Arrays of B-doped p-Si microwires, diffusion-doped with P to form a radial n+ emitter and subsequently coated with a 1.5-nm-thick discontinuous film of evaporated Pt, were used as photocathodes for H_2 evolution from water. These electrodes yielded thermodynamically based energy-conversion efficiencies >5% under 1 sun solar simulation, despite absorbing less than 50% of the above-band-gap incident photons. Analogous p-Si wire-array electrodes yielded efficiencies <0.2%, largely limited by the low photovoltage generated at the p-Si/H_2O junction

Atwater, Harry A.

Boettcher, Shannon W.

Brunschwig, Bruce S.

Kelzenberg, Michael D.

Lewis, Nathan S.

McKone, James R.

Putnam, Morgan C.

Santori, Elizabeth A.

Turner-Evans, Daniel

Walter, Michael G.

Warren, Emily L.

English

Caltech Authors - Main

Supporting Information  
 
 Photoelectrochemical hydrogen evolution using Si microwire arrays 
 
Shannon W. Boettcher, Emily L. Warren, Morgan C. Putnam, Elizabeth A. Santori, Daniel 
Turner-Evans, Michael D. Kelzenberg, Michael G. Walter, James R. McKone, Bruce B. 
Brunschwig, Harry A. Atwater*, Nathan S. Lewis* 
 
Supplementary Figures 
 
Figure S1. Panel (a) shows the J-E data collected for the electrode fabricated with ‘enhanced’ 
absorption due to light-trapping elements, in 0.5 M aq. H2SO4 under ELH-type W-halogen solar 
simulation. Panel (b) shows a cross-sectional SEM image of the same sample. Panel (c) 
compares the spectral response collected for the sample with light-trapping elements 
(‘enhanced’) versus the spectral response for the normal sample. The red response in the 
‘enhanced’ cell is significantly improved. Panel (d) shows the increased Jsc with reduced angle 
dependence, for the enhanced sample compared to the normal sample. Panel (e) shows a digital 
photograph of a normal Pt/n+p-Si wire-array electrode evolving hydrogen under ~ 1 sun 
illumination. Small bubbles can be seen nucleating on the wire-array surface. The larger bubbles 
are stuck on the epoxy, and are the result of the coalescence of many small bubbles. 
 
 
 S1
 
Figure S2. Spectral irradiance of solar simulator lamps compared to the ASTM AM1.5G (global 
tilt) reference spectra. 
 
Materials and Methods 
 
Wire-array growth. Si microwire arrays were grown using a process similar to that described 
previously.1, 2 Boron-doped p+-Si (111) wafers, having a resistivity of < 0.001 ·cm (Silicon 
Quest International), were used as growth substrates. The wafers were coated with 450 nm of 
thermal oxide that had been patterned with 4-μm-diameter circular holes arranged on a square 
lattice with a 7 μm pitch. The holes were defined in the oxide by photolithographic exposure and 
development of a photoresist layer (Microchem S1813), followed by a buffered HF(aq) (BHF) 
etch (Transene Inc.).  The holes were then filled with 600 nm of Cu (ESPI metals, 6N) via 
thermal evaporation onto the patterned photoresist, followed by liftoff. Patterned substrates 
approximately 1.5 cm × 1.5 cm in dimension were then annealed in a tube furnace for 20 min at 
1000 °C under H2 flowing at a rate of 500 sccm. Wire growth was initiated by flowing SiCl4 
(Strem, 99.9999+%), BCl3 (Matheson, 0.25% in H2), and H2 (Matheson, research grade) at rates 
of 10, 1.0, and 500 sccm, respectively, for 30 min. After growth, the tube was purged with N2 at 
200 sccm and was allowed cool to ~ 650 °C over the course of ~30 min. The resulting wires 
were typically between 40-60 m in length and ~2.8 m in diameter. These growth conditions 
have been shown to yield p-Si wires with active doping concentrations of ~ 1017 cm-3.1, 3 
 S2
 
Diffusion of the radial n+ emitter.  The Cu catalyst was removed from the as-grown wire arrays 
by etching in 10 % aq. HF for 10 s, 6:1:1 by volume H2O:H2O2 (30 % in H2O):conc. aq. HCl at 
75 °C for 15 min, 10% aq. HF for 10 s, and 20 wt % aq. KOH at 20 °C for 60 s. A conformal 
SiO2 diffusion-barrier that was ~ 200 nm in thickness was grown via dry thermal oxidation for 
2 h at 1100 °C under a pure O2 ambient. The wire-array samples were then coated with a solution 
that contained 4.4 g hexamethycyclotrisiloxane (Sigma-Aldrich), 1 g polydimethylsiloxane 
PDMS (Sylgard 184, Dow Corning), and 0.10 g of PDMS curing agent in 5 ml of 
dicholoromethane. These samples were then spun at 1000 RPM for 30 s and cured at 150 °C for 
30 min to produce a 10–20 μm thick PDMS layer selectively at the base of the wire array.6 After 
a ~ 2 s etch in a 1:1 mixture of 1.0 M tetrabutylammonium fluoride in tetrahydrofuran (Sigma-
Aldrich) and dimethylformamide (referred to as ‘PDMS etch’) and a H2O rinse, these partially 
in-filled arrays were immersed for 5 min in BHF to remove the exposed diffusion-barrier oxide. 
The PDMS was then completely removed by etching for 30 min in PDMS etch, which was 
followed by a 10 min piranha etch (3:1 aq. conc. H2SO4:H2O2) to remove residual organic 
contamination. After 5 s in 10 % aq. HF, thermal P diffusion was performed using solid-source 
CeP5O14 wafers (Saint-Gobain, PH-900 PDS) at 850 °C for 15 min under an N2 ambient, to yield 
a radial n+ emitter region in the regions of the wires unprotected by the thermal oxide.  A SEM 
image of these wires (after a 10 s etch in BHF to removed the thin dopant oxide) is shown in 
Figure 4d. Based on spreading-resistance measurements on planar control wafers, we estimate 
the dopant concentration in the n+ layer at ~1019 cm-3 with a junction depth of ~ 200 nm. 
 The wire array chip was then heated to 150 °C on a hot plate, and mounting wax 
(Quickstick 135, South Bay Tech.) was melted into the array.  Excess wax was removed from the 
array by applying gentle, even pressure with the flat surface of a glass cover slip to a tissue 
(KimWipe) that was draped over the sample on the hotplate.  The mounting wax was then etched 
in an O2 plasma (400 W, 300 mTorr) until ~ 10 - 20 m of the wire tips were exposed (~60 min) 
as shown in Figure 3d. 
 
Fabrication of planar n+p-Si. The process for pn-junction fabrication was adapted from 
Fahrenbruch and Bube.4 Boron-doped p-type (100)-oriented Si wafers (Silicon Inc.) with a 
resistivity of ~ 0.7  cm were unpacked in a clean-room and cleaved into chips ~ 3 cm x 3 cm. 
 S3
These chips were cleaned for 15 min in 6:1:1 by volume H2O:H2O2 (30 % in H2O):conc. aq. 
NH3OH at 75 °C (RCA 1) followed by 15 min in 6:1:1 by volume H2O:H2O2 (30 % in H2O):conc. 
aq. HCl at 75 °C (RCA 2). The chips were then etched for 30 s in BHF, rinsed in H2O, and dried.  
 These clean p-Si chips were then stacked in-between solid source CeP5O14 diffusion 
wafers (Saint-Gobain, PH-900 PDS) and heated at 850 °C for 15 min under an N2 ambient in a 
tube furnace. The n+p-Si wafers were then etched for 30 s in BHF to remove the dopant oxide. 
500 – 1000 nm of Al was then thermally evaporated onto the unpolished back surface of the 
wafers. The wafers were annealed for 10 min at 800 °C to drive the evaporated Al through the 
backside n+ layer to make ohmic contacts. The edges of the chips were cleaved off and discarded, 
to eliminate shunts from the backside Al ohmic contact around the edge to the diffused n+ emitter. 
 
Electrode fabrication. Four types of electrodes were fabricated, consisting of p-Si planar, p-Si 
wire-array, n+p-Si planar, and n+p-Si wire-array samples. In each case, the Si samples were 
cleaved into square ~ 0.5 cm × 0.5 cm pieces. Ohmic contact to the p-Si wire-array, p-Si planar, 
and n+p-Si wire-array chips were made by rubbing Ga-In eutectic on the back side of the chip. 
For the n+p-Si samples, the annealed Al back-contact served as an ohmic contact. Electrical 
connections were made to the samples by attachment of coiled tin-copper wire using conductive 
silver paint. The wire arrays were then sealed, using Hysol 1C epoxy, at the end of glass tubing 
through which the wire had been directed such that the surface-normal direction to the chip was 
perpendicular to the glass tube. A second type of epoxy (Hysol 9460) was used to define the 
active area of the electrode (~0.02 to 0.1 cm2), because Hysol 1C was found to wick in-between 
the Si microwires on the face of the electrode.  
Prior to Pt deposition and photoelectrochemical measurements, the p-Si wire-arrays 
samples (but not the others) were etched as follows: 10 s 10 % aq. HF, 30 min 30 wt. % aq. 
FeCl3, 10 s 10 % aq. HF, 1 min 20 wt. % aq. KOH, and 10 s 10 % aq. HF. After each step, the 
wires were rinsed with 18.3 MΩ cm resistivity H2O, and dried under a stream of N2 (g). This 
etching sequence removed the copper catalyst, the outer ~50 nm of Si, and the native oxide, 
leaving a clean surface for Pt deposition. 
 
Platinum deposition. Pt deposition on the p-Si electrodes was accomplished via a galvanic 
displacement reaction whereby Si is oxidized (and then etched by HF) and Pt is reduced onto the 
 S4
electrode surface.5 The planar samples were immersed for 3-4 ~2.5 min intervals in an aq. 
solution that contained 0.5 M HF and 1 mM K2PtCl6. The electrode was tested after each 
deposition interval (see below). The electrode performance typically increased after every 
deposition cycle up to the 3rd or 4th cycle, after which additional deposition of Pt did not improve 
the performance. Slightly improved performance could be achieved by sonicating the electrode 
to remove excess Pt and then re-platinizing for an additional 2.5 min. The data shown in Figure 
1a are representative of the best performance that was achieved using this procedure.  
The wire-array samples were similarly immersed in the platinization solution for 2.5 min 
intervals, with the optimum time around 7.5 min. Depositions for longer times were attempted, 
but led to lower photocurrents and hence worse performance. 
 For the n+p-Si samples, Pt was deposited by electron beam evaporation at a base pressure 
of ~ 6 x 10-6 torr onto the completed epoxy-sealed electrodes as the final step before testing. The 
thickness of Pt was monitored by a quartz crystal microbalance. 1 nm of Pt was deposited on the 
planar electrodes. For the wire arrays, 1.5 nm was deposited, and the wires were tilted along one 
axis during deposition to improve the sidewall coverage of the exposed wire tips. 
 
Photoelectrochemical characterization. All photoelectrochemical measurements were 
performed in a flat-sided Pyrex glass electrochemical cell. The solution was stirred rapidly to 
minimize bubble nucleation on the electrode and to reduce the associated variation of the 
measured J-E data. The measurements were performed using a three-electrode configuration 
with a saturated calomel reference electrode (SCE) and Pt-coil counter electrode, each separated 
from the main compartment by a medium porosity glass frit. After data collection, the data were 
shifted on the potential axis such that the potential of the reversible hydrogen electrode (RHE) 
was zero, but no other corrections were performed (i.e. the data were not iR corrected or 
corrected for any other extrinsic losses). For all measurements reported herein, research-grade H2 
was continuously bubbled through the solution in order to maintain a fixed Nernst potential for 
the H+/H2 redox couple and to remove dissolved oxygen.  
 The Pt/p-Si electrodes were tested in  0.5 M aq. K2SO4 adjusted to pH ~ 2 using H2SO4. 
To remove detrimental dissolved metal impurities from these standard-grade lab reagents, the 
electrolyte solution was pre-electrolyzed for > 12 h with ~3 V applied across two large carbon-
cloth electrodes under stirring. The electrodes were tested at pH ~ 2, because this pH appeared to 
 S5
be a good compromise between, (1) the reduced Pt activity and Si stability at higher pH’s and, 
(2) reduced Voc values at lower pH’s. Platinized p-Si wire-array electrodes with the base masked 
off with SiO2 (in an identical fashion to the n
+p-Si electrodes), did not show performance 
significantly different than p-Si wire-array electrodes without the SiO2 mask. 
 The Pt/n+p-Si electrodes were tested in ultrapure 0.5 M H2SO4 (Aristar Ultra) in 18.2 M 
resistivity water. The lower pH was selected due to the higher activity of the Pt catalyst in acidic 
media. 
During measurements, the cell was illuminated using either a 300 W tungsten-halogen 
ELH lamp (OSRAM) or a Newport Oriel Xe lamp with an AM1.5G filter set. The light intensity 
was calibrated using a Si photodiode to produce a photocurrent equivalent to that obtained under 
100 mW cm-2 of AM1.5 illumination at the working electrode. The spectral irradiance curves for 
both lamps are given in Figure S2. 
The quality factor (n) for the electrodes was extracted by a linear fit of the dependence of 
the photovoltage (Voc) on the logarithm of the light-intensity (and hence photocurrent, Jph). The 
analytical expression for the photovoltage is given by the ideal diode equation solved for zero net 
current: 
Voc = (nkBT/q) ln (Jph/Js),                       (1) 
 
where n is the diode quality factor, kB (m
2 kg s-2 K-1) is Boltzmann’s constant, T (in K) is the 
temperature, q (C) is the charge on an electron, Jph (A m
-2) is the photocurrent density, Js is the 
saturation current density (related to recombination pathways), and is the ratio of the actual 
junction area to the geometric surface area of the electrode (i.e. the roughness factor).1  
 
Angle-resolved photocurrent measurements. The short-circuit (E = 0 vs. RHE) photocurrent 
was measured under broad-area 100 mW cm-2 of ELH-type illumination while the electrode was 
rotated along one axis. Normal incidence was taken to be the angle at which the photocurrent 
was minimized. 
 
Spectral response. Spectral response measurements were made  using illumination from a 75 W 
Xe lamp that was passed through an Oriel monochromator (0.5 mm slits), chopped at ~13 Hz, 
and focused to a beam spot that was adjusted in size to slightly under-fill the electrode area. A 
 S6
calibrated Si diode (UDT UV-050) was used as a standard for the sample channel. Another Si 
photodiode was used to measure a beam-split portion of the illumination and hence serve as a 
continuous calibration of the output intensity. A potentiostat (Gamry Series G 300) was used to 
hold the potential of the Si working electrode at short circuit (E = 0 vs. RHE) and to record the 
sample current. The chopped components of the signals were measured with independent lock-in 
detection of the sample channel (potentiostat analog output) and of the calibration channel.  
 
Fabrication of electrodes with light-trapping features. Electrodes were also fabricated with 
light trapping features that included: (1) an a-SiNx:H antireflective coating (which also serves to 
enhance carrier collection from the non-pn-junction portion of the wire initially protected with 
SiO2)
6, (2) a Ag back reflector, and (3) Al2O3 light-scattering particles (Figure S1e). We have 
recently shown that these elements enhance the optical absorption7 and hence the efficiency of 
solid-state wire-array photovoltaics.8 After pn-junction fabrication, the wire arrays were etched 
for 5 min in BHF, to completely remove the remaining oxide diffusion barrier.  The wires were 
then cleaned for 15 min in 6:1:1 by volume H2O:H2O2 (30 % in H2O):conc. aq. HCl at 75 °C and 
30 s in BHF, prior to deposition of an a-SiNx:H layer (~ 140 nm thick at the wire tip and ~ 60 nm 
thick at the wire base) using plasma-enhanced chemical vapor deposition, as described 
previously.7 
1-m planar-equivalent thickness of Ag was then deposited in successive 500-nm-thick 
thermal evaporations at two different shallow angles (± ~5°) while the sample was slowly rotated.  
The array was then infilled with ~5 μm of PDMS using a process similar to the one described 
above. This PDMS etch barrier allowed the Ag at the wire tips and sidewalls to be selectively 
removed by etching for 6.5 min in 8:1:1 methanol: conc. aq. NH4OH: 30 wt.% aq. H2O2. 
Al2O3 light-scattering particles with an 80-nm nominal diameter (South Bay Technology) 
were then added to the wire array.  The wire-array was placed face-up in a flat-bottomed glass 
centrifuge tube and ~ 3 ml of an ethanolic dispersion of the particles (~0.3 mg/ml) was added.  
Centrifugation (~3000 RPM) for 5 min was used to drive the particles to the base of the wire-
array. The array was then infilled with wax and processed similarly to the devices without the 
added light trapping features. Prior to Pt deposition the a-SiNx:H layer at the exposed tips of the 
wires was removed with a ~2 min BHF etch.  
 
 S7
 S8
 
 
 
Supplementary References 
 
(1) 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, (5962), 
185-187. 
(2) Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Kelzenberg, M. D.; Lewis, N. S.; Atwater, H. 
A., Appl. Phys. Lett. 2007, 91, (10). 
(3) Putnam, M. C.; Turner-Evans, D. B.; Kelzenberg, M. D.; Boettcher, S. W.; Lewis, N. S.; 
Atwater, H. A., Appl. Phys. Lett. 2009, 95, (16). 
(4) Fahrenbruch, A. L.; Bube, R. H., Fundamentals of Solar Cells. Academic Press: Pg. 272, 
1983. 
(5) Lombardi, I.; Marchionna, S.; Zangari, G.; Pizzini, S., Langmuir 2007, 23, (24), 12413-
12420. 
(6) Kelzenberg, M. D.; Turner-Evans, D. B.; Putnam, M. C.; Boettcher, S. W.; Briggs, R. M.; 
Baek, J. Y.; Lewis, N. S.; Atwater, H. A., Energy Environ. Sci. 2010, Submitted. 
(7) Kelzenberg, M. D.; Boettcher, S. W.; Petykiewicz, J. A.; Turner-Evans, D. B.; Putnam, M. 
C.; Warren, E. L.; Spurgeon, J. M.; Briggs, R. M.; Lewis, N. S.; Atwater, H. A., Nat. Mater. 
2010, 9, (3), 239-244. 
(8) Putnam, M. C.; Boettcher, S. W.; Kelzenberg, M. D.; Turner-Evans, D. B.; Spurgeon, J. 
M.; Warren, E. L.; Briggs, R. M.; Lewis, N. S.; Atwater, H. A., Energy Environ. Sci. 2010, 
3, 1037. 
 


Photoelectrochemical Hydrogen Evolution Using Si Microwire Arrays

Supporting Information  
 
 Photoelectrochemical hydrogen evolution using Si microwire arrays 
 
Shannon W. Boettcher, Emily L. Warren, Morgan C. Putnam, Elizabeth A. Santori, Daniel 
Turner-Evans, Michael D. Kelzenberg, Michael G. Walter, James R. McKone, Bruce B. 
Brunschwig, Harry A. Atwater*, Nathan S. Lewis* 
 
Supplementary Figures 
 
Figure S1. Panel (a) shows the J-E data collected for the electrode fabricated with ‘enhanced’ 
absorption due to light-trapping elements, in 0.5 M aq. H2SO4 under ELH-type W-halogen solar 
simulation. Panel (b) shows a cross-sectional SEM image of the same sample. Panel (c) 
compares the spectral response collected for the sample with light-trapping elements 
(‘enhanced’) versus the spectral response for the normal sample. The red response in the 
‘enhanced’ cell is significantly improved. Panel (d) shows the increased Jsc with reduced angle 
dependence, for the enhanced sample compared to the normal sample. Panel (e) shows a digital 
photograph of a normal Pt/n+p-Si wire-array electrode evolving hydrogen under ~ 1 sun 
illumination. Small bubbles can be seen nucleating on the wire-array surface. The larger bubbles 
are stuck on the epoxy, and are the result of the coalescence of many small bubbles. 
 
 
 S1
 
Figure S2. Spectral irradiance of solar simulator lamps compared to the ASTM AM1.5G (global 
tilt) reference spectra. 
 
Materials and Methods 
 
Wire-array growth. Si microwire arrays were grown using a process similar to that described 
previously.1, 2 Boron-doped p+-Si (111) wafers, having a resistivity of < 0.001 ·cm (Silicon 
Quest International), were used as growth substrates. The wafers were coated with 450 nm of 
thermal oxide that had been patterned with 4-μm-diameter circular holes arranged on a square 
lattice with a 7 μm pitch. The holes were defined in the oxide by photolithographic exposure and 
development of a photoresist layer (Microchem S1813), followed by a buffered HF(aq) (BHF) 
etch (Transene Inc.).  The holes were then filled with 600 nm of Cu (ESPI metals, 6N) via 
thermal evaporation onto the patterned photoresist, followed by liftoff. Patterned substrates 
approximately 1.5 cm × 1.5 cm in dimension were then annealed in a tube furnace for 20 min at 
1000 °C under H2 flowing at a rate of 500 sccm. Wire growth was initiated by flowing SiCl4 
(Strem, 99.9999+%), BCl3 (Matheson, 0.25% in H2), and H2 (Matheson, research grade) at rates 
of 10, 1.0, and 500 sccm, respectively, for 30 min. After growth, the tube was purged with N2 at 
200 sccm and was allowed cool to ~ 650 °C over the course of ~30 min. The resulting wires 
were typically between 40-60 m in length and ~2.8 m in diameter. These growth conditions 
have been shown to yield p-Si wires with active doping concentrations of ~ 1017 cm-3.1, 3 
 S2
 Diffusion of the radial n+ emitter.  The Cu catalyst was removed from the as-grown wire arrays 
by etching in 10 % aq. HF for 10 s, 6:1:1 by volume H2O:H2O2 (30 % in H2O):conc. aq. HCl at 
75 °C for 15 min, 10% aq. HF for 10 s, and 20 wt % aq. KOH at 20 °C for 60 s. A conformal 
SiO2 diffusion-barrier that was ~ 200 nm in thickness was grown via dry thermal oxidation for 
2 h at 1100 °C under a pure O2 ambient. The wire-array samples were then coated with a solution 
that contained 4.4 g hexamethycyclotrisiloxane (Sigma-Aldrich), 1 g polydimethylsiloxane 
PDMS (Sylgard 184, Dow Corning), and 0.10 g of PDMS curing agent in 5 ml of 
dicholoromethane. These samples were then spun at 1000 RPM for 30 s and cured at 150 °C for 
30 min to produce a 10–20 μm thick PDMS layer selectively at the base of the wire array.6 After 
a ~ 2 s etch in a 1:1 mixture of 1.0 M tetrabutylammonium fluoride in tetrahydrofuran (Sigma-
Aldrich) and dimethylformamide (referred to as ‘PDMS etch’) and a H2O rinse, these partially 
in-filled arrays were immersed for 5 min in BHF to remove the exposed diffusion-barrier oxide. 
The PDMS was then completely removed by etching for 30 min in PDMS etch, which was 
followed by a 10 min piranha etch (3:1 aq. conc. H2SO4:H2O2) to remove residual organic 
contamination. After 5 s in 10 % aq. HF, thermal P diffusion was performed using solid-source 
CeP5O14 wafers (Saint-Gobain, PH-900 PDS) at 850 °C for 15 min under an N2 ambient, to yield 
a radial n+ emitter region in the regions of the wires unprotected by the thermal oxide.  A SEM 
image of these wires (after a 10 s etch in BHF to removed the thin dopant oxide) is shown in 
Figure 4d. Based on spreading-resistance measurements on planar control wafers, we estimate 
the dopant concentration in the n+ layer at ~1019 cm-3 with a junction depth of ~ 200 nm. 
 The wire array chip was then heated to 150 °C on a hot plate, and mounting wax 
(Quickstick 135, South Bay Tech.) was melted into the array.  Excess wax was removed from the 
array by applying gentle, even pressure with the flat surface of a glass cover slip to a tissue 
(KimWipe) that was draped over the sample on the hotplate.  The mounting wax was then etched 
in an O2 plasma (400 W, 300 mTorr) until ~ 10 - 20 m of the wire tips were exposed (~60 min) 
as shown in Figure 3d. 
 
Fabrication of planar n+p-Si. The process for pn-junction fabrication was adapted from 
Fahrenbruch and Bube.4 Boron-doped p-type (100)-oriented Si wafers (Silicon Inc.) with a 
resistivity of ~ 0.7  cm were unpacked in a clean-room and cleaved into chips ~ 3 cm x 3 cm. 
 S3
These chips were cleaned for 15 min in 6:1:1 by volume H2O:H2O2 (30 % in H2O):conc. aq. 
NH3OH at 75 °C (RCA 1) followed by 15 min in 6:1:1 by volume H2O:H2O2 (30 % in H2O):conc. 
aq. HCl at 75 °C (RCA 2). The chips were then etched for 30 s in BHF, rinsed in H2O, and dried.  
 These clean p-Si chips were then stacked in-between solid source CeP5O14 diffusion 
wafers (Saint-Gobain, PH-900 PDS) and heated at 850 °C for 15 min under an N2 ambient in a 
tube furnace. The n+p-Si wafers were then etched for 30 s in BHF to remove the dopant oxide. 
500 – 1000 nm of Al was then thermally evaporated onto the unpolished back surface of the 
wafers. The wafers were annealed for 10 min at 800 °C to drive the evaporated Al through the 
backside n+ layer to make ohmic contacts. The edges of the chips were cleaved off and discarded, 
to eliminate shunts from the backside Al ohmic contact around the edge to the diffused n+ emitter. 
 
Electrode fabrication. Four types of electrodes were fabricated, consisting of p-Si planar, p-Si 
wire-array, n+p-Si planar, and n+p-Si wire-array samples. In each case, the Si samples were 
cleaved into square ~ 0.5 cm × 0.5 cm pieces. Ohmic contact to the p-Si wire-array, p-Si planar, 
and n+p-Si wire-array chips were made by rubbing Ga-In eutectic on the back side of the chip. 
For the n+p-Si samples, the annealed Al back-contact served as an ohmic contact. Electrical 
connections were made to the samples by attachment of coiled tin-copper wire using conductive 
silver paint. The wire arrays were then sealed, using Hysol 1C epoxy, at the end of glass tubing 
through which the wire had been directed such that the surface-normal direction to the chip was 
perpendicular to the glass tube. A second type of epoxy (Hysol 9460) was used to define the 
active area of the electrode (~0.02 to 0.1 cm2), because Hysol 1C was found to wick in-between 
the Si microwires on the face of the electrode.  
Prior to Pt deposition and photoelectrochemical measurements, the p-Si wire-arrays 
samples (but not the others) were etched as follows: 10 s 10 % aq. HF, 30 min 30 wt. % aq. 
FeCl3, 10 s 10 % aq. HF, 1 min 20 wt. % aq. KOH, and 10 s 10 % aq. HF. After each step, the 
wires were rinsed with 18.3 MΩ cm resistivity H2O, and dried under a stream of N2 (g). This 
etching sequence removed the copper catalyst, the outer ~50 nm of Si, and the native oxide, 
leaving a clean surface for Pt deposition. 
 
Platinum deposition. Pt deposition on the p-Si electrodes was accomplished via a galvanic 
displacement reaction whereby Si is oxidized (and then etched by HF) and Pt is reduced onto the 
 S4
electrode surface.5 The planar samples were immersed for 3-4 ~2.5 min intervals in an aq. 
solution that contained 0.5 M HF and 1 mM K2PtCl6. The electrode was tested after each 
deposition interval (see below). The electrode performance typically increased after every 
deposition cycle up to the 3rd or 4th cycle, after which additional deposition of Pt did not improve 
the performance. Slightly improved performance could be achieved by sonicating the electrode 
to remove excess Pt and then re-platinizing for an additional 2.5 min. The data shown in Figure 
1a are representative of the best performance that was achieved using this procedure.  
The wire-array samples were similarly immersed in the platinization solution for 2.5 min 
intervals, with the optimum time around 7.5 min. Depositions for longer times were attempted, 
but led to lower photocurrents and hence worse performance. 
 For the n+p-Si samples, Pt was deposited by electron beam evaporation at a base pressure 
of ~ 6 x 10-6 torr onto the completed epoxy-sealed electrodes as the final step before testing. The 
thickness of Pt was monitored by a quartz crystal microbalance. 1 nm of Pt was deposited on the 
planar electrodes. For the wire arrays, 1.5 nm was deposited, and the wires were tilted along one 
axis during deposition to improve the sidewall coverage of the exposed wire tips. 
 
Photoelectrochemical characterization. All photoelectrochemical measurements were 
performed in a flat-sided Pyrex glass electrochemical cell. The solution was stirred rapidly to 
minimize bubble nucleation on the electrode and to reduce the associated variation of the 
measured J-E data. The measurements were performed using a three-electrode configuration 
with a saturated calomel reference electrode (SCE) and Pt-coil counter electrode, each separated 
from the main compartment by a medium porosity glass frit. After data collection, the data were 
shifted on the potential axis such that the potential of the reversible hydrogen electrode (RHE) 
was zero, but no other corrections were performed (i.e. the data were not iR corrected or 
corrected for any other extrinsic losses). For all measurements reported herein, research-grade H2 
was continuously bubbled through the solution in order to maintain a fixed Nernst potential for 
the H+/H2 redox couple and to remove dissolved oxygen.  
 The Pt/p-Si electrodes were tested in  0.5 M aq. K2SO4 adjusted to pH ~ 2 using H2SO4. 
To remove detrimental dissolved metal impurities from these standard-grade lab reagents, the 
electrolyte solution was pre-electrolyzed for > 12 h with ~3 V applied across two large carbon-
cloth electrodes under stirring. The electrodes were tested at pH ~ 2, because this pH appeared to 
 S5
be a good compromise between, (1) the reduced Pt activity and Si stability at higher pH’s and, 
(2) reduced Voc values at lower pH’s. Platinized p-Si wire-array electrodes with the base masked 
off with SiO2 (in an identical fashion to the n+p-Si electrodes), did not show performance 
significantly different than p-Si wire-array electrodes without the SiO2 mask. 
 The Pt/n+p-Si electrodes were tested in ultrapure 0.5 M H2SO4 (Aristar Ultra) in 18.2 M 
resistivity water. The lower pH was selected due to the higher activity of the Pt catalyst in acidic 
media. 
During measurements, the cell was illuminated using either a 300 W tungsten-halogen 
ELH lamp (OSRAM) or a Newport Oriel Xe lamp with an AM1.5G filter set. The light intensity 
was calibrated using a Si photodiode to produce a photocurrent equivalent to that obtained under 
100 mW cm-2 of AM1.5 illumination at the working electrode. The spectral irradiance curves for 
both lamps are given in Figure S2. 
The quality factor (n) for the electrodes was extracted by a linear fit of the dependence of 
the photovoltage (Voc) on the logarithm of the light-intensity (and hence photocurrent, Jph). The 
analytical expression for the photovoltage is given by the ideal diode equation solved for zero net 
current: 
Voc = (nkBT/q) ln (Jph/Js),                       (1) 
 
where n is the diode quality factor, kB (m2 kg s-2 K-1) is Boltzmann’s constant, T (in K) is the 
temperature, q (C) is the charge on an electron, Jph (A m-2) is the photocurrent density, Js is the 
saturation current density (related to recombination pathways), and is the ratio of the actual 
junction area to the geometric surface area of the electrode (i.e. the roughness factor).1  
 
Angle-resolved photocurrent measurements. The short-circuit (E = 0 vs. RHE) photocurrent 
was measured under broad-area 100 mW cm-2 of ELH-type illumination while the electrode was 
rotated along one axis. Normal incidence was taken to be the angle at which the photocurrent 
was minimized. 
 
Spectral response. Spectral response measurements were made  using illumination from a 75 W 
Xe lamp that was passed through an Oriel monochromator (0.5 mm slits), chopped at ~13 Hz, 
and focused to a beam spot that was adjusted in size to slightly under-fill the electrode area. A 
 S6
calibrated Si diode (UDT UV-050) was used as a standard for the sample channel. Another Si 
photodiode was used to measure a beam-split portion of the illumination and hence serve as a 
continuous calibration of the output intensity. A potentiostat (Gamry Series G 300) was used to 
hold the potential of the Si working electrode at short circuit (E = 0 vs. RHE) and to record the 
sample current. The chopped components of the signals were measured with independent lock-in 
detection of the sample channel (potentiostat analog output) and of the calibration channel.  
 
Fabrication of electrodes with light-trapping features. Electrodes were also fabricated with 
light trapping features that included: (1) an a-SiNx:H antireflective coating (which also serves to 
enhance carrier collection from the non-pn-junction portion of the wire initially protected with 
SiO2)6, (2) a Ag back reflector, and (3) Al2O3 light-scattering particles (Figure S1e). We have 
recently shown that these elements enhance the optical absorption7 and hence the efficiency of 
solid-state wire-array photovoltaics.8 After pn-junction fabrication, the wire arrays were etched 
for 5 min in BHF, to completely remove the remaining oxide diffusion barrier.  The wires were 
then cleaned for 15 min in 6:1:1 by volume H2O:H2O2 (30 % in H2O):conc. aq. HCl at 75 °C and 
30 s in BHF, prior to deposition of an a-SiNx:H layer (~ 140 nm thick at the wire tip and ~ 60 nm 
thick at the wire base) using plasma-enhanced chemical vapor deposition, as described 
previously.7 
1-m planar-equivalent thickness of Ag was then deposited in successive 500-nm-thick 
thermal evaporations at two different shallow angles (± ~5°) while the sample was slowly rotated.  
The array was then infilled with ~5 μm of PDMS using a process similar to the one described 
above. This PDMS etch barrier allowed the Ag at the wire tips and sidewalls to be selectively 
removed by etching for 6.5 min in 8:1:1 methanol: conc. aq. NH4OH: 30 wt.% aq. H2O2. 
Al2O3 light-scattering particles with an 80-nm nominal diameter (South Bay Technology) 
were then added to the wire array.  The wire-array was placed face-up in a flat-bottomed glass 
centrifuge tube and ~ 3 ml of an ethanolic dispersion of the particles (~0.3 mg/ml) was added.  
Centrifugation (~3000 RPM) for 5 min was used to drive the particles to the base of the wire-
array. The array was then infilled with wax and processed similarly to the devices without the 
added light trapping features. Prior to Pt deposition the a-SiNx:H layer at the exposed tips of the 
wires was removed with a ~2 min BHF etch.  
 
 S7
 S8
 
 
 
Supplementary References 
 
(1) 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, (5962), 
185-187. 
(2) Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Kelzenberg, M. D.; Lewis, N. S.; Atwater, H. 
A., Appl. Phys. Lett. 2007, 91, (10). 
(3) Putnam, M. C.; Turner-Evans, D. B.; Kelzenberg, M. D.; Boettcher, S. W.; Lewis, N. S.; 
Atwater, H. A., Appl. Phys. Lett. 2009, 95, (16). 
(4) Fahrenbruch, A. L.; Bube, R. H., Fundamentals of Solar Cells. Academic Press: Pg. 272, 
1983. 
(5) Lombardi, I.; Marchionna, S.; Zangari, G.; Pizzini, S., Langmuir 2007, 23, (24), 12413-
12420. 
(6) Kelzenberg, M. D.; Turner-Evans, D. B.; Putnam, M. C.; Boettcher, S. W.; Briggs, R. M.; 
Baek, J. Y.; Lewis, N. S.; Atwater, H. A., Energy Environ. Sci. 2010, Submitted. 
(7) Kelzenberg, M. D.; Boettcher, S. W.; Petykiewicz, J. A.; Turner-Evans, D. B.; Putnam, M. 
C.; Warren, E. L.; Spurgeon, J. M.; Briggs, R. M.; Lewis, N. S.; Atwater, H. A., Nat. Mater. 
2010, 9, (3), 239-244. 
(8) Putnam, M. C.; Boettcher, S. W.; Kelzenberg, M. D.; Turner-Evans, D. B.; Spurgeon, J. 
M.; Warren, E. L.; Briggs, R. M.; Lewis, N. S.; Atwater, H. A., Energy Environ. Sci. 2010, 
3, 1037. 
 


Photoelectrochemical Hydrogen Evolution Using Si Microwire Arrays

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