Strong Coulomb interactions in monolayer transition-metal dichalcogenides (TMDs) result in large exciton binding energies, such that photoexcited electrons and holes predominantly exist as bound excitons. In type-II MoSe₂/WSe₂ heterostructures, interlayer charge transfer can spatially separate electrons and holes into different layers. However, whether the transferred carriers remain as free separated charges or predominantly form charge-neutral interlayer bound states is an important issue for understanding the ultrafast carrier dynamics and transport properties of such heterostructures. In this work, optical-pump terahertz-probe (OPTP) spectroscopy is employed to investigate the transient low-frequency photoconductivity of monolayer MoSe₂, monolayer WSe₂, and MoSe₂/WSe₂ heterostructures. A pump wavelength of 780 nm, resonant with the MoSe₂ A exciton while lying below the WSe₂ A-exciton energy, is used to selectively excite the MoSe₂ layer. Under the same excitation conditions, monolayer WSe₂ exhibits no resolvable terahertz photoconductivity response, and monolayer MoSe₂ shows only a weak response. In contrast, the MoSe₂/WSe₂ heterostructure exhibits a significantly enhanced transient terahertz photoconductivity. This enhancement is attributed to exciton dissociation and interlayer charge separation driven by the type-II band alignment, with holes transferred from MoSe₂ to WSe₂ while electrons predominantly remain in the MoSe₂ layer. The pronounced terahertz photoconductivity provides evidence for the formation of free separated charges that can contribute to low-frequency transport, rather than exclusively charge-neutral interlayer bound states.
The transient photoconductivity dynamics of the heterostructure can be well described by a biexponential decay model, yielding a fast relaxation component of approximately 2-3 ps and a slower component on the order of several tens of picoseconds. Within the experimental uncertainty, neither relaxation time exhibits a pronounced dependence on pump fluence. The fast and slow relaxation components may be associated with defect trapping of electrons in the MoSe₂ layer and holes in the WSe₂ layer, respectively. To further investigate the low-frequency transport properties of the spatially separated carriers, the frequency-resolved complex photoconductivity is analyzed using the Drude-Smith model. The measured spectra exhibit pronounced non-Drude behavior and are well reproduced by the Drude-Smith model. With increasing pump fluence, the fitted plasma frequency increases, consistent with an increase in the density of photoexcited carriers contributing to the terahertz response, whereas the momentum-scattering time decreases. In contrast, the backscattering parameter c remains nearly unchanged at approximately -0.6 within the experimental uncertainty. Delay time-dependent measurements further show that the photoconductivity gradually decreases with increasing pump-probe delay, whereas the backscattering parameter remains nearly constant at approximately -0.7. These results indicate that although the population of terahertz-active carriers evolves substantially with excitation density and delay time, the degree of transport restriction remains relatively stable. The observed non-Drude behavior therefore suggests that the transport of the spatially separated carriers is strongly influenced by defects and other related factors.
These results provide a low-frequency transport perspective on the charge states formed following interlayer charge transfer in MoSe₂/WSe₂ heterostructures. In contrast to conventional optical measurements, OPTP spectroscopy directly probes the photoconductivity response of charge carriers. The present results therefore provide complementary experimental evidence for the formation of terahertz-active free separated charges following exciton dissociation and interlayer charge transfer, and further reveal the important influence of the local electronic environment on their subsequent ultrafast relaxation and restricted transport. This work provides new insight into the relationship among interlayer charge transfer, charge separation, and low-frequency transport in two-dimensional van der Waals heterostructures.