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shadow loss, and the sheet resistance and absorption losses
associated with planar layers that facilitate lateral carrier trans-
port to the grid fi
ngers.
[ 22,23 ]
For high effi
ciency silicon hetero-
junction (HIT) solar cells, contact design requires a trade-off
between grid fi
nger resistance and the sheet resistance and
transmission losses of the transparent conducting oxide (TCO)/
amorphous silicon structures coating the cell front surface.
[ 24 ]
In this paper, we describe a new front contact design prin-
ciple that overcomes both shadowing losses and parasitic
absorption without reducing the conductivity. By redirecting
the scattered light incident on the front contact to the solar cell
active absorber layer surface, micrometer-scale triangular cross-
section grid fi
ngers can perform as effectively transparent and
highly conductive front contacts. Previously, researchers have
designed light harvesting strings that serve to obliquely refl
ect
light, which is then redirected into the cell by total internal
refl
ection from the encapsulation layers.
[ 16 ]
By contrast our front
contact design does not require total internal refl
ection at the
encapsulation layer. Furthermore in our design, the contact fi
n-
gers are micrometer sized and can be placed very close together
such that a TCO with reduced thickness can be used—and in
some cases the TCO layer might possibly be omitted completely.
We demonstrate with simulations and experimental results that
designs utilizing effectively transparent triangular cross-section
grid fi
ngers rather than conventional front contacts have the
potential to provide 99.86% optical transparency while ensuring
effi
cient lateral transport corresponding to a sheet resistance
of 4.8
Ω
sq
−
1
due to their close spacing of only 40 μm. Thus
effectively transparent contacts have potential as replacements
for both the front grid and TCO layer used, e.g., in HIT solar
cells. While related schemes for contacts were envisioned early
in the development of photovoltaics technology,
[ 25 ]
they have
not found application in current photovoltaic technology, which
is increasingly dominated by high effi
ciency silicon photovol-
taics. Moreover, the effectively transparent front contact design
is conceptually quite general and applicable to almost any other
front-contacted solar cell or optoelectronic device. For example,
we obtained similar experimental results when applying our
structures to InGaP-based solar cells.
Figure
1
a,b shows the steady-state electric fi
eld magnitude
distribution of a freestanding triangular contact and a fl
at con-
tact, respectively, with 550 nm monochromatic plane wave illu-
mination normally incident at the top of the simulation cell. For
planar grid fi
ngers, part of the incident light is refl
ected back
toward the incidence direction, as is apparent from the high
electric fi
eld density above the contact plane. By contrast, the
triangular cross-section grid fi
nger does not exhibit a similar
back refl
ection, as indicated by the lack of an increased electric
Effectively Transparent Front Contacts for Optoelectronic
Devices
Rebecca Saive , Aleca M. Borsuk , Hal S. Emmer , Colton R. Bukowsky ,
John V. Lloyd ,
Sisir Yalamanchili , and Harry A. Atwater *
Dr. R. Saive, A. M. Borsuk, Dr. H. S. Emmer,
C. R. Bukowsky, J. V. Lloyd, S. Yalamanchili,
Prof. H. A. Atwater
Department of Applied Physics
and Materials Science
California Institute of Technology
Pasadena , CA 91125 , USA
E-mail: haa@caltech.edu
DOI: 10.1002/adom.201600252
Optoelectronic devices such as light emitting diodes, photodi-
odes, and solar cells play an important and expanding role in
modern technology. Photovoltaics is one of the largest opto-
electronic industry sectors and an ever-increasing component
of the world’s rapidly growing renewable carbon-free electricity
generation infrastructure. In recent years, the photovoltaics
fi
eld has dramatically expanded owing to the large-scale man-
ufacture of inexpensive crystalline Si and thin fi
lm cells and
modules. The current record effi
ciency (
η
=
25.6%) Si solar cell
utilizes a heterostructure intrinsic thin layer (HIT) design
[ 1 ]
to
enable increased open circuit voltage, while more mass-man-
ufacturable solar cell architectures feature front contacts.
[ 2,3 ]
Thus improved solar cell front contact designs are important
for future large-scale photovoltaics with even higher effi
ciency.
Improving the state of the art for optoelectronic device tech-
nology requires optimal photon management. For example,
in conventional solar cells or photodiodes with front and
rear contacts, a nonnegligible fraction of the incoming solar
power is immediately lost at the front contact either through
absorption, as in the case of transparent conductive oxides or
through refl
ection at contact grid fi
ngers. As a result, many
groups have recently proposed design schemes to mitigate
front contact losses, such as less absorbing transparent con-
ductive oxides,
[ 4–8 ]
or less refl
ective metal contacts such as
nanowire grids,
[ 9,10 ]
fractal contacts,
[ 11 ]
contacts with different
shapes,
[ 12–16 ]
and various other approaches.
[ 17–21 ]
Very high
contact transparency usually comes at the expense of reduced
conductivity, which in turn leads to series resistance and device
electrical losses. A comparison of the photonic designs for
different recently developed contacts is shown in Section S3
(Supporting Information).
For any fl
at plate solar cell, the front contact design process
involves a balance of the grid fi
nger resistance, grid fi
nger
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This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial License , which permits use, distribution
and reproduction in any medium, provided the original work is properly
cited and is not used for commercial purposes.
The copyright line of this paper was changed 1 August 2016 after initial
publication.
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fi
eld density in the incidence direction. However an enhance-
ment of the electric fi
eld is seen in the forward scattering direc-
tion, behind the contact, explaining its effective transparency.
Figure 1 c,d is cross-sections from 3D confocal scanning
microscopy measurements of a fl
at grid line grid fi
nger and
triangular cross-section grid fi
nger, respectively, on a Si het-
erojunction solar cell. The focused confocal illumination
laser was scanned in
x
-,
y
-, and
z
-direction and the displayed
images show a cross-section of the signal at constant
y
-value.
A dashed black line in each image marks the solar cell sur-
face. In Figure 1 c it can be seen that in the vicinity of the fl
at
contact (dashed black rectangle), the refl
ection signal is much
stronger than at the antirefl
ection-coated solar cell surface. In
Figure 1 d the position of the triangular grid fi
nger is marked by
a dashed white triangle. Along the contact sidewalls, it appears
black suggesting that there is no refl
ection back to the incident
light source from the sidewalls. Only the tip shows some refl
ec-
tion, which can be attributed to fi
nite radius of curvature of
the tip, as confi
rmed by optical simulations. Simulations and
experiments indicate that triangular cross-section grid fi
ngers
outperform fl
at grid fi
ngers between 0
°
and 55
°
incident angle
in a plane perpendicular to the grid fi
nger length. The perfor-
mance of the triangular cross-section grid fi
ngers is not altered
if the incident angle of the light is varied in a plane parallel to
the contact lines. Triangular grid fi
ngers show superior perfor-
mance for wavelengths between 250 and 1400 nm. Simulations
and experiments supporting these conclusions can be found in
the Section S1 (Supporting Information). In the simulations as
well as in the experiments, the grid fi
ngers were 2.5 μm wide
and the triangular cross-section grid fi
ngers were 7.0 μm high.
The period of the pattern was 40 μm.
Figure
2
shows spatially resolved measurements of refl
ection
(a and b) and the photocurrent (c and d) of an area spanned by
fl
at grid fi
ngers (a and c) and by triangular cross-section grid
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Figure 1.
Simulated steady-state electric fi eld magnitude distribution at 550 nm incident light for a free-standing a) fl
at contact and b) triangular
cross-section contact. Cross-sectional profi les from 3D confocal scanning microscope image of c) fl
at contact and d) triangular cross-section contact.
Figure 2.
Spatially resolved refl
ection of a) fl
at lines and b) effectively transparent triangular cross-section contacts and the corresponding spatially
resolved photocurrent for c) fl
at lines and d) effectively transparent triangular cross-section contacts determined by laser beam induced photocurrent
measurements at a wavelength of 543 nm. e) Line-scan profi les of the photocurrent taken across fl
at contact lines and across lines with triangular
cross-section. Photocurrent and refl
ection profi les taken over one f ) fl
at and g) triangular cross-section line.
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fi
ngers (b and d) on the same solar cell. In Figure 2 a, the dark
regions correspond to the antirefl
ection-coated cell substrate
while the bright regions correspond to fl
at silver contacts. In
Figure 2 b, lines with triangular cross-section are aligned with
and cover the silver grid fi
ngers in a different area on the same
cell. It can be seen that the triangular cross-section grid fi
ngers
appear much darker than the fl
at line grid fi
ngers, in some
regions exhibiting no measurable refl
ection. This has direct
infl
uence on the measured photocurrent. As can be seen in
Figure 2 c, the orange color represents the photocurrent meas-
ured in the areas between grid fi
ngers, while the green color
corresponds to the grid fi
ngers, illustrating that there is very
little photocurrent generated at the position of the fl
at contact
lines.
Figure 2 d illustrates the photocurrent in the vicinity of the
triangular cross-section grid fi
ngers. The photocurrent at the
position of the triangular cross-section grid
fi
ngers is relatively higher as seen by the
yellow color, while the photocurrent between
grid fi
ngers is the same as in Figure 2 b. The
difference in photocurrent collection near
the grid fi
ngers becomes very apparent when
comparing line-scan profi
les of the photocur-
rent taken across fl
at grid fi
ngers and across
grid fi
ngers with triangular cross-section, as
shown in Figure 2 e. Figure 2 f,g shows higher
resolution profi
les, plotted with similar
y
-
axis scales, over one fl
at and one triangular
cross-section line, respectively. We note that
in light beam induced current (LBIC) meas-
urements, the position of the laser light is
detected rather than the actual location of
the current generation. Thus a photocurrent
enhancement is visible at the position of the
triangular cross-section grid fi
nger although
the current is generated next to the grid
fi
nger.
Averaging generation photocurrent density
over the area shown in Figure 2 c indicates a
grid fi
nger transparency of 96.67% normal-
ized to the open regions between fi
ngers.
This value is higher than one would expect
from the areal surface coverage (
≈
6%) since
the 100 nm Ag thin fi
lm grid fi
ngers are thin
enough to be partially transmitting (as can
be seen by the nonzero photocurrent signal
at the position of the fl
at lines). (Compare
Section S4, Supporting Information) Aver-
aging over the area in Figure 2 d indicates a
grid fi
nger transparency of 99.74% while the
most transparent regions within this area
even reach 99.86% contact transparency. The
fabrication process was repeated numerous
times, and other fabricated samples were
found to exhibit slightly lower transparency,
but always well above 99%.
Calculations for an indium tin oxide
(ITO)-free HIT solar cell featuring closely
spaced triangular cross-section grid fi
ngers
with 40 μm period predict a series resistance in lateral trans-
port of
≈
1.0
Ω
cm
2
(Section S5,
[ 26 ]
Supporting Information).
This, for example, implies the possibility of a HIT cell fabrica-
tion process without any ITO layer needed. For other types of
solar cells with higher sheet resistance, the grid fi
ngers can in
principle be spaced even closer together without deteriorating
the transparency (Section S2, Supporting Information). The
40 μm period grid fi
ngers provide measured conductivity that
is similar to a homogeneous material
[ 10 ]
with a sheet resistance
of 4.8
Ω
sq
−
1
.
Prototype triangular cross-section grid fi
ngers were fabri-
cated using 3D writing by two-photon lithography (scanning
electron microscopy (SEM) image shown in
Figure
3
a), and
these prototypes were used as master samples for a gravure
printing process (see the Experimental Section). An example
of a structure printed with silica sol–gel is shown in Figure 3 b
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Figure 3.
Scanning electron microscope images of a) a triangle structure, prepared by two-
photon lithography, that acted as a master for the stamp used to print the triangular cross-
section structures shown in (b) and (c). Replicas were printed with b) silica sol–gel and
c) silver paste. Panels (d) and (e) illustrate conducting triangular cross-section lines formed via
an ink capillary fl
ow process; f ) Sol–gel replica formed on a commercial texture-etched silicon
solar cell. g) Master sample formation via triangular cross-section structure directly etched into
silicon. Images (a)–(f ) are taken under a 45
°
angle, (g) was taken under a 90
°
angle.
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and a silver paste replica is shown in Figure 3 c. In both cases
even the sidewall texture was reproduced. We have successfully
printed over areas up to 8 mm
×
8 mm using this technique.
If printing is performed with a conductive ink, the printed
structures could be used for current transport throughout the
whole triangular cross-sectional conductor, leading to very low
sheet resistance. However, silver residues on the substrate
as can be seen in Figure 3 c pose a challenge to this proposi-
tion. We were able to overcome this issue by use of a modifi
ed
stamping process in which the stamp is fi
rst applied to the sub-
strate and then loaded with silver ink via a capillary fl
ow pro-
cess (see method section). Using this procedure, we obtained
residue-free triangular grid fi
ngers composed of conductive
ink, as seen in Figure 3 d,e. Figure 3 f illustrates that contact
printing can be performed on a textured solar cell front surface;
a replica printed on a structured silicon substrate illustrates the
potential for implementation on cell surfaces similar to those
used in silicon solar cell manufacturing.
Another approach to triangular cross-section grid fi
nger
master fabrication is via directional dry etching. The SEM
image in Figure 2 g illustrates the high aspect ratio lines with
triangular cross-section formed by direct etching into silicon.
These high aspect ratio silicon structures could be candidates
for large-area master stamps for a large-scale gravure printing
process for effectively transparent contacts.
We have demonstrated a design for effectively transparent,
highly conductive front contacts for optoelectronic devices. Our
design is applicable to many types of devices, including, e.g.,
texture-etched silicon solar cells. Spatially resolved photocur-
rent measurements at normal incidence show transparency
of up to 99.86% in prototype structures, which exhibited a low
sheet resistance of 4.8
Ω
sq
−
1
. We have also demonstrated a
fi
rst feasibility step toward large-scale fabrication by gravure
printing of contacts using conductive inks.
Experimental Section
The contact design process began with optical simulations that were
performed by 2D rigorous coupled wave analysis simulations using
RSoft DiffractMOD software. Fabrication of heterojunction with intrinsic
thin layer (HIT) solar cells has been detailed in ref. [ 27 ] . For our
measurements, contacts to HIT cells were fabricated with only 18 nm
thin ITO layer to ensure high optical transmission while providing good
electrical contact to amorphous silicon. Note that this layer can be
thinned or removed if ohmic contact to the top layer is provided.
Prototype samples were prepared by fi rst lithographically defi ning
a fl
at aluminum fi nger grid with 2.5 μm width and 40 μm period on
planar HIT solar cells. An antirefl
ection coating of 50 nm TiO
2
and
100 nm SiO
2
was deposited on top by electron beam evaporation. 3D
two-photon lithography was then used to prepare triangular shaped
grid fi ngers with 2.5 μm width and 7 μm height. We used a Nanoscribe
Photonic Professional GT which operates at a wavelength of 780 nm
and the photoresist IP-Dip (by Nanoscribe). The writing was performed
in piezo mode leading to a voxel width of around 215 nm. A scanning
electron microscope image of such a structure is shown in Figure 3 a.
The triangular shaped lines were coated with silver by evaporation
under an angle such that only the sidewalls of triangular shaped grid
fi ngers became metalized, while the active cell surface remained free
of metal. Measurements presented in this paper were performed
on these prototype structures. Note, that even without metalization
a transparency of 99% was achieved. In this confi guration, the fl
at
fi nger grid geometry determines the sheet resistance. Calculating
the equivalent sheet resistance
[ 10 ]
for the geometry of the fabricated
samples (silver lines with 2.5 μm width, 100 nm thickness, and
40 μm distance) leads to 2.6
Ω
sq
−
1
along the direction of the fi ngers.
The experimental sheet resistance was determined from four-point
measurements of grids with known geometry that were contacted using
bus bars on opposite sides of the grids. We measured a higher value
(4.8
Ω
sq
−
1
), as our fabricated lines were not perfectly homogeneous
and uniform in width. In general, the equivalent sheet resistance value
can be straightforwardly modifi ed by altering thickness, width, and
distance of the contact lines.
Triangular cross-section structures prepared by two-photon
lithography were used as master samples to prepare stamps for a
gravure contact printing process (e.g., ref. [ 28 ] ). Stamps were fi lled with
a silica sol–gel or a silver paste and triangular shaped grid fi ngers were
stamped onto a substrate. An example of a structure printed with silica
sol–gel is shown in Figure 3 b, a silver paste replica is shown in Figure 3 c.
In both cases even the sidewall texture was reproduced. To date, we have
printed areas up to 8 mm
×
8 mm using this technique, and larger areas
are possible. When printing is performed using a conductive silver paste,
the printed structures could be used for current transport throughout
the whole triangular cross-sectional conductor, leading to very low
sheet resistance. However, as can be seen in Figure 3 c a conventional
imprint process leaves unwanted silver residues on the substrate. We
developed a method in which the stamp is applied to the sample and
is then infi lled from a cut side via capillary fl
ow. This results in residue-
free printed metal lines shown in Figure 3 d,e. The defects seen in
the lines at the bottom of Figure 3 d are perfect reproductions of the
(defective) master stamp and resulted from stitching errors during the
two-photon lithography writing process. To date, we have demonstrated
ink permeation into stamps over distances of
≈
5 mm via ink capillary
fl
ow into the stamp. Because commercially produced silicon solar cells
commonly feature texture-etched front surfaces to enable light trapping
and antirefl
ection, it is important to establish that our process is
compatible with such cells. Figure 3 f illustrates replication of effectively
transparent contacts printed on a commercial texture-etched silicon
solar cell. While effectively transparent contact fabrication via two-
photon lithography is currently limited to areas of
≈
1 cm
2
, we envision
that a future gravure printing process using master patterns fabricating
by the silicon dry etching method described above will enable stamp and
contact fabrication to be scaled to sizes comparable to contemporary
silicon solar cells (e.g., 156 cm
2
).
Triangular shaped lines were furthermore defi ned by direct etching
into silicon. The result is shown in the electron microscope image in
Figure 3 g. Dry etching of silicon is a possible route to fabrication of large-
area master samples for stamp formation. First, an etch mask composed
of Al
2
O
3
was defi ned by lithography. Then cryogenic inductively coupled
plasma reactive ion etching was performed using SF
6
as etching gas
and O
2
as passivation gas. The aspect ratio and taper of the triangular
shaped lines etched into the silicon sample can be adjusted by varying
the SF
6
/O
2
ratio in the plasma (for further details see refs. [ 29 ] and [ 30 ] ).
Here we started with a line pattern with
≈
2.5 μm width and the etching
was performed at 900 W inductively coupled plasma, 5 W capacitive
coupled plasma, 70 sccm SF
6
, and 9 sccm O
2
for 10 min at
−
120
°
C.
LBIC measurements were performed with a confocal scanning
microscope using a 543 nm wavelength laser source. Spatially
resolved refl
ection and photocurrent were simultaneously obtained
using an objective with ten times magnifi cation and 0.25 numerical
aperture (NA) resulting in a laser spot size of less than 500 nm. For
the 3D measurements shown in Figure 2 c,d an objective with 20 times
magnifi cation and 0.6 NA was used.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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Acknowledgements
The information, data, or work presented herein was funded in part
by the U.S. Department of Energy, Energy Effi ciency and Renewable
Energy Program, under Award Number DE-EE0006335 The
lithographic printing (C.R.B. and S.Y.) was supported by the Bay Area
Photovoltaics Consortium under Award Number DE-EE0004946. One
of us (A.M.B.) was supported by the National Science Foundation
(NSF) and the Department of Energy (DOE) under NSF CA No. EEC-
1041895. S.Y. acknowledges the Kavli Nanoscience Institute and the
Joint Center for Artifi cial Photosynthesis. The authors acknowledge
Mathieu Boccard (Arizona State University) for providing the
solar cells used to carry out this study. The authors thank Cristofer
A. Flowers (California Institute of Technology) for helpful discussion
and Lucas Meza (California Institute of Technology) for two-photon
lithography advice.
Received: April 7, 2016
Revised: May 17, 2016
Published online: June 10, 2016
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