Molybdenum Disulfide As a Protection Layer and Catalyst for Gallium Indium Phosphide Solar Water Splitting Photocathodes

Gallium indium phosphide (GaInP 2 ) is a semiconductor with a nearly ideal bandgap for solar water-splitting as the top absorber in a dual junction tandem absorber device. It has been used in conjunction with a gallium arsenide (GaAs) bottom absorber in an overall water splitting cell with 12.4% solar-to-hydrogen (STH) efficiency, one of the highest STH efficiencies for an integrated photoelectrochemical (PEC) water-splitting device reported to date. However, GaInP 2 suffers from one of the biggest challenges facing the field: instability due to electrochemical corrosion in aqueous electrolytes. Molybdenum disulfide (MoS 2 ) nanomaterials can be used to both protect GaInP 2 and significantly improve its catalytic ability since it is resistant to corrosion and also possesses high activity for the hydrogen evolution reaction (HER). In this work, we demonstrate that GaInP 2 photocathodes coated with thin MoS 2 surface protecting layers exhibit excellent activity and stability for solar hydrogen production and we probe the details of failure mechanisms using novel flow cell microscopic and spectroscopic techniques. Our GaInP 2 photocathodes demonstrated no loss in performance (photocurrent onset potential, fill factor, and light limited current density) until 60 hours of operation which represents a five-hundred fold increase in stability compared to bare p-GaInP 2 samples tested in identical conditions. We believe this to be one of the first successful attempts to stabilize GaInP 2 using a thin film protection layer scheme. Furthermore, as this protection scheme has previously been used successfully on silicon photocathodes, this work highlights the potential for MoS 2 to be used as a thin film protection layer for many different semiconductor water splitting devices that are unstable in acid. Using a custom-designed flow cell coupled with various microscopic and spectroscopic techniques (optical, Raman, FT-IR), we gained a greater understanding of the failure mechanisms of MoS 2 as a thin-film protection layer. We discovered that pinhole formation in the MoS 2 layer exposes the GaInP 2 substrate, which readily corrodes in the acidic conditions, ultimately leading to device degradation. The flow cell further allowed us to capture the time scale of this pinhole formation. These insights represent a deeper understanding of MoS 2 as a protection layer and can be leveraged to improve the stability of thin film protected semiconductor water splitting devices. Re