Dark trions govern the temperature-dependent optical absorption and emission of doped atomically thin semiconductors

We perform absorption and photoluminescence spectroscopy of trions in hBN-encapsulated WSe${}_2$, WS${}_2$, MoSe${}_2$, and MoS${}_2$ monolayers, depending on temperature. The different trends for W- and Mo-based materials are excellently reproduced considering a Fermi-Dirac distribution of bright and dark trions. We find a dark trion, ${\mathrm{X}}_{\mathrm{D}}^{-}$ 19 meV $\mathit{\text{below}}$ the lowest bright trion, ${\mathrm{X}}_1^{-}$ in WSe${}_2$ and WS${}_2$. In MoSe${}_2$, ${\mathrm{X}}_{\mathrm{D}}^{-}$ lies 6 meV $\mathit{\text{above}}$ ${\mathrm{X}}_1^{-}$, while ${\mathrm{X}}_{\mathrm{D}}^{-}$ and ${\mathrm{X}}_1^{-}$ almost coincide in MoS${}_2$. Our results agree with GW-BSE $\mathit{\text{ab-initio}}$ calculations and quantitatively explain the optical response of doped monolayers with temperature.