Metal-insulator-semiconductor (MIS) structures are promising candidates for integrated solar driven water splitting devices. Often, the metal serves a dual purpose, catalyzing water oxidation while also setting the built-in field for extracting photogenerated carriers from the semiconductor. Recently, incorporation of additional protection layers into the MIS junction has made it possible to use semiconductor materials, such as silicon, that would normally be unstable under the conditions required for water oxidation.¹ Although the theoretical maximum photovoltage for silicon photovoltaics is 700-800 mV,² there is a tradeoff between photocurrent and photovoltage in adventitiously-doped TiO₂.³ For example, maximum photovoltages of ~400 mV have been reported for nSi / TiO₂ / metal photoanodes protected by highly conductive TiO₂ layers,⁴ with some devices achieving only 200-250 mV.⁵

Our goal is to develop a more ideal protection layer using transition metal oxide-titania alloys synthesized by ALD. An ideal protection layer should be corrosion resistant, highly conductive, and enable a high photovoltage. For n-type silicon Schottky photoanodes, this means that the protection layer must also possess a high work function. We previously demonstrated that TiO₂-RuO₂ alloys composed of 13-46% Ru were highly conductive and consistently achieved photovoltages >525 mV without any post-processing anneals.⁶ The built-in field was set by the TiO₂-RuO₂ alloy, which possessed a sufficiently high work function of 5.02 eV. Although TiO₂-RuO₂ alloys were ideal Schottky contacts to nSi, they lacked the long-term corrosion resistance required of a protection layer. RuO₂ is known to be unstable at the conditions required for water oxidation. We also report on ALD alloys of TiO₂ and IrO₂, which exhibit high conductivity and photovoltage > 600 mV on nSi, but with improved stability. Additionally, TiO₂-IrO₂ alloys are capable of catalyzing oxygen evolution. Thus, TiO₂-IrO₂alloys have the potential to be an “all-in-one” catalyst, Schottky contact, and corrosion resistant protection layer.

References:

1. Chen, Y.C. et al. Atomic Layer-Deposited Tunnel Oxide Stabilizes Silicon Photoanodes for Water Oxidation. Nature Mater. 10, 539-44 (2011).

2. Green, M. A. Limits on the Open-circuit Voltage and Efficiency of Silicon Solar Cells Imposed by Intrinsic Auger Processes. IEEE Trans. Electron Devices ED-31,671–678 (1984).

3. Scheuermann, A.G. et al. Photovoltage Design for Metal Oxide Protected Solar-Water-Splitting Photoanodes. Nature Mater. 15, 99-105 (2016).

4. Hu, S. et al. Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation. Science (80-. ). 344,1005–1009 (2014).

5. McDowell, M. T. et al. The Influence of Structure and Processing on the Behavior of TiO ₂ Protective Layers for Stabilization of n-Si/TiO ₂ /Ni Photoanodes for Water Oxidation. ACS Appl. Mater. Interfaces 7,15189–15199 (2015).

6. Hendricks, O. L. et al. Isolating the Photovoltaic Junction : Atomic Layer Deposited TiO₂ − RuO₂ Alloy Schottky Contacts for Silicon Photoanodes. (2016). doi:10.1021/acsami.6b08558