  Moir\'e superlattices in van der Waals heterostructures have emerged as a
powerful tool for engineering novel quantum phenomena. Here we report the
observation of a correlated interlayer exciton insulator in a double-layer
heterostructure composed of a WSe2 monolayer and a WS2/WSe2 moir\'e bilayer
that are separated by an ultrathin hexagonal boron nitride (hBN). The moir\'e
WS2/WSe2 bilayer features a Mott insulator state at hole density p/p0 = 1,
where p0 corresponds to one hole per moir\'e lattice site. When electrons are
added to the Mott insulator in the WS2/WSe2 moir\'e bilayer and an equal number
of holes are injected into the WSe2 monolayer, a new interlayer exciton
insulator emerges with the holes in the WSe2 monolayer and the electrons in the
doped Mott insulator bound together through interlayer Coulomb interactions.
The excitonic insulator is stable up to a critical hole density of ~ 0.5p0 in
the WSe2 monolayer, beyond which the system becomes metallic. Our study
highlights the opportunities for realizing novel quantum phases in double-layer
moir\'e systems due to the interplay between the moir\'e flat band and strong
interlayer electron interactions.
