(Invited) P-Type Transparent Conducting Oxide Protection Layers for Sustainable Photoelectrochemical Water Oxidation

Achieving stable operation of high efficiency photoanodes used as components of solar water splitting devices is critical to realizing the promise of this renewable energy technology.  Here, we show that a p-type transparent conducting oxide (p-TCO) can function as a selective hole conduct and corrosion protection layer on photoanodes used for light-driven water oxidation. We selected NiCo₂O₄ with the inverse spinel structure was used as the p-TCO and show that this material, when integrated with Si-based photoanodes, has the requisite electronic structure, stability, transparency, and hole conductivity to achieve sustained and efficient solar water oxidation. 

NiCo₂O₄ was deposited on n-Si (100) and np-Si (100) by reactive sputtering at a substrate temperature of 70-100 °C.  The film structure was amorphous, as shown by the lack of XRD and Raman features. P-type conductivity was confirmed by measurement of a positive Seebeck coefficient. Light transparency of >70% (l > 400 nm) was achieved with a NiCo₂O₄ thickness of 40 nm.  The hole conductivity was 50-60 S/cm and, as expected, p-NiCo₂O₄ forms a rectifying contact to n-Si. 

Photoelectrochemical (PEC) evaluation was performed in aqueous 1M NaOH (pH 14) with simulated AM1.5 illumination; these conditions would rapidly corrode the n-Si photoanodes in the absence of a protection layer.  PEC performance of the np-Si/p-NiCo₂O₄ structures is excellent, particularly when a thin NiFe oxygen evolving catalyst is applied.  An onset potential of 0.95 V vs. RHE is observed, which is one of the lowest reported for a Si-based photoanode.  The current density at the reversible potential for water oxidation (+1.23 V vs. RHE) is >25 mA cm^-2, and the current rises to a limiting value of 30 mA cm^-2 at more anodic potentials, which demonstrates the attractive combination of transparency and low-resistance hole conductivity in the NiCo₂O₄.  Long-term testing indicates multi-day stability with minimal decrease in performance or observable corrosion of the Si photoanode.  In depth characterization of both the solid-solid interfaces between NiCo₂O₄ and light absorber/catalyst and of the solid/electrolyte interface will be discussed. 

This work demonstrates that p-TCOs are promising as corrosion protection layers for stable water oxidation photoanodes.  

This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.