Light Generation and Harvesting in a Van der Waals Heterostructure*

We report on the realization of light-emitting diodes based on heterojunctions with monolayer MoS$_{2}$. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of the A and B excitons and the A- trion resonance related to the optical transitions between the conduction and valence bands. Our pn diodes can also operate as solar cells with an external quantum efficiency higher than 2{\%}. Our work opens up the way to more sophisticated optoelectronic devices such as 2D LEDs and solar cells based on monolayer MoS$_{2}$.