Interfacial effects in Janus HfSSe/SnSSe van der Waals heterostructure: synergistic enhancement of optical absorption and thermoelectric performance.

Two-dimensional Janus monolayers hold great promise for energy-related applications but are limited by inefficient charge separation, restricted optical absorption bandwidth and relatively high lattice thermal conductivity. One effective strategy to overcome these shortcomings is to construct van der Waals (vdW) heterostructures from distinct Janus monolayers. This study systematically performs first-principles calculations to investigate the dual-Janus HfSSe/SnSSe vdW heterostructure. The heterostructure is thermodynamically and dynamically stable, featuring a type-II band alignment with an indirect bandgap of 0.90 eV (HSE06) and 0.91 eV (HSE06 + SOC). Benefiting from intrinsic dipole synergy and efficient interlayer charge transfer with a tunneling probability (TB) of 27.05%, photogenerated carriers are spatially separated effectively. The heterostructure exhibits strong and broad-spectrum optical absorption, with a coefficient on the order of 105 cm-1 in the visible region and exceeding 3.0 × 105 cm-1 in the ultraviolet region. Thermoelectrically, it delivers a high Seebeck coefficient of 490 µV K-1 at 300 K and an ultralow lattice thermal conductivity of 0.31 W m-1 K-1 at 700 K, yielding a maximum thermoelectric figure of merit of 1.05 at 700 K, which outperforms the standalone Janus monolayers. The excellent optoelectronic and thermoelectric properties render the HfSSe/SnSSe heterostructure a promising multifunctional candidate for next-generation optoelectronic and thermoelectric devices, and establish a feasible design paradigm for Janus-based vdW heterostructures.