Shift current in two-dimensional Janus transition-metal Dichalcogenides: The role of excitons

We investigate the shift current in two-dimensional (2D) Janus transition-metal dichalcogenides (TMDs). The shift current is evaluated using a real-time approach, where the coupling with an external field is described in terms of a dynamical Berry phase. This methodology incorporates electron-hole interactions and quasiparticle band structure renormalization through an effective Hamiltonian derived from many-body perturbation theory. We find that the shift current is strongly enhanced in correspondence with <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi>C</a:mi> </a:mrow> </a:math> excitons. An analysis in terms of the electron-hole pairs reveals that electron and hole are localized on different atoms and thus, following an optical excitation, the center of the electron charge is displaced, giving rise to a significant photocurrent. Janus TMDs, with their intrinsic out-of-plane asymmetry and tunable electronic properties, are particularly appealing for next-generation optoelectronic and energy-harvesting technologies. These results highlight the role of excitons in the shift-current response of Janus TMDs and demonstrate their potential as promising building blocks for future photovoltaic devices.