Shift current in 2D Janus Transition-Metal Dichalcogenides: the role of excitons

TL;DR

This work demonstrates that excitons play a pivotal role in the shift current of 2D Janus TMDs MoSSe and WSSe. By employing a real-time, dynamical Berry-phase framework that incorporates $GW$ quasiparticle corrections and GW+BSE excitons, it reveals a strong enhancement of the shift current at C-exciton resonances and suppression at A/B resonances, driven by a real-space separation of electron–hole pairs and the consequent shift of the charge center. The authors show that breaking $C_{3v}$ symmetry—through strain, heterostructures, or nanotube formation—is needed to realize sizable SC in devices, with WSSe exhibiting a maximum photocurrent around $1.2$ nA at C resonances. Overall, the paper provides a unified computational approach to predict nonlinear optical responses in low-dimensional materials and identifies Janus TMDs as promising platforms for next-generation photovoltaics and energy harvesting.

Abstract

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 C 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.

Figures (5)

• Figure 1: The electronic band structures of (a) MoSSe and (b) WSSe monolayers, at the Kohn-Sham level (blue solid lines), including the quasiparticle corrections from $\mathrm{G_0W_0}$ calculations (red dots) and interpolated bands (red lines).. See Supplemental Material for the coordinates of the high-symmetry points used.
• Figure 2: Projected band structure of WSSe on atomic orbitals. Panel (a): projection weight onto S and W; panel (b): projection weight onto Se and W; panel (c): projection weight onto S and Se.
• Figure 3: Exciton analysis in WSSe. Top left (right) panel: the projection onto the BZ of the A/B, C$_1$ and C$_2$ excitons. Bottom left (right) panel: the projection along the WSSe band structure of the A/B, C$_1$ and C$_2$ excitons.
• Figure 4: Shift current along the $yyy$ direction for MoSSe (upper panel) compared with the linear absorption (lower panel), at the TD-IP (gray dashed lines), TD-IP@GW (red dashed lines) and TD-aGW(GW+BSE) (blue solid lines) levels.
• Figure 5: Shift current along the $yyy$ direction in WSSe (upper panel) compared with the linear absorption (lower panel), at the TD-IP (gray dashed lines), TD-IP@GW (red dashed lines) and TD-aGW (GW+BSE) (blue solid lines) levels.. The positions of A/B, C1 and C2 excitons analyzed in Fig. \ref{['exc_analysis']} are labeled with magenta letters in black boxes.