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 WSe2, WS2,MoSe2, and MoS2 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, X−D, 19 meV below the lowest bright trion, X−1, in WSe2 and WS2. In MoSe2, X−D lies 6 meV above X−1, while X−D and X−1 almost coincide in MoS2. Our results agree with GW-Bethe-Salpeter equation (GW-BSE) ab initio calculations and quantitatively explain the optical response of doped monolayers with temperature.