Interface Engineering for High Performance Metal-Insulator-Semiconductor
Materials Science and Engineering, Stanford University
Photoelectrolysis is of interest for direct solar-driven production of chemicals and fuels. A persistent challenge in this field is the difficulty of simultaneously achieving high photovoltaic efficiency and chemical stability of semiconductor photoelectrodes during water oxidation. In 2011, Chen et al.1 demonstrated that atomic layer deposition (ALD) of a thin TiO2 layer can avoid oxidation of the surface of a Si photoanode during long-duration oxygen evolution. Using an ALD-TiO2 corrosion protection layer, it was possible to decouple efficient light absorption by the Si anode from the oxygen evolution reaction (OER) occurring on the surface of a catalyst layer overlying the TiO2. Metal oxide protection layers that block oxidative corrosion of highly efficient solar absorbers while permitting facile hole transport from the semiconductor to the OER catalyst layer have since become the standard approach for photoelectrochemical hydrogen synthesis. Reports of increasing photocurrent,2 photovoltage,3 and corrosion stability4 appear in the literature regularly.
For silicon photoanodes, the most favorable combination of photovoltage and photocurrent during water splitting has been achieved using thin ALD-TiO2 protection layers on a buried p+n junction. However, a buried homojunction electrode has several drawbacks compared to a simpler metal-insulator-semiconductor (MIS) Schottky junction. Formation of the buried junction involves extra processing steps and complexity. Furthermore, some interesting absorbers for tandem cells cannot form such junctions. Based in part on lessons learned from MIS gate stack engineering5 and from ultrathin oxide passivation of Schottky junction solar cells,6 we have investigated methods to boost the performance of MIS Schottky silicon photoanodes. This presentation will describe several approaches for engineering the interface region between the silicon surface and an overlying OER catalyst to build in high photovoltage and photocurrent, while inhibiting oxidative corrosion of the semiconductor.
This work is supported in part by the Stanford Global Climate and Energy Project and by National Science Foundation program CBET-1336844. The contributions of A. Scheuermann, P. Satterthwaite, K. Kemp, A. Meng, K. Tang, J. Lawrence, O. Hendricks, P.K. Hurley and C.E.D. Chidsey are gratefully acknowledged.
 Y.W. Chen et al., Nat. Mater. 10, 539 (2011).
 S. Hu et al., Science 344, 1005 (2014).
 A.G. Scheuermann et al., Nat. Mater. 15, 99 (2016).
 M.J. Kenney et al., Science 342, 836 (2013).
 H. Kim et al., J. Appl. Phys. 96, 3467 (2004).
 A.G. Aberle et al., Prog. Photovoltaics 2, 265 (1994).