  Excitons are key to the optoelectronic applications of van der Waals
semiconductors with the potential for versatile on-demand tuning of properties.
Yet, their electrical manipulation is complicated by their inherent charge
neutrality and the additional loss channels induced by electrical doping. We
demonstrate the dynamic control of valley polarization in charged biexciton
(quinton) states of monolayer tungsten disulfide, achieving up to a sixfold
increase in the degree of circular polarization under off-resonant excitation.
In contrast to the weak direct tuning of excitons typically observed using
electrical gating, the quinton photoluminescence remains stable, even with
increased scattering from electron doping. By exciting at the exciton
resonances, we observed the reproducible non-monotonic switching of the charged
state population as the electron doping is varied under gate bias, indicating a
coherent interplay between neutral and charged exciton states.
