Surface Engineering of Metal Oxide Photoelectrodes for Improved Band Alignment in Solar Water Splitting Cells

Abstract

Several earth-abundant transition-metal oxides (e.g. Fe2O3, CoO, and Cu2O) possessing suitable band gaps for solar water splitting exist, but energy level alignment is often sub-optimal, i.e. the conduction and valence bands do not straddle the water oxidation and reduction potentials. Here, using a nanocrystalline-TiO2-based photoelectrochemical cell as a model system, we investigate the effect of tuning the semiconductor energy levels by adding Li+ ions to the electrolyte. The effect of LiClO4 addition on band edges, interfacial recombination resistance, electron diffusion length, and charge-separation efficiency were quantified by impedance spectroscopy and analysis of incident photon-to-current efficiency spectra. We find that the TiO2 band edges are shifted toward positive potentials by the addition of Li+, and that this increases the apparent electron diffusion length without affecting the charge-separation efficiency, most likely due to a change in the driving force for O2 reduction. These results should prove useful in the modeling and optimization of solar water splitting cells employing metal oxide photoelectrodes.

Electron Diffusion Length and Charge Separation Efficiency in Nanostructured Ternary Metal Vanadate Photoelectrodes

“…Electron-hole pair separation in the bulk and/or space-charge region of the BiVO 4 , as well as hole transfer across the semiconductor-solution interface (i.e., water oxidation), are both accounted for by η sep ; their individual efficiencies cannot be separately determined by the IPCE analysis used here. A similar analysis has been extensively applied in the field of dye-sensitized solar cells [16] and was more recently applied to TiO 2 and BiVO 4 photoanodes in PEC water splitting cells [6,17,18].…”