Research Paper David LaFehr

ABSTRACT

Photoelectrochemical (PEC) watersplitting is an environmentally friendly way to obtain
hydrogen, a clean, energydense fuel with a wide range of applications. The highest
efficiency PEC watersplitting devices employ tandem absorbers, with a pGaInP top
junction in contact with electrolyte. However, bare pGaInP has low catalytic activity
toward hydrogen evolution and low stability. Previous work has shown that a thin film of
MoSx, applied to pGaInP by sputtering of Mo and a subsequent sulfidization anneal in
H2S at 250 °C for one hour, provides stability and activity for the electrode. We
investigated whether MoSx also acts as a protective and noblemetalfree catalytic coating
on pn GaInP, employed to yield a photovoltage sufficient for the top junction of a
tandem watersplitting device. Anticipating that 250 °C is not compatible with future
tandem device processing requirements, we also studied the performance of sulfidization
carried out at 150 °C for two hours. We found that 150 °Cformed MoSxcoated pn
GaInP is stable for at least 100 hours and has a photocurrent onset potential rivaling that
of PtRumodified pn GaInP. We discovered that there is no significant performance
difference between MoSx formed at 150 °C and 250 °C, verifying that future MoSx
processing can be carried out at 150 °C. We also show a proofofconcept that 150
°Cformed MoSxcoated pn GaInP can undergo constantcurrent treatment to yield a
muchimproved photocurrent while maintaining its impressive photovoltage. Ultimately,
our work suggests that 150 °Cformed MoSxcoated pn GaInP should be considered as a
replacement for PtRumodified pGaInP as the top junction in tandem watersplitting
devices.

I. INTRODUCTION

In standalone solar cells, solar energy is transformed into electricity, which is sent to the
grid. However, electricity derived from solar cells is often not commensurate with grid demand.
This misalignment motivates the development of a storage mechanism for solar energy so that it
may be called upon whenever it is needed. One promising concept is photoelectrochemical
(PEC) watersplitting, wherein solar energy is captured and simultaneously converted to
chemical energy in the form of hydrogen gas.

A PEC watersplitting device consists of an anode and a cathode in an aqueous
environment. At least one of the electrodes is sensitive to light in similar fashion to a solar cell:
when light is incident on the photoelectrode, there is a spatial separation of photogenerated holes
and electrons, which induces a voltage across the cell. If this voltage is at least 1.7 V, the
hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) occur on the
cathode and the anode, respectively. The oxygen bubbles are allowed to naturally exit the device,
while the hydrogen bubbles are collected and stored. As an energy carrier, hydrogen can later be
used on demand as a clean fuel for vehicles and in the production of ammonia for fertilizers,
among other applications.

The highest efficiency PEC watersplitting device has a photocathode consisting of
tandem absorbers1
. The top junction is ptype gallium indium phosphide (pGaInP). A standard

technique to increase the catalytic activity and stability of this photocathode is to sputter a
platinum ruthenium alloy (PtRu) on its surface2
. However, the stability of PtRumodified
semiconductors is limited. Previous work3
has shown that a coating of molybdenum sulfide
(MoSx), which is more abundant and less expensive than PtRu, applied via molybdenum (Mo)
sputtering and a subsequent anneal in hydrogen sulfide (H2S), on the top surface of pGaInP
provides catalytic activity and stability to the photocathode. There are two barriers to using this
approach in standalone watersplitting devices. The first issue concerns photovoltage. In a
tandem photocathode, the top junction should provide 1.21.4 V and the bottom junction should
provide 0.70.9 V. MoSxcoated pGaInP yields a photovoltage well short of the mark. This
shortcoming motivated us to evaluate the stability of MoSxcoated pn GaInP, which we believed
would have better photovoltage. The second issue revolves around future tandem device
processing. Tandem photocathodes have typically been grown bottomtotop, layerbylayer.
Recently, however, there has been a shift towards inverted metamorphic multijunction (IMM)
growth4
. Figure 1 shows an IMM cell. The epoxy presently used to mount the IIIV
semiconductor to the silicon (Si) wafer handle in an IMM cell has a thermomechanical stability
limit of 150 °C. The sulfidization anneal that converts Mo to MoSx occurs at 250 °C for one
hour. When the epoxy is subjected to such a high temperature, it swells and causes deleterious
wrinkles to form in the IMM structure, as shown in Figure 2. This problem motivated us to
investigate the use of a lower annealing temperature.

II. MATERIALS & METHODS

A. Sample Synthesis

In addition to bare pn GaInP control samples, three surface modifications were
characterized: 150 °Cformed MoSx, 250 °Cformed MoSx, and PtRu. Every sample initially
consisted of a pn GaInP absorber (the ntype material was the top layer), a p+
GaAs conductive
substrate, and a Ti/Au back contact. The thicknesses of nGaInP, pGaInP, and p+
GaAs were 25
nm, 1 µm, and 650 µm, respectively. No modifications were made to the bare samples. The

MoSx coatings were produced by first using a DC magnetron sputter coater to deposit Mo on
GaInP at a rate of 7.2 nm/s for a nominal Mo thickness of 3.6 nm. Then, Mo was sulfidized in
90% H2/10% H2S in a tube furnace. Each MoSx sample was annealed either at 250 °C for 1 hour
or at 150 °C for 2 hours. After the sulfidization anneal, the thickness of the MoSx coating was ~5
nm. For the PtRu samples, PtRu was deposited on GaInP in a vacuum chamber via flash
sputtering. The samples to be treated passed under the sputter plume twice for a duration of
~0.25 s each. The resulting PtRu particles were 25 nm in diameter and covered 2030% of the
pn GaInP top surface. The PtRu continuousfilmequivalentthickness was ~0.5 nm. Figure 3
illustrates the structure of each type of sample.

B. Electrode Construction

The samples were made into photoelectrodes after stereoscopic and microscopic images
of their surfaces were taken and UVvisible reflectance measurements were performed. The
procedure for constructing a photoelectrode containing each sample is as follows. The top 6 cm
of copper wire of length 20 cm was coiled. This copper wire was then placed in a glass rod
approximately 12 cm long, and the back contact of the sample was affixed to the copper wire
using Pelco Colloidal Silver Liquid. The photoelectrode was placed in an 80 °C oven for 2030

minutes. Following this step, Loctite EA9462 HySol epoxy was applied to the copper wire
around the sample so that only the top surface of the sample was visible. The next day, the
photoelectrode was placed in an 80 °C oven for 2 hours. Then, Resinlab EP1385 black epoxy
was applied on top of the existing epoxy at the very edges of the sample. A few hours later, the
photoelectrode was put in an oven for 2030 minutes. After this step, PEC characterization
measurements could be performed.

C. Photoelectrochemical Cell Setup

With the exception of UVvisible reflectance measurements, all characterization
techniques involved the threeelectrode PEC cell setup, which Figure 4 shows. The reference
electrode was mercury/mercurous sulfate (Hg/Hg2SO4, referred to as MSE) with 3 M sulfuric
acid (H2SO4) filling solution, the counter electrode was platinum (Pt), and the working electrode

was a constructed photoelectrode containing a sample. The reference, counter, and working
electrodes were submerged in 3 M H2SO4 with 1 mM Triton X100 surfactant, all in a glass
container. The surfactant was used to prevent hydrogen bubbles from sticking to the working
electrode. However, for durability testing, a fritted tube containing the counter electrode and 3 M
H2SO4 was placed in the glass cell. Electrolyte without surfactant was used in the counter
electrode compartment because the surfactant is known to foul the counter electrode after
extended operation. Each electrode was connected to the corresponding leads of a Solartron SI
1287 Electrochemical Interface potentiostat. The light source was a 250 W quartz

tungstenhalogen lamp (66883, Newport) with a waterfilled infrared filter. A lightshaping
diffuser was placed between the lamp and the glass cell. A GaInP reference cell (1.8 eV
bandgap) was used to calibrate the illumination intensity to oneSun equivalent flux density.

D. Characterization Techniques

i. Stereoscopy and Microscopy

Before its transformation into a photoelectrode, each sample was photographed with a
stereo microscope and any abnormalities were captured on a digital camera through a compound
optical microscope. Upon completion of durability testing, stereoscopic pictures of the sample
were taken both before and after deconstructing each photoelectrode.

ii. UVVisible Reflectance

A cable containing optical fibers guided light from a visible illumination source to an
opening in the other end, directly below which lay a reflection standard. Reflected light reentered
the cable, which carried it to a spectrometer. SpectraWiz software read the light intensity at
wavelengths up to 1100 nm. This spectrum was saved as the reference spectrum. Then the
reflection standard was taken away and each sample was moved into its place such that the
surface of the sample was in the same plane as the surface of the reflection standard. The
software read the ratio of light intensity to that of the reference spectrum on a scale of 0100%.
This spectrum was recorded as the visible reflectance of the sample. For UV reflectance, a UV
source was used instead of a visible source. For each sample, the UV reflectance data for 450 nm
and below, and the visible reflectance data for above 450 nm, were used.

iii. Incident PhotontoCurrent Efficiency

A 300 W xenon arc lamp (67005, Newport) coupled through a monochromator (SP50,
Acton) provided illumination for incident photontocurrent efficiency (IPCE) measurements. An
NRELcalibrated silicon photodiode (S13368BQ, Hamamatsu) was used to measure the
monochromatic light output while it was stepped from 300 nm to 720 nm in 10nm increments.
A 495 nm longwavelength pass filter was used when the light output was 550 nm and above.
For each wavelength, the shutter was open for two seconds and closed for two seconds. The last
few current measurements for each wavelength were averaged and recorded as the current for
that wavelength. The spectral response of the photodiode was recorded and used to obtain the
lamp spectrum. The photodiode was replaced by each photoelectrode in a threeelectrode PEC
configuration. In place of a glass container, a quartz container held the electrolyte. The procedure
used for the photodiode was then used for each constructed photoelectrode, with one change:
each sample was reversebiased to the most positive potential for which lightlimited
photocurrent was observed. The IPCE of each photoelectrode was obtained through the use of
ratios, represented in Equation 1.

q 1. IPCE E photoelectrode =
IPCEphotodiode
photocurrent responsephotodiode
photocurrent responsephotoelectrode

iv. Current DensityVoltage

For each sample in a threeelectrode PEC setup, two current densityvoltage (JV) sweeps
were performed and graphed. For the first sweep, the lamp output was blocked for the first 100
mV. The lamp output was then unblocked until the current density approached 0 mA/cm2
, at
which point the measurement was stopped. For the second sweep, the lamp output was blocked
for 100 mV and unblocked for 100 mV for a total of 800 mV. Then, the lamp output remained
unblocked until the current density approached 0 mA/cm2
. The starting voltage for each sweep
was chosen to allow for 200400 mV of constant current before the JV curve sloped upwards.

v. Durability

Prior to durability testing of each photoelectrode, a JV measurement was performed.
From the resulting JV curve, the operating point for the durability run was chosen. Each
durability run was either galvanostatic or potentiostatic. For every sample that underwent
galvanostatic durability testing, the current density was set to 10 mA/cm2
. If no point
corresponding to 10 mA/cm2
existed on the JV curve, the light intensity was increased until
that condition was satisfied. The operating voltage varied from photoelectrode to photoelectrode
for those that underwent potentiostatic durability testing. Each photoelectrode underwent
durability runs that lasted between 1 and 100 hours. Periodically, durability testing was paused
and a JV sweep was performed to determine whether the performance of the photocathode had
changed. The condition of the photoelectrode, including the visual appearance of its top surface,
was evaluated to determine whether durability testing should be resumed or permanently
stopped.

vi. Profilometry

Samples from deconstructed photoelectrodes were placed under an optical profilometer
(Wyko NT1100, Veeco) and a scan was performed. The profilometer’s software generated a 3D
image of the top surface each sample, allowing for easy identification of etching.

III. RESULTS & DISCUSSION

A. Improved Photovoltage

As Figure 5 conveys, the photocurrent onset potential of 250 °Cformed MoSxcoated pn
GaInP is significantly more positive than that of 250 °Cformed MoSxcoated pGaInP.
Impressively, pn GaInP coated with MoSx has better photovoltage than pn GaInP modified
with PtRu, which is the quintessential catalytic material for GaInP. Reported values for potential
are relative to the potential of the reversible hydrogen electrode (RHE), which is 0.69 V negative
of the potential of the MSE.

B. High Stability

Figure 6 shows that 150 °Cformed MoSxcoated pn GaInP held at a potential of 0.3 V
versus MSE (+0.39 V versus RHE) is stable for at least 100 hours. The sample retained over
90% of its initial current at the 100hour mark. One reason for the difference in photocurrents of
the 150 °Cformed MoSxcoated pn GaInP sample and the 250 °Cformed MoSxcoated pGaInP
sample is that the former’s absorber was optically thick while the latter’s was not. The data for
the bare pGaInP sample look erratic only because a JV measurement was performed every
hour.

C. Transparent MoSx: ProofofConcept via ConstantCurrent Treatment

Figure 7 shows that after 3 hours of galvanostatic operation (see Appendix A), the
photocurrent of 150 °Cformed MoSxcoated pn GaInP substantially increases (becomes more
negative) while the impressive photovoltage is unchanged. This result suggests that 150
°Cformed MoSxcoated pn GaInP should undergo constantcurrent treatment prior to
spontaneous watersplitting operation.

Figure 8 represents a possible explanation for the increased photocurrent and unchanged
photovoltage of 150 °Cformed MoSxcoated pn GaInP after a few hours of galvanostatic
operation. Our hypothesis is that a few hours of galvanostatic operation thins the MoSx coating to
the point that it is almost transparent. It follows that more light reaches the absorber and
generates charge carriers, resulting in a larger photocurrent. MoSx is still present, so there is no
change in the number and quality of active sites, imparting a photovoltage that does not vary
with the thickness of MoSx, but simply depends on the presence of MoSx.

D. Limitations

Although it has very impressive stability and catalytic activity, 150 °Cformed
MoSxcoated pn GaInP has a drawback. The lightlimited photocurrents for 150 °Cformed
MoSxcoated pn GaInP, 250 °Cformed MoSxcoated pn GaInP, and PtRumodified pn GaInP
are 6.6 mA/cm2
, 6.2 mA/cm2
, and 10.6 mA/cm2
, respectively. These values were obtained by
integrating IPCE data (see Appendix B) over the AM1.5G spectrum. They differ from the
photocurrents observed in JV measurements and durability runs because the IPCE
measurements were performed with much lower light intensity, and photocurrent scales
nonlinearly with light intensity when light intensity is sufficiently low. The difference between
the lightlimited photocurrents obtained from the IPCE data for 150 °Cformed MoSxcoated pn
GaInP (6.6 mA/cm2
) and PtRumodified pn GaInP (10.6 mA/cm2
) is 4 mA/cm2
, implying that
the MoSx coating is responsible for more losses relative to the PtRu particles. Data from
UVvisible reflectance and IPCE measurements can explain these losses.

Figure 9 shows that for all wavelengths, both 150 °Cformed MoSxcoated pn GaInP and
250 °Cformed MoSxcoated pn GaInP are more reflective than PtRumodified pn GaInP. 150
°Cformed MoSxcoated pn GaInP and 250 °Cformed MoSxcoated pn GaInP reflect 4055%
of incident light whereas PtRumodified pn GaInP reflects 3045% of incident light. Because
more light is reflected by the MoSx samples, less light reaches the GaInP absorber, and fewer
charge carriers are generated, corresponding to decreased photocurrent.

UVvisible reflectance data in conjunction with IPCE data show that parasitic absorption
in MoSxcoated pn GaInP is another reason for its lower photocurrent relative to PtRumodified
pn GaInP. IQE (internal quantum efficiency) is related to EQE (external quantum efficiency),
synonymous with IPCE, through Eq. 2:

where R is reflectance. Figure 10 shows that for all wavelengths, the IQEs of 150 °Cformed
MoSxcoated pn GaInP and 250 °Cformed MoSxcoated pn GaInP are lower than that of
PtRumodified pn GaInP. This result implies that the MoSx coating either absorbs more photons
or induces a higher recombination rate of photogenerated charges than the PtRu particles do. So,
fewer charges exit the photocathode, corresponding to decreased current.

Figure 11 offers a visual explanation of the lower photocurrents of 150 °Cformed and
250 °Cformed MoSxcoated pn GaInP relative to PtRumodified GaInP. MoSx reflects and
absorbs more light than PtRu, resulting in the incidence of fewer photons on GaInP and the
production of fewer photogenerated electrons and holes, corresponding to less current.

IV. CONCLUSIONS

A coating of 150 °Cformed MoSx on pn GaInP yields a photovoltage rivaling that of
PtRumodified pn GaInP and stabilizes the photocathode for at least 100 hours. The superior
performance and lower cost of MoSx suggest that it should replace PtRu as a catalytic and
protective material on pn GaInP, as long as its properties can be further optimized for
transparency. With very similar photocurrent onset potentials and lightlimited photocurrents,
150 °Cformed MoSxcoated pn GaInP and 250 °Cformed MoSxcoated pn GaInP show no
significant performance differences, verifying that tandem device processing can and should
occur at 150 °C. Finally, operation at constant current for a few hours substantially increases the
lightlimited photocurrent of 150 °Cformed MoSxcoated pn GaInP and is therefore a potential
treatment for improving the transparency of MoSx. Future work includes a quantitative
investigation of constantcurrent treatment and, ultimately, the incorporation of 150 °Cformed
MoSxcoated pn GaInP in a tandem watersplitting device.
V. ACKNOWLEDGEMENTS
I would like to thank James Young, Todd Deutsch, and Reuben Britto for their guidance.
This work was supported in part by the U.S. Department of Energy, Office of Science, Office of
Workforce Development for Teachers and Scientists (WDTS) under the Science Undergraduate
Laboratory Internships Program (SULI).

VI. APPENDICES
A. GALVANOSTATIC DURABILITY

B. INCIDENT PHOTONTOCURRENT EFFICIENCY

VII. REFERENCES

1
M. May, H. Lewerenz, D. Lackner, F. Dimroth, and T. Hannappel. Efficient direct
solartohydrogen conversion by in situ interface transformation of a tandem structure. Nature
Communications 6, 8286. (2015)

2
T. Deutsch and J. Turner. “Semiconductor materials for photoelectrolysis: 2014 U.S. DOE
Hydrogen & Fuel Cells Program review.” (2014)
https://www.hydrogen.energy.gov/pdfs/review14/pd035_deutsch_2014_o.pdf

3
R. Britto, J. Benck, J. Young, C. Hahn, T. Deutsch, and T. Jaramillo. Molybdenum disulfide as
a protection layer and catalyst for gallium indium phosphide solar water splitting photocathodes.
Journal of Physical Chemistry Letters 7, 20442049. (2016)

4
R. France, F. Dimroth, T. Grassman, and R. King. Metamorphic epitaxy for multijunction solar
cells. MRS Bulletin, 41, 202209. (2016)

Research Paper David LaFehr

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