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

Abstract

Gallium indium phosphide (GaInP2) 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, GaInP2 suffers from one of the biggest challenges facing the field: instability due to electrochemical corrosion in aqueous electrolytes. Molybdenum disulfide (MoS2) nanomaterials can be used to both protect GaInP2 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 GaInP2 photocathodes coated with thin MoS2 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 GaInP2 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-GaInP2 samples tested in identical conditions. We believe this to be one of the first successful attempts to stabilize GaInP2 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 MoS2 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 MoS2 as a thin-film protection layer. We discovered that pinhole formation in the MoS2 layer exposes the GaInP2 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 MoS2 as a protection layer and can be leveraged to improve the stability of thin film protected semiconductor water splitting devices. References Khaselev, O.; Turner, J. A., A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production Via Water Splitting. Science 1998, 280, 425-427. Benck, J. D.; Lee, S. C.; Fong, K. D.; Kibsgaard, J.; Sinclair, R.; Jaramillo, T. F., Designing Active and Stable Silicon Photocathodes for Solar Hydrogen Production Using Molybdenum Sulfide Nanomaterials. Advanced Energy Materials 2014, 4, 1-8 Britto R.J., Benck J.D., Young J.L., Hahn C., Deutsch, T.G., Jaramillo T.F. In Review (2016) Figure 1