of 4
S1
Supporting Information for:
520 mV, pH Independent, Open-Circuit Voltages of Si/Methyl Viologen
2+/+
Contacts Through Use
of Radial n
+
p-Si Junction Microwire Array Photoelectrodes
Emily L. Warren, Shannon W. Boettcher
, Michael G. Walter, Harry A. Atwater*, and Nathan
S. Lewis*
Division of Chemistry and Chemical Engineering, Division of Engine
ering and Applied Science,
Kavli Nanoscience Institute and Beckman Institute, 210 Noyes Labora
tory 127-72, California
Institute of Technology, Pasadena, California 91125
Current Address: Dept. of Chemistry, University of Oregon.
*haa@caltech.edu
,
*nslewis@caltech.edu
,
MATERIALS AND METHODS
VLS-Catalyzed Wire-Array Growth
(111)-oriented p
+
Si wafers with a resistivity of
ρ
<0.003

-cm, were coated with 300 nm of thermal oxide
(Silicon Quest International). The samples were then patterned w
ith square arrays of 3 μ m circular holes, with a
hole-to-hole pitch of 7 μ m, using a positive photoresist layer (Mic
rochem S1813). The exposed SiO
2
was etched
away with buffered HF(aq) (BHF, Transene Inc.). 300-500 nm of Cu
(EPSI 6N) was then thermally evaporated
onto the substrate and the excess material was lifted off with a
cetone. Growth substrates were cut into chips with
dimensions of ~1.5 cm x 2 cm and were annealed in a tube furnace at
1000 °C for 20 min under a flow of 500
sccm H
2
. Wire growth was performed by introducing SiCl
4
(Strem, 6N), and BCl
3
(0.25% in H
2
, Mattheson) into
the reactor chamber for 15-30 min. The wires were allowed to coo
l to ~650 °C under H
2
or He at ambient
pressure before the samples were removed from the reactor.
Photoelectrochemical Characterization
1. pH Buffer Solutions.
Three different buffer solutions were prepared by literatur
e methods to compare the
performance of the electrodes at different pHs: phthalate (pH
2.9), and phosphate (pH 5.9 and 8.9).
1,2
To improve
the ionic conductivity of the solutions, 0.5 M K
2
SO
4
(Aldrich) was added to each buffer, and the pH was adjusted
with 0.1 M KOH or 0.1 M H
2
SO
4
. The pH of each solution was measured with a NexSens WQ-pH me
ter.
Solutions were purged with Ar for a minimum of 10 min before additi
on of 50 mM methyl viologen (MV
2+/+
)
dichloride (Aldrich). The MV
+
radical cation species was generated by bulk electrolysis
using the large carbon
electrode as a cathode, and a Pt mesh counter electrode separat
ed by a glass frit,
to create well-defined Nernstian
solution potential of -0.60 V to -0.59 V vs. SCE.
2. Current Density vs. Potential Measurements.
Current density vs. potential (
J-E
) data were obtained at a scan
rate of 20 mV s
-1
with a Princeton Applied Research (PAR) Model 273 potentiostat us
ing CorrWare software. To
S2
remove any native oxide, all electrodes were dipped in BHF imm
ediately before use. The illumination source
was a 1 W, 808-nm diode laser (Thor Labs L808P1WJ). To calibrate the
light intensity incident upon the Si
sample, a Si photodiode (UDT UV-005) was mounted in solution parallel t
o each down-facing electrode. The
laser beam was expanded to fill the entire area of both the w
orking electrode and calibrated photodiode.
Photoelectrochemical data were compared at an incident 808-nm light
intensity of 60 mW cm
-2
, because the
maximum short-circuit current density obtainable for Si under suc
h conditions is similar to the maximum short-
circuit current density obtainable for Si under 100 mW cm
-2
of Air Mass 1.5 (AM 1.5) solar illumination.
3
For all
electrodes, the cathodic limiting current density measured in t
he solution containing 3 mM MV
+
was directly
compared to that measured in a solution that contained no MV
+
to ensure that light intensity was correctly
measured.
3. Conversion of Current Density into an External Quantum Yield.
When using a monochromatic illumination
source, it is appropriate to describe the current in terms of the quantum
yield. The external quantum yield (
Φ
ext
) is
the fraction of photons incident on the photoelectrochemical cell that
produce minority carriers that are collected
as current. The current density can be converted to
Φ
ext
using the following equation:
(S1)
where
i
is the current in mA,
q
is the unsigned electronic charge,
A
Si
and
A
PD
are the areas of the Si working
electrode and the photodiode detector, respectively,
P
is the power incident on the photodiode in mW,
λ
is the
wavelength in nm (808 nm in this work),
h
is Planck’s constant, and
c
is the speed of light. The number of
incident photons is calculcated from the current density of the c
alibrated Si photodiode parallel to the working
electrode.
Data Correction for Concentration Overpotenial and Uncompensated Resi
stance
To determine the intrinsic behavior of the semiconductor materi
al, the measured data were corrected for the
concentration overpotential and uncompensated resistances that are
present in unoptimzed electrochemical cell
designs. To calculate the fill factor
, ff,
and the photocathode efficiency,
η
808
, inherent to the photoelectrode, a
polished glassy carbon electrode was used to determine the limiti
ng anodic (
J
l,a
) and cathodic (
J
l,c
) current
densities, as well as the uncompensated resistance (
R
cell
) of the cell (eqn S2).
4,5
Raw
J-E
data were then corrected
for both losses (eqn S3), and the corrected data were used to calc
ulate the
ff
corr
and
η
808,corr
that are reported in
Table S1.
(S2)
S3
(S3)
The correction parameters were very similar for all th
ree of the buffered electrolytes:
R
cell
= 15 to 20

,
J
l,a
= 3 to
5 mA cm
-2
,
J
l,c
= -40 to -50 mA cm
-2
. While the data correction did not change the value of
V
oc
, it slightly
changed the
J
sc
used to calculate the
ff
corr
for samples that did not reach a saturation current density a
t the solution
potential (
E
corr
= 0).
V
oc
/ mV
J
sc
/ mA cm
-2
Φ
ext, sc
ff
raw
ff
corr
η
808, raw
η
808, corr
n
+
-p Si Wire Array (best)
pH 2.9
545
9.69
0.25
0.65
0.72
5.8%
6.4%
pH 5.9
537
9.62
0.25
0.65
0.71
5.6%
6.1%
pH 8.9
535
9.45
0.25
0.65
0.73
5.5%
6.2%
n
+
-p Wire Array Electrodes (average)
pH 2.9
527 ± 25
7.75 ± 2.74
0.20 ± 0.07
0.62 ±
0.05
0.67 ± 0.07
4.3 ± 2.0%
4.7 ± 2.3%
pH 5.9
519 ± 26
7.95 ± 2.37
0.20 ± 0.06
0.64 ±
0.01
0.69 ± 0.03
4.4 ± 1.6%
4.8 ± 1.8%
pH 8.9
518 ± 23
7.78 ± 2.36
0.20 ± 0.06
0.62 ±
0.05
0.68 ± 0.07
4.2 ± 1.8%
4.7 ± 2.1%
p-Si Wire Array Electrodes
pH 2.9
418 ± 14
7.31 ± 0.50
0.19 ± 0.01
0.45 ±
0.02
0.48 ± 0.02
2.3 ± 0.2%
2.5 ± 0.2%
pH 5.9
378 ± 15
8.21 ± 1.07
0.21 ± 0.03
0.44 ±
0.02
0.48 ± 0.03
2.2 ± 0.3%
2.5 ± 0.3%
pH 8.9
228 ± 8
8.31 ± 0.73
0.21 ± 0.02
0.40 ± 0
.01
0.53 ± 0.03
1.3 ± 0.1%
1.7 ± 0.3%
n
+
-p Si Planar Electrodes
pH 2.9
560 ± 19
24.0 ± 4.5
0.61 ± 0.12
0.45 ± 0
.02
0.58 ± 0.04
10.2 ± 2.0%
13.5 ± 2.6%
pH 5.9
550 ± 15
27.4 ± 1.0
0.70 ± 0.02
0.38 ± 0
.03
0.52 ± 0.04
9.5 ± 1.0%
13.5 ± 1.4%
pH 8.9
554 ± 13
25.8 ± 0.4
0.66 ± 0.01
0.41 ± 0
.00
0.61 ± 0.05
9.8 ± 0.1%
13.8 ± 0.5%
p-Si Planar Electrodes
pH 2.9
530 ± 34
22.2 ± 3.2
0.57 ± 0.08
0.55 ± 0
.05
0.63 ± 0.06
10.8 ± 2.0%
12.4 ± 2.6%
pH 5.9
371 ± 17
22.7 ± 4.6
0.58 ± 0.12
0.43 ± 0
.00
0.58 ± 0.03
6.0 ± 1.0%
8.2 ± 1.8%
pH 8.9
264 ± 8
20.1 ± 3.4
0.41 ± 0.18
0.24 ± 0.
05
0.41 ± 0.13
2.1 ± 0.8%
3.8 ± 1.9%
Table S1:
Photoelectrochemical performance data for each type
of electrode measured under 60 mW cm
-2
of 808 nm illumination in aqueous
solution with 50 mM MV
2+/+
REFERENCES
(1)
CRC Handbook of Chemistry and Physics
; CRC Press: Boca Raton, FL, 2010.
(2)
To compare the effect of the buffer, a pH 5.9 phthalate buffer
was made to directly compare the effect of
phosphate/phthalate. No appreciable difference in performance or
stability was discerned between these two
buffers.
(3)
Boettcher, S. W.; Spurgeon, J. M.; Putnam, M. C.; Warren, E. L.;
Turner-Evans, D. B.; Kelzenberg, M.
D.; Maiolo, J. R.; Atwater, H. A.; Lewis, N. S.
Science
2010
,
327
, 185.
(4)
Hamann, T. W.; Gstrein, F.; Brunschwig, B. S.; Lewis, N. S.
J. Am. Chem. Soc.
2005
,
127
, 7815.
(5)
Spurgeon, J. M.; Boettcher, S. W.; Kelzenberg, M. D.; Brunschwi
g, B. S.; Atwater, H. A.; Lewis, N. S.
Adv. Mater.
2010
,
22
, 3277.
S4