The electrocatalytic performance for hydrogen evolution has been evaluated for radial-junction n^+p-Si microwire (MW) arrays with Pt or cobalt phosphide, CoP, nanoparticulate catalysts in contact with 0.50 M H_2SO_4(aq). The CoP-coated (2.0 mg cm^(–2)) n^+p-Si MW photocathodes were stable for over 12 h of continuous operation and produced an open-circuit photovoltage (V_(oc)) of 0.48 V, a light-limited photocurrent density (J_(ph)) of 17 mA cm^(–2), a fill factor (ff) of 0.24, and an ideal regenerative cell efficiency (η_(IRC)) of 1.9% under simulated 1 Sun illumination. Pt-coated (0.5 mg cm^(–2)) n^+p-Si MW-array photocathodes produced V_(oc) = 0.44 V, J_(ph) = 14 mA cm^(–2), ff = 0.46, and η = 2.9% under identical conditions. Thus, the MW geometry allows the fabrication of photocathodes entirely comprised of earth-abundant materials that exhibit performance comparable to that of devices that contain Pt

Brunschwig, Bruce S.

Chorkendorff, Ib

Gray, Harry B.

Hansen, Ole

Lewis, Nathan S.

Pedersen, Thomas

Popczun, Eric J.

Read, Carlos G.

Roske, Christopher W.

Schaak, Raymond E.

Seger, Brian

Vesborg, Peter C. K.

Caltech Authors - Main

Page 1 
Supplementary Information for: 
 
Comparison of the Performance of CoP-Coated and Pt-Coated Radial-Junction 
n+p-Silicon Microwire-Array Photocathodes for the Sunlight-Driven Reduction of Water 
to H2(g) 
Christopher W. Roske†, Eric J. Popczun‡, Brian Seger∑, Carlos G. Read‡, Thomas 
Pedersen§, Ole Hansen§, Peter C. K. Vesborg∑, Bruce S. Brunschwig¶, Raymond E. 
Schaak*‡, Ib Chorkendorff*∑, Harry B. Gray*†, Nathan S. Lewis*†¶ ß 
† Division of Chemistry and Chemical Engineering, California Institute of Technology, 
Pasadena, CA, 91125, USA 
‡ Department of Chemistry and Materials Research Institute, The Pennsylvania State 
University, University Park, PA 16802, USA 
∑ Center for Individual Nanoparticle Functionality (CINF), Department of Physics, 
Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark 
§ Department of Micro- and Nanotechnology, Technical University of Denmark, DK-
2800 Kongens Lyngby, Denmark 
¶ Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA 
ß Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA 91125, 
USA 
 
 
 
Page 2 
Electrode Preparation 
 Reactive ion etching of Si(100) substrate (B-doped, resistivity of 14.6 ohm-cm) 
was used to form 4 μm diameter Si microwires (MWs) at an 11 μm pitch, with the wires 
being 50 μm in length. Radial n+ junctions on the p-Si microwires were formed by vapor-
phase P diffusion from phosphoryl chloride (POCl3) at 900 oC.  Ohmic contacts to the 
back of the Si(100) substrates were made by electron-beam evaporation of 150 nm of Al, 
followed by a 5 min anneal at 800 oC under forming gas (5% H2(g) in N2(g)). The Si MW 
arrays were etched for 10 s in 0.5 M HF(aq) (Transene, Inc.) and were dried under a 
stream of Ar(g).  Centrifugation was then promptly performed in 25 mL falcon tubes in a 
swinging-head centrifuge, with the tubes filled with ~15 mL of polydimethylsiloxane 
(PDMS, Dupont).  The PDMS was cured overnight while centrifuging at 3000 rpm.  Si 
MW arrays were then loaded into the bottom of the falcon tubes and centrifugation of the 
particles was performed at 3000 rpm with a suspension of the CoP nanoparticles (0.2 
mg/mL) in hexane for 5 min (the suspension was sonicated immediately prior to 
application of 15 μL for a 0.1 cm2 sample). A Cahn microbalance was used to determine 
the geometric mass loading (mg cm-2) by weighing samples before and after deposition of 
particles. After deposition of the particles, the catalyst was activated by annealing at 450 
oC for 30 min under forming gas. Ga-In eutectic was scratched onto the back side of the 
wire arrays, and Ag paint (SPI) was then applied to the back side of the arrays.  A Cu 
wire was affixed to the array using Ag paint.  The Cu wire was then threaded through a 
glass tube.  Hysol 9460 epoxy was used to adhere the glass tube to the back side of the 
electrode and to seal the back and sides of the electrode.   The epoxy was cured 
overnight. 
Page 3 
Synthesis of Pt nanoparticles: A solution of 0.10 mM H2PtCl6 (Sigma, ACS 
Reagent), 0.10 mM sodium polyacrylate (Mw 2100, Aldrich), and 0.50 mM ascorbic acid 
(Sigma, ACS Reagent) was prepared in 250 mL of degassed, deionized H2O with a 
resistivity of ~18.2 MΩ-cm obtained from a Barnsted Nanopure column. The solution 
was stirred for 1 h under N2(g), prior to cooling overnight with the subsequent addition of 
2.0 g of NaOH. The suspension was left overnight, centrifuged, and then washed with 
deionized H2O three times before suspending the particles in isopropanol.  
Synthesis of CoP nanoparticles: synthesized and characterized as previously 
reported.1 
Electrochemical Measurements 
Photoelectrochemical measurements were performed in 0.50 M H2SO4(aq).  
Experiments were performed in a Pyrex electrochemical cell in a conventional three-
electrode configuration, with a Ag/AgCl reference electrode (BASi) in saturated KCl(aq) 
and a high-purity carbon cloth counter electrode (FuelCellStore). During a typical 
acquisition with a Gamry Reference 600 potentiostat, H2(g) was constantly purged into 
the solution (except for the H2 determination case), to maintain a constant, reversible 
(H+/H2) Nernstian potential in the solution. The solution was rapidly stirred with a 
magnetic stir bar, to minimize mass-transport limitations and to minimize any effects due 
to bubble formation. Potentially detrimental oxidative processes were minimized by 
limiting the scans to potentials that were negative of the open-circuit potential. 
The cell was illuminated with an ELH-type W-halogen lamp equipped with a dichroic 
rear reflector. The light intensity was calibrated using a Si photodiode (Thorlabs UDT 
UV-005) that had been mounted in the same orientation as the Si working electrodes.  
Page 4 
The Si photodiode was calibrated, in turn, against a NIST-traceable secondary standard Si 
cell that had been calibrated and produced a specified short-circuit current density under 
100 mW cm-2 of illumination by Air Mass 1.5 G sunlight.  Prior to data collection, trace 
metal impurities were removed by soaking the cell overnight in aqua regia, then 
thoroughly rinsing the cell with 18 MΩ resistivity H2O. After each set of experiments, a 
Pt electrode was used to determine the reversible hydrogen potential, and thereby to 
calibrate the reference electrode.   
We have not extended the measurement time nor have we compared the stability 
of the Si/Pt system to that of the Si/CoP system for extended time periods. 
Spectral Response 
 Photoelectrochemical spectral response measurements were performed using 
illumination from a 150 W Xe lamp that was passed through an Oriel monochromator 
(0.50 mm slits), then chopped at 30 Hz, and focused to a beam spot that illuminated a 
portion of the electrode area in solution. A calibrated Si photodiode (Thorlabs UDT UV-
050) was used to measure the light intensity incident on the electrode. Another Si 
photodiode was used to measure the beam-split portion of the illumination, with this Si 
photodiode providing a continuous calibration of the light intensity from the 
monochromator. A Gamry G 300 potentiostat was used to maintain the potential of the Si 
working electrode at -0.20 V vs RHE and to record the current produced by the sample. 
The signal components were measured by use of independent lock-in detection of the 
sample channel and the calibration channel.  
Characterization 
Page 5 
 Scanning-electron microscopy (SEM) was performed using a Phenom Pro 
(Phenom World) electron microscope. The substrate was affixed to the sample chuck 
using Cu tape.  
A FEI model TF30ST transmission-electron microscope with 300 kV field 
emission, equipped with a HAADF STEM detector and a CCD camera, was used to 
image Pt nanoparticles that were dispersed on a copper mesh supported by a Cu grid.  
 Although the images in Figure 1 are displayed with different tilt angles for 
convenience of viewing, when these images were loaded into software such as ImageJ, 
with the appropriate calibration indicated by the scale bar, the diameters and lengths of 
the wires were found to be the same in all three cases. 
 The CoP nanoparticles were monodisperse (Figure S2), whereas the Pt 
nanoparticles (Figure S1) were dispersed in a carbonaceous conductive matrix that may 
more readily adhere to a hydrophobic Si-H surface than the CoP.  These factors may 
contribute to the differences in the distributions of the Pt and CoP nanoparticle on the 
MW arrays (Figure 1). 
In some cases, i.e. with a highly resistive radial emitter or with no emitter present, 
a more uniform distribution of catalyst on the tops, sides, and bottoms of the wire arrays 
would be more optimal than deposition only at the bottoms of the wire arrays as 
described herein.  Electrodeposition could be used to obtain a more uniform placement of 
the electrocatalyst onto microwire arrays.2 
 
 
 
Page 6 
H2 Faradaic Yield 
H2(g) generation was determined by quantitative volumetric collection of the gas 
evolved in the catholyte chamber in a two-electrode setup consisting of  an H-tube made 
from volumetric burets.  The gas formed at the cathode was collected after passage 
of -100 C with the electrode held at a current density of -10 mA cm-2.  The electrode area 
was 0.050 cm-2.  The volume of the collected gas was in excellent agreement with the 
amount of H2(g) expected assuming 100% Faradaic efficiency for H2(g) production. The 
counter electrode was an IrOx-coated Ti wire. 
Calculation of Photocathode Efficiency 
 The ideal regenerative-cell efficiency was calculated by using the measured three-
electrode characteristics of the photoelectrode in conjunction with an ideally 
nonpolarizable counterelectrode operating at the Nernstian potential E(O2/H2O).   Hence,  
ηIRC =  
ff ∙𝑉oc ∙𝐽𝑠𝑐
𝑃in
, where Pin is the input solar power, Voc is the difference between the 
open-circuit potential and E(O2/H2O), Jsc is the current density observed at E(O2/H2O)., 
and the fill factor is calculated as fractional maximum power with respect to the quantity 
Voc ⋅ Jsc. 
 
 
 
 
Page 7 
 
Figure S1. Transmission-electron micrographs of ~ 3 nm mean diameter crystalline Pt 
nanoparticles, with diameters ranging from 1 to 8 nm. The scale bar on the left image is 2 
nm, whereas the bar on the right indicates 10 nm. 
 
 
 
Figure S2. Powder x-ray diffraction (left) and transmission-electron micrograph (right) 
of ~13 nm diameter crystalline CoP nanoparticles. The scale bar indicates 100 nm. 
Page 8 
 
 
 
 
Figure S3. Spectral response for bare (top), Pt-coated (middle), and CoP-coated (bottom) 
n+p Si MW arrays in contact with H2(g)-saturated 0.50 M H2SO4. The data points that 
were acquired near complex Xe arc-lamp spectral features were omitted due to artifacts. 
 
 
Figure S4.  Dependence of photocurrent on the angle of illumination.  The blue data were 
obtained using a CoP-coated planar n+p Si photocathode (0.050 mg cm-2 mass loading), 
whereas the red data were obtained using a CoP-coated n+p Si microwire-array 
photocathode (2.0 mg cm-2 mass loading), both at 1 Sun of simulated AM1.5G solar 
illumination in 0.50 M H2SO4. 
Page 9 
Comparison Between Calculated Resistance and Measured Series Resistance 
 The sheet resistance of 100 Ω sq-1 for a 100 nm (𝑡) thick emitter yields a 
calculated resistivity of 0.001 Ω cm (ρ). The resistance for a single wire of radius 2 μm 
(𝑟) along the radial emitter on a 50 μm (𝐿) tall wire is 398 Ω (Rwire =
ρL
2πrt
,  where 𝐿, 𝑟 
and 𝑡 are the wire length, radius and emitter thckness, respectively). For an entire wire 
array, the total resistance decreases by the number of parallel resistors (Rarray =
Rwire
Nwires
). 
A 0.1 cm2 geometric array on an 11 μm pitch (8.26  105 wire cm-2 ) therefore produces a 
total calculated emitter-based resistance of 0.0048 Ω. The measured series resistance was 
determined by the measurement of the area (A = ∫ V dJ
Jsc
0
)  under the J-E data3, yielding 
(RS,∫ = 2 ⋅ (
Voc
Jsc
−
A
Jsc
2 −
nkbT
q
⋅
1
Jsc
))  a value of 285 Ω.  
 
 
 
 
 
 
 
 
 
 
 
Page 10 
 
Table S1. J-E parameters for n+p-Si planar and microwire photocathodes. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Catalyst 
 
Loading 
(mg cm-2) 
Jph  
(mA cm-2) 
ff Voc  
(V vs RHE) 
Illumination 
(mW cm-2) 
Bare (planar) 0 30 NA 0.44 100 
      
Pt (planar) 0.10 23 0.40 0.43 100 
 0.20 12 0.49 0.48 100 
      
Pt (MW) 0.50 7.8 0.49 0.37 30 
 0.50 14 0.46 0.44 100 
 0.50 30 0.43 0.46 330 
      
CoP (planar) 0.050 28 0.15 0.41 100 
 0.20 12 0.52 0.49 100 
 2.0 0.19 0.54 0.29 100 
      
CoP (MW) 2.0 8.5 0.35 0.45 30 
 2.0 17 0.24 0.48 100 
 2.0 21 0.23 0.52 330 
      
Page 11 
 
References 
 (1) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. 
Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide 
Nanoparticles. Angew. Chem. 2014, 53, 5427-30. 
 (2)  Saadi, F. H.; Carim, A. I.; Verlage, E.; Hemminger, J. C.; Lewis, N. S.; 
Soriaga, M. P. CoP as an Acid-Stable Active Electrocatalyst for the Hydrogen-Evolution 
Reaction: Electrochemical Synthesis, Interfacial Characterization and Performance 
Evaluation. J. Phys. Chem. C, 2014, 118, 29294. 
(3)  Pysch, D.; Mette, A.; Glunz, S.W. A Review and Comparison of Different 
Methods to Determine the Series Resistance of Solar Cells. Sol. Energ. Mat. Sol. C., 
2007, 91, 1698. 
 


English

Comparison of the Performance of CoP-Coated and Pt-Coated Radial Junction n^+p-Silicon Microwire-Array Photocathodes for the Sunlight-Driven Reduction of Water to H_2(g)

Page 1 
Supplementary Information for: 
 
Comparison of the Performance of CoP-Coated and Pt-Coated Radial-Junction 
n+p-Silicon Microwire-Array Photocathodes for the Sunlight-Driven Reduction of Water 
to H2(g) 
Christopher W. Roske†, Eric J. Popczun‡, Brian Seger∑, Carlos G. Read‡, Thomas 
Pedersen§, Ole Hansen§, Peter C. K. Vesborg∑, Bruce S. Brunschwig¶, Raymond E. 
Schaak*‡, Ib Chorkendorff*∑, Harry B. Gray*†, Nathan S. Lewis*†¶ ß 
† Division of Chemistry and Chemical Engineering, California Institute of Technology, 
Pasadena, CA, 91125, USA 
‡ Department of Chemistry and Materials Research Institute, The Pennsylvania State 
University, University Park, PA 16802, USA 
∑ Center for Individual Nanoparticle Functionality (CINF), Department of Physics, 
Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark 
§ Department of Micro- and Nanotechnology, Technical University of Denmark, DK-
2800 Kongens Lyngby, Denmark 
¶ Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA 
ß Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA 91125, 
USA 
 
 
 
Page 2 
Electrode Preparation 
 Reactive ion etching of Si(100) substrate (B-doped, resistivity of 14.6 ohm-cm) 
was used to form 4 μm diameter Si microwires (MWs) at an 11 μm pitch, with the wires 
being 50 μm in length. Radial n+ junctions on the p-Si microwires were formed by vapor-
phase P diffusion from phosphoryl chloride (POCl3) at 900 
oC.  Ohmic contacts to the 
back of the Si(100) substrates were made by electron-beam evaporation of 150 nm of Al, 
followed by a 5 min anneal at 800 oC under forming gas (5% H2(g) in N2(g)). The Si MW 
arrays were etched for 10 s in 0.5 M HF(aq) (Transene, Inc.) and were dried under a 
stream of Ar(g).  Centrifugation was then promptly performed in 25 mL falcon tubes in a 
swinging-head centrifuge, with the tubes filled with ~15 mL of polydimethylsiloxane 
(PDMS, Dupont).  The PDMS was cured overnight while centrifuging at 3000 rpm.  Si 
MW arrays were then loaded into the bottom of the falcon tubes and centrifugation of the 
particles was performed at 3000 rpm with a suspension of the CoP nanoparticles (0.2 
mg/mL) in hexane for 5 min (the suspension was sonicated immediately prior to 
application of 15 μL for a 0.1 cm2 sample). A Cahn microbalance was used to determine 
the geometric mass loading (mg cm-2) by weighing samples before and after deposition of 
particles. After deposition of the particles, the catalyst was activated by annealing at 450 
oC for 30 min under forming gas. Ga-In eutectic was scratched onto the back side of the 
wire arrays, and Ag paint (SPI) was then applied to the back side of the arrays.  A Cu 
wire was affixed to the array using Ag paint.  The Cu wire was then threaded through a 
glass tube.  Hysol 9460 epoxy was used to adhere the glass tube to the back side of the 
electrode and to seal the back and sides of the electrode.   The epoxy was cured 
overnight. 
Page 3 
Synthesis of Pt nanoparticles: A solution of 0.10 mM H2PtCl6 (Sigma, ACS 
Reagent), 0.10 mM sodium polyacrylate (Mw 2100, Aldrich), and 0.50 mM ascorbic acid 
(Sigma, ACS Reagent) was prepared in 250 mL of degassed, deionized H2O with a 
resistivity of ~18.2 MΩ-cm obtained from a Barnsted Nanopure column. The solution 
was stirred for 1 h under N2(g), prior to cooling overnight with the subsequent addition of 
2.0 g of NaOH. The suspension was left overnight, centrifuged, and then washed with 
deionized H2O three times before suspending the particles in isopropanol.  
Synthesis of CoP nanoparticles: synthesized and characterized as previously 
reported.1 
Electrochemical Measurements 
Photoelectrochemical measurements were performed in 0.50 M H2SO4(aq).  
Experiments were performed in a Pyrex electrochemical cell in a conventional three-
electrode configuration, with a Ag/AgCl reference electrode (BASi) in saturated KCl(aq) 
and a high-purity carbon cloth counter electrode (FuelCellStore). During a typical 
acquisition with a Gamry Reference 600 potentiostat, H2(g) was constantly purged into 
the solution (except for the H2 determination case), to maintain a constant, reversible 
(H+/H2) Nernstian potential in the solution. The solution was rapidly stirred with a 
magnetic stir bar, to minimize mass-transport limitations and to minimize any effects due 
to bubble formation. Potentially detrimental oxidative processes were minimized by 
limiting the scans to potentials that were negative of the open-circuit potential. 
The cell was illuminated with an ELH-type W-halogen lamp equipped with a dichroic 
rear reflector. The light intensity was calibrated using a Si photodiode (Thorlabs UDT 
UV-005) that had been mounted in the same orientation as the Si working electrodes.  
Page 4 
The Si photodiode was calibrated, in turn, against a NIST-traceable secondary standard Si 
cell that had been calibrated and produced a specified short-circuit current density under 
100 mW cm-2 of illumination by Air Mass 1.5 G sunlight.  Prior to data collection, trace 
metal impurities were removed by soaking the cell overnight in aqua regia, then 
thoroughly rinsing the cell with 18 MΩ resistivity H2O. After each set of experiments, a 
Pt electrode was used to determine the reversible hydrogen potential, and thereby to 
calibrate the reference electrode.   
We have not extended the measurement time nor have we compared the stability 
of the Si/Pt system to that of the Si/CoP system for extended time periods. 
Spectral Response 
 Photoelectrochemical spectral response measurements were performed using 
illumination from a 150 W Xe lamp that was passed through an Oriel monochromator 
(0.50 mm slits), then chopped at 30 Hz, and focused to a beam spot that illuminated a 
portion of the electrode area in solution. A calibrated Si photodiode (Thorlabs UDT UV-
050) was used to measure the light intensity incident on the electrode. Another Si 
photodiode was used to measure the beam-split portion of the illumination, with this Si 
photodiode providing a continuous calibration of the light intensity from the 
monochromator. A Gamry G 300 potentiostat was used to maintain the potential of the Si 
working electrode at -0.20 V vs RHE and to record the current produced by the sample. 
The signal components were measured by use of independent lock-in detection of the 
sample channel and the calibration channel.  
Characterization 
Page 5 
 Scanning-electron microscopy (SEM) was performed using a Phenom Pro 
(Phenom World) electron microscope. The substrate was affixed to the sample chuck 
using Cu tape.  
A FEI model TF30ST transmission-electron microscope with 300 kV field 
emission, equipped with a HAADF STEM detector and a CCD camera, was used to 
image Pt nanoparticles that were dispersed on a copper mesh supported by a Cu grid.  
 Although the images in Figure 1 are displayed with different tilt angles for 
convenience of viewing, when these images were loaded into software such as ImageJ, 
with the appropriate calibration indicated by the scale bar, the diameters and lengths of 
the wires were found to be the same in all three cases. 
 The CoP nanoparticles were monodisperse (Figure S2), whereas the Pt 
nanoparticles (Figure S1) were dispersed in a carbonaceous conductive matrix that may 
more readily adhere to a hydrophobic Si-H surface than the CoP.  These factors may 
contribute to the differences in the distributions of the Pt and CoP nanoparticle on the 
MW arrays (Figure 1). 
In some cases, i.e. with a highly resistive radial emitter or with no emitter present, 
a more uniform distribution of catalyst on the tops, sides, and bottoms of the wire arrays 
would be more optimal than deposition only at the bottoms of the wire arrays as 
described herein.  Electrodeposition could be used to obtain a more uniform placement of 
the electrocatalyst onto microwire arrays.2 
 
 
 
Page 6 
H2 Faradaic Yield 
H2(g) generation was determined by quantitative volumetric collection of the gas 
evolved in the catholyte chamber in a two-electrode setup consisting of  an H-tube made 
from volumetric burets.  The gas formed at the cathode was collected after passage 
of -100 C with the electrode held at a current density of -10 mA cm-2.  The electrode area 
was 0.050 cm-2.  The volume of the collected gas was in excellent agreement with the 
amount of H2(g) expected assuming 100% Faradaic efficiency for H2(g) production. The 
counter electrode was an IrOx-coated Ti wire. 
Calculation of Photocathode Efficiency 
 The ideal regenerative-cell efficiency was calculated by using the measured three-
electrode characteristics of the photoelectrode in conjunction with an ideally 
nonpolarizable counterelectrode operating at the Nernstian potential E(O2/H2O).   Hence,  
ηIRC =  
ff ∙𝑉oc ∙𝐽𝑠𝑐
𝑃in
, where Pin is the input solar power, Voc is the difference between the 
open-circuit potential and E(O2/H2O), Jsc is the current density observed at E(O2/H2O)., 
and the fill factor is calculated as fractional maximum power with respect to the quantity 
Voc ⋅ Jsc. 
 
 
 
 
Page 7 
 
Figure S1. Transmission-electron micrographs of ~ 3 nm mean diameter crystalline Pt 
nanoparticles, with diameters ranging from 1 to 8 nm. The scale bar on the left image is 2 
nm, whereas the bar on the right indicates 10 nm. 
 
 
 
Figure S2. Powder x-ray diffraction (left) and transmission-electron micrograph (right) 
of ~13 nm diameter crystalline CoP nanoparticles. The scale bar indicates 100 nm. 
Page 8 
 
 
 
 
Figure S3. Spectral response for bare (top), Pt-coated (middle), and CoP-coated (bottom) 
n+p Si MW arrays in contact with H2(g)-saturated 0.50 M H2SO4. The data points that 
were acquired near complex Xe arc-lamp spectral features were omitted due to artifacts. 
 
 
Figure S4.  Dependence of photocurrent on the angle of illumination.  The blue data were 
obtained using a CoP-coated planar n+p Si photocathode (0.050 mg cm-2 mass loading), 
whereas the red data were obtained using a CoP-coated n+p Si microwire-array 
photocathode (2.0 mg cm-2 mass loading), both at 1 Sun of simulated AM1.5G solar 
illumination in 0.50 M H2SO4. 
Page 9 
Comparison Between Calculated Resistance and Measured Series Resistance 
 The sheet resistance of 100 Ω sq-1 for a 100 nm (𝑡) thick emitter yields a 
calculated resistivity of 0.001 Ω cm (ρ). The resistance for a single wire of radius 2 μm 
(𝑟) along the radial emitter on a 50 μm (𝐿) tall wire is 398 Ω (Rwire =
ρL
2πrt
,  where 𝐿, 𝑟 
and 𝑡 are the wire length, radius and emitter thckness, respectively). For an entire wire 
array, the total resistance decreases by the number of parallel resistors (Rarray =
Rwire
Nwires
). 
A 0.1 cm2 geometric array on an 11 μm pitch (8.26  105 wire cm-2 ) therefore produces a 
total calculated emitter-based resistance of 0.0048 Ω. The measured series resistance was 
determined by the measurement of the area (A = ∫ V dJ
Jsc
0
)  under the J-E data3, yielding 
(RS,∫ = 2 ⋅ (
Voc
Jsc
−
A
Jsc
2 −
nkbT
q
⋅
1
Jsc
))  a value of 285 Ω.  
 
 
 
 
 
 
 
 
 
 
 
Page 10 
 
Table S1. J-E parameters for n+p-Si planar and microwire photocathodes. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Catalyst 
 
Loading 
(mg cm-2) 
Jph  
(mA cm-2) 
ff Voc  
(V vs RHE) 
Illumination 
(mW cm-2) 
Bare (planar) 0 30 NA 0.44 100 
      
Pt (planar) 0.10 23 0.40 0.43 100 
 0.20 12 0.49 0.48 100 
      
Pt (MW) 0.50 7.8 0.49 0.37 30 
 0.50 14 0.46 0.44 100 
 0.50 30 0.43 0.46 330 
      
CoP (planar) 0.050 28 0.15 0.41 100 
 0.20 12 0.52 0.49 100 
 2.0 0.19 0.54 0.29 100 
      
CoP (MW) 2.0 8.5 0.35 0.45 30 
 2.0 17 0.24 0.48 100 
 2.0 21 0.23 0.52 330 
      
Page 11 
 
References 
 (1) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. 
Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide 
Nanoparticles. Angew. Chem. 2014, 53, 5427-30. 
 (2)  Saadi, F. H.; Carim, A. I.; Verlage, E.; Hemminger, J. C.; Lewis, N. S.; 
Soriaga, M. P. CoP as an Acid-Stable Active Electrocatalyst for the Hydrogen-Evolution 
Reaction: Electrochemical Synthesis, Interfacial Characterization and Performance 
Evaluation. J. Phys. Chem. C, 2014, 118, 29294. 
(3)  Pysch, D.; Mette, A.; Glunz, S.W. A Review and Comparison of Different 
Methods to Determine the Series Resistance of Solar Cells. Sol. Energ. Mat. Sol. C., 
2007, 91, 1698. 
 


https://authors.library.caltech.edu/57042/2/jz5b00495_si_001.pdf

Comparison of the Performance of CoP-Coated and Pt-Coated Radial Junction n^+p-Silicon Microwire-Array Photocathodes for the Sunlight-Driven Reduction of Water to H_2(g)

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