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, $rm{X_D^-}$ 19 meV $textit{below}$ the lowest bright trion, $rm{X}_1^-$ in WSe$_2$ and WS$_2$. In MoSe$_2$, $rm{X_D^-}$ lies 6 meV $textit{above}$ $rm{X}_1^-$, while $rm{X_D^-}$ and $rm{X}_1^-$ almost coincide in MoS$_2$. Our results agree with GW-BSE $textit{ab-initio}$ calculations and quantitatively explain the optical response of doped monolayers with temperature.