  Transition metal dichalcogenide (TMD) monolayers are two-dimensional
semiconductors with two valleys in their band structure that can be selectively
addressed using circularly polarized light. Their photoluminescence spectrum is
characterized by neutral and charged excitons (trions) that form a chemical
equilibrium governed by the net charge density. Here, we use chemical doping to
drive the conversion of excitons into trions in $\text{WS}_{2}$ monolayers at
room temperature, and study the resulting valley polarization via
photoluminescence measurements under valley-selective optical excitation. We
show that the doping causes the emission to become dominated by trions with a
strong valley polarization associated with rapid non-radiative recombination.
Simultaneously, the doping results in strongly quenched but highly
valley-polarized exciton emission due to the enhanced conversion into trions. A
rate equation model explains the observed valley polarization in terms of the
doping-controlled exciton-trion equilibrium. Our results shed light on the
important role of exciton-trion conversion on valley polarization in monolayer
TMDs.
