Charge Separation and Recombination in Two-Dimensional MoS₂/WS₂: Time-Domain ab Initio Modeling

Abstract

With applications in high performance electronics, photovoltaics and catalysis, two-dimensional transition metal dichalcogenides (TMDs) attract strong attention. Isolated TMDs, which are already remarkably complex, can stack in sequence to make even more complex heterostructures. Surprisingly, charge separation is ultrafast in layered TMD heterostructures, even though the interlayer interaction is weak. Also surprisingly, the charge separated state is long-lived, despite the close proximity of electron and hole. Using real-time time-dependent density functional theory combined with nonadiabatic (NA) molecular dynamics, we model hole and electron transfer, and electron–hole recombination at a MoS2/WS2 heterojunction. Hole transfer is ultrafast, in excellent agreement with the experiment, due to significant delocalization of the photoexcited state between the donor and acceptor materials. Electron transfer is 1 order of magnitude longer, due to weaker donor–acceptor and NA couplings, lower density of acceptor states, and shorter quantum coherence. The electron–hole recombination is 3–4 orders of magnitude slower than the charge separation, because the initial and final states are localized strongly within different materials, rationalizing the long-lived charge separation. The computed recombination time scale agrees with the experimental data on the closely related MoSe2/WSe2 system. All electronic processes are coupled to the characteristic out-of-plane 400 cm–1 motion of the MoS2 and WS2 layers. The atomistic, time-domain methodology provides theoretical insights into the photoinduced electron–phonon dynamics in two-dimensional TMD heterostructures, and can be used for in silico design of novel functional materials operating under nonequilibrium conditions.