The development of efficient and stable photocathodes is crucial for advancing solar energy harvesting and sustainable chemical fuel production. In this context, selenium (Se) emerges as a promising material due to its favorable band alignment for hydrogen evolution reaction (HER), intrinsic p-type conductivity, and compatibility with scalable fabrication techniques such as co-evaporation. Additionally, Se offers advantages such as relative abundance, low toxicity, and tunable optoelectronic properties. Despite being the first photovoltaic material demonstrated in 1883, Se-based photoelectrochemical (PEC) devices remain largely underexplored, with significant potential for optimization.

This study presents a comprehensive investigation into Se-based photocathodes, focusing on optimizing key parameters to enhance activity, efficiency, stability, and sustainability. The effects of Se crystallization and thickness are systematically examined to balance photon absorption and charge extraction. Additionally, protective layers are studied to improve charge tunneling and overall device performance. Different back contacts and electrolyte formulations are explored to minimize series resistance and accelerate reaction kinetics.

Through these optimizations, this work achieves both the highest-ever efficiency and photocurrent density for a Se-based photocathode, setting a new benchmark in the field. Notably, these performances were achieved without the use of expensive nanoparticles, marking a significant step toward cost-effective and scalable PEC technology. Specifically, an all-time highest HC-STH efficiency of 2.76% and a photocurrent density of 11.35 mA/cm² at 0V(RHE) were obtained using  SLG/Mo/Se-based devices (previous literature benchmarks were 0.39% efficiency and J of 7.2mA/cm² at 0V(RHE) [1]). Further advancing the concept of sustainability and reduced toxicity, similar results were achieved in a H₂SO₄-free system, with SLG/Mo/Se/TiO₂ devices in phosphate buffer electrolyte, enhancing both environmental sustainability and device safety.

These findings provide deeper insight into selenium-based photocathodes and highlight their potential for scalable PEC applications. By improving not only performance but also material sustainability and device safety, this study aligns with ongoing efforts to develop next-generation inorganic materials for solar-driven fuel production.