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The luminescence yield of transition metal dichalcogenide monolayersfrequently suffers from the formation of long-lived dark states, which includeexcitons with intervalley charge carriers, spin-forbidden transitions, and alarge center-of-mass momentum located outside the light cone of dispersionrelations. Efficient relaxation from bright exciton states to dark statessuppresses the quantum yield of photon emission. In addition, the radiativerecombination of excitons is heavily influenced by Auger-type exciton-excitonscattering, which yields another nonradiative relaxation channel at roomtemperature. Here, we show that Auger-type scattering is promoted not onlybetween (bright) excitons but also between excitons and long-lived dark states.We studied the luminescence dynamics of monolayer WS2 capped with hexagonal BNover broad time ranges of picoseconds to milliseconds using carefully designedpump-and-probe techniques. We observed that luminescence quenching associatedwith Auger-type scattering occurs on 1-100 microsecond time scales, which thuscorrespond to the lifetimes of the relevant dark states. The broad distributionof the measured lifetimes implies the impact of various types of long-livedstates on the exciton annihilation process.

Dark-state impact on the exciton recombination of WS2 monolayers as revealed by multi-time-scale pump-and-probe spectroscopy