(Invited) Stabilizing Semiconductor-Solution Interfaces Via Chemically Stable but Electronically Defective Coatings

Recently, a photoelectrode protection strategy, which uses an amorphous “leaky” TiO₂ coating to form a complex semiconductor/TiO₂/Ni structure, has shown to stabilize semiconductor-solution interfaces against photoanodic corrosion.  Particularly, stabilizing semiconductor-aqueous solution interfaces for water oxidation is important because water oxidation is required for all solar fuel strategies, e.g. H₂ production and CO₂ reduction.  We show that this TiO₂protection strategy has stabilized a number of 1.1 – 1.9 eV band gap light absorbers including Si, group III-V, group II-VI semiconductors, their tandem structures and wire structures for up to 2000 hours continuous water oxidation in 1 M KOH(aq).  Because of this, various otherwise unstable materials can be brought back to the table again for building photoelectrochemical devices.  A water splitting reactor prototype of ~10% solar-to-hydrogen efficiency was also built.    

The n-doped Si/TiO₂/Ni structure also operates as a high barrier height junction for solar energy conversion.  To further understand the mechanism of “leaky” TiO₂ protection and the role of Ni electrocatalyst overlayers, we used Si/TiO₂/Ni structures as a model system to study solid-solution interfaces.  TiO₂ coatings of 3 – 143 nm in thickness introduce two interfaces: a TiO₂/liquid interface and a TiO₂/n-Si heterojunction.  We have employed operando ambient-pressure X-Ray photoelectron spectroscopy (AP-XPS) to study TiO₂/Ni/liquid and TiO₂/liquid interfaces, and directly observed their ohmic and rectifying junction behavior, respectively.  We also extensively investigated n-Si/TiO₂ heterojunction interfaces by employing photoelectrochemical, solid-state electrical, and photoelectron spectroscopic techniques.  The distinctive electrical behavior of n-Si/TiO₂ heterojunctions will be compared with conventional semiconductor/metal and semiconductor/liquid junctions.  Because electronic defect states exist in band gaps of “leaky” TiO₂, their effects on the formation of n-Si/“leaky”TiO₂ heterojunctions will be discussed.  This interfacial study reveal several strategies for improving the performance of thin-layer protected photoelectrodes as well as for further expanding selection of “leaky” protective coatings.