  Light emission from higher-order correlated excitonic states has been
recently reported in hBN-encapsulated monolayer WSe2 and WS2 upon optical
excitation. These exciton complexes are found to be bound states of excitons
residing in opposite valleys in momentum space, a promising feature that could
be employed in valleytronics or other novel optoelectronic devices. However,
electrically-driven light emission from such exciton species is still lacking.
Here we report electroluminescence from bright and dark excitons, negatively
charged trions and neutral and negatively charged biexcitons, generated by a
pulsed gate voltage, in hexagonal boron nitride encapsulated monolayer WSe2 and
WS2 with graphene as electrode. By tailoring the pulse parameters we are able
to tune the emission intensity of the different exciton species in both
materials. We find the electroluminescence from charged biexcitons and dark
excitons to be as narrow as 2.8 meV.
