Nonadiabatic Molecular Dynamics Simulation of Charge Separation and Recombination at a WS2/QD Heterojunction

Two-dimensional transition metal dichalcogenides (TMDs), such as WS2, are appealing candidates for optoelectronics and photovoltaics. The strong Coulomb interaction in TMDs is however known to prevent electron-hole pairs from dissociating into free electron and hole. The experiment demonstrates that combination of WS2 and quantum dots (QDs) can achieve efficient charge separation and enhance photon-to-electron conversion efficiency. Using real-time time-dependent density functional theory combined with nonadiabatic molecular dynamics, we model electron and hole transfer dynamics at a WS2/QD heterojunction. We demonstrate that both electron and hole transfer are ultrafast due to strong donor-acceptor coupling. The photoexcitation of the WS2 leads to a 75 fs electron transfer, followed by a 0.45 eV loss within 90 fs. The photoexcitation of QD results in 240 fs hole transfer, but loses only 0.1S eV of energy within 1 ps. The strong charge-phonon coupling and a broad range of phonon modes involved in electron dynamics are responsible for the faster electron transfer than the hole transfer. The electron-hole recombination across the WS2/QD interface occurs in several 100 ps, ensuing a long-lived charge-separated state. Particularly, the hole transfer is threefold magnitude faster than the electron-hole recombination inside QD, ensuing that QD can be an excellent light-harvester. The detailed atomistic insights into the photoinduced charge and energy dynamics at the WS2/QD interface provide valuable guidelines for the optimization of solar light-harvesting and photovoltaic efficiency in modern nanoscale materials.