Hot carrier diffusion-assisted ideal carrier multiplication in monolayer MoSe₂

Carrier multiplication (CM), the process of generating multiple charge carriers from a single photon, offers an opportunity to exceed the Shockley-Queisser limit in photovoltaic applications. Despite extensive research, no material has yet achieved ideal CM efficiency, primarily due to significant energy losses from carrier-lattice scattering. In this study, we demonstrate that monolayer MoSe₂ can attain the theoretical maximum CM efficiency permitted by the energy-momentum conservation principle, using ultrafast transient absorption spectroscopy. By resolving the scatter-free ballistic transport of hot carriers and validating our findings with first-principles calculations, we identify the cornerstone of optimal CM in monolayer MoSe₂: superior hot-carrier dynamics characterized by suppressed energy dissipation via minimized carrier-lattice scattering and the availability of abundant CM pathways facilitated by 2E_g band nesting. Comparative analysis with bulk MoSe₂ further emphasizes the enhanced CM efficiency in the monolayer, attributed by superior hot-carrier diffusion and access to additional CM pathways. These results position monolayer MoSe₂ as a promising candidate for high-performance optoelectronic applications, providing a robust platform for next-generation energy conversion technologies.