Atomically thin transition metal dichalcogenides can show a rich variety of bound exciton complex states, such as trions, biexcitons, Fermi polarons, and phonon replicas, because of the reduced dielectric screening and enhanced Coulomb interaction. To date, studies have mainly focused on the complexes of intralayer excitons, while the electrically tunable interlayer exciton (IX) complexes remain elusive. Here, we report the observation of IX complexes with large out-of-plane electric dipole, strong emission intensity, and giant valley responses in bilayer ${\mathrm{MoS}}_2$, through on-resonance photoluminescence spectroscopy. In sharp contrast to the small, positive circular dichroism of intralayer excitons, the circular polarization of IX complexes in bilayer ${\mathrm{MoS}}_2$ can basically reach the theoretical limit (100%) but is negative. Such highly unusual light-valley responses of IX complexes in bilayer ${\mathrm{MoS}}_2$ demonstrate the strongly suppressed valley depolarization and spin-preserving scattering of electrons during the formation. Remarkably, by breaking the time-reversal symmetry with an out-of-plane magnetic field, a record level of spontaneous valley polarization (7.7%/Tesla) is identified for IX complexes in bilayer ${\mathrm{MoS}}_2$. The giant valley polarization of IX complexes in bilayer ${\mathrm{MoS}}_2$, together with the feasibility of electrical/optical/magnetic control, provides a firm basis for the development of next-generation electronic and optoelectronic applications with valley functionalities.

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• Excitons
• Valleytronics
• Transition metal dichalcogenides
• Photoluminescence