Engineering Exciton Recombination Pathways in Bilayer WSe2 for Bright Luminescence

Exciton-exciton annihilation (EEA) in counterdoped monolayer transition metal dichalcogenides (TMDCs) can be suppressed by favorably changing the band structure with strain. The photoluminescence (PL) quantum yield (QY) monotonically approaches unity with strain at all generation rates. In contrast, here in bilayers (2L) of tungsten diselenide (WSe2) we observe a nonmonotonic change in EEA rate at high generation rates accompanied by a drastic enhancement in their PL QY at low generation rates. EEA is suppressed at both 0% and 1% strain, but activated at intermediate strains. We explain our observation through the indirect to direct transition in 2L WSe2 under uniaxial tensile strain. By strain and electrostatic counterdoping, we attain similar to 50% PL QY at all generation rates in 2L WSe2, originally an indirect semiconductor. We demonstrate transient electroluminescence from 2L WSe2 with similar to 1.5% internal quantum efficiency for a broad range of carrier densities by applying strain, which is similar to 50 times higher than without strain. The present results elucidate the complete optoelectronic photophysics where indirect and direct excitons are simultaneously present and expedite exciton engineering in a TMDC multilayer beyond indirect-direct bandgap transition.

Automated summary

Exciton-exciton annihilation in monolayer TMDCs can be controlled by strain. In bilayer WSe2, a nonmonotonic change in EEA rate at high generation rates is observed, accompanied by a significant enhancement in PL QY at low generation rates. By strain and electrostatic counterdoping, similar to 50% PL QY can be achieved in 2L WSe2 at all generation rates.