The low valley polarization of monolayer transition-metal dichalcogenides at room temperature poses a significant obstacle to the development of valleytronic devices, and a mechanistic insight into the valley depolarization in such systems is still lacking. In this study, we demonstrate that substitutional doping leading to alloyed monolayers offer an effective strategy for enhancing valley polarization at room temperature. The degree of valley polarization, as determined by helicity-resolved transient absorption spectroscopy, is 15% for Se-doped and 30% for Se- and V-doped monolayer layer $\mathrm{Mo}{\mathrm{S}}_2$ (referred to as MoSSe and VMoSSe, respectively). The valley polarization persists for longer durations in MoSSe (∼15 ps) and VMoSSe (>500 ps) compared to pristine $\mathrm{Mo}{\mathrm{S}}_2$ (∼1 ps). The prolonged valley depolarization in MoSSe is attributed to a reduction in long-range electron-hole exchange interactions due to thermal mixing of bright and dark excitons. However, the valley depolarization in VMoSSe takes place via intervalley scattering of carriers between hybridized and defect excitons. Our study elucidates that altering the exciton ground state through the inclusion of additional levels near the conduction-band or valence band edges via alloying can enhance and prolong valley polarization in $\mathrm{Mo}{\mathrm{S}}_2$. These findings are pivotal in realizing valleytronics devices at room temperature.

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• Spin-orbit coupling
• Valley degrees of freedom
• Valleytronics