(365d) Molybdenum Disulfide As a Protection Layer and Catalyst for Gallium Indium Phosphide Solar Water Splitting Photocathodes

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.¹ 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 a novel flow cell microscopy technique.

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.² 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 photocathodes³, 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 techniques, 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

(1) Khaselev, O.; Turner, J. A., A Monolithic Photovoltaic­Photoelectrochemical Device for Hydrogen Production Via Water Splitting. Science 1998, 280, 425­427.

(2) Britto R.J., Benck J.D., Young J.L., Hahn C., Deutsch, T.G., Jaramillo T.F., Molybdenum Disulfide as a Protection Layer and Catalyst for Gallium Indium Phosphide Solar Water Splitting Photocathodes. Journal of Physical Chemistry Letters. Accepted (2016)

(3) 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, 1­8