Role of Photogenerated Iodine on the Energy-Conversion Properties of MoSe2 Nanoflake Liquid Junction Photovoltaics.

Abstract

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.