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

Research paper by Yaqing Wei, Wei-Hai Fang, Qiu Fang, Run Long

Indexed on: 27 Mar '18Published on: 13 Mar '18Published in: Journal of Physical Chemistry C



Abstract

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 0.15 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.

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