The Effect of SPA-SiO2 Tunnel Oxide Thickness for Metal-Insulator-Silicon Photoelectrochemical Cells

Photoelectrochemical cells are becoming increasingly attractive for the on-site production of clean chemicals and fuels, both as end-use products and as an integrated energy storage solution for mitigating the intermittency of solar radiation.  One persistent challenge in this field is the difficulty of simultaneously achieving high photovoltaic efficiency and chemical stability of semiconductor photoelectrodes during water oxidation under wide-ranging pH conditions.  Wide band gap semiconducting metal oxides are frequently stable under water oxidation conditions but have inherent photocurrent limitations.  We have recently demonstrated that atomic layer deposition (ALD) of a thin and chemically-protective TiO₂ layer can avoid oxidation of the surface of a silicon photoanode during long-duration oxygen evolution.  Our initial work showed that it was possible to decouple efficient light absorption by the silicon anode from the oxygen evolution reaction (OER) occurring on the surface of a water oxidation catalyst layer by the presence of an interposed ALD-TiO₂ layer.  This conformal and pinhole-free oxide layer was found to be dense enough to block the solution from oxidizing the underlying semiconductor (greatly increasing its stability), but still thin enough to allow efficient hole transport between an n-type silicon substrate and the iridium OER catalyst [1]. In more recent work, we have showed that ALD-TiO₂ could be used with a variety of other catalysts to produce efficient devices with more than 600mV of open circuit voltage, high saturation currents, and similar stability.  In addition, by studying thicker ALD-TiO₂ films, we found that protection could be increased with a minor increase in anode resistance.  The additional resistance is associated with bulk conduction of electronic carriers through states in the TiO₂ band-gap between the catalyst and anode [2].  Together, these findings indicate that atomic layer deposited coatings in metal-insulator-semiconductor (MIS) photoanodes serve as a general protection method for a variety of semiconductor-catalyst systems.  In addition to summarizing prior reports and the most recent results on such MIS photoelectrodes, this presentation will touch on future directions for efficient artificial photosynthesis using ALD surface protection layers. 

[1] YW Chen, J.D. Prange, et al. Nat. Mater. 2011, 10, 539-44.

[2] A.G. Scheuermann, et al. Energy Environ. Sci. 2013, 6, 2487 – 96.