Thermalization loss of absorbed solar energy above the semiconducting active-material bandgap is the largest limiting factor of efficiency in solar energy conversion devices1. Extraction of highly excited (hot) charge carriers before thermalization loss can improve device efficiency by up to 33%2. Single monolayer transition metal dichalcogenides (2D TMDs) are a promising active material for hot carrier devices due to their broad visible light absorption, atomically thin structure, and slow hot carrier cooling3. A crucial step towards design of commercial hot carrier devices is understanding how operational device conditions affect hot carrier dynamics. Using an unprecedented combination of photoelectrochemical and in situ transient absorption spectroscopy measurements, we demonstrate ultrafast (<50 fs) hot exciton and free carrier extraction under applied bias in a proof-of-concept photoelectrochemical solar cell made from earth-abundant and potentially inexpensive monolayer MoS2. Our theoretical investigations of the spatial distribution of exciton states suggest that greater electronic coupling between hot C-exciton states located on peripheral S atoms and neighboring contacts likely facilitates ultrafast charge transfer. Additionally, we reveal that the broadening and shifting of optical spectra of 2D TMDs arises from the formation of negative trions. We do this by fitting an ab initio based, many-body model to our experimental photoelectrochemical data. Overall, our study sheds light on the mechanisms underlying charge carrier dynamics in 2D TMDs and provides insight for the design and optimization of hot carrier devices for solar energy conversion.

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