  Semiconductor heterostructures are backbones for solid state based
optoelectronic devices. Recent advances in assembly techniques for van der
Waals heterostructures has enabled the band engineering of semiconductor
heterojunctions for atomically thin optoelectronic devices. In two-dimensional
heterostructures with type II band alignment, interlayer excitons, where
Coulomb-bound electrons and holes are confined to opposite layers, have shown
promising properties for novel excitonic devices, including a large binding
energy, micron-scale in-plane drift-diffusion, and long population and valley
polarization lifetime. Here, we demonstrate interlayer exciton optoelectronics
based on electrostatically defined lateral p-n junctions in a MoSe2-WSe2
heterobilayer. Applying a forward bias enables the first observation of
electroluminescence from interlayer excitons. At zero bias, the p-n junction
functions as a highly sensitive photodetector, where the wavelength-dependent
photocurrent measurement allows the direct observation of resonant optical
excitation of the interlayer exciton. The resulting photocurrent amplitude from
the interlayer exciton is about 200 times smaller compared to the resonant
excitation of intralayer exciton. This implies that the interlayer exciton
oscillator strength is two orders of magnitude smaller than that of the
intralayer exciton due to the spatial separation of electron and hole to
opposite layers. These results lay the foundation for exploiting the interlayer
exciton in future 2D heterostructure optoelectronic devices.
