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Energ
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nv
ironmental
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COVER ARTICLE
Atwater and Lewis
et al.
Si microwire-array solar cells
PERSPECTIVE
Park and Holt
Recent advances in nanoelectrode
achitecture for photochemical
hydrogen production
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Si microwire-array solar cells
†
Morgan C. Putnam,
ab
Shannon W. Boettcher,
a
Michael D. Kelzenberg,
b
Daniel B. Turner-Evans,
b
Joshua M. Spurgeon,
a
Emily L. Warren,
a
Ryan M. Briggs,
b
Nathan S. Lewis
*
ac
and Harry A. Atwater
*
bc
Received 5th April 2010, Accepted 18th May 2010
DOI: 10.1039/c0ee00014k
Si microwire-array solar cells with Air Mass 1.5 Global conversion
efficiencies of up to 7.9% have been fabricated using an active
volume of Si equivalent to a 4
m
m thick Si wafer. These solar cells
exhibited open-circuit voltages of 500 mV, short-circuit current
densities (
J
sc
)ofupto24mAcm
-2
, and fill factors >65% and
employed Al
2
O
3
dielectric particles that scattered light incident in
the space between the wires, a Ag back reflector that prevented the
escape of incident illumination from the back surface of the solar
cell, and an a-SiN
x
:H passivation/anti-reflection layer. Wire-array
solar cells without some or all of these design features were also
fabricated to demonstrate the importance of the light-trapping
elements in achieving a high
J
sc
. Scanning photocurrent microscopy
images of the microwire-array solar cells revealed that the higher
J
sc
of the most advanced cell design resulted from an increased
absorption of light incident in the space between the wires. Spectral
response measurements further revealed that solar cells with light-
trapping elements exhibited improved red and infrared response, as
compared to solar cells without light-trapping elements.
Vertically aligned arrays of crystalline-Si (c-Si) microwires may
enable the fabrication of flexible c-Si solar cells with near unity
internal quantum yield that are c
apable of absorbing >85% of the
day-integrated (above band gap) direct solar illumination using
a volume of Si equivalent to a 2.8
m
m thick Si film.
1
Two advantages
conferred by the three-dimensional geometry of vertically aligned,
high-aspect ratio Si microwires are: (1) the ability to create high-
quality single crystal Si stru
ctures with passivated surfaces
via
a vapor
growth process;
2–4
and (2) enhanced absorpti
on relative to planar c-Si
absorbers.
1
These two advantages, in combination with the ability to
grow arrays of Si microwires over large areas (>1 cm
2
),
5
to peel the
wire arrays from the growth substrate in a flexible polymer,
6
and to
re-use the growth substrate,
7
offer the potential to fabricate flexible,
high efficiency c-Si solar cells.
8,9
Wire solar cells have been fabricated using c-Si,
10–20
amorphous-
Si,
21
GaAs,
22
III-nitride,
23
and InP,
24
via
a variety of growth
techniques, including vapor-
liquid-solid (VLS) growth,
10–16,19,20
metal-catalyzed chemical etching,
17,21
molecular beam epitaxy,
22
metal–organic chemical vapor deposition,
23,24
and deep reactive-ion-
etching.
18
In particular, the VLS growth method offers a materials-
efficient and scalable route for the synthesis of semiconducting wires.
However, the efficiencies of VLS-grown, c-Si, single-wire
13,14,16
and
wire-array
10–12,15,19,20
solar cells, up to 3.4%
13
and 1.8%
15
respectively,
have fallen short of the
15% photovoltaic efficiency predicted from
simple considerations.
8,9
In particular these solar cells have failed to
demonstrate open-circuit voltages (
V
oc
)inexcessof300mV,possibly
indicative of significant recombin
ation within the depletion region
and/or at the surfaces of the cells.
8,12,25
We report c-Si microwire-array
solar cells that have exhibited 7.9% conversion of simulated Air Mass
(AM) 1.5 Global (G) solar illumination to electrical energy with
negligible photovoltaic response from the growth substrate.
Square-tiled arrays of vertically aligned Si microwires (2–3
m
min
diameter on a 7
m
mpitch)weregrownonp
++
(resistivity,
r
, <0.001
U
cm) Si(111) wafers using the VLS growth method, as described
previously.
5
P-type doping of the Si microwires was achieved during
growth using BCl
3
as a gaseous dopant source.
4
Four-point electrical
measurements performed on individual Si wires from arrays grown
a
Division of Chemistry and Chemical Engineering, California Institute of
Technology, 1200 E. California Blvd, Pasadena, CA, 91125, USA.
E-mail: nslewis@caltech.edu; Fax: +1 626 395-8867; Tel: +1 626 395-6335
b
Thomas J. Watson Laboratories of Applied Physics, California Institute of
Technology, 1200 E. California Blvd, Pasadena, CA, 91125, USA. E-mail:
haa@caltech.edu; Fax: +1 626 844-9320; Tel: +1 626 395-2197
c
Kavli Nanoscience Institute, California Institute of Technology, 1200 E.
California Blvd, Pasadena, CA, 91125, USA
† Electronic supplementary information (ESI) available: Experimental
methods and additional solar cell characterization are presented. See
DOI: 10.1039/c0ee00014k
Broadercontext
Driven by the restructuring of Germany’s Renewable Energy Sources Act in 2000, the photovoltaics industry has grown tremen-
dously, demonstrating an average compound annual growth rate of 56% in the five-year period prior to 2008. As a result of this
growth and the subsequent development of the industry, the cost of photovoltaic electricity will likely reach grid-parity within the
next 6–10 years (without significant technological advances.) However, for photovoltaics to generate an appreciable fraction of
electricity, costs must be further reduced such that energy storage systems (batteries, hydrogen production coupled with fuel cells)
can be implemented. Recently, Si microwire-array solar cells have emerged as a promising new type of low-cost solar cell with the
potential for dramatically reduced Si consumption and flexible modules, while offering c-Si photovoltaic efficiencies. In this work we
demonstrate the fabrication of Si microwire-array solar cells with high open-circuit voltages, short-circuit current densities and fill
factors. These solar cells exhibit photovoltaic efficiencies of up to 7.9% and should achieve efficiencies of
15% with known
improvements in cell design.
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3
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COMMUNICATION
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under nominally identical conditions indicated that the wires were
p-type with
r
¼
0.05
U
cm, which corresponds to an electrically active
dopant concentration (
N
A
)of7
10
17
cm
3
, assuming a bulk hole
mobility of 1.8
10
2
cm
2
V
1
s
1
for Si.
Radial p–n junctions were fabricated within each wire, as illus-
trated in Fig. 1. First, the as-grown wire arrays (Fig. 1a) were
chemically etched to remove the Cu-catalyst and to remove a thin
layer (
50 nm) of surface Si, prior to the growth of a 200 nm thick
thermaloxide(Fig.1b).Thether
mal oxide was then selectively
removed in a hydrofluoric acid (HF) solution (aq.), using poly-
dimethylsiloxane (PDMS) as an etch barrier for the thermal oxide
located at the bases of the microwires (Fig. 1c). After removal of the
PDMS,
26
radial p–n junctions were formed in the upper region of the
Si microwires during a phosphorus diffusion (junction depth of
80 nm in a planar control), while the thermal oxide functioned as
a phosphorus diffusion barrier for the lower region of the wires
(Fig. 1d). We note that by appropriate choice of the PDMS layer
thickness, the p–n junction could be defined to approximate either
a radial or an axial p–n junction, or some combination of the two.
Three different types of Si microwire solar cells were fabricated.
The As-Grown cell contained no light trapping elements or surface
passivation. The Scatterer cell incorporated light-scattering Al
2
O
3
particles (nominally 80 nm in diameter) in-between the wires. The
PRS
cell utilized an a-SiN
x
:H passivation layer to minimize surface
recombination and to serve as an anti-reflection coating, a Ag back
reflector to prevent the loss of incident illumination into the growth
substrate, and Al
2
O
3
particles to scatter light incident between the Si
microwires. Following the inclusion of the selected light-trapping
elements (see ESI†), each wire array was filled to the tips of the wires
with
mounting
wax
(a
transparent,
non-conducting,
thermoplastic polymer). Indi
um tin oxide (ITO) (120–150 nm thick,
r
z
7
10
4
U
cm) was then sputtered through a shadow mask to
form a top-contact pad and t
o define individual cells.
Fig. 2 displays cross-sectional sca
nning electron microscope (SEM)
images of a wire array after p–n junction formation and of a micro-
wire solar cell for each cell type. As seen in Fig. 2a, the height of the
thermal oxide (and thus the extent of the radial p–n junction) was
uniform across the wire array. Wire heights ranged from 57–63
m
m,
71–78
m
m, and 43–49
m
m for the As-Grown (Fig. 2b), Scatterer
(Fig. 2c), and PRS (Fig. 2d) microwire solar cells, respectively. The
thermal oxide covered the lower 27–32
m
mofthewiresinthe
As-Grown and Scatterer solar cells, but was removed prior to the
desorption of the a-SiN
x
:H layer in the PRS solar cells. For both
the Scatterer and PRS solar cells, the 80 nm Al
2
O
3
particles were
observed to form micron-sized agglomerates that were located near
the base of the wires, as evidenced by the granular texture of the
mounting wax near the bottom of the wire array (Fig. 2c and d) and
at the wire tips and sidewalls (Fig. 2c and d, inset). In the PRS solar
cells, the 1000 nm thick Ag back reflector covered the growth
substrate and the tapered base of the wires (Fig. 2d and S1†). The a-
SiN
x
:H anti-reflection/passivation layer in the PRS cell is not visible
in Fig. 2d. However, the a-SiN
x
:H layer conformally coated the wires
and substrate prior to selective removal of the a-SiN
x
:H from the tips
of the wires, which allowed for the ITO to contact the n-Si emitter
(Fig. S2†). For all devices, the mounting wax uniformly infilled the
wire array, and the ITO conformally coated the mounting wax and
the wire tips, thereby providing a continuous top contact despite the
highly textured surface.
An important consideration for measurements of the photovoltaic
performance of wire-array solar cells is the contribution from the
growth substrate to the observed photocurrent. Though the fabrica-
tion of an appropriate control cell is not straightforward (even if the
emitter doping compensated the substrate doping, the n
+
emitter and
p
++
substrate would form a tunnel junction) significant photocurrent
from the substrate can be ruled out in our microwire-array solar cells.
For the As-Grown and Scatterer solar cells, scanning photocurrent
microscopy measurements indicated an effective minority-carrier
diffusion length < 0.5
m
m for electrons in the thermal-oxide-coated
bases of the wires.
3
Consequently, neither the growth substrate nor the
lower 27–32
m
m of the wires contributed significantly to the observed
photocurrent of the As-Grown and Scatterer solar cells. For the PRS
microwire solar cells, the removal of the thermal oxide, followed by the
deposition of the a-SiN
x
:H passivation layer, produced an effective
electron minority-carrier diffusion length
[
30
m
m in the p-type bases
of the wires.
3
Taken together, these results suggest that the bulk
minority-carrier diffusion length is
[
30
m
m throughout the wire but
that the thermal-oxide-coated bases of the wires, for the Scatterer and
As-Grown cells, exhibited very high surface recombination velocities,
limiting the effective diffusion length in the oxide-coated wire bases to
<0.5
m
m. Hence, a photovoltaic response from the entire length of the
wires was possible for the PRS solar cells. However, the photovoltaic
contribution from the substrate for the PRS cells should be negligibly
small, as the optically thick Ag back reflector coated the entire
substrate except for where the wire
s had grown, ensuring that only the
light guided through the Si microwires was able to reach the substrate.
Consequently, 95% of the illumination
#
800 nm should have been
absorbed over the 43–49
m
m length of the wires, by a simple Beer–
Lambert law analysis. The remaining illumination entered the p
++
Si
substrate (
r
< 0.001
U
cm), which has been shown to exhibit an
external quantum yield <0.05
for 800–1100 nm illumination.
1,2
Fig. 1
Schematic of the radial p–n junction fabrication process. (a) VLS-grown, p-Si microwire array. (b) Microwire array after catalyst removal,
growth of a thermal oxide and deposition of a PDMS layer. (c) Removal of the unprotected thermal oxide. (d) Removal of the PDMS and subsequent
phosphorus diffusion to complete the fabrication of a radial p–n junction.
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, 2010,
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, 1037–1041
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In total, 15 As-Grown microwire solar cells, 12 Scatterer microwire
solar cells, and 24 PRS microwire solar cells were fabricated. The area
of the fabricated cells spanned a range from 0.12 to 0.21 mm
2
,as
a result of variations in the gap between the top of the microwire
arrays and the shadow mask during the deposition of the ITO. For
each cell type, the majority of the cells were found to exhibit mutually
similar
V
oc
and fill factor (FF) values (see Table S1†). To convert the
measured short-circuit currents to short-circuit current densities (
J
sc
)
and to calculate the photovoltaic efficiency (
h
), scanning photocur-
rent microscopy (SPCM) was used to image the perimeter of 2–3 cells
from each cell type and thus accurately determine the photoactive cell
area (see Fig. S3†).
Fig. 3 plots the measured current density as a function of voltage
for the champion microwire solar cell of each cell type, in the dark
(Fig. 3a) and under 100 mW cm
2
of simulated AM 1.5G illumina-
tion (Fig. 3b), respectively. In the dark, the microwire solar cells
exhibited rectifying behavior with diode ideality factors between
1.7 and 2.2. The roll-off in the current density near 0.5 V in forward-
bias resulted from the series resista
nce of the solar cells, which ranged
from 300 to 3000
U
and was dependent upon the quality of the
contact between the electrical probe and the ITO.
Under simulated AM 1.5G illumination, the champion PRS solar
cell exhibited markedly higher pho
tovoltaic performance than the
champion Scatterer and As-Grown solar cells, as a result of a signif-
icant increase in
J
sc
(Fig. 3b). Table 1 displays the values of
V
oc
,
J
sc
,
FF, and
h
for all of the microwire solar cells whose cell areas were
Fig. 2
Si microwire array solar cell device geometry. (a) Cross-sectional
scanning electron microscope (SEM) image of a Si microwire array after
radial p–n junction formation. The white arrow denotes the height of the
thermal oxide (used as a phosphorus diffusion barrier in the radial p–n
junction fabrication process).
Inset
: top-down SEM image of the same Si
microwire array illustrating the pattern fidelity and slight variation in
wire diameter. Cross-sectional SEM image of (b) an As-Grown solar cell,
(c) a Scatterer solar cell, and (d) a PRS solar cell.
Insets
: higher magni-
fication SEM images of the wire tips coated with ITO. For (b) and (c) the
white arrow again denotes the height of the thermal oxide. For (d) the
white arrow denotes the presence of the Ag back reflector. For the inset of
(d) the white arrow denotes the ITO layer.
Fig. 3
Current density as a function of voltage for the champion
microwire solar cell of each cell type (a) in the dark and (b) under
simulated AM 1.5G illumination. The black line in (a) is an exponential
fit to the dark
J
–
V
data of the PRS solar cell and was used to extract an
ideality factor of 1.8.
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measured by SPCM.
V
oc
of
500 mV and FF > 65% were observed
for all three cell types. The champion PRS solar cell produced a
V
oc
of 498 mV,
J
sc
of 24.3 mA cm
2
, and FF of 65.4%, for an
h
¼
7.92%.
The champion Scatterer and As-Grown solar cells exhibited
h
¼
5.64% and
h
¼
3.81%, respectively, with similar
V
oc
and FF but
lower
J
sc
. For the PRS and Scatterer cells, the differences in
h
within
a cell type largely resulted from differences in
J
sc
,whichmayhave
resulted from variations in the incorporation of the Al
2
O
3
scattering
particles or from variations in the fraction of electrically contacted
wires (see Fig. 4b and c and S3†). We estimate the internal error in the
measurement of the cell area to be 5% and the internal error in the
AM 1.5G illumination intensity to be 5%, yielding a
7% internal
error in the measurement of
J
sc
and
h
.
To better understand the differences in
J
sc
between the PRS,
Scatterer, and As-Grown solar cells, scanning photocurrent micros-
copy was used to map the photocurrent produced by the wire-array
solar cells as a function of localized laser illumination (
l
¼
650 nm,
1.0
m
m beam waist), as seen in Fig. 4. To facilitate comparison
between the different types of cells, each scanning photocurrent image
was normalized to its maximum photocurrent. The measured
photocurrent was maximized when the laser illumination was
centered on a wire and was minimized when the illumination was
centered between four adjacent wires. The photocurrent cross-
sections shown below each scanning photocurrent image indicated
that the relative magnitude of the decay in photocurrent as the laser
moved from a peak (centered on a wire) to a valley (between two
adjacent wires) clearly decreased from the As-Grown cell (Fig. 4a) to
the Scatterer cell (Fig. 4b) and from the Scatterer cell to the PRS cell
(Fig. 4c). In particular, the PRS solar cell exhibited nearly uniform
photocurrent across the array, demonstrating that the Ag back
reflector and Al
2
O
3
dielectric scattering particles allowed for the
effective collection of light incident between the wires.
The spots of greatly reduced photocurrent in the Scatterer and
PRS solar cells arose from wires that were not electrically contacted
by the ITO (wire vacancies would be expected to produce a photo-
current similar to the valley photocurrent, whereas, uncontacted
wires parasitically absorb incident illumination). The small fraction of
electrically inactive wires seen for the PRS and Scatterer cells likely
results from the presence of Al
2
O
3
scattering particles at the wire tips
preventing the fabrication of a good electrical contact between the
n
+
-Si emitter and the ITO.
As seen in Fig. 5, the As-Grown and Scatterer solar cells exhibited
similarly shaped spectral response curves (though different in abso-
lute magnitude), both exhibiting a
decline in the external quantum
yield (EQY) at wavelengths >550 nm. By comparison, the PRS solar
cell exhibited nearly constant EQY between 500 nm and 800 nm. The
increased red and infrared response of the PRS cell presumably arose
from light incident between the wires that was scattered multiple
times from the Al
2
O
3
scattering particles and the Ag back reflector.
Integration of the observed EQY with the AM 1.5G solar
spectrum predicted
J
sc
values of 13.3 mA cm
2
, 18.0 mA cm
2
,and
23.3 mA cm
2
for the As-Grown, Scatterer, and PRS solar cells,
respectively, in good agreement with the measured
J
sc
values.
The three types of microwire solar
cells were fabricated to facilitate
a comparison between the cell types. However, three differences
between the cells are worth noting. First, the wire length and thermal
oxide heights translated to active wire lengths of 27–33
m
m,
41–48
m
m, and 43–49
m
m for the As-Grown, Scatterer and PRS solar
cells, respectively. Assuming no reflection losses and single-pass
absorption, the theoretical increase in
J
sc
from a 30
m
m thick Si wafer
to a 45
m
m thick Si wafer is 1.75 mA cm
2
, a 5.3% increase. Applying
a 5.3% increase to the 11.8 mA cm
2
J
sc
of the As-Grown champion
Table 1
Photovoltaic performance under simulated AM 1.5G illumi-
nation. The champion solar cell from each cell type is bolded
Sample
V
oc
/mV
J
sc
/mA cm
2
FF
(%)
h
(%)
As-Grown C2R3
482
11.2
69.4
3.75
As-Grown C4R6
477
11.8
67.5
3.81
Scatterer C2R4
499
16.6
68.0
5.64
Scatterer C3R3
504
15.2
68.8
5.28
PRS C2R5
503
22.2
66.1
7.38
PRS C3R5
500
22.8
67.2
7.65
PRS C4R5
498
24.3
65.4
7.92
Fig. 4
Scanning photocurrent microscopy (SPCM) images and associated photocurrent line profiles from the center of (a) an As-Grown solar cell, (b)
a Scatterer solar cell, and (c) a PRS solar cell. The SPCM images are 90
m
m
90
m
m and were normalized to the maximum measured photocurrent in
each image. The black lines on each SPCM image denote the cross-section used to produce the associated photocurrent line profiles. The black arrows
denote spots of greatly reduced photocurrent believed to result from wires that were not in contact with the ITO.
1040 |
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solar cell yields a
J
sc
of 12.4 mA cm
2
, well below the observed
16.6 mA cm
2
J
sc
for the Scatterer champion solar cell. Thus, the
additional active wire length alone cannot explain the increase in
J
sc
from the As-Grown solar cells to the Scatterer solar cells. Second, the
Al
2
O
3
scattering particles were largely located adjacent to the photo-
inactive, thermal-oxide-coated, bases of the wires. Consequently, the
full effect of the Al
2
O
3
scattering particles is unlikely to have been
seen in the Scatterer solar cells. Third, for the PRS and Scatterer cell
types,
2% of the wires in the center of the cell (Fig. 4) and 2–20% of
the wires near the perimeter of the cell (Fig. S3†) were not electrically
active. Thus, with improved conta
cting, the PRS and Scatterer cell
types would be expected to produce a still slightly higher
J
sc
and
h
.
Recently, we have demonstrated single-wire solar cells with
V
oc
of
up to 600 mV and FF of up to 82%.
3
Additionally, we have previ-
ously shown that wire-array photoelectrochemical cells can exhibit
near-unity internal quantum yields.
1
Basedonthesemeasurements,
efficiencies for wire arrays of
15%,
as compared to the simple theo-
retical expectation of 17%
, could potentially be achieved by increasing
the
J
sc
to 32 mA cm
2
(
e.g.
, by using longer wires and increasing the
electrically active wire fraction, while accounting for parasitic
absorption in the ITO and contact
shading), by increasing the FF to
80% (through the addition of a metallic grid on the top contact), and
by increasing the
V
oc
to 600 mV.
3
Separately, the design of our p–n
junction, which does not extend to the base of the wire array, should
prevent shunting of the p–n junction at the back contact in wire-array
solar cells that have been removed from the growth wafer.
Acknowledgements
This work was supported by BP and in part by the Department of
Energy (Basic Energy Sciences, Energy Frontier Research Center
under grant DE-SC0001293 and a
lso grant DE-FG02-07ER46405)
and made use of facilities supported by the Caltech Center for
Sustainable Energy Research, the Center for Science and Engineering
of Materials—an NSF Materials Research Science and Engineering
Center at Caltech (DMR 0520565), the Molecular Materials
Research Center of the Beckman Institute at Caltech, and the Kavli
Nanoscience Institute at Caltech. S.W.B. acknowledges the Kavli
Nanoscience Institute for fellowship support, and D.B.T.-E.
acknowledges the National Science Foundation for fellowship
support. The authors acknowledge Dr Michael Walter for helpful
discussions.
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Fig. 5
Spectral response of the champion Si microwire solar cell of each
cell type.
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The Royal Society of Chemistry 2010
Energy Environ. Sci.
, 2010,
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