Fundamental limits of exciton-exciton annihilation for light emission in transition metal dichalcogenide monolayers

摘要

We quantitatively illustrate the fundamental limit that exciton-exciton annihilation (EEA) may impose on the light emission of monolayer transition metal dichalcogenide (TMDC) materials. The EEA in TMDC monolayers shows a dependence on the interaction with substrates as its rate increases from 0.1cm~2/s (0.05cm~2/s) to 0.3 cm~2/s (0.1 cm~2/s) with the substrates removed for WS_2 (MoS_2) monolayers. It turns to be the major pathway of exciton decay and dominates the luminescence efficiency when the exciton density is beyond 10~(10)cm~(-2) in suspended monolayers or 10~(11) cm~(-2) in supported monolayers. This sets an upper limit on the density of injected charges in light-emission devices for the realization of optimal luminescence efficiency. The strong EEA rate also dictates the pumping threshold for population inversion in the monolayers to be 12-18 MW/cm~2 (optically) or 2.5-4 × 10~5 A/cm~2 (electrically).