductor optoelectronic
devices.
[0006] Glass is completely transparent for wavelengths longer than
400 nm. However, due to Fresnel reflection, it reflects about 4% of
the incident light from its surface (-8% from two surfaces). This
reflection is undesirable in many applications as it can degrade
the efficiency of the underlying device (e.g. efficiency of a solar
photovoltaic cell), reduce signal-to-noise ratio (e.g. in a
photodetector), and cause glare (e.g. from LCD screens, computer
monitors, and televisions). For these applications, it is desirable
not only to reduce reflectance but also to improve transmittance
through the surface, which is achieved through a coating material
that is non-absorbing and a coating surface that is specular.
[0007] Conventionally, a single-layer coating with optical
thickness equal to one quarter of the wavelength (.lamda./4) of
interest has been used as an AR coating. Preferably, such
single-layer .lamda./4 AR coating should have a refractive index,
n.sub..lamda./4 as given by
n.sub..lamda./4- {square root over
(n.sub.substrate.times.n.sub.air)}.
Often due to unavailability of materials with the desired, exact
value of the refractive index, the performance of such .lamda./4 AR
coatings deviates from the optimum. This is especially the case for
low-index substrates, such as glass. An ideal single-layer
.lamda./4 AR coating on glass surface in an air ambient would
require a material with refractive index of
(1.46).sup.1/2.apprxeq.1.2. There is no conventional inorganic
material that has such a low refractive index. Also, fundamentally,
these single-layer .lamda./4 AR coatings can minimize reflection
only for one specific wavelength at normal incidence and they are
inherently unable to exhibit spectrally broadband reduction in
reflectance over wide range of angles-of-incidence.
[0008] In 1880, Lord Rayleigh mathematically demonstrated that
graded-refractive-index layers have broadband antireflection
properties. Multi-layer stacks of materials with different
refractive indices have been used in order to achieve broadband
reduction in reflection. Anti-reflection (AR) coatings with
specular surface made of multiple discrete layers of non-absorbing
materials can exploit thin film interference effects to reduce the
reflectance while improving transmittance.
[0009] Optimization of multi-layer AR coatings is a difficult
challenge because of the extremely large and complex dimensional
space of possible solutions. Analytical methods to optimize AR
coatings are not feasible due to the complexity of the problem.
Heuristic methods such as needle-optimization, jump-elimination,
and genetic algorithm are commonly used. It is desirable to provide
a computational genetic algorithm method to achieve optimization of
the coatings.
[0010] Theoretically, it has been known for some time that Fresnel
reflection losses can be minimized between two media by varying the
index of refraction across the interface. Until recently, however,
the unavailability of materials with desired refractive indices,
particularly materials with very low refractive indices below
n=1.2, prevented the implementation of high-performance step graded
refractive index designs. Recently, however, Prof. Fred Schubert
and his group at Rensselear Polytechnic Institute (RPI) have
created a new class of materials comprising porous nanorods. In
particular, the RPI group has demonstrated that oblique-angle
deposition can be used to tailor the refractive index of a wide
variety of thin film materials. Therefore it is desirable to apply
this new material synthesis technique to the formation of coatings
that can minimize reflection losses and maximize the transmittance
through a wide variety of optical windows.
SUMMARY OF THE INVENTION
[0011] This invention overcomes the disadvantages of the prior art
by providing antireflection structures and a method of
manufacturing the antireflection structures to increase the
transmittance through a variety of different optical windows for a
variety of applications. The various illustrative embodiments
reduce reflection losses, thus maximizing transmittance through
optical windows. The various illustrative embodiments utilize
multiple layer optical coatings in which the refractive index is
varied between that of the window material and air in discrete
steps. It is possible to design antireflection (AR) coatings that,
due to interference effects, have a lower reflectivity than a
continuously graded AR coating. In one embodiment, the optical
antireflection coating comprised of at least two layers, up to any
plurality of layers, which have a similar chemical composition but
a different porosity and thus a different refractive index. In
another embodiment, the optical antireflection coating contains (i)
at least one layer of the AR coating comprising a single dense
material, (ii) at least one layer of the AR coating comprising a
solid solution of two different dense materials (that is a mixture
of two dense materials), and (iii) at least one layer of the AR
coating comprising a porous material. In yet another embodiment, a
pore-closure layer is employed that covers the top surface and
prevents moisture, or particles, from infiltrating the porous film.
The pore-closure layer is very thin (much smaller than .lamda.) so
as to be applied without influencing the reflectivity of the AR
coating. More particularly, the pore closure layer is constructed
and arranged to avoid negatively affecting the reflectivity.
[0012] In the illustrative embodiment, a high transmittance window
comprises a transparent substrate coated on both sides with a
multiple layer coating, such that each multiple layer coating
comprises a plurality of optical films. The multiple layer coating
defines a refractive index intermediate between the refractive
index of the transparent substrate and air. The window can
comprise, but is not limited to, glass, quartz, and sapphire
materials. The multiple layer coating can comprise a plurality of
various optical thin film materials, including but not limited to,
SiO.sub.2, TiO.sub.2, Si.sub.3N.sub.4, BaF.sub.2, CdTe, and diamond
like carbon materials. Conductive, transparent coatings can be
formed by using transparent conductive oxide (TCO) materials such
as indium tin oxide (ITO) and zinc oxide (ZnO). The individual
layers in the optical coating can comprise a single material of
varying porosity, or of a solid solution of two different, dense
materials, or any combination thereof. In a specific embodiment,
the window material comprises sapphire and the index of refraction
in each coating is varied from 1.5 to 1.1 over two steps, with the
plurality of deposited layers defining approximately 230 nm of
dense SiO.sub.2 (n.about.1.46) and approximately 300 nm of porous
SiO.sub.2 (n.about.1.18).
[0013] In another illustrative embodiment, a plurality of
antireflection layers of transparent refractive thin film are
deposited on the front, sun-facing surface of a photovoltaic
device. The purpose of the antireflection layers is to maximize the
number of incident photons that are directed into the active region
of an underlying semiconductor solar cell device. The
antireflection structure is formed of multiple layers of optical
thin film material on top of a transparent cover glass, while
having an index of refraction intermediate between that of the
glass and air. In the illustrative embodiment, the profile is
characterized by a step-graded profile that may or may not follow a
quintic profile to provide maximum photon transmission through the
antireflection layers. The exact thickness and index of refraction
of each of the layers in the antireflection layer can be adjusted
to further minimize reflection losses over a broad spectrum of
photon wavelengths and angles of incidence. The antireflection
coating can be built using a variety of different materials, either
in combination or with various degrees of porosity, including but
not limited to, SiO.sub.2, TiO.sub.2, Si.sub.3N.sub.4, BaF.sub.2,
CdTe, ITO or other TCO materials, and diamond like carbon
materials. In a specific embodiment, the index of refraction in the
topmost coating is varied from 1.5 to 1.1 over three steps, with
the plurality of deposited layers defining approximately 192 nm of
porous SiO.sub.2 (n.about.1.36), approximately 179 nm of porous
SiO.sub.2(n.about.1.19), and approximately 260 nm of porous
SiO.sub.2(n.about.1.10).
[0014] A method of constructing the improved antireflection
structures described herein comprises coating the top and sometimes
also the bottom surfaces of a transparent window with
nanostructured optical coatings. The nanostructured optical
coatings can be applied using the oblique angle deposition material
synthesis technique. According to the illustrative embodiment, a
transparent substrate is provided having a front surface and a back
surface. The transparent substrate is then coated on at least one
surface with a multiple layer ("multi-layer") coating comprising a
plurality of optical films, and the multi-layer coating defining a
refractive index intermediate between the refractive index of the
transparent substrate and the refractive index of air. The coating
can be performed by depositing porous SiO.sub.2 layers using
oblique-angle deposition. The coating can also be performed by
depositing layers comprising SiO.sub.2, TiO.sub.2, Si.sub.3N.sub.4,
BaF.sub.2, CdTe and diamond like carbon materials using oblique
angle deposition. In further embodiments, the coating is applied to
the front surface after forming a thin film solar cell device on
the back surface of the transparent substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The invention will be readily understood by the following
detailed description in conjunction with the accompanying drawings,
wherein like reference numerals designate like structural elements,
and in which:
[0016] FIG. 1 is a schematic side cross-sectional view of a
double-side coated optical window, according to an illustrative
embodiment;
[0017] FIG. 2 is a graph showing index of refraction versus
position for a two-layered step-graded antireflection coating
structure on a sapphire window to enhance optical transmission into
or out of the window, according to the illustrative embodiment;
[0018] FIG. 3 is a graph showing measured transmittance through an
uncoated sapphire window and through a sapphire window coated on
both sides with a 2-layer AR structure, according to the
illustrative embodiment;
[0019] FIG. 4 is a graph showing index of refraction versus
position for a three-layered step-graded antireflection coating
structure on a glass window to enhance optical transmission into or
out of the window, according to the illustrative embodiment;
[0020] FIG. 5 is a graph showing measured transmittance through an
uncoated glass slide and through a glass side coated on one and
both sides with a 3-layer AR structure, according to the
illustrative embodiment;
[0021] FIG. 6 is a graph showing index of refraction versus
position for a four-layered step-graded antireflection coating
structure on a glass window to enhance optical transmission into or
out of the window, according to the illustrative embodiment;
and
[0022] FIG. 7 is a schematic side cross sectional view of a
photovoltaic device employing a step-graded antireflection coating
on the top side of a glass window covering a semiconductor device
structure, configured and arranged to face the sun to enhance
optical transmission of photon energies into the active regions of
the underlying solar cell, according to an illustrative
embodiment.
[0023] The drawings are not necessarily to scale with emphasis
instead being placed upon illustrating embodiments of the present
invention.
DETAILED DESCRIPTION
[0024] Ultra-high, broadband transmittance through coated glass
windows is demonstrated over a wide range of incident angles. The
measured improvements in transmittance result from coating the
windows with materials consisting of porous nanorods. The use of
porous nano-materials fabricated by, for example, oblique-angle
deposition, enables a tunable refractive index, flexibility in
choice of material, simplicity of a physical vapor deposition
process, and the ability to optimize the coating for any
substrate-ambient material system. A multi-layer coating adapted
for a glass substrate, is fabricated and characterized as described
below. For multi-layer AR coatings, according to an illustrative
embodiment, the refractive index of the layers is step-graded (i.e.
decreased in discrete steps), from the substrate value, 1.46, to a
value of 1.18, according to the various illustrative
embodiments.
[0025] FIG. 1 details a cross-sectional view illustrating a high
transmittance window structure 100 comprising a transparent optical
window 120 having an antireflection structure 110 and 130 coated,
respectively, on the front and back sides. According to the
illustrative embodiment, the front coating 110 is deposited on a
device window 120 configured and arranged to face a light source,
which provides a readily available source of photons 140. The front
coating 110 is a multiple-layer coating comprising a plurality of
optical films, and the multiple-layer coating defines an index of
refraction between air 150 and the window 120. The multi-layer
coating can comprise two, three, or more layers, up to a plurality
of layers, defining refractive indices as appropriate to achieve
the desired transmittance. Refer to FIGS. 2, 4 and 6, showing
examples of the refractive index profile. A back coating 130 is
applied to the back side of the window 120 and comprises materials
possessing indices of refraction between that of the window 120 and
air 150. Although photons 140 are illustratively shown as a series
of a single direction of photon stream, it should be clear to those
skilled in the art that the various, illustrative, and alternate
embodiments will function with various varying degrees and/or
amount of incident of light or source of photon energies.
[0026] In various embodiments, front coating 110 and back coating
130 are configured and arranged with transparent antireflection
coating structures to reduce the reflection of incident photons at
the material interface between air 150 and the window 120. In the
various embodiments, front coating 110 and back coating 130 are
implemented in accordance with industry standard processes and
materials known to those skilled in the art. Transparent
antireflection coating structures can comprise a single layer or
multiple layers of materials having an index of refraction
intermediate between the window 120 and the media in which the
incident photons are delivered, which by way of example is
illustrated as air 150 in FIG. 1. Single-layer transparent
antireflection coating structures are generally characterized by
enhanced transmittance around a single wavelength of light when the
light is at normal incidence to the transparent antireflection
coating structure surface. In alternate embodiments, graded-index
coatings with variable-index profiles are utilized. By way of
example, a quintic profile is illustrated at near optimum profile
for a graded-index antireflection coating (see, for example, by way
of useful background information, U.S. Pat. No. 4,583,822, entitled
QUINTIC REFRACTIVE INDEX PROFILE ANTIREFLECTION COATINGS, by W. H.
Southwell, the teachings of which are expressly incorporated herein
by reference as useful background information). The various
illustrative and alternate embodiments utilize optical materials
with very low refractive indices that closely match the refractive
index of air, which historically have not been utilized.
[0027] Oblique-angle deposition is utilized as an effective
technique for tailoring the refractive index of a variety of thin
film materials (see for example, by way of useful background, J.-Q.
Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin,
W. Liu, and J. A. Smart, Optical Thin-Film Materials with Low
Refractive Index for Broad-Band Elimination of Fresnel Reflection,
Nat. Photon., vol. 1, pp. 176-179, 2007). Oblique-angle deposition
is a method of growing nanostructured, porous thin films, and hence
thin films with low-refractive index (low-n), enabled by surface
diffusion and self-shadowing effects during the deposition process.
In oblique-angle deposition, random growth fluctuations on the
substrate produce a shadow region that incident vapor flux cannot
reach, and a non-shadow region where incident flux deposits
preferentially, thereby creating an oriented rod-like structure
with high porosity. The deposition angle, defined as the angle
between the normal to the sample surface and the incident vapor
flux, results in the formation of nanorod structures that are
tilted relative to the sample surface. Given that the gaps between
the nanorods can be much smaller than the wavelength of visible and
infrared light, the nanostructured layers act as a single
homogeneous film with a refractive index intermediate between air
and the nanorod material, decreasing in refractive index with
increasing porosity.
[0028] Both conducting and non-conducting graded-index
antireflection coatings that are broadband and Omni-directional
have been demonstrated using this deposition technique. As taught
by Cho et al. in U.S. Pat. No. 7,483,212, by way of background,
both oblique angle deposition and co-sputtering are material
synthesis techniques that can be used to construct multiple layer,
graded refractive index coatings to minimize reflection losses. The
teachings of this patent are expressly incorporated herein by
reference as useful background information. It is contemplated in
illustrative embodiments that these processes can be adapted to
minimize reflection losses for optical windows.
[0029] The refractive index of a front coating 110 and/or a back
coating 130 on a window 120 is shown in the graph of FIG. 2
according to an illustrative embodiment. One example of step graded
profile 210 is shown, along with a continuously varying quintic
profile 200 of the index of refraction approximated as taught in
U.S. Pat. No. 4,583,822, which is incorporated by reference as
useful background information. In particular, the index of
refraction, herein referred to as "n", is varied from that of the
window 120, which in this case is composed of transparent sapphire
material 220 having a reflection value "n" of approximately 1.77,
to that of air 150, which is shown by way of example to be
approximately 1. Fresnel reflection from one surface of uncoated
sapphire generally varies from approximately 8% at normal incidence
to up to approximately 50% at an incident angle of 75.degree..
While dense SiO.sub.2 is an optically transparent material, it has
an index of refraction comparable to common silicon encapsulants
(n.about.1.47). Thus, in conventional implementations, SiO.sub.2 is
not typically used to reduce reflection losses. However, according
to the illustrative embodiment, oblique angle deposition is
employed to produce porous SiO.sub.2 layer with lower index of
refraction. More particularly, the index of refraction of the front
coating 110 is varied from 1.77 to 1 over two discrete steps which
can comprise a first approximately 230 nm layer 230 of dense
SiO.sub.2 material (n.about.1.46) and a second approximately 300 nm
layer 240 of porous SiO.sub.2 material (n.about.1.18).
[0030] Notably, all layers of a multi-layer AR coating are
constructed from a single material, porous silica (porous
SiO.sub.2), according to the illustrative embodiment. Silica is
particularly adept for use as AR coating on a glass, quartz, or
sapphire substrate, as it is native, stable and robust.
[0031] SiO.sub.2 coatings are well known for their long-term
stability and high transmittance over a wide spectral range.
Conventional, dense SiO.sub.2 has a refractive index of
approximately 1.46, and thus is not an effective antireflection
material for glass windows with a refractive index of approximately
1.5. However, the refractive index of porous SiO.sub.2 can be
reduced to values of 1.1 or lower by increasing the porosity.
Oblique angle deposition enables the creation of a wide variety of
step graded refractive index structures.
[0032] This particular combination of index of refraction and layer
thicknesses illustratively provides an appropriate approximation of
the quintic profile 200, as shown in the graph of FIG. 2. During an
operational embodiments of a design-optimization process, the
thickness as well as the porosity of each layer in the multi-layer
graded index AR coating is permitted to vary. In an embodiment, the
coatings are optimized in the wavelength range of 400 nm to 2500
nm, and the angle of incidence ranges from 0.degree. to 40.degree..
The thickness and refractive index values of each coating can be
measured using any conventional technique known to those of
ordinary skill, including variable angle spectroscopic ellipsometry
and scanning electron microscopy, among others. It should be clear
to those skilled in the art that the number of discrete steps and
the illustrated refraction index are only shown for illustrative
purposes and that the number of discrete steps and various values
of refractive index can be varied according to the various
embodiments. Furthermore, discrete antireflection coatings can
surpass the performance of continuously graded coatings by taking
of advantage of interference effects, which continuously graded
coatings are expressly designed to avoid, as taught by Martin F.
Schubert et al. in Appl. Phys. Express, volume 3, article no.
082502.
[0033] FIG. 3 shows a graph that compares the measured
transmittance of an uncoated sapphire (without AR) to sapphire
coated on two sides with a two-layered, nanostructured SiO.sub.2 AR
coating. The samples were prepared in an electron-beam evaporator
using two different deposition angles (.about.0.degree. and
60.degree.). In order to quantify the thickness and refractive
index of each individual layer, a sacrificial silicon substrate was
placed alongside the sapphire windows during each deposition step.
The thickness and refractive index of the single layer films on
silicon were measured with an ellipsometry-based measurement
system. The transmittance of the coated and uncoated glass slides
was then measured using an angle and wavelength dependent
transmittance measurement setup. The measurement setup for
characterizing transmittance versus wavelength includes a Xenon
lamp light source and an Ando AQ6315A optical spectrum analyzer.
The spectrum analyzer is calibrated to detect transmitted photons
over a broadband spectrum (400 nm-1800 nm).
[0034] The measured peak transmittance of the uncoated glass slide
is approximately 88%, in-line with the expected approximate 6-7%
reflection loss at each glass/air interface. The peak transmittance
increases to over 98% for the double-sided coated samples. As shown
in the graph of FIG. 3, the transmittance of the double-sided
two-layer antireflection coating is also significantly higher than
the sample without antireflection coating across a wide range of
incident angles. While the transmittance of the uncoated sapphire
falls to below 80% at an incident angle of approximately
60.degree., the sapphire with the double-sided coating still
maintains at a transmittance above 92%. The measured average
transmittance of the sample with double-sided 2-layer
antireflection coatings is 97% (between 0.degree. and 75.degree.
and between 400 nm and 1600 nm), which represents tremendous
increase over the 86% average transmittance of the uncoated
reference sample.
[0035] The refractive index of a front coating 110 or a back
coating 130 on a window 120 is shown in the graph of FIG. 4
according to an illustrative embodiment in which the window
material is glass and the coating comprises SiO.sub.2 of varying
porosity. One example of step graded profile 410 is shown, along
with a continuously varying quintic profile 400 of the index of
refraction approximated as taught in U.S. Pat. No. 4,583,822, which
is incorporated by reference as useful background information. In
particular, the index of refraction, herein referred to as "n", is
varied from that of the window 120, which in this case is composed
of transparent glass material 420 having a reflection value "n" of
approximately 1.5, to that of air 150, which is shown by way of
example to be approximately 1. The Fresnel reflection from one
surface of uncoated glass generally varies from approximately 4% at
normal incidence to up to approximately 40% at an incident angle of
75.degree.. According to the illustrative embodiment, oblique angle
deposition is employed to produce a porous SiO.sub.2 layer with a
lower index of refraction. More particularly, the index of
refraction of front coating 110 is varied from 1.5 to 1 over three
discrete steps, which can comprise one 192 nm layer optical
material 430 having a refractive index of n.about.1.36, a second
179 nm layer of optical material 440 having a refractive index of
n.about.1.19, and a third 260 nm layer of optical material 450
having a refractive index of n.about.1.10. This particular
combination of index of refraction and layer thicknesses
illustratively provides an appropriate approximation of the quintic
profile 200, as shown in the graph of FIG. 2. It should be clear to
those skilled in the art that the number of discrete steps and the
illustrated refraction index are only shown for illustrative
purposes and that the number of discrete steps and various values
of refractive index can be varied according to the various
embodiments.
[0036] FIG. 5 shows a graph that compares the measured
transmittance of an uncoated glass slide to the measured
transmittance of glass slides coated on either one side, or two
sides, with a three-layered, nanostructured SiO.sub.2 coating. The
samples were prepared in an electron-beam evaporator using three
different deposition angles (.about.60.degree., 72.degree., and
80.degree. respectively). In order to quantify the thickness and
refractive index of each individual layer, a sacrificial silicon
substrate was placed alongside the glass slides during each
deposition step. The thickness and refractive index of the single
layer films on silicon were measured with an ellipsometry-based
measurement system, yielding layers with n.about.1.10, 1.22, and
1.36 at a wavelength of 460 nm. The transmittance of the coated and
uncoated glass slides was then measured using an angle and
wavelength dependent transmittance measurement setup. The
measurement setup for characterizing transmittance versus
wavelength includes a Xenon lamp light source and an Ando AQ6315A
optical spectrum analyzer. The spectrum analyzer is calibrated to
detect transmitted photons over a broadband spectrum (400 nm-1800
nm).
[0037] The measured broadband transmittance of the uncoated glass
slide is approximately 92% at normal incidence, which is expected
given the approximate 4% reflection loss at each glass/air
interface. The broadband transmittance at normal incidence
increases to over 96% and 98%, respectively, for the single- and
double-sided coated samples. These results are dramatically better
than previous efforts to improve the transmittance through glass by
reducing reflection losses (for example, as shown in U.S. Pat. No.
7,642,199 by Paul Meredith and Michael Harvey). The transmittance
of the double-sided three-layer antireflection coating is also
significantly higher than the sample without antireflection coating
across a wide range of incident angles, as shown in FIG. 5. While
the transmittance of the uncoated glass slide falls to below 80% at
an incident angle of 65.degree., the glass slide with the
double-sided coating still maintains a transmittance above 95%. The
measured average transmittance of the sample with double-sided
double-layer antireflection coatings is 97% (between 0.degree. and
75.degree. and between 400 nm and 1600 nm), which represents
tremendous increase relative to the 86% average transmittance of
the uncoated reference sample.
[0038] In the illustrative embodiments discussed above, SiO.sub.2
materials have been employed for the coating material because of
its high transmission and stability. Window material can include
quartz, glass, and sapphire. Additional optical material can also
be employed in step graded AR coatings on optical windows,
including SiO.sub.2, TiO.sub.2, Si.sub.3N.sub.4, BaF.sub.2, CdTe,
and diamond like carbon materials. Conductive, transparent coatings
can be formed by using transparent conductive oxide (TCO) materials
such as indium tin oxide (ITO). The individual layers in the
optical coating can comprise a single material of varying porosity,
or of a solid solution of two different, dense materials, or any
combination thereof. A variety of different index profiles can be
employed, using two, three, four, or more index steps. While in
some cases these index steps and individual layer thickness can be
adjusted to approximate a continuous graded profile, in further
embodiments the index versus thickness profile can deviate from
that of a continuously graded profile in order to take advantage of
interference phenomena. Moreover, the index step profile can be
altered to minimize reflections and maximize transmittance through
the optical window over specific spectral regions or incidence
angles.
[0039] The refractive index of a front coating 110 or a back
coating 130 on a window 120 is shown in the graph of FIG. 6,
according to another illustrative embodiment. One example of step
graded profile 610 is shown, along with a continuously varying
quintic profile 600 of the index of refraction approximated as
taught in U.S. Pat. No. 4,583,822, which is incorporated by
reference as useful background information. In particular, the
index of refraction, herein referred to as "n", is varied from that
of the window 120, which in this case is composed of transparent
glass material 220 having a reflection value "n" of approximately
1.5, to that of air 150, which is shown by way of example to be
approximately 1. FIG. 6 depicts a profile for a four-layered
coating in which Layer 1 630 comprises a 70 nm layer having a
refractive index of n.about.1.43, Layer 2 640 comprises a 90 nm
layer having a refractive index of n.about.1.37, Layer 3 650
comprises a 150 nm layer having a refractive index of n.about.1.26,
and Layer 4 660 comprises a 350 nm layer having a refractive index
of n.about.1.09. In another embodiment, the index of refraction of
front coating 110 is varied from 1.5 to 1 over four discrete steps,
which can comprise one 75 nm layer optical material 630 having a
refractive index of n.about.1.35, a second 100 nm layer of optical
material 640 having a refractive index of n.about.1.29, third 160
nm layer of optical material 650 having a refractive index of
n.about.1.20, and a forth 210 nm layer of optical material 660
having a refractive index of n.about.1.09. It should be clear to
those skilled in the art that the number of discrete steps and the
illustrated refraction index are only shown for illustrative
purposes and that the number of discrete steps and various values
of refractive index can be varied according to the various
embodiments.
[0040] In yet another embodiment, a pore-closure layer is employed
that covers the top surface and does not allow moisture, or
particles, to enter the porous film. The pore-closure layer is very
thin (much smaller than .lamda.) so that it does not influence the
AR coating in terms of its reflectivity. That is, the pore closure
layer does not affect the reflectivity in a negative way. For
example, the topmost, low-index layer in the AR coating can be
capped with a thin (.about.10 nm), dense layer of SiO.sub.2.
[0041] Double coated windows as described hereinabove can be
applicable to a variety of different optical systems used for both
defense and commercial applications. Optical windows coated on a
single side are also of interest for a variety of applications,
including photovoltaic solar cells. FIG. 7 details a cross
sectional view illustrating a partial photovoltaic structure 700
comprising a semiconductor solar cell device structure 740 with a
bottom contact 750. An intermediate layer 730 connects the
semiconductor device structure 740 to a glass cover 720. The
photovoltaic structure 700 includes an antireflection structure 710
to enhance photon absorption within the active region of the
semiconductor structure 740. According to the illustrative
embodiment, a front coating 710 is deposited on a device covered by
a glass window 720 configured and arranged in a photovoltaic (PV)
system arranged to face the sun, which provides a readily available
source of photon energies 760 to the PV system. The front coating
710 is comprised of materials possessing optical characteristics
having index of refractions between air 770 and the glass window
720. Refer to FIGS. 4 and 6 showing examples of the refractive
index. Although photons 760 are illustratively shown as a series of
a single direction of photon stream, it should be clear to those
skilled in the art that the various, illustrative, and alternate
embodiments will function with various varying degrees and/or
amount of incident of light or source of photon energies.
[0042] It should now be apparent that a multi-layer, broadband,
omnidirectional AR coating made of a single material having
tailored-refractive-index layers on a glass substrate reduced
reflectance while improving optical transmittance. The availability
of the nanostructured low-n material and tunable-n materials
deposited by using oblique-angle deposition has allowed the
fabrication of highly effective AR coatings for low index
substrates such as glass. Antireflection coatings consisting of
three layers of nanostructured SiO.sub.2 have been shown to
significantly increase the transmittance of optical glass windows.
Double-sided coatings have achieved average transmittance values in
excess of 98% over a broad spectrum and range of incident angles,
which has benefits for a wide variety of specialized commercial and
military optical window applications. In addition, single-sided,
step graded-refractive index coatings can benefit from crystalline
silicon or thin film photovoltaic systems which employ either a top
cover glass or a glass superstrate.
[0043] The many features and advantages of the illustrative
embodiments described herein are apparent from the above written
description and thus it is intended by the appended claims to cover
all such features and advantages of the invention. Further, because
numerous modifications and changes will readily occur to those
skilled in the art, it is not desired to limit the invention to the
exact construction and operation as illustrated and described. For
example, the illustrative embodiments can include additional layers
to perform further functions or enhance existing, described
functions. Likewise, while not shown, the electrical connectivity
of the cell structure with other cells in an array and/or an
external conduit is expressly contemplated and highly variable
within ordinary skill. More generally, while some ranges of layer
thickness and illustrative materials are described herein. It is
expressly contemplated that additional layers, layers having
differing thicknesses and/or material choices can be provided to
achieve the functional advantages described herein. In addition,
directional and locational terms such as "top", "bottom", "center",
"front", "back", "above", and "below" should be taken as relative
conventions only, and not as absolute. Furthermore, it is expressly
contemplated that various semiconductor and thin films fabrication
techniques can be employed to form the structures described herein.
Accordingly, this description is to be taken only by way of example
and not to otherwise limit the scope of the invention.
* * * * *