Gate-Tunable Hot Electron Extraction in a Two-Dimensional Semiconductor Heterojunction.

Hot carrier solar cells (HCSCs), harvesting the excess energy of hot carriers generated by above-band gap photoexcitation, are crucial for pushing the solar cell efficiency beyond the Shockley-Queisser limit, which is challenging to realize mainly due to fast hot-carrier cooling. By performing transient reflectance spectroscopy in a MoSe2/hBN/WS2 junction, we demonstrate the gate-tunable harvest of hot electrons from MoSe2 to WS2. By spectrally distinguishing hot-electron extraction from lattice temperature increase, we find that electrostatically doped electrons in MoSe2 can boost hot-electron extraction density (nET) by a factor up to several tens. Such enhancement arises from the interaction between hot excitons and doped electrons, which converts the excess energy of hot excitons to excitations of the Fermi sea and hence generates hot electrons. Moreover, nET can be further enhanced by reducing the conduction band offset with an external electric field. Our results provide in-depth insights into the design of HCSCs with electrostatic strategies.