of 6
Comparison between the electrical junction properties of H-terminated and
methyl-terminated individual Si microwire/polymer assemblies for
photoelectrochemical fuel production
Iman Yahyaie,
a
Shane Ardo,
b
Derek R. Oliver,
*
a
Douglas J. Thomson,
a
Michael S. Freund
ac
and Nathan S. Lewis
b
Received 8th August 2012, Accepted 24th September 2012
DOI: 10.1039/c2ee23115h
The photoelectrical properties and stability of individual p-silicon (Si) microwire/
polyethylenedioxythiophene/polystyrene sulfonate:Nafion/n-Si microwire structures, designed for use
as arrays for solar fuel production, were investigated for both H-terminated and CH
3
-terminated Si
microwires. Using a tungsten probe method, the resistances of individual wires, as well as between
individual wires and the conducting polymer, were measured
vs.
time. For the H-terminated samples,
the n-Si/polymer contacts were initially rectifying, whereas p-Si microwire/polymer contacts were
initially ohmic, but the resistance of both the n-Si and p-Si microwire/polymer contacts increased over
time. In contrast, relatively stable, ohmic behavior was observed at the junctions between CH
3
-
terminated p-Si microwires and conducting polymers. CH
3
-terminated n-Si microwire/polymer
junctions demonstrated strongly rectifying behavior, attributable to the work function mismatch
between the Si and polymer. Hence, a balance must be found between the improved stability of the
junction electrical properties achieved by passivation, and the detrimental impact on the effective
resistance associated with the additional rectification at CH
3
-terminated n-Si microwire/polymer
junctions. Nevertheless, the current system under study would produce a resistance drop of

20 mV
during operation under 100 mW cm

2
of Air Mass 1.5 illumination with high quantum yields for
photocurrent production in a water-splitting device.
a
Department of Electrical and Computer Engineering, University of
Manitoba, Winnipeg, Manitoba, Canada. E-mail: derek.oliver@ad.
umanitoba.ca
b
Beckman Institute and Kavli Nanoscience Institute, Division of Chemistry
and Chemical Engineering, M/C 127-72, 210 Noyes Laboratory,
California Institute of Technology, Pasadena, California 91125, USA.
E-mail: nslewis@caltech.edu
c
Department of Chemistry, University of Manitoba, Winnipeg, Manitoba,
Canada. E-mail: michael.freund@ad.umanitoba.ca
† Electronic supplementary information (ESI) available: Metallic
catalyst removal procedure, microwire methylation procedure,
conductive polymer film preparation, microwire/polymer junction
formation, resistance
vs.
probe spacing measurement results for
CH
3
-terminated microwires, Au contact/polymer junction
I
V
character, initial XPS analysis results for H-terminated and
CH
3
-terminated Si microwires over time. See DOI: 10.1039/c2ee23115h
Broadercontext
Photoelectrosynthetic splitting of water to store solar energy in the simplest chemical bond, H–H, would provide a globally scalable
means of compensating for the intermittency of solar energy at a given region of the earth’s surface. A working device will require: a
light harvesting component; redox catalysts; and a membrane barrier, for separating the products of the oxidation and reduction
reactions, while maintaining efficient ionic conductivity to maintain charge neutrality. Given the useable solar energy range and the
energy requirements for both oxidation and reduction reactions, it is challenging to find a single light absorber that can function
efficiently. As in the natural photosynthetic system, it is possible to combine two light absorbers across a product-separating,
ionically conductive membrane; however, the membrane is then required to manage, with minimal resistance, electron transport
between the two light absorbers. We report herein the photoelectrical properties of a test structure of this type of device, incor-
porating an individual semiconductor photoanode and photocathode (with no redox catalysts) embedded into a candidate con-
ducting polymer membrane, to form a single functional unit.
This journal is
ª
The Royal Society of Chemistry 2012
Energy Environ. Sci.
, 2012,
5
, 9789–9794 | 9789
Dynamic Article Links
C
<
Energy &
Environmental Science
Cite this:
Energy Environ. Sci.
, 2012,
5
, 9789
www.rsc.org/ees
PAPER
Downloaded by California Institute of Technology on 03 January 2013
Published on 25 September 2012 on http://pubs.rsc.org | doi:10.1039/C2EE23115H
View Article Online
/ Journal Homepage
/ Table of Contents for this issue
I. Introduction
A membrane-bridged microwire structure (Fig. 1) is an inter-
esting approach to the production of hydrogen using sunlight.
1–7
In this approach, two semiconductors functioning electrically in
series would provide the necessary photovoltage (>1.23 V) to
split water into H
2
(g) and O
2
(g). Suitable electrocatalysts would
be used to drive the fuel-forming anodic and cathodic reactions
at low overpotentials.
6,7
The membrane would support and
incorporate the two semiconductor materials, and must also
allow for ionic and electronic transport, to complete the circuit
while separating the H
2
(g) and O
2
(g) with minimal product
crossover.
8–10
One important aspect of this proposed device
structure is the electrical behavior of the contacts between the
semiconducting microwires and the conducting polymer.
3,11–14
H-terminated Si microwires have relatively low surface recom-
bination velocities, and can provide an approach to producing the
photocathode portion of such a structure. H-terminated Si
microwires have been used in solar cell,
15–18
organic,
19
liquid
junction,
20,21
and inorganic solid-state
16
devices, as well as for
photocathodes in photoelectrochemical H
2
production from
water.
22
The electrical properties of Si microwires have been
studied in contact with polyethylenedioxythiophene/polystyrene
sulfonate (PEDOT:PSS):Nafion films, and in some cases the
junctions have been shown to be ohmic and of acceptably low
contact resistance to be useful in the proposed membrane struc-
ture. However, oxide formation within the Si microwire/conduct-
ing polymer membrane junction would significantly increase the
junction resistance, resulting in electrical losses during operation.
23
Such losses could compromise the functionality of the device,
considering the limited amount of photogenerated voltage avail-
able to drive the water-splitting reactions.
6,7,11,12,24
One approach to prevent oxide formation is to chemically
passivate the Si microwire surface through surface functionali-
zation reactions that exploit the kinetic stability of the Si–C
bond.
25–27
Alkylation of H-terminated Si using alkylmagnesium
reagents;
28,29
radical halogenation;
30
chemical free-radical acti-
vation;
31,32
irradiation with ultraviolet light;
33–37
thermal activa-
tion;
32,38–40
and hydrosilylation
41–43
has been reported for both
the Si(100)
39,44
and Si(111) surfaces,
23,26,45
as well as for porous
Si.
46
In some cases, such surface functionalization has been
shown to produce enhanced oxidative stability and to result in a
low surface recombination velocity.
27
We report herein the electrical behavior of the contact between
a conducting organic polymer and functionalized Si microwires.
Functionalization of Si microwires with methyl groups produces
surfaces that are electrically and chemically stable, with negli-
gible native oxide growth over time.
23,26,27,45
The CH
3
-terminated
Si microwires were used to form single microwire device struc-
tures that contain the main structural features involved with a
membrane-supported photoelectrosynthetic device.
47
Although
CH
3
–Si termination is expected to suppress oxide formation,
CH
3
-functionalization of Si is also expected to produce an
interfacial dipole that should result in a shift in the barrier height
of the Si/polymer contact. Specifically, relative to H-terminated
Si microwires, increased rectification is expected at CH
3
-func-
tionalized n-Si/polymer contacts and decreased rectification is
expected at CH
3
-functionalized p-Si/polymer contacts. In this
work, we have directly compared the electrical properties of
CH
3
-terminated and H-terminated Si microwires with the same
conducting polymer. The data include electrical parameters that
cannot be precisely determined from bulk measurements, such as
the Si microwire resistivity, the total series resistances in the
microwire/conducting polymer system, and the stability over
time of the wire/polymer junction.
II. Experimental
A. Si microwires
Si microwires were grown by vapor–liquid–solid (VLS) chemical
vapor deposition (CVD) methods. The substrates were Si(111)
wafers that were patterned photolithographically with Cu cata-
lysts. During growth, the wires were doped with either boron (B)
or phosphorus (P) to concentrations of 10
17
to 10
18
cm

3
.
13,20,48,49
The single crystalline Si microwires were 90

15
m
m in length,

1.5 to 1.7
m
m in diameter, and were arranged in a square
pattern with a 7.0
m
m pitch. Residual metallic growth catalyst at
the top of each microwire (and some small amounts on the sides)
was removed using a two-step etching procedure (ESI†). The
microwires were terminated with methyl groups (ESI†) using a
two-step chlorination–alkylation procedure.
26,27
The microwires were removed from the growth substrate either
by sonication
50
or by scraping a corner of the substrate using a
razor blade. The microwires were suspended in isopropanol or
acetonitrile, and a drop of the suspension was deposited onto the
glass substrate to facilitate measurements on individual micro-
wires. Prior to each set of measurements, surface oxides were
removed by etching the Si microwires in buffered HF(aq.). The
time interval between the oxide removal process and the
measurements was typically <15 min.
Fig. 1
Schematic diagram of a single unit cell of the proposed photo-
electrochemical device. Each cell would ideally include two semi-
conductors with sufficiently large bandgaps to provide the necessary
photovoltage for water splitting, and the semiconductors would have
electrocatalysts deposited onto the wire surfaces. The semiconducting
light absorbers (in this case Si microwires) would be embedded into an
electronically and ionically conductive membrane. The various DC series
resistances that contribute to the total resistance of the system are shown.
R
wire
and
R
polymer
represent the microwire and polymer resistances,
respectively, whereas
R
C-n
and
R
C-p
are the junction resistances of the n-
Si and p-Si microwire/polymer contacts, respectively. The maximum
acceptable photovoltage drop across the cell is 10 mV for 20 mA cm

2
of
photocurrent density. Reprinted with permission from ref. 47. Copyright
(2011) American Chemical Society.
9790 |
Energy Environ. Sci.
, 2012,
5
, 9789–9794
This journal is
ª
The Royal Society of Chemistry 2012
Downloaded by California Institute of Technology on 03 January 2013
Published on 25 September 2012 on http://pubs.rsc.org | doi:10.1039/C2EE23115H
View Article Online
To eliminate any variation in the results due to microwire
growth variability, all the microwires were taken from a common
set of growth runs. The microwire samples were divided into two
separate batches, with the first batch etched in HF(aq.) to remove
the native oxide, and the second batch methyl-terminated. The
junction behavior of H-terminated and CH
3
-terminated Si
microwires was evaluated for Si microwires having either p- or n-
type doping.
B. Conducting polymer films
Solutions of the conducting polymer, polyethylenedioxythiophene/
polystyrene sulfonate/Nafion (PEDOT:PSS:Nafion) with 12 wt%
PEDOT:PSS,
10
were prepared according to established procedures
(ESI†). Conducting polymer films (thickness: 150 to 200 nm) were
deposited by spin-coating a solution of polymer onto a glass
substrate that contained prepositioned Parafilm masks on opposite
ends of one side of the substrate, with the polymer film deposition
forming a lane on the substrate. After removal of the Parafilm
mask, p-type or n-type Si microwires were deposited onto the
exposed glass substrate. Using tungsten probes, single microwires
were then positioned perpendicular to the border between the
conducting polymer and the glass substrate. To ensure good elec-
trical contact between the polymer and the microwire, approxi-
mately 2–5
m
m of the microwire length that was in contact with
polymer was covered with a small amount (<10
m
L) of polymer.
Ohmic contacts to the conducting polymer films were then formed
by sputtering 32 nm thick pads of Au directly onto the polymer.
This p-Si microwire/polymer/n-Si microwire assembly (Fig. 2) was
used as a model for a single unit of the proposed solar water split-
ting microwire array cell (Fig. 1).
C. Instrumentation
Following the preparation of polymer membranes, an Edwards
s150b sputter coater was used to sputter Au pads onto the polymer
surface. A Fogale Photomap 3D optical profilometer and a KLA
Tencor AS-500 Alpha-Step were used for the polymer film
thickness measurements. X-ray photoelectron spectroscopic
(XPS) analysis on the microwire samples was performed using a
Kratos Axis Ultra DLD instrument. Current
versus
voltage (
I
V
)
measurements were performed in a standard probe station using
an Agilent 4155c semiconductor parameter analyzer. A Newport
model 96000 full spectrum solar source with a global Air Mass
(AM) 1.5G filter was used to simulate the standard solar irradi-
ance spectrum. Tungsten probes (with a diameter of

1
m
m), used
in the
I
V
measurements, were etched for

30 s in 2.0 M
KOH(aq.) immediately before the experiments, to remove the
tungsten native oxide and to improve the quality of the contacts.
III. Results
A. Single microwire measurements
The doping concentration
vs.
distance along the Si microwires
was investigated through resistance measurements (ESI†)
Fig. 2
(a) Schematic diagram of the measurement system. This system was
used as a representation for the photoelectrochemical device depicted in Fig. 1.
The PEDOT:PSS:Nafion film (

200 nm thick) was spin-coated onto a glass
substrate. H-terminated and CH
3
-terminated p-type and n-type Si microwires
were aligned at the polymer/glass border, with 2–5
m
m of the microwire
embedded into the polymer. Au contacts (32 nm thick) were sputtered onto
the polymer to provide ohmic contacts to the polymer membrane; (b)
photograph of the test structure, along with SEM micrographs of the
microwires alignedat each side. SEM im
ages were taken on a gold-coated slide
to minimize charging issues. The scale bars in the images are 20
m
m.
Fig. 3
I
V
data for a 100
m
m long freshly etched (a) H-terminated p-type
Si microwire and (b) H-terminated n-type Si microwire, both with
diameters of

1.5
m
m aligned at the PEDOT:PSS:Nafion/glass border
(figure insets). The total series resistance of the system in each case
(displayed in the figures) was used to extract the junction resistance,
R
C-p
or
R
C-n
, by incorporating the known values of
R
wire
and
R
polymer
.
This journal is
ª
The Royal Society of Chemistry 2012
Energy Environ. Sci.
, 2012,
5
, 9789–9794 | 9791
Downloaded by California Institute of Technology on 03 January 2013
Published on 25 September 2012 on http://pubs.rsc.org | doi:10.1039/C2EE23115H
View Article Online
obtained using a direct-contact technique.
14,47
The resistance per
unit length of the CH
3
-terminated microwires was

0.50 k
U
m
m

1
and 0.18 k
Um
m

1
for the p-type and n-type Si microwires,
respectively. As expected, these values are similar to those
reported for H-terminated Si microwires.
47
The contact resis-
tance between the tungsten probes and the microwires was also
calculated by performing a linear fit to the resistance
versus
probe
separation data,
14,47
and was verified to be a negligible contri-
bution (<1 k
U
) to the total measured resistance for the system.
The doping concentration of the microwires was uniform over
the length scales considered, and was estimated to be 10
17
to 10
18
cm

3
, well within the expected range of doping concentrations
based on the growth conditions.
13,20,48,51
B. Single Si microwire/PEDOT:PSS:Nafion system
Using the tungsten probes, the microwires were aligned on the
polymer films (Fig. 2b). The sputtered gold/polymer junction
exhibited well-defined ohmic behavior (ESI†), and the tungsten
probe/microwire junction also demonstrated ohmic character,
with negligible contact resistance when compared to the other
resistances in the system. To extract the contact resistance
between each type of Si microwire and conducting polymer, the
current that passed through the system for different applied
voltages was recorded with one tungsten probe in contact with
the microwire, while the other probe was placed on the gold
contact (insets of Fig. 3). The total measured resistance included
contributions from the conducting polymer,
R
polymer
(measured
using a 4-point probe technique), the microwire/polymer contact,
R
C
(denoted as
R
C-p
for p-type microwires and
R
C-n
for n-type
microwires), and the microwire,
R
wire
(denoted as
R
p-wire
for p-
type and
R
n-wire
for n-type microwires, measured as described in
Section A). The microwire/polymer contact resistance was then
calculated from the total measured series resistance by incorpo-
rating the known values (
R
wire
,
R
polymer
).
14
The
I
V
data for H-terminated Si microwires aligned at the
PEDOT:PSS:Nafion/glass border (Fig. 3) indicated ohmic
behavior at the junction between H-terminated p-Si microwires
and the PEDOT:PSS:Nafion and non-ohmic behavior between
H-terminated n-Si microwires and the polymer.
CH
3
-terminated p-Si microwires aligned at the polymer/glass
border exhibited ohmic character (Fig. 4a), in agreement with
prior measurements on planar CH
3
-terminated p-Si and
PEDOT.
7
Fig. 4b shows the junction properties of CH
3
-termi-
nated n-Si microwire/polymer junctions, demonstrating rectifying
behavior (Fig. 4b) and in effect showing a large junction resis-
tance. After one month under laboratory conditions, such
samples exhibitedless than 10% increase in the junction resistance.
This behaviour verifies the stability of these junctions as compared
to the H-terminated samples which demonstrated a more than 20-
fold increase in the junction resistance under the same conditions.
Under 1 Sun illumination (AM1.5G, 100 mW.cm

2
) an increased
conductivity was observed in the p-Si microwires (Fig. 4a), leading
to a slightly lower
R
wire/polymer
value. In these measurements,
microwires were lying flat on the substrate, as indicated in Fig. 2,
and were illuminated from above, so the illuminated photoactive
area was

1.5
m
m

100
m
m.
The electrical character of the model water-splitting cell,
formed using the combination of CH
3
-terminated n-Si and p-Si
microwires bridged by the PEDOT:PSS:Nafion conductive
polymer, was dominated by the rectifying behavior at the n-type
microwire/polymer junction (Fig. 5). Under illumination,

5nA
of short circuit current was measured for this rectifying junction
and in the conduction region, the dark and illuminated resistance
values were nearly identical. Table 1 summarizes the measured
resistances for both H-terminated and CH
3
-terminated samples.
IV. Discussion
Conventional approaches to form ohmic contacts to individual
Si microwires (
e.g.
thermal evaporation of contact metals)
50,52,53
are only applicable to a certain range of microwire diameters,
and are not compatible with many microwire/polymer struc-
tures, due to the complexity of the interactions between
Fig. 4
Measured
I
V
data for a 100
m
m long CH
3
-terminated (a) p-type
and (b) n-type Si microwire with a diameter
z
1.5
m
m aligned at the
PEDOT:PSS:Nafion/glass border, in dark and under solar AM 1.5G
illumination. As expected, ohmic behavior was observed at the p-Si
microwire/polymer contact. A strong rectifying behavior was observed at
the CH
3
-terminated n-Si microwire/polymer junction.
Fig. 5
Measured
I
V
response for a complete photoelectrosynthetic
water splitting cell with

100
m
m long CH
3
-terminated p-Si and n-Si
microwires with diameters of

1.5
m
m. The electrical character of the cell
is dominated by the rectifying behavior at the CH
3
-terminated n-type
microwire/polymer junction.
9792 |
Energy Environ. Sci.
, 2012,
5
, 9789–9794
This journal is
ª
The Royal Society of Chemistry 2012
Downloaded by California Institute of Technology on 03 January 2013
Published on 25 September 2012 on http://pubs.rsc.org | doi:10.1039/C2EE23115H
View Article Online
polymers, photoresists, and the etchant solutions, as well as the
high temperatures used during lithographic processes. Previous
reports have demonstrated that with the application of suffi-
cient local mechanical pressure, direct and reliable ohmic
contacts to the Si microwires can be made by the use of
tungsten probes.
14
The method also enables the basic electrical
properties of single Si microwires and of Si microwire/polymer
junctions to be extracted in a standard probe station, without
the need to thermally evaporate metallic contacts.
14,47
The characteristics of the n-type Si microwire/polymer junc-
tion can be ascribed to a work-function mismatch between the
n-type microwires and PEDOT:PSS:Nafion film, which has p-
type characteristics.
54,55
Methylation of Si shifts the energetics of
the Si band edges by creation of negative dipoles at the
surface,
6,26,27
inducing an increased barrier height at the n-Si
microwire/polymer junction. Approximately 500 mV is required
to overcome this energy barrier, as indicated in Fig. 4b.
Consistently, methylation produced lower resistance, more
ohmic, contacts between p-Si microwires and PEDOT:PSS:-
Nafion films.
Although the H-terminated Si microwires were etched imme-
diately prior to the measurements, a thin layer of native oxide
likely formed on the surface of the Si, because the alignment of
the microwires at the polymer–glass interface was conducted in
air. Quantum mechanical tunneling through the native oxide is
thus expected to contribute to the nonlinearity in the
I
V
profile
of the H-terminated Si microwire/polymer junctions. Consis-
tently,
I
V
measurements performed after aging of the H-
terminated microwire/polymer junctions for a month in air
revealed a more than 20-fold increase in the junction resistance,
and in some cases loss of electrical conduction at the junction,
presumably reflecting the deleterious effects of increased oxide
formation on the Si surface.
The increase in the junction resistance with time for CH
3
-
terminated microwires is potentially due to oxidation of the
backsides of the microwires where they had been cut off from the
growth substrate. However, as confirmed by XPS measurements,
CH
3
-terminated n-Si microwires showed significantly improved
oxidative stability (ESI†) compared to the H-terminated
microwires.
Absorption of all above-bandgap photons incident normal to
a7
m
m pitch size microwire array at 100 mW cm

2
(the global
AM 1.5 spectrum) should result in a maximum of

21 nA
short-circuit photocurrent in each microwire.
14,56
To minimize
the impact of iR losses in a functional system, voltage drops of
<

10 mV would produce losses that were

1% of total pho-
togenerated voltage required for water splitting. Assuming a
current of 21 nA flowing through the junction to the polymer
film, the maximum acceptable resistance to produce a 10 mV
voltage drop in the microwire–polymer system is

480 k
U
.
Hence, the p-Si microwire PEDOT:PSS:Nafion contacts meet
this criterion, whereas n-Si microwire PEDOT:PSS:Nafion
contacts exceed this target voltage drop by a factor of

2
(Table 1).
Fully ohmic behavior across all inner contact junctions in such
a model water-splitting cell could be achieved by combining two
separate conducting polymers, one that makes ohmic contact to
each electrode. Alternatively, increased junction conductance
would be expected from modification of the n-type microwire
base (in contact with the membrane), either by addition of a
metallic interfacial layer or by increased doping (degenerate
levels) to narrow down the depletion region and facilitate
quantum mechanical tunneling of charge carriers.
57
V. Conclusions
The electrical properties of a single cell of a proposed photo-
electrosynthetic fuel production system, as well as of each junc-
tion within the cell, were investigated both in the dark and under
simulated 1 Sun AM 1.5G illumination. H-terminated Si
microwire/polymer contacts exhibited poor long-term stability,
whereas CH
3
-terminated Si microwires demonstrated increased
stability toward oxidation behavior as compared to their H-
terminated counterparts. CH
3
-terminated p-Si microwires yiel-
ded ohmic contacts at PEDOT:PSS:Nafion junctions, whereas
CH
3
-terminated n-Si microwire/PEDOT:PSS:Nafion junctions
demonstrated rectifying behavior, potentially due to the shift in
the energy of the Si band edges at the interface, induced by
creation of a surface dipole.
Acknowledgements
Financial support from the Natural Sciences and Engineering
Research Council (NSERC) of Canada, the Canada Foundation
for Innovation (CFI), the Manitoba Research and Innovation
Fund, and the University of Manitoba is gratefully acknowl-
edged. The work reported made use of surface characterization
infrastructure in the Manitoba Institute for Materials. This work
was supported by a National Science Foundation (NSF) Center
for Chemical Innovation (CCI) Powering the Planet (grants
CHE-0802907, CHE-0947829, and NSF-ACCF) and made use
of the Molecular Materials Research Center of the Beckman
Institute at Caltech and the Kavli Nanoscience Institute at Cal-
tech. This research was undertaken, in part, thanks to funding
from the Canada Research Chairs Program. S. A. acknowledges
partial support from a U. S. Department of Energy, Office of
Energy Efficiency and Renewable Energy (EERE) Postdoctoral
Research Award under the EERE Fuel Cell Technologies
Program.
Table 1
Measured resistances (

1%) for both H-terminated and CH
3
-terminated Si microwire/polymer systems. The microwire length and diameters
were

100
m
m and

1.5
m
m, respectively. In the case of rectifying junctions, the measured resistance represents the series resistance in the turn-on region
R
(k
U
)

1%
R
wire-p
R
wire-n
R
C-p
R
C-n
R
PEDOT:Nafion
Expected
R
total
H-terminated (dark)
50
18

120

850
2
1040
CH
3
-terminated (dark)
50
18

100

900
2
1070
CH
3
-terminated
(Solar AM1.5G)

30

10

90

850
2
980
This journal is
ª
The Royal Society of Chemistry 2012
Energy Environ. Sci.
, 2012,
5
, 9789–9794 | 9793
Downloaded by California Institute of Technology on 03 January 2013
Published on 25 September 2012 on http://pubs.rsc.org | doi:10.1039/C2EE23115H
View Article Online
References
1 H. B. Gray,
Nat. Chem.
, 2009,
1
,7.
2 O. Khaselev and J. A. Turner,
Science
, 1998,
280
, 425–427.
3 S. Licht,
J. Phys. Chem. B
, 2001,
105
, 6281–6294.
4 S. Licht, B. Wang, S. Mukerji, T. Soga, M. Umeno and H. Tributsch,
J. Phys. Chem. B
, 2000,
104
, 8920–8924.
5 B. D. Alexander, P. J. Kulesza, I. Rutkowska, R. Solarska and
J. Augustynski,
J. Mater. Chem.
, 2008,
18
, 2298–2303.
6 M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher,
Q. X. Mi, E. A. Santori and N. S. Lewis,
Chem. Rev.
, 2010,
110
,
6446–6473.
7 J. M. Spurgeon, M. G. Walter, J. Zhou, P. A. Kohl and N. S. Lewis,
Energy Environ. Sci.
, 2011,
4
, 1772–1780.
8 X. Hu, B. S. Brunschwig and J. C. Peters,
J. Am. Chem. Soc.
, 2007,
129
, 8988–8998.
9 F. Jiao and H. Frei,
Angew. Chem., Int. Ed.
, 2009,
48
, 1841–1844.
10 S. L. McFarlane, B. A. Day, K. McEleney, M. S. Freund and
N. S. Lewis,
Energy Environ. Sci.
, 2011,
4
, 1700–1703.
11 M. D. Kelzenberg, D. B. Turner-Evans, M. C. Putnam,
S. W. Boettcher, R. M. Briggs, J. Y. Baek, N. S. Lewis and
H. A. Atwater,
Energy Environ. Sci.
, 2011,
4
, 866–871.
12 O. Gunawan, K. Wang, B. Fallahazad, Y. Zhang, E. Tutuc and
S. Guha,
Progress in Photovoltaics: Research and Applications
, 2011,
vol. 19, pp. 307–312.
13 S. W. Boettcher, J. M. Spurgeon, M. C. Putnam, E. L. Warren,
D. B. Turner-Evans, M. D. Kelzenberg, J. R. Maiolo,
H. A. Atwater and N. S. Lewis,
Science
, 2010,
327
, 185–187.
14 I. Yahyaie, K. McEleney, M. Walter, D. R. Oliver, D. J. Thomson,
M. S. Freund and N. S. Lewis,
J. Phys. Chem. Lett.
, 2011,
2
, 675–680.
15 L. Tsakalakos, J. Balch, J. Fronheiser, M. Y. Shih, S. F. LeBoeuf,
M. Pietrzykowski, P. J. Codella, B. A. Korevaar, O. Sulima,
J. Rand, A. Davuluru and U. Rapol,
J. Nanophotonics
, 2007,
1
,
013552.
16 K. Q. Peng, Y. Xu, Y. Wu, Y. J. Yan, S. T. Lee and J. Zhu,
Small
,
2005,
1
, 1062–1067.
17 L. Hu and G. Chen,
Nano Lett.
, 2007,
7
, 3249–3252.
18 B. Z. Tian, X. L. Zheng, T. J. Kempa, Y. Fang, N. F. Yu, G. H. Yu,
J. L. Huang and C. M. Lieber,
Nature
, 2007,
449
, 885–889.
19 W. U. Huynh, J. J. Dittmer and A. P. Alivisatos,
Science
, 2002,
295
,
2425–2427.
20 J. R. Maiolo, B. M. Kayes, M. A. Filler, M. C. Putnam,
M. D. Kelzenberg, H. A. Atwater and N. S. Lewis,
J. Am. Chem.
Soc.
, 2007,
129
, 12346–12347.
21 A. P. Goodey, S. M. Eichfeld, K. K. Lew, J. M. Redwing and
T. E. Mallouk,
J. Am. Chem. Soc.
, 2007,
129
, 12344–12345.
22 S. W. Boettcher, E. L. Warren, M. C. Putnam, E. A. Santori,
D. Turner-Evans, M. D. Kelzenberg, M. G. Walter, J. R. McKone,
B. S. Brunschwig, H. A. Atwater and N. S. Lewis,
J. Am. Chem.
Soc.
, 2011,
133
, 1216–1219.
23 W. J. Royea, A. Juang and N. S. Lewis,
Appl. Phys. Lett.
, 2000,
77
,
1988–1990.
24 M. C. Putnam, S. W. Boettcher, M. D. Kelzenberg, D. B. Turner-
Evans, J. M. Spurgeon, E. L. Warren, R. M. Briggs, N. S. Lewis
and H. A. Atwater,
Energy Environ. Sci.
, 2010,
3
, 1037–1041.
25 S. F. Bent,
Surf. Sci.
, 2002,
500
, 879–903.
26 L. J. Webb and N. S. Lewis,
J. Phys. Chem. B
, 2003,
107
, 5404–5412.
27 E. J. Nemanick, P. T. Hurley, B. S. Brunschwig and N. S. Lewis,
J.
Phys. Chem. B
, 2006,
110
, 14800–14808.
28 S. A. Mitchell, R. Boukherroub and S. Anderson,
J. Phys. Chem. B
,
2000,
104
, 7668–7676.
29 H.-Z. Yu, R. Boukherroub, S. Morin and D. D. M. Wayner,
Electrochem. Commun.
, 2000,
2
, 562–566.
30 A. Bansal, X. Li, I. Lauermann, N. S. Lewis, S. I. Yi and
W. H. Weinberg,
J. Am. Chem. Soc.
, 1996,
118
, 7225–7226.
31 M. R. Linford and C. E. D. Chidsey,
J. Am. Chem. Soc.
, 1993,
115
,
12631–12632.
32 M. R. Linford, P. Fenter, P. M. Eisenberger and C. E. D. Chidsey,
J.
Am. Chem. Soc.
, 1995,
117
, 3145–3155.
33 J. Terry, M. R. Linford, C. Wigren, R. Cao, P. Pianetta and
C. E. D. Chidsey,
Appl. Phys. Lett.
, 1997,
71
, 1056–1058.
34 J. Terry, R. Mo, C. Wigren, R. Cao, G. Mount, P. Pianetta,
M. R. Linford and C. E. D. Chidsey,
Nuclear Instruments and
Methods in Physics Research Section B: Beam Interactions with
Materials and Atoms
, 1997, vol. 133, pp. 94–101.
35 R. Boukherroub and D. D. M. Wayner,
J. Am. Chem. Soc.
, 1999,
121
,
11513–11515.
36 F. Effenberger, G. G
otz, B. Bidlingmaier and M. Wezstein,
Angew.
Chem., Int. Ed.
, 1998,
37
, 2462–2464.
37 R. L. Cicero, M. R. Linford and C. E. D. Chidsey,
Langmuir
, 2000,
16
, 5688–5695.
38 A. B. Sieval, R. Linke, G. Heij, G. Meijer, H. Zuilhof and
E. J. R. Sudh
olter,
Langmuir
, 2001,
17
, 7554–7559.
39 M. M. Sung, G. J. Kluth, O. W. Yauw and R. Maboudian,
Langmuir
,
1997,
13
, 6164–6168.
40 R. Boukherroub, J. T. C. Wojtyk, D. D. M. Wayner and
D. J. Lockwood,
J. Electrochem. Soc.
, 2002,
149
, H59–H63.
41 J. M. Buriak, M. P. Stewart, T. W. Geders, M. J. Allen, H. C. Choi,
J. Smith, D. Raftery and L. T. Canham,
J. Am. Chem. Soc.
, 1999,
121
,
11491–11502.
42 L. A. Zazzera, J. F. Evans, M. Deruelle, M. Tirrell, C. R. Kessel and
P. McKeown,
J. Electrochem. Soc.
, 1997,
144
, 2184–2189.
43 J. M. Schmeltzer, L. A. Porter, M. P. Stewart and J. M. Buriak,
Langmuir
, 2002,
18
, 2971–2974.
44 A. B. Sieval, A. L. Demirel, J. W. M. Nissink, M. R. Linford,
J. H. van der Maas, W. H. de Jeu, H. Zuilhof and
E. J. R. Sudh
olter,
Langmuir
, 1998,
14
, 1759–1768.
45 A. Bansal and N. S. Lewis,
J. Phys. Chem. B
, 1998,
102
, 1067–1070.
46 M. P. Stewart and J. M. Buriak,
Angew. Chem., Int. Ed.
, 1998,
37
,
3257–3260.
47 I. Yahyaie, K. McEleney, M. G. Walter, D. R. Oliver, D. J. Thomson,
M. S. Freund and N. S. Lewis,
J. Phys. Chem. C
, 2011,
115
, 24945–
24950.
48 B. M. Kayes, M. A. Filler, M. C. Putnam, M. D. Kelzenberg,
N. S. Lewis and H. A. Atwater,
Appl. Phys. Lett.
, 2007,
91
,
103110.
49 R. S. Wagner and W. C. Ellis,
Appl. Phys. Lett.
, 1964,
4
, 89–90.
50 M. D. Kelzenberg, D. B. Turner-Evans, B. M. Kayes, M. A. Filler,
M. C. Putnam, N. S. Lewis and H. A. Atwater,
Nano Lett.
, 2008,
8
,
710–714.
51 M. C. Putnam, D. B. Turner-Evans, M. D. Kelzenberg,
S. W. Boettcher, N. S. Lewis and H. A. Atwater,
Appl. Phys. Lett.
,
2009,
95
, 163116–163118.
52 E. Koren, Y. Rosenwaks, J. E. Allen, E. R. Hemesath and
L. J. Lauhon,
Appl. Phys. Lett.
, 2009,
95
, 092105–092107.
53 J. E. Allen, D. E. Perea, E. R. Hemesath and L. J. Lauhon,
Adv.
Mater.
, 2009,
21
, 3067–3072.
54 M. S. Freund and B. A. Deore,
Self-Doped Conducting Polymers
,
John Wiley & Sons, Ltd, 1st edn, 2007.
55 T. A. Skotheim and J. Reynolds,
Handbook of Conducting Polymers
,
CRC Press, 3rd edn, 2007.
56 M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop,
Progress in Photovoltaics: Research and Applications
, 2012, vol. 20,
pp. 12–20.
57 S. O. Kasap,
Principles of Electronic Materials and Devices
, McGraw
Hill, New York, NY, 3rd edn, 2006.
9794 |
Energy Environ. Sci.
, 2012,
5
, 9789–9794
This journal is
ª
The Royal Society of Chemistry 2012
Downloaded by California Institute of Technology on 03 January 2013
Published on 25 September 2012 on http://pubs.rsc.org | doi:10.1039/C2EE23115H
View Article Online