ArticlePDF Available

Abstract and Figures

Anti-reflective coatings (ARCs) have evolved into highly effective reflectance and glare reducing components for various optical and opto-electrical equipments. Extensive research in optical and biological reflectance minimization as well as the emergence of nanotechnology over the years has contributed to the enhancement of ARCs in a major way. In this study the prime objective is to give a comprehensive idea of the ARCs right from their inception, as they were originally conceptualized by the pioneers and lay down the basic concepts and strategies adopted to minimize reflectance. The different types of ARCs are also described in greater detail and the state-of-the-art fabrication techniques have been fully illustrated. The inspiration that ARCs derive from nature ('biomimetics') has been an area of major research and is discussed at length. The various materials that have been reportedly used in fabricating the ARCs have also been brought into sharp focus. An account of application of ARCs on solar cells and modules, contemporary research and associated challenges are presented in the end to facilitate a universal understanding of the ARCs and encourage future research.
Content may be subject to copyright.
Anti-reflective coatings: A critical, in-depth review
Hemant Kumar Raut,*
a
V. Anand Ganesh,
a
A. Sreekumaran Nair
b
and Seeram Ramakrishna*
acd
Received 15th March 2011, Accepted 7th June 2011
DOI: 10.1039/c1ee01297e
Anti-reflective coatings (ARCs) have evolved into highly effective reflectance and glare reducing
components for various optical and opto-electrical equipments. Extensive research in optical and
biological reflectance minimization as well as the emergence of nanotechnology over the years has
contributed to the enhancement of ARCs in a major way. In this study the prime objective is to give
a comprehensive idea of the ARCs right from their inception, as they were originally conceptualized by
the pioneers and lay down the basic concepts and strategies adopted to minimize reflectance. The
different types of ARCs are also described in greater detail and the state-of-the-art fabrication
techniques have been fully illustrated. The inspiration that ARCs derive from nature (‘biomimetics’) has
been an area of major research and is discussed at length. The various materials that have been
reportedly used in fabricating the ARCs have also been brought into sharp focus. An account of
application of ARCs on solar cells and modules, contemporary research and associated challenges are
presented in the end to facilitate a universal understanding of the ARCs and encourage future research.
1. Introduction
Light transmission and production have been an area of
profound intricacy that baffled scientists and inquisitives since
the proverbial Newtonian era. The conclusive explanation of
structural colour of silverfish and peacock offered by Hooke
1
and
Newton
2
and later, of the membranous wings of many insects by
Goureau,
3
modified the popular notion of coloration. Interac-
tion of light with different structures received a lot of research
attention and major breakthroughs such as explanation of highly
metallic to dull green colour of beetle’s cuticle and wings of
butterfly Arhopala Micale (interference), discovery of zero order
gratings in the cornea of Zalea Minor (Diptera) making it highly
anti-reflective (diffraction) and the bluishness of the skin of
Octopus Bimaculatus (scattering), to name a few, were accom-
plished.
3
It was this relentless quest for understanding nature’s
optical strategy for imparting conspicuousness or camouflaging
to the astonishingly diverse array of biological species that led to
the conceptualization of anti-reflectivity. The iridescent blue of
Morpho Rhentenor butterfly wings which are prominent from
great distance and the cornea of nocturnal moths that remain
absolutely lustreless to disguise predators were analyzed for the
first time with the aid of ultra-powerful scanning electron
microscopy. The cuticular protuberances called corneal nipple
array on the surface of these species held the key to a great deal of
research and commercialization that ensued thereafter.
In the world of physical sciences, the idea of anti-reflective
coatings was quite incidentally construed by Lord Rayleigh
(John Strutt) in the 19th century when he observed the tarnishing
on a glass increasing its transmittance instead of reducing it. This
led to the strategy of achieving anti reflectivity by gradually
varying the refractive index. However, the actual anti-reflective
coatings were produced by Fraunhofer
4
in 1817 when he noticed
that reflection was reduced as a result of etching a surface in an
atmosphere of sulphur and nitric acid vapours.
The ever growing demand of optical and opto-electronic
equipments in areas as diverse as space exploration to consumer
electronics has led to the search for ways to maximize light
collection efficiency. Anti-reflective coatings on top glass cover of
the solar panel have served quite well in this regard, by bringing
about better transmission and glare reduction. As for solar cells,
reflectance reduction is achieved by silicon nitride or titanium
dioxide coatings of nanometre scales produced by PECVD dis-
cussed later. Additionally, textured front surface of solar cells
especially in mono-crystalline silicon helps increase light coupled
into the cell. Moreover, as the world is witnessing the long
overdue transition to alternative sources of energy especially
solar; low conversion efficiency due to reflection losses in the
conventional photovoltaic modules poses a major bottleneck. It
has been reported that a normal solar panel absorbs approxi-
mately 25% of the incident solar radiation, thus, reflecting a third
of the incident radiation which could otherwise have contributed
to the overall efficiency. Moreover, conventional solar panels
require an integrated mechanized tracking system that keeps
a
Department of Mechanical Engineering, National University of Singapore,
Singapore, 117574, Singapore. E-mail: hemantraut@nus.edu.sg; seeram@
nus.edu.sg
b
Healthcare and Energy Materials Laboratory, National University of
Singapore, 2 Engineering Drive 3, Singapore; Tel: +6516 8596
c
Institute of Materials Research and Engineering, Singapore, 117602,
Singapore
d
King Saud University, Riyadh, 11451, Kingdom of Saudi Arabia
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3779
Dynamic Article LinksC
<
Energy &
Environmental Science
Cite this: Energy Environ. Sci., 2011, 4, 3779
www.rsc.org/ees REVIEW
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
/ Journal Homepage
/ Table of Contents for this issue
them aligned to the sun throughout the day though such systems
are not mandatory in case of Building Integrated Photovoltaic
(BIPV) systems.
To overcome these issues a great many anti-reflective coatings
have been commercially manufactured and used though the
improvement in overall efficiency gets eclipsed by durability,
debonding and high cost issues. In optical equipments especially
cameras and eye glasses ARCs have been commercially explored
by numerous manufacturers and the results are indeed spectac-
ular. This has led to product manufacturers of LCDs, cathode
ray tubes and aerospace sensors take stock and incorporate
ARCs to better the performance of their products as well. Yet
perfect antireflectivity and exceptional photon collection effi-
ciency even at dusk remain the forte of only a few biological
species, easily observed than emulated.
2. Basic concept
The optical phenomenon, reflection is born out of a transition in
the medium in which light is travelling. The medium (glass,
water, air, etc.) is characterized optically by the parameter,
refractive index (n) which quantifies the speed of light in the
current medium with respect to that in vacuum. Thus, as light
travels, eyes can spot an optical disturbance if there is a change in
refractive index (RI). It is the Fresnel equation that offers the
basic preliminary mathematical model of reflection and refrac-
tion. According to Augustin-Jean Fresnel, the fraction of inci-
dent light reflected at the interface is measured by reflectance, R
and the rest transmitted (refracted) is measured by trans-
mittance, T.
The mathematical model or the vector method
5
to deduce the
condition for anti-reflection considers a thin film (RI ¼n)on
a glass substrate (RI ¼n
s
) as shown in Fig. 1 (a) and makes the
following assumptions
The reflected waves have the same intensity and one reflected
wave per interface.
Other optical interactions such as scattering, absorption etc.
are negligible.
Therefore, from Fig. 1 (a) it’s quite obvious that if R
1
and R
2
,
the two reflected waves, undergo a destructive interference,
thereby cancelling each other, there would be no reflection. From
this follow the two essential criteria for Anti-Reflection.
The reflected waves are pradians out of phase or the phase
difference, dis np/2.
The thickness of the film (d) is an odd multiple of l/4 where l
is the wavelength of the incident beam.
As the equation governing phase difference is d¼2pnd cosq/l,
substituting the value of dand d, we get q¼0, that is normal
incidence. The reflectance at normal incidence is given by
r¼nairnsn2
nairnsþn22
Where, n
s
¼refractive index of the substrate
n
air
¼refractive index of air
n¼refractive index of the film
As the objective is to achieve zero reflectance, Ris set to zero
and the refractive index of the film (n) is found to be ffiffiffiffiffiffiffiffiffiffiffi
nairns
p. The
fact that reflectance (R) proposed by Fresnel is dependent on the
s and p-polarization of the light, the anti-reflective property is
also analyzed on the basis of those parameters, as we will
subsequently see.
The above analysis was done on the basis of a single layer on
the substrate. For multi layered ARC the basic principle remains
the same; the only difference lies in the mathematical model
which relies on vector analysis of the individual reflected rays.
The reflected light from the interface ij between the adjacent
layers iand jis given by
5
R
ij
¼|R
ij
| exp[2(d
i
+d
j
)]
Where, |R
mn
|¼[(n
i
n
j
)/(n
i
+n
j
)] and phase thickness of each
layer is given by d
i
¼2pn
i
cosq
i
d
i
/l(q
i
is the angle of refraction, d
i
is the physical thickness of the layer and lis the wavelength of
light. As shown in Fig. 1 (b) the reflection vectors originate from
each of the ij interface, the resultant reflection vector can be
expressed as
R
sum
¼R
01
+R
12
+R
23
+R
34
+R
4s
where R
01
¼|R
01
|
R
12
¼|R
12
| exp[2(d
1
)]
R
23
¼|R
23
|exp[2(d
1
+d
2
)]]
R
34
¼|R
34
|exp[2(d
1
+d
2
+d
3
)]
R
4s
¼|R
4s
|exp[2(d
1
+d
2
+d
3
+d
4
)]
By minimizing R
sum
, that is, by adjusting RI and film thickness
of each layer, a state of anti-reflection can be attained.
3. Strategies to achieving anti-reflection
Anti-reflection can be imparted to a medium in two ways as
demonstrated by researchers over the years. The basic model has
Fig. 1 Propagation of light rays through (a) a single layer film on
substrate (n
s
>n) (b) multilayer film on substrate.
3780 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
been imported from natural species such as moths
6,7
and it has
been improvised by a host of computational findings and models
to evolve into newer strategies.
3.1. Porous/patterned
In the previous section we determined the refractive index of
a single film for zero reflectance and the value was ffiffiffiffiffiffiffiffiffiffiffi
nairns
p.
However, the refractive index of the film can be lowered still by
imparting porosity to it. Porous/density graded silicon (PSi) is
the most widely explored material in this context.
8,9
Here the
refractive index is dependent on the volume fraction because
porous silicon (PSi) is a sponge like material marked with
nanovoids. The refractive index can be lowered by lowering the
fraction, f of Si, thereby increasing the proportion (1-f) of air.
Emulating best strategies of nature has been a worldwide
research priority off late and this has led to the evolution of
a whole new branch of science – biomimetics. In fact, the very
inspiration of ARCs also comes from the eye of the moth. In
order to disguise predators, the ‘‘moth’s eye’’ or the corneal lens
has evolved into absolutely lustreless surface thus, preventing
light to reflect off it.
6,7
Scanning electron microscopy (SEM)
reveals the fine nanostructure patterns or sub-wavelength struc-
tures (SWS) as they have dimensions smaller than the wavelength
of light
7
as shown in Fig. 2. It is these microscopic SWS that
interact with the incident light in a different way than the
macroscopic structures. Light incident on a macrostructure
would normally undergo reflection and scattering after being
absorbed partly (Fig. 3(a)). However, light interacts differently
when it falls on structures that have dimensions less than its
wavelength, essentially the SWSs. Light interacts with the rough
surface wholly as the wavelength of light is higher than the
dimension of the structure and as if the surface has a gradient RI
(Fig. 3(b)), light rays tend to bend progressively (discussed later,
Rayleigh effect). If the same dimensional constraints are applied
to the spacing and depth between nanostructures in textured or
grooved surface, light rays get trapped in the crevices resulting in
multiple internal reflections (Fig. 3(c)). It is this phenomenon
that results in maximum absorption of the incident radiation
thus, reducing the reflection in the visible range to 0.1%.
10
3.2. Gradient
An alternative way of reducing reflectance is to gradually reduce
the refractive index of the film from the refractive index of the
substrate (n
s
) to the refractive index of air (n
air
) (notice the
variation in step and gradually in Fig. 4 (a) and (b) respectively).
Gradient-index lenses are indeed naturally occurring and incor-
porate broad field vision in the eyes of antelope, aberration
reduction in eyes of human and high spatial resolution in the eyes
of eagles. In the area of anti reflectivity also this has a bearing to
the effect that it imparts a smooth transition of refractive index
from a higher value to the lower value of air.
The basic idea here comes from the seminal work by Lord
Rayleigh
11
in 1880 on problem of gradual transition where he
explained numerically that when density changes in a variable
medium, reflection reduces and the ray bends as shown in Fig. 4
(c). These structures are called graded refractive index (GRIN)
structures and the variation in refractive index follows several
profiles (parabolic, cubic etc.) as we will subsequently see. The
index matching or optical impedance matching as it is technically
called is a crucial job as the adjustment of gradually reducing
refractive index from substrate to the air is quite difficult. In fact,
attainment of unity refractive index at the film-air interface is
impossible. Therefore, the most practical approach in this field is
to reduce the packing density of the films.
12
However, this affects
the mechanical strength of the ARCs and thus, raises durability
issues especially in outdoor solar panels.
Fig. 2 The eye of the moth (left)
158
and the SEM image (right)
159
showing the nano bumps or protuberances on the outer surface of the
corneal lens.
Fig. 3 (a) Light being reflected away from a macrostructure, (b) light interacting with the whole rough surface due to comparable dimensions, (c) light
undergoing multiple internal reflections through a nanostructure pattern.
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3781
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
4. Effective medium theory (EMT)
The digression at this point is very much in keeping with the fact
that the EMT is an essential concept which is the cornerstone of
many contemporary models and computational work in the area
of anti-reflectivity. The basic idea to bear in mind is that mate-
rials with surface in-homogeneity (porous or a regular/irregularly
patterned, rough surface or GRIN anti-reflective surface) have
refractive index that depends on the topology because light gets
scattered by nanoscale inclusions at times. Garnett
13,14
and
Bruggeman
15
proposed the mathematical models to calculate the
effective refractive index in this scenario (Table 1). Maxwell-
Garnett (MG) proposed the technique to determine the ‘‘effective
refractive index’’ of a ‘‘homogeneous’’ mixture under wave
propagation. For a material mixture with volume fraction of
inclusion f, MG model can be extended to give the effective
refractive index as a function of f. Now if we have an inhomo-
geneous film the same can be considered to be consisting of layers
of these ‘‘homogeneous’’ mixtures thus, determining the effective
refractive index as a whole. A dichotomy can be established here
between the MG model and Bruggeman’s approximation.
Consider a medium consisting of two media with refractive
indexes n
1
and n
2
and volume fractions f
1
and f
2
(¼1f
1
)
respectively, the effective refractive index given by the two
models is as reproduced in Table 1.
5. Requirements for perfect anti-reflectivity
5.1. Broadband anti-reflectivity
The anti-reflective property must be reasonably consistent over
a broader spectrum of wavelength of the incident radiation. The
fact that an optical impedance matching, in the visible region of
the incident light doesn’t ensure a match in the ultraviolet (UV)
or the near infra-red (NIR) region, impairs the performance of
some anti-reflective in the UV or NIR regions. In fact, normal
incidence AR coating designs typically fail in spectral ranges
0.85 < {l
U
/l
L
} < 5 where l
L
and l
U
are the lower and upper
wavelengths, respectively.
16
In addition, we will also see subse-
quently how the behaviour of a single layer, double layer and
multi layer anti-reflective affect the very property of reflectance
minimization in different ways at times showing quite uneven
suppression of reflectance as well.
5.2. Omni-directional anti-reflectivity
It has been established by Fresnel that the angle of incidence
plays a decisive role in the determination of reflectance. In fact,
most glasses and plastics with RI around 1.5 show a 4% reflec-
tance at normal incidence but a 100% reflectance at grazing
angles. This same phenomenon is observed in anti-reflective films
and numerous designs catering to incident angles from 30to 60
from the normal have been proposed.
17,18
In fact, this poses
a challenge in case of silicon photovoltaic which need to be
mechanically oriented to face the sun throughout the day. This
obviously requires a control mechanism which involves addi-
tional overheads and consumes energy. Omni-directional anti-
reflectivity is one of the ways this issue can be addressed.
5.3. Polarization insensitivity
The effect of the two types, s and p polarization of light on the
ARCs also needs analysis. The s-polarization has the electric field
perpendicular to the incidence plane and p-polarization has the
electric field parallel to the incidence plane. Polarization plays
a very important role in antiglare coatings (AGCs) and ARCs
due to the fact that light reflecting at shallow angles has the
p-polarized light reflecting to the maximum and sunlight indeed
shows appreciable degree of polarization.
6. Types of anti-reflective coatings (ARCs)
6.1. Type I (based on the layer composition)
6.1.1. Homogeneous ARCs. A single homogeneous layer of
refractive index n, will impose restriction on the RI and thickness
that we have already discussed. The RI must obey n¼ffiffiffiffiffiffiffiffiffiffiffi
nairns
p
and thickness equal to l/4. However, if the substrate is sur-
rounded by air, n
air
is unity and nlargely depends on n
s
. Thus, as
discussed earlier, a porous or pattern layer lowers the nconsid-
erably thereby facilitating optical impedance matching
(Fig. 5 (b)). Moreover, multiple thin layers in a homogeneous set
Fig. 4 (a) Sharp drop in refractive index observed in single layer anti-reflective films (b) smooth drop in refractive index from n
s
to n
air
in case of graded
refractive index anti-reflective. (c) Light rays bending in the case of gradually varying RI medium.
Table 1 Maxwell’s model vs. Bruggeman’s model
Maxwell-Garnett model Bruggeman approximation
Effective RI can be evaluated
from the equation
n2n12
n2þ2n122
¼
ð1f1Þn22n12
n22þ2n122
Effective RI can be
calculated from the equation
f1n12n2
n12þ2n22
þf2n22n2
n22þ2n22
¼0
This value is based on the
assumption that the
medium n
2
is surrounded by n
1
.
Thus, the equation will
change if n
1
is surrounded by n
2
.
This can be extended for
klayers as
X
k
i¼1
fin12n2
n12þ2n22
¼0
3782 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
up also help achieve zero reflectance at certain wavelength(s)
though the RI rule is not strictly obeyed.
6.1.2. Inhomogeneous ARCs. Inhomogeneous ARCs follow
the gradient RI approach to achieving anti-reflectivity. For
a quick recall, the contribution to reflection from abrupt inter-
faces is progressively reduced with depth due to varying RI from
n
air
to n
s
. The overall thickness of the layer in this case is not
critical
19
from the design point of view though it’s better to have
thickness more than l
U
or 2l
U
, where l
U
is the upper wavelength
of the incident light.
21
The inhomogeneous ARCs can be
approximated to a large number of sub-layers where RI differ-
ence between adjacent sub layers is very small (Fig. 5 (f)). If we
consider n
air
¼1, the inhomogeneous layer can achieve zero
reflectance provided the layer thickness follows the aforemen-
tioned l
U
limit and the layer is incorporated with SWS as shown
in Fig. 5(f). The EMT discussed earlier can indeed help evaluate
the RI in this case.
6.2. Type I (based on the layer composition)
6.2.1. Single layer ARCs. These are used for moderate
suppression of reflectance to 2.5% for a very broad spectral
range, like 450–1100 nm at normal incidence.
159
However, the
selection of material is a challenge because the RI of glass is
1.5151 (BK7 at 633nm) and calculations would set RI to
approximately 1.22. Single layer magnesium fluoride (MgF
2
)is
the most commonly used ARCs under this type because its RI is
1.38 and it reduces surface reflectance of BK7 glass from
approximately 4% to less than 1.3% at the specified centre
wavelength and normal incidence.
160
Even though the perfor-
mance of single layer MgF
2
ARCs is not exceptional, the
advantage is its broad usable wavelength zone as can be seen in
Fig. 6(a).
6.2.2. Double layer ARCs. In order to reduce the reflectance
further still, the double layer ARCs can be employed. The
necessary and sufficient index condition for a double-layer
coating with equal optical thickness (n
1
d
1
¼n
2
d
2
¼l/4) to give
zero reflectance is:
22
n1
n2¼ffiffiffiffi
n0
ns
ror n1n2¼n0ns
The first equation produces one minimum (Fig. 7(a)) where
reflectance becomes zero and the second equation has two
minima and one weak central maximum. These double layer
coatings are called v-coatings due to the V-shape of their profile
and quarter–quarter coatings also due to their thickness rela-
tionship. These V-coatings are the perfect solution for especially
laser applications, where resistance to intensive laser radiation is
important and minimum reflection is required only at a specific
wavelength.
161
The discussion above was considering glass as the reflecting
surface. However, since the outdoor solar panels suffer from two
tier reflection one at the protective glass and the other on the Si
PV surface we would also take a glance at a research undertaken
by using single layer thin films of SiO (RI 1.85), CeO
2
(RI 2.2)
and ZnS (RI 2.3) on Si substrate and subsequently for
Fig. 6 Simulated transmissivity of (a) Si(RI 3.76)|SiO2(RI 1.455)|Si|
SiO2|Air, l/4 thick layers, (b) Si (RI 3.76)|Ge (RI 4.897)|Si|SiO2(RI
1.455)|Air, l/4 thick layers multi layer ARCs.
20
NB. Caption in the figure
has been enhanced for clarity.
Fig. 5 Homogeneous ARCs, single layer (a), patterned (b) and multi-
layer (c) and inhomogeneous ARCs, single layer (d), patterned (e) and
multi-layer (f). Notice the variation in refractive index with respect to film
thickness.
19
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3783
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
V-coatings, MgF
2
(RI 1.38) as outer layer and SiO (n
2
¼1.85),
CeO
2
(n
2
¼2.2) and ZnS (n
2
¼2.3) as an inner layer on silicon
(RI 3.5 4 at 400 1100 nm).
23
The results indicate
a phenomenal drop of reflectance from 56.8–37.5% (spectral
range 400–100 nm) for uncoated Si surface to below 5% by using
single layer ARCs made of SiO/CeO
2
/ZnS. The double-layer
quarter-quarter coatings also offer interesting results. MgF
2
(outer) + SiO (inner) have two reflectance minima at 540 nm
(reflectance 8.9%) and 1021 nm (reflectance 12.6%) separated by
one marginal maxima at 802 nm (reflectance 22.7%) (Fig. 7(b)).
However, when we consider MgF
2
(outer) + CeO
2
(inner) the
minima are observed at 601 nm (reflectance 4.9%) and 997 nm
(reflectance 6.9%) respectively, thus making this a better choice
over the MgF
2
+ SiO. At last, MgF
2
(outer) + ZnS (inner)
registers a drop in reflectance from 20% at 400 nm to 0.9% at
720 nm and a quick relook at Fig. 7 (b), (c), (d) would make it
quite obvious that MgF
2
+ ZnS has a wider profile than the
others, thus complying to broadband.
6.2.3. Multi layer ARCs. As we are gradually drifting
towards broadband anti-reflection (BAR), the multi layer ARCs
come into the reckoning. However, the trade-off between a larger
bandwidth and overall reflectivity is the challenge facing us. The
mathematical analysis of multi layer ARCs is just an extension of
the earlier destructive interference type vector model we had
discussed.
The mathematical model for the multilayer coatings has been
discussed in section 2 and just to recollect, the summation of all
the reflected vectors must be minimized by adjusting the RI and
the thickness of the film. A computational study
20
in this area
analyses the transmissivity curves in the far red zone (720
750 nm) as shown in Fig. 6 of multi layers on Si substrate. The
results reveal that the insertion of an extra layer of Si between the
high index Ge and the low index SiO
2
has maximum blue tran-
sitivity of approximately 96% (at 480 nm). Similarly, in many
biological species such as Coleoptera, alternate high and low RI
layers facilitate optical interference.
6.2.4. Gradient refractive index ARCs. As already discussed,
gradient RI coatings have their RI profile following different
curves such as linear, parabolic, cubic, quintic, exponential,
exponential-sinusoid etc. and comply with the Rayleigh effect.
Southwell has proposed in his study of gradient ARCs,
24
equa-
tions governing various gradient refractive index (GRIN)
profiles which are given below (RI of incident medium (e.g., air)
is n
i
and RI of substrate is n
s
):
Linear index profile, n¼n
i
+(n
s
n
i
)t,0#t#1
Cubic index profile, n¼n
i
+(n
s
n
i
)(3t
2
2t
3
)
Quintic index profile, n¼n
i
+(n
s
n
i
)t(10t
3
15t
4
+6t
5
)
Similarly, Sheldon and Haggerty have proposed some inter-
esting results
25
on GRIN profiles involving linear, concave and
convex-parabolic and cubic curves under transverse electric (s-
polarized) waves. Notice the oscillation in percentage reflectivity
vs (d/l) curve in Fig. 8(a) which turns out to have the same period
and varies with the angle of incidence. In 2007, Xi et al. have also
demonstrated a GRIN profile made of 450 inclined SiO
2
Fig. 7 (a) Comparative study of reflectivity vs wavelength of the single layer MgF
2
, V and broadband coatings
161
(b, c, d) Comparative study of
uncoated Si surface, single layer AR (SiO/CeO
2
/ZnS) and double layer (MgF
2
{outer} + SiO/CeO
2
/ZnS {inner}) coatings.
23
3784 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
nanorods that gives an overall RI of 1.05
26
and they propose
a quintic profile to be the best AR profile (Fig. 8(b)). Moreover,
three layers of TiO
2
nanorods and two layers of SiO
2
nanorods
above it, all having quintic profiles render a hybrid system of
multi-layer quintic RI coatings and it registers a RI of 2.03 for
the bottom layer and RI of 1.05 for the top layer and a reflectivity
of as low as 0.1%. In fact, something similar has also been studied
by Dobrowolski et al.
19
computationally where the GRIN profile
is likened to a 200 layers simulated ARCs where the RIs of
adjacent layers differ by 0.01. The average reflectance has been
computed to be less than 0.05 and 0.01 for all angles less than 85
and spectral region 5.0–8.0mm, respectively, and the comparison
with a 4-layer system is also shown in Fig. 8 (c), (d). However, the
findings of Schubert et al.,
27
2010 runs contrary to the popular
notion of GRIN outdoing multi layer ARCs in all ways.
The underperformance of GRIN is attributed to two factors (i)
there is a gap in the RI between the upper surface of GRIN
ARCs (minimum attainable 1.05) and ambient air (unity RI) and
(ii) at the coating-substrate interface, because the absorbing
substrate has a complex RI and transparent ARC has a real RI,
a mismatch exists. It is this discrepancy that violates the Rayleigh
effect and the fact that a multi layer ARCs basically facilitates
destructive interference type anti-reflection, the results are quite
explicable in Fig. 8(e).
6.3. Type III (based on the surface topography)
The development of this type was essentially triggered by the
requirement of omnidirectional AR property while retaining
broadband AR as well.
6.3.1. Porous ARCs. A great deal of research has been
undertaken in this area
8,28–30,130,131
especially relating to porous
silicon (PSi) because of its application in the field of solar energy
harvesting. PSi consists of nanometre-sized voids with a large
hydrogenated surface.
30
The relationship between refractive
index and porosity is proposed differently in different studies; for
example,
npc ¼"1P
100ðndc21Þþ1#1
2
where n
pc
,n
dc
and Pare the RI of the porous and dense media
and the porosity percentage respectively.
31
However, EMT is the
computational foundation for all RI calculations. The porosity
and consequently, reflectance depend on the solution composi-
tion, porous film growth rate
32
and film thickness. Reflective
properties of PSi films on the outer region of p/n
+
junction have
been studied with regards to solar cells
8
by etching the outermost
region in the presence of HF/HNO
3
. The study claims that
a 1500
A thick layer gives the best photovoltaic conversion effi-
ciency (12%) for 10 10 cm
2
multicrystalline devices. A
comparative study (Fig. 9(c)) involving PSi and other textured
surfaces, exemplifies PSi as the most effective medium to reduce
scattering. Complying results were also obtained (Fig. 9(a),(b)) in
case of tetraethyl orthosilicate (TEOS) + ethanol (EtOH) porous
layers with the transmittance pegged at 99.6% (AR-1) and 99.4%
(AR-2) and reflectance varying between 0.15–0.8%.
33
In addition,
low reflection silica films have also been reported to have a low
reflectance of 0.2% while being chemically inert, abrasion resis-
tant and durable.
28
A discussion on the manufacturing process would involve
state-of-the-art techniques not discussed thus far which is why
only a brief outline, in the context of porous films, would be
presented here. Glass used phase separation and etching
4,7,34
were
the formative technology for fabrication of porous layers. In
recent times, sol–gel,
35
adsorption of colloids into polyelectrolyte
multilayer
36
and selective dissolution of spin-coated films
37,38
have been adopted for fabrication of a nanoporous film.
6.3.2. Biomimetic photonic nanostructures (‘‘moth’s eye’’).
Nature has perfected the optical systems so well that it indeed
makes perfect scientific sense to imitate them and if examples are
to go by, scallop eyes inspired chromatic aberration minimiza-
tion, horsefly cornea inspired extreme UV-optics and human
deformable eye lenses inspired liquid lenses still strive to attain
the perfection nature has achieved.
The camouflaging strategy of some insects and the exceptional
photon collection capability of nocturnal creatures enabling
them to see at night motivated scientists to analyze the eyes of
such night-active insects, especially moths (Fig. 10(a)) and
butterflies
39,40
and transparent wings of hawkmoths.
41
It turned
out that the corneal surface in the eyes of moths has a hexagonal
array of sub wavelength (SW) conical or cylindrical nano-
structures known as nipple-array. The diameter of each of these
protuberances is found to be 100 nm and are 170 nm away from
one another
42
(Fig. 11(b)).
Not just broadband and omnidirectional AR, these nano-
structures are found to remove disparity in the reflection for
polarized lights as well though research now shows that butterfly
eyes are highly sensitive to polarized light.
43
Further investiga-
tion revealed that these nanostructures have a gradient RI
between chitin (polysaccharide RI 1.54, consider substrate) and
ambient (RI 1.0)
44
which makes it clear that AR is primarily due
to exceptional transmission (Rayleigh effect). The first repro-
duction of ‘‘moth’s eye’’ structure involved crossing of three
gratings at 120using ‘‘lithography’’.
45
Precision and scalability (onto lenses and solid plastics)
requirements led to the fabrication of highly accurate ‘‘moth’s
eye’’ structures using electron-beam etching. A very interesting
finding by Boden et al.
46
which involves the study of anti-reflec-
tivity throughout the day shows computationally that moth’s eye
arrays of 250 nm show similar performance as the double layer
ARCs we have already discussed. However, an increase of height
to 500 nm reduces the reflection losses by 25% thus, translating the
extra photon collection to 12% rise in energy of a solar cell.
Rigorous probe of eyes of other species revealed sinusoidal grat-
ings (RI ¼f[sin g(x)]) of 250 nm periodicity in the cornea of
amber
47
(Fig. 11(a)) which has outstanding broadband properties.
Several other profiles in the domain of ‘‘moth’s eye nipple
array’’ have also been analyzed,
48,49
namely, conical, parabo-
loidal (Fig. 11(d)) and Gaussian-bell shape and it has been
reported that paraboloidal nipple array displays best perfor-
mance among these (Fig. 11(b),(c)). Thus, it has been proposed
that tall paraboloids with base touching each other at hemi-
spherical facet surface results in the best reduction of reflec-
tance.
48
The rigorous study of corneal samples of 361 species by
Bernhard et al.
49
has been a landmark contribution in this area.
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3785
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
We have seen the height (< l/2), inter-nipple spacing and shape in
‘‘moth’s eye’ structures playing a pivotal role than the width of
the ‘‘moth’s eye’’ (Fig. 11(c)).
The idea has been extended onto quartz
50
and fused silica
substrate with conical array reducing reflectance to 0.5% in 400–
800 nm,
51
polymer
7
and GaSb
52
and many other materials.
Moreover, SWS have been found to have excellent compati-
bility with the substrate material compared to multi-layer thin
films discussed earlier. The latter reportedly has adhesion issues
due to thermal coefficient difference between the substrate and
the adjoining layer or between adjacent layers which predomi-
nantly gives rise to ‘‘debonding’’ problems. It is worth
Fig. 8 (a) Reflectance of s-polarized waves at 450 incidences for semi-bound a. linear b. concave-parabolic c. convex-parabolic d. cubic profile.
Semibound- notice the difference in N1 and N2,
25
(b) a. Linear, cubic and qunitic index profile (ns ¼2.05) showing RI vs height or thickness, reflectivity
vs wavelength and reflectivity vs incidence angle. Notice the superior performance of qunitic profile (lowest R),
26
(c) Simulated 200 layer AR coating and
(d) simulated 4 layer AR coating to analyze the GRIN effect through multi-layer approximation,
19
(e) (left) reflectivity vs angle and wavelength of ARC
with quintic profile and 4-layer ARC of thickness 400 nm under transverse electric (s-polarized) and transverse magnetic (p-polarized) flux. Notice the
13.59% reflectivity of quintic compared to 8.86% for 4-layer under p-polarized light and 10.76% reflectivity of quintic as opposed to 8.60% for 4-layer
under s-polarized light. (right) SEM image showing composition and arrangement of the four layers on Si substrate.
27
3786 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
mentioning the preference of some researchers
53
for rigorous
coupled wave analysis or RCWA (solutions to Maxwell equa-
tions) over EMT as the former predicts the dimensions of the
crucial parameters more effectively thus, offering better ways of
optimization.
6.3.3. Textured surface ARCs. Surface texturing also renders
the substrate anti-reflective as demonstrated by many scientists
since 1960s
54
and the reason for AR is the phenomenon of light
trapping and multiple internal reflections.
55,56,132,133
In an effort
to produce high efficiency solar cells with no ARCs, Smith et al.
57
report a new texturing geometry as shown in Fig. 12(b) (mech-
anism of light bouncing shown in Fig. 12(c)) which outperforms
the perpendicular slats geometry for texturing proposed by
Landis (1987) (Fig. 12(a)). This texture geometry has three
perpendicular planes (3PP) that facilitate multiple internal
reflections due to planes arranged in a way to make the incident
angle greater than the critical angle (Fig. 12(b)) which is how the
3PP geometry keeps the largest percentage of rays in optical
confinement (15 bounces). Lambertian texture on the rear
surface of the solar cell also produces the same light trapping
effect but when used with pyramid (Fig. 12 (f))
58
or inverted
pyramid textures
59,60
on the top surface, it shows performance
comparable to or better than 3PP). Note that the spacing of the
structures and depth of the same is an important tunable factor.
Surface texturing imparts a reduction of reflectance by approx-
imately 10%
61
in mono-crystal Si and reflection losses are
reportedly minimized to nearly 1% in case of amorphous Si
62
using surface texture.
A few other interesting surface texturing or light trapping
structures have been reported such as honeycomb-textured
structures
63
that reduce reflectance to 10% around 600 nm and
light trapping 2-mm silica microsphere coatings (Fig. 12 (d))
64
on
glass substrate that reduce reflectance (7.6%) and increase
transmittance (92.7%) around 400 nm (up to 1100 nm).
Fig. 12 (e) pictorially explains the underlying concept. In fact,
a honeycomb textured surface
60
has reportedly enhanced the
efficiency of multicrystalline and monocrystaline solar cells by
19.8 and 24.4%, respectively, against the author’s claim of the
highest reported lab statistics of 18.6 and 24%, respectively.
Rayleigh effect is also perceived in case of surface texturing
besides the dominant multiple internal reflections. A study by
Craighead et al.
10
shows that by making cross sectional dimen-
sions substantially smaller than l, spatial variation (linear) in RI
can be attained and the same has been employed in reflectance
reduction in several cases.
10,62
6.3.4. Anti-reflection grating (ARG). Broadband antireflec-
tion requirement over a large region even up to the terahertz
(0.1 10 THz, 30 mm to 3 mm wavelength
65
) range gets fulfilled
by the surface relief gratings or ARGs. The grating structures
work on the same principle of creating a continuous gradient of
RI (n
s
to ambient, 1) and their efficacy has been proved in the
solar,
66
microwave
67
and THz
68
wavelength ranges. However,
Fig. 9 (a) Transmittance, (b) reflectance of uncoated glass and coated AR samples. AR-1, AR-4 etc. show the ratio of TEOS:EtOH and film thickness.
33
(c) TIS vs angle for l¼980 nm comparing ultrafast laser textured Si {L}, annealed ultrafast laser textured Si{L1}, chemically textured Si with SiNx
{CT}, Porous Si{P}.
72
Fig. 10 (a) SEM image showing tiny bumps on moth’s eye,
162
(b) highly magnified drill down SEM images showing the hexagonal array of SW nano
structures.
42
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3787
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
ARGs have been found to be less effective in case of solar cells
because gratings basically help propagate the zeroth diffraction
orders (directly transmitted and reflected light) and don’t cater to
higher diffraction orders which apparently contribute to the total
energy collected in a solar cell. Computational results from
a study by Br
uckner et al.
68
show that a one dimensional ARG
with a triangular cross section and spacing of 50 mm and depth of
100 mm, produces a transmittance of over 99% (Fig. 13(a)) in the
0.75 to 3 THz range and if the same cross section with revised
dimensions, period ¼0.5 mm and depth ¼1 mm is subjected to
the flux, 0.1 0.4 THz, shows 99.5% transmission but beyond
0.4 THz, transmission reduces drastically (Fig. 13(b)). However,
there is a way to avoid the diffraction grating. The period pof the
gated structure should follow the relationship p<l/n
s
.
69
In this
scenario, an array of square base pyramids have been reported
70
to be optimum as the RI of the grated system can be
Fig. 11 (a) Sinusoidal gratings on the cornea of amber;
47
(b) variation of RI w.r.t height for three different nipple arrays; (c) reflectance vs wavelength
for the three arrays.
48
Notice the linearity and lowest reflectance for a parabolic shape. (d) Computer simulations of moth’s eye.
53
NB: Fig. 11 (b)&(c)
have been highlighted and illustrated for better understanding.
Fig. 12 (a) Perpendicular slat geometry for textured solar cells;
57
(b) 3PP geometry for textured solar cells displaying the multiple light bounce effect;
57
(c) mechanism of light bouncing off the 3PP geometry;
57
(d) SEM image of 2-mm silica microsphere on glass;
64
(e) multiple internal reflection through the
AR coating comprising the silica microsphere;
64
(f) pyramid texture on Si surface (apex angle 720, base length 110–30 mm).
58
3788 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
approximated to a quintic profile [n
i
+(n
s
n
i
)f(thickness
5
)] that
has already been proved effective in ARCs. Fig. 13(c) shows the
grated surface relief pattern on a substrate with RI 1.45.
Another such sub wavelength pyramid shaped surface relief
grating comprising a stack of eight binary layers
71
has been
imparted to multi-crystalline Si widely used in solar cell appli-
cations. It is reported that for a 100 nm depth and aspect ratio
(depth/periodicity) of 100 nm reflectivity scales up at less than 1%
in the visible range (Fig. 13(d)). Fabrication techniques such as
laser-interference lithography and nano-imprinting which are
used to produce the ARGs will be dealt with in the next section.
7. Fabrication techniques of anti-reflective coatings
(ARCs)
Most of the ARCs being nanostructure based can have their
fabrication techniques classified under the same broad subtrac-
tive top-down or additive bottom-up techniques of nanotech-
nology. There are indeed a few other unconventional techniques
which will be brought under the spotlight.
7.1. Conventional fabrication: bottom-up technology
7.1.1. Sol–gel processing. This is one of the most prevalent
technologies for the production of porous ARCs. This process
involves the use of inorganic salts or metal alkoxides as precursor
materials which when exposed to aqueous or organic solvent,
hydrolyses and condenses to form inorganic polymer comprising
metal-oxide-metal bond.
73
The different coating techniques
adopted to coat sol-gels will be discussed in brief here.
Dip coating. In this process the substrate is dipped and with-
drawn from the desired solution at a controlled rate (feed
velocity). The stages in the process can be enumerated as
immersion, start-up, deposition, evaporation and drainage (steps
in Fig. 14(a)).
Spin coating. It is a batch-production technique which
produces thin films on flat or marginally curved substrates. It
involves deposition of a small puddle of viscous film material and
subsequent spinning of the substrate at high angular speed (e.g.,
3000 rpm) thereby forcing the puddle to spread owing to
centripetal force (steps in Fig. 14(b)).
Meniscus coating. This technique involves flowing of coating
material through porous applicator onto the surface of the
substrate. The crux of the technique is to maintain menisci of the
Fig. 13 Reflectance and transmittance for triangular ARG of (a) period 50 mm and depth 100 mm (b) period 0.5 mm and depth 1 mm.
68
(Results of
rigorous coupled wave analysis) (c) optimum ARG profile for substrate RI 1.45
70
(d) grated pyramidal structures on multi crystalline Si (L¼periodicity,
100 nm, d¼thickness of the grating) & reflectivity vs wavelength for d/L.
71
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3789
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
coating material between the applicator and the substrate which
can produce micron thick films on the substrate. This technique
is used in flat panel displays, optical devices and Si, Ge wafers etc.
An enhancement of this used in paint industry is demonstrated in
Fig. 14(c).
Since 1964
134
sol–gel process has been industrially used to
produce ARCs. Production of porous ARCs is an area where
sol–gel is extensively used and studies show that the desired
minimum pore-size and maximum pore-volume is achieved by
varying the mixing ratio in the mixture of two insoluble materials
judiciously and subsequently dissolving one of them using
a solvent thereby rendering the other material porous.
37
For instance, a mixing of polystyrene and polymethyl-meth-
acrylate (PMMA) in the solvent tetra-hydrofuran (THF), fol-
lowed by spin-coating on a transparent substrate and subsequent
treatment with cyclohexane that dissolves polystyrene is a way of
producing a porous PMMA film.
37
This film has RI varying
between 1.225 and 1.285 and transmittance 99.7% in 400–680 nm
range. Interference type ARCs comprising TiO
2
/SiO
2
-TiO
2
-SiO
2
(3 layer) on glass
74,75
used in contrast enhancement filters for
monitors, SiO
2
coatings deposited on low emissivity glazing
(windows) by dip-coating resulting in 10% increase in trans-
mittance in visible region, colloidal solution being explored as
precursor material for porous coatings
76
and use of the same in
high power optics,
135,136
sol–gel processed ARCs for cathode
tubes (CRTs),
77,137
Al
2
O
3
porous low index layers for CRT
faceplates and solar collectors
138
are quite a few examples
indicative of the versatility of the sol–gel technique. A cursory
look at the developments in the field of solar cells with regards to
sol–gel technique again provides us with a wealth of information.
Sprayable TiO
2
ARCs for solar cells in 1980,
139
two-layer inter-
ference ARCs (layer 1: TiO
2
RI 2.4, layer 2: 10%TiO
2
, 90% SiO
2
RI 1.4) for photovoltaic resulting in a rise in conversion efficiency
by 49%,
140
bi-layer multi functional nanostructure material
incorporating anti-reflectivity, hydrophobicity, antifogging
properties via sol–gel method
78
just goes to shows the continued
interest in sol–gel processing of ARCs.
On the flip side, however, sol–gel prepared films at times have
un-reacted or un-removed solvent materials that might influence
the properties of AR. In addition, controlling the thickness of the
film apparently is the trickiest part.
7.1.2. Glancing angle deposition. This is basically a physical
vapour deposition technique involving condensation of vaporized
material onto a substrate. An improvement to this technique is the
glancing angle deposition(GLAD) that has thevapour flux incident
at an angleon a rotating substrate. Theadvantage in so doing is that
the thin film grows with a gradually decreasing density there by,
increasing porosity due to ‘‘atomic-shadowing’’ (Fig. 15(a)) and this
technique also gives high degree of control over the morphology of
the resultant nanostructure. However, it is obvious that the angle of
material deposition/growth, in turn, porosity of the film is depen-
dent on the angle of vapour incidence.
79
A relationship between the
oblique angle aand column angle bis give by Tait et al.
80
as b¼a
ksin[1 cos a/2]. Thus, if the material is deposited at large oblique
angles the porosity of the film will increase and vice-versa.Inorder
to counteract this, an enhancement was proposed by K. Robbie
79
which impart two substrate motions, rotational about the axis
perpendicular to the substrate as discussed and secondly, rotational
about an axis shown in Fig. 15(b).
Based on the sculptured thin film (STF) deposition,
81
GLAD
technique has been modified in different ways to engineer films
with slanted, chevron, helix, vertical and other morphology
based nanostructures (Fig. 15(c)–(f)). GLAD assisted ARCs
made of SiO
2
exhibit a Gaussian profile RI and register a peak
transmission of 99.9% (up to 460 nm).
82
Moreover, a five layer (3
TiO
2
layers and 2 SiO
2
layers over it) by depositing atoms at 87
forms a GRIN film with quintically varying RI (resultant RI ¼
1.05) showing a minimum reflectivity of 0.1%
26
(Fig. 8(b)).
7.1.3. Chemical vapour deposition (CVD):. As the name
suggests, this technique involves reaction and deposition of
volatile precursor(s) on a heated substrate and it is one of the
most widely used technique in the context of ARCs. In an effort
to fabricate near-perfect optically absorbing materials, long
(greater than 300 mm) and low-density (0.01–0.02 g cm
3
) verti-
cally aligned carbon nanotubes (VA-CNTs) arrays have been
produced by Yang et al.
83
using water assisted CVD. As is
evident from Fig. 16(a), the total reflectance at 633 nm has been
found to be an impressive 0.045%.
Plasma enhanced CVD (PECVD) is the most industrially
preferred technology for Silicon Nitride anti-reflective coatings
on crystalline solar cells, as we will see in section 10.
165
Fig. 14 Steps involved in (a) dip coating and (b) spin coating,
163
(c) technique of meniscus coating.
165
3790 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
In fact, a multi-purpose coating
85,156
that combines anti-
reflective, anti-scratch, easy-to-clean coating (of SiO
x
C
y
H
z
and
TiO
2
85
) has been developed via PECVD and quite interestingly
the coatings are tailor-made for polymers for example, PC
(polycarbonate) and PMMA (polymethylmethacrylate).
85
In this
technique the precursor molecules are decomposed by pulsed
microwave-induced plasma and for instance, can have an adhe-
sion layer followed by anti-reflective layer (TiO
2
/SiO
2
) and/or
anti-scratch layer (SiO
2
). Reflectance spectra
85
for the already
discussed multi-purpose coating via PECVD displayed in
Fig. 16 (b) shows that the anti-reflective coating produces
a steady profile while the anti-scratch coating gives rise to fluc-
tuations which the author logically attributes to the RI mismatch
between the polymer substrate and the anti-scratch layer. The
study also maintains that by using the PECVD technique the RI
of the anti-scratch layer can be lowered to 1.46 thereby flattening
the otherwise wavy reflectance vs. wavelength profile.
Radio frequency plasma enhanced CVD (RF-PECVD) has
been reported to be effective in producing uniform coating for
substrates of various shapes and sizes.
86
Diamond-like carbon
(DLC) films having potential use in protective coating for IR
windows and ARCs for solar cells have also been formed on Si
substrate via RF-PECVD.
87
If we are to understand the perfor-
mance of the film in real time solar applications such as solar
collectors etc. the results bear significance for a temperature rise
from 300 C to 900 C results in a change in reflectance from 8.75
to 18.27%, which brings into sharp focus the performance
degradation of coatings under real-life conditions vis-
a-vis
temperature fluctuation.
Fabrication of GRIN RI is also feasible through atmospheric
pressure CVD (APCVD) as shown by Neuman in his GRIN
layer comprising the widely favoured SiO
2
, TiO
2
duo
88
that has
a reflectance of 0.5%. Quite interestingly analogous of nano-
structures on Morpho butterfly wings
89
have been fabricated in
the form of Morpho Christmas tree structures via focused ion
beam CVD (FIB-CVD)
90
(Fig. 16(c)). However, the cost of
covering even micrometres with this nanostructure is quite high
whereas Morpho butterfly wings have these structures spread
across many centimetres. This is just one of the numerous cues to
be drawn from the exceptionally superior ‘‘nature’’. Apparently,
the research on ARCs using CVD is quite exhaustive which is
why a rather diverse perspective of ARCs using CVD and the
techniques thereof is presented.
7.2. Conventional fabrication: top-down technology
7.2.1. Etching. Etching technique can be understood as
a subtractive method that causes a selective dissolution or abla-
tion of a surface or substrate in this case. Basically wet etching
and dry etching are the two techniques relevant to our study and
both processes are either mask-assisted or mask-less.
Wet etching quite obviously uses a chemical solution for the
dissolution and is credited with advantages like precise selective
attack and faster etching rate. To recount a study, Si AR SWS
have been made by wet etching using single nanosized gold (Au)
nobel particle as the catalyst.
91
The fact that structure and
reflectivity of the Si surface is strongly affected by the shape and
selection of material as the metal catalyst is the very reason why
Fig. 15 (a) Schematic diagram of GLAD technique (b) modified GLAD to minimize the dependency between porosity and oblique angle,
79
(c) slanted
microstructure,
84
(d) zigzag morphology,
84
(e) helix morphology,
84
(f) double MgF
2
layers with a capping MgF
2
layer.
84
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3791
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
Au is used for rather finer nanostructures. This can be under-
stood by the idea that the size of each of the Au particle deposited
on the surface of the substrate is determinant of the size of the
grove that is produced after the Au particle is etched away from
the surface. The state-of-the-art technique involves preheating so
that Au particles attach on the substrate surface. This is followed
by soaking of the substrate in aqueous solution of HF and H
2
O
and subsequently etching using catalysis of Au. A 15 min etched
surface is this case bore a black appearance and registered
a reflectance of less than 5% in 300–800 nm range.
Studies have also proved that electrochemical etching (in this
category) of porous silicon, PSi is capable of adjusting the RI of
PSi continuously without damaging the already formed PSi on Si
substrate.
92
As the etching is performed at the interface (PSi–Si)
the current density plays a decisive role in RI adjustment. The
dynamic etching employed in this case has a steadily decreasing
etching current density applied for less than 10 s. The reflectance
plot also shows (Fig. 17(a)) reduction in reflectivity for PSi
applied on p+ Si wafers and a simulation using a GRIN PSi
layer, which in reality may be fabricated by applying a higher
current density in the beginning imparting high porosity and then
gradually decreasing the current density which will in turn lower
the porosity, gives satisfyingly lower reflectance plots.
Dry etching is performed in vacuum chambers and surface
removal is through ablation or volatilisation though plasma or
ion bombardment. Surface texturing of Si that we discussed in
the previous section also involves etching using femtosecond
lasers
93–96,166
in the presence of SF
6
or N
2
and height of these
spikes can be increased with increasing laser shots as starkly
presented in Fig. 17 (b) (c).
166
In addition, the gas used while the
etching is underway, say SF
6
or N
2
is quite an important factor as
N
2
has proved to be less absorptive than SF
6
which shows
appreciable performance in the IR region.
97
The scan speed of the
laser determines the height and density of the spikes.
96
Fig. 17(d),
(e) shows that microstructures in presence of SF
6
produce rather
sharp tips as compared to blunt tips in presence of N
2
. Moreover,
the reflectance vs wavelength plot (Fig. 17(f)) also establishes the
superiority of SF
6
which is mainly attributed to a combination of
surface morphology, density and presence of surface defects
(S and F in this case) that produce absorption states in the mid or
the band edges of Si thereby increasing the absorption in NIR,
thus, demonstrating the added advantage incorporating ARCs.
Chlorine trifluoride (ClF
3
) gas has been used as a dry etchant
to form honeycomb textured nanostructures on
substrates
60,141,142
and it has been reported that ClF
3
gas can etch
Si without plasma at temperatures near room temperature and
doesn’t affect SiO
2
or positive-type photoresist mask material.
63
Moreover, usage of ClF
3
doesn’t hamper the isotropic property
or crystal orientation of the substrate.
The corkscrew morphology of Si nanotips
99,143
explained in
detail in the section 8.1 also employs a two step dry etching
process. The technique is microetch mask assisted which can be
etched away along with the substrate to produce the desired
morphology. The first step is a reactive ion etching process (RIE)
in the presence of CF
4
that produces Fluorine based compounds
at random spots on the substrate. The second step is deep RIE
which enhances the earlier ‘‘polymer-grass’’ like structure
resulting in tall Si spike like structures (Fig. 21(h)).
7.3. Unconventional fabrication
7.3.1. Lithography. Patterning local surface of a substrate
with nanoscale features can be performed by techniques such as
scanning probe lithography (SPL), focused beam lithography
(FBL) and nano imprint lithography (NIL) and being formed by
a probe scanning the surface of the substrate, the features formed
are self-assembled for the most part. In fact, NIL
100
has been
demonstrated to have produced ‘‘moth’s eye’’ structures on
GaAs tandem solar cells.
101
First off, a replicated poly(vinyl
chloride), PVC mold is fabricated by Hot Embossing on a Ni
mold containing a hexagonal array of conical shaped structure
with 300 nm pitch and height and diameter 300 and 250 nm,
respectively. Next, this mold is lowered onto GaAs solar cells and
placed in a pressurized imprinting system for replication on
GaAs as shown in Fig. 18(a). The same technique has been used
for texturing protective low iron glass of solar cells
102
and
replicating SWS on large area polymer sheets.
103,104
NIL is also
Fig. 16 (a) Comparative study of reflectance vs wavelength plots for VA-CNTs, glassy carbon (regarded as black body) and gold mirror.
83
(b)
Reflectance spectra of PMMA with AR and AR + Anti-scratch coatings
85
NB: The pointers and description have been modified for legibility. (c) FIB-CVD
fabricated mimic of nanostructures on ‘‘morpho’’ butterfly.
90
3792 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
the most preferred technique due to its low cost and high
throughput advantages.
A study by Motamedi et al.
69
dealing with detectors that
operate at long wavelengths (10–30 mm) and thus, might require
thick ARCs which are susceptible to de-lamination during
‘‘repeated cycling to cryogenic temperatures’’ suggests pillar
arrays on Si substrate by binary optics technology.
105
The
nanostructure ensemble is fabricated using high-resolution
lithography to transfer the micro-relief pattern onto the substrate
followed by reactive-ion etching for appropriate depth and
feature attainment. This enhances the transmission of the
detector by approximately 90%.
7.3.2. Micro replication technique. Fig. 18 (c) explains the
technique of roll-to-roll micro-replication process (R2R MRP)
which entails replication of conical ‘‘moth’s eye’’ nanostructures
on thermoplastic polymer film, such as polyvinyl chloride (PVC)
by host embossing.
106
The master template made of polycarbonate has an array of
conical moth’s eye structures which when transferred to PVC
film at 100 C and 1 atm pressure, produces a tapered-hole
pattern on PVC. This PVC template is in turn used to replicate
the nanostructures on glass substrates. The interesting inference
drawn in this study is that double side patterned glass shows
a transmittance of 96% compared to 94% for one-sided. The
most significant advantage of this technique is that it can be used
to produce ARCs on wafer on a large scale. To enumerate the
steps involved in the micro replication technique, it basically
starts with an optical design with the help of an optical design
applications.
108,144
This design is developed on a Si wafer through
lithography and dry etching. The nanostructure pattern so
developed on Si is then transferred onto a Ni mould by electro-
less plating which serves as the master template for all roll-to-roll
micro replication done thereafter. This template when R2R is
imprinted on a flexible PET surface produces the desired array of
nanostructures on it.
106
Bio template assisted micro replication techniques have also
been reported. The surprisingly temperature resistant cicadia
wings have been used in the replication process to fabricate large
area AR SWS on polymethyl methacrylate (PMMA) polymer.
107
The authors of the research paper have quite illustratively
explained the technique of replication (Fig. 18(b)). The modus
operandi is a fabrication of a Au mould by depositing Au on
cicadia wings thermally. The replica of the nanostructures so
obtained on the Au template is in turn transferred to a PMMA
film. An advantage of this technique is that the problem of
PMMA sticking to the gold template doesn’t arise which has
been reasoned as the wax transmitted from the cicadia wings may
have been transferred to the Au film.
7.3.3. Miscellaneous technique. Photo-aligning,photo-
patterning which have been shown to have fabricated LCDs etc.
has been reportedly employed
38
on optical polymer films that
exhibit controlled surface topologies. The basic idea is that
phase-separation can be induced optically in monomer liquid-
crystal films on substrate such that nano-corrugated surface is
generated on the surface. This monomer corrugation by phase
separation of the polymer film produce uniform isotropic or
Fig. 17 (a) Reduction in reflectivity for PSi applied on p+ Si wafers confirming simulation profile.
92
(b) Surface texturing of Si showing height of the
spikes at 25 laser shots and (c) 1000 laser shots.
166
(d) Tip profiles of spikes in presence of SF
6
98
(e) in presence of N
2
.
98
(f) Reflectance vs wavelength plots
for Si textured in the presence of SF
6
or N
2
.
98
NB: Pointers added to Fig. 17(f)for better understanding.
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3793
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
ansiotropic nanostructures and it can be used on a broader array
of optical thin films. The results of applying the coating on glass
surface is evident in the picture. Fig. 19 in one-sided and both-
sided applications and the both-sided glass shows a very low
reflectivity of 0.1% in the 400–700 nm wavelength zone, thereby
maximizing the transmission to 99.1%.
8. The materials perspective of anti-reflectivity
8.1. Silicon based
Silicon is evidently one material that has been explored in all
types of anti-reflective coatings, starting from porous silicon
(PSi) on solar cells to Moth’s eye inspired biomimetic structures
and silicon nanotips. What makes it so indispensable is its use in
the photovoltaic industry for reflection losses minimization and
compatibility with Si photovoltaic.
A great many silica based ARCs especially the GRIN types
have been looked at in the previous sections and it can be recalled
that their exceptional hardness combined with very low RI
(1.52005) makes them such an apt choice for ARCs. To bring PSi
(Fig. 20(c)) back to our discussion, it has been reported that PSi
has little or no angular dependency on reflectance even compared
to ultrafast laser treated Si (Fig. 20(a)) with and without
annealing and chemically textured Si (Fig. 20(b) with SiN
x
72
(Fig. 20(d)–(g)). It can also be observed in Fig. 20(g) that ultra-
fast laser textured sample which is thermally annealed has total
internal scattering (TIS) values close to 100% which indicates
non-absorption with energies less than the band gap and PSi and
chemically textured Si samples also did not show any impressive
TIS results validating the annealing effects on Si based ARCs
(photon absorption reduced at higher temperature).
A quick relook at ‘‘biomimetics’’ ARCs also renders a slew of
Si based nanostructure. Huang et al.
109
have fabricated a random
array (packing density 610
9
cm
2
) of silicon nanotip
structures having an apex diameter of 3–5 nm and a base
diameter of 200 nm, length 1000–16000 nm by masked dry
etching that reportedly outperforms the existing Si microstruc-
tures on the broadband and polarization-insensitivity counts.
These SiNTs (Fig. 21(a),(b)) can be coated on wafers as
demonstrated by this current study. However, SiNTs being dark
arrays of nanostructures invoke curiosity with regard to their
application on transparent optical devices or photovoltaic. Yet
the study shows an impressive hemispherical reflectance (spec-
ular+diffuse) of 0.2% in 0.25–0.4 mm range and specular of less
Fig. 18 (a) Flow chart of nano-imprint lithography technique producing ‘‘moth’s eye’’ pattern on GaAs solar cells
101
(b) Stepwise bio-replication
technique using cicadia wing as bio template
107
(d) Illustration of R2R MAP technique involving replication on PET.
106
Fig. 19 Transmission vs. wavelength plot for a nano-corrugated surface
on optical thin films by photo-aligning, photo-patterning. Notice the high
glare in case of the uncoated film sample in (1).
38
NB:Image modified by
adding further information and images from the same paper for better
understanding.
3794 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
than 1% for 16 mm long SiNTs (Fig. 21(c),(d)). In fact, it can also
be seen in Fig. 21(e),(f) that the performance of SiNTs is quite
superior to Si wafers in case of s- and p-polarized lights and the
fact that all the analysis encompassing the UV-VIS-NIR regions
of the solar spectrum indicates the broadband properties of
SiNTs as well.
In the THz regime (discussed above) SiNTs show quite an
appreciable performance compared to other lithographically
produced SWS, which can be attributed to the rather aperiodic
or random arrangement of SiNTs as opposed to the other peri-
odic arrays and hexagonally packed ‘‘moth’s eye’
nanostructures.
Similarly, Si nanopillar arrays by a self-assembly process
involving self-assembled 2D polystyrene spheres as masks in
reactive ion etching has been proposed by Xu et al.
110
Fig. 21(g)
shows SEM image of these Si nanopillars which exhibit a reduc-
tion of reflectance in Si wafers from 32 to 8% and can be reduced
to 1% in the 700–800 nm range. Although having a reduced
reflectance, the Si nanopillars can be compared to the the SiNTs
and the plateau in the former against the tapper in the latter and
greater heights of SiNTs can conjure up a relationship between
the effectiveness of ARCs and the tip profile and height/depth of
the microstructures.
The fact that application of Si based ARCs has been reported
in the aerospace industry and space exploration, just goes to
show the diversity of its use. For instance, research under-
taken
99,143
to establish a technique for minimizing ghost image
formation on the micro sun sensors (MSS) for Mars Rover using
Si nanotips is a case in point. It has been reported that Si
nanotips, which have cork-screw morphology (Fig. 21(h)) coated
with Cr/Au (to enhance absorption) at the target wavelength of
1m have 30specular reflectance of the about 0.09% as opposed
to 35% for bare Si and hemispherical reflectance is 8%. These AR
when applied to the MSS helps correct the ghost image problem.
8.2. TiO
2
based
TiO
2
has been widely used as ARCs in silicon photovoltaics due
to its matching refractive index (anatase 2.49, rutile 2.903) and
low deposition cost. Numerous samples of ARCs using TiO
2
and
TiO
2
/SiO
2
multilayers, have already been given in the previous
sections (6.2.3) and it is the RI of TiO
2
that can be used to lower
the RI of glass slightly which eventually can be lowered further
using SiO
2
mono or multi layers. However, one important aspect
of TiO
2
worth mentioning here is that it doesn’t facilitate
substantial surface passivation which is desirable especially in
semiconductors as it prolongs the effective life time of the charge
career. This challenge has been squarely met by
approaches
111,112,145,146
that involve initially growing a thin (5–
30 nm) SiO
2
passivation layer and then depositing it with TiO
2
.
8.3. Polymer based
Antireflective polymer optical films with SWS are a new area
under exploration. Ting et al.
106
has reported a fabrication of
periodic conical SWSs on flexible polyethylene terephthalate
(PET) substrate using a nickel mould containing an array of
cylindrical nanostructures. Fig. 22(a) shows a SEM image of the
resulting nanostructure template containing conical and cylin-
drical array with a period of 400 nm, diameter 200 nm and height
350 nm. Reflectance and transmittance figures also stack up at
2.45 and 89.4% (450–700 nm range) respectively.
Surface porous polystyrene/poly(vinylpyrrolidone) (PS/PVP)
polymer films by phase separation in a dip-coating process with
precursors hydrophobic polystyrene (PS) and hydrophilic poly
(vinylpyrrolidone) (PVP) at a volume ratio of 7 : 3 have been
reported
113
to have tuneable film thickness, pore size and pore
depth by altering the concentration of the PS/PVP solution and
withdrawal speed during the fabrication. The films when applied
Fig. 20 (a) Ultrafast laser textured silicon surface in presence of SF
6
gas;
72
(b) chemically textured silicon surface;
72
(c) PSi surface;
72
(d)–(g) TIS vs angle
plots at aforementioned wavelengths comparing ultrafast laser textured Si{L}, annealed ultrafast laser textured Si{L1}, chemically textured Si with SiN
x
{CT}, Porous Si{P}.
72
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3795
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
on glass surface reportedly increased the transmittance by 3–4%
(Fig. 22(b)). Another paper reporting porous polymers based on
self-assembled supramolecular block copolymer polystyrene
terminated with carboxyl PS-COOH, poly(methyl methacrylate)
terminated with amide (PMMA-NH
2
)
114
demonstrated a 97.93%
transmittance in visible wavelength range and on increasing the
thickness from 152 to 203 nm, revealed an inhomogeneous 3
layered structure showing a transmittance of 99% when
applied on both sides of glass.
Apart from these ‘‘moth’s eye’’ structures fabricated on
PMMA
115
with reflectance pegged at 1% and 4% when applied on
one side and both sides of a glass substrate respectively and 2-D
array of polystyrene spheres coated with Ag
116
are examples to
show the growing interest in the field of polymer based anti-
reflective coatings.
8.4. Gallium based
Group III–V semiconductors which by the virtue of their high
career mobility and direct energy gaps are used in optoelec-
tronics. Most of these materials pose broad band challenges due
to their bandwidth disparity.
A survey of scientific works in this area uncovers mostly
Gallium and Indium based ARCs. For instance, Random GaN
nanopillars of 300, 550 and 720 nm height have been fabricated
by Chiu et al.
117
GaN is a wide bandgap material (3.4eV) and has
antireflective properties depending on structural roughness. The
angle-dependent reflectance for s- and p-polarized light as
showing in Fig. 23(a),(b) make it clear that the GaN nanopillars
of 550 and 720 nm have similar profiles with reflectance at less
than 3% up to 50. Further analysis by RCWA reveals that the
GaN nanopillars due to their cone-like or spearhead-like struc-
ture create a RI gradient (GaN, RI 2.5 to ambient) which
contributes to the omnidirectional AR property. Similarly,
random GaS based SWS have also been fabricated though Au
mask etching and analyzed by Leem et al.
118
has been predicted to
increase photonic absorption in GaAs based solar cells. The
study also makes an insightful analysis of the reflectance and
wavelength variation w.r.t etching time and an optimization of
height and sidewall shape of the SWS led to a surface reflectance
of less than 5% in the 350–900 nm range.
GaAs solar cells employing conductive Indium Tin Oxide
(ITO) nanocolumns by the GLAD through electron beam have
Fig. 21 (a) SEM image of 6-inch Si-NT wafer.
109
(b) Cross sectional SEM image of SiNT.
109
(c) Comparative study of hemispherical reflectance for Si
wafer and SiNT
109
and (d) specular reflectance for Si wafer and SiNT, specular reflectance.
109
(e) Comparision of SiNTs w.r.t s-polarization;
109
(f)
p-polarization of incident flux;
109
(g) SEM image of these Si nanopillars;
110
(h) corkscrew shaped Si nanotips
99
NB: Markers added to Fig. 21(e)and (f)for
better understanding.
Fig. 22 (a) SEM image showing periodic conical nanostructures on
PET.
106
[118] (b) Transmittance vs wavelength plot showing performance
of surface porous PS/PVP before/after etching and glass.
113
NB: Pointers
added to Fig.22 (b)for better understanding.
3796 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
reported to have enhanced the efficiency of GaAs solar cells by
23%.
118
‘‘Moth’s eye’’ nanostructures have also been incorpo-
rated on GaAs tandem solar cells via nano imprint lithography
102
discussed in section 7.3.1. The reflectance without and with these
nanostructures on GaAs solar cells are 14.34% and 9.08% as
shown in Fig. 23(c).
Last but not the least, conically tapered GaP nanorods have
been reportedly manufactured in the shape of ‘‘Moth’s eye’
nanostructures
119
and these serve as GRIN layers on the GaP
substrate. Quite interestingly these nanostructures were fabri-
cated by the vapour-liquid-solid (VLS) mechanism in a metallic
organic vapour phase epitaxy (MOVPE) reactor. The growth of
nanorods is facilitated by spin-coated gold particles on the
substrate. According to the authors, temperature (570 C) plays
a crucial role in producing the desired conical morphology apart
from the fact that the size of gold particles determines the base
diameter of the cone during the vertical growth process. The
transmission of the GaP substrate goes up by 15% due to this
nanostructure ensemble especially registering a sharp absorption
peak at GaP band-gap, 548 nm, as claimed by the study.
8.5. Carbon based
Amorphous diamond (a:DLC) like carbon films as hard AR
coatings for Si solar cells [139] exhibit a transparency of 90% in
VIS region and RI varying between 1.7 to 2.2 and even enable the
solar cells to operate in oxide or corrosive environments. Addi-
tionally carbon nanotubes (CNTs) ARCs have been discussed in
the section 7.1.3. The proverbial ‘‘moth’s eye’’ nanostructure has
been realized on the flat diamond surface
120
as well. The flat
diamond surface has reflectance of around 18% at 10 mm. When
this surface is replaced with the ‘‘moth’s eye’’ structure the
reflectance is reportedly reduced to 7% at the same wavelength
values. The fabrication basically involves etching a reverse
‘‘moth’s eye’ onto Si by lithographic technique. Subsequently,
diamond is grown on the etched surface by chemical vapour
deposition and on dissolution of Si the leftover diamond assumes
a moth’s eye structure.
8.6. Organic based
Fatty acid films have been reported to have anti-reflective
properties. Moreover they can act both as ARCs and protective
shield for solar cells. Organic films of long chain fatty acids
deposited by Langmuir–Blodgett (L-B)
121
technique on glass
have been extended onto solar cells.
147
The L-B films administer
control over their thickness, allows RI to be modified by
changing the number of CH
2
groups or replacing H
+
with metal
ions.
Intentional positioning of hydrophobic CH
3
on the outermost
layer can impart hydrophobicity to the L-B film and photo-
polymerized polydiacetylene [CH
3
(CH
2
)
n
ChC–ChC–
(CH
2
)
m
–COOH] films inclusion can impart thermal and
mechanical stability as studies
147
have proved. The research
undertaken by Buckner et al.
122
uses stearic acid thin films (20–
80 nm thick) coatings on Si solar cells by L-B technique shows
that in 450–750 nm, organic films bring down the reflectance
from 15% to 5–7% and in higher wavelength region, up to 5%.
9. Characterization techniques for anti-reflective
coatings
The refractive index, reflectance and transmittance are the vital
statistics that help deduce major inferences regarding the
broadband, ominidirectional and polarization-insensitivity
aspect of the ARCs. The optical characteristics of the films are
examined using ex-situ ellipsometer and optical spectra
measurements are done by UV-Vis-NIR spectrophotometer. In
certain cases, crystallinity and orientation of the films are
measured by high resolution X-ray diffractometer. Thickness
and roughness measurements are done by the help of profilers
such as Dektak stylus profiler. Chemical states, structures and
composition are determined through X-ray photoelectron spec-
troscopy (XPS), Raman spectrometry and Fourier transform
infrared spectroscopy (FTIR). All the images displaying topo-
graphical details and structural close-ups that we have seen so far
are captured by the inevitable transmission electron microscopy
(TEM), field emission scanning electron microscopy (FESEM)
and atomic force microscopy (AFM). In a case where measure-
ment of thermal conductivity of AR systems having diverse
physical characters is required, the transient line heat source
(TLHS) method is used. In addition, there is a large number of
computational models that provide refractive index computing
methodologies for different types of ARCs that we have dis-
cussed so far. These computational models relay on two of the
Fig. 23 (a) Reflectivty vs angle of incidence for GaN nanopillars subjected to s-polarized
117
and (b) p-polarized light.
117
(c) Reflectance vs wavelength for
GaAs solar cells with and without patterns.
101
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3797
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
already discussed landmark analytical techniques namely, the
effective medium theory and the rigorous coupled wave analysis
for refractive index computation. Durability of ARCs on solar
panels can be judged by accelerated aging tests in real time
condition. In fact, the accepted standards of adherence of coat-
ings are supplied by US Military specifications MIL-C-675C,
which involves MgF
2
anti-reflective coatings on glass substrates.
Some of the stipulations of the same are presented here
123
The coating should not peel-off, crack or blister after being
exposed to 95–100% humidity at 49 C for 24 h or after being
immersed in a sodium chloride solution (45 g L
1
) at room
temperature for 24 h.
Coating must withstand a 24 h salt spray fog test.
Abrasions test involves a standard pencil eraser to be rubbed
against the coated surface for 20 cycles (40 strokes) with a load of
10 N.
Adherence test involves a cellophane tape to be pressed
evenly and tightly to the coated substrate and removed quickly at
an angle normal to the substrate.
10. Antireflective coatings on solar cells and modules
Besides numerous references to antireflective coatings on solar
cells and enhancement in transmission by ARC in the previous
sections, a focused discussion on ARC on solar cells and modules
would bring forth the finer details such as the effect of encap-
sulants, cost-performance trade-off and accepted industry trends
etc. and review the latest developments in this field.
The coating of antireflective layer on solar cells dates back to
the 1960s when Elliot Berman observed that roughness on the
wafer surface serves as a suitable ARC.
171
PV industry experi-
mented with SiO
2
and TiO
2
ARC for some time though SiO
2
did
not match optically (RI ¼1.45) with the Si solar cell and TiO
2
despite being a better candidate for ARC did not contribute to
surface passivation. Over the years continuous research has led
to a notable industry trend—the application of silicon nitride
(Si
3
N
4
) ARC instead of TiO
2
because the former is found to have
excellent surface passivation properties. The Si
3
N
4
coatings
discourage carrier recombination at the surface of the solar cell
which is highly desirable. Plasma enhanced CVD (PECVD) has
been an industry approved fabrication technique for producing
Si
3
N
4
coatings on an N/P junction and it is this coating that
imparts the dark blue color to the crystalline silicon solar cells.
PECVD is a fast deposition technique and the deposition takes
place at 400 C
149,150
by the virtue of the following chemical
reaction:
3 SiH
4
+4NH
3
/Si
3
N
4
+24H
2
The by-product hydrogen (H
2
) is used to accomplish the
additional task of passivating the impurities and defects,
a process better known as impurity-gettering (remove majority of
the minority charge carriers).
148
Moreover, the nitride layer
imparts a low resistance ohmic contact to the Ag-based contact
metallization shown in Fig. 24(a). A well-designed layer for
ARCs would be multipurpose if it has the following properties:
148
1. Low reflectance over a broader solar spectrum
2. Low surface recombination velocity in case of a N
+
/P solar
cell
3. Provide a low ohmic resistance to the Ag-contact
metallization
4. Encapsulate H
2
within the surface of Si and promote further
diffusion to facilitate impurity-gettering
The optical property of the Si
3
N
4
coating is of great interest
and it has been reported that by lowering the proportion of Si in
the film (RI as low as 1.9), the optical losses can be reduced since
Si-rich film have higher refractive index and N-rich films have
lower refractive index and our objective is to lower the RI to
match the substrate RI.
148
A textured solar cell is deposited with
a SiN
x
:H coating, 750
A thick. Fig. 24 (b) shows the calculated
absorbance and reflectance of Si solar cell operating in air and
the encapsulated module. However, in real time when the cell is
encapsulated in ethylene vinyl acetate (EVA, RI ¼1.5) an ARC
of RI ¼2.2 to 2 is found to be the optimum.
148
The reported
calculated photocurrent density in the optimized coatings in case
of solar cells and encapsulated module is 40.97 mA cm
2
and
39.74 mA cm
2
, respectively.
Kuo et al.
151
have designed an ‘‘all-l– all-q’ AR design and
fabricated the same by depositing layers of slanted nanorods
layer-over-layer by means of glancing angle deposition (GLAD),
producing seven layers with refractive index varying from 2.6
(near substrate) to 1.09 (adjacent to air) (Fig. 24(c)). This entire
system can be treated as a gradient refractive index (GRIN) layer
and the experimental reflectance data plotted in Fig. 24(d) shows
a reflectance in the range of 1–6% (l¼400–1600 nm) for all
visible and near-infrared wavelengths. These multilayer coatings
also exhibit a reflectance of 2–5% over a broad angle of incidence
(0 to 60) and they have reported that a transition from
conventional coatings to the seven layers ARC on a Si wafer
results in a solar-to-electric efficiency improvement from 20.5 to
42.7%.
151
ARC coatings help in increasing the short-circuit current (I
sc
)
which translates into an increase in efficiency. The increase in I
sc
is brought about by reduction in reflection at the front surface,
absorption of base of photons closer to the collector junction and
trapping the weakly absorbed photons within the cell for
subsequent absorption.
The solar cells manufactured and characterized under ideal
laboratory conditions stipulated by ASTM, AM 1.5 spectrums,
perpendicular irradiance at 1000 W m
2
and temperature (25 C)
is not quite reflective of the real world conditions all the time and
of all the geographies. Accurate thin film measurement also
assumes paramount importance in the face of cost-efficiency
trade-off. Spectroscopic ellipsometry (SE) provides a non
destructive characterization technique by studying changes in
polarization of light after being reflected from a layer on a solar
cell.
152
SE measurement of nano-structured thin films is
successful at wavelengths in the visible and higher regions.
However, when textured silicon surface further coated with SiN
x
is subjected to SE measurements, optical characterization poses
a challenge because the reflected light from such surface is
significantly less. This requires special techniques to collect
adequate signals and from that model the interaction between the
beam of light and individual rough surface.
152
As we have already seen, solar modules have cells laid out
between encapsulant sheets (typically EVA) the optical property
3798 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
of encapsulants also merits a discussion. The reflectance of SiNx
coated cell (Fig. 25(a)), R
cell/air
or R
cell/EVA
is given by
154
R¼
m11
~
n0þm12
~
nsi
~
n0m21 m22
~
nsi
m11
~
n0þm12
~
nsi
~
n0þm21 þm22
~
nsi
2
where
m11 m12
m21 m22 ¼cos dSiNx
isin dSiNx
g
nSiNx
isin dSiNx g
nsiNx cos dSiNx
0
B
@1
C
A
and ~
n
i
is the complex refractive index for the incident medium
(can be used for air or EVA depending on presence or absence
of encapsulant respectively) which can be written as ~
n
i
¼n
i
ik
i
and dSiNx ¼2pd
lg
nsiNx . As complex refractive index has come
into the reckoning, it’s noteworthy that this consists of a real
part which indicates the phase speed and an imaginary part
which quantifies the amount of absorption loss. The imaginary
part kis also called the extinction coefficient and if k¼0, light
travels without any loss.
Grunow et al.
153
have measured the I
sc
for an ecapsulated multicell
as a function of the refractive index (RI) of SiN
x
layer and its
thickness. The results have confirmed the optimization parameters
outlined by Doshi et al.
155
(RI 2.23, l¼68 nm) and Ekai et al.
156
(RI
2.2, l¼67 nm) and the optimum values of SiN
x
coating on encap-
sulated solar cells is determined to be RI ¼2.22 at l¼632.8 nm and
thickness of coating 67 nm (Fig. 25(b)). EVA and cell texturization
for antireflective property have been analyzed in tandem by G runow
et al.
153
and the difference in reflectance between R
cell/air
and R
cell/EVA
is shown in Fig. 25(c). As the degree of texturization increases (0 to 50
to 100%) the difference in reflectance decreases though it comes at
a decrease in encapsulation gain (EQE
module
/EQE
cell
where EQE is
external quantum efficiency of a solar cell).
Pern et al.
157
have fabricated high quality single and bi-layer
diamond-like carbon coatings for solar cells by two fabrication
techniques, namely, ion-assisted plasma-enhanced deposition
(IAPED) and electron cyclotron resonance (ECR) deposition.
These DLC layers when deposited on Si or GaAs solar cells,
serve as an antireflective coating (tunable RI from 1.5 to 2.6) and
a protective encapsulant. The IV(current–voltage) and P–V
(power-voltage) graphs shown in Fig. 25(d) show that the DLC
layer aids in efficiency improvement. Moreover, DLC layers are
quite resistant to high humidity at high temperature, prevent
moisture ingress and protect against mineral acids such as HCl,
HNO
3
and H
2
SO
4
. These DLC films on Si wafers don’t register
any degradation in reflectance after having subjected to 762 h of
rigorous damp-heat exposure
157
(85 C and 85% relative
humidity) and they don’t peel-off and are highly scratch
resistant.
As for the effectiveness of the ARCs on the front glass cover of
the solar modules, a great deal of discussion has already been
made and texturization and ‘‘moth’s eye’’ nanostructure appli-
cation have been discussed at length in the previous sections.
11. Contemporary research, challenges and
conclusion
Anti-reflective coatings have been used in such diverse array of
optical and optoelectronic devices such as lenses, eye-glasses
Fig. 24 (a) A typical silicon solar cell with the layer configuration displayed (b) calculated reflectance and absorbance in Si solar cell operating in air and
encapsulated modules;
148
(c) scanning electron micrograph of gradient index ARC comprising seven layers;
151
(d) deduced experimental reflectance data
from top surface of GRIN multilayer coating.
151
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3799
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
(Fig. 26(c)), military equipments, lasers, mirrors, solar cells,
diodes, multipurpose narrow and broad band-pass filters,
cathode ray tubes, television screens, sensors for aeronautical
applications, cameras, window glasses and anti glare glasses for
automotive applications (Fig. 26(c)) that the list is literally
endless. Commercially, crystalline Si modules have all integrated
anti-reflective coatings, self-cleaning coatings, encapsulant, one
layer over the other on crystalline Si cells which is why there is
this growing interest in fabricating hybrid coatings that serve as
anti-reflective, self-cleaning, anti-microbial, low-emissivity coat-
ings. Moreover, areas such as electrochromism and green
architectural strategies have also led to the integration of ant-
reflective property and electrochromism into one. The research
on WO
3
-TiO
2
124
composite films which are known for their
electrochromic properties is one such example. Research on
thermochromic AR materials is also underway.
Ensembles of ‘‘moth’s eye’’ nanostructures have also been used
in the anechoic chambers to reduce reflection of micro waves,
wavelengths in the order of centimetres and millimetres.
123
As
these chambers are used to measure radar reflections from test
objects, radiations bouncing off the walls and causing distraction
needs to be minimized. The coatings can be fabricated on
aZr
0.75
TiSn
0.25
O
4
ceramic microwave optical element of RI ¼6.
The coatings are mostly Al
2
O
3
anti-reflection coatings with RI of
2.7 and thickness of 0.78 mm.
125
As the Nobel Prize for Physics, 2010 marked the emergence of
the ubiquitous, graphene, a lot of world-wide research effort has
gone into exploring its applications. A recent report claiming an
unprecedented feat of imaging graphene on gallium arsenide,
168
GaAs wafers is going to have far reaching impact on next
generation semiconductors and optoelectronic devices. The crux
of the matter is that the imagining is made possible by the use of
an anti-reflective filter made of optimized layers aluminum
arsenide on GaAs substrate. The reason why this is done is that
when a very thin almost transparent layer (graphene) is super-
imposed over another, the reflectivity of the underlying layer
(5 layers of AlAs) will change prominently, viz. our very own
concept of anti reflectivity.
A recent research on what is called inverse moth’s eye of
microwave or millimetre wavelength thickness has been reported
by Mirotznik et al.
129
The fabrication technique is quite inter-
esting in a way that they have suggested drilling multilevel sub-
wavelength holes of various diameters as shown in Fig. 27 into
a non-absorptive substrate and the modelling of the same relies
on RCWA. The objective is to minimize reflectance in the 30 to
40 GHz broadband regime and from 0to 60angle of incidence.
The grated multilayer hole array when formed on a Hik material,
which has a dielectric permittivity of 3
r
¼9.0–0.02j shows 86%
transmittance of incident power in the 32–38 GHz range. These
are best candidates for passive imaging components where
collection of more energy in the specified energy range and
suppression of Fresnel reflections is of paramount importance.
Anti-reflective coatings have also been tested for their appli-
cation on the new age organic solar cells which comprise many
Fig. 25 (a) Encapsulated solar cell module with losses depicted at various layers;
153
(b) plot of short circuit current (I
sc
) in an encapsulated multi-
crystaline cell coated with SiNx antireflective coating;
153
(c) effect of the degree of texturization on the reflectance loss and encapsulation factor.
153
(d)
Current–voltage and power-voltage curves for 100 cm
2
crystaline silicon solar cell.
171
3800 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
layers. The organic solar cell made of glass/ITO/PEDOT has
been shown to have minimized reflection losses by adding
another layer of ‘‘moth’s eye’’ nanostructure array and
improving the performance of the cell by 2.5–3%.
126
Reversibly erasable ARCs from polyelectrolyte PAH/PAA
multilayer has also produced interesting results.
127
PAH/PAA
multilayer have been repeatedly immersed in aqueous solution
for not more than 5 min thereby alternating the value of n
between 1.52 and 1.25 (non-porous to nano-porous). In anti-
reflective state the reflectance is calibrated at 0.01% (650 nm).
Fig. 26(a) shows the enhancement in transparency and reduction
of glare brought about by using this ARCs on one side of a Petri
dish. Additionally, patterned coatings with alternating anti-
reflective spots, shown in Fig. 26(b) have been a potential
application in microelectronic applications.
127
The drift from the
conventional ARCs vis-
a-vis this case is the fabrication technique
which indeed is environment friendly. And this just goes to show
that fabrication techniques of ARCs are undergoing major
overhaul thereby offering immense opportunity for research and
environmentally compatible technological innovation.
Anti-reflective coatings despite having exemplified their
application in the field of solar photovoltaic still pose significant
challenges with regard to their performance over a period of
time. Pertinent among these is the issue of debonding or peeling-
off of the ARCs applied not just on solar panels but eyeglasses
too.
A research by Druffel et al.
128
on ceramic thin-films and
polymer nanocomposites shows that the ceramic thin-films are
susceptible to brittle fracture. Moreover, brittle materials will
inherently exhibit features such as microcracks, crazings and
voids etc. that serve as the origin of crack propagation. Humidity
and difference in thermal coefficients at the interfacial zone
(between film and substrate) can also lead to gradual peeling-off,
what is otherwise technically known as debonding of the ARCs.
On the contrary, the materials that show higher propensity
towards ductile fracture tend to exhibit durability but to survive
the entire life of a solar cell with consistent performance is what
calls for rigorous research and real-time testing of the anti-
reflective coatings. In addition, most coatings are deposited at
a stress range of 100–1000 MPa and residual stresses in the
coating-substrate duo might set in due to disparity in thermal
coefficients. If a coating of low thermal expansion is deposited on
a substrate of high thermal expansion, due to higher contraction
of the substrate than the coating, the coating is under tension and
it results in a bend in the overall configuration as shown in
Fig. 28(a) which is indeed exaggerated for better visualization.
However, if the case is exactly the opposite that is, the coating
has a higher thermal coefficient than the substrate the coating
undergoes a contraction higher than the substrate which is why it
tries to compress the substrate thereby bending it as shown in
Fig. 28(b).
Besides, performance of the solar cell also gets impaired by
accumulation of dust, microbes and moisture. The emergence of
hydrophobic, hydrophilic and anti-microbial coatings based on
the well known ‘‘lotus-effect’’ has facilitated integration of both
the areas conceptualizing what is popularly known as ‘‘hybrid
multi-purpose coatings’’. We have already seen examples of the
same in the previous sections. Yet the trade-off between dura-
bility and affordability makes it really hard to strike a balance
when it comes to formulating high efficiency anti-reflective
coatings for solar cells at affordable price.
As for the materials, a particular family of materials called
Reststrahlen materials which when exposed to radiation of
wavelengths close to that which cause lattice vibrations in them,
the Reststrahlen frequency is attained. These result in rapid
change in the RI of the material, rise in extinction coefficient and
sharp rise in Fresnel reflection coefficient as well. Thus, the
reflectance increases sharply.
169
Dobrowolski et al.
19
in their
quest for perfect anti-reflective materials have proposed a tech-
nique to reduce the reflectance in such materials. According to
the computational study the lattice vibration might give rise to
sharp absorption band and less than unity RI despite high
extinction coefficient. A design of a 4-layer omnidirectional AR
has also been proposed with substrate RI of 3.0 and it has been
computed that for 7.2 mm wavelength the reflectance is 0.05 for
all angels less than 85.
A great deal of research on anti-reflective materials is
underway as there are numerous unmet challenges some of which
we discussed in the previous sections. More so because of the
Fig. 26 (a) Polystyrene Petri dish half coated with PAH/PAA ARC. Notice the glare on the right half and the enhanced transparency in the left (b)
patterned coatings showing 1-non-porous normal glass 2-anti-reflective zones.
127
(c) ARC reducing glare both from inside and outside in eye glasses and
automobile glasses.
167
Fig. 27 Multilevel sub-wavelength gratings model forming reverse
moth’s eye (left) ARC and the same hole array structure machined on
1200 1200 0.500 slab of Hik material
129
NB: Image modified slightly for
better understanding.
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3801
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
unprecedented surge in demand for highly efficient, durable and
cost effective ARC for numerous optical and electronic equip-
ments and to a greater extent for the solar cells as the paradigm
shift to alternative sources of energy stands close to realization.
New developments in optical devices also present immense
opportunity for customization of anti-reflective coatings to suit
the cutting edge technology and product improvisation. Last but
not the least, the predominant biological aspect of anti-reflection
is under close watch because of the intense biological research
and findings confirming existence of even more sophisticated
photonic nanostructures in many species (gyroids in butterflies,
colossal eyes of squids etc.), which have to be explored in greater
depth for their anti-reflective characteristics.
Acknowledgements
H.K.R and V.A.G thank National University of Singapore for
graduate research fellowship. A.S.N and S.R thank the National
Research Foundation, Singapore (Grant Number:
NRF2007EWT-CERP01–0531) for partially supporting the
program.
References
1 R. Hooke and F. o. t. R. Society, Micrographia or some physiological
descriptions of minute bodies, Martyn and Allestry, London, 1665.
2 I. Newton and Thomas Jefferson Library Collection (Library of
Congress), Opticks: or, A treatise of the reflections, refractions,
inflections and colours of light, 4th edn, Printed for W. Innys,
London., 1730.
3 A. R. Parker, J. Opt. A: Pure Appl. Opt., 2000, 2, R15–R28.
4 J. Fraunhofer, Joseph von Fraunhofer Gesannekte Schriften, Munich,
Germany, 1888.
5 H. A. Macleod, Thin-film Optical Filters, 2nd edn, McGraw-Hill,
1989.
6 C. G. Bernhard, Endeavour, 1967, 26, 79.
7 P. B. Clapham and M. C. Hutley, Nature, 1973, 244, 281–282.
8 L. Schirone, G. Sotgiu and F. P. Califano, Thin Solid Films, 1997,
297, 296–298.
9 A. Ramizy, W. J. Aziz, Z. Hassan, K. Omar and K. Ibrahim,
Microelectronics International, 2010, 27, 117–120.
10 H. G. Craighead, R. E. Howard, J. E. Sweeney and D. M. Tennant,
J. Vac. Sci. Technol., 1982, 20, 316–319.
11 L. Rayleigh, Proceedings of the London Mathematical Society, 1879,
s1–11, 51–56.
12 T. H. Elmer and F. W. Martin, Ceram. Bull., 1979, 58, 1092–1097.
13 J. C. M. Garnett, Philos. Trans. R. Soc. London, Ser. A, 1904, 203,
385–420.
14 J. C. M. Garnett, Philos. Trans. R. Soc. London, Ser. A, 1906, 205,
237–288.
15 D. A. G. Bruggeman, Ann. Phys., 1935, 24, 636–664.
16 J. A. Dobrowolski, Optical properties of films and coatings in
Handbook of Optics, M. Bass edn, McGraw-Hill, New York, 1995.
17 Y. Fink, J. N. Winn, S. Fan, C. Chen, J. Michel, J. D. Joannopoulos
and E. L. Thomas, Science, 1998, 282, 1679–1682.
18 J. A. Dobrowolski and S. H. C. Piotrowski, Appl. Opt., 1982, 21,
1502.
19 J. A. Dobrowolski, D. Poitras, P. Ma, H. Vakil and M. Acree, Appl.
Opt., 2002, 41, 3075–3083.
20 H. A. Macleod, Thin Film Optical Filters (Institute of Physics,
Bristol, UK), 2001.
21 K. Q. Salih and N. M. Ahmed, Int. J. Nanoelectron. Mater., 2009, 2,
109–118.
22 J. T. Cox and G. Hass, ‘‘Antireflection coatings for optical and
infrared materials’’ in Physics of Thin Films, Academic Press, New
York, 1968.
23 Y. G. Kavakli and K. Kantarli, Turk. J. Phys., 2002, 26, 349–354.
24 W. H. Southwell, Opt. Lett., 1983, 8, 584–586.
25 B. Sheldon, J. S. Haggerty and A. G. Emslie, J. Opt. Soc. Am., 1982,
72, 1049–1055.
26 J. Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen,
S. Y. Lin, W. Liu and J. A. Smart, Nature Photonics, 2007, 1(3),
176–179.
27 M. F. Schubert, D. J. Poxson, F. W. Mont, J. K. Kim and
E. F. Schubert, Appl. Phys. Express, 2010, 3, 082502–082503.
28 F. H. Nicoll and F. E. Williams, J. Opt. Soc. Am., 1943, 33, 434–435.
29 F. H. Nicoll, J. Opt. Soc. Am., 1952, 42, 241–241.
30 V. Lehmann and U. Gosele, Appl. Phys. Lett., 1991, 58, 856–858.
31 B. E. Yoldas and D. P. Partlow, Thin Solid Films, 1985, 129, 1–14.
32 K. H. Jung, S. Shih, D. L. Kwong, C. C. Cho and B. E. Gnade, Appl.
Phys. Lett., 1992, 61, 2467–2469.
33 R. Prado, G. Beobide, A. Marcaide, J. Goikoetxea and A. Aranzabe,
Sol. Energy Mater. Sol. Cells, 2010, 94, 1081–1088.
34 M. J. Minot, J. Opt. Soc. Am., 1976, 66, 515–519.
35 D. R. Uhlmann, T. Suratwala, K. Davidson, J. M. Boulton and
G. Teowee, J. Non-Cryst. Solids, 1997, 218, 113–122.
36 H. Hattori, Adv. Mater., 2001, 13, 51–54.
37 S. Walheim, E. Sch
affer, J. Mlynek and U. Steiner, Science, 1999,
283, 520–522.
38 M. Ibn-Elhaj and M. Schadt, Nature, 2001, 410, 796–799.
39 W. H. Miller, C. G. Bernhard and A. R. Moller, J. Opt. Soc. Am.,
1964, 54, 353.
40 A. R. Moller, C. G. Bernhard and W. H. Miller, Acta Physiol.
Scand., 1966, S68, 140.
41 A. Yoshida, M. Motoyama, A. Kosaku and K. Miyamoto, Zool.
Sci., 1997, 14, 737–741.
42 R. Brunner, A. Deparnay, M. Helgert, M. Burkhardt, T. Lohm
uller
and J. P. Spatz, Product piracy from nature: biomimetic
microstructures and interfaces for high-performance optics
Fig. 28 Effect of thermal-coefficient mismatch between the coating and the substrate (a) thermal coefficient of the coating is less than that of substrate
resulting in a concave bend (b) thermal coefficient of coating is higher than that of substrate resulting in a convex bend. NB: Images exaggerated for
better visualization.
3802 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
(Proceedings Paper): The Nature of Light: Light in Nature II, SPIE,
2008.
43 A. Sweeney, C. Jiggins and S. Johnsen, Nature, 2003, 423, 31–32.
44 A. R. Parker and H. E. Townley, Nat. Nanotechnol., 2007, 2, 347–
353.
45 M. Gale, Physics World, 1989, 2, 24–28.
46 S. A. Boden and D. M. Bagnall, Photovoltaic Energy Conversion,
Conference Record of the 2006 IEEE 4th World Conference on,
2006, 2, 1358–1361.
47 A. R. Parker, Z. Hegedus and R. A. Watts, Proc. R. Soc. London,
Ser. B, 1998, 265, 811–815.
48 D. G. Stavenga, S. Foletti, G. Palasantzas and K. Arikawa, Proc. R.
Soc. London, Ser. B, 2006, 273, 661–667.
49 C. G. Bernhard, G. Gemne and J. S
allstr
om, Journal of Comparative
Physiology A: Neuroethology, Sensory, Neural, and Behavioral
Physiology, 1970, 67, 1–25.
50 R. C. Enger and S. K. Case, Appl. Opt., 1983, 22, 3220–3228.
51 H. Toyota, K. Takahara, M. Okano, T. Yotsuya and H. Kikuta,
Jpn. J. Appl. Phys., 2001, 40, L747–L749.
52 W. L. Min, A. P. Betancourt, P. Jiang and B. Jiang, Appl. Phys.
Lett., 2008, 92, 141109.
53 S. A. Boden and D. M. Bagnall, Appl. Phys. Lett., 2008, 93, 133108.
54 V. V. Iyengar, B. K. Nayak and M. C. Gupta, Sol. Energy Mater.
Sol. Cells, 2010, 94, 2251–2257.
55 B. Dale and H. G. Rudenberg, Proc. 14th Annual Power Sources
Conf., US Army Signal Research and Develeopement Lab., Ft.
Monmouth, New Jersey, 1960, 22.
56 P. Campbell, J. Opt. Soc. Am. B, 1993, 10, 2410–2415.
57 P. Campbell, Sol. Energy Mater., 1990, 21, 165–172.
58 A. W. Smith and A. Rohatgi, Sol. Energy Mater. Sol. Cells, 1993, 29,
51–65.
59 Y. W. Chen, P. Y. Han and X. C. Zhang, Appl. Phys. Lett., 2009, 94.
60 P. Papet, O. Nichiporuk, A. Kaminski, Y. Rozier, J. Kraiem,
J. F. Lelievre, A. Chaumartin, A. Fave and M. Lemiti, Sol. Energy
Mater. Sol. Cells, 2006, 90, 2319–2328.
61 J. Zhao, A. Wang, M. A. Green and F. Ferrazza, Appl. Phys. Lett.,
1998, 73, 1991–1993.
62 B. L. Sopori, Sol. Cells, 1988, 25, 15–26.
63 J. I. Gittleman, E. K. Sichel, H. W. Lehmann and R. Widmer, Appl.
Phys. Lett., 1979, 35, 742–744.
64 Y. Saito and T. Kosuge, Sol. Energy Mater. Sol. Cells, 2007, 91,
1800–1804.
65 Y. Wang, L. Chen, H. Yang, Q. Guo, W. Zhou and M. Tao, Sol.
Energy Mater. Sol. Cells, 2009, 93, 85–91.
66 D. M. Mittleman, AIP Conf. Proc., 2005, 760, 25–32.
67 M. Karlsson and F. Nikolajeff, Opt. Express, 2003, 11, 502–507.
68 A. Gombert, K. Rose, A. Heinzel, W. Horbelt, C. Zanke, B. Bl
asi
and V. Wittwer, Sol. Energy Mater. Sol. Cells, 1998, 54, 333–342.
69 C. Br
uckner, B. Pradarutti, O. Stenzel, R. Steinkopf, S. Riehemann,
G. Notni and A. T
unnermann, Opt. Express, 2007, 15, 779–789.
70 M. E. Motamedi, W. H. Southwell and W. J. Gunning, Appl. Opt.,
1992, 31, 4371–4376.
71 W. H. Southwell, J. Opt. Soc. Am. A, 1991, 8, 549–553.
72 H. Sai, H. Fujii, K. Arafune, Y. Ohshita, M. Yamaguchi,
Y. Kanamori and H. Yugami, Appl. Phys. Lett., 2006, 88, 201116–
201113.
73 C. J. Brinker and A. J. Hurd, J. Phys. III, 1994, 4, 1231–1242.
74 H. Dislich and P. Hinz, J. Non-Cryst. Solids, 1982, 48, 11–16.
75 E. K. Hussmann, Key Eng. Mater., 1998.
76 H. G. Floch, P. F. Belleville, J.-J. Priotton, P. M. Pegon,
C. S. Dijonneau and J. Guerain, Am. Ceram. Soc. Bull., 1995, 74, 60.
77 P. Hinz and H. Dislich, J. Non-Cryst. Solids, 1986, 82, 411–416.
78 M. Faustini, L. Nicole, C. d. Boissie
ere, P. Innocenzi, C. m. Sanchez
and D. Grosso, Chem. Mater., 2010, 22, 4406–4413.
79 K. Robbie, J. C. Sit and M. J. Brett, J. Vac. Sci. Technol., B, 1998,
16, 1115–1122.
80 R. N. Tait, T. Smy and M. J. Brett, Thin Solid Films, 1993, 226, 196–
201.
81 R. Messier, T. Gehrke, C. Frankel, V. C. Venugopal, W. Otano and
A. Lakhtakia, J. Vac. Sci. Technol., A, 1997, 15, 2148–2152.
82 K. Robbie and M. J. Brett, J. Vac. Sci. Technol., A, 1997, 15, 1460–
1465.
83 S. R. Kennedy and M. J. Brett, Appl. Opt., 2003, 42, 4573–4579.
84 Z. P. Yang, L. J. Ci, J. A. Bur, S. Y. Lin and P. M. Ajayan, Nano
Lett., 2008, 8, 446–451.
85 M. Kuhr, S. Bauer, U. Rothhaar and D. Wolff, Thin Solid Films,
2003, 442, 107–116.
86 L. Martinu and D. Poitras, J. Vac. Sci. Technol., A, 2000, 18, 2619–
2645.
87 W. S. Choi and B. Hong, Renewable Energy, 2008, 33, 226–231.
88 G. A. Neuman, J. Non-Cryst. Solids, 1997, 218, 92–99.
89 S. Kinoshita, S. Yoshioka, Y. Fujii and N. Okamoto, Forma 17,
2002, 17, 103–121.
90 K. Watanabe, T. Hoshino, K. Kanda, Y. Haruyama and S. Matsui,
Jpn. J. Appl. Phys., 2005, 44, L48–L50.
91 S. Koynov, M. S. Brandt and M. Stutzmann, Appl. Phys. Lett., 2006,
88, 203107–203103.
92 C. C. Striemer and P. M. Fauchet, Appl. Phys. Lett., 2002, 81, 2980–
2982.
93 C. Wu, C. H. Crouch, L. Zhao, J. E. Carey, R. Younkin,
J. A. Levinson, E. Mazur, R. M. Farrell, P. Gothoskar and
A. Karger, Appl. Phys. Lett., 2001, 78, 1850–1852.
94 J. D. Fowlkes, A. J. Pedraza and D. H. Lowndes, Appl. Phys. Lett.,
2000, 77, 1629–1631.
95 T.-H. Her, R. J. Finlay, C. Wu, S. Deliwala and E. Mazur, Appl.
Phys. Lett., 1998, 73, 1673–1675.
96 M. Zhao, G. Yin, J. T. Zhu and L. Zhao, Chin. Phys. Lett., 2003, 20,
1789–1791.
97 R. Younkin, J. E. Carey, E. Mazur, J. A. Levinson and C. M. Friend,
J. Appl. Phys., 2003, 93, 2626–2629.
98 Y. Liu, S. Liu, Y. Wang, G. Feng, J. Zhu and L. Zhao, Laser Phys.,
2008, 18, 1148–1152.
99 C. Lee, S. Y. Bae, S. Mobasser and H. Manohara, Nano Lett., 2005,
5, 2438–2442.
100 S. Y. Chou, P. R. Krauss and P. J. Renstrom, Appl. Phys. Lett.,
1995, 67, 3114–3116.
101 K.-S. Han, J.-H. Shin, W.-Y. Yoon and H. Lee, Sol. Energy Mater.
Sol. Cells, 2011, 95, 288–291.
102 K.-S. Han, J.-H. Shin and H. Lee, Sol. Energy Mater. Sol. Cells,
2010, 94, 583–587.
103 C. J. Ting, M. C. Huang, H. Y. Tsai, C. P. Chou and C. C. Fu,
Nanotechnology, 2008, 19, 205301.
104 Y. Kanamori, E. Roy and Y. Chen, Microelectron. Eng., 2005, 78–
79, 287–293.
105 G. J. Swanson, Binary optics technology: the theory and design of
multi-level diffractive optical elements, Tech. Rep. 854, Lincoln
Laboratory, MIT, Lexington, Mass., 1989.
106 C. J. Ting, F. Y. Chang, C. F. Chen and C. P. Chou, J. Micromech.
Microeng., 2008, 18, 075001.
107 G. Y. Xie, G. M. Zhang, F. Lin, J. Zhang, Z. F. Liu and S. C. Mu,
Nanotechnology, 2008, 19.
108 T. K. Gaylord, W. E. Baird and M. G. Moharam, Appl. Opt., 1986,
25, 4562–4567.
109 Y.-F. Huang, S. Chattopadhyay, Y.-J. Jen, C.-Y. Peng, T.-A. Liu,
Y.-K. Hsu, C.-L. Pan, H.-C. Lo, C.-H. Hsu, Y.-H. Chang,
C.-S. Lee, K.-H. Chen and L.-C. Chen, Nat. Nanotechnol., 2007, 2,
770–774.
110 H. Xu, N. Lu, D. Qi, J. Hao, L. Gao, B. Zhang and L. Chi, Small,
2008, 4, 1972–1975.
111 G. Crotty, T. Daud and R. Kachare, J. Appl. Phys., 1987, 61, 3077–
3079.
112 B. S. Richards, J. E. Cotter and C. B. Honsberg, Appl. Phys. Lett.,
2002, 80, 1123–1125.
113 C.-Y. Kuo, Y.-Y. Chen and S.-Y. Lu, ACS Appl. Mater. Interfaces,
2009, 1, 72–75.
114 J. Gao, X. Li, B. Li and Y. Han, Polymer, 2010, 51, 2683–2689.
115 A. Kaless, U. Schulz, P. Munzert and N. Kaiser, Surf. Coat.
Technol., 2005, 200, 58–61.
116 P. Zhan, J. B. Liu, W. Dong, H. Dong, Z. Chen, Z. L. Wang,
Y. Zhang, S. N. Zhu and N. B. Ming, Appl. Phys. Lett., 2005, 86,
051108–051103.
117 C. H. Chiu, P. Yu, H. C. Kuo, C. C. Chen, T. C. Lu, S. C. Wang,
S. H. Hsu, Y. J. Cheng and Y. C. Chang, Opt. Express, 2008, 16,
8748–8754.
118 J. W. Leem, J. S. Yu, Y. M. Song and Y. T. Lee, Sol. Energy Mater.
Sol. Cells, 2011, 95, 669–676.
119 S. L. Diedenhofen, R. E. Algra, E. P. A. M. Bakkers and J. G. Rivas,
Next Generation (Nano) Photonic and Cell Technologies for Solar
Energy Conversion, Proc. SPIE–Int. Soc. Opt. Eng., 2010, 7772,
77720M–77720M.
This journal is ªThe Royal Society of Chemistry 2011 Energy Environ. Sci., 2011, 4, 3779–3804 | 3803
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
120 J. F. DeNatale, P. J. Hood, J. F. Flintoff and A. B. Harker, J. Appl.
Phys., 1992, 71, 1388.
121 K. B. Blodgett and I. Langmuir, Phys. Rev., 1937, 51, 964.
122 S. L. Buckner and V. K. Agarwal, Solar Energy Materials, 12, pp.
131–136.
123 D. C. Harris, Materials for Infrared Windows and Domes:
Properties and Performance, SPIE, 1999.
124 J. A. Hiller, J. D. Mendelsohn and M. F. Rubner, Nat. Mater., 2002,
1, 59–63.
125 E. O. Zayim, Sol. Energy Mater. Sol. Cells, 2005, 87, 695–703.
126 P. Osbond, Adv. Mater., 1992, 4, 807–809.
127 K. Forberich, G. Dennler, M. C. Scharber, K. Hingerl, T. Fromherz
and C. J. Brabec, Thin Solid Films, 2008, 516, 7167–7170.
128 T. Druffel, K. B. Geng and E. Grulke, Nanotechnology, 2006, 17,
3584–3590.
129 M. S. Mirotznik, B. L. Good, D. Wikner and J. N. Mait, Broadband
Antireflective Properties of Inverse Motheye Surfaces, IEEE Trans.
Antennas Propag., 2010, 58(9), 2969.
130 S. Bastide, M. Cuniot, P. Williams, N. Le Quang, D. Sarti and
C. Levy-Clement, 12th EPSECE, Amsterdam, 1994, p. 4A25.
131 K. Grigoras, A. Krotus, V. Pacebutas, J. Kavaliauskas and
I. Simkiene, EMRS Conf Proc., Strasbourg, Elsevier, Amsterdam,
1995.
132 R. A. Arndt, J. F. Allison, J. G. Haynes, and A. Meulenburg Jr.,
Prec. llth IEEE Photovoltaic Specialist Conf. (IEEE New York,
(1975) p. 40.
133 C. R. Baraona and H. W. Brandhorst, Prec. llth IEEE Photovoitaic
Specialist Conf. (IEEE, New York, 1975) p. 44.
134 Prospect Glass ohne Reflexe. Deutsche Spezialglas AG,
Grunenplan, FRG.
135 I. M. Thomas, SPIE Proc. 2114, 1993, 232.
136 I. M. Thomas, Appl. Opt., 1986, 25, 1481.
137 T. Kinoshita, K. Takahashi, T. Yanagisawa, M. Uehara, H. Kimata,
US patent 5446339, Sumitomo Cement Co.
138 B. E. Yoldas, US Patent. No. 4271210, 1981.
139 C. E. Tracy, W. Kern, R. D. Vibronek, US Patent No. 4241108,
1980.
140 B. E. Yoldas, US Patent No. 4361598 and 4346131, 1982.
141 J. Zhao, A. Wang, M. A. Green, in: Proceedings of the Second World
Conference and Exhibition on Photovoltaic Solar Energy Conversion,
1998.
142 O. Shultz, G. Emanuel, S. W. Glunz, G. P. Willeke, in: Proceedings
of the Third World Conference on Photovoltaic Solar Energy
Conversion, 2003, pp. 1360–1363.
143 H. Manohara, NASA Tech Briefs, vol. 28, (no. 11), pp. 62,
November 2004.
144 S. Sinzinger, J. Jahns. (2003). Microoptics, 2nd edn Wiley-VCH,
Weinheim (2003).
145 J. E. Cotter, B. S. Richards, F. Ferrazza, C. B. Honsberg,
T. W. Leong, H. R. Mehrvarz, G. A. Naik, and S. R. Wenham,
Proceedings of the Second World Conference on Photovoltaic
Energy Conversion, Vienna, Austria, 6–10 July 1998, p. 1511.
146 R. M. Swanson, P. J. Verlinden, and R. A. Sinton, US Patent
No.5,907,766, 1999.
147 D. BloorIn: J. Ladik, J. M. Angre and M. Seel, Editors, Quantum
Chemistry of Polymers—Solid State Aspects, Reidel, Dordrecht
(1984).
148 B. Sopori, Silicon nitride processing for control of optical and
electronic properties of silicon solar cells, J. Electron. Mater.,
2003, 32(10), 1034–1042.
149 K. Yasutake, Z. Chen, S. K. Pang and A. Rohatgi, Journal of, J.
Appl. Phys., 1994, 75, 2048.
150 J. W. Lee, R. Ryoo, M. S. Jhon and K. I. Cho, J. Phys. Chem. Solids,
1995, 56, 293.
151 M. Kuo, D. J. Poxson, Y. S. Kim, F. W. Mont, J. K. Kim,
E. F. Schubert and S. Y. Lin, Realization of a near-perfect
antireflection coating for silicon solar energy utilization, Opt. Lett.,
2008, 33(21), 2527–2529.
152 J. N. Hilfiker, R. A. Synowicki, J.A., Spectroscopic ellipsometry
characterization: silicon-based solar cells, Woollam Co., Inc.,
Photovoltaic International’s 2nd Event Supplement.
153 P. Grunow, S. Krauter, Modelling the encapsulation factors for
photovoltaic modules, Proceedings of the 4th World Conference on
Photovoltaic Energy Conversion (Joint congress of IEEE/PVSEC/
EUPVC), Waikoloa, Hawaii (USA), 12 May 2006.
154 O. S. Heavens, Dover Publications Inc. 1991.
155 P. Doshi et al. IEEE Trans. Electron Devices 44, 1997,
p. 1417.
156 R. Ekai., Proc. of the 2nd PV World Conference, Vienna, 1998,
1430.
157 F. J. Pern, Zh. Panosyan, A. A. Gippius, J. A. Kontsevoy,
K. Touryan, S. Voskanyan, Y. Yengibaryan, Diamond-like carbon
coatings as encapsulants for photovoltaic solar cells, Nat.
Renewable Energy Lab., Golden, CO, USA, Photovoltaic
Specialists Conference, 2005. Conference Record of the Thirty-first
IEEE, 3–7 Jan. 2005.
158 http://physicsworld.com/cws/article/news/37711/1/moth.
159 http://academic.udayton.edu/ShirleyWright/SEM/images/Samples/SEM_
MothEye.jpg.
160 http://www.optida.lt/home.php?lng ¼EN&id ¼249.
161 http://www.ultiquestcom.com/products/optics/coating_sar.html.
162 http://spie.org/x16144.xml?ArticleID¼x16144.
163 http://www.csmonitor.com/Science/2010/0525/How-a-moth-s-eye-
inspires-glare-free-TV-screens.
164 http://commons.wikimedia.org/wiki/File:SolGel_SpinCoating.jpg.
165 http://www.scielo.org.ar/scielo.php?pid¼S0327-
07932002000300006&script¼sci_arttext.
166 http://www.crystec.com/triarce.htm.
167 http://mazur-www.harvard.edu/sentFiles/MazurTalk_382.pdf.
168 http://www.allaboutvision.com/lenses/coatings.htm.
169 http://www.eetimes.com/electronics-news/4084575/Graphene-mixes-
it-up-with-GaAs.
170 http://www.crystran.co.uk/optics-definitions.htm.
171 http://en.wikipedia.org/wiki/Solar_cell.
3804 | Energy Environ. Sci., 2011, 4, 3779–3804 This journal is ªThe Royal Society of Chemistry 2011
Downloaded by State University of New York at Buffalo on 13 March 2013
Published on 05 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01297E
View Article Online
... To lower the reflection at the front side of the solar cell, the surface is textured [21] and coated with an anti-reflection coating (ARC) [22]. The refractive indices of the silicon and the surrounding medium (e.g.: air) are important, since they determine the optimum refractive index of the anti-reflection coating as stated in equation (2) [23]. ...
... 22: EDS line scan across a TEM lamella cut through the interface between the sputtered ITO contact layer and the poly-silicon. ...
... 22: Scatter plot of the connecting resistivity for p-and n-type bonded silicon test structures. In both sample structures, the bond-silicon interfaces were coated with a sputtered ITO contact layer. ...
Thesis
This work describes the development of a new ZnO-based adhesive for the application as interconnection layer for III V/silicon tandem solar cells. Tandem solar cells are a new generation of solar cells that allow photoconversion efficiencies beyond the theoretical limit of single junction solar cells. Within this work, III V/silicon tandem solar cells interconnected in series (resulting in a two-terminal device) are investigated. Options for the interconnection of the III V and silicon solar cells include direct wafer bonding and the direct growth, with both requiring sophisticated processing conditions as particle-free and smooth surfaces. Another option for the interconnection of III V top cells and silicon bottom solar cell is the implementation of a transparent conductive adhesive. All approaches based on transparent conductive adhesives for the interconnection of tandem solar cells published so far focus on the combination of non-conductive, polymer-based adhesives with embedded conductive particles. This work, however, describes the development of a ZnO-based adhesive (precursor: Zn acetylacetonate dissolved in methanol) where the transparent adhesive matrix itself becomes electrically conductive upon thermal annealing at a max. temperature of 290°C. By adding 3 at% of indium, the ZnO is doped to form the transparent conductive oxide (TCO) ZnO:In. The first attempts at interconnecting test structures showed that TCO contact layers deposited at the adhesive/subcell interfaces prior to the deposition of the adhesive significantly improved the conductivity of the formed interconnection. Both sprayed (ZnO:In) and sputtered (In2O3:Sn) TCOs were investigated and the differences in contact formation to the utilized semiconductor materials were analyzed with regard to the deposition process by analyzing the TCO/semiconductor interface with Transmission Electron Microscopy. The sprayed ZnO:In layer was further characterized for it’s crystalline structure by Transmission Kikuchi Diffraction to characterize the deposition process. The conductivity of the ZnO-based adhesive interconnection was analyzed in dependence of the process parameters (applied pressure/temperature) and a correlation between the bond‘s homogeneity and the connecting resistivity was found. The structure and homogeneity of the resulting bond was analyzed by Scanning Acoustic Microscopy (SAM). For a more detailed characterization of the bond layer, cross sections were analyzed by Scanning Electron Microscopy (SEM). The homogeneity of the bond was found to be mainly compromised by the outgassing from the adhesive layer during the annealing, as this leads to embedded gas within the bond layer and thus non-bonded areas. To improve the bond‘s homogeneity, and thereby the conductivity, the pressing program was adapted to match the stages of thermal decomposition of the precursors, resulting in a significantly improved homogeneity in bonded test structures. The lowest measured connecting resistivity of the developed ZnO-based adhesive (measured in planar silicon-silicon test structures) was 0.073 Ωcm². Such a low connecting resistivity will not limit a glued tandem solar cell (dual-junction or triple-junction, under 1 sun illumination). The optical effect of the ZnO-based adhesive interconnection on a glued III V/silicon tandem solar cell was investigated by both bonded test structures and optical simulations. An optical simulation of the bond showed a weighted reflectance at the bond layer of 16.5 % if sputtered In2O3:Sn contact layers are implemented. It was found that the anti-reflective effect of the contact layers is highest if the refractive index of the contact layer is n ≈ 2.5, which can be achieved by TiO2-based contact layers. Another option for reducing the reflectance at the bond is a textured silicon surface (random pyramid texture) as the optical simulation showed a further reduction of the weighted reflectance down to 10.6 %. A bonded test structure featuring one textured surface resulted in a connecting resistivity of 0.083 Ωcm², which is in the same range as the bonded planar reference. For the integration of the developed ZnO-based adhesive into III V/silicon tandem solar cells, it was found that the gluing process needed to be adjusted because of the difference in coefficient of thermal expansion between Si and GaAs. A first connecting step at a low temperature was complemented with a second step at high temperature, which was only possible at lower pressure without risking a breakage of the III-V material. The adjusted process resulted in a first working tandem solar cell which demonstrates the proof-of-concept for the developed ZnO-based adhesive. The glued tandem device was mainly limited by a low short-circuit current (non-optimized optics) and low fill factor. The latter is limited by the connecting resistivity of the bond, as the optimized pressing process could not be used in the cells. The voltage of the obtained device is close to the one reported for record wafer bonded devices, close to 3V, which indicates that the developed gluing process does not degrade the individual solar cells. The reflectance at the bond layer, even for In2O3:Sn-based contact layers and a textured silicon surface, would still limit tandem solar cells interconnected by the developed ZnO-based adhesive. To reduce this optical loss, a contact layer with a high refractive index (TiO2:Nb, n ≈ 2.5) was investigated for it’s application in III V/silicon tandem solar cells and found to be suitable. A simulation of the optical effect in a glued device showed a reduction in reflectance caused at the bond layer down to < 2 %, which should no longer limit the potential of a glued tandem solar cell.
... Traditionally, the color filters are made of multiple transparent dielectric thin films with quarter-wavelength thickness and different refractive indices, like SiO 2 , Al 2 O 3 [37,50]. However, such Fabry-Perot-type Bragg coatings are rather thick, usually with thicknesses up to several micrometers (Table 1). ...
Article
Full-text available
We design, fabricate, optically and mechanically characterize wearable ultrathin coatings on various substrates, including sapphire, glass and silicon wafer. Extremely hard ceramic materials titanium nitride (TiN), aluminium nitride (AlN), and titanium aluminium nitride (TiAlN) are employed as reflective, isolated and absorptive coating layer, respectively. Two types of coatings have been demonstrated. First, we deposit TiAlN after TiN on various substrates (TiAlN-TiN, total thicknesses <100 nm), achieving vivid and viewing-angle independent surface colors. The colors can be tuned by varying the thickness of TiAlN layer. The wear resistance of the colorful ultrathin optical coatings is verified by scratch tests. The Mohs hardness of commonly used surface coloring made of Si-/Ge-metals on substrates is <2.5, as soft as fingernail. However, the Mohs hardness of our TiAlN-TiN on substrates is evaulated to be 7-9, harder than quartz. Second, Fano-resonant optical coating (FROC), which can transmit and reflect the same color as a beam split filter is also obtained by successively coating TiAlN-TiN-AlN-TiN (four-layer film with a total thickness of 130 nm) on transparent substrates. The FROC coating is as hard as glass. Such wearable and color-tunable thin-film structural colors and filters may be attractive for many practical applications such as sunglasses.
... magnetron sputtering [4], and high refractive index titania thin films on polycarbonate (PC) substrates by ion beam assisted evaporation [5]. High refractive index thin films may provide plastics surface both with high reflective and anti-reflective performances, the latter of which is realized by laminating with low refractive index films [6]. High refractive index thin films can also modify plastics surface with a variety of interference colors through their thickness control [7]. ...
Article
Full-text available
In order to obtain the information of the fundamental properties of alkoxide-derived titania gel films on plastics, we prepared the films on polycarbonate (PC) substrates by spin-coating, dried them at 120 °C for 24 h, and evaluated their refractive index, uncracking critical thickness, flexibility, chemical durability and adhesion to the substrate. First, dried gel films were prepared from solutions of various types of alkoxides, i.e., from Ti(OR)4-HNO3-H2O-ROH solutions (R = C2H5, C3H7ⁱ, or C4H9ⁿ) with or without CH3COOH as a chelating agent (CA). Among them, those prepared from Ti(OC3H7ⁱ)4 had higher refractive indices than the others. Next, dried gel films were prepared from Ti(OC3H7ⁱ)4-HNO3-H2O-(C2H5OH or C3H7ⁱOH) with or without CA, where HCOOH, CH3COOH, CH3COCH2COCH3 were employed as CA. The addition of CA in solutions tended to lower the refractive index of the dried gel films. The uncracking critical thickness was ca. 30–80 nm, with larger critical thickness given by the addition of CA in solutions, which was attributed to the lower degrees of the increase in in-plane tensile stress during heating. The critical thickness on PC substrates was found to be smaller than that on Si(100) substrates (ca. 100–150 nm) possibly due to the larger thermal expansion coefficient of PC than Si(100). Cross-cut adhesion test showed that all these Ti(OC3H7ⁱ)4-derived dried gel films have excellent adhesion to PC substrates. Chemical durability was evaluated on these dried films by soaking them in water and ethanol at room temperature for 30 min. No significant changes in thickness or refractive index on soaking were detected under such a limited test condition. These dried films on PC substrates were shown to be bent without cracking until the radius of curvature decreased down to about 16 mm. Graphical abstract
... Besides attaining optimal light absorbance, light collection efficiency is another significant factor influencing the efficiency of photodetector performance. To this end, thin films have previously been adopted to improve carrier collection efficiency [9]. However, film thickness was found to be a critical parameter. ...
Article
Full-text available
Light reflectance mitigation is the most crucial factor for achieving optimal photodetector performance. In this respect, light-trapping mechanisms based on nanostructures or microstructures such as nanopillars, nanocones and nanopyramids have emerged as the most promising candidate for reducing overall light reflectance. This is because of their large effective irradiation area, multiple scattering of incident light and increased path length of incident rays in these nanostructures. This paper proposes an optical model of a GaAs/GaSb material-based vertically oriented core–shell cone-topped octagonal nanopillar structure with periodical trapezoidal nanotexturization over it to be deployed on a circular planar detector surface with a radius of 50 μm. The geometric analytical investigation of the proposed model reveals 0.999 overall absorbance, 0.995A/W photoresponsivity, and 87% EQE at 1 μm operating wavelength.
... The high-aspect-ratio structure arrays allow the insects to avoid tracking by predators. Inspired by their camouflage characteristics, artificial conical structure arrays, pillar-shaped structure arrays, pyramid-like structure arrays, and so on, have been developed via a large variety of top-down fabrication approaches to function as antireflective structures [15][16][17]. Nevertheless, current lithography-based technologies, such as interference lithography and photolithography, are costly, complex, and restricted to either a limited sample size or low resolution features. ...
Article
Full-text available
Most bio-inspired antireflective nanostructures are extremely vulnerable and suffer from complicated lithography-based fabrication procedures. To address the issues, we report a scalable and simple non-lithography-based approach to engineer robust antireflective structures, inspired by the longtail glasswing butterfly, in a single step. The resulting two-dimensional randomly arranged 80/130/180 nm silica colloids, partially embedded in a polymeric matrix, generate a gradual refractive index transition at the air/substrate interface to suppress light reflection. Importantly, the randomly arranged subwavelength silica colloids display even better antireflection performance for large incident angles than that of two-dimensional non-close-packed silica colloidal crystals. The biomimetic coating is of considerable technological importance in numerous practical applications.
... On the other hand, the optical reflection at the 532 nm wavelength decreases with increasing the gap size, as shown in Fig. S5 in the ESM. This is due to the reduction of the effective refractive index of the structure, which induces better impedance matching between the air and the substrate [52]. More free-space light can couple into surface plasmons to generate stronger hot spots [53]. ...
Article
Surface-enhanced Raman spectroscopy (SERS) is a powerful and promising analytical technique for fast, non-destructive, and sensitive analysis of trace analytes. However, the serious matrix interference effect of complex sample is a bottleneck that limited rapid SERS analysis. Therefore, it is essential to introduce proper sample preparation techniques to address the limitation for advancing SERS analysis. With adaptable sample preparation techniques, target substances could be separated and purified from complex matrices, matrix interference effect could be eliminated/reduced, and finally pure SERS spectra could be obtained. However, the process of traditional sample preparation usually requires the tedious and time-consuming operation, large solvent consumption, high labor intensity, as well as low efficiency. Given that SERS is an ultrafast analytical technique, there is a great desire to develop advanced/modern sample preparation techniques for fast and efficient analysis of complex samples. To attract much attention and raise research enthusiasm on the beginning, recent development on advanced sample preparation techniques for rapid SERS analysis of complex sample was reviewed. (1) ‘All-in-one’ strategy for simultaneous separation, enrichment, and in-situ SERS detection. (2) Integrated strategy for high-speed pretreatment and high throughput analysis. (3) Derivatization strategy switches on the SERS activity of molecules with weak responses. (4) Field-assisted strategy for preconcentration acceleration. (5) Instrument combination strategy for online processing and real-time SERS analysis. Finally, conclusions and perspectives on future development were briefly discussed.
... On the contrary, the transmittance has increased with the silica nanoparticles coating, which indicates its antireflective characteristic. This can be explicated through a single-layered anti-reflection film system [28], in which coating material with a lower refractive index than substrate is preferable. In our study, the silica film is porous and could be treated as a composite of silica and air [29]. ...
Article
Mechanical robustness is required for a wide range of protective coatings for their long-term usage. It is particularly crucial for the coatings on polymeric substrates to maintain the surface free of contamination such as dust and fingerprint. Here, we have developed an effective fabrication method to prepare amphiphobic anti-fingerprint coating on polycarbonate (PC) substrates. Thin films with porous silicon dioxide (SiO2) nanoparticles were deposited via pulse laser deposition on plasma pre-treated PC substrate. After the coating is applied, trichloro(1H,1H,2H,2H-perfluorooctyl) silane (PFTS) was assembled on the silica surface. The as-prepared PFTS-treated silica coating is superhydrophobic with water contact angle at 155.2 ± 1.8 ° and highly oleophobic with a contact angle of 133.6 ± 3.3 ° with diiodomethane. The coating also displays excellent adhesion with the PC substrate. The developed room temperature deposition process makes it potentially applicable for other low melting point polymeric substrates towards a wider range of functional surface applications.
... Therefore, one of the top priorities for solar cell researchers around the world is to increase the efficiency and reduce the cost of silicon-based solar cells. These goals are achieved by several ways, but the most effective are: optimization of antireflection and passivation coatings [1][2][3]. ...
Article
In this paper, the optimal thickness of SiC film for an antireflection coating was determined by computer simulations using Lumerical FTDT and SCOUT software. The simulation was carried out for the SiC/MgF2 system, where silicon carbide films were deposited at a magnetron power of 100, 150, 200, 250 W, while the thickness of the magnesium fluoride films remained unchanged and amounted to 130 nm. The simulation results showed that the optimal parameters for the synthesis of SiC antireflection layer are 100 W/50 nm. With these parameters, the reflection is less than 3% in the widest wavelength range of 475–1020 nm. The dependence of the physical properties of the synthesized films on the power of the magnetron is investigated. Using reflection and transmission spectroscopy it was experimentally revealed that a decrease in the magnetron power from 250 to 100 W leads to a decrease in the refractive index. According to our results the best antireflection effect can be achieved with SiC/MgF2 coatings when SiC films are deposited at 100 W magnetron power. The reflectance spectra are consistent with the simulation spectra, especially in the 475–1020 nm range, where the surface reflects only 0.2–3.0% of the incident light. The obtained results are explained by the correlation between the structural properties, composition of amorphous silicon carbide films and antireflection properties.
Article
To effectively improve the power conversion efficiency (PCE) of Si solar cells, vibration-assisted UV nanoimprint lithography based on piezoelectric driving is proposed to fabricate grating on Si solar cells. By applying piezoelectric vibration under the photoresist layer, vibration causes micro-displacement and micro-impact force, increasing the contact area between the photoresist and the grating's side wall and reducing surface tension. Meanwhile, the photoresist filling rate is increased by 25%. The effect of grating parameters on reflection is determined using Finite Difference Time Domain (FDTD), and grating with the best optical performance is optimized. In addition, the rationality of vibration introduction is verified by establishing a motion model, the influence of frequency and amplitude on the filling rate is analyzed using the finite element method, and a reasonable range of vibration parameters required by the experiment is obtained. Vibration-assisted UV nanoimprint is used to fabricate the periodic grating structures. PCE is increased by 25% when compared to bare Si solar cells. PCE of grating fabricated by vibration-assisted UV nanoimprint is increased by 4% when compared to traditional nanoimprint. The results show that vibration-assisted nanoimprint can efficiently and accurately fabricate periodic grating structures, resulting in improved Si solar cells performance.
Article
Full-text available
The rapid anthropomorphic emission of greenhouse gases is contributing to global climate change, resulting in the increased frequency of extreme weather events, including unexpected snow, frost, and ice accretion in warmer regions that typically do not encounter these conditions. Adverse weather events create challenges for energy systems such as wind turbines and photovoltaics. To maintain energy efficiently and operational fidelity, snow, frost, and ice need to be removed efficiently and rapidly. State‐of‐the‐art removal methods are energy‐intensive (energy density > 30 J cm⁻²) and slow (>1 min). Here, pulsed Joule heating is developed on transparent self‐cleaning interfaces, demonstrating interfacial desnowing, defrosting, and deicing with energy efficiency (energy density < 10 J cm⁻²) and rapidity (≈1 s) beyond what is currently available. The transparency and self‐cleaning are tailored to remove both snow and dust while ensuring minimal interference with optical light absorption. It is experimentally demonstrated a multi‐functional coating material on a commercial photovoltaic cell, demonstrating efficient energy generation recovery and rapid ice/snow removal with minimal energy consumption. Through the elimination of accretion, this technology can potentially widen the applicability of photovoltaics and wind technologies to globally promising locations, potentially further reducing greenhouse gas emissions and global climate change.
Book
Written by a world-renowned authority of optical coatings, Thin-Film Optical Filters, Fourth Edition presents an introduction to thin-film optical filters for both manufacturers and users. The preeminent author covers an assortment of design, manufacture, performance, and application topics. He also includes enough of the basic mathematics of optical thin films to enable readers to carry out thin-film calculations. This new edition of a bestseller retains most of the descriptions of older design techniques because of their importance in understanding how designs work. However, this edition includes a substantial amount of new material as well. A new chapter on color takes into account the increasing importance of color in optical coatings. In addition, a new section discusses the effects of gain in optical coatings. This comprehensive yet accessible book continues to offer valuable insight into the principles, techniques, and processes of successful coating design. It provides the sound foundation required to make further advances in the field.
Conference Paper
This paper discusses the various attempts to obtain the efficiencies of ideal solar cell by thermodynamic methods. The different methods that are discussed include the Landsberg efficiency which is obtainable from a simple model depicting the solar cell as a heat engine. The Shockley-Queisser detailed balance methods is also outlined briefly. We show that the Shockley solar cell equation implies different types of losses (reversible and irreversible) in different parts of the I-V characteristic. The application of thermodynamics to photovoltaic energy conversion includes also the open circuit voltage which is copared with experiment.
Article
What do a South American butterfly, an Australian banknote, a European moth and an international credit card have in common? The answer is they all use sub-micron grating microstructures as sophisticated optical components to achieve unique visual effects which are difficult to realize with more conventional optics. There is, however, a basic difference between these diffractive optical elements – the visual effect of the credit card hologram or the banknote diffraction grating is viewed in the first diffraction order, whereas the butterfly and moth structures are so fine that there is no first order under normal viewing conditions and the optical behaviour is seen primarily in the zero order (figure 1).
Article
The reflectance losses on alkali-borosilicate glasses can be drastically reduced by exposing heat-treated glasses to dilute acid solutions containing low concentrations of etchants, such as ammonium fluoride or ammonium bifluoride. The resulting surface layers are unique in that they maintain low reflectance losses not only in the visible but also in the near ir region of the spectrum. This characteristic is attributed to the formation of a porous, silica-rich graded refractive index film on the glass. 16 refs.
Article
In 1880, by studying light passing through Earth's atmosphere, Lord Rayleigh mathematically demonstrated that graded-refractive-index layers have broadband antireflection characteristics1. Graded-index coatings with different index profiles have been investigated for broadband antireflection properties, particularly with air as the ambient medium2, 3, 4. However, because of the unavailability of optical materials with very low refractive indices that closely match the refractive index of air, such broadband antireflection coatings have not been realizable. Here we report the fabrication of TiO2 and SiO2 graded-index films deposited by oblique-angle deposition, and, for the first time, we demonstrate their potential for antireflection coatings by virtually eliminating Fresnel reflection from an AlN–air interface over a broad range of wavelengths. This is achieved by controlling the refractive index of the TiO2 and SiO2 nanorod layers, down to a minimum value of n = 1.05 in the case of the latter, the lowest value so far reported.
Article
During the process of dip-coating, the substrate is withdrawn from the sol at a constant rate. After several seconds, the process becomes steady. The entrained film thins by evaporation of solvent and gravitational draining. Because the shape of the depositing film remains constant with respect to the reservoir surface, it is possible to use analytical methods such as ellipsometry and fluorescence spectroscopy to characterize the depositing film in situ. The microstructure and properties of the film depend on the size and structure of the inorganic sol species, the magnitude of the capillary pressure exerted during drying, and the relative rates of condensation and drying. By controlling these parameters, it is possible to vary the porosity of the film over a wide range.