Transient Reflection Spectroscopy on Ultrafast Interlayer Charge Transfer Processes in a MoS2/WSe2 van der Waals Heterojunction

Understanding the dynamical process of charge transfer (CT) across van der Waals heterostructures (VdWHs) is vitally important for improving photovoltaic conversion efficiency at the atomically thin interfaces. Here, ultrafast dynamics of VdWHs constructed by vertically stacking monolayer molybdenum disulfide (MoS2) onto tungsten diselenide (WSe2) with a multilayer hexagonal boron nitride (hBN) substrate were investigated by transient reflection measurements. MoS2 and WSe2 were selectively excited to obtain direct evidence of the CT processes. It is interesting to note that the bleaching band of MoS2/WSe2 is composed of two different contributions with different dynamics under excitation at 650 nm: direct excitation-induced bleaching of WSe2(with a decay time constant of similar to 63 ps) and bleaching signals arising from photo-induced hole transfer (with a decay time constant of similar to 303 ps). The interlayer CT process was found to occur on the subpicosecond time scale, resulting in long-lived interlayer spatially separated charge carriers (approximately hundreds of picoseconds). These results imply potentially high photovoltaic conversion efficiency of these heterostructures.