Single-nanoflake photoelectrochemistry of MoSe2 thin films
Date
2018
Authors
Isenberg, Allan Edward, author
Sambur, Justin, advisor
Neilson, James, committee member
de la Venta, Jose, committee member
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Abstract
Transition metal dichalcogenide (TMD) thin films represent promising materials for large-area, low-cost, and high-efficiency photoelectrochemical solar energy conversion applications. The outstanding efficiency of bulk TMD crystals has been well documented, which has driven interest in large-area exfoliated TMD thin film devices in recent years. Unfortunately, the solar energy conversion efficiency of nanoflake-coated electrodes is typically much worse than bulk crystal electrodes. It is currently unclear how the high degree of variability among nanoflakes (e.g. area, thickness, types structural features, etc.) contribute to the efficiency gap between nanoflake and bulk electrodes. It is also unclear if exfoliated nanoflakes can achieve the solar conversion efficiencies demonstrated by bulk crystals. The semiconductor-electrolyte dynamics of TMD/iodide photoelectrochemical cells has also been characterized in bulk systems. Bulk TMD electrodes in an iodide electrolyte will form adsorbed oxidation products at the TMD surface, which can cause sharp drops in efficiency in these systems. A clear understanding of how this phenomenon affects the local photoelectrochemical response has not been established. Additionally, it is not clear how the surface reaction kinetics of iodide oxidation products are affected by surface structural features (e.g. basal planes, perimeter-edges, and interior step edges) on TMD nanoflakes. Here, a single-nanoflake photoelectrochemical approach is used to establish the existence of highly active champion and inactive spectator nanoflakes in mechanically exfoliated MoSe2 thin films. In the samples studied, 7% of nanoflakes are highly active champions, whose solar conversion efficiencies exceed that of the bulk crystal. Though, 68% of the deposited nanoflakes are inactive spectators, and contribute substantially to the lower photocurrent efficiencies of nanoflake-coated electrodes compared to bulk electrodes. Structural features are also shown to have a significant effect on photocurrent collection efficiencies. Photocurrent collection response is shown to increase with nanoflake area and is more negatively affected by perimeter edges than interior step edges. Moreover, local photoelectrochemical spot measurements show that while adsorbed iodide oxidation products can form at any type of surface structure, these films preferentially form at the most catalytically active and thermodynamically favorable sites for iodide oxidation. These observations reveal previously hidden performance issues associated with exfoliated TMD thin films and highlights performance aspects that can be improved upon.
Description
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Subject
solar energy conversion
ultra-thin
transition metal dichalcogenide
photovoltaic