Hot carrier-based energy conversion systems could double the efficiency of conventional solar energy technology or drive photochemical reactions that would not be possible using fully thermalized, “cool” carriers, but current strategies require expensive multi-junction architectures. 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 (ML) MoS₂. Our approach facilitates ultrathin 7 Å charge transport distances over 1 cm² areas by intimately coupling ML-MoS₂ to an electron-selective solid contact and a hole-selective electrolyte contact. Our theoretical investigations of the spatial distribution of exciton states suggest greater electronic coupling between hot exciton states located on peripheral S atoms and neighboring contacts likely facilitates ultrafast charge transfer. Our work delineates future 2D semiconductor design strategies for practical implementation in ultrathin photovoltaic and solar fuels applications.

Transient absorption data were collected with at 390 nm pump pulse (50 fs) and broadband white light probe pulse. The delay time between the pump and probe was scanned from -1.2 ps to 201 ps. Spectra were imported into Matlab and plotted in contour plots as a function of delay time and wavelength. The spectra are given here in Matlab workspace format (.mat). To fit each exciton peak, the spectrum at each time delay was segmented into the A-exciton region (630 nm – 685 nm), B-exciton region (580 nm – 630 nm), and C-exciton region (413 nm – 460 nm). Each region was fit with a convolution of a gaussian and line to fit the exciton peak and background signal, respectively. The amplitudes of the gaussian fits were plotted as a function of delay time for each exciton. These plots were fit as described in detail in the Supplementary Information. All raw spectral data and processed data are saved in a Matlab workspace and uploaded here.

Potential-dependent absorbance measurements were taken by recording transmission microscope images of the sample while scanning the monochromatic light source from 375 nm to 700 nm. The image files were analyzed using Matlab script to select and track pixel trajectories, then sync these trajectories with each wavelength scanned with the monochromatic light source via the data acquisition card (DAQ). The absorbance was calculated using the intensity of chosen sample pixels and intensity of chosen background pixels at each wavelength. All data extracted from the images and the DAQ card are saved in a Matlab workspace and uploaded here.

The data files are Matlab Data workspace files (.mat). Scripts used to process the transient absorption data and the potential-dependent absorbance are Matlab files (.m). Codes used to do BSE-GW calculations are python scripts (.py).