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Abstract
<p class="first" id="d69109e77">Transition metal dichalcogenides (TMDs) such as MoSe2
and WSe2 are efficient materials
for converting solar energy to electrical energy in photoelectrochemical photovoltaic
cells. One limiting factor of these liquid junction solar cells is that photogenerated
oxidation products accumulate on the electrode surface and decrease the photocurrent
efficiency. However, it is unclear where the reaction products accumulate on the electrode
surface and how they impact the local photoelectrochemical response. This open question
is especially important for the structurally heterogeneous TMD nanoflake thin-film
electrodes that are promising for large-area solar energy conversion applications.
Here, we use a single-nanoflake photoelectrochemical and Raman microscopy approach
to probe how the photogenerated I2/I3- products impact the photocurrent collection
efficiency and the onset potential in MoSe2-nanoflake|I-/I2|Pt photoelectrochemical
solar cells. We observed localized I2/I3- deposition on all types of MoSe2 nanoflake
surface motifs, including basal planes, perimeter edges, and interior step edges.
Illuminated nanoflake spots with the highest photocurrent collection efficiency are
the first to be limited by I2/I3- formation under high-intensity illumination. Interestingly,
I2/I3- formation occurs on illuminated surface spots that have the lowest photocurrent
onset potential for iodide oxidation, corresponding to the highest open circuit voltage
( VOC). The VOC shifts could be attributed to variations in the surface reaction kinetics
or doping density across the nanoflake. Our results highlight important limiting factors
of nanoflake thin-film TMD liquid junction photovoltaics under concentrated solar
illumination intensities.
</p>