  The intriguing physics of carrier-carrier interactions, which likewise affect
the operation of light emitting devices, stimulate the research on
semiconductor structures at high densities of excited carriers, a limit
reachable at large pumping rates or in systems with long-lived electron-hole
pairs. By electrically injecting carriers into WSe$_2$/MoS$_2$ type-II
heterostructures which are indirect in real and k-space, we establish a large
population of typical optically silent interlayer excitons. Here, we reveal
their emission spectra and show that the emission energy is tunable by an
applied electric field. When the population is further increased by suppressing
the radiative recombination rate with the introduction of an hBN spacer between
WSe$_2$ and MoS$_2$, Auger-type and exciton-exciton annihilation processes
become important. These processes are traced by the observation of an
up-converted emission demonstrating that excitons gaining energy in
non-radiative Auger processes can be recovered and recombine radiatively.
