Defect-Engineered Reduction of the Carrier Multiplication Threshold in Monolayer WS ₂ to 1.6 <i>E</i> _g

Carrier multiplication (CM), a process generating two or more electron-hole pairs after absorbing one high-energy photon, holds great potential for breaking the Shockley-Queisser limit in photovoltaics. However, conventional CM in semiconductors is constrained by a high threshold (>2<i>E</i>_g), leaving the excess energy of sub-2<i>E</i>_g high-energy photons unutilized. Here, using transient-absorption spectroscopy, we report the systematic experimental realization of a low CM threshold of 1.6<i>E</i>_g in monolayer WS₂ by leveraging strong electron-phonon coupling (EPC) through engineering high-symmetric disulfur vacancies. Steady-state spectroscopic measurements (photoluminescence and Raman) and density functional theory calculations reveal that the reduced CM threshold arises from efficient phonon-assisted upconversion that enables the transition from sulfur divacancy-induced in-gap states to the conduction band minimum of WS₂ due to strong EPC. Our work enables a distinct strategy for reducing the CM threshold, which contributes directly to the pursuit of high-efficiency photovoltaics and next-generation optoelectronic technologies.