Light trapping in silicon nanowire solar cells.

Abstract

Thin-film structures can reduce the cost of solar power by using inexpensive substrates and a lower quantity and quality of semiconductor material. However, the resulting short optical path length and minority carrier diffusion length necessitates either a high absorption coefficient or excellent light trapping. Semiconducting nanowire arrays have already been shown to have low reflective losses compared to planar semiconductors, but their light-trapping properties have not been measured. Using optical transmission and photocurrent measurements on thin silicon films, we demonstrate that ordered arrays of silicon nanowires increase the path length of incident solar radiation by up to a factor of 73. This extraordinary light-trapping path length enhancement factor is above the randomized scattering (Lambertian) limit (2n(2) approximately 25 without a back reflector) and is superior to other light-trapping methods. By changing the silicon film thickness and nanowire length, we show that there is a competition between improved absorption and increased surface recombination; for nanowire arrays fabricated from 8 mum thick silicon films, the enhanced absorption can dominate over surface recombination, even without any surface passivation. These nanowire devices give efficiencies above 5%, with short-circuit photocurrents higher than planar control samples.

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Light Trapping in Silicon Nanowire Solar Cells
Erik Garnett and Peidong Yang*
Department of Chemistry, University of California, Berkeley, California 94720
ABSTRACT Thin-film structures can reduce the cost of solar power by using inexpensive substrates and a lower quantity and quality
of semiconductor material. However, the resulting short optical path length and minority carrier diffusion length necessitates either
a high absorption coefficient or excellent light trapping. Semiconducting nanowire arrays have already been shown to have low
reflective losses compared to planar semiconductors, but their light-trapping properties have not been measured. Using optical
transmission and photocurrent measurements on thin silicon films, we demonstrate that ordered arrays of silicon nanowires increase
the path length of incident solar radiation by up to a factor of 73. This extraordinary light-trapping path length enhancement factor
isabovetherandomizedscattering(Lambertian)limit(2n2∼25withoutabackreflector)andissuperiortootherlight-trappingmethods.
By changing the silicon film thickness and nanowire length, we show that there is a competition between improved absorption and
increased surface recombination; for nanowire arrays fabricated from 8 µm thick silicon films, the enhanced absorption can dominate
over surface recombination, even without any surface passivation. These nanowire devices give efficiencies above 5%, with short-
circuit photocurrents higher than planar control samples.
KEYWORDS Silicon, nanowires, solar cell, light trapping
O
the most feasible carbon-neutral route to displacing tera-
watts (TW) of nonrenewable power consumed worldwide.1
However, large-scale implementation is currently not eco-
nomically feasible because of the high cost as compared to
traditional power sources. One of the primary costs for
silicon photovoltaic cells is the starting silicon wafer, which
requires extensive purification to maintain reasonable
performance.2-4Therefore, reducing the required silicon’s
quality and quantity will help drive large-scale implementa-
tion of silicon photovoltaics. Using solar cells with nano-
structured radial p-n junctions may solve both of these
problems simultaneously by orthogonalizing the direction
of light absorption and charge separation while allowing for
improved light scattering and trapping.5Previously, the
highest efficiency silicon nanowire radial p-n junction solar
cells required vapor-liquid-solid (VLS) nanowire growth
from gold colloid particles, in situ thin-film deposition,
multipleelectron-beamlithography(EBL)stepsandcarefully
timed silicon etching to contact the p- and n-type layers
selectively.6Even with this complicated processing, the
resulting solar cells showed very low efficiency unless an
intrinsic silicon thin-film layer was inserted in between the
p- and n-type layers. With the p-i-n structure, the single
nanowire solar cells still showed a low open circuit voltage
(Voc) of 0.26 V. A moderate overall efficiency of 2.3-3.4%
was possible because of an extraordinarily high short-circuit
current density (Jsc), which was attributed to strongly en-
ver the last 50 years, commercial silicon photovol-
taics have been developed to convert sunlight into
electricity at efficiencies around 20% and provide
hanced absorption in the thin-film shell because of its
nanocrystalline nature.
In contrast, our approach has focused on large-area,
scalable, and simple fabrication methods for making solar
cells from arrays of silicon nanowires with radial p-n
junctions. We have already demonstrated a room temper-
atureaqueousetchingmethodfollowedbylow-temperature
thin-film deposition and a rapid thermal annealing crystal-
lization step to make wafer-scale radial p-n junction solar
cells, but the efficiency was only about 0.5% primarily
because of a low Vocand poor fill factor (FF).7In this report,
our improved process maintains the advantages of our
previous work but also dramatically reduces surface rough-
ness and improves control over the nanowire diameter and
density. This new method leads to greatly enhanced Voc, FF,
andJscvalues,ultimatelyyielding10timeshigherefficiencies
using thin silicon absorber films. We also quantitatively
measure strong light trapping effects, with path length
enhancement factors up to 73, which help improve Jscover
planar controls, despite the increased surface and junction
recombinationthatisassociatedwiththenanowiregeometry.
The fabrication method consists of four simple steps
illustrated in Figure 1: silica bead synthesis,8dip coating to
form a self-assembled monolayer of beads on the silicon
surface, deep reactive ion etching to form the nanowire
array,9and diffusion to form the p-n junction. Figure 2a
shows scanning electron microscopy (SEM) images of silica
bead monolayers assembled on silicon via dip coating. The
excellent packing shown in the SEM images extends over
large areas, up to 10 cm2, limited now by the size of the dip-
coating cell. The monolayer films of silica beads were used
as a mask for deep reactive ion etching (DRIE) with SF6to
form uniform periodic nanowire arrays (Figure 2b,c). The
nanowire pitch and diameter are controlled by the silica
*To whom correspondence should be addressed, p_yang@berkeley.edu.
Received for review: 01/17/2010
Published on Web: 01/28/2010
pubs.acs.org/NanoLett
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bead diameter, while the length is determined by the DRIE
time. Because of partial etching of the silica bead mask, the
final nanowire diameter is always smaller than the starting
bead diameter and is coupled to the etch time; for our
standard 8 min etch the diameter was 390 nm when using
530 nm beads and the length was about 5 µm. After DRIE,
the mask was removed with hydrofluoric acid and the radial
p-n junction was formed via boron diffusion into the
starting n-type silicon wafer. The junction depth was calcu-
latedtobeabout160nm.Ourpreviousworkhasshownthat
diffusioninsiliconnanowiresgivesradialdopingprofilesthat
match well with those calculated using standard diffusion
equations.10Photolithographyandaluminum/palladiumsput-
tering led to a top contact grid with metal filling completely
between the nanowires, which allowed for probing without
breaking any nanowires (Figure 2b, inset).
Figure 2d shows an optical image of the completed
devices on a 16 cm2substrate; the iridescent color gradient
is caused by white light interacting with the periodic nano-
wire array at different angles, demonstrating the excellent
FIGURE 1. Ordered silicon nanowire array fabrication scheme. The fabrication consisted of three major steps depicted above: dip coating an
n-type silicon wafer in an aqueous suspension of silica beads to get a close-packed monolayer; deep reactive ion etching (DRIE) using the
beads as an etch mask to form nanowires; bead removal in HF and boron diffusion to form the radial p-n junction. Standard photolithography
and metal sputtering were used for the top finger grid while metal evaporation provided the back contact (not shown).
FIGURE 2. Solar cell fabrication and structure. (a) Scanning electron micrograph (SEM) of a close-packed monolayer of silica beads assembled
on a silicon wafer using dip coating. Scale bars are 10 and 1 µm for the main picture and inset, respectively. (b) Plan view SEM of a completed
ordered silicon nanowire radial p-n junction array solar cell made by bead assembly and deep reactive ion etching. The inset shows the edge
of a top contact finger, demonstrating that the metal completely fills in between the nanowires. Scale bars are 1 µm. (c) Tilted cross-sectional
SEM of the solar cell in panel b. Scale bar is 1 µm. (d) Tilted optical image of 36 silicon nanowire radial p-n junction solar cell arrays from
(a-c). The color dispersion demonstrates the excellent periodicity present over the entire substrate. Scale bar is 4 cm.
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uniformity over large areas that can be achieved with this
technique. Each of the 36 solar cells located on the same
substrate is electrically isolated from the others by mechan-
icalsawing.Assubstratesforthedipcoatingandsubsequent
solar cell fabrication, we used very highly doped n-type
silicon wafers with two different doping levels, each with a
thin, lightly doped epitaxial layer on top. Since the minority
carrier diffusion length depends on the doping concentra-
tion,11these wafers allowed us to mimic the photovoltaic
response of very thin silicon solar cellss8 and 20 µm in our
caseswhileavoidingcomplicationsassociatedwithhandling
such thin silicon wafers or depositing and contacting mul-
ticrystalline silicon on a low-cost substrate. This simple
methodforvaryingthesiliconsolarcellthicknessalsoallows
for quantitative measurements of the nanowire array’s light
trapping efficiency, as will be discussed later. Even though
low-cost substrates were not used in this proof-of-concept
study, the fabrication procedure demonstrated here should
be transferable to multicrystalline silicon (or even other
semiconductor) thin films deposited on low-cost substrates,
such as glass, aluminum, or metallurgical grade silicon.12
Figure 3 shows the output characteristics of 5 µm nano-
wire and planar control solar cells under AM1.5G illumina-
tion for two different silicon absorber thicknesses. The
average Vocand FF values of 0.519 ( 0.003 V and 0.607 (
0.005 for the 20 µm absorbing layer and 0.525 ( 0.002 V
and 0.559 ( 0.02 for the 8 µm absorbing layer are higher
than previously reported values for silicon nanowire core-
shell p-n junction photovoltaics.6,7The average Jscvalues
of 16.82 ( 0.50 and 16.45 ( 0.19 mA/cm2give an overall
efficiency of 5.30 ( 0.19% and 4.83 ( 0.19% for the 20
and 8 µm cells, respectively. The 2% increase in Jscfor the
thicker nanowire cell is much smaller than the expected
20% increase calculated from the absorption for a single
pass through the different silicon layer thicknesses and the
22% increase shown in the planar control samples. This
suggests the periodic vertical nanowire arrays provide a
strong light trapping effect, which will be discussed in detail
later.
The 4.83% average efficiency for the 8 µm absorber
silicon nanowire array solar cells is about 20% higher than
results on 8 µm thick silicon ribbon solar cells, while the 20
µm absorber silicon nanowire cell average efficiency of
5.30% is about 35% lower than equivalent ribbon cells,13
demonstrating that the nanowire geometry provides supe-
riorlighttrappingbutalsodramaticallyincreasessurfaceand
junction recombination. For very thin absorbing layers the
light-trapping effects dominate, while for thicker cells that
already absorb a large fraction of the incident light, the
recombination effect is more important. Indeed, planar
controls fabricated in parallel on the 8 µm silicon absorbing
layerwafersshoweda4%lowerJscversusthenanowirecells,
but planar controls made using the 20 µm silicon absorbing
layers had a 14% higher Jscthan nanowire cells. Previous
reports advocating silicon nanowire array solar cells have
showntheyexhibitexcellentantireflectionpropertiesbuthave
not considered additional light-trapping effects.14-18In order
to explore the optical trapping advantages and recombina-
tion disadvantages of nanowire arrays, we have fabricated
arrays with the same periodicity but with different lengths
on several different silicon absorbing layer thicknesses and
measured their optical transmission spectra and photovol-
taic characteristics.
Figure 4 shows tilted cross-sectional SEM images and
corresponding transmission measurements of thin silicon
membranespatternedwithnanowirearraysoftwodifferent
lengths.Themembraneswerepreparedbyetchingwindows
in the handle of silicon on insulator (SOI) wafers through a
silicon nitride mask using KOH. The buried oxide acts as an
etch stop, giving windows with only the thin silicon device
layer remaining. The SOI device layer was chosen to have a
similar thickness to the 8 µm thin silicon absorber solar cells
toallowfordirectcomparison.Thesewaferswiththinsilicon
windows were used for nanowire fabrication following the
bead assembly and etching procedure discussed previously.
The standard 8 min etch yields nanowires that are about 5
µm long so that most of the membrane has a nanowire
structure, while the 4 min etch leads to about 2 µm short
FIGURE 3. Solar cell output characteristics. (a) 5 µm nanowire and planar control solar cells fabricated from an 8 µm thin silicon absorber.
(b) The same structures as in panel a, but using a 20 µm thin silicon absorber.
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nanowires that penetrate into only the top 25% of the
membrane (Figure 4). Optical measurements performed on
these membranes before and after nanowire fabrication
show that the transmission over the entire spectral range
from 600 to 1100 nm is reduced by factors of 2.9-7.8 and
12-29 for the 2 and 5 µm nanowires, respectively (Figure
4c). The planar control shows strong interference patterns
and is in excellent agreement with an optical model for a
7.5 µm thin silicon membrane (details in Supporting Infor-
mation). The insets in Figure 4c show backlit color images
of the windows with and without nanowires; clearly the
nanowire arrays reduce the intensity and red shift the
frequency of the transmitted light, as expected for a strong
light-trapping effect. It is possible that the nanowires simply
act as very effective scattering centers and that some (or
even a majority) of the light is coupled out of the membrane
at some angle so that it does not strike the detector. One
waytoconfirmthattheperiodicverticalnanowirearraytruly
traps light and leads to increased absorption is to monitor
the Jscas a function of nanowire length for thin silicon solar
cellsthatcanbenefitsignificantlyfromanincreasedeffective
path length.
Figure 5a shows the photovoltaic characteristics for 8 µm
cells fabricated with various nanowire lengths, leading to
different roughness factors. The roughness factor (RF) is
defined as the actual surface area of the structure divided
by the geometric area (e.g., RF of a planar cell is 1). The Voc,
FF, and Jsc trends show that the increased surface and
junction areas lead to both enhanced recombination (lower
Voc, FF) and improved light trapping (higher Jsc). It also
appears that the light-trapping effect dominates over the
surface and junction recombination effect for this silicon
absorber thickness, as the Jsccontinues to increase with
higher RF. The correlation between Jscand RF is especially
striking when considering that by increasing the nanowire
length,theamountofsiliconlefttoabsorblightisdecreasing
significantly and a much higher fraction of the remaining
silicon is in close proximity to the surface and p-n junction,
increasing the probability of carrier recombination. In order
to correct for the reduced volume, the Jscwas normalized
by the fraction of light that should be absorbed as compared
to the planar control (Figure 5a, normalized Jsc).
TheRF-dependentrecombinationeffectwasremovedby
comparing the Jscof silicon nanowire array solar cells with
the same RF, and thus the same surface and junction
recombination characteristics, but with different absorbing
layer thicknesses. In this way, we can quantitatively extract
light-trapping path length enhancement factors (EF) for
different nanowire geometries (full derivation in Supporting
Information).Figure5bshowstheminimumandmaximum
EF as a function of RF plotted on a semilog scale compared
to the normalized Jscfrom Figure 5a on a linear scale. The
strong correlation further supports the light-trapping effect,
as the absorption (and thus Jsc) should increase logarithmi-
cally with the path length. The transmission measurements
in Figure 4c can also be used to calculate an EF as a function
of RF. In this case, there is uncertainty as to how much of
the reduced transmission comes from absorption versus
how much is from scattered light that does not couple into
the cell and is ultimately lost. From the photocurrent data,
the light-trapping efficiency clearly increases with RF, so we
would expect that the fraction of scattered light that leads
toabsorptionwouldalsoincreasewithRF.Ifweassumethis
couplingefficiencyisproportionaltothelogarithmofRFand
set a coupling efficiency of 95% (after reflection losses) for
the longest nanowires, then we get EF of 2.0 and 62 for the
shortest and longest nanowires, respectively, in excellent
agreement with the maximum EF from the Jscdata that
varied between 1.7 and 73 (Figure 5). If instead we assume
an 85% coupling efficiency, we get EF of 1.2 and 8.3 for the
shortest and longest nanowires, respectively, close to the
minimum EF from the Jscdata of 0.96 and 11. This demon-
FIGURE 4.
windows with and without nanowires. Tilted cross-sectional SEM
images of (a) 2 µm nanowire arrays and (b) 5 µm nanowire arrays
on 7.5 µm thick silicon windows. Both scale bars are 10 µm. (c)
Transmission spectra of thin silicon window structures before (red)
and after etching to form 2 µm (green) and 5 µm (black) nanowires.
The spectra from an optical model for a 7.5 µm thin silicon window
is in blue and matches very well with the planar control measure-
ment. The insets are backlit color images of the membranes before
and after etching. Clearly there is a large intensity reduction and
red shift in the transmitted light after the nanowires are formed,
suggesting strong light trapping.
Optical transmission measurements on thin silicon
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stratesthatthereisverygoodcouplingandanextraordinary
light-trapping effect, exceeding the theoretical maximum of
2n2(∼25 for Si), where n is the refractive index, for ideal
randomizing (Lambertian) schemes without a back reflec-
tor.19-21Itisimportanttorecognizethattheseextraordinary
EF results include the entire AM1.5G spectrum (above the
siliconbandgap),suggestingthattheEFatwavelengthsnear
the band gap must be much higher. Indeed, using the
transmission measurements as described above gives EF of
156-210 at 1050 nm for 85-95% coupling efficiency.
Calculations have shown that certain periodic structures
are capable of exceeding the Lambertian EF limit,20,22-24
including orthogonal grooves on opposite sides of a silicon
slab (EF 50-70)20and photonic crystals coupled to a planar
absorber (EF >100 for certain wavelength ranges).22One
study predicted a maximum EF for solar cells coupled to
photonic crystals consisting of silicon pillars with a period
of approximately 500 nm and a diameter of 390 nm, nearly
identical to what we have produced here (though they
examined a square lattice and our structures have a hex-
agonal lattice).22However, without comparing arrays of
different periods and even random spacing, we cannot
conclusively say that our cells show a photonic crystal
enhancement effect. Despite the incredible light trapping,
these nanowire structures still do not exceed the overall
efficiency of planar cells, at least for silicon layers above 8
µm in thickness, due to enhanced surface and junction
recombination.Wedidnotperformanysurfacepassivation,
which is known to be important in high-performance planar
solarcellsandshouldbeevenmorecriticalforthesedevices.
Proper surface treatments and improved nanowire array
geometries that allow for lower roughness factors without
sacrificing light trapping will be critical to achieve high
efficiency ultrathin silicon solar cells.
In conclusion, we have demonstrated a simple and scal-
able method to fabricate large-area silicon nanowire radial
p-n junction photovoltaics. It requires dip coating a silicon
substrate to self-assemble silica spheres, DRIE to form
nanowires and diffusion to form the p-n junction. We
achievedefficienciesofbetween5and6%fortheseordered
vertical silicon nanowire array solar cells on 20 and 8 µm
silicon absorber layers with different roughness factors. By
comparing the photovoltaic output characteristics of these
different cells, we show that longer nanowires lead to both
increased recombination and higher absorption, with the
light-trapping effect dominating for 8 µm thin silicon absorb-
ing layers. We quantitatively measure a maximum light
trapping path length enhancement factor over the entire
AM1.5G spectrum between 1.7 and 73, depending on the
nanowire geometry, which agrees well with enhancement
factors between 2 and 62 extracted from optical transmis-
sion measurements. This light-trapping ability is above the
theoretical limit for a randomizing scheme, indicating that
there may be photonic crystal enhancement effects present
in our devices. This ordered vertical nanowire array geom-
etryrepresentsaviablepathtowardhigh-efficiency,low-cost
thin-film solar cells by providing a way to reduce both the
quantity and quality of the required semiconductor.
Acknowledgment. We thank Akram Boukai and Sarah
Brittmanforreviewingthemanuscript.NSF-COINSprovided
funding for this project. P.Y. would like to thank NSF for the
Waterman Award.
Supporting Information Available. Methods, enhance-
ment factor, and planar transmission model calculation
details. This material is available free of charge via the
Internet at http://pubs.acs.org.
FIGURE 5. Photovoltaic response and light-trapping effect as a function of roughness factor. (a) The Voc, FF, and Jscof periodic silicon nanowire
array solar cells fabricated from an 8 µm thick silicon absorber with three different roughness factors (RF) compared to a planar control (RF
) 1). The normalized Jscis determined by dividing the nanowire Jscby the fraction of light that should be absorbed compared to the planar
control, considering the loss in volume for the nanowire cells. The increasing Jscwith higher RF suggests that the light trapping effect induced
by ordered nanowire arrays is sufficient to overcome the increased surface and junction recombination that comes with longer nanowires.
(b) Light-trapping path length enhancement factor (EF) vs RF on a semilog scale derived by comparing the Jscfrom ordered silicon nanowire
arrays with the same RF but different absorber thicknesses (red and black) and from the optical transmission measurements in Figure 3c,
assuming 95% (blue) or 85% (green) of the reduced transmission ultimately leads to absorption. The normalized Jsc(brown) is also shown on
a linear scale for comparison. The strong correlation further supports the light-trapping effect since absorption (and ultimately photocurrent)
scales with the logarithm of the path length.
© 2010 American Chemical Society
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DOI: 10.1021/nl100161z | Nano Lett. 2010, 10, 1082-–1087