(Invited) Energy Level Alignment and Hot Carrier Extraction in Monolayer Semiconductor Photoelectrochemical Cells

Abstract

The fundamental problem that limits the solar energy conversion efficiency of conventional semiconductors such as Si is that all absorbed photon energy above the band gap is lost as heat. The critical question that our research addresses is: Can we avoid energy losses in semiconductors? Hot-carrier systems that avoid such losses have tremendous potential in photovoltaics and solar fuels production, with theoretical efficiencies of 66% (well above the detailed-balance limit of 33%). Ultrathin 2D semiconductors such as monolayer (ML) MoS2 and WSe2 have unique physical and photophysical properties that could make hot-carrier energy conversion possible. The specific knowledge gap in the field is how the energy levels of 2D semiconductors move with applied potential and/or illumination, making the driving force for charge transfer (DG 0´) unclear. Since DG 0´ governs the hot-carrier extraction rate (k ET), understanding how and why DG 0´ changes under solar fuel generation conditions is critical to controlling k ET relative to the cooling rate. Absence of this critical information is limiting our ability to perform hot-carrier photochemistry. 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.