1. Solar energy can only be generated when the sun is shining, but
the demand for energy exists without respite. This dilemma
motivates the development of solar energy storage technologies.
One method for storing energy from the sun is to transform
solar energy into chemical energy. PEC water-splitting devices
perform this transformation, outputting hydrogen gas. Hydrogen
is an energy carrier and can be used as a clean fuel and in the
production of fertilizers, among other applications.
Introduction
Results
MoSx thin films on hydrogen-evolving p-n GaInP photocathodes for improved activity and stability
David LaFehr1, James Young2, Reuben Britto1, Tom Jaramillo1, Todd Deutsch2
1Stanford University, Palo Alto, CA 2National Renewable Energy Laboratory, Golden, CO
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).
Abstract
I would like to thank James Young, Todd Deutsch, and Reuben Britto for their guidance. I have also appreciated
the professional development pointers from Marcus Giron and Madison Martinez.
Conclusions & Future Work
Materials & Methods
Characterization TechniquesThree Electrode Setup
n-GaInP (25 nm) p-GaInP (1 µm)
p+-GaAs (625 µm?) Ti/Au
MoSx (~5 nm) PtRu (<1 nm)
NREL is a national laboratory of the U.S.
Department of Energy, Office of Energy
Efficiency and Renewable Energy, operated
by the Alliance for Sustainable Energy, LLC.
Photoelectrochemical (PEC) water-splitting is an
environmentally friendly way to obtain hydrogen, an energy-
dense fuel with a wide range of applications. The highest
efficiency PEC water-splitting devices employ tandem
absorbers, with a GaInP top junction in contact with electrolyte.
However, bare GaInP has low catalytic activity and low
stability. Previous work has shown that a thin film of MoSx,
applied to p-GaInP 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 catalytic coating on p-n
GaInP, employed to yield a photovoltage higher than that of p-
GaInP. Anticipating that 250°C is not compatible with future
tandem device processing requirements, we also compared the
performance of sulfidization at 250°C for one hour vs 150°C for
two hours. We found that MoSx-covered p-n GaInP is stable and
has a photocurrent onset potential higher than that of MoSx-
covered p-GaInP and rivaling that of PtRu. We also 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. Finally, we show a
proof-of-concept that MoSx can be thinned to yield a much-
improved photocurrent while maintaining its impressive
photovoltage.
Objectives
1) Determine whether a coating of MoSx on p-n GaInP yields a stable and catalytic photocathode
2) Determine the impact of sulfidization at 150°C for two hours vs 250°C for one hour
The photocurrent onset potential of
250°C MoSx-covered p-n GaInP is
about 750 mV higher than that of
250°C MoSx-covered p-GaInP,
indicating more photovoltage and
better catalytic activity. There is
no difference in photovoltage
between MoSx formed at 150°C
and 250°C on p-n GaInP. This
photovoltage is higher than that for
PtRu-covered p-n GaInP, which is
impressive given that PtRu is the
standard catalytic coating for
GaInP.
Reflectance and IPCE measurements can explain the lower light-limited
photocurrent of MoSx-covered relative to PtRu-covered p-n GaInP. For all
wavelengths, both 150°C and 250°C MoSx-covered p-n GaInP have higher
reflectance and higher parasitic absorbance relative to PtRu-covered p-n GaInP.
Higher parasitic absorbance is indicated by lower internal quantum efficiency. The
reported internal quantum efficiency is not absolute, though, as reflectance
measurements were performed in air while IPCE data was taken with the sample
submerged in aqueous electrolyte.
A coating of MoSx on p-n GaInP stabilizes the electrode and yields a photovoltage approaching
and even surpassing that of PtRu-covered p-n GaInP. There is no significant performance
difference between MoSx formed at 150°C vs 250°C, verifying that tandem device processing
can occur at lower temperatures. Future work should focus on determining optimal thicknesses
of Mo and MoSx on GaInP.
The light-limited photocurrent for 150°C MoSx-covered p-n GaInP increases
substantially after three hours while the photocurrent onset potential shows no
change. So, there is a marked increase in solar-to-hydrogen efficiency after a few
hours. This is a proof-of-concept that the thickness of MoSx can be optimized to
yield increased current density with no drop in its catalytic activity.
Previous work has
shown that MoSx
formed at 250°C
provides stability to p-
GaInP for 70+ hours.
The same holds for
MoSx formed at 150°C
on p-n GaInP, as
evidenced by the
steady current in this
durability test*.
*Test is ongoing, data
will be available soon
Improving photocurrent and unchanging photovoltage
during operation
To do: add cartoon.
Wouldn’t the cartoon be
pretty similar to the one
I currently have
(reflection)?
LimitationsImproved stabilityImproved photovoltage
Background Motivation
Figure #. The
structure of an
inverted
metamorphic
multijunction cell.
Figure #. A stereoscopic image
of the surface of an IMM cell
that was annealed at 250°C.
The high temperature is
responsible for the wrinkles.2hν + H2O(l) → ½O2(g) + H2(g)
Figure #. Visualization of a PEC water-
splitting device. Light is incident on the
photoelectrode, and a the separation of
photogenerated holes and electrons creates
a voltage. If this voltage is above x V,
hydrogen evolution occurs on one
electrode and oxygen evolution takes place
on the other. The electrodes are submerged
in aqueous electrolyte to complete the
circuit.
Equation 1. Light and water react to
produce oxygen gas and hydrogen gas.
The most efficient PEC water-splitting device has a
photocathode consisting of p-GaInP/p+-GaAs, but it operates
for only a few hours before surface corrosion severely
degrades device performance. In the past, sputtered PtRu has
been employed to protect and provide activity to GaInP, but
its high cost makes it undesirable. MoSx, which is cheaper
than PtRu, has very recently been identified as a catalytic
coating on p-GaInP that stabilizes the device [1]. There are
two drawbacks to this setup, though: 1) MoSx-covered p-
GaInP does not have a very high photovoltage and 2) the
annealing of Mo to produce MoSx occurs at 250°C, a
temperature too high for future tandem device processing.
These drawbacks are the motivation for our work.
Figure #. The three electrode setup. Not shown are wires
that run from each of the electrodes to a potentiostat.
Photocathode Samples
Bare MoSx PtRu
Reflectance: Visible and
ultraviolet light were shined on the
sample. A photodetector measured
reflected light.
IPCE: Light of wavelengths 300-
720 nm, in increments of 10 nm,
was shined on the working
electrode. The steady-state current
for each wavelength was
measured. Same as EQE.
JV: The voltage between the
working and reference electrodes
was swept, each voltage yielding a
current between the working and
counter electrodes.
Durability: Galvanostatic and
potentiostatic measurements were
performed for 1-100 hours. JV
measurements were done
periodically.
Figures #a, b, and c. Figure
#c shows a possible
explanation for the relatively
low light-limited
photocurrent of MoSx-
covered p-n GaInP as seen
in Figure #a. In Figure #c,
due to reflection, less light
reaches the semiconductor
when it is coated with MoSx
(left) as opposed to PtRu
(right), corresponding to
generation of fewer charges.
Figure #b (IQE) suggests
that the poor photocurrent
can also be attributed to
parasitic absorbance.
a)
c)
b)To do: add
cartoon