Orbital hybridization induces fast photoelectron capture by graphene to promote high gain in transition metal dichalcogenide/graphene heterojunctions

Transition metal dichalcogenide/graphene (TMDC/Gr) heterojunction devices exhibit significantly higher photoresponsivity compared to TMDC devices alone, making them promising for optoelectronic applications. However, experiments demonstrated that graphene cannot prolong the photogenerated carrier lifetime of TMDC/Gr heterojunctions and, further, that the high density of sulfur vacancies in TMDCs complicates photogenerated carrier dynamics, leaving the underlying physical mechanism behind the high photoresponsivity unclear. Herein, we investigate photogenerated carrier transfer and recombination of $\mathrm{Mo}{\mathrm{S}}_{2}$/Gr and $\mathrm{W}{\mathrm{S}}_{2}$/Gr heterojunctions through nonadiabatic molecular dynamics simulations. Instead of conventional speculation that sulfur vacancies of TMDCs store photogenerated carriers to enhance optoelectronic performance, we find that the hybridization between defect states of TMDC and Dirac points of graphene induces fast photoelectron transfer from TMDC to graphene, promoting high carrier gain in TMDC/Gr heterojunctions. Fast carrier transfer of heterojunctions derives from the excitation of low-frequency in-plane phonon modes. Meanwhile, graphene does not drastically reduce the photogenerated carrier lifetime of TMDC/Gr heterojunctions. Therefore, the faster photogenerated electrons transfer and long carrier lifetime lead to photogenerated carrier gain, resulting in superior optoelectronic performance of TMDC/Gr heterojunctions. This study provides a comprehensive understanding of photogenerated carrier dynamics in TMDC/Gr heterojunctions, laying the foundation for design of high-performance TMDC optoelectronic devices.