The fundamental problem that limits the solar energy conversion efficiency of semiconductors such as CdTe and Si is that all excess solar photon energy above the band gap is lost as heat. Avoiding thermalization energy losses is of paramount significance for solar energy conversion because hot-carrier-based systems theoretically achieve 66% efficiency, which breaks the detailed balance limit of 33%. Of all the candidate materials, 2D semiconductors such as monolayer (ML) MoS2 have unique physical and photophysical properties that could make hot-carrier energy conversion possible. The knowledge gap in the field is that the electronic states of 2D materials move with carrier density, due to either light absorption or an applied electrochemical potential. The energy level movements are significant because the real fundamental driving force for charge transfer is unclear for a given reaction and applied potential. In principle, quantifying the driving force under working conditions opens up the possibility to tune the hot carrier extraction rate relative to the cooling rate. Our research team has employed photocurrent spectroscopy, steady-state absorption spectroscopy, and in situ femtosecond transient absorption spectroscopy as a function of applied potential to characterize underlying steps in a ML MoS2 photoelectrochemical cell. The rich data set informs us on the timescales for hot-carrier generation/cooling and exciton formation/recombination, as well as the magnitudes of changes in exciton energy levels, exciton binding energies, and the electronic band gap. These findings open the possibility of tuning the hot-carrier extraction rate relative to the cooling rate to ultimately utilize hot-carriers for solar energy conversion applications.

Energy level alignment in monolayer semiconductor electrochemical cells

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ACS Fall 2022

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