High-Efficiency Solar-To-Hydrogen Conversion and Complementary Thermoelectric Performance of GeSnS ₂ Monolayer

The development of multifunctional, low-dimensional materials is central to advancing sustainable energy technologies. In this work, we conduct a comprehensive first-principles investigation of the GeSnS₂ monolayer to evaluate its potential for integrated solar-to-hydrogen and thermoelectric energy conversion. The material is found to be dynamically, thermally, and mechanically stable, exhibiting a notable fracture strain of approximately 0.18 and strong in-plane stiffness. Electronic structure calculations reveal an indirect band gap of 1.34 eV and highly anisotropic carrier mobilities reaching approximately 1000 cm² V^-1 s^-1, supporting efficient charge transport. Optical analyses demonstrate strong in-plane absorption across the visible spectrum and favorable band edge alignment for spontaneous overall water splitting, resulting in a high solar-to-hydrogen efficiency of 17.44%. Moreover, thermoelectric assessments show a large Seebeck coefficient, a power factor surpassing 57.88 mWm^-1 K^-2, and a peak figure of merit (ZT) of 0.78 at moderate temperatures. The coexistence of excellent photocatalytic and thermoelectric performance highlights GeSnS₂ as a compelling two-dimensional material platform for next-generation, solar-driven, and sustainable energy harvesting systems.