Competition between Free Carriers and Excitons Mediated by Defects Observed in Layered WSe2 Crystal with Time-Resolved Terahertz Spectroscopy

The dynamics of photoexcited species is quite important for the development of next-generation ultrafast optoelectronic devices based on transition metal dichalcogenides (TMDs). Herein, time-resolved optical pump terahertz (THz) probe spectroscopy, which is sensitive to both bounded excitons and free electrons/holes, is employed to study the dynamics of photo-induced carriers in the typical layered TMDs crystal tungsten diselenide (WSe2). Initial photoexcitation could generate both free carriers and excitons. The free carriers decay followed by phonon-assistance (approximate to 30 ps) and defect-assistance (approximate to 200-280 ps). The excitons decay followed by the phonon-assisted recombination (approximate to 100 ps) and the defect-induced exciton separation (approximate to 40-200 ps). With the increasing of pump fluence, more free electrons and holes will bind to form excitons by many-body effect, while with the decay of time, more excitons will dissociate into free carriers by defects. The frequency-dependent transient complex THz photoconductivity of layered WSe2 crystal can be well described by Drude-Smith-Lorentz model, which suggests preferential free carriers backscattering due to the defects. The ratio of free carriers to excitons suggests that free carriers dominate after the photoexcitation, which is important for the optoelectronic devices such as solar cells and photodetectors.